Field of the Invention

The present invention relates to segregating coating compositions having a relatively high or high solids content, more especially those having a solids content of at least 75% and up to 100% by weight (corresponding in many instances to a solids content of 70% by volume and over) .

Background of the Invention

Coating compositions generally comprise a solid film-forming resin, usually with one or more pigments; fillers and other additives may also be included. The resins or binders can be thermoplastic but are more usually cross-linkable, in which case the final composi- tion incorporates two co-reactive film-forming resins or incorporates a curing agent for the film-forming resin. In solvent-based coating compositions the solid ingredients are mixed with one or more solvents. Compositions having a solids content of at least 80% by volume are generally known as "high solids" systems; these generally correspond to an approximate solids content of at least 85% by weight, although if very heavy pigments are present the weight % solids figure will be higher. Usually, such compositions conform to the E.C. recommended maximum solvent level of 250g/litre. The terminology "high solids systems" is also used in the art to include 100% solids coating systems, where the composition is applied in powder or liquid form.

In solvent-based coating compositions, the solvent, pigment and other additives are mixed with the binder or resin (typically, for example, an epoxy resin), and then ground; usually the minimum particle size is 2μm.

Finally, solvent is added to achieve the reguired volume of solids for a suitable application viscosity. The material is supplied together with a curing agent, which is generally in a solvent, usually as a two pack paint system, and the two components are mixed together prior to application. Application may be, for example, by spray, brush or roller.

Powder coating compositions are generally prepared by intimately mixing the ingredients, for example in an extruder at a temperature above the softening point of the film-forming resin but below the curing temperature of the composition (the process of extrusion) , and comminuting the mixture to the desired particle size in suitable grinding equipment (the process of micronising) . The powder is then applied to the substrate by various means, for example by the use of fluid beds, for example in the case of a "coil" substrate consisting of a metal strip wound on a coil, or most commonly by electrostatic spray gun, and is cured on the substrate by the applica- tion of heat (the process of stoving) ; the powder particles melt and flow, and a film is formed.

Other high solids systems (often referred to as 100% liquids and some as 100% solids epoxies or liquid epoxies, even though they may contain small quantities of solvent) are often applied under controlled temperature conditions, e.g., using hot twin feed equipment. That is, the resin and curing agent are supplied at a tempera¬ ture of 50 to 100°C, and the first time the two com¬ ponents come into contact is only shortly before they arrive at the substrate, i.e. at the tip of the applica¬ tion gun, seconds before application to the substrate, in the case of impingement mixing or, in the case of hot twin feed, mixing at the point in the line several metres before the tip, so that mixing can occur several minutes before application. This is particularly important for rapid curing or low temperature curing systems.

Another application technique for 100% solids systems is melt extrusion, in which a film-forming composition is applied in melt or plastified form to a substrate through an extrusion coating die, and the substrate is preferably stoved to cure the composition. Other melt application techniques are also possible.

100% solids coatings systems (powder, melt application and many hot twin feed and dual impingement systems) provide a number of advantages over their solvent-based counterparts. They are essentially free of solvent (the ultimate in an ultra high solids composi¬ tion) and are therefore more environmentally acceptable; and powder especially is economical in use of ingredients, with any powder not reaching the substrate being collected and re-used.

Very high and ultra high solids solvent-based systems (solids contents > 90% and > 95% by volume, respectively) are similarly more environmentally acceptable than other solvent-based systems, but are little used at this time because of difficulties associated with these systems: problems of application and problems in achieving good flow, low temperature cure and intercoat adhesion.

Some of the main uses of powder coatings and solvent-based systems are in the architectural field, for example for window frames or architectural cladding; other uses include use in the domestic appliance market, and examples include electrical accessories and furni¬ ture. A more recent use, and one expected to grow, is the use of powder in the automotive industry. For such uses the durability and stain resistance and, in the architectural and automotive areas, the weathering performance and acid rain resistance of the coating are very important. Solvent-based systems are also used for architectural purposes where the structures cannot be assembled easily after coating, for example for ships,

bridges and oil installations, and here weatherability and corrosion resistance are extremely important. Hot twin feed application of liquid epoxies is also especially useful for such purposes, more especially for coating oil installations or ships. The use of melt extrusion techniques for coatings on packagings, for example on metal cans, should also be mentioned; stain resistance is important here too.

Excellent stain resistance can be obtained, for example, with acrylic polymers, and fluorinated acrylics especially (but also other fluorinated polymers and chlorinated and silicon-containing polymers) provide durability and weathering performance, but are expensive and do not have optimum mechanical properties; poly- esters and epoxy resins, on the other hand, are cheaper and generally have good mechanical performance, but poorer durability. Systems exhibiting the best properties of polyester and acrylic polymers (good mechanical properties and good weatherability respectively) have been sought-after for some time.

Such properties can be achieved by applying two coats separately, but preferred is the use of a single coating composition that segregates to provide an enrichment of one polymer, having suitable top-coat properties, near the air interface, and of the polymer having suitable base-coat properties near the substrate; a bilayer system, in which there is a continuous top layer over a base layer, with an enriched content of the more weatherable polymer in the layer at the air interface and the polymer with the better mechanical properties in the base layer would, of course, be especially preferred. Ideal would be a complete stratification such that the top layer is composed almost completely of the desired polymer, and with low or zero content of that polymer in the base layer, but avoiding any delamination problems which might be expected with one coat on top of another.

Although with existing polyester-acrylic powder coating systems partial segregation of one component can often be obtained, formation of a true bilayer coating from a mixed polymer system has in practice been very difficult to achieve. Segregation is also known for solvent-based systems with normal and high solids contents, but would be expected to become increasingly difficult as the solvent content is reduced, because the shorter evaporation time reduces the time during which phase segregation can occur, and the higher viscosities would also retard the segregation process.

Summary of the Invention

We have found that, surprisingly, some systems segregate much more readily than others. We have surprisingly found a very marked difference between the extent of segregation of different fluorinated polymers: polymers of similar fluorine content exhibited different segregation abilities. Many, for example, produced complete stratification over polyester, whereas others were less successful. Our findings suggest that this variation in extent of segregation is related to the structure of the fluorinated polymer, more particularly on the position of fluorination and/or identity of the end groups. Non-polar end groups appear to favour segregation.

Accordingly, the present invention provides a coating composition, comprising two or more film-forming materials and having a relatively high or high solids content, more especially of at least 75% by weight, and up to 100% by weight, wherein the film-forming materials comprise

(A) a fluorinated, chlorinated and/or silicon-containing film-forming component having one or more non-polar end groups (but which film-forming material is

preferably free of chlorine) , and (B) a suitable other film-forming component.

The term "non-polar" denotes a non-ionic species, not readily polarisable; non-polar groups have no permanent charge and a charge is not readily induced.

Our results suggest that segregation is assisted by the presence of fluorine or silicon-containing groups at the end(s) of the main chain and/or, especially, in (for example at the end of) any pendant groups; compounds with such end groups segregated readily. Limited segregation was achieved, however, with various commer¬ cially available fluorinated polymers, and we believe this may be attributable to the absence of fluorinated groups at chain ends. Such commercially available polymers are generally prepared by addition polymerisa¬ tion, for example by free radical polymerisation (using initiators) , and accordingly are likely to have non- fluorinated and probably polar end groups, and the commercial polymers tested either had no pendant groups at all or had non-fluorinated pendant groups, a high proportion of which were terminated by polar groups. The presence of chlorinated groups may also assist segregation, but to a more limited extent than fluorina¬ ted groups. Segregation should also take place when the fluorinated compound has non-fluorinated hydrocarbon groups at chain ends, although in some cases the effect of short-chain hydrocarbon groups is not as favourable as the presence of even a single terminal fluorinated group, CF ₃ . Increasing the length of the hydrocarbon groups can favour segregation; similarly, increase in length of the fluorinated or chlorinated or silicon-containing end groups should favour segregation.

Suitably therefore an end group is a fluorinated, chlorinated, hydrocarbon and/or silicon-containing end group and the component A film-former may contain the same or different end groups. A hydrocarbon end group

preferably has at least 4 carbon atoms, and preferably these are arranged in a straight chain. Alkyl groups such as, for example, n-butyl or lauryl (dodecyl) , should especially be mentioned. Thus, the present invention also provides a coating composition comprising two or more film-forming materials and having a relatively high or a high solids content, more especially of at least 75% by weight and up to 100% by weight, in which the film-forming materials comprise (A) a fluorinated, chlorinated and/or silicon-containing film-forming component having one or more fluorina¬ ted, chlorinated, silicon-containing and/or hydrocarbon end groups, a hydrocarbon end group preferably comprising at least 4 carbon atoms arranged in a straight chain, and

(B) a suitable other film-forming component.

The term "end group" is used herein to denote a carbon- and/or silicon-containing group, not only at an end of the main polymer chain, but also a pendant group or a group at the end of a pendant chain. An end group may be straight-chained or branched and may include functional groups, but it is preferred that the term as used herein in relation to pendant groups is taken to exclude any functional link to the main chain. Thus the term "end group" may denote the end portion of the main polymer chain that does not contain the repeat units of the polymer, or a pendant (side) group or chain, or a group at the end of a pendant chain excluding any functional link to the main chain, for example an ester or ether linkage.

We have also found that stratification (that is, the formation of a bilayer coating, as distinct from a coating in which segregation occurs but leads only to enrichment of the top-coat polymer at the air interface) is improved when there are certain differences in surface energy and viscosity between the two segregating

systems .

Accordingly, the present invention provides a coating composition comprising two or more film-forming materials and having a relatively high or a high solids content, more especially of at least 75% by weight, and up to 100% by weight, wherein the film-forming materials comprise

(A) a fluorinated, chlorinated and/or silicon-containing film-forming component, (B) a suitable other film-forming component, such that

(i) the surface energy of component A at the time of maximum mobility of that component is no more than that of component B, preferably at least 2 dynes/cm, especially at least 6 dynes/cm, less than that of component B, at the time of maximum mobility of that component, and (ii) the zero shear viscosity of component A at the time of maximum mobility of that component is no more than 4 Ln Pa.s, preferably no more than 3 Ln Pa.s, especially no more than 2 Ln Pa.s, more than that of component B, more especially at least 2 Ln Pa.s less than that of component

B, at the time of maximum mobility of that component. The importance of viscosity in high solids systems of the present invention contrasts to the situation in prior art segregating systems, which utilise lower solids contents, and where the convection currents set up by the evaporating solvent create turbulence and assist in the enrichment of the lower surface energy component at the surface; viscosity differentials are therefore not the major driving force. With increased solids content, however, this effect will diminish, and the system will

be too viscous to create effective convection currents. Surprisingly, however, we have achieved surface enrich¬ ment in relatively high and high solids systems, which are of significantly higher viscosity than low solids systems: typically, a 70% by volume (generally a 75% by weight) solids system will have a zero shear viscosity at room temperature in the range of from 1 to 5 Poise, an 80% by volume solids system will have a viscosity in the range of from 3 to 10 Poise, and a higher solids system will have a correspondingly higher viscosity, for example at least 5 Poise at 90% solids and above. Most 100% solids systems will also, of course, have a viscosity of at least 5 Poise at room temperature.

In our high solids systems we have found that stratification can be made to occur, provided there is an appropriate differential viscosity; this differential is, however, unimportant in lower solids systems where the effect of evaporation of the solvent is significant. Coating systems having a zero shear viscosity of at least 3, for example at least 3.7, preferably at least 4, especially at least 5, and advantageously at least 10, Poise at room temperature should especially be mentioned.

Accordingly, the present invention also provides a coating composition comprising two or more film-forming materials and having a solids content of at least 75% by weight, and up to 100% by weight, wherein the film- forming materials comprise

(A) a film-forming component comprising a fluorinated, chlorinated and/or silicon-containing polymer, or a reactive diluent therefor,

(B) a film-forming component comprising a polyester, an epoxy resin, an acrylic resin, a polyurethane or a hybrid of two or more of these polymers, or one of more reactive diluents therefor, the relative surface energies and zero shear viscosities of the components being such that, when the composition

is applied to a substrate and a film is formed, enrich¬ ment of the fluorinated, chlorinated and/or silicon- containing polymer at the air interface is obtained.

Component A polymers having one or more non-polar end groups as specified above, preferably one or more end groups containing fluorine, chlorine, silicon and/or comprising a hydrocarbon group preferably having at least 4 carbon atoms arranged in a straight chain, should especially be considered in selecting the powder and solvent-based compositions with the specified surface energies and viscosities. Random copolymers with pendant groups should especially be mentioned.

Brief Description of the Drawings

In the accompanying Figures: Figures 1 to 4 show plots of the change of zero shear viscosity with temperature of various examples of fluorinated and non-fluorinated polymers;

Figures 5 to 7 show the variation of surface tension with temperature of various examples of fluorina- ted and non-fluorinated polymers and of two illustrative interface-modifying agents of the invention; and Figures 8(a) , 8(b) and 9 to 11 show scanning electron microscopy pictures of coated substrates obtained with coating compositions comprising mixtures of fluorinated and non-fluorinated components: in Figures 8 to 10 the coatings were produced using illustrative compositions of the present invention, and bilayer systems can be clearly seen; in Figure 11, in contrast, separate portions of fluorinated component are seen at the top of and within the non-fluorinated layer.

Detailed Description of the Invention

Usually in compositions of the invention each film-

forming component is polymeric. In any of the composi¬ tions of the present invention component A, the "top coat" component, may be thermoplastic or thermosetting. Component B, forming the "base coat", generally comprises a thermosetting polymer but, alternatively, a thermoplas¬ tic polymer may be considered. Usually component B com¬ prises a polyester (and, as will be understood, the term "polyester" includes alkyd resins) , an epoxy resin, an acrylic polymer or a polyurethane or a hybrid of two or more of these polymers, or one or more reactive diluents therefor, and component A may comprise, for example, an acrylic polymer, a polyolefin, a polyester, an epoxy resin, a polyvinyl ether or a polyurethane or a hybrid of two or more of these polymers, or one or more reactive diluents therefor.

Preferably, the component A film-former contains at least 2%, especially at least 4%, e.g. at least 5%, especially at least 10%, more advantageously at least 15%, of repeat units free of polar groups and having a non-polar pendant group. Advantageously, component A film-formers have up to 70 mol % or more of repeat units having non-polar end (pendant) groups; polymers with up to 50 mol % or up to 40 mol % of such units should also especially be mentioned. Preferably, if polar groups are present, the ratio of non-polar end groups, especially of end groups containing F, Cl, Si and/or a hydrocarbon group having at least 4 straight chain carbon atoms, to polar end groups is at least 0.5:1, and the ratio is preferably more than 1:1, especially at least 2:1. Advantageously, no more than 25%, especially no more than 15%, of repeat units within the component A film-former have a polar substituent. Advantageously, at least 60%, e.g., at least 80%, and even all or substantially all end groups are fluorine-, chlorine- and/or silicon-containing and/or hydrocarbon-containing end groups.

As examples of suitable (non-polar) end groups, there should be mentioned

R (C-_2)2~ pendant groups (each linked to the main chain by a COO group) where R _F denotes the fluorinated alkyl groups in the commercially-available Zonyl monomers,

CF ₃ CH ₂ pendant groups, each linked to the main chain by a COO group,

(CH ₃ ) ₃ Si(OSi(CH ₃ ) ₂ ) n" pendant groups (n = 0 to 50) , linked to the main chain by -0-, and long chain hydrocarbon or fluorocarbon groups, e.g. decyl, dodecyl, perfluorodecyl or perfluorododecyl, at the end of the main chain or as pendant groups on the main chain. Preferably the component A film-former has a number of pendant end groups, for example a plurality of R (CH2)2 ^{or a} plurality of CF3CF2 pendant groups or a plurality of (CH ₃ ) ₃ Si (OSi(CH ₃ ) ₂ ) _n - pendant groups and a plurality of Rp(CH ₂ )2~ pendant groups. These and structurally-related end groups are of low surface energy and their presence leads to a low surface energy polymer, which appears to assist segregation. Compositions of the present invention have given good segregation, and, as demonstrated by work in house, segregation of suitable acrylic polymers over polyester or epoxy coatings enhances weathering performance.

We believe, however, that the extent of segregation is influenced, not only by surface energy considerations, but also by considerations of viscosity in selecting suitable top and base coat components. We believe it is important, in high solids systems, to utilise a top¬ coat component of relatively low viscosity and/or to utilise a base-coat component of relatively high vis¬ cosity (consistent of course with capability of film- formation) ; preferably the top-coat component should have a zero shear viscosity below that of the base-coat

component or at most up to 4 Ln Pa.s higher than that of the base-coat component, during the critical period for segregation.

It should be noted that it is the surface energy and viscosity of the entire component, and not merely that of the polymer, that is important; thus the presence of other materials, more especially pigment/filler and curing agent, in either or both components will influence these properties (although of course the applicable viscosity and surface energy measurements must be before appreciable reaction with any curing agent) . In solvent- based systems, as will be appreciated, it is the surface energies and viscosities of the polymer solutions, including any curing agent, that should be considered. (As will be appreciated, for practical purposes it is appropriate to consider the surface energy and zero shear viscosity values of the individual components prior to mixing, including any solvent and other additives present with the polymer in the component to be mixed and including any crosslinking agent for that polymer.)

Thus, for example, in powder coating compositions we have found that segregation of various fluoroacrylic resins (top-coat resins) over a base-coat resin (a polyester) was not complete when, in the critical period before film formation, the surface energy of the base- coat polymer component was less than about 2 dynes/cm more than that of the top-coat polymer component at any given temperature and/or when the zero shear viscosity of the top-coat polymer component at any given temperature was more than about 3 or 4 Ln Pa.s more than that of the base-coat polymer component; improved results were obtained when the zero shear viscosity of the top coat polymer component was less than that of the base coat polymer component in the critical period. The critical period for this purpose is related to the stoving temperature for the powder coating

composition and this is itself dependent on the par¬ ticular polymers present in the composition, more especially (when component A is thermoplastic) on the identity of the component B polymer. For acrylic and polyester systems a stoving temperature of about 200°C may be used, but for other systems a temperature in the range of 150-260°C may, for example, be selected, and stoving at lower temperatures, e.g. 120°C, should also be considered, and in general the surface energy and viscosity differences in the sixty or even up to one hundred degrees C up to the stoving temperature are important, especially in the temperature range between sixty and twenty degrees C below the stoving temperature. Although preferably the surface energy and viscosity differentials should be maintained throughout these temperature ranges, some variation from the specified differentials may not be important.

The gellation time of component A is usually at least as long as that of B. Typically, gellation in powder coating compositions occurs at 50 to 70% cure, and preferably the specified relative surface energies and viscosities should apply up to 80% cure. Systems in which the viscosity differential applies between the softening point of the base-coat resin and up to 50% cure should especially be mentioned. Alternatively, there may be considered the average viscosity and surface energy of each component from the time of softening up to gella¬ tion. In practice, we have found that segregation occurs often in the first 2 minutes after stoving begins, and while the oven is still warming up to its full temperature.

In practice the surface energies and viscosities often vary in a linear manner with temperature. Alternatively, therefore, for convenience, the selection of the preferred component A and component B components may be made having regard to the surface energy and

viscosity differences at a single temperature or over a limited temperature range, defined either in relation to the temperature of stoving, e.g. at twenty degrees C below the stoving temperature, or defined in absolute terms, e.g. at 140°C, 150°C, 160°C or 170°C, or at a temperature in the range of from 120 to 150°C, which is an important temperature range in relation to segregation for most, if not all, powder coating systems; more especially the differentials are maintained at all temperatures in that range.

We found that where the component B was an unpigmented polyester resin, used with a conventional curing agent, and component A had a surface energy > 45 dynes/cm at 120°C, > 43 dynes/cm at 130°C, > 41 dynes/cm at 140°C, > 39 dynes/cm at 160°C, > 37 dynes/cm at 180°C,

> 35 dynes/cm at 200°C, and/or a zero shear viscosity

> 11.5 Ln Pa.s at 120°C, > 11 Ln Pa.s at 130°C, > 10.5

Ln Pa.s at 140°C, > 10 Ln Pa.s at 160°C, > 9.5 Ln Pa.s at 180°C, > 9 Ln Pa.s at 200°C complete stratification was not achieved.

Preferably, when component B is an unpigmented polyester resin, the surface energy of component A is < 19 dynes/cm at 120°C, < 16 dynes/cm at 140°C, < 14 dynes/cm at 180°C, < 13 dynes/cm at 160°C, < 11 dynes/cm at 200°C; and/or the zero shear viscosity is < 9.5

Ln Pa.s at 120°C, < 9 Ln Pa.s at 140°C, < 8.5 Ln Pa.s at 160°C, < 8 Ln Pa.s at 180°C, < 7.5 Ln Pa.s at 200°C. The use of a polyester component with a top-coat component having a zero shear viscosity at 200°C of Ln -2 to 0 or, for example, 0.1 to 1 at 200°C or 150°C, should especially be mentioned.

Accordingly, the present invention also provides a coating composition, especially a powder coating composi¬ tion, comprising (A) a fluorinated, chlorinated and/or silicon-containing acrylic or other film-forming component having a

surface energy < 25 dynes/cm at 140°C, and a zero shear viscosity of < 7 Ln Pa.s at 140°C, and preferably less than 2 Ln Pa.s at 200°C, and (B) a polyester or other film-forming component having a surface energy no less than 28 dynes/cm at 140°C and a zero shear viscosity of no more than 8.5 Ln Pa.s at 140°C.

Preferably, the surface energy of component A is at least 5 dynes/cm less than that of component B at 120°C, at least 4 dynes/cm less than that of component B at

140°C, at least 3 dynes/cm less than that of component B at 160°C, at least 2 dynes/cm less than that of component B at 180°C, and at least 2 dynes/cm less than that of component B at 200°C. Preferably, the zero shear viscosity of component A is no more than 3.5 Ln Pa.s more than that of component B at 120°C, no more than 3.2 Ln Pa.s more than that of component B at 140°C, no more than 3.0 Ln Pa.s more than that of component B at 160°C, no more than 2.8 Ln Pa.s more than that of component B at 180°C, and no more than 2.6 Ln Pa.s more than that of component B at 200°C.

With different component B polymers and/or with changes in curing agent and, more especially, in pigment content, the surface energy and viscosity of component B will vary, and the surface energy and viscosity of component A should be varied correspondingly. Thus, for example, where the component B polymer is an epoxy resin or a polyester-epoxy hybrid resin, the surface energy of component A may be, respectively, ten or five dynes/cm higher than where the component B polymer is a polyester, and where the base coat is an acrylic-polyester hybrid or an aerylic-epoxy hybrid, the surface energy of component A should, in general, desirably be at least five dynes/cm less than the corresponding figure when the base coat is a polyester.

In solvent-based systems the viscosity and/or

surface energy differences are especially important in the early stages of film formation; for example, for a 75% solids system, the differences should apply at 75% solids, and preferably up to 90% solids. More especially, however, the differentials are maintained for the entire period of evaporation of the solvent. Thus, preferably, the differences should apply at the solids content of the individual components when applied and up to the solids content achieved in the final coating (substantially 99%) .

In high solids controlled temperature application systems (dual impingement, hot twin feed) differentials are preferably maintained from the time of application to gellation. In melt application systems, viscosity and/or surface energy differentials should preferably be maintained between the softening points and gellation.

As in a powder system, however, some slight varia¬ tion is not of significance and, although it is preferred that the differentials apply up to 80% cure, selection of the components may be made by reference to average differences in surface energy and/or viscosity or differences at a particular point or over a limited time during the period of film formation, for example at a single temperature or limited temperature range defined in relation to the temperature of application (where this is other than ambient temperature) /at a particular solids content or range of solids contents for a solvent- containing system. For a melt application system, where stoving is often, for example, at 200°C, the differen¬ tials may be judged, for example, at a temperature in the range 90 to 180°C, e.g. , at 140°C or 150°C, or over the limited temperature range 120 to 150°C. For a controlled temperature application system, for example, the dif- ferentials may be judged, for example, at an individual temperature in the range of -5 to 85°C, (for a system

applied by hot twin feed, for example, at a temperature in the range of 30 to 85°C) , or at a particular tempera¬ ture in the range of from two to fifty-five degrees C above the softening point of both components; a tempera- ture of 70°C is a suitable reference point. For a solvent- based system, the differentials may be judged at the solids content of the components to be applied, and for this system, especially, a single reference point is most convenient, and assessment is suitably of the component (to be added to the system) at room temperature, i.e., 20°C.

Thus, the present invention also provides a coating composition, comprising two or more polymeric film- forming materials and having a relatively high or high solids content, more especially of at least 75% by weight, and up to 100% by weight, wherein the film- forming materials comprise

(A) a fluorinated, chlorinated and/or silicon-containing film-forming component having one or more end groups containing fluorine, chlorine, silicon and/or a hydrocarbon group, and

(B) a suitable other film-forming component, such that, measured for each component at the time of its maximum mobility in the composition when applied to the substrate (and preferably, in the case of a powder or melt application system, at all temperatures between the softening and gellation; in the case of a controlled temperature application system, at all temperatures in the range two to one hundred degrees C above the soften¬ ing of both components; in the case of a solvent-based system, at the solids content of the components applied and if the solids content of the system is <90% by weight, up to at least 90% solids content) then:

(i) the surface energy of component A is no more than, preferably at least 2 dynes/cm, preferably at least 6 dynes/cm, less than,

that of component B, and (ii) the zero shear viscosity of component A is no more than 4 Ln Pa.s, for example no more than 3 Ln Pa.s, preferably no more than 2 Ln Pa.s, more than that of component B, preferably at least 2 Ln Pa.s less than that of component B. Although the use of both surface energy and viscosity differentials as specified may be important to assist the formation of bilayer films, the use of combinations of components that fulfil neither or only one of these requirements may also be of value.

Compositions in which condition (ii) applies should especially be mentioned. As has been mentioned, in a thermosetting component, as will be readily appreciated, the polymer should reach its maximum potential mobility before appreciable reaction with the curing agent, and generally before 10% of reaction has occurred. Thus, each of the components softens before the curing of either, and remains mobile before any appreciable curing of either. It is important too that the times for each component to reach maximum mobility are not too dissimilar. These times may differ by a factor" of 10, but advantageously by a factor of 5 or less, and preferably by a factor of 2 or less. For example, if the base-coat reaches its maximum mobility after time t, the top-coat should preferably reach its maximum mobility at a time in the range of from 0.5 to 2t. As will be understood, the curing times also should be comparable; for example one may be up to 20% more than the other. Segregation, however, is often complete early, before cross-linking begins.

Preferably the surface energy of component A is at least 3, preferably at least 4, especially at least 6, advantageously at least 7, more especially at least 10 or at least 15, dynes/cm less than that of component B.

Preferably, the zero shear viscosity of component A is no more than 3, preferably no more than 2, Ln Pa.s more than that of component B, and especially the viscosity is no more than, for example at least 1 Ln Pa.s, advantageously at least 2, more especially at least 3, Ln Pa.s, less than, that of component B.

As will be understood in the art, the surface energy and zero shear viscosity of each component can be manipulated, not only by choice of polymer, but by other conventional means, for example by the choice of or adjustment of the amount of pigment, filler, curing agent or solvent, where appropriate, and/or, in the case of the base-coat component, by addition of a thixotropic agent. Top-coat components having a zero shear viscosity up to 3 Ln Pa.s, for example in the range of from -1 to 3, e.g. 0.5 to 3, Ln Pa.s at the point of application, and base-coat components having a zero shear viscosity of at least -1.6 Ln Pa.s, e.g. for solvent-containing systems up to -0.35 Ln Pa.s, at the point of application should especially be mentioned. In powder systems in practice, the use of a base-coat component having a zero shear viscosity of at least Ln 0.65 Pa.s at the point of application may ensure a suitable differential viscosity and hence good segregation. As an alternative or in addition to considering the difference in surface energy between component A and component B and/or the difference in zero shear viscosity of the two components when selecting components, the surface energy of the end groups of component A may also be considered. Our results suggest that segregation may be assisted by the presence of low surface energy end groups; low surface energy groups are generally of low polarity.

The Zonyl monomer, for example, contains the long fluorinated chain end (pendant) group, predominantly consisting of - (CH ₂ ) ₂ ( ^F 2) 5 ^F 3 groups having a relatively

low surface energy.

We have found that the presence of Zonyl end groups is especially advantageous even when present in relatively low proportions. Thus, with a low surface energy end group, especially a pendant group, less of the fluorinated polymer may be required to achieve the same degree of segregation/stratification. For example, we have found that, surprisingly, a 80/20 (w/w) copolymer of MMA and Zonyl, where there is one Zonyl monomer unit to every 22 MMA monomer units, nevertheless can give better segregation than 100% PTFEMA. Low surface energy end groups, especially pendant end groups, may therefore be highly efficient with respect to interfacial segregation. The surface energy of the end groups may be calcu- lated by the group contribution method (see D.W. Van Krevelen, Properties of Polymers, Their Estimation and Correlation with Chemical Structure, Elsevier, 1976, 2nd edition)) . Such calculations show that fluorinated, chlorinated and silicon-containing end groups have a lower surface energy than hydrocarbon end groups of corresponding length (and that all such groups have a lower surface energy than polar end groups) and that increasing the length of the fluorinated, chlorinated or silicon-containing groups decreases their surface energy.

The group contribution method may also be used to calculate the surface energy of the polymer backbone and of the polymer itself, and differences between the surface energy of the end group and the surface energy of the polymer may be calculated.

Accordingly, the present invention also provides a coating composition, comprising two or more film-forming materials and having a relatively high or high solids content, more especially of at least 75% by weight and up to 100% by weight, wherein the film-forming materials comprise

(A) a fluorinated, chlorinated and/or silicon-containing film-forming component, the component (A) polymer having one or more end groups having a surface energy less, preferably at least 2 dynes/cm less, than that of the polymer backbone, and

(B) a suitable other film-forming component. Advantageously, the surface energy of the end groups is at least 2 dynes/cm, for example at least 4 dynes/cm, preferably at least 5 dynes/cm, especially at least 10 dynes/cm, less than the surface energy of the polymer backbone units, the backbone unit being considered as the repeating unit minus the end group. With a surface energy less than that of the backbone, it follows that such end groups have a surface energy less than the polymer.

There should also be mentioned polymers in which the surface energy of the component A polymer end group is at least 2 dynes/cm, preferably at least 5 dynes/cm, especially at least 10 dynes/cm, less than the surface energy of the polymer itself.

The end groups are preferably the non-polar end groups mentioned above, preferably fluorinated, chlorinated and/or silicon-containing end groups, or hydrocarbon end groups, preferably having at least 4 carbon atoms arranged in a straight chain, and are especially pendant groups. Such pendant groups bring about a reduction in surface energy and viscosity of the component. Preferably, also, the surface energy and/or zero shear viscosity differentials mentioned above should also apply.

In a copolymer, account must be taken of the proportion of such end groups in the polymer and/or of other polymer units.

Preferably the average surface energy of all the end groups containing fluorine, chlorine and/or silicon or comprising a hydrocarbon group containing at least 4

carbon atoms arranged in a straight chain is less than the surface energy of the polymer backbone and the polymer as a whole, advantageously less by the amounts mentioned above. Preferably, the average surface energy of all end groups is no more than that of n-butyl, i.e. 34.3 dynes/cm; advantageously each end group has a surface energy no more than 34.3 dynes/cm.

Preferably, the specified end groups, whether specified as being non-polar end groups, or as end groups containing fluorine, chlorine, silicon and/or hydrocarbon groups preferably having a C ₄ or longer chain, or speci¬ fied as having a surface energy or on an average surface energy less than the polymer itself, constitute at least 2 weight %, advantageously at least 4 weight %, especially at least 12 weight %, more especially at least 30 weight %, of the polymer.

Alternatively, or in addition, a copolymer may advantageously contain at least 2 mol %, for example at least 2.5 mol %, advantageously at least 5 mol %, preferably at least 10 mol %, especially at least 25 mol %, e.g. at least 50 mol %, more especially at least 75 mol %, of the monomer containing the specified end groups.

As the length of the pendant group increases, the effect of these groups will be shown with a lower proportion of such groups. Preferably, the F, Cl, Si or hydrogen content of the total of the various end groups specified is, respectively, at least 4, 8, 4 or 7% by weight of the polymer. Preferably, the fluorine content of all pendant groups together is more than the fluorine content of the backbone, the chlorine content of all pendant groups together is more than that of the backbone, and/or the silicon content of all pendant groups together is more than that of the backbone.

Advantageously, the overall ratio of F:C, C1:C, Si:C

and/or H:C in all the variously specified end groups is greater than the corresponding ratio in the rest of the molecule.

Advantageously, end groups have, on average, an F:C ratio of at least 1.5:1 by weight, especially at least 2:1 by weight, or a Cl:C ratio of at least 1.5:1 by weight, especially at least 2:1 by weight, or a Si:C ratio of at least 0.5:1 by weight, especially at least 1:1 by weight, or a H:C ratio of at least 2:1 by weight, especially at least 2.25:1 by weight.

In an end group, for example, the fluorine content is advantageously at least 5%, preferably at least 25%, e.g. at least 50%, by weight; the chlorine content is advantageously at least 10%, preferably at least 20%, e.g. at least 50%, by weight; and/or the silicon content is advantageously at least 5%, preferably at least 10%, e.g. at least 25%, by weight; or (where there is no F, Cl or Si in the end group) the hydrogen content is advantageously at least 8%, preferably at least 12%, e.g. at least 15%, by weight.

Advantageously, for cost reasons, the fluorine content of the component A film-former is no more than 60 weight %, preferably no more than 50 weight %, especially no more than 40 weight %. F contents <30%, <25% and <20% should especially be mentioned. The presence of fluorine (or silicon) in pendant groups as specified herein represents a very efficient utilisation of the fluorine (or silicon) .

Preferably, however, the film-former contains at least 5 weight %, advantageously at least 7.5 weight %, especially at least 10 weight % or at least 15 weight %, of fluorine.

Similarly, the silicon content of the component A film-former is advantageously no more than 35 weight %, preferably no more than 25 weight %, especially no more than 15 weight %. Silicon contents <36%, <18% and <9%

should especially be mentioned. Preferably, however, the film-former contains at least 5 weight %, advantageously at least 10 weight %, especially at least 20 weight %, of silicon. Likewise, the chlorine content of the component A film-former is advantageously no more than 20 weight %, preferably no more than 10 weight %, especially no more than 5 weight %. Chlorine contents <18%, <16% and <8% should especially be mentioned. Preferably, however, the film-former contains at least 5 weight %, advantageously at least 10 weight %, especially at least 20 weight %, of chlorine.

For environmental reasons, chlorinated compounds are least preferred, and compositions having no chlorination should especially be mentioned. Preferably, the film- former of component A is fluorinated and/or contains silicon.

A fluorinated or chlorinated polymer (component A) may be fully or partially fluorinated or chlorinated; the component may alternatively contain both chlorine and fluorine atoms, being fully or partially halogenated. Suitable fluorinated and/or chlorinated monomers for component A are, for example, fluorinated acrylic monomers; fluoro-olefin monomers, for example vinylidene difluoride; fluorinated and/or chlorinated aliphatic esters; and fluorine-modified epoxy resin monomers. Suitable siliconised polymers are, for example, silicon-modified polyesters, silicon-modified acrylic polymers and silicon-modi ied epoxy resins. The polymer may, if desired, be a copolymer based on one or more of the above. Random or statistical (including 'blocky') copolymers are especially preferred, although block copolymers should also be mentioned. Preferably a copolymer has a content of at least 2 %, more especially at least 10 %, by weight fluorine, chlorine and/or silicon.

The composition may, if desired, include more than one such component A polymer. These may, for example, be mutually compatible or incompatible but co-extruded and partially reacted. Suitable end groups for component (A) are, for example,

CF ₃ , (CHF) _n CF ₃ where n = 1 to 100

CHF ₂ (CH ₂ ) _n CF ₃ where n = 1 to 100

CH ₂ F ( ^CF 2)n ^CF ₃ where n = 1 to 100 Si(CH ₂ CF ₃ ) ₃ (CHF) _n CF ₃ where n = 1 to 100 SiH ₃ (CH ) _n CH ₃ where n = 5 to 100

Si(CH ₃ ) ₃ Si(CH ₃ ) ₂ -(0-Si(CH ₃ ) ₂ ) _n where n = 4 to 100 (preferably n = up to 50, especially up to 25, for example 9) . Pendant end groups comprising a perfluorohydrocarbon group or a long chain hydrocarbon radical or a perfluoro- ether or perfluoropolyether or siloxane chain should especially be mentioned.

Preferably component A has Rp(CH ₂ ) ₂ pendant end groups where Rp denotes perfluorinated alkyl of at least 4, preferably at least 6, e.g. on average at least 8, carbon atoms, derived for example from an aerylate-type monomer, e.g. from a methacrylate, advantageously in an amount of 3 mol%. There should also be mentioned, in particular, end groups CF ₃ (CF ₂ )y(CH ₂ ) _x where x = 0-10, y = 0-20, and usually x is a maximum of 2 or 3 if y f o.

Such groups may be present at one or, preferably, both ends of the main chain and/or, more especially, at ends of any chain branches.

Preferred end groups are those having group contri¬ bution parameters comparable to that of the base-coat component; long pendant groups are preferred, for example those having at least 4 Si atoms or at least 4 F- substituted carbon atoms.

A suitable acrylic monomer is, for example,

trifluoroethyl ethacrylate (TFEMA) ; in the resulting polymer the chain has the structure

-C

COOCH ₂ CF ₃ n

Other commercially available monomers of similar structure are the mixed Du Pont Zonyl monomers (Zonyl is a Trade Mark) of structure CH ₂ =C(R) -COO(CH ₂ ) ₂ Rp and in the resulting polymer the chain has the structure

where R represents methyl, and

Rp represents (C ₄ -C ₂ o)perfluorinated alkyl. Others that should be mentioned are various fluorinated esters of unsaturated carboxylic acids available from

Hoechst: acrylic acid 2 , 2, 3 , 3-tetrafluoropropylester

(CH ₂ =CH-COO-CH ₂ CF ₂ CF ₂ H) , acrylic acid hexafluoroisopropylester

(CH ₂ =CH-COO-CH(CF ₃ ) ₂ ) , acrylic acid 2 , 2, 3 , 4 , 4 , 4-hexafluorobutylester

(CH ₂ =CH-COO-CH ₂ CF ₂ CFHCF ₃ ) , methacrylic acid 2 , 2 , 3 , 3-tetrafluoropropylester (CH ₂ =C(CH ₃ )-COO-CH ₂ CF ₂ CF ₂ H) , methacrylic acid hexafluoroisopropylester

(CH ₂ =C(CH ₃ )-COO-CH(CF ₃ ) ₂ ) , methacrylic acid 2 , 2 , 3 , , 4 , 4-hexafluorobutylester

(CH ₂ =C(CH ₃ )-COO-CH ₂ CF ₂ CFHCF ₃ ) , the fluoromonomer Fluorad FX-189 (Fluorad is a Trade

Mark) , available from 3M company, of formula CF ₃ (CF ₂ )4 ^_ 0-

CH ₂ CH ₂ 0-CO-C(CH ₃ )=CH2, the perfluoropolyether monomers, available from

Ausimont :

CH ₂ =C(CH ₃ ) -COO-CH ₂ CF ₂ 0- (CF ₂ CF ₂ 0) ₂ -CF ₂ CF ₃ , CH ₂ =C(CH ₃ ) -COO-CH ₂ CH2θ-CH ₂ CF2-(OCF ₂ CF ₂ ) _n -OCF ₂ CF ₃ , where n = 3 or 4 , and CF ₃ 0-(CF ₂ CF(CF ₃ )0) _m - (CF ₂ 0) _n -CF ₂ CH ₂ -0-COC(CH ₃ )=CH ₂ , having a molecular weight 719 or 538 (and known by the trade name Galden Methacrylate) and the ethoxylated analogue thereof (known by the trade name Galden TX Methacrylate) (Galden Methacrylate and Galden TX Methacrylate are Trade Marks) . The silicon monomer CH ₂ =C(CH ₃ )-C00-(CH ₂ ) ₃ -(0Si(CH ₃ ) ₂ ) _n -Si(CH ₃ ) ₃ available from Wacker-Chemie under the number SLM455127 with n = 10-20 (mean 15) should also be mentioned. Suitable functional monomers which may be included in a copolymer include glycidyl methacrylate and 2- hydroxyethyl methacrylate and the functional polyfluoro- polyethers available from Ausimont, for example HOOC-(CF ₂ CF ₂ 0) ₉ -C00H. Suitable copolymers may be, for example, based on Zonyl, TFEMA or the above silicone monomer, or on one of the above Hoechst, 3M or Ausimont monomers with, for example, one or more of methyl methacrylate, butyl methacrylate, butyl aerylate, 2-hydroxyethyl meth- acrylate, glycidyl methacrylate, methacrylic acid, acrylic acid or aldehyde-containing monomers (e.g. acrolein) , for example copolymers of Zonyl monomer and methyl methacrylate, Zonyl monomer and one of the above Ausimont monomers, or a copolymer of TFEMA and MMA, BA or BMA, or copolymers of different fluorinated/silicone monomers (and optional other monomer) , e.g. Zonyl plus TFEMA or Zonyl plus a Hoechst monomer, or Zonyl, the above silicone monomer and MMA.

Advantageously there are at least 8 and no more than 25 atoms between pendant groups. Such structures are typically random polymers, which are preferred.

Non-crystalline, or amorphous, structures (not block copolymers) are especially to be mentioned.

Component A film-formers having at least 11 con¬ tinuous atoms (excluding F or Cl or any F- or Cl-sub- stituted carbon atom or any silicon atom) separating

F-/Cl-substituted atoms and/or Si-containing groups in or comprising end groups should especially be mentioned.

The present invention further provides a coating composition comprising two or more polymeric film-forming materials and having a relatively high or high solids content, more especially of at least 75% by weight and up to 100% by weight, wherein the film-forming materials comprise

(A) a film-forming polymeric component in which the film-forming polymer is a polymer of one or more fluorinated monomers selected from trifluoroethyl methacrylate, the monomer of formula CH ₂ =C(CH ₃ )- C00(CH ₂ ) ₂ Rf, where Rf denotes straight chain perfluoroalkyl having at least 6 carbon atoms and the monomer CH ₂ =C(CH ₃ ) C00CH ₂ CH ₂ 0(CF ₂ ) ₄ CF ₃ , and

(B) a suitable other polymeric film-forming component. The component A polymer may also be a polymer of a suitable top-coat monomer having a moiety providing compatibility with component B; for example it may contain units B' mentioned below. For example, Zonyl or TFEMA may be copolymerised with polyethylene glycol methacrylate; a copolymer may, for example, contain

C00-(CH ₂ -CH ₂ -0) _n R' units where R' = H or methyl, in addition to polyZonyl or PTFEMA units.

Suitably, the polymer contains moieties which either through specific interactions, such as hydrogen bonding, or through entanglement interacts with the base-coat

polymer .

Random copolymers of a monomer of the formula CH ₂ =C(CH ₃ ) -C00(CH ₂ ) ₂ R _f and one or more comonomers selected from methyl methacrylate, butyl methacrylate, butyl acrylate, glycidyl methacrylate, 2-hydroxyethyl methacrylate, acrylic acid, or one or more other comono¬ mers including at least one functional monomer should especially be mentioned. Such copolymers per se are also included in the present invention. Generally, component (B) in powder or solvent-based systems will be a polyester, epoxy, polyester-epoxy hybrid, polyurethane or acrylic polymeric film-forming component, or a hybrid system: in powder systems, usually a polyester, acrylic, epoxy, polyester-epoxy hybrid or polyester-acrylic hybrid, and in solvent-based systems usually an epoxy resin, or, alternatively, in such a system an alkyd resin may be used; in melt application systems a polyester is most common, in impingement mixing systems, a polyurethane is most common, and in hot twin feed systems, epoxy resins are usually used.

Suitably, component B polymers have molecular weights Mn at leat 300, for example, at least 2,000 and suitably up to 15,000, for example up to 7,000, prefer- ably in the range 2000 to 7,000, for example 5000.

Examples for solvent-based systems include the epoxy resins Epikote 828, Epikote 1001 and Epikote 1004 (available from Shell Chemicals) and blends thereof; Epikote is a Trade Mark. For reasons of economy this component B is generally non-fluorinated, non-chlorinated and non-silicon-contain- ing. However, the use of such polymers for component (B) is not excluded, provided the components (A) and (B) form a segregating system. Suitably such systems may be achieved when component (B) has no fluorinated, chlorina¬ ted or silicon-containing or hydrocarbon end groups

and/or when (B) is a different chemistry from (A) . For example, component (A) may be a fluorinated acrylic component and component (B) a fluorinated polyester component. The chemistries may, however, be the same, if component (A) has fluorinated, chlorinated, hydrocarbon and/or silicon-containing end groups, and component (B) does not.

The composition may, if desired, include more than one such component B polymer; these will remain as one layer provided they are mutually compatible. Alterna¬ tively, two mutually incompatible polymers, for example a polyester and an acrylic polymer, may be used; if co- extruded and partially reacted with one another, these would not then stratify. Such polymers are incompatible with component A polymers, and in the absence of an interface-modifying agent, the component A and component B polymers cannot be blended or mixed to form a single (stable) phase. Such systems have such different surface tensions that gross defects are caused when one resin "contaminates" the other.

We have also found that results in some mixed systems can be improved by using a third component to control the interfacial energy at the bilayer interface. With this component it is possible to produce a bilayer system with uniform thickness of the top layer; such a possibility has not been previously disclosed.

Accordingly, the present invention provides a coating composition, having a relatively high or high solids content, more especially of at least 75% by weight and up to 100% by weight, and comprising two film-forming components of different UV durability and surface energies, and an interface-modifying agent having a surface energy between the surface energies of the other two components, and comprising, for example, one or more moieties providing compatibility with component A and one

or more moieties providing compatibility with component B.

With the different systems of the present invention, with and without the third component C, we have obtained bilayer coatings, that is, coatings having a "top" or

"surface" layer (that is, at the air interface) , at least 95% of which is composed of component A. For example, we have obtained coatings in which the top layer is at least 4.7 μm ± 0.5 μm up to 40 μm ± 4 μm uniformity as deter- mined by transmission electron spectroscopy (TEM) and optical microscopy. For the first time very accurate control of the thickness of the top layer becomes possible.

For example, one system investigated was a poly- ester/triglycidyl isocyanurate/fluorinated acrylic blend, the fluorinated acrylic material being composed mainly of poly-trifluoroethyl methacrylate ("PTFEMA") (having side-chains terminated in (CH ₂ ) _x (CF ₂ ) _y CF ₃ groups where x = 1 and y = 0) ; the system was tried with and without an interface-modifying agent, and with dry- blending of the components. We used X-ray photoelectron spectroscopy and attenuated total reflection Fourier transform infra-red spectroscopy (ATR-FTIR) as analytical techniques to investigate surface and bulk compositions of powder coatings produced from the mixed system. TEM micrograph sections were also taken to confirm the structure (i.e. the thickness and uniformity of the top coat) of the bilayer system.

Initial investigations were carried out on clear panels consisting solely of resins and cross-linkers, with no other additives present. By analysing the surface and then the mechanically-scraped surface of the resulting coating and comparing these results with a pure polyester and a pure fluorinated acrylic polymer, information was obtained on the behaviour of the two components during cure. The results showed very

efficient segregation of the fluorinated acrylic component to the polymer/air interface, with a concentra¬ tion of polyester in the bulk of the film. The use of the interface-modifying agent, however, gave improved surface topography and more uniform thickness of the top layer.

This work was then followed by probing the surface/ bulk compositions of pigmented samples (that is, having the composition of the above clears but with the inclusion of 35% of Ti0 ₂ in component B) . The results were the same as in the clears.

We produced an efficient segregation of fluoropolymer and polyester which resulted in an upper layer, several microns thick, composed almost entirely (>98%) of the fluoropolymer. Preferably in any segregat¬ ing system such layer is at least 4.5 μm, more especially at least 5 μm, and advantageously 10 to 15μm thick. Advantageously, also, in the top 10 μm, preferably the top 20μm, at least 50% is composed of component A. Investigations of the surface of segregating systems showed the coatings produced with the use of the interface-modifying agent (component C) gave a constant thickness of top coat at different points along the coating. Without the interface-modifying agent the thickness can vary in some cases depending on the structure of the top-coat polymer.

An interface-modifying agent (component C) prefer¬ ably has a surface energy of at least 15 dynes/cm and preferably up to 60 dynes/cm, advantageously at least 18 dynes/cm, and advantageously up to 50 dynes/cm, especially 20 to 40 dynes/cm, more especially 20 to 30 dynes/cm, at 130°C (powder systems) or at ambient temperature (e.g. 20°C) (in a solvent-based system measured at 80% solids) or, more generally, under the condition of application, which is, for example, at a temperature in the range of from -5 to 85°C (for a hot

twin feed system) or from -5 to 120°C for an impingement mixing system or from room temperature to 200°C for a melt application system. The surface energy of the interface-modifying agent should remain intermediate between those of components A and B during the gellation period. As described above, however, the surface energy difference requirement may conveniently be applied at only a single solids content or a single temperature or over a limited temperature range. Surface energy may be determined, for example, by Dunuoy ring or Wilhelmy plate (see Physical Chemistry of surfaces 4th Edition by Arthur W. Adamson and published by John Wiley & Sons) . The group contribution method mentioned above may also be used to calculate surface energy, but it does not enable the surface energy to be calculated as a function of temperature. If, however, the interface modifying agent has a surface energy inter¬ mediate between the surface energies of A and B at room temperature, our findings suggest that it generally remains intermediate at the higher temperature used for stoving of powder systems. Thus, for example, in all systems component A may have a surface energy in the range of from 6 to 35, for example 15 to 20, dynes/cm (at 130°C) , component B may have a surface energy in the range of from 30 to 80, for example 30 to 60, dynes/cm

(at 130°C) and component C for example 30 to 40 dynes/cm at ambient temperature or 10 to 20 dynes/cm at 130°C.

In powder and melt application systems, the interface-modifying agent should generally be mobile between the softening point of the polymers and gellation; in solvent-based and controlled temperature, e.g. hot twin feed, systems the requirement is for mobility at the time of application and, in the case of solvent-based systems, preferably up to 90% solids, e.g. up to 100% solids. The viscosity of the interface- modifying agent should preferably be less than that of

components A and B; a zero shear viscosity of, for example, up to 130 poise, advantageously 0.1 to 130 poise or -2.3 to 4.8 Ln Pa.s should be mentioned.

The interface-modifying agent may comprise one or more moieties generally compatible with component B and one or more moieties generally compatible with component A or less compatible with component B than with A; alternatively, it may comprise one or more moieties generally compatible with component A and one or more moieties less compatible with component A than with component B.

Thus, suitable interface-modifying agents include those compatible with components A and B prior to, and during, cross-linking, (that is, when mixed they give a generally clear, non-hazy, appearance) , but the invention is not limited to the use of such agents. Compatibility of different species may be defined, for example, as the co-existence of the species as, for example, in the common situation where a compatibiliser (such as a block copolymer) is added to a mixture of two incompatible polymers: a 'compatible' blend is formed; this is usually a dispersion of one of the polymers in the other, with the compatibiliser stabilising the system. In the process of the present invention, the species may be, but are not necessarily, miscible (i.e. mixing at a molecular level) .

Compatibility may be tested, for example, by optical or thermal methods. A common method of determining whether two polymers are compatible at a molecular level is Differential Scanning Calorimetry, DSC; generally, if the materials are compatible, only one glass or melting transition would be observed. Microscopy, more especially transmission electron microscopy, is another technique that is often used. Compatibility may come about, for example, by van der Waals forces, by covalent bonding or polar

interaction.

The interface-modifying agent may, for example, be oligomeric or polymeric.

It is often convenient to use an interface-modifying agent with moieties of, or chemically related to, a film- former in each of the components A and B; ^' generally medium- to long-chain moieties are present and/or the agent is an oligomer or polymer or contains one or more oligomeric or polymeric sections. For example, it may comprise one or more sections A' chemically related to a component A film-former, for example to the particular film-former used in the composition, and one or more sections B' chemically related to a component B film- former, for example to the particular film-former used in the composition. A block copolymer of component A and component B monomers should also be mentioned.

Although not wishing to be limited by theory, the applicants believe that the interface-modifying agent acts as an organic pin, becoming positioned during curing between the segregating layers, the or each A' section preferably positioning itself in the layer of component A and the or each B' section preferably positioning itself in the layer of component B.

An A' moiety may be, for example, a halogenated moiety (being fluorinated and/or chlorinated) and/or a silicon-containing moiety, irrespective of whether com¬ ponent A is itself halogenated or siliconised, and in the case of a halogenated component A and a halogenated moiety A' the identity of the halogen(s) in A and A' may be the same or different; a halogenated moiety A' may be partially or fully halogenated. An A ¹ moiety may also be a hydrocarbon chain, though this is less preferable, and two or more different A' moieties may be present in the interface-modifying agent. In some instances it may be convenient to use the same compound in the preparation of polymer A and of the interface-modifying agent.

A B' moiety may also be derived from a compound suitable for preparing component B or may be a modified form of this; it may be convenient to use the same com¬ pound(s) in preparing polymer B and the interface- modifying agent. For example, B' may denote a polyester, polyether, polyacrylic or polyglycidyl ether chain; two or more different B' moieties may be present in the interface-modifying agent, and ester and ether groups may both be present. A polymeric interface-modifying agent may be thermo¬ setting or thermoplastic.

It is also possible to use other (non-polymeric) substances having the specified surface energy and that may react with component A and/or component B. The use of the fluorinated surfactants available commercially from 3M, CF ₃ (CF ₂ ) _n -CH ₂ C00(CH ₂ ) _m Y, where n = 6 to 14, m = 5 to 20 and Y represents an amine or amide or glycidyl group, as interface-modifying agent, especially with an epoxy base coat, should be mentioned. The use of an interface modifying agent can improve uniformity in the segregation at different points along the coating surface and may prevent potential delamina- tion problems.

We have found also that those products that are suitable as interface-modifying agents that also have one or more fluorinated, chlorinated and/or silicon-contain¬ ing end groups can be used with other polymeric film- forming components to produce segregated coatings of very uniform thickness, even in the absence of a third component. In one embodiment, for example, an interface-modi ying agent may be copolymerised with a suitable base-coat monomer or, preferably, with a suitable top-coat monomer and used, respectively, as a base-coat or top-coat polymer. The interface-modifying agent may, for example, comprise

one or more sections comprising a fluorinated, chlorinated, hydrocarbon and/or siliconised aliphatic moiety, usually saturated or, if desired, containing unsaturation, for example (CF ₂ ) _n units where n represents a number from 1 to 40, e.g. at least 4 or 5, and one or more sections having polyester or polyether moieties, for example 1 to 10, e.g. at least 4, oligomeric units.

Examples of suitable A' units are as follows:

(CF ₂ -CH ₂ )m- ^• (CH ₂ -CH ₂ ) _m - (CF ₂ -CF ₂ ),-- •(CF ₂ CF ₂ -0-CH ₂ -CH ₂ ) _ιn - ^■ Si(CH ₃ ) ₂ 0-

(n = 2 to 20, e.g. 8)

-CH.-CR- where R is H or methyl, and

C00CF ₃ m

where m is, for example, from 1 to 20. Examples of suitable polyester units B' are as follows:

n

where n is, for example, from 1 to 20, preferably 3 to 7

Examples of suitable polyether units B ¹ are as follows:

where n is, for example, from 1 to 20, preferably 3 to 7

Examples of suitable combinations are as follows:

A fluorinated acrylic polymer B polyester C - (CF ₂ -CF ₂ ) _m - (m = 1 to 20) and polyester units;

2. A silicon-modified epoxy resin B epoxy

- (CF ₂ -CH ₂ ) _m - (m = 1 to 20) and polyglycidyl ether units;

3. A silicon-modified polyester B polyester

C -( ^CF 2~ ^CF 2)m~ (m = 1 to 20) and polyester units;

where, in (1) to (3) , m is preferably > 3;

4. A polyester based on isophthalic acid and having fluorinated end groups B polyester based on terephthalic acid C a copolymer of isophthalic acid and terephthalic acids and having fluorinated end groups;

5. A CF ₃ -terminated aliphatic polyester

B polyester based on terephthalic acid C a copolymer of isophthalic acid and terephthalic acids having fluorinated end groups.

The molar proportions of the A' and B' moieties may be, for example, 9:1 to 1:9, advantageously 5:1 to 1:5, more especially 2:1 to 1:2, for example substantially 1:1.

Suitably the interface-modifying agent has, for example, a molecular weight of at least 100, preferably at least 400, more especially at least 500, and for example up to 5000, preferably up to 2000, more especially up to 1000.

The interface-modifying agent may have symmetrical or asymmetrical structure; for example, it may have the structure A'B'A ¹ , B'A'B' or A'B' . The component may also be a block copolymer of A' and B' units (or a random copolymer may be possible) or a graft copolymer of A' and B' units; although a completely random structure should

generally be avoided, a block copolymer of variable block length may be advantageous to minimise the risk of formation of micelles. In all cases additional func¬ tional grouping(s) , for example an acid, epoxy or hydroxy grouping, may be present, for example on the A and/or B moieties and/or at the end(s) of the chain. Preferably the interface-modifying agent co-reacts with component A and/or B during curing; advantageously the interface- modifying agent has functional groups capable of reaction with both components A and B.

The interface-modifying agent may be, for example, a haloalkyl- and/or alkyl-terminated polyester or poly¬ ether (alkyl suitably having at least 10, e.g. 10 to 20, carbon atoms) , optionally carrying one or more functional groups, for example hydroxy, epoxy or carboxy groups. As indicated above, in this context halo denotes fluorine and/or chlorine, and haloalkyl denotes a partially or fully halogenated group. One or both ends of the polyester or polyether chain may have the specified termination. When both ends are terminated the terminal groups may be the same or different, but are usually the same. A functional group present may be at one or both ends of the chain and/or functional groups may be present on the polyester or polyether chain. For example, the polyester or polyether chain may have one or two fluoro- alkanol terminals.

Thus, for example, suitable interface-modifying agents are as follows:

A'B' structure

^F ( ^CF 2)m ^~CH 2 ^CH 2 OH

m = 2 to 20 n = 2 to 100

A'B'A' structure

CH. m = 2 to 20 n = 2 to 100 B'A'B' structure

Block copolymers of A'B' and A'B'A' structure and graft copolymers, for example polyesters containing

- O - CO - CH - CO - O - or - O - CO - CH - COO

I I s ^c n ^F 2n+l I

CO

I

CR

II

CH ₂ R - H or methyl

groups should also be mentioned.

The present invention also provides an interface- modifying agent which comprises (a) one or more moieties A', each moiety A' being an aliphatic hydrocarbon radical having a carbon chain of at least 2, e.g. at least 4 or 5, members, or a halogenated (fluorinated and/or chlorinated) or

silicon-containing chain having at least 2, e.g. at least 4 or 5, chain members, and (b) one or more moieties B', each moiety B' being a polyester and/or polyether chain comprising at least 2, e.g. at least 4, ester and/or ether groups.

Advantageously, an aliphatic hydrocarbon radical or halogenated or silicon-containing chain (A') comprises repeating units, and where more than one such radical or chain is present in the molecule these may be the same or different, advantageously being the same or differing only in chain length.

A polyester/polyether chain (B ¹ ) preferably also comprises repeating units, and where more than one such chain is present in the molecule these may be the same or different, advantageously being the same or differing only in chain length.

An end atom or group of an interface-modifying agent may be, for example, a hydrogen atom, a halogen atom (F or Cl) or a hydroxy or carboxy group. The interface-modifying agents may be prepared by conventional methods, for example by condensing a suitable derivative of an acid of the general formula

HOOC-B^COOH for example an acid chloride or methyl ester, with an alcohol of the general formula

HO-B ₂ -OH where (B ₁ -B ₂ ) _n together denote a B' chain, and with an alcohol of the general formula

A' -OH. If desired, the formation of the polyester component by reaction of the acid derivative with the alcohol HOB2OH may be carried out first and the polyester subsequently reacted with the alcohol A'OH.

Solution polymerisation using the acid chloride and triethylamine or pyridine should, for example, be mentioned.

A graft copolymer may be prepared, for example, using thiomalic acid (H0 ₂ CCH ₂ CH(SH) C0 ₂ H) as a chain- transfer agent in a free radical polymerisation; polymerisation of a solution of monomer (especially acrylic or methacrylic monomers, for example TFMA or

Zonyl), initiator (e.g. 2 , 2 ' -azobisisobutyronitrile) and thiomalic acid would give a polymer terminated at one end with a thiomalic acid group; the other end group would be from the initiator or a proton from the chain-transfer agent. For example

00 ₂ H

Me/H CH ₂

CH ₂ = ; + initiator + thiomalic acid > (meth)acrylic - S - CH

I '

O0 ₂ R 00 ₂ H

Adjustment of the degree of polymerisation may be made by altering the concentration of thiomalic acid.

The resultant thiomalic acid derivative may be further reacted to form the acid chloride or methyl ester which may then be copolymerised into a polyester chain using a suitable diol or diacid derivative to give the graft copolymer.

In a different method alkyl or perfluoroalkyl side- chains may be introduced onto a polyester backbone. A precursor monomeric diacid is synthesised by reacting dimethyl malonate, treated with sodium hydride and a brominated, or other similarly terminated, (perfluoro)- alkyl, for example, as follows:

RX

CH ₃ CO ₂ CH ₂ O0 ₂ CH3 + NaH —> CH3CO2CHCO2CH3 + H ₂ > CH3CO2CHCD2CH3 + NaX Na I

R (X=Br, I etc.)

Again, this monomer may be used directly to form a

polyester or it may be reacted further to form the acid or acid chloride prior to forming the polyester in the usual way.

We have found that an interface-modifying agent was not, however, required when the polymer of component A contained CF ₃ (CF ₂ )y (CH ₂ ) _x pendant end groups where x = 2, y >3 (i.e. the polymer included groups with y = 3, y = 5, y = 7 etc.), whereas CF ₃ CH ₂ pendant end groups (i.e. x = 1, y = 0) for example did not give as good uniform results without an interface-modifying agent. The presence in the polymer of (CF ₂ ) _m blocks where m >4 should be noted (equivalent silicon-containing block = (SiR ₂ 0) _ro ) , as should the presence of a non-fluorinated CH ₂ CH ₂ block in the end group, contributing in this case to at least 4 carbon atoms between the main polymer chain and the fluorinated moiety and to a chain of at least 11 continuous atoms separating fluorinated moieties. Although not wishing to be bound by theory, we believe the presence of a 10- or 11-atom or longer chain between the low surface energy groups may be sufficient to act as effective "compatibiliser" for the higher-surface energy base-coat.

Thus, for example, a Zonyl polymer, in which there are 11 carbon/oxygen atoms in the chain between the F- substituted carbon atoms in a homopolymer as follows

CHo CH->

(CF ₂ ) ^{( F} 2 ⁾ 5

CF ₃ CF ₃

and at least 11 such atoms in a copolymer, gives satis¬ factory results without a separate interface-modifying agent as compatibiliser, whereas TFEMA monomer units, in which there are 9 carbon atoms between the F-substituted atoms, may be less satisfactory.

The carbon or silicon-containing units or end groups may also if desired contain additional carbon or silicon- containing branches, and preferably the coating compo¬ sition also has the surface energy and viscosity differences specified.

Although high amounts are not required for the purpose of self-compatibilisation, for assisting segrega¬ tion/stratification, as discussed above, advantageously such end groups are present on at least 2.5%, preferably at least 5 or 6%, e.g. at least 9 or 10%, for example on at least 15%, of the monomer units. Monomer units having such end groups are preferably at least 10 weight %, advantageously at least 20 weight %, for example at least 35, or at least 50, weight %, of all the monomer units. Component A polymers may be prepared by methods known in the art. Thus, free-radical methods may, for example, be used, for example solution, suspension, dispersion and emulsion polymerisation methods. With emulsion polymerisation, however, particular attention should be paid to the molecular weight of the product and the use of chain transfer agents. The use of chain transfer agents in the preparation of the component A polymer may be especially advantageous to provide a low molecular weight polymer and hence lower viscosity product. As is well known, adjustment of molecular weight may also be made by adjustment of the amount of initiator and/or stabiliser used.

We have found preparation by suspension polymerisa¬ tion, more especially in the presence of a stabiliser which has a higher surface energy than the polymer

produced, to be advantageous, not only on environmental and cost grounds (it avoids the use of solvent and gives advantageous yields) , but also as it can lead to a high molecular weight product, and it may be especially advantageous if the stabiliser used in the polymerisation system can act as an interface-modifying agent as described here.

A suitable stabiliser is, for example, a poly(vinyl alcohol), molecular weight 25 x 10 ³ , degree of hydrolysis 88%. Such materials are commercially available, for example, from Hoechst AG and Rhone Poulenc. The use of polyvinyl pyrrolidone stabilisers should also be mentioned.

Polymerisation of the Zonyl monomer both by solution and suspension polymerisation methods gave a component A which in turn gave coatings with no appreciable dif¬ ference in the degree of uniformity of the top coat, but the suspension polymerisation product was more flexible. Thus, for example, when a stratified coating obtained with a component A polymer produced by suspension polymerisation was subjected to very severe shearing force it nevertheless did not delaminate. Although not wishing to be bound by theory, we believe that the stabiliser used in its preparation assisted in providing interlayer adhesion in the final coating.

The solution viscosity of products produced by solution and suspension polymerisation was found to be the same, but suspension polymerisation produced a product with a surprisingly high zero shear viscosity, varying in one case for example from 3.9 Ln Pa.s at 45°C to 1.0 Ln Pa.s at about 190°C, whereas solution poly¬ merisation of the same monomer in contrast gave a product having a zero shear viscosity varying from 1.5 Ln Pa.s at 140°C to -1.5 Ln Pa.s at 200°C. Both these products have a viscosity less than that of conventional base-coat polymers.

In the compositions of the present invention a thermosetting resin-containing component generally requires a curing agent for the thermosetting resin; alternatively two co-reactive film-forming thermosetting resins may be used. The same or different curing agents may be present for each polymer. Curing agents are well known, and are described, for example, in chapter 7 of the textbook "The Chemistry and Physics of Coatings", edited by R. Marrion and published by The Royal Society of Chemistry in its paperback series 1994. For component B, for example, the film-forming binder system may comprise a carboxy-functional polyester film-forming resin used with an epoxy-functional curing agent such as, for example, an epoxy resin, for example a condensed glycidyl ether of bisphenol A, or a low molecular weight tri-functional epoxy compound such as, for example, triglycidyl iεocyanurate, or with a beta-hydroxyalkyl- amide; or a hydroxy-functional polyester used with an isocyanate-functional curing agent; or an epoxy resin used with an amine-functional curing agent such as for example, dicyandiamide or with a thiol curing agent, for example the reaction product of pentaerythritol and mercaptopropanoic acid available from Grace Brothers or Hampshire Chemicals ("Penta/Mpa") ; or a functional acrylic resin, for example a carboxy-, hydroxy- or epoxy- functional resin, used with an appropriate curing agent. Compositions curing at ambient and sub-ambient temperature should especially be mentioned.

With a thermosetting top-coat polymer, the possibility for cross-linking allows a lower molecular weight polymer to be used, so a correspondingly lower viscosity can be achieved, which, we believe, compensates wholly or in part for the adverse effect on surface energy obtained by the presence of the functional (polar) group of the thermosetting polymer. The use of a lower molecular weight (and hence lower viscosity) analogue of

Lumiflon (Lumiflon is an alternating fluoro olefin-vinyl ether polymer availablefrom Asahi Glass Co. Ltd.) or of Fluorobase C (a chlorotrifluoroethylene-containing polymer available from Ausimont, Milan) , should especially be mentioned.

Each component A and B may be, for example, at least 1%, advantageously at least 2%, preferably from 5 to 95%, by weight of the total of components A and B. Advantageously component A is from 5 to 20%, and com- ponent B from 95 to 80%, but the use of 5 to 20% of component B with 95 to 80% of component A to provide good adhesion for a fluorinated coating composition should also be mentioned.

Advantageously, the segregating layer, whether top layer or base layer, is at least O.lμm, more usually at least 0.2 μm, preferably at least 10μm, and up to 50 μ , advantageously up to 20 μm.

In the case of a solvent-based system, the composi¬ tions of the invention also include one or more solvents, for example two or three solvents, for the film-forming components and optional interface-modifying agent.

Preferably, compositions of the invention have no more than 340 g/litre of solvent, more especially no more than 250 g/litre of solvent (these are North American and European recommended levels, respectively).

Compositions having a solids content of at least 80% by volume or 85% by weight should especially be mentioned, more especially whichever has the lower solvent level (i.e., a lower volatile organic content) . Preferably, a composition of the invention has a solids content of at least 90% by volume (or 95% by weight) , and may, for example, be at least 95% by volume. Compositions having a somewhat lower solids content, for example down to 70% or 65% or down to 60% or down to 50%, by weight, should also be mentioned.

As mentioned above, the composition may also include

one or more interface-modifying agents which may be present, for example, in an amount of at least 0.1% and usually up to 3% by weight, advantageously up to 1%, preferably 0.5 to 2%, by weight, of the major component B or A.

As indicated above, a compound having the structure mentioned above for an interface modifying agent and having one or more fluorinated end groups may segregate as a top coat from a system consisting only of that component and a component B polymer; desirably, however, the top coat polymer should be of a higher molecular weight, for example by including one or more functional groups in an interface-modifying agent polymer so that that polymer can be cross-linked. Systems containing 1% or more of such a modified interface-modifying agent as component A, for example 5- 10% of this component A, should for example be mentioned.

In powder or melt application systems a thermosetting resin is generally co-extruded with its curing agent or co-reactive other thermosetting resin; in solvent systems of the present invention where ambient cure is advantageous, the curing agent is generally separate, but mixed in immediately prior to application to the substrate. In hot twin feed systems or other dual impingement systems, the curing agent is also separate and mixed prior to application or metered in at the same times as the other components.

In each of components A and B mixtures of film- forming binders may be used; in each component of powder systems these will generally be co-extruded. For example a carboxy-functional polyester may be used with a carboxy-functional (non-fluorinated and non-chlorinated) acrylic resin, and with a curing agent such as, for example, a beta-hydroxyalkylamide which serves to cure both polymers.

In powder systems, each of components A and B may comprise the polymeric binder system and where appro¬ priate and desired one or more further ingredients selected, for example, from colouring agents, flow- promoting agents, degassing agents, catalysts, anti- oxidants, stabilising agents (for example a UV absorber or a stabiliser against UV degradation) , fillers and plasticisers.

For powder systems, components A and B are advan- tageously prepared separately, and are not co-extruded (although co-extrusion may be appropriate in some instances) , the segregating component A preferably being distributed as discrete particles, although it may also be coated on the surface of the film-forming particles of component B; component B may alternatively be coated on to component A particles. In a different embodiment, however, the components may be co-extruded to a limited extent, and then micronised. One or more other additives may also be present as separate components. In powder systems, an interface-modifying agent is advantageously melt-mixed, for example co-extruded, with either or each of components A and B. Thus, for example, components A and C or B and C may be melt-mixed together, and the co-extrudate and remaining component ground to suitable particle size. Alternatively, the interface- modifying agent may be present as a separate component, and dry blended with components A and B, in which case it may be a longer chain molecule.

Particle size may be of importance in allowing control of the thickness of each layer. For example, the particle size of component A may be smaller than the particle size for component B.

The particles comprising component B may be, for example, of conventional size, for example at least 90% by volume between 20μm and lOOμm and no more than 70% by volume <50μm, with a mean particle size at least 35μm,

and generally no more than 60μm, often in the range of from 35 to 55μm, usually 35 to 50μm. The particles comprising component A may be, for example, also of conventional size, or for example of reduced particle size, for example at least 90% by volume <50μm and more than 10% by volume >20μm, with a mean particle size in the range of from 15 to 35μm, more especially 20 to 30μm, for example substantially 25μm, or substantially smaller sizes, e.g. a mean size of 5 μm, may be used. Powder sizes as described in PCT Application No. PCT/GB93/02238 (British Patent Application No. 92.23300.6) should par¬ ticularly be mentioned. The mean particle size of component A may vary, for example, from 5 to 35μm; the mean particle size of component B may also vary, for example, from 5 to 35μm. The ratio of the mean particle sizes of component A to component B is advantageously in the range of from 1:6 to 3:1, with the maximum mean particle size in any composition being advantageously 35μm. It is believed that reduction of the particle size of the top-coat component A may lead to improved appearance.

Comminution may be carried out, for example, in conventional comminution devices or by jet milling in a fluid energy mill, and the powders are then mixed. Details of suitable mixing processes are given in WO 90/06345 and WO 91/18951.

A preferred method for mixing particles is dry mixing of the powders in a high-shear mixer. A simple example is a modification of the type of food mixer known as a liquidiser, as described for example in WO 90/06345; another example of a suitable high-shear mixer is described in GB Patent 2132128; mixers of this type are sold by Freund Industrial Co. Ltd.. The Herfeld mixer and that sold by Lodige-Morton Machines Ltd. as the 'Lodige Ploughshare' mixer and by Winkworth Engineering Ltd. under the trade name ' RT Mixer ¹ are also suitable.

Alternatively, comminuting and mixing may be carried out simultaneously by feeding a mixture of components to a comminuting apparatus such as, for example, a fluid energy mill, or by feeding such a mixture in aqueous dispersion to a bead mill or ball mill.

In an alternative mixing process the powders are mixed by an electrostatic mixing technique; a suitable apparatus for electrostatic mixing of powders is described by C.L. Tucker and N.P. Suh in 'Polymer Engineering and Science' , October 1976, Vol. 16, at pages 657 to 663.

The mixture may then be applied as a powder coating composition in the form of a dry mixture of powders; suitable application procedures are electrostatic spraying, electrostatic fluid beds, and fluid bed dipping.

In a different embodiment, before applying to the substrate the mixed powders may be thermofused and then reground, or agglomerated to form composite particles, for example as described in WO 90/06345. In such agglomerates the particles may be fused or bonded, for example as described in WO 91/18951, and in such circum¬ stances the particle size of B may be reduced to be within the range specified above for reduced-size component A, then agglomerated to ensure the resulting fused particles are fluidisable and suitable for application by electrostatic spray.

Thus, for example, an agglomerate particle size distribution of 0-120μm, preferably 5-110μm, more especially 10-100μm, with a mean particle size (by volume) in the range of from 15-80μm, preferably 20-75μm, especially 25-50μm, may be obtained.

After application to the substrate the powder coating composition is cured by heating ("stoving") , usually for a period of from 5 to 30 minutes and usually at a temperature in the range of from 150 to 220°C,

although in coil-coating processes shorter stoving times are generally used, for example 1 to 5 minutes at 200- 220°C. It may be advantageous to introduce a stepped temperature increase into the stoving schedule, for example 5 minutes at 130°C followed by 10 minutes at 200°C.

In melt extrusion technology, the bilayer coating will generally be a thermoplastic film-forming material over a thermosetting film-forming material or a thermo- setting material over another thermosetting material; the components will generally be pre-mixed. The composi¬ tion is then applied as a film by extrusion of the melt through an extrusion coating die onto a suitable sub¬ strate, there being relative movement between the substrate and the die such that successive areas of the substrate are coated with the composition. In the process the coating composition is supplied to the extrusion coating die in melt (or plastified) form. The melting or plastification may be carried out in any suitable melt mixing apparatus, which may be a static or dynamic mixer, for example a Banbury mixer or a Z-blade mixer. The melt-mixed composition may be supplied to the extrusion coating die by a suitable pump. For pigmented systems a conventional extruder may be used as a melt mixer. Stoving at a temperature in the range of 200 to 220°C is preferably carried out after application to the substrate; stoving times are typically about 30 sec to 1 minute. These techniques are well-known in the art. Reference should be made especially to the processes and compositions described in WO 94/01224.

A further melt application technique for compositions of the invention is as described in AU-A- 10071/92 for coil applications.

In solvent-based systems of the present invention the paint system would generally be supplied as a two- or even three-pack system. In the case of a two-pack

system, the component B polymer in solution containing pigments and fillers may be together with the component A polymer such that the component A polymer exists as discrete meso phases within the component B polymer until applied. The second pack would contain the curing agent(s) ; where required, the interface-modifying agent (e.g. an ABA block copolymer) may, for example, also be placed in the first pack. Examples are the use of an epoxy resin and a glycidyl-functional fluoropolymer, e.g. a Zonyl copolymer and optionally an interface-modifying agent in the first pack and an amine curing agent in the second pack, and a hydroxy-functional Fluonate polymer (available from Dainippon Inks & Chemicals) together with a suitable interface-modifying agent and an epoxy resin in the first pack and a curing agent for the epoxy and a curing agent for the Fluonate in the second pack. For situations where curing agents cannot co-exist, three pack systems can be considered.

Suitable solvents for solvent-based systems are, for example, methyl isoamyl ketone and trimethylbenzenes and blends thereof. Often in a two-pack system the solvents in the two packs will be similar, or the same solvents will be used but in different ratios.

Before application the components are generally combined and mixed sufficiently to produce a homogeneous dispersion as is common in the art. Particular attention should, however, be paid to application conditions to avoid dramatic alteration to the initial state of mixing which could lead to mixing at a molecular level/the formation of more intimate mixtures.

Suitable application temperatures are, for example in the range of from -5 to 30°C, although higher applica¬ tion temperatures are also possible, for example using hot twin feed technology, and indeed may be advantageous for application of the preferred more viscous compositions.

The preparation and application of high solids controlled temperature application compositions (hot twin feed, dual impingement, etc.) is known, and is described, for example, in commercial literature from Shell, Schering AG and International Paint Ltd. The terms "hot twin feed" and "dual impingement" are used herein, however, to denote the method of application whereby different materials in a coating composition are heated and fed separately, coming into contact only shortly before application to the substrate. The terms are not restricted to the supply of two separate materials, but encompass the supply of more than two materials in this way, and are more properly called "hot plural feed" and "plural impingement" . Powder and solvent-based coating compositions of the present invention find application in the architectural and domestic appliance fields, and in the coating of aircraft, and for solvent-based systems and hot twin feed systems especially in the coating of ships, bridges, oil installations and structural steel. Other possible uses of compositions that should particularly be mentioned are of powder compositions in the automotive industry, of solvent-based compositions for coating yachts or for decorative paints, and of melt extrusion compositions in the food and beverage industry.

We have evidence that acrylic segregation over conventional polyester systems reduces the rate of degradation; therefore sel -stratifying acrylic/ polyester systems are regarded as potentially superior on a cost performance basis, with respect to weatherability, to both polyester and acrylic individually. Moreover, blending of components after the extrusion stage offers additional attractions of ease and formulating flexibility. The use of a tri-component, preferably tri- layer, system, with two component A polymers, e.g. of different F/Cl/Si/hydrocarbon contents such that a higher

F- or Si- content polymer segregates to the air interface relative to a lower F-/Si- content polymer, which itself segregates over a suitable base coat, should also be mentioned. Also, for example, the possibility of inclusion of additives, e.g. a colouring pigment, in only one com¬ ponent should be mentioned. An anti-corrosion pigment, for example, may be in the base layer (component B) and a colouring pigment in only the surface layer (component A) . In one system, for example, a pigmented polyester component having fluorinated end groups may, for example, be used as component A, together with a non-pigmented non-fluorinated polyester as component B. In a different embodiment, a non-pigmented top-coat component A having F- and/or Si-containing end/pendant groups may be used with a pigmented base-coat component B, and a UV absorber may be included in component A. Alternatively, in powder compositions pigmented polyester particles coated with a fluorinated acrylic polymer may be used as component A, with an unpigmented polyester as component B, provided that, in this case, the fluorinated polymer has a suitable molecular weight so it does not melt before the polyester (thus avoiding the segregation of the two component A polymers) ; because of surface energy considerations, the acrylic polymer (carrying the pigmented polyester) will form the surface layer. For example, a small quantity (for example 2%) of fluorinated polymer may be added to a pigmented polyester for component A during the melt extrusion stage in powder preparation, and after comminution the temperature may be raised above the Tg of the fluorinated polymer, for example for 1 to 2 hours, to allow the fluorinated polymer to surface.

The following Examples illustrate the invention; unless otherwise stated ratios, parts and percentages are by weight.

The following abbreviations are used:

MMA = methyl methacrylate

BMA = butyl methacrylate

BA = butyl acrylate

HEMA = 2-hydroxyethyl methacrylate

GMA = glycidyl methacrylate AA = acrylic acid

SM = siloxane monomer SLM 455127, available from Wacker-Chemie

FRAD = Fluorad FX-189 monomer, available from 3M company

PFPE = perfluoropolyether monomer, Galden TX Methacrylate, available from Ausimont, Italy

PMMA = polymethyl methacrylate

PBMA = polybutyl methacrylate

PBA = polybutyl acrylate

PA = polyacrylic acid PHEMA = poly-hydroxyethyl methacrylate

PGMA = polyglycidyl methacrylate

PS = polysiloxane

PZ = polyZonyl

PTFEMA = poly(trifluoroethyl methacrylate) PFRAD = polyFluorad

PPFPE = polyperfluoropolyether

Examples

TOP-COAT POLYMERS USED IN THE EXAMPLES

SYNTHESIS AND CHARACTERISATION OF FLUORINATED ACRYLIC OR SILICONE POLYMERS

Zonyl Monomer

The Zonyl monomer used was the medium distribution Zonyl TM monomer product available from du Pont. Commercial literature gives the approximate weight percentages of components in the product as follows:

Results of our measurement of the distribution of fractions using gas chromatography - mass spectrometry are given below; in these measurements the concentration is in area % which, although not identical to weight % or mole % figures, nevertheless gives a rough indication of the amounts of each monomer chain length present.

Silicone Monomer

The silicone monomer used was the monomer SLM 455127 available from Wacker Chemie of structure

CH ₂ =C(CH ₃ )-COO-(CH ₂ ) ₃ -(OSi(CH ₃ ) ₂ ) _n -Si(CH ₃ ) ₃

where n has a Gaussian distribution from n = 10 to n = 20 with mean n = 15.

Fluorad Monomer

The Fluorad monomer used was the monomer Fluorad FX- 189 available from 3M company of formula

CH ₂ =C(CH ₃ ) -COO-CH ₂ CH ₂ 0-(CF ₂ ) ₄ CF ₃

PFPE monomer

The perfluoropolyether monomer used was Galden TX Methacrylate available from Ausimont, Italy, of molecular weight 858, equivalent weight 848 and being the ethoxylated perfluoropolyether chain analogue of Galden Methacrylate which itself has the formula

CH ₂ =C(CH ₃ ) -COO-CH ₂ -CF ₂ -(0-CF ₂ ) _n -(0-CF(CF ₃ )CF ₂ )-.0-CF ₃ the ethoxylated analogue having the C ₃ F ₆ 0/CF ₂ 0 ratio of 27.6 and average number of ethoxy units 1.5.

Solution Polymerisation methods

Method 1 - Pre-reaction of Zonyl Monomer General Procedure

A three-necked flask was fitted with a condenser and a thermometer. The flask was charged with isopropanol solvent, the appropriate amount of Zonyl and half the initiator, 2 , 2 '-azobisisobutyronitrile ("AIBN") (approx. lg) available from Wako. The mixture was blanketed with nitrogen and heated to the solvent reflux temperature while being stirred magnetically. After being refluxed for one hour, the other monomer(s), if any, and the remaining initiator (approx. lg) dissolved in solvent, were added over 1 hours via a peristaltic pump; the total monomer and total solvent in the system were each approximately 200g. After addition had been completed, the mixture was refluxed for a further X ₂ hours. The isopropanol solution was discharged slowly into cold water (lOOOg) , vigorously agitated for 1 hr, and the polymer was recovered by filtration and dried under vacuum at 50°C.

The products produced are shown in the Table below.

Fluorine analysis of Zonyl monomer 62.35% w/w.

This method tended to produce copolymers having two Tgs and which in subsequent experiments produced coatings that were revealed by scanning electron microscopy pictures of the cross-sections as having low molecular weight inclusions in the top layer, believed to be low molecular weight polymethyl methacrylate.

Methods 2-5 - No Pre-reaction of Monomer

Method 2

The procedure was the same as that used for Method 1. However, the reaction vessel was initially charged with Zonyl monomer, and all the initiator was added at the same time as the other monomer(s) over a period of 2 to 3 hours.

Using a 50:50 mixture of Zonyl monomer and methyl methacrylate, the polymer produced had a fluorine analysis consistent with the formation of the Zonyl polymer; the product appeared as a pale yellow solid powder.

Method 3 General Procedure (Zonyl polymers)

A 1 litre 3 necked flask was charged with solvent and the appropriate amount of monomer(s) used. The flask was fitted with a condenser and thermometer; the third neck was blanked off. Agitation was provided by means of an anchor stirrer close coupled with an electric motor. The mixture was blanketed in nitrogen. The solution was heated to the solvent reflux temperature. Initiator, 2, 2 '-azobisisobutyronitrile (AIBN) , was dissolved in an aliquot (10 ml) of solvent and added to the reaction mixture. The reaction was left for about 6 hours at reflux temperature. After this time the mixture was allowed to cool to room temperature. The polymer was precipitated out by adding to an approx. 5 times excess

of petroleum ether and recovered by filtration. The polymer was dried at 40°C. Results are given in the following Table.

* Reaction carried out for about 8.5 hours.

By analogous methods, the following polymers were prepared:

Trifluoroethyl methacrylate polymer: "DA 190"

A further polymer, an acid-functional poly(tri- fluoroethyl methacrylate) ("PTFEMA") was prepared by solution polymerisation using 96% w/w trifluoroethyl methacrylate and 4% acrylic acid, following the above Method 3.

Silicone Polymer

Monomer(s) , solvent (isopropanol, 200ml) and initiator (lauryl peroxide, 0.7g) were charged to a reactor and purged with nitrogen while being heated to reflux (80-85°C) with the stirrer set at 150 rpm. Refluxing was continued for 5 or 6 hours before cooling to below 30°C and removing solvent.

Polymerisation of 40g silicone monomer and 160g Zonyl monomer gave a 20:80 PS/PZ polymer with a melting

point of 105°C and Tg (determined by DSC) of 5l°C.

Fluorad-containing polymer

Fluorad FX-189 (20g) was polymerised with methyl methacrylate (20g) in isopropanol using 0.35% by weight of lauryl peroxide initiator and refluxing at 60-80°C. The 50/50 PFRAD/PMMA polymer was obtained in a yield of 68.75%.

Perfluoropolyether-containing polymers

Galden TX Methacrylate (an ethoxylated fluoro- polyether monomer of molecular weight 858) (20g) was polymerised with Zonyl monomer using 0.35% by weight lauryl peroxide initiator and refluxing in isopropanol at 60-80°C. The 10/90 PPFPE/PZ polymer obtained in a yield of 63.6% had a melting point <150°C. A 20/80 PPFPE/PZ polymer was obtained in a similar manner in a yield of 70% and melted also at <150°C.

Method 4 (Thermosetting polymers)

The monomers were mixed and added at a feed rate of 5.5g per minute to 250g xylene containing lOg t-butyl- peroxybenzoate initiator (Trigonox C available from Akzo) at reflux (140°C) . Refluxing was continued for 2 hours before adding a further l.25g of Triganox C as a diges¬ tion shot and refluxing for another 1 hour. The solvent was then stripped off under vacuum before drying at 180°C. Details are given in the Table below.

Monomer feed Equivalent Wt.

Zonyl MMA Functional Monomer ratio Theory Actual

Monomer mol/q

439.9g 28.15g 32. Og HEMA 88:5.6:6.4 2030 2035 346.9g 122.8g 30.3g GMA 61.4:24.6:6.1 2342 2572

496.3g - 3.75g AA 99.25:0.75 6381 5982

The polymer produced was then double extruded with the appropriate amount of cross-linker to yield a thermoset polymer system which could be ground, sieved and sprayed in the usual manner. The PZ/PMMA/PHEMA polymer was used with a caprolactam- blocked polyisocyanate cross-linker of equivalent weight 394 mol/g (IPDI - Adduct B1065 available from Htils) . The PZ/PMMA/PGMA polymer was used with the cross-linker dodecanedioic acid DDA of equivalent weight 115 mol/g. The PZ/PA polymer was used with the cross-linker PRIMID XL-552 of equivalent weight 84 mol/g (available from Rohm & Haas Co. ) .

Method 5

General Procedure The monomers and initiator (Trigonox C - t-butyl peroxyoctanoate available from Akzo Chemicals UK) , 3 mol%, were mixed and added slowly to refluxing methyl isoamyl ketone solvent over 4 hours under nitrogen to a total solids level of 70%. The polymer produced was worked up in the usual way.

The following polymers were prepared

Mn and Mw measured by gas permeation chromatography

Suspension Polymerisation General Procedure 1

A 2 litre water-jacketed reaction vessel was fitted with a solid carbon dioxide condenser, nitrogen inlet and overhead stirrer. The stabiliser was predissolved in double-distilled water. The stabiliser solution was added to the reaction vessel and heated to the required reaction temperature (80°C) with stirring, under nitro¬ gen. The initiator was dissolved in the monomer or monomer mixture and added to the stabiliser solution. The temperature was monitored throughout the reaction. Reaction was allowed to proceed for one hour after the exotherm, and after this time the mixture was cooled to about 30°C. Throughout the cooling period stirring was maintained. Once cool, stirring was stopped and the polymer beads allowed to settle. The liquid was decanted off. The beads were filtered and washed with water, then methanol, and dried in a fan oven at 40°C.

Details of the reactions and products produced are given in the following Table.

Initiators i) 2,2'-azobisisobutyronitrile ii) dimethyl 2,2'-azobisisobutyrate

Stabiliser: In all reactions Gohsenol GL05 available from Nippon Gohsei

Inherent viscosity measured in 1,3-di(trifluoromethyl)benzene as 0.5% w/v solution

800g water used in all runs except (b) where 6000g were used.

General Procedure 2

The stabiliser and water (600g) were charged to the reaction vessel and purged with nitrogen while heating to 60°C; thereafter a nitrogen blanket was maintained. The initiator was dissolved in the first monomer and then mixed with further monomer and/or chain transfer agent if required and added to the reactor at a steady rate with a stirrer speed of 500 rpm; a temperature of 60°C was maintained. Stirring at 500 rpm was continued for 15 minutes before reducing the stirrer speed to 150 rpm and stirring for a further 6 hours. The mixture was then cooled to below 60°C before washing thoroughly and drying the polymer beads obtained.

Details of reactions and products are given in the following Table

Stabiliser: Gohsenol GI-05 available from Nippon Gohsei Initiator: 2,2-azobis(2,4-dimethylvaleronitrile) available from Wa o (1.5g) Chain transfer agent (CTI) : octanethiol

Commercially available fluorinated polvmersinvestiqated in the Examples were as follows:

Fluorobase C AlOO (chlorotrifluoroethylene-containing polymer available from Ausimont (Milan) )

Teflon TF 9205 (polytetrafluoroethylene: PTFE) available from du Pont

Kynar 500 (polyvinylidenedifluoride: PVDF) available from Atochem, Philadelphia, U.S.A..

Fluorine contents of polymers used

Polymer Zonyl 100% 90/10 PMMA/PZ 80/20 PMMA/PZ 65/35 PMMA/PZ 50/50 PMMA/PZ 25/75 PMMA/PZ 50/50 PBMA/PZ 50/50 PBA/PZ 5/45/50 PBA/PMMA/PZ 88/5.6/6.4 PZ/PMMA/PHEMA 61.4/24.6/6.1 PZ PMMA/PGMA

continued....

F content with

Polymer cross linker % w/w 99.25/0.75 PZ/PA 41.6 Fluorobase C A 100 Teflon TF 9205 (PTFE) Kynar 500 (PVDF) DA 190 (PTFEMA) NPB 21 (see below)

BASE-COAT POLYMERS USED OR INVESTIGATED IN THE EXAMPLES Polyester polymers used or investigated were as follows:

RP 0434. RP 0436, RP 0479 and RP 0555: acid-functional polyester polymers prepared from different blends of terephthalic and isophthalic acids, and neopentyl glycol and trimellitic anhydride, all of International Paint Ltd. , England.

RP 0434 and RP 0479 were used with triglycidyliso- cyanurate (TGIC) cross-linking agent available from Ciba- Geigy AG; RP 0555 was used with PRIMID XL-552 cross- linking agent (bis(N, -di-(2-hydroxyethyl) )adipamide, a 4-functional hydroxamine) available from Rohm & Haas Co.

Epoxy resins used or investigated were as follows:

RE 0114 : available from Dow Chemicals, sold as DER 042U

Epikote 828 : a liquid epoxy resin available from

Shell Chemicals

RE 0114 was used with EPON P104 curing agent available from Shell Chemicals Epikote 828 was used with Versamide 125 (a curing agent available from Cray Valley Products, England) or with

Penta/Mpa (a curing agent available from Hampshire

Chemicals, England)

Eurepox RV-A, available from Schering AG.

SYNTHESIS AND CHARACTERISATION OF INTERFACE-MODIFYING AGENTS USED IN THE EXAMPLES

General Synthesis

A resin kettle was fitted with a five-necked lid to which was fitted a nitrogen inlet, an outlet to a water bubbler and overhead stirrer. The remaining necks were blanked off. The vessel was purged with nitrogen and charged with reactants terephthaloyl chloride (ex Lancaster Synthesis and Aldrich) , neopentyl glycol (NPG) (ex Aldrich) , and 1H, 1H, 2H, 2H-perfluorodecanol (ex

Fluorochem) or decanol, in the mole ratio x+1 : x : 2 respectively where the required degree of polymerisation is given by (x+0.5) .

The mixture was heated in an oil bath to approxi- ately 80°C and the reagents were stirred constantly. The mixture was held at this temperature until the vigorous evolution of HC1 ceased (about 2 hours) . The temperature was raised to approximately 150°C for a further 2 hours with the nitrogen flow maintained. After this time the nitrogen flow was reduced to a trickle and the outlet to the water bubbler was replaced with a con¬ nection to a vacuum and evacuated for a further 2 hours with the temperature maintained at approximately 150°C. The reaction mixture was left to cool to room temperature overnight. The product was dispersed in an excess of dichloromethane with gentle heating to yield an opaque solution. The solution was added dropwise to an excess of rapidly stirred methanol to give a dense white precipitate. The precipitate was recovered by filtration and dried under vacuum at 40°C to constant weight. Analysis

The degree of polymerisation (dp) was calculated from the C-13 NMR spectrum of the samples dissolved in a CDCI3/DMSO (approximately 2:1 vol ratio) mixture at approximately 5%. The concentration of terephthalate

units was determined from the aromatic CH carbon integral and the concentration of neopentyl glycol units was determined from the aliphatic CH ₂ carbon integral. The concentration of the monofunctional end group was determined from the aliphatic 0-CH ₂ CH - carbon integral. The presence of any acid or hydroxyl end groups could also be detected from the carbon NMR spectrum.

Perfluorodecane-tipped polyesters

CHT

"NPB21" : Formula I. n = 20

The general procedure above was followed, the reaction vessel being initially charged with 22 g of terephthaloyl chloride, 10 g of neopentyl glycol and

5.9 g of 1H, 1H, 2H, 2H-perfluorodecanol. 26 g of product were isolated. The dp was calculated to be approximately 20 and the ratio of perfluoro end groups to NPG hydroxyl end groups 4:1. No acid end groups were detected. "NPB30" : Formula I. n = 11

The general procedure above was followed, the reaction vessel being initially charged with 101.5 g of terephthaloyl chloride, 49.5 g of neopentyl glycol and 23.1 g of 1H, 1H, 2H, 2H-perfluorodecanol. 95.35 g of product were isolated. The dp was calculated to be approximately 11 and the ratio of perfluoro end groups to NPG hydroxyl end groups was 1:3.1. No acid end groups were detected. "NPB31" : Formula I. n = 11.2 The procedure above for the preparation of "NPB30" was repeated. 115.35 g of product were isolated. The dp

was calculated to be approximately 11.2 and the ratio of perfluoro end groups to NPG hydroxyl end groups 1:3.2. No acid end groups were detected.

Decane-tipped polyester

"NPB10" : Formula II. n = 9.2

The general procedure above was followed, but using methyl terephthalate (ex Aldrich) (62.45 g) in place of the terephthaloyl chloride and using 27.36 g neopentyl glycol and with 9.43 g decanol as the end capping group, and dibutyl tin laurate as catalyst. During this prepara¬ tion there were problems with sublimation of the methyl terephthalate. 58 g of product were isolated. The dp was calculated to be approximately 9.2 and the ratio of decyl end groups to NPG hydroxyl end groups 9:1. No acid end groups were detected.

SURFACE ENERGY AND VISCOSITY MEASUREMENTS OF POLYMERS

Surface energy measurements were taken at a set of temperatures using the Willhelmy plate technique on the molten polymeric material.

Results are given in the following Table 1.

Table 1

Table 1 continued

Table 1 continued

Notes for Table 1

1 Prepared by solution polymerisation, method 1

2 Figures for other PolyZonyl homopolymers approx. the same (to within experimental error)

Prepared by solution polymerisation, method 3

RP 0434 and RP 0479 figures the same (within experimental error)

5 Equivalent of Epikote 828 available from Schering 6 Measured in 50:50 xylene/methyl ethyl ketone blend

Viscosity data was obtained using a Bohlin VOR rheometer equipped with a high temperature unit and parallel plate geometry. The geometries used were dis¬ posable platten parallel plates accurately set with a predetermined gap width; this was set typically in the range 300-900μm. Experiments were run in continuous mode at a variety of shear rates for a set of temperatures in the range 130-200°C. For samples where the shear behaviour at low shear rates was generally Newtonian (that is where the viscosity was constant and independent of shear rate) the steady shear Newtonian (also termed zero-shear) viscosity of the samples was determined from a plot of viscosity against shear rate. Where the plots did not plateau to a constant value a simple mathematical model was

used to fit the data and thus the zero shear viscosities were estimated. The Moore model = viscosity at any shear rate

^ _. v _. = ' ^r \ ₊ V - +1 ^{Υ =} shear rate

^{ ι ' ^{l ∞} ° ^t °° _Q = viscosity at zero shear rate

1 + Cy ^ _oo = viscosity at infinite shear

^ rate

C = constant was found to fit the data satisfactorily.

In some instances ICI Cone and Plate viscosity was measured and equated with zero shear viscosity for polymers having Newtonian shear behaviour.

The results are shown in the following Table 2.

Table 2

Table 2 continued

Table 2 continued

Table 2 continued

Table 2 continued

Table 2 continued

Notes to Table 2

1 (too high to measure)

2 based on ICI Cone & Plate = 0.16 Pa.s

3 based on ICI Cone & Plate = 0.22 Pa.s

4 based on ICI Cone & Plate = 0.20 Pa.s

5 RP0434 figures comparable

6 RP 0434 figures comparable (within experimental error)

7 based on ICI Cone & Plate = 4.4 Poise = 0.44 Pa.s

8 based on ICI Cone & Plate = 4.0 Poise = 0.4 Pa.s

9 based on ICI Cone & Plate = 3.2 Poise = 0.32 Pa.s

10 prepared by solution polymerisation, method 5

11 very similar to Epikote 828

12 measured in 50:50 xylene/methyl ethyl ketone

13 commercial data

Zero shear viscosity plots of various polyZonyl homopolymers, of two polymethyl methacrylate/polyZonyl (PMMA/PZ) copolymers and one polybutyl methacrylate/ polyZonyl (PBMA/PZ) copolymer, as well as a representa¬ tive polyester (RP 0555) are shown in Figure 1. The polyZonyl homopolymers were prepared by solution polymer¬ isation (PZ-TM) and by suspension polymerisation (ZNBS-1 and ZNBS-8 - see procedure 1 in the Examples above (experiments (a) and (b) ) ; the polymers prepared by suspension polymerisation had a higher viscosity.

The Figure shows that increasing the proportion of polymethyl methacrylate in the polymethyl methacrylate/ polyZonyl copolymer increases viscosity. In comparison

with the 50/50 PMMA/PZ copolymer, the viscosity of the 50/50 PBMA/PZ copolymer is higher.

Zero shear viscosity plots of PTFEMA and two further polyZonyl copolymers (50/50 polybutyl acrylate/polyZonyl (PBA/PZ) and 5/45/50 polybutyl acrylate/polymethyl meth- acrylate/polyZonyl (PBA/PMMA/PZ) ) , the coπtmercially- available fluoropolymer Fluorobase C and a representative polyester (RP 0434) are shown in Figure 2. The highest viscosity is that of Fluorobase C. The Figure also shows that the viscosity of the PBA/PMMA/PZ copolymer is higher than that of PTFEMA and the PBA/PZ copolymer.

Figure 3 shows the zero shear viscosity plots of three Zonyl-containing homo- or co-polymers and two epoxy resins. The epoxy resins have, in general, lower vis¬ cosities than the polyester resins shown in Figures 1 and 2; thus although the 50/50 PMMA/PZ top-coat polymer shown in Figure 3 gives good segregation with standard polyesters, for use with an epoxy resin a much lower viscosity polymer is preferred, for example the 35/65 PZ/PBA polymer shown in the Figure.

The liquid epoxy resin Eurepox RV-A available from Schering AG is often used in solvent-containing compositions, which may be applied, for example at room temperature, and the graph of Figure 3 therefore includes viscosity figures at and below room temperature, as well as at temperatures appropriate for other application technologies.

Of course, when curing agent is added to the thermo¬ setting epoxy polymer, the viscosity of the component may decrease, and when solvent is present (and usually more solvent is used with the base-coat polymer than with the top-coat polymer) the viscosity of the component may decrease by approximately up to 3 or 4 Ln Pa.s, depending on the amount of solvent. However, the polyZonyl/ polybutyl acrylate polymer component will still have an appropriate zero shear viscosity relative to Eurepox.

Figure 4 shows the zero shear viscosity plots of various Zonyl/butyl acrylate/glycidyl methacrylate polymers, together with a representative epoxy resin, Eurepox RV-A. All the fluorine-containing polymers shown have lower zero shear viscosities than the Eurepox resin, and give good segregation with that epoxy resin.

Of the fluoropolymers shown in Figures 1 to 4 , as will be seen below only Fluorobase C failed to give appreciable segregation in our experiments. Viscosities of the commercially-available polymer Kynar 500 (PVDF) and Teflon (PTFE) and also of 90/10 and 65/35 PMMA/PZ copolymers are too high to appear on the graph. Although with the standard polyesters shown these may give some segregation, they do not lead to complete bilayer systems, in contrast to the other low-viscosity polymers shown.

The surface energy plot of various Zonyl/methyl methacrylate polymers are shown in Figure 5. It can be seen that the surface energy of the different Zonyl polymers increases with increased replacement of the Zonyl by polymethyl methacrylate. The 10/90 Zonyl/PMMA surface energy plot may represent the approximate limit for surface energy for a segregating system with standard polyesters.

The surface energy plot of PTFEMA, various polyZonyl copolymers and of a representative polyester (RP 0434) are shown in Figure 6. All the fluorinated polymers in the Figure have a surface energy less than that of the polyester.

The surface energy of two interface-modifying agents of the invention is shown in Figure 7; the surface energy of NPB 20 described in the Examples above is approximately between that of the representative poly¬ ester and the fluorinated acrylic polymers shown in Figure 6.

TESTING OF COATING COMPOSITIONS

Various polyester-fluorinated acrylic powder coating systems were prepared and electrostatically sprayed on to aluminium foil or panels and stoved vertically. Solvent-based compositions were applied using a drawn-down bar onto steel and aluminium substrates.

Coatings were examined visually and under an optical microscope. Finally, film thickness measurements were taken for each coating to calculate the thickest layer of acrylic possible at the surface for each system.

The resulting cured films were also analysed using a variety of spectroscopic techniques. X-ray photoelectron spectroscopy (XPS) , Fourier Transform Infra-Red Attenuated Total Reflectance (FTIR-ATR) and PhotoAcoustic Spectroscopy (FTIR-PAS) were used to determine the composition profile, as a function of depth, of the films over the range of 50A ^" to 20μm. Whereas XPS is the most surface-sensitive technique, probing to a depth of 5θA, the FTIR results, which were used to determine the composition over the sampling depth of 0.5-20μm, provided more information on the thickness of the acrylic layer to determine whether stratification had occurred.

The depths calculated using FTIR are determined using the properties for pure polyester or epoxy and pure acrylic. The extent of segregation observed was quantified by comparison of PAS-FTIR data with transmission infra-red spectra collected from a series of powder mixtures of differing composition. Since in most cases both polyester/ epoxy and acrylic were present in the depth measured, it was assumed that the actual penetration depth would lie between the depth calculated for pure polyester/epoxy and the depth calculated for pure acrylic.

EXAMPLES OF COATINGS FORMULATIONS

POWDER COATINGS

The two polymeric film-forming components used in each experiment were each prepared by micronising using a conventional coffee grinder and sieving through a 106μm mesh to give a powder of conventional particle size. Where the component consisted of a number of ingredients, these were generally co-extruded at 120°C before micronisation. Unless otherwise indicated the components were dry blended during micronisation and Corona-charged and electrostatically deposited onto aluminium panels and stoved for 15 mins at 200°C.

Effect of structure on segregation for powder systems; surface energy and viscosity considerations

Comparative Example 1

Fluorobase C (A 100) was dry blended with a polyester component (96% RP 0555 plus 4% PRIMID cross-linker) , in the proportions 5 : 95 and 10 : 90 by weight. After application and stoving, the concentrations of Fluorobase C at the surface of the coating were 6-8% and 15-20% respectively; no significant segregation had occurred. Comparative Example 2 Kynar 500 (PVDF) was dry blended with purple polyester component based on RP 0555 with PRIMID cross- linker in the proportions 10:90 and 20:80 by weight.

After application and stoving and FTIR-ATR analysis of the surface, 10 to 20% of PVDF was found to be present in the top 3 microns; no appreciable segregation of the surface had occurred.

Comparative Example 3

10% weight of Teflon TF 9205 (PTFE) was dry blended into a polyester component based on RP 0479 with TGIC curing agent and conventional additives. After conven- tional application and stoving, a surface concentration of about 20 weight % of Teflon was found in the top 3μm of the film (70-100 micron thick) .

The experiment was repeated with 20 weight % of Teflon dry blended into a polyester component based on RP 0555 using PRIMID as a cross-linker, but otherwise formulated as above. After application and stoving, a surface concentration of 20 to 40 weight % in the top 3μm of the 70-100 micron film was obtained.

Although these results represented some degree of segregation, segregation was not very satisfactory, and certainly not complete, despite the very high fluorine content of the polymer.

Example 1

The fluorinated acrylic polymer DA 190 (PTFEMA) was dry blended with a polyester component (RP 0555 plus 4.4% TGIC cross-linker) in the weight ratio 5 : 95. After application to an aluminium substrate and stoving as described above, a bilayer film was formed, with the top 2-3μm of the cured film being composed of 45% of fluorinated acrylic polymer, and the top 0.5 μm of 90 % fluorinated acrylic, for a 60-70μm thick film.

Example 2

A fluorinated acrylic component containing a Zonyl polymer (prepared by the method 1 solution polymerisation method) was dry blended with a polyester component (RP 0555 plus cross-linker and conventional additives) in a ratio 20:80 by weight.

Component 1 consisted of

Zonyl polymer 998g

Purple pigment 2g

Component 2 was as follows RP 0555 926.1 g

PRIMID XL-552 cross-linking agent 53.90g

Stabiliser IRGANOX 245 (polyphenol) 2.00g

Degassing agent (benzoin) 3.00g

Flow aid Perenol BYK 360 P 12.00g Flow aid carnauba wax 3.00g

After application to a substrate and stoving as described above, a bilayer film 70μm thickness with a surface acrylic layer of approximately I3μm was formed. The surface topography of the coating were excellent.

The structures of PTFEMA and polyZonyl, having fluorinated end pendant groups, clearly favoured segrega¬ tion in comparison to those of Fluorobase C, Teflon (PTFE) and Kynar 500 (PVDF) which contain a fluorinated backbone and no fluorinated pendant or end groups. Fluorobase C is a ter- or tetra-polymer including chlorotetrafluoroethylene (CTFE) units and having inter alia hydroxy and, optionally, acid functions as follows

F F H H H i I I I I

- C - C - CH- _> - C - CH _? - C - CH _? - C -

^' I I I l F Cl O CH _? R ₃

I I I

CO O COOH

I ^' I ^R l *2

I

OH

Fluorobase C AlOO, tested by us, contained no acid groups but nevertheless included a high proportion of polar end

groups. PTFE and PVDF contain no pendant groups and probably polar groups at the chain ends. End groups in all three polymers have a surface energy higher than that of the backbone/of the polymer itself.

Comparison of the zero shear viscosities of the different polymers used in Examples 1 and 2 and in Comparative Examples 1-3 also shows that the viscosities of Fluorobase C, Teflon and Kynar 500, which did not give adequate segregation, were substantially higher than (>>4 Ln Pa.s in most instances) that of the polyester component; the viscosities of PTFEMA and polyZonyl, however, were in each case less than that of the relevant polyester component at all relevant temperatures.

It should be noted also that the stratification improved in changing from a polymer having short-chain pendant end groups (PTFEMA) to one having longer pendant end groups (polyZonyl) . This change may also be a reflection of the lower surface energy of the Zonyl end groups in comparison with that of PTFEMA, the surface energy of the backbone remaining the same in each case.

In Fluorobase C, however, in contrast, the surface energy of the pendant groups is more than that of the backbone.

According to group contribution theory (see, for example, D.W. Van Krevelen, Properties of Polymers, Their Estimation and Correlation with Chemical Structure,

Elsevier, 1976, 2nd edition) , the surface energy of the end groups and backbone units are given by the equation

V ⁼ ( f) ⁴

where Ps^ = a constant for the group, called the parachlor and V _j _ = molar volume

For the PTFEMA and polyZonyl polymers used in Examples 1 and 2 the relevant surface energies can be calculated as follows:

PTFEMA

48 dyne cm -1

Y (end group) = 26.8 dyne cm -1

285.6

Y (whole polymer) = = 37.1 dyne cm -1

115.75

PZonyl

As indicated above, the Zonyl product used contained a mixture of fluorinated chain lengths. Considering the perfluorohexyl product:

Y (backbone unit) 48 dyne cm-1

(end group) 23.2 dyne cm -1

1 = 27.9 dyne cm ^-1

Considering the perfluorooctyl product: Y(end group) = 22.6 dynes/cm ^-1 Y(whole polymer) = 26.5 dynes/cm ^-1

Bearing in mind the full distribution of monomers, the surface energy of the end group and of the whole polymer will in fact be less than the figures calculated above.

Examples 3-7 The segregation of various Zonyl-methyl methacrylate copolymers prepared by the Route 1 solution polymerisa¬ tion method were compared by repeating Example 2. The components were dry blended in a ratio of 20:80 by weight. Details of the polymers used and the results obtained are given in the following Table

Example Polymer No.

3 PMMA/PZ 90:10

4 PMMA/PZ 80:20

5 PMMA/PZ 65:35

6 PMMA/PZ 50:50

7 PMMA/PZ 25:75

Good segregation was observed in all Examples, but

none of the films in Examples 3-5 exhibited a complete surface acrylic layer: the acrylic at the surface did not coalesce fully, and the films could not be described as bilayer coatings. Improved results were obtained as the proportion of Zonyl in the polymer increased, and Examples 6 and 7 gave full bilayer systems.

Comparison of Examples 3-7 showed that the ability to form a fully bilayer film is dependent on having a sufficiently high proportion of Zonyl-derived units in the fluorinated acrylic polymer. As can be seen from the plots of surface energy and viscosity measurements as a function of temperature, increasing the proportion of PZ decreases the surface energy and viscosity of the fluorinated acrylic polymer. Only those compositions having surface energies and viscosities no higher than those of 50/50 polyZonyl/PMMA formed a fully bilayer coating in the above system, where the polyester component had a surface energy varying from 38 to 24 dynes/cm over the temperature range 130-210°C and a viscosity varying from 7.2 to 2.1 Ln Pa.s over the temperature range 120 to 200°C.

The result obtained with the 50:50 copolymer (Example 6) and with the copolymer having the higher polyZonyl content (Example 7) was in each case better than that obtained with PTFEMA (Example 1). The 50:50 copolymer (corresponding to a molar ratio MMA:Zonyl of 5:1 to the nearest whole number) has a surface energy calculated by group contribution theory of approximately 40, which is higher than that of 100% PTFEMA (approximately 37) . Indeed, not only the 50:50 and 25:75 copolymers, but also the 80:20 and 65:35 copolymers undergo interfacial segregation to a greater extent than 100% PTFEMA.

Thus, the presence of even a relatively small molar proportion of Zonyl units in the polymer, with their low surface energy end groups, was sufficient to provide

efficient segregation.

In Examples 4 and 5 a good appearance was obtained, and in Examples 6 and 7 the appearance was excellent.

Examples 8-12 The above experiments were repeated with variations in the ratios of the components, with changes in pigment content of the components and with different methods of preparation of the component 1 polymer. Details of the components and bilayer coatings produced are given below.

Example 8 10% polyZonyl (unpigmented) prepared by solution polymerisation, method 3)

90% polyester component prepared from

RP 0555 916.65g

PRIMID 53.35g purple pigment lO.OOg other additives as in Example 2

1.5μm thick acrylic layer

47μm film thickness

Example 9 20% polyZonyl (unpigmented) prepared by solution polymerisation, method 3

80% polyester component as in Example 8 llμm thick acrylic layer 53μm film thickness

Example 10 15% acrylic component prepared from polyZonyl 990g purple pigment lOg

85% polyester component as in Example 2

(no pigment)

(PZ prepared by solution polymerisation, method 3) 4μm thick acrylic layer

75μm film thickness

Example 11 15% acrylic component as in Example 10

85% polyester component prepared from

RP 0555 600.72g

PRIMID 29.28g Ti0 ₂ 350. OOg

(other additives as in Example 2) approx. lOμm thick acrylic layer

75μm film thickness

Example 12 reproduced Example 2, but utilised the 50:50 PMMA/PZ copolymer prepared by the solution polymerisation method 2.

Approx. 12μm thick acrylic layer 65μm film thickness

Example 13 20% acrylic component prepared from polyZonyl 998g purple pigment 2g

80% polyester component prepared from

RP 0555 600.72g

PRIMID 29.28g Ti0 ₂ 350. OOg other additives as in Example 2

(PZ prepared by suspension polymerisation, procedure 1) lOμm thick acrylic layer

75μm film thickness

Comparison of Examples 8 and 9 shows that increasing the proportion of polyZonyl composition increased the thickness of the surface acrylic layer. The most efficient use of the fluoropolymer occurred when blended in the ratio 20:80 when essentially all the fluoropolymer added segregated to make up the top layer.

Examples 8-12 show that polymers prepared by some of the more desirable of the solution polymerisation procedures, that is, by procedures that do not involve pre-reaction of the Zonyl monomer, can be pigmented to

different levels and still segregate in the presence of a clear and pigmented polyester base coat. Comparison of Examples 10 and 11 shows that Example 11 gave better results because the presence of pigment in Component 2 increased the surface energy and viscosity of the component.

In Example 13 the bilayer coating containing polyZonyl prepared by suspension polymerisation was as uniform as those containing polyZonyl prepared by solution polymerisation. However, bilayer coatings containing polyZonyl prepared by suspension polymerisation appeared to have better mechanical properties than those containing polyZonyl prepared by solution polymerisation; in the case of solution polymerisation samples, polishing the sections tended to cause delamination.

Examples 14-22

The components 1 and 2 indicated in the following Table 3 were dry blended in the ratios shown and applied as thin films to aluminium foil. Results were based on FTIR-atr spectra of the top 2 microns, the ratio of fluoro-acrylic polymer to polyester in that thickness being assessed by the ratio of bands at 1130 and 1014 cm ^-1 respectively. In the Table:

d = RP 0555 600.72g

PRIMID 29.28g

Ti0 ₂ 350. OOg a - d all also contain the other additives as in Example 2

Table 3

Example Component 1

PolyZonyl prepared by

18 suspension white polyester (d) 20:80 50 100% 127μm 13μm polymerisation run (c)

19 brown polyester (b) 20:80 40 20 50/50 PBA/PZ purple polyester (c) 20:80 21 5/45/50 PBA/PMMA/PZ purple polyester (c) 20:80 22 50/50 PBMA/PZ purple polyester (c) 20:80

All systems having a content of at least 20 % of component 1 formed a bilayer system with the exception of the 50/50 polybutyl methacrylate/polyZonyl polymer prepared by suspension polymerisation (Example 22) . This segregated, doubling its concentration at the surface, but did not stratify fully.

The average surface energy of the pendant groups in the polymer can be calculated (approximately) considering the perfluorodecyl component of the Zonyl product (compare the average fluoroalkyl chain length for the monomer given above) as follows: Zonyl mol weight = 532 BMA mol weight = 142

50/50 weight % corresponds to 3.7:1 molar % BMA: Zonyl Surface energy Zonyl end group = 22.6 dynes/cm Surface energy butyl end group = 34.5 dynes/cm

(34.5 X 3.7) + 22.6

Average surface energy end groups =

4.7 = 32 dynes/cm

Surface energy polymer backbone = 48 dynes/cm The average surface energy of the end groups is therefore less than that of the polymer backbone and consequently less than that of the polymer as a whole. However, as shown by Figure 1, the zero shear viscosity of the polymer is about 4 Ln Pa.s higher than that of the polyester used.

Examples 23 and 24

Comparison was made between the segregation achieved with structurally similar polymers of different molecular weight. In each case the top coat polymer was a 50/50 PZ/PMMA polymer prepared by suspension polymerisation, procedure 2, and the base-coat component had the following composition

Wt % polyester RP 555 59.5

PRIMID XL-552 cross-linking agent 3.5 anti-oxidant IRGANOX 245 0.2 flow aid Perenol-BYK 1.2 degassing agent (benzoin) 0.3 carnauba wax hardener 0.3 pigment Ti0 35.0

Further details and results achieved were as follows

Example Suspn. procr. ratio compts. Result

Compt. 1 1: 2

23 procr. 2, 10:90 ^" ) complete strati- expt. c ( fication bilayer system 24 procr. 2, 30:70 expt. c procr. 2, 30:70 I incomplete expt. a I stratification procr. 2, 30:70 expt. b

Although the surface energies of the polymers were the same (or possibly higher) than the surface energies of the 50/50 PZ/PMMA polymers prepared by other methods, the viscosities of the polymers varied according to the molecular weights of the polymers. The polymer prepared in experiment c of Procedure 2, prepared using a chain transfer agent, was of much lower molecular weight than those prepared in experiments a and b, and had a corres¬ pondingly lower viscosity: the low molecular weight polymer had a zero shear viscosity at 200°C of 10 Pa.s (Ln = 2.3) , and the higher molecular weight polymer of experiment a had a zero shear viscosity at 250°C of >2500 Pa.s (Ln >7.82) (at 200°C the viscosity would be even higher) ; experiment b gave a lower particle size polymer

than experiment a but the polymer had a similarly high molecular weight and viscosity. However, the viscosity of the polyester used in the segregation Examples was 4 Pa.s at 200°C (Ln = 1.4) so that the zero shear viscosity of the low molecular weight polymer was <4 Ln Pa.s greater than that of the base-coat polymer, whereas the high molecular weight polymers had a zero shear viscosity >4 Ln Pa.s greater than that of the base-coat polymer. Thus, a complete bilayer system was produced with the low viscosity polymer (85% and >99% fluorinated polymer at the surface in Examples 23 and 24 respectively) , but with the higher viscosity polymers some segregation occurred but stratification was incomplete.

Examples 25 to 27 The thermosetting top-coat systems detailed below, and based on the polymers prepared by solution polymeri¬ sation, method 4, were tested for segregation with the polyester base-coat component of Examples 23 and 24 using a weight ratio 20:80. In Example 25,

300g of the PZ/PMMA/PHEMA polymer

58.08 g of cross-linker IPDI-Adduct B1065, equivalent weight 394 mol/g available from Hϋls 1.07 g of the degasser benzoin were coextruded to give a component with gel time 4 minutes 37 seconds at 200°C, 2 minutes 26 seconds at 220°C and 1 minute 30 seconds at 240°C. In Example 26,

250 g of the PZ/PMMA/PGMA polymer 11.18 g of cross-linker dodecanedioic acid, equivalent weight 115 mol/g 0.78 g benzoin were coextruded to give a component with gel time 4 minutes 3 seconds at 200°C, 3 minutes 1 second at 220°C and 2 minutes 16 seconds at 240°C.

In Example 27,

300 g of the PZ/PA polymer 4.21 g of cross-linker PRIMID XL-552, equivalent weight 84 mol/g 0.91 g of the degasser benzoin were coextruded to give a component with gel time 3 minutes 24 seconds at 200°C and 1 minute 43 seconds at 240°C.

FTIR-ATR results on the top 2-3μm of the coating from 20:80 mixtures with component 2 were as follows:

Example 25 >94% consisted of fluorinated material Example 26 >99% consisted of fluorinated material Example 27 >89% consisted of fluorinated material Thus good stratification can be achieved with thermoset- ting top-coat systems.

Example 28

The 50/50 PFRAD/PMMA polymer described above was tested for segregation with the white polyester base-coat component of Example 23 using a ratio of 20:80 by weight top-coat component to base-coat component. The PFRAD- containing polymer segregated to give an estimated 60% fluorinated polymer at the surface, based on IR measurement.

Example 29 Repeat of Example 28 using the 10/90 and 20/80

PPFPE/PZ top-coat polymers gave good segregation with, in the case of the 20/80 polymer, over 90% fluorinated polymer at the surface according to IR data.

Example 30

The 20:80 PS/PZ silicone polymer described above was cryogenically ground at the temperature of liquid nitrogen to give a powder which was sieved through a

106μm mesh as usual to give a powder of conventional particle size. This top-coat component was then mixed in a ratio of 20:80 with the polyester base-coat component of Example 23 and applied to a substrate as usual. IR data showed there to be >82% of top-coat polymer in the top 2μm of the resultant film: good segregation had been achieved.

Example 31

A 20:80 dry blend of the 50:50 PMMA/PZ component prepared by solution polymerisation method 3, code III with an epoxy component consisting of epoxy resin RE 0114 65.96g curing agent EPON P104 2.64g benzoin 0.22g flow aid l.OOg

Ti0 ₂ 30.OOg was tested for segregation in the usual way. The EPON curing agent was available from Shell US. The ratio of PMMA/PZ to epoxy in the top 2 microns was given by the ratio of IR bands at 1730 and 1501 cm ^-1 respectively, and was found to be 2.5:1, corresponding to an acrylic content of 50% in the top 2 microns.

Example 32

Example 31 was repeated using as component 1 the polyZonyl component of Example 18 prepared by suspension polymerisation, procedure 1, run c. The film gave a ratio of 2.5:1 acrylic:epoxy in the top 2 microns, corresponding to 50% acrylic in the top 2 microns.

Effect of thermomechanical fusion of mixed composition Example 33

The effect of thermomechanically fusing the powder particles was examined. Components 1 and 2 were mixed in a coffee grinder, then melted together at 80°C under a pressure of 125kg/cm ² for 30 sec and subsequently reground

to conventional particle size and passed through the usual 106μm sieve. The component 1 used was the 100% Zonyl polymer prepared by suspension polymerisation, procedure 2, (experiment d) ; component 2 was the polyester com- ponent of Examples 23 and 24. Dry-blending a 20:80 mixture gave a 28μm film with a top layer of thickness 0.2 to 3.0μm; ther ofusion of the same blend gave a 30μm film with a top layer of thickness 3 to 5μm. There appeared to be a greater amount of fluoropolymer present in total in the ther ofused film; that is, less of the fluoropoly¬ mer was lost in spraying in the case of the thermofused composition.

The experiments were then repeated, but applying a thicker layer of coating, and the coatings were examined for uniformity by measurement of minimum and maximum thicknesses of top layer and total film along a 3cm length of panel at each of eight different distances from the panel edge. Again, thermofusion was shown to give a thicker and more even top layer, and whereas the dry- blended sample gave an incomplete top layer in a number of places, seven out of eight of the representative areas of panel inspected with the thermofused sample showed complete bilayer coverage.

Effect of interface-modifying agent

Example 34

An acid-functional fluorinated acrylic polymer ("DA 190") , essentially PTFEMA, was dry blended with a second component consisting of

87.3% acid-functional polyester polymer ("RP 0434") 9.7% cross-linking agent (TGIC)

3.0% interface-modifying agent ("NPB 10") in a ratio 5:95 by weight. The final mixture was then sprayed on to an aluminium substrate and cured at 200°C for 15 min as usual.

A uniform bilayer film was formed, the top layer being 2.4μm ± 0.2μm, with 90% of the top layer being fluorinated acrylic polymer, whereas the top layer in the product produced from the corresponding system containing no interface-modifying agent (Example 1 above) varied from 0.2μm to 3μm in thickness. In comparison to that product, the surface topography of the film containing interface- modifying agent was much improved.

Impact tests were carried out on the coating on an aluminium panel. These tests involved dropping a weight from various heights which corresponded to different levels of impact; both sides of the panel were impacted. The panel survived impacts of 0.39 and 0.78 J/cm ² and met the required standard.

Examples 35 to 38

Example 34 was repeated with different compositions to investigate the use of a different interface-modifying agent, the presence of pigment (Ti0 ₂ ) , UV absorber (Tinuvin 900) , cross-linking agent (TGIC) and additives

(degasser, anti-oxidant, flow aid and catalyst) in the fluorinated acrylic component, the presence of pigment (Ti0 ₂ ) and additives (degas¬ ser, anti-oxidant and flow aid) in the polyester component, and the use of a different polyester. In Example 37, the pigment, the other additives and the acrylic component were melted together at 120°C before dry blending; in Example 36 the additional ingredients were incorporated in the respective component by coextrusion at 110°C. The results are summarised in the following Table.

component 1 component 2

The surface tension plots (Figs. 5 to 7) show how the surface tensions of the individual components and interface-modifying agents varied with temperature. The polyester had a value of 38 dynes/cm at 150°C, falling to 23 dynes/cm at 200°C, while the fluorinated acrylic had a value of approximately 14 dynes/cm at 150°C, falling to 4 dynes/cm at 200°C. The two interface-modifying agents NPB10 and NPB21 were found to have surface tensions of 28 and 16 dynes/cm at 150°C, falling to 23 and 11 dynes/cm at 200°C respectively.

The lower surface tension of the fluorinated acrylic component in comparison with the polyester component may provide the driving force for segregation. NPB10 and NPB21 had surface tensions intermediate between the polyester and acrylic. However, NPB21 had a much lower surface tension (slightly higher than the acrylic) than NPB10, which had a surface tension similar to the poly¬ ester. This might explain why the film of Example 36 (RP0434/TGIC/NPB21 coextruded + 5% dry blended acrylic)

had a better surface appearance than the other 5% hybrids. Since NPB21 has a much lower surface tension than the polyester but still higher than the acrylic it might have formed a thin layer between the polyester and acrylic. This should help to compatibilise the polyester and allow the acrylic to "wet out" more readily.

Example 39

Three further tests were carried out using the RP0434/TGIC/3% NPB10 extrudate dry-blended with 10% of the fluoroacrylic component. In each test a different stoving schedule was used to investigate the effect of time to gellation on segregation. Three schedules were used: a standard schedule of 15 minutes at 200°C, a reduced temperature schedule of 15 minutes at 140°C and a stepped schedule of 5 minutes at 140°C and 10 minutes at 200°C.

The results showed that increasing the time to gellation by altering the stoving schedule enhanced segregation. Using the stepped schedule of 5 min at 140°C and 10 min at 200°C, the depth to which the fluorinated acrylic could be determined as >95% increased such that essentially all of the fluorinated acrylic could be accounted for in the stratified layer at the surface.

SOLVENT-BASED SYSTEMS Example 40

Two initial polymer solutions were prepared as follows.

Component 1 consisted of the fluorinated acrylic polymer DA 190 (PTFEMA) in 17.8g methyl ethyl ketone and 17g xylene prepared as follows: lOg DA190 was placed in a beaker, to which 17. Og of xylene was added. The solution was heated but the polymer was observed to precipitate out of solution

rapidly on recooling. Methyl ethyl ketone (17.8g total) was added to keep the polymer in solution.

Component 2 consisted of 200g Epikote 828 liquid epoxy resin (100% solids) .

The system was formed by mixing the two liquids (components 1 and 2) and 135g of curing agent Versamide 125. This gave a total content of solids of 91.4% (acrylic polymer 2.6% and epoxy resin 88.2% (52.7% epoxy polymer + 35.5 % curing agent) by weight) .

This mixture was applied to degreased aluminium panels to give films with a nominal (wet film) thickness of 200μm. Separate samples were stoved at 530°K (257°C) , at 430°K (157°C) and at 350°K (77°C) for 1 minute.

The coated aluminium panels were cut down to 1cm ² for XPS analysis. Phase separation was followed by monitoring the level of fluorine present at the surface.

The analysis demonstrated unambiguously the rapid and efficient segregation of fluoropolymer to the surface. After 1 min. stoving at 530°K, the surface was found to be enriched in acrylic by a factor of 25 above the expected bulk value; after stoving at 430°K and 350°K, the corresponding enrichment was by factors of 22 and 15 respectively.

Example 41

The PGMA/PBA/PZ polymers prepared by solution poly¬ merisation method 5 as described above were mixed in a weight ratio of 10:90 fluorine polymer to epoxy resin with a solution of Epikote 828 in xylene containing the thiol curing agent Penta/Mpa and Dabco catalyst, both available from Hampshire Chemicals, in a weight ratio epoxy:curing agent of 3.5:1. The solids contents of the mixed compositions and the results achieved on applica¬ tion to a substrate and after curing at room temperature are shown below.

% fluorinated compound in top 2.5μm

70% 55% 65% 65% 55% 60% 75% 65%

Table 2 gives zero shear viscosity data, and Figures 3-4 show the zero shear viscosity plots, of various top¬ coat polymers included in these solvent-based composi¬ tions. Data for the base-coat polymer, Epikote 828, is given in Table 2, together with data for another rep¬ resentative epoxy resin for solvent-based systems, Eurepox RV-A. Comparison of the data for these two resins shows that a zero shear viscosity plot for Epikote 828 would follow closely that of Eurepox RV-A shown in Figure 4, and having regard to the Figure 4 plots it can be seen that at room temperature (the temperature of application) the zero shear viscosities of the represen¬ tative top-coat polymers are less than the zero shear viscosity of the base-coat polymer Epikote 828. Even allowing for the effect of solvent and curing agent + catalyst (see Table 2) , the differential in the zero shear viscosities of the components added is such as to permit good segregation.

All the compositions used in the Examples had a zero shear viscosity of at least 10 Poise at room temperature. All had a solids content of at least 80 % by volume, at least 85 % by weight, and contained less than 250 g/litre of solvent.

Illustrative scanning electron microscopy pictures of substrates coated with 20:80 mixtures of a fluorinated acrylic polymer (100% polyZonyl prepared by suspension polymerisation) with a polyester (RP 0555) are shown in Figures 8(a) and 8(b) . In 8(a) the acrylic component contained 2% purple pigment and the polyester was unpig¬ mented; in 8(b) the acrylic component was unpigmented and the polyester contained Ti0 ₂ pigment.

The very uniform bilayer structures formed on the aluminium substrate (white portion on the left-hand side) are immediately evident. The fluorine line scan in Figure 8 is diagnostic for fluorine, and as can be seen all fluorine appears in the top layer. This is confirmed also by measurement of the width of the "top" layer in relation to the width of the entire coating on both pictures.

Figure 9 shows a scanning electron microscopy picture of a substrate coated with a thermomechanically fused mixture of 20% by weight of a 50/50 (by weight) Zonyl/methyl methacrylate polymer prepared by solution polymerisation with 80 % by weight of a fully formulated polyester component containing a white pigment. As can be seen, a bilayer system was produced.

Figure 10 shows a corresponding bilayer produced with a dry-blended mixture of 30 % by weight of a low molecular weight 50/50 Zonyl/methyl methacrylate polymer prepared by suspension polymerisation procedure 2 run c in the presence of a chain transfer agent with 70 % by weight of the same white polyester, and should be contrasted with Figure 11, where a corresponding polymer of high molecular weight and high viscosity failed to give stratification.