|1.||Aqueous composition for use in heat sealing applications comprising a combination of at least one polyester polymer and at least one acrylic polymer, wherein said combination of polymers is in the form of an aqueous solution in said composition, and wherein said polyester has a number average molecular weight of below 2,500, preferably < 2100, and said acrylic polymer has a glass transition temperature Tg of from 5 to 105°C, preferably from 40 to 70°C, and wherein the solubility of said polymer combination in the aqueous medium is provided by or contributed to by the presence of carboxylate anion groups and/or sulphonate anion groups borne by at least the at least one polyester polymer.|
|2.||Aqueous composition according to claim 1 wherein a polyester polymer and an acrylic polymer in the composition have been brought together by the preparation of the acrylic polymer in the presence of the polyester polymer, optionally in the absence of a conventional emulsifying agent for the acrylic polymerisation, and optionally without employing a monomer(s) for providing acid anion groups in the acrylic polymer.|
|3.||Aqueous composition according to claim 1 wherein the polyester and acrylic polymers in the composition have been brought together by blending of the preformed polymers.|
|4.||Aqueous composition according to any one of claims 1 to 3 wherein said at least one polyester polymer is formed by the condensation of an acid component and a hydroxyl component in which the acid component contains at least 30 mole % of aromatic carboxylic acid(s) (or esterforming derivatives).|
|5.||Aqueous composition according to any one of the preceding claims wherein said at least one polyester polymer is formed by the condensation of an acid component and a hydroxyl component and bears chain pendant sulphonate anion groups for providing or contributing to the solubility of the polymer combination in the aqueous medium, said groups being present in the polyester by employing as part of the acid component an alkali metal salt of a sulphonic acidsubstituted aromatic dicarboxylic acid or ester.|
|6.||Aqueous composition according to claim 5 wherein said alkali metal salt of a sulphonic acidsubstituted aromatic dicarboxylic acid or ester in sodio5 sulphoisophthalic acid.|
|7.||Aqueous composition according to any one of the preceding claims wherein said at least one polyester polymer bears carboxylate anion groups for providing or contributing to the solubility of the polymer combination in the aqueous medium.|
|9.||Aqueous composition according to any one of the preceding claims wherein said at least one polyester incorporates hydrophilic nonionic segments, preferably polyethylene oxide chains, which are present as inchain and/or chainpendant and/or terminal groups, and said nonionic groups when polyethylene oxide chains preferably not exceeding 25 % by weight, more preferably not exceeding 15% by weight, 0 of the polyester.|
|10.||Aqueous composition according to any one of the preceding claims wherein said at least one polyester is wholly or predominantly sulphonate stabilised and has a sulphonate anion group content within the range of from 7.5 to 100 milliequivalents, s preferably 10 to 75 milliequivalents per 100 g, of polyester.|
|11.||Aqueous composition according to any one of claims 1 to 8 wherein said at least one polyester is wholly or predominantly sulphonate stabilised, and wherein sodio5sulphoisophthalic acid is used for providing the sulphonate anion groups and is 0 employed in the polyester synthesis in an amount within the range of 2 to 20% by weight, preferably 3 to 15% by weight, based on the weight of all the monomers used in the polyester synthesis.|
|12.||Aqueous composition according to either claim 9 or claim 10 wherein the 5 carboxylic acid value AV of the at least one polyester is within the range of from 0 to 100 mg KOH/ g, preferably 0 to 50 mg KOH/g.|
|13.||Aqueous composition according to any one of claims 1 to 8 wherein said at least one polyester is wholly or predominantly carboxylate anion stabilised and has a o carboxylic acid value AV within the range of from 20 to 140 mg KOH/g, preferably 30 to 100 mg KOH/g.|
|14.||Aqueous composition according to any one of the preceding claims wherein said at least one polyester has a number average molecular weight Mn within the 5 range of from 500 to < 2500, preferably 750 to 2100.|
|15.||Aqueous composition according to any one of the preceding claims wherein said at least one acrylic polymer is a polymer made from a monomer charge which comprises 40 to 100 weight %, preferably 50 to 100 weight %, of a (meth)acrylate 0 monomer(s) of formula CH2 = CRsCO2R6 where R5 is H or methyl and R6 is C1 20 alkyl (preferably C16 alkyl) or cycloalkyl of 514 ring carbon atoms (preferably 610 ring carbon atoms).|
|16.||Aqueous composition according to claim 14 wherein said (meth) acrylate 5 monomer(s) is selected from methyl acrylate, methyl methacrylate, isopropyl acrylate, isopropyl methacrylate, ethyl acrylate, ethyl methacrylate, nbutyl acrylate, nbutyl methacrylate, cyclohexyl acrylate, cyclohexyl methacrylate, isobornyl methacrylate and isobornyl acrylate.|
|18.||Aqueous composition according to any of the preceding claims wherein said at least one acrylic polymer is made from a monomer charge which comprises 0 to 20 weight %, preferably 0 to 10 weight % of at least one ethylenically unsaturated acid selected from sulphonic and carboxylic acid(s).|
|20.||Aqueous composition according to claim 16 wherein said at least one acid is a monofunctional carboxylic acid, preferably acrylic acid and/or methacrylic acid.|
|21.||Aqueous composition according to any one of the preceding claims wherein acid anion groups are formed from carboxylic acid groups, if present in the 0 acrylic polymer, subsequent to the formation of the acrylic polymer by neutralisation of at least some of these groups.|
|22.||Aqueous composition according to any one of the preceding claims wherein said acrylic polymer contains no units of an acid monomer. 5.|
|23.||Aqueous composition according to any one of the preceding claims wherein said acrylic polymer comprises polymerised units derived from an ethylenically unsaturated monomer(s) which is neither a (meth)acrylate of formula CH2=CRsCO2R6 nor an acid monomer, said monomer(s) preferably being selected from one or more of 1 ,3 o butadiene, isoprene, styrene, various substituted styrenes, preferably αmethylstyrene, divinyl benzene, acrylonitrile, methacrylonitrile, vinyl halides, preferably vinylidene chloride, vinyl chloride and vinyl fluoride, vinyl esters, preferably vinyl acetate, vinyl propionate and vinyl laurate, heterocyclic vinyl compounds, alkyl diesters of mono olefinically unsaturated dicarboxylic acids, preferably dinbutyl maleate and dinbutyl 5 fumarate, and ethylenically unsaturated functionalised monomer(s) bearing hydroxyl, glycidyl, amino or siloxane group, preferably of formula CH2=CR7CO2R8 where R7 is H or methyl and R8 is NH2, hydroxyalkyl or hydroxycycloalkyl of 1 to 20 carbon atoms, more preferably 1 to 6 carbon atoms, dialkylaminoalkyl and glycidyl.|
|24.||Aqueous composition according to any one of the preceding claims wherein said at least one acrylic polymer is derived from a monomer charge which comprises 40100 wt % of a monomer(s) of formula CH2=CR5CO2R6, preferably 50 to 100 wt %, 0 to 20 wt % of an ethylenically unsaturated carboxylic or sulphonic acid 5 monomer(s), preferably 0 to 10 wt %, and 0 to 60 wt % of a monomer(s) which is neither a (meth)acrylate of formula CH2=C5CO2R6 nor an acid monomer(s), preferably 050 wt %.|
|25.||Aqueous composition according to claim 21 wherein said at least one acrylic polymer is derived from a monomer charge comprising 90 to 100 wt % of C16, o preferably C1 4, alkyl (meth)methacrylate monomer(s) and 0 to 10 wt % of monofunctional ethylenically unsaturated carboxylic acid(s), preferably methacrylic acid and/or acrylic acid.|
|26.||Aqueous composition according to claim 22 wherein said at least one 5 acrylic polymer is an ethyl acrylate/ methyl methacrylate copolymer.|
|27.||Aqueous composition according to any one of the preceding claims wherein said at least one acrylic polymer has a number average molecular weight of at least 50,000, preferably at least 100,000. 0.|
|28.||Aqueous composition according to my one of the preceding claims wherein the weight ratio of the at least one polyester polymer to the at least one acrylic polymer therein is within the range of from 80/20 to 20/80, preferably 75/25 to 25/75.|
|30.||Aqueous composition according to any one of the preceding claims wherein said composition includes one or more of a hot slip agent, an antiblocking material, and a wetting surfactant.|
|31.||Use in heat sealing applications of an aqueous composition according to o any one of the preceding claims.|
|32.||Method of heat sealing a film(s) wherein an aqueous polymer composition according to any one of claims 1 to 26 is applied to the surface of the parts of the film(s) to be sealed, the aqueous polymer composition is dried, the coated film parts clamped or 5 crimped together under pressure and heat is applied for a time period to effect heat sealing.|
|33.||Method according to claim 28 wherein prior to applying the aqueous polymer composition to the surface of the film, the latter is treated to enhance the adhesion of the resulting coating after drying, said treatment being selected from film chlorination, treatment with oxidising agent, flame treatment, and electronic treatment.|
|34.||Method according to either claim 28 or claim 29 wherein an intermediate adhesionpromoting primer is applied to the surface of the parts of the film(s) to be sealed before application of the aqueous coating composition.|
|35.||Method according to any one of claims 28 to 30 wherein the film(s) to be sealed is a polypropylene film, preferably a molecularly oriented, isotactic polypropylene film.|
|36.||Coated film substrate suitable for heat sealing, wherein said coated film is obtained by applying an aqueous polymer composition according to any one of claims 1 to 26 to the regions of a film to be sealed, preferably a polypropylene film, and drying the aqueous polymer composition.|
|37.||Heatsealed package made from a film material(s) suitable for packaging, preferably polypropylene, wherein heatsealing has been effected using a coating on said film derived from an aqueous polymer composition according to any one of claims 1 to 26.|
The present invention relates to certain aqueous polymer compositions and their use in heat sealing. In the packaging industry using film materials such as paper, metal foil
(e.g. Al) and, in particular thermoplastics films (including metallised thermoplastics films), there is often a need for the films to be heat sealable, especially at their border regions, in order to effectively enclose the object(s) or substance(s) being packaged or wrapped. The parts of the film(s) to be sealed are crimped or clamped together under pressure and heat applied so as to effect a bond there between.
Particularly important film materials for packaging are thermoplastics such as polyesters, e.g. polyethylene terephthalate and polyolefines, e.g. polyethylene, polypropylene, and polybutene. In the case of certain of these thermoplastics films e.g. oriented polyesters and polyolefines, and particularly oriented polypropylene, while their properties for packaging purposes are otherwise excellent, in themselves (i.e. without treatment) their heat sealing properties are either not apparent or are highly unsatisfactory. Thus, oriented films such as polypropylene have very high heat-sealing temperatures and a very narrow heat-sealing temperature range, and tend to disorientate and shrink when the prerequisite heat sealing temperatures are applied to their surfaces. To ameliorate these problems, various coating materials have been applied to the film surface. For example coatings of various hydrocarbon resins and mixtures of such resins have been applied in order to provide improved heat seal properties. Unfortunately such prior art coatings have often imparted deleterious effects to the coated film product - such as poor blocking characteristics, poor optical properties (hazy or milky), poor heat sealability, bad aging characteristics, and a tendency of the coating to delaminate on exposure to or storage at high relative humidities.
Other prior art compositions that have been used for improving heat- sealability have been organic-solvent based (e.g. toluene-based) chlorine-containing polymers such as polypropylene chloride and vinyl chloride/vinyl acetate copolymer. However the use of organic solvents is becoming increasingly undesirable from the environmental point of view.
US 3753769 describes an aqueous coating composition comprising an acrylic copolymer for improving heat-seal properties, e.g. by reducing heat-seal temperature and increasing heat-seal temperature range, while exhibiting good blocking, optical and hot slip properties. This copolymer is derived from 2.5-15 (2.5-6) parts of (meth)acrylic acid(s) and 97.5-85 (97.5-94) parts of neutral (meth)acrylate(s), and the composition also includes a hot slip agent and a finely-divided wax for providing cold-slip and antiblocking properties. It is stated that the glass transition temperature (Tg) of the acrylic copolymer should be in the range 37 to 60°C in order for the heat-sealing temperature to be as low as possible while exhibiting non-blocking at storage conditions
(which can be up to 44°C). Additionally the acid groups are neutralized with ammonia so that the acrylic polymer is in aqueous solution; this is said to allow intimate blending with other components of the composition. US 3753769 also specifies that the heat-sealable resin should possess sufficiently low molecular weight in order to exhibit sufficient viscous flow at temperatures moderately above Tg to give a good seal, and also low molecular weight is desirable to result in ammonia water solutions of low viscosity. Such low molecular weight is achieved by using a chain transfer agent in the polymerisation, e.g. a mercaptan or halohydrocarbon.
US 4058645 describes heat-sealing compositions of typical formulations such as those described in US 3735769 but including low levels of a rosin ester (3-15 weight %) to reduce the tendency of heat seal failure at high humidity.
In order to obtain good heat sealability one requires in the coating resin low molecular weight in order to obtain acceptable viscous or melt flow at a low temperature just or moderately above the Tg (higher molecular weight will tend to incur chain entanglement and hence higher melt viscosity), and the Tg should itself be low in order to avoid the requirement for very high heat sealing temperatures. (The Tg of a polymer is that temperature at which it changes from a glassy, brittle solid to a plastic or rubbery state. At temperatures below Tg, or even slightly above Tg, polymers exhibit such great resistance to viscous flow that sealing does not take place irrespective of applied pressure or how long the surfaces are left in contact with each other). When such viscous flow does occur, it should take place quickly over a narrow temperature range (i.e. the polymer should melt crisply) in order to avoid inhomogenous sealing characteristics in different areas of the bonded film regions. The available heat-sealing temperature range above Tg may (and preferably should) still be broad of course (providing the upper heat-seal temperature employed is acceptably low).
On the other hand, for good "in-use" application properties, such as good block resistance (wet and dry, the former being that at high humidity), good heat-seal strength, and good abrasion resistance, high molecular weight in the polymer is required. Such desired properties dependent on either high or low molecular weight, are thus mutually incompatable, and so to incorporate them by the use of a single polymer leads to a series of compromises.
Similarly, low Tg is required (as mentioned above) for a low heat-seal threshold temperature, while high Tg is required for properties such as good resistance to aqueous plasticisation, improved hardness and good bond strength. Again, to achieve such properties when using a single polymer requires compromise.
In JP 60-147486 there is disclosed an aqueous polyester composition for heat-sealing which provides an improvement over the single polymer or single polymer type systems of the prior art publications discussed above. This aqueous composition is composed of an aqueous solution comprising a mixture of a polyester of molecular weight 2,500-30,000 containing suiphonic acid salt and/or carboxylic acid salt groups, a
(meth)acrylate copolymer with Tg of 5-105°C (preferably 30-80°C) and optionally containing (meth)acrylic acid salt groups, optionally a water-soluble organic solvent of boiling point 60-200°C, optionally a surfactant, and, of course, water. The presence of the organic solvent and/or wetting agent (if used) is to improve wetting properties on application of the composition to films and to facilitate dissolution of the higher molecular polyesters..
Although not explicitly stated or discussed in JA 60-147486, the use of a combination of polyester and acrylic polymers as defined therein, which as shown by the examples is intended to be a simple blend of the preformed polymers, does allow the problem of unwanted compromises incurred by the use of a single polymer type (as discussed above) to be ameliorated. Thus the use of the oligomeric polyester allows desired properties associated with low molecular weight such as good viscous flow just above Tg (as discussed above), while the presence of the acrylic polymer, which is not limited in molecular weight and so can be high (although no minimum level is actually specified) allows desired properties associated with high molecular weight to be achieved, such as good blocking resistance, good heat-seal bond strength and good abrasion resistance.
It is to be noted that the polyester and preferably the acrylic polymer components of the composition of JA 60-147486 bear acid salt groups, presumably to allow sufficient dispersibility of both as to achieve the necessary aqueous solubility to provide the required aqueous solution. Such an aqueous solution would be necessary to achieve or assist in obtaining transparancy for a simple blend of the polymers, and transparency is said to be a necessary condition for the heat-sealing coating.
While the composition disclosed in JA 60-147486 is certainly an advance over the previous systems taught in the art, it still leaves room for considerable improvement. Thus the minimum molecular weight for the polyester is 2,500. Below this value, it is said that mechanical properties (particularly flexibility) are poor.
We have now discovered that not only does the employment of a polyester of molecular weight < 2,500 in a composition of this type surprisingly (in view the teaching of JA 60-147486) not incur inferior mechanical properties, but such use achieves the advantage of allowing a distinctly lower heat seal threshold temperature in combination with good bond strength in comparison to currently available commercial products, as well as providing an improved heat-seal threshold temperature/bond strength balance in comparison to that allowed by the composition of JA 60-147486. Furthermore, it might have been expected that the use of such a low molecular weight polyester component would have incurred blocking problems (wet and dry) due to stratification or phase separation of any wax in the composition (see later), but surprisingly this drawback does not in fact materialize.
Accordingly, there is provided according to the invention an aqueous composition for use in heat sealing applications comprising a combination of at least one
polyester polymer and at least one acrylic polymer, wherein said combination of polymers is in the form of an aqueous solution in said composition, and wherein said polyester has a number average molecular weight of below 2,500, preferably < 2100, and said acrylic polymer has a glass transition temperature Tg of 5-105°C, more preferably 40-70°C, and wherein the solubility of said polymer combination in the aqueous medium is provided by or contributed to by the presence of carboxylate anion groups and/or sulphonate anion groups borne by at least the polyester polymer.
There is further provided according to the invention the use in heat sealing applications of an aqueous composition as defined above. There is further provided according to the invention a method of heat- sealing a film(s) wherein an aqueous polymer composition as defined above is applied to the surfaces of the parts of the film(s) to be sealed, the aqueous polymer composition is dried, the coated film parts clamped or crimped together under pressure and heat is applied for a time period (usually 0.5 to 5 seconds depending on the heat seal temperature) to effect heat-sealing. (The pressure and heat sources are then removed of course).
There is still further provided according to the invention a coated film substrate for heat sealing, wherein said coated film is obtained by applying an aqueous polymer composition as defined above to the regions of the film to be sealed, and drying the aqueous polymer composition.
There is yet further provided according to the invention a heat-sealed package made from a film material(s) suitable for packaging, wherein said heat-sealing has been effected using a coating on said film derived from an aqueous polymer composition as defined above. Aqueous compositions of the invention can provide an exceptionally advantageous combination of properties desirable in heat-sealing applications (as discussed above) such as low heat-seal threshold temperature, good viscous flow over a narrow range at the chosen heat seal temperature, strong heat-seal bonds, good abrasion resistance, good antiblocking (wet and dry), good optical properties (i.e. allowing clear and not milky/hazy coated films), and low sensitivity in the resulting seal to water or high humidity.
The polyester and acrylic polymers of the invention composition may be brought together when preparing the composition by a simple blending of the preformed polymers. For example, an aqueous solution of a polyester may be combined with an aqueous solution of an acrylic polymer, or a preformed acrylic polymer in solid form may be dissolved in an aqueous solution of the polyester. Where aqueous dissolution of the acrylic polymer is effected by neutralizing acid groups thereon to form acid anion groups, this may be done before, during or after combining the polyester and acrylic polymers. More preferably, however, a polyester polymer and an acrylic polymer in the composition are brought together by preparation of the acrylic polymer in the
presence of the polyester (i.e. in-situ preparation of the acrylic polymer). Usually the monomer(s) employed for making the acrylic polymer are emulsion polymerised in the presence of an aqueous solution of the polyester. Such in-situ polymerisation has the advantage that the amount of conventional (external) surfactant(s) (emulsifing agent(s)) which would normally have to be employed in the acrylic polymerisation process (i.e. in the absence of the polyester) may be much reduced or even omitted entirely, the polyester in effect acting as a stabilising entity in the acrylic polymerisation process. Another advantage is that there may be no need to incorporate monomer(s) for providing acid anion groups in the acrylic polymer in order to achieve water solubility; the in-situ process wherein the acrylic polymer becomes closely associated with the water soluble polyester may allow acrylic polymer solubility even without the presence of acid anion groups thereon. The reduction or elimination of such acid anion groups in the acrylic polymer will in turn provide decreased water sensitivity in the resulting coating, leading to improved properties such as wet blocking resistance and improved bond adhesion under wet or humid conditions.
The polymeric combination of the polyester and acrylic polymers should be present in the invention composition in the form of an aqueous solution. The requirement for solution characteristics with regard to the polymer particles is desirable for the achievement of good clarity (transparency) in the resulting coating (after drying), i.e. avoiding milky-looking or hazy coatings. While it is possible for the polymeric component to be completely dissolved in the aqueous medium, it may nevertheless be possible to achieve acceptable clarity with less than complete dissolution of the polymeric component. For example, one or both polymer types of the polymeric component may be partially rather than fully soluble in the aqueous medium, so that the polymeric component might exist as a colloidal dispersion, or intermediate a colloidal dispersion and a true solution, or could be partly dissolved and partly colloidally dispersed. Sometimes the distinction between colloidal dispersions, true solutions, and intermediate states such as those mentioned above are difficult to distinguish from a practical viewpoint. In any event the term "aqueous solution" is intended to extend to all of these situations described above and not only to complete dissolution of the polymeric component in the aqueous phase. The expression "solubility of said polymer combination in the aqueous medium" is also to be construed in like manner.
The polyester component of the composition can be prepared using conventional polymerisation procedures known to be effective for polyester synthesis. Thus, it is well known that polyesters, which contain carbonyloxy (i.e. -C(=O)-O-) linking groups may be prepared by a condensation polymerisation process in which an acid component (including ester-forming derivatives thereof) is reacted with a hydroxyl component. The acid component may be selected from one or more polybasic carboxylic acids such as di- or tri-carboxylic acids or ester-forming derivatives thereof such as acid halides, anhydrides or esters. The hydroxyl component may be one or more
polyhydric alcohols or phenols (poiyols) such as diols, triols, etc. (It is to be understood, however, that the polyester may contain, if desired, a proportion of carbonylamino linking groups -C(=O)-NH- (i.e. amide linking groups) by including an appropriate amino functional reactant as part of the "hydroxyl component"; such as amide linkages are in fact useful in that they are more hydrolysis resistant). The reaction to form a polyester may be conducted in one or more stages (as is well known). It would also be possible to introduce in-chain unsaturation into the polyester by e.g. employing as part of the acid component an olefinically unsaturated dicarboxylic acid or anhydride.
There are many examples of polybasic carboxylic acids (or their ester forming derivatives) which can be used in polyester synthesis for the provision of the acid component. One can for example mention C 4 to C 20 aliphatic dicarboxylic acids, and alicyclic and aromatic dicarboxylic acids (of C5-C12 ring carbons (or higher functionality acids) or their ester-forming derivatives (such as anhydrides, acid chlorides, or lower alkyl esters). Specific examples include adipic acid, fumaric acid, maleic acid, succinic acid, itaconic acid, sebacic acid, nonanedioic acid, decanedioic acid, 1 ,4- cyclohexanedicarboxylic acid, 1 ,3-cyclohexanedicarboxylic acid, 1 ,2- cyclohexanedicarboxylic acid, terephthalic acid, isophthalic acid, phthalic acid and tetrahydrophthalic acid. Anhydrides include succinic, trimellitic maleic, phthalic and hexahydrophthalic anhydrides. It is preferred that the acid component contains at least
30 mole % of aromatic polybasic carboxylic acid(s)(or ester-forming derivative^)) to provide the best possible heat-seal bond strength.
Similarly there are many examples of polyols which may be used in polyester synthesis for the provision of the hydroxyl component. The polyol(s) preferably have from 2 to 6 (2 to 4) hydroxyl groups per molecule. Suitable polyols with two hydroxy groups per molecule include diols such as 1 ,2-ethanedioI, 1 ,3-propanediol, 1 ,4- butanediol, 1 ,6-hexanediol, 2,2-dimethyl-1 ,3-propanediol (neopentyl glycol), the 1 ,2-, 1 ,3- and 1 ,4- cyclohexanediols and the corresponding cyclohexane dimethanols, diethylene glycol, dipropylene glycol, and diols such as alkoxylated bisphenol A products, e.g. ethoxylated or propoxylated bisphenol A. Suitable polyols with three hydroxy groups per molecule include triols such as trimethylolpropane (1,1 ,1-tris (hydroxymethyl)ethane). Suitable polyols with four or more hydroxy groups per molecule include pentaerythritol (2,2-bis(hydroxymethyl)-1 ,3-propanediol) and sorbitol (1 ,2,3,4,5,6-hexahydroxyhexane). The polyester should bear carboxylate anion groups and/or sulphonate anion groups for providing or contributing to the solubility of the polymeric combination in the aqueous medium. Such groups will be chain pendant and/or terminal in the polyester.
Chain-pendant sulphonate anion groups may be introduced into the polyester polymer molecules by using at least one monomer having two or more functional groups which will readily undergo an ester condensation reaction (i.e. carboxyl
groups, hydroxyl groups or esterifiable derivatives thereof) and one or more sulphonic acid groups (for subsequent neutralisation after polyester formation) or sulphonate anion groups (i.e. neutralisation of the sulphonic acid groups already having been effected in the monomer). (In some cases it may not be necessary to neutralize sulphonic acid groups since they may be sufficiently strong acid groups as to be considerably ionised in water and so provide anion groups without the addition of base). Often, the sulphonic acid or sulphonate anion containing monomer is a dicarboxylic acid monomer having at least one sulphonic acid salt group substituent (thereby avoiding any need to effect neutralization subsequent to polyester formation). (Alternatively, alkyl carboxylic acid ester groups may be used in place of the carboxylic acid groups as ester-forming groups). Such a monomer will therefore be part of the acid component used in the polyester synthesis.
The sulphonate anion groups will, of course, have the formula -SO 2 O\ with the counter ion usually being H + , alkali metal or alkaline earth metal cation (the latter being divalent and so being associated with two sulphonate anion groups instead of one), ammonium, organic amine cations such as those derived from trialkylamines (e.g. triethylamine, tributylamine), morpholine and alkanoldiamines and quaternary ammonium cations. It is preferred not to employ ammonium, or amine-derived cations in order to minimise odour in the resulting coating. It is particularly preferred that the cation is selected from Na + , Li + and K + .
Examples ofcompounds for providing sulphonate anion groups are the alkali metal salts of sulphonic acid substituted aromatic dicarboxylic acid such as those of formula:
where M is sodium, lithium, or potassium and R 1 is H or lower alkyl of 1 to 5 carbon atoms (such as methyl or ethyl). Preferred examples of sulphonic acid salt substituted aromatic dicarboxylic acids are the alkali metal salts of 5-sulpho-1 ,3-benzene dicarboxylic acid, which have the formula:
where M and R 1 are as defined above. Particularly preferred is the sodium salt (M=Na) where R 1 =H, this material being commonly known as sodio-5-sulphoisophthalic acid (SSIPA).
Other useful sulphonic acid containing monomers are the alkali metal salts of sulphonic acid substituted aromatic dicarboxylic acid-dihydroxyalkylesters which have the formula:
where M is sodium, potassium or lithium, and R 2 is an alkylene radical. (Such monomers will therefore, if used, form part of the hydroxyl component in the polyester synthesis). Preferred examples of sulphonic acid salt substituted aromatic dicarboxylic acid- dihyroxyalkylesters are the alkali metal salts of 5-sulpho-1 ,3-benzenedicarb oxylic acid -1 ,3-dihydroxyethylester which has the formula:
where M is sodium, lithium or potassium.
Carboxylate anion groups may be incorporated into the polyester by various means. For example, if the hydroxyl component of the reactants is stoichiometrically in excess of the acid component, a hydroxyl-terminated polyester can be formed, which may be subsequently converted to a carboxyl terminated polyester by reacting the hydroxyl groups with an appropriate reagent (such as an acid anhydride or a dicarboxylic acid). Alternatively terminal carboxyl functionality may be directly introduced by employing an appropriate stoichiometric excess of the acid component reactants. Yet further, chain-pendant carboxyl groups may be introduced by using reagents such as dimethylol propionic acid (DMPA) since if appropriate reaction condition are employed (e.g. polymerisation temperature below 150°C) the hindered carboxyl group thereof does
not take part to any significant extent in the ester-forming reactions during the polyester synthesis and the DMPA effectively behaves as a simple diol. Chain-pendant and/or terminal carboxyl groups could also be introduced by employing a tri- or higher functionality carboxylic acid or anhydride in the polyester synthesis such as trimellitic acid or anhydride. Combinations of each procedures could be used of course. It is thus seen that terminal or side-chain carboxyl groups or both can be introduced as desired. These can be fully or partially neutralized with an appropriate base to yield carboxylate anion groups. The counter ions used may be as for the sulphonate anions described above (apart from H + since the carboxylic acid groups themselves are normally insufficiently ionised to provide a significant amount of carboxylate anion groups - although F substituents would increase acid strength), with alkali metal ions such as Na + , Li + and K + again being particularly preferred, and ammonium and organic amine derived cations not being preferred on odour considerations.
The polyester may optionally incorporate hydrophilic non-ionic segments within the polyester backbone (i.e. in-chain incorporation), and/or as chain-pendant and/or terminal groups. Such groups may act to contribute to the dispersion stability or even water solubility of the polyester. For example, such polyethylene oxide chains may be introduced into the polyester during its synthesis by using as part of the hydroxyl component, ethylene oxide-containing mono, di or higher functional hydroxy compounds such as polyethlene glycols, and alkyl ethers of polyethylene glycols, examples of which include:
R 3 -O-(CH 2 CH 2 O) n -H HO-(CH 2 CH 2 O) m -H CH 2 -O-(CH 2 CH 2 O) p -H R -C-CH 2 -O-(CH 2 CH 2 O) p -H
CH 2 -o-(CH 2 CH 2 o) p -H
where R 3 is alkyl of 1 to 20 carbon atoms (e.g. methyl), R 4 is alkyl of 1 to 20 carbon atoms (e.g. ethyl), n is 1 to 500. m is 1 to 500, and p is 1 to 100. Preferably the number average molecular weight of the polyethylene oxide units (if used) should not exceed 10,000 in order not to encourage water-sensitivity in the resulting heat seal.
A small segment of a polyethylene oxide chain could be replaced by a propylene oxide or butylene oxide chain in such non-ionic groups, but should still contain ethylene oxide as a major part of the chain. The amount of sulphonate anion and/or carboxylate anion groups present in the polyester should be sufficient to provide or contribute to the solubility of the polymeric combination of polyester and acrylic polymers in the aqueous medium of the composition although it should not be so high as to render the resulting coating unacceptably water-sensitive. This amount will depend, inter alia, on factors such as the hydrophilicity/hydrophobicity of units provided by other monomers in the polyester
synthesis or any surfactants (if used), and also the relative proportions of sulphonate and carboxylate anion groups. With regard to the last mentioned point, sulphonate anions are more effective at providing or contributing to water-solubility than carboxylate anion groups, and so can be used at considerably lower levels in comparison to those of 5 carboxylate anion groups.
If the polyester is wholly or predominantly sulphonate stabilized (by which is meant the water solubility-providing groups are provided wholly or predominantely by sulphonate anion groups) the sulphonate anion group content is preferably within the range of from 7.5 to 100 milliequivalents (more preferably 10 to 75 millequivalents and 0 particularly 1 1 to 56 millequivalents) per 100 g of polyester. When using SSIPA as the monomer for providing the sulphonate anion groups, the amount of this monomer used in the polyester synthesis, based on the weight of all the monomers used in the polyester synthesis will usually be within the range of from 2 to 20% by weight (more usually 3 to 15% by weight). The carboxylic acid value AV of polyester which is predominantly s sulphonate stablized, i.e. an AV based on the carboxylic acid groups only (i.e. excluding sulphonate groups) will generally be within the range of from 0 to 100 mgKOH/g, more preferably 0 to 50 mgKOH/g, and particularly 0 to 25 mgKOH/g.
If the polyester is wholly or predominately carboxylate anion stabilized, the carboxylic acid value AV of the polyester is preferably within the range of from 20 to 0 140 mgKOH/g (more preferably 30 to 100 mgKOH/g.
Usually, the polyester is either wholly or predominantly sulphonate- stabilized or wholly or predominantly carboxylate stabilized (preferably the former).
If the polyester contains polyethylene oxide chains, the polyethylene oxide chain content should preferably not exceed 25% by weight (and more preferably 5 should not exceed 15% by weight), based on the total weight of the polyester, in order to avoid unacceptable water-sensitivity. Therefore the amount is preferably 0 to 25% by weight (more preferably 0 to 15% by weight) based on the total weight of the polyester.
The polyester has a number average molecular weight Mn of below 2500, preferably < 2100 and more preferably < 1900. From a practical viewpoint the minimum o value of Mn will usually be 500, more usually 750. Therefore a preferred range for Mn is
500 to <2500 (more preferred 750 to < 2500, still more preferred 750 to 2100, and particularly 750 to 1900. (The measurement of Mn is well known to those skilled in the art, and may e.g. be effected using gel permeation chromatography (gpc) in conjunction with a standard polymer such as polystyrene or polymethylmethacrylate of known 5 molecular weight).
Polyester Tg (the temperature at which the polymer changes from a glassy, brittle state to a plastic, rubbery state) is preferably in the range 20 to 105°C, more preferably 20 to 70°C.
The polyester preferably has a hydroxyl number within the range of 20 to o 300 mgKOH/g (more preferably 40 to 250 mgKOH/g).
The esterification polymerisation processes for making polyesters for use in the aqueous composition are well known and need not be described here in detail. Suffice to say that they are normally carried out in the melt using catalysts such as tin- based catalysts and with the provision for removing any water (or alcohol) formed from the condensation reaction.
An aqueous solution of the polyester, (which may be used as part of, or in the preparation of, the aqueous solution of the polyester/acrylic combination of polymers) may be readily prepared by dispersion of the solidified melt from the condensation polymerisation directly with agitation (stirring) into water. The solidified melt is preferably in a form such as flake (which can often be obtained directly from the melt) or comminuted solid (obtained for example by grinding). Alternatively, water can be added directly to the hot polyester melt until the desired solids content/viscosity is reached. Still further, the polyester may be dispersed in water by adding an aqueous pre-dispersion (or organic solvent solution) of the polyester to the water phase. Preferably the pH of the aqueous polyester solution is from 1.5 to 10, more preferably from pH 2.5 to 4.5 if predominantly sulphonate stabilized and pH 6.5 to 10 (particularly 7 to 9) if predominantly carboxylate stablized. Solids content is preferably from 5 to 55 wt % (on a total weight basis) more preferably 10 to 45 wt %. Solution viscosity is usually within the range of from 5 to 500 centipoise. (Brookfield viscosity at 25°C).
The polyesters normally do not require the essential use of an external surfactant (emulsifier) when being dispersed into water, although such emulsifiers may be used to assist the dispersion if desired and in some cases can be useful in this respect because additional surfactants reduce the required amount of the stabilizing groups (i.e. sulphonate and/or carboxylate and/or ethylene oxide chains if used).
The acrylic polymer component of the heat-sealable composition is typically a polymer made from the polymerisation of a monomer charge which comprises a high amount, e.g. 40-100 weight %, more usually 50 to 100 weight %, of a (meth)acrylate monomer(s) of formula CH 2 =CR 5 CO 2 R 6 where R s is H or methyl and R 6 is alkyl of 1 -20 carbon atoms (more preferably 1 -6 carbon atoms) or cycloalkyl of 5-14 ring carbon atoms (more preferably 6-10 ring carbons atoms). Examples of these are, in particular, methyl acrylate, methyl methacrylate, isopropyl acrylate, isopropyl methacrylate, ethyl acrylate, ethyl methacrylate, n-butyl acrylate, n-butyl methacrylate, cyclohexyl acrylate and cyclohexyi methacrylate, isobornyl methacrylate and isobornyl acrylate. The acrylic polymer may also contain polymerised units of at least one ethylenically unsaturated acid selected from sulphonic and carboxylic acid(s) (or ethylenically unsaturated monomers which have acid-forming groups which readily yield such acid groups, such as anhydrides and acid chlorides) preferably an ethylenically unsaturated carboxylic acid, and more preferably a monofunctional carboxylic acid(s) such as one(s) selected from acrylic acid and/or methacrylic acid.
The amount of ethylenically unsaturated acid(s) in the monomer charge will usually be from 0 to 20 weight % (more preferably 0-10% weight %, and particulariy 0- 5 weight %) based on the total weight of monomers charged. When such acid units are present in the acrylic polymer in the composition, they (or at least a proportion of them) they may be in sulphonate or carboxylate anion form in order to provide or contribute to the water solubility of the acrylic polymer. Neutralization of acid groups to form anion groups may take place prior to or after formation of the acrylic polymer; e.g. the acid monomer could be polymerised as the free acid and neutralized after polymerisation, or polymerised as a salt (e.g. the alkali-metal salts of ethyl methacrylate-2-sulphonic acid or 2-acrylamido-2-methylpropane sulphonic acid, or their corresponding free acids). It is preferred that the unsaturated acid(s) is in fact a carboxylic acid(s), and is particulariy preferably one or both of acrylic acid and methacrylic acid. Such a carboxylic acid(s) will normally be polymerised (when forming the acrylic polymer) as the free acid, and neutralization of at least some of the polymer-borne acid groups effected after polymerisation by neutralization (with a base such as LiOH, NaOH or KOH) before, during or after dispersing into/dissolving in water. In one of the preferred embodiments of the invention, the acrylic polymer contains no units at all of acid monomer(s), or if they are present they are in unneutralized form. Such an embodiment is usually associated with the case where the polymerisation to form the acrylic polymer has been carried out in- situ, i.e. in the presence of the preformed polyester; as explained above, the inter- polymer association achieved by such a technique between the polyester and acrylic polymer allows acceptable water solubility of the polymer combination as a whole (to achieve good clarity) to be achieved by virtue of the anion groups on the polyester only - although of course anion-bearing acrylic polymer may still be used. The absence of acid anion groups on the acrylic polymer is extremely advantageous in some ways as it results in a much less hydrophilic or water-sensitive polymer combination in the resulting heat seal (the carboxylic acid groups on a high molecular weight acrylic polymer are more liable to cause water sensitivity than relatively small amount of sulphonate anion groups on a low molecular polyester. This carries through into improvements in properties associated with lowered water sensitivity; such as improved wet blocking resistance, greater bond resistance to wet or humid conditions).
The acrylic polymer may also comprise polymerised units derived from an ethylenically unsaturated monomer(s) which is neither a (meth)acrylate as defined above, nor an acid-monomer as described above. Examples of such monomers include 1 ,3- butadiene, isoprene, styrene, the various substituted styrene such as α-methylstyrene, divinyl benzene, acrylonitrile, methacrylonitrile, vinyl halides such as vinylidene chloride, vinyl chloride and vinyl fluoride, vinyl esters such as vinyl acetate, vinyl propionate and vinyl laurate, heterocyclic vinyl compounds, and alkyl diesters of mono-olefinically unsaturated dicarboxylic acids (such as di-n-butyl maleate and di-n-butyl fumarate). This group may also include ethylenically unsaturated functionalized monomer(s) (the
functionality being other than acid) for providing functional groups in the polymer which impart properties such as curability and adhesion to the resulting hybrid polymer system (such as hydroxyl, glycidyl, amino and siloxane groups). Examples of such functionalised monomers include those of formula CH 2 =CR 7 CO 2 R 8 where R 7 is H or methyl and R 8 is NH 2 , hydroxyalkyl or hydroxycycloalkyl of 1 to 20 carbon atoms (more preferably 1 to 6 carbon atoms), dialkylaminoalkyl or glycidyl, examples of which include acrylamide, methacrylamide, 2-hydroxyethyl methacrylate, 2-hydroxyethyl acrylate, glycidyl acrylate, glycidyl methacrylate and dimethylaminoethyl methacrylate.
The amount of such non-(meth)acrylate (as defined), non-acid monomer(s) in the monomer charge is usually within the range of 0 to 60 wt %, more usually 0-50 wt %, particulariy 0-40 wt %, based on the total monomer charge. Frequently of course, the amount will be zero.
Preferred acrylic polymers are therefore derived from a monomer charge which comprises 40-100 wt % of a monomer(s) of formula CH 2 =CR 5 CO 2 R 6 (more preferably 50 to 100 wt %, particulariy 90 to 100 wt %), 0 to 20 wt % of an ethlenically unsaturated carboxylic or sulphonic acid monomer(s) (more preferably 0-10 wt %, particularly 0 to 5 wt %, and very often zero), and 0 to 60 wt % of a non-(meth)acrylate (as defined above), non-acid monomer(s) (more preferably 0-50 wt %, particularly 0-40 wt %, and often zero). Typical acrylic polymers are made from a monomer charge comprising 90 to 100 wt % of alkyl (meth)acrylate monomer(s) with 1-6C, preferably 1-4C, in the alkyl group, and 0-10 wt % of monofunctional ethylenically unsaturated carboxylic acid(s) (e.g. methacrylic acid and/or acrylic acid). As a specific example one may mention an ethyl acrylate/methyl methacrylate copolymer. The Tg of the acrylic polymer is within the range of from 5 to
105°C preferably from 40 to 70°C.
The number average molecular weight Mn of the acrylic polymer should be reasonably high in order to provide good properties associated with a high molecular weight polymer component (as discussed above). Usually the acrylic polymer Mn will be at least 50,000, more usually at least 100,000. The upper limit will not usually exceed
In principle, the acrylic polymer may be made using a bulk, organic solution, or aqueous suspension polymerisation process (all of such processes being well known to the art), wherein the solid product (granular or bead in the case of suspension polymerisation) is isolated from the polymerisation medium and dissolved in water (converting acid groups thereof to anion groups if necessary by neutralization). The dissolution may be performed before, during, or after combining with the polyester. For example separate aqueous solutions of acrylic and polyester may be mixed; or polyester and acrylic solids may be introduced to water and then dissolved by neutralization; or a
solid acrylic polymer may be dissolved in an aqueous solution of polyester to form the polymer combination.
Far more usual, however, is for the acrylic polymer to be made using an aqueous emulsion polymerisation process. This allows an aqueous emulsion or latex of the acrylic polymer to be formed, and the acid groups of the polymer (if present) can be neutralized if necessary to achieve water solubility. The polymerisation can be performed in the absence of the polyester, in which case the two polymers (or their solutions etc) are subsequently mixed (see above), or, more preferably the acrylic polymerisation process can be performed in the presence of the polyester, particularly in the presence of an aqueous solution of it. This usually allows the presence of acid groups to be omitted, since even if such groups are absent, the resulting composition has sufficient solution characteristics to allow acceptable optical properties to be achieved in the heat seal film (good clarity or transparency).
The actual emulsion polymerisation technique used to form the acrylic polymer may in itself be quite conventional and need not be described in great detail here. Suffice to say that such a process involves dispersing the monomer(s) in an aqueous medium and conducting polymerisation using a free-radical initiator (normally water soluble) and (usually) appropriate heating (e.g. 30 to 120°C) and agitation (stirring) being employed. The aqueous emulsion polymerisation can be effected with conventional emulsifying agents (surfactants) being used [e.g. anionic and/or non-ionic emulsifiers such as Na, K and NH 4 salts of dialkylsulphosuccinates, Na, K and NH 4 salts of sulphated oils, Na, K and NH 4 salts of alkyl sulphonic acids, Na, K and NH 4 alkyl sulphates such as sodium lauryl sulphate, alkali metal salts of sulphonic acids. C** 2 . 24 fatty alcohols, ethoxylated fatty acids and/or fatty amides, Na, K and NH 4 salts of fatty acids such as Na stearate and Na oleate; aryl-containing analogues of the alkyl-containing surfactants are also useful; other surfactants include phosphates. The amount used is preferably low, preferably 0 to 10% by weight, more usually 0 to 8% by weight based on the weight of total monomer(s) charged (the value 0 is possible because the polyester itself can sometimes function as an emulsifying agent - see following paragraph). The polymerisation can employ conventional free radical initiators [e.g. hydrogen peroxide, t- butyl-hydroperoxide, cumene hydroperoxide, persulphates such as NH 4 persulphate, K persulphate and Na persulphate: redox systems may be used; combinations such as t- butyl hydroperoxide isoascorbic acid and FeEDTA are useful; the amount of initiator, or initiator system, is generally 0.05 to 3% based on the weight of total monomers charged]. However, an additional and useful feature of the invention is that when the acrylic emulsion polymerisation is carried out in-situ (i.e. in the presence of preformed polyester), it is often possible to eliminate or much reduce the requirement for the addition of a surfactant to act as an emulsifier in the polymerisation to form the acrylic polymer, because the polyester itself can fulfil such a function (i.e. act as an emulsifying agent).Thus, in such a case, the aqueous emulsion (or rather solution) resulting from the
acrylic polymerisation preferably contains no or a very low level of such added emulsifier (not counting the polyester itself), with often less than 5% and sometimes zero), based on the total weight of monomers charged being used, and with the only surfactant present often being that remaining from the polyester polymerisation if used (not counting the polyester itself).
The acrylic polymerisation may use if desired a chain transfer agent such as one selected from mercaptans (thiols), certain halohydrocarbons and α-methyl styrene, as is quite conventional. However, the amount used, if present at all, will not be high since low molecular weight is not a requirement for the acrylic polymer. Such low or zero levels of mercaptan, possible because one does not require a low molecular weight acrylic polymer (unlike as in above-discussed prior art), since one already has present a low molecular weight polyester, allows compositions of improved odour to be made (another advantage of the invention). Other materials may also be present, e.g. water- soluble salts such as sodium bicarbonate may also be present. The acrylic polymerisation process may be carried out using an "all-in- one" batch process (i.e. a process in which all the components to be employed are present in the polymerisation medium at the start of polymerisation) or a semi-batch process in which one or more of the components employed (usually at least one of the monomers), is wholly or partially fed to the polymerisation medium during the polymerisation. Although not preferred, fully continuous processes could also be used in principle.
In general, the uncoated substrate films employed in the practice of the present invention are usually from 5 to up to 130 μm in thickness.
Before applying the coating composition to the appropriate substrate, the surface of the substrate film may optionally treated to enhance the adherence of the resulting coating after drying on the film, thereby lessening the possibility of the coating peeling or being stripped from the film. This treatment may be accomplished by employing known prior art techniques such as, for example, film chlorination, i.e. exposure of the film to gaseous chlorine, treatment with oxidizing agents such as chromic acid, hot air or steam treatment, flame treatment and the like. Although any of these techniques may be effectively employed to pretreat the film surface, a particularly desirable method of treatment has been found to be the so called electronic treatment method which comprises exposing the film surface to a high voltage corona discharge while passing the film between a pair of spaced electrodes. After electronic treatment of the substrate film surface it may be coated with the coating composition which will then exhibit a tendency to more strongly adhere to the treated film surface.
In applications where even greater coating-to-film adherence is desired, i.e. greater than that resulting from treatment of the film surface by any of the above- mentioned methods, an intermediate primer coating may be employed to increase the adherence of the coating composition of the present invention to the substrate film. In
that case the film is first treated by one of the foregoing methods, electronic treatment being a preferred method, to provide increased active adhesive sites thereon (thereby promoting primer adhesion) and to the thus treated film surface there is subsequently applied a continuous coating of a primer material. Such primer materials are well known in the prior art and for example include titanates and poly(ethylene imine), the latter being particularly effective. The imine primer provides an overall adhesively active surface for thorough and secure bonding with the subsequently applied coating composition of this invention. The primer may be applied to the electronically treated base film by conventional solution coating means such as mating roller application for example. It has been found that an effective coating solution concentration of the poly(ethylene imine) applied from either aqueous or organic solvent media such as ethanol, for example, is a solution comprising about 0.5 % by weight of the poly(ethyleme imine).
The coating composition may be applied to a film substrate in any convenient or known manner, such as by gravure coating, roll coating, dipping, spraying and painting. The excess aqueous solution may be removed by squeeze rolls, doctor knives, etc. The coating composition should be applied in such amount that there will usually be deposited upon drying, a smooth evenly distributed layer expressed as equivalent to about 0.5 to 2.0 grams per m 2 of film. In general, the thickness of applied coating is such that it is sufficient to impart the desired heat sealability and stiffness characteristics to the base film structure.
The wet coating on the film is subsequently dried using natural or accelerated drying (e.g. using hot air, or radiant heat).
The heat sealing composition may also optionally incorporate a hot-slip agent, in order to impart "hot slip", that is, satisfactory slip properties when the film material or partially wrapped package passes in contact with the heat sealing portions of wrapping or packaging apparatus such as heated platens, etc. It is usually defined quantitatively as the coefficient of friction at the temperature and pressure used. Suitable slip agents include freely divided, water insoluble inorganic materials such as colloidal silica. Other finely divided inorganic materials which can be used to enhance hot slip properties include such water insoluble solids as diatomaceous earth, calcium silicate, bentonite, and finely divided clays. In order to function most efficiently, it is desirable that this finely divided inorganic material have a particle size between 10 and 200 millimicrons, an alkali stabilized silica dispersion being the preferred material for use. The amount employed is usually from 0 to 60 % by weight (based on the polymer combination) of the slip agent and more preferably from about 0% to about 45% by weight (if present, preferably 30 to 60% by weight, more preferably 35 to 45% by weight).
"Blocking" is the tendency of film to adhere to itself when two or more surfaces of the film are held pressed together, for example when sheets or mill rolls of the film are stacked in storage. It is more pronounced at elevated temperatures and high relative humidities. Under normal storage conditions, the maximum temperatures
encountered will be between 38-42°C and the relative humidity may run as high as 90- 100%. It is desirable that under these conditions the coated film will not stick to itself - in other words, that it be resistant to blocking. Otherwise, when the film is stored in rolled form on cores, for example, the layers will stick together and the film cannot readily be unwound for use.
Anti-blocking materials which may be optionally incorporated into the composition include finely divided waxes and wax-like materials which melt at temperatures above the maximum temperatures encountered in the storage of the film and are not soluble in the polymer combination at these temperatures. Specific examples are natural waxes such as paraffin wax, microcrystalline wax, beeswax, camauba wax, japan wax, montan wax, and synthetic waxes such as hydrogenated castor oil, chlorinated hydrocarbon waxes and long chain fatty acid amides.
Such antiblocking agents are used in an amount of 0 to 20 wt % based on the weight of the polymer combination (more preferably 0 to 10 wt%. In addition to functioning as anti-blocking materials, the above-described wax materials when incorporated into the coating compositions also function to improve the "cold-slip" properties of the films coated therewith, i.e. the ability of a film to satisfactory slide across surfaces at about room temperatures (usually 15-25°C).
A particular type of thermoplastic film which can be advantageously coated with the coating compositions is a polypropylene film, and particularly a molecularly oriented, isotactic polypropylene film. After extrusion of the base polypropylene film utilizing conventional extrusion techniques the film is heated and molecularly orientented by stretching it in both longitudinal and transverse directions. The resultant oriented film exhibits greatly improved tensile and stiffness properties. However, it is difficult to heat seal by conventional techniques because at the requisite sealing temperature, i.e. of the order of about 180°C, film disorientation and shrinkage occur which results in the film rupturing and tearing apart. However, when such oriented films are subjected to surface treatment methods as hereinbefore described and subsequently coated with the present coating compositions they can then be sealed at temperatures sufficiently low to prevent shrinkage of the substrate, i.e. the oriented polypropylene film.
The composition used in the invention may optionally include a surfactant(s) (i.e. one(s) additional to that employed in the polymerisation processes to form the polyester and/or acrylic polymer) to improve the wetting characteristics. Such a material will usually be used at a level of 0 to 5 wt % (more preferably O to 3 wt % based on the weight of the polymeric combination of polyester and acrylic polymers. Examples of such surfactants include nonionic and anionic agents.
Although, not usually necessary, it is possible to incorporate an organic solvent into the composition, also to enhance wetting. The amount used is from 0 to 25 wt %, more preferably 0 to 15 wt %, based on the weight of polymeric material. Most
usually it is zero (i.e. none is used). Examples include water-soluble organic solvents of boiling point 60-200°C, specific examples of which are primary alcohols such as methanol, ethanol, n-propanol, i-propanol, n-butanol, i-butanol, sec-butanol, and tert- butanol, glycols such as ethylene glycol and propylene glycol, glycol derivatives such as methyl cellosolve, ethyl cellosolve, n-butyl cellosolve, t-butyl cellosolve, 3-methyl-3- methoxy butanol, and n-butyl cellosolve acetate, ethers such as dioxane and tetrahydrofuran, esters such as ethyl acetate, ketones such as methyl ethyl ketone, cyclohexanone, cyclooctanone, cyclodecanone, and isophorone.
The relative amount of the polyester and acrylic polymer to use in the composition may vary according to the particular compositions of the polymers being used. Generally speaking, however, the weight ratio of polyester/acrylic polymer in the composition (on a weight/weight basis) will usually be within the range of from 80/20 to 20/80, more preferably from 75/25 to 25/75, and particulariy from 70/30 to 30/70.
The amount of the polymeric combination (i.e. the polyester plus the acrylic polymer) in the composition will usually be in the range of 10 to 60 wt %, more usually 20 to 55 wt %, based on the total weight of the composition (including the water present).
The solids content of the compositions will usually be within the range of from 10 to 60, more usually 20 to 55 wt % (on a total weight basis). The present invention is now further illustrated, but in no way limited, by reference to the following examples. Unless otherwise specified all parts, percentages and ratios are on a weight basis. The prefix C before an example denotes that it is comparative.
In the examples, Tg is determined by differential scanning calorimetry run at 10 degrees Kelvin per minute, taking the peak of the derivative curve as Tg.
The heat seal strength was determined by using a static laboratory heat sealer at various temperatures (as indicated). A sealer dwell time of 2 sees and a pressure of 15 psi were used.
Examples 1 -3 and C4
Synthesis of an aqueous sulphonate-stabilised polyester solution (Polyester solution A).
To a glass reactor fitted with a distillation column and condensor, the following were charged with stirring under nitrogen:
Neopentyl glycol 1545g
Trimethylol propane 888g
Methoxy PEG750 * 450g
Isophthalic acid 1050g Sodio-5-sulphoisophthalic acid 600g
Fascat 4100 (Sn-based catalyst. ) 3g
( * polyethylene glycol monomethyl ether; Mn 750)
The contents were heated to a reaction temperature of 210°C until 340 mis of distillate was removed and held until a final acid value of 4.4mgKOH/g in dimethyl formamide DMF was obtained. To the reactor were charged:
Isophthalic Acid 926g
Adipic Acid 381 g
and the reaction continued for a further 1 hour, after which a vacuum of 200mbar was applied. After 1.5hrs under vacuum a total of 785mls of distillate was recovered and the resulting molten polyester resin was cast. The polyester was characterised by an acid value AV of 7.8mgKOH/g, an ICI cone and plate viscosity of 120 poises at 150°C, and a Tg of 25°C. 200g of the polyester resin were ground into a fine powder and slowly added to 600g of distilled water at 70°C with agitation until complete dissolution was obtained. The solution was characterised by a solids content of 25% w/w and a pH of 3.6.
Synthesis of polyester: acrylic composite solution compositions Examples 1-3.
As typical there is described the synthesis of a 50/50 w/w polyesteπacrylic composite based upon an acrylic composition ethyl acrylate EA/ methylmethacrylate MMA = 27.4:72.6. (This corresponds to Example 1).
To a reaction flask fitted with a stirrer and condenser were charged 400g of the above polyester solution A and 70g of distilled water; the charge was heated under nitrogen and agitation to a reaction temperature of 80-85°C. An acrylic monomer feed was prepared by mixing 27.4g ethylacrylate, 72.6g methylmethacrylate, 0.8g lauryl mercaptan and 0.1 g sodium bicarbonate. An aqueous initiator feed was prepared by dissolving 0.3g ammonium persulphate in 90g of distilled water.
To the reaction flask was added 10% of the monomer feed, and held there for 5 minutes. 30% of the initiator feed was added and held for a further 5 minutes before adding over 1 hour the remainder of the monomer and initiator feeds, whilst maintaining the reaction temperature. After this 3.7g of aq. i-ascorbic acid (20% w/w) and 1.0g t- butylhydroperoxide (70%) were added and the reaction held for a further 1 hour at 85°C. The product was cooled and filtered through a 125 μm mesh sieve.
The hybrid polymeric product was characterised by a solids content of 29.6% w/w, a pH of 3.11 and a dual Tg of 25°C and 52°C (the latter being the Tg of the acrylic polymer).
Examples 2 and 3 corresponding to 60/40 and 75/25 polyester/acrylic ratios, were similarly prepared.
The acrylic/polyester compositions were formulated with 8.2 wt % (based on the weight of polymers) of a proprietary wax/inorganic powder composition (antiblock/ hot slip composition)). They were coated using a laboratory reverse gravure coater onto polypropylene film (24μm) at a coat weight of 0.7-1.3 g m 3 . Heat seal bond strengths were evaluated against a standard commercial acrylic polymer ("Neocryl" BT35; Zeneca Resins) formulated in a similar fashion as a control (Example C4).
Example PolyesteπAcrylic Heat Seal (N): 15psi 2s dwell time (w/w)
90°C 100°C 110°C 120°C
1 50/50 120 264 135 280 2 60/40 NHS 157 240 280 3 75/25 NHS 189 265 280 C4 "Neocryl" BT 35 NHS 60 190 160
NHS: No heat seal.
"Neocryl" BT35: A Zeneca Resins commercial acrylic aqueous solution resin.
The polyester acrylic compositions show much lower heat seal temperature thresholds. Moreover, the films showed good transparency, with good wet and dry blocking characteristics.