Our earlier UK patent application 00265507.4 discloses a low melting point, low viscosity polypropylene containing adhesive suitable for use in high-speed manufacture of packaging.

W095/24449 discloses an adhesive composition comprising maleic anhydride-grafted propylene and grafted ethylene/vinyl acetate copolymer.

According to a first aspect of the present invention an adhesive comprises:- (a) about 80 to about100% of a melt blend of about 40 to about 98% polyalphaolefin having at least one monomer which has been grafted with about 0.05 to about 5% of at least 1 ethylenically unsaturated carboxylic acid or derivative thereof having a molecular weight distribution of 1.5 to 4.0 and a melt flow index not less than 20; (b) about 2 to about 60% of at least one copolymer of ethylene and vinyl acetate; (c) ) 0 to about 20% of an opacifier; (d) optional further ingredients; wherein the total amount of components (a) and (b) is from about 80 to 100% of the composition and wherein the percentages of the components are selected to total 100% According to a second aspect of the present invention there is provided a method of uniting surfaces of opposed layers using an adhesive composition in accordance with the first aspect of this invention, wherein the composition is in the form of a film, strands or pellets, comprising the steps of : contacting the film, strands or pellets with a first layer, contacting a second layer to the film, strands or pellets, causing the film, strands or pellets to melt and subsequently allowing the film, strands or pellets to solidify on cooling to form a united article.

Percentages and other proportions referred to in this specification are by weight unless indicated otherwise.

Other components may be added to the formulation to extend the range of applications such as organic or inorganic particulates to alter the modulus or ferromagnetic materials to allow dielectric heating.

The processing and properties of the adhesive films are aided by the addition of inorganic and/or organic micro/nano particulates alone or in combination at appropriate levels and in the range 0. 01-70% wt/wt. The mean particle size and particle size distribution of the particulate additives influence clarity of the film as well as processing end use properties.

There are optimal particle sizes and size distributions for the control of different properties at any given loading in the adhesive. For example, to retain clarity, monodisperse particulates with a mean diameter of less than 50, um are preferred. The rheological behaviour of the adhesive during film manufacture and during bond formation are influenced primarily by the loading of micro/nano particulates as well as particle size and size distribution.

An adhesive composition in accordance with the present invention may be applied onto a substrate by doctoring, melt casting, melt spraying, solution casting, emulsion casting or by other means. It may also be formed into a film or fibrous network which can be applied onto a substrate in a variety of ways such as compression, compaction and lamination. A film composed of an adhesive of the present invention can be employed as a free-standing film.

Alternatively, the adhesive composition maybe formed into rods, pellets or otherwise shaped portions to facilitate application to a substrate.

The adhesive might be pre-coated to substrates for subsequent use, e. g. a bi-layer or multiple layered film where the adhesive is, in the final product, the surface coating.

Films or other structures formed from compositions of this invention may have the advantage of being printable; the ink being applied as an aqueous, non-aqueous or molten form. They may also be paintable and accept sputtered metallic coatings. Paint formulations maybe applied as an aqueous suspension, anon-aqueous solution/suspension or powder form.

Physical forms of paint application may include brush, roller, air assisted spray, airless spray or electrostatic spray. The films or structures not containing particulate additives may be transparent, that is, having no apparent phase separation. Transparency is advantageous and unexpected, particularly because polypropylene blends containing ethylene vinyl acetate are commonly either translucent or opaque.

The polyalphaolefin is preferably a polymer of an alphaolefin where at least one of the monomers present in the polyalphaolefin has the formula (CHzCHR)", wherein R is Cl-C6 alkyl. The preferred polyalphaolefin is selected from polypropylene and copolymers thereof.

The rheological behaviour of the formulations described in this invention is important for film forming, doctoring and other processing methodologies for which they are used. The rheological characteristics of the formulations are also important in relation to their capacity to wet substrates. Typical shear and elongational viscosities of a preferred formulation are shown in Figure 1 and Figure 2, respectively.

The functionalised polypropylene maybe produced directly by reactive modification by using radical chemistry where the functional group can be isocyanate, anhydride, amine, alcohol or acid, preferably having a melt flow index of about 10 to about 120, preferably about 10 to about 95, more preferably about 50 to about 60 at 190°C. The functionalised polypropylene may also be made by copolymerisation of propylene with suitable diolefinc monomer and then subsequent modification of the pendant unsaturation.

Grafted polypropylenes which may be employed have a molecular weight distribution (Mw/Mn) of 1. 5 to 4.0, preferably 1. 5 to 3.0, more preferably 2.5 to 3.0, most preferably 2.7 to 2. 8.

In general, when micro/nano particulates are added to the adhesive, lower molecular weights and broader molecular weight distributions of the polymers can be employed effectively.

Preferred maleic anhydride grafted polypropylenes have a melt flow index (MFI) not less than 20, preferably not less than 70, e. g. 20 to 150, preferably 20 to 100, more preferably 40 to 80, most preferably 40 to 70, especially 50 to 60 at 190°C.

Preferred grafted polypropylenes have a weight average molecular weight (Mw) about 118, 000 and number average molecular weight (Mn) about 4, 300.

Maleic anhydride grafted polypropylenes are preferred.

A preferred maleic anydride grafted polypropylene is Exxelor P01015.

Alternatively, 2, 2'-dimethyl-1, 3-isopropenyl benzyl isocyanate (also known as dimethyl meta-isopropenyl benzyl isocyanate) (TMI) modified polypropylenes as disclosed in W096/34024, W096/34031 and W098/13398 may be employed. The ethylenically unsaturated carboxylic acid or derivative in the polalphaolefin is from about 0.05% to about 5%.

The unmodified ethylene/vinyl acetate copolymer is preferably an unmodified random or statistical copolymer having a melt flow index of not more than 800, preferably not more than 600, more preferably not more than 400 at 190°C. A vinyl acetate content of up to 40% in the copolymer may be employed, preferably around 28%. A preferred EVA copolymer is Escorene 40028.

Preferred adhesives in accordance with this invention do not incorporate an opacifier.

However, opacifiers which can be used include inorganic particulates, for example, mica, talc and inorganic sulphates, carbonates, halides and pigments. Organic opacifiers which can be used include insoluble particulate materials including polymers such as polystyrene, polyesters, polyamides, polyacrylates, polymethacrylates, inks, pigments and colorants.

Adhesives in accordance with this invention may be applied in solid form to a workpiece, for example, as a sheet of film, by spraying or as pellets and then heated to cause melting of the adhesive. Conventional hot compression and compaction using hot plates or a heated press may be employed. Alternatively, a metal workpiece or substrate may be heated to cause melting of the adhesive.

Bonding with these heat-activated adhesives is achieved by the application of heat.

Any conventional heating processes can be employed such as hot lamination (batch and continuous), infrared heating (heating efficiency may be enhanced by using infrared absorbent materials in the composition), hot calendering, hot air welding, ultrasonic welding, dielectric or induction heating (with the use of appropriate additives such as metal particles). With the advent of comparatively compact and'affordable'diode lasers, it has been successfully shown that high strength joints between thermoplastics and, for example, mild steel, aluminium, copper, titanium and metal alloys and that these are of an engineering standard. Less conventional methods are also applicable such as electrical heating through the use of embedded wires or mats. Extrusion or injection moulding of the adhesive onto heating wire or mats is a convenient means of producing tailored components such as gaskets or polypropylene spacer in engineering devices.

Bonding of polypropylene (PP) components with the adhesives in accordance with this invention is usually carried out at temperatures around 155 0C which is below the melting point of PP (approximately 165 C) and distortion problems with the substrate can be avoided completely when the bonding process is carried out under properly controlled conditions. The bond is formed as soon as the molten adhesive is cooled and solidified between the wetted

surfaces of the adherends. No curing is required. This is an improvement over prior processes for bonding polypropylene to polypropylene or other substrates which employ often hazardous wet chemicals and which require relatively long curing times. The other advantage of using the adhesives in accordance with this invention is that elaborate surface treatment of substrates (metals or non-metals) such as flaming, corona treatment or chemical etching is not required for bonding. Degreasing with organic solvents such as acetone may be advantageous in some cases.

The adhesives in accordance with this invention can be used in the form of pellets (e. g. for co-extrusion and injection moulding), strands or rods (e. g. for hot air and induction welding), and so on. However, it has been found that an effective way to use these adhesives is in film form for laminating large flat panels in a continuous manner. For laminating curved and shaped panels, a semi-continuous thermoforming process or compression moulding method can be adopted. These laminating processes can further be modified as coating processes by laminating (coating) the surface or surfaces of each substrate with the adhesive in this invention. The adhesive coated article so produced will have a reactive surface or surfaces for subsequent processes such as printing, painting or bonding.

The following combinations of adherends illustrate successful bonding using the adhesive of this invention: Compacted PP fabric to compacted PP fabric for engineering applications.

PP sheet to PP foam to give composite panels for general engineering and insulation purposes.

. PP sheet to PP honeycomb to give composites for lightweight automobile structures.

PP sheet to aluminium to give composite structures for use in the aerospace and allied industries.

'Aluminium sheet to phenolic thermoset decorative sheets or laminates e. g. Formica@ panels for decorative panels suitable for container, railcar, bus and coach construction.

One example of a thermoset decorative laminate is Formica@.

PP sheet to steel pipe for lagging and protection purposes.

Cotton fabric to cotton fabric for luggage and other textile applications that require adhesive bonding.

Cotton fabric to aluminium foil for laminated fabric structures for use in safety and other clothing applications.

PP moulded structures to leather for the footwear applications.

PP sheet or mouldings to PP sheet or mouldings for packaging purposes.

. Similar or dissimilar metals to each other to form an insulating layer.

'Metal-polymer and metal-fabric laminates for blast proof panels or containers.

The following results are illustrative of the good bonding which can be achieved using one method of heat application, namely hot pressing, and one physical form of the adhesive.

A film of about 10 microns to about 200 microns thickness can be used with a preferred thickness of about 20 microns to about 80 microns and a most preferred thickness of about 40 microns to about 60 microns. When used for the purposes of a tie layer in multi-layers film, adhesive thicknesses of 3 um + 1, um are sufficient. Similar bond strengths are achieved using other heat sources in an appropriate manner for the activation of the adhesive. The reduction in bond strengths as the ambient temperature is increased reflects the intrinsic change in modulus of the adhesive formulation. The higher the content of modified polypropylene in the formulation the better the high temperature bond strength. Those skilled in the art can design specific formulations to achieve optimal wetting, bond strength and processability for any given means of heat application to effect good bonds.

The invention also provides an article adapted to be secured to another article including a surface carrying a pre-coated layer of an adhesive in accordance with the present invention.

Compositions in accordance with this invention may be manufactured by melt blending of the key components in a batch blender or by using an extruder, for example, a twin-screw extruder. This is the preferred method as the adhesives can be pelletised for processing by standard injection moulding, extrusion or a range of other commonlyused polymer processing methods. The adhesive can be readily formed into different shapes e. g. gaskets for bonding shaped work pieces.

The adhesive formulation described is particularly well suited to the bonding of polypropylene to polypropylene and the substrates can be in the form of sheet, woven fabric or other physical forms. The adhesives in accordance with this invention may also be used for the bonding of many different substrates or combination of substrates including: aluminium, steel, copper, brass, zinc coated steel, iron, titanium, glass, ceramics, cellulosic materials,

melamine laminates, phenolic laminates, glass filled thermoplastic and thermoset composites, polyurethane composites, synthetic and natural fibre composites and leather.

The invention finds a particular application in manufacture of engineering mechanical components or articles wherein high bond strength is desired. Bonding of polypropylene surfaces is facilitated.

In a continuous process, a film in accordance with this invention, may be dispensed between surfaces to be bonded, nipped by compression of the surfaces and then heated to cause melt adhesion. Large flat surfaces can be bonded in a continuous process. Large curved or other shaped surfaces can be thermoformed in a semi-continuous process. This is an improvement over prior processes in which the adhesive is applied by spraying. Spraying results in the poor control of the thickness, integrity and density of the adhesive layer.

Laminated panels may be conveniently manufactured in a continuous manner using many different combinations of fabric, PP sheet, or metal films, or other adherends known to those skilled in the art.

The invention is further described by means of example but not in any limitative sense, with reference to the accompanying drawings of which: Figure 1 is a graph comparing shear viscosity vs. shear rate for an adhesive in accordance with this invention.

Figure 2 is a graph comparing elongational viscosity vs. shear rate for an adhesive in accordance with this invention.

Figure 3 is a graph showing the variation of bond strength, between polypropylene and polypropylene, with bonding temperature.

Figure 4 is a graph showing the variation of bond strength, between polypropylene and mild steel, with bonding temperature.

Example 1 An APV 2030,30 mm diameter screw, 40: 1 L/D, co-rotating twin-screw extruder was used in a continuous method of manufacturing of the adhesive. A mixture of maleic anhydride functionalised polypropylene having a melt flow index of 50-60 at 190°C, Mw about 118000, Mn about 43000 and MWD of 2.7 to 2.8 and ethylene/vinyl acetate copolymer (EVA), (Escorene 40028) pellets at a weight ratio of 80: 20 were fed continuously into the hopper of the extruder. The feed rate was approximately 17.5 kgh-'and the extrusion temperature was

approximately 170°C. The extrudate was then passed through a water bath. The solidified extrudate was finally cut up into suitably sized pellets for further applications. This process was later scaled up to manufacture adhesive pellets at a throughput rate of 250 kgh-'. The pellets so produced were used for conventional extrusion, injection moulding and a range of other commonly used polymer processing methods.

Example 2 Approximately 4 kg of the adhesive pellets as described in Example 1 was dried overnight in an oven at 60°C. The pre-dried pellets were then used to extrude into rolls of film of approximately 250 mm in width and 70 microns in thickness. The extrusion temperature was approximately 180°C and the wind-up speed was 1.5 mmini'. This process was later scaled up to manufacture adhesive film of 2.4 m in width and 30-150 microns in thickness.

Example 3 The thermal behaviour of the adhesive as described in Example 1 was examined by differential scanning calorimetry and melt rheometry. The onset and the peak of melting of the adhesive were found at approximately 130°C and 145°C respectively. Typical shear and elongational viscosities of the preferred formulation, measured at 170°C, are shown in Figure 1 and Figure 2 respectively.

Example 4 The bond strength of the adhesive as described in Example 1 was examined by lap-shear test of single overlap joints (with reference to BS 5350 : Part C5 : 1990, ASTM D 1002-94 and D3164-97) and T-peel test (with reference to BS 5350: Part C12 : 1994 and ASTM D 1876- 95). Sheets of glass-filled polypropylene, aluminium and mild steel were used to prepare all the test samples. The adhesive films, as described in Example 2, were used to prepare the bonds. The sample surface was degreased with acetone prior to bonding. No further surface treatment was carried out. The bonding temperature and contact time were 155°C and 1 minute respectively. A gentle pressure was applied to ensure a good contact between the adhesive and the bonding surfaces. Results are shown in Tables 1 and 2.

Table 1. Variation of shear strength (values in MPa) of the adhesive film, as described in Example 2, on various adherends with temperature.

Temperature (°C)-40-20 0 18 40 70 100 130 Mild steel 7.02 5.77 3.01 20. 80 20.30 18.32 14. 33 11.29 Aluminium 8. 88 7.23 2.61 2.37 12. 31 12.62 12.19 11.48 Glass-filled PP 11.37 Table 2. Variation of peel strength (values in N/mm) of the adhesive film, as described in Example 2, on various adherends with temperature.

Temperature (°C)-40-20 0 18 40 70 100 130 Mild steel 4. 18 3. 74 3. 92 6.36 5.78 3. 67 3. 37 1. 57 Aluminium 1. 86 1.97 2.58 4.78 5.67 4.13 2.60 1.38 Glass-filled PP---3. 70

If a conventional atactic polypropylene or EVA hot melt adhesive was used to bond the above substrates very poor shear and adhesive performance were observed.

Example 5 The effect of bonding temperature on the bond strength of the adhesive, as described in Example 1, was examined by T-peel test (with reference to BS 5350: Part C12: 1994 and ASTM D 1876-95). Sheets of polypropylene, 3 mm in thickness, were used to prepare all the test samples. The adhesive films, as described in Example 2, were used to prepare the bonds.

The sample surface was degreased with acetone prior to bonding. No further surface treatment was carried out. The bonding temperatures were 145,150, 153,155, 158,160 and 165°C respectively. The contact time was 5 minutes. A gentle pressure was applied to ensure a good

contact between the adhesive and the bonding surfaces. Results are shown in Figure 3. The best bond strength was found at 155°C. This temperature is conveniently below the melting temperature of polypropylene (around 165°C).

Example 6 The ability of the adhesive, as described in Example 1, to bond polypropylene to metals at various bonding temperatures was examined by T-peel test (with reference to BS 5350: Part C12: 1994 and ASTM D 1876-95). Sheets of polypropylene and mild steel were used to prepare all the test samples. The adhesive films, as described in Example 2, were used to prepare the bonds. The sample surface was degreased with acetone prior to bonding. No further surface treatment was carried out. The bonding temperatures were 145,150, 155,160 and 165°C, respectively. The contact time was 4 minutes. A gentle pressure was applied to ensure a good contact between the adhesive and the bonding surfaces. Results are shown in Figure 4. The best bonding temperature was found at 155°C.

Example 7 The bonding of polypropylene box-section to itself has been demonstrated by using the adhesive film, as described in Example 2, as bonding agent. A Heraus infra-red heating equipment, in the form of a chamber approximately 1 m in length, was used to activate the adhesive in the bonding process. The infra-red heating elements were positioned approximately 20 cm above the surfaces of the components being joined. The samples, which were two halves of a polypropylene box-section, were then heated for approximately 15-20 seconds. When the samples had emerged from underneath the heating elements, a sheet of adhesive film, as described in Example 2, was placed over the heated surface of one of the samples and melted. The second sample was then inverted so that the two heated surfaces could be placed in intimate contact with each other under pressure. A bond was formed when the molten adhesive had cooled down and solidified.

Example 8 Ultrasonic energy can be used to activate the adhesive film, as described in Example 2, for the bonding of polypropylene components. Single-lap-shear joints were prepared by ultrasonically joining two polypropylene coupons (each 2 mm thick) together, using the

adhesive film, as described in Example 2, as bonding agent. The top coupon was placed directly beneath the ultrasonic horn. The two coupons were clamped together to avoid slippage induced by the molten adhesive during bonding. The joint area was approximately 25 mm x 25 mm. The bond strength of these joints was then examined using a tensile tester with a cross-head speed of 100 mm min-1. The joints remained intact whilst the polypropylene coupons were necked and drawn during the test. Another set of joints was prepared and tested in the same way using glass reinforced polypropylene and hot-compacted-polypropylene as substrates. All the samples failed by delamination of the hot-compacted polypropylene, showing that the bond strength was stronger than the inter-laminar shear strength of the hot- compact-polypropylene.

Example 9 Semi-translucent isotropic polypropylene, approximately 3 mm thick, was laser-welded at different laser pass rates to 0.7 mm mild steel plate, using an adhesive film as described in Example 2. The effectively bonded area varied greatly dependent on the speed of the laser over the joint. The laser-welded samples were then cut into 25 mm parallel strips for testing their lap-shear strength. Results are shown in Table 3. The most effective laser speed over the material to effect good bonding in this example is in the region of 200 mmmin-l but this changes with laser power and the substrates being bonded. When the substrate is opaque but conducts heat rapidly, as in the case for most metals, it is possible to achieve similar bond strengths by applying the laser heating to the metal surface.

Table 3. Variation of shear strength of the laser-welded samples with laser speed. Laser speed (mmmin-l) 100 150 175 200 225 250 Maximum strength (MPa) 4. 2 6. 8 12. 1 16 10.3 1 9. 5

Example 10 Composites comprising a sandwich structure of hot-compacted-PP sheet/PP foam/hot- compacted-PP sheet or hot-compacted-PP sheet/PP honeycomb/hot-compacted-PP sheet were prepared in a continuous laminating process using the adhesive film, as described in Example 2, as bonding agent. The laminating temperature and speed were 160°C and 3 mmin-1 respectively. These composites exhibit good impact resistant properties and recyclability.