GB2192187A - Curable thermoplastic polymer compositions - Google Patents

Abstract

A thermoplastic resin is melted and mixed with a cross-linking agent and a catalyst. The mixture is extruded to form a thermoplastic film or subdivided and is useful as an adhesive or coating material which can be cured by heating to a temperature in excess of 90 DEG C. The resin and cross-linking agent are mixed in a mixer-extruder having an extruder head having an elongated body 31 along the length of which is a canal 32 open only into inlets 35 and outlets 34. The outlets exceed inlets in number by a factor of 10 or more and feed a channel 33 closed by a top 36 defining a slit 41 whose width is fixed by spacer 40. Adjustable valves 43 are provided in each outlet 34. <IMAGE>

Description

SPECIFICATION Adhesive composition, method of preparing an adhesive and apparatus for use in the method Description of invention According to a first aspect ofthe present invention, there is provided a method of preparing an adhesive wherein a thermoplastic material is heated, a curing agentforthe thermoplastic material is mixed with the heated material and the mixture is cooled to a solid state, before the thermoplastic material is cured to a solid state.

The adhesive thus prepared has a thermoplastic character, inthatthe mixture can be softened by heating, this being a reversible change which can be repeated. However, the presence ofthe curing agent in the mixture also imparts a latentthermo-setting characterto the adhesive, in that, if the mixture is heated to a sufficiently high temperature (which is a temperature above the temperature at which the mixturesoftens),thenthethermoplasticmaterial is cured bytheformation of crosslinks between polymer chains thereof and the composition assumes a solid state, even at an elevated temperature. This second change is irreversible.

Itwill be understood that, prior to and during mixing ofthe curing agent with the thermoplastic material, the thermoplastic material is not heated to a temperature at which curing will occur. Priorto mixing, the thermoplastic material is heated sufficiently to reduce the viscosity ofthe material to a value at which the thermoplastic material can be mixed with the curing agent to produce an homogeneous mixture which can be stored without loss of the curing agent and which, when cool, is self-supporting, so that the mixture can be handled, stored and transported without substantial degradation.

After mixing and before cooling of the mixture,the mixture of the thermoplastics material and the curing agent may be formed into a film. The film may beformed on orappliedto a substrate, priorto cooling. Alternatively, the mixture may be formed into thicker bodies prior to cooling. The thicker bodies must normally be subjected to further processing, prior to use ofthe adhesive. In a case where the mixture is formed into a film, it may be extruded onto a moving carrier and removed from the carrier after cooling. The film removed from the carrier may bewound into a roll.

The invention includes a method of adhering to each othertwocomponentsofan assembly,wherein an adhesive is prepared by a method according to the first aspect of the invention, the components are brought together with a layer of the adhesive between them, the adhesive is softened by heating either before or after the components and adhesive are broughttogetherandwherein,afterthe components are brought together, the adhesive is heated to bring about curing of the adhesive.

Between the steps of softening the adhesive by heating and curing the adhesive by heating,the assembly may assume a lower temperature, at which the adhesive is solid.

During heating of the adhesive in the assemblyto bring about curing of the adhesive, the components ofthe assembly may be subjected to pressure in localised areas only so that the layer of adhesive and at least one of said components assume an embossed form.

The invention also includes a method of coating a substrate wherein an adhesive is prepared by a method according to the first aspect of the invention, the adhesive is applied to the substrate to form a coating thereon and the adhesive is subsequently heated to bring about curing of the adhesive.

According to a second aspect ofthe invention, there is provided a method of applying a coating to a substrate wherein athermoplastic resin is heated to afirsttemperature, a curing agentforthe resin is mixed with the heated resin, the mixture is applied to the substrate and the mixture is cured by heating to a highertemperature.

According to a third aspect of the invention, there is provided a thermoplastic, resinous material in a solid condition comprising a mixture of a thermoplastic resin and a curing agentforthe resin, the curing agent being capable of cross-linking the resin at an elevated temperature. The material may be in the form of a powder, larger pieces such as chips or in the form of a film. In a casewherethe material has the form of a film, the film is preferably self-supporting and is without any supporting substrate.

According to a fourth aspect of the invention, there is provided apparatus for preparing a film of an adhesive comprising an extrusion head, means defining first and second reservoirs, heating means for supplying heat to the contents of the first reservoir, means defining a flow path from the first reservoirtothe extrusion head and pumping means for pumping a molten composition at a first rate from the first reservoir along the flowpath and for pumping a second composition from the second reservoir into the flowpath at a second rate related by a predetermined ratio to the first rate.

Pumping ofthe first composition from the first reservoir and of the second composition together along a part of the flowpath promotes mixing of these compositions. However, that part ofthe flowpath along which both compositions are pumped preferably includes a m ixer ca pable of producing an homogeneous mixture of the first and second compositions. The preferred form of mixer is a mixerwhich has no moving parts, mixing being achieved by movement of the compositions relative to the mixer.

According to a further aspectofthe invention, there is provided an extrusion head comprising an elongated body defining a canal extending along the body, a plurality of inlets to the canal, a channel extending along the body, a plurality of outlets from the canal leading to the channel and an extrusion slot extending along the body and communicating along substantially its entire length directly with the channel, the canal being closed, except four said inlets and outlets.

The extrusion head may be used as the extrusion head of apparatus in accordance with the fourth aspect of the invention.

The extrusion head is capable offorming into a film a moderately viscous composition,for example a composition having a viscosity within the range 1,000 to 10,000 poise.

It will be understood that, to facilitate start-up after a period when the apparatus has been out of use, it is desirable to minimise the volume ofthe cavities in the extrusion head which are occupied bythe composition to be extruded. When the apparatus is out of use for a substantial period, this composition will be allowed to cool and solidify and must be melted once more before the apparatus can be used.

An extrusion head in accordance with the sixth aspect ofthe invention is capable of delivering a film which is of uniform thickness along the entire length ofthe extrusion head, whilst containing a relatively small volume ofthe composition which is to be extruded.

The number of outlets from the canal preferably exceeds the number of inlets thereto by a factor of at leastten.To promote even distribution offlowfrom the extrusion head along the entire length thereof, there may be provided in each outlet from the canal a respective valve which causes a pressure drop within the outlet and/or impedes flow through the outlet to a variable degree.

An example of apparatus in accordance with the invention and which is used in a method according to the invention will now be described, with reference to the accompanying drawings, wherein Figure tis a diagrammatic representation ofthe apparatus; Figure2 shows a cross-section through an extrusion head ofthe apparatus in a plane perpendicularto the length ofthe extrusion head; and Figure 3 is a diagram illustrating use of an adhesive film produced by the apparatus of Figure 1.

The apparatus illustrated in Figure 1 includes an unwind station 10 having known means for supporting a roll of a web 11 andforpermitting unwinding ofthatwebfrom the roll undertension.

The unwind station may include known means for controllingthetension intheweb.

The apparatus further comprises a pair of support rolls 12 and 13 which are mounted for rotation about respective parallel axes, these axes being spaced apart by a distance somewhat greater than the diameter ofthe support rolls but less than fourtimes the diameter. These support rolls are conveniently arranged one vertically above the other. Spaced horizontally from the uppermost support roll 12 is a cooling roll 14 provided with known meansfor cooling a web which passes overthe roll. Morethan one cooling roll may be provided, where a single cooling roll would be inadequate. Drive means 15 is provided for driving the rolls 12, 13 and 1 4with the same peripheral speed. The drive means is adjustable so that a selected peripheral speed ofthe rolls can be attained.

Guide means is provided for guiding the web 11 along a pathwhich extendsfrom the unwind station 10 overthesupport rolls 12 and 13,overthecooling roll or rolls and then to a take-up station 16. By way of example, two guide rolls 17 and 18 are represented in Figure 1, but it will be understood that the guide means may comprise additional elements. A transducer 19 is associated with one of the rolls around which the web is guided, to provide an electrical signal representing the speed oftheweb.

The apparatus further comprises an elongated extrusion head arranged for contacting one face of the web 11 at a position between the support rolls 12 and 13. The extrusion head is arranged with its length extending horizontally across the web and is preferably supported on a carrier for movement between the operative position shown in Figure 1 and an inoperative position in which the extrusion head is withdrawn from the path which the web follows between the support rolls 12 and 13.

The apparatus further comprises respective containers defining first and second reservoirs 21 and22forcontaining bulksuppliesofmaterialstobe fed to and extruded through the extrusion head 20.

With at least the first reservoir 21 ,there is associated heating means 23 for supplying heatto the contents ofthefirst reservoir. The heating means preferably includes electrical heating elements, temperature sensing devices for sensing the temperature of the contents of the first reservoir and control means for controlling energisation ofthe heating elements in accordance with the outputofthetemperature sensing devices to heat the contents of the first reservoir to a pre-selected temperature and to avoid heating of the contents to a temperature higherthan this selected temperature. If required, similar heating means maybe provided forthe contents of the second reservoir, although this is not essential.

An outletfrom the first reservoir 21 leads to a positive displacement pump 24 having an outlet connected by a duct 25 which leads to the extrUsion head 20. Adjacent to the extrusion head, the duct 25 has a number branches which communicate with respective inlets to the extrusion head, these inlets being spaced apart along the extrusion head. With an outlet from the second reservoir, there is connected a further positive displacement pump 26, an outlet of which is connected with an injector 27 in the duct 25, through which injector a fluid from the second reservoir can be introduced into the material flowing from the first reservoir to the extrusion head.

Between the pump 22 and the injector is a non-return valve 28 and a non-return valve 29 is provided between the pump 24 and the injector. The duct 25 is adapted to provide mixing ofthe materials from the first and second reservoirs by incorporation in the duct downstream ofthe injector 27 of a mixer 30. The mixer may be a known mixer having no moving parts, but a number of vanes projecting from a circumferential wall into a flowpath through the mixer. To accommodate movement of the extrusion head between operative and inoperative positions, a part ofthe duct 25 between the mixer and the extrusion head may comprise a flexible hose.

Electrical heating elements are associated with the duct 25, the mixer 30, the injector 27, the valve 29 and the pump 24. These heating elements are used to heatuptheassociated components priorto operation of the apparatus and to compensate for heat loss from the components during operation.

The extrusion head 20 comprises an elongated body 31 defining a canal 32 which extends through the interior of the bodyfrom oneend thereof to the other. The bodyfurther defines a number of inlets to the canal, an open-topped channel 33 which extends parallel to the canal but spaced therefrom and a number of outlets 34, each extending from the canal 32 in a direction perpendicularto the length ofthe canal to the channel 33. The canal 32 is closed, except for the inlets thereto and the outlets 34.

Typically, there is one inlet to the canal 32 at each end ofthe body 31 and additional inlets, one of which is shown at 35, spaced from the ends ofthe canal and spaced from each other. The spacing of adjacent inlets is preferably no greaterthan 600 mm and is typically 400 mm.

The extrusion head further comprises a cover36 which covers the open upper boundary ofthe channel 33 entirely. The body 31 presents along opposite margins of the channel 33 flat, co-planar surfaces 37 and 38, the latter extending to a tip 39 of the body 31. Between the flat lower surface ofthe cover 36 and the surface 37, there is interposed a flat spacer40which maintains a corresponding slit 41 between the cover and the surface 38. This slit is the sole outlet from the channel 33 and communicates with the channel along the entire length of the latter.

The cover is releasably mounted on the body 31 by screws 42 which maintain the cover in firm contact with the spacer 40 and the spacer in firm contact with the surface 37 ofthe body.

Each of the outlets 34 leads into the channel 33 at the side thereof remote from the slit 41. There protrudes into each outlet a respective valve 43 which may partly obstruct the outlet and so establish a pressure differential between the canal 32 and a part of the outlet downstream ofthe valve. The valves are preferably adjustable and each valve may be constituted by a screw which extends through a threaded bore defined by the body 31 and communicates with the corresponding outlet 34.

The number of outlets 34 exceeds the number of inlets 35 by a factor of at least 10. Each outlet is preferably circular in cross-section, having a diameter which is substantially less than the width of the channel 33. The transverse cross-sectional area of the canal 32 preferably exceeds the transverse cross-sectional area of the channel 33 by a factor within the range 1 to 3.

The width of the slit 41, identified as the dimension bin Figure 2, can be varied bysubstituting a different spacerforthe spacer 40. Preferably, the width does not exceed 1 mm.The cover 36 has a nose 44which protrudes somewhat beyond the tip 39 of the body 31 by a distance identified in Figure 2 as the dimension a. This dimension preferably exceeds the dimension bandistypicallyapproximatelytwice that dimension, There is associated with the body 31 a heater comprising electrical heating elements and represented diagrammatically at 45 in Figure 2.

During useoftheapparatus,a bulksupplyofa thermoplastic resin is heated in the first reservoir 21 to a predetermined temperature, such thatthe viscosity of the resin is sufficiently low to permit mixing of the resin with a curing agent supplied from the second reservoir 22. The resin contained in the first reservoir is substantially free of water and other solvents. The viscosity of the resin, when h heated to the selected temperature in the first reservoir, is preferably below 10,000 poise and is typically in the region of 5,000 poise. The viscosity will usually exceed 2,000 poise. The composition in the extrusion head 20 is maintained atatemperaturewhich is sufficiently high to avoid the viscosity being in excess of 10,000 poise.However, the temperature is maintained below that temperature at which curing of the composition will occur. Typically, the temperature of the material in the extrusion head is within the range 80-85"C and the viscosity is in the region of 5,000 poise. There is maintained in the extrusion head a pressure of 1,000 psi, i.e. almost7 million newtons pew square metre.

The curing agent contained in the second reservoir 22 comprises a cross-linking agentforthe thermoplastic resin in the first reservoir and a catalystforthe reaction between the cross-linking agent and the resin. Appropriate quantities ofthe cross-linking agent and catalyst are introduced into the reservoir and thoroughly mixed. Since both the cross-linking agent and catalyst are typically liquids at ambienttemperatures, no heating ofthe contents of the second reservoir is essential.

With the extrusion head in its operative condition, the drive means 15 is operated to drawtheweb 11 through the apparatus at a predetermined speed. A signal representing the speed of the web is fed from the transducer 19 to control means 46 ofthe apparatus and signals are provided by the control meanstothe pumps 24and 26, causing these pumps to deliver materials from the reservoirs to the injector 27 at respective rates which are appropriate to the speed of the web and which are in a predetermined ratio. Mixing ofthecuring agentwith the thermoplastic resin commences at the injector 27 and is completed in the mixer 30. The mixerthen flows to and through the extrusion head 20, emerging from the extrusion slit 41 beneath the nose 44.In the operative position of the extrusion head, the nose 44 is immediately adjacentto the web 11 at a position between the support rolls 12 and 13 so that the material emerging through the extrusion slits 41 contacts the web, which is travelling upwardly past the extrusion head. A small volume ofthe extruded material accummulates at47 immediately beneath the nose 44 and a film ofthe extruded material is carried from this accummulation on the surface of the web. Afterthe web has passed around the support roll 12, it passes around the cooling roll 14 so that the temperature of the extruded material is reduced sufficiently to solidify the extruded material, whilst still carried on the web 11.

In a case where the film of adhesive is required for use on the web 11, the web, bearing the film of adhesive on one face thereof, is wound into a roll at the take-up station 16.1 n a case where the film of adhesive is required without any supporting web, the film is peeled from the web at the take-up station and the film and web are wound into separate rolls 48 and 49. To facilitate separation ofthe film from the web, the face of the web which bears the film may be pre-coatedwith a release agent.

The thickness of the film of adhesive deposited on the web 11 can be varied by varying the speed ofthe web, by means of the drive means 15. The control means 46 automatically adjusts the speed of the pumps 24 and 26to supply the adhesive to the extrusion head at a rate appropriate to the speed of the web.

In a case where the film of adhesive is required without any supporting web, the apparatus illustrated in Figure 1 may be modified to incorporate an endless web which is moved around the support rolls 12 and 13 and the cooling roll 14 and which is returned to the support rolls, after the adhesive film has been peeledfromtheweb.

Where the adhesive is not required in the form of a film, the extrusion head may be modified to extrude the adhesive in some otherform. For example, the head illustrated in Figure 2 may be modified merely by removal of the cover 36. The adhesive will then be extruded in ribbons, one ribbon through each ofthe outlets 34, and these ribbons can be carried away from the extrusion head on the supporting web and cooled. When solidified, the ribbons would be broken into small pieces, commonly referred to as chips. The chips may be stored and transported, before being heated to provide a bulk supply of molten adhesive. Alternatively, the chips may be ground two a powder, priorto use.

It will be understood that the adhesive prepared by use ofthe apparatus illustrated in Figure 1 has a thermoplastic character. At ambienttemperatures, the adhesive is in a solid condition but the adhesive can be softened by heating and will flow under its own weight when heated to a moderate temperature, for example 50"C. The film or powdered adhesive can be applied to a substrate by placing the adhesive on the substrate, whilst the adhesive is in a solid condition, and then heating the adhesive to a temperature within the range 50into 60"C. This causes the adhesive to melt and bond to the substrate.In a case where the substrate is permeable, the viscosity ofthe adhesive may be reduced by heating to a highertemperature,for example 80"C, so that the adhesive penetrates into the substrate. The adhesive may be permitted to cool, in which case the adhesive returns to a solid condition, still retaining its thermoplastic character.

A substrate to which the adhesive has been applied, either in the apparatus illustrated in Figure 1,as a film or powderorby melting a bulksupplyof adhesive produced by means of apparatus generally as shown in Figure 1 and applying the molten adhesive by known techniques used for the application of hot melt adhesives, may be laminated with a further layer by bringing the further layer into contact with the adhesive borne by the substrate, the adhesive being heated to a temperature within the range 50-85'C either priorto, during or subsequent to application ofthe further layer. The adhesive may then be permitted to cool, in which case it will firmly bond the additional layerto the substrate.

Alternatively, the molten adhesive may be subjected to treatment which promotes curing of the adhesive.

For example, the adhesive may be heated to a temperature in excess of 90"C or subjected to radiation which promotes curing. During treatment to promote curing,the laminate may be subjected to pressure at selected positions to emboss one or both ofthefurther layer and the substrate. The laminate may be shaped in a mould during curing.

In acasewherethe laminate is cooled without curing of the adhesive, the adhesive would be subjected subsequently to treatment which promotes curing. Thus, the laminate produced by melting ofthe adhesive may be stored and transported,forexample in a roll of laminate.

Shaping of the laminate and curing of the adhesive may be effected at a site different from that atwhich the laminate is produced.

Adhesive produced in the apparatus of Figure 1 may be used to provide a surface coating on a substrate. For this purpose, the adhesive may be applied to the substrate in powder form, in solution in a solvent or in a molten condition. In a casewhere the adhesive is applied as a powder, itwould subsequently be melted by heating to form a cohesive coating on the substrate. The powder may be applied by an electro-static coating process. The surface coating would be subjected to further treatment which causes curing of the adhesive.

During such further treatment, a pattern may be applied to the surface coating. The pattern may be applied by embossing the surface coating or by applying additional material to selected areas of the surface coating. Alternatively, additional material may be applied to the surface coating whilst the latter is molten and priorto curing. For example, flock fibres may be applied to the surface coating whilst the surface coating is molten, but before the surface coating is cured.

Examples of suitable materials for use as the resin which is melted in the reservoir 21 and as the curing agent which is contained in the reservoir 22 are given in GB 2,057,959A. There may be incorporated in the adhesive, in addition to the resin and the curing agent, additional components for modifying properties of the adhesive. For example, there may be incorporated one or more pigments for modifying the appearance of the adhesive.

Prior to curing ofthe adhesive, composite materials incorporating the adhesive can readily be shaped by heating sufficiently to soften the adhesive and the application of pressure to bring aboutthe required shaping. After curing, the adhesive has shape-holding ability, even at elevated temperatures and is capable of maintaining a strong bond at elevated temperatures, for example in excess of 300"C. Furthermore,the cured adhesive is resistant to solvents.

The adhesive hereinbefore described can be used for laminating flexible sheet materials, for example paper and other non-woven fabrics, woven and knitted textiles and plastics films and sheets. The adhesive is also suitable for producing less flexible laminations and for bonding coverings to relatively inflexible substrates. Furthermore, the adhesive is suitable for bonding together particles, fibres and other discreet pieces of material and for bonding particles, fibres and other pieces to continuous substrates. Whilst the adhesive is in a thermoplastic condition, assemblies comprising flexible components bonded by the adhesive can be subjected to shaping and curing of the adhesive will impart shape-holding properties so that the shaping is substantially permanent.

The cured adhesive provides a bond which is not adversely affected by heating to temperatures in excess of 1 00'C. Typically, the cured adhesive can withstand a temperature of 300"C without significant deterioration. Furthermore, the cured adhesive can withstand solvents such as are used in the dry cleaning of clothing. Application and use of the adhesive does notgive rise tothe emission of unpleasantortoxicfumes and there is no necessatity to provide equipmentforthe recovery of a solvent.

In the example of use of the adhesive illustrated in Figure 3, an unsupported film 50 of the adhesive prepared by use of the apparatus shown in Figure 1 is drawn from a roll 51 and is incorporated in a sandwich between webs 52 and 53 drawn from respective rol Is. The sandwich is subjected to heat and pressure at a bonding station defined in the example illustrated, by heated rolls 54 and 55. Atthe bonding station, the film 50 is heated to a temperature within the range 50 to 85"C and the sandwich is subjected to moderate pressure, to causethe webs 52 and 53 to be bonded together by the adhesive. Alternative means, for example heated plattens of a press, may be provided at the bonding station.Downstream ofthe bonding station, the bonded sandwich may be permitted to cool, rolled or cut up, stored and transported to another site.

Subsequently, the sandwich is subjected art a curing station 56 to treatme nt wh ich causes cross-linking ofthe adhesive. Typically, the treatment involves heating of the sandwich to a temperature of 1 200C. This may be achieved by heated plattens 57,58 of a press. One or both ofthe plattens may be provided with a raised pattern which embosses the product. Furthermore, the plattens may be shaped to give the product a non-flat configuration. Once the adhesive has cured, it will maintain the shape imparted to the product by the plattens 57 and 58.

The web 52 may be a release paper which is removedaftercuring of the adhesive, in which case the adhesive will be left as a surface coating on the material of the web 53.

The features disclosed in the foregoing description, orthefollowing claims, orthe accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for attaining the disclosed result, ora class orgroupof substances or compositions, as appropriate, may, separately or any combination of such features, be utilised for realising the invention in diverse forms thereof.

Claims (29)

1. A method of preparing an adhesive wherein a thermoplastic material is heated, a curing agent for the thermoplastic material is mixed with the heated material and the mixture is cooled to a solid state before the thermoplastic material is cured.

2. A method according to Claim 1 wherein, after mixing and before cooling, the mixture is extruded as a film onto a moving carrier.

3. A method according to Claim 2 wherein the film is cooled whilst on the carrier, is subsequently separated from the carrier and is then wound into a roll.

4. A method according to Claim 2 or Claim 3 wherein the speed of the carrier is varied from time to time to vary the thickness of the film deposited on the carrier.

5. A method according to Claim 1 wherein the mixture is formed into small pieces which are stored and/or transported and wherein a number ofthe small pieces are subsequently remelted together for use.

6. A method according to Claim 1 wherein the mixtu re is com minuted after cooling to form a powder,the powder is subsequentlyapplied to a substrate and the powder is then heated to meltthe adhesive and allow the particles ofthe powderto unite with each other on the substrate.

7. A method of adhering to each othertwo components of an assembly wherein an adhesive is prepared by a method according to any one of Claims 1 to 6, the components are brought together with a layer of the adhesive between them, the adhesive is softened by heating either before or after the components are brought together and wherein, after the components are brought together, the adhesive is subjected to treatment which brings about curing of the adhesive.

8. A method according to Claim 7 wherein, during treatmentto bring about curing ofthe adhesive, the components are subjected to pressure in localised areas so that the layer of adhesive and at least one of said components assume an embossed form.

9. A method of coating a substrate wherein an adhesive is prepared by a method according to any one of Claims 1 to 6,the adhesive is applied to the substrate to form a coating thereon and the adhesive is subsequently cured.

10. A method of applying a coating to a substrate wherein a thermoplastic resin is heated to a first temperature, a curing agentforthe resin is mixed with the heated resin, the mixture is applied to the substrate and the mixture is cured by heating to a higher temperature.

11. Athermoplastic, resinous material in solid condition comprising a mixture ofathermoplastic resin and a curing agent capable of cross-linking the resin at an elevated temperature.

12. Aself-supporting, thermoplasticfilm consisting of a mixture of a thermoplastic resin and a curing agent capable of cross-linking the resin at an elevated temperature.

13. Afilm according to Claim l2withoutany supporting substrate.

14. A material according to Claim 11 orafilm according to Claim 12 or Claim 13wherein the curing agent comprises a cross-linking agent capable of reacting with the resin to form cross-links between polymeric chains thereof and a catalystforthe reaction between the resin and the cross-linking agent.

15. Apparatus for preparing a film of an adhesive comprising an extrusion head, means defining first and second reservoirs, heating means for heating the contents of the first reservoir, pumping means for pumping a molten composition at a first rate from the first reservoir along a flowpath to the extrusion head and for pumping a second composition from the second reservoir into the flowpath at a second rate related by a predetermined ratio to the first rate.

16. Apparatus according to Claim 15further comprising a web for receiving the film from the extrusion head and means for moving the web past the extrusion head.

17. Apparatus according to Claim 16 further comprising separating means for separating the film from the web.

18. Apparatus according to Claim 16 comprising control meansforcontrolling the rate at which the pumping means pumps material from thefirstand second reservoirs and means providing to the control means a signal representing the speed at which the web moves past the extrusion head, the control means being adapted to maintain a predetermined relation between the speed ofthe web and the rate at which material is pumped from the first reservoir.

19. An extrusion head comprising an elongated body defining a canal extending along the body, a plurality of inlets to the canal, a channel extending along the body, a plurality of outlets from the canal leading to the channel and an extrusion slot extending along the body and communicating along substantially its entire length directly with the channel, the canal being closed, except for said inlets and outlets.

20. An extrusion head according to Claim 19 wherein settablevalves are provided in respective ones of said outlets for restricting the flow therethrough.

21. An extrusion head according to Claim 19 or Claim 20 wherein the number of outlets is at leastten times the numberofinlets.

22. An extrusion head according to Claim 21 wherein said outlets are at a pitch within the range 15mm to 50mm.

23. An extrusion head according to any one of Claims 19 to 22 wherein the extrusion slot extends laterally from the top ofthechannel.

24. An extrusion head according to Claim 23 wherein the outlets from the canal lead into the channel art a side thereof opposite to the extrusion slot.

25. Apparatus according to anyone of Claims 15 to 18 wherein the extrusion head is an extrusion head as defined in any one of Claims 19 to 24.

26. Apparatus according to Claim 25 wherein an adhesive composition in said flowpath is subjected to a pressure exceeding 3 x 106 newtons pew square metre.

27. Apparatus substantially as herein described with reference to and as illustrated in Figure 1 ofthe accompanying drawings.

28. A method substantially as herein described of preparing an adhesive.

29. Any novel feature or novel combination of features disclosed herein or in the accompanying drawings.