EP0415918A1

EP0415918A1 - A method for production of a cross-linked polymer film

Info

Publication number: EP0415918A1
Application number: EP88906147A
Authority: EP
Inventors: Christer BERGSTRÖM; Peter Nikko; Hans Martensson
Original assignee: Neste Oyj
Current assignee: Neste Oyj
Priority date: 1987-07-20
Filing date: 1988-05-27
Publication date: 1991-03-13
Also published as: DK16590A; FI900319A0; JPH03500784A; FI900320A0; SE457960B; WO1989000500A1; SE8702914D0; DK16590D0

Abstract

Procédé de production d'un produit (1) polymère réticulé par technique multicouche, ledit produit comportant au moins une couche (2, 4) de polymère contenant un groupe de silanes, réticulée au moyen d'un catalyseur. Le produit polymère réticulé consiste en une pellicule mince multicouche. Dan le procédé, on introduit le catalyseur dans une couche (3) située au niveau de la ou des couches (2, 4) à réticuler ou adjacente à celle(s)-ci, la réticulation étant obtenue par diffusion du catalyseur à partir de la couche (3) contenant celui-ci, dans la ou les couches (2, 4) à réticuler. La matière polymère réticulable est un copolymère d'oléfine contenant un groupe de silanes, tel qu'un copolymère de silane de triméthoxy vinylique d'éthylène, ou un terpolymère de silane triméthoxy d'éthylène/butylacrylate/vinyle, réticulable au moyen d'un catalyseur de condensation au silanol, tel qu'un composé de carboxylate d'étain, par exemple du dilaurate de dibutylétain.Method for producing a crosslinked polymer product (1) using a multilayer technique, said product comprising at least one layer (2, 4) of polymer containing a group of silanes, crosslinked using a catalyst. The crosslinked polymer product consists of a thin multi-layer film. In the process, the catalyst is introduced into a layer (3) situated at the level of the layer (s) (2, 4) to be crosslinked or adjacent to it (s), the crosslinking being obtained by diffusion of the catalyst from the layer (3) containing the latter, in the layer or layers (2, 4) to be crosslinked. The crosslinkable polymeric material is an olefin copolymer containing a silane group, such as an ethylene vinyl trimethoxy silane copolymer, or an ethylene trimethoxy silane terpolymer / butylacrylate / vinyl, crosslinkable using a silanol condensation catalyst, such as a tin carboxylate compound, for example dibutyltin dilaurate.

Description

A METHOD FOR PRODUCTION OF A CROSS-LINKED POLYMER FILM

The present invention relates to a method of producing a crosslinked polymer product by multilayer technique, said product incorporating at least one polymer layer crosslinked by means of a catalyst. Crosslinking different polymers by means of catalysts is previously known. The crosslinking enhances the properties of the polymer, such as mechanical strength, thermal resistance and other properties. Also polymers which are normally regarded as thermo- plastics and as non-crosslinkable, can be crosslinked by introducing crosslinkable groups into the polymer. One example of this is the crosslinking of polyolefins, such as polyethylene. As crosslinkable group, a silane compound can be introduced, for example by grafting the silane compound on the finished polyolefin, or by copolymerisation of the olefin and the silane compound. This is prior art technique, and for particulars in this respect reference is made to US patent specifications 4,413,066, 4,297,310, 4,351,876, 4,397,981, 4,446,283 and 4,456,704 which are included herein by reference.

However, the production of the crosslinked silanecontaining polymer material may cause difficulties, especially when the crosslinked polymer is in the form of a thin layer, as is the case in the present invention. By thin layer is here meant a thickness corresponding to film and foil, i.e. up to about 2 mm, preferably about 1 mm at most, and more preferred about 0.6 mm at most. in the production of a multilayer material, for example by extrusion, in which at least one layer is crosslinked, it is important that crosslinking occurs only after the mixture has left the extruder because premature crosslinking or precuring in the extruder interferes with the rate of production and causes the finished product to deteriorate in quality. Incipient crosslinking or precuring already in the extruder (or similar equipment) causes gel formation. and adhesion of polymer gel to the equipment surfaces with the ensuing risk of clogging. To prevent this, the equipment must be cleaned of adhering polymer gel, and for each cleaning operation the equipment must be shut down, which means a decline in production. A further disadvantage is that any gel lumps not clogging the equipment will be discharged and show up in the product as disfiguring undesired lumps which, if they occur in thin layers, such as films and foils, are unacceptable and usually make the product useless.

Undesired precuring may be prevented by incorporating in the polymer composition substances counteracting precuring, so-called precuring retarders. For polymers whose crosslinking is moisture dependent, for example the above-mentioned silanes, such precuring retarders may be in the form of drying agents. However, the use of precuring retarders implies that there is introduced into the polymer composition a further component, which makes the composition more expensive and, besides, may be undesirable, for example in packages in contact with food products. It therefore is an advantage if the addition of such further components as precuring retarders can be avoided.

The present invention aims at obviating the above- mentioned disadvantages encountered in the production of crosslinked silane-containing polymer products. The invention provides a process of producing a crosslinked polymer product comprising at least one polymer layer crosslinked by means of a catalyst, and the process is characterised in that a multilayered film is produced which comprises at least one layer of a silane group-containing olefin copolymer cross- linkable under the action of water and a silanol condensation catalyst, and at least one other layer free from crosslinkable silane and incorporating a silanol condensation catalyst, and that crosslinking of the silane group-containing layer is achieved by subjecting the film to the action of water and causing the silanol condensation catalyst to diffuse into the silane group-containing layer.

Further features of the invention will appear from the following description and the appended claims. The invention is restricted to silane-containing olefin copolymer materials that are crosslinked under the action of water and a silanol condensation catalyst, and the invention will be described below with reference to this application.

The reason why the invention is restricted to silane-containing olefin copolymer materials is that it was found, when the invention was in progress, that the aim of the invention cannot be achieved with all silane-containing olefin polymers. Thus, the desired result is not obtained with silane-containing graft polymers, even if the silanol condensation catalyst according to the invention is incorporated in another layer free from crosslinkable silane. Although the silanol condensation catalyst is originally provided in another layer, and undesired precuring thus should be precluded, such precuring still occurs and imparts to the film a grainy, unacceptable appearance. The cause of this must presumably be attributed to peroxide residues from the production of the graft polymer which initiate precuring of the polymer. The use of silane-containing graft polymers also leads to free monomer residues in the final product, resulting in an obnoxious smell and may constitute a health hazard, for example in food packagings. It was therefore found necessary, in the context of this invention, to utilise for the crosslinkable polymer a silane group-containing olefin copolymer and to provide the silanol condensation catalyst in layer separate from the polymer. The present invention thus is characterised by the combination of these two requirements. As has been mentioned, the crosslinkable polymer material according to the invention is a silane-containing copolymer by which is meant an olefin polymer, preferably an ethylene homopolymer or copolymer containing crosslinkable silane groups provided in the polymer by copolymerisation. The manner in which the crosslinkable silane groups are attached to the polymer chain thus is critical; according to the invention, for example unsaturated silane compounds can be copolymerised with olefins, or amino silane compounds can react with acrylate esters, whereas the invention does not include graft polymers in which silyl peroxides are decomposed and grafted on the finished polymer by direct reaction with the polymer chain.

The silane-containing polymer has preferably been obtained by copolymerisation of an olefin, preferably ethylene, and an unsaturated silane compound which is represented by the formula

RSiR'_nY_3-n in which R is an ethylenically unsaturated hydrocarbyl or hydrocarbyloxy group, R' is an aliphatic saturated hydrocarbyl group, Y is a hydrolysable organic group, and n is 0 , 1 or 2. If there is more than one Y-group, these need not be identical. Specific examples of the unsaturated silane compound are those in which R is vinyl, allyl, isopropenyl, butenyl, cyclohexenyl, or gamma-(meth)acryloxy propyl, Y is methoxy, ethoxy, formyloxy, acetoxy, propionyloxy or an alkyl or arylamino group, and R' is a methyl, ethyl, propyl, decyl or phenyl group.

An especially preferred unsaturated silane compound is respresented by the formula CH₂=CHSi(OA)₃ in which A is a hydrocarbyl group having 1-8 carbon atoms, preferably 1-4 carbon atoms.

The most preferred compounds are vinyl trimethoxy silane, vinyl bismethoxyethoxy silane, vinyl triethoxy silane, gamma-(meth)aeryloxypropyltrimethoxy silane, and gamma-(meth)acryloxypropyltriethoxy silane and vinyl triacetoxy silane.

The copolymerisation of the olefin (ethylene) and the unsaturated silane compound may be carried out under any suitable conditions causing copolymerisation of the two monomers.

Furthermore, polymerisation may be carried out in the presence of one or more further comonomers copolymerisable with the two monomers. Examples of such comonomers are: (a) vinyl carboxylate esters, such as vinyl acetate and vinyl pivalate; (b) (meth)acrylates, such as methyl (meth) acrylate, ethyl (meth) acrylate, and butyl (meth)acrylate; (c) olefinically unsaturated carboxylic acids, such as (meth)acrylic acid, maleic acid and fumaric acid; (d) (meth) acrylic acid derivatives, such as (meth)acrylonitrile and (meth)acrylamide; and (e) vinyl ethers, such as vinyl methyl ether and vinyl phenyl ether. Of these comonomers, vinyl esters of monocarboxylic acids having 1-4 carbon atoms are preferred, such as vinyl acetate, and (meth)acrylates of alcohols having 1-4 carbon atoms, such as methyl (meth)- acrylate. An especially preferred comonomer is butyl- acrylate. Two or more such olefinically unsaturated compounds may be used in combination. The expression "(meth)acrylic acid" is here intended to comprise both acrylic acid and methacrylic acid. The comonomer content in the copolymer may amount to about 40% by weight, preferably about 0.5-35% by weight, and most preferred about 1-25% by weight of the copolymer. The silane-containing polymer of the present invention contains the silane compound in a content of 0.001-15% by weight, preferably 0.01-5% by weight, and most preferred 0.1-3% by weight. Crosslinking of the polymer is carried out by so-called moisture hardening which means that the silane group, under the action of water, is hydrolysed and splits off alcohol to form silanol. The silanol groups are then crosslinked under the action of a so-called silanol condensation catalyst by a condensation reaction during which water is split off.

Generally, all silanol condensation catalysts may be used for the present invention. More particularly, they are selected among carboxylates or metals, such as tin, zinc, iron, lead and cobalt, organic bases, inorganic acids and organic acids.

Specific examples of silanol condensation catalysts are dibutyl tin dilaurate, dibutyl tin diacetate, dioctyl tin dilaurate, stannoacetate, stannocaprylate, lead naphthenate, zinc caprylate, colbalt naphthenate, ethyl amines, dibutyl amine, hexyl amines, pyridine, inorganic acids, such as sulphuric acid and hydrochloric acid, and organic acids, such as toluene sulphonic acid, acetic acid, stearic acid, and maleic acid. Especially preferred catalyst compounds are the tin carboxylates.

The amount of silanol condensation catalyst employed usually is of the order 0.001-10% by weight, preferably 0.01-5% by weight, especially 0.03-3% by weight, relative to the amount of silane-containing polymer in the composition.

The crosslinkable polymer may contain different additives, as is usually the case in polymer compositions. Examples of such additives are miscible thermoplastics, stabilisers, lubricants, fillers, colourants and foaming agents.

Among additives in the form of miscible thermoplastics, mention may be made of miscible polyolefins, such as polyethylene of low density, medium density and high density, polypropylene, chlorinated polyethylene, and various copolymers including ethylene and one or more other monomers (such as vinyl acetate, methyl acrylate, propylene, butene, hexene and the like). The above-mentioned polyolefin may be used alone or in mixture with several polyolefins. The polyolefin content of the composition may amount to 70% by weight, based upon the sum of the amounts of this polyolefin and the silane-containing polymer. As examples of fillers, mention may be made of inorganic fillers, such as silicates, for example kaolin, talc, montmorillonite, zeolite, mica, silica, calcium silicate, asbestos, glass powder, glass fiber, calcium carbonate, gypsum, magnesium carbonate, magnesium hydroxide, carbon black, titanium oxide and the like. The amount of this inorganic filler may be up to 60% by weight, based upon the sum of the weights of the filler and the silane-containing polymer. What has been said above concerns the composition of the preferred crosslinkable polymer of the multilayered polymer material in the context of this invention. The multilayered polymer material of the invention includes at least one crosslinkable layer, preferably of the preferred polymer, and at least one layer of another material. Such other materials are those usually employed in, for example, laminate films or foils, together with polyolefins, and as examples mention may be made of saturated polyesters, polyamides, saponified products of ethylene-vinyl acetate copolymers, polyolefins, polystyrenes, polyvinyl chlorides, polyvinylidene chlorides, and acrylic resins, paper, cellophane, textile fabrics, and the like. The multilayered polymer material of the present invention may further include a film or foil of metal, such as aluminium iron and copper. The multilayered polymer material of the present invention is produced by means of any of the conventional techniques, such as dry lamination and wet lamination, in which case an adhesive may be used between the layers, and extrusion coating, coextrusion etc. If necessary, the adhesion between the layers may be increased by providing an anchoring layer between the layers. Generally, the preferred lamination technique comprises a method step in which a resin composition including the above-mentioned silane-modified polyolefin, is melted. A technique of this character is especially useful for resin compositions containing this polymer.

As has been mentioned before, the present invention relates generally to a multilayered polymer material including at least one silane-containing polymer layer crosslinked by means of a catalyst. This polymer layer is thin, i.e. it has a thickness of at most about 2 mm, preferably at most about 1 mm. The invention is especially useful for coextruded and laminated multilayer structures, such as film, extrusion coatings, and bottles. The invention will be described in more detail below with reference to extrusion coatings and films. In the drawings. Fig. 1 is a cross-sectional view of a three-layered polymer film.

Fig. 1 shows a three-layer film 1. It is understood that the invention is not restricted to precisely three-layer films, and that the invention comprises laminates having from two layers up to, in principle, an infinite number of layers, provided that at least one of the layers is a silane-crosslinkable layer. In principle, the catalyst-containing layer may be positioned anywhere in the film structure, and it need not be positioned such that it adjoins the silanecrosslinkable layer or layers. However, it is a condition that the intermediate layer, if any, between the catalyst-containing layer and the silane-crosslinkable layer or layers does not absorb, react with or act as a barrier to the catalyst. The three-layer film illustrated comprises three layers 2, 3 and 4 which in the Figure are shown to have the same thickness, but which in actual practice may have mutually different thicknesses. The polymer layers 2 and 4 are silane-crosslinkable and consist of the previously described silane-modified polyolefin, preferably an ethylene vinyl alkoxy silane copolymer, a silane-modified ethylene polymer, or an ethylene/butylacrylate/vi- nyl alkoxy silane terpolymer crosslinkable under the action of moisture in the presence of a silanol condensation catalyst, such as dibutyl tin dilaurate, dibutyl tin diacetate, or dioctyl tin dilaurate. The intermediate layer 3 consists of a non-crosslinkable polymer, such as polyethylene, polyvinyl alcohol, ethylene vinyl alcohol copolymer, polyamide, polyethylene terephthalate, ethylene vinyl acetate, polypropylene, etc.

In the conventional production of a crosslinked film according to the above, a monofilm is extruded from a polymer composition containing all of the components, also the catalyst. In such production, where the catalyst is admixed from the outset to the polymer composition, there is a risk that crosslinking is initiated already in the extruder, which results in gel formation with the ensuing difficulties and disadvantages mentioned above. TO eliminate these disadvantages, the procedure according to the invention is such that the catalyst for the crosslinkable layers 2 and 4 is not admixed to the polymer composition for these layers prior to the extrusion, but is instead supplied to the composition for the layer 3. This means that the catalyst will be present in the layer 3 during extrusion so that it can then migrate by diffusion into the layers 2 and 4 and initiate crosslinking thereof. Since the crosslinking of the polymers in the layers 2 and 4 is not initiated until after the extrusion, the difficulties encountered with prior art technique are avoided. The layer 3 is a non-crosslinkable polymer composition, i.e. a polymer composition which is not affected by the admixed catalyst because this could result in a precuring and an ensuing formation of gel lumps, whereby one would create precisely the problems which one tries to avoid for the two other layers 2 and 4. In this connection, the statement that the layer 3 is not "crosslinkable" implies that it is not crosslinked by means of the admixed catalyst. Polymer layers

3 which are crosslinkable in other ways not affected by the admixed catalyst, may here by ranked equal with non-crosslinkable polymer compositions.

While the crosslinkable layers 2 and 4 in the context of this invention consist of the silane-containing polyolefin previously mentioned, the layer 3 may be selected substantially freely among other polymers, cellulose, textile fabrics and similar materials into which the catalyst can be introduced. Examples of such materials have been mentioned before.

When the catalyst after extrusion of the three-layer film 1 has diffused from the layer 3 into the cross-linkable layers 2 and 4, crosslinking can be initiated by the action of water in liquid or vapour form. Crosslinking is preferably carried out at a temperature of about 20-200°C, usually about 20-130°C, for a period of time of from about 10 sec. to 1 week, usually about 1 min. to 1 day. Crosslinking can be carried out at atmospheric pressure or elevated pressure.

To further illustrate the invention, the following nonrestrictive Examples are given. In these Examples, the materials, the testing techniques, the conversion equipment, the extrusion conditions employed, and the procedure were as follows. MATERIALS EMPLOYED;

SILANE 1 Ethylene butylacrylate vinyltrimethoxy silane terpolymer with 20% butylacrylate and 2.3% vinyltrimethoxy silane. Melt index 4.5, (w) = 132000 SILANE 2 Ethylene vinyltrimethoxy silane copolymer with 1.7% vinyltrimethoxy silane. Melt index 4.5, M(w) = 124000

SILANE 3 Ethylene methacryloxypropyl trimethoxy silane copolymer with 1.5% methacryloxypropyltrimethoxy silane. Melt index 2.5, M(w) = 120000

CAT.OCTYL 1% dioctyl tin dilaurate in LDPE 1 CAT.BUTYL 1% dibutyl tin dilaurate in LDPE 2 LDPE 1 High pressure polyethylene with: Melt index 4.0 g/10 min. Density 922 kg/m³ (w) 123800

LDPE 2 High pressure polyethylene with:

Melt index 4.5 g/10 min.

Density 919 kg/m³ (w) 202000 EVA Ethylene vinylacetate copolymer melt index 1.6 g/10 min.

Vinylacetate content 4% (w) 330000

EBA Ethylene butylacrylate copolymer Melt index 7.0 g/10 min

Butylacrylate content 17% (w) 89500

TESTING TECHNIQUES: 1. Crosslinking degree: The film sample was ground and screened to recover that part which passes through a 30 mesh sieve but stays on a 60 mesh sieve. The sample was placed in a 100 mesh sieve and put into a 500 ml glass flask. The glass flask was then filled with xylene, and 1% antioxidant ( 2 ,2-methylene-bis-4-methyl-tert.-butyl phenol) was added. The sample was boiled for 6 hours with reflux and then dried at 140ºC for 1 hour, cooled in an exsiccator and weighed. The crosslinking of the film was measured in three different phases.

1. Immediately after the film had been run (1-6 hours after running). 2. After storage in a conditioned room (about 14 days at 23°C and 50% relative humidity) in rolled-up condition. 3. After 1 day in water of 60ºC

2. Weldability: The weldability of the vinyl silane films was measured at three different temperatures (160°C, 180°C, 200°C) and at three different contact times (0.1 sec, 0.2 sec. and 0.5 sec). The weld was tested in accordance with the tensile strength test. 3. Dart drop ASTM D 1709

4. Tensile strength ISO R 1184

CONVERSION EQUIPMENT

1. Reifenhauser 3-layer film blowirιg equipment. 1st extruder: - Screw diameter 70 mm - Combined LDPE/LLDPE screw, length

25 D, with UCC mixing zone and mixing unit, length 3 D 2nd extruder: - Screw diameter 50 mm

- HDPE screw, length 20 D, with mixing unit, length 5 D

3rd extruder: - Screw diameter 50 mm

- Polyamide screw, length 23 D

3-layer revolving die, diameter 200 mm and slot 1 mm 2. Beloit Dual Slot, two-layer equipment for extrusion coating

Screw diameters 4 1/2". LDPE screws, length 24 D Die: Dual slot, length 2 x 850 mm, slot 2 x 0,75 mm EXTRUSION CONDITIONS:

FILM BLOWING: The extrusion temperature for layers containing SILANE 1 or SILANE 2 from the feeding zone to the filter during the tests had been set at: 130°C, 140°C, 150ºC. The filter temperature was maintained at 150ºC, as was the temperature in the adaptor. During the runs with LDPE 1, LDPE 2, EVA, CAT.OCTYL, CAT.BUTYL and mixtures thereof, the corresponding temperatures were: 140°C, 150°C, 160°C. The temperature in the filter and the adaptor was 160°C.

The temperature in the die was maintained at 160ºC. The film was run with a blowing ratio of 3 and with a frost line of 750 mm.

The total production rate was maintained at 60 kg film/hour.

The layer thicknesses for LDPE 1, LDPE 2, CAT.OCTYL, CAT.BUTYL and the mixtures thereof were maintained constant at 10 μm. For SILANE 1, SILANE 2, SILANE 3, EVA, EBA and mixtures thereof, layer thicknesses of 20 μm were used. Thus, the total film thickness for all samples was 50 μm.

EXTRUSION COATING: The temperature setting was the same for all layers and materials. From the feeding zone to the filter: 200°C, 240°C, 280°C, 280°C, 280°C. The temperature in the filter and the adaptor was maintained at 280°C. Web speed 100 m/min. For SILANE 2, a 40 μm layer was extruded, while the layer was 10 μm for LDPE 1 and CAT.BUTYL. PROCEDURE: FILM BLOWING: The extruder and die temperatures were set with LDPE 1 in the film blowing line. When constant conditions had been established, the selected layer or layers were charged with the silane polymer. When the silane polymer or polymers had displaced LDPE 1, the catalyst was charged into the remaining layer.

In this manner, a film free from defects and without gel formation could be produced. If, on the other hand, the silane polymer and the catalyst are charged into the same extruder, the silane polymer will be crosslinked already in the extruder, and there is considerable risk of obtaining a film of high gel content and total crosslinking in the extruder which, in that case, must be cleaned.

EXTRUSION COATING: The extruders were started with LDPE 1, and when constant conditions had been established, the silane polymer was charged into the desired extruder. When the silane polymer had displaced LDPE 1, the catalyst was charged into the extruder for the remaining layers. A coating without defects and without gel formation was obtained. If, on the other hand, the silane polymer and the catalyst were charged into the same extruder at the extrusion coating temperature, the risk that the extruder would jam due to crosslinking of the silane polymer in the extruder, was considerable. This risk is especially high when extrusion coating is carried out at high temperatures, and this again increases the risk of precuring. EXAMPLE 1

Using the above-mentioned materials, equipment and techniques, there were produced by coextrusion multilayer films having the general configuration Silane polymer/Catalyst-containing layer/Silane polymer, and were compared with a monolayer film of silane polymer and catalyst. More particularly, the multilayer films had the following composition:

Example 1.1: SILANE 1/25% CAT.BUTYL + 75% LDPE 2/SILANE 1 Example 1.2: SILANE 1/50% CAT.BUTYL + 50% LDPE 2/SILANE 1 Example 1.3: 50% SILANE 1 + 50% EBA/25% CAT.BUTYL 75% LDPE 2/50% SILANE 1 + 50% EBA Example 1.4: SILANE 2/25% CAT.BUTYL + 75% LDPE 2/SILANE 2 Example 1.5: SILANE 3/25% CAT.BUTYL + 75% LDPE 2/SILANE 3 Example 1.6: SILANE 2/LDPE 2/SILANE 2 Example 1.7: SILANE 2/20% CAT.OCTYL + 80% LDPE 1/SILANE 2 Example 1.8: SILANE 1/20% CAT.OCTYL + 80% LDPE 1/SILANE 1.

The monolayer film used for comparison had the following composition: Example 0: 80% SILANE 1 + 20% (25% CAT. BUTYL + 75% LDPE 2)

In Table 1, the test results of the various films have been compiled. In Table 1 and in the following Tables, the abbreviations A-E have the following meaning: A = Crosslinking immediately after extrusion (%) B = Crosslinking after 14 days (%)

C = Crosslinking after storage in water of 60°C (%) D = Dart drop immediately after film blowing (g) E = Dart drop after 14 days (g)

Example 0 was extremely difficult to run, as com- pared with the coextruded films of Examples 1.1-1.8. The film appearance was extremely bad, with large gels causing hose rupture during production. The pressure within the cylinder was high as compared with the silane polymer without catalyst, and the risk of total crosslinking in the extruder was obvious.

Examples 1.1, 1.2 and 1.3 gave a readily extruded gel-free film also at high production rates. Because of the smooth and gel-free film, the dart drop characteristics are vastly improved after crosslinking of the film (compare the dart drop characteristics of Example 0 with the remaining Examples of Table 1). There is a marked increase in the dart drop characteristics, concurrently with the crosslinking degree of the coextruded film. The total catalyst concentration does not affect the final crosslinking degree when a catalyst is present, but merely the crosslinking rate (Examples 1.1 and 1.2). Thus, the total crosslinking degree of the film depends solely on the vinyl silane content of the film, when the film contains a catalyst (Examples 1.3 and 1.1).

Examples 1.4-1.5 illustrate the effect obtained by using different silane-containing polyolefins. The Examples also show that copolymers of ethylene and various hydrolysable silanes are crosslinked in the same manner as the terpolymer of Examples 1.1-1.3, although the crosslinking degree is lower and the improvement in dart drop is less.

The effect obtained if the catalyst is omitted, is shown in Example 1.6. The dart drop-value must be compared with the Dart drop values of the other SILANE 1-containing films, i.e. Examples 1.1-1.3 and 1.8.

Examples 1.7 and 1.8 show the effect obtained by using a catalyst of higher molecular weight, and should be compared with Examples 1.1-1.3 in Table 1. Catalysts of higher molecular weight do not affect the mechanical test results. The crosslinking rate decreases somewhat because of a lower migration rate. EXAMPLE 2

In the same manner as in Example 1, a multilayer film having the general configuration Silane polymer/ Silane polymer/Catalyst-containing layer was prepared. More particularly, the multilayer film had the following composition. Example 2: SILANE 1/SILANE 1/25% CAT. BUTYL + 75% LDPE 2.

The test result of this film is shown in Table 2.

Example 2

It appears that the layer thickness of the silane polymer or the position of the catalyst layer does not appreciably affect the crosslinking process or the mechanical values. EXAMPLE 3 In the same manner as in Examples 1 and 2, a multilayer film having the general configuration Silane polymer/Polyolefin/Catalyst-containing layer was prepared. More particularly, the multilayer film had the following composition: Example 3.1: SILANE l/EVA/25% CAT.BUTYL + 75% LDPE 2. In addition, another multilayer film without catalyst was prepared which had the composition: Example 3.2: SILANE 1/EVA/LDPE 2 .

The test results of this film are shown in Table 3.

It appears that it is not necessary that the catalyst-containing layer adjoins the crosslinkable silane polymer layer, and this means that the catalyst-containing layer can be positioned anywhere in the multilayered film structure, provided that any intermediate layers do not absorb, react with or act as a barrier to the catalyst. EXAMPLE 4

Using the coating extrusion technique previously described, a coated paper was prepared. The following tests were made: Example 4.0: Paper/20% (25% CAT.BUTYL + 75% LDPE 2) +

80% SILANE 2 Example 4.1: Paper/25% CAT.BUTYL + 75% LDPE 2/SILANE 2 Example 4.2: Paper/LDPE 2/SILANE 2.

Example 4.0 comprising a mixture of catalyst and silane-containing polymer could not be run because the mixture was crosslinked in the extruder and the screw jammed so that production had to be shut down. When the silane-containing polymer and the catalyst were run in separate layers (Example 4.1) no cross- linking tendency occurred in the extruder, and a gel- free film could be coated on the paper web. The test result in respect of the crosslinking degree will appear from Table 4.

EXAMPLE 5

In this Example the weldability of SILANE 1 and SILANE 2 was tested. The test results are shown in Tabel 5 which indicates the weld strength in N/cm weld.

Table 5 shows that the weldability of SILANE 1 deteriorates as a function of the crosslinking degree. After 14 days, however, the film can be welded, although the weld strength will be slightly lower. For SILANE 2, however, no deterioration occurs.

As will appear from the above description, the present invention brings, inter alia, the following advantages:

1. No risk of crosslinking of materials in the extruder, although temperatures were used which in most cases would partly crosslink the material and render film production difficult. 2. A gel-free product can be produced (no crosslinking reaction during the extrusion phase). 3. A silane-containing polymer can be used as a weld- able layer in a structure of otherwise thermally stable layers (usually not weldable) such that the entire structure can withstand elevated temperature of use. 4. One or more crosslinked layers can be combined to form an otherwise non-crosslinked structure of functional layers in one step.

5. The silane-containing polymer layer can be welded prior to crosslinking. After crosslinking, welds are obtained which are resistant to surface active, dissolving and aggressive substances, which is important in the drug industry, the food industry and the chemical industry. 6. A silane-crosslinked product can be welded, which is not the case with peroxide or radiation crosslinked products. 7. In contrast to peroxide or radiation crosslinked products, the production of silane-crosslinked products requires no large investments in auxiliary equipment.

Claims

1. A process of producing a crosslinked polymer product incorporating at least one polymer layer crosslinked by means of a catalyst, c h a r a c t e r i s e d in that a multilayered film (1) is produced which comprises at least one layer (2, 4) of a silane group- containing olefin copolymer crosslinkable under the action of water and a silanol condensation catalyst, and at least one other layer (3) free from crosslinkable silane and incopbrating a silanol condensation catalyst, and that crosslinking of the silane group-containing layer (2, 4) is achieved by subjecting the film to the action of water and causing the silanol condensation catalyst to diffuse into the silane group-containing layer (2, 4).

2. A process as claimed in claim 1, c h a r a c t e r i s e d in that the silane group-containing olefin copolymer layer which is crosslinked consists of a copolymer of (a) ethylene, (b) ethylenically unsaturated silane compound, and optionally

(c) one or more further monomers copolymerisable with said monomers.

3. A process as claimed in claim 2, c h a r a c t e r i s e d in that the silane group-containing olefin copolymer layer which is crosslinked consists of a copolymer of

(a) ethylene

(b) an unsaturated silane compound having the formula

RSiR'_nY_3-n in which R is an ethylenically unsaturated hydrocarbyl or hydrocarboloxy group, R' is an aliphatic unsaturated hydrocarbyl group, Y is a hydrolysable organic group, and n is 0, 1 or 2, and optionally

(c) one or more further monomers copolymerisable with these monomers.

4. A process as claimed in claim 3, c h a r a c t e r i s e d in that the silane group-containing olefin copolymer layer which is crosslinked consists of a copolymer of (a) ethylene

(b) unsaturated silane compound having the formula

CH₂ = CHSi(OA)₃

in which A is a hydrocarbyl group having 1-8 carbon atoms, preferably 1-4 carbon atoms, and optionally

(c) one or more further monomers copolymerisable with these monomers.

5. A process as claimed in any one of claims 1 to 4, c h a r a c t e r i s e d in that the silane group- containing olefin copolymer layer contains one or more further monomers (c) selected among vinyl esters of monocarboxylie acids having 1-4 carbon atoms and acrylate or methacrylate of alcohols having 1-4 carbon atoms.

6. A process as claimed in any one of claims 1-5, c h a r a c t e r i s e d in that the multilayered polymer material is produced by coextrusion of said layers, and that the silanol condensation catalyst is introduced into the material of the catalyst-containing layer (3) prior to extrusion.

7. A process as claimed in claim 6, c h a r a c t e r i s e d in that the silane group-containing olefin copolymer is crosslinked under the action of a silanol condensation catalyst which is a tin salt of a carboxylic acid, preferably dibutyl tin dilaurate, dibutyl tin diacetate, or dioctyl tin dilaurate.

8. A process as claimed in any one of claims 1-7, c h a r a c t e r i s e d in that the silane group- containing olefin copolymer layer has a thickness of at most 2 mm.

9. A process as claimed in any one of claims 1-8, c h a r a c t e r i s e d in that the monomer constitutes 0.001-15% by weight of the silane group-containing copolymer.

10. A process as claimed in any one of the preceding claims, c h a r a c t e r i s e d in that the catalyst is introduced into a layer (3) adjacent the layer (2, 4) to be crosslinked.