WO1989000501A1

WO1989000501A1 - Gastight, heat-stable, multilayered material and a method for production thereof

Info

Publication number: WO1989000501A1
Application number: PCT/SE1988/000381
Authority: WO
Inventors: Christer BERGSTRÖM; Peter Nikko; Hans MÅRTENSSON; Maria Stjernberg
Original assignee: Neste Oy
Priority date: 1987-07-20
Filing date: 1988-07-15
Publication date: 1989-01-26
Also published as: EP0379487A1; DK16190A; DK16190D0; JPH03500747A

Abstract

A gastight, heat-stable, multilayered material and a method for the production thereof is described. The multilayered material consists of a layer of a first polymer which consists of a mixture of 99-1% by weight polyolefin and 1-99% by weight polyvinyl alcohol and, optionally, a plasticiser containing alcohol groups, and on at least one side of said layer a layer of a second polymer derived from an olefin copolymer containing hydrolysable silane groups. The first polymer consists of, for example, a mixture of polyethylene and polyvinyl alcohol, while the second polymer consists of, for example, a crosslinked copolymer of ethylene and vinyl alkoxy silane, such as vinyl trimethoxy silane which is crosslinked by hydrolysis and condensation of the silane groups under the action of water and a catalyst. In the production of the multilayered material, the catalyst is provided in a layer separate from the crosslinkable silane group-containing copolymer layer or layers, for example in the layer of polyolefin/polyvinyl alcohol mixture, the catalyst then migrating into the layer or layers of silane group-containing copolymer and crosslinking these layers. The multilayered material is preferably produced by coextrusion. The adhesion between the layers can be improved by providing adhesion layers between the layers, or by incorporating an adhesion-improving agent, such as benzoic acid or tetraisopropyl titanate, in any one of the layers.

Description

GASTIGHT, HEAT-STABLE, MULTILAYERED MATERIAL AND A METHOD FOR PRODUCTION THEREOF

The present invention relates to a gastight, heat- stable, multilayered material and to a method for pro¬ duction thereof.

Impermeable polymers containing vinyl alcohol are previously known and are in principle of two kinds: olefin/vinyl alcohol copolymers with random distribu¬ tion of the mer units, and olefin/vinyl alcohol block copolymers. The former are usually ethylene/vinyl alcohol copolymers, but propylene/vinyl alcohol copo- lymers have also been developed.

At present, ethylene/vinyl alcohol copolymers are produced by hydrolysis of ethylene/vinyl acetate copolymers. The hydrolysis (alcoholysis) takes place with the aid of alcohol, and the mixture may be either acid or basic. The hydrolysis may be carried out in the solid state, the molten state, or in solution.

A great many patent specifications exist which concern this technique for the production of ethylene/ vinyl alcohol copolymers, and a review is presented in the publication "Reactivity of Ethylene Vinyl Acetate Copolymers: A Critical Evaluation of the Comprehensive Patent Literature on the Acetoxy-Hydroxide Transforma¬ tion of Ethylene Vinyl Acetate Copolymers", R.J. Koopmas, R. van der Linden, and E.F. Vansant, Polymer Engineering and Science, July 1982, Vol. 22, No. 10, p. 645. There are two kinds of commercial ethylene/vinyl alcohol copolymers. The brands containing 20-30 mole% vinyl alcohol are used in the first place for injection mould¬ ing and for the powder coating of steel tubing, and are made from ethylene/vinyl acetate copolymer produced by high-pressure technique. Brands containing 60-80 mole% vinyl alcohol are also available on the market and are used mainly for gastight layers in multilayered products. They are produced by adding ethylene to the polyvinyl acetate process and hydrolysing the product in a manner similar to the production of polyvinyl alcohol (PVA) from polyvinyl acetate. If there is less than 60 mole% vinyl alcohol , the gas-tightness of the product goes down steeply. When the ethylene/vinyl alcohol copolymer contains more than 60 mole% vinyl alcohol, it forms o- noclinic crystals (the same as PVA), and when it contains less than 20 mole% vinyl alcohol, it forms rhombic crys- tals (the same as polyethylene). In the range between these limit values, a mixed crystal structure is formed. Only the monoclinic crystal structure is sufficiently impermeable for barrier applications.

Olefin/vinyl alcohol block copolymers can be produced by using reactive compounding technique (com¬ pound = a melt-homogenised mixture) as disclosed in our European Patent Application EP 187040. Here, a PVA chain is chemically bonded to a polyolefin chain, for example by means of a hydrolysable silane. The hydrolysable silane is first bonded to the polyolefin chain by copolymerising or grafting the unsaturated silane, or by bonding to the polyolefin chain another functional hydrolysable silane, such as a ino silane. The polyolefin modified with hydrolysable silane is then mixed in the molten state with PVA, whereby the silane undergoes hydrolysis and reacts chemically with PVA, and an olefin/vinyl alcohol block copolymer is formed.

Polyvinyl alcohol (PVA) is mainly used as a dls- persing agent in suspension polymerisations of PVC, and it is possible, by controlling the molecular weight of PVA and its degree of hydrolysation, to control the properties of the PVC. When PVA has a sufficiently high degree of hydrolysation, it is water-soluble. The water-solubility and difficulties encountered in process¬ ing restrict the use of PVA in the plastics industry. Dry PVA is interesting in itself because it has excellent barrier properties (gas-tightness). To avoid these shortcomings, the above-mentioned ethylene/vinyl al¬ cohol coplymer (EVOH) was developed. The gas-tightness of EVOH is, however, dependent on moisture, for which reason an EVOH film must be protected against air humidity, for example by applying a polyethylene film on both sides. Furthermore, EVOH does not adhere to polyethylene, and an adhesion plastic (for example Admer) must also be used between these components. However, a coextruded five-layer construction like this is highly exacting and expensive. Also, EVOH and adhesion plastics are very expensive.

EVOH may also be mixed with plastic and is then less susceptible to moisture. EVOH is mixed with, for example, polyethylene terephthalate (PET), whereby bottles of higher gas-tightness are obtained than by coextrusion. EVOH may also be mixed with polyolefins, whereby sufficient gas-tightness and adhesion to polyo¬ lefins are simultaneously obtained. In the present invention, an easier way of utilis¬ ing the good barrier properties of polyvinyl alcohol has now been found. It was observed in the invention that mixing of polyvinyl alcohol (PVA) with polyole¬ fins, such as polyethylene, polypropylene and polybu- tene, and copolymers thereof, yields products which have the good barrier properties of polyvinyl alcohol, but not its drawbacks.

To further improve the barrier properties of the product, and also to make the product heat-resistant, it comprises, in addition to a layer based on the above- mentioned mixture of polyvinyl alcohol and polyolefin, at least one layer of a polymer derived from an olefin copolymer containing hydrolysable silane groups.

The gastight heat-stable, multilayered material according to the invention is thus characterised in that it consists of a layer of a first polymer which consists of a mixture of 99-1% by weight polyolefin and 1-99% by weight polyvinyl alcohol and, optionally, a plasticiser containing alcohol groups, there being provided on at least one side of this layer a layer of a second polymer derived from an olefin copolymer containing hydrolysable silane groups.

The method for production of a gastight heat-stable, multilayered material according to the invention is characterised in that there is provided, on at least one side of a layer of a first polymer consisting of a mixture of 99-1% by weight polyolefin and 1-99% by weight polyvinyl alcohol and, optionally, a plasticiser contain¬ ing alcohol groups, a layer of a second polymer derived from an olefin copolymer containing hydrolysable silane groups, and that there is provided, before said first and second polymer layers are brought together, a sila- nol condensation catalyst in a layer separate from said second polymer.

Further characteristic features and advantages of the invention will appear from the following descrip- tion and the nonindependent claims.

As mentioned above, the layer based on a mixture of polyolefin and polyvinyl alcohol is characterised in that it includes, in mixture, 99-1% by weight poly¬ olefin and 1-99% by weight polyvinyl alcohol and, op- tionally, a plasticiser containing alcohol groups.

The material preferably contains at most 10% by weight plasticiser.

Polyolefin/PVA mixtures are not novel in themselves, but they have not been used in the form of tight multi- layered products in the manner taught by the present invention. For example, polyolefin/PVA mixtures are known in the art, use being made of the hydrophilic properties. In U.S. Patent 4,529,539 such mixtures have been impregnated with electrolyte, and elecrrical- ly conductive plastic products have been obtained in this manner. Japanese Patent 60147473 discloses electrically conductive plastic products consisting of polyolefin/PVA/carbon black mixtures, and U.S. Patent 3,984,358 discloses ion exchangers consisting of poly¬ olefin/PVA mixtures.

Japanese Patent 77024976 discloses batteries in which there is provided, between the anode and the cathode, a polyolefin/PVA mixture serving as a drying agent, and Japanese Patent 54020057 has developed PVA mixtures that can be successfully dispersed in water. In Japanese Patent 70001747 the stainability of polyolefin fibres has been improved by admixture of PVA. The hygroscopic properties of polyolefin/PVA mixtures have also be utilised for coating cement and strengthening cement with polyolefin/PVA fibres (JP 5922328 and JP 79036095). PVA also improves the adhesion to cement.

It has been found in the present invention that PVA can be mixed with polyolefins in the molten state in any proportions whatsoever, whereby a mixture is obtained which consists of two phases, a continuous phase and a dispersed phase. When the diameter of the dispersed phase is sufficiently small, a trans¬ parent film is obtained. Such a good dispersion is obtained only if the mixture is efficient enough. Normal extruders are not good enough in this respect, and compounding is required before the final product is extruded.

When the PVA component in a mixture of polyolefin and PVA forms the continuous phase, excellent tight¬ ness, e.g. oxygen-tightness, is obtained, and the pro- duct is clear and stable both in the molten state and in the solid state, in spite of its being composed of two phases, and it is readily plasticised by adding a plasticiser.

It has also been found that, when polyolefin con- stitutes the continuous phase, excellent adhesion to polyolefins was obtained, and it is possible in this manner to produce three-layer products by coextruding polyolefin/PVA mixture in the middle and equivalent poly¬ olefin, or other polyolefins, on either side. However, the tightness properties are not as good as in the case when PVA constitutes the continuous phase, in which case an adhesion plastic is required between the poly¬ olefin layers and the polyolefin/PVA layer (i.e. five- layer products). Factors influencing the phase struc¬ ture of PVA and polyolefins are: the proportion of the components, their viscosities, dispersing agents, if any, the compounding conditions, and the processing conditions.

According to the invention, the polyvinyl alcohol may be of the grade commonly used in suspension poly¬ merisation of PVC, and may be completely or partly hydrolysed (with vinyl acetate as comonomer) , or any other polymer containing 0.5-100% by weight vinyl alcohol .

It has also been found that polyolefin/PVA mix¬ tures are both clearer and more elastic if a plasticiser has been added to the mixture. Such plasticisers are usually alcohol compounds which enter the PVA crystals and reduce the degree of crystallinity of the polyvinyl alcohol and the rigidity and brittleness of PVA and of polyolefin/PVA mixtures. Suitable plasticisers are, for example, glycerol, trimethylol propane and tri- ethylene glycol. The plasticisers may be added by impregnating at elevated temperature either the poly¬ vinyl alcohol before compounding or the polyvinyl/PVA mixture after compounding, or they may be added in connection with the compounding, either before melting the polymer components or after melting, or in some other way, depending on the type of plasticiser.

The above-mentioned polyolefin/PVA mixtures may be produced by using as polyolefin high-pressure poly- ethylene (LDPE), low-pressure polyethylene (HDPE, LMDPE, LLDPE, VLDPE, ULDPE), polypropylene (PP) , poly-1-butene (PB), poly-4-methyl-l-pentene (TPX) or other polyolefin- based plastics, rubbers or additives. It is also possible to use copolymers of the above-mentioned polymers, such as ethylene-methylacrylate (EMA), ethylene-ethylacrylate ⁽EEA), ethylene-butylacrylate (EBA) and ethylene-vinyl- acetate (EVA), and propylene-ethylene copolymers or block copolymers.

The gastight polyolefin/PVA mixture according to the invention for improving the barrier characteristics may be produced by mixing 1-99% by weight polyolefin, 99-1% by weight polyvinyl alcohol and, optionally, a plasticiser containing alcohol groups. All requisite components may be added to a melt mixer simultaneuously in the form of a dry mix, premixed in the solid or molten state, or the different components may be added sepa- rately. Plasticising may also be carried out after com¬ pounding, and it is also possible to proceed such that the plasticiser is admixed to the PVA component, where¬ by one gains the advantage that the entire mixture need not be plasticised. in the gastight, heat-stable, multilayered product according to the invention, a first polymer consisting of the above-mentioned polyolefin/polyvinyl alcohol mixture is provided, on one or both sides, with a layer of a second polymer derived from a silane group-contain- ing olefin copolymer. The multilayered product may be produced by coextrusion, (co)extrusion coating, (co)ex¬ trusion lamination, glue lamination, or some other tech¬ nique, and these production methods may also be com¬ bined. Coextrusion is preferred at present. if it is desired to improve the adhesion between the first and second polymer layers, this may be done by providing in per se known manner an adhesion layer between the layers. Alternatively, it is possible to add to one of the layers an adhesion-improving sub- stance which is selected among substances catalytical- ly promoting a condensation of silanol with hydroxyl groups (for example the substances mentioned in the Dow Corning paper Silicones in Protective Coatings, p. 548, in the Treatise on Coatings, Vol. 1. part 3,^* Marcel Dekker, 1972).

As examples of adhesion improving substances that can be incorporated in the first and/or the second po¬ lymer, mention may be made of benzoic acid and tetra- isopropyl titanate.

The last-mentioned alternative of incorporating an adhesion-improving substance is preferred and con¬ stitutes a specific aspect of this invention because it eliminates the use of a separate adhesion layer, whereby the production of the multilayered material according to the invention is simplified and made less expensive (allows production of a three-layer material instead of a five-layer material).

Suitable products incorporating the gastight ma¬ terial according to the invention include films, blow- moulded bottles and containers, sheets, tubing, injec¬ tion moulded vessels, deep-drawn films and sheets, li- quid packaging cartons, etc. As a rule, such tight and multilayered products are used in packaging food¬ stuffs when oxygen impermeability is desired, but impermeability to carbon dioxide or some other gas may also be desirable. In addition, tightness against fats, chemicals and odour is required of foodstuff, packages as well as technical products.

The specific combination of materials in the mul¬ tilayered material according to the invention gives improved barrier properties, such as low permeability to oxygen and other gases and, furthermore, high heat stability. Thus, the combination of layers of cross¬ linked silane group-containing olefin copolymer makes it possible to achieve a heat stability to temperatures of up to about 170°C. Above all, the multilayered ma¬ terial is useful for different types of packages, for example of the type Bag-in-Box, for e.g. foodstuffs, and other products that are to be heat-treated, for example autoclaved. Since the material is transparent, the packaged goods can be viewed, and since it is nonmetal- lic, it can be used for packaging foodstuffs intended to be heated in a microwave oven. The second polymer layer of the multilayered mate¬ rial, which polymer is derived from a crosslinkable olefin copolymer incorporating hydrolysable silane groups, will now be described in detail.

The layer or layers of the silane group-contain- ing olefin copolymer are crosslinked after production of the multilayered material by so-called moisture hardening during which the silane groups are hydro- lysed under the action of water and split off alco¬ hol to form silanol groups. The silanol groups are then crosslinked under the action of a so-called sila¬ nol condensation catalyst by a condensation reaction during which water is split off.

However, the production of the crosslinked silane- containing 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 sheet, i.e. up to about 2 mm, preferably about 1 mm at most, and more pre- ferred 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 cf 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 nor 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 sheets, are unacceptable and usually make the product useless.

Undesired precuring may be prevented by incorporat¬ ing 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 intro- duced 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 re- tarders can be avoided. The present invention obviates the above-mentioned disadvantages by providing the silanol condensation catalyst in a layer separate from the second polymer, i.e. either in the layer consisting of a mixture of polyolefin and polyvinayl alcohol or in another separate layer, such as a layer provided between the first and the second polymer and containing the silanol condensation catalyst. The last-mentioned layer may consist of, for example, a so-called master batch layer of the silanol condensation catalyst. Alter- natively, the catalyst may be included in an optional adhesion layer of adhesion plastic (for coextrusion) or adhesive (for lamination).

Crosslinking of the second polymer, i.e. the silane group-containing olefin copolymer, is carried out by subjecting the second polymer to the action of water and causing the silanol condensation catalyst to diffuse into the layer or layers of the second po- 1ymer.

The layer or layers of the second polymer are re¬ stricted to polymers derived from silane group-contain- ing olefin copolymers. The reason for this 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 pre- curing still occurs and imparts to the film a grainy, unacceptable appearance. The cause of this must presum¬ ably be attributed to peroxide residues from the produc¬ tion 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. In addi¬ tion, free monomer may cause discoloration (yellowing) of the product. It was therefore found necessary, in the context of this invention, to utilise for the crosslink- able polymer a silane group-containing olefin copolymer and to provide the silanol condensation catalyst in layer separate from the polymer, i.e. either in the layer of the first polymer or in another separate layer between the layers of the first and the second polymer. The com¬ bination of these two requirements thus is important to the invention.

As has been mentioned, the crosslinkable polymer ma¬ terial according to the invention is a silane-containing copolymer by which is meant an olefin polymer, preferably an ethylene polymer 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 a ino silane compounds can react with acrylate esters, whereas the invention does not include graft polymers in which peroxides are decomposed and the silane compound is grafted on the finished polymer by radical reaction.

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

in which R is an ethylenically unsaturated hydrocarbyl group, R" is a hydrocarbyl group, Y is a hydrolysable 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 com- pound are those in which R is vinyl, allyl, isopropenyl, butenyl, cyclohexenyl, or gamma-(meth)acryloxy alkyl, Y is methoxy, ethoxy, formyloxy, acetoxy, propionyloxy or an alkyl or arylamino group, and R¹ is a methyl, ethyl, propyl, decyl or phenyl group. Among preferred compounds, mention may be made of vinyl trimethoxy silane, vinyl trismethoxyethoxy silane, vinyl triethoxy silane, gamma-(meth)acryloxy- propyl trimethoxy silane, gamma-(meth)acryloxy propyl triethoxy silane, and vinyl triacetoxy silane. An unsaturated silane compound especially preferred at present is represented by the formula

CH₂=CHSi(0A)_

in which each A which is the same or different, is a hydrocarbyl group having 1-8 carbon atoms, preferably 1-4 carbon atoms. 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 copoly- merisable with the two monomers. Examples of such como¬ nomers are: (a) vinyl carboxylate esters, such as vinyl acetate and vinyl pivalate; (b) (meth)acrylates, such as methyl Cmeth)aerylate, ethyl (meth)acrylate, and butyl (meth)acrylate; (c) olefinically unsaturated car- boxylic acids, such as (meth)acrylic acid, aleic 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 pre¬ ferred, 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 0.1-4% by weight.

Generally, all silanol condensation catalysts may be used for the present invention. More particu¬ larly, they are selected among carboxylates of 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 naph¬ thenate, zinc caprylate, colbalt naphthenate, ethyl amines, dibutyl amine, hexyl amines, pyridine, inorganic acids, such as sulphuric acid and hydrochloric acid, and orga¬ nic 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 com¬ position. The crosslinkable polymer may contain different addi¬ tives, 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 thermo¬ plastics, mention may be made of miscible polyolefins, such as polyethylene of low density, medium density and high density, polypropylene, chlorinated polyethy¬ lene, 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, magne¬ sium 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.

It is pointed out that what has been said above concerns the composition of the preferred crosslinkable polymer of the multilayered polymer material according to the invention.

The following nonrestrictive Examples illustrate the production, the properties and the use of tight multilayered products of polyolefin/PVA mixtures.

The oxygen permeability of the films was determined by means of an OX-TRAN 1000 apparatus (ASTM D 3985). The brands of polyolefin, polyvinyl alcohol and silanol condensation catalyst used in the tests of the Examples are stated in the Table below.

Pol olefins (PO)

Remarks

LDPE with 1.7% vinyl tri¬ methoxy silane LDPE with 2% vinyl tri¬ methoxy silane and 17% butylacrylate

Homopolymer

Methylacrylate content 10% Butylacrylate content 17%

Vinylacetate content 5%

Abbreviations: LDPE Low density polyethylene

HDPE High density polyethylene

LMDPE Linear medium-density polyethylene

PP Polypropylene

PB Polybutylene

EMA Ethylene-methylacrylate copolymer

EBA Ethylene-butylacrylate copolymer

EVA Ethylene-vinylacetate copolymer

Visico 1 = Ethylene-vinyl trimethoxy silane copolymer

Visico 2 = Ethylene-butylacrylate vinyl tri¬ methoxy silane copolymer

DOTDL Dioctyl tin dilaurate DBTDL Dibutyl tin dilaurate Polyvinyl alcohol (PVA)

PVA Degree of hydrolysis Viscosity Plasticiser of basic polymer Type ?ό by weight mole_ mPa-s

PVA1 88 PVA2 88 Glycerol/water (85:15)

PVA3 88 Triethylene glycol

PVA4 88 Trimethylol propane

EXAMPLE 1

First polymer (polyolefin/polyvinyl alcohol mixture)

A barrier compound was produced by mixing polyvinyl alcohol (PVA) and polyolefin (PO) in the molten state with the aid of Berstorff mixer, applying the following temperature profile (°C): 190-210-220-230-230-160-200- 200-200-200; the yield of the mixer was 80 kg/h.

From the compound mixes thus obtained, monolayer films with a thickness of 25 μm were made in a Brabender laboratory extruder. In Table 1 are given the composi¬ tions of the films thus obtained, the manufacturing con¬ ditions and the oxygen permeability characteristics. The ASTM standard was applied in the measurement, whereby the result is not dependent on whether monolayer or mul¬ tilayer films are used, when the measurements are car¬ ried out in dry condition (humidity 0).

Note: In Test 2 the compound mix was impregnated with 0.6% by weight of glycerol/water mixture (85:15) before the film was run.

The results clearly show the lowering effect of PVA addition on the oxygen permeability characteristics of the polyethylene film. The oxygen permeability can be controlled by controlling the amount of PVA.

EXAMPLE 2 First polymer (polyolefin/polyvinyl alcohol mixture)

Using a Buss Kokneader mixer (temperature profile 200-200-200°C) , polyolefin/polyvinyl alcohol mixtures were prepared, and monolayer films were run from these mixtures with a Brabender laboratory extruder. The composition of the films, the production conditions and the oxygen permeability characteristics are given in Table 2.

TABLE 2

First polymer (polyolefin/polyvinyl alcohol mixture) As in Example 2, compound mixes were prepared in a Buss Kokneader. From the compound mixes, films with a thickness of 100 μm were made by pressing. The compo¬ sition of the films obtained, the production conditions and the oxygen permeability characteristics are given in Table 3.

Note: In Test 13 the compound mix was, prior to film pressing, impregnated with 5% by weight of glycerol/ water mixture (85:15).

The results of Examples 2 and 3 show that low per¬ meability is also achieved with other polyolefins and plasticisers.

EXAMPLE 4

First polymer (polyolefin/polyvinyl alcohol mixture) and second polymer (olefin copolymer with hydrolysable silane groups)

70% PVA-1 and 30% LDPE-1 were compounded in the same manner as in Example 1, and the compound mixture was co¬ extruded with Visico 1 or Visico 2 and the corresponding modified polymers. Three-layer films with the film thick¬ nesses 25 μm Visico/25 μm barrier compound/25 μm Visico were extruded such that the temperature profile in all extruders was 160-185-200-200°C. Then the two Visico layers were crosslinked by storing the coextruded films for 2 weeks at 23°C and 50% RH. The heat resistance of the coextruded films was tested (5 min. at 125°C silicone oil) before and after crosslinking, and the dimensional stability was observed (+ = dimensionally stable,

- = not dimensionally stable). Furthermore, the oxygen barrier and the adhesion between the different layers was measured (peel test with 50 mm/min. ) after cross- linking. The results are shown in Table 4. TABLE 4

Test Recipe Heat stab . Adhesion Oxygen perm.

Barrier Visico Before After N/cm ml/ 2 x 24 h x bar

15 PVA-l/LDPE-1 Visico-1 0 4.6 16 PVA-l/LDPE-1 Visico-2 0 4.2 17 PVA-l/LDPE-1 Visico-2 + 0 3.8 O . IK DOTDL

18 PVA-l/LDPE-1 Visico-2 0 3.4 0.38 D0TDL

19 PVA-l/LDPE-1 Visico-1 1.7 2.6 + 0- 3S5 benzoic acid

20 PVA-l/LDPE-1 Visico-2 2.8 1.9 + 0.38 benzoic acid

21 PVA-l/LDPE-1 Visico-2 1.4 2.4 + 0.38 ben- zoic acid

22 PVA-l/LDPE-1 Visico-2 2.6 2.3 + 0.3% D0TDL + 0.355 benzoic acid

23 PVA-l/LDPE-1 Visico-1 2.3 2.5 + 0.38 DOTDL + 0.38 benzoic acid 24 PVA-l/LDPE-1 Visico-2 2.1 2.1 + 0.38 DBTDL + 0.38 benzoic acid

25 PVA-l/LDPE-1 Visico-2 3.4 1.6 + 0.38 DOTDL + 0.38 tetraiso- propyl titanate

It appears from Table 4 that a barrier of plastic based on a mixture of PVA and polyolefin can be coextruded with a polyolefin modified with hydrolysable silane, such as ethylene-vinyl trimethoxy silane copolymer, and that an improved adhesion between the layers is obtained under certain conditions. Thus, chemical substances exist (benzoic acid, tetraisopropyl titanate etc. ) which accelerate the condensation reaction between silanol (in e.g. Visico) and hydroxy groups (in e.g. PVA), and in this manner the adhesion between barrier plastic, such as EVOH or PVA/polyolefin mixture, and Visico is increased. The adhesion is improved no matter to which layer the adhesion-improving substance is added. It can also be seen that adding a silanol condensation catalyst (such as DOTDL or DBTDL) to any one of the layers causes crosslinking of the layers modified with hydrolysable silane,^' whereby heat stability is obtained. However, adding the silanol condensation catalyst to a layer modified with hydrolysable silane will cause gel formation upon extrusion. Regardless of whether there is adhesion or no adhesion between the layers, or whether the layers modified with hydrolysable silane are crosslinked or not, a satisfactory oxygen barrier is obtained, and this barrier is mainly dependent on the composition of the barrier layer.

Claims

1. A gastight, heat-stable, multilayered material, c h a r a c t e r i s e d in that it consists of a layer of a first polymer which consists of a mixture of 99-1% by weight polyolefin and 1-99% by weight polyvinyl alcohol and, optionally, a plasticiser con¬ taining alcohol groups, there being provided on at least one side of this layer a layer of a second poly¬ mer derived from an olefin copolymer containing hydro¬ lysable silane groups.

2. A gastight, heat-stable, multilayered material as claimed in claim 1, c h a r a c t e r i s e d in that an adhesion-improving substance catalytically promoting a condensation of silanol with hydroxyl groups is incorporated in at least one of the layers of the material.

3. A gastight, heat-stable, multilayered material as claimed in claim 2, c h a r a c t e r i s e d in that it consists of a layer of a first polymer sur¬ rounded on each side by a layer of a second polymer.

4. A gastight, heat-stable, multilayered material 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 second polymer is derived from a copolymer of (a) ethylene, (b) an ethylenically unsaturated silane compound, and optionally

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

5. A gastight, heat-stable, multilayer material as claimed in claim 4, c h a r a c t e r i s e d in that the second polymer is derived from a copolymer of

(a) ethylene,

(b) an unsaturated silane compound having the formula RSiR' n Y 3,-n in which R is an ethylenically unsaturated hydrocarbyl group, R' is a hydrocarbyl group, Y is a hydrolysable group, and n is 0, 1 or 2, and optionally

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

6. A gastight, heat-stable, multilayered material as claimed in claim 5, c h a r a c t e r i s e d in that the second polymer is derived from

(a) ethylene,

(b) an unsaturated silane compound having the formula

CH₂=CHSi(OA)₃

in which each A which is the same or different, is a hydrocarbyl group having 1-8 carbon atoms, preferably 1-4 carbon atoms, and optionally

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

7. A gastight, heat-stable, multilayered material as claimed in any one of claim 4-6, c h a r a c t e r ^¬ i s e d in that the second polymer contains one or more further monomers (c) selected among vinyl esters of monocarboxylic acids having 1-4 carbon atoms and acrylates or methacrylates of alcohols having 1-4 car¬ bon atoms.

8. A method for the production of a gastight, heat- stable, multilayered material, c h a r a c t e r i s e d in that there is provided, on at least one side of a layer of a first polymer consisting of a mixture of

99-1% by weight polyolefin and 1-99% by weight polyvinyl alcohol and, optionally, a plasticiser containing alcohol groups, a layer of a second polymer derived from an olefin copolymer containing hydrolysable silane groups, and that there is provided, before said first and second polymer layers are brought together, a silanol condensation catalyst in a layer separate from said second polymer.

9. A method as claimed in claim 8, c h a r a c - t e r i s e d in that the catalyst is provided in the layer of the first polymer.

10. A method as claimed in claim 8, c h a r a c ¬ t e r i s e d in that the catalyst is provided in a layer between the layer of the first polymer and the layer or layers of the second polymer.