GB2043533A

GB2043533A - Laminate including a gas barrier layer

Info

Publication number: GB2043533A
Application number: GB8006929A
Authority: GB
Original assignee: Kureha Corp
Current assignee: Kureha Corp
Priority date: 1979-03-03
Filing date: 1980-02-29
Publication date: 1980-10-08
Also published as: FR2450690A1; NL183506C; JPS55117659A; DE3008111C2; FR2450690B1; DE3008111A1; NL8001244A; JPS5933306B2; GB2043533B

Abstract

A laminated film for packaging foods comprises at least two resin layers including a first gas barrier layer containing a copolymer of vinylidene chloride having a reduced viscosity of 0.030 to 0.050, heat- stabilizer and a plasticizer, and a second, strengthening, layer containing an organic additive as plasticizer and a polymer of vinyl chloride, the additives being present in specified proportions. The organic additive is an oligomer or rosin derivative which has a number-average molecular weight of 800 to 10,000 and a melting point lower than that of the vinylidene chloride copolymer.

Description

SPECIFICATION Laminate including a gas barrier layer This invention relates to a laminated resin layer film having a high gas-barriering property and a favorable strength at a low temperature. More in detail, the present invention relates to a laminated resin layer film comprising at least two layers of which one is a gas-barriering layer prepared from a resin compound of 100 parts by weight of a copolymer of vinylidene chloride mainly composed of vinylidene chloride units, which is a type of lower average molecular weight, 0 to 3 parts by weight of a plasticizer and 0.5 to 3 parts by weight of a heat-stabilizer and the other is a strengthening layer prepared from a resin compound of 100 parts by weight of a polymer of vinyl chloride and 5 to 70 parts by weight of an organic additive selected from oligomer(s) and derivative(s) of rosin with a number average molecular weight of 800 to 10,000 determined osmometrically, and having a high gas-barriering property and strength at a low temperature, which properties do not change with the passage of time.

Hitherto, the extruded and shaped film-material prepared by compounding and extruding a resin compound of a copolymer of vinylidene chloride with a suitable amount of stabilizer and various modifying agents such as plasticizers, etc., has been used as an important packing material for wrapping hams, sausages, cooked foods and other foods and materials to be packed for a long period of time within gas-barriering packagings, because of the film-material's excellent luster, transparency, pliability and high barriering property against gases and water vapour.

However, for the package of food products easily denaturizable by a minute amount of permeating oxygen, and of processed meat food products and cooked foods affected by a minute amount of water vapour or by drying, the above-mentioned film-material sometimes is insufficient in intercepting oxygen and water vapour, and accordingly, the appearance of a packaging material having a higher gas-barriering property has been desired eagerly.

In order to give a sufficient gas-barriering property and an impermeability to water vapour to a single layer of film or sheet prepared from a copolymer of vinylidene chloride, it is considered that the content of vinylidene chloride units in the composition of the copolymer is increased or the amount of the plasticizer is reduced form the resin compound.

However, in such cases, the melting point of the resin of the film comes near to the decomposition point thereof, and thereby easily causing thermal decomposition and coloring of the film. Accordingly, in order to safely process the resin using an ordinary extruder by a meltextruding process industrially without the thermal decomposition and the coloration, it has been absolutely necessary to use a considerable amount of modifiers such as plasticizer, stabilizer, etc. The larger the amount of the modifier, the lower the melting point of the resin and the melt viscosity of the resin, and the heat stability of the resin is also improved as well as the strength at a low temperature. However, on the other hand, the gas-barriering property and the impermeability to water vapour are deteriorated by the increase of the amount of the modifier.

Accordingly, it is usual, in practice, to control the amount of addition of the low molecular weight plasticizer in the range of 4 to 1 5 parts by weight to 100 parts by weight of the copolymer according to the usage and the object of the processed film.

In short, according to the customary recognition, in the case of copolymer of vinylidene chloride of high solution viscosity, the smaller amount of incorporated plasticizer makes the film prepared from the copolymer more excellent in gas-barriering property, however, it also reduces the processability of the resin compound and the strength of the film at a low temperature.

Therefore, the lower limit of the amount of addition of the plasticizer is 4 parts by weight to 100 parts by weight of the copolymer for a smooth industrial production, 3 parts by weight of the plasticizer being the lowest limit for production of the film, however, not accompanied with smooth production, and the less amount making the production of the film satisfying simultaneously the processability of the resin compound itself, the strength at a low temperature and the gas-barriering property of the processed film impossible.

Accordingly, the main object of the present invention is to produce a film of copolymer of vinylidene chloride, which shows a high gas-barriering property overcoming the abovementioned limit and in the same time fulfilling the request for strength at a low temperature, the resin compound for the film being excellent in processability on extruding.

As a method of producing a film of copolymer of vinylidene chloride consisting mainly of vinylidene chloride units satisfying simultaneously the high gas-barriering property and the strength at a low temperature, a method of combining, for instance, laminating a highly gasbarriering film layer (however, with a small strength at a low temperature) with a film layer having a large strength at a low temperature (however, low gas-barriering) is considered.For instance, a laminate resin layer film is considered, which comprises a film layer of high gasbarriering property prepared from a resin compound made of a copolymer of vinylidene chloride added with an extremely small amount of a plasticizer (hereinafter referred to as the gas barriering layer) and a film layer of a high strength at a low temperature prepared from a resin compound made of polyvinyl chloride added with a large amount of a plasticizer (hereinafter referred to as the strengthening layer) laminated together.

However, there has been the following problem to be solved: That is, the thus obtained laminated resin layer film is a film showing temporary the high gasbarriering property and the high strength at a low temperature just after its production, however, because of the migration of a large amount of the plasticizer from the strengthening layer into the gas-barriering layer with the lapse of time after the co-extrusion of the two layers or the heattreatment of the laminate film, the gas-barriering property becomes deteriorated with the deterioration of the strength at a low temperature, and as a result, it is impossible to obtain a film or sheet showing the high gas-barriering property and the strength at a low temperature for a long time period, which were expected at first.

In order to solve the above-mentioned problem, the inventors of the present invention have made efforts in studying the process for preparation of the above-mentioned film having the desired properties, and as a result has found that in the case where at least one organic additive selected from oligomers, rosin derivatives and hydrogenated rosin derivatives and having a number average molecular weight of 800 to 10,000, in place of the conventionally employed plasticizer, is used in the strengthening layer, the diffusion or migration of such an organic additive from the strengthening layer into the barriering layer is extremely small, even if a large amount of such an organic additive is used, with the lapse of time after the co-extrusion or at the time of heat-treatment of the laminated film.Moreover, the inventors have found that it is most suitable for obtaining a high barriering property to reduce the amount of plasticizer in the barriering layer, and that by using a copolymer of vinylidene chloride having a lower reduced viscosity, that is, using a copolymer of vinylidene chloride having a smaller molecular weight, the extruding process can be operated without any trouble to prepare the film layer of the copolymer in spite of the smaller amount of plasticizer than usual.

According to the present invention, there is provided a laminated resin layer film comprising at least two layers of which one layer is a gas-barriering layer containing a copolymer of vinylidene chloride having a reduced viscosity of 0.030 to 0.050, a plasticizer and a heatstabilizer in a ratio of 100 parts by weight of said copolymer of vinylidene chloride to O to 3 parts by weight of said plasticizer and 0.5 to 3 parts by weight of said heat-stabilizer provided that the total amount of said plasticizer and said heat-stabilizer is lower than 3 parts by weight to 100 parts by weight of said copolymer of vinylidene chloride, and the other layer is a strengthening layer containing a polymer of vinyl chloride and at least one organic additive in a ratio of 100 parts by weight of said polymer of vinyl chloride to 5 to 70 parts by weight of said organic additive, said organic additive having a number average molecular weight of 800 to 10,000 and a lower melting point than that of said copolymer of vinylidene chloride and being selected from the group consisting of polyester oligomers, epoxy resin oligomers, oligocopolymer of styrene, oligomers of butadiene, oligomers of butene, oligomers of ethylene, oligoamides, rosin esters and hydrogenated rosin esters.

As copolymers of vinylidene chloride for use in the present invention, copolymers comprising mainly vinylidene chloride units and other monomeric compound copolymerizable with vinylidene chloride are mentioned, and particularly, those having 65 to 95% by weight of vinylidene chloride units are preferable. Smaller content of vinylidene chloride units leads to a lesser barriering property, however, a larger content than 95% by weight leads to the difficulty of processing.As the monomeric compound copolymerizable with vinylidene chloride, for instance, one or more comonomer selected from unsaturated monomers such as vinyl chloride, vinyl acetate, acrylonitrile, styrene, acrylic acid, alkyl(C, to C,8) acrylates, methacrylic acid, alkyl(C, to Ca8) methacrylates, itaconic acid, alkyl(C, to C,8) itaconates, maleic acid, maleic anhydride, alkyl(C, to C,8) maleates, etc. are mentioned. From the view points of the polymerizability, the barriering property of the film, and the processability of the resin compound of copolymer, particularly, the copolymers with vinyl chloride are preferable.

Moreover, concerning the reduced viscosity of the copolymer of vinylidene chloride, since it is necessary to extrude the resin compound of the copolymer and to process to be a film without any problem by using a plasticizer as small as possible in amount or by completely without using any amount of the plastizicer, in this case, the copolymer having a smaller reduced viscosity, that is, those having the reduced viscosity of 0.30 to 0.050, preferably of 0.035 to 0.045 are used in the present invention. The copolymer having the reduced viscosity of less than 0.030 gives a film having a reduced anti-pressure strength not to be processed for the barriering layer. The copolymer having the reduced viscosity more than 0.050 is hardly extrudable after the addition of a small amount of the plasticizer because of its high melt viscosity. The film processed by adding a larger amount of the plasticizer shows a reduced barriering property.

The reduced viscosity, herein mentioned, is a parameter representing the magnitude of the molecular weight of a copolymer of vinylidene chloride, determined by the following method on a solution of a copolymer of vinylidene chloride in cyclohexanone of a concentration of 4 g/liter at a temperature of 30"C: METHOD FOR DETERMINATION OF REDUCED VISCOSITY: (i) Eighty milligrams of a purified and dried specimen of a copolymer of vinylidene chloride is weighed into a conical flask provided with a ground glass stopper, and an exact amount of 20 ml of warmer cyclohexanone to 30"C is introduced into the flask, which is placed in a water bath of 70"C. Under agitation, dissolution of the specimen takes 60 minutes.

(ii) Five milliliters of the solution prepared according to (i) and controlled to be a temperature of 30"C is taken into an Ubbelohde's viscometer, and after 5 minute's immersion of the viscometer in a thermostat at 30"C, the time period for the meniscus of the solution passing the two base lines is determined by a stop-watch in the usual procedures. The determination is repeated 3 times, and the mean value of the time period (sec) is taken as the time period necessary for the meniscus passes the lines.Reduced viscosity is calculated by the following formula: 1 t2 Reduced viscosity isp/c = -(- - 1) C t, wherein C: the concentration (4 g/liter), t,: time period for passing, of pure cyclohexanone at 30"C, and t2: time period for passing, of test solution at 30"C.

Further, a copolymer of vinylidene chloride having a reduced viscosity in the above-mentioned range is obtainable by copolymerization in an usual apparatus for polymerization according to the procedure of an ordinary polymerization under the optionally selected conditions such as monomer ratio at charging, kind and amount of the polymerization initiator, temperature of polymerization, extent of polymerization and adjuvants for polymerization. For instance, the copolymer having a lower value of reduced viscosity than the value of ordinary copolymers, that is, the copolymer having a lower molecular weight than ordinary copolymers is obtained by setting the temperature of polymerization in a higher side than in polymerization for obtaining ordinary copolymers.

As the polymer of vinyl chloride for the strengthening layer, for instance, homopolymer of vinyl chloride, copolymers of vinyl chloride with one or more comonomers selected from vinyl acetate, acrylonitrile, ethylene, esters of acrylic acid, esters of methacrylic acid, alkyl vinyl ethers, etc. and chlorinated polyvinyl chloride are used.

Furthermore, the magnitude of the molecular weight of polymers of vinyl chloride for use in preparing the strengthening layer governs the kind and the amount of the oligomer and the derivative of rosin used for preparation of the strengthening layer, however, the molecular weight of the polymer is not particularly specified, being freely chosen. Preferably, from the view point of the easiness in its extrusion, in case of homopolymer of vinyl chloride, those having an average degree of polymerization of 700 to 1,300 are used. The average degree of polymerization, herein used, is obtained by calculating using a solution viscosity in nitrobenzene determined by using an Ubbelohde's viscometer according to the method indicated in Japanese Industrial Standard (JIS)-K-6721/1966.

In the next place, the plasticizer for use in the barriering layer is not particularly specified but plasticizers usually employed in forming a film of copolymer of vinylidene chloride may be used.

For instance, the plasticizers for use in the barriering layer are exemplified by those with low molecular weight such as esters of dibasic aliphatic acids, phthalic esters, hydroxy polycarboxylic esters and glycerol esters and those with high molecular weight of a number average molecular weight lower than 10,000 and having a lower melting point than that of the copolymer of vinylidene chloride such as polyester oligomers, epoxy resin oligomers, oligo-copolymers of styrene, oligoamides, rosin esters and hydrogenated rosin esters.

However, the amount of the plasticizer admixed with the copolymer for the barriering layer is O to 3 parts by weight to 100 parts by weight of the copolymer. Over 3 parts by weight, it is difficult to obtain the barriering layer having an extremely high barriering property just after production, or it is difficult to obtain the layer having an extremely high barriering property without the successive treatments such as thermal-treatment. Accordingly, for obtaining the layer with an extremely high barriering property, it is preferable to reduce the plasticizability of the resin compound by reducing the amount of plasticizer to zero.

The organic additive for use in the strengthening layer, selected from oligomers, rosin esters and hydrogenated rosin esters has a number average molecular weight determined by the osmometry of 800 to 10,000, preferably 1 ,000 to 3,000. In cases of the number average molecular weight of less than 800, there is an unfavorable tendency of migration of the organic additive from the strengthening layer into the barriering layer.

In addition, the melting point of the organic additive is lower than the melting point of the copolymer of vinylidene chloride. In cases where the melting point of the organic additive is higher than that of the copolymer of vinylidene chloride, the additive does not melt at the time of extrusion of the resin compound and accordingly, it remains dispersed in the resin layer and appears as fish-eye in the processed film and sheet to deteriorate the appearance of the film and sheet. Such an organic additive having a high melting point exudes onto the surface of the film or sheet after processing, and causes clouds to deteriorate the appearance or to be the cause of peeling off of the laminate, and accordingly, the organic additive is preferably liquid at room temperature.

Organic additives fulfilling the above-mentioned conditions are rosin esters, hydrogenated rosin esters, polyester oligomers of co-polycondensates of dibasic acid such as adipic acid, sebacin acid, azelaic acid, etc. with polyols such as ethyleneglycol, propyleneglycol, butyleneglycol, etc., the terminal group of the polyester oligomers being, if necessary, a monobasic acidic group or an alcoholic group, epoxy resin oligomers of copolymers of epichlorhydrine and bisphenol, oligocopolymers of styrene with isobutylene, butadiene, isoprene, maleic acid, etc., oligomers of 1,2-butadiene, of butene, of ethylene, of 1,4-butadiene, oligoamide, etc.

Among these substances, those preferably used are polyester oligomers having a number average molecular weight of 1,000 to 3,000, preferably, 1,500 to 2,000, which is formed from aliphatic dicarboxylic acid having six to ten carbon atoms and polyol having two to six carbon atoms. In the present invention, 5 to 70 parts by weight, preferably 10 to 60 parts by weight of at least one organic additive selected from these oligomers and derivatives of rosin are added to 100 parts by weight of polymer of vinyl chloride to form resin compound from which the strengthening layer is prepared. When the amount of the organic additive is less than 5 parts by weight to 100 parts by weight of polymer of vinyl chloride, there is a problem in the process of extruding.On the other hand, in cases where the amount is more than 70 parts by weight, the organic additive's solubility in polymer of vinyl chloride is saturated to make the exudation of the additive conspicuous onto the surface of the film or sheet, as a result, a problem of a stickiness arises. This stickiness causes the sticking or blocking of films to each other when brought into contact to reduce the packaging aptitude of the film, and reduces the sliding property of the film against the metallic surface, as a result of reducing the packaging machine-aptitude.

In addition, in the present invention, a heat-stabilizer is applied in preparing the barriering layer, and in accordance to necessity, a heat-stabilizer is also applied in preparing the strengthening layer, and moreover, inorganic powdery materials, organic lubricants and colouring materials may be admixed, if necessary into the resin compounds for both layers.

As the heat-stabilizer, commercial heat stabilizers are utilizable, and particularly, epoxystabilizers are effective.

Epoxy-stabilizer has epoxy-groups comprising a three membered ring of carbon-oxygen-carbon in their molecule, and acts as a catcher or hydrogen chloride which generates on the thermal degradation of copolymer of vinylidene chloride. Accordingly, the stabilizer is added to copolymer of vinylidene chloride for the purpose of preventing the deterioration of copolymer of vinylidene chloride. In the present invention, the stabilizer is differentiated from the plasticizer.

Concrete examples of the epoxy-stabilizer for use in the present invention are epoxydized vegetable oils such as epoxydized-soybean oil, -safflower oil, -sunflower oil, -linseed oil, and -cotton seed oil, monoesters of epoxydized fatty acid represented by octyl epoxydized stearate, diesters of epoxydized fatty acid obtained by epoxydizing a glycol ester of unsaturated fatty acid, and cycloaliphatic epoxides represented by esters of epoxy-hexahydrophthalic acid. Among the above-mentioned epoxy-stabilizer, the epoxydized vegetable oils are preferably selected from the view point of low volatility, low migratoriness, oil-resistivity and from sanitary consideration.An amount of 0.5 to 3 parts by weight of the epoxy-stabilizer, however, such that the total amount of the plasticizer and the epoxy-stabilizer does not exceed 3 parts by weight to 100 parts by weight of copolymer of vinylidene chloride, is preferably added to the resin compound for preparing the barriering layer.

The above-mentioned epoxy-stabilizer may be added to either of both barriering layer and strengthening layer, however, in the case where the barriering layer is prepared without utilizing any plasticizer, 0.5 to 3 parts by weight of the epoxydized vegetable oil is preferably used in the barriering layer.

In addition to the above-mentioned additives, for the purpose of preventing the mutual sticking of films and of improving the slipping property against the metallic surface to improve the aptitude for the mechanical treatment in packaging, inorganic powdery substances such as diatomaceous earth, talc, silica, calcium carbonate and organic lubricants such as fatty acid am ides, higher alcohols, metallic soaps, sorbitan esters of fatty acids, etc. may be incorporated into the resin compound for preparing both of the layers in total amount of 0.01 to 3 parts by weight, preferably, 0.1 to 0.5 parts by weight to 100 parts by weight of the copolymer of vinylidene chloride or the polymer of vinyl chloride.

Furthermore, as the colouring agent, inorganic or organic pigment may be incorporated into the above-mentioned resin compounds in an amount of 0.05 to 1.0 part by weight to 100 parts by weight of the copolymer of vinylidene chloride or the polymer of vinyl chloride.

As has been stated, the laminated resin layer film according to the present invention comprises at least two layers adjacent to each other with or without an adhesive layer therebetween, of which one is the barriering layer, which may contain an extremely small amount of plasticizer, and the other is the strengthening layer, which contains oligomer(s) or derivative(s) of rosin, and such a laminated resin layer film is possibly prepared by either of publicly known methods such as coextruding and laminating, or by the combined method of them, and if necessary, an adhesive may be used between the layers. In cases where such an adhesive is used, the thickness of the layer of the adhesive is not specified, however, in order to have an effective adhesion, it should be thinner as possible, preferably thinner than 5 microns.

As the adhesive, for instance, known adhesives such as copolymer of ethylene and vinyl acetate, thermo-setting polyurethane, block-copolymer of styrene and isoprene, etc. are used.

According to the invention, provided is a laminated resin layer film comprising multiple layers more than two i.e. the gas barriering layer and the strengthening layer, in which other resin layer(s) comprising, for instance, a resin of a low softening point selected from, for instance, polyethylene of low density, copolymers of ethylene and vinyl acetate, copolymers of ethylene and an unsaturated organic acid and their ionomers is formed on one side or both sides of each of the above-mentioned two layers.The multiple-layered film thus obtained may be an ideal film for vacuum packaging under heating in the following meaning because of the remarkable elastic recovery shown by the strengthening layer comprising soft polymer of vinyl chloride resin at a temperature of higher than 60"C, preferably higher than 80"C: That is, the packaging of articles or food products with such a laminated resin layer film is carried out as follows:: At first, the laminated layer film is processed by shaping using a metal mold to have a depression almost resembling the contour of the article or the food product to be packed, and after placing the article or the food product to be packed into the depression, another base film separately prepared is placed on the laminated resin layer film having the article to be packed in its depression with the edges of both films coincided, or after placing the article on the base film prepared separately, the laminated resin layer film is placed on the separatedly prepared base film so as to match the depression against the article to be packed, as in the process of blister packaging.

Then, the whole system is placed in a vessel and the gaseous space of the vessel is evacuated to a reduced pressure to make the gaseous space in the depression also to a reduced pressure.

In the next place, air is introduced into the vessel to make the space between the metal mold and the laminated resin layer film having the articles placed in the evacuated depression to be the ordinary pressure. Then the laminated resin layer film sticks tightly to the article without producing wrinkles by the change of atmospheric pressure and the elastic recovery of the shapeprocessed laminated resin layer film together with the base film which is maintained falt. In the above-mentioned case, since the deformation stress under heating is reduced remarkably by the influence of the strengthening layer of low softening point, the laminated layer film is able to act substantially as a rubber-elastic body.

Furthermore, at an ordinary temperature, since the gas-barriering layer of copolymer of vinylidene chloride is hardly deformed plastically and is so strong that it can withstand the residual stress of the strengthening layer of polyvinyl chloride, a large compressive force is not applied on the article packed.

Accordingly, even when the article to be packed is soft material such as hams and sausages, it is not squashed or squeezed to expel its juice because only the pressure necessary for tight packing is applied on the article. There is no fear of coming off the adhered films (the base film and the laminated layer film) at the sealed parts due to the residual strain of the laminated layer film.

It is also possible to carry out the vacuum packaging without shape-processing by using the metal mold, only by wrapping the article to be packed with the laminate resin layer film of the present invention together with or without the base film. Accordingly, the laminate resin layer film of the present invention is really epochmaking as a film for use in vacuum packaging.

Furthermore, the laminated resin layer film of the present invention is possibly utilized in various purposes other than the vacuum packaging. For instance, in the case of the conventional laminated resin layer film comprising a layer of polyvinyl chloride plasticized by a low molecular weight-plasticizer and a layer of copolymer of vinylidene chloride, there have been various problems originated from the migration of the plasticizer from the layer of polyvinyl chloride into the layer of copolymer of vinylidene chloride such as not only the reduction of gas-barriering property, but also the sweating of plasticizer and the reduction of adhesiveness to other material caused by the sweating, etc.

On the other hand, by the present invention, the gas-barriering property is remarkably improved with the improvement of the pliability and of the adhesiveness to other material, and the sweating of plasticizer is largely suppressed. Thus, the laminated resin layer film of the present invention is possibly used in Bag in Box, pouch-packaging, blister-pack, stretch-packing, etc., thus proving to be a remarkably useful material.

Thus, the present invention has brought the unfavorable combination of polyvinylidene chloride layer with polyvinyl chloride layer, hitherto not having been compatible, into a favorable combination and has brought the combination into an industrially very important and very useful execution.

The followings are the more concrete explanation of the present invention while referring to the non-limitative examples.

EXAMPLE 1: A copolymer of vinylidene chloride and vinyl chloride of a reduced viscosity of 0.039 was obtained by copolymerization at the charged monomer weight ratio of 71.0/29.0 in an autoclave. A compound prepared by mixing 1.5 parts by weight of epoxyfied linseed oil as an epoxy stabilizer with 100 parts by weight of the copolymer was melt-extruded by an extruder provided with a tube-like nozzle, and according to the conventional method of inflation for preparing films of copolymer of vinylidene chloride the extruded material was formulated to be a single film A of 1 2 microns in thickness.

Separately, another copolymer of vinylidene chloride and vinyl chloride of a reduced viscosity of 0.041 was obtained by copolymerization at the charged monomer weight ratio of 80.0/20.0 in an autoclave. To 100 parts by weight of the copolymer, 1.0 parts by weight of expoxyfied soy-bean oil and 1.7 parts by weight of dioctyl adipate (abbreviated as DOA, hereinafter) as a low-molecular weight plasticizer were admixed, and the thus prepared compound was processed as in the preparation of the single film A to obtain a single film B of 20 microns in thickness.

As a comparative example, a copolymer of vinylidene chloride and vinyl chloride of a reduced viscosity of 0.060 was obtained by copolymerization at the charged monomer weight ratio of 71.0/29.0, the copolymer being a resin which is used for preparing a single film after compounding with 1.5 parts by weight of a conventional stabilizer and 7.0 to 10.0 parts by weight of DOA. To 100 parts by weight of this copolymer, 1.5 parts by weight of epoxyfied soybean oil and 1.7 parts by weight of DOA were admixed, and the mixture was tried to be an extruded film by an extruder provided with a tube-like nozzle. However, owing to the excess load on the extruder and to the continuous extrusion of coloured products of degradation, a satisfactory extrusion-processing was impossible without obtaining any film suitable for the purpose.

In another process, to 100 parts by weight of a commercial polyvinyl chloride of a polymerization degree of 1,000, 1.5 parts by weight of epoxyfied soy-bean oil and 55 parts by weight of an esterified oligo-condensate of adipic acid and butyleneglycol of a number average molecular weight of 1,700 with acetylated chain ends were admixed, and the compound was processed by an extruder provided with a tube-like nozzle and the inflation process to be a single film C of 50 microns in thickness.

In still another process, to 100 parts by weight of the above-mentioned polyvinyl chloride, 1.5 parts by weight of epoxyfied soy-bean oil, 9.0 parts by weight of DOA were admixed, and the compound was formulated to be a single film D of 25 microns in thickness according to the method used in preparing the single film C.

After applying a thermosetting polyurethane bonding agent at a thickness of 3 microns on the one or the two sides of the single film A or the single film B, the following laminate combinations of the films were prepared by laminating under pressure: (1) C/A (2) C/A/C (3) C/B (4) C/B/C (5) D/A (6) D/A/D (7) D/B (8) D/B/D The laminated sheets were heat-treated at a temperature of 80"C for 5 hours in a state of wound-up around a bovin just after the lamination. The gas-permeability of the thus prepared laminates was determined at a temperature of 30"C at a relative humidity of 100%. The results of determination are shown in Table 1.

Table 1 DOA content of each layer (% by weight) Laminate Permeability to construction oxygen* Outer layer Center layer Outer layer Experiment Outer Center Outer just after after heat- just after after heat- just after after heat- just after after heat No. layer layer layer lamination treatment lamination treatment lamination treatment lamination treatment (1) C A - 17.0 16.7 0 0 0 0 0 0 (2) C A C 16.0 17.0 0 0 0 0 0 0 (3) C B - 18.0 13.0 0 0.2 1.63 1.1 0 0.2 (4) C B C 18.5 12.2 0 0.1 1.63 1.0 0 0.1 (5) D A - 16.5 165.0 8.3 7.1 0 2.3 - 7.3 (6) D A D 15.0 170.0 8.3 7.1 0 2.4 8.3 7.1 (7) D B - 18.5 52.0 8.3 7.7 1.63 2.1 - 7.8 (8) D B D 18.0 60.0 8.3 7.9 1.63 2.3 8.3 7.9 Notes: Experiments (1) to (4) belong to the present invention.Experiments (5) to (8) are of Comparative Examples. *cc/m2.24 hrs. 1 atm. determined at 30 C and R.H. of 100% As seen in Table 1, among the laminates obtained according to the process of the present invention (that is, the laminates in Experiments (1) to (4)), those in Experiments (1) and (2) show that the permeability to gaseous oxygen is the same between just after lamination and after the heat-treatment, and those in Experiments (3) and (4) show that although the permeability to gaseous oxygen reduces a little in tendency after heat-treatment as compared to that of just after lamination, at any rate; their barriering property to gaseous oxygen is maintained almost unchanged.

Whereas the laminates of the comparative examples, that is, all the laminates in Experiments (5) to (8) show a larger permeability to gaseous oxygen after the heat-treatment as compared to those just after lamination, they showing the conspicuous deterioration of gas-barriering property after the heat-treating.

In order to elucidate the reason of the deterioriation, the content of DOA in each single layer of the laminates was determined just after lamination and after the heat-treatment. The determination was carried out by extracting DOA from each single layer with n-hexane and quantitatively analysing the content of DOA with gaschromatography. The results of determination are shown in Table 1.

It has been known from the results that in the laminates according to the process of the present invention, DOA remained in the layer of copolymer of vinylidene chloride, which comprises the gas-barriering layer, in an amount scarcely detectable by the gaschromatography, and on the contrary, the DOA was detected in the outer layer comprising polyvinyl chloride and the oligomer. According to the above-mentioned findings, it has been elucidate that the migration of DOA from within the layer of copolymer of vinylidene chloride to within the outer layer of polyvinyl chloride by the heat-treatment caused the improvement of gas-barriering property of the layer of copolymer of vinylidene chloride.In this case, the migration of the oligomer from within the outer layer for the strength of the laminate to the layer of copolymer of vinylidene chloride was inspected by extracting the oligomer with n-hexane and developing it with paper chromatography and found to be negligibly small.

In contrast to the above-mentioned tendency, in the cases of the laminates in Experiments (5) to (8) in the comparative examples, the content of DOA in the center layer of copolymer of vinylidene chloride showed an increase as compared to that before the heat-treatment, and such a migration of DOA from within the layer of polyvinyl chloride to the gas-barriering layer or copolymer of vinylidene chloride caused the deterioration of gas-barriering property of the laminate after the heat-treatment.

In addition, the impact strength of the laminates in Experiments (1) to (4) of Example was over 5 keg cm at a temperature of 23"C assuring the sufficient practical strength (by TBS-type Impact-tester).

EXAMPLE 2: By using a multi-layer extruder, and a copolymer of ethylene and vinyl acetate containing 25% by weight of vinyl acetate and of a melting index of 3 as the interlayer bonding agent, instead of the extruder and the interlayer bonding agent in Example 1, the same laminates as those obtained in Example 1 were respectively produced by co-extrusion and inflation. These laminates were heat-treated under the same conditions as in Example 1, and their permeabilities to gaseous oxygen were determined just after extrusion and after the heat-treatment. The thus obtained results were quite the same as those shown in Table 1. The fact confirms that the laminates of polymer layers having a high gas-barriering property of the present invention could be produced also by the coextrusion.

EXAMPLE 3: By using a multi-layer extruder, a 5-layered laminate tube was extruded and inflated to be a 5layered laminate film E comprising an outer layer, a bonding agent layer, the central layer, a bonding layer and an outer layer with the respective thicknesses of 35, 3, 20, 3 and 35 microns, one of the outer layers comprising a mixture of 100 parts by weight of a copolymer of vinyl chloride and vinyl acetate containing 5% by weight of vinyl acetate, 50 parts by weight of a polyester oligo-condensate of ethylene glycol and sebacic acid of a number average molecular weight of 2,000 having butoxylated chain ends and 2.0 parts by weight of epoxyfied soy-bean oil, the other of the outer layers comprising a copolymer of ethylene and vinyl acetate containing 15% by weight of vinyl acetate, the center layer comprising a mixture of 100 parts of a copolymer of vinylidene chloride and vinyl chloride of a reduced viscosity of 0.040 obtained by copolymerizing a monomer weight ratio of 78/22 and 1.5 parts by weight of epoxyfied soybean oil and a bonding agent layer between the outer layer and the center layer, comprising a copolymer of ethylene and vinyl acetate containing 25% by weight of vinyl acetate and having a melt index of 3.

The thus produced laminate E showed a permeability to gaseous oxygen at 30"C just after the production of 12 cc/m2 during 24 hours under 1 atm. The permeability to gaseous oxygen of the same laminate E after keeping at room temperature for 43 days was 14 cc/m2 during 24 hours atm, a little larger than the former value. The increment was, however, very small, telling the maintenance of the high gas-barriering property.

For the purpose of comparison, another laminate F was produced by using dibutyl sebacate, a low-molecular-weight plasticizer, instead of the oligo-condensate in the laminate E with other components unchanged from the laminate E. The permeability of gaseous oxygen of the laminate F was 1 6 cc/m2 during 24 hours under 1 atm. just after production and as large as 208 cc/m2 during 24 hours under 1 atm. after keeping at room temperature for 43 days, showing the remarkable deterioration of gas-barriering property. In this case, the content of dibutyl sebacate of the center layer of the copolymer of vinylidene chloride was 6.2% by weight according to the gaschromatographic determination.

As is shown by Examples, even the same layer of copolymer of vinylidene chloride is used as the outer layer, the gas-permeability of the laminate film changes with the lapse of time according to the kinds and the property of its plasticizer in its broader definition. The fact is caused by the easy migration of the low-molecular-weight plasticizer admixed with the outer layer of resin to the center layer of copolymer of vinylidene chloride to deteriorate the gasbarriering property of the center layer of copolymer of vinylidene chloride.

On the contrary, in the case where a higher-molecular weight plasticizer, for instance, an oligo-condensate of molecular weight of higher than 800, is used, the migration of such a plasticizer is extremely small and accordingly the gas-barriering property is kept for long time period.

Claims

1. A laminate comprising at least two resin layers including a first, gas-barrier, layer containing a copolymer of vinylidene chloride having a reduced viscosity of from 0.030 to 0.050, a plasticizer and a heat-stabilizer in a weight ratio of said copolymer:plasticizer:heat- stabilizer of 100:0 to 3:0.5 to 3 provided that the total amount of said plasticizer and said heat-stabilizer is not more than 3 parts by weight per 100 parts by weight of said copolymer, and a second, strengthening, layer containing a polymer of vinyl chloride and, as plasticizer, at least one organic additive in a weight ratio of said polymer:said at least one organic additive of 1 00: 5 to 70, said organic additive having a number of average molecular weight of from 800 to 10,000 and a melting point lower than that of said copolymer of vinylidene chloride and being selected from polyester oligomers, epoxy resin oligomers, oligo-copolymers of styrene, oligomers of butadiene, oligomers of butene, oligomers of ethylene, oligoamides, rosin esters and hydrogenated rosin esters.

2. A laminate according to claim 1, wherein said copolymer of vinylidene chloride is prepared from 60 to 95% by weight of vinylidene chloride and 40 to 5% by weight of at least one monomer copolymerizable with vinylidene chloride.

3. A laminate according to claim 2, wherein said monomer is selected from vinyl chloride, vinyl acetate, acrylonitrile, acrylic acid, methacrylic acid, maleic acid, maleic anhydride, styrene, itaconic acid, alkyl (C, to C,8) esters of acrylic acid, alkyl (C, to C,8) esters of methacrylic acid, alkyl (C, to C18) esters of maleic acid and alkyl (C1 to C18) esters of itaconic acid.

4. A laminate according to any one of the preceding claims, wherein said copolymer of vinylidene chloride has a reduced viscosity of from 0.035 to 0.045.

5. A laminate according to any one of the preceding claims, wherein said polymer of vinyl chloride is a homopolymer of vinyl chloride, a copolymer of vinyl chloride or a chlorinated polymer of vinyl chloride.

6. A laminate according to any one of the preceding claims, wherein said organic additive is a polyester oligomer with a number average molecular weight of from 1 ,000 to 3,000 formed from an aliphatic dicarboxylic acid having an alkyl group of six to ten carbon atoms and a polyol having two to six carbon atoms.

7. A laminate according to any one of the preceding claims, wherein the content of said organic additive in said second, strengthening, layer is from 10 to 60 parts by weight per 100 parts by weight of said polymer of vinyl chloride.

8. A laminate according to any one of the preceding claims, wherein said heat-stabilizer is a epoxydized vegetable oil.

9. A laminate according to any one of the preceding claims, wherein said first, gas-barrier, layer contains no plasticizer.

10. A laminate according to any one of the preceding claims, wherein a thin layer of an adhesive is interposed between said first, gas-barrier, layer and said second, strengthening, layer.

11. A laminate according to claim 10, wherein said adhesive is selected from copolymers of ethylene and vinyl acetate, thermo-setting polyurethane and block copolymers of styrene and isoprene.

1 2. A laminate according to claim 10 or 11, wherein the thickness of said thin layer of adhesive is less than 5 microns.

1 3. A laminate according to any one of the preceding claims, further comprising at least one other layer of a resin selected from low density polyethylene, copolymers of ethylene and vinyl acetate, copolymers of ethylene and an unsaturated organic acid and ionomers thereof.

14. A laminate substantially as hereinbefore described with reference to any one of Experiments (1) to (4) of Example 1, Example 2 and any one of Experiment (1) to (4) of Example 1, and laminate E of Example 3.

15. A material packaged using a laminate as claimed in any one of the preceding claims.

16. A material according to claim 1 5 which is a foodstuff.

17. A material according to claim 1 5 or 1 6 which has been vacuum packaged.

18. A material according to claim 1 5 or 1 6 contained in a blister package.