A resin composition and food packaging materials formed therefrom

Abstract: An article suitable for packaging food which comprises at least one layer of 3 to 90 microns thickness which comprises: i) a composition comprising a polyvinyl alcohol resin having a degree of saponification of 60 to 95 mole% (A); and ii) an alkylene glycol or hydroxy acid monomer or a polymer thereof, having a carbon dioxide/oxygen solubility ratio (SCO₂/SO₂) of 30 or greater (B₁); B₁ being present in an amount of from 3 weight parts of B₁ per 100 weight parts of A to the saturated solubility of B₁ in A; and a resin composition which comprises: (i) a polyvinyl alcohol resin having a degree of saponification of 60 to 95 mole% (A); (ii) an alkylene glycol monomer or polymer thereof having a carbon dioxide/oxygen solubility ratio (SCO₂/SO₂) of 30 or greater (B₂); and (iii) a saponified ethylene/vinylacetate copolymer resin (C); the blending ratio of B₂ being from 3 weight parts of B₂ to 100 weight parts of A to the saturated solubility of B₂ in A and the blending ratio of C being from 20 to 50 weight parts of C to 100 weight parts of an aggregate of A and C. the blending ratio of B₂ being from 3 weight parts of B₂ to 100 weight parts of A to the saturated solubility of B₂ in A and the blending ratio of C being from 20 to 50 weight parts of C to 100 weight parts of an aggregate of A and C.

Description: The present invention relates to a resin composition and articles obtained therefrom such as films, and to the use of the articles in packaging food. In the food packaging industry there is a desire for optimal performance of each packaging material for a given kind of food. Taking cheese, which is a fermented foodstuff, for example, a wax-coated packaging material has been conventionally used for packaging natural cheese, whereas plastic materials have also been adopted to improve the appearance, to increase the eatable portion and to reduce manufacturing cost. Live bacteria are added to natural cheese to permit ripening. The bacteria generate carbon dioxide not only during the ripening step, but throughout the later stages of physical distribution. Since such cheese packaged with an ordinary kind of plastic bag swells up with generated carbon dioxide, the introduction of a packaging material which will readily transmit carbon dioxide has been awaited. On the other hand, since oxygen has adverse effects such as accelerating the growth of mould on the cheese and oxidizing the fat in the cheese, the trade desires a packaging material which suppresses transmission of oxygen. For foodstuffs other than cheese, e.g. coffee beans, packaging materials that readily permit permeation of carbon dioxide but considerably restrain permeation of oxygen have been desired. Although it is desirable that plastic packaging materials intended for use with, for example, cheese products and coffee beans have a higher gas transmission rate (hereinafter abbreviated to "GTR") to carbon dioxide and a lower GTR to oxygen, plastic packaging materials that have a high GTR to carbon dioxide almost without exception have a high GTR to oxygen. Packaging materials that have a low oxygen GTR and a high carbon dioxide/oxygen GTR ratio are desired. There is described in Published Japanese Patent Sho 44-2576 a process for producing a polyvinyl alcohol (PVA) film which is insoluble in cold water and soluble in warm water at 40°C or higher by extruding a specific PVA with a plasticizer, as a separation membrane. While glycerin is generally used as the plasticizer, polyethylene glycol can be also used. The film thus obtained, however, is used as a unit package material for a detergent and agricultural chemicals and for laundry bags. It is not used as a food packaging material for which a selective gas permeability is particularly required. There is described in Japanese Published Patent Sho 54-15029 (US-A-4073733) a process for producing a web possessing gas permeability by coagulating polyethylene glycol-containing PVA in a liquid phase. The web thereby obtained is used for separation of urea and a vitamin, and is not used as a food packaging material, for which selective gas permeability is required. It is desired to provide packaging materials having a low oxygen GTR and, additionally, a high carbon dioxide/oxygen GTR ratio. The present invention provides a composition which comprises: i) a PVA resin having a degree of saponification of 60 to 95 mole% (A); ii) an alkylene glycol monomer or polymer thereof having a carbon dioxide/oxygen solubility ratio (SCO 2 /SO 2 ) of 30 or greater (B 2 ); and iii) a saponified ethylene/vinyl acetate copolymer resin (C); the blending ratio of B 2 being from 3 weight parts of B 2 to 100 weight parts of A to the saturated solubility of B 2 in A and the blending ratio of C being from 20 to 50 weight parts of C per 100 weight parts of an aggregate of A and C. B 2 is preferably polyethylene glycol. The present invention also provides a melt-extruded article which comprises at least one layer comprising a resin composition as defined above. The melt-extruded article is preferably a food packaging film. The preferred kind of food to be packaged is cheese. The present invention further provides the use of an article as defined above in packaging cheese. Fig. 1 illustrates an apparatus for measuring the solubilities of carbon dioxide and oxygen to organic compounds. The degree of polymerization of the PVA(A) used in the present invention is preferably 300 or higher as measured according to JIS-K6726 in order to produce a strong film and preferably 3500 or lower to facilitate processing and, furthermore, particularly preferably in the range of 500 to 2000. While the degree of saponification of the PVA which is used is within the range of 60 to 95 mole%, it is preferably in the range of 60 to 90 mole%, and more preferably in the range of 60 to 80 mole%. From the viewpoint of the oxygen GTR, the degree of saponification of PVA of 60 mole% or higher and 95 mole% or lower are both preferable, since the former reduces the oxygen GTR of the obtained film and the latter keeps the saturated solubility of the compound (B 2 ) from being reduced excessively. Moreover, a PVA having a degree of saponification of 95 mole% or lower is preferable in that it gives a wide temperature differential between the melting point and the thermal decomposition initiating temperature, namely, the processable temperature range. The compound (B 2 ) used in the present invention is an alkylene glycol monomer or polymer thereof and its carbon dioxide/oxygen solubility ratio (SCO 2 /SO 2 ) is 30 or higher, or preferably 50 or higher. Examples of alkylene glycol include such monomers as ethylene glycol, propylene glycol, 1,2-butanediol, 1,3-butanediol, 1,4-butanediol, and 2,3-butanediol. As a typical example of their polymers, polyethylene glycol (hereinafter abbreviated as "PEG") can be mentioned. It is preferable that among alkylene glycols, 1,2-butanediol and 1,4-butanediol be used. It is preferable that PEG, as the preferable polymer, has an average molecular weight of 400 or more for such reason that it is then not readily lost during the film forming step by the melt extrusion technique. On the other hand, there is not any particular upper limit for the average molecular weight of PEG. However, it is preferable that the average molecular weight be about 2000 or less from the viewpoint of miscibility with PVA. The resin composition of the present invention comprises said PVA (A), said composition (B 2 ) and a saponified ethylene/vinyl acetate copolymer resin (hereinafter abbreviated as "EVOH") (C). The compound (B 2 ) is preferably PEG. The PVA (A)/EVOH (C) blending ratio is preferably within the range of 20 to 50 weight % of EVOH to an aggregate of PVA and EVOH, particularly within the range of 30 to 40 weight %. Said blending ratios are desirable because the oxygen GTR is not excessively large as long as EVOH is present at 20% by weight or more and because the carbon dioxide/oxygen GTR ratio is increased if EVOH is present at 50% by weight or less. The lower limit of the blending ratio of the compound (B 2 ) is 3 weight parts per 100 weight parts of PVA, and is preferably 5 weight parts. The carbon dioxide/oxygen GTR ratio of the film is improved by blending the compound (B 2 ) by 3 weight parts or more. On the other hand, as regards the upper limit of the blending ratio of the compound (B 2 ), the carbon dioxide/oxygen GTR ratio of the film increases with an increase in the blending ratio of the compound (B 2 ). However, since it is principally PVA that dissolves the compound (B 2 ) in the composition, and the amount of lost compound (B 2 ) during the film forming step (described in the following) is minimal and the 'bleed-out' (exudation) of the compound (B 2 ) from the obtained film is suppressed so long as the compound (B 2 ) is within the saturated solubility of the compound (B 2 ) in PVA. The upper limit of the blending ratio of the compound (B 2 ) is set at the saturated solubility of the compound (B 2 ). Such saturated solubility decreases with an increase in the degree of saponification of PVA. In addition to the said three components, there may be added to said composition, if necessary, various kinds of stabilizers, antiblocking agents, lubricants, colorants and fillers to the extent that they do not impair the effect of the present invention. While there is no particular limitation with respect to the method for mixing the components of PVA (A), the compound (B 2 ) and EVOH (C), an example of a preferable method is that PVA impregnated with the compound (B 2 ) with heating at about 60°C is left to stand for several hours to undergo aging and the obtained substance is dry blended with EVOH (C). Where the compound (B 2 ) used is of a type which is solid at ambient temperature, those three components may be dry blended directly. The mixture is formed into film and the like. It is preferable that a composition pelletized beforehand by a twin-screw extruder or the like be used. The obtained impregnated mixture or pellet is formed into articles such as film, sheet or tray. While melt extrusion is preferable as the forming method, the forming method is not limited thereto alone. In the case of film, which is a particularly preferable formed article, film may be oriented monoaxially or biaxially. Methods such as compression molding, extrusion by T-die, coextrusion by a circular die and deep draw forming may be employed as the forming method. Particularly, in cases where laminated films are formed by combining a core layer made from the composition of the present invention with other kinds of resin as mentioned in the following; methods such as extrusion lamination and coextrusion lamination may be employed. The film is formed so that the core layer will have a thickness of 3 to 50 microns, preferably 5 to 30 microns, although the optimal thickness varies with whether the film produced from the composition of the present invention is used independently, or the film made from the composition of the present invention is used as the core layer for producing a laminate with films made from other kinds of resin. Where foodstuff which copiously generates carbon dioxide and does not tolerate extensive exposure to oxygen like cheese products or coffee beans is packaged, it is preferable that the oxygen GTR of the core layer of the package film comprising the composition of the present invention is 400 cc/m 2 .day.atm. (30°C, dry) or less, preferably 300 cc/m 2 .day.atm. (30° C, dry) or less, and a carbon dioxide/oxygen GTR ratio of 5 or more, preferably 6 or more. While the core layer may be used independently as described above, it is particularly preferable with a view to improving the water repellency, heat sealability and securing a practical strength that it is used in the form of a laminate obtained by laminating onto it layers of other kinds of thermoplastic resin. The laminate may be constructed of two layers consisting of the core layer and a layer of another resin, or three or more layers consisting of the core layer and layers of other kinds of thermoplastic resins laminated on both sides of the core layer. Although the two or more outside layers laminated onto both sides of the core layer may be of the same kind or different kinds of resin, it is preferable that the material of the innermost layer contacting directly to the foods be constructed of a heat sealable resin and the outside layer be constructed of a resin having a reinforcing effect. For the thermoplastic resin layer to be laminated onto the core layer, a type of resin is selected which does not impair the selective gas permeability of the core layer. In particular, it is preferable that the laminated thermoplastic layer has an oxygen GTR of 1000 cc/m 2 .day.atm., 30 microns thickness (30°C, dry) or more. There can be mentioned as examples of preferable kinds of thermoplastic resin to be laminated onto the core layer at least one kind of ethylene copolymer wherein its comonomer being selected from among vinyl acetate, and acrylates such as ethyl acrylate, methyl acrylate, and methyl methacrylate, or polyolefin resins such as high pressure process low-density polyethylene, linear low-density polyethylene (L-LDPE), polypropylene, and ionomer, and, furthermore, polyamide resins. An adhesive resin layer may be used for laminating the external layer onto the core layer. There can be mentioned as examples of the adhesive resin the thermoplastic polymer containing carboxyl group or its derivative, such as polyolefin graft-modified with maleic acid, fumaric acid or maleic anhydride, the resins obtained by copolymerizing those monomers with olefins, and, moreover, adipic acid polyesters having urethane-combined carboxyl group. While the film provided by the present invention may be used for packaging foodstuff as a whole, it is particularly suitable for use as packaging material for cheese products such as natural cheese or coffee beans and the like. Particularly, for the use of packaging natural cheese, there is no need to change the packaging throughout the whole step, from the ripening stage to the latter stages of physical distribution. The present invention is specifically illustrated by examples as follows: Reference Example 1 The amount of carbon dioxide generated and the extent of growth of surface molds in the presence of oxygen during the ripening of natural cheese vary with the kind of natural cheese, the kind of live bacteria and conditions of ripening. Four specimens of packaging material having differentiated thicknesses and compositions (the amount of plasticizer) of the core layer were compared with one another with respect to such performances as the swelling of the packaging material due to generation of gas and growth of surface molds during the ripening step, the carbon dioxide GTR, the oxygen GTR and the carbon dioxide/oxygen GTR ratio as opposed to four typical kinds of cheese. The result of this study is shown in Table 1. Packaging material Oxygen GTR 50 300 500 600 Carbon dioxide GTR 200 1000 2000 3000 Permeability ratio 4.0 3.3 4.0 5.0 Cheddar cheese 1/No 0/No 0/Yes 0/Yes Edam cheese 2/No 1/No 0/Yes 0/Yes Gouda cheese 2/No 2/No 1/Yes 0/Yes Emmentaler cheese 2/No 2/No 2/No 1/No It is seen from the results set forth in Table 1 that the preferable oxygen GTR is about 400 cc/m 2 .day.atm. or less and the preferable carbon dioxide/oxygen GTR ratio is 5 or more, although there can be few exceptions to this general tendency. It is seen from Table 1 that when PVDC (polyvinylidene chloride resin) is used as the oxygen gas barrier, increasing the amount of plasticizer added to the PVDC results in an increased carbon dioxide GTR and also an increased oxygen GTR, hence it becomes difficult to thereby obtain a packaging material having a low oxygen GTR and at the same time a large carbon dioxide/oxygen GTR ratio. The Method for Measuring Gas Transmission Rates of Oxygen and Carbon Dioxide: The gas transmission rates to both oxygen and carbon dioxide were measured simultaneously at 30°C, using a gas GTR testing apparatus GTR-10A manufactured by Yanagimoto Manufacturing Co. Reference Example 2 Measured values of the solubilities of carbon dioxide and oxygen and the solubility ratio between the two (SCO 2 /SO 2 ) of the compound (B) and other compounds as comparative specimens are given in Table 2. Organic compounds Solubility (ml/ml) Solubility ratio (SCO 2 /SO 2 ) SCO 2 SO 2 n-propanol 0.88 0.12 7 Glycerin 0.04 0.002 20 Isooctane 1.60 0.20 8 Dioctyl adipate 1.64 0.14 12 Dibutyl sebacate 2.80 0.56 5 1,4-butanediol 0.85 0.007 127 Ethylene glycol 0.81 0.013 62 PEG ( ) 1.63 0.031 53 Lactic acid 1.10 0.008 137 The Method for Measuring Solubilities of Oxygen and Carbon Dioxide: The solubilities of carbon dioxide and oxygen were measured in accordance with the saturation method as described in the New Series of Lectures on Experimental Chemistry (Shin Jikken Kagaku Koza) [I] pages 124-243 (Maruzen Co., Ltd., Tokyo, Japan, 1985). The solubility of carbon dioxide and the solubility of oxygen in those organic chemicals set forth in Table 2, respectively, were measured at 23°C, using the apparatus illustrated in Fig. 1. The solubility was calculated by the following equation: alpha = (a x V 0 )/(76 x V 1 ) x 76/(76 - a) where "alpha" is the solubility, "V 0 " is the space volume (ml), "V 1 " is the amount of specimen (ml), and "a" is the pressure differential on a mercury manometer (cmHg) Explanation on Fig. 1: Operating procedures for the apparatus for measuring the solubilities of carbon dioxide and oxygen in organic compounds as illustrated in Fig. 1 are as follows: 1. Close the valve I and open the vacuum valves A, D, E, and F to evacuate air through A, using a vacuum pump. G is a vessel containing compound(B). 2. Close the valve A after evacuation is complete and introduce the specimen gas through the valve B, with the prevailing pressure adjusted to 0 kg/cm 2 gage P. 3. After the specimen gas has been introduced, quickly close the valves D and F, then open the valve I and the vacuum in the system is measured by the mercury manometer. 4. In measuring the vacuum, the reading is taken when the pressure has become constant, namely, when the saturation is reached. Reference Example 3 Measurement of the degree of saponification of each kind of PVA and the saturated solubility of PEG, etc. in PVA: 10 g. each of PVA (the degree of polymerization: 500) varying in the degree of saponification was precisely weighed out and 20 g. each of PEG was added to it and the the samples were left to stand for 24 hours at 60°C. This impregnated substance was formed by a hot press (200 Deg. C, 100 kg/cm 2 , 2 minutes). After wiping off with paper towel PEG which had exudated from the formed article, a compression molded sheet was obtained. The sheet thus obtained was weighed and the saturated solubility was calculated on the basis of the amount of PEG contained in the sheet. Likewise, the saturated solubility of lactic acid and 1,4-butanediol in PVA (the degree of polymerization: 500, the degree of saponification: 80 mole%) was measured. Results of the measurement are set forth in Table 3. Degree of saponification (mole%) Saturated solubility (PHR) 60 PEG 170 80 PEG 27 88 PEG 8.5 98.5 PEG 4.7 80 Lactic acid 70 80 1,4-butanediol 57 Examples 1-4 and Comparative Example 1 PVA (manufactured by Kuraray Co., the degree of saponification of 60 mol%, the degree of polymerization: 500) and PEG (the average molecular weight: 600) were mixed together by the weight parts set forth in the footnote of Table 8, and the mixture was aged overnight in a dry atmosphere maintained at 60°C. Certain amounts of the obtained composition and EVOH, a saponified ethylene/vinylacetate copolymer (the ethylene content = 44 mole%, the degree of saponification: 99 mole%, and the melt viscosity at 210°C and a shear rate of 10 2 sec -1 = 0.6 x 10 4 poise) were dry blended, melt kneaded by a twin-screw extruder (BT-30, 30 mm diameter), and then pelletized by a chill roll-type pelletizer. The obtained blended pellet (for the core layer), an ethylene/vinylacetate copolymer resin (EVA)(for the outside layer), an ionomer resin (for the inside layer) and an adhesive resin ("Admer" SF750, manufactured by Mitsui Petrochemical Industries, Ltd., MI = 4.7)(for the adhesive layer) were melt kneaded individually in separate extruders, and were melt coextruded by a coextruding apparatus with the resin temperature maintained at 200° C and then stretched 2.5 x 3.2 times to obtain a film constructed of 5 layers, namely, the ethylene/vinylacetate layer, the adhesive layer, the core layer, the adhesive layer, and the ionomer layer. Results of measurement of the gas GTR, etc. of the obtained film are shown in Table 4. It can be seen from Table 8 that the GTR ratio of this laminated film has increased if the composition of the invention disclosed in the present application is used. Comparative Examples 2-3 A five-layer oriented film (Comparative Example 2) was obtained by the same procedure as employed in Example 1, except that the below-specified weight parts of PVA (manufactured by Kuraray Co., the degree of saponification: 55 mole%, the degree of polymerization: 500) and PEG (the average molecular weight: 600) were mixed together. Results of the measurement of GTR of the obtained ratio are shown in Table 5. Likewise, the same procedure as employed in Example 1 was followed in order to obtain data as Comparative Example 3, except that PVA (the degree of polymerization: 500) having a degree of saponification of 98.5 mole% was used. However, the attempt to melt extrude the mixture ended in failure, as the crystalline melting point of PVA (220°C) is higher than the thermal decomposition initiating temperature (190°C) and said two temperature levels are close to each other. Examples 5-8 The below-specified weight parts of PVA (manufactured by Kuraray Co., the degree of saponification: 80 mole%, the degree of polymerization: 300) and PEG (the average molecular weight: 600) were mixed together and the mixture was aged in a dry atmosphere maintained at 60°C overnight. Then, the obtained material was dry blended with a specified amount of the same EVOH as used in Example 1 into pellets. Successively, this blended pelletized material (the core layer), low-density polyethylene (LDPE)(the outside layer and the inside layer) and adhesive resin ("Admer" SF 750, manufactured by Mitsui Petrochemical Industries, Ltd. MI=4.7) were individually melt kneaded by separate extruders, and then were melt coextruded through a T-shape die with the resin temperature maintained at 200°C. Thus was obtained a 5-layer unoriented film constructed of LDPE/Admer/the core/Admer/LDPE. Measured GTR's of the obtained film are shown in Table 6. Examples 9-10 Using the 5-layer unoriented film obtained in Example 5 (Example 9) and Example 7 (Example 10), package tests were carried out on two kinds of shred-type cheese. The following two kinds of cheese were used as specimens: "Samsoe", a produce of Denmark, characterized by a high carbon dioxide generation rate "Maribo", a produce of Denmark, characterized by a low carbon dioxide generation rate Results of the visual inspection made on the samples are shown in Table 7. In either sample, there was not observed visible growth of surface molds. The Test Method: 200 g each of the aforementioned cheese was packed in 5 bags, each of which being prepared by heat sealing a piece of film cut out from each kind of the sample packaging material along both sides and one end. After displacing the interior of the bag with a mixture gas consisting of 70% oxygen gas and 30% carbon dioxide gas, the remaining open end of the bag was heat sealed. The size of the packed bag was 140 x 200 mm. The samples thus prepared were stored at 15°C and 68% relative humidity, and after the lapse of given storage periods the packed products were observed with respect to visible growth of surface molds and swelling of the bag. Comparative Example 4 Using a multilayer film constructed of EVA/Adhesive/ PVDC/Adhesive/Ionomer (the thicknesses:20/2/7/2/40 microns), cheese was packed according to the same procedure as employed in Examples 9-10, and the packaged product was visually inspected. The oxygen GTR of this film was 180 cc/m 2 .day.atm. (30°C, dry) and the carbon dioxide GTR was 690 cc/m 2 .day.atm. (30°C, dry), and the carbon dioxide/oxygen (GTR) ratio was 3.8. Results of the visual inspection are shown in Table 7. In either sample, there was not observed visible growth of surface molds. Description of cheese Packaging material Storage period (days) 1-6 7 13 15 20 Samsoe Example 9 00000 00000 00000 00000 00000 Example 10 00000 00000 00100 00001 00101 Comparative Example 4 00000 11111 12222 22222 22222 Maribo Example 9 00000 00000 00100 00100 00101 Example 10 00000 00000 00000 00100 00101 Comparative Example 4 01000 01000 12122 22222 22222 [Note] 0 = No swelling of bag; serviceable 1 = Slight swelling of bag; serviceable 2 = Conspicuous swelling of bag, not serviceable As can be seen from Table 7, the film did not undergo such practical problems as swelling of the bag due to generation of carbon dioxide gas and growth of surface molds. On the other hand, the film described in Comparative Example 9 was practically inadequate, inasmuch as there occurred conspicuous swelling of the packaging bag due to its low carbon dioxide GTR.

Claims: A resin composition which comprises: i) a polyvinyl alcohol resin having a degree of saponification of 60 to 95 mole% (A); ii) an alkylene glycol monomer or polymer thereof having a carbon dioxide/oxygen solubility ratio (SCO 2 /SO 2 ) of 30 or greater (B 2 ); and iii) a saponified ethylene/vinylacetate copolymer resin (C); the blending ratio of B 2 being from 3 weight parts of B 2 to 100 weight parts of A to the saturated solubility of B 2 in A and the blending ratio of C being from 20 to 50 weight parts of C to 100 weight parts of an aggregate of A and C. A resin composition according to claim 1 wherein B 2 is polyethylene glycol. A melt extruded article which comprises at least one layer comprising a resin composition as defined in claim 1 or 2. A melt extruded article according to claim 3 which is a food packaging film. Use of an article as defined in claim 3 or 4 in packaging cheese.

Inventors: FUNABASHI SHINICHIRO, HIROSE KAZUHIKO, HISAZUMI NOBUYUKI, OHBA HIROYUKI, TANAKA HIDEAKI, FUNABASHI, SHINICHIRO, HIROSE, KAZUHIKO, HISAZUMI, NOBUYUKI, OHBA, HIROYUKI, TANAKA, HIDEAKI
Applicants: KUREHA CHEMICAL IND CO LTD, KUREHA CHEMICAL INDUSTRY CO., LTD.
Application year: 1992
Main classification: C08K5
IPC codes: C08J5, C08L29, C08K5, B65D65