COMPOSITION, PROCESS FOR PRODUCING THE SAME AND MULTILAYER STRUCTURE

Abstract: A composition comprises a matrix of an ethylene-vinyl alcohol copolymer having dispersed therein a drying agent in a particulate state. The volume-area average diameter of the grains of the drying agent having a long diameter of not less than 10µm is not greater than 30µm. A multi-layer structure comprises layers of such a composition. Gas barrier property of the multi-layer structure is hardly reduced after a retort treatment and thus, the multi-layer structure is extremely useful for packaging of food, expecially packaging of retorted food.

Description: The present invention relates to a composition having a high resistance to gas permeation resulting from an improved dispersion of the grains of a drying agent which are present in an extremely particulate state in the blend of an ethylene-vinyl alcohol copolymer (hereafter simply referred to as EVOH) whereby the drying agent does not form an aggregated mass, a process for producing the same and a multi-layer structure comprising the same. Compositions comprising EVOH and drying agents are disclosed in US-A-4,425,410 and 4,464,443, indicating that multi-layer structures comprising the compositions have a small increase in oxygen permeability even after a retort (steam sterilization) treatment, as compared to EVOH alone. The compositions are preferred packaging materials. When a container is prepared in accordance with the method disclosed in the above US patents and used for the storage of food which requires a retort treatment, gas permeation resistance is improved as compared to the use of EVOH alone but the degree of the improvement is still poor and the container is unsatisfactory yet for the purpose of greatly improving the storability of food which requires a steam sterilization treatment. On the other hand, as methods for preventing an increase of the oxygen permeability upon retort treatment of multi-layer structures comprising EVOH, there are known methods which comprise blending a drying agent in an adhesive resin layer, adjacent to EVOH , as shown in US-A-4,407,897 or JP-A-170748/82. In these cases, the adhesive resin layer containing the drying agent should be used in a great thickness for satisfactory results. This causes problems from an economical viewpoint and in addition, the effects are not satisfactory from a practical standpoint, either (Comparative Example 4). Under these circumstances, the present inventors have made extensive investigations on molding techniques for the purpose of controlling the blended state of a blend composition and studied the relationship between the blended state of the obtained composition and the rate of oxygen permeability of multi-layer structures upon a retort treatment. As a result, it has been found that the graininess and dispersed state of the drying agent grains in an EVOH matrix greatly affect the rate of oxygen permeability. They succeeded in obtaining the composition free from the defects as described above. The present invention is directed to a composition comprising a matrix of an ethylene-vinyl alcohol copolymer having dispersed therein a drying agent in a particulate state, which is characterized in that the volume-area average diameter of the grains of the drying agent having a long diameter of not less than 10 µm is not greater than 30 µm. Further subject matters of the invention are a process for producing the composition and a multi-layer structure comprising the composition as(a) layer(s). Structures, especially multi-layer structures, obtained using the composition of the present invention show after a retort treatment a gas permeation resistance that is comparable to the state prior to the retort treatment. This is unexpected. Containers prepared using such a multi-layer structure are extremely useful for packaging a wide range of food. The ethylene-vinyl alcohol copolymer (EVOH) as used in the present invention can be any product obtained by hydrolysis of the vinyl acetate units of a copolymer of ethylene and vinyl acetate. Copolymers which are suited for the purposes of the present invention particularly include those in which the ethylene unit content is 25 to 60 mol% and the saponification degree in the vinyl acetate component is at least 96%, preferably at least 99%. The value of the melt index (190°C, 2160 g), may range from 0.2 to 60 g/10 min. Further EVOHs referred to in the present invention may be those modified up to 5 mol% with a copolymerizable monomer. Examples of such monomers for modification include propylene, 1-butene, 1-hexene, 4-methyl-1-pentene, acrylic acid esters, methacrylic acid esters, maleic acid, fumaric acid, itaconic acid, higher fatty acid vinyl esters, alkyl vinyl ethers, N-(2-dimethylaminoethyl)methacrylamides or quaternized products thereof, N-vinylimidazole or quaternized products thereof, N-vinylpyrrolidone, N-n-butoxymethylacrylamide, vinyltrimethoxysilane, vinylmethyldimethoxysilane and vinyldimethylmethoxysilane. As the EVOH matrix, two or more EVOH resins having a different composition can also be mixed and used as a mixture. Further additives, for example, coloring agents such as pigments and dyes; antistatic agents, UV absorbants, plasticizers, heat stabilizers and lubricants, can be incorporated into the matrix within such a range that does not impair the effects of the present invention. Further, as the drying agent referred to in the present invention, there are suitable salts capable of forming hydrates, namely, salts which absorb water as water of crystallization, in particular phosphates such as monosodium phosphate, disodium phosphate, trisodium phosphate, trilithium phosphate and sodium pyrophosphate. Particularly, the anhydrides thereof are most suited for the present invention in view of these effects. Other hydrate-forming salts, for example, salts such as sodium borate and sodium sulfate, and particularly anhydrides thereof are also suited for the present invention. Further, other hygroscopic compounds, for example, sodium chloride, sodium nitrate, sugar, silica gel, bentonite, molecular sieve and highly water-absorbing resins may also be used. These drying agents can be used in admixture of two or more simultaneously. In the present invention, it is necessary that the drying agent by dispersed in the matrix of EVOH as fine grains and that the grains of the drying agent with a long diameter of 10 µm or more have a volume-area average diameter not greater than 30 µm, preferably 25 µm or less, most preferably 20 µm or less. By forming such a fine dispersion state, a multi-layer structure can be prepared which has so high a gas permeation resistance as has never been achieved before. The composition having such a fine dispersion state can be obtained only by careful combination of particular processing steps suited for that purpose. First, special attention should be paid to the drying agent in order to obtain a grain diameter as fine as possible upon precipitation from an aqueous solution of the salts through e.g. spray drying. The grains of the drying agent can be classified into sizes of 30 µm or less, preferably 10 µm or less and then provided for use, but in general, the dried salts are subjected to pulverization using a jet grinder, an impact grinder, a ball mill or a vibration ball mill. The ground grains are classified into ultrafine grains having a size of 30 µm or less, preferably 10 µm or less, using a classification machine such as an air classification machine. 30 µm or less as used herein means that the grains exceeding 30 µm are less than 0.1% in the volume fraction, namely, the fine grains of 30 µm or less are present in an amount of at least 99.9%. The graininess of the ultrafine grains is a value determined by the Coulter counter method. To determine the grain size, the grains are preliminarily sieved through a sieve having a mesh of 10 to 75 µm and the size of the coarse grains remaining on the sieve is determined using a Coulter counter. In this way the size of the coarse grains can be measured with high accuracy. Next, the ultrafine grains of the drying agent described above are mixed with EVOH. Mixing can be done by a method where the fine grains of the drying agent are mixed with powders, grains or pellets of EVOH using a conventional mixer, for example, a Henschel mixer, a super mixer or a tumbling mixer, or by a method which comprises mixing the fine grains of the drying agent with a melt of EVOH to make a master batch and mixing the master batch with powders, grains, pellets or melts of EVOH. Then, the mixture is kneaded at temperatures higher than the melting point of EVOH to prepare the composition. EVOH and the drying agent grains may also be directly fed into a kneader to effect kneading, without preliminary mixing of EVOH and the drying agent grains as described above. In this kneading operation, however, the fine grains of the drying agent tend to aggregate to each other; even if fine grains of 10 µm or less are used, the remarkable effects of the present invention cannot be obtained if the fine grains agglomerate and agglomerates exceeding the volume-area average diameter of the present invention are formed. Accordingly, the kneading operation is extremely important in the present invention. As kneader giving a composition having a high dispersion state, there are most suited continuous twin rotor kneaders such as a continuous intensive mixer or a kneading type twin screw extruder (same directions or different direction) but, there may also be used batch type twin rotor extruders such as a Banbury mixer, an intensive mixer or a pressure kneader. Further, as another continuous mixing devices, there may also be used rotary disks having an attrition mechanism such as a stone mill, e.g., KCK Kneader Extruder manufactured by KCK Co., Ltd. Kneaders conventionally used, include a single screw kneader equipped with a kneading part (Dulmage, CTM, etc.) or handy type kneaders such as a Brabender mixer. In the case of using such kneaders, however, it is difficult to obtain the excellent composition of the present invention. Of these kneaders, the most preferred one for the purposes of the present invention is a continuous intensive mixer. Commercially available models are FCM manufactured by Farrel Co., Ltd., CIM manufactured by The Japan Steel Works, Ltd., and KCM, NCM, LCM or ACM manufactured by Kobe Steel, Ltd. From a practical standpoint, it is preferred that a device equipped with a kneader having mounted a single screw extruder beneath the kneader be used to perform kneading and extrusion pelletization simultaneously. For the kneading purpose of the present invention, there may also be used a twin screw kneading extruder having kneading disks or kneading rotors, for example, TEX manufactured by The Japan Steel Works, Ltd., ZSK manufactured by Werner & Pflenderer Co., Ltd., TEM manufactured by Toshiba Machine Co., Ltd., or PCM manufactured by Ikegai Iron Works, Inc. In these continuous kneaders, the shape of the rotor or disk plays an important role. In particular, a gap (tip clearance) between the mixing chamber and the rotor tip or the disk top is important; if the gap is too narrow or two wide, the composition having good dispersion of the present invention cannot be obtained. Most preferably the tip clearance should be in the range of 1 to 5 mm. In order to obtain the composition having good dispersion of the present invention, it has been found that kneading should be performed at a unit work of the kneader of at least 0.1 kwh/kg, preferably 0.2 to 0.8 kwh/kg. The unit work is determined by dividing the energy (quantity of electric power consumed, kw) used for the kneading by the amount (kg) of substances subjected to the kneading treatment per hour. Its dimension is kwh/kg. In order to produce the composition of the present invention, it is required that the kneading be performed at a unit work value higher than that of conventional kneading. To render the unit work at least 0.1 kwh/kg, it is insufficient to merely increase the rotation number of a kneader but it is preferred to cool the composition during kneading, e.g. with a jacket, thereby to lower the temperature and increase the viscosity. If the kneading is conducted in such a state that the viscosity is low, it is difficult to obtain the composition contemplated in the present invention. Accordingly, kneading is suitably conducted at a temperature ranging from the melting point of EVOH to a temperature higher than the melting point by 60°C, more preferably ranging from the melting point of EVOH to a temperature higher than the melting point by 40°C,determined as the temperature of the discharged resin at the outlet of the kneading part. It is desired that the rotation number of the rotor of the kneader be in the range of 200 to 1200 rpm, more preferably 400 to 1200 rpm. The inner diameter(D) of the chamber in the kneader is generally 30 mm or more, preferably in the range of 50 to 400 mm . It is preferred that L/D of the kneader be 4 to 10(L/D means the ratio of the length (L) of the mixing chamber(s). to the inner diameter (D) of the mixing chamber(s)). Further, the kneader may be one kneader or two or more kneaders combined together may also be used. The longer the kneading time, the better the results; in view of thermal deterioration or change of EVOH or from an economic standpoint, the kneading time is, however, generally in the range of 10 to 600 seconds, preferably 20 to 200 seconds, most preferably 20 to 100 seconds. There is no particular restriction to the ratio of EVOH to the drying agent used but the ratio of 97 : 3 to 50 : 50, particularly 95 : 5 to 70 : 30, by weight, is preferred. The graininess of the drying agent grains in the composition is measured by the microscopic method; in general, the graininess is determined by visual observation or using an image analysis device on a photograph of the grains. In the present invention, it is required that among the dispersed grains, the volume-area average diameter of the grains having a long diameter of 10µm or more is not greater than 30 µm. The long diameter as used herein means the distance between two parallel lines which is the longest distance when a projected image of each grain is inserted between the parallel lines. Regarding the grains having a long diameter of 10µm or more, an average grain diameter must be determined. Various methods are known to determine the average grain diameter; a convenient method suited for the purpose of the present invention is that the average value D of the long diameter L and a diameter B in the direction rectangular to L is taken as the average diameter. This method is one of the methods often used in this technical field. Thus, when the average diameter D N is determined with respect to a number of N grains in an appropriate measurement range (200 µm x 200 µm), the volume-area average diameter D AV is defined as follows: D AV = ΣD N 3 / ΣD N 2 As has been made clear in the present invention, the volume-area average diameter of the grains of the drying agent in the composition of the present invention having a long diameter of at least 10 µm, greatly affects the gas permeation resistance of a multi-layer structure comprising this composition as (a) layer(s). The reason is not totally clear but is it assumed that grains having a large grain diameter would be particularly disadvantageous for the hygroscopic effect or gas permeation resistance of EVOH. From a practical viewpoint, it is most effective to use the thus obtained composition generally in a multilayered state with other thermoplastic resins. Examples of suitable thermoplastic resins include a polyolefine resin, a polyamide resin, a polyester amide resin, a polyester resin, a polystyrene resin, a polyvinyl chloride resin, an acrylic resin, a polyvinylidene chloride resin, a polyurethane resin, a polyacetal resin and a polycarbonate resin. Among them, particularly important for the present invention in their effects and practical efficiencies are a polyolefine resin; a polyamide resin, a polyester resin and a polystyrene resin are also important. Examples of the polyolefin resin include polyethylene of high density, medium density or low density; polyethylene copolymerized with vinyl acetate, an acrylic acid ester, or α-olefins such as butene, hexene or 4-methyl-1-pentene; ionomer resins, polypropylene homopolymer, polypropylene graft-copolymerized with ethylene, or polypropylene copolymerized with α-olefins such as ethylene, butene, hexene or 4-methyl-1-pentene; poly-1-butene, poly-4-methyl-1-pentene, or modified polyolefins obtained by reacting the aforesaid polyolefins with maleic anhydride. Examples of the polyamide resin include polycapramide (nylon-6), poly-ω-aminoheptanoic acid (nylon-7), poly-ω-aminononaic acid (nylon-9), polyundecanamide (nylon-11), polylaurine lactam (nylon-12), polyethylenediamine adipamide (nylon-2,6), polytetramethylene adipamide (nylon-4,6), polyhexamethylene adipamide (nylon-6,6), polyhexamethylene sebacamide (nylon-6,10), polyhexamethylenedodecamide (nylon-6,12), polyoctamethylene adipamide (nylon-8,6), polydecamethylene adipamide (nylon-10,6), polydodecamethylene sebacamide (nylon-10,8), or a caprolactam/laurin lactam copolymer, caprolactam/hexamethylene diammonium adipate copolymer, laurine lactam/hexamethylene diammonium adipate copolymer, hexamethylene diammonium adipate/hexamethylene diammonium sebacate copolymer, ethylenediammonium adipate/hexamethylene diammonium sebacate copolymer, ethylene diammonium adipate/hexamethylene diammonium adipate copolymer and caprolactam/hexamethylene diammonium adipate/hexamethylene diammonium sebacate copolymer. Representative examples of the polyester resin include poly(ethylene terephthalate)(PET), poly(butylene terephthalate), poly(ethylene terephthalate /isophthalate), poly(ethylene glycol/cyclohexane dimethanol/terephthalate), and further include these polymers which contain as a copolymer component a diol such as ethylene glycol, butylene glycol, cycolhexane dimethanol, neopentyl glycol and pentanediol; a dicarboxylic acid such as isophthalic acid, benzophenonedicarboxylic acid, diphenylsulfone dicarboxylic acid, diphenylmethane dicarboxylic acid, propylenebis(phenylcarboxylic acid), diphenyloxide dicarboxylic acid, oxalic aid, malonic acid, succinic acid, glutaric acid, adipic acid, pimelic acid, suberic acid, azelaic acid, sebacic acid and diethylsuccinic acid. Examples of the polyvinyl chloride resin include a homopolymer of vinyl chloride alone and in addition thereto, a copolymer with vinyl acetate, a maleic acid derivative or a higher alkyl vinyl ether. Examples of the polystyrene resin include a homopolymer of styrene alone and in addition thereto, polystyrene obtained by graft copolymerization with butadiene, a styrene-butadiene rubber mixture or a styrene-maleic anhydride copolymer. The thermoplastic resins used in the present invention may also be used as admixture of two or more. It has been experimentally confirmed that the dispersion state of the fine graines of the drying agent in the composition does not principally change during the step of forming the multi-layer structure of the composition of the present invention in combination with the other thermoplastic resins described above. The multi-layer structure of the present invention is molded into films, sheets, cups, bottles, tubes, etc. by subjecting the composition of the present invention and the other thermoplastic resin to molding processing such as a co-extrusion molding method, a multilayered co-injection molding method or a heat molding method. It is generally preferred that the composition of the present invention be used as an intermediate layer and the inner and outer layers be composed of thermoplastic resins such as polyolefins. In the case of such a multi-layer structure, it is preferred to use an interlayer adhesive resin. There is not particular restriction to such an interlayer adhesive resin but representative examples include resins obtained by modifying (e.g. addition or grafting) thermoplastic resins (e.g. polyethylene, polypropylene, an ethylene-vinyl acetate copolymer or an ethylene-acrylic acid ester copolymer) with an ethylenically unsaturated carboxylic acid or an anhydride thereof (e.g. maleic anhydride). In addition, there may be used polyesters having bound thereto aluminum and a monocarboxylic acid as described in JP-A-115327/84 (EP-A-112718). When the thermoplastic resins, the EVOH composition and the adhesive resin are designated A, B and C, respectively, the layer construction of the multi-layer structure can be A/B, A/C/B, A/B/A, A/C/B/C/A, A/B/A/B/A, A/C/B/C/A/C/B/C/A, but is not limited thereto. The multi-layer structure of the present invention is characterized by containing the drying agent grains in the layer of EVOH in a highly dispersed state; layers other than the EVOH layer, for example, a layer of the adhesive resin, may also contain the drying agent. The multi-layer structure of the present invention is readily distinguishable from conventionally known plastic materials because its gas permeation resistance, especially gas permeation resistance when a retort treatment is performed, is much greater than that of the known plastic materials. The retort treatment is performed generally at 120°C by putting a container having packed therein food in an autoclave called a retort pot. The treatment time varies depending upon the kind of food; for some kind of food a treatment of 20 minutes is sufficient whereas others may require a treatment for 120 minutes. Further the multi-layer structure of the present invention is extremely useful even for food containers which require so called boiling sterilization in which sterilization is effected in boiled water under normal pressure. It has also been confirmed that the multi-layer structure of the present invention exhibits gas permeation resistance to a high degree also when used in cases where neither a retort treatment nor a boiling treatment is performed. Particularly in the case that the inner and outer layers are composed of resins having high moisture permeability such as polystyrene, polyvinyl chloride or polyester, there is a tendency that moisture in the container (or outside the container) permeates through the inner and outer layers to reduce the gas permeation resistance of the EVOH in the case that the multi-layer structure has the shape of a film. However, in the multi-layer structure containing the composition of the present invention, the retention of gas permeation resistance is high, the period of storage of the food can be markedly prolonged and the industrial significance is great. Next, there are various processes for producing the multi-layer structure of the present invention as described above. In particular, the production of the multi-layer structure by multi-layer injection molding is described hereafter. For the multi-layer injection molding of the present invention, various known processes are used including (1) simple multi-layer injection molding, (2) multi-layer injection direct blow molding and (3) multi-layer injection drawing blow molding. In the case of (1), the process comprises injection-molding a plasticized resin directly into a container mold in a multi-layer form as a concentric multi-layer structure and dwelling, which is per se known from JP-B-39174/86 and 9007/87. The processes (2) and (3) comprise injection-molding into a parison (also called prefoam sometimes) mold in a multi-layered form to mold a concentric multi-layered closed-end parison, transferring the closed-end parison into a blow mold and subjecting to blow foaming by a pressure fluid such as compressed air or compressed nitrogen. Differences between (2) and (3) lie in that in (2) the parison is transferred into the blow mold together with a core having a pressure fluid blowing inlet while the parison is not completely cooled but substantially in the molten state, whereas in (3), the parison is generally cooled to separate it from the core, reheated to a temperature higher than the glass transition temperature (generally the softening point) but lower than the melting point (or the plasticizing temperature) and then transferred to the blow mold, and a blowing core equipped with a drawing rod is newly inserted to draw the closed-cell parison in the blow mold to the axis direction and at the same time or thereafter, a pressure fluid is blown to effect blow forming. From the aspect of physical properties, it is different in that orientation of the resin hardly occurs in (2) whereas in (3), orientation occurs. These differences are known for example, from JP-B-8971/83,WO 81/230 (JP-A-501082/81) and JP-A-34819/85, with respect to (2). Also with respect to (3), the differences are known from JP-A-128520/82, 240409/85, 152411/86, 173924/86, 203332/86, 219644/86, 235126/86 and 152412/86. The significance of the co-injection of the present invention is described below in more detail. The co-injection is a method for molding in a closed operation using a plurality of injection cylinders. According to the known method, large mold cavities are sequentially used for production of the different layers; first, injection molding is performed using a mold cavity for a first layer. Then, the mold cavity for the first layer is opened and the first molded article is put in a larger mold cavity for a second layer. A resin for the second layer is injected into a gap formed between the mold cavity for the second layer and the primary molded article followed by thermal fusion of the primary molded article and the resin for the second layer. The thus obtained two-layer secondary molded article is subjected to injection molding using a mold cavity larger than the second molded article thereby to mold a third layer. Such a multi-stage injection molding involves increased numbers of molds and steps and a long operation time. For brevity of explanation, the co-injection will be described, taking as an example a container composed of 2 kinds of resins and 3 layers using thermoplastic resin A in the innermost and outermost layers and EVOH having dispersed therein the drying agent (hereafter referred to as EVOH composition) as the intermediate layer. As the multi-layer injection device, a device equipped with 2 injection cylinders is used. First, the resin A is primarily injected in part into a mold from a nozzle through a hot runner block via a mold gate to fill the mold halfway with the resin A. The EVOH composition constituting the intermediate layer is concentrically injected simultaneously or sequentially during of after the primary injection to form the intermediate layer. After completion of the injection of the EVOH composition, the resin A is injected (secondary injection) singly to completely envelope the EVOH composition layer therein. With respect to the structure of the hot runner, the order of injection such as simultaneous injection or sequential injection and the timing of the injection, there are various modifications and the present invention is not limited to the example described above. In summary, the key of the co-injection according to the present invention is that while forming a skin layer by primarily injecting the resin for forming the innermost and outermost layers, the EVOH composition as the core is injected (in this case, the resin A is generally injected in parallel to spread the EVOH composition) and finally the resin for forming the innermost and outermost layers thereby to fully seal the EVOH composition therein. When the edge surface of the EVOH composition is exposed, moisture is absorbed from the edge surface so that a poor appearance such as whitening is caused. This is not preferred. Hereafter the present invention will be described more specifically with reference to the examples, wherein parts are by weight unless otherwise indicated. Example 1 Anhydrous disodium phosphate powders were micronized and classified using a jet grinder (Micron Jet Model MJ-3, manufactured by Hosokawa Micron Co., Ltd.) and an air classification machine (Micron Separator, manufactured by the same company). By observation of raw powdery grains with a test glass using methanol as a dispersant, it was confirmed that a number of grains of 500µm or more were contained and the average grain diameter (medium diameter) was 86 µm according to the Coulter counter. After pulverization was performed in a similar manner, the maximum grain diameter (according to the Coulter counter) was 13µm and the medium diameter was 6.4µm ( grains having a grain diameter exceeding 13µm were less than 0.1% in volume fraction). After premixing a mixture of 20 parts of the anhydrous disodium phosphate fine powders and 80 parts of EVOH [ethylene unit content of 32 mol%, melt index of 1.3 g/10 mins. at 190°C under a load of 2160 g, melting point of 181°C {main endothermic peak temperature at DSC (scanning speed) 10°C/min}] pellets in a Henschel mixer, high speed mixing was performed to give the mixture. Then, kneading, extrusion and pelletization were performed using a counter-rotating continuous kneader (LCM-50, manufactured by Kobe Steel, Ltd.) with two-stage mixing rotors having an inner diameter(D) of the mixing chamber of 54 mm , L/D of 5.8 (first stage) and L/D (whereby L/D has the same meaning as described on page 10) of 4.2 (second stage) and having a vent at the second-stage screw section, having connected therewith a single screw extruder. The mixer rotor used in this case has a tip clearance with the mixing chamber of 3 mm. The operation was performed at a kneading temperature (outlet temperature) of 206 to 220°C for a kneading time of 30 to 40 seconds at a rotor rotation number of 530 to 650 rpm and a unit work of 0.3 to 0.6 kwh/kg. The composition (pellets) obtained was designated Composition 1. The thus obtained pellets were treated in a hot press machine at 220°C to give a thin layer having a thickness of approximately 100 µm. With an optical microscope, the dispersion state of the powder in this thin layer was examined on a photograph having an enlargement of 800 times. For this purpose a negative having an enlargement of 50 times was first prepared in order to ensure a high depth of the focus. Then a photograph having an enlargement of 800 times was made from the negative. With respect to 10 samples prepared in the same way, the average diameter of the grains having a long diameter of 10 µm or more was measured in a region of 200 µm x 200 µm and the volume-area average diameter was calculated to be 17.6 µm. Next, using a co-extrusion sheet molding device equipped with 3 extruders and having a 3 kinds-5 layers type feed block, a die for sheet molding and a puller, co-extrusion was performed to give a multi-layer structure of a polypropylene/adhesive resin/the above-described composition/adhesive resin/polypropylene (thickness: 600/50/100/50/600 µm). Polypropylene and the adhesive resin used herein were UBE POLYPRO® E-103D manufactured by Ube Industries, Ltd. and ADMER® QF-500 (polypropylene modified with maleic anhydride) manufactured by Mitsui Petrochemical Industries, Ltd., respectively. Next, using a vacuum air pressure forming machine (manufactured by Asano Seisakusho Co., Ltd.), heat forming of this sheet was performed to prepare a cup container (opening diameter of 72 mm, bottom diameter of 65 mm, height of 35 mm). The cup was subjected to a steam heating treatment at 120°C in a retort pot for 30, 60 and 120 minutes, respectively, and the cut was withdrawn. Then water was charged in the inside of the cup, the opening of the cup was connected with a device for measuring oxygen gas permeability (manufactured by Modern Controls Co., Ltd.) and the rate of oxygen permeability was measured (20°C, 100% RH at the inside, 65% RH at the outside). The results are shown in Table 1. The rate of oxygen permeability after the retort treatment was approximately twice or less than that of a container which was not retorted, and gas permeation resistance was as good as sufficient for storage of food. A part of the cup container prior to the retort treatment was cut, taken out and heated in xylene at 120°C, whereby the polypropylene and the adhesive resin were melted to leave the film of the composition of the invention. The dispersion state of the powder in this film was observed on a photograph obtained from an optical microscope having an enlargement of 800 times. For this purpose a negative having an enlargement of 50 times was first prepared in order to ensure a high depth of the focus. Then a photograph having an enlargement of 800 times was made from the negative. (Figure 1-1 shows a photograph of the same objective enlarged to 200 times and Figure 1-2 shows its model wherein numeral 1 denotes the matrix and numeral 2 denotes drying agent grains). With respect to 10 samples collected from the wall of the container at different locations, the average diameter of the grains having a long diameter of 10 µm or more was measured within a region of 200 µm x 200 µm and the volume-area average diameter was calculated to be 17.7 µm. Comparative Examples 1 to 3 Co-extrusion molding, thermoforming and retort treatment were performed in a manner similar to Example 1 except for using EVOH (that shown in Example 1) in place of the composition shown in Example 1. The rate of oxygen permeability was measured. The results (Comparative Example 1) are also shown in Table 1. Notwithstanding that the oxygen permeability rate was the same as that prior to the retort treatment, the oxygen permeability rate after the retort treatment was 10 to 100 times that of Example 1. On the other hand, 20 parts of anhydrous disodium phosphate fine powders shown in Example 1 and 80 parts of EVOH in Example 1 were premixed and then the mixture was subjected to kneading, extrusion and pelletization at a temperature of 220°C using an ordinary full-flighted type single screw extruder (inner diameter of the cylinder, 50 mm) to give pellets of the composition (this was designated Composition 2). Further kneading was performed in a manner similar to Example 1 except that the unit work of the continuous twin rotor kneader upon kneading was 0.08 kwh/kg (this is designated Composition 3). Co-extrusion molding, heat forming and retort treatment were performed in a manner similar to Example 1 except for using Composition 2 or Composition 3 described above in place of the composition of Example 1. Thereafter, the oxygen permeability rate was measured and the results are also shown in Table 1 as Comparative Examples 2 and 3. The oxygen permeability rates in Comparative Examples 2 and 3 were 10 to 40 times that of Example 1 and gas barrier property was inferior to that of Example 1. A part of the cup container prior to the retort treatment was cut, taken out and heated in xylene at 120°C, whereby the polypropylene and the adhesive resin were melted to leave the film of the composition of the invention. The dispersion state of the powder in this film was observed on a photograph obtained from an optical microscope having an enlargement of 800 times. For this purpose a negative having an enlargement of 50 times was first prepared in order to ensure a high depth of the focus. Then a photograph having an enlargement of 800 times was made from the negative. (Figure 2-1 shows a photograph of the same objective enlarged to 200 times and Figure 2-2 shows its model wherein numeral 1 denotes the matrix, numeral 2 denotes drying agent grains and numeral 3 denotes drying agent grains having large graininess). With respect to 10 samples collected from the wall of the container at different locations, the average diameter of the grains having a long diameter of 10 µm or more was measured and the volume-area average diameter was calculated to be 56.9 µm. Further measurement was made also with respect to Comparative Example 3 in a similar manner and the volume-area average diameter was 36.8 µm. Example 2 to 10 Various drying agents were micronized and used in place of anhydrous disodium phosphate fine powders of Example 1. Kneading and extrusion were performed together with EVOH (shown in Example 1) in a manner similar to Example 1 to give pellets of the respective compositions. Further co-injection extrusion molding, thermoforming and retort treatment were performed and the oxygen permeability rate of the containers obtained was measured The intermediate layer of each cup was taken out and the volume-area average diameter (D AV ) of the grains having a long diameter of 10 µm or more was determined by microscopic observation. The results are shown in Table 2. The gas barrier property of the compositions of the present invention, especially the gas barrier property of the cups using the drying agents of Examples 2 to 5 after the retort treatment is extremely high and sufficient for storage of most foodstuffs. Further in Examples 6, 7 or 8 to 10, remarkable gas permeability resistance is noted with the retorting time of 30 to 60 minutes. These are useful for many foodstuffs for which a retort treatment at this level is sufficient. However, with a retort treatment over a long period of time of 120 minutes, there is a tendency that gas barrier property decreases. Example 11 and Comparative Examples 4 and 5 Co-extrusion was carried out using a co-extrusion sheet molding device as used in Example 1 to give a multi-layer structure sheet composed of polypropylene/adhesive resin/Composition 1/adhesive resin/polypropylene (thickness: 217/40/48/37/225µm) (Sheet C). After this sheet was subjected to a steam heating treatment at 120°C in a retort pot (60 and 120 minutes), one surface was exposed to a RH of 100% and the other surface to a RH of 65%. The oxygen permeability rate (OTR) was measured at 20°C in the course of time. The change in OTR during storage for 12 weeks is shown in Figure 3-C. The sheet shows an OTR of 0.2 to 0.3 cm³/m².day. 0.98 bar (1 atm) 3 hours after the retort treatment. On the other hand, 90 parts of adhesive resin (Admer® QF-500) and 10 parts of anhydrous disodium phosphate fine powders used in Example 1 were premixed and then the mixture was subjected to kneading, extrusion and pelletization using a continuous extruder as used in Example 1 to give pellets of the adhesive resin composition containing the drying agent (this was designated Composition 4). A sheet having a thickness and construction corresponding to Sheet C of Example 11 was obtained in a manner similar to Example 11 except that EVOH (ethylene unit content of 32 mol%, melt index at 190°C of 1.3 g/10 mins.) was used in place of Composition 1 and Composition 4 described above was used in place of the adhesive resin ( Comparative Example 4, Sheet B ). After this sheet was subjected to a steam heating treatment at 120°C in a retort pot for 120 minutes, OTR was measured in the course of time under the same conditions as in Example 11. The results are shown in Figure 3-B. For control, a sheet having a thickness and construction corresponding to Sheet C of Example 11 was obtained in a manner similar to Example 11 except that EVOH described above was used in place of Composition 1 in Example 11 (Comparative Example 5, Sheet A). After this sheet was subjected to a steam heating treatment at 120°C in a retort pot for 30, 60 and 120 minutes, respectively, the cup was withdrawn and OTR was measured in the course of time under the same conditions as in Example 11. The results are shown in Figure 3-A. In Figure 3, Sheet A of Comparative Example 5 shows the OTR values of 2.6 (retorted for 30 minutes), 9.5 (retorted for 60 minutes) or 30 (retorted for 120 minutes) cm³/m².day. 0.98 bar (1 atm) which gradually decreases during storage; on the other hand, the multi-layer structure or Sheet C of Example 11 of the present invention shows an OTR of less than 0.3 cm³/m².day. 0.98 bar(1 atm) for three hours to 12 weeks after the retort treatment . Further Sheet B of Comparative Example 4 containing the drying agent in the adhesive resin shows a lower OTR than that in Comparative Example 5 retorted for 120 minutes but the OTR after the retort treatment is 10 times or more than that of the sheet of the present invention. It is thus evident that the multi-layer structure of the present invention shows a markedly low OTR after the retort treatment. Example 12 and Comparative Example 6 A composition similar to Composition 1 of Example 1 was obtained except that EVOH (ethylene unit content of 32 mol%, melt index at 190°C under load of 2160 of 4.4 g/10 mins., melting point of 181°C) was used in place of Composition 1 in Example 1 (Composition 5). A multi-layer draw-blowing container having an inner volume of 700 ml, which had an intermediate layer of Composition 5 and inner and outer layers of polyethylene terephthalate resin (hereafter simply referred to as PET (1)) having an intrinsic viscosity [η] (dissolved in a solvent mixture of 50 wt% of phenol and 50 wt% of tetrachloroethane and measured at a temperature of 30°C) of 0.75 was prepared by co-injection molding. Namely, using a co-injection molding device equipped with two injection cylinders A and B, PET (1) was charged in cylinder A (inner diameter of 38 mm) at a barrel temperature of 285°C and Composition 5 was charged in cylinder B (inner diameter of 16 mm) at a barrel temperature of 240°C. Then a part of PET (1) was injected first into a parison cavity set at a temperature of 20°C from a nozzle through a hot runner set at 280°C via a mold gate. The injection of PET (1) was discontinued for 1 to 2 seconds after the initiation of the injection and at the same time, Composition 5 was concentrically injected into the aforesaid parison cavity from the nozzle through the hot runner via the mold gate. After discontinuation of the injection for 0.1 second, PET (1) was continued to inject again together with Composition 5. By setting the amount of PET (1) injected in this case larger than the resin amount of Composition 5 injected simultaneously, filling was made in such a manner that the fluidizing tip of Composition 5 was finally enveloped in the fluidizing tip of PET (1). Then, the injection of Composition 5 was discontinued and somewhat thereafter, the injection of PET (1) was discontinued. Thus, Composition 5 could be completely enveloped by PET (1) by fully filling with the resin in the parison cavity. The total injection time was 2.8 seconds. After dwelling, the mold was opened and the obtained multi-layer parison was transferred to a temperature-controlled pot. The temperature of the multi-layer parison was set at 110°C and then the thus temperature-controlled multi-layer parison was transferred to a blow mold and drawn twice with drawing rods to the axis direction; almost simultaneously, the multi-layer parison was drawn 3 times with compressed air of approximately 10 bar to the peripheral direction to fit along the shape of the mold. After cooling, the product was withdrawn to prepare a container having a weight of 26 g and a volume of 700 ml. The total thickness of the thus obtained multi-layer draw blowing container was 300 µm at the peripheral side wall of the waist and the layer construction was inner layer of PET (1) (160µm) intermediate layer of Composition 5 (40µm)/outer layer of PET (1) (100µm). At the same time, a multilayer container was prepared in almost a similar manner except that a resin of EVOH alone (ethylene unit content of 32 mol%, melt flow index at 190°C under a load of 2160 g of 5.1 g/10 min) was used in place of Composition 5 and comparison was made. The total thickness of the thus obtained multi-layer draw blowing container was 300µm at the peripheral side wall of the waist and the layer construction was inner layer of PET (1) (160µm/ intermediate layer of EVOH (40µm /outer layer of PET (1) (100µm). Next, water containing carbon dioxide was filled up in these bottles and carbon dioxide gas permeability was measured with the passage of time. The results are shown in Table 3. Method of Evaluation (1) Permeability of Carbon Dioxide Gas Saturated aqueous carbon dioxide solution having a vapor pressure of 4 bar at 20°C was filled in a container and sealed. The container was put in a sealed box and the content of the box was purged with nitrogen gas, the moisture content of which had been adjusted to a RH of 65%. The carbon dioxide gas permeating through the container wall was detected by a Permatran C-IV carbon dioxide gas permeability measurement device manufactured by Modern Controls Co., Ltd. and the number of days until the carbon dioxide gas in the container reached 15% loss was measured. From the above table, it is understood that the storability of the container according to the present invention is improved. Example 13 and Comparative Example 7 Using a co-injection molding device equipped with three injection cylinders A, B and C, polypropylene (melt flow index at 230°C under a load of 2160 g = 7.0, specific gravity at 23°C = 0.91) was charged in cylinder A, maleic anhydride-graft modified polypropylene (amount of maleic anhydride for modification = 0.01 mol%, melt flow index at 230°C under a load of 2160 g = 7.0, specific gravity at 23°C = 0.91) was charged in cylinder B and Composition 5 prepared in Example 12 was charged in cylinder C. A mold for 4 cups (cup opening diameter of 72 mm, bottom diameter of 65 mm, height of 35 mm, wall thickness of 285 µm) was mounted and the temperature was set at 20°C. The barrel temperature of cylinders A and B was set at 240°C and that of C at 220°C. First, polypropylene was injected from cylinder A so as to fill 85% of the mold volume set at a temperature of 240°C through a hot runner block, a nozzle through and a gate. Next, the modified polypropylene was injected from cylinder B in a similar manner so as to fill 7.5% of the mold volume. Then Composition 5 was injected from cylinder C in a similar manner so as to fill 7.5% of the mold volume, and finally polypropylene was injected in a small quantity to seal the bottom outer layer with polypropylene. After dwelling and cooling, the mold was opened and the obtained 4 multi-layer cup-like containers were withdrawn. The construction was, starting from the outermost layer, polypropylene (225µm)/modified polypropylene (20µm)/Composition 5 (40µm)/modified polypropylene (20µm)/polypropylene (225µm) and the total thickness was 530 µm. Next, a multilayer container was prepared in almost a similar manner except that EVOH (ethylene unit content of 32 mol%, melt flow index at 190°C under a load of 2160 g of 5.1 g/10 mins., melting point of 181°C) was used in place of Composition 5 (Comparative Example 7). The layer construction was, starting from the outermost layer, polypropylene (225µm)/modified polypropylene (20µm)/EVOH (40µm)/modified polypropylene (20µm/polypropylene (225µm). The container was sealed with an aluminum foil lid and steam-treated at 120°C in a retort pot for 30, 60 and 120 minutes, respectively. Then, in such a state that water was charged in the inside of the cup, the opening of the cup was connected with a device for measuring oxygen gas permeability and the rate of oxygen permeability was measured. The results are shown in Table 4. The rate of oxygen permeability after the retort treatment was approximately three times or less than that of a container which was not retorted, and gas permeability resistance was as good as sufficient for storage of food. Example 14 and Comparative Example 8 Polyethylene terephthalate resin (melt flow index at 270°C under a load of 2160 g of 2.0 g/10 mins., 265°C), Composition 1 (melt flow index at 190°C under a load of 2160 g, 0.7 g/10 mins.) of Example 1 described above and,as an adhesive resin, modified ethylene-vinyl acetate resin having a vinyl acetate content of 24 wt% and a maleic anhydride modification degree of 1.1 wt% were supplied to 3 extruders to mold a multi-layer pipe having an outer diameter of 25 mm, composed of 3 kinds/3 layers of polyethylene trephthalate resin/adhesive resin/Composition 1/adhesive resin/polyethylene terephthalate resin. This pipe was cut into a length of 130 mm and a bottom was formed at one end of the pipe. Thereafter the neck was processed so as to make a cap mountable to prepare a closed-end parison. This closed-end parison was heated to 110°C and stretched by 2.2 times with stretching rods to the axis direction and almost simultaneously, nitrogen gas under a pressure of 12 bar was blown to stretch by 2.8 times to the peripheral direction. Thus, a biaxially stretched blow bottle having a volume of 1.5 liters was formed. In this case, the blow mold was kept at 15°C by chilled water. Then after dwelling, the mold was opened to give a multi-layer bottle (Example 14). For purpose of comparison, a multi-layer bottle having the same size prepared in a similar manner except for using EVOH (ethylene unit content of 32 mol%, melt flow index at 190°C under a load of 2160 g, 1.3 g/10 mins.) in place of Composition 1 (Comparative Example 8). The layer construction of the wall and the time required until the carbon dioxide loss reached 15% are shown in Table 5. It is evident that the bottle according to the present invention has extremely excellent carbon dioxide gas barrier properties. Example 15 and Comparative Example 9 Three extruders A, B and C were used; isotactic polypropylene resin (melt index at 190°C under a load of 2160 g = 0.8 g/10 min, specific gravity at 23°C = 0.91) was charged in extruder A, maleic anhydride-graft modified polypropylene (amount of maleic anhydride for modification = 0.001 mol%, melt index at 190°C under a load of 2160 g = 1.0, specific gravity at 23°C = 0.91) was charged in extruder B at a barrel temperature of 220°C and Composition 1 prepared in Example 1 was charged in extruder C at a barrel temperature of 220°C. The mixture was combined in a die for a multi-layer circular parison kept at a temperature of 240°C so as to form 3 kinds/5 layers of isotactic polypropylene/maleic anhydride-graft modified polypropylene/Composition 1/maleic anhydride-graft modified polypropylene/isotactic polypropylene to form a multi-layer parison. The parison was subjected to known direct blow molding (blow molding of melt parison) to form a multi-layer bottle (Example 15). Next, a multi-layer bottle was molded in almost a similar manner except for using EVOH Composition 1 (ethylene unit content of 32 mol%, melt index of 1.3 g/10 min) (Comparative Example 9). The bottle had a weight of 22 g and an inner volume of 500ml. The average thickness and construction was polypropylene (1-50 µm)/maleic anhydride-graft modified polypropylene (20 µm), Composition 1 (30µm)/maleic anhydride-graft modified polypropylene (20 µm)/polypropylene (150µm) (Example 15) and polypropylene (150µm)/maleic anhydride-graft modified polypropylene (20µm)/EVOH (30µm)/maleic anhydride-graft modified polypropylene (20µm)/polypropylene (150µm) (Comparative Example 19). Next, water was charged in each bottle and sealed with a cap and an inside plug. After it was steam-treated at 120°C in a retort pot for 30, 60 and 120 minutes,respectively, the bottle was withdrawn and the opening of the cup was connected with a device for measuring oxygen gas permeability and the rate of oxygen permeability was measured. The results are shown in Table 6. The rate of oxygen permeability after the retort treatment of Example 15 was approximately comparable to that of a container which was not retorted, and the gas barrier property was as good as sufficient for storage of food. Example 16 Using a multi-layer injection molding machine equipped with 3 injection cylinders, 3 kinds of resins were injected into an inversed conical mold having an opening of 65 mm and a height of 65 mm to give a parison having a 5 layer structure of polypropylene/adhesive resin/composition/adhesive resin/polypropylene. Polypropylene and the adhesive resin used herein were E-103D manufactured by Ube Industries, Ltd. and Admer QB530 manufactured by Mitsui Petrochemical Industries, Ltd. The composition was EVOH containing the drying agent grains shown in Example 1 (Composition 1). The parison was subjected to direct blow to prepare an almost cylindrical container having an opening of 65 mm and a height of 70 mm. The construction thickness was 300/8/45/8/300 µm. Water was charged in the container and a metal-made lid was mounted by double-seaming. After the container was treated in a retort pot at 120°C for 30, 60 and 120 minutes, respectively, water was charged in the inside of the cup. In such a state, 2 pipes were mounted to the lid of the container and connected with a device for measurement of oxygen permeability. The results are shown in Table 7. A part of the cup container prior to the retort treatment was cut, taken out and heated in xylene at 120°C, whereby polypropylene and the adhesive resin were melted out to give a film of the composition in the container. The dispersion state of the powders in this film was observed by an optical microscope. With respect to 10 samples collected from the wall of the container at different locations, grains having a long diameter of 10 µm or more were measured within a region of 200 µm x 200 µm with their average diameter, respectively, and the volume-area average diameter was calculated to be 17.5 µm. Example 17 Molding was performed in a manner similar to Example 16 except that a parison having a 3 layer structure of polypropylene/Composition 1 of Example 1/polypropylene was prepared using a multi-layer injection molding machine equipped with 2 injection cylinders. Thus a container having a thickness construction of 300/45/300 µm was prepared. Also with respect to the container, a metal-made lid was mounted and the rate of oxygen permeability after the retort treatment was measured. The results are shown in Table 7. Comparative Examples 10 and 11 A container having a thickness construction of 300/45/45/45/300 µm was obtained in a manner similar to Example 16 except that EVOH (ethylene unit content of 32 mol%, melt index of 1.3 g/10 min , melting point of 181°C) was used in place of Composition 1 (Comparative Example 10). Further a container having a thickness construction of 300/45/45/45/300 µm was obtained in a manner similar to Comparative Example 10 except that adhesive resin (Admer QB530) containing 10 wt% of fine powder of drying agent (disodium phosphate) was used in place of the adhesive resin of Comparative Example 10 (Comparative Example 11). With respect to the containers of Comparative Examples 10 and 11, a metal-made lid was mounted thereto in a manner similar to Example 16 and the rate of oxygen permeability after the retort treatment was measured. The results are also shown in Table 7. The rates of oxygen permeability of the containers of Examples 16 and 17 after the retort treatment were lower than those in Comparative Examples 10 and 11 and they were excellent in storability.

Claims: A composition comprising a matrix of an ethylene-vinyl alcohol copolymer having dispersed therein a drying agent in a particulate state, characterized in that the volume-area average diameter of the grains of the drying agent having a long diameter of not less than 10 µm is not greater than 30 µm. A composition as claimed in claim 1 wherein the amount of said drying agent used in said composition is 3 to 50 wt%. A composition as claimed in claim 1 wherein said ethylene-vinyl alcohol copolymer is a saponified ethylene-vinyl acetate copolymer having an ethylene unit content of 25 to 60 mol% and a saponification degree in the vinyl acetate component of at least 96%. A composition as claimed in claim 1 wherein said drying agent is a salt capable of forming a hydrate. A composition as claimed in claim 1 wherein said drying agent comprises as a main ingredient monosodium phosphate, disodium phosphate, trisodium phosphate, trilithium phosphate, sodium pyrophosphate or a mixture of at least two of them. A process for producing the composition according to claim 1 which comprises kneading an ethylene-vinyl alcohol copolymer and a drying agent comprising fine grains wherein at least 99.9% (volume fraction) of the grains having a size of not greater than 30 µm in a weight ratio ranging from 97 : 3 to 50 : 50 at a unit work of at least 0.1 kwh/kg using a continuous twin rotor kneader. A process for producing a composition as claimed in claim 6 wherein said unit work is 0.2 to 0.8 kwh/kg. A process for producing a composition as claimed in claim 6 wherein said ethylene-vinyl alcohol copolymer is a saponified ethylene-vinyl acetate copolymer having an ethylene unit content of 25 to 60 mol% and the vinyl a saponification degree in the vinyl acetate component of at least 96%. A process for producing a composition as claimed in claim 6 wherein said drying agent is a salt capable of forming a hydrate. A process for producing a composition as claimed in claim 6 wherein said drying agent comprises as a main ingredient monosodium phosphate, disodium phosphate, trisodium phosphate, trilithium phosphate, sodium pyrophosphate or a mixture of at least two of them. A multi-layer structure comprising at least one layer of the composition according to any one of claims 1 to 5.

Inventors: MORITANI, TOHEI
Applicants: KURARAY CO., LTD.
Application year: 1987
Main classification: B32B27
IPC codes: C08L29, C08J3, C08K3, B32B27, C08L23