THERMOFORMABLE VINYLIDENE CHLORIDE RESIN COMPOSITION AND RESIN LAMINATE PREPARED THEREFROM

Abstract: This improved thermoformable vinylidene chloride resin compsn. comprises (A) vinylidene chloride resin composed of 90 to 99 wt.% of vinylidene chloride and 1 to 10 wt.% of acrylic or methacrylic monomer, (B) a low-molecular or high -molecular plasticiser (0.5 to 5 wt.% of the resin) and (C) a condensed phosphate (0.2 to 5 wt.% of the resin). The condensed phosphate is an alkali metal salt having a structure in which phosphoric acid gps. are linked to each other linearly (1) or in a ring (2). The linear condensed phosphate (1) is represented by the general formula (I) where M is an alkali metal, esp. sodium or potassium, n is at least 2. The specific linear condensed phosphate can be pyrophosphate, tripolyphosphate, tetrapolyphosphate, etc. The ring condensed phosphate (2) is represented by the general formula (MPO3)m where M is an alkali metal esp. sodium or potassium; m is at least 3. Specifically, the ring condensed phosphates can be trimetaphosphate, tetrametaphosphate or hexametaphosphate. The condensed phosphates are fine powders in which the mean particle size lies between 5 and 80 microns. The resin laminate comprises layers of the vinylidene chloride resin compsn. (described above) and layers of thermoplastic resin having a thermoforming temp. higher than that of the vinylidene chloride resin.

Description: The present invention relates to a thermoformable vinylidene chloride resin composition. More particularly, the present invention relates to a vinylidene chloride resin composition in which decomposition foaming or discoloration is controlled at the forming step and which has an excellent gas permeability and is valuable as the material for the production of a packaging material. Moreover, the present invention relates to a resin laminate comprising this resin composition. A vinylidene chloride resin has excellent oxygen-barrier property, and among various oxygen-barrier resins, the vinylidene chloride resin has an extremely low water vapor permeability and the humidity dependency of the oxygen permeability of this resin is extremely small. Therefore, the vinylidene chloride resin is widely used as a gas-insulating packaging material. In the case where the vinylidene chloride resin is used in the field of a packaging material, a so-called aqueous latex of the vinylidene chloride resin is applied to the formation of a vessel because the vinylidene chloride resin is easily thermally decomposed. One example is that a latex of the vinylidene chloride resin is coated on a finished formed body of other resin (see, for example, the specification of U.S Patent No. 4,370,368). And another example is that an aqueous latex of the vinylidene chloride resin is coated on a polyester preform (parison) for draw-blow forming of a polyester bottle (see the specification of U.S. Patent No. 4,393,106). It has already been known that a vessel such as a bottle is prepared by thermoforming a vinylidene chloride resin, and for example, the specification of U.S. Patent No. 4,041,235 teaches that a vinylidene chloride copolymer containing 0.5 to 10% by weight of glycidyl methacrylate as an internal heat stabilizer is extruded into a parison and the parison is blow-formed into a bottle. Furthermore, the specification of U.S. Patent No. 4,123,477 teaches that a film having excellent transparency and gas-barrier property is prepared by the inflation film-forming process from a resin composition comprising 100 parts by weight of a copolymer comprising 75 to 95% by weight of vinylidene chloride and 5 to 25% by weight of a comonomer and 3 to 60 parts by weight of a chlorinated ethylene/vinyl ester copolymer. US 4 418 168 discloses a thermoformable vinylidene chloride resin comprising 70 to 96 wt% vinylidene chloride and 5 to 30wt% of one or more monoethylenically unsaturated monomers. The composition comprises a plasticizer such as 0.2 to 3wt% epoxidized soybean oil and a polyphosphate salt. GB 1 238 216 discloses a resin laminate comprising a vinylidene chloride resin layer and a thermoplastic resin layer. The vinylidene chloride resin comprises at least 70 wt% vinylidene chloride. 0.4 to 1 wt% of the vinylidene chloride of inorganic phosphate and up to 15% of the total weight of the mixture of ethylene/vinyl acetate copolymer can be present. Use of a vinylidene chloride resin in the form of an aqueous latex for the production of a packaging material is not advantageous from the economical viewpoint, because troublesome operations such as coating and drying are necessary and a large quantity of heat energy is necessary for evaporation and removal of water. A resin composition formed by incorporating a low-molecular-weight or high-molecular-weight lubricant at a high concentration into a vinylidene chloride resin can resist the thermoforming operation conducted at such a relatively low temperature as 170 to 180°C, but at the extrusion molding or injection molding conducted at higher temperatures, foaming or discoloration is caused by dehydrohalogenating thermal decomposition, and the inherent gas-barrier property is lost. In the case where a multi-layer laminate formed by combining a vinylidene chloride resin as a gas-barrier layer with other resin is used as a packaging material, simultaneous melt-extrusion molding of the vinylidene chloride resin with other resin, or co-injection molding or sequential injection of the vinylidene chloride resin with other resin, becomes necessary. Accordingly, the vinylidene chloride resin layer is exposed to a high temperature possessed by other resin, and therefore, the above-mentioned troubles such as foaming, discoloration and reduction of mechanical properties are caused. Incorporation of a large quantity of a plasticizer for imparting a low-temperature thermoformability to a vinylidene chloride resin is not preferred, because the gas-barrier property of a final packaging material is degraded and especially, the gas-barrier property of the final packaging material after the retort sterilization is drastically degraded, and therefore, it is preferred that the amount incorporated of the plasticizer be controlled to a level as low as possible. The present invention seeks to provide a thermoformable vinylidene chloride resin composition in which foaming, discloration and reduction of mechanical properties by thermal decomposition are controlled even if the composition is exposed to a high temperature, and which is therefore used advantageously for the production of a packaging resin laminate. According to the invention there is provided a thermoformable vinylidene chloride resin composition comprising: A) a vinylidene chloride resin comprising 90 to 99 wt% of vinylidene chloride and 1 to 10 wt % of an acrylic or methacrylic monomer' B) 0.5 to 5 wt% of plasticizer, and C) 0.2 to 5% of a condensed phosphoric acid salt characterised in that the plasticizer comprises: B1 an expoxy plasticizer and B2 an ethylene/vinyl acetate copolymer the weight ratio of B1/B2 being from 100/5 to 100/400. The present invention also found that when this vinylidene chloride resin composition is used for the lamination with a thermoplastic resin having a thermoforming temperature higher than the thermoforming temperature of the vinylidene chloride resin, a packaging resin laminate having excellent gas-barrier property is obtained. The present invention is based on the finding that when a condensed phosphoric acid such as sodium tripolyphosphate is incorporated, together with a small amount of a plasticizer, into a thermoformable vinylidene chloride resin, even if the resulting composition is exposed to a high temperature, thermal decomposition is controlled and foaming, discoloration or other deterioration can be effectively prevented. Thermal decomposition of the vinylidene chloride resin is the dehydrochlorination of the polymer chain, which is an autocatalytic reaction in which hydrogen chloride formed by thermal decomposition acts as a catalyst for the decomposition. Moreover, if a minute amount of a metal such as iron, zinc or chromium is present, the metal acts as a catalyst for this decomposition reaction. In the present invention, the condensed phosphoric acid salt exerts a function of controlling thermal decomposition of the vinylidene chloride resin at high temperatures. It is construed that this function is due to the fact that the condensed phosphoric acid salt effectively catches hydrogen chloride isolated from the vinylidene chloride resin and controls the chain transfer in the dehydrochlorination of the vinylidene chloride resin, and the condensed phosphoric acid salt sequesters metals incorporated from a kneader, a mold or the like. The vinylidene chloride resin used in the present invention comprises 90 to 99% by weight, especially 92 to 98% by weight, of vinylidene chloride and 1 to 10% by weight, especially 2 to 8% by weight, of an acrylic or methacrylic monomer. This feature is important for maintaining a good balance between the thermoformability and the barrier property to various gases. Namely, if the amount of the comonomer is smaller than the lower limit of the above range, thermoforming becomes difficult, and if the amount of the comonomer exceeds the upper limit of the above range, the gas-barrier property is degraded. The kind of the comonomer is important from the viewpoint of the gas-barrier property. When vinyl chloride or other vinyl monomer is used, the gas-barrier property is insufficient. A low-molecular-weight or high-molecular-weight plasticizer is incorporated in the resin composition of the present invention for maintaining a good melt-formability. One of the characteristic features of the resin composition of the present invention is that the content of the plasticizer is relatively low and 0.5 to 5% by weight, especially 1 to 3% by weight, based on the vinylidene chloride resin. Therefore, the trend toward reduction of the gas-barrier property by the presence of the plasticizer can be controlled. This reduction of the amount of the plasticizer can be attained by incorporation of the condensed phosphoric acid salt into the vinylidene chloride composition. The condensed phosphoric acid salt is incorporated in an amount of 0.2 to 5% by weight, especially 0.5 to 3% by weight, based on the vinylidene chloride resin. If the amount of the condensed phosphoric acid salt is smaller than the lower limit of the above range, foaming, discoloration and reduction of physical properties by thermal decomposition can hardly be avoided. If the amount of the condensed phosphoric acid salt exceeds the upper limit of the above range, no particular effect is attained with respect to the heat stability but because of the hygroscopic property of the condensed phosphoric acid salt, the gas-barrier property is adversely influenced. By dint of the foregoing functions, according to the present invention, foaming, discoloration and reduction of physical properties by thermal decomposition on exposure to high temperatures are prominently controlled, and a high melt-formability can be maintained with a reduced amount of the incorporated plasticizer and a packaging resin formed article excellent in the barrier property to various gases can be easily obtained by melt-forming. Moreover, if the vinylidene chloride resin composition of the present invention is used, lamination with other thermoplastic resin having a thermo-forming temperature higher than the thermo-forming temperature of the vinylidene chloride resin can be advantageously performed without thermal deterioration of the vinylidene chloride resin. Preferred Embodiments of the Invention Condensed phosphoric acid salt As the condensed phosphoric acid salt, there can be mentioned salts, especially alkali metal salts, in which phosphoric radicals are connected linearly or in a ring. As the linear condensed phosphoric acid salt, there can be mentioned condensed phosphoric acid salts represented by the following formula: wherein M stands for an alkali metal, especially sodium or potassium, and n is a number of at least 2, such as pyrophosphoric acid salts, tripolyphosphoric acid salts and tetrapolyphosphoric acid salts. As the cyclic condensed phosphoric acid salt, there can be mentioned condensed phosphoric acid salts represented by the following formula: (MPO₃) m (2) wherein M is as defined above, and m is a number of at least 3, such as trimetaphosphoric acid salts, tetrametaphosphoric acid salts and hexametaphosphoric acid salts. Potassium metaphosphate and sodium metaphosphate include a variety of salts ranging from salts having a low molecular weight to salts having such a high molecular weight as 1 x 10⁴ or more, for example, Kurrol's salt and Graham's salt. Any of these salts can be used for attaining the object of the present invention. In view of the dispersibility of the condensed polyphosphoric acid salt in the vinylidene chloride resin, it is preferred that the condensed phosphoric acid salt be used in the form of a fine powder having an average particle size of 5 to 80 µm, especially 10 to 30 µm. Vinylidene chloride resin The vinylidene chloride resin used in the present invention is a copolymer comprising 90 to 99% by weight, especially 92 to 98% by weight, of vinylidene chloride and 1 to 10% by weight, especially 2 to 8% by weight, of an acrylic or methacrylic monomer. As the acrylic or methacrylic monomer, there can be mentioned monomers represented by the following formula: wherein R₁ stands for a hydrogen atom or a methyl group and X stands for a nitrile group (-C≡N) or a group (in which R₂ is an organic group). As preferred examples, there can be mentioned acrylonitrile, methacrylonitrile, methyl acrylate, ethyl acrylate, butyl acrylate, 2-hydroxyethyl acrylate, glycidyl acrylate, methyl methacrylate, ethyl methacrylate, 2-hydroxypropyl methacrylate, glycidyl methacrylate, β-methoxyethyl methacrylate and β-aminoethyl methacrylate, though monomers that can be used in the present invention are not limited to those mentioned above. As the vinylidene chloride resin especially preferable for attaining the object of the present invention, there can be mentioned (i) a copolymer of vinylidene chloride with at least one monomer selected from the group consisting of acrylic acid esters and methacrylic acid esters, and (ii) a copolymer of vinylidene chloride with at least one monomer selected from the group consisting of acrylic acid esters and methacrylic acid esters and at least one monomer selected from the group consisting of acrylonitrile and methacrylonitrile. The molecular weight of the vinylidene chloride resin is not particularly critical, so far as the vinylidene chloride resin has a film-forming molecular weight. It is preferred that under conditions of a temperature of 20°C and a relative humidity of 100%, the oxygen permeation coefficient of the vinylidene chloride resin be smaller than 9 x 10⁻¹⁴ cm ³ ·cm/cm²·sec·cmHg and the water vapor permeation coefficient (JIS Z-0208) be smaller than 3 x 10⁻³ g·cm/m²·day. In the present invention, in order to impart a melt-formability to the vinylidene chloride resin, a plasticizer comprising a first component B-1 of an epoxy plasticizer and an ethylene/vinyl acetate copolymer component B-2 is incorporated into the vinylidene chloride resin. The epoxy type plasticizer preferably comprises epoxidized soybean oil, epoxidized castor oil, epoxidized linseed fatty acid ester, epoxidized methyl stearate, epoxidized butyl stearate or epoxidized octyl stearate. However, phthalic acid ester type plasticizer, adipic, sebacic or azelaic acid diester type plasticizers, phosphoric acid ester type plasticizers, hydroxy polyvalent carboxylic acid, ester type plasticizers, fatty acid ester type plasticizers, polyhydric alcohol type plasticizers and polyester type plasticizers may be used in combination with epoxy type plasticizers. Components B-2 comprises an ethylene/vinyl acetate copolymer. An ethylene/vinyl acetate copolymer having an ethylene content of 50 to 95% by weight, especially 65 to 90% by weight, a vinyl acetate content of 5 to 50% by weight, especially 10 to 35% by weight, and a melt flow rate (MFR) of 0.2 to 20 g/10 min, especially 1 to 10 g/10 min, is most preferred. The weight ratio of the plasticizer (B-1) to the plasticizer (B-2) a (B-1)/(B-2) is from of from 100/5 to 100/400, especially from 100/30 to 100/300. Composition According to the present invention, the above-mentioned vinylidene chloride resin (A), low-molecular-weight or high-molecular-weight plasticizer (B) and condensed phosphoric acid salt (C) are mixed at the above-mentioned mixing ratios to form a composition for forming a packaging material. More specifically, these components are dry-blended by using a mixing machine such as a ribbon blender, a conical blender or a Henschel mixer, and the dry blend can be directly supplied to a melt-kneading device of an extruder or a melt-kneading device of an injection machine. Alternatively, the above components can be melt-kneaded and formed into pellets in advance by a Banbury mixer, a kneader, a roll or an extrusion type pelletizer. Known additives such as lubricants, other heat stabilizers, colorants, fillers and antioxidants can be incorporated according to known recipes into the resin composition of the present invention, if desired. The vinylidene chloride resin composition of the present invention can be solely used for the production of packaging vessels. Namely, the resin composition can be formed into various film vessels, cup-shaped vessels and tray-shaped vessels by extruding the resin composition into a film or sheet and, if necessary, subjecting the film or sheet to plug-assist forming, air-pressure forming, vacuum forming or press forming. Furthermore, a hollow vessel such as a bottle or tank can be obtained by extruding the resin composition into a cylinder and blow-forming the cylinder in a split mold. Moreover, a vessel having the wall biaxially molecularly oriented can be obtained by injection-molding the resin composition into a bottomed parison and biaxially draw-forming the parison. Laminate As pointed out hereinbefore, the vinylidene chloride resin composition of the present invention is characterized in that the thermal deterioration is prominently controlled even at high temperatures. By utilizing this property, the vinylidene chloride resin composition of the present invention can be used for the production of a resin laminate comprising a vinylidene chloride resin layer as a gas-barrier layer. As the resin having a thermoforming temperature higher than the thermoforming temperature of the vinylidene chloride resin, there can be mentioned olefin resins having an excellent heat resistance, such as polypropylene, a crystalline propylene/ethylene copolymer and a crystalline propylene/butene-1 copolymer, aromatic polyesters such as polyethylene terephthalate and poly(bisphenol A terephthalate/isophthalate), polycarbonates and polyamides. The resin laminate can be prepared according to known customary procedures, except that the above-mentioned vinylidene chloride resin composition is used. For example, a layer of the vinylidene chloride resin (PVDC) and a layer of the high-melting-point resin (HMR) are simultaneously melt-extruded, if necessary together with an intermediate layer of an adhesive (AD), through a multi-layer multi-ply die to obtain a multi-layer film, sheet, pipe or parison. In the formed multi-layer structure, the vinylidene chloride resin composition forms a gas-barrier layer and the high-melting-point resin acts as a stress support. Even if the vinylidene chloride resin composition is exposed to extremely high temperatures in the multi-layer multi-ply die, thermal decomposition is not caused but an excellent gas-barrier property can be maintained. Preferred examples of the laminate structure are described below, though laminate structures that can be adopted are not limited to those exemplified below. Two-resin/two-layer structure PVDC/HMR Two-resin/three-layer structure HMR/PVDC/HMR Three-resin/three-layer structure PVDC/AD/HMR Three-resin/five layer structure HMR/AD/PVDC/AD/HMR Three-resin/eight-layer structure HMR/AD/PVDC/AD/HMR/AD/PVDC/AD/HMR Four-resin/six-layer structure HMR/(HMR+PVDC+AD)/(AD/PVDC/AD/HMR) As the adhesive layer, there can be used an ethylene/vinyl acetate copolymer, an ethylene/acrylic acid ester copolymer, an ionomer and an acid- or acid anhydride-modified propylene type polymer for the propylene type resin layer, and there can be used a copolyester hot adhesive for the polyester layer. The multi-layer sheet can be punched into an appropriate shape and formed into a multi-layer cup-shaped or tray-shaped vessel by plug-assist forming or press forming. The multi-layer parison can be formed into a hollow vessel such as a bottle by blow-forming the parison in the molten state in a split mold. Furthermore, the multi-layer pipe can be formed into a biaxially drawn vessel by closing one end of the multi-layer pipe to obtain a bottomed preform, stretch-drawing the preform in the axial direction at a drawing temperature and simultaneously expansion-drawing the preform in the circumferential direction. From the viewpoint of the gas-barrier property, it is preferred that in the so-obtained multi-layer vessel, the thickness of the vinylidene chloride resin layer is 5 to 400 µm, especially 10 to 300 µm. The thickness of the high-melting-point resin layer is changed according to the kind of the resin or whether or not the resin is molecularly oriented, but it is preferred that the thickness of the high-melting-point resin layer be 50 to 5000 µm, especially 100 to 4000 µm. The resin laminate can also be prepared according to the so-called multi-layer injecction method. For example, there can be adopted a process in which a number, corresponding to the number of the kinds of the resins, of injection cylinders are used and a multi-layer resin flow is injected into an injection mold through a multi-layer multi-ply orifice, and a process in which the high-melting-point resin is injected to the midway of a cavity in an injection mold and the vinylidene chloride resin composition is injected as an intermediate layer, whereby a multi-layer container or a preform for formation of a multi-layer container can be obtained. The vinylidene chloride resin composition of the present invention shows excellent heat resistance at a melting heat treatment for molding, a heat treatment for drawing and a heat treatment for orientation and fixation. Moreover, the vinylidene chloride resin composition of the present invention can sufficiently resist heat treatments for sterilizing a content to be filled in a container, such as a retort sterilization treatment, a thermofilling treatment and a germ-free filling treatment. Examples 1 through 4 and Comparative Examples 1 through 10 A composition comprising amounts (% by weight), shown in Table 1, of a vinylidene chloride/methyl acrylate copolymer resin (PVDC) having a methyl acrylate content of 7% by weight, epoxidized soybean oil (ESO), an ethylene/vinyl acetate copolymer resin (EVA) having a vinyl acetate content of 35% by weight and anhydrous sodium pyrophosphate (TSPP) having an average particle size of 30 µm was formed into a film having a thickness of 30 µm by a film-forming apparatus comprising an extruder having a screw diameter of 50 mm, a T-die, two vertical chill rolls and a take-up device. At this film-forming operation, the temperature of the extruder was set so that the resin extruded from the T-die was maintained at a temperature shown in Table 1. With respect to each of the resin compositions shown in Table 1, the film-forming property, the yellowing degree (JIS K-7103-1970) of the obtained film, the oxygen gas permeation coefficient of the film at a temperature of 23°C and a relative humidity of 60% and the oxygen gas permeation coefficient of the film after the treatment with steam at 120°C for 30 minutes in a high-temperature high-pressure kettle were determined. The obtained results are shown in Table 1. Example 5 A sheet having a symmetric three-resin/five layer structure (PP/AD/PVDC/AD/PP) and a total thickness of 0.8 mm (thickness ratio = 43/3/8/3/43) was formed by the co-extrusion method using the vinylidene chloride resin composition (PVDC) used in Example 1 as the intermediate layer, an ethylene/vinyl acetate copolymer having a melt flow rate of 1.0 g/10 min and a vinyl acetate content of 36% by weight as the adhesive layer (AD) and isotactic polypropylene (PP) having a melt flow rate of 0.5 g/10 min as the inner and outer surface layers. The temperatures of three extruders were set so that the extrusion temperature for the vinylidene chloride resin was 175°C, the extrusion temperature for the adhesive resin was 192°C, and the extrusion temperature for the polypropylene was 225°C. In the formed sheet, foaming or scorching by decomposition of the vinylidene chloride resin layer was not observed at all, and the oxygen gas permeation constant of the sheet was 4.3 x 10⁻¹⁴ cc·cm/cm²·sec·cmHg as determined at a temperature of 23°C. Comparative Examples 11, 12 and 13 Formation of a sheet was tried in the same manner as described in Examle 5 except that the vinylidene chloride resin composition used in Comparative Example 1, 3 or 5 was used instead of the vinylidene chloride resin used in Example 4. In each case, yellowing (yellowing degree higher than 60) was caused within 10 to 20 minutes from the point of initiation of the extrusion, and after passage of 30 minutes, scorching and foaming were caused by thermal decomposition and forming of the sheet became impossible.

Claims: A thermoformable vinylidene chloride resin composition comprising: A) a vinylidene chloride resin comprising 90 to 99 wt% of vinylidene chloride and 1 to 10 wt % of an acrylic or methacrylic monomer, B) 0.5 to 5 wt% of plasticizer, and C) 0.2 to 5% of a condensed phosphoric acid salt characterised in that the plasticizer comprises: B1 an epoxy plasticizer and B2 an ethylene/vinyl acetate copolymer the weight ratio of B1/B2 being from 100/5 to 100/400. The resin composition of claim 1 wherein the condensed phosphoric acid salt is an alkali metal salt of condensed phosphoric acid in which the phosphoric acid radical is linked linearly or cyclically. The resin composition of claim 1, wherein the condensed phosphoric acid salt is represented by the following formula: wherein M stands for an alkali metal and n is a number of at least 2. The resin composition of claim 3 wherein the linear condensed phosphate is a pyrophosphate, tripolyphosphate or tetrapolyphosphate. The resin composition of claim 2 wherein the cyclic condensed phosphate is represented by the following general formula: (MPO₃) m wherein M represents an alkali metal, especially sodium or potassium, and m is a number of at least 3. The resin composition of claim 2 wherein the cyclic condensed phosphate is a trimetaphosphate, a tetrametaphosphate or hexametaphosphate. The resin composition of any one of claims 1 to 6 wherein th condensed phosphate is used in the form of a fine powder having an average particle diameter of 5 to 80 µm. A thermoformable vinylidene resin composition as claimed in claim 1, wherein said epoxy plasticizer is selected from the group consisting of epoxidized soybean oil, epoxidized castor oil, epozidized butyl linseed fatty acid ester, epoxidized methyl stearate, epoxidized butyl stearate and epoxidized octyl stearate. A thermoformable vinylidene chloride resin composition as claimed in claim 1, wherein said ethylene/vinyl acetate copolymer has an ethylene content of 50 to 95 % by weight, a vinyl acetate content of 5 to 50 % by weight and a melt flow rate (MFR) of 0.2 to 20 g/10 min. A resin laminate comprising a layer of vinylidene chloride resin as claimed in any one of the preceding claims and a layer of a thermoplastic resin having a thermoforming temperature higher than the thermoforming temperature of the vinylidene chloride resin.

Inventors: KANO, FUMIO, YAMADA, MUNEKI
Applicants: TOYO SEIKAN KAISHA, LTD.
Application year: 1987
Main classification: C08L27
IPC codes: C08K3, C08L27, C08K5, B32B27