JPH0449861B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a polystyrene resin foam sheet suitable for forming containers such as fruit trays and fresh fish trays by vacuum forming or pressure forming. Since the foam sheet of the present invention is highly flexible, it is useful as a cushioning material used as a partition between fruits. Polystyrene resin foam sheets (so-called styrene paper) are used as a material for forming trays for fresh fish and fruits (Utility Model Application No. 58-92040). Normally, such styrene paper is produced by supplying a composition in which polystyrene pellets and a nucleating agent such as silica sand, talc, calcium carbonate, etc. are mixed in a mixer from a hopper of an extruder, and then using the extruder to
Melt-kneaded at 250℃, butane, pentane,
After adding blowing agents such as methylene chloride and dichlorodifluoromethane, the cooling zone of the same extruder was heated to 160°C.
℃, then extruded from a die into a tubular foam.
After cooling, the tubular foam sheet is cut into two sides using a cutter. (Tokuko Showa 43-
No. 15709). In addition, this sheet is sometimes manufactured using separate extruders for kneading and cooling (Japanese Patent Publication No. 25781/1989). The thus produced polystyrene paper having a wall thickness of 1 to 3 mm and an apparent density of 0.05 to 0.2 g/cm ² is vacuum formed and used as a tray for fresh fish, vegetables, meat, etc. It is also laminated with other sheets, such as crosslinked polyethylene foam sheets, and vacuum formed to be used as trays for fruits such as peaches and pears. When this polystyrene paper tray is used as a packaging material for meat and seafood, it is usually wrapped in a wrapping film, but since it lacks flexibility (impact resistance), the tray is wrapped when wrapping this film. If the force is too strong, it may be damaged. The present invention attempts to solve the drawbacks of conventional styrene paper by adding a styrene-butadiene-styrene block copolymer and a metal salt of a higher fatty acid to polystyrene to improve its flexibility. That is, the present invention includes (A) polystyrene 100 parts by weight (B) styrene-butadiene-styrene block copolymer 5-20 parts by weight (C) nucleating agent 0.2-5 parts by weight (D) metal salt of higher fatty acid 0.05-1 Parts by weight A sheet obtained by foaming a resin composition containing the above components (A), (B), (C), and (D) using a foaming agent, the sheet having an apparent density of 0.05 to The present invention provides a polystyrene resin foam sheet characterized by a weight of 0.2 g/cm ² . In carrying out the present invention, the polystyrene used as component (A) has a number average molecular weight (Mn) of 8 x ¹⁰⁴ to 2.2 x ¹⁰⁵ and a weight average molecular weight (Mw) of 10 x ¹⁰⁴ to ₅ x 105. can. The styrene-butadiene-styrene block copolymer of component (B) generally has an average molecular weight of 25,000 to 1,000,000.
It is sold under the trade names TR1111 and TR1112 and by Asahi Kasei under the trade name Toughprene. This styrene-butadiene-styrene block copolymer is rubbery. As an impact modifier for polystyrene, SBR,
Rubber materials such as NBR, buna rubber, HIPS, and butyl rubber are disclosed, but styrene-butadiene-styrene block copolymer has the best compatibility with polystyrene. The styrene-butadiene-styrene block copolymer is blended in an amount of 5 to 20 parts by weight per 100 parts by weight of component (A). If the amount is less than 5 parts by weight, the effect of improving impact resistance will be low. On the other hand, if it exceeds 20 parts by weight, the bending strength of the resulting sheet will drop significantly, which is not preferable. Next, the nucleating agent (C) is useful for uniformly adjusting the cell diameter of the foam, and includes talc, magnesium carbonate, perlite, calcium silicate, calcium carbonate, silicon oxide, paraita, and vermiculite. Inorganic nucleating agents such as citric acid, tartaric acid, boric acid,
Organic acids such as oxalic acid are used. This nucleating agent is used in a proportion of 0.2 to 5 parts by weight per 100 parts by weight of polystyrene as component (A). The metal salts of higher fatty acids as component (D) include metal salts of higher fatty acids having 12 to 22 carbon atoms, such as capric acid, stearic acid, palmitic acid (Ca, Na,
K, Li, Zn, Al). Particularly suitable are zinc stearate, aluminum stearate and calcium stearate. This metal salt of higher fatty acid improves the processability during extrusion molding of the sheet and also improves the flexibility of the foam sheet. This metal salt of higher fatty acid is
It is used in a proportion of 0.05 to 1 part by weight. If the amount is less than 0.05 part by weight, the effect of improving the flexibility of the sheet is small. If the amount exceeds 1 part by weight, there is a drawback that the film forming stability of the sheet deteriorates. The resin composition containing components (A), (B), (C), and (D) is produced by heating and melting a low-boiling organic compound as a foaming agent in an extruder and extruding it into a cylindrical shape through a die. Formed into a sheet. The foaming agent may be press-injected into the extruder through a pipe as described above, but the polystyrene resin composition is impregnated with the foaming agent in advance, and then this is supplied from the hopper of the extruder, melt-kneaded, and extruded. It may be foamed and molded. The tray obtained by thermoforming the foam sheet of the present invention has good impact resistance. Hereinafter, the present invention will be explained in more detail with reference to Examples. Example 1 (A) 100 parts of polystyrene having the following physical properties Number average molecular weight 2.0×10 ⁵ Weight average molecular weight 1.0×10 ⁵ (B) 10 parts of styrene-butadiene-styrene block copolymer “Califlex 1102” (C) Talc 2.5 parts (D) Zinc stearate 0.3 parts The above resin composition was supplied to the first extruder, and the extruder melted and kneaded the resin at 230°C and 150Kg/cm ² G, and then poured butane at 230°C. 160Kg/ ^cm2 from the entrance
A mixture containing 4% butane gas is transferred to the cooling zone of the extruder using a screw in the extruder, and in this cooling zone the mixture of butane gas and molten polystyrene is screw-kneaded to make it more uniform. Cooled to ℃. Next, an inflation sheet with a diameter of 100 mmφ is extruded from a cylindrical die, and this sheet is internally cooled and expanded in diameter with a mandrel. Then, one place of this inflation sheet is cut away from the mandrel to form a single foam. It was collected as a sheet (collection speed 5m/min). The foam sheet had an average wall thickness of about 2.3 mm, a sheet width of 790 mm, and an apparent density of 0.09 g/cm ² . This foam sheet was made into a 170mm sheet using a vacuum forming machine.
Heat and melt at ℃, vacuum mold (450mmHg) using a 15-cavity mold, and then trim to obtain an average wall thickness of 3.0mm, length 195mm, width 126mm, and depth 22mm.
15 mm trays were obtained. Grasp both sides of this tray with your hands and instantaneously bend it 90 degrees in the longitudinal direction.The degree of breakage is then quantified into the following four levels, and used as a guideline for cell resistance (the lower the number, the better). The result was 0. 0=No cracking. 1 = Cracks occur only on either the bottom or both sides. 2 = Cracks occur on both sides or either side and the bottom. 3 = Cracks occur on both sides and the bottom. Further, a three-point bending test was conducted using an Autograph manufactured by Shimadzu Corporation, with the longitudinal center of the tray pushed up from the bottom surface. The bending strength at this time is 2.0Kg
It was hot. Furthermore, using the autograph described above, the repulsive force of the tray was determined when the tray was deformed by 12.5 mm by sandwiching the center of both sides in the longitudinal direction.
It was 0.8Kg. If this repulsive force is large, when packaging using a tray, it becomes easier to cover the wrap film over the tray and resist the tightening force when the wrap film is stretched. That is, even a thin sheet can be used, resulting in cost reduction. Furthermore, the flexibility is determined by applying a load of 1Kg/cm ² to a 10mm thick material (sheets are stacked so that the thickness is 10mm).
It is expressed in compressive strain (mm) when applied to It can be said that the material with a larger value is softer. The result is 0.8mm
It was hot. Examples 2 to 4, Comparative Examples 1 to 3 Foam sheets were prepared in the same manner as in Example 1, except that the compositions of components (A), (B), (C), and (D) were changed as shown in Table 1. A tray was obtained by vacuum forming. Table 1 shows the physical properties of the sheet and tray. Comparative Example 4 A foam sheet was prepared in the same manner as in Example 1, except that styrene-butadiene random copolymer rubber (SBR) was used instead of the styrene-butadiene-styrene block copolymer (SBS) of component (B). A tray was obtained by vacuum forming. Table 1 shows the physical properties of the sheet and tray.

【table】

[Table] * Polystyrene

Claims (1)

[Claims] 1 (A) Polystyrene 100 parts by weight (B) Styrene/butadiene/styrene block copolymer 5 to 20 parts by weight (C) Nucleating agent 0.2 to 5 parts by weight (D) Metal salt of higher fatty acid 0.05 ~1 part by weight A sheet obtained by foaming a resin composition containing the above components (A), (B), (C), and (D) using a foaming agent, the sheet having an apparent density of A polystyrene resin foam sheet having a weight of 0.05 to 0.2 g/cm ² . 2. The foam sheet according to claim 1, wherein the metal salt of higher fatty acid is zinc stearate, calcium stearate, or aluminum stearate.