JPS6326770B2 - - Google Patents

Description

[Detailed Description of the Invention] Polyolefin foams such as polyethylene are used in a wide range of applications such as packaging materials, cushioning materials, and insulation materials, but when it is necessary to obtain foams with a high magnification and a complex shape, Generally, a method is used in which a pre-foamed body of cross-linked small particle diameter pellets is prepared and then fusion-molded in a mold. At this time, it is desirable that the cells of the pre-expanded particles be uniform and fine, since the uniformity of the cells of the pre-expanded particles affects the visual appearance of the cross section of the product, the mechanical strength of the product, the cooling time during molding, etc. In order to make the cells of foams, including polystyrene foam, finer,
Various organic or inorganic additives are used. When foaming polyethylene or the like, inorganic additives such as talc are often kneaded and added to the resin and used as a nucleating agent for foaming. As described below, conventionally reported additives such as talc are more effective in making bubbles finer than additive-free additives to some extent.

However, in recent years, the quality of polyolefin foams has improved more and more than ever before, and higher quality is now required, and conventional additives are not always sufficient to make the cells uniform and fine.

In order to obtain a foam having uniform and fine cells in a method for producing a foam using a polyethylene/polystyrene composite as a base material particle, the present inventors have conducted intensive studies to find that inorganic calcium salts and higher fatty acid metal salts We have found that the combination of

The base particles used in the present invention are polyethylene/polystyrene composite particles containing 30 to 70 wt% polystyrene obtained by impregnating and polymerizing polyethylene particles with a styrene monomer.

2-10wt instead of ethylene homopolymer
Ethylene-vinyl acetate copolymer, ethylene-acrylic acid ester copolymer containing % comonomer,
It may also be an ethylene-α-olefin copolymer, or a mixture of two or more of these.

Polyethylene/polystyrene composite particles are produced by gradually adding styrene monomer containing a polymerization initiator to a suspension of polyethylene particles dispersed in water, as described in Japanese Patent Publication No. 52-33156. It can be obtained by impregnation and polymerization.

As described in Japanese Patent Publication No. 47-27336, there are many known inorganic additives used for bubble control, such as alumina, talc, zinc white, and silica. However, in the present invention, inorganic calcium salts such as calcium sulfate, calcium carbonate, and calcium silicate are used in combination with higher fatty acid metal salts such as calcium stearate. As a result, it has been found that a foam having uniform, fine cells, which cannot be obtained using conventional inorganic additives alone, can be obtained.

Inorganic calcium salt accounts for 50% of cumulative particle size distribution
It is desirable that the particle size is about 3 to 10 μm, and if the particle size is smaller or larger than this, the bubble regulating effect will be reduced.

As the higher fatty acid metal salt, salts of calcium, barium, zinc, etc. such as palmitic acid, stearic acid, oleic acid, and erucic acid can be used.

It is desirable that the total amount of the above two types of additives be in the range of 0.1 to 1.0 parts based on 100 parts of polyethylene. The inorganic calcium salt and the higher fatty acid salt are most effective in uniformly making the bubbles fine when added in a weight ratio of 2:1 to 6:1.

These additives are preferably kneaded and added during the granulation of polyethylene pellets with a small particle size.

As the crosslinking agent used in the present invention, organic peroxides such as dicumyl peroxide, 2,5-dimethylperoxyhexane, di-t-butyl perphthalate, and t-butyl hydroperoxide can be used. The amount added varies depending on the type of crosslinking agent and the type of base material particles, and is appropriately determined so that the gel fraction determined by hot xylene extraction is 10 to 70%.

The method of adding the crosslinking agent is to solubilize or finely disperse the crosslinking agent in water containing a surfactant and add it to a suspension of base material particles dispersed in water.
Either a method of impregnating the polymer at a temperature of 0.degree. C. or a method of adding the polymerization initiator to the styrene monomer in advance is used.

As the evaporative blowing agent, saturated or unsaturated aliphatic hydrocarbons such as butane, propane, isopentane, and isobutylene, and halogenated hydrocarbons such as dichlorofluoromethane and dichlorotetrafluoroethane are used. Foams can be obtained by exposing the particles to liquid heating media such as water vapor, heated air, and silicone oil.

The present invention will be specifically explained below using examples, but the present invention is not limited to the examples unless it departs from the gist.

Example 1 High-pressure polyethylene (Sumikasen manufactured by Sumitomo Chemical)
Calcium silicate (SW400 manufactured by Shiraishi Kogyo Co., Ltd.; particle size (D ₅₀ ), 4) per 100 parts of F1103-1)
After adding 0.3 parts and 0.1 parts of calcium stearate (manufactured by Kyodo Yakuhin Co., Ltd.) and thoroughly kneading,
The mixture was granulated to obtain pellet-like polyethylene particles.

Take 100 parts of these particles and 200 parts of water in a pressure-resistant container, and add 0.04 parts of the surfactant sodium dodecylbenzenesulfonate.
1 part and 0.1 part of a suspending agent magnesium pyrophosphate were added, and the temperature was raised to 85°C while stirring. In this system,
A solution of 1 part of peroxybenzoate and 0.7 part of dicumyl peroxide dissolved in 100 parts of styrene monomer was gradually added dropwise to impregnate and polymerize the styrene monomer.

After the dropwise addition was completed, the temperature of the system was raised to 140°C, and stirring was continued for 4 hours to effect crosslinking. Thereafter, the pellets were cooled to room temperature and washed with a large amount of water acidified with hydrochloric acid to obtain polyethylene/polystyrene composite base particles. The gel fraction of this particle (in boiling xylene)
The weight fraction of the residue extracted for 8 hours was 20.1%.
Take 100g of this base material particle in a pressure container and add 300g of butane.
g was added and the system was held at 60°C for 4 hours to reduce the
Expandable base material particles containing butane were obtained. Place this in a polyethylene glycol bath at 100℃ and
By heating for a minute, foamed particles with an expansion ratio of about 60 times were obtained. The cross section of these foamed particles is extremely uniform and made up of fine bubbles, and the bubble diameter is 0.1
At ~0.3mm, there was almost no difference between the area near the epidermis and the center of the foam.

Example 2 Polyethylene/polystyrene composite base material particles were foamed in the same manner as in Example 1 except that 0.5 part of calcium silicate and 0.1 part of calcium stearate were used, resulting in foamed particles having the same cell state as in Example 1. was gotten.

Comparative Example 1 Polyethylene/polystyrene composite base material particles were foamed in the same manner as in Example 1 except that the raw material polyethylene was not added with any inorganic additives, and the cells were coarse with a diameter of 0.8 to 1.5 mm. Only non-uniform foams were obtained.

Comparative Example 2 Polyethylene/polystyrene composite base material particles were foamed in the same manner as in Example 1 except that only talc (Micron White #5000S manufactured by Hayashi Kasei) was used as an additive. In the cross section of this foam, the particles were non-uniform and the cells were larger than in Example 1.

Comparative Example 3 Polyethylene/polystyrene composite base material particles were foamed in the same manner as in Example 1, except that only silica (Dicalite, manufactured by Hayashi Kasei; particle size (D ₅₀ ), 3) was used as an additive. . In the cross section of this foam, the particles were non-uniform and the cells were larger than in Example 1.

Claims (1)

[Claims] 1 Polyethylene/polystyrene composite obtained by kneading and adding a total of 0.1 to 1.0 parts by weight of inorganic calcium salts and higher fatty acid metal salts to 100 parts by weight of polyethylene and forming pellets, impregnating and polymerizing styrene monomer. The particles are used as base particles, and the base particles are treated with an organic peroxide to have a gel fraction of 10 to 10.
A method for producing polyethylene foam particles having uniform fine cells, which comprises crosslinking to 70wt% and then foaming using an evaporative foaming agent.