JPS62109834A - Production of expandable styrene-modified polyolefin particle - Google Patents

Abstract

PURPOSE:To obtain the titled particles capable of giving a foam having improved elasticity and cushioning property, by impregnating a specific foaming agent in polymerizing styrene monomer in the coexistence of polyolefin particles containing a bisamide compound in an aqueous suspension system. CONSTITUTION:(A) Styrene monomer and (B) 40-500pts.wt., based on 100pts.wt. styrene monomer, polyolefin, preferably particles containing 0.01-10pts.wt., based on 100pts.wt. low-density polyethylene, bisamide expressed by the formula (R<1> and R<2> are 1-22C alkyl; R<3> and R<4> are H or 1-22C alkyl; n is an integer 1-4), preferably stearic acid ethylenebisamide are suspended in an aqueous medium and polymerized. In the process, (C) a foaming agent consisting of an organic solvent having a lower boiling point than the softening temperature of the formed polystyrene, e.g. propane or butane, is impregnated in the styrene monomer whether it is before or after the polymerization to afford the aimed expandable polyolefin particles.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field of Application] The present invention relates to a method for producing expandable polyolefin particles that provide foams with excellent elasticity and cushioning properties. More particularly, the present invention relates to a method for producing modified expandable polyolefin particles modified with styrene. From another perspective,
The present invention relates to a method for producing expandable polystyrene particles modified with polyolefins.

[Prior art]

Polymerize styrene monomer in an aqueous suspension system, and impregnate the styrene monomer or the resulting polystyrene with a blowing agent consisting of an organic solvent with a boiling point lower than the softening point of the resulting polystyrene, such as butane or pentane, to produce expandable polystyrene particles. This is well known. It is also known to coexist polyethylene particles in this aqueous suspension system to obtain polyethylene-modified expandable polystyrene particles. It is also known to obtain expandable polyolefin particles by impregnating particles of polyolefin, particularly low-density polyethylene, high-density polyethylene, and polypropylene, with the above-mentioned blowing agents.

Generally, polystyrene foam is molded by heating such expandable polystyrene particles with steam to pre-foam them, and then going through an aging process to seal the resin but not the heating fluid such as steam. This is done by filling the container with water, heating it with steam, and foaming it.

However, this polystyrene foam has a drawback in that it has a low elastic density and has a weak ability to recover from strain against repeated external stresses, resulting in deformation of its shape.

Polyolefin foams have the advantage of being highly elastic and have a large ability to recover from repeated stresses, but mold foam molding can only form simple plate shapes; In order to obtain a shaped foam, it is necessary to perform operations such as cutting, gluing, and die-cutting the low-density foam formed by extrusion foaming or mold foaming, and the resulting product is expensive.
The drawback is that it is not very practical.

Expandable polyolefin particles that can be molded by steam heating and foaming have been developed, but because of the rapid gas diffusion during foaming, it is difficult to set the molding conditions, and it is necessary to cure the particles after foaming.

If the purpose is simply to restore the elasticity of the polystyrene foam, it may be possible to blend pre-foamed polystyrene particles and pre-foamed particles made by impregnating polyethylene with a foaming agent and perform foam molding. The fusion of different types of beads is poor, making it impossible to produce a practical foam molded product.

In consideration of these conventional drawbacks, we will produce modified polystyrene particles that can be formed by steam-heated foam to provide a foam with excellent elasticity and cushioning properties. There is a method of producing expandable styrene-modified polyethylene particles by dispersing styrene in an aqueous medium, compressing a physical blowing agent in the form of a gas or liquid in a normal state, and subjecting the styrene to suspension polymerization under heat and pressure. proposed (see IP!j Publication No. 44-2469 and Special Publication No. 45-32623). In addition, polyethylene particles, styrene monomer, and polymerization catalyst are dispersed in an aqueous medium and subjected to turbid polymerization under heating and pressure.The resulting styrene-modified polyethylene particles are impregnated with a physical blowing agent to produce expandable styrene. A method for producing modified polyethylene particles has also been proposed (JP-A-48-101457, JP-A-49
-85187 and Japanese Unexamined Patent Publication No. 49-97884).

The expandable styrene-modified polyethylene particles produced by these improved methods have a structure in which polystyrene particles are dispersed in polyethylene, and the styrene-grafted polyethylene produced during polymerization acts as an interpolymer that binds polyethylene and polystyrene. By using these expandable styrene-modified polyethylene particles, it is possible to foam to a certain degree of desired ratio and to obtain a foam with even foam and bubbles. However, when these expandable styrene-modified polyethylenes have a high styrene content, for example, when polyethylene is used at 5 to 30 tJlg (7) per 100 parts by weight of styrene, they give foams with relatively uniform cells; As the amount of polyethylene used increases, the uniformity of the cells in the resulting foam deteriorates, reducing its commercial value.Additionally, the rate of advancement of the blowing agent increases, making it impossible to foam at a high ratio. There tends to be no.

In order to solve the problems of the conventional improved methods, a method was proposed (Japanese Patent Application No. 108738/1982).

The method uses styrene monomer FI! in an aqueous suspension system in which styrene monomer and polyethylene particles are dispersed. match,
At that time, when manufacturing polyethylene-modified expandable polystyrene particles by impregnating the styrene monomer with a blowing agent consisting of an organic solvent having a boiling point lower than the softening temperature of the polystyrene produced, the amount of the polyethylene particles is 50 to 400 parts by weight per 100 parts by weight of monomer, the organic solvent contains 40% by weight or more of isobutane, and the styrene monomer is added to the polyethylene particles in the aqueous suspension system at a temperature at which the polyethylene softens. After impregnation, the temperature of the aqueous suspension system is further raised to polymerize the styrene monomer.

According to this prior invention, isobutane is used as a blowing agent at 40%
It has been confirmed that when the styrene monomer is used in a weight percent or more, and by providing a pre-process in which the styrene monomer is sufficiently impregnated into the polyethylene particles before being substantially polymerized, the effect of reducing the dissipation of the blowing agent and making the cells of the foam uniformly fine has been confirmed. Ta.

[Problem that the invention seeks to solve]

However, according to the method of this prior invention, when isobutane is pressure-injected into the reactor during or after polymerization, the pressure inside the reactor increases if the temperature inside the reactor is high, so the pressure resistance of the reactor used is insufficient. Must be high.

This problem can be solved by using a blowing agent with a relatively high boiling point, such as normal butane or pentane, but in that case, a foam having uniform, fine cells cannot be obtained.

[Specific measures to solve the problem]

The present invention provides a solution to the above-mentioned problems and provides an expandable styrene foam that provides a foam with uniform and fine cells even when using not only a low-boiling point blowing agent such as inbutane or methyl chloride, but also a relatively high-boiling point blowing agent. This method aims to produce high-quality polyolefin particles, and provides uniform expandable styrene-modified polyolefin particles with small cell diameters by incorporating a specific bisamide compound having a nucleating effect into the polyolefin particles dispersed in water. be.

(Structure of the Invention) The present invention involves polymerizing a styrene monomer in an aqueous suspension system in which the styrene monomer and polyolefin particles are dispersed, and in which an organic compound having a boiling point lower than the softening temperature of the polystyrene produced regardless of before or after the polymerization of the styrene monomer. In manufacturing expandable styrene-modified polyolefin particles by impregnating them with a blowing agent consisting of a solvent, the polyolefin particles
Per part by weight, a bisamide compound represented by the following formula [wherein R1 and R2 are alkyl groups having 1 to 22 carbon atoms;
3 and R4 are hydrogen atoms or alkyl groups having 1 to 22 carbon atoms, and are integers of nu1 to 4. ] The present invention provides a method for producing expandable styrene-modified polyolefin particles, characterized in that the particles contain 0.01 to 10 parts by weight of styrene-modified polyolefin particles.

(Styrene Monomer) As mentioned above, it is known to produce expandable polystyrene particles by suspension polymerization of styrene monomer, and this known technique can be used as long as it does not go against the spirit of the present invention.

Therefore, in addition to styrene, the "styrene monomer" includes styrene and/or side-chain substituted styrene such as chlorostyrenes, vinyltoluenes, α-methylstyrene, etc. alone or in mixtures thereof, or in small amounts that can be copolymerized. Other monomers such as acrylonitrile, alkyl methacrylate (alkyl portion has about 1 to 4 carbon atoms), acrylic acid alkyl ester (alkyl portion has about 1 to 4 carbon atoms), mono- to dialkyl maleate (alkyl portion has about 1 to 4 carbon atoms), 4), divinylbenzene, mono- or diacrylic acid or methacrylic acid ester of ethylene glycol, and mixtures with others are used.

(Polyolefin particles) Examples of polyolefins include low-density polyethylene, linear low-density polyethylene, high-density polyethylene, ethylene-propylene copolymer, ethylene-propylene copolymer, ethylene-butene-1 copolymer,
Ethylene-based resins such as ethylene-acetate-vinyl copolymer, ethylene-acrylic acid copolymer; propylene homocopolymer, propylene-containing ethylene copolymer L 7'robylene-butene-1 copolymer, propylene-containing ethylene-butene -1
Copolymers, propylene resins such as propylene/4-methylpentene-1 copolymer, mixtures of two or more of these, or these olefin resins as the main component,
In addition, mixtures containing polystyrene, polyamide, polyvinyl chloride, polyethylene terephthalate, etc. can be used.

Among these, the density is 0.91 to 0.94 P/cd
Melt index (MI) is 0.5 to 20 y/1
Polyethylene of 0.05°C is preferred. This is because such polyethylene has the most remarkable effect of improving the elasticity and cushioning properties of polystyrene foam, and polyethylene having a density and melt index within the above range is easily available commercially. .

The polyolefin particles are present in the suspension polymerization system of styrene monomers, and are in such a way that they themselves become foamable or (and) facilitate impregnation of the styrene monomers into the polyolefin. Therefore, it is preferable that the particles have a relatively small particle size. Generally, powder or pellets having an average particle size of about 0.05 to 31 are preferable. If the particle size is too large, not only will it be difficult to disperse during polymerization, but the speed at which the styrene monomer impregnates the polyolefin during polymerization will be slow, resulting in a longer reaction time. When using polyethylene gold with a large size of 4 to 8 g, the time required for the polyolefin to swell with the styrene monomer becomes longer, and the resulting foamable resin mass only needs to be crushed, so the particle size of the polyolefin should be adjusted accordingly. It is not necessarily critical to the invention.

Addition of polyolefin to styrene monomer t t
ri, styrene monomer 100! [40 against Okibe
-500 parts by weight, preferably 80-200 parts by weight. If it is less than 40 parts by weight, it will not exhibit the desired elasticity and elastic recovery, and if it exceeds 500 parts by weight, sufficient fusion between particles will not be obtained during steam molding.

(Bisamide Compound) A bisamide compound having a nucleating agent effect has the following properties: [In the formula, R' and 1 (2 are alkyl groups having 1 to 22 carbon atoms; Rj and R4 are H or an alkyl group having 1 to 22 carbon atoms] and 10 is a number from 1 to 4.Specifically, ethylene bisamide stearate, methylene hisamide stearate, ethylene/bisamide oleate, etc. are used.Among these, Stearic acid ethylene bisamide is preferred from the viewpoint of cost.

This bisamide compound is added to polyolefin/
Contained in particles. If the amount is less than 0.01 parts by weight, the cells of the resulting foam will be too large, while if more than 10 parts by weight is added, the increase in the effect will be small.

The bisamide compound-containing polyolefin particles can be obtained by melting and dispersing the bisamide compound in a polyolefin and granulating it.

Specifically, for example, a mixture of a polyolefin and a bisamide compound may be melt-extruded into strands and cut into pellets.

As nucleating agents for foamed resin, inorganic fillers such as talc and silica; metal soaps such as zinc stearate and aluminum stearate are known, and although they have a certain degree of fine cell effect, their use is limited to polyolefins. It is not used for polystyrene.

This bismaleimide compound may be used in combination with other nucleating agents such as silica, talc, and metal soap.

(Blowing Agent) As is well known, a physical blowing agent is a compound that is a liquid at room temperature or a normal gas that forms under pressure (in this invention, these compounds are referred to as organic solvents). It does not dissolve polystyrene made from styrene monomer, but only swells it, and its boiling point is below the softening point of polystyrene. In the present invention, since polyolefin is present in the suspension polymerization system, this blowing agent must not dissolve the polyolefin used or only swell it at most, and must have a boiling point below the softening point of the polyolefin used. It is desirable that there be.

The blowing agent in the present invention can be any compound as defined above. Specifically, for example, aliphatic hydrocarbons such as propane, normal butane, isobutane, normal pentane, impentane, neopentane, and hexano;
Examples include alicyclic hydrocarbons such as cyclobuta/cyclopentane, halogenated hydrocarbons such as methyl chloride and dichlorodifluoromethane, and other types or mixtures.

(Polymerization of styrene monomer and introduction of blowing agent) The mode of polymerization of the styrene monomer and the mode of introduction of the blowing agent (regardless of before or after polymerization) of the present invention are as follows. The manufacturing method is essentially the same.

To suspend the polyolefin particles, bisamide compound and styrene monomer in an aqueous medium, water-soluble polymers such as polyvinyl alcohol, polyvinylpyrrolidone, methyl cellulose, etc. It is common to use suspension stabilizers such as substances or sparingly soluble inorganic substances such as calcium phosphate or magnesium oxide. The concentration of this aqueous suspension agent is between 0.1 and 5! with respect to the polyolefin particles. It is about i%.

Impregnation is carried out by heating this aqueous suspension at a temperature at which the polyolefin used softens. Suitably, the softening temperature of the polyolefin is much higher than that, especially a temperature at which the polyolefin can maintain its particle shape. It is generally carried out at a temperature of 50 to 140°C. At excessively high temperatures, there is a greater tendency for the polyolefin particles to fuse together and form agglomerates, and there is also a greater tendency for premature polymerization of the styrene monomer to occur. The pressure may be self-generated pressure, increased pressure or reduced pressure, or any other arbitrary pressure.

Such nesting is carried out until at least 80% by weight, preferably at least 90% by weight, of the styrene monomer is impregnated or adhered to the polyolefin particles, that is, the free styrene monomer droplets are less than 20% by weight, preferably 10% by weight or more. The aqueous suspension is preferably left under stirring to the extent that the amount is less than the weight ratio.

The stirring time is about 1 to 4 hours. Evidence of free styrene monomer can be determined by the following method. That is, an arbitrary amount of the aqueous suspension is sampled and this is
Separate the polyolefin particles and the liquid phase by quickly using a wire mesh of about 0.00 mesh to separate the polyolefin particles and the liquid phase, measure the amount of styrene monomer in the liquid phase, and calculate the ratio of the amount of free styrene monomer from this value and the styrene monomer charging chamber. Calculate 〇 A polymerization initiator for the styrene monomer (details will be described later) should be introduced in this step, and it is preferable to dissolve the polymerization initiator in the styrene monomer.

Other modifiers are also preferably introduced in this step. For example, plasticizers such as toluene, xylene, ethyl acetate, dioctyl phthalate, and tetratalolethylene are effective in stabilizing the expansion rate during pre-expansion when molding modified expandable polystyrene particles, which are chemicals. However, it can be impregnated into polyolefin particles together with styrene monomer.

Virtually does not decompose (up to about 10'Ntht) during the styrene monomer impregnation process for polyolefin particles,
Polymerization initiator tl that decomposes in the polymerization step at a higher temperature than this step
- It is better to use selectively.

Such polymerization initiators are generally radical polymerization initiators having a decomposition temperature of 50 to 110° C. to obtain a half-life of 10 hours, and specifically include, for example, t-butyl peroxyvivalate, lauroyl Peroxide (benzoyl peroxide, cyclohexanone peroxide), t-butyl peroxybenzoate, t-butyl peroxy-2-ethylhexanoate, t-butyl peroxyhexahydroterephthalate, n-butyl-4
, 4-his(t-butylperoxy)valerate, 1.
Examples include 1-bis(t-butylperoxy)-3,3°5-trimethylcyclohexane, di-t-butylperoxybutane, and t-butylperoxyisopropyl carbonate.

The polymerization initiator should be oil-soluble and is preferably used dissolved in the styrene monomer.

The amount of polymerization initiator used is usually 0.1 to 5 parts by weight, preferably 0.3 to 3 M parts per 100 parts of styrene monomer.

The temperature of the aqueous suspension thus prepared is raised to reach a temperature at which the polymerization initiator used is sufficiently decomposed. 1 The moisture content may not be constant throughout the process; in any case, the heating is continued for a sufficient period of time to ensure complete polymerization of the styrene monomer.

The heating time is, for example, about 7 to 20 hours.

Sufficient stirring should be carried out at least at the beginning of the polymerization process.

The polymerization pressure can be a self-generated pressure or a deviation of the applied pressure.AO (Introduction of blowing agent) The above-mentioned blowing agent is impregnated into the styrene monomer before, during and after polymerization. be able to. If the styrene monomer is impregnated into the polyolefin particles in advance, it will be present in the polyolefin particles regardless of whether before or after polymerization, so the impregnation of the blowing agent can also be said to be the impregnation of the polyolefin particles.

As mentioned above, one typical example of the time of impregnation with a blowing agent is when the polyolefin particles are impregnated with a styrene monomer. Another typical example of the time of impregnation with the blowing agent is when styrene-modified polyolefin particles are obtained after the completion of polymerization, and this may be carried out according to a conventional method, including the case where it is introduced during polymerization.

The blowing agent is usually supplied so that the content in the produced particles is about 5 to 30% by weight.

When impregnating the styrene-modified polyolefin particles with the blowing agent, heating or the use of a swelling agent or any other auxiliary means may be used to facilitate the impregnation of the blowing agent into the styrene-modified polyolefin particles. It is normal to do so.

As mentioned above, plasticizer substances can also be introduced.

(Product) The expandable styrene-modified polyolefin particles according to the present invention essentially consist of expandable polystyrene particles and polyolefin coexisting in a suspension polymerization system. Not the same as polyolefins. That is, in the process of polymerizing the styrene monomer of the present invention, the styrene monomer is not only polymerized to form polystyrene, but also partially graft-polymerized to the polyolefin. Therefore, the obtained product is a homogeneously dispersed composition consisting of polyolefin/polyolefin-polystyrene copolymer (including graft products) and polystyrene.

Polyolefin/polystyrene copolymers or graft products are compatible with both polystyrene and polyethylene, so foam-molded products using the expandable beads obtained by carrying out the present invention are free from fusion between beads. In good condition.

The degree of fusion referred to here is the degree to which the expanded particles have softened and the particle surfaces have fused to each other, which is quantified by folding a test piece, observing the cross section, and visual judgment. be.

0% means that all of the fractures in the test piece are due to surface peeling of particles. 100% means no surface peeling and cohesive failure of the particles.

(Experimental Example) In the following experimental example, the apparent density, bending strength, compressive strength, and cell structure of the foam were determined as follows.

1) Fill a container with an apparent density of pre-expanded particles with a total internal volume of 21, and reduce the weight (
Wy) was calculated, and the apparent counterfeiting W (V/11) (=T) was calculated.

2) Bending strength According to JISA9511-1969, 25mm thickness x 75
(2) Measurements were made using a test piece with a width of 300 m and a length of 300 m.

3) Compressive strength JI8A9511-1969, 50■×50■X
Using a test piece of 50+a+, the load was measured when compressed at 25 inches.
The diameter of the bubbles was determined by measuring the diameter of the bubbles under 0x magnification.

5) Fusion degree: When a foam product is broken by hand, it indicates the percentage of particles remaining on the fracture surface.

Examples 1 to 3 Low density polyethylene Yucalon HE-60 manufactured by Mitsubishi Yuka ■
A mixture of 100 parts by weight of (trade name, specific gravity 0.91) and 0.2 parts of ethylene bisamide stearate was melt-kneaded in an extruder and extruded into strands through a goo,
After cooling, it was cut to obtain polyethylene particles each having a weight of about 0.8119 and having ethylene bisamide stearate uniformly dispersed therein.

Next, in a pressure-resistant polymerization container, 500 parts by weight of water, 10 parts by weight of tribasic calcium phosphate, 0.015 parts by weight of sodium dodecylbenzenesulfonate, 100 parts by weight of the above polyethylene particles containing ethylene bisamide stearate, and 6 parts by weight of xylene were added. was added and stirred to obtain an aqueous suspension.

Next, while stirring this aqueous suspension, t-butylperoxy-2-ethylhexanoate l, which is a polymerization initiator, was added.
After adding dropwise a solution of 1.2 parts by weight and 1.2 parts by weight of t-butylperoxybenzoate in 200 parts by weight of methione,
The polyethylene particles were heated at 72°C for 6 hours to soften the styrene into the particles, and then the aqueous @
The suspension was heated at 80°C for 4 hours, then raised to 125°C, and then heated at 125°C for 2 hours to polymerize styrene.

The aqueous suspension was then cooled at 60'C''1 and then treated with butane (
45 parts by weight of n-butane (70 cm) and isobutane (30 cm) were pressurized into the aqueous suspension, maintained for 3 hours, cooled, and the polymer particles were separated from the aqueous phase, pickled, washed with water, and dried. The blowing agent content was about ioZ of the polymer particle width.

In addition, the addition ratio of polyethylene particles containing stearic acid ethylene bisamide to 100 parts by weight of Methyref was 13
0 parts by weight (Example 2) and 200 parts by weight (Example 3), one cup was carried out to obtain each polymer particle (in addition,
The content of the blowing agent is approximately 8.6 to 8.91i-% for each particle.
The amount of blowing agent added was adjusted so that

Pre-foam the polymer particles with steam using a batch-type pre-foaming machine with a steam pressure of 0.4 ky/caG,
The apparent density and number of bubbles of the obtained pre-expanded particles were measured.
It was filled into a mold with a mold cavity of 400 mm x 50 mm and a large number of vapor permeation holes, and the steam pressure was 1.0 kg/c-
The pre-expanded particles were heated for 30 seconds with jQ to expand and fuse together, and then cooled to obtain a molded article.

Samples for strength measurement were cut out from these molded products, and the compressive strength and bending strength were measured. The measured values were as shown in Table 1.

(Space below) Example 4 Polymerization was carried out in the same manner as in Example 1 except that the amount of ethylene bisamide stearate added was changed from 0 to 5 parts by weight per 100 parts by weight of polyethylene particles. Particles were obtained.

The blowing agent content t of each obtained polymer particle was 9.5±0
．． It was 5%. These particles were pre-foamed in the same manner as in Example 1, the apparent density of the pre-foamed particles was determined, and the diameter of the cells in the pre-foamed particles was also examined.

The results are shown in Table 2.

(The following is a blank space) Example 5 When polymerization was carried out using pentane instead of butane in Example 1, the cell diameter of the pre-foamed product was 0.3 mm.

Example 6 In Example 1, propylene containing INM% of ethylene bisamide stearate was used instead of polyethylene.
Ethylene (4.5%) lantum copolymer “Mitsubishi Yuka (-
100 parts by weight of Mitsubishi Noprene FX-4' (trade name) particles, 1 part by weight of styrene dissolved gxool containing 1 part by weight of t-butyl peroxybenzoate, 400 parts by weight of water, 8 parts by weight of tribasic calcium phosphate, dodecylbenzenesulfone 0.012 ml of sodium chloride was charged into a pressure-resistant reactor, and heated at 90°C for 3 hours and at 105°C for 2 hours with stirring.
After polymerization at 25°C for 4 hours, 40 parts by weight of butane was injected, the valve at the bottom of the autoclave was opened, and the slurry was discharged into a wire mesh container at room temperature and pressure.
Pre-expanded particles of r/J were obtained.

After acid-washing and water-washing the pre-expanded particles, they were dried for one day.
It is filled into a mold with a mold cavity of 400 m in length, 400 m in width and 50 m in thickness, and has steam permeation holes, and the steam pressure is 3 k.
g/dG steam for 20 seconds to fuse each other, and then cooled to obtain a good foamed molded product.

The cell size of this molded product was as good as 0.2.

Claims (1)

[Scope of Claims] 1) A styrene monomer is polymerized in an aqueous suspension system in which styrene monomer and polyolefin particles are dispersed, and an organic compound having a boiling point lower than the softening temperature of polystyrene produced regardless of before or after the styrene monomer is polymerized. When manufacturing expandable styrene-modified polyolefin particles by impregnating them with a blowing agent consisting of a solvent, per 100 parts by weight of the polyolefin particles, a bisamide compound represented by the following formula ▲ There are mathematical formulas, chemical formulas, tables, etc. ▼ [Formula In the formula, R^1 and R^2 are alkyl groups having 1 to 22 carbon atoms; R^3 and R^4 are hydrogen atoms or hydrogen atoms having 1 to 2 carbon atoms;
2 alkyl group; n is an integer from 1 to 4. ] A method for producing expandable styrene-modified polyolefin particles, characterized in that the particles contain 0.01 to 10 parts by weight. 2) The method according to claim 1, wherein the bisamide compound is ethylene bisamide stearate. 3) The method according to claim 1, wherein the polyolefin particles are used in an amount of 40 to 500 parts by weight per 100 parts by weight of the styrene monomer. 4) The method according to claim 1, wherein the polyolefin particles are low density polyethylene particles.