JP2021054931A - Styrene composite polyethylene based resin foam particulate, production method thereof, and foam molded article - Google Patents

Abstract

To provide a foam molded article excellent in appearance, dimensional stability or the like and foam particulates for producing the foam molded article.SOLUTION: Styrene composite polyethylene based resin foam particulates contains a polyethylene based resin and a polystyrene based resin as base resins. The polyethylene based resin has a content of vinyl acetate of 1 mass% to 15 mass% relative to the total mass of the polyethylene based resin. The polyethylene based resin has a melting point of 95°C or higher and lower than 110°C. The styrene composite polyethylene based resin foam particulates have 0 to 2 large air bubbles having a bubble diameter of three or more times the average bubble diameter in the central part of the particulate.SELECTED DRAWING: Figure 1

Description

The present invention relates to styrene composite polyethylene-based resin foamed particles, a method for producing the same, and a foamed molded product.

Foam molded products made of polyolefin resins generally have high elasticity and high strain recovery against repeated stresses, as well as excellent oil resistance and crack resistance. Have. Therefore, it is widely used as a packaging material and an automobile member (eg, a cushioning material for automobiles). However, the polyolefin-based resin foam molded product has a disadvantage that the rigidity is low and the foamed molded product easily shrinks after in-mold molding. As a method for improving such a disadvantage, a method of impregnating a polyethylene resin with a styrene monomer and polymerizing the styrene composite polyethylene resin obtained by impregnating the polyethylene resin with a styrene monomer and using the styrene composite polyethylene resin as a base resin is known.

It is known that the physical properties and appearance of the foamed molded product are affected by the state of bubbles in the foamed particles. For example, bubble control of a styrene composite polyolefin resin has been proposed in Japanese Patent No. 5493606 (Patent Document 1) and Japanese Patent No. 6081266 (Patent Document 2). In specific examples of these documents, an organic gas such as butane or pentane is used as a foaming agent for producing foamed particles. However, such organic foaming agents are difficult to use from the viewpoints of safety, environment and the like.
Therefore, Japanese Patent No. 6298624 (Patent Document 3) proposes a method for producing styrene composite polyethylene-based resin foamed particles using an inorganic gas having a low environmental load as a foaming agent.

Japanese Patent No. 549360 Japanese Patent No. 6081266 Japanese Patent No. 6298624

It is an object of the present invention to provide a foamed molded product having excellent appearance, dimensional stability and the like, and foamed particles for producing the foamed molded product.

As a result of diligent studies to solve the above problems, the present inventors have found many large bubbles in the foamed particles obtained by using an inorganic gas as a foaming agent, and the large cells are the appearance of the foamed molded product. , I noticed that it causes a decrease in dimensional stability. When the vinyl acetate content of the polyethylene-based resin constituting the base resin is within a predetermined range and the melting point of the polyethylene-based resin is 95 ° C. or higher and lower than 110 ° C., even if an inorganic gas is used as the foaming agent. , The number of large bubbles having the same bubble diameter is 2 or less per foamed particle, and the appearance, dimensional stability, etc. of the foamed molded product obtained by using the foamed particle are improved. I found out to do. The present invention has been completed based on such findings. A typical invention is as follows.

Item 1.
Styrene composite polyethylene-based resin foamed particles containing a polyethylene-based resin and a polystyrene-based resin as a base resin.
The vinyl acetate content of the polyethylene-based resin is 1% by mass to 15% by mass with respect to the total mass of the polyethylene-based resin.
The polyethylene-based resin has a melting point of 95 ° C. or higher and lower than 110 ° C., and the styrene composite polyethylene-based resin foamed particles have 0 to 0 large bubbles having a bubble diameter of 3 times or more the average bubble diameter at the center of the particles. Including 2
Effervescent particles.
Item 2.
Item 2. The styrene composite polyethylene-based resin foamed particles according to Item 1, wherein the vinyl acetate content is 1% by mass to 10% by mass.
Item 3.
Item 2. The styrene composite polyethylene-based resin foamed particles according to Item 1 or 2, wherein the average bubble diameter in the central portion is 100 μm to 400 μm.
Item 4.
Item 2. The styrene composite polyethylene resin foamed particles according to any one of Items 1 to 3, wherein the content of the polystyrene resin is 120 to 1000 parts by mass with respect to 100 parts by mass of the polyethylene resin.
Item 5.
Items 1 to 4 in which the melt flow rate of the polyethylene resin is 2.0 g / 10 minutes or less and / or the density of the polyethylene resin is 0.910 g / cm ^{3 to} 0.935 g / cm ^3. The styrene composite polyethylene-based resin foamed particles described.
Item 6.
Item 2. The styrene composite polyethylene-based resin foamed particles according to any one of Items 1 to 5, which contain 0.01% by mass to 0.2% by mass of calcium carbonate.
Item 7.
Item 2. Styrene composite polyethylene-based resin foaming according to any one of Items 1 to 6, which contains 0.01% by mass to 0.2% by mass of at least one compound selected from the group consisting of diisobutyl adipate and glycerin diacet monolaurate. particle.
Item 8.
Item 2. The method for producing styrene composite polyethylene-based resin foamed particles according to any one of Items 1 to 7, which comprises a polyethylene-based resin and a polystyrene-based resin as a base resin.
The vinyl acetate content of the polyethylene resin is 1% by mass to 15% by mass with respect to the total mass of the polyethylene resin, and the melting point of the polyethylene resin is 95 ° C. or higher and lower than 110 ° C.
A production method comprising a step of press-fitting an inorganic gas into styrene composite polyethylene-based resin particles to obtain foamable particles, and a step of foaming the foamable particles with steam to obtain styrene composite polyethylene-based resin foamed particles.
Item 9.
In the step of obtaining the foamable particles, the press-fitting of the inorganic gas into the styrene composite polyethylene-based resin particles is a dry impregnation method, and in the step of obtaining the styrene composite polyethylene-based resin foamed particles, in the absence of liquid water, the above. Foaming effervescent particles with water vapor,
Item 8. The manufacturing method according to Item 8.
Item 10.
A foamed molded product composed of a fused body of a plurality of styrene composite polyethylene-based resin foamed particles containing a polyethylene-based resin and a polystyrene-based resin as a base resin.
The styrene composite polyethylene-based resin foamed particles are the foamed particles according to any one of Items 1 to 7.
Foam molded product.
Item 11.
Item 10. The foamed molded product according to Item 10, wherein the foamed molded product is for a cushioning material for a vehicle.

According to the present invention, even if an inorganic gas is used as a foaming agent, it is possible to provide styrene composite polyethylene-based resin foamed particles in which the number of large bubbles present is 2 or less per foamed particle. Further, according to the present invention, by foam-molding the foamed particles, it is possible to provide a foamed molded product having excellent appearance, dimensional stability and the like. Furthermore, according to the present invention, it is possible to provide a method for producing styrene composite polyethylene-based resin foamed particles in which the number of large bubbles present is 2 or less per foamed particle, using an inorganic gas as a foaming agent.

It is a cross-sectional photograph of the foamed particles obtained in Examples 1, 4 to 6. In FIG. 1, A1 to A4 are cross-sectional photographs of the entire foamed particles, and B1 to B4 are cross-sectional photographs of the central portion of the foamed particles. Large bubbles cannot be confirmed. The magnifications of A1, A2, A3, A4, B1, B2, B3, and B4 are 30 times, 30 times, 20 times, 22 times, 100 times, 100 times, 100 times, and 100 times, respectively. It is a cross-sectional photograph of the foamed particles obtained in Comparative Examples 1 and 3. In FIG. 1, A5 and A6 are cross-sectional photographs of the entire foamed particles, B5-1 and B6 are cross-sectional photographs of the central portion of the foamed particles, and B5-2 is a cross-sectional photograph of the foamed particles from the surface to the inside. .. In A5, B5-1 and B5-2, 5 large bubbles can be confirmed. In A6 and B6, 5 large bubbles can be confirmed. The magnifications of A5, A6, B5-1, B5-2, and B6 are 25 times, 22 times, 70 times, 100 times, and 100 times, respectively.

Styrene-based polyethylene-based resin foamed particles Styrene-based polyethylene-based resin foamed particles (hereinafter, may be simply referred to as “foamed particles”) include a polyethylene-based resin and a polystyrene-based resin as a base resin.

(1) Polyethylene-based resin The polyethylene-based resin is not particularly limited, and known resins can be used. Further, the polyethylene resin may be crosslinked.
Examples of the polyethylene-based resin include branched low-density polyethylene, linear low-density polyethylene, medium-density polyethylene, high-density polyethylene, ethylene-vinyl acetate copolymer, ethylene-methylmethacrylate copolymer, and ethylene-propylene random copolymer. Examples thereof include polymers, ethylene-propylene-butene random copolymers, and crosslinked products of these polymers. These polyethylene-based resins may be used alone or in combination.
The polyethylene-based resin preferably contains an ethylene-vinyl acetate copolymer from the viewpoints of appearance of the foamed molded product, dimensional change rate, etc. and the number of large bubbles present in the foamed particles, and contains vinyl acetate of the polyethylene-based resin. The amount is more preferably 1% by mass to 15% by mass, and even more preferably 1% by mass to 10% by mass.

In the above example, low density ^is preferably ^{0.91g / cm 3 ~0.94g / cm 3} , more preferably ^{0.91g / cm 3 ~0.93g / cm 3} . High density ^is preferably ^{0.95g / cm 3 ~0.97g / cm 3} , more preferably ^{0.95g / cm 3 ~0.96g / cm 3} . Medium density is an intermediate density between these low and high densities. The polyethylene-based resin can be preferably selected from ethylene-vinyl acetate copolymer, high-density polyethylene, linear low-density polyethylene, and mixtures thereof.
Polyethylene resin, from the viewpoint of deformation of the resin during molding property and styrene impregnation ^{polymerization,} preferably has a density of ^{0.910g / cm 3 ~0.935g / cm 3} , 0.910g / cm 3 ~0. More preferably, it is 920 g / cm ^3.

The polyethylene-based resin preferably has a melting point of 95 ° C. or higher and lower than 110 ° C., preferably 97 ° C. or higher, from the viewpoint of the appearance of the foamed molded product, the rate of dimensional change, and the number of large bubbles present in the foamed particles. It is more preferable to have a melting point of less than 110 ° C., and even more preferably to have a melting point of 100 ° C. or higher and lower than 110 ° C.

The polyethylene-based resin has a melt flow rate of 2.0 g / 10 minutes or less (hereinafter referred to as "MFR") in terms of the appearance of the foamed molded product, the rate of change in dimensions, and the number of large bubbles present in the foamed particles. There is).

(2) Polystyrene-based resin The polystyrene-based resin is not particularly limited as long as it is a resin containing a styrene-based monomer as a main component, and examples thereof include styrene or a styrene derivative alone or a copolymer.
Examples of the styrene derivative include α-methylstyrene, vinyltoluene, chlorostyrene, ethylstyrene, isopropylstyrene, t-butylstyrene, chlorostyrene, dimethylstyrene, bromostyrene and the like. These styrene-based monomers may be used alone or in combination.

A vinyl-based monomer copolymerizable with the styrene-based monomer may be used in combination with the polystyrene-based resin.
As the vinyl-based monomer, for example, an alkyl ester having 1 to 10 carbon atoms of acrylic acid such as methyl acrylate, ethyl acrylate, propyl acrylate, butyl acrylate, and -2-ethylhexyl acrylate should be used. Can be done. Examples of vinyl-based monomers include alkyl esters and acrylonitriles having 1 to 10 carbon atoms of methacrylic acid such as methyl methacrylate, ethyl methacrylate, propyl methacrylate, butyl methacrylate, and -2-ethylhexyl methacrylate. , Methacrylic acid and other nitrile group-containing unsaturated compounds, o-divinylbenzene, m-divinylbenzene, p-divinylbenzene and other divinylbenzene, ethylene glycol di (meth) acrylate, polyethylene glycol di (meth) acrylate and other alkylene Examples thereof include polyfunctional monomers such as glycol di (meth) acrylate. The vinyl-based monomer may be used alone or in combination of two or more.
Among these, alkyl esters having 1 to 10 carbon atoms of acrylic acid are more preferable, and butyl acrylate is particularly preferable. The content of the vinyl-based monomer is preferably 0.5 to 10 parts by mass as the amount of the monomer used with respect to 100 parts by mass of the foamed particles. When the content of butyl acrylate is within the above range, foamed particles having good foamability can be obtained. The content of the vinyl-based monomer is more preferably 0.5 to 8 parts by mass and further preferably 0.5 to 5 parts by mass as the amount of the monomer used with respect to 100 parts by mass of the foamed particles.

(3) Content ratio of resin component Polystyrene resin is preferably contained in an amount of 120 to 1000 parts by mass as a monomer usage amount with respect to 100 parts by mass of polyethylene resin from the viewpoint of strength and molding processability. More preferably, it is ~ 800 parts by mass. The content of the polystyrene-based resin is substantially the same as the amount of the styrene-based monomer added at the time of producing the foamed particles.

(4) Other components In the foamed particles, a plasticizer, an antistatic agent, a filler, a lubricant, a fusion accelerator, a spreading agent, and a coloring agent are included within the range that does not impair the physical properties of the foamed particles and the foamed molded product. , Flame retardants, flame retardants, antistatic agents and the like may be added.

It is preferable that the foamed particles contain a plasticizer having a boiling point of more than 200 ° C. under 1 atm. By containing the plasticizer, good foamability can be maintained even if the pressure of water vapor during heat foaming is low, and the appearance of the foamed molded product is improved.
Examples of the plasticizer include phthalates such as bis (2-ethylhexyl) phthalate and diisononyl phthalate, glycerin fatty acid esters such as glycerin diacet monolaurate, glycerin tristearate, and glycerin diacet monostearate, and diisobutyl adipate. Examples thereof include plasticizers such as adipic acid ester and coconut oil.
The content of the plasticizer in the foamed particles can be, for example, 0.1% by mass to 3.0% by mass, preferably 0.1% by mass to 2.0% by mass.

Examples of the bond inhibitor include calcium carbonate, silica, zinc stearate, aluminum hydroxide, ethylene bisstearic acid amide, calcium tertiary phosphate, dimethyl silicon and the like.
Examples of the filler include synthetic or naturally produced silicon dioxide and the like.
Examples of the lubricant include paraffin wax, zinc stearate and the like.
Examples of the fusion accelerator include stearic acid, stearic acid triglyceride, hydroxystearic acid triglyceride, stearic acid sorbitan ester, polyethylene wax and the like.
Examples of the spreading agent include polybutene, polyethylene glycol, silicone oil and the like.

Colorants include furnace black, ketjen black, channel black, thermal black, acetylene black, carbon black such as graphite and carbon fiber, chromate such as yellow lead, zinc yellow and barium yellow, and ferrocyanide such as dark blue. , Cadmium yellow, sulfides such as cadmium red, oxides such as iron black and red husks, silicates such as ultramarine, inorganic pigments such as titanium oxide, monoazo pigments, disazo pigments, azolakes, condensed azo pigments, chelate azo Examples thereof include azo pigments such as pigments, and organic pigments such as phthalocyanine-based, anthraquinone-based, perylene-based, perinone-based, thioindigo-based, quinacridone-based, dioxazine-based, isoindolinone-based, and quinophthalone-based polycyclic pigments. These colorants may be a masterbatch blended with a resin. When carbon black is used as the colorant, the content of carbon black in the foamed particles is preferably 1.5 to 5.0% by mass.

Flame retardants include tri (2,3-dibromopropyl) isocyanurate, bis [3,5-dibromo-4- (2,3-dibromopropoxy) phenyl] sulfone, tetrabromocyclooctane, hexabromocyclododecane, and tris. Examples thereof include dibromopropyl phosphate, tetrabromobisphenol A, tetrabromobisphenol A-bis (2,3-dibromo-2-methylpropyl ether), tetrabromobisphenol A-bis (2,3-dibromopropyl ether) and the like.
Examples of the flame retardant aid include organic peroxides such as 2,3-dimethyl-2,3-diphenylbutane, 3,4-dimethyl-3,4-diphenylhexane, dicumyl peroxide, and cumene hydroperoxide. Can be mentioned.
The contents of the flame retardant and the flame retardant auxiliary in the foamed particles are preferably 1.0 to 5.0% by mass and 0.1 to 2.0% by mass, respectively.

(5) Physical Properties The foamed particles are large cells having a cell diameter that is three times or more the average cell diameter at the center of the foamed molded product from the viewpoint of appearance, dimensional change rate, etc., from 0 to 0 per particle. It is preferable to include 2 particles, more preferably 0 to 1 particle, and particularly preferably 0 particle, that is, not to contain large bubbles.
The average bubble diameter at the center of the foamed particles can be, for example, 70 μm to 400 μm, preferably 80 μm to 370 μm. When the average cell diameter at the center is within this range, the bubble film becomes thin and the bubble film is less likely to break during secondary foaming, the proportion of open cells is reduced, and the dimensional stability of the foamed molded product is reduced. Can be improved, and foam moldability can be improved.

The average cell diameter at the center of the foamed particles is measured from a cross-sectional photograph of the center of the foamed particles (example: 50 to 200 times cross-sectional photograph), and is the average cell diameter per cell existing in the center. Is. Details of the method for determining the average cell diameter will be described in Examples.
The central portion of the foamed particles is a region of 30% in the radial direction from the center point of the foamed particles.
A large cell is a cell whose diameter is three times or more the average cell diameter at the center of the foamed particle.
The bubble diameter of a large bubble is measured from a cross-sectional photograph of the entire foamed particle (eg, a 20-fold cross-sectional photograph).
The cross-sectional photograph used when measuring the average cell diameter of the central portion and the bubble diameter of the large cell is a photograph of a cross section cut out by cutting the foamed particles from the surface so as to pass through the center.
The magnification of the cross-sectional photograph used for measuring the average cell diameter in the central portion is not particularly limited as long as the bubble diameter of the air bubbles in the central portion can be measured, but is preferably 50 to 200 times.
Further, the magnification of the cross-sectional photograph used for measuring the bubble diameter of the large bubble is not particularly limited as long as the bubble diameter of the large bubble can be measured, but is preferably 15 to 35 times.
Unless otherwise specified, the number of large cells in the present invention is the number of large cells per foamed particle (10 average number of large cells) obtained from 10 foamed particles. Specifically, the average value D of the average cell diameter d is determined from the cross-sectional image of the central portion of the 10 foamed particles by the method described in Examples described later, and is 3 times the average value D of the 10 foamed particles. It is the number of large cells per foamed particle obtained by dividing the number of bubbles (that is, large cells) having the above cell diameter (that is, the number of large cells per 10 foamed particles) by 10.
D = ( _{sum of d 1 to d 10} ) / 10
d _{1 to d 10} are the individual average cell diameters (μm) of the 10 foamed particles.
In another embodiment, the number of large cells in the present invention determines the average cell diameter d from the cross-sectional image of the central portion of one foamed particle, and is three times or more the average cell diameter d in one foamed particle. It can also be the number of bubbles having a bubble diameter (that is, large bubbles). In this case, the number of large bubbles can be determined for each foamed particle, which is useful when the number of foamed particles is less than 10.

The foamed particles preferably have a volatile component content of 1% by mass or less. When the content of the volatile component is 1% by mass or less, the surface pressure at the time of molding does not increase excessively, so that the cooling time can be shortened and the molding cycle can be shortened. In addition, problems of safety (suppression of combustion rate of foamed molded product) and environmental load caused by volatile components can be reduced.
The content of the volatile component is preferably 0.5% by mass or less, more preferably 0.3% by mass or less, and particularly preferably 0% by mass.
The shape of the foamed particles is not particularly limited. For example, spherical, substantially spherical, columnar and the like can be mentioned. Of these, it is preferable that it is as close to a spherical shape as possible. That is, it is preferable that the ratio of the minor axis to the major axis of the foamed particles is as close to 1 as possible.

The foamed particles can have various bulk densities. The bulk density is more preferably is preferably 0.05 g / cm ³ or ^less, is ^{0.01g / cm 3 ~0.04g / cm 3} .
The average particle size of the foamed particles is, for example, 1.0 mm to 8.0 mm, preferably 1.0 mm to 5.0 mm, more preferably 1.0 mm to 3.0 mm, from the viewpoint of moldability and appearance of the molded product. preferable. Here, the average particle size of the foamed particles is such that about 5 g of a sample is classified with a JIS standard sieve (JIS Z8801-1: 2006) for 10 minutes, the sample weight on the sieve net is measured, and the cumulative weight distribution is obtained from the obtained results. A curve is created, and the particle size (median size) at which the cumulative weight is 50% is taken as the average particle size.
The mass average molecular weight of the base resin: Mw is about 250,000 to 450,000. The mass average molecular weight can be measured using gel permeation chromatography (GPC).

The foamed particles are preferably foamed particles obtained by foaming foamable particles containing an inorganic gas as a foaming agent.
The foamed particles are preferably foamed particles obtained by foaming foamable particles containing an inorganic gas impregnated by a dry impregnation method (in the absence of water). Further, the foamed particles are preferably foamed particles obtained by steam-foaming foamable particles containing an inorganic gas impregnated by a dry impregnation method in the absence of liquid water.

(6) Production Method The foamed particles are not particularly limited, but styrene composite polyethylene-based resin particles (composite resin particles) are impregnated with a foaming agent to obtain foamable particles, and the foamable particles are foamed to obtain foamed particles. Can be manufactured by The effervescent particles are resin particles impregnated with a foaming agent, and the effervescent particles are particles obtained by foaming effervescent particles.
Further, the composite resin particles are not particularly limited, but can be produced by, for example, a seed polymerization method. The composite resin particles produced by the seed polymerization method are sometimes called styrene-modified polyethylene-based resin particles (modified resin particles).
Other components such as plasticizers, bubble modifiers, cross-linking agents, etc. listed in the column of foaming particles may be added and used at the time of polymerization or impregnation with a foaming agent, or may be mixed with polyethylene resin particles in advance. It may be used by inserting it.

As the bubble adjusting agent, either an organic bubble adjusting agent or an inorganic bubble adjusting agent can be used. Examples of the organic bubble adjusting agent include aliphatic bisamides such as methylene bisstearic acid amide and ethylene bisstearic acid amide, stearic acid amides, and polyethylene wax. Examples of the inorganic bubble adjusting agent include talc, silica, calcium silicate, calcium carbonate, sodium borate, zinc borate and the like.
Examples of the cross-linking agent include organic peroxides such as 2,2-di-t-butylperoxybutane, dicumyl peroxide, and 2,5-dimethyl-2,5-di-t-butylperoxyhexane. Be done.

(A) Seed polymerization method The seed polymerization method can be carried out under an aqueous suspension. The aqueous suspension refers to a state in which seed particles and droplets of monomers are dispersed in an aqueous medium by stirring or the like. A water-soluble surfactant or monomer may be dissolved in the aqueous medium, and a dispersant, an initiator, a cross-linking agent, a bubble conditioner, a flame retardant, a plasticizer, etc., which are insoluble in water, are dispersed in the aqueous medium. You may. The mass ratio of the composite resin particles / aqueous medium is preferably 1 / 0.1 to 1/20, more preferably 1 / 0.6 to 1/3.
In the seed polymerization method, generally, a monomer (styrene-based monomer and optionally copolymerizable with a styrene-based polymer) is added to an aqueous suspension charged in a container equipped with a stirrer. , The composite resin particles can be obtained by allowing the seed particles to absorb the particles, and then polymerizing the monomers after or while absorbing the particles. It is also possible to obtain foamable particles by impregnating the composite resin particles with a foaming agent after or while polymerizing.

Specifically, first, in an aqueous suspension, the seed particles absorb the styrene-based monomer, and after or while absorbing the styrene-based monomer, the composite resin particles are polymerized to form the composite resin particles. obtain.
For the styrene-based monomer, it is not necessary to supply all the constituent monomers to the aqueous suspension at the same time, and all or a part of the monomers are supplied to the aqueous suspension at different timings. You may. When the additive is contained in the composite resin particles, the additive may be added to the styrene-based monomer or the aqueous suspension, or may be contained in the seed particles.
The mass average molecular weight of the composite resin particles can be adjusted by arbitrarily selecting the addition amount, addition rate, etc. of the styrene-based monomer.
The amount of the monomer for producing the composite resin particles and the amount of the resin component constituting the composite resin particles are almost the same.

The polymerization temperature and the polymerization time are not particularly limited as long as the polymerization proceeds, but can be, for example, 2 hours to 40 hours at 60 ° C. to 150 ° C.
In the polymerization step, it is preferable to keep the polymerization temperature for a long time, that is, to anneal.
In the steps leading up to the annealing step, the styrene-based monomer and the polymerization initiator absorbed in the seed particles may not have completely completed the reaction, and there are few unreacted substances inside the composite resin particles. May exist. By annealing, it is possible to secure a time for the unreacted product to undergo a polymerization reaction. By annealing, the mechanical properties of the foamed molded product are lowered, the heat resistance is lowered, and the odor caused by the volatile unreacted material is caused by the unreacted material having a low molecular weight such as styrene-based monomer. Can be reduced.

As the styrene-based monomer, those exemplified in the section of composite resin particles can be used, and the amount used can be within the range described in the section of composite resin particles.
The impregnation and polymerization of the styrene-based monomer are preferably carried out in the following embodiments. That is, a part of the styrene-based monomer to be added, for example, 30 parts by mass to 150 parts by mass of the styrene-based monomer with respect to 100 parts by mass of the polyethylene-based resin particles, at a temperature at which polymerization essentially does not proceed. This is an embodiment in which the mixture is added and impregnated, and the remaining styrene-based monomer is continuously added at a temperature at which polymerization proceeds. Here, the "temperature at which polymerization essentially does not proceed" is a temperature equal to or lower than the 10-hour half-life temperature of the main polymerization initiator used (for example, the polymerization initiator on the lower side of the 10-hour half-life temperature). Means. In this embodiment, the viscosity of the polyethylene-based resin particles, which is the polymerization site, can be changed by adding and impregnating a part of the styrene-based monomer to be added at a temperature at which the polymerization essentially does not proceed. Therefore, there is an advantage that the mass average molecular weight of the composite resin particles can be easily adjusted.

(B) Seed particles The seed particles include a polyethylene-based resin. The seed particles can be obtained, for example, by mixing and melt-kneading a polyethylene resin using an extruder, a kneader, a Banbury mixer, a roll, or the like, extruding it into a strand shape, and cutting it to a desired particle size.
The particle size of the seed particles can be appropriately adjusted according to the average particle size of the composite resin particles.
The average mass of the seed particles is preferably 0.1 mg / grain to 3 mg / grain from the viewpoint that the diffusion of the foaming agent is suppressed and the magnification can be increased and the filling property at the time of molding is improved.
The shape of the seed particles is preferably powder-like, pellet-like, or the like. More specific shapes include a true sphere, an elliptical sphere (egg-like), a columnar shape, a prismatic shape, and the like.

(C) Aqueous medium Examples of the aqueous medium include a mixed medium of water, water and a water-soluble solvent (for example, a lower alcohol such as methyl alcohol or ethyl alcohol).

(D) Dispersant As the aqueous medium, a dispersant may be used to prevent coalescence of composite resin particles and to stabilize the dispersibility of monomer droplets and seed particles. The dispersant can be used alone or in combination of two or more. Examples of the dispersant include polymer dispersants such as polyvinyl alcohol, polyacrylate, polyvinylpyrrolidone, carboxymethyl cellulose, methyl cellulose, and polyacrylamide, magnesium pyrophosphate, calcium pyrophosphate, calcium phosphate, hydroxyapatite, calcium carbonate, and magnesium phosphate. , Magnesium carbonate, magnesium oxide, kaolin and other poorly water-soluble inorganic dispersants. Among these, an inorganic dispersant is preferable, and magnesium pyrophosphate is more preferable, because a more stable dispersed state can be maintained. The dispersant may be added before the polymerization, during the polymerization, or before and during the polymerization.
When a poorly water-soluble inorganic dispersant is used, it is preferable to use an anionic surfactant in combination in order to improve the dispersion stability. Examples of such anionic surfactants include sodium dodecylbenzenesulfonate, sodium α-olefin sulfonate, etc., which may be used alone or in combination of two or more.

(E) Polymerization Initiator A styrene-based monomer and other monomers used arbitrarily are usually polymerized in the presence of a polymerization initiator. The polymerization initiator is usually impregnated into seed particles at the same time as the styrene-based monomer.
As the polymerization initiator, a radical generation type polymerization initiator generally used in the production of a thermoplastic polymer can be used. Typical examples are, for example, benzoyl peroxide, dicumyl peroxide, t-butyl peroxybenzoate, lauroyl peroxide, t-butyl peroxy-2-ethylhexanoate, t-butyl perpivalate, t. -Butylperoxyisopropyl carbonate, di-t-butylperoxyhexahydroterephthalate, 1,1-di (t-butylperoxy) 3,3,5-trimethylcyclohexane, 1,1-di (t-butylperoxy) ) Organic peroxides such as cyclohexane and azo compounds such as azobisisobutyronitrile and azobisdimethylvaleronitrile can be mentioned. These polymerization initiators may be used alone or in combination of two or more.

The amount of the polymerization initiator used is not particularly limited as long as the desired polymerization reaction is achieved, but is preferably 0.05 parts by mass to 1.0 part by mass with respect to 100 parts by mass of the styrene-based monomer. When the amount used is within this range, the polymerization proceeds sufficiently to reduce the residual monomers, and the difficulty of temperature adjustment of the reaction system due to heat generation due to the rapid progress of the reaction is reduced. The amount used is more preferably 0.1 parts by mass to 0.8 parts by mass.
In order to uniformly absorb the polymerization initiator from the seed particles or the growing particles, the polymerization initiator was suspended or emulsified and dispersed in the aqueous medium in advance when the polymerization initiator was added to the aqueous medium. It is preferable to add the above to the dispersion, or to dissolve the polymerization initiator in the styrene-based monomer in advance and then add it to the aqueous medium.

(F) Others In the polymerization of styrene-based monomers, it is generally used for the polymerization of mercaptan-based chain transfer agents such as n-octyl mercaptan, n-dodecyl mercaptan, and t-dodecyl mercaptan, and acrylonitrile-styrene-based resins. You may use α-methylstyrene dimer which is a chain transfer agent.
A water-soluble polymerization inhibitor may be used to prevent coalescence of the composite resin particles. Examples of the water-soluble polymerization inhibitor include sodium nitrite, 3,5-dibutyl-4-hydroxytoluene (BHT) and the like. The water-soluble polymerization inhibitor is preferably used so that the concentration in water is 150 ppm or less because it is difficult to induce polymerization inhibition.
The composite resin particles preferably have an average particle size of 1.0 mm to 2.0 mm. The average particle size is more preferably 1.2 mm to 1.6 mm. The average particle size of the composite resin particles is obtained as follows.
<Average particle size of composite resin particles>
The average particle size is a value expressed by D50.
Specifically, using a low-tap type sieve shaker (manufactured by Iida Seisakusho), the mesh opening is 4.00 m.
m, 3.35 mm, 2.80 mm, 2.36 mm, 2.00 mm, 1.70 mm, 1.4
0mm, 1.18mm, 1.00mm, 0.85mm, 0.71mm, 0.60mm, 0
.. 50 mm, 0.425 mm, 0.355 mm, 0.300 mm, 0.250 mm, 0.
Approximately 25 g of the sample is classified with a 212 mm and 0.180 mm JIS standard sieve (JIS Z8801-1: 2006) for 10 minutes, and the sample weight on the sieve net is measured. A cumulative weight distribution curve is created from the obtained results, and the particle size (median diameter) at which the cumulative weight is 50% is defined as the average particle size.

(G) Foamable Particles Foamable particles contain composite resin particles and an inorganic gas as a foaming agent, and can be produced by impregnating composite resin particles with a foaming agent by a known method.
The effervescent particles are preferably effervescent particles containing an inorganic gas impregnated by a dry impregnation method (in the absence of water). Further, the effervescent particles are preferably steam-foamed in the absence of water.

(G-1) Foaming Agent The foaming agent is not particularly limited as long as it has been conventionally used for foaming polystyrene-based resins, and is, for example, air, nitrogen gas, carbon dioxide gas (CO ₂ ), water and the like. Inorganic gas can be mentioned. These foaming agents may be used alone or in combination of two or more. Among these foaming agents, carbon dioxide gas is particularly preferable because it has excellent foamability and stability against cell rupture.

The content of the foaming agent in the foamable particles is 0.5 parts by mass to 15 parts by mass with respect to 100 parts by mass of the composite resin particles because the foaming power is unlikely to decrease and the shrinkage during foaming can be reduced. It is preferable to have. A more preferable content of the foaming agent is 1 part by mass to 8 parts by mass. Within this range, it is possible to achieve both appropriate foamability and suppression of breakage of the bubble film due to the bubbles becoming too fine.

(G-2) Manufacturing Method As a method of impregnating the composite resin particles with a foaming agent, a wet impregnation method in which the composite resin particles are dispersed in an aqueous system and the foaming agent is press-fitted while stirring, or a sealable container is used. Examples thereof include a dry impregnation method (a method of impregnating in the absence of liquid water; a vapor phase impregnation method) in which resin particles are charged and a foaming agent is press-fitted to impregnate the mixture. Further, the composite resin particles are dispersed in an aqueous dispersion medium in a pressure-resistant container, a foaming agent is put in the pressure-resistant container, and the composite resin particles are heated to a temperature equal to or higher than the softening point of the composite resin particles under pressure equal to or higher than the vapor pressure of the foaming agent. After impregnating the composite resin particles with a foaming agent, the mixture of the composite resin particles and the aqueous dispersion medium is discharged to a lower pressure region than in the pressure resistant container while keeping the temperature and pressure in the pressure resistant container constant, so-called decompression foaming. There is also a law. Of these methods, a dry impregnation method that can be impregnated without using water is preferable. The impregnation pressure, impregnation time, and impregnation temperature when the composite resin particles are impregnated with the foaming agent are not particularly limited.
From the viewpoint of efficiently impregnating and obtaining even better foamed particles and foamed molded product, the impregnation pressure is preferably 0.1 to 4.0 MPa (gauge pressure based on atmospheric pressure).

(H) Production conditions of foamed particles As a method of foaming foamable particles impregnated with a foaming agent to obtain foamed particles, a method of heating the foamable particles with steam (steam) or the like to foam them is preferably used. .. It is preferable to use a closed pressure-resistant foam container for the foaming machine for foaming. The steam pressure is preferably 0.004 MPa to 0.2 MPa (gauge pressure), more preferably 0.01 MPa to 0.15 MPa. The foaming time may be any time required to obtain a desired foaming multiple. The foaming time is preferably 5 seconds to 180 seconds in that the shrinkage of the foamed particles can be suppressed.
In foaming, air may be introduced at the same time as steam when foaming, if necessary. Also, foaming is preferably carried out in the absence of liquid water.
When the desired foaming ratio is not reached during the production of the foamed particles, the unreached foamed particles are pressurized with air in a closed container to add an internal pressure to the unreached foamed particles, and then heated by steam or the like. Thereby, the desired foaming ratio may be reached by further foaming (two-stage foaming).
The foamed particles have good foamability even when foamed by an inorganic gas (for example, carbon dioxide gas), and are safer in the production process than organic foaming agents such as butane and pentane. In addition, it is excellent in moldability, and the foam molded product has sufficient dimensional stability and appearance.
Further, since the foamed particles do not use an organic foaming agent, the volatile component can be 1% by mass or less.

Foam molding The foam molding is composed of a fused body of a plurality of foam particles containing a polyethylene resin and a polystyrene resin as a base resin. The foamed particles are the foamed particles of the present invention.
Therefore, the foamed particles are styrene composite polyethylene-based resin foamed particles containing a polyethylene-based resin and a polystyrene-based resin as a base resin, and the vinyl acetate content of the polyethylene-based resin is relative to the total mass of the polyethylene-based resin. The polyethylene-based resin has a melting point of 95 ° C. or higher and lower than 110 ° C., and the styrene composite polyethylene-based resin foamed particles have three times the average cell diameter at the center of the particles. It is a particle containing 0 to 2 large bubbles having the above bubble diameter.
The foamed molded product has the following effects when it is composed of the fused product of the foamed particles of the present invention.
(I) Since the bubble diameters of the foamed particles are uniform, the appearance (particularly, the smoothness of the surface) is good, and the fusion rate of the particles is high.
(Ii) Since the bubble diameters of the foamed particles are uniform, the dimensional stability over time is high (for example, the rate of change in size with respect to the mold is low).
(Iii) Since the content of the volatile component in the foamed particles is small, the surface pressure during molding does not rise excessively, so that the cooling time can be shortened and the molding cycle can be shortened.

The foam-molded product can be obtained by foam-molding the foam particles in a mold by a known method. In in-mold foam molding, foam particles are heated in a molding machine built in a foam molding machine to be fused and integrated. Specifically, the foamed particles are filled in the mold of the foam molding machine and heated to obtain a foamed molded product by heat-sealing the foamed particles while foaming. As the foam molding machine, an EPS molding machine used when manufacturing a foam molded product from polystyrene-based resin foam particles and a high-pressure specification molding used when manufacturing a foam molded product from polypropylene-based resin foam particles. Machines and the like can be used. As the heating medium, the foamed particles may shrink or have poor fusion when the heating time is long. Therefore, a heating medium capable of giving high energy in a short time is desired, and steam is suitable as such a heating medium. ..
The pressure of water vapor is preferably 0.02 MPa to 0.2 MPa (gauge pressure). The heating time can be appropriately determined depending on the shape, size, etc. of the foamed molded product, but is preferably 10 seconds to 90 seconds, more preferably 20 seconds to 80 seconds.

At the time of producing the foamed molded product, the foamed particles may be impregnated with a foaming agent to impart foaming power (internal pressure). As the foaming agent used here, the foaming agent described in the above item (g-1) can be used. Among them, it is preferable to use one of nitrogen gas, air and carbon dioxide gas, or to use two or more in combination. It is desirable that the pressure for applying the internal pressure is such that the foamed particles are not crushed and the pressure is within a range in which the foaming force can be applied. The pressure is preferably 0.05 MPa to 4 MPa (gauge pressure), more preferably 0.1 MPa to 3 MPa.

Expanded molded article, slow-retardant, from the viewpoint of impact resistance and light ^weight, preferably have a density in the range of ^{0.02g / cm 3 ~0.2g / cm 3} . The density of the preferred foam moldings ^is in the range of ^{0.02g / cm 3 ~0.1g / cm 3} .

The foam molded body can be used for various purposes, for example, a shock energy absorbing material such as a core material for a vehicle bumper, a cushioning material for interior doors, a shock absorbing material for a lower limb, and a cushioning material for an automobile vehicle such as a floor raising material. , Electronic parts, various industrial materials including glass, tool boxes, food cushioning materials, transport containers, etc. In particular, it can be suitably used as a cushioning material for automobiles.

Hereinafter, the present invention will be specifically described with reference to Examples and the like, but the present invention is not limited to the embodiments shown therein.

<Material>
PE1: Ethylene-vinyl acetate copolymer resin (manufactured by Japan Polyethylene Corporation, product number LV115A, density 0.930 g / cm ³ , MFR 0.3 g / 10 minutes, melting point 108 ° C, vinyl acetate content 4% by mass)
PE2: Ethylene-vinyl acetate copolymer resin (manufactured by Japan Polyethylene Corporation, product number LV211A, density 0.930 g / cm ³ , MFR 0.3 g / 10 minutes, melting point 103 ° C, vinyl acetate content 6% by mass)
PE3: Ethylene-vinyl acetate copolymer resin (manufactured by Asahi Kasei Corporation, product number EF-0910, density 0.930 g / cm ³ , MFR 1.0 g / 10 minutes, melting point 97 ° C, vinyl acetate content 9% by mass)
PE4: Linear low-density polyethylene resin (manufactured by Japan Polyethylene Corporation, product number NF444A, density 0.912 g / cm ³ , MFR 2.0 g / 10 minutes, melting point 121 ° C, vinyl acetate content 0% by mass)
PE5: Low-density polyethylene resin (manufactured by Japan Polyethylene Corporation, product number YF30, density 0.920 g / cm ³ , MFR 1.1 g / 10 minutes, melting point 108 ° C, vinyl acetate content 0% by mass)

<Vinyl acetate content of polyethylene resin>
The vinyl acetate content in the polyethylene resin was measured according to the method described in JIS K6924-2: 1997.

<Melting point of polyethylene resin>
The melting point was measured by the method described in JIS K7122: 1987 “Method for measuring transition heat of plastic”.
That is, using a differential scanning calorimeter device DSC6220 (manufactured by SI Nanotechnology Co., Ltd.), about 6 mg of a sample was filled so that there was no gap in the bottom of the aluminum measuring container. Then, under a nitrogen gas flow rate of 20 mL / min, the temperature was lowered from 30 ° C to -40 ° C, held for 10 minutes, raised from -40 ° C to 220 ° C (1st heating), held for 10 minutes, and then from 220 ° C to −. A DSC curve was obtained when the temperature was lowered to 40 ° C. (Cooling), held for 10 minutes, and then raised from −40 ° C. to 220 ° C. (2nd heating). All temperature raising and lowering were performed at a rate of 10 ° C./min, and alumina was used as a reference material.
The melting point was defined as the value obtained by reading the temperature at the top of the melting peak observed in the 2nd heating process using the analysis software attached to the device. When there are two or more melting peaks, the temperature at the top of the lowest peak is defined as the melting point (° C.).

<MFR (melt flow rate) of polyethylene resin>
MFR was measured at 190 ° C. with a 2.16 kg load according to JIS K6922-1: 1998.

<Density of polyethylene resin>
Density was measured according to JIS K7112: 1999.

<Volume density and bulk multiple of foamed particles>
The bulk density of the foamed particles was measured as follows.
First, the foamed particles were filled in a graduated ^{cylinder up to a scale of 500 cm 3.} However, when the measuring cylinder is visually inspected from the horizontal direction and even one of the foamed particles ^{reaches the scale of 500 cm 3} , filling is completed. Next, the mass of the foamed particles filled in the graduated cylinder was weighed with two significant figures after the decimal point, and the mass was defined as W (g). The bulk density of the foamed particles was calculated by the following formula.
Bulk density (g / cm ³ ) = W ÷ 500
The bulk multiple was calculated by the following formula.
Bulk multiple = 1 / bulk density (g / cm ³ )

<Average cell diameter at the center of foamed particles>
The average bubble diameter at the center of the foamed particles was measured by the following test method.
Any foamed particles obtained in the foamed particle manufacturing process were subjected to the test. The foamed particles are cut from the surface through the center, and the central part of the cut out cross section (range of 30% in the radial direction from the particle center point) is measured with a scanning electron microscope (JSM-6360LV manufactured by JEOL Ltd.). The image was taken at a magnification of 50 to 200 times so that the number of bubbles existing on a straight line of 60 mm, which will be described later, was about 10 to 20. One of the captured images was printed on A4 printing paper in a size of 7 cm in length × 10 cm in width. For the printed image, a total of 6 arbitrary straight lines were drawn, 3 in the vertical direction and 3 in the horizontal direction. The number of bubbles on each straight line (length 60 mm) was counted. The number calculated by dividing the total number of bubbles on each straight line by 6 was defined as the average number of bubbles, and the average chord length (t) of the bubbles was calculated from the average number of bubbles by the following equation. The image shooting magnification was adjusted between 50 and 200 times so that the number of bubbles existing on a straight line of 60 mm was about 10 to 20.
Average chord length t (mm) = 60 / (average number of bubbles x image magnification)
However, when the number of bubbles for 60 mm length could not be counted, the number of bubbles for 30 mm or 20 mm was counted and converted into the number of bubbles for 60 mm, and the average number of bubbles was calculated. Arbitrary straight lines were designed so that bubbles would not touch only at the contacts as much as possible. If it touches, it is included in the number of bubbles.
The magnification of the image was calculated by measuring the scale bar on the image to 1/100 mm with a "Digimatic Caliper" manufactured by Mitutoyo Co., Ltd. and using the following formula.
Image magnification = Scale bar actual measurement value (mm) / Scale bar display value (mm)
Then, the average cell diameter d at the center of one arbitrary foamed particle was calculated by the following formula.
d (μm) = 1000t / 0.616
By the above method, the average cell diameter d at the center of each of the 10 arbitrary foamed particles is calculated, and the average value of the average cell diameter d at the center of 10 foamed particles is calculated as the average value of the average cell diameter d at the center of the foamed particles. The average cell diameter was set to D. In the present invention, the average cell diameter at the center of the foamed particles is this D.

<Number of large bubbles of foamed particles>
The number of large bubbles was counted using the cross section of the foamed particles used to determine the average cell diameter in the center. Specifically, the cross section of the foamed particles was photographed with a scanning electron microscope (JSM-6360LV manufactured by JEOL Ltd.). The magnification was set so that large bubbles having a bubble diameter three times the average cell diameter D in the central portion could be confirmed. In the captured image (20 times), large bubbles having a bubble diameter of 3 times or more the average bubble diameter D in the central portion were counted. The number of large bubbles was counted for each of the 10 foamed particles, and the average value was taken as the number of large bubbles present. The number of large bubbles in the present invention is the average number obtained from 10 foamed particles.

<Amount of volatile components of foamed particles>
After producing the foamed particles, the particles were allowed to stand at room temperature for 30 days. After standing, about 2 g of the foamed particles were heated and dried in a dryer at 150 ° C. for 30 minutes to disperse the volatile components. After cooling, the mass of the foamed particles after the volatile component was dissipated was measured, and the content of the volatile component was determined from the following formula.
Volatile component (% by mass) = (W1-W2) x 100 / W1
W1 (g): Weight of foamed particles before drying W2 (g): Weight of foamed particles after drying

<Density of foam molded product and foaming multiple>
The density of the foam molded product was measured by the method described in JIS K7222: 1999 "Foam plastics and rubber-Measurement of apparent density".
From the obtained foamed molded product, ^{a test piece of 50 cm 3 or more (100 cm 3} or more in the case of a semi-hard and soft material) was cut so as not to change the original cell structure, and the mass thereof was measured. The density was calculated by the following formula.
Density (g / cm ³ ) = test piece mass (g) / test piece volume (cm ³ )
The test piece for measurement shall be cut from a sample 72 hours or more after molding and left in an atmospheric condition of 23 ° C ± 2 ° C × 50% ± 5% or 27 ° C ± 2 ° C × 65% ± 5% for 16 hours or more. Then, the condition of the test piece was adjusted.
The foaming multiple of the foamed molded product was calculated by the following formula.
Foaming multiple = 1 / density (g / cm ³ )

<Surface elongation of foamed molded product>
The surface of the foam molded product was visually evaluated according to the following criteria. Sufficient quality is provided with 4 points and 5 points.
5 points: No gaps are observed between the foamed particles on the surface.
4 points: A gap is observed only in a part between the foamed particles on the surface, but it is not conspicuous.
3 points: A gap is observed in a part between the foamed particles on the surface, and the gap is conspicuous.
2 points: There are gaps in many of the foamed particles on the surface, and the gaps are conspicuous.
1 point: There are gaps in many of the foamed particles on the surface, and the gaps are large.

<Fusion rate of foam molded product>
After making a cut of about 3 mm in the foam molded product (400 mm × 300 mm × thickness 20 mm) with a cutter knife, the foam molded product was broken along the cut and the fracture surface was observed.
An arbitrary range including 50 or more foamed particles is set in the fracture surface, and the number of foamed particles (strongly heat-sealed foamed particles) that are broken inside the fractured surface instead of the surface of the foamed particles (a). The number (b) of the foamed particles (weakly heat-sealed foamed particles) broken at the interface between the foamed particles was counted, and the fusion rate F (%) was calculated by the following formula.
F (%) = (a / (a + b)) × 100

<Appearance of foam molded product>
The appearance of the foam molded product was visually evaluated according to the following criteria. ○ has sufficient quality, but Δ and × are insufficient.
◯: The surface is fine and smooth (good appearance)
Δ: The texture of the surface is fine, but unevenness due to voids is seen on the surface. ×: The texture of the surface is rough and the unevenness of the surface is remarkable.

<Rate of change in mold size of foam molded product>
The dimensions of the predetermined portion of the mold were measured, the dimensions of the foam molded product corresponding to the predetermined portion were measured, and the dimensional change rate was determined by the following formula (1). The foam molded product to be measured was stored in an environmental atmosphere having a temperature of 23 ° C. and a relative humidity of 50% for 21 days or more after molding, and then measured in the same environmental atmosphere. This time, the dimensions of the 400 mm wide portion of the foam molded product having a length of 300 mm, a width of 400 mm and a thickness of 30 mm were measured. The mold size was 404 mm.
Dimensional change rate U = (mold size-mold size) ÷ mold size x 1000
From the obtained mold dimensional change rate U, good or bad is judged according to the following criteria.
U ≦ 10/1000: Good (○)
U> 10/1000: Defective (×)

(Example 1)
[Preparation of composite resin particles]
PE1 is supplied to an extruder as a polyethylene resin (PE resin), melt-kneaded at 230 to 250 ° C., granulated by an underwater cutting method, cut into elliptical spheres (oval), and polyethylene resin particles (seed particles). ) Was obtained. The mass of the seed particles was 0.4 mg / grain.
Next, 315 g of magnesium pyrophosphate (dispersant) and 8.2 g of sodium dodecylbenzenesulfonate (surfactant) were dispersed in 43 kg of pure water in a 100 liter autoclave equipped with a stirrer to obtain an aqueous medium. 10.5 kg of seed particles were dispersed in an aqueous medium at 30 ° C. and held for 10 minutes, and then the temperature was raised to 70 ° C. to obtain an aqueous suspension. Further, this aqueous suspension was heated to 65 ° C., and 5.4 g of dicumyl peroxide (polymerization initiator; 10-hour half-life temperature 116.4 ° C.) was dissolved in 4.5 kg of styrene. The seed particles were impregnated with styrene by dropping over a minute and holding at 65 ° C. for 30 minutes. After impregnation with styrene, the temperature was raised to 135 ° C., and polymerization (first polymerization) was carried out at this temperature for 1 hour.

Next, a dispersion in which 47 g of sodium dodecylbenzenesulfonate was dispersed in 500 g of pure water was added dropwise to the reaction solution cooled to 90 ° C. over 10 minutes. Next, in 6.5 kg of styrene, 53 g of benzoyl peroxide (polymerization initiator; 10-hour half-life temperature 73.6 ° C.) and 5 g of t-butylperoxybenzoate (polymerization initiator; 10-hour half-life temperature 104.3 ° C.). A solution prepared by dissolving 0.53 kg of butyl acrylate (polymer) and butyl acrylate (polymer) was added dropwise over 2 hours. After the dropping, 13 kg of styrene was further dropped over 2 hours, and the seed particles were impregnated with styrene by holding at 90 ° C. for 1 hour. Then, the temperature was raised to 143 ° C., and the temperature was maintained at this temperature for 2.5 hours for polymerization (second polymerization).
After cooling, it was washed, dehydrated and dried to obtain styrene composite polyethylene-based resin particles. The polyethylene-based resin / polystyrene-based resin is 30/70 (mass ratio).

[Preparation of foamed particles (dry impregnation and steam foaming)]
The composite resin particles were placed in a 5 L autoclave, and then 0.2 parts by mass of calcium carbonate was added as a binding inhibitor per 100 parts by mass of the composite resin particles, and the mixture was stirred so as to adhere to the particle surface. Effervescent particles were obtained by introducing carbon dioxide gas into the autoclave from a carbon dioxide gas cylinder and holding it at 2.5 MPa (gauge pressure) at 20 ° C. for 12 hours to impregnate the composite resin particles with carbon dioxide gas (dry impregnation). ). After the impregnation was completed, the pressure vessel was decompressed, the foaming particles inside were taken out, and the foaming particles were put into a foaming machine equipped with a stirrer as quickly as possible. ^{After charging, particles having a bulk density of 0.083 g / cm 3} were obtained by foaming (steam foaming) using steam of 0.10 MPa (gauge pressure) for 120 seconds while stirring.
The obtained foamed particles were placed in a 10 L autoclave, the inside of the autoclave was adjusted to 0.7 MPa (gauge pressure) with air, and the particles were impregnated with air at 20 ° C. for 12 hours (dry impregnation). After the impregnation was completed, the pressure vessel was decompressed, the foamed particles inside were taken out, and the foamed particles were put into a foaming machine equipped with a stirrer as quickly as possible. ^{After charging, foamed particles having a bulk density of 0.033 g / cm 3} were obtained by foaming (steam foaming) using steam of 0.02 MPa (gauge pressure) for 180 seconds while stirring.
The surface of the obtained foamed particles was washed with 0.01N-hydrochloric acid to remove calcium carbonate adhering to the surface to obtain foamed particles for molding. A cross-sectional photograph of the foamed particles is shown in FIG.

[Preparation of foam molded product]
The obtained foamed particles were allowed to stand at room temperature for 3 days, then put into a 10 L autoclave, and internal pressure was applied to the foamed particles at 0.2 MPa (gauge pressure) for 1 hour with air. The foamed particles are taken out and filled in a foamed bead automatic molding machine (DABO Japan, DPM-7454) equipped with a mold having a length of 400 mm, a width of 300 mm, and a thickness of 30 mm at a cracking rate of 10%, and then at a pressure of 0.10 MPa. The steam was introduced under the heating conditions of mold heating for 5 seconds, one-sided heating for 10 seconds, reverse one-sided heating for 5 seconds, and double-sided heating for 15 seconds to foam the foamed particles. Next, water cooling was performed for 20 seconds, and then when the surface pressure value of the foamed molded product dropped to 0.01 MPa by vacuum cooling, it was taken out from the ^{mold and foam molded with a density of 0.033 g / cm 3} (foaming ratio 30 times). I got a body.

(Example 2)
The amount of butyl acrylate used was 1.05 kg, and after the polymerization (second polymerization) was completed, the mixture was cooled to 70 ° C., and then 0.35 kg of glycerin diacet monolaurate as a plasticizer (1.0 mass% with respect to the composite resin particles). Foamed particles and a foamed molded product were obtained in the same manner as in Example 1 except that the composite resin particles were impregnated by being charged and held for 1 hour.

(Example 3)
The polyethylene-based resin (PE-based resin) was replaced with PE2, the amount of butyl acrylate used was 0.35 kg, and after the completion of the polymerization (second polymerization), the temperature was cooled to 70 ° C., and then 0.35 kg of diisobutyl adipate (composite) was used as a plasticizer. Foamed particles and a foamed molded product were obtained in the same manner as in Example 1 except that the composite resin particles were impregnated by adding 1.0% by mass with respect to the resin particles and holding the mixture for 1 hour.

(Example 4)
Foamed particles and foamed molded article were obtained in the same manner as in Example 1 except that the amount of PE1 used was 13.07 kg and the amount of butyl acrylate used was changed to 0.28 kg.

(Example 5)
The amount of PE1 used was 13.2 kg, the amount of butyl acrylate used was 0.57 kg, and after the completion of the polymerization (second polymerization), the mixture was cooled to 70 ° C., and then 0.18 kg of diisobutyl adipate as a plasticizer (to the composite resin particles). Foamed particles and a foamed molded product were obtained in the same manner as in Example 4 except that the composite resin particles were impregnated with 0.5% by mass) and held for 1 hour.

(Example 6)
The polyethylene-based resin (PE-based resin) was replaced with PE2, the amount of PE2 used was 14.9 kg, and the amount of butyl acrylate used was 0.29 kg. Obtained.

(Example 7)
Foamed particles and foamed molded article were obtained in the same manner as in Example 6 except that the amount of PE2 used was 15.5 kg and the amount of butyl acrylate used was 1.22 kg.

(Example 8)
The polyethylene-based resin (PE-based resin) was replaced with PE3, the amount of PE3 used was 14.9 kg, and the amount of butyl acrylate used was 0.39 kg. Obtained.

(Comparative Example 1)
[Preparation of composite resin particles]
Composite resin particles were obtained in the same manner as in Example 1 except that the polyethylene-based resin (PE-based resin) was replaced with PE4 and the amount of butyl acrylate used was 0.35 kg.
[Preparation of foamed particles (wet impregnation and decompression foaming)]
300 parts by weight of water, 0.5 parts by weight of tertiary calcium phosphate (dispersant), 0.028 parts by weight of n-paraffin sulfonic acid sodium (surfactant), and 100 parts of the obtained composite resin particles (seed particles) in a 10 L autoclave. 0.75 parts by weight and 0.75 parts by weight of dibutyl sebacate were charged and pressurized to 1.0 MPa with carbon dioxide. After heating the inside of the container to 155 ° C., carbon dioxide gas was introduced into the charged liquid, the pressure inside the container was pressurized to 3.0 MPa, and the container was held for 30 minutes to be impregnated with carbon dioxide gas (wet impregnation). While maintaining the temperature and pressure, the particles are foamed (excluded) by opening the valve at the bottom of the pressure-resistant container and discharging the charged liquid through an orifice plate with an opening diameter of 3.6 mmφ into a cylinder filled with saturated steam at 94 ° C. Pressure foaming) was performed to obtain foamed particles.
[Preparation of foam molded product]
A foamed molded product was obtained in the same manner as in Example 1 except that no internal pressure was applied to the foamed particles.

(Comparative Example 2)
Foamed particles and foamed molded article were obtained in the same manner as in Comparative Example 1 except that the amount of PE4 used was 13.07 kg and the amount of butyl acrylate used was changed to 0.28 kg.

(Comparative Example 3)
Foamed particles and a foamed molded product were obtained in the same manner as in Comparative Example 1 except that the polyethylene-based resin (PE-based resin) was replaced with PE5.

(Comparative Example 4)
Foamed particles and a foamed molded product were obtained in the same manner as in Comparative Example 2 except that the polyethylene-based resin (PE-based resin) was replaced with PE5.

The physical properties of the foamed particles and the foamed molded product obtained in Examples and Comparative Examples were measured. The measurement results are shown in Table 1.

In Examples 1 to 8 in which the foamed particles obtained by dry impregnation using an ethylene-vinyl acetate copolymer containing a small amount (4, 6 or 9% by mass) of vinyl acetate were used, the amount per foamed particle was large. Since the number of bubbles was 0, it was confirmed that there was no significant change in the size of the bubbles.

In Examples 1 to 8, excellent results were obtained in terms of appearance and surface elongation, and it was confirmed that unevenness due to voids and gaps between surface particles on the surface of the foamed molded product were extremely suppressed, and the smoothness was high. Was done.
On the other hand, in Comparative Examples 1 and 2 using the polyethylene-based resin containing no vinyl acetate, there were many large bubbles per foamed particle, both the appearance and the surface elongation were evaluated low, and the smoothness of the surface of the foamed molded product was low. Was inadequate. As a result, it was confirmed that when an inorganic gas is used as the foaming agent, the size of the bubbles in the foamed particles fluctuates greatly, which adversely affects the surface of the foamed molded product.

Further, in Comparative Examples 1 and 2, since the fusion rate was low, the particles were easily peeled off at the interface between the particles, and the heat fusion was insufficient. On the other hand, in Examples 1 to 8, the fusion rate was high, and it was confirmed that the heat fusion between the foamed particles was sufficient.

In the evaluation of the dimensional change rate with respect to the mold, the dimensional change rate was as small as 10/1000 or less in all of Examples 1 to 8. On the other hand, in Comparative Examples 1 to 4, the dimensional change rate was as large as 11/1000 to 13/1000, and the dimensional stability was insufficient. From this, it was confirmed that the foam molded product of the present invention has high dimensional stability. In particular, from the comparison between Example 3 and Comparative Examples 1 and 3, and the comparison between Example 4 and Comparative Examples 2 and 4, it can be seen that the dimensional change rate of the foamed molded product obtained from the foamed particles of the present invention is small. ..

Claims (11)

Styrene composite polyethylene-based resin foamed particles containing a polyethylene-based resin and a polystyrene-based resin as a base resin.
The vinyl acetate content of the polyethylene-based resin is 1% by mass to 15% by mass with respect to the total mass of the polyethylene-based resin.
The polyethylene-based resin has a melting point of 95 ° C. or higher and lower than 110 ° C., and the styrene composite polyethylene-based resin foamed particles have 0 to 0 large bubbles having a bubble diameter of 3 times or more the average bubble diameter at the center of the particles. Including 2
Effervescent particles. The styrene composite polyethylene-based resin foamed particles according to claim 1, wherein the vinyl acetate content is 1% by mass to 10% by mass. The styrene composite polyethylene-based resin foamed particles according to claim 1 or 2, wherein the average bubble diameter in the central portion is 100 μm to 400 μm. The styrene composite polyethylene resin foamed particles according to any one of claims 1 to 3, wherein the content of the polystyrene resin is 120 to 1000 parts by mass with respect to 100 parts by mass of the polyethylene resin. Any of claims 1 to 4, wherein the melt flow rate of the polyethylene resin is 2.0 g / 10 minutes or less, and / or the density of the polyethylene resin is 0.910 g / cm ^{3 to} 0.935 g / cm ^3. The styrene composite polyethylene-based resin foamed particles described in 1. The styrene composite polyethylene-based resin foamed particles according to any one of claims 1 to 5, which contain 0.01% by mass to 0.2% by mass of calcium carbonate. The styrene composite polyethylene-based resin according to any one of claims 1 to 6, which contains 0.01% by mass to 0.2% by mass of at least one compound selected from the group consisting of diisobutyl adipate and glycerin diacet monolaurate. Foamed particles. The method for producing styrene composite polyethylene-based resin foamed particles according to any one of claims 1 to 7, which comprises a polyethylene-based resin and a polystyrene-based resin as a base resin.
The vinyl acetate content of the polyethylene resin is 1% by mass to 15% by mass with respect to the total mass of the polyethylene resin, and the melting point of the polyethylene resin is 95 ° C. or higher and lower than 110 ° C.
A production method comprising a step of press-fitting an inorganic gas into styrene composite polyethylene-based resin particles to obtain foamable particles, and a step of foaming the foamable particles with steam to obtain styrene composite polyethylene-based resin foamed particles. In the step of obtaining the foamable particles, the press-fitting of the inorganic gas into the styrene composite polyethylene-based resin particles is a dry impregnation method, and in the step of obtaining the styrene composite polyethylene-based resin foamed particles, in the absence of liquid water, the above. Foaming effervescent particles with water vapor,
The manufacturing method according to claim 8. A foamed molded product composed of a fused body of a plurality of styrene composite polyethylene-based resin foamed particles containing a polyethylene-based resin and a polystyrene-based resin as a base resin.
The styrene composite polyethylene-based resin foamed particles are the foamed particles according to any one of claims 1 to 7.
Foam molded product. The foamed molded product according to claim 10, wherein the foamed molded product is used as a cushioning material for a vehicle.

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