JP2010043195A - Method for producing pre-expanded particle of styrene-modified polyethylene-based resin and foamed molding of styrene-modified polyethylene-based resin - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a pre-expanded particle of styrene-modified polyethylene-based resin, being excellent in molding processability even after dissipation of an expanding agent of the pre-expanded particle, and having high crack resistance. <P>SOLUTION: This method for producing the pre-expanded particle of styrene-modified polyethylene-based resin including a gel component insoluble in xylene includes: obtaining a particle of styrene-modified polyethylene-based resin by impregnating and polymerizing a particle of polyethylene-based resin with a styrene-based monomer; dispersing the particle of styrene-modified polyethylene-based resin in an aqueous dispersion medium with a dispersing agent in a pressure-resistant container; adding the expanding agent into the pressure-resistant container; heating the dispersed material to the temperature higher than the softening point of the particle of styrene-modified polyethylene-based resin; expanding the particle of styrene-modified polyethylene-based resin by opening one end of the pressure-resistant container and releasing the particle of styrene-modified polyethylene-based resin to an area having lower pressure than the pressure-resistant container; wherein the amount of oxygen in system in the pressure-resistant container is maintained at a predetermined amount. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a method for producing styrene-modified polyethylene resin pre-expanded particles. Furthermore, the present invention relates to a styrene-modified polyethylene resin foam molded article having excellent crack resistance and molding processability.

  Polyolefin resin foam moldings are generally used as packaging materials because they are generally highly elastic and have a high strain recovery capability against repeated stresses, as well as being excellent in oil resistance and split resistance. ing. However, it has the disadvantages of low rigidity, easy shrinkage of the foamed molded product after in-mold molding, and low compressive strength.

  As a method for improving such a defect, a foam molded article made of a styrene-modified polyethylene resin obtained by polymerizing a polyethylene resin impregnated with a styrene monomer is known.

  For example, in Patent Document 1, in a modified thermoplastic resin particle obtained by polymerizing a polyethylene resin by impregnating a styrene monomer, the polyethylene particle is added to the styrene monomer for the purpose of improving the weather resistance. An initiator having a 10-hour half-life temperature of 60 ° C. to 105 ° C. of an organic peroxide having no benzene ring as a polymerization catalyst when adding, polymerizing and crosslinking is used, and having no benzene ring as a polyethylene crosslinking agent Polymerization / crosslinking reaction is performed using an initiator having an organic oxide 10-hour half-life temperature of 100 ° C. to 125 ° C. Furthermore, a method is disclosed in which pre-expanded particles are obtained by impregnating the resin particles obtained above after the polymerization / crosslinking reaction with a foaming agent.

  However, in the styrene-modified polyethylene resin pre-expanded particles obtained by the above method, good moldability of the pre-expanded particles is obtained, and the molded body formed by molding the pre-expanded particles has high crack resistance. It is difficult to achieve both, and various attempts have been made.

  For example, Patent Document 2 describes a gel amount and a molecular weight range for achieving both crack resistance and moldability, and pre-expanded particles having excellent moldability are obtained. However, the evaluation of the pre-expanded particles in Patent Document 2 is merely evaluating the molding and surface properties of the pre-expanded particles cured at room temperature for one day after the pre-expansion.

  As described above, in the styrene-modified polyethylene resin pre-foamed particles and the styrene-modified polyethylene resin foam molded article, many studies have been made on improving crack resistance, but there is almost no molding processability. There has been a demand for styrene-modified polyethylene resin pre-foamed particles having excellent moldability even after the foaming agent escapes from the foamed particles.

  In the production of these conventional styrene-modified polyethylene resin pre-foamed particles, a gel component is generated at the stage of preparing the resin particles before foaming, and the gel component is generated at the time of impregnation with the foaming agent. I didn't come.

Patent Document 3 describes a method of improving the secondary foamability of the pre-expanded particles obtained by decomposing the surface portion of the polypropylene resin particles using a peroxide in a pressure-resistant container when impregnated with the foaming agent. ing. In this document, a reaction using a peroxide is performed, but a gel component is not generated in the pre-expanded particles.
JP-A-4-126726 JP 2006-29895 A JP 2005-97412 A

  In view of the circumstances as described above, the present invention is to provide styrene-modified polyethylene resin pre-foamed particles that are excellent in moldability even after the foaming agent escapes from the pre-foamed particles and have high crack resistance. .

  As a result of intensive studies to solve the above problems, styrene-modified polyethylene resin particles are dispersed in an aqueous dispersion medium together with a dispersant in a pressure vessel, and a foaming agent is placed in the pressure vessel. After heating to a temperature equal to or higher than the softening point of styrene-modified polyethylene resin particles impregnated with a foaming agent, one end of the pressure vessel is opened and a mixture of styrene-modified polyethylene resin particles and an aqueous dispersion medium is prepared. When the amount of oxygen in the system in the pressure vessel and the amount of the crosslinking agent are in a predetermined relationship when released into the low pressure region than in the pressure vessel, the resulting styrene-modified polyethylene resin pre-expanded particles are obtained. A styrene-modified polyethylene resin foamed molded article with good surface properties can be produced even immediately after the foaming agent has passed through the sun, and a steel with excellent crack resistance. It found that the modified polyethylene resin expanded molded article is obtained, thereby completing the present invention.

That is, according to the first aspect of the present invention, polyethylene resin particles are impregnated with a styrene monomer and polymerized to form styrene modified polyethylene resin particles, and the styrene modified polyethylene resin particles are dispersed in a pressure vessel. After being dispersed in an aqueous dispersion medium together with the agent, a foaming agent is placed in the pressure vessel and heated to a temperature equal to or higher than the softening point of the styrene-modified polyethylene resin particles, and then one end of the pressure vessel is opened to open the styrene-modified polyethylene. A method for producing styrene-modified polyethylene resin pre-expanded particles comprising a gel component insoluble in xylene, wherein the resin particles are discharged into a lower pressure region than the pressure-resistant container and foamed. The present invention relates to a method for producing styrene-modified polyethylene resin pre-expanded particles, wherein the amount of oxygen in the system in the pressure vessel satisfies the following formula (1).
0 ≦ (X / Y) × 100 ≦ 35 (1)
(Wherein, X is the amount of oxygen in the system (mol), Y is the amount of crosslinking agent in the resin after polymerization (mol))

As a preferred embodiment, the present invention relates to a method for producing styrene-modified polyethylene resin pre-expanded particles as described above, wherein the amount of oxygen in the system in the pressure vessel satisfies the following formula (2).
5 ≦ (X / Y) × 100 ≦ 15 (2)
(Wherein, X is the amount of oxygen in the system (mol), Y is the amount of crosslinking agent in the resin after polymerization (mol))

  The second aspect of the present invention relates to styrene-modified polyethylene-based foamed resin pre-foamed particles obtained by the above-described production method. As a preferred embodiment, the gel component insoluble in xylene in the styrene-modified polyethylene-based resin pre-foamed particles is used. The styrene-modified polyethylene resin pre-expanded particles described above, wherein the amount is 10 wt% or more and 50 wt% or less.

  The third aspect of the present invention relates to a styrene-modified polyethylene resin foamed molded article obtained by molding the styrene-modified polyethylene resin pre-expanded particles described above.

  The styrene-modified polyethylene resin pre-expanded particles obtained by the present invention are styrene-modified polyethylene resin foam molded articles having good surface properties even after the foaming agent escapes from the styrene-modified polyethylene resin pre-expanded particles. Is obtained. Therefore, the bead life is extended, which is industrially advantageous. Further, the obtained styrene-modified polyethylene resin foamed molded article has high crack resistance.

The method for producing styrene-modified polyethylene resin pre-expanded particles of the present invention comprises impregnating a polyethylene resin particle with a styrene monomer and polymerizing it to form a styrene-modified polyethylene resin particle. Resin particles are dispersed in an aqueous dispersion medium together with a dispersant in a pressure vessel, and a foaming agent is placed in the pressure vessel and heated to a temperature equal to or higher than the softening point of the styrene-modified polyethylene resin particles. And a styrene-modified polyethylene resin comprising a gel component insoluble in xylene, wherein the styrene-modified polyethylene resin particles are discharged into a low-pressure region and foamed while maintaining the pressure constant. This is a method for producing pre-expanded particles, and the amount of oxygen in the system in the pressure vessel satisfies the following formula (1).
0 ≦ (X / Y) × 100 ≦ 35 (1)
Here, in the formula, X is the amount of oxygen in the system (mol), and Y is the amount of crosslinking agent (mol) in the resin after polymerization.

Preferably, the following formula (2)
5 ≦ (X / Y) × 100 ≦ 15 (2)
Satisfied.

  In the present invention, the amount of oxygen in the system refers to the amount of oxygen existing in the space volume of the pressure vessel (hereinafter sometimes referred to as “space oxygen amount”) and the amount of oxygen dissolved in the aqueous dispersion medium ( Hereinafter, it may be referred to as “dissolved oxygen amount”).

  The amount of space oxygen is calculated from the equation of state of the ideal gas. The amount of dissolved oxygen is determined by taking out the aqueous dispersion medium from the pressure vessel and immediately using a dissolved oxygen meter (for example, Iijima Electronics Co., Ltd .: dissolved oxygen meter B-505 / main body, Wagnet GU-AM / sensor). taking measurement.

  In order to adjust the amount of oxygen in the system, the amount of oxygen in the system can be adjusted by adjusting one or both of the amount of spatial oxygen and the amount of dissolved oxygen.

  As a method for adjusting the space oxygen and the dissolved oxygen, there are a method of repeatedly deoxidizing and nitrogen substitution after sealing the pressure vessel and reducing the amount to a desired amount and a method of using a deoxidizer.

  Also, as a method of reducing the amount of dissolved oxygen in the aqueous dispersion medium, in addition to using an aqueous dispersion medium with a small amount of dissolved oxygen from the beginning, a method in which the amount of dissolved oxygen is reduced by heating the aqueous dispersion medium is used. There are ways to do this.

  In the present invention, the polyethylene resin particles are impregnated and polymerized with a styrene monomer, preferably 150 parts by weight to 400 parts by weight, more preferably 180 parts by weight to 300 parts by weight. Within this range, the moldability is good, and the resulting styrene-modified polyethylene resin foamed molded product tends to be pre-expanded styrene-modified polyethylene resin particles having good crack resistance.

  Polyethylene resins constituting the polyethylene resin particles used in the present invention are ethylene homopolymers such as high-density polyethylene and low-density polyethylene, polyethylene, and propylene, 1-butene, 1-pentene, 1-hexene, for example. And a copolymer with α-olefin such as vinyl acetate, acrylic acid ester, vinyl chloride and the like. Among these, a copolymer of ethylene and vinyl acetate is preferable. Furthermore, an ethylene / vinyl acetate copolymer having a melt flow rate (hereinafter sometimes referred to as MFR) of 1.5 g / 10 min or less and a vinyl acetate content of 10 wt% or less is preferable. If the MFR exceeds 1.5 g / 10 min, the development of crack resistance tends to be difficult. If the vinyl acetate exceeds 10% by weight, the melting point is low, so that resin deformation tends to occur during polymerization. The MFR is a value measured according to JIS K 6924.

  The polyethylene resin is made into polyethylene resin particles by melting in advance using, for example, an extruder, a kneader, a Banbury mixer, a roll or the like. The shape is preferably a particle state such as powder or pellet. The average particle weight of these particles is preferably in the range of 0.1 mg / particle to 3 mg / particle. If it is less than 0.1 mg / grain, the foaming agent may dissipate rapidly, making it difficult to increase the magnification. If it is greater than 3 mg / grain, the moldability during molding may be deteriorated. At this time, various additives such as a plasticizer and a bubble regulator may be added as necessary.

  As the styrene monomer used in the present invention, styrene and styrene derivatives such as α-methyl styrene, paramethyl styrene, t-butyl styrene and chlorostyrene can be used as main components. In addition, components capable of copolymerization with styrene derivatives such as esters of acrylic acid and methacrylic acid such as methyl acrylate, butyl acrylate, methyl methacrylate, and ethyl methacrylate, acrylonitrile, dimethyl fumarate, ethyl fumarate, etc. These various monomers may be used alone or in combination of two or more. Furthermore, polyfunctional monomers such as divinylbenzene and alkylene glycol dimethacrylate can also be used.

  In the polymerization, it is preferable to use a polymerization initiator. As the polymerization initiator that can be used, radical generating polymerization initiators generally used for the production of thermoplastic polymers can be used. Typical examples include benzoyl peroxide, lauroyl peroxide, t- Butyl perpivalate, t-butyl peroxyisopropyl carbonate, di-t-butyl peroxyhexahydroterephthalate, 1,1-di (t-butylperoxy) 3,3,5-trimethylcyclohexane, 1,1-di Examples thereof include organic peroxides such as (t-butylperoxy) cyclohexane, and azo compounds such as azobisisobutyronitrile and azobisdimethylvaleronitrile. These polymerization initiators can be used alone or in admixture of two or more. The weight average molecular weight can be adjusted by the amount of the polymerization initiator and the reaction temperature.

  The amount of the polymerization initiator used is preferably 0.05 parts by weight or more and 1.0 parts by weight or less, more preferably 0.1 parts by weight or more and 0.5 parts by weight or less with respect to 100 parts by weight of the styrene monomer. Part or less.

  In the present invention, as a method of polymerizing styrene-modified polyethylene resin particles, a styrene monomer is continuously or intermittently added to an aqueous suspension containing polyethylene resin particles charged in a container equipped with a stirrer. By adding to the polyethylene resin particles, the polyethylene resin particles are impregnated with a styrene monomer and polymerized. In the polymerization, the weight average molecular weight of the styrene-modified polyethylene resin pre-expanded particles can be adjusted by arbitrarily selecting the addition rate of the styrene monomer to be added. A polymerization temperature of 70 ° C. or higher and 90 ° C. or lower is preferable because styrene-modified polyethylene resin pre-expanded particles having a desired weight average molecular weight can be obtained.

  In the present invention, it is necessary that the styrene-modified polyethylene resin pre-expanded particles contain a gel insoluble in xylene, and it is preferable to use a cross-linking agent for forming the gel. Specifically, it is preferable to use a radical species-generating crosslinking agent having a 10-hour half-life temperature of 100 ° C. or more and 125 ° C. or less. Examples of such radical species-generating crosslinking agents include di-t-butyl peroxide (10-hour half-life temperature: 123 ° C.), dicumyl peroxide (10-hour half-life temperature: 116 ° C.), and t-butyl peroxy. Benzoate (10-hour half-life temperature: 104 ° C.), t-butyl peroxyacetate (10-hour half-life temperature: 102 ° C.), 2,2-bis-t-butyl peroxybutane (10-hour half-life temperature: 103 ° C.) Etc. These crosslinking agents can be added to the polymerization system before addition of the styrene monomer or together with the styrene monomer.

  In the present invention, when the amount of decomposition of the crosslinking agent is preferably 50% or less, more preferably 40% or less, the foaming agent is 10 to 60 parts by weight with respect to 100 parts by weight of the styrene-modified polyethylene resin particles. It is preferable to carry out a crosslinking reaction at a temperature of 120 ° C. or higher and 150 ° C. or lower in the presence of a blowing agent. Styrene-modified polyethylene resin pre-foamed particles with excellent moldability when the foaming agent escapes from the styrene-modified polyethylene resin pre-foamed particles when the foaming agent is added after the decomposition amount of the crosslinking agent exceeds 50% May not be obtained.

  In addition, if a radical species-generating crosslinking agent having a 10-hour half-life temperature lower than 100 ° C. is used, the crosslinking reaction proceeds too much during the polymerization, and crosslinking occurs when crosslinking is performed at 120 ° C. or more and 150 ° C. or less in the presence of a foaming agent. When the 10-hour half-life temperature exceeds 125 ° C., it tends to take time to advance the crosslinking reaction at a temperature of 120 ° C. or more and 150 ° C. or less.

  In the present invention, the decomposition amount of the crosslinking agent is determined based on the half-life at the polymerization / crosslinking temperature using the Arrhenius equation from the 1 minute half-life temperature, 1-hour half-life temperature, 10-hour half-life temperature and activation energy of the crosslinking agent. And the amount of decomposition at the time of polymerization / crosslinking can be calculated therefrom.

  Examples of the blowing agent that can be used in the present invention include known ones such as aliphatic hydrocarbons such as propane, isobutane, normal butane, isopentane, normal pentane, and neopentane, difluoroethane, tetrafluoroethane, and the like. Examples thereof include volatile foaming agents such as hydrofluorocarbons having an ozone depletion coefficient of zero, and inorganic gases such as nitrogen and carbon dioxide. These foaming agents can be used in combination.

  The amount of the foaming agent varies depending on the type of styrene-modified polyethylene resin to be used, the composition of the base resin, the type of foaming agent, the target foaming ratio, etc. The amount is preferably 10 parts by weight or more and 60 parts by weight or less, more preferably 15 parts by weight or more and 30 parts by weight or less, based on 100 parts by weight of the modified polyethylene resin particles. If it is less than 10 parts by weight, a sufficient foaming ratio cannot be obtained, and it may be difficult to obtain prefoamed grains having good moldability. If it exceeds 60 parts by weight, the dispersion state of the resin particles when impregnated with the foaming agent becomes unstable, and the resins tend to aggregate. In some cases, a method of using water contained in the aqueous dispersion medium as a foaming agent is used.

  The styrene-modified polyethylene resin pre-expanded particles according to the present invention are obtained by dispersing the styrene-modified polyethylene resin particles and the dispersant as described above in an aqueous dispersion medium, and placing the foaming agent in a pressure-resistant container. After heating to a temperature equal to or higher than the softening point of the resin particles, the styrene-modified polyethylene-based resin particles are discharged and foamed in a lower pressure atmosphere than in the pressure vessel.

  Specifically, for example, in a pressure vessel, an aqueous dispersion medium containing styrene-modified polyethylene resin particles, a foaming agent, a dispersing agent, and, if necessary, a dispersion aid is charged and stirred at a predetermined temperature (hereinafter, The temperature is raised to “foaming temperature.”), The resin particles are impregnated with a foaming agent, and additional foaming agent is added as necessary, and the inside of the pressure vessel is referred to as a constant pressure (hereinafter referred to as “foaming pressure”). In some cases, the mixture is released from the lower part of the pressure vessel under a pressure lower than the pressure inside the pressure vessel. There is no particular limitation on the pressure vessel to be used, and any vessel that can withstand the vessel pressure and the temperature in the vessel at the time of producing the pre-foamed particles may be used. For example, an autoclave type pressure vessel may be mentioned.

  The aqueous dispersion medium in the present invention is not particularly limited as long as it does not dissolve the resin particles. Examples thereof include water, methanol, ethanol, glycerin, and ethylene glycol, and these may be used in combination. In particular, it is preferable to use water. The amount of the aqueous dispersion medium used is preferably 50 to 1000 parts by weight with respect to 100 parts by weight of the styrene-modified polyethylene resin particles.

  The dispersant in the present invention may be a known one, and examples thereof include poorly water-soluble inorganic salts such as tricalcium phosphate, magnesium carbonate, calcium carbonate, aluminum oxide, hydroxyapatite, magnesium pyrophosphate, and kaolin.

  In the present invention, a dispersion aid may be used to efficiently disperse the styrene-modified polyethylene resin particles with a small amount of dispersant.

  Examples of the dispersion aid include surfactants, and specifically, anionic surfactants such as sodium dodecylbenzene sulfonate, higher alcohol sodium sulfate, n-paraffin sulfonic acid soda, alkyl naphthalene sulfonic acid soda, and chloride. Examples include cationic surfactants such as benzalkonium, alkyltrimethylammonium chloride, and dialkyldimethylammonium chloride. In particular, an anionic surfactant is suitable.

  The amount of these dispersants and dispersion aids used varies depending on the type thereof, the type and amount of styrene-modified polyethylene resin, the type of foaming agent, etc., but is usually based on 100 parts by weight of the aqueous dispersion medium. The dispersant is preferably 0.05 part by weight or more and 3 parts by weight or less, and the dispersion aid is 0.0001 part by weight or more and 0.2 part by weight or less.

  Further, for the purpose of reducing the amount of dispersant adhering to the styrene-modified polyethylene resin pre-expanded particles, an acid may be mixed with the aqueous dispersion medium to make the aqueous dispersion medium acidic.

  The mixture containing the styrene-modified polyethylene resin particles prepared in the pressure vessel as described above is heated to a predetermined foaming temperature, usually above the softening point of the styrene-modified polyethylene resin particles, with stirring, While maintaining the temperature for a certain period of time, usually 5 to 180 minutes, preferably 10 to 60 minutes, the pressure in the sealed container is increased, and the styrene-modified polyethylene resin particles are impregnated with the foaming agent. Thereafter, the foaming agent is additionally supplied until a predetermined foaming pressure is reached, and the foaming agent may be maintained for a certain period of time, usually 5 minutes to 180 minutes, preferably 10 to 60 minutes. The styrene-modified polyethylene resin particles maintained at the foaming temperature and the foaming pressure are released into the low-pressure atmosphere (usually atmospheric pressure) by opening the valve provided at the bottom of the pressure-resistant container. Polyethylene resin pre-expanded particles are obtained.

  When the styrene-modified polyethylene resin particles are discharged under a low-pressure atmosphere, they can be discharged through an opening orifice of 1 to 10 mmφ for the purpose of adjusting the flow rate and reducing the variation in magnification. In some cases, the low-pressure atmosphere is filled with saturated steam for the purpose of increasing the expansion ratio.

  The styrene-modified polyethylene resin pre-expanded particles obtained as described above comprise a gel component insoluble in xylene. The amount of the gel component insoluble in xylene is preferably 10% by weight or more and 50% by weight or less, more preferably 15% by weight or more and 40% by weight or less. Within this range, when performing in-mold molding, there is no need for high-pressure or long-time steam heating, and a tendency to obtain a styrene-modified polyethylene resin foam molded article that is easy to increase the magnification and has good crack resistance. It is in.

  The amount of gel component insoluble in xylene is measured as follows. Place 0.4 g of pre-expanded resin particles in a 200-mesh wire mesh bag, immerse in 450 ml of xylene boiled under atmospheric pressure for 2 hours, take it out after cooling, and then remove the resin in new boiled xylene. For 1 hour and after cooling, it is taken out from xylene. Thereafter, the immersion and elution were repeated for 2 hours and 1 hour in the same manner. After that, the liquid was drained overnight at room temperature, dried in an oven at 150 ° C. for 1 hour, allowed to cool naturally to room temperature, and the residue after cooling was removed. The weight ratio of the amount of the gel component to the initial amount of pre-expanded particles is used as the gel component amount.

  The styrene-modified polyethylene resin pre-expanded particles obtained by the production method of the present invention preferably have a weight average molecular weight of 150,000 or more and 350,000 or less of components soluble in tetrahydrofuran. Within this range, when performing in-mold molding, there is no need for high-pressure or long-time steam heating, and a tendency to obtain a styrene-modified polyethylene resin foam molded article that is easy to increase the magnification and has good crack resistance. It is in.

  Here, the weight-average molecular weight of the component soluble in tetrahydrofuran refers to a component extracted by immersing 0.02 g of styrene-modified polyethylene resin pre-expanded particles in 20 ml of normal temperature tetrahydrofuran for 24 hours through a 0.2 μm filter. This is a value obtained by gel permeation chromatography GPC (Tosoh HLC-8220 GPC, detector: RI8020, column: TSKgel-GMHHR × 2) based on a standard polystyrene sample.

  The styrene-modified polyethylene resin pre-expanded particles thus obtained are molded by a general in-mold molding method. Specifically, it is filled in a mold that can be closed but cannot be sealed, and is heat-sealed to obtain a styrene-modified polyethylene resin foam molded article. The obtained styrene-modified polyethylene resin foam molded article exhibits excellent fusing properties.

  In addition, the styrene-modified polyethylene resin pre-expanded particles of the present invention can be used when the foaming agent escapes after the production, and the foaming agent in the styrene-modified polyethylene resin pre-expanded particles becomes 1% by weight or less. Even if it exists, it shows favorable moldability, and the obtained styrene-modified polyethylene resin foam molded article shows good surface properties.

  Examples and comparative examples will be described below, but the present invention is not limited thereby. In addition, about measurement evaluation, it implemented as follows.

<Surface condition of molded body>
The surface state of the styrene-modified polyethylene resin foamed molded article was evaluated by visual observation. The surface with the larger numerical value is the surface state with fewer gaps between the particles, and 3 or more expressed with a perfect score of 5 was regarded as acceptable.
5: Gaps are not found 4: There are some gaps but almost unknown 3: There are gaps in some places, but the tolerance level as a whole 2: The gaps are conspicuous 1: There are many gaps

<Crack resistance (measurement of half fracture height)>
Based on JIS K 7211, a 321 g steel ball was dropped with a sample piece obtained by cutting the foamed molded product into 200 × 20 × 20 (t) mm, and the half fracture height was measured. 35 cm or more was accepted.

<Surface condition of the molded body molded when the foaming agent escapes>
The surface state of the styrene-modified polyethylene resin foam molded article was evaluated by visual observation after being stored in a drying room at about 35 ° C. for 1 day after molding. The one where the numerical value is larger is the surface state where there are few gaps between the particles, and 3 or more was regarded as acceptable.

<Measurement of gel amount>
Place 0.4 g of pre-expanded resin particles in a 200-mesh wire mesh bag, immerse in 450 ml of xylene boiled under atmospheric pressure for 2 hours, take it out after cooling, and then add the resin into the newly boiled xylene. Immerse for 1 hour and remove from xylene after cooling. Thereafter, the immersion and elution were repeated for 2 hours and 1 hour in the same manner. After that, the liquid was drained overnight at room temperature, dried in an oven at 150 ° C. for 1 hour, naturally cooled to room temperature, and the remaining content after cooling. Was the gel component, and the weight ratio of the amount of the gel component to the initial pre-expanded particles was the gel component.

<Molecular weight of tetrahydrofuran solubles>
A component extracted by immersing 0.02 g of styrene-modified polyethylene resin pre-expanded particles in 20 ml of normal temperature for 24 hours with a 0.2 μm filter was obtained by gel permeation chromatography (GPC). : Tosoh HCL-8220GPC, detector: RI8020, column: TSKgel-GMHHR × 2), based on a standard polystyrene sample.

(Production method)
Using "Evaate F1103-1" manufactured by Sumitomo Chemical Co., Ltd. as the polyethylene resin, 0.2 part by weight of talc is mixed with 100 parts by weight of the polyethylene resin, melt-mixed in the extruder, granulated, and submerged in water. By cutting immediately after extrusion, spherical polyethylene resin particles having a particle weight of about 1 mg / particle were produced.

  Subsequently, 150 parts by weight of water, 1 part by weight of tribasic calcium phosphate, 0.024 part by weight of sodium α-olefin sulfonate, and 30 parts by weight of polyethylene resin particles are suspended in a 6 L autoclave, and a polymerization initiator is added to 15 parts by weight of styrene. 0.26 parts by weight of benzoyl peroxide (10-hour half-life temperature: 74 ° C.) and 0.60 part by weight of t-butyl peroxybenzoate (10-hour half-life temperature: 104 ° C.) as a radical species-generating crosslinking agent The dissolved solution was added. Thereafter, this aqueous suspension was heated to 70 ° C. and maintained for 30 minutes to impregnate the polyethylene resin particles with the styrene monomer solution. Furthermore, the temperature was raised to 85 ° C., 55 parts by weight of a styrene monomer was dropped into the reaction system over 3 hours and 40 minutes to perform polymerization, and the temperature was further raised to 125 ° C. and held for 30 minutes. Styrene-modified polyethylene resin particles were obtained by dehydration and drying. The amount of crosslinking agent decomposed at this point was 26%.

Example 1
A 10 L autoclave was charged with 330 parts by weight of water, 1.0 part by weight of tricalcium phosphate, 0.0175 part by weight of sodium n-paraffin sulfonate, and 100 parts by weight of styrene-modified polyethylene resin particles. Thereafter, the amount of oxygen in the system was adjusted by pressurizing with nitrogen to 2.50 MPa and then reducing to -0.088 MPa. After adding 22 parts by weight of normal rich butane (normal butane / isobutane = 70/30) as a foaming agent to the autoclave, the temperature was raised to 140 ° C. and held for 50 minutes to advance the impregnation of the foaming agent and the crosslinking reaction. After that, at a foaming pressure of 2.30 MPa (gauge pressure), styrene-modified polyethylene resin particles together with an aqueous dispersion medium are discharged under atmospheric pressure from an autoclave through an orifice with an opening diameter of 5 mm, and a styrene-modified foam having a foam volume ratio of 35 times is released. Polyethylene resin pre-expanded particles were obtained. During the discharge under atmospheric pressure, the pressure in the autoclave was adjusted to be kept constant by introducing high-pressure nitrogen (hereinafter sometimes referred to as a foaming step).

  The obtained styrene-modified polyethylene resin pre-foamed particles were dehydrated and dried, and then pre-foamed particles cured at room temperature for 2 days, and the amount of residual foaming agent in the drying chamber at about 35 ° C. was 1 with respect to the pre-foamed particles. Two types of pre-expanded particles dried to less than wt% are prepared, heated with water vapor at a pressure of 0.10 MPa (gauge pressure), and molded with a 300 x 450 x 25 (t) mm size mold. Then, a styrene-modified polyethylene resin foam molded article was obtained and evaluated. The evaluation results were as shown in Table 1. The amount of remaining foaming agent in the styrene-modified polyethylene resin pre-expanded particles is approximately 2 g of the pre-expanded particles, weighed again after cooling to room temperature after heat-treating in an oven at 150 ° C. for 30 minutes. And was calculated by calculating the weight percentage of the dissipation.

（実施例２）
１０Ｌオートクレーブでの発泡工程にて、窒素加圧を行わず、−０．０９２ＭＰａまで減圧することによって系内酸素量を調整した以外は実施例１と同様にし、スチレン改質ポリエチレン系樹脂予備発泡粒子を得た。得られたスチレン改質ポリエチレン系樹脂予備発泡粒子は、実施例１と同様に成形して評価した。

(Example 2)
Styrene-modified polyethylene resin pre-expanded particles in the same manner as in Example 1 except that the amount of oxygen in the system was adjusted by reducing the pressure to -0.092 MPa without performing nitrogen pressurization in the 10 L autoclave foaming step. Got. The obtained styrene-modified polyethylene resin pre-expanded particles were molded and evaluated in the same manner as in Example 1.

(Example 3)
Styrene-modified polyethylene resin pre-expanded particles in the same manner as in Example 1 except that the amount of oxygen in the system was adjusted by reducing the pressure to -0.084 MPa without performing nitrogen pressurization in the foaming process in a 10 L autoclave. Got. The obtained styrene-modified polyethylene resin pre-expanded particles were molded and evaluated in the same manner as in Example 1.

(Comparative Example 1)
Styrene-modified polyethylene resin pre-expanded particles in the same manner as in Example 1 except that the amount of oxygen in the system was adjusted by reducing the pressure to -0.074 MPa without performing nitrogen pressurization in the foaming process in a 10 L autoclave. Got. The obtained styrene-modified polyethylene resin pre-expanded particles were molded and evaluated in the same manner as in Example 1.

(Comparative Example 2)
Styrene-modified polyethylene resin pre-expanded particles were obtained in the same manner as in Example 1 except that nitrogen pressurization and decompression were not performed in the foaming step in a 10 L autoclave. The obtained styrene-modified polyethylene resin pre-expanded particles were molded and evaluated in the same manner as in Example 1.

  The styrene-modified polyethylene resin pre-expanded particles obtained by the production method of the present invention are excellent in molding processability even if not immediately after the pre-expansion, and the obtained styrene-modified polyethylene resin foam molded article is crack resistant. Since it has excellent properties, it can be suitably used especially for automobile members and cushioning materials.

Claims (5)

Polyethylene resin particles are impregnated with a styrene monomer and polymerized to form styrene-modified polyethylene resin particles. The styrene-modified polyethylene resin particles are dispersed in a water-based dispersion medium together with a dispersant in a pressure vessel. After the foaming agent is placed in the pressure vessel and heated to a temperature equal to or higher than the softening point of the styrene-modified polyethylene resin particles, one end of the pressure vessel is opened so that the styrene-modified polyethylene resin particles are more than in the pressure vessel. A method for producing styrene-modified polyethylene resin pre-expanded particles containing a gel component insoluble in xylene, which is characterized by being released into a low pressure region and foamed, wherein the amount of oxygen in the system in the pressure vessel is A method for producing styrene-modified polyethylene resin pre-expanded particles, wherein the following formula (1) is satisfied.
0 ≦ (X / Y) × 100 ≦ 35 (1)
(Wherein, X is the amount of oxygen in the system (mol), Y is the amount of crosslinking agent in the resin after polymerization (mol)) The method for producing styrene-modified polyethylene resin pre-expanded particles according to claim 1, wherein the amount of oxygen in the system in the pressure vessel satisfies the following formula (2).
5 ≦ (X / Y) × 100 ≦ 15 (2)
(Wherein, X is the amount of oxygen in the system (mol), Y is the amount of crosslinking agent in the resin after polymerization (mol))   A styrene-modified polyethylene-based foamed resin pre-expanded particle obtained by the production method according to claim 1 or 2.   The styrene-modified polyethylene resin pre-expanded particles according to claim 3, wherein the amount of the gel component insoluble in xylene in the styrene-modified polyethylene resin pre-expanded particles is 10 wt% or more and 50 wt% or less.   A styrene-modified polyethylene resin foam molded article obtained by molding the styrene-modified polyethylene resin pre-expanded particles according to claim 3 or 4.