Method for producing expandable polystyrene resin particles

Abstract

To provide a method for producing foamable styrenic resin particles which is excellent in dispersion stability at the time of suspension polymerization, has a small amount of a residual monomer, and can give a foam molding of low thermal conductivity.SOLUTION: A method for producing foamable styrenic resin particles includes a polymerization step of suspension-polymerizing a monomer containing 50 wt.% or more of a styrenic monomer and graphite together with a polymerization initiator in an aqueous medium, and obtaining a polymer, and impregnating the obtained polymer with a volatile foaming agent, wherein a used amount of the graphite is 1 pts.wt. or more and 10 pts.wt. or less with respect to 100 pts.wt. of the monomer, and the polymerization step adds an azo-based initiator into the aqueous medium, starts polymerization, subjects the monomer to polymerization reaction until a polymerization conversion rate becomes 80-90%, and then adds a peroxide, and further subjects the monomer to polymerization reaction.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to a method for producing low thermal conductivity expandable polystyrene resin particles.

Background Art

[0002] A polystyrene resin foam molded body obtained using expandable polystyrene resin particles is a foam having excellent balance such as light weight, heat insulation, and cushioning properties, and has been widely used as a food container box, a cooler box, a cushioning material, and a heat insulating material for houses and the like.

[0003] In recent years, in relation to various problems such as global warming, energy saving by improving the heat insulation of buildings such as houses has been aimed at, and higher heat insulation performance has been required for polystyrene resin foam molded bodies. As a polystyrene resin foam molded body having high heat insulation performance, a polystyrene resin foam molded body containing graphite as a radiation inhibitor is known.

[0004] As a method for producing a polystyrene resin foam containing graphite, there is a method in which expandable polystyrene resin particles containing graphite are foamed into foamed particles, and the foamed particles are molded in a mold to produce a foam. As a method for producing expandable polystyrene resin particles containing graphite, for example, as described in Patent Documents 1 and 2, there is a method of polymerizing a styrene monomer in a suspension aqueous liquid in the presence of graphite particles (suspension polymerization method).

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0006] In Patent Document 1, peroxide is used as a polymerization initiator to produce expandable styrene resin particles containing graphite. However, when peroxide is used as a polymerization initiator, the oxygen radicals generated by the cleavage of the peroxide may be trapped by the graphite, resulting in the polymerization reaction not proceeding and the occurrence of an induction period. Also, after the induction period, the polymerization rate may increase rapidly, causing the polymerization reaction to run out of control and the dispersion of the styrene polymer to become unstable. As a result, coarse particles may be generated due to particle coalescence, and styrene polymer particles having a desired particle size may not be obtained in good yield.

[0007] In Patent Document 2, in order to stabilize styrene suspension polymerization in the presence of graphite, it has been proposed to use a combination of a low-temperature decomposable azo initiator and a high-temperature decomposable azo initiator as a polymerization initiator. The nitrogen radicals, which are the cleavage products of the azo initiator, can react with the oxygen functional groups on the graphite surface, enabling stable suspension polymerization. However, the amount of monomer remaining in the expandable styrene resin particles tends to increase. The present invention provides a method for producing expandable styrene resin particles that are excellent in dispersion stability during suspension polymerization, have a small amount of residual monomer, and can provide a foam molded article with low thermal conductivity.

Means for Solving the Problems

[0008] As a result of intensive studies, the present inventors have found that in the presence of graphite, a monomer containing a styrene monomer is suspension polymerized using an azo initiator until the polymerization conversion rate reaches 80% or more, and then a peroxide is added to further proceed with the polymerization reaction to produce expandable styrene resin particles, thereby enabling the stable production of expandable styrene resin particles with low thermal conductivity and low VOC content, and thus completing the present invention.

[0009] That is, one embodiment of the present invention includes the following configuration. [1] A polymerization step of obtaining a polymer by suspension-polymerizing a monomer containing 50% by weight or more of a styrene monomer and graphite in an aqueous medium together with a polymerization initiator, and a foaming agent impregnation step of impregnating the obtained polymer with a volatile foaming agent. The amount of the graphite used is 1 part by weight or more and 10 parts by weight or less with respect to 100 parts by weight of the monomer. In the polymerization step, an azo initiator is added to the aqueous medium to initiate polymerization, and the monomer is subjected to a polymerization reaction until the polymerization conversion rate reaches 80% to 90%. Then, a peroxide is added to further polymerize the monomer. A method for producing foaming styrene resin particles. [2] The method for producing foaming styrene resin particles according to [1], wherein the average particle size of the graphite is 1 to 10 μm. [3] The method for producing foaming styrene resin particles according to [1] to [2], wherein the azo initiator contains 2,2'-azobisisobutyronitrile. [4] The method for producing foaming styrene resin particles according to [1] to [3], wherein the 10-hour half-life temperature of the peroxide is 80 to 110°C. [5] The method for producing foaming styrene resin particles according to [1] to [4], wherein 10 parts by weight or less of a styrene resin is added to 100 parts by weight of the monomer in the aqueous medium. [6] The method for producing foaming styrene resin particles according to [1] to [5], wherein 0.5 part by weight or more and 5 parts by weight or less of a flame retardant is added to 100 parts by weight of the monomer in the aqueous medium. [7] The method for producing foaming styrene resin particles according to [6], wherein the flame retardant contains a bromine-containing organic compound having a 2,3-dibromo-2-alkylpropyl group. [8] The method for producing foaming styrene resin particles according to [1] to [7], wherein the monomer contains a monomer copolymerizable with the styrene monomer. [Effects of the Invention]

[0010] According to the present invention, foaming styrene resin particles having low thermal conductivity and low VOC can be produced by a suspension polymerization method. [Embodiments for Carrying Out the Invention]

[0011] Hereinafter, the present invention will be described in detail. The method for producing expandable polystyrene resin particles of the present invention includes a polymerization step of subjecting a monomer containing 50% by weight or more of a styrene monomer and graphite to suspension polymerization in an aqueous medium together with a polymerization initiator to obtain a polymer, and a foaming agent impregnation step of impregnating the obtained polymer with a volatile foaming agent. The amount of graphite used is 1 part by weight or more and 10 parts by weight or less with respect to 100 parts by weight of the monomer. In the polymerization step, an azo-based initiator is added to an aqueous medium to initiate polymerization, and the monomer is subjected to a polymerization reaction until the polymerization conversion rate reaches 80% to 90%. Then, a peroxide is added to further polymerize the monomer. This is a method for producing expandable polystyrene resin particles.

[0012] [1. Polymerization step] The polymerization step is a step of adding graphite and a polymerization initiator to an aqueous medium and subjecting a monomer containing 50% by weight or more of a styrene monomer to suspension polymerization to obtain a polymer. An azo-based initiator is added to initiate polymerization, and the monomer is subjected to a polymerization reaction until the polymerization conversion rate reaches 80% to 90%. Then, a peroxide is added to further polymerize the monomer. In the present invention, the amount of graphite used is 1 part by weight or more and 10 parts by weight or less with respect to 100 parts by weight of the monomer. Note that the styrene monomer contained in the entire monomer used in the present invention may be 50% by weight or more, and may be 100% by weight.

[0013] (Aqueous medium) The method for producing expandable polystyrene resin particles of the present invention is a method of suspending and polymerizing a monomer containing a styrene monomer in an aqueous medium, that is, so-called suspension polymerization. It is preferable that the aqueous medium contains a dispersant. Examples of the dispersant that can be used in suspension polymerization include hardly water-soluble inorganic salts such as tricalcium phosphate, magnesium pyrophosphate, hydroxyapatite, and kaolin, and polyvinyl alcohol, methyl cellulose, polyacrylamide, polyvinyl Examples of the water-soluble polymer include pyrrolidone. When a hardly water-soluble inorganic salt is used as the dispersant, an anionic surfactant such as sodium α-olefin sulfonate or sodium dodecylbenzenesulfonate is preferably used in combination because the dispersion stability is increased. These dispersants are preferably contained in the aqueous medium from the start of polymerization, and may be further added at any point in the polymerization process as necessary.

[0014] The amount of the dispersant used depends on the type of the dispersant. When a hardly water-soluble inorganic salt is used as the dispersant, the amount of the dispersant used is preferably 0.1 part by weight or more and 1.5 parts by weight or less with respect to 100 parts by weight of water. When a water-soluble polymer is used as the dispersant, the dispersant is preferably used so as to be 30 ppm or more and 100 ppm or less in the aqueous medium. Further, when an anionic surfactant is used in combination with the hardly water-soluble inorganic salt, the anionic surfactant is preferably used so as to be 30 ppm or more and 100 ppm or less in the aqueous medium.

[0015] (Styrene monomer) In the production method of the present invention, at least a styrene monomer is polymerized. Therefore, by the production method of the present invention, expandable styrene resin particles composed of a resin containing a component derived from the styrene monomer can be obtained. The styrenic monomer used in the present invention is not particularly limited as long as it is a styrenic compound having one ethylenically unsaturated group, and styrene and / or styrene derivatives can be used. As the above styrene derivatives, any known styrene derivatives can be used. For example, α-methylstyrene, vinyltoluene (for example, a mixture of m-vinyltoluene and p-vinyltoluene), chlorostyrene (for example, 3-chlorostyrene, 4-chlorostyrene, etc.), ethylstyrene (for example, 3-ethylstyrene, 4-ethylstyrene, etc.), isopropylstyrene (for example, 4-isopropylstyrene), dimethylstyrene (for example, 3,5-dimethylstyrene, 2,3-dimethylstyrene, etc.), bromostyrene (for example, 2-bromostyrene, 3-bromostyrene, 4-bromostyrene, etc.) and the like can be used. These styrenic monomers may be used alone or in combination of two or more.

[0016] (Monomer copolymerizable) In the production method of the present invention, a monomer copolymerizable with the above styrenic monomer may be used together with the above styrenic monomer for copolymerization. The copolymerizable monomer may be any vinyl monomer copolymerizable with the styrene monomer. For example, divinylbenzene such as o-divinylbenzene, m-divinylbenzene, p-divinylbenzene, etc., polyfunctional vinyl monomers such as alkylene glycol di(meth)acrylate like ethylene glycol di(meth)acrylate, polyethylene glycol di(meth)acrylate, etc.; monofunctional vinyl monomers such as (meth)acrylate esters like (meth)acrylonitrile, methyl (meth)acrylate, butyl (meth)acrylate, etc. These copolymerizable monomers may be used alone or in combination of two or more. In these application documents, "vinyl monomer" means a compound having at least one ethylenically unsaturated group (vinyl group in a broad sense), "polyfunctional vinyl monomer" means a vinyl monomer having two or more ethylenically unsaturated groups, and "monofunctional vinyl monomer" means a vinyl monomer having one ethylenically unsaturated group. Also, in these application documents, "(meth)acrylate" means acrylate or methacrylate, "(meth)acrylonitrile" means acrylonitrile or methacrylonitrile, and "(meth)acrylic" means acrylic or methacrylic.

[0017] When using the above copolymerizable monomer, the amount used is less than the amount of the above styrene monomer used. In other words, the content of the component derived from the above copolymerizable monomer in the above expandable styrene resin particles is such that the component derived from the styrene monomer in the above expandable styrene resin particles becomes the main component of the polymer component (the total of the component derived from the styrene monomer and the component derived from the above copolymerizable monomer), that is, less than 50% by weight.

[0018] (Graphite) The graphite used in the present invention acts as a radiation inhibitor and imparts low heat insulation properties. In the present invention, when the expandable styrene resin particles produced by suspension polymerization contain graphite, the timing and mode of adding the graphite are not particularly limited. For example, the graphite may be added to the aqueous medium simultaneously with the monomer at the start of polymerization, or may be added to the aqueous medium at a different time from the monomer, but it is preferably added to the aqueous medium in a state where the graphite is dispersed in the monomer.

[0019] The above-mentioned graphite is not particularly limited, and any known natural and artificial graphite can be used. As the above-mentioned graphite, graphite having various shapes such as flaky, lamellar, spherical, etc. can be used. Among them, graphite having a flaky or lamellar shape is preferred. By using graphite having a flaky or lamellar shape as the above-mentioned graphite, the thermal conductivity of the polystyrene resin foam molded body formed by foam molding the expandable styrene resin particles can be further reduced.

[0020] As the above-mentioned graphite, graphite having an average particle diameter in the range of 1 to 100 μm is preferred, and graphite having an average particle diameter in the range of 1 to 30 μm is more preferred. This is because when the average particle diameter of the graphite is smaller than the above numerical range, the heat insulation property of the polystyrene resin foam molded body obtained by foaming the expandable styrene resin particles may decrease. On the other hand, when the average particle diameter of the graphite is larger than the above numerical range, the bubble film is likely to break when the expandable styrene resin particles are foamed, and it may not be possible to achieve a high expansion ratio of the polystyrene resin foam molded body.

[0021] The amount of the graphite used is 1 to 10 parts by weight, preferably 1 to 6 parts by weight, and more preferably 1 to 4 parts by weight, based on 100 parts by weight of the monomer used. The monomer used may be only a styrene-based monomer or a monomer copolymerizable with the styrene-based monomer. That is, the amount of the graphite used is 1 to 10 parts by weight based on 100 parts by weight of the total amount of the monomers used. When the amount of the graphite used is less than 1 part by weight, the thermal conductivity of the polystyrene-based resin foam molded body obtained by foaming the expandable styrene-based resin particles may not be sufficiently reduced. On the other hand, when the amount of the graphite used exceeds 10 parts by weight, the bulk expansion ratio and the mechanical strength of the polystyrene-based resin foam molded body obtained by foaming the expandable styrene-based resin particles may decrease.

[0022] (azo initiator) When the polymer radicals generated by the thermal decomposition of an azopolymer having an azo group in the main chain of carbon black are reacted, it is known that the polymer radicals are efficiently trapped by the condensed aromatic rings and the polymer is grafted onto the carbon black surface. From this, it is presumed that the azo initiator used in the present invention reacts with the oxygen-containing groups on the surface of the graphite particles, protects the surface of the graphite particles, and allows the suspension polymerization to proceed stably.

[0023] For example, oil-soluble azo initiators such as 2,2'-azobis-(4-methoxy-2,4-dimethylvaleronitrile) (10-hour half-life temperature: 30°C), 2,2'-azobis(2,4-dimethylvaleronitrile) (10-hour half-life temperature: 51°C), 2,2'-azobis(isobutyronitrile) (10-hour half-life temperature: 65°C), 2,2'-azobis(isobutyric acid) dimethyl (10-hour half-life temperature: 66°C), 2,2'-azobis(2-methylbutyronitrile) (10-hour half-life temperature: 67°C), 1,1'-azobis-(cyclohexane-1-carbonitrile) (10-hour half-life temperature: 88°C), 2,2'-azobis[N-(2-propenyl)-2-methylpropionamide] (10-hour half-life temperature: 96°C), 1-[(1-cyano-1-methylethyl)azo]formamide (10-hour half-life temperature: 104°C), 2,2'-azobis(N-butyl-2-methylpropionamide) (10-hour half-life temperature: 110°C), etc. are included. Here, the "10-hour half-life temperature" refers to the decomposition temperature at which a half-life of 10 hours is obtained. These azo initiators may be used alone or in combination of two or more.

[0024] The polymerization temperature and the polymerization time are selected from the 10-hour half-life temperature of the azo initiator used, and the polymerization temperature is preferably +5°C to +20°C higher than the 10-hour half-life temperature. If it is less than 5°C, the polymerization rate is slow and the productivity decreases. If it exceeds 20°C, the polymerization rate becomes fast and the heat removal of the polymerization machine becomes impossible. For example, when carrying out suspension polymerization, if the polymerization temperature range that can be stably controlled is 60 to 100°C, the 10-hour half-life temperature of the azo polymerization initiator is preferably in the range of 40 to 95°C. 2,2'-azobis(isobutyronitrile) (10-hour half-life temperature: 65°C) is preferable not only in terms of low cost but also in terms of easy storage and handling.

[0025] The usage amount of the azo initiator is 1% by weight or more and 100% by weight or less based on the amount of graphite (100% by weight), and the usage amount is appropriately selected according to the amount of graphite used. If the usage amount of the azo initiator is less than 1% by weight of the amount of graphite, many oxygen-containing groups on the graphite surface will remain, causing an induction period in suspension polymerization. If it exceeds 100% by weight, it will easily react with the oxygen-containing groups on the graphite surface, resulting in a high reaction heat, exceeding the cooling capacity of the polymerization machine, and tending to become uncontrollable.

[0026] (Peroxide) As the peroxide used in the present invention, the half-life temperature for 10 hours is preferably 80 to 110 °C, and more preferably 90 to 105 °C. For example, as the peroxide, 1,1-bis(t-amylperoxy)-3,3,5-trimethylcyclohexane (half-life temperature for 10 hours: 92 °C), 1,1-bis(t-butylperoxy)cyclohexane (half-life temperature for 10 hours: 97 °C), 2,2-di(t-amylperoxy)butane (half-life temperature for 10 hours: 97 °C), t-butylperoxy-2-ethylhexyl monocarbonate (half-life temperature for 10 hours: 99 °C), t-amylperoxy-2-ethylhexyl monocarbonate (half-life temperature for 10 hours: 98.5 °C), etc. can be mentioned. 2,2-di(t-amylperoxy)butane, t-butylperoxy-2-ethylhexyl monocarbonate, and t-amylperoxy-2-ethylhexyl monocarbonate, which have an effect of reducing the remaining monomer, are preferred.

[0027] These peroxides (initiators) are preferably 0.1 part by weight or more and 0.5 part by weight or less, more preferably 0.15 part by weight or more and 0.3 part by weight or less, based on 100 parts by weight of the monomer. When the amount of the polymerization initiator is small, the remaining monomer tends to remain, and when it is large, it becomes difficult to adjust the molecular weight. When the half-life temperature for 10 hours is less than 80 °C, the peroxide is consumed quickly, and the effect of reducing the remaining monomer in the foaming agent impregnation step in the next process is reduced.

[0028] (Additive) In the polymerization process, additives such as flame retardants, flame retardant aids, styrene resins, chain transfer agents, plasticizers, and bubble regulators may be appropriately added in addition to the monomer, graphite, and azo initiator.

[0029] (Flame retardant) The flame retardants that can be used in the present invention are not particularly limited, and any of the flame retardants conventionally used in styrene resin foamed moldings can be used. Among them, brominated flame retardants with a high flame retardancy-imparting effect are desirable. Examples of the brominated flame retardants used in the present invention include brominated organic compounds having a 2,3-dibromo-2-alkylpropyl group such as 2,2-bis[4-(2,3-dibromo-2-methylpropoxy)-3,5-dibromophenyl]propane (alias: tetrabromobisphenol A-bis(2,3-dibromo-2-methylpropyl ether)), 2,2-bis[4-(2,3-dibromopropoxy)-3,5-dibromophenyl]propane (alias: tetrabromobisphenol A-bis(2,3-dibromopropyl ether)), and brominated butadiene-vinyl aromatic hydrocarbon copolymers such as brominated styrene-butadiene block copolymer, brominated random styrene-butadiene copolymer, and brominated styrene-butadiene graft copolymer (for example, disclosed in Japanese Patent Application Laid-Open No. 2009-516019). These brominated flame retardants may be used alone or in combination of two or more.

[0030] Among the brominated flame retardants, in particular, brominated organic compounds having a 2,3-dibromo-2-alkylpropyl group are preferable because they are easily soluble in styrene monomers, easy to handle, and do not reduce the heat resistance of the final product.

[0031] The amount of the bromine-based flame retardant used should be appropriately selected according to the expansion ratio of the foamed molded article which is the final product, but it is preferably 0.5 parts by weight or more and 8.0 parts by weight or less, more preferably 0.5 parts by weight or more and 5.0 parts by weight or less, based on 100 parts by weight of the monomer. When the amount of the bromine flame retardant is less than 0.5 parts by weight, the flame retardancy-imparting effect tends to be small, and when it exceeds 8.0 parts by weight, the strength of the resulting styrene resin foamed molded article tends to decrease.

[0032] (Flame retardant aid) As the flame retardant aid that can be used in the present invention, radical generators such as peroxides are used, and peroxides having a half-life temperature of 120 °C or higher, such as t-butyl peroxybenzoate (125 °C), dicumyl peroxide (136 °C), 2,3-dimethyl-2,3-diphenylbutane (234 °C), and 2,5-dimethyl-2,5-di(t-butylperoxy)hexane (138 °C), are preferred. If the half-life temperature is within this range, the decomposition of the flame retardant aid can be suppressed with respect to the polymerization temperature of 60 to 100 °C used in the present invention, and the suspension polymerization system will not become unstable. The amount of the flame retardant aid used is preferably 0.3 parts by weight or more and 1.5 parts by weight or less based on 100 parts by weight of the monomer. If the amount of the flame retardant aid is small, the flame retardant performance deteriorates, and if it is large, the heat resistance tends to deteriorate.

[0033] (Styrene resin) When the monomer is polymerized with an azo-based initiator in the presence of graphite, a large amount of the azo-based initiator needs to be added to protect the graphite surface, and the molecular weight of the resulting foaming styrene resin particles tends to be low. For adjusting the molecular weight, the styrene resin is dissolved in the monomer for polymerization, which serves to increase the molecular weight of the foaming styrene resin particles. The styrene resin used in the present invention is dissolved in the styrene-based monomer and subjected to suspension polymerization, and is used for adjusting the molecular weight of the polymer. The styrene resin is not particularly limited, and examples include styrene resin, styrene-acrylonitrile copolymer, styrene-acrylonitrile-α-methylstyrene terpolymer, and the like.

[0034] The weight average molecular weight (Mw) of the styrene resin to be used is 250,000 or more and 400,000 or less. If it is less than 250,000, the effect of increasing the molecular weight is small, and if it exceeds 400,000, the solution viscosity tends to increase. By increasing the amount of the styrene resin used, the molecular weight of the expandable styrene resin particles can be increased. The amount of the styrene resin used is appropriately selected according to the molecular weight of the expandable styrene resin particles, but it is preferably 10 parts by weight or less, more preferably 5 parts by weight or less, based on 100 parts by weight of the monomer. With the addition amount within this range, the molecular weight of the expandable styrene resin particles is adjusted to 150,000 to 300,000. If it exceeds 10 parts by weight, the solution viscosity of the monomer becomes high and the polymerization system becomes unstable.

[0035] (Chain transfer agent) The chain transfer agent that can be used in the present invention is used for molecular weight adjustment. The chain transfer agent is not particularly limited. For example, alkyl mercaptans such as n-hexyl mercaptan, n-octyl mercaptan, tert-octyl mercaptan, n-dodecyl mercaptan, tert-dodecyl mercaptan; phenolic compounds such as α-methylstyrene dimer, 2,6-di-tert-butyl-4-methylphenol, styrenated phenol; allyl compounds such as allyl alcohol; halogenated hydrocarbon compounds such as dichloromethane, dibromomethane, carbon tetrachloride, etc. These chain transfer agents may be used alone or in combination of two or more.

[0036] (Plasticizer) The plasticizer that can be used in the present invention includes high-boiling plasticizers having a boiling point of 200 °C or higher. For example, fatty acid glycerides such as triglyceride stearate, triglyceride palmitate, triglyceride laurate, diglyceride stearate, monoglyceride stearate; vegetable oils such as coconut oil, palm oil, palm kernel oil; aliphatic esters such as dioctyl adipate, dibutyl sebacate; organic hydrocarbons such as liquid paraffin, cyclohexane, etc. However, since the heat resistance tends to deteriorate due to the use of these, it is preferably not used.

[0037] (Bubble regulator) Examples of the bubble regulator that can be used in the present invention include aliphatic bisamides such as methylene bisstearic acid amide and ethylene bisstearic acid amide, and polyethylene wax.

[0038] (Polymerization conversion rate) In the polymerization step of the present invention, in the presence of an azo initiator, the monomer is polymerized with graphite until the polymerization conversion rate reaches 80% - 90%, and further, a peroxide is added to proceed with the polymerization reaction. Hereinafter, in the polymerization step, the polymerization step until a peroxide is further added during the polymerization may be referred to as the first polymerization step, and the polymerization step of further proceeding with the polymerization reaction after adding the peroxide may be referred to as the second polymerization step.

[0039] In the first polymerization step, an azo initiator is used as the polymerization initiator. In the first polymerization step, the polymerization reaction of the monomer is initiated in the presence of an azo initiator, and the monomer is polymerized until the polymerization conversion rate reaches 80% - 90%, preferably 83% - 90%. Here, the azo initiator only needs to be present in the aqueous medium at the start of polymerization, and may be added separately into the aqueous medium before the start of polymerization, apart from the monomer, or may be added into the aqueous medium as a monomer mixture in a state dispersed in the monomer. If the polymerization conversion rate of the polymer in the first polymerization step is 80% or more, the graphite surface is sufficiently protected by the styrene-based resin, so that the delay of the polymerization reaction can be suppressed. On the other hand, if the polymerization conversion rate is less than 80%, there are many oxygen-containing groups on the graphite surface, which delays the polymerization rate and deteriorates productivity. Also, when the polymerization conversion rate exceeds 90%, the time of the first polymerization step is prolonged and productivity decreases. Note that the polymerization initiator used in the first polymerization step is not particularly limited as long as it contains an azo initiator as the main component, but it may contain a polymerization initiator other than the azo initiator. The usage amount of the polymerization initiator other than the azo initiator is preferably 30% or less of the total amount of the initiators used in the first polymerization step, and in particular, it is preferably free of peroxides.

[0040] In the second polymerization step, a peroxide is used as a polymerization initiator. In the second polymerization step, the peroxide is added to an aqueous medium in which polymers having a polymerization conversion rate of 80% to 90% are dispersed, and the monomers in the polymerization system are further reacted. The polymerization conversion rate of the polymer at the end of the second polymerization step is not particularly limited, but it may exceed 90%, and preferably it is 98% or less. If the polymerization conversion rate exceeds 90%, even if a volatile foaming agent is added in the next step, the dispersibility of the suspension polymerization system will not become unstable and stable production can be achieved, and the remaining monomers can be reduced.

[0041] [2. Foaming Agent Impregnation Step] This is a step in the next step of the polymerization step, in which a volatile foaming agent is added and the volatile foaming agent and, if necessary, a foaming aid are impregnated into the polymer particles.

[0042] (Volatile Foaming Agent and Foaming Aid) In the production method of the present invention, by adding and impregnating a volatile foaming agent into a polymer (polystyrene-based resin particles) obtained by polymerizing a monomer containing the above styrene-based monomer, foaming polystyrene-based resin particles containing the volatile foaming agent can be obtained. The impregnation of the volatile foaming agent into the foaming polystyrene-based resin particles may be carried out after the above polymerization, or may be carried out during the above polymerization (while polymerizing). The impregnation of the volatile foaming agent can be carried out by a method known per se. For example, the impregnation of the volatile foaming agent during the above polymerization can be carried out by carrying out the polymerization reaction in a sealed container and pressuring the volatile foaming agent into the sealed container during the polymerization. The impregnation of the volatile foaming agent after the polymerization is completed can be carried out by carrying out the polymerization reaction in a sealed container and pressuring the volatile foaming agent into the sealed container after the polymerization.

[0043] The above-mentioned volatile foaming agent is not particularly limited as long as it is a volatile foaming agent (physical foaming agent) conventionally used for foaming polystyrene-based resins. Examples of the above-mentioned volatile foaming agents include aliphatic hydrocarbons having 5 or less carbon atoms such as propane, isobutane, n-butane, isopentane, neopentane, n-pentane, and the like. Among these volatile foaming agents, butane-based foaming agents such as isobutane and n-butane are preferred. The content of the volatile foaming agent in the expandable polystyrene-based resin particles is preferably in the range of 2 to 8% by weight, and more preferably in the range of 3 to 7% by weight. If the content of the volatile foaming agent is less than the above range, it may not be possible to obtain a polystyrene-based resin foam molding having a desired density when the expandable polystyrene-based resin particles are foam-molded. Also, if the content of the volatile foaming agent is less than the above range, the effect of enhancing the secondary foaming force during in-mold foaming is reduced when the expandable polystyrene-based resin particles are in-mold foam-molded, and thus the appearance of the polystyrene-based resin foam molding may deteriorate. Further, if the content of the volatile foaming agent is more than the above range, the time required for the cooling step in the process of manufacturing a polystyrene-based resin foam molding from the expandable polystyrene-based resin particles becomes longer, and thus the productivity of the polystyrene-based resin foam molding may decrease.

[0044] In the production method of the present invention, a known foaming aid may be used in combination with the foaming agent. Examples of the above-mentioned foaming aids include toluene, xylene, cyclohexane, diisobutyl adipate, and the like. When producing a polystyrene-based resin foam molding used as a building material or the like from expandable polystyrene-based resin particles, it is preferable to use diisobutyl adipate having a high boiling point as the foaming aid in order to avoid the occurrence of sick house syndrome due to the foaming aid. When impregnating polystyrene resin particles with a volatile foaming agent (and a foaming aid used as necessary), the temperature is preferably in the range of 80 to 130°C, more preferably in the range of 100 to 120°C. If the temperature during impregnation is lower than the above range, the time required to impregnate the polystyrene resin particles with the volatile foaming agent may become longer, or the monomer may not be sufficiently consumed, and low VOC may not be achieved. On the other hand, if the temperature during impregnation is higher than the above range, the polystyrene resin particles may fuse together to generate bonded particles.

[0045] (Shape and particle size of expandable polystyrene resin particles) The shape of the expandable polystyrene resin particles obtained by the production method of the present invention is not particularly limited, but is preferably spherical from the viewpoint of ease of molding. Further, considering the fillability into the mold and the like, the particle size of the expandable polystyrene resin particles is preferably in the range of 0.3 to 2.0 mm, more preferably in the range of 0.5 to 1.4 mm. The obtained expandable polystyrene resin particles are used to produce an expanded molded article. The expanded molded article is produced by using pre-expanded particles obtained by pre-expanding the expandable polystyrene resin particles.

[0046] [3. Pre-expanded particles] The expandable polystyrene resin particles obtained above are brought into contact with heated steam in a pre-expansion apparatus to produce pre-expanded particles having a desired expansion ratio. In the production of the pre-expanded particles, a blocking inhibitor and a fusion promoter are preferably attached to the surface of the expandable polystyrene resin particles in advance using a mixer such as a super mixer, a Nauta mixer, a universal mixer, a Proshear mixer, an Apex mixer, a Henschel mixer, a Lodige mixer, etc.; a blender such as a ribbon blender, a tumbler type blender.

[0047] (Blocking inhibitor) The anti-blocking agent is used to prevent the foamed particles from coalescing (blocking) with each other in the pre-foaming device. Examples include silicate minerals such as kaolinite, mica, talc, zeolite, chlorite, glauconite, smectite, etc., and fatty acid metal salts such as zinc stearate and magnesium stearate.

[0048] (Fusion promoter) The fusion promoter is used to increase the fusion strength of the foamed molded body. Examples include fatty acid triglycerides such as tricaprin, tristearin, trilinolein, and hydroxystearic acid triglyceride; fatty acid diglycerides such as dilaurin, distearin, and dilinolein; fatty acid monoglycerides such as monolaurin, monostearin, and monolinolein; and vegetable oils (hydroxy fatty acid triglycerides) such as hydrogenated castor oil (hydroxystearic acid triglyceride).

[0049] (Pre-foaming device) As a method for pre-foaming, for example, an ordinary method such as using a cylindrical pre-foaming device and heating and foaming the expandable thermoplastic resin particles using a heating medium such as steam can be adopted. The conditions of the pre-foaming device and the pre-foaming process may be appropriately set according to the base resin type of the expandable styrene-based resin particles, the desired pre-foaming ratio, etc., and are not particularly limited.

[0050] [4. Foamed molded body] The foamed molded body is obtained by heating and foaming (secondary foaming) the above-mentioned pre-foamed particles and then molding them. As a method of heating and foaming pre-expanded particles and then molding them, for example, ordinary methods such as in-mold foaming molding method can be adopted, in which the pre-expanded particles are filled into a mold and a heating medium such as steam is blown in for heating. As a specific in-mold foaming molding method, there is a method in which pre-expanded particles are filled into a mold that can be closed but not sealed airtight, and the pre-expanded particles are heated and fused by a heating medium to form an in-mold foamed molded body. The device used for heating and foaming and the conditions for heating and foaming may be appropriately set according to the composition of the expandable styrene resin particles, the desired foaming ratio, etc., and are not particularly limited.

[0051] The above-mentioned foamed molded body, particularly the in-mold foamed molded body, is suitable for, for example, packaging materials (trays) such as food containers, transport packaging materials such as fish boxes, and particularly for cushioning materials for household electrical appliances and precision parts that require low charging performance, etc., because it is easy to produce a molded body of a desired shape.

Examples

[0052] Examples and comparative examples are given below, but the present invention is not limited thereto. The molecular weight of the resin, the residual monomer amount in the resin, and the evaluation of the molded body in the examples and comparative examples were measured by the following methods. In addition, "parts" and "%" are based on weight unless otherwise specified.

[0053] (Molecular weight measurement method) 0.02 g of expandable resin particles was dissolved in 20 ml of tetrahydrofuran and measured by gel permeation chromatography (GPC) (HLC-8020 manufactured by Tosoh Corporation, column: TSKgel Super HZM-H, column temperature: 40 °C, flow rate: 0.35 ml / min).

[0054] (Measurement of residual monomer) 0.25 g of the expandable resin particles was dissolved in 20 ml of methylene chloride together with the internal standard cyclopentanol, and the amount of monomer contained in the expandable resin particles was measured using a gas chromatograph GC-2014 manufactured by Shimadzu Corporation (capillary column: Rtx-1 manufactured by GL Sciences, column temperature condition: heated from 50°C to 80°C at a heating rate of 3°C / min, then heated from 80°C to 180°C at a heating rate of 10°C / min, carrier gas: helium).

[0055] (Measurement of polymerization conversion rate) Styrene resin particles were collected from the lower part of the pressure-resistant container, and after wiping off the moisture on the surface of the resin particles with filter paper, they were measured by gas chromatography according to the above method for measuring the remaining monomer. The polymerization conversion rate was calculated from the amount of the remaining monomer component.

[0056] (Evaluation of the foamed molded body) The foamed molded body obtained by the production method described in Example 1 below was dried at room temperature for 24 hours, and then the following evaluations (1) to (4) were carried out.

[0057] (1) Evaluation of the fusion rate The foamed molded body was broken and its fracture surface was observed, and the ratio (fusion rate) at which the expandable particles were broken rather than at the interface of the expandable particles was determined. When the fusion rate was 80% or more, it was evaluated as qualified.

[0058] (2) Surface property The surface states of four parts of the foamed molded body were visually observed and evaluated in the following five grades. The average value of the four parts was taken as the score of the surface property. The larger the numerical value, the smaller the gap between the expandable particles and the more beautiful the surface state, and it was determined as qualified when it was "4" or more. 5: No gap is found 4: There are gaps partially, but hardly noticeable 3: There are gaps here and there, but it can be tolerated as a whole 2: The gaps are conspicuous 1: There are many gaps.

[0059] (3) Flame retardancy Five test pieces cut from the foamed molded body to a length of 200 mm, a width of 10 mm, and a thickness of 25 mm were cured in an oven at 60 °C for 12 hours, and then measured in accordance with Measurement Method A of JIS A9511:2006R. The average value of the five test pieces was obtained and taken as the anti-inflammatory time. An anti-inflammatory time within 1 minute was considered qualified.

[0060] (4) Heat insulation property (thermal conductivity) Test pieces cut from the foamed molded body to a length of 300 mm, a width of 300 mm, and a thickness of 25 mm were left in a dryer at 50 °C for 48 hours and 96 hours, and the weight loss of the molded body was measured and taken as the gas amount in the molded body. A calibration curve of the thermal conductivity of the foamed molded body with respect to each gas amount was calculated, and the thermal conductivity of the foam at a gas amount of 0 was extrapolated to obtain the thermal conductivity of the foam. The thermal conductivity was measured in accordance with JIS A9511 using a thermal conductivity measuring device HC-074 (Eihong Seiki Co., Ltd.).

[0061] The graphite, azo initiator, chain transfer agent, flame retardant, flame retardant aid, peroxide, etc. used in this example and the comparative example are as follows. (Graphite) · Flaky graphite SGP-40B: particle size 5 μm (manufactured by Marutoyo Casting Co., Ltd.) (Azo initiator) · AIBN; 2,2‘-azobisisobutyronitrile: half-life temperature: 65 °C: (manufactured by Fujifilm Wako Pure Chemical Corporation) · ADVN; 2,2‘-azobis(2,4-dimethylvaleronitrile): half-life temperature: 51 °C: (manufactured by Fujifilm Wako Pure Chemical Corporation) · ACHCN; 1,1’-azobis-(cyclohexane-1-carbonitrile): half-life temperature: 88 °C) (manufactured by Fujifilm Wako Pure Chemical Corporation) (Chain transfer agent) · α-methylstyrene dimer (MSD (manufactured by NOF Corporation)) (Flame retardant) · Tetrabromobisphenol A-bis(2,3-dibromo-2-methylpropyl) ether (Pyrogard SR-130 (manufactured by Dai-ichi Kogyo Seiyaku Co., Ltd.). (Flame retardant aid) · Dicumyl peroxide (Parkmyl D (manufactured by NOF Corporation)). (Peroxide) ·t-BEH; t-butyl peroxy-2-ethylhexyl monocarbonate: 10-hour half-life temperature 99 °C (Perbutyl E (NOF Corporation)) ·BPO; Benzoyl peroxide: 10-hour half-life temperature 74 °C (Niper BW (NOF Corporation)) (Volatile blowing agent) ·n-butane / iso-butane = 70 / 30 (Iwatani Corporation) (Anti-blocking agent) ·Zinc stearate (NOF Corporation) (Fusion promoter) ·Triglyceride hydroxystearate (Castor Wax (NOF Corporation)) Next, the manufacturing method of the styrene resin used in a part of the present examples and comparative examples will be described. (Manufacturing method of styrene resin) Into a polymerization vessel with a stirrer having an internal volume of 10 liters, 200 parts by weight of water, 0.1 part by weight of tricalcium phosphate, 0.003 part by weight of sodium α-olefin sulfonate, 0.23 part by weight of benzoyl peroxide (Niper BW (NOF Corporation)), and 0.05 part by weight of 1,1-bis(t-butylperoxy)cyclohexane (Perhexa C (NOF Corporation)) were stirred, and 100 parts by weight of a styrene monomer was added. Then, the temperature was raised, and polymerization was carried out at an internal temperature of 98 °C for 5 hours. Subsequently, polymerization was carried out at an internal temperature of 120 °C for 3.5 hours, followed by cooling, discharging, and drying to obtain styrene resin particles (polystyrene). The weight average molecular weight is 360,000.

[0062] (Examples 1 to 3) (Manufacture of foamed styrene resin particles) Using only a styrene monomer as the monomer, with respect to a total of 100 parts by weight of the monomer, the amounts of styrene resin, graphite, chain transfer agent, azo initiator, flame retardant, and flame retardant aid shown in Table 1 were dispersed and dissolved to obtain a monomer mixture. Into a polymerization vessel with a stirrer having an internal volume of 10 liters, while stirring an internal liquid (aqueous medium) consisting of 200 parts by weight of water, 0.1 part by weight of tricalcium phosphate, and 0.003 part by weight of sodium α-olefin sulfonate with respect to a total of 100 parts by weight of monomers, the above monomer mixture was added to the above internal liquid to prepare an aqueous suspension.

[0063] The internal temperature of the above polymerization vessel was raised, and polymerization was carried out at a polymerization temperature of 75 °C for 3.5 hours. The polymerization conversion rate was 84%. Then, 0.1 part by weight of tricalcium phosphate was added to the content, and 0.15 part by weight of peroxide (t-butylperoxy-2-ethylhexyl monocarbonate) was added. Then, the internal temperature was raised to 98 °C and polymerization was carried out for 1 hour. The polymerization conversion rate was 93%, and the polymerization process was completed.

[0064] Next, with respect to a total of 100 parts by weight of monomers, 7 parts by weight of a volatile foaming agent (n-butane / iso-butane = 7 / 3 (Iwai Gas Co., Ltd.)) was charged, the temperature was raised to 115 °C, and a foaming agent impregnation process was carried out for 8 hours. Then, it was cooled to 40 °C to obtain foamed styrene-based resin particles. The foamed styrene-based resin particles had no generation of coarse particles due to particle coalescence during polymerization (hereinafter, this state may be referred to as agglomeration), and polymerization was successfully completed.

[0065] (Production of pre-expanded particles) With respect to 100 parts by weight of the foamed styrene-based resin particles obtained above, 0.2 part by weight of zinc stearate as an anti-blocking agent and 0.07 part by weight of triglyceride hydroxystearate as a fusion promoter were mixed with a universal mixer (EM15 type, Tsukishima Machine Sales Co., Ltd.) for 2 minutes to obtain a mixture. 1000 g of the obtained mixture was charged into a pressure-type pre-expander equipped with a stirrer (manufactured by Daikai Kogyo Co., Ltd., CH-100), and pre-expansion (primary foaming) was carried out using steam (injected steam pressure 60 kPa) as a heating medium to obtain pre-expanded particles with an expansion ratio (apparent magnification) of 50 times. Then, the obtained pre-expanded particles were left at room temperature for 24 hours for curing and drying.

[0066] (Production of foamed molded article) The pre-expanded particles after curing and drying were heated using a molding machine (manufactured by Daisen Co., Ltd., KR-57) and a mold (box-shaped with a length of 450 mm, a width of 300 mm, and a depth of 25 mm) with steam (injected steam pressure of 80 kPa) as the heating medium for 10 seconds for the pre-expanded particles in the mold. Subsequently, they were cooled with water for 2 seconds and then cooled under vacuum. When the surface pressure gauge (pressure of the foam molded body) provided in the mold reached 30 kPa, the mold was opened and the foam molded body was taken out. Various measurements and evaluations described above were performed on the obtained foaming styrene resin particles, pre-expanded particles, and foam molded body, and the results are shown in Table 1.

[0067] (Examples 4 - 5) As monomers, styrene monomer and acrylonitrile monomer were used. 76 parts by weight of styrene monomer and 24 parts by weight of acrylonitrile monomer were used based on a total of 100 parts by weight of monomers. A monomer mixture in which a styrene-based resin, graphite, chain transfer agent, azo-based initiator, flame retardant, and flame retardant aid in the amounts described in Table 1 were dispersed and dissolved was obtained based on a total of 100 parts by weight of monomers. Using the same operation method as in Example 1, foaming styrene resin particles, pre-expanded particles, and foam molded body were produced, and various measurements and evaluations described above were performed, and the results are shown in Table 1.

[0068] (Examples 6 - 7) As monomers, styrene monomer, α-methylstyrene monomer, and acrylonitrile monomer were used. 71 parts by weight of styrene monomer, 5 parts by weight of α-methylstyrene monomer, and 24 parts by weight of acrylonitrile monomer were used based on a total of 100 parts by weight of monomers. A monomer mixture in which a styrene-based resin, graphite, chain transfer agent, azo-based initiator, flame retardant, and flame retardant aid in the amounts described in Table 1 were dispersed and dissolved was obtained based on a total of 100 parts by weight of monomers. Using the same operation method as in Example 1, foaming styrene resin particles, pre-expanded particles, and foam molded body were produced, and various measurements and evaluations described above were performed, and the results are shown in Table 1.

[0069] (Comparative Examples 1 - 3) A monomer mixture was obtained by dispersing and dissolving 100 parts by weight of styrene monomer as a monomer, styrene resin, graphite, chain transfer agent, flame retardant, flame retardant aid, and two azo initiators in the amounts shown in Table 1. Into a polymerization vessel equipped with a stirrer having an internal volume of 10 liters, while stirring an internal solution of 200 parts by weight of water, 0.1 part by weight of tricalcium phosphate, and 0.003 part by weight of sodium α-olefin sulfonate with respect to a total of 100 parts by weight of monomers, the above monomer mixture was added to the above internal solution to prepare an aqueous suspension. The internal temperature of the polymerization vessel was raised, and polymerization was carried out at a polymerization temperature of 75 °C for 4 hours or 7 hours to complete the polymerization step. That is, in Comparative Examples 1 to 3, the polymerization step was completed without adding a peroxide. Next, in the foaming agent impregnation step, the same operation as in Example 1 was performed except that the temperature was raised to 120 °C to obtain foamed styrene resin particles. Various measurements and evaluations of the foamed styrene resin particles, pre-expanded particles, and foamed molded articles were carried out, and the results are shown in Table 1. The amount of residual monomer is large.

[0070] (Comparative Examples 4 to 5) A monomer mixture was obtained in the same manner as in Example 3 or Example 5, except that an azo initiator was not used and a peroxide in the amount shown in Table 1 was used. Into a polymerization vessel equipped with a stirrer having an internal volume of 10 liters, while stirring an internal solution of 200 parts by weight of water, 0.1 part by weight of tricalcium phosphate, and 0.003 part by weight of sodium α-olefin sulfonate with respect to a total of 100 parts by weight of monomers, the above monomer mixture was added to the above internal solution to prepare an aqueous suspension. When the internal temperature of the polymerization vessel was raised and polymerization was carried out at a polymerization temperature of 90 °C, agglomeration of particles occurred at the 5th hour of polymerization, and the polymerization was terminated.

[0071] (Comparative Example 6) Foamed styrene resin particles were produced in the same manner as in Example 1, except that the amount of the azo initiator used was changed to 0.5 part by weight with respect to a total of 100 parts by weight of monomers. At the 3.5th hour of polymerization, the polymerization conversion rate was 70%. Then, after adding a peroxide, agglomeration of particles occurred at the 1st hour, and the polymerization was terminated.

[0072] (Comparative Example 7) Except for not using peroxides, expandable polystyrene resin particles, pre-expanded particles, and foamed molded articles were produced by the same operating method as in Example 1, and various measurements and evaluations described above were performed, and the results are shown in Table 1. The amount of residual monomer increased.

[0073]

Table 1

[0074] From Table 1, it was found that the expandable polystyrene resin particles of Examples 1 to 7 according to one embodiment of the present invention had a small amount of residual monomer, and furthermore, a foamed molded article with low heat insulation properties could be produced. On the other hand, in Comparative Examples 1 to 3 and Comparative Example 7 where only an azo-based initiator was used as the polymerization initiator, the amount of residual monomer increased, and in Comparative Examples 4 and 5 where only a peroxide was used as the polymerization initiator, agglomeration of particles occurred and abnormal polymerization occurred. Furthermore, as shown in Comparative Example 6, when a peroxide was added before the polymerization conversion rate of the first polymerization step (Step 1) reached 80% or more, agglomeration of particles occurred and abnormal polymerization occurred.

Industrial Applicability

[0075] According to one embodiment of the present invention, expandable polystyrene resin particles with excellent low thermal conductivity, flame retardancy, and low VOC can be stably produced. Therefore, one embodiment of the present invention can be used for heat-insulating foams used in the automotive and building material fields.

Claims

1. A polymerization step of obtaining a polymer by suspension-polymerizing a monomer containing 50% by weight or more of a styrene monomer and graphite in an aqueous medium together with a polymerization initiator, and a foaming agent impregnation step of impregnating the obtained polymer with a volatile foaming agent. The amount of graphite used is 1 part by weight or more and 10 parts by weight or less with respect to 100 parts by weight of the monomer. In the polymerization step, an azo initiator is added to the aqueous medium to initiate polymerization, and the monomer is subjected to a polymerization reaction until the polymerization conversion rate reaches 80% to 90%. Then, a peroxide is added to further polymerize the monomer. A method for producing expandable styrene resin particles.

2. The method for producing expandable styrene resin particles according to claim 1, wherein the average particle size of the graphite is 1 to 10 μm.

3. The method for producing expandable styrene resin particles according to claim 1 or 2, wherein the azo initiator contains 2,2'-azobisisobutyronitrile.

4. The method for producing expandable styrene resin particles according to any one of claims 1 to 3, wherein the 10-hour half-life temperature of the peroxide is 80 to 110°C.

5. In the aqueous medium, 10 parts by weight or less of a styrene resin is added with respect to 100 parts by weight of the monomer. The method for producing expandable styrene resin particles according to any one of claims 1 to 4.

6. In the aqueous medium, 0.5 part by weight or more and 5 parts by weight or less of a flame retardant is added with respect to 100 parts by weight of the monomer. The method for producing expandable styrene resin particles according to any one of claims 1 to 5.

7. The method for producing expandable styrene resin particles according to claim 6, wherein the flame retardant contains a bromine-containing organic compound having a 2,3-dibromo-2-alkylpropyl group.

8. The method for producing the expandable styrenic resin particles according to any one of claims 1 to 7, wherein the monomer contains a monomer copolymerizable with the styrenic monomer.

Images