JP7693532B2 - Method for producing expandable styrene resin particles - Google Patents

Description

The present invention relates to a method for producing expandable styrene-based resin particles with low thermal conductivity.

Polystyrene resin foam molded articles obtained using expandable styrene resin particles are well-balanced foams that have light weight, heat insulation, and cushioning properties, and have been widely used as heat insulation materials for food containers, cooler boxes, cushioning materials, and housing, etc.

In recent years, in relation to various issues such as global warming, there has been a trend towards energy conservation through improved insulation of buildings such as homes, and higher insulation performance is being demanded of polystyrene-based resin foam molded products. Polystyrene-based resin foam molded products containing graphite, a radiation suppressant, are known as polystyrene-based resin foam molded products with high insulation performance.

One method for producing a polystyrene-based resin foam containing graphite is to expand graphite-containing expandable styrene-based resin particles to form expanded particles, which are then molded in a mold to produce a foam. One method for producing graphite-containing expandable styrene-based resin particles is to polymerize a styrene-based monomer in a suspension aqueous liquid in the presence of graphite particles (suspension polymerization method), as described in Patent Documents 1 and 2, for example.

Special Publication No. 2001-522383 JP 2014-62191 A

In Patent Document 1, graphite-containing expandable styrene-based resin particles are produced using a peroxide as a polymerization initiator. However, when a peroxide is used as a polymerization initiator, oxygen radicals generated by cleavage of the peroxide are captured by the graphite, which may cause the polymerization reaction to not proceed and an induction period to occur. Furthermore, after the induction period, the polymerization rate increases rapidly, causing the polymerization reaction to go out of control and the dispersion of the styrene polymer to become unstable. As a result, coarse particles are generated due to particle coalescence, and styrene polymer particles having the desired particle size may not be obtained in good yield.

Patent Document 2 proposes the combined use of a low-temperature decomposition type azo initiator and a high-temperature decomposition type azo initiator as a polymerization initiator in order to stabilize styrene suspension polymerization in the presence of graphite. Nitrogen radicals, which are cleavage products of the azo initiator, react with oxygen functional groups on the graphite surface, enabling stable suspension polymerization, but the amount of monomer remaining in the expandable styrene resin particles tends to increase.
The present invention provides a method for producing expandable styrene-based resin particles which has excellent dispersion stability during suspension polymerization, leaves a small amount of residual monomer, and can give foamed molded articles having low thermal conductivity.

After extensive research, the inventors discovered that expandable styrene-based resin particles with low thermal conductivity and low VOC content can be stably produced by suspension polymerizing monomers containing a styrene-based monomer in the presence of graphite using an azo-based initiator until the polymerization conversion rate reaches 80% or more, and then adding a peroxide to further advance the polymerization reaction to produce expandable styrene-based resin particles, thereby completing the present invention.

That is, one embodiment of the present invention includes the following configuration.
[1] A method for producing a polymer by suspension polymerization of a monomer containing 50% by weight or more of a styrene-based monomer and graphite together with a polymerization initiator in an aqueous medium, and a blowing agent impregnation step of impregnating the obtained polymer with a volatile blowing agent,
the amount of the graphite used is 1 part by weight or more and 10 parts by weight or less based on 100 parts by weight of the monomer,
In the polymerization step, an azo initiator is added to an aqueous medium to initiate polymerization, and the monomer is polymerized until a polymerization conversion rate reaches 80% to 90%, and then a peroxide is added to further polymerize the monomer.
[2] The method for producing expandable styrene-based resin particles according to [1], characterized in that the graphite has an average particle size of 1 to 10 μm.
[3] The method for producing expandable styrene-based resin particles according to [1] to [2], wherein the azo-based initiator contains 2,2'-azobisisobutyronitrile.
[4] The method for producing expandable styrene-based resin particles according to any one of [1] to [3], characterized in that the 10-hour half-life temperature of the peroxide is 80 to 110° C.
[5] The method for producing expandable styrene-based resin particles according to any one of [1] to [4], wherein 10 parts by weight or less of a styrene-based resin is added to the aqueous medium per 100 parts by weight of the monomer.
[6] The method for producing expandable styrene-based resin particles according to any one of [1] to [5], wherein 0.5 parts by weight or more and 5 parts by weight or less of a flame retardant is added to the aqueous medium per 100 parts by weight of the monomer.
[7] The method for producing expandable styrene-based 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 expandable styrene-based resin particles according to any one of [1] to [7], wherein the monomer includes a monomer copolymerizable with the styrene-based monomer.

According to the present invention, expandable styrene-based resin particles with low thermal conductivity and low VOCs can be produced by the suspension polymerization method.

The present invention will be described in detail below. The method for producing expandable styrene-based resin particles of the present invention comprises a polymerization step of suspension-polymerizing a monomer containing 50% by weight or more of a styrene-based monomer and graphite together with a polymerization initiator in an aqueous medium to obtain a polymer, and a blowing agent impregnation step of impregnating the obtained polymer with a volatile blowing agent,
the amount of the graphite used is 1 part by weight or more and 10 parts by weight or less based on 100 parts by weight of the monomer,
In the polymerization step, an azo initiator is added to an aqueous medium to initiate polymerization, and the monomer is polymerized until a polymerization conversion rate reaches 80% to 90%, and then a peroxide is added to further polymerize the monomer, in this method for producing expandable styrene-based resin particles.

[1. Polymerization process]
The polymerization step is a step of adding graphite and a polymerization initiator to an aqueous medium, and suspending and polymerizing a monomer containing 50% by weight or more of a styrene-based monomer to obtain a polymer, in which an azo-based initiator is added to initiate polymerization, the monomer is polymerized until the polymerization conversion rate reaches 80% to 90%, and a peroxide is added to further polymerize the monomer. The amount of graphite used in the present invention is 1 part by weight or more and 10 parts by weight or less per 100 parts by weight of the monomer. The amount of the styrene-based monomer contained in the entire monomer used in the present invention may be 50% by weight or more, and the styrene-based monomer may be 100% by weight.

(aqueous medium)
The method for producing the expandable styrene-based resin particles of the present invention is carried out by suspending monomers including a styrene-based monomer in an aqueous medium and polymerizing the monomers, that is, by so-called suspension polymerization. The aqueous medium preferably contains a dispersant.
Examples of dispersants that can be used in suspension polymerization include poorly water-soluble inorganic salts such as tricalcium phosphate, magnesium pyrophosphate, hydroxyapatite, and kaolin, and water-soluble polymers such as polyvinyl alcohol, methylcellulose, polyacrylamide, and polyvinylpyrrolidone. When poorly water-soluble inorganic salts are used as dispersants, it is preferable to use them in combination with an anionic surfactant such as sodium α-olefin sulfonate or sodium dodecylbenzene sulfonate, in order to increase the dispersion stability. 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.

The amount of dispersant used depends on the type of dispersant. When a poorly water-soluble inorganic salt is used as the dispersant, the amount of dispersant used is preferably 0.1 parts by weight to 1.5 parts by weight per 100 parts by weight of water. When a water-soluble polymer is used as the dispersant, the dispersant is preferably used so that its concentration in the aqueous medium is 30 ppm to 100 ppm. When an anionic surfactant is used in combination with a poorly water-soluble inorganic salt, the anionic surfactant is preferably used so that its concentration in the aqueous medium is 30 ppm to 100 ppm.

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

(Copolymerizable monomer)
In the production method of the present invention, a monomer copolymerizable with the above-mentioned styrene-based monomer may be used together with the above-mentioned styrene-based monomer to be copolymerized.
The copolymerizable monomer may be a vinyl monomer that is copolymerizable with a styrene monomer. Examples of the copolymerizable monomer include polyfunctional vinyl monomers such as divinylbenzenes, such as o-divinylbenzene, m-divinylbenzene, and p-divinylbenzene, alkylene glycol di(meth)acrylates, such as ethylene glycol di(meth)acrylate and polyethylene glycol di(meth)acrylate, and monofunctional vinyl monomers, such as (meth)acrylic acid esters, such as (meth)acrylonitrile, methyl (meth)acrylate, and butyl (meth)acrylate. These copolymerizable monomers may be used alone or in combination of two or more. In the present application, the term "vinyl monomer" refers to a compound having at least one ethylenically unsaturated group (vinyl group in the broad sense), the term "polyfunctional vinyl monomer" refers to a vinyl monomer having two or more ethylenically unsaturated groups, and the term "monofunctional vinyl monomer" refers to a vinyl monomer having one ethylenically unsaturated group. In addition, in the present application, "(meth)acrylate" means acrylate or methacrylate, "(meth)acrylonitrile" means acrylonitrile or methacrylonitrile, and "(meth)acrylic" means acrylic or methacrylic.

When the copolymerizable monomer is used, the amount used is less than the amount used of the styrene-based monomer. In other words, the content of the component derived from the copolymerizable monomer in the expandable styrene-based resin particles is an amount in which the component derived from the styrene-based monomer is the main component of the polymer component (the sum of the component derived from the styrene-based monomer and the component derived from the copolymerizable monomer) in the expandable styrene-based resin particles, that is, less than 50% by weight.

(Graphite)
The graphite used in the present invention acts as a radiation inhibitor and imparts low heat insulation. In the present invention, as long as the expandable styrene-based resin particles produced by suspension polymerization contain graphite, the timing and manner 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 preferable to add the graphite to the aqueous medium in a state where the graphite is dispersed in the monomer.

The graphite is not particularly limited, and any of the known natural and artificial graphites can be used. As the graphite, graphite having various shapes such as flakes, flaky, and spherical shapes can be used. Among these graphites, graphite having a flake or flaky shape is preferable. By using graphite having a flake or flaky shape as the graphite, the thermal conductivity of the polystyrene resin foam molded product obtained by foaming the expandable styrene resin particles can be further reduced.

The graphite preferably has an average particle size in the range of 1 to 100 μm, and more preferably has an average particle size in the range of 1 to 30 μm. This is because if the average particle size of the graphite is smaller than the above numerical range, the heat insulating properties of the polystyrene resin foam molded product obtained by expanding the expandable styrene resin particles may decrease, while if the average particle size of the graphite is larger than the above numerical range, the air bubble membrane may easily break when the expandable styrene resin particles are expanded, making it impossible to achieve a high expansion ratio for the polystyrene resin foam molded product.

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

(Azo initiator)
It is known that when a polymer radical generated by the thermal decomposition of an azopolymer having an azo group in the main chain of carbon black is reacted, the polymer radical is efficiently captured by a condensed aromatic ring, and the polymer is grafted onto the carbon black surface. From this, it is presumed that the azo-based initiator used in the present invention reacts with the oxygen-containing group on the graphite particle surface, protecting the graphite particle surface and allowing the suspension polymerization to proceed stably.

For example, 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), Examples of oil-soluble azo initiators include 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), and 2,2'-azobis(N-butyl-2-methylpropionamide) (10-hour half-life temperature: 110°C). 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.

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

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

(Peroxide)
The peroxide used in the present invention preferably has a 10-hour half-life temperature of 80 to 110° C., more preferably 90 to 105° C. For example, the peroxide may be 1,1-bis(t-amylperoxy)-3,3,5-trimethylcyclohexane (10-hour half-life temperature: 92° C.), 1,1-bis(t-butylperoxy)cyclohexane (10-hour half-life temperature: 97° C.), 2,2-di(t-amylperoxy)butane (10-hour half-life temperature: 97° C.), t-butylperoxy-2-ethylhexyl monocarbonate (10-hour half-life temperature: 99° C.), t-amylperoxy-2-ethylhexyl monocarbonate (10-hour half-life temperature: 98.5° C.), etc.
Preferred are 2,2-di(t-amylperoxy)butane, t-butylperoxy-2-ethylhexyl monocarbonate, and t-amylperoxy-2-ethylhexyl monocarbonate, which are effective in reducing the amount of remaining monomers.

The amount of these peroxides (initiators) is preferably 0.1 parts by weight or more and 0.5 parts by weight or less, more preferably 0.15 parts by weight or more and 0.3 parts by weight or less, based on 100 parts by weight of the monomer. If the amount of the polymerization initiator is small, residual monomers tend to remain, and if it is large, it becomes difficult to adjust the molecular weight.
If the 10-hour half-life temperature is less than 80° C., the peroxide will be consumed too quickly, and the effect of reducing the amount of remaining monomer in the next blowing agent impregnation step will be reduced.

(Additives)
In the polymerization step, in addition to the monomer, graphite, and azo-based initiator, additives such as a flame retardant, a flame retardant assistant, a styrene-based resin, a chain transfer agent, a plasticizer, and a bubble regulator may be appropriately added.

(Flame retardant)
The flame retardant that can be used in the present invention is not particularly limited, and any flame retardant that has been conventionally used in styrene-based resin foam moldings can be used, but among them, bromine-based flame retardants that have a high flame retardancy-imparting effect are preferable. Examples of the bromine-based flame retardant used in the present invention include bromine-containing 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 (also known as tetrabromobisphenol A-bis(2,3-dibromo-2-methylpropyl ether)), 2,2-bis[4-(2,3-dibromopropoxy)-3,5-dibromophenyl]propane (also known as tetrabromobisphenol A-bis(2,3-dibromopropyl ether)), and brominated butadiene-vinyl aromatic hydrocarbon copolymers, such as brominated styrene-butadiene block copolymers, brominated random styrene-butadiene copolymers, and brominated styrene-butadiene graft copolymers (for example, as disclosed in JP-T-2009-516019). These bromine-based flame retardants may be used alone or in combination of two or more.

Among bromine-based flame retardants, bromine-containing organic compounds with 2,3-dibromo-2-alkylpropyl groups are particularly preferred because they are easily soluble in styrene monomers, are easy to handle, and do not reduce the heat resistance of the final product.

The amount of bromine flame retardant used must be appropriately selected depending on the expansion ratio of the final product, the foamed molded article, but it is desirable that the amount is 0.5 to 8.0 parts by weight, and more preferably 0.5 to 5.0 parts by weight, per 100 parts by weight of the monomer. If the amount of bromine flame retardant is less than 0.5 parts by weight, the effect of imparting flame retardancy tends to be small, and if it exceeds 8.0 parts by weight, the strength of the resulting styrene resin foamed molded article is likely to decrease.

(Flame retardant synergist)
The flame retardant auxiliary usable in the present invention is a radical generator such as a peroxide, and preferably a peroxide having a one-hour half-life temperature of 120° C. or more, such as t-butyl peroxybenzoate (125° C.), dicumyl peroxide (136° C.), 2,3-dimethyl-2,3-diphenylbutane (234° C.), 2,5-dimethyl-2,5-di(t-butylperoxy)hexane (138° C.). If the half-life temperature is within this range, decomposition of the flame retardant auxiliary is suppressed at the polymerization temperature of 60 to 100° C. used in the present invention, and the suspension polymerization system does not become unstable.
The amount of the flame retardant synergist 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 synergist is too small, the flame retardancy tends to deteriorate, whereas if the amount is too large, the heat resistance tends to deteriorate.

(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 is added to protect the graphite surface, and the molecular weight of the resulting expandable styrene-based resin particles tends to be low. In order to adjust the molecular weight, the styrene-based resin is dissolved in the monomer and polymerization is carried out, which serves to increase the molecular weight of the expandable styrene-based resin particles.
The styrene resin used in the present invention is dissolved in the styrene monomer and subjected to suspension polymerization, and is used to adjust the molecular weight of the polymer. The styrene resin is not particularly limited, and examples thereof include styrene resin, styrene-acrylonitrile copolymer, and styrene-acrylonitrile-α-methylstyrene terpolymer.

The weight average molecular weight (Mw) of the styrene-based resin 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 be high.
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 depending on the molecular weight of the expandable styrene resin particles, and is preferably 10 parts by weight or less, more preferably 5 parts by weight or less, relative to 100 parts by weight of the monomer. If the amount added is 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 increases and the polymerization system becomes unstable.

(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, and examples thereof include alkyl mercaptans such as n-hexyl mercaptan, n-octyl mercaptan, tert-octyl mercaptan, n-dodecyl mercaptan, and tert-dodecyl mercaptan; phenolic compounds such as α-methylstyrene dimer, 2,6-di-tert-butyl-4-methylphenol, and styrenated phenol; allyl compounds such as allyl alcohol; and halogenated hydrocarbon compounds such as dichloromethane, dibromomethane, and carbon tetrachloride. These chain transfer agents may be used alone or in combination of two or more.

(Plasticizer)
Plasticizers that can be used in the present invention include high-boiling plasticizers having a boiling point of 200° C. or higher. Examples of such plasticizers include fatty acid glycerides such as stearate triglyceride, palmitate triglyceride, laurate triglyceride, stearate diglyceride, and stearate monoglyceride; vegetable oils such as coconut oil, palm oil, and palm kernel oil; aliphatic esters such as dioctyl adipate and dibutyl sebacate; and organic hydrocarbons such as liquid paraffin and cyclohexane. However, it is preferable not to use these plasticizers because their heat resistance tends to deteriorate.

(Foam Regulator)
Examples of the cell regulator that can be used in the present invention include aliphatic bisamides such as methylene bis-stearic acid amide and ethylene bis-stearic acid amide, and polyethylene wax.

(Polymerization Conversion Rate)
In the polymerization step in the present invention, a monomer is polymerized together with graphite in the presence of an azo initiator until the polymerization conversion rate reaches 80% to 90%, and then a peroxide is added to further advance the polymerization reaction. Hereinafter, in the polymerization step, the polymerization step up to the further addition of peroxide during the polymerization is sometimes referred to as a first polymerization step, and the polymerization step in which the polymerization reaction is further advanced after the addition of peroxide is sometimes referred to as a second polymerization step.

In the first polymerization step, an azo-based initiator is used as a polymerization initiator. In the first polymerization step, the polymerization reaction of the monomer is started in the presence of the azo-based initiator, and the monomer is polymerized until the polymerization conversion rate reaches 80% to 90%, preferably 83% to 90%. Here, the azo-based initiator only needs to be present in the aqueous medium at the start of polymerization, and may be added to the aqueous medium separately from the monomer before the start of polymerization, or may be added to the aqueous medium as a monomer mixture in a state where it is 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 in the polymerization reaction can be suppressed. On the other hand, if the polymerization conversion rate is less than 80%, many oxygen-containing groups are present on the graphite surface, which delays the polymerization rate and deteriorates productivity. Also, if the polymerization conversion rate exceeds 90%, the time for the first polymerization step is long, and productivity decreases. The polymerization initiator used in the first polymerization step is not particularly limited as long as it contains an azo-based initiator as the main component, but may contain a polymerization initiator other than an azo-based initiator. The amount of the polymerization initiator other than an azo-based initiator used is preferably 30% or less of the total amount of the initiator used in the first polymerization step, and in particular, it is preferable that it does not contain a peroxide.

In the second polymerization step, a peroxide is used as a polymerization initiator. In the second polymerization step, a peroxide is added to an aqueous medium in which a polymer with a polymerization conversion rate of 80% to 90% is 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 is sufficient that the polymerization conversion rate is more than 90%, and preferably 98% or less. If the polymerization conversion rate is more than 90%, the dispersion of the suspension polymerization system does not become unstable even if a volatile blowing agent is added in the next step, allowing stable production and reducing the amount of remaining monomers.

[2. Foaming agent impregnation process]
The step following the polymerization step is a step of adding a volatile foaming agent to impregnate the polymer particles with the volatile foaming agent and, if necessary, with a foaming assistant.

(Volatile foaming agent and foaming assistant)
In the manufacturing method of the present invention, a volatile foaming agent is added to a polymer (polystyrene resin particles) obtained by polymerization of a monomer containing the styrene monomer, and the volatile foaming agent is impregnated to obtain expandable styrene resin particles containing the volatile foaming agent. The impregnation of the expandable styrene resin particles with the volatile foaming agent may be performed after the polymerization, or during the polymerization (while polymerizing). The impregnation of the volatile foaming agent can be performed by a method known per se. For example, the impregnation of the volatile foaming agent during the polymerization can be performed by performing the polymerization reaction in a closed container and pressurizing the volatile foaming agent into the closed container during the polymerization. The impregnation of the volatile foaming agent after the completion of the polymerization can be performed by performing the polymerization reaction in a closed container and pressurizing the volatile foaming agent into the closed container after the polymerization.

The volatile foaming agent is not particularly limited as long as it is a volatile foaming agent (physical foaming agent) that has been conventionally used for foaming polystyrene-based resins. Examples of the volatile foaming agent include aliphatic hydrocarbons having 5 or less carbon atoms, such as propane, isobutane, n-butane, isopentane, neopentane, and n-pentane. 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 styrene resin particles is preferably within the range of 2 to 8% by weight, more preferably within 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 resin foam molded product of the desired density when the expandable styrene resin particles are foam molded. If the content of the volatile foaming agent is less than the above range, the effect of increasing the secondary foaming power during in-mold foam molding may be reduced when the expandable styrene resin particles are foam molded in-mold, so that the appearance of the polystyrene resin foam molded product may be reduced. If the content of the volatile foaming agent is more than the above range, the time required for the cooling step in the step of producing a polystyrene resin foam molded product from the expandable styrene resin particles is increased, so that the productivity of the polystyrene resin foam molded product may be reduced.

In the manufacturing method of the present invention, a known foaming assistant may be used in combination with the foaming agent. Examples of the foaming assistant include toluene, xylene, cyclohexane, and diisobutyl adipate. When producing a polystyrene resin foam molded product to be used as a building material from the expandable styrene resin particles, it is preferable to use diisobutyl adipate, which has a high boiling point, as the foaming assistant in order to avoid the occurrence of sick house syndrome caused by the foaming assistant.
The temperature at which the polystyrene resin particles are impregnated with the volatile blowing agent (and the foaming assistant used as necessary) is preferably within the range of 80 to 130° C., and more preferably within the range of 100 to 120° C. If the temperature at which the impregnation is performed is lower than the above range, the time required to impregnate the polystyrene resin particles with the volatile blowing agent may be long, or the monomer may not be sufficiently consumed, and low VOC may not be achieved. On the other hand, if the temperature at which the impregnation is performed is higher than the above range, the polystyrene resin particles may fuse together to generate bonded particles.

(Shape and particle size of expandable styrene resin particles)
The shape of the expandable styrene 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. In addition, the particle diameter of the expandable styrene resin particles is preferably within the range of 0.3 to 2.0 mm, more preferably within the range of 0.5 to 1.4 mm, taking into consideration the filling ability into a molding die, etc.
The obtained expandable styrene-based resin particles are used to produce a foamed molded article. The foamed molded article is produced by using pre-expanded particles obtained by pre-expanding the expandable styrene-based resin particles.

3. Pre-expanded particles
The expandable styrene resin particles obtained above are brought into contact with heated steam in a pre-expansion device to produce pre-expanded particles with a desired expansion ratio. In the production of pre-expanded particles, it is preferable to apply an antiblocking agent and a fusion promoter to the surface of the expandable styrene resin particles in advance using a mixer such as a Super Mixer, Nauta Mixer, Universal Mixer, Proshare Mixer, Apex Mixer, Henschel Mixer, or Loedige Mixer; or a blender such as a ribbon blender or a tumbler type blender.

(Anti-blocking agent)
The anti-blocking agent is used to prevent the foam particles from coalescing with each other (blocking) in the pre-expansion device, and examples of the anti-blocking agent include silicate minerals such as kaolinite, mica, talc, zeolite, chlorite, glauconite, and smectite, and fatty acid metal salts such as zinc stearate and magnesium stearate.

(Fusion promoter)
The fusion promoter is used to increase the fusion strength of the foamed molded body, and examples thereof include fatty acid triglycerides such as lauric acid triglyceride, stearic acid triglyceride, linoleic acid triglyceride, and hydroxystearic acid triglyceride; fatty acid diglycerides such as lauric acid diglyceride, stearic acid diglyceride, and linoleic acid diglyceride; fatty acid monoglycerides such as lauric acid monoglyceride, stearic acid monoglyceride, and linoleic acid monoglyceride; and vegetable oils (hydroxy fatty acid triglycerides) such as castor hardened oil (hydroxystearic acid triglyceride).

(Pre-expansion device)
As a method for pre-expanding, a conventional method can be used, for example, using a cylindrical pre-expanding device and heating and expanding the expandable thermoplastic resin particles with a heating medium such as steam.
The pre-expansion device and the conditions of the pre-expansion step may be appropriately set depending on the type of base resin of the expandable styrene-based resin particles, the desired pre-expansion ratio, etc., and are not particularly limited.

[4. Foam molded article]
The foamed molded article can be obtained by heating and foaming (secondary foaming) the above-mentioned pre-foamed particles, followed by molding.
The method of heating and expanding the pre-expanded particles to form them can be, for example, a conventional method such as an in-mold foaming method in which the pre-expanded particles are filled into a mold and heated by blowing in a heating medium such as steam. A specific in-mold foaming method includes a method in which the pre-expanded particles are filled into a mold that can be closed but cannot be sealed, and the pre-expanded particles are heated and fused by a heating medium to form an in-mold foamed article.
The apparatus used for heat expansion and the conditions for heat expansion may be appropriately set depending on the composition of the expandable styrene-based resin particles, the desired expansion ratio, etc., and are not particularly limited.

The foamed molded products, particularly in-mold foamed molded products, have the advantage of being easy to produce molded products in the desired shape, and are therefore ideal for use as packaging materials (trays) for food containers and the like, transport packaging materials for fish boxes and the like, and particularly as cushioning materials for home appliances and precision parts that require low static electricity performance.

The following are examples and comparative examples, but the present invention is not limited to these. The molecular weight of the resin in the examples and comparative examples, the amount of residual monomer in the resin, and the evaluation of the molded products were measured using the following methods. Note that "parts" and "%" are by weight unless otherwise specified.

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

(Measurement of remaining 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 conditions: heating from 50° C. to 80° C. at a heating rate of 3° C./min, and then heating from 80° C. to 180° C. at a heating rate of 10° C./min, carrier gas: helium).

(Measurement of polymerization conversion rate)
The styrene-based resin particles were collected from the bottom of the pressure vessel, and the moisture on the surface of the resin particles was wiped off with filter paper. The amount of the remaining monomer was measured by gas chromatography according to the above-mentioned method for measuring the amount of the remaining monomer. The polymerization conversion rate was calculated from the amount of the remaining monomer component.

(Evaluation of foamed molded products)
The foamed molded article obtained by the manufacturing method described in Example 1 below was dried at room temperature for 24 hours, and then evaluated in the following items (1) to (4).

(1) Evaluation of fusion rate The foamed molded product was broken and the broken surface was observed to determine the percentage of the foamed beads that were broken rather than the foamed bead interface (fusion rate). A fusion rate of 80% or more was evaluated as acceptable.

(2) Surface Properties The surface condition of four parts of the foamed molded article was visually observed and evaluated on the following five-point scale. The average value of the four parts was taken as the score for surface properties. The larger the value, the fewer the gaps between the foamed particles and the more beautiful the surface condition, and a score of "4" or higher was judged to be acceptable.
5: No gaps found 4: There are some gaps, but they are barely noticeable 3: There are gaps here and there, but overall it is acceptable 2: The gaps are noticeable 1: There are many gaps.

(3) Flame retardant: Five test pieces measuring 200 mm in length, 10 mm in width and 25 mm in thickness were cut out from the foamed molded article and cured in an oven at 60° C. for 12 hours. Then, measurements were carried out in accordance with the measurement method A of JIS A9511:2006R, and the average value of the five test pieces was calculated as the flame extinction time. A flame extinction time of 1 minute or less was considered to be acceptable.

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

The graphite, azo initiator, chain transfer agent, flame retardant, flame retardant assistant, peroxide, etc. used in the present examples and comparative examples are as follows.
(Graphite)
・Flake graphite SGP-40B: particle size 5 μm (Marutoyo Foundry Manufacturing Co., Ltd.)
(Azo initiator)
AIBN: 2,2'-azobisisobutyronitrile: Half-life temperature: 65°C: (Fujifilm Wako Pure Chemical Industries, Ltd.)
ADVN: 2,2'-azobis(2,4-dimethylvaleronitrile): half-life temperature: 51°C: (Fujifilm Wako Pure Chemical Industries, Ltd.)
ACHCN; 1,1'-azobis-(cyclohexane-1-carbonitrile): half-life temperature: 88°C (Fujifilm Wako Pure Chemical Industries, Ltd.)
(Chain Transfer Agent)
α-Methylstyrene dimer (MSD (NOF Corp.))
(Flame retardant)
Tetrabromobisphenol A-bis(2,3-dibromo-2-methylpropyl)ether (Pyroguard SR-130 (Dai-ichi Kogyo Seiyaku Co., Ltd.)
(Flame retardant synergist)
Dicumyl peroxide (Percumyl D (NOF Corporation)).
(Peroxide)
t-BEH: t-butylperoxy-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 Corp.))
(Volatile Blowing Agent)
・n-butane/iso-butane = 70/30 (Iwatani Gas Co., Ltd.)
(Anti-blocking agent)
・Zinc stearate (NOF Corporation)
(Fusion promoter)
・Hydroxystearic acid triglyceride (Casterwax (NOF Corporation))
Next, a method for producing the styrene-based resin used in some of the Examples and Comparative Examples will be described.
(Method for producing styrene-based resin)
Into a 10-liter polymerization vessel equipped with a stirrer, 200 parts by weight of water, 0.1 parts by weight of tribasic calcium phosphate, 0.003 parts by weight of sodium α-olein sulfonate, 0.23 parts by weight of benzoyl peroxide (Niper BW (NOF Corp.)), and 0.05 parts by weight of 1,1-bis(t-butylperoxy)cyclohexane (Perhexa C (NOF Corp.)) were added 100 parts by weight of a styrene-based monomer while stirring the contents. Thereafter, the temperature was raised and polymerization was carried out at an internal temperature of 98° C. for 5 hours, and then at an internal temperature of 120° C. for 3.5 hours, followed by cooling, removal, and drying to obtain styrene-based resin particles (polystyrene). The weight-average molecular weight is 360,000.

(Examples 1 to 3)
(Production of expandable styrene-based resin particles)
Only a styrene-based monomer was used as the monomer, and a styrene-based resin, graphite, chain transfer agent, azo-based initiator, flame retardant, and flame retardant assistant in the amounts shown in Table 1 were dispersed and dissolved relative to a total of 100 parts by weight of the monomers to obtain a monomer mixture.
In a 10-liter polymerization vessel equipped with a stirrer, a liquid (aqueous medium) containing 200 parts by weight of water, 0.1 parts by weight of tricalcium phosphate, and 0.003 parts by weight of sodium α-olein sulfonate per 100 parts by weight of monomers in total was placed, and the monomer mixture was added to the liquid while stirring 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 3.5 hours, with a polymerization conversion rate of 84%. After that, 0.1 parts by weight of tribasic calcium phosphate was added to the contents, and 0.15 parts by weight of peroxide (t-butylperoxy-2-ethylhexyl monocarbonate) was added. The internal temperature was then raised to 98°C, and polymerization was carried out for 1 hour, with a polymerization conversion rate of 93%, and the polymerization process was completed.

Next, 7 parts by weight of a volatile blowing agent (n-butane/iso-butane = 7/3 (Iwatani Gas Corporation)) was added to 100 parts by weight of the total monomers, and the temperature was raised to 115°C, and a blowing agent impregnation process was carried out for 8 hours.The mixture was then cooled to 40°C to obtain expandable styrene-based resin particles.The expandable styrene-based resin particles had not generated coarse particles due to particle coalescence during polymerization (hereinafter, this state may be referred to as agglomeration), and the polymerization had been completed satisfactorily.

(Production of pre-expanded particles)
100 parts by weight of the expandable styrene-based resin particles obtained above were mixed with 0.2 parts by weight of zinc stearate as an antiblocking agent and 0.07 parts by weight of hydroxystearic acid triglyceride as a fusion promoter in a universal mixer (EM15 type, Tsukishima Machine Sales Co., Ltd.) for 2 minutes to obtain a mixture. 1000 g of the obtained mixture was put into a pressurized pre-expander (manufactured by Daikai Kogyo Co., Ltd., CH-100) equipped with a stirrer, and pre-expanded (primary expansion) was performed using steam (blowing steam pressure 60 kPa) as a heating medium, to obtain pre-expanded particles with a bulk magnification (apparent magnification) of 50 times. The obtained pre-expanded particles were then left at room temperature for 24 hours to be cured and dried.

(Production of foamed molded article)
The pre-expanded particles after curing and drying were heated in a molding machine (KR-57, manufactured by Daisen Co., Ltd.) and a mold (box-shaped, 450 mm long x 300 mm wide x 25 mm deep) using steam (blowing steam pressure of 80 kPa) as a heating medium for 10 seconds. The pre-expanded particles were then water-cooled for 2 seconds and allowed to cool in a vacuum. When a surface pressure gauge (pressure of the foamed molded product) installed in the mold reached 30 kPa, the mold was opened and the foamed molded product was taken out.
The expandable styrene-based resin particles, pre-expanded particles, and foamed molded articles obtained were subjected to the above-mentioned various measurements and evaluations. The results are shown in Table 1.

(Examples 4 to 5)
Styrene monomer and acrylonitrile monomer were used as monomers, and 76 parts by weight of styrene monomer and 24 parts by weight of acrylonitrile monomer were used per 100 parts by weight of monomers in total, to obtain a monomer mixture in which styrene-based resin, graphite, chain transfer agent, azo-based initiator, flame retardant, and flame retardant assistant were dispersed and dissolved per 100 parts by weight of monomers in total in the amounts shown in Table 1. Expandable styrene-based resin particles, pre-expanded particles, and foamed molded articles were produced in the same manner as in Example 1, and the above-mentioned various measurements and evaluations were carried out, and the results are shown in Table 1.

(Examples 6 to 7)
Styrene monomer, α-methylstyrene monomer, and acrylonitrile monomer were used as monomers, and 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 relative to 100 parts by weight of the total monomers, and a monomer mixture was obtained in which a styrene-based resin, graphite, chain transfer agent, azo-based initiator, flame retardant, and flame retardant auxiliary were dispersed and dissolved in the amounts shown in Table 1 relative to 100 parts by weight of the total monomers. Expandable styrene-based resin particles, pre-expanded particles, and foamed molded articles were produced in the same manner as in Example 1, and the above-mentioned various measurements and evaluations were performed, and the results are shown in Table 1.

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

(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-based initiator was not used and the amount of peroxide shown in Table 1 was used. A 10-liter polymerization vessel equipped with a stirrer was charged with 200 parts by weight of water, 0.1 parts by weight of tribasic calcium phosphate, and 0.003 parts by weight of sodium α-olein sulfonate per 100 parts by weight of monomers in total, and the monomer mixture was added to the content while stirring the content to prepare an aqueous suspension. The internal temperature of the polymerization vessel was raised and polymerization was carried out at a polymerization temperature of 90° C., and agglomerations between particles occurred 5 hours into the polymerization, and the polymerization was discontinued.

(Comparative Example 6)
Except for changing the amount of the azo-based initiator to 0.5 parts by weight per 100 parts by weight of the total monomers, expandable styrene-based resin particles were produced in the same manner as in Example 1. At 3.5 hours into the polymerization, the polymerization conversion rate was 70%, and then, after 1 hour from the addition of peroxide, agglomerations between particles occurred, and the polymerization was discontinued.

(Comparative Example 7)
Expandable styrene resin particles, pre-expanded particles, and foamed molded articles were produced in the same manner as in Example 1, except that no peroxide was used, and the above-mentioned various measurements and evaluations were carried out, and the results are shown in Table 1. The amount of remaining monomer was increased.

From Table 1, it was found that the expandable styrene-based resin particles of Examples 1 to 7 according to one embodiment of the present invention have a small amount of residual monomer, and furthermore, can produce a foamed molded article with low thermal insulation. On the other hand, in Comparative Examples 1 to 3 and Comparative Example 7, in which only an azo-based initiator was used as the polymerization initiator, the amount of residual monomer was large, and in Comparative Examples 4 and 5, in which only a peroxide was used as the polymerization initiator, agglomeration occurred between the particles, resulting in abnormal polymerization. Furthermore, as shown in Comparative Example 6, when peroxide was added before the polymerization conversion rate in the first polymerization step (step 1) reached 80% or more, agglomeration occurred between the particles, resulting in abnormal polymerization.

According to one embodiment of the present invention, it is possible to stably produce low-VOC expandable styrene-based resin particles that have low thermal conductivity and excellent flame retardancy. Therefore, one embodiment of the present invention can be used for insulating foams used in the automotive and building material fields.

Claims (8)

The method includes a polymerization step of suspension-polymerizing a monomer containing 50% by weight or more of a styrene-based monomer and graphite together with a polymerization initiator in an aqueous medium to obtain a polymer, 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 based on 100 parts by weight of the monomer,
In the polymerization step, an azo initiator is added to an aqueous medium to initiate polymerization, and the monomer is polymerized until a polymerization conversion rate reaches 80% to 90%, and then a peroxide is added to further polymerize the monomer. The method for producing expandable styrene-based resin particles according to claim 1, wherein the average particle size of the graphite is 1 to 10 μm. The method for producing expandable styrene-based resin particles according to claim 1 or 2, wherein the azo-based initiator contains 2,2'-azobisisobutyronitrile. The method for producing expandable styrene-based 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. The method for producing expandable styrene-based resin particles according to any one of claims 1 to 4, wherein 10 parts by weight or less of a styrene-based resin is added to the aqueous medium per 100 parts by weight of the monomer. The method for producing expandable styrene-based resin particles according to any one of claims 1 to 5, wherein 0.5 parts by weight or more and 5 parts by weight or less of a flame retardant is added to the aqueous medium per 100 parts by weight of the monomer. The method for producing expandable styrene-based resin particles according to claim 6, wherein the flame retardant contains a bromine-containing organic compound having a 2,3-dibromo-2-alkylpropyl group. The method for producing expandable styrene-based resin particles according to claim 1 , wherein the monomer comprises a monomer copolymerizable with the styrene-based monomer.