JP7588511B2 - Method for producing expandable styrene resin particles - Google Patents

Description

The present invention relates to a method for producing expandable styrene-based resin particles containing a flame retardant by suspension polymerization of a styrene-based monomer.

The styrene resin expanded particle molded product (hereinafter referred to as "expanded molded product") obtained from expandable styrene resin particles has excellent heat insulating properties and is therefore used in residential heat insulating materials, cold storage boxes, etc. In addition, because the expanded molded product is lightweight and has excellent shock-absorbing properties, it is also used as an automobile interior material such as a shock-absorbing material or a floor raising material. In such applications, a flame-retardant expanded molded product containing a flame retardant is used. In addition, it is desirable to reduce the content of volatile organic compounds (i.e., VOCs) such as unreacted styrene monomer (hereinafter referred to as "residual styrene") in the expanded molded product.

In particular, for interior materials for vehicles such as automobiles, foamed molded articles that emit less volatile organic compounds are required, even more so than residential insulation materials. However, when suspension polymerization of styrene-based monomers is carried out in the presence of a flame retardant to obtain foamed molded articles with excellent flame retardancy, polymerization inhibition occurs, making it difficult to sufficiently reduce the amount of residual styrene in the expandable styrene-based resin particles.

In response to this, for example, Patent Documents 1 and 2 disclose a method for producing expandable styrene-based resin particles that exhibit excellent flame retardancy and have a low styrene monomer content by performing suspension polymerization in the presence of a flame retardant consisting of a brominated styrene-butadiene copolymer. Patent Document 3 discloses a method for producing expandable styrene-based resin particles that use a specific polymerization initiator to provide an expanded molded product that has flame retardancy, a low styrene monomer content, and reduced unpleasant odors.

JP 2017-71669 A JP 2020-147668 A JP 2016-37573 A

The manufacturing method of Patent Document 1 has difficulty in sufficiently reducing the amount of residual styrene in the expandable styrene resin particles, and also leaves a problem with the flame retardancy of the resulting foamed molded article. The manufacturing method of Patent Document 2 leaves a problem with the flame retardancy of the resulting foamed molded article. In addition, the manufacturing method of Patent Document 2 requires the use of an organic peroxide having a special structure as a polymerization initiator, and is also inferior in productivity. The manufacturing method of Patent Document 3 can sufficiently reduce the amount of residual styrene in the expandable styrene resin particles, and also provides excellent flame retardancy of the resulting foamed molded article. However, the expandable styrene resin particles obtained by the manufacturing method of Patent Document 3 may have benzene detected in the particles, and there is room for improvement depending on the application.

The present invention has been made in view of the above background, and aims to provide a method for producing expandable styrene-based resin particles that can easily produce expanded styrene-based resin bead moldings that have excellent flame retardancy and low styrene and benzene contents.

One aspect of the present invention is a method for producing expandable styrene-based resin particles containing a flame retardant and a blowing agent, comprising the steps of:
a polymerization step of adding a styrene-based monomer containing styrene and having the flame retardant dissolved therein to an aqueous medium, and carrying out suspension polymerization of the styrene-based monomer in the aqueous medium to obtain styrene-based resin particles containing the flame retardant,
In the polymerization step, a first organic peroxide of an aliphatic peroxyester type and/or an aliphatic peroxyketal type having a 10-hour half-life temperature of 65° C. or more and 90° C. or less and a second organic peroxide having a 10-hour half-life temperature of more than 90° C. and 110° C. or less are used as a polymerization initiator,
The flame retardant is a brominated styrene-butadiene copolymer,
The blending amount of the flame retardant is 0.05 to 2 parts by mass per 100 parts by weight of the styrene-based monomer,
The method for producing expandable styrene-based resin particles is characterized in that the expandable styrene-based resin particles have a styrene content of 25 ppm by mass or less (including 0) and a benzene content of 1.5 ppm by mass or less (including 0).

In the manufacturing method of the present invention, the specific first organic peroxide and second organic peroxide are used as polymerization initiators, and the specific flame retardant is used while performing suspension polymerization of a styrene-based monomer in which the flame retardant is dissolved in an aqueous medium. This makes it possible to manufacture expandable styrene-based resin particles containing a flame retardant, with a styrene content of 25 mass ppm or less and a benzene content of 1.5 mass ppm or less. Such expandable styrene-based resin particles make it possible to manufacture styrene-based resin expanded bead moldings having excellent flame retardancy and low styrene and benzene contents.

Next, a preferred embodiment of the expandable styrene resin particles will be described. In this specification, expandable styrene resin particles are appropriately referred to as "expandable particles". Expanded styrene resin particles obtained by expanding expandable styrene resin particles are appropriately referred to as "expanded particles". Expanded styrene resin beads molded by molding styrene resin expanded beads in a mold are appropriately referred to as "expanded molded bodies". In this specification, when a numerical value or a physical property value is expressed by using "~" before and after it, it is used to include the values before and after it. In addition, when a numerical value or a physical property value is expressed as a lower limit, it means that it is equal to or greater than that numerical value or physical property value, and when a numerical value or a physical property value is expressed as an upper limit, it means that it is equal to or less than that numerical value or physical property value. In this specification, "parts by mass" and "parts by weight", "% by mass" and "% by weight" are substantially synonymous. "ppm" means "ppm by mass" unless otherwise specified.

In the polymerization step, the styrene monomer with the flame retardant dissolved therein is added to an aqueous medium, and the styrene monomer is suspended in the aqueous medium. In other words, the styrene monomer is suspended in the presence of the flame retardant. This produces styrene resin particles containing the flame retardant. The flame retardant contains a brominated styrene-butadiene copolymer. In the following description, the brominated styrene-butadiene copolymer is referred to as "Br-SBC" as appropriate. As the Br-SBC, for example, a brominated styrene-butadiene block copolymer can be used. As the flame retardant, a brominated flame retardant other than Br-SBC can be used in combination as long as the intended object of the present invention is achieved.

The suspension polymerization of the styrene monomer in the polymerization process is carried out in an aqueous medium containing, for example, a suspending agent. The aqueous medium is a liquid such as water. The flame retardant is added to the aqueous medium in a state in which it has been dissolved in the styrene monomer beforehand.

In the polymerization step, suspension polymerization of the styrene monomer is performed. Therefore, unlike the so-called seed polymerization method in which, for example, a styrene monomer is impregnated and polymerized into a core particle containing a styrene resin, there is no need to produce a core particle. Therefore, the production efficiency of the expandable particles is high. In addition, in the polymerization step, the styrene monomer in which a specific flame retardant is dissolved is added to an aqueous medium as described above, and suspension polymerization is performed using a specific polymerization initiator. As a result, in a state in which the polymerization of the styrene monomer progresses and the concentration of the styrene monomer decreases, the generation of benzene derived from the polymerization initiator, for example, is suppressed, and the polymerization of the styrene monomer progresses sufficiently. As a result, it is possible to produce expandable particles that contain a flame retardant, have a styrene content of 25 mass ppm or less (including 0), and a benzene content of 1.5 mass ppm or less (including 0). The reason for this is considered to be as follows.

In general, when suspension polymerization of styrene-based monomers is performed in the presence of a flame retardant, polymerization inhibition is likely to occur, and styrene-based monomers tend to remain in the expandable particles. As described above, when a styrene-based monomer in which a flame retardant is dissolved is added to an aqueous medium to perform suspension polymerization, polymerization inhibition is suppressed, the amount of remaining styrene is reduced, and flame retardancy is improved by using Br-SBC as a flame retardant. However, when Br-SBC is used as a flame retardant in the suspension polymerization of styrene-based monomers, benzene was sometimes detected in the expandable particles. The inventors have found that this is due to decarboxylation of the polymerization initiator radical. Furthermore, they have found that this decarboxylation is likely to occur when an aromatic polymerization initiator such as benzoyl peroxide is used in the suspension polymerization, and the polymerization of the styrene-based monomer progresses and the amount of the styrene-based monomer is reduced to an extremely small amount. Specifically, it was found that when benzoyl peroxide is used as a polymerization initiator, in an environment where the concentration of styrene-based monomers is extremely low, the reaction between the benzoyl peroxide radical and the styrene-based monomer is difficult to occur, and the radical undergoes decarboxylation to produce benzene. As in the manufacturing method disclosed herein, by using the above-mentioned specific first organic peroxide and second organic peroxide as polymerization initiators, the production of benzene is suppressed even in an environment where the concentration of styrene-based monomers is extremely low, and expandable particles with a low benzene content can be obtained.

The amount of flame retardant per 100 parts by weight of styrene monomer is preferably 0.05 to 2 parts by weight. In this case, it is possible to further improve the flame retardancy and prevent the increase in production costs associated with an increase in the amount of flame retardant. Furthermore, it is possible to further prevent the inhibition of polymerization of the styrene monomer and further reduce the styrene content in the foamed molded product. From the viewpoint of further reducing the styrene content, it is preferable that the amount of flame retardant per 100 parts by weight of styrene resin is less than 1 part by weight, more preferably less than 0.5 parts by weight, and particularly preferably 0.1 parts by weight or less. The amount of the flame retardant is usually equal to the amount of flame retardant in the foamed molded product.

As the styrene-based monomer, styrene can be used. In addition to styrene, styrene compounds such as α-methylstyrene, o-methylstyrene, m-methylstyrene, p-methylstyrene, p-ethylstyrene, 2,4-dimethylstyrene, p-methoxystyrene, p-n-butylstyrene, p-t-butylstyrene, o-chlorostyrene, m-chlorostyrene, p-chlorostyrene, 2,4,6-tribromostyrene, divinylbenzene, styrenesulfonic acid, and sodium styrenesulfonate can be used. The styrene-based monomers may be used alone or in combination of two or more types. From the viewpoint of product cost and ease of processing of the foamed molded product, it is preferable that the main component of the styrene-based monomer is styrene.

In addition, a vinyl monomer that is copolymerizable with the styrene monomer may be used in combination with the styrene monomer. Examples of such vinyl monomers include (meth)acrylic acid, (meth)acrylic acid esters, vinyl monomers containing a nitrile group, organic acid vinyl compounds, olefin compounds, diene compounds, halogenated vinyl compounds, halogenated vinylvinylidene compounds, and maleimide compounds. Note that "(meth)acrylic acid" is a concept that includes "acrylic acid" and "methacrylic acid", and means either or both of these.

Examples of (meth)acrylic acid esters include methyl acrylate, ethyl acrylate, propyl acrylate, butyl acrylate, 2-ethylhexyl acrylate, methyl methacrylate, ethyl methacrylate, propyl methacrylate, butyl methacrylate, 2-ethylhexyl methacrylate, 1,4-butanediol diacrylate, 1,6-hexadiol diacrylate, trimethylolpropane trimethacrylate, hydroxyethyl acrylate, hydroxypropyl acrylate, hydroxyethyl methacrylate, and hydroxypropyl methacrylate. Examples of vinyl monomers containing a nitrile group include acrylonitrile and methacrylonitrile. Examples of organic acid vinyl compounds include vinyl acetate and vinyl propionate. Examples of olefin compounds include ethylene, propylene, 1-butene, and 2-butene. Examples of diene compounds include butadiene, isoprene, and chloroprene. Examples of halogenated vinyl compounds include vinyl chloride and vinyl bromide. Examples of vinylidene halide compounds include vinylidene chloride. Examples of maleimide compounds include N-phenylmaleimide and N-methylmaleimide.

The phenylacetylene concentration in the styrene-based monomer is preferably 5 ppm by mass or more and 20 ppm by mass or less. By having a phenylacetylene concentration of 20 ppm by mass or less, the styrene content can be further reduced. Furthermore, even if the phenylacetylene concentration is 5 ppm by mass or more, the manufacturing method disclosed herein makes it possible to sufficiently reduce the styrene content and the benzene content, so that it is possible to avoid the use of high-cost styrene-based monomers with a phenylacetylene concentration of less than 5 ppm by mass (including 0).

In the polymerization step, for example, a styrene-based monomer with a flame retardant dissolved therein can be dispersed in an aqueous medium in a sealed container equipped with a stirrer to initiate the polymerization reaction. The polymerization step can include a first polymerization step in which polymerization is performed using a specific first organic peroxide as a polymerization initiator, and a second polymerization step in which polymerization is performed using a specific second organic peroxide as a polymerization initiator. The first polymerization step is a step in which most of the styrene-based monomer is polymerized at a relatively low temperature, and the second polymerization step is a step in which the remaining unreacted styrene-based monomer is polymerized at a temperature higher than the first polymerization step. Specifically, the polymerization step preferably includes a first polymerization step in which the styrene-based monomer is polymerized at a temperature of 110°C or less until the polymerization conversion rate reaches 90% by mass or more, and a second polymerization step in which the styrene-based monomer is further polymerized at a temperature higher than 115°C and 135°C or less until the content of the unreacted styrene-based monomer is 25 ppm by mass or less (including 0). In this case, it is possible to suppress the decrease in the weight average molecular weight of the styrene-based resin, improve the mechanical strength of the foamed molded product, and further reduce the amount of residual styrene.

The polymerization initiator is an organic peroxide, and as described above, a first organic peroxide and a second organic peroxide are used. The first organic peroxide has a 10-hour half-life temperature of 65°C or more and 90°C or less, and is an aliphatic peroxyester type and/or aliphatic peroxyketal type organic peroxide. If the 10-hour half-life temperature of the first organic peroxide is higher than 90°C, it may be difficult to reduce the amount of residual styrene. In addition, the mechanical strength of the resulting foamed molded product may be reduced. From the viewpoint of further reducing the amount of residual styrene and further improving the mechanical strength of the foamed molded product, the 10-hour half-life temperature of the first organic peroxide is preferably 85°C or less, more preferably 80°C or less. On the other hand, if the 10-hour half-life temperature of the first organic peroxide is less than 65°C, it may be difficult to reduce the amount of residual styrene. From the viewpoint of further reducing the amount of residual styrene, the 10-hour half-life temperature of the first organic peroxide is preferably 68°C or more, more preferably 70°C or more. The first organic peroxide is used, for example, mainly as a polymerization initiator in the first-stage polymerization step. The first organic peroxide is appropriately referred to as the "first polymerization initiator."

The second organic peroxide is an organic peroxide having a 10-hour half-life temperature of more than 90°C and not more than 110°C. If the 10-hour half-life temperature of the second organic peroxide is higher than 110°C, it may be difficult to reduce the amount of residual styrene. From the viewpoint of further reducing the amount of residual styrene, the 10-hour half-life temperature of the second organic peroxide is preferably 105°C or less, and more preferably 102°C or less. On the other hand, if the 10-hour half-life temperature of the second organic peroxide is 90°C or less, it may be difficult to reduce the amount of residual styrene. From the viewpoint of further reducing the amount of residual styrene, the 10-hour half-life temperature of the second organic peroxide is preferably 92°C or more, and more preferably 95°C or more. The second organic peroxide is used, for example, mainly as a polymerization initiator in the latter polymerization step. The second organic peroxide is appropriately referred to as a "second polymerization initiator".

The second organic peroxide preferably has a 10-hour half-life temperature 10°C or more higher than that of the first organic peroxide. In this case, the second organic peroxide is more suitable as a polymerization initiator (i.e., a second polymerization initiator) in the latter polymerization step. The second organic peroxide is preferably an aliphatic peroxydicarbonate-type and/or an aliphatic peroxyester-type organic peroxide. Since aliphatic peroxydicarbonate-type and/or aliphatic peroxyester-type organic peroxides are general-purpose polymerization initiators and can be obtained inexpensively, the manufacturing cost of the expandable particles can be reduced. In addition, in the manufacturing method of the present disclosure, even if an aliphatic peroxydicarbonate-type and/or an aliphatic peroxyester-type organic peroxide is used as the second polymerization initiator, the styrene content and the benzene content can be sufficiently reduced.

The 10-hour half-life temperature is defined as the temperature at which 50% of the organic peroxide charged in an inert solvent is thermally decomposed in 10 hours. The 10-hour half-life temperature can be measured, for example, as follows. First, the organic peroxide is dissolved in benzene to obtain a solution with a concentration of 0.1 mol/L. This solution is sealed in a glass tube in which the air inside has been replaced with nitrogen in advance. Next, the glass tube is immersed in a thermostatic bath set to a predetermined temperature to thermally decompose the organic peroxide. Here, the relationship kt=ln[PO]0/[PO] _t holds, where k is the decomposition rate constant, t is the time, [PO] ₀ is the initial concentration of the organic peroxide, and [PO] _t is the concentration of the organic peroxide after time t. Therefore _{, if the relationship between time t and ln[PO]0 /} [PO] _t is plotted on a graph, the decomposition rate constant k can be obtained from the slope.

Since the relationship [PO] ₀ /[PO] _t = 2 holds for the half-life time t1 _/2 , the half-life time t1/ ₂ at a certain temperature can be calculated from the equation t1 _/2 = ln2/k. The 10-hour half-life temperature can be obtained by calculating the half-life times t1 _/2 for multiple temperatures and plotting the relationship between lnt1 _/2 and 1/T on a graph. T is the absolute temperature (unit: K).
In addition, as the value of the 10-hour half-life temperature, data on the 10-hour half-life temperature listed in a catalog or technical document issued by a manufacturer of the organic peroxide can also be used.

Examples of the first organic peroxide include t-butylperoxy-2-ethylhexanoate, t-hexylperoxy-2-ethylhexanoate, 1,1,3,3-tetramethylbutylperoxy-2-ethylhexanoate, t-amylperoxy-2-ethylhexanoate, 1,1-bis(t-hexylperoxy)cyclohexane, 1,1-bis(t-butylperoxy)-2-methylcyclohexane, and 1,1-bis(t-butylperoxy)cyclohexane.
Examples of the second organic peroxide include t-butylperoxy-2-ethylhexyl carbonate, t-butylperoxy isopropyl carbonate, t-butylperoxy acetate, t-butylperoxy-3,5,5-trimethylhexanoate, t-butylperoxy laurate, t-amylperoxy-2-ethylhexyl carbonate, t-hexylperoxy acetate, 2,2-bis(4,4-di-t-butylperoxycyclohexyl)propane, and 2,2-bis(t-butylperoxy)butane.

Among the above organic peroxides, it is preferable to use a first organic peroxide made of t-butylperoxy-2-ethylhexanoate as the polymerization initiator and a second organic peroxide made of t-butylperoxy-2-ethylhexyl monocarbonate as the polymerization initiator. In this case, the amount of residual styrene in the expandable particles can be further reduced, and the generation of benzene in a state where the styrene-based monomer is in an extremely low concentration during the polymerization process can be further suppressed, so that the benzene content can be further reduced. Furthermore, the variation in the average particle size of the expandable particles (specifically, the degree of dispersion) can be reduced.

The amount of organic peroxide added is preferably 0.01 to 2 parts by mass per 100 parts by mass of styrene-based monomer. In this case, it is possible to increase the polymerization rate and sufficiently increase productivity while suppressing the increase in production costs associated with the use of organic peroxide. From the same viewpoint, it is more preferable that the amount of organic peroxide added is 0.1 to 1 part by mass per 100 parts by mass of styrene-based monomer.

When t-butylperoxy-2-ethylhexanoate is used as the first organic peroxide, it is preferable to add a polymerization inhibitor to the aqueous medium within a range that does not inhibit the polymerization of the styrene monomer. In this case, the variation in the average particle size of the expandable particles can be reduced. Examples of the polymerization inhibitor include oil-soluble polymerization inhibitors such as 4-t-butylcatechol (p-TBC), hydroquinone, p-benzoquinone, chloro-p-benzoquinone, 2,5-dichlorobenzoquinone, 2,6-dichlorobenzoquinone, 2,3-dimethyl-p-benzoquinone, 2,5-dimethyl-p-benzoquinone, methoxy-p-benzoquinone, tetrabromo-p-benzoquinone, tetrachloro-p-benzoquinone, dimethyl-p-benzoquinone, o-dinitrobenzene, m-dinitrobenzene, p-dinitrobenzene, 2,4-dinitrotoluene, 1,3,5-trinitrobenzene, 1,3,5-trinitroanisole, and 2,4,6-trinitrophenol; and water-soluble polymerization inhibitors such as sodium nitrite, potassium nitrate, ammonium nitrite, L-ascorbic acid, and citric acid.
The timing of adding the polymerization inhibitor is not particularly limited, but it is preferable to add the styrene-based monomer in which the flame retardant, the polymerization initiator, and the polymerization inhibitor are dissolved to the aqueous medium in the polymerization step. The amount of the polymerization inhibitor added is preferably 0.0001 to 0.01 parts by mass per 100 parts by mass of the styrene-based monomer.

In addition, as described above, Br-SBC is used as a flame retardant in the polymerization process. Compared to conventionally used bromine-based flame retardants such as 1,2,5,6,9,10-hexabromocyclododecane and 2,2-bis[4'-(2",3"-dibromo-2"-methylpropoxy)-3',5'-dibromophenyl]propane, Br-SBC is less likely to inhibit the polymerization of styrene monomers, and enables the reduction of unreacted styrene monomers by increasing the polymerization temperature in the latter stage, and does not significantly reduce the molecular weight of the styrene resin even if the polymerization temperature in the latter stage is increased. Therefore, by performing the multi-stage polymerization process as described above, it is possible to obtain resin particles that contain a flame retardant but have a lower content of unreacted styrene and a high molecular weight. The polymerization temperature in the latter polymerization process is appropriately referred to as the "final polymerization temperature."

In addition, in the manufacturing method of the present disclosure, the styrene-based monomer in which the flame retardant is dissolved is added to an aqueous medium to carry out suspension polymerization. This can increase the stability of the suspension polymerization. In contrast, for example, if the flame retardant and the polymerization initiator are dissolved in an aqueous medium and the styrene-based monomer in which the flame retardant is not dissolved is added to the aqueous medium to carry out suspension polymerization, the flame retardant may not be uniformly impregnated into the expandable particles, and the flame retardancy of the foamed molded product may be impaired. From the viewpoint of further reducing the content of unreacted styrene (i.e., the amount of remaining styrene), it is preferable to add the styrene-based monomer in which the flame retardant and the polymerization initiator are dissolved to an aqueous medium and carry out suspension polymerization. Note that the styrene-based monomer in which the flame retardant is dissolved and the styrene-based monomer in which the polymerization initiator is dissolved can also be added separately to an aqueous medium to carry out suspension polymerization.

From the viewpoint of suppressing the decrease in the weight average molecular weight of the styrene resin and improving the mechanical strength of the foamed molded product, the polymerization temperature in the first polymerization step is preferably 110°C or less, as described above, and more preferably 105°C or less. On the other hand, from the viewpoint of polymerization efficiency, the lower limit is about 70°C. Also, from the viewpoint of more sufficiently reducing unreacted styrene monomer in the second polymerization step performed following the first polymerization step, it is preferable to carry out polymerization in the first polymerization step until the polymerization conversion rate of the styrene monomer becomes 90% by mass or more, as described above, more preferably until it becomes 95% by mass or more, and even more preferably until it becomes 98% by mass or more.

From the viewpoint of efficiently reducing the amount of unreacted styrene monomer and further preventing a decrease in the weight average molecular weight of the styrene resin, the final polymerization temperature in the second polymerization step is preferably greater than 115°C and equal to or less than 135°C, as described above, and more preferably 118°C to 130°C. The amount of unreacted styrene monomer in the second polymerization step can be controlled by the holding time of the final polymerization temperature.

In the polymerization step, it is preferable to carry out suspension polymerization under conditions where the polymerization rate is 14%/hr or more and 18%/hr or less until the polymerization conversion rate reaches 90%. In this case, the amount of residual styrene can be further reduced, and the variation in the average particle size of the expandable particles can be reduced. The polymerization rate can be controlled within the above range by changing the polymerization temperature, the type and amount of the polymerization initiator, etc.
The polymerization rate until the polymerization conversion rate reaches 90% can be calculated by dividing the polymerization conversion rate of 90% by the time (hr) required for the polymerization conversion rate to reach 90%.

In addition, in the polymerization process, a flame retardant assistant can be used as necessary. As the flame retardant assistant, a radical generator with a one-hour half-life temperature exceeding 130°C can be used. The mass ratio of the flame retardant to the flame retardant assistant is preferably 1:0.1 to 1:5 (where flame retardant: flame retardant assistant). In this case, the flame retardancy of the foamed molded article can be further improved. As the flame retardant assistant, dicumyl peroxide, di-t-butyl peroxide, cumyl hydroperoxide, 2,3-dimethyl-2,3-diphenylbutane, 3,4-dimethyl-3,4-diphenylhexane, poly(1,4-diisopropylbenzene), etc. can be used.

In the suspension polymerization of styrene-based monomers, a suspending agent can be used. That is, the polymerization of styrene-based monomers can be carried out in a dispersion medium such as water to which a suspending agent has been added. As the suspending agent, for example, a hydrophilic polymer such as polyvinyl alcohol, methyl cellulose, or polyvinylpyrrolidone can be used. As the suspending agent, a poorly water-soluble inorganic salt such as calcium triphosphate, magnesium pyrophosphate, hydroxyapatite, aluminum oxide, talc, kaolin, or bentonite can also be used. In addition, a surfactant can be used in combination with the suspending agent as necessary. When a poorly water-soluble inorganic salt is used as a suspending agent, it is preferable to use an anionic surfactant such as sodium alkylsulfonate or sodium dodecylbenzenesulfonate in combination.

The amount of the suspending agent used is preferably 0.01 to 5 parts by mass per 100 parts by mass of the styrene-based monomer. When using a suspending agent made of a poorly water-soluble inorganic salt in combination with an anionic surfactant as described above, it is preferable to use 0.05 to 3 parts by mass of the suspending agent and 0.0001 to 0.5 parts by mass of the anionic surfactant per 100 parts by mass of the styrene-based monomer.

In addition, additives such as bubble nucleating agents (bubble regulators), plasticizers, chain transfer agents, phosphorus-based flame retardants, inorganic flame retardants, antistatic agents, antioxidants, UV absorbers, light stabilizers, conductive fillers, organic antibacterial agents, and inorganic antibacterial agents can be added to the styrene-based monomers as long as they do not impair the effects of the present invention.

As the bubble nucleating agent, for example, polyethylene wax, talc, silica, ethylene bisstearylamide, methyl methacrylate copolymer, silicone, etc. can be used.
As the plasticizer, for example, liquid paraffin, glycerin diacetomonolaurate, glycerin tristearate, di-2-ethylhexyl phthalate, di-2-ethylhexyl adipate, etc. can be used.
As the chain transfer agent, for example, octyl mercaptan, dodecyl mercaptan, α-methylstyrene dimer, etc. can be used.

Examples of phosphorus-based flame retardants that can be used include trimethyl phosphate, triethyl phosphate, tributyl phosphate, trioctyl phosphate, tributoxyethyl phosphate, triphenyl phosphate, and tricreadyl phosphate.
As the inorganic flame retardant, aluminum hydroxide, magnesium hydroxide, calcium carbonate, antimony trioxide, expandable graphite, red phosphorus, etc. can be used.

As the antistatic agent, alkyldiethanolamine, glycerin fatty acid ester, sodium alkylsulfonate, etc. can be used.
As the antioxidant, phenol-based, phosphorus-based, sulfur-based, and other antioxidants can be used.

As the ultraviolet absorbing agent, a benzotriazole type, a benzophenone type, or the like ultraviolet absorbing agent can be used.
As the light stabilizer, a hindered amine-based light stabilizer or the like can be used.

As the conductive filler, conductive carbon black, graphite powder, copper-zinc alloy powder, copper powder, silver powder, gold powder, etc. can be used.
As the organic antibacterial agent, for example, silver-based, copper-based, zinc-based, titanium oxide-based, etc. antibacterial agents can be used.
Furthermore, a rubber component such as butadiene rubber, styrene-butadiene rubber, isoprene rubber, ethylene-propylene rubber, etc. may be added to the styrene-based monomer as long as the effect of the present invention is not impaired.

Next, in the foaming agent impregnation step, a foaming agent is added during and/or after the suspension polymerization, and the styrene-based resin particles are impregnated with the foaming agent to obtain expandable particles. The foaming agent impregnation step may be performed on resin particles in the middle of polymerization, on resin particles after polymerization, or on both. In other words, the "resin particles" in the foaming agent impregnation step are a concept that includes at least either resin particles in the middle of polymerization or resin particles after polymerization. It is preferable to add the foaming agent when the polymerization conversion rate of the styrene-based monomer is 80 mass% or more. In this case, the content of unreacted styrene can be reduced.

In the foaming agent impregnation step, it is more preferable to add the foaming agent when the polymerization conversion rate of the styrene monomer is 90% by mass or more. In this case, the polymerization inhibition by the foaming agent is suppressed, and the amount of styrene in the expandable particles is further reduced. From the viewpoint of improving this effect, it is more preferable to add the foaming agent when the polymerization conversion rate is 95% by mass or more, and particularly preferably when it is 98% by mass or more. On the other hand, from the viewpoint of shortening the polymerization time and preventing, for example, the occurrence of surface depressions and improving the appearance of the foamed molded product, it is more preferable to add the foaming agent when the polymerization conversion rate is 99.5% by mass or less, and even more preferably when it is 99% by mass or less.

As organic physical blowing agents, hydrocarbons having 3 to 6 carbon atoms such as propane, n-butane, isobutane, n-pentane, isopentane, cyclopentane, n-hexane, and cyclohexane can be used. These blowing agents can be used alone or in combination of two or more types.

In the foaming agent impregnation step, it is preferable to add butane and pentane as the foaming agent. In this case, the foaming agent is highly retained, the bubbles in the foamed molded product do not become too fine, and the surface is less likely to collapse.

The foaming agent content in the expandable particles is preferably 2 to 10% by mass. In the foaming agent impregnation process, a foaming agent is supplied, for example, into a sealed container in an amount such that the foaming agent content in the expandable particles is 2 to 20% by mass.

The styrene content in the expandable particles is 25 ppm by mass or less (including 0). In this case, it is easy to reduce the styrene content in the foamed molded body to, for example, 10 ppm by mass or less (including 0). From the viewpoint of more reliably reducing the styrene content in the foamed molded body to 10 ppm by mass or less (including 0) and further reducing the amount of styrene emitted from the foamed molded body, the styrene content in the expandable particles is more preferably 20 ppm by mass or less (including 0), and even more preferably 18 ppm by mass or less (including 0). There is no particular limit to the lower limit of the styrene content in the expandable particles, but it is generally 5 ppm by mass or more. The styrene content in the expandable particles can be measured using a headspace gas chromatograph mass spectrometer. As a measurement sample, a solution obtained by dissolving the expandable particles in dimethylformamide can be used.

The benzene content in the expandable particles is 1.5 ppm by mass or less (including 0). In this case, it is easy to reduce the benzene content in the foamed molded body to, for example, 0.4 ppm by mass or less (including 0). From the viewpoint of more reliably reducing the benzene content in the foamed molded body to 0.4 ppm by mass or less (including 0) and further reducing the amount of benzene emitted from the foamed molded body, the benzene content in the expandable particles is more preferably 1 ppm by mass or less (including 0), and even more preferably 0.8 ppm by mass or less (including 0). There is no particular limit to the lower limit of the benzene content in the expandable particles, but it is generally 0.1 ppm by mass or more. The benzene content in the expandable particles can be measured using a headspace gas chromatograph mass spectrometer. As a measurement sample, a solution obtained by dissolving the expandable particles in dimethylformamide can be used.

The moisture content in the expandable particles is preferably 1.5% by mass or less. By using Br-SBC as a flame retardant, the moisture content in the expandable particles can be reduced to within the above range. In this case, it becomes easier to suppress the coarsening of the bubbles in the expandable particles obtained by using the expandable particles, and the maturation period until the size of the bubbles becomes uniform can be shortened. Even if the maturation period of the expandable particles is shortened, it is possible to further suppress the decrease in the mechanical strength of the foamed molded body and to further improve the appearance of the foamed molded body. The maturation period is the period during which the expandable particles are stored in a sealed container in a low-temperature environment of, for example, 10°C or less. The moisture content of the expandable particles is more preferably 1% by mass or less, even more preferably 0.5% by mass or less, and even more preferably 0.3% by mass or less. The moisture content in the expandable particles can be determined by a Karl Fischer moisture meter equipped with a heating vaporizer.

The expandable particles are used to manufacture expanded particles. Specifically, the expanded particles can be obtained by carrying out an expansion process in which the expandable particles are expanded. In the expansion process, the expandable particles are expanded by heating them with a heating medium such as steam, thereby obtaining the expanded particles. The expanded particles are also generally called pre-expanded particles. The expansion process can be carried out, for example, by supplying steam to the expandable particles introduced into a batch-type pre-expanding machine equipped with a stirrer.

The bulk density of the expanded beads can be controlled by adjusting the expansion conditions. Examples of the expansion conditions include the steam pressure (specifically, the gauge pressure) and the heating time with steam. Increasing the bulk density of the expanded beads improves the strength of the expanded molded article, while decreasing the bulk density makes the article lighter and reduces the raw material cost. From this viewpoint, the bulk density of the expanded beads is preferably 12 to 100 kg/ ^m3 , more preferably 16 to 50 kg/ ^m3 , and even more preferably 20 to 50 kg/ ^m3 .

The bulk density of the expanded beads can be determined by the following method: The expanded beads that have been air-dried for a day and night are filled into a 1-L measuring cylinder up to the 1-L standard position, and the weight of the expanded beads (=Wp) is measured to the first decimal place. The bulk density of the expanded beads (kg/ ^m3 ) can then be calculated from the weight Wp (g) of the expanded beads using the following formula (1).
Bulk density of expanded beads (kg/m ³ )=Wp (g)/1 (L) (1)

The expanded beads are used to produce expanded molded articles. The molding process can be carried out by in-mold molding. Specifically, expanded beads are fused together in a mold and molded into the desired shape to obtain an expanded molded article. In the in-mold molding method, expanded beads filled in a mold are heated by supplying a heating medium such as steam to the expanded beads to obtain an expanded molded article.

Next, a preferred embodiment of the foam molded product will be described. The foam molded product uses a styrene-based resin as the base resin, which is obtained by suspension polymerization of a styrene-based monomer in the presence of a flame retardant as described above. In the foam molded product, a large number of foam particles containing the flame retardant are fused together. The flame retardant contained in the foam molded product is as described above, and contains at least a brominated styrene-butadiene copolymer.

The weight average molecular weight Mw of the styrene resin in the foamed molded product is preferably 150,000 or more. In this case, the strength of the foamed molded product is sufficiently high. From the viewpoint of further improving the strength of the foamed molded product, the weight average molecular weight Mw is preferably 180,000 or more, and more preferably 200,000 or more. From the viewpoint of improving the expandability of the expandable particles, the weight average molecular weight Mw is preferably 350,000 or less, and more preferably 300,000 or less. From the viewpoint of improving the strength of the foamed molded product, the number average molecular weight Mn of the styrene resin is preferably 80,000 or more, more preferably 85,000 or more, and even more preferably 90,000 or more. From the viewpoint of improving the expandability, the number average molecular weight Mn of the styrene resin is preferably 150,000 or less, more preferably 140,000 or less, and even more preferably 120,000 or less. From the viewpoint of improving the strength of the foamed molded product, the z-average molecular weight Mz of the styrene resin is preferably 300,000 or more, more preferably 400,000 or more, and even more preferably 450,000 or more. From the viewpoint of improving the foaming property, the z-average molecular weight Mz of the styrene-based resin is preferably 700,000 or less, more preferably 600,000 or less, and even more preferably 550,000 or less. The weight average molecular weight Mw, the number average molecular weight Mn, and the z-average molecular weight Mz are standard polystyrene equivalent values measured by gel permeation chromatography (i.e., GPC method).

The content of styrene in the foamed molded product is preferably 10 ppm by mass or less. In this case, the amount of styrene emitted from the foamed molded product can be sufficiently reduced. This makes the foamed molded product more suitable for use in narrow spaces such as automobile interior materials. From the same viewpoint, the content of styrene in the molded product is preferably 5 ppm by mass or less. In addition, the amount of styrene emitted from the foamed molded product is preferably 50 μg/m ³ or less. The content of styrene in the foamed molded product can be measured using a headspace gas chromatograph mass spectrometer, similar to the above-mentioned expandable particles. As a measurement sample, a solution obtained by dissolving a test piece taken from the foamed molded product in dimethylformamide can be used.

The benzene content in the foamed molded product is preferably 0.4 mass ppm or less. In this case, the amount of benzene emitted from the foamed molded product can be sufficiently reduced. This makes the foamed molded product more suitable for use in narrow spaces such as automobile interior materials. From the same viewpoint, the benzene content in the foamed molded product is preferably 0.3 mass ppm or less. In addition, the amount of benzene emitted from the foamed molded product is preferably 11 μg/m ³ or less. The benzene content in the foamed molded product can be measured by the same method as that for the above-mentioned expandable particles. As a measurement sample, a solution obtained by dissolving a test piece taken from the foamed molded product in dimethylformamide can be used.

The apparent density of the foam molded article is preferably 12 to 100 kg/m ^3. In this case, it is possible to achieve both physical properties such as strength and flame retardancy of the foam molded article and light weight. From the viewpoint of improving this effect, the apparent density of the foam molded article is more preferably 16 to 50 kg/m ³ , and further preferably 20 to 50 kg/m ³ .
The apparent density of the foamed molded article can be determined by dividing the mass of the foamed molded article by the volume determined from its outer dimensions.

Foam molded products are used for interior and exterior materials for vehicles such as automobiles, insulation materials for homes, and cooler boxes. Foam molded products have excellent flame retardancy and a low content of styrene and benzene, making them ideal for interior materials for vehicles, and are particularly suitable for debia pads and floor spacers.

The following describes examples and comparative examples of expandable particles and foamed molded products.

Example 1
The manufacturing method of the expandable particles and the expanded molded article of this example will be described below. First, 16 kg of deionized water, 14.4 g of tribasic calcium phosphate (suspending agent), 0.6 g of sodium α-olefin sulfonate (surfactant), 0.2 g of disodium alkylbiphenyl disulfonate (surfactant), and 24.4 g of sodium acetate (electrolyte) were charged into an autoclave with an internal volume of 50 L and equipped with a stirrer. Next, 43.2 g of t-butylperoxy-2-ethylhexanoate ("Perbutyl O" manufactured by NOF Corp., first polymerization initiator), which is an organic peroxide having a 10-hour half-life temperature of 72.1° C., 25.6 g of t-butylperoxy-2-ethylhexyl monocarbonate ("Perbutyl E" manufactured by NOF Corp., second polymerization initiator), which is an organic peroxide having a 10-hour half-life temperature of 99° C., and 10.0 g of brominated styrene-butadiene block copolymer ("Emerald Innovation" manufactured by Chemtura Japan, Ltd.) were mixed with the 10-hour half-life temperature initiator. 12.8 g of dicumyl peroxide (NOF Corp.'s "Percumyl D", flame retardant aid), 51.2 g of dicumyl peroxide, an organic peroxide with a 1-hour half-life temperature of 136 ° C., 128 g of liquid paraffin (MORESCO Corp.'s "Moresco White P-60", plasticizer), and 3.2 g of polyethylene wax powder (Toyo Adle Corp.'s "Polyethylene Wax 1000", foam regulator), and 0.32 g of 4-t-butylcatechol (DIC Corp.'s "DIC-TBC", polymerization inhibitor) were mixed with 16 kg of styrene, and the mixture was stirred at a rotation speed of 210 rpm and charged into an autoclave. The flame retardant used in this example will be referred to as "E3000" hereinafter.

Next, the air in the autoclave was replaced with nitrogen, and the temperature was raised to 90°C over 1.5 hours. After reaching 90°C, the temperature in the autoclave was raised to 100°C over 6 hours and 30 minutes (first polymerization step). The temperature in the autoclave was then raised to 120°C over 2 hours, and the autoclave was kept at 120°C for 5 hours (second polymerization step). The temperature was then cooled to 30°C over about 6 hours. During the temperature rise from 90°C to 100°C, 5 hours and 30 minutes after reaching 90°C, 320g of pentane (a mixture of 80% n-pentane and 20% isopentane) and 880g of butane (a mixture of 70% n-butane and 30% isobutane) were injected into the autoclave as a blowing agent over 30 minutes. At the start of the addition of the blowing agent, i.e., 5 hours and 30 minutes after the temperature reached 90°C, the polymerization conversion rate of styrene was 98% by mass. In addition, at the end of the first polymerization step, i.e., when the temperature reached 100°C, the polymerization conversion rate of styrene was 99% by mass. The method for measuring each polymerization conversion rate is described below. In addition, the polymerization rate up to a polymerization conversion rate of 90% was 15%/hr. The method for measuring the polymerization rate is described below.

After cooling, the contents (expandable particles) were removed from the autoclave, and in order to remove the tricalcium phosphate adhering to the surface of the expandable particles, nitric acid was added to the expandable particles, which were then dehydrated in a centrifuge and the water adhering to the surface was removed using a fluidized bed dryer. In this way, expandable particles with an average particle size of approximately 1.0 mm were obtained.

Next, the expandable particles were sieved to select and remove particles of 0.5 to 1.4 mm. Thereafter, 0.005 parts by mass of N,N-bis(2-hydroxyethyl)alkylamine (antistatic agent) was added to 100 parts by mass of the expandable particles to coat the expandable particles with an antistatic agent. Furthermore, 0.1 parts by mass of zinc stearate, 0.05 parts by mass of glycerin tristearate, and 0.05 parts by mass of glycerin monostearate were added to 100 parts by mass of the expandable particles to coat the expandable particles with this mixture. Thereafter, the expandable particles were placed in an airtight container and stored in a refrigerator at 6°C to allow maturation.

The polymerization conditions for the expandable particles are shown in Table 1. The polymerization conversion rate was measured as follows.

"Polymerization conversion rate at the end of the first polymerization step"
A separate first-stage polymerization step was carried out under the same conditions as the first-stage polymerization step carried out when the expandable particles were produced. At the same time as the completion of this first-stage polymerization step, the temperature of the contents of the autoclave was rapidly cooled to 30°C or less within 10 minutes to stop the polymerization reaction. After cooling, the styrene-based resin particles in the middle of polymerization were taken out of the autoclave, dehydrated by a centrifuge, and the moisture attached to the surface was removed by a fluidized drying device. In this way, the styrene-based resin particles at the end of the first-stage polymerization step were obtained, and the content of unreacted styrene-based monomer in the obtained styrene-based resin particles was determined by gas chromatography. The method for measuring the content of styrene-based monomer by gas chromatography will be described later. Then, the polymerization conversion rate was calculated from the following formula (2). This operation was carried out three times, and the arithmetic average value of each polymerization conversion rate was calculated.
Polymerization conversion rate (mass%) = 100 - styrene monomer content (mass%) (2)

"Polymerization conversion rate when foaming agent is impregnated"
Polymerization was carried out separately under the same conditions as those for producing the expandable particles, and the temperature of the contents of the autoclave was rapidly cooled to 30°C or less within 10 minutes immediately before the start of the addition of the blowing agent to terminate the polymerization reaction. The polymerization conversion rate at the time of impregnation with the blowing agent was determined by carrying out the same operation as that for measuring the polymerization conversion rate at the end of the above-mentioned first-stage polymerization step. This operation was carried out three times, and the arithmetic average value of each polymerization conversion rate is shown in the table.

"Polymerization rate up to 90% polymerization conversion"
Polymerization was carried out separately under the same conditions as those for producing the expandable particles, and after the temperature of the autoclave was raised, the temperature of the contents of the autoclave was rapidly cooled to 30° C. or less within 10 minutes after an arbitrary time had elapsed, to stop the polymerization reaction. The polymerization conversion rate was then determined by the same method as the method for measuring the polymerization conversion rate at the end of the first-stage polymerization process described above. This operation was carried out at multiple time intervals, and a diagram was created in which the relationship between the polymerization conversion rate and the time required to reach that polymerization conversion rate was plotted. The time (hr: hour) required for the polymerization conversion rate to reach 90% was then determined from the relationship between the obtained polymerization conversion rate and the time required to reach that polymerization conversion rate. The polymerization rate until the polymerization conversion rate reached 90% was determined by dividing the polymerization conversion rate of 90% by the time (hr: hour) required for the polymerization conversion rate to reach 90%.

The expandable particles of this example were measured for the blowing agent content, styrene content, benzene content, water content (i.e., WC), average particle diameter d63, average particle diameter variation, average molecular weight of the styrene-based resin, and expandability. The results are shown in Table 3.

"Foaming agent content"
The content of the blowing agent was measured by performing gas chromatography of the solution obtained by dissolving the expandable particles in dimethylformamide (DMF). Specifically, first, about 5 g of cyclopentanol was precisely weighed out to the third decimal place in a 100 ml volumetric flask. This weight is hereinafter referred to as Wi (g). Furthermore, DFM was added to the volumetric flask to make the total volume 100 mL. This DMF solution was further diluted 100 times with DMF. This was used as the internal standard solution. Next, about 1 g of the expandable particles to be measured was precisely weighed out to the third decimal place. This weight is hereinafter referred to as Ws (g). The precisely weighed sample of the expandable particles was dissolved in about 18 ml of DMF, and then 2 ml of the internal standard solution was added using a whole pipette. 1 μL of the solution thus obtained was collected using a microsyringe and introduced into gas chromatography to obtain a chromatogram. The peak areas of the foaming agent components and the internal standard were determined from the obtained chromatogram, and the concentrations of each component were calculated from the following formula (4).

Component concentration (mass%) = (Wi/10000) 2 x (An/Ai) x Fn/Ws x 100 (4)
Wi: weight of cyclopentanol (g) when the internal standard solution was prepared, Ws: weight of sample dissolved in DMF (g), An: peak area of each component measured by gas chromatography, Ai: peak area of each component measured by gas chromatography Fn: correction coefficient for each component determined from a calibration curve prepared in advance.

The analysis conditions for the gas chromatograph are as follows:
Equipment used: Gas chromatograph GC-6AM manufactured by Shimadzu Corporation
Detector: FID (Flame Ionization Detector)
Column material: Glass column with an inner diameter of 3 mm and a length of 5,000 mm Column packing material: [Liquid phase name] FFAP (free fatty acid), [Liquid phase content] 10% by mass, [Carrier name] Diatomaceous earth for gas chromatography Chomasorb W, [Carrier particle size] 60/80 mesh, [Carrier treatment method] AW-DMCS (washed with water, calcined, acid treated, silane treated), [Packing amount] 90 ml
Inlet temperature: 250℃
Column temperature: 120 ° C.
Detector temperature: 250°C
Carrier gas: _N2
Liquid volume: 40ml/min

"Styrene content, benzene content"
The contents of unreacted styrene monomer and benzene in the expandable particles were measured by a gas chromatograph mass spectrometer using the headspace method. As a sample, a solution obtained by dissolving the expandable particles in dimethylformamide (i.e., DMF) was used. Specifically, a standard solution was first prepared so that the styrene or benzene concentration in DMF was 5 mass ppm, 50 mass ppm, or 500 mass ppm. Next, 0.2 g of the standard solution was precisely weighed into a 20 ml vial, and 1 ml of DMF was added and sealed. The vial was kept warm in a headspace sampler, and the gas phase was measured by a gas chromatograph mass spectrometer, and a calibration curve was created from the obtained chromatogram. Next, 0.2 g of the sample was precisely weighed into a 20 ml vial, and 1 ml of DMF was added and sealed, and the vial was kept at room temperature for one day to completely dissolve it. The mixture was kept warm in a headspace sampler, and the gas phase was measured with a gas chromatograph mass spectrometer. The contents of unreacted styrene and benzene were determined from a calibration curve previously prepared from the obtained chromatogram.

The conditions of gas chromatography mass spectrometry for measuring the styrene content and the benzene content are as follows.
Gas chromatograph mass spectrometer: Shimadzu Corporation GCMS-QP2020
Headspace sampler: Shimadzu Corporation HS-20
Capillary column: Stabilwax, GL Sciences, Inc., inner diameter 0.32 mm, length 30 m
Headspace sampler incubation conditions: 90°C for 1 hour Column temperature: 50°C, 2 minutes ⇒ (heating rate: 10°C/min) ⇒ 90°C ⇒ (heating rate: 5°C/min) ⇒ 120°C ⇒ (heating rate: 20°C/min) ⇒ 230°C, 2 minutes Ion source temperature: 200°C
Carrier gas: Helium Carrier gas column flow rate: 2 ml/min Split ratio: 1/10

"Moisture content"
The moisture content of the expandable particles was measured using a Karl Fischer moisture meter. Specifically, about 0.28 g of a sample of the expandable particles was precisely weighed. Next, the sample was heated at a temperature of 160° C. using a moisture vaporizer CHK-501 manufactured by Kyoto Electronics Manufacturing Co., Ltd. to vaporize the moisture, and the moisture content was measured using a Karl Fischer moisture meter (coulometric titration method) MKC-610 manufactured by Kyoto Electronics Manufacturing Co., Ltd.

"Molecular weight"
The molecular weight (number average molecular weight, weight average molecular weight, Z average molecular weight) of the styrene resin in the expandable particles can be measured by gel permeation chromatography (GPC) using polystyrene as a standard substance. Specifically, the measurement was performed using HLC-8320GPC EcoSEC manufactured by Tosoh Corporation under the following measurement conditions: eluent: tetrahydrofuran (THF), THF flow rate: 0.6 ml/min, sample concentration: 0.1 wt%. As the column, one TSKguard column SuperH-H and two TSK-GEL SuperHM-H columns connected in series were used. That is, the test piece taken from the expandable particles was dissolved in tetrahydrofuran (THF), and the molecular weight was measured by gel permeation chromatography (GPC). The measured value was then calibrated with standard polystyrene to determine the number average molecular weight, weight average molecular weight, and Z average molecular weight, respectively.

"Average particle size, dispersity"
The 63% volume average particle diameter d63, the 90% volume average particle diameter d90, and the 10% volume average particle diameter d10 of the expandable particles were measured. The 10% volume average particle diameter d10 is the particle diameter at volume cumulative values of 63%, 90%, and 10%, respectively, in the particle size distribution obtained by a laser diffraction/scattering method. showed a value of 63% volume average particle diameter d63.
The specific measurement method is as follows. The particle size distribution of the expandable particles was measured using a particle size distribution measuring device "Militrack JPA" manufactured by KISO Co., Ltd. 30 g of expandable granules were allowed to fall freely from a supply feeder, and the projected image was captured by a CCD camera. Then, the captured image information was subjected to calculation and combination processing in sequence, and the particle size distribution and shape index results were output. In this manner, the particle sizes (d10, d63, d90) mm at the volumetric integrated values of 10%, 63%, and 90% in the particle size distribution were calculated. From these values, the following formula ( The 63% average particle size dispersion was calculated based on the above formula (5). The average particle size was measured for the expandable particles immediately after production, and was not classified by sieving or the like. The smaller the dispersity value, the smaller the variation in particle size.
Dispersion degree of 63% volume average particle size=(d90−d10)/d63 (5)

"Effervescent"
The expandable particles that had completed the aging were expanded in a shelf-type expansion machine by heating with steam at 3 kPa (gauge pressure) for 270 seconds. The expanded particles were then air-dried for one day and night. The expanded particles were then filled into a 1 L graduated cylinder up to the 1 L standard position, and the weight of the expanded particles (=Wp) was measured to one decimal place. The bulk density (kg/ ^m3 ) of the expanded particles was calculated from the weight Wp (g) of the expanded particles using the following formula (5). The expandability was determined from the bulk density of the expanded particles.
Bulk density of expanded beads (kg/m ³ )=Wp (g)÷1 (L) (5)

Next, 3.6 kg of the aged expandable particles were placed in a pressurized batch expansion machine (DYHL-500U, Daisen Kogyo Co., Ltd.), and heated for 130 seconds by supplying steam at a gauge pressure of 0.15 MPa so that the pressure inside the expansion machine (gauge pressure) became 0.023 MPa, thereby obtaining expanded particles with an expansion ratio of approximately 50 times (bulk density of 20 kg/ ^m3 ).

Next, the foamed beads were aged at room temperature (specifically, 23°C) for one day, then filled into the mold of a molded product molding machine manufactured by DABO Co., Ltd. and heated for 15 seconds at a steam pressure of 0.08 MPa (gauge pressure). Next, after cooling for a specified time, the foamed molded product was removed from the mold. In this way, a 300 mm x 300 mm x 25 mm plate-shaped foamed molded product was obtained in which many foamed beads were fused together. Next, the apparent density, the content of unreacted styrene and benzene, the amount of styrene and benzene emitted, and the bending strength of the foamed molded product were measured, and the flame retardancy (specifically, burning rate and self-extinguishing property) and moldability were evaluated. The results are shown in Table 3.

"Apparent density"
The apparent density was calculated by dividing the mass of the foamed molded article by the volume determined from its outer dimensions.

"Styrene and benzene content"
The sample was measured in the same manner as in the case of the expandable granules, except that a solution obtained by dissolving a part of the foamed molded article in dimethylformamide (ie, DMF) was used.

"Styrene and benzene emissions"
A foam molded body was produced under the same molding conditions as above, except that a molded die for molding a plate-shaped foam molded body with dimensions of 100 mm x 80 mm x 15 mm was used. The foam molded body was dried at 40°C for one day. Thereafter, the foam molded body was placed in a 10 L Tedlar (registered trademark) bag, and the gas (specifically, air) in the bag was expelled by nitrogen purging three times, after which 4 L of nitrogen was injected into the bag and the temperature was kept at 40°C for two hours. Styrene and benzene generated from the foam molded body were collected in a Tenax-TA collection tube. The flow rate to the collection tube was 0.1 ml/min, and the collected amount was 1.0 L. Next, the styrene and benzene in the collection tube were quantified by a thermal desorption-gas chromatograph mass spectrometer, and the amount of styrene and benzene per 1 ^m3 of gas (unit: μg/ ^m3 ) were obtained. In addition, when the amount of emission was less than 5 μg/ ^m3 , it was indicated as "<5" in the table.

"Flame retardancy (burning speed)"
The evaluation of the burning rate was performed in accordance with FMVSS No. 302. Specifically, the foam molded body was first left at a temperature of 40°C for 3 days, and then left at room temperature for 1 day to be aged. Thereafter, the foam molded body was left at a temperature of 40°C for 3 days, and then left at room temperature for 1 day to be cured. Thereafter, five flat test pieces measuring 356 mm long x 102 mm wide x 13 mm thick were cut out from the foam molded body, and a burning test was performed using an MVSS flammability tester MVSS-2 manufactured by Suga Test Instruments Co., Ltd. Specifically, the test piece was fixed horizontally to a U-shaped fixture, and the free end of the test piece was exposed to the flame of a Bunsen burner with the flame length adjusted to 38 mm for 15 seconds to ignite the test piece. Then, the time required for the test piece to burn from the fixed end located on the opposite side of the free end to a point 38 mm away was measured, and the burning rate (mm/min) was calculated from the burning distance and the burning time. If the specimen did not reach the specified end, the burning speed was calculated from the burning distance to the point where the flame stopped and the burning time. If the specimen did not ignite, the burning speed was set to 0 mm/min. The arithmetic average of the burning speeds of the five specimens was taken as the burning speed, and the suitability for FMVSS No. 302 (burning speed of 102 mm/min or less) was judged.

"Flame retardant (self-extinguishing)"
The self-extinguishing property of the foam molded article was evaluated based on the combustion test (method A) of JIS A 9511:2006R. Specifically, the foam molded article was first left at a temperature of 40°C for 3 days, and then left at room temperature for 1 After that, five rectangular parallelepiped test pieces measuring 200 mm long x 25 mm wide x 10 mm thick were cut out from the foamed molded article. Then, the ignition limit indicator line and After the test piece was ignited up to the flammability limit indicator line, the candle was quickly removed from the test piece. The time from the moment the candle was removed to the time the flame of the test piece went out (flame extinguishing time) was measured. The arithmetic mean of the flame extinction times of the three test pieces was calculated. The self-extinguishing property (flame extinction time of 3 seconds or less) was judged from this arithmetic mean of the flame extinction time. If the flame extinction time exceeded 10 seconds, the flame extinction time was judged to be 3 seconds or less. The number ">10" is written inside.

"Bending strength"
The foam molded article was cut to prepare a rectangular parallelepiped test piece having a length of 300 mm, a width of 75 mm, and a thickness of 25 mm. The test piece was subjected to a three-point bending test in accordance with JIS K 7221-2, Appendix 1, to measure the bending strength and evaluate the mechanical strength of the foam molded article.

"Moldability"
The surface of the foamed molded article was visually inspected, and the absence of depressions was rated as "◯ (absent)", while the presence of depressions was rated as "× (present)".

(Examples 2 to 9)
The polymerization conditions were changed as shown in Tables 1 and 2, and the same measurements and evaluations as in Example 1 were performed. In Example 4, the expansion ratio of the expanded beads was set to 30 times, and the expanded molded article was evaluated. In Example 5, the expansion ratio of the expanded beads was set to 20 times, and the expanded molded article was evaluated. In Example 6, the first polymerization initiator was changed from t-butylperoxy-2-ethylhexanoate ("Perbutyl O" manufactured by NOF Corp.) having a 10-hour half-life temperature of 72.1°C to 1,1,3,3-tetramethylbutylperoxy-2-ethylhexanoate ("Perocta O" manufactured by NOF Corp.) having a 10-hour half-life temperature of 65.3°C. In Example 7, the first polymerization initiator was changed from t-butylperoxy-2-ethylhexanoate ("Perbutyl O" manufactured by NOF Corp.) having a 10-hour half-life temperature of 72.1°C to 1,1-bis(t-butylperoxy)cyclohexane ("Perhexa HC" manufactured by NOF Corp.) having a 10-hour half-life temperature of 87.1°C. In Example 8, the temperature was raised to 100°C over 1.5 hours, and after reaching the temperature of 100°C, the temperature inside the autoclave was further raised to 110°C over 6 hours and 30 minutes. Thereafter, the temperature inside the autoclave was further raised to 120°C over 2 hours, and the autoclave was maintained at the temperature of 120 for 5 hours. Thereafter, the temperature was cooled to 30°C over about 6 hours. During the temperature increase from 100° C. to 110° C., 5 hours and 30 minutes after reaching 100° C., the flow was changed to pressure-feeding 320 g of pentane (a mixture of 80% n-pentane and 20% isopentane) and 880 g of butane (a mixture of 70% n-butane and 30% isobutane) into the autoclave over a period of 30 minutes.

(Comparative Example 1)
In this example, the same procedure as in Example 1 was carried out except that no polymerization inhibitor was used, and t-butylperoxy-2-ethylhexanoate ("Perbutyl O" manufactured by NOF Corp.) was used as the first polymerization initiator, and benzoyl peroxide ("Niper BW" manufactured by NOF Corp., water-dilutable powder product, polymerization initiator) was used.

(Comparative Example 2 and Comparative Example 3)
Comparative Examples 2 and 3 are examples in which the amount of foaming agent added was changed from that of Comparative Example 1. Specifically, Comparative Example 2 was carried out in the same manner as Comparative Example 1, except that no polymerization inhibitor was used, and 320 g of pentane (a mixture of 80% n-pentane and 20% isopentane) and 928 g of butane (a mixture of 70% n-butane and 30% isobutane) were used as the foaming agent. Comparative Example 3 was carried out in the same manner as Comparative Example 1, except that no polymerization inhibitor was used, and 320 g of pentane (a mixture of 80% n-pentane and 20% isopentane) and 976 g of butane (a mixture of 70% n-butane and 30% isobutane) were used as the foaming agent.

(Comparative Example 4)
No polymerization inhibitor was used, and the air in the autoclave was replaced with nitrogen, after which the temperature was raised to 90° C. over 1.5 hours, and after the polymerization temperature reached 90° C., the temperature in the autoclave was raised to 100° C. over 6 hours and 30 minutes, all of which were carried out in the same manner as in Comparative Example 1. The temperature was then further raised to 115° C. over 2 hours, and the temperature was maintained at 115° C. for 5 hours. Thereafter, the temperature was cooled to 30° C. over approximately 6 hours. The cooling operation and subsequent steps were carried out in the same manner as in Comparative Example 1.

(Comparative Example 5)
In this example, the same procedure as in Comparative Example 1 was carried out except that no polymerization inhibitor was used and 160 g of 2,2-bis(4'-(2",3"-dibromo-2"-methylpropoxy-3',5'-dibromophenyl)propane was used as the flame retardant. The flame retardant used in this example will hereinafter be referred to as "SR130" as appropriate.

(Comparative Example 6)
In this example, the same procedure as in Comparative Example 1 was carried out except that 160 g of SR130 was used as the flame retardant and Perbutyl O was used as the first polymerization initiator.

(Comparative Example 7)
In this example, the same procedure as in Comparative Example 1 was carried out except that no polymerization inhibitor was used, and the flame retardant, the first polymerization initiator, and the second polymerization initiator were added to the aqueous phase without being dissolved in the styrene-based monomer.

(Comparative Example 8)
In this example, the flame retardant, the first polymerization initiator, and the second polymerization initiator were added to the aqueous phase without being dissolved in the styrene monomer, and Perbutyl O was used as the first polymerization initiator. After the air in the autoclave was replaced with nitrogen, the temperature was raised to 90° C. over 1.5 hours, and after the polymerization temperature reached 90° C., the temperature in the autoclave was raised to 100° C. over 6 hours and 30 minutes in the same manner as in Comparative Example 1. Next, the temperature was further raised to 115° C. over 2 hours, and the temperature was kept at 115° C. for 5 hours. Then, the temperature was cooled to 30° C. over about 6 hours. The cooling operation and subsequent operations were performed in the same manner as in Comparative Example 1.

(Comparative Example 9)
Comparative Example 9 is an example in which the timing of adding the foaming agent was changed from that of Comparative Example 1. Specifically, Comparative Example 9 was carried out in the same manner as Comparative Example 1, except that the timing of adding the foaming agent was changed from 5.5 hours to 6.5 hours after the temperature reached 90°C.

(Comparative Example 10)
In this example, the same procedure as in Example 1 was carried out except that t-butylperoxyisopropyl monocarbonate ("Perbutyl I" manufactured by NOF Corporation) was used as the first polymerization initiator.

The details of the organic peroxides in the table are as follows:
Perbutyl O is t-butylperoxy-2-ethylhexanoate, and its structure is represented by the following structural formula (1).

Perhexa HC (manufactured by NOF Corporation) is 1,1-di(t-hexylperoxy)cyclohexane, and its structure is represented by the following structural formula (3).

BPO (specifically, NOF Corp.'s "Niper BW", a water-dilutable powder product) is dibenzoyl peroxide, and its structure is represented by the following structural formula (3).

Perbutyl I (manufactured by NOF Corporation) is t-butylperoxyisopropyl monocarbonate, and its structure is represented by the following structural formula (4).

Perbutyl E (manufactured by NOF Corporation) is t-butylperoxy-2-ethylhexyl monocarbonate, and its structure is represented by the following structural formula (5).

Perocta O (manufactured by NOF Corporation) is 1,1,3,3-tetramethylbutylperoxy-2-ethylhexanoate, and its structure is represented by the following structural formula (6).

For each example and comparative example, the polymerization conditions, properties of the expandable particles, and the results of the expansion and molding evaluation are shown in Tables 1 to 6.

As can be seen from Tables 1 to 3, Examples 1 to 5 and Example 8 are examples in which a polymerization inhibitor was used and t-butylperoxy-2-ethylhexanoate was used as the first polymerization initiator, Example 9 is an example in which a polymerization inhibitor was not used and t-butylperoxy-2-ethylhexanoate was used as the first polymerization initiator, Example 6 is an example in which a polymerization inhibitor was used and 1,1,3,3-tetramethylbutylperoxy-2-ethylhexanoate was used as the first polymerization initiator, and Example 7 is an example in which a polymerization inhibitor was used and 1,1-di(t-hexylperoxy)cyclohexane was used as the first polymerization initiator. The expandable particles and expanded molded products of Examples 1 to 9 have low styrene and benzene contents. Therefore, the amount of styrene and benzene emitted from the expanded molded products is low. In addition, since the weight average molecular weight of the styrene resin is maintained at 150,000 or more, it has excellent bending strength, contains a brominated styrene-butadiene copolymer as a flame retardant, and complies with the flammability test of FMVSS No. 302. Example 3 is an example in which the amount of flame retardant is increased, and also passed the flammability test (Method A) of JIS A 9511:2006R.

In particular, when t-butylperoxy-2-ethylhexanoate is used as the first polymerization initiator and t-butylperoxy-2-ethylhexyl monocarbonate is used as the second polymerization initiator, the degree of dispersion is small and the variation in particle size of the resulting expandable particles is small. Also, when t-butylperoxy-2-ethylhexanoate is used as the first polymerization initiator and a polymerization inhibitor is added, the degree of dispersion is small and the variation in particle size of the resulting expandable particles is small.

On the other hand, as can be seen from Tables 4 to 6, Comparative Examples 1 to 4, 7, and 9, which used benzoyl peroxide (NOF Corp.'s "Niper BW", a water-dilutable powder product) as the first polymerization initiator, had low styrene contents and low amounts of styrene-based monomers emitted, but had high benzene contents and high amounts of benzene emitted.

Comparative Examples 5 and 6 are examples in which 2,2-bis(4'-(2",3"-dibromo-2"-methylpropoxy-3',5'-dibromophenyl)propane was used as the flame retardant. In this case, the polymerization of the styrene monomer in the polymerization process was easily inhibited, and the styrene content in the expandable particles increased. The styrene content in the foamed molded product also increased. In addition, since the weight average molecular weight of the styrene resin is low at 140,000, the mechanical strength of the foamed molded product, such as bending strength, is low. In Comparative Examples 5 and 6, no high benzene content was detected.

Comparative Examples 7 and 8 are examples in which a flame retardant was added to the aqueous phase and suspension polymerization was carried out. In these cases, the burning rate was high and the flame retardancy was insufficient. This is thought to be because the flame retardant was difficult to be sufficiently incorporated into the interior of the expandable particles. In addition, the moldability was not evaluated sufficiently. Comparative Example 10 is an example in which an organic peroxide with a high 10-hour half-life temperature was used as the first polymerization initiator. As a result, polymerization inhibition was more likely to occur and the styrene content in the expandable particles was high.

Claims (9)

A method for producing expandable styrene-based resin particles containing a flame retardant and a blowing agent, comprising:
a polymerization step of adding a styrene-based monomer containing styrene, in which the flame retardant and a polymerization initiator are dissolved, to an aqueous medium, and carrying out suspension polymerization of the styrene-based monomer in the aqueous medium to obtain styrene-based resin particles containing the flame retardant,
In the polymerization step, a first organic peroxide of an aliphatic peroxyester type and/or an aliphatic peroxyketal type having a 10-hour half-life temperature of 65° C. or more and 90° C. or less and a second organic peroxide having a 10-hour half-life temperature of more than 90° C. and 110° C. or less are used as a polymerization initiator,
The flame retardant is a brominated styrene-butadiene copolymer,
The blending amount of the flame retardant is 0.05 to 2 parts by mass per 100 parts by weight of the styrene-based monomer,
The method for producing expandable styrene-based resin particles, wherein the expandable styrene-based resin particles have a styrene content of 25 ppm by mass or less (including 0) and a benzene content of 1.5 ppm by mass or less (including 0). The method for producing expandable styrene-based resin particles according to claim 1, wherein the phenylacetylene concentration in the styrene-based monomer is 5 ppm by mass or more and 20 ppm by mass or less. The method for producing expandable styrene-based resin particles according to claim 1 or 2, wherein the second organic peroxide is an aliphatic peroxydicarbonate-type and/or aliphatic peroxyester-type organic peroxide having a 10-hour half-life temperature 10°C or higher than that of the first organic peroxide. The method for producing expandable styrene-based resin particles according to any one of claims 1 to 3, wherein the polymerization process includes a first-stage polymerization process in which the styrene-based monomer is polymerized at a temperature of 110°C or less until the polymerization conversion rate reaches 90% by mass or more, and a second-stage polymerization process in which the styrene-based monomer is polymerized at a temperature of more than 115°C and not more than 135°C until the content of unreacted styrene reaches 25 ppm by mass or less (including 0). The method for producing expandable styrene-based resin particles according to any one of claims 1 to 4, wherein the first organic peroxide is t-butylperoxy-2-ethylhexanoate, and the second organic peroxide is t-butylperoxy-2-ethylhexyl monocarbonate. The method for producing expandable styrene-based resin particles according to any one of claims 1 to 5, further comprising a blowing agent impregnation step of adding the blowing agent when the polymerization conversion rate of the styrene-based monomer is 90% by mass or more, and impregnating the styrene-based resin particles with the blowing agent. The method for producing expandable styrene-based resin particles according to claim 6, wherein butane and pentane are added as the blowing agent in the blowing agent impregnation step. The method for producing expandable styrene-based resin particles according to any one of claims 1 to 7, wherein in the polymerization step, suspension polymerization is carried out under conditions in which the polymerization rate until the polymerization conversion rate of the styrene-based monomer reaches 90% is 14%/hr or more and 18%/hr or less. The method for producing expandable styrene-based resin particles according to any one of claims 1 to 8, wherein the styrene-based monomer is subjected to suspension polymerization in the presence of a polymerization inhibitor.