Preparation of polystyrene using 1,3 1,4 bis(tert-alkylperoxydiisopropyl)benzenes

Abstract

The present invention relates to a method for the preparation of polystyrene, in particular expandable polystyrene, by suspension polymerisation, and to a polystyrene obtained by such a method. The invention also relates to the use of an organic peroxide of formula (I) as a styrene polymerisation initiator and as a flame retardant synergist, as well as to a composition comprising at least one organic peroxide of formula (I) and a halogenated flame retardant.

Description

[0001] Description

[0002] Title: Preparation of polystyrene using 1,3-1,4-bis(tert-alkylperoxydiisopropyl)benzenes

[0003] Field of invention

[0004] The present invention relates to a process for preparing polystyrene, in particular expandable polystyrene, by suspension polymerization, as well as to a composition comprising a polystyrene obtained by such a process. The invention also relates to a composition comprising at least one organic peroxide of formula (I) and a halogenated flame retardant.

[0005] Technical background

[0006] One of the known methods for producing polystyrene-type polymers, and in particular expandable polystyrene-type polymers (also referred to as EPS in the following) is aqueous suspension polymerization. This is generally a batch-type process in which two or more monomer-soluble polymerization initiators are employed, with a temperature profile increasing in steps, in a gradient, or by a combination of both. The initiators in the process are chosen according to their half-life temperature, to feed the polymerization with radicals in a controlled manner at selected points in the temperature profile, so that an efficient conversion is achieved in an acceptable time.

[0007] Traditionally, suspension polymerization to prepare polystyrene is carried out in a process using two different temperature levels and two initiators, a first-level or "conversion" initiator and a second-level or "finishing" initiator. These two initiators have different half-life temperatures, each adapted to the corresponding temperature level. In such a process, dibenzoyl peroxide (BPO) is often employed as the conversion initiator at a reaction temperature between about 82 °C and 95 °C. Other conversion initiators that can be employed in this temperature range include tert-butyl peroxy-2-ethylhexanoate, tert-amyl peroxy-2-ethylhexanoate, and 2,5-dimethyl-2,5-di(2-ethylhexanoylperoxy)hexane.Liquid initiators such as tert-butyl peroxybenzoate (TBP), tert-butyl (2-ethylhexyl)monoperoxycarbonate (TBEC) and tert-amyl (2-ethylhexyl)monoperoxycarbonate (TAEC), or solid dicumyl peroxide (DCP), are often used as finishing initiators, at a temperature between 115 °C and 135 °C. This finishing step serves to minimize the amount of residual monomer in the polystyrene.

[0008] Expanded polystyrene is widely used as a thermal insulation material in the construction industry. Expanded polystyrene panels typically contain one or more flame retardants to limit the risk of fire. These flame retardants are generally small halogenated molecules or, more often now, halogenated oligomers, which reduce the risk of flame retardant migration from the expanded polystyrene. In order to play their role as flame retardants, these halogenated molecules require the presence of a high-temperature radical-generating compound (also called a "synergist") which, when a fire occurs, generates halogen radicals through its decomposition, which can slow the spread of the fire.

[0009] During the synthesis of polystyrene, the temperature rise to carry out the finishing step requires the complete closure of the polymerization reactor, the synergists must be introduced into the reaction medium before or at the start of polymerization or at the latest before the temperature rise for finishing and / or closing of the reactor.

[0010] This role of synergist is generally fulfilled by an organic peroxide. This organic peroxide must therefore decompose as little as possible under the conditions of styrene polymerization (which is conventionally carried out at temperatures ranging from approximately 90 to 130°C, for a duration that can exceed ten hours in the case of a batch suspension polymerization process), in order to be able to perform its role in the final material. Generally, the peroxide synergist is chosen to be solid to limit its migration out of the cells of the expanded polystyrene during its period of use. For example, the use of dicumyl peroxide as a flame retardant synergist is widespread.

[0011] Until now, in the processes for preparing suspension polystyrene, either different organic peroxides have been used to fulfill each of the functions of finishing initiator and flame retardant synergist, or dicumyl peroxide has been used, which is capable of fulfilling the function of flame retardant synergist in the final material while contributing to the conversion of styrene in the finishing step of the styrene polymerization process. However, dicumyl peroxide has recently been recognized as reprotoxic (classified H360D according to European Regulation (EC) No. 1272 / 2008, i.e. as being capable of harming the unborn child).

[0012] Another family of halogenated flame retardant synergists used is bis-cumyl (or 2,3-diphenyl-2,3-dimethylbutane such as Curox® CC-D marketed by United Initiators) and its derivatives, poly(1,4-diisopropylbenzene) such as Curox® CC-P3 marketed by United Initiators. These non-peroxide compounds decompose at very high temperatures and can withstand not only the temperature conditions of polystyrene polymerization but also the extrusion of the material at temperatures above 150°C. However, since their thermal triggering can only occur at temperatures above approximately 300°C, these hydrocarbons, which have sites with high steric hindrance allowing the generation of radicals by breaking carbon-carbon bonds at very high temperatures (> 300°C), cannot participate in the finishing stage of a suspension styrene polymerization process.

[0013] There is therefore a real need to provide a process for preparing polystyrene using an organic peroxide which is non-reprotoxic and which can act both as a polymerization initiator, in particular a finishing initiator, in the preparation process and fulfill the function of a flame retardant synergist in the final product.

[0014] Summary of the invention

[0015] The invention relates firstly to a process for preparing polystyrene, comprising the following steps: a) adding, into an aqueous suspension comprising styrene monomer, from 0.2% to 0.8% by weight relative to the total weight of styrene monomer of at least one organic peroxide of formula (I): [Chem 1] in which:

[0016] - Ri is a linear or branched alkyl group comprising from 1 to 6 carbon atoms, and

[0017] - R2 is a linear or branched alkyl group comprising from 1 to 6 carbon atoms; and b) heating the suspension.

[0018] Preferably, the polystyrene is an expandable polystyrene, the method further comprising adding a blowing agent, preferably to the suspension.

[0019] Preferably, the method according to the invention comprises adding to the suspension at least one other organic peroxide A, preferably having a half-life temperature of one hour lower than that of the organic peroxide of formula (I), more preferably chosen from the group consisting of dibenzoyl peroxide, tert-butyl peroxy-2-ethylhexanoate, tert-amyl peroxy-2-ethylhexanoate, 2,5-dimethyl-2,5-di(2-ethylhexanoylperoxy)hexane, 1-alkoxy-1-tert-alkylperoxycyclohexanes, in which the alkoxy group comprises between 1 and 4 carbon atoms, the tert-alkyl group comprises between 4 and 12 carbon atoms, and the cyclohexane ring may optionally be substituted by 1 to 3 alkyl groups, each independently comprising between 1 and 3 carbon atoms, in particular 1 -methoxy-1-tert-amylperoxycyclohexane, and mixtures thereof.

[0020] Preferably, the heating step b) comprises at least a first step b1) of heating the suspension to a temperature of 80 to 115°C, preferably 85 to 110°C, and a second step b2) of heating the suspension to a temperature of 120 to 140°C, preferably 120 to 135°C.

[0021] Preferably, the organic peroxide of formula (I) is added before the step of heating the suspension.

[0022] Preferably, R1 and R2 are identical and represent a linear or branched alkyl group comprising from 1 to 6 carbon atoms. More preferably, R1 represents a -CH3 or -CH2-CH3 group and R2 represents a -CH3 group.

[0023] Particularly preferably, the organic peroxide of formula (I) comprises a peroxide selected from the group consisting of a,a'-bis(tert-butylperoxy)-diisopropylbenzene (also called bis-a,a'-(tert-butylperoxy)-isopropylbenzene), a,a'-bis(tert-amylperoxy)-diisopropylbenzene, a-(tert-butylperoxy)-a'-(tert-amylperoxy)-diisopropylbenzene and mixtures thereof, and preferably comprises, more preferably consists of, a,a'-bis(tert-butylperoxy)-diisopropyl benzene.

[0024] Preferably, the method according to the invention comprises a step of adding to the suspension a flame retardant, preferably a halogenated flame retardant, more preferably a brominated flame retardant.

[0025] The invention also relates to a polystyrene obtained by a process according to the invention, said polystyrene comprising a flame retardant, preferably a halogenated flame retardant, more preferably a brominated flame retardant.

[0026] The invention also relates to the use of at least one organic peroxide of formula (I):

[0027] [Chem 1] in which:

[0028] - Ri is a linear or branched alkyl group comprising from 1 to 6 carbon atoms, and

[0029] - R2 is a linear or branched alkyl group comprising from 1 to 6 carbon atoms, as an initiator for the polymerization of styrene.

[0030] The invention also relates to a composition comprising at least one organic peroxide of formula (I): [Chem 1] in which:

[0031] - Ri is a linear or branched alkyl group comprising from 1 to 6 carbon atoms, and

[0032] - R2 is a linear or branched alkyl group comprising from 1 to 6 carbon atoms, and at least one halogenated flame retardant, preferably brominated, said composition comprising less than 0.1%, preferably less than 0.01%, still preferably does not comprise organic peroxide chosen from the group consisting of tert-butyl perbenzoate, tert-butyl (2-ethylhexyl)monoperoxcarbonate (or TBEC), tert-amyl (2-ethylhexyl)monoperoxcarbonate (or TAEC) and dicumyl peroxide and mixtures thereof.

[0033] Preferably, the organic peroxide of formula (I) is as defined below.

[0034] Preferably, the halogenated flame retardant is selected from the group consisting of brominated polymers, hexabromocyclododecane, tetrabromobisphenol A, decabromodiphenyl ether, pentabromodiphenyl ether, octabromodiphenyl ether, tris-(dibromopropyl)phosphate, carbon tetrabromide, beta-dibromopropionate, tetrabromoethylene, 1-2-dibromo-1,1,2,2-tetrachloroethane, 1,1,2,2-tetrabromoethane, dibromodichloroethane, 1,2-dibromo-1,1-dichloroethane, 1,2-dibromo-1,2,2-trichloroethane, 1,2,3,4-tetrabromobutane, 1,2,3-tribromopropane, pentabromoethane, tribromotrichlorocyclohexane, 1,2,4-tribromobutane, tetrabromopentane, hexabromoethane, tetrabromodichlorocyclohexane, pentabromomonochlorocyclohexane, 1,2-di-(dibromomethyl)benzene, alpha, beta-dibromoethylbenzene, alpha, beta-dibromopropionate, brominated monomers, such as 1,1'-(isopropylidene)bis[3,5-dibromo-4-(2,3-dibromo-2-methylpropoxy)benzene]), and mixtures thereof, more preferably is a brominated polymer.,

[0035] The present invention makes it possible to meet the need expressed above. More particularly, it provides a process for preparing suspended polystyrene in which an organic peroxide is used which can, during said preparation of the polystyrene, generate radicals to enable the conversion of the styrene, in particular during the finishing stage of the process, and at the same time confer on the final product obtained, in combination with a flame retardant, effective flame retardancy. In addition, the polystyrene prepared according to the process of the invention has good mechanical properties, including after expansion, and a sufficient weight-average molecular weight (Mw).

[0036] This is accomplished by the use in the suspension of a particular organic peroxide, namely an organic peroxide of formula (I) as defined above, particularly as a finishing initiator.

[0037] Detailed description

[0038] The invention is now described in more detail and in a non-limiting manner in the following description.

[0039] Unless otherwise indicated, all percentages relating to quantities are percentages by mass, relative to the mass of monomer(s) in the composition.

[0040] In this text, the quantities indicated for a given species may apply to this species according to all its definitions (as mentioned in this text), including the more restricted definitions.

[0041] For the purposes of the present invention, “a temperature of X°C to Y°C” means a temperature in the range of X°C to Y°C and / or a temperature gradient in the range of X°C to Y°C.

[0042] According to a first aspect, the invention relates to a process for preparing polystyrene, comprising the following steps: a) adding, into an aqueous suspension comprising styrene monomer, from 0.2% to 0.8% by weight relative to the total weight of styrene monomer, at least one organic peroxide of formula (I): [Chem 1] in which:

[0043] - Ri is a linear or branched alkyl group comprising from 1 to 6 carbon atoms, and

[0044] - R2 is a linear or branched alkyl group comprising from 1 to 6 carbon atoms; and b) heating the aqueous suspension.

[0045] Preferably, the invention relates to a process for preparing polystyrene, comprising the following steps: a) adding, into an aqueous suspension comprising styrene monomer, from 0.2% to 0.8% by weight relative to the total weight of styrene monomer, at least one organic peroxide of formula (I):

[0046] [Chem 1] in which:

[0047] - Ri is a linear or branched alkyl group comprising from 1 to 6 carbon atoms, and

[0048] - R2 is a linear or branched alkyl group comprising from 1 to 6 carbon atoms; and b) heating the aqueous suspension, said heating step b) comprising a step b1) of heating to a temperature of 80°C to 115°C, preferably from 85 to 110°C, for a period of 30 minutes to 5 hours, preferably from 30 minutes to 4 hours, more preferably from 1 hour to 3 hours. The process according to the invention may comprise a step a'), of preparing such an aqueous suspension. The suspension may be prepared in any suitable device, for example a reactor. This step a') comprises the introduction of water into the reactor and then adding the styrene monomer to the water.

[0049] Step a') is advantageously prepared at a temperature ranging from 10 to 60°C, preferably from 30 to 50°C, for example at approximately 40°C.

[0050] The aqueous suspension preferably comprises the styrene monomer in an amount of 30% to 70%, more preferably 40% to 60%, even more preferably 45% to 55% by weight, relative to the total weight of the suspension.

[0051] The aqueous suspension may optionally comprise monomers other than styrene, preferably these monomers are monomers bearing ethylenic unsaturations and are copolymerizable with styrene.

[0052] These other monomers may, for example, comprise, or be, alkylstyrenes, such as alpha-methylstyrene, acrylic acid esters and / or acrylonitrile. Most preferably, the amount of these other monomers is at most 15% by weight relative to the total weight of styrene monomer.

[0053] Preferably, step a') is prior to step a)). Less preferably, step a') and step a) are carried out simultaneously, that is to say that the organic peroxide of formula (I) can be added simultaneously with the addition of the styrene monomer to the reaction medium.

[0054] The organic peroxide of formula (I) preferably has a one-hour half-life temperature of 120 to 150°C, preferably 125°C to 140°C. Preferably, the one-hour half-life temperature is measured for the peroxide diluted to 0.1 M in 1,2,4-trichlorobenzene. The one-hour half-life temperature of an organic peroxide represents the temperature at which half of the amount of the organic peroxide has decomposed in one hour.

[0055] The organic peroxide of formula (I) may be the meta isomer (i.e., the substituents of the benzene ring are in the meta position to each other) or may be the para isomer (i.e., the substituents of the benzene ring are in the para position to each other), or a combination of both isomers.

[0056] R 1 and R 2 may be the same or different. Preferably, they are the same and each comprise at least 1 carbon atom. Preferably, R 1 is an alkyl group comprising from 1 to 6 carbon atoms, more preferably from 1 to 5 carbon atoms, more preferably from 1 to 4 carbon atoms, more preferably from 1 to 3 carbon atoms, even more preferably 1 carbon atom.

[0057] In other preferred embodiments, R 1 is an alkyl group comprising from 1 to 5 carbon atoms, more preferably from 1 to 4 carbon atoms, more preferably from 1 to 3 carbon atoms, even more preferably 1 or 2 carbon atoms.

[0058] Preferably, R2 is an alkyl group comprising from 1 to 5 carbon atoms, more preferably from 1 to 4 carbon atoms, more preferably from 1 to 3 carbon atoms, even more preferably 1 carbon atom.

[0059] Advantageously, R1 and R2 are independently of each other an alkyl group comprising from 1 to 6 carbon atoms, more preferably from 1 to 5 carbon atoms, more preferably from 1 to 4 carbon atoms, more preferably from 1 to 3 carbon atoms, even more preferably 1 to 2 carbon atoms.

[0060] Preferably, R 1 is a linear alkyl group. Preferably, R 2 is a linear alkyl group. More preferably, R 1 and R 2 are both a linear alkyl group.

[0061] Particularly advantageously, R1 represents a -CH3 or -CH2-CH3 group and R2 represents a -CH3 group.

[0062] The organic peroxide of formula (I) advantageously comprises a peroxide selected from the group consisting of α,α'-bis(tert-butylperoxy)-diisopropylbenzene, α,α'-bis(tert-amylperoxy)-diisopropylbenzene, α-(tert-butylperoxy)-α'-(tert-amylperoxy)-diisopropylbenzene, and mixtures thereof.

[0063] More preferably, the organic peroxide of formula (I) is selected from the group consisting of α,α'-bis(tert-butylperoxy)-diisopropylbenzene, α,α'-bis(tert-amylperoxy)-diisopropylbenzene, α-(tert-butylperoxy)-α'-(tert-amylperoxy)-diisopropylbenzene, and mixtures thereof.

[0064] Particularly preferably, the organic peroxide of formula (I) comprises, or consists of, α,α'-bis(tert-butylperoxy)-diisopropylbenzene.

[0065] The at least one organic peroxide of formula (I) may consist of a single organic peroxide of formula (I), or a mixture of several (two or more) organic peroxides of formula (I), for example a mixture of at least two, or at least three, or at least four, or at least five, or at least six, organic peroxides of formula (I). The mixture of organic peroxides of formula (I) may comprise a mixture of meta and para isomers and / or a mixture of peroxides of formula (I) having different substituents R 1 and / or R 2 . The mixture of organic peroxides of formula (I) may be a mixture of any of the organic peroxides described above. Preferably, the at least one organic peroxide of formula (I) consists of a single organic peroxide of formula (I),

[0066] The organic peroxide of formula (I) can be prepared by reacting a dialcohol of formula HO-C(CH3)2-Ar-C(CHs)2-OH, Ar representing a benzene ring, with one or more hydroperoxides of formula HOO-C(CHs)2-R1 and / or HOO-C(CH3)2-R2.

[0067] Preferably, heating step b) comprises, or is, a heating step at a temperature of 80 to 140°C, preferably 90 to 135°C.

[0068] More preferably, the heating step b) comprises a heating step b1) at a temperature of 80 to 115°C, preferably 85 to 110°C.

[0069] More preferably, heating step b) comprises a heating step b2) at a temperature of 120 to 140°C, preferably 120 to 135°C.

[0070] Preferably, the heating step comprises a gradual rise in temperature between steps b1) and b2). Preferably, said gradual rise in temperature between steps b1) and b2) is carried out between 30 minutes and 2 hours, preferably between 40 minutes and 1 hour 30 minutes.

[0071] Even more preferably, the heating step b) comprises: b1) a heating step at a temperature of 80 to 115°C, preferably 85 to 110°C; and b2) a heating step at a temperature of 120 to 140°C, preferably 120 to 135°C.

[0072] Even more preferably, the heating step b) comprises, even more preferably consists of, the following successive steps: b1') a gradual rise in temperature up to step b1); b1) a heating step to a temperature of 80 to 115°C, preferably 85 to 110°C; b2') a gradual rise in temperature between steps b1) and b2); and b2) a heating step to a temperature of 120 to 140°C, preferably 120 to 135°C. Preferably, the heating step b1) is carried out before the heating step b2), the heating step b1) also being referred to in the present text as the "conversion step" and the heating step b2) also being referred to in the present text as the "finishing step".

[0073] Preferably, step b1) is carried out over a period of 30 minutes to 6 hours, preferably 30 minutes to 5 hours, preferably 30 minutes to 4 hours, more preferably 1 hour to 3 hours.

[0074] Preferably, step b2) is carried out over a period of 1 to 5 hours, preferably 1 to 4 hours.

[0075] Preferably, said heating step b) comprises a step b1) of heating to a temperature of 80°C to 115°C, preferably 85 to 110°C, for a period of 30 minutes to 5 hours, preferably 30 minutes to 4 hours, more preferably 1 hour to 3 hours and a step b2) of heating to a temperature of 120 to 140°C, preferably 120 to 135°C, said step b2) preferably being carried out for a period of 1 to 5 hours, preferably 1 to 4 hours.

[0076] Preferably, step b) does not include heating to a temperature of 80°C to 115°C, preferably 85 to 110°C, other than in step b1).

[0077] Preferably, step b) does not include heating to a temperature of 120 to 140°C, preferably 120 to 135°C, other than in step b2).

[0078] In embodiments, the organic peroxide of formula (I) is added before the heating step b). In these embodiments, the addition of the organic peroxide of formula (I) to the suspension may be carried out at a temperature identical to that applied during the preparation of the suspension. The addition of the organic peroxide of formula (I) to the suspension may be carried out at a temperature of 10 to 60°C, preferably 30 to 50°C, for example at about 40°C. The organic peroxide of formula (I) may be added in one or more times and / or continuously.

[0079] In other embodiments, the organic peroxide of formula (I) may be added continuously, the addition of organic peroxide of formula (I) taking place more particularly at least partly simultaneously with the heating step b). The organic peroxide of formula (I) may be added continuously over a period of 1 to 6 hours, preferably 1 to 2 hours.

[0080] In other embodiments, the organic peroxide of formula (I) may be added to the suspension intermittently (or incrementally), i.e., the addition of the organic peroxide is interrupted and then resumed, one or more times, the addition of organic peroxide of formula (I) taking place more particularly at least partly simultaneously with the heating step b). When the organic peroxide of formula (I) is added intermittently, each addition of peroxide may be independently carried out at once, or continuously, over a determined period of time.

[0081] Preferably, the total amount of organic peroxide of formula (I) added to the reaction medium ranges from 0.3% to 0.6%, more preferably from 0.35% to 0.5% by weight relative to the total weight of styrene monomer.

[0082] Very advantageously, the process according to the invention comprises the addition to the reaction medium (preferably the aqueous suspension of styrene monomer) of at least one other organic peroxide A, i.e. an organic peroxide other than an organic peroxide of formula (I). The organic peroxide A preferably has a half-life temperature of one hour lower than that of the organic peroxide of formula (I) added in the process. The organic peroxide A preferably has a half-life temperature of one hour lower than 110°C, more preferably a half-life temperature of between 80 and 105°C, even more preferably of between 85 and 100°C.

[0083] The organic peroxide A is preferably a “conversion” initiator, the organic peroxide of formula (I) preferably being a “finishing” initiator. Thus, when the process comprises the introduction of an organic peroxide A as described above, the heating step very advantageously comprises a heating step b1) and a heating step b2) as described above.

[0084] Preferably, the organic peroxide A is selected from the group consisting of dibenzoyl peroxide (BPO); tert-butyl peroxy-2-ethyl hexanoate; tert-amyl peroxy-2-ethyl hexanoate; 2,5-dimethyl-2,5-di(2-ethylhexanoylperoxy)hexane; 1-alkoxy-1-tert-alkylperoxycyclohexanes, wherein the alkoxy group comprises between 1 and 4 carbon atoms, the tert-alkyl group comprises between 4 and 12 carbon atoms, and the cyclohexane ring may optionally be substituted with 1 to 3 alkyl groups, each independently comprising between 1 and 3 carbon atoms, in particular 1-methoxy-1-tert-amyl peroxycyclohexane (TAPMC); 1,1-di(tert-amyl peroxy)-cyclohexane; and mixtures thereof.BPO, tert-butyl peroxy-2-ethylhexanoate, tert-amyl peroxy-2-ethylhexanoate, 2,5-dimethyl-2,5-di(2-ethylhexanoylperoxy)hexane are for example marketed by the company ARKEMA under the trade names “Luperox® A75”, “Luperox® 26”, “Luperox® 575” and “Luperox® 256”, respectively.

[0085] The organic peroxide A is preferably added to the reaction medium after the preparation of the aqueous suspension of styrene monomer (the reaction medium is therefore the aqueous suspension of styrene monomer). It may be added to the reaction medium (preferably the aqueous suspension of styrene monomer) before, after and / or simultaneously, or partly simultaneously, with the addition of the organic peroxide of formula (I). It may be added before and / or simultaneously, or partly simultaneously, with heating step b), preferably heating step b1). The organic peroxide A may be added all at once. Alternatively, the organic peroxide A may be added continuously, preferably over a period of 1 to 6 hours, preferably 1 to 5 hours and / or intermittently (or in increments, each increment being able to be added all at once or continuously).

[0086] Preferably, the amount of organic peroxide A added to the reaction medium is from 0.25% to 0.55%, preferably from 0.28% to 0.45% by weight, relative to the total weight of the styrene monomer.

[0087] The method according to the invention may optionally comprise a step of adding at least one third organic peroxide C, having a half-life temperature at one hour of 110 to 135°C, preferably of 115 to 130°C. The organic peroxide C may act as a “finishing” initiator, in addition to the organic peroxide of formula (I). This organic peroxide C may be chosen from the group consisting of tert-butyl perbenzoate; tert-alkyl alkyl monoperoxycarbonates, in which the alkyl group comprises between 3 and 12 carbon atoms, preferably 8 carbon atoms, and the tert-alkyl group comprises between 4 and 12 carbon atoms, preferably between 4 and 5 carbon atoms, more particularly tert-butyl (2-ethylhexyl) monoperoxycarbonate (or TBEC) and / or tert-amyl (2-ethylhexyl) monoperoxycarbonate (or TAEC) and mixtures thereof.Tert-butyl perbenzoate, TBEC and TAEC are for example marketed by the company ARKEMA under the trade names “Luperox® P”, “Luperox® TBEC” and “Luperox® TAEC”, respectively. However, preferably, only the organic peroxide(s) of formula (I) are used as “finishing” initiator. Preferably, the process according to the invention involves the use of one or more organic peroxides of formula (I) as “finishing” initiator and one or more organic peroxides A with a half-life temperature of one hour between 80 and 105°C as “conversion” initiator, to the exclusion of any other organic peroxide.

[0088] Preferably, the suspension of the process according to the invention comprises less than 0.1%, preferably less than 0.01%, still preferably does not comprise organic peroxide chosen from the group consisting of tert-butyl perbenzoate, tert-butyl (2-ethylhexyl)monoperoxcarbonate (or TBEC), tert-amyl (2-ethylhexyl)monoperoxcarbonate (or TAEC) and dicumyl peroxide and mixtures thereof.

[0089] Preferably, the suspension of the process according to the invention comprises less than 0.1%, preferably less than 0.01%, still preferably does not comprise organic peroxide chosen from the group consisting of:

[0090] - tert-butyl perbenzoate;

[0091] - dicumyl peroxide; and

[0092] - tert-alkyl alkyl monoperoxcarbonates, in which the alkyl group comprises between 3 and 12 carbon atoms, preferably 8 carbon atoms, and the tert-alkyl group comprises between 4 and 12 carbon atoms, preferably between 4 and 5 carbon atoms.

[0093] Preferably, the suspension of the process according to the invention comprises less than 0.1%, preferably less than 0.01%, still preferably does not comprise organic peroxide having a half-life temperature at one hour of 110 to 140°C, preferably of 115 to 130°C, other than the organic peroxide of formula (I).

[0094] Preferably, the suspension of the process according to the invention comprises less than 0.1%, preferably less than 0.01%, and still preferably does not comprise “finishing” organic peroxide, in addition to the organic peroxide of formula (I).

[0095] The peroxide(s) used in the process according to the invention may be added to the reaction medium (in particular the aqueous suspension of styrene monomer) in pure form or in the form of a dispersion (for example a suspension or an emulsion in styrene) or in the form of a solution, diluted in one or more solvents.

[0096] Very advantageously, the method according to the invention further comprises a step of adding a blowing agent. The polystyrene prepared is then an expandable polystyrene. Preferably, the blowing agent comprises, or is, one or more alkanes comprising from 4 to 6 carbon atoms. In particular, the blowing agent may comprise, or be, a blowing agent selected from the group consisting of butane, 2-methylbutane, n-pentane, cyclohexane and mixtures thereof. A mixture of n-pentane and 2-methylbutane is preferred.

[0097] The blowing agent is preferably added in an amount of 5 to 10% by weight relative to the weight of the styrene monomer.

[0098] Preferably, the blowing agent is added to the reaction medium (preferably the aqueous suspension of styrene monomer). It may be added before, after and / or simultaneously with the addition of one (or more) of the organic peroxides of the process, and / or before, after and / or simultaneously with the heating step b) (in particular the heating steps b1) and / or b2).

[0099] In other embodiments, the blowing agent may be added after polymerization of the styrene, into the polystyrene thus prepared, preferably into the polystyrene beads obtained at the end of the polymerization.

[0100] Preferably, the process according to the invention comprises a step of adding, to the reaction medium (preferably the aqueous suspension of styrene monomer), a flame retardant (or flame retardant).

[0101] The flame retardant is preferably added in an amount of 0.5 to 1% by weight relative to the weight of the styrene monomer.

[0102] The flame retardant used in the invention preferably comprises, or is, a halogenated flame retardant, more preferably a brominated and / or chlorinated flame retardant, more preferably a brominated flame retardant.

[0103] In particular, the flame retardant agent may comprise, or be, a flame retardant agent selected from the group consisting of brominated polymers, hexabromocyclododecane (HBCD), tetrabromobisphenol A (TBBPA), decabromodiphenyl ether (Deca-BDE), pentabromodiphenyl ether (Penta-BDE), octabromodiphenyl ether (Octa-BDE), tris-(dibromopropyl)phosphate, carbon tetrabromide, beta-dibromopropionate, tetrabromoethylene, 1-2-dibromo-1,1,2,2-tetrachloroethane, 1,1,2,2-tetrabromoethane, dibromodichloroethane, 1,2-dibromo-1,1-dichloroethane, 1,2-dibromo-1,2,2-trichloroethane, 1 ,2,3,4-tetrabromobutane, 1,2,3-tribromopropane, pentabromoethane, tribromotrichlorocyclohexane, 1,2,4-tribromobutane, tetrabromopentane, hexabromoethane, tetrabromodichlorocyclohexane, pentabromomonochlorocyclohexane, 1,2-di-(dibromomethyl)benzene, alpha, beta-dibromoethylbenzene, alpha, beta-dibromopropionate,brominated monomers, such as 1,1'-(isopropylidene)bis[3,5-dibromo-4-(2,3-dibromo-2-methylpropoxy)benzene]), and mixtures thereof. More preferably, the flame retardant agent is chosen from brominated polymers, in particular brominated polystyrenes and brominated styrene-butadiene block copolymers such as Emerald Innovation ® 3000 marketed by Lanxess (CAS 1195978-93-8).,

[0104] The flame retardant agent may be added before, after and / or simultaneously with the addition of one (or more) of the organic peroxides of the process. It is preferably added before heating step b) (in particular heating steps b1) and / or b2), and / or simultaneously with it.

[0105] The process according to the invention may further comprise the addition, into the reaction medium (preferably the aqueous suspension of styrene monomer), of one or more additives chosen from the group consisting of inorganic suspension stabilizers, such as magnesium pyrophosphate or calcium phosphate, preferably tricalcium phosphate; organic suspension stabilizers, such as polyvinylpyrrolidone, polyvinyl alcohol and hydroxyethylcellulose; surfactants; chain transfer agents; nucleating agents; expansion aids; lubricants; anti-static additives and plasticizers. These additives may be added into the suspension at any time during the process, preferably before the heating step b) (in particular the heating steps b1) and / or b2), and / or simultaneously therewith.

[0106] In a particular embodiment, the method according to the invention may further comprise the addition, to the reaction medium (preferably the aqueous suspension of styrene monomer), of one or more athermal additives such as carbon black, graphite or metal oxides.

[0107] Preferably, the reactor is the same as that in which the aqueous suspension of styrene monomer is prepared (in other words, the suspension is prepared directly in the polymerization reactor).

[0108] The polystyrene prepared according to the process of the invention advantageously has a weight-average molecular weight (Mw) of at least 120,000 g / mol, more preferably at least 130,000 g / mol, more preferably at least 150,000 g / mol, even more preferably at least 160,000 g / mol. The weight-average molecular weight can be measured by the size exclusion chromatography (SEC) method with polystyrene standards.

[0109] According to another aspect, the invention relates to the use of at least one organic peroxide of formula (I) for the preparation of polystyrene, preferably expandable polystyrene. More particularly, the invention relates to the use of at least one organic peroxide of formula (I) as a styrene polymerization initiator, preferably as a styrene polymerization initiator and as a flame retardant synergist. By the expression "as a styrene polymerization initiator and as a flame retardant synergist" is meant that the organic peroxide of formula (I) is capable of acting both as a styrene polymerization initiator and as a flame retardant synergist within the same composition. What has been described above in relation to the preparation process (in particular with regard to the organic peroxide of formula (I) and the expanded polystyrene prepared) can be applied in a similar manner to said use.

[0110] According to yet another aspect, the invention relates to a polystyrene, preferably expandable, obtained or capable of being obtained by the process described above.

[0111] According to yet another aspect, the invention relates to a composition comprising a polystyrene, preferably expandable, obtained or capable of being obtained by the process described above and a flame retardant. The flame retardant may be as described above.

[0112] The polystyrene according to the invention or the composition according to the invention can be used in insulation applications. For example, the polystyrene (preferably expandable) according to the invention, or the composition according to the invention, can be used in the manufacture of insulation parts, for example in the transport industry and construction.

[0113] Another aspect of the invention relates to insulation parts comprising a polystyrene (preferably expandable) according to the invention, or a composition according to the invention.

[0114] The invention also relates to a composition comprising at least one organic peroxide of formula (I) and at least one halogenated, preferably brominated, flame retardant. What has been described above concerning the organic peroxide of formula (I), the suspension and the halogenated flame retardant can be applied in a similar manner to this aspect of the invention. Examples

[0115] The following examples illustrate the invention without limiting it.

[0116] Example 1: Polymerization in aqueous suspension with second stage at 130°C

[0117] In a 2 liter reactor, 680 g of demineralized water are introduced at a temperature of 40°C, then 320 g of styrene monomer, 0.45% by weight of dibenzoyl peroxide (Luperox® A75 marketed by the company ARKEMA) relative to the weight of styrene monomer, a second peroxide in the quantities indicated in the table below, 100 ppm of sodium dodecylsulfonate relative to the weight of styrene monomer and 0.5% by weight of tricalcium phosphate relative to the weight of styrene monomer.

[0118] The organic peroxides B used as “finishing” initiators in the examples are dicumyl peroxide (DCP) (Luperox® DCP marketed by ARKEMA) and α,α'-bis(tert-butylperoxy)-diisopropylbenzene (Luperox® F marketed by ARKEMA).

[0119] [Table 1]

[0120] Polymerization conditions (1): Aqueous suspensions 1 and 2 are heated to 90°C for 1 h (conversion step), the temperature is then gradually increased from 90 to 130°C for 1 h 15 min, under an inert nitrogen atmosphere, then the temperature is maintained at 130°C for 3 h 30 min (finishing step). The reaction medium is then cooled to a temperature of 50°C. The polystyrene beads obtained are recovered by filtration and then dried.

[0121] Polymerization conditions (2): The aqueous suspension 2 is heated to 90°C for 6 hours (conversion step), the temperature is then gradually increased from 90°C to 130°C, under an inert nitrogen atmosphere, then the temperature is maintained at 130°C for 3.5 hours (finishing step). The reaction medium is then cooled to a temperature of 50°C. The polystyrene beads obtained are recovered by filtration and then dried.

[0122] The polymerization process using suspension No. 1 is a comparative process, the process using suspension No. 2 under the polymerization conditions (1) is according to the invention.

[0123] The following properties and quantities were measured or calculated:

[0124] - Residual styrene content: measured according to gas chromatography of the free styrene monomer obtained by dissolving 0.5g of expanded polystyrene in 25ml of THF, left stirring for 1 hour. The solution is injected twice and the amount of styrene is determined by external styrene calibration;

[0125] - Weight-average molecular mass (Mw) of polystyrene: measured by size exclusion chromatography (SEC) with polystyrene standards

[0126] - Polydispersity index of polystyrene, defined as the ratio of the weight-average molar mass (Mw) of polystyrene to the number-average molar mass (Mn) of polystyrene. The weight-average (Mw) and number-average (Mn) molecular masses are measured by size exclusion chromatography (SEC) with polystyrene standards.

[0127] - Quantity of second organic peroxide that has not reacted, i.e. not decomposed (in % by weight relative to the weight of initial peroxide): measured by gas chromatography of the liquid phase after reprecipitation with methanol of the polymer dissolved in methylene chloride. The peak corresponding to the unreacted organic peroxide B (dicumyl peroxide (Luperox® DCP) or a,a'-bis(tert-butylperoxy)-diisopropylbenzene) is quantified by external calibration with standards of each of the peroxides to be quantified.

[0128] The results are presented in the table below.

[0129] [Table 2]

[0130] The molecular masses obtained for suspensions 1 and 2 under polymerization conditions (1) are comparable. In addition, the use of a,a'-bis(tert-butylperoxy)-diisopropylbenzene, under polymerization conditions (1), allows the obtaining of a low residual styrene monomer content.

[0131] Example 2: Evaluation of flame retardant properties

[0132] Example 2 was repeated, except that during the preparation of the suspensions, 0.80% by weight of a brominated flame retardant relative to the weight of styrene monomer was also introduced into the suspension. The flame retardant is Emerald Innovation® 3000 marketed by LANXESS.

[0133] The flame retardant properties of polystyrenes obtained from suspensions No. 1 and No. 2 under polymerization condition (1) were evaluated: polystyrene prepared using α,α'-bis(tert-butylperoxy)-diisopropylbenzene as a second-stage initiator exhibits flame retardant properties similar to those of polystyrene prepared using DCP. α,α'-bis(tert-butylperoxy)-diisopropylbenzene used as a finishing initiator is therefore also capable of acting as a synergist for the brominated flame retardant agent.

Claims

Claims 1. Process for the preparation of polystyrene, comprising the following steps: a) addition, into an aqueous suspension comprising styrene monomer, of at least one organic peroxide of formula (I): [Chem 1] in which: - Ri is a linear or branched alkyl group comprising from 1 to 6 carbon atoms, and - R2 is a linear or branched alkyl group comprising from 1 to 6 carbon atoms; and b) heating the suspension, said heating step b) comprising a step b1) of heating at a temperature of 80°C to 115°C, preferably from 85 to 110°C, for a period of 30 minutes to 5 hours, preferably from 30 minutes to 4 hours, more preferably from 1 hour to 3 hours.

2. The method of claim 1, wherein the polystyrene is expandable polystyrene, the method further comprising adding a blowing agent, preferably to the suspension.

3. Method according to one of claims 1 to 2, comprising the addition to the suspension of at least one other organic peroxide A, preferably having a half-life temperature of one hour lower than that of the organic peroxide of formula (I), more preferably chosen from the group consisting of dibenzoyl peroxide, tert-butyl peroxy-2-ethyl hexanoate, tert-amyl peroxy-2-ethyl hexanoate, 2,5-dimethyl-2,5-di(2- ethylhexanoylperoxy)hexane, 1-alkoxy-1-tert-alkylperoxycyclohexanes, in which the alkoxy group comprises between 1 and 4 carbon atoms, the tert-alkyl group comprises between 4 and 12 carbon atoms, and the cyclohexane ring may optionally be substituted by 1 to 3 alkyl groups, each independently comprising between 1 and 3 carbon atoms, in particular 1-methoxy-1-tert-amylperoxycyclohexane and mixtures thereof.

4. Method according to one of claims 1 to 3, in which the heating step b) comprises a step b2) of heating the suspension to a temperature of 120 to 140°C, preferably 120 to 135°C.

5. Method according to one of claims 1 to 4, in which step b2) is carried out over a period of 1 to 5 hours, preferably 1 to 4 hours.

6. Method according to one of claims 1 to 5, in which the organic peroxide of formula (I) is added before the step of heating the suspension.

7. Method according to one of claims 1 to 6, in which R1 and R2 are identical and represent a linear or branched alkyl group comprising from 1 to 6 carbon atoms.

8. Method according to one of claims 1 to 7, in which Ri represents a -CH3 or -CH2-CH3 group and R2 represents a -CH3 group.

9. A method according to one of claims 1 to 8, wherein the organic peroxide of formula (I) comprises a peroxide selected from the group consisting of a,a'-bis(tert-butylperoxy)-diisopropylbenzene, a,a'-bis(tert-amylperoxy)-diisopropylbenzene, a-(tert-butylperoxy)-a'-(tert-amylperoxy)-diisopropylbenzene and mixtures thereof, and preferably comprises, more preferably consists of, a,a'-bis(tert-butylperoxy)-diisopropylbenzene.

10. Method according to one of claims 1 to 9, further comprising a step of adding to the suspension a flame retardant, preferably a halogenated flame retardant, more preferably a brominated flame retardant.

11. Polystyrene obtained by a process according to one of claims 1 to 11, comprising a flame retardant, preferably a halogenated flame retardant, more preferably a brominated flame retardant.

12. Use of at least one organic peroxide of formula (I): [Chem 1] in which: - Ri is a linear or branched alkyl group comprising from 1 to 6 carbon atoms, and - R2 is a linear or branched alkyl group comprising from 1 to 6 carbon atoms, as a styrene polymerization initiator and as a flame retardant synergist.

13. Composition comprising at least one organic peroxide of formula (I): [Chem 1] in which: - Ri is a linear or branched alkyl group comprising from 1 to 6 carbon atoms, and - R2 is a linear or branched alkyl group comprising from 1 to 6 carbon atoms, and at least one halogenated, preferably brominated, flame retardant, said composition comprising less than 0.1%, preferably less than 0.01%, still preferably does not comprise organic peroxide chosen from the group consisting of tert-butyl perbenzoate, tert-butyl (2-ethylhexyl)monoperoxcarbonate, tert-amyl (2-ethylhexyl)monoperoxcarbonate and dicumyl peroxide and mixtures thereof.

14. The composition of claim 12 or 13, wherein the halogenated flame retardant is selected from the group consisting of brominated polymers, hexabromocyclododecane, tetrabromobisphenol A, decabromodiphenyl ether, pentabromodiphenyl ether, octabromodiphenyl ether, tris-(dibromopropyl)phosphate, carbon tetrabromide, beta-dibromopropionate, tetrabromoethylene, 1-2-dibromo-1,1,2,2-tetrachloroethane, 1,1,2,2-tetrabromoethane, dibromodichloroethane, 1,2-dibromo-1,1-dichloroethane, 1,2-dibromo-1,2,2-trichloroethane, 1,2,3,4-tetrabromobutane, 1 ,2,3-tribromopropane, pentabromoethane, tribromotrichlorocyclohexane, 1,2,4-tribromobutane, tetrabromopentane, hexabromoethane, tetrabromodichlorocyclohexane, pentabromomonochlorocyclohexane, 1,2-di- (dibromomethyl)benzene, alpha, beta-dibromoethylbenzene, alpha, beta-dibromopropionate, brominated monomers, such as 1,1'-(isopropylidene)bis[3,5-dibromo-4-(2,3-dibromo-2-methylpropoxy)benzene]), and mixtures thereof, more preferably is a brominated polymer.

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