WO2006061571A1

WO2006061571A1 - Expandable polystyrene composition

Info

Publication number: WO2006061571A1
Application number: PCT/GB2005/004607
Authority: WO
Inventors: Jean-Marc Galewski; Karine Labastie Coeyrehourcq
Original assignee: Ineos Europe Limited
Priority date: 2004-12-06
Filing date: 2005-12-01
Publication date: 2006-06-15

Abstract

Expandable polystyrene composition comprising: (1) 100 parts by weight of a polymer of styrene preferably having a weight-average molecular mass Mw ranging from 150 000 to 450 000 daltons, (2) from 2 to 20 parts by weight of at least one blowing agent, and (3) from 0.05 to less than 1.0 part by weight of a carbon black having a BET specific surface area, measured according to the method ASTM D 6556, ranging from 550 to 1600 m2/g is disclosed.

Description

EXPANDABLE POLYSTYRENE COMPOSITION

The present invention relates to an expandable polystyrene composition comprising a carbon black, to a process for preparing the composition and its use, and to the expanded articles resulting from the use.

Carbon black has been known for a long time to be used as a filler or pigment in large amount, for example of the order of 50% by weight, or else as a nucleating agent, in a lesser amount, in polymer foams such as polyethylene, polypropylene, polystyrene or polyurethane foams, though generally without the type of carbon black used being specified. It is known, for example, from Kirk-Othmer, Encyclopedia of Chemical Technology, published by John Wiley & Sons, 4th edition, Vol. 4, pages 1037 to 1074, that carbon black exists in various physical forms, depending in particular on the manufacturing process. Accordingly, among the various types of carbon black, a distinction is made between oil furnace black (petroleum black), gas furnace black, acetylene black, lamp black, flame black (smoke black), channel black (carbon black obtained by small-flame combustion), thermal black, and electrically conductive carbon black. Electrically conductive carbon black differs from the other carbon blacks in particular in an extremely high specific surface area.

Also known is the use of carbon black in polymer foams, in a relatively large amount, of up to 25% by weight for example, as a thermal insulator or as an absorber/reflector of infrared radiation. For instance, American patents US 4,795,763 and , US 5,149,722 disclose polyethylene, polypropylene and polystyrene foams and especially, according to the examples, polyurethane foams containing from 1% to 20%, preferably from 2% to 10%, by weight, relative to the polymer, of a carbon black which may be selected from all of those referred to above, but preferably from electrically non-conductive carbon blacks, having in particular a low specific surface area, of less than 200 m²/g, for example, and selected in particular from lamp black, channel black, oil furnace black, gas furnace black and thermal black. All of the examples show polyurethane foams containing from 2.5% to 10% by weight of a carbon black selected from electrically non-conductive carbon blacks. International patent application WO 90/06339 discloses a polystyrene foam containing from 1.0% to 25% by weight, relative to the polymer, of a carbon black having a specific surface area of from 10 to 1500 m²/g. In particular, a polystyrene foam is prepared by extruding a mixture comprising a melted polystyrene, a blowing agent and 5.0% by weight, relative to the polymer, of a carbon black having a specific surface area of 1475 m²/g. However, international patent application WO 94/13721 shows that, in order to obtain a polystyrene foam having the greatest thermal insulation property for a given charge of carbon black, in particular from 1.0% to 25% by weight, the recommendation is to use a thermal black having a low specific surface area, of 8 m²/g, for example, in comparison in particular with a carbon black (especially furnace grade carbon black) having a relatively high specific surface area, of 80 m²/g, for example. The same teaching appears in international patent application WO 2004/087798, which advises using, in an expandable polystyrene, from 0.01% to 20% by weight of a carbon black having a low specific surface area ranging from 5 to 40 m²/g. The international application shows in the examples how it is preferable to use a carbon black (Carbon black T990^® sold by Concarb (USA)) having a low specific surface area of 10 m²/g in place of a carbon black (Printex 85 TM^® sold by Degussa (Germany)) having a high specific surface area of 200 m²/g.

International patent application WO 97/45477 discloses an expandable polystyrene containing from 0.05% to 25%, preferably from 2% to 8%, by weight, relative to the polymer, of a carbon black which can have a BET specific surface area of from 10 to 500 m²/g. Such a polystyrene exhibits improved thermal insulation and flame retardancy properties. By way of example an expandable polystyrene is shown which is prepared by extruding a mixture comprising a melted polystyrene, a blowing agent and 6% by weight, relative to the polymer, of a flame black (or lamp black) known under the trade name Flammruss FL 101^® sold by Degussa (Germany).

European patent application EP 0 620 246 discloses a polystyrene foam prepared from polystyrene particles containing a blowing agent and enveloped with an infrared absorber selected from aluminium powder, graphite and carbon black, in a proportion of from 0.5% to 5% by weight relative to the polymer. However, the type of carbon black used is not specified. It has been observed, moreover, that the envelopment of polystyrene particles with an absorber of this kind makes it possible to obtain neither a uniform dispersion of the agent in the particles nor satisfactory thermal insulation properties.

One of the objects of the present invention is to provide an expandable polystyrene composition capable of producing a polystyrene foam having maximum thermal insulation for a given charge of carbon black. In other words the object is to provide an expandable polystyrene composition capable of producing expanded articles having the lowest possible charge of carbon black for a given level of performance in terms of thermal insulation. It has been observed that good results may be obtained in particular when the carbon black is uniformly distributed in the composition and, moreover, when it is employed during the manufacture of the polystyrene. It has also been observed that the presence of certain types of carbon black, having in particular a very high specific surface area, during the manufacture of the polystyrene may disrupt the polymerization reaction of the styrene, and may in particular inhibit the reaction and lead to styrene polymers having a high residual monomer content and an inappropriate weight-average molecular mass Mw. The presence of a large amount of residual monomer in the expandable polystyrene composition may give rise both to problems of uncontrolled plastification in the manufacture of expanded articles, particularly with inappropriate cell structures, and to substantial toxicological and environmental problems. Furthermore, other advantages may appear in the subsequent description of the present invention.

The present invention first provides an expandable polystyrene composition comprising:

( 1 ) 100 parts by weight of a polymer of styrene having in particular a weight-average molecular mass Mw ranging from 150 000 to 450 000 daltons,

(2) from 2 to 20 parts by weight of at least one blowing agent, and

(3) . from 0.05 to less than 1.0 part by weight of a carbon black having a BET specific surface area, measured according to the method ASTM D 6556, ranging from 550 to l600 m²/g. It has in effect been found, surprisingly, that by virtue of the selection of a carbon black selected from those having a very high specific surface area it is possible to achieve the abovementioned objects, contrary to the teaching of international patent applications WO 94/13721 and WO 2004/087798.

According to the present invention the expandable polystyrene composition comprises a carbon black having a very high BET specific surface area, ranging from 550 to 1600 m²/g, preferably from 600 to 1500 m²/g, and in a very low proportion, ranging from 0.05 to less than 1.0 part, preferably from 0.05 to 0.98 part, in particular from 0.05 to 0.95 part, especially from 0.1 to 0.9 part by weight per 100 parts by weight of the styrene polymer.

The carbon black may be selected from electrically conductive carbon blacks, and more specifically from carbon blacks having in particular a very high pore volume, as measured by dibutyl phthalate absorption (DBPA) according to method ASTM D 2414, having for example a value of more than 300 ml/100 g, in particular ranging from 310 to 600 ml/100 g, preferably from 310 to 550 ml/100 g, or else having a very high iodine adsorption number, measured according to method ASTM D 1510, having for example a value of more than 500 g/kg, preferably more than 600 g/kg, or ranging from 700 to 1300 g/kg, in particular from 740 to 1150 g/kg. The carbon black may be in the form of solid aggregates with in particular a relatively low size, measured according to method ASTM D 3849, having for example a value ranging from 10 to 100 nm, preferably from 10 to 50 nm, in particular from 10 to 30 nm. By way of example it is possible to use a carbon black from among the electrically conductive carbon blacks sold under the trade names Ketjenblack EC300J^® and Ketjenblack EC600JD^® by Akzo Nobel (Netherlands).

The composition according to the invention is termed expandable, which is to say that it comprises a blowing agent and that the styrene polymer^'is capable of expansion by virtue of the presence of the blowing agent. The composition is preferably a homogeneous composition, i.e. one in which the carbon black is distributed uniformly throughout the composition. The composition may be in the form of particles or, preferably, of expandable beads. By beads are meant, generally, spherical or substantially spherical particles, in particular spheroidal particles which may have a large diameter and a small diameter, with a ratio between the large diameter and the small diameter ranging in particular from 1.0 to 1.3, preferably from 1.0 to 1.2. The expandable particles or beads may have an average size ranging from 0.3 to 3 mm, preferably from 0.3 to 2 mm, in particular from 0.4 to 1.5 mm. They may also have a bulk density (or apparent density), measured according to method ASTM D 1622, ranging from 550 to 720 kg/m³, preferably from 580 to 710 kg/m³, in particular from 600 to 770 kg/m³. The composition may advantageously be in the form of particles or beads and may be preferably homogeneous, such that the carbon black is distributed uniformly inside each expandable particle or bead.

The composition comprises a styrene polymer, which may be a homopolymer or a copolymer of styrene, containing at least 50%, preferably at least 80% and in particular at least 90% by weight of styrene. The comonomer or comonomers present in the styrene copolymer may be selected from vinylaromatic compounds, in particular from alpha- methylstyrene, a styrene halogenated on the aromatic ring or a styrene alkylated on the aromatic ring, (meth)acrylic acid, C₁ to C₄ alkyl esters of (meth)acrylic acid such a methyl acrylate, methyl methacrylate, ethyl acrylate, ethyl methacrylate, isopropyl acrylate or butyl acrylate, amides and nitriles of (meth)acrylic acid such as acrylamide, methacrylamide, acrylonitrile or methacrylonitrile, butadiene, ethylene, divinylbenzene and maleic anhydride. Preference is given to using a homopoly styrene or a copolymer of styrene with acrylonitrile or methyl methacrylate. The weight-average molecular mass, Mw, of the styrene polymer may be in a range extending from 150 000 to 450 000 daltons, preferably from 160 000 to 400 000 daltons, in particular from 170 000 to 300 000 daltons. The distribution of the molecular masses of the styrene polymer, calculated by the ratio between Mw and the number-average molecular mass Mn of the polymer, may range from 1.5 to 4.0, preferably from 1.7 to 3.5, in particular from 1.8 to 3.0. The styrene polymer may advantageously have a relatively low level of residual monomer and, where appropriate, of residual comonomer(s), despite the presence of the carbon black, which is known to be a polymerization inhibitor: for example, the amount may be less than or equal to 2000 parts by weight per million (ppm), preferably equal to or less than 1000 ppm, in particular equal to or less than 800 ppm or equal to or less than 500 ppm. The composition comprises, per 100 parts by weight of styrene polymer, from 2 to 20 parts, preferably from 3 to 15 parts and in particular from 3 to 10 parts by weight of at least one blowing agent. The blowing agent may be selected from aliphatic or cyclic, optionally fluorinated, hydrocarbons, carbon dioxide, water and mixtures of two or more of these compounds. More particularly it may be selected from Freons, linear or branched saturated hydrocarbons and cyclic saturated hydrocarbons, preferably C₃ to C₇ hydrocarbons, in particular C₄ to C₆ hydrocarbons, such as n-butane, isobutane, n-pentane, isopentane, n-hexane or isohexane, carbon dioxide, water and mixtures of two or more of these compounds, especially mixtures of two or more of these hydrocarbons, mixtures of carbon dioxide with water, mixtures of carbon dioxide with at least one of these hydrocarbons, or mixtures of water with at least one of these hydrocarbons and, optionally, carbon dioxide.

The composition may further comprise at least one additive selected from flame retardants, nucleating agents, plasticizers and agents which facilitate the demoulding of the moulded and expanded articles. In particular it may comprise at least one flame retardant selected in particular from halogenated hydrocarbons, preferably brominated hydrocarbons, in particular C₆ to C₁₂ hydrocarbons, such as hexabromocyclohexane, penta- bromomonochlorocyclohexane or hexabromocyclododecane, in an amount which can range from 0.05 to 2 parts, preferably from 0.1 to 1.5 parts, by weight, per 100 parts by weight of the styrene polymer. The composition may further comprise at least one nucleating agent selected in particular from synthetic waxes, in particular Fischer-Tropsch waxes and polyolefin waxes such as polyethylene waxes or polypropylene waxes, in an amount which can range from 0.05 to 1 part, preferably from 0.1 to 0.5 part, by weight per 100 parts by weight of the styrene polymer. The composition may likewise comprise at least one plasticizer, selected in particular from mineral oils and petroleum waxes such as paraffin waxes, in an amount which can range from 0.1 to 1 part, preferably from 0.1 to 0.8 part, by weight per 100 parts by weight of the styrene polymer. The composition may additionally comprise at least one agent which facilitates the demoulding of the moulded and expanded articles, selected in particular from inorganic salts and esters of stearic acid, such as glycerol mono-, di- or tristearates and zinc stearate, calcium stearate or magnesium stearate, in an amount which can range from 0.05 to 1 part, preferably from 0.1 to 0.6 part, by weight per 100 parts by weight of the styrene polymer.

The present invention pertains, furthermore, to a process for preparing the composition, characterized in that it is carried out by mixing the styrene polymer in the melted state with the blowing agent or agents and the carbon black, in particular in amounts identical to those given above for the composition. Mixing may be carried out in a chamber equipped with at least one mechanical stirrer means, under temperature and pressure conditions which are capable of preventing expansion of the composition. Mixing may preferably be carried out in an extruder, in particular a single-screw or twin-screw extruder, at a temperature above the glass transition temperature, Tg, of the polymer and ranging, for example, from 120 to 250⁰C, preferably from 150 to 230⁰C, and under an absolute pressure of from 0.1 to 10 MPa, preferably from 0.2 to 5 MPa. The resulting mixture may be subsequently extruded directly and may thus form extruded and expanded articles as described subsequently.

The present invention also pertains to another process for preparing the composition which is in the form, in particular, of expandable particles or, preferably, expandable beads. The process comprises a step of polymerizing styrene and optionally at least one comonomer as mentioned above in aqueous suspension, followed by a step of separating off and eliminating the aqueous phase, and isolating the composition in particular in the form of expandable particles or beads. The polymerization in aqueous suspension may be carried out with stirring, in the presence of at least one free-radical polymerization initiator, at least one suspension stabilizer, blowing agent or agents in an amount which can range from 3 to 23 parts by weight, and carbon black in an amount which can range from 0.05 to less than 1.0 part, preferably from 0.05 to 0.98 part, in particular from 0.05 to 0.95 part, especially from 0.1 to 0.9 part by weight per 100 parts by weight of styrene and optionally of comonomer(s) used in the polymerization.

Polymerization in aqueous suspension may be carried out at a temperature ranging from 80 to 150⁰C, preferably from 85 to 140⁰C. It may be carried out with a weight ratio between the water and the styrene and optionally the comonomer(s) ranging from 0.2/1 to 5/1, preferably from 0.5/1 to 4/1. The polymerization may be continued for a time such that the amount of residual monomer and, where appropriate, of residual comonomer(s) is less than or equal to 2000 ppm, preferably less than or equal to 1000 ppm, in particular less than or equal to 800 ppm or even than/to 500 ppm.

Polymerization in aqueous suspension is carried out in the presence of one or more free-radical polymerization initiators, in an amount which can range from 0.01 to 2 parts, preferably from 0.05 to 1 part by weight, per 100 parts by weight of monomer and optionally of comonomer(s). The free-radical polymerization initiator may be selected from mono-, di- and polyfunctional free-radical initiators, and in particular from peroxides, hydroperoxides, peroxycarbonates, perketals, peresters and azo compounds. It is preferably selected from difunctional or polyfunctional free-radical initiators and more particularly from peresters. It has been found particularly advantageous to carry out the polymerization in the presence of at least one free-radical polymerization initiator selected from peresters, such as tert-butyl peroxy-2-ethylhexanoate, tert-butyl peroxyneodecanoate, tert-butyl peroxypivalate, tert-butyl peroxydiethylacetate, tert-butyl peroxyisobutyrate, tert-butyl peroxy-3,5,5-trimethylhexanoate, cumyl peroxyneodecanoate, tert-amyl peroxy- 2-ethylhexanoate, tert-amyl peroxyneodecanoate, tert-amyl peroxypivalate, 2,5-bis(2-ethylhexanoylperoxy)-2,5-dimethylhexane or 2,4,4-trimethylpentyl- 2-peroxyneodecanoate, or else from difunctional or polyfunctional free-radical initiators, such as l,l-bis(tert-butylperoxy)-3,3,5-trimethylcyclohexane, l,l-bis(tert- butylperoxy)cyclohexane, 2,5-bis(tert-butylperoxy)2,5-dimethylhexane, 2,2-bis(tert- butylperoxy)butane or bis(tert-butylperoxyisopropyl)benzerie. In effect, these initiators make it possible to improve the yield of the polymerization reaction in spite of the presence of the carbon black. It is possible, accordingly, to manufacture a styrene polymer containing a very low amount of residual styrene and, where appropriate, of residual comonomer(s). The free-radical polymerization initiator or initiators may be introduced in its or their entirety at the beginning of polymerization or else part at the beginning and the remaining part in one or more portions during the polymerization.

Polymerization in aqueous suspension is preferably carried out in the presence of at least one suspension stabilizer, in particular at least one organic suspension stabilizer, selected in particular from polyvinyl alcohols, hydroxyethylcellulose, methylcellulose, sodium dodecylbenzenesulphonate, starch, polyacrylamides and polyvinylpyrrolidones, or at least one inorganic suspension stabilizer, selected in particular from alumina, magnesium silicate, magnesium oxide, zinc oxide, calcium carbonate, calcium fluoride and inorganic salts of (pyro)phosphoric acid such as tricalcium phosphate, barium phosphate, aluminium phosphate or magnesium pyrophosphate. The amount of suspension stabilizer employed may be from 0.05 to 6 parts, preferably from 0.1 to 4 parts, by weight per 100 parts by weight of styrene and optionally of comonomer(s).

In order to improve the stability of the suspension it is also possible to carry out the polymerization in the presence of a styrene (pre)polymer different from or, preferably, identical to the polymer of the composition, in ati amount which can range from 1 to

30 parts, preferably from 5 to 20 parts, by weight per 100 parts by weight of monomer and optionally of comonomer(s) employed. Thus it is possible, before beginning the polymerization, to form a solution, by addition and dissolution of the (pre)polymer of styrene in the monomer or, where appropriate, the comonomer(s). Thus it is possible to carry out prepolymerization in bulk or in a solution of styrene and, where appropriate, of one or more comonomers, until the abovementioned proportion of (pre)polymer is attained, and then to continue the polymerization in aqueous suspension in the presence in particular of the aforementioned reactants and additives.

Polymerization in aqueous suspension may be carried out in the presence of other additives, selected from chain transfer agents such as mercaptans and the dimer of alpha-methylstyrene, flame retardants such as halogenated hydrocarbons, preferably brominated hydrocarbons, in particular those mentioned above, crosslinking agents such as butadiene or divinylbenzene, plasticizers and nucleating agents, especially those mentioned above.

Polymerization in aqueous suspension is carried out in particular in the presence of at least one blowing agent, in an amount ranging from 3 to 23 parts, preferably from 4 to 17 parts, in particular from 4 to 12 parts by weight, per 100 parts by weight of styrene and optionally of comonomer(s) employed in the polymerization. Generally, a small portion of the blowing agent employed during the polymerization is lost and is not recovered in the expandable polystyrene composition after the polymerization. The blowing agent or agents may be selected from those mentioned above. They may be introduced in their entirety at the beginning or during the polymerization, or else partly at the beginning and the remaining part in one or more portions during the polymerization.

Polymerization in aqueous suspension is carried out in the presence of the carbon black, in an amount ranging from 0.05 to less than 1.0 part, preferably from 0.05 to 0.98 part, in particular from 0.05 to 0.95 part, especially from 0.1 to 0.9 part by weight, per 100 parts by weight of styrene and optionally of comonomer(s). The carbon black may be introduced at the beginning and/or during the polymerization, preferably before a degree of polymer conversion of 70%, preferably 50%, in particular of 40%, has been reached. It may be introduced in particular before the moment in polymerization known by the expression "particle identity point" (PIP) is reached, as defined in Ullmann's Encyclopedia of Industrial Chemistry, Vol. A 21 (1992), page 364, which corresponds to the moment in the polymerization where the suspended particles in water attain a constant size which no longer changes during the remainder of the polymerization.

Advantageously the aqueous suspension polymerization may include (i) a preliminary step of preparing an aqueous mixture comprising water and the suspension stabilizer(s) and, separately, (ii) a preliminary step of preparing an organic suspension comprising the styrene and, where appropriate, the comonomer(s), the carbon black, some or all of the free-radical polymerization initiator(s), and where appropriate some or all of the blowing agent(s). The aqueous mixture and the organic suspension may be advantageously preheated separately to identical or different temperatures, such that no substantial polymerization begins in the organic suspension, and such that subsequently, by mixing of the organic suspension and the aqueous mixture thus preheated, an aqueous suspension is formed, with stirring, which is immediately in reaction, at a temperature greater than or equal to the temperature at which the polymerization commences , spontaneously, for example at a temperature of at least 80⁰C, preferably at least 85°C and in particular of at least 90⁰C.

The present invention likewise pertains to the use of the expandable polystyrene composition for producing an expanded article, in particular an extruded or moulded article, having in particular a bulk density (or apparent density), measured according to method ASTM D 1622, ranging from 5 to 200 kg/m³, preferably from 50 to 180 kg/m³ and in particular from 5 to 150 kg/m³. The expanded article may in particular have a low or very low open cell content, measured according to method ASTM D 2856, of less than 10% for example, preferably less than 5% and in particular less than 2%.

More particularly, the invention pertains to the use of the expandable polystyrene composition for producing an extruded and expanded article, having a bulk density (or apparent density) as mentioned above, ranging for example from 20 to 200 kg/m³, preferably from 30 to 180 kg/m³ and in particular from 50 to 150 kg/m³. The extruded and expanded article is produced in particular by a process consisting in (a) employing the composition by mixing the styrene polymer in the melted state with the blowing agent(s) and the carbon black in an extruder, in particular a single-screw or twin-screw extruder, and under temperature and pressure conditions capable of preventing expansion of the composition, in particular at a temperature greater than the glass transition temperature,

Tg, of the polymer, for example a temperature ranging from 120 to 150°C, preferably from 150 to 23O⁰C, with stirring and under a pressure greater than atmospheric pressure, for example an absolute pressure ranging from 0.1 to 10 MPa, preferably from 0.2 to 5 MPa, then (b) extruding the mixture from the extruder, for example under a pressure equivalent to atmospheric pressure, so as to cause the composition to expand to the desired bulk density (or apparent density) and so form the extruded and expanded article.

The invention likewise pertains to another use of the expandable polystyrene composition, in the form in particular of expandable particles or, preferably expandable beads, for producing a moulded and expanded article having a bulk density (or apparent density) as mentioned above, for example ranging from 5 to 200 kg/m³, preferably from 5 to 100 kg/m³, in particular from 5 to 50 kg/m³, and especially from 5 to 30 kg/m³. The moulded and expanded article is produced in particular by a process comprising the following steps: (i) a step of pre-expansion (or prefoaming) by contacting and mixing the composition, in particular in the form of expandable particles or expandable beads, with water vapour, in particular in a stirred tank and in particular under pressure and temperature conditions capable of forming expanded particles or beads having in particular a bulk density (or apparent density) ranging from 5 to 200 kg/m³, preferably from 5 to 100 kg/m³ and in particular from 5 to 50 or to 30 kg/m³, for example at a temperature ranging from 80 to 110°C, or from 85 to 105⁰C, and under an absolute pressure which can range from 20 to 160 kPa, or from 50 to 15O kPa, (ii) a step of stabilizating (or maturating) the particles or beads thus expanded, by contacting them with ambient air, in particular at a temperature ranging from 0 to 4O⁰C, and under an absolute pressure which can range from 50 to 130 kPa, preferably from 80 to 120 kPa, for a time which can range from a few hours to a few days, for example from 2 hours to 3 days, such as in particular to attain an equilibrium between the ambient air and the internal atmosphere of the expanded particles or beads, and

(iii) a step of moulding the particles or beads thus stabilized, by introducing them into a mould and by heating the mould, in particular so as to weld the particles or beads to one another, for example at a temperature ranging from 80 to 12O⁰C, and so to produce a moulded and expanded article having in particular the desired bulk density (or apparent density) which, preferably, is substantially identical to that of the expanded particles or beads obtained in step (i). ^'

Thus it has been found that, by virtue of the specific choice of the carbon black, it is possible to provide an expandable polystyrene composition in the form in particular of particles or, preferably, of beads, this composition being capable of producing an expanded article containing a low proportion of carbon black, in particular less than 1.0% by weight relative to the styrene polymer, while having thermal insulation properties superior to those known to date for a similar charge of carbon black. Accordingly the present invention likewise pertains to an expanded article having a bulk density (or apparent density), measured according to method ASTM D 1622, ranging from 5 to 200 kg/m³, this article being obtained by employing the expandable polystyrene composition described above or prepared by one of the processes described above. The expanded article may have in particular a low or very low open cell content (measured according to method

ASTM D 2856) for example less than 10%, preferably less than 5%, in particular less than 2%. The expanded article advantageously exhibits a very low thermal conductivity, ranging for example from 25 to 50 mW/m.K, preferably from 30 to 45 mW/m.K, and generally lower on average by 10% to 20% than that of an expanded article of identical bulk density which contains no carbon black or contains an identical proportion of a carbon black having a low specific BET surface area, in particular less than 550 m²/g.

More particularly, the invention relates to an extruded and expanded article having in particular a bulk density (or apparent density), measured according to method ASTM D 1622, ranging from 20 to 200 kg/m³, preferably from 30 to 180 kg/m³ and in ^■ particular from 50 to 150 kg/m³, this article being obtained by employing the expandable polystyrene composition as described above, or prepared by one of the processes described above, or else used according to one of the uses described above, by extrusion. The extruded and expanded article may advantageously have a low or very low open cell content, as described above, and also a very low thermal conductivity as mentioned above.

The invention likewise pertains to a moulded and extruded article having in particular a bulk density (or apparent density), measured according to method ASTM D 1622, ranging from 5 to 200 kg/m³, preferably from 5 to 100 kg/m³, in particular from 5 to 50 or 30 kg/m³, this article being obtained by employing the expandable polystyrene composition as described above, or prepared by one of the processes described above, or else used according to one of the uses described above, by moulding. The moulded and expanded article may advantageously have a low or very low open cell content, as described above, and a very low thermal conductivity as mentioned above.

The examples which follow illustrate the present invention.

Example 1

A composition of expandable polystyrene according to the invention is prepared in a reactor equipped with a stirring device and a double envelope connected to a heating and cooling device, by the introduction into the reactor, under agitation, of 5,000 parts _,by weight of water and 70 parts by weight of a polyvinyl alcohol sold under the trade name

^• "PVA 224"® by Kuraray Co. Limited (Japan) at ambient temperature (20 ⁰C), in such a way as to obtain an aqueous mixture. Separately, in a receptacle under stirring and at ambient temperature, an organic suspension is prepared by mixing 2700 parts by weight of styrene with 300 parts by weight of a polystyrene of a mean molecular weight Mw equal to 215,000 daltons, 3 parts by weight of carbon black sold under the trade name "Ketjenblack EC300J"® by Akzo Nobel (Netherlands), with a specific surface BET of 800 m2/g (in accordance with the ASTM D 6556 method), a pore volume (DBPA) of 330 ml/10Og (in accordance with the ASTM D 2414 method), an iodine adsorption number of 790 g/kg (in accordance with the ASTM D 1510 method), 8.3 parts by weight of tertiary-butyl peroxy- 2-ethylhexanoate and 5.4 parts by weight of tertiary-butyl peroxy-2-ethylhexylcarbonate. The organic suspension prepared in this way is kept under agitation at ambient temperature for 30 minutes. The organic suspension is introduced into the reactor containing the aqueous mixture, under stirring (300 rev/min) and at ambient temperature, in such a way as to obtain an aqueous reaction suspension ready for use. The temperature of the reactor is then raised in 1 hour and 10 minutes from 20 ⁰C to 90 ⁰C and kept at 90 ⁰C for 4hours and 20 minutes. At the end of this time the temperature of the reactor is again raised over 1 hour from 90 ⁰C to 120 ⁰C, while 240 parts by weight of a 75/25 mixture by weight of, respectively, n-pentane and isopentane is introduced into the reactor. At the end of this period the temperature of the reactor is maintained at 120 ⁰C for a period such that the content of residual styrene is equal to 2000 ppm, and the reactor is then cooled again to ambient temperature. An aqueous suspension of expandable polystyrene beads is thus obtained. After the separation and elimination of the aqueous phase, a composition of expandable polystyrene is isolated in the form of beads, consisting of 100 parts by weight of a polystyrene having a mean molecular weight Mw equivalent to 291 000 daltons, a

- molecular weight distribution Mw/Mn equal to 3.6, and a residual styrene content of 1758 ppm, 6.2 parts by weight of the mixture of n-pentane and isopentane, and 0.1 parts by weight of the carbon black.

Example 2

The procedure according to Example 1 is repeated exactly, except that in the preparation of the organic suspension use is made of 14.8 parts by weight of tertiary-butyl peroxy-2-ethylhexanoate and 11.2 parts by weight of tertiary-butylperoxy-2- ethylhexylcarbonate, and the duration of polymerisation is identical to that of Example 1. Under these conditions a composition of expandable polystyrene is obtained in the form of beads, identical to that of Example 1, except that the polystyrene has a residual styrene content of 316 ppm instead of 1758 ppm.

A moulded and expanded object is then prepared in the form of a parallelepipedic panel (0.36 m x 0.44 m x 0.14 m), according to a procedure comprising the following stages: (i) a step of pre-expansion by mixing and contacting the expandable beads obtained before with with water vapour in a stirred tank, at a temperature of 100⁰C and under an absolute pressure of 128 kPa, for a time sufficient to give expanded beads having a bulk density (or apparent density), measured according to method ASTM

D 1622, of l3.3 kg/m³. (ii) a step of stabilization by contacting the beads thus expanded with ambient air, at a temperature of 20⁰C and under an absolute pressure of 100 kPa for 1 day, and (iii) a step of moulding, by introducing the beads thus stabilized into a mould intended to give the parallelepipedal panel and by heating the mould so as to weld the beads to one another and to give the panel a bulk density (or apparent density), measured according to method ASTM D 1622, of 13.3 kg/m³

The moulded and expanded article has a thermal conductivity (measured according to draft European standard prEN 13163T) of 36.16 mW/m.K.

Example 3

A composition of expandable polystyrene according to the invention is prepared in a reactor equipped with a stirring device and a double envelope connected to a heating and cooling device, by the introduction into the reactor, under agitation, of 4,000 parts by weight of water and 85 parts by weight of a polyvinyl alcohol sold under the trade name "PVA 224"® by Kuraray Co. Limited (Japan) at ambient temperature (20 ⁰C), in such a way as to obtain an aqueous mixture. Separately, in a receptacle under stirring and at ambient temperature, an organic suspension is prepared by mixing 3600 parts by weight of styrene with 400 parts by weight of a polystyrene of a mean molecular weight Mw equal to 215 000 daltons, 4 parts by weight of a chain transfer agent of the type di alpha methyl styrene, 10.8 parts by weight of tertiary-butyl peroxy-2-ethylhexanoate, and 7.2 parts by weight of tertiary-butylperoxy-2-ethylhexylcarbonate. The organic suspension obtained in this way is maintained under stirring at ambient temperature for 30 minutes. The organic suspension is introduced into the reactor containing the aqueous mixture, under stirring (200 rev/min) and at ambient temperature, in such a way as to obtain an aqueous reaction suspension ready for use. The temperature of the reactor is then raised in 1 hour and 10 minutes from 20 ⁰C to 90 ⁰C. After 15 minutes at 90 ⁰C, 12 parts by weight of carbon black sold under the trade name "Ketjenblack EC300J"® by Akzo Nobel (Netherlands), with a specific surface BET of 800 m²/g (in accordance with the ASTM D 6556 method), a pore volume (DBPA) of 330 ml/10Og (in accordance with the ASTM D 2414 method), and an iodine adsorption number of 790 g/kg (in accordance with the ASTM D 1510 method), is introduced into the reactor. The polymerisation is then carried out at 90 ⁰C for an additional 4 hours and 45 minutes. 1 hour before this stage, at 90 ⁰C, 3.2 parts by weight of a polyvinyl alcohol sold under the trade name "PVA 220E"® by Kuraray Co. Limited (Japan) is introduced into the reactor. The temperature in the reactor is then raised in 1 hour from 9O⁰C to 120 ⁰C. 30 minutes after the start of this increase in temperature, one begins to introduce into the reactor, over a period of 2 hours, 240 parts of a mixture of 75/25 parts by weight of n-pentane and isopentane respectively, increasing the stirring to 250 rev/min. The temperature of the reactor is maintained at 120 ⁰C for about 3 hours in total in order to reduce the residual styrene, and the reactor is then cooled to ambient temperature, reducing the stirring to 200 rev/min. An aqueous suspension of expandable polystyrene beads is thus obtained. After the separation and elimination of the aqueous phase, a composition of expandable polystyrene is isolated in the form of beads, consisting of 100 parts by weight of a polystyrene having a mean molecular weight Mw equivalent to 224 000 daltons, a molecular weight distribution Mw/Mn equal to 2.6, and a residual styrene content of 2545 ppm, 3 parts by weight of the mixture of n-pentane and isopentane, and 0.3 parts by weight of the carbon black.

A moulded and expanded object is then prepared in the form of a parallelepipedic panel (0.36 m x 0.44 m x 0.14 m), according to a procedure set out in Example 2 except that the bulk density of the beads in step (i) and in step (iii) is 19.4 kg/m³. The moulded and expanded object exhibits a thermal conductivity (measured in accordance with the draft European Standard prEN 13163T) equal to 34.21 mW/m.K. Example 4

A composition of expandable polystyrene according to the invention is prepared in a reactor equipped with a stirring device and a double envelope connected to a heating and cooling device, by the introduction into the reactor, under agitation, of 5,000 parts by weight of water and 85 parts by weight of a polyvinyl alcohol sold under the trade name "PVA 224"® by Kuraray Co. Limited (Japan) at ambient temperature (20 ⁰C), in such a way as to obtain an aqueous mixture. Separately, in a receptacle under stirring and at ambient temperature, an organic suspension is prepared by mixing 4000 parts by weight of styrene with 500 parts by weight of a polystyrene of a mean molecular weight Mw equal to 215 000 daltons, 4.5 parts by weight of a chain transfer agent of the type di alpha methyl styrene, 16.65 parts by weight of tertiary-butyl peroxy-2-ethylhexanoate, and 12.6 parts by weight of tertiary-butylperoxy-2-ethylhexylcarbonate. The organic suspension obtained in this way is maintained under stirring at ambient temperature for 30 minutes. The organic suspension is introduced into the reactor containing the aqueous mixture, under stirring (200 rev/min) and at ambient temperature, in such a way as to obtain an aqueous reaction suspension ready for use. The temperature of the reactor is then raised in 1 hour and 10 minutes from 20 ⁰C to 90 ⁰C. After 15 minutes at 90 ⁰C, 18 parts by weight of carbon black sold under the trade name "Ketjenblack EC300J"® by Akzo Nobel (Netherlands), with a specific surface BET of 800 m²/g (in accordance with the ASTM D 6556 method), a pore volume (DBPA) of 330 ml/10Og (in accordance with the ASTM D 2414 method), and an iodine adsorption number of 790 g/kg (in accordance with the ASTM D 1510 method), is introduced into the reactor. The polymerisation is then carried out at 90 ⁰C for an additional 4 hours and 5 minutes. 1 hour before this stage, at 90 ⁰C, 4 parts by weight of a polyvinyl alcohol sold under the trade name "PVA 220E"® by Kuraray Co. Limited (Japan) is introduced into the reactor. The temperature in the reactor is then raised in 1 hour from 9O⁰C to 120 ⁰C. 20 minutes before the increase in the temperature in the reactor, one begins to introduce into the reactor, over a period of 1 hour and 30 minutes, 240 parts of a mixture of 75/25 parts by weight of n-pentane and isopentane respectively, increasing the stirring to 250 rev/min. At the end of the introduction of the pentane, the stirring is reduced to 150 rev/min. The temperature of the reactor is maintained at 120 ⁰C for about 3 hours in order to reduce the residual styrene before being reduced to ambient temperature. An aqueous suspension of expandable polystyrene beads is thus obtained. After the separation and elimination of the aqueous phase, a composition of expandable polystyrene is isolated in the form of beads, consisting of 100 parts by weight of expandable polystyrene having a mean molecular weight Mw equivalent to 174 000 daltons, a molecular weight distribution Mw/Mn equal to 2.6, and a residual styrene content of 874 ppm, 3.6 parts by weight of the mixture of n-pentane and isopentane, and 0.4 parts by weight of the carbon black.

Claims

1. Expandable polystyrene composition comprising:

(1) 100 parts by weight of a polymer of styrene preferably having a weight- average molecular mass Mw ranging from 150 000 to 450 000 daltons,

(2) from 2 to 20 parts by weight of at least one blowing agent, and

(3) from 0.05 to less than 1.0 part by weight of a carbon black having a BET specific surface area, measured according to the method ASTM D 6556, ranging from 550 to 1600 m²/g.

2. Composition according to Claim 1, characterized in that it contains from 0.05 to 0.98 part, preferably from 0.05 to 0.95 part and in particular from 0.1 to 0.9 part by weight of carbon black.

3. Composition according to Claim 1 or 2, characterized in that it is a homogeneous composition in which the carbon black is distributed uniformly throughout the . composition.

4. Composition according to any one of Claims 1 to 3, characterized in that it is in the form of expandable particles or, preferably expandable beads having in particular a density ranging from 550 to 720 kg/m³.

5. Composition according to any one of Claims 1 to 4, characterized in that the styrene polymer contains an amount of residual monomer and, where appropriate, of residual comonomer(s) of less than or equal to 2000 parts by weight per million

(ppm), preferably less than or equal to 1000 ppm and in particular less than or equal to 800 ppm or to 500 ppm.

6. Composition according to any one of Claims 1 to 5, characterized in that it comprises from 3 to 15 parts, preferably from 3 to 10 parts, by weight of at least one blowing agent.

7. Composition according to any one of Claims 1 to 6, characterized in that the blowing agent is selected from aliphatic or cyclic hydrocarbons, optionally fluorinated hydrocarbons, carbon dioxide, water and mixtures of two or more of these compounds.

8. Process for preparing the composition according to any one of Claims 1 to 7, characterized in that it is carried out by mixing the styrene polymer in the melted state with the blowing agent or agents and the carbon black. ^'

9. Process according to Claim 8, characterized in that mixing is carried out in a chamber equipped with at least one mechanical stirring means and under temperature and pressure conditions which are capable of preventing expansion of the composition, preferably in an extruder, in particular a single-screw or twin- screw extruder, at a temperature greater than the glass transition temperature of the polymer, in particular a temperature ranging from 120 to 250°C and under an absolute pressure ranging from 0.1 to 10 MPa.

10. Process for preparing the composition according to any one of Claims 1 to 7, in the form in particular of expandable particles or, preferably, expandable beads, said process being characterized in that it comprises a step of polymerization in aqueous suspension of the styrene and, where appropriate, of at least one comonomer, followed by a step of separation and removal of the aqueous phase and isolation of the composition in particular in the form of expandable particles or, preferably, expandable beads.

11. Process according to Claim 10, characterized in that the polymerization is carried out with stirring in the presence of at least one free-radical polymerization initiator, at least one suspension stabilizer, blowing agent or agents and carbon black.

12. Process according to Claim 11, characterized in that the free-radical polymerization initiator is selected from mono-, di- and polyfunctional free-radical initiators, in particular from peroxides, hydroperoxides, peroxycarbonates, perketals, peresters and azo compounds, preferably from difunctional or polyfunctional tree-radical initiators and more particularly peresters.

13. Use of the composition according to any one of Claims 1 to 7 for producing an expanded article having in particular a bulk density ranging from 5 to 200 kg/m³.

14. Use according to Claim 13, characterized in that an extruded and expanded article is produced, in particular with a bulk density ranging from 20 to 200 kg/m³, preferably from 30 to 180 kg/m³ and in particular from 50 to 150 kg/m³.

15. Use according to Claim 14, characterized in that the extruded and expanded article is produced by a process consisting in (a) employing the composition by mixing the styrene polymer in the melted state with the blowing agent or agents and the carbon black in an extruder with stirring and under temperature and pressure conditions capable of preventing expansion of the composition then (b) extruding the mixture from the extruder so as to cause the composition to expand to the desired bulk density and so form the extruded and expanded article.

16. Use according to Claim 13, characterized in that a moulded and expanded article is produced, in particular with a bulk density ranging from 5 to 200 kg/m³, preferably from 5 to 100 kg/m³ and in particular from 5 to 50 kg/m³.

17. Use according to Claim 16, characterized in that the moulded and expanded article is produced by a process comprising the following steps:

(i) a step of pre-expansion, by contacting and mixing the composition, which is in the form in particular of expandable particles or, preferably, expandable beads, with water vapour, in particular in a stirred tank, under pressure and temperature conditions capable of forming expanded particles or expanded beads having in particular a bulk density ranging from 5 to 200 kg/m³, preferably from 5 to 100 kg/m³ and in particular from 5 to 50 kg/m³, (ii) a step of stabilizing the particles or beads thus expanded, by contacting them with ambient air, and (iii) a step of moulding the particles or beads thus stabilized, by introducing them into a mould and by heating the mould so as to weld the particles or beads to one another and so to produce a moulded and expanded article having in particular the desired bulk density and, preferably a bulk density substantially identical to that of the expanded particles or expanded beads obtained in step (i).

18. Expanded article having in particular a bulk density ranging from 5 to 200 kg/m³, obtained by employing the composition described according to any one of Claims 1 to 7, or prepared by the process according to any one of Claims 8 to 12, or used according to one of Claims 13 to 17.

19. Extruded and expanded article having in particular a bulk density ranging from 20 to 200 kg/m³, preferably from 30 to 180 kg/m³, in particular from 50 to 150 kg/m³, obtained by employing the composition described according to any one of Claims 1 to 7, or prepared by a process according to Claim 8 or 9, or used according to Claim 14 or 15.

20. Moulded and expanded article having in particular a bulk density ranging from 5 to 200 kg/m³, preferably from 5 to 100 kg/m³, in particular from 5 to 50 kg/m³, obtained by employing the composition described according to any one of Claims 1 to 7, or prepared by the process according to any one of Claims 10 to 12, or used according to Claim 16 or 17.