Patent Description:
High impact monovinylidene aromatic polymers have been known for a long time and have numerous applications in many fields such as food packaging, including disposable trays, coffee cups, and yogurt cups, toys, bicycle components, television and computer houses, automotive instrument panels and gas tanks.

The utility of a particular high impact monovinylidene aromatic polymer depends on the polymer having some combination of mechanical, thermal and/or physical properties that render the material suitable for a particular application, yet for a majority of applications, mechanical properties are the key feature.

Various techniques for improving the mechanical properties, such as impact strength, of compositions based on monovinylidene aromatic polymers and rubber have been disclosed. Among them, incorporation of a blend of polybutadiene rubbers in the monovinylidene aromatic polymer matrix has been proposed.

<CIT> discloses a polymer composition comprising a vinylaromatic polymer and a rubber containing polybutadiene, wherein the rubber is derived from a high-viscosity polybutadiene and from a low-viscosity polybutadiene,.

The polymer composition contains between <NUM> and <NUM>% by weight of poybutadiene.

<CIT> discloses a rubber-modified monovinylidene aromatic polymer composition comprising:.

<CIT> discloses an impact-resistant polystyrene based resin, with improved impact strength, rigidity and surface gloss, comprising between <NUM> and <NUM>% by weight of a particular polybutadiene blend and between <NUM> and <NUM>% by weight of a polystyrene based resin.

<CIT> discloses an impact-resistant thermoplastic moulding material which essentially contains.

<CIT> discloses a method for preparing continuously an ultra-high impact rubber-modified polystyrene resin which contains a dispersive rubber particle whose particle size distribution has a bimodal shape, and has a high grafting efficiency, a narrow distribution of molecular weight and excellent Izod impact strength and practical impact strength. Preferably the three different kinds of synthetic butadiene polymers comprise <NUM>-<NUM>% by weight of a styrene-butadiene block copolymer and <NUM>-<NUM> % by weight of a low cis-polybutadiene rubber.

<CIT> discloses a method for preparing continuously a rubber-modified polystyrene resin which contains a dispersive rubber particle whose particle size distribution has a trimodal shape, and has a high grafting efficiency, a narrow distribution of molecular weight and excellent Izod impact strength and practical impact strength. The method comprises the steps of reacting a liquid supply material composition comprising three different kinds of synthetic butadiene polymers dissolved in a styrene-based monomer. Preferably the three different kinds of synthetic butadiene polymers comprise <NUM>-<NUM>% by weight of a styrene-butadiene block copolymer, <NUM>-<NUM>% by weight of a low cis-polybutadiene rubber, and <NUM>-<NUM>% by weight of a high cis-polybutadiene rubber.

<CIT> discloses a high impact high gloss bimodal polystyrene material and preparation method thereof, comprising the mixing of polybutadiene rubber component with polystyrene monomers; wherein, the polybutadiene rubber component is selected from high cis-polybutadiene and/or low cis-polybutadiene, the cis-content of high cis-polybutadiene is not less than <NUM>% by mole and the cis-content of low cis-polybutadiene is between <NUM>% to <NUM>% by mole. The invention also provides a preparation method of the high impact high gloss bimodal polystyrene.

<CIT> discloses a high impact polystyrene composition comprises (a) as the continuous phase, <NUM>-<NUM> parts by weight of polystyrene and (b) as a discontinuous elastomer gel phase dispersed there through, <NUM>-<NUM> parts by weight of a diene elastomer-styrene inter-polymer phase wherein (<NUM>) <NUM>-<NUM>% by weight of the dispersed phase have particle sizes of <NUM>-<NUM> diameter and (<NUM>) <NUM>-<NUM>% by weight of the dispersed phase have particle sizes of <NUM>-<NUM> diameter. The composition may be prepared by blending together two high impact polystyrene compositions, one of which contains the dispersed elastomer phase having the smaller particles and the other of which contains the dispersed elastomer of larger particle size. In another embodiment, styrene is inter-polymerized with two different elastomers to yield the two different particle size ranges in the dispersed phase.

Without contesting the associated advantages of the state of the art systems, it is nevertheless obvious that there is still a need for high impact monovinylidene aromatic polymers, in particular high impact styrene polymers, exhibitting improved mechanical properties.

The present invention aims to provide rubber-modified monovinylidene aromatic polymer compositions, such as high impact polystyrene (HIPS) compositions showing a unique combination of mechanical properties and thermal resistance.

The present invention aims to provide high impact polystyrene with improved Notched Izod impact strength at room temperature, which substantially remains unmodified after extruding at high temperature (≥<NUM>).

The present invention discloses a rubber-modified monovinylidene aromatic polymer composition comprising:.

Preferred embodiments of the present invention disclose on or more of the following features:.

The present invention further discloses a method for the preparation of the rubber-modified monovinylidene aromatic polymer composition comprising the steps of:.

Preferred embodiments of the method for the preparation of the rubber-modified monovinylidene aromatic polymer composition disclose one or more of the following features:.

The present invention additionally discloses a rubber-modified monovinylidene aromatic polymer composition characterized by:.

said characteristics applying to the monovinylidene aromatic polymer composition as obtained after the devolatilization step and on the devolatized monovinylidene aromatic polymer composition after three extrusions at a temperature of at least <NUM> up to <NUM>, in a twin-screw extruder with a LID value of <NUM>.

For the purpose of the invention the following definitions are given.

As used herein, a "polymer" is a polymeric compound prepared by polymerizing monomers, whether of the same or a different type. The generic term polymer thus embraces the term homopolymer, usually employed to refer to polymers prepared from only one type of monomer, and the terms copolymer and interpolymer as defined below.

As used herein, a "copolymer", "interpolymer" and like terms mean a polymer prepared by the polymerization of at least two different types of monomers. These generic terms include polymers prepared from two or more different types of monomers, i.e. terpolymers, tetrapolymers, etc..

As used herein, "blend", "polymer blend" and like terms refer to a composition of two or more compounds, typically two or more polymers. As used herein, "blend" and "polymer blend" also include "reactor blends," such as where a monomer is polymerized in the presence of a polymer. For example, the blend may initially be a blend of a first polymer and one or more monomers which are then polymerized to form a second polymer. A blend may or may not be miscible. A blend may or may not be phase separated. A blend may or may not contain one or more domain configurations, as determined from transmission electron spectroscopy, light scattering, x-ray scattering, or any other method known in the art. Preferred blends (i.e. preferred reactor blends) include two or more phases. For example, the blend may include a first phase including some or all of the monovinylidene aromatic polymer and a second phase including some or all of the rubber.

As used herein, "composition" and like terms mean a mixture or blend of two or more components. The composition of this invention is the rubber-modified monovinylidene aromatic polymer. The composition may include other components, polymeric or non-polymeric (i.e., additives), necessary or desirable to the end use of the composition.

The recitation of numerical ranges by endpoints includes all integer numbers and, where appropriate, fractions subsumed within that range (e.g. <NUM> to <NUM> can include <NUM>, <NUM>, <NUM>, <NUM> when referring to, for example, a number of elements, and can also include <NUM>, <NUM>, <NUM> and <NUM>, when referring to, for example, measurements). The recitation of end points also includes the recited end point values themselves (e.g. from <NUM> to <NUM> includes both <NUM> and <NUM>).

The present invention relates to high impact monovinylidene aromatic polymers comprising a monovinylidene aromatic polymer matrix comprising rubber particles, said rubber particles comprising a blend of at least two polybutadienes as well as graft- and block copolymers of polybutadiene and monovinylidene aromatic polymer segments, said rubber particles exhibiting a particle size distribution by volume within the range of from <NUM> and <NUM>.

Monovinylidene aromatic polymers suitable for use as the matrix in the composition of the present invention are those produced by polymerizing a monovinylidene aromatic monomer. Monovinylidene aromatic monomers include, monomers of the formula:.

wherein R is hydrogen or methyl, Ar is an aromatic ring structure having from <NUM> to <NUM> aromatic rings with or without alkyl, halo, or haloalkyl substitution, wherein any alkyl group contains <NUM> to <NUM> carbon atoms and haloalkyl refers to a halo substituted alkyl group.

Preferably, Ar is phenyl or alkylphenyl, wherein alkylphenyl refers to an alkyl substituted phenyl group, with phenyl being most preferred.

Typical vinyl aromatic monomers which can be used include styrene, alpha-methylstyrene, all isomers of vinyl toluene, especially paravinyltoluene, all isomers of ethyl styrene, propyl styrene, methyl-<NUM>-styrene, methyl-<NUM>-styrene, methoxy-<NUM>-styrene, hydroxymethyl-<NUM>-styrene, ethyl-<NUM>-styrene, ethoxy-<NUM>-styrene, dimethyl-<NUM>,<NUM>-styrene, chloro-<NUM>-styrene, chloro-<NUM>-styrene, chloro-<NUM>- methyl-<NUM>-styrene, tert-butyl-<NUM>-styrene, dichloro-<NUM>,<NUM>-styrene, dichloro-<NUM>,<NUM>-styrene, vinyl biphenyl, vinyl naphthalene, vinyl anthracene and the like, and mixtures thereof. It would not depart from the scope of the invention to use more than one monovinylidene aromatic monomer. Preferably, the monovinylidene aromatic monomer includes or consists of styrene.

The monovinylidene aromatic polymer is the monovinylidene aromatic polymer matrix in the rubber-modified monovinylidene aromatic polymer. The concentration of the monovinylidene aromatic monomers (i.e. the concentration of styrene) preferably is <NUM> % by weight or more, more preferably <NUM> % by weight or more, even more preferably <NUM> % by weight or more, even more preferably <NUM> % by weight or more, even more preferably <NUM> % by weight or more, based on the total weight of the rubber-modified monovinylidene aromatic polymer.

The monovinylidene aromatic monomers may also be combined with other copolymerizable monomers. Examples of such monomers include, but are not limited to, acrylic monomers such as acrylonitrile, methacrylonitrile, (meth)acrylic acid, C1-C18 alkyl(meth)acrylate; fumaronitrile; methyl methacrylate or n-butyl acrylate; maleimide, phenylmaleimide, maleic anhydride and/or n-aryl maleimides such as n-phenyl maleimide, and conjugated and nonconjugated dienes and alkyl esters of acrylic or methacrylic acid. Representative copolymers include styreneacrylonitrile (SAN) copolymers.

The polymerization of the monovinylidene aromatic monomer is conducted in the presence of pre-dissolved elastomer to prepare impact modified, or grafted rubber containing products.

In a preferred embodiment, the rubber-modified monovinylidene aromatic polymer is a rubber-modified polystyrene (HIPS) or a rubber-modified poly(styrene-acrylonitrile) (ABS). More preferably, the rubber-modified monovinylidene aromatic polymer is a rubber-modified polystyrene (HIPS).

The molecular weight of the monovinylidene aromatic polymer may be characterized by the weight average molecular weight (Mw), and a dispersity (DM = Mw/Mn).

The molecular weight of the monovinylidene aromatic polymer influences its mechanical strength and its rheological properties. In the invention, the molecular weight should be sufficiently high so that the composition has good impact strength, despite having a low concentration of the rubber (i.e. at most <NUM> % by weight based on the total weight of the rubber-modified monovinylidene aromatic polymer) and/or a generally high concentration of monovinylidene aromatic polymer (i.e. at least <NUM> % by weight based on the total weight of the rubber-modified monovinylidene aromatic).

The monovinylidene aromatic polymer of the present invention is characterized by a weight average molecular weight (Mw) comprised between <NUM>,<NUM> and <NUM>,<NUM>/mol, preferably between <NUM>,<NUM> and <NUM>,<NUM>/mol, more preferably between <NUM>,<NUM> and <NUM>,<NUM>/mol and a dispersity (DM = Mw/Mn) comprised between <NUM> and <NUM>, preferably between <NUM> and <NUM>.

The rubber present in the composition of the present invention is in the form of discrete, dispersed rubber particles comprising a blend of:.

The rubber particle comprising a blend present in the composition of the present invention comprises.

The polybutadiene blend comprising one or more polybutadiene(s) (A) and one or more polybutadiene(s) (B), used as starting material for the preparation of the rubber-modified monovinylidene aromatic (co)polymer and prior to being incorporated in the rubber-modified monovinylidene aromatic (co)polymer, is characterized by a dynamic solution viscosity, as determined by Brookfield viscometer according to ISO <NUM> at a concentration of <NUM>% by weight in toluene and at <NUM> , of from <NUM> to <NUM> mPa. s, preferably of from <NUM> to <NUM> mPa. s, more preferably of from <NUM> to <NUM> mPa.

The polybutadiene blend comprising one or more polybutadiene(s) (A) and one or more polybutadiene(s) (B) is present in the monovinylidene aromatic polymer matrix at a concentration of from <NUM> to <NUM>% by weight, preferably of from <NUM> to <NUM>% by weight, more preferably of from <NUM> to <NUM>% by weight, most preferably of from <NUM> to <NUM>% by weight based on the total weight of the composition comprising the monovinylidene aromatic polymer matrix and the polybutadiene blend.

The polybutadiene blend, present in the monovinylidene aromatic polymer matrix is present in the form of discrete, dispersed rubber particles characterized by a particle size distribution by volume within the range of from <NUM> and <NUM>, preferably of from <NUM> and <NUM>, more preferably of from <NUM> and <NUM>, most preferably of from <NUM> to <NUM>.

The dispersed rubber particles are characterized by:.

Preferably the particle size distribution is monomodal.

Particle size distribution is determined by the laser light scattering granulometry technique using the particle size analyzer (HORIBA <NUM>) from (Horiba Scientific). This technique is used to characterize rubber particle size distribution in high impact polystyrene (HIPS) since more than <NUM> years (<NPL>)).

The technique of laser diffraction is based on the principle that particles passing through a laser beam will scatter light at an angle that is directly related to their size: large particles scatter at low angles, whereas small particles scatter at high angles. The laser diffraction is accurately described by the Fraunhofer approximation and the Mie theory, with the assumption of spherical particle morphology.

Concentrated suspensions, comprising about <NUM>% by weight of rubber-modified monovinylidene aromatic polymer are prepared, using suitable wetting and/or dispersing agents.

Suitable solvents are for example water or organic solvents such as for example ethanol, isopropanol, octane or methyl ethyl ketone. A sample presentation system ensures that the material under test passes through the laser beam as a homogeneous stream of particles in a known, reproducible state of dispersion.

Particle size measurements are performed on pure solvent, e.g. <NUM> of methyl ethyl ketone, to which the concentrated suspension of polybutadiene particles is added drop by drop until the concentration of rubber particles is such that a transmission, as displayed by the particle size analyzer (HORIBA <NUM>), comprised between <NUM> and <NUM> % is obtained.

The compositions of the invention can further comprise one or more fillers and/or additives as long as they do not detrimentally affect the desired property combinations that are otherwise obtained, or, preferably, they would improve one or more of the properties.

For example, in an embodiment, the compositions of the present invention further may comprise one or more flame retarding agents such as decabromodiphenyl oxide, decabromodiphenyl ethane, <NUM>,<NUM>-bis(tri-bromophenoxy) ethane, ethylene-<NUM>,<NUM>-bis(pentabromophenyl), tris(tribromophenoxy)triazine, deca bromodiphenyl amine, decabromodiphenyl oxide, pentabromobenzyl acrylate, tetra bromobisphenol A, N,N'-bis(tetrabromophthalimide), N,N'-ethylenebis(tetra bromo phthalimide), pentabromo benzyl acrylate, brominated polystyrene, and brominated epoxy oligomers and polymers; one or more inorganic flame retardant synergists such as iron oxide, tin oxide, zinc oxide, aluminum trioxide, alumina, antimony tri- and pentoxide, and boron compounds, antimony silicates and ferrocene; one or more anti-dripping agent such as polytetrafluoroethylene; and conventional ingredients, such as fillers, pigments, colorants, UV stabilizers, heat stabilizers, lubricants, antioxidants (i.e., hindered phenols such as, for example, IRGANOX™<NUM> ) plasticizers, mould release agents, processing aids other than mineral oil (such as other oils, organic acids such as stearic acid, metal salts of organic acids).

The concentration of each of the conventional additives is typically in the range up to <NUM>% by weight, and more preferably up to <NUM>% by weight, of the total weight of the rubber modified monovinylidene aromatic polymer formulation.

The compositions of this invention can comprise polymers other than the monovinylaromatic polymers and the rubber. Representative other polymers include, but are not limited to ethylene polymer (i.e. low density polyethylene (LDPE), ultra-low density polyethylene (ULDPE), medium density polyethylene (MDPE), linear low density polyethylene (LLDPE), high density polyethylene (HDPE), homogeneously branched linear ethylene polymer, substantially linear ethylene polymer, graft-modified ethylene polymers, ethylene vinyl acetate interpolymer, ethylene acrylic acid interpolymer, ethylene ethyl acetate interpolymer, ethylene methacrylic acid interpolymer, ethylene methacrylic acid ionomer, and the like), conventional polypropylene (i.e. homopolymer polypropylene, polypropylene copolymer, random block polypropylene interpolymer and the like), polyether block copolymer (i.e. PEBAX), polyphenylene ether, (co)polyester polymer, polyester/polyether block polymer (i.e. HYTEL), ethylene carbon monoxide interpolymer (i.e., ethylene/carbon monoxide (ECO), copolymer, ethylene/acrylic acid/carbon monoxide (EAACO) terpolymer, ethylene/methacrylic acid/carbon monoxide (EMAACO) terpolymer, ethylene/vinyl acetate/carbon monoxide (EVACO) terpolymer and styrene/carbon monoxide (SCO), polyethylene terephthalate (PET), chlorinated polyethylene, styrene-butadiene-styrene (SBS) interpolymer, styrene-ethylene-butadiene-styrene (SEBS) interpolymer, and the like and mixtures of two or more of these other polymers. The polyolefins that can comprise one or more of the other polymers include both high and low molecular weight polyolefins, and saturated and unsaturated polyolefins.

If the composition comprises one or more other polymers, then the other polymers typically are present in an amount of no more than <NUM>% by weight of the total weight of the composition, preferably no more than <NUM>% by weight, ore preferably no more than <NUM> by weight, more preferably no more than <NUM> % by weight, and most preferably no more than <NUM>% by weight of the total weight of the composition.

Another subject of the invention relates to a process for the manufacture of the polymer compositions described above.

The polymerization stage may be a suspension or bulk polymerization process, the principle of these two techniques being well known to a person skilled in the art.

Preferably the polymerization stage is a bulk polymerization process.

In the bulk polymerization process, the polybutadienes are first ground and dissolved in the at least one monovinylidene aromatic monomer, optionally in the presence of an organic solvent. The polymerization will be generally conducted between <NUM> and <NUM> and preferably between <NUM> and <NUM>, optionally in the presence of one or more polymerization initiator(s) and optionally one or more transfer agent(s). During this polymerization stage the vinylaromatic monomer is polymerized either by itself or with a proportion of the polybutadienes and in this latter case grafting is said to take place between the polybutadienes and the vinylidene aromatic monomer and, in addition, the polybutadienes are partially crosslinked.

Suitable polymerization initiators are chemical initiators including peroxide initiators such as peresters, e.g. tertiary butyl peroxybenzoate and tertiary butyl peroxyacetate, tertiary butyl peroxyoctoate, dibenzoyl peroxide, dilauroyl peroxide, <NUM>-bis tertiarybutyl peroxycyclohexane, <NUM>-<NUM>-bis tertiarybutylperoxy-<NUM>,<NUM>,<NUM>-trimethyl cyclohexane, ethyl <NUM>,<NUM> di- t-buty peroxy butyrate, dicumyl peroxide, and the like. Photochemical initiation techniques can be employed if desired. Preferred initiators include tertiary butyl peroctoate, tertiary butyl isopropyl percarbonate, dibenzoyl peroxide, tertiary butyl peroxybenzoate, <NUM>,<NUM>-bistertiarybutylperoxycyclo hexane and tertiarybutylperoxy acetate.

Preferably chemical initiators are used in an amount of from <NUM> to <NUM> ppm, more preferably of from <NUM> to <NUM> ppm, most preferably of from <NUM> to <NUM> ppm based on the total weight of monovinylidene aromatic monomer(s) and polybutadienes.

Organic solvents, optionally used in the bulk polymerization process include aromatic and substituted aromatic hydrocarbons such as benzene, ethylbenzene, toluene, xylene or the like; substituted or unsubstituted, straight or branched chain saturated aliphatics of <NUM> or more carbon atoms, such as heptane, hexane, octane or the like; alicyclic or substituted alicyclic hydrocarbons having <NUM> or <NUM> carbon atoms, such as cyclohexane; and the like. Preferred solvents include substituted aromatics, with ethylbenzene and xylene being most preferred.

In general, the solvent is employed in amounts sufficient to improve the processability and heat transfer during polymerization. Such amounts will vary depending on the rubber, monomer and solvent employed, the process equipment and the desired degree of polymerization. If employed, the solvent is generally employed in an amount of up to <NUM> weight percent, preferably from <NUM> to <NUM> weight percent, based on the total weight of the solution.

Transfer agents include n-octyl mercaptan, t-octyl mercaptan, n-decyl mercaptan, n-dodecyl mercaptan, t-dodecyl mercaptan, tridecyl mercaptan, tetradecyl mercaptan, n-hexadecyl mercaptan, t-nonyl mercaptan, ethyl mercaptan, isopropyl mercaptan, t butyl mercaptan, cyclohexyl mercaptan, benzyl mercaptan and mixtures thereof.

The one or more transfer agent(s), if employed, is (are) generally added in an amount of up to <NUM> ppm, preferably between <NUM> and <NUM> ppm, more preferably between <NUM> and <NUM> ppm.

Preferably the polymerization is carried out in a continuous multi-reactor process, performed in a multiple series-connected stirred vessels with continuous flow, each stirred vessel having an optimum temperature range.

Preferably the polymerization is carried out in at least two connected stirred reactors.

An example of the polymerization process comprises sequences of:.

The first reactor is preceded by a mixing/dissolving vessel wherein a mixture/solution is prepared, comprising:.

Optionally one or more chemical intiator(s) and/or one or more transfer agent(s) can be added at any stage of the polymerization.

The composition with a monomer conversion comprised between <NUM> and <NUM>%, preferably between <NUM> and <NUM>%, as obtained in the last polymerization reactor, is transferred to a <NUM> steps devolatilizing system (DV1 and DV2), downstream of the last reactor, wherein devolatilizing, to remove the unreacted monomer and a solvent, if present, is carried out at a temperature comprised between <NUM> and <NUM>, preferably between <NUM> and <NUM>, at a pressure comprised between <NUM> and <NUM> mbar. abs, preferably between <NUM> and <NUM> mbar. abs, most preferably between <NUM> and <NUM> mbar. abs for DV1 and at a pressure comprised between <NUM> and <NUM> mbar. abs, preferably between <NUM> and <NUM> mbar. abs, most preferably between <NUM> and <NUM> mbar. abs for DV2.

Upon completion of the polymerization up to a monomer conversion comprised between <NUM> and <NUM>%, preferably between <NUM> and <NUM>%, and prior to transferring to the devolatilizer, one or more retarding agent(s) optionally is (are) added to the composition in an amount up to <NUM> ppm, preferably between <NUM> and <NUM> ppm, more preferably between <NUM> and <NUM> ppm, most preferably between <NUM> and <NUM> ppm, said retarding agent including a free radical scavenger, a polyfunctional (meth)acrylic monomer, an allylic compound, a metal salt of an unsaturated monocarboxylic acid, a tertiary amine oxide, an aromatic tertiary amine oxide and a tertiary amine.

High impact polystyrene (HIPS) according to the present invention and prepared according to the process of the present invention, is characterized by an improved Notched Izod impact strength, according to ISO <NUM>/1eA, when compared to commercial available grades of HIPS, with an identical polybutadiene concentration and prepared according to an identical polymerization process, said improved impact strength remaining substantially unchanged over a temperature range of from <NUM> to -<NUM>, contrary to the decrease in Notched Izod impact strength characteristic to the commercial available grades of HIPS.

The high impact polystyrene of the present invention further is characterized in that Notched Izod impact strength and other mechanical properties and thermal resistance are substantially maintained after successive extrusions.

The inventors have surprisingly found that incorporation of one or more flame retardants, one or more flame retardant synergists, one or more antioxidants and one or more lubricants has a negligible influence on the mechanical properties of the high impact polystyrene compared to the same high impact polystyrene not comprising said additives. In other words, contrary to the prior art high impact polystyrenes, the incorporation of said additives substantially does not modify/ deteriorate its mechanical properties.

The articles of the invention (i.e. made from the composition of the invention) are selected from films, fibers, sheet structures, moulded objects (such as appliance and automobile parts), hoses, refrigerator and other liners, clothing and footwear components, gaskets and the like. The articles are made by any forming and/or shaping process, i.e. extrusion, casting, injection moulding, blow moulding, thermoforming, etc.

The following illustrative examples are merely meant to exemplify the present invention but is not intended to limit or otherwise define the scope of the present invention.

Molecular weight: The molecular weight may be measured using gel permeation chromatography. Different solvents can be used, a typical solvent is tetrahydrofuran. Polystyrene standards may be used for calibration.

The melt index of the composition is measured according to ISO <NUM>. For polystyrene, the melt index (MI5) is measured according to ISO <NUM> conditions H at <NUM> under a load of <NUM>.

The glass transition temperature was determined by the method according to ISO <NUM>-<NUM>:<NUM>.

D10(v), D50(v), D90(v) µm: The volume average diameter of the rubber particles was measured by laser light scattering using the particle size analyser HORIBA <NUM> from Horiba Scientific. The samples were suspended in methyl ethyl ketone at a concentration of about <NUM> % by weight.

Notched Izod impact of the composition was determined according to ISO <NUM>/1eA.

Vicat Softening temperature B50 was measured according to ISO <NUM> at a heating rate of <NUM>/hour and under a load of <NUM> N.

Elongation at break was performed according to ISO <NUM>-<NUM>.

Young-modulus was determined according to ISO <NUM>-<NUM>.

Tensile strength at yield and at break was determined according to ISO <NUM>-<NUM>.

The Heat Deflection Temperature (HDT) was determined in accordance with ISO <NUM>-<NUM>/A conditions <NUM>, <NUM>, <NUM> MPa, annealed.

Into a reactor fitted with a mechanical stirrer and a temperature control were introduced <NUM> parts of styrene, <NUM> parts of a blend of two polybutadienes along with <NUM> ppm of ethyl <NUM>,<NUM> di-t-buty peroxy butyrate, wherein <NUM> parts of the polybutadiene blend consist of <NUM> parts of Budene® <NUM> from Goodyear (= polybutadiene (A) and <NUM> parts of Asaprene™ 730AX from Asahi_Kasei Corp. (= polybutadiene (B), said blend being characterized by a dynamic solution viscosity, as determined by Brookfield viscometer according to ISO <NUM> at a concentration of <NUM>% by weight in toluene, of <NUM> mPa.

The reactor while stirred at <NUM> rpm was heated to a temperature of <NUM> and maintained at that temperature for about <NUM> hours to obtain a monomer conversion of about <NUM> %; subsequently the temperature was increased to <NUM> and maintained at that temperature for about <NUM> hours to obtain a monomer conversion of about <NUM>%; thereafter the temperature was increased to <NUM> and maintained at that temperature for about <NUM> hour to obtain a monomer conversion of about <NUM>%; finally the temperature was increased to <NUM> and maintained at that temperature to obtain a monomer conversion rate of about <NUM> %.

To the composition thus obtained <NUM> ppm of Genox®EP (tertiary amineoxide) were added, <NUM> minutes before being transferred to a devolatizing unit, where unreacted monomers were removed at a temperature of <NUM> and a pressure of <NUM> mbar.

The rubber particles were characterized by a volume average particle size (D50) of <NUM>, a D90 of <NUM> and a span of <NUM>, as measured by laser light scattering.

The high impact polystyrene of example <NUM> was characterized by a Weight average Molecular weight (Mw) of <NUM>,<NUM>/mol and a dispersity (DM = Mw/Mn) of <NUM> wherein Mn is the number average molecular weight, as measured by Gel Permeation Chromatography, and a Melt Index, at <NUM> under a load of <NUM>, of <NUM>,<NUM>/<NUM>. and a glass transition temperature (DSC, <NUM>/min. ) of <NUM>.

The product obtained was granulated in a manner which is known to a person skilled in the art. Specimens were produced by injection moulding for carrying out the mechanical tests.

A high impact polystyrene was prepared, wherein <NUM> parts of styrene, <NUM> parts of a Buna® CB 550T from Arlanxeo, along with <NUM> ppm of ethyl <NUM>,<NUM> di- t-buty peroxy butyrate were reacted according the method as disclosed in example <NUM>.

The rubber particles were characterized by a monomodal particle size distribution with a volume average particle size by volume (D50) of <NUM>, a D90 of <NUM> and a span of <NUM>, measured by laser light scattering.

The high impact polystyrene of comparative example <NUM> was characterized by a Weight average Molecular weight (Mw) of <NUM>,<NUM>/mol and a dispersity (DM = Mw/Mn) of <NUM>, wherein Mn is the number average molecular weight, as measured by Gel Permeation Chromatography, a Melt Index, at <NUM> under a load of <NUM>, of <NUM>,<NUM>/<NUM>. and a glass transition temperature (DSC, <NUM>/min. ) of <NUM>.

The HIPS of example <NUM> and the comparative example <NUM> were subjected to the Notched Izod Impact test, according to ISO <NUM>/1eA, at a temperature of +<NUM>, <NUM>, -<NUM> and -<NUM> respectively. The values, in kJ/m<NUM> are represented in table <NUM>.

In order to verify thermal stability, upon extrusion, the Notched Izod Impact strength was measured at a temperature of +<NUM> and -<NUM> respectively for HIPS extruded at <NUM> and <NUM> in a twin-screw extruder, wherein extrusion was performed three times in succession. The values for example <NUM> and comparative example are given in table <NUM> and <NUM> respectively.

Other characteristics generally reported for high Impact polystyrene, such as tensile strength at yield (MPa) and tensile strength at break (MPa) (ISO <NUM>-<NUM>), elongation at break (%) (ISO <NUM>-<NUM>), Young modulus (MPa) (ISO <NUM>-<NUM>), Heat Deflection Temperature (°C) (ISO <NUM>-<NUM>/A) and Vicat softening @ 50N (°C) (ISO <NUM>), are substantially comparable, within the experimental error, for example <NUM> and comparative example <NUM>, measured on the high impact polystyrenes as coming out of the devolatizer (DV1 + DV2) and after <NUM> extrusions in a twin screw extruder (L/D=<NUM>), at a temperature of <NUM> up to <NUM>.

The inventors have surprisingly observed that less Volatile Organic Compounds are detected for HIPS of Example <NUM> relative to HIPS of Comparative Example <NUM>, as coming from the devolatizer and as extruded at <NUM> and <NUM> in a twin-screw extruder, wherein extrusion was performed three times in succession.

Values, in ppm, are reported in table <NUM>.

Volatile organic Compounds are measured by Gas Chromatography - Flame Ionisation Detection (GC-FID). The rubber-modified polystyrene is solubilized in dichloromethane, and subsequently precipitated through the addition of ethanol comprising n-butylbenzene and n-hexadecane as internal standard. The solution is injected in the GC-FID; the different components are identified based on their retention time, and are quantified by referring to the internal standard.

Claim 1:
A rubber-modified monovinylidene aromatic polymer composition comprising:
I) a matrix comprising monovinylidene aromatic polymer, and
II) from <NUM> to <NUM>% by weight of rubber in the form of discrete rubber particles dispersed within the matrix, wherein the rubber particles comprise a blend of at least two polybutadienes as well as graft- and block copolymers of polybutadiene and monovinylidene aromatic polymer segments,
said rubber particles exhibit:
- an average particle size by volume (D50) comprised between <NUM> and <NUM>, as measured by laser light scattering;
wherein said blend of at least two polybutadienes comprises:
- at least <NUM> % by weight of one or more polybutadiene(s) with a cis-<NUM>,<NUM> structure content of at least <NUM>% by weight and
- at most <NUM>% by weight of one or more polybutadiene(s) with a trans-<NUM>-<NUM> structure content of at least <NUM>% by weight and a <NUM>,<NUM>-vinyl content of at least <NUM>% by weight;
said blend of at least two polybutadienes, as such and prior to being part of the rubber-modified monovinylidene aromatic (co)polymer, being characterized by a dynamic solution viscosity, at <NUM>, comprised between <NUM> and <NUM> mPa.s, as determined by Brookfield viscometer according to ISO <NUM>, at a concentration of <NUM>% by weight in toluene.