Improved Expandable Vinyl Aromatic Polymers

Abstract

The present invention is related to expandable vinyl aromatic polymers comprising comminuted coke, said comminuted coke having an average stack height (Lc) of carbon crystallites at least 4 nm, a volume median particle diameter (D50) comprised between 1 and less than 5 m and being characterized by a span (D90D10)/D50 below 2.5 Molded parts produced from the expandable vinyl aromatic polymers prove low thermal conductivities for a low foam density.

Claims

1.-13. (canceled)

14. Expandable vinyl aromatic polymers comprising at least one blowing agent and a plurality of dispersed coke particles, said dispersed coke particles being characterized by: an average stack height (Lc) of carbon crystallites of at least 4 nm; wherein Lc is given by the formula $L_c\left(\mathrm{nm}\right)=\frac{0.89*0.154}{*\cos \left(\right)}$ and is estimated from the full width at half maximum of diffraction peak [002]; a volume median particle diameter (D50) comprised between 1 and less than 5 m, as obtained from laser light scattering measurements according to ISO 13320, and a span (D90D10)/D50 below 2.5 as obtained from laser light scattering measurements according to ISO 13320.

15. The expandable vinyl aromatic polymers according to claim 14 wherein the dispersed coke particles are characterized in that the volume percentage of particles with a diameter of less than 1 m is lower than (3613 ln D50) (36 minus 13 times the natural logarithm of D50).

16. The expandable vinyl aromatic polymers according to claim 14 wherein the dispersed coke particles are characterized in that the volume percentage of particles with a diameter of less than 1 m is lower than (3212 ln D50) (36 minus 12 times the natural logarithm of D50).

17. The expandable vinyl aromatic polymers according to claim 14 comprising between 3 and 8% by weight of coke.

18. The expandable vinyl aromatic polymers according to claim 14 comprising between 2 and 10% by weight of a C3-C6 alkane.

19. The expandable vinyl aromatic polymers according to claim 14 additionally comprising: between 0.1 to 5% by weight, preferably between 0.2 and 3.0% by weight, of an halogenated polymer; between 0.05 and 5% by weight of a flame retardant synergist; between 0.01 and 1.0% by weight, more preferably between 0.05 and 0.50% by weight of polyethylene wax, as cell regulator, characterized by a weight average molecular weight comprised between 1500 and 5000 Da.

20. A process for the preparation of beads or granules of expandable vinyl aromatic polymers comprising the steps of: a. producing a polymer melt stream of expandable vinyl aromatic polymers comprising at least one blowing agent and a plurality of dispersed coke particles, said dispersed coke particles being characterized by: an average stack height (Lc) of carbon crystallites of at least 4 nm; wherein Lc is given by the formula $L_c\left(\mathrm{nm}\right)=\frac{0.89*0.154}{*\cos \left(\right)}$ and is estimated from the full width at half maximum of diffraction peak [002]; a volume median particle diameter (D50) comprised between 1 and less than 5 m, as obtained from laser light scattering measurements according to ISO 13320, and a span (D90D10)/D50 below 2.5 as obtained from laser light scattering measurements according to ISO 13320; b. deriving a part of said polymer stream and creating main polymer melt stream (1) and a side loop with an additional polymer melt stream (2); c. dispersing the coke particles and the foam cell regulator into said additional polymer melt stream (2); d. joining the additional polymer stream (2) and the main stream (1) and forming a new polymer melt stream; e. introducing a blowing agent into the new polymer melt stream; f. cooling the polymer melt comprising all ingredients down to a temperature of 200 C. or less; g. discharging the melt stream through a die plate with holes and pelletizing the melt under water with a pressure above 3 bar, preferably above 5 bar.

21. The process according to claim 20 comprising the additional step of introducing flame retardant agent and synergist into the new polymer melt stream between step f) and step g).

22. The process according to claim 20, wherein between 5 and 30% of the polymer stream is derived in step b) to form the additional polymer stream (2).

23. The process according to claim 20, wherein in step c), the coke particles and the foam cell regulator are dispersed in the additional polymer stream (2) by means of an extruder.

24. The process according to claim 20, wherein the dispersion in step c) is performed in the polymer melt at a temperature comprised between 180 and 250 C.

25. The process according to claim 20, wherein in step g) one or more thermal stabilizer(s) and anti-acid(s) are added.

26. Polymer foams obtained from the molding of expanded vinyl aromatic polymers comprising at least one blowing agent and a plurality of dispersed coke particles, said dispersed coke particles being characterized by: an average stack height (Lc) of carbon crystallites of at least 4 nm; wherein Lc is given by the formula $L_c\left(\mathrm{nm}\right)=\frac{0.89*0.154}{*\cos \left(\right)}$ and is estimated from the full width at half maximum of diffraction peak [002]; a volume median particle diameter (D50) comprised between 1 and less than 5 m, as obtained from laser light scattering measurements according to ISO 13320, and a span (D90D10)/D50 below 2.5 as obtained from laser light scattering measurements according to ISO 13320; said foams being characterized by a thermal conductivity (k), measured according to ISO 8301, of: 32.5 mW/m.Math.K or less at a foam density of 12.5 kg/m.sup.3 or lower; or 31 mW/m.Math.K or less at a foam density of 15 kg/m.sup.3 or lower; or 31 mW/m.Math.K or less at a foam density of 18 kg/m.sup.3 or lower.

Description

DRAWINGS

[0022] FIG. 1 represents a flow-sheet for the production of expandable vinyl aromatic polymer wherein: [0023] (A) is the polymerization reactor producing a polymer stream; [0024] (B) is the branching point where part of the polymer stream is derived creating a polymer side stream (2) main polymer stream (1); [0025] (C) is the mixing unit, preferably an extruder, where comminuted coke particles and foam cell regulator, preferably polyethylene wax are dispersed in the derived polymer side stream (2); [0026] (D) is the merging point where both polymer streams (1, 2) join through a static mixer forming a new polymer stream; [0027] (E) is the unit for the addition of blowing agent, preferably n-pentane and/or isopentane, to the new polymer stream; [0028] (F) is the extruder where flame retardant agent and synergist are blended, optionally with vinyl aromatic polymer, before being fed into the new polymer stream through (G) to form the expandable vinyl aromatic polymer melt; [0029] (H) is the under-water pelletizing unit; [0030] (I) is the drying unit; [0031] (J) is the packaging unit.

SUMMARY OF THE INVENTION

[0032] The present invention discloses expandable vinyl aromatic polymers comprising at least one blowing agent and homogeneously dispersed coke particles, said coke particles being characterized by an average stack height (Lc) of carbon crystallites of at least 4 nm and a volume median particle diameter (D50) comprised between 1 and less than 5 m, preferably between 1.5 and 4.5 m and a span (D90D10)/D50 below 2.5, preferably below 2.3, more preferably below 2.2, as obtained from laser light scattering measurements according to ISO 13320.

[0033] Preferred embodiments of the present invention disclose one or more of the following features: [0034] the coke particles are characterized by a volume percentage of particles with a diameter of less than 1 m of lower than (36-13 In D50); [0035] the coke particles are characterized by a volume percentage of particles with a diameter of less than 1 m of lower than (32-12 In D50); [0036] the expandable vinyl aromatic polymers comprise between 3 and 8% by weight, preferably 4 and 6% by weight of coke; [0037] the expandable vinyl aromatic polymers comprise between 2 and 10% by weight, preferably between 3 and 7% by weight of a C3-C6 alkane, preferably a C4-C5 alkane blowing agent. [0038] the expandable vinyl aromatic polymers additionally comprise: [0039] between 0.1 to 5% by weight, preferably between 0.2 and 3.0% by weight of an halogenated polymer, preferably a brominated styrene butadiene block copolymer with a bromine content comprised between 40 and 80% by weight; [0040] between 0.05 and 5% by weight, preferably between 0.1 and 3.0% by weight, more preferably between 0.1 and 1.0% by weight of flame retardant synergist, preferably a thermal free radical generator of the type comprising a CC or a COOC thermo-labile bond; and [0041] between 0.01 and 1.0, preferably between 0.05 and 0.5% by weight, more preferably between 0.08 and 0.40% by weight of polyethylene wax, as cell regulator, characterized by a weight average molecular weight comprised between 1500 and 5000 Da.

[0042] The present invention further discloses a method for the preparation of beads or granules of the expandable vinyl aromatic polymer, comprising the steps of: [0043] a) producing a polymer melt stream after the polymerization process; [0044] b) deriving a part of said polymer melt stream and creating a main polymer melt stream (1) and an additional polymer melt stream (2); [0045] c) incorporating the coke particles and the foam cell regulator into the additional polymer melt stream (2); [0046] d) merging the main (1) and the additional (2) polymer melt streams, forming a new polymer melt stream; [0047] e) introducing a blowing agent into the new polymer melt stream; [0048] f) cooling the vinyl aromatic polymer melt comprising all necessary ingredients to a temperature of 200 C. or less; [0049] g) discharging through a die plate with holes, the diameter of which at the exit from the die is comprised between 0.3 and 1.5 mm, preferably between 0.5 and 1.0 mm; and [0050] h) pelletizing the melt comprising blowing agent directly downstream of the die plate under water at a pressure above 3 bar, preferably above 5 bar.

[0051] Preferred embodiments of the process for the preparation of beads or granules of the expandable vinyl aromatic polymer disclose one or more of the following features: [0052] the method comprises an additional step of introducing flame retardant agent and synergist into the new polymer melt stream between step f) and step g). [0053] between 5 and 30% of the polymer stream is derived in step b) to form the additional polymer stream (2); [0054] in step c), the coke particles and the foam cell regulator are dispersed in the additional polymer stream (2) by means of an extruder; [0055] the dispersion in step c) is performed in the polymer melt at a temperature comprised between 180 and 250 C., preferably between 200 and 240 C., more preferably between 210 and 230 C.; [0056] in the additional step one or more thermal stabilizer(s) and anti-acids are added.

[0057] The present invention additionally discloses polymer foams obtained from the molding of the expanded vinyl aromatic polymers, said foams being characterized by a thermal conductivity, (A) measured according to ISO 8301, of: [0058] 32.5 mW/m.Math.K or less, preferably of 31 mW/m.Math.K or less at a foam density of 12.5 kg/m.sup.3 or lower; or [0059] 31 mW/m.Math.K or less, preferably of 30.5 mW/m.Math.K or less at a foam density of 15 kg/m.sup.3 or lower; or [0060] 31 mW/m.Math.K or less, preferably of 30 mW/m.Math.K or less at a foam density of 18 kg/m.sup.3 or lower.

DETAILED DESCRIPTION OF THE INVENTION

[0061] It is an object of the present invention to provide expandable vinyl aromatic polymers, in particular styrene polymers containing homogeneously dispersed coke particles which can be processed to expanded foams which have both a low density and a low thermal conductivity.

[0062] We have found that this object is achieved by expandable vinyl aromatic polymers, in particular styrene polymers, comprising homogeneously dispersed coke particles characterized by an average stack height (Lc) of carbon crystallites higher than 4 nm, a volume median particle diameter (D50) comprised between 1 and less than 5 m and a span (D90D10)/D50 lower than 2.5.

[0063] By homogeneously dispersed coke particles, the present invention means that particles with an equivalent diameter of more than 40 m are observed by less than 1%, preferably by less than 0.5%, more preferably by less than 0.1% in a method analogous to those described in ISO 18553.

[0064] The homogeneity of the dispersion of the athermanous particle is quantified on compression-moulded film with a thickness comprised between 10 and 30 m, preferably of about 20 m. The film is obtained after melting a few EPS beads at 200 C. to allow release of blowing agent, applying pressure for 15 minutes and cooling to 35 C. under pressure. The film is examined in transmission with an optical microscope (Nikon LV100) using a 20 lens. The dispersed black particles are counted and measured (area and perimeter) with image analyser NIS Element AR. The percentage of area covered by particles with equivalent diameter above 40 m is calculated on the basis of 10 different image fields (10290000 pmt or 2.9 mm.sup.2). The criterion for an inhomogeneous dispersion is that the percentage area of particles with an equivalent diameter above 40 m is more than 1%.

D.sub.eq=(4A/).sup.0.5, wherein A is the projected area of the particle.

[0065] The expandable vinyl aromatic polymers of the present invention can be processed to foams with a density of 18 kg/m.sup.3 or less, preferably of 16 kg/m.sup.3 or less which are characterized by a thermal conductivity of 32 mW/m.Math.K or less in accordance to ISO 8301 and self-extinguishing properties, for the case that a suitable flame retardant additive package is present, to pass the burning test B2 (in accordance with DIN 4102).

[0066] Expandable vinyl aromatic polymers, in particular styrene polymers, are vinyl aromatic polymers comprising blowing agent, preferably n-pentane and/or isopentane. The size of the expandable polymer beads is preferably in the range from 0.2 to 2 mm, preferably from 1 to 1.5 mm. Molded polymer foams can be obtained via prefoaming and sintering of the appropriate expandable vinyl aromatic polymer beads, in particular of the styrene polymer beads.

[0067] The vinyl aromatic polymers preferably used in the present invention comprise general-purpose (also known as glass-clear) polystyrene (GPPS), high impact polystyrene (HIPS), anionically polymerized polystyrene, styrene-alpha-methylstyrene copolymers, acrylonitrile-butadiene-styrene polymers (ABS), styrene-acrylonitrile polymer (SAN), acrylonitrile-styrene-acrylate (ASA), styrene acrylates, such as styrene-methyl acrylate and styrene-methyl methacrylate (SMMA), styrene maleic anhydride (SMA), methyl methacrylate-butadiene-styrene (MBS), methyl methacrylate-acrylonitrile-butadiene-styrene (MABS) polymers, styrene-N-phenylmaleimide copolymers (SPMI) or a mixture thereof.

[0068] The weight average molecular weight of the expandable vinyl aromatic polymers, in particular styrene polymers, of the present invention is preferably in the range from 100 kDa to 400 kDa, particularly preferably in the range from 150 kDa to 300 kDa, measured by means of gel permeation chromatography against polystyrene standards. The molar mass of the expandable vinyl aromatic polymers, after the extrusion processes is generally below the molar mass of the vinyl aromatic polymers, before the extrusion process, because of the degradation caused by shear and/or by heat. The molar mass difference due to extrusion can be up to 10 kDa.

[0069] The above-mentioned vinyl aromatic polymers can be blended with thermoplastic polymers, such as polyamides (PA), polyolefins, e.g. polypropylene (PP) or polyethylene (PE), polyacrylates, e.g. polymethyl methacrylate (PMMA), polycarbonate (PC), polyesters, e.g. polyethylene terephthalate (PET) or polybutylene terephthalate (PBT), polyphenylene ethers (PPE), polyether sulfones (PES), polyether ketones, or polyether sulfides (PES), or a mixture thereof, generally in total proportions of up to at most 30% by weight, preferably in the range from 1 to 10% by weight, based on the polymer melt, in order to improve mechanical properties or heat resistance, optionally with use of compatibilizers. Mixtures within the above-mentioned ranges of amounts are also possible with, for example, hydrophobically modified or functionalized polymers or oligomers, rubbers, e.g. polyacrylates or polydienes, for example styrene-butadiene block copolymers, or biodegradable aliphatic or aliphatic/aromatic copolyesters.

[0070] The coke to be used with the expandable vinyl aromatic polymers of the present invention is obtained from grinding coke, preferably petroleum coke. The petroleum coke used in the present invention is a residue of petroleum distillation and is produced in so-called crackers. The petroleum coke is liberated from the volatile components through calcination, as a result of which a carbon with a degree of purity of about 99% is obtained. Therefore, coke may be regarded as a carbon, but is not included in the allotropic forms. Calcined petroleum coke is neither graphite nor can it be included in the amorphous carbons, like carbon black.

[0071] Grinding is preferably performed in a delamination mill, such as for example an air jet mill and preferably a spiral flow mill, in such a way that a particle size distribution (before incorporation in the expandable vinyl aromatic polymer), as determined by the laser light scattering granulometry technique and calculated using the Fraunhofer/Mie model, with a volume median particle diameter (D50) comprised between 1 and 7 m, preferably between 1 and 5 m is obtained.

[0072] 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.

[0073] Concentrated suspensions, comprising about 1.0% by weight of carbon based particles, are prepared using suitable wetting and/or dispersing agents.

[0074] 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.

[0075] In the present invention, the particle size distribution was measured by laser light scattering using the particle size analyzer (HORIBA 920) from (Horiba Scientific) according to ISO 13320. The samples of expandable vinyl aromatic polymers with coke as filler are dissolved in methyl ethyl ketone at a concentration of about 1% weight, without the use of ultrasonication. This technique is used to characterize rubber particle size distribution in high impact polystyrene (HIPS) since more than 30 years (R. A. Hall, R. D. Hites, and P. Plantz, Characterization of rubber particle size distribution of high-impact polystyrene using low-angle laser light scattering, J. Appl. Polym. Sci. 27, 2885, (1982)).

[0076] Particle size measurements are performed on pure solvent, e.g. 150 ml of methyl ethyl ketone, to which either the concentrated suspension of carbon based particles or the solution of expandable vinyl aromatic polymer comprising carbon based particles is added drop by drop until the concentration of carbon based particles is such that a transmission, as displayed by the particle size analyzer (HORIBA 920), comprised between 75 and 90% is obtained.

[0077] The petroleum coke to be used in the present invention is selected from a coke being characterized by a particle size distribution which is monomodal or polymodal and by a significant graphitization degree, as revealed by an average stack height (L.sub.c) of coke carbon crystallites of at least 4 nm.

[0078] It was demonstrated (WO 2014102137) that extrusion conditions of the vinyl aromatic polymer comprising the comminuted coke and optionally other components used for the production of expandable vinyl aromatic polymers are the key for the thermal conductivity reduction capabilities of the incorporated comminuted coke particles.

[0079] It has been observed that particular soft dispersion conditions (dispersion into the polymer matrix, through addition into the molten polymer stream) to a large extent improves the delamination of the comminuted coke, while further fragmentation is hardly observed.

[0080] These soft extrusion conditions ensure that the absolute value of the difference of the span (D90D10)/D50 of the coke particles as received from the supplier and the span of the coke particles incorporated into the foam is low, preferably less than 1.5, more preferably less than 1.3, most preferably less than 1. Deviation from these specific extrusion conditions (extrusion of the polymercoke mix wherein the polymer is in a non-molten state under harsh shear stress conditions) causes mostly fragmentation of the comminuted coke particles resulting in a sizeable increase of the coke fraction with a diameter of less than 1 m, meanwhile delamination is observed to a lesser extent. On the other hand, too mild extrusion conditions can lead to a non-homogeneous coke dispersion in EPS with agglomerates being formed. Those coke agglomerates (D.sub.eq>40 m) are readily quantified by image analysis on thin compression molded films (20 m thick) observed by optical microscopy in a fashion similar to that described for carbon black dispersion in ISO 18553.

[0081] Using OCS (Optical Control System), it has been demonstrated that there is less gel-formation in the vinyl aromatic polymer, when prepared according to proper extrusion conditions. These gels, in general having a diameter of 100 m or more, result from the agglomeration of the small coke particles. Contrary to the small coke particles, these agglomerates reflect less infrared irradiation, thus negatively influencing the insulation properties of the derived foam panels.

[0082] Further, it has been observed that the presence of gels has a negative influence on the proportion of closed cells in the foam, once more negatively influencing the insulation properties of the foam.

[0083] It has been demonstrated that improper dispersion conditions result in a gel-formation that is about ten times that of using proper dispersion conditions easily reached by dispersing the coke particles directly into the molten polymer matrix.

[0084] Thus, within the scope of the present invention, it has been observed that the dispersion of the comminuted coke particles directly into the molten polymer matrix generates so called proper dispersion conditions (for example extrusion conditions) of the vinyl aromatic polymer comprising the comminuted coke and optionally other components used for the production of expandable vinyl aromatic polymers.

[0085] The coke particles for being dispersed in the molten polymer are characterized by a volume median particle diameter (D50) comprised between 1 and 7 m, preferably between 1 and 5 m and a span (D90D10)/D50 lower than 3.0, preferably lower than 2.7, more preferably lower than 2.5, said characteristics being typical to the coke particles after grinding, i.e. as obtained from the supplier.

[0086] Proper dispersion conditions result in homogeneously dispersed coke particles as revealed by optical microscopy observations on compression molded films as alluded above, said coke particles being characterized by a monomodal or polymodal particle size distribution, with a volume median particle diameter (D50) comprised between 1 and less than 5 m, preferably between 1.5 and 4.5 m and a span lower than 2.5, preferably lower than 2.3, as obtained from laser light scattering measurements according to ISO 13320.

[0087] The inventors have further observed that coke particles, incorporated into the foam according to soft conditions, preferably are characterized in that the volume percentage of particles with a diameter of less than 1 m is lower than 3613 In D50 (36 minus 13 times the natural logarithm of D50) preferably lower than 3212 In D50.

[0088] For expandable foams comprising particles having a volume median particle diameter (D50) comprised between 1 and less than 5 m, preferably between 2 and 4.5 m, said criteria contributes to the thermal conductivity of the expanded foam, whereas values exceeding 3613 In D50 have a negative influence (increased thermal conductivity).

[0089] Improper dispersion conditions, for example where solid polymer and comminuted coke particles are mixed and molten via a master batch in an extruder and subsequently extruded in a second step on an extruder (often with a milder screw profile), result in a too high degree of fragmentation of the comminuted coke causing an increase of the volume percentage of particles with a diameter of less than 1 m.

[0090] For the particular case of expanded polystyrene comprising coke particles with a D50 value within the range of from 2 to 4 m, it has been observed that optimal reduced thermal conductivity values are obtained for a volume percentage of particles with a diameter of less than 3 m, of more than 40.

[0091] Foams with a density of less than 18 kg/m.sup.3 and even less than 16 kg/m.sup.3 obtained from the expansion and molding of expandable polystyrene (EPS) obtained from the above proper extrusion conditions are characterized by a thermal conductivity of less than 32 mW/m.Math.K and even less than 31 mW/m.Math.K.

[0092] Thermal conductivity is determined by ISO 8301 using a heat flow meter device, with a mean temperature of 10 C. and a temperature difference of 20 C. The samples are kept in an oven at 70 C. for 7 days before measuring the A-value.

[0093] The particle size distribution has been measured by laser light scattering using the particle size analyzer (HORIBA 920) from (Horiba Scientific) and calculated using the Fraunhofer/Mie model.

[0094] The measurement method is analogous to the protocol used for the determination of the apparent diameter of styrene-butadiene-styrene particles in high impact polystyrene (HIPS).

[0095] Concentrated suspensions of carbon-based particles are prepared in methyl ethyl ketone at a concentration of about 1% by weight, without the use of ultrasonication. The expandable vinyl aromatic polymer particles are dissolved in the solvent while the carbon-based particles are dispersed; the dissolution time is about 30 minutes.

[0096] Coke with a crystallite stacking height (Lc) of at least 4 nm (mostly of the needle coke variety), homogeneously dispersed in the continuous phase of the expandable vinyl aromatic polymer, preferably is characterized by a carbon crystallite stacking height (L.sub.c) comprised between 4 and 10 nm, a graphitization degree (g) comprised between 0.21 and 0.0, and an interlayer spacing (d.sub.002) comprised between 0.344 and 0.3458 nm.

[0097] The characterization of the carbon structure of coke is conducted on the basis of the analysis of powder X-ray diffraction (XRD) patterns. XRD patterns from coke powders inside a spinning probe holder are measured (intensity versus 2) using a diffractometer Bruker D8 Discover operating with a divergence slit having a fixed aperture of 0.3. The X-ray source emits Cu-K radiation with an acceleration voltage of 40 kV and a current of 40 mA. The performances of the diffractometer (proper diffracted angle and intensities) are controlled with NIST standards SRM660b (LaB6) and SRM 1976b (sintered alumina disc prepared from alumina powders with corundum structure) to insure that they remain within Bruker' specifications.

[0098] After the coke powders are filled into an annular sample holder (1-2 mm depth) resting on a glass slide, the powders are packed using a manual press and excess powders scraped off; then a cover lid is put on the top of the packed coke powders and the whole assembly (glass plate, ring with packed powder inside, cover lid) turned over, followed by removal of the glass slide revealing a smooth flat surface for subsequent X-ray diffractometer analysis.

[0099] Diffracted intensities are recorded, starting at 2 10 and ending at 85. A step-scan procedure is performed with 0.02 step size and an acquisition time of 1 s at each angle. This acquisition procedure is very close to those used by F. R. Feret and Iwashita et al. The size of carbon crystallites (average stack height, Lc) is estimated from FWHM (full width at half maximum) of diffraction peak [002] using Scherrer's formula. The software DIFFRAC.EVA (XRD software DIFFRAC.SUITE from Bruker) is used to determine the angular full width at half maximum (FWHM) in radians.

[00001]$L_c\left(\mathrm{nm}\right)=\frac{0.89*0.154}{*\cos \left(\right)}$ [0100] F. R. Feret, Determination of the crystallinity of calcinated and graphitic cokes by X-ray diffraction, Analyst, 123, 595 (April 1998). [0101] N. Iwashita, C. Rae Park, H. Fujimoto, M. Shiraishi, and M. Inagaki, Specification for a standard procedure of X-ray diffraction measurements on carbon materials, Carbon, 42, 701 (2004)

[0102] The graphitization degree g, as introduced by Maire and Mring, is calculated from diffraction peak [002] position. A powder silicon standard NIST (National Institute of Standards & Technology) SRM 640d is blended (5%) with coke powder. The certified value for [111] diffraction plane of silicon SRM 640d with Cu K X-ray source is 2=28.441. A corrected value for [002] peak position is obtained by adding to the observed [002] peak position the difference between certified silicon peak position (28.441) and observed peak position.

[0103] The graphitization degree g is expressed according to Maire and Mring's formula:

g=(0.344d.sub.002)/(0.3440.3354)

wherein d.sub.002 is the lattice spacing in nm. [0104] J. Maire and J. Mering, in Chemistry and Physics of Carbon, Marcel Dekker, New York, 1970; See also: J. Maire and J. Mring, Proc. 4.sup.th Conference on Carbon, p. 345, Pergamon Press, New York 1960 based on C. R. Houska and B. E. Warren, X-Ray study of the graphitization of carbon black, J. of Applied Physics, 25, 1503 (1954).

[0105] It is preferred to use full width at half peak maximum [002] as a criterion to differentiate coke graphitic crystallinity since the peak values are not very sharp and addition of an internal standard (silicon) with a sharp well-defined peak is mandatory for precise determination of d.sub.002 spacing, as reported by N. Ishigawa et al. Carbon, 42, 701 2004.

[0106] The inventors have observed that incorporation of the coke particles in the vinyl aromatic polymer foam does not affect the X-ray diffraction (XRD) pattern.

[0107] In the manufacturing process of the present invention, the vinyl aromatic polymer leaves the polymerization reactor in a molten state. The obtained melt stream is then divided in a main polymer melt stream and an additional polymer side stream.

[0108] The additional polymer melt stream is deviated to a dynamic or a static mixing equipment, preferably an extruder, where the comminuted coke and foam cell regulator are fed to and dispersed in the polymer melt, at a temperature comprised between 160 and 250 C., preferably between 170 and 240 C., more preferably between 180 and 230 C., whereupon the additional polymer melt stream recombines with the main polymer melt stream to result in a polymer melt comprising, with respect to the overall weight of the resulting polymer melt, between 3 and 8% by weight, preferably between 3 and 7% by weight, more preferably between 4 and 6% by weight of comminuted coke and between 0.05 and 5% by weight and preferable between 0.05 and 3.5% by weight of foam cell regulator.

[0109] Various kinds of cell regulators working according to various mechanism are known in the field of polymer foams. Inert particles at polymer melt temperatures such as talc; titanium dioxide; clays such as kaolin; silica gel; calcium polysilicate; gypsum; metal particles; calcium carbonate; calcium sulfate; magnesium carbonate; magnesium hydroxide; magnesium sulfate; barium sulfate; diatomaceous earth; nano-particles such as nano-particles of calcium carbonate, nano clay and nano-graphite work by adsorbing microscopic (mainly liquid) blowing agent particles and improving the dispersion of those particles in the polymer matrix.

[0110] Other cell regulators, called chemical cell regulators, such as citric acid, sodium bicarbonate or other products with decomposition temperatures in the range of the polymer melt temperatures work as bubble initiators in the polymer matrix trough the decomposition of the salt releasing CO.sub.2.

[0111] Finally, various molecules, the so called nucleating agents, are also known as cell regulators as they influence the crystallinity of a polymer and thereby the solubility of the blowing agent (n-pentane/iso-pentane) in the polymer matrix. Typical products considered as nucleating agents, in particular for semi crystalline or crystalline polymers are esters of abietic acids, polyoxyethylene sorbitan monolaurate, Montan wax, Candelilla wax, Carnauba wax, Paraffine wax, Ceresine wax, Japan wax, Petrolite wax, Ceramer wax, polyethylene wax, polypropylene wax and mixtures thereof.

[0112] A foam cell regulator of particular interest within the scope of the present invention comprises polyethylene wax.

[0113] Examples of polyethylene wax, particularly suitable for being used in the expandable vinyl aromatic compositions, are high density polyethylene waxes characterized by a weight average molecular weight in the range of from 1000 to 5000 Da and a polydispersity (Mw/Mn) of less than 2.0, preferably less than 1.3, more preferably less than 1.2.

[0114] The foam cell regulator is added in such a way that the final expandable vinyl aromatic polymer comprises between 0.01 and 1.0% by weight, preferably between 0.05 and 0.50% by weight of polyethylene wax.

[0115] The use of a molten vinyl aromatic polymer stream as a matrix for the coke particles allows a particular soft treatment of those particles during the dispersion process.

[0116] Alternatively, a master batch of vinyl aromatic polymer, comprising between 15 and 50% by weight of comminuted coke particles is prepared accordingly the above proper dispersion conditions.

[0117] Subsequently the master batch along with vinyl aromatic polymer and the other optional components are fed to the extruder in such an amount that the resulting extrudate, comprises between 3 and 10% by weight, preferably between 3 and 8% by weight, more preferably between 4 and 6% by weight, with respect to the overall weight of the resulting extrudate, of coke particles.

[0118] The foams derived from the expanded vinyl aromatic polymers, comprising the coke particles, are used, not only because the thermal insulating effect is significantly improved, but also because they allow the addition of a reduced amount of fire retardant agent for the case that fire retardancy with B2 rating, in accordance to DIN 4102-1, is required.

[0119] The components that are used as the flame retardant agentsusually halogenated productsare relatively expensive, so that the molded bodies comprising the comminuted coke particles, according to the invention, are considerably less expensive and can also be produced so as to be more environmentally compatible.

[0120] The flame-retardant agents particularly suitable for being used in the expandable vinyl aromatic compositions are chlorinated and/or brominated polymers, aliphatic, cyclo-aliphatic and aromatic brominated compounds, such as hexabromocyclododecane, pentabromomonochlorocyclo hexane, tetrabromobisphenol A bis(allyl ether) and pentabromophenyl allyl ether; among the above tetrabromobisphenol A bis(allyl ether) and brominated polymers are preferred.

[0121] The flame-retarded polymer foams obtained from the vinyl-aromatic polymers of the present invention generally comprise, based on the polymer foam, an amount in the range from 0.2 to 10% by weight, preferably in the range from 0.2 to 5% by weight of aliphatic, cyclo-aliphatic and aromatic brominated compounds. Amounts of from 1 to 10% by weight, based on the polymer foam, ensure adequate flame retardancy, in particular for foams made of expandable polystyrene.

[0122] The flame-retarded polymer foams obtained from the vinyl-aromatic polymers of the present invention alternatively comprise, based on the polymer foam, an amount in the range from 0.1 to 5% by weight, preferably in the range from 0.2 to 3% by weight of the halogenated polymers.

[0123] The weight average molecular weight, determined by means of gel permeation chromatography against polystyrene standards, of the halogenated, preferably brominated polymers, used as flame retardant, is preferably in the range from 5 kDa to 300 kDa, in particular from 30 kDa to 200 kDa.

[0124] The weight loss from the halogenated polymer in thermo-gravimetric analysis (TGA) is 5% by weight at a temperature of 250 C. or higher, preferably in the range from 270 to 370 C.

[0125] The bromine content of preferred halogenated polymers is in the range from 20 to 80% by weight, preferably from 40 to 80% by weight.

[0126] Halogenated polymers preferred as flame retardant are brominated polystyrene or styrene-butadiene block copolymer having bromine contents in the range from 40 to 80% by weight.

[0127] Other halogenated polymers preferred as flame retardant are polymers having tetrabromobisphenol A units (TBBPA), for example tetrabromobisphenol A diglycidyl ether compounds.

[0128] The effectiveness of the halogenated fire retardant agent can be still further improved via addition of suitable flame retardant synergists, examples being the thermal free-radical generators dicumyl, dicumyl peroxide, cumyl hydroperoxide, di-tert-butyl peroxide, tert-butyl hydroperoxide, or a mixture thereof. Another example of suitable flame retardant synergist is antimony trioxide. Flame retardant synergists are generally used in amounts of from 0.05 to 5% by weight, preferably of from 0.1 to 3% by weight, based on the polymer foam, in addition to the halogenated flame retardant agent.

[0129] Preferably, the halogenated fire retardant agent and the flame retardant synergist are added to the vinyl aromatic polymer via a separate mixing unit, such as an extruder, at a temperature comprised between 170 and 210 C., preferably between 180 and 200 C., the mix (extrudate) subsequently joining the polymer stream, for example through a static mixer.

[0130] Expandable vinyl aromatic polymers are vinyl aromatic polymers comprising blowing agent. The vinyl aromatic polymer melt generally comprises from 2% to 10% by weight, preferably from 3% to 7% by weight of one or more blowing agents distributed homogeneously. Suitable blowing agents are the physical blowing agents usually used in expandable styrene polymers e.g. aliphatic hydrocarbons having from 2 to 7 carbon atoms, alcohols, ketones, ethers, or halogenated hydrocarbons. Preferred blowing agents are isobutane, n-butane, isopentane, or n-pentane, preferably blends of isopentane and n-pentane. On the other hand sustainable blowing agents such as water or supercritical carbon dioxide may be used.

[0131] The expandable vinyl aromatic polymers further can comprise the usual and known auxiliaries and additives, examples being, fillers, UV stabilizers, chain-transfer agents, plasticizers, antioxidants, soluble and insoluble inorganic and/or organic dyes and pigments.

[0132] It is advantageous that the molded foams have a density of 18 kg/m.sup.3, or less preferably of 16 kg/m.sup.3 or less.

[0133] The molded foam can exhibit a thermal conductivity of 32 mW/m.Math.K or less, even of 31 mW/m.Math.K or less.

[0134] Various processes can be used to produce the particularly preferred expandable vinyl aromatic polymers. After the polymerization process, the melt stream is divided into a main polymer stream (1) and an additional polymer side stream (2) (FIG. 1). The side stream constitutes a loop to take up the first additive package, for example coke and foam cell regulator.

[0135] In a preferred embodiment, comminuted coke particles are taken as starting point together with foam cell regulator. These components are simultaneously fed into the additional polymer side stream of the vinyl aromatic polymer via a mixing unit, preferably via an extruder. After dispersion of the first additive package, said additional polymer stream joins again the main polymer stream, both in the molten stage, preferably through a static mixer, whereupon blowing agent is added.

[0136] The vinyl aromatic polymer melt comprising blowing agent, coke particles, foam cell regulator and in a later stage flame retardant agent and synergist, after homogenization, is rapidly cooled under pressure in order to avoid foaming. It is therefore advantageous to carry out underwater pelletizing in a closed system under pressure.

[0137] Particular preference is given to a process comprising the steps of: [0138] a) producing a polymer melt stream after the polymerization process; [0139] b) deriving a part of said polymer melt stream and creating a main polymer melt stream (1) and an additional polymer melt stream (2); [0140] c) using an extruder for incorporating the comminuted coke particles and the foam cell regulator into the additional polymer melt stream (2) at a temperature of at least 160 C., preferably comprised between 180 and 250 C., more preferably between 200 and 240 C., most preferably between 210 and 230 C.; [0141] d) merging the main (1) and the additional (2) polymer melt streams, forming a new polymer melt stream containing the dispersed coke and foam cell regulator; [0142] e) introducing a blowing agent into the new polymer melt stream; [0143] f) cooling the vinyl aromatic polymer melt comprising all necessary ingredients to a temperature of 200 C. or less, preferably a temperature comprised between 120 C. and 200 C.; [0144] g) introducing the flame retardant agent and the synergist into the new polymer melt stream; [0145] h) discharging through a die plate with holes, the diameter of which at the exit from the die is comprised between 0.3 and 1.5 mm, preferably about 0.5-1.0 mm; and [0146] i) pelletizing the melt comprising blowing agent directly downstream of the die plate under water at a pressure above 3 bar, preferably above 5 bar.

[0147] The pellets (beads, granules) can then further be coated and processed to give expanded vinyl aromatic polymer foams, in particular polystyrene foams.

[0148] In a first step, the expandable vinyl aromatic polymer pellets of the invention can be prefoamed by using hot air or steam, in what are known as prefoamers, to give foam beads of density in the range from 8 to 200 kg/m.sup.3, in particular from 10 to 50 kg/m.sup.3 preferably from 10 to 20 kg/m.sup.3. Eventually, in order to reach the lower densities a second prefoaming step can be applied. After maturation, in a next step the prefoamed beads (to which a coating has been applied) are placed in molds where they are treated with steam and where they are further expanded and fused to give molded foams.

Examples

[0149] The examples in Tables 1 and 2 illustrate the invention; they are merely meant to exemplify the present invention but are not destined to limit or otherwise define the scope of the present invention. Examples 1 to 6 are according to the present invention; Examples 7 to 9 are comparative.

[0150] All the self-extinguished (SE or FR) examples (Examples 2 to 9) comprise 0.15% by weight of wax with a weight average molecular weight Mw of 2 kDa, 1.2% by weight of Emerald Innovation 3000 (Chemtura) and 0.33% by weight, of 2,3-dimethyl-2,3-diphenylbutane (synergist). The foam panels derived all have B2 rating (DIN 4102).

[0151] The expandable foam of Example 1 is free of flame retardant agent and flame retardant synergist yet further being identical (composition and processing parameters) to the expandable foam of Example 2; minor to no influence on thermal conductivity values is perceived.

[0152] In Table 1, the thermal conductivity (A, in mW/m.Math.K) is given for a foam density () of respectively 12.5, 15 and 18 g/I for a foam comprising respectively 6% by weight (value before the slash symbol) and 4.5% by weight (value after the slash symbol) of coke.

[0153] In this table is additionally reproduced: [0154] D10: diameter where 10% by volume of the distribution has a smaller particle diameter and 90% by volume has a higher particle diameter; [0155] D50: diameter where 50% by volume of the distribution has a smaller particle diameter and 50% by volume has a higher particle diameter; [0156] D90: diameter where 90% by volume of the distribution has a smaller particle diameter and 10% by volume has a higher particle diameter; [0157] (D90D10)/D50: the span quantifying the distribution width; [0158] % vol.<1 m: the volume percentage of particles with a diameter of less than 1 m; [0159] % vol of particles with a diameter of less than 1 m calculated according to 3613*(ln D50) (value before the slash symbol) and to 3212 ln D50 (value after the slash symbol).

[0160] In Table 2, the particle size and the particle size distribution of the coke particles before introducing into the vinyl aromatic polymer (as received from the supplier) is compared to the particle size and the particle size distribution of the coke particles after being dispersed into the polymer melt according to the method of the present invention. In the same table, the structure characteristics of the coke particles are given, wherein: [0161] the degree of graphitization is given by (d.sub.0020.344)/(0.33540.344) with d.sub.002 being the interlayer spacing; 0.3345 being the interlayer spacing of highly graphitized carbons and 0.344 being the interlayer spacing for a typical non-graphitic carbon; [0162] L.sub.c is the crystallite stacking height; [0163] d.sub.002 is the average interlayer spacing.

[0164] All examples answer the criteria for volume median particle diameter (D50), span (D90D10)/D50), volume percentage of particles with a diameter of less than 1 m and structure characteristics (g, Lc, d.sub.002). As can be seen in Table 1, the expanded polystyrene foams of Examples 1 to 6, according to the invention, all are characterized (for coke percentage of 4.5% and more) by a thermal conductivity of 32 mW/m.Math.K or less for a foam density of 12.5 kg/m.sup.3, by a thermal conductivity of 31 mW/m.Math.K or less for a foam density of 15 kg/m.sup.3 and by a thermal conductivity of 30.5 mW/m.Math.K or less for a foam density of 18 kg/m.sup.3.

[0165] For Examples 7 to 9 (comparative examples), thermal conductivity values of more than 32 mW/m.Math.K are measured for expanded foams with a foam density of 12.5 kg/m.sup.3 and of more than 31 mW/m.Math.K for expanded foams with a foam density of 15 kg/m.sup.3 each comprising 6% by weight of coke.

[0166] In: [0167] Example 7, the volume median particle diameter (D50) is higher than 5 m (9.15 m); [0168] Example 8, the span is higher than 2.5 (2.73) and the volume median particle diameter (D50) is higher than 5 m (7.85 m) while the volume percentage of particles with a diameter of less than 1 m (10.6) is higher than 36-13 In D50 being 9.2; [0169] Example 9, the volume median particle diameter (D50) is higher than 5 m (6.1 m) while the crystallite stacking height (Lc) is lower than 4 nm (2.6 nm).

[0170] As appears from Table 2, the soft dispersion conditions result in a particle size distribution for which the span differs by less than 1, in absolute value, from the span of the particles before dispersion (as received). Processing in extruders induces an increase of the percentage of particles with a diameter below 1 m. Soft dispersion conditions result in a reduced increase of said percentage.

TABLE-US-00001 TABLE 1 Thermal conductivity () in mW/m .Math. K a.f.o. foam density () in g/l) D10 D50 D90 (D90 D10)/ % vol. < 36-13 InD50/ 12.5 15 18 Example (m) (m) (m) D50 1 m 32-12 InD50 kg/m.sup.3 kg/m.sup.3 kg/m.sup.3 1 1.09 4.08 8.25 1.75 9 17.7/15.1 30.9/31.5 30.2/30.7 29.7/30.1 2 0.51 2.62 5.78 2.01 20.2 23.5/20.4 30.6/31.3 30.0/30.5 29.6/30.0 3 0.52 2.47 5.7 2.10 23.0 24.2/21.2 30.6/31.2 30.0/30.5 29.6/30.0 4 0.59 2.00 5.02 2.22 20.9 27.0/23.7 30.8/31.5 30.3/30.8 29.9/30.3 5 1.13 4.18 8.42 1.74 8.7 17.4/14.8 30.9/31.6 30.3/30.8 29.8/30.1 6 0.95 4.07 8.34 1.82 10.6 17.8/15.2 30.9/31.6 30.2/30.7 29.7/30.1 7 1.84 9.15 18.55 1.83 4.7 7.2/5.4 32.8 31.8 31.1 8 0.94 7.85 22.37 2.73 10.6 9.2/7.2 32.9 31.9 31.2 9 2.39 6.10 10.30 1.30 6.0 12.5/10.3 34.5 33.1 31.9

TABLE-US-00002 TABLE 2 Particle size distribution (PSD) of coke as received PSD coke in EPS foam D10 D50 D90 (D90 D10)/ % vol. < Degree of Lc d.sub.002 D50 (D90 D10)/ Example (m) (m) (m) D50 1 m graphitization (nm) (nm) (m) D50 1 1.54 4.76 8.80 1.53 4.9 0.12 7.7 0.345 4.08 1.75 2 0.87 3.07 6.29 1.77 12.1 0.12 5.2 0.345 2.62 2.01 3 0.83 2.67 5.46 1.73 13.1 0.12 5.2 0.345 2.47 2.10 4 1.12 2.85 6.81 2.00 7.9 0.00 5.5 0.344 2.00 2.22 5 1.33 4.27 8.23 1.62 6.4 0.01 7.7 0.344 4.18 1.74 6 1.33 4.27 8.23 1.62 6.4 0.01 7.7 0.344 4.07 1.82 7 3.49 10.32 18.57 1.42 0.7 0.12 5.1 0.345 9.15 1.83 8 2.77 12.28 27.51 2.01 2.3 0.12 4.3 0.345 7.85 2.73 9 4.66 6.80 9.69 0.74 1.0 0.30 2.6 0.3466 6.10 1.30