Method to start-up a process to make non-expandable vinyl aromatic polymers

Abstract

The present invention is a method to start-up a process to make expandable vinyl aromatic polymer pellets comprising, a) providing a pelletizer (S) containing means to introduce the molten vinyl aromatic polymer comprising the expandable agent and optionally additives, a die plate having a plurality of holes of small diameter, typically in the range 0.8 to 1.6 mm and cutting means to make pellets, b) providing a pelletizer (L) containing means to introduce the molten vinyl aromatic polymer comprising the expandable agent and optionally additives, a die plate having a plurality of holes of large diameter, typically in the range 3 to 5 mm and cutting means to make pellets, c) sending the expandable vinyl aromatic polymer pellets comprising an expandable agent and optionally additives to the pelletizer (L) until the polymer flow rate is in the operating range of the pelletizer (S) and provided the proportion of expandable agent and optional additives are in the specifications, d) switching the molten vinyl aromatic polymer stream comprising the expandable agent and optionally additives to the pelletizer (S) and operating said pelletizer (S) at conditions effective to produce expandable vinyl aromatic polymer pellets, e) recovering from pelletizer (S) the expandable vinyl aromatic polymer pellets, f) recovering the pellets produced at step c) for optional subsequent recycling in the molten state at step d). In another embodiment while the pelletizer (S) is in production and troubles happen in the introduction of the expanding agent and/or the optional additives or in any equipment or even an equipment needs maintenance the production is switched from the pelletizer (S) to one or more pelletizers (L). When the troubles are over, the production is switched from the pelletizer (L) to the pelletizer (S). In an embodiment while the pelletizer (S) is in production the die plate of the pelletizer (L) having a plurality of holes of large diameter is removed and replaced by a die plate having a plurality of holes of small diameter to convert said pelletizer (L) into a pelletizer (S) capable to produce expandable vinyl aromatic polymer pellets. By way of example said established pelletizer (S) is used during maintenance of the other pelletizer (S).

Claims

1. A method for production of non-expandable vinyl aromatic polymer pellets comprising: during start-up of the production of the non-expandable vinyl aromatic polymer pellets: introducing a vinyl aromatic polymer and optionally additives to a pelletizer (L) comprising a first die plate having a plurality of holes with a diameter ranging from 2 mm to 5 mm and a first cutting means; and producing pellets in the pelletizer (L) until a polymer flow rate is in an operating range of a pelletizer (S); when the polymer flow rate is in the operating range of the pelletizer (S), switching the introduction of the vinyl aromatic polymer and optionally the additives from the pelletizer (L) to the pelletizer (S), and operating the pelletizer (S) at conditions effective to produce the non-expandable vinyl aromatic polymer pellets, wherein the pelletizer (S) comprises a second die plate having a plurality of holes with a diameter ranging from 0.5 mm to 1.9 mm and a second cutting means; recovering the non-expandable vinyl aromatic polymer pellets from the pelletizer (S); and recovering the pellets from the pelletizer (L); recycling the pellets produced by the pelletizer (L), wherein the recycling of the pellets produced by the pelletizer (L) comprises: introducing the pellets produced by the pelletizer (L) to a side extruder; and mixing the pellets produced by the pelletizer (L) with the vinyl aromatic polymer and optionally the additives prior to introducing the vinyl aromatic polymer and optionally the additives into the pelletizer (S).

2. The method of claim 1, wherein the vinyl aromatic polymer is introduced to the pelletizer (S) from a vinyl aromatic polymer source.

3. The method of claim 1, wherein the pellets produced in the pelletizer (S) are crystal polystyrene or high impact polystyrene.

4. The method of claim 1, wherein the diameter of the plurality of holes of the second die plate ranges from 0.8 mm to 1.4 mm, and wherein the diameter of the plurality of holes of the first die plate ranges from 3 mm to 4 mm.

5. The method of claim 1, further comprising incorporating flame retardant into the pellets produced with the pelletizer (L).

Description

DETAILED DESCRIPTION OF THE INVENTION

(1) As regards the vinyl aromatic polymer, mention may be made of: polystyrene, elastomer-modified polystyrene, copolymers of styrene and acrylonitrile (SAN), elastomer-modified SAN, in particular ABS, which is obtained, for example, by grafting (graft polymerization) of styrene and acrylonitrile on a backbone of polybutadiene or of butadiene-acrylonitrile copolymer, mixtures of SAN and ABS, copolymers with styrene blocks and blocks made of butadiene or isoprene or of a mixture butadiene/isoprene, these block copolymers can be linear blocks copolymers or star blocks copolymers, they can be hydrogenated and/or functionalized. These copolymers are described in ULLMANN'S ENCYCLOPEDIA OF INDUSTRIAL CHEMISTRY, fifth edition (1995) Vol A26, pages 655-659, They are sold by Total Petrochemicals under the trade mark Finaclear, by BASF under the trade mark Styrolux, under the trade mark K-Resin by Chevron Phillips Chemical, SBR (Styrene butadiene rubber),

(2) Possible examples of the abovementioned elastomers are EPR (the abbreviation for ethylene-propylene rubber or ethylene-propylene elastomer), EPDM (the abbreviation for ethylene-propylene-diene rubber or ethylene-propylene-diene elastomer), polybutadiene, acrylonitrile-butadiene copolymer, polyisoprene, isoprene-acrylonitrile copolymer and copolymers with styrene blocks and blocks made of butadiene or isoprene or of a mixture butadiene/isoprene. These block copolymers can be linear blocks copolymers or star blocks copolymers, they can be hydrogenated and/or functionalized (see above).

(3) In the above vinyl aromatic polymer just mentioned, part of the styrene may be replaced by unsaturated monomers copolymerizable with styrene, for example alpha-methylstyrene or (meth)acrylates, Other examples of styrene copolymers which may be mentioned are chloropolystyrene, poly-alpha-methylstyrene, styrene-chlorostyrene copolymers, styrene-propylene copolymers, styrenebutadiene copolymers, styrene-isoprene copolymers, styrene-vinyl chloride copolymers, styrene-vinyl acetate copolymers, styrene-alkyl acrylate copolymers (methyl, ethyl, butyl, octyl, phenyl acrylate), styrene-alkyl methacrylate copolymers (methyl, ethyl, butyl, phenyl methacrylate), styrene methyl chloroacrylate copolymers and styrene-acrylonitrile-alkyl acrylate copolymers.

(4) In a specific embodiment the vinyl aromatic polymer comprises:

(5) i) from 60 to 100 weight % of one or more C.sub.8-12 vinyl aromatic monomers; and

(6) ii) from 0 to 40 weight % of one or more monomers selected from the group consisting of C.sub.1-4 alkyl esters of acrylic or methacrylc acid and acrylonitrile and methacrylonitrile; which polymer may be grafted onto or occluded within from 0 to 20 weight % of one or more rubbery polymers.

(7) By way of example rubbery polymers can be selected from the group consisting of:

(8) a) co- and homopolymers of C.sub.4-6 conjugated diolefins,

(9) b) copolymers comprising from 60 to 85 weight % of one or more C.sub.4-6 conjugated diolefins and from 15 to 40 weight % of a monomer selected from the group consisting of acrylonitrile and methacrylonitrile and

(10) c) copolymers comprising from 20 to 60, preferably from 40 to 50 weight % of one or more C.sub.8-12 vinyl aromatic monomers which are unsubstituted or substituted by a C.sub.1-4 alkyl radical and from 60 to 40, preferably from 60 to 50 weight % of one or more monomers selected from the group consisting of C.sub.4-6 conjugated diolefins.

(11) The rubber may be prepared by a number of methods, preferably by emulsion or solution polymerization. These process are well known to those skilled in the art. The vinyl aromatic polymers may be prepared by a number of methods. This process is well known to those skilled in the art.

(12) If present, preferably the rubber is present in an amount from about 3 to 10 weight %. Polybutadiene is a particularly useful rubber.

(13) In the specific embodiment in which the vinyl aromatic polymer is polystyrene, it could be a crystal polystyrene or a rubber modified polystyrene. The rubber modified polystyrene is called HIPS (High Impact Polystyrene). The process for making HIPS is well known to those skilled in the art. The rubber is dissolved in the styrene monomer (actually the rubber is infinitely swollen with the monomer). This results in two co-continuous phases. The resulting solution is fed to a reactor and polymerized typically under shear. When the degree of polymerization is about equal to the weight % of rubber in the system it inverts (e.g. the styrene/styrene polymer phase becomes continuous and the rubber phase becomes discontinuous. After phase inversion the polymer is finished in a manner essentially similar to that for finishing polystyrene. The polymer is prepared using conventional bulk, solution, or suspension polymerization techniques.

(14) The vinyl aromatic polymers of the present invention may be co- or homopolymers of C.sub.8-12 vinyl aromatic monomers. Some vinyl aromatic monomers may be selected from the group consisting of styrene, alpha methyl styrene and para methyl styrene. Preferably the vinyl aromatic monomer is styrene. The vinyl aromatic polymer may be a copolymer comprising from 60 to 100 weight % of one or more C.sub.8-12 vinyl aromatic monomers; and from 0 to 40 weight % of one or more monomers selected from the group consisting of C.sub.1-4 alkyl esters of acrylic or methacrylc acid and acrylonitrile and methacrylonitrile. Suitable esters of acrylic and methacrylic acid include methyl acrylate, ethyl acyrlate, butyl acrylate, methyl methacrylate, ethyl methacrylate and butyl methacrylate. The vinyl aromatic polymers of the present invention may be rubber modified.

(15) Advantageously the vinyl aromatic polymer is a monovinylaromatic polymer.

(16) As regards the expanding agent, it is selected from aliphatic or cyclo-aliphatic hydrocarbons containing from 3 to 6 carbon atoms such as n-pentane, iso-pentane, cyclopentane or blends thereof; halogenated derivatives of aliphatic hydrocarbons containing from 1 to 3 carbon atoms, such as, for example, dichlorodifluoromethane, 1,2,2-trifluoroethane, 1,1,2-trifluoroethane; carbon dioxide and water.

(17) As regards the additives, one can cite any material capable to reduce the thermal conductivity of the expanded vinyl aromatic polymer. One can cite carbon black, graphite, mica, talc, silica, titanium dioxide and barium sulfate. One can cite carbon black with a surface area, measured according to ASTM D-3037/89, ranging from 5 to 200 m2/g.

(18) The expandable vinyl aromatic polymer may also 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 or brominated flame-retardant grafted on polymer chains 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 vinyl aromatic 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 vinyl aromatic 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 vinyl aromatic 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 vinyl aromatic polymer.

(19) As regards the process to make said expandable polymer, it is carried out by mixing the vinyl aromatic polymer in the melted state with the expanding agent or agents and optionally the additives. In an advantageous embodiment the mixing is carried out in a chamber equipped with at least one 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, or in one or more static mixers 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. Such processes are described in WO 2008 041766, WO 2009 052898, EP 2062935, US 2008 203597 and U.S. Pat. No. 6,783,710 the content of which is incorporated in the present application.

(20) According to an embodiment the present invention relates to a process for preparing in mass and in continuous, expandable vinyl aromatic polymers, which comprises the following steps in series: (i) feeding the vinyl aromatic polymer, as described above, to an extruder, optionally together with fillers, (ii) heating the vinyl aromatic polymer to a temperature higher than the relative melting point; (iii) injecting the expanding agent and possible additives into the molten polymer before extrusion through a die; and (iv) forming expandable pellets, through a die, with an average diameter ranging advantageously from 0.5 to 1.9 mm.

(21) According to a specific embodiment, the process includes the incorporation, in a first polymeric stream (hereinafter referred to as main stream), of a second polymeric stream (hereinafter referred to as side stream) containing the expanding system and additives. Alternatively, the expanding system can be directly incorporated into the main stream.

(22) The resulting composition, in the molten state, is then homogenized and finely sieved by one or more filtering steps which either remove or disgregate the polymeric aggregates and the non-dispersed inorganic fillers. The polymeric composite product is then extruded through a die and granulated.

(23) According to a preferred embodiment, the polymer forming the main polymeric stream is taken in the molten state from a continuous polymerization process. The polymer, coming from one or more polymerization steps, is typically removed from the possible dilution solvent, the non-reacted monomer and the oligomers, in a section called devolatilization.

(24) The so purified polymer is used directly, in the molten state, as the main polymeric stream of the process of the present invention. For this purpose, the polymer coming from the devolatilizer preferably contains no more than 2,000 ppm of monomers and 8,000 ppm of dimers, trimers and oligomers, so as to prevent damage to the structure of the foam obtained after expansion of the resulting expandable particle polymers.

(25) According to an alternative embodiment, the polymer used as the main stream is in the shape of pellets. Said pellets are melted in a suitable device (a single-screw or twin-screw extruder, for example).

(26) In both embodiments, the molten polymeric material is pressurized and then pushed into the subsequent process section, by means of any suitable device, typically a gear pump.

(27) Advantageously, the additives are incorporated in a secondary polymer stream which subsequently joins, in the molten state, the main polymer stream.

(28) In a preferred embodiment, the additives are metered in a twin-screw extruder together with the granules of the polymer. Expediently, the extruder, after the melting section, contains mixing elements which allow a better distribution of the additives in the polymeric phase. The mass fraction of the polymeric phase must be at least equal to 20%, more preferably at least 40% with respect to the content of the polymer in the side fraction, to process the resulting molten mass successfully.

(29) Advantageously, the extruder contains a degassing phase to remove possible solvents contained in the additive blend.

(30) The temperature of the molten stream must be kept within a prefixed range. Typically, the minimum temperature is equal to the maximum temperature selected among the solidification temperatures of the molten components, plus 20 C., whereas the maximum temperature is the same plus 150 C.

(31) Optionally, before entering the extruder, the additives and polymer in granules can be premixed in a suitable mixer for solids, in order to favour a homogeneous distribution of the components. The preferred device for this operation is a screw mixer.

(32) When liquid or gaseous additives are used, an efficient means to incorporate them is to inject the same into a side feeding point of said extruder, located down stream the melting and degassing section.

(33) The solid additives which do not melt at the extrusion temperature of the molten stream must consist of fine particles. In particular, considering the population of non-meltable particles, the d90, i.e. the dimension under which lies 90% of the population, typically must not be larger than half of the diameter of the holes of the die plate.

(34) Preferably, d90 must not be larger than th of the diameter of the die holes. Dimension means the diameter as calculated by means of laser diffraction measurement on the non-meltable materials.

(35) The recycling of the pellets produced on the pelletizer (L) during the start up or during the troubles in the introduction of the expanding agent and/or the optional additives or in any equipment or even when an equipment needs maintenance can be made by any means. Said pellets can be melted in a suitable device (a single-screw or twin-screw extruder, for example) and then mixed with the molten vinyl aromatic polymer comprising the expandable agent and optionally additives feeding the pelletizer (S).

(36) The pellets produced with the pelletizer (L) during start-up, during the troubles and/or the change of specifications are used to incorporate Flame Retardant and/or one or more additives and ensuring a good dispersion of said Flame Retardant and/or one or more additives.

(37) The recycled pellets can be premixed with the flame retardant additive, flame retardant synergist such as peroxide and other specific additives prior to the introduction on the main stream to facilitate and ensure a good dispersion of those additives in the polymer, and in the same time avoiding or decreasing the need of virgin polymer.

(38) As regards the removal of the die plate to be replaced by a die plate having small holes, this is an operation known to the man skilled in the art.

(39) The expandable beads (pellets) produced are subjected to pre-treatment generally applied to conventional expandable beads and which essentially consists in: 1. coating the beads with a liquid antistatic agent such as amines, tertiary ethoxylated alkylamines, ethylene oxide-propylene oxide copolymers, etc. The purpose of this agent is to facilitate both the adhesion of the coatings 2. applying the coating to the above beads, said coating essentially consisting of a mixture of mono-, di- and triesters of glycerin (or other alcohols) with fatty acids and of metallic stearates such as zinc and/or magnesium stearate.

(40) The expandable vinyl aromatic polymer pellets are used to make expanded articles, in particular insulation boards.

(41) In an embodiment the moulded and expanded article is produced by a process comprising the following steps:

(42) (i) a step of pre-expansion, by contacting and mixing the composition, which is in the form in particular of expandable pellets 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,

(43) (ii) a step of stabilizing the particles or beads thus expanded, by contacting them with ambient air, and

(44) (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).