More specifically, the present invention relates to compositions based on polystyrene, with an improved expand- ability.

Vinylaromatic polymers, and among these, polystyrene in particular, are known products which have been adopted for a long time for preparing compact and/or expanded arti- cles which can be used in various applicative sectors, among which the most important are household appliances, the transport and building industries, office machines, etc. A particularly interesting sector is the field of thermal insulation, where vinylaromatic polymers are essen- tially used in expanded form.

These expanded products are obtained by pre-foaming in a pre-expander beads of expandable polymer previously im- pregnated with an expanding agent and molding the pre-

foamed particles contained inside a closed mould by means of the contemporaneous effect of pressure and temperature.

The foaming of the particles is generally effected with va- pour, or another gas, maintained at a temperature slightly higher than the glass transition temperature (Tg) of the polymer.

It is known that the expandability of vinylaromatic polymers, referring to the possibility of obtaining low density products without their collapsing, can be improved by means of suitable additives.

According to European patent 217, 516, for example, the expandability of vinylaromatic polymers can be improved by adding plasticizers such as rubbers or oils, to the poly- mer. Additives contained in resins effectively cause a good, immediate expansion but, as they remain englobed in the polymeric matrix, they cause the products to collapse with a consequent deterioration in the density.

The use of oligomers of aliphatic olefins, as de- scribed in U. S. patent 5,783, 612, also improves, for exam- ple, the expandability of polystyrene but reduces the proc- essability range.

It has been verified that the reduction of the molecu- lar weight of polymers to values lower than 50,000 also im- plies an excellent expandability but with the detriment of the processability and mechanical characteristics of the

end-product.

Object of the present invention is to provide an ex- pandable composition based on vinylaromatic polymers with an improved expandability which can be processed with technologies and operating conditions analogous to those of equivalent products available on the market.

The Applicant has now found that this and other ob- jects, which will appear evident from the following de- scription, can be obtained through certain expandable poly- meric mixtures containing a polymer of at least one vi- nylaromatic monomer having an average molecular weight Mw ranging from 50, 000 to 170,000, which comprise: a) 0-50% by weight of a copolymer obtained by polymerizing one or more vinylaromatic monomers and 0. 1-15% by weight of an a-alkylstyrene ; b) 0-10% by weight of a compatible polymer having a crys- tallinity lower than 10% and a glass transition tempera- ture (Tg) higher than 100°C, the remaining percentage consisting of the vinylaromatic polymer; c) 2-10% by weight, calculated with respect to the total, of an expanding agent; with the proviso that at least one of (a) or (b) is present in the polymeric mixture.

The term"vinylaromatic monomer", as used in the pres-

ent description and claims, essentially refers to a product which corresponds to the following general formula: wherein n is zero or an integer ranging from 1 to 5 and Y is a halogen, such as chlorine or bromine, or an alkyl or alkoxyl radical having from 1 to 4 carbon atoms.

Examples of vinylaromatic monomers having the general formula defined above are: styrene, methylstyrene, ethyl- styrene, butylstyrene, dimethylstyrene, mono-, di-, tri-, tetra-and penta-chlorostyrene, bromo-styrene, methoxy- styrene, acetoxy-styrene, etc. Styrene is the preferred vi- nylaromatic monomer.

The vinylaromatic monomers having general formula (I) can be used alone or in a mixture of up to 50 by weight with other copolymerizable monomers. Examples of these monomers are (meth) acrylic acid; C1-C4 alkyl esters of (meth) acrylic acid, such as methyl acrylate, methyl- methacrylate, ethyl acrylate, ethylmethacrylate, aisopropyl acrylate, butyl acrylate ; amides and nitriles of (meth) acrylic acid such as acrylamide, methacrylamide, ac-

rylonitrile, methacrylonitrile; butadiene, ethylene, divi- nylbenzene, maleic anhydride, etc. Preferred copolymeri- zable monomers are acrylonitrile and methylmethacrylate.

The vinylaromatic polymer or copolymer which is ob- tained has a molecular weight Mw ranging from 50,000 to 170,000, preferably from 70,000 to 150,000.

The vinylaromatic monomers are also copolymerized with an a-alkylstyrene in quantities ranging from 0.1 to 15% by weight, preferably from 2 to 10%, to give the copolymer (a). The preferred a-alkylstyrene according to the present invention is a-methylstyrene, a-ethylstyrene or a- propylstyrene. a-methylstyrene is particularly preferred.

Any expanding agent capable of being englobed in the polymeric matrix can be used in a combination with the vi- nylaromatic polymers object of the present invention. In general, liquid substances can be used, with a boiling point ranging from 10 to 100°C, preferably from 20 to 80°C.

Typical examples are aliphatic hydrocarbons, freon, carbon dioxide, water, etc.

The expanding agent (c) can be added to the polymeric mixture during the polymerization phase, as described be- low, or, alternatively, by impregnation of the beads pro- duced at the end of the polymerization or by injection into the molten product. At the end of the addition, a. polymer is obtained, which can be transformed to produce expanded

articles with a density ranging from 5 to 50 g/1, prefera- bly from 8 to 25 g/1, with an excellent thermal insulation capacity. In order to favour the retention of the expanding agent in the polymeric matrix, additives capable of forming bonds both of the weak type (for example hydrogen bridges) or strong type (for example acid-base adducts) can be used with the expanding agent. Examples of these additives are methyl alcohol, isopropyl alcohol, dioctylphthalate, di- methylcarbonate, derivatives containing an amine group.

The compatible polymer (b) with a crystallinity degree lower than 10%, preferably equal to or lower than 5%, and a Tg higher than 100°C, is preferably used in quantities ranging from 2 to 8% by weight. Examples of these polymers are polyphenylethers, such as polyphenyleneoxide (PPO) and polycarbonates such as polycarbonate from tetramethyl- bisphenol A.

Conventional additives, generally used with commercial materials, such as pigments, stabilizers, flame-retardant agents, mineral fillers of athermanous materials, such as graphite or carbon black, or refracting and/or reflecting agents such as titanium dioxide, antistatic agents, detach- ing agents, shock-resistant agents, etc. , can be added to the expandable vinylaromatic polymers, object of the pres- ent invention.

Vinylaromatic polymers with an improved expandability

can alternatively comprise those consisting of: d) a matrix obtained by polymerizing one or more vinylaro- matic monomers and 0.1-14. 5% by weight of an a- alkylstyrene; e) 2-10% by weight, calculated with respect to the polymer (a), of an expanding agent englobed in the polymeric ma- trix; f) 0-10% by weight, preferably 2-80-. calculated with respect to the polymer (a), of a compatible polymer with a crys- tallinity lower than 10% and a glass transition tempera- ture (Tg) higher than 100°C.

A further object of the present invention relates to the process for the preparation of mixtures with an im- proved expandability based on vinylaromatic polymers.

In particular, a further object of the present inven- tion relates to a process for preparing expandable composi- tions based on vinylaromatic polymers which comprises po- lymerizing in aqueous suspension a composition containing one or more vinylaromatic monomers, 0-50% by weight of a copolymer (a), obtained by polymerizing one or more vi- nylaromatic monomers and 0. 1-15% by weight of a- alkylstyrene, and 0-10% by weight of a compatible polymer (b) with a crystallinity lower than 10% and a glass transi- tion temperature (Tg) higher than 100°C, (a) and (b) being pre-dissolved in the monomeric mixture, in the presence of

an expanding agent (c) added during or after the end of the polymerization, with the proviso that at least one of (a) or (b) is present in the reagent mixture.

At the end of the polymerization, substantially spherical polymer beads are obtained, with an average di- ameter ranging from 0.2 to 2 mm, inside which the expanding agent is homogeneously dispersed.

During the polymerization in suspension, polymeriza- tion additives, typically used for producing vinylaromatic polymers, are adopted, such as polymerization catalysts and chain transfer agents, through which it is also possible to regulate the molecular weight of the end-polymer, stabiliz- ing agents of the suspension, expanding aids, nucleating agents, plasticizers, mineral fillers, etc.. In particular, it is preferable to add, during the polymerization, min- eral fillers of athermanous materials, such as graphite, or refracting agents, such as titanium dioxide, in quantities ranging from 0. 05 to 25% by weight, calculated with respect to the resulting polymer.

The expanding agents are preferably added during the polymerization phase and are selected from aliphatic or cy- cloaliphatic hydrocarbons containing from 3 to 6 carbon at- oms such as n-pentane, isopentane, cyclopentane or their mixtures; the halogenated derivatives of aliphatic hydro- carbons containing from 1 to 3 carbon atoms such as, for

example, dichlorodifluoromethane, 1,2, 2-trifluoroethane, 1,1, 2-trifluoroethane; carbon dioxide and water.

To improve the stability of the suspension, it is pos- sible to increase the viscosity of the reagent solution by dissolving some vinylaromatic polymer therein, in a concen- tration ranging from 1 to 30% by weight, preferably from 5 to 20%, calculated with respect to the polymer alone. The solution can be obtained either by dissolving a preformed polymer (for example fresh polymer or the waste products of previous polymerizations and/or expansions) in the reagent mixture or by pre-polymerizing the monomer, or mixture of monomers, in mass, in order to obtain the above concentra- tions, and then continuing the polymerization in aqueous suspension in the presence of the remaining additives, in particular (b) and (c).

Another object of the present invention relates to a process for preparing in continuous mass, expandable mix- tures based on vinylaromatic polymers, which ccmprises the following steps in series: i. feeding to an extruder a vinylaromatic polymer, to- gether with 0-50% by weight of a copolymer (a), ob- tained by polymerizing one or more vinylaromatic mono- mers and 0.1-15% by weight of an a-alkylstyrene, and 0-10% by weight of a compatible polymer (b) with a crystallinity lower than 10% and a glass transition

temperature (Tg) higher than 100°C, with the proviso that at least one of (a) or (b) is present in the mix- ture; ii. heating the polymers to a temperature higher than the relative melting point; iii. injecting the expanding agents into the molten product before extrusion through a die; and iv. forming beads, optionally expandable, through a die, in a substantially spherical form with an average di- ameter ranging from 0.2 to 2 mm.

A detailed method for preparing expandable vinylaro- matic polymers in continuous mass is provided in European patent EP 126,459.

At the end of the polymerization of the expandable vi- nylaromatic polymers, object of the present invention, the beads, either prepared in suspension or continuous mass process, are subjected to pre-treatment generally applied to traditional materials and which essentially consists in: 1. coating the beads with a liquid antistatic agent such as amines, tertiary ethoxylated alkylamines, ethylene ox- ide-propylene oxide copolymers, etc. The purpose of this agent is to facilitate both the adhesion of the coating and also the screening of the beads prepared in suspen- sion; 2. applying the"coating"to the above beads, said"coat-

ing"essentially consists of a mixture of mono-, di-and tri-esters of glycerin (or other alcohols) with fatty acids and of metallic stearates such as zinc and/or mag- nesium stearate.

Some illustrative but non-limiting examples are pro- vided for a better understanding of the present invention and for its embodiment.

EXAMPLE 1 98 parts of polystyrene granules having an MFI (meas- ured at 200°C/5 kg) of 25 g/10', a Mw of 130,000 and 2 parts of polyphenylene-ether (having a weight average mo- lecular weight of 262, 000), are fed into an extruder, through a hopper, and 500 ppm of polyethylene wax are added as nucleating agent.

6. 5% of a mixture of n/i-pentane 80/20 is fed to the extruder, as expanding agent, through a specific injection line.

The polymer obtained containing the expanding agent is extruded through the holes of a die, cut by means of knives, baked at 60°C, dried and lubricated with 0. 1% by weight of magnesium stearate and 0. 3% by weight of glyc- erylmonostearate.

The beads are then expanded at various vaporization times and subsequently moulded into end-products. The ex- pandability results are indicated in the following table.

EXAMPLE 2 Example 1 is repeated, feeding 40 parts of styrene/a- methyl-styrene copolymer and 60 parts of the polystyrene used in Example 1.

6. 2% of a mixture of n/i-pentane 80/20 is fed to the extruder as expanding agent. The copolymer contains 6. 5% of a-methyl-styrene and has an Mw of 155,000. The expandabil- ity results are indicated in the following table.

EXAMPLE 3 (comparative) Example 1 is repeated but using 98 parts of a polysty- rene having an MFI of 8 (Mw of 180, 000), available on the market under the trade-name of Edistir N 1782 of ENICHEM S. p. A.

As can be seen from the table below, the product has only a very slight expansion, even if it does not collapse.

EXAMPLE 4 (comparative) Example 1 is repeated but using only polystyrene gran- ules (therefore 100 parts).

As can be seen from the table below, the product ex- pands very much but collapses.

EXAMPLE 5 (comparative) Example 4 is repeated but using only granules of poly- styrene Edistir N 1782.

As can be seen from the table below, the product has only a very slight expansion, even if it does not collapse.

TABLE 1 Vapourization time Density after 24 hours (min.) (g/l) EXAMPLE 1 1 16.5 2 14 3 12. 5 5 11 7 10. 9 EXAMPLE 2 1 17 2 13.8 3 12.4 5 11.1 7 10.8 EXAMPLE 3 2 19 3 16. 6 5 14. 8 7 13. 6 10 12.7 EXAMPLE 4 1 13 2 12 3 13(*) EXAMPLE 5 1 18 2 156 3 13.9 5 12. 7 7 12. 5 (*) Strong collapsing of the end-product