DETAILED DESCRIPTION OF THE INVENTION In accordance with the invention, a variety of expandable thermoplastic homopolymers and copolymers can be made to have reduced flammability.
The polymers may be derived from vinyl aromatic monomers, such as styrene, vinyltoluene, isopropylstyrene, alpha-methylstyrene, chlorostyrenes, tert-butylstyrene, etc.
, as well as copolymers prepared by the copolymerization of at least 50 percent by weight of a vinyl aromatic monomer with monomers such as butadiene, alkyl methacrylates, alkyl acrylates, acrylonitrile and maleic anhydride.
For the purpose of convenience, these polymers and copolymers are referred to herein as styrene polymers.
The styrene polymers can, of course, be produced by any of the known techniques, for example by suspension or mass polymerization, to obtain particles in the form of beads or pellets.
To render the polymer particles expandable, the blowing agent is incorporated into the particles, as for example in U.
S.
Pat.
No.
2,983,692, by suspending the particles in water with the aid of suspending agent systems such as tricalcium phosphate in combination with an anionic surfactant.
The blowing agents are compounds which are gases or which will produce gases on heating.
Suitable agents include aliphatic hydrocarbons containing from 4-7 carbon atoms in the molecule, such as butane, pentane, cyclopentane, hexane, heptane, cyclohexane, and the hologenated hydrocarbons which boil at a temperature below the softening point of the polymer.
Mixture of these agents may also be used, such as a 50/50 mixture of isopentane/n-pentane, or a 55/45 mixture trichlorofluoromethane/n-pentane.
Usually from 4 to 20 percent of blowing agent per 100 part of polymer is incorporated by the impregnation.
The impregnation is conventionally carried out at temperatures ranging from about 60.
degree.
to 150.
degree.
C.
Increasing the temperature makes the impregnation proceed at a faster rate.
After the impregnation is completed, the suspension of polymer particles is cooled to room temperatures to allow separation of the impregnated beads from the aqueous phase
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