Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Cellular products or processes of preparing a cellular...
Patent
1997-05-08
1999-04-27
Foelak, Morton
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Cellular products or processes of preparing a cellular...
521139, C08J 914
Patent
active
058980393
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a method of preparing foamed articles formed from a blend of a styrenic polymer and at least one acrylic polymer.
It is well known that polystyrene may be readily foamed by a variety of processes, including extrusion. The density of such industrially produced polystyrene foam, absent any particular treatment is usually of the order of 32 kg.m.sup.-3.
GB 2146941 discloses the preparation of a polystyrene foam by an extrusion process.
EP 0411923 is directed towards the preparation of polystyrene foam having a density from 32 to 160 kg.m.sup.-3 using an extrusion process in which liquid carbon dioxide is employed as a blowing agent instead of a conventional halogenated hydrocarbon blowing agent. As is well known, the use of such halogenated hydrocarbons presents certain risks to the environment and is therefore increasingly unacceptable. However, a process which uses carbon dioxide as a blowing agent has the attendant complication of requiring the refrigeration of the liquid carbon dioxide.
The preparation of normal to low density polystyrene foam, i.e. foam having a density of 32 kg.m.sup.-3 or less, using a physical blowing agent which is neither non-halogenated hydrocarbons nor liquid carbon dioxide is difficult to achieve.
U.S. Pat. No. 3,554,932 is directed towards the use of nucleating agents to improve the foaming of variety of polymers, including styrene homopolymer, using a hydrocarbon blowing agent. The density of the styrene foam is about 64 kg.m.sup.-3. In order to prepare a normal density foam from such a material it is typically necessary to either foam the material directly into a sub atmospheric pressure environment or else to subject the material to some form of controlled re-expansion, for example U.S. Pat. No. 4,552,904 discloses the preparation of a polystyrene foam having a density of less than 32 kg.m.sup.-3 by the re-expansion of an extruded foam. Such requirements add to the complexity of the foam making process and the expense of the hydrocarbon blown foam.
It is an object of the present invention to provide a normal or low density polystyrene foam using a conventional extrusion foaming process using a hydrocarbon blowing agent, without the need to use re-expansion or an other such technique.
Accordingly, in a first aspect the present invention provides a process for forming a foamed article having a closed cell structure comprising (a) a polymer blend comprising at least 50% by weight of a styrenic polymer and up to 50% by weight of an acrylic polymer which has an average molecular weight which is less than 90% of that of the styrenic polymer and (b) a hydrocarbon blowing agent, which process comprises supplying the styrenic polymer and acrylic polymer to a heated extruder, heating and mixing the styrenic polymer and acrylic polymer in the extruder so as to form a molten polymer blend, incorporating a hydrocarbon blowing agent into the molten polymer blend within the extruder to form a molten mixture and thereafter releasing the from the extruder so as to cause the blowing agent to expand the molten mixture to form the foamed article.
In a second aspect the present invention provides a foamed article having a closed cell structure comprising (a) a polymer blend comprising at least 50% by weight of a styrenic polymer and up to 50% by weight of an acrylic polymer which has an average molecular weight which is less than 90% of that of the styrenic polymer and (b) a hydrocarbon blowing agent.
The styrenic polymer may be a homopolymer or a copolymer of at least one of styrene, methyl styrene, acrylonitrile. Preferably at least 50% of the monomer units of the styrenic polymer are derived from styrene and in particular the styrenic polymer is a homopolymer of styrene.
Suitably, the styrenic polymer has an average molecular weight from 180000 to 250000, and preferably from 180000 to 230000, for example about 200000.
The acrylic polymer is a homopolymer or a copolymer of at least one C.sub.1-8 alkyl(C.sub.0-4 alkyl)acrylate, for example methyl methacrylate, me
REFERENCES:
patent: 4196267 (1980-04-01), Watanabe et al.
Database WPI, Week 9208, Derwent Publications Ltd., London, GB; AN 92:061823 `Foamed polystyrene with uniform, fine cell structure. `& JP ,A, 4 008 738 (Chisso Corp.) Jan. 13, 1992 see abstract.
Cross Bernard John
Smith Peter John
Foelak Morton
Imperial Chemical Industries plc
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