Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Cellular products or processes of preparing a cellular...
Patent
1998-02-05
1999-11-16
Foelak, Morton
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Cellular products or processes of preparing a cellular...
521 51, 521 79, 521 81, 521139, C08J 902, C08J 934
Patent
active
059859466
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to 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.
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.multidot.m.sup.-3 using an extrusion process in which liquid carbon dioxide is employed as a blowing agent.
U.S. Pat. No. 4,552,904 discloses the preparation of a polystyrene foam by the re-expansion of an extruded foam. The thermal conductivity of the materials produced according to the invention of U.S. Pat. No. 4,552,904 are shown to have thermal conductivities of 0.032 Kcal.multidot.(m.multidot.hr.multidot..degree.C.).sup.-1, i.e. 37.2 mW.multidot.(mK).sup.-1 or above. Furthermore, commercial products are shown to have thermal conductivities of 0.029 Kcal.multidot.(m.multidot.hr.multidot..degree.C.).sup.-1, i.e. 33.7 mW.multidot.(mK).sup.-1 or above.
For a foam to be considered a "good" insulating material it should have as low a thermal conductivity as possible and the value of the thermal conductivity should remain stable with time.
The thermal conductivity of a foam is largely associated with the bubble structure within the foam, the density of the foam and the material which fills the bubbles within the foam. In foams that have been blown by gaseous blowing agents, typically carbon dioxide, CFCs and HCFCs, the gaseous blowing agent tends to leach out of the foam to be replaced by air. The thermal conductivity of a foam which contains air is much inferior to that of one within which is retained the gaseous blowing agent.
The present invention provides foams which have lower initial thermal conductivities and retain gaseous carbon dioxide blowing agent over longer periods thereby maintaining beneficial thermal properties. Additional benefits may also include the reduced emission of gaseous blowing agents to the environment and the ability to prepare foams without the need for the inclusion of fire retardants or plasticisers within the foam.
Accordingly, in a first aspect the present invention provides a foamed article having a closed cell structure formed from
(i) a polymer blend comprising at least 80% by weight of a styrenic polymer and up to 20% by weight of an acrylic polymer; and
(ii) gaseous carbon dioxide blowing agent.
The styrenic polymer may be a homopolymer or a copolymer of at least one of styrene, methyl styrene and 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 (number average) from 180000 to 250000, and preferably from 180000 to 230000, for example about 200000. Adventitiously, the present invention can also be used with styrenic polymers which are obtained at least in part from recycled or scrap materials.
The acrylic polymer may be a homopolymer or a copolymer of at least one C.sub.1-8 alkyl (C.sub.1-4 alkyl)acrylate, for example methyl methacrylate, methyl acrylate, ethyl acrylate, propyl acrylate and butyl acrylate. Preferably the acrylic polymer is a copolymer, particularly a copolymer containing a methacrylate, e.g. methyl methacrylate, and an acrylate, such as methyl acrylate or ethyl acrylate. The acrylic polymer preferably contains from 50 to 100%, particularly from 80 to 95, and especially 90 to 95 by weight of monomer units which are derived from a methacrylate, e.g. methyl methacrylate, with the balance of the monomer units being derived from at least one acrylate. Preferably, the acrylic polymer has an average molecular weight (number average) which is less than 90%, preferably less than 80%, particularly less than 75% and especially from 50 to 60%, that of the styrenic polymer. Suitably, the acrylic polymer has an average molecular weight from 50000 to 180000, preferably from
REFERENCES:
patent: 4196267 (1980-04-01), Watanabee 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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