Saturday, December 3, 2011

Styrene/Acrylonitrile (SAN) Copolymer

Copolymerization of styrene with a moderate amount of acrylonitrile (AN) provides a clear, amorphous polymer (SAN).20 The addition of the polar AN group gives increased heat deflection temperature and chemical resistance compared to PS. Other benefits of SAN are stiffness, strength, and clarity. Like PS the impact resistance is still poor. SAN is chemically resistant to hydrocarbons, bases, most detergents, and battery acid. However, SAN has poor resistance to chlorinated and aromatic solvents, esters, ketones, and aldehydes. The composition and molecular weight can be varied to change properties. An increase in AN will
increase SAN’s physical properties but will make melt processing more difficult and will decrease the transparency. In general, the AN level in SAN does not exceed 30% for molding applications. Typical properties appear in Table 7.  Uses. SAN is utilized in typical PS-type applications where a slight increase in heat deflection temperature and/or chemical resistance is needed. Such applications include appliances
(refrigerator compartments, knobs, blender and mixer bowls), electronics (cassette cases, tape windows, meter lenses), packaging (bottle jars, cosmetic containers, closures), medical (syringe components, dialyzer housings), and other (glazing, battery cases, pen barrels).



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    Monday, November 14, 2011

    Thermosetting Plastics

    A thermosetting plastic is produced by a chemical reaction which has two stages. The first stage results in the formation of long chain-like molecules similar to those present in thermoplastics, but still capable of further reaction. The second stage of the reaction (cross-linking of chains) takes place during moulding, usually under the application of heat and pressure. The resultant moulding will be rigid when cooled but a close network structure has been set up within the material. During the second stage the long molecular chains have been interlinked by strong bonds so that the material cannot be softened again by the application of heat. If excess heat is applied to these materials they will char and degrade.
    This type of behaviour is analogous to boiling an egg. Once the egg has cooled and is hard, it cannot be softened again by the application of heat. Since the cross-linking of molecules is by strong chemical bonds,
    thermosetting materials are characteristically quite rigid materials and their mechanical properties are not heat sensitive. Examples of thermosets are phenol formaldehyde, melamine formaldehyde, urea formaldehyde, epoxies and some polyesters.

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  • PLASTICS ENGINEERING Third Edition
    R.J. Crawford, BSc, PhD, DSc, FEng, FIMechE, FIM
    Department of Mechanical, Aeronautical
    and Manufacturing Engineering
    The Queen’s University of Belfast



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    Monday, September 19, 2011

    Syndiotactic Polystyrene (SPS

    Syndiotactic polystyrene is a semicrystalline polymer and is produced via metallocenecatalyzed polymerization of styrene.21 By comparison to general-purpose, amorphous polystyrene (PS, HIPS), SPS has  tereoregularity in its structure, which facilitates crystallization. In SPS the phenyl groups alternate above and below the polymer chain. Hence SPS has a high crystalline melt point and good chemical resistance.
    The slow rate of crystallization and high Tg of SPS typically require an oil-heated tool when injection molding and longer cycle times to maximize physical propereties. Typical properties appear in Table 6. Uses. SPS is targeted at automotive under-the-hood and specialty electronic applications. It will compete with polyamides, thermoplastic polyesters, and polyphenylene sulfide (PPS).



    Edward N. Peters
    General Electric Company
    Selkirk, New York

    Mechanical Engineers’ Handbook: Materials and Mechanical Design, Volume 1, Third Edition.
    Edited by Myer Kutz
    Copyright  2006 by John Wiley & Sons, Inc.



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