Compounding processes such as mixing, kneading, extrusion, pelletization are of great importance because the properties, processing, and use of thermoplastic polymers are substancially determined by the additives The additives include nucleating agents, fillers, flame retardants, stabilizers, and pigments
The properties of thermoplastic polyesters depend primarily on the starting compounds (dicarboxylic acids, diols), their molecular weight, and added fillers
PETP and PBT are partially crystalline polymers. They have high hardness and rigidity, good strength, high dimensional stability, and very good slip and wear behavior PETP can be processed into amorphous molded bodies with high transparency. On heating to 70100°C this transparency is lost due to postcrystallization.
More than 85% of PETP is processed into fibers. A large proportion is used to produce gastight bottles for carbonated beverages. Highly stressed technical molded parts such as bearings, gearteeth, cam wheels, connectors, bolts, screws, and washers are produced from PETP by injection molding.
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The history of thermoplastic polyester goes back to 1929 with the pioneering work of Carothers. The first thermoplastically processible polyesters synthesized from adipic acid and ethylene glycol were described by him in 1932. Polyesters only became of industrial interest in 1941, with the synthesis of high melting point products based on terephthalic acid.
The rapid industrial development of polyesters after World War II was initially restricted to polyester fibers based on polyethylene terephthalate. It was subsequently used in the production of films. Thermoplastic polyesters were first employed as construction material in 1966. In 1970 the more readily processible polybutylene terephthalate (PST), polytetrmethylene terephthalate was introduced into the market A short time later copolyesters were introduced for powder coating, paint binders and hot-melt adhesive applications.
Recently a new class of fully aromatic thermoplastic polyesters has been developed – liquid crystalline polyesters. These polymers have outstanding mechanical and thermal properties.
Four most important raw materials are Dimethy Terephthalate (DMT), Terphthalic Acid (T A), Ethy!ene Glycol and 1,4-Butanediol. Raw materials are required with very high purity because impurities can either interfere with polycondensation via chain termination or branching or can lead to undesirable secondary reactions and discoloration under high reaction temperatures.[ad#aryshi1]
Polyesters are defined as all polymers containing repeating units of ester group
in the polymer chain. Polyesters are generally produced by condensation of polyfunctional carboxylic acids with polyfunctional alcohols.
Polyesters with widely varying properties can be produced depending on the starting materials:
1) High molecular mass linear polyesters (>10000) are produced from difunctional alcohols and dicarboxylic acids They can be processed thermoplastically to molding materials.
2) Low molecular mass polyesters «10000) from saturated aliphatic or aromatic dicarboxylic acids and mixtures of di- and trifunctional alcohols. These are linear or slightly branched intermediates for polyurethanes.
3) Low molecular mass polyesters «10000) from di-, tri-, and polyfunctional alcohols and polyfunctional (usually aromatic) carboxylic acids. These are classified as alkyd resins
4) Unsaturated polyesters are polyesters which can copolymerize with unsaturated compounds and are formed from polyfunctional alcohols and polyfunctional unsaturated carboxylic acids. After copolymerization with monomers such as styrene they can also be classified as thermosets
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