subject: Chemical Composition of Polyethylene terephthalate [print this page] Chemical Composition of Polyethylene terephthalate
As far as it's processing and history is concerned, polyethylene terephthalate exists both as an amorphous (transparent) and as asemi-crystalline polymer. The semicrystalline material might appear transparent (particle size < 500nm) or opaque and white (particle size up to a few microns) depending on its crystal structure and particle size. Its monomer (bis--hydroxyterephthalate) can be synthesized by the esterificationreaction betweenterephthalic acidandethylene glycolwith water as a byproduct, or bytrans-esterificationreaction betweenethylene glycolanddimethyl terephthalatewithmethanolas a byproduct. Polymerization is through a polycondensationreaction of the monomers (done immediately after esterification/transesterification) with ethylene glycol as the byproduct (the ethylene glycol is directly recycled in production). you can get more information about Polyethylene terephthalate at www.worldofplastic.net
A transparent, heat resistant polyester composition having improved mechanical strength is provided by comprises (A) 20 to 80% by weight of a polyalkylene terephthalate predominantly comprising ethylene terephthalate and having an intrinsic viscosity of from 0.6 to 1.2 dl/g, and (B) 20 to 80% by weight of a bisphenol-A polycarbonate having an intrinsic viscosity of from 0.6 to 1.2 dl/g, the composition having an intrinsic viscosity of from 0.6 to 1.2 dl/g and a single glass transition temperature of from 80 to 130 C.
Recently, Poly (trimethylene terephthalate) (PTT) has been introduced to the same family of poyesters as polyethylene terephthalate (PET) and polybutyllene terephthalate (PBT). Owing to its outstanding characteristics such as high elastic recovery, chemical resistance and resilience, PTT has attracted increasing attention. Like PET and PBT, the applicability and processing condition of PTT are determined by its thermal properties, such as crystallization, melting etc.
In addition to pure (homopolymer) PET, PET modified bycopolymerizationis also available. In some cases, the modified properties of copolymer are more desirable for a particular application. For example,cyclohexane dimethanol(CHDM) can be added to the polymer backbone in place ofethylene glycol. Since this building block is much larger (6 additional carbon atoms) than the ethylene glycol unit it replaces, it does not fit in with the neighboring chains the way an ethylene glycol unit would. This interferes with crystallization and lowers the polymer's melting temperature. Such PET is generally known as PETG (Eastman Chemical and SK Chemicals are the only two manufacturers). PETG is a clear amorphous thermoplastic that can be injection molded or sheet extruded. It can be colored during processing.
