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25038-59-9

Name POLY(ETHYLENE TEREPHTHALATE)
CAS 25038-59-9
Molecular Formula (C10H8O4)n
MDL Number MFCD00084422
Molecular Weight 228.199
MOL File 25038-59-9.mol
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Chemical Properties

Melting point  250-255 °C
Boiling point  >170 °C(Press: 10 Torr)
density  1.68 g/mL at 25 °C
storage temp.  Room Temperature
form  pellets
color  black
Uses
The first stage to produce Polyethylene Terephthalate (PET) is the reaction of ethylene glycol with terephthalic acid or dimethyl terephthalate. After the initial reaction, two or three polymerization steps are then performed, depending on the required molecular weight. The chemical structure of PET is composed of repeated units. Each unit has a physical length of about 1.09 nm and a molecular weight of ~200. PET monomer consists from an aromatic ring coupled with a short aliphatic chain that makes PET a stiff molecule as compared to other aliphatic polymers such as polyolefin or polyamide. The lack of segmental mobility in the polymer chains results in relatively high thermal stability. A textile grade polymer will have an average number of 100 repeat units per molecule so that the extended length of the typical polymer chain is about 100 nm with a molecular weight of ~20,000. Higher levels of polymerization produce higher strength fibers but the melt viscosity and stability of the melt to even tiny amounts of moisture causes hydrolytic degradation.
Photo-degradation of PET occurs after exposure to near-ultraviolet light resulting in either chain scission by Norrish I and II reactions. Cross-linking also takes place and polymer becomes brittle, discolored, and with uneven surface. PET when exposed to UV light degrades rather rapidly leading to deterioration in physical and mechanical properties and develops intense yellow color. It has been suggested that the photo-oxidation of PET involves the formation of hydroperoxide species through oxidation of the CH2 groups adjacent to the ester linkages and the hydroperoxide species involving the formation of photoproducts through several pathways. The ester moieties in the terephthalate moiety as well as CH2 groups are strongly involved in the photo-degradation of PET. The vinyl ester ends also act as cross-linkers and gelling agents. They polymerize and the polymers thermally degrade to give yellow or brown polyenes that discolor the final polymer. The formation of highly conjugated species is catalyzed by carboxyl groups. The formation of colored species is followed by increase of more carboxyl terminated species. Hence, the product having higher carboxyl value is subject to more discoloration. Furthermore, as the carboxyl content increases, the thermo-oxidative stability decreases. The carboxyl end groups act as catalysts for further degradation.
EPA Substance Registry System Poly(ethylene terephthalate) (25038-59-9)

Safety Data

Hazard Codes  Xi
Risk Statements 
Safety Statements 
WGK Germany  3
RTECS  TR2725000
HS Code  3921905050
Hazardous Substances Data 25038-59-9(Hazardous Substances Data)

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