Comonomers - the property changers

This specialist monomer (diacid) is used in various concentrations to increase the Tg of the polyester or to increase its oxygen barrier properties. The stiffness of the naphthalene (two benzene rings) reinforces the backbone of the polyester and inhibits bending at lower temperatures it also increases the pathway for oxygen molecules and hence provides an increased gas, parhcularly oxygen, barrier. A fairly new and naturally expensive monomer to manufacture, in the United States it is approved for use for some applications by 21 CFR 177.1637 and for others via the FDA premarket notification system and is 

This monomer (diol) is used to broaden the processing window, increase line speeds, slow the crystallisation rate and reduce stress in biaxially orientated structures. It is used in concentrations ranging from 1% up to about 18%. The higher concentration PET is usually referred to as PETG and is mainly used for extrusion blow moulding and sheet extrusion. Polymers from PTA and CHDM are approved in the US by 21 CFR 177.1315 recently the use of CHDM as comonomer in PTA-EG polymers has been listed in the US FDA regulation for PET (21CFR 177.1630). Moreover, the use of CHDM has been approved in Europe for plastics intended for food contact applications since the first directive 90/128/EC. 

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