A reactive melt modification of polyethylene terephthalate

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After this stage, PET pellets are transferred to hopper reactors, maintained at a
temperature of about 216 °C and swept by a continuous stream of nitrogen under reduced
pressure, resulting in increased MW at the end of the solid-stating. The disadvantages in
this method are the very slow reaction rates, unfavorable economics, and the need for
special equipment such as special heaters, hopper driers, hopper reactors for the largescale
operation (Aharoni, 2002).
Another method proposed to increase MW and broaden the MWD, particularly
applicable to foaming and extrusion blow molding applications, is to use the so-called
"chain extenders", these are able to react with a polymer end groups more effectively
either during solid stating or, alternatively when added during a reactive extrusion
process.
The production of extrusion foamable polyesters modified by solid state
polyaddition with a premixed multifunctional modifier, e.g. pyromellitic dianhydride
(PMDA) is discussed in a series of patents and publications from Sinco Engineering (Al
Ghatta, 1993, Al Ghatta and Pizzetti, 1993, Al Ghatta et a1., 1993). These resins are
distinguished from non-foamable resins by increased MW, as measured by intrinsic
viscosity I.V. (>0.9dL/g), increased melt strength, high extrudate swell, increased
complex viscosity with higher shear sensitivity and increase in storage modulus. Postreactor
modification by reactive extrusion of PET carried out using PMDA or other
branching additives also results in significant increase in zero shear viscosity, increase in
melt strength and die swell, increase in molecular weight and increase in polydispersity
ratio Mz/Mn from about 4.5 up to 11. Property changes depend on the choice of process