May 15, 2000 Quiz Question

Thermal Degradation on a Film Extrusion Line

Perspective Meter:

Poly(ethylene terephthalate) film is being cast using a 4 inch dia. single screw extruder running at 150 rpm. Substandard physical properties of the film product indicate that thermal degradation is occuring during the processing operation. However, process temperature measurements are well below the threshold at which thermal degradation is known to occur. Specifically, careful laboratory measurements have shown that thermal degradation does not occur with this formulation unless the temperature exceeds 350C. The extruder has five temperature zones set as follows:

(Feed Section) 200C, 275C, 285C, 285C, 285C (Die)

The melt thermocouple at the die reads steady at 290C. As a check, a sample of the extrudate was collected in an insulated bucket and its temperature was directly measured at 293C using a thermocouple.

How can thermal degradation of the poly(ethylene terephthalate) be occuring despite the fact that all process temperatures are well below the degradation threshold?

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