Thermal Degradation on a Film Extrusion Line
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
How can thermal degradation of the poly(ethylene terephthalate) be occuring despite the fact that
all process temperatures are well below the degradation threshold?