Answer to the May 15, 2000 quiz question at PolymerProcessing.com

Answer to the May 15, 2000 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?

There are many reasons why local temperatures within an extrusion operation may be much higher then the average temperature. This is due to the low thermal conductivity of polymer melts along with high rates of viscous dissipation in regions with high deformation rates. Degradation due to local heating is the most likely cause of the problem. The most straightforward solution is use of a more gentle screw and/or opening up the channels in the die in order to reduce viscous dissipation.

It is important to recognize the limitations of temperature measurements available on typical processing equipment.

In most cases, so called "melt thermocouples" actually measure an average of the temperatures of the materials they are in contact with, including the melt, the thermocouple sheath, and the barrel. In order to accurately measure a real melt temperature a more sophisticated system like an infrared system is required.

An experiment in which the extrudate is collected in an insulated container and the temperature is measured produces a result which is called the "cup mixing temperature." This can be more accurate than a melt thermocouple; however, this result is confounded with the velocity field which produces the flow into the cup. Specifically, the cup mixing temperature is the result of an integral of the temperature over the flow domain.

High local heating often occurs during the processing of polymer melts due to viscous dissipation. In regions where the shear rate is high mechanical energy is quickly transformed to heat. Due to the low thermal conductivity of polymers heat is transferred only slowly to the surrounding melt or equipment walls.

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