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A foam with acceptable cell
structure and low density cannot be produced with all materials. A balance
between viscous and elastic characteristics is necessary. Usually a lower
viscosity material does not provide high enough pressure drop at the die
and foaming may start before the material exits the die land. The cells,
created because of premature nucleation,
deform at the high shear region of the die and collapse. On the other hand,
processing of very high viscosity material is very difficult. Materials
with medium viscosity and high elasticity are generally preferred for extrusion
foaming. Sometimes it is possible to chemically modify the material in
the extruder before foaming.
The picture on the left shows the die swell of a 0.7 IV PET; the picture on the right shows the die swell of the same PET with reactive modifier. The material on the right has higher melt strength and is easily foamable. The actual foaming process is more complicated. The material going through the die is not polymer alone but a solution of blowing agent in the polymer. Normally the blowing agent acts as a plasticizer for the polymer and reduces melt viscosity, melt elasticity, glass transition temperature and polymer solidification temperature. Research is under way at PPI to understand the actual mechanism of foaming. Traditional capillary viscometer, cone and plate or plate and plate viscometers cannot be used for measuring the viscosity of the blowing agent polymer solution. It is necessary to measure viscosity under hydrostatic pressure. PPI has developed a new rheometer, the Helical Barrel Rheometer (HBR ) which can be used for this application. This information is very useful in the die design for the foaming process. For more information, please contact Dr. Niloufar Faridi (973) 596-5665
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