Dependence of Rheology on Extrusion Foaming

A foam with acceptable cell structure and low density cannot be produced with all materials. A balance between the viscous property and elastic property is necessary. Usually lower viscosity material does not provide high enough pressure drop at the die and the foaming of the material starts before it 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. Material with medium viscosity and high elasticity is generally preferred for extrusion foaming. Some times it is possible to modify the material during foaming.

 

 

The picture on the left shows the melt strength of 0.7 IV PET and the picture on the right shows the melt strength of the same PET with reactive modifier. 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 all the melt viscosity, melt elasticity, glass transition temperature and polymer solidification temperature. Some work has been done along these lines at PPI to understand the actual mechanism of foaming. More work is in the way.

 

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 this viscosity under hydrostatic pressure. PPI has developed a new rheometer, Helical Barrel Rheometer ( HBR ) which can be used for this application. Using this device, one can measure the viscosity of the blowing agent polymer solution as a function of shear rate and hydrostatic pressure and of course temperature. This information is very useful in the die design for the foaming process.

 

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For more information please call

Subir K. Dey

subirdey@yahoo.com

Tel. # (973) 642 4996

Polymer Processing Institute

at

New Jersey Institute of Technology

Dept. PEC/CMS

218 Central Avenue

Newark, NJ 07102