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Blog
What is the relationship between shrinkage and orientation?
June 28, 2008
Anandh Balakrishnan of the University of Oklahoma's Nanocomposites lab asks: What is the exact relationship between shrinkage and orientation? What thicknesses are we talking about in such cases?
Well, there is no unique relationship between orientation and shrinkage because of variations in stretching and annealing (stress relaxation) conditions. What needs to be done for every manufacturing situation is to measure the shrinkage as a function of temperature, and this can then be correlated with the amorphous phase orientation. Basically the recoverable strain (shrinkage) will depend on the stretching rate, cooling rate and any post quenching thermal history (annealing). Also, if the part is restrained or unrestrained during annealing.
Another thing to remember is that the recoverable strain is in the amorphous portion of the part as the crystals are already formed. Crystals will serve to restrict the shrinkage by acting as physical restraints to the relaxation of the amorphous polymer chains. This is how oriented PET is heat-stabilized. In the PET orientation process, the film is formed from a predominantly amorphous sheet. After stretching, if the film is quenched, it will be a shrink film. Therefore, to inhibit the shrinkage, the film is crystallized by heating at the crystallization temperature while the film is restrained in the tenter frame. This will increase the PET film crystallinity from perhaps 15-25% to about 55%.
Shrinkage should be measured as a function of temperature because it is possible that a film or part will grow in one direction as it shrinks in another direction. I commented about this in an earlier posting on April 9 and on March 24 and March 20, which you might like to review.
As far as thickness goes, most films are in the range of 12 to 250 microns (0.5 to 10 mils) but all polymer parts will shrink if oriented and reheated regardless of their dimensions. All that is necessary is uniform heating to a particular temperature.
Posted by Eldridge M. Mount on June 28, 2008 | Comments (0)