Cold-drawing natural draw ratio

Table 2. Lamellar thickness 1, as function of annealing temperature at 5.3 Kbar for PE cold-drawn to a natural draw ratio X 8.

In seetion 8.2.2 the phenomenon of cold drawing and the associated natural draw ratio are described. This process is referred to again in section 10.2.2 and discussed further in section 10.2.4. 

The natural draw ratio is the length to which a unit length of fiber has been stretched by cold-drawing and subsequent relaxation. 

As noted, once the material has been drawn into the craze it appears to stabilize, and little, if any, further extension takes place. As with cold drawing, the fibrillar material is seen to take on a natural draw ratio, Xn, the value of which is characteristic of the individual polymer. Kramer (116,141) found that the natural draw ratio correlated well with the maximum theoretical extension expected for an entangled network, Xmax ... 

Cold-drawing to a length I2 gives a natural draw ratio... 

The natural draw ratio for amorphous polymers is very sensitive to the degree of preorientation, i.e. the molecular orientation in the polymer before cold-drawing. This was reported for polyethylene terephthalate by Marshall and Thompson [20] and for PMMA and polystyrene by Whitney and Andrews [26]. 

It is, of course, possible that the natural draw ratio is determined directly by the strainhardening requirements. This does not invalidate the hypothesis that cold-drawing involves the extension of a molecular network, but suggests that strain hardening increases very rapidly as the network reaches its limiting extensibility. 

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