Polymers cold-drawing

An oriented crystalline polymer usually has a much higher tensile strength than the unoriented polymer. Cold drawing is an important step in the production of synthetic fibers. 

There are two major methods used to produce very highly oriented, high-modulus polymers cold drawing and hydrostatic extrusion. Although the highest moduli may be reached by way of the cold-drawing technique, hydrostatic extrusion provides better control over the final product dimensions than does drawing. 

Fig. 23.10. Cold-drawing of o linear polymer the molecules ore drown out and aligned giving, after o draw ratio of about 4, o material which is much stronger in the draw direction than it was before.

Obviously, the discrepancy between the experimental data [238-241] and predictions of the theory [236,237] can be attributed to the difference of the coefficients of thermal expansion. The polymer exerts pressure on the filler, thereby masking the effect of the strength of adhesion on the modulus. The pressure on the filler may be sufficiently high. In [243] it was found, for example, that in PP, quartz particles experienced a compression force of about 100 MPa after cold drawing of the composite the force reduces to 50 MPa in the direction of drawing but at the same time increases to 300 MPa in the perpendicular direction. 

Thermoplastic polymers subjected to a continuous stress above the yield point experience the phenomenon of cold-drawing. At the yield point, the polymer forms a neck at a particular zone of the specimen. As the polymer is elongated further, so this neck region grows, as illustrated in Figure 7.7. 

Figure 7.7 Successive stages in cold-drawing of a polymer...

Figure 7.8 Stress-strain curve for polymer during cold-drawing...

Polymers <100% crystalline Regularity of structure needed for high crystallinity Stretching Cold drawing... 

Prepare the polymer samples for the cold drawing experiment by cutting six to eight specimens about 2 mm X 2 to 3 cm from the film described in the preceding section. Exact width is unimportant, but the samples must have a constant width and edges must be free from nicks or irregularities. 

Dilatometric studies have demonstrated the negative thermal expansivity for many oriented crystalline polymers 64,170 176). The results of these experimental studies may be summarized as follows. Cold-drawing of PE below Tm 172) and solid-state extrusion under elevated pressure 170 1711 lead to a monotonous decrease of the positive thermal expansion coefficient with increasing draw ratio. At a certain degree of orientation, dependent on temperature, PM becomes negative with Pi < Pell (Fig. 16). This is the second way of reaching negative expansivity applied, e.g. to POM (w = 63 % Tdr = 423 K) 173>. 

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