Orientation and shrinkage

As previously stated, molecular orientation occurs during melt processing of polymers. On removal of the deforming stresses the molecules start to coil up again but the process may not go to equilibrium before the polymer cools to below its Tg. This leads to residual orientation (frozen-in strain) and corresponding frozen-in stresses. 

The resulting mouldings and extrudates are consequently anisotropic and mouldings can be four to five times as strong in one direction as in another direction. This can lead to planes of weakness and easy fracture when subject to shock (impact) stresses. Generally such orientation is undesirable but there are at least two instances of its being of value  

In general it may be said that the amount of frozen-in orientation will depend on  

With crystalline polymers the more orderly molecular packing leads to much greater shrinkage. Variations in moulding conditions can lead to large variations in shrinkage and need to be closely controlled. The main factors which cause an increase in shrinkage are  

Shrinkage is often different along the lines of flow and perpendicular to them. It is commonly found that an increase in the melt temperature reduces the along-flow shrinkage but increases the shrinkage across the flow . Volumetric shrinkage is, however, virtually unaffected by melt temperature. 

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Figure 12.9 Effect of spinning speed on fiber orientation and shrinkage [14]. From Brunnschweiler, D. and Hearle, J. (Eds), Polyester - 50 Years of Achievement, 1993, p. 193, and reproduced by permission of The Textile Institute, Manchester, UK...

When processing thermoplastic materials, material properties not only dictate drying and processing conditions, viscosity, orientation, and shrinkage, but also the processing techniques and equipment that can be used. The next section reviews material properties with an emphasis on processing temperatures, particle properties, melt viscosity and elasticity, and orientation, relaxation, crystallization, and shrinkage. 

Fig. 8. Orientation and shrinkage of spun yarn vs spinning speed (18).

Film Formation and Propoties.— Work on film preparation and subsequent processing of 2GT has been limited to studies on the transverse constriction and stresses observed during uniaxial orientation and the problems associated with the maintenance of planarity during heat relaxation. Orientation and shrinkage have been discussed in a number of papers with studies on orientational setf-ieinforcement and relaxation and its effects on orientation. The effect of the interaction of films with organic liquids has promoted studies on the structural arrangements in liquid-induced crystallization of cold-drawn 2GT films, and the effect of crystallization on adhesion and cohesion. ... 

Crimp. The tow is usually relaxed at this point. Relaxation is essential because it gready reduces the tendency for fibrillation and increases the dimensional stabiUty of the fiber. Relaxation also increases fiber elongation and improves dye diffusion rates. This relaxation can be done in-line on Superba equipment or in batches in an autoclave. Generally saturated steam is used because the moisture reduces the process temperatures required. Fiber shrinkage during relaxation ranges from 10 to 40% depending on the temperature used, the polymer composition used for the fiber, and the amount of prior orientation and relaxation. The amount of relaxation is also tailored to the intended apphcation of the fiber product. 

In general, more highly oriented and therefore higher modulus fibers tend to exhibit lower shrinkage and less creep. Creep is an important factor in the control of tire dimensions during service and in certain aspects of tire appearance (30). 

X-ray diffraction and thermomechanical analysis are used, respectively, to examine structural order and shrinkage behaviour for oriented samples of rigid and flexible PVC. Results are compared with previously measured tensile... 

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