Amorphous plastic orientation

The most desirable annealing temperatures for amorphous plastics, certain blends, and block copolymers is just above their glass transition temperature (Tg) where the relaxation of stress and orientation is the most rapid. However, the required temperatures may cause excessive distortion and warping. 

The optimum stretching heat for amorphous plastics (PVC, etc.) is usually just above its glass transition temperature (Tg Chapter 1). Generally the orientation temperature is 60 to 75% between the Tg and Tm (melt temperature). For crystalline plastics (PE, PET, etc.) generally it is below the Tg. Stretching can take place in-line or off-line with or without tenter frames using the appropriate temperature-pull rates as the plastic travels first through a series of heated rolls. For unidirectional orientation just the rolls are used. 

Basically, birefringence is the contribution to the total birefringence of two-phase materials, due to deformation of the electric field associated with a propagating ray of light at anisotropically shaped phase boundaries. The effect may also occur with isotropic particles in an isotropic medium if they dispersed with a preferred orientation. The magnitude of the effect depends on the refractive index difference between the two phases and the shape of the dispersed particles. In thermoplastic systems the two phases may be crystalline and amorphous regions, plastic matrix and microvoids, or plastic and filler. See amorphous plastic coefficient of optical stress compact disc crystalline plastic directional property, anisotropic ... 

Semiciystalline Polymers There are two cases to be considered in the stress-strain relationships of semicrystalline plastics. If the amorphous portion is rubbery, then the plastic will tend to have a lower modulus, and the extension to break will be very large see polyethylene. Table 11.1. If the amorphous portion is glassy, however, then the effect will be much more like that of the glassy amorphous polymers. Orientation of semicrystalline polymers is also much more important than for the amorphous polymers. A special case involves fibers, where tensile strength is a direct function of the orientation of the chains in the fiber direction. 

Copolymer type and melt flow rate also influenee the creep behaviour. Copolymer grades of PP have substantially lower ereep modulus than the homopolymer grades. PP has a similar modulus to high density PE, but its resistance to creep is much better and, at a equivalent time under similar load, the creep modulus of PP is more than that of high density PE. However, the ereep resistance of amorphous plastics is much better than the semi-crystalline plastics such as PP and PE. Creep resistance of PP could be further improved by addition of fillers or reinforcements. The creep behaviour of moulded artefacts is affected by the residual stress or orientation effect in the moulded article. 

Plastics shrink in molds during the cooling process. Shrinkage for amorphous plastics is less than for semi-crystalline plastics. Shrinkage is larger in the thickness direction than other directions. Orientation of plastic occurs in the direction of flow. 

The properties of elastomeric materials are also greatly iafluenced by the presence of strong interchain, ie, iatermolecular, forces which can result ia the formation of crystalline domains. Thus the elastomeric properties are those of an amorphous material having weak interchain iateractions and hence no crystallisation. At the other extreme of polymer properties are fiber-forming polymers, such as nylon, which when properly oriented lead to the formation of permanent, crystalline fibers. In between these two extremes is a whole range of polymers, from purely amorphous elastomers to partially crystalline plastics, such as polyethylene, polypropylene, polycarbonates, etc. 

---