Blend uniformity history

If the FDA chooses to follow the stratified sampling recommendation of the PQRI, blend uniformity testing and the large problem of sampling may soon become history. Until then, however, guidelines should be followed at all times, awareness should be raised to minimize errors, and the importance of sampling should not be underestimated. Testing must be carried out frequently to ensure that each blend reaches the standards set. 

Russell and Stein [9] also reported on the effects of thermal history on blend morphology. When samples were heated to just above the PCL melting point they found that recrystallisation resulted in the formation of rod-like crystals but if samples were heated above 90 °C recrystallisation gave spheruhtes. The implication of these data is that crystalline polymer, on melting, cannot diffuse into the amorphous material to form a uniform homogeneous phase at low temperature but can at high temperatures. This effect was not observed in earlier studies, possibly because of the influence of the low molecular weight fraction and nucleation in the blends. 

Even at the end of mixing, the system is not uniform and consists of parts having different viscoelastic history. The model described here represents only macroscopic changes. Microscopic change such as growth of bound rubber must be accommodated later. This type of phase-modelling is in progress in the field of polymer blends and a similar approach may be adopted here. 

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