Higher shear processing

Higher stabilizer level required, due to higher shear processing... 

Because of the rotation of the N—N bond, X-500 is considerably more flexible than the polyamides discussed above. A higher polymer volume fraction is required for an anisotropic phase to appear. In solution, the X-500 polymer is not anisotropic at rest but becomes so when sheared. The characteristic viscosity anomaly which occurs at the onset of Hquid crystal formation appears only at higher shear rates for X-500. The critical volume fraction ( ) shifts to lower polymer concentrations under conditions of greater shear (32). The mechanical orientation that is necessary for Hquid crystal formation must occur during the spinning process which enhances the alignment of the macromolecules. 

Important for polymer processing is the fact that when the concentration of a hard filler is increased in the composite, the unsteady flow (in the sense of large-scale distortions) of the extrudate occurs at higher shear rates (stresses) than in the case of the base polymer [200, 201,206]. Moreover, the whirling of the melt flow is even suppressed by small additions of filler [207]. 

The dispersions used in this process are formulated in such a way that they wet the carrier sufficiently and tolerate higher shear at the wire-wound bars due to a relatively high coating belt speed. This is accomplished by a combination of nonionic surfactants (e.g., octyl phenoxy polyethoxy ethanol) and fluorosurfactants. This subject is discussed in detail in Reference 14. 

In other cases, several discrete relaxation times or distributions of relaxation times can be found [39]. This is typically the case if the stress relaxation is dominated by reptation processes [42 ]. The stress relaxation model can explain why surfactant solutions with wormlike micelles never show a yield stress Even the smallest applied stress can relax either by reptation or by breakage of micelles. For higher shear rates those solutions typically show shear thinning behaviour and this can be understood by the disentanglement and the orientation of the rod-like micelles in the shear field. 

In PEI the DCG process, as in any polymer, is active. The epsilon CTPZ, however, was not observed. No plane strain shear bands have yet been observed. Some form of localized crack tip shear process can be activated, however, as evidenced by the inversion transition that occured at higher stresses (at the higher temperatures). The fracture surface did not show fracture to occur on a slanted 45 degree plane. The fracture plane was still normal to the leading direction. The fracture surface, however, was not smooth, as seen with craze fracture, but has a definite roughened texture which is associated with active localized shearing. This texture is often described as honeycomb or tufted. 

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