Highly oriented polymers state extrusion

NEW METHODS OF PRODUCTION OF HIGHLY ORIENTED POLYMERS BY SOLID STATE EXTRUSION... 

Zachariades, A. E., Mead, W. T., Porter, R. S. Recent developments in ultramolecular orientation of polyethylene by soldid-state extrusion, in Ultra-High Modulus Polymers (eds.) Ciferri, A., Ward, I. M., p. 77, London, Applied Science 1979... 

Table 2.5 clearly Indicates that the mechanical properties of solid-state extruded HOPE are much superior to the melt extruded HDPE. In fact, the tensile strength of solid-state extruded HDPE is about the same as carbon steel There are some other interesting benefits associated with solidstate extrusion of polymers. There is essentially no die swell at high extrusion ratios (extrusion ratio is the ratio of the area in the cylinder to the area in the die). Thus, the dimensions of the extrudate closely conform to those of the die exit. The surface of the extrudate produced by hydrostatic extrusion has a lower coefficient of friction than that of the un-oriented polymer. Above a certain extrusion ratio (about ten), polyethylene and polypropylene become transparent. Further, solid-state extruded polymers maintain their tensile properties at elevated temperatures. Polyethylene maintains its modulus up to 120°C when it is extruded in the solid state at a high extrusion ratio. The thermal... 

Practical polymer processing operations, such as extrusion and injection, are complex processes highly dependent on the conditions of shear, extensional stress and temperature. It has now been realized that shear stress and extensional stress have different effects on the orientation of PLGs. Nematic PLCs have a polydomain texture and each domain consists of mesogens with the same local orientation. The directions of these domains are randomly aligned while in a quiescent state. Only if a stress is applied are the domains oriented in one direction. As Ide and Ophir [6] and Viola et ah [7] have pointed out, shear stress is related to rotational motion (torque), and its application will result in a tumbling flow of PLC domains. Only once the shear stress has reached a critical value will it break down the domains and lead to a uniform molecular orientation. In contrast, extensional stress tends to orient the domains in one direction without breaking down the domains even if the stress is low. Viola et ah [7] also considered that shear flow induces sheet-like textures while extensional flow induces fiber-like textures. Therefore there will be differences in the hierarchical and fibrillar structures acquired in different fibrication processes. 

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