Orientation effects polyethylene films

While we are concerned mainly with solutions, it is interesting that the parentage relationship appears also in the other possible passage of a solid polymer into a liquid phase, namely by melting. Already in 1945 Charlesby (17) pointed out the existence of what he called the memory effect in polyethylene films orientation was preserved even after prolonged heating... 

Biaxial orientation effects are important in the manufacture of films and sheet. Biaxially stretched polypropylene, poly(ethyleneterephthalate) (e.g., Melinex) and poly(vinylidene chloride) (Saran) produced by flat-film extrusion and tentering are strong films of high clarity. In biaxial orientation, molecules are randomly oriented in two dimensions just as fibers would be in a random mat the orientation-induced crystallization produces structures which do not interfere with the light waves. With polyethylene, biaxial orientation often can be achieved in blown-film extrusion. 

The CG model (Cook-Gordon model) of crack blunting does provide a plausible explanation for macroscopically observed effects. In fact, in oriented polypropylene and polyethylene films a tendency to flbrillize following storage under UV... 

Indeed, at very high polymer concentrations enhancement of these effects occurs. Recent studies by Franks et al. 23) on the rheological behaviour and freeze fracture electron microscopical analysis of several synthetic linear flexible polymers, including poly(vinylpyrrolidone) and polyethylene glycol) in concentrated solutions, suggest that these molecules do not form a network mesh but rather exhibit aggregation. Anionic polysaccharides, on the other hand, are known to form an anisotropic packing array in condensed films. These films may be stretched to enhance orientation and be used for X-ray diffraction studies... 

The range over which the state of orientation can be changed in the film blowing operation and the resulting effects on mechanical properties have been demonstrated by Bates.He investigated the effect of both transverse and longitudinal orientation on the impact strength of low density polyethylene. 

W. Chinsirikul, T. C. Hsu, and I. R. Harrison, Liquid crystalline polymer (LCP) reinforced polyethylene blend blown film Effects of counter-rotating die on fiber orientation and film properties, Polym. Engr. and Sci., vol. 36, No. 22, 2708-2717, November (1996). 

Initial die gaps are set to about 20 percent greater than the final film thickness, and then adjusted to accommodate changes in polymer flow which are resin and rate sensitive. Higher screw speeds increase extruder output, overall film thickness, the tendency toward melt fracture, and may alter the flow pattern. Thus, extruder speed is not a recommended control. In contrast, increased chill-roll speeds decrease film thickness, reduce film width due to increased neck in, increase uniaxial orientation, and alter the optimum air gap or drawdown distance. The optimum air gap, which produces the best orientation, crystallization, and surface properties, depends on the material and chill-roll speed. At 23 to 30 m/min (75 to 100 ft/min), the air gap for low-density polyethylene is about 100 mm (4 in), but when the line speed increases, the air gap is found by trial and error, Since the chill-roll speed controls film stretching, the take-off speed has little effect on the film dimensions. 

GMS behaves anomalously. In polypropylene it forms a surface film, with its polar groups (stearate) oriented within the polymer, and its non-polar groups (glyceryl) oriented away from the surface, the opposite of the usual trend. It is fast-acting but its antistatic effect dies away, typically after a few weeks. On the other hand ethoxylated amines can be slow to achieve their antistatic effect in low-density polyethylene. 

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