Processing polyethylene films, effect

In the first stage in order to test the process of various types of polymer films were surface-fluorinated. From 1990 to 1994 it was shown that XeF2 could be used effectively for surface fluorination of a variety of plastics. Polyethylene film and plates,18 aromatic polysulfone,19 polyvinyltrimethylsilane,20 and polycarbonate,21 among other polymeric materials, were fluorinated successfully. 

Phenolic antioxidants are well known for being melt processing stabilizers as well as long-term thermal stabilizers. In the chemiluminescence measurements on the polyethylene films under oxygen (Fig. 3.3), the antioxidant effect of the phenols is clear when these induction times are compared with those of the free additive polyethylene film (0.73 h) (Table 3.2). The results showed that the structure of the phenolic moiety will be a cmcial factor influencing the stabilization performance. 

Table II. Effect of Processing Conditions on the UV Lifetime of Polyethylene Films Containing Synergistic UV Stabilizers0...

One effect that sustained exposure to elevated temperature has is to stabilize the structure of the material. The processing of polymer materials frequently leaves residual stresses of various types in the material. In addition, the cooling rate on a polymer with a crystalline structure may be such that the equilibrium is not reached during processing. Sustained exposure to heat will result in relaxation of the residual frozen-in stresses and/or the recrystallization of the material. Both of these effects involve movement of the material on a molecular or micro scale with a consequent distortion of the overall shape of the object. In the case of a quenched polyethylene film which has a high degree of clarity, the effect of the recrystallization would be to increase the haze level so that the material becomes opalescent or even opaque. 

Fig. 3.15. The effect of processing at 150°C in air on the thermal oxidative stability of low density polyethylene films. The numbers on the curves are processing times in minutes. (Reprinted with permission from [414], Pergamon Press Ltd, Oxford, England.)...

Section 11.17 shows many data characterizing performance of slip agents in polyethylene film. The discussion on bulk and surface concentrations, migration to surface, distribution between layers of different composition, and the effect of these essential parameters on coefficient of friction is important for selection of additives and process design of film manufacture. 

Figure 10.24 shows the effect of erucamide concentration on the coefficient of friction of linear low density polyethylene film. Two different trends are present. If the coefficient of friction is measured as soon as film is made it has linear relationship with concentration of erucamide. But for measurements done after 1 hour this relationship shows that coefficient begins to approach constant value, independent of concentration beginning at 400 ppm emcamide. This shows that different properties are to be expected during the process and in material during storage. These different properties may in some cases require different concentrations of additive. 

Polyethylene. The most straightforward process for the production of polymers from ethylene is that of the direct polymerization of the olefin. The polymerization process usually requires pressures and temperatures of 15,000 to 30,000 pounds per square inch and 200° to 300° C., and may be effected in either gas or liquid phase reactions (9). The polymer of molecular weight above 20,000 is the white, translucent plastic, polyethylene, widely used in electrical insulation, packaging material for foods, cosmetics and pharmaceuticals, liners for paper bags, etc. Articles molded from polyethylene are semirigid or rigid, depending on their thickness, but in thin films the material has excellent flexibility, even at relatively low temperatures. 

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