Stretching temperature

As temperature increases, the layer expands and the orientation of endbeads smears. This is shown in Figure 1.46. The observed expansion of layer thickness is attributed mainly to the temperature dependence of the intermolecular interaction and excluded volume effect not due to bond stretching. Temperature dependence, which is pertinent to the annealing process of thin polymeric lubricant films, has been carefully examined by Hsia et al. [171]. 

At present most PVC films, both HT and LT, are stretched on this type of equipment. For the improvement of the mechanical properties by molecular orientation, the stretching temperature is in the range of 100-130°C in order to use the thermoelastic properties of the material. 

It seems to be possible to split each curve into 2 individual melting peaks whose positions shift with the stretching temperature to higher values. Furthermore, while the high-temperature peak is nearly constant the low-temperature peak increases with stretching temperature. 

The web passes through an inlet nip, over the preheat rolls and enters the stretch section (Fig. 5). A high speed nip pulls the film in the machine direction. The amoimt of orientation in the film will be determined by the strain rate, stretch temperature, amoimt of stretch, and how quickly the film is cooled. Stretching between two rolls is a very high strain rate process in industrial settings, often of the order of 5000%/s. The level of orientation attainable is quite high. 

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