هدی بیاضیان

In this study, the influence of line speed on the crystallinity of linear low-density polyethylene (LLDPE) stretch films manufactured in a cast extrusion line was examined using differential scanning calorimetry (DSC) and wide-angle X-ray diffraction (WAXD). The multilayer LLDPE films were prepared at a wide range of line speeds. The DSC results showed that there was an increase in the crystallinity of films at higher line speeds. The crystallinity increased from 24.5 to 39.8 % while the line speed changed from 200 to 1000 m/min. Evaluating melting endotherms showed that the size of crystals was more uniform as the line speed increased. The crystallinity of films obtained from WAXD analysis exhibited the similar trend of DSC results, though their values were different. Additionally, there was a reduction in the crystal size calculated from WAXD data upon increasing the line speed. The observed increase in crystallinity and decrease in crystal size were due to the enhanced flow induced crystallization (FIC) as a result of greater shear stresses the polymer melt encountered at higher line speeds.

As a commercially available biodegradable and bio-based polymer, Poly (lactic acid) is accepted to be a promising potential to replace conventional polymers in packaging applications. Oriented PLA films are obtained through stretching at temperatures above the glass transition temperature. The present study evaluates the effects of both temperature and drawing speed during stretching on the tensile behavior and surface morphology of PLA films. Increasing the drawing speed enhances the strain hardening behavior. Conversely, there is a noticeable reduction in strain hardening as stretching temperature increases from 70 to 80 °C. Besides, stretching at 90 °C eliminates, thoroughly, the strain hardening behavior at lower drawing speeds. However, stretching the PLA film at 90 °C and at a high drawing speed (30 mm/sec) stimulates the strain hardening behavior demonstrating chain extension exceeds the chain relaxation at such a high drawing speed. The surface morphology of PLA films observed by scanning electron microscopy (SEM) reveals no crystalline structures on the film surface of PLA films stretched at 70 and 80 °C. The SEM image of PLA film stretched at 90 °C indicates formation of crystalline lamellae which is attributed to cold crystallization during stretching.