Online measurements of microstructure using Raman spectroscopy during blown film extrusion of polyolefins

An understanding of how the microstructure develops along a blown film is important for gaining an insight into this extrusion process, potentially allowing for better control and optimization. The objective of this study was to conduct real-time Raman spectroscopy measurements during the film blowing of polyolefins and to relate these online microstructure results to the final film properties.; Four different polyolefin resins were studied: high-density polyethylene (HDPE), linear-low density polyethylene (LLDPE), low-density polyethylene (LDPE), and isotactic polypropylene (i-PP). The take-up ratio (TUR), blow-up ratio (BUR), and freeze line height (FLH) values were considered as the processing variables, and their effect on the crystallinity development was studied. Characteristic crystalline peak intensities at 1418 cm-1 and 809 cm-1 were analyzed to obtain the crystalline phase content for the polyethylenes and isotactic polypropylene, respectively. The results for all polyolefins indicated that the crystallization process begins at the freeze line, increases along the axial distance before reaching a plateau. The differences in the profiles for crystallinity were also evident with changing processing conditions. As the TUR or BUR was increased, with the remaining processing conditions kept nominally constant, the rate of crystallization (as measured using crystallization half-time, the time taken for a polymer to reach half of its equilibrium crystallinity) increased dramatically before reaching saturation. These results clearly signified the role of flow-enhanced crystallization during film extrusion. For a given processing condition, the rate of crystallization was also found to be equivalent for the three polyethylenes.; Real-time polarized Raman spectroscopic measurements were conducted on LLDPE to measure the crystalline phase orientation during blown film extrusion. The principal Raman tensor components (alphaii) for the crystalline phase were initially calculated for solidified films from offline right-angle and backscattering Raman measurements. By assuming an invariance of these ratios, the equations for different polarization intensities were then solved using real-time Raman measurements obtained from backscattering mode alone. The results showed the evolution with axial distance of the P2 and P4 values, which plateau as the film reaches the nip rolls. The P2 values obtained from Raman spectroscopy were higher for the final films than those obtained from X-ray studies. The uniaxial orientation assumption for the blown films coupled with the assumed invariance of the ratios of principal Raman tensor components may contribute to the difference. (Abstract shortened by UMI.)...