Investigation of molecular weight distributions during extrusion process of polypropylene by rheometry experiment

Edge waste is an inevitable consequence of the cast film extrusion process, since the edges of the film are trimmed due to dimensional distortion and unpleasant layer distribution. On the other hand, most of the materials which are used in film production are not biodegradable. Therefore, for protecting the environment, the waste output should be reduced. For this reason, nowadays film production lines are equipped with inline recycling units which are also acceptable from the economic aspect, although, it caused degradation in polymer materials. Upon degradation, the film quality decreases which is associated with the molecular weight and molecular weight distribution (MWD) alterations. Despite this, film quality should be kept at a high rate as much as possible to be acceptable in the market. Hence, chasing MWD which is changed during the recycling process, is one of the hints for controlling the film quality. In this study, the degradation of the film grade polypropylene through recycling by twin screw extruder at 170°C and 220°C was investigated. It was assumed the material in the extruder was degraded by extremely high temperature and shear rate. The degradation of polypropylene subjected to multiple extrusions was evaluated by following the changes in the MWD curves in each run, which were indirectly obtained by rheometry based on the generalizing mixing rule. In this equation, Kernel function describes the relaxation behavior of a monodisperse component of molecular weight, and time is related to molecular weight, and the exponent ß characterizes the mixing behavior of the chains. As its calibration, gel permeation chromatography (GPC) was also used. In this regard, two values of ß (ß=2.5 and ß=3) and different kernel functions such as Tumilino, single exponential and Doi-Edward were used. According to the MWDs obtained from rheometry and GPC data, chain scission during the recycling processes was evaluated.