On -line monitoring of grafting monomers onto low density polyethylene in a model twin screw extruder by fiber optic Raman spectroscopy

An on-line fiber optic Raman spectroscopy technique was developed to monitor the grafting reaction of glycidyl methacrylate (GMA) onto low density polyethylene (LDPE) during reactive processing in a model non intermeshing counter rotating twin screw extruder. A series of glass windows along each sectional chamber along with a data acquisition system attached to a Raman spectrometer were employed to monitor and investigate the dynamics of grafting reaction and melting mechanism. The experimental results obtained reveal that the grafting reaction began at the second chamber of the extruder because a drastic change in the monomer concentration was observed, while the LDPE spectra indicated melting behavior.;Second, prior to investigation of the grafting level by on-line Raman spectroscopy a model study to determine the effect of temperature on Raman spectra of LDPE was conducted using a hot stage. The changes in intensity and peak positions related to disappearance of crystalline peaks and appearance of amorphous peaks were observed above the melting temperature of LDPE. It was found that the melting behavior at high temperature which corresponds to reactive processing temperature presented a difficulty in following the grafting reaction of GMA onto LDPE due to presence of overlapping peaks. The epoxy peak at 1255 cm-1, which is a characteristic peak of GMA was found to be overlapping with CH2 peak at 1295 cm -1, a characteristic of main chain LDPE. However, the grafting level determined by the on-line measurement was determined to be sensitive enough to overcome the overlapping problems when the grafting level reached 12 wt%.;In the first stage of this research, studies on the grafting level using a titration method, FTIR, and off-line Raman spectroscopy were conducted. Three initiators were investigated in the grafting process and all were found to have an effect on the grafting level, rheological properties, and gel content of the system. The addition of styrene as a comonomer increased the grafting level; however, it was also found to initiate crosslinking or chain branching reactions as signified by the increase in viscosity and gel content. The crystallinity of grafted LDPE was determined to be slightly lower than that of LDPE due to chain branching.