Methods for inline detection of the influence of molecular mass and history of wall slipping plastic melts on their velocity profile

摘要

Wall slippage is a rheological phenomenon which occurs within the flow of some polymer melts. During measurements via extrusion rheology the wall slippage is generally analyzed in a die located after the extruder. Polylactidacide (PLA) is processed on a co-rotating twin screw extruder, while the influence of humidity, the addition of chain extender and the variation of processing parameters are evaluated. PLA is hydrophilic, so it absorbs air humidity. If it is not dried before processing, water molecules react with the polymer at higher processing temperatures, which leads to polymer degradation. Chain extender is an additive used in polymer processing to increase the molecular weight of polymers. Since parameters as screw speed, screw geometry and humidity influence the molecularmass and distribution of the extrudate, these are determined after processing via gel permeation chromatography (GPC) to obtain information on the relationship between wall slippage and the molecular mass.To investigate wall slippage Pulsed Wave Sonography is used. This is an image-creating measuring procedure via ultrasound which allows measuring of the velocity profile of the melt within a die that was developed specifically for this measuring technology. It can be done inline, during the extrusion, and is therefore much faster and more practical than usual extrusion rheology. Additionally, the residence time distribution is measured via fluorescence spectroscopy.