Entwicklung eines kontinuierlichen Extraktionsverfahrens zur Reinigung von Kunststoffschmelzen mittels überkritischem Kohlendioxid

摘要

The latest legal requirements call to an increasing degree of purification for plastics, for both virgin and recycled material. Abidance by the law demands new processing technologies for purifying plastics. The extraction with supercritical carbon dioxide could be an approach. Supercritical carbon dioxide is characterized by a high diffusion coefficient, a low viscosity and a high density. These properties are determining the outstanding solubility in scCO2 even for higher molecular substances. Furthermore CO2 is non flammable, ecologically safe, inexpensive and non toxic. One disadvantage of CO2 is its low polarity and thus its low solubility for polar substances. The polarity of the extracting agent can be influenced specifically by dosing additional cosolvents like methanol in scCO2. Carbon dioxide also is a gas which causes the greenhouse effect and should therefore be recycled. The usage of scCO2 as an extracting agent in a twin screw extruder offers high potential for decreasing the amount of contaminations in the polymer melt. The extraction process is examined in the present paper for cleaning polymer melts. Main objective of the extraction is to remove foreign substances as monomers, solvent residues, processing aids and contaminations, which are diffusing into the interior material by using the plastics. These contaminations are dispersed or dissolved in the polymer. The applicability of a twin screw extruder for purifying polymer melt is analysed for both virgin material (two types of polyamide 12) to remove monomers and specifically contaminated high density polyethylene. To implement the extraction process on a twin screw extruder the compounding process has to be modified regarding this process technology, in order to keep the CO2 in its overcritical state at high pressures > 75 bar. This is an additional requirement for common twin screw extruders. Pressure tightness was achieved by using a copper sealing in combination with a sealing paste. For dimensioning the extruder for extraction purposes, the processing unit should feature few linkage points. Besides the pressure tightness of the extruder the screw configuration has to be determined and optimised. A reflow zone following the zone for the CO2-outlet is proved to provide the possibility for the carbon dioxide, which is swept away with the polymer, to flow against the polymer flow back to the CO2 outlet. After elaborating a sufficient process for taking samples, dependences of different processing parameters the extraction effect were researched. In general the highest extraction effect can be realised by increasing pressure, screw speed, processing temperature and the CO2-flow. Contrary to this an increased temperature level results in charred melt and a high CO2 flow has a negative effect on the economy of the extraction process. The integration of a degassing unit and a counter flow process were also investigated. To do so an extruder cascade is inevitable due to safety reasons. Furthermore the extruder cascade facilitates stable processing conditions and simplified process control. Regarding the counter flow processing, the used screw configuration is inappropriate. Due to this improvement of the screw design is inevitable. Industry is reluctant using supercritical fluids despite of the considerable advantages. Reasons for the scepticism can be found in the high investment cost al well as using hegh pressure processing technology. However the increasing relevance of legal requirements and the progressive development will promote this technology.