How is HDPE made
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Technical synthesis of HDPE
Polyolefins such as HD polyethylene are commercially available either with the heterogeneous classic Ziegler-Natta catalyst on MgCl2Carriers or with the chrome-based Phillips catalyst (CrO3 on silica or Al2O3Carriers).
Ziegler polymerizations take place in an inert gas atmosphere such as nitrogen and a hydrocarbon solvent such as hexane at moderate temperatures (50-120 ° C) and moderate pressures (1 atm to 10-20 atm). The exact reaction conditions depend on the catalyst, the desired product and the solvent. Hydrogen is used to control the chain growth. The catalyst is fixed on a carrier material, e.g. magnesium chloride. These are porous grains into which the monomer penetrates and polymerizes there. Due to the increase in volume, the polymer breaks the grain open at the end (see spider web formation of polyethylene).
Another catalyst system was developed by Phillips (Phillips catalyst). It is chromium trioxide on the surface of SiO2. The production of HDPE with so-called metallocene catalysts has become more and more important in recent years.
HDPE is made using three different processes:
All processes are operated continuously and are divided into three steps:
- Catalyst preparation
- Polymer insulation
Making the catalyst is the most difficult step in the process. The polymerization is started by adding the gaseous ethene to the catalyst solution. The ethene, the solvent and the reactor must be free of H2O, O2, CO2, Sulfur compounds, ethyne and dienes. These compounds deactivate the catalyst. After the polymerization has stopped, the catalyst still present is deactivated and the polymer is dried. As a rule, however, the catalyst is included in the polymer. This powder can then be mixed with additives such as stabilizers and pigments by extrusion.
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