Meaning
Mass transfer operation extracts volatile organic compounds and residual monomers from a polymer melt by applying vacuum or inert gas stripping during the extrusion process. Effectiveness of devolatilization is measured by the reduction in the concentration of small molecules that could otherwise cause bubbles, odours or safety issues in the final part. This process typically occurs in a specialized section of the extruder where the screw geometry creates a large surface area for the melt.
It is a necessary step for materials like polystyrene or polyamides where unreacted components can compromise the quality of the product. The operation stops once the desired purity level is achieved before the melt enters the die.
Volatile Removal
Extraction of low boiling point substances from a viscous melt requires both heat and low pressure. During devolatilization, the volatile components migrate from the bulk of the polymer to the surface where they evaporate into the gas phase. This migration is driven by the difference in chemical potential between the liquid and the gas.
Increasing the residence time in the vacuum zone allows more of the unwanted material to escape. In many cases, a stripping agent like water or nitrogen is injected to increase the vapor pressure of the volatiles. This technique is useful for removing substances with high affinity for the polymer matrix.
The result is a more stable material that does not release gas during subsequent moulding or thermoforming steps.
Vacuum Efficiency
Performance of the degassing system depends on the level of vacuum and the rate at which the vapors are removed from the barrel. A high vacuum level lowers the boiling point of the contaminants, making it easier for them to leave the polymer melt. Sensors monitor the pressure in the vent port to ensure that the devolatilization process remains within the specified parameters.
If the vacuum is too weak, residual monomers can remain trapped, leading to splay or silver streaks on the surface of the moulded part. The vent must be designed to prevent the polymer from backing up and clogging the vacuum line. Proper maintenance of the filters and pumps is necessary to maintain the required flow rate of extracted gases.
This efficiency is a primary factor in the production of food grade resins where low residual content is mandatory.
Polymer Purity
Quality of the final resin is a direct outcome of the thoroughness of the degassing stage. Through effective devolatilization, manufacturers can produce materials that meet strict regulatory standards for migration and odour. This purity is important for medical devices and food packaging where any leachables could have health implications.
The process also improves the mechanical properties of the plastic by removing molecules that act as internal plasticizers. It reduces the risk of degradation during high temperature processing because fewer reactive small molecules are present. Moulders who use well degassed resins experience fewer issues with mould deposits and corrosion.
This consistency in material quality leads to better part performance and longer tool life. The thorough removal of volatiles ensures that the polymer remains stable throughout its intended service life.
