Meaning
A polymer processing technique combines chemical reactions with extrusion compounding to modify polymer structures or synthesize new compounds in a continuous melt process. This process, known as reactive extrusion, utilizes the extruder as a continuous flow reactor where high temperatures and intensive mixing facilitate chemical reactions. It is widely used for polymer grafting, chain extension, and controlled rheology modifications.
Process Mechanism
Reactants like initiators, monomers, and catalysts are fed into the extruder barrel alongside the polymer resin. The screw configuration is designed with specialized mixing elements that generate high shear to distribute the reactants uniformly throughout the melt. This design ensures that the chemical reactions occur rapidly before the polymer exits the die.
The residence time and temperature profile within the barrel must be tightly controlled to achieve the target degree of chemical modification.
Rheology Modification
In the production of controlled-rheology polypropylene, peroxide initiators are injected into the melt to induce controlled chain scission. This reaction breaks down the longest chains, narrowing the molecular weight distribution and increasing the melt flow rate. The resulting polymer has lower melt elasticity, which reduces die swell during extrusion and allows for faster injection moulding cycles.
Recycling Applications
Post-consumer resins often exhibit variable molecular weights and low melt strength due to previous degradation. Using reactive extrusion with chain extenders can reconstruct the polymer chains by linking their active end groups together. This chemical reconstruction restores the melt viscosity and toughness of the recycled material, enabling its use in demanding structural applications.
