Meaning
Transport phenomena in fluid or polymer phases establish a localized region of high resistance to the flow of diffusing molecules near an interface. The mass transfer boundary layer represents the thin zone next to a polymer surface where concentration gradients are steep and molecular diffusion dominates over bulk flow. In polymer devolatilization, the thickness of this zone determines how quickly volatile byproducts can escape.
Devaporization Rate
Mechanical agitation of the polymer melt breaks up stagnant surface regions to enhance gas removal. By reducing the thickness of the mass transfer boundary layer, reactors can increase the rate of polymerization. High-vacuum devolatilizers use twin screws to generate thin polymer films and renew the melt surface continuously.
This action ensures that volatile molecules do not accumulate near the surface.
Transport Optimization
Processing parameters such as screw speed and vacuum levels govern the efficiency of byproduct removal during extrusion. In the melt phase, a thinner mass transfer boundary layer increases the transport rate of water or ethylene glycol. This minimizes the residence time required to reach the target molecular weight.
Reduced residence time in the extruder prevents thermal degradation and polymer yellowing. Moulders can therefore process the resin at higher throughput rates without compromising the physical properties of the finished parts.
Processing Boundary
Solid-state polymerization processes rely on gas flow rates to control boundary layers around polymer pellets. When processing solid pellets, the velocity of the inert carrier gas determines the resistance of the mass transfer boundary layer. Low gas flow allows volatile byproducts to build up around the pellets, which slows down the polycondensation reaction.
