Meaning
The physical migration of volatile byproducts through the molten or solid polymer matrix acts as the rate-limiting step during the condensation polymerization of polyester resins. Because the reaction is reversible, ethylene glycol diffusion must occur rapidly to allow the chemical chains to grow. If this byproduct cannot escape the polymer melt or pellet, the forward reaction stops and the material remains at a low molecular weight.
The rate of this migration depends on both the temperature of the system and the size of the polymer pellets.
Mass Migration
At the molecular level, small glycol molecules must move through the spaces between the long polymer chains. Higher temperatures increase the kinetic energy of these chains, which facilitates ethylene glycol diffusion by creating larger transient gaps in the amorphous regions. This process is much slower in the crystalline regions because the tightly packed structure restricts molecular movement.
Pellet Geometry
Pellet size and shape are critical design factors in solid-state polymerization because they dictate the transport distance for the volatile byproducts. Smaller pellets offer a higher surface-area-to-volume ratio, which accelerates ethylene glycol diffusion and reduces the processing time required to reach the target viscosity. However, very small pellets can create gas flow blockages in the reactor.
Reactor Performance
Operating reactors at high temperatures under a deep vacuum or using a dry nitrogen gas sweep accelerates the removal of the volatile glycol from the pellet surfaces. This action maintains a high concentration gradient that continuously drives ethylene glycol diffusion out of the material. If the purge gas flow or vacuum level is insufficient, the byproduct accumulates at the surface, which causes the reaction to stall and results in resins with inconsistent intrinsic viscosity.
