Karl Fischer Moisture Determination Standards for Recycled Polyethylene Terephthalate Flakes
Coulometric Karl Fischer with oven desorption at 170°C isolates true flake moisture from volatiles to prevent hydrolytic intrinsic viscosity loss.
Solid State Condensation
Meaning
Polymerization processes that occur below the melting point of the resin increase the molecular weight through the removal of reaction byproducts from the solid phase. Solid state condensation is primarily used for polyesters and polyamides to reach the high viscosity required for bottle grade or industrial fiber applications. This technique involves heating the polymer pellets in a vacuum or under a flow of inert gas.
The process stops when the diffusion of byproducts to the pellet surface becomes the rate limiting step.
Viscosity Enhancement
Higher molecular weight improves the mechanical properties and the environmental stress crack resistance of the plastic. Applying solid state condensation allows a manufacturer to upgrade the quality of a base resin or a recycled stream. The increased chain length results in a melt that is more stable during the stretch blow moulding of containers.
This adjustment is essential for producing parts that must withstand internal pressure.
Thermal History
The duration and temperature of the treatment determine the final properties of the material.
Byproduct Removal
Efficiency in the reactor depends on the pellet size and the purity of the inert gas stream. Smaller pellets provide a larger surface area for the release of water or ethylene glycol, speeding up the reaction. Solid state condensation also helps in removing volatile organic compounds that could affect the taste or smell of the contents in food packaging.
Monitoring the color change of the resin ensures that the thermal exposure does not lead to yellowing.