Vaporization Coulometry Moisture Analysis in Industrial Polyesters
Coulometric vaporization titration under ISO 15512 Method B2 quantifies trace moisture in industrial polyesters to prevent hydrolytic degradation during melt extrusion.
Oligomer Sublimation
Meaning
Thermal phase transition describes the direct vaporization of low-molecular-weight polymer chains from the solid or melt state into the surrounding gas phase during high-temperature processing. In the extrusion and injection moulding of polyesters and polyamides, oligomer sublimation causes the deposition of white powder on cold mould vents and vacuum extraction lines. This phase change occurs because oligomers have higher volatility than the long polymer chains that make up the bulk resin.
Mould Deposition
Repetitive thermal cycling in injection moulding causes vaporized oligomers to accumulate on the polished surfaces of the mould cavity. When polymer melts enter the hot mould, oligomer sublimation occurs, and the resulting vapors condense upon the cooler steel inserts. This condensation forms a crust that alters the dimensions of the moulded part.
It also blocks the escape of air, which causes burn marks and incomplete cavity filling.
Maintenance Frequency
Frequent cleaning is necessary to prevent surface finish defects on moulded parts. Because the rate of oligomer sublimation increases with melt temperature, running the process too hot accelerates the buildup on mould vents. Production runs must be paused periodically to wipe the tooling.
Resin Selection
Raw material quality determines the volume of sublimated deposits generated during production. High-grade virgin resins contain fewer low-molecular-weight fractions, which minimizes oligomer sublimation. Moulders often specify resins with narrow molecular weight distributions to reduce tooling downtime.