Standard Test Methods for Measuring Volatiles in Recycled PET Flake
Accurate rPET flake volatile testing pairs ASTM D6980 sensor drying for moisture with ASTM F2013 headspace gas chromatography for acetaldehyde and solvents.
Cyclic Trimers
Meaning
Oligomeric byproduct molecules often emerge during the condensation polymerization of polyester resins. These cyclic trimers are the most abundant low-molecular-weight species found in polyethylene terephthalate. They can migrate to the surface of the polymer during melt processing.
Processing Deposit
Sublimation of these molecules occurs at high melt temperatures, leading to a white powder deposit on the mould surface or the die lips. This buildup from cyclic trimers disrupts the continuous production of bottles or fibers because it leaves indentations on the part surface. Frequent stops for mould cleaning reduce overall plant efficiency.
Material Specification
Resin suppliers measure the concentration of these low-molecular-weight species to guarantee the purity of the incoming material. High levels of cyclic trimers in the raw polyester chips increase the frequency of spinning breaks in textile applications. Standard datasheets typically show values below one percent by weight to prevent migration.
When regrind polyester is blended with virgin resin, the overall concentration of these oligomers can rise due to thermal degradation during the previous molding cycles.
Part Quality
Surface haze develops on finished bottles when the oligomers crystallize on the part exterior after injection molding. This migration of cyclic trimers also reduces the adhesion of barrier coatings applied to the container. Poor adhesion compromises the shelf life of carbonated beverages.