Ester Bond Hydrolysis Kinetics in Molten Polyethylene Terephthalate
Polyethylene terephthalate ester bonds hydrolyze rapidly in melt above 270 °C, requiring drying below 30 ppm moisture to prevent intrinsic viscosity collapse.
Moisture Concentration
Meaning
Quantitative value expresses the mass of water molecules contained within a specific quantity of polymer resin before processing. Often measured in parts per million, this metric dictates whether a plastic is sufficiently dry to withstand the high temperatures of an injection barrel. High moisture concentration results in structural defects such as internal voids or severe hydrolytic degradation of the molecular chains.
Sampling Accuracy
Reliable testing involves extracting a sample from the bottom of the hopper where the material is supposedly at its driest. Variations in moisture concentration can occur if the drying air is not evenly distributed through the bed of pellets. Small inconsistencies in these numbers lead to massive fluctuations in melt pressure and flow behavior.
Verification through loss-on-drying or chemical titration methods confirms that the material is within safety limits.
Quality Impact
High concentrations of water turn into vapor under heat and expand into the part geometry during the injection phase. These vapors cause silvery streaks on the surface or cloudy patches in transparent resins. If the moisture concentration stays high, the molecular weight of the plastic drops permanently as the chains are severed.
Control Limit
Datasheets specify the maximum level permitted for reliable molding without property loss. Drying equipment is programmed to hit these targets based on initial ambient readings. Consistently low values ensure part strength remains identical batch over batch.