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.
Polymer Chain Scission
Meaning
Molecular failure occurs when the long primary backbones of a plastic break into smaller and separate segments during molding. This reduction in atomic length drastically lowers the viscosity of the melt and reduces the structural durability of the finished item. Polymer chain scission is frequently caused by excessive shear inside the barrel or chemical reactions with moisture at high temperatures.
Fragment Formation
Mechanical stress from a fast-turning screw physically tears the structures apart if the temperature is too low to permit easy flow. Chemical events like oxidation or hydrolysis achieve the same result by targeting the internal bonds at a molecular scale. The resulting shorter sequences cannot entangle effectively which leads to parts that are brittle under high load.
Detection Method
Measuring the melt flow index provides a standard check on the extent of this internal damage. A significantly higher flow rate indicates that polymer chain scission has lowered the resistance of the resin to movement. Operators use this data to confirm when temperatures or screw speeds have exceeded the material safe zone.
Correcting the process requires lowering energy inputs to protect the backbone.
Physical Outcome
Final components lose their toughness and flexibility when the chain length drops below the critical entanglement threshold. This decay leads to field failures where parts shatter rather than deform when dropped or stressed. Maintaining the longest possible sequences ensures consistent mechanical performance.