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.
Melt Degradation
Meaning
Material deterioration describes the irreversible loss of physical and chemical properties that occurs while a thermoplastic is in its liquid state. High temperatures combined with oxygen or residual moisture cause the long polymer chains to break down or recombine into unwanted structures. Melt degradation is characterized by changes in viscosity, the onset of discoloration and the release of volatile compounds that interfere with molding quality.
Structural Decline
Chain molecules lose their length and entanglement density when exposed to extreme shear or excessive heat within the screw barrel. This internal destruction reduces the mechanical integrity of the final part including its impact resistance and tensile elongation capacity. A noticeable shift toward yellow or brown hues usually accompanies the early stages of this decay.
Indicator Shift
Viscosity measurement serves as the standard way to detect these hidden failures during a production run. If the melt index rises significantly, the material has degraded enough to lose its technical specification. This change causes the plastic to flash across the mold parting line or drool from the nozzle.
Maintaining tight control over temperatures prevents these shifts from ruining entire production lots.
Processing Limit
Residence time dictates the timeframe in which melt degradation remains manageable for a specific grade of resin. Small injection machines with oversized barrels often struggle to move plastic fast enough to avoid thermal breakdown. Proper machine selection eliminates the stagnant areas where plastic hangs up and burns.