Quantifying Regrind Degradation through Gel Permeation Chromatography and Rheology
High-temperature GPC and oscillatory shear rheology reveal chain scission and elasticity losses in regrind that standard melt flow testing consistently misses.
Thermal Mechanical Degradation
Meaning
Irreversible molecular breakdown occurs when polymers experience excessive heat and shear during processing. The occurrence of thermal mechanical degradation shortens the polymer chains and reduces the molecular weight of the material. This leads to a permanent loss of physical properties such as impact strength and elongation at break.
Molecular breakdowns are a common problem in recycled materials or in processes with long residence times.
Molecular Chain
Mechanism of thermal mechanical degradation involves the snapping of molecular bonds. Heat provides the energy to vibrate the bonds while the mechanical shear from the screw provides the physical force to break them. Breakdown is permanent.
Processing Shear
High screw speeds and small gates increase the risk of thermal mechanical degradation by generating localized heat through friction. Even if the barrel temperature is correct, the actual melt temperature can rise above the safe limit. Monitoring the pressure drop across the tool helps identify where the shear is highest.
Reducing the injection speed can often stop the damage from occurring.
Property Loss
Parts affected by thermal mechanical degradation often show visible signs like yellowing and black specks. Beyond these visual defects, the structural integrity of the part is compromised. Brittle failure occurs at much lower loads than what is specified on the material datasheet.
This makes the parts unsuitable for load-bearing or safety-critical applications where high performance is required.