Regrind Fractions Nobody Writes into the Grade Specification
Undeclared regrind degrades melt stability and impact strength; verify batch history using melt flow rate delta and oxidative induction time limits.
Tie Molecule Degradation
Meaning
Molecular breakdown of long-chain polymer segments that bridge adjacent crystalline regions reduces the mechanical integrity of semi-crystalline plastics. In semi-crystalline materials, tie molecule degradation occurs under environmental stress or thermal oxidation, leading to premature brittle failure. It directly affects the polymer’s ability to transmit loads between the lamellar crystals, making the material highly susceptible to cracking.
Molders observe a sharp drop in impact strength and environmental stress crack resistance when these molecular links are damaged. This molecular breakdown is restricted to semi-crystalline polymers and does not apply to amorphous grades.
Degradation Mechanism
Thermal energy, ultraviolet light, or chemical attack can cleave the long polymer chains that extend from one crystal to another. Since these tie molecules are under tension, they are more susceptible to chemical attack than the folded chains within the crystals. Once these links are broken, the crystalline regions can easily separate.
This propagates cracks through the material.
Property Loss
Reduced ductility is the most direct result of this molecular breakdown. Molded parts that were once tough will fail under low loads without showing any signs of plastic deformation. This change is particularly dangerous because the external appearance of the part remains unchanged.
It is a leading cause of unexpected product failures.
Processing Influence
High melt temperatures and excessive shear in the barrel can initiate this degradation before the part is even molded. Molders must optimize their process parameters to minimize the thermal and mechanical stress on the polymer. Proper drying also prevents hydrolytic cleavage of the tie molecules in sensitive polymers.
This helps preserve the long-term durability of the molded parts.