Meaning
Mechanical degradation occurs when chemical cleavage of polymer chain backbones by water molecules severely reduces molecular weight and converts ductile thermoplastics into brittle solids. Moisture-sensitive engineering resins such as polyamides, polyesters, and polycarbonates undergo hydrolytic embrittlement when exposed to ambient moisture at elevated temperatures or when melted without adequate drying. Hydrolysis severs ester, amide, or carbonate linkages along the chain, sharply reducing degree of polymerization and molecular entanglement.
This transformation terminates plastic deformation capability, causing components to shatter under loads far below design specifications.
Chain Scission
Water molecules diffuse into the amorphous regions of solid polymers or mix into molten resins within the barrel of an injection press. During processing, hydrolytic embrittlement proceeds through direct chemical attack where water cleaves ester or amide bonds, replacing structural chains with short fragments terminated by carboxyl, amine, or hydroxyl groups. This loss of high molecular weight species prevents chains from forming load-bearing entanglements under mechanical stress.
The material loses its ability to yield, shifting from tough ductile behavior to sudden, catastrophic brittle fracture when subjected to tensile or impact forces.
Moulding Evidence
Moisture-induced chain degradation reveals itself through severe processing instabilities and visible cosmetic defects on moulded surfaces. Parts experiencing hydrolytic embrittlement often display silver splay marks, foaming around gates, and drooling at the injection nozzle caused by escaping steam and volatile chain fragments. Melt viscosity drops dramatically, causing excessive flash at parting lines even when injection pressures are reduced.
Moulders measuring melt flow index observe significant increases compared to virgin resin datasheets, confirming that excessive hopper moisture destroyed molecular weight inside the heating cylinder.
Regrind Hazard
Blending post-industrial or post-consumer regrind into virgin feedstocks introduces high risks of mechanical failure if moisture histories are uncontrolled. Because ground parts carry surface moisture, internal humidity, and legacy degraded chains, uncontrolled blending accelerates hydrolytic embrittlement throughout the entire batch. Virgin resin possesses sufficient average molecular weight to buffer trace moisture, but regrind blends lack this margin and fail impact tests unpredictably.
Molders must apply rigorous desiccant drying protocols down to moisture levels below two hundred parts per million before processing regrind polyesters to preserve elongation and impact performance in critical components.
