Hydrolytic Chain Scission

Meaning

Chemical degradation processes in polymers involve the cleavage of covalent bonds in the polymer backbone due to the action of water at high temperatures. In condensation polymers such as polyamide, polycarbonate, or polyester, hydrolytic chain scission reduces the molecular weight by splitting the polymer chains into shorter segments. This reaction occurs rapidly in the melt phase of the resin if moisture is present during extrusion or molding.

The loss of molecular weight leads to a significant reduction in impact strength and elongation at break.

Degradation Chemistry

High melt temperatures provide the activation energy for water to attack the carbonyl carbon or ester groups in the polymer chain. This nucleophilic attack results in the breaking of the ester or amide bond and creates shorter polymer chains with acid or amine end groups. The rate of hydrolytic chain scission increases with moisture content, meaning that even trace amounts of water can cause severe damage.

Performance Cost

Molded parts made from degraded resin suffer from brittleness and premature failure in service. This issue is particularly costly because the molded parts may look visually perfect but fail during drop tests or load bearing applications. The financial impact includes part recalls, customer rejection, and increased scrap rates on the production floor.

Prevention Method

Thorough drying of the resin before molding is the primary defense. Desiccant dryers must reduce the water content to below the critical threshold, which for many polyesters is less than zero point zero two percent.

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