Question 1

What is the Meaning of Hydrolytic Cleavage?

Accepted Answer

Chemical degradation of polymer chains occurs when water molecules react with sensitive functional groups at elevated processing temperatures. Through hydrolytic cleavage, the molecular weight of polymers like polyamides, polyesters, and polycarbonates is reduced as their backbone links are broken. This reaction takes place rapidly in the melt zone of the extruder if the resin is not dried. Molders observe a sharp increase in melt flow index and a corresponding drop in the mechanical strength of the molded parts. The reaction is limited to condensation polymers and does not occur in simple polyolefins.

Question 2

What is the Chemical Reaction of Hydrolytic Cleavage?

Accepted Answer

Water molecules act as nucleophiles that attack the carbonyl carbon atoms in the polymer chain. This reaction splits the ester or amide bonds, creating shorter chains with acid or alcohol end groups. The rate of this cleavage increases with higher temperature and moisture levels. It permanently alters the chemical structure of the resin.

Question 3

What is the Processing Damage of Hydrolytic Cleavage?

Accepted Answer

Shortened polymer chains reduce the melt viscosity, causing the material to drool from the nozzle or flash the mold. The molded parts exhibit low impact resistance and are prone to brittle failure in the field. This defect cannot be corrected by post-molding treatments. It renders the affected parts unusable.

Question 4

What is the Moisture Control of Hydrolytic Cleavage?

Accepted Answer

Desiccant dryers must reduce the moisture content of the resin to below the critical limit before molding. For polycarbonates, this limit is often below zero point zero two percent by weight. Molders measure the moisture content with specialized instruments to confirm that the drying process is complete. This step is necessary to prevent hydrolytic cleavage during the run.

Hydrolytic Cleavage

Meaning

Chemical Reaction

Processing Damage

Moisture Control

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