Why Must Hydrolysis-Sensitive Plastics Be Dried Before Processing?

Hydrolysis-sensitive polymers are plastics whose molecular chains can be degraded by water at processing temperature. Moisture does not only create bubbles or silver streaks; it can chemically attack the polymer backbone.

Typical hydrolysis-sensitive materials include: - PET, PBT, and PETG - PA6, PA66, PA610, and PA612 - PC - TPU and some polyester elastomers - PLA and other biodegradable polyesters

These materials must be dried according to validated conditions before injection molding, extrusion, or compounding.

Hydrolysis is a chemical reaction where water breaks polymer chains. Under high temperature, even small amounts of moisture can react with ester, amide, carbonate, or urethane groups.

The result is chain scission. The average molecular weight drops, melt strength decreases, and the material can no longer deliver its rated mechanical performance.

This is why hydrolysis is more serious than simple surface moisture. Once polymer chains are broken, later drying cannot repair them.

Polymer performance depends strongly on molecular weight. When hydrolysis cuts long chains into shorter segments, the material loses toughness, ductility, fatigue resistance, and long-term durability.

Common effects include: - Lower tensile strength - Reduced impact resistance - Brittle fracture - Lower melt viscosity - Reduced hydrolytic aging resistance - Poor weld-line strength

In critical applications, a part can look acceptable after molding but fail during assembly, shipping, or customer use.

Insufficient drying can produce visible and invisible defects.

Visible defects: - Silver streaks - Bubbles - Splay marks - Surface haze - Flow instability

Invisible defects: - Molecular weight reduction - Lower mechanical strength - Reduced heat resistance - Poor long-term reliability - Unstable viscosity during production

For hydrolysis-sensitive materials, a clean surface appearance does not guarantee that the material has not degraded.

Polyester materials such as PET and PBT are highly sensitive to moisture during melt processing. PC can also suffer significant degradation if processed with excessive moisture.

Examples of typical drying risks: - PET: severe intrinsic viscosity loss and brittleness - PBT: reduced toughness and surface defects - PC: molecular degradation, bubbles, silver streaks, and loss of impact strength - TPU: viscosity drop and unstable processing - PLA: rapid molecular weight reduction during melting

The exact drying temperature and time must follow the resin supplier's technical data sheet, because over-drying or overheating can also damage some materials.

In engineering practice, drying performance is controlled by moisture content, drying temperature, drying time, airflow, and dryer dew point.

For many moisture-sensitive engineering plastics, a dry-air dryer with dew point around -30°C to -40°C is preferred. The target residual moisture depends on the polymer type and application requirement.

Important variables: - Material type and hydrolysis sensitivity - Pellet size and regrind percentage - Hopper residence time - Dryer dew point - Airflow and hopper loading - Exposure time after drying

Regrind absorbs moisture faster than virgin pellets because it has higher surface area and may contain microcracks. Opened bags and partially used material can also pick up moisture from ambient air.

Production controls should include: - Sealed storage after drying - Controlled regrind percentage - Short exposure time before molding - Moisture checks for critical materials - Separate handling rules for PA, PC, PET, PBT, TPU, and PLA

For export or safety-critical parts, drying records should be treated as part of quality documentation.

Moisture problems are often misdiagnosed as mold venting, injection speed, or material quality issues. A useful troubleshooting method is to compare parts molded from freshly dried material with parts molded after air exposure.

Warning signs include: - Silver streaks that change with drying time - Sudden viscosity drop - Unstable filling pressure - Brittle parts despite correct grade selection - Reduced impact strength - Surface defects that appear across different molds

If the defect improves after stricter drying, moisture and hydrolysis should be treated as primary causes.

For hydrolysis-sensitive plastics, drying is not optional. It protects molecular weight, processing stability, and long-term part performance.

The most reliable production systems define: - Drying temperature - Drying time - Dew point requirement - Maximum regrind ratio - Maximum exposure time after drying - Moisture testing method for critical batches

This is especially important for automotive, electronics, appliance, industrial, and precision injection molded components.