Hello and welcome to a new Rule of Thumb post.
For condensation polymers—such as PET, PC, PA, and PU proper resin drying is absolutely critical before processing. Why? These polymers contain reactive groups in their backbone (ester, amide, carbonate, or urethane) that can interact with even tiny amounts of moisture.
If not thoroughly dried, water can trigger chain scission during processing at high temperatures (250–300°C), leading to a significant drop in molecular weight (Mw) and, ultimately, a loss of part performance and properties.
Even as little as 0.01% water can have a major impact, so special care must also be taken with regrind material, which can introduce additional moisture into the system.
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| Figure 1: Polyester, Polyamide, Polycarbonate, and Polyurethane - Example of Chain Scission Due to Hydrolysis |
As a rule of thumb, the molecular weight of condensation polymers should not decrease by more than 10%, since a 5% drop in Mw can already reduce properties by 10%.
In contrast, addition polymers like PE and PP are more tolerant to molecular weight reduction, showing only minor property losses (5% property drop).
Understanding and controlling moisture is key to ensuring the quality and durability of your finished parts and prevent part failure.
Check out my deep dive on functional groups here.
Thanks for reading & #findoutaboutplastics!
Greetings,
Herwig
Literature:
[1] Ezrin M.: Plastics Failure Guide https://www.hanser-elibrary.com/doi/book/10.3139/9783446428829
[2] https://www.findoutaboutplastics.com/2026/04/the-11-functional-groups-of-polymers.html
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