There are plastics (like Polyamide) which are brittle in the dry condition (water proportion < 0.2 %).
The plastic part gets more elastic when water up to 3.5 % is absorpt.
The storage and water absorption in moistured athmosphere or in a water bath until the weight is consistent is called conditioning.
The conditioning occurs by diffusion and may take several days.
The process is reversible.
A polyamide is elastic after conditioning.
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This glossary of plastic industry is provided by PLEXPERT Canada Inc.