Meaning
Thermodynamic properties defining the maximum percentage of water a polymer absorbs from its surrounding environment at a specific temperature and relative humidity determine the long-term dimensional stability of molded parts. The equilibrium moisture uptake dictates how much a finished component will expand and soften as it adapts to ambient conditions over time. Polyamides and other polar plastics exhibit high sensitivity to this environmental behavior.
Polymer Absorption
Water molecules penetrate the amorphous regions of the polymer matrix by forming hydrogen bonds with polar groups. This absorption continues until the chemical potential of the water inside the polymer equals that of the surrounding atmosphere. The equilibrium moisture uptake is therefore a reversible material property rather than a permanent chemical change.
Plasticizing Consequence
Moisture absorption acts as an internal plasticizer by increasing the free volume between polymer chains and lowering the glass transition temperature. This softening effect alters the mechanical performance, yielding a significant decrease in tensile strength alongside an increase in impact resistance and elongation at break. Designers must calculate these altered values to ensure the part functions properly in humid end-use environments.
Dimensional Expansion
Volumetric changes occur as the absorbed water molecules force the polymer chains further apart. This swelling directly affects the precision of molded components, as parts can grow in all directions and fall outside design tolerances. In high-precision assemblies, understanding the equilibrium moisture uptake allows engineers to design tools with appropriate tolerances that accommodate both initial mold shrinkage and subsequent environmental swelling.
Moulders often subject parts to pre-conditioning baths to reach this equilibrium state before shipping them to assembly plants, preventing unexpected post-molding dimensional changes.
