ASTM D570 Water Absorption of Plastics

ASTM D570 Water Absorption of Plastics Overview

ASTM D570 defines the test method for measuring the amount of water absorbed by a plastic specimen after immersion in distilled water for a specified time at a controlled temperature. Water absorption is expressed as the percentage increase in mass relative to the dry specimen mass. Although the test itself is straightforward – weigh, immerse, reweigh – the result has significant practical consequences because water absorption changes the dimensional, mechanical, and electrical properties of plastic materials in ways that can compromise the performance of finished parts.

The standard defines three conditioning periods before the initial dry weighing – 24 hours at 50 degrees Celsius, 1 hour at 105 to 110 degrees Celsius, and equilibration over anhydrous calcium chloride – to establish a reproducible dry baseline. Immersion is typically performed for 24 hours at 23 degrees Celsius (the standard condition), but ASTM D570 also covers long-term immersion and elevated temperature immersion for materials used in hot or prolonged wet environments. The 2-hour immersion at 100 degrees Celsius condition provides accelerated saturation data for materials with very low absorption rates.

Water absorption matters because absorbed water acts as a plasticizer in many polymers, reducing the glass transition temperature and lowering the elastic modulus and tensile strength. In glass-fiber-reinforced composites, water at the fiber-matrix interface weakens adhesion and causes hydrolytic degradation of the sizing. In electrical insulating applications, absorbed water raises the dielectric constant and loss factor, degrading signal integrity and potentially reducing dielectric breakdown voltage. Dimensional swelling from absorbed water causes tolerance problems in precision plastic components and can generate stresses in constrained assemblies.