WATER/MOISTURE ABSORPTION
Plastic materials absorb water to varying degrees. Moisture content may result in changes in dimensions and some other properties. Factors such as the type of material, additives, temperature, and length of exposure can affect the amount of water absorbed.
Certain materials require drying prior to moulding. After moulding it may be necessary to condition parts. Nylon components with critical dimensions used to be immersed in tanks of hot water after moulding for anything up to 24 hours in order to condition them. However, modern grades of Nylon are more stable so this conditioning process is less common nowadays.
Water absorption is measured by immersing a test sample of specified dimensions in water for a specified time at a specified temperature. Prior to this the specimens are dried and cooled. Results are given either as milligrams of absorbed water or as a percentage increase in weight.
In this instance the two test standards are essentially the same.
ASTM D570
Three testing procedures are commonly employed. Only data from the same testing procedures are readily comparable.
| Procedure | Test Description |
|---|---|
| Water Absorption @ 24 hrs | Test specimens are immersed in distilled water at a specified temperature for 24 hours. Testing is most commonly done at 23°C |
| Water Absorption @ Saturation | Test specimens are immersed in distilled water at a specified temperature until the water absorption essentially ceases. |
| Water Absorption @ Equilibrium. | Test specimens are exposed to a humid environment at a specified temperature for 24 hours. Testing is most commonly performed at 50% relative humidity (RH) and 23°C. |
ISO 62
As with ASTM, three testing procedures are commonly employed. Only data from the same testing procedures are readily comparable.
| Procedure | Test Description |
|---|---|
| Water Absorption 24hrs @ 23˚C | Test specimens are immersed in distilled water at room temperature for 24 hours. |
| Water Absorption Saturation @23˚C | Test specimens are immersed in distilled water at a room temperature until the water absorption essentially ceases. |
| Water Absorption 23˚C @ 50% RH | Test specimens are exposed to 50% relative humidity (RH) room temperature air for 24 hours. |
