Change in mechanical properties and dimensions
As a result of this moisture uptake, dimensional changes, which are usually very small, occur. Also the mechanical properties, mainly in creep behavior, will be affected. Dimensional changes are ascribed to expansion of the molecular chains by (physical) attachment of water molecules to the imide groups of the polymer through van der Waals interactions. The water molecules act as plasticizers, making ROHACELL® more ductile. Apart from this degradation in mechanical properties, processing at temperatures above 100°C results in the formation of steam, which adversely affects the bonding of the skins with the structural foam core.
Reversible by drying
Moisture uptake by ROHACELL® is a reversible process. Redrying the material restores the original mechanical properties and, within certain limits, reverses dimensional changes. We therefore recommend that ROHACELL® be dried before it is processed at higher temperatures. Dried or heat-treated (-HT) ROHACELL® is available ex works. It is packaged in diffusion-proof aluminum bags.
In general ROHACELL® should always be stored dry.