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Similar to any other foams, ROHACELL® absorbs moisture from the environment. Due to the fact that ROHACELL® is a 100% closed cell foam, the mechanism of moisture absorption is a pure diffusion process. This process depends heavily on the ambient temperature and atmospheric humidity.
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.
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.
More information is available in the section Heat Treatment and the download Conditions for Drying and Heat Treatment of ROHACELL®.