Similar to any other foams, Rifeng® PMI foam cores absorbs moisture from the environment. Due to the fact that Rifeng® PMI foam is a closed-cell foam, the mechanism of moisture absorption is a pure diffusion process. This process depends heavily on the ambient temperature and atmospheric humidity.
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 an 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 Rifeng® PMI foam 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 Rifeng® PMI foam is a reversible process. Re-drying the material restores the original mechanical properties and, within certain limits, reverses dimensional changes. We therefore recommend that Rifeng® PMI foam be dried before it is processed at higher temperatures. Dried or heat-treated (-HT) Rifeng® is available ex works. It is packaged in diffusion-proof aluminum bags.
The sheets should be dried at minimum 3hrs at 130°C (2-3°C/min) by using a heating cabinet with air circulation. During this drying process, each panel shall be separated from adjacent panels by not less than 35mm. All precautions regarding placement and accurate temperature control shall be strictly maintained. It’s necessary to prolong the drying duration when the sheet thickness is more than 25.4mm.
In general Rifeng® PMI foam should always be stored dry.