|MadSci Network: Chemistry|
Foams are a very useful class of materials. They are used as thermal and acoustic insulator materials, furniture, floatation materials etc. Foams could be of the closed or open cell type. Both types are characterized by a significant fraction of the material being voids. When these voids are connected to one another it is called an open-cell structure. If the voids are individually surrounded by the plastic matrix such as polyurethane, then it is a closed-cell structure.
Cell structure determines certain properties, thereby influencing the type of application of the foamed plastic. Open-cell foams offer little resistance to the passage of liquids and gases through them. A general principle is that flexible foams have open-cell structure and rigid foams closed cells. The cell structure depends on the process used for the production of the foamed plastic; in some cases both flexible and rigid foams may be produced with either open or closed cells (e.g., PVC). Generally, no foam has entirely one type of cell structure (open- or closed-cell structure implies that the number of cells in the foam is predominantly open or closed, respectively). For example, most rigid polyurethane foams have high closed-cell contents, usually 85 to 95 percent for a 2-lb/ft³ foam.
In an open-cell foam the gas phase is inevitably air. Open-cell foams have sound absorbing properties and, when flexible, cushioning characteristics. This makes them suitable for use as sound absorbing materials and in cushioning applications (e.g.. flexible urethane foam).
In a closed-cell foam the resin membrane, which forms the cell walls, acts as a barrier to gases and liquids, although gases may pass through the membrane by the slow process of diffusion. Closed-cell foams, therefore, have lower water absorption and lower water vapour permeability than open-cell foams. If the gas phase has low thermal conductivity and is captive, closed-cell foams can usually provide higher thermal resistance than open-cell foams that are air filled. The cell size also influences thermal resistance.
References and acknowledgment: Canadian Building Digest
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