Flexible polyurethane foam, used in chair cushions, consists of networks of tiny cells, assembled in a honeycomb structure. Cells consist of struts, or cell walls, and voids, or window-like areas, through which air passes easily when force is applied. The elasticity of the struts absorb force, allowing the foam to recover its shape when the force is removed.
Foam consists of several ingredients. The main ingredients are polyol, toluene diisocyanate (TDI), and water, which react together, producing the foaming agent carbon dioxide. The chemical mix also contains emulsifiers, which stabilize rising foam, and catalysts, which control chemical reaction rates. Optional ingredients may include colors, combustion-modifying additives, auxiliary blowing agents, fillers, or any other materials needed to produce special properties.
Colors are primarily used to distinguish one foam grade from another. Flame retardants resist igniting and burning, while antistatic additives reduce static electricity buildup. Additionally, antimicrobials protect foam from the buildup of bacteria, yeast, or other fungi. Also, other fillers may be added, to increase the weight of foam, and to add support. Fillers, however, may have a negative effect on durability.
A foam's density rating is its mass per unit of volume. The only structures in foam are its struts and voids, and the only one which has mass is the strut. The firmness of a flexible foam's surface does not depend on its density, because very dense foams may also be very soft. Firmness is a measurement of the surface feel, while support measures foam's ability to push back when force is applied.
The chemical mix becomes foam by one of two methods. In the slabstock form process, the chemical mix is poured onto a moving conveyor, where it reacts and expands, until it is between two and four feet high. After being cut into buns, foam is allowed to cure for twenty-four hours, and then fabricated into useful shapes. Most furniture, bedding, and carpet pads are made with the slabstock process.
Another manufacturing process is called the molded foam process. In this process, the chemical mix is poured into specially-made molds, in which the foam reaction takes place. Automotive cushioning is primarily made this way, although the process does have some residential applications.
All flexible foam applications have to meet three basic criteria. The foam should demonstrate adequate support, so that cushions do not bottom out when supporting a person's weight. Also, the foam should demonstrate comfort, which is a measure of the foam's ability to deflect at the surface, to conform to body shape. In addition, the foam should demonstrate durability, which affects how well the foam retains its shape, and its load-bearing capacity.
Flexible polyurethane foam fills chair cushions, but is also used for other types of furniture, for bedding, and for automotive cushioning. The chemicals in flexible foam have an exothermal reaction, which means that they give off considerable heat when reacting. The industry used to use freon to combat the heat, but due to the Clean Air Act, CFCs have been completely eliminated from the production process.
