ABSTRACT
I. INTRODUCTION A foam is a two-phase system made up of gas bubbles trapped in a liquid: the desired quantity of bubbles of the requisite shape and size may be generated by any one of several methods. Often the gas volume fraction is very large, and this converts the gas bubbles into cells of a regular or irregular shape. Such cellular plastic foams can be formulated using either thermoplastic or thermosetting polymers. ln the latter case, the processes of foaming and curing usually occur simultaneously. The final solid product can be rigid, semirigid, or flexible. Indeed, we are all familiar with semirigid polystyrene foam used in coffee cups and flexible urethane foam employed in seat cushions and carpet backing. The cells in the foam may be open (interconnected), permitting the transport of nuids. or closed and therefore impermeable. Open cell foams are good acoustical insulators, and open cell foams can also be used as filter media. Closed cell foams. on the other hand. are good thermal insulators, especially when the average bubble size is of the order of the mean free path of the gas. An example of closed celL but 1igid, foam is microcellular structural foam used to make automobile bumpers. Here, the foam contains a very large number of tiny bubbles (having a size less than or equal to 10 11m) of a gas such as carbon dioxide or nitrogen, and this makes the solid foam lightweight and resilient. and energy absorbing. The gas is typically injected in the supercritical state into the melt zone of the extruder barrel (see, for example. Gael and Beckman [1[). The resulting single-phase solution has a significantly lowered viscosity, and it can be processed at a lower temperature. An important application area for structural foams is equipment housing and thinwalled parts. and injection molding is the preferred manufacturing process. al304
though structural foam can be thermoformed. blow molded, and also extruded into rods, tubes, and sheets. Common polymers, both filled and unfilled, used in structural foam are nylons, polyolefins. polyurethane, polystyrene. ABS, and PPO. and the main attraction of structural foam is the high strength-to-weight ratio [2]. However, transparent parts cannot be produced. since the cells act as opacifiers.
