Cryogenic Foam Insulations: Polyurethane and Polystyrene

Abstract

Increased use of cryogenic temperatures in the recent past may be credited in large part to military and aerospace endeavors. However, the impending energy shortage, advanced surgical techniques, refrigeration of food stuffs, and scientific needs are rapidly projecting the use of cryogenics into the mainstream of modern living. In order for these applications of cryogenics to be economically feasible, associated technologies have had to be developed or extended. Thermal insulation is one of the areas where both materials and techniques have been developed to meet the growing needs. Indeed, the requirements for thermal insulation are sufficiently diverse that several rather independent technologies or methods of insulation have developed. Generally speaking, the particular application dictates the generic type of thermal insulation or, at least, reduces the number of viable candidates. Some of the widely used materials and their attendant thermal conductivity ranges are depicted in Fig. 1.1 As seen in this figure, the evacuated insulations are the most thermally efficient. Most large-scale needs, however, preclude vacuum insulation on the grounds of cost and durability. The three classes of unevacuated insulation are reasonably competitive in thermal effectiveness at low temperatures; all are used, for instance, in large liquefied natural gas (LNG) storage tanks and LNG ship tankers. The materials to be discussed in this paper, polystyrene (PS) and polyurethane (PU) foam, are widely used in the present generation of LNG ships. These foams, also known as cellular plastics or expanded plastics, offer both a high strength-to-weight ratio and low thermal conductivity. The principal disadvantages are low maximum temperature limits, lack of mechanical integrity upon thermal cycling2 and degradation of insulating properties with time. Each of these items will be addressed below. The low temperature thermal and mechanical behavior of these two foam systems can be better understood if the basic structure, chemistry, and processing techniques are briefly reviewed. The latter two areas tend to be highly pro-prietary, so that their discussion is based on well-known principles rather than on specific, contemporary production practices.