Insulating materials, thermal

Eig. 1. Thermal conductivity components vs density for a typical thermal insulation material at 300 K A, total conductivity B, air conduction C, radiation ... 

An Assessment of Thermal Insulation Materials and Systemsfor Building Applications, DOE Report, BNE-S0862 UC-9Sd, U.S. Dept, of Energy, Washington, D.C., 1978 R. P. Tye and D. L. McElroy, eds., ASTM STP 718, Thermal Insulation Peformance, American Society for Testing and Materials, Philadelphia, Pa., 1980, pp. 9—26. 

W. Tobiasson, A. Greatorex, and D. VanPelt, "Wetting of Styrene and Urethane Roof Insulations ia the Laboratory and on a Protected Roof Membrane," F. J. PoweU and S. L. Matthew, eds., ia Thermal Insulation Materials and Systems STP922, ASTM, Philadelphia, Pa., 1987. 

An additional topic to discuss from an introductory standpoint is thermal insulating materials. These materials are used to reduce the flow of heat between hot and cold regions. The sheathing often placed around steam and hot-water pipes, for instance, reduces heat loss to the surroundings, and insulation placed in the walls of a refrigerator reduces heat flow into the unit and permits it to stay cold. 

Powell, F. J., and Matthews, S. L., eds. (1984). Thermal Insulation Materials and Systems (STP 922). West Conshohocken, PA American Society for Testing and Materials. 

A thermal insulation material is one that frustrates the flow of heat. It will slow down the rate of heat loss from a hot surface and similarly reduce the rate of heat gain into a cold body. It will not stop the loss or gain of heat completely. 

In solids, metals are good conductors - gold, silver and copper being among the best. The range continues downwards through minerals such as concrete and masonry, to wood, and then to the lowest conductors such as thermal insulating materials. 

The heat loss by radiation increases with increasing temperatures, being proportional to T4. This means there is a steep increase with increasing temperature. It was found that about 50 percent of heat loss of a battery is caused by radiation. In order to reduce the transmission of radiation through the thermal insulating material, opacifiers are added to the insulating material. 

The heat conductivity in solids occurs via phonons. This conductivity is ideal in single crystals and is considerably reduced in porous solids, by one to two orders of magnitude. Therefore thermal insulation materials are built up of small particles which should touch each other at only a few points. This effect is of course enhanced by a low density of the material. 

The conduction through residual gases can be reduced by the application of porous structures. The convection within a single pore is minimal if pore sizes are small. In small pores the temperature difference at the walls of the pore are negligible and no convection occurs. The convection is further reduced by the evacuation of the thermal insulating material. 

Table 17. Glass fiber board thermal insulating material...

Other thermal insulation materials are known besides the three described above. These materials are compared in Table 19, which shows that the thermal conductivity can be reduced by a factor of 2-A by evacuation in the evacuated state it is below lOmWirT1 K l for all materials. The lowest conductivity is shown for foil insulations, but this requires an extremely low pressure of 10 5 mbar which is difficult to obtain in practical applications which ought to last for ten years. 

Polystyrene can be moulded, cast or extruded in sheets, rods and tubes. It can be easily pigmented. It finds wide, application in making household goods, lenses, plastic moulds, toys, wood laminates, films, battery boxes and electrical components. Expanded polystyrene is used as thermal insulating material and packaging. It is also used in lifejackets and floats. 

The thermal insulation materials used in the walls, ceilings, and floors of refrigerated storage are usually combustible, such as polyurethane and polystyrene foam, and could present an additional fire hazard. The insulation should be covered with a noncombustible barrier material a V2-in (1.3-cm) thickness of Type-X gypsum board or cement plaster is often used for this purpose. Where the floor is insulated, the floor insulation should be covered with a layer of concrete. 

The applications for addition polyimides are for electronic/electrical materials such as printed circuit boards and insulators, as matrix resins for structural composites in aircraft and as thermal insulation materials. Recently the market for polyimide based structural composites has suffered from the termination and reduction of military aircraft programs. However, the possible emergence of the High Speed Civil Transport program (HSCT) may offer an opportunity for addition polyimides (2). 

Most common thermal insulating materials are acceptable for ethyleneamine service. However, porous insulation may introduce the hazard of spontaneous combustion if saturated wilh ethyleneamincs from a leak or exlemal spill. 

ASTM Committee C-16 on Thermal Insulating Materials delines thermal insulation us a material or assembly of materials used primarily to resist heat flow-, The reference to assembly of materials indicates (hat the concern is thermal insulating systems, because it is not limit materials have been designed imo systems that pciformunce cun be estimated. Thermal insulating systems include mu only the basic materials, but also the auxiliary materials and the methods of application and protection in service. 

On the contrary, a beneficial aspect of the low heat conductivity is the possibility to apply plastics as thermally insulating materials, in particular as foams. The heat conduction of a polymer foam is composed of four components ... 

---