S. F. Wang and S. J. Grossman Department of Plastics Engineering, University of Lowell, Lowell, MA 01854 Today, as energy shortages become more critical and costs of heating and cooling continue to escalate, the use of energy conservation methods has become economically attractive. One aooroach to conservation is the use of thermal insula.. tion. The choice of material for heat insulation purposes is determined essentiallv. though not exclusivelv, bv its ability to restrict the flow of heat. polymer materials, particularl; in a foamed or expanded form, provide excellent insulation and are available-at low cost in a convenient, structurally adequate form ( I ) . The thermal conductivity, k value, of a uniform substance is defined as the quantity of heat that passes in unit time through unit area of a slab of indefinite extent and of unit thickness when unit difference of temperature is established between its faces (2). The units of measurement most commonly used are (Btu)(in.)/(hr)("F)(ft2).I t is indicated from this definition that the material with lower k value will provide lower heat conduction and consequently offer hetter thermal insulation. Table 1 lists the common materials that provide varying degrees of insulation and applications. Table 2 indicates the k value for several classes of materials. I t can he seen from these tables that foamed plastic materials are one of the most suitable materials for thermal insulation up to moderate levels of temperature (300 OF).However, recent developments with foamed olastic materials have indicated that h:gher tempersturr applicntions may soon be avnilnhlr r.1). I'lnsric mnttrinls used in the aonlirarion of thtmnnl insulation are in their foamed state. l hey are made basically from the same resins that provide solid plastics. The difference is that, in the manufacturing process, air or some other gas (blowing agent) is introduced so that gas-filled cells are distributed throughout the mass. The gas in a foam can be
Table 1.
Thermal Insulation ( 4 )
Temperature Range
Applications
InsulationType
Above 2000 OF
Aerospace reentry vehicles
Above t200PF
Furnaces. kilns, roasters,
Refractory metals and oxides; aluminosilicate fibers Diatomite, silica bricks, fireclay brick Asbestos
flues, direct flame Pipe boilers, steam lines. furnaces Ovens, bailers, stills, pipe wrapping
600-1200°F
300-600 OF
Home insulation, roofs commercial buildings Cryogenic chemical and food processes, liquified gases. pipe insulation
50-300 OF Below 50 OF
Table 2.
Magnesia-asbestos fiber, calcium silicate, felted aSbeStOS paper Mineral wool, plastic foams Cork, mineral wool blankets, plastic foams
Thermal Conductivity of Commercial Insulating Materials ( 5 )
Material
Density (Ib ft3)
Thermal Conductivity @ 68 OF (Btu) (in.)/(h) (OF) (HZ)
Sawdust Rock Wool Glass Fiber Polystyrene Foam Polyurethane Foam
12.0 10.0 2.0 2.0 2.0
0.41 0.27 0.23 0.26 0.16
Volume 64
Number 1 January 1987
39
typically 5-7% (wt),may he incorporated before polymerization or used to impregnate the head under heat and pressure in a vost-nolvmerization vrocedure (7). The foaming operationis then cknpleted b;placing the beads into a g o l d and applying heat generally in the form of steam, causing the beads to expand and knit together. The molding operation is relativelv simvle but requires a steam source and fairly heavy molds tb witbstandthe internal pressures devleoped. In addition to some of the previously mentioned applications, these foams are used-greatly in the food packaging industry. The high resistance of polystyrene foam to heat flow makes it an excellent material to use as a hot beverage container. thought of as a lightweight filler or extender of the plastic phase. Gases are known to possess the lowest values for heat transfer ( 6 ) so that their incorporation into the foamed structure provides enhanced insulating capability. This can he emphasized by recognizing that foamed plastics are primarily solid polymers by weight but are primarily gaseous by volume (7). In a plastic foam the trapped gas provides both the lowest thermal conductivity and the greatest distance through which heat must flow, and thus exerts by far the major effect on thermal insulation (8). A number of forces have been identified that contribute to the final arrangement of the solid and gas in a plastic foam. These forces are (1)the vressure of the gas inside the voids or cells forcing the plastii in the cell wails to flow as the cell volume increases, (2) surface tension forces that cause the flow of the plastic from the cell walls to the points at which they intersect, and (3) the counterbalancing viscoelastic retractive force of the plastic, restricting its flow ( 1 ) . There is a wide variety of plastic foams, which can he hard or soft, rigid or flexible, or intermediate between these extremes. There can he varying degrees of open(interconnectinr) -. cells or closed (non-interconnedine) cells. Closed-cell for an insulat?ng application hestructures are cause they greatly reduce the convection of gas in the cells. As a consequence, optimum thermal insulation will he develoved at low foam densities and a t higher of - -vrovortions ciosed cells. Currently, there are several types of cellular plastic foams. Polymers that have been successfully used in production of plastic foams include polystyrenes, polyurethanes, polyisocyanurates, epoxies, phenol-formaldehydes, polyethylenes, urea-formaldehydes, silicons, cellulose acetates, and polyimides ( I , 3). We will consider a few of these in this present discussion. Polystyrene Foam Polystyrene is produced from the polymerization of styrene monomer:
One of the maior in its foam form is as an . applications -. insulating material in commercial/industria1 and residential buildings in ceilings, roofs, walls, floors, and foundations. The extruded foam has been prepared by flowing a hot mixture of polystyrene and volatile solvent through a slit into the atmosphere (see figure). As a result of the pressure drop a t the slit opening, the gas (produced from the volatile solvent) expands resulting in a closed cell structure. The polystyrene foam as described above can he produced in the form of planks from which boards can be cut. Another type of polystyrene foam is molded into boards. This operation begins with polystyrene beads, which contain a gaseous hydrocarbon blowing agent, e.g., pentane. This blowing agent, 40
Journal of Chemical Education
Polyurethane Foam Polvurethane foam has also been developed as an insulating material in the commercial/industri~and residential buildine industry. In contrast to polystyrene, this foam is produced at the-same time that ihe~polymeris produced. This provides for some additional versatility in application. Polyurethanes are prepared from the reaction of diisocyanates (e.g., diphenylmethane diisocyanate, MDI) with hydroxyl-terminated polymers (e.g., polyethers or polyesters) in the presence of base catalysis (e.g., tertiary amines). OCN-R-NCO
+ HO-R'OH
H20or blowing agent catalysts
Equation 2 indicates that the blowing agent can he developed in two ways. Originally, carbon dioxide was generated in situ by the retention of isocyanate with water, R-NCO
+ H,O
-
RNHCOOH
-
RNH,
+ CO,
(3)
It is still common practice today to rely largely on this method of gas generation for flexible foam material. Rigid polyurethane foams for insulation (which contain additonal cross-linking agents) are now produced using volatile liquids (e.g., CClaF or CC12Fp) that act as blowing agents, producing gas during the exotherm of the reaction mixture. The main advantage of the fluorocarbon blowing agent is their lower thermal conductivitv in comvarison to carbon dioxide. The k facttx d a fluoroca;bon.blu'wn fusm will he reduced up to 4Ur; in com~arisonto a m r h m dioxide-hlown s w e t n . Additional advantages of fluorocarbon blowing agents (e.g., lower moisture vapor transmission, better adhesion of the foam to metal, higher proportion of closed cells) has been extensively reviewed (10). Two general types of processes have been developed for producing polyurethane foams on a commercial scale. The two processes are commonly called the "one-shot" process and the "two-shot" process. In the one-shot process all of the necessarv ineredients for oroducine the foam are mixed together and then discharged from &e mixer onto a suitable surface. In a two-shot process the hydroxy-terminated component is separately reacted with diisocyanate before the foamina operation. The "two-shot" system is more tolerant to varia%orwin processing conditionsand can be used where the final resin does not easily adapt itself to a "one-shot" nrocess.
About half of the rigid polyurethane foam consumed is in the form of board or laminate and the other half has been used as liquid systems iur in i i t u applicatiuns (pour-in-place and sorav inrludc refriaeration . . fonml.. 'l'v~>irnl .. auvl~cations .. insulation, refrigerated truck and trailer insulation, insulation of pipes and tanks, insulation of solar energy systems, and pour-in-place wall and roofing insulation. ~
~
Polylsocyanurate Foam Recently developed rigid polyisocyanurate foams demonstrate superior thermal stability and combustibility charac-
teristics. These properties have been achieved by cyclotrimerizing the isocyanate group into a polyring structure,
These foams have been formed in situ for retrofit applications in building cavity walls. Incorrect installation techniques, such as improper metering of components, led to an odor problem with these materials and their withdrawal from the market. Finally, reaction of a tetracarboxylic acid with a diamine produces a polyimide structure, The presence of this highly cross-linked polyring structure provides for applications where sewice temperatures may be as high as 150 "C. Furthermore, like a polyurethane foam, when foamed in the presence of a fluorocarbon, they demonstrate low thermal conductivity. Their one major drawback has been their brittle or friable nature (as a result of high cross-link densities) so that there has been a move toward ptrlyisocyanurate-polyurethane combinatione. The key to this dereloomrnt has been the nnoror)riatc choice of catalvst to control the polymerization-t&e&ation reactions (eds 2 and 4). Ships transporting liquified natural gas have been insulated with polyisocyanurate foam laminates that provide temperature stability from -180 "C to 150 O C . The main fuel tank of the space shuttle has been insulated with polyisocyanurate foam (9). Miscellaneous Polymeric Foams
As mentioned earlier. a number of other oolvmers have . . beenrowerted tofuam form and haveseen application asan insulating mntrrial. Cellulose acetate foam is produced in a manner similar to the one used for the production of polystyrene foam. A mixture of molten cellulose acetate polymer is dissolved in a volatile solvent and extruded &provide a foamed board. I t is similar in performance, but i t does have a higher operating temperature range. Phenolic foams are made by catalysis of a liquid phenolic resin (formed by the reaction of phenol and formaldehyde in which a small amount of blowing agent has been added),
The multiple ring structure built into the main chain of the polymer provides for thermal stability approaching the 500 OF level. In foamed form, these new materials should find manv high-tem~erature insulatine" a~nlications. " .. In addition to providing thermal insulation, cellular plastic materials have satisfied other standards necessitated by construction codes. These include resistance to degradative effects caused by moisture, minimal increase in the fire risk of a structure, and providing some structural stability upon application. Cellular foams show much greater resistance to dimensional changes with time (i.e., settling out) than most loose-fill cellulose or fiber class materials. Such settling presents hoth structural andconductivity problems, for, as a material settles, the increase in density resulting a t the base is usually accompanied by an increasein thermal conductivity. The energy-conserving contributions of foamed plastic materials are meeting the demands of our energy-conscious societv. The insulation markets that have been estahished for foamed plastic resins, and those that appear to contain tremendous ootential. bridee hoth the commercial and residential sectdr.
-
Literature Clted
These foams have low k factors and are used for thermal insulntim. Ar present, they are relntively more expensive than other foam systrm*. Foams have also been prepared from urea-formaldehvde nolvmers. In this foamine Drocess the components are mixedwiih a foamingdetergen< air, and an acid catalyst,
1. Baar, E., Ed. Enpinee
