Thermal conductivity epoxies Table

Due to the dependence on mean free path as described in Eq. (4.40), the thermal conductivity of heterogeneous systems is impossible to predict on heat capacity alone. As in previous sections, we do know that disorder tends to decrease thermal conductivity due to mean free path considerations, and this is indeed the case for fillers with high thermal conductivities, such as copper and aluminum in epoxy matrices (see Table 4.12). The thermal conductivity of the epoxy matrix increases only modestly due to the addition of even high percentages of thermally conductive fillers. 

Table 4.12 Thermal Conductivity of Epoxy Resin Filled with Various Compounds...

TABLE 3.33 Thermal Conductivity of Filled Epoxy Resins, Btu/[(ft -hr-"F)/ft]... 

Table 3.3-29 Epoxy resin, EP thermal conductivity and coefficient of thermal expansion...

Table 2 lists thermal conductivity values for several metals as well as for beryllium oxide, aluminum oxide, and several filled and unfilled resins. Fig. 2 shows the thermal conductivity for an epoxy resin as a function of volume fraction of heat-conductive filler. 

The best current 100% solids epoxy adhesives contain about 70% aluminum oxide by weight and give thermal conductivities in the range of 0.8-1 in the English units shown in Table 2. For convenience, a conversion chart is included in Table 2 to permit conversion to any other set of units. The k values for the best alumina-filled epoxies are 10-12 times greater than for unfilled epoxy resins, but are still much lower than for pure metals or solders. Nevertheless, heat flow is adequate for bonding most components. For example, an adhesive with a thermal conductivity of 0.91 and a bond thickness of 3 mils would be able to transfer about 20 W/cm of surface area, with a AT only about 10 C above the heat sink temperatures ... 

The data of this table show that unfilled polymers and plastics have rather low X values and are very good thermal barriers or insulators. Furthermore, the thermal conductivity of most polymers falls in the tight range indicated for unfilled epoxy resins, about 0.2 W m which is more than three orders... 

A number of examples of reinforced or filled polymer systems are shown in Table 6. As a general rule, as the thermal conductivity or the amount of the non-polymeric phase increases, so does the thermal conductivity of the composite. Examples listed in Table 6 include glass-reinforced polycarbonate and pol3Kethylene terephthalate), and mica-filled epoxy resins. On the other hand, plasticizers... 

Conductivity is not easily increased. A high concentration of a metal in powder or fiber form can raise it perhaps tenfold. The thermal conductivity of the base resins in Table 11.3 can be increased by aluminum or copper metal. These also increase the electrical conductivity. If low electrical conductivity, of the order of 10" (ohm cm)", must be combined with high thermal conductivity, the mixture of aluminas (Table 11.3) will increase the former by a factor of only 1.5 over that of the base epoxy resin, whereas the latter is increased by a factor of 12. Foaming with air or some other gas is used to decrease the thermal conductivity. A foamed polystyrene with a density of 15 kg/m and = 0.040 W/m °C is useful as insulation for a variety of applications, from picnic baskets to boxcars. 

Epoxy systems used in structural applications, whether as adhesives or the matrix of fibre-reinforced composites, are normally cured under some pressure and can be regarded as in closed containers. Studies on the kinetics and mechanisms of cure chemistry are often conducted without pressure in containers essentially open to the atmosphere. Extensive thermal analysis examinations at this Laboratory on a range of epoxy formulations have shown that for such fundamental quantities as the heat of reaction substantially different values can be obtained by using open or hermetic pans (Table 2). These differences are apparent with both the TDI-DMA adduct and dicyandiamide as curing agent but not with DDS. 

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