Thermal conductivity plastics Table

In this research, thermal insulation properties of the wood sawdust/PC composites were evaluated by thermal conductivity analysis. Table 1 shows the thermal conductivity of sawdust/PC composites in various treatments. Thermal conductivity is defined as the quantity of heat transmitted through a unit thickness in a direction normal to the surface of that unit area, due to a unit temperature gradient under steady state conditions [6]. The addition of wood can improve thermal insulation of neat PC because thermal conductivity (k) of wood materials (k 0.08 W/m K [9] was normally lower than plastics. Moreover, the lower thermal conductivity was associated with discontinuous phases which were a result from poor compatibility between the wood sawdust and PC matrix [7]. 

The other principal thermal properties of plastics which are relevant to design are thermal conductivity and coefficient of thermal expansion. Compared with most materials, plastics offer very low values of thermal conductivity, particularly if they are foamed. Fig. 1.10 shows comparisons between the thermal conductivity of a selection of metals, plastics and building materials. In contrast to their low conductivity, plastics have high coefficients of expansion when compared with metals. This is illustrated in Fig. 1.11 and Table 1.8 gives fuller information on the thermal properties of pl tics and metals. 

In order to select materials that will maintain acceptable mechanical characteristics and dimensional stability one must be aware of both the normal and extreme thermal operating environments to which a product will be subjected. TS plastics have specific thermal conditions when compared to TPs that have various factors to consider which influence the product s performance and processing capabilities. TPs properties and processes are influenced by their thermal characteristics such as melt temperature (Tm), glass-transition temperature (Tg), dimensional stability, thermal conductivity, specific heat, thermal diffusivity, heat capacity, coefficient of thermal expansion, and decomposition (Td) Table 1.2 also provides some of these data on different plastics. There is a maximum temperature or, to be more precise, a maximum time-to-temperature relationship for all materials preceding loss of performance or decomposition. Data presented for different plastics in Figure 1.5 show 50% retention of mechanical and physical properties obtainable at room temperature, with plastics exposure and testing at elevated temperatures. 

The thermal conductivities of various commercial insulating materials are given in Table 10.7. Plastic foams containing a captive blowing agent have much lower thermal conductivities than other insulating materials. 

Fillers usually increase the thermal conductivity of polytetrafluoroethylene as with other plastics (Table 3.38). Specific heat of PTFE at various temperatures is given in Table 3.39. Enthalpy of molded PTFE is given in Fig. 3.33. 

The thermal conductivity of solids varies considerably (Table 15.2). Metals have a high thermal conductivity, with silver having the highest room-temperature thermal conductivity, at 430 W m K . Alloys have lower thermal conductivities than pure metals. Ceramics are even lower, especially porous porcelains or fired clay products (Figure 15.3). The lowest thermal conductivities are shown by plastic foams such as foamed polystyrene. As would be expected, the thermal conductivity of crystals varies with direction. For example, the thermal conductivity of the hexagonal metal cadmium Cd, (A3 structure), is 83Wm K parallel to the c axis and 104 W m parallel to the a axis. At 25 °C, the oxide quartz, which has a hexagonal unit cell, has a thermal conductivity parallel to the c axis of 11 W m K , and 6.5 W m K paraUel to the a axis. 

Plastics are, in general, good insulators. Gases are much better insulators than solids, so in a foam, the combination of the plastic and the gas pockets within it provides very low thermal conductivity. Each phase, gas and solid, contributes an amount roughly proportional to its volume fraction. Thermal conductivities of some common packaging foams are given in Table 13.1. 

Table 2.7 Thermal conductivities of adhesives and plastics commonly used in electronic devices and assemblies ...

With practically all machines, only the cylinder temperature is directly controlled (see Chapter 1). The actual heat of the melt, within the screw and as it is ejected from the nozzle, can vary considerably, depending on the efficiency of the screw design and the method of operation. Factors affecting melt heat include the time plastic remains in the cylinder the internal surface heating area of the cylinder and the screw, per volume of material being heated the thermal conductivity of cylinder, screw, and plastic (Table 1-6) the heat differential between the cylinder and the melt and the amount of melt turbulence in the cylinder. In designing the screw, a balance must be maintained between the need to provide adequate time for heat exposure and the need to maximize output most economically. 

Thermal conductivity coefficients and densities of some plastics are collected in Table 2.4. The relation between the thermal conductivity coefficient and the bulk density of a rigid polyurethane foam is presented in Figure 2.10. 

Materials having the highest thermal conductivities are the metals, metal alloys, and some ceramics while plastics, polymeric materials, and glasses have the lowest thermal conductivities (Tables 2.5,2.6, and 2.7). 

Table 2.7. Thermal Conductivities of Adhesives and Plastics Commonly Used in Electronic Devices and Assemblies... 

Table 35-12. Densities, p. Tensile Strengths, ob. Compressive Strengths, ac, at 10% Deformation, and Thermal Conductivities, X, of Various Foams. (After Data from the Modern Plastics Encyclopedia 1976/1977). )Structural foam...

S.2.5.2 Thermal Conductivity. Thennal conductivity can be increased to shorten molding cycles and to avoid overheating of electrical equipment. Silver, copper, and aluminum have conductivities 1000 times that of unfilled plastics loading them into plastics can increase conductivity considerably, in proportion to their volume fraction (Table 5.22). Beryllium oxide, boron nitride, aluminum oxide, aluminum nitride, and graphite are also quite effective. 

In general, thermal conductivity is low for plastics and the plastic s structure does not alter its value significantly. To increase it the usual approach is to add metallic fillers, glass fibers (Fig. 2-16), or electrically insulating fillers such as alumina (refer to Table 2-13). Thermal conductivity can also be decreased, by foaming (see Chapter 6). 

The economics of processing depend on the thermal characteristics of the material. The thermal properties of PP are compared with some other plastics in Table 23. The heat transfer requirement for cooling from the melt temperature to mould temperature in the case of PP is much higher than those in the case of amorphous polymers such as ABS and PS. Hence, the processing of PP is costly. In addition, thermal conductivity determines the cooling time of the material in the mould. It can be seen that the thermal conductivity of PP is less than HOPE. It would require more cooling time, and hence, a slower production rate. 

In order to use this unit also for catalyst particles of any shape, the standard method was modified (method lie in Table 3) in the same way as described for the determination of the thermal conductivity of nonporous plastic powders in [13]. The concept is to compare the thermal conductivity of the catalyst pellet X with the conductivity of a binary liquid mixture. If the liquid mixture can be adjusted such that it has the same conductivity as the pellet, the conductivity of a fluid/solid system A (consisting of catalyst particles and the liquid mixture) will be independent of the solid content, and... 

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