Thermal properties selected materials

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. 

Several material properties exhibit a distinct change over the range of Tg. These properties can be classified into three major categories—thermodynamic quantities (i.e., enthalpy, heat capacity, volume, and thermal expansion coefficient), molecular dynamics quantities (i.e., rotational and translational mobility), and physicochemical properties (i.e., viscosity, viscoelastic proprieties, dielectric constant). Figure 34 schematically illustrates changes in selected material properties (free volume, thermal expansion coefficient, enthalpy, heat capacity, viscosity, and dielectric constant) as functions of temperature over the range of Tg. A number of analytical methods can be used to monitor these and other property changes and... 

LIG. 34 Schematic illustrations of changes in selected material properties (free volume, thermal expansion coefficient, enthalpy, heat capacity, viscosity, and dielectric constant) as functions of temperature over the range of Tg. 

Of the three general categories of transport processes, heat transport gets the most attention for several reasons. First, unlike momentum transfer, it occurs in both the liquid and solid states of a material. Second, it is important not only in the processing and production of materials, but in their application and use. Ultimately, the thermal properties of a material may be the most influential design parameters in selecting a material for a specific application. In the description of heat transport properties, let us limit ourselves to conduction as the primary means of transfer, while recognizing that for some processes, convection or radiation may play a more important role. Finally, we will limit the discussion here to theoretical and empirical correlations and trends in heat transport properties. Tabulated values of thermal conductivities for a variety of materials can be found in Appendix 5. 

Since the discovery of Teflon by Roy Plunkett in 1937 a number of fluorinated plastics have reached commercial status. These plastics, exemplified by polytetrafluoroethylene (PIPE), have outstanding electrical, chemical, and thermal properties. AU these commercial materials are either crystaUine or semicrystalline. Teflon AF is a family of amorphous copolymers that retain the desirable electrical, chemical, and thermal properties of semicrystalline fluorinated plastics and also have such properties associated with amorphous materials as optical clarity, improved physical properties, and solubility in selected fluorinated solvents. 

While thermal insulations are selected first for their resistance to heal flow, their other properties need evaluation for each application. Hence there is no "best" insulation because a material well suited to one service may be poorly suited to another. Economics must be studied in detail because in some services the cost of a highly efficient material per unit of thickness may be overcome by additional thickness of a less efficient material, provided there is room for the grealet thickness. All properties of materials must he considered for each exposure, even within the same system. Recognize that when different materials are used or different parts nl insulation sv -.tents that arc close together, the probability of using the wrong material on a particular surface is increased appreciably. Unless there is specific reason for wide use ol multiple types of materials, it may be prudent to accept some compromises of properties. 

Table 2.1 Thermal Properties for Selected Polymeric Materials...

Typical values of thermal properties for selected polymers are shown in Table 6.1 [7, 17]. For comparison, the properties for stainless steel are also shown at the end of the list. It should be pointed out that the material properties of polymers are not constant and may vary with temperature, pressure or phase changes. This section will discuss each of these properties individually and present examples of some of the most widely used polymers and measurement techniques. For a more in-depth study of thermal properties of polymers the reader is encouraged to consult the literature [24,46, 66],... 

The most common advanced composites are made of thermosetting resins, such as epoxy polymers (the most popular singlematrix material), polyesters, vinyl esters, polyurethanes, polyimids, cianamids, bismaleimides, silicones, and melamine. Some of the most widely used thermoplastic polymers are polyvinyl chloride (PVC), PPE (poly[phenylene ether]), polypropylene, PEEK (poly [etheretherketone]), and ABS (acrylonitrile-butadiene-styrene). The precise matrix selected for any given product depends primarily on the physical properties desired for that product. Each type of resin has its own characteristic thermal properties (such as melting point... 

With optimum selection of fibres and matrices, favourable residual stress conditions can be established in the matrix, which lead to increased A Tc. Above A Tc, matrix cracks appear but the presence of crack-deflecting fibre-matrix interfaces ensures minimal effect on mechanical properties as the fibres remain largely unaffected. As damage is also confined mostly to the surface of the materials, changes in mechanical and thermal properties are more readily identified by means other than mechanical testing. 

Selecting materials for scale-up based on separate flammability criteria for HRC, HR, and p increases the likelihood that a sample will pass the requirements of 14 CFR 25. However, it has been shown that these three properties are not independent [27], For a single-step thermal decomposition reaction at a constant heating rate, these thermal combustion properties are related ... 

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. 

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