Thermal conductivity glass transition temperature

What are heat capacity, specific heat, thermal conductivity, coefficient of thermal expansion, glass transition temperature, melting temperature, and degradation and decomposition ... 

The glass-transition temperature, T, of dry polyester is approximately 70°C and is slightly reduced ia water. The glass-transitioa temperatures of copolyesters are affected by both the amouat and chemical nature of the comonomer (32,47). Other thermal properties, including heat capacity and thermal conductivity, depend on the state of the polymer and are summarized ia Table 2. 

Thermal Properties. Thermal properties include heat-deflection temperature (HDT), specific heat, continuous use temperature, thermal conductivity, coefficient of thermal expansion, and flammability ratings. Heat-deflection temperature is a measure of the minimum temperature that results in a specified deformation of a plastic beam under loads of 1.82 or 0.46 N/mm (264 or 67 psi, respectively). Eor an unreinforced plastic, this is typically ca 20°C below the glass-transition temperature, T, at which the molecular mobility is altered. Sometimes confused with HDT is the UL Thermal Index, which Underwriters Laboratories estabflshed as a safe continuous operation temperature for apparatus made of plastics (37). Typically, UL temperature indexes are significantly lower than HDTs. Specific heat and thermal conductivity relate to insulating properties. The coefficient of thermal expansion is an important component of mold shrinkage and must be considered when designing composite stmctures. 

Composite-based PTC thermistors are potentially more economical. These devices are based on a combination of a conductor in a semicrystalline polymer—for example, carbon black in polyethylene. Other fillers include copper, iron, and silver. Important filler parameters in addition to conductivity include particle size, distribution, morphology, surface energy, oxidation state, and thermal expansion coefficient. Important polymer matrix characteristics in addition to conductivity include the glass transition temperature, Tg, and thermal expansion coefficient. Interfacial effects are extremely important in these materials and can influence the ultimate electrical properties of the composite. 

The volume coefficient of expansion of Teflon AF is linear with temperature and quite low. The coefficients are 280 ppm/°C and 300 ppm/°C for AF-1600 and AF-2400, respectively. Above the glass transition temperature these values increase sharply. Thermal conductivity is quite low, increasing from only 0.05W/mK at 40°C to 0.2 W/mK at 260°C. Many of these properties are believed to be related to the very low (1.7-1.8 g/ml) densities of these dioxole... 

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. 

After isothermal cure, temperature scans are conducted in order to measure the Tg after cure and Tg . However, due to thermal degradation, postcures can lead to lower glass transition temperatures than those obtained after cure. Thus, the determination of Tg , for high T, systems is a difficult problem. One approach is to establish a relationship between Tg and theoretical crosslink density for systems of lower Tgoc and similar chemical structure, and extrapolate to the system with higher crosslink density, thereby obtaining an estimate of Tg, ... 

The polymer heat capacity data bank is, as outlined in the Introduction, is only the first step towards the establishment of a comprehensive Thermal Properties Data Bank. Presently we are expanding our efforts to include glass transition temperatures, melting temperatures and heats of fusion. In the planning stage are specific volune, compressibility, and thermal conductivity data banks, as well as the expansion to non-equilibrium properties. 

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