Thermal conditions, performance-influencing factors

The main purpose for the heating and air conditioning of work spaces is to provide an environment that is acceptable and does not impair the health and performance of the occupants. During production processes and in the external environment it may be necessary to work in unacceptable conditions for a limited time period. However, it must be ensured that these conditions do not impair the health of the employees. Light, noise, air quality, and the thermal environment are all factors that influence the acceptability of conditions for and performance of the occupants. This section will only deal with the thermal environment. Several standards dealing with methods for the evaluation of the thermal environment have been published by international standard organizations such as ISO and CEN. 

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. 

Establishing the process sensitivity with respect to the above-mentioned factors is crucial for further scale-up considerations. If the sensitivity is low, a direct volume scale-up is allowed and the use of standard batch reactor configurations is permitted. However, many reactions are characterized by a large thermal effect and many molecules are very sensitive to process conditions on molecular scale (pH, temperature, concentrations, etc.). Such processes are much more difficult to scale up. Mixing can then become a very important factor influencing reactor performance for reactions where mixing times and reaction times are comparable, micromixing also becomes important. 

Processing conditions required to attain desirable composite properties can be defined more easily if the factors controlling composite microstructure are understood. Such factors include type of precursors employed and the composite s processing history. The microstructure of the matrix may contribute to the performance of the fibers and influence the properties of the composite. Reviewed are experiments to determine matrix micro-structural features, how microstructural variations are achieved, and ways in which thermal expansion and fracture behavior relate to microstructure. 

The broad capabilities of dielectric thermal analysis make possible the definitive evaluation of many influences on measured dielectric properties and, therefore on the performance of finished products. These influences include crystallinity, cooling rates, annealing temperatures, and post-curing. Experiments to evaluate these factors are possible because of the ability of this technique to measure and control force and plate spacing (sample thickness). These data will help the scientist and engineer correlate end-use performance with such vital factors as chemical structure, molecular relaxations, and materials storage and processing conditions. 

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