Thermal properties, of plastics

In order to select materials that will maintain acceptable mechanical characteristics and dimensional stability designers must be aware of both the normal and extreme operating environments to which a product will be subject. Plastics properties and processes are influenced by their thermal characteristics such as melt temperature (T ), glass-transition temperature (7 ), dimensional stability, thermal conductivity, thermal diffusivity, heat capacity, coefficient of thermal expansion, and decomposition (7J. Table 2-18 provides some of these data on different plastics. 

All these thermal properties relate to how to determine the best useful processing conditions to meet product performance requirements. 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. This section therefore reviews important thermal properties. More details about ASTM standards are given in Chapter 9. The effects of temperature on plastics are discussed throughout this book, particularly in Chapters 3-6. 

The Tfn occurs at a relatively sharp point for crystalline materials. Amorphous materials basically do not have a they simply start melting as soon as the heat cycle begins. In reality there is no single melt point, but rather a range, which is often taken as the peak of a DSC curve (see Chapter 9). 

A plastic s thermal properties, particularly its Tg, influence its processability in many different ways. The selection of a plastic should take these properties into account. A more expensive plastic could cost less to process because of its shorter processing time, requiring less energy for a particular weight. 

The 7 is unique to amorphous TPs. It occurs at a specific temperature that depends on pressure and specific volume and is lower than the melting point. Designers should know that above Tg the mechanical properties are reduced. Most noticeable is a reduction in stiffness by a factor that may be as high as 1,000. Therefore, the operating temperature of an amorphous TP is usually limited to below its Tg. Amorphous TPs generally have several transitions. 

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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. 

Figure 5-6 and Tables 5-3 to 5-5 provide an introductory guide to the different thermal properties of plastics. Heat resistance properties of plastics retaining 50% of properties obtainable at room temperature with plastic exposure and testing at elevated temperatures are shown in Fig. 5-6 for the general family or group type. 

Of course in general the mechanical properties of plastics film and sheet will be rather different from those of metal foil and strip, whether ferrous or non-ferrous, and equipment and conditions may have to be adapted to allow for this. Sometimes cold techniques are modified to take advantage of the thermal properties of plastics but in their simplest forms they are limited to the compounds with which cold working is possible—that is, those of suitable hardness, ductility, and finish—and that can be fed in the forms of sheet, roll, or strip. So as to avoid frequent interruptions of the cycle the stock must be uniform and any strain distributed in a regular manner. Fast processes requiring the application of pressure are not suitable for hard, brittle materials, nor for any produced in such a way as to give concentrations of strain, perhaps in areas that differ with circumstances. 

The useful thermal properties of plastics include specific heat, thermal expansion, thermal conductivity, and thermal softening [32]. 

Chapter 4 presents an overview of the chemical, optical, physical and thermal properties of plastics which are most relevant to conservation. Properties determine which plastics are suited to particular functions and also why some are no longer in use. The chapter starts with a basic description of the types of bonding and structure which determine the properties of polymers. Details of the chemical, optical, physical and thermal properties for plastics most often encountered in collections are presented in tables, one for each material, in Appendix 1. 

In another research, Wang et al. [36] investigated the effect of carbon black (CB) on mechanical and thermal properties of plasticized PLA. They used two different plasticizer, acetyl tributyl citrate (ATBC) and poly( 1,3-butylene adipate) (PBA) and revealed the plasticizers improved interaction existed between PLA and CB. The FT-IR spectra revealed that with increasing CB content in each... 

THERMAL PROPERTIES OF PLASTICS. SUNDSTROM DW MODERN PLASTICS... 

Van, S., Yin,)., Yang,)., and Chen, X. (2007) Structural characteristics and thermal properties of plasticized poly(L-lactide)-silica nanocomposites synthesized by sol-gel method. Mater. Lett., 61 (13), 2683-2686. 

As discussed in Chapter 4, in addition to the mechanical properties, incorporation of reinforcing agents or fillers can affect the thermal properties of plastics, such as heat resistance. 

Z. Ren, L. Dong, Y. Yang, Dynamic mechanical and thermal properties of plasticized poly(lactic acid), J. Appl. Polym. Sci. 2006,101, 1583-1590. 

The thermal properties of plastics are generally those expected for organic substances, modified in some cases by the polymer structure. Thus, partially... 

Table 4. Effect of the PEG 1000 content on the thermal properties of plasticized PLA filled with 3 wt% of Cloisite 30B ...

The principles of temperature modulated DSC are described, and a number of examples are presented of its application to studies of the thermal properties of plastics. Some limitations of the technique are also examined. 6 refs. 

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