Heat release capacity
18. Average heat release rate versus heat release capacity of polymers.
Fig equation 70 becomes
eR^niYi/Mi)
with Hi, Vi, Ei, Yi, and Mi being the molar heat of combustion, mole fraction of fuel, molar activation energy, molar thermal decomposition function, and molar mass of component i, respectively. Expanding the summations in equation 72 and retaining only the noninteracting terms for which i = j = k... (ie, neglecting terms containing products and quotients with mixed indices), the heat release capacity on a mass basis is nc = M =
T^nMi ENiM
Experimental heat release capacities nc have been measured for over 200 polymers and the results are used to generate over 30 additive molar group contributions by treating the ^i as adjustable parameters in equation 73 for polymers with known chemical structures (29). Table 8 contains for a few dozen polymers. Figure 19 compares calculated and measured heat release capacities for 50 polymers using
- Fig. 19. Polymer heat release capacities: calculated versus measured values.
the 30 empirical ^i determined by Walters and Lyon (2003). Overall agreement between measured and calculated values is about 15%, which is encouraging at this early stage of research. Additive contributions to the heat release capacity are being updated as more polymers are tested and the measurement technique improves. As an example of the additive procedure for calculating the heat release capacity, the chemical repeat unit for PET is for which the most recent chemical group contributions are listed in Table 13. Combining the Ni, Mi, and ^i for PET as per equation 73,
= EN^i= (1X28.8) + (2)(0) + (2X16.7) _ 62.2 kJ/(mol ■ K) nc = T^NiMi = (1X76.10) + (2)(44.01) + (2)(14.03) = 192.18 g/mol
The predicted heat release capacity for PET compares favorably with typical experimental values for this polymer, nc = 326 ± 52 kJ/(kg ■ K) (see Table 8),
Table 13. Group Contributions to the Heat Release Capacity of Poly(ethylene terephthalate)
Structural group, i
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