THERMAL ANALYSIS 



Thermogravimetric analysis (TGA) and differential thermal analysis (DTA) are 

 commonly used to assess the thermal behavior of materials. The rate at which a 

 material volatilizes as the temperature is raised in a controlled environment is shown 

 by dynamic TGA. A small sample (usually <1 g.) is placed in the pan of an electronic 

 balance suspended in a heated tube. As the system is heated, the weight loss of the 

 sample is continuously recorded against temperature. In DTA a small sample and an 

 inert reference material are heated in a silver block. As the heating continues at a 

 constant rate, the sample undergoes various exo- or endothermic reactions or phase 

 changes, which are detected in comparison with the inert material. The combination of 

 DTA and TGA in conjunction with chemical methods provides a tool for detecting the 

 nature of the pyrolytic reactions . 



The thermal analysis curves can be analyzed through the following characteristics: 



1. The initiation temperature, (T- ) , or the onset of pyrolysis, as indicated by 

 the beginning of weight loss on TGA, ana/or the appearance of a peak on DTA; 



2. The rate of weight loss, (R) , shown by the slope of the TGA curve; 



3. The residue (char), (Cj) , remaining at a given temperature, (T) , but normally 

 after the period of rapid pyrolysis; 



4. The temperature at which the peaks representing the endo- or exothermic reac- 

 tions occur and the relative value of the enthalpy indicated by the area under the peak. 



Thermal analysis is used for detecting the chemical kinetics and the energy of 

 activation of the pyrolysis process. This would be valid if it is assumed that 

 pyrolysis is a single reaction. However, nothing could be further from the truth. 

 What actually is measured is the rate of weight loss caused by evaporation of numerous 

 pyrolysis products. 



Despite the fact that the data from thermal analysis describe the net effect, the 

 characteristics which are tabulated above could be used for describing the general 

 pyrolysis behavior of the substituents . The main objective of this analysis is to 

 point out these characteristics and indicate how they are related to the thermal be- 

 havior of the original material. 



Tang and others (Tang and Eickner 1968; Shafizadeh 1968) have found that the flame 

 retardants generally decrease the rate of pyrolysis, R, in a dynamic environment, lower 

 the initiation temperature, T^, and increase the char, C-p. 



Recently, Philpot has shown that the amount of silica-free minerals (SFA) in 

 plant material could be correlated with the pyrolysis characteristics. A direct log- 

 log relationship was found between SFA and maximum pyrolysis rate. The temperature at 

 which the characteristic cellulose endotherm appeared showed a high correlation (R 2 = 

 0.91) with the SFA of the plant material. The char remaining at 400° C. could also be 

 correlated with mineral content. There was some indication that the effect of the 

 inorganic materials on the course of the pyrolytic reaction could vary according to 

 the composition of the minerals. 



The following procedures were used for obtaining the thermal analysis data. The 

 thermal properties of Cottonwood and its components were investigated by thermogravi- 

 metric analysis (TGA) and differential thermal analysis (DTA) . These data were obtained 



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