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Thermal Analysis - Fundamentals and Applications to Polymer Science Part 6

Tham khảo tài liệu 'thermal analysis - fundamentals and applications to polymer science part 6', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Page 67 Figure 4.17. Characteristic temperatures and masses for a single-stage mass loss curve. The rate of mass loss Ml is given by the equation In the case of increasing mass Figure 4.19 illustrates the procedure for calculating the maximum mass attained and the rate of increase in mass. The residue rate R mf ms x 100 where mf and ms are the sample mass before heating and following completion of the final stage respectively. WL2 t A2 meai msJ X 100 Tamper aturtì K Figure 4.18. Characteristic temperatures and masses for a multi-stage mass loss curve. The rate of mass loss at each stage is given by the equation Page 68 ta - spmsl X tOG Tamperaiur8 . K Figure 4.19. Characteristic masses for a single-stage mass gain curve. The rate of mass gain MG is given by the equation 4.9 Derivative Thermogravimetry DTG Overlapping reactions are sometimes difficult to resolve and in some cases resolution may be improved by hindering the escape of evolved gases from the sample by placing a loosely fitting lid on the crucible changing the packing or form of the sample choosing a different crucible or by varying the heating rate. Altering the experimental conditions may change the relative rates of the overlapping reactions and lead to better resolution. Often it is undesirable to alter optimized experimental conditions and in this case overlapping reactions may be more clearly resolved by plotting the derivative TG curve. In DTG the mass change with respect to temperature dm dT is plotted against temperature or time Figure 4.20A . A point of inflection in the mass change step becomes a minimum in the derivative curve and for an interval of constant mass dm dT is zero. A peak in the DTG curve occurs when the rate of mass change is a maximum. DTG peaks are characterized by the peak maximum temperature Tmax and the peak onset temperature Te . Figure 4.20B shows how a DTG curve can be used to resolve overlapping reactions. The area under a DTG curve is proportional to the mass change .