Hello dear for it you have to employ different heating rate. Please you see this paper: Polymer Degradation and Stability 91 (2006) 1903-1908

In the case of single heating rate, you can use FREEMAN-CAROL method to determine the kinetic parameters like Activation energy, order of the reaction and frequency factor.  You can refer the Encyclopedia of Polymer Engineering (may be 14 th or 15th volume) under Thermogravimetric Analysis.  In this article, different methods of determining kinetic parameters are described.

Thank you Rajulu for your answer, however, according to Liu and Fan 1999 (Critical consideration on the Freeman and Carroll method for evaluating global mass loss kinetics of polymer thermal degradation) they concluded THAT the FC approach in determining the Ea is unreliable and proposed another  approach which hitherto has not been widely adopted as the FC method. Please which one of these approaches do you suggest I go with? Thanks

If the primary thermograms are steep, then it may be difficult to calculate the slope at different temperatures and application of FC method will be complicated.  There are another 3 methods described in the Encyclopedia of Polymer Engineering.  For example, variable rate method, multiple thermograms of the same sample with different rates etc.  Though multiple rate method is time consuming, it will be more accurate as it avoids the finding of slopes.  Long back we used these methods but now unfortunately, I do not remember the other methods as I retired from service nearly 6 years back.  But you can refer the Encyclopedia or the book entitled THERMAL ANALYSIS OF POLYMERS by Turi.  I wish you every success in your calculations.