Reich Stivala 1983 method

An iteration method for calculation of E and n using computer data reduction techniques was also described by Reich and Stivala (127) as well as various other algorithms (128-133, 137-139) and graphical methods to determine the reaction mechanism (134-136). 

Criado and Ortega(140) concluded that Reich and Stivala s method makes possible the assignment of an n order to a reaction that follows quite a different mechanism. In such a case, the method gives erroneous values of E. 

For two pairs of given values of a7/AT and T, values of E/R can be calculated from equation (5.104) for various arbitrarily selected values of n. However, assuming uniqueness, only one pair of E and n values will be pertinent. Equation (5.104) does not apply if n = l. but Reich and Stivala state that this value is rare in practice, and hence the equation is considered to be of general validity. An iteration method was also presented whereby values of E and n are computer calculated using a single DTA curve (165). 

An interesting variation on the Coats and Redfern method has been developed by Reich and Stivala (1980). The method makes use of an iterative technique to arrive at the best value of n to fit the a,T data to a rate law. It is best employed using a computer to perform aU of the computations. The integrated rate equation is written in the form... 

The Coats and Redfem equation (either in original form or as modified by Reich and Stivala) provides the basis for several numerical procedures for determining a value of n. One such method requires only three (a,T) data pairs when starting with an equation in the form derived by Reich and Stivala. An examination of Eq. (8.16) reveals that when calculations are performed using pairs of (o ,T) data a constant value for Ea/R will be obtained only if n has the correct value. Any other (incorrect) value for n will result in the calculated values for Ej k showing a trend. 

Many of these methods are based on the rate law shown in Eq. (8.6), which is not a general rate law because it can not be put in a form to describe diffusion control or Avrami rate laws (see Chapter 7). In 1983, Reich and Stivala removed the constraint imposed by Eq. (8.6) by developing a kinetic analysis procedure that tests most of the common types of rate laws including Avrami, diffusion control, and others not covered by Eq. (8.6). The method is based on a computer program that fits the (a,T) data to the rate laws and computes the standard error of estimate (SEE) for each so that the rate law that provides the best fit to the data can be identified. It is stiU true that when data from a large number of runs are considered, it is rare that a given rate law fits the data from all the runs. It is still necessary to make a large number of runs and examine the results to determine the rate law that fits the data from most of the runs. 

Reich, L., Stivala, S. S. (1980). Thermochim. Acta, 36, 103. The computer method of applying the Coats and Redfem method iteratively. 

Reich, L. and Stivala, S. S. Elements of Polymer Degradation. McGraw-Hill, New York 1971 Tkac, A. Radical processes in polymer burning and its retardation. I. ESR methods for studying the thermal decomposition of polymers in the preflame and flame zones. J. Polym. Sci. Polym. Chem. Ed., 19, 1475 (1981)... 

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