Arrhenius plot degradation

Data was collected over a two-year period on the effect of water on DuPont s Zytel 101. In an Arrhenius plot of this data the failure point was the time when the elongation and impact strength started to decrease. This is not a chemical degradation, but rather a permeation or diffusion rate phenomenon. It shows that high temperature water tests can be used to predict normal temperature exposure results. 

As practiced by the UL, the procedure for selecting an RTI from Arrhenius plots usually involves making comparisons to a control standard material and other such steps to correct for random variations, oven temperature variations, condition of the specimens, and others. The stress-strain and impact and electrical properties frequently do not degrade at the same rate, each having their own separate RTIs. Also, since thicker specimens usually take longer to fail, each thickness will require a separate RTI. 

Arrhenius plots permit the determination of activation energies (Ea) associated with a particular pathway of degradation that allows one to estimate reaction rates as a function of temperature. Such information, if demonstrated to accurately model... 

FIGURE 3 Arrhenius plot for first-order degradation. 

FIGURE 5 Arrhenius plot for zero-order degradation. 

Figure 4 Arrhenius plot for cefaclor monohydrate degradation.

Stability studies to support a requested shelf life and storage condition must be run under real-time, real-temperature conditions [16,17], The prediction of shelf life by using stability studies obtained under stress conditions and Arrhenius plots is not meaningful unless it has been demonstrated that the chemical reaction accounting for the degradation process follows first-order reaction. 

Figure 5.40 shows the effect of the temperature (Arrhenius plot) on the degradation of gemcitabine at pH 3.2 (acetate buffer). 

Previous studies of the decomposition of cellulose reported Ea for absorbent cotton as 54.3 kcal/mol at a high-temperature range of 270-310 °C (23). For temperatures below pyrolysis, Ea = 20 kcal/mol reflects the low-temperature degradation effects of loss of H and OH from adjacent carbon atoms in cellulose (dehydration) and the concomitant creation of C=C bonds (24). In another work Ea = 21 kcal/mol was estimated from Arrhenius plots of the degree of polymerization versus time for cellulose heated in air at 150-190 °C (25). 

These apparent contradictions can be rationalized in terms of a model which incorporates plasma-induced polymerization along with depolymerization. PBS has long been known to exhibit a marked temperature-dependent etch rate in a variety of plasmas. This is clearly seen in the previously published Arrhenius plots (3,7) for two different plasma conditions (Figure 1). This dependence is characteristic of an etch rate that is dominated by an activated material loss as would occur with polymer depolymerization. The latter also greatly accelerates the rate of material loss from the film. Bowmer et al. (10-13) have shown in fact that poly(butene-l sulfone) is thermally unstable and degrades by a depolymerization pathway. A similar mechanism had been proposed by Bowden and Thompson (1) to explain dry-development (also called vapor-development) under electron-beam irradiation. 

Figure 13 Arrhenius plots of degradation of tetracycline hydrochloride in phosphate buffer at pH 7 under longwave ultraviolet from a black-light tube radiation. O without glutathione with glutathione. Source. From Ref. 71.

Figure 3. Arrhenius plots for PP degradation. 9, thermal photothermal Y, photo.

The essentially linear Arrhenius plot for the temperatures 130 t 150 , and 1T0 C demonstrates that random chain scission for polyethylene in the melt does behave in an Arrhenius manner. While we are not able to determine if the degradation behaves Arrhenlusly in the solid state, we can conclude that there is a definite discontinuity in the linear behavior between the two states. This discontinuity precludes the use of high temperature melt experiments to determine rate constants at temperatures below 90°C. 

Degradation rate constants were obtained by linear regression least squares analysis of plots of log % EDB remaining vs time. Pseudo-first order rate constants were used to generate Arrhenius plots (log rate constant vs 1/T °K) to estimate activation energies (E ) and to make extrapolated estimates of rate constants and half-life values at ambient temperature. 

Figure 3. Arrhenius plot of the tmax times for HTPB at different temperatures showing the faster degradation when infected by the degrading PP at 150°C.

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