Prediction degradation

Polylactic Acid. Polylactic acid (PLA) was introduced in 1966 for degradable surgical implants. Hydrolysis yields lactic acid, a normal intermediate of carbohydrate metaboHsm (23). PolyglycoHc acid sutures have a predictable degradation rate which coincides with the healing sequence of natural tissues. 

Before releasing a process column for chromatography, it is advisable to perform some test to measure efficiency, such as calculating height equivalent theoretical plates (HETP), both to forestall any problems in the column bed and to provide a benchmark by which to measure column reproducibility and predict degradation of the bed or material. Examples of compounds that are relatively innocuous for use in pharmaceutical applications are 1% NaCl (for gel filtration), concentrated buffer solutions (for ion exchange), and benzyl alcohol and parabens for reverse phase LC.10... 

The practical conclusion is that if comparisons are to be made between accelerated and natural ageing results or attempts made to predict degradation at lower temperatures from accelerated tests, the existence of degradation gradients is likely to have significant effect. Also, predictions made from tests on thin test pieces may be misleading if applied to thick products. 

In some simple cases an additive approach can be successful. The service life is divided into stages, for example moderate exposure for one year, severe exposure for one month and low exposure for five years. The predicted degradation effects for the three periods can be summed and the condition of the product at the end, and hence residual lifetime, estimated. If it is assumed that degradation is proportional to the time of exposure and that the damage from different exposure conditions can be cumulatively added then the so called Miner s rule could be used ... 

Safety factors are used in engineering design to reduce the design load (or equivalent parameter) to allow for the predicted degradation, to allow for a confidence limit based on statistical evaluation, or to allow simply for uncertainty. It is clear that any predicted degradation should be allowed for in full. Confidence limits can be calculated. Uncertainties must be estimated. For example, to allow for the extrapolation necessary... 

Zee, M.E. van der, Stoutjesdijk, J.H., Fell, H. and Feijen, J. (1998b). Relevance of aquatic biodegradation tests for predicting degradation of polymeric materials during biological waste treatment. Chemosphere, 36(3), 461 73. 

Hammett s equation was also established for substituted phenols from the elementary hydroxyl radical rate constants. The Hammett resonance constant was used to derive a QSAR model for substituted phenols. The simple Hammett equation has been shown to fail in the presence of electron-withdrawing or electron-donating substituents, such as an -OH group (Hansch and Leo, 1995). For this reason, the derived resonance constants such as o°, cr, and o+ were tested in different cases. In the case of multiple substituents, the resonance constants were summed. Figure 5.24 demonstrates a Hammett correlation for substituted phenols. The least-substituted compound, phenol, was used as a reference compound. Figure 5.24 shows the effects of different substituents on the degradation rates of phenols. Nitrophenol reacted the fastest, while methoxyphenol and hydroxyphenol reacted at a slower rate. This Hammett correlation can be used to predict degradation rate constants for compounds similar in structure. 

The QSAR models can be used to estimate the treatability of organic pollutants by SCWO. For two chemical classes such as aliphatic and aromatic compounds, the best correlation exists between the kinetic rate constants and EHOMO descriptor. The QSAR models are compiled in Table 10.13. By analyzing the behavior of the kinetic parameters on molecular descriptors, it is possible to establish a QSAR model for predicting degradation rate constants by the SCWO for organic compounds with similar molecular structure. This analysis may provide an insight into the kinetic mechanism that occurs with this technology. 

This predictive degradation strategy was demonstrated for biomolecules as a tool to accelerate drug development (Kleintop et al., 1998). Biomolecule samples are exposed to various accelerated stressed environments such as acid, base, heat, high intensity light, and humidity. Molecular weight information obtained from... 

Kleintop, B. Volk, K. Hail, M. Sanders, M. Reddick, C. Bolgar, M. Warrack, B. Liu, X Detlefsen, D. Kerns, E. Lee, M. 1998. Predictive degradation profiling of biomolecules using LC/MS methodologies. Proc. 46th ASMS Conf. Mass Spectrom. and Allied Topics (Orlando, Florida), 240. 

A third section on algorithms and software attempts to overview the currently available, preferably computerized, predictive degradation/transformation models, as well as their applicability and reliability. Any recommendations for particular programs are dependent on the suitability of the program for specific questions (e.g., specific compounds and transformation), and also continuous improvements and new developments. 

A model has been developed to predict degradation during moulding based on a spiral mould and the use of mathematical equations (249). 

Run i Degradation Rate t Constant K[Pg.230]

Predicted degradation rate from baseline correlation ... 

This book discusses biological and geological cydes of various chemical substances and their potential exposure from cradle-to-grave. Contents indude physico-chemical properties, predicting degradation, partition coeffidents, kinetics of distribution phenomena, validating predictions, and hazard assessment. 

Each of these parameters was experimentally established by Carstensen and co-workers and utilized to predict the degradation curve.277-287-288 The predicted degradation curve diverged from the observed curve, indicating that the adsorbed moisture layer theory cannot fully explain the degradation of aspirin (Fig. 28). 

The predicted degradation half-life of tetrachloroethylene in the atmosphere indicates that long-range global transport is likely (Class and Ballschmiter 1986). Indeed, monitoring data have demonstrated that tetrachloroethylene is present in the atmosphere worldwide and at locations far removed from anthropogenic emission sources (see Section 5.4.1). 

Application of Redox Chemical Information to Predict Degradation Rates... 

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