Kinetics degradation

The fundamental rate equation used in all kinetic studies is generally described as  

Combination of Equation (21.7) with the term described previously gives  

For a dynamic TGA process, introduction of the heating rate, = dT/dt, into Equation (21.8), yields  

Equations (21.8) and (21.9) are the fundamental expressions of the analytical methods used to calculate the kinetic parameters based on TGA data [28,35,36]. The most common model-free methods used are summarized in Table 21.1. 

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As previously stated, GPC is the method of choice for studying polymer degradation kinetics. The GPC trace, as given by the detector output, does not provide the true MWD due to various diffusion broadening processes inside the different parts of the equipment. The first step is to correct for instrument broadening if a precise evaluation of MWD is desired. Even with the best columns available, this correction may change the MWD significantly as can be visualized... 

It should be stressed that the observed critical strain-rate for bond fracture (sf) in the case of a polydisperse fraction refers to the longest chain present in the sample. This quantity is significantly different from the critical strain-rate (r ) defined with respect to an average molecular mass whose value could be determined only after careful consideration of the degradation kinetics. 

The parameter R in Eq. (92) can be derived empirically from the experimental SEC traces with a minimum of computational effort and without regard to the details of the degradation kinetics by using the following arguments. Once a macromolecule is fractured, the moieties are immediately driven into another region of space. Due to the decrease in MW, a considerably higher strain rate... 

Lartiges SB, Garrigues PP. 1995. Degradation kinetics of organophosphorus and organonitrogen pesticides in different waters under various environmental conditions. Environ Sci Technol 29 1246-1254. 

Polymers have inherently high hydrocarbon ratios, making liquefaction of waste plastics into liquid fuel feedstocks a potentially viable commercial process. The objective is to characterise the thermal degradation of polymers during hydrogenation. LDPE is studied due to its simple strueture. Isothermal and non-isothermal TGA were used to obtain degradation kinetics. Systems of homopolymer, polymer mixtures, and solvent-swollen polymer are studied. The significant variables for... 

Penuela GA, Barcelo D. 1998. Application of C18 disks followed by gas chromatography techniques to degradation kinetics, stability and monitoring of endosulfan in water. J Chromatogr 795 93-104. 

Elucidation of degradation kinetics for the reactive extrusion of polypropylene is constrained by the lack of kinetic data at times less than the minimum residence time in the extruder. The objectives of this work were to develop an experimental technique which could provide samples for short reaction times and to further develop a previously published kinetic model. Two experimental methods were examined the classical "ampoule technique" used for polymerization kinetics and a new method based upon reaction in a static mixer attached to a single screw extruder. The "ampoule technique was found to have too many practical limitations. The "static mixer method" also has some difficult aspects but did provide samples at a reaction time of 18.6 s and is potentially capable of supplying samples at lower times with high reproducibility. Kinetic model improvements were implemented to remove an artificial high molecular weight tail which appeared at high initiator concentrations and to reduce step size sensitivity. 

Many investigators have studied the in vivo degradation kinetics of lactide/glycolide materials (5,35-39). There has been some confusion in the interpretation of results primarily because of lack of consistency in nomenclature and careful attention in describing the specific stereoisomers evaluated. Nevertheless, the overall degradation kinetics are fairly well established for the entire family of homopolymers and copolymers. At the present, this common knowledge of the in vivo lifetimes of various lactide/glycolide polymers is a primary reason for their popularity. 

Nisha, R, Singhal, R.S., and Pandit, A.B., A study on the degradation kinetics of visual green colour in spinach (Spinacea oleracea L.) and the effect on salt therein, J. Food Eng., 64, 135, 2004. 

Since it is easier to control and change the conditions of carotenoid studies carried out in model systems, information on degradation kinetics (reaction order model, degradation rate, and activation energy) and products formed are often derived from such studies. 

Rios, A.O., Borsarelli, C.D., and Mercadante, A.Z., Thermal degradation kinetics of bixin in an aqneons model system, J. Agric. Food Chem., 53, 2307, 2005. 

The degradation kinetics of malvidin 3-glucoside in ethanolic solutions under conditions simulating wine accelerated with the increase of ethanol concentration, probably because the extent of anthocyanin self-association decreased with elevated ethanol concentration. ... 

Reyes, L.R and Cisneros-ZevaUos, L., Degradation kinetics and colour of anthocyanins in aqueous extracts of purple- and red-flesh potatoes Solanum tuberosum L.), Food Chem., 100, 885, 2007. 

Kirca, A. and Cemeroglu, B., Degradation kinetics of anthocyanins in blood orange juice and concentrate. Food Chem., 81, 583, 2003. 

Tseng, K.C. et al.. Degradation kinetics of anthocyanin in ethanohc solutions, J. Food Process Preserv., 30, 503, 2006. 

Acero JL, K Stemmier, U van Gunten (2000) Degradation kinetics of atrazine and its degradation products with ozone and OH radicals a predictive tool for drinking water treatment. Environ Sci Technol 34 591-597. 

Hwang H-M, RE Hodson, DL Lewis (1989) Microbial degradation kinetics of toxic organic chemicals over a wide range of concentrations in natural aquatic systems. Environ Toxicol Chem 8 65-74. 

No discussion of models will be found in this volume, either for the analysis of degradation kinetics or for the prediction of biodegradability. For these, the interested reader should consult monographs by experts. 

What materials are likely to contain pesticide residues following application What are the known degradation kinetics of the crop protection product being evaluated ... 

The CAT model considers passive absorption, saturable absorption, degradation, and transit in the human small intestine. However, the absorption and degradation kinetics are the only model parameters that need to be determined to estimate the fraction of dose absorbed and to simulate intestinal absorption kinetics. Degradation kinetics may be determined in vitro and absorption parameters can also be determined using human intestinal perfusion techniques [85] therefore, it may be feasible to predict intestinal absorption kinetics based on in vitro degradation and in vivo perfusion data. Nevertheless, considering the complexity of oral drug absorption, such a prediction is only an approximation. 

Not only do we need to know how susceptible a polymer is to degradation, we also need to understand how quickly the polymer will succumb to its effects. From a processing standpoint, we need to see how long the polymer can be held at elevated temperatures before excessive degradation occurs. From an end-use perspective, we need to predict the lifetime of the final product as a function of the environment it will encounter. For these reasons degradation kinetics are extensively studied. 

Figure 3. Degradation kinetics of Copolymer C at 80°C 1 without oxygen 2 with oxygen ...

Under these conditions, the degradation kinetics of sample H is similar,with a different value of k in relation (4). However, the origin of the phenomenon is different for sample H, the degradation is due to the decomposition of chain hydroperoxides by traces of Fem. The sample C does not contain peroxides nor Fem and we have explained its instability by the presence of catalyst residues. 

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