Thermal degradation modeling steps

An important step in the thermal degradation of many polymers such as PMMA involves depropagation. Here the polymer molecule or radical fragment rapidly unzips in such a way that produces mainly a monomer. This process may be modeled simply by a rapid reaction where an z-mer is converted to an (i - l)-mer plus a monomer P —> P x + Px. In reality, this is a gross oversimplification of the process and a more sophisticated model will be discussed subsequently and also later in the chapter in relation to the thermal degradation of PMMA. 

Diffusion of solutions of Cr, Co and Mn ions through a PTFE membrane allows for separation of Cr from the remaining ions. Thermal stability of polymeric materials is a significant consideration in many poly(phosphazene) applications. The kinetics of the thermal degradation of PTFE are best fit with a model requiring a two step initiation for depolymerization. These steps involve formation of defect units, such as =P(0)NH- and =P(0)N(CH2CFj)-, which become active centers for depolymerization. Mixed... 

As a final consideration, it is relevant to discuss the behaviour of mixtures of different plastics. In fact, one possible process for recovering valuable chemical and petrochemical products from plastic waste is the stepwise thermal degradation of polymer mixtures. This potentially allows the step-by-step simultaneous separation of the different fractions generated by the polymers of the blend. The effect of the mixing scale of PE and PS and their interactions in the melt on the basis of several hypotheses was recently investigated (Faravelli et al., 2003). The first and simplest approach was a completely segregated model which... 

Taking this as a reasonable approximation for PE and PE-n-MMT, the fits with the aid of nonlinear regression were attempted by the model (5), where an one-dimensional diffusion type reaction was used for the first step and the nth-order (Fn) reaction - for the two subsequent steps of the overall thermal degradation process (Figure 6, Table 1). 

Table 2. The kinetic parameters for the three-step thermal degradation of PE and PE-n-MMT as obtained by the multiple-curve analysis of the experimental TGA-data (heating rates 3,5,10 and 20 K/min) in frames of the reaction model...

Competitive hydrogen abstraction reactions by phenoxy and benzyl radicals on the a-or )8-carbon of PPE are the core reactions of the radical chain propagation (Scheme 7.2) in the thermal degradation of PPE and are the only reactions needed in the analytical kinetic model derived in Section 7.3.2.3 to calculate overall a/)8-selectivities. Because hydrogen abstractions are the rate-determining steps, effects of substituents on hydrogen abstraction have the most impact on overall product distributions and pyrolysis rates. 

Kinetic studies have been made on the thermal decomposition of a poly(oxypropylene)triol-toluene di-isocyanate copolymer foam. Following a diffusion rate-controlled step, the cellular structure collapses to a viscous liquid and degradation then occurs on a random scission basis. Products of degradation of A-monosubstituted and A A-disubstituted polyurethanes have been analysed by direct pyrolysis in the ion source of a mass spectrometer. The mono-substituted polymers depolymerize quantitatively to di-isocyanates and diols, whereas the disubstituted materials decompose selectively to secondary amines, olefins, and carbon dioxide. The behaviour of the monosubstituted polymers has been confirmed in an i.r. study of the degradation of model compounds. A study of the thermal degradation in vacuum of polyurethanes prepared from butanediol, methylene bis(4-phenylisocyanate), and hexanedioic acid-ethylene glycol-propylene glycol polyesters has been reported and reaction mechanisms proposed. ... 

Thermal degradation of blends based on recycled PP and an elastomeric additive, ethylene/ a-octene copolymer (EOC) or synthetic rubber, in low concentration is usually a one-step process and can be kinetically described by using the autocatal)U ic model [a.3]. Figures 43 and 44 show the dynamic TG results obtained. 

Camino, G., Martinasso, G., Costa, L., Gobetto, R., 1990a. Thermal degradation of pentaerythritol diphosphate, model compound for fire retardant intumescent systems Part 11-lntumescence step. Polymer Degradation and Stability 28, 17—38. 

The FDS5 pyrolysis model is used here to qualitatively illustrate the complexity associated with material property estimation. Each condensed-phase species (i.e., virgin wood, char, ash, etc.) must be characterized in terms of its bulk density, thermal properties (thermal conductivity and specific heat capacity, both of which are usually temperature-dependent), emissivity, and in-depth radiation absorption coefficient. Similarly, each condensed-phase reaction must be quantified through specification of its kinetic triplet (preexponential factor, activation energy, reaction order), heat of reaction, and the reactant/product species. For a simple charring material with temperature-invariant thermal properties that degrades by a single-step first order reaction, this amounts to -11 parameters that must be specified (two kinetic parameters, one heat of reaction, two thermal conductivities, two specific heat capacities, two emissivities, and two in-depth radiation absorption coefficients). 

Instability of the polymer is responsible for the primary step in decomposition and is attributed either to fragments of initiator or to branched chains or to terminal double bonds. The appearance of branching is the result of reactions of chain transfer through the polymer, while that of unsaturated terminal groups results from reaction of disproportionation and chain transfer through the monomer. During thermal and thermo-oxidative dehydrochlorination of PVC, allyl activation of the chlorine atoms next to the double bonds occurs. In this volume, Klemchuk describes the kinetics of PVC degradation based on experiments with allylic chloride as a model substance. He observed that thermal stabilizers replace the allylic chlorine at a faster ratio than the decomposition rate of the allylic chloride. 

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