Thermal degradation depolymerisation

Hydrolysis to L-lactic acid and thermal degradation (depolymerisation) to the cyclic dimer L-lactide (LLA) is the general procedure for PLLA recycling [a.246]. For depolymerisation. 

CFP are normally quite resistant to thermal degradation, under dinitrogen, and depolymerisation starts in the 280-320 °C range, in the dry state [21]. This makes M°/ CFPs quite suitable for operations in the liquid phase occurring at mild to moderate temperatures. CFPs... 

One important thermal degradation mechanism of cellulose fibres (cotton, rayon, linen, etc.) is the formation of the small depolymerisation product levoglucosan (Fig. 8.7). Levoglucosan and its volatile pyrolysis products are extremely flammable materials and are the main contributors to cellulose combustion. Compounds that are able to hinder levoglucosan formation are expected to function as flame retardants for cellulose. The crosslinking and the single type of esterification of... 

The estimated Td,o-temperature giving an indication of the polymers thermal stability, holds only for polymers with the more or less simple thermal degradation processes of chain depolymerisation and/or random decomposition. S. Shkolnik and E.D. Weil [30] reported, however, that for non-stabilised PK copolymer a process of furan ring formation can start at temperatures of about 250 °C. Water, one of the reaction products, is coming free during this intramolecular... 

Thermal degradation of polymers usually proceeds via a number of possible mechanisms, which can generally be grouped into three classes (a) random scission, (b) depolymerisation, and (c) side group elimination. 

They found that thermal degradation of poly(4- -alkyl styrenes) followed mainly a free radical depolymerisation mechanism. The main product is a monomer similar to unsubstituted PS, i.e., 59% to 92% monomer from poly(4-n alkyl styrenes) ranging from 136,500-737,000 and = 37,000-99,000. The amoxmts of this monomer decrease with increasing length of alkyl sidechain from hexyl to decyl. This behaviour is connected with the stability of monomer under isothermal pyrolysis conditions at 600 °C. 

The thermal degradation of PMMA is accompanied by two reactions (random break of the C-C bonds and depolymerisation via a chain mechanism). The former reaction is thought to proceed with a lower rate and thus to determine both the rate of the whole process and the reaction order. 

The characteristics of the thermal degradation have been studied most fully for aliphatic polyamides [1-3], from which cyclic monomers are split rather readily. Thus, of the volatile products of the thermal degradation of polycaproamide at temperatures above 300 °C, mainly e-caprolactam is observed in a dry inert atmosphere. Simultaneously the molecular mass of the polycaproamide decreases. The monomer is supposed to be released via a depolymerisation reaction proceeding from terminal COOH groups ... 

The decomposition behaviour of PPS has been found to be far from simple, as products that correspond to both (a) cyclisation and (b) depolymerisation from radical chain-end reactions are found as shown in Equation 6.7 and Equation 6.8. The conditions used are obviously important and show that if a detailed and accurate knowledge of the thermal degradation behaviour of a thermoplastic is desired, it is important to use conditions that, as accurately as possible, reflect the thermal environment where they will be used ... 

The observed products from the thermal degradation of a wide range of thermoplastics can be accounted for by the differences in the rate constants ki, fed, fef, fes, and fet. For example, if fed > fef, the polymer tends to depolymerise to the monomer in almost quantitative yield, whereas if fed fef the products will include large fragments arising from the chain scission reaction. 

Volatile products can be clipped from the end of a polymer chain during the very start of a reaction, with a distribution that is not random, or by a process of end scission or backbiting - a process of unzipping can regenerate the monomer. Thermal degradation of polymers can follow three major pathways side-group ehmination, random scission and depolymerisation that can occur randomly and/or at the end of the polymer chain. ... 

TVA has been used mainly for the study of the basic degradation patterns of depolymerisation [910,942]. McNeill et al. [949] have studied thermal degradation of PS and PS/IDBPby means of TVA, SATVA and GC-MS. In the presence of 4,4 -isopropylidene-bis-(2,6-dibromophenol) (IDBP), the main products were similar, except for propiophe-none and phenylpropanoyl bromide in the presence of IDBP. Similarly, TG and TVA have been used to study the thermal stabilities of PET, PBT and PDMT [950]. In this case, the amounts of the main product fractions (residue, cold ring fraction, volatile products) have been determined quantitatively and the various materials present in the volatile and cold ring fractions have been separated and identified. McNeill et al. [939] also fractionated PVC/DOP by... 

Pyrolysis can be split into two types, both carried out in an oxygen depleted environment. Low temperature pyrolysis is a depolymerisation technique, whilst high temperature pyrolysis is a thermal degradation technique. 

However, recent research agrees that the main route of thermal degradation of PA-6 is the formation of caprolactam with yields as high as 85% - the presence of oligomeric products with nitrile and vinyl chain ends, which are formed as a result of depolymerisation, has been confirmed 890179 [a.l7, a.ll4j. The increase of reaction order of the overall decomposition of PA-6 above 420 C is correlated with the formation of by-products. Especially, the formation of the cyclic dimer seems to be a second-order reaction, which is responsible for the increase of the overall reaction order. The observed first-order reaction of E-caprolactam formation (Table 3) is consistent with the mechanism of cfs-elimination suggested, whereby the ds-elimination proceeds via a six-membered intermediate product (Scheme 12) [a.ll4] Table 3 [a.ll5] provides kinetic data. 

Scheme 17. (a) Thermal degradation of PHB and (b) unzipping depolymerisation from... 

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