Polymers, burning pyrolysis

The intumescent char acts essentially as a physical barrier to heat and mass transfer between the flame and the burning material. Thus, the process of pyrolysis of the polymer that produces combustible volatile products to feed the flame is reduced by a decrease in temperature, caused in turn by a lower heat supply from the flame. The diffusion of the volatile products towards the flame is hindered with further reduction of the flame feed. Furthermore, whatever may be the role of oxygen in the combustion process, its diffusion towards the polymer burning surface is also hindered, This series of events can lead to an interruption in the self-sustained combustion process because the flame is starved. 

N.S. Cohen et al, A1AA J 12 (2), 212-18 ( qia QQt 135471 (1974V The effects of inert polymer binder properties on composite solid proplnt burning rate are described. Surface pyrolysis data for many polymers over a wide range of conditions are used to derive kinetics constants from Arrhenius plots and heat of... 

Goethite is used in flame retardants and smoke suppressants. Both laboratory and large scale pilot tests showed that goethite is the most active smoke suppressant when polymers and plastics are burned (Carty and White, 1999 Carty et al., 1999). It reduces the amount of smoke produced during pyrolysis in air of chlorinated PVC plasticized with dioctylphthalate, by changing the decomposition pathway followed by phthalate, so that benzene, which is produced in the absence of the smoke suppressant, is not formed (Carty et al., 1999). 

Because of the ease of formation of these flammable pyrolysis products, polyesters have LOI values of 20-22 vol% (see Table 2.4), and hence, burn readily and because of the styrene content, give heavy soot formation. As these resins are cured at room temperature, bromine-containing flame retardants, which would decompose in melt-processed, thermoplastic polymers, may be effectively used. 

Learmonth, G. S. Nesbitt, A. Thwaite, D. C. Flammability of plastics I. Relation between pyrolysis and burning, British Polymer Journal, 1969, 1, 149-153. 

Burns and co-workers (i 7) prepared a series of alkyl-, aryl-, and arylalkyl-substituted polysilazane polymers (equation 12), and a mechanistic study of pyrolysis was carried out to determine the effect of substituents on char yield, char composition, and stability of the resulting ceramic powders. 

Wood burns because the cell wall polymers undergo hydrolysis, oxidation, dehydration, and pyrolysis reactions with increasing temperature to give off volatile, flammable gases. The lignin component contributes more to char formation than do the cellulose components, and the charred layer helps insulate the wood from further thermal degradation see Chapter 13). 

The study of polyethylene thermal decomposition is important in relation to the polymer resistance to heating [2], to various attempts to use waste containing polyethylene as a combustible or as a source of other useful materials [3-7], or to environmental issues when polyethylene is burned [8]. Various other studies on polyethylene pyrolysis were reported [9-22], etc. 

Thermal analysis experiments have clearly shown that tin-based fire retardants markedly alter both the initial pyrolysis and the oxidative burn off stages that occur during polymer breakdown These changes have been interpreted as being indicative of an extensive condensed phase action for the tin additive, in which the thermal breakdown of the polymer is altered to give increased formation of a thermally stable carbonaceous char at the expense of volatile, flammable products. The consequent reduction in the amount of fuel supplied to the flame largely accounts for the beneficial smoke-suppressant properties associated with zinc stannates and other tin-based fire retardants. 

The third route is defined as substractive (lUPAC), in that certain elements of an original structure are selectively removed to create pores. Examples include the formation of porous metal oxides by thermal decomposition of hydroxides, of porous glasses by chemical etching, of activated carbons by controlled pyrolysis, of ceramic foam membranes by burning off a polymer (e.g. polyurethane), of alumina by anodic oxidation of aluminium to give oriented cylindrical pores with a narrow size distribution. 

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