Polymers, formaldehyde Thermal decomposition

Why is the thermal decomposition of polymers of formaldehyde called unzipping ... 

Important in combustion is not so much the thermal stability of the material itself but rather the amount and nature of the decomposition products. It is sufficient to compare the LOI of poly(vinyl chloride), whose thermal decomposition begins at 160-175 °C with that of heat resistant phenol-formaldehyde fibers (Kynol). The thermodynamic approach to the problem seems to be most reasonable. It allows to consider the polymer structure to explain the details of the combustion reactions and to estimate the heat of combustion of polymers. 

Carbonaceous materials (CMs) are sometimes also named polymeric carbons. They are mostly prepared by thermal decomposition of organic precursors. One strategy is pyrolysis of gaseous or vaporized hydrocarbons at the surface of heated substrates, a second is heating (pyrolysis) of natural or synthetic polymers, both in an inert atmosphere. The latter is of special interest, and according to Miyabayashi et al. [374], precursors such as condensed polycyclic hydrocarbons, polymeric heterocyclic compounds, phenol-formaldehyde resins, polyacrylonitrile or polyphenylene are heated to 300-3000 °C for 0.15-20 h. Sometimes, a temperature/time profile is run. The temperature range must be divided into two domains, namely... 

Several reports are available In literature regarding thermal decomposition of poly(ethylene oxide) not in flash pyrolysis conditions. One such report indicates that at temperatures between 324° C and 363° C, the polymer generates 9.7% of volatile compounds (at 25° C), 3.9% monomer with smaller amounts of CO2, formaldehyde, ethanol, and saturated C1-C7 compounds [4]. Another report indicates that at temperatures between 225° C and 250° C, the polymer generates CO2, HCHO,... 

Polyacetals form a different subclass of compounds with oxygen in the backbone chain. In this group are included polymers that contain the group -0-C(R2)-0- and can be formed from the polymerization of aldehydes or ketones. A typical example of a polymer from this class is paraformaldehyde or polyformaldehyde or polyoxymethylene (CH20)n. Polyoxymethylene can be prepared by anionic catalysis from formaldehyde in an inert solvent. Acetylation of the -OH end groups of the polymeric chain is common since it improves the thermal stability of the polymer. Some results reported in literature regarding thermal decomposition of these polymers are indicated in Table 9.2.1 [1]. 

The information from Table 12.1.2 mainly refers to thermal decomposition in conditions different from those used in flash pyrolysis. However, the nature of pyrolysis products during flash pyrolysis is not usually very different from that resulting from thermal decomposition at slower rates but at similar temperatures. Also, the decomposition mechanisms are similar, and cleavages occur at the weaker bonds in the polymer. For example, for thiohenol/formaldehyde resin the benzylic C-S bond is the weakest in the resin, and its cleavage is the key step in the formation of most pyrolysis products. 

A number of low molecular weight products, for example, benzaldehyde, formic acid, formaldehyde, phenylglyoxal, phenylglyoxalic acid, and carbon dioxide, were identified among the products of thermal decomposition of the ozonides. A determination of the detailed structure of the polymer is possible from the qualitative analysis of these decomposition products. 

The Tt, Td and of the materials based on CFD increase with the introduction of Cl except for VNHL-20, which shows a reduced CFD Tt, and Td, evidently due to its own low thermal resistance. The inhibitors are likely to assist in binding of formaldehyde formed during CFD destruction and lead to decomposition of the copolymer macromolecular chain. In compositions on a polyamide base. Cl inhibit thermally oxidative processes that accelerate thermal decomposition of the polymer, elevating Tt and Td by 5-10°C. 

As stated previously, condensation between urea and formaldehyde leads to the formation of the polymer urea-formaldehyde. A third member in this series is the melamine, which will also react with formaldehyde leading to the polymer melamine-formaldehyde. Melamine may also be derived by thermal decomposition of urea. 

Later Dudina and Enikolopyan [4-12] expressed considerations in favor of the possible occurrence of the process of thermal destruction of polyformaldehyde according to ionic and radical mechanisms. Moreover, the authors established that in the thermal decomposition of the polymer, the natural gaseous product is monomeric formaldehyde. The participation of oxygen in the thermal oxidation of polyformaldehyde is manifested only in an initiation of depolymerization, which proceeds according to the "laws of chance" for the polymer with blocked terminal groups, and in the fact that the oxidative direction of the reaction is absent in this case. 

Phenol formaldehyde Resins, nickel, copper, and cobalt [95-97] all cause thermal decomposition of polymers. 

DuPont s method for making polyacetal yields a homopolymer through the condensation reaction of polyformaldehyde and acetic acid (or acetic anhydride). The acetic acid puts acetate groups (CH3COO—) on the ends of the polymer as shown in Figure 3.1, which provide thermal protection against decomposition to formaldehyde. 

Thermal degradation takes place in three stages. Up to 300°C the polymer releases only some water and remaining traces of phenol and formaldehyde. Decomposition starts above 300°C, when water, carbon monoxide, carbon dioxide, methane, phenol, cresols, and xylenols are expelled. The third stage begins above 600°C, again involving the release of water, carbon dioxides, methane, benzene, toluene, phenol, cresols, and xylenols. 

However, the introduction of stabilizers is essential, since processes of thermal and thermooxidative destruction develop extremely intensively during the use and reprocessing of such polymers, leading to a sharp deterioration of their physicomechanical and dielectric properties. Thus, an extremely urgent problem at the present time is a detailed investigation of the processes of decomposition of condensation polymers for developing a theory of their stabilization. Works in the field of the study of the thermal and thermooxidative destruction of certain condensation polymers (epoxide, phenol-formaldehyde resins, polycarbonate, and polyarylates) are outlined below. 

The most commonly used polymer precursors for carbon membranes have been reported to be polyimides, polyfurfuryl alcohol, phenol formaldehyde resins and cellulose. Their common characteristic is that they do not melt during pyrolysis at high temperature, which keeps their original shape and structure during the thermal heating and decomposition process. In this sense, the commercially available Matrimid and Kapton are the fully imidized polyimides with high values. They do not abruptly change their... 

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