SynopsisThe degradation of phenol-formaldehyde polycondensates has been investigated a t temperatures as high as 1OOO"C. By employing infrared spectrophotometric techniques and vapor-phase chromatographic methods, as well as thermogravimetric and x-ray analyses, it has been possible to examine the oxidation chemistry of this resin system. It has been found that the primary degradation route of phenol-formaldehyde polycondensates, regardless of whether the resin is exposed to elevated temperatures in air, argon, or nitrogen, is oxidation. At elevated temperatures, products are observed from thermal pyrolysis in addition to those from the oxidation path. However, throughout the temperature region studied, the oxidative degradation is always more pronounced. The mechanism of oxidation has been found to agree favorably with the proposals formulated earlier by Conley and Bieron. It is also possible to extend this mechanism to include the formation of graphitelike char. This extension is postulated on the simultaneous formation of the char and carbon monoxide. By infrared studies of the intermediate stages in char formation, the postulation that these products arise from quinone-type intermediates is justified. In this study, it was found that phenolformaldehyde polycondensates of unusual thermal stability could be produced by high temperature postcuring. The roinparison of the oxidation rates of the resin cured a t low temperature and this material indicates greatly improved oxidative stability, as well as resistance to thermal pyrolysis. This undoubtedly is due to the complete crosslinking of the resin system.

Furan resins were prepared by the polymerization of furfuryl alcohol with either acid or thermal catalysis. The effect of catalysts, solvents, polymerization time, and atmosphere were studied for their effects upon the chemical structure of the polymer produced. It has been found that, regardless of the technique employed, the resins contained appreciable amounts of ketonic species. Examination of the infrared spectrum of these resins indicated that the relative amount of ketonic material to furan ring–containing species was the same, regardless of the resin viscosity. It is proposed that the ketonic species arise during the polymerization by ring–opening of the furan unit, forming γ‐diketone functional systems as part of the polymer unit. The resins were separated into crude fractions by vacuum distillation, base extraction, and fractional precipitation. From heat‐catalyzed resins a lactonic component identified as 5‐hydroxy‐3‐pentenoic γ‐lactone was isolated. The curing of furan resins in nitrogen was shown to proceed through further condensation of furan methylol groups with furan rings having an available α‐hydrogen. β‐Hydrogen crosslinking reactions were not supported by infrared examination of the curing process.

An infrared spectrophotometric examination of the oxidative degradation of acid‐ and base‐catalyzed phenol‐formaldehyde polycondensates, novolaks and resoles, respectively, has been carried out in the temperature range form 100–200°C. The existence of structural moieties such as quinone methides and dibenzyl ethers in the cured phenolic resins systems could not be substantiated. The oxidation of phenolic resins was shown to be a stepwise degradation. Attack of oxygen, a surface reaction, was shown to be at the doubly activated methylene bridge linkage to form a substituted dihydroxybenzophenone system. This species was substantiated by the synthesis of polymers containing the ketonic linkage and their spectral identity to the degrading resin. The initial oxidation was shown to continue through the formation of quinone structures and secondary oxidation was shown to continue through the formation of quinone structures and secondary oxidation at these functional linkages to produce carboxylic acids as one of the fragments during chain scission. This degradation mechanism is in good agreement with other supporting experimental data concerning phenolic resin degradation.

The oxidative thermal degradation of polyacrylonitrile has been examined using infrared spectroscopy. It was found that the generally accepted mechanism for the thermal degradation of polyacrylonitrile in air, which involves direct interaction of neighboring nitrile groups, and alternate proposals, involving azomethine crosslinks through reaction of the nitrile group and a neighboring tertiary hydrogen atom, do not satisfactorily represent an initial degradation scheme accounting for the observed infrared spectral changes. Rather, it must be concluded that these reactions take place after the initial degradation, to produce highly complex pyridinoid systems, and are not observable under the experimental conditions employed here. By detailed interpretation of the spectral changes using, in part, a difference spectral examination technique, an alternate route for the oxidative degradation involving the introduction of double bonds in the polymer chain could be formulated. This reaction can best be visualized as occurring via the attack of molecular oxygen at the activated tertiary carbon–hydrogen bond adjacent to the electron‐withdrawing nitrile group. The nitrile group was shown to remain unreacted during the course of the initial oxidation. After the introduction of the double bond, a number of secondary degradative reactions, of which the previously proposed routes of degradation are representative, take place; these include degradation reactions of the nitrile groups to form acid and amide groups. It was observed that the accumulation of oxygenated intermediates on the surface of the polymer film is a rapid reaction and that the initial degradation process is controlled by the concentration of these species.