Phenol-formaldehyde resins degradation

Knstkova, M., Filip, P. et al. Influence of metals on the phenol-formaldehyde resin degradation in friction composites. Polymer Degradation and Stability, 84 (2004), p. 49 - 60... 

Recently, several reports of the flame-retardant properties of boron-containing bisphenol-A resins have appeared from Gao and Liu.89 The synthesis of a boron-containing bisphenol-A formaldehyde resin (64 and 65) (Fig. 42) from a mixture of bisphenol-A, formaldehyde, and boric acid, in the mole ratio 1 2.4 0.5, has been reported.893 The kinetics of the thermal degradation and thermal stability of the resins were determined by thermal analysis. The analysis revealed that the resin had higher heat resistance and oxidative resistance than most common phenol-formaldehyde resins. 

The mechanical degradation and production of macroradicals can also be performed by mastication of polymers brought into a rubbery state by admixture with monomer several monomer-polymer systems were examined (10, 11). This technique was for instance studied for the cold mastication of natural rubber or butadiene copolymers in the presence of a vinyl monomer (13, 31, 52). The polymerization of methyl methacrylate or styrene during the mastication of natural rubber has yielded copolymers which remain soluble up to complete polymerization vinyl acetate, which could not produce graft copolymers by the chain transfer technique, failed also in this mastication procedure. Block and graft copolymers were also prepared by cross-addition of the macroradicals generated by the cold milling and mastication of mixtures of various elastomers and polymers, such as natural rubber/polymethyl methacrylate (74), natural rubber/butadiene-styrene rubbers (76) and even phenol-formaldehyde resin/nitrile rubber (125). 

These polymers may be compared with the more detailed discussion of thermal degradation pathways for phenol-formaldehyde resins described in Section 2.3.3 earlier, which explains the... 

A condensation polymer is one in which the repeating unit lacks certain atoms which were present in the monomer(s) from which the polymer was formed or to which it can be degraded by chemical means. Condensation polymers are formed from bi- or polyfunctional monomers by reactions which involve elimination of some smaller molecule. Polyesters (e.g., 1-5) and polyamides like 1-6 are examples of such thermoplastic polymers. Phenol-formaldehyde resins (Fig. 5-1) are thermosetting condensation polymers. All these polymers are directly synthesized by condensation reactions. Other condensation polymers like cellulose (1-11) or starches can be hydrolyzed to glucose units. Their chemical structure indicates that their repealing units consist of linked glucose entities which lack the elements of water. They are also considered to be condensation polymers although they have not been synthesized yet in the laboratory. 

The structures of the lignins are amenable to conversion to several classes of substituted phenols by thermochemical and thermal degradation methods. The displacement of pyroligneous tars by coal tars eliminated much of the demand that existed for the lignin-based products. Creosote oil or cresylic acid, a mixture of o-, m-, and p-cresols, is now manufactured mainly from coal tars, while only small amounts of cresols are made from wood tars. The use of wood tars and other biomass-derived tars as substitutes for a major portion of the phenol and formaldehyde in phenol-formaldehyde resins could reverse this trend (Himmelblau, 1995). The key to this process seems to be that the... 

Formation of stable free radicals in the hardening and thermal degradation of phenol-formaldehyde resins. Visokomol. Soed. 2, 1167 (1960). 

WeatherBest solid 28% HDPE, 60% wood flour, 12% talc + phenol-formaldehyde resin solid, density 1.20 g/cm3 No degradation effect No failure in 40 min... 

BSH is used for foaming rubbers, polystyrene, epoxy resins, polyamides, PVC, polyesters, phenol-formaldehyde resins, and polyolefins. However, the thermal decomposition of BSH yields not only nitrogen but also a nontoxic residue (disulfide and thiosulfone) which may degrade to give thiophenol and thus an unpleasant odor to the foams. 

Prokai [2] used pulse probe mass spectroscopy (MS) and pyrolysis-gas chromatography-mass spectroscopy (Py-GC-MS) to study the thermal decomposition of high molecular weight phenol-formaldehyde resins. He showed that degradation occurred by cleavage of the phenol-methylene bond and subsequent hydrogen abstraction to form phenol and methyl substituted phenols. 

Figure 9.3 shows the proposed degradation mechanism of phenol-formaldehyde resin in the presence of copper and iron catalysts. 

Figure 9.3 Proposed degradation reaction of phenol-formaldehyde resin with copper and iron as catalysts. Reproduced with permission from M. Kristkova, P. Filip, Z. Weiss and R. Peter, Polymer Degradation and Stability, 2004, 84, 49. ...

However, densely cross-linked plastics (thermosets) are sometimes deliberately used. Reasons tend to be nonmechanical increased solvent and swelling resistance, high-temperature resistance, and reduced flammability are cited. The last arises because the polymer chars rather than degrades with concomitant reduction of volatile (and flammable) components. Examples include epoxies, phenol-formaldehyde resins, and some polyurethanes see Section 14.2. 

MS has been widely used to identify the products and their formation kinetics in the degradation of filled reaction layers, e.g., phenol-formaldehyde resins [53, 54], epoxide resins, polyesters and polyacrylates [55]. 

Madorsky, S. L., Thermal Degradation of Organic Polymers , Interscience, New York, 1964. A compilation of the existing knowledge on polymers and copolymers of styrene, alkenes, halo-carbons, vinyl acetate, acrylonitrile, butadiene, isoprene, poly(ethylene terephthalate), polybenzyl, polyxylene, phenol, formaldehyde resin and cellulosic polymers Polym, Rev, vol. 7). 

---