Polymer degradation phenolic resins

Epoxides can react with alcohols via acidic or basic catalysed reaction mechanisms. However, since both strong acids and bases will degrade the cell wall polymers of wood, the reaction is usually catalysed via the use of amines, which are more strongly nucleophilic than the OH group. For example, whereas the production of epoxy-phenolic resins requires temperatures in the region of 180-205 °C, reaction between epoxides and primary or secondary amines takes place at 15 °C (Turner, 1967). Reaction of epoxides with wood often involves the use of tertiary amines as catalysts (Sherman etal., 1980). The sapwood is more reactive towards epoxides than heartwood (Ahmad and Harun, 1992). 

Very recently, attempts have been made to develop PP/EOC TP Vs. In order to make TPVs based on PP/EOC blend systems, phenolic resin is ineffective because the latter needs the presence of a double bond to form a crosslinked network structure. Peroxides can crosslink both saturated and unsaturated polymers without any reversion characteristics. The formation of strong C-C bonds provides substantial heat resistance and good compression set properties without any discoloration. However, the activity of peroxide depends on the type of polymer and the presence of other ingredients in the system. It has been well established that PP exhibits a (3-chain scission reaction (degradation) with the addition of peroxide. Hence, the use of peroxide only is limited to the preparation of PP-based TPVs. Lai et al. [45] and Li et al. [46] studied the fracture and failure mechanism of a PP-metallocene based EOC based TPV prepared by a peroxide crosslinking system. Rajesh et al. 

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. ...

Peroxide degrades some plastics, such as polypropylene (PP). Consequently, in peroxide crosslinked TPVs, polymers like PP degrade and polymers like polyethylene crosslink. In phenolic resin cured TPVs, thermoplastics like PP do not degrade but moisture is always an issue, because the stannous chloride or zinc chloride that are used as Lewis acid are hygroscopic. 

Liu, L., Ye, Z. Effects of modified multi-walled carbon nanoUrbes on the curing behavior and thermal stability of boron phenolic resin. Polym. Degrad. Stab. 94, 1972-1978 (2009)... 

The formation of DPQ not only reduces the yield of polymer but also degrades its quality. Thus, it is imporant to remove both the DPQ and metallic catalyst residues from the reaction solution because they could result in discoloration and degradation of resin while processing. In typical large scale operations, the reaction mixtures are usually treated with a combination of the dihydric phenol and a mild reducing agent, e.g. hydroquinol and sodium sulfite for 20-30 minutes to minimize the residues before isolation of the polymer (Figure 2.2.). 

Like phenol resins, epoxy resins can be identified rapidly with the help of a number of relatively simple, yet reliable chemical spot tests, but they serve mostiy only for establishing the class of polymer. More subtle analytical techniques are required to characteri.se technical epoxy resin formulation, especially in the presence of additives, fillers or perhaps even of spurious materials. The decisive breakthrough in the systematic characterisation of epoxy resins came with the introduction of thermal degradation procedures into polymer analysis. The polymer composition can be established beyond doubt from the separated and identified products of thermal degradation. The problems of separation and detection of these products have been solved to practice by GLC - and... 

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... 

Lyim and co-workers [31] studied the effect of PETP on the thermo-oxidative stability of Novolac-type phenolic resins using TGA, DSC and FTIR. It was found that the incorporation of PETP in hlends improves stability. Degradation of polymer is avoided up to 370 °C. 

T., Chemical recycling of carbon fibre reinforced epoxy resin composites in subcritical water synergistic effect of phenol and KOH on the decomposition efficiency, Polym. Degrad. Stab., 97 (3), 214-220 (2012). 

Commercially important thermosets include epoxies, polyesters, and phenolic resins. Each of these materials starts out as (often viscous) liquids that set by curing into a final shape. Because these materials set the first time they are made, they cannot be reheated after the polymer is formed without degrading the structure. 

Thermosets are heavily cross-linked polymers which are normally rigid and intractable. They consist of a dense three-dimensional molecular network and, like rubbers, degrade rather than melt on the application of heat. Common thermosetting polymers include phenol-formaldehyde or urea-formaldehyde resins and high-performance adhesives such as epoxy resins. 

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