Curing novolacs

Phenolic Resins. Phenohc resins (qv) are formed by the reaction of phenol [108-95-2] C H O, and formaldehyde [50-00-0] CH2O. If basic conditions and an excess of formaldehyde are used, the result is a resole phenohc resin, which will cure by itself Hberating water. If an acid catalyst and an excess of phenol are used, the result is a novolac phenohc resin, which is not self-curing. Novolac phenohc resins are typically formulated to contain a curing agent which is most often a material known as hexamethylenetetraamine [100-97-0] C H22N4. Phenohc resin adhesives are found in film or solution... 

Polymerization and curing rates of novolacs depend strongly on the acidity of the reaction mixture. Fig. 16 depicts the general pH dependence. Fig. 17 shows a partial structure for a hexa-cured novolac. Incorporation of amine is widely, though not universally, reported in hexa-cured novolac structures. In addition to the structure shown in Fig. 17, A, A -dibenzyl and A, A, A -tribenzylamine linkages have been reported [185-192]. The main by-products of hexa-curing conditions are water and ammonia, though formaldehyde is also produced. The structure and abundance of the amino portions of the cured polymer vary considerably with conditions. 

Fig. 17. The structure one linkage in a hexa-cured novolac. Many other linkages are also found.

Benzoxazines are heterocyclic compounds obtained from reaction of phenols, primary amines, and formaldehyde.98,99 As described previously, they are key reaction intermediates in the HMTA-novolac cure reaction.40,43 Crosslinking benzoxazine monomers at high temperatures gives rise to void-free networks with high Tgs, excellent heat resistance, good flame retardance, and low smoke toxicity.100 As in HMTA-cured novolac networks, further structural rearrangement may occur at higher temperatures. 

Novolac network degradation mechanisms vary from tiiose of resole networks due to differences in crosslinking metiiods. Nitrogen-containing linkages must also be considered when HMTA (or other crosslinking agent) was used to cure novolac networks. For example, tribenzylamines, formed in HMTA-cured novolac networks, decompose to cresols and azomethines (Fig. 7.50). 

Phenolic fibers based on spun and cured novolac resins are infusible. These fibers possess excellent flame and chemical resistance, and they evolve only low smoke or toxic gases. These materials are useful for fire protection suits, insulating gloves, curtains, clothing for foundry workers, and flame resistant papers and felts. Phenolic fibers are rigid and they carbonize in the flame while retaining their form. Therefore, phenohc fibers are useful precursors for carbon fibers [1],... 

Novolac epoxy resins, phenolic or cresol novolacs, are reacted with epichlorohydrin to produce these novolac epoxy resins which cure more rapidly than the epi-bis epoxies and have higher exotherms. These cured novolacs have higher heat-deflection temperatures than the epi-bis resins as shown in Table 2.8. The novolacs also have excellent resistance to solvents and chemicals when compared with that of an epi-bis resin as seen in Table 2.9. 

For this reason, HMTA cured novolac networks are generally brittle in nature and have low fracture energies and impact strengths, probably due to the voids combined with the high crossliiik densities. Curing novolac resins with epoxy resins can produce networks without volatile by products. These can be reinforced with glass or carbon fibers to yield tough, void-fiee composites, which also retain much of the flame-retardant properties of phenolics [39],... 

The phenol/formaldehyde molar ratio, coupled with the type of catalyst used, determines whether the polymer is phenol terminated or methylol terminated phenol-terminated resins are referred to as novolacs, or two-step resins. Because phenol is nominally trifunctional and formaldehyde is difunctional, novolacs are produced from a reaction mixture having a formaldehyde/phenol molar ratio between 0.5 and 0.8 in the presence of an acid catalyst. Such resins are not heat reactive until a second ingredient is added that supplies additional formaldehyde needed for a cure. Novolacs are employed as solid products. 

Rather detailed studies using DSC have been performed by Katovic " who isolated a 3 1 phenol/hexa adduct regardless of initial ratio of reactants. This adduct then undergoes a slow chemical change if kept above its melting temperature (130°C). Thermogravimetric studies have shown that ortho-ortho linked structures are of lower thermal stability. Hexa-cured novolac resins do not attain their thermosetting properties until the exother-... 

Increased bonding strength has been achieved by priming the wood with a polymeric isocyanate prior to the application of the phenolic adhes-ive. A mixture of an alkaline novolac/resole polymer has shown improved dryout resistance. " Recently hexamethylene-cured novolacs have been also used for metal-metal bonding. " ... 

Phenolic resins such as resol-type resins generate water and formaldehyde during curing as by-products of condensation reactions. Hexamethylenetetramine (HMTA) can be used as a source of formaldehyde for curing novolac-type resins, producing formaldehyde and ammonia as by-products. During curing. 

The substitution of HMTA, normally used to cure novolac-type resins, with epoxies has reduced void content. To retain the fire-retardant properties of phenolics, a network was formed with a higher phenol than epoxy content. 

Cured novolacs show a more or less slightly yellow color. There is some indication in the literature that the benzylamine type bridges are converted to azomethines by hydrogen elimination under heating conditions applied in the curing reaction [20]. 

Ammonia is reacted with formaldehyde to produce hexamethylene tetramine, which is used as a methylene donor in the HRH adhesion system for rubber. It is also used to cure novolac phenolic resins in rubber compounds to increase hardness. 

Phenolic resins are obtained by polymerizing phenol with formaldehyde. When polymerized at low pH (i.e., acidic reaction medium), the resultant material is a straight-chain polymer, normally called novolac. However, under basic conditions, a higher-branched polymer called resole is formed. To cure novolac, a cross-linking agent, hexamethylenetetramine, is required, which has the following chemical formula ... 

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