Novolacs curing reactions

The number of remaining ortho reactive sites versus the number of para reactive sites can also be calculated using 13C NMR (Table 7.4). Since the rates of novolac cure reactions differ with the amount of ortho versus para reactive sites available, it is of great interest to calculate these parameters. 

Biernath et al. concluded that phenolic novolac and epoxidized cresol novolac cure reactions using triphenylphosphine as the catalyst had a short initiation period wherein the concentration of phenolate ion increased, followed by a (steady-state) propagation regime where the number of reactive phenolate species was constant.85 The epoxy ring opening was reportedly first order in the steady-state regime. 

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 formation occurs under acidic conditions at 100°C with the excess of phenol (about one mole phenol to 0.7 to 0.85 mole formaldehyde is reacted). Novolac curing is accomplished by addition of crosslinking agents, usually HEXA (hexamethylenetramine) or HMTA whose reaction products form urea and formaldehyde when heated (110°C). 

Novolacs for wood bonding are solid resins cured in the presence of about 10 to 15% hexamethylenetetramine, which is added as a fine powder. Upon heating, both powders melt and the curing reaction starts. The use of these so called two-step resins in wood bonding is only minor and most resins contain resorcinol as a modification. 

Curing reactions applied to epoxy prepoiymers, unsaturated polyesters, resoles, and novolacs make use of three general classes of prepoiymers which are distinguished by the number and location of sites of functional groups available for subsequent crosslinking reactions. These three general classes have been defined as discussed in the following sections. 

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]. 

Recently, interesting results were obtained for BMIs modified with eugenol (biobased and synthetic) and eugenol-modifled novolac (Shibata et al. 2013). These new resins underwent the ene reaction and subsequent thermal addition copolymerization in the same way as the curing reaction of DBA and BMI, and no phase separation was noticed for the cured resins. 

Novolacs with some branched molecules [61] are produced by the reaction of 1.0 mole of phenol and 3.0 moles of formaldehyde (37% aqueous solution) using 0.06 mole of calcium hydroxide and heating at SO C until 2.4 moles of formaldehyde have reacted. Then 2.6 moles of phenol are added and the mixture is neutralized with dilute sulfuric acid. Then more acid is added and a novolac is prepared in the usual manner, as described above. This novolac cures faster with hexamethylenetetramine (Hexa) and does not produce a foamy mass [61]. 

The furfural-phenol novolacs are stable under neutral or alkaline conditions but readily undergo a thermoset reaction on the acid side to give a black resin [74]. It is possible that the furfuryl ring system also takes part in the curing reaction to give in addition furan-type polymers [75]. 

Some other reported curing agents are quinone [81], chloranil [81], anhydroformaldehyde aniline [82], methylolureas and melamines [83], ethylenediamine-formaldehyde products, and paraformaldehyde. Resorcinol novolac resins can be cured with paraformaldehyde [84] and the mildly acid nature of resorcinol helps to catalyze the curing reaction [84]. Ammonium salts have also been reported as accelerators for the curing of phenolic resins [85]. 

Curing with phenolic resins has been shown to provide the excellent gloss retention and durability of alkyds together with the excellent adhesion and chemical- and heat-resistance properties of phenolics. Curing reactions mainly involve condensation reactions between the hydroxyl groups of the alkyd/polyester resins and the phenolic resins. The novolac type of phenolic resins are generally used for these purposes. The reaction mechanisms and their structural implications together with their influence on final paint properties will be treated in Section 6.3.3. 

Using dynamic mechanical analysis the curing reactions of typical phenol-formaldehyde novolac resins were followed. The evolution of various rheological parameters was recorded for samples of the resins on cloth. A third-order phenomenological equation described the curing reaction. The influences of the structure, composition and physical treatment on the curing kinetics were evaluated. 20 refs. 

Two-Stage Resins. The ratio of formaldehyde to phenol is low enough to prevent the thermosetting reaction from occurring during manufacture of the resin. At this point the resin is termed novolac resin. Subsequently, hexamethylenetetramine is incorporated into the material to act as a source of chemical cross-links during the molding operation (and conversion to the thermoset or cured state). 

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. 

Reaction pathways involved in the curing of novolacs with HMTA have been extensively investigated by Solomon and co-workers.43-50 In a series of model studies where 2,6-xylenol and/or 2,4-xylenol was reacted with HMTA, these workers found that the types of linkages formed were affected by the initial... 

Two-stage resins (novolacs) are produced by the acid-catalyzed reaction of phenol and a portion of the required formaldehyde. Tire resin product is brittle at room temperature. It can be melted, but it will not crosslink. Novolacs can only be cured by the addition of a hardener, almost always formaldehyde supplied as hexamethylene tetramine. Upon heating, the latter compound decomposes to yield ammonia and formaldehyde. 

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