Resole cure accelerators

Resoles are usually those phenolics made under alkaline conditions with an excess of aldehyde. The name denotes a phenol alcohol, which is the dominant species in most resoles. The most common catalyst is sodium hydroxide, though lithium, potassium, magnesium, calcium, strontium, and barium hydroxides or oxides are also frequently used. Amine catalysis is also common. Occasionally, a Lewis acid salt, such as zinc acetate or tin chloride will be used to achieve some special property. Due to inclusion of excess aldehyde, resoles are capable of curing without addition of methylene donors. Although cure accelerators are available, it is common to cure resoles by application of heat alone. 

Helmut Orth first reported the use of laetones to accelerate phenolic resole cure in 1957 [161]. A year later, Orth discovered that this effect could be extended to aliphatic esters as well [162], Despite the dramatic nature of the acceleration seen, Orth s observations were not applied in industry for a decade. In 1967, Sumitomo and BASF applied esters to soil grouting and wood uses [133,163, 164]. Neither of these applications were commercially successful, however, and commercial success would not occur until 1980 when Borden introduced ester-cured sand binders for foundry [165]. This technology was highly successful in UK and spread to the US, where it was applied immediately to foundry in 1981 and eventually to wood products in 1990 [119,166-173]. Esters are capable of reducing the gel times of resoles from several weeks to less than 30 s at room temperature. Both gaseous and liquid esters are applicable [119,166]. 

As resoles cure simply by heat activation, no hardener components need to be added to the formulation, which will cure in the region of 140 to 150 °C in 30 to 60 min resoles are stUl active, albeit to a low degree, at ambient temperatures. Simple acceleration of cure cau be achieved by... 

The mechanism of this reaction has been studied by several groups [133,174-177]. The consensus is that interaction of ester with the phenolic resole leads to a quinone methide at relatively low temperature. The quinone methide then reacts rapidly leading to cure. Scheme 11 shows the mechanism that we believe is operative. This mechanism is also supported by the work of Lemon, Murray, and Conner. It is challenged by Pizzi et al. Murray has made the most complete study available in the literature [133]. Ester accelerators include cyclic esters (such as y-butyrolactone and propylene carbonate), aliphatic esters (especially methyl formate and triacetin), aromatic esters (phthalates) and phenolic-resin esters [178]. Carbamates give analogous results but may raise toxicity concerns not usually seen with esters. 

Youngquist etal. (1988) found improvements in both resin distribution and IB values when suitable emulsifiers were used in conjunction with waterborne resins, but considered it unlikely that the improvement in performance could be justified on cost grounds. It was postulated that acetylated wood interfered with the polymerization of the resin, so that it was not fully cured. It has also been suggested that acetic acid, which may be released during board pressing, could accelerate resin curing of resol type resins. In a study to determine whether this was so, acetylation of wood was found to slightly reduce... 

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