Curing resoles

Early phenoHc resins consisted of self-curing, resole-type products made with excess formaldehyde, and novolaks, which are thermoplastic in nature and require a hardener. The early products produced by General BakeHte were used in molded parts, insulating varnishes, laminated sheets, and industrial coatings. These areas stiH remain important appHcations, but have been joined by numerous others such as wood bonding, fiber bonding, and plywood adhesives. The number of producers in the 1990s is approximately 20 in the United States and over 60 worldwide. 

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. 

Due to the presence of reactive CH2OH groups, resol oligomers may be converted into highly crosslinked products without the addition of hardeners. Heat curing is conducted at T 130 200°C. The polycondensation mechanisms are complex and different bridges are possible CH2-0-CH2-and CH2. The latter is thermodynamically the most stable. Therefore the methylene bridges are the prevalent crosslinks in cured resols. 

Cured Resole resins are hard and insoluble, which makes it difficult to study the reaction by conventional analytic techniques. By using model compounds which have two of the three reactive sites on the aromatic ring blocked, the products of the reaction become relative simple to separate, analyse and characterize. Solomon and coworkers used the model compounds 2,4-dimethylphenol (21), 2,6-dimethylphenol (23) and 2-hydroxymethyl-4,6-dimethylphenol (27), which contain some of the functional groups found in resole resins, to gain an insight into the curing process for ortho-hydroxymethyl groups. 

Many different structures have been identified within cured resole resins. The most common crosslink is the methylene bridge, though ethers can also be present in significant amounts . Phenoxy bridge , and carbonyl and methyl groups " , have also been identified within the cured structure. 

Decomposition of Cured Resoles and Novolaks. Above 250°C, cured phenolic resins begin to decompose. For example, dibenzyl ethers such as 9 disproportionate to aldehydes (salicylaldehyde) and cresols (o-cresol). The aldehyde group is rapidly oxidized to the corresponding carboxylic acid. In an analogous reaction in hexa-cured novolaks, tribenzylamines decompose into cresols and azome-thines, which cause yellowing. 

It has been found that whilst methylene compounds are generally rather stable, dibenzyl ethers are not so stable and at temperatures above about 150°C (i.e., at temperatures commonly used for curing resols) they undergo a number of ill-defined reactions. The mechanisms of these secondary reactions have been extensively studied but are still rather poorly understood. Various workers have put forward conflicting suggestions, which are briefly summarized below. 

As indicated in Section 12.4.4, casting resins have a resol-type structure but contain a greater proportion of methylol groups. It may therefore be anticipated (cf.. Section 12.5.1) that an appreciable number of ether links are produced when a casting resin is cured. Further, since castings are normally cured at relatively low temperatures these ether links are likely to persist in the final product. It may be noted that it is possible to prepare castings which are colourless, whereas resols cured at about 150°C are dark coloured. This observation is in accordance with the suggestion that it is the thermal decomposition of ether links into quinone methides which leads to colour formation in cured resols. 

Usually hexamethylenetetramine (hexa) is used at 0% of the novolac resin. Resin and hexa are ground together before curing the resin. These mixes are stable in the dry state. The physical properties of the cured resole or novolac reach a maximum degree of cross-linking at or near approximately 1.5 1.0 ratio of formaldehyde to phenol. 

Rapidly curing resole phenolic resins have been reported to use barium acetate and zinc nitrate and organic peroxides [105],... 

Acid-curing resoles can be processed by hand-lamination, spray-up lamination, winding, and by infusion-technology, as well as by processes familiar from fiber-composite polymer (FCP) technology. Acid-resistant machine components and molds have to be used, and the shorter open times compared to polyester resins have to be taken into consideration. Compression processing, which is suitable for novolacs and resoles, requires ventilation strokes — as in most processing methods with closed molds - to let the developing water vapor escape. 

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