Resole resin solubility

The setting chemistry of resole resins is complex, and experimentally difficult to study, mainly because the cured product, being insoluble, is not amenable to ready chemical investigation. Part of the information on these materials has come from studies of model systems such as mononuclear methylphenols, which give soluble products. These products present fewer difficulties in chemical analysis. 

The package stability of alcohol-soluble resole resins and their compatibility with epoxy resins can be improved by partial conversion of the methylol groups to ethers. Allyl ethers have been used with epoxy resins in interior can coatings. Low molecular weight butyl ethers are used with acid catalysts to cross-link epoxy resins and other hydroxy-substituted resins, by etherification and transetherification reactions (148). 

The physical properties of a resole resin prepared with hexa catalyst are shown in Table 5. Compared to the resin catalyzed with NaOH, this resin has higher molecular weight, less free phenol, lower water solubility, and a higher Tg. 

The in situ process is simpler because it requires less material handling (37) however, this process has been used only for resole resins. When phenol is used, the reaction system is initially one-phase alkylated phenols and bisphenol A present special problems. As the reaction with formaldehyde progresses at 80-100°C, the resin becomes water-insoluble and phase separation takes place. Catalysts such as hexa produce an early phase separation, whereas NaOH-based resins retain water solubility to a higher molecular weight. If the reaction medium contains a protective colloid at phase separation, a resin-in-water dispersion forms. Alternatively, the protective colloid can be added later in the reaction sequence, in which case the reaction mass may temporarily be a water-in-resin dispersion. The protective colloid serves to assist particle formation and stabilizes the final particles against coalescence. Some examples of protective colloids are poly(vinyl alcohol), gum arabic, and hydroxyethylcellulose. 

In 1907, Baekeland produced an ethanol-soluble novolac, which was used as a substitute for shellac, from the condensation of an excess of phenol with formaldehyde under acid conditions. In 1908, he produced resole prepolymers from the condensation of phenol and formaldehyde under mild alkaline conditions. These prepolymers, called A-stage, could be converted into infusible C-stage insoluble eastings or coatings in the presence of strong acids. Nevertheless, neither the novolac or resole resins could be substituted for natural resins in oleoresinous varnishes. 

Alkaline Catalysts, Resoles. Resole-type phenoHc resins are produced with a molar ratio of formaldehyde to phenol of 1.2 1 to 3.0 1. For substituted phenols, the ratio is usually 1.2 1 to 1.8 1. Common alkaline catalysts are NaOH, Ca(OH)2, and Ba(OH)2. Whereas novolak resins and strong acid catalysis result in a limited number of stmctures and properties, resoles cover a much wider spectmm. Resoles may be soHds or Hquids, water-soluble or -insoluble, alkaline or neutral, slowly curing or highly reactive. In the first step, the phenolate anion is formed by delocali2ation of the negative charge to the ortho and para positions. 

Two classes of resol are generally distinguished, water-soluble resins prepared using caustic soda as catalyst, and spirit-soluble resins which are catalysed by addition of ammonia. The water-soluble resins are usually only partially dehydrated during manufacture to give an aqueous resin solution with a solids content of about 70%. The solution viscosity can critically affect the success in a given application. Water-soluble resols are used mainly for mechanical grade paper and cloth laminates and in decorative laminates. 

In contrast to the caustic soda-catalysed resols the spirit-soluble resins have good electrical insulation properties. In order to obtain superior insulation characteristics a cresol-based resol is generally used. In a typical reaction the refluxing time is about 30 minutes followed by dehydration under vacuum for periods up to 4 hours. 

Phenolic resins are useful surface coating materials. Resols are useful for stoving lacquers for coating chemical plant, textile equipment, razor blades, brassware cuid food cans. Phenolic resins are used with poly(vinyl formal) as a flexible, tough and solvent-resistant wire enamel. Oil-soluble resins based on synthetic phenols form the basis of some gloss paints. 

Initial or early stage in the reaction of some thermosetting resins in which the material is still soluble in certain liquids and fusible referred to as resol. 

Although many types of compounds have been tested as sensitizers in phenolic host resins (Novolacs, Resols, etc.) (S), all commercial positive resists employ aromatic diazoquinones of some type which photochemically generate base soluble products via Wolff rearrangement initiated by the loss of nitrogen (6). A staggering variety of diazoketones have been synthesized and evaluated for lithographic purposes, but derivatives of J[ and 2 are most commonly employed (5). 

Waterborne systems based on resol-epoxy resin precondensates are already at an advanced development stage. Carboxyl groups are introduced into the preformed resols and made water soluble by salt formation with amines. These systems offer a significant saving in solvent compared with that of conventional high-solvent, high-viscosity products. 

Resins that are water soluble and have a low molecular weight are finished at as low a temperature as possible, usually around 40 to 60°C. It is important that the liquid, water-soluble resols retain their ability to mix with water easily when they are used as wood adhesives. Resols based on phenol are considered to be stable for 3 to 9 months. Properties of a typical resin are a viscosity of 100 to 200 cP at 20°C, a solids content of 55 to 60%, a water mixibility of a minimum of 2500%, and a pH of 7 to 13, according to the application for which the resin is destined. 

In the A stage, simple PF resins are readily soluble in alcohol, esters, ketones, phenols, and some ethers, and insoluble in hydrocarbons and oils. As a class, resols tend to be more soluble in alcohols and water, and novolaks tend to be more soluble in hydrocarbons. In the early stages of condensation, resols are often soluble in water, owing to the presence of methylolphenols, especially polyalcohol. This is more pronounced with resols that are derived from phenol. Cresilic resols are less soluble, and xylenolic resols are almost insoluble in water. The solubility of A-stage resins in dilute aqueous sodium hydroxide or in mixtures of water and alcohols follows the same trend. 

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