Step polymerization resole resins

Phenolic novolacs are produced by condensation reactions (Step polymerization), and a typical schematic is given below. It is clear that one of the major differences between novolacs and resoles is that, in their initially advanced stage, novolacs do not contain the methylol groups, which are so essential to the thermal cure of phenolic resole resins and adhesives. 

Simple resoles are based on phenol and catalysed with NaOH or a caustic/anunonia combination. They generally utilize phenol to formaldehyde ratios (P F ratios) in the region of 1 1.4 to 1 1.6, which appear to be optimum for structural adhesives ratios higher than 1 1.6 produce resins, which, for example, are used in plywood manufacture, where excellent moisture resistance is required. Phenolic resoles are produced by condensation reactions (Step polymerization), and a typical schematic is as follows ... 

Adhesives of the aminoplastic (see Step polymerization) and phenol formaldehyde (see Phenolic adhesives single-stage resoles and Phenolic adhesives two-stage novolacs) types are most widely used. Although basically similar, an adhesive for plywood manufacture will require a different formulation to one for particle board, or medium-density fibre board (MDF) since methods of application and processing differ. Thus, in plywood, large sheets of veneer must be uniformly coated with adhesive, usually by a roller or curtain coater in particle board, chips or wafers must be coated with very fine adhesive droplets, while small bundles of wet fibres must be sprayed with adhesive in the manufacture of MDF. Hence, formulation and production of resins has become a mixture of art and science, with resin manufacturers able to produce resins tailored for use in a particular board-manufacturing plant, or with a particular species of timber. 

Alkaline Catalysts, Resoles. Resole-type phenolic 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. While novolak resins and strong acid catalysis result in a limited number of structures and properties, resoles cover a much wider spectrum. Resoles may be sohds or liquids, water-soluble or -insoluble, alkaline or neutral, slowly Cluing or highly reactive. In the first step, the phenolate anion is formed by delocahzation of the negative charge to the ortho and para positions. Alkaline catalysts are also effective in the polymerization-depolymerization of methylene... 

The chemistry of the phenolics is complex. Polymerization involves heating the reactants with acidic or basic catalysts until condensation begins. Depending on the ratio of reactants and the catalyst type, materials with different properties are formed. If the molar ratio of formaldehyde to phenol is 1 1 or greater, and a basic catalyst such as sodium or calcium hydroxide is used, then a reactive resin called a resole is formed. The initial condensation step is carefully controlled to produce a resin that is easily handled during the final fabrication step. 

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