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Coloring Finishing And Painting Wood
The pure phenol-formaldehyde resins vary in characteristics, depending on the relative amounts of phenol and formaldehyde which are used and whether an acid or alkaline catalyst is employed. Some of these resins are always soluble in alcohol and are fusible. Another group to which Bakelite belongs is soluble in alcohol in the early stages of condensation, yet becomes insoluble on higher heat treatments. Resins that are soluble in drying-oils, or which can be made soluble thru heat treatments or processing, are needed for oil-varnishes. In order to secure such oil solubility more complex phenols are sometimes substituted for the ordinary phenol in the manufacture of phenol-formaldehyde resins. Cresol is a chemically related compound which sometimes replaces phenol. Another method of making a phenol-formaldehyde resin oil soluble is to introduce into the mixture some soluble substance, such as rosin or a fatty acid which will dissolve in drying-oils or mineral thinners. The manufacturers of pure or 100% phenolics claim that several years of experience with varnishes made from such resins have shown that they are superior under outdoor exposure when tried where moisture was excessive and salt spray was possible. The reaction to chemicals such as acids, alkalies, and gases was not rapid.
3. The alkyd groups which include the Glyptals, Rezyls, and Teglacs especially, together with some of the modified Amberols and Beckasols. The Glyptals of the General Electric Company are important resins which are chemically glycerol phthalic-anhydride condensates. Some of the Beckasols are of the straight alkyd type, while others are modified, especially with phenol.
The phenol-alkyds are a composite type of important resins that are sometimes placed in a separate group or type. Good examples of the modified phenol-alkyd type are some of the Beckasols, which in pure form are alkyds; and some of the Amberols, which in unmodified form are of the phenol-formaldehyde class.
Experience with the alkyd resins has shown that they are valuable in films that must retain colors—especially all whites and light tints.
4. The Cumar or Coumarone type of resins which is based on the polymerization of indene, coumarone, and similar compounds found in some of the coal-tar naphthas. The Neville resin, called Nevindene, is an example of the Coumarone type.
5. Other groups of synthetic resins might be mentioned, such as urea resins, vinyl resins, chlorinated diphenyls, and toluol sulfonamide resins.
The urea resins are valuable for use in plastic moldings in the formation of colorless glass. In varnishes they lack the needed waterproofness, but they produce films of unusual transparency and good gloss. They have not proved satisfactory for general use because of their lack of flexibility and other defects.
