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Coloring Finishing And Painting Wood
A few difficulties in the use of some of the synthetic varnishes are known, but improvements have corrected many of these faults already and the few remaining objections to their use will probably soon be removed.
324. Meaning of Term "Synthetic Resins."—In appearance synthetic-varnish resins somewhat resemble natural or fosil resins, which are formed by natural processes thru long years and are dug out of the ground. Synthetic resins are not chemically the same as natural resins but are manufactured products that can be substituted partly or entirely for them. The old varnish formulae are usually greatly modified when synthetic gums are used. Synthetic resins are manufactured resin-like materials which are made from chemicals that are usually simple in form and, thru treatment and processing, are converted into new chemical compounds which somewhat resemble natural resins though often with greatly improved and changed properties.
The known synthetic resins at present are numerous, though only a few of them are really important commercially. The various resins have a molecular structure that is somewhat indefinite as a result of polymerization or condensation reactions. The final synthetic resin varies with such factors in manufacturing as the following: (1) relative amounts of the original materials used; (2) the acid or alkali chosen as a catalyst which forms new unstable compounds; (3) the temperature and length of time allowed in processing the materials; and (4) the amount and kind of other substances that are incorporated into the mixtures during manufacture.
325. Types of Synthetic Resins Used.—Several types of synthetic resins are manufactured that might be listed as follow:
1. Resin-esters or ester-gums which are partly synthetic. They are produced by the reaction of such natural resins as rosin with glycerine or some other higher alcohol. Chemical treatment or processing greatly changes the properties and reactions of the final product—ester-gum—from the original rosin, which is highly acid. Ester-gum is really a glyceryl ester of rosin. Fig. 23 pictures various types of this gum.
2. The phenol-formaldehyde resins, which are purely synthetic, are called phenolics and are really condensates of one of the phenols with formaldehyde. Many resins of this type are really modified phenolics and are combinations often with ester-gum, and sometimes with rosin or some fatty acid. The following resins belong in this group of pure and modified phenolics: Bakelite, Amberol, Beckacites, Paranol, Durez, and Durite.
