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A Short Encyclopedia
By Rutherford J. Gettens, George L. Stout
Dover Publications, Inc.Copyright © 1942 D. Van Nostrand Company, Inc.
All rights reserved.
MEDIUMS, ADHESIVES, AND FILM SUBSTANCES
Acrylic Resins (see also Synthetic Resins). The polyacrylic resins have been recently developed. Neher has outlined the history of the work on this class of compounds and he credits their industrial development to Otto Röhm of Darmstadt. Chemically, they are closely related to the vinyl resins (see Vinyl Resins), for they have a CH2 = CH-group in common. Although solid polymers can be made from acrylic acid, CH2 = CH·COOH, and from methacrylic acid, CH2:C(CH3)COOH, it has been found that the esters of these acids lend themselves better to the formation of useful resins. Most useful is that made by the polymerization of methyl methacrylate, CH2:C(CH3)COO[CH.sub.3], often referred to as methacrylate resin.
Methyl methacrylate monomer is a volatile liquid of low viscosity which boils at 100.3º C. Polymerization is autocatalytic and is easily effected by light, heat, and oxygen. The polymer is a hard, strong resin which has the clarity of glass. It is a linear polymer and is thermoplastic, although its softening temperature is high (125º C.). Now it is used chiefly as a plastic for clear or light-colored, molded articles. For these it is more suitable than polyvinyl acetate, because it is harder, is less rubbery, and has little cold flow. It can be worked well mechanically. The solid resin is so clear that printed matter can be read through masses of it several inches thick with perfect visibility. It is insoluble in water, alcohols, and petroleum hydrocarbons (Anonymous, 'Methacrylate Resins,' p. 1163), and is soluble in esters, in ketones, in aromatic and in chlorinated hydrocarbons. Lacquers and protective coatings may be made by dissolving the clear resin in these solvents singly or in combination. In general, the solubility is lower than that of pulyvinyl acetate. The acrylic resins are characterized by their strong adhesion to most surfaces, and advantage may be taken of their thermoplastic properties to effect good adhesion. Ultra-violet transmissibility and stability to light are high. The refractive index is 1.482 to 1.521. Polymerized methyl methacrylate is supplied as a molding powder and in made-up forms under the trade name, 'Lucite.'
In addition to methyl methacrylate, other methacrylic ester polymers are available, including ethyl, n-propyl, isobutyl, and n-butyl. These have become commercially important as materials for protective coatings and lacquers. Strain, Kennelly and Dittmar supply data on their physical properties, solubilities, and compatibilities with resins and plastics. As the molecular weight of the esterified alcohol radical increases, the polymers become softer and more plastic. Film-forming and adhesive properties, as well as solubility and compatibility, also change markedly along the series from methyl esters to the higher esters. The higher esters become increasingly more miscible with aliphatic type solvents, the butyl and isobutyl esters being soluble in petroleum solvents. Strain presents data which show wide variations in viscosities of methyl methacrylate polymers made from different solvents in the same concentrations. Toluene gives lower viscosity for the polymer than any other single solvent tested.
It has been suggested ('Methacrylate Resins,' p. 1163) that the monomeric ester, since it has such low viscosity and can be polymerized so easily, may be used as an impregnating agent which can be polymerized in situ. Porous, fibrous, and cellular materials, which are ordinarily difficult to impregnate because of the viscosity of the organic solutions of the polymers, may be treated for protection and stiffening in this way. It is also reported (ibid.) that ' monomeric methyl methacrylate has been used to protect wood to give a final product containing as much as 60 per cent by weight of resin.'
Albumen (see Egg White).
Alkyd Resins (see also Synthetic Resins). The alkyd resins are obtained by the elimination of water from polyhydric alcohols (glycol and glycerol) with dibasic acids (phthalic, etc.). These resins have been prepared from a number of different ingredients leading to widely differing properties. There are many so-called ' alkyd resins.' Combined with drying oils, they are now much used in the industrial preparation of paints, lacquers, and enamels which are durable and flexible and do not yellow. Some of the resins are thermosetting and are used for making molded articles. The alkyd resins are the most important of the synthetic resins in the industrial paint and lacquer field today. Incorporation of alkyd resins in cellulose nitrate and cellulose ester coatings has helped to overcome some of the disadvantages of the latter.
Amber (see also Resins). The name 'amber' in early times was given to many hard resins. It is, properly, a fossil resin found chiefly on the shores of the Baltic Sea but also in Denmark, Sweden, Norway, France, and along the coast of England. A dark variety has been found near Catania, Sicily. Aristotle was the first to record that amber was not a mineral but a fossil tree resin. It is mostly known in its natural state as jewelry. Beads of it have been found in early English graves and good specimens are still highly valued for ornamental purposes. It has been used, also, as a varnish ingredient, undoubtedly when adulterated with other hard resins.
The chief distinguishing feature of true amber is its yield of succinic acid when heated, and the name, 'succinite,' is now commonly used in scientific writings to denote the real Prussian amber. There are several ways to distinguish between amber and copal with which it is often confused or adulterated. One is the presence of succinic acid in the distillate of amber; another is the insolubility of amber in cajuput oil which completely dissolves copal; amber, when heated quickly, splits up and then fuses into a viscous liquid, the drops of which rebound when falling on a cold surface; copal resin does not have this characteristic.
Amber is practically insoluble in ordinary resin solvents. When made into a varnish, it is melted or distilled and the residue is dissolved in amber oil, oil of turpentine, or a fatty oil. It makes a very dark, slow-drying varnish, unsuitable for paintings, and there is doubt that it was ever employed alone for this purpose.
Animal Waxes (see also Waxes and Vegetable Waxes). These are obtained from a great variety of sources and have little in common, except their absence of glycerides. Small deposits may be found in many parts of animals and are also present in the cell contents of their tissues. Hydrocarbons do not seem to be of so frequent occurrence as in the vegetable kingdom; among the alcohols there are cholesterol and allied substances, which replace the phytosterols of the plants, and higher aliphatic alcohols containing, as a rule, fewer carbon atoms than the aliphatic plant alcohols. They have, in fact, the same carbon content (16, 18, 20) as the most common fatty acids (Hilditch, p. 127).
Balsam (see also Resins). This general term has been used to designate the resinous exudate from trees of the order Coniferae. It is also spoken of as oleoresin, turpentine, or gemme. The flow of balsam is quite profuse from shallow incisions, except for larch balsam, and for that the heart of the tree is pierced. The composition of balsams varies with the habitat of the tree. Those containing the largest amount of essential oil come from trees growing in sandy soil near the sea. Balsam is a soft, semi-liquid consisting of terpenes associated with bodies of resinous character. By distillation, turpentine and the residue, colophony, are obtained. The balsams most used in varnishes or as paint mediums are Venice turpentine, Strasbourg turpentine, Canada balsam, and copaiba balsam. Balsams flow easily on a surface and give a lustrous, pleasing quality when first applied. Unless a harder resin is mixed with them, however, they deteriorate easily.
Beeswax (see also Waxes) is produced by the common bee, Apis mellifica, and also by some allied species. It is not collected by the bee, but is the secretion of organs situated on the underside of the abdomen of the neuter or working bees, and is used by them in forming the cells of the honeycomb. They are said to consume about ten pounds of honey in order to secrete one pound of wax. The wax may be obtained by melting the combs in hot water and by straining to free it from impurities, or by pressure extraction. A further yield may be obtained by the use of volatile solvents. The industry is carried on in many parts of the world and, naturally, the waxes from widely different localities vary considerably in texture, color, and, to some extent, in chemical composition. The color ranges from light yellow to dark, greenish brown. Those of light color are used directly in many cases but the darker colored varieties are more frequently bleached. This may be done by treatment with bleaching earths or charcoal, or by chemical means such as simple exposure to light and air, or by treatment with ozonized air or hydrogen peroxide; the use of oxidizing acids such as chromic acid tends to cause deterioration. Beeswax is fairly brittle, but is plastic when warm; bleached beeswax, 'white wax,' is heavier, more brittle, and has a smoother fracture. Like other waxes, beeswax is somewhat complex in composition and contains about 10 per cent of hydrocarbons in addition to alcohols, acids, and esters. It consists principally of melissyl (myricyl) palmitate(C15 H31COOC80H61) and there are also present small proportions of a number of other alcohols and acids, including ceryl and melissyl alcohols, palmitic, cerotic, melissic, and probably other higher fatty acids. Beeswax is very likely to be adulterated. In some districts it is the custom to place artificial combs in the hives. These are frequently composed of paraffin wax or stearic acid, or a mixture of the two, and the resulting wax will thus be largely adulterated. Besides its use in the arts (see Waxes, history in painting), and it has doubtless been the principal wax used by painters, beeswax is mainly used in candle manufacture and in the preparation of wax polishes.
Benzoin (see also Resins) is a dark, resinous substance obtained from trees (Styrax Benzoin and other species) growing in Siam and in Sumatra. Siamese benzoin has a characteristic odor which results partly from the presence of 1 per cent vanillin. It has frequently been used as a plasticizer for varnishes and lacquers. It was imported into Europe at an early period, but Merrifield (1, cclx) says that it does not appear to have been used as an ingredient in varnish until the middle of the XVI century when it became a spirit varnish, but did not figure in the preparation of oil varnishes. It is mentioned in various mediaeval MSS.
Binding Medium (see Medium).
Bitumen Waxes form a link between the vegetable waxes and the mineral waxes. In this respect they resemble lignite and peat, the parent substances which are bodies intermediate between vegetable and mineral in character (see also Waxes and Montan Wax).
Blown Oil. The usual procedure for preparing blown oil is to pass an air current through the oil (see Oils and Fats), at about 120º C., in the presence of traces of cobalt driers. Blown linseed oil is used somewhat instead of stand or polymerized oils which are more expensive to manufacture. By prolonged blowing, drying oils yield jelly-like or even solid, elastic masses. Fatty oils belonging to the class of semi-drying oils lend themselves especially to the manufacture of blown oils. Rape oil and cotton-seed oil are blown in order that the products may be mixed with mineral oils to produce specific lubricants, while other blown oils find various technical applications.
Boiled Oil is linseed or other drying oil which has been heated with the addition of lead, manganese, or cobalt oxides, or other suitable siccative compounds of those elements. Formerly it was usual to heat the oil at 260º to 290º C., to add a metallic oxide, and to continue heating for a few hours until a homogeneous solution was obtained. The modern practice is to operate at lower temperatures (130º to 150º C.) and to employ 'soluble driers' such as the metallic resinates or linoleates. If the oil is blown with air, the driers may be incorporated at temperatures as low as 100º C., for slight oxidation of the oil facilitates dispersion of the driers. These are probably colloidally dispersed, not truly dissolved. Boiled oils have the property of absorbing oxygen from the air at a much more rapid rate than does raw linseed oil, and the time required for the formation of a skin is thereby much shortened (see Oils, drying process). They are used largely for industrial paints, varnishes, and enamels, and for waterproofing, for electrical insulation, and for patent leather. Doerner (pp. 105-106) says that commercial boiled oil is not of much use for artistic purposes because it dries with a sleek, greasy sheen and easily forms a skin.
Bone Glue is impure gelatin prepared from bones (see also Gelatin and Glue).
Canada Balsam (see also Balsam) is derived from a fir (Abies balsamea Mill.) which grows widely in the eastern United States and Canada. It is obtained from small blisters in the bark and only a small amount can be collected at a time. The balsam is relatively pure and is valuable for its transparency and its high refractive index (1.5194 to 1.5213 at 20º C.). It was introduced into Europe in the XVIII century.
Candelilla Wax (see also Waxes) is obtained from the stem of the leafless Mexican plant, Pedilanthus pavinia, and from other Mexican genera of the Euphorbiaceae. It is a brownish, brittle mass which may be bleached. Although of a lower melting point than carnauba wax, it finds application in similar industries.
Candlenut Oil is obtained from the seeds of Aleurites moluccana, a tree covering large areas in the western tropics. For use in paints and varnishes, it is recommended by some and condemned by others. It is closely related to tung oil.
Carnauba Wax (see also Waxes) is obtained from the Brazilian palm, Corypha cerifera (the carnauba tree), on the leaves of which it forms a deposit. The young leaves are cut and dried and the wax powder is scraped off and melted in boiling water. It is bleached with fuller's earth or charcoal or by a chemical oxidant such as chromic acid. It is a yellowish, hard, brittle material of exceptionally high melting point (83º to 86º C.) which increases somewhat with age. The major component of the wax is melissyl (myricyl) cerotate (C25H51COOC30H61) with minor amounts of hydrocarbons, wax alcohols, and higher fatty acids. Owing to its hardness and high melting point, it takes a fine, hard gloss when rubbed. It has been recommended (Rosen, p. 115) as a coating material for paintings, when mixed with other waxes.
Casein (see also Casein Tempera), usually referred to as a glue, is an organic compound belonging to the class known as proteins, the most complex compounds with which chemists have to deal. Furthermore, it belongs to one of the more complex subdivisions, the phosphoproteins. It consists of carbon, hydrogen, oxygen, nitrogen, sulphur, and phosphorus, and, although it has been the subject of many investigations, a great deal of information is still lacking with regard to the amino-acids of which it is composed. Like all proteins, it is amphoteric, i.e., it functions both as an acid and as a base. It has, however, decided acid properties and exists in milk as calcium caseinate. Casein is prepared from skimmed milk by heating it at 34.5º to 35º C. and adding hydrochloric acid till the mixture reaches a pH of 4.8. It is then allowed to settle and, after separation from the supernatant liquid, is washed with hydrochloric acid, also with a pH of 4.8. Casein so prepared is technically pure, and is a snow-white, slightly hygroscopic powder with a specific gravity of 1.259. It reacts as a weak acid, is insoluble in water, alcohol, and other neutral organic solvents, and is soluble in the carbonates and hydroxides of the alkali and alkaline earth metals and in ammonia.
Excerpted from Painting materials by Rutherford J. Gettens, George L. Stout. Copyright © 1942 D. Van Nostrand Company, Inc.. Excerpted by permission of Dover Publications, Inc..
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