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Adhesives, Paints, and Varnishes (II)
ADHESIVES - PASTES - MUCILAGE-GLUE - CEMENT-SPECIAL AD- HESIVES-SPECIAL PURPOSE CEMENTS-PAINTS AND PAINTING -USES OF PAINT-SPECIFICATIONS FOR PAINTING-CARE OF PAINTS AND BRUSHES - SPECIAL KINDS OF PAINT-VARNISH- ING - FIXED-OIL VARNISHES - SPIRIT OR LAC VARNISHES - VOLATILE-OIL VARNISHES-SPECIAL VARNISHES-SEALING WAX -OILS, LUBRICATORS, ETC.-FURNITURE POLISH-SOLDER AND SOLDERING ADHESIVES Adhesives.-Adhesives are a class of substances capable of attaching themselves to the surface of solid bodies, and thus, when interposed between them, of uniting such bodies. Various substances and compounds have adhesive or sticky properties.
Among these are the gums arabic, tragacanth, and senegal; dextrin, gelatin or glue, isinglass or fish glue; various resins, as shellac, rosin, etc.; casein, from the curd of milk and cheese; India rubber, gutta percha, litharge, and other substances too numerous to mention.
Various preparations of these substances may be loosely classed.according to their composition in the order of their adhesive power, as paste, mucilage, glue (either solid or liquid), and cement. But these terms hardly have a definite meaning.
Dextrin. A substance formed from starch, rice flour, or cornstarch, also known as British gum or starch gum. It is used as a substitute for gum arabic, as a size for mucilage, and especially for the backs of postage stamps and sealing envelopes. -- Gluten. A substance believed to be produced by the action of a ferment. It is formed in flour by uniting or mixing it with cold water. Gluten is the substance which retains the carbonic-acid gas in bread making, and thus assists in the process of raising bread. Gelatin. Gelatin is produced from certain animal membranes by the action of hot water. Isinglass, calf'sfoot jelly, and glue are chiefly composed of gelatin. It absorbs water, which causes it to swell, and may be dissolved in hot water or acetic and other acids. The addition of alcohol, corrosive sublimate, or tannic acid to a solution of gelatin in water causes the gelatin to be thrown down.
Glue. The glue of commerce is dry gelatin having a more or less brownish color according to its purity. White or pale glue is the best. It is a hard, brittle, glossy substance which usually comes in thin sheets.
Glue is obtained by cleansing scraps of hides, hoofs, and horns with lime, and boiling them until changed into gelatin.
Isinglass.-Dry gelatin is prepared from the air bladder of sturgeon and other fish, such as cod, weakfish, hake, etc. It is used in preparing jellies, blancmange, gum drops, etc.; in making court-plaster, as a size for delicate fabrics, and as an adhesive.
Fish Glue is an inferior isinglass made from the offal of fisheries.
To melt isinglass, beat up 1/2 teaspoonful of white of egg in 1 pint of water. Add 4 ounces of isinglass, and melt over a slow fire.
To detect adulteration by gelatin, drop a sample of the suspected isinglass into vinegar. Pure isinglass will swell like jelly, while gelatin will become hard.
Or put a sample in cold water.
Pure isinglass becomes cloudy and white, and the adulteration becomes jellylike and clear.
Resins. For the various resins having adhesive qualities including rosin, shellac, and the like, see under "Varnish."
Gutta Percha. -The hardened milky juice of a large tree growing in the East Indies. It is insoluble in water, slightly soluble in alcohol and ether, and readily dissolved in bisulphide of carbon, benzol, chloroform, and oil of turpentine. It deteriorates rapidly when exposed to the air, and becomes brittle and useless. It is chiefly used for coating submarine telegraph wires and other metallic articles under water.
Caoutchouc, gum elastic, or India rubber is the juice or sap of several tropical plants growing in the East Indies and South America. It is obtained by cutting the bark and drying the juice over smoky fires, which impart its black color. It is elastic and waterproof. When combined with about 25 per cent of sulphur and raised to a temperature of about 270° F., it is converted into soft vulcanized rubber; by the addition of 50 per cent of sulphur and heating to 300° F., it becomes hard vulcanized rubber or ebonite.
To Choose Adhesives. - An adhesive should be selected according to the nature of the substances to be united and the use to which they are to be put. If the right cement is employed, the hardest and smoothest surfaces, as glass and polished metals, may be united so firmly that they will break anywhere rather than where the parts are cemented.
To Use Adhesives. The object of using adhesives is to bring two surfaces into such intimate contact as to make them practically one, and not to interpose between them any perishable layer or thickness of the adhesive itself. Most adhesives are more brittle than the substances which they unite; hence the best work is done when the adhesive penetrates into the pores of the materials on both sides and brings the particles of both surfaces closely together, so that the strength of the materials themselves is added to that of the adhesive in a union which may be stronger than the adjacent parts.
The presence of any foreign substance, as dirt, grease, or bubbles of air, hinders adhesion. Heating the surfaces to be joined promotes it by expanding the pores and thus enabling them to absorb more of the adhesive.
Heating the adhesive itself also assists.
Moreover, heat tends to drive away the air. Hence the hotter one can handle the parts and the adhesive the closer they can be brought into contact and the less adhesive will be required. To Use Glue. To get the best results from glue, it should be thin and hot, and the parts should be at least warm enough to prevent the glue from being chilled by them. As glue is gummy and elastic, the parts, when possible, should be squeezed together by means of a vise or under clamps tightened by a screw, so as to squeeze out the excess of glue and bring the parts into intimate contact. The articles should be left in the vise until the glue is set.
To Use Cement. - Resinous cements which are used in a melted state will not do good work unless the adjacent parts are heated above the point at which the resins melt.