A BRIEF ACCOUNT OF GLUE AND ITS PLACE IN INDUSTRY
Glue i s a substance of considerable industrial importance and but slight academic interest. In contrast to its analog, gelatin, it is not a chemical entity but a family of hydrolytic products. It i s made by a n extraction process from the connective tissue of animals, different sources giving rise to different kinds of glue. Its unique "setting" properties are of great technological significance. I t i s the only strong adhesive which changes reversibly from solution to jelly a1 or near room temperature. Although its jellies tend to dry out on standing, they can be prevented from doing so, and permanent flexible jellies made therefrom have many every-day uses. Glue finds its way into a long list of industries and innumerable articles of commerce. Its widespread use i s attributable to its powerful adhesive action, its suitability for "sizing" purposes, and its value as a binder, a stiffener, a plastic, a n elas&icjelly, and a protective colloid.
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The man in the street (i.e., the average, none-too-well-informed citizen) knows little or nothing about glue. If pressed, he may venture the opinion that it is made from horns and hoofs and he is likely to add-this time a little more emphatically-that it smells badly. Inasmuch as both these statements would assay not more than a tiny kernel of truth to a large bulk of falsity, it can be seen that so far as the man in the street and his hearers are concerned, the general knowledge of glue comes some distance short of being adequate. Most people never have taken the trouble to inform themselves about glue. They have left it almost entirely to the cbedsts. And the chemists, in spite of their reputation for being tireless investigators of matter in all its forms, have left glue to a few specialists. Glue has long been regarded as an object of mystery. It has been suspected of not being amenable to natural law-not even to the occasional regularities of the somewhat lawless class of colloids to which it belongs. Owing to the vagueness and uncertainty which surround it, many misconceptions have arisen. Some of them are harmless, like the examples cited in the first paragraph, but there may be others of greater consequence. I t is the aim of this article to furnish a few important basic facts about glue, so that the reader will be able to form his own opinion concerning its nature and its industrial significance and possibilities. The word glue arouses diierent responses in different individuals. Naturally one thinks of the substance with which his experience has made him most familiar. Originally there was but one g l u e e e . , animal glue. Some authorities, arguing on historical grounds and with the blessing of the lexicographers, contend that animal glue is the only adhesive to which the term, glue, rightfully belongs. Most people, however, being very much 1553
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in the dark as to sources and origins, make no such distinction but with a sweeping verbal gesture throw everything in together and designate any substance of marked adhesive action as a glue. In the confusion which has thus arisen glue suggests to many people a certain thick, brown fluid which is displayed in cans and tubes on paint and hardware dealers' shelves. This fluid is a liquid fish glue, and the literature states that such products are usually made from the heads, skins, and other parts of the cod, haddock, cusk, hake, etc. Among the materials, often inaccurately referred to as glues, are mucilage and library paste. These are of varying composition. The best of them are solutions of gum arahic, dextrin and the like, and they have nothing in common with animal glue except a certain parallelism in regard to use. Some of the newer preparations for general repair work and household use consist of a cellulose ester, a plasticizer, and orgailic solvents. Such compounds often go under the name of pyroxylin cement. In the industries a variety of starch pastes, casein and other protein compounds, rubber, bitumen, etc., are used adhesively. Although many of these have interesting applications, the present discussion will be confined to animal glue, known also as hot glue and cabinetmaker's glue, but more often as just glue. An Early Chemical Industry Perhaps few people realize that glue-making is one of the oldest arts. From a wall carving in the tomb of Rekhmara in Thebes we learn that the Egyptians were familiar with the preparation and use of animal glue about 3.500 years ago. Very few chemical'industries can boast of a greater antiquity. Glass-making is a very old art; it had origins in both China and Egypt. Von Meyer thmks that the discovery of glass in Egypt was probably accidental, soda having been added as a flux with sand containing gold, for the purpose of extracting the latter. The metallurgy of gold, silver, copper, iron, lead, and tin are thought to be older than glue-making. The art of pottery is certainly older. The Egyptians knew how to fix dyes on cloth by means of mordants, how to calcine lime, how to prepare certain medicines and antiseptics, how to make bronze and temper iron. But with these few exceptions glue-making seems to have had a longer history behind it than any other chemical industry. The earliest record of the manufacture of glue in Europe goes back to the year 1690, in Holland. The industry was established in England about 1700. The beginnings of glue manufacture in the United States can be traced to the first decade of the nineteenth century. A compilation of manufactures taken by the government in 1810 lists seven small gluemakmg establishments. The first American glue factories were located in Massachusetts, New
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York, Pennsylvania, and Maryland. No doubt they sprang up in connection with the leather industry, which was doing a flourishing $20,000,000 business in the year 1810. It is conjectured that the early glue factories were built in proximity to the tanneries and with the idea of utilizing their hide trimmings and other (untanned) leather scrap, which are believed to have been so plentiful a by-product as to he almost a burden. The industry grew a t a rapid rate, keeping pace, no doubt, with the rise of furniture manufacture and other woodworking activities. In the decade 1860-70 the large meat-packing establishments, located for the most part in the Middle West, came into prominence. Many of the meat packers included within the scope of their enterprise plants for the production of glue. Today glue is manufactured from coast to coast. The states which have the largest output are Illinois, Massachusetts, Pennsylvania, and New York. The value of the plants and equipment devoted to the making of glue is estimated a t around $50,000,000. Glue is not made from horns and hoofs, for these structures contain n:, glue. Horn pith, the inner bony core of the horn, is a source of ossein (see below). The hoofs are of absolutely no value to the glue maker. The principal raw materials from which glue is made are: hide trimmings, fleshings, pates, ears, tails, sinews, and bones. As might be expected, the quality of the glue depends in large measure on the type and condition of the material from which it is made. The manufacturer's success is governed largely by his ability to obtain a stgady supply of glue stock of a uniform good quality. In this respect the AmeFcan glue men are exceptionally fortunate on account of the of the American cattle industry, which sends carloads of prime beef cattle to the packing houses and shipments of calfskins (which are in great demand by the producers of leather and glue) to the tanneries. No other industrial nation is quite so favorably situated. How Glue Is Made Before passing to a description of the interesting properties and uses of glue it may be helpful to give a brief account, first, of how i t is made. In essentials the story is simplicity itself. Yet no glue manufacturer could truthfully say that his job is an easy one. The parent substances of glue are collagen (which is present in animal skins) and ossein (which is present in bones). Collagen yields hide glue, ossein yields bone glue. Since the glues from these two sources are somewhat dierent, it will be best to discuss them separately. Take hide glue first. The starting point is pieces of skin which have been trimmed off the hide on account of unsuitability as regards size, shape, or texture for conversion into leather. The skin pieces are subjected to a long preliminary treatment with lime, the purpose of which
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is to remove the hair, dissolve out certain constituents which are not wanted, and cause the collagen to swell. The reaction by which collagen is transformed into glue is a hydrolysis. Since one of the main requirements of a hydrolytic process is water, and since it is obvious that the water and collagen should be intimately mixed, a swelling treatment is the logical first step in the process. Secondly, the swollen skins are steeped in hot water. The conversion of collagen into glue now takes place and the reaction products, being quite soluble in hot water, pass out of the skins into the medium that bathes them. The resulting solution (called glue liquor) is filtered and then run into vacuum pans, where it is concentrated. From here i t is sprinkled onto an endless rubber belt, chilled t o a jelly, cut to convenient size, and spread out on wire nettings which are supported on frames. The frames bearing the jelly are then placed in a drying tunnel, through which they pass slowly, giving up their moisture to a counter-current of dry air. What were originally slices of jelly come out of the tunnel as plates of dried glue. These are cracked up into small pieces having an area of a couple of square inches. In this case flake glue is the proper name for them. But if the material is crushed to particles that will pass through an 8-mesh screen or thereabouts, it is designated as ground glue. For the making of bone glue there are two alternative processes. I n choosing between them the manufacturer is governed somewhat by the condition in which the stock is recgived. Both, however, are based on the fact that bony structures consist of mineral matter (mainly calcium phosphate) and ossein, intermingled with &nor constituents of no value. In the first, or ossein, process the aim is to eliminate the mineral matter and leave a tough, porous residue which is mainly ossein. This is accomplished by digesting the bones with dilute hydrochloric acid. The ossein which is obtained in this way can then be swollen, steeped in hot water, evaporated, chilled, dried, and finished, just as in the hide glue process. In the alternative bone glue process the glue is simply extracted by heating, in somewhat the same way as the housewife extracts the nutriment in making soup from meat bones. The manufacturer crushes and washes the bones. Then he frees them from grease, first by a brief preliminary boiling, and later, in some cases, by extraction with an organic solvent such as benzine. After that the glue is dissolved out by treating the grease-free bones with hot water. The glue liquor then goes t o the vacuum pans, and from that point on this process is the same as the other two.
A Substance of Unique Properties Let us now assume that we have some flakes of glue before us in order that we may observe some of their properties. The flakes are irregularly shaped flat pieces, usually about to inch thick. Their color ranges
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from light tan to dark brown. In some cases they are almost transparent, but more often they are merely translucent. Glue is hard and very tough. Some flakes are so tough that they can be bent almost double without breaking; this is especially true of thin flakes of high-grade glue. A few glues are brittle. Brittleness is not necessarily a sign of poor quality; moisture content has an important effect on elasticity. When the moisture content is high, all glue flakes are flexible. But if the flakes are dried completely, as in an oven, they will be rather brittle. The mechanical properties of animal glue are remarkable. When it is cast in the form of thin flat films and tested a t a moisture content of, say, 13 per cent., the films give a strength value which indicates that a bundle of them having a cross-sectional area of one square inch would withstand a tensile load of about 20,000 pounds, which is not far from the tensile strength of cast iron. Because large masses of glue jelly warp badly and set up internal stresses on drying, a strength of 20,000 pounds probably could not be realized in one-inch bars. Since in use glue ordinarily reduces to a film and never occurs in large masses in manufactured articles, there is nothing fictitious about the value given. Strength of this magnitude is exhibited only by the better glues. The strength of the less select glues is not so high but is still considerable, being in the neighborhood of 12,000 to 15,000 pounds a t normal moisture content. When a glue of any grade is moistened its tensile strength takes a decided drop. Glue behaves in a most interesting way when immersed in water. In hot water it dissolves very slowly and without much swelling. In cold water, however, it swells, absorbing ten (or more) ti&es its weight of water. The colder the water, the greater the swelling. Swollen glue is very pliable but it retains its original shape. Some jellies containing as little as 5 per cent. of dry substance show an appreciable rigidity. Rather curiously, the volume of a glue jelly is smaller than the sum of the volumes of the dry glue and water. In other words, there has been a contraction in total volume. A small amount of heat is liberated at the time of swelling. This explains, in accordance with the principle of Le Chatelier, why low temperatures favor and high temperatures oppose the absorption of water by glue. When a glue jelly is heated to, say, 40 degrees C., it melts to a slowrunning liquid. Glue solutions exhibit the property of viscosity (the reverse of fluidity) to a marked degree. If the solution is allowed to cool to 20 to 35 degrees, it will form a jelly again. The alternate melting and jellying of glue can be repeated indefinitely; it is a reversible process. This reversibility distinguishes it from egg albumin, which when once a jelly remains a jelly. If a glue jelly is allowed to stand exposed to dry air, it will gradually lose water and return to the solid state. When a glue has been soaked and melted in the proper manner it is capable of acting as a powerful adhesive. The adhesiveness of animal glue
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is a great puzzle to the scientific investigator, but the practical man has no difficulty in taking advantage of it. He simply spreads a coating of the hot solution on the blocks of wood or other objects which he wishes to unite, presses the moistened surfaces together and holds them in contact until the enclosed glue film has cooled to a jelly. From that point on there is nothing to do except wait until the water has evaporated from the film. The strength of the joint does not reach its maximum until the moisture content of the glue has fallen to a normal value of about 10 to 12 per cent. Gluing is simply the process of transferring solid glue to the spot where it is needed, using water as the vehicle. As yet there has been no explanation of the adhesive properties of animal glue that would fit all the facts and satisfy all the investigators. A stumbling block to all attempts to formulate an acceptable theory is the fact that animal glue adheres strongly to a great variety of objects, such as wood, compressed wood fiber, glass, sand, leather, rubber, etc. It does not adhere well to oily surfaces, especially oily metals. The only helpful generalization which can be offered a t this time is that glue will adhere well to those surfaces which its solutions are able to wet. The Characteristics of Glue Jelly As stated above, glue jellies tend to lose their water and return to the dry state. Often permanent jellies are sought by those who need a flexible, ~ b b e r ycomposition. As a rule, jt is not feasible to prevent the water from escaping, but the same objective is attained by adding a liquid that does not evaporate readily. There arebeveral such liquids. Glycerin is the one which is most commonly used. Recently diethylene glycol has been proposed as a substitute. Glue jellies containing glycerin or the like are used wherever a flexible composition or a flexible adhesive is required. Bookbinders use large quantities of flexible glue. It finds further application in printers' rollers, hectograph plates, and sandblaster's stencils; also in the manufacture of pads and paper boxes. The ability of animal glue solutions to go into a jelly on cooling is one of their most unusual characteristics. It is easy enough to destroy this property, should one wish to do so. Various chemicals have the power to annul the gelatinizing ability of animal glue. By adding acids or certain salts or other compounds it is possible to prepare a glue solution that will not form a jelly a t any ordinary temperature. Such solutions are, in effect, liquid glues and are not unknown in commerce. Glue is an exceedingly reactive substance, especially when in solution. Even water reacts with it a t high temperature. In fact, water and heat are able to destroy the gelatinizing ability of glue just as surely, though not as spectacularly, as acids. If a glue solution is heated for a long time a t a high temperature, i t will suffer a serious decline not only in gelatinizing power
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but in viscosity. The change is due to a hydrolysis, which results in a breaking down of the molecules into smaller molecules. A n impairment in quality and adhesiveness accompanies this molecular breakdown. It is important to both the maker and the user of glue that severe heating should be avoided. The manufacturer is well aware of this and he takes such precautions as he can, but the user is often deceived, because the thickening of the solution by evaporation masks its decline in real quality. The susceptibility of glue to the effect of moisture and humidity has already been mentioned. Sometimes this is felt to be a drawback. Animal glue can be treated in such a way as to increase its moisture resistance. The means which are used suggest the process of tanning; indeed, the chemicals most often employed are tanning agents in a broad sense. Chromates and dichromates are always named among the. waterproofing agents, but their applicability is so limited that it is difficult to see why they are recommended. Formaldehyde is more suitable, although its action is too rapid for most purposes. Apparently the best compound to use is paraformaldehyde. This liberates formaldehyde quite slowly if the temperature is kept low. The following formula was proposed by the Forest Products Laboratory a few years ago: animal glue, 100; water, 225; oxalic acid, 5.5; paraformaldehyde, 10. If the temperature is not allowed to go above 120 degrees this glue should last about eight hours. Formaldehyde-tanned glues swell slightly in water, but they cannot be liquefied by heating. This place is as good as any to correct the impression that glue must of necessity smell badly. Animal glues as a class do not have an objectionable odor. If a glue room really is malodorous, it is afkost a sure sign that the operator has allowed putrefaction to set in. Every one who uses glue should realize that it is an excellent medium on which to grow bacteria. Moisture, warmth, and long standing are all that is needed to ensure bacterial decomposition. A clean, well-kept glue room is not only more pleasant but more efficient, because decomposing glue is always weak glue. A fresh, hot hide-glue solution is as nearly odorless as any one could wish. Many bone glues have an odor like that of soup-which is only natural, considering how they are made. Only a small percentage of the domestic animal glues have a pronounced odor of any kind.
The Place of Glue in Industry The uses of animal glue are numerous and varied. It plays an important part in the manufacture of furniture and woodwork. The wooden framework of automobile bodies is usually joined with animal glue. I t has properties which commend it to the maker of fine veneered work. Wooden airplane propellers are laminated boards made one by animal glue. It is part of the composition on match heads; it also holds the head on to the
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stick. As indicated above, it is in great demand among bookbinders, printers, and box and carton makers. It is used for "sizing" purposes in many fields, espeaally in the various textile and paper industries. The stiffness of fiber reed furniture is attributable to the glue with which the paper tubing (the so-called reeds) is impregnated and coated. Sandpaper and emery cloth depend on animal glue to hold the abrasive grains in place in spite of the rough usage t o which they are subjected. Certain kinds of abrasive wheels are built up with animal glue. Glue is often blended with rubber. It might surprise you to know that automobile tires sometimes coritain animal glue. Various composition and molded articles, such as furniture ornaments, dolls' heads, billiard balls, and cork bottle caps utilize glue as a binder. It is the chief ingredient of some of the best leather belt cements and other special adhesives. There is glue in some plastic paints. It is in demand for plastering, wall-sizing, and calcimining. Gummed tape owes its stickiness to animal glue and a softener. Its affinity for glass makes it useful as a roughening agent in the manufacture of frosted glass. Animal glue is sometimes added to electroplating solutions, as it has been found that it tends to prevent crystallization, thereby aiding in the formation of a smooth coating of metal. Because it is a powerful adhesive, an indispensable "sizing" agent, a good binder and filler, a plastic, and a source of flexible jellies, animal glue bas gained a widespread commercial importance. Its broad range of properties makes it a factor in the production of commodities which touch our lives at innumerable points. It s-es as a bridge between agriculture and industry. It provides an additional ouffet for a product of the farm and cattle range, and it enables many a manufacturer to turn out a superior kind of merchandise. Industrial research on glue has always been fairly active. The principal problems on which interest appears to center today are: manufacturing improvements, especially a continued search for methods of obtaining maximum yields with minimum hydrolysis and "de-grade," and for new methods of finishing off glue liquors without going through the tedious processes of chilling and tunnel drying. I n the field of utilization glue chemists are at work on: warp correction (i. e., the preparation of flexible glues that will impart no twist or wrinkle to flimsy surfaces); waterproofing; the control of foaminess; and the application of glues by new methods, such as spraying. A new development which seems to be needed is a means of preparing a colloidal suspension of glue in nou-aqueous solvents. At present the adhesive action of glue is manifested only toward surfaces which its solutions (in water) will wet. If a way could be found for getting animal glue to act from acetone solution, for example, it would probably adhere to surfaces from which it is now repelled.
