The Properties of Animal Glue H . B. SWEATT Animal Glue Information Service, New York, New York
A
NIMAL glue is a good illustration of an age-old product whose use has been widely extended by the application of scientific principles to an ancient
the molecule, no acceptable structural formula has been arrived at. We may draw, however, upon the great amount of research directed a t protein in general, to art. -... explain some of the structural features of the glueThere are evidences that animal glue has been known gelatin molecule. X-ray diffraction data, combined and used for over 30 centuries. Stone carvings of the with analysis of amino acids produced by decomposing ancient city of Thebes include one a t least 3300 years gelatin, have yielded the best, but still highly conjecold, depicting the gluing of a thin piece of rare wood to tural, structural formula. Like other proteins, glue is a yellow plank of sycamore. A pot of adhesive is being amphoteric, reacting as a weak base or weak acid. heated over a fire and several samples of veneered and Hide glue made with alkali precursor has an isoelectric inlaid woods can be seen. One of the figures is spread- point of 4.7. Bone glue made with an acid precursor ing glue with a brush, and a piece of dry glue with its has an isoelectric point of 8.3. characteristic concave fracture is shown. When "dry," animal glue in its commercial forms of Other evidences of early gluing practice include furni- powder, pearls, and sheets is an odorless solid, tan to ture pieces taken from the tombs of Egyptian Pharoahs. dark brown in color, with a specific gravity of approxiThese were dovetailed and joined with glue and nails. mately 1.36, a t a moisture content of 10 to 14 per cent. That glues were used a t the beginning of history for Tensile strength tests on better glues have shown them purposes other than woodworking is demonstrated in to rnn from 5000 to 10,000 pounds per square inch, or the writings of Pliny, the Roman, who refers to their considerably higher than the 2000 to 3000 pounds per use in Egyptian paints. square inch of most commercial woods. The adhesive Animal glue has been mentioned in literature since strength of these glues is also generally above the tensile 200 A.D., but i t was not until 1690, in Holland, that the strength of most woods. Anhydrous animal glue is unknown, since on dehyfirst commercial glue plant was erected. Ten years later a factory was established in England. A half cen- dration irreversible changes occur. The important tury later patent literature began to register glue ad- properties of glue, however, are those exhibited in soluvances. The first reference in this literature to bone tion. Glue is so obviously a colloid that the term "colglue (all early animal glues had been made from the loid," which is Latin for glue-like, was coined by the hides or skins of wild animals) appeared in 1814. English physicist, Graham, to describe materials of The manufacture of animal glue began in the United like behavior. Glues have been used in the laboratory States in 1808. For almost 140 years i t has been a probably more than any other material in demonstratbasic industry linking farm and factory. Despite its ing the colloidal phenomenon. long history, however, animal glue did not realize its The most striking and useful properties of glue aside maximum potentialities until certain fundamentals from its adhesive character, are those dependent upon about its technical nature were fully developed. These its being a reversible emulsoid colloidal gel. Highfundamentals, clarified by progress in protein and col- grade glues will form a gel containing as little as one loid chemistry, and accompanied by the development per cent solid a t temperatures below 15% Above of definitive testing methods, have established animal 35'C. gelation takes place a t varying glue concentraglue as a fully engineered material holding promise for tions. Below 15'C. glue shows a specific rotation of research and practical applications in the plant. a = -313', while above 3 5 T . i t exhibits a specific rotation of a~ = - 141' with intermediate specific roCHEMICAL AND PHYSICAL PROPERTIES tations between 15' and 35'C. These properties, The terms "animal glue" and the more familiar "gela- among others, indicate that glue exists in two formstin" are interchangeable when discussing their chemical a sol form stable above 35"C., and a gel form stable properties inasmuch as they differ only in that animal below 15'C., with mixtures a t intermediate temperaglue has not been refined as fully as g e l a h . tures. These forms are completely reversible and are Generally, animal glue can be defined as a protein de- obtained a t will. This property of glue is made use of rived by hydrolysis from collagen, the chief organic in many industrial processes. constituent of skin and bones; however, i t is still not Animal glue is completely insoluble in most organic certain whether glue is the result of a hydrolysis of solvents. An analytical procedure for gelatin, using 95 collagen or an intramolecular rearrangement. One per cent alcohol as a precipitant, is based on this propempirical formula of animal glue that has achieved erty. There are various reagents which react upon some acceptance is (C,02H159039N3~)N with a molecular glue lilms to render them moisture-resistant, formalweipht of about 34,900. Owing, to the complexity of dehyde and potassium dichromate being the best known.
Dried glue will keep indefinitely. Glue solutions should be held a t working temperature of approximately 140°F. As the temperature rises beyond 140°F., hydrolysis proceeds more rapidly, breaking i t down into its component parts. Inasmuch as adhesive strength is related to the average molecular weight of glue, further hydrolysis of the protein with the resultant lowering of the average molecular weight results in a lowered adhesiveness. Like other organic matter, glue is subject to structural breakdown. Whether chemical change is caused by overheating or other factors, it follows the same general path of progressive breakdown of the protein molecule into proteoses, peptones, amino acids, and finally ammonia. Sixteen amino acids have been identified in the degradation products of glue, of which glycene and proline are the most important. Glue does not occur as such in nature. It is formed by heating animal skins, connective tissues, and bones in the presence of water. The reaction is chemical, and not simply one of solution. Through hydrolysis a portion of the water molecule combines chemically with the animal matter. The speed of this reaction varies with the temperature. By-products of the glue-making process include grease for soap makers, nitrogen feed for poultry and certain livestock, and fertilizer of high ammonia content. The manufacturing process is a complicated one, calling for rigid technical controls throughout. Today the manufacture of animal glue is classified as a process industry employing chemical engineering unit operations. As the processing of glue stock requires substantial capital investment in raw materials and fixed plant equipment, glue manufacturing is carried on by a relatively small number of companies. The glue industry is currently consuming over a million dollars worth of chemicals in processing 1,500,000,000 pounds of raw stock to produce 150,000,000 pounds of finished dry glue. Measured in terms of employment, capital invested, value of output, and diversity of product application, animal glue is the most important member of the adhesives family. GLUE TESTING
Laboratory testing of glue may involve as many as ten distinct pxysical and chemical tests, each related in some fashion to the performance of glue in the applications for which it is intended. A host of uses have been developed for animal glue, the common denominator of which is their reliance on its adhesive properties. Many methods of directly determining the adhesive strength have been devised. It has been found, however, that the most reliable and the most easily duplicated results could be obtained by determining two physical properties, jelly strength and viscosity. Both have a direct relationship to adhesive performance. Apparatus used to determine jelly strength is the specialized Bloom Gelometer, in which a 12.5 per cent glue jelly brought to a temperature of 10°C. is
tested for resistance to deformation. A quantity of shot is caused to flow into a pan supported by a spring. The ddection of the spring is transmitted to a blunt probe extending from the underside of the pan to the glue gel. When the pan has moved down 4 mm., an electrical contact stops the flow of shot. The weight of shot needed to obtain this deflection is a function of jelly strength and is readily duplicated. The Bloom Gelometer is a lineal descendant, through many stages, of the empirical tests made by gluemakers during the past hundred years. In 1844 Peter Cooper, one of the earliest American glue makers, proposed a system of testing the jelly consistency of glue. For lack of equipment, finger probing was used. Cooper retained standard glue samples corresponding to the grades he established and measured all glue runs against them. Prior to the advent of more scientific testing methods, the Cooper standards were widely used. The other property of animal glue having an important relationship to its adhesive quality is viscosity. This is determined by the use of a calibrated viscosity pipet. Viscosities are reported in millipoises and jelly strength in grams of shot. Table 1indicates the glue grades adopted as standard by the National Association of Glue Manufacturers in 1923. Jelly Gadc
0 27 47
70 95 122 150 178 207 237 267 299 331 363 395 428 461 495
When the jelly-strength, viscosity relationship of a sample is such that i t does not fit into any of the standard grades, covering customer requirements, the glue is blended with other batches to produce a regular grade. This blending enables the glue industry to use a wide range of raw materials and still produce standard products. While the tensile strength of glue is of importance in many applications, testing of this property has not been extensive because of the difficulty of specimen preparation. This same difficulty has militated against extensive direct tests of adhesive strength as a control procedure. Such tests, however, have been made continually by the Forest Products Laboratory and glue manqfacturers as guides for industrial users. The melting point, which for glue has been defined as that point where less than one per cent of the gel form is present, is a function which generally parallels the jelly strength. This test, however, is rarely used.
of glue even in small amounts prevents crystallization and thus makes for smooth, hard deposits of copper, tin, silver, etc. Glue is also used in acid pickle baths to remove scale from iron and steel prior to galvanizing. As an illustration of the new uses to which animal glues have been put, we might examine the process of warp slashing in rayon weaving. Unlike silks and other natural yarns, rayon possesses none of the natural oils required as lubricants during the weaving process. Satisfactory trade acceptance of the synthetic yarn depended upon the discovery of a coating or size that possessed high adhesive properties, did not hinder the natural elongation of the yarn, and was easily removable a t the dyers' and finishers'. This was one of the most important of a host of chemical and mechanical problems created by the advent of synthetic yarns. Rayon consists of many parallel filaments. Running rapidly through the weaving looms, these filaments are subject to high friction and elongation. The yarn may be "picked" as many as 180 times a minute a t the looms. As each pick results in a stretch of two inches, the INDUSTRIAL USES OF ANIMAL GLUE elongation per minute is 360 inches. Animal glues are Animal glue is known traditionally as a woodworking almost universally used in rayon mills for "warp slashadhesive; however, three-fourths of the glue that is ing." Administered through size baths prior to weavproduced today is used outside this field-as coatings ing, the glue coating binds the filaments together to and sizings, bindings, adhesives, and composition and prevent abrasion or chafing a t the looms. Glue sizings are highly elastic, allowing the yarns to elongate natucolloidal agents. As an adhesive, animal glue is characterized by high rally. In addition, they are easily removable a t the sheer strength and has long been used with paper, felt, dyers' by low temperature scouring, thus eliminating leather, fiber, wood, and similar materials. Millions of the danger of adverse effects on the dyeing process. pounds of animal glue are used each year for sizings Today glue manufacturers maintain extensive laboraand coatings. Protective coatings are applied to tex- tory testing programs, designed specifically to solve tiles, papers, floor coverings, packaging materials, and the sizing problems of the rayon mill. gaskets. Glue coatings are used both in the weaving of THE CHEMIST I N GLUE MAKING yarn and in textile finishing processes. Bank, writing, ledger, and currency papers are sized with glue to enAnimal glue manufacture, like many other indusable them to resist water and ink, withstand wear, and tries that had their beginnings in rule-of-thumb procalso to facilitate erasure. Twisted paper, sized with esses, is now a field to which the activities of the chemglue, can be fashioned into strong and durable reed ist and the chemical engineer, through the exercise of furniture. science and technical skill, have become indispensable. Animal glue also possesses ready miscibility. It Further advances in colloidal and protein chemistry as mixes readily with such materials as clay, sawdust, and well as chemical engineering should make the role of the various chemicals. This quality finds application in chemist and the chemical engineer even more important match heads, doll head composition, frescoes, wood in this industry. composition, cork composition, and so on. Animal glue manufacturers contemplate a busy As a colloidal gel, glue is used in the manufacture of period during reconversion and after. Huge quantities printing rollers, shoe polishes, and hectograph plates. of glue will go into many products and processes and Useful also as a protective colloid, animal glues are will call for further experimentation with controls in valuable in electro-metallurgic processes. The addition glue manufacture and application. The pH of glue solutions is regularly determined as a n aid in manufacturing control and as a guide to glue utilization in various applications such as paper and textile sizings. Several other properties of g l u e s n c h as optical rotation, swelling capacity, and setting rate--have been used a t various times as indicative of grade, but have been largely abandoned in favor of the tests for jelly strength and viscosity. Glue destined for mechanical spreader application is tested for foaming by a simple test tube shaking procedure, or by use of an electrical beater that tests the amount, the type, and the building quality of foam. Foaming properties are then controlled by the addition of antifoam agents. A test for grease is made by observing the presence of undyed droplets in a smear made by a mixture of glue and water-soluble dyes. Grease-free glue is required for certain sizing applications. For adhesive uses, small inclusions of grease are tolerated.
