^ U. S. Department of Agriculture, Forest Service FOREST PRODUCTS LABORATORY In cooperation with the University of Wisconsin MADISON, WISCONSIN ANIMAL GLUES: THEIR MANUFACTURE, TESTING, AND PREPARATION £-4?z. 2x c- ^ o B* •-7 ^ t | o Revised & October 1937 ANIMAL GLUES: THEIR MANUFACTURE, TESTING, AND PREPARATION Varieties and Classes of Animal Glue The term "animal glue" refers "broadly to all glues made from the skins, "bones, etc., of cattle, goats, sheep, deer, horses, pigs, rabbits, or other mammals, unless otherwise specified; however, the term usually refers to cattle glue only. The market value attached to the glues from the seven sources mentioned is a"bout in the order named. Cattle glues are divided into three main classes: hide glue, sinew glue, and bone glue. Hide glue is the strongest and most reliable of the three. Bone glue, as a rule, is quite inferior. Sinew glue lies "between hide and bone glue as regards strength and value. Although these are the three main classes of cattle glue, it is manufactured frroai a wide variety of raw materials. Hides, sinews, tendons, horn pith, bones, head pieces, ears, trimmings, fleshings, tannery waste, etc., are utilized for glues of different kinds. The raw material has an important influence on the character of the resulting glue. Each class of glue is sold in cake, flake, ground, pearl, shredded, and other forms, hut the form of the glue is no reliable indi- cation of quality. The chief difference between the various forms is in the quickness and convenience with which they can "be put in solution. The finely divided forms absorb water more rapidly and can be dissolved more easily than the cake and flake forms. The higher-grade glues, in flake form, are usually light in color and nearly transparent. Inferior glues tend to be dark in color and opaque. Color and transparency, however, are not safe indications as to the quality, for low-grade glues are sometimes bleached. On the other hand, foreign substances such as zinc white, chalk, etc., are frequently added to transparent glues to produce what are technically known as opaque glues. The added materials, while they apparently do no harm, do not increase the adhesive qualities. Aside from the fact that they give an inconspicuous glue line in a joint, the "opaque" or whitened glues have no apparent advantage over other glues of the same grade. Glues are graded according to a variety of tests which will be described later. In the past no two manufacturers have tested their glues in exactly the same way, nor have they followed the same system of grades. The oldest known system of classification is that of Peter Cooper, a manufacturer whose product was early established in the American trade. His grades, arranged in order of value, are: A Extra, I Extra, 1, lx moulding, lx, 1-1/4, 1-3/8, 1-1/5, 1-5/8, 1-3/4, 1-7/8, and 2. Later R492 the National Glue Manufacturers' Association (8)— adopted standard methods of testing and a uniform system of grades. Animal glue can "be used anywhere in the woodworking industry except where a strongly water-resistant adhesive is required. High-grade hide glues are used mostly in joints of thick stock; lower grade glues for veneer work. Manufacture of Animal Glue The process of making animal glue is, "briefly, as follows (2): The stock, after certain preliminary treatments to remove foreign matter and to get it into suitable condition for rapid conversion, is heated in water. The collagen, a complex insoluble protein "body present in the raw materials, is thus hydrolyzed into the soluble, jelly-forming substances constituting glue. The hydrolysis continues "beyond the formation of these products and gives rise to compounds having little or no jelly-forming power and therefore of slight value as adhesives. For this reason the liquor is drawn off after a certain period of heating and a fresh supply of water added for a second run. A number of successive "cooks" may be made, "but the glue obtained from the first is generally of the highest grade. There are many and varied details in the steps above outlined, depending upon the kind of stock used and the plant in which the glue is made. Any detail of manufacture may be expected to have its effect upon the character of the resultant glue. The solution of glue from the "boiling kettles is next concen- trated by "boiling off the water in vacuum dryers until the percentage of glue in the solution is high enough to make a firm jelly on cooling. If too high a temperature is reached during the concentrating process, the quality of the glue may "be lowered. When sufficiently concentrated, the glue solution is cooled "by refrigeration, either after "being run into pans or as it is carried upon a traveling "belt. As it cools it forms a jelly firm enough to handle. From the pans, the jelly is removed, sliced with wires or a knife, and placed upon screens to dry. If a belt is used, the jelly is formed in a contin- uous sheet; this is cut into sections and placed on screens as it travels along. The screens are then placed in a drying chamher and left until the glue is dry. The glue may easily "be injured during the drying process if the temperature conditions are not properly controlled. The form of the glue when dry depends upon the shape in which it was placed upon the screens. If carefully sliced to the proper thickness, cakes of regular shape will be formed. Sheet glue from the belt "breaks into thin, irregular pieces as it comes from the drying screens. It is then commonly -Figures in parentheses refer to references at the end of the discussion. Rk32 _2- run through a machine to "break it into smaller pieces, in which shape it is shipped as flake glue. In this or any other form, the dried glue may subsequently be ground and sold as ground glue. By varying the method of handling the glue as it comes from the evaporators the pearl glues or the shredded glues are produced. Testing Animal Glue The past 10 years have seen a marked improvement in the standardization of methods for grading animal glues for woodworking. The National Association of Glue Manufacturers has adopted a standard method of grading based upon tests of viscosity and jelly strength (8_). In 1931 a federal specification was adopted for procurement of animal glues for woodworking. The grades set up in the federal specification are based on tests of viscosity and jelly strength (lj>_). It includes also tests for moisture content, pH, foaming, odor, and keeping quality and specif ..es the minimum requirements, considered adaptable to glues for woodworkings The definitely established tests and specifications afford the user of animal glues a method of insuring uniformity in the glue and of securing a product suited to his operating needs. The following methods for testing animal glue are taken from the federal specification ( 13 )» Viscosity Solutions of the glue for viscosity and jelly strength deter- mination shall be prepared as follows: Sufficient of the glue sample shall be taken (calculated from the apparent moisture content) to give 13»2 grams of dry glue and placed in a standard wide-mouthed bottle of 150 cc. capacity with an inside diameter of 59 mm., an outside diameter of 66 mm. and a height over all of 85 mm., and fitted with a No. 9 rubber stopper. To the glue shall be added enough distilled water to make a total of 106.8 grams of water, including that naturally contained in the glue. This will give an 11 percent solution based on approximately anhydrous glue. The glue and water shall be thoroughly mixed with a stirring rod and then allowed to stand at a temperature of 10 to 15° C. for at least k hours. The glue shall then be melted by bringing the samples to a temperature of 62° C. in a water bath in which the temperature of the v/ater shall not exceed 70° C. The time required to bring the sample to 62° C. shall not exceed 15 minutes. The viscosity determination shall be made in a standard jacketed pipette viscometer or other suitable viscometer capable of expressing viscosity in the absolute unit of "poise." The instrument must be equir; to hold the glue solution at approximately constant temperature. The Rl+92 -3- Digitized by the Internet Archive in 2013 http://archive.org/details/glueseirOOunit temperature of the glue solution shall be 60° C. and the determination made as soon as the solution has uniformly reached the required temperature. Jelly Strength The samples from the viscosity test shall "be placed in the standard containers and used for jelly strength determinations. The glue solution should he free of air or foam. The samples shall he allowed to cool to approximately U5° C. and any water of condensation shall he mixed thoroughly with the solution. The samples shall then "be placed in a chamber or hath at 10 i 0.1° C. for a period of l6 to 18 hours. The determination of jelly strength shall he made on a Bloom gelometer adjusted to give h mm. depression and to deliver a load of UO grams per second to the surface of the jelly. The test must he completed within 2 minutes from the removal of the sample from the cooling chamber. Apparent Moisture Content The loss in weight of a 10-gram sample, previously reduced to a size that will pass a No. k mesh screen, placed in a well-ventilated oven and heated at 105° - 3° C. for l6 hours, shall be not less than 9 nor more than 15 percent of the original weight of the glue. Neutrality Portions of the solutions upon which the viscosity and jelly strength tests were made or other samples of equal concentration shall be used to determine the acidity or alkalinity of the glue. The acidity or alkalinity shall be determined in terms of pH and must be within the range of 5*5 to 7»5» Foam Portions of the samples used in the viscosity and jelly strength tests or other samples of equal concentration shall be used for a deter- mination of the tendency to foam. Seventy-five cc. of the solution at 60° C» shall be placed in the standard container and agitated with a stirrer or propeller. The stirrer shall be placed in the lower one-half of the solution and the speed of the stirrer during test shall be between 13,000 and 15,000 r.p.m. The solution shall be agitated for 1 minute and then allowed to stand at room temperature for 10 minutes, at the end of which there shall be not more than 2.5 cm. foam. RU92 -L- Odor and Keeping Qualities A 50 cc » portion of the glue solution used in previous tests shall "be placed in an uncovered container and held in an incubator at 37°-38° C. for US hours. At the end of this time it shall he free from any odor of decomposition. Grease, Ash, and Other Miscellaneous Materials If for any reason it seems desirable, tests may also he made for ash and grease content and the presence of materials that give a fictitious value to a glue. The presence of more than U percent ash in clear glues and 10 percent in opaque (white) glues, or 1 percent of grease (determined "by the Kissling or other satisfactory extraction method that shows total grease), or the presence of materials which increase the viscosity or jelly strength may he cause for rejection. Wood-Joint Tests Wood-joint tests are not a part of the federal specification. They are not suitable for the grading of animal glues "because more direct and accurate evidence of the physical properties of the glue is afforded hy the viscosity and jelly strength tests. Strong joints may be made with a number of grades of animal glue but different gluing conditions must be used according to the grade of the glue. If wood- joint tests are made with glues of different grades under a uniform set of gluing conditions, the grade of glue that gives the best results is the one best adapted to the particular gluing conditions used and the results are not an accurate measure of the inherent strengths of the grades tested (12). Wood-joint tests are, however, of decided value in determining the combination of gluing conditions that should be used with any given grade of glue. The Forest Products Laboratory has adopted the following procedure for making the wood- joint test (ll_): Selected pieces of v:ood, usually sugar maple, are stored in an atmosphere at 30 percent relative humidity until they have reached an equilibrium moisture content with this condition. Matched pieces, approximately 3A" x 2.5" x 12", are carefully surfaced to insure smooth, plane, and parallel surfaces. The pieces are glued under controlled conditions of temperature, amount of glue spread, duration of assembly period, and amount of pressure applied. They are allowed to remain under pressure over night and then returned to 30 percent relative humidity for seasoning. A seasoning period of at least a week is allowed between gluing and testing. The glued pieces are then cut into specimens and tested in shear. The unit pressure required to break the joint and the amount of wood fiber torn in breaking are measures of the quality of the joint. The effect of gluing conditions on joint strength is discussed in detail in U. S. Department of Agriculture Bulletin 1500 (11). RU92 Selection and Preparation of Animal Glue for Use Making strong joints depends primarily upon having the proper correlation of gluing pressure and glue consistency at the moment pressure is applied. With animal glue solutions, the consistency depends on the cooling and drying effects. For the first few minutes after an animal glue has been spread on the wood the cooling effect is much more important than the drying and this temperature-viscosity relationship varies with the grade and with the concentration of the glue solution. High-grade animal glues thicken to the proper pressing consistency quicker and at higher tempera- tures than do low-grade glues of equal concentration. Assuming glues of equal grade, one mixed with less water will thicken more rapidly than one mixed with a greater quantity of water. Warm animal glues, as they normally exist in the spreader, are too thin for pressing and some thickening must occur "before pressure is applied. The best consistency for pressing exists when the glue is thick enough to form short, thick strings when touched with the finger but not thick enough to resist an imprint or a depression readily. The thickening time or assembly period is usually fixed by the operating conditions that dictate how much time shall elapse between spreading and pressing. The grade of glue and the proportion of water added in mixing become, therefore, the variables by which the manufacturer can fit the glue mixture to his operating conditions. When once established, the grade and proportion of water should be adhered to except as temperature changes in the gluing room or wood require a change in the mixture. The following tabulation shows the grades, with the corresponding ratings for jelly strength and viscosity, as established by federal specifi- cations for animal glue for woodworking ( 1 3 ) ; Grade VI V2 Jl J2 SI S2 As a general rule, the following division of grades is recommended for different uses and service conditions: VI and V2 for general use in veneering where assembly periods may be somewhat prolonged. Jelly strength Viscosity (grams ) (millipoises) 150-194 57-68 195-239 69-81 240-284 82-95 285-329 96-1C9 33C-37U 110-124 375-419 125-139 R492 -6- Jl and J2 for general use in joint work where assembly periods are short. SI and S2 for special use in warm and humid climates where a quick chilling glue may he required. As a further guide in selecting the grade and concentration of glue for a particular job, the following schedule (ll_) will pre luce good joints when an animal glue equivalent to the J2 grade is mixed in the pro- portions of 1 - 2-l/h. Temperature of Assembly room and wood period (°F.) (minutes) 70 1/2 to 1 80 3 to 5 90 10 to 18 When the assembly period is fixed by the operation and the temperature in the glue room rises, an adjustment must be made to accelerate the speed of thickening. This adjustment can usually be made most easily by mixing less water with the glue. The above schedule of conditions can be met with a lower grade, as Jl for example, by mixing less water with the glue than recommended for the J2 grade. Likewise, a higher grade, as SI or S2, may be made to conform by mixing more water with the glue. Within limits, therefore, a user may pur- chase a glue of higher rating and mix it with more water or one of lower rating and mix it with less water to fit a particular operation. In preparing animal glue for use a number of precautions must be observed if satisfactory results are to be obtained. The proportion of glue and water should be varied to meet manufacturing conditicr.s (11 ). When the right proportions have been worked out they should be adhered to consistently. The glue and the water should be weighed out whenever a batch is prepared. Clean, cold water should be used and the mixture thoroughly stirred at once to allow a uniform absorption of water by the dry glue and prevent the formation of lumps. The batch should then stand in a cool place until the glue is thoroughly water soaked and softened. The soaking may take only an hour or two, or it may take several hours, the time depending upon the size of the particles. The glue should then be melted over a water bath at a temperature not higher than I5O F. High temperatures and long continued heating reduce the strength of glue solutions and are, therefore, to be avoided. The glue pot should be kept covered as much as possible in order to prevent the formation of a skin or scum over the glue surface. Strict cleanliness of glue pots and apparatus and of the floors and tables, of the glue room should be observed. Old glue _socn becomes foul RUQ2"' -7- and affords a "breeding place for the "bacteria of decomposition, exposing the fresh "batches to the constant danger of "becoming contaminated. Glue pots should "be washed every day and cnly enough glue for a day's run should be mixed at a time. If these simple, sanitary precautions are not observed, poor joints are more than likely to result. In a brief review, it is not possible to cover thoroughly all the points touched upon, or even to mention many facts which the glue user should know. For more detailed treatment of the subject, with information on animal glues and other types as well, the reader is referred to the publications listed on page 9» Liquid Glues Most of the commercial liquid glues are made from fish stock — heads, bones, skins, and swimming bladders — although several brands of animal glue in liquid form are on the market. Liquid glue can be used cold and spread without any preparation whatsoever. Commercial liquid glues vary widely in strength; some are so weak as to be suitable only for use on paper jT cardboard, while others are almost as strong as high-grade dry animal glue. Liquid glues are used to a certain extent in joint work, but not of the highest class. They are used also for repair work and miscellaneous purposes « Liquid glues would be used more extensively if they were less expensive and if the better grades could be readily and accurately distin- guished by the user. Testing Liquid Glues Methods of testing liquid glues have not been standardized. Tests of joint strength on liquid glues have indicated that the very thin, watery liquid glues tend to produce weak joints, but no definitely con- clusive relation appears to exist between viscosity and joint strength. Tressler (10) has suggested the following tests for liquid glues from fish stock: Viscosity, ash content, drying properties, hygroscopicity, joint strength, color, permanence, and pH value. Jennings (6) holds that liquid fish glues should have the following properties: Be neutral to litmus, slightly acid to phenolphthalein, remain unaltered after one month's storage at 37°-3 8 ° C., set to a jelly between 7. 5° and 10° C, be free from tackiness when exposed in film form at 25° C. to 80 percent relative humidity, and contain not more than 0.2 percent chlorides. Since the importance or value of the various chemical and physical properties of liquid glues are not well established and since standardized test procedure for their determination is not available it is necessary to RU92 -g- place more reliance on joint tests. The procedure described for making joint tests on animal glues is also applicable to liquid glues. Here again joint tests must "be carefully made to "be of value. References Alexander, J. 1923» Glue and gelatin, Chem. Catalog Co., New York. Bogue, R. H. 1922. The chemistry and technology of gelatin and glue, McGraw- Hill Book Co., New York, Fernbach, R. L. 1907« Glues and gelatine, D. Van Nostrand, New York. Gill, A. H. 1925* A comparison of various tests upon glue, Indus. 8c Eng. Chem. 1L 597-8. Houseman, P. A. 1917* Glue for use on airplanes, Indus. 8c Eng. Chem. % 359~60. Jennings, D. 1933. Quality of fish glues, Chem. Trade Jour. 22_, 105-7* Linder, 0,, and Frost, E. C. 191U. Specifications and tests for glue, Amer. Soc. Testing Materials. Proc . Y^ 508-19 (v. lU, pt. 2). National Association of Glue Manufacturers. 1930« Standard methods (revised) for determining viscosity and jelly strength of glue, Indus. & Eng. Chem., Anal. Ed. 2, 3^-8-51 • Rideal, A. 1926. Glue and glue testing, Scott, Greenwood £ Son, London. Tressler, D. K. 192H. Examination of liquid glue, Indus. & Eng. Chem. 16, 9^3~5» Truax, T. R. 1929. The gluing of wood, U. S. Dept. of Agr. Bull. 1500. j Browne, F. L., and Brouse, Don. 1929« Significance of mechanical v/ood-joint tests for the selection of woodworking glues, Indus. £ Eng. Chem. 21, 7^-9* United States Federal Specifications Board. 1931« Federal specification for glue: Animal, woodworking No. CG k^l y May 26, 1931. Rl+92 -9- UNIVERSITY OF FLORIDA 3 1262 08927 9193