Modern glues and glue testing (other than water proof glues)

Front matter

PERIODICAL | PUBLISHING CO) SUCH. |

Sone ae oe

~

Book I (Other than Waterproof Glues)

BY CLYDE H. TEESDALE, A. B.

Formerly in charge, Section of Wood Preservation, Forest Products Laboratory, Madison, Wisconsin

With an Appendix

Methods of Testing Animal Glues at the Forest Products Laboratory, Madison, Wis., in which is set forth the standards of the Bureau of Aircraft Production, U. S. Army

Published By THE PERIODICAL PUBLISHING CO.

GRAND RAPIDS, MICHIGAN ss Copyrighted by ee eae ae cus The Periodical Publishing Co. — eRe ea ee Grand’ Rapids, Michigan > Rome Pea at 2> a

Preface

When the United States entered the great war and it became necessary to build airplanes by thousands, very definite specifications and satisfactory methods of testing glues were sorely needed. The problem of developing specifications and methods of test was turned over to the Forest Products Laboratory and was worked out under my direction.

This book is based largely on the results that were obtained, and which were applied in practically all purchases of glue during the war by both the Army and Navy Departments. General information on the manufacture and use of glues is given, and a description of the methods of testing that are described in the literature on the subject.

Acknowledgement is made to Mr. A. T. Deinzer of Monroe, Mich., who contributed valuable information used in compiling this book; also to Mr. Geo. M. Hunt of the Forest Products Laboratory for valuable suggestions and criticisms and for contributions to parts of the text, and to Mr. L. J. Markwardt, Mr. A. C. Knauss, and Mr. Wilbur Lloyd Jones, of tie Forest Products Laboratory, for contributions to the text.

THE AUTHOR.

| 4 1 } ie

BOOK I TABLE OF CONTENTS

CHAPTER VIII—Strength of the Glued Joint

Joint Tests Indicate Quality of Workmanship Rather than Glue Strength—Selection of Woods for Glue Tests —Variation in Application of Glue and Manufacture of Joint—A pplication of the Load—The Spandau Test— British Royal Aircraft Test—Aeronautical Inspection —Directorate (England) Method—Shear Test and Tension Test—A Widely Used Method Discarded at Madison, Wis.—The Forest Products Laboratory Shear Test —A Modification of Madison Standard Test—Miscellaneous Tests—Plaster Rod Test—Setterburg Test— Gill Test i... ak a Ee 41

Methods of Testing Animal Glue at the Forest Products Laboratory, Madison, Wis., 1919 Standards of the Bureau of Aircraft Production, U. S. Army.................... 87

Chapter I

GENERAL DISCUSSION thirty-three hundred years. Furniture found

in tombs of the ancient Egyptians of the period of the Exodus was dovetailed, and joined with glue and nails. Romans, of the time of Cicero, were familiar with glue, and it was in extensive use in England during the reign of Queen Elizabeth.

The term glue is used in its broadest sense to denote an adhesive. Used in this sense it includes a great variety of substances. The oldest and best known of these is animal ‘glue. Animal glue comes in an almost infinite variety, due to the differences in methods of manufacture and the source and quality of the materials used. Glues or adhesives are also produced from starches, casein, blood albumin and sodium silicate.

In general, animal glue is a substance akin to gelatine, though not identical with it, and is produced from the bones, sinews, hides, etc., of animals. It is difficult - to define the difference between glue and gelatine because these substances may be very similar in their composition, appearance and properties. Gelatine, however, is usually used for purposes in which absence of taste, odor and color, together with a firmness of jelly are desired. It is often used in foods. Consequently it is prepared from selected materials free from decomposition. Glue; on the other hand, is usually used for its adhesiveness, and lower grade materials and less care may be employed in its production.

It is rather important for a glue user to have a clear idea of the difference between glue and gelatine and of the method of manufacture of glue. A true ie is said that glue has been known and used for conception of this may cause him to change his.

methods of glue handling. The frequently heard statement that gelatine is a pure form of glue is not strictly correct. Neither glue nor gelatine occurs as such in

EARLIEST RECORD OF GLUES.

DISTINCTION BETWEEN GLUES AND GELATINE.

GENERAL PROCESS OF ANIMAL GLUE

MANUFACTURE.

VARIATIONS IN METHODS OF

MANUFACTURE.

nature. They are formed by heating certain animal tissues in the presence of water. The action is chemical, not simply one of solution. A portion of the water molecule, according to recent ideas, combines chemically with the animal matter, the technical term describing the action being “hydrolysis.” This action proceeds rapidly at a high temperature, and slows down as the temperature is lowered. Hence both time and temperature are important factors in glue making. As the action proceeds, a series of products is formed, each one differing slightly from the one from which it was produced. While neither glues nor gelatines are produced in this way commercially, we may consider, for our purpose, that the first one formed is gelatine. Continued action produces first very strong viscous glue, then weaker and weaker glues, until finally, if the action is continued long enough, a product is obtained that will not form a jelly when cold, and which has no adhesive properties. It makes no difference whether the heating occurs in the glue maker’s boiler or in the wood user’s shop room. Whenever the glue is heated in the presence of water, chemical action occurs which steadily reduces the quality of the product.*

The process of making animal glue is, briefly, as follows: The stock is washed and treated to remove dirt and grease, then boiled to convert the glue-forming substances into a glue solution, which is concentrated by evaporation until it will form a jelly on cooling. The jelly is then dried and the resulting product is the glue.

There are many details and variations in the steps above outlined which depend upon the kind of stock used and the plant in which the glue is made, all of which have more or less effect upon the character of the resultant glue. For instance, bones are sometimes boiled without first removing either dirt or grease. This naturally fails to produce a high-quality glue. It is common practice to treat bones with acid to remove the calcium salts before cooking, but this is not always done. In cooking, the temperature and time have to be

*See Rideal (Glue and Glue Testing), page 10.

GENERAL DISCUSSION 13 carefully watched, as over-cooking may materially reduce the strength of the glue. The stock is usually boiled several times, using fresh water each time. The first boiling or first run gives the best glue, the strength of the glue obtained from the succeeding boilings being less each time. Sometimes two or more boilings from the same kettle, or boilings from different kettles, are mixed together or “blended” before the liquor is concentrated.

The solution of glue from the boiling kettles is too weak to form a jelly which can be handled, so it must be concentrated. This is done by boiling off the water in vacuum pans until the percentage of glue in the solution is high enough to make a firm jelly on cooling. If the temperature gets too high 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 runs upon a traveling belt. As it cools it forms a jelly firm enough to handle. The jelly in the pans 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 continuous sheet, which is cut into sections and placed on screens as it travels along. The screens are then placed in a drying chamber and left until the glue is dry. The glue may be easily 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 regular shaped cakes will be formed. The sheet glue from the belt breaks into thin irregular shaped pieces as it comes from the drying nets. This is commonly run through a machine to break it into smaller pieces, in which shape it is shipped as flake glue. Other miscellaneous forms of glue are also made, and any of them may be subsequently ground and sold as ground glue.

Sometimes mineral matter such as barium sulphate, white lead, chalk, or whiting is added to the glue after it has been concentrated, but before it is cooled.

METHODS OF COOLING.

MISCELLANEOUS FORMS OF GLUE.

PETER COOPER GRADES.

This gives it a light color and makes it opaque, which is a feature desired by some consumers. .

From the above very brief description of various steps in the manufacturing process it can readily be seen that the variety in character, color, form, strength, etc., of glues can be almost without limit. A system of classification, based chiefly on the jelly strength, was devised a long time ago by Peter Cooper, by which it is possible to group the great variety of glues into a relatively few classes or grades. The grades established by Cooper, beginning with the strongest, were designated, respectively, A Extra, 1 Extra, 1, IX, 1144, 1%, 114, 15%, 134, 1%, 2. There are now glues stronger than A Extra, and glues weaker than 2, for which there is no standard Peter Cooper grade. This system of grading, however, appears to be but little used today by manufacturers, except sometimes for comparative purposes. Each manufacturer has his own system of grading which he keeps more or less secret, and to the general buying public the grades mean little or nothing. It is sometimes claimed also that the Peter Cooper grades of today are not the same as those established long ago.

Chapter II. Miscellaneous Glues

ISH glue is made from the skins, bones, bladders,

etc., of fish. The finest grades are made from

the swimming bladders, and come on the market in the product known as isinglass. It is said that fish glues can be made that are practically equal in adhesiveness to hide or bone glues.

Liquid glues are marketed in liquid form ready for use. Most glues are so prepared that they can be used cold. They are quite popular and have a large sale. Usually they are higher in price than ordinary animal glues. They may be prepared by special treatment of hide or bone glue, but very often they are made from fish glues.

In recent years vegetable glues have been used in increasing quantities, especially for gluing veneers. At the start of the war the price of animal glue soared, and forced a very wide use of vegetable glues, especially in the veneer trade, where the use of animal glues was practically eliminated. The commercial importance of vegetable glue is now obvious to practically every large user of glue.

Most vegetable glues are made from starch and alkali. Some consumers seem, however, to be successfully gluing with starch boiled in water, without the addition of alkali. The most commonly used vegetable glues are produced from cassava flour and caustic soda (sodium hydroxide). During the war the shortage of shipping facilities prevented the importation of cassava flour, and much potato starch was used, usually where possible in mixtures of as much of the cassava flour as possible. Corn starch has been used. Other materials than caustic soda, such as tri-sodium phosphate, sodium carbonate, etc., have been used. Sometimes the starch is treated with sulphuric acid (hydrolized or processed) before mixing with the alkali. The best qualities of vegetable glues, when properly made, show strength properties which com-

FISH GLUES.

LIQUID GLUES.

STARCH GLUES.

POTATO STARCH GLUE

PERKINS GLUE.

WATERPROOF CASEINS AND BLOOD ALBUMIN GLUES.

pare favorably with the medium to good grades of animal glues. They also have certain advantages of use, especially that of applying cold, that are well known to most of our readers. Vegetable glues are often criticised because of their rather common property of staining wood. Recent work indicates that this difficulty is being overcome.

The Perkins Glue Company, well known manufacturers of vegetable glue, made the following statement:

“Vegetable glue is manufactured by taking a carbohydrate such as cassava flour, the water absorptive property of which has been decreased or is already sufficiently low, but not too low, and mixing therewith the necessary small amount of water and dissolving the starch in the water by stirring in a solvent such as caustic soda, whereby a viscous, colloidal semi-fluid glue is formed having substantially the properties of animal glue for veneering. Some heat may be used in making the solution, in which case the amount of alkali used may be decreased.”

Ordinary glues were not equal to the demands of war time airplanes, and as a result the development of waterproof glues, especially for veneer parts of airplanes, received a tremendous impetus during the war. The successful airplane waterproof glues were all made either from casein or blood albumin. The wide use of these glues in the industries will without doubt result after the war in their widespread adoption for industrial purposes. Not only may they replace bone or vegetable glues for some purposes, but they make possible the use of veneered and built-up wood products in exposed locations, formerly not possible because of the lack of a suitable glue.

Casein glues in general are made from ground casein, calcium hydrate (slaked lime), and some form of caustic soda. Sometimes sodium silicate or sodium phosphate are used. Blood albumin glues are made usually from black soluble blood albumin, calcium hydrate, with a small quantity of sodium silicate, sodium hydrate, or ammonia. Often casein is mixed with the blood albumin. Casein glues set hard when

MISCELLANEOUS GLUES 17 cold, and are consequently termed cold glues. They may be used in cold press equipment. Blood albumin glues depend upon the application of heat to coagulate the albumin, and hence require heated presses for their application.

Silicate of soda is successfully used as an adhesive in the paper box industry. There is a widespread feeling that it may some day be used to advantage in certain lines of wood work. Without doubt the very low cost of silicate of soda makes it an attractive material with which to experiment, and the results thus far obtained are promising.

The Bureau of Plant Industry, United States Department of Agriculture, has patented an adhesive _ produced by boiling corn cobs in water under pressure. The resultant solution is concentrated and used as an adhesive. The product is not as yet developed to a commercial success, but gives great promise of succeeding as a substitute for sodium silicate in the paper box industry. It is said that it will be lower in cost than sodium silicate.

WATER GLASS.

CORN COB ADHESIVE,

Chapter III

USES OF GLUE aside from its use in joining wood. Among

these may be mentioned paper sizing, where its purpose is to make the surface of paper less porous, or to give it “glaze.”’ Animal glue, gelatine and casein are used, dependent to some extent on the relative market costs and avuilability at the moment, and upon the quality of the product being made.

Glue for sizing paper is usually bone glue, of high grade, free from foam, light in color, and containing no more than the normal amount of grease. It should show no perceptible odor after heating a solution in water at 100° F. for 100 hours. Alum is sometimes added to this glue by the paper manufacturer to make a free flowing solution.

Paper box makers prefer a quick setting hide glue for “setting up.” For ‘‘covering” a slow setting bone glue is used. Mixtures of animal glues with vegetable glues are often used, one advantage being lower cost.

Here a product is required that is strong, flexible and free from color and odor.

Straw hat manufacturers desire a hide glue free from lime, color and odor.

In wall paper, freedom from grease and objectionable odor is desired.

In match making, uniformity is desirable. The glue is used in combination with phosphorous to assist in preventing atmospheric oxidation of the phosphorous.

. For leather goods and belting, flexibility, tenacity and resistance to moisture are the desirable qualities -in glue for leather and belting. High grade hide glues with linseed oil and glycerine added in small amounts are often used. Glycerine is said to increase flexibility, and linseed oil, resistance to moisture.

( LUE is used in a great variety of industries,

PAPER SIZE.

PAPER BOX ADHESIVE.

BOOK BINDING.

MISCELLANEOUS USES.

USED IN TEXTILES.

JOINING WOOD.

HIGH GRADE GLUE MAY BE THE MOST ECONOMICAL.

KEEPING QUALITY OF GLUES.

Glue is used in dressing and finishing colored yarns and threads, sizing of worsted and woolen wraps, and in the printing of fabrics. High grade hide glues free from odor and color are required.

Tons of glue are used for writing tablets. Almost any grade, except the very poorest, will answer.

A glue is valued primarily for its power of resisting rupture. This includes absence of brittleness, and it should have the power of yielding or stretching slightly before it ruptures. Glues may be found which are very strong if the strain is applied steadily, but which break under the impact of a suddenly applied load. A weaker but more elastic sample is superior to such a glue for joining wood. In general however, a glue may be valued on a basis of its strength properties.

Intimately related to the strength property is the covering capacity of glue. Its value is largely controlled by these two factors. High strength usually is accompanied by great covering capacity, and hence higher priced glues may actually cost less to use than lower priced ones, which require more glue to accomplish the same purpose. Covering capacity is related to three factors—the water absorption, the jelly strength, and the viscosity of a solution of known strength. These tests made with accuracy will usually give a close indication of the grade or the value of a — glue.

Color is important in some lines of wood work, being especially undesirable in high grade furniture. As a rule, however, a dark color should not be considered detrimental, as absence of color may represent high cost or the use of undesirable and weakening chemical bleaching agents.

Keeping qualities are of some importance. Alkaline glues are more liable to spoil or decompose, due to bacterial action, than acid glues. Hence a test for acidity and alkalinity give an indication of keeping properties. Glues made from partly decomposed stock do not keep well, and have a bad odor when the glue solution is heated. Hence odor has a bearing on keeping qualities.

USES OF GLUE all

A recent development is the use of glue as a filler in automobile tires. The lower grades of bone glue that formerly went largely to the veneer trade, are preferred. This new use of glue is of tremendous importance to the animal glue industry, especially in view of the inroads made on their market by vegetable glues. At the present time, only a few tire concerns are using glue, but the demand from this source promises to expand to very large proportions.

AUTOMOBILE TIRES.

Chapter IV. Glue Testing (preliminary)

testing glue has never been developed, due largely to the variable and uncertain chemical composition of glue. It is exceedingly desirable for glue users to test their glues rather than trust wholly to the promises of the salesman. The technique of glue testing requires a trained observer and good equipment to accomplish very much, though there are a few simple tests and observations that can be made by any one and which will give some idea of quality. Chemical analysis of glue does not enable one to select glues according to quality, and has never been successfully applied. Glue testing consists largely of physical tests, such as viscosity, jelly strength, strength of the glued joint, ash, moisture, odor, etc. War made necessary the development of methods of testing and specifying glue, and centered the attention of experts in this kind of work on the glue testing problem, not only in the United States, but in the European countries. The glue testing methods selected by the warring countries were naturally the best available, and hence are worthy of the most serious consideration of any reader interested in this subject. One of the most successful attempts thus far made to develop a uniform practice of glue testing was accomplished in the United States, where all glues for airplanes for both the army and navy was given a most thorough inspection. This work was done at the Forest Products Laboratory at Madison, Wis. Intensive investigation by a large force over several months on methods of testing was carried out under the guidance of a glue chemist of national reputation. All of the testing methods that might seem to have any merit whatsoever were tried. The resultant methods selected in this way were incorporated in a tentative specification which was submitted to a conference of glue manufacturers, and as finally evolved, had the

A UNIFORM and generally adopted method of

GLUE TESTING REQUIRES A’

SKILLED OPERATOR.

GLUE TESTING SCIENCE ADVANCED BY THE WAR.

TESTS BY AN AUTHORITY.

SOME SIMPLE BUT SUPERFICIAL TESTS.

SIGNIFICANCE OF COLOR.

approval of all. They may therefore be considered as the best that this country has developed thus far.

In the following pages a few simple tests will be outlined that are possible to make without great experience or complete equipment. The more refined tests, however, require a trained analyst, familiar with the manipulation of testing apparatus, and a well equipped laboratory. The methods of test recommended by the Forest Products Laboratory are mentioned in outline only in the text, but are grouped together and given in full in the appendix.

Glue should not become damp or sticky (hygroscopic) in the air, or it may mold. A hygroscopic glue indicates adulteration with sugar, molasses, etc. The appearance, hardness and manner of breaking are points which may be used to judge quality after some experience. Some glues may have a cloudy or milky appearance, due to the presence of calcium phosphate. Glues of excellent strength may be warped or twisted, and very dark in color. Ground glue should not lump together in warm, humid weather.

Samples should be taken from several parts of the package, as glues are very often blended from more than one run and the mixing may not have been thorough. Special attention should be given ground glue, which is very easy to adulterate. White bubbles on the surface of the glue are an indication of decomposition. With such samples be sure to test the odor, as described later. Decomposed or sour glues are to be avoided. Bubbles appearing deep in the glue are not necessarily an indication of putrefaction. An expert glue buyer is able to judge somewhat by examining a flake of glue by looking through it at a strong light. .

Gloss on the surface does not necessarily indicate high quality. On the contrary, low grade glues may be highly glossed, while good quality glues may be dark, though the surface should be smooth. A uniform color and surface appearance is desirable,

Color is not a reliable indication of quality. The first runs of glue from the glue stock are lighter in color than the last ones. But it is easy to clarify glues

GLUE TESTING (PRELIMINARY) 25 chemically, greatly weakening them in the process. Bone glues are usually darker than hide glues, unless they have been bleached. With any particular lot, non-uniformity of color indicates either adulteration or blending. The addition of zinc oxide gives a white opaque glue, and one that sets quickly. It is also said to add somewhat to its water resistance. In moderate quantities zinc oxide is not injurious. Sometimes inert matter of no value is added as an adulterant. Hence the color of such samples may well be reason for suspicion.

Breaking a sample of the glue with the thumb and forefinger of each hand gives an indication of glue quality. The condition of the air must be considered, as a dry day will give a different indication than a humid one. If the glue fractures evenly and bends but little, low strength and brittleness are indicated. If a thin sheet bends well and in case it breaks, shows a splintery fracture, good strength is indicated. Bone glues show a glossy fracture; hide glues of high grade never do.

Foam in a glue is undesirable. <A solution of glue in water may be stirred vigorously, with a rod, or better, with an egg beater, and a fair indication of its foaming properties will be obtained. Particular attention should be paid to the rate at which the foam subsides. If it persists for long, the glue should be avoided. :

Odor is a sign of poor keeping quality. Usually offensive odor is the result of using spoiled or decomposed stock, or else it may mean that the glue itself has started to decay.

Something may be learned by smelling of a moistened flake of glue warmed in the hand. A more reliable method is to heat a solution of glue in water, about 1 part glue to 12 of water, at a temperature of 100° for 48 hours. It should remain sweet during this period. A similar sample should be able to stand at room temperature for four or five days without developing a bad odor, or showing the presence of mold or decomposition.

SIMPLE TEST FOR QUALITY.

ODOR SHOULD BE SWEET.

COAPTER.YV.

GLUE TESTING*

HILE the tests just described are useful guides

in purchasing glue in the absence of better

methods, they are, at best, a makeshift. Wherever possible advantage should be taken of more refined methods.

The reader should, however, have a rather clear idea of the limitations of glue testing. It is usually not possible to obtain a very clear idea of glue value from any single test that may be applied. In an article describing the testing and grading of glues, the following statement is made by E. G. Clayton (Jour. Soc. Chem. Ind., 1902, 21, 670):

“In conclusion, the observations seem to show that while it would be rash to form a judgment on glue from a single test, the evidence afforded by a number may be irresistible. The expert’s surest system appears to be not to rely on single short cut tests of general quality, but to employ a number of methods, including any having especial bearing on the prospective or present uses of the glue, and then to base his conclusions on a consideration of all the results together.”

Ordinary glue of good quality contains from 8 to

16 per cent. of water. Too much water indicates poor:

keeping quality, and of course means that the buyer is paying for that much water. Too little water may indicate an over-dried glue, which is injurious to its strength properties. Moisture as low as 5 per cent. is not uncommon, but a good glue should have not less than 8 per cent.

Moisture is determined by heating a ground sample at a temperature of 110° C. to a constant weight. The sample being hygroscopic, must be cooled in a desiccator.

After making the moisture determination, the same sample may be used for determining ash. It

*See Appendix for Forest Products Laboratory methods.

SINGLE TESTS ARE MISLEADING,

MOISTURE DETERMINATION.

ASH DETERMINATION

REACTION TO LITMUS.

INSOLUBLE.

GREASE.

WATER ABSORPTION.

should be incinerated carefully in platinum, or preferably in vitreosil, care being taken to prevent free ignition. The ash usually varies from 114 to 3 per cent. A fused ash indicates a bone glue, one not fused a hide glue. A large ash indicates adulteration, and a chemical examination will show what it is. Calcium phosphate is found in the ash of bone glues.

By its reaction to litmus a glue shows whether it is acid, alkaline or neutral. The test is made by dipping strips of red and blue litmus paper in a glue solution, and noting the color change. (The solution left after the viscosity test may be used.) An acid glue turns blue litmus red, and a neutral glue will not change the color of either red or blue litmus. Alkaline glues will turn red litmus blue. A slightly acid glue is preferable to a neutral or alkaline glue, because it is not quite so favorable a medium for the growth of the organisms which cause the decay of glue. Strong alkalinity usually means an over-limed glue, which will almost surely cause trouble.

To determine insoluble matter dissolve 30 grams of glue in one liter of hot water, and allow to stand for 12 hours in a warm place. Filter the hot solution through a weighed filter paper or good crucible, wash well with hot water, dry and weigh. In hide glues the insoluble matter is rarely over 2 per cent. In bone glues it may be more.

A little aniline color is added to the solution of glue in water (some of the solution remaining from the viscosity test may be used). The mixture is painted on unsized white paper and the appearance noted. Grease is visible in round, characteristic spots, and the relative amount present is indicated by comparison with other glues or the experience of the operator.*

The water absorption test was proposed by Schattermann in 1845. Glue does not dissolve in cold water, but has the property of absorbing several times its weight of water. Some glues will absorb eight to ten times their weight of water and still retain a rather firm consistency. Good glues will absorb at least six