Or\' ADHESIVES AND ADHESIVE ACTION1
0 BY J. W. MCBAIN AND D. G. HOPKINS
No systematic study of the action of adhesives appears to have been published, although various isolated suggestions have been put forward from time to time. In the endeavour to find a rational approach to the study of these phenomena and the general factors upon which they are dependent we have carried out many hundreds of qualitative and quantitative tests with the most diverse materials and adhesives. It is our experience that joints are of two kinds; namely, mechanical and specific. Mechanical joints are possible only with porous materials, whereas specific adhesion occurs with smooth non-porous surfaces whether these are vitreous, polished or tiue crystal planes. We find that a joint results between porous materials whenever any liquid material solidifies in situ to form a solid film embedded in the pores. The most importart and rather surFrising example of a purely mechanical joint is wood joined with gelatin or glue, where apparently even adsoiption does not occur. An excellent model of a mechanical joint is obtained by spot welding silver gauze to silver plates so that a porous surface is formed in which embedding can occur. Such surfaces are strongly joined by gelatin whereas smooth silver surfaces are joined but feebly. In mechanical joints the none of the bedding is often the chief source of weakness. I n the specific type of joint some species of interaction is involved between the smooth non-porous surface and the adhesive. It may be chemical or adsorption or mere wetting. A liquid which wets a surface and is then solidified possikly always makes a joint. The strength of many joints will of course be the resultant of both factors, mechanical and specific. The chief problem is to relate this specific factor to the general properties of the materials involved, and a suggestion in this direction will be made. A further factor into which enquiry is necessary is the nature and strength of the adhesive film itself, which has to transmit and withstand the stress. Another that was soon shown to be of importance is the effect of the solid materials in the formation of a coherent adhesive film. Finally, it is impossible to overemphasize that the method of employing an adhesive is very frequently as important as the properties of the adhesive itself. Unsuitable procedure with a really good adhesive easily results in the production of unsatisfactory joints. Qualitative Experiments A qualitative study of a great variety of glued and cemented joints was mzlde with the object of accumulating sufficient data to enable a satisfactory The present investigation was undertaken for the ,Idhesives Research Committee of the Department of Scientific and Industrial Research and the authors are indebted to the Department for permission to publish the results. A more detailed account will be published by that Committee in their forthcoming Second Report.
ADHESIVES AND ADHESIVE ACTION
189
explanation of adhesive action t o be put forward. These tests were made simply by hand. The surfaces of the various materials were in most cases roughly plane but were thoroughly clean. In many instances the time allowed for the drying of the adhesive was insufficient. In addition, cemented areas varied considerably fioin material to material so that it was difficult t o make an appropriate allowance, Notwithstanding these limitations the results undoubtedly convey some idea of the strength of a joint made under ordinary conditions. The following adhesives and materials were used in this series of qualitative tests. Adhesives. Canada balsam, casein and borax cement, commercial glue B, fish glue, a high grade commercial gelatin, commercial starch paste, glycerine cement, gum arabic, commercial nitrocellulose cement A. nitrocotton (alcoholether sol), shellac (ordinary and wax free), silicates of soda, starch. Mwterials. Aluminum, calcite, charcoal, copper, ebonite, gas carbon, glass, lead, mica, nickel, porcelain (unglazed), pumice, rubber, silica (fused), steel, tin, wood. The first result was to show that the weakness of many joints is due to the inability of the adhesive to dry between two non-porous surfaces. Subsequent quantitative experiments have shown that gelatin and many other adhesives between closely fitting discs of metal 0.75” in diameter require as much as three weeks for thorough drying and even then there is the possibility that the maximum strength had not been attained. Drying was effected in numerous instances by the device of employing a “mixed” joint of the type material-adhesive-wood. As a rule the adhesive-wood linkage does not yield first so that the strength of the “mixed” joint depends on the degree of adhekion between the material and the adhesive, assuming of course that the adhesive film is sufFiciently strong in itself to withstand the applied stress.
Observations on the Experimental Results Porous bodies such as wood, unglazed porcelain, pumice, charcoal, etc., yield strong joints with the majority of recognized adhesives. It is obvious that a good joint must iesult whenever a strong continuous film of partially embedded adhesive is formed in situ. Formation of such joints would appear in many cases to be independent of adhesive action, and should then be a function of the mechanical strength of the embedded film. The failure of a few adhesives such as starch paste to unite certain porous materials was seen to be due to the complete soaking away of the adhesive or to the farmation of excessively thin films incapable of filling the gap between the surfaces. Investigations into the thickness and strength of adhesive films showed that the thickness of a dried coating of starch paste was less than one thousandth of an inch (cf. footnote to Table 111). Further corroboration of the mechanical explanation of glued wooden joints was found when comparing stained and unstained wood joined with gelatin. Three successive coatings of Stephens’ Ebony Stain were applied. With
190
J. IT’. M c B h I N 4 X D D. G. HOPKINS
the soft wood (deal) neither the stained nor the unstained joints could be broken by hand but with hard wood (mahogany) the stained joints easily parted. Calcite with gelatin is an illustration of a strong joint with a true crystal surface, and may serve as a model of true specific action. Under favorable conditions of drying most adhesives form strong joints with polished glass and fused silica. This is probably not unconnected with the fact that we are here dealing with surfaces which are very nearly plane. These joints with vitreous surfaces cannot possible be due to the purely mechanical action operative in the case of porous bodies. There must be specific interaction between the adhesive and the material whether mere wetting, adsorption or chemical combination. The results with metals were of interest in that they showed that good adhesives may be rendered ineffective unless appropriate conditions are ensured. If the films incompletely bridge the gap between the surfaces or if the adhesive dries imperfectly then no strong joint is possible. It will be shown later that extremely strong metal-adhesive joints are obtainable under suitable conditions. I n order to explain the action of lubricants W. B. Hardy assumes that the unsaturated molecules of the lubricant unite physico-chemically with the metallic surfaces to form a composite interface of oil and metal, or that the lubricant is adsorbed by the metal. Hence the interaction is truly specific. If, therefore, the fluid film of lubricant can be rendered tenacious by subsequently treating the joint with suitable refrigerants or by other methods a satisfactory joint should result. Experiments carried out with (i) vaseline between steel and fused silica surfaces (ii) oleic acid between steel surfaces, showed that strong joints were obtained if the lubricants were cooled in situ with solid carbon dioxide. It was also found possible to produce strong joints with water (frozen) and sulphur (molten and subsequently cooled) between fused silica and glass plates. The above results are of special interest in that whilst they support Hardy’s theory of specific interaction they furnish examples of adhesives of definite chemical composition. Quantitative Tests of Various Joints The quantitative tests comprise four series; namely:(a) Shear tests with walnut surfaces. (b) Shear tests with various smooth metal surfaces. (c) Tension tests with various metal surfaces. , (d) Tension tests with various materials mounted on metal pieces. The chief sources of uncertainty to be guarded against are incomplete drying, incomplete (discontinuous) films, varying thickness of adhesive layer, and bubbles. The latter must be excluded as far as possible by sliding or rubbing the coated surfaces together under pressure. Certain adhesives never dried except with porous bodies and many took weeks to dry. a) Shear Tests with Walnut Surfaces. Very carefully prepared walnut test pieces (2S’’X 2 ” X X’’) were supplied by the Royal Aircraft Establishment, Farnborough. They were of a type used
ADHESIVES A S D -4DHESIVE ACTION
191
and recommended by Professor Andrew Robertson’, to whom our thanksare due for his constant advice and the invaluable facilities afforded us in the preparation of test pieces and the testing of joints. The walnut test pieces had vertical faces parallel to the grain as found by splitting, and whole series were made from a single plank. The top and bottom surfaces ( 2 ” X X’’) are accurately at right angles to the other faces. With these test pieces, joints of the type indicated in Fig. 1 were set up. The adhesive was applied in every instance io the vertical 2 ” X 2%” surface Each of the opposing surfaces was coated with a film of the STEE adhesive applied with the finger, and the surfaces were then brought together under hand pressure. (In certain few cases the STEEL BLOCK instructions given by the manufacturers necessitated a slightly different mode of application of the adhesive), The resulting joint was placed in a pressure device the total pressure applied to each joint being 28 lbs., or about 7 Ibs. per sq. in. The joints were kept under this pressure until thoroughly dry. The end surfaces of the joint to be PLANE STEEL PLATE tested rest on a plane steel plate. The load on the projecting end surface is FIG.I gradually increased until the joint gives Form of Test Piece for Shear Tests way. The break usually takes place on Between Kooden Surfaces. one side only. The strength of the joint is obtained by dividing the load applied, by the total area of contact between the surfaces. Several instructive test pieces have been found where occasionally a joint has not broken under a pressure of 4 or 5 tons but where instead the upper part of the middle wooden piece has flowed in such a manner as to show that the breaking of the joint with such a strong adhesive as glue is not due to shear but to the local stresses set up by the bulging of the central piece. Kevertheless, this appears to be the best form of test piece for all weaker adhesives. b ) Shear tests-Adhesive between .Metal Surfaces. The metal pieces employed in these tests are cylindrical in form, being 2 in length and %’/in diameter, The plane surfaces are accurately at right angles to the length of the test-pieces. When the joint has been set up and has dried thoroughly it is inserted (very tight fit) in a specially constructed device which is then screwed into the axial loading grips. By a proper adjustment of the joint, a sheaiing force can be applied across the adhering surfaces in a plane perpendicular to the common axis of the test pieces forming the joint.
I
i
I
“Report on Materials of Construction used in Aircraft and Aircraft Engines”, p. 135 (I~zo).By Lt. Col. C. F. Jenkin.
192
J. 1%'. McBAIiX A N D I). G. HOPXINS
c ) Tenston Tests with Varzous Metal Surfaces. The metal test pieces used in these tests are of the form indicated in Fig. 2. The tin and lead test pieces differ from the otheis, the diameter of the plane surface being 0.750") giving a superficial area of 0.442 sq. in., whereas in all the other test pieces the diameter is 0.846" with an area of 0.562" sq. in. The plane surface is perpendicular to the axis of the screw. All the test pieces were prepared in the same way. After being turned in the lathe the remaining slight irregularities were removed on the facing stone. All traces of grease and grit were subsequently got rid of by washing o a!+(" -, the surfaces successively with methylated spirits, and freshlyredistilled mixture of alcohol and ether.
-
d ) Tenston Tests with Various Muterials mou?ated on
M e t a l Pieces. I n the pieliininary experiments, coinpound joints were set up by sandwiching a strip of walnut with parallel faces between the two metal test pieces with FIG.2 the object of accelerating drying. It was assumed that I'orm of Metalllc Test the joint would yield first at themetal-adhesive junction Piece for TensionTests. but the walnut-adhesive linkage was often the weakest of the system. I n subsequent experiments on metal joints the walnut strip was therefore omitted and the metal surfaces were joined directly. The adhesive was applied to each of the surfaces and the two test pieces mere pressed together with a slight sliding motion in order to squeeze out the surplus cement and air bubbles. With certain cements, the surfaces were coated and allowed to dry before joining with further adhesive. All joints were made only under hand pressure. The adhesive was allowed to dry or set thoroughly before testing. The joints were tested in a specially adapted Denison Testing Machine; axial loading grips being employed. A gradually increasing load was applied at approximately the same rate with all the joints; viz, 30 lbs. per sq. in. per see. Quantitative adhesion tests with metals necessarily precede all other tests because the metal test pieces can readily be adapted to enable us to determine the strength of various other joints, e.g. glass-adhesive, if suitable adhesives for metals are known. In order to find the strength of a material-adhesive joint two types of joints may be set up, vie., I ) Metal-Adhesive-Material-Adhesive-Metal, Le., a piece of material such as glass, ebonite, etc., with parallel surfaces is cemented to the metal surfaces with the adhesive to be tested. 2 ) Metal-Cement-Material-Adhesive-~~aterial-Cement-~~etal, i.e., a piece of material is cemented to each of two metal surfaces with a suitable stronger adhesive for metal and material and the adhesive to be tested is then applied in the usual way to each of the material surfaces. It is obvious that this method can only succeed with those adhesives which form a linkage with the glass or other intermediate material weaker than the
I93
ADHESIVES AND ADHESIVE ACTION
linkages of the system. As the result of a few preliminary experiments, the second type of joint mas found to be more satisfactory. In nearly all cases joints were made at least in quadruplicate and in the summarised data the “representative value” is usually the mean of the two or three highest values rounded off to the nearest hundred pounds per sq. in.’ This gives a measure of the strength obtainable under favourable conditions. The materials used in making joints were chemically pure tin, lead, nickel, best commercial copper and aluminum, brass, cast iron and niild steel, oxidised and amalgamated copper, walnut, ebonite, crystals of calcite, mica, fused silica and glass, etc. The adhesives used were (a) gelatin, highly purified by Professor Schryver and also a high grade commercial gelatine, also a high grade commercial glue A (b) fish glue (c) commercial glue B (d) a liquid commercial glue C (e) gum arabic (f) casein and borax cement (gj casein and silicate cement (hj sodium silicates of different molecular ratios (i) commercial nitrocellulose cement h ( j ) starch paste (k)shellac (ordinary and wax-free, molten and alcoholic) (1) an American commercial cement B (of shellac basis) in three grades (in) various shellac creosote cements (n) a commercial wax (used for high vacuum work) io) commercial cement C (a metallic jointing compound,(p) a marine glue. In the shear and tension tests with the metals or with various materials niounted on the metal surfaces the method of “double coating” with the adhesive was allowed to dry for about 24 hours before applying a second coating when bringing the opposite surfaces together. This procedure yields a much stronger joint. TABLE I Tension and Shear Tests Shellac Creosote Cements of Various Composition* between Metal Surfaces. Representative values are given in Ibs./sq. in.--Period of Setting I to 4 days. The compositions are given in parts by weight. ‘
Cement
Shellac, brown Shellac, orange Creosote Ammonia (0.880) Turpentine Terpineol Forinanilide
I
I1
TI1
50 5 0.5
50
50
5
3
5
0. j
4400
SHEAR
3400
4900 4800
-
VI
-
SO
2
TENSIO?U’
T
-
50
5
IV
50 5 1
2
5
-
4000
3300
4000
4600
2500
4300
2
4400 3400
*Beechwood creosote mas used in compoiniding each cement. In cements V and VI terpineol was employed instead of turpentine as it ifi a solvent for shellac whereas the turpentine used in I1 is not. The adhesive was applied in exactly the same way as molten shellac. 1
T o convert these to kilos per sq. cm. divide by
14.22.
J . W . McBAIN 9 N D D . G. HOPKIXS
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O N a C C N r i
ADHESIVES AND ADHESIVE .4CTIOlY
TABLE 111 Shear Tests-Adhesive between Walnut Surfaces Representative values given in lbs./sq. in. Period of drying. (days)
Adhesive
Fish glue Liquid commercial glue C A high grade commercial gelatin Gelatin, highly purified by Schryver Fish glue, with added bone gelatin Casein and borax cement Gum arabic Commercial glue B French Adhesive (silicate basis) Casein and silicate cement Commercial nitrocellulose cement A Starch Silicates of soda Grade D molar ratio 2.0 Grade K molar ratio 2.45 Grade C ” ” 2.9 Grade J ” ” 3.0 Grade A ” ” 3 Experimental I molar ratio 4.08 Experimental I1 ” ” 4.2 9
3
Strength of joint. Lbs./sq. in.)
7 7 6 6 7
I 400
6 6
600
I2
400
8 8 30**
400 300
9***
i200
1200* I IO0
700 400
300
300
28 28
28 28 28
15 15
*This value is reduced to about one third if the joints be heated to 100’ for four days when securely clamped t o prevent distortion. **This adhesive dries very slowly between walnut surfaces. I t is a matter of some difficulty to set up wood joints with it on account of its quick setting capacity when exposed to the atmosphere. ***Three coatings instead of one raises the strength of joint to 1600 although the film is still incomplete. S.M. Seale, “Shirley Institute Memoirs” 3,207,(1924),has shown that’ the tensile strength of a dried starch film may reach 9000 lbs. per sq. in. of cross section.
J. W. McBAIN AND D. G. HOPKINS
196
TABLE IT’ Tension Tests-Adhesive between Glass Surfaces Representative values are given except where otherwise stated. Maximum values are recorded where the results have been so discordant that a representative value could not be fixed. Period of drying (days)
Adhesive
Commercial nitrocellulose cement A A high grade commercial g e l a h Fish glue Fish glue, with added bone gelatin Alcoholic shellac (wax free) Silicates of Soda Grade D Molar ratio 2.0 Grade K ” ” 2.45 Grade C ’’ 2.9 Grade J ’’ 3.0 Grade A ” 3.3 Experimental saniple molar ratio 4.08 “Hard” American commercial cement B
Single coating of adhesive. Ibs.,’sq. in.
1000
19 I9
250
i max.
22
500
22
499 max.
21
2 coatings of adhesive lbs./sq. in.
m
T IOO
max.
*
m* 300
)’
)’
)’
M
2 00
1000
500
600
800
600
800
567 (max.)
600
3 700
i-film incomplete. m-film moist. *-no teste possible as the joints were too weak to withstand ordinary handling.
TABLE V Miscellaneous Tension Tests
(I)
(2)
(3)
Time of drying. (dags)
Representative value. lbs./sq. in.
Joint
Adhesive
Glass cover slip between two metal test pieces.
A commercial nitrocellulose cement
I8
IO00
French Adhesive
22
I100
Ebonite plate between two metal test pieces.
A commercial nitrocellulose cement
18
800
Gum arabic 63 Commercial glue B 63 Liquid commercial glue C 63
1000
Calcite crystal between metal test pieces *One test only.
A commercial nitrocellulose cement
20
400
800 200*
ADHESIVES AND ADHESIVE ACTIOK
=97
Discussion of the Results of the Quantitative Tests The results of the preceding tests suffice to give a reasonably clear notion of the strength of a great variety of joints in both tension and shear. An examination of the individual results from which the representative values of the summarised data are computed demonstrates that a number of factors were not sufficiently controlled and that improved manipulative procedure in certain might result in the production of even stronger joints. Perhaps that most striking result is the remarkable strength of some of the joints set up with shellac cements between smooth metal surfaces. h few such joints have attained a strength of 2% tons per sq. in. both in tension and shear whereas the best glued wood joints rarely exceed 1 2 0 0 lbs. per sq. in. In many instances the strength of the joints was limited by the flow of the adhesive, as for example, the commercial wax and the commerical nitrocellulose cement A between metal surfaces. In other cases, such as tin, lead, and mica, failure occurred far below the strength of the adhesive or its attachment on account of the yielding of the metals themselves. A few preliminary experiments have proved that film thickness is a most important factor in adhesion phenomena. Crow1, in a systematic study of soft soldered joints, has shown that by making the film of solder very thin the strength of the resulting joints may be as great as I I tons per sq. in., the tensile strength of the solder being exceeded several fold. However, such joints were apparently not strengthened very appreciably when the thickness was diminished from 0.2 mm. to the thinnest attainable. We have had occasion to note the importance of this factor and a precise investigation is now in progress. That thin films are very much stronger than an adhesive en masse is established by Crow’s result for soft solder and our much more striking instance of wax-free shellac. Joints made with this shellac, which is quite soft and pliable, actually withstood a pull of nearly 4000 lbs. per sq. in. when a thin film was used between nickel surfaces. It may be summarized that the thinnest films yield the strongest joints provided, of course, that complete contact is maintained with the surfaces. Using a shellac-creosote cement between surfaces of mild steel and taking no precautions to secure very thin films the maximum strength of joint attainable was just over 3000 lbs. per sq. in. in tension. When care was taken to ensure that the adhesive films were as thin as possible the joints could withstand a pull of 5000 lbs. per sq. in., the difference of 2000 lbs. per sq. in. being apparently due to decreased thickness of the film, although the precautions taken in obtaining excessively thin films result in a more rigorous exclusion of air bubbles and consequently in a greater completeness of film. Reference may conveniently be made a t this point to the opinim expressed by Crow that in soldered joints it is essential that diffusion (alloying) should take place. The whole weight of the evidence brought forward in the present inquiry runs contrary to this conception for there would appear to be no valid J. SOC.Chem. Ind. 43, 6 j , (1924).
J . W. McBAIN AND D. G. HOPKISS
198
reason for assuming any fundamental difference between soldered and glued or cemented joints. Shear test, carried out with a large number of adhesives between walnut surfaces, show that gelatin preparations are far in advance of other adhesives and a high grade gelatin is best of all. It is very interesting to note that highly purified ash-free gelatin is little, if at all, inferior to a first grade commercial gelatin or glue. I n those wood joints we regard the adhesive film as embedded in the pores and surface irregularities. It is somewhat significant, however, that their strength is only a small fraction - a quarter of a sixth, possibly - of the tensile strength of the adhesive itself. Experiments to be described later show the tensile strength of a high grade commercial gelatin may greatly exceed 7000 lbs. per sq. in. Of course, with glued joints failure almost invariably takes place in the wood itself, or in the layer where the glue film is weakened by the embedding in the mood. With less tenacious adhesives such as gum arabic, silicate of soda, etc., where the failure occurs in the adhesive, the strengths of wood joints ought to run parallel with the tensile strengths of the adhesives; and determinations of these are in progress. The results obtained with silicates of soda of different molecular ratios (mols SiOe/mols NazO) between *;‘ walnut surfaces are noteworthy. Table I11 and Fig. 3 show that the FIG.3 ShearTests. Silicate of Soda of Various Corn- so-called “neutral” silicate of composipositions between Walnut Surfaces tion Naz0.3Si02 exhibits maximum (Maximum Values Plotted) tenacity, that a higher proportion of SiOz rapidly diminishes this whereas the addition of alkali causes a less marked reduction. A similar maximum is observed with the silicates between glass surfaces although not nearly so pronounced since more acid and more alkaline silicates still yield strong joints. The most alkaline silicates dry only with difficulty. When the joints between metal surfaces are set up with gelatin, commercial nitrocellulose cement A, etc., merely by bringing together the surfaces with a single coating of adhesive the results almost invariably fluctuate considerably. If,however, the first thin coating of adhesiveis allowed to dry thoroughly before applying the second coating just previous to bringing the surfaces together the results are generally higher and more concordant than those obtained in the simple mode of application of the adhesive. The probable explanation is that the method of “double coating” gives rise to more complete adhesive films. It is probable that better results would be obtained with all adhesives if this modified procedure were adopted. L700
100
I $
2 0
2 5
ADHESITES A h D ADHESIVE ACTIOh-
199
Experiments carried out with the object of comparing the strength of joints made between pairs of surfaces of different roughness prove conclusively that the degree of roughness exerts little influence on the strength of joints, provided that the roughing process has been carried out without pitting the surface appreciably If, however, the metal surfaces are rather deeply furrowed in the process of employing a round nosed roughing tool, then the joints are decidedly weaker than those obtained with smooth surfaces, the lower results presumably being attributable to (i) an increase jn the average thickness of the adhesive film (ii) the presence of insufficient adhesive completely to bridge the gap between the two surfaces. Relation of Strength of Joints to Materials The qualitative tests show with certainty that the strength of a joint with any one adhesive depends upon the nature of the solid materials joined, even in the entire asbence of chemical action. It is very desirable to identify the properties of the solid materials which are parallel with the strength of joint although we can as yet do this only in a very tentative manner. For the present purpose it is necessary to omit from consideration any adhesives which react chemically with the surfaces or which induce corrosion, so that only a few isolated data remain besides the series tests with molten shellac and commercial cement B. With these cements the adhesive films generally broke away cleanly froin the metal showing that true adhesion was being measured. In these cases there is recognisable a manifest parallelism between the strength of joint and the atomic volume, compressibility, tensile strength, elastic limit, elasticity and even the hardness of the metals concerned. Table VI, giving data mostly from Landolt-Bornstein Tabellen, furnishes a comparison between those related properties and the strength of joints. The results with very many adhesives such as fish-glue, gelatin, liquid commercial glue C, gum arabic, etc., are omitted because of the corrosion that occurred. The same parallelism does not occur with these adhesives but this does not necessarily deprive the generalisation of its validity. The higher the values of the atomic volume and compressibility the weaker the joint, and the higher the tensile strength, elastic limit and elasticity (or even hardness), the stronger the joint. These results show that joints with nickel, iron and copper are of practically the same strength, those with aluminum weaker, tin and lead joints being much weaker still. With tin and lead quite appreciable distortion of the metal occurred in the shear tests. Nickel, tin and ebonite joints made with commercial nitrocellulose cement A and glass with American commercial cement B (3700 Ibs. per sq. in.) give results which might have been anticipated from our generalization. It is also in line with this hypothesis that mica formsveryweak joints parallel to its cleavage plane and other favourable evidence may also be found in the tests we have made. If the strength of a joint with a given adhesive depends on the tensile strength of the metal it may be that the strength of a joint with a given metal similarly depends on the tensile strength of the adhesive film itself and the necessary data are now being sought.
200
J. W. McBAIK A N D D. G. HOPKINS
ADHESIVES AND ADHESIVE ACTION
201
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M c B A I K AND D. G . HOPKINS
The tensile strength of films of a number of adhesives, such as gelatin and glue is found to be many times greater than that of the strongest wooden joints made from them. Adsorption and Adhesion According to the preceding inquiry both smooth and porous materials yield strong joints. I n the former case Bancroft would postulate adsorption, whereas in the latter neither adsorption nor chemical interaction is requisite if the joint is due solely to the mechanical embedding of the film. The adsorption experiments about to be described were carried out both with porous and with smooth impervious materials. Change in the concentration of an adhesive solution when an insoluble and unreactive material is introduced into it would be ascribed to the adsorption of solvent and adhesive in relative quantities differing from those in which they are present in the original solution. If the adhesive only is adsorbed from the solution then the residual solution is less concentrated than the original. If, on the other hand, both the solvent and the adhesive are adsorbed, then there might be an increase or decrease in concentration according to the relative quantities adsorbed. Thence, in order to ascertain whether the adhesive is adsorbed two distinct determinations have in general to be made, viz., (a) The amount of solvent the material is capable of adsorbing. (bj The change of concentration of the adhesive solution brought about by t)he adsorptive action of the material of the same quality and in the same physical condition as used in (a). The method adopted to determine the amount of water adsorbed by the various materials (filter paper, fused silica fibres, etc.j was a modification of that developed in this laboratory by Bakr and King1, (since superseded by the sorption balance of McBain and Bakr2). The effect of bringing various materials into contact with the adhesive solution was determined in the most straight forward case by placing a known weight of material (thoroughly washed and dried) in a known volume of the adhesive solution where it was allowed to remain until equilibrium was established. The change of concentiation was deteimined by means of a Zeiss Interferometer, measuring the refractive index of the solution to eight significant figures. A blank experiment is carried out with the solvent in place of the adhesive sol in order to determine the effect of possible traces of soluble impurities in the material upon the apparent concentration of the solution. Allowance is made in all calculations for this very small source of erroi, although in most cases it is quite negligible. I n the experiments with filter paper the presence of very small amounts of peptised paper would cause an increase in the refractive index of the adhesive solution and this would be sufficient to render the results valueless, for the observed changes of concentration are exceedingly small. In order to eliminate this source of error the filter paper is taken in the form of extraction thimbles, so that, after remaining in contact with the adhesive solution it is possible to filter the residual solution J. Chem. SOC.,119, 454 (1921) J. Am. Chem. SOC.,46, 2781 (1924)
ADHESIVES AXD SDHFEIVE A C T I O 3
203
through it. When dealing with wood-meal and certain other materials such as viscose such a direct procedure is not possible. In these instances an additional extraction thimble is employed and allowed for. The results of the experiments are summarized in Table T'II. Note that there is no evidence of adsorption of the various adhesives by the porous materials employed. Positive although slight adsorption of gelatin by fused silica fibres is shown to occur in accordance with expectation for smooth surfaces exhibiting adhesion, but with gum arabic the indications were within the limits of experimental error. Effect of Heat on the Adhesive Power of Glue and Gelatin Solutions There seems to be a general impression based upon observations of viscosity and jelly strength that the heating of glue or gelatin even for periods of a few hours brings about a marked decrease in the adhesive powers. To test the truth of this popular idea aqueous solutions of a high grade commercial gelatin (16%) and a high grade commercial glue A ( 2 sVc and 4 0 y ) were maintained at various temperatures (60'~ So', IOO', 130OC) in a thermostat,evaporationbeing prevented by using carefully ground glass stoppered bottles a t 6ooC and sealed tubes a t the higher temperatures. At vaiioiis intervals three joints were set up with portiocs of the adhesive. Two forms of walnut joint were employed the usual type already described and another form of joint known as the double cover plate type. The maximum values obtained for each set are given in Table VI11 in lbs. per sq. in. except for the double cover plate joints wheie the total load is given. The results prove quite conclusively that whatever change or degradation takes place at 60' or 80'c does not effect the strength of the wcod joints as measured by the present methods even aftei heating has proceeded for a month at the lower, and five days at the higher temperature. Summary Qualitative and quantitative examination of a large variety of adhesives and materials have led to general conclusions expressed in the opening paragraphs of this communication. Further specific points are as follows: (a) Although gelatin and glue adhesives including highly purified ash-free gelatine are by far the strongest for wooden joints, theie is no evidence of any specific interaction such as adsorption with wood, filter paper or viscose. Heating to 60' for a month does not diminish the adhesive power of glue although on heating to 130' for one day its adhesive power is almost completely lost. Gelatin is adsorbed by fused silica and unites smooth silica surfaces. Other sorption experiments have been carried out with sodium silicate and gum arabic. (b) Silicate of soda used between walnut surfaces exhibits a sharp maximum strength a t the composition Sa20.3Si02. This maximum is by no means so pronounced with glass surfaces but occurs at approximately the same composition.
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J. W. McBAIN AXD D. G . HOPKINS
(c) Most adhesives join smooth metallic surfaces, and even soft pliable shellac yields joints withstanding two tons to the square inch. There would appear to be a certain parallelism between the strength of joints between metals and the mechanical properties (such as tensile strength, compressibility, atomic volume, etc.) of the metals themselves. Department of Physical Chemistry, Uniziersity of Bristol, England.
Nociember , 2984.
