ADHESIVES AND ADHESION: GUMS, RESINS AND WAXES BETWEEN POLISHED METAL SURFACES* BY JAMES W. McBAIN AND W. B. LEE
It is of interest to collate some of the definite regularities which are emerging from our investigations on the laws of adhesion.’ True adhesion to smooth, polished or crystalline surfaces, where the adhesion is necessarily specific, is the most interesting type. This is exhibited not only by the common recognised adhesives, but by pure crystalline chemical substances, and the magnitude of the effect may amount to one or more tons per square inch. Such joints often exceed by several fold the tensile strength of the adhesive itself. It is illuminating that there is an unmistakable although imperfect parallelism between the strength of such joints and the intrinsic properties of the materials joined and those even of the adhesive itself. Such properties include internal pressure, tensile strength, elasticity, hardness, and conversely compressibility and atomic volume. Probably these regularities extend to the adhesives themselves where these are homogeneous; but adhesives involving a solvent, such as glues and silicates, are not directly comparable with such others as shellac or tri-nitro-toluene. The thinner the layer of adhesive the stronger the joint; the strength is increasing very rapidly when the adhesive layer is reduced to a millionth of an inch in thickness. It is evident that liquids are modified and immobilised in the immediate neighborhood of solid surfaces and that either the range of molecular attraction approaches the magnitude named, or that more probably not only are the molecules which actually touch a surface oriented, but that through a sort of chain effect (or in certain cases micellar linkage) this influence extends rather deeply into the body of the liquid or subsequently solidified liquid.2 The effect of such orientation and the resulting disparity of properties with change of direction is illustrated by the antibatic relation between lubricants and adhesives and by the similarity between the strength of joints in tension and in shear, It appears that disorderly arrangement of molecules results .in maximum strength, and that there is a close connection between adhesion and the architecture of the molecule. *Investigation carried out in the Department of Physical Chemistg,, University of Bristol for the Adhesives Research Committee of the Department of cientific and Industrial Research, Great Britain. Published by permiasion of the Department of Scientific and ,Industrial Research. Our thanks are due to Principal Andrew Robertson for the facilities placed a t our disposal in the Faculty of Engineering, University of Bristol. ‘For previous pa era see McBain and Hotkina: J. Phys. Chem., 29, 188-204(1925); 30, 114-125 (1926); g c o n d Report of the Ad eswes Research Committee, A endix IV, 34-89 ( I 26)’MoBain and Lee: Proc. Roy. SOC.,l l 3 A , 606-620 (1927); J. SOC.s t e m Ind., (1927), fnd. kng. Chem. Sept. (1917). * I n a separate communication by McBain and G. P. Davies (J. Am. Chem. Soc., 49, Sept. (1927)) absolute measurements of adsorption in the air-water interfaceareadduced and this conception elaborated as a result. For example, it explains the rigidity of liqujds in which inert particles are suspended a8 shown by the coefficient of viscoslty varying wlth rate of shear, a phenomenon stressed by Hatschek, for which he has been a t a loss to account.
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I675
Other regularities which have been found are not so directly connected with the subject matter of the present paper because they relate to mechanical joints, to the great practical importance of mechanical properties other than tensile strength, and to the effect of humidity upon the tensile strengths of joints using aqueous adhesives such as glue. The present paper adduces some preliminary measurements to show the way in which joint strength increases with thinness of adhesive layer over a wide range, especially when the film is the thinnest possible. I n addition data are given with regard to the strength of joints between metals using such adhesives as waxes, shellacs, gums and resins which are of increasing importance owing to the development of the lacquer industry. I n nearly all cases the adhesive was employed in the molten state, the temperature of the metal test-pieces being only just above that of the molten adhesive. The test-pieces were generally of the same form as those used by McBain and Hopkins (1.c.); but, in a few experiments, test-pieces of larger area were prepared with a view of minimising the influence of experimental errors on the final results. In the experiments with optically polished surfaces, test-pieces of somewhat different form and dimensions were used. After the joints had been made, (taking care to keep the test-piece axial) and allowed to set, they were tested in a specially adapted Denison testing-machine. Determinations have been made both in tension and in shear. Relation of Joint Strength to Thinness of Layer of Adhesive I n the previous communications many isolated data have indicated that the thinnest layers of adhesives give the strongest joints and this is confirmed by the preliminary study below. The best joints are obtained where the layer of adhesive is as thin as possible provided always that after drying or setting they are perfectly continuous and completely in contact with both the surfaces to be united. Where solvents are employed it is often necessary to use double or triple coatings in order to fulfill these conditions. In the present work however no solvents were involved. Experzment 1 . Alumznium and a proprietary shellac adhesave. A cylindrical stick of the cement of the same diameter as the test pieces was cut into slices of difTerent thicknesses for use in coniparative experiments. Each slice was allowed to stick to a hot metal test piece and when this had cooled the other half of the test piece was heated and applied. With all but the thinnest slices the central adhesive layer was left unmelted. The results are collected in Table I. I t is a difficult matter to examine where rupture begins; but as far as these and many other tension tests are concerned, an inspection of the fractured joints nearly always reveals a final breakdown of both adhesive and adhesion: and of these failure of the adhesive itself seems to predominate. Where failure of adhesion preponderated the explanation indicated by ad hoc experiments was that the temperature of the test pieces had been somewhat lower than usual during the preparation of the joint. I t has to be remembered that the adhesive here used is not a substance of definite melting point, and
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JAMES
W. M c B A I N A N D W. B. L E E
TABLE I Relation of joint strength to thickness Tension tests : aluminium and a commercial shellac adhesive Thickness of adhesive (ins.)
Joint strength (Ibs./sq. in.)
Usual*
2250
Time of setting (days)
Several days.
0.04
750
6
0 05
500
I
0.08 0.13 0.17
350 400
8 8
350
IO
Rate of loading (Ibs./sq. in./sec.)
Less than 38 25 25 21 (25)
21
*The adhesive film melted completely and the metal pieces were pressed together by hand.
that unless the metal test-pieces are hot enough no strong joint is possible between metal and shellac. Experiment 2. Nickel and the same shellac adhesive. In this experiment the whole of the adhesive was kept molten during the preparation of the joint, being kept in place by a rubber tube surrounding the test-pieces. The rubber is readily removed from the shellac. The average thickness of adhesive layer was measured roughly by means of a micrometer microscope and is recorded in Table I1 in micrometer divisions, each of which is approximately O.OOOIZ inches. Failure of the adhesive itself seemed to predominate as in the first experiment, but some failure of adhesion even though slight, seemed always to occur. Out of nine tests (Table 2) with nickel surfaces, only one failure was
TABLE I1 Relation of joint strength to thickness Tension tests: nickel and the same shellac adhesive as for Table I pmP
C
Thickness of adhesive (arbitrary units)
Extremely thin * ,J
21
17
,,
I7 21 17
Jl
JJ
Joint strength (lbs./sq. in.)
3600
60
3000
60 60 30 (30)
30
2430
90
2150
IO0
2030
IO0
I950
150
1550
400
I075 1000
400
Rate of loading (Ibs./sq. in./sec.)
33 50 54 50
'Optically polished test-pieces, which had been used more than once.
mostly in adhesion, and this was probably due to a slightly lower temperature of the upper half of the test-piece when preparing the joint. The general form of the curve showing the relation between joint strength and thinness of adhesive layer is clearly indicated in Fig. I .
hDHEYIYES .ISD ADHESIOS
16; j
I n a previous paper' it was mentioned that a curve of siniilar foriii for the joint strength of polished metal surfaces held together by film of liquid down to only a few hundred molecules thick had been obtained in the Sational Physical Laboratory. Crow? obtained a strikingly similar curve for various thicknesses of lead-tin solder between copper surfaces. It is noteworthy that the greatest dependence of joint strength upon thinness of adhesive layer is Ehown in the very thinnest layers. For example in measurements at the Sational Physical Laboratory the greatest effect \vas shown in layers less than one millionth of an inch thick. This gives rise t o tvio important practical
FIG.I Relation between joint strength and thinness of laver of adhesive
difficulties. In the first place, the effect is largest, and of the greatest throretical interest, just in the regions where the practical difficulties of controlling and measuring the thinness of the film are greatest. In the second place it follows that if serious fluctuations are to be climinatcd the thickness of adhesive layer must always be measured and taken into account. This has not been done either in our previous papers or in the general comparison of the adhesives studied in the second half of this paper, although a beginning has been made in subsequent work by designing a test piece in which such precision measurements have been shown to be possible. K i t h thick filins: that is those more than a tenth of an inch thick, the effect of thickness seeins to become unimportant. It is seen in Fig. I that for each filin of definite thickProc. Roy. Soc., 113 A, 617 (1927). J. SOC.Chem. Ind., 43, 65-70 (1924).
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JA.MES W. M c B A I S A S D W. B. LEE
ness between nickel and aluminium surfaces respectively the joint strength is always greater with the nickel than with the aluminium. Experiment 3. Rubber solution between surfaces of mild steel. Confirmatory experiments have incidentally been obtained when showing that double and multiple coating is not advantageous in any “specific joint” between smooth surfaces provided that the first coating completely fills the space between the metal surfaces. Rubber solution was applied in successive coatings to steel surfaces and each coating allowed to dry before uniting t,he surfaces with a final coating. When the number of such coatings in four series of experiments was 2, 3, 5 and 8 respectively the mean of the two highest values for the joint strength in tension in each case was 300, 283, 160 and 115 lbs./sq. in. Experiment -4. Sealing wax and J a p a n w a x between optically polished steel surfaces. In experiments with steel surfaces optically polished at the National
TABLE I11 Joint strengths in tension (maximum values found, lbs./sq. in.) using adhesives between metal surfaces Adhesive
Sickel Steel Cast, iron Copper Brass Aluminum Tin
Shellac creosote’ 4000 Sealing wax 3150 Marine glue Ij50 Pitch 1540 Phenol formaldehyde3 1300 (dried 6 days) Phenol formaldehyde 1900 (dried 14 days)4 Phenol formaldehyde 800 (dried 5 days) Coumarone resin 1600 Ester gum Guaiacum resin 1350 Gum Dammar 1050 Carnauba wax Japan wax Paraffin wax ,, ’I (set 5 days) I,
”
C26H64(set
(
2-3
”
6
”
days)*
1’
4600
4000
3150
3000
1600 (450)
1650
1850
1350
32502
(200) 2500
Iojo 1300
1300 1800
1400
1700 2400
2200
I200
1200
2300
(500)
I500
I500
900
700
Lead IO00
500
1 9 5 0 ~1700 1600 1600 850 1450 1 6 5 0 ~ 1 2 0 0
950
950
850
600 680 460
880 540
430
400
400
I 400
(500) 900
0800
950
800
( 1300)
900 1950
490 300
450
250
320
jjo
500
450
300
450
400
420
370
370
250
Supplied by Sir H. Jackson and containing the following parts by weight: hard shellac 50, creosote 5 , ammonia 0.5, turpentine 2 . *Thisis the highest of four values, viz,, 3250, 3100, 3000,2j50; the single experiment with optically polished surfaces gave nearly 3500. 3 Commercial liquid. Fusible resin. 5 Very thin film. 8 Between optically lane surfaces, failure wholly within the Japan wax. 7 From Scottish s h a g . 8 Synthetic normal saturated paraffin of crystal spacing 3 j . 75 Angstrom units. 1
ADHESIVES AND ADHESION
1679
Physical Laboratory, exceptionally thin and therefore strong films were obtained, stronger than those obtainable with any ordinary polished surfaces of metal. Similarly McBain and Hopkins (1.c.) obtained with shellac creosote cement between optically polished nickel surfaces a tensile strength of no less than 6400 lbs./sq. in. I n all cases the failure was within the adhesive layer and not between metal and adhesive. The results are given for comparison in Table 111.
Tension on Tests : Various Adhesives between Metal Surfaces I n order to provide further comparative data towards our survey of the strength of various adhesives with typical materials the measurements summarised in Table I11 have now been performed. The results are arranged in the order of the metals nickel, steel, cast iron, copper, brass, aluminium, tin and lead which is the order of the internal pressure, tensile strength, elasticity and (in reverse order) compressibility and atomic volume. To save space maximum values only are given in the table although usually three or four separate tests were made. A mixture of equal amounts of shellac and phenol formaldehyde resin between cast iron surfaces gave a value of 1000 lbs./sq. in. which is much lower than that obtained with either ingredient separately. Thicker films gave much lower values still. A series of experiments with beeswax showed that it was distinctly weaker than paraffin wax. Although the general trend of the results is in agreement with the order of the metals as given, there are many exceptions and the effect seems to disappear with the weakest adhesives. Some of the many discrepancies in the table are however to be ascribed to the lack of strict comparability between the various tests. For example, metals are not equally corroded and the highest results are obtained where there is no corrosion. McBain and Hopkins found that where corrosion was extensive the rule as to the order of the metals broke down. Another example is provided by guaiacum resin where if only those joints are compared in which failure took place solely in adhesion between metal and adhesive the results for nickel, copper, alumiriium and tin are without exception in the predicted order; namely, 1100, 1050, 950 and 850 lbs. sq. in. Likewise, with coumarone resin and phenol formaldehyde resin the low result with nickel can be attributed to the corrosion which occurred. With the waxes failure is mostly within the wax itself; in such cases one would expect to find the influence exerted by the various metals only with the thinnest films. Paraffin wax was found to be extremely brittle. The marine glue tended to segregate and likewise became brittle after long storage. In testing the strongest adhesives the elastic limits of such metals as tin and lead are reached or exceeded, a fact which must influence joint failure. Attention may be directed to pitch as an inexpensive ingredient for use in adhesives. In Table I V are collected the results obtained with adhesives between metal test pieces when the joints were broken in shear. Here again the test pieces were in the form described in previous papers.
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J A M E S W. McBAIN AND W. B. LEE
TABLE IV Joint strength in shear (maximum values found, lbs./sq.in.) using adhesives between metal surfaces Nickel
Steel
Shellac reosote 4900 Sealing wax 2900 Phenol formaldehyde* I I O O (set j days) Gum Dammar Paraffin wax (set 1 2 days) Paraffin wax** (set I day)
3500
Adhesive
2500
Cast iron Copper 2750
1500
joo
Brass Aluminum Tin
4900
850
2450
2300
2350
2100
1200
850
1000
650
700
250
200
I050
450
1000 250
50
200
200
*Commercial fusible resin. **Result with lead zoo lbs./sq. in.
I t is interesting to note that in general the joint strength in shear is the same as in tension. Summary I. The results obtained from our studies of adhesion have been briefly reviewed. The thinner the layer of adhesive the stronger the joint. The effect 2. of thickness is not appreciable with very thick films but is rapidly increasing when the thinnest possible films are studied. 3. Data are provided which show t.he adhesive power of various materials for metals. 4. On the whole the rule is confirmed that the parallelism exists between the strengt,h of a joint between smooth metallic surfaces and the mechanical and intrinsic properties of the metals themselves. Cnioersity of BTistd, England and Stanford l'nive~sity,Calijornia. J u l y 2, 1927.
