INDUSTRIAL AND ENGINEERI-VG CHEMISTRY
778
Vol. 22, No. 7
Adhesives and Adhesion Some Mechanical Properties of Materials and Glued Metal Joints‘ W. B. Lee STANFORD UNIVERSITY, CALIF.
r
The tensile strength of celluloid, Cellophane, and fishing gut, like that of “lithographic” gelatin, depends upon the rate of loading employed. Stress-strain relations and the influence of humidity have also been studied. I t is further confirmed that long-continued stress reduces the ultimate strength of metal joints glued with recognized adhesives. A phenol-formaldehyde resin (Bakelite “C”) has been studied in compression and in bending. Young’s modulus was of the order 35,000 kg. per sq. cm. A shellac-creosote adhesive was investiga’tedin compression and showed the phenomenon of “creep.” Joints between sand-blasted metal surfaces made with
the same shellac adhesive are weaker the greater the pressure employed in sand-blasting. The relative strength of joints between smooth nickel surfaces made with various common adhesives is summarized graphically. Optically polished quartz surfaces wrung together in air with a trace of redistilled alcohol required a force of 70 pounds per square inch (4.9 kg. per sq. cm.) to separate them normally. Experiments with a few highly purified liquid 1-ethyl-R-carbinols gave somewhat lower values. Improvements in the technic for the preparation and testing of glue films are described.
.... ... .. . . . . . HllS paper extends previous investigations on adhesives
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and adhesion (6 to 11). Data are presented on the mechanical properties of certain materials either in bulk or in the form of thin strips. These include recognized adhesives such as shellac-creosote cement and celluloid, together with materials such as phenol-formaldehyde infusible resin, Cellophane, and fishing gut. The influence of such factors as humidity and rate of loading upon tensile strength is investigated. Improved Technic of Testing Free Films
I n precise measurements of the tensile strength of glue films, the films should be of uniform thickness and the rate of loading accurately controlled. The latter requirement has been met by using a new Schopper dynamometer of the pendulum type fitted with accessories for uniform application of the load a t a rate which can be determined. Attempts to prepare films of uniform thickness by pouring 10 or 20 per cent aqueous glue solutions on mercury were unsuccessful. Satisfactory films were, however, obtained in this way by the addition of glycerol to various grades of animal glue. If too much glycerol is added, the films when under test seem to behave more like liquids than solids and draw out with great extension before fracture. I n certain glue films containing added glycerol a pronounced opalescence began to appear as the breaking load was approached (4). The white enamel appearance (pearl disease) which these originally transparent films developed under the appropriate stress was very striking and uniform over the entire film. h marked improvement on the ferrotype plate method was effected by using Bakelite plates2 which had high polish and were sufficiently rigid for use on leveling screws. Uniform glue films prepared from 20 and 30 per cent animal glue solutions were readily detached from the Bakelite. Burnished aluminum and tin plates were also satisfactory. On the other hand, plate glass, either clean or slightly greased, was useless. Ferrotype plates (slightly greased) hitherto used are not entirely suitable; in any case they should be firmly glued to a rigid base which can be leveled accurately. At present the 1 2
Received March 3, 1930. Obtained from the Federal Telegraph CO , Palo Alto, Calif.
tangular strips when loaded near to the point of fracture. They were all parallel to the axis of pull-in c o m p l e t e contrast to similar lines perpendicular t o t h i s axis which have been
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INDUSTRIAL AND ENGIiVEERING CHEMISTRY
July, 1930
was practically linear up to a stress of about 140 kg. per sq. cm. This corresponded to a Young’s modulus of about 23,000 kg. per sq. cm., or about half that of the best animal glue. This result is of the same order of magnitude as that of Coker and Chakko (3)for bars of Xylonite.
had the following composition: 50 parts orange shellac, 5 parts beechwood creosote, 1 part ammonia, and 2 parts terpineol. A sample was turned in the lathe, the length and diameter of the cylindrical specimen being 1.839 and 1.214 cm., respectively. One plane face of the specimen rested on a flat steel plate and pressure was applied axially to the other end and a Denison testing machine was used. Load, kg. per sq. cm. 25.778 Per cent compression 1 . 3 8
0
500
IO00
L o o d , 49 Figure 2-Compression of a Cylindrical Rod of PhenolFormaldehyde R e s i n
CELLoPHrlxE-The mean tensile strength of Cellophane (in approximate equilibrium with air at 6 i per cent humidity) was found to be 1200 kg. per sq. cm. for a rate of loading of 8.3 kg. per sq. cm. per second. For a five-fold increase in the rate of loading the tensile strength Tvas 6.75 per cent greater, all the strips (17.78 x 1.59 X .0127 cm.) being cut from the same sheet and in the same direction. An increase of 5 per cent in the relative humidity caused a drop of about 10 ppr cent in the tensile strength. E$ect of wetting. Cellophane strips were dipped in t a p water for 2 minutes. drained for 1 minute, and tested a t once. I n two tests, 45 and 47 seconds, the stretch was 33.7 per cent and over 33.7 per cent, and the tensile strengths referred to the original cross section were 437 and 422 kg. per sq. cm., respectively. Since the cross section of the swollen Cellophane was not measured, the tensile strength was much less, being between one-third and one-sixth of that of‘ dry Cellophane. The elongation, however, increased only about onequarter. FISHING GuT--,\t the suggestion of Professor Francis, of Bristol University, some of the trial runs with the new Schopper dynamometer were made n-ith various samples of fishing gut. which he kindly supplied. Results nhich are of interest from the point of view of mechanical testing may be summarized as follows: ( a ) The tensile strength of the gut was higher than that of any high-grade animal glue. ( b ) The tensile strength was dependent upon the rate of loading, being higher the higher this rate. In this respect the gut resembles “lithographic” gelatin, Cellophane, and celluloid. ( c ) Fishing gut soaked in water had a lower ultimate strength in tension than the dry material. ( d ) In those cases where a “knot” was present in tv,e gage length, fracture almost invariably occurred a t the “knot
C o m p r e s s i o n a n d B e n d i n g Tests
SHELLAC-CREOSOTE CEJIEST-This cement, which is a good adhesive for metals. as supplied by Sir Herbert Jackson, and
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4 3 . 0 6 6 60,200 77.489 5.53 0.90 1 2 , 4 to 4 1 . 4 during 1 hour
The lever of the machine was constant’ly adjusted so it tended to fall and the load was kept constant until a fairly steady state was reached. Readings were continued the next day. The load of 91.4 kg.-i. e., equivalent to 77.489 kg. per sq. cm.-was kept on as before and the compression slo~vlyincreased to 0.914 cm. (50 per cent compression). To obtain a rough idea of the mode of collapse of this adhesire, a very large but “fictitious” load was then applied. The last reading of compression before collapse was about 1.78 cm. The cement was squashed out flat’ like lead. A second sample 1.206 cm. in diameter and 1.702 cm. in length was tested, load and compression readings (to 0.01 inch or 0.0254 cm.) being taken at intervals of 5 minutes. Under the first four loads in the times given the deflection finally became approximately constant, but with a load of 43.221 kg. per sq. cm. the deflection was not quite constant. Considerable creeping occurred even under rather small loads. Hooke’s law does not appear to hold and there seems to be no definite elastic limit, as is seen from the following data: Time under each load, minutes Load,kg.persq.cm. Per cent compression
20 10 10 45 8 . 6 4 4 17.288 2 5 . 9 3 3 34.577
TO 4 . 4 8
4.486.00
150 75 4 3 . 2 2 1 51.865
7 . 4 7 - 10.520.943.39.00 19.4 43.3 53.7
Lood, Kg. Figure 3-Bending
a Cylindrical Rod of Phenol-Formaldehyde Resin
PHEXOL-FORM.4LDEHYDE IXFUSIBLE RESIN (B.4RELITE /IC”)-Special rods of this material were made by Messrs. Reichwald of London. I t was of a light yellowish lirown color.
ISDUSTRIAL A S D E S G I N E E R I S G CHEMISTRY
780
* * * * *
ADHESIVE Shellac-creosote cement I American commercial cement (“Hard”) 1 Wax-free shellac L High-grade sealing wax I Commercial nitrocellulose cement 1 -L Coumarone resin Marine glue
* * High-grade rosin * Ester gum * Guaiacum resin
(Colophony)
TENSION^
I I
0
Gum arabic Phenol-formaldehyde fusible resin
j -1
* Gum dammar
0 0
High-grade commercial gelatin Dextrin -4silicate of soda
0 0
* Fiber wax
0
* Adhesive applied molten. a
IN
-
Fish glue
*
RELATIVE STRENGTH OF JOINTS
211 kg. per sq. cm. = t w o
Figure 4-Various
strongest joints.
Adhesives between S m o o t h Nickel S u r f a c e s
A more delicate extensometer was used in these coniprcssibility experiments. Readings were taken to 0.001 inch (0.00254 em.). The results, plotted in Figure 2, give a straight line over the first part of the curve, and therefore Hooke’s law holds. This is followed by a large deviation from the linear relationship in the middle part. The cylindrical sample (length, 3.835 cm; diameter, 1.280 cm.) finally broke longitudinally a t about 1400 kg. load. From the earlier, linear part of the curve a value for Young’s modulus can be deduced:
strain
E
=
e/l
34,500 kg. per sq. cm.
TV/e is obtained from the slope in the compressibility graph.
To obtain an accurate value for Young’s modulus on the basis of the compressibility tests, determinations in triplicate mould have been desirable. It was decided, however, to evaluate E in another may-viz., by bending a long rod of the material which at the same time supplied information regarding the behavior of the Bakelite under a different kind of deformation. The depression of the beam was measured with a scale and vernier device, reading to 0.0254 em. (0.001 inch). The results, with a rod of 1.27 em. diameter and a span of 30.48 cm., are plotted in Figure 3. The curve on the left refers to the depression of the midpoint of the rod with increasing loads; that on the right to the depressions on unloading. The material suffered about 0.12 cm. of permanent set. The type of diagram here reproduced is very similar to that obtained with some specimens of concrete. E (by bending) = 34,600 kg. per sq. cm. The agreement of the values obtained by two independent methods is very good and of the same order Of magnitude as E for high-grade animal glue. Relative Strength of Joints in Tension Figure 4 shows at a glancethe values of COmlllOn adhesives for smooth nickel surfaces. It summarizes an important part of the work carried out for the British Adhesives Research Commit,tee. The adhesives are compared, as far as possible, under similar conditions. Shellac adhesives give the strongest joints with metals. The results obtained for the strength of many joints made with recog-
Vol. 22, s o . 7
nized adhesives are appreciably lower if the load is applied very slowly-e. g., during a day or week-instead I ComI of during 5 minutes as in ordinary testing work. mon pitch and shellac-creosote cements illustrate this point. Sand-Blasted Metal Joints
Experiments carried out a t the Royal Aircraft Establishment, Farnborough, England, have shown that the previous sandblasting of smooth wood surfaces raises the joint strength obtainable with aqueous glue solutions. Corresponding experiments are here reported with sandblasted metal surfaces, showing the opposite effect for metals as compared with wood. The surfaces chosen were mild steel with a shellac adhesive which was always applied molten and allowed to mature for 3 days. The rate of loading was 2.8 kg. per sq. cm. per second. Under similar conditions ordinary smooth steel surfaces gave a breaking strength in tension of 218 and 267 kg. per sq. cm., a much thicker film giving only 140 kg. per sq. em. Results show that not only does sand-blasting greatly diminish the strength, but the higher the air pressure used in sand-blasting, and hence rougher the surfaces, the more seriously are the joints affected, possibly because of the increase in the thickness of the adhesive layer. Thus with air pressures of 15, 20, 23, and 30 pounds per square inch (1.C54, 1.41, 1.62, and 2.12 kg. per sq. em.) the joint strengths were 190, 148, 91, and 63 kg. per sq. em., respectively. Pure Liquids as “Glues” Even chemically pure liquids may unite two surfaces strongly. Several highly purified ethyl R-carbinols (R = alkyl) used as adhesives between optically polished quartz surfaces required pulls varying from 40 to 70 pounds per square inch (2.8 to 4.9 kg. per sq. em.) to separate the joint normally. Redistilled alcohol withstood n force of 70 pounds per square inch (or 4.9 kg. per sq. cm.) without breaking. The correction for the effect of atmospheric pressure is only about 5 pounds per square inch (0.36 kg. per sq. em.) in such cases, as was shown by Budgett (1). Pure liquid p-cymene was used to wring together two smooth surfaces, which although originally of “optical” quality had deteriorated soniewhat. A . force of 20 pounds per square inch (1.41 kg. per sq. em.) was required to separate the pieces normally. It is evident that the attractive fields of the quartz or steel surfaces exert a powerful influence on the molecules of the extremely thin liquid glue film extending oyer many molecular diameters, whether directly or through chain effects. Acknowledgment The writer is indebted to Professor J. W. McBain for his helpful criticism and advice during the course of this investigation. Literature Cited (1) Budgett, Proc. R o y . SOC.( L o n d o n ) , AS6, 25 (1912). (2) C1krnent and ~ i ~ i “La + ~Cellulose,” ~ , Libraire Polytechnique, Paris, 1920. (3) Coker and Chakko, Trans. R o y . SOL.(London), A221, 139 (19301. (4) Hauser, IND.ENG.CHEM.,21, 249 (1929). ( 5 ) Hvden, Ibid.. 21, 405 (1929). ( 6 ) McBain and Hopkins, J . Phrs. Ciiem., 29, 188 (1925); 30, 114 (1926). (7) McBain and Hopkins, 2nd Rept. Adhesives Research Committee, App. I V , 34 (1926). ( 8 ) XcBain and Lee, PWC.ROY. S O L . (London), AIIS, 606 (1937). (9) McBain and Lee, J. Soc. Chem. I n d . , 46, 321 (1927). (10) McBain and Lee, I X D . E K G . CHEM., 19, 1005 (1927). (11) McBain and Lee, J . Phys. Chem., 31, 1674 (1927); 32, 1178 (1938). 1244 (1925), (12) Sheppard and Carver, Ibid,, (13) Sheppard, Carver, and Sweet, IND. ENG.CHEII., 18, 76 (1936).
