FILMS OF ADHESIVES”
_____
*:
BY J. W. MCBAIN AND D. G. HOPKINS
Our previous work1has divided joints into two classes, specific and mechanical. Joints between smooth surfaces are necessarily specific, that is, true adhesion must occur between the surfaces and the film of adhesive. We have noticed that the strength of such joints is parallel to the mechanical conetants of the materials joined and the question immediately arises as to whether there should not be a similar definite relation between their strength and the mechanical properties of the film of adhesive itself. The second class of joints has a purely mechanical explanation in that the adhesive is embedded in the pores and surface irregularities of the materials joined. Here the film of adhesive acts as a solidified casting holding the materials together. Glued wooden joints appear to be a conspicuous example of this class. It is evident that for any given material the strength of such a joint must depend upon the mechanical properties of the film of adhesive, its penetrating power in the liquified state, its rigidity and tensile strength and absence of brittleness in the solidified state. For both classes of joint the tensile strength of the film itself imposes an upper limit on the strength of joint obtainable, since the film in both cases must transmit the strain. Thus an investigation of adhesive action necessarily involves a study of the properties of adhesive films. Even as a routine test the study of a film of the adhesive itself should give more direct information than could be obtained in any other way. The difficulty in the way of most investigators haE been to procure suitable test specimens of the dried adhesive, and has been overcome only in a very few instances by following an elaborate and troubleeome procedure2. We find that it is possible to employ a very rapid and simple method by adopting *This invest,igation was undertaken for the Adhesives Research Committee of the Department of Scientlfic and Indust,rial Research and the authors are indebted l o the Department for permission t o publish the rcsults. Table 111 of the previous paper ( J . Phge. Chcni., 29, 1 9 j (192j); si.e also Serond Report of Adhesives ltesearch Committee, 192j ) should have been completed by the insertion of the following particulars with regard to the various grades of silicate of soda employed. Grade p = (density of % Na?O % Si02 solutions) D I .70 18.5 36.0 1.56 13 8 33.9 C 1.50 11.65 32.85 J 1.375 9.06 26.41 A 1.39 9 . I4 2(3,36 Experimental sample I .225 4.94 19.53 In footnote (**:F) of the same table the statement “three coatings instead of one raised the strength of joint with starch to 1600 lbs” should be amplified by pointing out that this special result WBS obtained wit,h a double-cover plate type test piece, where the total load is given and not the strength in lbs per sq. in. The corresponding value for il high-grade glue is about 3000 lks total load. 2 Thus Hopp (J.Ind. Eng. Chem., 12,365 (1920)) and Gill (17, 297 (192j ) )both obtained values of 12,000 to 13,000 lbs. per sq. in. for a high grade glue.
x
FILMS O F ADHESIVES
11.5
the device of using very thin films, which can be tested with an accuracy far surpassing that of any known wooden join. Furthermore, the strength of such films is far greater than that of any wooden join and is equaled only by the end grain strength of the strongest woods. Up to the present no really satisfactory type of wood joint has been evolved, either as regards of accuracy of measurement or absolute strength as compared with the glue itself. It appears that a glue may be appreciably deteriorated before there is any falling off observable in the strength of joins made there from, probably because the strength of the joins is such a small fraction of that of the glue itself. These thin films also allow a ready method of investigating not only the tensile strength but also such important mechanical properties as rigidity and brittleness and the limit of deformability. The necessity for study of the film cannot be obviated by merely studying the properties of the liquid or dissolved adhesive such as jelly strength, viscosity etc., although this is commonly assumed in the case of glue. On the other hand, the writers are well aware that tensile strength, although an essential factor, is only one of the many mechanical properties of the film itself which have to be considered. Further, there are other important qualities which must be possessed by the solution (prior to the formation of the film) such as penetralibility, rate of setting etc. Paper Strip Method Before describing our work with thin films some data may be given which show that the method of using strips of paper impregnated with glue’ cannot be considered satisfactory, although it has recently been advocated by Bechhold. This method is incapable of giving an absolute measure of the strength of adhesive films because the true cross-sectional area of the film cannot be determined even approximately, and further the effect of the presence of the paper cannot be gauged, nor the effect of the adhesive upon the paper itself. The method is limited to obtaining the order of magnitude of the tensile strength, A slight modification of Millar’s method was adopted in the following experiments carried out to investigate the possibilities of the method. Pure cellulose paper was cut into dumb-bell shaped strips j ” long and I ” wide in the narrow part. These test pieces were dipped into the adhesive solution and then hung up by the end to dry. When dry the process was repeated but this time they were allowed to dry in the reverse position so that a fairly uniform coating of adhesive was obtained, Tension tests were then carried out, the breaking loads being measured to the nearest pound and the mean of four determinations taken. The thickness of the strip a t the point of fracture was letermined in six places with a micrometer reading to O.OOI”. Table I gives .he order of magnitude of the tensile strengths of a number of adhesive films, nade from aqueous solutions. Millar: J. SOC.Chem. Ind., 18, 16 (1899).
116
J . W. MCBAIN AND'D. G. H O P K I N S
TABLEI Tensile strength of f i l m s of adhesive made from aqueous solutions. Strip
Paper only Paper adhesive H*** Paper gelatine X*** Paper starch Paper gum arabic Paper silicate?
+ + + + +
Concn. Heated Thickness Total Net.* Teneile** % t o ("Cj of strip breaking breaking strength (1o-k) load (lbfi) load (lbsj (lbs/sq. in.)
-
-
IO
60 60 60 18 I8
IO
2 IO
3 4
5 4 4
7
7
0
36 41 I4
29
7000
34 7
2000
1.5
16
8 9
(2000)
7000
2000 1000
*Breaking load for impregnated strip less breaking Ioad (7 lbfi) for paper. **Order of magnitude only = adhesive - paper. ***See p. 119 for information regarding these adhesives. ?Molar ratio 3.0, density 1.375
It is evident from these results that the paper strip method is not satisfactory for comparing the tensile strengths of different adhesives. Kot only has the paper itself an unknown influence upon the structure and mode of fracture of the film but its own strength is too large a fraction of that of the film to allow the data to possess any quantitative significance. Some of these adhesives are many times stronger than would here be indicated. T h e preparation of test pieces of adhesive f i l m . The most trustworthy method of testing the strength of films of adhesive is by measuring the strength of thin films which have been formed by allowing them to dry on a surface to which they will not adhere. The thinnest films obtainable should be used in order to minimise strains which may develop in drying. Such films also approach most nearly to the condition of the adhesive in a joint. This simple method of procedure has been adopted by which thin films may be readily prepared and tested. The results are sufficiently concordant to warrant the assertion that this method will supplement and to some extent supersede those which are a t present in use. It also affords a ready means of investigating the effect of addition of soluble substances, or powders (fillers) upon the strength of glue and gelatine films. This method has already supplied new and highly significant information. Farrow and Swani, using a suggestion of J. H. Sturgess, prepared thin films of starch by pouring starch solutions on t o very thinly greased ferrotype plates, where they were allowed to dry. The films were readily detachable from the plates. Neale2 has thus found that such starch films possess considerable tensile strength ( 2 4 to 43 tons per square inch). Glue or gelatine was soaked in the requisite amount of water overnight and a solution prepared by heating a t about 60°C. This solution was poured on to ferrotype plate coated with a trace of Vaseline. On drying, which usually 1 2
J. Textile Inst. 14, T465 (1923). J. Textile Inst. 15, KO.9.
FILMS O F ADHESIVES
117
takes a few days, the film very frequently separated spontaneously from the greased surface but if it had adhered it could be removed by upraising the edges with a knife. A few films (e.g. gelatine with added phenol, chloral hydrate, sodium fluoride or sodium iodide) adhered strongly to the greased plate and it was not possible to detach them in portions large enough for the making of test pieces. Celluloid was occasionally used in place of the ferrotype plate allhough it showed a tendency to warp. Films of gum arabic are particularly interesting because as they dried they became so brittle that they spontaneously broke into small pieces from internal strain. They could be rendered soft and flexible by slightly humidifying them but were as brittle as ever when dried again. I t is possibly for this reason that glycerine is frequently recommended as an addition to gum arabic adhesives and indeed small quantities of non-volatile solvents are added to most adhesive compositions. Care has to be taken in order to obtain really good films. One of the principal difficulties to be overcome is the elimination of air bubbles, for fractures originate at such points. In order to get a perfectly homogeneous adhesive solution it is necessary to shake or stir very thoroughly. Each of these operations has a pronounced tendency to produce air bubbles which are extremely difficult to get rid of on account of their great stability. In practice it has been found most satisfactory to give a whirling motion to the solution. When solids are added to the adhesive solutions the difficulty is still further increased because the powdered materials themselves enclose considerable quantities of air. The powder also tends to FIG.I separate out both in the vessel from which the suspension is poured and on the ferrotype plate. Particles of certain sub3tances, such as barium sulphate, agglomerated in glue and gela,tine solutions. When studying the effect of the addition of soluble substances it is necessary to make the addition in such a way that the resulting solution is quite iomogeneous. For instance, alum could not be satisfactorily dissolved in a [oojG gelatine solution. It was found better to soak the gelatine in the alum ,ohtion. On the other hand, formalin could not be added to the water in sontact with the gelatine as it rendered the latter insoluble; it was added t o he warm gelatine solution. When finally prepared the film was slightly rehumidified t o prevent it rom splitting and then cut with scissors into test pieces, two types of which Lave been employed. The first or earlier type is shown in Fig. I. Parallel nes ab and cd are drawn lightly in black lead a t right angles to the central xis XY so as to facilitate fixing in the testing grips in such a way that the
9
I 18
J. W , McBAIN A N D D . G . HOPKINS
load is applied axially. The two wide end portions are reinforced with stiff paper so that a satisfactory grip can be secured in testing. This type of test piece may perhaps be criticised on the ground that it is easy to initiate a rupture when forming the curved edges by cutting with scissors, To obviate this the second and final type of test piece was designed (Fig. 2 ) . It consisted of a straight strip the long sides of which could be cut out with one stroke of the scissors. The ends were reinforced with stiff paper as before, the paper being pressed on to the wetted adhesive strip. The results obtained with the
Y I
edge.
ptece
W
Y FIG.2
Load FTG.
3
two different forms of test piece were practically identical but the second type is to be prefered on account of its simplicity and from the point of view of economy of material. The tensile strength of glue and gelatine films is greatly influenced by the hygrometric state of the atmosphere in which the tests are made. However, the variation of humidity in a room maintained at approximately the same temperature and otherwise under the same general conditions is often so small that it can be ignored in the case of most films. When certain soluble substances are added to the glue however, it becomes necessary to take the humidity factor into account and for accurate determinations, absolute 01 relative, it was necessary to carry out the experiments in an atmosphere ol constant humidity. The humidity of the testing room was recorded in a1 later experiments being measured by means of a hair hygrometer. The pattern of the grips employed is indicated in Fig. 3. The wholf system was supported on a horizontal knife edge and the load applied b j
FILRIS O F ADHESJVRS
119
placing weights and sand in a bucket suspended from the lower grip, the rate of loading being maintained constant as far as possible. In the majority of films the mean thickness may be taken in calculating the cross sectional area but in those cases where the film was not of uniform thickness the mean of six readings, three on each side of the point of fracture has been used in calculating the tensile strength. All measurements mere malde with a micrometer gauge reading to a ten thousandth of an inch. Particulars of the solutions from which the films have been prepared are given jn Tables 11-IV. A few notes are however necessary to indicate the nature of the adhesives employed which appear in the tables under arbitrary designations.
( a ) Gelatine and glue (without additions) Gelatine X- a high grade commercial gelatine. Highlv purified gelatine-supplied by Prof. Schryver. Isinglass-refined. Adhesive H--a high-grade commercial glue. Glue size---a proprietary brand. Glue with added solids Lead sulphate-ordinary Zinc oxide-Kahlbaum’s Aloxite-finely ground Fine water ground flint
(powders). qua1it)y. heavy. (fired)
supplied by Sir Herbert Jackson.
Glue OT gelatine with added soluble subsiances Potassium dichromate-the films were exposed to diffused sunlight for seven days. -these substances increase the viscosity of glue and Formalin Potassium alum gelatine solutions, Sodium benzoate-this salt lowers the jelly strength. Sodium formate -thesc had been said to increase the adhesive power Sodium salicylate of the glue. Glucose
3
I’he effect of the prolonged heating qf a glue solution o n the tensile strength of the resulting f i l m s . The loss of adhesive power of a glue solution by continued heating a t various temperatures has already been followed by determining the strength of wood joints set up with the heated glue solutions1. It was pointed out that the wood joint method could be fidly sacisfactory for this purpose only in the case of adhesives which are less tenacious than the wood. I n the present series of experiments the cause of the degradation has been followed by measurement of the tensile strength of the glue films. MrBain and Hopkins:
