470
ANALYTICAL EDITION
Minor adjustments which may be made with each assembly are: moving the position of the barrel by sliding it through the pipe clamps, C, until proper release of the air is made at the end of the stroke, increasing the number (but not the size) of the holes where air is released, increasing the size of the air inlet, and adjusting the tension and the position of the platform. When correctly assembled and regulated, the machine should start promptly with admission of air a t d, run continuously, and require no further adjustment for loads which vary from a gram to 1.5 kg. Higher weight limits have not been tried. The arrangement given in this description is for an apparatus in which only a few changes have been made with the starting materials. 1 ‘Since the stroke in this machine need
Vol. 6, No. 6
not be more than 2 inches (5 cm.) long a t the most, it should be possible to cut down the length of the barrel and piston rod to proportionate size, reducing the apparatus to almost half the space formerly occupied. It should also be possible to mount the barrel under the shaking platform by use of suitable connections. The use of smaller springs will give a longer stroke. The same system of springs may be used with some advantage on motor-driven shakers. ACKNOWLEDGMENT The author is indebted to E. C. Curtis for valuable aid in the construction and assembling of this apparatus. RECEIYED June 20, 1934.
Apparatus for Measuring Adhesion of Dried Films R. P. COURTNEY AND H. F. WAKEFIELD, Bakelite Corporation, Bloomfield, N. J.
T
HE force of adhesion between two solids, limited in particular to the molecular attraction exerted a t the interface, has been and still is subject to more empirical than exact information, particularly when the materials concerned are organic films and metal surfaces. While it has been comparatively simple to determine whether two substances adhered to each other, it has been in only rather vague terms that we could describe how well or why they adhered. Further, there has been only empirical differentiation of the two factors of molecular and mechanical adhesion. The well-known classical researches of McBain and his coworkers gave us our first, most complete, and only systematic analysis of the fundamentals underlying adhesion. The determinations of adhesive strength were made by breaking joints by hand or by pulling apart in a tensile-strength machine. Within the limits of the determinations the data could be applied to the paint and varnish industry. This industry has been particularly concerned with these problems as they affect both wood and metal surfaces. Not only does adhesion play an important part in the weathering and age-resistance of a film but it is of great importance where mechanical flexing or abuse is encountered. Very satisfactory advances in the practical development of adherent coatings have been made using empirical tests, such as scratching with a thumb nail, lifting with a knife, or severe bending and crimping. The authors at one time tested the adhesion of enamels to tin plate by pounding the sample with a certain hammer on a specific wooden laboratory floor. If the coating held until objections came from others in the laboratory, it was considered very adherent. It has been desirable to set up for the coating industry a testing method which would disclose the more minute variations in adhesiveness, would give more easily reproduced results, and would allow a more fundamental study of the phenomena to be made. An ideal test method would meet the following requirements : 1. It should be easily reproducible with similar coatings under identical conditions. 2. It should be adaptable to any organic or even inorganic film, 3. Normal drying or baking of the film should be possible. 4. The film should be uncha,nged by subsequent treatment prior to test.
5 . The test uhould not delorm or mechanically alter the film. 6. The test should be independent of the rate of application
of stress. 7. It should show adhesion at all parts of the area under test and should give a measure of the adhesion at each differential unit of the area whether of uniform adhesion or not. 8. Adhesion expressed in numerical terms should not be affected by changes in hardness, flexibility, elongation, or plasticity of the film or by the effect of internal friction as shown in the relation of slow or sudden bending or shock.
Several proposals have been made for measuring the adhesion of films to solid surfaces in mechanical units of force. Van Heuckeroth and Gardner (9)imbed a fabric strip in the film and measure the force required to strip the film from the underlying base. Gelva (8), in a method now being developed, cements a metal plug to the film after the film has dried on the base and measures the necessary rupturing force. Schmidt (6, 7 ) glues a wood block to the film, cuts the film a t the edge of the block, and, using a balance with water as the tare, obtains from a direct pull the force necessary for release in grams per square centimeter. Douglas and Pettifor (3) describe an “energy-absorption” test for the adhesiveness of glue to wood. A strip of strong webbing is glued to the wood panel and is then pulled off by a pendulum-type machine measuring the energy required for detachment. Christopher (8) describes the U. S. Bureau of Standards method for determination of the adhesiveness of bituminous coatings to steel in which a straight-pull method is used. Hart (6)has designed an apparatus which gives a measured impact to a coated panel and affords a comparative indication of the adhesion and brittleness. For determining the adhesive strength of surgical plaster there is a standard test in which a tensile-strength machine measures the force necessary to strip the tape loose from a plate, the force being applied to the tape in a direction parallel with the adhesive surface (1,6). These methods, valuable as applied to specific industries and giving in general interesting comparative results, have nevertheless certain limitations. In some the drying or baking of the film cannot be carried out under normal conditions. In others, the dried film may be changed by solvent or adhesive. In attempts to apply a load normal to the surface, the angle of applied force is apt to vary. The tensilestrength-of-bond method dependent on sudden application of stress may confuse brittleness with lack of adhesion. I n most
November 15,1934
I N D U STR IAL AND E N GIN E E R I N G CHEMISTRY
of the methods the assumption is made that adhesion is uniform over the area under test. Deformation of the film itself, which may be plastic and may require variable stresses, may give variable results. Some of these methods are not applicable to brittle films.
recording spring balance, H . This balance carries a pen, I , which traces a record on chart J as the foil is removed. Ball bearings are used throughout and adjusted to run as freely as possible. The end of the spring on the balance is connected by a cord, K , to a pulley, L , to apply the load. This pulley is connected through a reducing gear and clutch to a motor for constant-speed drive.
PROPOBED METHOD With the hope of eliminating some of the variables and obtaining a more accurate measure of the energy involved in separation of a film from a metal, a somewhat different method is proposed, which consists essentially of stripping the metal, in the form of a thin foil, from the immobilized film. The following advantages are anticipated : 1. No variable is introduced as the nature of the film is changed from brittle to flexible, rigid to plastic, or amorphous to crystalline. For instance, as certain polymers change in properties with variation in the number of polymeric units, the change in adhesiveness may be followed without correction for changes in other properties. 2. The energy involved seems to be independent of the speed of separation. 3. Variations in adherence over the area under test are indicated. 4. A permanent record is available showing total energy required and any characteristic variations. 5. With a possible correction for the energy of deformation of the metal foil, results are independent of the angle of pull, at least within the limits imposed by the test method.
471
Y
FIGURE1. DIAGRAM OF APPARATUS
Any sensitive tensile-strength apparatus could be adapted to this purpose, but since none delicate enough was available in the laboratory, the one described was constructed. A Scott tester, especially if provided with a recording mechanism, would be ideal. TESTPROCEDURE. The prepared test iece is secured to the table and the end of the foil attached to tEe clamp. The clutch is thrown in so that the recording balance, pulling the foil after it, is drawn across the chart. The force required to rupture the bond between film and metal is then recorded as ordinate, while the abscissa shows twice the length of foil removed. Typical charts are shown in Figure 2. The spring balance wm calibrated to give grams load necessary for an extension of 1 mm., the calibration being done on the apparatus itself in order to compensate for friction.
The use of metal foil in this manner allows the pull to be With a chart of the type shown in Figure 2, knowing the applied to the material having the better combination of tensile strength and elasticity with maximum flexibility. spring calibration (in this case 63.1 grams per mm.), we are Foils of tin, aluminum, zinc, copper, silver, and lead are able to calculate the energy, or force times distance, which is available and may be given any mechanical, chemical, or required to separate a unit area of metal-film interface. electrolytic pretreatment desired. The method is offered, For absolute values a correction must be applied to these however,. principally in the hope of developing a better means figures for the energy required to deform the metal foil and of studying the fundamentals of adhesion between organic straighten it again. For practical laboratory varnish testing where comparative results alone are satisfactory, such a films and metal surfaces. It is expected that complete data will later be secured, using correction is not necessary. All adhesion figures the same mechanical arrangement and pulling the films from in this paper are exthe rigidly held metal base, using the method of Gardner (9) pressed in gram centifor holding the film. Such data may show (1) that for some meters of energy per range of flexible films the two methods of either flexing the b.Typica.1 good adhesion test. square centimeter- of film or flexing the base are interchangeable, (2) that for very interface without brittle films the films should be held rigid and the base made c. Erratic sample correction for the foil, flexible, and (3) that for very flexible films it may be a definite a l t h o u g h t h e foil advantage to work with a rigid base. This preliminary paper deals with data on the use of a - 6 correction will be discussed below. Y flexible base metal while the film is held rigid. For most r o u t i n e e. Showing accuracy of check tests. PREPARATION OF SAMPLES.The varnish to be tested is work the total energy 2 x length of test piece in cm. applied by any convenient method to one surface of a section of could readily be demetal foil. In the present tests in order to avoid unnecessary FIGURE2. TYPICAL ADHESION termined by removvariables the authors have standardized upon aluminum foil of CHARTS i n g the foil with a 0.127-mm. thickness obtained from the Aluminum Company of America. This is coated in 10 X 25 em. sections by spraying. pendulum mechanism Three coats have been used in all the following tests. After by a method similar to that proposed by Douglas and Pettidrying, the surface of the varnish film and also the surface of a 7.6 X 25 cm., 18-gage steel panel are coated with cement (sold for (3) for glue bonds. Such a method would not, however, commercially as Bakelite cement BC-6052). After drying for give a complete record of the entire test specimen. Gardner 24 hours the cement surfaces are pressed together and passed (9) states that the pull is usually constant for the entire manually through steam-heated rollers in approximately30 seconds length of the test, although in two tests where adhesions of 0 to secure contact. For this purpose an ordinar clothes wringer provided with steam-heated brass rollers has geen used. The are given in his tables, footnotes indicate that once the force temperature of the roll surfaces is approximately 170" C. reached 400 and 1400 grams, respectively. Uniformity After cooling it is usually found that as the test load is applied undoubtedly exists in such adhesion as is obtained with the foil can be pulled off , leaving the varnish film firmly cemented lacquers to metals, and the authors have found it also with to the steel panel. For test the foil is trimmed even with the varnishes of poor adhesion. (a, Figure 2.) As the adhesion edge of the steel panel or slit with a razor blade to strips of suitimproves, on the other hand, differences may become more able width. APPARATUS. A rigid level supporting table A is provided apparent (b, c, d, Figure 2). With varnishes of strong upon which the prepared specimen may be secured. In Figure 1 adhesion the value may or may not be uniform (Figure 2). are shown steel panel B, cement layer C, varnish film D, and It was the original intention to measure the area of these metal foil E. The loosened end of the foil is attached by means of clamp F adhesion diagrams by means of a planimeter and compute the to cord G, the opposite end of which is attached to the movable average energy. So far the authors have been unable to
I,
-
-
A
ANALYTICAL EDITION
472
Vol. 6 , No.6
explain many of the variations and have taken the maximum adhesion for their values rather than the average, being not yet sure which figure is of primary importance. Fortunately, in most cases, the curves have been fairly flat. One interesting type of curve is shown in Figure 2, d. A few varnishes seem to have the property of strongly resisting the initial bond rupture, but once it is started, continue to rupture under a much lower stress, producing the jerky curve shown here. This difficulty is not often encountered but is being investigated further with hope of eliminating it.
EXPERIMENTAL DATA
A series of seventy different varnishes have so far been tested and tentatively evaluated. It is well recognized that aging and weathering have profound influences upon the thorough drying and adhesive properties of varnish films, but thus far the authors have not had time to follow these characteristics. It is the purpose of this paper to outline a method by which adhesion may be measured, giving a few observations on its use with varnishes of varying composition. Studies are in progress to determine the effect on adhesion of variations in cooking, compounding, aging, weathering, etc. Since their studies of these factors are so incomplete at present, the authors wish to avoid confusing discussion by giving only a few figures to show that the method can differentiate practical variations in adhesion and that it is reproducible and accurate. For this purpose in Table I adhesion values are given for a number of varnishes, airdried for 3 weeks. No. 9, an oldfashioned, fossil resin varnish of accepted good adhesion, is given as a sort of bench mark, while the other figures give some idea of the wide variation to be expected from synthetic resins by reason of variations in compounding and cooking. TABLEI. ADHESIONVALUESOF AIR-DRIED RESINS [All varnishes 45 kg. ot resin to 170 kg. of oil (50 q l . ) using various oil8 and resins cooked by various methods. Varruahes dried and aged 3 weeka.] No. DESCBIPTION ADHESION Q.-cm./aq. em. 9 Old-fashioned fossil resin varnish, oongo-linseed 967 10 Commercial resin No. 7, tung oil 568 16 Commercjal resin No.8, tung oil 736 673 17 Commercial resin No. 9 tung oil 18 Commercial resln No. lb, tung oil 757 19 Commercial re~binNo. 11, tung oil 799
6 e
1400
i::/G
5
Varnish Na3
800 ,/'
Varnfsh No.1
c
6033..'"
4 .:,?
as i n t h e case of v a r n i s h N o . 1. Consequently more e n e r g y i s used in deforming the metal and the correction w o u l d have to be different f o r e a c h degree of adhesion. It would be possible
In Table I1 are given values for a number of varnishes which for lack of time had to be force-dried. Here as a bench mark a rosin-ester gum varnish was used, which rather surprisingly shows the same value as the congo-linseed varnish No. 9 in Table I. The other varnishes are all made with synthetic resins, These figures are given only to illustrate the way in which varnishes may be classified to show differences of adhesion under d e h i t e conditions. TABLE11. ADHESIONVALUESOF FORCE-DRIED RESINS [Varnishes 45 kg. of rqsin to 170 kg. of oil 50 a1 ) air-dried 24 hours, forcedried 24 hours at l6ODb.h:l0 C.)] NO. D~SCRIPTION ADHESION Q.-cm./aq. em. 20 Rosin-aster gum tung oil 967 21 commercial spar' NO.1 799 84 1 22 Commercial spar No.2 23 Commercial resin No. 1 tung oil 715 24 Commercial resin NO.2: tung oil 757 25 Commercial resin No. 3, tung oil 1051 26 Commercial resin No. 4 tung oil 883 27 Commercial resin No. 5' tung oil 1051 70 Commercial resin No. 6: tung oil 1304
Fteproducibility of results is best shown by Figure 2, e, which gives curves from duplicate determinations. Determinations must be conducted under controlled conditions as to temperature, humidity, time of drying, etc., to secure check results such as are indicated. Under such conditions the characteristic curves of Figure 2, e, give values of 2902 and 3028 gram-cm. per sq. cm. or a check within 2 per cent.
0.025 0.037 0.075 0.126 0.025 0.037 0.076 0.125
631
758 1006 1172 715 841 1176 1472
In so far as varnish testing is concerned, it is of advantage to use foil at least 0.127 mm. thick without correction. This is strong and sturdy enough to be easily handled without wrinkling or tearing, and in addition (Figure 3) its use magnifies differences in adhesion, which simplifies testing and evaluating. While the figures obtained are only relative, for varnish plant work that is all that is needed. As for the effect of speed of separation of the interface, the authors have operated at rates between 2.5 and 3.5 om. per second, the limits for their instrument as it stands, with check-
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I N D U ST R I A L A N D E N G I N E E R I N G C H E M I S T R Y
473
The film is unchanged by ing results, but in the case treatment before test. of most very green films the The test does not deform varnish next to the interor alter the film. face remains tacky, or in the Results are independent nature of a viscous liquid. of rate of a p p l i c a t i o n of Very light loads slowly apstress. plied by hand will strip off The test, being graphic, such films, leaving a thin will show inequalities of adlaver of v a r n i s h o n t h e metal. In such cases not FIGURE 4. APPARATUS SHOWING TEST IN PROCRESS hesion over the length of the sample. As the width of the adhesion but cohesion of the partially dried varnish is measured and testing a t these speeds stripsisdecreased and their number on a single panel isincreased, no doubt gives abnormally high results. The same type of we can to that extent localize areas of varying adhesion. variation was noted by Douglas and Pettifor (8) in their The test is independent of hardness, brittleness, flexibility, study of glue bond strength where their energy-absorption etc. method was used after exposure of specimens to atmospheres LITERATURE CITED of different humidities. This condition offers no difficulty, (1) h Y , u. 8.9 Med. De&, Tentative Specifications, NO. 2248, however, with the thoroughly dried films here discussed, with June 14, 1933. which the separation is clean and speed of test does not in- (2) Christopher, Western G ~8, ~16 ,(1932). fluence the results, a t least within any accuracy of importance (3) Douglas and Pettifor, Third and Final Rept. of the Adhesives Research Comm., Dept. Soi. Ind. Reaearch, England, Appbnin varnish testing. dix 1, p. 5 (1932). Another question of vital interest refers to the relation (4) Hart, circ, 435 (June, 1933). Am. Paint Vsrnish Mfre. between adhesion to aluminum and to other metals. Study is (6) pharmaoopeia,U.s.,loth ~ ~ ~ v vinsi^^,i p. ~128 (1925). 1 in progress with foils of tin, lead, copper, and zinc. Iron and (6) Schmidt, Farben-Ztg., 37, 376 (1931). (7) Schmidt, Z. angezo. C h m . , 46, 625 (1933). steel are of course of first importance, but means have not yet (8) Shaw, T. P. G., private communication to Gardner, “Physical been found for studying them. and Chemical Examination of Paints, Varnishes, Lacquers and CONCLUSIONS The method described meets the requirements of a factory test method as follows: It is reproducible and adaptable to any type of iilm. Films may be dried Or baked under any conditions before
test.
Colon,” p. 220, Inst. of Paint 85 Varnish Research. Washing-
ton, 1933. Reuckeroth, A. w.,and Gttrdner, H. A., SCi. Section Education Bureau, Am. Paint Varnish Mfrs. Assoc., Circ. 323,
(9) Van
p. 155 (1928).
RWBXVZD April 18, 1934. Presented before the Division of Paint and Varnish Chemistry at the 87th Meeting of the American Chemical Society, St. Peteraburg, ma., Maroh 26 to 30. 1934.
Stability of Aqueous Solutions of Boric Acid Used in the Kjeldahl Method ABNER EISNERAND E. C. WAGNER Harrison Laboratory of Chemistry, University of Pennsylvania, Philadelphia, Pa.
I
N A RECENT paper [“Titration of Ammonia in F’resence The solution kept in Pyrex glass gave after 197 days a color of Boric Acid,” Meeker and Wagner, IND. ENG.CHEM., not distinguishable from that of the freshly prepared color Anal. Ed., 5 , 396 (1933)l it was mentioned that boric acid standard. The solution kept in lime glass showed no d e solutions suffered deterioration on standing, finally failing to yield a pink color with methyl red. This behavior has now been investigated and is apparently attributable to lack of purity of the distilled water used, revealed by the fact that one lot of freshly prepared boric acid solution was practically neutral to methyl red, the water itself being alkaline. This does not account for the progressive deterioration of other solutions, which could not be studied because none of the water giving such a result was available for examination a t the time its probable responsibility was discovered. A freshly prepared 4 per cent solution of c. P. boric acid in redistilled water was transferred to two bottles, one of lime glass and the other of Pyrex glass, both previously cleaned by steaming out. The fresh solution gave a normal pink color with methyl red under the conditions specified for the ammonia titration. The bottles were kept at room temperature and protected from dust and light. Portions of the solutions were removed at intervals and tested under the conditions of the microtitration: 5 cc. of boric acid solution were diluted with 25 cc. of redistilled water, boiled to remove carbon dioxide, and cooled, 2 drops of 0.05 per cent methyl red indicator were added, and tthe color was com ared with that of a color standard similarly prepared from a Eeshlp made solution of boric acid (using the same lots of boric acid and redistilled water).
terioration for 91 days; after 70 days more, the weakening of the color with methyl red was just visible; after a final interval of 36 days the deterioration was measurable, addition of 0.02 cc. of 0.01 N acid to the test portion being required to equal the color intensity of the standard. At the end of the test period the pH of each solution was determined using a potentiometric titration outfit with calomel half-cell. The two stored solutions and a fresh solution all showed the same. pH. A colorimetric determination of the pH of a fresh 4 per cent solution gave the value 4.1. This demonstration of the stability of boric acid solution, especially if stored in Pyrex glass, removes a possible objection to the boric acid modification of the Kjeldahl method using methyl red indicator. However, the method d e scribed for restoration of solutions deteriorated as kst reported has been used repeatedly and is satisfactory. The necessity for such treatment will probably not arise if boric acid solutions are made with distilled water of the quality ordinarily available in analytical laboratories. RECEIVED June 28,1934.
