580
INDUSTRIAL AND ENGINEERING CHEMISTRY
vulcanizates absorb oxygen much more rapidly than do those of GR-S, but also the amount of oxygen required t o reduce the tensile strength below a useful limit is much less in the case of the Hevea stocks. SUMMARY
The rate of oxygen absorption of both GR-S and Hevea polymers is influenced by the nature of the electrolyte used t o coagulate the latex. The influence is, in general, reduced by compounding and vulcanizing and, in certain cases, may even be reversed. Therefore, the oxidation behavior of uncompounded and unvulcanized polymers does not give a reliable indication of the relative resistance of the vulcanizates t o oxidation and aging. The soaps of aluminum, sodium, magnesium, zinc, lead, and calcium are effective in partially protecting GR-S polymer against the absorption of oxygen. GR-S vulcanizates prepared from polymers coagulated with salt-acid, aluminum, and zinc salts are most resistant, while those containing soaps of sodium, calcium, and magnesium are somewhat less resistant. Cobalt, copper, and iron promote the oxidation of GR-S. Of the metal soaps observed t o impart some protection to GR-S polymer during drying and storage, only those of aluminum and zinc appeared to be without detrimental effect upon the oxidation of the vulcanizates. Salt-acid vulcanizates are essentially equivalent t o the alum GR-S vulcanixate from the standpoint of resistance t o oxidation. The coagulation of Hevea latex with sulfuric acid and with zinc sulfate solutions produced the most stable polymers : coagu$ation with aluminum and magnesium sulfates and acetic acid gave polymers of intermediate resistance t o oxidation; and cobalt and copper sulfate solutions produced coagula which oxidized more rapidly. The nature of the electrolyte has very little effect upon the oxidation resistance of the Hevea vulcanizates, except in the case of cobalt and copper sulfates. These latter agents are active ccatalysts for the oxidation. Hevea vulcanixates absorb oxygen
Vol. 44, No. 3
much more rapidly than GR-S vulcanizates, and only about one fourth as much oxygen is required t o reduce the tensile strength as much as 50%. Except for the effect of copper soap in GR-S and cobalt in Hevea, the rate of degradation of the properties of both GR-Sand Hevea vulcanizates are a function of the amount of oxygen absorbed and are independent of both the time required for the absorption and the nature of the coagulant. Copper soap is a strong catalyst for the absorption of oxygen by GR-S vulcanizates, and also causes a substantial increase in the amount of degradation per unit of oxygen absorbed. Copper apparently favors the oxidation reactions of Hevea polymer leading t o chain scission, and cobalt catalyzes those leading,to cram linking or gelation. ACKNOWLEDGMENT
Part of the work reported here was carried out under the sponsorship of The Firestone Tire and Rubber Co. iY.V. C. Rao was a Scholar of the Ministry of Education of the Government of India. LITERATURE CITED
(1) Albert, Gottschalk, and Smith, IND. ENG. CHEM.,40, 482 (1948).
Blum, Shelton, and Winn, Ibid., 43,464 (1951). (3) Bruni, Pelizzola, and Giorn, Chim. ind. applicata, 3, 451 (4)Eaton, Agri. Bull., St. Fed. M a l a y St., 17 (1912). ( 5 ) Fickendy, Kolloid-Z., 9, 81 (1911). (2)
(1921).
(6) Kirchoff, Kautschuk, 3, 239, 256 (1927).
Rossem, van, and Dekker, IND.ENG.CHmi., 18, 1152 (1926). Shelton, Am. Soo. for Testing Materials, Special Tech. Publication, No. 89, 12-26 (March 1949). (9) Shelton and Winn, IND. ENG.CHEM.,38, 71 (1946). (10) Taylor and Jones, Ibid., 20, 132 (1928). (11) Thomson and Lewis, Chem. News, 64, 1G9 (1891). (12) W n n and Shelton, ISD. ENG.CHEM.,40, 2081 (1948). (7) (8)
RECEIVED for review July 27, 1961.
ACCXPTXD October 8, 1951.
Carbon Black Flocculation in Rubber to Metal Cements G. M. SHEEHAN, GERARD KRAUS, AND A. B. CONCIATORI' Applied Science Research Laboratory, University of Cincinnati, Cincinnati, Ohio
R
E C E X T work on the nature of the carbon black surface (8, 1 0 ) indicates strongly that carbon blacks are aggregates of large polynuclear benzenoid hydrocarbons arranged in a layerlike structure with various organic functional groups situated at the edges of the carbon planes. Some of the organic groups which have been identified are the hydroxyl, hydroperoxide, carboxyl, and carbonyl groups as well as ethylenic double bonds. It has been shown that these groups are capable of entering into chemical reactions and quite recently Stearns and Johnson (8) proposed a mechanism of rubber reinforcement based on chemical interaction between polymer and carbon black involving sulfur bridges between the double bonds of the carbon black and those of the polymer. The chemical activity of the carbon black surface should make carbon black an interesting material in adhesive technology, 1
Present address, Celanese Corp. of America, Summit, N . J.
particulaily in the field of rubber to metal adhesion. In view of this it is somewhat surprising that, with the exception of two patents of Brams ($), little use appears to have been made of carbon black as a main constituent in rubber to metal cements. The work reported here was undertaken in order to establish the role of carbon black in adhesive cements for rubber to metal bonding and, in doing so, secure additional information with regard to the chemical activity of the carbon black surface. PREPARATION OF CARBON BLACK CEMENTS
-411of the work reported here was conducted with a basic GR-Scarbon black cement formulation which is given in Table I. Two different methods were used in the preparation of the cement. The first of these consisted in adding the solvent to the dry batch a t an approximate rate of 1 ml. per minute in a Baker-
March 1952
INDUSTRIAL AND ENGINEERING CHEMISTRY
TABLE I. COMPOUNDING INGREDIENTS Parts by Weight GR-9 100.0 Carbon black Variable Zinc oxide 3.2 Sulfur 8.0 3.2 Accelerator0 Antioxidantb 3.2 32.0 Plasticizer 0 Solventd 1000.0 a 2 2’-benzothiazyl disulfide. b Phenyl-B-na hthylsmine. 0 Dibutyl phtfalate. d Xylene. For evaluations over chlorinated rubber primers an aliphatic solvent, Amsco “Special Naphtholite,” was substituted.
Perkins laboratory-type mixer. I n the second method all materials were charged into a 1-gallon capacity ball mill, loaded to 40% of its volume with 1/2- and 3/~-inchsteel balls, and allowed to mill for 16 hours. Cements obtained by the two methods vary considerably in degree of flocculation of the pigment and in adhesion. Table I1 shows results obtained with the basic cement formulation containing 100 parts of hard processing channel black (Micronex) and 20 parts of color black (Royal Spectra). All adhesion tests were carried out using the standard A.S.T.M. tensile procedure ( 1 ) . Each of the results reported represents an average of a t least four test specimens with a n average standard deviation of approximately 5%. Evaluations were made on brass-plated steel and on steel covered with various resinous primers.
TABLE11. ADHESIONRESULTSWITH GR-S-CHANNELBLACK CEMEKTS~
0:
On
On On Brass Chlorinated Chlorinated Plate Rubberb, Pol is0 reneb, Lb./Sq. Lb./S& Eb.fiq. Inch Inch Inch
ButadieneMethacrylic AcidCopolymerc, Lb./Sq. Inch
Ball-milled carbon black cement 1225 1180 1220 1350 Dough-mixed carbon black cement 950 645 700 Without carbon 320 290 935 black cement 850 5 All evaluations are against a standard natural rubber tread stock of 52 hardness. Cure, 30 minutes at 298O F. b 6 0 7 chlorine. c 22% methacrylic acid. The copolymerization of dienes with unsaturated acids and use of resulting copolymers as adhesives for rubber to metal will be described in a later publication.
...
From the results of Tqble I1 it is apparent that the ball-milled, highly flocculated preparation produces a very pronounced, general increase in adhesion while the analogous deflocculated or very weakly flocculated cement results only in a moderate improvement in two of the cases reported. Another characteristic feature of the ball-milled cements is the occurrence of failure entirely in the rubber stock or a t the stock-cement interface. On the other hand dough-mixed cements frequently exhibit partial failure between the substrate and the carbon black cement. FLOCCULATION AND ADHESION
The photomicrographs shown in Figure 1 display the great difference in the degree of flocculation of the two preparations under discussion. The small particles visible in the dough-mixed preparation are believed t o be small flocculates, as the particle size of the carbon blacks used is well below the resolving power of the optical microscope. It is questionable if with the high carbon black loading employed, complete deflocculation could ever be achieved. The clustered appearance of the particles in Figure 1, A is in itself no criterion for flocculation as a static picture does not
581
allow differentiation between flocculates, “hard” aggregates, or groups of dispersed particles merely pushed together. The significance of these types of aggregation has been discussed in detail by Green (6),who also describes a simple test for differentiating between them. The test consists of placing a drop of the dispersion between a microscope slide and a cover glass and observing under the microscope the changes produced on manipulation of the cover glass with a needle point. Flocculation is indicated if, on working the cover glass, the aggregates disperse and then reform when the convection curr e n t s c e a s e . All b a l l milled dispersions in this work gave a strongly posiA tive cover glass test for flocculation. The difference in flocculation of the two types of preparations are perhaps most clearly illustrated by the yield values obtained for two sets of dispersions on an Interchemical rotational viscometer. These yield B values are presented in Table 111. Further evidence for the effect of flocculation on adhesion can be g a t h e r e d f r o m Table I11 where . . adhesion __ . Figure 1. Photomicrographs values on brass plate are of Carbon Black Cements given for cements con(250 X ) taining 100 aarts of the A . Ball-mixed preparation specifiid blaik. It was B. Dough-mixed preparation found that a semiquantitative measure of flocculation could be obtained by measuring the per cent light reflectance of the d r y cement coated on a glass surface, the more highly flocculated cements reflecting the least amount of light.
-
OF FLOCCULA TABI 11. DEGREE .TION AND
A DlHESION Adhesion Flocculation on Brass by Per Cent Plat ea, Yield Value, Light Lb./Sq. Carbon Preparation Dynes/Sq. Cm. Reflected Inch HPC 1.4 1296 (Micronex) 10.6 952 EPC 4.6 1110 (Micronex) 12.1 760 SRF 2.5 925 9.6 800 L (Statex Y n e x A) ) 2.4 755 mixer 11.6 357 FT Ball. mill 81 6.1 720 mixer 18 10.6 (P-W 700 a Failures in all cases predominantly a t the cement-stook interface.
While adhesion is dependent on the particular carbon used, the more highly flocculated preparation generally gives the higher adhesion. P-33 is so readily dispersed t h a t the ball-milled cement shows relatively little flocculation as evidenced by the low yield value and high reflectance reading. Consequently, there is no appreciable difference in the adhesion of the two P-33 cements. EFFECT OF CARBON BLACK P R ~ P E R T I E S
Table I11 does not allow a critical comparison of carbon blacks as the cements are compared a t constant weight loading of carbon. Since the five blacks used differ widely in specific surface area, a comparison a t constant area loading furnishes a better criterion.
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
582 ~
TABLE IV. ADHESION OF FLOCCULATED CARBONBLACK CEXENTSAT APPROXIIIATELY CONSTAIFT AREA LOADING” Specific Adhesion Surface on Brass drertb Platec, Bq. Afete&/G. pHb Lb./Sq. Inch 286 2.8 1090 Highest color carbon (Royal Spectra) 1055 Medium oolor carbon (Superba) 143 ’ 3.5 4.0 12963 H P C (Micronex Mark 11) 107 89 4.8 1000 E P C (Micronex ‘Ar-6) 31 8.9 800 S R F (Furnex) 67 8.7 975 FF (Stater B) 75 9.6 710 CF (Statex A) Acetylene 65 8.2 915 37 8.2 540 FT (P-33) a Carbon black loading t o give a total surface area equivalent t o 100 parts of HPC. b Data compiled from references (4, 9 , 11). Since the present investigation was made, more reliable data on surface areas of oarbon blacks have become available. These are, however, not sufficiently different from the areas given here to invalidate a n of the conclusions reached. c Failures predoininantly a t tge cement-etock interface. ~
The results, given in Table IV, of such a correlation reveal a definite though qualitative relation between the p H of the carbon ( 1 1 ) and adhesion. The decrease in adhesion with increasing p H is demonstrated further by the result obtained with medium color black which had been heated to 1500” F. for 2 hours. For a cement containing 100 parts of black the following adhesion values were obtained:
Medium color black, untreated Xedium color black, heat treated
PH 3 5 8 9
Adhesion (Brass Plate). Lb./Sq. Inch 1345 875
DISCUSSIORi
The increase in adhesion caused by flocculation of the carbon suggests that the action of these cements is a t least in part due to adhesion of bare carbon black surface to the substrate and rubber Stock. In producing a largely deflocculated dispersion the carbon surface is well wetted by the polymer, thus becoming unavailable for bonding by direct interaction with substrate or stock. The term ILbare”is perhaps used somewhat loosely, since it has been shown that carbon blacks are capable of sorbing GR-S from solution (6); the carbon surface, even in flocculated cements, can therefore not be entirely free of polymer. I t appears reasonable, however, to presume that a molecular layer of sorbed polymer would not offer a complete barrier to the interaction of the carbon surfaces with the substrate. The variety of functional groups present on the carbon black surface would provide ample explanation for the observed increases in adhesion. In the case of brass plate the interface limiting the strength of the bond is between cement and stock, as evidenced by the type of failure observed. Since flocculation generally raises adhesion on brass plate, it appears that the interaction between carbon black and polymer is actually larger than the polymerpolymer bond effected by vulcanization across the interface, the exact magnitude of the carbon-rubber bond depending on the nature and quantity of the carbon black employed. At constant surface area loading, acidic carbons produce the strongest bond. This, however, should not be interpreted as a quantitative relation between p H and adhesion as the p H alone does not furnish an adequate picture of the number of acid groups on the surface or their distribution with regard to availability to contribute to the bond. Furthermore a number of variables, such as structure, degree of flocculation, and cure, undoubtedly share in determining the ultimate strerigth of the bond. Of these factors the state of cure could conceivably exert an appreciable effect on the comparative behavior of the various cements, much as carbon black properties affect the state of cure and the resulting physical properties in conventional rubber compounds. However, in the flocculated cements giving the highest adhesion, the carbon black
Vol. 44, No. 3
is actually too poorly dispersed for the development of superior bulk properties of the cement’, so that it is difficult to conceive of the bond strength being affected by the cohesive strength of the cement film or any variable contributing to it. On the ot,her hand tightness of cure between the carbon black particles of tile cement and the rubber of the stock must be expected t o contribute to the adhesion; such a picture is consistent with both the adhesion mechanism proposed in this paper and the theory of rubber reinforcement of Stearns and Johnson (8). The highest bond strengths have been obtained with flocculated cements containing 100 parts of channel black or a combination of 100 parts of channel with 20 parts of color carbon. These cements give almost equally good results on brass, chlorinated natural or synthetic polyisoprene, or the butadiene-methacrylic acid copolymer (Table 11). The latter polymer is capable of vulcanization to rubber and its adhesion when used in a single cement process is quite high. This adhesion is furt’her raised by the use of an intermediate coat of flocculated carbon cement, suggesting the possibility of hydrogen bonding bet’neen carboxyl groups of the copolymer and oxygen-containing functional groups on the carbon surface. Hydrogen bonds bet,lveen the carbon black surface and the oxide film may also contribute t,o the brass to cement bond, although the bulk of this adhesion is most likely furnished by sulfur bridges (3). The most difficult phenomenon to explain is the very great increase in adhesion realized from the use of flocculated carbon cements over chlorinated rubber and polyisoprene; in fact, no explanation can bc offered a t t,hist’ime. In conclusion it should be pointed out that in all cases considered here adhesion is ascribed either to covalent or to hydrogen bonds, This conclusion is not based on the effects of flocculation alone, for this could readily lead to increased physical interaction as well. However, while physical forces of the van der J$-aals type are ca,pable of producing high adhesion in some instances (Y),it is believed that, under the extremely unfavorable stress distribution of the A.S.T.M. test involving large shear stress concentrations a t the edge of the test specimen, adhesion values of the order of 1000 pounds per square inch cannot reasonably be ascribed to physical forces alone. ACKNOWLEDGMENT
The aut.hors wish to express their thanks to the Inland Manufacturing Division of the General Motors Corp. for the fellowship grants under which this work was performed. LITERATURE CITED
A.S.T.M. S t a n d a r d s for R u b b e r Products, Designation D 429-39 (Adopted 1939). Brams, S. L., U. S.P a t e n t s 2,388.037 (Oct. 30. 1945), 2,424,336 (July 29, 1947). B u c h a n , S., “ R u b b e r to M e t a l Bonding,” C h a p t e r XIII, London, Croshy Lockwood 8 Son, L t d . . 1948. Columbian Colloidal C a r h o n s , Vol. 111, “The Surface Area. o f Colloidal C a r b o n s , ” Kew York, Colunibian Carbon Co., 1942. Green, H., J.A p p l i e d Phys., 13,611-22 (1943). Kolthoff, I. M.,a n d Kahn, A , J . Phus. &. Colloid Chenl., 54, 251 (1950). Kraus, G., a n d M a n s o n , J. E., J . Pdgrner ,Sei., 6, 625 (1951). Stearns, R. S.,a n d ,Johnson, 3 . L., IND.ENG.CHEN..43, 147 (3951). Sweitzer, C . W., a n d Goodrich, W. C., Rubher A Q P ,55, 469 ( I944). T’illsrs, D. S., J . A m . Chem. Soc.. 70, 3656 (1948). Wiegand, 14’.B., IND, EXG. CHEJI.,29, 953 (10371. RECEIVED for review May 19, 1951. ACCEPTED October 17, 1951, Based upon a thesis submitted by G. M. Sheehan in partial fulfillment of t h e requirements f o r the degree of master of srience, University of Cincinnati.
