INDUSTRIAL AND ENGINEERING CHEMISTRY
1600
CONCLUSIONS
Comparison of the three phthalic acid isomers in oil-modified alkyd resins shows clearly that phthalic anhydride is less reactive than either of the other two isomers, even though phthalic anhydride has the advantage of nearly instantaneous formation of half ester. The cause of this lower reactivity of phthalic anhydride is probably a combination of steric hindrance, intraester reaction, and a tendency t o de-esterification a t elevated temperatures. The intraester reaction has the effect of making phthalic anhydride less functional than the other two isomers, and the result is that considerably less phthalate ester is required in an isophthalic or terephthalic alkyd resin than is required in a comparable phthalic anhydride resin. Other advantages of making alkyd resins from isophthalic and terephthalic acids are the low acid losses by sublimation and the greater thermal stability a t oil bodying temperatures. Superior film properties of alkyd resins made from isophthalic and terephthalic acids were observed. Resistance to water and alkali is better, drying times are faster, and the films appear t o be more durable to weathering. A comparison of isophthalic and terephthalic acids shows that &heacids differ primarily in their relative solubilities in reaction mixtures. Because of this difference in solubility, isophthalic acid appears t o be better than terephthalic acid for use in the manufacture of oil-modified alkyd resins. The work that has been done with isophthalic and terephthalic acids suggests t h a t many new and interesting polymer products
Vol. 44, No. 7
can be developed from these dibasic acids, using any of a number of diols and diamines as chain builders and modifying the structure with cross-linking materials such as polyhydric alcohols, polybasic acids, polyisocyanates, and unsaturated acids, alcohols, and aldehydes. ACKNOWLEDGMENT
The authors wish to express their appreciation t o H. L. Wampner, Reichhold Chemicals Co., and to many members of the California Research Corp. for their assistance, advice, and encouragement during the course of this work. LITERATURE CITED
(1) Biggs, B. S., Frosch, C. J., and Erickson, R. H., ISD. ENO. CHEY.,38, 1016 (1946). (2) Carothers, W. H., Trans. Faraday SOC.,32, 39 (1936). (3) Graebe,C., Ann., 149,18 (1868). (4) Hill, R., and Walker, E. E., Polymer Sci., 3 , 609, (1948). (5) Hovey, A. G., and Hodgins, T. S.,Paint, Oil, Chem. Rev., 102, No. 2, 9, 1940. (6) I. G . Farbenindustrie A. G., Brit. Patent 414,665 (Aug. 7, 1934). (7) Kienle, R. H., Van der Meulen, P. A., and Petke, F. E., J . Am. Chem. SOC.,61,2258 (1939). (8) Kolb, H. J., and Izard, E. F., J . Applied Phys., 20, 564 (1949). (9) Whinfield, J. R., and Dickson, J. T., Brit. Patent 578,079 (June 14, 1946).
RECEIVED for review h'ovember 7, 1951. ACCEPTEDFebruary 9, 1952. Presented before the Division of Paint, Varnish, and Plastics Chemistry at t h e 1 2 1 s t Meeting of t h e A X E R I C ~CNH E M I C A L S O C I E T Y , Milwaukee, Wie.
Butadiene-Methacrylic Acid Copolymers as Rubber-to-Steel Adhesives CHARLES E. FRANK, GERARD KRAUS, AND ALBERT J. HAEFNER Applied Science Research Laboratory, University of Cincinnati, Cincinnati, Ohio
T
HE development of high strength, rubber-to-steel adhesives
long has been an important goal in the rubber industry. A number of products now available are based largely on chlorinated or cyclized rubber ( 5 ) . While some of these have been highly successful, they h a v e not been able t o supersede the older brass-plating process entirely (S), especially in applications where maximum strength and durability of the bond are desired. This is particularly true of vibration engine mounts used by the automotive industry. I n addition, many of the adhesives for which superior rubber-to-steel bonds have been claimed are actually multicoat cement systems ( 7 ) , the complexity of which does not warrant their use in large scale production. The copolymers of butadiene and acrylic or methacrylic acid esters have been studied under the Rubber Reserve program. Ternary copolymers of butTdiene, acrylonitrile, and acrylic or methacrylic acid also have been reported (14). However, to the authors' knowledge, there has been no information on thc binary copolymers of dienes and acrylic or methacrylie acids. I n view of the highly polar nature of the carboxyl group and its known ability t o promote metal adhesion (6, 11), these dieneunsaturated acid copolymers were considered particulzrly attractive candidates as rubber-to-steel adhesives. The work reported in this paper has been concerned with the preparation and properties of this interesting class of products; most of the investigation has involved the copolymers of butadiene and methacrylic acid.
T H E POLYMERIZATION R E 4 C T I O N
Butadiene (Phillips Petroleum Co.) and methacrylic acid (E. I. du Pont de Nemours & Co.) both were distilled before use Polymerization was carried out in a n emulsion system in crowncapped bottles rotated end-over-end in a constant temperature bath a t 50' C. Of a number of representative dispersing agents investigated, Rohm & Hans' Triton X-301 ( a 20% solution of a sodium alkyl aryl polyether sulfate) was found most effective The following table gives a typical polymerization recipe: Distilled water, boiled Triton X-301 Potassium persulfate Tertiary octyl mercaptan Methacrylic acid Butadiene
Grams 108
3 0 0 18 0 04 10 0 47 5
At the conclusion of the reaction the bottles were cooled, opened, and hydroquinone was added as a shortstop. The polymer was coagulated with saturated sodium chloride solution, and washed with water a t 40" t o 50' C. until the washings gave a negative test for chloride ion. The product then was dried t o constant weight in a vacuum oven a t 45' t o 50" C. The monomer reactivity ratios for the methacrylic acid and butadiene radicals calculated from the Q and e values of Price (IS) are 0.526 and 0.201, respectively. Because of the Iarye
INDUSTRIAL AND ENGINEERING CHEMISTRY
July 1952
difference in the distribution coefficients of these monomers between the oil and aqueous phases, these values do not allow the exact calculation of copolymer composition in an emulsion polymer (16); they d o indicate, however, that t h e methacrylic acid will enter the copolymerization much more rapidly than butadiene. This is borne out in Figure 1, where acid content is plotted as a function of conversion, and is further reflected in the over-all rate of copolymerization. With the monomer charge given in the table, a 75% conversion t o copolymer of 22y0 methacrylic
501
s\
I
Z4Ot
1601
As might be expected from the polarity of methacrylic acid, the rubbery characteristics of the copolymers decrease with increasing acid content, until at about 45% acid the copolymer has little elasticity and can be milled only with great difficulty. Typical rubber solvents swell but fail t o dissolve these materials, the extent of swelling decreasing with increasing acid content. The best solvents for the butadiene-methacrylic acid copolymers are cyclohexanone and dioxane; a t about 3% concentration, solutions of the modified copolymer containing 20% acid are of a viscosity suitable for brushing. RUBBER-TO-STEEL ADHESION
For maximum rubber-to-steel adhesion, the copolymer should possess a sufficient diene content for vulcanization to the rubber, and a sufficient number of carboxyl groups for securing high adhesion t o steel. Above this acid content the bonds should fail a t the cement-rubber interface; below this acid content failure should occur a t the cement-metal interface. Figure 2 shows t h e adhesion results obtained from a series of copolymers of various acid contents. I n these experiments a single coat of the unmodified, unformulated, butadiene-methacrylic acid cement was applied to grit-blasted steel inserts, and vulcanized (cure 30 minutes a t 298" F.) against the rubber stock, following the standard ASTM testing procedure (1). 0 1000I
z
acid is obtained in 7 hours a t 50" C. On the other hand the copolymerization (unmodified) of butadiene and methacrylic acid charged in a weight ratio of 19:l required 47 hours t o reach 77% conversion; t h e corresponding copolymerization of a 3:2 mixture gave a 91% conversion in less than 7 hours. The chemically heterogeneous nature of the copolymer is indicated by the results of methanol extractions conducted on. products of three different conversions (Table I). There can be no doubt, however, that these products are true copolymers, since their physical and chemical properties are quite different from those of polymethacrylic acid-polybuta diene mixtures.
=
00c-
4
P E 600L u1 2
; 400-
2
E
200-
0;
5
IO
15
Figure 2.
TABLE I. Conversion,
%
Acid,
%
41
30
35
40
,
ACID
Effect of Acid Content on Adhesion
EXTRACTIONS WITH METHANOL"
Extractable,
%
Acid in MethanolSoluble Polymer,
%
Acid in MethanolInsoluble Polymer,
%
36 50 25 20 34 22 93 22 16 34 19 Polymers were prepared from identical charges of 21.7% acid and 78.3% 22 72
a
POLYMER
25
20 PERCENT
64
butadiene b y weight.
The attainment of high strength rubber-to-metal bonds from butadiene-methacrylic acid copolymers requires the formation of the highest molecular weight products which will remain sufficiently soluble for the preparation of cements of 3 t o 4% solids content. One such product is the unmodified copolymer a t about 75% conversion. However, this polymer contains anywhere from 30 t o 50% gel and is not readily reproducible. This difficulty may be overcome by the use of a mercaptan modifier in the polymerization. Of several mercaptans investigated, tertiary octyl mercaptan proved to be the most satisfactory; when used in * amounts of 0.07% based on the total monomer charge, the resulting copolymers were sufficiently soluble in cyclohexanone for the preparation of adhesive cements. Higher degrees of modification enhance solubility, - e . g . , 0.21yo tert-octyl mercaptan-further but the products are apparently of toa low molecular weight to yield a cured bond of the desired strength,
The stock used throughout this work was a diphenylguanidineaccelerated natural rubber compound of 52 Shore hardness containing 38% zinc oxide, suitable for use in truck engine mounts. Each point in Figure 2 represents a n average of four teat specimens, and while the data show the characteristio scatter commonly observed in tensile tests, they clearly indicate the optimum acid content t o lie in the range of 15 t o 24%. On either side of this range the type of failure-i.e., a t the rubber or steel surface-was as expected. The success of the unformulated, unmodified polymers suggests that the butadiene-methacrylic acid copolymers are self curing, for it is believed that cure of the cement film is a necessary requirement for a superior rubber-to-metal bond, This was substantiated in a number of experiments using slab cures oi the solid copolymer. Attempts t o improve the cure by a variety of vulcanization methods gave little or no increase either in tensile strength or adhesion. The modified butadiene-methacrylic acid copolymers do not undergo self cure under these conditions. However, organic peroxides were found t o be effecthe curing agents for these products. By the use of a peroxide cure, the adhesive strength of the modified polymers became equal or superior to that of the unmodified. With the self-curing, unmodified copolymers, t h e incorporation of peroxides had little effect; this may mean t h a t these unmodified, partially gelled products already contain sufficient peroxide nuclei t o accomplish the addi-
1602
INDUSTRIAL AND ENGINEERING CHEMISTRY
tional cross linking necessary for cure. Actually, the presence of small amounts of peroxides in such cements is detectable. Solutions of the unmodified polymer undergo fairly rapid oxidative degradation as do the modified polymer cements t o which additional peroxide has been added, Since the oxidat,ion of diene polymers may result in either scission or cross linking (4,18), it appears t h a t in the bulk polymer and a t curing temperat,ures the cross-linking reaction is dominant, while a t room temperature and in solution scission is the prevailing oxidation mechanism. It is likely that the presence of carboxyl groups in t,he butadiene-mebhacrylic a,cid copolymers facilitates oxidative attack. This effect of the carboxyl group is superimposed on the well-known activating influence of t.he double bond, Peroxides investigated as curing agents include cumene hydroperoxide, lauroyl peroxide, tert-but,yl hydroperoxide, cyclohexanone peroxide, met8hy1ethyl ketone peroxide, and hydrogen peroxide. ,411 of these in amounts of 10 to 20%, based on the copolymer, gave higher strength bonds than could be obtained by the modified polymer alone. Cumene hydroperoside was most effective (Table 11), showing improved bond strength over t'he complete range of 5 t o 25%. All data reported in Table I1 are for one cure, in t'his case the optimum cure for the stock. A limited amount of work under other conditions of vulcanization failed t o reveal any crit.ical dependence of adhesion on temperature arid time of cure.
TABLE 11. EFFECT OF CUMENEHYDROPEROXIDE UPOX BOND STRENGTHOF BUTADIENE-METHACRYLIC ACID CEMENT (Each result represents t h e average of a minimum of 12 test pieces, with an average standard deviation of t h e mean of 2 . 3 % ; . polymer employed contained 2 3 7 , acid at 70% conversion, a n d was modlfied wlth 0.07 parts t e r t octyl mercaptan; cure 30 minutes at 298O F.) ASTM Pull, Cuniene Hydroperoxide Lb./Sq. In. o n Polymer, 70 0 965 3 945 5 1000 IO 1160 25 1165
The question arises whether chemical changes in the polymer upon peroxide cure are directly responsible for the adhesion of the modified polymers t o steel. While such a n effect would not be entirely unexpected, it is nevertheless believed t o be secondary t o the pIomotion of adhesion by the carboxyl groups. T h e fact t h a t these are present in the copolymer in much higher concentration than polar groups introduced from the oxidation, as well as the clearly established dependence of bond strengch on carboxyl content, would appear t o bear this out. The high bond strengths obtained with the butadiene-methacrylic acid copolymers are suggestive of a n adhesion mechanism involving hydrogen bonding with the oxide film of the steel. I t is, however, necessary t o exercise a certain amount of caution in such a n interpretation, for the relation between specific adhesion and effective adhesion (ultimate joint strength) is as yet not clearl;. understood. I t is certainly true that, in general, polarity alone is neither a necessary nor a sufficient condition for the attainment of a strong adhesive bond ( 2 , I O ) , even though many superior adhesive compositions do contain polar groups. The contribution of polar groups t o the specific adhesion to oxide-free metals has been shown to be relatively small, both experimentally (8) and by an approximate theoretical calculation (10). It is only with hydrogen bonding groups and oxide-coated metals that the physical attraction is greatly increased over the alm-ays present dispersion forces (8). Despite the complex relation betveen specific adhesion and joint strength, it appears reasonable t o expect a rough parallelism of the two quantities, particularly where differences in molecular forces are large. This leads t o the conclusion t h a t hydrogen bonding is indeed the responsible; factor in securing metal adhesion with butadiene-
Vol. 44, No. 7
methacrylic acid cements. This conclusion is in accord with the results of McLaren and collaborators on adhesion to cellulose and aluminum (9, 1 I). While the present cement was developed for use mith a particular natural rubber compound, adhesion tests also have been conducted on a number of other types of natural rubber and GR-S stocks; in general, somewhat lower results were obtained. A common component particularly deleterious to butadiene-methacrylic acid adhesion is stearic acid. For example, a GR-S stock containing a high percentage of stearic acid gave an rlSTM pull of 260 pounds per square inch; t h e identical stock compounded from stearic acid-free copolymer gave an adhesion of 715 pounds per square inch. It is possible t o extend the adaptability of the present butadiene-methacrylic acid cement t o other stocks by the use of an intermediate coat of a high carbon black GR-S cement in which the carbon is strongly flocculated. Such cements have the property of raising adhesion of butadiene-methacrylic acid t o all stocks; their general use in rubber-bo-metal bonding has been described in an earlier paper (15) *4n alternative course for adapting bu tadiene-methacrylic acid cements t o various natural and synthetic rubber compounds would be through variation of acid content and curing system. T h e latter possibilities have not yet been investigated. As already stated, t h e unmodified polymers undergo relatively rapid oxidative attack, suggesting a catalytic effect of the acid groups on the attack of the hydrocarbon portion of the molecule. This attack may result either in degradation a s apparent from the reduction in viscosity of the polymer solutions, or in cross linking as in the "cure" of the cement film. While these Oxidative processes are retarded by the incorporation of a modifier in t h e polymerization, it was considered t o be of interest t,o investigate stabilization of the butadiene-methacrylic acid cements. Some 70 amines, phenols, and miscellaneous compounds were tested a t 2y0 concentration (based on the polymer) in a n accelerated oxidation test which consisted of bubbling oxygen through the cement a t 75' C. conversion The basic materials, notably p-hydroxydiphenylamine, m-phenylenediamine, and the mono- , di- , triethanolamines proved most effective. Sodium hydroxide itself had a considerable Etabilizing action, affording additional indication of the catalytic action of the acid groups on the drgradation process. As would be expected, these stabilizers also tend t o retard the cure, and accordingly reduce adhesion. This effect can be largely overcome by the use of sufficient peroxide in the cure; however, unless prolonged storage of the cement is anticipated, the use of a stabilizer is not desirable. REL4TED COPOLYMERS
Related diene-unsaturated acid copolymers were prepared for comparison with the butadiene-methacrylic acid copolymers; analogous polymerization recipes were used. Acrylic acid-butadiene and acrylic acid-isoprene copolymers were similar t o t h e butadiene-methacrylic acid copolymers in appearance, but in general were less soluble and gave lower adhesion values. As would be expected, the weight per cent acrylic acid required for optimum adhesion was somewhat. lower than of methacrylic acid. The rate of polymerization of a mixture containing acrylic acid was considerably slower than that of the corresponding methacrylic acid mixture. Of the various analogs prepared, the methaciyiic. acid-isoprene copolymers resembled the butadiene-methaciylic acid copolymers most closely. However, for the most part, bolubilitv was better while adhesive strength was somewhat poorer. Both the solid polymer and the cement also were subject to more rapid autoxidative degradation than the butadiene-methacrylic acid copolymers. This parallels the tendency of polyisoprene itself t o undergo oxidative degradation rather than ovidative cross linking, while with polybutadiene these tendencies are 1 eve1sed
(4, 12).
INDUSTRIAL AND ENGINEERING CHEMISTRY
July 1952
ACKNOWLEDGMENT
1603
(S) Harkins, W. D., and Loeser, E. H., J . Chem. Phus., 18, 556
The authors are indebted t o the Inland Division of the General Motors Corp. for sponsoring this research. LITERATURE CITED
(1) “A.S.T.M. Standards on Rubber Products,” test designation D 429-39, Philaelphia, Am. SOC.Testing Materials, 1939. (2) Bikerman, J. J., Surface Chemistry,” p. 361, New York, Academic Press, 1948. (3) Buchan, S.,“Rubber to Metal Bonding,” London, Crosby Lockwood and Son, Ltd., 1948. (4) Cole, J. O., and Field, J. E., IND. ENG.CHEM.,39, 174 (1947). (5) Del Monte, J., “Technology of Adhesives,” pp. 195, 205, 435, New York, Reinhold Publishing Corp., 1947. (6) Doolittle, A., and Powell, G., Paint, Oil, Chem. Rev., 107, 40, 9011 (1944). (7) Gossot, J., Rev. yen. caoutchouc, 26, 273 (1949); Rubber Chem. and Technol.,23,281 (1950).
(1950). (9) Hofrichter, C. H., and McLaren, A. D., IND. ENG.CHEM.,40, 329 (1948). (10) Kraus, G., and Manson, J. E., J. Polymer Sci.,6 , 625 (1951). (11) McLaren, A. D., and Seiler, C. J., Ibid., 4 , 63 (1949). (12) Mesrobian, R. B., and Tobolsky, A. V., IND. ENG.CHEM.,41, 1496 (1949). (13) Price, C. C., J. Polymer Sci., 3, 772 (1948). (14) Semon, W. H. (to B. F. Goodrich Co.), U. S. Patent 2,395,017 (February 1946). (15) Sheehan, G. M., Kraus, G., and Conciatori, A B., IND.ENQ. CHEM.,44,580-2 11952). (16) Wall, F. T., Florin, R. E., and Delbecq, C. J., J . Am. Chem. SOC., 72,4769 (1950). RECEIVED for review October 1, 1031. A C C B P T E D February 21, 1952. Abstracted from the thesis of -4. J. Haefner submitted in partial fulfillment of the requirements for the M.S. degree, University of Cincinnati.
Color Formation in Copper Chloride-Sw eetened Distillates JACK H. KRAUSE AND THEODORE B. TOM Research Department, Standard Oil Co. (Indiana),Whiting, Znd.
M
ERCAPTANS occurring in petroleum fractions are frequently converted to disulfides to improve odor. Refinery sweetening processes designed t o accomplish this objective involve mild oxidation reactions. The general reaction can be summarized by the following equation: PRSH
+
1/202
+RzS2
+ HzO
(1,
During the past decade such a process in which copper chloride serves as catalyst has found wide favor. Copper chloride sweetening is said t o proceed by the following route (6):
SRSH 2CuCI
+ 2CuCI2 +RzSz + ZCUCI + 2HC1 + 2HC1 + ---+ 2CuC12 + HzO ‘/202
(2) (3)
Equation 3 is almost instantaneous; hence the corper salt is reoxidized without the need for a separate regeneration step. This process can be carried out in conventional refinery cquipment a t low cost and, in contrast t o several other sweetening processes, does not require the addition of sulfur to the oil. Low boiling fractions of crude petroleum such as gasolinc and kerosene are especially amenable t o the copper chloride-sweetening process, and some cracked gasolines can be handled satisfactorily. After treating, these stocks occasionally undergo deterioration catalyzed by traces of dissolved copper bompounds, but this can be easily remedied by the addition of a metal deactivator. In contrast, the sweetening of distillates boiling above kerosene-particularly those from high-sulfur crudes-results in color-unstable products that cannot be adequately stabilized by even large amounts of deactivator. Color of distillates is a very rough indication of burning performance in critical space heaters; dark oils generally give more carbon deposits. Moreover, color is intimately related t o sales appeal. For these reasons it was felt desirable t o determine why distillates differ in their behavior after copper chloride sweetening. EXPERIMENTAL
Sweetening experiments were carried out in bench-scale equipment of both batch and continuous types. Commercial catalyst-a slurry comprising 72% Attapulgus clay fines, 8% cupric chloride and 20% water-was used, Immediately after sweeten-
ing, the products were washed with water and subjected to an accelerated aging test. Color stability was determined by measurement of colors after accelerated aging. PROPERTiES O F FEEDSTOCKS. This study was mainly concerned with virgin distillates derived from Mid-Continent and West Texas crudes. Some of the essential physical properties are given in Table I. These stocks are high boiling kerosencs sometimes called range, or heater, oils. Such fuels are commonly classified as No. 1 Grade fuels (ASTM D 396) (1). Hereinafter the terms Mid-Continent KO. 1 distillate and West Texas No. 1 distillate will be used t o designate these stocks.
TABLEI. PROPERTIES OF VIRGIN DISTILLATEFEEDSTOCKS Mercaptan N0.a Sulfur, % Color, original Saybolt True Color, aged Saybolt
MidContinent 13 0.11
West Texas 70 0 85
+22 0.02
+“.02
+
+ 140 . 0 7
14 True 0.07 Gravity, OAPI 42.5 ASTM distillation, F. Initial 346 10% 377 50% 439 90% 526 Maximum 569 a Milligrams of mercaptan sulfur per 100 mi. of distillate.
40.3 330 365 421 502 570
BATCHSWEETENING.Studies concerned with the effect of added polar compounds and experiments involving catalysts other than copper chloride were conducted in batch equipment. This apparatus consisted of a 1-liter 3-necked flash equipped with a thermometer, stirrer, oxygen inlet tube, and a stopcock a t the bottom. Heat was supplied by a fiber-glass mantle. After 300 ml. of the sour oil was heated to reaction temperature, 3 grams of the copper chloride slurry was added and t h e mixture was stirred vigorously for 2 minutes. Then the oil was quickly drawn off, separated in a beaker from the slurry, water-washed, dried, and aged.
