of -OL= 1.06 wt. p.p.m. as H,O, = 2.69 wt. p.p.m. per liter a t 0” = 0.58 as (C4H602)= 0.65 mg. of -Oeper liter at 50” = 1.7 mole p.p.m. of -02mg. of -OrH 2 0 2 or , -C,HeO2--. Literature Cited
Alexander, D . S., Znd. Eng. Chem. 51, 733 (1959). Batt, L., Benson, S.W., J . Chem Phys. 36, 895 (1962). Bell, E. R., Rust, F. F., Vaughn, W. E., J . Chem. Phys. 72, 337 (1950). Bowden, F. P., “Fast Reactions in Solids,” Academic Press, New York, 1958. Bowden, F. P., Yoffe, A. D . , “Initiation and Growth of Explosions in Liquids and Solids,” Cambridge University Press, London, 1952. Braithwaite, B., Penketh, G. E., Anal. Chem. 39, 1471 (1967). 69, 1332 Brown, H. C., Taylor, M. D., J . Am. Chem. SOC. (1947). Chambre, P. L., J . Chem. Phys. 20, 1795 (1952). Cook, M. A., “The Science of High Explosives,” Reinhold, New York, 1958. Cross,D., Amster, A. B., 8th Symposium on Combustion, Pasadena, Calif., 1960, p. 728; CA 57, 10093g (1962). Frank-Kamenetskii, D. A., “Diffusion and Heat Exchange in Chemical Kinetics,” Princeton University Press,
Princeton, N. J., 1955; Acta Phys. Chem. USSR 10, 365 (1939). “Handbook of Chemistry and Physics,” 45th ed., p. D-76, Chemical Rubber Publishing Co., 1964. Handy, C. T., Rothrock, H . S., J . Am. Chem. Soc. 80, 5306 (1958); (to E. I. du Pont de Nemours and Co.) U. S. Patent 2,898,377 (Aug. 4,1959). Hendry, D . G., Mayo, F. R., Schuetzle, D., IND. CHEM. ENG.,PROD. RES.DEVELOP. 7,136 (1968). Henkin, H., McGill, R., Ind. Eng. Chem. 44, 391 (1952). Liquid Propellant Information Agency, The Johns Hopkins University, Baltimore, Md., “Liquid Propellant Test Methods, Test No. 4 Drop-Weight Test.” Mair, R. D., Graupner, A. J., Anal. Chem. 36, 194 (1964). Mayo, F. R., Miller, A. A., J . A m . Chem. SOC.78, 1023 (1956). Morse, B. K., J . Am. Chem. SOC.79, 3375 (1957). Robey, R. F., Wiese, H. K., Morrell, C. E., Znd. Chem Eng. 36, 3 (1944). Wagner, C . D., Smith, R. H., Peters, E. D . , Znd. Eng. Chem., Anal. Ed 19, 976 (1947). Wenograd, J., Trans. Faradaq SOC57, 1612 (1961). Wu, C.-H., Hammond, G. S., Wright, J. M., J . Am. Chem. SOC. 82, 5386 (1960). RECEIVED for review August 9,1967 ACCEPTED April 8,1968
REACTION OF RESORCINOL AND FORMALDEHYDE IN LATEX ADHESIVES FOR TIRE CORDS G
,
E
.
V A N G I L S , The General Tire & Rubber Co., Akron, Ohio
44309
The thin-layer chromatography (TLC) technique was used to study the reaction of resorcinol and formaldehyde. Two different consecutive reactions occur. Under the reported experimental conditions, methylol resorcinols are formed in the first hour. These reactions are then gradually overtaken by a second process, the formation of high molecular weight condensation products. The reaction was also studied by viscometry, calorimetry, and cryoscopy. The results are consistent with the TLC findings.
condensates in cord dip adhesives were used back as far as 1935 (Baumann, 1936; Charch and Maney, 1938; du Pont Rayon Co., 1937; Mighton, 1951). Since that time, many other patents have been issued and many publications have appeared, but basically the process has not changed. I n this process, resorcinol and formaldehyde are prereacted in aqueous solution in the presence of NaOH catalyst, and after a certain degree of condensation has been reached, the product is added to the vinylpyridine copolymer latex. Many authors have stressed the importance of controlling the resorcinol formaldehyde (RF) reaction, specifically the determination of the right time before mixing with the latex. For this reason, we have to look more closely into the kinetics of this reaction. Resorcinol reacts with formaldehyde essentially in the same way as phenol. However, whereas the phenol reaction has been studied very extensively, comparatively little work has been done on the resorcinol reaction, not only because phenol resins are economically and technically more important, bLt also because the reaction between resorcinol and formaldehyde is so fast that isolation and identification of intermediate products are very difficult. Sen and Sarkar (1925) were able to prepare one of the many possible condensation products, monomethylolRESORCINOL-FORMALDEHYDE
resorcinol, which they found discolored rapidly to a brick red-brown color even in a vacuum desiccator and became insoluble in alcohol. Similar results were obtained by the author as reported later. Stedry (1951) recommended ultraviolet absorption measurement as a tool for studying the R F reaction kinetics. He used the 365-mp wavelength which is less susceptible to colored oxidation products. A conceivable way to study the kinetics of the phenolformaldehyde reaction is to follow the disappearance of the formaldehyde during the reaction and also t o measure the reactive sites of the phenolic compound. This method, which consists of measuring the amount of bromine taken up by phenol, has been applied to resorcinol by Doyle (1960). In the first hours of reaction, methylol resorcinols are formed, hut the number of reactive sites as determined by the bromine method remained the same, since the methylol group is just as replaceable to Br atoms as an active H atom of the resorcinol. On further reaction, di-, tri-, and polynucleic compounds result through formation of methylene bridges, so that the number of reactive sites drops. Doyle found that, in conjunction herewith, the adhesive properties of the dip diminish. Levitin et al. (1962a) followed the R F reaction polarographically and correlated the gradual disappearance of VOL. 7 N O . 2 JUNE 1 9 6 8
151
resorcinol with adhesion. After 110 minutes, when almost no resorcinol remained, the adhesion was at a maximum. Levitin et al. (196210) also measured the change of the absorbance of RF solutions with time and derived conclusions about the progress of the reaction. T h e author thinks however, that since visible light was used, their results must he affected by the gradual brown coloring of the resin due to oxidation by atmospheric oxygen. By reaction of resorcinol and formaldehyde in the molecular ratio 1 to 1, under nitrogen, with a small amount of NaOH catalyst, Bohin (1959) obtained a crystalline product, probably a mixture of methylolresorcinols. These were quickly washed with cold water and dried in vacuum. By reaction of these products with diazomethane the phenolic OH groups were converted to -OCH, groups. The stable compounds can now he fractionated by distillation and examined by infrared. On oxidation the -CH,OH groups were converted into -COOH groups. These oxidation products could also he identified. They concluded that the original crystalline product consisted of the following compounds: two parts of hydroxymethyl-4-dihydroxy-1,3-henzene (I) and one part of dihydroxymethyl1,4-dihydroxy-l,3-benzene(11) and a small amount of unreacted resorcinol.
GOH
HOHzc$oH
CH20H
CHzOH
I
U
These compounds, in the proportion as they appear in the crystals, easily combine to a relatively stable trimer with a molecular weight of 415.
I n studying the reaction of resorcinol and formaldehyde, the author has applied thin-layer chromatography. viscosity, calorimetric,and cryoscopic measurements. Thin-layer Chromatography
Because of the very rapid reactions involved, the TLC technique as is commonly used had to he modified to obtain almost instantaneous, results. Silica gel G from Research Specialties Co. was used. A slurry in water was prepared by grinding in a mortar, and a 4-ounce bottle was filled with it. Two microscopic glass slides pressed together were dipped into the slurry. After withdrawal, the slides were inverted to distribute the silica as uniformly as possible, then the two slides were separated and dried in horizontal positions a t 105°C. for 10 minutes. A droplet of the solution to he tested was applied on the silica layer 5 mm. above the edge of the slide by means of a micropipet. Without drying, to prevent further reaction, the slides are placed in a 4-ounce capped bottle, the bottom of which contains ethyl acetate to a height of 3 mm. This solvent moves up the silica layer very quickly, reaching the top edge after about 5 minutes. The slide is then taken out, dried on a platen a t W C . , sprayed with an acetone solution of p-nitrohenzenediazoniumfluoroborate (Freeman, 1952), and subsequently sprayed with a clear lacquer solution to give the silica layer some strength. Because the whole procedure does not take more than 10 minutes, the RF reaction could he followed closely. The presence of low molecular weight phenolic compounds is indicated by yellow or yellow-brown spots, The presence of higher molecular weight compounds, which
Figure 1. Crystals of mono- and dimethylolresorcinol Approximately 500x
152
I&EC PRODUCT RESEARCH A N D DEVELOPMENT
do not move up in the silica layer and which are designated as "resin," is indicated by a more or less intensely colored dot at the place where the solution was deposited. As reference materials, a dilute solution of mono- and dimethylolresorcinol was used, prepared in crystal form according to Bohin. Depending on the conditions of crystallization, needles or lenticular shapes are obtained. The polarized light extinction is in the direction of the longest axes (Figure 1). The crystals eventually become more and more insoluble in water. On exposure to air, a rusty brown color develops. Crystals a few hours old can be divided into a watersoluble and a methanol-soluble fraction, the rest being soluble in ethanol. On TLC analysis, however, these different fractions show the same spots. The author confirmed Bohin's observation that, on aging, the crystals become insoluble on the outside and that by crushing and powdering, the solubility increases. Under the conditions used here, resorcinol travels the fastest and is deposited immediately behind the line reached by the upward moving solvent. Manomethylol resorcinol comes about % inch after the resorcinol spot and the dimethylol compound about 1 inch lower. The standard RF solution used in cord dips, the so-called MD-2 resin, is made according to the recipe in Table I with the temperature thermostatically controlled a t 25" C. Small samples were taken a t %-hour intervals and diluted with water to % b the original reactant concentration. The diluted material was used for making TLC plata. Figure 2 shows the products present at different reaction times. After % hour, mono- and dimethylol compounds are present. After 1 hour, more of the compounds have been formed, and after 1!4 hours, a third compound becomes visible. This is more conspicuous after 2 hours. At the same time, a faint spot of a fourth compound can be seen and also a resin spot is now visible. With increased reaction time, the resin spot becomes more and more conspicuous whereas the resorcinol and methylolresorcinol spots decrease in intensity. The last slide, taken after 1.5 hours, still shows a faint resorcinol spot. In factory practice, the MD-2 resin solution is considered sufficiently "matured" after 6 hours and ready to he mixed with the vinylpyridine copolymer latex. Such a solution, according to TLC, consists mainly of a watersoluble "resin" plus smaller amounts of a number of methylol resorcinols and a trace of unreacted resorcinol. The TLC procedure as described is a convenient and quick method for checking the composition of MD-2 resin solutions and is, therefore, recommended for control purposes. The TLC method has enabled the author to study some factors affecting the rate of reaction. I t is increased when temperature and Concentration are increased and decreased when the ratio formaldehyde-resorcinol is de-
Figure 2. TLC of MD-2 resin at different times of reaction creased. An addition of NaOH speeds up the reaction. Ammonia addition causes the formation of insoluble RFA polymer as long as free resorcinol and free formaldehyde are present (van Gils, 1967). When added to a 4-hourold MD-2 resin, where primarily methylolresorcinols are present, a sudden increase in the rate of formation of higher molecular weight compounds is found. Viscometric Meosurernents The MD-2 solution will gradually increase in viscosity on maturing because of resin formation. Since TLC has shown that high molecular weight compounds are formed only after 2 hours, we can expect the greatest viscosity rise t o occur after 2 t o 3 hours. T o make the viscosity changes more conspicuous, the MD-2 resin was prepared at a 10% total solids content instead of the usual 6.590, realizing a rate increase from increased concentration of the reactants. The viscosities were measured by a Hoeppler viscometer a t 25°C. (Figure 3). The viscosity increased sharply after 3 hours. After a total maturing time of 6 hours, the solution was of a sirupy consistency. Addition of ammonia after 6 hours speeded up the thickening even more. The next day, the solution with ammonia had solidified to a soft gel, and the solution without ammonia was a hard gel. Colorimetric Measurements The reaction was carried out in a 500-ml. Dewar beaker immersed in a thermostatically controlled bath with the heat of the reaction being followed by a thermistor coupled
Table l. Recipe for MD-2 Resin
Grams Water Resorcinol 37% formaldehyde NaOH
2300 110 160 3
Moles
I 1
2 0.075
.
.
1
2
3
4
5
6
MATURING TIME, h o u r i
Figure 3. lncreore of resin viscosity with time VOL. 7 NO. 2 JUNE 1 9 6 8
153
-~
1/3 213
1
2 TIME
3
4
houri
Figure 4. Temperature rise with time and rate of temperature change with time The temperature is recorded in millivolts
Cryoscopic Measurements
By measuring the freezing point of MD-2 resin solution a t intervals, we can obtain an idea about the number of molecules and thus about the change in composition (Figure 6). The composition of the solution is given in Table I. When calculating the number of moles, a correction was made for the contribution to the freezing point depression caused by the catalyst. Starting with 3 moles (in 2300 ml.), the number of molecules drops rapidly to 153 moles in 1 1 2 hours and then slowly to 1.3 moles after 6 hours which is consistent with the author’s previous findings.
U
ar
2 2
5
:
to a Wheatstone bridge, recording output in millivolts. In the first experiment, the MD-2 recipe was used. The reactants were brought together as dilute solutions at the same temperature (Figure 4 ) . The differential curve has a sharp bend after a 1-hour reaction period. This is in agreement with the TLC findings that at 25°C. after 1 hour the so-called resins start to form. In another experiment (Figure 5) only the catalyst concentration was varied. The curves show that this only affects the first part of the curve but that after 1 hour the rates of reaction are the same. Thus, the catalyst concentration primarily affects the methylol formation. With very high catalyst concentration, 2 moles of NaOH per mole of resorcinol, the reaction is very fast. The heat curve levels off after 1.5 minutes and the heat generated is 26.8 kcal. per mole of resorcinol.
Acknowledgment
+ l
10
20 3C 40
130
50 TIME
H. Piotrowski from this laboratory and Gene Wise from the Virginia Military Institute performed some preliminary work employing the TLC technique for the study of the resorcinol-formaldehyde reaction.
m,nu:es
Figure 5. Influence of NaOH on the
RF reaction
Curves A, 8 and C represent, respectively, 1, 2, and 3 mmoles of N a O H per mmole of resorcinol. Temperature change recorded in millivolts
1
2
3
MATLPING TIME
4
5
literature Cited
6
hcLfs
Figure 6. Change of total moles in solution with increasing maturing time
154
I & E C PRODUCT RESEARCH A N D DEVELOPMENT
Baumann, P. (to Continental Gummi-Werke A. G.), Ger. Patent 725,499 (Feb. 24, 1936). Bohin, G., Ph.D. thesis, University of Paris, 1959. Charch, W. H., Maney, D. B (to E. I. du Pont de Nemours & Co.), U.S. Patent 2,128,229 (Aug. 30, 1938). Doyle, G. M., I R.I. Trans. 36, 177 (1960). D u Pont Rayon Co., Brit. Patent 477,380 (Dec. 23, 1937). Freeman, J. H., Anal Chem. 24,955 (1952). Levitin, L. A., Poloskin, E. N., Kalacheva, A. V., Doronina, T. P., Kautchuh i Rezina 21, No. 7,42 (1962a). Levitin, L. A., Poloskin, E. IK.,Bytrova, Z. L., Ladynina, E. F., Doronina, T. P., Kautchuh i Rezina 21, No. 1, 41 (196213). Mighton, C. J. (to du Pont Co.), U.S. Patent 2,561,215 (July 17, 1951). Sen, R. Ii., Sarkar, N. N., J . Am. Chem. Soc. 47, 1084 (1925). Stedry, P. J., Ind. Eng. Chem. 43, 2372 (1951). Van Gils, G. E., Abstracts, Vol. 8, p. 508, Division of Polymer Chemistry, 153rd Meeting, ACS, Miami Beach, Fla., April 1967. RECEIVED for review July 17, 1967 ACCEPTED February 9, 1968
