2372
INDUSTRIAL AND ENGINEERING CHEMISTRY
triangles of Figures 2, 3, and 4 will separate into three phases of compositions represented by the apexes of these triangles. According to Gibb’s phase rule, F = C - P 2, there can be 4 degrees of freedom for any single phase and as temperature and pressure are fixed, the concentration of two of the three components in the phase adequately describes the system. the solid figure in Figure 1 where three phases exist, there can be 2 degreees of freedom, and as pressure is fixed temperature adequately describes the system. Table I1 shows the observed tramition temperatures between
+
Vol. 43, No. 18
two and three phases. These Kere used to plot the closed curve ABCDC‘B’A’E’ shown in the space model. LITERATURE CITED
(1) Claffey, J. B.,Badgett, C. O., Skahmera, J. J., and Phillips, G. T.T’. M., I N D . E S G . C H E M . , 42, 166-71 (1950). ( 2 ) Hudson, C. S.,Z. p h y s i k . Chem., 47, 113 (1903). RECEIVED February 23, 1951. The appearance of trade names in this manuscript does not constitute a recommendation by the Government over any other brand not mentioned.
esoreinol-Fo
a1
OBSERVED BY ULTRAVIOLET ABSORPTION MEASUREMENTS PAUL J. STEDRY Minnesota Mining & Manufacturing Co., S t . Paul, >Ifinn.
Synthetic fibers used in reinforcement of molded rubber goods are usually treated with an adhesive which is in part a condensation product of resorcinol and formaldehyde. This product is prepared in dilute water solution, using sodium hydroxide as catalyst. Because established methods of kinetics cannot be readily applied to control the manufacture, a convenient and accurate index of reaction velocity was sought. Ultraviolet transmittance of the solution decreases gradually and reproducibly as the condensation progresses. The change in transmittance was used to compare reaction rates between 1 mole of resorcinol and 1, 2, or 3
moles of formaldehyde in the temperature range from 55” to 110” F. Temperature coefficient and activation energy were found to be independent of the molar ratio, and the reaction time to a given end point could be expressed as an exponential function of temperature. The effects of various catalysts and concentrations were similarly studied. Transmittance data in the near-ultraviolet thus provide a tool for the rate study of phenolic condensations which take place in a single phase. Before this method can be applied to theoretical w-orli, however, several error-introducing factors must be appraised more thoroughly.
I
ing viscosity changes during condensation. In ensuing ycars, Russian chemists (7-10, 19) made extensive use of relative viscositr, refractive indeu, and electrical conductivity data to follow the formation of phenol-formaldehyde resins catalyzed by acid or alkali, and thus n’ere able to calculate some fundamental phr;sicochemical constants. Simultaneously in England, Megson and collaborators ( d J 1 I , 1 2 ) introduced the concept of “resinification time,” measured from start of reaction to appearance of permanent turbidity and taken as a measure of reactivity of the phenolic mixture employed. As an extension of this method, Finn and Rogers ( 3 ) followed the initial phenol-formaldehyde condensation stage by observing the temperatures a t which the reaction niixtuic just becomes turbid. Still another method of rate study is the determination of phenols by broniination, used by Sprung ( 1 4 , l i ; ) and other authors. The only spectrophotometric approach to the kinetic study of phenolic condensation was briefly described by Engeldinger (W), who measured light absorption a t 490 mp during acidcatalyzed reaction b e k e e n resorcinol and formaldehyde. He ] m a thus able to iollow micelle growth up to the time of flocculation, and to draw conclusions regarding the effect of p H and of conceiitration of reactants.
N USING continuous synthetic filaments for reiniorcing
molded rubber goods, it iq necessary to pretreat the fiber with an adhesive to assure good bonding to rubber. An adhesive commonly used for this purpose, known as the RFL treatment, is prepared by partially condensing 1 mole of resorcinol with 3 moles of formaldehyde in dilute sodium hydroxide solution, and adding this solution to a mixture of natural rubber and GR-S latices, The adheslve is aged for 18 to 24 hours at room temperature before use. Production experience has shown that the initial condensation between resorcinol and formaldehyde niust be controlled rigidly. An improper end point of the reaction often causes coagulation of the latex on mixing and is nearly always reflected in the properties of the finished rubber article. A reaction time of 6 hours a t 5 5 O F. has been found empirically to give good results, but this standard is difficult to maintain because of varying atmospheric and water temperatures In order to eliminate costly temperature control equipment, it was desirable to find a rapid and convenient method for ascertaining the status of the condensation a t any given time, regardless of temperature. HISTORICAL
The condensation of phenols with formaldehyde is one of the oldest polymerization reactions known to modern chemistry, and its kinetics have been studied by many investigators. In the earliest published attempts, Jablonower (6) determined reaction velocity by making density measurements a t regular intervals, and Drummond (1) made the method more sensitive by measur-
RESORCINOL-FORMALDEHYDE REACTION IN DILUTE SODIUM HYDROXIDE SOLUTION
None of the described methods is conveniently suitable to determine the end point in the preparation of RFL adhesive. Refractive index, measured during a simulated production run
INDUSTRIAL AND ENGINEERING CHEMISTRY
October 1951
at 75" F. (40.5 pounds of resorcinoll 39.0 pounds of 37% formalin, and 0.54 pound of sodium hydroxide in 100 gallons of solution), remains practically constant. The pH decreases slowly and consistently, but not reproducibly. Only a slight and erratic viscosity increase can be detected by a sensitive Ostwaldtype viscometer, and a laborious mathematical analysis of a large number of viscosity data is required to obtain an index of reaction velocity. The only good and fairly rapid method of formaldehyde analysis is a polarographic determination in 0.1 N lithium hydroxide and 0.01 N lithium chloride (& but I even ), these results are not sufficiently accurate in view of the low formaldehyde concentration and its relatively slow rate of disappearance. The "resinification time" and "cloud point" methods of determining reaction velocity, as well as the light-absorption method of Engeldinger, are dependent on the appearance of turbidity a t some stage of the condensation. The sodium hydroxidecatalyzed resorcinol-formaldehyde reaction in dilute solution takes place in a single phase throughout, finally forming a homogeneous gel if allowed to proceed far enough. No cloudiness appears even if the solution is cooled to the proximity of its frcczing point. 100
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2373
Broad absorption bands given in the ultraviolet by condensation products of phenols and formaldehyde have also been described (17, 18). During the reaction between resorcinol and formaldehyde. transmittance in the range from 500 to 300 mp becomes weaker as shown, and the peak appearing at approximately 245 mp shifts slightly toward a higher R'ave length and decreases in magnitude. The most significant change, occurring in the 400 to 300 mp region, varies with the reaction time and temperature, and hence can be used to follow the rate of condensntion.
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IIO~F.IOO*F. W F .
301
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I
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Figure 1. Transmittance Spectra of Resorcinol Solution
I t was found that an extension of Engeldinger's method into the near-ultraviolet region affords a convenient, rapid, and reasonably accurate means for following the course of this reaction. ULTRAVIOLET ABSORPTION METHOD
Figure 1 contains a simple transmittance spectrum of a resorcinol solution in the ultraviolet region, extending partially into the visible. Per cent transmittance of the solution, corrected for absorption of cell and water, is plotted against the wave length. Because the concentration of reactants and products during condensation is generally not known, it is neither practical nor necessary to plot the more conventional molar absorptivity. Consequently the measured and plotted transmittance of the solutions is dependent on concentration and the curves become discontinuous whenever the concentration is changed. To obtain the two segments of Figure 1, 0.57 and 0.0057% resorcinol solutions were used. Addition of alkali such as ammonia or sodium hydroxide nioves the curve in the near-ultraviolet toward a higher wave length, and a point of inflection appears. The dependence of the spectrum of phenols on solution pH is well known (6, 13, 16) and has been ascribed variously to ionization a t the hydroxyl group or to formation of quinonoid bonds. The theoretical reasons are of little importance to this investigation. Presence of free formaldehyde in the solution does not change the spectrum because this compound absorbs only in the far-ultraviolet region.
EXPERIMENTAL
Reactions were carried out in a constant temperature bath with slow agitation, using analytical grade reagents and distilled water. The investigation was limited to a temperature range from 55' to 110" F. and to 1 to 1, 1 to 2, and 1 to 3 mole ratios of resorcinol to formaldehyde. Unless otherwise indicated, 0.04 mole of sodium hydroxide per mole of resorcinol was used as catalyst, and the Concentration of all reactants in solutions was 10%. Samples were withdrawn a t regular intervals and diluted tenfold with ice-cold distilled water. This slowed down the reaction sufficiently to permit accurate transmittance measurements. Transmittance data were obtained by means of a Beckman spectrophotometer, Model DU, using a tungsten lamp light source for the near-ultraviolet region and a hydrogen discharge tube source for wave lengths shorter than 320 mp. Diluted samples were placed in quartz cells, 1 cm. in thickness, and the customary blank correction for solvent and cell absorption was applied. Wave lengths of 390, 365, and 330 mp were chosen somewhat arbitrarily to follow the course of the reaction. DISCUSSION OF RESULTS
Figures 2, 3, and 4 are typical curves of transmittance versus reaction time during condensations of 1 mole of resorcinol with 1, 2, and 3 moles of formaldehyde, respectively. All three selected wave lengths were used to measure the decrease in transmittance for each of the three molar ratios, and excellent agreement in results was obtained. In order to illustrate this point while avoiding unnecessary duplication, a different wave length is plotted in each graph. The transmittance decreases more rapidly a t higher reaction temperatures, and the slope of the curves can be taken to represent a relative index of the condensation rate. This affords a rapid and convenient method of comparing reaction velocities a t a series of temperatures. Once the per cent transmittance a t any desired stage of condensation is established, it is simple to duplicate this end point under all temperature conditions. In this work, the conversion accomplished in 6 hours at 55" F. was taken as the reference point. For example, in the reaction of equimolar quantities of resorcinol and formaldehyde a t 55' F.
INDUSTRIAL A N D ENGINEERING CHEMISTRY
2374
the transmittance at 365 nip drops froin 85 to 48.7% in 360 minutes The same change talres place in 199 minutes a t 65" F., in 112 minutes at 75" F., in 63 minute9 a t 85" F., and so on; these periods represent the time required to reach the same end point a t vaiious temperatures. In plotting the reaction time on a logarithmic scale against temperature on a
Val. 43. No. 10
RESORCINOL-FORMALDEHYDE MOL RATIO
1
B = 0.025618 i 20
+- I
4 0 [ . -
~
0
60
120
180
REACTION TIME
1
240
300
I
360
J
420
IN MINUTES
linear scale for the three molar ratios investigated (Figure 51, it is seen that all points fall on the same straight line whose equation can be readily found. This shows that the reaction rate increases by a factor of 1.80 for each 10' F. rise in temperature, or by a factor of 2.89 for each 10" C. By substituting this latter temperature coefficient in the well-known Arrhenius equation, an activation energy of 19,100 calories per gram-mole can be calculated. These values are very similar to those obtained by different methods for a phenol condensation (7, 8, IO), but no comparable constants for resorcinol could be found in the literature.
ALDEHYDE MOL RATIO
I
1
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100
200
300
REACTION TIME IN MINUTES
500
only while the solution remains clear. This imposev a serious restriction, eliminating the study of some catalysts as well as a majority of reactions using phenol. The presence of quinones in the fitarting material and their formation by oxidation of resorcinol during the reaction are the most frequent sources of (mor. as the quinonoid bond absorbs very strongly in the nearultraviolet-.for exaniplp, a t 75' F. the oxidation of resorcinol causes approxiniat~ely15% of the total decrease in transmittancc during the first. 2 hours, after which time no further change takes place. However, only the error due i o t'he difference in the rate ol" quinone formation under different conditions needs to be considered, and this was found to be small and well within the esperiniental error, Quinone-type impurities in resorcinol account for differcnt initial transinittancc in Figures 2 and 6: in the first case, the materials were used as received, the solution initially giving 85% transrnitt'ance at 365 mG; when purified in the second rase, initial transrnittance of a similar solution was 97%. Obviously, it is necessary to use the same raw materials and to follow a standardized procedure in order to obtain reproducible ie.sults. Though the data calculated from measurements at three different wave lengths generally agreed closely, it was noted that quinones cause the least interference at 365 mp. This m v e length also covers the widest range of transmittance for most applications, and is therefore preferred. OTHER APPLICATIONS
Ultraviolet nieavurements can also be applied to study the effect of catalyst concentration or of total concentration in solution, This is shown in Figure 6 for n reaction between equimolar quantities of resorcinol and formaldehyde a t 75" F. The reaction rate can be increased inorc by doubling the amount of sodium
0
It is conceivable that the ultraviolet transmittance data could serve to establish the order of reaction and to calculate velocity constants. To accomplish this it would only be necessary to h d a relationship between the transmittance and the concrntration of reactants or products in the solution. As that wap not done during this research work, the method remains o n l relatire. ~~ MOL RESORCINOL
LIMITATIONS AND SOURCES OF ERROR
In contrast with other recent means of following the course
of phenolic condensation, which depend on appearance of turbidity, the ultraviolet transmittance measurements are reliable
0
20
40 60 80 100 REACTION TIME IN MINUTES
I20
140
INDUSTRIAL AND ENGINEERING CHEMISTRY
October 1951 100
ESORCINOL-FORMALDEHYDE
E" P ul
REACTION TEMPERATURE 7 80
CONC OF REAGENTS 0 0 4 MOL
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MOL RATIO I
CATALYST
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Bo
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20
60
40
80
120
100
140
REACTION TIME IN MINUTES
hydroxide than by increasing the total concentration of reagents in solution from 10 to 20%. In the latter case, the solution samples were diluted twentyfold to provide an equal basis for comparison. It was also shown by blank determination that the concentration of base or other reactants, within the limits employed here, has no appreciable effect on the rate of quinone formation and thus introduces no error.
80
-
2375
sation products formed with amine catalysts are probably different in structure from those formed in the presence of strong inorganic bases, and such products may absorb much more strongly in the ultraviolet region. This question must be answered before any direct comparison can be made. Figure 8 shows the ultraviolet spectrum of phenol, which is very similar to that of resorcinol. The bands also shift on addition of alkali and a reaction cycle with formaldehyde again produces an increase in absorption of the solution. In the case of phenol, however, the study is complicated by the appearance of turbidity a t some stage of condensation and finally by precipitation of resin. These effects would have to be taken into arcount in trying to use the data as an index of reaction velocity. Though this line of work was not carried to a useful conclusion, Figure 8 is presented as a possible starting point in the study of phmolic condensations by transmittance measurements. Though this method is very limited in some respects, it has flexible features which can be adjusted to fit the case under investigation. These include the choice of proper wave length and suitable dilution of samples withdrawn for measurement. ACKNOWLEDGMENT
The author is indebted to D. R. Husted and A. F. Schmelele for valuable assistance during this investigation, and to J. 0. Hendricks and H. N. Stephens for permission to report thp results. A major share of the experimental work was carried out b j PHENOL
IN
ALKALINE
Ella Langevin.
SolN.iij
LITERATURE CITED
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BY
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Drummond, A. A., J . SOC.Chem. Ind., 43,323T (1924). Engeldinger, M., Compt. rend., 202, 842 (1936). Finn, S. R., and Rogers, L. R., J . SOC.Chem. I d . , 67,51 (1948). Holmes, E. L., and Megson, N. J. L., Ibid., 52,415T (1933). (5) Jablonower, B., J . Am. Chem. SOC.,35,811 (1913). (6)Lemon, H. W., Ibid., 69,2998 (1947). (7) Lomakin, B. A., and Guseva, V. I., Narodnyl Komissariat Tyazhelol Prom. S.S.S.R., Nauch.-Issledovatel. Inst. Plasticheskikh. Mass., Plasticheskie Massy, Sbornik, 2, 281 (1937) (8) Lomakin, B. A., and Kutuzova, N. I., Ibid., 2, 111 (1937). (9) Lomakin, B. A., and Kutuzova, N. I., Plastichmkie Massui, 1934, No. 3,25. (10) Medvedkov, E. S., and Sukhina, A. F., Org. Chim. I n d . (C- S.S.(1) (2) (3) (4)
60
170'F.
I
.
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Figure 8. Transmittance Spectra of Phenol Solutions
Figure 7 illustrates the effect of several basic catalysts (0.04 mole per mole of resorcinol) on rate of condensation between 1 mole of resorcinol and 2 moles of formaldehyde a t 75' F. In the presence of ammonia, precipitation of resin takes place almost immediately and no data could be obtained. With triethanolamine and triethylamine, turbidity appears after 59 and 110 minutes, respectively. The order of apparent catalytic effect ehown in Figure 7 is contrary to expectations, but any possibility of interference from undesirable side reactions or from quinone formation was again eliminated by blank determinations. It was also ascertained by direct light-scattering measurements that there is no loss in transmitted energy due to invisible turbidity. However, one cannot hastily conclude that triethanolamine is a better catalyst than sodium hydroxide. The conden-
. R . ) , 6,564 (1939). (11) Megson, N. J. L., Brit. Plastics, 20, 361 (1948). (12) Megson, N. J. L., J . SOC.Chem. I n d . , 57, 189 (1938). (13) Saint-Maxen, A,, and Dureuil, E., Compt. rend., 197, 1411 (1933). (14) Sprung, M. M., IND. ENG.CHEJI.,ANAL.ED.,1 3 , 3 5 (1941). (15) Sprung, M. M., J . Am. Chem. SOC.,63,334 (1941). (16) Stenstrom, W., and Reinhard, M., J . Phys. C h e m , 29, 1477 (1925).
(17) Sugimoto, S., Repts. I m p . I n d . Research I n s t . Osaka ( J a p a n ) , 7 , No. 1, 1 (1926). (18) Sugimoto, S., and Kasai, E., Ibid., 7, No. 14, l(1927). (19) Veidenbaum, 2. V., and Medvedkov, E. k, Org. Chim I n d . (U.S.S.R.),6,437 (1939). (20) Whitnack, G. C., and Moshier, R. W., IND.ENG.CHEM., ANAL. ED., 16, 496 (1944). RECEIVBDOctober 25, 1950. Presented before the Division of Paint, Varnish, and Plastics Chemistry a t the 118th Meeting of the .\MBRICAN CHEMICALSOCIEPY, Chicago, I11 Contribution 28 from the Central Research Department, Minnesota Mining & Manufacturing Co
