November 1952
INDUSTRIAL AND ENGINEERING CHEMISTRY
mixtures of either different aliphatic or different aromatic hydrocarbons. For example, the hydrocarbons listed in the second section of Table I11 require approximately the same time interval t o reach the same maximum temperature when reacted with anhydrous nitric acid but their minimum ignition temperatures are markedly different. It is probable that the differences are related t o the combined effects of the thermal instability and the concentration of the most unstable chemical compounds formed during the reaction. The importance of the concentration of the unstable intermediate is deduced from a comparison of the minimum ignition temperatures for mixtures of n-decane and dicyclopentadiene; the latter compound forms very unstable compounds with nitric acid. To ignite n-decane and nitric acid required a block temperature of 340’ C. before combustion occurred, whereas mixtures of 25 and 35% dicyclopentadiene were ignited a t 154’ and 85’ C., respectively. Figure 7 presents the minimum ignition temperature as a function of per cent dicyclopentadiene. B y means of the curve the minimum ignition temperature for a mixture of 45% dicyclopentadiene was predicted t o be 12’ C.; the observed temperature was 8’ C. The correlation between minimum ignition temperature and ’the thermal instability of the reaction intermediates closely parallels the conclusions of Reutenaur (1.4) who studied the spontaneous ignition behavior in air of several hydrocarbons. ACKNOWLEDGMENT
The authors wish t o acknowledge Project Squid, the Office of Naval Research, and the Office of Air Research whose financial support made this work possible.
2673
LITERATURE CITED
Adams, R., ed., “Organic Reactions,” Vol. 5, p. 48, New York, John Wiley & Sons, 1949. Ellis, Carleton, “The Chemistry of Petroleum Derivatives,” Vol. 2, pp. 31-6, 140-5, New York, Reinhold Publishing Corp., 1945. Forsythe, W. R., and Giauque, W. F., J . Am. Chem. Soc., 64, 48 (1942).
Fournier, I$., Bull scc. chim. France, 13, 884 (1895). Gilman, H., “Organic Chemistry, An Advanced Treatise,” Vol. 1, p. 204, New York, John Wiley & Sons, 1945. Gunn, 8.V., M.S. thesis, Purdue University, 1949. Haitinger, Ann., 193, 366 (1878). Henne, A. L., and Greenlee, K. W., J . Am. Chem. Soc., 65, 2020 (1943).
Kuster, F. W., and Munch, S., 2. anorg. Chem., 43, 350 (1905). Lauer, W. M., and Gender, W. J., J . Am. Chem. Soc., 67, 1174 (1945).
Levy, N., Scaife, C. W., and Wilder-Smith, A. E , J. Chem. SOC.. 1948,52.
Michael, H., and Csrlson, G. H., J . Am. Chem. Soc., 57, 1268 (1935).
Rand, M. J., and Hammett, L. P., Ibid., 72, 287 (1950). Reutenaur, G., Pubs. sci. et. tech. direction inds. aeronaut. (France), Bull. services tech., No. 177,93 pp. (1942).
Sachanen, A. N., “The Chemical Constituents of Petroleum,” pp. 197-274, New York, Reinhold Publishing Corp., 1945. Wieland, H., and Rahn, F., Ber., 54, 1770 (1921). Wieland, H., and Sakellarious, E., Ibid., 53, 201 (1920). RECEIVED for review February 26, 1952.
ACCEPTED July 9, 1952.
Resin Forming Reactions of
Furfural and Phenol LLOYD H . BROWN The Quaker Oats Co.,Merchandise Mart Plaza, Chicago 5 4 , I l l . OST of t h e work t h a t has been published on the reactions of phenol and furfural has been confined to the patent literature; as such, it is concerned primarily with t h e preparation of specific resins (3, 7 ) . A few attempts have been made to isolate intermediate reaction products and to describe t h e course of t h e reaction ( 1 , 4 ,6). The purpose of this work has been to investigate some of the variables involved and to describe their effects in terms of yield and properties of t h e reaction products obtained. I n making resins from phenol and furfural one must accept t h e fact t h a t furfural is a slower reacting aldehyde than formaldehyde. Also, our work has thus far been confined t o alk‘aline catalysts. Even so, it is possible to prepare most of t h e types of resins which can be made from phenol and formaldehyde, such as both one and two step resins and intermediate types. Some of these resins possess unusual properties, such as (in t h e Novolak series) oil solubility and acid reactivity. REACTIONS IN AQUEOUS SOLUTIONS
T h e variables which affect t h e rate of reaction in aqueous systems and which have been explored in this work are: the furfural-phenol ratio, catalyst concentration, and amount of water. Strong alkaline catalysts are required to obtain high .yields in a reasonable time a t reflux temperatures. Alkaline
earth hydroxides and alkali carbonates are too weak; ammonium hydroxide is also too weak, and in addition, it reacts with t h e furfural to form hydrofuramide. Sodium hydroxide is the preferred catalyst, and it is needed in such high concentrations as to appear to be a reactant. Figure 1 shows the rate of reaction with 5 and 10% sodium hydroxide based on the phenol in a system containing an amount of water equal t o t h e volume of phenol. Indications are t h a t increasing t h e catalyst concentration above 10% sodium hydroxide increases the reaction rate still further; however, t h e effect is not as pronounced as it is below 10%.
EXPERIMENTAL. Two moles of furfural are refluxed with one mole of phenol, in the presence of 5 or 10% sodium hydroxide based on the phenol, in an amount of water equal t o the phenol. Zero time refers t o the moment the system reaches reflux. At intervals thereafter, samples are withdrawn, neutralized, and tested for nonvolatile solids. T h e reaction rate is also affected by the concentration of furfural (Figure 2). T h e rate increases with increasing proportions of furfural to phenol. EXPERIMENTAL. These reactions are run as described with 10% sodium hydroxide: 10-ml. samples are removed a t intervals, diluted with methanol, and neutralized t o p H 5 ( p H meter) with 1N hydrochloric acid. After diluting t o 200 ml. with methanol, 2 ml. aliquots are tested for furfural using the sodium bisulfite method ( 2 ) . The curves are not corrected for furfural consumed
INDUSTRIAL AND ENGINEERING CHEMISTRY
2674
TABLE I. EFFECT OF COiVTINUOUS RE>fOVAL OF
I30
'.s 120}
Resin
I10
3
E 90
0.5
Reaction ~ i Hours
WATEH
Yield, ~qo of~ Phenol
Melting point, , 'C.
300° F.
4
106
82
5
1.75
4
142
95
4
1.3
4.5
146
92
4
1.3
Stroke Cure, 1Iin. 330" I.'
1
/
*oc
Method of Water Removal None Distillation column Breotropic distillation
1
2
6
5
Vol. 44, No. 11
1.0
1.5
EXPERIMENTAL. Eight granis of potassium carbonate are dissolved in 400 grams of phenol by heating t o 135" C. in a 1-liter three-necked flask equip ed with stirrer, thermometer, and reflux condenser. Furfura?(300 grams) is added dropwise over a period of 30 minutes, the temperature being maintained a t 135' C. The systems are refluxed for the time indicated in Table I. I n Resin KO.1, a water-cooled reflux condenser i s used, and owing to the effect of water of condensation, the temperature falls slowly to 116' C. before the reaction is stopped. With resin No. 2 , a steam-jacketed column is used t o reflux unreacted materials and remove water. I n resin No. 3, a Bidwell trap is used, with benzene (70 t o 150 ml.) t o remove water by azeotropic distillation. I n both the last two runs, the temperature is held a t 135" C. throughout. After refluxing, the resins are vacuum distilled a t a minimum pressure of 20 mm. and a maximum temperature of 135' C. until comparable melting points are attained. The melting point is determined by the capillary tube method. Stroke cures are taken after adding 2% calcium oxide and 10% hexamethyleneamine.
2.0
HRS Figure 1. Reaction Rates i n Aqueous Sodium Hydroxide Tlk*F
by the Canniazaro reaction; this appears to be about 1 t o 2% of the furfural charged. By carrying reactions of this type to a high degree of condensation, resins may be produced which are quite similar in appearance and curing properties to the standard phenol-formaldehyde adhesive resins used in gluing Douglas fir plgwood. Homogeneous sohtions are obtained with 5 to 10% sodium hydroxide, and additional caustic may be added toward the end of the reaction to improve compatibility with water and to assist in reacting the last traces of furfural. I n general, molar ratios of furfural to phenol in this type of application are between 1 and 2 . Water has a strong inhibiting action in making a lump resin from furfural and phenol. For example, G . Z two resins may be made with equimolar mopor- tions of furfural and phenol using 2% sodium carbonate as the catalyst. One system contains 75 grams of water and the other only 10 grams. After refluxing 101/2 hours, the former gives a 75% yield (based on the weight of the phenol charged), whereas the latter gives 115% yield. Another example of the inhibiting action oi water may be seen in Figure 3. The two resins are made with 2 moles of furfural and 1 mole of phenol. One is catalyzed with 5% sodium hydroxide in an amount of water equal to the phenol; t h e other contains the same amount of catalyst $ but only a small amount of water. The rate of reaction in the latter is more rapid. 0
NONAQUEOUS SYSTEMS
Except for the specific preparation of alkalisoluble-adhesive resins, the best way to react Figure 2. phenol and furfural is probably in the absence of waterorperhapswith continuous removal of water during the reaction. This is borne out by the folloxirig exaiiiples: A much-quoted patent on phenol-furfural resins calls for 0.75 mole of furfural per mole of phenol, with potassium carbonate as a catalyst (5). Under reflux a t atmospheric pressure, such a system gives slow and incomplete reactions. Table I compares such a resin, using 2% potassium carbonate as the catalyst, Bith similar systems in which water of condensation is removed continuously. Water removal, either by a steam-jacketed distillation column which strips out water and returns unreacted furfural and phenol or azeotropic distillation with benzene, speeds t h e reaction considerably.
Increasing the ratio of furfural to phenol raises the melting point and speeds t h e stroke cure. A resin similar to KO.2, except t h a t the ratio of furfural to phenol is 0.G instead of 0.75, gives a yield of 158%, melting point 110" C.. and stroke cure of 3.5minutes a t 300" F., and 1 minute a t 330" F. Kone of these resins is a true iiovolak for although they ale made with less than 1 mole of aldehyde per mole of phenol, they are not permanently fusible. However, they are sufficiently fusible to allow removal of most of the unreacted materials. Similar phenol-formaldehyde resins are not true iiovolaks either. ~
,
s
,
l
,
l
,
,
,
,
,
,
,
,
,
,
,
RATIO I N I N I T I A L REACTION SYSTEM. TIME, MINUTES
Reaction Rates at Various Furfural/Phenol Mole Ratios
A series of permanently fusible potentially reactive resiris can be made by lowering still further the ratio of furfural to phenol. Under these conditions the furfural is reacted almost quantitntively; the excess phenol may he recovered as pure, crystalline phenol by vacuum distillation. Table I1 describes a series of Sovolaks which can be made bgiiiethods similar to run No. 3. Toluene is used, with a Bidwell trap, to remove water of reaction as formed. This incidentally is a convenient means of following the course of t'he reaction. With 1% sodium hydroxide based on the phenol, 3 to 5 hours are required, a t approximately 135' C. to reach completion. Figure
INDUSTRIAL AND ENGINEERING CHEMISTRY
November 1952 40
I
I
I
h
I
2675
T h e reaction is less exothermic if furfural is used as a solvent, but a final cure is attained more rapidly with furfuryl alcohol, Furfural-phenol Novolaks are unusual also in their solubilities. For example, the lower molecular weight members of the series are soluble in drying oils such as tung and dehydrated castor and in aromatic solvents. I n t h e presence of a small quantity of tung oil, they also are compatible with Gilsonite. For example, 45 parts of resin No. 4, Table 11, 45 parts of Gilsonite, and 10 parts of tung oil will cook in about a n hour at 225' C., to a brittle, grindable resin. Such a mixture can be made very high-melting with acids, with or without hexamethyleneamine; however, i t does not give a true cure at 160" C. I
Figure
I
3. Effect of Water Content on Reaction Rate
4 shows the rates of formation of water for two members of the series. Since we can demonstrate t h a t furfural reacts quantitatively, yields are determined after distillation of excess phenol, and as t h e amount of water evolved is known, it is possible t o calculate the ratio of furfural to phenol reacted. This is shown in Table 11, eolumn 3. 0
0
TABLE 11. SERIES OF FURFURAL-PHENOL NOVOLAK RESINS Resin NO.
Ratio of Furfural t o Phenol in Charge
Ratio of Furfural to Phenol Reacted
Melting Fint, C.
stroke Cure, Mine. 300' F. 330' F.
I
05
I O
I 5
Po
I5 TIME,
Figure 4.
30
3 5
4 0
4 5
I
50
HRS.
Rate of Water Evolution-Phenol-Furfural Novolaks
The lower molecular weight phenol-furfural Novolaks may be cooked with drying oils. Although their red color is a disadvantage for many applications, their added functionality, as compared to resins from formaldehyde and substituted phenols, may open u p new commercial possibilities.
I t is possible to make similar resins without continuous water removal, but higher catalyst concentrations and/or reaction times are needed. Stroke cures are determined under the same conditions as described previously. The resins gel somewhat more slowly than those shown in Table I. These resins can be made t o yield shorter stroke cures by acid treatment of the resins either befoie or after distillation. For example, treatment of resin No. 4, Table 11, for 40 minutes with 1% sulfuric acid (based on t h e weight of t h e lump resin and added as 5% solution in alcohol) at 90" to 100" C., followed by neutralization and distillation of t h e alcohol, reduces t h e stroke cure (at 330" F.) from 2.25 to 1.5 minutes. At t h e same time t h e melting point is raised from 59" to 97' C. Similar results are obtained if t h e acid treatment is applied at the end of t h e refluxing period, before distillation. However, because of the increased melting point, it is very difficult to remove unreacted phenol when the stroke cure is below 1.3 minutes. Treatment with sulfuric or hydrochloric acids, at pH 1.5 (Hydrion paper) for a n hour a t 100" C., produces this type of product. Similar results are obtained by oxidation instead of acid treatment. Presumably, furan rings are opened to furnish additional functional groups. Furfural-phenol Novolaks are unique in their reactions to acids. Although stable under neutral or alkaline conditions, they may be thermoset on the acid side without addition of aldehyde. Thus, if an acid alcoholic solution of a Novolak is baked as a film, i t advances t o a black, thermoset resin. By dissolving t h e Novolaks in Ieactive solvents, such as furfuryl alcohol or furfural, i t is possible to prepare high-solids, cold-setting casting sirups. For example,' a solution of 70 grams of resin No. 4,Table 11, in 30 grams of furfuryl alcohol will harden in a few minutes at room temperature with 2% p-toluenesulfonic acid.
ACKNOWLEDGMENT
The author wishes to thank E. A. Reineck and A. P. Dunlop for advice and encouragement and Bessie Roiniotis and Xorman Stephens for technical assistance. LITERATURE CITED
(1) Dunlop, A. P., and Peters, F.N., in Alexander's, "Colloid Chemistry," Vol. 6, p. 1048, New York, Reinhold Publishing Corp., 1946. (2) Dunlop, A. P., and Trimble, F., IND.ENG.CHEM.,ANAL.ED., 11, 602 (1939). (3) Ellis, C., "Chemistry of Synthetic Resins,'; 1st ed., Vol. 1, p. 522, New York, Reinhold Publishing Corp.. 1935. (4) Hachihama, Y., et al., J . SOC.C h e k . I&. Japan, 45, 1054 (1942); 46, 520, 808 (1943); 47, 178, 180, 183, 188, 220, 359, 824 (1944); 49, 61 (1946). (5) Novotny, E. E., and Kendall, D. S. (to John S. Stokes), U. S. Patent 1,705,495 (March 19, 1929). (6) Porai-Koshita, A. E., Kudryavtaev, N. A., and Mashkileiaon, B. E., J . Applied Chern., (U.S.S.R.), 6, 685 (1933). (7) The Quaker Oats Co., Chicago 54, Ill. Digest of Furan Resin Patents (available on loan basis from Chemicals Department). RECEIYED for review February 1, 1952.
ACCEPTED June 13, 1952.
