May 1953
INDUSTRIAL AND ENGINEERING CHEMISTRY
1063
(3) Ibid., 2,499,983 (March 7, 1950). (4) Biggs, B. S., Erickson, R. H., and Fuller, C. S., IND. ENG. CHEM.,39,1090 (1947). * ( 5 ) Boyer, R. F., T a p p i , 34, 357 (1951). (6) Bruson, H. A., U. S. Patents 2,386,736 (Oct. 9, 1945)’ 2,342,606 (Feb. 22, 1944). (7) Carothers, W. H., Chem. Reus., 8,353 (1931). (8) Conyne, R. F., and Felske, F. G., unpublished results: Brit. Patents 586,826 (April 1 , 1947), 588,574 (May 28, 1947). (9) Flory, P. J., J . Am. Chem. SOC.,61, 3334 (1939); Chem. Reus., 39,137 (1946). (10) Flory, P. J., J. Am. Chem. SOC.,62, 1057 (1940) (11) Ibid., 64, 2205 (1942). (12) Hanson, W. E., and Bowman, J. R., IND.ENQ.CHEM.,ANAL. ED., 11, 440 (1939). (13) Hurwits, M. D., Conlon, D. R., Mason, H. F., Meunier, V C., and Auten, R. W., “Approximate Molecular Weights of Alco-
the normal amount of ring formation for conventional polyesters should be approached. The divergence illustrated in Figure 2 increases not only in absolute magnitude with molecular weight, but also percentagewise; this suggests that the ring content actually increases with molecular weight in the case of these terminated polyesters. Polymers 35-3 and 3K-3 (derived from P,P’-thiodipropionic acid and ?-methyl-r-acetopimelic acid, respectively) are poorly accredited members of the series because they have undergone other reactions as well as the intended polyesterification. Precipitation of zinc sulfide in the first case is evidence of “betacleavage.” The melt viscosity of polymer 3K-3 did not reach the normal constant value during the polyesterification, but rather continued to rise throughout a long reaction period. This might indicate t h a t the keto group in methylacetopimelic acid activated a n intermolecular condensation, perhaps resembling the Dieckmann reaction. ACKNOWLEDGMENT
The authors wish to express their appreciation t o Louis P. Hammett, who consulted with them throughout the course of the study of which this is a part. LITERATURE CITED (1) Baker, W. O., Fuller, C. S.,and Ileiss, J. H., J . Am. Chem. SOC., 63,2142 (1941). ( 2 ) Reavers, E. M., U. S. Patent 2,445,553 (July 20, 1948).
hol-Modified Urea-Formaldehyde and Melamine-Formaldehyde Resins,” presented before Division of Paint, Varnish, and Plastics Chemistry, Symposium on Urea, Melamine, and Related Resins, 119th Meeting, AM. CHEM.SOC., Boston 1951. (14) Jacobson, H., Beckmann, C. O., and Stockmayer, W. H., J . Chem. Phys., 18, 1607 (1950). (15) Jacobson, H., and Stookmayer, W.H., Ibid., 18,1600 (1950). (16) Keyssner, E., Ger. Patent 669,961 (Jan. 7 , 1939). (17) Rothrock, D. A,, Jr., and Conyne, R. F., U. S. Patent 2,437,046 (March 2, 1948); Brit. Patent 588,834 (June 4, 1947). (18) Tribit, S., U. S. Patent 2,413,803 (Jan. 7, 1947). (19) Verley, A., and Bolsing, F., Be?., 34, 3354 (1901). (20) Whitmore, F. C., Popkin, A. H.. Bernstein, H. I., and Wilkins, J. P . , J . Am. Chem. SOC., 63,124 (1941). RECEIVED for review October 7, 1952. ACCEPTED January 17, 1953.
Phenol-, Urea-, and Melamine= Formaldehyde Plastics J
RECENT DEVELOPMENTS P. 0. POWERS Pennsylvania Industrial Chemical Corp., Clairton, Pa.
T
HESE plastic materials are now well established items and the past 5 years, which are taken as including recent developments, have seen little that was revolutionary. Many workers have indicated there were no new developments. However, we find an expanding volume of products, new applications, and, what is of particular interest, a renewed activity in the chemistry of these plastics. Phenolic resins were important industrial materials long before the principles of polymer chemistry were formulated. Perhaps for t h a t reason, the chemistry of the reactions of phenol and formaldehyde has nevkr been completely clarified. Excellent work now going on in this field has added greatly t o our knowledge and promises t o make our understanding more nearly complete. PHENOL AND FORMALDEHYDE
Phenol is an important ,organic chemical and the plastic industry is the largest consumer. Recent surveys (8) have indicated t h a t the present capacity in the United States of 344,000,000 pounds annually is not sufficient] and further capacity is planned or building t o bring the total to 624,000,000 pounds. Much of the new production will employ a novel method, involving the peroxidation of cumene. Because neither sulfuric acid nor chlorine, materials which have recently been in short supply, is required, the process is particularly attractive. Other phenols can be made b y this method and i t is contemplated t h a t p-cresol
will be produced by the peroxidation of cymene, obtainable b y the dehydrogenation of dipentene or other terpenes. Cumene is oxidized with air t o the hydroperoxide (7), which on decomposition in the presence of acids and a catalyst yield8 phenol and acetone (Figure 1).
H HsCCCH3
Benzene
Propylene
(CHa)&OOH
(R)
(CH3) Cumene (Cymene) OH
f)
v
+$H3COCH3
(CHI) Phenol (p-Cresol ) Figure 1
&
Cumene Hydroperoxide
Acetone
1064
INDUSTRIAL AND ENGINEERING CHEMISTRY
The simplest method for making phenol would be air oxidation of benzene. This process has been further considered ( I S , 36) and an ultimate yield of 35% reported. It is estimated t h a t (8) considerably more formaldehyde will be required, and some of this increase is already planned. Formaldehyde from the oxidation of butane has become an import a n t source of the material. n-Propyl alcohol and isobutyl alcohol are also formed in the oxidation and have recently been used t o modify the urea resins. Trioxane (48), the cyclic trimer of formaldehyde, has been described and particularly recommended for chloromethylation reactions. New production facilities for manufacture of formaldehyde from methanol have recently been described ($3).
OH
Phenol
Dibenzylamine
H2
i
“Hexa”
OH
OH
Figure 2
APPLICATIONS.In recent years, and particularly for television cabinets, much larger moldings are now made than was formerly thought possible. The plastic must have a long flow and furfural (34) has been useful here. Foamed phenolic resins receive attention periodically, and recently foamed phenolic resins have found an interesting application as a packaging material ( 3 1 ) . Fragile articles can be pressed into the foam and shipped safely. Foams weighing 0.3 t o 0.4 pound per cubic foot can be made, which have only one seventh the density of shredded paper. Phenolic resins have been used several years for bonding sand in foundry cores. Further development ( 4 6 ) has resulted in a shell molding process which greatly reduces the necessary finishing operations and is especially desirable when tough alloys are cast. It is expected t h a t this will develop into a large market for phenolic resins. Phenolic resins are widely used as adhesives. Resorcinol has been used in this field, particularly for curing adhesives a t lower temperatures (S9), and is often uwd in conjunction with phenol. Phenolic resins have also been used with a variety of thermoplastic resins, such as polyvinyl formal (6), in the preparation of thermosetting adhesives for metals, and with the “epon” coating resins ($0)as curing agents. REACTIONPRODUCTS. It is surprising t h a t a reaction apparently as simple as t h a t of phenol and formaldehyde, and as industrially important, has not been better understood. At present there is a renewed interest in this field and considerable clarification has already occurred. Until recently surprisingly little was published on the kinetics of the reaction. Recent studies have reported on the reaction catalyzed by ammonia and caustic soda (12). Kinetics of the uncatalyeed reaction of resorcinol and formaldehyde have been published (33). It has been found t h a t the cloud point ( 1 7 ) can be used t o follow the condensation. It has often been assumed t h a t the formals are intermediates in the reaction of phenol and formaldehyde. Sprung has (44)
Vol. 45, No. 5
shown t h a t ethers are formed in an uncatalyzed reaction, b u t are not formed in the presence of triethanolamine. It is felt that the ether is not an intermediate in the catalyzed reaction, since condensation is slower than normal when catalyst is added t o the ether. Saligenin and phenol react very slowly in the absence of catalyst, and the reaction is apparently second order. Saligenin reacts more readily with itself than with phenol. Investigators a t Graz have studied the reaction of phenol and hexamethylenetetramine, and find a dibenzylamine derivative ( 4 9 ) is formed, comparable to the dibenzyl ether formed from formaldehyde (Figure 2). On heating, ammonia and formaldehyde are liberated t o form diphenylmethane derivatives. With 2,4-xyIenol a tribenzylamine was formed (60). The alkylated phenols (62) also give dibenzylamines with “hexa.” The reaction of 3,4-xylenols with formaldehyde has been studied ( 5 1 ) . PHENOLIC STRUCTURE.The composition of the methylol (hydroxymethyl) phenols formed in alkaline condensation was determined by alkylation of the phenolic group and oxidation of the methylol groups t o carboxylic acids ( 4 3 ) ; 10 to 15% saligenin, 35 t o 45% p-methylolphenol, 30 t o 35% dimethylolphenol, and 4 to 870 trimethylolphenol were found present in addition t o 5 t o 10% unreacted phenol. Subsequent development of chromatographic methods showed (18) t h a t 2,6-dimethylolphenol is also present. Paper chromatography is employed and characteristically colored dyes are formed on addition of p-nitrobenzene diazonium fluoroborate. The diazonium compound will displace a p-methylol group if other positions for coupling are not open. Trimethylsilyl derivatives ($6)have been used for separation of methylol phenols. The compounds are volatile and can be fractionated, subsequently hydrolyzed, and recrystallized. p-Methylolphenol and 2,4di- and 2,4,6-trimethylolphenol were isolated in this way.
”
C H3
Figure 3
It has been shown t h a t the sodium salt of trimethylolphenol (87) will react with alkyl or alkenyl halides t o form saturated
or unsaturated ethers. These ethers are low-shrinkage casting resins or coating materials. Considerable progress has been made in the separation of many of the possible derivatives with three or four rings (5, 41, 51). I n some cases i t has not been possible t o isolate crystalline products, although many two-ring compounds have been crystallized, The rings in diphenylmethane derivatives are not in the same plane; x-ray and infrared studies (9, 16, 84) have shown this to be the case. T h e materials become resinous in character when four or five rings are present in the molecule. Sprengling has found t h a t the o,o‘-dihydroxydiphenylmethane compounds are much more acidic than phenol b u t less acidic than oleic acid, Only one acidic group is present in a molecule, even though two or three hydroxyls are present. Hydrogen bonding occurs in these compounds ( 4 1 ) and results in higher acidity. It is suggested t h a t the acidic group may cause greater reactivity in t h a t
INDUSTRIAL A N D ENGINEERING CHEMISTRY
May 1953
portion of the molecule. It has been shown that strongly hindered diphenylmethane derivatives are entirely in the cis arrangement of the hydroxyl groups, and t h a t this form is less acidic (9) than the trans (Figure 3). The quinone methide structure has been suggested as the intermediate in the polymerization of the methylolphenols. Such a structure would explain the formation of aldehydes and diphenylethane compounds (Figure 4).
OH
11
Aldehyde
4
1
Styrene
CH2-CH;. €Io()-
Diphenylethane
Phenyl Chromane Figure 4
It has been used t o explain (40)the addition of methylolphenols t o unsaturated compounds with the formation of a chromane ring. Persuasive evidence has been presented for the formation of such an addition compound from saligenin and oleic acid ( 4 6 ) . UREA AND MELAMINE-FORMALDEHYDE
Urea is produced in large quantities a t the present time and further production capacity is being built. Melamine is produced in increasing quantities from dicyanodiamide (63). There is, however, considerable interest in the preparation of melamine from urea ( I , 14). A variety of materials will produce melamine when heated a t high temperatures, b u t of these urea is the most promising (Figure 5).
jUHC-N
“&NO Ammonium Cvanate
C=NH
“€I2 Dicvanodiamide
Ammeline H~NCONHC~NH~ (H\OGN)( Biuret Cyanuric Acid Figure 5
1065
APPLICATIONS. Considerable effort has been exerted t o obtain urea-formaldehyde solutions of increased stability; methyl ethers of methylol compounds possess considerably greater stability than the methylol compounds. Very, high ratios of formaldehyde have been used. This is also effective but more urea is added before using the resin to take up the excess. Amine salts have been offered in preference to ammonium salts as accelerators. Foamed urea resins (SO)can be produced with very low densities. Urea resins modified with alcohols are widely used in coatings, particularly with alkyds, New ureas with better compatibility with short oil alkyds have been developed. The alcohol-modified resins vary in formaldehyde content, alcohol content, and the type of alcohol employed, long-chain alcohols giving a wider range of compatibility (47). %-Butyl alcohol has been widdy used, b u t recently n-propyl alcohol and isobutyl alcohol have been successfully used in modifying the ureas. One advantage of these alcohols is a somewhat more stable price structure. Webstrength paper has been an important field of application for urea resins, b u t in recent years melamine resins have possessed better properties. Urea resins have now been developed a t least equivalent t o the melamine resins (3, 69). The resin particle should be positively charged for complete absorption, and with urea resins this has been accomplished b y carrying out the condensation in the presence of dimethylaminoethanol (6) or aliphatic diamines (46). A new type of condensate for wet-strength paper (11) has recently been made b y the condensation of formaldehyde with polyamines, which are prepared b y the ammonolysis of polyketones from ethylene and carbon monoxide, or by the reduction of polyacrylonitrile. High-frequency curing of urea resins has been a problem and the introduction of special catalysts has given better results. Urea resins find application in the textile field; recently the use of water repellents with the methylol ureas has given fabrics with repellency, shrinkproof properties and some stiffening. I n the adhesive field urea resins are often used with starch; new resins have been developed for this use. Urea resins have been used to insolubiliae polyvinyl alcohol (65). Many attempts have been made t o impart some flexibility to urea resins. Recently 1,4-butylenediurea or 1,3-naphthylene dithiourea has been condensed with formaldehyde t o give flexible condensates (19). GR-S rubber latex has been suggested as a flexibilizing agent (16). Plants for the manufacture of urea and melamine resins have been described (38, 36). Plastic tableware is becoming increasingly popular and now represents an important outlet for melamine resins. Their light weight and strength have been factors in their acceptance, Melamine resins are being used in the plywood adhesive field and‘their durability and ease of handling have been advantageous. Cationic-type melamine resins are widely used for wet-strength paper. Alcohol-modified melamines are used with alkyds for baked coatings. Triazines, other than melamine, have been reported to have superior.properties for coatings (21). A new development is solid soluble melamine and triazine resins for use in coatings. KINETICSAND STRUCTURE.Several studies have been made on the kinetics of the reaction of urea and formaldehyde (10, 37, 38). It has been found t h a t hydrogen ion concentration has a greater effect on the rate of the reaction than does temperature. Methylene glycol does not react with urea ( 3 7 ) , and methylol groups are slowly completely removed in the presence of hydroxylamine hydrochloride. The structure of urea resins is not entirely agreed upon. Marvel has suggested t h a t a cyclic trimer of methylene urea is formed (28) and has shown that urea reacts like an amine-amide (Figure 6). T h e second NH2 or amide group is assumed to condense to form a bisamide. Such behavior would explain ths condensation of 3 moles of formaldehyde with 2 of urea.
1066
INDUSTRIAL AND ENGINEERING CHEMISTRY
The structure of melamine-formaldehyde resins is not entirely clarified, b u t the occurrence of methylene bridges in the polymer is recognized.
H O H
I
I1
O
II
I
HOH~C-X-C-S-CHLOH Dimethyl01 Urea
e R-C--?;=CH, Methylene Urea
HZC
CH,
\
/
‘K’ I
C-X-CHgOH I
O
I
H
Figure 6 iMIXED CONDENSATES
Melamine and urea have been condensed with formaldehyde t o give resins wit,h intermediate properties. Acetone-formaldehyde has been condensed with phenol ( 4 ) . Phenol has been used with both urea and melamine t o produce condensates with formaldehyde. The structure of phenol-, urea-, and melamine-forinaldehyde resins still presents a problem t h a t is only partially solved. Work in this field promises t o extend our understanding and assure better utilization of these resins. ACKNOWLEDGMENT
Invaluable help in the preparation of this paper has been provided b y R. W. Auten. R. W. Auxier, F. H. Carman, E. E. Kimmell, J. 31.Sanderson, and J. M. Simons. LITERATURE CITED
American Cyanamid Co., Brit.. Patent 598,175 (Feb. 12, 1948). Auten, R. W., U. S. Patent 2,471,188 (May 24, 1949). Auten, R. W.,and Rainey, J. L., Ibid., 2,407,599 (Sept. 10, 1946).
Bakelite Corp., Brit. Patent 655,689 (Aug. 1, 1951). Bender, H. L., Farnham, A. G., Guyer, J. W., 4pe1, F. N., and Gibb, T. B., Jr., IND. EKG.CHEM.,44, 1619 (1952). Bruyne, N. A. de, U. S. Patent 2,499,134 (Feb. 28, 1950). Chem. Eng., 58, No. 10, 215 (1951). Chem. Eng. ‘Vews, 30, 3452 (1952). Coggeshall. N. D., J . Am. Chem. Soc., 72, 2836 (1950). Crowe, G. A., Jr., and Lynch, C. C., Ibid., 70, 3795 (1948); 71, 3731 (1949).
Cupery, bI. E., U. S. Patents 2,526,637, 2,526,638 (Oct. 24, 1950).
Vol. 45, No. 5
(12) Debing, L. M., Murray, G. E., and Schatz, R. J., IND. ESG. CHEM.,44, 354 (1952). (13) Denton, W.I., Doherty, H . G., and Kiieble, R. H., Ibad., 42, i 7 7 (1950). (14) Du Pont de Kemours & Co., E. I., Brit. Patent 583,504 (Dee. 19, 1946). (15) Edgar, K. L., Stavely, F. W., and Kovotny, C . K., U. S. Patent 2,424,923 (July 29, 1947). (16) Finn, S. R., Megson, N. J. L., and Whittaker, E. J . W., Chemist r y and I n d u s t i y , 1950, 849. (17) Finn, S. R., and Rogers, L. R., J . Soc. C h e m I d , 67, 51 (1948). (18) Freeman, J . H , Division of Paint, Varnish, and Plastics Chemistry, 121st Meeting AM. CHEY.Soc., Milwaukee, 1952. (19) Garner, P. J., and Bowman, R. E., U. S.Patent 2,572,256 (Oct. 23, 1951). (20) Greenlee, S.O., Bid.,2,521,911 (Sept. 12, 1950). (21) Grinsfelder. H.. ISD. ENG.CHEni.. 44. 563 11952). (22) Harder, R. K., Wallace, R. D., and McKinney, R. W., Ihid., 44, 1508 (1952). (23) Hoover, hI. M., Chem. Eng., 57, No. 4, 132 (1950). (24) Hunter, R. F., and Vand, V. J., Applied Chem. ( L o n d o n ) , 1, 298 (1951) (25) Lowe, W. G., U. S. Patent 2,481,676 (Sept. 13, 1949). (26) Martin, R. R., J. Am. Chem. Soc., 74, 3024 (1982). (27) Martin, R. W., U. S. Patents 2,579,329, 2,579,330, 2,579,331 (Dee. 18, 1951). (28) Marvel, C. S.,Elliot, J. R., Boettner, F. E., and Yuska, H., J . Am. Chem. Soc., 68, 1681 (1946). (29) Maxwell, C. S.,U. S.Patent 2,582,840 (Jan, 15, 1952). (30) Meyer, L. S., Ibid., 2,559,891 (July 10, 1951). (31) M o d e r n Plastics, 29, N o . 12, 83 (1952). (32) O’Connor, J. A., Chem. Eng., 56, No. 12, 89 (1949). (33) Raff, R. A. V., and Silverman, B, H., IND.ENG.CHEM.,43, 1423 (1951). (34) Reineck, E. A , , M o d e r n Plastics, 29, KO,10, 122 (1952). (35) Sawyer, F. G., Hodgins, T. S.,and Zeller, J. H., IND. EKG. CHEM,40, 1011 (1948). (36) Schlesman, C. H., Denton, W. I., and Bishop, R. R., U. S. Patent 2,456,597 (Dec. 14, 1948). (37) Smythe, L. E., J. Am. Chem. Soc., 73, 2735 (1961). (38) Smythe, L. E., J . P h y s . Colloid Chem., 51, 369 (1947). (39) Spahr, R. J., .Moffitt, W.R., and Pryde, E. H., U. S. Patent 2,489,336 (Nov. 29, 1949). (40) Sprengling, G. R., Division of Paint, Varnish, and Plastics Chemistry, 118th Meeting AM. CHEY.SOC., Chicago, 1950. (41) Sprengling, G. R., Division of Paint, Varnish, and Plastics Chemistry, 121st Meeting A M , CHEM. SOC., Milwaukee, 1952. (42) Sprengling, G. R., J . Am. Chem. Soc., 74, 2937 (1952). (43) Sprengling, G. R., and Freeman, J. H., Ibid., 72, 1982 (1950). (44) Sprung, M. M., and Gladstone, M. T., Ibid., 71, 2907 (1949). (45) Suen, T. J., and Daniel, J. H., Jr., U. S.Patent 2,554,475 (May 22, 1951). (46) TenHoor, R. E., Dow D i a m o n d , 15, KO.2, 81 (1952). (47) Thurmond, C. P., P a i n t a n d Vamish Production, 30, No. 5, 10 (1950). (48) Walker, J. F., and Chadwick, A. F., IND.ENG.CHEM.,39, 974 (1947). (49) Zigeuner, G., and Gabriel, O., Monalsh., 81, 952 (1950). (50) Zigeuner, G., Schaden, W., and Wiesenberger, E., Ibid., 81, 1017 (1 > - %50 \. (51) Zigeuner, G., Ziegler, E., Schaden, TT‘., and Wiesenberger, E., Ibid., 81, 326 (1950). (62) Zinke, iz., Zigeuner, G., Weiss, G., Leopold-Lowenthal, W., and Wiesenberger, E., Ibid., 81, 1098 11950).
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RECEIVED for review September 24, 1052.
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