ORGANIC ISOCYANATES-VERSATILE CHEMICAL INTERMEDIATES
0
R. 6. ARNOLD, J. A. NELSON, and J. J. VERBANC E I. du Pont de Nemours and Compsny, Wilmington, Delaware
I~OCVANATES (R-N=C=O) are as old as the field of organic chemistry. Wurta (1) synthesized the first isocyanate in 1849, Hofmann (2). the first aromatic member of the series in 1850. 'Ibis early work also disclosed their reactions with water, amines, and elcohols as well as the formation of dimers and trimers. I n subsequent years these compounds were not entirely neglected, but intensive study of isocyanate chemistry did not begin until their potential importance in the polymer field was recognized. The knowledge that certain basic raw materials such as cellulose, rubber, wool, silk, and proteins are polymers served as the foundation for a new chemical era. Chemists visualiaed the potentialities of synthetic high polymers designed to overcome inherent limitations of the natural materials. The search began for quantitative reactions, which could be used for the preparation of high polymers, and for the necessary pure, bifunctional intermediates. This research resulted in the development of such well-known products as the polyolefin elastomers and plastics, the polyamide and polyester fibers and plastics, the phenol- and urea-formaldehyde resins. It also focused attention npon the versatile isocyanate family not only as polymer inter-
mediates but also as modifiers for polymers prepared from other materials (8, 4). REACTIONS
The isocyanate function (N=C=O) is reactive toward a wide variety of reagents. I t reacts with nucleophilie componnds, i.e., those which have a pair of electrons which can be shared. These nucleophilic compounds such as amines, amides, alcohols, phenols, water, earboxylic acids, thiols, and structures containing active methylene groups attack isocyanates in a manner which may be generalized: 0
Isocyanates are reduced to secondary amines (R2NH) catalytically, or with lithium aluminum hydride. With Grignard reagents they react ~moothlyto form amides 0 R'
I1
I
(RC-NH). Under the influence of tertiary amines (RIN) or phosphines the aromatic derivatives dimeriae to form 1,3-suhstituted uretidinediones (see Figure 2). I n the presence of strong alkalies or carhoxylates both aliphatic and aromatic isoeyanates trimerize to form tri-N-substituted isoeyanurates. An outline showing some of the typical reactions of isoeyanates is given in Figure 1. Reviews are available which treat the chemistry of isoeyanates in considerable detail (5,6). Isocyanates are often described as "reactive," but their activity varies considerably depending npon the specific conditions. With amines reaction rates are very high though they fall off rapidly with decreasing base strength of the amine. Water, alcohols, phenols, OC N
R&
NCO
0 NH
NH-&
CH3 OCN-{-C~~Q CH3
70
0 NH~NH-@H~~-NMJ
CH3
OCN
/o
R
b
+
C+A~NH!CI ~ C I - A ~ N C OHCI
riwm
1.
Typical 1.ong.n.t.
Reaction.
JOURNAL OF CHEMICAL EDUCATION
and thiols are less reactive than most amines. Rcactivity toward a given attacking reagent also depends upon the structure of the isocyanate. Withdrawal of electrons from the isocyanate function by the structure to which it is attached enhances its activity. Thus, the reaction rates of negatively substituted aromatic isocyanates may be lo5-lo6times as great as those ohserved for the alkyl derivatives. Reaction kinetics and mechanisms represent a fruitful area of research in the isocyanate field. The seemingly simple reactions of isocyanates are often rendered complex by catalytic effects. Not only can known catalysts be added deliberately, but catalysis may be due to the reactants, the products, or traces of impurities. Some work has been done in this area but much is still required before conclusive descriptions can he given. SYNTHESIS
Numerous methods for the preperation of organic isocyanates have been developed based on such diverse reactions as the Lossen (7) and Curtius rearrangements ( 8 ) ,double decomposition between a-cyanate salt and an ester of an inorganic acid (9, I ) , the thermal decomposition of ureas (10, 11, 12) and urethanes (13, 1 4 , and the phosgenation of amines (15-20).
covered by distillation. Yields are generally high as is the purity of the product. Industrial facilities are available today for the manufacture of millions of pounds of such products as toluene-2,4-diisocyanate. POLYISOCYANATES
It is the polyisocyanates which are of particular interest to the industrial chemist as versatile intcrmediates, the hasis of a growing segment of the polymer field. However, their chemistry can be, for the most part, defined by studies with monoisocyanates. One problem which does arise in the case of aromatic polyisocyanates is the prediction of relative reactivities. This subject has received some attention (26). If two isocyanate functions are attached to the same aromatic ring, each activates the other. When one of the functions reacts, for example, with an alcohol to form a carbamate ester, it is transformed from a mildly electronegative to a mildly electropositive suhstituent and the remaining isocyanate group is somewhat deactivated. To illustrate the complications caused by this factor, fcur rate constants are required to describe the reaction between toluene-2,4-diisocyanate and an alcohol to form a urethane.
NHCOOR
A large number of aliphatic, aromatic, and heterocyclic isocyanates have been reported, and a picture of the divers it,^ of the structures involved can best be gained by reference to a review on their preparation by the Curtius rearrangement (8). The recent literature, particularly the recent patent literature, discloses a further variety of structures including isocyanates derived from fluorocarbons (21, 22), alkylphosphonyl isocyanates (23) and carbonyl- (24) and sulfonylisocyanates (25). While the Curtius rearrangement is an elegant laboratory method and various other procedures are useful in individual cases, the method generally chosen for the largescale manufacture of isocyanates, particularly of the high boiling aromatic di- and polyisocyanates which are today assuming industrial significance, is the phosgenation of amines or amine hydrochlorides. The chemistry of this method involves, essentially, two steps: (a) the formation of a carhamoyl chloride in the presence of an excess of phosgene followed by (b) thermal dissociation of this intermediate to the isocyanate and hydrogen chloride. RNH* amine
+
-
COCI, phosgene
+
RNHCOCl HCI carbamoylchloride
130'-30O'C. RNHCOCl A RNCO
+ HCI
At the elevated temperatqres employed, the HC1 is stripped from the liquid reaction mass, an inert, high hoiling solvent is usually used, and the isocyanate is reVOLUME 34, NO. 4, APRIL, 1957
~ N H C O O R + ROH
\/ I
NCO
In certain cases advantage can be taken of this complex reaction rate picture. Some of the diisocyanates bear functions which differ sufficiently in reactivity to permit high yield reactions involving only one of the isocyanate groups It is thus possible to prepare a secondary series of new high melting, storage-stable, relatively insoluble diisocyanato uretidinediones and ureas. The synthesis of these derivatives is schematically outlined in Figure 2 (27,28). I n the preparation of uretidinediones (dimers) advantage is taken of the fact that hindered aromatic and aliphatic isocyanate functions do not dimerize under mild conditions or in the presence of the catalysts which lead to dimerization of the unhindered isocyanate groups attached to the aromatic nucleus. Similarly, the simple urea compounds can be prepared by reaction with limited amounts of water when one of the isocyanate functions is sufficiently deactivated by steric hindrance or attachment to an aliphatic carbon. Because of their extremely low solubility these compounds do not react
ADHESIVES
significantly with water a t room temperature. This fact makes possible the preparation of stable water dispersions (99). The voluminous formal and patent literature which has appeared since 1940 is an index of the scope and intensity of research in the field of isocyanate chemistry. Detailed variations are so numerous that a complete discussion is not warranted. However, the major areas in which isocyanates are today finding application are schematically outlined in Figure 3. A gen eralized treatment of the chemical technology involved in the synthesis of representative products will serve to illustrate isocyanate versatility.
useful polyurethanes have an average molecular weight of 7000-12,000. The best known polymer of this series is a textile fiber, Perlon U. While. the major use is in the textile field, additional applications as a thermoplastic molding composition have been developed. These polymers are of interest because of their strength, abrasion resistance, low water absorption, good electrical properties, and resistance to outdoor exposure and acids. FOAMS
An early stimulus to the use of diisocyanates in the polymer field was the appearance of the polyamide fibers, the nylons. Shortly thereafter work was initiated on the preparation of spinnable polyurethanes and polyureas from glycols, diamines, and diisocyanates. The polymerization of two simple bifunctional intermediates, specifically, 1,4-butanediol and 1,6-hexane diisocyanate, is the basis for a series of well established synthetic polyurethanes developed in Germany (3, SO) :
Diisocyanates, in their reactions with water and carboxylic acids, produce as a byproduct of polymerization a gas, carbon dioxide. As a consequence the polymerizing mass is expanded to form a porous voluminous structure. When this reaction is properly controlled, foams with excellent structure and almost any desired density can be obtained. Depending upon the structure of the intermediates used, the resulting product will be either rigid or elastic. Foam prepolymers are viscous fluid reaction p~oductsof low molecular weight made from polyalcohols and polyisocyanates. These prepolymers are mixed with water or dicarboxylic acids
These products can be produced either in a solvent system or as melts from which the products are isolated. Depending upon the mole ratio of the reactants, temperature, time, and the use of monofunctional modifiers, the molecular weight of the polyurethane can be controlled within specific limits. The technically
just prior to use, the mixture is poured into a mold, and chain extension, and foaming and crosslinking occur simultaneously (Sf). The fundamental chemistry involved in the formation of rigid and elastic products is similar as illustrated in the followingequations (39, SS) :
FIBERS
160
JOURNAL OF CHEMICAL EDUCATION
11. Rigid Foam
I. Elastic Foam
Basic intermediates:
Basic intemediates:
-
HO-R'-OH (mol. at. 1000)
+ n OCN-R-NCO
I
OH
1
urethane formation
I
urethane formrttion
P~epolpner:
0
+ n OCN-R-NCO
HO-OH
Prepolpner:
0
I
H20, catalyst chain extension (R3N) by urea farmetion Polymer:
+ (2n - 2)COz t
1 crosslinking reaction
I
R
Crosslinked foam:
OCN-R-NCO
Elastic foams are prepared from low melting polymeric glycols such as polyesters, polyethers, or polyesteramides of molecular weights of a t least 1000, and the finished, crosslinked polymer is an open network with long flexible chain segments between crosslinks. Rigid foams are prepared from low molecular weight polyalcohols, and the final polymer is densely crosslinked. The elastic or resilient foams exhibit excellent ozone, oxygen, and oil resistance and moderate resistance to water. They have greater load-bearing capacity than natural rubber foams of equal density. This characteristic combined with their high tensile strength and ease of fabrication suggests their use in many applications now dominated by natural rubber. The rigid foams are tough, dimensionally stable (below 100°C.) cellular materials with good resistance to shock and vibration, and insulating properties equivalent to cork. Since the product can be applied as a pourable liquid and foamed in place and cured without the application of external heat, it can be used without the aid of elaborate VOLUME 34, NO. 4, APRIL, 1957
equipment (Figure 4). As it foams and cures, it will form strong bonds to metals, wood, paper, and textiles making possible the construction of strong, lightweight laminated structures faced with a variety of materials. Such products are of interest to the construction industry. ELASTOMERS
Polyurethane elastomers represent one of the largest potential outlets for isocyanates. Interest in these products stems primarily from their excellent resistance to abrasion, solvents, and oxygen. The intermediates employed include polyesteramides, polyester glycols, and polyether glycols of molecular weights in general of 1000 or greater. Aromatic and aliphatic diisocyanates have been used in the polymerizations. The products range from liquids to solids, some of which must be immediately molded and cured and some of which are stable t o storage. A number of experimental elastomers have been announced, Vulcollan ($2, $3) and Chemigum SL (34),
both derived from polyester glycols, and "Adiprene" urethane rubber (55),prepared from polyether gl~cols. Vulcaprene (36) which appeared on the market a number of years ago was based on polyesteramides. The chemistry involved in the preparation of Vulcollan has been described in some detail W). One member of the Vulcollan family is based on a polyester glycol, a diisocyanate and wateras depicted in the following equations. Three primary steps are involved in the synthesis.
The water in the sequence (see Step 111) serves both to chain-extend the 'Lisocyanate-polyester" to a high molecular weight polymer and to produce a crosslinking reaction. Other bifunctional compounds may be substituted for the water, ~ l ~ diamines, ~ ~ dil ~ , carboxylic acids, and amino alcohols have been used, The finished product is a transparent rubbery solid, usually amber in color. An example of a solid ~ 0 1 ~ urethane elastomer is shown in Figures 5 and 6.
Step I Synthesis of polyester glycols:
HO--CH2CH,-OH,
I-
0 Ho[cH2cH20-
A 70/30 mixture of polyethylene and polypropylene adipates is preferred in practice. The elastomer derived from polyethylene adipate is strong and resists abrasion but freezes s t spproximately 25% The elastomer prepared from the mixture retains this strength and abrasion resistance hut has a lower freezing point and hence is of greater practical interest. Step I1 Synthesis of the "isoeyanatepolyeste~":
-
+3
(7-NC0
2 HO-(polyester)-OH (mol. wt. 2000)
OCN
d -
0
0
OCN
I
C)
-NHAO-(polyester)-Ol!!NHo
C>_yco Step I11 Chain e z t a s i o n and erosslinking:
"Isocyanate-polj,ester2'
* 0
C =0
0
o
D-NH80-(polye8ter)-o
+ co,
-SpNd
iw
'
-O&NH-C)
I
OCN-NCO
JOURNAL OF CHEMICAL EDUCATION
hose, belting, aud coated fabrics. The resin isocyanate combinations are applicable where flexibility of the bond is not required as in t,he manufacture of solid and cellular wood laminates. Adhesives derived from polyols and isocyanates show promise also for bonding rigid members such a s metals, wood, and glass, and probably will find their greatest outlet in the fabrication of lightweight structural or decorative units. The mechanism by which one component is bonded to another by polyisocyanates or their derivatives is not kno7r-n. It is unlikely that a siugle mechanism can explain the adhesion of the numerous diverse substrates. Considerable effort will he required to elucidate the chemist,ry and to realize the full pot~entialof polyisocyanates in the adhesive field. ISOCYANATE GENERATORS
=igure 4.
Variety of Products Prodused from the V e r s a t i l e Urethane roams
Hieh strength with light weight, reaistsnee to hotmehold chemicals. and wasl~abilitvmake the new foams idcalfor numerous uses in the home. These inelado sillows s n d oaahiana. rug underlays, sponge%, atending mats, absorbent susg dirbes. ironing-board cov?ia. mattress and table pads, olotheshanger pads. and other products. For clothing there are now shoulder pads toade of urethane form. ss well as foam-lined fabrics for cold-weather wear. The foalns also onerhieh energy-absorption and *re umd ior dashboard and sunvisor crash pads on nev model automobiles ( s h o r n in backmuend). The material is foamed-in-,,1aeet o eliminate seDarate cementine and t o D m vide sleek, wric~kle-free rarfaees.
ADHESIVES
The union of two or more substrates by means of an adhesive is increasing in importance as a commercial fabricating technique. The demand for economical, durable unions in varied fields has catalyzed the search for adhesives. Without adhesives certain artirles of commerce such as tires, hose, belting, etc., which involve such diverse materials as textiles and rubber, would not be practical. Thus the combined advantages of such materials of construction as nylon, rayon, etr., and the modern synthetic elastomers could not be realized. The desire to lower fabrication costs by circumventing classical techniques such as nailing, riveting, soldering, and welding and to produce composite structures having uniform stress distribution further stimulated development and acceptance of adhesives. Polyisocyanates because of their compatibility and reactivity with a variety of materials have been used successfully in numerous adhesive applications. In a few instances the polyisocyanates are used without auxiliary agents. In the majority of applications, however, the polyisocyanate is combined with a second component such as a diene hydrocarbon elastomer (V), a resin or a polyfunctional alcohol (SS), the isocyanate being generally used in sufficient quantity that the finished adhesive contains unreacted isocyanate funct.ions. The adhesives prepared from natural or synthetic rubbers are most useful in the bonding of elastomer compositions to substrates such as cotton, rayon, nylon, metal, wood, leather, and ceramics. A particularly important application is in the fabrication of the reinforced flexible laminated structures found in tires, VOLUME 34, NO. 4, APRIL, 1957
Industrial interest in isocyanaies stems from their' extreme reactivity with a host of compounds containing a displaceable hydrogen atom. Many of the potential applications, such as the modification or rrosslinking of polymeric systems, have been impeded by the poor storage stability of certain desirable isocyanates and by their premature reaction when combined \vit,h the substrate. Some control of the reactivity of isocyanates has been achieved by ( a ) introduring neighboring substituents which by steric hindrance or elertrooic effects serve to decrease the activity of the adjacent isocyanate group and ( h ) by the synthesis of compounds which thermally dissociat,e to produce the parent isoryanate. The main disadvantage of the latter system in some instances is the contamination of the finished product with the non-isocyanate fragment.
Figure 5 .
A Thin Sheet of Raw P d y v r e t h a n e Elastomez
The concept of isocyanate "generatorsn is based on the fact that many of the reactions which isocyanates undergo with compouuds having a displaceable hydrogen atom are reversible. Emphasis has, therefore, been directed toward the preparation of those derivatives which dissociate or exchange readily. In order to minimize cost or decrease dilution of the finished product by the uon-isocyanate fragment, active hydrogen compounds of low molecular weight are generally used. Two examples will serve to illustrate the principle of an isocyanate generator. A prime example is that already disrussed for the preparation of isocyanates by the commercially attractive phosgenation route. The intermediate carbamoyl chloride dissociates readily a t elevated temperatures to produce isocyanate and hydrogeo chloride. Carbamoyl chlorides, however, are not suitable for the modifiration or crosslinking of polymeric substrates because of the undesirable hyproduct, hydrogen chloride. A more practical compound is exemplified by the orethane derived from an isocyanate and phenol.
-
The preparation of a crosslinked macromolecule IS accomplished by the reaction of a diisocyanate with a polyfunctional alcohol. Glycerol, 1,2,4butane trial, triinethylol propane, polyvinyl alcohol, and acetyl cellulose have been used. These products are useful in lacquer formulations and also for the fabrication of molded objects. Since the finished products are insoluble and infusible, they must be formed in place. Generally the mixture of intermediates is deposited and the final reaction achieved by heating or baking (50).
n
/I
100"-150" C.
R-NH-c-o-C>
F========? R-NCO
+ C,H,OH
Xumerous compounds of the lat,ter type have been investigated (3, 11). They include the mono-, di-, or poly-reaction products of mono-, di-, or polyisocyanates with serondary aromatic amines, tertiary alcohols, amides, lactams, monohydric phenols, mercaptans, enolizable hydrogen compounds, heterocyclics, hydrogen cyanide, and sodium bisulfite.
Figilrr 6. A Tare Constructed w i t h an Experimental Polyurethane
Elastomer Tmad
PLASTICS
CONCLUSION
Plastics are becoming increasingly importaut as articles of commerce. For the most part they are hoinopolymers and copolymers derived from one or more unsaturated intermediates. I n contrast to these materials, limited somewhat by availability of suitable monomers and their ability to copolymerize, an infinite numher of segmented polymers are readily available via the addition polymerization of di- or polyisocyanat,es and difunctional components. Not only can the stmct,ure of the diisocyanate be varied ad infiniturn, but. the size, structure, reactivity, aod functionality of t,he coupling component can be varied also. As a result of this apparently unlimited choire of reactants, t,he desigo of plastic compositions for any specific application can be visualized. Research in this field has not been exteusive thus far. Low molecular weight or crystalline high molecular weight intermediates have been investigated. These include such functional units as glycols, diamines, and dicarboxylic acids. The technology is similar to that used in the synt,hesis of the fiber, Perlon V. I n the case of the thermoplastic, Igamid U, the intermediates are identical. Further variations are commonly introduced by changes in the glycol structure or by combinations of diols. The synthesis of a soluble linear product, Igamid UL, is as illustrated:
Other miscellaneous uses for isocyanates or polymers derived from them have been disclosed. These include the fixation of pigments on textiles and paper, and the bonding together of or coating of fibers in order to develop water repellency or other novel effects. Modification of leather and other naturally occurring products such as wool and silk have been reported also. Their use in the synthesis of organic intermediates has been limited thus far to pharmaceuticals and herbicides. In a short span of twenty-five years organic isocyanates have grown from the stage of obscure laboratory curirsities to commercial products. The versat,ility of these compounds has been demonstrated on a laboratory and semiworks scale and a host of end use items are being developed and t,ested.
OCN(CH*),NCO 0
I
LITERATURE CITED WURTB, A,, Ann., 71, 326 (1849). HOFMANN, A. W., Ann., 74, 9 (1850). BAYER,O., Angew. Chen~.,A59, 257 (1047). W. I., AND D. F. HOLMES, U. S. Patcnt 2,281,HANFORD, 896; Chem. Abs., 36, 6706 (1942). (5) SAUNDERS, J. H., A N D R. J. SLOCOMBE, Chem. Revs., 43, 203 (1948). (6) ARNOLD, R. G., J. A. NELSON,AND J. J. VERBANC, Chem. Reus., forthcoming puhlioation. (7) YALE,H. L., Chem. Revs., 33, 209 (1943). (1) (2) (3) (4)
+ '/,HO-(CH,kOH + '/?HO-CH2CH(CHs),CH,-OH
0
I
0
CH,
0
0
I1 I ocx(cH8)6NH~-o~(cH2)4-o-~xH(c~2)aNH~-o-cHz~H(cH2)4o-cNH(cH2~axH~-o~(c~2l4-oH L -17, 164
JOURNAL OF CHEMICAL EDUCATION
(8) SMITH,P. A. S., "Org. Reactions," Val. 3, John Wiley & Sons, Inc., New York, 1946, p. 337. (9) BIEBER,T. I., J. Am. Chem. Soe., 74, 4700 (1952). (10) BENNET, W. B., J. H. SAUNDERS, AND E. E. HARDY, J. Am. Chem. Soe., 75, 2101 (1953). (11) PETERSEN, S., Ann., 562, 205 (1949). (12) STROHMENGER, L., German Patent 748,714 (Feb. 5, 1945). ((3) CUPERY,H. E., U. S. Pstent 2,346,665; Ckem. Abs., 38, 5845 (1944). (14) SIEFKIN.W.. Ann.. 562. 75 (19491. (15i ~arhenfibriken ~ & e r'A. G., krench Patent 1,098,303 (July 22, 1955). (16) Farbenfabriken Bayer, A. G., French Patent 1,098,492 (July 27, 1955). (17) IRWIN,C. F., U . S. Patent 2,683,160; C h a . Abs., 49, 9034 (1955). French Pstent 1,088,275 (18) IRWIN,C. F., A N D F. W. SWAMER, (March 4, 1955). (19) LONG,J . R., French Patent 1,102,430 (Oct. 20, 1955). (20) Monsanto Chemical Co., British Patent 737,487 (Sept. 29, 1955). ~~~(21) A N D E A NF. , A,, B. BAK, 11. J. CALLOMON, A N D H. W. THOMPSON, J . Chem. Soc., 1953, 3709. (22) REID,T. S., U. S. Patent 2,706,733 (April 19, 1955). (23) HAVEN,A. C., J. Am. Chem. Soc., 78, 842 (1956). (24) WALTMANN, E., AND E. WOLF,U. 8. Patent 2,346,202. (2.5) KRZILKALLA. H.. U. S. Patent 2.666.787: Chem. Abs.. 48.
VOLUME 34, NO. 4, APRIL, 1957
(26) BAILEY,M. E., V. KIRSS, A N D R. G. SPAUNBUAGH, Ind. Eng. Chem., 48, 794 (1956). (27) BALON, W., E. 0. LANGERAK, D. M. SIMONS, AND 0 . STALLMANN, Division of Paint, Plastics, and Printing Ink Chemistry, 16, 67 (1956). (28) BARTHEL, E.; C. L. KEHR,E. 0. LANGERAX, R. L. PELLEY, AND K. C. SMELTE. Diuision of Paint.' Plastics. and Printing Ink Chemistry,' 16, 78 (1956). (29) CHRISTOPH, F. J., Du Pont Co., private communication. (30) HOPFF,H., A. MULLER,AND F. WENGER,"Die Polysmide," Springer-Verlsg, Berlin, Gottingen, Heidelberg, 1954, p. 42 ff. (31) STEVENSON, A. C., Rubber Age, 77, 63 (1955). (32) BAYER,O., E. M ~ L L E R 8., PETERGEN, H. F. PIEPENBRINK, E. WINDEMUTH, Angew. Chem., 62, 57 (1950). (33) MULLER,E., 0.BAYER,S. PETERSEN, H. F. PIEPENBRINK, F. SCHMIDT,E. WEINBRENNER, Angew. Chem., 64, 523 (19521. (34) SE~GER,'N. V., T. G. MASTIN,E. E. FAUSER, F. S. FARSON, A. F. FINELLI,A N D E. A. SINCLAIR, Ind. Eng. Chem., 45, 25RR (1U.5R) ---,----,. (35) HILL,F. B., C. A. YOUNG, J. A. NELSON, AND R. G. ARNOLD, Ind. Eng. Ckem., 48, 927 (1956). (36) HARPER,D. A., W. F. SMITH,AND H. G. WHITE,Rubbe? Chem. and Teehnol., 23, 608 (1950). (37) NEAL,A. M., AND J. J . VERBANC, U. S. Pstellt 2,415,839. (38) Monsanto Chemical Co., Australian Painnt Application 2192/54 (Aug. 5, 1954).
