September, 1942
INDUSTRIAL AND ENGINEERING CHEMISTRY
1091
rectly determined from the difference of the refractive index TABLEIV. VAPOR AND LIQUIDEQUILIBRIA AND RELATIVE readings for liquid samples drawn from above and below the OF 0-DICHLOROBENZENE-DIETHYLBENZENE VOLATILITIES column under test. MIXTURES ~ ~ l Mole % o-DiohloroVolatility benzene in: Liauid VaDor U -5 0 Millimeters97.70 1.143 97,38 92.40 1.192 91.07 1,085 82.77 83.90 1.068 77.05 75.87 1,105 71.10 73.10 64.56 1.081 62.76 1.063 64.02 62,60 56.90 1.094 54.68 1.079 49.97 48,07 1,150 61.20 47.70 42.37 1.118 39.67 36.00 1.156 32.73 1.100 34.65 32,53 31.15 1.135 28.50 1.141 24.17 21.83 1.115 17.05 18.65 1.095 11.20 10.33 1.162 8.55 9.80 1.298 5.40 6.90 1.361 4.73 6.33 2.90 3.25 1.125
~ Mole t 7% i o-Diohloro~ ~ ~ ~ l benzene in: Volatility Liquid Vapor LY -lo Millimeters97.23 97.67 1.194 95.45 1.097 95.03 91.88 1.092 91.20 1.119 91.00 90.04 81.58 1.091 80.23 79.95 81.84 1.130 70.27 73.70 1.186 73.02 1.146 70.26 63.17 1.125 60.40 61.68 1.079 59.87 54.30 1.146 50.90 53.28 1.142 49.97 40.05 42.93 1.126 42.74 1.144 39.48 34.12 1.200 30.14 32.60 1.190 28.90 1.229 20.98 24.60 1.161 20.56 23.10 1.274 15.16 18.54 11.60 i.im 10.33 8.55 1.218 7.13 6.54 1.279 6.19 3.24 1.125 2.89
~
t
i
~
~
Acknowledgment The authors wish to acknowledge the services of L. X I S sen, P. N. Heere, and S. Finelli in obtaining the data and preparing the plots.
Literature Cited (1) Beatty, H. A., and Calingaert, G., IND. ENG.CHEM.,26, 504-8 (1934). (2) Cornell, Wallace, and Montonna, R. E., Ibid., 25, 1331-5 (1933). (3) Pahlavouni, E., Bull. SOC. chim. Belg., 36, No. 11, 533-47 (1927). (4) Rosanoff, M. A., and Easley, C. W., J . Am. Chem. SOC.,31, 953-87 (1909). ( 5 ) Smith, E. R., and Matheson, H., J . Research Natl. Bur. Standards, 20, 641-50 (1938). (6) Tables annuelles de constantes e t donn6s numbriques, Vol. V I I I , p. 270 (1931). (7) Zawidski, Jan von, 2. physik. Chem., 35, No. 2 , 129-203 (1900).
Nitroparaffins as Solvents in the Coating Industry CHARLES BOGIN AND H. L. WAMPNER Commercial Solvents Corporation, Terre Haute, Ind. Four nitroparaffins-nitromethane, nitroethane, 1-nitropropane, and 2-nitropropane-are now produced on a commercial scale. Their mild odor and low degree of toxicity, coupled with high solvent strength and medium rate of evaporation, offer a combination of properties heretofore not obtainable in any series of solvents. While these materials are nitro-containing organic compounds, they are not so flammable as hydrocarbon solvents of equal rates of evaporation. The nitroparaffins are among the most powerful solvents known for the organic esters of cellulose, such as cellulose acetate and cellulose acetobutyrate. Solutions of these materials tolerate appreciable proportions of cheap diluents, such as alcohols and hydrocarbons. Their ideal rates of evaporation make it possible to formulate finishes which duplicate those of nitrocellulose for drying
T
HE commercial production of four nitroparaffinsnitromethane, nitroethane, and 1- and 2-nitropropanes -was started in the summer of 1940. Since that time they have become of considerable importance in the chemical industry. The synthesis of these materials and of many of their derivatives, their physical properties, and many suggested uses have been described in previous articles (47, IO, 11). The present paper will be limited to considering certain of the solvent uses of the nitroparaffins. Organic liquids find use as solvents in two large fields: the extraction of various materials by selective-solvent action,
speed and leveling properties. It is no longer necessary to use the customary combinations of fast- and extraslow-evaporating solvents to apply satisfactory films. The nitroparaffins show unusual solvent strength for vinyl copolymer resins and are being used in large quantities for this purpose. They are also of interest in practically every other coating problem. Thus, while they show no advantage over the ester-type solvents in most nitrocellulose coatings, there are certain specialty applications where their mild odor and different solvent properties have given them commercial applications. The nitroparaffins are also of interest in a number of miscellaneous coating applications-with shellac, synthetic and processed rubber, paint and varnish removers, alkyd resins, and other high-polymer coatings.
and the dissolving of materials to aid in carrying out reactions or to produce desired effects such as the deposition of films of protective coatings. In any of these applications the physical properties of the solvent are important. The nitroparaffins belong to the medium-boiling class of solvents, and their rates of evaporation lie between those of toluene and butyl acetate. Table I, which compares the evaporation rates of the nitroparaffins with the more commonly used lacquer solvents, gives a better picture of their exact position. The nitroparaffins are definitely not hygroscopic, and these materials and water have very low mutual solubilities.
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
1092
TABLE I. ET-APORATION RATESOF NITROPARAFFIXS AKD LACQUER SOLVENTS Solvent Acetone Ethyl acetate Methyl ethyl ketone Dioxane Ethyl alcohol Toluene Nitromethane 0
B.P.,
Evapn. Rate"
56.1 77.1 79.6 101.5 78.1 110.8 101.0
720 525 465 215 203 195 180
C.
Solvent Nitroethane 2-Nitropropane Butyl acetate 1-Nitropro ane Methyl Cefiosolve Butylalcohol Ethyl lactate Diacetone alcohol
B. P., Evapn. C. Rate' 114.0 120.0 126.5
145 124 100
124.3 117.7 154.0 166.0
55 45 22 15
132.0
100
Compared to butyl acetate a8 100.
Although the nitroparaffins contain both nitrogen and oxygen-a combination often associated with explosives and other highly flammable compounds-their flash points show that they are much less flammable than the common hydrocarbons, ketones, and esters of similar evaporation rate. Unsuccessful attempts have been made t o cauBe the nitroparaffins t o explode with detonating caps (6). A detailed study has been made of the toxicity of the nitroparaffins, and in this respect they are similar t o the ester-type solvents of approximately equal vapor pressure (6, 8, 9). They therefore offer no unusual hazards, either with respect to toxicity or flammability. The ordinary precautions eser-
TABLE 11.
cised in handling the common lacquer solvents are sufficient for the nitroparaffins. Table I1 shows the solubilities of a large number of substances in 1- and 2-nitropropane. It fails to show, however, that there are qualitative and quantitative differences between the solubilities in individual members of the nitroparaffin series. Thus, while 1- and 2-nitropropane are miscible with petroleum naphtha, nitromethane is immiscible with even low-boiling petroleum hydrocarbons, and the miscibility of nitroethane depends on the boiling range of the hydrocarbon. Because of these differences, it is logical to look for the use of the nitroparaffins as selective solvents in the petroleum industry, and developments of considerable magnitude have been forthcoming. The use of the nitroparaffins by the coating industry is, perhaps, of even greater interest
CELLULOSEORGANOESTERS Cellulose acetate has been known for many years, and a t one time was thought to be potentially the most useful of the various cellulose derivatives. It failed t o fulfill its original promise as a protective coating because of the lack of suitable solvents, plasticizers, and compatible resins. As a result, investigation of other cellulose esters has been pursued extensively.
SUBSTAXCES S O L U B L E AXD IXSOLUBLE IN
SOLUBLE SUBSTANCES~ Waxes Bayberry Ceresin (SS) J a p a n (SS) Spermaceti (SS) Natural Resins and Gums Barbados Manjak No. 54 Cumar Dammar (PSI Dammar (dewaxed) (SA) Kauri white (SA) Manila (SA) Pontianak (SA) Rosin (SS) Tars and Pitches Blown asphalt (SS) Bone pitch (SS) Gilsonite (SS) Petroleum asphalt (SS) Refined coal tar (5s) Road tar Synthetic Resins Arochlor 1242 Arochlor 4465 (PSI Bakelite BI-5900 Bakelite XR-3180 Bakelite XR-9366 Beckacite 1001 Beckacite 1100 Beckacite 1110 Beckacite 1112 Dures 500 Dures 525 Duiea 550 Ester gum Glyptal 2471 Mowalith N Paraplex 5B Paraplex R G 2 Phaflex Resyl 14 Resyl 19 Resyl 114 (SS) Santolite RIH
(SS)
Synthetic Resins (Cont'd) Santolite M H P Teglac 65 Uformite Vinylite H Vinylite "-44 (SS)
Oils and Fat* ( C o n t ' d ) Linseed oil Petroleum ether Petroleum oil Soybean oil Tung oil
IXSDLUBLE SUBSTANCES
(SS)
Organic Chemicals Acetic acid Acetone Aniline Benzene Butanol Butyl acetate Butyl butyrate Butyl lactate Butyl propionate Butyric acid Cottonseed oil f a t t y ncitls Dibutyl phthalate E t h y l alcohol Ethyl acetate Ethyl ether Ethylpropylacrolein Isopropyl alcohoi Maleic anhydride Mesityl oxide Waphtha, aliphatic Naphthalene Phenol Phthalic anhydride Picric acid Ricinoleic acid Toluene Tributyl phosphate Zinc stearate Oils and Fats Castor oil Coconut oil Lanolin (SS)
1- A S D %I\rTITROPROPANE SOLUBLE SUBSTANCES (Cont'd)
Coating Materials Benzylcelluloae Cellulose acetate (SA) Cellulose acetobutyrate Cellulose acetopropionate Chlorinated rubber Ethylcellulose Nitrocellulose Dyes Oil-soluble Spirit-soluble
Vol. 34, No. 9
Waxes Beeswax Candelilla Carnauba Montan Orocerite Paraffin Natural Resins and Gums Batu Boea Chicle Congo (dark amber) East India Shellac
Tars and Pitches Cottonseed pitch (m. p . 69' C.) Cottonseed pitch (m. p. 71" C.) Mixed pitch Paving pitch Roofing pitch Soft coal Coating Materials Alkagel Casein Dextrin Egg albumen Gelatin Methylcellulose Zein (cured) Zein (uncured)
Dyes Water-soluble Organic Chemicals Adipic acid Aluminum stearate Ammonium linoleate Anthracene Citric acid Fumaric acid Glucose Glycerol Glycerol boriborste Hydroquinone Maleic acid Oxalic acid Pyrogallic acid Salicylic acid Sebacic acid Sodium alginate Starch, corn Starch, potato Stearic acid Succinic acid Sugar Tannic acid Tartaric acid
Soluble t o the extent of a t least 10 grams per 100 ml., unless marked b y letters in parentheses as follows: SA = soluble in presence of alcohols; SS = soluble b u t leas t h a n 10 grams per 100 ml.; P S = only p a r t of substance soluble.
.
September, 1942
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
1093
The precipitating tendency of ethyl alcohol-toluene mixAT 25“ C. OF CELLULOSE ACETOBUTY- tures is shown in Table IV. The tolerance or dilution ratio of TABLE111. SOLUBILITY RATE I N MIXTURES O F NITROPR PROPANE AND DILUENTS cellulose acetobutyrate solutions in 1-nitropropane for ethyl --Compn. of Solvent Mixt., % by V 0 1 . p alcohol is 3.3 and for toluene is 2.1; but for a mixture of 1-NitroButyl DroDane Toluene Ethanol Butanol acetate Solubility ethyl alcohoI and toluene it is several times as high as for .. Sol. .. .. .. 100 either separately. The data in these tables also indicate that Sol. .. 40 60 butanol has a somewhat greater precipitating power than Sol. 50 50 ., P. 501.” .. .. 60 40 ethyl alcohol and is therefore more of a diluent; however, ,. P. so1.a .. 70 30 ., P. s 0 1 . a .. 60 40 the well-known antiblush characteristics of butanol make it a , , Sol. 30 40 30 worth-while constituent of solvent mixtures. ,, Sol. 32.5 35 32.5 Sol. 40 35 25 Table V is a series of viscosity measurements for solutions Sol. 25 40 35 .. Sol. 35 30 35 of cellulose acetobutyrate in various types of solvent mixtures. Sol. 25 30 45 It is of interest to note that in certain mixtures as much toluSol. 37.5 .. 25 37.5 Sol. 25 . . 50 25 ene can be used as is customarily employed with nitrocellulosc Sol. .. .. 30 40 30 P. sol. 35 30 35 Furthermore, comparison of mixture 19 with mixtures 6, 9, 50 P. sol. .. .. .. 50 and 16 shows that it is unnecessary to use large amounts of 30 Sol. 30 .. 40 .. 35 Insol. .. 35 30 materials such as acetone or ethyl acetate to produce low 0 At somewhat higher tern eratures the “psrtly soluble” mixtures disviscosities in cellulose acetobutyrate solutions. solved the oelluloae ester and &e solutions remained homogeneous when the temperature was reduced. One of the chief drawbacks to the development of cellulose acetate lacquers has been the dearth of suitable resins and plasticizers. This difficulty has not been so serious with cellulose acetobutyrate, since it is compatible with many of the Cellulose Acetobutyrate. The outstanding developcommon nitrocellulose plasticizers, and suitable formulations ment in the field of cellulose esters has been the producalso permit the incorporation of some of the resins which are tion of mixed cellulose acetobutyrates. The most imfrequently used with nitrocellulose. Table VI is a list of portant member of this series is probably the derivative resins that give compatible films with cellulose acetobutyrate containing twice as much acetyl (CHaCO) as butyryl (CsH7when deposited from nitroparaffin solvent mixtures. CO). This material is superior to nitrocellulose in that it Coatings are often sold commercially as concentrated offers no fire hazard, and it is superior to cellulose acetate in stocks to be diluted with a thinner by the user. For ease of water resistance and solubility in various organic solvents. solution and for stability, it is desirable to prepare such conThe resistance of cellulose acetobutyrate to sunlight and to ultraviolet light makes it an excellent material for use in exterior coating compositions. Before the development of the nitroparaffins, no solvents were available that would contribute to cellulose organoester TABLE IV. DILUTION RATIOSOF N NITRO PRO PANE" SOLUTIONS coatings the characteristics given by butyl acetate to nitroOF CELLULOSE ACETOBUTYRATE cellulose lacquers. For example, a typical solvent mixture Final Concn. of of the following composition has been used in cellulose acetoCellulose AcetobutyCompn. of Diluent, yo Dilution rate, G. 100 MI. of butyrate coatings: diacetone 15 per cent, ethyl acetate 35, by Vol. Ratio Voiatiles acetone 30, toluene 20. The rapid evaporation of the acetone 100 ethanol 3.3 7.0 and the ethyl acetate from such formulas tends to gel the film 100 toluene 2.1 4.5 70:30 ethanol: toluene 5.6 6.0 before it has a chance to flow out, and a t the same time, suffi60:50 ethano1:toluene 9.6 4 2 30:iO ethanol: toluene 2 5 10+ cient cooling is produced to cause an undesirable condensation 100 butanol 2.0 6 5 of moisture on the film. On the other hand, the very slow 70:30 butanol: toluene 3.5 8 5 50:50 butanol: toluene 5.4 6 0 evaporation of the diacetone tends to keep the film soft for 30:70 butano1:toluene 10+ 2 7 70:30 ethano1:naphtha 3.5 2 5 an extended period ( 2 ) . 30:iO ethanol: naphtha 3.0 2 5 Table I shows, however, that the four nitroparaffins evapoa The values for 2-nitropropane are only slightly different rate in the approximate range of butyl acetate, l-nitropropane having exactly the same rate of evaporation as the normal ester, Nitromethane, the fastest evaporating of the TABLE V. VISCOSITIES OF 5 PERCENTSOLUTIONS OF CELLULOSE group, is similar to toluene in its rate of evaporation. All of ACETOBUTYRATE IN VARIOUSSOLVENT MIXTURES these materials are solvents for cellulose acetobutyrate of the Mixt. Viscosity grade specified in the preceding paragraph; however, owing NO. Compn. of Solvent Mixt., % by Vol. Centipoisks to the limited supply of nitromethane and nitroethane, ex1 1-Nitropropane 100 74.5 2 1-Nitropropani:ethyl alcohol, 60:40 69.0 perimental work on the nitroparaffins as solvents for cellulose acetobutyrate has been restricted to 1- and 2-nitropropanes. 1-Nitropropane :ethanol:toluene 3 60:20:20 57.0 A grade of cellulose acetobutyrate which meets the above 30:35:35 4 56.0 5 i 0 :40:40 55.0 specifications has been studied extensively; while there are 6 20:25:56 47.0 less well-defined grades available which differ slightly in per7 20:50:30 65.0 15:40:45 8 55.0 formance from this product, information on this particular 1 6 . 9 6 . Rn _9 47.0 10 10:45:45 L0:30:60 Gelled grade is given here. 11 Partly gelled Table I11 shows that combinations of alcohol and toluene, 1-Nitropropane :butanol :toluene both of which might be normally considered diluents for solu60:20: 20 12 63.0 tions of cellulose acetobutyrate in a true solvent, actually 18 40:30:30 66.0 14 30:35:35 68.0 contribute to the solvent power of the nitropropanes. Thus, 20:40: 40 15 70.0 16 20: 25:55 52.0 1-nitropropane solvent mixtures containing 60 per cent by 17 20:50:30 82.0 volume of ethyl alcohol or 30 per cent by volume of toluene 15:25:60 18 56.0 do not dissolve cellulose acetobutyrate, whereas a l-nitroDiacetone:ethyl acetate:toluene:aoet6he propane solvent containing 75 per cent of a 50:50 mixture of 19 15:35:20:30 62.5 ethyl alcohol and toluene works satisfactorily.
INDUSTRIAL AND ENGINEERING CHEMISTRY
1094 TABLE VI.
RESINCOMPATIBILITY
Resina Compatible with Cellulose Acetobutyrate Ratio. Resin: Resin ester Santolite MS 4:1 Santolite M P H 4:1 Reayl 14 4:1 Reayl 337-1 4:1 Reayl 337-2 4:l Superbeckacite 2000 3:l Vinsol 2:l Hercolyn 1.5:l Bakelite 4357 1.5:1
50:50 Mixt. of Santolite MHPO with
Following Resins Ratio. Sentolite Resin MPH:resin Ester gum 2:1 Dammar 2:1 Rezyl 19 2:l Reayl 53 2:1 Rezyl 127 2:1 Petrex A21 2:l Beckosol 1323 2:1 Superbeckacite 1001 2:l Teglac 15 2:l Staybelite A-1 2:1 In some cases the Sentolite hIHP can be replaced by Reayl 14, Reayl 337-1, Santolite MS, or Bakehte 4357. (1
centrated stocks with solutions rich in nitropropanes. This can then be diluted with a thinner that is lean in nitropropanes so that the final solution has the composition selected by the manufacturer as most suitable to his purposes. Thus, in the formulation of a cellulose acetobutyrate lacquer containing the resins listed in Table VI, it is suggested that the concentrated stock solution be prepared with the solvent mixture given in Table VIIA. Prior to application, this concentrate would be diluted with the proper amount of the thinner given in Table VIIA to produce a lacquer having the solvent composition shown in the final column. The exact ratio of stock t o thinner would depend on the concentration of solids in the stock. When sprayed under the usual conditions for applying nitrocellulose coatings, lacquers of this type dry rapidly to films which are practically free of defects, Cellulose Acetate. The grades of cellulose acetate generally used in coatings consist of partially hydrolyzed materials. Often about 2.5 acetyl radicals are present per glucose residue, whereas theoretically 3.0 are possible. This hydrolysis of the cellulose acetate increases the solubility of the final product, but at the same time it increases its sensitivity to water. As a result, there has been some tendency within recent years to use a cellulose triacetate grade, which approaches the theoretical acetyl content. For the partially hydrolyzed type of cellulose acetate, a typical and commonly used solvent formula is the following: acetone 36 per cent, ethyl acetate 15, methyl ethyl ketone 35, ethyl lactate 15. This is similar in type to the formula specified above for cellulose acetobutyrate. I t is now possible through the use of nitroparaffin solvent mixtures to produce compositions which approach the ideal in rate of evaporation instead of the unbalanced mixtures for-
T.4BLE
VII. FORMULATION O F CELLCLOSE ACETOBUTYRATE, ACETATE,AND TRIACETATE LACQUER SOLVEKTS Solvent for concd. stock
Solvent Constituent
soln
yc Composition Thinner
Cellulose Acetobutyrate 45 5 5 15 15 15 35 65
A. 1- or 2-Nitropropane Butanol Ethanol Toluene
B.
10 15 50
15 20 20 45
35 15 15 35
Cellulose Triacetate 45 45 30 .. 25 25
45 10
10 10 25
C.
25
Cellulose Acetate 55
1-Nitropropane Butanol Ethanol Toluene
Solvent in lacquqr a t spraying viscosity
30
26
20
Vol. 34, No. 9
merly used. As with cellulose acetobutyrate, the solubility depends on the mixture of alcohol and diluents used with the nitroparaffins. One difference exists, however. The nitroparaffins, with the exception of nitromethane, are not true solvents for the usual grades of cellulose acetate but require the presence of small proportions of alcohol to activate them. Furthermore, the proper ratio of alcohol to nitroparaffin must exist during the entire drying of the composition. Because of this requirement, the formulation of solvent mixtures for cellulose acetate is more critical than for cellulose acetobutyrate. The desired effect is achieved by using mixtures of alcohols. With l- and 2-nitropropane a mixture of butanol and ethanol in equal proportions is satisfactory. However, since alcohols form constant-evaporating mixtures with hydrocarbons, the exact ratio of ethanol to butanol may vary somewhat, depending on the hydrocarbon used. A comparison of mixtures 1 to 4 of Table VI11 with those of Table IV shows that the solvent strength of 1-nitropropane for cellulose acetate is somewhat lower than for cellulose acetobutyrate. Of more practical interest are the data on mixtures 5 to 11, shoii-ing the dilution ratios when 50: 50 mixtures of ethanol and butanol are used. Solutions of cellulose acetate in the proper mixtures of nitroparaffins and alcohols will tolerate quite large proportions of hydrocarbon diluents. It has been found, however, in the preparation of stock lacquers with mixed solvents that there is some tendency toward gelling when concentrated stocks are prepared in mixtures containing high proportions of hydrocarbons. This gelling is not encountered if the proportion of hydrocarbon is reduced below the proportion that can be used
TABLE VIII.
1095
I N D U S T R I A L A N D ENGINEERING CHEMISTRY
September, 1942
DILUTION RATIOS OF 1-NITROPROPANE SOLUTIONS OF CELLULOSE ACETATE" Com n. of Diluent, by Vol.
Mixt. No.
Dilutjon Ratio
8
Ethanol: toluene 20:80 30:70 50 :50 1oo:o
1 2 3 4
1.2 3.0 5.9 2.3
Ethanol: butanol: toluene 0.7 12.5:12.5:75 3.1 15:15:70 20:20:60 4.0 25:25:50 4.2 4.0 30 :30:40 9 40 :40:20 10 2.8 ~. 11 60:s 0 : o 2.1 a Combined acetic acid, 54.5 per cent; A. S.T. M. visposity, 2.6-8 seconds: concentration at end point, 8 grams per 100 ml. volatiles.
5 6 7 8
to produce normal spraying viscosities. The solvent mixtures used in preparing concentrated stock solutions of cellulose acetate should therefore be rich in nitroparaffins. Such stocks can be diluted with a thinner which is lean in nitroparaffins, so that the desired solvent composition is obtained in the final lacquer. This parallels the recommended procedure for cellulose acetobutyrate. The recommended solvent compositions are given in Table VIIB. The use of nitroparaffin solvent mixtures does not change the resin compatibility of cellulose acetate to any great degree. The most noticeable effect has been to increase the quantity of resin that can be used rather than to enlarge the selection of compatible resins. Santolite MS and MHP are
compatible with cellulose acetate, and Rezyl 14 and Bakelite resins 4357 and 3180 show some compatibility. The usable proportions of the last three resins are substantially higher with nitroparaffin solvent mixtures than with the common mixtures containing acetone and ethyl acetate. As with cellulose acetobutyrate, resins which are themselves compatible with cellulose acetate have some power to improve the compatibility of less compatible resins. Cellulose Triacetate. The development of cellulose triacetate as a coating rpaterial has been seriously impeded b y a lack of suitable solvents. The only materiaIs that have been available for use with the triacetate are the chlorinated hydrocarbons, which unfortunately are both corrosive and toxic. However, since the degree of toxicity depends on the proportion of toxic substance present in the solvent vapors, it has been found possible to use the chlorinated compounds by reducing their concentration in the solvent mixtures. To accomplish this reduction, the nitropropanes have been found especially suitable; for although the nitropropanes themselves are not true solvents for cellulose triacetate, mixtures of the nitropropanes with a small proportion of chlorinated hydrocarbon have been found to be good solvents, and satisfactory films are deposited by the resulting solutions. As with the acetobutyrate and acetate, concentrated stock solutions should be rich in true solvents. A series of solvent mixtures recommended for cellulose triacetate is given in Table VIIC. While the concentrated stock contains 30 per cent tetrachloroethane, the final spray lacquer contains only 10 per cent of this material. The content of toxic material has therefore been greatly reduced. Cellulose Acetopropionate. This material was developed to supply the need for a grade of cellulose mixed esters which would be soluble in cellulose racquers solvents, such as esters and ketones. However, the proportion of these materials which must be used with cellulose acetopropionate is higher than with cellulose nitrate. The strong solvent power of the nitroparaffins for such cellulose mixed esters is evidenced by the high proportions of diluents that can be used. A comparison of dilution ratios of ester-type and nitroparaffin-type solvents is given in Table IX. It has been observed that the usable solvent formulas for depositing films from cellulose acetopropionate dissolved in the nitroparaffins may contain more diluents than are sometimes found in nitrocellulose lacquers made with ester-type solvents. TABLE IX. DILUTION RATIOS OF CELLULOSE ACETOPROPIONATE SOLUTIONS
Solvent Compn., % by Vol. 1-Nitropropane 1-Nitropropane: butanol, 66:33 1-Nitropropane:butanol, 66:33 Butyl acetate Butyl aoetste :butanol, 66 :33 Butyl acetate: butanol, 66:33
I
40
I
50
I
60
I
70
I
BO
VOLUME%TOLUOL IN SOLVENT MIXTURf
Diluent Toluene Toluene Xylene Toluene Toluene Xylene
Dilution Ratio 3.5 7.1 3.8 1.9 3.3 0.9
Final Concn. of Cellulorte Aceto ropionate, G.,8OO M1. Volatiles 3.5 6.3 6.7 6 2 5.6 6 9
Nitropropane solvent mixtures produce cellulose acetopropionate solutions of lower viscosity than ester-type solvents. This effect is shown graphically in Figure 1. Other Cellulose Derivatives. The nitroparaffins are also strong solvents for nitrocellulose and cellulose ethers. For most uses, however, they offer no economical advantage over the more common solvents, and therefore the use of nitroparaffins is generally restricted to those applications where the specific properties of the nitroparaffins are required. A number of specialty applications have been
INDUSTRIAL AND ENGINEERING CHEMISTRY
I096
TABLEX. DILFTIOARATIOSOF NITROCELLULOSE IN NITROPROPANE SOLVENTS" Dilution Ratio with Following Diluents: 50:60
Solvent
0
Toluene
toluene: naplrtlia
Kaphtha
Concentration, 8 grams per 100 ml. volatiles.
developed taking advantage of the fact that they ha\-e a mild odor, are water insoluble, and impart certain characteristics to the finished product which cannot be obtained with more common solvents. However, since practically all the developments of this type have been made by purchasers of the nitroparaffins, it is not possible to mention specific illustrations. The dilution ratios of nitropropane solutions of nitrocellulose are given in Table X. SYNTHETIC POLYMERS
I n the last few years the number of synthetic polymers of interest as coating materials has increased considerably. Several are already being used in large amounts. With the ever-increasing number of these compounds, most of which exist as several modifications, it is impossible in an article of this scope to give detailed results on each individual. A few examples have therefore been selected t o illustrate the strong solvent power of the nitroparaffins for these new materials. Vinyl Polymers. The copolymerization of vinyl chloride and vinyl acetate yields a series of resins which are being utilized in large volumes for protective coatings, especially as can liners. The most common of these materials is the product containing approximately 85-87 per cent of the chloride. The small amount of the acetate present improves the solubility of the product. RATIOS O F FORMVBR TABLE XI. DILUTION Dilution Ratio Diluent 1.25 Toluene 0.95 Ethanol 1.05 Ethyl acetate 0.5 Methyl ethyl ketone 1.2 Butanol 0.7 Butyl acetate 5.8 Ethanol: toluene, 5 0 : 50 1.3 Ethanol: ethyl acetate, 50: 50 1.1 Ethanol: methyl ethyl ketone, 50: 50 >5.0 Toluene: butanol 50: 50 >5.0 Toluene: ethyl aietate. 50: 50 1.1 Toluene: methyl ethyl ketone, 5 0 : 50 a Solvent composition, 90:10 I-nitropropane. ethanol. I
SOLUTIOSB"
Final Concn. of Formvar, G./100 hI1. of Volatiles 6.7
7.7 7.3 10.0
6.8 8.8 2.6
6.5 7.2 2,6
2.5 7.2
The nitroparaffins are among the most powerful solvents for such vinyl copolymer resins. Some idea of the solvent power of the nitroparaffins in comparison with methyl isobutyl ketone, which is often used in making solutions of the vinyl copolymer, is given in Figure 2. At a given solids content and viscosity, much more hydrocarbon diluent can be used \Tit11 the nitroparaffins than with the ketone. While Figure 2 shoJIs results for only one grade of Vinylite, parallel results have been found for other grades. The unusual properties of the acetals of polyvinyl alcohol have found widespread application for these materials. As an illustration of the solvent power of the nitroparaffins, it has been found that the polyvinyl formal sold under the name of Formvar can be dissolved in mixtures of the nitroparaffins and ethanol. Table XI shows that the nitroparaffins are
Vol. 34, No. 9
among the few solvents for this material, since most of the other compounds commonly regarded as solvents are precipitants for this polymer. As with the cellulose derivatives, mixtures of nitroparaffins with alcohols and hydrocarbons are the best solvents for Formvar. Rubber. The use of solutions of synthetic rubbers as cements and impregnating media is becoming of considerable importance. It has been found that the nitroparaffins are true solvents for synthetic rubbers such as Hycar OR, and are slower evaporating and far less toxic than the chlorinated hydrocarbons, which are the only other materials of comparable solvent power. Solutions of Hycar in the nitroparaffins have high tolerances for aromatic hydrocarbons. The use of the nitroparaffins does not eliminate the necessity for milling the polymer, but there is less tendency for the prepared solution t o gel when these solvents are used. The nitroparaffins and certain derivatives are also useful in cements made of natural rubber (3). MISCELLANEOUS APPLICATIONS
Owing to the decreased availability of aromatic solvents a t present, the replacement of these materials with aliphatic hydrocarbons is of great interest. For certain alkyd resins which are normally insoluble in petroleum naphthas, a previous article ( I ) suggested mixtures of butanol and naphtha for partial replacement of the aromatic solvent. The objection to replacing all of the aromatic, however, is that butanol itself, or its combination with naphtha, is not a solvent for many resins. If the nitroparaffins are used in the mixture to replace part or all of the butanol, a much higher proportion of the aromatic solvent can be replaced without sacrificing either solids content, viscosity, or performance. The nitroparaffins also contribute to the performance of paint and varnish removers. For example, the use of 20 per cent nitroethane or 2-nitropropane in the familiar benzeneacetone-alcohol type of remover produces a satisfactory attack on finishes which are otherwise little affected. Such improved varnish removers are of special interest for stripping baked alkyd or urea-formaldehyde finishes. CONCLUSION
A brief outline of certain solvent applications of the nitroparaffins has been made. The high power of these materials in producing solutions of a wide range of substances makes them of primary importance to the coating industry. They mill not, however, fill every solvent need, although no solvent investigation is complete unless it includes the nitroparaffins and mixtures of the nitroparaffins with other volatiles which might mutually assist in producing the desired result. Additional information of a more detailed nature on the nitroparaffins as solvents will be published in the near future. LITERATURE CITED (1) Bogin, Charlee, P a i n t , Oil Chem. Rev., 97, 45 (1935). ENG. CHEM., 29, (2) Bogin, Charles, and Wampner, H . L., IND. 1012 (1937). (3) Campbell, A. W., I b i d . , 33,809 (1941). (4) Gabriel, C. L., Chem. Industries, 45, 664 (1939). ( 5 ) Gabriel, C.L., IND. ENG.CHEM., 32, 887 (1940). (6) Hass, H. B.,Hodge, E. B., and Vanderbilt, B. M., I b i d . , 28, 339 (1936). (7) Lippincott, S.B., and Hass, H.B., Ibid., 31, 118 (1939). (8) Machle, Willard, Scott, E. W., and Treon, Joseph, J . I n d . Hug. Tozicol., 22, 315 (1940). (9) Ibid., 24, 5 (1942). (10) Senkus, Murray, J.Am. Chem. Soc., 63,2635 (1941). (11) Vanderbilt, B. M., and Hass, H. B., IND.ENO.CHEM., 32, 34 (1940). PRESENTED before the Division of Paint, Varnish, and Plaatios Chemistry Memphis, a t the 103rd Meeting of the AMBRICANCHEMICALSOCIETY, Tenn.
