968
INDUSTRIAL A N D ENGINEERING CHEMISTRY
there is an ideal outlet for impurities which tend to accumulate in such cyclic systems. Assuming the sales value of coke-oven tar to be 5 cents per gallon, the writer estimates the cost of the light-oil solvent to be less than 8 cents per gallon. This cost includes operating and fixed charges and assumes the small loss of tar volume a t the price of the tar with no credit taken for the ammonia liquor recovered from the tar.
Vol. 19,
XO.
9
Coke and gas plants which do not have light-oil recovery equipment can now utilize the ammonia-liquor extraction process for the elimination and recovery of phenols from still wastes. This has not heretofore been feasible because such plants could not afford to purchase supplies of motor-fuel solvent as the process would operate a t prohibitive costs. Applications have been made for patents covering the improved process.
Solvent Balance’ By Bruce K. Brown and Charles Bogin COMMERCIAL SOLVENTS CORPORATION, TERRE HAUTE,IND.
The proper balancing of solvents and non-solvents in OR several decades ranges which represent norlacquer formulation is discussed. Empirically deternitrocellulose solvents mal room conditions. mined rates of evaporation for individual solvents proand diluents have been A direct comparison of the vide more accurate indices of evaporating tendencies classified according to boiling vapor pressures of solvents at than are afforded by boiling point or vapor pressure point, those boiling below the room temperature is not a determination. boiling point of water being true index of their comparaThe amount of non-solvent tolerated by a nitrodesignated as low boilers and tive volatility. Vapor prescellulose solution varies with the concentration, solthe others as high boilers. sure determinations are based vent, and non-solvent. Dilution ratios are deterDespite its popularity, this on the number of molecules mined by titrating a solution of nitrocellulose with nonclassification is extremely inleaving the liquid state rather solvent until precipitation of some of the nitrocellulose accurate and the erroneous than on the weight of the occurs. The technic of the determination is described idea that the boiling point of molecules. Two solvents of and a number of dilution ratios for common solvents a solvent is an index of its identical vapor pressure will are given. behavior in a lacquer has not evaporate at the same rate tended to obstruct scientific a t a given temperature, unlacquer formulation. less their molecules are of the same weight and their latent The term “boiling point” has no scientific significance heats of vaporization are identical. The solvent having except that it describes the temperature at which the vapor the greatest molecular weight will evaporate most rapidly. pressure of the liquid under consideration is equal to one While it is possible to determine the vapor pressures of lacquer atmosphere of pressure. The ratio of the volatility of two solvents at room temperature, and then to correct these liquids at room temperature cannot be predicted by a com- figures for molecular weight and latent heat, so that the data parison of their boiling points. For example, there is a dif- for various solvents can be directly compared, such a proference of only 2 degrees in the boiling points of toluene and cedure is too complicated for everyday use-particularly since isobutyl alcohol, yet a t room temperature the former evap- new solvents whose physical properties are not well known orates almost three times as rapidly as the latter. While are being added monthly to the range of selection for lacquer xylene and normal amyl acetate boil at practically the same manufacture. temperature, xylene evaporates 125 per cent as rapidly as As yet, no better means of determining the volatility of the ester. Similarly, butanol boils at a temperature about solvents has been developed than the employment of purely 22 degrees below xylene, yet it evaporates only about 80 per empirical evaporation tests of the type reported by Davidson2 and Gardner.3 cent as fast. A factor representing rate of evaporation, whether it is Recognition of the disparity between the boiling point relations of various solvents and their volatility a t room tem- derived by physico-chemical calculation or by empirical test, perature has led to a study of vapor pressure curves. The is but an approximation of the behavior of the solvent in vapor pressures of a number of lacquer solvents a t varying question in so far as practical lacquers are concerned. Every temperatures hare been plotted as curves, from which data mixture of solvents and diluents which might be employed attempts have been made to compare the volatility of the as the liquid portion of a lacquer may produce binary, ternary, solvents. I n a number of cases the curves for various sol- or quaternary minimum vapor pressure mixtures of such vents will be found to cross during their ascent. At these complexity as to defy any prediction based merely on knowlpoints the solvents have the same vapor pressure, though edge of the vapor pressures of the unit liquids. For example, they may differ conversely above and below that critical point. so simple a mixture as 30 per cent benzene and 70 per cent’ ethyl alcohol evaporates (at room temperature) about 25 Importance of Evaporation Rate per cent more rapidly than the rate that might be derived The use of vapor pressure curves may be conceded to be from the formula for the evaporation of “perfect mixtures.” a vastly more accurate method of solvent evaluation than a I n general, substances which form constant-boiling mixtures study of boiling points, but in the opinion of the writers such also form mixtures of minimum vapor pressure at room temcurves will assist the formulator of lacquers only in EO far perature, but the proportions of ingredients present in the as they improve his knowledge of the principles of evaporation. mixture volatilized a t room temperature may differ from It may fairly be said that the only sectors of vapor pressure the proportions present a t the boiling point. curves that are of practicaI interest are those temperature Water blush, gum blush, cotton blush, improper drying 1 Presented as a p a r t of t h e Symposium on Lacquers, Surfacers, and time, poor flow, orange peel, and a variety of other lacquer Thinners before t h e Section of Paint and Varnish Chemistry a t the 73rd
F
Meeting of rhe American Chemical Society, Richmond, Va., April 11 t o 16, 1927.
2 8
THISJOURNAL, 18,669 (1926). P a n t M f ~ s . ’Assocn. U . S., Tech. Car. 218 (1924)
INDUSTRIAL A N D ENGINEERING CHEMISTRY
September, 192i
imperfections may be caused by an improper balance of the evaporation rates of solvents and diluents. The achievement of a proper evaporation balance is manifestly of great importance, and it is t o be regretted that adequate scientific data are not yet available for the accurate and rapid adjustment of the evaporation rate of a complete lacquer by means of predetermined proportioning. Dilution Ratio
A solution of nitrocellulose will tolerate the addition of some quantity of a non-solvent, but after a critical point is reached the further additions will cause a precipitation or gelling of the nitrocellulose. I n the industry this limit of tolerance has been described as “dilution ratio,” and it is ordinarily expressed as the number of cubic centimeters of non-solvent required to gel or coagulate some of the nitrocellulose in 1 cc. of a solution of definite concentration. Like many other useful indexes in the lacquer art, the dilution ratio is wholly empirical and is affected by the following variables : nature of solvent, nature of non-solvent, grade of nitrocellulose, temperature of tests, and final concentration of nitrocellulose in solution. I n the same manner, solutions of some gums in hydrocarbons or alcohols will tolerate the addition of only a certain quantity of ester (nitrocellulose solvent), and a critical limit of tolerance may be reached in such instances also. It may readily be seen that the amount of diluent that it is possible to add t o a nitrocellulose solution to form a lacquer may depend largely on the limit of tolerance of the nitrocellulose solvent for the diluent, and that hence the limit of tolerance (dilution ratio) is indeed an important property. Balance of Solvent Khile the amounts of diluents that can be added to lacquers in manufacture without causing incompatibility or gell-
969
While the proper balance of solvents and diluents to produce lacquers of desirable evaporation rates is incapable of definite prediction, the balance of solvent and non-solvent requisite to a complete compatibility of the ingredients, both in the liquid lacquer and during the drying period, is more easily ascertained. Determination of Dilution Ratio The method of determining the dilution ratio (limit of tolerance) of a solvent consists simply in titrating a nitrocellulose solution until so much non-solvent has been added that the nitrocellulose becomes gelled or precipitated. This determination is adaptable to back-titration, and if an excess of non-solvent has been added the mixture may be “brought back” by merely adding a little more of the nitrocellulose solvent and retitrating until a sharp end point is reached. The amount of non-solvent tolerable by a nitrocellulose solution varies with the concentration of the solution, and in the past various experimenters have fallen into the error of comparing dilution ratios determined for solutions of varying concentration. For example, Davidson2 performed tests on nitrocellulose solutions of initial concentration of 10 per cent. While the figures thus obtained are scientifically accurate, since solvents vary widely in their tolerance for nonsolvents, the final mixtures of solvent and non-solvent vary just as widely in their volume and in the consequent concentration of nitrocellulose therein. Furthermore, since all dilution ratios vary with the conceiitration of nitrocellulose, and since the purpose of determining the dilution ratio is t o learn the amount of non-solvent that will be tolerated in a completed lacquer, the only dilution ratios of real value are those that are determined from mixtures which approximate the concentration of true lacquers a t the time the end points of the tests are reached. An ordinary nitrocellulo3e lacquer contains from 8 to 12 per cent of nitrocellulose.
Table I - - - D i l u t i o n R a t i o s 5 of C o m m o n Solvents I
1
TOL u E N E
SOLVENT Ratio
E t h y l acetate Isopropyl acetate sec-Butyl acetate n-Butyl acetate “Medium-boiling solvent acetate” (A. D . Little) Amyl acetate from fusel oil n-Amyl acetate Acetate of ethyl ether of ethylene glycol “High-boiling solvent acetate” (-1. D. Little) n-Butyl propionate (water-white) E t h y l lactate Acetone Diacetone alcohol Mesityl oxide Dibutyl phthalate Tricresyl phosphate Dibutyl t a r t r a t e Diatol” E t h y l ether of ethylene glycol Diethyl carbonate
3.73 3.27 2.60 2.93
Final Concn. h-itrocellulose
Pev cent 7.0 7.0 8.3 8.1
TOLUENE--50’50 ~IIX‘CLTRE SOLVENT A N D BUTAWL Ratio
320 2 io 220 2 40
Final Concn. h-itrocellulose
Per cent 9 0
7.6 7.9 8.6
2.15 2.25 2.00
7.3
2 60
8 2
2.50
8 0
1.93 2 30 5.80 4.90 3.53
6 8 8.0 8 2 8.1 9.0 7.2 7.2 8.1 8.9 8.1 8.0 10.0
1 80 2 00 4 30 5 30 4 00 3 90 2 70 3 44 5 94 1 33 3 10 1 50
6 5 8.5 7.6 7.3 7.2 7.6 8.2 5.7 6.5 7.6 7.3 8.0
4.80
2.77 3.95 8.00
1.20 5.30 0.76
Ratio
I
Final Concn. Sitrocellulose
8.8
1
Ratio
Final Concn. Nitrocellulose
Per 8 8 8 8
cent 8 0 3 6
V. A I . P. NAPHTHA
Ratio
Final Concn. Nitrocellulose
Per cent
840 885
Per cent 8 5 7:
6 75 7.28 7 28
8 1 i.O 7.0
2.26 2.40 2.20
7.6 8.2 8.7
7 20
8 7
2.46
8,5
1.12
8.1
5.40 7.50 10 20 7.00 8.13 9.20 8.00 4.91 15.00 6.10 7.50 6.20
S.6 t . 8 8 9 7.2 7.5 7.8 8.9 7.7 8.0 7.0 8.0 7.0
1.70 2.26 4.80 4.00 2.86 4.30 2.93 5.00 7.70 1 12 4.70 0.70
7.0 8.2 8.3 7.2 8.0 7.0 7.7 6.7 7.0 8.7 8.4
0 80
7 8 7.6 8.3 8.3 7.6 9.0 8.5 7.0
9 0 9 0
2 93 2 53 2.20
XYLENE
BUTAKOL
3 3 2 2
30 27 60 73
8.2
1.25 125 1.20 1.75
7.8 7.8 8.0 8 0
1.00 1.50 1.4u
8.7 7.0 7.3
1.30
0.68 0.68 0.53 0.95 1.70 0.67 1.40 0.57 1,05 0.27
1.3 ,.a 8.6 9.2
Expressed a s number of cubic centimeters of non-solvent t h a t m a y be added t o 1 cc. of nitrocellulose solution without causing coagulation or precipitation.
ing may be readily determined from a study of the limits of tolerance of the ingredients, it will be remembered that the liquid ingredients of lacquer evaporate a t varying rates and that the balance of solvent, diluent, gum, and nitrocellulose may be disturbed during the dIying period. If one type of ingredient evaporates much more rapidly than the other, the limit of tolerance of the nitrocellulose solutions for the diluents or of the gum-diluent solution for the solvents may be exceeded and a gelling or precipitation may occur.
Table I presents the dilution ratios of a number of common nitrocellulose solvents as determined with “1/2-second cotton” a t room temperature. I n each case the concentration of nitrocellulose in the final mixture of solvent and non-solvent is approximately 8 per cent. This uniformity makes the ratios of different solvents directly comparable. I t was accomplished by means of preliminary determinations of the dilution ratios employing a nitrocellulose solution of known initial concentration, the data being used in adjusting the
I N D U S T R I A L A N D ENGINEERING CHEMISTRY
970
initial concentration of nitrocellulose in the solvent, so that after all of the non-solvent tolerable had been added the concentration of nitrocellulose in the resultant mixture would approximate 8 per cent. I n the past it has been generally assumed that the dilution ratio of a solvent for the typical non-solvent-tolueneaffords a fair index for solvent evaluation. But the dilution ratio of a solvent, like its evaporating quality, may be affected by the presence of other liquids in the completed lacquer. Table I clearly indicates this, and shows the utter unreliability of dilution ratios based on toluene alone.
FfGUR€ 1 EffEC T O f FINAL CONCENTRATION OF NI TRKLLLULO X ON DILU TION RA TI@,
For
\
E/UO/)
Dibutyl Tartrate
Vol. 19, No. 9
viously applied coats of lacquer. Whereas the hydroxyl-besring solvents show marked superiority in dilution ratios for toluene over the simple esters, in the case of naphtha the condition is completely reversed, and the simple esters tolerate from 50 to 100 per cent more naphtha than do such materials as ethyl lactate, ethyl ether of ethylene glycol, and diacetone alcohol. Data on the dilution ratios for naphtha of mixtures of solvents and butanol are not yet available, but it is hoped that they may soon be published. The effect of the presence of gums in the nitrocellulose solutions employed in the study of dilution ratios also merits attention. Effect of Final Concentration of Nitrocellulose on Dilution Ratios
In presenting these dilution ratios it was remarked that, unless the final concentration of the nitrocellulose in the mixture approximates the quantity present in a commercial lacquer the figures are of little value. This point is illustrated in Figures 1 and 2. I n Figure 1 the dilution ratios for toluene of dibutyl tartrate, ethyl ether of ethylene glycol, diacetone alcohol, and normal butyl acetate, determined for final concentrations of nitrocellulose, are reported. (Dibutyl tartrate is a plasticizer; it is included in the data because its dilution ratios are uniformly higher than those of any solvent known to the authors.) At a point corresponding to 2 per cent nitrocellulose in the final mixture the dilution ratios for ethyl ether of ethylene glycol, diacetone alcohol, and butyl acetate are, respectively, 7.0, 4.5, and 3.5-a maximum difference amounting to 100 per cent. At 8 per cent
Butyl Aceiate
I
/ 2 3 . 4 5 6 7 8 PIO//LZJ3/4J. Final Br Cenf Come&-ahon o f Ntroce//uhw (& Sec ~~Jcos/&) /n the Mlxfure .
For toluene the average dilution ratio of the simple ester solvents is about 2.9 (Table I) whereas hydroxyl-bearing solvents such as ethyl lactate, ethyl ether of ethylene glycol, and diacetone alcohol, show a markedly superior tolerance, ranging from 3.5 to 5.0. A second series of determinations gives the dilution ratio, for toluene, of a 50:50 mixture of solvent and butanol. The final mixture approximates the average lacquer more closely than does the mixture obtained with toluene alone, since alcohol diluents, as well as hydrocarbons, are employed in commercial lacquers. The addition of butanol reduced the dilution ratios of the simple ester solvents, such as the acetates, only slightly, whereas the hydroxyl-bearing solvents, particularly ethyl ether of ethylene glycol, were profoundly affected. The dilution ratios of the pure solvents for butanol are notably high since most solvents bear considerable admixture with butanol. I n general, the ratios for xylene are slightly lower than those for toluene. The figures for V. M. P. naphtha are interesting for several reasons. While the unavailability of closely boiling fractions of petroleum hydrocarbons has hindered the use of these materials in the past, this difficulty has now been overcome and fractions of almost any reasonable boiling range are available. Petroleum hydrocarbons are less expensive than coal-tar diluents a t the present time, and in the brush lacquer field they are especially valuable, since lacquers containing these materials in place of benzene and its homologs have a much retarded solvent action on pre-
EFFECT OF FINAL CONCENTRATION O f
NI TROCEL 1 U l OSE ON DlL I/ TION RATIOS ( F o r KM.P Naptha) 1 . 2 3 4 5 6 7 8 9 10 // l 2 / 3 / 4 /.5 find Per Cenf Cancenfrahan o f N/roce//u/ose(t&c V&-os/fy) in /he Mixf ure
concentration the figures for the same solvents are 5.2, 3.5, and 2.9-a gross difference of about 76 per cent. At 12 per cent concentration the corresponding figures are approximately 4.2, 3.2, and 2.7-a maximum difference of about 55 per cent, It is thus seen that differences of dilution ratio are unduly magnified a t low final concentrations of nitrocellulose. The curves for V. hf. P. naphtha (Figure 2) show that the dilution ratios of the solvents depend on the final concentration of nit,rocellulose in the mixture. None of the solvents reported are as tolerant, of petroleum hydrocarbons as they are of toluene. The same general rule applies-an increase in final concentration causes a reduction in dilution ratio. I n the case of naphtha ratios, however, the simple ester sol-
INDUSTRIAL A N D ENGINEERING CHEMISTRY
September, 1927
vents, such as butyl acetate, will stand the most dilution with non-solvent. These curves also illustrate the fallacy of comparing dilution ratios obtained for different solvents where a uniform initial concentration of nitrocellulose-say, 10 per cent-is employed, and the final concentration is disregarded. I n such cases the natural difference of dilution ratios is exaggerated. For example, with a solvent having a high dilution ratio the final concentration of the nitrocellulose, after all the noli-solvent has been added, will be less than the final concentration of nitrocellulose obtained in testing a solvent having a lower dilution ratio, to which less non-solvent may be added. The curves clearly indicate that a t low final concentrations of nitrocellulose the dilution ratios of the solvents may be expected to be larger than a t the higher concentrations, which are more comparable with actual lacquers. In the preparation of a satisfactory lacquer the use of diluent ratios must be tempered by the evaporation balance of the liquid mixture. Diluent ratios represent merely the
97 1
limit of tolerance of solvent for non-solvent in the complete lacquer, and are no indication of tolerance during the drying period. If solvent and non-solvent present are adjusted in evaporation rates so that neither is completely evaporated before the lacquer film sets during drying, the diluent ratio mill not be exceeded and a satisfactory film should result. This is not always the case. For example, the data presented show that a mixture comprising 1 part of ethyl acetate and 3 parts of xylene will retain 8 per cent of nitrocellulose in homogeneous colloidal solution. But if such a mixture is employed as a lacquer a badly blushed film of poor adherence and low tensile strength will result, since all of the solvent will evaporate before the film dries and the remaining xylene will coagulate (blush) the nitrocellulose. Acknowledgment
C. W. Simms, of this laboratory, performed many of the laboratory tests on which this paper is based and the authors' thanks are due him.
Lacquer Surfacers' By F. M. Beegle and C. M . Simmons T H E CLEVELAND VARNISHCOMPANY, CLEVELAND, OHIO
K DET'ELOPISG a suitable lacquer surfacer for use in automobile production, it was thought desirable to grind the pigments on a burr stone mill, such as all paint factories possess, thereby obviating the necessity of purchasing special equipment. Various combinations of chemical plasticizers, oils, and gum were therefore tried in order to secure a liquid that would wet sufficient pigment to give satisfactory filling and feed through the mill. A mixture of castor oil, linseed oil, and dibutyl tartrate gave a very good grinding mixture when Titanox, Keystone filler, talc, and iron oxide formed the bulk of the pigment.
I
Gums 111 accordance with good varnish reasoning it was believed that the less gum used the tougher and more waterproof would be the product. Consequently, a batch of surfacer was made with the ingredients mentioned above, without any gum. This surfacer was very elastic and held out the finishing lacquer in a very satisfactory manner, but on spraying an automobile body with it and sending to the rubbing deck after sufficient drying, it rubbed very tough, and in spots where the water lay for a time it blistered from the primer. Considerable work was then done to find the cause of the blistering, retaining the idea that no gum should be used. Grinds were made with the same pigment combinations and oils, but substituting Lindol, dibutyl phthalate, and diamyl phthalate for the dibutyl tartrate. A11 combinations were better than the one using dibutyl tartrate, because this plasticizer seemed to allow the water to pass quickly through the film. From this point considerable stress was placed on a water test and all batches were sprayed on, force-dried 1 hour a t 66" C. (150" F.), sanded to a smooth surface, and stood in a pail of tap water overnight. The formula for any surfacer showing a tendency to blister was discarded. The varnish theory had to be discarded because it was found necessary to include gum in the formulas in order to get adhesion and a material to stand a satisfactory water 1
Received April 23, 1927.
test. The tendency t o blister was found to decrease as the gum increased up to an optimum point. Fortunately, the gum content giving optimum waterproofness was about the same as the gum content giving satisfactory rubbing properties. The addition of the gum solution t o the oils and chemical plasticizer gave a very satisfactory grinding liquid. After the proper ratio of oil, chemical plasticizer, gum, total amount of pigments, and cotton was established to give proper adhesion, waterproofness, sufficient filling and sanding, work was started to determine the gum or combination of gums to be used to secure the best results. Shellac, dammar, ester gum, and kauri gum solutions were used. On all tests that could be completed in a week or 10 days it was decided that the gums gave the best results in the order named above. However, very good results could he obtained by using a combination of dammar and ester gum. It was found that a very valuable test could be made on lacquer surfacers by applying two coats on top of a good, long oil-baked primer, force-drying 1 hour a t 66" C. (150" F.),rubbing to a surface, then coating with two double coats of lacquer. After the film had dried about an hour, the panels were hung in an electric oven and baked overnight at 121" C. (250" F.). This treatment caused many lacquer surfacers to crack open and pull the finishing lacquer with them, giving the appearance of sun-baked mud. Optimum Cotton Solution
Surfacers were made using the forrnula with the best oil, chemical plasticizer, pigments, and gum conibinations with l/r-second cotton solution in one case, and with 7-second cotton solutions in other cases, after reducing the latter to the viscosityof the '/Tsecond solution by use of various catalysts. The same number of grams of cotton per liter of solvents was used in all cases. The surfacer made from '/*-second cotton was the best of the series, irrespective of the catalyst used with the 7-second cotton, even though the catalyst were only heat and pressure. A weather test on the roof bore out the conclusions of the quick tests in this case.
