December, 1926
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Interpretation of Stress-Strain Curves on Lacquer and Lacquer Components' By W. A. McKim JOHNSON
I
MCKIMJOHNSON Co., LINDEN, N. J.
T IS generally known that lacquers are fairly resistant to
moisture or humidity but quite sensitive to temperature and sunlight. In order to depict graphically the changes that lacquers undergo when subjected to variations in temperature, humidity, and light, tensile strength measurements were made. It was found that the method used was sufficiently sensitive to indicate the relative ralue of many of the various materials employed in lacquers. The instrument used was the stress-strain machine developed by Nelson2 for oleoresinous films except that heavier springs are required to provide the higher loading rate necessary for lacquer films. I t is the purpose of this paper to interpret certain graphic results obtained in the study of lacquers by the stress-strain method. Procedure
Test films were prepared by flowing the lacquers on amalgamated tinned sheets, set up a t a slight angle from the vertical. Flowed films were more satisfactory than sprayed films because of the ease in maintaining uniform film thickness and freedom from flocculates, pinholes, and other film defects. When sufficiently dry for handling, the films were stripped, cut by means of a suitable die, and the specimens measured for film thickness. The specimens were then placed in a ventilated case until a few days prior t,o measurement. At this point, unless otherwise specified, they were placed in a humidor maintained a t 20' C., and 60 per cent relative humidity. The specimens were transferred from the humidor direct to the machine and tested as quickly as possible. Check determinations are necessary, the number varying with the condition of the films, soft films usually requiring five or less and hard films as high as ten. It is possible to measure some lacquer films within a few hours after flowing, but it was generally found that curves obtained from premature measurements were not indicative. Practical information is not derived unless periodical measurements are made on the lacquer in question oi'er a period of eight or more weeks. This makes it advisable to consider the age of a lacquer when comparing its curve with that of another lacquer. Temperature exerts considerable effect on the distensibility of lacquers, fluctuations of a few degrees while measuring sometimes preventing concordant results. These general characteristics must be kept in mind while considering the results.
they have apparently very little plasticizing effect and may be judged as having left the film entirely. Among the plasticizers, diethyl phthalate, dibutyl phthalate, butyl tartarate, and the phosphates (tricresyl and triphenyl) impart the greatest distensibility in the order named. Apparently, no direct relationship can be pointed out between the so-called distensibility imparted and the chemical or physical characteristics of the plasticizers, although, were it not for one or two exceptions, we could probably say that the plasticizing action is greatest in materials possessing the lowest vapor pressure. It would be very convenient if later we were able to prove that some such relationship does exist. However, we have not sufficient data to prove this although theoretically some such law must seem to hold. It is unfortunate that the study on the plasticizers has not been completed, but it is obvious that smaller proportions of the phthalates are necessary in lacquer to obtain a distensibility equal to that of the other plasticizers. With the phosphates the trend has been to try mixtures with other plasticizers, '
Plasticizers and High-Boiling Solvents
Figure 1 shows results on several of the plasticizers and so-called high-boiling solvents, ranging from 4 to 107 days in age. It will be noticed that an appreciable change has taken place in the films marked 4, 5, 6, and 7, after 35 days. These high-boiling solvents apparently exert a semiplasticizing effect for about a month, but after 3 to 4 months' aging 1 Presented before the Midwest Regional Meeting and the Meeting of the Section of Paint and Varnish Chemistry of the American Chemical Society, Madison, Wis., May 27 to 29, 1926. * Proc. A m . SOC.Tcsfing Materials, 21, 1111 (1921);Nelson and Rundle, "Some Physical Properties of Paint and Varnish Films," Research Bulletin, New Jersey Zinc Company
0
Per c e d L/ongalion Figure 1
and from preliminary results mixed plasticizers seem to possess marked advantages over their individual components. Detailed results of this study will be published later. Since vapor pressure of the plasticizers seemed to have some influence on the behavior of the stress-strain curve, it was thought that the results could be accelerated by maintaining the lacquer films at slightly elevated temperatures.
IhrDUSTRIAL A N D EN 'GINEERI .VG CHE-VISTRY
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Vol. 18, No. 12
(U) 224 /5
30 37
87 23 2 10 9
13 2 109
i s 2 Figure 3
Per cm# Eionyohon Figure 2
Per cent E/ongaf/on
Per r e n t L/onjat/ao Figure 4
A very interesting reference to the effect of temperature on certain lacquer films is reported in the German literature.3 This report states that nitrocellulose coatings in general possess lower distensibility a t low temperatures and can be determined qualitatively by bending lacquer-coated metal panels that hare been subjected to different temperature exposures. Invariably a coating about 10 days old when brought to 0" C. and bent rapidly will crack, whereas a duplicate panel brought t o 35" C. and bent will show no cracking whatever. 3 Herzog, Hildesheimer, and Medicus, tell, 57 (1921).
Z. angew. Chcm., 84, Aufsatz-
Figure 5
That relatively small variations in temperature affect nitrocellulose coatings is shown in Figure 2. Herzog, Hildesheimer, and Medicus3 found that certain plasticizers crystallized out of the film, causing the film to suffer a loss of distensibility and strength unless an additional "protective" plasticizer was introduced. Apparently, some such effect has been brought about in the films shown in this graph. The two lacquers were prepared in identical formulas except that one contained triphenyl phosphate, a solid, and the other tricresyl phosphate, a liquid. The increase in distensibility with temperature brings u p the question of the time factor. Preliminary determinations indicated that nitrocellulose films soften progressively when
INDUSTRIAL A N D ENGINEERING CHEMISTRY
December, 1926
Per cent L/onyat/on
Figure 6
Per r e 4 E/onyaf/on Figure 8
held a t slightly elevated temperatures. Continuing the study, it was found that the limit of softening s t 48" C. is reached sometimes during the second day of heating. After that the distensibility is greatly influenced by the plasticizer used. It is obvious that the distensibility of the film containing triphenyl phosphate gradually drops, first softening,
124 1
Per cenf €/onyuf/on
Figure 7
Per cent €/onyafion Figure 9
then hardening again, until it becomes brittle. The curves for the tricresyl phosphate-bearing film indicate the limit of the softening effect and the stability of the film under prolonged heating. The slight variation in the sequence of these curvw may be considered within the limits of experimental error. The great difference in the behavior of these
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two films under a prolonged exposure to slightly elevated temperatures may be due- to the difference in the boiling points of the two plasticizers-300” C. for tricresyl phosphate and 245’ C. for triphenyl phosphate. If we assume that slow evaporation takes place, leaving the higher boiling plasticizer longest in the film, it is possible that at the temperature mentioned, which is 1” C. below the melting point of triphenyl phosphate, the latter decomposes or mechanically reverts to a crystal solid and in this state may not function as a plasticizer. This belief was substantiated by the formation of a slight cloud in the triphenyl phosphate film during the heating period. I t is quite possible that the addition of a “protective” plasticizer that would keep the triphenyl phosphate type of plasticizer from crystallizing as recommended by the German investigators, might change the temperature effect on the lacquer film. The sensitivity of nitrocellulose films to ultra-violet light is well and it is a generally accepted fact that nitrocellulose films exposed to sunlight tend to chalk very rapidly, thereby indicating decomposition of the organic matter by the active rays of light. A clear cotton solution was prepared and in this a small amount of zinc oxide was ground to determine whether the pigment that was opaque to ultraviolet light actually accomplished protective action. From Figure 3 it is evident that the gradual exposure of the clear nitrocellulose film to ultra-violet light causes considerable change in the strength of the film. This change is represented by a 100 per cent reduction in distensibility whereas in the pigmented film the reduction in elongation is only 60 per cent. The breaking strength of the clear film increases from nine to eighteen units and in the pigmented film from thirty to thirty-six units. It may be difficult to realize that the pigmented film is superior, but the stabilization of the film which has been rendered by the zinc oxide and the lack of this property in the unpigmented film is quite evident. 4
Stutz, J. Franklin Inst., 100,87 (1925).
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Vol. 18, No. 12
Gums
In studying the gums or gum-like materials some very interesting results were produced. Films were prepared without any plasticizer, containing in one case Oregon balsam, in another Canadian balsam, and in a third case no balsam but rather the equivalent amount of refined castor oil. Figure 4 shows the effect of humidity on these lacquers. At the end of one week the two balsam-bearing films show no difference between a film kept desiccated and film kept a t 80 per cent relative humidity. However, the castor oil type of film shows an appreciable difference and consequently brings us to realize that an oil-bearing film is not necessarily water-resistant whereas the balsam gums seem to impart that property. However, in considering the effect of aging, the results are reversed. The balsam-bearing films (Figure 5 ) show practically no distensibility a t the end of 18 days, whereas the film containing castor oil, although it undergoes a great change in load resistance, still possesses considerable elongation. Practically all the other gums show a slight sensitivity to moisture (Figures 6 and 7). A gum-free film shows practically no change whether the film is kept dry or moist. Rosin; as would be expected, shows the greatest sensitivity to moisture and is unable to stand exposure to humid conditions. Elemi stands out as a practical gum for most cases where extreme distensibility is desired. A high rosin-bearing film undergoes a great loss in distensibility on aging (Figures 8 and 9). Ester gum, when used in appreciable amounts, likewise produces considerable change in distensibility on aging, although not so much as rosin. Damar gum and elemi hold up very satisfactorily. Elemi at equal weight imparts a greater elongation than damar. One of the most interesting materials included in this series is zinc resinate (Figure 9), which, as will be noted from the formula, was used in a very much less quantity than the others. This material is a great hardener and requires only a small amount to render a film quite brittle.
Isopropanol as a Substitute for Ethanol’ 111-The Determination of Acid Numbers By H. A. Schuette and Maybelle P. Smith UNIVERSITY O F WISCONSIN, MADISON, WIS.
BOUT five years ago there was made available in c o m m e r c i a l quantities a homolog of ethyl
A
tion, but on the other hand rectification is easily accomplished. ~ i ~ t afol-~ t i ~ lowing a short digestion with lime gives a practically anh y d r o u s alcohol of boiling point 82-40 C. and specific gravity 0,7855 at 200/40 c.2 its chemicalproperties i t has much in with ethyl alcohol, a marked property of both being free miscibility with water. It has, however, a wider range of solvent properties than has ethyl alcohol, which is particularly marked by the readiness with which fats, oils, and waxes dissolve in it. Its odor is described as being not unpleasant and its vapors lack the irritating qualities O f its higher homologs. Earlier inVeSti-
An exhaustive test of a contemplated “all-isopropanol” procedure for the determination of acidities of fatty oils and waxes, in which was represented a wide variety of materials of divergent degrees of acidity, casts Some doubt upon the general applicability of the method, although for fatty oils and waxes of low acidity results were obtained which are as satisfactory as other published methods. Evidence is presented which supports the view that partial saponification of the fatty oil or wax during neutralization is a contributing cause Overlooked by Other Of high resultsl a point investigators in this field.
alcohol, the isopropyl form. the United States it is a by-product of the petroleum and natural gas industries. It is made by absorbing in s u l f u r i c a c i d those olefin fractions which consist largely of propylene. Hydro~ysisof t h e r e s u l t i n g alkylsulfuric acid causes the formation of crude isopropyl alcohol, which yields on purification a constant-boiling mixture containing about 91 per cent of alcohol by volume. Thus, like ethyl alcohol, it cannot be made anhydrous by simple distilla-
1 Presented before the Midwest Regional Meeting and the Meeting of the Section of Paint and Varnish Chemistry of the American Chemical Society, Madison, Wis., May 27 to 29, 1926.
3
Lebo, J. A m . Chem. Soc., 43, 1005 (1921).
