I N D U S T R I A L A N D EA’GINEERISG CHEMISTRY
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Vol. 17, No. 6
The Use of Plasticizers in Lacquers‘ A Brief Survey By Bruce K. Brown COMMBRCI.4L SOLVENTS CORP., TBRRBHAUTE, IND.
T
H E present tremendous expansion of the nitrocellulose lacquer industry has quite naturally been accompanied by a quest for raw materials. The basic nitrocellulose lacquer actually consists of a solution of nitrocellulose in a suitable ‘honblushing” solvent mixture, which on evaporation leaves a deposited film or coating. But a nitrocellulose film, per se, does not often produce a desirable protective or ornamental coating. To increase the adherence of the film to the under surface, to increase the body and covering power of the lacquer, and to impart certain other desirable properties, it is common practice in the industry to introduce gums into the mixture. The natural gums such as dammar, kauri, and shellac, and certain synthetic materials, notably ester gum, are often added to lacquer solutions in quantities approximating the amount of nitrocellulose employed. Pigmented lacquers are also important compositions, particularly in the finishing of automobile bodies and metal articles. Purpose of a Plasticizer
For many purposes these lacquer compositions are not sufficiently plastic for successful use. Although some gums in proper admixture with solvents and nitrocellulose will produce sufficiently plastic, and in some cases actually soft, lacquers, the attainment of the desired lacquer body and adherence by the introduction of gums frequently produces a brittle lacquer. Likewise, the additon of considerable quantities of pigment to a lacquer invariably causes the film produced to become more brittle than before. A properly plastic lacquer may be produced by the use of a “plasticizer.” The term “plasticizer” has a broad significance in the nitrocellulose industry, and in this usage it generally refers to a nonvolatile neutral substance miscible with nitrocellulose, to which it lends plasticity. In the lacquer industry, however, a plasticizer imparts additional properties to the film and the terms are far from synonymous. For example, camphor is the standard plasticizer in the pyroxylin plastic industry; yet it is unsuitable for lacquer films on account of its volatility and odor. Likewise, castor oil is the standard plasticizer in the artificial leather industry, but, although employed to some extent in lacquers, it is not well suited for this use on account of the softness of the films thus produced, and on account of its tendency to become rancid on aging. I n general, a plasticizer for use in pyroxylin lacquers should have the following properties : (a)It should be nonvolatile. ( b ) It should be completely miscible with lacquer solvents. ( c ) It should be neutral in character. ( d ) I t should be stable on aging in a lacquer film. ( e ) It should be a solvent for nitrocellulose. (f) I n admixture with pyroxylin and with pyroxylin-gum mixtures, it should lend plasticity to the composite without greatly diminishing the film strength. Plasticizers in Use
A search for suitable plasticizers has been a part of the recent expansion of the lacquer industry. The old lists of literally thousands of substances that were proposed or 1
Received April 4, 1925.
patented as “camphor substitutes” in the pyroxylin plastics industry have been combed by chemists in search of suitable lacquer plasticizers. The present trend of the lacquer industry is to settle on a few standard plasticizers to the exclusion of hundreds of others. The most satisfactory plasticizers yet found for nitrocellulose lacquers are high-boiling esters of organic acids and of phosphoric acid. Over thirty years ago dialkyl phthalates1**were employed in pyroxylin plastics, and lacquers containing these substances were in use almost twenty years ago. I n 1894 Nobel2patented the use of dialkyl esters of phthalic acid in admixture with celluloid and with volatile solvents for the production of “varnishing solutions.’’ Goldsmith3 in 1900 patented the “esters of phthalic acid” as total or partial substitutes for camphor in the manufacture of celluloid. I n 1906 a German patent covering very similar ground was issued to Meister, Lucius, and B r ~ n i n g . ~The use of phthalic acid, phthalic anhydride, and the esters of phthalic acid was generally claimed by Zuh16 in an English patent issuing in 1901. In the same year the use of diamyl phthalate as nitrocellulose solvent in the explosive industry was patented by Lundholm.‘j In 1907 diethyl phthalate was definitely proposed by Lilienfeld’ as a plasticizer for use in lacquers compounded for application to textiles. In addition to the increased softness and plasticity of a nitrocellulose film containing a dialkyl phthalate, Lilienfeld noted the fact that the presence of diethyl phthalate in the lacquer film imparted a superior resistance to water. Dibutyl phthalate and diethyl phthalate are now Tvidely employed as lacquer plasticizers. Triphenyl phosphates and tricresyl phosphateg-both widely used in pyroxylin plastics-have likewise stood the tests of the modern lacquer industry and are also employed as “plasticizers.” Butyl tartrate, which has been employed as a camphor substitute, also finds a considerable use in lacquers. The wide use of butyl esters of organic acids as plasticizers is easily explainable. High-boiling esters are inherently suitable plasticizers as they are without exception nitrocellulose solvents and are sufficiently stable for satisfactory use. In general, also, the esters of primary alcohols are less volatile than those of the secondary and tertiary alcohols and are also more stable. Method of Functioning
Despite the widespread employment of plasticizers in lacquers, the method of their functioning is still disputed. This dispute has been enhanced by the fact that precise definitions have not yet been developed for many of the phenomena observed. For example, it is commonly stated that the addition of a plasticizer produces an “elastic” lacquer. An illustration of this phenomenon will prove interesting. If 227 grams (8 ounces) of nitrocellulose (28 second viscosity lacquer cotton), 255 grams (9 ounces) of dammar gum, and 57 grams (2 ounces) of e&er gum are dissolved in 3.78 liters (1 gallon) of a solvent m i x t u r e 2 5 per cent butyl acetate, 10 per cent butanol, 15 per cent ethyl acetate, 8 per cent xylene, and 42 per cent toluene-an unpigmented enamel base lacquer is produced. If 1300 to 1800 grams (3 to 4 pounds) of zinc white pigment are ground in with this mixture during
* Numbers in text refer to bibliography at end of article.
June, 1925
I,VD USTRIAL A-VD ELVGIiVEERIiVGCHEXISTRY
compounding (the grinding is preferably done in a ball mill in a viscous nitrocellulose solution to which the hydrocarbon diluents are afterwards added), a good grade of “white enamel” is produced. This enamel is suitable for application to flat surfaces which do not encounter much bending or vibration and which do not undergo large expansion and contraction. But if such an enamel is applied to a thin brass plate, and after drying and aging the plate is S~OWIYbent double, the enamel will chip and splinter a t the point of strain. While the effect is enhanced by the presence of pigment, the same phenomenon is observable to a lesser degree in “clear” or unpigmented lacquers. Such a test is suitable for laboratory experiment; the conditions are merely accentuated to a slight degree beyond those ordinarily met in practice. Under less rigorous conditions, smaller proportions of plasticizers may be employed. If there is added to the initial lacquer mixture, dibutyl phthalate in quantity equal to about 80 per cent of the weight of the nitrocellulose, the brass plate may be bent double and straightened out a number of times without marring the enamel or perceptibly loosening it from the metal. At the same time the enamel containing the plasticizer is not softer than the unplasticized enamel-i. e., it is not easily marred, nor is it “sticky.” This, then, is the “plasticizing” function. But careful measurement of the film itself has not yet disclosed a true “elasticity.” When such a film is broken on a tensile strength machine it is observed that the presence of the plasticizer has reduced the tensile strength of the film and has increased the measurable elongation before fracture. The reduction of the tensile strength is of no practical significance in most cases, as a good lacquer film 76 X 13 X 0.08 mm. (3 X 0.5 X 0.003 inch) will show a tensile strength of from 350 to 1050 grams per square centimeter (So00to 15,OOO pounds per square inch) of cross section a t the breaking point. On the other hand, although the elongation of the film before fracture is increased by the presence of the plasticizer, there is no perceptible “snap back” or true elasticity. By using suitable plasticizer- it i q even possible to lacquer
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flat metal plates in such a manner that they may later bc bent in fabrication without marring the lacquer. In addition to the increased plasticity induced by the presence of plasticizers in lacquer, these substances in general improve the flow or smoothness of the lacquer films, functioning, no doubt, in the same manner as high-boiling ester solvents such as butyl acetate. In some cases plasticizers are stated to improve the “gloss” or shine of lacquers, and this is perhaps partially due to their retardant effect on solvent evaporation. Durability is, of course, the final test of a lacquer, and the extremes of heat, cold, and moisture met by a lacquer film on automobile body constitute a severe test. The presence of a plasticizer in lacquer film imparts the plasticity necessary to permit the extremes of expansion and contraction without in any way marring or loosening the film. The presence of moisture produces a further difficult test. Some substances which seem ideally suited as plasticizers, and which do, in fact, produce a plastic lacquer and tend to render the film homogeneous and nonporous, nevertheless have the property of absorbing water. The action of such a film on exposure is most discouraging. The film itself alternately takes on and loses water with humidity changes and is thereby much deteriorated. On the other hand, dibutyl phthalate, butyl tartrate, the phosphate esters, and other suitable lacquer plasticizers actually add considerably to the life of the exposed film. Bibliography 1-Graebe, B e y , 16, 860 (1883) 2-Nobel (Newton), English Patent 15,914 (1894). 3-Goldsmith (British Xylonite Co 1, English Patent 13,131 (1900) 4-hfeister, Lucius, and Bruning, German Patent 127,816 (September 12, 1906) b Z u h l , English Patent 4326 (1901). 6-Lundholm (Nobel Explosives Co ), English Patent 14,231 (1901). ;-Lilienfeld, English Patent 492 (1907). 8-Zuh1, English Patent 8072 (1901) * 9-Zuhl, English Patent 4383 (1902) 1: See also Worden, “Technology of Cellulose Esters,” where data on the phosphate plasticizers are completely re\ reued
*
A Special Vacuum Distillation Flask’ By R. L. Shriner U N I V E R S I T Y OF ILLINOIS, U R B A N A , ILL.
THE course of some work involving the separation of ItnKvacuo certain solid fatty acids, it became necessary to distil a large amount of crude material. The ordinary Claissen flask could not be used because of the high boiling points of the compounds, since the hot vapors attacked the rubber stoppers, making it impossible to maintain c y ~ L a vacuum for any length of time. The flask shown in the diagram was designed and has proved useful in the vacuum distillation of high-boiling products. The apparatus2was constructed throughout of Pyrex glass carefully blown so as to avoid any thin spots a t seals or indentations. The ground-glass joints should possess considerable taper to prevent jamming and are lubricated with a mixture of talc and beeswax in order to hold the vacuum. The length of the fractionating column depends on the boiling points of 1
Received February 26, 1925 The apparatus was blown by Paul W. Anders of this laboratory.
the substances distilled and the degree of fractionation desired, a short column giving more rapid distillation but less fractionation than a long column. For material boiling a t 200’ to 300” C. a t 7 mm., 12 cm. of indentations was sufficient. The set-in side arm permits only vapor to pass over and prevents any liquid condensing on the side walls from running over into the distillate. The fractionating column of large diameter is placed in the center of the flask, instead of a t the side as in the Claissen type, in order to prevent the mechanical carrying over of liquid due to condensation with subsequent plugging of the side arm, which usually occurs a t the beginning of a distillation. I n this apparatus 2 kg. of material have been distilled under a vacuum of 5 to 7 mm., the final temperature of the Wood’s metal bath rising above 400’ C. with no sign of the flask collapsing. I,~nd
