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JOURNAL OOPCHEMICAL EDUCATION
APRIL,1926
LACQUERS AS PROTECTIVE COATINGS* ARTHURORR.C O ~ E R CSOLVENTS UL CORPORATION. TERRE HAUTE, INDIANA
The subject that has been assigned to me suggests some preliminary questions. The phrase "protective coating" has been used for the last few years as a general synonym for paint, varnish, enamel, and similar coverings. But what is really a protective coating? What may i t protect? What is the attacking agency against which protection is needed? In nature we find that nearly everything has a protective coating. The bark of a tree, the husk surrounding an ear of corn, an onion-skin, pea-pod, banana peel, and the shell of a walnut are all different, but admirably designed, protective coatings. In the animal kingdom, we find an equal or greater variety. What could be more ingenious for their special purposes than the lobster's shell, the scales of a fish, and perhaps most wonderful of all, the skins of mammals? There is a tendency to assume that a protective coating acts only in the interest of the object to which it is attached; that outside agencies, both physical and chemical, are destructive fie7 se, and that a barrier must be put in their way which will check their ravages. This is often the case, but I hope to be able to convince you that other considerations are always involved and in some cases predominant; that a protective coating may even be a benefit t o the observer or to some outside agency, as well as to the object on which it occurs. I would also point out that occasionally protection is needed against internal enemies, that, in fact, under some conditions outside agencies may be beneficial rather than destructive. In any event, we must bear in mind that a protective coating may render a great variety of benefits in a great variety of ways. Naturalists speak of protective coloring in plants and animals. This kind of a protective coating carries us beyond the mere establishment of a physical barrier. It involves design and color.. The beauty which we may see in the skin of some rare fish, in the plumage of a brilliant bird, or in the marking of a leopard's skin, is not accidental. The evolution of these designs and colors is no doubt connected with the idea of protection. In some cases this may mean protection against destruction by a more powerful animal through relative invisibility; in other cases, protection against extinction through inability to attract. Emphasized visibility and beauty, in this instance, distinguished the individual fittest to survive. It is evident that the laws of esthetics have their foundation in Nature, that these laws have a profound bearing on the color and design of natural protective coatings, and that man's use of artificial coatings is also influenced largely by esthetic as well as physical and chemical considerations. The earliest use of pigments by man was, in fact, prompted solely by *Address before Group 24tudents' Course, Tenth Exposition of Chemical Industries, New York, October 2, 1925.
his desire to make designs and drawings, and not by any intention to protect physically the surfaces on which these materials were applied. I think i t is safe to say that the principal function of so-called protective coatings applied by man has, up to about the last century, been decorative rather than physically protective. A culture or civilization whose habitations and miscellaneous equipment are made of stone or earth obviously has little need for the purely protective properties of paint. Even today, if our sole object were merely to obtain maximum physical protection of underlying surfaces, our houses, our cities, and even our countryside would present a dreary aspect indeed, as we would have little use for other than black pigments. I will not press the point farther as I feel you must agree that the decorative or estheticvalue of a protective coating plays a verylarge part in the development and use of man-applied materials, and when I deal in the rest of my talk principally with the physical and chemical aspects of lacquer as a protective coating, I will assume that its merit from an esthetic point of view is not forgotten in spite of the fact that we have no accepted units by which this most important characteristic can be measured. Nitrocellulose lacquers are applied under ordinary conditions of temperature, pressure, and humidity. I will, therefore, in any comparisons refer only to materials similarly applied and will exclude those which require special conditions in connection with their application. The following is a rough outline of the considerations to be taken into account in passing on the practical merit of any coating material: (1) Its availability, which involves the sources of raw material, the equipment and processes used in its manufacture, price, etc. (2) Its general characteristics in liquid form, such as inflammability, toxicity, odor, keeping qualities, possibilities in color range, gloss range, etc. (3) Its application, including the preparation of undersurfaces, speed of drying, cost of application, etc. (4) Its performance after application. All the ingredients of the modern lacquer are obtainable in practically unlimited quantities a t fairly stable prices. There is reason to believe that the supply of these materials will constantly increase and that their cost will diminish. They are obtained either synthetically or from raw materials such as cotton, corn, coal, etc., of which there will always be an adequate supply. Some fossil gums are now used in many lacquers, but this is not obligatory, and there is certainly nothing to indicate that the use of lacquer will ever be limited through the exhaustion of natural deposits of these resins. The manufacture of lacquer, while requiring technical knowledge and experience of a high order, does not call for as large an initial investment or as much floor space per unit of output as does the manufacture of oleo-resinous coatings.
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APRIL, 1926
The cost per gallon to the consumer of industrial lacquers now on the market is greater than the gallon cost of corresponding oleo-resinous materials. The real finishing cost, which takes account of lessened floor space, greater output per man hour, much reduced inventory tie-up, increased durability, and lessened sales resistance, is much less. The general characteristics of lacquer during manufacture and before application are not unlike those of other materials. Its friendly enemies have dwelt on its inflammability. The fact that nitrated cotton is one of its ingredients seemed to present a delightfully easy point for attack. But there are different kinds of nitrated cotton (depending on the degree of nitration), and pyroxylin, the particular kind used in lacquers, although inflammable, is not explosive. When mixed with solvents and with the usual-gums and pigments it is perhaps as dangerous as a can of cut shellac. The most dangerous common lacquer ingredient both from the point of view of inflammability and of toxicity, is undoubtedly benzene, but as this has been used for many years in non-lacquer coatings I will not dwell on its properties. Aside from benzene and similar coal tar products, all of the common lacquer ingredients are practically non-toxic. There has been much talk about the odor of lacquer. In this connection let me emphasize the fact that odor and toxicity are two very different things. Many highly poisonous gases are completely odorless; and certainly the epicure who prefers his Camembert soft would repudiate the suggestion that he is a poison addict. As to odor we must, in dealing with lacquers, make a clear distinction between odor of the vapors during evaporation of the solvents and residual odor which may remain in the film after it is in use. In industrial finishing, odor of evaporation is not a factor because fortunately the sense of smell is highly accommodating and our noses soon cease to register a smell which on first exposure may have seemed even nauseating or irritating. After working for a few days in the usual concentration of these vapors men cease to notice the smell a t all. It is, therefore, rather to guard against toxic effects than to diminish odor that manufacturers equip their finishing rooms with ventilative fans and spraying hoods. I n the case of brush lacquers odor of evaporation is less, other things being equal, than with spray lacquers because the need for longer flow and slower drying calls for the use of solvents whose rate of evaporation is relatively slow. Furthermore, it is quite possible to obtain a solvent mixture of proper volatility which has little, if any odor, even during evaporation. Residual odor in a lacquer film is entirely due to impurities in one or more of its ingredients. All the usual lacquer ingredients are available today in such purity as will obviate any risk of residual odor. The keeping qualities of lacquer are about the same as with other materials. Exposure to heat tends to reduce viscosity, and, of course, im-
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pure ingredients may result in the formation of an excess acidity, but these difficulties are unimportant or can be avoided. Lacquer may be clear, stained, or pigmented. The possibilities in color are at least as wide as in the field of oleo-resinous materials. There is, perhaps, a popular idea that a lacquer finish must be less glossy than an oleo-resinous film. It is true that a polished film of pigmented lacquer has less depth than an enamel film on which one or more coats of transparent varnish have been applied, but the diierence cannot be detected by casual observation. Depth of finish is a function of transparency and thickness. On automoMles, in order to get full lacquer durability, and because a pigmented lacquer film is much more resistant to sunlight than a clear film, we usually dispense with the clear final coats. In its application, owing to rapid drying, lacquer should be put on evenly and rapidly. Fortunately, mechanical appliances for suitable application of this material have recently been brought to a high state of perfection. The modem air gun not only enables the industrial finisher to obtain perfect application, but also greatly increases the area which his men can cover per unit of time, as compared to what they might have covered with a brush. Lacquers have recently appeared on the market designed for brush application. This will make lacquer available to small users, housewives, etc., who could not have considered its application by mechanical means. The apparent obstacles in the way of this development have been largely overcome, and it is now possible to obtain brushing lacquers which not only have enough flow to leave a smooth surface, but which also have that property which permits successive coats to be applied without damage to the preceding coat of similar material. Although there will probably, in the aggregate, be a wide demand for brush lacquer, it is my opinion that mechanical application has advantages in the industrial field which will restrict the use of brush lacquer to non-industrial application. A lacquer film has much higher tensile strength per unit area of cross section than other similar materials. It is not uncommon to find lacquer films with a tensile strength of about 10,000 pounds per square inch. This characteristic, while of great advantage in many ways, makes it necessary to strengthen other links in the chain to an almost corresponding degree. Adhesion, for instance, is very important However strong a lacquer film may be, it becomes inferior to a much weaker film if it fails to adhere to its underlying foundation. Expansion and contraction caused by changes in humidity and temperature must be met in a lacquer film by sufficientplastiaty or extensibility to permit it to remain in place; but even aside from expansion and contraction, it is necessary that a lacquer film have such adherence that it will not continue to peel or chip off if some accidental mechanical injury breaks the film at any point.
In order to get sufficient extensibility, we may sacrifice some of our tensile strength and hardness, but just as steel is stronger and a t the same time more elastic than lead, so a lacquer film is stronger and more elastic than an oleo-resinous film, although its plasticity is far less. Poor adhesion is often the result of bad surface preparation. As the balance of a lacquer formula is upset by the presence of even small amounts of certain incompatible substances, it is important that the surface to be covered be cleaner than is necessary when oleo-resinous materials are applied. The manufacturer who takes up lacquer for the first time occasionally is slow in realizing the' importance of surface preparation; but this trouble will gradually disappear. A coat of lacquer dries in a few minutes. This is one of its most useful characteristics, but here again the user may have to readjust his old ideas. The common type of blush (the appearance of a white cloud in the drying film) is an indirect result of this speed in drying. Lacquers can be made which will dry without blushing, even in air which is almost saturated but such lacquers would be unnecessarily expensive. With commercial lacquers, i t is, therefore, customary to avoid extreme humidity and to maintain a room temperature of between 65" and 90' Fahrenheit. Recently developed materials and methods for polishing give us as high a degree of surface luster with lacquer films as with other equally pigmented materials. The greater strength, toughness, and resistance to wear of a lacquer film enable us to use i t in thinner layers than would be a t all practicable with oleo-resinous materials. This diminished thickness naturally means less filling power. It is, therefore, usual to put lacquer on smooth undersurfaces. Where much building up is necessary it is customary to use old style materials for the undercoats as their greater thickness per coat and lesser resistance to sanding permits economies in sanding time and material. But it is likely that even these old style materials will soon be replaced by newly developed pyroxylin primers and undercoats which dry rapidly a t room temperature. The actual cost of application varies greatly with the nature of the work. If dipping lacquer is used the cost should be the same as with other materials. Elimination of drying ovens is sometimes an important cost factor. The fact that lacquer dries entirely through evaporation rather than through oxidation is an advantage not only in its practical use, but it also enables us experimentally to determfhe in a much quicker time just what its continued performance is likely to be. The best indication of how lacquer behaves after application is the remarkable way in which its uses have been recently extended. Lacquer is now used on a great variety of surfaces. Many locomotives, passenger
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cars, tank cars, trolley cars, buses, trucks, and nearly all American makes of automobiles are being lacquered today. Steel frame-work and even exterior sandstone and skylights have been lacquered, a t least experimentally. In the Book-Cadillac Hotel a t Detroit, above the mezzanine floor, all the wood and metal trim is finished in lacquer. A textile mill in New England has been refinished in white lacquer enamel. A considerable percentage, possibly forty per cent of the furniture now made in this country is being finished in lacquer, both clear and pigmented. Many ice-boxes, filing cabinets, and a great variety of miscellaneous objects and novelties are now being lacquered. Lacquer gives a beautiful and a durable finish and its use saves time. We have, in these characteristics, the explanation of its recent growth and future promise.
