Almost all of the large paint and varnish manufacturers in the United States are now making lacquers, which in some cases are being unwisely advertised as suitable for nearly all purposes. In the hands of a skilled finisher they produce satisfactory effects, but when a novice attempts to spread them with a brush over newly varnished surfaces, there is sure to be trouble because the solvents in the lacquer are similar to those contained in some of the paint and varnish removers and the newly applied coating reacts with the old, resulting in a spoiled finish.
If a varnished or enameled surface is old, hard, and well preserved, a lacquer finish may adhere satisfactorily soon after it is applied. However, checking is apt to occur later on. Lacquer finishes have certain limitations and cannot be used over other finishes indiscriminately with assurance of success. Lacquer finishes have grown in popularity on wood frames, fireplace designs, and designer window toppers and, when used properly by one who understands them and their limitations, they are attractive and durable and possess many good qualities.
Lacquer enamels, which are pigmented lacquers, are successful and popular finishes for automobiles. Lacquers of various kinds are used extensively on metalwork, particularly building-hardware, electric-light fixtures, metal spinning and stamping objects, and often for toys, artwork, and even jewelry. They are also used satisfactorily on high grade furniture. Pigmented lacquers are used on electric light fixtures, novelties, and machinery, and also almost universally on the highest priced automobiles.
Composition of Nitrocellulose Lacquers
Clear or transparent lacquers contain five types of ingredients:
o nitrocellulose or pyroxylin
o varnish resins
o dilutents or thinners
Pigmented lacquers (for use on everything from fireplace accessories and wood frames to frieze boards) are often called lacquer enamels. These enamels contain one extra ingredient. They must give obscuration or hiding power and to give this opacity various kinds of pigments and coloring matter are added to the lacquer. Pigments and coloring-matter may therefore be considered as the sixth type of ingredient found in lacquers.
Nitrocellulose, Soluble Cotton, or Pyroxylin
The distinctive ingredient in a modern lacquer, that makes it quite different in behavior from other varnishlike materials, is given various names, such as nitrocellulose, cellulose nitrate, soluble cotton, and pyroxylin. The words nitrocellulose and nitrocotton mean that the substance is made of cellulose or cotton that has been nitrated. Cotton is one of the purest forms of cellulose fibers and is the chief source of raw material from which nitrocellulose is manufactured.
Long-fibered cotton which is valuable for making cotton-cloth is usually too expensive for nitrating; therefore, the short-fibered material, called “cotton linters,” is generally used. In the factories the cotton gin removes the long fibered cotton from the linters, hull fibers, dirt, and other impurities. The linters are separated from the waste material by boiling, chemical treatment, bleaching, washing, and drying and are then nitrated through treatment with a mixture of nitric and sulphuric acids.
The particular function of the sulphuric acid is to remove the moisture from the cotton fibers during the nitrating process. The amount of nitrogen given to the cotton is very carefully controlled, and depends upon the use for which the nitrocellulose is intended. The nitrogen content of nitrocellulose used in lacquer varies from 11.5 to 12.4 percent, with a viscosity of a half-second up to about 80 seconds. Nitrocellulose that has between 12.5 to 13.5 percent of nitrogen is called gun-cotton and is used in the manufacture of smokeless powder and various explosives.
Nitrocellulose for use in making lacquer for fireplace accessories, frieze boards, and wood corner blocks is made wet with denatured alcohol, in order to remove traces of water, and then dried out to a 30 per cent alcohol content mixture, which is not explosive though it is flammable. The gun cotton type of nitrocellulose has having between 12.5 to 13.5 percent of nitrogen by weight is very explosive when dry and is soluble only in acetone, while the nitrocelluloses which contain a lower amount of nitrogen are easily soluble in a mixture of anhydrous ethyl alcohol, anhydrous ethyl acetate and other well-known solvents.
The viscosity of the early types of nitrocellulose used in making lacquers for metal was relatively high and the amount of solvents and thinners were great in proportion. The result was a very thin film when the mixture was applied. The lacquers of the present day usually contain a nitrocellulose of rather low viscosity and certain resins which, when combined, produce a film that is much like a varnish in body or solid matter.