Components and types of Varnishes
Varnish is traditionally a combination of a drying oil, a resin, and a thinner or solvent. However, different types of varnish have different components. After being applied, the film-forming substances in varnishes either harden directly, as soon as the solvent has fully evaporated, or harden after evaporation of the solvent through curing processes, primarily chemical reaction between oils and oxygen from the air (autoxidation) and chemical reactions between components of the varnish. Resin varnishes "dry" by evaporation of the solvent and harden almost immediately upon drying. Acrylic and waterborne varnishes "dry" upon evaporation of the water but will experience an extended curing period. Oil, polyurethane, and epoxy varnishes remain liquid even after evaporation of the solvent but quickly begin to cure, undergoing successive stages from liquid or syrupy, to tacky or sticky, to dry gummy, to "dry to the touch", to hard. Environmental factors such as heat and humidity play a very large role in the drying and curing times of varnishes. In classic varnish the cure rate depends on the type of oil used and, to some extent, on the ratio of oil to resin. The drying and curing time of all varnishes may be sped up by exposure to an energy source such as sunlight, ultraviolet light, or heat. Drying oil There are many different types of drying oils, including linseed oil, tung oil, and walnut oil. These contain high levels of polyunsaturated fatty acids. Drying oils cure through an exothermic reaction between the polyunsaturated portion of the oil and oxygen from the air. Originally, the term "varnish" referred to finishes that were made entirely of resin dissolved in suitable solvents, either ethanol (alcohol) or turpentine. The advantage to finishes in previous centuries was that resin varnishes had a very rapid cure rate compared to oils; in most cases they are cured practically as soon as the solvent has fully evaporated. By contrast, untreated or "raw" oils may take weeks or months to cure, depending on ambient temperature and other environmental factors. In modern terms, "boiled" or partially polymerized drying oils with added siccatives or dryers (chemical catalysts) have cure times of less than 24 hours. However, certain non-toxic by- products of the curing process are emitted from the oil film even after it is dry to the touch and over a considerable period of time. It has long been a tradition to combine drying oils with resins to obtain favourable features of both substances. Resin Many different kinds of resins may be used to create a varnish. Natural resins used for varnish include amber, kauri gum, dammar, copal, rosin , sandarac, balsam, elemi, mastic, and shellac. Varnish may also be created from synthetic resins such as acrylic, alkyd, or polyurethane. A varnish formula might not contain any added resins at all since drying oils can produce a varnish effect by themselves. Solvent Originally, turpentine or alcohol was used to dissolve the resin and thin the drying oils. The invention of petroleum distillates has led to turpentine substitutes such as white spirit, paint thinner, and mineral spirit. Modern synthetic varnishes may be formulated with water instead of hydrocarbon solvents.
Types of Varnishes
Varnishes may be divided into the following four categories, depending upon the type of solvent used:
Oil varnishes
These varnishes use linseed oil as solvent in which hard resins such as amber and copal are dissolved by heating. These varnishes dry slowly, but form hard and durable surface. Sometimes, small quantity of turpentine is added to make the varnish more workable. Oil varnishes are recommended for all external wood work, and for joinery and fittings.
Spirit varnishes or lacquers
These varnishes are methylated spirit of wine as solvent in which soft resins such as lac or shellac are dissolved. They dry quickly, but are not durable. French polish is a variety of this type varnish. It is commonly used on furniture.
Turpentine varnishes
These varnishes use turpentine as solvent in which soft resins such as gum dammer, mastic and rosin are dissolved. The varnish dries quickly, but is not so durable. These are cheaper than oil varnishes.
Water varnishes
These varnishes are formed by dissolving shellac in hot water, using enough quantity of ammonia, borax, potash or soda. Water varnishes are used for varnishing wall papers, maps pictures, book jackets, etc.