Post 337 – by Gautam Shah
The art of Oil painting developed in Europe in the late Middle Ages. It was widely accepted as a better art medium, as it was easier to work with and allowed a greater variety of effects in comparison to the (than available) other media, like encaustic paint, tempera, water colour, fresco, etc. Oil painting dried relatively slowly with little change in colour. Tones are therefore easy to match, blend, or grade, and corrections easy to make. With this medium the painter was not limited to linear brush strokes, but could apply paint in glazes, washes, blobs, trickles, spray, or impasto (textured effect). The painter was no longer restricted to a prearranged design, as the slow drying system allowed change, improvisation and over work. Rich effects were possible with colours, tonal contrasts, and chiaroscuro (shading). Jan Van Eyck, a Flemish painter in early 15th C. explored the oil paint as medium to work over the tempera style of paintings. The Venetians took it the further with oil painting on canvas.
Early oil paints were made by modifying the drying oils like walnut or linseed with natural gums such as amber, copal, pine-rosin, etc. The oils were cooked, or exposed to air and sunlight. But later, cooking was conducted in closed kettles under pressure and in absence of oxygen. These ‘stand oils’ were modified with natural gums, resulting in viscous substance. The viscous substances were reduced (diluted) with turpentine.
Modified drying oil mediums were found to be good for wood surfaces, unlike the tempera colours. The grains and texture of wood surfaces helped in binding the medium. It was also realized that the medium should have a better body (high viscosity compared to pure oil materials) and drying properties, otherwise would run (drip down).
OILS AND OLEO-RESINOUS BINDERS
OILS: Vegetable oils and animal fats are the oldest binders known to man, some of which are still in use today. The suitability of an oil as a film forming substance depends on the highly unsaturated fatty acids. To make raw oils usable, three treatments are carried out 1. alkali refining to reduce the acidity and improve the colour, 2. kettle bodying -heating oils for a prolonged period at high temperatures to increase the viscosity and polymerizes it, 3. blowing air or oxygen through oil at elevated temperature to increase its oxidation.
Types of Oils: Drying oils (Linseed oil, Tung oil -china wood oil, Dehydrated castor oil), Semi drying oils (Safflower oil, sunflower oil), and Non drying oils (olive, castor, cotton seed, coconut oil). For coatings drying oils and some semi drying oils are commonly used.
Oils have been treated with many types of processes and substances. This included metallic salts. There were many problems with oil modified film forming substances. There was colour darkening of coating on ageing. It was not possible to create long lasting lighter colour shades. Over oxidation made the film very weak. Presence of natural gums made the film tacky in moist weather. The process of polymerization was not clearly understood.
OLEO-RESINOUS BINDERS: These substances are often combined with oils, bodied oils or their resins. Rosin is a low-cost resin derived from sap of mainly Pine trees (abietic acid). It is neutralized with lime (to form calcium Rosinate), glycerin (ester gum) or penta-erythritol (penta). These substances are now used in low cost paints and varnishes, or as cost effecting additives. Coumaro-indene (cumar) resins are derived from coal tar naphtha, and are used in leafing aluminum paints and other low cost systems. Phenolic resins are produced by reacting synthetic phenolics or phenolic compounds (cashew nut shell liquid) with formaldehyde.
The technology of oil coating, in spite of its many drawbacks was very popular. It first came into being as the art media, but soon began to be used extensively in upgrading the interiors of mansions and palaces. It was the beginning of oil based architectural coating system.
With the onset of industrial revolution the use of iron and ferrous alloys became extensive. Not only coating materials in large quantities, but for specific surfaces were now required. Rusting was a major problem with metal surfaces. Many protective coating systems were developed.
During world war II, availability of natural oils such as linseed, castor, etc., which were of Asiatic and transatlantic origin, was affected. At the same time several petroleum-based products and derivatives began to be produced. These prompted research on non oil resins. Such chemically polymerized resins had faster drying time, better toughness, finer gloss and superior colour fastness, then heat bodied mediums. Polymeric resins both, thermo-plastic and thermo-setting varieties were produced.
Alkyd resins first came into use in the 1920s. These first alkyd resins (first polymerized polyester resins) were merely the reaction products of phthalic anhydride and glycerine. These products were too brittle to make a satisfactory film. The use of oils or unsaturated fatty acids in combination with the brittle alkyds resulted in the air-drying coatings which revolutionized the coating industry.
Alkyd resins form the basis of most of the commercially available glossy paints, flat paints and primers. These are produced by reacting drying and semi drying oils with a phthalic anhydride (acid), glycerin and penta-erythritol. Resins which have smaller proportion of oil -called short oil alkyds, or have oils of semi drying type, are used in baking finishes or as plasticizers. While long oil alkyds are used for air drying finishes or GP Enamels. Typically a long oil alkyd will take longer to dry out but may have higher strength, and a short oil alkyd may require baking to aid the drying.
A variety of alkyds are produced by modifying them with other polymeric compounds. These modifiers are like: Rosinated alkyds, phenolated alkyds, styrenated alkyds, silicone modified alkyds, epoxy modified alkyds, acrylic modified alkyds, vinyl-toluene modified alkyds, and urethane alkyds.