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What is Alkyd Resin ?
Alkyd is a kind of polyester synthetic resin created by the poly-condensation reaction of polyhidric alcohols and dibasic acids and modified with oil or fatty acids. Considered as the first polyester study, Berzelius’s (1847) synthetic resin study started with the reaction of tartaric acid and glycerin, then sped up with alkyd resin studies after the World War II and began to be used widely in paint and varnish manufacturing. The word alkyd derived from alcohol and anhydride and the polyester part in alkyd resin helps the resin’s hardness while the oil and fatty acid part helps the flexibility, adhesion, pigment wetting and its’ solubility with solvents. Alkyd resin is the most important building block of paint and varnish systems which directly effects the film forming as well as determining the important properties of coating films like physical and chemical resistance, adhesion and durability.
Today conventional alkyds are widely used and because their chemical structures are suitable they can be used by cold blending with most of the resins or radical groups or by modified as “hot blends”. It’s possible for the properties of alkyd polymer to be improved. While differently modified alkyd resins can be produced with new designs, today the trend toward the high solid systems and especially emulsified alkyds is rapidly increasing.
Alkyd resins are composed of three groups according to their oil lengths:
- Long oil alkyds: Alkyds, of which oil percentage (oil length) in alkyd resin is above %55 are called long oil alkyds. They have a wide area of use in all types of construction and decoration purposed, air drying paint systems. They are thinned with aliphatic solvents. The application method is by brush or roller.
- Middle oil alkyds: Alkyds, of which oil percentage (oil length) in alkyd resin is between %45-55 are called middle oil alkyds. They are preferred in decorative purposed paints, marine and machine paints and auto mechanic paints. They are thinned with aliphatic or aromatic solvents. The application method is by brush, roller or spray.
- Short oil alkyds: Alkyds, of which oil percentage (oil length) in alkyd resin is below %45 are called short oil alkyds. There are types which are made of drying or undrying oil/ fatty acids. The types which are made of drying or undrying oil/ fatty acids will generally be modified and take place in the rapid alkyd group and widely used in industry. The types which are made of undrying oil are used in metal or furniture industry as single or two components. The application method is by spray and dipping and they are thinned with aromatic solvents.
Alkyd resin is produced by the esterification of polyhydric alcohols and dibasic acids and through modification with oil and fatty acids in 200-240 °C. There are two methods of alkyd resin production process; process with solvent and process without solvent (fusion). In the process with solvent, after the reactants are loaded the water came out from the esterification reaction will be removed from the environment by making an azeotropy mixture with the help of a solvent and poly-condensation reaction will be carried out until the desired acidity and viscosity properties are achieved. The process without solvent will be made with the help of an inert gas, by removing the water that came out from reaction by vacuum. This method is especially used in long oil alkyds.
There are four kinds of alkyd production methods but two of them are more widely used.
- Alcolysis ( Monoglyceride) method: In this method the oil which are the triglycerides of fatty acids will be turned into the monoglyceride formation as a result of the ester exchange reaction with polyalcohol by the help of a catalyst. By adding anhydride (polyacid) esterification reaction will goes on in 200-240°C until the desired acidity and viscosity is achieved. This is the most preferred method because it’s economic.
- Fatty acid method: Fatty acid will be loaded in the polyacid and polyalcohol reactor. Esterification reaction will goes on in 200-240 °C until the desired acidity and viscosity properties are achieved and as a result alkyd resin will be produced. It’s a widely used method.
- Oil-polyacid (acidolysis) method: After the reaction of oils mainly with isophthalic acid in 260-270°C, polyhydric alcohol will be added. As a result of the condensation reaction alkyd resin will be gotten. It’s preferred for some special products.
- Fatty acid-oil method: It’s a method where polyalcohol and polyacid are added after the use of fatty acid together with oil, but not preferred much.
A great part of the physical and chemical properties of the paint is given by the binder in its structure, resins. Using alkyd based resin adds following features to the paint:
- Perfect durability, flexibility and hardness
- Perfect gloss and color retention
- Wetting the pigment
- Water resistance– wash-off feature
- Heat resistance
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