In the coatings industry, fatty acid or oil-modified polyester resins are commonly referred to as Alkyd Resins. Alkyd resins are polymeric structures formed by the polycondensation of polyols and polyacids, typically using fatty acids (oil) as the modifying agent. The molecular structure of alkyd resins consists of a main chain derived from polyols and side chains derived from fatty acids.
Chemical Structure of Alkyd Resin
In terms of production methods, before the 1960s, the melt process was predominantly used for alkyd resin production. However, in the 1960s, the solvent process gained widespread adoption in the coatings industry. By the late 1980s, alkyd resin production achieved commercialization and large-scale manufacturing. At the time, the fatty acid process of alkyd resins became more prevalent in the production technology.
Alkyd resins come in a variety of types due to their versatile adjustability in composition and properties. When formulating alkyd resin, different polyols and polyacids can be chosen, and the ratio of functional groups between alcohols and acids, as well as branching, can be adjusted. The molecular structure of alkyd resins, featuring hydroxyl, carboxyl, double bonds, and ester groups, serves as a foundation for chemical modification. Moreover, the resin molecule's polar main chain and non-polar side chains allow for physical modification.
Classified based on the dryness of vegetable oil or oil used for modification, alkyd resins can be categorized into drying oil alkyd resin, semi-drying oil alkyd resin, and non-drying oil alkyd resin.
Drying Oil Alkyd Resin: This type of resin is typically modified using drying oils such as linseed oil or sunflower seed oil. These oils contain a higher amount of polyunsaturated fatty acids, resulting in the resin forming a hard and weather-resistant coating during drying. Drying oil alkyd resins are often used in the production of coatings that require high hardness and weather resistance.
Semi-Drying Oil Alkyd Resin: This resin is modified using semi-drying oils like soybean oil or cottonseed oil. The fatty acid content in semi-drying oils falls between that of drying oils and non-drying oils. Semi-drying oil alkyd resins typically exhibit good elasticity and adhesion in coatings, along with a certain level of weather resistance.
Non-Drying Oil Alkyd Resin: This category of resin is modified using non-drying oils such as peanut oil or soybean oil. Non-drying oils have a lower content of polyunsaturated fatty acids, resulting in a relatively soft coating when the resin dries. They are suitable for applications where higher flexibility in coatings is required.
Oils include tung oil, flaxseed oil, soybean oil, cottonseed oil, tung tree seed oil, safflower oil, dehydrated castor oil, castor oil, coconut oil, etc. Vegetable oil fatty acids are a mixture of various saturated and unsaturated fatty acids. Oils are generally classified based on their iodine value into: drying oils, non-drying oils, and semi-drying oils.
Alkyd resins can be classified based on the content of fatty acids or oil in the resin into long oil length, medium oil length, and short oil length alkyd resins. The oil length of an alkyd resin, also known as oil content, is expressed as a percentage and can be calculated using the following formula:
Oil Length=Weight of Oil/Total Weight of Alkyd Resin×100%
The content of oil or fatty acids in long oil length alkyd resins is typically between 60% and 70%, in medium oil length alkyd resins it ranges from 40% to 60%, and in short oil length alkyd resins, it falls between 30% and 40%.
The coheating of oil (triglycerides, i.e., glycerol esters of fatty acids) with alcohol, due to an excess of hydroxyl groups, leads to the phenomenon of carboxyl group redistribution. Common polyols used in the production of alkyd resins include glycerol and pentaerythritol. Because of carboxyl group redistribution, varying amounts of a mixture of oil, glyceryl monoesters, and glyceryl diesters are formed depending on the quantity of polyol and reaction conditions. Similar results are obtained when other polyols react with oil.
Oil cannot be used directly in alkyd resin manufacturing. It must undergo Alcoholysis reaction to become incomplete esters that can dissolve in a mixture of phthalic anhydride and glycerol, forming a homogeneous reaction. Alcoholysis reaction is crucial for the manufacture and modification of alkyd resins and is typically conducted at elevated temperatures with the aid of catalysts such as red lead and litharge.
Esterification reaction is the primary chemical process in the production of alkyd resins. It involves the condensation of the hydrogen atom on the hydroxyl group of alcohol molecules with the hydroxy group of acid molecules, resulting in the formation of water and esters. Esterification is a reversible reaction, and to drive it to completion, it is essential to remove the byproduct, which is water, from the system. This is a critical aspect of the alkyd resin production process. Esterification occurs slowly at room temperature, and typically, the esterification temperature for alkyd resins is in the range of 180 to 240 degrees Celsius. The rate and extent of esterification are influenced by the structures of the acid and alcohol components.
Single-component alkyd paints prepared from alkyd resins exhibit excellent wetting and penetration, good leveling, ease of application, strong adhesion, corrosion resistance, high gloss, and good decorative qualities. However, conventional alkyd paint films tend to be relatively soft before complete curing, exhibit average initial water resistance, and have limited alkali resistance (ester bonds are susceptible to hydrolysis in alkaline environments). As a result, they are seldom used in heavy corrosion protection for steel structures.
Alkyd resins can be combined with amino resins to produce amino-baking paints. When combined with isocyanate curing agents, they result in alkyd polyurethane acid-resistant anticorrosive paints. Introducing acrylic acid monomers during alkyd resin synthesis can yield acrylic-modified alkyd resins, used in the preparation of quick-drying acrylic-modified alkyd paints. These paints find wide applications in corrosion protection coatings for steel structures, machinery, chemical plants, bridges, automobiles, trains, wood, and other areas.