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Summary Note
Polyester resin is a versatile resin that balances cost, durability, hardness, and flexibility. However, it’s important to be cautious about its water resistance.
1. What is Polyester Resin?
Polyester resin is a synthetic resin connected by ester bonds, which are formed through a reaction that removes water from carboxyl groups and hydroxyl groups (condensation reaction). This resin is produced when monomers with hydroxyl groups and monomers with carboxyl groups react. Polyester resin is a polymer created when monomers with two or more hydroxyl groups (hydroxyl monomers) and monomers with two carboxyl groups (acid monomers) react with each other. The quantity of hydroxyl groups and carboxyl groups within the resin is expressed as hydroxyl value and acid value, respectively.
2. What is Alkyd Resin?
Alkyd resin is a modified polyester resin that primarily uses vegetable oils as raw materials, incorporating fatty acids. Alkyd resins are classified into long oil, medium oil, and short oil alkyd resins based on the oil content, and there are also alkyd resins with double bonds depending on the type of fatty acids. The quantity of double bonds is indicated by a value known as iodine value. Alkyd resin is a type of polyester resin and is historically common, sometimes referred to as “oil-free polyester” to distinguish it from polyester resins that don’t contain oils.
3. Characteristics
The characteristics of polyester resin depend on the types of monomers used. Specifically, using monomers with benzene rings, low methylene chain lengths (n), and aliphatic dicarboxylic acid monomers can result in a hard resin. On the other hand, a high methylene chain length (n) leads to a softer resin. Alkyd resin is relatively cost-effective and offers good pigment dispersion, gloss, and durability. Among straight-chain oil-free polyester resins, those with medium molecular weights (average molecular weight of 2,000 to 30,000) can achieve a balance between flexibility and hardness.
However, it’s important to note that the ester bonds in polyester resin are sensitive to water, so water resistance should be considered. To enhance water resistance, it is recommended to use hydroxyl monomers and branched diol monomers instead of straight-chain diols.
4. Usage
Polyester resin, due to the presence of hydroxyl and carboxyl groups, can form a strong cross-linked coating by reacting with curing agents like polyisocyanates or melamine resin. For this purpose, oil-free polyester and short oil alkyd resins are used. Additionally, high iodine value long and medium oil alkyd resins can cure at room temperature through oxidative polymerization of double bonds and are used in one-component air-drying paints (catalysts known as dryers are required).