Polyester resin (C4H10O2 )is a high-molecular-weight compound formed through the condensation polymerization of diols or diacids or polyols and polyacids. Polyester resins are classified into saturated polyester resins and unsaturated polyester resins based on the presence of unsaturated bonds. Saturated polyester refers to synthetic materials that do not contain unsaturated bonds other than the benzene ring.
The fundamental raw materials for synthesizing saturated polyester resin are polyols and polybasic acids. To ensure the normal progress of resin production and processing, solvents are used to aid in dehydration and dilution. In industrial production, catalysts are employed to accelerate reaction rates for the purpose of reducing processing time.
Polyols refer to compounds in which multiple hydroxyl groups (-OH) are directly attached to aliphatic carbon chains within a molecule. The chemical properties of polyols are primarily determined by the hydroxyl functional groups, and are also influenced to some extent by the hydrocarbon groups.
Due to the different nature of the carbon atoms to which the hydroxyl groups are attached—such as primary (1°), secondary (2°), or tertiary (3°) carbons—these hydroxyl groups can be categorized as primary hydroxyls, secondary hydroxyls, and tertiary hydroxyls. The varying structural features of aliphatic carbon chains within the hydrocarbon groups will have different effects on the reactivity of the hydroxyl groups.
Polybasic acids are those acids in which molecules contain multiple carboxyl groups directly linked to hydrocarbon radicals. Depending on the type of hydrocarbon radical, they can be classified into aliphatic acids, cycloaliphatic acids, and aromatic acids. Based on whether the hydrocarbon radical contains unsaturated bonds, they can be categorized as saturated carboxylic acids and unsaturated carboxylic acids. While their chemical properties are primarily determined by the carboxyl functional group, the structural characteristics of the hydrocarbon radical also influence the reactions of polybasic acids, imparting different properties to the resulting polyesters.
The chemical reactions of polybasic acids mainly occur at the carboxyl groups. The synthesis of saturated polyester resins primarily involves esterification reactions between carboxyl groups and hydroxyl groups in polyhydric alcohols.
Solvents play a dual role in polyester resin production. They help remove water by co-boiling during the reflux dehydration stage, and they also dissolve and dilute the resin.
Industrial production of saturated polyester resin needs to be completed within a specific timeframe. However, different polybasic acids and polyhydric alcohols exhibit varying reaction activities. If the formulation includes ingredients with lower reactivity (such as phthalic acid), the esterification reaction rate will be slow. This means it would take a long time to achieve the required acid value and viscosity, making it economically inefficient. Catalysts are often used to shorten the reaction time.
Saturated polyester resin has rapidly developing in the coating and adhesive applications in recent years. When combined with suitable cross-linking agents, it forms coatings with excellent weather resistance, UV light resistance high hardness, good flexibility, and adhesion.
When paired with amino or polyurethane, they are primarily used in coil coatings and automotive coatings. Applications also include powder coatings when used with epoxy resin.
The basic structure of polyester resin includes ester bonds, formed through esterification reactions between polybasic acids and polyhydric alcohols. It contains carboxyl and hydroxyl groups. While epoxy resin is synthesized through the reaction of epoxy compounds with acids or bases.
PET (Polyethylene Terephthalate): PET can be processed into fibers, films, and plastic products. Polyester fibers are a significant category of synthetic fibers, mainly used in apparel. Epoxy resin is primarily utilized in the coatings and electronics industries. Epoxy used in the molding of composite materials (mainly applied in the electronics industry for printed circuit boards) constitutes a quarter of its applications.