Despite the different chemical structures of resins, common indicators for evaluating resin properties include acid value, hydroxyl value, iodine value, amine value, epoxy equivalent, isocyanate equivalent, molecular weight, viscosity, and glass transition temperature.
◆Main Indicators of Resin Properties
Even resins with different chemical structures, like polyester and acrylic, have common indicators that show their properties.
1. Indicators related to the amount of functional groups involved in reactions like curing
Resins are evaluated based on indicators like acid number, hydroxyl value, iodine value, amine value, epoxy equivalent, and isocyanate equivalent. These are defined by JIS and need careful consideration depending on the resin form. When designing two-component polyurethane paints, the ratio of hydroxyl groups in polyol resins to isocyanate groups in polyisocyanates helps study curing conditions. For epoxy resins cured with amine hardeners, the ratio of epoxy to amine groups is useful. In organic solvent systems, high polymer interacts with pigments through acid-base interactions. A high acid value indicates an acid polymer, which can improve the dispersibility of basic pigments, while a high amine value indicates a base, which can improve the dispersibility of acid pigments during dispersion design.
2. Molecular Weight
Generally measured by gel permeation chromatography(GPC), the larger the molecular weight of a resin, the stronger the coating film will be, but the viscosity of the paint will also increase. A certain viscosity is needed when applying paint, and using high molecular weight resins results in a lower solid content concentration and consequently, more volatile solvents.
Bubble viscometry is used to measure the viscosity of resins. The rising speed of bubbles in a sample tube is compared with a standard liquid, and the viscosity is indicated by symbols A to Z, A5 to A1 for low viscosity, and Z1 to Z10 for high viscosity. Each standard tube symbol has a defined kinematic viscosity (viscosity divided by the liquid’s density).
4. Glass Transition Temperature
Amorphous polymers like coating films are soft at high temperatures but harden rapidly as the temperature decreases. This hardening temperature is called the glass transition temperature (Tg), and at Tg, physical properties (specific heat, thermal expansion coefficient, compressibility, elasticity, etc.) change significantly. Tg is determined by the movement of polymer chains and is affected by the resin’s chemical structure, hardeners, plasticizers, and crosslinking density. Typically, resins with a low Tg form soft coating films, while those with a high Tg form hard films.