Film-former are also referred to as resins, binders. Typically, natural oils, natural, or synthetic resins would act as the film-former. They are the main materials that create the paint film, and thus determining the properties of the coating.
The primary function of resin or binder is to form a robust film and provide adhesion. During the curing process of coatings, resin combines all coating components together, forming a cohesive and uniform layer or film. Resin also contributes to wetting, penetration the substrate or primer to generate adhesion for a robust film.
The coating film-forming process is highly complex. Most coatings undergo the transformation from a liquid wet film to a solid coating (powder coatings also melt into a liquid state before cooling and solidifying). The chemical composition and structure, molecular weight and distribution, solubility parameters, polarity, and distribution of polar groups in organic film-forming resin, as well as the content and distribution of active groups in cross-linking resin, and the glass transition temperature (Tg) directly determine the coating's performance. These properties are closely related to the dispersion stability, rheological characteristics, and overall uniformity of the film formation in liquid dispersion systems. Choosing the appropriate film-former and understanding its characteristics is a critical step in developing new coating products.
Resins can be categorized into two groups based on their organic and inorganic properties.
Organic Film-formers: Natural and synthetic polymers, chemically modified natural resins, etc., serve as the main components of coatings, constituting organic coating materials.
Inorganic Film-formers: Primarily based on polymeric silicates or phosphates, such as high modulus potassium silicate, silicic acid lithium, and zinc polyphosphate, act as the main binders in coatings.
Thermoplastic resins are high-molecular-weight natural or synthetic polymer resins. Examples include asphalt, nitrocellulose, chlorinated rubber, and chemically modified resins such as acrylics, chlorinated polyolefins like polyvinyl chloride (PVC), polyethylene terephthalate (PET), and polyvinyl acetate (PVAc). Typically, these are dissolved in solvent to create resin solutions for coating preparation. The coatings solidify into films through solvent evaporation. The chemical structure of the resin remains largely unchanged before and after film formation (though there might be changes in physical state and molecular entanglement).
Thermoplastic resins feature a limited solubility, making it challenging to formulate high solids content coatings. These coatings often have higher VOC (volatile organic compound) content, which can’t be used in applications with strict environmental regulations.
However, thermoplastic solvent-based coatings offer various advantages such as fast-drying, simple installation, excellent adaptability to environmental conditions. Despite challenges, thermoplastic solvent coatings still hold a significant market share.
Thermosetting resins are oligomeric substances with low molecular weights, containing functional groups for cross-linking reactions. During the film formation process, thermosetting resins undergo cross-linking reactions with curing agents (such as epoxy, polyurethane, unsaturated polyester, and polyurea coatings). Alternatively, they can undergo oxidative cross-linking by absorbing oxygen from the air, particularly in the presence of unsaturated bonds in resinous compounds.
Thermosetting resins react to form three-dimensional network structures, resulting in polymers with molecular weights approaching infinity. The coatings produced are insoluble and do not melt. Compared to thermoplastic coatings, thermosetting coatings exhibit higher mechanical strength, enhanced protective properties, and superior decorative performance. These resins find extensive use in high-performance industrial coatings and specialty functional coatings.
With lower molecular weights and excellent solubility, thermosetting resins can be processed into coatings with high solid content, low volatile organic compound (VOC) emissions, and solvent-free formulations. Most of the thermosetting coatings are two-component, with resins and curing agents being packaged separately.
Waterborne resins are prepared through emulsion polymerization of vinyl monomers, utilizing water as the dispersion medium. Waterborne resins exhibit low volatile organic compound (VOC) levels and are associated with higher molecular weight thermoplastic resins. During the water evaporation process, resin latex particles coalesce and form a film. However, the water resistance of these coatings is not as robust as solvent-borne resin coatings, due to the presence of hydrophilic emulsifiers or hydrophilic groups.
Waterborne resin can also be prepared through a post-emulsification process, where the resin is first dissolved in a solvent and then diluted by removing the solvent after mechanical dispersion with the addition of emulsifiers. In recent years, there has been a trend towards developing self-emulsifying resins by introducing hydrophilic groups (such as -COOH, -OCH2CH2-) into the resin structure. Simultaneously, the application of thermosetting resin emulsions in industrial coatings has seen rapid development.
Waterborne resins is used as the one componnet for waterborne paints.
Water reducible resins are typically synthesized by first dissolving monomers in water-friendly solvents like acetone or butanol, followed by polymerization. Neutralization and dilution with water follow this step. These coatings have lower VOC content compared to corresponding solvent-borne coatings but are higher than emulsion-type coatings.
This category involves dissolving resins in strong solvents, followed by dilution with fatty alcohols in the presence of special surfactants to create organic emulsions. These coating systems demonstrate superior film-forming properties compared to water emulsions. The main dispersion medium consists of low-toxicity hydrocarbons, allowing the formulation of coatings with lower VOC. This film-forming system is currently under active development.