Acrylic monomers such as HEMA, HPMA, HEA, HPA, IBOA, and IBOMA are pivotal in the production of high-performance resins and coatings. These monomers offer outstanding control over glass transition temperature (Tg), exceptional balance between hardness and flexibility, and superior adhesion properties. By integrating these monomers, manufacturers can significantly enhance resin performance to meet the stringent demands of various industrial coatings, thereby boosting the competitiveness of their products.
Structural Characteristics
Monomers like HEA, HEMA, and HPMA, which contain hydroxyl groups, introduce hydroxyl functionalities into the side chains of resins. This enables cross-linking with polyurethane hardeners or amino resins, thereby improving the durability and adhesion of coatings. On the other hand, HPA, IBOA, and IBOMA enhance resin flexibility and weather resistance through their distinct molecular structures, leading to improved lightfastness and anti-aging properties in coatings.
In solvent-based coatings, monomers such as HEMA, HPMA, HEA, and HPA are used to prepare high-solids coatings and thermosetting acrylic resins, providing higher cross-linking density and better mechanical properties. IBOA and IBOMA, with their moderate Tg, adjust the hardness and flexibility of coatings, improving application performance.
Surface Tension Variations
- Low surface tension enhances wetting and adhesion, where the coating film should have lower surface tension than the substrate for effective adhesion.
- Monofunctional monomers exhibit the lowest surface tension.
- Methacrylates have lower surface tension compared to acrylates.
- Ethoxylation increases surface tension.
- Propoxylation decreases surface tension.
Refractive Index Variations
- Cyclic monomers have a higher refractive index.
- Increasing functional groups raises the refractive index.
- High molecular weight monomers exhibit a higher refractive index.
- Methacrylates have a higher refractive index than acrylates.
- High refractive index products result in higher gloss and clarity.
Functional Group Quantity and Structural Differences
- Monofunctional monomers significantly reduce system viscosity while providing some flexibility (unidirectional polymerization).
- Bifunctional monomers promote branching or cross-linking, enhancing strength or heat resistance.
- Tri-functional and higher monomers, even in small amounts, can increase cross-linking density, improving heat resistance, chemical resistance, hardness, curing speed, and abrasion resistance.
Selection and Ratio of Soft and Hard Monomers
Design of Tg (Glass Transition Temperature): Tg is the lowest temperature at which polymer segments can move. Higher chain flexibility results in lower Tg, while greater chain rigidity leads to higher Tg. The appropriate Tg for acrylic resins is determined by considering the type of coating, performance requirements, and special properties:
- High Tg results in harder films (though care must be taken to avoid brittleness) and greater scratch resistance.
- Higher Tg provides better surface drying and solvent release post-application.
- Higher Tg increases the final viscosity of the resin reaction.
- Higher Tg improves solvent and corrosion resistance.
- For topcoats on thermoplastic plastics, resins generally require a Tg above 70°C.
- Primer resins for plastic coatings should have a Tg between 45°C and 60°C.
- Topcoats for electronic devices (TVs, mobile phones, computers) typically require a Tg between 90°C and 110°C.
- ABS plastic coatings demand high-performance resins with Tg ideally between 100°C and 110°C.
- PP plastic primers use modified thermoplastic acrylic resins with Tg ranging from 50°C to 65°C.
Characteristics of Waterborne Coating Resin Film Formation and Curing
The formation and curing of waterborne coating films involve several stages:
- Aggregation and coalescence of emulsion particles, a common mechanism for all emulsion-based coatings during surface drying.
- Evaporation of water and coalescing agents, which reveals the intrinsic properties of thermoplastic resins, marking the second stage of curing.
- In some formulations, cross-linking mechanisms are introduced during emulsion preparation or coating application to further enhance the film's hardness. Common cross-linking mechanisms include oxidative cross-linking (e.g., in alkyd resins), Michael addition cross-linking (e.g., in self-crosslinking emulsion systems), and nucleophilic substitution cross-linking (e.g., in epoxy or polyurethane systems). These reactions are influenced by factors such as temperature and pH, requiring a balance between curing requirements and other performance criteria during formulation.
Key Considerations in Waterborne Coating Formulation
- Resin Solid Content: Higher solid content in emulsions usually leads to faster drying but can introduce challenges such as reduced application workability.
- Emulsion Particle Size: Smaller particles generally result in better film formation and higher gloss but may require more careful formulation adjustments.
- Glass Transition Temperature (Tg): A higher Tg generally correlates with better film performance, though it may increase drying time due to the need for additional coalescing agents.
- Emulsion Particle Morphology: The core-shell structure is a classic example, where the shell's lower Tg requires fewer coalescing agents but may impact film hardness.
- Surfactant Type and Quantity: Surfactants play a crucial role in particle stabilization and film formation, with varying impacts on drying speed and film hardness based on their solubility in the resin.
