Factors Affecting the Drying Efficiency of Waterborne Polyurethane Paint

Waterborne polyurethane (WPU) paints are gradually replacing traditional solvent-based paints due to environmental concerns. Unlike organic solvent-based paints, waterborne paints use water as a dispersing and diluting agent. However, due to water’s significantly lower volatility compared to organic solvents, drying waterborne paints requires heating during industrial production. Only a few large-scale facilities unable to heat dry or manufacturers with lower production efficiency requirements opt for natural air drying. What are the intrinsic factors affecting the drying efficiency of waterborne polyurethane paints? Today, we will delve into this topic with a focus on waterborne polyurethane.

Waterborne polyurethane, also known as water-dispersible polyurethane or water-based polyurethane, replaces organic solvents with water as the dispersing medium. It offers advantages such as non-toxicity, safety, excellent mechanical properties, good compatibility, and ease of modification. Polyurethane (PU) is a polymer compound containing urethane links in its main chain. Waterborne polyurethane (WPU) refers to PU emulsions dissolved or dispersed in water. Due to its water-based nature, WPU is non-toxic, non-flammable, and environmentally friendly, making it suitable for applications in textiles, leather, paper, rubber, automotive, and furniture industries for decorative and protective purposes. It has gradually replaced solvent-based PU paints.

In waterborne polyurethane paints, water molecules exist in three forms: free water, bound water, and gel water. Compared to solvent-based PU paints, WPU drying process is characterized by a higher initial temperature point for water evaporation. In environments with certain humidity, the vapor pressure of water within the paint film must exceed the environmental vapor pressure for evaporation to occur. These characteristics contribute to slower drying rates of waterborne polyurethane paints compared to solvent-based counterparts, which has partly hindered their broader adoption. Therefore, improving the drying rate of waterborne paints is essential for market expansion and necessitates further research.

1. Effect of Solid Content: The content of free water is closely related to the solid content. Higher solid content in the emulsion reduces free water content, thereby shortening the evaporation time and improving the drying rate of WPU. Studies have shown that drying rates of WPU can match those of conventional solvent-based PUs when solid content exceeds 50%, and heating with carbon fiber heating elements raises the ambient temperature to 40°C-60°C. Control of solid content through paint formulation is crucial in production.
2. Impact of Hydrophilic Group Content: To enhance dispersion of WPU, hydrophilic ion groups are often introduced into the PU molecular chain. However, increased hydrophilic group content complicates drying and reduces drying rates. Bound water formed through hydrogen bonding and intermolecular forces with PU hydrophilic groups evaporates at a much slower rate than free water. Therefore, higher hydrophilic group content leads to increased bound water in the emulsion, hindering evaporation. To ensure emulsion stability, it is important to minimize hydrophilic groups.
3. Influence of Hard Segment Content: Increasing hard segment content accelerates drying rates by enhancing compatibility between soft and hard segments, facilitating easier displacement of water molecules during molecular chain movement. However, excessive hard segment content (approximately 46%) can lead to surface skin formation during drying, creating a barrier that slows down moisture evaporation and may result in defects such as orange peel or poor adhesion.
4. Effect of Neutralization Degree: Carboxylic acid groups exhibit weak hydrophilicity, while carboxylic acid anions formed through neutralization possess stronger hydrophilicity. Higher neutralization degree enhances hydrophilicity of molecular chains, increasing bound water content and thereby affecting WPU drying rates. Additionally, higher neutralization degree increases paint viscosity, promoting surface skin formation and impeding internal moisture evaporation, thus affecting both drying rate and product quality.
5. Substrate Influence on Film Formation: Surface properties of substrates significantly influence intermolecular forces between water molecules, thereby affecting paint drying rates. For instance, WPU paint dries faster when applied on glass substrates compared to composite panels due to differences in surface tension and interaction between emulsion and substrate. Substrates with lower surface tension, such as wood, exhibit stronger interaction with water, making moisture evaporation more challenging.

Now that we’ve explored the intrinsic factors affecting the drying efficiency of waterborne polyurethane paints, it’s evident that optimizing these factors through paint formulation adjustments is crucial for achieving high production efficiency and economic viability of these innovative paints. Future discussions will cover external factors influencing waterborne polyurethane paint drying rates.

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