Does water-based paint become less water-resistant after adding silica sol?
Release date:
2026-02-18
Water-based coatings inherently suffer from poor water resistance, as they contain hydrophilic or water-soluble groups that make them prone to dissolution or swelling in water. Typically, the medium of water-based coatings has a slightly alkaline pH ranging from 7.5 to 8.5. Meanwhile, the ester bonds in the resin are easily hydrolyzed, leading to degradation of the polymer chains. This, in turn, can affect both the stability of the coating and the bath solution as well as the performance of the resulting coating film.
Water-based coatings inherently suffer from poor water resistance, as they contain hydrophilic or water-soluble groups that make them prone to dissolution or swelling in water. Typically, the medium of water-based coatings has a slightly alkaline pH ranging from 7.5 to 8.5. Meanwhile, the ester bonds in the resin are easily hydrolyzed, leading to degradation of the polymer chains. This, in turn, can affect both the stability of the coating and the bath solution as well as the performance of the resulting coating film.
I. The main reason for the poor water resistance of water-based coatings themselves
Chemical affinity of hydrophilic groups
After water-based coatings form a film, the hydrophilic groups remaining in the coating film—such as carboxyl, hydroxyl, and amide groups—adsorb water molecules via hydrogen bonding or ionic coordination, causing the coating film to absorb water and swell. This chemical adsorption is the common underlying cause of poor water resistance in water-based coatings, particularly evident in environments with high humidity.
Chemical degradation by ester bond hydrolysis
Acrylic resins commonly used in waterborne coatings contain ester bonds (-COO-), which, under the catalysis of water, are prone to hydrolysis, leading to chain scission, softening of the paint film, or blistering. For example, the hydrolysis of ester bonds in waterborne alkyd resins is the primary chemical mechanism responsible for their deterioration in water resistance.
Physical defects in the film-forming process
During the drying of water-based coatings, if excessive residual volatile additives (such as polyether film-forming aids) remain, they can reduce the cross-linking density of the paint film, leading to the formation of microscopic pores or cracks that provide channels for moisture penetration.
II. The Key Reason Why It Becomes Water-Insensitive After Adding Silica Sol
The inherent hydrophilicity of silica sol
The surface of silica sol particles is rich in silanol groups (-OH), which exhibit strong hydrophilicity. If added directly without hydrophobic modification, these silanol groups will form a hydrogen-bonding network with water molecules, significantly increasing the water absorption of the coating film and even negating the densification benefits that silica sol is supposed to provide.
The compatibility issue between silica sol and organic systems
The polar surface of silica sol tends to undergo phase separation when mixed with organic emulsions, leading to an uneven microstructure in the paint film. This phase separation can create hydrophilic microregions, accelerating water penetration. For example, poor compatibility between silica sol and polyurethane emulsions is a common contributing factor.
Structural defects caused by improper addition of silica sol
Excessive silica sol can cause the paint film to become brittle or develop a rough surface, disrupting the continuity of the film formation; whereas insufficient addition will fail to create an effective hydrophobic barrier. The optimal addition level must strike a balance between the density and flexibility of the paint film.
III. Mechanism of Synergistic Action
Chemical level: Silanol groups synergistically adsorb water molecules together with carboxyl and hydroxyl groups in the coating, thereby enhancing the hydrophilic tendency of the paint film.
Physical level: If the nanoparticles in silica sol are unevenly dispersed, capillary pores will form, and capillary forces will accelerate water penetration.
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