In paint production, opacity defines both cost efficiency and product aesthetics. Many engineers face the situation where the same TiO₂ level gives different results in wet and dry states. The difference between wet opacity and dry opacity reflects the complexity of light scattering, solvent evaporation, and film structure. Understanding this mechanism helps optimize formulas, reduce cost, and improve quality.
WET OPACITY AND DRY OPACITY
Wet opacity – coverage in wet state
Wet opacity is the ability of a coating to hide the substrate right after application, while water or solvent is still present in the film. At this stage, TiO₂ particles are dispersed in the liquid phase, giving a temporary cloudy look. The applicator often perceives the paint as covering well. However, this is only the initial state and does not represent the final performance. The measured value is only a reference for process control.
Dry opacity – coverage in dry state
Dry opacity appears after solvent has fully evaporated, binder hardens, and the film stabilizes. In this state, TiO₂ and extenders are rearranged within the polymer network. Dry opacity reflects the true value, which determines long-term economy and quality. It is also the basis for evaluating TiO₂ efficiency in a formula. International standards rely on dry opacity for testing and certification.
WHY WET OPACITY AND DRY OPACITY DIFFER
Pigment crowding in dry film
When the film dries, solvent evaporation reduces the volume significantly. TiO₂ particles that were evenly dispersed start moving closer together. This phenomenon, called pigment crowding, reduces light scattering efficiency. As a result, dry opacity is much lower than wet opacity. This is a common challenge in paint formulation.
Uneven TiO₂ dispersion
Opacity strongly depends on the quality of pigment dispersion in the system. If TiO₂ is not well dispersed, particles will agglomerate as solvent evaporates. This creates uneven areas, reducing hiding power in the dry state. In contrast, the wet film may still look acceptable. Controlling dispersion is therefore a critical factor in formulation design.
Refractive index difference between wet and dry
The refractive index of solvent is close to that of TiO₂, so in wet state light scattering is limited. This creates a “false” opacity, making the film look more opaque than it really is. Once dry, the binder’s refractive index differs greatly, changing the scattering efficiency. Depending on the system, it may increase or decrease. This explains why the same TiO₂ level gives different results.
Transparency change of binder
Binder not only forms the film but also controls light transmission through the layer. Some binders are clear when wet but become less transparent when dry, allowing more light to pass through. This reduces dry opacity compared with the initial value. On the other hand, binders that keep high clarity minimize this difference. Choosing the right binder is therefore vital.
COMPARISON OF WET AND DRY OPACITY
Wet opacity reflects a temporary state right after application, when TiO₂ is dispersed in solvent. It often gives the impression of better coverage than reality. Dry one, in contrast, shows the true efficiency once the film has stabilized. It is the key parameter in product performance and quality testing.
The difference between the two is a natural result of drying, not a test error. Factors like pigment crowding, binder properties, refractive index, and extender use define the level of variation. Recognizing these differences allows manufacturers to optimize formulas, avoid wasting TiO₂, and ensure economic efficiency. This is the key to stronger market competitiveness.
HOW TO OPTIMIZE DRY OPACITY IN PAINT
Optimize TiO₂ dispersion
The way TiO₂ is dispersed directly determines coverage in the dry film. Well-separated particles remain evenly distributed and maintain scattering efficiency. This requires suitable dispersing additives and proper milling conditions. As a result, agglomeration and crowding are minimized. This is a crucial step to increase dry opacity and reduce cost.
Choose a binder with stable transparency
Binder forms the film and controls how light passes through it. A binder that maintains high clarity when dry prevents light from penetrating the film, letting TiO₂ work at full efficiency. Poorly transparent binders reduce the effect. Selecting the right binder balances coverage, durability, and appearance. This is a sustainable solution for modern formulations.
Use high-performance rutile TiO₂
In many coating systems, rutile TiO₂ offers far better hiding than anatase. With its high refractive index, rutile scatters light strongly and remains photochemically stable. It also helps films stay white longer under sunlight. Although more expensive, rutile provides economic benefit by reducing the TiO₂ level required. It is the preferred choice for premium paints.
Please refer to official TiO₂ products distributed by Vichem:
VICHEM – AUTHORIZED DISTRIBUTOR OF TITANIUM DIOXIDE IN VIETNAM
Vichem is an authorized distributor of plastic additives with full legal compliance in Vietnam. We not only supply high-quality products but also deliver after-sales services that meet international standards. Our services include:
- Technical consulting: Supporting customers in selecting the right products for their needs.
- Color matching tests: Vichem’s specialists conduct tests to ensure results meet design requirements.
- Application support: Assisting customers in integrating products into production lines for optimal efficiency.
For more information, please contact us via:
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