Basic Knowledge of Waterborne Coating Formulation, Part 3
Film-Forming Aids
Polymers that make up emulsions or dispersions typically have glass transition temperatures above room temperature. To ensure good integration of emulsion particles into a uniform paint film, film-forming aids must be used to lower the minimum film-forming temperature (MFFT). Film-forming aids are a class of small-molecule organic compounds that eventually escape and volatilize from the paint film.
Most film-forming aids are a significant component of volatile organic compounds (VOCs) in coatings; therefore, the less film-forming aid used, the better.
When selecting film-forming aids, prioritize compounds that are not subject to VOC restrictions but have moderate volatility and high film-forming efficiency.
The amount of film-forming aid depends on the amount of emulsion or aqueous dispersion in the formulation and the glass transition temperature. For emulsions or aqueous dispersions with high Tg values, a larger amount of film-forming aid is required, and vice versa. When designing a formulation, the film-forming aid should ideally comprise approximately 3%-5% of the emulsion or aqueous dispersion, or 5%-15% of the solids content.
However, for polymer emulsions with Tg values exceeding 35°C, the amount of film-forming aid may need to be increased to ensure reliable low-temperature film formation. In this case, the amount of film-forming aid should be gradually increased until a uniform, non-cracking, non-powdering paint film can be formed at low temperatures (around 10°C or lower), thus determining the minimum required amount.
Using film-forming aid at 15% or higher of the emulsion or dispersion is not advisable; alternative film-forming aids should be considered. Besides lowering the minimum film-forming temperature and increasing film density, film-forming aids can also improve workability, increase leveling properties, extend open time, and improve storage stability, especially low-temperature antifreeze properties.
Film-forming aids in water-based coatings are generally alcohol ether solvents, most commonly diethanol ethers, propylene glycol ethers, and N-methylpyrrolidone, which vary in boiling point.
During summer application, water-based coatings dry relatively quickly, meaning some moisture may remain trapped within the coating film before it is fully dry, leading to whitening or poor leveling. Therefore, adding a small amount of appropriate high-boiling-point solvent can slow down the drying process, extend the open time of the film, and improve its application properties and appearance.
In winter, due to lower temperatures, water-based coatings dry more slowly, meaning water evaporates more slowly. However, film-forming aids evaporate relatively faster than water, and some may not evaporate with the water. This can prevent the water-based coating from forming a dense layer, resulting in whitening and cracking of the film.
Therefore, when adding film-forming aids, it is essential to consider overcoming the application challenges of water-based coatings in different seasons, while also ensuring the fastest possible drying speed. This is the most difficult technical aspect of water-based coatings, especially for acrylic dispersions with high Tg. Therefore, using a combination of multiple film-forming aids is the best option.
Film-forming aids have a compatibility issue with resin systems. A film-forming aid that works well in one system may cause instability, severe problems, or poor recoatability in another water-based wood coating. This must be fully considered during formulation design, and the optimal film-forming aid and its dosage should be selected through testing.
Defoamers and Antifoaming Agents
Defoamers are among the most difficult additives to control in water-based coatings, preventing defoaming and leveling. Air bubbles are generated during paint application due to agitation. If these bubbles are not eliminated promptly, they will form unacceptable defects after the paint film dries. Air bubbles are also generated during the paint production process, pumping, and filling.
Foaming is an indispensable factor in formulation design. The defoaming mechanism of defoamers involves wetting and penetrating the foam film generated by the system, continuously diffusing across the film, causing an imbalance in surface tension and ultimately breaking the bubbles.
Generally, defoamers are divided into two types: foam suppressants and foam defoamers. Foam suppressants inhibit foam formation in water-based coatings under external force, while foam defoamers rapidly disperse on the surface of existing foam films, disrupting the elastic membrane of the bubbles and causing them to rupture. Often, a combination of both is more effective; otherwise, at least one effective defoamer should be selected.
Most defoamers, especially silicone defoamers, can cause pinholes in the wet paint film if used in excessive amounts. Therefore, the amount of defoamer used should be sufficient to essentially eliminate bubbles; excessive pursuit of defoaming effects should be avoided to prevent side effects such as pinholes. For water-based wood coatings, mineral oil-based defoamers are more forgiving than silicone defoamers; adding slightly more does not easily lead to severe pinholes, and they can be considered as a preferred choice.
Many defoamers are available on the market, and their selection requires careful consideration. Several aspects should be taken into account:
1. Controlled compatibility: Achieving a balance between defoaming and surface finish is crucial. Defoaming should be effective while preventing defects such as pinholes and poor leveling.
2. Defoaming should be tailored to application conditions and methods. For example, brushing, spraying, and roller coating present different levels of foaming, necessitating different defoamer choices.
3. The balance between foam suppression and defoaming should be achieved through appropriate combinations based on different processing and application methods.
4. Long-lasting defoaming performance. Many defoamers gradually weaken their defoaming properties during paint storage. Therefore, the defoamer content in the paint should be relatively high when designing the formulation.
The amount of defoamer used should be 0.05%–0.5% of the total formulation, ideally around 0.1%. If a good defoaming effect is only achieved with an amount exceeding 0.5%, the defoamer should be considered for replacement. Different resin systems have varying sensitivities to defoamers. The type and dosage of water-based defoamer must be determined experimentally based on different systems.
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