Abstract: This paper elucidates the selection rationale for water-based resins and various additives in the formulation of high-solids, high-transparency water-based wood base coats, aiming to develop a product with excellent sandability, transparency, and fullness.
Introduction
With the issuance of national industry standards for "Indoor Water-based Wood Coatings" and Shanghai regulations on "Construction Methods for Water-based Wood Coatings," the development and popularization of water-based wood coatings have been further promoted, and people have gained a certain understanding of the concept of water-based wood coatings. However, the fullness and application tolerance of water-based wood coatings have restricted their rapid development. In particular, the contradiction between sandability, high solids content, and high transparency in water-based wood base coats is an urgent issue to be addressed in water-based technology. Below, we discuss the design concepts for the formulation system of high-solids, high-transparency water-based wood base coats in response to the above technical issues.
1. Formulation Design Principles
Water-based wood coatings are water-dispersed systems with water as the dispersing medium. Since most of the system is water, which has a surface tension of 72 mN/m, about 2-3 times that of general organic solvents, coupled with the specificity of the system, the formulation design process is significantly challenging, especially for the development of high-solids, high-transparency water-based wood base coats, which presents a pair of conflicting properties in water-based technology. To achieve an ideal balance, it is necessary to make a reasonable choice of raw materials. Meanwhile, the stability of the system, low-temperature film-forming properties, and other basic requirements, as well as the ease of production processes, must be considered for testing and screening to truly meet the requirements of formulation design. Sometimes, cost considerations should be set aside to broaden the formulation design approach and boldly seek a formulation framework.
1.1 Selection of Water-based Resins
Water-based resins are a crucial component of water-based wood coatings, typically accounting for 70%-80% of the system, or even higher. Therefore, selecting a good water-based resin is almost half the battle in formulation design. The main characteristic of water-based resins is that the particle size of the dispersed phase is between 10-100 nm, a polymer dispersion. Consequently, their stability and comprehensive performance limit their solids content to not too high, generally between 30%-45%. Of course, for different applications or special systems, the solids content may be higher. Therefore, for the selection of water-based resins for high-solids, high-transparency water-based wood base coats, it is advisable to choose those with higher solids content indicators. Meanwhile, the glass transition temperature (Tg) is also a technical parameter that cannot be ignored. A high Tg results in high hardness and good sandability but poor encapsulation of fillers and easy cracking. Conversely, a low Tg allows for greater formulation flexibility for systems with multiple fillers but significantly affects sandability. Therefore, a Tg of 30-50°C is optimal, and soap-free water-based acrylic resins offer the best cost-performance ratio.
1.2 Selection of Substrate Wetting Agents
Water-based coatings require wetting agents to reduce surface tension and achieve wetting of substrates and pigments/fillers. Thus, wetting agents are essential for the basic performance of water-based coatings. Especially for high-solids (high-filler) base coat systems, without good wetting properties, the stability, substrate wettability, and application leveling of low surface tension systems cannot meet the basic technical parameter requirements of formulation design. The water resistance of wetting agents is also crucial. Good water resistance of the primer layer directly contributes to the surface finish of the topcoat. Generally, wetting agents have poor water resistance, and modified silicone-type wetting agents are preferred.
1.3 Selection of Rheological Additives
Rheological additives are crucial for the application tolerance and long-term storage stability of coatings. Improper use of rheological additives can lead to many drawbacks in the application process, directly affecting the surface finish and stability of the coating, such as film leveling and sagging, anti-settling properties upon dilution, and the coating's inherent anti-settling properties. Especially for high-filler, high-transparency water-based wood base coats, due to the specificity of water-based resin dispersions and the low viscosity of the primer itself, it increases the difficulty of anti-settling properties in water-based wood base coats. Often, two or more rheological additives are required to achieve anti-settling effects. Water-based polyurethane rheological additives are preferred due to their good water resistance and dense film formation. Combining rheological additives that provide medium-to-low shear viscosity with thixotropic rheological additives can truly address the anti-settling properties and application requirements of water-based wood base coats with conflicting high solids content and high transparency. Of course, there are significant differences depending on the system and application range.
1.4 Selection of Fillers
Fillers ensure the thickness of the paint film after a single application and, to a certain extent, improve the sandability of the coating, making them an important material in the formulation system of high-solids, high-transparency water-based wood base coats. Fillers are typically added at 5%-15%, with common examples including talc, calcium carbonate, silica glass powder, and special mineral fillers. When the addition of talc or calcium carbonate reaches 10%, whitening may occur during application on dark substrates due to formulation design. The use of glass powder is challenging to address anti-settling properties in water-based wood base coat dispersions due to its high density. Although it can be mitigated through the combination of rheological additives mentioned above, it does not completely eliminate anti-settling but only slows down the settling speed of the fillers. In general, as the addition of these fillers increases to about 10%, it restricts the application tolerance of water-based wood base coats. Special mineral fillers have a lower density than glass powder but also face challenges in anti-settling. However, due to differences in mineral content, they are much better than glass powder and are a suitable choice for high-solids, high-transparency water-based wood base coat formulations.
1.5 Selection of Sanding Aids
The sandability of water-based wood base coats is difficult to compete with solvent-based coatings due to the structure of their formulations. The main component contributing to sandability is derived from one of the properties of water-based resins. Therefore, sandability is far from meeting the basic requirements of practical applications, necessitating the use of sanding aids for improvement. Commonly used sanding aids are zinc stearate powders or slurries. From a production process perspective, it is recommended to add zinc stearate slurry for better dispersion and ease of operation, typically at 2%-5%. Of course, the addition amount should also consider the system's transparency, adhesion, recoatability, and the substrate being applied to.
1.6 Selection of Defoamers
Defoamers are one of the most difficult additives to control in water-based wood coatings to balance defoaming and leveling. Water-based wood coatings, being water-dispersed systems, have a root cause of bubbling in the numerous types of surfactants present, which readily produce and stabilize bubbles during production and application. Especially in water-based wood base coat systems, the addition of sanding aids typically easily causes bubbling with strong foam stabilization, posing a technical challenge in selecting defoamers to achieve a balance point for defect-free surfaces with good defoaming properties. To achieve ideal defoaming performance, it is necessary to use a combination of two defoamers: one added initially to inhibit foam generation during production and application, and another to eliminate foam already present in the system. Additionally, considering the recoatability, adhesion, and transparency of the water-based wood base coat system, mineral oil and silicone-type defoamers should be avoided, with non-silicone, non-oil, long-lasting defoamers being the preferred choice.
1.7 Selection of Film-forming Additives
Film-forming additives are essential in water-based wood coatings, dictated by the specificity of the water-based resins within. The Tg of water-based resins is a key indicator of the sandability of water-based wood base coats. To prepare primers with better sandability, it is necessary to choose water-based resins with a higher Tg, typically 30-50°C, which increases the difficulty of low-temperature film formation in water-based wood coatings. Therefore, film-forming additives are necessary to ensure the film-forming performance of water-based resins in low-temperature environments. The type of film-forming additive also affects the sanding time of water-based wood base coats. Low-boiling, fast-drying film-forming additives, due to their rapid drying nature, aid in the sandability of water-based wood base coats. However, to effectively ensure the low-temperature film-forming properties of water-based resins, a relatively higher amount is often required. The addition of film-forming additives often affects the stability of water-based wood coatings, caused by different dispersion coefficients in the aqueous phase and within the water-based resin polymer particles. Therefore, a combination of two or more film-forming additives is necessary to ensure a balance between low-temperature film-forming properties and system stability in water-based wood coatings.
1.8 Others
High-Solid, High-Transparency Water-Based Wood Furniture Primer is a water-based coating that is susceptible to contamination and decomposition during production. To ensure long-term preservative stability during transportation and storage, it is necessary to add 0.1% to 0.2% of preservatives. During the addition process, attention should be paid to the operating procedures of the production process and the compatibility with other additives in the formula.
Formula Examples, Production Process, and Performance Comparison Results
2.1 Formula Examples
Table 1 presents an example of a formula design for high-solid, high-transparency water-based wood furniture primer that can be applied through multiple coats of spray or brush on dark woods (such as black walnut), exhibiting excellent transparency, no whitening phenomenon, and superior sanding performance.
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2.2 Production Process
First, add material A. Gradually add materials B to D under medium-low speed stirring. Stir at medium speed for 5 minutes. Then, slowly add materials E and G under medium speed stirring, scraping the edges. Use a small amount of material N to clean the edges of the tank. Stir at medium speed for another 5 minutes. Next, slowly add materials H and I (pre-dissolved with polar film-forming additives) under medium speed stirring. Stir at medium-high speed for 10 to 20 minutes. Use a 100μm fineness gauge to check the fineness, ensuring it is ≤30μm. After passing the fineness test, slowly add materials H to O (materials I, J, and L are pre-diluted with water) under medium-low speed stirring. Stir at medium speed for 10 minutes before conducting various performance tests.
2.3 Physical Performance Comparison
Table 2 presents a comparison of the physical performance of different formulas for high-solid, high-transparency water-based wood furniture primer.
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Note: Performance tests were conducted according to the chemical industry standard "Indoor Water-Based Wood Coatings".
2.4 Results and Analysis
From Tables 1 and 2, it can be seen that the formula design of high-solid, high-transparency water-based wood furniture primer requires the optimal selection of various additives to develop a product with superior overall performance. Comparing Formula 1 and Formula 2 in Table 2, it is evident that different fillers added in the same base material result in significant differences in transparency. Formula 1 serves as the foundation for the formula design of high-solid, high-transparency water-based wood furniture primer. Comparing Formula 1 and Formula 3, it is clear that the addition of sanding additives significantly improves the sanding performance of water-based wood coatings but introduces the defect of stable foam. Comparing Formula 3 and Formula 4, it can be seen that the stable foam caused by the addition of sanding additives can be controlled by increasing the amount of defoaming agents added beforehand, achieving good results. Comparing Formula 4 and Formula 5, it is evident that fast-drying and slow-drying film-forming additives significantly affect the drying time of water-based wood primers. The fast-drying film-forming additive system exhibits better sanding performance, while the slow-drying film-forming additive system exhibits poorer sanding performance. Due to the low efficiency of fast-drying film-forming additives in reducing the film-forming temperature of resins in water-based wood coatings, a relatively larger amount must be added to ensure low-temperature film-forming performance. Formula 6 in Table 2 demonstrates that due to the high filler loading and relatively low viscosity of the prepared water-based wood primer, anti-settling is challenging. This issue is addressed by adding a combination of thixotropic rheological additives and general rheological additives. It is clear from Table 2 that Formula 6 exhibits the best overall performance.
Conclusion
High-solid, high-transparency water-based wood furniture primer is an inevitable product in the development of water-based wood coatings. For a long time, issues such as poor fullness, multiple coats required, long application time, and poor sanding performance of water-based wood coatings have restricted their widespread application and promotion. Therefore, high-solid, high-transparency water-based wood furniture primer is a product urgently needed in the market today. However, due to the unique nature of water-based wood primers, the formula design process must follow the principles of high transparency, high solid content, good sanding performance, and ease of application to select various additives that are compatible. Moreover, it is necessary to select additives that can achieve optimal effects with the least possible amount without compromising water resistance, formulating a product with high application tolerance and excellent cost performance.
Guangzhou Zhenghao New Material Technology Co., Ltd.