Enhancing Block Resistance in Low-VOC Acrylic Paints Using Silane-Modified Colloidal Silica
Inside this Article
- Colloidal silica improves block resistance in low-VOC acrylic paints by enriching the coating surface and reducing film-to-film adhesion.
- Silane-modified colloidal silica provides better stability and compatibility in latex coatings compared with non-modified silica nanoparticles.
- Drying time and formulation factors including resin hardness, rheology modifiers and coalescent levels strongly influence anti-blocking performance.
- Combining colloidal silica with HEUR thickeners improved block resistance compared with cellulose-based rheology modifiers in the evaluated acrylic systems.
Numerous studies have explored the use of silica nanoparticles in resin-based systems. Water-free systems of silane-modified colloidal silica particles have been shown to enhance the mechanical properties of coatings.¹² Silane-modified fumed silica has also been used to improve coating properties. Organosols have been employed to enhance scratch resistance in clear coatings through surface enrichment. Aqueous colloidal silica has been utilized in the copolymerization of resins and copolymerized colloidal silica resin hybrids are known to improve various coating properties such as hardness, anti-blocking and reduced dirt pickup. Recently, silica particles derived from tetraethylorthosilicate (TEOS) have been investigated for their role in the copolymerization of hybrid coatings, where they were found to enhance hardness and adhesion properties. Non-surface-modified colloidal silica has been tested as a nanofiller in latex coatings, yielding promising results in terms of mechanical properties. However, there have been few studies on the use of silane-modified water-based colloidal silica in waterborne coating formulations.³
In paint production, various materials are employed to enhance anti-blocking properties. Non-colloidal silica, both natural and synthetic, is commonly used to create a micro-rough surface that reduces the contact area between layers, thereby minimizing blocking. Talc is another effective anti-blocking agent, working similarly to silica by increasing surface roughness. Calcium carbonate is often utilized when clarity is not a primary concern, as it effectively reduces blocking but can affect the transparency of the coating. Diatomaceous earth, a natural form of silica, is valued for its high porosity and ability to prevent layers from sticking together. Polyethylene wax is added to modify the surface properties of coatings, reducing friction and preventing adhesion. Fatty acid amides can migrate to the surface, creating a lubricating layer that reduces blocking. However, each of these materials can present challenges: silica and talc may impact the smoothness and gloss of the final coating, calcium carbonate can reduce clarity, diatomaceous earth might introduce porosity issues, polyethylene wax can affect the coating's hardness and fatty acid amides may lead to surface blooming or migration issues over time.
Additionally, environmental concerns have led to the banning of certain materials, such as quartz, due to its link to silicosis in workers and PTFE, a type of PFAS, due to its persistence in the environment and potential health risks since some waxes can contain them.
Recently, there has been significant interest in colloidal silica particles for waterborne paints since their small particle size with high surface area offers benefits such as anti-blocking, sanding and reinforcement in acrylic emulsion-based wood coatings without compromising gloss or clarity. From an environmental perspective, using colloidal silica in latex-based coating formulations is advantageous. The incorporation of colloidal silica enables the use of softer resins with better film-forming properties and hence reduces the need for coalescing agent, thereby lowering the overall number of VOCs. Additionally, softer resins can be utilized, as the addition of colloidal silica enhances their mechanical properties to match those of harder resins. This improvement eliminates the need for potentially hazardous film-forming agents such as NMP or glycol ethers.
Concentrated silane-modified colloidal silica, available as aqueous dispersions, is one of the most accessible sources of nanoparticles for coatings. These colloidal silica dispersions are characterized by high solids content, up to at least 50 wt. % silica depending on particle size, which ranges from about 5 nm to 100 nm. Compared to conventional non-surface-modified colloidal silica, silane-modified colloidal silica offers greater stability against aggregation and gelling both in their original form and in latex-based coating formulations.
Although the main motivation of this present study is to gain insight into the anti-blocking behavior of commercial nanoparticle silica systems, it was also found important that drying time has a significant role since other chemistries can provide such properties. The blocking properties of coatings are influenced by several key factors. The type of resin used impacts blocking properties, with softer resins being more prone to blocking than harder resins. Additionally, the amount of pressure applied during stacking or storage and the duration of contact between coated surfaces can increase the likelihood of blocking. Relative humidity and temperature are additional factors to be considered. Understanding and controlling these factors can help in formulating coatings with improved anti-blocking properties.
Results and Discussion
The commercial silica colloidal dispersions (Levasil® colloidal silica) were water-based anionic products supplied by Nouryon, Sweden. In this study, Levasil® CC301, particle size of 7 nm and 28 wt. % solids, an epoxy-silane-modified colloidal silica, was selected for these experimental studies due to extensive paint stability. Non-modified colloidal silicas are mostly used in industrial applications where the demands on formulation stability and resin compatibility is limited. A non-surface-modified colloidal silica can give very good dirt-pickup resistance results, but the commercial paints evaluated with this product gelled after 2 weeks.
To prove the concept, three commercial semigloss paints were evaluated for blocking properties. The block performance indicating face-to-face adhesion of two paint films pressed together was rated on the scale of 0 to 10 as defined by ASTM D4946-89 (Table 1). The test paints were prepared on the Leneta 3B opacity charts using a 3-mil bird drawdown bar. The films were dried in a constant temperature and humidity (CTCH) environmental chamber, 23 °C and 50 % RH. For the elevated temperature (ET) block test, the paint strips after a certain time of drying at CTCH were placed in a 50 °C oven under 1000 g weight for 30 min. The weight was transferred to the paint films via a 1-inch diameter rubber stopper, generating approximately 2.2 psi on the film strips. The films were then allowed to cool for 30 minutes before the ratings of film separation were given. The test was run in triplicate and the average value was reported.
Table 1. ASTM block rating.
Tests were conducted after 3 hr, 24 hr and 72 hr drying time with 0 (control), 5 and 10 wt. % product loading level of colloidal silica and as comparison 3 wt. % of 35 wt. % solid paraffin wax emulsion. Scanning electron microscopy mapping (SEM-EDX) has been used to visualize the surface distribution of the inorganic component embedded in the organic matrix. Figures 1, 2 and 3 display the distribution of the inorganic component embedded in semigloss paint with varying dosage levels (1 %, 5 % and 10 %) of colloidal silica. From the images, we can confirm that the coating surfaces are increasingly enriched with silica at each higher loading level, which corroborates the results of anti-blocking tests.
Figures 4, 5 and 6 show the anti-blocking rate of coatings after 3, 24 and 72 hours, respectively. The efficiency of colloidal silica in extending the blocking resistance of water-based systems is dependent on drying time of the system and the concentration used. It is important to note that the open time (coalescence process) can be influenced by several factors including the Tg of the polymer and the content of the coalescing agent (MFFT), the nature and type of stabilization of the polymeric particles (anionic, non-ionic), polymeric particle size and particle size distribution and additives such as wax, dispersing agents, anti-foaming agents/defoamers and thickeners (both their nature and the amount added), as well as PVC. All these factors impact the coalescence process to varying degrees, thereby affecting the open time to a greater or lesser extent.
Figure 1. Overlay SEM image of the surface and Si element map of 1 wt.% loading.
Figure 2. Overlay SEM image of the surface and Si element map of 5 wt.% loading.
Figure 3. Overlay SEM image of the surface and Si element map of 10 wt.% loading.
Figure 4. Anti-blocking rate after 3 hrs. dry time.
Figure 5. Anti-blocking rate after 24 hrs. dry time.
Figure 6. Anti-blocking rate after 72 hrs. dry time.
The use of colloidal silica significantly extends the open time of coatings. The presence of colloidal silica does not appear to affect surface tension, as the dynamic surface tensions for all paints were consistently around 28 mN/m throughout the drying process. Therefore, the extension of the open time cannot be attributed to changes in surface tension during the coating drying process. This further supports the hypothesis that the extended open time is related to a reduction in skin formation caused by the presence of silica at the coating surface.⁴ Extending the open time allows the emulsion particles to fully coalesce and the enrichment of the surface of coating with silica particles improves block resistance.
Addition of colloidal silica will reduce viscosity of the paints, especially those with 10 wt. % samples. To evaluate impact on block resistance at “constant viscosity,” several formulations were put together with two types of thickeners: cellulosic based (EHEC), urethane (HEUR) and hydrophobically modified EHEC with HEUR combination (Table 2). Measured Brookfield viscosity for all paint samples were around 3000–3500 cP and Krebs around 95–100 KU.
Table 2. Semigloss paint formulation examples.
It can be noted from the results of anti-blocking properties testing of formulations that silane-modified colloidal silica in combination with HEUR thickeners appeared to be beneficial for block resistance in contrast to EHEC.
Figure 7. Anti-block resistance rate of formulations.
Figure 7 displays the block resistance of the acrylic latex with MFFT > 5 °C modified with different rheology additives. At the same loading level HEUR provided improved anti-blocking property compared to EHEC chemistry. The slight tackiness of the specimen with EHEC might be explained by the interaction between colloidal silica and cellulose, which could lead to the entrapment of colloidal silica in the film.
Conclusions
The effect of colloidal silica particles strongly depends on the resin and other components such as rheology modifiers of the coating formulation. In this study we focused on enhancements in block resistance but other properties like sanding properties and hardness can be influenced by colloidal silica. Additionally, the silane surface modification of colloidal silica ensures good compatibility with the resins, maintaining or even enhancing the aesthetic properties of the coating (gloss and haze) in some formulations.
References
¹ C. Vu, O. La Ferte, A. Eranian, “High Performance UV Multi-Layer Coatings Using Inorganic Nanoparticles,” Proc. RadTech Europe 2005
² P. Greenwood, B. Gevert, “Aqueous silane modified silica sols: theory and preparation,” Eka Chemicals AB, AkzoNobel, Gothenburg, Sweden; Department of Chemical and Biochemical Engineering, Chalmers University of Technology, Gothenburg, Sweden
³ B. Chisholm, J. Resue, “UV-curable, Hybrid organic-inorganic Coatings,” Proc. 30th International Waterborne High-Solids and Powder Coatings Symposium, New Orleans, USA, 2003
⁴ P. Greenwood, “Impacts of silane modified colloidal silica on waterborne clear coatings,” Akzo Nobel Pulp and Performance Chemicals AB, Sweden
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