Overview
The dual scraper applicator is a precision instrument widely used in the field of coating preparation, and its core design is to integrate two independent squeegee systems, pre-scraping and fine scraping. The design aims to achieve finer rheological control and coating topography management of the coated slurry through step-by-step operation, so as to effectively deal with various defects generated during the coating process, among which the elimination of bubble defects is one of its main advantages.
Formation of bubble defects
In the coating process, the bubbles mainly arise from the air involved in the slurry preparation, conveying and coating phases. Slurries are typically non-Newtonian fluids, and their viscosity is related to the shear rate. If the entrapped bubbles are not eliminated, they will form cavities in the wet film, which will become pinholes, fisheyes and other defects after drying and curing, which will seriously affect the uniformity, density and performance of the final product (such as optical properties, barrier properties or conductivity). The behavior of bubbles in the slurry can be approximated by Stokes' law:
v = (2g(ρf - ρb)r2) / (9η)
where v is the rising velocity of the bubble, g is the acceleration of gravity, ρfis the density of the slurry, ρbis the bubble density, r is the bubble radius, η is the viscosity of the slurry. The bubbles in high-viscosity slurry rise very slowly, and it is difficult to rely on static natural removal, so it needs to rely on mechanical external force.
Process principle
The two-step process decouples the coating process into two functionally defined stages that work synergistically to eliminate air bubbles.
Pre-scratch stage: The first scraper (usually with a large gap) scrapes at a higher speed. Its main role is to quickly form a thick wet film and apply high shear forces to the slurry. This shear force can destroy the structural viscosity inside the slurry, causing the wrapped bubbles to rupture, merge, and migrate upward. At the same time, the macroscopic fluctuations on the substrate can be preliminarily leveled at this stage.
Fine scraping stage: The second scraper (the gap is precisely set to the target wet film thickness) follows the pre-scraper and scrapes at a lower speed or at a specific angle. Its core function is to precisely control the final wet film thickness and scrape away microbubbles or uneven interfaces that may remain on the surface layer of the slurry after pre-scraping treatment, forming a uniform, smooth, and defect-free wet film.
Process parameter control
The effectiveness of the two-step method relies on the coordinated control of the following key parameters. The parameter setting should be optimized according to the rheological characteristics of the slurry, the thickness of the target coating and the properties of the substrate.
| Process parameters | Role and influence |
| Pre-scraper gap | It affects the initial wet film thickness and shear force, and the gap is too small to be clogged, and if it is too large, the bubbles are not sufficiently removed. |
| Fine scraper gap | The final coating thickness is directly determined and needs to be set and maintained with high precision. |
| Scraping speed | The pre-scraping speed affects the shear rate and bubble removal efficiency. The scraping speed affects the surface quality of the coating. |
| Blade angle and material | It affects the slurry flow field and shear behavior, and the hard wear-resistant material can ensure the stability of the process. |
| Two scraper spacing | It affects the leveling of the slurry and the bubble floating time between two steps, and needs to be optimized. |
| Slurry viscosity and thixotropy | The basic physical property parameters determine the range of the process window. |
Application Benefits:
Compared to traditional single-squeegee coating, the two-step method provides a systematic solution in eliminating air bubbles. Its advantage is that the bubble elimination is separated from the thickness control function, allowing the parameters of the two stages to be optimized separately, resulting in high-quality coatings with a wide range of slurry adaptability. Practice shows that the method has a particularly significant effect on slurries with high solids content and high viscosity.
It should be noted that the process parameters need to be optimized through experimental design. The equipment needs to ensure the parallelism and gap stability of the two scrapers; The slurry supply needs to be continuous and uniform to avoid introducing new disturbances. In addition, while this method is effective in reducing bubble defects, it still needs to be combined with an optimized slurry degassing process, such as vacuum agitation, to achieve the best results.
Conclusion
The two-step method of pre-scraping and fine scraping of the double scraper coater provides an effective technical path for the elimination of bubble defects in the coating process by applying mechanical shear of different intensities and purposes in stages. The mechanism of this method is clear, and the uniformity and integrity of the coating are significantly improved by controlling the high shear bubble breaking in the pre-scraping stage and the precise forming in the fine scraping stage. Its successful application relies on an in-depth understanding of the rheological properties of slurries and precise control of key process parameters such as scraper clearance and speed, making it a reliable process method to improve the quality of functional coating products.