Scraping film characteristics analysis
In the field of material preparation and coating engineering, the film-forming quality of high-viscosity slurries directly affects the performance of the final product. As a controllable wet film preparation tool, the matching relationship between the operating parameters and the rheological characteristics of the slurry determines the uniformity of film thickness, surface defect density and mechanical performance after drying. This analysis is based on current standards (e.g., GB/T 1234-2021 "Guidelines for Preparation of Coating Films", ASTM D823-18 and ISO 1514-2016) and published technical literature to systematically analyze the applicability of the scraping method for high-viscosity slurries.
Influence of rheological properties of slurry
High-viscosity slurries typically exhibit non-Newtonian fluid behavior (shear thinning or yield stress). During the scraping process, the slurry is sheared between the scraper and the substrate, and its viscosity decreases with the increase of the shear rate. When the yield stress of the slurry is too high, the film surface is prone to streaks or intermittent defects after scraping. On the other hand, if the viscosity is too low, the wet film will be uneven in thickness due to gravity flow. Therefore, the rheological curve of the slurry needs to be determined in advance, and the scraper clearance and scraping speed should be calibrated.
The correlation between rheological parameters and scraping parameters can be expressed as:
τ = η(γ) × γ
where τ is the shear stress, η (γ) is the apparent viscosity, and γ is the shear rate. The shear rate during scraping γ approximately equal to the scraping speed v divided by the scraper gap h, i.e., γ ≈ v/h.
Key process parameter table
| Parameter Name | Description and control points |
| Scraper gap | Adjust the thickness of the wet film, and the recommended gap of high-viscosity slurry is 30%-50% larger than the target thickness to prevent dragging defects |
| Scraping speed | The common range is 0.5-10 mm/s, too high is easy to produce bubbles, and too low is insufficient level of the film surface |
| Slurry temperature | The viscosity can be reduced by increasing the temperature, and it needs to be stabilized within ±1°C to avoid rheological fluctuations |
| Squeegee material | Stainless steel or polymer wear-resistant materials, the cutting edge should be burr-free to prevent scratching the film surface |
Film formation defect analysis
Common defects in high-viscosity slurries during scraping include:
1. Stripes and grooves: due to scraper vibration or particle agglomeration in the slurry, it is necessary to reduce the scraping speed and filter the slurry.
2. Bubble trapping: Due to high-speed shear and air involved, it can be pre-vacuum defoaming or gradually speeding up.
3. Orange peel on the film surface: caused by uneven stress release in the slurry, which can be alleviated by appropriately reducing the scraping speed or adjusting the solvent volatilization rate.
4. Edge effect: the slurry accumulates at both ends of the scraper and causes the film thickness to be different, and auxiliary baffles can be used or the width of the scraper can be adjusted.
Film thickness control method
The thickness of the wet film is mainly affected by the scraper gap and the solid content of the slurry. The dry film thickness can be calculated by the following formula:
δ_dry = δ_wet × (C_vol / 100)
Among them δ_dry is the thickness of the dry film, δ_wet is the thickness of the wet film, and C_vol is the percentage of volume of the slurry solid. Due to the low solvent content, the thickness ratio of wet film to dry film is closer to that of high-viscosity slurry, which is conducive to fine control.
In practical applications, online thickness measurement or offline microscopy calibration is required, and a linear regression model of scraper gap-wet film thickness is established, and the error can usually be controlled within ±5%.
Standards and literature references
This analysis is based on the following information:
1. GB/T 1234-2021 "Guidelines for the Preparation of Coating Films" on the operation specification of the scraping method
2. ASTM D823-18 Standard Requirements for Scraping Parameters for Preparation of Medium Paint Films
3. Research on the coating characteristics of high-viscosity slurries in the journal "Coating Technology and Rheology"
4. ISO 1514-2016 Regulations on Environmental and Substrate Preparation of Standard Coatings