In many industrial and scientific research fields such as coatings, inks, adhesives, and functional coatings, the preparation of coating samples is the basic link for subsequent performance testing and analysis. The standardization of the preparation process directly determines the accuracy, reproducibility and comparability of the test results. As a critical tool, the specification use of coating film preparers is essential for obtaining standard coating samples with uniform thickness, flat surfaces, and free defects. This article will systematically describe the standardized operation process, key parameter control and its impact on the detection results of coating film preparation.
Principles and types
The core principle of the coating film preparation machine, usually the scraping type, is to use a scraper or scraper with a fixed gap to move at a uniform speed on the flat substrate to scrape the excess liquid or slurry coating into a uniform thickness of wet film. Wet film thickness (WFT) is mainly determined by the gap height of the preptor, which is dried or cured to form a dry film (DFT). The main types include manual, automatic, and different cell designs for different viscosity ranges. Selection should take into account the viscosity of the sample, the desired film thickness, the properties of the substrate and the budget.
Preparation process
A standardized preparation process is a prerequisite for ensuring sample consistency and can be broken down into the following steps:
1. Preliminary Preparation: Clean the preparer and substrate to ensure that there is no dust, oil and residue. Depending on the standard or experimental design, the substrate (such as glass plate, tinplate, polyester film, etc.) is fixed horizontally on the level table. According to the target dry film thickness and the volume solids (VS) of the coating, the theoretical wet film thickness is calculated by the formula, and the preparation clearance is selected or set accordingly.
2. Paint application and scraping: Pour a sufficient, pre-matured, or well-agitated sample onto one end of the substrate in front of the scraper starting line. For manual operation, the prep should be pulled at a time at a uniform, constant speed (usually in the range of 100-300 mm/sec) without pause. Automatic coaters can precisely control speed and pressure.
3. Post-Processing: After scraping, the sample is immediately placed in a standard environment (e.g., temperature 23±2°C, relative humidity 50±5%) to stand horizontally for flash drying, baking, or curing for the specified time. After curing, film thickness measurement and performance testing can be carried out.
Parameter control
Multiple parameters interact to affect the final coating quality, which needs to be strictly controlled:
Scraping speed: Too fast speed may cause the wet film to be discontinuous or streaking; Too slow may cause edge thickening or sagging due to paint fluidity. It needs to match the rheological characteristics of the coating.
Preparer clearance: It is the primary factor that determines the thickness of the wet film. Regular calibration is required to ensure its accuracy. The clearance setting should consider the rebound and leveling characteristics of the paint after shear.
Coating viscosity and rheology: The viscosity behavior under different shear rates directly affects the uniformity of film formation. Highly thixotropic coatings need to pay attention to the leveling time after scraping.
Environmental Conditions: Temperature and humidity affect the viscosity, volatilization rate and curing reaction of the coating, which must be recorded and controlled within the standard range.
The relationship between dry film thickness (DFT) and wet film thickness (WFT) and volumetric solids (VS) can be approximated as follows:
DFT = (WFT × VS) / 100
This formula can be used for preliminary estimation, but the actual DFT is affected by many factors such as solvent volatilization and curing shrinkage, and the actual measurement shall prevail.
FAQs
| Problem phenomenon | Possible causes and adjustment directions |
| The film thickness is uneven or streaking | uneven scraping speed; impurities or wear in the gap of the preparer; The paint is not sufficiently dispersed or gel particles are present. |
| Edges are thickened or serrated | the scraping speed is too slow; The viscosity of the coating is too high or the thixotropic is strong; Too much paint is applied. |
| The coating film has bubbles or pinholes | Paint agitated or poured into air; The surface energy of the substrate is high and the wettability is poor. The solvent evaporates too quickly. |
| The actual film thickness deviates greatly from the set value | the preparer clearance is not calibrated; poor flatness of the substrate; The leveling of the paint is very different from the theoretical calculation model. |
The significance of standardized preparation
The significance of standardized sample preparation using a coating preparer is to provide a reliable and comparable reference sample for a series of subsequent tests such as adhesion, hardness, abrasion resistance, chemical resistance, gloss, color and electrochemical properties. It reduces the variables introduced by human operation, so that the testing data between different laboratories and batches can be effectively compared and analyzed, which is the basis for supporting product quality control, formula research and development, and compliance with relevant standards (such as ASTM, ISO, GB series standards).
Conclusion
The standardized application of coating film preparers is a systematic technical practice. It requires operators to have a deep understanding of instrument principles, the interaction of material properties and environmental factors. Through strict implementation of standard operating procedures, fine control of various parameters, and supplemented by regular instrument maintenance and calibration, high-quality testing samples can be continuously and stably, so as to ensure the scientific and reliable data of the entire testing system and provide solid support for product development and quality assurance.
References
ASTM D823-18, Standard Practices for Producing Films of Uniform Thickness of Paint, Varnish, and Related Products on Test Panels.
ISO 1514:2016, Paints and varnishes — Standard panels for testing.
GB/T 1727-1992, General preparation method of paint film.
Coating Technology (4th ed.), Chemical Industry Press.