Introduction
Pigment coverage is one of the key indicators to evaluate its optical properties, which refers to the ability of pigments to cover the color difference of the substrate. In the coatings, inks and coatings industry, accurate determination of coverage force is important for product formulation design and quality control. Traditional measurement methods mainly rely on the black-and-white grid method, while modern techniques widely use reflectometry for quantitative analysis. This article will systematically discuss the principles, operation processes, data interpretation, and applicable scenarios of these two methods, providing reference for technical personnel in related fields.
Black and white grid method
The black-and-white grid method is a classic method of comparative determination based on visual assessment. The pigment to be measured is evenly applied to the surface of the standard black and white grid, and the covering power is determined by observing the degree to which the black and white grid is covered. When the black and white boundaries on the black and white grid completely disappear, the pigment layer is considered to have reached complete coverage. This method usually expresses the masking force in terms of the minimum mass of pigment required per unit area (grams per square meter), with lower values indicating stronger coverage.
During operation, use a scraper or applicator to evenly coat the pigment paste on the black and white grid and observe it under standard lighting conditions (such as D65 light source). The coating thickness needs to be tightly controlled, usually by wet film preparer. Record the amount of coating when the black and white grid boundary is completely covered, and calculate the occlusion force using the following formula:
Occlusion = (m × 10⁴) / (A × ρ)
where m is the pigment mass (grams), A is the coating area (square centimeters), and ρ is the solid content of the pigment slurry (percentage).
Reflectometer method
The reflectometer method quantifies the occlusion force by measuring the reflectance ratio of the coating on the black and white substrate, and the results are more objective and accurate. Instruments typically use diffuse reflectance geometry to measure the reflectance of coatings applied to blackboards and whiteboards at specific wavelengths (e.g., 550 nanometers). Occlusion force can be expressed as a pair ratio or occlusion force index.
The formula for calculating the contrast ratio is:
Contrast ratio = (RBlack / RWhite) × 100%
Among them, RBlackis the reflectivity of the coating on the black background, RWhiteis the reflectivity of the coating on the white background. The closer the contrast ratio is to 100%, the stronger the coverage. International standards such as ISO 6504-3 and ASTM D2805 specify specific test conditions.
During operation, a uniform thickness coating is prepared on a standard black and white card, and after drying, it is measured separately using a reflectometer. The instrument needs to be pre-calibrated to ensure the accuracy of the measurement. This method is suitable for rapid laboratory screening and production line quality control.
Method comparison
The black and white grid method relies on subjective visual judgment and is easily affected by lighting conditions and observer experience, but the equipment is simple and low-cost, and it is suitable for rapid on-site evaluation. The reflectometer method provides digital results with good repeatability for accurate research and standardized testing. The data obtained by the two methods should be comprehensively interpreted in combination with factors such as coating thickness and pigment dispersion status.
The following is a comparison of the characteristics of the two methods:
| Black and white grid method | Rely on visual assessment, and the results are subjective |
| Reflectometer method | Based on optical measurements, the results are quantitative |
| Black and white grid method | The equipment is simple and suitable for rapid testing in the field |
| Reflectometer method | Requires special instruments for precise laboratory analysis |
| Black and white grid method | Susceptible to light and human factors |
| Reflectometer method | High repeatability and compliance with standard specifications |
In practical applications, the appropriate method can be selected according to the test purpose, accuracy requirements and resource conditions. For high-precision quality control, the reflectometer method is recommended; For preliminary formula screening or on-site debugging, the black and white grid method still has its application value.
Influencing factors
Pigment coverage is affected by a variety of factors, including pigment particle size, refractive index, degree of dispersion, coating thickness, and substrate properties. During testing, it is necessary to control ambient temperature, humidity and drying conditions to ensure that the results are comparable. Relevant domestic and foreign standards such as GB/T 23981, ISO 6504-3, ASTM D2805, etc. have made detailed provisions on the test procedures, instrument parameters and report formats, and it is recommended to follow the corresponding standard version when testing.
The coating thickness had a significant effect on the measurement of the masking force. Coatings that are too thin may result in incomplete coverage, while those that are too thick may waste material and do not meet practical application scenarios. Standard methods typically specify a specific range of wet or dry film thicknesses, which are controlled by an applicator or thickness gauge during testing.
Conclusion
The black and white grating method and the reflectometer method are the two main means of determining the masking power of pigments, each with its own applicable scenarios and advantages. The black and white grid method is known for its simplicity and intuitiveness, while the reflectometer method is known for its accurate quantification. In actual work, they can be selected according to test accuracy, efficiency requirements and resource conditions, or used in combination to verify each other. With the development of detection technology, the application of reflectometer method is becoming more and more popular, but the traditional black and white grating method still has practical value in specific occasions. It is recommended that testers have a deep understanding of the method principles and strictly follow standard practices to ensure the reliability and comparability of the measurement results.
References
GB/T 23981-2023, Colored paints and varnishes - Determination of masking force
ISO 6504-3, Paints and varnishes — Determination of hiding power
ASTM D2805, Standard Test Method for Hiding Power of Paints
Coating Process Editorial Board. Coating process. Chemical Industry Press