Hiding power measurement: Equivalence of mass per unit area method and reflectance contrast method.

Introduction

Masking power is a key indicator for evaluating the optical properties of coating materials such as coatings, inks, and pigments, which reflects the ability of the material to mask the color of the substrate. In industrial quality control and R&D, accurate determination of occlusion force is critical. At present, there are two main types of measurement methods: mass per unit area based on physical coating, and reflectance comparison based on optical measurement. For a long time, the two methods have been used in parallel in different standard systems and application scenarios, and exploring the equivalence between the two is of great technical significance for unifying the evaluation system, improving the detection efficiency and comparability of results. This paper aims to systematically analyze the principles, operation and influencing factors of the two methods, and discuss their equivalence.

Method principle

The core principle of the unit area mass method is to determine the mass of the wet or dry film consumed by evenly coating the specimen on a standard black and white contrast substrate until the black and white area is completely covered, and calculate the minimum mass required to cover a unit area. This method is directly related to the actual amount of material used, and the physical significance is clear.

The law of reflectance comparison is based on optical measurements. The specimen is coated on the black and white contrast substrate to form a uniform film layer of specified thickness, and the reflectivity of the coating on the black and white backgrounds is measured separately using a reflectance analyzer. The contrast ratio is calculated by the Kubelka-Munch formula or its simplified form, and the ratio value is considered to be fully occluded when the ratio value reaches the specified requirement (usually 0.98). This method quantifies the optical effect of occlusion.

Testing process

The test flow for both methods includes several key steps: sample preparation, coating, conditioning, and measurement, but the specific operation and determination basis are different.

The key to the operation of the mass per area method is to use a precision applicator (such as a wire rod applicator) to ensure uniform film thickness and to accurately weigh the mass after drying. The final result is expressed as the mass required to cover a unit area, often in grams per square meter.

The reflectance comparison law requires measurements to be made using a reflectometer under standard light sources and geometric conditions. The key calculation involves the contrast ratio, which is simplified as:
CR = R_B / R_W
where CR is the contrast ratio, R_Bis the reflectance on the black background, R_Wis the reflectivity on the white background. When CR ≥ 0.98, it is considered fully masked.

Equivalence analysis

The equivalence of the two methods is not a simple numerical conversion, but the consistency of the evaluation results of the material's masking performance under specific conditions. The analysis showed that its equivalence was influenced by various factors.

First of all, the optical properties of the material are fundamental. For materials with the same coloring power and light scattering properties, the corresponding coating quality per unit area is usually within an expected range when achieving the same optical masking effect (i.e., a contrast ratio of 0.98). However, for materials with different particle size distributions, refractive index, or oil absorption, the correlation between the results of the two methods may be biased.

Secondly, the influence of coating and film formation process is significant. The mass method per unit area is extremely sensitive to the uniformity of the coating, while the reflectance contrast method has high requirements for the surface flatness and thickness uniformity of the film layer, and any factor affecting the optical uniformity of the film layer will affect the reflectance reading.

Finally, standardized test conditions are the basis for achieving comparability of results. Including the black and white reflection value of the substrate, coating tools, drying conditions, calibration and geometric conditions of measuring instruments, etc., the provisions of the corresponding standards must be strictly followed.

Data comparison and discussion

To visualize the relationship between the two methods, the following summarizes the key influencing factors and considerations.

Taken together, the two methods have a good positive correlation for most conventional coating materials under standardized laboratory conditions. That is, the smaller the mass per unit area (the higher the masking efficiency), the thinner the film thickness required to achieve the specified contrast ratio, and the higher the contrast ratio value measured by the reflectance contrast method. However, there are difficulties in directly converting the results of one method to the exact values of the other, as the conversion coefficient relies on the specific optical properties of the material.

Conclusion

The mass per unit area method and the reflectance comparison method are two effective means to evaluate the occlusion force, each with its own emphasis. The former focuses on the economy of practical applications, while the latter focuses on the accurate evaluation of optical properties. Under the premise of strictly following their respective standard test procedures, the two methods are generally consistent in ranking the advantages and disadvantages of masking properties for the same class of materials, indicating their functional equivalence in quality control. It is recommended that laboratories choose the appropriate method based on specific product standards, R&D goals, or customer requirements, and indicate the methods and conditions used in the report. Promoting the accumulation of test data and correlation research between the two methods will help establish a more reliable correlation model on a wider scale.

References

GB/T 1726-1979 Determination of coating covering force

GB/T 23981.1-2019 Colored paints and varnishes - Determination of covering force - Part 1: Determination of the ratio of white and light paints

ASTM D2805-11(2018) Standard Test Method for Hiding Power of Paints by Reflectometry

ISO 6504-1:2019 Paints and varnishes — Determination of hiding power — Part 1: Kubelka-Munk method for white and light-coloured paints