Reflectometer for measuring the hiding power of colored paint coatings

Overview

The covering power of colored paint is one of the key performance indicators to evaluate its quality, which reflects the ability of the coating to mask the color difference of the substrate. In industrial production and quality control, accurate and objective measurement of coverage force is essential to ensure product consistency and final coating results. Traditional visual comparison methods are greatly affected by ambient light and observer subjective factors, while reflectometers can provide quantifiable and reproducible data. This method is based on the principle of optics, which scientifically evaluates the occlusion by measuring the reflectivity of the coating on the black and white contrast substrate and calculating its contrast ratio.

Principle of determination

The core principle of this method is to evenly apply the color paint to be measured on the standard substrate with high reflectivity (whiteboard) and low reflectivity (blackboard) to form a wet film of consistent thickness, and after drying, the reflectance value of the coating on the whiteboard and blackboard is measured separately using a reflectometer. Occlusion force is usually expressed as a contrast ratio, which is calculated as follows:

CR = (R_Black / R_White) × 100%

where CR stands for Contrast Ratio, R_BlackRepresents the reflectivity of the coating on the black substrate, R_WhiteRepresents the reflectivity of the coating on the white substrate. The closer the contrast ratio is to 100%, the stronger the coverage power of the coating. Similar methods are stipulated in several domestic and international standards, such as the national standard GB/T 23981.1 and the international standard ISO 6504-1, which provide a detailed operational framework for ensuring consistency and comparability of tests.

Instrument and material preparation

The following main instruments and materials are required for testing: a reflectometer (which usually contains a standard light source and receiver geometry), black and white contrast cardboard or glass plate (whose reflectivity must meet the standard specifications), a wet film preparer (such as a wire rod applicator or scraper for the preparation of a wet film of a specified thickness), a constant temperature and humidity curing box, and a standard paint sample. All instruments need to be calibrated regularly to ensure measurement accuracy.

Test steps

The testing process is carried out under standard ambient conditions, with a generally recommended temperature of 23±2°C and a relative humidity of 50±5%. First, a wet film preparer is used to coat the well-agitated paint sample on the black and white contrast substrate at a constant speed and pressure to form a flat, uniformly thick wet film. Record the wet film thickness. The coated sample is then dried horizontally under specified conditions until fully cured. After the sample was prepared, the reflectivity values of the coating covering the center of the white and black substrates were measured separately using a reflectance meter. Each area should be averaged multiple times to reduce accidental errors.

Interpretation of the results

The average of the measured reflectance is substituted into the above ratio formula for calculation. The result is usually expressed as a percentage. To clearly demonstrate the performance differences between different formulations or batches, the key test conditions and results can be summarized as follows:

The ratio results can be used directly to determine whether the masking force meets the specific application requirements. Generally speaking, when the ratio is not lower than the specified value (e.g. 95% or 98%), the coating is considered to have sufficient coverage capacity under normal coating thickness. This quantitative data aids in recipe optimization, production process control, and product quality acceptance.

Influencing factors

The accuracy of test results is influenced by various factors. Coating thickness is a critical factor, and insufficient thickness can lead to low contrast ratios that do not truly reflect the product's potential. The black and white reflectance of the substrate must meet the standard range, otherwise it will affect the calculation baseline. The state of the reflectometer, including the stability of the light source, the calibration, and the selection of the measurement aperture, must be strictly controlled. In addition, the dryness of the sample, the flatness of the surface, and the alignment accuracy of the measurement are also important for obtaining reliable data. It is recommended to document all these condition parameters in detail in the test report.

Application:

It is widely used in product development and quality control in the fields of architectural coatings, industrial coatings, automotive coatings, and pigment manufacturing. It provides an efficient and objective means of testing, helping to reduce quality disputes between supply and demand. With the improvement of instrument automation and intelligence, this method may be more closely integrated with online monitoring systems in the future to achieve real-time feedback and adjustment of coverage force in the production process, further improving production efficiency and product stability.

References

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

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