Introduction
In coating performance evaluation, contrast ratio is one of the key indicators to measure its coverage ability. It reflects the ability of the paint to cover the color difference of the substrate after drying, which directly affects the construction effect and material dosage. As a classic method for determining contrast ratios, the black and white grid method is widely used in the industry because of its easy operation and intuitive results. This article will elaborate on the technical details of the black and white lattice method of masking force jamming, covering the principle, equipment, steps, calculations and influencing factors, in order to provide reference for related testing work.
Method principle
The black and white grid method is based on the principle of optical reflection. The coating is evenly applied to a special cardboard with black and white grids printed on the surface, and after drying, the reflectivity of the coating on the black and white grids is measured separately. If the coating does not have enough coverage, the color of the substrate will affect the reading through the coating when measuring the reflectivity between the black and white grids. By calculating the ratio of reflectance between the two, the masking effectiveness of the coating can be quantified. The closer the ratio is to 1, the stronger the coating coverage and the higher the contrast ratio.
Equipment & Materials
The following main equipment and materials need to be prepared for the experiment: the black and white lattice coverage jam that meets the requirements of the standard should have clear regulations on the reflectivity of the black and white area; The accuracy of the instrument must meet the relevant standards. Standard film makers, such as wire rod applicators or scrapers, are used to prepare wet films of uniform thickness; constant temperature and humidity environment box for sample drying and state adjustment; analytical balances for coating weighing; and the paint sample to be tested with the appropriate diluent.
Procedure:
First, stir the paint sample well and dilute it if necessary according to the product regulations. Use an analytical balance to weigh the appropriate amount of sample. Fix the black and white checkered paper flat on a solid countertop. Using the selected film maker, the paint is evenly applied to the surface of the cardboard, ensuring that the coating completely covers the black and white grid area. Record the wet film thickness or coating amount. Subsequently, the prepared test plates are placed horizontally in a constant temperature and humidity environment and dried according to the time and conditions specified by the standard. After drying, multiple positions were selected on the test plate coating, and the reflectivity values of the black grid area and the white grid area were measured using a reflectance meter. Each area should be measured several times and averaged to reduce error.
The result is calculated
The formula for calculating the Contrast Ratio (CR) is as follows:
CR = RB / RW
Among them, RB Represents the average reflectivity of the coating on the black grid, RW Represents the average reflectivity of the coating on the white grid. The calculation is usually kept to two decimal places. According to the relevant criteria, the contrast ratio values can be divided into different grades to evaluate the masking performance.
Influencing factors
The results of the assay are influenced by a variety of factors. Coating thickness is a key factor, too thin will lead to insufficient coverage, too thick may cause sagging or cracking, which needs to be strictly controlled according to the standard. The uniformity of the coating directly affects the accuracy of reflectance measurements. Ambient temperature and humidity affect the drying process and film formation state of the coating. The calibration status of the instrument and the selection of the measuring aperture are also crucial. In addition, the quality of the black and white checkered paper itself, such as the reflectivity stability and wear resistance of the black and white area, has a direct impact on the reproducibility of the results.
Notes:
During operation, it is necessary to ensure that the coating speed is uniform to avoid streaking. The film maker should be cleaned immediately after application. The dry environment should avoid dust contamination. When measuring reflectance, ensure that the measuring head of the instrument fits closely with the coating surface to prevent ambient light interference. Parallel tests should be performed on the same batch of samples to verify the repeatability of the results. If the paint color is dark, pay attention to whether the instrument measurement range is applicable.
Summary
The black-and-white lattice method is an effective method for determining the ratio of paints. Through standardized operation processes and strict condition control, reliable and comparable test data can be obtained, providing an important basis for the research and development, quality control and construction application of coating products. Understanding and mastering its technical details can help improve the accuracy and efficiency of inspection work.
References
The technical details of this paper refer to the chapters on reflectance measurement and sample preparation in the general test method standards for the determination of coating coverage force at home and abroad, instrument operation manuals and related technical literature.