Definition and background
Black and white masking paper, as the name suggests, is a standard test base used to assess the ability to cover coatings, inks, or similar materials. It usually consists of a black-and-white area, with one side pure black and the other pure white, or presented as black-and-white stripes. This paper isn't particularly advanced in the lab, but without it, many coating tests can't be done accurately. Its core value lies in providing a stark contrasting background, making covering, which seems like a subjective metric, quantifiable.
Principle
The basic principle of opposing power is simple: when light reflects off the coating being tested, if the coating is thick enough or the pigment concentration is high enough, it can mask the difference in the underlying black and white background. In theory, we estimate the shielding power by measuring the difference in reflectivity of the coating across the black and white areas. The smaller this difference, the stronger the coverage. In specific calculations, the contrast ratio (CR) is usually used, with the formula being:
CR = (Rblack / Rwhite) × 100%
where RblackAnd RwhiteThese are the reflectivities of the coating on a black and white background. When CR approaches 100%, the coating almost completely covers the background—in other words, the color underneath is no longer visible.
Measurement method
In actual measurement, the process is quite straightforward. First, you apply the material to be tested to a standard thickness (usually controlled by wet film thickness) onto black-and-white masking paper, then use a colorimeter or spectrophotometer to read the reflectance on the black and white areas. When reading, pay attention to the misalignment of the dots, or the data will drift—this is a common beginner mistake. The substrate surface must be clean and the coating must be even; otherwise, the results cannot be trusted. International standards such as ASTM D2805 or ISO 2814 provide detailed steps. Although some provisions are somewhat tongue-twisting, just follow them.
Here's a small detail: if the coating transparency is too high, the measured contrast ratio may be very low. In this case, you first need to estimate the dry film thickness and then convert it to the contrast ratio at the standard film thickness—don't mind the hassle; the data is meaningful.
Case studies and analysis
For example: an ink factory develops a new type of white printing ink, and the customer reports that the printed packaging box always shows the color of the backing paper. The technician tested it with black-and-white masking strength paper and found the contrast ratio was only 91%, far below the 95% requirement. After investigation, it was found that the problem lay in the uneven dispersion of titanium dioxide, resulting in low pigment concentrations in some areas. After adjusting the grinding process, the ratio rose to 96%, and the customer passed. This example isn't a twist, but it shows one thing—data doesn't lie, provided you measure the right place.
Applications:
The applications of black-and-white masking paper extend far beyond the coatings and ink industries. It is useful for assessing the coverage of plastic coloring, building material coatings, and even cosmetics (such as foundation). Whenever it involves the scenario of "covering one layer of something on another," this paper becomes a cross-industry universal referee. For example, in automotive topcoat development, engineers use it to verify whether the paint film can cover the color difference of the primer. How to put it, some things may seem inconspicuous, but once you use them, you realize how much they can carry a burden.
Selection guide
When choosing black or white masking paper, don't just look at the price. You need to pay attention to several indicators: whether the reflectance difference between black and white areas is large enough (typical white areas have reflectance above 85%, black areas below 5%), whether surface flatness affects coating, and whether the paper size fits your coater. Some paper coatings on the market turn yellow, and after prolonged use, the data will drift—you need to be cautious about this. It's recommended to buy the same batch each time to avoid batch differences that could "hurt" your experiments. If possible, regularly calibrate the measuring equipment with a standard color swatch—this is much more reliable than obsessing over which paper is "best."