Coverage is a key performance for evaluating the decorative and protective functions of paints, inks and other coating materials. It is directly related to the uniformity of the coating appearance, the fullness of color, and the minimum amount of material required to achieve the expected effect in practical application, and has a clear guiding role in the design optimization of product formulation, production cost control and construction process formulation.
This paper aims to systematically elaborate the core concepts, physical mechanisms, standard measurement methods and main influencing factors of occlusion force.
1. Definition of coverage force
In the field of coatings and inks, masking power refers to the quantitative characterization of the ability of the dry film to cover the base color of the coated surface after the color paint or ink is uniformly coated. Its physical nature is the common result of the scattering and absorption of incident light by pigment particles. When light hits the paint film, the pigment prevents light from penetrating to the bottom layer and reflecting back, thus "masking" the color and state of the substrate.
Depending on the application scenario, the requirements for coverage force also vary. For example, for colored rubber products, the colorant is usually required to have a high covering power to prevent the rubber background color from affecting the color vividness; For transparent products, too high coverage means reduced transparency.
2. Standardized measurement methods
In order to obtain accurate, repeatable and comparable coverage data, domestic and foreign standardization organizations have developed detailed test methods. These methods are divided into two main categories: the minimum coating dosage assay and the optical contrast ratio-based instrumental assay.
1. Black and white grid method
This is a traditional and classic method that involves applying a paint specimen evenly to the surface of a standard black and white grating, and visually observing the coverage force by visually characterizing the minimum amount of paint required to be exactly masked. The results of this method are expressed in grams per square meter (g/m²), with smaller values indicating stronger coverage.
our country standard GB/T 23981.2-2023 "Determination of Covering Force of Colored Paints and Varnishes - Part 2: Black and White Grating Method" regulates this method. The specific operation can be divided into brushing method and spraying method. Testing is typically performed in a standard camera obscura to ensure consistent observation conditions.
2. Reflectance measurement method (contrast ratio method)
In order to overcome the subjectivity of the visual method, the objective instrumental measurement method is more often used for white and light paints. The method is carried out according to GB/T 23981.1-2019. The core step is to make the paint into a uniform paint film of the specified thickness, dry it and place it on the black and white standard substrates, and use a reflectance meter to measure its reflectance (RB) on the black subplate and the reflectance (RW) on the white substrate.
By calculating the contrast ratio (RB/RW), the occlusion force can be quantified. When the contrast ratio reaches 0.98, it is generally considered to have achieved complete occlusion. The closer the contrast ratio is to 1, the stronger the covering power of the paint film and the better the shielding effect on the base color.
Comparison of core features of test methods
Black and white grid method Characterized by "minimum paint usage", the results are intuitive and directly related to cost.
Reflectometry Characterization by "optical contrast ratio", objective and accurate, suitable for quality control of white and light paints.
3. Key factors affecting the coverage power
The strength of the covering force is not determined by a single factor, but is the result of the combined effect of the pigment itself, the formulation system and the construction conditions.
1. Key characteristics of pigments
The following inherent properties of pigments are the internal factors that determine the strength of their covering power -
Refractive index - the greater the difference between the refractive index of the pigment and the refractive index of the film-forming resin, the stronger the scattering ability of light and the higher the hiding power. This is the main reason why titanium dioxide (rutile-type with high refractive index) is the best white masking pigment.
Light absorption - the stronger the absorption of light, the stronger the coverage. Carbon black is a typical representative.
Particle size and dispersion – The particle size distribution of the pigment (especially D50) and the degree of dispersion in the system are critical. The scattering efficiency of visible light is highest at particle sizes of 0.2-0.4 μm. The better the dispersion, the more effective the pigment will be.
Crystal morphology: Generally speaking, the pigment covering power of regular crystal shape is better than that of amorphous form.
2. Formulation and construction factors
At the application level, the following factors also significantly affect the final coverage effect -
Pigment Volume Concentration (PVC) – In coating formulations, PVC has an optimal range (usually 20%-30%), where the coverage is optimal. When PVC exceeds a critical value, a large number of microscopic air bubbles are introduced into the paint film, creating a "dry masking" effect but may sacrifice other properties.
Paint Film Thickness – Without a doubt, increasing the thickness of the paint film is the most direct way to improve the coverage effect.
Uniformity of application – Uneven coating can lead to local film thickness being too thin, forming a covering defect.
4. Application of masking force in different fields
While the core principles are similar, different industries have their own emphasis on coverage testing and focus.
Coatings industry – focus on quantitative control of coverage force as it directly correlates with the cost of materials required to cover a unit area of coverage. The test method is mainly based on the national standard GB/T series, and at the same time, it pursues to achieve a balance between cost and performance through formula optimization (such as titanium dioxide compounding, PVC control).
The ink industry, especially printing inks, often focuses on the concept of "opacity" in addition to hiding power. There are special test method standards in the world, such as GOST 25117-1982 "Method for Determination of Coverage Force of Printing Ink". The test needs to consider specific conditions such as printing method and substrate absorbency.
Pigment manufacturing – As an upstream raw material supplier, the core of its R&D is to improve the coverage of core pigments such as titanium dioxide. The research focuses on maximizing the light scattering efficiency by controlling particle size distribution, crystal morphology, and surface treatment.
In summary, occlusion force is a comprehensive technical parameter that connects material science, optical principles and practical processes. From microscopic pigment refractive index to macroscopic paint film contrast ratio testing, in-depth understanding and precise control run through the entire industry chain from pigment research and development, coating formulation design to final construction application. Mastering its influencing factors and standard evaluation methods is an indispensable foundation for product development, quality control and process optimization for technicians working in related fields.