ISO 6504 Paints and Varnishes - Determination of Opacity

The “ISO 6504 Paints and varnishes – Determination of opacity” standard, published by the International Organization for Standardization (ISO), consists of the following sub-sections published under the same title:

• The standard “ISO 6504-1 Part 1: Kubelka-Munk method for white and light-colored dyes” describes a method for determining the opacity (spread rate required to achieve 98% opacity) of white or light-colored dyes. This standard is applicable to dye films with a tri-stimulus value of at least 70 and opacity exceeding 80%. It is not applicable to fluorescent or metallic dyes.

For the purposes of this standard, light-colored paint refers to a coating with tristimulus values ​​greater than 25, as measured by a spectrophotometer on a black and white substrate.

In principle, this method is based on the Kubelka-Munk equations, which relate the scattering and absorption coefficients of pigmented films to their color and opacity. To determine the hiding power, both the reflectance and reflectivity coefficients of a paint film of a specific thickness on a black background must be included in the Kubelka-Munk equations.

In the world of coatings and surface finishes, the performance of a paint or varnish isn't just about color or durability; it's also about how well it conceals the underlying surface. This property, known as hiding power or opacity, is crucial for providing even coverage, aesthetic appearance, and functional protection. Providing a precise method for measuring this essential property in white and light-colored paints and varnishes, the ISO 6504-1 standard is based on decades of research in color science and optics to help manufacturers, quality control laboratories, and researchers reliably measure hiding power.

While the ISO 6504-1 standard focuses on the Kubelka-Munk model for laboratory-based analyses, other sections of the standard address practical applications for specific surfaces such as masonry or concrete.

Hiding power refers to a coating's ability to cover the underlying surface, preventing the substrate from being visible. This is especially important for white and light-colored paints, as these shades are often used in primers, topcoats, and decorative finishes where uniformity is crucial. Low hiding power can result in visible streaks, color variations, or glare effects.

Factors that affect concealment power include:

• Pigment concentration and type: Higher pigment volume can increase opacity.
• Particle size and dispersion: Finer particles disperse light more effectively.
• Film thickness: Thicker layers generally provide better concealment, but standards regulate this.
• Differences in refractive index: Between pigments and binders.

At the heart of the ISO 6504-1 standard lies the Kubelka-Munk theory, a mathematical model that explains the interaction of light with obscure media such as paint films, taking into account absorption and scattering. It is particularly suitable for white and light colors because they reflect most visible light, enabling precise measurements.

• The standard “ISO 6504-3 Part 3: Determination of the covering power of paints for wall, concrete and interior use” describes methods for determining the covering power of white or light-colored paint coats with tristimulus values ​​greater than 25, applied to a black and white panel or a colorless transparent foil. Tristimulus values ​​are measured on the black and white panels. Then, the covering power is calculated from these tristimulus values.

This standard also includes a simple method for calculating the diffusion rate for volatile paints with low evaporation rates (e.g., interior wall and ceiling coatings as specified in EN 13300).

For the purposes of this standard, the following definitions apply:

• Coverage rate: The surface area that a given amount of coating material can cover to obtain a dried film of the desired thickness.
• Practical spread rate: This is the spread rate that is practically achieved on a given coated surface.
• Theoretical diffusion rate: This is the diffusion rate calculated solely from the volume of non-volatile matter.
• Concealment power: The ability of a coating to mask discoloration or color variations on a surface.
• The three stimulus values ​​(of a color stimulus): In a given tricolor system, these are the quantities of the three reference stimuli required to match the color of the stimulus under consideration.

This standard is based on the following principles:

• For Method A, coated transparent foils are fixed onto a black and white panel. The spread rate is calculated from measurements of surface bulk density and theoretical dry film thickness. The tristimulus values ​​of the coated foils are measured on black and white areas. The hiding power is calculated as a percentage of these values ​​for each coated foil.
• For Method B, black and white graphics are coated and dried or baked. The dispersion rate is calculated from surface bulk density and theoretical dry film thickness measurements. The tristimulus values ​​of each coated graphic are measured over the black and white areas. The hiding power is calculated as a percentage of these values ​​for each coated graphic.
• For Method C, black and white graphs are coated. The spreading ratio is calculated from measurements of the coated area (all graphs), the density of the coating material, and the mass difference between the uncoated and coated graphs. The coating is then dried. The tristimulus values ​​of each coated graph are measured on the black and white areas. The hiding power is calculated as a percentage of these values ​​for each coated graph.

In the coatings industry, ensuring that paints effectively conceal surfaces such as walls or concrete is critical for both aesthetic appearance and functional performance. The ISO 6504-3 standard provides a standardized method for measuring the hiding power of paints and varnishes specifically designed for concrete and similar porous surfaces. Unlike the ISO 6504-1 standard, which uses Kubelka-Munk theory for spectral analysis, this standard focuses on practical, real-world opacity assessment at a given spreading ratio.

The ISO 6504-3 standard applies to water-based and solvent-based paints and varnishes designed for walls, concrete, and similar cement-based surfaces. It specifies a method for determining covering power at a constant spread rate, reflecting typical application conditions. The standard is designed for:

• White and light-colored coatings (tristimulus value at least 25).
• Laboratory or field testing for quality control and product certification.
• Compliance with specifications in the construction and architecture sectors.

For many years, our organization, with its competent and experienced staff, has been understanding the needs of its clients, providing the advanced testing services they require, and assisting in the establishment, implementation, and improvement of management systems. Within this framework, we also provide testing services to businesses in accordance with the "ISO 6504 Paints and varnishes - Determination of opacity" standard.