Opacity – How to Get the Desired Hiding in Your Coatings?

When a paint or coating is applied onto the substrate, the first and foremost requirement is “how effectively a coating hides the substrate?”. Coatings that are black in color completely hide most substrates in a single coat. While yellows or reds may take multiple coats to get full coverage.

Opacity determines the ability of a coating film to completely mask the underlying substrate. Given the same thickness of applied coating film, not all coatings have the same opacity.

So, what does opacity mean? It is a simple phenomenon that results from the complex interplay of:

• various components (pigments, extenders, dispersants, mineral fillers, etc.), and
• parameters (pigment dispersion, film thickness, etc.) in a coating formulation

These factors help in determining the opacity of coatings and how to obtain the desired opacity in your formulation.


Get a clear understanding of:

1. How are opacity and hiding power related?
2. What are the techniques to measure the opacity?
3. Which ingredients help you get the right opacity in your coatings?
4. What factors influence the opacity of coatings?

How are opacity and hiding power related?

Opacity is a material’s optical property related to the refractive index (n). The refractive index is a measure of the ability of the material to refract or bend light as it passes through the material. When a coating is opaque the pigment particles present in a coating scatter and/or absorb light to prevent it from reaching the substrate.

The higher the refractive index of the pigment, the higher the paint opacity it delivers to a coating. The pigment diffuses/reflects light before it can reach the underlying substrate. This makes the coating film appear opaque and vibrant. For example, titanium dioxide has a very high refractive index of 2.7 (Rutile) and 2.55 (Anatase). It is the most widely used white pigment to impart opacity and whiteness to paints and coatings. Typical binders have a refractive index of 1.49.

Opacity is one of the crucial characteristics in determining the quality of paint. It quantifies how effectively paint covers the surface. On the other hand, hiding power is the ability to hide the surface of an object. Opacity is a film property while hiding power is a property of the whole coating. Hiding power is also known as opacity and contrast ratio.

What are the techniques to measure the opacity?

There are several ways to measure the opacity & hiding power of a coating. The most common is using opacimeters and spectrophotometers which use the "contrast-ratio method". Contrast ratio is defined as the apparent reflectance of the material backed by a perfectly absorbing surface (black) divided by its apparent reflectance when backed by a white surface.



The ratio between these two reflectances is the contrast ratio. Opacity (%) is the Contrast Ratio multiplied by 100 to get a percentage, where 100% is complete hiding.


The test method consists of spreading the paint over a Leneta Chart (paper with Black & White strip). Then measure the reflectance of the dried paint film in the white (Rw) and black areas (Rb). The paint is applied with a drawdown bar (a metallic bar with a gap between the bar itself and the surface beneath). The paint is placed on top of the card. Moving the drawdown bar at a constant velocity causes the paint to be spread over the card, in a thick film. The thickness of this film is directly related and assumed to be equal to the gap depth of the drawdown bar used. The paint is left to dry, and the CR is then determined.

These determinations are more precise and objective when done using an opacity tester (opacity meter or opacity gauge). It is a device used to measure opacity or opacity-related properties of a material. An opacity tester works on a basic principle of shining light through the coating and measuring the amount of light that is reflected. They are used during quality control processes. They provide quantitative measurements of the material's opacity and hiding power as per industry standards.


Important to Note:

The greater the hiding power, the less coating is required per unit area to obtain adequate hiding. Therefore, knowledge of hiding power is important for coating costs and for comparing coating values.

Contrast ratio plays a significant role in paint films and powder coatings. It is usually associated with the wet thickness (i.e., the thickness of the film immediately after application).

Standard Test Methods to Measure Paint Opacity

Contrast ratio measurements have been standardized by the ASTM, ISO and other agencies. Common standards (not limited to) used to determine contrast ratio/hiding power of coating are listed below.

• ASTM D2805 - 11(2018) Standard Test Method for Hiding Power of Paints by Reflectometry
• ASTM D6441 - 05(2016) Standard Test Methods for Measuring the Hiding Power of Powder Coatings
• ASTM D6762 - 18 Standard Test Method for Determining the Hiding Power of Paint by Visual Evaluation of Spray Applied Coatings
• ASTM D5007 - 99(2017) Standard Test Method for Wet-to-Dry Hiding Change
• ASTM D5150 - 92(2017) Standard Test Method for Hiding Power of Architectural Paints Applied by Roller 
• ISO 6504-3:2019 Paints and varnishes — Part 3: Determination of hiding power of paints for masonry, concrete & interior use
• BS EN ISO 2814:2006, BS 3900-D4:2006 Paints and varnishes. Comparison of contrast ratio (hiding power) of paints of the same type and color 

Which ingredients help you get the right opacity in your coatings?

There are many additives and polymers available today that are used to improve or achieve the right opacity/hiding power in paints and coatings formulations. The pigment and filler particles within the coating are key to achieving the ideal optical properties for the coating’s intended purpose.

Pigments

There are many additives and polymers that improve the opacity in the formulation. This is done to achieve the right opacity in paints and coatings formulations. The pigment and filler particles within the coating are key to achieving the ideal optical properties for the coating’s intended purpose.

Pigments are the most important constituents to impart color, but their role involves many more benefits. Pigments are organic or inorganic, colored, white, or black materials that give color (all pigments), opacity (TiO₂), weatherability & conductivity (carbon black), corrosion resistance, etc.

• Inorganic pigments are easy to disperse. They provide excellent opacity, enhanced durability, and low cost as compared to organic pigments.
• Organic pigments are primarily used to achieve bright clean colors.

A key factor in the opacity of a pigment is its refractive index (RI), which measures the ability of a substance to bend light. The opacifying effect is proportional to the difference between the refractive index of the pigment and that of the medium in which it is dispersed.

Some of the common opacifying pigments are:

• Titanium dioxide (TiO₂)
• Zinc oxide (ZnO)
• Zinc sulfide (ZnS), and 
• Antimony oxide (Sb₂O₃)

The most commonly used opacifying pigment in paint formulations is titanium dioxide (TiO₂) which provides exceptional hiding power to the paint film owing to high refractive index. TiO₂ produces a bright, opaque coating at lower levels of pigmentation compared to other white pigments.

Titanium dioxide

Pigmentary titanium dioxide is found in virtually all modern coatings, except those that are clear or extremely dark. This is because of its high opacity. Efficient use of TiO₂ requires the particles be well dispersed in the final paint film, since close particle-particle interactions decrease the light scattering efficiency of TiO₂ and, thereby, its opacity.

Rutile TiO₂ has the highest refractive index (RI=2.7) of all the white pigments. Therefore, Rutile TiO₂ is the first choice used to maximize the opacity and whiteness of paint.

Titanium dioxide is an expensive component. But partial substitution of TiO₂ can be achieved using opacifying extenders in paint formulation. These extenders can improve the effectiveness of the titanium dioxide to:

• Achieve required covering power, and
• Use less titanium dioxide – reducing overall costs

This can be achieved in several ways and using extenders is among them. The refractive index of most extenders is too low to directly affect the opacity of coatings. But their small particle size materials improve the spacing and thus the optical efficiency of titanium dioxide.

Opacifying Extenders

As mentioned, extender pigments are mineral compounds of relatively low refractive index. They differ in composition, particle size, and shape.

• They develop very little hiding in gloss and semi-gloss paints. But they contribute "high flat hiding" (air pigmented interface) to paints at low cost.
• They are used to control gloss, texture, suspension, viscosity, etc.

The main type of extenders are carbonate, silicates, sulphates, and oxides.

Calcium carbonate

Calcium Carbonate is a white powdery substance primary serves as replacement for the binder material. This is due to lower cost of filler materials or formulation above critical pigment volume concentration (CPVC). It is used as an extender and functional filler which can significantly influence paints and coatings:

• opacity,
• brightness,
• reflectance,
• scrub resistance, etc.

Kaolin

Kaolin (hydrous and calcined) is the common name for the mineral products comprised of the aluminum silicate clay mineral kaolinite. Its chemical composition is Al₂O₃ · 2 SiO₂ · 2 H₂O. Calcined kaolin is used to extend titanium dioxide in aqueous matt or low-sheen coatings. Kaolin plays a vital role in the manufacture of coatings by offering:

• Increased opacity and tint strength
• Improved pigment suspension
• Stain and scrub resistance

While kaolin is not an absolute replacement for titanium dioxide, it is an excellent and economical extender. It helps keep manufacturing costs down in paints and coatings.

More mineral-based opacifying extenders chemistries include:

• diatomaceous earth,
• talc,
• silicate,
• wollastonite,
• feldspar, and
• nepheline syenite

They are used to enhance hiding power in different paints and coatings formulations.

Microspheres

Microspheres are known in the coatings industry for their use as low-surface-area fillers that offer benefits in:

• viscosity and density control,
• solids content,
• application, and
• flow characteristics

Opaque microspheres made up of polyethylene are available. They provide good opacifying properties and hiding power with as small as 40 microns in diameter.

Polymeric Opacifiers

Polymeric opacifier is another option to increase TiO₂ efficiency. Opaque polymers are used to improve hiding and whiteness in paints and coatings. They function as a partial replacement for titanium dioxide (TiO₂) and as extender. Thus, improving the efficiency of titanium dioxide (TiO₂) through better spacing for improved light scattering.

For example, polymeric opacifiers are voided latex particles with a polymeric shell. The difference in refractive index between the shell and the air void allows for improved light scattering & thus, better opacity.

How to change opacity in paints?

Here are some ways you can reduce or modify the paint opacity:

• Application Technique Modification: The technique used to apply the paint can influence the opacity of your paint. If the opacity is too thick you can add thinners and make it thinner so that the underlying colors of the surface are more visible. If more thickness is needed, you can try applying multiple coats instead of one thick coat.
• Mixing: The Opacity can be adjusted by mixing the paint with other materials like a transparent base, solvent, or water. The more the paint is mixed with these materials the more transparency you can achieve.

What factors influence the opacity of coatings?

The opacity of paint is influenced by several factors, such as:

Difference of the refractive indices of the pigment and the medium

When the refractive index of dispersed pigment is equal to that of medium, the pigment becomes transparent. This means no hiding action because the light passes through it unaffected. However, when the pigment refractive index is greater than that of the medium, pigments’ hiding power appears stronger. This is because of the scattering of the incident light.

The higher the refractive index of the pigment compared to that of the medium. The nearer the particle size to the optimum, the greater the scatter and greater the opacity of the paint.

Particle size and dispersion of the pigment

The particle size of pigmented coatings directly affects optical performance by influencing:

• opacity,
• gloss,
• hue, and 
• tinting power

As the particle size increases, the ability of the particle to scatter light increases, up to a maximum. It then starts to decrease. This ability to scatter light increases the hiding power of the pigment. Therefore, the hiding power also reaches a maximum and then decreases as the particle size increases.


As the refractive index of a compound cannot be altered, the pigment manufacturer can influence the particle size of pigments. So, particle size selection has become one of the principal developments in pigment technology in recent years.

The paint opacity is also greatly determined by the dispersion status of the pigment particles in the liquid medium. Complete dispersion of the pigments is required for:

• optimum color strength,
• good opacity, as well as 
• improved mechanical characteristics of the coating film

In general, hiding power is improved by grinding the pigment down to a fine, controlled particle size.

Proportion of pigment in the vehicle (PVC, CPVC)

The proportion of pigment in a coating formulation is determined by PVC (Pigment Volume Concentration). Pigment volume concentration or PVC is a measure of how much volume of pigment there is in a paint compared to the volume of solid binder.

Paints with high pigment volume concentration (PVC) levels above critical pigment volume concentration (CPVC) usually have high opacity at low to very low pigment concentration. In case of high PVC, the opacity is controlled by both pigment concentration and an extender/filler package.

Thickness of the applied film

Film thickness is an essential parameter that directly affects the:

• paint opacity,
• appearance, and
• protective properties of the coating

A liquid paint usually has a substantial quantity of volatiles. So its dry film thickness (DFT) is substantially less than the original wet film thickness (WFT).

Uniform application of all paint coatings with the correct tools and at the specified spreading rate is critical. It helps to achieve acceptable coverage and opacity levels.

Suitable opacifiers or hiding agents for coating formulations