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Coatings Ingredients
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Coatings Ingredients

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

Opacity in Paints and Coatings
  1. Masking the Substrate to Determine Opacity
  2. Key Optical Properties – Opacity, Hiding Power and Refractive Index
  3. Popular Methods to Measure the Opacity and Hiding Power
  4. Ingredients & Opacifiers – Get the Right Opacity in Your Coating Formulations
  5. What are the Factors That Influence the Opacity of Coatings?


Masking the Substrate to Determine Opacity


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

This ability of a coating film to completely mask the underlying substrate is determined by the opacity of the coating. Given the same thickness of applied coating film, not all coatings have the same opacity.

Opacity is a simple phenomenon that results from the complex interplay of various components pigments, extenders, dispersants, mineral fillers, etc. and parameters, such as pigment dispersion, film thickness, etc. in a coating formulation.


We will discuss how these factors determine the opacity of coatings and how to obtain the desired opacity in your formulation. Let us first begin by understanding opacity in general.


Key Optical Properties – Opacity, Hiding Power and Refractive Index


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 sufficiently to prevent it from reaching the substrate.

The higher the refractive index of the pigment, the higher opacity it delivers to a coating. The pigment diffuses/reflects light before it can reach the underlying substrate making 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 while 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

Now that you have learned what opacity & hiding power are, let us walk through the test methods used to determine the opacity of paints and coatings.


Popular Methods to Measure the Opacity and Hiding Power


Digital Tool to Ease Coating R&D Formulation

There are several ways to measure the opacity & hiding power of a coating, the most common being using opacimeters and spectrophotometers which use the so-called "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.

Checking the Opacity on Leneta Chart
Credits: Dacrylate Paints Ltd


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

Opacity (%) = (Rb / Rw) x 100 

The test method consists of spreading the paint over a Leneta Chart (paper with Black & White strip) and measuring the reflectance of the dried paint film in the white (Rw) and black areas (Rb). The application of the paint is done 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 normally 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.


Important to Note:

The greater the hiding power, the less coating is required per unit area to obtain adequate hiding. Knowledge of hiding power is therefore important regarding coating costs and for comparing coating value.

Contrast ratio, also referred to as “hiding power,” and opacity play a significant role in various industries, including both paint films and powder coatings. The contrast ratio is usually associated with the wet thickness, that is, the thickness of the film immediately after being applied.



Standard Test Methods


Contrast ratio measurements have been standardized by the ASTM (American Society for Testing Materials), 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 — Determination of hiding power — 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 


Ingredients & Opacifiers – Get the Right Opacity in Your Coating Formulations


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


Pigments: Types & Selection Pigments are the most important constituents to impart color, but their role involves much 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, provide excellent opacity, enhance durability, and are lower in 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, 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 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.

However, titanium dioxide is an expensive component, partial substitution of TiO2 can be achieved using opacifying extenders in paint formulation which 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 by 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 improves the spacing and therefore the optical efficiency of titanium dioxide.

TiO2 by Chemours


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 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 influences opacity, brightness, reflectance, scrub resistance, etc. in paints and coatings formulations.


Kaolin


Kaolin (hydrous and calcined) is the common name for the mineral products comprised totally or substantially of the aluminum silicate clay mineral kaolinite with chemical composition Al2O3 · 2 SiO2 · 2 H2O. Calcined kaolin 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, helping keep manufacturing costs down in paints and coatings.

More mineral-based opacifying extenders chemistries include diatomaceous earth, talc, silicate, wollastonite, feldspar, and nepheline syenite which are widely used to enhance hiding power in different paints and coatings formulations.

Boost Opacity with ChameleoBoost™ by Omya


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 that 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. Polymeric opacifiers function as a partial replacement for titanium dioxide (TiO2) and as extender to improve 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.


What are the Factors That Influence the Opacity of Coatings?


The hiding strength (or opacity) of paint is influenced by several factors, such as:


These factors determine the opacity of a particular paint.


Difference of the Refractive Indices of the Pigment and the Medium


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

The higher the refractive index of the pigment compared to that of the medium and the nearer the particle size to the optimum, the greater the scatter and the 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 parameters such as 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, and therefore the hiding power also reaches a maximum and then decreases as the particle size increases.

Effect of particle size on scattering
Effect of Particle Size on Light Scattering

Whereas 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 that its dry film thickness (DFT) is substantially less than the original wet film thickness (WFT).

Rheology Modifier Selection

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


Suitable Opacifiers or Hiding Agents for Coating Formulations



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