Paints are mainly characterized by three optical properties – opacity, gloss, and color. The opacity of any paint is directly related to the hiding power of the paint.
Hiding power is associated with two phenomena:
- light scattering and
- absorption of light
The hiding power in white paint comes from a high level of light scattering. While the hiding power in black paint comes from the perfect absorption of light.
Here, we will focus on white paint because white is part of most of the paint colors. The difference in refractive index of pigments, extenders/fillers, and binders leads to scattering of light.
Get answers to all your questions related to hiding power:
- What is the hiding power of paint?
- What is the mechanism behind hiding power?
- What factors affect the hiding power of paint?
- What technologies improve hiding power?
- What test methods determine the hiding power of paint?
What is the hiding power of paint?
Hiding power is the ability of the paint to hide any substrate (black, white, or any painted substrate). It is the optical property of the paint.
Influence of rheology on hiding power
Hiding power also considers the
rheology of the paint. For example, paints with the same opacity but different rheology will show different hiding power. This is due to different application characteristics/film build of the paint. Paint with higher shear viscosity may show higher hiding power. This is due to the higher film build.
Influence of opacity on hiding power
The
opacity of any paint is directly related to the hiding power of the paint. Opacity is defined as the ratio of light reflectance over an absorbing black surface to the light reflectance over a white surface. It is the optical property of the coating. The opacity/contrast ratio is usually determined using a light spectrophotometer.
Opacity = Rb/Rw x 100
where R
b/R
w is the Contrast Ratio
Relative hiding power is determined when the paint opacity is compared with the standard/benchmark paint at the same spreading rate. Thus, it is important to understand the opacity of the paint film/coating in the first place.
What is the mechanism behind hiding power?
Light scattering occurs at interfaces between:
- the resin and pigment particles
- the air and pigment particles.
The more the difference in refractive index for materials at the interface, the more the light scattering. For example, the air has a refractive index of 1.0, most resins about 1.5-1.6, and inert extenders or fillers about 1.55 -1.65.
| Substance/Pigments |
Absolute Refractive Index (589.3 nm) |
Substance/Pigments |
Absolute Refractive Index (589.3 nm) |
Diatomaceous earth
|
1.45 |
Kaolin
|
1.56 |
| Quartz |
1.55 |
Dolomite
|
1.60 |
Calcium sulfate
|
1.59 |
Barium sulfate |
1.64 |
Magnesium silicate
|
1.59 |
Zinc oxide |
2.00 |
Zinc sulfide + barium sulfate (30:70)
|
1.84 |
Zinc sulfide |
2.34 |
Antimony trioxide
|
2.20 |
Anatase TiO2 |
2.55 |
Zirconium dioxide
|
2.40 |
Rutile TiO2 |
2.70 |
The Refractive Indices of Different Pigments
Hiding Power of Different Pigments (Extenders and TiO2)
The indices of refraction for air and extenders are far enough apart. This generates sufficient light scattering to develop hiding. Resins and extenders have about the same refractive index. Thus, the
extender is generally transparent within the resin. Resins and TiO
2 are sufficiently far apart in refractive index to produce good hiding. Thus, the preference for TiO
2 as a white hiding pigment.
Difference Between Light Scattering by TiO2 and Extender Pigments
What factors affect the hiding power of paint?
Due to the high refractive index, TiO
2 pigment contributes to the scattering of light. As a result, it imparts hiding power to the paint. Thus, we are going to discuss factors related to TiO
2, which affect the hiding power of paint.
1. Crowding of TiO2
The term "crowding" of TiO
2 refers to the
TiO2 particles in a paint formulation that becomes densely packed. This leads to reduced hiding power and other performance issues. This can occur when the TiO
2 particles are not distributed, dispersed, or stabilized within the paint matrix.
In high pigment volume concentration (PVC) paint formulations, TiO
2 pigment particles are distributed in the paint matrix. It depends upon the particle size of the extender/filler pigments. Here, the inorganic pigments which are low in cost as compared to TiO
2 are used.
The lower particle size of the extenders helps TiO
2 particles to distribute uniformly. This results in high scattering efficiency of TiO
2 particles. The crowding of pigments can be studied using SEM/TEM images.
TiO2 Crowding by Extenders
2. Dispersion and flocculation of TiO2
There are
different grades of TiO2 available in the market. Most of the TiO
2 grades are treated at the surface for better dispersion. It is done with:
- Al2O3 (inorganic treatment) and
- Hydrophobic/hydrophilic organic treatment.
It is important to disperse TiO
2 particles at the iso-electric point (IEP). It is done by using the correct
dispersant as per the surface treatment of TiO
2 (refer to the below table to see TiO
2 with different surface treatments and their IEP).
| Sample |
IEP |
Surface Area (m2/g) |
Description of TiO2 Pigments (Surface Concentration wt %) |
| 1 |
9.0 |
15.0 |
4.5 Al2O3 | TiO2 |
| 2 |
6.3 |
15.0 |
3.6 Al2O3 - 6.5 SiO2 | TiO2 |
| 3 |
4.51 |
12.2 |
1.25 ZrO2 | 2.0 Al2O3 | 6.0 SiO2 | TiO2 |
| 4 |
6.36 |
14.3 |
2.0 Al2O3 | 0.5 ZrO2 - 2.0 SiO2 | TiO2 |
| 5 |
8.49 |
14.1 |
2.0 Al2O3 | 0.5 ZrO2 - 1.0 Al2O3 | TiO2 |
| 6 |
8.28 |
15.0 |
1.0 ZrO2 - 1.0 Al2O3 | TiO2 |
Characteristics of Different Surface-treated TiO2 Pigments
Achieving effective pigment dispersion — Key stages & processing conditions
There are different stages of effective pigment dispersion. They are:
Effective pigment dispersion is achieved by processing the right combination of materials. For example, TiO
2 pigment, extenders, and dispersants. Processing is done in a high-speed disperser processing unit. This allows the breaking of the agglomerates. The surface treatment on TiO
2 makes it easier to break down the agglomerates. Thus, making the pigment easier to disperse. All TiO
2 pigment manufacturers use
aluminum oxide for this purpose. Also, the organic surface treatment helps better wetting of pigment surfaces by binders.
For effective dispersion, the
dispersant is selected based on the zeta potential of pigment particles. The dispersant demand is calculated based on the BET surface area. It also depends on the volume of the pigment particles.
Types of flocculation
The ineffective dispersion may lead to flocculation. There are two types of flocculation:
- Bridging — In this type, polymer molecules connect pigment particles. Thus the joined pigment particles flocculate together. Examples of polymer molecules are binder/polymer or other polymeric materials like surfactants/dispersants.
- Depletion — TiO2 particles become squeezed out of the thickener solution. This results in a lower gloss and contrast ratio (CR). Thus, it causes low hiding power.
The dispersion quality of any mill base (pigment dispersion) can be checked using a
Hegman gauge. A gauge of 7-7.5 on the Hegman scale suggests good dispersion of pigments.
Idealized Dispersion Phase Diagram
3. Rheology of paint
Most of the time, although paints might have the same opacity, they would show different hiding power. This is due to different spread rate/application characteristics. The spread rate of any paint can be correlated with high shear viscosity and weight% solids of the paint.
What technologies improve hiding power?
1. Extenders and fillers acting as TiO2 spacers
In high PVC formulations, pigments with lower refractive indices are added. This reduces the cost of the paint. This also maintains several properties including:
As the TiO
2 content in the paint starts increasing, the opacity starts increasing initially. But it starts decreasing as it tends towards CPVC.
Pigment composites have an average particle size of 1 micron. Each particle of opacity pigment contains a minimum of 3-6 particles of TiO
2. They are statistically placed from each other by the optimum distance for the scattering of 280 nm. Different mechanisms that these pigment composites work for maximum light scattering are:
- Optimally spacing TiO2 inside the opacity pigment particle
- Spacing loose TiO2 around the particle
- Air voids inside and on the surface of the particle
- Diffraction from the surface of the rough particle
TiO2 Spacing within Pigment Composites
Factors for the selection of TiO2 spacer pigments
- Median particle size within ±20% of TiO2 median particle size
- Similar oil absorption to that of TiO2 (14-45 gm of linseed oil per 100 g of pigment)
- Similar particle size distribution curve
- Compatibility with different resin types
Alternatives of TiO2 spacer pigments
- Recently, an alumina hydrate pigment has been developed. This is in accordance with the present invention. Its median particle size and particle size distribution curve very closely approach that of titanium dioxide.
- Alumina trihydrate is also preferred within the present invention. It exhibits similar oil absorption characteristics as titanium dioxide. It is also compatible with all resin types. It has good exterior durability to facilitate its use in exterior formulations.
- Kaolins (hydrous and flash calcined), calcium carbonate, and talcs optimize titanium dioxide in a formulation. They do so by spacing the TiO2 particles for maximum efficiency across a range of pigment concentrations.
- Opacity pigments have inherent scattering functionality and RI of 1.9 and can partially replace TiO2 (5-30%) on a weight basis. This depends upon the type of formulation & application.
2. Core-shell opaque polymers
They are special materials having a core-shell structure. They contain water inside the shell of polymeric material (
acrylic/styrene-acrylic). As the water inside the shell dries, an air pocket is generated. This creates a high refractive index at the interface of air void & shell polymer.
A high refractive index at the interface causes scattering and thus hiding. Also, these opaque polymers are half the wavelength of visible light. This helps in optimal TiO
2 spacing and improves scattering.
TiO2 Spacing within Core-shell Opaque Polymers
3. Pigment adsorbing composite binder polymer
Many suppliers are developing polymer nanoparticles. One such example is Dow Chemicals. These nanoparticles adsorb onto TiO
2 pigment via phosphate groups onto TiO
2 and act as a spacer. Moreover, it ensures more uniform TiO
2 dispersion within the paint film. It allows either greater hiding power and/or less TiO
2 to be used, so it produces cost-effective paint.
Pigment Adsorbing Composite Binder Polymer
What test methods determine the hiding power of paint?
ASTM D 2805 11(2018)
It determines the hiding power of paints by reflectometry. This is a precise instrumental method. It gives results having an absolute physical significance. It has no reference to a comparison paint. It should be used when maximum precision and minimum subjectivity are required. For example in testing specification coatings or evaluating the hiding efficiency of pigments.
ASTM D344
This test method determines the relative hiding power of paints by the visual evaluation of brush outs. It evaluates the hiding power of a test paint relative to a comparison paint. The results have significance only within that relationship. It may be used for production control or quality comparisons.
This method gives results that are relative to a material standard instead of absolute. It is less precise than the D2805 test method. But it is more closely aligned with practical painting procedures.
- When the paint is applied by brush or any other practical method, the opacity of the film is affected by variations in film thickness. This is related to the application procedure and application characteristics of the paint.
- Two paints that hide well by this method might thus differ when applied with a doctor's blade. This is because the latter method gives perfect leveling.
- Different brushes or surface application conditions can likewise give different results.
Credits: Spektrochem Paint Technical Center
Conclusion
In summary, the hiding power of any paint refers to its ability to completely cover the white, black, or any painted substrate. It is applicable to all colors including white, black & other colors. White paint is given most of the attention because it is part of pastel colors too. Hiding power is a function of the opacity and spread rate of the paint.
Hiding power can be improved by:
- Avoiding crowding of TiO2 pigment particles
- Effective dispersion and spacing of TiO2 particles
- Use of opaque polymers
- Use of special binder resins which adsorb onto TiO2 pigment particles
- Controlling the rheology of paint
References
- Spacer/extender for titanium dioxide in pigment systems for coatings
- Optical properties of coating pigments: Absorption and scattering
- TiO2 efficiency FP-Pigments- Innovative Technology - FP-Pigments
- Effects of TiO2 Partial Substitution by Various Extenders on Architectural Interior Paints
- D5007 Standard Test Method for Wet-to-Dry Hiding Change
About Author
Young dynamic & collaborative leader with a detail-oriented approach towards solving problems, Sameer brings in 14 years of experience in coatings & materials development. In his role as coatings & materials expert, he has been able to help companies to steer through all sorts of technical problems including those at the customer end with his strong analytical & problem-solving skills. He has helped companies to save millions of dollars through different development strategies (for materials) such as localization, contract manufacturing and create- -at-customer.
Since 2021, Sameer has been an independent consultant offering his services in the development of coatings & materials for applications such as printing & packaging, paints, OEM graphics & labels. He has helped different companies to develop end-to-end solutions (development to end-applications) for their customers and thoroughly analyze technical problems with customers. His interest lies in the development of sustainable solutions for every problem that he faces.
Sameer graduated in Polymer Science & Engineering from a renowned chemical institute called the Institute of Chemical Technology (ICT formerly UDCT, Mumbai). He has varied experience from bench to top in the formulation and development of different materials & products.
He worked on the development of interior & exterior paints during his tenure at Behr Process Paints. Later on, at Classic Stripes Pvt Ltd, he went on to complete different assignments such as backward integration, international projects, and development & technical services to automotive OEMs. He leveraged his experience & skills in the development of industrial coatings, conductive inks, and materials such as adhesives & polymeric films.