Tinting Strength of Paints, Coatings, and Inks - All You Need To Know

Tinting is the process of adding two or more colors to a system. Tint machines or color dispensing machines are used to tint the standard paint bases.

The tinting strength of a paint film is the ability of a colorant or a pigment by which it can alter the:

• Color intensity and
• Appearance

It helps standardize the paint formulations on the basis of tint levels. It also produces various shades and colors after adding titing colors.

Solve all your queries related to tinting strength:

1. What is tinting strength?
2. What is the formula of tinting strength?
3. What is the process of developing tinting strength?
4. What are the factors affecting tinting strength?
5. How to measure tinting strength?
6. What are the benefits of evaluating tinting strength?

What is tinting strength?

Definition in terms of hiding power

The tinting strength of paint refers to the strength of Titanium dioxide (TiO₂) in the paint. It is also referred to as the hiding power of the TiO₂ on the top layer of paint.

Definition in terms of reduction strength

The tinting strength in paint bases (whites) is generally referred to as reduction strength. It is checked by adding colorant paste to evaluate the reduction in color strength of the final solution.

Definition in terms of color pigment

The tinting strength can also be defined as the relative power of coloring a paint, coating, or ink system. This is also referred to as tint color. The higher the tinting strength the higher will be the reduction of the whiteness (Rutile or TiO₂) of the paint. In many cases, the tinting strength is also relative to the opacity of the paint or coating system.

What is the formula of tinting strength?

It is determined by the Absorption and Scattering coefficients of the trial (batch) and reference standard. It is denoted by K/S value, and it is expressed in percentage (%). The equation is as follows:


where,

• K is the absorbance
• S is the scattering

What is the process of developing tinting strength?

The tinting strength is dependent on the:

• pigment, and
• processing factors like shear rate, temperature, type of mixing equipment, etc.

Pigments are the key components in a paint or ink formulation which are responsible for tinting strength development.

• The pigments are broken down from aggregates to agglomerate and finally to primary particle size.
• They are broken sometimes lower than that as well by means of external shear or impact. The processes can be of different types like:
• high shear dispersion,
• milling or
• high-speed grinding.
• When the particles are stacked in a compact manner the refraction is higher. This also depends on the refractive index of the pigment as well.

Generally, pigments with a refractive index of more than 1.5 are considered hiding or opacifying pigments. Pigments below 1.5 are considered extenders or fillers.

What are the factors affecting tinting strength?

There are multiple factors that affect tint strength and color development as well.

Process-related factors

The multiple factors related to the process which controls tint strength include:

• Shear rate & tip speed is directly proportional to tint strength.
• Optimum temperature & viscosity aids the dispersion process. This improves tint strength or pigment strength.
• Choice of grinding media or dispersion (cowl blade, bead mill, ball mill), etc. facilitates tint strength development.

All these factors have an impact on the tinting strength development of the system.

Pigment type and nature

It is the most crucial factor which decides the tint strength development. Pigment nature means the:

• Morphology of the pigment (e.g., Packing ratio, hardness, shape, etc.),
• Surface charge on pigment,
• Type of pre-treatment on pigment,
• Particle size of pigment (smaller or finer particle sizes results in higher tinting strength),
• Reflectance value or the refractive index of the pigment etc.

These factors have a great influence on tinting strength development.

Particle Size Influence on Different Properties (Credits: Vibron)

Degree of Dispersion vs Optical Properties of Paint or Ink (Credits: Intechopen)

Formulation of the paint or ink system

There are many components in a paint or ink formulation which contribute to pigment dispersion and particle size reduction. Components like:

• wetting agents or surfactants,
• dispersing agents also known as dispersants,
• rheology modifiers,
• advanced pigment spaces like kaolins and special additives help in improving tinting strength by 5 to 10% by titanium dioxide spacing.

These majorly impact on tint strength development of any paint or ink system.

For instance, A good dispersing agent helps in reducing the dispersion time. It also increases the tinting strength by uniformly dispersing and stabilizing the pigment in the paint or ink system.

How to measure tinting strength?

Key instruments for evaluation

There are many optical phenomena occurring when light falls on a substrate. These phenomena include:

• reflection,
• transmission,
• absorption, and
• scattering.

For a good understanding and standardizing of the analytical process, the maximum absorption and scattering coefficients are used for control and batch both.


where,

• K is the absorbance
• S is the scattering

The instrument used to measure tint strength is called Spectrophotometer. It reads the incidence and reflectance of light of a particular wavelength. We can relate K/S with reflectance by Kubelka-Munk theory, which states:


where R is reflectance at a given wavelength

In some systems of Pigment quality control and assurance, the mechanical or automatic muller can also be used for Tinting strength analysis.

Test methods to evaluate tinting strength

• The test methods adopted for Tinting strength for paints are ASTM E308, ASTM D387, ISO 787
• For inks ASTM D 6531, ASTM D 2066
• ASTM D387 - This method refers to the testing using mechanical or automatic muller instrument. However, this is just a Quality control test.

What are the benefits of evaluating tinting strength?

There are a lot of important analytical and statistical inferences driven by tinting strength evaluation. A few of them are as follows:

• Process standardization - Optimizing and controlling TS can be done only when the process is standardized and rationalized. Therefore, when we are constantly controlling TS, also means we are working on the process control as well.


• Pigment standardization - The pigment is the reason for Tinting strength development in any paint or ink system. Tinting strength standardization is a result of efficient pigment dispersion. Hence, pigment dosage can be standardized by TS control.


• Quality control and assurance - Tinting strength is one key parameter, which is used in quality control and assurance. It determines that the paint/ink has been processed in an effective manner.


• Better color delivery in paints and inks - Tinting strength is the degree of color development in any system. Paints and inks are often subjected to colorants or color mixing. The control over tinting strength leads to:
  
  • consistency in resulting shades and
  • superior color delivery


• Cost optimization - A standardized process, pigment loading, and other parameters can bring in a considerable amount of cost-cutting to the product.

References:

1. https://www.intechopen.com/chapters/58535
2. https://www.byk-instruments.com/en/industrial-coatings-determination#:~:text=Tinting%20strength%20is%20the%20ability,S%20of%20standard%20and%20batch
3. https://www.echemi.com/cms/823899.html
4. http://www.xycarbonblack.com/aspcms/news/2012-12-5/260.html
5. https://pubs.acs.org/doi/pdf/10.1021/j150183a004
6. https://www.pcimag.com/articles/110186-how-light-and-pigment-interactions-affect-hiding-tint-strength-and-more