Titanium dioxide extenders for coatings – calcined kaolin or hydrous kaolin?

In Paints and Coatings, kaolins are primarily used to improve the optical performance of rutile titanium dioxide, but they can be used to extend other pigments as well. Titanium dioxide has excellent light scattering properties and high whiteness/brightness, making it the pigment of choice for white coatings, plastics, papers etc.  It can also used to tint coloured systems and provide opacity to formulations. Usually, titanium dioxide will be the most expensive white inorganic in the formulation - especially when you consider its cost on a volume basis (due to its high S.G.) - and kaolin offers a way to reduce the amount of titanium dioxide in a formulation with minimal impact on optical properties.

Kaolin types

Kaolin can be broadly classified as either Primary or Secondary based on how they were formed millions of years ago:

• Primary deposits are those that formed in situ from the weathering of granite or feldspar and can be found in the UK and mainland Europe.  Primary deposits are low in kaolin (15 to 30%) and contain a large amount of impurities of mainly quartz, feldspar and mica, in addition to iron oxides which result in discolouration. 
• Secondary kaolins are much higher in kaolin (85 to 90%) and are those that were transported and laid down as sediments and can be found in the southeastern United States and Brazil.  They are naturally fine (<2 microns), purer, softer and less abrasive than those found in Primary deposits.

Figure 1: Hydrous kaolin, delaminated kaolin and calcined kaolin

In addition to characterising kaolin by the deposit type, commercial grades can be further divided into the following categories based on how the material has been treated after mining (Figure 1):

• Hydrous kaolins such as the  ASP® products, have been water-washed and treated to remove any residual impurities before spray-drying or pulverisation.  Hydrous grades have a stacked platy structure and are typically 0.1 µm to 4 µm in size.
• Separation or delamination of the stacked platelets yields delaminated kaolins such as ASP® NC and NCX1 which have a more platy structure and an aspect ratio as high as 10:1.
• Calcined kaolins such as Satintone®, Mattex® and Ultrex® have been dehydroxylated at high temperatures resulting in an amorphous fused structure with internal air voids.
• Both hydrous and calcined grades can be surface-treated with various silanes, stearates or polyacrylates to provide hydrophobicity and ease of dispersion such as the Translink® products from KaMin
• Metakaolin such as MetaMax® has been treated at temperatures over 500°C to produce a reactive and pozzolanic material used in Portland cement-based products and concrete.

Table 1: Typical properties of kaolin

When to use Hydrous or Calcined kaolin in a coating formulation

The choice of whether to use hydrous or calcined kaolin in a coating formulation largely depends on the formulation itself and what the formulator is trying to achieve.  In general hydrous kaolins are the primary choice for gloss and semigloss paints due to their transparency and small particle size where they can be used in both decorative architectural paints and industrial coatings.  Calcined kaolin grades on the other hand find use in matt and eggshell paints where they can contribute not only to dry hiding but also provide additional properties such as gloss reduction. 

Table 2: Types of kaolin and addition levels used in paint formulations

Hydrous kaolin in gloss or semi-gloss coatings

When used in a coating formulation, hydrous kaolin acts as a physical spacer for titanium dioxide particles to prevent optical overcrowding (Figure 2).  Due to the small particle size, often less than 1µm, hydrous kaolin finds use in gloss, semi-gloss and some eggshell paints as it has minimal impact on gloss levels compared to the typically larger calcined grades. Hydrous kaolin is also transparent so can be used in transparent coatings as an easy to disperse filler.  Typical loading levels can be between 3 to 7% by weight although a higher titanium dioxide content in a formulation allows for a higher level of replacement with hydrous kaolin. 

Figure 2: Fine particles of hydrous kaolin are highly efficient at spacing titanium dioxide particles so that each particle intercepts a wavelength of light

Calcined kaolin in matt coatings

Calcined kaolin on the other hand typically has a coarser particle size distribution and works particularly well in medium to high PVC coatings where it contributes to opacity by enhancing dry hiding.  Calcination introduces air voids within each particle which are sealed from resins, solvents or water in a liquid paint.  Incoming light is then scattered due to the differing refractive indices of air, the binder and calcined kaolin (Figure 3).  Calcined kaolins are primarily used in flat or matt paints formulated above the critical pigment volume loading (CPVC) so addition levels are higher than with hydrous grades, in the range of 10 to 20% by weight, and newer calcined grades offer an excellent balance of high opacity and low sheen whilst providing excellent touch-up properties.

Figure 3: Calcined kaolin introduces inaccessible air voids which contribute to light scattering and opacity due to the differing refractive indices.

Addition levels of kaolin in paints

When replacing titanium dioxide in paints it is important to consider that TiO₂ and kaolin have different densities and therefore any replacement TiO₂ should be carried out on an equal volume basis to preserve PVC; for every 10kg of TiO₂ which is removed from the formulation, only 6.6kg of kaolin is required to replace it therefore ensuring that a good cost saving on total formulation can be achieved. 

Titanium dioxide extension in gloss and semi-gloss paints

In gloss and semi-gloss, the higher the TiO₂ content, the higher the replacement level that can be achieved.  As a general rule:

• if TiO₂ content is >25 wt%, then a 10% replacement with hydrous kaolin is possible
• if TiO₂ content is 20 to 25 wt%, then an 8 to 10% replacement with hydrous kaolin is possible
• if TiO₂ content is 10 to 20 wt%, then a 5 to 8% replacement with hydrous kaolin is possible

Hydrous grades from KaMin, such as ASP® G90, 170, 172 and 600 are often used for gloss and semi-gloss paints due to their small particle size (0.4 to 0.6 µm).

Figure 4: In matt paints, calcined kaolin is used to provide opacity and dry hiding

Titanium dioxide extension in matt paints

In matt and low sheen paints it is more difficult to provide a rule of thumb as other factors such as air voids and particle packing have an effect but in general the maximum addition calcined kaolin would be up to 20 wt%. 

KaMin offers a range of calcined kaolin grades which provide an excellent balance of high opacity and low sheen, and excellent touch-up properties.  Satintone® Whitetex and Satintone® 5HB find use in many decorative and industrial paint formulations due to their excellent opacity, hiding power and high tint strength.  For matt coatings, the structured particles of Mattex® combine high opacity and low sheen properties in one product, eliminating the need for additional matting or flattening agents. 

Summary

In general, hydrous grades such as such as ASP® G90, 170, 172 and 600 products from KaMin are used in gloss and semi-gloss paints due to their small particle size and low influence on sheen.  They are primarily used to extend out titanium dioxide but can also be used in transparent waterborne and solventborne coatings as an easy to disperse filler.  Replacement levels depend on the levels of other pigments in the formulation but it would not be unusual to see a 2 to 5% loading based on weight.  Calcined kaolin on the other hand like the Satintone® Whitetex and Satintone® 5HB or Mattex®  products from KaMin are used in flatt or sometimes satin coatings as the larger more irregular particles can also be used to reduce gloss and provide excellent dry hiding properties. Calcined kaolin additions can be much higher, up to 20% by weight in a formulation.