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

Blog Archive | 10 minutes  | Author: Adam Morgan , Ph.D.

This article looks at why hydrous kaolin and calcined kaolin are such popular extenders for pigmented coatings. Not only do they improve various facets of coating performance, but they enable cost enhancement of the formulation as well. If you would like more information on how to choose the right grade for your coating then read on.

Kaolin extenders 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 is also used to tint coloured systems. 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.).

In this technical article:


What are the key differences and similarities between hydrous and calcined kaolin?

In the following sections, we will look at which grades are best for gloss/matt and aqueous/non-aqueous coatings. First, let’s take a quick look at the main differences between hydrous kaolin and calcined kaolin:

1). Mechanism - Hydrous kaolin works by spacing the titanium dioxide apart in an efficient manner, because of it’s much finer particle size of around 200 nm – similar to rutile titanium dioxide. This helps to prevent optical crowding. Calcined kaolin has a coarser particle size distribution and works particularly well in medium to high PVC coatings where it contributes to opacity through enhancing dry hiding.

2). Gloss level – Due to the fine particle size, hydrous kaolins are used in gloss/semi-gloss systems. Calcined kaolins are typically used more in satin or matt systems (although hydrous kaolins are also used in these sometimes as well).

3). CPVC – Typically you would use hydrous kaolin in systems below CPVC and calcined kaolin near or above CPVC.

4). Burnish resistance – This can be enhanced with speciality calcined kaolin grades, such as Mattex® PRO. These particles are large enough to protrude from the surface and have a higher Mohs hardness than standard calcined kaolins. Hydrous kaolins can be used in clear coat varnishes to improve scuff and mar resistance.

So, what are the similarities? Both hydrous kaolin and calcined kaolin are easy to handle and disperse in your system. There are various particle sizes and compositional forms available. They have a low abrasion on equipment. Our product ranges offer cost enhancement to aqueous and non-aqueous coating formulations.

BASF are the largest kaolin producer in the world. In partnership with Lawrence Industries, we bring many years of expertise to the table. Why not get in touch after reading this article, to see how our technical sales team can help you with your formulation challenges?


Hydrous kaolin used to extend titanium dioxide in aqueous gloss and semi-gloss coatings

ASP® G90 and ASP® G92 are high-performance hydrous kaolin grades that are unique to BASF. These are some of the finest grades available (around 180 nm primary particle d50).

You can replace approximately 10% titanium dioxide by volume in low PVC aqueous gloss/semi-gloss coatings, with no appreciable difference in opacity or tint strength. This can afford a significant cost saving whilst also reducing VOC through resin reduction. These grades are suitable for both architectural and industrial coatings.

They offer excellent gloss maintenance when compared with competitive offerings.

Both are easy to disperse, in particular, ASP® G92. This is a spray dried bead for low dusting and easy incorporation.

Let’s look at a real-life example of an interior/exterior acrylic gloss enamel with ASP® G92 as the extender. You will find the starting point formulation given in Figure 1.

Starting point formulation for acrylic gloss enamel

Figure 1. Low VOC interior/exterior aqueous acrylic gloss enamel, with ASP® G92 as the titanium dioxide extender. 18.7 PVC. VOC< 100 g/L.

Opacity and gloss measurements were compared with the control sample containing no titanium dioxide extender. As you can see from the results, opacity is almost identical whilst gloss is significantly improved (Figure 2). Bear in mind that this is also reducing the cost of the formulation.

Graph showing optical properties of coatings containing ASP G92

Figure 2. Optical characterisation of two aqueous coatings (with and without kaolin extension). Extension with ASP® G92 not only makes it more cost effective but actually improves the gloss levels appreciably.

Hydrous kaolin is not just limited to spacing titanium dioxide – they can also improve the efficiency of coloured pigments as well. This is a great way to reduce the cost of expensive pigments and improve the overall efficiency of the formulation. See the technical handout relating to the use of ASP® 170 in water-reducible industrial enamels.

For more formulation examples with ASP® G90 and ASP® G92, please see the brochure.


Calcined kaolin used to extend titanium dioxide in aqueous matt or low sheen coatings

BASF offers three calcined kaolin product ranges for extending titanium dioxide in aqueous matt coatings. These products are popular as they can confer excellent hiding power, increased tint strength, sheen control, burnish resistance – all whilst reducing formulation cost.

Each grade in the family offers a different balance of values due to the range of particle morphologies, sizes and calcination processes. Let us look at some of the key grades from these ranges:

  • Satintone® W (Whitex) – This is the workhorse of the calcined kaolin portfolio. It has a combination of good hiding, scrub resistance and sheen control. It is widely used in interior and exterior matt coatings as an opacifying extender. Due to the high level of pulverization, it is easy to disperse. Satintone Whitex gives the dry film a good balance of mechanical and optical properties.

  • Satintone® 5HB – This grade is best suited to eggshell and satin formulations. It offers the best hiding power and tint strength out of the Satintone® range, primarily due to its ultrafine particle size distribution – in that it spaces titanium dioxide and provides dry hiding. Both high and low PVC formulations benefit from its excellent hiding power. It is generally used at relatively low loadings in combination with another Satintone® grade, to bump up the hiding power whilst maintaining scrub resistance.

  • Mattex® – This is a medium particle size grade featuring high brightness. It is a structurally calcined material which allows for the optimization of both low sheen and high opacity. It can be used in exterior and interior formulations, often avoiding the need to add additional matting agent. This gives it excellent touch up properties along with good scrub resistance.

  • Mattex® PRO – This is an engineered kaolin for architectural coatings, giving outstanding scrub and burnish resistance whilst also offer excellent hiding power and very low sheen knockdown. It is ideal for paints requiring touch up properties without the use of matting agents. In this sense it helps to enhance the coatings stain and scrub resistance.


Comparison of calcined kaolin optical performance in paint

Figure 3. Visual comparison of the resultant film’s optical properties, imparted by using calcined kaolin extenders at the same volume loading, in different PVC level formulations.

Ultrex® HP is one of the newer innovations from BASF kaolin. It gives a very high level of opacity and tint strength when used in decorative coatings. High brightness levels result in whiter paints and bases that provide cleaner colours. It is suitable for us in both matt and satin paint formulations. Figure 4 compares the optical properties of Ultrex® HP with some commonly used competitor grades in a 65% PVC aqueous formulation. It is clear that large reductions in titanium dioxide loading are possible. For more information see the brochure on the Ultrex® HP product page.

Comparison of Ultrex HP optical performance in coatings

Figure 4. Charts demonstrating the high optical performance of ULTREX HP vs. competitor grades, in a 65% PVC aqueous coating.


Hydrous kaolin in non-aqueous gloss and semi-gloss formulations

Usually, the function of an extender in non-aqueous coatings is to reduce costs and increase solids loading, whilst providing specific functional benefits to the coating. This could be titanium dioxide extension, barrier properties, adhesion to the substrate, gloss control, ease of sanding and maintenance of corrosion and humidity resistance.

ASP® G90 is an excellent titanium dioxide extender in non-aqueous coating formulations. It offers high gloss levels, excellent opacity retention and the potential to extend the resin at equivalent gloss levels.

The starting point formulation in Figure 5 is for a non-aqueous alkyd resin, containing emulsified water. Lorama™ LPR 76 is used to achieve this stable water-in-oil emulsion and is also represented by Lawrence Industries. Visit the product page for more information. Gloss and opacity are then compared to a major competitor grade in Figure 6. It is clear that ASP® G90 offers outstanding opacity and gloss retention.

starting point formulation for alkyd coating

Figure 5. Starting point formulation for a non-aqueous alkyd enamel, with ASP® G90 as titanium dioxide extender.

Figure 6. Optical performance of the dry film made from the coating outlined in Figure 5, with ASP® G90 as titanium dioxide extender and gloss enhancer.


Why choose BASF and Lawrence Industries?

BASF is the world’s largest producer of calcined kaolin products and is able to offer a portfolio of innovative kaolin products. BASF and Lawrence Industries have many years of experience with these products and the formulations that they can be effectively utilised in. Get in touch with our sales team today to discuss your goals and let us see how we can help you realise them. Either give us a call or put in a sample request through the ASP®, Mattex®, Satintone® or Ultrex® product pages. You will also find more literature there for your reference.

BASF kaolin is committed to sustainable development. A prime example of this is the land reclamation initiative in Georgia. After kaolin deposits are extracted, and mining has completed, the land must be returned to its natural state and must produce renewable resources including timber, agriculture, fish and recreation areas.


General advice for titanium dioxide extension with kaolin

When working out the formulation, always replace titanium dioxide on a volume basis - as the densities are different. For 10 kg of titanium dioxide, you want to substitute with about 6.6 kg kaolin. Work out cost savings on a volume basis and then back-calculate the weight saving.

For gloss and semi-gloss paints – the higher the titanium dioxide content, the higher the replacement. Typically you are replacing up to 10% of the titanium dioxide on a volume basis. Max wt.% of hydrous kaolin is usually 5 wt%.

For flat and low sheen paints - generally more calcined kaolin is used but hydrous is also sometimes. Maximum wt. % of kaolin usually 20%.

Author: Adam Morgan , Ph.D.

Adam studied chemistry at the University of Warwick for 8 years, where he obtained a Ph.D. in the field of polymer and inorganic colloid science. He has been with Lawrence Industries since 2014 as a technical sales manager covering all industry areas. He is now responsible for marketing within the company as well.