TiO2 dispersion in water-based coatings

A few years ago, Hunstman announced they were closing their titanium dioxide plant in South Africa. This put the industry into a bit of a spin. Paint chemists suddenly had their hands full scrambling to approve alternatives for their product range.

Then just when they thought their TiO2 work was over, there are price increase announcements every month!

This keeps the focus firmly on how best to utilise that costly TiO2. Is it being dispersed properly and are there still ways to reduce TiO2 without loss of properties?

As Paul Dietz of FP PIGMENTS puts it, there is no alternative to TiO2. It is by far the most effective white, inert, non-toxic pigment for producing opacity. The rutile form is better at that than the anatase form. However it is in waterbased coatings that TiO2 is often poorly dispersed, and flocculation can occur. How can we prevent this happening so that we do not lose that expensive opacity?

The first thing to check is the pH. The titanium dioxide particle has a different surface charge depending on its surface treatment and the pH of the dispersion. It also has no charge at all at a certain pH. That point is called the isoelectric point.

 

This diagram shows why it is important to keep the pH high enough to keep all the particles negatively charged. Then they will they repel each other and not flocculate.

It pays to be aware of how the pH is affected as each component of the formulation is added to the millbase. For example a low-pH binder added during let-down can cause localised pH shock. This may push some of the TiO2 particles through their isoelectric point and cause them to flocculate.

 

These days however, formulators don’t just rely on pH control for dispersion. They will also make use of commercial dispersants to disperse and stabilise their suspension.

Which dispersant should one use then, and how much should one add?

There are so many dispersants on the market that this can be a daunting task. To simplify matters, we can divide dispersants for TiO2 into two groups: those providing charge stability (usually anionic) and those providing steric stability. Often both types are used. This makes finding the optimal dose quite challenging.

Fortunately, there is a pretty reliable technique for this. You need to draw up a dispersion demand curve for each dispersant in your particular dispersion. This involves adding incremental amounts of dispersant, milling, measuring the viscosity, then plotting the curve as shown in Figure 3.

 

The dispersant demand is the amount of dispersant at the lowest point of the curve. Normally in practice this figure gets doubled. However, if you use the same ratio of pigments and extenders as in your formulation, you only need to increase by around 10-20%. Typical dispersant levels are 0.5-1% of the TiO2 weight. We find that higher levels of dispersant are required when hydrophobic extenders like talc are used.The dispersant demand when using two dispersants can be measured by first adding one until you find lowest viscosity. Then immediately add the other one and continue plotting the graph as shown here: 

 

Finally a word of advice about the mixer. The guys at FP PIGMENTS in Finland have found that to get efficient milling – a rolling doughnut effect if you are lucky – the geometry of the mixer is very important. See Figure 5.

 

Most operators will base their milling on the blade size. They may, however, overlook increasing the blade size when they increase the pot size. It is important to maintain the above ratios when changing anything. Bearing this in mind, it is still advisable to follow the recommendations of the mill supplier.

Once you’ve paid attention to the above issues, the next question arises. Are there still ways to reduce TiO2 when you’ve already optimised formulations to the hilt, using the best extenders on the market? 

The answer is Yes. The opacifying pigment FP 440 can offer further TiO2 savings without loss of performance. It does not replace other extenders; it replaces up to 20% TiO2.