While we tend to see this as a disadvantage, there are in fact some applications where it can be an advantage – take making paint, for example. The paint chemist’s nightmare is to have his paint settling in the can, so he makes sure to add a thickener to avoid this at all costs. However if he chooses to use a filler which has good natural viscosity, this helps to keep the solids in suspension. He can then use a bit less thickener, so there’s a bit of a cost saving to be enjoyed, plus the continued peace of mind about that dreaded “in-can” settling problem.
So a high viscosity goes hand-in hand with better suspensibility, which makes sense really. We have a customer making slips for ceramic applications – this is a suspension of solid particles including kaolin, which is poured into a mould to form a skin and eventually the ceramic body of the final article. Once the skin is formed, the excess slip is poured out and the article dewaters (dries) inside the mould. Here it is also vital that the solids do not separate and settle out before or during the slip casting process – again viscosity plays an important role. Interestingly, they find that increasing the pH also helps to keep the solids in suspension.
However big sanitaryware factories that are under production pressure may find that if the slip is too viscous, the sanitaryware dewaters too slowly. So they will avoid a highly viscous kaolin in favour of a less viscous one from another kaolin deposit.
What causes kaolin from one deposit to have a higher viscosity than kaolin from another deposit? Well this is quite a complex topic, but it seems a disordered kaolin will be more viscous than an ordered kaolin – some kaolin is disordered, which means the kaolin platelets are all higgledy-piggledy and not arranged in nice neat booklets. So when forced into close proximity, the platelets start to interact with each other in almost a gel-like fashion, forming bonds and starting to get some sort of structure going in solution, hence the resulting viscosity. But give this solution some SHEAR, like blast it with a high-speed mixer, and hey-presto, the bonds are broken and the solution goes all watery again. This is typical of a shear thinning, non-Newtonian fluid such as a kaolin slurry – or a water-based paint for that matter. For comparison, a Newtonian fluid’s viscosity is not affected by shear (e.g. maple syrup, which does not get thinner the quicker you stir it).
Well you gotta either love it or hate it, this viscosity thing. You just can’t get away from it. Even the adhesive chemist needs to keep his filler in suspension- he doesn’t want a layer of solids forming in the bottom of his can either!
So if you want to increase your viscosity, the answer is obvious – add more kaolin (I’m joking, – that is only one possible solution). However if you are using kaolin already and you want to reduce the viscosity, some things to try are:
- Add or optimize your dispersant/deflocculant
- Apply some shear
- Add a pinch of salt. No really, we stumbled on this unexpected result quite by accident. The viscosity of a stirred kaolin slurry goes down as you add NaCl or Na2CO3, and then plateaus out (now you learnt something!).
You know by now to Contact Us to chat about your filler requirements.