Everyone is talking about nanoparticles these days. We hear about the exciting new uses opening up for minerals in the nanoparticle size range, from revolutionary new sunscreens to self-cleaning windows, to amazing new uses inside the human body. It is almost like a new industrial age – the world is going sub-microscopic!
A nanoparticle is usually defined to be in the size range 0 – 100 nanometers (1nm = 10-9m) and 100 nanometers is 0.1 micron. At present, the average particle size of two of our typical kaolin grades is 2 microns and 1 micron. The following schematic diagram gives you some idea of the relative scale of the particle sizes we are talking about:
We do not have a nanoparticle kaolin grade to offer commercially, but we have made some interesting discoveries about kaolin nanoparticles. For example:
Actually, all nanoparticles are invisible if spread into a film that is roughly one particle thick. The particles are simply too small to be seen by the human eye (or more correctly, nanoparticles are 20 times smaller than the wavelength of visible light, so they scatter light to an insignificant extent). This is why nanoparticles of titanium dioxide and zinc oxide are used in sunscreens these days – when spread out onto the skin, you cannot see them. Surprisingly, since titanium dioxide is so famous for its opacifying power! But put a whole lot of nanoparticles together in a pile, and you will certainly be able to see them. A thin film of dirt-rejecting nanoparticles on glass will not affect the transparency of the glass. A mineral that is generally opaque (like kaolin and titanium dioxide), can be put into wood finishes when in nanoparticle form, and the wood finish will remain transparent when applied.
When the particles this small and are suspended in water, the effect gravity has on them becomes smaller than effect of the Brownian motion of the particles – they just never settle! They are endlessly moving around and interacting in perpetual motion….
Water-washed kaolin is usually filtered using a fine filter cloth to produce a filter cake for further processing. The rest of the water passes through the filter cloth.
If you try to filter a slurry of nanoparticles, they immediately clog up the filter cloth. Try as you might, no more filtration can occur. So a whole new thinking is required if you want to get a dry powder from a slurry of nanoparticles. Spray drying is probably the only way to go.
Due to their tiny size, a kilogram of nanoparticles will have a very high surface area. This is something that allows nanoparticles in general to have many interesting new properties, most importantly a much higher reactivity than larger particles. This is sure to revolutionize many chemical reactions, which will take place far quicker than ever before. Since kaolin is generally unreactive, the implication for kaolin relates more to having increased powers of absorption. However a high oil absorption is not always desirable. In applications where the clay is mixed with a polymer, the increased surface area means that the clay functions as a specialized additive rather than a filler and allows the composite to have increased strength at lower addition rates. An added advantage is that the material is then less brittle.
I am sure you can imagine how easily such small particles would be launched into the air when you are opening a bag of them or pouring it into your vessel. Breathing such fine particles is not good for the lungs. Careful handling and dust extraction systems must therefore be considered if you want to use nanoparticles.
Some producers have overcome the health and safety issues by encapsulating the nanoparticles inside another material, such as a gel.
So before we get excited about all the fascinating new uses for nanoparticles, we have to step back and consider some of the practical implications. The producer has challenges like how to dry the powder and the user has challenges like how to handle such a product.
New companies are springing up dealing solely with nanoparticles. For example, has a look at: www.nanophase.com
We are watching developments in this field with interest!