Halloysite Clay Nanotubes
Halloysite is a member of the kaolinite family of aluminosilicates but, whereas kaolin particles are platy in shape, halloysite particles are nanotubes, where the aluminosilicate sheets are rolled up like a scroll. The well-known mineral kaolin and the lesser known halloysite share the exact same chemistry and can be difficult to distinguish from one another without the use of special chemical tests, or x-ray methods.
While synthetic nanotubes have been the subject of much attention, they have proven expensive to manufacture. The idea of a natural nanotube has great appeal to the scientific community but also to industry. It would seem clear that a nanotube, with a price similar to kaolin, would be a hit.
The tubes vary depending on the deposit from which they come. By far the most well-known is Imerys’ New Zealand deposit. Imerys is a world-leading producer of minerals and they have sold high purity halloysite, mainly for fine china and porcelain, for many years. Many other deposits have been reported all around the world. During my tenure as CTO of a halloysite company, I counted around 50 deposits, of which perhaps five or so seemed potentially viable. I collected and read over 700 articles and patents on halloysite to become the leading expert on the subject. Some of that information will be outlined here.
Fine china and porcelain
Halloysite has long been used and is a ideal because of high whiteness, which gives a translucent article once fired. The tubular shape helps prevent sag, although it can also increase shrinkage. Large amounts of halloysite are supplied into the Asian market from the New Zealand deposit.
Reinforcement for plastics
Minerals are well-known as fillers for plastics. Those with a high aspect ratio, i.e. ratio of length to width, are effective at increasing both stiffness and strength of the polymer. In terms of aspect ratio, halloysite is similar to for example, a standard talc or wollastonite and inferior to high aspect ratio talc, mica, or glass fiber. Quite simply, it cannot compete in cost or performance with those latter products. In a few instances, halloysite does give potentially viable reinforcement, particularly at very low loadings, around 1 weight% in nylon 6. That is believed to be a result of alteration in the crystallization behaviour of the nylon.
Nucleation of polymer crystallization
As mentioned above, halloysite can nucleate crystal growth in thermoplastics. In fact, it nucleates a very wide range of plastics including PE (very rare), PP, polyamides and some polyesters (PBT, PLA). Nucleating agents are big business so this effect could be commercially applicable assuming the halloysite can be competitive with incumbent solutions. There is a patent on nucleating polyethylene using halloysite and the only other commercial nucleating for polyethylene is rather costly.
Additive to polymer foams
It is common to add mineral particles, such as fine calcium carbonate, or talc, to facilitate manufacture of polymer foams used for insulation, cushioning and so on. The filler particles provide a surface for the gas bubbles to grown on. This encourages more and finer bubbles, thus engendering a more even (homogeneous) foam with better mechanical properties, insulation performance and lower density. Due to its asymmetrical shape with sharp edges and high surface area, halloysite seemed an excellent candidate for this application.
Extended release of active ingredients
Halloysite tubes are effectively nanocapillary tubes. Capillaries are well known for their ability to suck up liquid, in much the same way a paper towel does. It turns out that the finer the capillary, the more force there is to attract and hold the liquid. The nanosized capillary inside each halloysite tube can be filled with liquid by simply putting the tubes in the liquid and applying mild vacuum to remove the air from inside the tubes. Once inside, the liquid is held there by capillary action. As an example, glycerol, a common humectant in cosmetics, can be released over a period of 24 hours or more to create an all day moisturizing effect.
Fire retardant for plastics
Halogen free, mineral based fire retardants for polymers are big business. Popular products include aluminium trihydrate (ATH), magnesium hydroxide (MDH) and huntite/hydromagesite (HMH). They operate primarily by liberating water when heated and absorbing heat during their decomposition. Both effects help to quench the flame. Halloysite does release some water, although much less than the other minerals mentioned. Halloysite is also a char former because the surface is catalytic causing the polymer to cross-link and char. Although initially considered a promising use for halloysite, it has not proven commercially attractive and interest has dwindled.
Halloysite was sold in the thousands of tons per year amounts as a cracking catalyst, until it was superseded by newer technology. Clearly then, it has a catalytically active surface. This turns out to be a major problem in polymer applications leading to discoloration (yellowing) and even degradation of the polymer. The problem is the extreme acidity of the mineral surface which may seem odd because the pH in water is almost neutral. Although not widely known, it turns out that as the surface of kaolin is dried, the acidity of the surface sites increases to reach levels equivalent to 90% concentrated sulfuric acid. Although the results published are for kaolinite, halloysite has the same chemistry but higher surface area, further exacerbating the problem.
The applications where halloysite has proven to be commercially viable, are ones where the addition level is very low, i.e. 1 weight % or so, or the application itself is niche. So, although halloysite is rare compared to commodity minerals, to date there has been more than sufficient supply to meet demand. Pricing is an important topic.
Halloysite is an unusual and rare natural nanotubular material. Although it does have interesting and unusual properties, new applications in polymers and coatings have been low volume and easily satisfied by existing supply capabilities. So far, the larger potential applications have been thwarted by degradation of the polymer or failure to outperform other incumbent alternatives.