A bright future for halloysite?

Halloysite can be used in sun screens, nail varnish and toothpaste. (Source: Gerry Lauzon/Flickr)

Halloysite is a naturally occurring alumino-silicate clay mineral with the chemical formula Al₂Si₂O₅ (OH) ₄. It is similar to kaolinite, but crystalises as hollow tubes rather than flat plates, or "books" of plates. Because the tubes are very small they are known as halloysite nanotubes (HNT). These are usually formed by the alteration of feldspars and/or micas in acidic granites or volcanic rocks by hydrothermal and meteoric water.

Tube morphology varies from one halloysite deposit to another. Tube length is typically 0.1 to 5 microns, but can reach 10 microns or longer. External diameters can be anything from 50 to 200 nanometres, while the hollow lumen is typically 15-20 nanometres.

The purity of commercial halloysite varies depending on the geology of the deposit and refining techniques employed. Properties such as tube morphology, chemistry and brightness determine the mineral’s markets. Traditional whiteware ceramics, for example, require low iron oxide (Fe₂O₃) and titanium dioxide (TiO₂) content, while medical applications require highly pure, non-toxic material with a high (typically >90%) halloysite content, low levels of heavy metals and zero dioxins.

TEM and SEM images of HNTs. (Source: I-Minerals Inc.)

Commercial halloysite suppliers

The number of halloysite suppliers globally is relatively small. Some well-established halloysite are shown in Table 1.

Large scale suppliers in China, Brazil, Thailand and Argentina sell halloysite or halloysite/kaolin blends, mainly for ceramic applications. Small deposits have been identified in Kosovo, Korea, Japan and elsewhere.

In terms of new supply, US-based Applied Minerals Inc. recently commenced halloysite refining activity at the Dragon Mine in Utah, while Canada-headquartered I-Minerals Inc. is developing a new deposit in Idaho, with first production expected in early 2018.

High quality tableware ceramics are the most important market for halloysite (Source: Christoph Kaiser/Flickr)

Traditional ceramics applications

The most important market for halloysite is high quality tableware ceramics, including porcelain, bone china, fine china and glazed items. Its low Fe₂O₃ and TiO₂ content provides high whiteness and translucency after firing, which, along with chip resistance, texture and porosity, determine the finished quality of ceramics.

Virtually all manufacturing companies prefer to use a blend of halloysite and kaolin, rather than 100% halloysite, in their recipes, to reduce costs and improve workability. Typical raw material mixes and firing temperatures for ceramic tableware are shown in Table 2.

Porcelain is manufactured using high purity kaolin and halloysite and has a low feldspar content compared to bone china. It is thus more refractory and is fired twice at higher temperatures. Finished porcelain is non-porous and translucent with a cool white colour.

Bone china comprises approximately 50% bone ash produced from de-gelatinised cattle bones, which have been calcined at 1,250⁰C and milled to powder. It is possible to make thinner, more delicate pieces from bone china than from porcelain, which are white and translucent but comparatively brittle.

Fine china is manufactured using a similar raw material blend to porcelain but is only fired once at a lower temperature. The finished product is opaque with a warmer white colour.

Halloysite can also be used in glaze preparation for all types of tableware. The fine particle size provides good suspension properties and the low Fe₂O₃ and TiO₂ content helps to produce a clear transparent glaze.

Halloysite is an established raw material for making honeycomb catalyst supports for vehicle exhaust systems and molecular sieves (often as a mixture with zeolite) for applications such as water purification and separation of gaseous and liquid mixtures. 

Prices range from approximately $250/tonne for high volumes of Chinese material up to around $650/tonne for high performance, white firing New Zealand halloysite.

China is the world’s largest producer of porcelain and Asia is the largest market for halloysite, with high demand growth expected in China, India and the Middle East. French industrial minerals producer Imerys SA supplies significant tonnages to Europe, but volumes have declined since 2008 as many end users exited the market, moved to Asia or switched from halloysite to less expensive kaolin.

Demand prospects for HNTs are changing as potential applications in non-ceramic industries are recognised. During the last 10 years, there has been a significant amount of new research across the world into HNTs, with special attention being paid to their morphology. New markets have been identified where HNTs can provide significant benefits and command higher prices, notably as an alternative to carbon nanotubes (CNT), which are expensive and come with health concerns linked to respiratory conditions.

Two of the largest opportunities for HNTs are in polymers and life sciences. Prices for HNTs used in these applications are expected to be around $2-3/kg, compared to prices for CNT which can be more than $500/kg.

Polymers

The long tubes and high aspect ratio of HNTs provide significant strength improvements when added to polymers such as polypropylene and nylon. Fire resistance is also improved due to a barrier effect and release of water of hydration. Where HNTs replace glass or rock fibre additives, there is a reduction in energy consumption, as these materials require melting temperatures of around 1,400⁰C prior to spinning or extruding the fibres.

HNTs added to polyethylene terephthalate (PET), high-density polyethylene (HDPE), polyvinyl chloride (PVC) and polystyrene used for food packaging acts as a barrier which helps to keep carbonated drinks fizzy and prevents ingress of air and water vapour to products such as milk, extending shelf life. Tubes can also be loaded with anti microbial chemicals.

Plastic pellets containing halloysite (Source: Dreamstime)

The zeta potential (a measure of the magnitude of the electrostatic or charge repulsion/attraction between particles, affecting the stability of a material expressed as mV" of HNTs is low at approximately -45mV, which allows for good colloidal stability and dispersion in polar polymers, without the need for expensive surface modification.

NaturalNano, a US nanomaterials company, imports halloysite to its plant in Rochester, New York, where it is further refined to remove impurities prior to incorporation into its Pleximer range of polymers. Some Pleximer grades contain 30% HNT and are sold to plastics manufacturers who re-melt and dilute to their required specifications.

Life sciences include the cosmetics, medical, environmental, agriculture and animal feed industries.

HNTs are a "generally regarded as safe" (GRAS) group of materials and are biocompatible. They meet EU regulations for use in cosmetics and animal feedstuffs and are Registration, Evaluation, Authorisation and Restriction of Chemicals (REACH)-exempt. HNTs are not biodegradable and therefore not suitable for intravenous injections, however.

The ability to load the hollow tubes for slow release of chemicals has led to opportunities in numerous fields, including pharmaceuticals (delivery of drugs); paints (delivery of corrosion inhibitors); cosmetics (delivery of moisturisers and fragrances); and marine oil spills (delivery of surfactants).

HNTs carry a negative charge on the outer tube, which contains silicon dioxide (SiO₂) and a positive charge on the inner lumen surface, which is composed of aluminium oxide (Al₂O₃), which promotes loading with negative molecules. Lumen diameters are also the right size to absorb large molecules such as proteins.

Researchers at Louisiana University in the US have developed techniques to increase the volume of the hollow lumen by etching it with acid and to extend release time by capping the ends of the tubes.

Elsewhere, a team at the James Hutton Institute in Scotland has come up with a statistical procedure using a scanning electron microscope (SEM) to accurately measure tube length and diameter.

UltraHallopure impregnated onto textile fibre for wound dressings.  Plastic pellets containing halloysite (Source: Dreamstime)

According to I-Minerals, its HNTs are longer and have bigger internal and external diameters than competitive commercial products, offering potential advantages through their higher aspect ratio for use in polymers and higher loadings of chemical solutions for prolonged slow release in life science applications.

In the cosmetics sector, HNTs are suitable for use in face creams owing to their high oil absorption capacity, which helps to remove grease and toxins to cleanse facial pores, while the hollow tubes can release moisturiser such as glycerol. They can also be used in sun screens, nail varnish (to improve adhesion and flow) and toothpaste.

In May 2012, NaturalNano announced a three-year exclusive agreement and licence to supply French nail polish makers Fiabila with HNTs for use in nail-care products.

There is growing demand for non-woven textiles used in sanitary items such as nappies and incontinence products. Demand for the latter is increasing in line with rising life expectancies. According to packaging producer Smithers Pira, global sales of disposable nonwovens rose from $10bn in 2010 to $14.1bn in 2014, at an annual growth rate of 7.1%. Hygienic applications represent the largest sector of disposable nonwovens and HNTs offer a number of improvements to these fabrics, including odour removal by gas absorption, particularly effective for ammonia, fluid absorption and potential for slow release of fragrances or medicines.

In February this year, I-Minerals announced that its UltraHallopure halloysite had been used in the development of a new wound treatment by the Institute of Diabetes in Karlsburg (IDK), Germany. This treatment utilises a new technology developed by DURTEC GmbH in Neubrandenburg, allowing HNTs to be impregnated into textile fibres. 

HNTs can also be added to polymers such as polymethylmethacrylate (PMMA), which are used to make dental implants. HNTs form a scaffolding structure, improving mechanical strength as well as helping the implants bond to the gum and accelerating UV cure, but HNTs face strong competition from synthetic TiO₂ nanotubes in this field.

Research suggests that adding 5-8% HNTs to orthopedic-grade PMMA bone cement used in hip and knee replacement surgery can enhance structural integrity. Additionally, HNTs can be loaded with an antibiotic such as gentamicin for sustained release for up to 400 hours. Adding HNTs to calcium phosphate cements offers similar improvements.

Tissue engineering is a new approach to treating damaged or lost organs using patients’ own cells, grown on a polymer support so that tissue parts can be regenerated. This scaffold serves as an adhesive substrate for the implanted cells and as a physical support to guide the formation of the new organs.

HNTs can be added to biodegradable polymers like PVA chitosan or poly-lactic acid for the preparation of tissue engineering scaffolds to improve mechanical strength. Research into this area is being conducted at the Slovak Institute of Technology in Bratislava. 

In cancer treatment, researchers at Cornell University in the US have improved the capture rate of circulating tumour and leukemic cells using HNTs coated onto special micro tubes, which selectively target, capture and destroy cancer cells, delivering chemotherapeutics to circulating tumour cells in the bloodstream, avoiding healthy cells.

Environmental protection

Geomembrane installation is used as part of the construction of a base liner system of a landfill. (Source: Alan Levine/Flickr)

Due to their hollowness, high surface area and high porosity, HNTs have the ability to absorb toxic materials from both fluids and gases. They can be incorporated into honeycomb catalyst supports used in the exhaust systems of motor vehicles to clean exhaust gases, molecular sieves used to separate liquids and gaseous mixtures and for water purification.

HNTs can absorb toxic gas such as carbon dioxide, hydrogen sulphide (H₂S) and ammonia (NH₃) and examples of application areas include biogas plants to remove H₂S and NH₃; foundries for the fixation of amine emissions; and farms for reduction of sulphur compounds, amines and pathogenic bacteria.

In fluids, HNTs can help remove dyes used in textile, paper or leather making from watercourses, as well as heavy metals from mining activities.

In landfill management, geosynthetic clay liners consisting of two layers of woven fabric usually filled with bentonite absorb multivalent toxic metals from runoff and reduce permeability, slowing the rate of leakage. However, if the water flow stops, the bentonite may dry and crack, allowing contaminated water to pass through untreated, once the flow resumes. Blends of bentonite and halloysite have been reported to prevent this problem and provide improved metal removal, as HNTs have a strong affinity for monovalent cations, unlike bentonite.

HNTs can be added to polymers such as polyacrylonitrile in water filtration membranes to provide reinforcement and improved filtration performance. Research suggests that HNTs can also be effective in containing certain forms of nuclear contamination and in cleaning up marine oil spills by pre-loading the tubes with surfactants, which are slowly released to reduce surface tension and help disperse the oil as very small droplets, which eventually biodegrade.

Agriculture

HNTs loaded with fertilisers allow nutrients to be released slowly, potentially reducing problems associated over use and run off. They can also be used for the application of pesticides and herbicides.

When added at low dose rates of approximately 0.5% into animal feedstuffs, mycotoxins are absorbed and safely removed from the animal’s system. HNTs and kaolins are especially effective in removing Aflatoxin, the most prevalent and toxic mycotoxin.

By absorbing ammonia contained in animal faeces, HNTs can prevent leg burns in poultry and reduce odours.

^{*Frank Hart is the owner and director of First Test Minerals Ltd.}

^References:

^{Saif and Asif from Journal of the Chilean Chemical Society}

^{Clay Nanotube/Poly(methyl methacrylate) Bone Cement Composites with Sustained Antibiotic Release by Wenbo Wei, Elshad Abdullayev, Anne Hollister, David Mills and Yuri M. Lvov}

^{Selected properties of the halloysite as a component of Geosynthetic Clay Liners (GCL) by P. Sakiewicz, R. nowosielski, W. Pilarczk, K. Golumbek & M. Lutynski}