HALLOYSITE CLAY NANOTUBES
Most natural clays consist of stacks of clay nanoplatelets that can be separated from each other (exfoliated). A vast amount of literature exists about all aspects of the science, technology, and applications of platy clays.
Halloysite, by contrast, consists of bundles of clay nanotubes that can be separated from each other (exfoliated). It is found in much fewer locations than platy clays. It is much less familiar than platy clays. Its applications are at an earlier stage of development than the applications of platy clays.
The historic Dragon Mine deposit is the only known measured resource of halloysite in the Western Hemisphere significant enough for large scale commercial production. Applied Minerals, through its ownership of this mine, is the leading global source of halloysite. Its products are marketed under the Dragonite™ tradename. The Dragonite™ Handbook is a valuable comprehensive resource for information about halloysite. All of the numbered figures and tables in this post were reproduced from this handbook. The sources of the remaining images will be identified while discussing them.
Figure 5 shows samples of exceptionally pure halloysite ore and powder from the Dragon Mine. Figure 1 shows a scanning electron micrograph (SEM) of a sample of pure, as-mined halloysite, from which the presence of clay nanotubes can be observed. Figure 21 shows a transmission electron micrograph (TEM) from which the fact that these nanotubes are hollow can be observed. Figure 2 illustrates the structure and chemistry and Table 14 summarizes the properties of halloysite.
Many applications of halloysite have either been developed or are under development:
- Polymer reinforcement. Some of the effects of incorporating halloysite into various types of polymers are summarized in the Table pasted below.
- As a non-halogenated reinforcing flame retardant.
- Has the ability to provide flame retardancy that reaches the V0 standard.
- Can be used by itself.
- Can be used as a synergist in conjunction with other non-halogenated materials such as magnesium hydroxide, aluminum trihydrate, glass fibers, and organoclays.
- Provides benefits as a reinforcing agent along with the benefits of flame retardancy.
- As a carrier and controlled release medium for active pharmaceutical ingredients.
- The tubular nature of halloysite enables a product to maintain uniform and sustained release rates of an active agent, thereby reducing application frequency and initial overdosage.
- Release rates can be controlled through selection of tube diameter, length, and encapsulation.
- As a non-irritating skin cleanser with a hollow inner tube capable of serving as a carrier and controlled release medium for cosmetic agents for long-lasting application.
- As an eco-friendly carrier and controlled release medium for agricultural agents.
- Utilizing its inner lumen as a natural reservoir, halloysite nanotubes can load, store, and achieve controlled release of a range of agricultural agents uniformly and in appropriate dosages.
- Release rates can be tuned to match the duration of a growth or reproductive cycle, eliminating the need for frequent spraying or application by the end-user.
- The agricultural agents may include pesticides, fertilizers, essential oils, insecticides, fungicides, herbicides, nutrients, growth stimulants, vitamins, and hormones.
- Environmental remediation.
- Halloysite performs as an effective sieve to sequester pollutants released from a variety of sources such as oil spills, power plants, or mine sites.
- Once captured, the pollutant is immobilized in a solid form where it can be collected and disposed in an eco-friendly manner.
- As an additive which can provide many benefits in paint and coating
- A drop-in additive that creates sustained release protection against microbial growth and corrosion. Utilizing its inner lumen as a natural reservoir, Dragonite can load, store, and control-release a range of biocides uniformly and in appropriate dosages.
- The product disperses easily in a wide range of carrier materials and can release single or multiple agents at predetermined rates.
- Serving a dual-purpose, the addition of halloysite may also simultaneously provide the benefits of a functional filler, dramatically improving the mechanical properties.
- In addition, halloysite acts within UV curable systems as an optical fiber to conduct light into the coating to insure proper cure throughout the material depth.
- Molecular sieves, hydrocracking catalysts, and catalyst supports.