Properties and Applications
Molecular structure of Kaolin ©molekuul.be/fotolia.com
Nanoclays originate from naturally occurring clays, which are mainly made up of finely divided mineral particles. The clay particles consist of layered silicates, which are compounds based on the elements silicon, oxygen and other elements. They can be thought of as a "stack" of two-dimensional single, double or multi layers, which are formed by corner-linked silicate (SiO4) tetrahedrons. Due to the lack of chemical bond between the layers, the stacks are able to either absorb or dissipate water and other substances between the different layers causing swelling or shrinking of the clays. This is a major difference between (nano)clays and the more rigid framework silicates or tectosilicates (see also zeolites).
The layered silicates include different clay mineral groups such as mica (K Al2 [AlSi3O10 (OH) 2]), talc (Mg3 [Si4O10 (OH)2]), kaolin (Al2 [Si2O5 (OH)4]), serpentine (Mg3 [Si2O5 (OH)4]), montmorillonite (Mg0.33Al1.67 [Si4O10 (OH)2] (Ca, Na) x (H2O)n) or sepiolite (Mg4 [Si6O15] (OH)2 4 H2O). They differ, among other things, in size and sequence of the regions in which the silicate tetrahedrons are oriented upwards or downwards in the planes. Furthermore, they differ in the nature of the intercalated ion.
Montmorillonite is the main component of bentonite and the technically most significant clay mineral. It is composed of double layers of silicate tetrahedra with embedded octahedron layers of aluminium, hydroxide and iron ions. A typical montmorillonite particle consists of approximately 1 nm thick aluminium silicate layers with lateral dimensions in the range of several hundred nanometres to about 10 micrometres, which are combined into large stacks.
Nanoclays are non-combustible materials, which therefore are used in flame and fire protection for plastics. Mixing them into the plastic reduces the overall amount of combustible material. If the plastic burns, a separating layer forms, which hinders burning progress. Nanoclays are indispensable nowadays for the electrical and electronics industry.
Due to their ability to bind caesium, nanoclays are used to reduce radioactive contamination.
For use as cat litter, a mixture of mostly zeolites, sepiolites or bentonites or montmorillonites is used. They show a high absorption capacity for water and odour-causing components from the cat urine and faeces.
Food packaging films and plastic surfaces are other application areas for nanoclays as plastic additives (not approved in the EU). The addition of nanoclays to the plastics leads to higher tensile strength, improved barrier and abrasion properties, excellent surface qualities, low thermal expansion and very good flow and processing properties.
This outstanding property profile qualifies plastic-nanoclay-composites as an interesting alternative to conventionally reinforced materials. In addition, chemically incompatible plastics can be mixed with the help of nanoclays, resulting in a plastic that optimally combines the properties of both plastics.
Powder from nanoscale nanoclays is not self-igniting. Nanoclays mixed with air (dust) are also non-flammable under the influence of an ignition source, so there is no possibility of dust explosion.
Occurence and Production
Montmorillon ©Jean Paul Bounine / Fotolia.com
Naturally occurring sedimentary rock bentonite, which consists of 60 to 80 percent of montmorillonite, is the major source for industrial nanoclay production. The name "bentonite" originates from a clay deposit near Fort Benton in the US state Wyoming and describes an extra-fine clay mineral produced by the weathering of volcanic ash. Important European deposits for bentonite are found on the Greek island of Milos, in Turkey and in Germany. One of the largest German mining areas for the mineral is located in Bavaria. Montmorillonite is named after a deposit at Montmorillon in southern France. Montmorillonite clays are formed by the weathering and transformation of volcanic ash, tuff and alkaline rocks (e.g. basalt).
Many applications and products from the cosmetics and agricultural industry or even cat litter producers preferably use natural forms of bentonites.
The swelling capacity in water makes natural bentonites ideal materials for applications in foundry, construction and drilling industries as well as in the food sector. Most often, alkaline activation is carried out to further improve the desired properties. In this process, the cations (magnesium, aluminium, calcium) of the intermediate layers of montmorillonite are replaced by sodium ions. Industrial production steps include first the kneading activation of bentonite with soda (NaCO3) followed by a drying process and subsequent grinding of the sodium montmorillonite.
Bentonites are used in their acid-activated form in paper production and as catalysts. These bentonite forms are used whenever a large surface area is required for their adsorption properties. The reaction with acids facilitates the removal of cations (magnesium, aluminium, iron) from the intermediate layers, which are then replaced with hydrogen cations. Depending on the degree of acid activation, specific surface areas of up to 450 m2/g can be achieved, which is very important for the use as catalyst.
Organophilic modified bentonites are used to adjust the flow properties of oils, fats, ointments and plastics. Due to the organophilic modification, the surface of the bentonite is converted from originally water attracting to water-repellent, which allows for a complete distribution/dispersion of the individual layered silicate platelets in organic (water-repellent) solvents. This is important if you want to use water-repellent nanoclays for adjusting the flow properties of solvent-based paints and varnishes or integrate them into in plastics.
Naturally occurring halloysite nanoclays could be used e.g. as carrier for controlled drug delivery or as a means for adjusting flow characteristics. The mineral consists of double-walled tubes of aluminium silicates with dimensions of 15 × 1000 nm and is chemically similar to kaolin.
The global market for nanoclays is growing and is estimated to amount to more than 3.3 billion US dollars in 2023.