Home Education Fillers for Polymer Applications
Now largely removed from the market because ofsevere health issues, asbestos is an example of a naturally occurring nanofiber. Asbestos is the name given to a family of hydrated metal silicates, of which only two, chrysotile and anthophyllite, have been of any major significance in composites. Chrysotile is a highly hydrated magnesium silicate, with the formula Mg6[(OH)4Si2O5]2 and anthophyllite has the formula (MgFe)7[(OH)Si4On]2. They are both crystalline materials with fibrous particles which can be broken down to very fine fibrils 15-90 nm in diameter.
The health issues associated with asbestos are now well known, but help to account for the great caution exercised over the safety of new nanofibers. There is no useful recent literature on asbestos in composites, but Axelson (1987) is a useful introduction to the topic.
Halloysite (CAS: 1332-58-7)
Halloysite is one of a number of silicate minerals which exist as hollow tubes (Pasbakhsh and Churchman 2015) and has received some interest as a polymer additive.
Halloysite is an aluminosilicate mineral, the same as kaolinite in composition, but with a more disordered stacking of the plates and the ability to form rolled up, hollow tubes. While relatively rare, significant workable deposits are found in several places including the USA and New Zealand.
The shape and purity vary from deposit to deposit and according to the processing used. The tubular, high aspect ratio, particles of most interest for composites production are ~1-2 pm long, 50 nm across, and with a 15 nm hole or “lumen.” The surface area depends on processing and is in the range ~50-150 m2g-1. The outside of the tubes has a siloxane (Si-O-Si) surface chemistry while the inside has an hydroxylated aluminum surface chemistry.
Unlike carbon nanotubes, halloysite tubes are open ended, allowing some molecules to enter. The central lumen accounts for 20% of the particle volume and this has an effect on the effective density when it is encapsulated within a matrix. For example, if the polymer fills the inside of the tube then the effective density of the halloysite is 2.54 gcm~3. In the case where it is not filled by the polymer, then the effective density of the halloysite is 20% less, i.e., ~2.00 gcm~3.
Appropriately processed halloysite particles can be used to reinforce all types of polymer and also to produce special effects such as polymer crystal nucleation and flame retardancy. More information can be found in the review by Du et al. (2010) and in the Dragonite Handbook from Phantom Plastics (http://phantomplastics.com).
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