Is there a change in flammability and explosiveness of a material in its nanoform?
Yes. Hazardous properties like flammability or explosiveness of certain substances or materials also apply to their nanoform. Here the material reacts with the oxygen from the air (oxidization) and releases energy in form of radiation / heat or as a shock wave (explosions). Especially oxidable metals and metal powders have this pyrophoric (spontaneous combustion) property.
The smaller the particle size of a flammable substance is, the easier it is to ignite the material thereby increasing its flammability. Nanomaterials have a lower ignition temperature compared to microscale particles and can be oxidized faster due to the higher specific surface area. The same is true for the explosiveness of flammable powdered nanomaterials. First the flammable nanomaterials and their respective agglomerates have to be finely distributed in the air (dust formation). Then, the air-nanomaterial-mixture is ignited resulting in an explosion. Usually, significantly less minimum energy is required to ignite nanoscale materials compared to their macroscale form.
Standardised testing methods like DIN EN 13501 are used to assess the flammability o different materials. In 2016, a new ISO standard was published for the testing of explosive materials (ISO/IEC 80079-20-2:2016).
Further information on this topics can be found online:
- German Federal Institute for Occupational Safety and Health (BAuA) – Fire and explosion hazards
- German Social Accident Insurance (DGUV) – Nanotechnology
- Swiss Federal Office of Public Health (FOPH) – Nanotechnology
How can it be explained that nanoparticles change their optical properties?
Question: “Why, for example, are titanium dioxide nanoparticles transparent once they have reached a certain size?”
This is due to a physical effect. Any object that is clearly smaller than the wavelength of the visible light is invisible. Visible light is composed of wavelengths in the range of approximately 380 – 790 nanometers.
Particles that measure e.g. 100 nanometers are not visible anymore. This, however, happens only under extraordinary conditions: As soon as several particles are found one in front of or beside the other, they (normally) take on a white color and become visible again due to e.g. diffraction or dispersion. In spite of this, not all of the particles’ chemical and physical properties change at the nanolevel. Absorption properties, for example, persist i.e., the particles do not reflect light anymore, thus are transparent but actually absorb UV radiation.