Expandable graphite
Expandable graphite , also called expandable graphite , is made from the naturally occurring mineral graphite . A graphite flake consists of layers of honeycomb carbon atoms . Within the layers, the atoms are very firmly connected by covalent bonds . There are only weak binding forces between the layers, so that molecules can be embedded ( intercalated ) between the graphite layers . The storage of acids, usually sulfuric acid , converts graphite into expandable graphite.
properties
If expandable graphite is heated, the graphite flakes expand, depending on the quality, from a temperature of approx. 140 ° C to a multiple of the original volume. As the embedded compounds evaporate, the graphite layers are driven apart like an accordion. The expanded flakes have a "worm-like" appearance and are usually several millimeters long.
| Physical Properties | |
| Carbon content | 85-99% |
| Expansion rate | 30-400 cc / g |
| Particle size | 80% <75 µm - 80%> 500 µm |
| Starting temperature | 140-230 ° C |
Manufacturing
To produce expandable graphite, natural graphite flakes are treated in a bath of acid and oxidizing agent . Commonly used oxidizing agents are hydrogen peroxide, potassium permanganate and chromic acid, although the latter can have after-effects. Concentrated sulfuric acid or nitric acid are mostly used as the compound to be stored, the reaction taking place at temperatures of 30 ° C. to 130 ° C. for up to four hours. After the exposure time, the flakes are washed with water and then dried. The starting temperature and expansion rate depend on the fineness of the graphite.
Applications
Flame retardant
One of the main applications of expandable graphite is flame protection. When exposed to heat, expandable graphite expands and forms an intumescent layer on the material surface . This slows down the spread of the fire and counteracts the most dangerous consequences of fire for humans, namely the formation of toxic gases and smoke.
Graphite foil
Foils made of pure graphite can be produced by compressing expanded graphite. These are mainly used as thermally and chemically highly resistant seals in chemical plant construction.
Expandable graphite for metallurgy
Expandable graphite is also used in metallurgy to cover melts and molds. The material serves as an oxidation protection and insulator.
Expandable graphite for the chemical industry
Expandable graphite is included in the chemical processes for lacquers and paints.
literature
- Simon Moores: Industrial Minerals: Natural Graphite Report 2012, Specialist grades / Expandable Graphite, pp. 319-324.
Individual evidence
- ^ A b Vijay J. Bhagat: Behavior of expandable graphite as a flameretardant in flexible polyurethane foam Presented at: Polyurethane Foam Association (PFA) Arlington, Virginia, USA, May 10, 2001
- ↑ European patent specification EP 1 323 670 B1 of July 8, 2009
- ↑ Mineral flame protection with expandable graphite - the highly effective and REACH-compliant intumescence generator. Press release Georg H. Luh GmbH.