Menthol (Menthanol-3), 1-Methyl-4-isopropyl-cyclohexanol-(3)
Menthol is a terpene in the menthol class, in nature it is a major component of the peppermint oil. It can also be produced synthetically, then being a racemic product. Menthol builds long, lance-like crystals.
Chemical Characteristics
The secondary OH-group at the C-3 makes the molecule acid-sensitive under certain circumstances, because it can form double bonds when splitting off water. Therefore you need to take care to have no acid conditions in or around the object.
Physical Characteristics Melting point 30,5-32°C Boiling point von 216°C Flash point ca. 80°C Fire point ? Vapor pressure (20°C) ? Specific gravity at 65°C ? Viscosity at 65°C ?
Solubility
Menthol is nearly non-soluble in water. But a 50% mixture of ethane and water already changes this and makes it soluble easily. In contrast to other solvents menthol doesn't have any protective effect.
Non-polar solvents very high solubility (saturated, aromatic and halogenated hydrocarbons)
Polar Solvents except in water soluble in all usual solvents. Non-soluble water 0,4mg/l
The insolubility in polar solvents like water is of central importance for any application in the field of restoring. It is only through this characteristic that a volatile sealing, or adding of hydrophobic features as a protection against water or other polar solvents, becomes possible. This protection is not reached because of the insolubility of menthol in water alone. The formation of the film and its characteristics are the most critical factors for the production of the film.
Formation of the film and its characteristics
The way menthol forms a film is best compared to wax or stearin. Basically it forms an absolutely transparent and homogeneous film when solidifying from the melt, having a slightly sticky surface. As the evaporation time is comparably slow as CCD, you can apply very thin films which will stay longer than just a few minutes.
In addition menthol can be used in polar as well as non-polar solvents, having a quite film after the required time of drying. On top of this films from menthol have the best adhesive characteristics.
Due to the very irritating effects to the mucous membrane it should not be used with very large objects.
Films from a melt
Mechanical Characteristics
Very resistant against pressure, very good elasticity, very good resistance to abrasion.
Density of the film:
A very thick film offers a very good protection against water.
Reversibility:
Takes a long time, because of the low melting point it is easily removed with a hot-air blower.
Tips and Tricks
Excellent usage when used as a melt. Heat it in a water-bath to a temperature of over 50°C. (33°C would be enough of course, but it doesn't make any sense to work too close to the melting point. The menthol would solidify too fast in the brush, working would be very difficult.) The higher the water-temperature, the better the menthol will flow and penetrate.
Films from a solution
Films from a solvent produces a film that is hardly less than from a melt. The key rule for the crystallization also applies:
The slower the temperature sinks from above the melting point to a temperature below it, the more distinctive the felt formed by the crystal needles will be. A fast transition on the other hand leads to rather more homogeneous and denser films.
This leads to select a solvent that is highly volatile and has a low melting point in order to end up with films as dense as possible.
Mechanical Characteristics
The mechanical characteristics hardly show any difference compared to the pure melt. High resistance to pressure, very good elasticity, very good resistance to abrasion.
Density of the Film:
Very dense film, offers a very good protection against water, but not sufficient against ethanol, isopropanol or acetone!
Reversibility:
Takes a long time, because of the low melting point it is easily removed with a hot-air blower.
Tips and Tricks
The solidification of porous and sucking surfaces is achieved by using solvents. Nevertheless you must keep in mind, that a lower boiling point of the solvent also means that the solvent penetrates less deep.
A solvent of menthol in petroleum spirit has been tested successfully.