Flammability of liquid mixtures calculation

The flammability of liquid mixtures is based on an assessment of the flash point and the initial boiling point.  Both of these can be tested, but it is also possible to calculate the flash point of a liquid mixture.

Calculating the flash point of liquid mixtures

There is more than one acceptable method for calculating the flash point of a liquid mixture.  The CLP regulation explicitly mentions a paper written by Gmehling and Rasmussen, Ind. Eng. Fundament, 21, 186, (1972), which is called “Flash points of flammable mixtures using UNIFAC”.  This paper can be obtained from http://pubs.acs.org/doi/abs/10.1021/i100006a016 (note that this will be chargeable).

It is likely that the methods in this paper, or updated versions of the method, would be considered valid for CLP classification, but we recommend that you should contact your Competent Authority (see Competent Authority list for CLP and REACH) before using a calculation method for flash point.

This method is described in CLP as being suitable for mixtures containing up to 6 volatile components.  However, it is not suitable in these circumstances:

• mixtures containing halogenated sulphurous, and/or phosphoric compounds as well as reactive acrylates
• if the calculated flash point is less than 5 °C greater than the relevant classification criterion, the calculation method may not be used and the flash point should be determined experimentally.

Problems with using calculation methods for flash point

Overall, this type of calculation methodology depends on having the “right” sort of components which fit within the calculation parameters, eg suitable solvents; full data on the component substances; and enough time and expertise to carry out the calculations.

Calculated results will always be less valuable than test data, as they are in effect a model, rather than an experimental result.

Unlike e.g. the equations to work out the flammability of gas mixtures, which are well understood and accepted, the calculation of flash point of liquid mixtures is not as well accepted.  The cost of carrying out flash point tests  may be considerably less than using a calculation method which is acceptable to the regulators.

Physical tests can be carried out by an external test house, or internally, if you have the equipment (in this case, a closed cup flash point apparatus such as Pensky-Martens).  Although physical tests must be carried out in conformance with an accepted standard,they are not required to be carried out to GLP standards, or by a toxicology lab.  This means that the cost of tests may be cheaper than you might anticipate.