Solid, liquid, solution, dispersion, suspension, paste, gel – so what exactly is it?

If it is not obvious at first whether the mixture, formulation or the like that you’re dealing with is a liquid or, in fact, a solid, the UN Manual requires that a penetrometer test be carried out. This ultimately determines whether the product in question is a solid or a liquid substance. Where the manual is concerned, there is only the one form or the other and no form at all in between.

A penetrometer according to ISO 2137 consists of a penetration vessel for holding the test substance, a sieve disk made of duralumin with conical bores, and a guide rod attached to the sieve disc.

To carry out the penetrometer test, the test substance is placed in the penetration vessel, the vessel is tightly closed, and the test substance is left to rest for at least half an hour. The test substance is then brought to a temperature of 35 °C ± 0.5 °C in the penetration vessel shortly before the start of the test (max. 2 min). The tip of the sieve disk is then placed on the surface of the test substance and the penetration depth is measured at the specified temperature as a function of time.

A substance is considered a solid if, after the tip of the sieve disk has been placed on the surface of the test substance, the penetration read on the gauge

a) is less than 15 mm ± 0.3 mm after a loading time of 5 s ± 0.1 s, or,
b) is more than 15 mm ± 0.3 mm after a loading time of 5 s ± 0.1 s, but additional penetration after a further 55 s ± 0.5 s is less than 5.0 mm ± 0.5 mm.

For samples with a yield point, it is often not possible to achieve a stable surface in the penetration vessel and thus to create definite starting conditions for the measurement when the tip is placed. In addition, with some samples, elastic deformation of the surface may occur as the sieve disc makes impact, simulating greater penetration during the first few seconds. In all such cases, it may be advisable to assess the results according to paragraph (b).