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Painting without runs

Painting without runs

If we have ever tried to paint any surface ourselves, there is one thing we all must have tried to avoid – runs. This applies to both amateurs and professionals, decorators and car painters. No matter what paints we use and whatever their application, the phenomenon allowing us to prevent runs is called thixotropy. They say that stirring won’t make your tea sweeter, but stirring will for sure make your paint thinner.

A bit of theory

The term thixotropy was first used in 1935 by a German chemist Herbert Max Finlay Freundlich, and the word itself derives from the Greek noun thixis – touch. The phenomenon involves a change in viscosity caused by stirring or grinding the paint intensively with a brush, as a result of which the paint becomes easy to apply, but it does not drip or trickle even if we are painting a ceiling. After some time of rest following the stirring, the liquid sets again, i.e. its viscosity increases back to the normal value. A reverse phenomenon may also occur though, where the viscosity of a substance temporarily increases as a result of stirring. The phenomenon is a reversible process, and supplying energy is necessary to destroy the thixotropic structure of a liquid (see the figure below).

How is the phenomenon used in practice

From the perspective of our industry, what a painter wants to receive is a product which is easy to apply using an airbrush – that is why spray viscosity plays such an important role. On the other hand, we want to be sure that the paint we have sprayed does not trickle down the surface creating runs, and we want to be able to apply a thick layer of paint in a single pass thus increasing the opacity. Using the phenomenon in question, we are able to produce paints with such properties. The necessary work should start as early as the stage of designing the product, and the task itself is not easy at all as we expect the final product to combine a number of advantages which are often difficult to combine due to the properties of the individual components.

Przebieg zmian lepkości układu tiksotropowego podczas ścinania i przy powrocie do stanu równowagi (model procesu)

Combining optimum spray viscosity with high opacity (in order to prevent runs) is a true challenge for a paint manufacturer. However, we have been able to achieve this even in the case of such demanding products as base coat water-dilutable paints, e.g. Profix Aqua 2G.