18 April 2013

About viscosity and surface tension

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If you are looking for a sealing material that should be able to penetrate small and narrow gaps, it is not always obvious that a liquid material with a very low viscosity is the best choice.

The picture shows a drop of water and a drop of silicone oil. Both drops have exactly the same mass. The water holds together, but the silicone oil quickly flows out and forms a very thin "lens". Water has a viscosity of 1.0 mPas, while the silicone oil in the picture has a viscosity of 4000 mPas. You could say that the silicone oil flows 4000 times slower, and that's a big difference. Even so, the silicone oil covers the surface much better by far. This effect depends on surface tension, i.e. which forces strive to hold the surface of the material together.

High and low surface tension respectively

This effect has a certain significance when choosing a suitable adhesive or sealing material. If you want a sealant that will fill small gaps, a water-based agent would not give such good results. The water's high surface tension means that it strives to form drops in the surface layer and then it does not flow into the smallest cavities. This effect becomes especially clear if you put water on plastic surfaces. However, a silicone-based sealant would work much better.

Now, for all intents and purposes, water has an unusually high surface tension and silicone has a very low surface tension. But it is also the case that silicone has a significantly lower surface tension than many other types of polymers, including most thermoplastics. This means that lacquers, glues and silicone sealants can "flow" over most surfaces, increasing the possibility of creating strong intermolecular bonds. This is one of the explanations why silicone glue can glue plastics that are otherwise difficult to glue, without first having to raise the surface tension on the substrate.

Do you have an unsolved glue problem? Feel free to contact us at Tribotec. Maybe we have a material that works completely without primer or elaborate pre-treatment.