What is viscoelastic

What is visco-elasticity?

This is a set of particles that due to their shape can temporarily connect to each other which causes elastic properties, and due to their flexibility, can easily slide along each other which causes the fluid properties. From our high-school lessons in physics, we learn that molecules are in a state that is considereds either a gas, a fluid or a solid. But there are materials that do not fit in this tight definition and thus behave as both a fluid and a solid material.

This is not a very uncommon or rare phenomenon, but it occurs frequently in nature. Actually, any material that consists of long flexible fibre like particles is in nature viscoelastic. As long as these particles can temporarily connect to and can easily slide along each, it will behave in a visco elastic way. Typical examples of viscoelastic materials are spaghetti, shag (tobacco), and polymers. Polymers are always viscoelastic because they consist out of long molecules which can be entangled with their neighbors.

Visco elastic coating systems

Viscoelastics are novel materials based on polyisobutene and or pure polyisobutylene–the same material used in chewing gum. But there is difference between bubble gum (PB) and for instance Wrigley Chewing gum (PIB). They adhere to almost any surface except water and sillicone, coldflow under pressure or heat, and are insensitive to extensive surface preparation except for millscale. In addition, they are highly moisture repellent, immune to polar solvents and bacteria, and can be applied at very low temperatures. The material is normally sold as a roll, profiling sheet or tape with an internal or external mesh /backing layer for support and a release film. But it is also sold as an injectable liquid. The material is used with a rigid outerwrap (PVC, PE, Sillicone) to provide rigidity and impact/indentation resistance to the whole system. PVC or PE may pass CP (cathodic protection) current at the overlap. Some meshbacked version can be overcoated by mesh-reinforced GRE, GRU, or GRP (Epoxy, Polyurethane, or Polyester). Extra mechanical protection can be applied, by adding polyester sheets, or rockshield

The visco-elastic material due to its fluid-like nature can prevent air enclosures during application. Extra attention should be taken care at the overlap of minimally 1 centimeter. The innerlayer of the visco-elastic materials should pass the 15Kv holiday test before the outerwrap will be applied. This ensures that the complete system is “CD resistant” and “CP cooperative.” The latter is because in the event of a minor penetration the material will flow around and out of the hole, effectively healing the 6mm puncture. That is, the outer wrap is wound sufficiently tightly that the viscoelastic is placed under compression. In other words, CD values of zero or even negative values of -3 (pure pib) are possible!

The second point is that the material is so sticky that it tends to fail cohesively, rather than by disbondment (adhesively). You can recognize this from this picture.

Cohesive failure (source: handbook of polymers)

A result of this cohesive failure is that significant quantities of material will remain to adhere to the steel substrate, and that it will remain it’s functional properties to corrosion resistance and as a consequence, no additional CP current will be required.

PIB based materials have grown into a very common commercial product for corrosion protection. Also scientific based field data suggest that this material has excellent corrosion resistance. A relevant specification is CSA Z245.30. Viscoelastics can be used as maintenance coatings, for mainline pipes, GWs, valves, flanges, and other complicated shapes.

Scientific information on Visco elastic coating systems

If you would like to learn more about the special aspects of visco elastic materials, we would like to give you a warm recommendation to read the handbook written by George Wypych (handbook of polymers).

This article was based on the extensive explanation from Hans Overeijnder, he has investigated the sputtering of alkali halides with ions and electrons at “Stichting Fundamenteel Onderzoek der Materie”. He has made an informative website on viscoelasticity. 

I would like to apply a visco elastic coating, which brand do I need to use

We have written an article about different manufacturers on the market. And be aware that the price of the tape or coating system is just a small part of the integrated cost for corrosion protection and asset manegement. Surface preparation and application of a coating are important influencers of the lifetime and quality.

More about application can be found here.

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