Expanding properties

Plastic materials that can expand on contact with water are finding increasing applications in industry. Justin Cunningham reports.

A range of plastics called hydrophilic elastomers have been developed by Elasto, a thermoplastic manufacturer. The material is unusual as it blends a super-absorbent polymer into the base elastomer compound, which causes it to swell up to eight times its volume when immersed in water. The materials have been developed so that, when they come in to contact with water they expand and form a positive seal, preventing the ingress or exit of any moisture.

The company has a range of formulations that offer swell rates between 150% and 1000% when they are immersed in water. As soon as water is no longer present, the compound then shrinks back to its original size. This process of expansion and contraction can be repeated an 'unlimited' number of times, says the company, with absolutely no loss in material performance. Its structural integrity means that it will not erode or shatter over time, which can happen to hydrophilic clay materials.

The material is finding a wide range of uses in industry. An obvious use would be as seals around a car, but it is finding uses in a multitude of industries. Applications so far include water stops, building and construction seals, water treatment plants, tunnels, drains, sewers, tanks, automotive sealant parts, glazing, headlights and cable protection.

David Mann, key account manager at Elasto UK, says: "Like a lot of our products, we find that we have the 'answers', and that although customers have the 'questions' or problems, they don't necessarily realise materials like this exist. So that is part of the challenge."

The temperature, the salt concentration in the water, and the elastomer compound all affect the swell rate.

The material known as Dryflex WS expands uniformly at a controlled rate to a maximum percentage, which can be set when the material is produced. It has the added advantage that it can be made in any colour, is 100% recyclable and can be processed using conventional fabrication methods; including extrusion and injection moulding.

The material can also be made to be antimicrobial; meaning that it kills or inhibits the growth of micro organisms such as bacteria, fungi, or protozoans. Additionally, it can be made to be suitable for saline and alkaline environments making it potentially suitable for medical applications. In an unstressed state the material can operate from -50 to +75°C.

A similar innovation has also recently been published by the Royal Society of Chemistry. A fabric has been made hydrophilic on one side by coating it with nano-titania, a substance which has the property of photo-induced hydrophicity. This essentially means that its hydrophilic property – the ability to absorb and attract water – is controlled by light. When light is taken away the fabric becomes hydrophobic.

The most immediate application is likely to be sportswear, but the coating has a massive potential to be transferred in to engineering for industrial uses. However, researchers are looking at the potential to use the fabric to wick sweat away from the human skin. Water will be transported in a controllable manner from the dark hydrophobic side (next to the skin) to the hydrophilic side on the outside where it can be spread out.

Current materials work by creating a surface energy gradient across the fabric by a pressure difference, whereas this introduces nano and smart elements into the system, taking advantage of titania's properties.

Justin Cunningham

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