Stain-shedding coating gets tough

Chemists have devised a better method of coating fabrics with a water-repellent, “self-cleaning” coating.

Many such “super-hydrophobic” coatings have been made before, but a report in the journal Langmuir describes coated fabrics that are far more durable.

The trick was to engineer a multi-layered coating whose layers, when struck with UV light, bond more firmly to each other and to cotton.

The technique also may also be put to use in medical antibacterial coatings.

Super-hydrophobic surfaces have fascinated scientists for years; they are behind the lotus plant’s self-cleaning leaves and the gecko’s super-dry and thus super-sticky feet.

These surfaces are practically impossible to wet – water beads on them and dirt and particulates do not stick to them, leading to the self-cleaning description.

Chemists looking for the next best thing in clothing coatings have tried several tricks in recent years to create a coating with similar properties in the laboratory.

The new work hinges on what is known as layer-by-layer self-assembly – basically dipping a fabric into a solution over and over again to deposit multiple layers on it.

The team from the Australian Future Fibres Research and Innovation Centre at Deakin University made their solution with tiny particles of silica – the same material as sand.

Crucially, they added a few chemical steps to coat the particles with long chemical tails ending in what are known as azido groups.

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via BBC – Jason Palmer

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Changing the Texture of Plastic Instantly

Creating surfaces that are self-cleaning and water-repellant

Just as a chameleon changes its color to blend in with its environment, Duke University engineers have demonstrated for the first time that they can alter the texture of plastics on demand, for example, switching back and forth between a rough surface and a smooth one.

By applying specific voltages, the team has also shown that it can achieve this control over large and curved surface areas.

“By changing the voltage applied to the polymer, we can alter the surface from bumpy to smooth and back again,” said Xuanhe Zhao, assistant professor of mechanical engineering and materials science at Duke’s Pratt School of Engineering. “There are many instances, for example, when you’d want to be able to change at will a surface from one that is rough to slippery and back again.”

Scientists have long been able to create different patterns or textures on plastics through a process known as electrostatic lithography, in which patterns are “etched” onto a surface from an electrode located above the polymer. However, once the patterns have been created by this method, they are set permanently.

“We invented a method which is capable of dynamically generating a rich variety of patterns with various shapes and sizes on large areas of soft plastics or polymers,” Zhao said.  The results were published online in the journal Advanced Materials.

“This new approach can dynamically switch polymer surfaces among various patterns ranging from dots, segments, lines to circles,” said Qiming Wang, a student in Zhao’s laboratory and the first author of the paper. “The switching is also very fast, within milliseconds, and the pattern sizes can be tuned from millimeter to sub-micrometer.”

The findings follow Zhao’s earlier studies, which for the first time captured on videotape how polymers react to changing voltages. Those experiments showed that as the voltage increases, polymers tend to start creasing, finally leading to large craters. This explained in physical terms, for example, why polymers used to insulate electric wires tend to fail over time. The new lithography strategy takes useful insights from this failure mechanism.

On a more fanciful note, Zhao described the possibility of creating rubber gloves whose fingerprints could be changed on demand.

“The changeable patterns we have created in the laboratory include circles and straight and curved lines, which are basic elements of fingerprints,” Zhao said. “These elements can be dynamically patterned and changed on a glove surface that covers fingertips.

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Self-cleaning cotton nanoparticle coating invented

Clothes and crowds at London Mela

Efforts to create self-cleaning cotton fabrics are bearing fruit in China.

 
Engineers have created a chemical coating that causes cotton materials to clean themselves of stains and remove odours when exposed to sunlight.

The researchers say the treatment is cheap, non-toxic and ecologically friendly.

Retail experts say the innovation could prove a hit with retailers thanks to a growing demand for “functional clothing”.

The research was carried out by engineers at Shanghai Jiao Tong University and Hubei University for Nationalities, and is published in the latest issue of the Applied Materials and Interfaces journal.

The study focuses on titanium dioxide – a chemical known to be an “excellent catalyst in the degradation of organic pollutants”.

The substance is already used in self-cleaning windows, odour-free socks and stay-clean kitchen and bathroom tiles.

Initial efforts to extend its use to cotton fabrics proved limiting because the substance’s self-cleaning properties could only be “excited” under ultraviolet lights, making it impractical for everyday use.

Creating the coating

The team’s breakthrough was to create a nanoparticle alcohol-based compound made up of titanium dioxide and nitrogen.

The mixture was added to triethylamine, an acid neutraliser commonly used in dyes. After being stirred for a 12 hours at room temperature, the liquid was heated at 100C (212F) for a further six hours.

The cotton fabrics were then immersed in the mixture before being squeezed dry, heated and immersed in hot clean water.

Finally the coated materials were treated with silver iodide particles, which aid light-based reactions.

To test the effectiveness of their invention, the engineers marked the fabrics with an orange dye stain and exposed them to the sun. After two hours in the light, the team said 71% of the stain had been removed – a “dramatic” improvement over previously trialled techniques.

Long-lasting

The experiment was repeated on the same cloth five times with no loss of activity – suggesting that the enhancement was stable. Washing and drying the material did not reduce its effectiveness.

Clothes industry experts said there should be huge interest in the process if it could be rolled-out on an industrial scale.

“This kind of functional clothing has already proved very popular, especially in Japan where the authorities ordered a crackdown on air conditioning use after March’s earthquake caused power shortages,” said Isabelle Cavill, a clothing analyst at Planet Retail.

“It is also likely to prove popular in other parts of Asia where the heat causes sweat problems.”

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‘Self cleaning’ kitchen work tops could be a reality

A :en:glass-ceramic cooktop. Model of :en:2004...

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Kitchen work tops and mirrors that effectively clean themselves could become reality thanks a new range of “smart” materials, according to scientists.

Coating the surfaces with the new materials allows them to be wiped clean with nothing more than water, rather than detergents and solvents.

The materials could also soon be used to produce self-cleaning floors, walls and windows.

“You add water, and the oil just comes right off like magic,” said Dr Jeffrey Youngblood, the project team leader from Purdue University in Indiana, US.

“These are eco-friendly coatings – environmentally ‘green’ in the sense that they eliminate the need for harsh detergents and solvents in settings ranging from home kitchens to industrial machine shops that must contend with heavy oil spills.”

The plastic coatings have a bottom layer which attracts water and a Teflon-like upper layer that prevents the passage of oil.

The result is an oil-repellent surface that holds a film of water.

The materials could be used in household cleaners, paints, water filters, and sealants that would allow concrete floors and walls to keep themselves clean, said Dr Youngblood.

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