US engineers electrospin oil-repellent materials

Advances in Textiles Technology, Feb, 2008

Materials that can repel oils and could have many applications in aviation, space travel and hazardous waste cleanup have been developed at the Massachusetts Institute ofTechnology (MIT) in Cambridge,USA.

Such materials could be used to help protect parts of planes or rockets that are vulnerable to damage from being soaked in fuel, such as rubber gaskets and o-rings.

St Laurent Professor of Chemical Engineering at Massachusetts Institute ofTechnology (MIT),and an author of a paper on the work that appeared in Science magazine,Robert Cohen said"These are vulnerable points in many aerospace applications. It would be nice if you could spill petrol on a fabric or a gasket or other surface and find that instead of spreading, it just rolled off."

Creating a strongly oil-repelling (oleophobic) material, has been challenging for scientists, and there are no natural examples of such a material.

Another member of the research team-Professor of Teaching Innovation in the Department of Mechanical Engineering,MIT School of Engineering,Gareth McKinley-added "Nature has developed a lot of methods for waterproofing, but not so much oil-proofing. The conventional wisdom was that it couldn't be done on a large scale without very special lithographic processes."

The tendency of oils and other hydrocarbons to spread out over surfaces is due to their very low surface tension (a measure of the attraction between molecules of the same substance).Water,on the other hand, has a very high surface tension and tends to form droplets. For instance, beads of water appear on a freshly waxed car, but over a period of time,oil and grease contaminate the surface and the repellence fades. That difference in surface tension also explains why water will roll off the feathers of a duck, but a duck coated in oil must be washed with soap to remove it.

The MIT team overcame the surface-tension problem by designing a nonwoven composed of specially prepared microfibres that essentially cushion droplets of liquid, allowing them to sit, intact, just above the material's surface.When oil droplets land on the nonwoven, they rest on top of the fibres and pockets of air trapped between them. The large contact angle between the droplet and the fibres prevents the liquid from touching the bottom of the surface and wetting it.

The microfibres are a blend of a specially synthesized fluoropolymer, called fluorinated polyhedral oligomeric silsesquioxane (fluoroPOSS),which has an extremely low surface energy, and a common polymer. They can be readily deposited on many types of surfaces, including metal, glass, plastic and even biological surfaces such as plant leaves, using the electrospinning process.

The researchers have also developed some dimensionless design parameters that can predict how stable the oleophobicity between a particular liquid and a surface will be. These design equations are based on structural considerations, particularly the re-entrant nature (or concavity) of the surface roughness, and on three other factors:

* the liquid's surface tension;

* the spacing of the fibres; and

* the contact angle between a liquid and a flat surface.

Using these relationships, the researchers can design fibre mats that are optimized to repel different hydrocarbons. They have already created a nonwoven that can separate water and octane (jet fuel),which they believe could be useful for cleaning up hazardous wastes.

The US Air Force,which funded the research and developed fluoroPOSS, is interested in using the new materials to protect components of planes and rockets from jet fuel.

For further information, contact: AnneTrafton, Massachusetts Institute of Technology (MIT),News Office,Room 11-400, 77MassachusettsAvenue, Cambridge,MA 02139-4307,USA. Tel: 1-617-253-2700.E-mail: newsoffice@mit.edu; http://web.mit.edu