L. Ratnasinghe, E. Lanza
Keywords: oleophobic, superoleophobic, membranes, filters, perfluoropolyether
Summary:We will present data on the performance of oleophobic and superoleophobic surfaces for a variety of commercial membrane and filter applications. It is well known that oleophobic surfaces create oil resistant, anti-fingerprint surfaces that are impervious to dirt, dust, oils, and other particulates resulting in a surface that is easy-to-clean and maintain. Fluorocarbon coated surfaces are excellent oleophobic coatings, since their surface energies are extremely low, generally less than 20 dynes/cm. The same fluorocarbon materials typically used on glass and metal surfaces as anti-finger print coatings are also used to produce oleophobic coatings on membranes. These oleophobic membranes are widely utilized as venting membrane in the electronics and automotive industries. The oleophobicity of a membrane is generally rated on a scale of 1 to 8 according to AATCC test 118-1992. This test evaluates the membrane's resistance to wetting by eight standard oils. The #1 oil is mineral oil (surface tension: 31.5 dynes/cm @25 degrees. C.) and the #8 oil is heptane (surface tension: 14.8 dynes/cm @25 degrees. C.). Long chain perfluoroalkyl fluorocarbons with greater or equal to C8 have typically produced the highest oleophobic rating of 8. Due to environmental concerns that fluoroalkyl groups that contain C8 chains or longer can degrade to perfluorooctanoic acid, various approaches have been used to increase the oleophobicity of shorter chain C6 fluoroalkyl materials. We will present data showing comparison of properties of membranes fabricated with different C8 and C6 materials. Both the chemical composition of the surface and the surface texture affect the CA (contact angle) of oil on a surface. Treating a surface with plasma can increase surface roughness. A comparison of metal mesh filters coated with a perfluoropolyether and a fluoroalkyl silane with and without plasma treatment will be reported. Superoleophobic surfaces with contact angles in excess of 150°, and droplet sliding angles approaching zero, have been developed, however, even superoleophobic surfaces are not adequate for separating oil from wastewater. Conventional pressure-driven membranes experience little success for treating fracking wastewater because of either severe membrane fouling or incapability of desalination. The ideal membrane, especially for use in fracking wastewater applications requires both hydrophilic (or superhydrophilic) and oleophobic (or superoleophobic), both in air and when submerged under water. We are conducting experiments with water and organics mixtures using perfluoropolyether coated ceramic filters, treated with plasma and nanoparticles to create superoleophobic and superhydrophilic surfaces. Fabrication procedures, CA, and flow profiles will be discussed.