Water repellent ability

For many applications the wetting behavior of a network of fibres is important. An example is the water repellent ability of clothing. As a simple model we consider a bundle of parallel cylinders which are separated by a certain spacing. This spacing is assumed to be small compared to the capillary constant, so that the shape of the liquid surface is assumed to be determined only by the Laplace equation. Practically, this leads to cylindrical liquid surfaces. 

Unlike the thermal and hydrothermal stabilities, the mechanical stability seems less dependent on the nature of mesoporous materials. All materials gradually collapse with the increase of pressure, accompanied with the decrease of surface area and pore volume. Recent studies show that cubic SBA-1 and MCM-48 are more mechanically stable than hexagonal mesoporous materials such as MCM-41 and SBA-15. Hydrolysis of Si-O-Si bonds by water adsorbed onto the silanol groups under compression was found as the main reason for mechanical instability. Organically functionahzed materials are more hydrophobic than unmodified counterparts, and thus show enhanced mechanical stability due to the water repelling ability. " ... 

The hydrophobicity of several highly fluorinated surfaces was found to increase with the number of layers, and their water-repellent abilities were directly correlated to the surface topologies (number of layers of silica nanoparticles and their organization on the glass support). 

Hydrophilic Refers to a substance or chemical that has a high affinity for moisture or water. Hydrophobic Refers to a substance or chemical that is poorly soluble in water water repellant. A hydrophobic agent is a chemical having the ability to resist wetting by water. It can be used in the treatment of synthetic sorbents to decrease the amount of water absorbed, hence increasing the volume of oil they can absorb before becoming saturated. 

Water Repellency and Water Resistance. Water repellency is defined as the ability of a textile fiber, yam, or fabric to resist wetting, whereas water resistance is a general term applied to a fabric s ability to resist wetting and penetration by water (2). A third term, waterproof, is applied to those fabrics that do not allow any water penetration at all. Waterproof fabrics are generally coated with an impermeable surface layer that does not allow air permeability. Water-repellent finishes are hydrophobic compounds that are applied to fabrics to inhibit water penetration while still allowing air permeability. 

Chemical modification of the wax can improve smear resistance (5). Silicones, which do not harm furniture finishes (6), are incorporated as film-forming ingredients in furniture polishes. The lubricant properties of silicones improve ease of application of the polish and removal of insoluble soil particles. In addition, silicones make dry films easier to buff and more water-repellent, and provide depth of gloss, ie, ability to reflect a coherent image as a result of a high refractive index (7). Wax-free polishes, which have silicones as the only film former, can be formulated to deliver smear resistance (8). Another type of film former commonly used in oil-base furniture polishes is a mineral or vegetable oil, eg, linseed oil. 

When used as water repellents, the very marked water-repellent effect of the silicone is utilized. The active silicon agents are oriented by using special catalysts so that the optimum water-repellent effect is obtained. The textiles thus treated retain the ability to "breathe" in full. 

Soap is one example of a broader class of materials known as surface-active agents, or surfactants (qv). Surfactant molecules contain both a hydrophilic or water-Kking portion and a separate hydrophobic or water-repelling portion. The hydrophilic portion of a soap molecule is the carboxylate head group and the hydrophobic portion is the ahphatic chain. This class of materials is simultaneously soluble in both aqueous and organic phases or preferential aggregate at air—water interfaces. It is this special chemical stmcture that leads to the ability of surfactants to clean dirt and oil from surfaces and produce lather. 

Some sorbents are treated with oleophilic (oil-attracting) and hydrophobic (water-repelling) agents to improve the ability of the material to preferentially absorb oil rather than water. As natural sorbents often recover large amounts of water along... 

Galbraith et al [20] compared cotton, water repellent cotton, and acrylic garments through wearing tests and concluded that the major factor causing discomfort was the excess amount of sweat remaining on the skin surface. Niwa [21] stated that the ability of fabrics to absorb liquid water (sweat) is more important than water vapour permeability in determining the comfort factor of fabrics. 

Many of the unique properties of silicone oils are associated with the surface effects of dimethylsiloxanes, eg, imparting water repellency to fabrics, antifoaming agents, release liners for adhesive labels, and a variety of polishes and waxes (343). Dimethylsilicone oils can spread onto many solid and liquid surfaces to form films of molecular dimensions (344,345). This phenomenon is gready affected by even small changes in the chemical structure of siloxane in the siloxane polymer. Increasing the size of the alkyl substituent from methyl to ethyl dramatically reduces the film-forming ability of the polymer (346). The phenyl-substituted silicones are spread onto water or solid surfaces more slowly than PDMS (347). 

In related work, Williams and Feist (148) showed that a trivalent chromium compound was capable of adhering to wood to produce nearly the same weathering protection and water repellency as hexavalent chromium compounds. The critical factor was the ability of the chromium to fix or become unleachable, not the oxidative action of the hexavalent chromium ion on the wood. A further study by Williams and Feist (152) proved that Cr03 fixes to both wood and pure cellulose. With both materials, complete fixation of chromium resulted in a highly water-repellent surface. The similarity between treated wood and treated cellulose indicated that chromium-cellulose interactions should be included in defining the mechanism for wood surfaces stabilized with Cr and that previously proposed chro-... 

Association of Textile Chemists and Colorists recently [3] described the incorporation of small amoimts of fluorocarbon derivative in a polymeric material normally used to treat textiles for water repellency. They observed that the fluorocarbon preferentially adsorbed at the interfaces and decreased the values to 16 to 18 dynes per cm. Their films clearly showed the ability to self-heal, for when the initially adsorbed layer was deliberately scraped off, additional molecules quickly adsorbed at the interface when the polymer matrix was recured at an elevated temperature. The usefulness of adsorbed films of surface active molecules is thus apparent, and one may expect wide application of this technique to specific problems. The present study, in combination with previous investigations of wettability and surface activity in organic liquids, forms an excellent guide for the design and synthesis of further surface active agents for polymeric systems. 

Uses Surfaefanf, mar resist, aid, water repellent, gloss aid, flow control agent, pigment dispersant for paints and protective coatings Features High purity stable sol n. effective at low levels improved wash-ability... 

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