Water-repellency

Complementary to the matter of wetting is that of water repellency. Here, the desired goal is to make 6 as large as possible. For example, in steam condensers, heat conductivity is improved if the condensed water does not wet the surfaces, but runs down in drops. 

Apparently a negative AP with Q 90° can be found for particular pore geometries [53]. A different type of water repellency is desired to prevent the deterioration of blacktop roads consisting of crushed rock coated with bituminous materials. Here the problem is that water tends to spread into the stone-oil interface, detaching the aggregate from its binder [54]. No entirely satisfactory solution has been found, although various detergent-type additives have been found to help. Much more study of the problem is needed. 

Contemporary concern about pollution has made it important to dispose of oil slicks from spiUs. The suitable use of surfactants may reverse the spreading of the slick, thereby concentrating the slick for easier removal. 

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Fig. Xm-3. Effect of contact angle in determining water repellency of fabrics.

J. L. Moilliet, ed.. Waterproofing and Water Repellency, Elsevier, New York, 1963. 

A drop of an aqueous solution of the mixture to be separated is now placed near the bottom of the paper strip and allowed to evaporate in the air. The strip is now again suspended in the closed cylinder, but with the bottom of the strip just immersed in the solvent. The capillary action of the paper will cause the solvent to rise steadily up the strip, and during this process the solvent, which now contains the mixture in solution, is continuously extracted by the retained water molecules in the paper. A highly hydrophobic (water-repellent) solute will move up closely behind the solvent-front, whereas a highly hydrophilic solute will barely leave the original point where the drop of the mixed solutes in solution has been dried. In an intermediate case,... 

The silicone oils and silicone resins find application as (i) lubricants (their change of viscosity with temperature is small), (ii) hydraulic fluids (they are unusually compressible), (iii) dielectric fluids, (iv) for the pro duction of water-repellant surfaces, and (v) in the electrical industry (because of their high insulating properties). 

Waxes are water repelling solids that are part of the protective coatings of a number of living things including the leaves of plants the fur of animals and the feathers of birds They are usually mixtures of esters m which both the alkyl and acyl group are unbranched and contain a dozen or more carbon atoms Beeswax for example contains the ester triacontyl hexadecanoate as one component of a complex mixture of hydrocar bons alcohols and esters... 

Wax (Section 26 5) A mixture of water repellent substances that form a protective coating on the leaves of plants the fur of animals and the feathers of birds among other things A principal component of a wax is often an ester in which both the acyl portion and the alkyl portion are characterized by long carbon chains... 

Methyl groups impart water repellency, surface hardness, and noncombustibility. 

The thin dispersion rapidly thickens into a gelled matrix and coagulates into a water-repellent agglomeration that doats on the aqueous medium as the mechanical agitation is continued. The agglomeration is dried gentiy shearing must be avoided. 

Titanium alkoxides are used for the hardening and cross-linking of epoxy, siUcon, urea, melamine, and terephthalate resins in the manufacture of noncorrodable, high temperature lacquers in the sol-gel process as water repellents and adhesive agents (especially with foils) to improve glass surfaces as catalyst in olefin polymeri2ation, and for condensation and esterification. 

Most commercial aluminum formate is monobasic aluminum diformate because of the difficulties involved in triformate preparation. The main appHcation is in textile waterproofing. Aluminum formate reacts with casein to form a water-soluble complex, which can emulsify paraffin and certain other waxes. Fabrics immersed in these emulsions are rendered water repellent (26—28). 

Paper (qv) is a material of tremendous versatility and utility, prepared from a renewable resource. It may be made soft or stiff, dense or porous, absorbent or water repellent, textured or smooth. Some of the versatility originates with the fibers, which may vary from short and supple to long and stiff, but the contribution of chemicals should not be underestimated (see Papermaking materials and additives). 

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-liking 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 aUphatic 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 abiUty of surfactants to clean dirt and oil from surfaces and produce lather. 

Early waterproofing treatments consisted of coatings of a continuous layer impenetrable by water. Later water-repellent fabrics permitted air and moisture passage to improve the comfort of the wearer. Aluminum and zirconium salts of fatty acids, siUcone polymers, and perfluoro compounds are apphed to synthetic as well as natural fibers. An increase in the contact angle of water on the surface of the fiber results in an increase in water repeUency. Hydrophobic fibers exhibit higher contact angles than ceUulosics but may stiU require a finish (142). 

Water Repellency and Water Resistance. Water repeUency is defined as the abihty of a textile fiber, yam, or fabric to resist wetting, whereas water resistance is a general term appHed to a fabric s abiUty to resist wetting and penetration by water (2). A third term, waterproof, is appHed 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 permeabihty. Water-repellent finishes are hydrophobic compounds that are appHed to fabrics to inhibit water penetration while still allowing air permeabihty. 

The polymeric acyl titanate esters are viscous Hquids or waxes that are soluble ia hydrocarbon solvents and can be used as Ti02-dispersiag agents, water-repellent agents for textile fabrics, and mst inhibitors for steel. 

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