Water-repellent polymers

In contrast to water-soluble polymers, such as polyacrylamide, which has a relatively high critical surface tension (35 dyne/cm), water-repellent polymers, such as the silicones and ptfe, have relatively low critical surface tensions (24 and 19 dyne/cm, respectively). The presence of hydroxyl groups in polymers, such as polyvinyl alcohol and polyacrylic acid which tend to... 

A microporous membrane is fabricated from a hydrophobic polymer. As the name implies, the membrane is highly porous (the average pore size is less than 1 /xm) and allows the free passage of gases at the same time, the water-repellent polymer prevents water and solute ions from entering the pores. The thickness of the membrane is about 0.1 mm. 

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). 

Admixtures are sometimes used to reduce permeabiUty of concrete (80—82). These iaclude pore-filling materials such as chalk. Fuller s earth, or talc water repellents such as mineral oil, asphalt, or wax emulsions organic polymers (acryflc latexes, epoxies) and salts of fatty acids, especially stearates. 

A/-substituted, long-chain alkyl monomethylol cycHc ureas have also been used to waterproof cotton through etherification. Other water repellent finishes for cotton are produced by cross-linked siHcone films (56). In addition to the polymeri2ation of the phosphoms-containing polymers on cotton to impart flame retardancy and of siHcone to impart water repeUency, polyduorinated polymers have been successfuUy appHed to cotton to impart oil repeUency. Chemical attachment to the cotton is not necessary for durabUity oU repeUency occurs because of the low surface energy of the duorinated surface (57). 

The silicones have established their value as water-repellent finishes for a range of natural and synthetic textiles. A number of techniques have been devised which result in the pick-up of 1-3% of silicone resin on the cloth. The polymer may be added as a solution, an emulsion or by spraying a fine mist alternatively, intermediates may be added which either polymerise in situ or attach themselves to the fibre molecules. 

Silicone fluids containing Si—H groups are also used for paper treatment. The paper is immersed in a solution or dilute emulsion of the polymer containing either a zinc salt or organo-tin compound. The paper is then air-dried and heated for two minutes at 80°C to cure the resin. The treated paper has a measure of water repellency and in addition some anti-adhesive properties. 

Water repellant To impart water-resistant properties, particularly in greases Aliphatic amines, hydroxy fatty acids and some organic silicone polymers... 

Silicones This term is given to a wide range of polymers including fluids, rubbers and thermosetting resins. Although rather expensive relative to most other plastic they are particularly noted for their thermal stability and their water repellency. 

In a random copolymer, different monomers are linked in no particular order. A graft copolymer consists of long chains of one monomer with shorter chains of the other monomer attached as side groups. For example, the polymer used to make hard contact lenses is a nonpolar hydrocarbon that repels water. The polymer used to make soft contact lenses is a graft copolymer that has a backbone of... 

Approximately 70,000 tons of silicone polymers are produced each year In the United States. Silicones are used as greases, caulking, gaskets, biomedical devices, cosmetics, surfactants, antifoaming agents, hydraulic fluids, and water repellents. 

Mention has already been made of the effectiveness of corona or plasma treatment in increasing the influence of subsequent or concurrent polymer treatment. As examples of polymers used in this way, mention can be made of reactive cationic polysiloxane [294] and polymerisation on the fibre of tetrafluoroethylene or hexafluoropropylene [299]. Water repellency was also improved by the fluorinated polymers. Tetrafluoroethylene gave superior shrink resistance this polymer covered the scale edges of the wool, whereas this did not occur with poly(hexafluoropropylene). 

Table 10.49 Oil and water repellency of cotton fabrics treated with perfluorinated acrylic polymers [502]...

They are hydrophobic and resistant to hydrolysis because of their hydrophobic nature. This change to a hydrophobic nature is positive for applications requiring water stability, resistance and repellency and is typical of most water soluble polymers (such as poly(acrylic acid), polyethyleneimine and poly(vinyl alcohol)... 

Since the polar groups repel each other, the expanded random coil molecules tend to become stiff rods. The nonpolar portions of the water-solubilized polymer face toward the organic phase at the organic-aqueous interfaces, and the polar portions preferentially point away from the organic phase. 

TABLE 2. Water Repellency and Water Slip Testing Results for Selected Polymers"... 

Exemplary water-repellent treatments for masonry surfaces include metal stearates, oils, waxes, acrylates (both polymers and monomers), silicones (solvent-based and emulsion), siliconates, silanes and, fluorochemicals. In contrast, to waterproofing coatings, water-repellent coatings, because they are permeable to water vapor, do not trap moisture and, therefore they can reduce spalling. In addition most water-repellent coatings do not alter the appearance of a porous masonry. 

By varying the nature of the side chain, R, various elastomers, plastics, films, and fibers have been obtained. These materials tend to be flexible at low temperatures, and water and fire resistant. Some fluoroalkoxy-substituted polymers (R = CHXFJ are so water repellent that they do not interact with living tissues and promise to be useful in fabrication of artificial blood vessels and prosthetic devices. 

---