Polyurethanes Permeability

Synthetic leather products are also produced using a urethane binder. These poromeric materials are produced from textile-length fiber mats impregnated with DMF solutions of polyurethanes. Permeability to moisture vapor is the key property needed in synthetic leather. In addition to shoe applications, poromerics are... 

First, we consider the experimental aspects of osmometry. The semiperme-able membrane is the basis for an osmotic pressure experiment and is probably its most troublesome feature in practice. The membrane material must display the required selectivity in permeability-passing solvent and retaining solute-but a membrane that works for one system may not work for another. A wide variety of materials have been used as membranes, with cellophane, poly (vinyl alcohol), polyurethanes, and various animal membranes as typical examples. The membrane must be thin enough for the solvent to pass at a reasonable rate, yet sturdy enough to withstand the pressure difference which can be... 

Water-Vapor Permeability. Water-vapor permeabiUty depends on the polymer used for the coating layer and its stmcture. Vinyl-coated fabrics have Httie water-vapor permeabiUty due to the coating layer. Although polyurethane polymer is water-vapor permeable, urethane-coated fabrics also have low permeabiUty values due to their soHd layer stmcture. On the other hand, man-made leathers have good permeabiUty values as high as that of leather due to their porous layer stmcture. The permeabiUty of grain-type is lower than that of suede-type, influenced by finishing method. 

Foamed or Cellular CeUular plastics such as polyurethane and polystyrene do not hold up or perform well in the ciyogenic temperature range because of permeation of the cell strnc tnre by water vapor, which in turn increases the heat-transfer rate. CeUular glass holds up better and is less permeable. 

WA Hunke, LE Matheson. Mass properties of Co(polycthcr) polyurethane membranes. Part 2. Permeability and sorption characteristics. Int J Pharm 1313-1318, 1981. 

Additional semipermeable membrane—forming polymers are selected from the group consisting of acetaldehyde dimethyl cellulose acetate, cellulose acetate ethyl carbamate, cellulose dimethylamino acetate, semipermeable polyamides, semipermeable polyurethanes, or semipermeable sulfonated polystyrenes. Semipermeable cross-linked selectively permeable polymers formed by coprecipitation of a polyanion and a polycation also can be used for this purpose.22 23 Other polymer materials such as lightly cross-linked polystyrene derivatives, semipermeable cross-linked poly(sodium styrene sulfonate), and semipermeable poly (vinylbenzyltrimethyl ammonium chloride) may be considered.24,25... 

Introducing fluorine into polyurethane resins results in changes in properties similar to those seen with other polymers. Chemical, thermal, hydrolytic, and oxidative stability is improved. On the other hand, the polymer becomes more permeable to oxygen. Surfaces become more biocompatible and the capability to bond to other substances in contact with them is diminished.33... 

Soft and hard contact lenses with good oxygen permeability, optical clarity, flexibility, and biocompatibility dental composites denture linings surgical adhesives catheters and hydrophobic microporous membranes are examples of applications of fluorinated polyurethanes in the medical and dental fields.53... 

The rate of reversion, or hydrolytic instability, depends on the chemical structure of the base polymer, its degree of crosslinking, and the permeability of the adhesive or sealant. Certain chemical linkages such as ester, urethane, amide, and urea can be hydrolyzed. The rate of attack is fastest for ester-based linkages. Ester linkages are present in certain types of polyurethanes and anhydride cured epoxies. Generally, amine cured epoxies offer better hydrolytic stability than anhydride cured types. 

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