Polyurethane water-swelling

It is resistant to most hydrocarbons, although polar solvents may extract plasticizer if present. Water swells the PVC polymer imparting an opacity which reverses on drying. PVC can be adhered using polyester, epoxy or polyurethane adhesives. 

The water-swelling polyurethane is a prepolymer obtained from polyoxyalk-ene (polyether) polyol reacted with isocyanate (ratio 1.1 to 1.7 of isocyanate groups to reactive hydrogens in polyol). 

Reservoir capacity is, in our view, an attempt by a polymer to dissolve. Because of cross-linking and molecular weight, the system does not fully dissociate into a true solution. Rather than dissolving in the normal sense, the polymer is said to swell in the solvent. Absorption of a solvent, water or organic, is a volumetric phenomenon controlled by the relative polarities of polymer and solvent. A nonpolar backbone is preferred for absorbing nonpolar solvents. The molecule we call polyurethane, however, is not entirely nonpolar but is close enough for use as an absorbing matrix. 

Much of the work related to environmental and medical devices required hydro-philicity. This property is inexorably bound to the swelling of the polymer in water. The amount of swell or hydrophilicity is controlled by the polyol used to build the polyurethane. In many cases, the design of the system requires a compromise of hydrophilicity and physical strength, and the choice of polyol is the chief tool. We stated earlier that block copolymers were suitable for intermediate levels of hydrophilicity. Table 3.5 lists a series of polymers and their equilibrium values. Each polymer is the result of proper selection of an EO or PO copolymer. The table can be used as a guide in designing polymers of intermediate hydrophilicity. 

Polyurethane hydrogels are prepared by reaction of a polyisocyanate with polyethyleneglycol. These hydrogels can swell with water and are used in medical applications, such as wound dressings. Although dialkyltin catalysts are claimed in this application, the use of organotin compounds in medical applications with potential contact with blood is questionable, and they are probably not the safest catalysts for this application. 

NBC (Nyrim) is more expensive than RIM polyurethane but it may be heated to 200°C without softening. NBC moldings are produced by the reaction injection molding (RIM) of polypropylene glycol and caprolactam in the presence of a catalyst. The tendency for this copolymer to swell in the presence of water is reduced by reinforcing with glass fibers. 

Network Synthesis (4) Solid MDI was weighed into a flask and an equivalent amount of polyol added. The mixture was heated to about 40°C to dissolve the MDI. The mixture was then cooled to room temperature and degassed for several minutes under vacuum in order to remove dissolved air. Catalyst was then added and the contents of the flask mixed under vacuum to ensure uniformity and then poured into a mold. All operations were carried out in a dry glove bag to minimize reaction with atmospheric water. The cross-linking process was also carried out in dioxane solution at 70% volume fraction of solids. Polyurethane networks with different crosslink densities were prepared by varying the ratio of difunctional and trifunctional polyols. All samples were extracted with dioxane to remove unreacted and uncrosslinked materialbefore swelling. 

Polyurethane rubbers, in general, and the Vulkollan-type rubbers, in particular, possess certain outstanding properties. They usually have higher tensile strengths than other rubbers and possess excellent tear and abrasion resistance. The urethane rubbers show excellent resistance to ozone and oxygen (in contrast to diene rubbers) and to aliphatic hydrocarbons. However, they swell in aromatic hydrocarbons and undergo hydrolytic decomposition with acids, alkalis, and prolonged action of water and steam. 

The sensitivity to hydrolysis is a key issue in many applications. The ester bond in 4GT-PTMO copolymers is sensitive to hydrolysis however, it is fairly protected since most of the ester is contained in a crystalline structure. The addition of a small amount (1-2%) of a hindered aromatic polycarbodiimide substantially increases the lifetime of this material in the presence of hot water or steam (Brown et al., 1974). Polyurethanes are susceptible to hydrolytic attack, especially those with polyester soft segments. However, polyester soft segment polyurethanes are generally more resistant to oils, organic solvents, and thermal degradation. lonomers will swell when exposed to water in fact, a commercial hydrated perfluorosulfonic ionomer (Nation) is used as a membrane separator in chlor-alkali cells. Styrene-diene copolymers and polyolefin TPEs are insensitive to water. 

Monomers act as plasticizers. They diffuse much more slowly than water molecules. A similar plasticizing effect of water is also observed with polyvinyl alcohol (PVAL) and linear polyurethane (TPU). This applies in general to swelling of plastics in solvents. 

To date, MIP microstructures have been successfully fabricated with dimethyl-formamide (DMF) as a solvent in both an acrylate-based [55] and a polyurethane based [59] imprinting system. While this solvent still swells PDMS, its effect is small. Other imprinting systems that utilize water and alcohols as the solvent may be possible since these solvents do not swell the PDMS stamps. Alcohols are the best as they have alow surface tension and wet the surface of the PDMS well. Water, however, has a high surface tension and does not wet the surface of the hydrophobic PDMS stamps. Possible solutions may be to render the stamp more hydrophilic by O2 plasma or UY irradiation to add a small percentage of alcohol ( 5 %) to the aqueous solution to wet the surface, or to use a small vacuum-assisted pump to fill the channels. 

Polyurethanes based on the incorporation of a polyethylene oxide soft segment exhibit substantial water uptake and, related high moisture vapor transmission rates (MVT). The variable and controlled degree of swelling with the related water and solute permeability could be useful in certain biomedical applications, including controlled release of medications from the water swollen polymer. In earlier work Tobolsky and coworkers (7) described the... 

Figure 7.1.10. Swelling of ciosslinked polyurethane in water and ice. [Adapted, by permission, from B Hladik,...

Figure 7.1.10. Swelling of erosslinked polyurethane in water and lee. [Adapted, by permission, from B Hladik, S Frahn, W Borchard, Polym. Polym. Compos., 3, No.l, 21-8(1995).]...

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