Polyurethane hydrophobic

To further explore the effect of the polarity of the molecule on its extractability, we studied the extraction of a water-soluble dye molecule by polyether polyurethane (hydrophobic) and a hydrophilic polyurethane (HPUR). The dye was bromothymol 2005 by CRC Press LLC... 

Water-borne polyurethane coatings are formulated by incorporating ionic groups into the polymer backbone. These ionomers are dispersed in water through neutrali2ation. The experimental 1,12-dodecane diisocyanate (C12DI Du Pont) is especially well suited for the formation of water-borne polyurethanes because of its hydrophobicity (39). Cationomers are formed from IPDI, /V-methyIdiethan olamine, and poly(tetramethylene adipate diol)... 

Polyurethane dispersions (PUD s) are usually high-performance adhesives based on crystalline, hydrophobic polyester polyols, such as hexamethylene adipate, and aliphatic diisocyanates, such as methylene bis(cyclohexyl isocyanate) (H12MDI) or isophorone diisocyanate (IPDI). These PUD s are at the more expensive end of the waterborne adhesive market but provide excellent performance. 

Even though poly(ortho esters) contain hydrolytically labile Linkages, they are highly hydrophobic materiads and for this reason are very stable and can be stored without careful exclusion of moisture. However, the ortho ester linkage in the polymer is inherently thermally unstable and at elevated temperatures is believed to dissociate into an alcohol and a ketene acetal (33). A possible mechanism for the thermal degradation is shown below. This thermal degradation is similar to that observed with polyurethanes (34). 

Liquid membranes consist of an organic phase, which by its hydrophobic nature is relatively impermeable to ions. Originally organic solvents such as decanol were used in conjunction with a porous hydrophobic membrane. These have been replaced by plasticized polyvinyl chloride membranes which behave like liquids yet have improved mechanical properties Other polymers such as silicone, polyurethane and ururshi, a... 

Covering the Ag/AgCl reference electrode surface with hydrophobic polymer, such as a solvent-processible polyurethane (PU), was reported to provide a stable potential... 

Interfacial behavior of different silicones was extensively studied, as indicated in Section 3.12.4.6. To add a few more examples, solution behavior of water-soluble polysiloxanes carrying different pendant hydrophilic groups, thus differing in hydrophobicity, was reported.584 A study of the aggregation phenomena of POSS in the presence of amphiphilic PDMS at the air/water interface was conducted in an attempt to elucidate nanofiller-aggregation mechanisms.585 An interesting phenomenon of the spontaneous formation of stable microtopographical surface domains, composed primarily of PDMS surrounded by polyurethane matrix, was observed in the synthesis of a cross-linked PDMS-polyurethane films.586... 

Owing to their improved stability towards hydrolysis and oxidation, dimer diol polyethers (and dimer diol polycarbonates) are used as soft segments in the preparation of thermoplastic polyurethanes. Polyurethanes prepared from such oleo-chemical building blocks are very hydrophobic and show the expected stability. 

Because the forces of attraction prevail when molecules are brought into sufficiently dose proximity under normal conditions, release is best effected if both the strength of the interaction and the degree of contact are minimized. Aliphatic hydrocarbons and fluorocarbons achieve the former effect, finely divided solids the latter. Materials such as microcrystalline wax [64742 42-3] and hydrophobic silica [7631-86-9] combine both effects. Some authors refer to this combined effect as the ball bearing mechanism. A perfluoroalkylated fullerene nanosphere would perhaps be the ultimate example of this combined effect (17). These very general mechanistic remarks can be supplemented by publications on the mechanism of specific classes of release agents such as metallic stearates (18), fatty acids and fluorinated compounds (19), and silicone-coated rdease papers (20,21). The mechanism of release of certain problem adherents, eg, polyurethanes, has also been addressed (22,23). 

This paper describes a method for improving hydrophilicity by using radiation-induced grafting of flexible polyurethane (polyether) foam with polar vinyl monomers. By this procedure, the normally hydrophobic material can be converted into a remarkably water-wettable sponge. 

The current libraiy of polyurethanes has some utility, and we will illustrate their uses with examples from our laboratory and from others. Currently, hydrophobic polyurethanes can be used to extract nonpolar pollutants, for example, from some pesticides. At the other end of the spectrum, hydrophilic polyurethanes can be used to extract sparingly soluble organic pollutants from groundwater. We will illustrate this with the extraction of methyl-tert-butylether. 

Our proposal is not theoretical. Researchers have used reticulated hydrophobic polyurethanes as liver assist devices with some success. We will discuss this research and future work in detail later. For now, it is useful to present an overview. Matsushita et al. inoculated a reticulated polyurethane with porcine hepatic cells, "fhe device functioned as noted, but it was necessary to separate the plasma from the blood because conventional hydrophobic polyurethanes are not hemocompatible. In addition, the technique made no provision for cell attachment. Workers in our laboratory grafted a hydrophilic polyurethane to the structural members of a hydrophobic reticulated foam in an effort to make the composite hemocompatible. Additionally, this gave us the opportunity to add cell attachment proteins. 

We will cite more examples of polyurethanes based on polyethers than on polyesters. The polyethers are more easily designed when the polarity of the backbone is important. For instance, one can use polyethers to construct polyurethanes that are hydrophilic or hydrophobic or react to water at all levels between these extremes. Polyethers permit the development of biocompatible and hemocompatible devices. Lastly, they are more hydrolytically stable and so are more appropriate for environmental studies. 

Special rules apply to the world of hydrophilic polyurethanes. These alternate rules are based on the fact that hydrophilic polyurethanes can and should be processed in water. Rather than emulsifying a prepolymer with a polyol, as would be done with a hydrophobic polyurethane, hydrophilics are mixed with water. While the properties of the foam are governed loosely by the guidelines described above, one has more flexibility and control of the formulation and process by which the polyurethane is made. For example, the water can serve as a heat sink to closely control the temperature of the foam the water controls the rate of reaction. 

As we will also show, polymers built on hydrophobic polyols and isocyanates are appropriate for the extraction of hydrophobic pollutants. The intent of most polyurethane research is to develop polymers of sufficient strength to meet the requirements of a particular application, for example, designing a polymer to extract an aromatic hydrocarbon from the air. As we begin to develop applications, we will use the principles of solvent extraction and use specific polyols matched to extractants. 

FIGURE 4.5 Extraction of Bromothymal Blue from water by a hydrophobic polyether polyurethane, indicating little or no extraction. 

While the properties of polyurethane as an extractant are useful, several problems make it less than ideal. Polyurethanes are far more specific than activated charcoal in removing contaminants. Charcoal separates particles by size, and while it has some specificity, it is well suited for mixtures of diverse chemistries (PAHs vs. halogenated hydrocarbons). Polyurethanes, however, operate on the principle that like dissolves like. They consist of hydrophobic isocyanates and hydrophobic polyalcohols. Thus, the molecules are hydrophobic. The polyalcohol backbone has some polarity, but it is hindered and therefore has a low net polarity. Inasmuch as the extraction effect is based, at least in part, on polarity, polyurethanes are most effective for nonpolar pollutants... 

It is, of course, possible to prepare a molecule that has both polar and nonpolar characteristics. This is the basis of surfactant chemistry. Typically, a nonpolar molecule is modified by sulfonation. The well-known Pluronic family of surfactants is based on block polymerization of polypropylene oxide (the hydrophobe) and polyethylene oxide (the hydrophUe). It is conceptually possible to build a polyurethane 2005 by CRC Press LLC... 

To address this paradox, we investigated and developed a number of composite materials. The most useful was produced by grafting a hydrophilic polyurethane onto the inside structure of a hydrophobic polyurethane. The process by which this is done is described in Chapter 3. We combined the polar nature of the hydrophilic polyurethane with the nonpolar nature of the conventional polyurethane. 

In the case studies to follow, both hydrophilic and hydrophobic polyurethanes are used to affect remediation of polluted air and water. We will not discuss conventional wastewater systems because they represent large public works projects that dot the developed world. The first three case studies cover the use of reticulated foam as a scaffold for the remediation of polluted air. Another involves the use of a hydrophilic foam as a scaffold for a biofilter to treat aquaculture wastewater, permitting its return to the system (closing the recycle loop). Lastly, we will review our work on a composite of hydrophilic polyurethane grafted onto a reticulated foam to treat VOC-contaminated air. 

Both of the above case studies used hydrophobic polyurethane as the substratum. The next study was intended to determine the effectiveness of a biofilter based on hydrophilic polyurethane cubes in permitting recycling of water in a closed-loop aquaculture facility. 

The effectiveness of this technology was reported in a study we sponsored at the University of California, Riverside, under the direction of Professor M.A. Deschusse.5 While the overall project was unsuccessful, the ability to support biological growth in a flow-through structure was confirmed. The effectiveness of the composite compared to a combination of hydrophobic polyurethane and the Zander material showed the method to have great potential. 

With hydrophobic polyurethanes, the only option is adhesion. The polyol and isocyanate environments producing hydrophobic polyurethanes by the prepolymer method are thought to be too severe for hving cells. As noted earher, the weaker adhesion of adsorption to a hydrophobic polyurethane can be an advantage. Sanroman compared die adsorption and entrapment techniques and determined that the adsorption technique was superior based on citric acid productivity and operational stability. 

Perhaps the most interesting application of polyurethane foam as a substratum for cell growth was studied by Bailliez et al. While not specifically a remediation study, their work compared hydrophobic and hydrophilic polyurethanes, TDI- and MDI-based prepolymers, and entrapment and adsorption methods, and also investigated the production of hydrocarbons by Botryococcus braunii. An unfortunate feature of biotechnical research in the use of polyurethanes is that the chemistry is rarely explained. While Bailliez includes some detail, much of their work simply designates products without specific references to the polyols. It is, of course, part of the mission of this book to show that polyurethanes are specialty chemicals. It cannot be assumed... 

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