Reticulated foam

Textile uses are a relatively stable area and consist of the lamination of polyester foams to textile products, usually by flame lamination or electronic heat sealing techniques. Flexible or semirigid foams are used in engineered packaging in the form of special slab material. Flexible foams are also used to make filters (reticulated foam), sponges, scmbbers, fabric softener carriers, squeegees, paint appHcators, and directly appHed foam carpet backing. 

Otner Collectors Tarry particulates and other difficult-to-handle hquids have been collected on a dry, expendable phenol formaldehyde-bonded glass-fiber mat (Goldfield, J. Air Pollut. Control A.SSOC., 20, 466 (1970)] in roll form which is advanced intermittently into a filter frame. Superficial gas velocities are 2.5 to 3.5 m/s (8.2 to 11.5 ft/s), and pressure drop is typically 41 to 46 cm (16 to 18 in) of water. CoUection efficiencies of 99 percent have been obtained on submicrometer particles. Brady [Chem. Eng. Prog., 73(8), 45 (1977)] has discussed a cleanable modification of this approach in which the gas is passed through a reticulated foam filter that is slowly rotated and solvent-cleaned. 

A variety of photocatalyst supports has been examined experimentally. Dip-coated glass slides or plates have been used in many experimental systems as a simple lab-scale supported photocatalyst system. Coated glass offers many of tlte important features of a supported photocatalyst while still offering relatively simple preparation. Honeycomb monoliths, widely used as commercial catalyst supports for a variety of gas-phase applications, have also been examined as photocatalyst supports (Fig. 3). Although these monoliths offer good stability and excellent throughput, providing illumination for the photocatalyst inside the monolith channels can be problematic [41,42]. Randomly structured support materials, like fiber-based filters, reticulated foams, and similar materials, have been used... 

Fynn, G.H. and Whitmore, T.N., Colonization of polyurethane reticulated foam biomass support particle by methanogen species, Biotechnol. Lett. 1982. vol. 4, no. 9, pp. 577-582. 

In recent years, we have become integrated into the much larger world of polyurethanes, but we have always begun our investigations with a focus on the surface chemistry. While our studies have been on the full range of polyurethane chemistries and the full range in which polyurethanes are produced, the chemical aspects in which we are most interested are foams (the bulk of polyurethane production), specifically open-celled foams, and more specifically products known in the industry as reticulated foams. 

An important theme of this book is impressing upon the reader the possibilities that are opened by adding aspects of chemical reactivity to the structure shown in Figure 1.1. In addition to describing how reticulated foams are produced and their physical parameters are varied, we will describe the ways we and others have used such structures. As noted area, the high surface area and low void volume make the reticulated foam a unique structure in material science. 

A reticulated foam is the end result of a manufacturing technique as opposed to a chemistry. In the next chapter, we will introduce readers to the reactions and components that yield this class of polymer. It is important to note that most, if not all, of the foam formulations we will discuss can be converted into reticulated foams to take advantage of the properties of their unique structures. 

Consider the procedure for immobilizing an enzyme using polyurethane technology. A solution of the enzyme is produced in water. The solution is then emulsified with a hydrophilic polyurethane prepolymer. The emulsion is applied to the structural members of a reticulated foam by means of nip rollers. After curing... 

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 have described the need for a device that could assist a compromised liver or even serve as a bridge until a transplant became available. We have compared the properties of an ideal scaffold for such a device with the structure of a reticulated foam and reported results of research into its use. Lastly, we have postulated improvements in current research that could lead to an efficacious solution. 

This chapter introduces readers to the versatility of polyurethane polymers without spending too much time on the chemistry. The next chapter will discuss a more classical view of the molecule and how it is developed. Our point, however, is to present a functional view of this system. We have examined its physical characteristics, focusing our attention on the uniqueness of reticulated foams. All the chemical points we have made apply to all polyurethane polymers, whether they are open-celled foams, closed-cell foams, or thermoplastic elastomers. 

FIGURE 2.14 Conversion of open-cell foam to reticulated foam. 

FIGURE 2.15 Surface area of reticulated foam as a function of porosity. 

While the primary reason for reticulation is to improve flow-through characteristics, it provides a further benefit by making the surface available to fluids passing through. The technology also produces a remarkable degree of uniformity in cell size. This contributes to the predictability of both flow and surface characteristics. If the surface is activated in some way, it is easy to see why this aspect of reticulation could be beneficial in designing functional devices. Table 2.4 and Table 2.5 show typical physical properties of commercially available reticulated foams. 

If we imagine that the surface has an effect on fluids that pass through it, the kinetics of the action in part serve as a function of the surface area. Again, reticulated foam presents some advantages. 

The big question is whether the seemingly ideal properties of a reticulated foam will be maintained when we start to change the chemistry (for cell adhesion, extraction, etc.). Changes in the polyol or isocyanate will inevitably affect its physical properties. A balance of chemical activity would have to be established. In many cases, this balance will degrade the desirable properties. An answer is the recent development of a composite of a chemistry designed according to desirable chemical features grafted to a reticulated scaffold. " Such a composite was developed and patented and it will be cited as an example for several applications. 

The chemistry function is produced by the prepolymer technique. Examples will follow in later chapters. The prepolymer is mixed with a reactant (polyol and/or water) and immediately pressed into a reticulated foam using a nip roller arrangement as shown in Figure 2.16. 

Density is an important property because, all other things being constant, it is colinear with the compressive and tensile strength of the foam. It is interesting to note that it is not cohnear with pore size. By visual examination, one would assume that a 100-ppi foam would have a higher density than a 10-ppi foam. In fact, they can be made to have the same density because the bars and struts that form the foam matrix become thinner as pore size gets smaller. Table 3.1 lists the porosities and densities of several commercially available reticulated foams. ... 

Air flow is a measure of the resistance that a foam presents to air passing through it. Intuitively one would expect that large pore size presents less of an impediment to flow than small cells. This is indeed true and it is this method that quantifies the effect. It is obvious by now that air flow is an important property in the context of this book. Unfortunately, the ASTM tests were designed for the measurement of resistance to air flow only. Our interests focus on the flow of other fluids, specifically water, through the foam. Nevertheless, air flow represents a quick and precise way to determine the quahty of a foam. If you are not sure of the difference between a reticulated foam and an open-cell foam, this test will differentiate them. Reticulated foams offer much less resistance to flow than open-cell foams. 

In the production of a hydrophilic polyurethane, the choice of surfactant or emulsifying agent is the design tool of choice to develop a specific type of cell structure. While flow-through is not an important factor for hydrophilic polyurethanes, control of the cell structure in composites based on use provides an opportunity to develop multiples of the surface area in standard reticulated foams. 

It is important to note that it is common in the reticulated foam industry to use air flow-through to define pore size. Manufacturers calculate average pore sizes from air flow measurements. Presumably, one would do this via a microscope either by counting pores or using an optical scanning device, but given the problems of sampling a bulk material with a microscope, it has become standard to use an empirically derived correlation between air flow and pore size. This serves as an indirect confirmation of the effect of the quality of the foam and its dynamic... 

Once a polymer is fuUy saturated, the physical tests described above can be conducted with confidence. Naturally, minimizing the evaporation of water should be considered. The one exception in this new category of testing is flow of water through the foam. This is not covered in the standard but will be very important for some applications, particularly in environmental remediation. If the intent is to build a biofilter or a continuous flow enzyme reactor, we must know the hydrodynamic properties of the materials we produce. Since polyurethanes are rarely used in these environments, the flow of water even through a reticulated foam is not described by the manufacturers. Furthermore, if we are to make composites of reticulated foams, the amount of polymer grafted to the surface will have a dominating effect on the flow of water. In a later chapter, we will describe our work in this area. 

The production of an open-cell foam by the techniques described above only partially covers the polyurethanes considered most useful in the context of this book. Open-cell foams are converted to reticulated foams by a postprocessing technique. Two techniques are used in the U.S. The oldest involves immersing the foam in a... 

FIGURE 3.15 Transition fiom open-cell to reticulated foam. 

We attempted to develop a multipurpose extraction system by combining the carbon adsorption and polyurethane extraction techniques. Our research continues. Our first experiments focused on the production of a carbon-impregnated reticulated foam. Several companies promote this technology, but in our tests, the method of incorporation of the carbon significantly reduced the activity of the carbon. 

The bottle was charged with butane again and a sample of the ungrafted reticulated foam was placed in the bottle. Figure 4.14 shows the kinetics of extraction. The extraction ability of the foam is significantly lower than the ability of the carbon. 

A similar result was seen in the performance of a sample of hydrophilic poly-urethane-grafted reticulated foam without carbon. (Figure 4.15). Figure 4.16 shows the analysis of the effect of the carbon-impregnated foam. It is clear from these data that the carbon-impregnated foam showed significant improvement in effectiveness. In all the extractions cited below, the CoFoam contained about 1.7 g carbon. 

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. 

One of the problems encountered in the entrapment of activated charcoal in a polymer matrix is the blinding-off of the pores of the charcoal, thus inactivating it. Because the pores of the carbon are responsible for its ability to adsorb organics, any pores that are filled or coated with a polymer matrix reduce its effectiveness. Conventional treatments involve blending the carbon into an acrylic latex and then applying the slurry to a reticulated foam. Upon drying, a coalescence encapsulates the carbon. 

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