Advantages of Polyurethanes

These general comments apply to polymer science in general, but we are concerned with the specific advantages of polyurethanes. In the remainder of this chapter, we will discuss the components of a polyurethane, the process and other factors that affect the polymer, and the chemical factors that confer efficacy for environmental, medical and other applications. 

Special advantages of polyurethane adhesives are their excellent adhesion strength on many surfaces even on those otherwise difficult to bond, for example, flexible... 

The main advantage of polyurethane-modified mortar is an improvement in the strength development at low temperatures or under wet conditions, good adhesion to wet substrates, waterproofness, and abrasion resistance. The properties of the polyurethane-modified mortar are shown in Table... 

Some of the advantages of polyurethane elastomers vs. conventional rubber are improved abrasion resistance, better tear resistance, transparency, ozone resistance, and pourability for cast molding and curing. On the other hand, polyurethane elastomers possess disadvantages such as poor high-temperature service, poor resistance to certain chemicals, poor resistance to hot, moist environments, and sometimes higher material costs. 

Ehie to the polar groups on flie polymer chain, polyurethane adhesives will adhere to many different surfaces, including metals, textiles, wood, plastics, and concrete. An advantage of polyurethane adhesives over epoxies is in flieir much greater flexibility on curing due to their relatively low crosslink density. 

Electrical Properties. CeUular polymers have two important electrical appHcations (22). One takes advantage of the combination of inherent toughness and moisture resistance of polymers along with the decreased dielectric constant and dissipation factor of the foamed state to use ceUular polymers as electrical-wire insulation (97). The other combines the low dissipation factor and the rigidity of plastic foams in the constmction of radar domes. Polyurethane foams have been used as high voltage electrical insulation (213). 

Thermosetting-encapsulation compounds, based on epoxy resins (qv) or, in some niche appHcations, organosiHcon polymers, are widely used to encase electronic devices. Polyurethanes, polyimides, and polyesters are used to encase modules and hybrids intended for use under low temperature, low humidity conditions. Modified polyimides have the advantages of thermal and moisture stabiHty, low coefficients of thermal expansion, and high material purity. Thermoplastics are rarely used for PEMs, because they are low in purity, requHe unacceptably high temperature and pressure processing conditions. 

There is persisting interest in nylon-RIM materials as alternatives to polyurethane-RIM. Advantages of the nylon materials are the better shelf life and lower viscosity of the reaction components, ability to mould thick-walled articles, absence of a need for mould lubrication and the ability to avoid using isocyanates with their associated hazards. The main disadvantages of nylon-RIM are the need to have heated storage tanks and elevated temperature reactions, difficulties in catalyst handling and the high water absorption of the product. Possible markets include exterior car body components and appliance and business machine components. 

The morphology of polyurethanes varies widely depending on the molecular characteristics of their components. We take advantage of this variability by selecting components that give us the properties that we desire for specific applications. In general, the lower the fraction of isocyanate residues in a polyurethane, the closer its morphology and properties will match... 

When a thermoplastic polyurethane elastomer is heated above the melting point of its hard blocks, the chains can flow and the polymer can be molded to a new shape. When the polymer cools, new hard blocks form, recreating the physical crosslinks. We take advantage of these properties to mold elastomeric items that do not need to be cured like conventional rubbers. Scrap moldings, sprues, etc. can be recycled directly back to the extruder, which increases the efficiency of this process. In contrast, chemically crosslinked elastomers, which are thermosetting polymers, cannot be reprocessed after they have been cured. 

In this contribution, we report equilibrium modulus and sol fraction measurements on diepoxidet-monoepoxide-diamine networks and polyoxypropylene triol-diisocyanate networks and a comparison with calculated values. A practically zero (epoxides) or low (polyurethanes) Mooney-Rivlin constant C and a low and accounted for wastage of bonds in elastically inactive cycles are the advantages of the systems. Plots of reduced modulus against the gel fraction have been used, because they have been found to minimize the effect of EIC, incompleteness of the reaction, or possible errors in analytical characteristics (16-20). A full account of the work on epoxy and polyurethane networks including the statistical derivation of various structural parameters will be published separately elsewhere. 

Although a majority of these composite thermistors are based upon carbon black as the conductive filler, it is difficult to control in terms of particle size, distribution, and morphology. One alternative is to use transition metal oxides such as TiO, VO2, and V2O3 as the filler. An advantage of using a ceramic material is that it is possible to easily control critical parameters such as particle size and shape. Typical polymer matrix materials include poly(methyl methacrylate) PMMA, epoxy, silicone elastomer, polyurethane, polycarbonate, and polystyrene. 

If toughness is not the limiting consideration, then the simple addition of PEG soft segment appears to offer an attractive route to the production of polyurethanes with 25 to 30 wt.% lignin. It is important to note that both of the studies considered here have employed modified lignins. The relative advantage of hydroxypropylation (16) over fractionation (19) would depend on the relative cost of either procedure. 

Our approach to the chemistry of the polyurethanes has no such limitations, and we use it to some advantage. While we take advantage of the physical properties of PURs, our focus is on what happens to a fluid (gas or liquid) when it passes through or otherwise comes in contact with a polyurethane chemistry. It has been part of the polyurethane tradition to consider the material inert. By removing the traditional restraints of conventional raw materials and a limited range of end uses, we allow the chemistry to affect the fluid or components of the fluid. 

However, we will not ignore physical properties. A section of the book will focus on structure-property relationships. PURs fonn devices that have chemical and physical features. The great value of polyurethanes as we will show in this book is the freedom to take advantage of their chemical and physical features and efficacies. While much of the book focuses on foams, we will also discuss coatings, membranes, elastomers, and their application to the problems addressed. 

While the use of these polyethers is widespread, the goal of discussion is to create a specialty chemical. Propylene- and ethylene-based polyols are produced for physical reasons and will serve as the backbone. Researchers should note, however, that the scope of polyethers and polyesters is much broader when they are willing to sacrifice some physical strength to gain a chemical advantage. To illustrate, we cite a particularly interesting example. Castor oil was a conunon polyol for the production of polyurethanes. It was replaced by less expensive and more predictable polyols in commercial production. Readers should be aware that mixed polyols can be used to advantage. 

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