Prepolymer design

Features High-performance prepolymer designed for long wear and abrasion resist. provides exc. wetout of and adhesion to SBR and EPDM prods, will yel. on exposure to UV light Properties Clear, pale yel. liq. sp.gr. 1.12 vise. 1900 cps vapor pressure < 0.0003 mm Hg (20 C) flash pt. (COC) > 227 C 10% NCO content... 

Figure 15.9 Predictions of the NIR model for prepolymer carboxyl ends over an eight-day period. Three states of a designed experiment, as well as a period of process upset and the return to lined-out operation, are indicated. Reprinted with permission from Brearley and Hernandez (2000). ...

Note 1 This term was used originally because of its analogy with a natural resin (rosin) and designated, in a broad sense, any polymer that is a basic material for plastics, organic coatings, or lacquers. However, the term is now used in a more narrow sense to refer to prepolymers of thermosets (thermosetting polymers). 

A hydrocarbon prepolymer containing terminal carboxyl groups (28) is available to the propellant chemist. These polymers were synthesized to eliminate some of the variables found in the copolymers. The carboxyl groups can be made of the same types with like reactivity. These linear non-branched polymers impart greater extensibility to elastomeric formulations. The chemistry in propellants is similar to the random functionality polymer. As 20 years of the chemistry of crosslinked propellant binders is reviewed, one familiar with the art cannot fail to predict solid propellant formulations using these polymers tailored to the specific requirements of the solid rocket design with the confidence that any discipline of science can be practiced. 

It is not known if HDI affects reproductive tissues in males or females however, given its short half-life in biological fluid, this seems unlikely. HDI has been reported to bind to biological tissues (protein) (Ted and Pesce 1979) however, the relevance of this observation to reproductive toxicity is not known. The toxicity of the HDI metabolite (HDA) is not known. Toxicological studies should be designed to answer questions about the potential reproductive toxicity of HDI or its prepolymers in both male and female himians and laboratory animals. 

If one were to design a new polyurethane, the prepolymer method is doubtless the method that would be used. Prepolymer is a term of art that designates an intermediate process in the production of polyurethanes as we know them. Prepolymers are quite easy to produce in a laboratory. The isocyanate is slowly added to... 

In the reaction shown in Figure 2.6, assume that we are beginning with a PPG with a molecular weight of 1000. There are two hydroxyls (the equivalent weight of the PPG is 500). Two moles of TDI per mole of PPG are required. For 1 kg of PPG, therefore, 542 grams of TDI are required. Excess is usually added to maintain a reasonable reaction rate. Very often, if foam is the desired final product, excess isocyanate is intentionally added to improve the foam characteristics. In that case, the prepolymer is designated as a quasi- or semiprepolymer. 

They are typically sold as two-part systems. By convention, the isocyanate prepolymer is designated Part A while the polyol, selected catalysts, and perhaps even water comprise Part B. 

In designing a prepolymer for a specific use, this technique provides for more control of the chemistry. For elastomers, for instance, including some three-dimensional character changes the physical properties of the polymer. More cross-linking lowers elongation and increases strength. Cross-linking is not necessary for elastomers, but it is required for foams. 

When a solvent is polar, a reservoir that reflects that must be developed. Hydrophilic polyurethanes were specifically designed to serve as reservoirs for polar solvents, although the inventors did not express their ideas in that manner. In many commercial cases, reservoir capacity ( the bottle ) is too large. The container swells as it absorbs. The nature of the material is to lose most of its physical strength as it swells. While we need the polarity, we also want to optimize the size of the bottle and strength of the material. The current library of hydrophilic prepolymers does not provide that flexibility, but we now know that we can build our own prepolymers with copolymers of propylene oxide (PO) and ethylene oxide (EO). 

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... 

In this regard, polyurethane technology offers a product designer a particular advantage. An aqueous solution or dispersion can be emulsified conveniently with a hydrophilic prepolymer and thus incorporated into the polyurethane matrix. The incorporation is accomplished by covalently bonding within the polyurethane backbone and by entrapping it within the matrix. Both methods are evident in foams produced by this technique. 

Thermocouple probe pockets also need to be fitted. Their location needs to be in conjunction with the design of the agitator and the need to obtain, as accurately as possible, the true temperature of the prepolymer as it is... 

Vessels used for the degassing of prepolymers must be designed to withstand the 250 to 280 Pa gauge vacuum applied. The exhaust fumes from the... 

There are two main methods to "de-gas" the prepolymer. The first is a simple batch system composed of a vacuum pump and a pot designed to... 

Compression set is controlled by the cross-linking in the material. This may be carried out during the design of the prepolymers or by using some short-chain cross-linking agents. TDI-based polyurethanes have better compression set than the MDI-based materials. 

New prepolymers based on 3,3-bis (azidomethyl) oxetane (BAMO) and 3-nitratomethyl- 3-methy-loxetane (NMMO) are used in advanced pintle-controllable solid rocket motor applications. These polymers yield favorable propellant energy with the combination of nitrato esters and ammonium nitrate, and allow solid propellant designs to compete with liquid propellants in arenas of energy management, approaching true start-stop-restart operation. 

Monomers and prepolymers that are useful to form shellwalls were reacted with interface modifying molecules to create novel surfactants in-situ leading to stable emulsions [23], The reactions are designed to create polymers with surfactant properties rather than encapsulating shellwalls at the OAV interface. The emulsions were formed without conventional emulsifiers, and were shown to have long term stability without creaming. Absence of classical emulsifiers reduces the tendency toward foaming. 

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