Polyurethane-forming systems

Behaviour similar to that shown by the polyester-forming systems is shown by the several polyurethane-forming systems which have been studied(3,4,6,15,23-28), and Figure 6 and Table II give the results(29) for polyurethane-forming systems from which network materials have been formed at complete reaction. 

Table II. Values of b for polyurethane-forming systems derived(29) from Figure 6 and plots of Aab versus (cac + cbc)-1 according to equations(7). v i/v - fractional length of diisocyanate residue in the chain of v bonds (i) cext = cao + cbo-(ii) cext = cac + cbc- For explanation of reactants see text and Figures 5 and 6.

The reactants used to form the networks studied are generally of lower molar mass than those used by other workers to form perfect networks (e.g. (35)). However, the present results do indicate that very little pre-gel intramolecular reaction can have a marked effect on modulus. For example, for pr,c = 0.05, or ac = 0.58, with a trifunctional polyurethane-forming system of Me = 635g mol l, the modulus is reduced by a factor of five below that calculated on the basis of the small-strain(affine) behaviour of the perfect network. As a result, it is recommended that the effective absence of pre-gel intramolecular reaction in endlinking reactions to form perfect networks be confirmed by experiment rather than be assumed. 

Equations of an analogous type can be used for other polyurethane-forming systems, although the numerical values of the constants may be different, because Eq. (2.29) must be treated primarily as empirical. It is reasonable to expect that an adequate approximation for other systems would be n equal to 2 (which is typical for polyaddition reactions) and m also equals to 2. Such values of the constants m and n are valid, for example, for RIM-2200 (produced by Union Carbide ), which consists basically of polyester and 4.4-diphenylmethane diisocyanate.4... 

Superabsorbents. Water-sweUable polymers are used extensively in consumer articles and for industrial appUcations. Most of these polymers are cross-linked acryUc copolymers of metal salts of acryUc acid and acrylamide or other monomers such as 2-acrylamido-2-methylpropanesulfonic acid. These hydrogel forming systems can have high gel strength as measured by the shear modulus (134). Sometimes inorganic water-insoluble powder is blended with the polymer to increase gel strength (135). Patents describe processes for making cross-linked polyurethane foams which contain superabsorbent polymers (136,137). 

The GMP s process eliminates the use of sheet metal for the skin of the refrigerator door. In this application, the thermoplastic film forms a durable, protective outer skin with a wide choice of color options that are applied directly to the film. In addition more innovations exist apart from the film and thermoplastic interior liner, the doors consist entirely of polyurethane. GMP backs the thermoplastic film with an approximately 4 mm thick layer of the Baydur 110 structural foam polyurethane RIM system from Bayer AG that creates a rigid, dimensionally stable outer shell with no need for sheet metal. Then, GMP fills the space between this shell and the inner liner with insulating polyurethane foam, a rigid, low-density foam. The result is a self-supporting door that satisfies all stability, thermal insulation, and surface finish requirements. 

Other synthetic approaches to the kinetic problem have been taken. Variations in catalyst concentration for the formation of each component network from linear polyurethanes and acrylic copolymers have been used along with a rough measure of gelation time (5) to confirm the earlier (2-3.) results. Kim and coworkers have investigated IPNs formed from a polyurethane and poly(methyl methacrylate) (6) or polystyrene (7) by simultaneous thermal polymerization under varied pressure increasing pressure resulted in greater interpenetration and changes in phase continuity. In a polyurethane-polystyrene system in which the polyurethane was thermally polymerized followed by photopolymerization of the polystyrene at temperatures from 0 to 40 C, it was found (8.) that as the temperature decreased, the phase-... 

Figure 2 illustrates results obtained from tri- and tetra-functional polyurethane-forming reaction systems, with X ab plotted against (cgo + the initial dilution of reactive... 

There has been much interest in stabilizers based on vitamin E. which, in its bioactive form, is a-tocopherol. For applications in contact with food, FDA and EGA regulations recommend liquid anti-oxidants based on vitamin E. These have been developed as patented systems and also open up new areas of application in polyolefins and polyurethane foam systems. 

The initial, linear portions of the experimental plots in Figure 2 were used to determine the values of b for each PDMS-forming system, and the results are listed in Table 3. The values of b for the polyurethane- and polyester-forming systems are also shown in Table 3. It can be seen that, in spite of the very large ranges of covered (148... 

Figure 4. versus Hcm for the six PDMS-forming systems listed in Table 2 the corresponding data for the polyurethane- and polyester-forming systems analysed previously are also plotted, for comparison (+). 

Reaction of isocyanates with different active H-compounds that form the fundamental basis for polyurethane coating systems are shown in Figure 2.60. The order of relative rates for these reactions is -NH -OH > H O > -COOH. Many types of catalysts are available that are used to control these reactions. It should be noted that while urethanes are produced only from reaction of isocyanates with alcohols, other reactions are also effectively controlled and used in different polyurethane coating systems. 

One of the technically and commercially most interesting applications of silicone surfactants is their use in the production of polyurethane (PU) foams (Fig. 13). These foams are formed by the reaction of polyols and isocyanates. The finished foams typically have cell sizes in the millimeter range and below with densities mostly less than 50 kg/m, thus forming systems with very large surfaces. The process of foam formation is complex and consists of different phases, which require a variety of properties of the used surfactants. The flexibility of silicone chemistry, especially the broad variety of silicone polyether chemistry, is particularly suited to meet these different requirements. 

As discussed by Woods (1990), polyurethane resin systems are usually formed by addition polymerization involving the reaction between alcohols with two or more reactive hydroxyl groups per molecule (polyol or diols) and isocyanates that have more than one reactive isocyanate group per molecule (a diisocyanate or polyisocyanate). 

Applications. The principal use for rigid polyurethane foams is for iasulation ia various forms utilized by a variety of iadustries. Lamiaates for resideatial sheatiag (1.2 to 2.5 cm thick with aluminum skins) and roofing board (2.5 to 10.0 cm thick with roofing paper skins) are the leading products with about 45 metric tons of Hquid spray systems also ia use. Metal doors iasulated by a pour-ia-place process coastitute another substantial use. 

Polymers. The molecular weights of polymers used in high energy electron radiation-curable coating systems are ca 1,000—25,000 and the polymers usually contain acryUc, methacrylic, or fumaric vinyl unsaturation along or attached to the polymer backbone (4,48). Aromatic or aUphatic diisocyanates react with glycols or alcohol-terrninated polyether or polyester to form either isocyanate or hydroxyl functional polyurethane intermediates. The isocyanate functional polyurethane intermediates react with hydroxyl functional polyurethane and with acryUc or methacrylic acids to form reactive p olyurethanes. 

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