Urethane-urea systems

Ionic polymers are also formulated from TDI and MDI (43). Poly(urethane urea) and polyurea ionomers are obtained from divalent metal salts of /)-aminohen2oic acid, MPA, dialkylene glycol, and 2,4-TDI (44). In the case of polyureas, the glycol extender is omitted. If TDI is used in coatings apphcations, it is usually converted to a derivative to lower the vapor pressure. A typical TDI prepolymer is the adduct of TDI with trimethyl olpropane (Desmodur L). Carbodiimide-modified MDI offers advantages in polyester-based systems because of improved hydrolytic stabihty (45). Moisture cure systems based on aromatic isocyanates are also available. 

Thermoset polyurethanes are cross-linked polymers, which are produced by casting or reaction injection molding (RIM). For cast elastomers, TDI in combination with 3,3,-dichloro-4,4,-diphen5lmethanediamine (MOCA) are often used. In the RIM technology, aromatic diamine chain extenders, such as diethyltoluenediamine (DETDA), are used to produce poly(urethane ureas) (47), and replacement of the polyether polyols with amine-terminated polyols produces polyureas (48). The aromatic diamines are soluble in the polyol and provide fast reaction rates. In 1985, internal mold release agents based on zinc stearate compatibilized with primary amines were introduced to the RIM process to minimize mold preparation and scrap from parts tom at demold. Some physical properties of RIM systems are listed in Table 7. 

Figure 16.15 shows the readout result of the four-layered optical memory. The readout system was a reflection-type confocal microscope. For reading, a He-Ne laser was used as a light source, because the urethane-urea copolymer has little absorption for red light. The scattered light at the recorded bit data... 

In rigid urethane foaming systems using the CO2 formed by the water-isocyanate reaction a balance of the relative rates between the urea... 

Core/wall ratio cannot only decide the wall thickness of a microcapsule but also the effectiveness of the microencapsulation." The work" on the microencapsulation of xylitol by poly (urethane-urea), which was performed in a water-in-oil system, indicates that the most proper core/wall ratio is 77/23 in terms of high encapsulation yield and xylitol loading content. Another work on the microencapsulation of perfume by polyurea, which was performed in a oil-in-water system, found that with the decrease of the core/wall ratio, the size of the formed microcapsule increased even the original droplet size is roughly the same. An outward diffusion mechanism was proposed to explain... 

Details of the TDI and MDI foam model systems have been previously published [2]. The models require the use of mono-functional reactants that are quantitatively analysed to correlate structure-activity relationships for various classes of catalysts. A realistic thermal profile is produced through the imposition of an external exotherm. Urethane, urea, allophanate and biuret reaction products are quantified by liquid chromatographic analysis of quenched reaction samples. The models effectively account for such nonideal conditions as reactant depletion at variable rates, temperature and concentration-dependent catalyst activity, and catalyst selectivity as a function of isocyanate distribution. 

Most polyurethane sealant systems are based on the reaction of polyhydroxy materials (and diamines) with polyisocyanates in situ to produce a polyurethane (or urethane-urea) rubber. By varying backbone compositions, NCO/OH ratios, catalyst content, etc., a wide range of... 

This also shows the development of hydrogen bonding in the system. The urethane and urea bands will split into bonded and nonbonded bands. Work by Seymour, Estes, and Cooper has shown how the percentage of hydrogen bonding can be calculated (Seymour et al., 1970). 

---