Rigid polyurethane formation

Urethane network polymers are also formed by trimerization of part of the isocyanate groups. This approach is used in the formation of rigid polyurethane-modified isocyanurate (PUIR) foams (3). 

Mechanical cleaning of a multitubular stainless steel condenser (blocked by a rigid polyurethane foam) by rodding the tubes was laborious, so chemical cleaning with cone, nitric acid was attempted. When the initial vigorous reaction ( fireworks ) subsided, owing to crust formation, the rod was again inserted, but a sudden explosion occurred which ruptured the condenser. 

Use Organic intermediate, cross-linking of rigid polyurethane foams, chelating agent, humectant, gas absorbent, resin formation, detergent processing. 

Propoxylated derivatives of sorbitol are used as the polyols in the formation of rigid polyurethane foams. Sorbitol can be dehydrated and esterified with stearic acid to give sorbitol mono- or tristearate. The ester then can be ethoxylated with about 20 ethylene oxide units to give an ethoxylated surfactant. Hydrolysis of sorbitol results with glycerin, which is used in the pharmaceutical and personal care products field [108,109],... 

Rigid polyurethane foams have a special problem when it comes to making a fair statement about their R-values. Essentially, all of these products are expanded with chlorofluorocarbon 11 (CFC-11), which is trichlorofluoromethane. At the time of formation, all of these materials have essentially the same R-value of about 7.5 to 8.0 per inch of thickness. At one time, that is how these products were marketed. The initial thermal resistance, however, changes with time. Where the foamed plastic is exposed to air, the air migrates into the cells, diluting the chlorofluorocarbon gas. The thermal resistance decreases when this takes place. This is a slow process and may go on for years. To the extent that the foam is sandwiched between air impervious skins, the process is all but halted. 

Hilado conducted a comparative study on the flammability of great many flame-retarded polyurethane foam grades. The flammability of rigid polyurethane foams are markedly reduced by formation of an isocyanurate structure. The exploitation of this possibility is detailed in Section 5.1.5. 

Rigid polyurethane foams have been synthesized from industrial raw materials and phosphorus-containing flame retardants, obtained by the interaction of dialkyl H-phos-phonates with amino alcohols. The phosphorus-containing compounds 1 and 2 are used as modifiers [105]. During polyurethane synthesis, both modifiers speed up the times for foaming, gel formation, and surface drying, and decrease the time for foam growth. 

The polyols used include PO adducts of polyfiinctional hydroxy compoimds or amines (see Table 4). The amine-derived polyols are used in spray foam formulations where high reaction rates are required. Crude aromatic polyester diols are often used in combination with the multifunctional polyether polyols. Blending of polyols of different functionality, molecular weight, and reactivity is used to tailor a polyol for a speciflc application. The high functionality of the polyether polyols combined with the higher functionality of PMDI contributes to the rapid network formation required for rigid polyurethane foams. 

Much work has been done on the incorporation of castor oil into polyurethane formulations, including flexible foams [64], rigid foams [65], and elastomers [66]. Castor oil derivatives have also been investigated, by the isolation of methyl ricinoleate from castor oil, in a fashion similar to that used for the preparation of biodiesel. The methyl ricinoleate is then transesterified to a synthetic triol, and the chain simultaneously extended by homo-polymerization to provide polyols of 1,000, 000 molecular weight. Polyurethane elastomers were then prepared by reaction with MDl. It was determined that lower hardness and tensile/elongation properties could be related to the formation of cyclization products that are common to polyester polyols, or could be due to monomer dehydration, which is a known side reaction of ricinoleic acid [67]. Both side reactions limit the growth of polyol molecular weight. 

The formation of cellular products also requires surfactants to facilitate the formation of small bubbles necessary for a fine cel] structure. The most effective surfactants are polyoxyalkylene-polysiloxane copolymers. The physical properties of polyurethanes are derived from their molecular structure and determined by the choice of building blocks as well as the suprainolecular structures caused by atomic interaction between chains. The ability to crystallize, the flexibility of the chains, and spacing of polar groups are of considerable importance, especially in linear thermoplastic materials. In rigid cross-linked systems, e.g., polyurethane foains, other factors such as density determine the final properties. 

---