Generation of urethanes and isocyanates

Generation of Urethanes and Isocyanates from Amines and Carbon Dioxide... 

In 1991 Vandichel and Appleyard (15) described a new promising approach for the production of "soft" flexible slabstock urethane foam blown exclusively by COj generated by the water-isocyanate reaction. These workers found that by the addition to the formulation of certain hydrophilic materials a substantial hardness reduction is obtainable, thereby permitting a considerable reduction, or even total elimination, of CFC-11 from some "conventional" foam formulations. The hydrophilic additive is called CARAPOR 2001. An example is a foam produced with an ILD value of 80N at a density of 21.5 kg/m" (1.34 Ib/ft ) (15). 

Since polyurethanes are frequently used in household objects, their thermal degradation and products generated during burning were studied frequently [3-5]. Among these can be included studies on polyester-urethanes [6], polyether-urethanes [7], phenol-formaldehyde urethane [8], studies on the influence of fire retardants on polyurethane decomposition [9, 10], generation of isocyanates during decomposition [11], and other studies [12-17]. Some reports on thermal decomposition of polyurethanes are summarized in Table 14.1.1. 

The last major difference between urethane and nylon RIM systems lies in the extent of cure obtained in the mold. Many urethanes are polymerized just far enough in the mold to generate sufficient green-strength for demolding and handling the part. Parts are then oven post-cured to complete the reaction before atmospheric moisture reacts with the residual isocyanate In the part. Nylon RIM parts are fully cured in the mold. 

Since 1-propanol is monofunctional and reacts with "matched dissociation isocyanate" (at internal urethane chain positions) as well as "unmatched isocyanate" (at polymer chain terminal positions) its effects include cleavage of some polyurethane chains in the process of generating more urethane groups. As a result, polymer DP, melt viscosity, and torque drop until the shortstop is consumed, or escapes the mixture by volatilizaton. Figure 9 shovjs that the more 1-propanol used to shortstop the polymerizations, the more pronounced the polymer reversion was. 

These materials are often used in a partially polymerized form, known as prepolymers. These are reacted with glycols to form high-molecular-weight polymers, or with mixtures containing controlled amounts of water to form rigid foams (note that water reacts with isocyanates to form CO2, which is generated in situ to form the foam). R and R may be aromatic or aliphatic, to form aromatic, aliphatic, or mixed polyurethanes. Also, different polyurethane prepolymers are produced from diisocyanates and diamines, giving rise initially to a urea bond (NH —CO—NH—) instead of urethane ... 

Finally, in this regard, it is worthwhile to recall that the substituted isocyanate need not be converted to the amine. That is, reaction of the isocyanate with an alcohol yields the corresponding carboxyamic acid ester (the urethane) and reaction with an amine generates an A -substituted urea. Because water is ubiquitous, the urea is often isolated as a side product (Equation 10.66). 

In any case, the principal chemistry used in making a polyurethane foam is the formation of urethane linkages from hydroxyl groups and isocyanate groups and the simultaneous generation of a gas that will enormously expand the reacting mass. This gas highly dilutes the polymeric network as it forms and remains as either open or closed gas cells in the final foamed product. 

One-part urethane sealants (Table 3) are more compHcated to formulate on account of an undesirable side reaction between the prepolymer s isocyanate end and water vapor which generates carbon dioxide. If this occurs, the sealant may develop voids or bubbles. One way to avoid this reaction is to block the isocyanate end with phenol and use a diketamine to initiate cure. Once exposed to moisture, the diketamine forms a diamine and a ketone. The diamine reacts with the isocyanate end on the prepolymer, creating a cross-link (10). Other blocking agents, such as ethyl malonate, are also used (11). Catalysts commonly used in urethane formulations are tin carboxylates and bismuth salts. Mercury salt catalysts were popular in early formulations, but have been replaced by tin and bismuth compounds. 

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