Water/isocyanate reaction

The reaction of water with isocyanate is shown in the third item of Fig. 1 [5]. The water/isocyanate reaction is the major curing mechanism for the one-component urethane adhesives. Most one-component urethanes are based on an isocyanate-terminated prepolymer (I). Usually, the moisture in the air is used to cure the adhesive, but in some instances, a fine mist of water may be introduced on top of the adhesive before the bond is closed, in order to facilitate cure ... 

For all its benefits, the water/isocyanate reaction can be troublesome as well. All raw materials that go into a one-component moisture-cured adhesive must have very low moisture content, usually less than 0.05% water. If higher water levels are present, the adhesive can start curing in the reactor, causing an increase in viscosity, or, in extreme cases, gelation. 

The most common catalyst used in urethane adhesives is a tin(lV) salt, dibutyltin dilaurate. Tin(IV) salts are known to catalyze degradation reactions at high temperatures [30J. Tin(II) salts, such as stannous octoate, are excellent urethane catalysts but can hydrolyze easily in the presence of water and deactivate. More recently, bismuth carboxylates, such as bismuth neodecanoate, have been found to be active urethane catalysts with good selectivity toward the hydroxyl/isocyanate reaction, as opposed to catalyzing the water/isocyanate reaction, which, in turn, could cause foaming in an adhesive bond line [31]. 

The prepolymer used for this study was an MDI-based preparation (Suprasec 1002) from Huntsman PU, Brussels, Belgium. Immobilization of the enzyme was accomplished by coating the inside structure of a reticulated foam with an acetone solution of a hydrophilic polyurethane prepolymer. A 45-pore/in. reticulated foam cut into sheets 0.25 in. thick was used. The coated reticulated foam was immersed in a catalase solution (5 pg/ml) at 4°C and left in the solution for 1 hour to ensure a complete solution. It is known that both the water and some functionality in the catalase react with the isocyanate groups to cause polymerization (the water-isocyanate reaction) and chain termination (the catalase reaction). Controlling the relative concentrations and temperature permits control of the physical properties of the composite and the ability of the foam to function as an enzyme. 

In connection with their study of the water-isocyanate reaction, Shka-penko et al. 51) found that at 80° o-tolyl isocyanate reacts with sj/m-ditolylurea in dioxane at 0.2 M concentration and forms biuret at approximately the same rate as that of the reaction between the isocyanate and water takes place. 

Chemical Blowing Agents. The conventional gas-generation reaction for flexible urethane foams is the water-isocyanate reaction which was first described in a German patent (122). Its chemical reaction is shown as follows ... 

In contrast, tertiary amine catalysts (Table 9) mainly accelerate the water-isocyanate reaction, that is, the gas-generation reaction, but are also catalysts for the hydroxyl-isocyanate reaction. 

Polyurethane formation is mainly accelerated by a tin catalyst, and gas generation, supplied by the water-isocyanate reaction, is mainly promoted by a tertiary amine catalyst. Therefore, if collapsed foams are obtained, increased tin catalyst can solve the foam collapse. Likewise, if... 

Some examples of urea foams prepared by the water-isocyanate reaction are as follows. ICI disclosed the foam prepared by the water-isocyanate reaction in the jn-esence of imidazole compounds (137). PRB NV disclosed a foam j epared in the presence of water-soluble saccharide and polyol (139). Bayer AG disclosed a foam prepared by using 1.5 to 50 parts of alkanolamine with water and 100 parts of polyisocyanate (138). Schaum Chemie disclosed foams prepared by using lower alkanols and alkylene diols (140). 

Flexible polyurethane foams are blown with water, methylene chloride, and chlorofluorocarbons (CFCs). Carbon dioxide from the water/isocyanate reaction functions as the blowing agent. The methylene chloride and CFCs assist in the blowing and contribute properties such as added softness and resilience. The CFCs also contribute to the insulation properties of rigid urethane foams. 

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). 

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

Polyester- and polyether-based rigid urethane foams generally require a surfactant, whether expanded with COj from the water-isocyanate reaction, or with an inert blowing agent such as fluorocarbon. Without surfactant the foam may collapse or have a coarse cell structure. Castor-oil-based systems generally do not require surfactants, but better results will be obtained if they are used (20). 

As stated, tertiary amines catalyze both the hydroxyl/isocyanate and the water/isocyanate reactions. One-shot foams utilizing primary hydroxyl-terminated polyesters as well as all types of prepolymer foams require tertiary amine catalysis only. Polypropylene ether one-shot foam formulations based on triols, in part, because of their low viscosity (about 300 cP versus 10000-30000 cP for polyesters or prepolymers) require the use of tertiary amine-metal catalyst combinations, even if the percentage of primary hydroxyl groups in the polyether is increased by capping with ethylene oxide. This is because of the relatively low polypropylene glycol activity. 

Combinations of one or more tertiary amines and a tin catalyst are commonly used in the production of polyurethane, especially in the RIM process. This not only ensures rapid reaction and complete cure by utilization of the synergistic effect, but also allows adjustment of a proper balance between hydroxyl/isocyanate and water/isocyanate reactions resulting in cellular structures without splits, voids or shrinkage. 

In the manufacture of flexible foams, carbon dioxide generated from the water-isocyanate reaction has replaced the external blowing agent, yet methylene chloride is still widely used with acetone and liquid carbon dioxide. Cyclopentane has become the main blowing agent for refrigeration appliances, w-pentane has only been exploited for sandwich panels and other building insulation. 

The water-isocyanate reaction forming carbon dioxide is commonly used to form open-cell flexible foams. Trichlorofluoromethane (TCFM) is a physical blowing agent relying on heat from the urethane reaction exotherm to boil this is used to form rigid closed-cell foams and most self-skinned moulded products. There is considerable concern over the effect of fluorocarbons on the Earth s ozone layer, and suitable replacements are actively being sought. 

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