Catalysis isocyanate-hydroxyl reaction

The preceding sections have shown the effect of changes in structure of isocyanate and hydroxyl compound, solvent and temperature on the isocyanate/hydroxyl reaction, and have indicated the role of basic catalysts. The largest commercial polyurethane processes utilize catalysed reactions, especially the preparation of foams. For this reason the catalysis of the isocyanate/hydroxyl reaction has been the object of extensive research. 

PTAd. We believe that this comparison illustrates the known weak catalysis of the isocyanate-hydroxyl reaction by acids (2 19, etc.). 

Tri-n-butylstannylpropanol was found to be an excellent catalyst, superior to DBTDL, in catalyzing the reaction of aromatic isocyanates with tertiary alcohols. Generally DBTDL and other dialkyltin catalysts show diminished catalysis with secondary and tertiary hydroxyl groups. 

H12MDI exists in three isomeric forms, the trans-trans, trans-cis, and cis-cis forms. Due to the fact the cis isomer is more sterically hindered, the trans isomer reacts considerably faster with a hydroxyl group than the cis isomer (27). As with other aliphatic isocyanates, catalysis (generally organotln catalysts) is usually used for both the prepolymer preparation and the cure reaction (cross-linking). 

Catalysis plays an important role in the deblocking or thermal dissociation of the blocked isocyanates. Notably organometallie compounds and tertiary amines are capable of lowering both the deblocking temperature and time as compared to the uncatalyzed systems. Wicks (61) has pointed out that since most of the deblocking reactions are carried out in the presence of hydroxyl... 

Organometalhc compounds based on lead, tin, bismuth and zinc are also used to catalyse a urethane reaction. Bismuth and zinc carboxylates are used because of the toxicity and disposal problems of lead and tin. Nowadays, alkyl tin carboxylates, oxides and mercaptide oxides such as dibutyltin dUaurate (DBTDL),dioctyltin mercaptide, stannous octoate and dibutyltin oxide are used successfully in all types of polyurethane applications (Table 6.4), among which DBTDL was found to be the most widely used catalyst. The catalytic effect of organometaUic compounds is due to their capacity to form a complex with the isocyanates and polyols. The catalysis mechanism involves interaction of the metal cation with isocyanate and hydroxyl groups, followed by rearrangement of the resulting complex to yield the final urethane product. 

Two general classes are very well known. The first is composed of tertiary amines, of which DABCO, tertiary alkylamines, substituted morpholines, piperazines, guanidines, and substituted hydroxy amines are representative types. A wide range of activities is represented in this class, but stronger catalysts are sometimes needed to promote the reaction of secondary hydroxyl groups with isocyanates. Stronger catalysis is supplied by catalysts of the second class, organotin compounds like dibutyltin dilaurate... 

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. 

As often happens with polymerization reactions, simple rate laws can seldom describe the whole course of reaction because of catalysis or inhibition by the urethane groups formed or by the initial reagents. Self-association of metallic catalysts, or their loose complexation by products or reagents, also prevents correlation of rates of reaction by simple proportionality or even power-law relations. Catalysis of isocyanate reaction with hydroxyls [250, 251] is by far the best understood. 

Thermosetting acrylic binder systems utilize copolymers of functional and nonfunctional acrylic (or similar) monomers. The functional monomers are incorporated for reactivity with crosslinkers. The most common functional monomer for reactions is the hydroxyl group. The hydroxyl groups on the acrylic copolymers react with melamine and urea resins (amino resins) and with polyisocyanates. These reactions are shown in Figure 11. The reaction of hydroxy functional polymers with amino resins require acid catalysis and heat. The reaction with polyisocyanates can occur at room temperature as well as at higher temperatures. A number of materials will catalyze the hydroxyl/isocyanate reaction (organotin compounds, acids, amines, metal salts, etc.)(9). 

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