Urethane reaction mechanism

The number of earlier publications can be a measure of the importance of a scientific topic. Numerous authors tried to predict more accurately the influence of hydrogen bonding on the PUs reactants reactivity and on the urethane reaction mechanism [89, 101-103]. The complexity of the hydrogen bonding systems determined by their high instability and the lack of proper information on the nature and number of associated alcohol species, maintained however so far a high degree of incertitude. 

Each of these product types will be discussed, including the method of application, the type of substrates that are bonded by each approach, and one or more typical adhesive formulations. In order to understand the adhesive cure mechanism, a brief review of the common urethane reactions is needed. 

The urethane reaction is particularly useful for solid propellant applications because of its quantitative nature, convenient rate which can be adjusted by proper choice of catalysts, and the availability of many suitable hydroxyl compounds which permit the tailoring of propellant mechanical properties. Despite the quantitative nature and apparent simplicity of the urethane reaction R NCO + ROH - R NHCOOR, its exact course has not been fully explored yet. It does not follow simple second-order kinetics as the above formula would suggest since its second-order rate constant depends on many factors, such as concentration of reactants and the nature of the solvent. Baker and co-workers (2) proposed that the reaction is initiated through the attack of an alkoxide ion on the carbon atom of the isocyanate group... 

Poly butadienes. Hydroxyl-terminated polybutadienes are comparatively late comers and are still in the development stage. They combine the high specific impulse of the well-proved carboxy-terminated polybutadienes with the clean, stoichiometric urethane reaction yielding propellants with unsurpassed mechanical properties. 

Analytical Tests. The properties of the binder network and thus the mechanical properties of the final propellant depend largely on the stoichiometry of the urethane reaction. Reproducibility requires a host of analytical tests such as the determination of hydroxyl number, acid... 

The presence of the methyl group at a double bond therefore involves two new effects in the reaction with carbethoxynitrene—namely the predominance of substitution reactions over addition reactions and the formation of a product in which the double bond is displaced (about 20% of the total mixture). In addition to the products of Structures D, Ei, E2, and E3 expected as the result of Lwowski s work, another product, probably of Structure E4, is formed (Figure 6). This-is now being checked and necessitates the separation of the products either at the urethane stage or more effectively at the amine stage. Once the structure of this product is established definitely, the conditions affecting its formation remain to be studied, and this may yield an explanation of the reaction mechanism involved. 

Based on the presented data, the probable mechanism of the catalysis of urethane reaction by the DBTDL catalyst is depicted in Figures 6 and 7. 

The reactivity of different isocyanates varies widely, and the most reactive NCO groups can react with almost any compound that contains an active hydrogen [1, 2, 16]. The reactivity of the nucleophilic groups also varies primary amines are more reactive towards NCO than primary alcohols, followed by water, secondary and tertiary alcohols, other urethanes, carboxylic acids, and carboxylic acid amides in that order [16]. The isocyanate will, of course, react with the water present in the EPI formulation to form amines followed by further reactions producing urea and biuret. The mechanism of this reaction is shown in Pig. 5. As can be seen from the reaction mechanism CO2 is a byproduct of this reaction. 

Various catalysts are used to prepare polyurethane at a relatively low temperature and with a much faster rate of polymerisation than would be the case with an uncatalysed reaction. Catalysts may be classified into two broad categories namely, amine (basic) compounds and organometalhc complex compounds. Tertiary amine is stiU one of the most frequently used urethane catalysts. Commonly used amine catalysts are triethylenedi-amine (TEDA), l,4-diazabicyclo[2.2.2]octane (DABCO), triethylamine (TEA), dimethylethanolamine (DMEA) and dimethylcyclohexylamine (DMCHA). The catalysis mechanism of tertiary amine catalysed urethane reaction involves complexation of the amine with isocyanate groups, followed by reaction of the complex with alcohol to produce polyurethane. A list of catalysts used in polyurethane preparation is given in Table 6.4. 

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. 

Scheme 6 Mechanism of the photobase-catalyzed urethane reaction.

Therefore, in general, a thermal curing resin contains a radical curable function that is activated by light and a hardening agent Acrylic resins such as urethane acrylate and epoxy acrylate are mainly used for the radically curable resin, and an epoxy resin is mainly used for thermal curing. The reaction mechanism due to the heat and UV irradiatiOTi are shown in Fig. 7.16. 

The kinetic features were exposed for two cases of IPN formation via reaction injection molding (RIM) and via usual molding. Kinetic measurements were performed at isothermal conditions using the DSC method and have shown that due to a different reaction mechanism (polyaddition and polymerization), urethane formation always begins after the mixing of all components. The reaction proceeds under conditions when the polyester component is not yet reacted and urethane formation proceeds in a solution of polyester network components. Polyester network begins to form when PU is either fully or partially formed, i.e., in the medium of the other network. Again, in this work the question about phase separation had not been considered. 

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 ... 

Problem 31.9 Show the mechanism of the nucleophilic addition reaction of an alcohol with an isocyanate to yield a urethane. 

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