Urethanes , reactions

Catalysts such as dibutyl tin dilaurate or tertiary amines are added to promote the urethane reaction and/or subsequent moisture cure. Dimorpholine diethyl ether is particularly effective at promoting moisture cure without promoting allophanate side reactions at the application temperature (which leads to instability in the hot melt pot) [29]. 

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. 

Over 40 chemical reactions are used in urethane chemistry. The six most common urethane reactions that are relevant to adhesives are shown in Fig, 1. The monomeric forms of the reactions are shown for simplicity s sake however, most commercially useful products for polyurethanes are based on polyfunctional isocyanates and polyfunctional alcohols or polyols . 

The monomeric form of the urethane reaction is shown in the first item of Fig. 1. The urethane or polyurethane reaction for an adhesive would be based... 

The first urethane reaction in Fig. 1 is used in two major ways in adhesives. In one case, a two-component adhesive usually employs a polyol and polyisocyanate with catalyst. This can react at room temperature to form the polyurethane. The second use of this reaction is to make an isocyanate-terminated prepolymer. Reacting a stoichiometric excess of isocyanate with polyol can produce an isocyanate-terminated prepolymer. A prepolymer is often made with an NCO/OH ratio of 2.0, as shown below, but the isocyanate ratio can range from 1.4 to over 8.0, depending upon the application ... 

The allophanate linkage is formed by the reaction of urethane with isocyanate, as shown in the fourth item of Fig. 1 [7], Isocyanates can react with many active hydrogen compounds. The active hydrogen of the urethane linkage is not very reactive, but if reaction temperatures get high enough (usually in excess of 100°C), or in the presence of certain allophanate catalysts, this reaction can actually become favored over the urethane reaction (see pp. 180-188 in [6]). 

Open times of two-component urethanes can vary widely, depending on the level of catalyst. Reaction times can vary from 90 s to over 8 h. Dibutyltin dilaurate is the most common catalyst employed to catalyze the urethane reaction. This is normally added to the polyol side. A tertiary amine may also be added in small amounts. Tin catalysts do not catalyze the amine/isocyanate reaction very well. Acids, such as 2-ethyl hexanoic acid, may be employed to catalyze the amine/isocyanate reaction where needed. 

Polyester polyols (Scheme 4.4) are prepared by condensation polymerization of dicarboxylic acids and diols. An excess of diol ensures OH functional product, minimizing die possibility of residual acid groups which react with isocyanates to generate C02 and act as inhibitors in catalyzed urethane reactions. The reactants are heated at 200-230°C under vacuum to remove the water by-product and drive the reaction to completion. The most common coreactants include adipic... 

The formation of Intermediate compounds (e.g. carbamlc acid) Is not described in the model. So, the formation of urethane (reaction 1) and the hydrolysis of the isocyanate (reaction 2) are the rate-determining steps. 

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. 

Figure 1. Dependence of the rate of the Fe(AA)3-catalyzed urethane reaction on HAA addition...

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... 

Bonnet [3] prepared aqueous dispersions of bitumen and asphaltenes using the urethane reaction product of 4,4 -diphenylmethane diisocyanate and PolyBd diol. 

The reaction between an isocyanate and the hydroxyl group in a polyol will take place without a catalyst, but at too slow a rate to be practical. Without a catalyst a foam may expand, but it may not cure adequately to give good physical properties. The urethane reaction can be catalyzed by basic materials such as the tertiary amines (20). 

Delayed-action catalysts have been made successfully. Buffered amine catalysts, where the activity of the amine has been reduced by the presence of an acid, have also been used. Acidic materials can be used to retard the urethane reaction. Hydrogen chloride and benzoyl chloride have been used in combination with amine-type catalysts to control reaction rates. A small percentage of acid can increase foaming time from 2.2 to 6 minutes (20). 

For the non-catalyzed urethane reactions It was established (23) that the relative reactivities (ratios of rate constants) of substituted aromatic Isocyanates correlated with the structural parameter a of the substituent R (measuring the electron withdrawing ability of the substituent R) according to the Hammett equation ... 

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. 

Effect of Hydrolyzable Chlorine on Activity of Tin Catalysts. The effect of small amounts of hydrolyzable chlorine on the catalytic activity of DBTDL was studied on the model aliphatic system—isocy-anatoethyl methacrylate and n-butanol. The presence of the hydrolyzable chlorine in isocyanate usually decreases the reactivity of isocyanates in the urethane reaction. The results of measurements of the chlorine effect on the change of the rate constant is summarized in Figure 8. It was determined that the very small amounts of the hydrolyzable chlorine, especially in the form of carbamoyl chloride, increased at the beginning the rate constant for the urethane reaction catalyzed by DBTDL and after achieving the maximum at 500 ppm of chlorine the reactivity decreased. This effect was not observed when benzoyl chloride was used in place of carbamoyl chloride. It was assumed that the activation effect of the chlorine was due to the interaction of the carbamoyl chloride with the DBTDL catalyst. In order to understand this effect, the interaction of DBTDL with carbamoyl chloride of hexamethylene diisocyanate (with and without the presence of n-butanol) was studied using the IR technique. Results are summarized in Figure 9. 

The success of urethane reaction injection molding (RIM) has led to a search for other chemistry with suitable characteristics for RIM which can yield products with useful properties and good economics. Lactams, especially caprolactam, are potentially interesting candidates. 

Figure 1. DSC curve of urethane reaction formulation (extended), 18 mg specimen heated at 10°C/min in nitrogen.

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