Diphenylmethane diisocyanate reaction rate

The catalysts of reactions between 4,4 -diphenylmethane diisocyanate (MDI) and alcohols in N,N-dimethylformamide (DMF) by dibutylin dilaurate has been investigated. The reaction rate of the catalyzed urethane formation in DMF is proportional to the square root of dibutylin dilaurate concentration. This result differs from that of similar studies on apolar solvents. The catalysis in DMF can be explained very well by a mechanism in which a small amount of the dibutylin dilaurate dissociates into a catalytic active species. 

The decrease in rate after 50% reaction is quite apparent in the reaction of tolylene diisocyanate with the polyester at 29°C (Fig. 11). This change in rate illustrates the reduced reactivity of the 2-position isocyanate group, having steric hindrance from the ortho methyl substituent, as well as the lesser activating influence of a meta urethane substituent compared to a meta isocyanate substituent. An increase in reaction temperature favours the slow reaction more than the fast, as would be expected if differences in activation energy accounted for at least part of the difference in rates. Thus at 100°C there was little decrease in rate of reaction with TDI. In the case of 4,4 -diphenylmethane diisocyanate (Fig. 12), there was little change in rate after 50% reaction at any of the temperatures studied. 

Di-n-butyltin catalysts are being used in the preparation of polyurethane foams. Most polyurethane foams utilize aromatic isocyanates such as toluene diisocyanate (TDI) or diphenylmethane diisocyanate (MDI) as the isocyanate, and a polyester or polyether polyols as the coreactant. Tertiary amine catalysts are used to accelerate the reaction with water and formation of the carbon dioxide blowing agent. To achieve a controlled rate of reaction with the polyol, an organotin catalyst can be used. Polyurethane foams are not only applied in place, but are also cast in a factory as slabstocks. These foam slabs are then cut for use in car seats, mattresses, or home furnishings. DBTDL is an excellent catalyst in high resiliency slabstock foams. DBTDL shows an excellent reaction profile for this application replacement for DBTDL in such an end-use is difficult and requires a substantial reformulation of the foam. 

Fig. 4.1. Temperature effects on reaction rate in a urethane. (Reaction of 4,4 -diphenylmethane diisocyanate with glycol adipate polyester in chlorobenzene.) (From Wright Gumming, 1969).

Wik et al. [26] reported the reaction of a natural polyol derived from castor oil and polymeric 4,4 -diphenylmethane diisocyanate (pMDl) used to prepare solid materials reinforced with nanocellulose. The polyol was obtained by the reaction of castor oil with triethanolamine. The reaction between the castor oil-based polyol and the isocyanate started almost immediately after mixing these main components, even without the addition of a catalyst. This fact forced the use of an organic solvent to reduce the initial reaction rate, which was evaporated during the curing process. Nanocellulose obtained by acid hydrolysis of commercial microcrystalline cellulose was added to the polyol (0.5, 1, and 3 wt%) and dispersed by sonication. It was found that the addition... 

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