Process Investigating And Trimerization Of Toluene Diisocyanate-based Prepolymer

For two-component polyurethane materials, toluene diisocyanate(TDI)-based polyisocyanate prepolymer imparts excellent mechanical and thermal properties, high reactivity and good corrosion resistance, and it is widely used in coating, fibers, foams and elastomer. However, most of researches in TDI-based polyisocyanate prepolymer usually focus on providing a method for preparation and catalysts choosing, and seldom study the optimization of the reaction conditions, the reaction process and mechanism in TDI self-polymerization reaction. In this paper, the conditions and process and the alcohol-modified in preparation of TDI based-polyisocyanate prepolymer was investigated, and series of characterization techniques were employed to monitor the polymerization process and characterize the structure.Firstly, the effects of type and concentration of catalyst and the reaction temperature were investigated, and then FT-IR, MS and 13C-NMR were utilized to character the structure of isocyanurate group. The results show that when 0.5 wt% of the Mannich base(M-01) catalyst was added, high catalytic activity was obtained and excellent selectivity in trimers and pentamers was also presented. In addition, the content of finally residual TDI monomer is suitable for post-treatment in this situation. The trimers and pentamers were consumed rapidly to form large amounts of higher-molecule-weight oligomers at high temperature, and the product is yellow. While, at a low reaction temperature, the selectivity in producing the trimers and pentamers is better, and the reaction is moderate and easy for controlling the polymerization. Therefore, the reaction at the temperature of 50 oC with 0.5 wt% M-01 catalyst is a preferred condition for the preparation of polyisocyanate prepolymer.Secondly, in order to control the reaction and predict the content of individual oligomer in TDI self-polymerization, GPC, GC and back titration methods were employed to monitor the reaction process. Experimental results show that the mechanism of step-growth polymerization could explain TDI self-polymerization well, and the formation of individual component could be interacted with each other. Moreover, the content of trimer and pentamer increased firstly and then decreased means that the steric hindrance would affect in formation and consumption of trimer and pentamer. Thus, we could control the reaction to obtain optimal performance of the prepolymer when the reaction time is 1 h. In addition, a kinetic simulation was established based on polymerization mechanism and the simulated data were in good agreement with the experimental results.Finally, the reaction activity and selectivity of alcohol with branched chain and different lengths of carbon chain were investigated in the process of prepolymer modification, and the structure of the final products was characterized by FT-IR and MS analyses. The results showed that the small molecular alcohols with a shorter carbon chain had a higher activity in reaction of NCO groups, and they presented an excellent selectivity in the reaction of free monomers. The opposite results would be obtained when the prepolymer was modified by an alcohol with a long chain. When TDI-based prepolymer was modified by isopropanol at NCO: OH =11:1, the contents of trimers and pentamers were the highest and accounted for 40% of the final prepolymer, and the content of free TDI was 0.4%. Similarly, the tolerance of the modified product was improved with the addition of isopropanol.