Synthesis And Reaction Mechanism Of HDI Biuret Curing Agent

Hexamethylene diisocyanate (HDI) biuret is widely used in the application ofhigh-performance coatings, adhesives and elastomers, with excellent weather resistance, nicecompatibility with polyols or butyl acetate solvent and nice solvent resistance after curingwith polyols. On the basis of predecessors’ work, synthesis and characterization of HDIbiuret are studied deeply. The research mainly focuses on the following contents:1. Compared with the traditional two-step temperature-controlling bulk polymerization,HDI biuret prepared by bulk polymerization of one-step temperature-controlling has lesswhite insoluble polyurea. The introduction of polar aprotic solvents PMA with high boilingpoint improves the compatibility between HDI and water, and completelyinhibites the generation of white insoluble polyurea byproduct. Specifically, whentemperature remains at130℃during the reaction, the PMA is added to the system by onetime originally with the amount of10%of the total amount of reactants, and the water isadded dropwise within2hours, the clear HDI biuret prepolymer with44.28%mass fraction,11.03of the relative content of biuret and no polyurea, has been obtained.2. A combination of back titration, gas chromatography (GC), nuclear magneticresonance (1H-NMR), mass spectrometry (MS) and gel permeation chromatography (GPC)were adopted to explore the effect of temperature on the chemistry of HDI with water andstructure of HDI biuret in bulk polymerization. Back titration, GC and1H-NMR indicate that,in a certain temperature range, with increasing temperature, the chemical reaction rate and theHDI biuret yield increases, and the relative content of biuret groups increases rapidly. GPCtest results show that each reaction step has its own activation energy, and parallel to the ureaand biuret bond formation, such as urea crosslink, biuret, and the higher molecular weightoligomer can also be prodeced at the same time. In addition, the more higher molecularweight oligomer is produced from120℃to130℃. However, the higher molecular weightoligomer would decomposite at160℃, resulting in the increasing of urea crosslink. MSanalysis results show that: Not only the main reaction exists with the products of molecularweight of142,310and478, but also the side reaction occurs with the byproducts of themolecular weight of284,452,620and788.The synthesis process of TDI trimer has been studied in this paper. The effects of typesand dosage of catalyst, reaction temperature and the mole ratio of NCO/OH on the massfraction of-NCO and the free TDI content of prepolymer is investigated. Therefore, under theconditions of the solid content of50%, when the mole ratio of NCO/OH is11, the amount of catalyst is0.5%of TDI and the temperature is60℃, TDI trimer with low free TDI (0.5%) andhigh-NCO mass fraction (8.48%) has been obtained.