Research On The Phosgene-free Process For The Preparation Of Hexamethylene Diisocyanate (HDI)

Diisocyanates are used to produce polyurethane(PU)adhesives, elastomers, rigid and flexible foames, and organic coatings. Hexamethylene Diisocyanate(HDI)based coatings have a unique combination of toughness, flexibility, solvent resistance, and light stability which has led to their widespread and continually increasing usage. At present, large industrial scale production of HDI has been accomplished by the phosgenation of the diamines ,using phosgene. Phosgene is notoriously difficult to manage industrially due to its high toxicity , corrosivity and severely corrosive hydrochloric acid as a byproduct. The phosgene-free routes have been developed and become the focus because of the environment problem. We have developed a substitute phosgene-free route using urea as the carbonylation regent. The technological process has many distinct advantages, such as economical, simple, and elimination the pollution in raw materials. The clean process of HDI synthesis mainly has two stages. As for the first stage, Hexamethylene Diurethane(HDU)is prepared by reacting hexamethylene diamine with urea and n-butyl alcohol in the presence of catalysts. HDU decomposes on heating to give HDI and alcohol in the subsequent process. The following research concerning about the process of HDI has been systematically done in this thesis. As for the process of HDU preparation. Firstly, the reaction equipment has been devised according to the results of the previous experiments. And then, the technological conditions to synthesize HDU were determined by using orthogonal testing method, the optimum conditions: reaction temperature 225℃, reaction time 5h, n(alco.):n(urea):n(HDA)=8:2.5:1. The influences of reaction temperature,reaction time and the proportion of the catalyst on the efficiency were discussed, and the temperature is major factor. Based on the possible mechanism, several catalysts were proposed and investigated, and results shows that the organic metal complex is an efficient kind of catalyst. Lastly, for the continuous preparation of HDU, the byproducts can be recycled completely. As for the process of HDU catalytic decomposition. Firstly, based on the specialty of the HDU decomposition, we devised a stable and efficient reactor for cracking in the liquid phase. And investigation on the several solvents shows that naphthenic oil is a good high boiling solvent for the reaction. Then the catalysts used in the decomposition reaction were researched, and there are good results using Zn and ZnO or combination as catalysts. The further research on the catalyst shows that Chromium Picolinate is a highly efficient catalyst for the reaction. Lastly, the effects of the temperature, the pressure, the concentration of reactant and the dosage of catalyst were investigated. Based on the feasibility analysis of the continuous process to produce HDI, a set of small scale continuous apparatus draft is made by cooperating with Hunan Chemical & Pharmaceutical Design Institute.