Studies On Synthesis Of Linear Polyurethane By Non-isocyanate Melt Transesterification

Various synthetic processes of polyurethane were reviewed in this paper. With CO2emission reduction as a final target, tansesterification process as the main synthesis process, a industrial process chain was identified which began from using carbon dioxide for the synthesis of dimethyl carbonate(DMC), then synthesizing1,6-hexamethylene carbamate (HDU) and polycarbonate diol (PCDL) from dimethyl carbonate(DMC) and finally synthesizing polyurethane (PU) by the melt transesterification from HDU and PCDL. PU was the green chemical product listed at the end of the process chain. With the melt transesterification synthesis of polycarbonate-type polyurethane (PCPU) process from HDU and PCDL(Mn=2000) as the research key, optimization of raw materials------HDU synthesis technology, analysis of catalytic principle, kinetics of prepolymerization reaction, optimization of polymerization process and performance of the product and so on were carried on the exploration and research.(1) With1,6-hexamethylenediamine (HDA) and dimethyl carbonate (DMC) as raw material, reaction distillation as reinforcement method,1,6-hexamethylene carbamate (HDU) was prepared. The process was optimized and the optimal process conditions were as follows: raw material ratio of n(DMC):n(HDA)=8:1; catalyst dosage m(Na-MCM-41)/m(HDA)=0.05%; reflux ratio:3; full reflux time:30min; formic acid amount m(formic acid)=m (HDA)*0.3%; injecting time of formic acid:0.5h later when tower kettle’s temperature got80℃; Control the temperature of tower kettle not higher than93℃. Under these conditions, the conversion of1,6-hexamethylenediamine could reach99.55%, the product selectivity was87.93%and the yield was87.54%. Products can be used in PU synthesis after purification and recrystallization.(2) The reaction mechanism of carbamate and polyols were analyzed, and the results show that there were no-NCO groups existed during the reaction process which proved that this reaction was a real melt tansurethane reaction. Further research showed that the reaction occurred must be under the condition of catalyst.(3) Different kinds of catalysts were using for this polycondensation reaction system. Organic tin-catalysts and titanium-catalysts can effectively catalyze the reaction. Potassium acetate was also a kind of effective transesterification catalysts, but because of its strong alkaline the product would be in poor appearance. Organic tin——dibutyl tin oxide (DBTO) had the best catalytic effect best in Sn-catalysts. Relatively, organic titanium——four ethyl ester titanate (TET) was the best. Organic tin and organic titanium catalysts had different catalytic effect. The results showed that organic tin had the better catalytic effect than organic titanium. Choose dibutyl tin oxide (DBTO) for this process as the best catalyst.(4) The mechanism of reaction catalyzed by DBTO was invesgated and discussed. It was found that Sn cation in DBTO molecule can activate carbonyl oxygen atom of HDU molecule in the process of reaction and a kind of the intermediate——SnA was formed which has good catalytic activity for this reaction.(5) With SnA as catalyst, tansurethane reaction kinetics of HDU and poyol at different temperature were studied. The results showed that the model for transurethane reaction kinetics of HDU with polyethylene glycol was secondary reaction which was consistent with hypothesis. When SnA concentration was0.1%wt the reactants, the relation formula between reaction rate constant and the reaction temperature was k=2.51×1012e99.62×103/RT.The reaction of the apparent activation energy was99.62KJ/mol. Under the condition of catalyst SnA,the reaction enthalpy was101.34KJ/mol. As the results showed, this transurethane reaction was endothermic reaction and the equilibrium constant increasedwith reaction temperature.(6) The polymerization process catalyzed by DBTO was optimized. The optimum process conditions were as follows:the time of preploymerization was1h (atmospheric pressure for20min and0.08MPa micro vacuum for40min), the preploymerization temperature was100℃, catalyst dosage was0.125wt%, polycondensation temperature was185℃, mole ratio of raw material was n(HDU):n(PCDL T-6002)=1:0.99, the optimal polycondensation time was4h.(7) Under the optimal process conditions, polycarbonate type polyurethane product was synthesized of which the intrinstic viscosity was61.30ml·g-1, shaw hardness was35.97D, Mn=50000, Mw=79000, Pd=1.55. Additionally, three kinds of PCPU sample were prepares from different PCDL&HDU which were characterized by FTIR, XRD and TGA. All the products’initial thermal decomposition temperatures were around300℃.(8) Using different chain extender, different kinds of polycarbonate type polyurethane were prepared and their material mechanical properties, thermal stability, hydrolytic stability, etc were characterized. Compared with no chain extender product, all the products’properties were similar. These linear polymer products have excellent water resistance. The BDO-HDU-PCDL and HDO-HDU-PCDL were the higher crystallinity products. The tensile strength of the product was from10to16MPa, elongation at break was of1000-1400%. The initial thermal decomposition temperature was about320℃.