Study On The Preparation And Performance Of Nylon-64 In Supercritical CO₂

Polyamide-64 (nylon-64) is one of polymer materials with high melting point. Compared with nylon-66, nylon-64 has many outstanding performances, and can be used as engineering plastics in fields of electronic, electrical, automotive industries and so on. However, the research on its synthesis has seldom been reported in the word. The thesis has reported the synthesis of nylon-64 by using a green medium-supercritical CO2 (SC-CO2). SC-CO2 has some advantage such as lower critical temperature and pressure, the density similar to liquid and fluidity similar to gas. SC-CO2 can produce the swelling and the plasticization to the carbon chain or the carbon heterochain polymer, which promote the reactivity activeness of terminated fumtional groups. Based on the research of polymerization for nylon-64, the following conclusions can be obtained:First, by using SC-CO2 medium, the samples of nylon-64 are synthesized successfully through investigating different process conditions. The chemical structure and physical performance of the sampres are tested. The intrinsic viscosity of the samples is measured and the value of its intrinsic viscosity is in the range of 0.7-1.47. In addition, the thermal properties of the samples are measured by DSC and TG. The melting point is 290℃-302℃and initial decomposition temperature is 322℃-420℃. These results show that the samples have a higher thermal stability.Second, in SC-CO2 medium, the effect of different synthesis technologies on the molecular weight of nylon-64 prepared by one-step method (including one-step method with two stages, one-step method with three stage and the increasing gradually temperature method) is investigated. The intrinsic viscosity of nylon-64 prepared by the increasing gradually temperature method is up to 1.21, its reference molecular weight is 29,600 and the melting point is up to 300℃. Based on studying different prepolymerizing time, post-polycondensation temperature and pressure, the results show that the appropriate condition is determined as follows:the prepolymerizing time is lh-2h, the post polycondensation temperature is 240 ℃-280℃and the time is 3h. And the molecular weight increases with increasing the pressure.Third, the effect of excess hexamethylene diamine on the pH value of nylon-64 salt and the molecular weight of nylon-64 is discussed. When hexamethylene diamine excesses from 3% to 8%, the pH value of nylon-64 salt is between 7.35 and 8.15. When hexamethylene diamine excesses 4% (pH value of 7.43), the highest molecular weight of nylon-64 can be obtained. If a certain amount of water is added into the nylon-64 salt, decomposition of nylon-64 in the early stage of polymerization can be avoided and the molecular weight of nylon-64 is imp oved effectively.Fourth, i order to further increase the molecular weight, the effect of meltingness-solid phase conde sation within SC-CO2 on the molecular weight of nylon-64 is studied. The result shows that.he highest molecular weight can be obtained when the solid phase condensation temperatue is 230℃and the time is at 4h. For example, the intrinsic viscosity of nylon-64 prepared oy the anhydrous nylon-64 salt is 1.34 (reference molecular weight is 33,800), the melting point is 302℃; and the intrinsic viscosity of nylon-64 prepared by nylon-64 salt co(?)aining 50% water is 1.47 (reference molecular weight is 38,000), the melting point is 301℃.Fifth, in order to break the condensation equilibrium in closed system and further increase the molecular weight of nylon-64, the method of coordination reaction between nylon-64 and nanoparticles within SC-CO2 is applied. And nylon-64 and particles of SiO2 are prepared successfully. The reference molecular weight of the samples can be up to 28,400, and the melting point is 296℃. The particle of SiO2 ranges from several hundred to several microns.Sixth, the nonisothermal crystallization kinetics of nylon-64 is also studied by using DSC. By the Avrami equations modified by Jeziomy, the results show that the nonisothermal crystallization kinetics of nylon-64 includes two stages. In main crystallization stage, when the cooling rate is below 10℃/min, the nucleations process is mainly homogeneous and the growth of the crystal particles is three dimension; And when the cooling rate is below 20℃/min, the growth of the crystal particles is restricted and is between two dimensions and three dimensions. In the inferior crystallization stage, the growth of the crystal particles is mainly the one dimension. When the relative crystallinity ranges from 20% to 80%, the value of a obtained by Mo method is from 1.26 to 1.33, and the F(T) is from 6.35 to 13.02. In addition, the activation energy of nonisothermal crystallization for nylon64 obtained by Kissinger method and Takhor method are-192.05 kJ/mol and-176.21 kJ/mol respectively.Seventh, the results of the melting crystallization morphology of nylon-64 show that only a small amount of nuclei can generate when the temperature decreases quickly; And when the temperature decreases slowly, it will help produce a more complete spherulite. Extending crystallization time is good for the growth of large spherulite. For example, the size of nylon-64 spherulite obtained after 3 hours at constant temperature is up to 80μm-100μm.