| Adding nucleating agents is a simple but effective way of improving the crystalline behavior and mechanical properties of polypropylene (PP). Since nucleating agents act as heterogeneous nucleus in the PP crystallization process, the crystalline properties of nucleating agents is crucial when it is used to controlling the PP crystalizing. Organic phosphate nucleating agents, which has excellent thermal stability, will not change color and has no peculiar smell under high temperature. They can improve the thermal stability and mechanical property. Among these organic phosphate nucleating agents, sodium 2,2’-methylene-bis-(4,6-di-t-butylphenylene) phosphate (NA40) is one of the most popular type, which can increase the strength, flexural modulus and distortion temperature of PP. This thesis studied the effects of crystal structure and particle sizes of NA40 on the mechanical and crystalline properties of PP.Firstlyl, using methanol and dimethylformamide (DMF) as solvent, the single crystal of NA40 was prepared through solvent evaporation method. Cell parameters was got from single crystal X ray diffraction. When mathanol was choosen as solvent, single crystal NA40 was obtained, whose space group was I b a 2 (Orthorhombic, a=1.7314 nm, b=3.6715 nm, c=1.1950 nm; α=90°,β=90°, γ=90°); while we choosed DMF, the space group was C 12/c 1 (Monoclinic, a=3.209 nm, b=1.774 nm, c=1.262 nm; α=90°, β=106.4°,γ=90°)。The length of c axis of NA40 from methanol solution was two times of the length of iPP c axis; and the length of c axis of NA40 from DMF solution was two times of the length of iPP c axis. The disregistries of both kinds of single crystal were under the upper limit considered by Wittmann et al.Secondly, the influence of nucleating agent crystal structure on its nucleating efficiency was studied. Mechanical and crystalline properties changes of iPP was analyzed. Results shows that both kind of nucleating agents could significantly enhance the mechanical properties. Spherulite size was much smaller when nucleating agents had been added in iPP. Comparing with neat iPP, the nucleating iPP shows 19.6% and 10.6% growth in flexural molecular and tensile strength, respectively. Nonisothermal crystallization kinetics of iPP was studied with Avrami equation by DSC, which tells that nucleating agents changed the growth pattern of iPP with Avrami index turning from about 4 to 3.Thirdly, we produced NA40 with different particle sizes through anti-solvent method, by controlling rotation rate, solvent ratio (solvent/anti-solvent) and solution concentration. Increase in rotation rate and solvent ratio and decrease in solution concentration all contribute to reducing of crystal particle size. Moreover, freeze drying could effectively improve monodispersion of the powder, and reduce particle size at the same time.Finally, nucleating agent with different particle sizes shows different effects on improving mechanical and crystalline properties of iPP. Despite great rise in Flexural modulus and tensile strength comparing to neat iPP, increase in these figures diminished with the growth of particle size of nucleating agents. Analyzing the nonisothermal crystallization kinetics of neat iPP and nucleating iPP, when cooling rate was low, crystal peak temperature was almost the same for those iPP with different particle sizes NA40, but when cooling rate reached 100℃/min, the smaller particle size nucleating agents had, the more effective it was, and the difference of iPP with smallest and largest particle size nucleating agents could be as much as 2℃. The reason for this phenomenon is NA40 with smaller size distribute evenly in iPP, while those with larger size will aggregate, which weakened their efficiency while iPP was nucleated, and also lowered their effects on improving the mechanical and crystalline properties of iPP. |
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