| Selective laser sintering(SLS)has become a mature technology in 3D printing.Various materials are used in SLS.Nylon 12 is the most widely used polymer materials,but its development are limited by less resource and expensive price.As a large nylon type,nylon 6 has better mechanical properties than nylon 12,but due to the bad thermo-oxidative stability of nylon 6 powder,yellow agglomeration will occur during SLS process,and the mechanical properties of the molded part will decrease,which limits its application in SLS.Therefore,it is necessary to increase thermo-oxidative stability of nylon 6 powder before sintering.According to the high temperature thermal aging mechanism of nylon 6,the addition of antioxidant is the most effective solution to improve stability.Compound antioxidant HD498P4,composite yellowing resistance agent KBG779,copper salt antioxidant H3373 as well as the compound antioxidants of hindered phenol 1098 and phosphite antioxidant 2921 T are added in this experiment.The effects of these four different types of antioxidants on the high temperature stability of nylon 6 powder were investigated in detail.The experimental results show that when the temperature is set at 200 ° C,after aging for 1 h,the yellow index(YI)of pure nylon 6 powder is increased by 46.82 from 16.43 before aging,the yellowing is very serious,and the addition of 0.5wt% of antioxidant will eliminate the yellowing.The best antioxidant of anti-yellowing performance is HD498P4,?YI<11.The composite yellowing resistance agent KBG779 can greatly improve mechanical properties of nylon 6 powder material,after 2 hours of aging,the tensile strength retention rate is 86.87%,and the bending strength retention rate is 90.21%,the second one is HD498P4,notched impact strength increased by 7.9%.Through TGA and DSC test analysis,the addition of HD498P4 can slightly increase the melting temperature and crystallinity to nylon 6 powder.Through SEM observation,after 2 hours of aging,the surface crack of the nylon 6 powder will be significantly reduced.According to the analysis of comprehensive mechanical properties and thermal performance,the most suitable antioxidant for PA6 powder in thermal oxygen aging is HD498P4.The aging environment is changed from thermal air to thermal vacuum.Nylon 6 powder will undergo anaerobic aging during vacuum aging.The molecular weight rises as nylon 6 undergoes polycondensation and the molecular chains are entangled.The melt index shows a downward trend over time.After 4 h,the melt index increased significantly to 28.95 g/10 min,which was increased by 30.4% compared with the unaged state,indicating a decrease in molecular weight and a heat resistance extreme value of 4 h.Adding an antioxidant in a vacuum environment did not significantly improve color.After aging for 4 hours,the addition of copper salt antioxidant H3373,on the contrary,increased the PA6 powder yellow index to 35.71,which is 9.17 higher than that of the pure PA6 powder.HD498P4 and KBG779 can improve the mechanical properties of nylon 6 powder.The tensile strength after aging for 4h increased by 28.78% and 15.66% respectively,the bending strength increased by 52.21% and 59.49%.It is necessary to add an antioxidant under anaerobic conditions,and the antioxidant can enhance the thermal stability of the nylon 6 powder.The effect of thermo-oxidative stability with nano-metal oxide on nylon 6 powder was studied.The addition of 4wt% titanium dioxide can reduce the YI of nylon 6 powder after thermal aging for 2h by 28.69%,and the yellow eliminating is obvious.The optimum addition amount of nano-tin dioxide is 1-2%.After aging for 2 hours,the tensile strength of nylon 6 powder with 2wt% nano-tin dioxide added increased by 26.97%,and the bending strength increased by 8.38%.After aging for 2 h with 2 wt% nano zinc oxide,the bending strength increased by 18.37% and the tensile strength increased by 8.45%.The addition content of the nanoparticles should be controlled within 2wt%,otherwise unable to disperse evenly in nylon 6 powder due to the special agglomeration effect of the nanoparticles,and the fluidity will decrease,which will increase the difficulty of the sintering process. |
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