Crystal Structure Regulation And Characterization For Polypropylene Based On Nucleating Agent Addition

Due to outstanding advantages such as low price,non-toxicity,good corrosion resistance,and easy processing and molding,polypropylene has been widely used in many fields and become one of the five most commonly used plastics.Although polypropylene has many outstanding advantages,it also has some defects.The poor resistance to photo-oxidation is one of the more serious defects,which limits polypropylene application as outdoor open-air environmental engineering material.Therefore,the study on the photo-oxidation aging resistance for polypropylene has become an important research topic and hotspot.In this paper,combining the photo-oxygen aging mechanism of polypropylene,the modification of polypropylene with photo-oxidation aging additives andβ-nucleating agent were studied.The main results and conclusions as follows:For the modification of hindered amine photo-oxygen stabilizers,the results show that the combination of high/low molecular weight hindered amine photo-oxygen stabilizers（HALS）and UV absorbers is beneficial to improve the photo-oxidation resistance of the modified polypropylene.The application of UV absorbers in combination with high and low molecular weight HALS can exhibit the good synergistic effect in the aging process,which promotes the long-lasting resistance to photo-oxidation of polypropylene.In particular,when the content of HALS783 is 0.1%,Tinuvin144 is0.1%and UV327 0.3%,the photo-oxidation resistance of polypropylene is better.After 672 h artificial accelerated aging,the tensile strength retention of polypropylene still reaches 99.3%.As mentioned above,HALS783 is the 1:1 mixture of Poly-（N-β-hydroxyethyl-2,2,6,6-tetramethyl-4-hydroxy-piperidyl succinate）and Poly-{6-[（1,1,3,3-tetramethylbutyl）amino-1,3,5-triazine-2,4-diyl][（2,2,6,6-tetramethyl-4-piperidyl）imino]-1,6-hexanediyl[（2,2,6,6-tetramethyl-4-piperidyl）imino)}.Tinuvin144 is[[3,5-di-tert-butyl-4-hydroxyphenyl]Methyl（1,2,2,6,6-pentamethylpiperidyl）methyl]butyl malonate.UV327 is（2-（2’-hydroxy-3’,5’-ditert-butyl）phenyl）-5-chlorobenzotriazole.For the crystalline structure modification of polypropylene by rare earthβ-nucleating agents（WBG,calcium and strontium organic complexes）,the results show that the nucleation behavior of nucleating agents has a great relationship with its molecular interaction.When the temperature increases,the organic ligand of theβ-nucleating agent undergoes hydrogen bond dissociation,and some organic macromolecules become free macromolecules.When cooled again,the organic ligand free macromolecules re-form hydrogen bond association,and which forms a specific aggregate structure.When the content is 0.1%and 0.2%,theβ-nucleating agent forms the needle-like structure.When the content is 0.3%and 0.4%,a dendritic structure is formed,and the induction ability of theβcrystal for polypropylene is the most significant.At the same time,the addition amount ofβ-nucleating agent has a certain influence on the crystallization process of polypropylene,and the optimum addition amount is 0.4%.In this case,the Avrami index n was 2.46,the crystallization rate constant Z_t is 1.53,and the half-crystallization period t_1/2 is 0.72 min.According to the influence of process conditions on the crystallization behavior ofβ-nucleating agent modified polypropylene,the results show that the process conditions,especially cooling temperature and draw ratio,can effectively control the crystal structure and crystal form distribution of modified polypropylene.The process conditions,especially the change of the cooling temperature and the draw ratio,can effectively control the crystalline structure and the layered crystalline structure of the modified polypropylene.When the cooling temperature is raised from 20°C to 100°C,theβ-crystal content of the outer and inner regions in the modified polypropylene increases from 43.07%and 53.05%to 55.27%and 57.57%,respectively.With the combined action ofβ-nucleating agent WBG and tensile shear induction,when the draft ratio is 3,theβ-crystal content of the outer and inner regions of the modified polypropylene reached the maximum,which were 52.58%and 60.32%,respectively.For the research on the regulation of the crystal structure ofβ-nucleating agent modified polypropylene by ultrasonic wave,the results show that Ultrasonic can control the crystal structure of modified polypropylene to a certain extent,especially for the regulation of modified polypropylene crystal form distribution.When the cooling temperature is 20°C and the ultrasonic frequency is increased from 0 kHz to 80 kHz,the difference in crystallinity between the outer and inner regions of the modified polypropylene increases from 1.08 to 1.35 times.When the cooling temperature is 40°C and 80°C,the difference between the outer and inner regions is weakened.Under different cooling temperatures,with the increase of ultrasonic frequency,theβ-crystal absolute content in each region of modified polypropylene increased,and the difference inβ-crystal absolute content between outer and inner regions increased.At 20°C,when the ultrasonic frequency is increased from 0 kHz to 80kHz,the ratio of theβ-crystal absolute content for outer region to the inner region in the modified polypropylene is increased from 0.59 to 0.63.However,when the cooling temperature is 40°C and80°C,the difference in the ratio of theβ-crystal absolute content for outer region to the inner region in the modified polypropylene is relatively larger,and increases from 0.67 to 0.88 and from 0.86 to0.93,respectively.