Study On Phtoaging Property And UV Resistant Modification Of Para-aramid Fibers

Poly(terephthaloyl terephthalamide)(PPTA)fibers are widely used in the fields of ballistic protection,aerospace,fiber optic cables and offshore cables because of their high strength,high modulus,light weight,high temperature resistance and comprehensive and outstanding performance.However,as a light-sensitive polymer material,PPTA is susceptible to aging and degradation by several aging factors such as solar radiation(especially UV light in the range of 280-320 nm)or seawater erosion in the environment,which affects the service life of its products,therefore,functionalized modification of PPTA fibers for UV protection is needed.In this thesis,the degradation patterns of PPTA fibers under different aging conditions such as sunlight aging,seawater aging and combined sunlight/seawater aging were investigated.And three methods,namely waterborne polyurethane(WPU)coating,chemical grafting ZnO and ZnO coating by immersion glue method,were used to modify PPTA fibers with UV resistance respectively.The main work of the paper and the research results obtained are as follows:(1)The effects of sunlight aging,seawater aging and combined sunlight/seawater aging on the surface morphology,chemical structure,crystalline structure and mechanical properties of PPTA fibers were systematically studied.The results showed that the surface of PPTA fibers gradually deteriorated with the increase of sunlight aging time,the amide bond in the molecular chain was broken,the crystallinity and grain size of fibers decreased,the crystalline structure was destroyed,the thermal stability decreased,and the mechanical properties of fibers showed a slow and then fast decreasing trend,the fracture strength and elongation at break of PPTA fibers after 35 d of sunlight aging were 73.7% and 69.2%,respectively.The surface morphology,chemical structure and mechanical properties of the fibers were damaged by seawater aging and sunlight/seawater compound aging,and the surface grooves of the fibers were deeper after 35 d of sunlight/seawater compound aging,and the retention rates of breaking strength and breaking elongation of the fibers after 35 d of seawater aging were 63.0% and 72.4%,respectively.The retention of breaking strength and elongation at break were only 58.7% and 65.9% of the initial ones.(2)Three methods,WPU coating,chemical grafting ZnO and ZnO coating by immersion glue method,were used to modify PPTA for UV resistance,respectively.The results showed that the anti-UV effect of coating a layer of WPU fibers on the fiber surface was not good.The surface modification of PPTA fibers by chemical grafting of ZnO,using the coordination reaction between-COOH and ZnO to produce PPTA-ZnO NW fibers,the results show that: PPTA-ZnO NW degree surface uniform growth of a layer of ZnO nanorods,modified fiber UV aging for 72 h after the fracture strength retention rate increased from 76.9 % to 81.2 % of unmodified fibers The decrease in the mechanical properties of the modified fibers was mainly attributed to the fact that the alkali treatment would destroy the native structure of PPTA fibers,resulting in a decrease in the initial strength of the modified fibers from 24 c N/dtex to 21.02 c N/dtex.The physical coating of ZnO was used to modify PPTA fibers for UV resistance.First,the silane modification of nano-ZnO was carried out with 20% concentration of KH550,and then the ZnO/WPU composite emulsion was configured with waterborne polyurethane(WPU)to produce PPTA/ZnO/WPU fibers,and the effects of silanized ZnO addition content and particle size in the dipping solution on the UV resistance effect of the modified fibers were investigated.The results showed that the best anti-UV effect of PPTA/ZnO/WPU fibers was achieved when the nano-ZnO addition content was 5% and the particle size was 600 nm in the impregnating solution,and the retention rates of fracture strength and elongation at break of the modified fibers were 83.96% and74.40%,respectively,after 168 h of artificially accelerated UV aging,while the retention rates of fracture strength and elongation at break of the unmodified PPTA fibers were only 48.75% and57.5%,respectively.The retention rates were only 48.75% and 57.01%,indicating that the ZnO/WPU modification could effectively improve the UV aging resistance of PPTA fibers.