| Polyester industrial fiber has been widely used in industrial fields such as tire cords,seat belts,airbags and geotechnical materials,due to mature and environmentally friendly production technology,good dimensional stability,weather resistance and mechanical properties.With the expansion of the application field,in addition to the basic characteristics of its breaking strength,modulus and thermal stability,the performance of polyester industrial fiber under post-processing and service conditions which related to its service cycle and service is also important.The prediction of safety is worthy of in-depth exploration from both academic research and application.In view of the insufficient research on the service characteristics and mechanism of polyester industrial fiber,typical service characteristics of polyester industrial fibers are selected,such as thermal stability under post-processing heating conditions,the fatigue and creep characteristics during long-term service are studied in detail.And from the multi-scale microstructure change to explain the mechanism causing the change of service characteristics,which provides a basis for the improvement of industrial fiber processing technology and the expansion of application fields.High modulus low shrinkage(HMLS)polyester industrial fiber is usually subjected to specific temperature and tension during the post-processing process of dyeing-drying,or dipping-heat setting.Therefore,HMLS was subjected to heat treatment without tension at different temperatures(120-180 °C)for 5 min and heattreatment with different tensions(0-0.10 c N/dtex)at 150 °C,respectively.The changes in fineness and mechanical properties were tested in detailly.The results show that,with the increase of the heat-treatment temperature or the decrease of the pre-tension,the breaking strength remains basically unchanged,the tenacity decreases slightly,the initial modulus,Lase-5(tenacity at specific elongation of 5.0%)decreased obviously as well as the ultimate elongation and Easl-4(elongation at specific load of 4.0 c N/dtex)increased significantly.The microstructure changes caused by the heat-treatment mainly occur in the amorphous chains.When the heat-treatment is carried out at a higher temperature without tension,the amorphous molecular chains with higher and lower degree of orientation will move and curl.Resulting in the molecular conformation transformed from the trans to the gauche,so that the fiber presents the characteristics of low amorphous region orientation,low trans conformation content,small long period thickness of lamellae and amorphous thickness.Heat-treatment under higher tension conditions,the presence of pre-tension can effectively offset the shrinkage stress of the fiber,so that the mobility of molecular chains in the amorphous region is reduced,and the degree of mechanical properties and structure changes in the amorphous region is reduced.Aiming at the problem of fiber flexibility,which is prone to large differences in test results caused by uneven test conditions,an experimental method suitable for the fatigue testing of industrial fibers has been established by optimizing the fatigue frequency.The fatigue strain,fatigue life time and ultimate elongation retention are used as the indicator of the fatigue characteristics of polyester industrial fiber.The fatigue deformation of polyester industrial fiber mainly occurs in the initial fatigue strain after stress is applied.In order to illustrate the relationship between the fatigue performance and microstructure of polyester industrial fiber in the cyclic stretching process,the high-tenacity(HT)and low-shrinkage(LS)fibers are subjected to different loads at room temperature.The fatigue performance under the conditions was compared.And through wide-angle X-ray scattering(WAXS),small-angle X-ray scattering(SAXS),birefringence testing and infrared spectroscopy(FTIR),the microstructure changes of industrial fibers before and after fatigue are studied at different length scales.The results show that there is no obvious change in the crystalline orientation and crystallinity of the fiber after fatigue,while the structure of the amorphous region changes with fatigue stress.The fatigue deformation strain of HT industrial fiber is smaller,and the proportion of recoverable rapid elastic deformation is higher.After fatigue test with 70%ABL(average breaking load)fatigue stress,the amorphous molecular chain has undergone a slight conformation transformation from the gauche to trans,and the lamellar thickness,long-period and amorphous thickness are slightly increased.The mechanism of this slight change is that the molecular chains of the amorphous regions with a higher degree of orientation and smaller sizes are further oriented under the fatigue tensile stress,while the most of oriented amorphous regions recovered after the stress is unloaded.In contrast,LS has a large fatigue deformation strain and a lower proportion of the elastic part.The degree of orientation of the amorphous region and the long-period both increased significantly after the fatigue test.LS has a lower degree of orientation and a larger thickness in the original amorphous region,and it is easier to further oriented along the stretching direction under the fatigue stress.At the same time,the degree of conformation transformation from gauche to trans is more obvious.The oriented amorphous region causes obvious changes in the structure of the amorphous region after the fatigue test.This shows that the amorphous region is a key structural factor that restricts the fatigue characteristics of polyester industrial fibers.To improve the fatigue resistance,it is necessary to increase the orientation of amorphous molecular chains and reduce the thickness of the amorphous region of polyester industrial fibers appropriately.HT fiber with a high strength and a low elongation is a commonly used fiber material in applications such as cables and geotextiles.Creep deformation occurs under constant stress conditions.Therefore,HT fiber was selected as a research object to establish a suitable method for the creep test.The creep strain,creep rupture time were selected to evaluate quantitative creep properties.Compare the creep properties of polyester industrial fibers with significant differences in the four morphological structures of HMLS,HT,LS,and SLS(super-low shrinkage)and the structural changes at different length scales before and after creep process,to illustrate the microscopic morphological structure of low creep HT fiber.The results show that the deformation strain of HT fibers in the creep process is smaller than that of LS and SLS,and the main thing that occurs is elastic deformation,and the creep recovery rate is higher.The crystallinity and crystalline orientation of the four industrial fiber did not change.The amorphous orientation increased with the increase of creep load.Compared with other types of polyester industrial fiber,the HT has a higher amorphous orientation and a smaller amorphous thickness.The number of molecular segments that can undergo conformational transformation under stretching is very small.The activity of the amorphous molecular chains is limited,and the phase transition into the crystal lattice will not occur,the lamella structure is basically unchanged before and after the creep load.In order to investigate the creep mechanism of HT polyester industrial fibers with different creep conditions.In-situ small-angle X-ray scattering(SAXS)and wide-angle X-ray scattering(WAXS)were conducted on a HT fiber during the room temperature creep and creep recovery process with a low load(15N)and a medium load(50N),as well as the creep and creep rupture process with a low temperature(30,80 °C)and a high temperature(200 °C),respectively.The creep strain-time curves were divided into tensile zone(?),creep deformation zone(?)and creep recovery(creep rupture)zone(?).The SAXS indicated that the macroscopic initial creep strain in zone ? and creep deformations in zone ? were attributed to conformation transition from gauche to trans in amorphous region,increasing the amorphous orientation and long period.Irreversible portion of conformation changes accounts for the small unrecoverable plastic creep strain after removing 15 N load in zone ?.The initial creep strain in zone ? and creep deformations in zone ? of the creep process with 50 N were bigger than the microscopic long period strain,because amorphous layers had conformational transformation and microfibril slippage,which also produced a higher unrecoverable plastic creep strain in zone ?.For the low-temperature creep fracture process,a small part of lamellar surfaces is transformed into normal surface,and the tilting angle of the inclined lamellar surface was increased with time in the first two zones.In zone ?,the full-stretched amorphous molecular chain was broken,which resulting in the disappearance of the periodic lamellar structure.During the high-temperature creep and creep fracture process at 200 °C,the surface of the lamellar structure always maintained an inclined state and the lamellar tilting angle gradually decreased in the first two zones.Highly-oriented molecular chains in the amorphous region exerted stress on the crystal region,which causes the surface chains between the amorphous and the crystal region to be pulled out.Resulting in the fracture of crystalline structure and the lamellar stack structure was damaged in zone ?.The above descriptions indicate that the main structural factors determined the creep behavior of polyester industrial fibers at room temperature are the degree and size of the amorphous region.Increasing the orientation of the amorphous region and reducing the stacking size of the amorphous region is to increase the creep resistance of industrial fibers.The failure of fiber creep rupture under high temperature conditions originates from the rupture of molecular chains in crystal regions,and the crystal structure is the key structural factor for high temperature creep performance. |
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