Preparation Of Creep Resistant Ultra High Molecular Weight Polyethylene Fibers

Ultra high molecular weight polyethylene(UHMWPE)fibers have high strength high modulus,light weight and high softness,abrasion resistance,impact resistance,chemical resistance and many other excellent properties.They have been widely used in military bulletproof,safety protection,ropes and fiber reinforced composite materials.However,UHMWPE fibers have weak molecular interactions,creep will occur under higher temperature and stress conditions,which greatly shortened their service life and restricted their application areas.Therefore UHMWPE fibers needed to be modified by cross-linking.Silane cross-linking modification was a commonly used method to improve the creep resistance of UHMWPE fibers.In this paper,high concentration UHMWPE solution was prepared by twin-screw swelling and dissolving process,and UHMWPE fiber was prepared by gel spinning-ultra drawing method.The twin-screw assembly and the temperature combination used in the swelling and dissolving process were optimized and their effect on the molecular weight of resultant UHMWPE gel fibers was investigated.Then the UHMWPE gel fibers were modified using extracting modification method with vinyl trimethoxy silane(VTMS)and benzoyl peroxide(BPO)as crosslinking agent and initiator,respectively.The grafting and crosslinking reactions of silane on UHMWPE molecules were initiated during subsequent hot drawing process,and then the UHMWPE fibers with improved creep resistance were obtained.The thermal,crystallizing and mechanical properties of resultant fibers before and after modification were characterized by differential scanning calorimeter(DSC),X-ray diffraction(XRD)and tensile tester.This thesis was divided into two parts.In the first part,the thread element combination of the twin-screw extruder was determined and the twin-screw swelling,dissolving and subsequent gel spinning temperatures were optimized to reduce the molecular degradation and prepare high strength UHMWPE fibers.By replacing the screw thread element combination,removing the large-angle shearing element,reducing the number of shearing elements and using the small-lead conveying element which was conducive to boost pressure and melting,the shear induced molecular degradation was successfully reduced.By reducing the temperature of first several sections of the twin-screw extruder,raising the temperature of middle sections and greatly reducing the temperature of extruding sections,the thermal degradation of UHMWPE was greatly reduced.The mechanical properties of UHMWPE fibers were closely related to their molecular weight.Higher strength will be obtained after ultra-drawing with higher molecular weight of UHMWPE gel fibers.When the UHMWPE concentration was 25% and the molecular weight was 1.6 million,the UHMWPE molecular weight degradation rate was less than 50% during the process of gel spinning via optimized technology,the strength of the resultant UHMWPE fiber was 21.88 cN/dtex.In the second part,the silane crosslinking method was employed to modify UHMWPE fibers and improve fibers' creep resistance.Firstly,quantitative analysis was carried out to determine the ultrasonic extraction time for silane crosslinking modification of UHMWPE gel fibers.The third extraction was determined as 10 min.When the content of the modifier in the extraction solution was 3%,the adsorption amount of modifier per gram UHMWPE fiber can reach 37.1 mg.The gel content of VTMS modified UHMWPE fibers increased with the increase of VTMS content in the extraction modification liquid.After the modified fiber was further treated by acidic water bath,the gel content of fibers was further increased.The optimum acidic water bath temperature was 30°C,and the treatment time was 12 hours.The melting temperature,crystallinity,crystallizing temperature and degree of orientation of VTMS modified UHMWPE fibers increased first and then decreased with the increase of VTMS content in the extraction modification liquid.The average crystal grain size in the normal direction of the fiber surface showed the opposite trend.The tensile properties of silane crosslinking modified UHMWPE fibers decreased slightly.After the modified fiber was further treated by the acidic water bath,the mechanical properties of the obtained fibers were decreased with the increase of the treatment temperature or the prolongation of the treatment time.Microfibril bifurcation on the surfaces of higher VTMS concentration modified UHMWPE fibers were observed,which affected the mechanical properties of the fibers.When the concentration of VTMS in the extraction solution was 4%,the gel content of the modified UHMWPE fiber was 8.12% and a breaking strength of 18.78 cN/dtex was obtained after the modified fiber was subjected to three-stage hot drawing and further acidic water bath treatment.The creep resistance of VTMS modified UHMWPE fibers was significantly improved,and the creep resistance of fibers after acid water bath treatment was further improved.At higher temperatures,cross-linking modification had a greater effect on the creep resistance of the fibers.