Structural Evolution Mechanism Of Ultra-High Molecular Weight Polyethylene High-performance Fiber During Spinning Process

Because of the Ultra-high molecular weight polyethylene(UHMWPE)fiber's excellent mechanical properties,chemical,impact and wear resistance,it is widely used in various civilian and military fields.Along with aramid fiber and carbon fiber,it is listed as the world's three high-performance fibers.In recent years,high-performance fibers industry in China has made considerable progress,due to the blockade of foreign technologies and the gap in R&D(research and development)level between domestic and abroad,domestic products are mostly concentrated in the low-end market,and high-performance fibers still need a lot of imports.The existing fiber processing uses the method of introducing foreign equipment and imitating the foreign technology.The research on UHMWPE fiber processing principle is weak,and it is impossible to fundamentally realize the optimization of the fiber processing technology and the improvement of the performance.The gel spinning-hot stretching method was currently used to prepare UHMWPE high-performance fiber.The method is mainly divided into three parts:1)swelling process;2)extrusion to prepare gel filaments;3)multi-stage hot stretching of gel filaments.Swelling process is related to the success or failure of the follow-up process,temperature,time and stirring method are the main parameters of the swelling process.At this stage,the molecular chain reaches a certain degree of disentanglement,forming a preliminary loose network structure.During extrusion spinning,the molecular chains are further disentangled by means of the high temperature of the cartridge and the shear agitation of the screw.The melting filaments formed after extrusion through the spinneret are rapidly cooled and the highly disentangled state is fixed to form gel filaments.In the multi-stage hot stretching stage,the non-oriented gel filaments are gradually transformed into highly oriented along the stretching direction,and the disentangled molecular chains are also converted from the folded-chain to the extended-chain.The key to obtaining high-performance UHMWPE fibers is that most of the long chains of macromolecules in the material participate in the formation of highly oriented extended chains.Therefore,how to get disentangled gel filaments and convert the molecular chains in the UHMWPE fiber into extended chains as much as possible is crucial.UHMWPE fiber processing is a multi-scale structural evolution process driven by coupled multi-field such as temperature and stretching.According to the characteristics of UHMWPE fiber processing,this thesis developed a gel spinning device and a continuous stretching device to prepare the gel filaments and simulate the UHMWPE fiber processing process.Combining a home-made biaxial stretching device and high-time,space resolved,synchrotron radiation X-ray scattering technology,coupled with scanning electron microscopy(SEM),mechanical testing,and other research methods,studied systematically the structural evolution of gel filaments and post-extraction gels during the process of filament stretching.The structure evolution mechanism of the gel filament in the temperature and strain parameter space was revealed,and the pre-stretch/post-stretch parameters-structure-property relationship was constructed.This work can provide basic data for the regulation of UHMWPE high-performance fiber processing technology.The main results and conclusions of this thesis are summarized as follows:In this paper,UHMWPE with a weight-average molecular weight of 4.5 million was selected as the research object.Gel-spinning method was used,and domestic mineral oil(white oil)was selected as the solvent.After swelling and extrusion spinning,the gel fibers with molecular chains in the disentangled state were obtained.The gel fibers were then through hot drawing to preparing high performance fibers.1)Combining industrial production and laboratory conditions,design a modular continuous stretching device.The device can simulate industrial production processes and perform the spinning process under laboratory conditions.Experiments have proved that the device can carry out small-scale fiber tensile test under the conditions of laboratory research.The innovation of this device is that it is modular and can be combined differently according to different needs.Three different tasks,continuous stretching,guided pull rolls,and an extensional rheometer,can be accomplished by a set of equipment through different combination.2)The structural evolution of the initial gel filament during the continuous stretching at 110 ? was tracked by ex situ SAXS and WAXS.It was found that the long period of the lamellae arrangement in the meridian direction gradually increased with the stretching,while the equatorial direction showed a faint reduction.With the progress of stretching,the degree of crystallinity(Xc)showing a gradual increase in the early stage and a gradual decline in the later stage.It indicates that the structural evolution during the continuous stretching process is:In the pre-stretch stage,a microfibril structure was formed due to stretching-induced crystallization.Further stretching,the molecular chains in the amorphous region are extended first,and then the Imolecular chains in the crystal block are gradually pulled out from the folded chain lamellae,eventually destroying the folded chain lamellar structure and generating the extended-chained fibers.3)The structural evolution of the jelly filaments during stretching away from the melting point and near the melting temperature was tracked by in situ SAXS and WAXS.In the stretching process at 25?,the lamellae rotate toward the stretching direction before the strain hardening,while the folded-chain crystals break and the extended-chain fiber formation that occurs during the strain hardening.During the stretching.process at 100?,the stretching induced the melting of the crystals before the stress plateau region,and the formation of fiber crystals at the beginning of the stress plateau region.Further stretching causes a shear displacement of the crystal mass and eventually breaks the folded-chain lamellae to form extended-chain fibers.4)The gel filaments were extracted first,and then stretched.The deformation mechanism of the initial filaments of different draw ratios during the spinning process was studied using ex situ WAXS and SEM.Based on the experimental results of 2D WAXS patterns with different draw ratios,orientation parameter(f)of the sample,Xc,and crystal size perpendicular to the(200)plane(L(200))it was found that the degree of orientation(f)transformed rapidly from isotropic before stretching to a highly oriented molecular chain along the stretching direction..The degree of crystallinity is reduced from about 60%at the beginning to about 40%rapidly and remains unchanged.From this we propose a crystal slip model that interprets the deformation behavior through melting or breaking.Stretching of the extracted fiber at 120? is a solid stretching.In the stretching process,the rapidly oriented molecular chain is accompanied by melting and destruction,and with further stretching,slippage occurs between the platelets.