Research On The Silane Crosslinking Modification Of Ultra-high Molecular Weight Polyethylene Fibers

Ultrahigh molecular weight polyethylene(UHMWPE)fibers have been widely used in high strength ropes and fiber reinforced composite materials owning to their outstanding properties of high strength,high modulus,light weight,excellent chemical resistance,corrosion and impact resistance.However,the heat and creep resistance of UHMWPE fibers are poor due to the line structure of UHMWPE molecular chains.In addition,the interfacial adhesion between UHMWPE fibers and matrix is poor due to fiber's smooth surface and the absence of polar groups in UHMWPE molecular chains,which greatly restrict their application in the fields of composite materials.Therefore,the modification of UHMWPE fibers is extremely important.In the thesis,a simple and easy to be industrialized method were find to significantly improve the creep resistance without damaging the excellent mechanical properties of UHMWPE fibers.Meanwhile,the interfacial adhesion of UHMWPE fibers have been improved to some extent.In this thesis,vinyl trimethoxy silane(VTMS)was used as crosslinking agent,benzoyl peroxide(BPO)or lauroyl peroxide(LPO)were used as initiator,UHMWPE fibers were modified with the method of extracting modification.The grafting reactions of silane on UHMWPE molecules were initiated during subsequent hot drawing process,and then crosslinking structure was formed in UHMWPE fibers after silane hydrolysis and condensation.The scanning electron microscopy(SEM)and UV spectrophotometer was used to study the adsorption of modifying agents by UHMWPE gel fibers.The gel content of UHMWPE fibers was measured according to ASTM standard 2756.Differential scanning calorimeter(DSC)and thermal gravimetric analyzer(TG)were used to characterize the thermal properties and heat resistance of fibers.Infrared spectrometer(FTIR)was used study the chemical properties of fibers surface.Two dimensional wide angle X-ray diffraction(2D-WAXD)was used to measure the crystalline and crystal orientation degree of fibers before and after modification.The DXLL-20000 tensile tester and dynamic mechanical analyzer(DMA)were used to test the mechanical properties and creep resistance of UHMWPE fibers.Contact angle and interfacial shear strength(IFSS)between UHMWPE fibers and epoxy resin were used to characterize the surface adhesion properties of different fibers.Firstly,the adsorption capacity of UHMWPE gel fibers for the modifying agents and the changes in the structure and properties of modified fibers were investigated.The results show that UHMWPE gel fibers could absorb some amount of modifying agents during extracting process.For the composite extracting solution containing 3%VTMS and 0.12%BPO,109.1 mg VTMS and 13.7 mg BPO could be absorbed by every grams of UHMWPE fibers.The initiators are decomposed to initiate the silane grafting reaction and crosslinking structure was gradually formed during hot drawing process.For both initiators modified UHMWPE fibers,the DSC crystallinity,melting point,crystallizing temperature and re-crystallinity become higher than those of unmodified fibers.These exists skin and core structure in UHMWPE fibers,and the crystallinity and degree of crystals orientation in fiber's skin layer are higher than that in fiber's core layer.Compared with unmodified UHMWPE fibers,BPO modified fibers have increased crystallinity,while LPO modified fibers have decreased crystallinity,and the degree of crystal orientations of both modified fibers are decreased.BPO modified UHMWPE fibers have better mechanical properties compared to LPO modified fibers.Therefore,BPO is more suitable initiator for silane crosslinking modification of UHMWPE fibers.Then,the effects of modifiers content in extraction solution on the structure and properties of UHMWPE fibers have been investigated.The results show that the gel fraction of modified UHMWPE fibers increases and then decreases with increasing of BPO/VTMS mass ratio.The optimal BPO/VTMS mass ratio is 1:25 at which the maximum gel fraction of modified fibers is obtained.The gel fraction of modified fibers increases with increasing of VTMS content and it could be increased further after water treatment.The crystallinity,melting point,crystallizing temperature and re-crystallinity of modified fibers increase with increasing of VTMS content and then decrease when the VTMS content is more than 3%.The mechanical properties of modified fibers are lower than that of unmodified fibers.3%VTMS and 0.12%BPO in the extraction solution is the optimal choice for silane crosslinking modification of UHMWPE fibers,the gel fraction of modified fibers is 5.7%,and the modified fibers has the least tensile strength loss.Lastly,the thermal stability,surface wetting properties,surface adhesion properties and the creep behavior under different temperature and stress of modified UHMWPE fibers are characterized.The results show that silane modified UHMWPE fibers have better wetting property,the water contact angle of modified fibers decreases with increasing VTMS content.The interfacial shear strength between fibers and epoxy resin is improved by 11.6%after modified by the extracting solution containing 3%VTMS and 0.12%BPO.After silane modified,the thermal stability of UHMWPE fibers decreases,while the maximum decomposing rate decreases and the residual fraction is increased.Under different environmental temperature and different applied stress,the creep resistance of silane crosslinking modified fibers is improved evidently.For UHMWPE fibers,the effect of temperature is larger than the effect of stress on creep behavior.With the increasing environmental temperature,the creep behavior of UHMWPE fibers becomes more sensitive to applied stress.