Research On Preparation And Properties Of Modified MF/PAN Fibers

Melamine formaldehyde（MF）fibers are halogen-free and environmental friendly flame retardant materials due to their low thermal conductivity,no smoke toxicity during burning and low shrinkage in the fire or at high temperature.Thus MF fibers have increasingly attracted people’s attention.However,MF molecules are easy to be crosslinked to form three-dimensional structure,which makes MF materials hard and brittle,and limits its spinnability.So it is essential to modify MF to improve its spinning property.It is well known that polyacrylonitrile（PAN）is a kind of flexible fiber prepared via the mature wet spinning process.In this thesis,PAN resin was used as the fiber forming agent and blended with MF resin via solution blending to prepare MF/PAN fibers,and MF was modified to improve the comprehensive properties of MF/PAN fibers.In this article,PAN was dissolved in aqueous Na SCN solution and then blended with aqueous pre-MF solution.MF was modified to improve the compatibility between MF and PAN phase via polyethyleneglycol（PEG）graft-copolymerization and nano-TiO₂ blending,respectively.Modified MF/PAN fiber was prepared via wet spinning process and the optimum modification method was determined.The stability of mixed MF/PAN spinning solution was investigated by means of optical microscope and rotational viscometer.The chemical structure of PEG modified MF was determined by means of infrared spectrum and nuclear magnetic resonance（NMR）spectroscopy.The MF/PAN film and fibers were characterized by means of scanning electron microscopy,elemental analyzer,X-ray diffraction,thermogravimetric analyzer,tensile tester and limiting oxygen index in order to compare the fiber’s surface morphology,MF loss rate,mechanical properties,crystallinity,thermal stability and flame retardant performance.The optimal conditions was determined to prepare the MF/PAN spinning solution.The effect of wet spinning,coagulation and drawing process on the mechanical and flame retardant properties of resultant MF/PAN fibers were investigated.Firstly,MF/PAN fibers were prepared via wet spinning from MF/PAN blend solution.The optimal preparing conditions of MF/PAN spinning solution were determined with measurements on the solution viscosity,MF loss rate during MF/PAN film forming process and mechanical properties of resultant film.The optimal conditions to prepare MF/PAN spinning solution were: the M/F molar ratio was 1:3,the blending weight ratio of MF/PAN was 7:3,the blended MF/PAN solution was stored and reacted for 16 h at 60℃.The resultant MF/PAN spinning solution presented "sea island structure",the MF phase dispersed in the continuous PAN phase.The solution had optimal film forming property with least MF loss rate.The obtained MF/PAN solution was used to prepare fibers via wet spinning,the effect of drawing process on the structure and properties of MF/PAN fibers was investigated.The resultant MF/PAN fibers showed homogeneous structure,many MF particles were existed on the fiber’s surface.With increasing of boiling water stretch ratio,fiber’s crystallinity and mechanical properties increased,and MF aggregates also increased on the fiber’s surface which resulted in higher flame retardant properties of MF/PAN fibers.As the jet stretch rate was decreased from-30% to-60%,the MF loss rate,fiber’s crystallinity and mechanical properties all decreased first and then increased,while the changing trends of fiber’s flame retardance and thermal stability were opposite absolutely.When the jet stretch rate was-50% and the boiling water stretch ratio was up to 3,the comprehensive properties of resultant MF/PAN fibers reached optimum value: the fiber strength was 1.17 c N/dtex,the breaking elongation was 8.80%,and the limiting oxygen index of fibers was up to 32.2%.Secondly,PEG graft-copolymerization and nano-TiO₂ blending were used to modify MF to improve the mechanical properties of MF/PAN fibers.When the MF was modified with PEG graft-copolymerization method,the modifying feasibility of PEG graft-copolymerization was studied,and the film forming properties of PEG modified MF/PAN solution was discussed.Orthogonal experiments of three levels and four factors were used to determine the optimal conditions to prepare PEG modified MF/PAN solution,using the film’s mechanical properties as the inspection standards.The optimal conditions to prepare PEG modified MF/PAN spinning solution were: the melamine and PEG modified melamine was mixed in the weight ratio of 10:1,and then reacted with formaldehyde in molar ratio of 1:3 for 20 minutes under 85℃.The resultant PEG modified MF was blended with PAN in the weight ratio of 7:3 via solution blending,and then stored and reacted for 20 h under 60℃.The tensile strength of the resultant MFPEG/PAN film was 15.78 MPa,the breaking elongation was 12.12%,the LOI value was 31.8%.When the nano-TiO₂ was used to modify MF/PAN solution,the nano-TiO₂ was first modified by KH570 to improve its dispersing ability,then the effect of added weight content of KH570-TiO₂ on the film forming properties was investigated.The results showed that when the addition of KH570-TiO₂ was 1.5%,the comprehensive properties of resultant MF/PAN fibers reached the optimal value: the film strength was 14.89 MPa,the breaking elongation was 8.24%,the limiting oxygen index was 28.1%.From above comparisons it could be concluded that the PEG modified MF/PAN film had comprehensive optimal mechanical and flame retardant properties.Thus the PEG modified MF/PAN solution was used to prepare modified MF/PAN fibers via wet spinning process.Finally,the wet spinning process of PEG modified MF/PAN solution was investigated,the effects of the coagulation bath concentration,jet stretch rate and boiling water stretch ratio on the fiber’s surface morphology,crystallinity,mechanical and flame retardant properties were discussed.The results showed that resultant MFPEG/PAN fibers had the least MF loss rate when the coagulation bath concentration was 10%.During wet spinning process of MF/PAN solution,the main MF losing occurred in coagulation bath followed by the preheated stretching bath,the fibers lost the least MF in boiling water stretching bath.With increasing of boiling water stretch ratio,fiber’s crystallinity and mechanical properties increased,the MF loss rate increased a little while the LOI value of resultant fibers obviously increased.After PEG modified,fiber’s boiling water stretch ratio reached 4,which will help to improve the fiber’s mechanical properties.When the jet stretch rate was-50% and the boiling water stretch ratio was 4,the fiber diameter of resultant MFPEG/PAN fibers was about 20μm,there were many MFPEG particles on fiber’s surface,fiber’s morphology structure was uniform and compact,the fiber strength was 1.85 c N/dtex,which was increased by 58.1% compared with that of unmodified MF/PAN fibers.Meanwhile,the thermal stability and flame retardant properties of PEG modified fibers were obviously improved,the limiting oxygen index of PEG modified MF/PAN fiber was up to 34.2%.