Study On Liquid Crystalline Properties And Fiber Forming Technology Of Poly (p-phenylene Terephthalamide) Solution

Poly(p-phenylene terephthalamide)(PPTA)fiber is one of aromatic polyamide fibers with high performances and used widely in aviation,national defense, composite material and other industries.The resolving process of PPTA and its appearance of liquid crystalline(LC)were observed by micropolariscope with plate heating.Effect of molecular weight(MW)and weight concentration of PPTA on its melting temperature(T_m)and phase transition temperature(T_trans)were analyzed by differential scanning calorimetry(DSC).Dynamic rheological properties of LC solution,effect of solution temperature,molecular weight(MW)and its distribution (MWD)of PPTA,weight concentration on complex viscosity(η(?)),loss tangent (tanδ),storage modulus(G′),loss modulus(G″)and so on,were studied with plate rotational rheometer,and parameters of PPTA-H₂SO₄ LC solution,non-Newton Index (n),zero shear viscosity(η₀),relaxation time(λ)and viscous activation energy(△E_η) were concluded and an empirical formula betweenη₀ and average viscosimetric molecular weight(M_W)was founded by fitting the experiment results with mathematic formulas.All of these conclusions are important references for the future spinning process.PPTA-H₂SO₄ LC solution was prepared by twin-screw for spinning. Effect of PPTA weight concentration,pressure,spinneret orifice number,draw ratio, wind-up speed,temperature and concentration of coagulation bath,et al,on fiber properties were discussed.Technology of heat-treating to get high-modulus fiber was investigated according to temperature,staying time and tension.In dissolving process,PPTA particles was swelling,then dissolving,and a few minutes later LC was appeared and more clearly with shearing.There was no obvious difference in micropolariscope photos of LC solution between imports and own.The T_transwas more accurate in process of heating,cooling and heating again by DSC,and the endothermic peak moved to higher temperature with raise of weight concentration and MW of PPTA.Phase transition was undergoing higher than T_m if weight concentration is larger than 19.5%,otherwise,melting peak and phase transition peak overlapped in melting process.LC solution is shearing sensitive liquid,η(?)decreased immediately,G″and tanδrose and G′was almost never changed,when shear rate((?))increased.The curves ofη(?),G″,G′and tanδwere higher with raise of weight concentration and MW,and lower with raise of temperature.The flow curves of LC solution fit with the Power Law and n,obtained by the Power Law,is very small,which means the solution deviated the Newton Liquid seriously.The n decreased when weight concentration and MW increased,when temperature decreased,and all these factors made tittles effects on it.The flow curves of LC solution fit with the Cross-Williamson Equation,andη₀ andλ,obtained by the equation,increased when PPTA weight concentration and MW increased,when temperature decreased.△E_ηof LC solution is large and larger a little as PPTA weight concentration and MW increased.The maximum scanning frequency(ω)of flow curves is 100rad·s^-1extending near to 1000rad·s^-1using the WLF Equation according to the Time-Temperature Equivalence Principle.Relation betweenη₀ and M_W were received according to the figures of logη₀～logM_W in different temperature. In vGP figure,minimum of phase angle is smaller as M_W is larger,plate modulus G_N⁰=1.92E5,and MWD wouldn't change the vGP shape,and own's is a little narrow than imports'.Breaking tenacity and initial modulus of PPTA fiber rose and elongation at break declined,when draw ratio rise,when spinning speed declined,when spinning temperature,weight concentration andη_inhof PPTA increased.Effect of spinning speed and draw ratio on fiber orientation is not obvious,and effect of spinning speed on fiber crystallinity is also not distinct,while fiber crystallinity increased,stress relaxation decreased with raise of draw ratio.The favorable parameters in spinning process are:dope temperature is 80℃;PPTA weight concentration is 19%;diameter of spinneret orifices is 0.07mm;spinning speed is 25～40m/min;draw ratio is 6～7; coagulation concentration is 8～10%H₂SO₄;Coagulation temperature is 5℃; spinning dope should be used as soon as possible.Breaking tenacity and elongation at break of fiber declined when heat-treating temperature rose,while initial modulus and its CV(coefficient of variation)increased. Breaking tenacity and initial modulus increased,then decreased,and elongation at break declined,its CV increased when staying time rose.Breaking tenacity and initial modulus reach the maximum more easily if heat-treating temperature rise and staying time shorten.When heat-treating tension increased,breaking tenacity and initial modulus increased,then decreased,and elongation at break decreased and its CV increased;when heat-treating temperature increased,appropriate tension of fiber heat-treating decreased.Relationship between initial modulus and heat-treated modulus is almost linear.Effect of heat-treating temperature and tension on fiber orientation and crystallinity is very little,its crystallinity increased a little with raise of staying time.Fiber stress relaxation decreased,then increased with increase of heat-treating tension.The new ideas mainly brought forward in this paper were as follows:(1) Melting and phase transition process of PPTA-H₂SO₄ LC solution is studied by the way of DSC.(2)Dynamic rheological properties and its influence factors are analyzed systemically.(3)An empirical formula betweenη₀ and M_W is founded.(4) PPTA fiber with high-tenacity,high-modulus is gained whose spinning dope is prepared in twin-screw.(5)PPTA fiber with higher modulus is prepared by means of heat-treating directly.