| It is difficult to obtain ordinary flame retardant fibers with excellent mechanical properties and thermal stability at the same time.How to obtain polymer fiber with excellent mechanical properties,thermal stability and flame retardant properties at the same time is an urgent problem to be solved in the academic community.Thermotropic liquid crystal copolyester(TLCP)is widely used in national defense,military and industrial and agricultural production fields because of its excellent mechanical properties,thermal stability and chemical resistance.Moreover,TLCPs have the characteristics of intrinsic flame retardant and anti-melt droplets.Thus,it is expected to obtain high-performance liquid crystal polymers with both intrinsic flame retardant and excellent thermodynamic properties by introducing flame retardant elements via molecular design.In this paper,10-(2,5-dihydroxyphenyl)-10-hydro-9-oxa-10-phosphophenanthrene-10-oxide(DOPO-HQ)and 2-hydroxy-6-naphthoic acid(HNA),4,4-dihydroxydiphenyl ether(OBBA),and 4,4’-dihydroxybiphenyl(BP)were copolymerized by melt polymerization method for synthesizing Series I phosphorus-containing flame retardant liquid crystal polymers;DOPO-HQ,HNA,BP and sebacateic acid(Se A)were used as monomers to synthesis series II phosphorus-containing flame retardant liquid crystal polymers by melt polymerization method.The chemical structure,thermal properties and flame retardancy of phosphorus-containing liquid crystal polymers with different compositions were studied.The spinnability and mechanical properties of the new flame retardant liquid crystal polymer were discussed,and finally the flame retardant mechanism of phosphorus-containing liquid crystal polymer was analyzed and established.The main contents are as follows:(1)Series I and II phosphorus-containing flame retardant liquid crystal polymers were prepared via continuous melt polymerization process of acetylation-polycondensation-viscosification.The effects of molecular structure on thermal behavior,microstructure,liquid crystal behavior and flame retardant properties were systematically studied as compared with model copolyester.The experimental results showed that the two series copolyesters had wide temperature range of nematic liquid crystal phase.The introduction of phosphorus-containing DOPO-HQ units increased the rigidity of the molecular chain,which significantly increased the glass transition temperature(T_g)of flame-retardant liquid crystal copolyester,the T_g of I and II series flame retardant copolyesters were between 117-162°C and 61-103°C,respectively;and the two series of phosphorus-containing liquid crystal copolyester had no obvious melting peak,and the crystallinity ability decreased significantly.I series and II series copolyester exhibited44%-53%and 1%-22%,26-36%and 2-22%carbon residue at 700°C under nitrogen and air,respectively.The introduction of DOPO-HQ can significantly improve the thermal stability of liquid crystal copolyester.With the increasing of phosphorus-containing monomer addition,the limiting oxygen index gradually increased,and the limiting oxygen index(LOI)of the I series and II series was 58.5%-68.5%and 52.8%-65.5%,respectively,which were both higher than their model copolyesters.The introduction of phosphorus-containing DOPO-HQ units could significantly improve the thermal stability and flame retardant property of liquid crystal copolyesters.(2)The fiber spinning was guided by studying the rheological behavior of liquid crystal copolyester,and the effects of spinning parameter and molecular structure on the mechanical properties of as-spun fibers were studied to obtain flame retardant liquid crystal polymer fibers with excellent flame retardant properties and mechanical properties.The experimental results showed thatboth series I and series II copolyesters showed shear thinning behavior,and the apparent viscosity decreased with the increasing of shear rate in the dynamic rheology test,indicating that they had good melting fluidity and wide processing temperature range.The surface of as-spunfiber was smooth,and the hierarchical fibrillated structure along the axial distribution of the fiber was observed,and the orientation degree of the as-spun fiber of the I series was between 75.3%and 80.2%,the orientation degree of representative II series copolyester fiber was 80.32%.The breaking strength and elastic modulus of flame retardant liquid crystal polymer fibers increased with the increasing of the spinneret draw ratio(SDR),and the strength and modulus of I-OBBA-P20 as-spun fiber with SDR of 139 were 0.9 GPa and35.7 GPa,respectively;and the strength and modulus of II-SA-P5 as-spun fiber with high molecular chain flexibility at SDR of 66 were 0.45 GPa and 18.59 GPa,respectively,both of which had excellent mechanical properties.In addition,higher spinning temperature also favored the orientation of the molecular chain.Therefore,the property regulation of flame-retardant liquid crystal copolyester fiber can be realized by adjusting the rigidity and flexibility of the molecular chain and the spinning process parameters.(3)The flame retardant mechanism of I and II series liquid crystal copolyester was elaborated by studying the degradation kinetics and mechanism,and the flame retardant liquid crystal copolyesters with excellent thermal stability and flame retardant performance were obtained.The experimental results were as follows:the kinetics of thermal degradation of phosphorus-containing liquid crystal copolyesters were studied by thermogravimetric analysis(TGA),Kissinger and Friedmen methods,and it was found that the I series had relatively higher degradation activation energy(E)than the II series under nitrogen and air atmosphere,but the E values of two series of copolyesters were both higher than that of model copolyester,indicating that phosphorus-containing liquid crystal copolyesters had good thermal stability under dynamic heating conditions.The degradation mechanism of phosphorus-containing liquid crystal copolyesters was studied by TG-FTIR and Py-GC-MS technology,and the I series phosphorus-containing copolyesters exhibited mainly flame retardant mechanism as H·and HO·radicals captured by DOPO-HQ units in vapor-phase,as well as complementary promoting formation of carbon layer for isolating oxygen and heat radiation.The II series phosphorus-containing copolyester conformed to the flame retardant mechanism of condensed phase,and the DOPO-HQ units existed in the form of phosphate in the later stage of thermal decomposition to promote the pyrolysis of copolyester into carbon,thereby improving the flame retardant performance of phosphorus-containing liquid crystal polymers.As compared with the phosphorus-free model M-I and M-II copolyester,the phosphorus-containing copolyester prepared by DOPO-HQ reached UL-94 V0 grade,the heat release rate(HRR)of I-OBBA-P20and II-SA-P20 decreased by 60.43%and 22.78%,and the total heat release(THR)decreased by 63.6%and 46.6%,respectively.In summary,the prepared flame retardant liquid crystal polymer has excellent thermal stability,spinnability,mechanical properties and flame retardant properties,and can be used as high-performance flame retardant material with broad application prospects. |
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