Effect Of Phosphorus-containing Flame Retardants On Structure And Performance Of LGF/PBT Composites

Poly(butylene terephthalate)(PBT)as one of the five major engineering plastics are widely used in household electronic appliances,automobile industry,aerospace and other fields.However,both strength and modulus of PBT resin are poor,which hinder its application.Thus,it is necessary for PBT resin to improve the mechanical properties.The glass fiber are widely used in reinforced polymer materials due to its high performance cost ratio;and long glass fiber reinforced PBT(LGF/PBT)composite has many advantages,such as high strength,good rigidity,high dimensional stability,but LGF/PBT composite is easily burning.Thus,flame retardant additives were used in LGF/PBT composite to improve the flame retardant properties.In this article,the efficient and environmentally friendly phosphorus-contai ning flame retardants,such as red phosphorus(RP)and 10-(2,5-dihydroxyphenyl)-10H-9-oxa-10-phosphaphenanthrene-10-oxide(DOPO-HQ)were used in LGF/PBT composite to improve the flame retardant performance.Through thermogravimetr ic analysis(TG),the thermal stability of LGF/PBT/DOPO-HQ and LGF/PBT/RP composites was investigated;through limiting oxygen index(LOI)and vertical burning(UL-94),calorimeter test(CONE),the flame retardant properties of LGF/PBT/DOPO-HQ and LGF/PBT/RP composites were investigated;through the analysis of thermogravimetric(TG)-infrared spectrometry(TG-FTIR)and Fourier infrared(FTIR)and the observation of scanning electron microscope(SEM)on the residual char microstructures,the flame retardant mechanism of LGF/PBT/DOPO-HQ and LGF/PBT/RP composites was explored;through optical microscope analysis and SEM observation,the distribution of fiber glass effective length and the dispersion of glass fiber were examined;the mechanical properties of LGF/PBT/DOPO-HQ and LGF/PBT/RP composites were carried out by mechanical properties test.The results showed as follows:(1)the addition of DOPO-HQ to LGF/PBT composite to induce the thermal decomposition of PBT/LGF composite ahead of time,and reduce the rate of maximum thermal decomposition,and improve the thermal decomposition activation energy of residual char to increase the thermal stability of residual char and the amount of residual char;with the addtion of DOPO-HQ increasing,the average heat release rate(Av-HRR),peak heat release rate(PHRR)and total heat release(THR),flame growth rate(FIGRA)and maximum average thermal radiation rate(MAHRE)of LGF/PBT/DOPO-HQ composites decreased,while the limited oxygen index(LOI)of LGF/PBT/DOPO-HQ composites showed a trend of increase;the addition of 15% DOPO-HQ,LGF/PBT/DOPO-HQ composite achieved UL-94 V-0 ranking,and the LOI value of it increased to 26.4%;the mechanical performance of LGF/PBT/DOPO-HQ composite remained excel ent;These behaviors showed that DOPO-HQ has excel ent flame retardant effect on LGF/PBT composite,and less influence on its mechanical properties.With the addtion of DOPO-HQ increasing,the residual char after LGF/PBT/DOPO-HQ composites burning became more compact,and the phosphoric acid compound in residual char increased;moreover,some gaseous phosphorus compounds released in the thermal decomposition process of LGF/PBT/DOPO-HQ composites,which can effectively inhibited the decomposition of LGF/PBT/DOPO-HQ composites;These showed DOPO-HQ played both condensed phase and gas phase flame retardant effect on LGF/PBT composite.(2)the addition of RP to LGF/PBT composite to hinder the thermal decompositio n of PBT/LGF composite,and reduce the rate of maximum thermal decomposition,and improve the thermal decomposition activation energy of residual char to increase the thermal stability of residual char and the amount of residual char;with the addtion of RP increasing,Av-HRR,PHRR,THR,FIGRA and MAHRE of LGF/PBT/RP composites decreased,while the LOI of LGF/PBT/RP composites showed a trend of increase;the addition of 4.5% RP,LGF/PBT/RP composite achieved UL-94 V-0 ranking,and the LOI value of it increased to 27.4%;the mechanical performance of LGF/PBT/RP composite remained excel ent;These behaviors showed that RP is an excel ent flame retardant additive for LGF/PBT composite.With the addtion of RP increasing,the residual char after LGF/PBT/RP composites burning became more compact,and the phosphoric acid compound in residual char increased;this behavior showed RP mainly played condensed phase flame retardant mechanism on LGF/PBT composite.(3)Comparing with the thermal decomposition of RP,thermal decomposition of DOPO-HQ was lower.Thus,the thermal decomposition DOPO-HQ was eaier than RP to produce phosphorus-containing compounds which inhibited the thermal decomposition of LGF/PBT composite and resulted in the formation of more stable and compact residual char to improve effectively the flame retardant properties of LGF/PBT composite,thus,comparing with LGF/PBT/RP-4.5 composite(containing 4.5% P),the flame retardant properties of LGF/PBT/DOPO-HQ-14 composite(containing 0.66% P)was more better.(4)With initial glass fiber length increasing,the distribution of fiber glass residual effective length of LGF/PBT/DOPO-HQ(RP)composites firstly moved towards long glass fibe area,then moved towards shorter glass fibe area.With initial glass fiber length increasing,the LOI value of LGF/PBT/DOPO-HQ(RP)composites firstly increased then decreased,and both Av-HRR and THR firstly decreased then increased;when the initial glass fiber was 16 mm,the flame retardant properties of LGF/PBT/DOPO-HQ(RP)composite was superior to another.These behaviors showed that initial glass fiber length has effect on flame retardant properties of LGF/PBT/DOPO-HQ composite.when the initial glass fiber was less 12 mm,the glass fiber dispersion in LGF/PBT/DOPO-HQ(RP)composite was uniform;when the initia l glass fiber was over 12 mm,the glass fiber dispersion in LGF/PBT/DOPO-HQ(RP)composite tended to be reunited;moreover,when the initial glass fiber was 12 mm,the mechanical properties of LGF/PBT/DOPO-HQ(RP)composite was best.