Chain-extended And Synergistic Flame-retarded Modification Of Poly(Butylene Terephthalate) Resin

Poly(butylene terephthalate)(PBT) is a major member of engineering plastics, it hasbeen widely used for industrial applications thanks to its attractive mechanical performance,dimensional stability, thermal properties and processing advantages. Nevertheless, the highflammability of neat PBT restricts its further use in the wider scope of application. Theutilization of traditional halogenated flame retardants in PBT are restrained due to the releaseof the toxic and corrosive smoke during combustion. And the employment of commonnon-halogen flame retardant will greatly impair the other key properties of PBT. Therefore,the development of novel halogen-free flame-retarded PBT has become the mainstreamnowadays.Owing to the difficulty in processing the blend of PBT and magnesium hydroxide (MH),the fluidity improver (maleic anhydride grafted polypropylene (PP-g-MAH)) was employedto prepare the halogen-free flame-retarded PBT/MH composites. The processability ofPBT/PP-g-MAH/MH composite was improved. With the addition of50wt%MH and6wt%PP-g-MAH, PBT/PP-g-MAH/MH composite attained UL943.2mm V-1rating and a LOI of35.0%. The tensile strength and Izod notched impact strength of the correspondingPBT/PP-g-MAH/MH composite were45.0MPa and22.2J/m, respectively. According to theTGA and SEM analysis, there existed a good dispersity of MH particle in PBT matrix. Withthe good MH particle dispersity, PBT/PP-g-MAH/MH composite showed high efficiency inflame retardancy.In addition, PBT/PP-g-MAH composite flame retarded by the combination of MH andresorcinol bis(diphenyl phosphate)(RDP) or expandable graphite (EG) was also investigated.The combination of MH and EG exhibited an obvious synergistic flame-retardant effect inPBT/PP-g-MAH composite. With the loadings of35wt%MH and10wt%EG,PBT/PP-g-MAH/EG/MH blend obtained UL943.2mm V-0classification and a LOI of33.3%.Nevertheless, there did not exist synergism in the flame retardancy ofPBT/PP-g-MAH/RDP/MH compound. The flame-retardant mechanism ofPBT/PP-g-MAH/EG/MH composite was analyzed through the TGA-FTIR and SEM characterization. The introduction of EG improved thermal stability and barrier effect of theresidual char layer, and the protection for the polymer matrix was enhanced.Aluminum diethylphosphinate (AlPi) was applied for flame retarding PBT. The influenceof AlPi in the overall properties of PBT was examined. With the combination of differentcontent for AlPi (11,13and20wt%), the corresponding PBT/AlPi composites passed UL94V-0rating (3.2,1.6and0.8mm) and achieved the LOI with34.3%,35.4%and39.3%,respectively. AlPi acted mainly through the flame inhibition in gas phase by the realease ofdiethylphosphinic acid, and an enhanced condensed-phase flame-retardant action was alsoobserved. Correspondingly, the smoke release was intensified significantly. The presence ofAlPi would provoke the degradation of PBT, resulting in the deterioration of mechanicalperformance. The tensile strength, elongation at break, Izod notched impact strength andflexural modulus of PBT/AlPi blend (AlPi:20wt%) were68.6%,10.9%,51.0%and104.4%of the corresponding value of neat PBT, respectively. A promotion in crystallization rate,crystallization temperature and crystallinity for PBT/AlPi was perceived, which was related tothe heterogeneous nucleation effect of AlPi. The heat resistance of PBT was enhanced withthe introduction of AlPi concluded from its rising heat distortion temperature (HDT). ForPBT/AlPi (AlPi:11wt%), the HDT value was65.7°C with an increment of10°C comparedto that of neat PBT.With the purpose of suppressing the degradation and enhancing the mechanicalproperties for PBT/AlPi composite, an epoxy-functional polysiloxane (EPM) was synthesizedand employed as chain extender to prepare the PBT/AlPi/EPM composite. Additionally,another chain-extended PBT/AlPi composite was prepared by using a commercialpolyfunctional chain extender (ADR) for comparative study. Based on the results of torquerheometer measurement, both EPM and ADR could induce evident chain extension reactionin PBT/AlPi composite. With appropriate loadings of EPM and ADR, the modified PBT/AlPicomposites not only demonstrated significant enhanced mechanical properties, but also muchbetter integrated properties including flame retardancy, crystallization performance and heatresistance. Comparatively speaking, the effect of EPM was much better than ADR due to itshigh functionality and polysiloxane chain structure.The synergistic flame-retarded mechanism of PBT/AlPi/EPM and PBT/AlPi/ADR system mainly related to the enhanced condensed-phase action. Small percentages of EPMand ADR could interact with PBT and AlPi intensively during the decomposition process,subsequently the formation of more stable cross-linking char residue occurred. Morephosphorus element retained in the solid phase whereas the release of phosphorus into the gasphase diminished. Logically, the smoke release was greatly suppressed. Moreover, Thecross-linking residue containing silicon showed the better thermal stability and oxidationresistance, which was accounted for the greater flame-retardant efficiency existed inPBT/AlPi/EPM composite.