Synergistic Effect Of Nano-Sb₂O₃ And Brominated Epoxy Resin On The Crystallization And Flame Retardancy Of Poly（Butylene Terephthalate）

Polybutylene terephthalate（PBT）is a semi-crystalline thermoplastic polymer with the characteristics of stable size,high strength,chemical corrosion resistance and fast crystallization rate.It has become an ideal choice in many engineering applications because of its good mechanical,thermal and chemical properties.However,PBT materials are easy to burn and a lot of heat will be released during the combustion process,which leads to the increase of fire accidents and limits its further application.With the development of flame retardant preparation technology,micron-scale flame retardants used in industy fail to meet the needs of the market.It has become an urgency to develop retardants with high flame retardancy and improved mechanical properties and processing properties of matrix,through nano-technology.Nano-flame retardants with excellent properties have attracted extensive attention of researchers since the addition of a small amount of Nano-flame retardants will improve the mechanical properties,thermal stability and flame retardancy of composites significantly.In this paper,nano-Sb₂O₃ particles were prepared and modified by mechanochemical method.Nano-Sb₂O₃ particles,brominated epoxy resin（BEO）flame retardant and PBT-based powder materials were blended by high energy ball mill.Then,the mixtutre were melt-blended and extruded and PBT/BEO/nano-Sb₂O₃composites were prepared.The crystallization kinetics,thermal degradation kinetics and flame retardant mechanism of nano-Sb₂O₃/BEO/PBT composites were systematically studied by differential scanning calorimeter（DSC）,scanning electron microscope（SEM-EDX）and cone calorimeter（CCT）.The main research contents and results are summarized as follows:1.Nano-Sb₂O₃ particles were prepared and modified by mechanochemical method.The morphology of nano-Sb₂O₃ particles prepared by high-energy ball milling was irregular and there was soft agglomeration between the particles.After modification by cationic surfactant CTAB,nano-Sb₂O₃ particles have higher dispersibility due to the steric hindrance of long alkyl chains on the surface of nano-Sb₂O₃ particles.On the basis of CTAB modification,nano-Sb₂O₃ particles were modified by silane coupling agent KH550.The dispersion effect of nano-Sb₂O₃particles was further improved.The average particle size of nano-Sb₂O₃ particles was59.36 nm.The surface activity of nano-Sb₂O₃ particles was reduced by the compound modification,and the contact angle reached 132.5 degrees.The surface hydrophilicity and interfacial compatibility of nano-Sb₂O₃ particles was improved.2.The effects of nano-Sb₂O₃ particles and BEO flame retardant on the crystallinity of PBT matrix composites were studied based on the successful preparation and modification of nano-Sb₂O₃ particles.The results show that the addition of nano-Sb₂O₃ particles and BEO flame retardant does not change the alpha crystal structure of PBT matrix composites,but changes their nucleation and crystal growth mode,which affects the structural regularity of PBT composites.The grain size of PBT composites decreases gradually,with the increase of nano-Sb₂O₃ particle content.The crystallinity of BEO/PBT composites is lower than that of pure PBT composites after adding only BEO flame retardant.With the increase of BEO content,the crystallinity of BEO/PBT composites decreases.The enthalpy of PBT composites decreases with the addition of nano-Sb₂O₃ particles,which is conducive to the crystallization of PBT composites.The more nano-Sb₂O₃ particles are added,the faster the crystallization rate of PBT composites is.Although the addition of nano-Sb₂O₃ particles improves heterogeneous nucleation,it hinders the regular movement of PBT molecular chains during crystallization,increases the energy required for PBT molecular chains to be discharged into the lattice,and increases the crystallization activation energy of nano-Sb₂O₃/BEO/PBT composites.3.The crystallinity of PBT composites and the dispersibility of nano-Sb₂O₃particles in matrix materials are very important to the mechanical properties of PBT composites.The flame retardancy of PBT composites is studied under the condition that the mechanical properties are not affected.The results of mechanical properties test show that the tensile strength and Young’s modulus of BEO/PBT composites increase,but the elongation at break and notch impact strength decrease significantly.When modified nano-Sb₂O₃ particles were added into BEO/PBT composites,the tensile strength and Young’s modulus of PBT composites increased first and then decreased with the increase of nano-Sb₂O₃ particles content.The dispersion of nano-Sb₂O₃ particles in PBT matrix has an important influence on the mechanical properties of the composites.When the content of nano-Sb₂O₃ particles is less than 3wt%,the nano-Sb₂O₃ particles disperse in PBT composites,and the tensile strength and impact strength of PBT composites are improved.However,nano-Sb₂O₃ particles can’t improve the strength of PBT composites significantly.The notched impact strength of nano-Sb₂O₃/BEO/PBT composites increases only by 7.2%,compared with that of BEO/PBT composites.4.It is of great significance to study the thermal degradation of nano-Sb₂O₃/BEO/PBT composites when analyzing of flame retardant mechanism of composites.The thermal degradation of PBT composites can be divided into two stages:rapid degradation and slow degradation of stable carbon layer.The synergistic effect between BEO flame retardant and nano-Sb₂O₃ particles is obvious.With the increase of nano-Sb₂O₃ particle content,the peak thermal degradation rate of PBT composites decreases,the thermal stability increases and the residual carbon rate increases.With the increase of nano-Sb₂O₃ content,the thermal degradation activation energy of PBT composites increases at first and then decreases.When nano-Sb₂O₃ content is 3 wt%,the thermal degradation activation energy of PBT composites is at the highest and the thermal degradation of PBT composites is the most difficult.The addition of nano-Sb₂O₃ particles hinders the movement and orderly stacking of molecular chains and enhances the thermal stability of PBT composites.5.The flame retardant properties of nano-Sb₂O₃/BEO/PBT composites were studied and the flame retardant mechanism was discussed,based on the analysis of the previous chapters.The results show that the flame retardancy of PBT composites can not meet the requirements,when only adding nano-Sb₂O₃ particles or only adding BEO flame retardants.When BEO is 16 wt%,the flame retardancy of PBT composites increases gradually with the increase of nano-Sb₂O₃ particles content,and the LOI value increases from 22.9%to 29.8%.When the nano-Sb₂O₃ particle content is 3 wt%,the vertical combustion（UL-94）grade of the sample reaches V-0 grade.The HRR,PHRR,THR,PCO₂P and PCOP values of PBT composites decreased significantly,with the maximum reduction rates of 57.6%,75.5%,43.7%,76.7%and 10.1%,respectively.Br free radicals and nonflammable gases are produced in the process of thermal cracking of PBT composites.Br free radicals not only capture the active free radicals in the combustion zone to inhibit the combustion reaction,but also quench the free radicals at the end of the chain to prevent the further cracking of the composites.The flammable gases and oxygen in the combustion zone are diluted by the nonflammable gases,and the heat generated in the combustion process is taken away because of gas phase flame retardant effect.At the same time,nano-Sb₂O₃particles promote the cross-linking of the condensed phase to form carbon.The carbon atom structure in the carbon layer changes into graphite crystal structure.The carbon layer has compact morphology and good thermal stability,which effectively prevents the volatile matter from emitting and heat transfer,and produces better flame retardant effect of the condensed phase.