Synthesis And Properties Of Environmentally Friendly Layered Double Hydroxides Based Polypropylene Flame Retardant Composites

Polypropylene(PP)is one of the most widely used commodity thermoplastics and is regarded as an important part in economic construction.But it has a low thennal stability and is a kind of flammable material.During combustion,PP can produce large amounts of smoke which will be harmful to environment and human health.In this study,layered double hydroxides(LDHs),which are eco-friendly,halogen-free and highly efficient,were used as the flame retardant for PP.Then the LDHs/oxidized carbon nanotubes(OCNTs)and LDHs/ammonium polyphosphate(APP)/zinc borate(ZB)composites were synthesized and used as the flame retardant for PP as well.The effect of the morphologies,particle diameters,and the interlayer anions of the LDHs on the flame retardant performance,thermal stability and mechanical properties of the PP were investigated systematically.LDHs with different morphologies and different particle diameters were synthesized using the coprecipitation and hydrothermal methods.The effect of morphologies and particle diameters of the synthesized LDHs on the flame retardant performance and thermal stability property of PP were studied firstly.The results showed that the plate-like LDHs nanoparticles had a better performance on both flame retardant and thermal stability of the PP/LDHs composites.The flame retardant performance of the PP/LDHs composites decreased with increasing the LDHs particle sizes.When the LDHs particle diameters were smaller than 300 nm,the value of peak heat release rate(PHRR)decreased by 57%(40 wt%of the LDHs loading).Moreover,the smaller particle diameters showed a less effect on decreasing the mechanical properties of PP.The flame retardant performance increased with increasing the LDHs loading,while the thermal stability and mechanical properties decreased.Those indicated that the plate-like LDHs with smaller particle diameters(<300 nm)are the best flame retardant additives for PP.Mg3Al-X LDHs with different interlayer anions(X=CO32-、NO3-、Cl-and SO42-)has been synthesized.The influence of the anions in the LDHs on the flame retardant properties of the PP/Mg3Al-X LDH composites was investigated as well.When the Mg3Al-X LDHs loading was 40 wt%,the PHRR value of the PP/Mg3Al-C03,PP/Mg3Al-N03,PP/Mg3Al-Cl and PP/Mg3Al-SO4 LDH composites decreased by 49%,58%,32%,and 58%,respectively,which indicated that the NO3-,CO32-and SO42-showed a better performance than Cl-.Mg3Al-CO3 LDHs has a good performance on improving the thermal stability of the PP.When the Mg3Al-CO3 LDHs loading was 15 wt%,the temperature at 50%weight loss(T0.5)of the PP/Mg3Al-C03 LDH composites increased by 54℃.Those suggested that the C032-was the suitable interlayer anions of Mg3Al LDHs for improving the performance of PP.The OCNTs were introduced into the LDHs,preparing the LDHs/OCNTs composites.The aim of introducing OCNTs was to decrease the LDHs loading in the PP matrix,improving the thermal stability and mechanical properties of PP.A proper OCNTs loading can significantly improve the flame retardant of PP/LDHs/OCNTs composites.Compared to PP/LDH composites with 10 wt%LDHs loading,the PHRR reduction value of PP/LDHs/OCNTs composites with 10 wt%LDHs and 1 wt%OCNTs loading was increased from 10%to 40%,which was much higher than the PP/LDH composites with 20 wt%LDHs loading(31%).The optimal OCNTs loading in the composites was 1 wt%.A higher OCNTs loading would lead to a high heat conduction,which will restrict the PHRR reduction of the PP based composites.LDHs-OCNTs hybrids were demonstrated to have a better dispersion in PP matrix,which was a key reason for the high flame retardant properties.In additionally,When the OCNTs loading in PP was 1 wt%,the tensile strength of PP/LDHs/OCNTs composites increased from 25.6 to 27.5 MPa,the elongation at break increased from 7.5%to 17.9%.The synergistic effect between the LDHs,and the commonly used flame retardant,such as the APP and ZB,was further investigated.The addition of APP could interact with the metal oxides generated from LDHs decomposition,promoting the rapid dehydration and char formation of the PP.As a result of this,Mg3Al-APP LDHs performed a better flame retardant performance than Mg3Al-CO3 LDHs.When the additives loading was controlled at 20 wt%,the PHRR reduction of PP/Mg3Al-APP LDH composites increased from 31%to 55%,compared with PP/Mg3Al-CO3 LDH composites.ZB could generate a char layer and Zn containing high temperature resistant materials after heating,which can prevent transferring the heat between inside and outside of the composites,and decrease the burning rate.When adding 2 wt%ZB,10 wt%Mg3Al-APP LDHs,the value of PHRR reduction of PP/Mg3Al-APP LDHs composites increased from 27%to 42%.