Preparation And Research Of Short Glass Fiber Reinforced Flame-retardant PET Composites

Glass fiber reinforced resin matrix composites are widely used due to their superior physical and chemical properties.Among them,glass fiber as the reinforcement has the characteristics of high strength,high modulus,chemical corrosion resistance,no combustion,and polymer resin as the matrix material has the characteristics of easy processing,strong plasticity.Due to the extensive use of fiber-reinforced resin matrix composites,their flammability has received much attention.The use of flame-retardant additives is an effective solution to this problem.Common flame-retardant additives are halogen,phosphorus and silicon.Although halogenated flame retardants have excellent flame-retardant properties,they release large amounts of toxic and harmful gases when burned.Phosphorous and silicon flame retardants have been the focus of research in the field of flame retardants due to their low smoke,low toxicity,and environmental protection compared to halogenated flame retardants.However,the inclusion of flame-retardant additives can affect the mechanical properties of the material.The mechanical properties of fiber reinforced resin matrix composites are affected by the fiber length,content and interfacial bonding.Therefore,it is important to investigate these influences in order to improve the mechanical properties of fiber reinforced resin matrix composites.In addition,when glass fiber reinforced resin matrix composites are burned,the candle wick effect is created due to the material properties of the glass fiber and resin,which leads to a reduction in the flame-retardant properties of the composites.Therefore,in the flame-retardant research of glass fiber reinforced resin matrix composites,not only the matrix flame retardant,but also through the interface flame retardant to suppress the candle wick effect,so as to improve the flame-retardant property of composite materials.In this paper,a new type of silicon flame retardant（Li-Ph-POSS）was synthesized,which was compounded with commercial phosphorus flame retardant（O-Zn（H₂PO₂）₂）,and then fused with PET resin to study its synergic flame-retardant mechanism on PET resin.The effect of the fiber length on the glass fiber reinforced PET composite during the processing process was analyzed by adjusting the processing parameters and fiber content of the twin screw,and the effect of the fiber length on the mechanical properties of the composite was further analyzed.By chemical grafting on the glass fiber surface,the interfacial bonding between the fiber and the resin can be improved to improve the mechanical properties of the composite.Based on the above research,appropriate matrix flame retardants and processing parameters were selected,and interfacial flame retardants（DOPO-VTS）were synthesized to modify the glass fiber surface and new flame retardants（Na-Ph-POSS）were used to further improve the flame retardancy of glass fiber reinforced PET resin composites.The results of particular studies are as follows:（1）Li-Ph-POSS flame retardant was prepared by one-step method,and the effect of Li-Ph-POSS flame retardant and commercial phosphorus flame retardant（O-Zn（H₂PO₂）₂）on the flame-retardant property of PET composites was studied.When the content of O-Zn（H₂PO₂）₂ is 10%,the limiting oxygen index（LOI）value of the composite is 27.5%,and there is droplet phenomenon during combustion.For a Li-Ph-POSS content of 1.5%,the LOI value of the compound was 25.5%and no droplet formation occurred.However,when the Li-Ph-POSS content reaches 3%,the flame retardance of the composite drops dramatically,with the LOI value of only 21.8%.When the PET resin was co-retardants with 8.5%O-Zn（H₂PO₂）₂ and 1.5%Li-Ph-POSS（PET/8.5%O-Zn（H₂PO₂）₂/1.5%Li-Ph-POSS）,the flame retardancy of the composite was significantly improved,the LOI value was 32%,and no melting drops occurred during combustion.UL 94 is rated at V-0.The calculated synergistic value of flame retardant of PET/8.5%O-Zn（H₂PO₂）₂/1.5%Li-Ph-POSS composites is 1.84,indicating that they have good flame-retardant synergistic effect.（2）The effect of the fiber length on the mechanical properties of the composite was further investigated by studying the effect of the fiber content and the screw rotation velocity on the fiber length.The results show that the fiber length of the glass fiber reinforced PET composite gradually decreases during the process of the conical twin screw extruder,which is also caused by the increase of the fiber content and the rotation speed of the screw.When the screw rotation speed is 30 rpm,glass fiber content increased from 10%to 30%,the average fiber length（L）reduced from 0.394 mm to 0.250 mm.The average fiber length decreases from 0.292 mm to 0.120 mm when the composite has a20%glass fiber content and the screw speed is increased from 20 rpm to 40 rpm.The tensile strength of the composite also decreased from 98.9MPa to 77.2 MPa,a decrease of 21.9%.In order to further improve the mechanical properties of the composite,the surface of the glass fiber was modified by silane coupling agent（KH-550）.When the content of KH-550 is 3 wt%,the tensile strength of the composite is increased by 21.4%compared with the unmodified 73.7 MPa to 89.5 MPa.（3）Based on previous studies,we synthesize the interfacial flame retardant DOPO-VTS,which has been used to modify glass fiber surfaces.When the DOPO-VTS content was 5 wt%,the tensile strength of modified glass fiber reinforced PET composites（PET/10%VGF₅）was increased by 9.4%,and the LOI value was 21.5%.Additionally,Co-flame retardants were further added to PET/10%VGF₅ composites（PET/10%VGF₅/P-Li）and PET/10%PGF composites（PET/10%PGF/P-Li）.The LOI of PET/10%VGF₅/P-Li composites increased from 26.5%to 30.0%compared to PET/10%PGF/P-Li composites.To improve the flame retardance of the composites,Na-Ph-POSS flame retardants were synthesized.PET/10%VGF₅/P-Na was found to be a better flame-retardant,with a LOI value of 32.5%.Cone calorimetry showed that PET/10%VGF₅/P-Li and PET/10%VGF₅/P-Na composites reduced the instantaneous heat release peak by55.8%and 55.4%respectively compared to PET/10%PGF composites.The total heat release of PET/10%VGF₅/P-Li and PET/10%VGF₅/P-Na composites is 39.4%and 27.7%lower,respectively,than that of PET/10% PGF composites.