An Investigaton On Structure And Properties Of Flame Retardanted Polypropylene Composites Reinforced By Montmorillonite And Fibers

Polypropylene (PP) has been widely used in various fields, such as construction, electronic and electric, communication and household appliance, due to its overwhelming advantages such as low density, ease of processing, low hygroscopic, nice overall mechanical properties, resistance to chemical erosion and excellent electric insulation. However, PP is rather flammable with a limited oxygen index (LOI) value of 17.4% because of its wholly carbon-hydrogen structure. Moreover, PP burns rapidly and releases plenty of heat accompanying melt-dripping, which cannot meet the requirements in many cases, and therefore it has attracted more and more attention to reduce its flammability. The addition of flame retardants can usually cause negative effects to the mechanical properties of PP composites. Therefore, it is of great significance to improve the flame retardancy and mechanical properties simultaneously. Three kinds of reinforcing materials including nano-montmorillonite, aramid fibers and glass fibers and two types of flame retardants were introduced to PP composites. The intercalative effect of OMMT in PP composites was analyzed by wide X-ray diffraction (WXRD) and transition electron microscopy (TEM). The mechanical properties of PP composites were conducted by tensile, flexural and notched impact tests. The crystallinity of PP was characterized by differential scanning calorimetry (DSC) and polarizing optical microscope (POM). The thermal behavior, flammable property, char morphology and structure were investigated by thermogravimetric (TG), differential thermal analyzer (DTA), limited oxygen index (LOI), vertical burning test (UL-94), cone calorimetry (CONE), scanning electron microscope (SEM) and Fourier transition infrared spectroscopy (FTIR). The flame retardant mechanism was also proposed and discussed.This paper can be divided into three parts:the mechanical, thermal, crystalline property and flame retardancy of PP/OMMT composites, PP/AF composites and PP/GF composites containing different flame retardant systems respectively.In the first part, PP/OMMT, PP/D-S/OMMT and PP/IFR/OMMT composites were prepared by using organic-montmorillonite (OMMT) as reinforcing material, decabromodiphenyl ethane/antimony trioxide (DBDPE/Sb2O3, D-S), intumescent flame retardant (melamine polyphosphate and pentaerythritol, MPP/PER, IFR) as flame retardants. The results show that the addition of PP-g-MAH, D-S and IFR can increase the interlayer spacing and improve the dispersion condition of OMMT. It has been demonstrated that intercalated or even exfloliated structure has been formed in PP/OMMT composites. OMMT can act as heterogeneous nucleation which increases the crystallization speed and degree of PP, and thus make PP spherulites grow smaller and align more regular. Moreover, the nanosize effect and reinforcement of OMMT can not only improve the tensile and notched impact properties of PP/D-S/OMMT and PP/IFR/OMMT composites, but also can enhance their thermal property and flame retardancy. The T50% of PP/D-S/OMMT and PP/IFR/OMMT composites are 20～30℃higher than that of pure PP, while the maximal char yield of the two composites can reach up to 13.4% and 24.4% respectively. The LOI value has been increased up to 24.2% and 28.5% and the UL-94 test can reach V-0 and V-1 respectively after the addition of D-S and IFR. Moreover, comparing with pure PP, the peak heat release rate (PHRR), average heat release rate (AHRR), total heat release (THR), average mass loss rate (AMLR) of PP/30phrD-S/5phrOMMT and PP/30phrIFR/5phrOMMT composites are reduced by 60.2% and 58.3%,52.7% and 46.0%,34.7% and 52.5%,31.2% and 63.2% respectively. The char residue of above two composites can remain its original morphology with firm structure. Hence, there is a nice synergistic effect on flame retardancy between OMMT and D-S or IFR in PP composites.The second part of this paper studies the properties of PP/AF, PP/AF/D-S and PP/AF/IFR composites composed of AF and D-S, IFR. AF was modified by phosphate coupling agent and the best modification condition is as follows:the mass ratio of ethanol and water is 9:1, the immersion time is 1h and the concentration of coupling agent is 1%. DSC and POM results show that AF can improve the crystallization speed and degree of PP but decrease the size of PP spherulites by acting as a heterogeneous nucleating agent. The PP/AF composite with 20phr AF has optimal mechanical properties. The T50% of PP/AF/D-S and PP/AF/IFR composites are 25～30℃higher than that of pure PP, while the maximum char yield is raised up to 17.7% and 24.4% respectively, the maximum exothermal fusion is reduced by 60% and 59.3% respectively, the maximum rate in UL-94 is V-0 and V-1 respectively. The PHRR,AHRR,THR,AMLR,AEHC of PP/20phrAF/30phrD-S and PP/20phrAF/30phrIFR composites are decreased by 78.8% and 75.8%,71.2% and 50.6%,75.0% and 69.7%,36.8% and 60.5%,68.5% and 23.1% respectively, and there was almost no smoke released in above two composites, when comparing with that of pure PP. Moreover, the morphology of above two samples before or after being heating is almost the same and the residue char is strong. AF with superior mechanical and thermal properties can act as "skeleton" materials and hinder the move of PP chains and thus improve the thermal stability and char yield of PP composites. The protective effect of D-S in gas phase together with the improved char protective effect in solid phase by IFR and AF make the flame retardancy of PP/AF/D-S and PP/AF/IFR improved greatly.The third part is about the properties of PP/GF, PP/GF/D-S and PP/GF/IFR composites when GF was used as reinforcing material while D-S and IFR were added as flame retardants. The surface of GF was also modified by phosphate coupling agent showing a better interface cohesion between GF and PP proved by SEM and FTIR tests. The optimal modification condition is listed below:the mass ratio of ethanol and water is 9:1, the immersion time is 1h and the concentration of phosphate coupling agent is 1.5%. The crystallization speed and degree of PP are improved while the size of PP spherulite decreases in the presence of GF in PP/GF composites. The mechanical properties of PP/GF/D-S and PP/GF/IFR composites are higher than that of PP/D-S and PP/IFR, but the reinforcing effect on mechanical properties of GF is lower than that of AF. The T50% of PP/GF/D-S and PP/GF/IFR composites are about 20～40℃higher than that of pure PP, while the maximum char yield is improved to 17.7% and 30.7% respectively, the exothermal fusion is also decreased a lot. The results from cone calorimetry tests show that:the PHRR and AHRR of PP/20phrGF/30phrD-S and PP/20phrGF/30phrIFR composites are decreased slightly compared with that of pure PP; however, the AHRR and AMLR are increased by 24.6% and 19.3%,59.4% and 105.0% respectively when compared with that of PP/D-S and PP/IFR composites. Moreover, the char morphology of above two composites changes greatly, especially for the latter one which has been melted completely during combustion. The LOI values of PP/GF/D-S and PP/GF/IFR composites are both lower than that of PP/D-S and PP/IFR, and the UL-94 rate of the former is V-2 but the latter is failed. Therefore, it is concluded that GF can cause negative effect on flame retardancy of PP composites.