Study Of Nano-fillers/Intumescent Flame Retardant Polypropylene Composites

Polypropylene (PP), which is widely used in building materials, automotive, electronics/electrical appliances and many other industries, is one of the world's largest categories of the resin. Unfortunately, flammable PP with a low limited oxygen index value (17～18%) as well as molten droplets produce during the combustion could easily trigger a fire and lead to rapid spread of the fire, which extremely restricts its wide application. Therefore, to develop flame retarded PP composites is particularly necessary and important. To ensure environment-friendly and flame retarded efficiency, intumescent flame retardant (IFR) is widely used in the field of flame retarded polymers. However, poor compatibility between IFR and polymer matrix leads to some deterioration of mechanical property of materials. Therefore, the novel nano-intumescent flame retardant coordinated system has become the dominant topic in the field of flame retardant. Based on the combustion mechanism of PP, effects of different nano-fillers on the intumescent flame retarded PP composites were discussed.Effects of organically modified montmorillonite (OMMT) on the flame retardancy and mechanical properties of intumescent flame retarded PP were investigated. XRD results indicated that OMMT particles were partially exfoliated. Incorporating 1.8 wt % OMMT into PP/intumescent flame retardant (IFR) considerably enhanced the thermal stability and char residues. Limiting oxygen index (LOI) tests revealed that OMMT led to a further increase in LOI value of PP/IFR. Compared to PP/IFR, incorporation of OMMT can not only further reduce the first peak heat release rate (PHRR1), but delay the time to the secondary combustion (PHRR2) of samples. Unlike the plainland-like structure of residue of PP/IFR, the char residue of PP/IFR/OMMT displayed a folded structure morphology because of the deposition of carbonaceous residue on the surface of clay mineral layers. This interesting morphology of residue was responsible for the enhanced thermal stability, improved flame retardancy, and higher char yields.Thermal and flame performances of intumescent flame retarded PP composites with OMMT or Sodium dodecyl sulfonate intercalated LDH (SDS-LDH) were studied. OMMT particles were partially exfoliated in the matrix, while intercalation/aggregation was obtained for LDH. Incorporation of OMMT and LDH improved the thermal stability and flame retardancy of PP/IFR composites in the early and middle-later stage of heating and combustion respectively. Differences in degradation pathway of clay and LDH were responsible for the above phenomenon which bore important implication for the barrier mechanism. The introduction of OMMT into PP/IFR not only increased the char residue, but also formed compact and folded morphology of char residue which provided more effective protect for underlying materials against heat and oxygen relative to LDH, thus improved the flame retardancy of intumescent flame retarded PP samples more efficiently.PP/IFR/Nano-fillers (carbon nanotubes (CNTs), OMMT. LDH or polysilsesquioxane (POSS)) were prepared by melt blending. XRD results showed that all nano-fillers dispersed in matrix uniformly. To POSS, its volatilization and conversion to SiO₂ occurred in lower and higher temperature respectively led to different thermal degradation stages for PP/IFR/POSS, excellent thermal stabilization of POSS can only be obtained in high temperature. In addition, except for CNTs which led to deterioration in thermal stability of materials in high temperature; optimization of thermal stability of other nano-fillers became more obvious with increasing temperature and in which OMMT showed the best performance. In cone calorimetry tests of PP/IFR/CNTs, the presence of CNTs resulted in loss of swelling characteristics of intumescent flame retarded system and showed poor flame retarded property. Further investigation demonstrated that to endow materials excellent flame retardancy optimal ratio between CNTs and IFR was needed.