Flame Retardation Of Polyethylene With Carbon Nanomaterials And The Rare Earth Compounds

Polyethylene (PE) is one of the most widely used polyolefins due to its balanced mechanical properties, chemical resistance and ease of processing. However, its inherent flammability has not only limited its applications in some fields where excellent flame retardancy is required, but also caused thousands of fire hazards. In view of the special property and structure of carbon nanomaterials and rare earth compounds, the effects of both on the flame retardancy of PE were investigated.Firstly, PE/brominated flame retardants (BFR)/graphene nanoplatelets (GNPs) composites were prepared via melt blending and the effects of GNPs and BFR on the thermal stability and flame retardancy of PE were discussed. The results indicated that there were combinatorial effect between GNPs and BFR. Thermogravimetric analysis coupled with FTIR spectroscopy (TG-FTIR) and residue analysis by scanning electron microscopy (SEM) revealed that the improvement of flame retardancy was especially attributed to the tortuous pathway effects of GNPs. In this manner, evolution of degradation products was slowed down, which increased the effectiveness of radical entrapment in the gas phase. However, GNPs tend to agglomerate themselves in polymer matrix. A Lewis acid catalyst, aluminium chloride anhydrous (AlCl3) was added to initiate Friedel-Crafts reaction for promoting the dispersion of GNPs in PE. Raman spectroscopy and rheometer test (ARES) revealed that the electrostatic attraction between the macrocarbocations formed by Friedel-Crafts reaction and π electrons of graphene sheets caused the better dispersive states of GNPs. The uniform dispersion of GNPs was crucial for forming continuous and compact char layer, which isolated the underlying materials from flame and prevented heat transfer, leading to the improvement of thermal stability and flame retardancy. At high content, however, AICl3 may iniciate chain scission of PE as a Lewis acid. So the influence of AICl3 content on the performance of PE/BFR/GNPs composites was further studied. At low AICl3 content (<0.8 phr), the improved dispersion of GNPs made the barrier effect and protective properties of graphene sheets better which was responsible for the improved flame retardant effectiveness, thermal oxidation stability and tensile strength. At high AICl3 content (1.5 phr), AICl3, as a strong Lewis acid, caused the degradation of composites, which was not in favor of the performance of composites.Secondly, multiwalled carbon nanotubes (MWNTs) bridged cerium phenylphosphonate (CeHPP) hybrids (Ce-MWNTs) were facilely prepared through in situ co-precipitation. Transmission electron microscopy (TEM) showed that due to the good dispersion of CeHPP and the acting force between CeHPP and MWNTs, the hybrids were dispersed uniformly to resist the strong intermolecular attractions. The hybrids led to a reduction in the peak heat release rate (PHRR) of brominated flame retardant PE composite and improved the UL-94 grade from V-2 to V-0, indicating that they could confer better flame retardancy on PE compared to CeHPP or MWNTs alone. The morphology of chars gave the evidence that Ce-MWNTs could enhance the physical barrier effect to retard the vaporization of flammable gases and the transfer of heat because of the mutual complementarity of CeHPP and MWNTs.Finally, effects of two kinds of rare earth trifluoromethanesulfonate (RE(OTf)3), ytterbium (Yb(OTf)3) or lanthanum (La(OTf)3), on the flame retardancy and thermo-oxidative stability of PE were investigated. Electron spin resonance (ESR) spectroscopy proved both of Yb(OTf)3 and La(OTf)3 could improve the thermo-oxidative stability of PE. However, Yb(OTf)3 was more efficient than La(OTf)3. Very low Yb(OTf)3 loading (0.5 wt%) in HDPE could increase the onset degradation temperature in air from 334 to 407 ℃, delay the the oxidative induction time (0IT) from 11.0 to 24.3 min, and decrease the enthalpy (ΔHd) from 61.0 to 13.0 J/g remarkably. Meanwhile the cone calorimeter result indicated that the effect of Yb(OTf)3 on the flame retardancy of PE/IFR was better than La(OTf)3. The improvement on the flame retardancy and thermo-oxidative stability was due to the action of anion. However, the different effects were attributed to the different reactivity of rare earth elements.