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Herein we investigate the influence of carbon additives with different particle sizes and shapes on the flame retardancy and mechanical properties of isotactic polypropylene. Thermally reduced graphite oxide (TRGO) and multi-layer graphene (MLG250), consisting of few graphene layers, are compared with spherical, tubular and platelet-like carbon fillers such as carbon black (CB), multiwall nanotubes (MWNT) and expanded graphite (EG). The different morphologies control the dispersion of the carbon particles in PP and play a key role in structure–property relationships. Uniformly dispersed CB, MLG250 and TRGO shift the onset temperature of PP decomposition to temperatures around 30 °C higher, induce a flow limit in the composites' melt viscosity and change drastically their fire behaviour. The prevented dripping and significantly increased heat absorption result in decreased time to ignition and hardly any change in the reaction to a small flame. Under forced-flaming conditions reductions in the peak heat release rate of up to 74% are achieved due to the formation of a protective layer of residue during combustion. The described effects of carbon nanomaterials on the properties of PP composites are most pronounced for well-exfoliated graphenes, making them preferable to less exfoliated, micron-sized expanded graphite or conventional spherical and tubular carbon nanoparticles.
Herein, we examine the influence of adding functionalized graphene (FG), distinct expanded graphites and carbon nanofillers such as carbon black and multiwall carbon nanotubes on mechanical properties, morphology, pyrolysis, response to small flame and burning behavior of a V-2 classified flame-retarded polypropylene (PP). Among carbon fillers, FG and multilayer graphene (MLG) containing fewer than 10 layers are very effectively dispersed during twin-screw extrusion and account for enhanced matrix reinforcement. In contrast to the other fillers, no large agglomerates are detected for PP-FR/FG and PP-FR/MLG, as verified by electron microscopy. Adding FG to flame-retardant PP prevents dripping due to reduced flow at low shear rates and shifts the onset of thermal decomposition to temperatures 40°C higher. The increase in the onset temperature correlates with the increasing specific surface areas (BET) of the layered carbon fillers. The reduction of the peak heat release rate by 76% is attributed to the formation of effective protection layers during combustion. The addition of layered carbon nanoparticles lowers the time to ignition. The presence of carbon does not change the composition of the evolved pyrolysis gases, as determined by thermogravimetric analysis combined with online Fourier-transformed infrared measurements. FG and well-exfoliated MLG are superior additives with respect to spherical and tubular carbon nanomaterials.
Thermally reduced graphite oxide (TRGO), containing only four single carbon layers on average, was combined with ammonium polyphosphate (APP) and magnesium hydroxide (MH), respectively, in polypropylene (PP). The nanoparticle's influence on different flame-retarding systems and possible synergisms in pyrolysis, reaction to small flame, fire behavior and mechanical properties were determined. TRGO has a positive effect on the yield stress, which is decreased by both flame-retardants and acts as a synergist with regard to Young's modulus. The applicability and effects of TRGO as an adjuvant in combination with conventional flame-retardants depends strongly on the particular flame-retardancy mechanism. In the intumescent system, even small concentrations of TRGO change the viscosity of the pyrolysing melt crucially. In case of oxygen index (OI) and UL 94 test, the addition of increasing amounts of TRGO to PP/APP had a negative impact on the oxygen index and the UL 94 classification. Nevertheless, systems with only low amounts (≤1 wt%) of TRGO achieved V-0 classification in the UL 94 test and high oxygen indices (>31 vol%). TRGO strengthens the residue structure of MH and therefore functions as a strong synergist in terms of OI and UL 94 classification (from HB to V-0).
Functionalized graphene nanosheets TRGO and MLG 250, prepared from thermally reduced graphite oxide, represent attractive carbon additives for improving the performance of flame retardant polypropylene (PP-FR). The influence of carbon nanofiller type and content on morphology, thermal, mechanical, and electrical properties as well as the fire behavior of melt-extruded PP-FR is investigated. In contrast to conventional nano- and micron-sized carbon fillers such as expanded graphite (EG 40), nano-scaled carbon black (CB), and multiwall carbon nanotubes (CNT), only TRGO and MLG 250 afford uniform dispersion combined with simultaneously improved stiffness (+80%), electrical conductivity (3 × 10-5 S · cm-1) and enhanced flame retardancy of PP-FR, as expressed by lower peak heat release rate (-76%).