Carbon black, multiwall carbon nanotubes, expanded graphite and functionalized graphene flame retarded polypropylene nanocomposites

Abstract

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.

Flame-Retardancy Properties of Intumescent Ammonium Poly(Phosphate) and Mineral Filler Magnesium Hydroxide in Combination with Graphene

“…The TRGO particles had no influence on the kind of volatiles of PP and APP decomposition, and increased the amount of intumescent residue in proportion to their concentration. This inert behavior has been reported before when TRGO particles were used in PP and halogenated flame-retarded PP [16,19]. (Table 2) thanks to fast extinguishment of the sample bars.…”

“…Increasing amounts of TRGO increased the melt viscosity of the already optimized commercial intumescent system APP in PP (Figure 4a). The increase in melt viscosity by nanoparticles is a generally known feature and has already been observed for many particles like TRGO, carbon nanotubes and layered silicate [16,[19][20][21]. Nevertheless, the increase in viscosity for low concentrations of 0.5 wt% TRGO appears to be tolerated without altering the swelling behavior of APP.…”

“…The residue consisting of MgO and TRGO was not effective in hindering the diffusion of the gaseous pyrolysis products. The effect of a shift in only Tonset is also known for TRGO and other carbon particles like carbon black and carbon nanotubes [16,19,20]. The combination of 59 wt% MH with TRGO shifted the entire decomposition step to higher temperatures.…”

“…Despite their success in the cone calorimeter, nanoparticles are often not efficient in reducing the overall flammability of polymers. The UL 94 classification and oxygen index remain similar or are even worsened by the addition of nanoparticles [19][20][21][22][23]. An increase in nanocomposites' melt viscosity hinders the polymer material from dripping or flowing away from the pyrolysis zone and thus from cooling it.…”

“…An increase in nanocomposites' melt viscosity hinders the polymer material from dripping or flowing away from the pyrolysis zone and thus from cooling it. The latest proposals for flame-retardant nanofillers are graphene particles, which have already proved to have the strongest influence on non-charring polymer properties and especially on the burning behavior within the group of carbon (nano)particles [16,19,20].…”