TY - JOUR A1 - Hofmann, D. A1 - Wartig, K.-A. A1 - Thomann, R. A1 - Dittrich, Bettina A1 - Schartel, Bernhard A1 - Mülhaupt, R. T1 - Functionalized graphene and carbon materials as additives for melt-extruded flame retardant polypropylene N2 - 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%). KW - Extrusion KW - Flame retardance KW - Graphene KW - Nanocomposite KW - Polypropylene PY - 2013 DO - https://doi.org/10.1002/mame.201200433 SN - 1438-7492 SN - 1439-2054 VL - 298 IS - 12 SP - 1322 EP - 1334 PB - Wiley-VCH Verl. CY - Weinheim AN - OPUS4-29817 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - CONF A1 - Dittrich, Bettina A1 - Schartel, Bernhard A1 - Hofmann, D. A1 - Wartig, K.-A. A1 - Mülhaupt, R. ED - Wilkie, C. T1 - Carbon black, multiwall nanotubes and graphene - promising approach to flame retarded nanocomposites? T2 - 24th Annual conference on recent advances in flame retardancy of polymeric materials CY - Stamford, NY, USA DA - 2013-05-20 KW - Graphene KW - Flame retardancy KW - Carbon nanoparticle KW - Nanocomposite PY - 2013 SN - 1-56965-218-X SP - 1 EP - 9(?) CY - Wellesley, MA, USA AN - OPUS4-28637 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - JOUR A1 - Dittrich, Bettina A1 - Wartig, K.-A. A1 - Hofmann, D. A1 - Mülhaupt, R. A1 - Schartel, Bernhard T1 - Flame retardancy through carbon nanomaterials: carbon black, multiwall nanotubes, expanded graphite, multi layer graphene and graphene in polypropylene N2 - 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. KW - Polypropylene KW - Flame retardancy KW - Nanocomposites KW - Graphene KW - Carbon nanomaterials KW - Thermally reduced graphite oxide PY - 2013 DO - https://doi.org/10.1016/j.polymdegradstab.2013.04.009 SN - 0141-3910 SN - 1873-2321 VL - 98 IS - 8 SP - 1495 EP - 1505 PB - Applied Science Publ. CY - London AN - OPUS4-28638 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - JOUR A1 - Dittrich, Bettina A1 - Wartig, K.-A. A1 - Hofmann, D. A1 - Mülhaupt, R. A1 - Schartel, Bernhard T1 - Carbon black, multiwall carbon nanotubes, expanded graphite and functionalized graphene flame retarded polypropylene nanocomposites N2 - 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. KW - Graphene KW - Flame retardancy KW - Nanocomposites KW - Polypropylene KW - Carbon nanoparticles PY - 2013 DO - https://doi.org/10.1002/pat.3165 SN - 1042-7147 SN - 1099-1581 VL - 24 IS - 10 SP - 916 EP - 926 PB - John Wiley & Sons, Ltd. CY - Chichester AN - OPUS4-29337 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - JOUR A1 - Dittrich, Bettina A1 - Wartig, K.-A. A1 - Mülhaupt, R. A1 - Schartel, Bernhard T1 - Flame-retardancy properties of intumescent ammonium poly(phosphate) and mineral filler magnesium hydroxide in combination with graphene N2 - 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). KW - Graphene KW - Intumescence KW - Ammonium polyphosphate KW - Magnesium hydroxide KW - Synergy KW - Polypropylene PY - 2014 UR - https://nbn-resolving.org/urn:nbn:de:kobv:b43-320685 DO - https://doi.org/10.3390/polym6112875 SN - 2073-4360 VL - 6 IS - 11 SP - 2875 EP - 2895 PB - MDPI CY - Basel AN - OPUS4-32068 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - JOUR A1 - Dittrich, Bettina A1 - Wartig, K.-A. A1 - Hofmann, D. A1 - Mülhaupt, R. A1 - Schartel, Bernhard T1 - The influence of layered, spherical, and tubular carbon nanomaterials' concentration on the flame retardancy of polypropylene N2 - The characteristic influences of increasing concentrations of graphene, expanded graphite (EG), carbon black (CB), and multiwall carbon nanotubes (MWNT) are investigated on pyrolysis, reaction to small flame, burning behavior, and on electrical, thermal, and rheological properties of flame retarded polypropylene (PP-FR). The property-concentration dependency is different for the various material properties, as threshold, linear, and leveling off functions were observed. Increasing concentrations of carbon nanoparticles resulted in a decrease in the electrical resistivity of the polymer by crossing the percolation threshold. The developing nanoparticle network changes melt flow behavior for small shear rates, increases thermal conductivity and therefore, affects the UL 94 classification and oxygen index. The onset temperature of PP decomposition is shifted to temperatures up to 37°C higher; the peak heat release rate is reduced by up to 74% compared to PP-FR. Both effects leveled off with increasing particle concentration. Among the four carbon nanomaterials tested, graphene presents superior influence on composite properties over the tested concentration range and outperforms commercial CB, MWNT, and EG. POLYM. COMPOS., 36:1230–1241, 2015. KW - Graphene KW - Flame retardancy KW - Concentration dependency KW - Nanocomposite KW - Carbon nanomaterial PY - 2015 DO - https://doi.org/10.1002/pc.23027 SN - 0272-8397 SN - 1548-0569 VL - 36 IS - 7 SP - 1230 EP - 1241 PB - Society of Plastics Engineers CY - Manchester, NH AN - OPUS4-33619 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - JOUR A1 - Meyer-Plath, Asmus A1 - Beckert, F A1 - Tölle, FJ A1 - Sturm, Heinz A1 - Mülhaupt, R T1 - Stable aqueous dispersions of functionalized multi-layer graphene by pulsed underwater plasma exfoliation of graphite N2 - A process was developed for graphite particle exfoliation in water to stably dispersed multi-layer graphene. It uses electrohydraulic shockwaves and the functionalizing effect of solution plasma discharges in water. The discharges were excited by 100 ns high voltage pulsing of graphite particle chains that bridge an electrode gap. The underwater discharges allow simultaneous exfoliation and chemical functionalization of graphite particles to partially oxidized multi-layer graphene. Exfoliation is caused by shockwaves that result from rapid evaporation of carbon and water to plasma-excited gas species. Depending on discharge energy and locus of ignition, the shockwaves cause stirring, erosion, exfoliation and/or expansion of graphite flakes. The process was optimized to produce long-term stable aqueous dispersions of multi-layer graphene from graphite in a single process step without requiring addition of intercalants, surfactants, binders or special solvents. A setup was developed that allows continuous production of aqueous dispersions of flake size-selected multi-layer graphenes. Due to the well-preserved sp(2)-carbon structure, thin films made from the dispersed graphene exhibited high electrical conductivity. Underwater plasma discharge processing exhibits high innovation potential for morphological and chemical modifications of carbonaceous materials and surfaces, especially for the generation of stable dispersions of two-dimensional, layered materials. KW - Exfoliation KW - Graphene KW - Aqueous dispersion KW - Solution plasma KW - Functionalization KW - Electrohydraulic effect KW - Shear exfoliation KW - Oxide-films KW - Water PY - 2016 DO - https://doi.org/10.1088/0022-3727/49/4/045301 SN - 0022-3727 VL - 49 IS - 4 SP - 045301-1 EP - 045301-11 AN - OPUS4-35796 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER -