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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%).
The flame retardancy mechanisms of aluminium diethylphosphinate (AlPi) and its combination with melamine cyanurate (MC) in glass-fibre-reinforced poly(butylene terephthalate) (PBT/GF) were analysed using TGA including evolved gas analysis (TGA-FTIR), cone calorimeter measurements using various irradiations, flammability tests (limited oxygen index, LOI, UL 94) and chemical analyses of residues (FTIR, SEM/EDX). AlPi decomposed mainly through the formation of diethylphosphinic acid and aluminium phosphate and influenced the decomposition of the PBT only slightly. AlPi acted mainly through flame inhibition. A halogen-free V-0 PBT/GF material was achieved with a LOI of 44%. Additional charring influenced the flammability. MC decomposed independently of the polymer and showed some fuel dilution effects.
The influence of distinct carbon based nanofillers: expanded graphite (EG), conducting carbon black (CB), thermally reduced graphene oxide (TRGO) and multi-walled carbon nanotubes (CNT) on the thermal, dielectric, electrical and rheological properties of polybutylene terephthalate (PBT) was examined. The glass transition temperature (Tg) of PBT nanocomposites is independent of the filler type and content. The carbon particles act as nucleation agents and significantly affect the melting temperature (Tm), the crystallization temperature (Tc) and the degree of crystallinity of PBT composites. PBT composites with EG show insulating behaviour over the tested concentration range of 0.5 to 2 wt.-% and hardly changed rheological behaviour. CB, CNT and TRGO induce electrical conductivity to their particular PBT composites by forming a conducting particle network within the polymer matrix. CNT reached the percolation threshold at the lowest concentration (<0.5 wt.-%), followed by TRGO (<1 wt.-%) and CB (<2 wt.-%). With the formation of a particle network, the flow behaviour of composites with CB, CNT and TRGO is affected, i.e., a flow limit occurs and the melt viscosity increases. The degree of influence of the carbon nanofillers on the rheological properties of PBT composites follows the same order as for electrical conductivity. Electrical and rheological results suggest an influence attributed to the particle dispersion, which is proposed to follow the order of EG<< CB<TRGO<CNT.
Novel DOPO-based flame retardants in high-performance carbon fibre epoxy composites for aviation
(2011)
Two novel, halogen-free, phosphorus-based oligomeric flame retardants are investigated in the commercial epoxy resin RTM6 and ~70 wt.% carbon fibre RTM6 composites (RTM6-CF) with respect to pyrolysis and fire behaviour. The flame retardants are based on 9,10-dihydro-9-oxy-10-phosphaphenanthrene-10-oxide (DOPO) units linked to the star-shaped aliphatic ground body tetra-[(acryloyloxy)ethyl] pentarythrit (DOPP), or heterocyclic tris-[(acryloyloxy)ethyl] isocyanurate (DOPI), respectively. The glass transition temperature is reduced by adding DOPP and DOPI, but the mechanical properties of the composites (e.g. interlaminar shear strength (ILSS) and Gc in mode I and II) remain unchanged. Decomposition models are proposed based on mass loss, evolved gas analysis (TGFTIR) and condensed product analysis (hot stage cell within FTIR). The fire behaviour is investigated comprehensively (UL 94, limiting oxygen index (LOI) and cone calorimeter). Both flame retardants act in the gas phase through flame inhibition and in the condensed phase through charring. The UL 94 of RTM6 is improved from HB to V-1 and V-0; the LOI from 25% to 3438%. Peak heat release rate (PHRR) and total heat evolved (THE) are lowered by 31-49% and 4044%, respectively. Adding CF increases the residue, reduces the THE, but suppresses the charring due to RTM6 and flame retardants. Thus the THE of RTM6-CF is reduced by about 25% when DOPI and DOPP are added. However, UL 94: V-0 and LOI of 45% and 48% are achieved with ~0.6 wt.% phosphorus.
Flame retarded poly(butylene terephthalate) (PBT) is required for electronic applications and is mostly achieved by low molar mass additives so far. Three phosphorus-containing polyesters are suggested as halogen-free and polymeric flame retardants for PBT. Flame retardancy was achieved according to cone calorimeter experiments showing that the peak heat release rate and total heat evolved were reduced because of flame inhibition and condensed-phase activity. The presented polymers containing derivatives of 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide form immiscible blend systems with PBT. Shear-rheology shows an increase in storage moduli at low frequencies. This is proposed as quantitative measure for the degree of phase interaction. The phase structure of the blends depends on the chemical structure of the phosphorus polyester and was quite different, depending also on the viscosity ratio between matrix and second phase. A lower viscosity ratio leads to two types of phases with spherical and additionally continuous droplets. Addition of the flame retardants showed no influence on the dielectric properties but on the mechanical behavior. The polymeric flame retardants significantly diminish the impact strength because of several reasons: (1) high brittleness of the phosphorus polyesters themselves, (2) thermodynamic immiscibility, and (3) weak phase adhesion. By adding a copolymer consisting of the two base polymers to the blend, an improvement of impact strength was obtained. The copolymer particularly acts as compatibilizer between the phases and therefore leads to a smaller phase size and to a stronger phase adhesion due to the formation of fibrils.
Highly soluble 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide-(DOPO)-based diamino hardener (2), bearing its amino groups directly on the DOPO framework, is investigated with respect to its use as a reactive flame retardant in thermosets. A mechanism for decomposition of the corresponding phosphorus-modified epoxy resin system based on a diglycidylether of bisphenol A DGEBA and 2 (DGEBA/2) is proposed and compared to the systems using DGEBA and 4,4'-diaminodiphenylsulfon (DGEBA/DDS) and to a similar system based on the structurally comparable non-reactive DOPO-based compound (DGEBA/DDS/1). Additive 1 changed the decomposition characteristics of the epoxy resin only slightly and phosphorus was released. Incorporating 2 induces two-step decomposition and most of the phosphorus remains in the residue. Furthermore, the fire behaviour of neat epoxy resin systems and a representative carbon fibre-reinforced composite based on DGEBA, DDS and 2 (DGEBA/DDS/2) were examined and compared to that of the analogous composite systems based on DGEBA/DDS and DGEBA/DDS/1. Based on different flame retardancy mechanisms both the reactive compound 2 and the additive compound 1 improve flammability (increase in LOI >13% and achieving V-1 behaviour) of the epoxy resin and composites. Under forced flaming only the flame inhibition of the additive compound 1 acts sufficiently. Lastly, the superior key mechanical properties of the epoxy resin and composite based on 2 are sketched.
A novel phosphorus-modified polysulfone (P-PSu) was employed as a combined toughness modifier and a source of flame retardancy for a DGEBA/DDS thermosetting system. In comparison to the results of a commercially available polysulfone (PSu), commonly used as a toughness modifier, the chemorheological changes during curing measured by means of temperature-modulated DSC revealed an earlier occurrence of mobility restrictions in the P-PSu-modified epoxy. A higher viscosity and secondary epoxy-modifier reactions induced a sooner vitrification of the reacting mixture; effects that effectively prevented any phase separation and morphology development in the resulting material during cure. Thus, only about a 20% increase in fracture toughness was observed in the epoxy modified with 20 wt.% of P-PSu, cured under standard conditions at 180 °C for 2 h. Blends of the phosphorus-modified and the standard polysulfone (PSu) were also prepared in various mixing ratios and were used to modify the same thermosetting system. Again, no evidence for phase separation of the P-PSu was found in the epoxy modified with the P-PSu/PSu blends cured under the selected experimental conditions. The particular microstructures formed upon curing these novel materials are attributed to a separation of PSu from a miscible P-PSuepoxy mixture. Nevertheless, the blends of P-PSu/PSu were found to be effective toughness/flame retardancy enhancers owing to the simultaneous microstructure development and polymer interpenetration.
10-Ethyl-9-oxa-10-phosphaphenanthrene-10-oxide (1) can be nitrated using acetic anhydride and fuming nitric acid. The nitro group is reduced using palladium on charcoal and hydrogen. These reaction conditions are used for the synthesis of an analogous DOPO-based diaminic hardener (7). An evaluation of the curing behavior, mechanical properties and flammability of a neat resin made of DGEBA and 7 (DGEBA + 7) and of a carbon fiber-reinforced resin made of DGEBA, 4,4-diaminodiphenylsulfon (DDS) and 7 (DGEBA + DDS + 7) shows the potential of this hardener to lead to flame-retardant systems while keeping relevant properties on a high level; especially when compared to a similar system (DGEBA + DDS + 1).
In den letzten Jahren werden zunehmend Nanopartikel als Füllstoff für Polymere vorgeschlagen und auch erfolgreich in Elastomer-Nanocomposites eingesetzt. In dieser Arbeit wird Multilayergraphen (MLG) als Nanofüllstoff näher untersucht, der sich bereits bei geringen Konzentrationen als effizient erweist. MLG besteht aus nur etwa zehn Graphenlagen.
Chlorbutylkautschuk (CIIR)/MLG-Nanocomposites mit verschiedenen MLG-Gehalten wurden mit Hilfe eines ultraschallunterstützen Mischverfahrens in Lösung hergestellt und auf einem Walzwerk weiterverarbeitet. Das Einmischen von MLG führt zu einer deutlichen Verbesserung der rheologischen und mechanischen Eigenschaften, des Vernetzungsverhaltens sowie der Barrierewirkung gegenüber Gasen. Bereits der Zusatz von 3 phr MLG zu CIIR führt zu einem mehr als zweifach höheren E-Modul und zu einer Reduktion der Permeabilität von O2 und CO2 um 30 %. Höhere Konzentrationen an Nanofüllstoff resultieren in einer weiteren Verbesserung der Eigenschaften der Nanocomposites. Weiterhin zeigten die CIIR/MLG-Nanocomposites auch eine geringere Entflammbarkeit.