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Communication: Fire retardant coatings are deposited on polyamide-66 using plasma polymerisation. Chemical composition and thickness of deposits are adjusted varying the plasma treatment based on hexamethydisiloxane mixed with oxygen. The fire retardancy performances are evaluated using a cone calorimeter. The correlation between fire retardancy and thickness as well as chemical composition is discussed.
Polymeric nanocomposites are discussed as one of the most promising advanced materials whose nanoscale effects can be exploited for industry. Layered silicate polypropylene-graft-maleic anhydride nanocomposites are investigated as a model to clarify the potential of such materials in terms of fire retardancy. The nanostructure is characterized using transmission electron microscopy (TEM) and shear viscosity. The fire behavior is characterized using different external heat fluxes in cone calorimeter, limiting oxygen index and UL 94 classification. A comprehensive fire behavior characterization is presented which enables an assessment of the materials’ potential with respect to different fire scenarios and fire tests. The influence of morphology and the active mechanisms are discussed, such as barrier formation and changed melt viscosity. To our knowledge, it is the first attempt to illuminate the concept’s strengths, such as the reduction of flame spread, and weaknesses, such as the lack of influence on ignitability, in a clear, comprehensive and detailed manner.
Phosphonium-modified layered silicate epoxy resin nanocomposites were evaluated by testing the thermal/thermo-mechanical properties [differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), torsional pendulum, Sharpy toughness], flammability (limiting oxygen index LOI) and fire behavior (cone calorimeter with different irradiations). The morphology of the composites was determined using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The drying conditions of phosphonium-modified layered silicate were varied in order to improve the nanocomposite formation and properties. The results were compared with using a commercial ammonium-modified montmorillonite. Enhanced nanocomposite formation was found for the commercial systems due to the amount of excess surfactant, but this effect was overcompensated through the advanced morphology of the phosphonium-modified systems. Several fire retardancy mechanisms and their specific influence on the different fire properties, such as ignitability, flammability, flame spread, total heat release (fire load), and the production of CO and smoke were discussed comprehensively. The main mechanism of layered silicate is a barrier formation influencing the flame spread in developing fires. Several minor mechanisms are significant, but important fire properties such as flammability or fire load are hardly influenced. Hence combinations with aluminum hydroxide and organo-phosphorus flame retardants were evaluated. The combination with aluminum hydroxide was a promising approach since it shows superposition in properties such as the fire load and only in some properties very little antagonism. The combination with an organo-phosphorus flame retardant disillusions, since it was characterized mainly by antagonism.
Barrier formation and increasing the melt viscosity are addressed as the two main general fire retardancy mechanisms of polymer nanocomposites. They result in specific impacts on fire properties that consequentially cause varying flame retardancy efficiency in different fire tests. The barrier formation retards mainly flame spread (peak of heat release rate) in developing fires, but does not reduce fire load (total heat evolved), ignitability or flammability (limiting oxygen index, UL 94). Furthermore, this flame retardancy effect increases with increasing irradiation and vanishes with decreasing irradiation. The increased melt viscosity prevents dripping, which is beneficial or disadvantageous depending on the fire test used. In some test, it become the dominant influence, transforming self-extinguishing samples into flammable materials or causing wicking. Advantages and the limits are sketched comprehensively for exploiting the main general fire retardancy mechanisms of polymer nanocomposites. It is concluded that barrier formation and changing the melt viscosity in nanocomposites are not sufficient for most applications, but must be accompanied by additional mechanisms in special systems or in combination with other flame retardants.
Summary: The decomposition of novel phosphorus-containing poly(oxyphenylene-sulfonyl-phenylene-oxy-diphenyl phenylene phosphine oxide) (PSU_I), 2,5-dihydroxy-1-biphenylene-phosphine oxide based polysulfone (PSU_II), poly(sulfonyl-diphenylphenylene phosphonate) (PSU_P) and bisphenol A-based polysulfone (PSU) is studied. The influence of the chemical structure, charring and phosphorus release is discussed based on the mass loss, kinetics and products. The pyrolysis and fire behaviour of blends with epoxy resin (EP) are studied. For EP-PSU_II, phosphorus initiates water elimination and changes the decomposition pathway of EP. The fire behaviour of EP-PSU shows some improvements, whereas the heat release rate is crucially reduced for EP-PSU_II due to simultaneous char formation and flame inhibition.
A comparative evaluation of systematically tailored chemical structures of various phosphorus-containing aminic hardeners for epoxy resins was carried out. In particular, the effect of the oxidation state of the phosphorus in the hardener molecule on the curing behavior, the mechanical, thermomechanical, and hot-wet properties of a cured bifunctional bisphenol-A based thermoset is discussed. Particular attention is paid to the comparative pyrolysis of neat cured epoxy resins containing phosphine oxide, phosphinate, phosphonate, and phosphate (with a phosphorus content of about 2.6 wt %) and of the fire behavior of their corresponding carbon fiber-reinforced composites. Comparatively faster curing thermosetting system with an enhanced flame retardancy and adequate processing behavior can be formulated by taking advantage of the higher reactivity of the phosphorus-modified hardeners. For example, a combination of the high reactivity and of induced secondary crosslinking reactions leads to a comparatively high Tg when curing the epoxy using a substoichiometric amount of the phosphinate-based hardener. The overall mechanical performance of the materials cured with the phosphorus-containing hardeners is comparable to that of a 4,4-DDS-cured reference system. While the various phosphorus-containing hardeners in general provide the epoxy-based matrix with enhanced flame retardancy properties, it is the flame inhibition in the gas phase especially that determines the improvement in fire retardancy of carbon fiber-reinforced composites. In summary, the present study provides an important contribution towards developing a better understanding of the potential use of such phosphorus-containing compounds to provide the composite matrix with sufficient flame retardancy while simultaneously maintaining its overall mechanical performance on a suitable level.
The pyrolysis of an epoxy resin and the fire behavior of corresponding carbon fiber-reinforced composites, both flame-retarded with either 10-ethyl-9,10-dihydro-9-oxa-10-phosphaphenanthrene 10-oxide or 1,3,5-tris[2-(9,10-dihydro-9-oxa-10-phosphaphenanthrene 10-oxide-10-)ethyl]1, 3,5-triazine-2,4,6(1H,3H,5H)-trione, are investigated. The different fire retardancy mechanisms are discussed, and their influence on the fire properties assessed, in particular for flammability (limiting oxygen index, UL 94) and developing fires (cone calorimeter with different external heat fluxes of 35, 50, and 70 kW m-2). Adding the flame retardants containing 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide affects the fire behavior by both condensed phase and gas phase mechanisms. Interactions between the additives and the epoxy resin result in a change in the decomposition pathways and an increased char formation. The release of phosphorous products results in significant flame inhibition. The fire properties achieved are thus interesting with respect to industrial exploration.
The pyrolysis and fire behavior of glass-fiber reinforced poly(butylene terephthalate) (PBT/GF) with two different metal phosphinates as flame retardants in combination with and without melamine cyanurate (MC) were analyzed by means of thermogravimetry, thermogravimetry coupled with infrared spectroscopy, flammability, and cone calorimeter tests as well as scanning electron microscopy/energy dispersive X-ray spectroscopy and X-ray fluorescence spectroscopy. In PBT/GF, dosages of 13-20% of the halogen-free flame retardant aluminum phosphinate or aluminum phosphinate in combination with MC fulfill the requirements for electrical engineering and electronics applications (UL 94 = V-0; LOI > 42%), whereas the use of the same amount of zinc phosphinate or zinc phosphinate in combination with MC does not improve the fire behavior satisfactorily (UL 94 = HB; LOI = 27-28%). The performance under forced flaming conditions (cone calorimeter) is quite similar for both of the metal phosphinates. The use of aluminum and zinc salts results in similar flame inhibition predominantly due to the release of the phosphinate compounds in the gas phase. Both metal phosphinates and MC interact with the polymer changing the decomposition characteristics. However, part of the zinc phosphinate vaporizes as a complete molecule. Because of the different decomposition behavior of the metal salts, only the aluminum phosphinate results in a small amount of thermally stable carbonaceous char. In particular, the aluminum phosphinate-terephthalate formed is more stable than the zinc phosphinate-terephthalate. The small amount of char has a crucial effect on the thermal properties and mechanical stability of the residue and thus the flammability.
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.
In this first of two papers, the thermal decomposition of bisphenol A bis(diphenyl phosphate)-flame retarded polycarbonate (PC) blends with different impact modifiers was studied. The impact modifiers were an acrylonitrile-butadiene-styrene (ABS), a poly(n-butyl acrylate) (PBA) rubber with a poly(methyl methacrylate) (PMMA) shell and two silicone-acrylate rubbers consisting of PBA with different amounts of polydimethylsiloxane (PDMS) and different shells (PMMA and styrene-acrylonitrile, SAN). The focus of this work was to study the impact of the acrylate and silicon-acrylate rubbers with respect to pyrolysis and flame retardancy in comparison to common ABS. Thermogravimetry (TG) was performed to investigate the pyrolysis behaviour and reaction kinetics. TG in combination with FTIR identified the pyrolysis gases. Solid residues were investigated by FTIR-ATR. PC/ABS shows two-step decomposition, with PC decomposing independently from ABS at higher temperatures. Pure acrylate rubber destabilises PC due to interactions between the rubber and PC, which leads to earlier decomposition of PC. Using silicone-acrylate rubbers led to similar results as PC/ABS with respect to pyrolysis, reaction kinetics and analysis of the solid residue; hence the exchange of ABS for the silicone-acrylate rubbers is possible.