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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.
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.
On the basis of two examples, temperature measurements are proposed within burning polymer specimen during the cone calorimeter test; especially to gain deeper insight into the actual pyrolysis conditions and flame retardancy mechanism. The heating and pyrolysis within a poly(methyl methacrylate) specimen were characterized, discussing the characteristic maximum heating rates (165-90°Cmin-1 decreasing with depth within the specimen and >275°Cmin-1 at the initial surface), pyrolysis temperature (454-432°C decreasing in accordance with decreasing heating rates), thickness of the pyrolysis zone (0.5-1.3 mm) and its velocity (1.2-2.1 mm min-1) as a function of sample depth and burning time. Thermally thick behaviour corresponds to a pyrolysis zone thickness of 0.74 mm and a velocity of 1.51 mm min-1 and occurs until the remaining specimen thickness is less than 8 mm. The shielding effect against radiation occurring in a layered silicate epoxy resin nanocomposite was investigated. It is the main flame retardancy effect of the silicate-carbon surface layer formed under fire. The reradiation from the hot surface is increased by a factor of around 4-5 when an irradiance of 70kWm-2 is applied. The energy impact into the pyrolysis zone is crucially reduced, resulting in a reduction of fuel production and thus heat release rate.
The pyrolysis and flame retardancy of a bisphenol A polycarbonate/silicon rubber/bisphenol A bis(diphenyl phosphate) (PC/SiR/BDP) blend were investigated and compared to those of PC/BDP and PC/SiR. The impact modifier SiR consists mainly of poly(dimethylsiloxane) (PDMS > 80 wt %). The pyrolysis of PC/SiR/BDP was studied by thermogravimetry (TG), TG–FTIR to analyze the evolved gases, and a Linkam hot stage cell within FTIR as well as 29Si NMR and 31P NMR to analyze the solid residue. The fire performance was determined by PCFC, LOI, UL 94, and a cone calorimeter under different external irradiations. The fire residues were studied by using ATR-FTIR as well as the additional binary systems PC + PDMS, PC + BDP, and BDP + PDMS, focusing on the specific chemical interactions. The decomposition pathways are revealed, focusing on the competing interaction between the components. Fire retardancy in PC/SiR/BDP is caused by both flame inhibition in the gas phase and inorganic-carbonaceous residue formation in the condensed phase. The PC/SiR/BDP does not work as well superimposing the PC/SiR and PC/BDP performances. PDMS reacts with PC and BDP, decreasing BDP's mode of action. Nevertheless, the flammability (LOI > 37%, UL 94 V-0) of PC/SiR/BDP equals the high level of PC/BDP. Indeed, SiR in PC/SiR/BDP is underlined as a promising impact modifier in flame-retarded PC/impact modifier blends as an alternative to highly flammable impact modifiers such as acrylonitrile–butadiene–styrene (ABS), taking into account that the chosen SiR leads to PC blends with a similar mechanical performance.
The reactivity of the flame retardant and its decomposition temperature control the condensed-phase action in bisphenol A polycarbonate/acrylonitrile–butadiene–styrene/polytetrafluoroethylene (PC/ABSPTFE) blends. Thus, to increase charring in the condensed phase of PC/ABSPTFE + aryl phosphate, two halogen-free flame retardants were synthesized: 3,3,5-trimethylcyclohexylbisphenol bis(diphenyl phosphate) (TMC-BDP) and bisphenol A bis(diethyl phosphate) (BEP). Their performance is compared to bisphenol A bis(diphenyl phosphate) (BDP) in PC/ABSPTFE blend. The comprehensive study was carried out using thermogravimetry (TG); TG coupled with Fourier transform infrared spectrometer (TG-FTIR); the Underwriters Laboratory burning chamber (UL 94); limiting oxygen index (LOI); cone calorimeter at different irradiations; tensile, bending and heat distortion temperature tests; as well as rheological studies and differential scanning calorimeter (DSC). With respect to pyrolysis, TMC-BDP works as well as BDP in the PC/ABSPTFE blend by enhancing the cross-linking of PC, whereas BEP shows worse performance because it prefers cross-linking with itself rather than with PC. As to its fire behavior, PC/ABSPTFE + TMC-BDP presents results very similar to PC/ABSPTFE + BDP; the blend PC/ABSPTFE + BEP shows lower flame inhibition and higher total heat evolved (THE). The UL 94 for the materials with TMC-BDP and BDP improved from HB to V0 for specimens of 3.2 mm thickness compared to PC/ABSPTFE and PC/ABSPTFE + BEP; the LOI increased from around 24% up to around 28%, respectively. BEP works as the strongest plasticizer in PC/ABSPTFE, whereas the blends with TMC-BDP and BDP present the same rheological properties. PC/ABSPTFE + TMC-BDP exhibits the best mechanical properties among all flame-retarded blends.
The pyrolysis and fire performance of bisphenol A polycarbonate/acrylonitrile-butadiene-styrene (PC/ABS) flame-retarded by a mixture of two aryl bisphosphates were investigated by thermogravimetry-coupled with FTIR, oxygen index (LOI), UL 94 and cone calorimeter. Both flame retardants, bisphenol A bis (diphenyl phosphate) BDP and hydroquinone bis (diphenyl phosphate) HDP, show gas-phase and condensed-phase actions. When mixed together at different ratios, a synergy is observed in terms of pyrolysis and fire residues as well as in effective heat of combustion (THE/ML). The synergisms were quantified and confirmed mathematically by the evaluation of the synergistic effect index (SE). All LOI values for the flame-retarded blends are between 29% and 32%, as opposed to 23% for PC/ABS, and UL94 testing results in V-0 at 1.6 mm instead of HB. Investigations on the binary system BDP + HDP reveal that BDP and HDP interact with each other, yielding stable intermediate products which are proposed to increase the thermal stability of the PC/ABS + BDP/HDP blends. Oligomeric phosphate esters are presumed to form via transesterification.
Biomass pre-treatments for bio-oil quality improvement are mainly based on thermal and chemical methods which are costly and hence reduce the sustainability of pyrolysis-based refineries. In this paper, anaerobic digestion (AD) and dark fermentation (DF) are proposed as alternative ‘green’ pre-treatments to improve this situation. For this purpose, three seaweeds namely Sargassum polycystum, (Phaephyta), Gracilaria tenuistipitata, (Rhodophyta) and Ulva reticulata, (Chlorophyta) with high ash and oxygen contents were pre-treated to improve their composition and structure prior to pyrolysis. The results reveal that both biological pre-treatments affected, positively, the composition and structure of the seaweed biomass with AD pre-treatment reducing N and S contents by 86% and 63%, respectively. DF was more efficient in terms of ash and moisture reduction with 25% and 70%, respectively. In addition, oxygen (O) reduction by 27% was observed after DF which was evidenced by FTIR spectroscopy indicating the reduction of most oxygen-containing functional groups in the biomass. On the other hand, the carbon (C) content increased in DF pre-treated seaweeds up to 42%, almost two times higher relative content than C in the raw seaweed. The changes in the composition of pre-treated seaweeds resulted in changes in their thermal degradation and the volatile profiles produced during pyrolysis. Interestingly, anhydrosugars and furans which account for some 70% (by area) in raw seaweeds markedly declined or become undetectable after DF pre-treatment and correspondingly more acetic acid and hydrocarbons were produced while after AD more aromatics with high toluene content (ca.17%) were generated. The results indicate that biooil with profiles more similar to petroleum-based composition i.e. rich in hydrocarbons and low in anhydrosugars, N and S can be generated by AD and DF pre-treatments and opens up the possibility of these approaches to effect cost reduction in the overall generation of bio-based fuels.
Condensed-phase mechanisms play a major role in fire-retardant polymers. Generations of development have followed the concept of charring to improve fire properties. Whereas the principal reactions are believed to be known, the specific description for multicomponent systems is lacking, as is the picture across different systems. A two-step approach is proposed in general, and also presented in greater detail. The second step covers the specific reactions controlling charring, whereas the actual reactants are provided in the preceding step. This model consistently incorporates the variety of structureproperty relationships reported. A comprehensive case study is presented on seven phosphorus flame retardants in two epoxy resins to breathe life into the two-step approach.
The impact of the chemical structure of four different aryl bisphosphates on the flame retardancy of bisphenol A polycarbonate/acrylonitrile–butadiene–styrene blends (PC/ABS) was investigated. The impact of the bridging unit was studied, by comparing bisphenol A bis(diphenyl phosphate) BDP with biphenyl bis(diphenyl phosphate) BBDP and hydroquinone bis(diphenyl phosphate) HDP; as well as the influence of an aromatic substitution by comparing BBDP with biphenyl bis (di-2,6-xylyl phosphate) BBXP. The blends were investigated in terms of pyrolysis (thermogravimetry TG, TG coupled with Fourier transformed infrared spectroscopy (FTIR) and mass spectrometry (MS)) and fire performance (cone calorimeter, LOI, UL 94). The decomposition temperature of the flame retardant is a main parameter enabling a condensed phase interaction with PC decomposition products. The phosphate esters reacting with phenolic groups during pyrolysis were shown to increase cross-linking and reduce the hydrolysis/alcoholysis of the carbonate group. Variation of the aromatic substitution with the use of biphenyl bis (di-2,6-xylyl phosphate) led to reduced performance, highlighting the importance of the reactivity of the flame retardant with the decomposing PC.
Flame Retardant Mechanisms of Red Phosphorus and Magnesium Hydroxide in High Impact Polystyrene
(2004)
The flame retardant mechanisms of red phosphorus, magnesium hydroxide and red phosphorus combined with magnesium hydroxide were studied in high impact polystyrene by means of comprehensive decomposition studies and combustion tests. The study is intended to illuminate prerequisites and the potential of red phosphorus as a fire retardant for hydrocarbon polymers in the condensed phase and in the gas phase. Thermal and thermo-oxidative decomposition, decomposition kinetics and the product gases evolved were characterized using thermogravimetry coupled with Fourier transform infrared spectroscopy and mass spectroscopy, respectively. Fire behaviour was investigated with a cone calorimeter using different external heat fluxes, whereas the flammability was determined by limited oxygen indices. The combustion residues were analysed using XPS. Red phosphorus reduced the heat release in HIPS due to radical trapping in the gas phase. Magnesium hydroxide influenced fire behaviour by heat sink mechanisms, release of water and the formation of a magnesia layer acting as a barrier. The combination of both flame retardants in HIPS nearly resulted in a superposition. A slight synergy in barrier characteristics was due to the formation of magnesium phosphate, whereas a slight anti-synergism occurred in flammability and in the gas phase action. The latter effect is controlled by a decreased fuel rate due to the barrier layer rather than by an initiation of red phosphorus oxidation in the condensed phase.