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Eingeladener Vortrag
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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 systematic and comparative evaluation of the pyrolysis of halogen-free flame-retarded epoxy resins containing phosphine oxide, phosphinate, phosphonate, and phosphate (phosphorus contents around 2.6 wt.%) and the fire behaviour of their carbon fibre composites is presented. Decomposition pathways are proposed based on the thermal analysis (TG), TG coupled with evolved gas analysis (TG-FTIR), kinetics and analysis of the residue with FTIR and XPS. All organophosphorus-modified hardeners containing phenoxy groups lead to a reduced decomposition temperature and mass loss step for the main decomposition of the cured epoxy resin. With increasing oxidation state of the phosphorus the thermally stable residue increases, whereas the release of phosphorus-containing volatiles decreases. The flammability of the composites was investigated with LOI and UL 94 and the fire behaviour for forced-flaming conditions with cone calorimeter tests performed using different irradiations. The flame retardancy mechanisms are discussed. With increasing oxidation state of the phosphorus additional charring is observed, whereas the flame inhibition, which plays the more important role for the performance of the composites, decreases. The processing and the mechanical performance (delamination resistance, flexural properties and interlaminar bonding strength) of the fibre-reinforced composites containing phosphorus were maintained at a high level and, in some cases, even improved. The potential for optimising flame retardancy while maintaining mechanical properties is highlighted in this study.
Starting from trivalent 10-alkoxy-10H-9-oxa-10-phosphaphenanthrenes, a broad range of DOPO derivatives was synthesized via transesterification with aliphatic alcohols and subsequent Michaelis-Arbuzov rearrangement using catalytic amounts of p-toluenesulfonic acid methylester. Due to the considerable differences in the nature of the alcohols employed, several procedures for processing them are presented.
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.
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.
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.
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).
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.
PET-P-DOPO is a phosphorus-containing polyester prepared from the glycol ether of the hydroquinone derivative of 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) and dimethyl terephthalate. PET-P-DOPO and a blend of PBT with PET-PDOPO were investigated with respect to pyrolysis and fire behavior. PET-P-DOPO achieves a V-0 rating in the UL 94 test and exhibits a high LOI of 39.3%. The outstanding flame-retardant properties of PET-P-DOPO are the result of three different mechanisms (flame inhibition, charring and a protection effect of the intumescent char) that are active in PET-P-DOPO. The fire load and the peak of heat release rate (pHRR) are reduced to 34% and 17%, respectively. The char exhibits an intumescent multicellular structure enabling it to act as an efficient protection layer. As PET-P-DOPO is immiscible with PBT, the blend shows a lower breaking elongation than pure PBT. Compared to pure PET-P-DOPO, the flame retardancy of the blend is decreased according to the fraction of PET-P-DOPO used. Nevertheless, the flame-retardancy of PET-P-DOPO in the blend was good enough to compete with PBT flame-retarded by AlPi-Et (aluminum diethylphosphinate) that was used as a Benchmark.
The pyrolysis and fire behavior of halogen-free flame-retarded DGEBA/DMC, RTM6 and their corresponding 60 vol.-% carbon fibers (CF) composites were investigated. A novel phosphorous compound (DOPI) was used. Its action is dependent on the epoxy matrix. DGEBA/DMC and DOPI decompose independently of each other. Only flame inhibition occurs in the gas phase. RTM6 shows flame inhibition and a condensed phase interaction increasing charring. Both mechanisms decrease with increasing irradiance, whereas in RTM6-CF charring is suppressed at low ones. RTM6+DOPI shows a higher LOI (34.2%) than DGEBA/DMC+DOPI and a V-0 classification in UL 94. Adding CF only enhances the LOI, DOPI+CF leads to a superposition in LOI for DGEBA/DMC-CF+DOPI (31.8%, V-0) and a synergism for RTM6-CF+DOPI (47.7%, V-0).
Pyrolysis, fire behaviour and mechanical properties of a blend of poly(butylene terephthalate) (PBT) with a phosphorus polyester (PET-P-DOPO) are investigated and compared with PBT/aluminium diethylphosphinate (AlPi-Et) composites. The PBT/PET-P-DOPO is immiscible and exhibits gas-phase and condensed-phase activity, whereas AlPi-Et in PBT results mainly in flame inhibition. Only higher loadings of AlPi-Et yield significant condensed-phase activity. Using the same phosphorus content, PBT/PET-P-DOPO and PBT/AlPi-Et exhibit similar reductions in fire load (22%) and flame spread (17% assessed by fire growth rate, FIGRA), compared with PBT. In contrast to AlPi-Et, the addition of PET-P-DOPO does not decrease the tensile strength of PBT. Thus, PET-P-DOPO is an interesting alternative to low-molecular-weight flame retardants.
Flame retardancy for thermoplastics is a challenging task where chemists and engineers work together to find solutions to improve the burning behavior without strongly influencing other key properties of the material. In this work, the halogen-free additives aluminum diethylphosphinate (AlPi-Et) and a mixture of aluminum phosphinate (AlPi) and resorcinol-bis(di-2,6-xylyl phosphate) (AlPi-H + RXP) are employed in neat and reinforced poly(butylene terephthalate) (PBT), and the morphology, mechanical performance, rheological behavior, and flammability of these materials are compared. Both additives show submicron dimensions but differ in terms of particle and agglomerate sizes und shapes. The overall mechanical performance of the PBT flame-retarded with AlPi-Et is lower than that with AlPi-H-RXP, due to the presence of larger agglomerates. Moreover, the flow behavior of the AlPi-Et/PBT materials is dramatically changed as the larger rod-like primary particles build a percolation threshold. In terms of flammability, both additives perform similar in the UL 94 test and under forced-flaming combustion. Nevertheless, AlPi-Et performs better than AlPi-H + RXP in the LOI test. The concentration required to achieve acceptable flame retardancy ranges above 15 wt %.
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.
Aluminium diethylphosphinate (AlPi-Et) and inorganic aluminium phosphinate with resorcinol-bis(di-2,6-xylyl phosphate) (AlPi-H+RXP) were compared with each other as commercially available halogen-free flame retardants in poly(butylene terephthalate) (PBT) as well as in glass-fibre-reinforced PBT (PBT/GF). Pyrolysis behaviour and flame retardancy performance are reported in detail. AlPi-H+RXP released phosphine at very low temperatures, which can become a problem during processing. AlPi-Et provided better limiting oxygen index (LOI) values and UL 94 ratings for bulk and PBT/GF than AlPi-H+RXP. Both flame retardants acted via three different flame-retardancy mechanisms in bulk as well as in PBT/GF, namely, flame inhibition, increased amount of char, and a protection effect of the char. AlPi-Et was more efficient in decreasing the total heat evolved of PBT in the cone calorimeter test. AlPi-H+RXP reduced the peak heat release rate of PBT more efficiently than AlPi-Et. An optimum loading of AlPi-Et in PBT/GF was found, which was below the supplier's recommendation. This loading provides a maximum increase in LOI and a maximum decrease in total heat evolved.
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.
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.
Effect of different sulphur-based crosslink networks on the nitrile rubber resistance to biodiesel
(2017)
Biodiesel possesses some comparable physical properties to petroleum diesel in addition to its improved environmental benefits. Nonetheless, both fuels differ greatly with respect to their chemical compositions.
Therefore, the compatibility of the materials, which are commonly employed in contact with diesel, must also be assured for biodiesel. This paper assessed the influence of sulphur-based curing systems on the resistance of nitrile rubber to soybean biodiesel. Formulations were prepared using highacrylonitrile-content nitrile rubber by employing a two-level experimental design. The amounts of two different accelerators and sulphur were varied to achieve different types of vulcanisation systems.
Thermal analyses, mechanical tests and microscopy analyses were conducted to evaluate the behaviour of the material after contact with biodiesel. The results showed that the choice of the accelerator played an important role on the resistance of the rubber to the biofuel, and crosslink density was not a key factor with respect to the resistance.