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Thermoplastic polyurethane - How thespecific two-stage decompositioncontrols its fire behavior
(2018)
Thermoplastic polyurethanes (TPUs) are one of the major classes of thermoplastic elastomers used in the cable and wire industry. Due to their chemical structure, they are considered as linear block copolymers with alternating hard and soft segments. This specific structure is not only responsible for the excellent mechanical properties of TPU like high tensile strength or abrasion resistance, but it also influences the pyrolysis and burning behavior. TPU is a highly flammable polymer which forms liquid pool fires or burning drops which results in intensive flame spread. Because of that, TPU often does not meet the strict requirements for indoor applications given by the cable market.
Under thermal stress, TPU decomposes in a specific way. At first, the hard segments decompose and the material becomes soft and melts. Subsequently, pool fires are formed and melted material burns extensively. There are not many works addressing this problem in detail especially in terms of suitable flame retardant solutions. It is crucial to understand at which stage the flame retardants are working and how they affect the thermal decomposition of TPU. Hence, the focus of this work was put on the identification of the reactions occurring during each stage of TPU decomposition and the contribution of released products to the effective heat of combustion (EHC). Moreover, the detailed analysis of the melt dripping behavior was performed to better understand the influence of the decomposition degree on the viscosity.
The use of melamine cyanurate (MC) in combination with other additives was reported to be a promising flame retardant solution for TPU, especially in cable jacket applications. Hence, the combinations of MC with various flame retardants were used in TPU (Elastollan® 1185A10) and were investigated in terms of fire behavior, flammability, pyrolysis and decomposition products. As flame retardants, common ones were used including aluminum trihydrate (ATH), melamine polyphosphate (MPP), or aluminum diethylphosphinate (AlPi). Moreover, commercial product TPU-FR (Elastollan® 1185A10FHF) was investigated as a reference.
Synergy in flame-retarded epoxy resin - Identification of chemical interactions by solid-state NMR
(2017)
The potential synergists aluminium diethylphosphinate (AlPi), boehmite (AlO(OH)) and melamine polyphosphate (MPP) were compared in flame-retardant epoxy resin (EP)/melamine poly(magnesium phosphate) (S600). The pyrolysis, the fire behaviour as well as the chemical interactions in the gas and condensed phases were investigated by various methods. Flammability was investigated by cone calorimeter and oxygen index (OI). The thermal and thermo-oxidative decomposition were studied by thermogravimetric analysis coupled with FTIR spectrometer. The special focus was on the Investigation of structural changes in the condensed phase via solid-state NMR of 27Al and 31P nuclei. By the comparison of epoxy resin with only one additive or with S600 in combination with AlPi, AlO(OH) or MPP, it was possible to calculate the synergy index. The best performance in terms of fire behaviour was observed for EP/S600/MPP with a PHRR (Peak heat release rate) of 208 kW m-2 due to slight synergy. In the case of THE (total heat evolved), clear synergy occurred for EP/S600/AlPi and EP/S600/AlO(OH). By solid-state NMR, different phosphates and aluminates were identified, indicating the chemical interactions between S600 and AlPi, AlO(OH) or MPP. The systematic multi-methodical approach yielded insight into the synergistic effects in the flame-retarded epoxy resin.
Advanced analysis of flame retarded epoxy resin/melamine poly(metal phosphate)/synergist systems
(2017)
Synergistic multicomponent systems containing melamine poly(metal phosphate) have been proposed as flame retardants. The potential synergists: aluminium diethylphosphinate (AlPi), boehmite (AlO(OH)) and melamine polyphosphate (MPP) were compared in flame retardant epoxy resin (EP)/melamine poly(magnesium phosphate) (MMgP) and EP/melamine poly(zinc phosphate) (MZnP). A multi-methodical approach was used to investigate the pyrolysis, fire residues, flame retardant modes of action, and synergistic effects. The decomposition and the fuel released into the gas phase were investigated by thermogravimetry coupled with Fourier transform infrared spectroscopy (TG-FTIR). A detailed analysis of the condensed phase was done by FTIR and comprehensive solid-state nuclear magnetic resonance spectroscopy (NMR). The fire behaviour was investigated by the cone calorimeter and the morphology of the fire residues with micro-computed tomography (µCT) and scanning electron microscopy (SEM). The best performance is observed for EP/MMgP/MPP and EP/MZnP/MPP based on a synergistic effect between the flame retardants. Particular the formation of a highly voluminous fire residue plays a major role and is stabilized by the interaction between the flame retardants. As a result highly effective protection layer was formed which led to the interesting results.
Solid-state NMR Identifying the Chemistry in Multicomponent Flame-retarded Polyolefin Systems
(2017)
Understanding of the interaction in multicomponent flame-retarded polymeric system is crucial to obtain the best performance at the possible lowest load of additives. In this work, polyolefine based systems are investigated by solid state NMR and cone calorimetry. As a polymer matrix thermoplastic elastomer based on styrene (TPE-S) was chosen. Different combination of additives was investigated in order to identify the chemistry occurring during the pyrolysis. As additives aluminium diethylphosphinate (AlPi), ammonium polyphosphate, zinc borate (ZB), poly(phenylene oxide) (PPO), magnesium hydroxide (MH) and dimethyl- methylvinyl siloxane (Si) were used. Fire residues remained after the cone calorimeter test were analyzed by solid state NMR (31P, 27Al, 13C, 11B, 29Si). The formation of different phosphates and aluminates was identified, indicating the chemical interaction between the additives. Detailed investigation delivered meaningful insights into the chemistry controlling flame retardancy.
Synergistic multicomponent systems containing melamine poly(metal phosphate)s have been recently proposed as flame retardants. This work focuses on the decomposition pathways, molecular mechanisms and morphology of the fire residues of epoxy resin (EP) flame retarded with melamine poly(zinc phosphate) (MPZnP) to explain the modes of action and synergistic effects with selected synergists (melamine polyphosphate (MPP) and AlO(OH), respectively). The total load of flame retardants was always 20 wt.%. The decomposition pathways were investigated in detail via thermogravimetric Analysis coupled with Fourier transform infrared spectroscopy. The fire residues were investigated via elemental analysis und solid-state nuclear magnetic resonance spectroscopy. The morphology of intumescent fire residues was investigated via micro-computed tomography and scanning electron microscopy.
EP + (MPZnP + MPP) formed a highly voluminous residue that showed structural features of both EP + MPZnP and EP + MPP, resulting in a highly effective protection layer. EP + (MPZnP + AlO(OH)) preserved the entire quantity of phosphorus content during combustion due to the Formation of Zn₂P₂O₇ and AlPO₄.
A systematic comparison of chemical interactions and fire behaviour is presented for the thermoplas-tic elastomer (block copolymer styrene-ethylene-butadiene-styrene) (TPE-S)/diethyl- and methylvinylsiloxane (Si)/poly(phenylene oxide) (PPO), flame-retarded with aluminium diethylphosphinate (AlPi)and with ammonium polyphosphate (APP), respectively. TPE-S/APP/Si/PPO performed better in the conecalorimeter test (reduction in peak heat release rate from 2042 to 475 kW m−2), but TPE-S/AlPi/Si/PPO inthe flammability tests (oxygen index (OI) and UL 94). This difference was caused by the different modes ofaction of APP (more in the condensed phase) and AlPi (mainly in the gas phase). Thermogravimetry cou-pled with Fourier transform infrared spectroscopy (TG-FTIR) was used to analyse the mass loss and theevolved gas products, while a Linkam hot-stage cell to investigate the decomposition in the condensedphase. Moreover, a detailed analysis of the fire residues was done using solid-state NMR.13C MAS NMRshowed that both flame-retarded compositions form graphite-like amorphous carbonaceous char, orig-inating from PPO.31P MAS NMR and29Si MAS NMR delivered important information about interactionbetween phosphorus and the siloxane. For TPE-S/AlPi/Si/PPO aluminium phosphate and silicon dioxideoccurred, while also silicophosphate was produced in TPE-S/APP/Si/PPO. The direct comparison of two ofthe most prominent halogen-free flame retardants containing phosphorus delivered meaningful insightsinto the modes of action and molecular mechanisms controlling flame retardancy.
Distinct approaches are used to reduce the fire risks of polymers, a key issue for many industrial applications. Among the variety of approaches, the use of synergy in halogen-free multicomponent systems is one of the most auspicious. To optimize the composition of such flame-retardant systems it is essential to understand the mechanisms and the corresponding chemistry in the condensed phase. In this work different methods are used, including cone calorimeter, thermogravimetry (TG), and TG-FTIR, with the main focus on the solid-state NMR analysis of the solid residues. The structural changes in the condensed phase of two thermoplastic elastomer systems based on copolymer styrene-ethylene-butadiene-styrene (TPE-S) were investigated: TPE-S/aluminium diethylphosphinate (AlPi)/magnesium hydroxide (MH) and TPE-S/AlPi/zinc borate (ZB)/poly(phenylene oxide) (PPO). Strong flame inhibition is synergistically combined with protective layer formation. 13C-, 27Al-, 11B- and 31P MAS NMR (magic angle spinning nuclear magnetic resonance) experiments using direct excitation with a single pulse and 1H31P cross-polarization (CP) were carried out as well as double resonance techniques. Magnesium phosphates were formed during the pyrolysis of TPE-S/AlPi/MH, while for the system TPE-S/AlPi/ZB/PPO zinc phosphates and borophosphates were observed. Thus, the chemistry behind the chemical interaction was characterized unambiguously for the investigated systems.
Multicomponent flame retardant systems containing aluminum diethylphosphinate in thermoplastic styrene–ethylene–butylene–styrene elastomers are investigated (oxygen index, UL 94, cone calorimeter, and mechanical testing). Solid-state nuclear magnetic resonance, scanning electron microscopy, and elemental analysis illuminate the interactions in the condensed phase. Thermoplastic styrene–ethylene–butylene–styrene elastomers are a challenge for flame retardancy (peak heat release rate at 50 kW m-2 > 2000 kW m-2, oxygen index = 17.2 vol%, no UL-94 horizontal burn rating) since it burns without residue and with a very high effective heat of combustion. Adding aluminum diethylphosphinate results in efficient flame inhibition and improves the reaction to small flame, but it is less effective in the cone calorimeter. Its efficacy levels off for amounts >~25 wt%. As the most promising synergistic system, aluminum diethylphosphinate/melamine polyphosphate was identified, combining the main gas action of aluminum diethylphosphinate with condensed phase mechanisms. The protection layer was further improved with several adjuvants. Keeping the overall flame retardant content at 30 wt%, aluminum diethylphosphinate/melamine polyphosphate/titanium dioxide and aluminum diethylphosphinate/melamine polyphosphate/boehmite were the best approaches. An oxygen index of up to 27 vol% was achieved and a horizontal burn rating in UL 94 with immediate self-extinction; peak heat release rate decreased by up to 85% compared to thermoplastic styrene–ethylene–butylene–styrene elastomers, to <300 kW m-2.