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Most fluorinated hydrocarbons that shall replace refrigerants with high GWP, like R134a, are flammable. For evaluating inertization measures for explosion protection, flammability of low-GWP refrigerants R1234yf, R32 and R1132a blended with carbon dioxide, nitrogen and argon were studied experimentally in a closed autoclave at atmospheric conditions. Furthermore, a calculation method was adapted to reduce the experimental costs for flammability studies on these gas mixtures. For igniting R1234yf in the closed autoclave a newly developed ignition system was used that allows generating electric arcs with high ignition energy. Gas mixtures containing the mildly flammable R1234yf and R32 could be inerted by adding much less inert gas than mixtures containing R1132a, which is more similar to unfluorinated hydrocarbons regarding the explosion regions. By using the adapted model of constant adiabatic flame temperature profiles estimating the explosion limits of fluorinated hydrocarbons was possible with similar accuracy as for unfluorinated hydrocarbons.
Keywords: Explosion Protection, Inertization, Flammability, HFOs, HFC., R1234yf, R32, R1132a
Nowadays, various polymeric materials are used in E&E applications with sufficient flame retardance by adding rather different flame retardants. It doesn’t matter whether cables are used outdoor or are installed indoor as building products, the weathering exposures such as UV radiation, humidity and variation in temperature occur and influence the flame-retardant property. Recently, the lifetime of the flame retardance itself becomes an increasingly important factor. In this work, several devices were used to perform accelerated artificial ageing simulating different environment exposures.
The comprehensive and global understanding of the durability of flame retardance in dependence on the weathering or ageing conditions is still a matter of discussion. Therefore, the weathering resistance of various halogen-free fire-retarded polymers was investigated in this work. Polymeric systems with different kinds of fire retardants were chosen, including various fire retardant mechanisms. Ethylene Vinyl Acetate (EVA) blends with high amounts of inorganic flame retardant such as aluminum hydroxide (ATH), boehmite and synergists, which mainly dilutes the polymer resin work as heat sink and cooling agent, and enhance residue formation was examined. Thermoplastic Polyurethane (TPU) was modified with melamine cyanurate (MC), which mainly acts by changed melt flow and dripping behavior as well as fuel dilution. Additionally, aluminum diethylphosphinate and boehmite are induced as assistant flame retardant. Furthermore, glass fiber reinforced Polyamide 66 (PA) was investigated containing different kinds of aluminum diethylphosphinate based flame retardant mixtures, which acts by flame inhibition and additional char formation.
The degradation of the surface was analyzed after the different weathering conditions. Most of the specimens exhibited an intensive material degradation at the top surface accompanied by a distinct discoloration, e.g. getting darker or showing yellowing. The weathering of the EVA samples lead to numerous cracks (already) after 4000 h. The corresponding changes in the chemical structure was investigated by ATR FT-IR for all materials.
The flammability was investigated by cone calorimeter, UL-94 burning chamber, and oxygen index (LOI) using plate and bar specimens. The flame retardance of most of the materials studied degrades only slightly or were rather stable for the investigated exposure times. Interestingly, also some opposite results were found. EVA modified by different inorganic flame retardants such as ATH achieved higher LOI after exposing in the humidity chamber and the accelerated oxidation under water in the autoclaves. It is suggested that the particle size of ATH and boehmite plays an important role, when these flame retardants agglomerate at the surface during accelerated weathering.
Both materials, EVA and TPU, were also investigated as cable jackets. While EVA modified with inorganic flame retardants exhibits low-smoke and non-dripping fire behavior, TPU flame-retarded with MC yields cables with pronounced melt-dripping. Cone calorimeter tests were carried out using cable rafts of the size of 100 mm * 100 mm as well as our self-made cable module test, which simulates the vertical full-scale test of a bundle of cables at the bench-scale. Both methods were used to investigate the weathering resistance of the flame retardance in cables. The results of the cable module test for the flame-retarded EVA cables were only slightly affected even when a long time hydrothermal ageing was carried out. This is because of inorganic residue which just delays the fire growth but does not extinguish. However, for the flame-retarded TPU cable jackets, the cable module test exhibited an accelerated fire spread and a melt-dripping behavior which was promoted by weathering exposure.
Flame-retarded biocomposites of thermoplastic starch and natural fibres are successfully processed according to state-of-the-art extrusion and injection moulding. Using agave fibres and henequen fibres recovered from local industrial waste is a convincing contribution to sustainability. A systematically varied set of biocomposites is investigated comprehensively, e.g. electron microscopy is used for characterizing the morphology, rheology for the melt viscosity, tensile and impact resistance for the mechanical properties, thermal analysis for the pyrolysis, UL 94 burning chamber and oxygen index for the flammability, and cone calorimeter for the fire behaviour. Achieving sufficient mechanical properties was not the goal in our pre-competitive study but may be tackled by adding compatibilizer in future. The combination of well-dispersed natural fibres, aluminium diethylphosphinate (AlPi) and a special silicone synergist (Si) is proposed as promising innovative route for V-classified biocomposites. The flame-retardancy modes of action in the gas phase (fuel dilution and flame inhibition) and in the condensed phase (charring, protective layer formation) are discussed in detail, as is the role of combining the ingredients. This work is a convincing proof of principle of how to prepare industrial-waste fibres biocomposites, to apply the synergistic combination of AlPi and Si for future flame-retarded technical polymer materials that are based on renewable resources and compostable.
This paper is based mainly on the results of two different projects performed in the group of the author recently (2016-2019). The three external partners involved in these two projects are competent in the preparation of FPUF (ICL IP America), RPUF (Department of Industrial Engineering, Padova University), and TPU (Fraunhofer-Institut für Betriebsfestigkeit und Systemzuverlässigkeit LBF, Darmstadt) as well as for the specimen preparation. Systematically varied sets of materials were prepared as the key basic for scientific discussion, varying the kind and combination of flame retardant, PUR structure, density, and blowing agent.
A multimethodical approach based on thermogravimetry (TGA), TGA coupled with evolved gas analysis (TGA-FTIR) and pyrolysis GC-MS was used for investigating the pyrolysis. The flammability was addressed using oxygen index (OI) and testing in UL 94 burning chamber in vertical and horizontal set-up. The fire behaviour was addressed by using a cone calorimeter. Beyond these methods according to the state of the art, key experiments were performed. We addressed the dripping and the two-stage burning of TPU using a self-designed apparatus and specific data evaluation, the foam burning through quenching burning samples, using different special sample holders, and measuring temperature profiles within the burning foams. The investigation is made round by intensive analysis of the fire residues, such as comprehensive investigation of the morphology.
Result on the pyrolysis (TGA-FTIR, Pyrolysis-GC/MS), flammability (UL 94, LOI), and fire behaviour (cone calorimeter) of TPU and flame retardant TPUs are shown. We discuss in detail the characteristic of PUR decomposition: the low tendency to char, and the specific two step decomposition and how these characteristics control the regimes in fire behaviour. We demonstrate that the different burning regimes are controlled by different pyrolysis products and effective heat of combustions. The resulting formation of pool fires as well as the formation of dripping is discussed in detail. The latter quite important to understand the flame retardancy applied with respect to achieve the UL 94 classification V0 nondripping or V0 non-flaming dripping.
Rigid and flexible PUR foams and their flame retarded versions are investigated for different densities. Water and pentane-blown foams are compared as well as PUR and polyisocyanurate-polyurethane (PIR) foams. Horizontal testing in the cone calorimeter is used and the vertical foam specimen holder as well. Self-designed set-ups within the cone calorimeter enable a better inside in the pyrolysis front running through the foam samples as well as the development of the temperature gradient inside the foam during the fire test. The morphology change during burning was characterised by the means of quenching burning foams with liquid nitrogen and investigating the cross sections with scanning electron microscope. In sum, a rather comprehensive study was performed to work out the principle fire phenomena controlling the fire behaviour of PUR foams in a very systematic and significant way.
Promising flame retardancy approaches are discussed. The importance of either combining the drain of fuel and flame inhibition or charring into an effective protection layer/multicellular structure is underlined.
This contribution focusses the general conclusions and trends. It tries to increase the understanding of the specific and demanding challenge to develop flame retardant PUR materials.
The paper provides a summary of safety characteristics of hydrogen and hydrogen fuel gases. The inves-tigations focus on water electrolysis, the feeding of hydrogen into the natural gas grid and the use ofhydrogen for the fermentation process in biogas plants. The safety characteristics of hydrogen such as avery low minimum ignition energy, very large explosion range and high flame velocity with the resultingrapid pressure increase of hydrogen explosions are of particular importance for explosion protection andthey differ strongly from those of natural gas. Explosion ranges of hydrogen-methane-carbon dioxidemixtures have been measured for the use of hydrogen in biomethane production. The paper also showsand discusses explosion ranges of hydrogen and hydrogen-natural gas mixtures. Pressure and tempera-ture dependencies of the explosion limits of mixtures were investigated. Furthermore, pressure rise rates(KGvalues) were measured with regard to constructive explosion protection. The maximum experimen-tal safe gaps were determined for the classification of the mixtures and assignment to explosion groupsaccording to the European ATEX directives. It was found that admixture of 10% hydrogen to natural gashas only a minor influence on the safety characteristics of gas explosions.
Developing halogen‐free flame retardants with reasonably high efficiency, which thus function at limited loadings in polypropylene‐based wood/plastic composites (WPC), is still a challenge. Cost‐effective flame‐retarded WPC have been identified as a way to open the door to an interesting, broader spectrum of application in the building and transportation sectors. This work imparts a systematic comprehensive understanding and assessment of different basic routes to halogen‐free flame‐retarded WPC, taking into account economic and environmental considerations. Cheap, halogen‐free single‐component flame retardants and their multicomponent systems are investigated at reasonable filling grades of 20 wt%. The basic routes of promising synergistic multicomponent systems are discussed, and their potential and Limits assessed. Optimizing the consistency of fire residue; closing the surface of inorganic‐organic residual layers; the thermal stabilization and design of the residue, eg, synergistic combination of ammonium polyphosphate and expandable graphite; and the combination of different flame‐retardant mechanisms, eg, intumescence and flame inhibition, are proposed as promising routes to boost the flame‐retardant efficiency.