7.5 Technische Eigenschaften von Polymerwerkstoffen
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The multicomponent flame retardant system of melamine polyphosphate (MPP), melamine cyanurate (MC) and aluminum diethylphosphinate (AlPi) is proposed and investigated for thermoplastic polyurethane (TPU). The synergy between those additives and the resulting superior fire performance are discussed. Systematically varied sets of flame retarded TPU with various MPP/MC/AlPi ratios were investigated in terms of fire behavior, pyrolysis products and mechanical properties. The total amount of the additives was always 30 wt.-%. Further, the influence of various AlPi concentrations was investigated. The optimal MPP:MC ratio was determined while keeping the amount of AlPi constant. The combination of 8 wt.-% MPP, 12 wt.-% MC and 10 wt.-% is proposed as the most promising halogen free flame retardant formulation for TPU, because it yielded a reduction in PHRR from 2660 kW/m2 (TPU) to 452 kW/m2 and enabled V-0 classification in the UL 94 test. Combinations of MPP and MC as well a high concentration of AlPi are beneficial for the mechanical properties e.g. tensile strength and elongation at break of the formulations and could be a strong competitor to commercial flame retarded TPUs.
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.
The rapid mass calorimeter (RMC) was used as a screening tool based on accelerated fire testing to assess flame-retarded thermoplastic polyurethane (TPU). The reliability of RMC results was proven with the cone calorimeter as reference fire test. The influence of melamine cyanurate (MC) concentration on the fire performance of TPU was investigated, along with some flame-retardant combinations such as MC with aluminium diethylphosphinate (AlPi), aluminium trihydrate (ATH), and melamine polyphosphate (MPP). The two-stage burning behaviour of TPU was investigated in detail; the first stage corresponds mainly to the hard segments' decomposition and has a much lower effective heat of combustion (EHC) than the second stage, in which mainly the soft segments decompose and an intensive liquid pool fire is observed in the cone calorimeter set-up. In addition to fire testing with the cone calorimeter, RMC, and UL 94 flammability tests, the decomposition of the materials was investigated using thermogravimetric analysis coupled with infrared spectrometry (TGeFTIR). TPU/MC/AlPi shows the most promising results, achieving V-0 classification in UL 94 and reducing the extreme peak heat release rate (PHRR) of the liquid pool fire from 3154 kW/m2 to 635 kW/m2. Using MC/AlPi/MPP enhances the latter PHRR reduction further. The decomposition products identified in the gas phase via TGeFTIR reveal specific MCeAlPi eMPP interactions, as they differ from products seen in systems with MC/AlPi or MC/MPP. Correlations between RMC and cone calorimeter results were examined and presented in the final part of the paper. Several characteristics correlate strongly, pointing out that RMC is a reliable high-throughput fire testing method to screen multicomponent flame-retardant solutions in TPU.