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Polyurethanes (PU) represent one of the most versatile classes of plastics. They are processed and used as thermoplastic, elastomer, and thermoset. The requirements regarding flammability are correspondingly versatile. Depending on the material and the field of application, specific fire tests have to be fulfilled. This paper describes the different concepts used to fulfil these requirements by choosing the right raw materials and flame retardants.
Rigid polyurethane foams (RPUFs) exhibit short times to ignition as well as rapid flame spread and are therefore considered to be hazardous materials. This paper focuses on the fire phenomena of RPUFs, which were investigated through a multimethodological approach. Water-blown polyurethane (PUR) foams without flame retardants (FRs) as well as waterblown PUR foams containing triethyl phosphate as a gas phase-active FR were examined. The aim of this study is to clarify the influence of the FR on the fire phenomena during combustion of the foams. Additionally, materials’ densitieswere varied to range from 30 to 100 kg/m3. Thermophysical properties were studied bymeans of thermogravimetry; fire behavior and flammability were investigated via cone calorimeter and limiting Oxygen index, respectively. During the cone calorimeter test, the temperature development inside the burning specimens was monitored with thermocouples, and cross sections of quenched specimens were examined visually, giving insight into the morphological changes during combustion.
The present paper delivers a comprehensive study, illuminating phenomena occurring during foam combustion and the influence of a FR active in the gas phase. The superior fire performance of flameretarded PUR foams was found to be based on flame inhibition, and on increased char yield leading to a more effective protective layer. It was proven that in-depth absorption of radiation is a significant factor for estimation of time to ignition. Cross sections investigated with the electron scanning microscope exhibited a pyrolysis front with an intact foam structure underneath. The measurement of temperature development inside burning specimens implied a shift of burning behavior towards that of non-cellular materials with rising foam density.
Degradation of polyurethanes in various environments – Effects on molecular mass and crosslinking
(2019)
The increasing application of polyurethanes (PU) in safety relevant sectors (fire protection, insulation, medicine technique) requires detailed knowledge of the stability and reliability of these materials. Different climate factors are supposed to induce diverse and overlapping degradation reactions. The knowledge of these degradation mechanisms is necessary for an estimation of the period of application depending on usage of the material. An essential property of a polymeric system is represented by the molecular weight. Since a change of the molecular weight is a measure for the chemical stability of a polymer, size-exclusion chromatography (SEC) was used to monitor changes of the molecular weight of thermoplastic polyether- and polyester urethane (TPU) exposed to thermal, hydrolytic and photo-oxidative (UV) degradation conditions for several days. Thermal treatments were performed at elevated temperatures (100 - 200 °C) under oxidative (air) as well as non-oxidative (nitrogen) conditions to evaluate the specific influence of oxygen on the degradation. At higher temperatures (≥ 175 °C) a fast decrease of the molecular masses of both PU accompanied by a high degree of crosslinking was found. At lower temperatures (≤ 150 °C) the polymers remained widely unaffected by thermal degradation within the investigated degradation interval of up to two weeks, which was already known from FTIR spectroscopy[1]. In contrast to that, UV treatment at 25 °C at less than 10 % rel. humidity (RH) resulted in a fast crosslinking, whereas the molecular masses of both PU decreased slower than during the thermal treatments. The depth of penetration of the UV radiation was determined using 3D printed PU samples with different thicknesses. Hydrolysis based degradation effects were less significant. Only slight molecular mass changes were detected at temperatures ≤ 80 °C within a time span of 14 days, while no crosslinking could be measured. Considering the degradation results at the investigated exposure parameters, it could be shown that ester-based PU in general exhibits a significant higher stability compared to ether-based materials.
Size-exclusion chromatography (SEC) was used to monitor changes of the molecular masses of thermoplastic polyether – and polyester urethane (TPU) exposed to thermal, hydrolytic, and photo-oxidative (UV) Degradation conditions for several days. The thermal treatment was performed at elevated temperatures (100–200 °C) under oxidative (air) as well as non-oxidative (nitrogen) conditions to evaluate the specific influence of oxygen on the degradation. At higher temperatures (≥175 °C) a fast decrease of the molecular masses of both PU accompanied by a high degree of crosslinking was found. At lower temperatures (≤150 °C) the polymers remained widely unaffected by thermal degradation within the investigated degradation interval of up to two weeks. Surprisingly, the influence of oxygen (air) was found to be less distinct. In contrast to that, UV treatment at 25 °C at less than 10% rel. humidity (RH) resulted in a fast crosslinking, whereas the molecular masses of both PU decreased slower than for thermal treatments. The depth of penetration of the UV radiation was estimated using 3D printed PU samples with different thicknesses. Hydrolysis based degradation effects were less significant. Only slight molecular mass changes were detected at temperatures ≤80 °C within a time span of 14 days, while no crosslinking could be measured. Considering the degradation results at the investigated exposure parameters, it could be shown that esterbased PU in general exhibits a significant higher stability compared to ether-based materials.
Polyether and -ester urethanes (PU) were exposed to artificial weathering at 40 °C and artificial UV radiation in a weathering chamber. In 3 parallel exposures, humidity was varied between dry, humid, and wet conditions. Material alteration was investigated by various analytical techniques like size exclusion chromatography
(SEC), liquid chromatography-infrared spectroscopy (LC-FTIR), thermal-desorption gas chromatography-mass spectrometry (TD-GC-MS), fluorescence mapping and dynamic mechanical analysis (DMA). Our results show that depending on the weathering conditions, different degradation effects can be observed. By means of SEC an initial strong decrease of the molar masses and a broadening of the mass distributions was found. After a material dependent time span this was followed by a plateau where molar mass changes were less significant. A minor moisture-dependent degradation effect was only found for polyester PU. Fluorescence measurements on two materials revealed an increase in the luminescence intensity upon weathering process reaching a saturation level after about 500 h. The changes in the optical properties observed after different exposure conditions and times were very similar. The TD-GC-MS data showed the fate of the stabilizers and antioxidant in the course of weathering. LC-FTIR measurements revealed a change in peak intensities and the ratio of urethane and carbonyl bands.
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.
Atomic force microscopy based Infrared spectroscopy (AFM-IR) is a quickly evolving technique that provides chemical analysis and compositional mapping with spatial resolution far below conventional optical diffraction limits. This is possible since the detection method is based on a very sharp AFM tip which starts to oscillate when the sample starts to thermally expand (the changed is caused by the absorption of IR wavelength) where the thermal expansion is related to the IR absorption. This presentation briefly described the application of that new technique from polymer characterization and utilization of AFM-IR in material research, up to life science applications.
Several expandable graphites (EGs), differing in Expansion volume but with the same mean size, are compared as flame retardants in polyurethane (PUR) foams. Not only common sulfur-intercalated graphites are investigated but also a new one intercalated with phosphorus. The main aim of this article is to understand which properties of EG are important for its flame retardancy effectiveness in PUR foams. Thermal stability, flammability, and fire behavior are analyzed through limiting oxygen index and cone calorimeter tests. Detailed characterization of the phosphorus-intercalated graphite is also provided as well as physical–mechanical characterization. The results show that the well-known sulfur-intercalated graphites and the one with phosphorus both enhance the residue yield, induce a protective layer, and thus efficiently flame-retard PUR foams. While the expansion volume of the EGs had a surprisingly limited influence on the performance of the foams, at least in the range tested, the most important feature Controlling the effectiveness of EG in terms of flame retardant PUR foams was the type of intercalant. The presence of EG affected the physical–mechanical properties of the foams; however, no significant effect of the expansion volume or intercalant type has been revealed on the physical–mechanical properties of the foams.
This work investigates the fire phenomena of rigid polyurethane foams (RPUF) in detail. To elucidate structure-property relationships systematically varied sets of foams were prepared. RPUF were synthesized with different densities using water and pentane as blowing agent. What is more, a flame retarded RPUF and rigid polyisocyanurate-polyurethane foams were examined. The comprehensive understanding of the processes taking place during combustion is the foundation of customized development of successful flame retardant approaches.
Advanced cone calorimeter investigations provide insight into the fire behavior under forced flaming conditions. Thermocouples in the inside of specimens give information about the temperature gradient and temperature of the pyrolysis zone during combustion. Furthermore, fire phenomena were characterized using SEM, LOI and thermal analysis. By using a multi-methodological approach and systematically varied sets of foam materials, new insights into the burning of RPUF were won. The detailed knowledge of fire phenomena is essential for future development of tailored flame retardant strategies for RPUF.