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INTRODUCTION: Most polymeric materials in most fire scenarios burn based on an anaerobe pyrolysis feeding the flame with fuel. Understanding the thermal decomposition in the condensed phase is key to tailor flame retardancy. Adjusting the decomposition temperature of flame retardant and polymer as well as providing the desired chemical structure for specific reactions determine the molecular mechanisms and thus the flame-retardant modes of action. Hereby, it is not only charring in the condensed phase and radical scavenging in the gas phase, but also physicochemical and physical effects such as melt flow and protective layer formation. This paper delivers thought-provoking impulses on how the understanding of the pyrolysis can be used for evidenced-based development and optimization of flame-retardant polymeric materials. Some rather overseen details are picked up as well as rethinking of concepts memorized long ago is encouraged to discover something new. The talk tries to fill some gaps between flame-retardant mechanisms, flame-retardant modes of action, and fire performance. EXPERIMENTAL: Materials - This paper deduces its conclusions from results of several research projects performed in the working group of the author. For a detailed description of the materials, their compounding, and the preparation of test specimens please go for the comprehensive description in the original papers.[1-10] Methods - A multi-methodical approach based on thermogravimetry (TGA), TGA coupled with evolved gas analysis (TGA-FTIR), hot stage FTIR, pyrolysis GC/MS, and residue analysis was used for investigating the pyrolysis. The flammability in the fire scenario ignition was addressed using oxygen index (OI) and testing in the UL 94 burning chamber. The fire behaviour in developing fires was investigated using a cone calorimeter. Additional efforts complete the studies, such as other fire tests, advanced analysis of the fire residue, melt rheology, or particle finite element simulations (PFEM). Tailored and self-designed experiments and advanced data evaluation described fire phenomena or modes of action. For a detailed description of the experimental the reader is relegated to the original papers.[1-10] RESULTS AND DISCUSSION: One important aspect in achieving, adjusting, and optimising flame retardancy is exploiting specific chemical reactions in the condensed phase between the pyrolyzing polymer and the flame retardant at the right place, time, and temperature.[1-3] At the same time, these reactions of partly decomposed or hydrolysed flame retardants in the condensed phase competes with releasing as volatile into the gas phase. Based on three comparisons this field is illuminated in detail: reactive phosphine oxide, phosphinate, phosphonate, and phosphate are compared in the same epoxy resins evaluating the influence of the oxidation state,[1] different phosphorous flame retardants in different epoxy resins underlining the specific reactions between the partly decomposed or hydrolysed flame retardant and the partly decomposed polymer,[3] and the comparison of three aryl phosphates with different volatility highlights the competition of chemical reaction in the condensed phase and gasification.[2] Any fire residue reduces fire risks, when the release of hydrocarbon fuel into the gas phase is replaced by storing fuel in the condensed phase as carbonaceous char. Thus, charring describing crosslinking, dehydration, aromatization, and graphitization is the flame retardancy mechanisms, whereas charring describing the reduction in fire load the flame-retardant mode of action. The phenomenon charring belongs to a complete pyrolysis or complete pyrolysis step; the char yield indicates the amount of fuel stored in the residue. Further, any fire residue works as protective layer. The barrier properties depend on the physical properties of the residue but not necessarily on its amount.[4] Usually, a residue design such as a tailored morphology of the fire residue is demanded. The mass loss rate and heat release rate are reduced. The main mechanisms are heat shielding and thermal insulation.[5] Sometimes the protective layer is good enough to cause incomplete pyrolysis due to extinguishing before the pyrolysis front went through the whole sample.[6.7] Analogous to charring also incomplete pyrolysis can result in efficient reduction in fire load. Proper data evaluation and key experiments are used to sort out and understand these different phenomena. Flame retardant polyurethane foams passing the heat release and smoke toxicity requirements of EN 45545 are discussed as evidence-based development using charring and incomplete pyrolysis due to an efficient protective layer.[7,8] The thermal decomposition into liquid intermediate products increases crucially the melt flow during burning,[9] whereas charring and the ablation of the polymer matrix increasing the content of fillers yield melt viscosities enlarged by orders of magnitude.[10] Violent burning of some polymers at the end of a cone calorimeter test can be understood as pyrolysis enabling a pool fire. The understanding of the thermal decomposition of the polymeric material harbours the explanation of non-flaming dripping extinguishing the flame via sufficient cooling, retreat effects preventing ignition, and efficient nondripping flame retardancy. This paper leads the audience from chemistry over complex macroscopic fire phenomena of physicochemical nature to fire performance. Thought-provoking impulses are given how the scientific understanding of the pyrolysis in the condensed phase can be used for research and evidenced-based development of future flame-retardant polymeric materials. Acknowledgement: The talk uses results from distinct projects; thus, thanks go to the German Research Foundation DFG SCHA 730/6-1, SCHA 730/8, SCHA 730/10-1, and Scha 730/19-1, Bayer MaterialScience AG, and the BMWi (BMWK) AiF: IGF No.: 19078 N/2 for financial support. Many thanks to former working group members U. Braun, Y. Y. Chan, B. Perret, S. Rabe, K. H. Richter, A. Weiß, and G. Wu, and to our co-operation partners A. Hartwig (IFAM), M. Döring and M. Ciesielski (at that time KIT), and J. M. Marti (CIMNE) as well.

In the context of the increasing effect of carbon dioxide emissions on the global climate biodiesel produced from renewable sources has emerged as a promising contender replacing fossil fuels, especially in long-range transport vehicles, using existing engines and infrastructure. High-density polyethylene is one of the prevailing materials for pipe and container applications for storage and transport of such fuels, both, from fossil and renewable resources. The contact with the respective fuels raises questions concerning material compatibility as biodiesel exhibits significant differences compared to conventional diesel fuel affecting its sorption and plasticization behavior in polyethylene. In this study, its behavior with respect to environmental stress cracking, considered one of the most frequent damage mechanisms leading to failure of polymer parts and packaging, was evaluated using the well-established Full Notch Creep Test. This approach allows for a detailed fracture surface analysis using imaging techniques, such as optical and laser scanning microscopy, as well as infrared spectroscopy. Comparing the environmental stress cracking behavior in standard surfactant solutions with that in biodiesel and diesel, respective crack propagation rates, showing different levels of acceleration, were determined and details of the underlying mechanisms could be revealed. Furthermore, the specific infrared absorption of the biodiesel’s ester functionality allows its semi-quantitative determination on the fracture surface of the tested specimens after failure. Thus, a preferred uptake of sorptive fluids in the fracture zone due to local morphological changes of the polyethylene could be directly evidenced by infrared spectroscopy.

Applying synergistic multicomponent systems is often key to efficient flame retardancy. Different flame retardants are combined or used together with fillers, adjuvants, or synergists to enhance their efficiency, reduce the worsening of other properties, or reduce the costs. Further, fibres and other reinforcing fillers contribute to fire properties crucially. Although the main flame-retardant modes of action are known, the scientific understanding usually falls short, when it comes to complex multicomponent systems, the crucial tiny optimizations, or quantifying in terms of specific fire properties. This book chapter illuminates the need for the multicomponent approach, the concept of synergistic flame retardants, and the main phenomena. Multicomponent systems are discussed in their capacity as general powerful strategy for achieving and optimizing future flame retardant polymeric materials.

Considering the existing challenges involved in the transfer of flame retardant (FR) formulations from epoxy (EP) resins to glass fiber reinforced composites (GFRCs), obtaining data on the post-fire flexural properties of such composites is even more challenging as this involves balancing test parameters with potential composite delamination. In this study, solvent-free FR additives: ammonium polyphosphate (APP), and inorganic silicate (InSi) were added at 10, 30% and 50% w/w loading to a Bisphenol A diglycidyl ether (DGEBA)-dicyandiamide (DICY)-Urone resin matrix. These resin formulations were transferred to bidirectional (BD) glass fiber composites via prepregs. A novel, but facile approach was developed to prepare the composite samples for furnace tests at 400 oC. The composites were also subjected to fire exposure at different heat fluxes and times via a bench-scale test and subsequently tested via three-point bending. At approximately 3.5% P content, the FRs significantly improve the fire performance of both the resins and composites. However, they also degrade the systems’ pre- and postfire flexural modulus and strength. Therefore, improving the flame-retardant mode of action of the FRs in the composites, contrastingly reduces their structural integrity post-fire leading to a trade-off effect.

The residual post-fire mechanical properties of fiber-reinforced epoxy (EP) composites are influenced by their fire residues after burning. This study uses intumescent/low-melting glass flame retardants (FRs) to tailor fire residues in epoxy resin. Processibility of prepregs, and their quality are analysed for transfer of the flame-retardant epoxy resins to layered glass-fiber reinforced composites (GFRCs). Minimal effects were found on the pre-fire flexural strengths of the composites due to low loading of the FRs. However, when transferred to GFRCS, the fire residues diminish significantly. Process, testing, and material adaptations are required to improve theoretical and experimental estimations of the post-fire mechanics of the composites.