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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.
Short overview is given of BAM's research within the topic "Flame Retardancy of Polymeric Materials". Fire science is identified as a crucial compentenc of BAM following our mission "Safety in Technology and Chemistry". In the area of polymers we work interdisciplinary, in the dimensions from nm to 2m, and we love to combine experiment and simulation. Multimethodical examples are given to describe the burning phenomena and flame-retardant modes of action. Further some examples are presented for tailored bench-scale fire testing and assessing concepts. Our goal is to provide the fundaments for a evidenced-based development of future materials.
Leather is among the most ancient, widely used materials worldwide. Industrial-scale leather production produces large quantities of organic waste attained during shaving and buffing steps during processing. In this study, leather wastes (LW) are used as fillers in flame retarded polymer composites. LW is investigated as a multifunctional bio-filler that enhances the fire performance of flame retarded poly(ethylene–vinyl acetate) (EVA) containing phosphorus flame retardants (P-FRs) ammonium polyphosphate (APP) or a melamine-encapsulated APP (eAPP). Using LW from tanneries as adjuvants to enhance P-FRs in EVA reduces industrial wastes that otherwise require costly waste management solutions. Materials are characterized multi-methodically via mechanical tests, electron microscopy, rheology, thermogravimetric analysis, evolved gas analysis, and condensed phase FTIR, also reaction-to-small-flames and cone calorimeter tests. EVA containing 10 wt-% LW and 20 wt-% P-FRs achieve 20% reductions in fire loads versus EVA, and up to 10% reduction in effective heats of combustion versus EVA with equal (30 wt-%) P-FR loadings. Enhanced char stabilization of EVA composites with LW and P-FRs lowered peaks of heat release rates up to 53% compared to EVA, and up to 40% compared to equal P-FRs loadings. Synergisms between LW and P-FRs in EVA are quantified. A chemical decomposition mechanism is proposed.
The main flame retardant modes of action are known, nevertheless in practise the detailed scientific understanding usually falls short, when it comes to modern multicomponent systems, the important tiny optimizations, or quantifying in terms of specific fire properties. The description of the flame retardant modes of action remains usually vague and fragmentary. This talk tries to deliver thought-provoking impulses how the understanding of the fire behaviour and flame retardancy can be utilized to direct the development of future flame retardant polymer products. Some overseen details are picked up as well as rethinking of concepts memorised long ago is encouraged to discover something new. Furthermore, the talk tries to fill the gap between flame retardant modes of action and fire performance constituting a product. This talk promotes the evidence-based development of flame retardant polymers.
The main flame retardant modes of action are well known, but in practise the detailed scientific understanding usually falls short, when it comes to modern multicomponent systems, the important tiny optimizations, or quantifying in terms of specific fire properties. This talk delivers thought-provoking impulses, picking up some overseen details as well as raising basic questions. A detailed scientific insight in flame retardant modes of action much more than an overview is presented.
Although the main flame retardant modes of action are known, in practise the detailed scientific understanding usually falls short, when it comes to modern multicomponent systems, the important tiny optimizations, or quantifying in terms of specific fire properties. Thus instead of a textbook-like overview of different flame retardant modes of action, this talk tries to deliver thought-provoking impulses. Some overseen details are picked up as well as basic questions raised. Rethinking of concepts memorised long ago is encouraged to discover something new. Furthermore, the talk tries to fill the gap between flame retardant modes of action and fire performance.
Condensed-phase mechanisms play a major role in fire-retardant polymers. Generations of development have followed the concept of charring to improve fire properties. Whereas the principal reactions are believed to be known, the specific description for multicomponent systems is lacking, as is the picture across different systems. A two-step approach is proposed in general, and also presented in greater detail. The second step covers the specific reactions controlling charring, whereas the actual reactants are provided in the preceding step. This model consistently incorporates the variety of structureproperty relationships reported. A comprehensive case study is presented on seven phosphorus flame retardants in two epoxy resins to breathe life into the two-step approach.
Phosphorus-based flame retardancy mechanisms - Old hat or a starting point for future development?
(2010)
Different kinds of additive and reactive flame retardants containing phosphorus are increasingly successful as halogen-free alternatives for various polymeric materials and applications. Phosphorus can act in the condensed phase by enhancing charring, yielding intumescence, or through inorganic glass formation; and in the gas phase through flame inhibition. Occurrence and efficiency depend, not only on the flame retardant itself, but also on its interaction with pyrolysing polymeric material and additives. Flame retardancy is sensitive to modification of the flame retardant, the use of synergists/adjuvants, and changes to the polymeric material. A detailed understanding facilitates the launch of tailored and targeted development.