Test method SEM/EDX: Advanced investigation of structured fire residues and residue formation H. Sturm, B. Schartel * , A. Weiß, U. Braun BAM Federal Institute for Materials Research and Testing, Unter den Eichen 87, 12205 Berlin, Germany article info Article history: Received 25 January 2012 Accepted 23 March 2012 Keywords: Fire residue SEM/EDX Fire retardancy PA 66 Layered silicate Diethylaluminium phosphinate abstract Heterogeneous, gradual or structured morphology of fire residues plays an important role in fire retardancy of polymers. A scanning electron microscope with an attached energy dispersive X-ray spectrometer (SEM/EDX) is highlighted as a powerful tool for the advanced characterization of such complex fire residues, since it offers high resolution in combination with both good depth of field and analysis of chemical composition. Two examples are presented: First, comprehensive SEM/EDX investigation on a complex structured fire residue of glass fibre reinforced polyamide 6,6 (PA 66-GF) flame retarded by diethylaluminium phosphinate, melamine polyphosphate and some zinc borate. A multilayered surface crust (thickness w 24 mm) covers a rather hollow area stabilized by GF glued together. The resulting efficient thermal insulation results in self-extinguishing before pyrolysis is completed, even under forced-flaming combustion. Second, sophisticated, quasi online SEM/EDX imaging of the formation of residual protection layer in layered silicate epoxy resin nanocomposites (LSEC). Burning specimens were quenched in liquid nitrogen for subse- quent analyses. Different zones were distinguished in the condensed phase characterized by distinct processes such as melting and ablation of organic material, as well as agglomeration, depletion, exfoliation and reorientation of the LS. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction Pyrolysis in the condensed phase plays a major role in fire behaviour of polymeric materials. The mass loss rate and the effective heat of combustion of the volatiles determine the heat release rate, and thus fire risks such as flammability, flame spread and heat release. Increasing char yield is equivalent to decreasing fuel production; back in the 70s van Krevelen reported the correlation between the char yield of pure halogen-free polymers and their fire properties such as LOI [1]. This approach has been refined since by considering additionally the effective heat of combustion of the fuel [2,3]. However, when the fire behaviour is considered, especially of polymers containing fillers and flame retardants, this is not the whole story. Physical mechanisms such as re-radiation, thermal insu- lation, mechanical stability of the char, transport of volatile products, melt flow, dripping and wicking are additional significant factors. Indeed, in some systems they even become the controlling mechanisms [4–7]. All residue formed during a fire also acts as barrier against heat and mass transport. The effectiveness of such a residual protection layer depends not only on the amount of char, but also on its properties, such as morphology, which determines gas permeability and thermal conduc- tivity. Indeed, such protection properties were observed quite independently of the amount of thermally stable char [8]. Further, the properties of residue may be tailored by adding inorganic adjuvants and synergists [9–12]. Hence, the complex, heterogeneous or gradual morphology of the fire residue plays a key role. The design of a fire residue consisting of multicellular structures or closed glassy * Corresponding author. Tel.: þ49 30 8104 1021; fax: þ49 30 8104 1027. E-mail address: bernhard.schartel@bam.de (B. Schartel). Contents lists available at SciVerse ScienceDirect Polymer Testing journal homepage: www.elsevier.com/locate/polytest 0142-9418/$ – see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymertesting.2012.03.005 Polymer Testing 31 (2012) 606–619