Elsevier

Fire Safety Journal

Volume 120, March 2021, 103068
Fire Safety Journal

Effect of phosphorus on soot formation and flame retardancy in fires

https://doi.org/10.1016/j.firesaf.2020.103068Get rights and content

Highlights

  • The main retardancy mechanism of phosphorus is through promoting soot formation.

  • Phosphorus loading increases soot surface reactivity.

  • Phosphorus increases direct aromatic structure condensation in soot formation.

Abstract

The mechanism by which phosphorus imparts flame retardancy to a burning polymeric solid remains controversial because of the complex coupling of gas and condensed phase reactions. To address that, an organophosphorous compound was mixed into epoxy at different concentrations and burned under diffusion flame conditions. The yield of carbon monoxide and soot was found to increase in the presence of phosphorus, significantly reducing the combustion efficiency. The reduction in heat release was attributed to a heterogeneous mechanism that promotes soot formation. Phosphorus does not exhibit a noticeable effect on soot morphology. Element analysis reveals that 75% of phosphorus in the virgin sample is incorporated into soot particles. More aromatic structures were identified under the influence of phosphorus. Soot surface reactivity was found to decrease as phosphorus loading increases. The main flame retardancy mechanism of phosphorus is through promoting soot formation in the gas phase. It is speculated that it increases direct condensation of aromatic structures in the soot formation path and decreases the number of the active sites on the particle surface for soot oxidation. The increased soot yield cools the flame by radiative heat loss, further hindering the dehydrogenation process that converts nascent soot to mature soot.

Introduction

Phosphorus is widely used as an alternative to halogens in flame retardant additives. Phosphorus-containing compounds have been drawing more attention recently as flame retardant additives because of their supposed gas phase activity as well as condensed phase char promoting reactions [1,2]. The effect of phosphorous flame retardant (FR) is sensitive to fuel type and combustion environment. A variety of phosphorous compounds have been tested in premixed hydrogen or hydrocarbon flames [[3], [4], [5], [6]], and phosphorus was found to inhibit combustion mostly in hydrocarbon flames. These studies are narrowly focused on well-characterized liquid or gaseous fuels at a steady fuel supply and the findings cannot be easily extended to solid polymeric fuels. The inhibition effect was mostly expressed as reduced flame temperature or reduced premixed flame speed. In the detailed chemistry modeling [[4], [5], [6]], the effect on soot formation and oxidation was never discussed.

Stoliarov et al. [7] and Guo et al. [1] used Microscale Combustion Calorimetry (MCC) to isolate gas-phase activity of FR from its condensed-phase activity. Incomplete combustion was achieved by reducing the combustor temperature. In Refs. [1], a two-step gas phase reaction model was used to investigate both halogen- and phosphorus-containing compound effects on polymer burning. It was found that bromine significantly inhibits the chemical reaction whereas phosphorus does not have a noticeable effect on the rate of premixed homogeneous combustion. Unfortunately, the MCC combustor temperature is typically below 1000 °C, which is insufficient to obverse the soot formation.

In larger scale non-premixed flames, the reaction is more limited by the supply of oxygen into the fuel stream either through diffusion or by turbulent mixing. The gas phase flame speed effect becomes less important, and is often simplified to be infinitely fast during the modeling of large scale fires. More attention is given to the peak heat release rate, total heat output, and the smoke yield. It is recognized that for a specific polymer type, heat release can be suppressed through either char formation or incomplete combustion products formation (CO, soot, etc.). In conditions where char formation is not significant, suppressing heat release is typically achieved at a sacrifice of more smoke yield, and vice versa [1,8]. Both bromine and phosphorous compounds were found to reduce the heat release mainly by promoting incomplete combustion products [1]. The incomplete combustion products is of particular interest to the aviation industry because it is be hazardous to flight crew and passengers. Soot, as the primary radiative emitter, also affects the fire spread through the radiative component of heat transfer. In the work of Tewarson [9], the radiative efficiency depends on the fire size, combustion efficiency, and fuel type.

Halogen additives were found to substantially increase the tendency of fuels to soot under diffusion flame conditions, as a homogeneous catalyst for hydrogen atom recombination [10]. Soot formation is hence promoted as a result of the enhanced hydrogen abstraction/acetylene addition (HACA) mechanism [11]. In contrast, metal and organometallic compounds have been found effective in suppressing the yield of soot through a catalyzed soot oxidation process [8,12], with a large amount of metallic compound on the soot nanoparticles [8]. The heat release rate is substantially increased under the influence of metallic compound.

Unfortunately, the mechanism through which phosphorus promotes soot formation is still not well understood. And there lacks sufficient characterization of phosphorus affected post-combustion soot. In order to isolate the effect of phosphorus on incomplete combustion products in the gas phase, char formation in the condensed phase should be avoided or minimized. In this study, our objectives are to: examine the impact of phosphorous compound on soot formation in fire tests; perform detailed characterization of soot collected in the post-combustion gas stream; and propose a possible soot formation path under the influence of phosphorus.

Section snippets

Material and methods

In this study, a crosslinked diglycidyl ether of bisphenol-A epoxy (DER-332) was used as the blank thermosetting polymer because it mixes easily with additives. The DER-332 resin was hardened by hardener of triethylenetetramine (TETA). The epoxy systems were mixed with triphenylphosphine oxide (TPPO). The epoxy and its phosphorus blends were then pyrolyzed under nitrogen in a MCC and the mass fraction of char remaining at 900 °C was measured to obtain the char yield. The combination of DER-332

Fire calorimetry

The representative fire calorimeter test results of epoxy and epoxy mixed with TPPO are shown in Fig. 1. The heat release rate, mass loss rate, soot production rate, and CO production rate are reported. It is seen that the peak heat release rate is reduced from 1680 to 850 kW/m2 when 2% phosphorus is added. Integrating yields the total heat release that shows a similar decrease. However, there is not significant change in the mass loss rate. The char yield is changing from 12% to 20%, which

Conclusions

Different amounts of triphenylphosphineoxide were mixed with epoxy and burned in a fire calorimeter to examine the effect of phosphorus on gas phase flame retardancy. The phosphorus was completely vaporized with the epoxy decomposition products during the burning process and significantly promoted the formation of soot, which reduced the release of heat. Detailed characterizations of soot collected in the post-flame region were performed including: diameter, element analysis of C, H, N, O, and

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

We acknowledge the FTIR support of Dr. Mark Beach and Dr. Mark Richard from Dow Chemical.

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