Non-flaming Conditions Research Articles

Relevant model complexity for non-charring polymer pyrolysis

The choice of the heat, mass and chemical mechanisms included in a pyrolysis model is often subjective, and detailed justifications of the inclusion or exclusion of the different mechanisms are infrequent. The implicit assumption that models with a higher number of mechanisms reproduce more accurately the reality has led to the recent growth of complexity in pyrolysis modelling seen in the literature. However, as we show in this work, the comparison of several conceptual models predicting the same experimental results does not support this assumption, but reveals the presence of unnecessary complexity and multiple sources of uncertainty. Using a novel approach corresponding to a mechanism sensitivity, the influence of the heat, mass and chemical mechanisms on the transient predictions of surface temperature and mass loss rate (non-flaming conditions) for PolyMethylMethAcrylate (PMMA) samples is investigated. While a small change in the chemical degradation mechanism has a large effect on the predictions of the mass loss rate, the surface temperature is not affected. The heat transfer mechanisms appear to have however a significant effect on both quantities of interest. This study demonstrates that the use of complex chemical mechanisms (e.g. multi-step reaction scheme or oxidative reaction) is not justified if the mechanisms of the heat transfer are kept simple. It is therefore recommended to use consistent levels of crudeness dictated by the heat transfer.

Validation of additive models for lethal toxicity of fire effluent mixtures

Fractional Effective Dose (FED) calculation models for incapacitation and lethality are based upon a concept that overall toxicity results from the effects of a few key toxic gases and that the interactions between the component gases are essentially additive (with a possible multiplicative [synergistic] effect of CO2). A body of experimental data exists for common polymeric materials for lethal toxicity in rats (30 min LC50 + 14 days post exposure observation) combining animal data with chemical test atmosphere analysis. This data set has been used to compare LC50 concentrations calculated using two multi-gas FED models, the Levin N-Gas model and the Purser rat LC50 model, with the measured animal LC50 results. The Purser rat LC50 model has then been used to calculate LC50 concentrations for a range of common materials decomposed under conditions considered typical of four types of compartment fires as specified in ISO 19706: non-flaming oxidative, well-ventilated flaming, under-ventilated flaming and post-flashover under-ventilated flaming. It is concluded that both FED models give reasonably good predictions of overall toxicity for flaming fires, while the Purser LC50 model, which includes a term for the toxicity of organic irritants, provides a good fit for both flaming and non-flaming decomposition effluent toxicities. The results therefore confirm that for a range of common polymers, toxicity can be predicted in terms of the additive effects of a small number of key toxic gases, especially for flaming fires, while the generalized mix of organic compounds make a contribution to the toxicity mainly under non-flaming conditions, in which they form a significant proportion of the overall effluent mixture.

A model for the oxidative pyrolysis of wood

A generalized pyrolysis model (Gpyro) is applied to simulate the oxidative pyrolysis of white pine slabs irradiated under nonflaming conditions. Conservation equations for gaseous and solid mass, energy, species, and gaseous momentum (Darcy’s law approximation) inside the decomposing solid are solved to calculate profiles of temperature, mass fractions, and pressure inside the decomposing wood. The condensed phase consists of four species, and the gas that fills the voids inside the decomposing solid consists of seven species. Four heterogeneous (gas/solid) reactions and two homogeneous (gas/gas) reactions are included. Diffusion of oxygen from the ambient into the decomposing solid and its effect on local reactions occurring therein is explicitly modeled. A genetic algorithm is used to extract the required material properties from experimental data at 25 kW/m 2 and 40 kW/m 2 irradiance and ambient oxygen concentrations of 0%, 10.5% and 21% by volume. Optimized model calculations for mass loss rate, surface temperature, and in-depth temperatures reproduce well the experimental data, including the experimentally observed increase in temperature and mass loss rate with increasing oxygen concentration.

An alternative method for fire smoke toxicity assessment using human lung cells

An alternative method for fire smoke toxicity testing using human lung cells has been investigated. A laboratory-scale vertical tube furnace was used for the generation of combustion products. Experiments were conducted under isothermal oxidative non-flaming conditions. A range of building and train interiors including polyethylene (PE), polypropylene (PP), polycarbonate (PC), polyvinyl chloride (PVC), fiberglass reinforced polymer (FRP), and melamine-faced plywood (MFP) were studied. The exposure of combustion toxicants to human lung cells (A549) at the air/liquid interface was acquired using a Harvard Navicyte Chamber. Cytotoxic effects on human cells were assessed based on cell viability using the MTS assay (Promega). Cytotoxicity results were expressed as NOAEC (No Observable Adverse Effect Concentration), IC 10 (10% inhibitory concentration), IC 50 (50% inhibitory concentration) and total lethal concentration (TLC) values (mg/L). These cytotoxicity results were compared to published combustion toxicity data. Mass loss data and toxic product yield were also determined. The following toxicity ranking was observed from the most toxic to the least toxic: PVC>PE>PP> FRP-10>PC> FRP-16>MFP. The method described here could potentially be an alternative to current fire toxicity standard test methods.

Studies on evolved gases and smoke generated by flame-retarded phosphorylated cellulosics

Samples of flame-retarded durable cotton fabrics, obtained by converting cotton to cellulose ammonium phosphate (CAP)-metal complexes, were pyrolysed in air in the temperature range 225–375°C. The limiting oxygen index (LOI) values of these fabrics were found to range from 28·5 for the Ni(II) complex to 53·6 for the Cu(II) complex. The evolved gases and smoke were collected in a Stanton-Redcroft FTB accessory which enabled smoke obscurations and percentage concentrations of carbon monoxide and carbon dioxide to be determined. In a separate series of experiments under non-flaming conditions, the Fourier transform infrared spectra obtained for the evolved pyrolysis gases did not show the presence of any gaseous phosphorus species indicating that phosphorus was retained in the condensed phase. Infrared spectra of the char residues did show some evidence for the presence of phosphorus species. Overall the studies showed that, compared to untreated cotton, the CAP-metal complex fabrics yielded less smoke under non-flaming conditions but more carbon monoxide and carbon dioxide under flaming conditions. Thus, the CAP-metal

Smoke, CO2 and CO Evolution from Cotton and Flame Retarded Cotton: Part 2. Behaviour of Single Layer Fabrics in Air at Elevated Temperatures

Smoke, CO and CO2 emissions have been determined from un treated cotton and flame retarded cotton fabrics at elevated temperatures. The flame retardants studied at commercial levels of applications were ammonium polyphosphate (Amgard TR), ammonium polyphosphate—ammonium bromide (Amgard CD), a phosphonium salt-urea-polycondensate (Proban CC), a phos phono-propionamide (Pyrovatex CP) and an antimony (III) oxide-aliphatic bro mide (Flacavon H14/587) formulation. Combustion product analyses were undertaken at elevated temperatures (225-300°C) in air under burning conditions (above the respective temperature oxygen index) and non-flaming pyrolysis. Under both conditions smoke densi ties and CO concentrations increased with temperature. Carbon dioxide con centrations under non-flaming conditions showed similar increases but under burning conditions, decreased with increasing temperature. At a given temper ature, the P- and N-containing retardants reduced smoke density under burn ing conditions and increased it under pyrolysis relative to pure cotton below 350°C. These same flame retardants reduced CO and CO2 formation at each temperature from burning fabrics but produced little change with respect to cotton under non-flaming conditions. The presence of aliphatic bromine in the Sb2O3—bromine synergistic system caused significant increases in smoke density and CO and CO2 concentrations with respect to all other fabrics under pyrolysis in air. The observed trends are analysed in terms of current knowledge of pyrolysis and combustion mechanisms.

Polymer degradation during combustion

AbstractA novel driven‐rod, radiant pyrolysis technique has been used to study the steady‐state linear regression of vertically mounted poly(methyl methacrylate) and polypropylene rods degrading from the top under high‐flux, nonflaming conditions that simulate those of combustion. With this technique reliable temperature‐depth and oxygen‐depth profiles have been obtained and used to calculate the observed mass loss rates and to elucidate the mode of polymer degradation during combustion, respectively. For poly(methyl methacrylate) burning in this rod configuration, condensed phase oxidation is unimportant and the degradation mechanism is similar to that under more conventional slow‐heating conditions in the absence of air. In contrast, for burning polypropylene condensed phase oxidation occurs; the lack of oxygen incorporation suggests that the polymer degrades by oxygen‐promoted pyrolysis.

Continuous Monitoring of Hydrogen Chloride in Combustion Atmospheres and in Air

A continuous analyzer for hydrogen chloride gas, based on a French standard method, has been utilized for measurement of HCl in laboratory-generated combustion atmospheres and in air. The method is based on continuous titra tion of chloride with silver in aqueous solution. The process is monitored by commercially available automated titration equipment, a mass flowmeter and a dedicated Apple IIe microcomputer. Pure HCl in air was monitored in a steady-state flow-through system and compared to sampling by soda lime tubes. Hydrogen chloride in PVC smoke (both flaming and nonflaming combustion conditions) was monitored in both flow-through and static environments, in cluding an apparatus designed for steady-state combustion. The advantages, disadvantages and recommendations for further study of this technique are discussed.

A Statistical Comparison of Test Protocols Used in the Assessment of Combustion Product Toxicity

Previously we reported a criterion for choosing an appropriate statistical model to describe the tolerance distribution for estimation of the median effec tive dose [1]. In the present paper we have shown how this statistical approach can be used to assess the precision of toxicity test data and the predictive abili ties of test methods. Specifically, we conducted statistical analyses of data generated by using the University of Pittsburgh (UP) and the National Bureau of Standards (NBS) combustion toxicity protocols. These evaluations revealed that 30% of the LD50 values from the UP test were not valid while with the NBS test 26% and 5% of the LD50 values under flaming and non-flaming conditions, respectively, were not valid. Comparisons of the two test methods also showed that there was not agreement on ranking across protocols except for the very few with extreme toxicities. These results indicated that caution should be exercised in the review and interpretation of combustion toxicity data and that these tests are probably only useful in identifying consistently super-toxic chemicals.

Use of an Approximate Lethal Exposure Method for Examining the Acute Inhalation Toxicity of Combustion Products

An approximate lethal exposure (ALE) method for acute inhalation toxicity testing of thermal decomposition products was developed and evaluated. In comparison to "traditional" LC 50 calculation methods, the ALE method re quired 70% fewer animals and 50% fewer test runs. The ALE procedure relied on analysis of combustion gases and their limited animal testing (F-344 rats). The procedure was used to evaluate eight materials under flaming (5.0 W/cm2) and nonflaming (2.5 W/cm2) conditions. The complete 30-minute L(Ct)50's were also calculated for five of those eight materials and compared to the ALE's. The ALE's were within eleven percent of the calculated L(Ct)50's at 5.0 W/cm 2 and ranged within thirteen to fifty-five percent of the L(Ct) 50's at 2.5 W/cm2. A microcomputer data acquisition and control system to improve data collection and Ct product calculation is also described for the radiant furnace test method for improved data collection and concentration-time product calculation.

Toxicity Of The Combustion Products From A Flexible Polyurethane Foam And Polyester Fabric Evaluated Separately And Together By The Nbs Toxicity Test Method

Representative specimens of two materials, a flexible polyurethane foam and a polyester, wer,e thermally decomposed separately and together in order to compare the toxicological effects of the combustion products from the combined materials with those from the single homogeneous materials. Gas concentrations (CO, CO 2 , O2 and HCN) , blood carboxyhemoglobin, and LCs o values [the concentration of material necessary to kill 50% of the test animals (Fischer 344 male rats) during a 30 minute exposure and a 14 day post-exposure observation period] were determined for the separate and combined materials under both flaming and non-flaming conditions. The results of the combined experiments indicated that under nonflaming conditions, both materials contributed in an additive manner to the concentration of the combustion products. However, under flaming conditions, the generation of HCN and CO is greater than that predicted from the addition of the maximum amounts produced by the materials separately.

Polystyrene degradation during combustion

AbstractA novel driven‐rod, radiant pyrolysis technique has been used to study steady‐state linear regression of vertically mounted polystyrene rods degrading under high‐flux, nonflaming conditions that simulate those of combustion. With this technique a reliable, reproducible temperature‐depth profile (identical for combustion and radiant pyrolysis) for polystyrene has been obtained and used to successfully calculate the observed mass loss rate. Residues and volatiles have been analyzed. Mechanistic treatment of these results indicates that degradation of polystyrene under these rapid‐heating conditions is similar to that under more conventional slow‐heating conditions in the absence of oxygen. Oxidation is unimportant in the condensed‐phase degradation of polystyrene in this configuration. This radiant pyrolysis technique has also been used to obtain the heat of gasification of polystyrene.

Aspects of the mechanisms of smoke generation by burning materials

This paper is concerned with the investigation of the detailed aspects of smoke generation during the burning of natural and synthetic solid materials under simulated fire conditions. With this objective in mind, the first portion of the paper is devoted to a review of relevant basic smoke formation studies. This review identifies the complex physical, chemical, and electrical processes that are believed to be important in smoke particulate production. The remainder of the paper presents measured experimental data and discusses observed trends in terms of many of the processes set forth in the previous section. The resulting increase in particle concentrations as heating rates are increased in nonflaming combustion is related to nucleation and condensation processes. Significant fundamental differences in smoke characteristics were observed between flaming and nonflaming conditions, and this is attributed to specific differences in controlling mechanisms and resulting pathways leading to particulate formation. Measured differences in smoke properties resulting from increased environmental temperature are similarly explained. The effects of polymer substrate properties and effects of various additives for a given substrate on observed smoke particulate properties are also discussed in terms of basic processes, although the determination of the actual mechanisms responsible for the observed smoke characteristics require further study. Finally, results of chemical analyses of smoke particulates from a variety of materials conducted as part of this study indicate that notable differences in particulate formation mechanisms are likely among the materials studied.

Analysis of the products of thermal decomposition of an aromatic polyamide fabric

AbstractThe thermal degradation of an aromatic polyamide was studied under conditions of pyrolysis and oxidative degradation at 550°C and of flaming combustion. Techniques described elsewhere were used to determine the volatile compounds quantitatively by gas chromatography‐mass spectrometry (GC–MS). The condensible material and the solid residue were characterized by infrared spectroscopy and MS, and in pyrolysis experiments 28 compounds were identified (CO, CO2, H2O, and C6H5CN were the primary products). Collectively, these compounds accounted for 79% of the sample weight loss. The remaining 21% was a condensible material that contained at least 17 compounds; the two major components were 1,3‐dicyanobenzene and 3‐cyanobenzoic acid. Most of the nitrogen content of the polymer remained as involatile residue. This study was sufficiently detailed to obtain a mass balance between the composition of the original polymer and the sum of the observed pyrolysis products. The major products observed in pyrolysis experiments supported a mechanism that involved the cleavage of an aromatic‐NH bond and the loss of H2O to form aromatic nitriles. Hydrolysis of the amide linkage, followed by decarboxylation of the product acid, accounted for the high concentrations of CO2 observed. Oxidative degradation at 450°C yielded ten identifiable compounds and an additional 19 volatile compounds were formed at 550°C. The condensible fraction, which contained at least 20 compounds, was similar in composition to the fraction collected from the pyrolysis experiments. The sum of the carbon content from the two major volatile products of oxidative degradation (CO and CO2) and from the solid residue quantitatively accounted for the carbon content in the original sample. Flaming combustion studies revealed a markedly different product distribution than was observed under nonflaming conditions, especially in regard to the higher‐molecular‐weight species.

Investigation of smoke particulates generated during the thermal degradation of natural and synthetic materials

This paper describes results obtained in a program investigating the physical and chemical properties of particulates generated during the burning of three commonly used materials. Small samples of wood, rigid urethane foam and polyvinyl chloride plastic have been burned under non-flaming conditions at room temperature in a ventilated combustion products test chamber. Continuous measurements have been made of smoke particle average size, particle size distributions, particulate optical density, particulate volume and mass concentrations, and particulate index of refraction utilizing an aerosol sampling system and an in situ scattered and attenuated light optical system. Non-flaming tests have been run in different ventilation gas compositions and at three radiant heating levels in an attempt to simulate real-fire situations. A review of previous results obtained by the aerosol sampling system is followed by presentation of new data taken with the in situ optical system. This data shows that average smoke particle sizes tend to vary during a test. Furthermore, particle sizes are generally larger for tests run at the highest heating rates. Available data also indicates that the physical characteristics of the smoke particles are relatively for non-flaming conditions. These results are consistent with previously reported aerosol sampling data. Also, continuous optical density measurements are presented, which allow the determination of the relative tendency of a smoldering material to obscure vision under different conditions. Finally, scanning electron microscope photographs provide new information on the detailed nature of smoke particles generated during non-flaming combustion.