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Polyethylene (PE) is one the most used plastics worldwide for a wide range of applications due to its good mechanical and chemical resistance, low density, cost efficiency, ease of processability, non-reactivity, low toxicity, good electric insulation, and good functionality. However, its high flammability and rapid flame spread pose dangers for certain applications. Therefore, different flame-retardant (FR) additives are incorporated into PE to increase its flame retardancy. In this review article, research papers from the past 10 years on the flame retardancy of PE systems are comprehensively reviewed and classified based on the additive sources. The FR additives are classified in well-known FR families, including phosphorous, melamine, nitrogen, inorganic hydroxides, boron, and silicon. The mechanism of fire retardance in each family is pinpointed. In addition to the efficiency of each FR in increasing the flame retardancy, its impact on the mechanical properties of the PE system is also discussed. Most of the FRs can decrease the heat release rate (HRR) of the PE products and simultaneously maintains the mechanical properties in appropriate ratios. Based on the literature, inorganic hydroxide seems to be used more in PE systems compared to other families. Finally, the role of nanotechnology for more efficient FR-PE systems is discussed and recommendations are given on implementing strategies that could help incorporate flame retardancy in the circular economy model.

The modelling of the behavior of a material exposed to fire is very complex and needs the coupling of fluid dynamics, combustion, heat and mass transfer, kinetics and so forth. A growing amount of ...

The influence of additives on the interaction between radiation from fires and single water droplets has been investigated in detail. A literature study was performed on available information of radiation spectra from different types of fires. Based on this, four reference spectra were proposed that cover most of the different types of radiation that can be expected from fires. These reference spectra were used to compare the effect of different water additives and droplets sizes. Using Mie-theory it was found that increased atomization, down to a diameter limit of 1–10 μm, gives a better volumetric absorption efficiency. Decreasing the diameter further does not lead to improved volumetric absorption since the Rayleigh (small droplet) limit is reached, where the volumetric absorption is independent of diameter. Different additives were investigated with respect to increased absorption in the droplets. It was found, however, that it is not trivial to find non-flammable and non-toxic additives that give a significant improvement in absorption. Carbonated water was a potential candidate but the increased absorption was limited to a very weak band centered at 2300 cm −1 . Since this coincides with the strong CO 2 emission band an effect could be seen when carbonated water interacted with radiation from clean flames. The maximum increase in volumetric absorption for carbonated water was 4%, occurring for a droplet diameter of 10 μm. Other additives gave better effects but they were either combustible (carbon nanopowder) and/or toxic to some degree.

Mid-infrared polarization spectroscopy : A tool for in situ measurements of toxic gases in smoke-laden environments

Measurements of the ion current between two electrodes have been conducted in order to assess whether the ion current can be used for the detection of ignition in a cone calorimeter, and for fire dynamics investigations in general. The measured ion current clearly indicates when ignition occurs, in agreement with traditional results based on visual inspection. It was also seen that soot deposits on the electrodes do not adversely affect the ability of the ion current to indicate ignition. However, short circuiting may occur if the electrodes are kept in a sooty flame long enough for the soot to completely bridge the gap between the electrodes. The latter scenario is not a problem from an ignition detection point of view since soot growth occurs after ignition has taken place. Measurement of the ion current between the electrodes was also conducted in a premixed propane flame in order to characterize the dependence of the current on applied voltage and on combustion conditions. The ion current obeys Ohm's law since the current increases linearly with applied voltage in the investigated voltage range. It was also seen that the ion current gave unique results based on its position in a propane flame and the fuel/air ratio of the flame. Copyright © 2010 John Wiley & Sons, Ltd.

The rotational temperature and concentration of OH in the gas-phase outside a polycrystalline platinum catalyst has been measured using cavity ringdown spectroscopy. A mixture of H2 and O2 gases was used forming a stagnation point flow field outside the catalyst. The temperature of the catalyst was 1500 K, the total pressure in the chamber was 26 Pa, and the relative hydrogen concentration, αH2, was set to 8%. From a Boltzmann plot, the rotational temperature was determined to be 775±24 K, and the amount of OH was calculated from that temperature to be 1.5±0.2×1012 cm−3, 6.5 mm outside the catalyst. A similar experiment has also been performed using a polycrystalline palladium catalyst. However, due to lower amount of OH in the gas-phase outside the Pd catalyst, the number density could not successfully be determined. In this study we have shown that cavity ringdown spectroscopy successfully can be used to quantify desorbed intermediates within a catalytic reaction.

Sensitivity analysis has been used to study the gas-phase chemistry near a hot catalytic wall. The gas mixture was H 2 /O 2 in different proportions and the studied pressure range was 1 to 10 5 Pa. The reaction mechanism for a polycrystalline Pt surface was used to model the catalyst surface, at a temperature of 1300 K. It is found that the chemistry of water production is relatively simple. For pressures of 1 to 10 3 Pa it is the transport of reactants to the surface, via sticking, which determines the water concentration in the gas-phase. For these low pressures almost all water is produced on the surface since there is no combustion zone in the gas. The water which is produced on the surface will desorb and then escape from the reactor without any further reactions. The situation is much more complex when an intermediate species, such as the O atom, is considered. O atoms in the gas-phase are prone to react with other species. This gives a complicated interaction between the surface and the gas-phase chemistry. The consumption of O atoms in the reaction O + H 2 ⇌ OH + H is a major inhibitor for the O-atom concentration. It is found that the O atoms which are created in the gas originate mainly from desorbed OH, since the reaction H 2 + OH ⇌ H 2 O + H yields H atoms which contribute to O-atom production in the reaction H + O 2 ⇌ OH + O. For the higher pressure range 10 4 to 10 5 Pa, there is a flame front at a certain distance from the surface, and the surface reactions are far less important than the gas-phase reactions. The reactions differ considerably in importance depending on whether the gas is fresh or burnt.

The H2/O2 reaction on a polycrystalline palladium foil has been studied. The experimental methods used were Laser-Induced Fluorescence (LIF) and microcalorimetry. The reaction was also simulated with the Chemkin software package. The water production maximum occurs at 40% H2 and the OH desorption has its maximum at 10% H2, at a total pressure of 100 mTorr (13 Pa) and a catalyst temperature of 1300 K. It is concluded, in agreement with the simulations, that the main water-forming reaction at these experimental conditions is OH+H→H2O. From the water production maximum the quotient of the initial sticking coefficient for hydrogen and oxygen (SH2(0)/SO2(0)) is calculated to be 0.75(±0.1).

The H2 + \( - \frac{1}{2}\)O2 ⇌ H2O reaction on platinum at 700 and 1300 K has been studied. A stagnation flow geometry was used with a gas mixture of H2 and O2 at pressures between 0.10 and 10 Torr. Comparing SHG results with simulations using different reaction parameters, it was concluded that \(E_{{\text{H}}_{\text{2}} {\text{O}}}^{\text{f}} \approx 0.7{\text{eV,}}\;E_{{\text{OH}}}^{\text{f}} \approx 0.7{\text{eV}}\), and \(E_{{\text{OH}}}^{\text{d}} \approx 2.6{\text{eV}}\). LIF measurements showed an ambiguity in the choice of main water-producing channel. Both hydrogen addition with low sticking coefficients and hydroxyl disproportionation with high sticking coefficients are plausible.

The OH concentration outside a Pt catalyst at 1300 K, in a stagnation flow of 90% O2 and 10% H2, has been studied by planar laser-induced fluorescence (PLIF), and compared to measurements outside a heated glass surface. The total pressure in the system was varied from 0.2 to 120 torr. At low pressure, surface reactions were observed for the Pt surface, but not for the glass. At higher pressure, gas-phase ignition occurred for both systems, but not at the same pressure: ignition occurred at a lower pressure outside the inert glass surface. Computer modeling using CHEMKIN confirmed these results. The difference in gas-phase ignition is also seen in the modeling results, and it is due to the removal of atomic O and H from the gas by adsorption and reaction on the catalytic surface. The catalytic reaction mechanism on the surface plays an important role as it enhances the removal of radicals, compared to a surface where only radical recombination back to reactants is allowed.

Projected stringent emissions legislation will make tough demands on engine development. For diesel engines, in which combustion and emissions formation are governed by the spray formation and mixing processes, fuel injection plays a major role in the future development of cleaner engines. It is therefore important to study the fundamental features of the fuel injection process. In an engine the fuel is injected at high pressure into a pressurized and hot environment of air, which causes droplet formation and fuel evaporation. The injected fuel then forms a gaseous phase surrounding the liquid phase. The amount of evaporated fuel in relation to the total amount of injected fuel is of importance for engine performance, i.e. ignition delay and mixing rate. In this paper, the fraction of evaporated fuel was determined for sprays, using different orifice diameters ranging from 0.100 mm up to 0.227 mm, with the aid of a high-pressure spray chamber. The conicity of the orifice was also varied. Three injection pressures and two chamber temperatures were also used. The amount of evaporated fuel was determined by laser-induced exciplex fluorescence measurement of the gaseous and liquid phases of the fuel spray. The measurement of the phases consists of time-resolved, two-dimensional images. The fuel in this study consisted of a mixture of 1-cyanonaphthalene, N,N-dibutylaniline and n-dodecane. The fuel was excited by a laser wavelength of 355 nm. The results show a strong reduction of fuel penetration for one of the conical orifices. It was also found that both increased injection pressure and reduced orifice diameter reduce the liquid volume fraction of the spray, i.e. the gas volume fraction increases.