Gasification of silicone fluids under external thermal radiation. Part 2. Gasification products — characterization and quantitation

The gasification behavior for a wide range of polydimethylsiloxane fluids in a nitrogen atmosphere was investigated. Part 1 of this study addressed the measurement of the energy (global heat of gasification) required for the gasification of a wide range of dimenthylsiloxanes. Several significant corrections were required to reconcile measured gasification energy(s) with calculated heat(s) of gasification based on fundamental thermochemical data. The identification of the dominant mode(s) of gasification via the characterization of pyrolysis products provided a firm basis and rationale for understanding and directing efforts at quantifying these correction factors. In Part 2, the gasification products were identified and quantified at various stages of the gasification process corresponding to ignition, fire growth, and steady-state burning. Pyrolysis of methylated siloxanes occurs via two modes: (1) the volatilization of short chain and intermediate chain length species native to the polymer, and (2) the volatilization of short chain and intermediate chain length species resulting from thermal degradation via siloxane rearrangement. The former process is the dominant gasification mechanism for short chain oligomers and low viscosity fluids (η<10 cS) and the latter process is dominant in all higher molecular weight polymers (η>100 cS). Both gasification mechanisms are evident in all polymers (η>20 cS); the dominant mechanism is dependent upon polymer size and distribution thereof, the gasification stage, and the presence of trace catalysts in the polymer. Because of their structural similarity, the combustion of all gasification products emanating from PDMS regardless of the stage of the pyrolysis process or the dominant mode of gasification will result in virtually identical combustion products, i.e. SiO₂, CO₂, and H₂O. Copyright © 1998 John Wiley & Sons, Ltd. This paper was written under the auspices of the US Government and is therefore not subject to copyright in the US.     

Experimental study on the radiative ignition of silicones

Silicones comprise a wide variety of materials such as fluids, elastomers, resins, and foams. This paper reports the ignitability of some typical silicones under various external radiant heat fluxes. The ignitability of silicones was studied using a cone calorimeter under radiant heat flux levels of 0.5–60 kW m⁻². The time to ignition of the silicones was found to be proportional to a power of the incident heat flux that varies from −1.33 to −2.84. For silicone fluids, viscosity (or molecular size) is the key variable in controlling the ignitability. For silicone elastomers, the fillers play an important role in controlling the ignitability, especially at incident heat fluxes lower than 35 kW m⁻². The ignitability of silicone resins depends on the chemical structure of the resins: the pure trifunctional resin has the lowest ignitability. The ignitability of the silicone foams having the same density depends on the foam thickness, especially at incident heat fluxes lower than 30 kW m⁻². © 1998 John Wiley & Sons, Ltd.     

Studies of cyclic and linear poly(dimethyl siloxanes): 6. Effect of heat

Cyclic and linear poly(dimethyl siloxanes) were heated under vacuum in the temperature range 623–693K for periods of hours or days in the absence of catalysts. The products were analysed by gas-liquid chromatography and gel permeation chromatography. The results for the linear poly(dimethyl siloxanes) were in full agreement with the published work of Thomas and Kendrick. The effect of heat on the cyclic poly(dimethyl siloxanes) was that predicted, assuming that similar siloxane bond interchange reactions take place to those believed to occur in the linear polymers. The cyclic poly(dimethyl siloxanes) produced mixtures of cyclic oligomers, together with polymeric products which have considerably higher molecular weights than the starting materials. It is proposed that these polymeric products consist of mixtures of ring molecules [(CH₃)₂SiO]_x. Some of these cyclic polymers are estimated to contain (on average) more than 10 000 skeletal bonds. Similar mixtures of cyclic oligomers and high molecular weight polymeric products were obtained by heating hexamethylcyclotrisiloxane and octamethylcyclotetrasiloxane.     

A theoretical basis for flammability properties

Theoretical formulations are presented for the fire growth processes under external radiant heating. They included ignition, burning and energy release rate, and flame spread. The behaviour of these processes with external heating is described along with the critical conditions that limit them. These include the critical heat fluxes for ignition, flame spread and burning rate. It is shown how these processes and their critical conditions depend on a limited number of properties measurable by a number of standard test methods. The properties include heat of combustion, the heat of gasification, ignition temperature and the thermal properties of the material. Alternatively, the properties could be related to parameters easily found from data; namely: (1) the critical heat flux (CHF) for ignition; (2) the slope of the energy release rate with externally imposed flux, defined as heat release parameter (HRP); and (3) the ignition parameter, defined as thermal response parameter (TRP). It is further shown that the flame heat flux differences between small laminar flame ignition sources and larger turbulent flames can affect flame spread due to heat flux and ignition length factors. Finally, it is found that the critical energy release rates theoretically needed for ignition, sustained burning, and turbulent upward flame spread are roughly 13, 52, and 100 kW/m², respectively, and independent of material properties. Copyright © 2005 John Wiley & Sons, Ltd.     

Ultra‐high‐molecular‐weight functional siloxane additives in polymers. Effects on processing and properties

Two new types of solid siloxane additives for plastics are described which give improved benefits compared to previous silicone additives. Ultra‐high‐molecular‐weight (UHMW) siloxanes are used in the new additives; traditional silicone plastic additives have used much lower molecular‐weight silicones. The siloxane is converted into solid forms, either masterbatch pellets or powders, that are easy to feed, or mix, into plastics during compounding, extrusion, or injection molding. Ultra‐high‐molecular‐weight siloxanes can be compounded into masterbatch pellets at higher siloxane concentrations than previously possible, e. g., up to 50%. They impart improved processing and release, lower coefficient of friction, and broader performance latitude compared to conventional lower‐molecular‐weight silicones. These benefits can be delivered at reduced siloxane levels with increased concentration at the surface interface with a new functionalized UHMW siloxane which provides unique surface segregation characteristics. Ultra‐high‐molecular‐weight siloxanes have been formulated into powders that can also act as processing aids and mechanical property modifiers for highly filled polymers such as fire‐retardant systems. This paper uses polyolefins as a model. However, many of the effects shown in polyolefins have also been seen in other resin systems.     

Note: measuring the effective heats of combustion of transformer‐insulating fluids using a controlled‐atmosphere cone calorimeter

The effective heats of combustion of two commonly used transformer‐insulating fluids, a high molecular weight hydrocarbon fluid and a 50 cS silicone fluid, have been measured using a controlled‐atmosphere cone calorimeter. The study shows that the cone calorimeter is a good tool to measure the effective heats of combustion of transformer‐insulating fluids. Copyright © 2002 John Wiley & Sons, Ltd.     

Burning behaviors of selected chlorosilanes and SiH‐containing siloxanes

Chlorosilanes are silanes containing the Si‐Cl functional group and SiH‐containing siloxanes are siloxanes containing the Si‐H functional group. Some chlorosilanes and SiH‐containing siloxanes present potentially high fire or explosion hazards during handling, storage, transport and process operations. Cone calorimeter tests have been used to study the burning behaviors of selected chlorosilanes and SiH‐containing siloxanes at various incident heat fluxes to simulate pool fire burning. The peak heat release rate of a silicon intermediate obtained from the cone calorimeter at 15 kW/m² incident heat flux was very close to that measured by a relatively large‐scale field test. The flammability of monochlorosilanes was similar to that of organic hydrocarbons having comparable volatility. The flammability of chlorosilanes descends in the order of monochlorosilanes, dichlorosilanes and trichlorosilanes. SiH‐containing siloxanes ignited faster than non‐SiH‐containing siloxanes because of the reactive silicon‐hydrogen linkages. The ignition of SiH‐containing siloxanes was much more violent than the ignition of non‐SiH‐containing siloxanes. The SiH‐containing siloxanes exhibited a lower peak heat release rate, less total heat released and a lower peak smoke extinction coefficient compared with non‐SiH‐containing siloxanes having comparable volatility. Copyright © 2004 John Wiley & Sons, Ltd.     

Preparation of poly(IPDI‐PTMO‐siloxanes) and influence of siloxane structure on reactivity and mechanical properties

Siloxane‐modified polyurethanes were prepared through isophorone diisocyanates (IPDI), poly(tetramethylene oxide) (PTMO), and siloxanes. IPDI served as the hard segment in the structure. Both PTMO and siloxanes were diols and served as the soft segments. In addition, different chemical structures of siloxanes were used, in which siloxane chains would remain in the main chain of polyurethanes (PU) or become the side chain of PU. First, the reactivities of PTMO and siloxanes to react with IPDI in bulk system were studied through DSC, in which the reaction heat was related to their reactivities. Copolymerization of IPDI, PTMO, and siloxanes in bulk were also studied. The results showed that hydrophobicity and steric hindrance of siloxane diols led to their low reactivities. Next, a series of siloxane‐modified PU in toluene solvent were synthesized, and the conversion of NCO groups was determined by the method of chemical titration. In the synthesis of PU copolymers in a solution polymerization, because of low reactivity of siloxanes, a two‐step procedure was adopted. The siloxane diol was first reacted with IPDI in toluene to form NCO‐terminated prepolymer. Then PTMO was added to form final PUcopolymers. The addition of side‐chain siloxanes resulted in PU copolymers with higher molecular weight than main‐chain siloxanes. Both main‐chain and side‐chain siloxanes increased the elongation at break and tensile strength of final PU copolymers. The microphase‐separation of siloxane segments was observed by SEM, which was the main cause for the improved mechanical properties. POLYM. ENG. SCI., 47:625–632, 2007. © 2007 Society of Plastics Engineers.     

Thermal sensor performance and fire characterisation during short duration engulfment tests

A small-scale reproduction of the ISO 13506-1 thermal manikin was constructed to enable the assessment of manikin sensor performance, the partitioning of energy, and the variability of the fire generated during short duration heat and flame engulfment tests. The cylindrical test apparatus simultaneously housed four total heat flux (THF) sensors, one radiant heat flux sensor, and three manikin sensors. Calibrated manikin sensors were provided by nine laboratories and were categorised as buried thermocouple, copper-based, or surface-mounted thermocouple sensors. The test apparatus was exposed to fire generated by four propane torches for three exposure durations. All sensors presented similar profiles in net heat flux over time, which could be divided into four distinct phases: transient increase, pseudo-steady state, transient decrease, and post-exposure. Over pseudo-steady state, the mean THF over all exposure durations was 88 ± 8 kW/m², and the ratio of convective to radiant energy was approximately 50:50, but highly variable. For a 4-second exposure, manikin sensors from five laboratories had a bias in heat flux greater than ± 5% during pseudo-steady state when compared with the THF sensors. This bias can primarily be attributed to the sensitivity of the manikin sensors to convective heat or heat loss due to sensor design.     

Flame‐retardant mechanism of silica: Effects of resin molecular weight

The effects of resin molecular weight on the flame‐retardant mechanism of silica were studied with two different molecular weights of poly(methyl methacrylate) (PMMA), 122,000 and 996,000 g/mol, and two silicas, fused silica with a small surface area and silica gel with a large surface area. A total of six different samples were studied, with a mass fraction of 10% silica. The mass loss rate of the six samples in nitrogen and the heat release rate from burning in air were measured at an external radiant flux of 40 kW/m². The addition of silica gel to the low‐molecular‐weight PMMA significantly reduced the mass loss rate and heat release rate; addition to the high‐molecular‐weight PMMA provided the largest reductions of these quantities in this study. For fused silica, some reduction in mass loss rate and heat release rate was observed when it was added to the high‐molecular‐weight PMMA; addition to the low‐molecular‐weight PMMA did not reduce either loss rate. Chemical analysis of the collected residues and observation of the sample surface during gasification reveal the accumulation of silica near the surface; the larger its coverage over the sample surface was, less the mass loss rate and heat release rate were. Both the level of accumulation and its surface coverage depended strongly not only on the silica characteristics but also on the melt viscosity of the PMMA. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 87: 1541–1553, 2003     

Mechanical properties of fire‐damaged glass‐reinforced phenolic composites

Changes to the mechanical and physical properties of a glass‐reinforced resole phenolic composite due to intense radiant heat and fire are investigated. Fire testing was performed using a cone calorimeter, with the composite exposed to incident heat fluxes of 25, 50, 75 or 100 kW/m² for 325 s and to a constant flux of 50 kW/m² for different times up to 1800 s. The post‐fire tensile and flexural properties were determined at room temperature, and these decreased rapidly with increasing heat flux and heat exposure time due mainly to the chemical degradation of the phenolic resin matrix. The intense radiant heat did not cause any physical damage to the composite until burning began on exposure to a high heat flux. The damage consisted of cracking and combustion of the phenolic matrix at the heat‐exposed surface, but this only caused a small reduction to the mechanical properties. The implication of the findings for the use of glass‐reinforced resole phenolic composites in load‐bearing structures for marine craft and naval ships, where fire is a potential hazard, is discussed. Copyright © 2000 John Wiley & Sons, Ltd.     

Solar gasification of carbonaceous waste feedstocks in a packed‐bed reactor—Dynamic modeling and experimental validation

Thermochemical gasification of carbonaceous waste feedstocks (specifically: scrap tire powder, industrial sludge, and sewage sludge) for high‐quality syngas production is numerically modeled and experimentally validated using concentrated solar process heat. The solar reactor consists of two cavities separated by a radiant emitter, with the upper one serving as the solar radiative absorber and the lower one containing the reacting packed bed. The reactor is modeled by considering combined heat transfer coupled to the reaction kinetics, driven by the applied solar flux at the reactor's aperture. Model validation is accomplished in terms of converted mass, reactor temperatures, efficiency, and solar upgrade based on experiments with an 8‐kW reactor subjected to solar fluxes up to 2560 suns and packed bed temperatures up to 1490 K. The transient operation of a 200‐kW pilot‐scale reactor for gasifying industrial sludge is simulated for a solar day, yielding a maximum solar‐to‐fuel energy conversion efficiency of 89%. © 2011 American Institute of Chemical Engineers AIChE J, 2011     

Comparison of cone and OSU calorimetric techniques to assess the flammability behaviour of fabrics used for aircraft interiors

Two test methods for measuring the heat release rate, HRR have been compared on fabric composites used for aircraft interior materials as side‐wall panels. These methods are based on the principles of direct measurement of the convective and radiant heat by thermopiles using an Ohio State University (OSU) calorimeter, and oxygen consumption using a cone calorimeter. It has been observed when tested by standard procedures, cone results at 35 kW/m² incident heat flux do not correlate with OSU results at the same heat flux. This is because in the cone calorimeter, the sample is mounted horizontally whereas the OSU calorimetric method requires vertical sampling with exposure to a vertical radiant panel. A further difference between the two techniques is the ignition source—in the cone it is spark ignition, whereas in the OSU it is flame ignition; hence, samples in the OSU calorimeter ignite more easily compared to those in the cone under the same incident heat fluxes. However, in this paper we demonstrate that cone calorimetric exposure at 50 kW/m² heat flux gives similar peak heat release results as the 35 kW/m² heat flux of OSU calorimeter, but significantly different average and total heat release values over a 2 min period. The performance differences associated with these two techniques are also discussed. Moreover, the effects of structure, i.e. type of fibres used in warp/weft direction and design of fabric are also analysed with respect to heat release behaviour and their correlation discussed. Copyright © 2005 John Wiley & Sons, Ltd.     

Thermal response characteristics of fire blanket materials

The thermal response characteristics of over 50 relatively thin (0.15–3.7 mm) fire blanket materials from four different fiber groups (aramid, fiberglass, amorphous silica, and pre‐oxidized carbon) and their composites have been investigated. A plain or coated fabric sample was subjected to a predominantly convective or radiant heat flux (up to 84 kW/m²) using a Meker burner and a cone heater, respectively. In addition to conventional thermal protective performance ratings for protective clothing, two transient thermal response times (for the fabric back‐side temperature to reach 300 °C and for the through‐the‐fabric heat flux to reach 13 kW/m²) and a steady‐state heat‐blocking efficiency (HBE) were introduced for both convective and radiant heat sources. For most woven fabrics, the HBE values were approximately 70 ± 10% for both convection and radiation and only mildly increased with the fabric thickness or the incident heat flux. Nonwoven (felt) fabrics with low thermal conductivity exhibited significantly better insulation (up to 87%) against convective heat. Highly reflective aluminized materials exhibited exceptionally high HBE values (up to 98%) for radiation, whereas carbon and charred aramid fabrics showed lower HBEs (down to 50%) because of efficient radiation absorption. A relatively thin fire blanket operating at high temperatures can efficiently block heat from a convective source by radiative emission (enhanced by its T⁴‐dependence and high surface emissivity) coupled with thermal insulation and from a radiant heat source by surface reflection while the aluminum surface layer remains. Copyright © 2013 John Wiley & Sons, Ltd.     

Particle‐Scale Investigation of Heat Transfer in Radiation‐Driven Char Gasification

A model of the steam gasification of a single char particle driven by high‐intensity radiation was developed and experimentally verified with available measurements in the literature. This was used to explore the sensitivity of the particle surface temperature and heat‐transfer mechanisms to variations in particle diameters, radiative heat flux, and the concentration of the gasification agent H₂O under typical conditions for solar gasification reactors. The results highlight the importance of the particle diameter in influencing solar‐to‐chemical energy conversion efficiency and assist in the selection of appropriate feedstock particles to match the conditions in specific solar gasification reactors.     

Influence of molecular rigidity on interfacial ordering in diphenyl-based polysiloxane films

Synchrotron X-ray reflectivity (XRR) shows significant differences between the ordering in thin films of diphenyl-based siloxane oligomers with single versus double backbones of -Si-O- repeating groups. We show that the more restricted conformational arrangement of twofold-skeleton molecules results in a higher degree of molecular ordering indicated by 2-2.5 times higher value of intensity of the corresponding Bragg peak in thin solid films of poly(phenylsilsesquioxane) than in films of poly(diphenylsiloxane), regardless of the solvent used for film casting. In both cases, the ordered molecules are located within 40-50 Å of the substrate surface. The results indicate unambiguously that the chain stiffness of siloxanes governs the degree of ordering in the restricted geometry of the interfacial region.     

Gasification of pelletized renewable fuel for clean energy production

The main aim of the study was to develop and investigate a small-scale experimental gasification technique for the effective thermal decomposition of pelletized renewable fuels (wood sawdust, wheat straw). The technical solution of the biomass gasifier for gasification of renewable fuels presents a downdraft gasifier with controllable additional heat energy supply to the biomass using the radial propane flame injection into the bottom part of the biomass layer. From the kinetic study of the mass conversion rate of pelletized biomass and variations of the composition of produced gas it is concluded that the process of biomass gasification is strongly influenced by the amount of additional heat energy and air supply into the biomass. The results of experimental measurements of the composition of produced gas have shown that under the conditions of the sub-stoichiometric air supply into the layer of pelletized wood biomass (α < 0.3) increasing additional heat energy supply in a range from 60 kJ up to 130 kJ leads to an enhanced mass loss of pelletized biomass and enhanced formation of volatiles (CO, H₂) in the flaming pyrolysis zone. For the wood biomass the average content of CO in the products can be increased from 73 g/m³ up to 97 g/m³, while the average content of H₂ increases from 4.7 g/m³ up to 6.2 g/m³. Similar variations of the composition of products are observed during the enhanced gasification of the wheat straw. At constant rate of additional heat energy supply and the sub-stoichiometric combustion conditions (α ≈ 0.17 − 0.30), a faster thermal decomposition of the pelletized biomass and larger average amount of the produced volatiles (CO, H₂) can be obtained by increasing the air supply rate from 0.27 to 0.43 g/s, determining the variations of air-to-fuel ratio in a range from 1.3 up to 1.6.     

Controlled-atmosphere cone calorimeter studies of silicones

A controlled-atmosphere cone calorimeter was used to investigate the burning of a silicone fluid and two silicone elastomers. The silicone materials were tested at 50 kW/m² incident heat flux in environments containing 15–30% oxygen. The test results were compared with a high molecular weight hydrocarbon fluid and an ethylene propylene rubber in terms of time to ignition, peak heat release rate and total heat released, carbon monoxide yield and carbon monoxide production rate, and smoke production and smoke production rate. The data from this study show that when materials burn in oxygen-enriched, normal, and vitiated atmospheres, silicone-based materials have a comparatively low peak heat release rate, total heat released, average CO production rate, and average smoke production rate as compared with organic-based materials. The smoke production and smoke production rate of silicone elastomers can be significantly reduced by adding appropriate smoke suppressants and additives. © 1997 John Wiley & Sons, Ltd.     

Studies of cyclic and linear poly(dimethyl siloxanes): 13. Static dielectric measurements and dipole moments

The static dielectric permittivities, refractive indices and densities of undiluted oligomeric cyclic and linear dimethyl siloxanes and narrow fractions of cyclic and linear poly(dimethyl siloxanes) have been measured for number-average molar masses M?_n in the range $\text{160 <}\text{M}\text{?}{}_{\mathrm{n}}\text{< 7700}$ at temperatures from 298 to 313 K. Measured total dielectric polarizations have been resolved into their electronic, atomic and orientation components and dipole moments have been derived. The dipole moments of cyclic oligomers ((CH₃)₂SiO)_x (for example, with x = 4, 5) are markedly lower than the dipole moments of the corresponding linear oligomers containing the same number of siloxane bonds. However, for x ? 10, the dipole moments of cyclic dimethyl siloxanes are identical, within experimental error, to those of the corresponding linear dimethyl siloxanes. Measured static dielectric permittivities of the dimethyl siloxanes and poly(dimethyl siloxanes) in solution in cyclohexane are markedly different from the corresponding values for the undiluted siloxanes. These differences are interpreted as resulting from the specific solvent effects.