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Organisationseinheit der BAM
Wood products are often treated by different techniques to improve their longevity when used as building materials. Most of the time, the goal is to increase their resistance to weathering effects, deformations in material dimensions or biotic decomposition. These wood treatment techniques have a significant impact on pyrolysis and burning behavior. The general effects of three different common wood treatments on flame retardancy were investigated by comparing treated woods with their untreated counterparts and with other kinds of wood. While the acetylation of beech leads to a slightly increased fire hazard, the thermal treatment of wood and crosslinking of cellulose microfibrils dimethyloldihydroxy-ethyleneurea show a limited flame retarding effect. Switching to woods with a higher lignin content, and thus higher char yield, however, results in a more pronounced improvement in flame retardancy performance. This article delivers a comprehensive and balanced assessment of the general impact of different wood modifications on the fire behavior. Further, it is a valuable benchmark for assessing the flame retardancy effect of other wood modifications.
Fire retardancy effects in single and double layered sol-gel derived TiO2 and SiO2-wood composites
(2012)
Sol–gel derived TiO2 and SiO2-wood inorganic composites are prepared by direct vacuum infiltration of silicon and titanium alkoxide based precursors in pine sapwood in one or two cycles followed by a controlled thermal curing process. The resulting flame retardancy effect is investigated under two different fire scenarios using cone calorimetry and oxygen index (LOI). Heat release rates (HRR) especially the values for the second peak, are reduced moderately for all single layered composites. This effect is more pronounced for double layered composites where HRR was reduced up to 40 % showing flame retardancy potential in developing fires. Beside this, smoke release was lowered up to 72 % indicating that these systems had less fire hazards compared to untreated wood, whereas no meaningful improvement is realized in terms of fire load (total heat evolved) and initial HRR increase. However impressively, the LOI of the composites were increased up to 41 vol% in comparison to 23 vol% for untreated wood displaying a remarkable flame retardancy against reaction to a small flame. An approximate linear interdependence among the fire properties and the material loading as well as fire residue was observed. A residual protection layer mechanism is proposed improving the residue properties for the investigated composites.
Most of the polymeric materials used are easy to ignite and show extensive flame spread along their surfaces. Apart from extensive heat release rates, their short time to ignition (tig), in particular, is a key fire hazard. Preventing ignition eliminates fire hazards completely. Protection layers that shift tig by more than an order of magnitude are powerful flame retardancy approaches presenting an alternative to the usual flame retardancy concepts.
Coatings are proposed that consist of a three-layer system to ensure adhesion to the substrate, acting as an infrared (IR) mirror and protecting against oxidation. The IR-mirror layer stack is realised by physical vapour deposition in the sub-micrometre (<1 µm) range, reducing heat absorption by up to an order of magnitude. Not only is the ease of ignition diminished (tig is increased by several minutes), the flame spread and fire growth indices are also remarkably reduced to as little as 1/10 of the values of the uncoated polymers open for further optimization. Sub-micrometre thin IR-mirror coatings yielding surface absorptivity <0.1 are proposed as a novel and innovative flame retardancy approach.
A quantitative experimental assessment of flame retardancy by the heat shielding in epoxy layered silicate nanocomposite (EP/TPPMMT) is presented. Online heat flux measurements and temperature monitoring within the specimen are performed during the burning in the cone calorimeter. For EP the surface layer equals a pyrolysis front. The reradiation by the hot surface corresponds to the fourth power of the pyrolysis temperature. The surface reradiation (around 10 kW m-2) is thus fairly invariable over burning time and different external heat fluxes. Further, the thermal feedback of the flame is approximated to 20 kW m-2 for both EP and EP/TPPMMT and invariable over different irradiations. Thus the net heat fluxes transformed to the fuel release rate within the pyrolysis front of EP are increased to 45–80 kW m-2 when irradiations of 35–70 kW m-2 are applied. For a residue-forming EP/TPPMMT the surface temperature and thus the reradiation (42–68 kW m-2) crucially increases compared to EP and with increasing irradiation. The net heat fluxes are reduced to 13–22 kW m-2 accordingly. This quantitative assessment of the heat shielding in EP/TPPMMT goes along with proportional and consistent improvement in the fire performance, such as the pyrolysis front velocity, the heat release rate (HRR) characteristics such as averaged and quasi-steady-state HRR and the peak HRR (PHRR). The heat shielding is proven to be the only major flame retardancy effect occurring in nanocomposites based on non-charring polymers.
Sol–gel technology was applied in tailoring
novel wood-made-inorganic composites with improved
thermal and fire properties. In practice, composites materials
were prepared by impregnating pine sapwood wood with
nano-scaled precursor solutions derived from titanium(IV)
isopropoxide followed by a thermal curing process. Thermal
and fire properties were evaluated by thermal analysis and
cone calorimetry, whereas flammability was specified by
oxygen index (LOI) and UL 94 test. Peak heat release rates
were moderately reduced indicating fire retardance potential
in terms of flame spread attributed to the appropriate protection
layer action of the titania-based depositions. LOI
(oxygen index) values of these composites were increased up
to 38 vol.% in comparison to 23 vol.% for untreated wood.
The flame retardancy performance depends on the fire scenario
and is strongly influenced by wood loading and crackfree
deposition of the titania layers inside the composite.
The influence of weathering on the fire retardancy of polymers is investigated by means of a cone calorimeter test, before and after artificial weathering. The surface degradation was monitored using different techniques (ATR–FTIR, microscopy, colour measurement). Different kinds of polymeric materials were chosen, all as they are used in practice: polycarbonate (PC) blends, polyamide (PA) and polypropylene (PP) flame-retarded with arylphosphate, melamine cyanurate (MC) and intumescent formulation based on ammonium polyphosphate (APP), respectively.
All samples show material degradation at the surface due to weathering. No significant weathering influence occurs on the flame retardancy when it is a bulk property, as was observed for aryl phosphates in PC blends and MC in PA. When the fire retardancy is dominated by a surface mechanism, dependence on the duration of weathering is detected: for intumescent formulations based on ammonium APP in PP, a worsening in the formation of the intumescent network was observed.
Eyewitnesses describe burning pavement surfaces in extreme fire scenarios. However, it was believed that the pavement plays a negligible role in comparison to other items feeding such an extreme fire at the same time. The asphalt mixtures used differ widely, thus raising the question as to whether this conclusion holds for all kinds of such materials. Three different kinds of asphalt mixtures were investigated with the aim of benchmarking the fire risks. Cone calorimeter tests are performed at an irradiance of 70kWm-2. All three investigated asphalts burn in extreme fire scenarios. The fire response (fire load, time to ignition, maximum heat release rate and smoke production) is quite different and varies by factors of up to 10 when compared to each other. The fire load per mass is always very low due to the high content of inert minerals, whereas the effective heat of combustion of the volatiles is quite typical of non-flame retarded organics. The heat release rate and fire growth indices are strongly dependent on the fire residue and thus the kind of mineral filler used. Comparing with polymeric materials, the investigated Mastic Asphalt and Stone Mastic Asphalt may be called intrinsically flame resistant, whereas the investigated Special Asphalt showed a pronouncedly greater fire risk with respect to causing fire growth and smoke. Thus the question is raised as to whether the use of certain kinds of asphalts in tunnels must be reconsidered. Apart from the binder used, the study also indicates varying the kind of aggregate as a possible route to eliminate the problem.
The fire retardancy mechanism of aluminium diethyl phosphinate (AlPi) and AlPi in combination with melamine polyphosphate (MPP) was investigated in glass-fibre reinforced polyamide 6 (PA6/GF) by analysing the pyrolysis, flammability and fire behaviour. AlPi in PA6/GF-AlPi partly vaporises as AlPi and partly decomposes to volatile diethylphosphinic acid (subsequently called phosphinic acid) and aluminium phosphate residue. In fire a predominant gas-phase action was observed, but the material did not reach a V-0 classification for the moderate additive content used. For the combination of both AlPi and MPP in PA6/GF-AlPi-MPP a synergistic effect occurred, because of the reaction of MPP with AlPi. Aluminium phosphate is formed in the residue and melamine and phosphinic acid are released in the gas phase. The aluminium phosphate acts as a barrier for fuel and heat transport, whereas the melamine release results in fuel dilution and the phosphinic acid formation in flame inhibition. The higher amount of aluminium phosphate in PA6/GF-AlPi-MPP stabilised the residue in flammability tests in comparison to PA6/GF-AlPi, so that this material achieved a V-0 classification in the UL 94 test.