Flammability and burning behaviour of fire protected timber

Decarbonization has driven the construction industry to rediscover biobased, but inherently combustible, materials like timber. To avoid compromising fire safety, reaction to fire of timber can be effectively reduced by fire retardant surface coatings or impregnation treatments. Using a combination of simultaneous thermal analysis, microscale combustion calorimetry and cone calorimetry, vacuum-pressure impregnated (boron free, phosphorus-based) plywood was tested against plywood coated with a thin layer of water-based fire retardant intumescent coating (melamine free, phosphorus-based). Comparing the peak heat release rate (pHRR) and total heat release (THR) of the three plywood samples, the impregnated was lowest, and the coated was lower (pHRR −34% and −20%, THR -45% and −21% respectively) relative to the untreated plywood. In contrast, the coating layer postponed sustained flaming for longer than impregnated wood and delayed burnthrough, effects critical to the growth rate of a developing fire. Better understanding of the assessment of flammability of fire protected timber has been obtained using cone calorimeter data supported by microscale analyses. The results challenge the simple flammability ranking based on total heat release, and highlight the need for further development of a methodology for comparing different fire protection strategies for timber

Smoke toxicity of fire protecting timber treatments

Most fire deaths arise from inhalation of toxic gases. The fire toxicity of untreated plywood, pressure impregnated plywood and surface coated plywood was investigated under a range of fire conditions. Using the steady state tube furnace individual fire stages were replicated, fire effluent sampled, and toxic product yield determined by high performance ion chromatography and spectrophotometry. Despite different phosphorus loadings, both treatments hindered developing fires with less than 50% mass loss during non-flaming oxidative pyrolysis and did not readily undergo steady flaming. During under-ventilated flaming, all samples produced hydrogen cyanide (HCN), and both fire protecting treatments produced phosphoric acid (H3PO4). Assessment of smoke toxicity as fractional effective dose for incapacitation was based on asphyxiants carbon monoxide (CO) and hydrogen cyanide (HCN). This work demonstrates that due to emission of large amounts of CO, the predicted smoke toxicity of impregnated timber is significantly higher than coated timbers when a growing fire reaches the under-ventilated stage. The results have clear implications for those selecting products to ensure fire safety in enclosures such as buildings

Quantification of Hydrogen Cyanide in Fire Effluent

Hydrogen Cyanide (HCN) is often the most toxicologically significant component in fire effluents from nitrogen-containing materials. Unlike the other major asphyxiant, carbon monoxide, sensors for continuous HCN quantification, at and above dangerous concentrations, are not commercially available. This paper investigates the analysis of fire effluent captured in bubbler solutions, by colorimetric quantification of HCN using chloramine-T/isonicotinic acid. The bubbler samples were mixed with colorimetric reagents to give a blue dye in response to cyanide ions. A novel reaction scheme accounting for the formation of the blue dye from cyanide ions is presented. Dilute, standard cyanide solutions were found to be stable after storage for up to one year. Alkaline bubbler solutions, through which the fire effluent has passed, showed consistent cyanide concentrations, for samples stored between 5°C and 35°C, for up to 31 days after sampling. The effect of other common ions likely to be present in fire effluent solution samples (CO32-, SO32-, SO42-, NO2- and NO3-) was investigated for their potential interference. The most significant interference was sulphite which reduced the apparent cyanide concentration by 13% at 10 mg L-1 SO32- concentration