Effects of fire retardants on the pyrolysis of Pinus halepensis needles using microscopic techniques

Two microscopic techniques, transmission light microscopy (LM) and stereoscopy, were used for studying the effect of fire retardants on the pyrolysis of Pinus halepensis needles. Pure (NH4)(2)HPO4 and a commercial product (Fire Trol), based on polyphosphates as the active chemical retardant, were tested as fire retardants. With increasing pyrolysis temperature, the loss of the cuticle layer, the cracking of the epidermal cells, the charification of the inner part and the degradation of the lignin content of the needles were observed by LM. LM showed that all these phenomena were accelerated (shifted to lower temperatures) in those treated with fire-retardant needles compared with the untreated ones. Stereoscopy was used for monitoring changes on the external surface of the pyrolyzed needles such as changes concerning the retardant itself and the tar and char formation. It appeared that, in the presence of the retardants, tar and char formed at lower pyrolysis temperatures and at a greater extent compared with the untreated needles. In all cases studied, the commercial product appeared to be more effective than the pure (NH4)(2)HPO4. The recorded observations were associated with the chemical and physical phenomena occurring during the pyrolysis procedure and the theory of the fire retardation action of the chemicals. (C) 2002 Elsevier Science B.V. All rights reserved.