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Fire retardants may considerably improve the 5re properties of wood products, but the durability, e.g. in exterior applications, has not been addressed fully. This paper reviews the existing knowledge and experience mainly from the USA with the aim of supporting further development in Europe. The review is concentrated on pressure impregnated 5re retardant treated wood products which have the best opportunities for increased durability.

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Beech wood boards were studied with a cone calorimeter. As the specific density of these materials is greater, the total amounts of CO and CO 2 produced were bigger than values measured on spruce specimens. Also, the shape of the heat release rate (HRR) curve was different as its second peak was bigger than the one observed on spruce wood. It was shown that impregnation of wood with water solutions of H 3 BO 3 resulted in decreased HRR values with increasing acid concentrations. Mass loss and CO and CO 2 production rates also decrease in the same way. Impregnation with water-soluble NaOH-silica resulted in improved data, when compared to untreated specimens, but were not better than those obtained with H 3 BO 3 . Simple mixing of both fire retardants gave no improvement, nor was the two-step impregnation with drying effective after the first step. When the second step-treatment with acid was followed without drying after impregnation with NaOH-silica in the first step, then improvement was evident. This is explained by precipitation of the silica content inside the material when the NaOH was neutralized with H 3 BO 3 , which could penetrate inside when the material was not dried before the second step. The improvement resulted in lower smoke production and longer times to ignition.