Innovative Water-Resistant Fire-Retardant Wood incorporating Ammonium Phosphate-based salts for Exterior Use Conditions

Abstract: Wood is a naturally based material and plays an important role as a renewable resource when aiming for a sustainable society. Nevertheless, its inherently combustible property needs to be enhanced to comply with modern construction methods and regulations. Ammonium phosphate-based additives are often used to effectively increase the fire-retardancy (FR) of wooden products when needed. However, their water-solubility make them unsuitable for exterior use unless such properties are overcome. This PhD thesis focuses on the development of methodologies to alleviate consequences from water-leaching of wood treated with ammonium phosphate-based additives, enabling further development of new types of fire protected wood construction material for exterior uses. As such, two methodologies were explored in this thesis: 1) a composite-type fixation, involving the introduction of a hydrophobic polymer matrix for entrapping the FR additives, 2) a reactive-type fixation, which is to fix the FR additives by creating covalent bonding with the wood polymeric constituents.In order to determine the influence of composite-type fixation systems, melamine-formaldehyde (MF) prepolymer, furfuryl alcohol (FA), and kraft lignin, respectively, were studied for immobilising ammonium phosphate-based FR-additives in wood by pressure impregnating Scots pine (Pinus sylvestris L.) sapwood with a solution of the prepolymer and fire-retardant additives (FRs), followed by drying and heating steps. Through the analysis of the treated wood materials, involving scanning electron microscopy energy dispersive spectroscopy (SEM-EDX) and thermal gravimetric analysis (TGA), the formation of a stable polymeric network structure entrapping the additives inside the wood with alleviating FRs’ water-leachability was proposed. In particular, MF-resin was able to encapsulate guanyl-urea phosphate (GUP) in the lumen of the wood. Further details on the distribution and structural features of FR additives and matrices within the wood structure are described in the thesis. Furthermore, the ability of FRs comprising ammonium dihydrogen phosphate (ADP) and urea to be fixed within the wood structure without the addition of polymeric materials was also investigated. This approach was accomplished by impregnating an aqueous solution containing the aforementioned additives, followed by drying and further heat treatment at 150°C. By analysis of the treated wood material by solid-state nuclear magnetic resonance (NMR), X-ray photoelectron spectroscopy (XPS), and FTIR, the formation of a reactive-type fixation of FR to the wood structure was realised. By this way, phosphate and carbamylate groups from treated ADP/urea were found to have reacted with hydroxyl groups of the wood polymers. The fire performance of FR-treated wood materials was studied with a series of modern techniques, namely, limited oxygen index (LOI), microscale combustion calorimeter (MCC), and cone calorimeter tests. LOI and MCC were used as a simple test of fire stability of FR-treated and subsequently water exposed wood. By applying the cone calorimeter test, the predicted reaction-to-fire classification of FR-MF, FR-FA, and phosphorylated/carbamylated wood was established and, actually, reached the highest possible classification, class B. This classification was held even after the accelerated ageing test according to the European standard EN 84. In summary, this further suggested that these methodologies, which enhanced the water-leaching resistance of ammonium phosphate-based salts, have the potential to give a fire-retardant wood suitable for exterior uses.