Fire behaviour of waterborne intumescent coatings on timber substrate for bushfire exposure

The fire behaviour of radiata pine timber, treated with intumescent coatings and exposed to heat flux conditions equivalent to a high-risk bushfire attack level (BAL) is investigated. The experiment is designed for coated timber exposed to bushfire attack levels greater than 19 kW/m² and up to 29 kW/m² (BAL-29) in accordance with AS 3959. The timber was coated with a DFT of 400 μm of the developed intumescent formulations based on expandable graphite (EG), ammonium polyphosphate (APP) and a waterborne acrylic resin with high adhesion strength. The coated timbers were tested according to AS3959, for a BAL-29 rating, using a cone calorimeter at 25 kW/m² for 10 min. Most of the coated samples reported a substantial reduction in the peak heat release rate (p-HRR) and average HRR with values not greater than 100 kW/m² and 60 kW/m², respectively. These results satisfy the requirements for bushfire-resistant timber at BAL-29 rating. Formulation 4 (F4), with EG:APP ratio of 3:1 and resin content of 45 wt% was identified as the most cost-effective formulation. Coatings with high loading of EG and APP provided the best fire protection to the timber substrate, however, a larger char with a less compact structure is formed.