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.

---

The materials for developing the intumescent coatings are shown in Table 1. The additive Dysperbyk-199 was added to improve the dispersion of the fillers in the resin and wettability of the coating. A low concentration of titanium dioxide (TiO2) was used as a pigment in the intumescent coatings as it is commonly used in paints. In addition, TiO2 can potentially enhance the performance of the intumescent coating [[29], [30], [31]]. The radiata-pine timber was provided by Bunnings Warehouse in Sydney-Australia with a thickness of 1.9 cm, density of 0.53 gr/cm3 and moisture content of around 12% The waterborne acrylic resin was supplied by Unitex, Australia.

---

Enhanced Fire Protection:The development of intumescent coatings that can achieve or exceed the BAL-29 rating provides better protection for timber structures in bushfire-prone areas, potentially saving lives and reducing property damage.

Cost-Effective Solutions:The study identifies formulations that are cost-effective, such as the EG ratio of 3:1 with lower resin content, making these solutions more accessible for widespread use, particularly in rural and economically constrained areas.

Environmental Sustainability:The use of waterborne acrylic resins and eco-friendly components in the coatings aligns with environmental sustainability goals, reducing the ecological footprint compared to traditional solvent-based coatings.

Scientific Advancement:The research contributes to the scientific understanding of the interactions between different flame retardants and their effects on the char formation and overall fire resistance of intumescent coatings.

Reduction in Economic Losses:By improving the fire resistance of timber structures, the implementation of these coatings can significantly reduce economic losses from bushfires, which have been substantial in recent years.

Increased Safety:Enhanced fire protection for wooden houses can lead to fewer casualties and safer living conditions for people in bushfire-prone areas.

Market Growth:The findings can stimulate the market for advanced fire-retardant coatings, encouraging further research and development, and creating economic opportunities in the fire protection industry.

---

The residual char of the coated samples was examined by FESEM and illustrated in Fig. 9. The images of the char residue were taken from the section closer to the substrate. The FESEM images revealed features of the char generated by the intumescent coatings. Agglomeration and other features are observed, suggesting an incomplete reaction of the fillers in the formulations. The coating features, shown in Fig. 9 B2, are related to an incomplete reaction of the APP, as it melts at temperatures above 280 °C, releasing ammonia and remaining the crosslinked polyphosphoric acid in a melted state and a glassy surface condition when it solidify. The degradation of the polyphosphoric acid occurs at temperatures above 550 °C [25]. This temperature might not be reached along the char thickness, and the remaining of this acid in the char can potentially contribute to the char strength. The bottom of the char in coatings F6 to F9 (Fig. 9F-I) show a more incomplete reaction of the FR fillers, despite the large char expansion.

Increasing Frequency and Severity of Bushfires:Bushfires are becoming more frequent and severe, causing significant damage to properties and the environment. This increasing risk drives the demand for effective fire protection solutions.

Economic Impact:The economic impact of bushfires is substantial, as evidenced by the $148.5 billion impact of the 2018 California wildfire. Investing in preventive measures like fire-retardant coatings can be a cost-effective strategy to mitigate such losses.

Current Limitations of Existing Solutions:The article highlights that current intumescent coatings have limitations in providing effective protection at higher bushfire attack levels (BAL-40 and BAL-FZ). This presents an opportunity for the development of more advanced coatings that can offer better protection.

Cost-Effectiveness:The study identifies cost-effective formulations, such as those with an EG ratio of 3:1. Cost-effective solutions are crucial for widespread adoption, especially in rural areas with wooden dwellings.

Environmental Considerations:The development of eco-friendly, waterborne intumescent coatings aligns with the increasing emphasis on sustainable and environmentally friendly building materials.