This study examined the feasibility of citric acid (CA) as a formaldehyde free and non-toxic binder for the fabrication of ultra-low-density hemp hurd particleboard (ULHPB). Panels were produced using three particle size categories, two CA ratios and two pressing times. Physico-mechanical characteristics were evaluated following Australian standard AS/NZS 1859.1 (2017) for reconstituted wood-based panels. Thermo-chemical analyses (Py-GC/MS, FTIR, and TGA) were conducted to understand the binding mechanisms facilitated by CA. Furthermore, both retted and unretted hemp hurd (not utilised in ULHPB) were analysed to assess the impact of provenance and maturity stage.
In most CA-ULHPB variants particle size had a significant impact on panel expansion (springback), internal bond strength, water absorption, and thickness swelling. Superior strength properties and dimensional stability were observed in panels with greater CA ratio and longer pressing time. The thermo-chemical analyses indicated the formation of ester linkages with carbohydrate-derived OH-groups and the involvement of lignin moieties in the CA-ULHPB and thereby confirmed the hypothesised binding mechanisms.
With regards to the raw materials, thermo-chemical analyses identified significant influences of cultivar, maturity stage, and processing method on the hurd’s chemical composition, functional groups, and thermal stability. For example, Py-GC/MS revealed variations in the lignin-to-carbohydrate ratio and absence of fatty acids in some cultivars. FTIR confirmed consistent functional groups, but differences in peak intensities reflected carbohydrate variations potentially affecting OH-group availability for interparticle bonding in bio-composites. Shorter retting enhanced thermal resistance initially, but prolonged retting accelerated degradation. Variances identified underscore the importance of investigating provenance, maturity, and processing to ensure effective agronomic practices that align the raw material characteristics with the specific requirements of intended applications.
The expertise acquired through this PhD research seamlessly integrates and contributes to the objectives of the Hemp CRC-P “Healing Australian Carbon Wounds Using Hemp Plantation and Construction”. Present efforts are focused on investigating the impact of storage and processing on the quality of raw hemp biomass, as well as conducting fundamental research to ensure that the characteristics of the raw material are optimally suited for their intended application in final building components. Emphasis is placed on developing eco-responsible composites through the exploration of e.g. raw material pre-treatments, enhancing bio binders and/or additives and optimising processing parameters. Throughout the research, it is ensured that the final products uphold desired characteristics that meet established performance standards, are adaptable for scalability, and seamlessly fit into existing logistics and industry infrastructure.