Effects of Particle Dimension and Constituent Proportions on Internal Bond Strength of Ultra-Low-Density Hemp Hurd Particleboard

Abstract

The recent legalisation of hemp seeds for human consumption has revitalised the cultivation of hemp in Australia. This provides opportunities for the valorisation of the stem’s residual xylemic core (hemp hurd). This study investigated the effect of particle dimension and constituent proportions on the internal bond strength (IB) of single-layer, ultra-low-density hemp hurd particleboard (ULHPB) with densities between 219 to 304 kg/m3. Particle size distributions (PSD) and granulometry assessments were conducted on three particle size classes (fine (F), medium (M), coarse (C)) based on digital image analysis using ImageJ. Subsequently, four particle size mixes (100% C, 100% M, 50/50% CM, 25/50/25% CMF) were considered for the ULHPB manufacture with bio-epoxy (EPX), phenol resorcinol formaldehyde (PRF) and emulsifiable methylene diphenyl diisocyanate (MDI) adhesives, respectively. The effect of particle loading and adhesive content varied significantly per adhesive type. Internal bond (IB) performance increased in most ULHPB comprising coarse particles and declined with the addition of smaller particle sizes. The granulometry assessment showed the smallest mean elongation amongst particles in the coarse PSD. The IB results confirmed a strong interdependence of particle size and constituent proportions and indicated that various MDI-ULHPB variants can surpass the minimum IB strength requirement of 0.30 MPa stipulated for standard particleboard (>12–22 mm) in AS/NZS 1859.1. Utilising residual hemp biomass as an alternative, renewable lignocellulosic feedstock in the manufacture of engineered lightweight panel products is a key principle of circular economy and an environmentally friendly strategy to address the increasing resource scarcity in the wood-based panel industry.