Camellia oleifera shell powder and palm kernel meal as an environmentally-friendly, low-cost compound filler in MUF adhesive for plywood preparation

Industrial flour served as a classical filler in wood industry for many years. However, its high-cost and competition for use as food stuff paved the way to a novel substitution tactic by using agriculture and forestry waste residues. Camellia oleifera shell and palm kernel meal are both by-products of oil-refining with advantages such as wide availability, great-reserve, renewability, and low-cost. Herein, a compound filler strategy was proposed by combining Camellia oleifera shell powder (Cosp) and palm kernel meal (PKM) to solve some drawbacks while Cosp as a single filler. The results showed that the bonding performance of plywood incorporating 20% single Cosp modified MUF was inferior with respect to MUF modified by same amount of industrial flour, due to the significantly increased viscosity, leading to a decrease in wet shear strength (1.63 MPa and 1.47 MPa for cold and hot-water soaking pretreatment, respectively). Encouraged, the overall status (including the viscosity, gel time, and solid content) of MUF modified by the compound filler displayed is regarded the best formulation for Coaps to PKM weight ratio of 5:5. Therefore, the wet shear strength for derived plywood samples with the adhesive incorporating 20% filler content, can reach 1.90 MPa and 1.88 MPa for cold and hot-water soaking pre-treatment, respectively. Interestingly, this bonding performance is similar to MUF resin modified by industrial flour. Thermogravimetric analysis (TGA), fourier transform infra-red spectroscopy (FT-IR), and scanning electron microscopy (SEM) were utilized to further confirm the potency of the compound filler modification strategy. Furthermore, the formaldehyde emission of MUF resin modified by the compound filler was effectively reduced by 8.24–14.12%, compared to industrial flour as filler. Cosp and PKM, as agricultural and forestry waste residue, exhibited the cost advantage for this approach based on calculation, which can save the cost of adhesive by over 40% for filler. The strategy for employing this compound filler provides a sustainable and environmentally friendly approach for green application of waste residues obtained from agriculture and forestry.