Effect of Steam-Treatment Duration on the Physical and Mechanical Properties of Three-Layer Betung Bamboo (Dendrocalamus asper) Plybamboo Board

Betung bamboo (Dendrocalamus asper) is a fast-growing, carbon-storing lignocellulosic resource that can relieve harvesting pressure on tropical hardwood, but its high starch and extractive content impairs adhesion and weakens bonded bamboo panels. Steam treatment offers a solvent- and chemical-free route to degrade these components, yet the exposure time best suited to a three-layer, poly(vinyl acetate) (PVAc)-bonded plybamboo board (PBB) has not been established for this sheet-based, cross-laminated architecture. We manufactured cross-laminated PBB (30×30×1.5 cm) from betung slats steamed with saturated steam at 126 °C for 0, 30, 45, and 60 min and measured density, moisture content, water absorption, thickness swelling, delamination, and static bending properties parallel and perpendicular to the grain. A short 30 min treatment gave the best mechanical profile, raising density by 11%, lowering water absorption by 19% and thickness swelling by 42%, and increasing parallel-grain modulus of elasticity and rupture by 82% and 56% relative to the untreated control; perpendicular-grain bending was statistically unchanged. The stiffness gain persists after density normalisation: the density-normalised (specific) modulus rose 64%, and panel density explained only 40% of the cross-panel variance in modulus. Longer treatment continued to improve dimensional stability (thickness swelling down 65% at 60 min, significantly below the 30 min value, and delamination down 54%) but progressively lowered density and strength. Measured moisture contents were below the SNI 8032:2014 reference limit (≤14%), and parallel-grain bending strengths exceeded the SNI 01-5008.7-1999 reference value, but none reached the intended 0.7 g cm−3 target density, and modulus of elasticity and perpendicular-grain strength fell short of the structural thresholds. Within this green, water-only process panel densification and adhesive formulation and application warrant further investigation, indicating where future optimisation of sustainable plybamboo should be directed.