Pressure agglomeration of raw, milled and cut-milled pine and poplar shavings: assessment of the compaction process and agglomerate strength

The aim of the study was to investigate the densification characteristics of raw, milled, and cut-milled pine and poplar shavings and determine the strength parameters of pellets, pastilles, and granules. In producing agglomerates from hard pine shavings compared to plastic poplar shavings, 19% more specific compaction work was required with over 2-times more specific work to push the agglomerate out of the die opening. Pine agglomerates exhibited lower linear expansion than poplar agglomerates, achieving a higher single density. Due to the elevated content of thermoplastic lignin in the wood (30.7 and 18.4%, respectively), pine agglomerates demonstrated superior radial compression strength parameters, including specific deformation energy, maximum tensile stresses at which agglomerates cracked, and the highest modulus of elasticity. Agglomerates made of cut-milled shavings had the highest single density, but their tensile strength was significantly lower than that of agglomerates made from raw shavings. The susceptibility to densification of the shavings during sequentially repeated densification of small doses during pellet production was the highest, resulting in pellets characterised by the smallest linear and radial expansion, as well as the highest single density of 1081 kg·m^–3 and tensile strength among agglomerates. The smallest single density and strength were observed in granules produced with parameters recommended for particleboard production: a temperature of 170 °C and an agglomeration pressure of 12 MPa, compared to 93 °C and 70 MPa for pellets and pastilles, respectively. The higher temperature did not compensate for the much lower pressure. Shavings compaction parameters for granules are not recommended for particleboard production without a binder, typically urea–formaldehyde resin.