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

Abstract

Abstract The aim of the research was to develop densification characteristics of raw, milled and cut-milled pine and poplar shavings and to determine the strength parameters of pellets, pastilles and granules. To produce agglomerates from hard pine shavings vs. plastic poplar shavings, 19% more specific compaction work was needed and more than 2-times more specific work of pushing the agglomerate out of the die opening. Pine agglomerates had a lower linear expansion than poplar agglomerates and reached a higher single density, and due to the higher content of thermoplastic lignin in the wood, 30.7% and 18.4%, respectively, they were characterised by better radial compression strength parameters; specific deformation energy, maximum tensile stresses at which agglomerates cracked and the largest modulus of elasticity. Milled shavings with a size of 0.62 mm, and especially cut-milled shavings with a size of 0.48 mm and lower cohesion and internal friction angle, were characterised by better compaction parameters than raw shavings with a size of 6.07 mm. Agglomerates made of cut-milled shavings had the highest single density, but the tensile strength was much lower than agglomerates made of raw shavings. The susceptibility to densification of the shavings during sequentially repeated densification of small doses of shavings during the production of pellets was the highest, and the pellets were characterised by the smallest linear and radial expansion and the highest single density of 1081 kg·m–3 and tensile strength among agglomerates. 67% more shavings dose when making pastilles vs. pellets was unfavourable, as it required much more compaction work, and the single density of pastilles was lower by 9.5% and their strength under radial load was significantly reduced. The smallest single density and strength were found in granules produced with the parameters recommended for the production of particleboards; temperature of 170°C and agglomeration pressure of 12 MPa vs. 93°C and 70 MPa for pellets and pastilles, respectively. The higher temperature did not compensate the much lower pressure. Shavings compaction parameters for granules cannot be recommended for the production of a particleboard without a binder, which is usually urea-formaldehyde resin. Along with the die opening diameter, the radial expansion and modulus of elasticity decreased, but other compaction and strength parameters practically did not change.