The effect of recycling on chemical properties of thermomechanical pulps

With the enhanced use of recycled fibre, it has become increasingly important to know how fibre properties are affected by recycling. This study was designed to elucidate the effect of recycling on chemical properties of thermomechanical pulp of mature and juvenile wood of black spruce (Picea mariana), jack pine (Pinus banksiana), eastern white cedar (Thuja occidentalis), and Douglas fir (Pseudotsuga meniziesii). All the pulps were recycled four times. Recycling process included disintegration, sheet formation, pressing, and drying. No chemical was added during recycling. Amount of extractives, holocellulose, alphacellulose, lignin and ash were tested after each cycle. Trace elements (Na, Al, Mn, Cl, and Ca) were also examined on each recycle. The extractives contents were reduced with every recycle with an overall reduction of 70 to 85%. In the case of lignin, trend was different from one cycle to another cycle but was more or less same for all the pulps. The amount of lignin was reduced by 5% after four cycles. The reverse trends were found with holocellulose with an overall increase of 4%. No statistical significant trend was found for ash contents. Calcium and manganese concentrations were reduced for all pulps while chlorine and sodium had no specific trend. Aluminium concentration was too small to infer any trend. No interspecies and interwood effect were found on major chemical constituents of pulp.