Reinforcement ability of mechanical pulp fibres

The objective of this study was to find out the reasons why the long fibres of mechanical pulp do not seem to reinforce paper as effectively as chemical reinforcement pulp. A preliminary laboratory trial showed that artificially increasing the average fibre length of TMP pulp by adding long fibres extracted from the same pulp increased the tear index, but decreased the tensile strength, internal bond strength and the fracture energy. Increasing the average fibre strength with chemical (NBSK) pulp fibres improved all of those properties considerably. In the second trial fibre properties and reinforcement ability of various mechanical pulps were investigated. It was shown that fibre dimensions of mechanical pulp fibres did not differ essentially from chemical pulp fibres. The biggest differences were in the properties characterizing the cell wall structure. This was clearly seen in fibre flexibility and fibre swelling (WRV), for instance. Mechanical pulp fibres are evidently more damaged than chemical pulp fibres which is seen as a much lower fibre strength (zero-span tensile strength). The reinforcement potential, on the grounds of fracture energy, tear strength and tensile strength of handsheets was much lower for mechanical pulp fibres than for chemical pulp. In the third trial, mechanical (MRP) and chemimechanical reinforcement pulp (CMRP) was manufactured from Norway spruce (P.abies) on a pilot scale. The focus was to increase fibre flexibility, bonding ability and maintain the fibre length and strength. The runnability of LWC base paper made from the trial pulps was tested using the KCL AHMA runnability tester. In spite of the good strength properties of the trial pulps, they did not have the same overall reinforcement ability than chemical pulp. The sulphonated trial pulp (CMRP) gave the same tensile stiffness and tensile strength as the chemical pulp. However, the fracture properties and extensibility of the paper was worse with it. The lower average length of the trial pulps did not explain the difference totally. Scaling the fibre length with the zero-span tensile strength improved the explanatory power essentially. It was concluded that the low fibre strength is the basic reason for the poorer reinforcement ability of mechanical pulps fibres over chemical ones.