The aim of this work was to gain a better understanding of the effect of reinforcement pulp beating on the strength of mechanical pulp-dominated paper. The main purpose of reinforcement pulp beating is to improve the runnability of paper. The first objective of this study was to maximize the runnability related strength properties by beating. It was assumed that the flaw-resisting ability of paper correlates with the runnability of the dry paper web. In-plane fracture properties were assumed to describe the flaw-resisting ability. The second objective was to understand the mechanism how beating affects paper strength and structure (e.g. the size of the fracture process zone).It was found that the reinforcement pulp ranking and optimisation of beating depend on the criteria used. The selection of critical strength properties affects the pulp ranking - sometimes different methods give opposite results (e.g. neutral sulphite vs. kraft). It was found that beating does not increase the in-plane fracture energy of mechanical pulp-dominated paper. Fracture toughness and elastic breaking strain increase only a little. However, tensile strength, elastic modulus and z-directional strength properties increase. It was concluded that beating does not significantly improve the flaw-resisting ability of mechanical pulp-dominated paper, while the strength of unflawed paper does increase.It was found that the fracture properties of mixture sheets cannot be estimated based only on fracture properties of pure components. The additivity behaviour of fracture properties was strongly non-linear. The fracture energy of pulp mixtures seems to correlate mainly with the average fibre length (and not with the beating level). Beating increases the fracture energy per fracture zone area in the pure chemical pulp sheets but not in the mixture sheets. Evidently beating does not increase the energy needed to open interfibre bonds in mechanical pulp-dominated paper.The results can be explained by the beating induced increase in both interfibre bonding and fibre segment activation. It was also found necessary to divide bonding into in-plane and out-of-plane components. The results largely support the activation theory originally presented in the 1960's. However, no direct measurement of activation was used in this study. Low-freeness mechanical pulp seems to have as high (in-plane) interfibre bonding as but lower activation than beaten reinforcement pulp. Therefore reinforcement pulp beating improves bonding-related properties such as elastic breaking strain or in-plane fracture energy only marginally. On the other hand, properties strongly dependent on activation, such as elastic modulus and tensile strength, are increased by reinforcement pulp beating.
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