Abstract
The effect of wet-combination of paper plies on some basic strength properties was investigated by using a series of wet-combined papers from lightly and heavily beaten pulps. As the basic in-plane and out-of plane strength properties, tensile, compressive, and internal bond strength were measured. Microfailures which are detectable as acoustic emissions (AEs) and occurred during the tensile deforming process were further examined to investigate the effect of wet-combination of paper plies on in-plane tensile strength. The results showed that an increase in paper ply by wet-combination brought about an increase in the plastic deformation region, leading to higher strain at breakage and an increase in tensile strength as a result of the higher strain at breakage rather than that of the increase in fiber bonding. Compared with homogenous paper made by pulp mixing, the presence of a strong paper ply within paper layer by wet-combination, irrespective of its z-directional position, may increase the period of gradual increase in microfailures at the plastic deformation region and delay the acute occurrence of microfailures just before the maximum load. The increase in microfailures accompanying the increased period of the plastic deformation by wet combination had little effect on the origin of microfailures. Strong paper plies located at both outer sides could prevent the development of microfailures connecting to paper sheet failure, and eventually resulting in higher strain at breakage and an increased tensile strength. In contrast to tensile strength, compressive strength in the X-Y direction was simply determined by the content of highly beaten pulp, regardless of the manner of combination, i. e., wet-combination or pulp mixing. Tensile strength in the Z-direction, which is evaluated as internal bond strength, was determined by that of the weakest paper ply and wet combination brought about a further decrease in the Z-directional tensile strength.
