Strength variations in paper containing a sharp notch

This Study investigates the hypothesis that the local material distribution around a defect determines the variation in individual critical strength of notched paper. Three paper grades, SC, newsprint and copy paper were Studied. We used 40 samples, each 100 x 50 mm from each paper grade, each sample given a 6 mm edge cut. The samples were analyzed using a beta-formation apparatus (Fujifilm FORMEX/BAS-1800). Areas of different sizes and shapes at the crack-tip were then analyzed with respect to various basis weight statistics. Statistical parameters of the local basis weights such as the mean, the standard deviation and the minimum value were correlated to the strength of the individual paper samples. For copy paper, a highly significant linear correlation was found between strength and the local mean and minimum basis weights, for areas in the range 3-10 mm(2). Correlations were found to a smaller degree for the newsprint and SC grade. An explanation for this difference in correlation has been found in the distribution of fiber floes and anti-flocs. In the SC and newsprint grades, floes and anti-flocs are smaller and tend to be oriented more perpendicular (MD) to the crack. In the copy grade the floes are larger and their orientations are more uniformly distributed. This method of studying paper strength might give novel information about small scale paper properties - not necessarily indicated by the formation - that is important for fracture properties of paper. The method may also be developed to a tool for the paper-maker to assess the contribution of formation to runnability.