The Effect of Moisture and Structure on Wet Web Strength and Its Variation – A Pilot Scale Approach Using Dry and Rewetted Mill Made Papers

In spite of extensive research on wet web strength properties and rheology, knowledge of the effect of web structure, e.g. formation and fibre orientation, on wet web strength properties has been limited. Therefore the topic was studied by running the re-wetted mill-made paper reels on a pilot runnability device at low dry solids contents of 56% and 68%. In addition, one trial was conducted by measuring the wet web strength properties in situ on the press section of a pilot Fourdrinier. The bene t of both these approaches is the ability to measure the strength properties in more realistic conditions compared with standard laboratory methods.

In order to differentiate between the effect of formation and fibre orientation on strength properties, the variables should not be correlated. This requirement was met in the main mill trial by suitably selecting the headbox and wire section parameters. Formation was measured using a -radiographic method and local grammage variation was examined as standard deviation in different wavelength bands and size classes. In addition, formation was also measured with Ambertec formation tester. Fibre orientation was determined using layered fibre orientation measurements.

It was shown that formation has an influence on the tensile strength variation and the effect depends on the scale of formation and dry solids content. In contrast to dry strength, the wet strength does not follow the Weibull distribution, but rather the Gaussian one. In addition, the distribution of wet strength is sensitive to centimetre-scale variability in paper structure instead of millimetre-scale in dry paper. When formation is good, as it typically is on modern paper machines, further improvement does not improve average wet strength properties. Only when large scale formation is poor, it has an influence on average wet web tensile strength and tensile stiffness. Presumably this would be the situation on a Fourdrinier type machine. Unlike formation, anisotropy does not affect the strength variation but it has an influence on the average tensile strength and tensile stiffness of wet and dry papers. The anisotropy profile in z-direction has no influence on the mentioned properties.