Browsing by Author "Maloney, Thaddeus, Professor, Aalto University"

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The effect of web structure on wet web runnability
(2012) Ora, Markku
School of Chemical Technology | Doctoral dissertation (monograph)
The effect of paper structure on wet web strength properties was studied by running the re-wetted mill-made paper reels on a pilot runnability device at the dry solids contents (DSC) typical to the press section and the first part of the drying section. In order to run those reels with low DSC, a new re-wetting procedure was developed. In addition, one trial was conducted with measuring the wet web strength in situ on the press section of a pilot Fourdrinier. The benefit of these approaches is the ability to measure the strength properties in more realistic conditions compared with standard laboratory methods. According to the results, if formation is good - as it presumably is on modern paper machines - further formation improvement does not improve average wet strength properties. Only when formation is poor - as it can be on a Fourdrinier-type machine - does improved formation result in improved average wet tensile strength and tensile stiffness. Instead, the fibre orientation had a strong effect on these strength properties. Interestingly, the orientation profile in the z-direction did not affect the wet strength properties if the average orientation remained constant. This suggests that it does not matter whether the machine runs the jet-wire-ratio in the rush or drag side. Formation has an influence on the tensile strength variation of wet and dry papers. In addition, the effect depends on the scale of formation and dry solids content. In wet papers, large scale formation has the strongest effect on the strength variation. Conversely, in dry papers the strength variation is controlled by small scale formation. In addition, it is suggested that the poorer the formation the lower the DSC at which the variability due to the effect of formation appears. Contrary to formation, fibre orientation does not affect the strength variation. The strength distribution of dry paper can be described by an extreme value model, such as the 2-parameter Weibull. Instead, the strength distribution of wet webs appears Gaussian, meaning that the wet papers do not show either weak-link behaviour or weak-link scaling. In addition, even large defects do not impair wet web runnability on the press section or in the first part of the drying section, at least if the defects are not located just at the edge of the web.