The effect of microfibrillated cellulose on the wet-web strength of paper

Abstract

The wet-web strength of paper immediately after the press section of a paper machine is a critical factor in determining machine runnability. However, it is difficult to determine at commercial scale, because the web has to be broken and production interrupted in order to obtain a sample for measurement. The use of microfibrillated cellulose (MFC) is believed to increase wet-web strength, as it has allowed filler level increases of 10% or more on many commercial paper machines. In this paper, we describe a laboratory method for estimating the effect of MFC on wet sheet strength after pressing, as well as actual measurements of wet-web strength from a pilot paper machine trial. These experiments have demonstrated the positive effect of MFC. At solids contents in the range typically observed after pressing, sheets with MFC at fixed filler content are significantly stronger, but also wetter, than those without it. When the use of MFC is combined with a typical increase in filler content, the wet web remains slightly stronger, but also becomes drier than the reference condition. These results are compatible with the theory put forward by van de Ven that wet-web strength is mainly a result of friction between entangled fibers, and they also suggest that the presence of MFC increases this friction.

Invited Perspective: Current and Potential Use of Highly Fibrillated Cellulose in the Paper and Board Industry

“…Note that whilst both MFC products have similar weight-averaged mean lengths (Lc(w) ISO) and % of fines, they differ greatly in the % of high aspect ratio fines. Figure 5 shows correlations between the high aspect ratio fines and the tensile index of sheets made from 80% filler grade calcium carbonate and 20% MFC, as well as the air permeability of sheets of the pure MFC [14]. Both of these properties are strongly enhanced by a high proportion of long, thin fibrils.…”

“…Studies show that, at low doses of MFC in paper, bonded area increases proportionately to the amount of MFC added. Figure 8 shows the calculated bonded area as a function of MFC addition for handsheets made with bleached Kraft pulp at low and high CaCO 3 filler contents [29]. Sheets made entirely from MFC or nanocellulose (often referred to as 'nanopaper') show almost zero air permeability and very low internal void space and can have densities approaching that of pure cellulose [30,31].…”

“…A consequence of this is that surface smoothness and formation may also be improved. However, in the presence of high loadings of mineral fillers, this does not lead to a loss in light scattering, as the filler particles prevent complete collapse of the fibril structure, resulting in a net increase in solid/air interface [29,36]. This is shown in Figure 7, where the scattering coefficient is shown as a function of tensile strength for a range of filler loadings, refining levels and MFC addition.…”

“…One of the most important effects of MFC addition at the papermachine wet end is its positive influence on wet web strength [38]. Given that a paper web contains too much water before entering the dryer section for hydrogen bonding to occur, the mechanism for this is believed to be entanglement friction between the fibres, which MFC increases [39].…”

“…Firstly, the creation of fibrils is decoupled from increasing fibre flexibility, and thus fibrillation can be pursued separately without the risk of losing fibre length and bulk. In some cases, this can enable tensile strength to be developed with less loss of tear strength that usually occurs with refining [29]. Secondly, studies have shown that MFC addition has a more profound effect on wet web strength than on dry strength, so that, for a given dry tensile strength, adding MFC will lead to a stronger web than refining.…”