Experimental Imaging of fluid penetration into papers

We use cryo-scanning electron microscopy to investigate capillary dominated fluid penetration into a range of papers including unfilled kraft and TMP laboratory sheets, PCC and talc-filled kraft papers with variable filler loadings, laboratory sheets with variable fines contents and a range of papers with differing degrees of sizing. We find that in all unsized samples the penetration of the wetting fluid is along the largest channels that form a connected pathway for fluid flow. In most cases in the form of bulk liquid films along channels formed by overlap of fibre components. This is in contrast to the description of fluid penetration, where the primary flow mechanism is based on the bulk filling of pores. Results on hard sized papers show that the only mechanism is via intra-fibre flow. In intermediate sized papers a mechanism combining intra-fibre flow, break-down of the sizing effect and subsequent infilling of channels and pores is observed. INTRODUCTION The structure of the pore space of paper, together with local surface energy considerations are the chief determinants of fluid penetration processes [1, 2]. Understanding these relationships and their implications for paper performance will assist in the design of products and troubleshooting problems. We describe a study based on cryo-scanning electron microscopy to investigate capillary dominated fluid penetration into a range of papers including simple cellulose laboratory sheets, PCC and talc filled papers with variable filler loadings, laboratory sheets with variables fines contents and a range of sized papers. Unfilled test papers were prepared from softwood bleached Kraft fibres (Figure 1a) and TMP (Figure 1b). TMP fibres are pulped mechanically and therefore have a higher rigidity than chemically pulped Kraft fibres. Due to their high rigidity they become less intertwined than Kraft fibres when paper sheets are formed. As a consequence, the average number of fibre overlaps per unit area (which we will call fibre overlap density) in TMP paper is lower than in Kraft paper. To eliminate any influence of selfsizing, papers were rendered hydrophilic by treating them with water plasma prior to the cryo-SEM experiment. Two filled papers made from the bleached kraft pulp were studied with filler; one with 30% talc filler and one with 30% precipitated calcium carbonate (PCC) filler. The distribution of the fillers differs. With PCC filled papers the filler tended to aggregate into large clumps up to 30μm in diameter and was heterogeneously distributed throughout the paper (Figure 1c). Many fibre surfaces exhibit no filler coverage while others show a complete coverage. This was not the case for talc filled papers where the filler tended to be concentrated around the edges rather than within the pores (Figure 1d). Furthermore we studied test papers, where Kraft fibres were mixed with various amounts of fines. It was found that fines accumulate in the fibre overlap channels and in the pore spaces of the paper sheet. In samples with fine contents of up to 15% the actual fibre network is visible and appears to be obstructed with fines, whereas in samples with fine contents of 45% the fibre network is completely embedded in fines and appears to be lost. The tests of sized papers included lab samples that were internally sized with AKD at a low (0.04%) and a high solid AKD content (0.1%). Cobb60 tests were undertaken on the samples and those with a lower solids content of AKD were shown to be weakly or medium sized whereas samples containing a higher solid content of AKD were strongly sized. EXPERIMENTAL The experimental method had to ensure that 1) droplets were maintained in a frozen state for long periods, 2) samples had to be frozen quickly and 3) a high contrast liquid had to be used to ensure differentiation from fibres. Cryo-SEM was chosen as Figure1 (left). SEM images of laboratory paper a: unfilled kraft, b: unfilled TMP, c: with 30% PCC. Note tendency for the PCC particles to aggregate into clumps in pores and d: with 30% talc. Note tendency for the talc particles to be distributed around and not in the larger pores. it would satisfy all of these requirements. The microscope was a Cambridge Instruments S360 Stereoscan SEM fitted with a high brightness LaB6 electron source, a backscattered electron detector fitted with an Oxford CT1500B Cryotrans coldstage/coating unit. The test paper was cut to approximately 10 x 20 mm and glued on a sample stub with a small amount of electroconductive tissue freezing medium. A small drop of 2M CsI solution was then placed on the test paper. After a period determined by the type of paper the drop spread a noticeable distance, at which point the stub was plunged rapidly into a nitrogen slush. The rapidly frozen sample was then transferred into the cryogenic stage of the SEM and maintained at –80°C under vacuum in order to remove adventitious ice crystals that result from the condensation of atmospheric moisture. Figure 2. a: A low magnification image of wetting of kraft and b: TMP paper. Scale bar = 1mm c