X-ray nanotomography of individual fibre bonds: The effect of fibre wall thickness on contact area

Fibre bonds play an essential role in optical and mechanical properties of paper. Much research has focused on the strength of the bonds but the area of the individual fibre bonds has recently been of interest as well. Many of the research methods rely on optical tools which are restricted by the used wavelength of light. Since the bonding happens in the nanometer scale the highest optical resolution of 200 nm may not be accurate enough to capture the smallest variations in distance between the fibres in the intersection area.X-ray computed tomography utilises X-rays in studying the inner structure of materials. The method is based on X-ray absorption imaging that produces a spatial distribution of attenuation according to the density variation in the sample. After imaging the sample from different angles a reconstruction algorithm is used to recreate a three-dimensional model of the imaged sample. Novel X-ray devices surpass the optical methods in terms of resolution and, in particular, facilitate obtaining a detailed three dimensional image of the sample.X-ray nanotomography has previously been found to be a competent tool in the study of individual fibre bonds . As a continuation to the previous study, the method was used to successfully image 26 individual cellulose fibre bonds made of springwood and summerwood fibres of bleached softwood kraft pulp refined to 25o SR. Various dimensional properties of spring-to-springwood, summer-to-summerwood and spring-to-summerwood bonds were measured, most importantly the relative contact area (RCA, apparent contact area/overlapping area). The RCA values showed a wide variation from 6.4% to 85% with an average value of 57.7 %. Although the summerwood bonds had a somewhat smaller overlapping area and contact area than springwood bonds there were no significant differences in the RCA between the bond types. This suggests that the effect of absolute and relative contact area on the strength differences between the bond types seems to be minor. Typically the contact area in the bond was composed of several separate areas and the edge areas had a clearly higher probability to remain unbonded. The total thickness of the overlapping area correlated with the contact probability probably due to the effect of varying wet pressing pressure.