Porous structure of fibre networks formed by a foaming process: a comparative study of different characterization techniques

Abstract

Recent developments in making fibre materials using the foam-forming technology have raised a need to characterize the porous structure at low material density. In order to find an effective choice among all structure-characterization methods, both two-dimensional and three-dimensional techniques were used to explore the porous structure of foam-formed samples made with two different types of cellulose fibre. These techniques included X-ray microtomography, scanning electron microscopy, light microscopy, direct surface imaging using a CCD camera and mercury intrusion porosimetry. The mean pore radius for a varying type of fibre and for varying foam properties was described similarly by all imaging methods. X-ray microtomography provided the most extensive information about the sheet structure, and showed more pronounced effects of varying foam properties than the two-dimensional imaging techniques. The two-dimensional methods slightly underestimated the mean pore size of samples containing stiff CTMP fibres with void radii exceeding 100 μm, and overestimated the pore size for the samples containing flexible kraft fibres with all void radii below 100 μm. The direct rapid surface imaging with a CCD camera showed surprisingly strong agreement with the other imaging techniques. Mercury intrusion porosimetry was able to characterize pore sizes also in the submicron region and led to an increased relative volume of the pores in the range of the mean bubble size of the foam. This may be related to the penetration channels created by the foam-fibre interaction.

Foam forming of fiber products: a review

“…The normalized frequency of various diameters gives the volume-weighted pore-size distribution. [21,124] As said earlier, the structure of the wet foam leaves special traces in the formed fiber network. This is seen in Figure 28 where we show the pore-size distribution characterized with X-ray microtomography for a foam-formed CTMP pulp sheet.…”

“…Direct microscopic imaging is often an effective way of getting an idea of the overall structure of material. There are both 2D and 3D imaging methods whose differences have been discussed in detail by Al-Qararah et al [124] The average bubble diameter of typical wet foams is of the order of 100 mm, which leads to a slightly smaller average pore size of the formed fiber network. Thus, the 1 mm resolution of today's X-ray microtomography is usually quite sufficient to obtain a good 3D image of the porous structure ( Figure 24) and to benchmark other methods against the obtained results.…”

“…In general, the 2D methods such as SEM, light microscopy, or direct CCD imaging yield similar results as X-ray microtomography. [124] However, because of the cumbersome sample preparation procedures, the available data are much more Figure 23. Operation principle (a) and image (b) of the bubble size analyzator, [119] together with measured distributions (c) from online images (d-e) of foam in foam-forming pilot conditions.…”

“…limited when using these methods than with X-ray microtomography, and therefore, for example, the 2D pore size analysis is less accurate. On the other hand, indirect methods such as mercury porosimetry [124] can deform a low-density structure during the measurement, and extreme care is needed in interpreting such results.…”

Foam Processing of Fibers As a Sustainable Alternative to Wet-Laying: Fiber Web Properties and Cause–Effect Relations

“…Various formulations and techniques have been proposed to control the pore structure of foam-laid webs. 19 Among these, carboxymethylated lignin was proposed as a biobased surface-active molecule suitable for foam-laying. 26 However, due to the different foam-generating methods and criteria for surfactant addition, it is difficult to compare the physical properties of webs to design foam-laid products.…”