X-ray Microscopic Examination of Consumer Paper Products

Figure 1: Volumetric rendering of a baby wipe imaged on the SkyScan 2214 nano-CT

Within the home care line of sanitary products, there’s been a rise in the prevalence and use of moist towels as part of the cleanup process in the bathroom with several wipe products marketed to consumers as tested by plumbers or safe to flush. Traditionally, products of this type were marked as solid waste disposal only, but for convenience different brands have begun marketing their products as safe to flush based on their manufacturing process and design. Many municipalities and homeowners may disagree based on news reports of sewer backups and damage at wastewater treatment plants that are attributed to the prevalence of wipes in the waste stream compared to standard waste products like toilet paper.

For this month’s article, we compared a standard wet baby wipe marked as “safe for solid disposal in trash only” as well as a wet wipe marked as “safe for direct flush into the sewer system”, using our ultra-high resolution SkyScan 2214 nano-CT at an isotropic voxel size of 750nm. Micro and Nano-CT excel as comparative tools to examine changes in 3D structure among samples.

X-Ray Microscopic Imaging of Consumer Paper Products

For this study, we are presenting the results of imaging both a “flushable wipe” and a traditional baby wipe at an isotropic voxel size of 750nm using the SkyScan 2214 nano-CT with both a standard tungsten filament and with a high resolution Lab6 filament. Unlike our desktop SkyScan instruments, the SkyScan 2214 utilizes a user-serviceable X-ray source with two choices of filament. For most work, and to achieve the highest power possible on the SkyScan 2214, the default tungsten filament is required. To assess the best resolutions possible on the instrument, an optional Lab6 filament is accessible, which further reduces the X-ray spot size to provide even sharper details on high resolution imaging projects.

Figure 2: Planar 2D views of baby wipe (top) and flushable wipe (bottom)

As shown in Figure 2, the SkyScan 2214 nano-CT with a standard tungsten filament installed provided the necessary resolution to obtain structural detail on individual fibers used in the construction of both sanitary products. Clear differences are visible between the two samples with the baby wipe comprised of small, cylindrical structures loosely woven together. The flushable wipe has a more organic looking structure with thin ribbons of material intertwined into the structure.

We should also note that both structures were imaged in a dried state and their hydrated structure will likely be different. Imaging hydrated organic samples can prove challenging due to contrast limitations, so additional contrast agents are often required if the goal is to image the samples in a fully hydrated state.

Figure 3: 3D volumetric rendering of baby (top) and flushable (bottom) paper samples

When moving out to a volumetric view of each dataset, the differences between the two products are more evident than within the 2D views (Figure 3). The compact, ribbon-like fibers of the flushable wipe stand in contrast to the more sparse, cylindrical fibers used in the baby wipe. To lend some credibility to the claims of the manufacturer of the wet flushable wipe, the fibrous structure shown in their product is more like what would be expected for a traditional paper product, such as toilet paper, which is generally considered safe to flush.

Figure 4: 3D volumetric rendering of baby wipe structure using the tungsten (top) and Lab6 (bottom) filaments at the same nominal voxel size

Replacement of the tungsten filament for the Lab6 filament within the nano-CT unlocks additional resolution even at the same nominal pixel size due to improvements in the coherence of the X-ray beam and the emission spot from the X-ray source transmission target (Figure 4).

Figure 5: Quantitative analysis of fiber thickness for both samples showing the percent of the total fiber volume that lies within the quantified thickness ranges

After segmenting the fiber signal from the air signal in the datasets, CTAnalyzer extracts quantitative data from our qualitative reconstructed images. In line with what we see qualitatively in the results, the average fiber thickness for the flushable wipe is only 5.59 ± 1.84 micrometers while the average for the baby wipe is 9.54 ± 2.04 micrometers (Figure 5). While the fibers for the flushable wipe are qualitatively wider than the fibers used for the baby wipe, their thin planar structure results in the average fiber diameter being approximately half of what is observed for the cylindrical baby wipe fibers.

Figure 6: Volumetric CTVox 3D rendering of the fiber samples color coded by relative orientation comparing the baby (top) and flushable (bottom) structure orientation patterns

In analyzing the fibers within CTAnalyzer, we can also extract some basic fiber orientation data and map these results within CTVox to create a color-coded representation of the data (Figure 6). Colors were assigned to the orientation values and the random distribution of colors within the baby wipe sample tells us the fibers in general are randomly distributed in 3D space as part of the structure of the sample. In contrast, the flushable wipe has clear regions of similar coloration which suggests a more ordered array of fibers in 3D space.

Conclusion

The SkyScan 2214 nano-CT is a versatile instrument geared towards ultrahigh-resolution imaging of medium and low-density samples like the paper products examined in this article. As part of user maintenance, the electron beam filament can be selected by the operator to optimize for either power (tungsten) or resolution (Lab6) providing additional levels of versatility.

We hope you found this Image of the Month informative and encourage you to subscribe to our newsletter and social media channels in preparation for the continuation of our Image of the Month series next month.

Scan Specifications

Filament Tungsten Lab6
Voltage (kV) 30 40
Current (µA) 133 166
Filter None None
Pixel Size (µm) 0.75 0.75
Rotation Step 0.2 0.2
Exposure Time (ms) 7000 3000
Rotation Extent (deg.) 180 180
Scan Time (HH:MM:SS) 14:46:35 06:27:30

 

These scans were completed on our SkyScan 2214 nano-CT system at the Micro Photonics Imaging Laboratory in Allentown, PA. Reconstructions were completed using NRecon 2.0 while visualization and volumetric inspection of the 2D and 3D results were completed using DataViewer and CTVox. Isolation of the individual fiber metrics utilized CTAnalyzer.

Would you like your work to be featured in our monthly newsletter? If so, please contact us by calling Seth Hogg at 610-366-7103 or e-mailing seth.hogg@microphotonics.com.

References

https://www.washingtonpost.com/local/trafficandcommuting/flushable-wipes-clogging-sewers/2021/04/23/5e8bbc82-a2c9-11eb-a774-7b47ceb36ee8_story.html

https://www.cityofalbany.net/pw/wastewater/wipes

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