Micro-CT of Adhesive Bonds in Wood, Using the Bruker SkyScan 1275

Animation of Superglue in Wood 3D Rendering

Animation of Wood Glue in Wood 3D Rendering

Using Micro-CT to Examine Material Structural Properties of Adhesive-Bonded Wood

Adhesive bonding of wood is important in the manufacture of building materials, including plywood, particleboard, structural composite lumber, doors, windows, and factory-laminated wood products. The role of adhesives is to transfer and distribute loads between the components, which increases the strength and stiffness of the final wood products. The performance of an adhesive bond depends upon how well multiple complex factors are controlled during assembly, meaning the wood, adhesive, and the interphase regions determine the strength of the bond. Increasingly, engineers are studying the penetration of adhesive into the porous structure of wood for developing new and better adhesives, designing bonding systems, and assessing bond performance. Micro-CT is instrumental in quantifying adhesive penetration as well as in creating 3D models to simulate adhesive bond performance. The information gained helps inform manufacturers of adhesives and bonded wood products regarding formulation decisions and adhesive selection.

For our demonstration in exploring differences in adhesive bonding to wood samples, we chose to utilize our SkyScan 1275 system because of its short scan times and good spatial resolution.

Micro-CT Scans of Adhesives Bonds in Wood

After reconstructing the X-ray attenuation data for each of three glue types — epoxy, superglue, and wood glue — we can explore calculated volumetric information by viewing two-dimensional slices from any of the coordinate axis directions (Figure 1). In these trans-axial views, we observe the grain and pore structure of our wood samples running perpendicular to the slice along with the glue depositions between the two layers of wood. From these images, it is clear the epoxy showed the most deposition and penetration into the wood pores. The superglue sample is spotty and did not spread well across the sample but still showed some penetration into pores. The wood glue shows fairly uniform distribution across the joint but little penetration into wood pores. Deeper infiltration of glue into wood pores leads to increased bond strength while the uniformity of a coating also contributes to the bond.

Using the three-dimensional volumetric rendering, we attempted to subtract the signal from the wood alone and instead visualize only the results arising from the glue. While some segmentation of the glue phase from the wood was possible, it should be noted that a full separation of the two phases in this case is not straightforward because of a small percentage of wood grains with sufficient X-ray density to generate a signal similar to the glue. Examining the results from the epoxy sample demonstrates agreement with our observations on glue infiltration from the two-dimensional slices (Figure 2). While we can see some faint signal from wood grains, we primarily observe signal arising from the epoxy itself in this view. The coverage is more consistent on both edges than the middle, but covers the entire sample fairly well. Interestingly, we can even see epoxy deposits within the length of several vertical channels arising from the pore structure of the wood, which provides evidence of an appreciable amount of epoxy infiltration.

Taking a closer look at the superglue sample (Figure 3), we note that the distribution of superglue appears to be much less regular than observed for epoxy. However, due to the lack of appreciable X-ray attenuation differences between the cyanoacrylate monomer comprising the superglue and the cellulose of the wood fibers, it is possible that subtracting the wood signal from this image is also removing some signal which should be present from the superglue. This demonstrates the need to be mindful of the need for contrast in samples being selected for micro-CT applications. Often times, natural contrast may be insufficient in samples and visualization will be greatly enhanced through the addition of a contrast agent to one portion of the sample.

Finally, if we examine the wood glue sample (Figure 4), we note that, as we observed in the two-dimensional slices, the distribution of glue across the sample appears to be of similar uniformity to that observed for epoxy. Regions of air entrapment within the glue are also visible and coupled with the information on the relative lack of pore infiltration compared to epoxy, may lead us to conclude that the viscosity of this wood glue could be reduced to improve adhesion for this particular application.

Conclusion

Micro-CT excels as a comparative technique to explore differences among samples under study. In this case, we were able to use identical sets of wood to compare three different adhesive classes. With suitable contrast between adhesive and substrate, we could also utilize our CTAnalysis software to quantitate glue volume and surface area as well as produce volumetric models suitable for three-dimensional modeling and analysis. We hope you found this image of the month illustrative. If you have an image of the month sample that you would like us to scan, please contact us by calling Seth Hogg at 610-366-7103 or e-mailing seth.hogg@microphotonics.com

Scan Specifications

All scans completed on our high speed SkyScan 1275 micro-CT system at the Micro Photonics Imaging Laboratory in Allentown, PA. Reconstructions were completed using NRecon and visualization of 2D and 3D results were completed using DataViewer and CTVox.

Works Cited

1. Kamke, Frederick A. et al, “Micro X-ray Computed Tomography of Adhesive Bonds in Wood,” Wood and Fiber Science, Vol 48 (2016).
2. Paris, Jesse Loren, “Wood-adhesive bondline analyses with micro X-ray computed tomography,” Oregon State PhD Dissertation, 10/27/2017.
3. Schwarzkopf, M. et al, “Integrating optical measurement and modelling for quantitative analysis of the micromechanical load transfer in the wood-adhesive bond interphase,” International Wood Products Journal, 7:4, 231-234, (2016).