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Synchrotron-based analysis of densified wood impregnated with curing resins

Summary of the Project

Synchrotron-based analysis of densified wood impregnated with curing resins. There has been renewed interest in the production and use of modified wood. Consumers and policy makers are promoting the usage of natural and sustainable materials for various reasons including: low environmental impact, linkage to local cultural heritage, and benefits to human health. Wood modification techniques have been used to help valorise under-utilised wood materials and increase their performance with respect to durability, mechanical characteristics, and to achieve new forms and functions desired by consumers and designers alike [1]. One such modification technique is thermal-hydro-mechanical (THM) treatments. THM uses only heat, water, and mechanical pressure to compress and densify the wood material. This results in increased density, hardness, abrasion resistance, and some strength properties. During the THM process, wood is softened and compressed, resulting in densification without fracturing the cell walls. The compression takes place in a hot press between 120 °C to 180 °C, in which heat is transferred to the material’s interior through contact with heated plates. The densified wood will maintain its shape if it is cooled under pressure. One of the largest challenges faced with this modification technique is dimensional stability. If this treated wood is exposed to high levels of moisture or water, it can revert partially to its original dimensions. One technique that has been proposed to increase dimensional stability has been to impregnate the wood micro-structure with various curing resins. Once these resins reach a certain temperature, they polymerize throughout the micro-structure. This polymerized polymer improves the dimensional stability of the wood by chemically bonding with the wood cell wall, within the cell wall, and through mechanical interlocking in the cellular wood structure. To date there is little known about the fundamental process of adhesive mobility/penetration into the microstructure of densified wood. This behaviour is a significant component of the overall effectiveness of adhesive bonds and impregnated curing resins. In addition, the load transfer efficacy and mechanical performance of these modified wood materials on a micro-scale is not well understood and is of high importance to develop better treatments that are more efficient and have lower environmental impacts. The project collaborators have been part of a research team originating at Oregon State University (USA) and the Forest Products Laboratory (USA) which developed a methodology to answer similar questions which combined X-ray computed microtomography scans (XCT), X-ray fluorescence microscopy (XFM), experimental validation, and numerical modelling [2,3,4,5]. Modified adhesives were used to achieve high attenuation contrast for material separation. Being able to separate the various phases of the scanned material made it possible to use this information to directly inform the physical morphology of the developed numerical model. This numerical model was developed as a predictive tool for industry prototyping of new adhesive resins. XFM was used in the same experiments to determine the penetration of the adhesives into the wood cell wall These same techniques can now be applied to densified wood materials to learn more about the curing resin mobility/penetration and mechanical performance of densified wood that has been impregnated to help solve the issue of dimensional stability in THM modified wood.

Relevance of the results expected from research project

The motivation for this project is to more fully understand the mechanisms behind dimensional stability and mechanical performance of impregnated, densified wood. More specifically this research will address the lack of fundamental knowledge regarding: 1) adhesive and curing resin penetration into the densified wood micro-structure; 2) mechanical performance and load transfer efficacy of impregnated, densified wood on a micro-scale. Building off the project collaborators’ previous experience and expertise in the fields of wood modification, XFM, XCT, and synchrotron-based micro-scale analysis, the project will merge the Slovenian and American research groups to create a tool that will be used for the technical assessment of adhesives/curing resins and densification methods used for wood with respect to dimensionally stability.

The basic experimental work plan is as follows:

  • Impregnate wood specimens with iodophenol-formaldehyde (Slovenia).
  • Densify the specimens and in the same process, polymerize the resin within the wood (Slovenia).
  • Scan the wood specimens using the synchrotron facility located in Trieste, IT (beam time is already secured).
  • While scanning, apply tensile loading, Map and quantify adhesive penetration into the cell walls using XFM at the Advanced Photon Source synchrotron (Argonne, IL, USA) using methods established in [5] Perform nanoindentation of wood cell walls to measure how adhesive penetration affects mechanical properties of the cell walls. Nanoindentation methods will follow those developed in [5] and be performed at the FPL Nanoindentation Lab (Madison, WI, USA).
  • Analyse the adhesive penetration and stress distribution using data gathered from XCT scans, XFM, nanoindentation, and numerical modelling (Slovenia/Madison, WI, USA).

References:

[1] Sandberg D, Kutnar A, Mantanis G (2017) Wood modification technologies – a review. iForest – Biogeosciences For 10:895–908. doi: 10.3832/ifor2380-010

[2] Kamke FA, Nairn JA, Muszyński L, Paris JLL, Schwarzkopf M, Xiao X, Muszyński L, Paris JL, Schwarzkopf M, Xiao X (2014) Methodology for micromechanical analysis of wood adhesive bonds using x-ray computed tomography and numerical modeling. Wood Fiber Sci 46:15–28.

[3] Paris JL, Kamke FA, Mbachu R, Gibson SK (2014) Phenol formaldehyde adhesives formulated for advanced X-ray imaging in wood-composite bondlines. J Mater Sci 49:580–591. doi: 10.1007/s10853-013-7738-2

[4] Schwarzkopf M, Muszynski L (2015) Stereomicroscopic optical method for the assessment of load transfer patterns across the wood-adhesive bond interphase. Holzforschung 69:. doi: 10.1515/hf-2014-0098

[5] Schwarzkopf M, Burnard M, Martínez Pastur G, Monelos L, Kutnar A (2017) Performance of three-layer composites with densified surface layers of Nothofagus pumilio and N. antarctica from Southern Patagonian forests. Wood Mater Sci Eng 1–11. doi: 10.1080/17480272.2017.1366945

[6] Zauner M (2014) In-situ synchrotron based tomographic microscopy of uniaxially loaded wood: in-situ testing device, procedures and experimental investigations. ETH Zurich.

Bibliography