Moisture durability is essential for wood composites, especially those used in building construction, where products are prone to weathering. The primary focus of this research was to determine if adhesive penetration into the cell wall has a positive influence on adhesive bond durability. To fully understand the measureable effects of adhesive penetration on moisture resistance, a multi-scale investigation was conducted.
The study used bonded Douglas-fir lap-shear test specimens (6mm x 14 mm x 64 mm), varying in bonded surface cell type (earlywood vs. latewood) and adhesive type. The adhesives of interest were a low and high molecular weight phenol-formaldehyde (PF), a low molecular weight PF with added filler and extender, and a methylene diphenylene diisocyanate (MDI). Half of each group of test specimens was subjected to accelerated weathering and the rest remained dry. All samples were mechanically tested in lap-shear. During the test, Digital Image Correlation (DIC) was used to track displacements across the surface of the sample. The failure load, effective shear modulus, energy to failure, percent wood failure, failure mode, and DIC shear strain maps were used to evaluate the mechanical performance of the dry and weathered specimens.
Smaller samples (2 mm x 2 mm x 10 mm) were cut from the previously tested lap-shear samples and scanned at the Advanced Photon Source (APS), Argonne National Laboratory, using micro-X-ray Computed Tomography (XCT). All adhesive types were previously tagged with iodine to increase contrast during the XCT scans. This technique gave high resolution (1.3 µm³/voxel), 3D images for which adhesive penetration was analyzed. Further, additional samples were cut from the previously scanned micro-XCT samples for nano-XCT scanning at the APS. The nano-XCT data gave a resolution of 60 nm³/voxel, allowing for detection of adhesive penetration into the cell substance and sub-micron cell features. Complementary analyses using Energy Dispersive Spectroscopy (EDS) and X-ray Fluorescence Microscopy (XFM) were used to confirm the presence of iodine in areas containing adhesive, and to further observe adhesive penetration into the cell substance.
Statistical analysis showed two of the adhesive formulations had significant differences in mechanical properties after weathering. Weathering had a negative effect on the mechanical properties. DIC shear strain maps revealed that the same two adhesive formulations had more strain around the bonded interphase at the same applied load. Adhesive penetration into the cell substance was observed, but cannot be quantitatively compared between the four adhesives, due to inadequate image quality from nano-XCT and the low number of specimen replications. Overall, there was more effective adhesive penetration in the lower molecular weight adhesive and less shear strain calculated from the DIC macroweathered analysis. This indicated that adhesive penetration could have a positive influence on moisture durability, although the variability is too high to make a definite conclusion and there were not enough replicates to compare the amount of adhesive penetration into the cell wall.