Evaluation of microscopy techniques to analyze the interphase of Schizolobium amazonicum glued-laminated timber with different adhesives

The use of hardwood species in glued laminated timber (glulam) production enables the utilization of native resources, with Paricá ( Schizolobium amazonicum ) showing promising properties comparable to commonly used species in Brazil. The structural performance of glulam beams is strongly influenced by the quality of the interface, which is often assessed through macroscale tests. However, microscopic evaluation of the glue line is essential to fully understand bonding characteristics affecting performance. Despite the availability of several microscopy techniques, including polarized light microscopy (LM), fluorescence microscopy (FM), confocal laser scanning microscopy (CSLM), scanning electron microscopy (SEM), and X-ray microtomography (microCT), comparative studies on their effectiveness for glulam characterization are rare. This study compares five microscopy techniques for evaluating adhesive penetration in Paricá glulam, analyzing three different adhesives. Results indicate that the microscopy methods are complementary, with each offering distinct advantages and limitations. Notably, microCT and CSLM provided the most comprehensive details about the glue line. The research presents a comparative overview and suggests a protocol for microscopy-based characterization of glued interfaces. To enhance assessment of bonding quality, it is recommended that, beyond conventional macroscale tests, at least two complementary microscopy analyses be carried out for a more complete structural evaluation of glulam beams.