Adhesive penetration analysis in birch plywood using X-ray densitometry

Non-destructive and rapid analysis of adhesive penetration in engineered wood products ensures bond integrity, enabling the development of novel adhesives and quality manufacturing. The present study evaluated the X-ray densitometry method for studying the adhesive penetration in birch plywood. In this approach, it was quantified as a function of distance by measuring the X-ray density profile section where the adhesive bond-line density was above the baseline wood density. Accordingly, untreated and esterified birch veneers were pressed into plywood using both liquid and solid-type phenolic adhesives, resulting in four types of five-layered plywood of 7-8 mm in thickness. Optimization of X-ray beam attenuation through the plywood was done by preliminary trials and identified the specimen dimensions as 50 mm in height and 25 mm in width using a scanning speed of 0.5 mm min ⁻¹ . The X-ray densitometry measured densities of cured phenolic adhesive to be 1150 ± 50 kg m ⁻³ and esterification-modified birch plywood to be 750 ± 25 kg m ⁻³ . These density values are consistent with gravimetric density measurements. Density profile observations indicate that both liquid and solid forms of phenolic adhesives exhibited similar levels of adhesive penetration in plywood. X-ray densitometry analysis determined a total interphase thickness ranging from 0.50 to 0.75 mm, encompassing the central adhesive bond-line and the effective adhesive penetration into both adjacent veneers. These values are comparable with those measurements obtained from microscopy imaging analysis. Therefore, X-ray densitometry could be a swift method for quantifying adhesive penetration, although further studies are needed to validate its accuracy and reliability.