Wood nails to fix softwoods: characterization of structural deformation and lignin modification
Softwoods can be nailed with high density solid wood like Santos Rosewood (Machaerium scleroxylon) or nails made of densified laminated beech wood composite by hammering or shooting, for example with air nail guns, without pre-drilling. Shooting with about 30 m/s causes friction between nail surface and softwood matrix, heating up the interface to temperatures high enough to soften lignin. Re-condensation of softened lignin causes wood welding which doubles pull out strength in comparison to wood nails fixed without wood welding. Changes in lignin UV-absorption within the single cell wall layer and deformation of main anatomical features of spruce wood tissue are characterized by scanning UV microspectrophotometry (UMSP) whereas structural deformations of spruce wood tissue are revealed by X-ray micro-computed tomography (μ-CT).