Laser Ablation of Machined Wood Surfaces. 1. Effect on End-Grain Gluing of Pine (Pinus silvestris L.) and Spruce (Picea abies Karst.)

Abstract

This report describes two attempts to test the effect of removing the layer of damaged cells -the mechanical weak boundary layer. In both trials, the wood has been laser ablated using different types of lasers with different wavelengths. The goal has been to determine whether the glue joint strength is influenced by the mechanical weak boundary layer and to show how the laser wavelength affects the glue joint strength. The statistical evaluation of the results shows however no great differences between glue joints made of ablated and glue joints made of unablated surfaces. Nor are there any real differences between the different lasers and different wavelengths. This may be because there are indeed no differences between ablated and unablated surfaces, in terms of a mechanical weak boundary layer, but it may also be due to cracks which occurred in the wood specimens, and it is also possible that the glue itself was too weak. An ESEM (Environmental Scanning Electron Microscope) -analysis showed that fracture occurred primarily in the glueline and not in the boundary layer. In addition to the ESEM analysis and the statistical evaluation, a theoretical FEM (Finite Element Method) -analysis has been used to explain the crack initiation in the second trial. Holzforschung 53 (1999) 93-103 Holzforschung / Vol. 53 / 1999 / No. 1

Bond strength of end-grain joints and its dependence on surface roughness and adhesive spread

“…Concerning surface roughness resulting from machining processes, the following points should be considered: machining leads to a weak boundary layer with nonfi xed or only slightly fi xed cell wall fragments which compromise formation of a stable bond line since cell wall fragments that adhere to the surface act as a barrier to the adhesive penetrating into the intact wood. [13][14][15] On the other hand, this barrier could also help to prevent starving of the adhesive bond line. In comparison, the rough and open end-grain surface enhances penetration of adhesive into wood pores 23 and constitutes a possible source of mechanical interlocking by enlarging the bond area, which improves the tensile strength.…”

“…4). It can be assumed that the damaged cell walls of the tracheids as well as the cell wall fragments were reinforced by the low-molecular-weight fractions of the adhesives, 13 which are able to diffuse into the cell wall where their curing leads to an increased hardness of the cell wall. 24,25 This is the case for phenol-formaldehyde (PF) or melamine-urea-formaldehyde (MUF) adhesives, for example, but is less applicable to PVAc.…”

“…These fragments, stemming from and adhering to the damaged surface, can act as a barrier to the adhesive penetrating into the intact wood. [13][14][15] In order to improve the penetration of the adhesive into the wood pores and to get a smooth surface on the macro scale, the damaged cells and cell wall fragments can be removed by a microtome 16 or by laser light. 13,14 Alternatively, an increase in the spreading quantity of the adhesive has also been shown to exhibit a positive effect, e.g., on the tensile strength of polyurethane-bonded butt end joints.…”

“…[13][14][15] In order to improve the penetration of the adhesive into the wood pores and to get a smooth surface on the macro scale, the damaged cells and cell wall fragments can be removed by a microtome 16 or by laser light. 13,14 Alternatively, an increase in the spreading quantity of the adhesive has also been shown to exhibit a positive effect, e.g., on the tensile strength of polyurethane-bonded butt end joints. 17 In the present study, the infl uence of surface roughness, created by fi ve different machining processes, as well as of the spreading quantity of two different adhesives was studied by investigating the tensile bond strength of an endgrain joint.…”

157 nm fluorine laser ablation of wooden surfaces as an improved preparation technique for microscopy

“…It was Seltman who in 1995 first published results with respect to UV laser-irradiation as a means of ablating the surface of damaged cells prior to the application of a protective layer [6]. This layer displaying damage to the wood structure averages 60 µm in depth and is now commonly referred to as the Mechanical Weak Boundary Layer (MWBL) by Seltman, Stehr, et al [7][8][9]. This model is based on the concept of the Weak Boundary Layers (WBL) introduced by Bikerman [10] in 1967.…”