Adhesion to machined and laser ablated wood surfaces

Abstract: This thesis is a contribution to the R&D-program ValueActivation at KTH,Division of Wood Technology and Processingand containsfive papers related to the evaluation and development ofmachining techniques (mechanical and irradiation treatments)whereby weak boundaries at the wood surfaces in the form of(mechanical) weak boundary layers and (micro) cracks due tomechanical treatment can be avoided.The first paper describes the development of a method?using a UV laser?to study the mechanically weakenedboundaries. The purpose is to achieve a sample preparationtechnique for microscopy that minimizes artefacts.Scientifically, the second, third and fourth papers,concerning the interaction between wooden surfaces andsynthetic polymers, are based on the relatively modern adhesiontheory (at least in the field of wood adhesion science) of weakboundary layers. The papers introduce and evaluate the(mechanical) weak boundary layers in relation to wood adhesionscience. The first of these three papers provides thetheoretical basis for the following two. The paper suggests adivision of the weak boundary layer concept into chemical(CWBL) and mechanical (MWBL) weak boundary layers. Thesubsequent two papers evaluate the influence on the glue jointstrength of a reduction in the (mechanical) weak boundarylayer.The last paper deals with (tip-)cracks on a wood surface dueto machining.The results presented in the first paper can be used toachieve samples for microscopic investigations with a minimizednumber of artefacts. The results presented in the fourth papercould be utilized to achieve stronger adhesive joints. Withhelp of a laser ablation technique, it is shown that the "lawof nature" saying that end-grain surfaces cannot be glued ismaybe not so obvious. Methods of eliminating or reducing theweak boundaries have been identified. The results indicate thatit is the adhesive itself that is the weak point in anend-grain joint.It is also possible that the laser ablation techniqueapplied on flat sides could lead to better adhesion between acoating and wood, and the results presented in the last papercould increase the adhesive performance particularly related tocoatings.Keywords:ablation, adhesion, checks, coating, cracks,excimer laser, gluing, IR laser, mechanical machining,mechanical weak boundary layer, microscopy, notched specimens,pine, planing, sample preparation, sawing, scanning electronmicroscopy, spruce, tip-cracks, UV laser, weak boundary layer,wood