Nanoindentation of the wood-adhesive interface

The increasing economical importance and technological complexity of wood composites require a continuous optimisation of adhesive bonding technology. However, in particular with regard to the natural material wood, a number of fundamental questions remains to be understood, before the significantly different mechanical performance of different adhesive systems can be fully explained. Nanoindentation, a method of hardness testing in the micro-Newton range, was shown to be very suitable in the context of wood adhesive bonds by the group of the applicant and recently also by other researchers. In the current project it is planned to further develop nanoindentation technique with the aim of performing direct measurements of the adhesion energy between the wood cell wall and the adhesive for the first time. In order to perform such experiments, a wedge- or cone-shaped indenter tip will be driven into the interface between wood cell wall and adhesive of specially prepared specimens. Through simultaneous measurement of load on the indenter tip and tip displacement, the energy of adhesion will be determined. This will enable to directly compare the cell-wall adhesion of different adhesive systems without the biasing effects of wood micro and macrostructure, as is the case in macroscopic standard specimens. Based on new findings on adhesion mechanisms acquired with this new method, suggestions for a further improvement of wood adhesive bonding systems will be made.

The use of wood in construction, furniture, and other areas is increasing due to the many favourable properties associated with this both traditional and current material, among which strong mechanical performance at comparably low weight and a favourable carbon dioxide balance feature prominently. In order to realise an optimum use of this precious resource, wood is not only used in its most basic shape, i.e. solid wood beams and boards, but more complex materials, shapes and structures are made possible by adhesive bonding of wood. Even though this process is very old, the details of interaction between wood and adhesive are not yet fully understood, which is particularly true for the immediate interface between a wood surface and an adhesive polymer. Here, the completed project Nanoindentation of the wood-adhesive interface contributes completely new insights: by means of a new micromechanical method it was made possible to perform a direct assessment of the strength of adhesion between a wood cell wall and wood adhesive for the first time.Due to the complexity of the wood surface, where cellular structures with sizes of few thousandths of millimetres prevail, it is necessary to perform mechanical experiments at the same scale in order to obtain information unbiased by surface roughness and other sources of heterogeneity. Novel information on variability of adhesion at this small level was obtained within this project. Firstly, this is highly relevant from the academic point of view, since it promotes a better understanding of wood adhesion. Secondly, it was found that the findings of this project can be quite directly transferred to shed light on current highly relevant open questions in an application-oriented context. Thus new recommendations for optimum performance of specific adhesives were derived from an initially very basic-science oriented research programme.In the view of the project team, the project Nanoindentation of the wood-adhesive interface is therefore a very good example for successful knowledge transfer from basic science projects to application.