UNDERSTANDING NEW INTERFACES – A STRATEGIC DISSERTATION
A strategic dissertation gave important insights into the effects of different adhesives and wood modification.
Knowledge on the interface between wood and adhesive is of fundamental importance to improve binders, processes and composites. "Understanding new interfaces", a project of common interest to our partners, aimed at an increased fundamental understanding of surface properties and their interaction with adhesives. Therefore, this project was organized as "strategic dissertation". Such dissertation was largely independent of short-term company interests and characterized by high independence, an open right to publish, and an especially deep integration with academics.
The performance of adhesives depends on its cohesion as well as its adhesion to the wood surface. These surfaces are, however, inhomogeneous and complex. Wood mainly consists of parallel hollow tubes, the wood cells. The cell walls are built up of a layered structure, where each layer differs in structure and chemical composition. Adhesive may bond to the compound middle lamella ("the outside of the tube"), a cut surface across the cell wall (mainly the S2-layer), or the inner cavity termed lumen ("the inside of the tube").
In order to compare the strength of an adhesive to all possible surfaces, an elaborative method was needed. Single wood fibres were isolated, straightened, frozen onto an ice cube, cut along its longitudinal axis, embedded into adhesive, and finally cut perpendicular to the fibre axis. Thus, samples of half wood cells with adhesive contact to all three surfaces were obtained. These samples were then investigated by nanoindentation: a very small cone diamond tip was pressed into the interface. The energy spent is related to the adhesive strength. For example, no differences in adhesion were found at the interfaces of the different surfaces and MUF adhesive. MUF penetrated into the cell wall leading to a strengthening superimposing adhesion differences.
Furthermore, influences of technical processes were investigated. MDF fibres were produced in a thermomechanical refining process, leading to higher content of relatively apolar lignin at the outside fibre surface. These fibres were halved and embedded in adhesive as well, followed by nanoindentation. A trend indicating higher adhesive strength between less polar PUR adhesive and the less polar compound middle layer was observed. Nanoindentation was also performed at the interface of PUR with the relatively polar surface of cellulosic fibres now leading to a lower adhesive strength.
Impact and effects
In conclusion, the adhesive strength can be steered by adapting the polarity of the wood fibre. This knowledge was also implemented. Cellulose nanofibers containing residual lignin were obtained by mechanical fibrillation and were used to stabilize aqueous styrene emulsions. Upon polymerization of the styrene, polystyrene microspheres coated in fibres were obtained. Also polyester microspheres were obtained this way.
The work of the Ph.D. student, who also completed an outgoing research stay at the University of Maine (USA) for 4 months, was published in six publications. The impact of this fundamental work is illustrated by the fact that these have been already cited several times in only a short period of time.