Cellulose fibre-reinforced wood adhesives

Adhesive bonding is one of the key process steps in wood industry. The stability of adhesive bonds is of vital importance to the performance of almost all wood products. To ensure continuing technological progress in wood industry it is therefore necessary to explore new possibilities of improvement in adhesive bonding. The proposers of the planned project "Cellulose fibre-reinforced wood adhesives" conclude from own preliminary experiments that the reinforcement of wood adhesives with cellulosic fibres presents an up to now little investigated possibility for significant performance improvement. It is planned to broadly investigate the potential effects of cellulose fibre reinforcement on the mechanical performance of wood adhesive bonds. Two adhesives widely used in wood industry will be used as representative adhesive systems: urea-formaldehyde (UF), which is commonly very brittle, and on the other hand ductile polyvinylacetate (PVAc). The adhesives will be compounded with cellulosic fibres derived from pulp fibres having undergone various degrees of fibrillation, ultimately down to nanofibrils. Specimens prepared with reinforced and unreinforced adhesive will be characterised with regard to their mechanical performance, in order to identify promising combinations of adhesive and fibre. In addition, pure adhesive films, both reinforced and unreinforced, will be produced and characterised in order to study the mechanism underlying eventual performance improvements.

Modern wood-based materials such as glue-laminated timber, plywood, or particleboard rely on wood adhesive bonds for their structural integrity. Depending on the type of wooden material, between 1% and 15% adhesive per unit mass are used. Therefore, the cost of adhesive plays a significant role in wood industry, which is why it is desirable to study novel ways for improving adhesive performance. In the present project, the fundamentals of a bio-based route to solving this challenge, i.e. the modification of wood adhesives with nano-scale cellulosic fibres (nanocellulose), were studied. Nanocellulose is relatively novel reinforcement fibre produced from wood pulp, which due to its nano-scale dimensions disposes of high strength and high specific surface area, which makes it an ideal reinforcement for all kinds of polymers. Furthermore, nanocellulose can be easily compounded with water-based systems, which is the case for most wood adhesives. Experiments focussed on urea-formaldehyde, which is the most widely used wood adhesive. It was found that nanocellulose disperses well in urea-formaldehyde, and that it does not negatively affect the cure characteristics of the adhesive. By adding as little of 1% nanocellulose per unit mass adhesive, the characteristics of this usually very brittle polymer changed towards a more ductile behaviour. It is proposed that the propagation of micro-cracks, which are frequently observed in cured urea-formaldehyde, is hindered by the nanocellulose reinforcement. Since ductility is an adhesive property which favours strong gluing performance, the overall performance of several wood-based materials was significantly improved when nanocellulose-modified adhesive was used.The findings of the present project are being used in an application-oriented follow-up project aiming at the development of an industrially applicable nanocellulose-modified wood adhesive. As a result, it is expected to enable the production of wood composites using less adhesive, thus reducing both cost and potential formaldehyde emission.