Cellulose nanofibrils (NFC) in wood coatings
Wood is often used as building material for outdoor applications. To protect it against weather conditions and microorganisms, surfaces are covered with coatings whose components and composition are continuously optimised. NFC could be used in this context and serve as an important new additive.
Project description (completed research project)
Coatings are used on wooden façades to maintain the natural appearance of the wood and ensure the serviceability of the façade. To avoid the failure of the coating due to environmental impacts such as UV light, humidity, hail or microorganisms a scheduled service of the coating is required. Transparent and semi-transparent systems are particularly susceptible to failure due to weathering and are less durable than pigmented coatings. As a result, they need to be renewed frequently and thereby cause high maintenance costs for home owners. To improve the performance and durability of wood coatings, we added nanofibrillated cellulose to aqueous, transparent binder systems based on acrylic and alkyd resin.
The durability of exterior wood coatings is often insufficient and thus regular renovation adds to the life-cycle costs of the wood product. By improving the functionality and durability of coatings, it is possible to reduce maintenance costs and make the exterior use of wood more competitive. The implementation of NFC into wood coatings is expected to improve those properties.
The aim of the project was to evaluate the potential of nanofibrillated cellulose (NFC) as a novel, biological and renewable additive to improve the performance and durability of wood coatings in external applications. On the one hand, we characterised the rheological and mechanical properties of the coatings and showed how they changed by the addition of NFC. On the other hand, we investigated if NFC could act as a carrier substance. To improve the performance of the coating, functional agents such as UV absorbers, organic biocides or nanoparticles were attached to the fibril network to provide a better distribution and more efficient fixation in the coating matrix.
The research project provides new knowledge on the possible fields of application, ability and function of pure or modified NFC as an additive in wood coatings. NFC is a multifunctional additive that improves different properties of wood coatings and influences their functionality positively. The results of the study make it possible to optimise selected properties such as UV stability, resistance to microorganisms or mechanical properties.
Our experiments with aqueous acrylic and alkyd binder systems showed the following results:
NFC can act as a thickener. It changes the flow characteristics of the binders from linear viscous to shear thinning behaviour.
NFC influences the film formation. A rougher surface structure results in reduced gloss, while the transparency and colour of the coating stayed nearly unchanged.
By the addition of NFC to the binder the films became stiffer and showed an increased tensile strength. At the same time an increased storage modulus allows a better recovery of their initial shape after deformation. Brittle binders are less prone to cracking if NFC was added.
NFC can act as carrier substance and as dispersing agent for protective components such as ZnO nanoparticles. By attaching the nanoparticles to the NFC network, agglomeration and sedimentation of the nanoparticles during the film formation can be reduced.
Modified nanofibrillated cellulose in wood coatings (MoNaCo)