Freeze drying suspensions of nanocellulose in water under the right conditions result in well-ordered networks of aligned fibres. This study shows that the ice-templating method is a promising way for sustainable composites and new human-made wood materials.
Human-made or engineered wood is regularly used in furniture, buildings and appliances. The material usually consists of some component of natural wood in a mix with an adhesive. The advantage of engineered wood is that its properties such as weight, strength, or size are predictable and can be engineered to meet specific standards. Scraps from wood manufacturing can be used as a component of engineered wood, leading to less waste.
Bio-mimetic wood is made by taking this concept one step further. The naturally hierarchical structure of wood and plants is used as inspiration to create materials with similar characteristics. A project on bio-mimetic wood taking advantage of the NanoMAX beamline was started in 2019. The project involves researchers from University of Oulu, Luleå University of Technology, Research Institutes of Sweden (RISE), and University of Toronto. The researchers recreated the hierarchical wood structure and aligned nanocellulose fibres with the cell wall, together with an epoxy adhesive. The paper, which is the first one published from the project, is concerned with the alignment of the cellulose nanofibres. Aligning of cellulose is of interest also for sustainable material reinforcement. The study looks into two challenges – controlling the structure of the ice-templated pre-form and its interfacial adhesion to an epoxy matrix. It is essential that the cellulose nanofibres are aligned in the loading direction to increased materials mechanical properties.
“With our experiments, we have been able to calculate the orientation of the cellulose fibrillar angle within the material. We could confirm that with the process we used, the fibres are aligned in the vacuum-infused epoxy composites,” says professor Kristiina Oksman, one of the authors of the paper.
Nanocellulose can be made from plants or synthesised by bacteria, and it has outstanding mechanical properties. Nanocellulose can form different types of fibre-networks. In the present study, the researchers used ice-templating to create a structure similar to the wood cells. The ice crystals that form from the freezing becomes the mould for the nanocellulose network. After drying, the result is a porous network of aligned nanocellulose fibres. The pores can be filled with resin by vacuum-infusion to form a composite material. It is crucial for optimal material properties that the cellulose fibres align properly and bind to the resin. The researchers used several different methods to study the material. The NanoMAX beamline was used to check the cellulose molecules’ alignment, with a method called Wide Angle X-ray Scattering or WAXS for short.
“This was the first MAX IV experiment for us, and we got help with planning the experiment from Kim Nygård, project manager at the upcoming ForMAX beamline,” says Oksman. “With the beam brightness and sensitivity at NanoMAX, we could see if there are differences on a very small scale. It is a fast method, and the beam area can be tiny. So one can exactly scan the area of interest.”
More publications are soon to follow from the project.
“The next step is to publish the work where we prepared human-made ‘wood’ from lignin and cellulose nanofibrils. The fibrils were prepared by ice-templating, just as in the present paper,” concludes Oksman.
Tuukka Nissilä, JiayuanWei, Shiyu Geng, Anita Teleman, and Kristiina Oksman, Ice-Templated Cellulose Nanofiber Filaments as a Reinforcement Material in Epoxy Composites, Nanomaterials 11, 490 (2021), DOI: 10.3390/nano11020490