Printing composite parts with a high cellulose content

Materials World magazine
,
24 Apr 2020

Structural materials may be produced with a higher cellulose content. Idha Valeur asks how it was done and why it matters. 

Printing complex 3D composite parts with a higher cellulose content is now possible, say Swiss scientists, maximising the benefit of the material’s mechanical properties. 

The team at ETH Zurich and Swiss Federal Laboratories for Materials Science and Technology claim to have successfully printed objects with double the cellulose content at a volume fraction of 27%. 

Cellulose has been of interest to researchers for years due to its capability of constructing intricate structures with sought after mechanical properties in its usual habitat – nature. ‘As cellulose-based materials can be extracted from natural resources, they make a potential building block to fabricate sustainable materials. Moreover, these building blocks exhibit outstanding mechanical properties with stiffness that are comparable to steel and superior than other engineering polymers such as Nylon and Kevlar,’ says Rafael Libanori, Senior Scientist at ETH Zurich.

To achieve the higher cellulose content, the researchers combine 3D printing – via direct ink writing – with a process for densifying the object. The ink is made up of water with cellulose particles and fibres a few hundred nanometres in size. ‘Cellulose nanofibres are employed in a much lower amount, about 1% in volume, and significantly enhance the shape retention of the printed filament. To fabricate the composites, we selected photocurable polymer resins that exhibit varied mechanical performances and consolidated them into a solid by irradiating with UV light,’ explains Libanori. 

The printed item is then added to a solvent bath. In response to the organic solvent, the cellulose particles bulk together, causing shrinkage. ‘After printing, we immerse the part into a bath containing organic solvents that exhibit poor interaction with the cellulose-based materials but are still miscible with water,’ adds Libanori. ‘The organic solvent replaces the water within the printed part and enhances the attractive interactions between the cellulose particles, inducing the densification of the 3D-printed part. By applying this trick, we are able to almost double the cellulose content from 14% to about 27% in volume.’

The team has created a range of objects using the technique from both hard and soft composite materials with high structural complexity. Libanori explains that the shrinkage poses no problem, as long as the part’s wall thickness is no greater than 5mm.

‘We believe that we can push this limit a bit further, up to 10mm, but this still requires further investigations. The research is still at its initial stage and we are doing our best to explore the design space. If the wall thickness is kept below 5mm, the overall size of the manufactured part is only limited by the dimensions of the 3D printer,’ he says. 

‘In addition to complex shapes, we are also able to control the alignment of the cellulose particles within the printed part, which allows us to fabricate composite parts [where the] microstructure resembles those of natural materials, such as wood,’ Libanori adds. ‘By selecting a stiff and strong polymer, we demonstrate the fabrication of a hook that can bear mechanical loads as high as 737 times its own weight. On the other hand, employing soft polymers allowed us to fabricate more deformable materials such as the human ear model.’

The team is now exploring other possible applications as well as searching for commercial partners to take the work further and adapt it to market demands. Libanori adds, ‘We are also interested in extending the densification process to other types of materials’.