Harbin, China – Cellulose offers an enticing option as a source of renewable raw materials. It offers numerous advantages for bio-based polyurethane, as they have a large number of hydroxyl groups, and is biodegradable. A group at the Northeast Forestry University has been looking at the potential of cellulose-based polyurethanes using ethyl cellulose in conductive adhesives for transparent electronic devices.
Few studies have thus far looked at complete replacement of polyether polyols with cellulose, however, because of compatibility and dispersion issues. Cellulose fibres are both hydrophilic and polar, making them less compatible with hydrophobic and non-polar matrices. They also have alternating amorphous and crystalline regions, greatly reducing the accessibility and, therefore, reactivity of the hydroxyl groups.
An alternative lies in a modified cellulose. Ethyl cellulose is amphiphilic and therefore should improve both compatibility and dispersibility, and the Harbin group looked at its potential in making polyol with no added polyether or polyester polyol. The PU they made had good shape memory behaviour, tensile strength and reprocessability and, importantly, biodegradation half-life of 38 days.
Most thermally conductive adhesives are black because of the nature of the polymers, which is not the case here. They suggest that it could be useful in making adhesives for transparent electronic devices, as it had a high transparency, with 84% transmittance, and good shape memory behaviour and tensile strength.
The PU film also had good adhesive bonding with various substrates. When Al2O3 nanoparticles were added, the transparency and adhesive strength remained high, and its thermal conductivity also improved. At 2.5% nanoparticles by weight, the film had a transmittance of 74%, with high thermal conductivity and diffusivity. They speculate that the PU has potential in high-performance bio-based plastics, including for transparent electronics.
The work has been published in the journal Industrial Crops and Products.