Plasma spray turns lignin waste into functional coatings in one step

A study published in the Journal of Bioresources and Bioproducts reports a novel approach for converting technical lignin into functional coatings through low-energy suspension plasma spraying (LE-SPS). The researchers used a colloidal lignin particle suspension deposited onto glass substrates using a modified plasma spraying system with radial suspension injection. Scanning electron microscopy revealed continuous and dense coatings with an average thickness of approximately 4 μm. Comprehensive spectroscopic analyses including X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, and nuclear magnetic resonance indicated substantial plasma-induced chemical restructuring, with increased oxygen-containing functionalities such as carbonyl and carboxyl/ester groups, alongside changes in aromatic signals. The coatings exhibited strong UV attenuation across UV-C, UV-B, and most of the UV-A range, with transmittance near zero at 270 nm. Additionally, the hydrophilic surface state, evidenced by a water contact angle of approximately 17°, demonstrated promising anti-fogging behavior in preliminary tests. The findings establish LE-SPS as a feasible green and scalable route for fabricating plasma-sprayed lignin coatings.
Lignin, a complex and highly branched biopolymer, represents one of the most abundant renewable alternatives to fossil-based materials, yet approximately 98% of it is currently combusted for energy recovery. Conventional wet-chemical coating methods typically require organic solvents, long processing times, and external crosslinkers to achieve stable films. The new LE-SPS approach overcomes these limitations by utilizing a liquid carrier that stabilizes lignin particles for consistent mass transport while shielding them from extreme thermal conditions. During the process, the aqueous suspension is injected into the plasma afterglow region, where atomized droplets undergo fragmentation and solvent evaporation before deposition. Optical emission spectroscopy confirmed the presence of reactive nitrogen and oxygen species, while gas temperature measurements showed that the water carrier substantially mitigated thermal load on the particles. The research team noted that this marks the first time LE-SPS has been applied to deposit lignin or other biopolymer-based coatings, representing a previously unexplored application of the process. The study was financially supported by zukunft.niedersachsen, the joint science funding program of the Lower Saxony Ministry of Science and Culture and the Volkswagen Foundation.