Chitosan-based coatings as a substitute for PFAS

Fluorinated compounds have become a critical issue due to their harmful effects on human health and the environment. The European Union is also aware of this and is imposing increasingly strict regulations. Polysaccharides, especially chitosan, are gaining importance as potential alternatives to fluorinated substances, as they can be chemically modified in many ways and have the ability to form films. In our research, we have investigated the chemical modification of chitosan with fatty acid chains, followed by solvent-free deposition on glass and textiles. Physico-chemical properties of the modified coating were analyzed and it was tested for durability, biodegradability and toxicity.

The replacement of fluorinated compounds has become a critical issue due to their harmful effects on both human health and the environment, a concern acknowledged by the European Union, which is enacting progressively stringent regulations. Polysaccharides, particularly chitosan, are gaining attention as potential alternatives to fluorinated substances, thanks to their versatility in chemical modification and ability to form films. In our research, we explored the chemical modification of chitosan with fatty acid chains, followed by a solvent-free deposition on glass and fabrics. The physicochemical properties of the resulting coating were analyzed to confirm the successful modification and tested in its durability, biodegradability and toxicity.

Superhydrophobic and self-cleaning surfaces are crucial for applications such as energy systems, electronics, building facades and antifouling. Surfaces inspired by natural models such as lotus leaves achieve high contact angles and low roll-off angles by combining hydrophobicity with nano- and micrometric structures. Despite advances in micro- and nanotechnology, most superhydrophobic technologies are still based on per- and polyfluoroalkyl substances (PFAS), which pose health and environmental risks. With increasing EU regulations on PFAS, safer alternatives are needed.

Chitosan, a polysaccharide derived from chitin, is a promising candidate due to its mechanical properties, biodegradability and ease of modification. Since chitin is the second most abundant polysaccharide after cellulose and the main component of the claws of crustaceans, chitosan can be used as a way to reuse waste derivatives from aquaculture.

Fig. 1: PFAS-free hydrophobic coating based on chitosan. Chitosan is derived from chitin, the second most abundant polysaccharide on earth, which is obtained from aquaculture waste. Chitosan is chemically modified and deposited using a solvent-free process to ensure transparency and superhydrophobicityAchitosan-based, PFAS-free coating has been developed and applied to glass [1] and textiles [2] at the Surface Engineering and Fluid Interfaces Laboratory (SEFI Lab) of the University of Milano-Bicocca. Through chemical modification of the chitosan and a solvent-free deposition, transparency and superhydrophobicity were achieved, taking into account both performance and environmental concerns(Fig. 1). The process to modify the chitosan functionality starts with esterification and secondary amide formation reactions using stearoyl chloride, followed by purification and freeze-drying. The modified chitosan powder is then deposited onto glass slides or textiles using a solvent-free technique, where the powder is sieved through a 100 µm metal mesh and subjected to heat treatment at 150 °C. Various characterization methods were used to analyze the chitosan modification and the resulting coating, including molecular weight distribution, 1H-NMR, 13C-DD-MAS solid-state NMR, infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), contact angle measurement and scanning electron microscopy (SEM).

On glass substrates, transparency was evaluated by UV-Vis analysis and durability by abrasion resistance, contact with water and acidic environments and adhesive tape tests. By adjusting the amount of modified chitosan during deposition, coatings with different hydrophobicity and transparency can be produced. A chitosan amount of 3.2 mg/cm2 provides the optimal balance between high hydrophobicity and transparency.

For fabrics, a deposition of only 1.6 ^mg/cm2 on polyester and cellulose acetate provided superhydrophobicity and water, coffee, juice and dye repellency.

SEM analysis of glass and fabrics shows that the freeze-dried modified powder has a lamellar structure and the coating thickness increases as more chitosan is deposited. The superhydrophobicity is enhanced by the roughness in two orders of magnitude, which is due to the fibrous form of the modified chitosan. Durability tests carried out on glass show that the coatings are well resistant to abrasion, water and acidic environments. On textiles, the coating proved to be durable over several washes, while it deteriorated under very intense UV radiation and a tear test. In addition, we demonstrated the effective biodegradability of the coating, confirming that the chemical modification of chitosan does not alter its biodegradability. Finally, we demonstrated the excellent biocompatibility of the coating on human keratinocytes and fibroblasts, allowing safe contact with the skin.

In summary, this work demonstrates the successful development of a PFAS-free chitosan-based coating that is applied to glass and textiles and provides excellent water repellency. Further improvements could focus on optimizing the mechanical properties of the coating and light transmission by adjusting the surface morphology or adding crosslinkers. This work contributes to the development of bio-derived polysaccharide materials that offer a sustainable alternative to conventional oil-based polymers and support a more sustainable circular economy. These findings pave the way to extend the applicability of polysaccharide-based materials to other sectors - including semiconductors and electronics, which rely on the widespread use of PFAS.

References

[1] Tagliaro, I., Seccia, S., Pellegrini, B., Bertini, S., & Antonini, C. (2023). Chitosan-based coatings with tunable transparency and superhydrophobicity: A solvent- and fluorine-free approach by stearoyl derivatization. Carbohydrate Polymers, 302, 120424.
[2] Tagliaro, I., Mariani, M., Akbari, R., Contardi, M., Summa, M., Saliu, F., ... & Antonini, C. (2024). PFAS-free superhydrophobic chitosan coating for textiles. Carbohydrate Polymers, 333, 121981.