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The release of formaldehyde and the lack of comfort during wearing are the shortcomings of high-grade flame-retardant cotton textiles. Empa researchers successfully circumvented these problems by developing a chemically and physically independent flame retardant network within the fiber.

This method retains the basic positive properties of cotton fibers, which account for three-quarters of the global demand for natural fibers for home textiles and clothing. Cotton is skin-friendly because it can absorb a lot of moisture and maintain a pleasant microclimate on the skin.

Protective clothing provides the most important barrier for firefighters and other emergency service personnel. For this reason, cotton is mostly used as an inner textile layer that requires supplementary properties. For example, it must be able to resist biological toxins or fire.

However, it cannot be hydrophobic, otherwise it will produce an unpleasant microclimate. These supplementary properties can be integrated into cotton fibers through appropriate chemical modification.

Until now, making cotton fire-resistant has always required compromises.

Sabyasachi Gaan, chemist and polymer expert, Advanced Fiber Lab, Empa

Washable flame-retardant cotton is made by treating fabrics with flame retardants, which are chemically combined with the cellulose in the cotton.

Currently, the textile industry has no choice but to use formaldehyde-based chemicals-where formaldehyde is classified as a carcinogen. This has been an unresolved issue for decades.

Although formaldehyde-based flame retardant treatments are strong, they also have other disadvantages: the -OH groups of cellulose are chemically hindered, which significantly reduces the water absorption capacity of cotton and makes textiles uncomfortable to wear.

Gaan understands the chemistry of cotton fiber very well because he has worked at Empa for several years and developed flame retardants based on phosphorus chemistry, which have been used in a variety of industrial applications. Currently, he has managed to find an elegant and simple way to anchor phosphorus in cotton as an independent network.

Gaan and his colleagues Rashid Nazir, Dambarudhar Parida, and Joel Borgstädt used a trifunctional phosphorus compound (trivinylphosphine oxide) that only reacts with specific added molecules (nitrogen compounds such as piperazine). So as to form its own network in cotton. This makes the cotton long-lasting fire resistance without blocking the much-needed -OH groups.

Moreover, the physical phosphine oxide network also likes water. This flame-retardant treatment does not involve carcinogenic formaldehyde that can cause harm to textile manufacturing workers. The phosphine oxide network produced in this way will not be washed away. Even after 50 washings, 95% of the flame-retardant network remains in the material.

In order to provide other protective functions to the flame-retardant cotton manufactured by Empa, the team also added silver nanoparticles generated in situ to the material. This is suitable for a single-step process and also produces a phosphine oxide network. Silver nanoparticles give the fiber antibacterial properties and can withstand 50 washing cycles.

We used a simple method to fix the phosphine oxide network inside the cellulose. In our laboratory experiments, we first treat cotton with an aqueous solution of phosphorus and nitrogen compounds, and then steam it in a ready-made pressure cooker to promote the cross-linking reaction of phosphorus and nitrogen molecules.

Sabyasachi Gaan, chemist and polymer expert, Advanced Fiber Lab, Empa

The application process is very matched with the equipment used in the textile industry.

“The cooking of textiles after dyeing, printing, and finishing is a normal step in the textile industry. Therefore, our process can be applied without additional investment,” adds the Empa chemist.

This newly formed phosphorus chemistry and its application are protected by patent applications.

There are still two important obstacles. For future commercialization, we need to find a suitable chemical manufacturer that can produce and supply trivinylphosphine oxide. In addition, trivinyl phosphine oxide must be REACH registered in Europe.

Sabyasachi Gaan, chemist and polymer expert, Advanced Fiber Lab, Empa

Nazir, R., etc. (2021) In-situ phosphine oxide physical network: a simple strategy to achieve long-lasting flame retardancy and antibacterial treatment of cellulose. Journal of Chemical Engineering. doi.org/10.1016/j.cej.2020.128028.

Source: https://www.empa.ch

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