Author: Swiss Federal Laboratory for Materials Science and Technology (EMPA) September 18, 2021
Cost-effective: Empa scientist Sabyasachi Gaan uses steam from a commercial pressure cooker to flame-retardant cotton fabric samples. Credit: Empa
A new chemical process developed by Empa turns cotton into a fire-resistant fabric, but still retains the skin-friendly properties of cotton.
The most advanced flame-retardant cotton textiles release formaldehyde and are uncomfortable to wear. Empa scientists have circumvented this problem by creating a physically and chemically independent flame retardant network inside the fiber. This method retains the inherent positive characteristics of cotton fiber, which accounts for three-quarters of the world's natural fiber demand for clothing and home textiles. Cotton is skin-friendly because it can absorb a lot of moisture and maintain a favorable microclimate on the skin.
For firefighters and other emergency service personnel, protective clothing is the most important barrier. For such purposes, cotton is mainly used as an inner textile layer that requires additional properties: for example, it must be fireproof or protected from biological contaminants. However, it should not be hydrophobic, otherwise it will cause an uncomfortable microclimate. These additional properties can be imparted to cotton fibers through appropriate chemical modification.
"Until now, making cotton fire-resistant has always required compromises," said Sabyasachi Gaan, a chemist and polymer expert who works in Empa's Advanced Fibers laboratory. Industrial washable flame-retardant cotton is produced by treating fabrics with flame retardants, which are chemically connected to the cellulose in the cotton. Currently, the textile industry has no choice but to use chemicals based on formaldehyde-and formaldehyde is classified as a carcinogen. This has been an unsolved problem for decades. Although formaldehyde-based flame-retardant treatments are durable, they have other disadvantages: the -OH groups of cellulose are chemically blocked, which greatly reduces the ability of cotton to absorb water, causing textiles to be uncomfortable.
Gaan understands the chemical properties of cotton fiber very well and has worked at Empa for many years to develop flame retardants based on phosphorus chemistry, which have been used in many industrial applications. Now, he has successfully found an elegant and simple way to fix phosphorus inside the cotton in the form of an independent network.
The new phosphorus chemistry can also be used to develop other materials, such as making hydrogels that release drugs when the pH changes. This gel can be used to treat slow-healing wounds. In such wounds, the pH of the skin surface rises, which can trigger a new phosphorus-based gel to release drugs or dyes, reminding doctors and nurses to pay attention to the problem. Empa has also applied for a patent for the production of such hydrogels.
Gaan and his colleagues Rashid Nazir, Dambarudhar Parida, and Joel Borgstädt used a trifunctional phosphorus compound (trivinylphosphine oxide) that can only react with specific added molecules (nitrogen compounds, such as piperazine). Thus forming its own network within cotton. This makes the cotton permanently fire-resistant without blocking the favorable -OH groups. In addition, the physical phosphine oxide network also likes water. This flame-retardant treatment does not include carcinogenic formaldehyde, which can endanger textile workers during the textile manufacturing process. The phosphine oxide network formed in this way will not be washed off: after 50 washings, 95% of the flame-retardant network is still present in the fabric.
In order to provide additional protection for the flame-retardant cotton developed by Empa, the researchers also added silver nanoparticles generated in situ to the fabric. This works well with the formation of the phosphine oxide network in a one-step process. Silver nanoparticles provide the fiber with antibacterial properties and can withstand 50 washing cycles.
"We used a simple method to immobilize the phosphine oxide network inside the cellulose," Gaan said. "In our laboratory experiments, we first treated cotton with an aqueous solution of phosphorus and nitrogen compounds, and then steamed it in a ready-made pressure cooker to promote the cross-linking reaction of phosphorus and nitrogen molecules." The application process is used in the textile industry. The equipment is compatible. "The cooking of textiles after dyeing, printing and finishing is a normal step in the textile industry. Therefore, no additional investment is required to apply our process," said Empa chemist.
At the same time, this newly developed phosphorus chemistry and its application are protected by patent applications. "There are still two important obstacles," Gaan said. "For future commercialization, we need to find a suitable chemical manufacturer to produce and supply trivinyl phosphine oxide. In addition, trivinyl phosphine oxide must be REACH registered in Europe."
Reference: "In-situ Phosphine Oxide Physical Network: A Simple Strategy to Realize Durable Flame Retardant and Antibacterial Treatment of Cellulose", Authors: Rashid Nazir, Dambarudhar Parida, Joel Borgstädt, Sandro Lehner, Milijana Jovic, Daniel Rentsch, Ezgi Bülbül, Anja Huch, Stefanie Altenried, QunRen, Patrick Rupper, Simon Annaheim and Sabyasachi Gaan, December 9, 2020, Journal of Chemical Engineering. DOI: 10.1016/j.cej.2020.128028
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