The release of formaldehyde and lack of comfort when worn are the downsides to advanced flame-retardant cotton textiles. Researchers at Empa have succeeded in circumventing these issues by developing a chemically and physically independent network of flame retardants within the fibers.
Cost-effective: Sabyasachi Gaan uses steam from a commercial pressure cooker to flame retard samples of cotton fabric. Image Credit: Empa.
This method retains the fundamentally positive properties of cotton fibers, which make up three-quarters of the demand for natural fibers in home textiles and clothing around the world. Cotton is skin-friendly as it can absorb significant amounts of water and sustain a pleasant microclimate on the skin.
Protective clothing offers the most vital barrier for firefighters and other emergency service personnel. To this end, cotton is mostly used as an inner textile layer that needs supplementary properties. For instance, it must protect against biological toxins or be fireproof.
However, it must not be hydrophobic, which would create an unpleasant microclimate. These supplementary properties can be integrated into the cotton fibers by appropriate chemical modifications.
Durability vs. Toxicity
Until now, it has always taken a compromise to make cotton fireproof.
Sabyasachi Gaan, Chemist and Polymer Expert, Advanced Fibers Lab, Empa
Wash-durable flame-retardant cotton is manufactured by treating the fabric with flame retardants, which chemically integrate with the cellulose in the cotton.
At present, without any other option, the textile sector uses formaldehyde-based chemicals — where formaldehyde is categorized as a carcinogen. This has been an unanswered issue for decades.
Although formaldehyde-based flame retardant treatments are sturdy, they have other drawbacks: The -OH groups of cellulose are chemically obstructed, which significantly decreases cotton’s capability to absorb water, resulting in a textile that is not comfortable to wear.
Gaan is well aware of the chemistry of cotton fibers as he has spent several years at Empa creating flame retardants based on phosphorus chemistry, which are already used in several industrial applications. Currently, he has managed to find an elegant and simple way to anchor phosphorous in the form of an independent network within the cotton.
Independent Network Between Cotton Fibers
Gaan and his colleagues Rashid Nazir, Dambarudhar Parida, and Joel Borgstädt employed a tri-functional phosphorous compound (trivinyl phosphine oxide), which would react only with specifically added molecules (nitrogen compounds, such as piperazine) to develop its own network within cotton. This makes the cotton lastingly fire-resistant without blocking the much-needed -OH groups.
Moreover, the physical phosphine oxide network is also fond of water. This flame-retardant treatment does not involve carcinogenic formaldehyde, which would cause harm to textile manufacturing workers. The phosphine oxide networks, thus created, do not wash out. Even after 50 washes, 95% of the flame-retardant network remained in the material.
To give other protective features to the flame-retardant cotton created at Empa, the team also added in situ generated silver nanoparticles into the material. This adapts well in a single-step process while also producing the phosphine oxide networks. Silver nanoparticles render the fiber with antimicrobial properties and endure 50 laundry cycles, too.
A High-Tech Solution from the Pressure Cooker
We have used a simple approach to fix the phosphine oxide networks inside the cellulose. For our lab experiments, we first treated the cotton with an aqueous solution of phosphorus and nitrogen compounds and then steamed it in a readily available pressure cooker to facilitate the crosslinking reaction of the phosphorus and the nitrogen molecules.
Sabyasachi Gaan, Chemist and Polymer Expert, Advanced Fibers Lab, Empa
The application process is well-matched with equipment used in the textile sector.
“Steaming textiles after dyeing, printing, and finishing is a normal step in textile industry. So it doesn’t require an additional investment to apply our process,” adds the Empa chemist.
This newly formed phosphorus chemistry and its application are safeguarded by a patent application.
Two important hurdles remain. For future commercialization, we need to find a suitable chemical manufacturer who can produce and supply trivinylphosphine oxide. In addition, trivinylphosphine oxide has to be REACH-registered in Europe.
Sabyasachi Gaan, Chemist and Polymer Expert, Advanced Fibers Lab, Empa
Journal Reference:
Nazir, R., et al. (2021) In-situ phosphine oxide physical networks: A facile strategy to achieve durable flame retardant and antimicrobial treatments of cellulose. Chemical Engineering Journal. doi.org/10.1016/j.cej.2020.128028.