The US Army has put the call out for new homegrown uniforms that are not only fire retardant but also insect repellent, and a team led by Dr. Ramaswamy Nagarajan at the University of Massachusetts Lowell Center for Advanced Material has answered with a new multifunctional material made by treating a commercially available nylon-cotton blend with non-toxic chemicals.
Military uniforms have come a long way from the days when their main function was to identify which side a soldier was on while intimidating the enemy. In the old days, uniforms may have been impressive and colorful, but they were also uncomfortable, hard to clean and maintain, and not very practical for either combat or everyday duties.
Today, uniforms are becoming increasingly high-tech, with digital camouflage, protection against biological and chemical agents, blast protection, and even self-heating gloves. The frustrating thing is that the armed forces with global projection, like the US Army, ideally want uniforms that will be comfortable in all climates, are easy to wash yet durable, are fire retardant, and can repel insects.
Add to that the fact the US government is keen to keep as much of the supply train as possible on domestic soil while keeping the uniforms inexpensive and eco-friendly and things get a bit more difficult.
"The Army presented to us this interesting and challenging requirement for multi-functionality," says Nagarajan. "There are flame-resistant Army combat uniforms made of various materials that meet flame retardant requirements. But they are expensive, and there are problems with dyeing the fabrics. Also, some of the raw materials are not produced in the US. So, our goal was to find an existing material that we could modify to make it flame retardant and insect repellent, yet still have a fabric that a soldier would want to wear."
For the base material, Nagarajan’s team chose a commercially available 50-50 nylon-cotton blend that had already proven itself to be inexpensive, comfortable, strong, and resistant to wear. They then made it more fire retardant by doping the fibers with a non-toxic, phosphorus-containing compound called phytic acid that is derived from seeds, nuts, and grains.
"We started with making the fabric fire retardant, focusing on the cotton part of the blend," says team member Sourabh Kulkarn. "Cotton has a lot of hydroxyl groups (oxygen and hydrogen bonded together) on its surface, which can be activated by readily available chemicals to link with phosphorus-containing compounds that impart flame retardancy."
The next step was to make the material insect repellent by adding an everyday non-toxic insect repellent known as permethrin, which is a synthetic chemical that acts like natural extracts from the chrysanthemum flower and was attached to the fibers using plasma-assisted deposition.
Under testing to determine heat release capacity and total heat release, along with a vertical flame test, the new material showed itself to be at least 20 percent more fire retardant than the untreated version. Meanwhile, live insect tests with mosquitoes demonstrated a 98 percent increase in repellency. Despite these improvements, the fabric remained breathable according to air permeability tests.
"We are very excited because we’ve shown we can modify this fabric to be flame retardant and insect repellent – and still be fairly durable and comfortable," says Nagarajan. "We’d like to use a substance other than phytic acid that would contain more phosphorus and therefore impart a greater level of flame retardancy, better durability and still be non-toxic to a soldier’s skin. Having shown that we can modify the fabric, we would also like to see if we can attach antimicrobials to prevent infections from bacteria, as well as dyes that remain durable."
The results will be presented at the American Chemical Society (ACS) Fall 2020 Virtual Meeting & Expo.
Source: American Chemical Society
