Polar bears are known for their warm fur, which protects them from the cold even in deep frost. Researchers have now developed a new type of fiber made from an airgel material that has a similarly good heat-insulating effect as polar bear fur. Unlike previous aerogels, the new material is also durable and stretchy at the same time, as the scientists demonstrate. This means it can easily be knitted or woven into textiles such as sweaters and other clothing items. Their properties also qualify the so-called EAFs for many other areas of application.
Materials scientists have been trying to produce textiles from aerogels for a long time. These materials, which were developed back in 1931, are so named because during their production the liquid in gels is exchanged for gases (sol-gel process). Aerogels have excellent thermal insulation because this process makes them very porous and conducts extremely little heat. NASA therefore uses these materials for its spacecraft. However, fibers made from aerogels are generally very fragile and are therefore not yet suitable for processing in textiles. They lack the strength and stretch needed to be woven or knitted into practical garments. In addition, airgel fibers are not machine washable and lose their insulating effect when wet. Attempts to combine various aerogels or airgel fibers in low concentrations with other materials have so far only resulted in textiles with rather poor thermal insulation.
Model polar bear fur
To overcome these shortcomings, a research group led by Mingrui Wu from Zhejiang University in Hangzhou looked for models for robust insulating fibers in nature. Many animals in very cold areas have special fur that keeps them warm and dry at the same time. These include polar bears, whose insulating fur consists of hair with a porous, largely hollow core and a dense shell. This makes the hair flexible and robust at the same time, effectively traps air and keeps moisture out.
Wu and his colleagues recreated this structure in the laboratory. Using a so-called freeze-spinning method, they first produced polymer airgel fibers with countless tiny lamellar cavities. To do this, they pressed the polymer solution into long frozen rods. The scientists freeze-dried these and then covered them with a thin, stretchy rubber layer, which they allowed to dry using heat. The researchers used different colored chitosan aerogels for the core fibers and a transparent thermoplastic polyurethane for the shell. The result was colored encapsulated airgel fibers (EAF), the structure of which resembled the hair of polar bears under the microscope.
Stretchy and warming sweater made from airgel fibers
The scientists then tested the properties of these new fibers. The result: They insulate and insulate heat excellently and are also water-repellent, robust and flexible. The results were best when the protective layer was 80 micrometers thick and the inner airgel fiber had a diameter of 600 micrometers. For example, such a fiber can be stretched up to a thousand times its original length without tearing, wearing out or losing its insulating effect, as the team reports. For comparison: traditional airgel fibers can only be stretched by around two percent. A single fiber with a diameter of 500 micrometers was also able to support a weight of 500 grams without breaking.
These properties qualify the new material for processing into knitted or woven thermal clothing for everyday use in extremely cold regions. To demonstrate this, Wu and his colleagues wove a thin sweater using the fibers they developed with the same knit structure as a sweater made from wool yarn. The EAF sweater insulated heat just as well as a down jacket five times as thick, the scientists report. Overall, the sweater warmed significantly better than comparable items of clothing made from down, wool or cotton. On top of that, it could be washed in a washing machine without changing.
Other areas of application are conceivable
Another advantage: the production of EAF fibers is cheaper than other yarns, write Zhizhi Sheng and Xuetong Zhang from the Chinese Academy of Sciences in a comment on the study. Their porous structure and associated properties make such encapsulated airgel fibers ideal for numerous other industrial applications, for example for the adsorption or separation of substances through membranes or for energy-storing materials. To do this, however, the production of EAFs would first have to be optimized for mass products. “By simultaneously advancing the materials and their production, airgel fibers could have many potential applications,” they write.
Source: Mingrui Wu (Zhejiang University) et al., Science, doi: 10.1126/science.adj8013