Polyethylene is one of the most common plastics in the world, but it is seldom found in clothing because it cannot absorb or carry away water. (Imagine wearing a plastic bag – you would feel very uncomfortable very quickly.) Now, however, researchers in the US have developed a new material spun from polyethylene that not only “breathes” better than cotton, nylon or polyester, but also has a smaller ecological footprint due to the ease with which it can be manufactured, dyed, cleaned and used.
The textile industry produces about 62 million tonnes of fabric each year. In the process, it consumes huge quantities of water, generates millions of tonnes of waste and accounts for 5–10% of global greenhouse gas emissions, making it one of the world’s most polluting industries. Later stages of the textile use cycle also contribute to the industry’s environmental impact. Textiles made from natural fibres such as wool, cotton, silk or linen require considerable amounts of energy and water to recycle, while textiles that are coloured or made of composite materials are hard to recycle at all.
Hydrophilic and wicking
Researchers led by Svetlana Boriskina of the Massachusetts Institute of Technology (MIT) set out to produce an alternative. They began by melting powdered low-density polyethylene and then extruding it into thin fibres roughly 18.5 μm in diameter (as measured using scanning electron microscopy and micro-computed tomography imaging techniques). This process slightly oxidizes the material’s surface so that it becomes hydrophilic – that is, it attracts water molecules – without the need for a separate chemical treatment.
Next, the researchers passed the fibres through a second extruder, creating a yarn made from bunches of 200+ PE fibres. The bunching process leaves spaces between the individual fibres in the yarn, forming capillaries through which water molecules can travel and allowing strips of fabric woven from this yarn to wick moisture when dipped in a liquid. When the researchers measured how long it took the liquid to travel up test strips, they found that the new PE material was faster than cotton, nylon and polyester samples of the same size.
To better understand the wicking process, and thus design higher-performance PE-based fabrics, the team modelled the internal structure of the PE yarn as an infinite assembly of identical parallel fibres with a circular cross-section, tightly packed into a periodic structure. The yarn fibres are arranged either in a hexagonal or square lattice with the wicking process occurring in the direction along the yarn. The model predicted that for fibres that contacted the water at an angle of 71.3°, the optimal fibre radius and porosity in both lattice shapes would be 15–20 μm and 45%.
Lower environmental impact
In addition to the new material’s promising moisture-transport properties, the researchers note that it can be dyed in a “completely dry fashion” by incorporating colour particles – either of conventional dyes or unconventional inorganic nanoparticle colourants – into the PE powder before the melting/extruding stages. In such a process, dye particles would be encapsulated within the fibres from the start, avoiding the need for traditional dyeing methods that require fabrics to be immersed in solutions of harsh chemicals. At the end of the fabric’s life, the dye particles could even be recovered for reuse by melting the material down and centrifuging it.
Members of the MIT team say that this dry-colouring process helps make the PE fabric more environmentally friendly than conventional textiles. They add that PE has a lower melting point than other synthetic polymer materials, meaning that it can be spun into yarns at lower temperatures. Synthesizing PE from raw materials also releases less greenhouse gases and waste heat than producing polyester or cultivating cotton. The latter, especially, requires a lot of land, fertilizer and water.
Textile gates infrared radiation to keep cool
PE fabric might also have a lower environmental impact while it is being used because it is easier to wash and dry than other textiles. “It doesn’t get dirty because nothing sticks to it,” Boriskina says. “You could wash polyethylene on the cold cycle for 10 minutes, versus washing cotton on the hot cycle for an hour.” It can even be “refreshed” by rubbing it against itself or exposing it to UV light – a process that helps maintain its hydrophilic properties too.
The researchers say they are now exploring ways to incorporate PE fabrics into lightweight, passively cooling sportswear and military uniforms. Spacesuits might be another possibility, they add, since PE shields against harmful radiation.
The PE fabric is described in Nature Sustainability.
