Can we recycle plastic bags into materials of the future?

When considering materials that could become the fabrics of the future, scientists have largely rejected one widely available option: polyethylene.

The stuff of plastic wrap and grocery bags, polyethylene is thin and lightweight, and can keep you cooler than most textiles because it lets heat through instead of trapping it. But polyethylene also retains water and sweat because it cannot drain and evaporate moisture. This anti-winding property has largely deterred the acceptance of polyethylene as a wearable textile.

Now MIT engineers have spun polyethylene into fibers and yarns designed to wick moisture away. They weaved the yarns into silky soft, lightweight fabrics that absorb and evaporate water faster than normal textiles such as cotton, nylon and polyester.

They also calculated the carbon footprint that polyethylene would have if it were produced and used as a textile. Contrary to most assumptions, they estimate that polyethylene fabrics may have a smaller impact on the environment during their life cycle than cotton and nylon.

The researchers hope that fabrics made from polyethylene can be an incentive to recycle plastic bags and other polyethylene products into wearable textiles, which contributes to the durability of the material.

“Once someone throws a plastic bag into the ocean, that’s a problem. But those bags can be easily recycled, and if you can make a sneaker or hoodie from polyethylene, it would make economic sense to pick up and recycle these bags says Svetlana Boriskina, a research scientist in the Mechanical Engineering Department at MIT.

Boriskina and her colleagues detailed their findings today in Sustainability of nature.

Water wick

A molecule of polyethylene has a backbone of carbon atoms, each with a hydrogen atom attached to it. The simple structure, repeated many times, forms a Teflon-like architecture that does not stick to water and other molecules.

“Everyone we spoke to said that polyethylene might keep you cool, but it doesn’t absorb water and sweat because it repels water, and therefore wouldn’t work as a textile,” says Boriskina.

Still, she and her colleagues tried to make woven fibers from polyethylene. They started with polyethylene in its raw powder form and used standard textile manufacturing equipment to melt and extrude polyethylene into thin fibers, much like twisting strands of spaghetti. Surprisingly, they found that this extrusion process slightly oxidized the material, changing the surface energy of the fiber, so that polyethylene became weakly hydrophilic and able to pull water molecules to the surface.

The team used a second standard extruder to bundle multiple polyethylene fibers together to make a weave yarn. They found that, within a strand of yarn, the spaces between fibers formed capillaries that allowed water molecules to be passively absorbed as soon as they were attracted to the surface of a fiber.

To optimize this new moisture-wicking ability, the researchers modeled the properties of the fibers and found that fibers with a certain diameter, aligned in specific directions through the yarn, improved the moisture-wicking ability of the fibers.

Based on their models, the researchers created polyethylene yarn with more optimized fiber arrangements and dimensions, and then used an industrial loom to weave the yarn into fabrics. They then tested the moisture-wicking ability of polyethylene over cotton, nylon and polyester by dipping strips of the fabrics in water and measuring the time it took for the liquid to soak up or climb up each strip. They also placed each substance on a scale over a single water droplet and measured its weight over time as the water evaporated and evaporated through the substance.

In each test, polyethylene fabrics were drained and the water evaporated faster than other normal textiles. The researchers did see that polyethylene lost some of its hydrophilic properties by repeatedly getting wet, but by simply applying some friction or exposing it to ultraviolet light, they caused the material to become hydrophilic again.

“You can freshen up the material by rubbing it against itself, so it retains its moisture-wicking ability,” says Boriskina. “It can continuously and passively pump out moisture.”

Eco cycle

The team also found a way to incorporate color into the polyethylene fabrics, which was challenging, again due to the material’s resistance to binding with other molecules, including traditional inks and dyes. The researchers added colored particles to the powdered polyethylene before extruding the material into fiber form. In this way, particles were encapsulated in the fibers, successfully giving them color.

“We don’t have to go through the traditional process of dyeing textiles by immersing them in solutions of harsh chemicals,” says Boriskina. “We can color polyethylene fibers in a completely dry way and at the end of their life cycle we can melt, centrifuge and recover the particles for reuse.”

The team’s dry dyeing process adds to the relatively small carbon footprint that polyethylene would have if it were used to make textiles, the researchers say. The team calculated this footprint using a life cycle analysis tool widely used by the textile industry. Taking into account the physical properties of polyethylene and the processes required to make and color the fabrics, the researchers found that it takes less energy to produce polyethylene textiles compared to polyester and cotton.

“Polyethylene has a lower melting temperature, so you don’t have to heat it up as much as other synthetic polymer materials to make yarn,” Boriskina explains. “The synthesis of raw polyethylene also releases less greenhouse gas and waste heat than the synthesis of more conventional textile materials such as polyester or nylon. Cotton also takes a lot of land, fertilizers and water to grow and is treated with harsh chemicals, all of which have a huge ecological footprint. “

In the use phase, polyethylene fabric could also have a smaller impact on the environment, she says, because it takes less energy to wash and dry the material compared to cotton and other textiles.

“It doesn’t get dirty because nothing gets stuck to it,” says Boriskina. “You could wash polyethylene on the cold cycle for 10 minutes, instead of cotton on the hot cycle for an hour.”

The team is exploring ways to incorporate polyethylene fabrics into lightweight, passive-cooling athletic apparel, military apparel and even next-generation space suits, as polyethylene protects against the harmful X-rays in space.