Can we recycle plastic bags into materials of the future?

When considering materials that could become the materials of the future, scientists rejected one of the available options: polyethylene, for the most part.

The material plastic packaging and grocery bags, polyethylene, are thin and lightweight and can keep you cooler than most textiles because it lets heat through, rather than trapping it. But polyethylene can also include water and sweat because it can not pull away and moisture evaporates. This antifouling property has made it an important deterrent to the acceptance of polyethylene as a portable textile fabric.

Now MIT engineers have spun polyethylene into fibers and yarns designed to absorb moisture. They weave the yarns into silky, lightweight materials that absorb water and evaporate faster than ordinary textiles such as cotton, nylon and polyester.

They also calculated the ecological footprint that polyethylene would have if it were manufactured and used as a textile. Contrary to most assumptions, they estimate that polyethylene fabrics may have a smaller environmental impact over their life cycle than cotton and nylon textiles.

The researchers hope that polyethylene fabrics can provide an incentive to recycle plastic bags and other polyethylene products into wearable textiles, which could increase the material’s sustainability.

“Once someone throws a plastic bag into the ocean, it’s a problem. But the bags can be easily recycled, and if you can make polyethylene in a sneaker or a hoodie, it makes economic sense to put those bags on. count and recycle, “says Svetlana Boriskina, a research scientist in MIT’s mechanical engineering department.

Boriskina and her colleagues presented their findings today in Nature Sustainability.

Water fuse

A polyethylene molecule has a backbone of carbon atoms, each with a hydrogen atom. The simple structure, which is repeated repeatedly, forms a Teflon-like architecture that resists sticking to water and other molecules.

“Everyone we spoke to said that polyethylene can keep you cool, but it will not absorb water and sweat because it rejects water, and therefore it will not work as a textile,” says Boriskina.

Nevertheless, she and her colleagues tried to make wearable fibers from polyethylene. They started with polyethylene in its raw powder form and used standard textile manufacturing equipment to melt and express polyethylene into thin fibers, similar to spinning spaghetti strands. Surprisingly, they found that this extrusion process slightly oxidized the material, changing the surface energy of the fiber, so that polyethylene became poorly hydrophilic and could attract water molecules to the surface.

The team used a second standard extruder to join multiple polyethylene fibers together to make a wearable yarn. They found that the spaces between fibers within a wire yarn form capillaries through which water molecules can be passively absorbed as soon as they are attracted to the surface of a fiber.

To optimize this new drainage capacity, the researchers modeled the properties of the fibers and found that fibers with a certain diameter, aligned in specific directions in the yarn, improve the fiber capacity of the fiber.

Based on their modeling, the researchers made polyethylene yarns with more optimized fiber arrangements and dimensions, and then used an industrial fabric to weave the yarn into fabrics. They then tested the drainage capacity of polyethylene fabric on cotton, nylon and polyester by dipping strips of the fabrics in water and measuring the time it took the liquid to pull up, or to climb up each strip. They also placed each substance on a scale over a single drop of water and measured its weight over time as the water flowed through the substance and evaporated.

In each test, polyethylene fabrics evaporated the water faster than other ordinary textiles. The researchers did observe that polyethylene lost its water-attracting ability with repeated wetting, but by simply applying friction or exposure to ultraviolet light, they caused the material to become hydrophilic again.

“You can refresh the material by rubbing it against itself, thus retaining its drainage capacity,” says Boriskina. “It can pump moisture away continuously and passively.”

Ecocycle

The team also found a way to incorporate color into the polyethylene fabrics, which was a challenge because of the material’s resistance to bonding 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 are encapsulated in the fibers, which successfully color them.

“We do not need the traditional process of textile painting by painting it in solutions of harsh chemicals,” says Boriskina. “We can dye polyethylene fibers in a completely dry way and at the end of their life cycle we can melt, centrifuge and recycle the particles for reuse.”

The team’s dry dyeing process contributes to the relatively small ecological footprint that polyethylene would have if used to make textiles, the researchers say. The team calculated this footprint using a life cycle assessment tool commonly used by the textile industry. Taking into account the physical properties of polyethylene and the processes required to make and dye the material, the researchers found that it required 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 polymeric materials to make yarn,” Boriskina explains. “Synthesis of raw polyethylene also releases less greenhouse gas and waste heat than synthesis of more conventional textile materials such as polyester or nylon. Cotton also takes in a lot of soil, fertilizer and water to grow and is treated with harsh chemicals, all of which come with a large ecological footprint. ‘

In the use phase, polyethylene fabric can also have a smaller environmental impact, as it requires less energy to wash and dry the material compared to cotton and other textiles.

“It does not get dirty because nothing stays with it,” says Boriskina. “You can wash polyethylene for ten minutes on the cold cycle, as opposed to washing cotton for an hour.”

The team is investigating ways to incorporate polyethylene fabrics into lightweight, passively cooling athletic wear, military clothing and even next-generation spacecraft, as polyethylene protects against the harmful X-rays of space.