Confucious say, "the green reed which bends in the wind is stronger than the mighty oak which breaks in a storm." The same concept applies to packaging materials, which must protect their contents from the rough and tumble of transport without breaking. Petroleum-based plastics that can take centuries to break down remain the go-to material for such applications, but researchers have found that adding broken eggshells to a bioplastic mix results in a biodegradable material with the strength and flexibility required for packaging purposes.
Despite cracking easily in the middle, when laid end to end eggshells are remarkably strong. Pound for pound, they are as strong as the stone, brick and concrete arches that support Rome's ancient aqueducts. Scientists at Alabama's Tuskegee University have leveraged this strength to increase the flexibility and strength of bioplastic through the addition of tiny shards of eggshell.
"We're breaking eggshells down into their most minute components and then infusing them into a special blend of bioplastics that we have developed," says Vijaya K. Rangari, Ph.D. "These nano-sized eggshell particles add strength to the material and make them far more flexible than other bioplastics on the market. We believe that these traits – along with its biodegradability in the soil – could make this eggshell bioplastic a very attractive alternative packaging material."
After experimenting with various plastic polymers, the team arrived at a mix of 70 percent polybutyrate adipate terephthalate (PBAT), a petroleum polymer, and 30 percent polylactic acid (PLA), a polymer derived from renewable resources, such as corn starch. Although PBAT is an oil-based plastic polymer, it begins degrading in as little as three months after being placed in soil.
While this mixture offered the desired strength and biodegradability, it was too rigid for the researchers' liking. To rectify this, they created nanoparticles made of eggshells, a material they chose for its porosity, light weight and calcium carbonate composition that means it decays easily.
Creating the nanoparticles involved first washing the the eggshells and grinding them up in polypropylene. The shell fragments were then exposed to ultrasonic waves which broke them down into nanoparticles over 350,000 times smaller than the diameter of a human hair.
A small fraction of these particles was then added to the 70/30 mix of PBAT and PLA, resulting in a bioplastic the researchers say was 700 percent more flexible than other blends, making it ideal for use in retail packaging, grocery bags and food containers – including egg cartons.
The team will present their research at a meeting of the American Chemical Society, and is also examining the potential for eggshell nanoparticles to improve wound healing, bone regeneration and drug delivery.
Source: American Chemical Society
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