Researchers from Purdue University, West Lafayette, Ind., developed tough, flexible, biodegradable films from cellulose—the main component of plant cell walls—for food packaging.
Food scientists Srinivas Janaswamy and Qin Xu engineered the cellophane-like material by solubilizing cellulose using zinc chloride, a common inorganic salt, and adding calcium ions to cause the cellulose chains to become tiny fibers known as nanofibrils, thus greatly increasing the material's tensile strength. The zinc chloride and calcium ions work together to form a gel network, allowing the researchers to cast the material into a transparent, food-grade film.
"We're looking for innovative ways to adapt and use cellulose—an inexpensive and widely available material—for a range of food, biomedical and pharmaceutical applications," says Janaswamy, research assistant professor of food science and principal author of the study. "Though plastics have a wide variety of applications, their detrimental impact on the environment raises a critical need for alternative materials. Cellulose stands out as a viable option, and our process lays a strong foundation for developing new biodegradable plastics."
Cellulose's abundance, renewability and ability to biodegrade make it a promising substitute for petroleum-based products. While a variety of products such as paper, cellophane and rayon are made from cellulose, its tightly interlinked structure and insolubility—qualities that give plants strength and protection—make it a challenging material to work with.
Janaswamy and Xu loosened the cellulose network by adding zinc chloride, which helps push cellulose's closely packed sheets apart, allowing water to penetrate and solubilize. Adding calcium ions spurs the formation of nanofibrils through strong bonds between the solubilized cellulose sheets. The calcium ions boost the tensile strength of the films by about 250%.
The production process preserves the strength and biodegradability of cellulose while rendering it transparent and flexible. And, because the zinc chloride can be recycled to repeat the process, the method offers an environmentally friendly alternative to conventional means of breaking down cellulose, which tend to rely on toxic chemicals and extreme temperatures.
"Products based on this film can have a no-waste lifecycle," says Xu, research assistant professor of food science and first author of the study. "This process allows us to create a valuable product from natural materials, including low-value or waste materials such as corn stover or wood chips that can eventually be returned to the Earth."
The next step in the project is to find ways of making the cellulose film insoluble to water while maintaining its ability to biodegrade.
Purdue University graduate student Tianming Yao and then-undergraduate students Chen Chen and Katelyn Rosswurm also contributed to the research.