Sustainable Polymers and Plastic Alternatives
In 2019, the global production of plastics was 260 million tons, an increase of 230x compared to 1950. This growth in plastic utilization continues today. While plastics have provided tremendous advantages in processing energy and mass (for shipping) compared to traditional materials like glass, there are challenges associated with their disposal. In addition, the majority of plastics in use today are not recycled and many are not recyclable, and the production of new virgin plastic to replace those disposed of is associated with greenhouse gas releases.
Packaging is the dominant form of plastic waste, as seen above, and is responsible for nearly half of global plastic waste. A significant fraction of single-use packaging is multilayers of different materials. Processes to recycle these complex multicomponent materials do not exist. Our research is focused on addressing these problems.
To address this multimaterial challenge, our research is focusing on developing biodegradable, compostable, or recyclable multilayer barrier materials derived from renewable sources. Nanomaterials from cellulose and chitin (above) are versatile options for biobased packaging materials because of their superior physical properties and ability to be assembled into different shapes. Two project thrusts are described below.
Improving Recyclability of Multimaterial Plastics
Multilayer films consisting of nanocellulose and nanochitin as oxygen barrier coatings and a conventional petroleum-based moisture barrier plastic as the substrate have been widely studied for food packaging applications. A challenge with enabling recycling of such hybrid multilayer materials is separating the coating from the substrate. We are studying methods to carry out this recycling. One example is multilayer films consisting of poly(ethylene terephthalate) (PET) as a model substrate with chitin nanowhiskers (ChNWs) and cellulose nanocrystals (CNCs) as bilayer barrier coatings. In a recent publication, we showed that the barrier CNC and ChNW can be removed in aqueous media at different pH values with only small losses in barrier properties after recycling and reapplying the coatings in multiple cycles.
Renewable and Biodegradable Barrier Polymers
The demand for packaging materials with low gas permeabilities is increasing, but commonly used petroleum-derived single-use plastics are not renewable, biodegradable, or easy to recycle. Nanomaterials composed of chitin and cellulose, which are abundant in nature, have high crystallinities and low oxygen permeabilities (OP), provide a viable alternative. In this project, we are exploring how processing of the nanoscale chitin fibers through deacetylation can be used to tune the charge and size of resulting chitin nanowhiskers (ChNWs), and to control the OP of layered ChNW-CNC (cellulose nanocrystal) film structures.
Link to NBC Nightly News Story on this research.
We showed recently that ChNWs prepared under aggressive deacetylation conditions had shorter lengths and higher surface charge. These were coated onto a renewable substrate, cellulose acetate (CA) (above). With optimal deacetylation, CA-ChNW-CNC films resulted in ~20% decrease in WVTR in comparison to uncoated CA. The optimization of process conditions resulted in CA-ChNW-CNC films with up to 99% decrease in OP into a range useful for food preservation.
Accelerating Materials Discovery
The development of high-throughput experimentation (HTE) methods and integrating them with databases and datamining enables accelerating the discovery of high-performance multicomponent materials for sustainable plastics for a wide range of applications, including electronics, packaging and others. The generation and characterization of gradient thin-film library samples is a common approach to enable materials HTE, but the ability to study many systems is impeded by the need to overcome unfavorable solubilities and viscosities among other processing challenges under ambient conditions.
Our group has developed a solution coating approach for the deposition of composition gradient polymer libraries in a range of solvents at elevated temperature, described here. Residence time distribution modeling was employed to predict the coating conditions necessary to generate composition gradient films using a poly(3-hexylthiophene) and poly(styrene) model system. Poly(propylene) and poly(styrene) blends were selected as a first demonstration of high temperature gradient film coating: the blend represents a polymer system where gradient films are traditionally difficult to generate via existing coating approaches due to solubility constraints under ambient conditions. The methodology developed here is expected to widen the range of solution processed materials that can be explored via high-throughput laboratory sampling and provides an avenue for efficiently screening multiparameter materials spaces and/or populating the large data sets required to enable data-driven materials science.