Biodegradable Polymer-Clay Nanocomposites for Food Packaging

Abstract

Plastic pollution is everywhere. We can see the buildup growing in our landfills and oceans in the recent decades, but perhaps more dangerous is the micro-sized plastic that we cannot see. The resistance of conventional plastics to degradation means that the physical breakdown of bulk plastic in the environment leaves behind tiny plastic fragments that are mobile and hazardous to ecosystems and human health. It is this same resistance to degradation, as well as other favorable physical characteristics, that have made plastics indispensable to society. The largest consumption of plastic is for single-use, disposable packaging. Alternative materials in this sector that would not persist in natural systems, i.e. capable of undergoing biodegradation, would mitigate meaningful amounts of plastic waste from accumulating in the environment. The challenge is matching the properties of the conventional materials with ones that are also susceptible to biodegradation. Focusing in on single-use food packaging, the requirements of a suitable material include a barrier to gases that cause deteriorative reactions, like oxygen and water vapor, and to be mechanically apt for the given application (flexible film vs. rigid container). Moreover, for a material to be truly feasible as a replacement to conventional materials, it should be processed on a large scale and available at a reasonable cost. In this work, the combination of biodegradable polymers with clay nanosheets was explored to meet the requirements of a food packaging material without sacrificing rapid environmental biodegradation. Nanosheets within a polymer matrix act as impermeable obstacles, creating a tortuous path against the diffusion of small molecules through the material. Synthetic sodium fluorohectorite (Hec) is particularly adapt for this purpose due to its exceptionally high aspect ratio and its rare ability to osmotically swell in water. Osmotic swelling allows for near effortless conversion from the bulk clay into individual platelets that form a stable, liquid crystalline suspension. A polymer solution can then be processed with the clay suspension to create high barrier films. The manuscripts in this thesis demonstrate strategies to process clay nanosheets with biodegradable polymers to obtain films for food packaging, and the effects that this combination may have on relevant film properties beyond barrier. The first study sought to improve commercial poly(lactide) (PLA); a biodegradable polymer that suffers from poor barrier properties, low flexural strength, and slow biodegradation in aqueous environments. The direct combination of this hydrophobic polyester with liquid crystalline clay suspensions had been restricted by the requirement of water to achieve osmotic swelling. By the addition of crown ethers to complex interlayer cations and provide steric pressure, osmotic delamination of Hec was achieved and finally allowed for solution processing of a PLA/Hec nanocomposite. This nanocomposite, prepared from layered slot-die coating, exhibited excellent barrier to oxygen and resistance to swelling under humid conditions, although mechanically brittle. Once immersed in water, the clay tactoids swell, physically fragmenting the film. The accelerated biodegradation rate in wastewater observed for the PLA/Hec film was in part attributed to the increased surface area after fragmentation. The following study took a step away from commercial polymers to focus on a promising novel polyester for flexible films that exhibited rapid hydrolysis and stretchable mechanical performance, however its gas barrier was unsuitable. Rather than the direct combination of the polyester with a barrier clay, which would cause embrittlement, a nanocomposite coating was applied by spray coating an aqueous glycol chitosan/Hec suspension. The nanocomposite coated film maintained the polyethylene-like mechanical behavior and rapid biodegradation in wastewater, although slightly slower than the polyester without a coating. Regardless of polymer choice, the high-quality synthetic Hec imparts steep prices on the nanocomposite film. To substitute the expensive synthetic Hec, a natural and abundant vermiculite clay was investigated for the production of barrier nanosheets in solution. Although vermiculite materials have historically required long procedures to obtain osmotically swollen states, by complexation of the interlayer Mg2+ cation with an appropriate anion, a quick and sufficient ion exchange is facilitated with yields as high as 84%. To demonstrate the effectiveness of these nanosheets as a barrier filler, a PLA/vermiculite nanocomposite coating was applied to a porous cellulose nanofiber substrate. Dramatic increase in the barrier to oxygen and water vapor were obtained and the low costs of natural vermiculite bring down the total price of the nanocomposite film. Lastly, biodegradable packaging beyond food applications was investigated. Films applied for water-soluble packets of single portion detergents, pesticides etc., are typically made from poly (vinyl alcohol), despite its poor biodegradation in the intended medium of disposal: wastewater. Biopolymers hydroxypropyl methylcellulose and sodium alginate were investigated as replacements for this application. Sandwich layered films of each polymer were prepared with a simulated roll-to-roll processing scheme. A pure Hec center layer in the films acted as an impermeable barrier wall that also provided mechanical reinforcement. The layered structure with the barrier filler center did not significantly impede biodegradation, although only the alginate-based film exhibited rapid mineralization in wastewater.