Utilization of Ultrasonication and Plasticizers to Increase the Thermo-Processability of Lignocellulose

Polymeric materials are essential in today’s modern society and their production has increased exponentially since their commercialization in the 1940s. One of the largest industrial sectors is packaging, estimated to 146 million tons in 2015 [1]. A large fraction of the packaging material is single use and ends up in landfill or is incinerated, as most plastics used are non-degradable and recycling is still challenging in large parts of the world. Material research has focused on decreasing the environmental impact of these kinds of materials by reducing and replacing them with, for example cellulose-based materials like cardboard, but also more innovative materials like bio-based foams. Cellulose is an abundant biopolymer already widely used, while also being bio-degradable and recyclable. One limiting factor for conventional cellulose-based materials is the limitations when shaping cardboard and paper-based materials. Although cellulose is a polymer it does not show typical polymeric behaviors such as thermo-processability which most synthetic polymers do. Our approach, inspired by plasticization of gluten [2], is to incorporate a small amount of plasticizer into the lignocellulose matrix. Our hypothesis is that the small plasticizing molecules will reduce the strong intermolecular interactions within cellulose and enhance the molecular mobility. Successful implementation will increase the lignocellulose’s thermo-processability and yield a material which can be processed through conventional thermo-processing such as extrusion or injection moulding allowing more advanced shaping opportunities than existing methods.<br/><br/>A step on the way has been the incorporation of different bio-based plasticizers through ultrasonication as a means to open up the fibers, aiding the sorption of glycerol, urea, and citric acid. The formed materials have been evaluated in regard to plasticizer content, thermal behavior, mechanical properties, degree of crystallinity and interaction with water. In this work, the primary focus was to understand the effect of ultrasonication on plasticizer sorption and its correlation to the highly improved physical properties of the formed materials. The sonication itself, a high energy treatment, peels off fibrils from the fiber surface, breaks down the crystal region and exposes hydroxyl groups which in turn increase the ductility and ultimate strength compared to the reference material [3]. The combination of plasticizer and sonication aids the sorption of urea and glycerol which leads to an increase in the ductility and ultimate strength further. As a result, a 100% bio-based material consisting of more than 75 % lignocellulose is under evaluation as an extrudable material for packaging applications with the potential to be bio-degradable and/or recyclable.<br/><br/>Using renewable resources to produce a thermo-processable lignocellulosic material as a potential alternative to single use plastics in the packaging industry has the potential to reduce our need for fossil-based materials, promote circular materials and reduce CO₂ emissions. This in turn contributes towards the fulfillment of circular bioeconomic principles and the United Nations Sustainable Development Goals 2030.<br/><br/>1. Geyer, R., Jambeck, J. R., & Law, K. L. (2017). Production, use, and fate of all plastics ever made. Science Advances, 3(7), e1700782.<br/>2. Özeren HD, Wei X-F, Nilsson F, Olsson RT, Hedenqvist MS. Role of hydrogen bonding in wheat gluten protein systems plasticized with glycerol and water. Polymer (Guilford). <b>2021, </b>232, 124149.<br/>3. An X, Liu J, Liu L, Zhang H, Nie S, Cao H, et al. Improving the flexibility of bamboo mechanical pulp fibers for production of high soft tissue handsheets. Industrial Crops and Products. 2020;150:112410