Researchers at KU Leuven University in Belgium report that they have developed a new type of biobased thermoplastic vulcanisate (TPV). And it’s based on gluten—the same substance thought by many to be responsible for ills ranging from chronic fatigue to irritable bowel syndrome when ingested.
What led the researchers to investigate gluten as a possible source of bioplastic? In the first pace, there is no shortage of it: gluten is the seed storage protein in mature cereal seeds. More specifically, it is a protein composite, meaning it is a substance made up of several different proteins, in this case gliadin and a glutenin. The cross-linking of gliadin molecules and glutenin molecules creates the primary properties associated with gluten. And as a co-product of industrial gluten-starch separation or bioethanol production, gluten is available in Europe in quantities of up to1 million tons on an annual basis. Only part of this gluten is used as a high-value bakery ingredient, while the excess is mostly used in animal feed.
Secondly, unlike most other proteinaceous resources, gluten contains high molar mass constituents and unique network forming properties, which means it can readily be converted into a variety of biobased materials. Homing in on gluten’s special network-formaing capacity, the Belgian gluten team has focussed its efforts on the development of gluten-based TPVs. Colorable and easily processable on conventional processing equipment, what sets these TPVs apart is the fact that unlike the olefin-based rubbers in conventional TPVs, no additional chemical crosslinker is needed: wheat gluten intrinsically crosslinks under the influence of heat. Gluten-based TPVs combine the typical properties and functional performance of rubbers with the melt processability of thermoplastic polymers, resulting in recyclable materials.
“The gluten TPV consists of (non-recyclable) crosslinked gluten particles within a thermoplastic matrix,” says Dr. Lien Telen, a postdoctoral researcher at KU Leuven, who has worked on the development of gluten-based bioplastic for thepast five years. “The main advantage of these TPVs is that they have elastomeric characteristics at room temperature combined with the melt processability of thermoplastic materials. The rubber particles are very small (a few µm) and will flow in the melt of the thermoplastic matrix making the entire material recyclable using standard thermoplastic polymer processing equipment such as extrusion and injection molding.”What is more, the gluten-based TPVs offers the possibility of combining elastomeric behavior and biodegradability in a single material. Depending on the choice of thermoplastic component, the gluten TPV’s can be designed to be fully biodegradable. Obviously, TPVs with a polyethylene or polyamide matrix are only partly biodegradable, as the matrix remains intact, making them unsuitable for composting. The team has already developed completely biodegradable and (home) compostable TPVs using a biodegradable and (home) compostable matrix. Applications for these materials could include indoor soft touch materials, or functional biodegradation applications in the agricultural and horticultural sectors.
The new materials are not yet ready to replace conventional materials, however. Telen says that the properties of the gluten-based TPVs “fall short on water-resistance, oil and chemical resistance and operational temperature range”. The team is working to improve performance in this area. At the same time, they have their eye on other materials, as well. “In the absence of a plasticizer, the heat induced crosslinking results in a glassy, rigid material with material properties comparable to polystyrene (PS)”, said Telen. “Gluten composites: rigid gluten bioplastic reinforced with flax fibers are another focus. Research on these materials is ongoing and very promising.”