Global warming potential (GWP), from cradle-to-grave, of two benchmark materials and two nanocellulose-reinforced composites.'Green' composites using cellulose fibers as reinforcement promise a sustainable and renewable alternative to petroleum-based plastics. But how green are these composites? Koon-Yang Lee, at Imperial College London, and colleagues at University College London led by Paola Lettieri have quantified the environmental impact of these materials [Hervy et al., Composites Science and Technology 118 (2015) 154].
Derived from biomass like wood pulp or synthesized by bacteria, nanoscale cellulose fibers (or nanocellulose) offer an environmentally friendly alternative for composite reinforcement without sacrificing performance. Spectroscopy measurements indicate that single cellulose fibers boast tensile moduli – or resistance to deformation – of 100-160 GPa, as well as lower toxicity and density than conventional glass fibers.
So Lee and Lettieri compared the environmental burden of epoxy reinforced with bacterial cellulose (BC) or cellulose derived from wood fiber (nanofibrillated cellulose or NFC) with conventional glass fiber-reinforced polypropylene (GF/PP) and the best performing bio-derived polymer, polylactide (PLA).
The researchers’ life cycle assessment (LCA), which spans every stage of production from extraction of raw ingredients to manufacture of final products (or ‘cradle-to-gate’), found a higher environmental burden associated with BC- and NFC-reinforced epoxy composites than GF/PP and PLA.
“It came as a surprise to us as nanocellulose-reinforced epoxy composites might not be as environmental friendly as we initially thought,” says Lee.
One of the problems with nanocellulose-reinforced epoxy composites, he explains, is the use of vacuum assisted resin infusion (or VARI) in manufacturing, which requires non-environmentally friendly consumables. And while producing NFC might be expected to be an energy intensive process, the team was also surprised to find that BC has an even greater environmental burden. The synthesis of BC has a few so-called environmental impact ‘hot spots’ such as the production of glucose for the bacterial growth medium and the cleaning and purification of BC after culturing.
However, when the entire life cycle – including use and disposal – are taken into account, nanocellulose-based composites come into their own. Automotive parts, for example, where lower weight parts can significantly reduce fuel consumption, could be a particularly attractive option for nanocellulose-based composites. And where higher volumes of nanocellulose are used, the cradle-to-grave comparison to conventional PLA materials is even more favorable.
“[Our findings] suggest that nanocellulose-reinforced epoxy composites with high nanocellulose loading is desirable to produce materials with ‘greener credentials’ than the best performing commercially available bio-derived polymers,” says Lee.
To make nanocellulose-reinforced polymer composites ‘truly green’, he suggests, more energy efficient and higher-yield cellulose manufacturing processes, as well as lower impact composite manufacturing methods such as lamination, are needed.
“[We are] looking to develop the next generation of nanocellulose-reinforced polymers by applying green engineering principles to reduce the use of solvents and energy,” Lee adds.