Abstract
As the second most abundant biopolymer on earth, and as a resource recently becoming more available in separated and purified form on an industrial scale due to the development of new isolation technologies, lignin has a key role to play in transitioning our material industry towards sustainability. Additive manufacturing (AM), the most efficient-material processing technology to date, has likewise made great strides to promote sustainable industrial solutions to our needs in engineered products. Bringing lignin research to AM has prompted the emergence of the nascent "lignin 3D printing" field. This review presents the recent state of art of this promising field and highlights its challenges and opportunities. Following a review of the industrial availability, molecular attributes, and associated properties of technical lignins, we review R&D efforts at implementing lignin systems in extrusion-based and stereolithography (SLA) printing technologies. Doing so underlines the adage of lignin research that "all lignins are not created equal," and stresses the opportunity nested in this chemical diversity created mostly by differences in isolation conditions to molecularly select and tune the attributes of technical lignin systems towards desirable properties, be it by modification or polymer blending. Considering the AM design process in its entirety, we finally propose onward routes to bring the full potential to this emerging field. We hope that this review can help promote the unique value and overdue industrial role of lignin in sustainable engineered materials and products.
Highlights
While additive manufacturing (AM) emerged in the late 1970s to 1980s with the pioneering work of Charles Hull for stereolithography (SLA), Scott Crump for extrusion-based 3D printing and Ross Householder for powder bed fusion, the last two decades have seen exponential research and development in the technology and its implementation in various application fields such as rapid prototyping, aerospace, tooling, spare parts, customized medical tooling and some mass production.[1]
Among the bio-based polymers considered, lignocellulosic polymers have attracted particular attention for AM over the past 5 years.[4,5,6]. This stems in part from their large natural abundance—cellulose and lignin are the two most abundant biopolymers on earth—and from their recent availability in purified and/ or nanoparticulate colloidal form from traditional pulp and paper processing and from biorefineries.[6]
They attributed this to inefficient dispersion of the unmodified lignin, which tended to settle to the bottom of the printer reservoir during printing. They discovered that the addition of unmodified lignin hinders the curing process during printing. They found that the addition of 1 wt% lignin to the resin leads to less than half of the material to be cured by printing, with 54% of the noncrosslinked, uncured resin removed from partially printed parts by washing with acetone
Summary
While additive manufacturing (AM) emerged in the late 1970s to 1980s with the pioneering work of Charles Hull for stereolithography (SLA), Scott Crump for extrusion-based 3D printing and Ross Householder for powder bed fusion, the last two decades have seen exponential research and development in the technology and its implementation in various application fields such as rapid prototyping, aerospace, tooling, spare parts, customized medical tooling and some mass production.[1]. The aim is to present research, which uses lignin as an additive (more than 30% of total volume), rather than a filler (up to 5% of total volume).[8] After briefly presenting the commercial availability and structural features of technical lignins, this review attempts to highlight the recent advances in AM of lignin-based feedstocks. The advances in implementing lignin systems in each of these processing technologies is examined, with a special focus on the desirable properties of lignin for each technology. This enables the delineation of challenges and opportunities to further add value to technical lignin's through AM, as discussed
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