Recyclability of additively manufactured bio-based composites

Abstract

As the flexibility, efficiency, and application space of additive manufacturing continues to grow, many have begun to investigate more sustainable feedstocks as well as options for the end of life of additively manufactured parts. This study examines the effects of mechanical recycling on additively manufactured parts from bio-based feedstock. Articles were printed on the Big Area Additive Manufacturing (BAAM) system at the Oak Ridge National Laboratory using poly (lactic acid)/wood flour (PLA/WF) pellets. These parts were shredded and granulated, and the granulate was fed directly back into the BAAM system for re-printing, skipping the costly and energy-intensive steps of extrusion and pelletization. The chemical, mechanical, thermal, and rheological changes to PLA/WF before and after recycling were investigated. Additionally, the energy savings from directly printing granulate on the BAAM system without extrusion and pelletization is reported. It is shown that PLA/WF is an excellent candidate for recycling of large format additively manufactured parts, and value of these parts can be reclaimed while saving cost and energy through mechanical recycling.

Introduction

Interest in additive manufacturing (AM, also known as 3D printing) has exploded in recent years, with constant improvements from emerging robotics and machinery, process control and simulation, and feedstocks. Fused deposition modeling (FDM) of thermoplastics is among the most popular 3D printing methods, and commercially available systems can range in size from the desktop scale, ∼0.3 m³, to 90+ m³ build volumes [1]. In addition to the complex geometries and high-fidelity parts achievable with AM, it typically produces less waste than traditional manufacturing methods and can enable a distributed manufacturing system in which production occurs at or near the location of the end-user. The distribution of manufacturing could also encourage the use of local materials and resources, improve supply chain stability, enhance local economies, and ultimately enable more sustainable manufacturing practices [2].

Large format additive manufacturing (LFAM) systems such as the Big Area Additive Manufacturing (BAAM) co-developed by Oak Ridge National Laboratory (ORNL) and Cincinnati, Inc., have opened a variety of new application spaces for AM in areas such as large-scale tooling, aerospace, building and construction, marine, and wind energy [3]. These systems are often pellet-fed and can be used to print neat polymers as well as fiber-reinforced thermoplastic composites. Typical feedstocks for LFAM systems include engineering plastic resins such as acrylonitrile butadiene styrene (ABS), polyethylene terephthalate glycol (PETg), poly(lactic acid) (PLA), and such high-performance polymers as polyether ether ketone (PEEK) and polyetherimine (PEI), which are often reinforced with carbon or glass fibers [4]. Of those feedstocks, ABS and PLA are the most commonly used for both small and large scale printers [5].

With the increasing attention on sustainability and the effects of industrial processes and waste on the environment, plastic waste and pollution has become a subject of particular scrutiny. Research interest has accordingly risen in the development of bio-based alternatives to petroleum-derived plastics as well as biodegradation and recycling of plastics [6]. As mentioned, PLA is a popular material used in AM and is currently one of the only completely bio-based polymers available for AM feedstocks on a commercial scale [7]. To enhance the properties and lower the cost of PLA while maintaining its sustainable and bio-based qualities, natural particles or fibers such as wood flour (WF), bamboo, flax, and hemp fibers are often added as reinforcing agents [[8], [9], [10], [11]]. Wood-plastic composites in particular have been used widely as long-lasting and sustainable alternatives to timber products for construction materials such as decking, cladding, and framing [12]. Using these sustainable materials as feedstocks for LFAM can further reduce its carbon footprint and expand the potential applications for bio-based plastics and composites. Fiber-reinforced composite LFAM feedstocks have found notable success in the production of tooling, and bio-based composites such as PLA/WF are excellent candidates to produce low temperature molds. The potential to recycle and re-print such materials for new tooling could further lower their cost and environmental impact [13]. This study explores the potential to recycle bio-based LFAM parts to determine if recycled feedstock could be used again and retain its value in the same application.

A typical mechanical recycling process for thermoplastic polymers and composites consists of shredding or grinding and re-extrusion into a filament, which can be further cut to pellets [14,15]. Numerous studies have been conducted on the recycling of thermoplastic composites, including PLA-based composites and small-scale AM parts, but very few studies are available on either the recycling of LFAM parts or on the ability of printers to process nonuniformly-sized feedstock [10,14,[16], [17], [18], [19], [20]]. As the BAAM system was designed to accommodate pellets of varying shapes and sizes, this study explored printing articles directly from recycled granulate, skipping the costly and time-consuming extrusion and pelletization steps that can also degrade fibers and polymer matrices [10]. The material properties of samples printed from virgin feedstock and recycled granulate on the BAAM system were characterized and compared, and the energy savings of mechanical recycling without extrusion and repelletization was calculated.