The Progress in Using 3-D Printing Wastes Towards a Circular Economy

. Additive manufacturing (AM) is a growing technology due to its ability to improve contemporary production techniques. However, resultant waste from the involved processes is a growing concern. In this mini-review, we explore on the strategies that can be employed to incorporate 3D printing waste and in particular, plastics in a circular economy (CE) to reduce and alleviate their negative environmental effects. Linking CE into AM in this context is a new tendency aimed at promoting eco-friendliness considering the negative pollution effects of plastics particularly, the polymers used in 3D printing. The field is less explored hence the novelty in this mini-review. Some of the proposed CE strategies possible to apply in AM include, recycling, refurbishing, recycling, remanufacturing, repurposing, reuse and reinventing associated products. The raw materials used, product-use and product effects can be modified to enhance reductive, avoidance and restorative tendencies. The waste could also be used directly, reprocessed or chemically recycled to prevent its pollution threat. Evidently, the potential to incorporate CE in AM is huge and should be prioritized for sustainable production processes.


Introduction
A circular economy (CE) is an economic approach aimed at tackling the growing issues of pollution, waste management, biodiversity loss and climate change through principles of efficient resource use, recycling, low emissions and consumption [1].Being a regenerative and restorative process, CE targets biomass, bioproducts, food waste, essential raw materials, construction waste and plastics in its action plans [2].To comprehensively alleviate the waste problems, CE strategies must be employed in all economic sectors including the manufacturing sector to extend the life cycle of raw materials used and their resultant wastes from production processes.
The principles of CE are applicable in 3-D printing or additive manufacturing (AM), which is a technology that enables the sequential layering of materials mainly plastic polymers to make three-dimensional products [3].The technology is transformative in contemporary industrial processes since it involves additional of materials to build unlike conventional approaches that were subtractive and wasteful in nature [4][5][6].Authors cite the advantages of AM as enabling cost efficient manufacture of complex products, providing diversity of products as shown in Figure 1 with little or no assembly and customized delivery of products based on customer requirements [7][8][9][10].Applications of the technology span in the medical, aerospace, pharmaceutical, automotive, packaging, jewelry, food, toy and clothing industries [3-4, 7-8, 11].Assessing the environmental effects of 3-D printing technology is an essential criterion to quantify its sustainability in reference to various principles of CE.For this reason, the technology has been studied in reference to its ability to enhance a CE as Giurco et al. [12] detailed.Supplementing AM technologies with CE principles offers a number of opportunities; it enables the use of locally sourced materials [13], supports in-situ recycling through material recovery, remanufacture and redesigning [14], which reduces the quantity of generated wastes and their resultant environmental impacts [15]. 2 Zhu et al. [16] agreed with these suggestions stating that circular development of additive manufacturing can be implemented through enhanced reuse and recycling of recoverable materials and improved remanufacturing and redesigning of target products to avoid wastage.Of particular evaluation in this study is waste material generated during additive manufacturing in the form of beds and supports used during complex geometry of various printable objects [3] and how they can be infused into the CE as a form of reducing emissions, wastes and pollution affiliated with the technology.This study therefore focuses on the wastes produced during 3D printing processes and in particular, plastic waste and the strategies that can be employed to enhance a CE.

Commercial Polymers for 3D Printing
Additive manufacturing technology is fast evolving and its associated market is growing exponential according to Shah et al. [17].The relatively new technology is becoming popular due to its simplicity and revolutionary ability to transform manufacturing of a variety of prototypes at customized, small and large-scales [1,[3][4].The filaments used in the technology are made of thermoplastics or polymers.Some of the commercially available polymers for AM include polylactic acid (PLA) and acrylonitrile butadiene styrene (ABS) that are the most popular [18].Other types of plastics include polyetheretherketone (PEEK), polycarbonate (PC), polycaprolactone (PCL), polystyrene (PS) and polyetherimide (PEI) [1].Additionally, various polyethylene (PE) filaments such as high density-PE (HDPE), linear low-density-PE (LLDPE) and low-density-PE (LDPE) are commercially available for 3D printing [1].Other specialized plastics used to introduce specialized properties and supplement the common polymers during AM include nylon, high impact polystyrene (HIPS), polyethylene terephthalate (PET) such as T-Glase, polypropylene (PP) [19].Based on the specifications of the input materials and if it is in powder, solid or liquid various methods such as gluing, Selective Laser Sintering (SLS), Fused Deposition Molding (FDM), Stereo Lithography Appearance (SLA) and Laminated Object Manufacturing (OLM) are applied to make 3D prints [16].

Wastes Produced in the 3D Printing Process
Depending on the AM method used, support structures are used and, in some cases, prints fail based on the input settings on the printer result to wastes [18].Additionally, unwanted or test prints can be produced adding to the waste load.As the demand for the technology grows, it is evident that quantities of generated wastes will also grow as Zhu et al. [16] noted.Nyika et al. [4] also observed instances where resultant polymer waste from 3D printing was even more compared to the final product.The quantity of wastes generated are dependent on the printing machine used where FDM printers usually results to near zero wastes since support is not needed unlike inkjet printers that waste about 40% of the material used excluding the support material [20].Authors like McAlister and Wood [20] refuted claims that AM results to reduced wastes as widely advocated in literature [14,21] and observed that the technology does not compare favorably with conventional manufacturing approaches such as computer numerical control (CNC) machining.To alleviate the waste problem resulting from AM processes advances to reuse and recycle waste filament are being advocated [1-4, 10,14,16].Some of the recyclable polymers are discussed below.

Recyclable Polymers for 3D Printing
Plastics used in 3D printing along with those disposed to the environment from domestic and industrial activities have become a growing challenge in contemporary society [22].With the plastic load in the world at 359 million tons in 2019 according to the Plastics Europe Market Research Group (PEMRG), there is need to seek alternative ways to manage such wastes considering their long half -life and capacity to bioaccumulate and pollute the environment [22][23].The largest quantities of plastic wastes found in landfills are made up of PP, polyvinyl chloride (PVC), LDPE and HDPE [1].In reference to AM, polymers such as PLA, PC, ABS, PET, PS, LDPE, HDPE, PVC and PP have been investigated for potential recyclability since with 3D printing the consumption of these plastics 3 had risen to 18, 500 tons in 2020.[1,11,16].PLA waste compared to the other polymers has been extensively researched on as a recyclable 3D printing material [11,[23][24][25].The studies mainly focused on analyzing the mechanical and thermal features of the polymer waste following reextrusion to filament to improve the properties of such recyclable material to be comparable to virgin filaments.Although the specific quantities of 3D printing waste produced have not been established, it is necessary to reduce it and ensure its life cycle is extended towards a CE.

CE Strategies Applicable in 3D Printing and their Benefits
Additive manufacturing promotes efficient production processes by alleviating manufacturing scrap since the technology is additive rather than subtractive [26].3D printing technology enables localized and decentralized manufacturing as well as reduces inventory associated costs [27].AM has be applied to offer repair facilities for broken spare parts and in some cases, locally manufacture them.Such a move improves the response time and also reduces costs of logistics as Rahito et al. [28] noted.Recycling of unmixed recyclates such as filament waste and plastic bottles [29] and remanufacturing of printable spare parts from old ones [29] are addition AM strategies that enhance CE.

Applications of 3D Printing Plastic Waste in Circular Economies
Having established that 3D printing processes produce polymer wastes, it is essential to know how such plastics can be introduced to circular applications.In this mini-review, four approaches are discussed.These include: -1) the direct use of 3D printing plastic waste, 2) reprocessing and reuse of the waste, 3) biodegrading the waste to render it harmless and 4) , 01 (2023) E3S Web of Conferences ICMPC 2023 https://doi.org/10.1051/e3sconf/202343001237237 430 using solvents or catalysts to make the waste innocuous.Table 2 summarizes these applications.

Direct Use of 3D Printed Waste Polymers
The reuse of 3D printed plastic waste for other purposes has been studied considering that some of the polymers are discarded after a single use.The waste could also be 3D printed but repeated printing results reduced strength of the polymers and such an undertaking is therefore limited.ABS is preferred for reuse due to its rigidity, cheapness and toughness.A 3D printed ABS framework coated with a metal coordination polymer (Cu-BTC) was directly reused to adsorb methylene blue [36].AM waste from PET was directly reused to make circuit boards and the quality of the product compared with virgin PET [36].The performance of the former was concluded to be better than that of pure PET hence increased advocacy for 3D printing waste reuse.After adding carbon fiber to PLA to reinforce it, more than three quarters of the waste was found reusable while the remaining 25% was recyclable [37].PLA waste can also be blended with virgin PLA enhance its performance in 3D printing rather than discarding it [38].Elsewhere, wastes of PLA based composites were directly reused as 3D printing materials and showed similar thermal properties as virgin PLA though their mechanical properties deteriorated [39].Most of the studies on direct reuse of 3D printed waste focus on PLA owing to its biodegradability unlike other polymers and its extensive use for AM processes.

Processing and Reuse of 3D Printed Waste
Contaminated 3D printed wastes cannot be reused directly unless after processing to enhance their performance.This is because as previously established, repeated reuse of polymer waste depreciates their mechanical and thermal properties.The performance of ABS and PET 3D printed waste was evaluated following their processing using the fused particle fabrication (FPF) technique [40].The resultant products matched the quality of 3D prints produced by fused filament fabrication (FFF) method.Using the same approach, PP and PLA wastes showed low performance unless the printer aperture and particle sizes were optimized [40].A comparative study of 3D prints of virgin PLA and waste PLA that was ground showed similar printing performance though the latter had reduced properties compared to the former [41].Addition of 10 wt% of silica to PLA wastes enhanced material properties in aspects of elastic modulus, flexibility and tensile strength and hence better application for 3D printing [41].The extrusion and pelletizing processes for 3D printed wastes improve performance during printing as well as enhance the thermal and mechanical properties of such materials.
In events where pelletizing and extrusion is impossible, additives are supplemented in the waste polymers to improve their performance.For instance, PET waste was combined with carbon fiber and graphite to make a new 3D printing material whose performance was excellent [42].Using the same material, secondary printing showed reductions in performance by only 25% hence, great advantages in reduced printing costs and environmental efficiency with minimal compromise on quality [42].Nylon waste produced after SLS printing was melted, extruded and combined with graphite to a new composite whose printing using the FDM approach showed no considerable performance change [43].The performance of the material was further enhanced by adding 10 wt% thermoplastic polyurethane (TPU) [43].

Degradation of Wastes Using Solvents and Catalysts
Repeated use of 3D printed plastic waste is not possible due to depreciating performance characteristics hence the need to device technologies to degrade them.The ester polymers in particular can be degraded to their monomeric or polymeric forms using solvents and/ or catalysts.AM mostly uses PLA as the extrusion material because it is biodegradable.However, its rate of degradation depends on the specific environmental conditions and can take some time.However, at low temperatures and using a solvent such as alcohol and a catalyst such as malondiamine Zn (II) could degrade PLA 3D printed waste to smaller molecules faster [43].Polymers can also be degraded using ionic liquid solvents and catalysts that are non-volatile and non-flammable in nature.Poly β-hydroxybutyrate (PHB), a form of PLA used in medical scaffolding prints can be degraded using iron (II) and (III) chloride ionic liquids [43].Such chemical recycling can result to pure monomers of PHB compared to alternative degradation approaches that are slow, energy inefficient and result to impure molecules [44].3D printed wastes made of PET were degraded using choline acetate ionic liquid to form bis hydroxyalkyl terephthalate (BHET) at an efficiency rate of more than 80% [45].The product, BHET can be used in the manufacture of paints.The use of ionic liquids to degrade 3D printed waste contributes to the CE because the chemicals are halogenated salts with good electrical conductivity, thermal stability and do not volatilize hence unlikely to cause environmental pollution [16].

Biodegradation of 3D Printed Wastes
Plastics have a long-life cycle, which enhances their pollution potential due to environmental bioaccumulation.However, through biodegradation, the pollution capacity can be reduced by converting the polymers to innocuous products usually carbon dioxide and water.Through AM, a composite of PLA, PCL and polyhydroxyalkanoates (PHA) used to manufacture a medical scaffold was found to grow many cells for organ repair [46].Eventually, the composite undergoes degradation, which avoids removal.Composite scaffolds 3D printed using PCL and alginate also showed in vivo degradation ability [47].3D printed medical cryogenic sheets made of thermoplastics are completely biodegradable in soils within 16 months [48].PLA can also be biodegraded using radiation, thermal, enzymatic and nonenzymatic means as Maharan et al. [49] supposed.The advantages of biodegradation in this case include its cost effectiveness and ability to promote environmental sustainability by reducing 3D printing waste.To adsorb methylene blue [36] ii.
To make circuit boards [37] iii.
Reprocessing of 3D waste prior to reuse v.
Processing of ABS and PET, PP and PLA using FPF before reuse [40] vi.
Enhancing PLA waste with silica before reuse to make 3D prints [41] vii.
Addition of graphite and carbon fiber to PET waste before reuse in AM [42] viii.
Addition of graphite to nylon waste for FDM printing [42] , 01 Enhanced performance of nylon waste with TPU for 3D reprinting [43] x.
Degradation using catalysts and plastics i.
Using alcohol and malondiamine Zn (II) catalyst to degrade PLA waste from 3D prints [43] ii.
Using catalysts and solvents made from ionic liquids to degrade PLA [44] and PET [45] 3D printing wastes xi.Biodegradation i.
Self-degradation of medical scaffolds made from PCL, PLA and PHA [46] ii.
In vivo degradation of scaffolds made of PCL and alginate [47] iii.
Use of thermal and enzymatic means to degrade PLA waste [49]

Summary and Future Directions
The use and production of plastics in the manufacturing sector has grown exponentially lately due to the high strength, durability, light weight and low production costs affiliated with the material [30].Consequently, plastic wastes have also increased, which necessitates the use of eco-friendly measures to valorize such material and prolong its life cycle for environmental conservation [1].For 3D printing polymer wastes, different valorization approaches can be done as discussed herein.These include direct use in printing or for other purposes, re-extrusion and pelletizing wastes for reuse [42].Recycling and reprocessing of such AM wastes is however influenced by market demands [50] hence the need to adopt a closed-loop recycling approach to enable sustainable use of such wastes in CE [51].Using other materials on 3D polymer wastes to make composites of enhanced performance in terms of mechanical, thermal and esthetic characteristics is highly recommended [1][2][3][4][5][50][51].This is usually done with the help of recyclebots and after careful assessment of variations in the characteristics using techniques such as spectroscopy, thermogravimetry and mechanical analysis [52].Owing to the necessity of 3D waste recycling from its ecofriendly advantages of reduced energy and emissions, manufacturers should invest more on machines to reextrude and pelletize such wastes.However, they should consider lowering the prices of the resultant products of secondary 3D printing because of reduce raw material input.Investments in community awareness should also be done to alleviate notions that recycled products including those from 3D printing wastes are not as effective or better compared to those produced from virgin polymers.Such consumer knowledge will expand the market and preference for recyclables even in AM.

Conclusion
Disposal of plastics including wastes resulting from 3D printing processes is a growing environmental concern due to slow biodegradability and bioaccumulation ability of the polymers.Therefore, incorporating such wastes in a CE is a growing research interest and a solution to the plastic pollution menace.In this research, we highlight a number of approaches that 3D printed wastes can be infused in the circular economy.This is through enhanced direct reuse or through reprocessing and recycling.The wastes can also be remanufactured, refurbished or recovered for other purposes.3D printed polymer waste can also be degraded biologically, using solvents or catalysts or enzymatically to convert it to useful innocuous products.Usually, these methods compromise some of the original features of the targeted polymer and hence their repetition for secondary printing is limited.The review concludes that there is high potential to make AM more adaptive to CE.However, further research should be invested on the effects of 3D print waste reuse on the quality of resultant prints and the number of times such re-printing can be done without significant property deterioration compared to using virgin filaments.

Table 1 .
Table1details some CE strategies that are applied in 3D printing and their associated benefits.The circular economy strategies applicable in 3D printing and their advantages

Table 2 .
A summary of the applications of 3D polymer wastes into circular economies based on their