There is worldwide consensus among governments, companies, consumers, and public opinion on the necessity to improve sustainability in manufacturing, use of natural resources, economy, living standards, etc. Plastics that can contribute to sustainability are known as SPs and sustainable plastics for AM can accelerate the transition to more sustainable plastics manufacturing.

Plastics are polymers plus additives. Polymers are man-made or natural compounds consisting of very long chains of a given molecule. Sustainable plastics (SPs) are basically biobased plastics that are plastics formulated from renewable resources, such as plants and animals (besides by-products, and organic residues), instead of fossil resources, such as crude oil and gas. SPs have advantages and disadvantages. SPs features the following benefits vs. fossil-based plastics: they are unaffected by crude oil price fluctuation, lightweight, less expensive, a source of revenue for farmers, and use of excess and waste of crops (peas, green beans, chickpeas. etc.), and of waste.

However, SPs also face some shortcomings (Ferreira-Filipe et al. 2021): costly manufacturing, narrow processing window, potential food competition, brittleness, not all SPs are biodegradable, etc. Other shortcomings are: (a) some low-degrading or nondegradable bioplastics only break-down at elevated temperatures or when treated in municipal digesters or composters; (b) some biodegradable plastics only degrade in landfill sites under certain specific conditions; (c) decomposition during composting generates methane gas that is many times more potent than CO₂, and contributes to global warming (Atiwesh et al. 2021).

In conclusion, SPs do not solve all the environmental problems caused by plastics but represent a significant improvement in that regard over fossil-based plastics.

Sustainable plastics

The market segments of SPs in 2020 were the following, in decreasing order: fibers, and packaging (same amount), automotive and transportation, building and construction, consumer goods, and others (including additive manufacturing, 3D printing, film, medical, aquaculture), and (at 3%) agriculture and horticulture, electric and electronics, functional (adhesive, coatings, cosmetics) (Skoczinski et al. 2021).

SPs constitute only a very small portion of the volume of plastics globally consumed, namely 1% of 3.9 million tonnes in 2019. However, the production of SPs in million tonnes is expected to raise from about 4.25 in 2019 to about 4.8 in 2024, according to nova Institute, a German research institute focusing on renewable carbon.

In 2020 biobased polymers were derived from glycerol, starch, sugars, non-edible plant oil, cellulose, and edible plant oil, in decreasing order (Skoczinski et al. 2021)

Sustainable plastics for AM

As feedstocks, SPs have been formulated for a family of manufacturing and prototyping processes best known as Additive Manufacturing (AM) or 3D printing. AM processes are basically characterized by the fact that making any object starts with a CAD-based 3D computer model that is ultimately fabricated adding layer upon layer of material. The most popular AM processes for plastics fall into three families: vat photopolymerization, powder bed fusion, and material extrusion. The plastics for these families are provided as liquid, powder, and filament, respectively.

Currently, feedstocks for AM encompass a number of commercial grades of filled and unfilled SPs, and an even larger number of experimental grades of SPs and fossil-based plastics filled with biobased ingredients, such as cellulose and natural fibers.

AM can be more sustainable than some conventional processes and when utilized in specific conditions. F. e., if AM is compared to metal casting and plastic injection molding and is used during design development or when making a limited number of parts, AM is more sustainable, that is, it requires less overall resources, because it requires no mold. Therefore, sustainability significantly gains from the combination of AM and SPs.

Commercial sustainable plastics for AM

Table 1 collects the commercial SPs for AM, along with their suppliers and compatible AM technology. Most of these SPs are sold in form of a filament compatible with desktop and industrial printers that are based on material extrusion, being these printers the most affordable among the most employed AM processes. The number of suppliers reported for each material reflects the fact that PLA is more sold than the other biobased plastics for AM. Most suppliers are currently located in Europe, USA, and China, and we expect a growing number of providers in Asia. The filament suppliers typically buy their PLA from formulators, such as f. e. NatureWorks (USA) and Total Corbion (The Netherlands), and combine it with additives, such as plasticizers, colorants, lubricants, etc. that ease the filament fabrication, reduce cost, and achieve the desired functional and aesthetic properties.

Table 1: Commercial SPs for AM

Source: A. Paesano, 2022. The Handbook of Sustainable Polymers for Additive Manufacturing. Boca Raton: CRC Press. First edition.

It is worth noting that:

• The PHA filaments by colorFabb and 3D Printlife contain other ingredients: the former includes PLA, and the latter PLA and PBS.
• The PBS filament by 3D-Fuel contains PLA and PHA.
• The algae filament contains PLA.
• Food includes pizza, spaghetti, burgers, meat, icing, chocolate, pastries, pancake, dough, cheese, biscuit, red and green bean pastes, etc.
• Bamboo, cork, and wood filaments contain PLA and other plastics.

As examples of the actual performance of SPs for AM, in Figures 1 to 4 mechanical properties of a representative sample of (a) commercial filled and unfilled SPs for AM and (b) commercial fossil-based plastics for AM are compared. The former ones perform well in terms of tensile strength and modulus, but not as to impact resistance (notched Izod impact strength) and temperature resistance (heat distortion temperature at 66 psi). As Figure 5 illustrates, in 2017 the price (USD/lb.) of a sample of filled and unfilled PLA for AM was more attractive than some representative fossil-based polymers for AM, including polycarbonate (PC).

Along with ABS, PLA is also the most popular feedstock for AM filaments for the following reasons: (a) it is easy to print with PLA because it has a lower printing temperature than that of ABS; (b) it does not warp as easily as ABS, and hence it does not always require a heated bed; (c) it is odorless when printing, at most emitting fumes smelling like sweet candy instead of an unpleasant smell like that of ABS; (d) it is more environmentally friendly than most types of AM filaments, being biobased. The PLA purchased by providers of AM filaments has a number of suppliers, led by NatureWorks® (USA) in the amount of volume produced.

Table 2 lists commercial PLA-filled filaments for AM and their suppliers, and confirms the dominance of PLA among SPs for AM, although mostly 3D printed for personal and educational printing (hobbies, home-made spare parts, schools, libraries, etc.) than for industrial and functional applications.

Table 2: Commercial PLA-filled filaments for AM

Source: A. Paesano, 2022. The Handbook of Sustainable Polymers for Additive Manufacturing. Boca Raton: CRC Press. First edition.

The mechanical properties of PLA-filled filaments fall markedly below the maximum values theoretically expected based on the contribution of the fillers when the latter are stiffer and stronger than PLA, and we attribute this shortcoming mainly to porosity and imperfect interfacial adhesion between PLA and fillers.

Present and possible applications of unfilled and filled SPs for AM include hobbies, prototyping, education, furniture, medicine/health care, architecture, construction, consumer goods, automotive, and arts.

Experimental sustainable plastics for AM

The R&D work being conducted on SPs for AM is remarkable in terms of creativity and volume, and leverages, on one hand, ancient materials, such as bamboo and cork, and, on the other hand, new materials like cellulose nanofibers and collagen hydrogels. Obviously, not all the ideas and innovations will be commercially successful, but the wide breadth of ideas behind this R&D effort proves that AM technologies, especially those based on extrusion, allow experimenting with new feedstocks in a time- and cost-effective way. There have been many experimental SPs for AM, and Table 3 only includes a sample of them. Paesano (2022) describes a comprehensive number of them, along with their properties, AM processes used, and applications.

Table 3: Examples of experimental SPs for AM

Abbreviations: CFs carbon fiber, EBB extrusion-based bioprinting, ME material extrusion, MWCNTs multi-walled carbon nanotubes, PBF powder bed fusion, PE polyethylene, PLLA poly(L-lactide), PP polypropylene.
Source: A. Paesano, 2022. The Handbook of Sustainable Polymers for Additive Manufacturing. Boca Raton: CRC Press. First edition.

Challenges and near future of SPs for AM

Experts agree that SPs will grow in the near future in terms of anoutput quantity and innovative materials. The following factors influence the market growth of SPs (Paesano 2022):

• Greater consumers’ demand for SPs arising from awareness of their environmental benefits, and esthetic and functional appeal
• Government policies and legislations advantageous to SPs
• Feedstocks’ prices competitive with that of fossil-derived feedstocks
• Ability to meet the expected performance at a competitive cost, without reducing resources for food crops in specific locations
• Being processed through routes compatible with current industrial infrastructures and supply chains to produce and sell monomers and polymers.

Switching from SPs in general to SPs for AM, the market of the latter ones is only a small fraction of the market of the former ones, which, in turn, as we recall from earlier, represented 1% of the volume of plastics globally consumed in 2019. However, the near-future market of SPs for AM may be auspicious due to the following reasons:

• The AM market is expected to expand at a compound annual growth rate of 20.8% from 2022 to 2030 (Grand View Research, n. d.);
• The small size of investments required for developing and launching new feedstocks for AM compared to investments for the same goal for conventional fabrication processes;
• Numerous customers of personal printers prefer green products, and like experimenting with new sustainable feedstocks; this phenomenon will raise awareness and will promote interest for commercial applications of SPs;
• The current policies and legislations at the national and local levels, international agreements, and public opinion reflect a pro-environment trend;
• Increasing production of SPs by world-leading chemical corporations derives from the availability of additional cost-effective production routes.

The price and performance of SPs for AM may drop and raise, respectively, if the production and properties of SPs for conventional processes and AM (such as PLA, PHA, and PBS) increase and improve, respectively (Paesano 2022). Those involved with the development of further SPs for AM should not only consider their environmental performance but also measure toxicity and conduct a life-cycle assessment.

When it comes to engineering applications, candidate materials have to demonstrate through test data continuous and reliable service performance at a competitive price, in order to be accepted by designers, and possibly displace incumbent feedstocks. R&D work is required to improve the physical and mechanical properties of PLA-filled SPs for AM and take advantage of the fillers’ strength and stiffness, f. e. by formulating compounds (sizings) to enhance the interfacial adhesion between PLA and fillers. The more demanding the engineering uses, the more extensive and costly are the test programs to evaluate the material’s performance.

We expect that automotive, construction and consumer goods will be the leading drivers of applications of SPs for AM.

Finally, the adoption of SPs for AM will also be stimulated by the job openings in sustainability and AM that are increasing globally, and the growing interest in sustainability, promoted by courses focused on sustainability that is spreading in universities internationally.

This article represents a succinct overview of current SPs for AM. The commercial grades listed here constitute the tip of the iceberg that includes also experimental formulations investigated for various AM processes, especially ME, PBF and bioprinting.

One proven feature of AM is that new or improved processes and feedstocks are being introduced at rapid pace, and SPs for AM may become more common earlier than expected.

References

• Atiwesh G., Mikhael A., Parrish C.C., et al. 2021 Environmental impact of bioplastic use: A review. Heliyon 7, e07918. https://doi.org/10.1016/j.heliyon.2021.e07918
• Bourell, D., Kruth, J. P., Leu, M., et al. 2017. Materials for additive manufacturing. CIRP Annals. Manuf. Technol. 66:659–681.
• Ferreira-Filipe, D.A., Paço, A., Duarte, A.C., at al. 2021. Are Biobased Plastics Green Alternatives?—A Critical Review. J. Environ. Res. Public Health 18, 7729. https://doi.org/10.3390/ijerph18157729
• Gibson, I., Rosen, D., Stucker, B. 2015. Additive Manufacturing Technologies. New York: Springer, 2nd edition.
• Grand View Research n. d. Additive Manufacturing Market Size, Share & Trends Report (accessed July 25, 2022).
• Kabasci, S. ed. 2014. Bio-Based Plastics – Materials and Applications. London: Wiley.
• Ligon, S. C., Liska, R., Stampfl, J., Gurr, M., Mülhaupt, R. 2017. Polymers for 3D printing and customized additive manufacturing. Rev. 117:10212–10290.
• nova-Institute 2021 (accessed June 25, 2022)
• Paesano, A., 2022. The Handbook of Sustainable Polymers for Additive Manufacturing. Boca Raton: CRC Press. First edition.
• Skoczinski, P., Carus, M., de Guzman, D., et al., 2021. Bio-based Building Blocks and Polymers – Global Capacities, Production and Trends 2020 – 2025 (accessed July 25, 2021)
• Wohlers Associates (USA)

Article co-author: Kathleen McHugh

Kathleen McHugh is a plastics professional and founding principal of FivePs Innovation, LLC.  She consults in the areas of materials and application development, market and competitive analysis, and plastics recycling & sustainability.  Her experience in the plastics industry spans 30+ years, including product, application & market innovation, and in-depth materials knowledge of polyolefins, fluoropolymers, engineering thermoplastics, and polymer compounds.  Her leadership experience in customer-focused product design, product development, process scale-up, commercial launch, and technical service has been practiced across global organizations and markets, specifically in Europe, Asia, South America, and NAFTA. She earned a BS in chemical engineering from Drexel University, an MBA from Villanova University, and an MS in Polymer Science from the University of Ferrara (Italy). She holds five patents. She can be contacted at kathleen@fiveps.org