Sustainability challenges and future prospects of plastic 3D printing

Across the world, governments, companies, consumers, and public opinion have reached a consensus on the necessity of improving sustainability in areas such as manufacturing, natural resource utilization, economy, and living standards. Plastics that can promote sustainability are called SPs, and sustainable plastics used for additive manufacturing will also accelerate the transition to more sustainable plastic manufacturing.

△Ingeo polymer 3D printed PLA images from Natureworks, the world's largest polylactic acid (PLA) manufacturer.

Plastic is a material made by mixing polymers and additives. Polymers are artificial or natural compounds composed of specific long chains

of molecules. Sustainable plastics (SPs) are essentially biobased plastics, plastics formulated from renewable resources such as plants and

animals (excluding by-products and organic residues), rather than fossil resources such as crude oil and natural gas. SPs also have advantages

and disadvantages. Compared to fossil based plastics, SPs have the following benefits: they are not affected by fluctuations in crude oil prices,

are lightweight, have low prices, are a source of income for farmers, and the residues and waste of crops (peas, green beans, chickpeas, etc.)

are recyclable.

However, SPs also face some drawbacks: high manufacturing costs, narrow processing windows, potential food competition, brittleness, not all

SPs are biodegradable, and so on.

Other shortcomings include:

(a) Some low degradable or non degradable bioplastics can only decompose when treated at high temperatures or in urban biogas

digesters or composting machines;

(b) Some biodegradable plastics can only degrade in landfills under certain specific conditions;

(c) The decomposition during composting produces methane gas, which is many times more potent than carbon dioxide and contributes to global warming.

In short, SPs cannot solve all environmental problems caused by plastics, but compared to fossil based plastics, SPs have significant improvements in this regard.

Sustainable plastics

The segmented markets of SPs in 2020 are as follows: fiber and packaging (with the same proportion), automotive and transportation, construction and construction, consumer goods and other industries (including additive manufacturing, 3D printing, film, medical, aquaculture), and (3%) agriculture and horticulture, electrical and electronics, functional (adhesives, coatings, cosmetics).

SPs only account for a small portion of the global plastic consumption, which is 1% of the 3.9 million tons in 2019. However, according to data

from the Nova Institute, a German research institution focused on renewable carbon, the production of SPs in millions of tons is expected to

increase from around 4.25 in 2019 to around 4.8 in 2024. In 2020, bio based polymers came from glycerol, starch, sugar, non edible vegetable

oil, cellulose, and edible vegetable oil, decreasing in order.

Sustainable plastics for additive manufacturing

As raw materials, SPs have been applied to a range of manufacturing and prototyping processes, with the most famous being additive manufacturing (AM) or 3D printing. The basic characteristic of additive manufacturing process is that the manufacturing of any object starts from a 3D

computer model based on CAD, and ultimately involves adding layer after layer of materials for manufacturing. The most popular plastic 3D

printing processes are divided into three series: barrel photopolymerization, powder bed melting, and material extrusion. These series of

plastics are provided in liquid, powder, and filament forms.

At present, the raw materials for 3D printing include some commercial grade filled and unfilled SPs, as well as more experimental grade SPs

and fossil based plastics filled with biobased components such as cellulose and natural fibers.

Compared to some traditional processes, additive manufacturing can be more sustainable when used under specific conditions. For example, if 3D printing is compared to metal casting and plastic injection molding and used during design and development or when manufacturing a

limited number of parts, 3D printing is more sustainable, meaning it requires fewer overall resources because it does not require molds.

Therefore, sustainability benefits significantly from the combination of 3D printing and SPs.

Commercial sustainable plastics for additive manufacturing

Table 1 collects commercial SPs used for additive manufacturing, as well as their suppliers and compatible additive manufacturing technologies. These sustainable plastics are mostly sold in the form of long filaments, compatible with material extrusion based desktop and industrial printers, which are the most affordable among the most widely used 3D printing processes. The reported number of suppliers for each material reflects the higher sales volume of PLA in additive manufacturing compared to other bio based plastics. Most suppliers are currently located in Europe, the United States, and China, and it is predicted that the number of suppliers in Asia will continue to increase.

Filament suppliers typically purchase polylactic acid from formulators such as NatureWorks (USA) and Total Corbion (Netherlands), and combine it with additives such as plasticizers, colorants, lubricants, etc. to facilitate filament manufacturing, reduce costs, and achieve desired functional and aesthetic properties.

Table 1: Commercial SPs for Additive Manufacturing

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

It is worth noting that:

（1)The PHA wires of colorFabb and 3D Printlife contain other components: the former includes PLA, while the latter includes both PLA and PBS.

（2) 3D Fuel's PBS filament contains PLA and PHA.

（3) Seaweed filaments contain PLA.

（4) Food includes: pizza, spaghetti, hamburgers, meat, sugar coating, chocolate, pastries, pancakes, dough, cheese, cookies, red beans and green bean sauce, etc.

（5) Bamboo, cork, and wood filament contain PLA and other plastics.

Figure 1: Fracture tensile strength of raw materials used for 3D printing: filled and unfilled SP (blue) and fossil based polymer (red)

As an example of the actual performance of SPs used for 3D printing, the mechanical properties of representative samples of (a)

commercial filled and unfilled SPs used for additive manufacturing and (b) commercial fossil based plastics used for additive manufacturing

are compared in Figures 1 to 4. The former performs well in tensile strength and modulus, but performs poorly in impact resistance

(notch cantilever beam impact strength) and temperature resistance (thermal deformation temperature at 66 psi). As shown in Figure 5, in 2017,

the prices (in US dollars per pound) of filled and unfilled polylactic acid samples used for additive manufacturing were more attractive than some

representative fossil based polymers used for additive manufacturing, including polycarbonate (PC).

Figure 2: Tensile modulus of raw materials used for additive manufacturing: filled and unfilled SP (blue) and fossil based polymer (red).

Along with ABS, PLA is also the most popular additive manufacturing silk material for the following reasons: (a) it is easy to print with PLA because its printing temperature is lower than ABS; (b) It is not as easily deformed as ABS, so it does not necessarily require a heated bed; (c) When printing, there is no odor, at most it emits smoke like sweet candy, not an unpleasant odor like ABS; (d) It is more environmentally

friendly than most types of AM silk and is a biobased material. There are many suppliers of PLA purchased by additive manufacturing filament suppliers, and NatureWorks has a high production volume ® Led by the United States.

Figure 3: Izod impact strength of gaps in additive manufacturing raw materials: filled and unfilled SP (blue) and fossil based polymer (red).

Figure 4 Impact strength of notched cantilever beams for raw materials used in additive manufacturing: filled and unfilled SP (blue) and fossil based polymer (red).

Figure 5: Raw material prices for additive manufacturing (2017 figures): filled and unfilled SP (blue) and fossil based polymer (red).

Table 2 lists the commercial PLA filling filaments and their suppliers used for additive manufacturing, and confirms the dominant position

of PLA in SPs used for additive manufacturing, although most 3D printing is used for personal and educational printing (hobbies, homemade

spare parts, schools, libraries, etc.) rather than industrial and functional applications.

Table 2: Commercial PLA filled filaments for AM

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

When the filling material is harder and stronger than polylactic acid, the mechanical properties of polylactic acid filled filaments are significantly lower than the theoretical expected maximum based on the contribution of the filling material. We mainly attribute this defect to the pores and imperfect interface adhesion between polylactic acid and the filling material.

The existing and potential applications of unfilled and filled SPs in additive manufacturing include hobbies, prototyping, education, furniture,

medicine/healthcare, architecture, construction, consumer goods, automotive, and art.

Experimental sustainable plastics for additive manufacturing

The ongoing research and development of SPs for additive manufacturing is remarkable in terms of creativity and quantity. On the one hand,

it uses ancient materials, such as bamboo and cork, and on the other hand, it uses new materials, such as cellulose nanofibers and collagen

hydrogels. Obviously, not all ideas and innovations will achieve commercial success, but the broad idea behind this research and development

work proves that 3D printing technology, especially those based on extrusion, allows for experiments with new materials in a time and

cost-effective manner. There are already many experimental additive manufacturing SPs, and Table 3 only includes one sample of them.

Paesano (2022) described their comprehensive quantity, as well as their characteristics, additive manufacturing processes used, and applications.

Table 3: Examples of Experimental SPs for Additive Manufacturing

Abbreviations: CFs carbon fiber, EBB extrusion bioprinting, ME material extrusion, MWCNTs multi walled carbon nanotubes, PBF powder bed fusion,

PE polyethylene, PLLA polylactic acid, PP polypropylene.

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

Challenges and Recent Prospects of Plastic Additive Manufacturing