“I shall say one word to you, just one word. Are you listening? Plastics.” – The Graduate, 1967
Naturally occurring organic plastics have been a part of human culture for thousands of years, including rubbers, as well as egg, milk and blood proteins. It wasn’t until the mid-19th century, however, that synthetic plastics first made an appearance in the history of our species. By the mid-20th century, developments in chemical technologies resulted in an explosion of new plastics and the ability to mass manufacture plastic products.
It seems that these materials are now so ubiquitous that it’s hard to imagine a time without them, and new technologies, like 3D printing, are wholly dependent on them. But what are these materials that are being used to additively manufacture parts, layer by layer using light and heat? Where do they come from?
We’ve already delved into photopolymers in terms of the variety of materials available on the market for photopolymer 3D printing, but what we didn’t actually examine were the base chemistries that make these light-sensitive chemicals possible. A UV-curable resin is made up of three main components: binders, monomers and photoinitiators, which are often combined with additives, chemical agents, colorants and plasticizers.
A 3D print made with photopolymer resin. (Image courtesy of Wikipedia.)
Binders usually make up about 50 to 80 percent of a resin. Monomers (usually made from acrylates or methacrylates) constitute between 10 and 40 percent of the formula. Photoinitiators make up a small proportion of the resin chemistry.These parts can largely be broken down into the families below.
Binders and Monomers
- Styrene: A derivative of benzene, a hydrocarbon obtained from crude oil.
- Acrylate and methacrylate: The former is made using propene (obtained from oil and gas). The latter combines propene and benzene(another hydrocarbon obtained from oil)—and cyanide, most often generated using a reaction between methane—the main component in natural gas—and ammonia, also obtained from natural gas, as well as petroleum, heavy oils and coal.
- Polyvinyl alcohol (PVA): ethylene (a hydrocarbon obtained from natural gas and oil) and acetic acid, made by introducing carbon monoxide (often made with coal and water) to methanol (made from natural gas, oil or coal).
- Olefine/Alkene: Made from natural gas or naptha, a hydrocarbon obtained from natural gas and other fossil fuels.
- Glycerol: Typically derived from plant or animal fats, like soy, palm and rendered beef fat.
- Polypropylene Glycol: Made from propylene oxide, beginning with propene, obtained from oil and gas.
- Polyamides: Made from benzene.
- Polyisoprene: Made from natural rubber or through the polymerization of isoprene, a byproduct of oil.
- Epoxies: There are a wide variety of epoxies used to make photopolymer resins, but the most common are made from epichlorohydrin (ECH) and bisphenol-A (BPA). ECH is most often obtained using hypochlorous acid (a combination of water and chlorine, made most frequently using rock salt) and chloride, made from chlorine and propene. BPA is synthesized using acetone and phenol, which comes from benzene and propene.
- Nitrile rubber: Typically made of acrylonitrile, derived from propene, and butadiene.
- Isopropylthioxanthone: Made from dithiosalicylic, created using benzene, and isopropylbenzene, obtained from crude oil.
- Diaryliodonium salts: Iodine derivatives obtained through the mining of caliche rock (primarilyin Chile) or extraction from brine occurring in natural gas fields, with the largest iodine brine producing field being located in Japan.
- Benzophenone: Derived from benzene and/or methanol, made via a reaction between carbon monoxide and hydrogen over a catalyst.
- Triarylsulfonium salts: Made using diaryliodonium salts and a catalyst.
- 2,2-azobisisobutyronitrile: Made using methane, ammonia (obtained from natural gas), propene, salt (obtained from sea water, brine and rock salt), and sulfur (obtained from petroleum, natural gas or mined from natural sources).
As with photopolymers, we have previously covered thermoplastics and their use in selective laser sintering and fused deposition modeling in in-depth articles, but have not explored the roots of these materials. The chemistry here is much simpler than what’s described above as 3D printing with thermoplastics only requires the melting of a single material, which cools and hardens. No need for photoinitiators! Below are the most common thermoplastics and their constituent chemistries:
- Polyamide: Benzene.
- Polyaryletherketone: Made from benzene and propene.
- Acrylonitrile butadiene styrene (ABS): Made from propene, ammonia, petroleum, ethylene and benzene.
- Polycarbonate: Made through a reaction between BPA and phosgene, made using carbon monoxide, chlorine and activated carbon, usually derived from charcoal.
- Polyactic acid (PLA): Made from the sugar in corn starch, sugarcane, as well as cassava roots, chips or starch.
- Polyesters: Ethylene and naptha, obtained from natural gas, petroleum, coal tar and crude oil.
- ·Styrenes: Benzene.
- Thermoplastic polyurethane: Benzene and phosgene or benzene and methanol.
- Acrylonitrile Styrene Acrylate: Propene and benzene.
- Polypropylene: Propene.
- Polyvinyl Alcohol: Ethylene and methanol.
The Implications of Plastic Chemistries
As one examines the constituent chemistries of all of these materials, it becomes apparent that the overwhelming majority are made from fossil fuels. Because plastics are often made using the byproducts of fossil fuel extraction, “the two product chains [plastics and fossil fuels] are intimately linked,” according to the Center for International Environmental Law. Additionally, the production of plastics makes up 1 percent of U.S. greenhouse gas (GHG) emissions and 3 percent of the country’s primary energy use.
The hyrdocarbons that serve as the basis for these plastics aren’t burned, so they don’t emit greenhouse gasses on their own, but scientists from the University of Hawaii at Manoa have recently found that plastics release the GHGs methane and ethylene when they are exposed to sunlight, left underwater for an extended period of time, or even in air alone. This is particularly troubling given the fact that almost 80 percent of waste plastic ends up in landfills or the environment (8 million tons finds its way into oceans annually).
In addition to any health or environmental effects associated with the materials themselves, the longevity of many of these plastics may be called into question as the fossil fuel industry is challenged by society’s response to climate change.
Both China and the U.S. are increasing investments in plastics made from fossil fuels, with production capacity for polyethylene expected to grow by up to 75 percent by 2022. Markets and Markets anticipates a compound annual growth rate for the 3D printing plastics market to be 26.1 percent between 2018 and 2023.
This comes despite the fact that the United Nations Intergovernmental Panel on Climate Change has projected that global civilization has to cut total emissions 45 percent by 2030 in order to avoid devastating warming-related effects, which doesn’t account for tipping points widely believed to exist in the ecosystem.
If this is the case and nations do decide to seriously regulate the fossil fuel industry, many of the plastics in the 3D printing industry may not survive the global transformation toward clean energy. In turn, we may actually see a growth in the bio-based plastics that exist in the industry.
PLA is the dominant bioplastic in 3D printing, although Polish 3D printer manufacturer Zortrax does produce a filament made from polyhydroxyalkanoate (PHA), a plastic made from microbes that are fed organic material.
According to a 2017 study, replacing plastic derived from fossil fuel with PLA could cut GHG emissions by 25 percent. And because the process to convert sugars into plastic is energy intensive, GHG emissions would be further reduced if PLA was made using renewable energy. Moreover, bioplastics produce GHGs over their lifetime, compared to fossil fuel-based plastics.
Though that may be the case, a 2010 study showed that bioplastics result in the release of more pollutants than fossil fuel-based plastics due to the fertilizers and pesticides used to grow the crops from which they’re made and the chemicals used to convert them to plastic. Given current health concerns associated with ingesting pesticides through our crops, this could serve as reason to explore the production of organic bioplastic.
Other issues include the fact that the production of bioplastics will increasingly compete with food production, which is predicted to suffer due to increased drought, shifting weather patterns and more pests caused by a warming climate.
PLA also requires industrial composting facilities to break it down properly, which is an energy-intensive process. If this bioplastic ends up in a landfill without oxygen, it can release methane gas, which is 23 times more potent than carbon dioxide when it comes to warming the planet.
The Future of 3D Printing Plastic
Fortunately, there are numerous groups researching and developing new plastics that may not have the same negative effects as those mentioned above. These include converting wastewater, food waste, solid waste and crop residue into PHA with microorganisms.
To avoid the issue of bioplastic crops competing with agricultural land, a team at Michigan State University is engineering blue-green algae to produce sugar that are then converted into plastic by bacteria. A startup called Mango Materials is working to convert methane from wastewater treatment plants and landfills into PHA using methane-eating bacteria. And a team at the University of Bath is working to replace fossil fuel-based polycarbonate with a BPA-free version made from sugars and carbon dioxide.
We’ve also interviewed Javier Gomez Fernandez, assistant professor at the Singapore University of Technology and Design (SUTD), who has developed a method for 3D printing with naturally derived cellulose and chitin. Oak Ridge National Laboratory is also developing 3D printable lignin—made from trees, plants, and agricultural crops—which it has then combined with other materials like carbon fiber to create composites.
There are also numerous companies that create 3D printing plastic from recycled goods, as well as those producing machinery for recycling plastic objects to make 3D printing filaments.
All of this means that there is the potential for 3D printing to become more environmentally sustainable as it and its feedstocks evolve. Regardless of what form that takes, it will still be crucial to understand the materials being used in 3D printing because they don’t exist in a vacuum, but as a part of an interconnected ecosystem.