Taylor Made: Life in plastic is not so fantastic
Plastic has become a ubiquitous material in our consumable products, but with its presence comes the call to consider sustainability. Once manufactured, plastic can take an estimated 400+ years to fully break down, which is why there has been a growing concern about plastic trash in our ecosystems. Plastic can be a very cost-effective and efficient material to use; as designers and engineers, it is our responsibility to make informed decisions regarding its best use.
In the late eighties, the Society of the Plastics Industry (SPI) devised a coding system for plastic products. Called resin ID codes, these numbers get stamped on products to indicate what type of plastic the product is made from so that recycling centers can better sort for proper recycling (when it comes to ease of recycling, plastics are not created equal). These codes double as a guide for consumers to know which products they can put into a single stream or other recycling program, if their community offers it.
The first six SPI codes are pretty common. 1 = polyethylene terephthalate (PETE), which is used for bottles and other food packaging, and considered the easiest to recycle. PETE typically gets recycled back into bottles or into polyester fibers for clothing. 2 = high-density polyethylene (HDPE), another easily recycled type of plastic, and again used to make containers, but this time also for more industrial uses like oil. HDPE also gets recycled back into bottles. 3 = polyvinyl chloride (PVC), aka the white piping we’ve all seen used for plumbing, as well as projects like bows and bikes. PVC is hard to recycle and can pose more of an environmental threat when not disposed of properly. 4 = low-density polyethylene (LDPE), which can be made into a very thin, flexible material. 5 = polypropylene (PP), which also gets recycled into fibers. And 6 = polystyrene (PS), aka Styrofoam, which is notoriously hard to recycle (for more on this, see my blog “Answering my own questions”). Then there are other plastics like polyvinylidine chloride, which is used to make saran wrap and medical and meat packaging, polyurethane, which goes into flexible foam for cushions, and polytetrafluoroethylene, which goes into Teflon. And let’s not forget nylon, which goes into all sorts of things, from parachutes to guitar strings.
Taking a closer look at some of these plastics reveals numerous characteristics, many overlapping, that make choosing the right plastic for the job a more complex decision than one might expect. For example, LDPE can form a lightweight, transparent plastic that has low toxicity, making it suitable for food packaging like bread loaf bags. However, it does not go into cardboard box liners (think cereal and crackers), which are made of PP because it resists penetration and corrosion from moisture and grease. Additionally, bags made of LDPE are harder to recycle and have to be kept separate from single stream containers (plastic bag recycling is not offered everywhere, but can typically be found in department stores like Wal-Mart), but even so, LDPE is considered more commercially viable to recycle than PP. HDPE holds up well in UV and extreme temperature exposure, making it a good material for plastic lumber. PP also resists heat and has good strength, but typically gets used for products that don’t take heavy loading, like packing tape, carpeting, and even disposable diapers.
Many of these plastics go into products that can’t be recycled without being disassembled, such as PP in the aforementioned diapers. Additionally, while it is possible to recycle plastics from all six codes, the plastics that are hard to recycle, like PVC and polystyrene, typically end up in the landfill (or ocean) because it’s difficult to find locations that have the capability to break them down. A product’s possibility of being recycled does not guarantee the outcome; thus, it’s not enough to say that a product simply made from recyclable plastic is sustainable, when in reality, experts are finding that the U.S. recycles less than 10% of the plastic it produces.
And where does plastic come from? Plastics are polymers generated from limited natural resources. With the exception of silicones, which are built from silicon, plastics are built from carbon atoms that bond to other common building block atoms like hydrogen, oxygen, and nitrogen. These atoms are derived from resources like coal and natural gas, while other materials like benzene, which goes into nylon and polystyrene, are derived from petroleum. There are lots of ways to categorize types of plastics; for example, thermoplastics can be melted, and thermoset plastics can’t.
With all the recent hype about plastic and the need for more sustainable materials, LEGO has begun moving toward changing over its iconic toys. Historically, LEGO pieces have been made of acrylonitrile butadiene styrene (ABS), a common thermoplastic polymer mainly derived from petroleum. In 2018, LEGO announced its move to begin manufacturing pieces from sugar cane-based plastic. The company aims to use sustainable materials in its products and product packaging by 2030, which is easier said than done; an article published by the Wall Street Journal in June discussed the company’s on-going struggle to find a sustainable blend of plastic. LEGO has invested millions of dollars in research to find a way to produce sustainable plastic that mimics the qualities of ABS.
Plastic gets the job done well, but at a higher cost than meets the eye. As engineers, thinking beyond the mechanical properties and aesthetic of the material we’re considering is imperative for managing the impacts, both positive and negative, that we make in the world.
Thank you to MechSE's Leon Liebenberg for the tip about Lego’s change in plastic material.