Plastics by the (recycling) numbers
The house I grew up in is next to the town dump. That is what we called it anyway, but to be honest, it was a compactor and recycling transfer station for a town of roughly 1,500 people. Drive up, chuck the garbage bags in the compactor, watch the grumpy old guy in the shack push the control knob to compress the garbage into the trailer, and off you go. It was only open on Tuesday, Thursday and Saturday, and while close to my house, it never smelled. Many people work in office complexes that produce more weekly tonnage of garbage than what goes through the West Stockbridge, Massachusetts, compactor.
In my teen years it was a scarlet letter to be the kid who lived next to the dump, but prior to that it was a convenient extension of my backyard. The looping driveway provided a convenient place to ride bikes and drive R/C cars, and we used to catch frogs in the adjacent swampy area. On Saturday mornings, my sister and I would strap grocery bags with the weekly garbage to our handle bars and ride over to launch them into the compactor. Some urban planners may reap high praise on such an eco-friendly arrangement.
Google maps aerial view of my parents house (blue) and the West Stockbridge compactor (red).
At some point, the compactor added recycling bays. My sister was a budding conservation enthusiast, and this gave her great delight. We dutifully separated out paper and plastics with the right recycling number and added those to the ends of our handlebars every Saturday, too. All we knew at the time was that certain numbers on the plastic containers were recyclable and others were not. I gave it very little thought as to why or how plastics were different. It was not until I was a degreed engineer that I had any idea what the numbers meant or the magic to the formulations of the plastics that are part of our daily lives. The following is a guide to the plastics behind the recycling numbers and how they can be used in your next prototype.
Polyethylene terephthalate, PET
Polyethylene terephthalate, often abbreviated PET is very popular in the food industry. Most beverage bottles are made from PET as its natural color is clear and does not leech into the liquids it contains. It is one of the easiest plastics to recycle and is taken by most recycling programs. It is also important in the garment industry as it is the primary ingredient in polyester fibers. Most fleece jackets are made from non-woven polyester fibers and are noted for their water resistant properties.
PET is a great prototyping material if only for its availability. Soda bottles are tough and are great for prototypes, especially if there is fluid handling component to the innovation. The drawback is that it can be difficult to bond together, as many super glues will adhere it. However, hot glue is always an option. It can also be heat formed with an industrial heat gun if you need to make a special shape.
A selection of PET containers.
High Density Polyethylene, HDPE
HDPE is another plastic that is commonly used for bottling. Its natural color is a hazy white, which makes it less suitable for packaging liquids that need to be seen. However, it is used in milk jugs, laundry detergent containers, and shampoo bottles. It has a very high strength to weight ratio, is very tough, and chemical resistant so it gets a lot of use in industry, too. It is used to make pipes, fuel tanks, and even hard hats. It has a slick surface, and is useful to make gears and bearings.
HDPE is useful as a prototype material for handling corrosive fluids. If you have a prototype that uses a fuel or chemical, it is a great way to house it safely. Since it is so smooth, it can be used as a sliding surface between parts, and tubes of it can be easily cut to make bearings.
Vinyl and PVC
For music buffs, the word vinyl brings back fond memories of the smell of old record jackets and the unmistakable scratch of a record needle. As an engineering material, vinyl and its sister Polyvinyl Chloride (PVC) is a very versatile family of plastics. Vinyl is typically not used in its virgin form. It is normally polymerized with other chemicals like chloride to make PVC which is much tougher. It can be made rigid or flexible depending on the formulation, which makes its applications wide ranging. In its flexible form it is made into adhesive wraps, signs, and clothing. It’s most popular rigid form is pipes although it is used for plastic fencing, and gutters too.
PVC is one of the best prototyping materials. It is inexpensive and widely available. PVC pipe and fittings can be used as a modular building system and it is easily locked into place with PVC cement. PVC cutters make it easy and mess free to cut. Just be sure to use low VOC cement and wear a mask when PVC cement. PVC should never be burned or laser cut as it will release toxic chlorine gas.
PVC tube fittings on an early concept model for the SockSync.
Low Density Polyethylene, LDPE
LDPE is a low strength polyethylene that is flexible but not as strong as HDPE. It is most commonly used as a film to make bags and cling wrap. Thicker LDPE parts are still flexible, so they are also used in sports water bottles and other squeezable bottles.
LDPE is not a very common, so has limited use as a prototyping material. PET and polypropylene have better properties and often used in its place.
Polypropylene, or polypro, is inexpensive, strong, flexible and tough. It has widespread use from food packaging to furniture to toys. It is resistant to fatigue and is used in applications that require a living hinge like the top of a Parmesan cheese container. It is food safe, but since it tends to be hazy, it is not used in high visibility packaging. It is often used in reusable food storage containers like Tupperware.
Despite its amazing properties and widespread use, polypro is difficult to prototype with. It is hard to drill and usually results in polygonal holes. It has a high molecular surface energy which makes it nearly impossible to glue to. Painting it is really difficult and requires a special primer to get it to stick at all, and even then it is not guaranteed to work.
Polystyrene is very inexpensive to manufacture, but it is hard to recycle and many municipalities exclude it from their programs. It is slow to biodegrade, and it has largely been replaced with paper products or other plastics in food packaging. Foamed styrene (Styrofoam) makes for cheap food packaging, and is often the tray that meat is packed on in the grocery store. It was popular as fast food packaging but has largely been phased out for paper packaging. Styrene sheet is tough and flexible. It flows really well in a mold and is used to make plastic model kits to get the high surface detail.
Styrene is a great prototyping material. Sheets of it are inexpensive and up to about .060″ thick can be cut with regular scissors. It can be bonded easily with super glue or model cement, and it holds paint well. I use a sheet as a photography backdrop for taking photos of prototypes or selling on Ebay.
Highly detailed model parts from a Tamiya model sprue shot on a piece of white styrene backdrop.
Polycarbonate, PC and others
Recycling symbol 7 is most often polycarbonate, but also serves as a catch-all for other uncategorized plastics like some nylons and BPA. Polycarbonate is known for its impact resistance and is used in R/C car bodies, sunglasses, and water bottles like the ever popular Nalgene brand bottles. Thicker forms are also used to make bullet proof glass.
PC is a useful prototype material. Thin sheets can be cut with scissors and it can be bonded together with super glue. PC bottles can be cut with a Dremel tool or saw and can even be threaded to accept pipe fittings. Since it is popular for R/C applications, small sheets can often be found at local hobby stores. Specially formulated R/C car paint sticks well to it, and the inside surface can be painted to yield a glossy outer finish.
The Perky Collar was molded from polycarbonate for its toughness and impact resistance.
The old saying goes that “One man’s trash is another man’s treasure.” However, it is difficult to understand the value of the plastics we recycle without understanding what they are or what kinds of properties they have. Hopefully this guide will help you upcycle some of your waste into a successful new innovation.