You have a plastic part in your hand. How do you determine what plastic it is made of? Here are four good places to start:
- Plastic Resin Identification Codes
- Burn Test
- Floats or Sinks in Water
- Unique Appearance
PLASTIC RESIN IDENTIFICATION CODES
The plastics identification codes were developed in 1988 by the SPI Society of Plastic Industries. These codes are intended to be recycling codes. #1 is polyethylene terephthalate (PETE, PET), and #2 is high-density polyethylene (HDPE). #3 polyvinyl chloride (PVC), #4 low-density polyethylene (LDPE), #5 Polypropylene (PP) #6 Polystyrene (PS) while #7 Misc. includes polycarbonate (PC), nylon, ABS, acrylic, and polylactic acid (PLA).
Indeed knowing what object you have is an important clue to knowing what plastic it is made from. For example, if you are holding a milk jug the material is probably high-density polyethylene. But a plastic container that once held laundry soap and most colored containers are also made of HDPE. Often trash bags are made from low-density polyethylene.
However if the object has a code molded into it or written on its label, you will know what plastic it is made of for sure. For example, the objects below are coded number five, so we know they are made from polypropylene. And of course, a Styrofoam cup will usually have a 6 on the bottom of it.
THE BURN TEST
The burn test is another way to get information about what kind of plastic a part is made of. It is important however that this test be performed in an industrial setting with the correct equipment as some burning plastics can release carcinogens. Also, molten plastics are very hot, and some will drip. Heat the plastic with a lighter until it burns using the proper ventilation equipment.
| Material | Odor | Color of Flame | Dips | Floats in Water |
| ABS | acrid. black smoke w soot | orange | no | no |
| PETE, (PET) | light smoke | yellow | yes | yes |
| PTFE | burnt hair | ——- | no | no |
| Nylon | burnt wool or hair | blue, yellow tipped | yes | no |
| Polycarbonate | faint, sweet aroma, black smoke w soot | orange | yes | no |
| Polyethylene | candle wax | blue, yellow tipped | yes | yes |
| Polypropylene | sweet odor | blue, yellow tipped | no | yes |
| Polystyrene | gas odor, (naptha) | yellow, black smoke | yes | no |
| PVC | hydrochloric acid | yellow w green spurts | no | no |
| Polyesters | hydrochloric acid | yellow | no | no |
FLOATS OR SINKS IN WATER
Plastics all have a particular density. Not all plastics float on water. If the density of the material is greater than water, it will sink. If it is less dense, it will float. We have all read about the amount of plastic waste floating in the earth’s oceans. The Great Pacific Garbage Patch in the north-central Pacific Ocean is the largest accumulation in the world. So it must be made of plastics that float on water. Sadly, not one square mile of the surface ocean is free from plastic waste.
PLASTICS THAT FLOAT
There are three plastics that float on water. And all have a density of less than that of water. These are HDPE, LDPE, and PP. See the chart below. Since plastics can vary in density when they are manufactured, there is a range of values indicated for each plastic.
Density Table (G/mL)
| Water 1.00 |
| (1) PETE (PET) 1.38-1.39 |
| (2) HDPE 0.95-0.96 |
| (3) PVC 1.16-1.35 |
| (4) LDPE 0.92-0.94 |
| (5) PP 0.90-0.91 |
| (6) PS 1.05-1.07 |
| (7) Others. Nylon 1.14, polycarbonate 1.2, acrylic 1.15, etc. |
PLASTICS THAT SINK
To understand what plastic the object you are holding is made from knowing if it will sink in water is auseful. So higher-density plastics such as PET (polyethylene terephthalate), PVC (polyvinyl chloride), and PS (polystyrene), do sink. PETE (or PET) is used to make single-use plastic drink bottles. Since the density of PETE is 1.38-1.39 many of those bottles we use and do not recycle may wind up sinking to the bottom of the oceans.
PVC is made into sheeting, rope, blood bags, and medical tubing as well as siding and flooring to name just a few of its uses. After polyethylene, PVC is the most widely used plastic in the world. And PVC also sinks. Finally, polystyrene is made into hot cups and clam shells for hamburgers and other food we eat on the go. With a density of 1.05 to 1.07, PS will also sink.
NYLON
Most nylon produced is used to make clothing and carpet although all clothing and carpet are not necessarily made of nylon. But with a density of 1.14, it will sink. Additionally, if the smell the material gives off in the burn test is that of burnt wool, if the flame is blue with a yellow tip, if the molten plastic drips, and it does not continue to burn when the flame source is removed, what you have is pretty sure to be nylon.
TEFLON (PTFE)
Teflon (PTFE) is a polymer known for its chemical resistance and ability to withstand high heat applications. It also has a characteristic appearance. It is generally very white in color and has a waxy feel. Additionally Teflon is notably soft, so much so that you can use your thumbnail to make an indent in it.
