What Plastic is this Part Made Of?
There are several ways to determine what plastic the object you have is made of:
*Plastic Resin Identification Codes
*Floats or sinks in water
Plastic Resin Identification Codes
The plastics identification codes were developed in 1988 by SPI Society of Plastic Industries. These codes are intended to be recycling codes. #1 is polyethylene terephthalate (PETE, PET), #2 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). (See our blog dated 10/15/21 for more on these.)
Indeed knowing what the object you have is an important clue to knowing what type of 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. But if the object has a code molded into it or written on its label you will know for sure. For example, the object below is coded number five so we know it is made from polypropylene. And of course a Styrofoam cup will usually have a 6 on the bottom of it.
The Burn Test
Another way to get information about what kind of plastic you have is to do a burn test. However, it is important 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. Therefore, heat the plastic with a lighter until it burns using the proper ventilation equipment.
|Dips||Floats in Water|
smoke w soot
|PETE, (PET)||light smoke||yellow||yes||yes|
|Nylon||burnt wool or
smoke w soot
|Polyethylene||candle wax||blue, yellow
|Polypropylene||sweet odor||blue, yellow
|yellow w green
Floats or Sinks in Water
Plastics all have a particular density. But 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 on and 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 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 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)
|(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
However, higher density plastics such as PET (polyethylene terephthalate), PVC (polyvinyl chloride), and PS (polystyrene), do sink. PETE (or PET) is used to make single used 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.
Is it 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 this material will sink. Additionally, if the smell the material gives of 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.