The Chemistry of Plastic Bottles (PET, HDPE, LDPE)

Introduction

  • Plastic bottles are part of our daily life, it help to make our lives easier. From water to prescription drug to shampoo, we use plastic bottles to make them easier to handle and transport. When I look around at all the plastic bottles we use every day, I found myself extremely curious about how these plastic bottles were made and how they were first discovered.Composition of ...
    • PET (polyethylene terephthalate), mono ethylene terephthalate is C10 H8 O4
    • HDPE (polyethylene), ethylene C2 H4
    • LDPE (polyethylene), ethylene C2 H4

Main Chemicals, Compounds, Components

The main component of polyethylene terephthalate, which is considered a part of the polyester family. Polyethylene terephthalate contains many mono ethylene terephthalate (C10 H8 O4) units that link together to create the PET plastic we see every day.

The main components of HDPE and LDPE is both polyethylene, which contains many ethylene (C2H4) units. The carbon atom has a bond with two hydrogen atoms and a double bond with the other carbon atom. This is the basic chemical compound of polyethylene, with different pressure, temperature, they can be created into either HDPE or LDPE.

Chemistry's Role

Plastic are used in our everyday life, and it is a product of chemical reaction. They are discovered by chemist when they mix different chemicals together, which will create a gooey texture and eventually be created to what we call plastic today. The properties of these of the plastic could be changed when different method or materials are used.

PET bottles are one of the most common bottles we use everyday and its main compound is polyethylene terephthalate. When you combine terephthalic acid and ethylene glycol together to create polyethylene terephthalate, you will get water as the byproduct, this is called the Esterification Reaction. You may also combine ethylene glycol and dimethyl terephthalate to get polyethylene terephthalate, but in this case, you will get methanol as the byproduct. After the reaction, it will go through a polycondensation with the monomers, which will give you water as a byproduct. After polymerization, the plastic will go through molting to form into a PET bottle.

Ethylene is the main compound of both LDPE and HDPE. LDPE was made by putting <10 ppm oxygen and ethylene in extremely high pressure, about 1000-3000 bars, and 80 to 300 degrees Celsius. The polyethylene is highly branched, making it hard to pack together. HDPE is created by using a much lower pressure, about 10-80 bars, an Al-based catalyst, and 70 to 300 degrees Celsius. The polyethylene created has just the opposite of LDPE, a low level of branching (a lot more linear), which help it to pack together easier, which makes it stiffer and higher tensile strength. After the plastic has been created, it would also need to go through molting to form into a bottle container.

Resources

About the Author

Lin is a Junior at Billings Senior High. He enjoy challenges from everyday life. He enjoys spending time with friends and family. Although Lin is planning on attending college, he does not know what career he would like to pursue.