Difference Between Polyethylene and Polypropylene

Main Difference – Polyethylene vs Polypropylene

Both polyethylene and polypropylene are types of plastics, and they are polymers. The word ‘poly’ means ‘many’ and, therefore, polymers are materials made up of lots of smaller units put together. Most of the polymers can be broken down into distinct parts which act as the building blocks of the polymer, and these single units are called ‘monomers.’ The monomer unit of polyethylene is ethylene whereas the monomer unit of polypropylene is propylene. The main difference between polyethylene and polypropylene is that polyethylene is formed through the polymerization of ethylene monomer units whereas polypropylene is formed through the polymerization of propylene monomer units.

What is Polyethylene

As mentioned above, polyethylene is a polymer made up of the polymerization of ethylene molecules which are two Carbon alkane units [-CH₂CH₂-]. It is categorized as a thermoplastic polymer. Most of its physical properties depend on its molecular weight. High-Density Polyethylene (HDPE), Medium-Density Polyethylene (MDPE) and Low-Density Polyethylene (LDPE) are the most common types found. And they are known for their chemical resistivity. That is, they do not react and degrade in the presence of strong acids and strong bases. Polyethylene is inert and translucent. This means that it allows light to pass through, but does not facilitate image formation, unlike in the case of transparent material.

Ethylene can undergo co-polymerization. In this case, its purity is lost. However, polyethylene undergoes co-polymerization to a lesser extent when compared to other plastics. Therefore, due to its purity it is often costlier. There have been serious environmental concerns regarding polyethylene as it doesn’t degrade naturally unless it has been treated. However, many methods have been developed and are been used to solve this issue. Polyethylene is now made from feedstock such as sugar cane, wheat grain, and sugar beet.

Difference Between Polyethylene and Polypropylene

Ball-and-stick model of part of the crystal structure of polyethylene

What is Polypropylene

Polypropylene is also a thermoplastic polymer which is more rigid when compared to polyethylene. As mentioned above, polypropylene is made up of propylene monomer units, which are three Carbon alkane units [–CH₂(CH₃)CH₂-]. Due to its rigid nature, it is often used for making of molded material. Propylene is often co-polymerized with ethylene molecules in order to improve its flexibility. i.e. ethylene propylene rubber. Polypropylene is not as translucent as polyethylene, but can be made translucent by un-colouring.

Polypropylene also comes in certain grades depending on its molecular weight. However, most of the polypropylenes come in between the weights of High-Density Polyethylene and Low-Density Polyethylene. It undergoes a chain degradation when exposed to light and gives rise to oxidations reactions forming free radicals which raise further concerns regarding health and safety.