What determines the strength of a plastic bag?

Plastic bags are made from a ubiquitous polymeric material called polyethylene. This starts with ethylene, usually extracted from natural gas, which is then processed into a polymer, forming long chains of carbon and hydrogen atoms. These chains can vary depending on the type of polyethylene used, but they all contribute to the manufacture of various types of plastic bags.

Materials used to make plastic bags

Low Density Polyethylene Plastics

LDPE plastic is made from low-density, branched polymeric materials. Rather than remaining linear, these polyethylene chains unfold in many different combinations of lines. This produces a very light, almost film-like plastic used to make the tearable bags dry cleaners use to wrap cleaned clothes.

High Density Polyethylene Plastics

HDPE stands for high density polyethylene and is the most common type of polyethylene used to make shopping bags. This plastic consists of straight molecular chains with few molecular chains that remain linear from end to end. This linear structure creates a very strong material, which is why the average grocery bag is light but can withstand many times its own weight without tearing.

Medium Density Polyethylene Materials 

If you are looking for a material that contains a small amount of LDPE and a small amount of HDPE, medium density polyethylene (MDPE) may be the solution for you. This resin has unique properties for very specific applications. 

For example, it is not as opaque as HDPE materials, is not as clear as LDPE, and is not dense enough for high volume storage needs. Fortunately, MDPE materials have excellent chemical resistance, ease of stretch and low stretch properties. As a result, MDPE is often used in packaging films. 

LLDPE Plastics

With linear low density polyethylene, these plastics do not branched, but they also do not have the same strength as the HDPE version. This means that shopping bags made from LLDPE plastic need to be thicker and heavier than traditional shopping bags. The shiny bags used in clothing stores are a common example of bags made from this substance.

What determines the strength of a plastic bag?

Not all plastic bags have the same strength capacity. The strongest type of plastic bag depends on how the plastic is manufactured. In fact, different types of plastic bags were developed just for their strength properties. The strongest plastic bags are made of HDPE or high-density polyethylene. Material changes that determine thickness and strength occur during the polymerization process.

The strength of plastic bags depends on the way ethylene reacts during the polymerization process. To make polyethylene, small molecules of ethylene combine together to form a chain of molecules called a polymer. From this main chain, branches can begin to form. The more branches that are formed, the thinner and less dense the resulting polyethylene is. In order to make denser, stronger polyethylene, fewer branches must be formed.

For example, if the polymer has a relatively straight main chain with few branches, it is called high-density polyethylene. HDPE is made of tightly packed molecules, which not only makes it denser but also less flexible. It can be used to make plastic bags that are stronger and more durable than lower density plastics.

If the polymer chain has many branches that form along the main chain, it may be MDPE or LDPE. medium density polyethylene and low density polyethylene are more flexible and have less resistance than HDPE.