How To Identify Polyethylene Plastic

Polyethylene plastic is commonly used in various applications due to its durability, versatility and relatively low cost. Given its recyclability, it also has a relatively low impact compared to other types of plastic. Made from the monomer ethylene, this thermoplastic polymer is one of the most widely produced plastics in the world, having multiple uses.

Polyethylene plastic is found in all kinds of products, ranging from grocery bags to bottles. To know how to identify polyethylene plastic, keep on reading this article. 

Characteristics of Polyethylene Plastic

For proper identification of polyethylene plastic, it’s important to understand its physical characteristics. Polyethylene has different forms like linear low-density polyethylene (LLDPE), low-density polyethylene (LDPE), ultra-high-molecular-weight polyethylene (UHMWPE) and high-density polyethylene (HDPE). Each form has unique physical characteristics that can help to identify it.

• Transparency and Clarity

Polyethylene is known for its transparency and clarity. The polymer chains in the plastic are long and flexible, allowing light to pass through them relatively easily. It is especially true for LDPE and LLDPE, which are commonly used for packaging and bags. On the other hand, HDPE is often opaque and can range in colour from white to black.

• Density and Flexibility

The density of polyethylene plastic can range from low to high depending on the specific form of the plastic. This again has to do with the molecular structure of the specific plastic, along with the processing conditions. LDPE is generally the least dense and most flexible type of polyethylene plastic, whilst UHMWPE is the densest and has the least flexibility. HDPE falls somewhere between them.

• Appearance and Surface Texture

Polyethylene can have a smooth or textured surface depending on its intended use. For example, HDPE is often used for pipes and is smooth, whilst LDPE is used for bags and can have a textured surface. Additionally, polyethylene can be moulded into different shapes, affecting its appearance.

How to Identify Polyethylene Plastic

Various testing methods can be used to determine if a material is made of polyethylene plastic. Regardless of the method, the goal of testing polyethylene plastic is to determine its composition and properties to ensure that it meets the requirements of a specific application. 

The following methods can be used to identify polyethylene plastic:

• Density Test

The density test method is a laboratory test used to determine the density of polyethylene. As a measure of its mass per unit volume, density is an important property that affects the strength, rigidity and other physical characteristics of polyethylene plastics.

One way of testing the density is by weighing the sample whilst it is suspended in a liquid of known density. The sample’s density can then be calculated by comparing its regular weight to its weight in the liquid. 

Another method is to place the sample in a sealed container. Its volume is then determined by measuring the pressure change as the container is filled with a gas. The sample’s density can then be calculated from its mass and volume. Sometimes, X-rays are used to measure the spacing between atoms in the sample. The sample’s density can then be calculated from its atomic structure. 

The outcome of a density test of polyethylene plastic will depend on the specific type of polyethylene being tested and the method used to test it. For example, LDPE has a lower density than HDPE, typically in the range of 0.91–0.93 g/cm3. On the other hand, HDPE has a higher density, typically in the range of 0.94–0.97 g/cm3.

• Burn Test

The burn test is a simple method used to test the flammability of material, including polyethylene plastic. This test involves heating a small sample of the material with a flame and observing how it burns.

In the case of polyethylene plastic, the burn test can be used to determine its flammability and whether it will emit toxic fumes when burned. When polyethylene is burned, it produces a bright yellow flame and gives off a characteristic odour, which can be used to identify the material. 

The burn test is a simple and quick method for testing the flammability of polyethylene plastics, but it does not provide a complete picture of the material’s properties and performance. It is hard to differentiate between plastics solely using this test. Additional tests may be necessary for a more comprehensive evaluation of the material’s properties and behaviour.

• Stretch Test

The stretch test is one of the methods used to identify polyethylene plastics. It involves stretching a thin film of the plastic material until it begins to resist stretching. Based on the behaviour of the strip during the test, you can identify the type of polyethylene as follows:

• Low-density polyethylene (LDPE): The strip can be easily stretched and is tough to break. It will not return to its original shape after stretching.

• Linear low-density polyethylene (LLDPE): The strip stretches easily, but breaks at a lower point than LDPE. It will not return to its original shape after stretching.

• High-density polyethylene (HDPE): The strip is difficult to stretch and will break before reaching its elastic limit. It will return to its original shape after stretching.

Keep in mind that the stretch test is not a definitive test and should be used in conjunction with other tests to confirm the type of polyethylene plastic.

• Recycling Code

The recycling code, also known as the Resin Identification Code (RIC), is a system used to identify the type of plastic material used in a product or packaging. The code consists of a number from 1 to 7, within a triangle made of arrows, which is usually located on the bottom of the item.

LDPE has a recycling number code of 4, whilst HDPE has a recycling number code of 2. This code helps to indicate to consumers and recyclers which plastics are recyclable and how they should be processed. However, not all recycling programs accept all types of plastics, including polyethylene, so it’s important to check with your local recycling centre.

The use of the recycling code is also helpful in identifying and separating different types of plastics during the recycling process. Polyethylene plastic is one of the most commonly recycled plastics, and it can be recycled into a variety of products, including plastic bags, trash cans and plastic lumber.

In addition to the recycling code, some manufacturers and distributors may include additional information on the packaging or label about the recyclability of the product or how to properly dispose of it. It’s important to follow these instructions to ensure that the plastic is disposed of or recycled properly and to minimise its impact on the environment.

McNeall Plastics: Helping Businesses Through Plastic Engineering and Manufacturing

With over 50 years of experience, McNeall Plastics is unrivalled at providing your business with the best quality plastic solutions. Our passion for plastic manufacturing and access to superior technology allows us to bring you high-grade polyethylene plastics and other materials.