Jul 25 2012
Image Credit: Quietword/Shutterstock.com
Article updated on 18/02/20 by Kerry Taylor-Smith and Laura Thomson
High-Density Polyethylene, usually shortened to HDPE or PEHD, is a plastic polymer with flexible properties that make it ideal for a wide range of applications.
High-density polyethylene, as the name suggests, has a higher specific density than low-density polyethylene, though this difference is only marginal. What makes the difference in the physical properties of HDPE is the lack of branching, meaning it is light with high tensile strength. As there is no branching, the structure is more closely packed, making HDPE a linear polymer. The branching can be controlled and reduced by using specific catalysts during production.
Properties and Applications of HDPE
HDPE has many advantageous properties that make it important in the manufacturing of different products. HDPE has a comparatively high density compared to other polymers, with a specific gravity of 0.95. HDPE is relatively hard and resistant to impact and can be subjected to temperatures of up to 120 °C without being affected.
HDPE is not autoclavable, unlike Polypropylene (PP). Autoclaving conditions are used to sterilize products using high pressures and temperatures. For more information regarding PP, please read the following article.
HDPE is recognizable by its opaque or translucent appearance.
These durable properties make it perfect for heavy-duty containers and HDPE is primarily used for milk containers, as well as Tupperware, shampoo bottles, bleach bottles, and motor oil bottles. HDPE also does not absorb liquid readily, making it a good barrier material for liquid containers. Almost a third (around eight million tons) of HDPE produced worldwide is used for these types of containers.
Furthermore, HDPE is an extremely resistant material to many chemicals, hence its widespread use in healthcare and laboratory environments. It is resistant to many acids, alcohols, aldehydes, esters, bases and oils.
Other uses for HDPE include:
- Plastic shopping bags
- Trays
- Tanks
- Food containers (HDPE is approved for contact with food by many food advisory boards)
- Pipe fittings
- Hinges
- Cutting boards
- Wear plates
How is HDPE Recycled?
HDPE is accepted at most recycling centers in the world, as it is one of the easiest plastic polymers to recycle. Most recycling companies will collect HDPE products and take these to large facilities to be processed.
First, the plastic is sorted and cleaned, to remove any unwanted debris. The plastic then needs to homogenized, so that only HDPE will be processed. If there are other plastic polymers in the batch, this can ruin the recycled end-product.
HDPE has a specific density of 0.93 to 0.97 g/cm3. This is much lower than that of PET which is 1.43-1.45 g/cm3, meaning that these plastic polymers can be separated by using sink-float separation. However, HDPE has a similar specific density to PP, which means the sink-float separation cannot be used. In this case, Near-Infrared Radiation (NIR) techniques can be used, unless the plastic is too dark and absorbs the infrared waves.
HDPE is then shredded and melted down to further refine the polymer. The plastic is then cooled into pellets which can be used in manufacturing.
Recycling plants can also benefit from the use of a baler, which can compress the post-consumer waste to minimize the energy used in transport.
Small steps at home can also be taken to recycle HDPE. With regards to milk bottles, these can easily be reused if washed out thoroughly first. To reduce packaging waste, buying plastic bottles in bulk is another good option.
Equally, carrier bags can also be reused when going shopping. Many large supermarkets also offer collection points for used carrier bags to be recycled. Some plastic films contain a message to recycle these with carrier bags at the supermarket and not to leave ‘kerbside’.
Recycling of HDPE is aided by the resin code on the product, which is an indiscriminate number assigned to different plastic polymers to help separate plastics at the recycling stage. The resin identification code for high-density polyethylene is ‘2’.
The Environmental Benefits of Recycling HDPE
The worldwide market for HDPE is huge, with a market volume of around 30 million tons per year.
The amount of plastic used in plastic bags has reduced by around 70% in the last 20 years thanks to the introduction of reusable canvas bags and using biodegradable materials, but the majority of bags are still produced from HDPE. Furthermore, there is a growing market for HDPE containers in China and India due to increased standards of living, as well as higher demand for HDPE pipes and cables due to rapidly growing industries.
HDPE is non-biodegradable and can take centuries to decompose, so it is imperative that these bags and containers are recycled and used again.
Recycling HDPE has many benefits. For example, it is more cost-efficient to produce a product from recycled HDPE than it is to manufacture ‘virgin’ plastic.
HDPE, like many plastic polymers, is produced using considerable amounts of fossil fuels and it takes a total of 1.75kg of oil to manufacture just 1kg of HDPE.
Many new products can be manufactured using recycled HDPE, including:
- Rope
- Toys
- Piping
- Recycling bins
- Trash cans
Scientists Prove HDPE can be Recycled at Least 10 Times
Scientists have often questioned why the recycling rate of HDPE and PET is not higher when it is one of the easiest materials to recycle and makes up 97% of plastic bottles in North America. In November 2017, a report made by the Association of Plastic Recyclers (APR) and American Chemistry Council (ACC) found that the total pounds of plastic bottles recycled decreased by 71 million pounds in 2016 compared with 2015. The report stated that compared with 2015, HDPE bottle collection fell by 31.7 million pounds to 1112.1 million pounds, with the HDPE bottle recycling collection rate decreasing to 33.3% from 34.4%.
ESE World B.V. carried out a test to demonstrate that HDPE can be recycled at least 10 times. ESE found that the plastics injection molding and shredding techniques do not alter the material properties over the entire period of reuse. As the plastic bottles or containers have a service life of around 10 to 20 years, the study indicates that the availability of material for sustainable production from the same plastic is secured for around 100 to 200 years.
The experiment, therefore, points researchers towards future developments in the recycling of HDPE.
References and Further Reading