A Guide to Understanding HDPE

What is HDPE?

Most commonly known by its identification mark of number 2 in the center of chasing arrows, this grade 2 or second type of plastic is called HDPE or high density polyethylene.

HDPE, just like any other plastic, is a thermoplastic polymer produced from petroleum and is known for being a strong, durable and very versatile form of plastic material.

Its versatility is evident from the fact that it is popularly used in a number of daily use applications such as milk jugs, shampoo bottles, cutting boards, liquid chemical bottles, anti-corrosion piping and plastic lumber.

Practically proven reasons for the popularity of HDPE or high density polyethylene

The popularity of high density polyethylene can be attributed to a number of its features that does not only offer a much diversified application but which have also been practically proven to be more beneficial when compared with their counterparts. Some of these reasons have been briefly described here.

1. Anti-corrosive and weather resistant

High density polyethylene (HDPE) can survive atmospheric, environmental and natural factors that can cause normal wear and tear or corrosion.

For example, pipes made from HDPE which are used for delivering water and which might be buried underground or remain outside with exposure to any external threat, can easily survive rotting, insect invasion, mildew and mold and can thus remain in service for more years as compared to pipes made of rubber or any metal.

2. Light weight but heavy duty

Although products made from high density polyethylene are light weight, they still have the strength to perform heavy duty tasks.

For example, you often use light weight but reasonably huge milk and juice bottles, big enough to easily hold a couple of liters or up to 3 to 5 gallons of liquid.

Aside from serving as food containers, many automakers use fuel tanks made from high density polyethylene in their cars to reduce the weight of the vehicle and increase fuel efficiency.

Image by PublicDomainPictures from Pixabay

3. Impact bearing quality

Products made from high density polyethylene also have the tendency to bear impact and any strike.

For example from the second bullet point above, you might have noticed that even if a milk or juice filled bottle falls from your hand it still remains intact and does not damage or leak. In fact, it often seems to bounce back after hitting the ground.

4. Usage as plastic lumber

Due to its long lasting and weather resistant quality, high density polyethylene (HDPE) is used to make plastic lumber as well.

For example, plastic lumber is widely used in play grounds and parks in the form of garden furniture including park benches, light poles, outdoor physical activity toys and dustbins, etc.; and at your home to make outdoor decking and fencing.

5. Easy to mold

High density polyethylene (HDPE) is super ductile or malleable.

For example, it can easily be molded into any shape and put to various uses.

6. Adheres to sustainability goals

Products made from high density polyethylene are known to be environmentally friendly and adhere to sustainability goals.

For example, packaging products made from high density polyethylene, especially those which are used in the food industry, have been known to have less impact on the environment in terms of contributing less waste, low energy usage and global warming threats, when compared with products made from other materials for the same purpose.

7. Recyclable

As mentioned in the beginning of this article about the number 2 mentioned in the center of chasing arrows at the bottom of many bottles, is also an indication that any product made from HDPE can be recycled.

Products made from recycled high density polyethylene have also turned out to be as good as made from HDPE for the first time.

Some examples of product manufacturing industries which commonly use recycled high density polyethylene (HDPE) include

  • producers of children’s outdoor physical play equipment for use in parks and backyards;
  • automobile makers for producing car dashboard panels, lock handles, different liquid tanks such as for brake oil, gear oil, radiator water tank, etc.;
  • plastic lumber to use for fencing, park benches and outdoor decking;
  • plastic storage containers such as waste containers, dust bins and recycling bins, etc.

Pros and Cons of using high density polyethylene

Just like any other product, high density polyethylene (HDPE) also has some advantages and disadvantages.

Advantages of high density polyethylene (HDPE)

  • The melting point of high density polyethylene ranges between 120⁰C to 140⁰C or 266 degrees Fahrenheit. Even when it gets near to its melting point, it does not completely melt. Rather it turns sticky and forms as solid blocks without discharging any fumes or harmful gases.
  • High density polyethylene (HDPE) has a slightly higher strength to density ratio than low density polyethylene or LDPE. With a density of 0.93 to 0.97 g/cm₃, it has an increased tensile strength than low density polyethylene (LDPE).

Photo by mali maeder from Pexels

  • High density polyethylene is in accordance with FDA compliance code which means that it is safe to be used in direct contact with food items.
  • Although high density polyethylene (HDPE) can typically survive direct contact with most chemical compounds, its chemical resistance levels vary in accordance with the type of chemical that it comes in contact with. While it is excellent in most cases, for liquids like diluted acids, alkalis and alcohol, its resistance level is rated much better. When containing oils and similar greasy products, its resistance levels are moderate. For any form of hydrocarbons, which means any organic compound consisting solely of hydrogen and carbon such as methane, ethane and propane etc., it has shown poor resistance.
  • Other than having an increased tensile strength than low density polyethylene (LDPE), high density polyethylene generally has a higher tensile strength when matched with that of other forms of polyethylene.
  • High density polyethylene is known for being a low cost polymer with a good processing ability
  • It can easily resist low temperatures.
  • Its water absorption rate is very low which makes it one of the best options to store liquids in it, especially food related.
  • High density polyethylene is known for having excellent properties as a non-conductor of electricity.
  • Between temperatures of -50⁰C and +60⁰C, high density polyethylene is known to mostly remain stiff.
  • By adding ultra violet absorbing additives or carbon black, the reinforcing quality and weathering resistance of high density polyethylene (HDPE) can be increased.

Disadvantages of high density polyethylene (HDPE)

  • High density polyethylene resistance level against ultra violet and low heat is low.
  • High density polyethylene is prone to crack under conditions of stress and pressure.
  • High density polyethylene has less stiffness rate which is less than that of polypropylene.
  • Mold shrinkage of high density polyethylene is high.
  • High frequency joining and welding cannot be applied on high density polyethylene (HDPE).

Some other applications of high density polyethylene (HDPE)

Aside from all of the above mentioned applications of high density polyethylene (HDPE), it can also be engineered to serve a few other purposes as per the specific requirements of its user base.

Application as consumer goods

The ability of high density polyethylene in terms of being easy to process and under low cost makes using it a preferred choice for making numerous household or end user products such as toys, ice boxes, water bottles, garbage containers, homeware items, etc.

Application an fibers and textiles

Due to its high tensile strength characteristics, high density polyethylene (HDPE) is widely used for making heavy duty ropes such as those used for hiking and climbing mountains or for pulling and lifting objects, nets used for fishing and in sports such hockey and football, nets used in the agricultural sector, and decorative fabrics used for industrial purposes, etc.

Application as a packaging material

To provide reasonable strength against any external impact, packaging made from high density polyethylene is used to make bottles for containing drinkable water, juices and milk, crates, trays, disposable food containers, jerry cans and bulk containers used for industrial purposes, etc.

Application in infrastructure enhancements and equipment

High density polyethylene is also used to replace and enhance infrastructural equipment because of its capability to stand wear and tear and environmental threats.

For example, pipes and fittings made from high density polyethylene (HDPE) are used for making supply networks to transport water, gas, sewage, drainage from main supply stations to households and end users. Many factories and manufacturing plants also use pipes and fittings made from high density polyethylene for transmission of gas, fuels and other liquids from storage units to consumption points.

Other than this, high density polyethylene is also used to coat or used as a shield cover of various underground cables and wires for protection against any form of chemical or hydrolysis. High density polyethylene is also used to make large inspection chambers and manholes for drainage and sewage use. In the automobile sector, fuel tanks made from high density polyethylene are also installed in vehicles to increase their fuel consumption efficiency.

Common questions related to high density polyethylene

While reading this article, many questions of common nature might have popped into your mind. While we cannot exactly know what they actually are, we have still managed to compile a list of some of the most generally asked questions by the public on the topic of high density polyethylene.

What does the number 2 mean on a high density polyethylene product?

Most specifically, the number 2 within the chasing arrows on any high density polyethylene product actually denotes its resin identification code. Among the total seven types of plastic, high density polyethylene is ranked at number 2.

Why is high density polyethylene used?

Most commonly, high density polyethylene is used for its high strength to density ratio which makes it light weight and heavy duty.

Is high density polyethylene dangerous?

While it does not bear any kind of danger in its end form, during the manufacturing process, high density polyethylene does pose danger and can be hazardous to the health of the persons involved in the process. For example, the dust, resin and vapors that emit during the heating process can cause eye irritation. Therefore to prevent any such issues, it is always recommended to take proper care and precautionary measures.

Is high density polyethylene good for the environment?

Aside from any kind of health and environmental hazards that exist during its manufacturing process, overall high density polyethylene is considered to be one of the most stable forms of plastic that are in use today.

Is high density polyethylene waterproof?

Yes, completely. In fact high density polyethylene is best for the waterproofing of building foundations. Aside from being waterproof, high density polyethylene also protects the foundation against root intrusion. And due to its non-biodegradable nature, high density polyethylene made from one hundred percent virgin resin will definitely last for a few hundred years.

Are all high density polyethylene plastics free from BPA?

Although both BPA as well as high density polyethylene are the derivative products of petroleum, HDPE is totally free from BPA and is therefore safe for being used directly with products for human consumption.

Conclusion

Although high density polyethylene can be put to various uses and is recyclable as well, it is still a non-biodegradable substance in nature and is viewed by environmentalists as a cause of significant contribution to the world’s waste products in the form of plastic.

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