Know Your Plastics Series: High-Density Polyethylene (HDPE)

Identifying Synbol for HDPE/PEHD resin.

Sometime in the 1930s, during the Second World War, the first HDPE was created, in the United Kingdom.

Its first application? High-frequency radar cables.

Fast forward 20plus years, the United States took on the commercial production of HDPE and, the polymer plastic has garnered popularity ever since.

What is HDPE?

A petroleum-based thermoplastic (click for more on thermoplastics), High-Density Polyethylene (HDPE) AKA Polyethylene High Density (PEHD) is widely renowned for its excellent strength levels –an outstanding strength-to-density ratio, tensile strength and specific strength. This strength is possible because HDPE polymers have little or no branching. The "little or no branching" characteristic results in significantly stronger intermolecular forces that make HDPE harder and more resistant to temperatures of up to 120 ⁰C (248 ⁰F) for short periods. This property also makes HDPE resistant to a lot of acids, caustics and solvents.

Sample HDPE bottles and jugs.

Essentially, HDPE is a strong, sturdy plastic resin used in producing short-term storage containers like bottles and jugs, which can hold varying liquids, from milk to bleach and even soaps. Of the three polyethylene plastic types (Polyethylene Terephthalate, High-Density Polyethylene and Low-Density Polyethylene), HDPE is very popular for its versatility in applications which include:

- HDPE as a food-grade plastic is excellent for storing perishable food and drink i.e. food that should not be stored for the long term. Any long term storage may result in HDPE plastic absorbing odours and residue that is tough to expel.

- HDPE is used in both flexible and corrugated pipes. Pipes made from HDPE are among the most commonly used for potable water and wastewater from hazardous manufacturing, sewage and storm drainage. HDPE here is often called polythene or alkathene.

- HDPE is useful in plastic surgery such as, in the reconstruction of the skeletal structure. For example, in the face.

Other applications include bread bags, liners of cereal boxes, cutting boards, ropes, fishing nets, gas pipes, toys and playground equipment, plastic mailing envelopes, plastic bags, plastic lumber, bottle crates, fireworks, snowboards, stone paper, 3D printing filament, laboratory equipment (beakers, test tubes and flasks), motor oil, jerry cans used for fuels, vehicle fuel tanks, swimming pool installation, storage sheds, plastic chairs and stools for outdoor use.

Applications of HDPE

GRADES

Based on the manufacturing process used, there are two grades of HDPE plastic, Injection grade and extrusion grade.

- Injection grade HDPE is a product of the injection moulding process used to produce hard plastics like buckets and crates.

- Extrusion grade HDPE is a product of the extrusion process that produces bottles, sheets and films. In recycling, this grade is favoured because it is easily recycled into high commercial end-value pellets as well as plastic packaging and pipes.

PROPERTIES

HDPE plastics are easily one of the most common in manufacturing due to the following qualities:

- It is a strong material, making it resistant to tearing and bursting.

- It is lightweight.

- It is resistant to UV rays.

- It is moisture resistant. Rotting, mould and mildew do not affect HDPE.

- It is resistant to oil, salt and fading.

- It is resistant to most strong mineral acids and bases. Also, it can withstand chemicals that occur naturally in soil which is why it is popular in underground piping.

- It does not contain BPA, phthalates, heavy metals and allergens.

- It is resistant to high impact. A toy truck dropped from a height, for instance, will more likely bounce than break.

- HDPE boards tend to have textured surfaces that allow for a proper grip when used to transport food.

- It is an excellent electrical insulator.

- It can be translucent or opaque in colour – Coloured bottles and jugs tend to be stronger than translucent ones. Opaque is best for household cleaners because they are corrosion resistant.

- It has a high resistance to low temperatures.

- It has a high melting point.

- It is highly malleable at its melting point so, easily moulded for various applications.

- It is easy to process by most methods.

Depending on the moulding process used in producing a particular type of HDPE plastic, its physical properties may vary from sample to sample. Inadvertently, this may affect the international standardized testing methods used to identify properties such as the environmental stress crack resistance of an HDPE sample.

These many properties have some great benefits:

- It is generally inexpensive to make so, it is cost-effective.

- It is designed to be safe, long-lasting, weatherproof and low maintenance. This is why the plastic lumber deck of a house can withstand wear and tear.

- It is corrosion-resistant and, most commonplace chemicals, acids, solvents, cleaning fluids and, powders cannot permeate.

- It does not leach into the environment.

- Virgin and recycled HDPE are environmentally friendly because they do not release harmful fumes and, their impact is minimal on the environment

- It is dishwasher and microwave safe.

- It is easily moulded into shapes.

- It is easily recycled into a composite material similar to the material from which the plastic was recovered. They are most common in bike racks, picnic tables, furniture, waste and recycling containers.

- Its lightweight makes it a perfect replacement for heavier materials in the manufacturing industry. For example, HDPE fuel tanks in cars allow for lighter weight which increases fuel efficiency.

- Its resistance to high temperatures makes boiling an acceptable method of sterilizing HDPE.

- It is easily adaptable. For example, it is the only plastic used for both pasteurised milk and sterilised milk. The only difference being the layers in the different bottles.

- It is the only plastic that can have a seal to preserve the product, prevent leakage and alert the consumer to tampering.

- It can be used in both rigid and flexible products.

DISADVANTAGES

- It is not biodegradable so, it cannot be in compost.

- It is highly flammable.

- Oxidizing acids and chlorinated hydrocarbons can affect HDPE.

- It is susceptible to stress cracking.

- It has a high propensity to shrink during moulding.

HDPE vs. PET – Are there similarities? Differences?

Comparing HDPE and PET.

HDPE and PET are known as two different types of plastic but, the differences between them are ever so slight, to be honest (Read PET Characteristics Here).

When compared side-by-side, HDPE has a higher resistance to impact, cold, heat, mineral oils, acetone and, hydrogen peroxide. On the other hand, PET has higher rigidity, clarity, resistance to gas, stress cracks, alcohol, essential oils and sodium chloride. On a scale of 1-5, the differences in individual properties may be one or two points each.

HDPE and PET are at par in their resistance to moisture and chemicals like weak acids, ethyl acetate and methyl ethyl ketone. They are both safe options for consumables.

Also, both plastics can be colour matched to any colour but are most popular in white because it blocks out light.

The beautiful thing about these plastic types is that you can choose whichever one best suit your product.

SOURCE POINTS

https://www.acplasticsinc.com/informationcenter/r/common-uses-of-hdpe

https://www.plasticsmakeitpossible.com/about-plastics/types-of-plastics/professor-plastics-high-density-polyethylene-hdpe-so-popular/

https://sciencing.com/hdpe-plastic-5839257.html

https://www.acmeplastics.com/what-is-hdpe#:~:text=High%20Density%20Poly%20Ethylene%20(HDPE,%2C%20cutting%20boards%2C%20and%20piping.

https://en.wikipedia.org/wiki/High-density_polyethylene

https://www.acplasticsinc.com/informationcenter/r/common-uses-of-hdpe

https://areteindustries.us/hdpe-plastics/

https://omnexus.specialchem.com/selection-guide/polyethylene-plastic

https://www.bpf.co.uk/plastipedia/polymers/hdpe.aspx