What Is HDPE?

High density polyethylene is a thermoplastic polymer usually used to make pipes, bottles and synthetic lumber. HDPE is sometimes called plastic pipe material due to its popular use in plumbing applications. It has several other uses like packaging, film and storage tanks. It was invented in the 1930’s. Its resin identification code is 2. Resin identification codes identify the material and the ways to dispose of it. The two primary uses of HDPE are packaging and piping. Packaging uses include milk jugs, juice cartons, water bottles, soda cans and so on. Piping uses include irrigation tubing, sinks, shower curtains, vinyl siding, roofing and so on. High density polyethylene or HDPE is made out of the monomer ethylene. Monomers are molecules, mostly organic, that react with other molecules to form long chains called polymers. High density polyethylene is one such polymer. It was used in World War 2 as high frequency radar cables. In 2022, HDPE production is projected to reach 67 million metric tonnes.

Properties of HDPE

HDPE is cost-effective and moisture resistant. It can withstand great temperature extremes- from -148 to 176 degrees Fahrenheit. It is also UV resistant and is safe to use in dishwashers. Higher the density, the stiffer the plastic is. HDPE is resistant to most chemicals.

High density polyethylene has a high density of 930 to 970 kg/m³. Its melting point is 130.8 °C. It is hard and opaque. You can check the structure of HDPE by holding it under a microscope. It will have a linear structure with little branching. Due to lesser branching as compared to low density polyethylene, HDPE has higher intermolecular forces. It has higher specific strength, tensile strength and can withstand comparatively higher temperatures for a short while in comparison to LDPE. The physical properties can vary depending upon the manufacturing process. It is also chemically resistant to several solvents, which makes it an excellent candidate for carrying drinking water.

Uses Of HDPE

HDPE can be used to make a wide variety of products. It has great use in the industry of food packaging. HDPE bottles and containers are strong, durable and moisture resistant. It is also resistant to several chemicals, so you do not have to fear that packaged food might be contaminated. It is recyclable, and thus helps to combat the waste disposal problem. HDPE can also be used to make toys and buckets, as it is UV resistant and hard. It would not discolor and break easy. It also makes great containers for different types of solvents and products including shampoo bottles, motor oil, disinfectant and so on. HDPE is highly useful for outdoor and industrial uses due to its strength; UV, heat and chemical resistance and cost effective nature.

Disadvantages

You must also consider the disadvantages of HDPE before choosing between high density polyethylene and polyethylene. HDPE does not weather well. It is sensitive to stress and may crack. It reacts in the presence of acids and chlorinated hydrocarbons. It is highly flammable. It cannot be naturally composted. It is non biodegradable.

What Is PET?

PET is an aliphatic polyester. PET is one of the most commonly used polymers, over 80 million metric tonnes of PET is produced every year. It is used to make fibers, containers and resins. Bottle and fiber production accounts for over 90% PET produced each year. It is a thermoplastic polymer. The resin identification code of PET is 1. The monomer unit of PET is polyethylene terephthalate C₁₀H₈O₄. It is produced by the reaction of terephthalic acid with ethylene glycol. Polyethylene terephthalate can exist both as an amorphous and a semi crystalline solid. The polymerization is done by a polycondensation reaction that occurs immediately after esterification between terephthalic acid and ethylene glycol or trans-esterification reaction between ethylene glycol and dimethyl terephthalate.

Properties Of PET

Polyethylene terephthalate is hard and stiff. It is strong and its absorbance is low. It resists most gasses and chemical solvents. It can be semi crystalline or amorphous. Amorphous PET has high transparency and lower physical strength. It is usually used in the manufacture of packaging products. Semi crystalline PET is harder and has greater physical strength. PET used in bottle manufacture is amorphous and will not crystallize. Crystallization affects both transparency as well as impact resistance. PET has good dimensional stability and temperature stability. PET is strong and inert. It is chemically resistant to food stuff and also immune to degradation due to microorganisms. It will not break easily and it is lightweight and hence makes excellent packaging material. PET has excellent electrical insulation properties. PET can be used across a wide range of temperatures, from -60 to 130°C. PET has higher heat distortion temperature. It is almost shatter-resistant and a suitable glass-replacement. It is very recyclable and transparent to microwave radiation.

Uses Of PET

PET has a high strength to weight ratio. It is transparent, shatterproof and above all, it can easily be recycled. This makes PET a common choice in the manufacture of day to day products. One of the greatest uses of PET is in both rigid and flexible packaging. Commonly it is used in the production of bottles. The PET bottle was invented by Nathaniel Wyeth. It was later patented by DuPont. It can be used along with other layers such as polyvinyl alcohol layers in order to enhance packaging capabilities. PET can be aluminized with a thin layer of metal in order to improve flexibility and thermal insulation. It is used in photovoltaic modules and thin film solar cells. PET is combined with glass fibers to make thermoplastic resins that go in things like electrical appliances. It is also widely used as water, weather and oxygen barrier and liner materials. Yet another use of this versatile material is in the production of water resistant paper.

Disadvantages of PET

PET is extremely versatile and useful but it does have certain drawbacks. Amorphous PET is easily affected by boiling water and chemicals like alkali. It has low heat resistance. PET resins get oxidized at a faster rate. In the case of food packaging, when oxidation occurs the taste of food is affected. The production process of PET is partially dependent on crude oil and it is non biodegradable.

HDPE vs PET: What To Choose? 

Both PET and HDPE are great for packaging. They are strong, lightweight and moisture resistant. However there are certain differences to consider. HDPE is more cost effective, but PET has a greater strength to weight ratio. This reduces the amount of packaging needed to be produced and in consequence, it reduces transportation costs. PET is transparent and clear whereas HDPE is not. PET is less susceptible to stress cracking as compared to HDPE. However HDPE is more durable. It also has a higher temperature resistance. PET is more recyclable, and is thus a more sustainable choice.