What is High Density Polyethylene HDPE Definition Structure Preparation and Uses
Whole world is under the threat of novel coronavirus or COVID-19. Governments and individuals are taking many measures to protect the nation and themselves, respectively. Even during this time of corona crisis, doctors are doing their duty diligently. You must have seen doctors wearing a special disposable suit during the treatment of a corona patient. You will be amazed to know that high density polyethylene or HDPE is one of the key ingredients of those special disposable suits. Not only this, if you look around, you will find at least five items composed of high density polyethylene or HDPE. Therefore, it becomes necessary for you to know about HDPE.
What is High Density Polyethylene?
High density polyethylene is a thermoplastic polymer whose monomer unit is ethylene. Ethylene or ethene (IUPAC name) is a hydrocarbon with the formula C2H4. High density polyethylene is also called PEHD (polyethylene high density), alkathene or polythene.
If any plastic material has a RIC (Resin Identification Code) - 2 written on it, then it means that plastic material is composed of HDPE. As RIC -2 stands for HDPE. High density polyethylene was discovered by Karl Ziegler of the Kaiser Wilhelm Institute in 1953. Karl Ziegler received the Nobel Prize for Chemistry in 1963. Soon, HDPE became the key raw ingredient for producing many materials. During World War -II, it was used in many vital military operations as critical insulating material, underwater cable coating etc. This made HDPE a very important resource during the WW – II. Due to its versatile uses, HDPE is still one of the largest volume polymers produced in the world. It still accounts over 34% of the global plastic market.
Production of High - Density Polyethylene
HDPE is generally produced by gas phase polymerization of ethylene. Under controlled conditions ‘cracking’ (A process of applying intense heat) is done by petroleum. Which leads to the production of ethylene gas. Now these ethylene molecules get polymerized and produce HDPE. After this HDPE goes through the process of separation and drying. It can also be produced by various catalytic processes such as Ziegler – Natta catalysis or Cr – silica catalysis.
What are the Properties of High - Density Polyethylene?
HDPE has numerous applications due to its unique properties. To understand its various uses in the industry, you need to know its properties first. Therefore, various properties of HDPE are listed below –
It can be molded again and again. Thus, it is a thermoplastic polymer.
It is well known for its high, strength to density ratio.
Its density is 940 kg/m3.
It has little branching in its structure.
It has stronger intermolecular forces and tensile strength than low density polyethylene.
Its melting point is 130.8℃.
Its temperature of crystallization is 111.9℃.
It is harder and more opaque than low density polyethylene.
It is resistant to many different solvents.
Its specific heat capacity is 1330 – 2400 J/kg – K
Its latent heat of fusion is 178.6 kJ/kg.
It shows chemical and electrical resistance.
It is able to resist lower temperatures than LDPE.
It is a strong and lightweight plastic polymer.
What are the Uses of High - Density Polyethylene?
All of the above stated desirable properties of high – density polyethylene make it suitable for numerous applications. Few applications of HDPE are listed below –
It is used to make disposable suits for various purposes.
It is applicable to make pipes which can be used for potable water and sewage water as well.
Its fibers can be spun into a rope.
It is used as house wrap to protect buildings.
Plastic envelopes which are generally used in mailing are made up of HDPE.
It is used in making chairs, stools, ice cube trays, bottles etc.
Bottle crates are composed of HDPE.
Many types of toys and playground equipment are made up of HDPE.
It is used to make milk jugs, jerrycans as it resists corrosion.
It is used in doormats.
It is a strong and long - lasting material so it is used in construction as well.
High density polyethylene is used in cutting boards.
High density polyethylene film is used in food packaging.
HDPE is used in wood plastic composites.
It is used in plastic surgery especially facial and rhinoplasty.
It is used in 3 – D printing filaments.
Food and beverage containers are composed of high density polyethylene.
HDPE is easily recyclable which makes it even more useful in today’s world. Recycled high – density polyethylene is almost as versatile as virgin or recently produced HDPE.
High – density polyethylene and low - density polyethylene, both are thermoplastic polymers of ethylene monomer unit. But differ in their various properties and have different uses.
Difference Between High – Density Polyethylene and Low – Density Polyethylene
This ends our coverage on the topic “High Density Polyethylene”. We hope you enjoyed learning and were able to grasp the concepts. We hope after reading this article you will be able to solve problems based on the topic. If you are looking for solutions to NCERT Textbook problems based on this topic, then log on to Vedantu website or download Vedantu Learning App. By doing so, you will be able to access free PDFs of NCERT Solutions as well as Revision notes, Mock Tests and much more.
FAQs on High Density Polyethylene HDPE Structure Properties and Applications
1. What is High Density Polyethylene (HDPE)?
High Density Polyethylene (HDPE) is a thermoplastic polymer made from the polymerization of ethene (ethylene) with a linear structure and high strength-to-density ratio. HDPE consists of long chains of the repeating unit (–CH2–CH2–)n, formed by addition polymerization of ethene CH2=CH2. Because its chains have minimal branching, they pack closely together, giving HDPE high density, strength, and chemical resistance. It is widely used in bottles, pipes, and containers.
2. What is the chemical formula of High Density Polyethylene?
The chemical formula of High Density Polyethylene is (C2H4)n, representing repeating ethene units. Each repeating unit is –CH2–CH2–, derived from the monomer CH2=CH2. The value of n is very large (thousands to millions), which gives HDPE its high molecular mass and strong mechanical properties.
3. How is HDPE produced from ethene?
HDPE is produced by addition polymerization of ethene using Ziegler–Natta or Phillips catalysts under controlled conditions. The reaction can be represented as:
n CH2=CH2(g) → (–CH2–CH2–)n(s)
Key points:
- Uses low pressure compared to LDPE production.
- Employs catalysts such as Ziegler–Natta (TiCl4/Al(C2H5)3) or chromium-based Phillips catalysts.
- Produces mostly linear polymer chains with very little branching.
4. Why does HDPE have a higher density than LDPE?
HDPE has a higher density than LDPE because it has a more linear polymer structure with minimal branching. In HDPE:
- Polymer chains are nearly straight.
- Chains pack closely due to stronger intermolecular (van der Waals) forces.
- This close packing increases density (about 0.94–0.97 g cm-3).
5. What are the properties of High Density Polyethylene?
High Density Polyethylene is strong, chemically resistant, and has a high melting point compared to LDPE. Key properties include:
- High tensile strength and stiffness
- Density: about 0.94–0.97 g cm-3
- Melting point: about 120–130°C
- Excellent resistance to acids, bases, and many solvents
- Good electrical insulating properties
6. What is the difference between HDPE and LDPE?
The main difference between HDPE and LDPE is the degree of branching in their polymer chains.
- HDPE: Linear structure, high density, higher melting point, stronger and more rigid.
- LDPE: Highly branched structure, lower density, lower melting point, more flexible.
7. What are the common uses of High Density Polyethylene?
High Density Polyethylene is commonly used for making strong, durable plastic products. Major uses include:
- Milk and detergent bottles
- Water and gas pipes
- Plastic containers and storage tanks
- Cutting boards
- Geomembranes and liners
8. Is HDPE a thermoplastic or thermosetting polymer?
HDPE is a thermoplastic polymer, meaning it softens on heating and hardens on cooling without chemical change. The polymer chains in HDPE are held together by intermolecular forces rather than covalent cross-links. This allows:
- Melting and reshaping multiple times
- Recycling by remelting
- Processing by extrusion and injection molding
9. Is High Density Polyethylene recyclable?
Yes, High Density Polyethylene is recyclable and is identified by resin identification code #2. Because HDPE is a thermoplastic:
- It can be melted and remolded.
- Recycled into pipes, plastic lumber, and containers.
- Maintains good mechanical properties after recycling.
10. What type of polymerization reaction forms HDPE?
HDPE is formed by addition polymerization (chain-growth polymerization) of ethene monomers. In this reaction:
- The double bond in CH2=CH2 opens.
- Monomers join to form long chains.
- No small molecule (like H2O or HCl) is eliminated.
n CH2=CH2 → (–CH2–CH2–)n
This chain-growth mechanism produces the linear structure characteristic of HDPE.
