Polyethylene (PE) and polypropylene (PP), two commonly encountered plastics in our everyday lives, are derived from petroleum. While they share similarities, these plastics possess distinct characteristics that make them individually suited for different industrial applications.
As a developer of plastic fabric technology that reduces greenhouse gas emissions, Renegade Plastics knows a thing or two about these polymers. The discussion of polyethylene vs polypropylene has become one of the most pressing topics due to the omnipresence of plastic waste.
This blog explains polypropylene vs polyethylene, examining their key differences and pros and cons.
Properties of Polyethylene and Polypropylene
Due to their favorable chemical and physical properties, affordability, and processing ease, polyethylene and polypropylene have emerged as global manufacturing favorites.
Both polypropylene (PP) and polyethylene (PE) are members of the thermoplastic family. Using standard thermoplastic equipment, they can be processed by common forming techniques such as injection molding, blow molding, extrusion, and thermoforming.
Polyethylene is a thermoplastic polymer comprising charcoal, hydrogen, and ethylene monomer units. Its molecular structure consists of a long chain of carbon atoms with two hydrogen atoms attached to each carbon atom.
The chain is composed of repeating carbon and hydrogen atoms. Due to the strong bonds of these units, Polyethylene molecules are generally insoluble in water and other solvents, giving the material strength and high chemical resistance.
Polypropylene is a linear hydrocarbon polymer with many repeating propylene monomer units. These propylene monomers are linked together by forming carbon-carbon bonds, forming a strong chain.
In other words, the molecular structure of Polypropylene is characterized by a backbone of connected carbon atoms, with two hydrogen atoms attached to each of the side carbon atoms. This results in a strong, lightweight material with excellent chemical resistance and flexibility.
In terms of physical properties, polyethylene and polypropylene exhibit striking similarities. However, while polyethylene can be manufactured to achieve optical clarity, polypropylene, on the other hand, is limited to a translucent appearance akin to a milk jug.
Polyethylene has a low density of 0.91-0.965 g/cm3. It is a strong and tough material with good chemical resistance. Polyethylene has a melting point of 105-115°C and a glass transition temperature of -100°C. It is a good electrical insulator and has a low thermal conductivity.
The physical properties of polyethylene:
- Density: 0.91-0.965 g/cm3
- Lower melting point: 105-115°C
- Glass transition temperature: -120 to -100°C
- Tensile strength: 20-30 MPa
- Yield strength: 10-20 MPa
- Elongation at break: 100-300%
- Impact strength: 15-25 kJ/m2
- Hardness: 40-60 Shore D
- Static charge: Lower
- Purity: Often 100% pure
Polypropylene has a low density of 0.91 g/cm3. It is a strong and tough material with good chemical resistance.
Polypropylene has a melting point of 165-175°C and a glass transition temperature of 0°C. It is a good electrical insulator and has a low thermal conductivity.
The physical properties of polypropylene:
- Density: 0.91 g/cm3
- Higher melting point: 165-175°C
- Glass transition temperature: -20-0°C
- Tensile strength: 25-40 MPa
- Yield strength: 15-25 MPa
- Elongation at break: 100-300%
- Impact strength: 20-30 kJ/m2
- Hardness: 55-65 Shore D
- Static charge: Higher
- Purity: Often co-polymerized with ethylene
Both polyethylene and polypropylene have a wide range of temperature stability and impact resistance. The primary difference between polyethylene and polypropylene is each material’s temperature and chemical resistance level.
- Chemical resistance: Good chemical resistance to various acids, bases, and organic solvents. It is resilient against many acids, such as hydrochloric acid, sulfuric acid, diluted acetic acid, alcohol, detergent, and alkalis.
- UV radiation: Low resistance to ultraviolet radiation.
- Oxidation: The oxidation resistance of polyethylene is similar to that of polypropylene. It is stable over a wide range of temperatures and is unaffected by atmospheric oxygen.
- Chemical resistance: Excellent chemical resistance, unaffected by most acids and alkalis, oxidizing agents, reducing agents, and organic solvents. It is also resistant to corrosion.
- UV radiation: Resistant to ultraviolet radiation and has high weather resistance.
- Oxidation: Unaffected by atmospheric oxygen, making it highly resistant to oxidation. It is also highly resistant to thermal oxidation at temperatures up to 375° F (190° C).
Industrial Applications of Polyethylene and Polypropylene
Both polypropylene and polyethylene plastics have various commercial and industrial applications and are constantly being developed for new uses.
Common Uses of Polyethylene
One of the most common uses for polyethylene in heavy industries is to create corrosion-resistant pipe liners used in the oil and gas extraction and refining industries, the chemical industry, and the water and wastewater treatment industry.
Other uses include:
- Packaging: Milk jugs, food containers, and crates that are lightweight, strong, and inexpensive.
- Construction: Pipes and roofing materials that are durable with weather-resistant insulation.
- Automotive: Bumpers, fuel tanks, and interior components that are strong, lightweight, and chemical resistant.
- Medical: Non-toxic and inert syringes, bottles, and tubing that are easy to sterilize.
- Electronics: Polyethylene is a good insulator and can be easily molded into complex shapes to make circuit boards and housings.
Common Uses of Polypropylene
Polypropylene is widely used in the automotive industry for exterior and interior parts like bumpers, fenders, skirts, wheel covers, air vents, mudguards, and engine parts, to body panels, mats, weather strips, and carpets.
Other uses include:
- Packaging: Bags, bottles, food containers, and crates that are strong, durable, and chemical-resistant.
- Agriculture: Covers and tarps are more resistant to wear and tear than natural fabrics.
- The high-temperature resistance of polypropylene means it can also be used in the manufacture of plastic greenhouses and gardening sheets, which protect delicate crops from harsh weather conditions.
- Textiles: Carpets, geosynthetics and landscaping fabrics.
- Other applications: Used in furniture, toys, and appliances.
Advantages and Disadvantages of Polyethylene
Polyethylene is processed in three main grades – low-density polyethylene (LDPE), linear low-density polyethylene (LLDPE), medium-density polyethylene (MDPE), and high-density polyethylene (HDPE) – each with advantages and disadvantages.
We’ll focus on polyethylene’s general pros and cons for brevity’s sake.
One significant advantage is polyethylene’s exceptional chemical resistance, allowing it to withstand contact with a wide range of substances without significant degradation. It is also known for its lightweight nature, making it an ideal choice for weight reduction applications.
Good impact strength provides durability and is relatively low-cost compared to other materials, making it an economical option for large-scale industrial use. Polyethylene’s fabrication versatility allows easy molding, extrusion, and shaping into different forms to meet specific application requirements.
Lastly, polyethylene is moisture-resistant, providing excellent performance in wet environments.
Despite its versatility, polyethylene has some limitations that should be considered in industrial applications. One significant limitation is its relatively low melting point, which makes it unsuitable for high-temperature environments where thermal stability is crucial.
Additionally, while polyethylene offers a good strength-to-weight ratio and impact resistance, it may not possess the mechanical strength required for heavy load-bearing applications. Furthermore, its susceptibility to UV degradation makes it less ideal for outdoor applications without proper UV stabilization.
The production of polyethylene demands substantial energy and results in significant carbon dioxide emissions, contributing to global warming and climate change. While recycling is technically feasible for most plastic polymers, the presence of numerous types of polyethylene necessitates expensive and complex sorting processes.
Advantages and Disadvantages of Polypropylene
Several grades of PP exist based on molecular structures and additives used. The most common is a homopolymer, used for injection molding, melt-blowing, sheet/film extrusion, and automotive parts, followed by copolymer PP, consisting of two strands, used for packing material, automotive parts, and industrial fibers.
Just like before, we’ll stick to basic pros and cons.
For starters, this thermoplastic polymer is strong, lightweight, and durable enough to withstand a wide range of temperatures and chemical exposures. It is also non-corrosive, so it can be used even in unpleasant environmental conditions. Its stiff nature makes it suitable for fabricating threads, ropes, and wires.
The material is also waterproof and resistant to insect and rodent infestations, making it ideal for many outdoor applications. Finally, and perhaps most importantly, it can be recycled, making it a more eco-friendly plastic choice for many industries wanting to invest in a circular economy.
Polypropylene has some disadvantages that should be considered when using it. These include its limited temperature resistance and relatively low modulus of elasticity.
At higher temperatures, polypropylene may become deformed or even melt, and its low modulus of elasticity limits its ability to absorb stress and vibration. It also lacks the ability to withstand heavy loads, making it less suitable for heavy-duty applications.
Additionally, because of its non-polar nature, polypropylene can be difficult to bond to other materials and may require specialized bonding agents.
Conclusion: PP vs PE
The choice between polypropylene and polyethylene depends mainly on what unique qualities you require from the material. Polyethylene works great when you need a strong, clean material resistant to moisture and water absorption.
It also performs best when used with food-contact applications, as it is generally considered food safe. Unlike polyethylene, polypropylene is an ideal choice when you need a material with a high melting point and long-term durability.
Both materials offer great strength and rigidity, but knowing each material’s unique characteristics can help you choose between polyethylene or polypropylene plastic.
At Renegade Plastics, we’d recommend polypropylene – not only is it strong and durable, but it’s recyclable and better for the environment. Contact us to find out more about our polypropylene products.