Polyethylene and polypropylene belong to the class of thermoplastics called polyolefins. While they are closely associated, there are several distinct characteristics that set them apart. It is essential to note that specific properties and characteristics of both will vary depending on:
- specific manufacturing techniques and
- grades from different suppliers.
In terms of availability, Polyethylene is more widely produced and available. Polypropylene might have limited availability in certain regions.
Both PE and PP find extensive use in several industries including automotive parts, packaging materials, and consumer goods, to name a few. However, understanding their unique properties is crucial for optimal performance and meeting industry requirements within specific applications.
Let’s have a look at some of their distinct features based on:
Polyethylene vs Polypropylene – Chemical Composition & Major Types
What is polyethylene?
Polyethylene is a polymer composed of repeated ethylene monomer units (-CH
2-CH
2-) which are derived from petroleum or natural gas. It is available as:
If you want a complete range of polyethylene grades, resume your PE resin selection »
What is polypropylene?
Polypropylene is a polymer composed of repeated propylene monomer units (-CH
2-CH(CH
3)-) derived from petroleum. They are well known for their high strength & stiffness. It is mostly available as:
Another form is called Thermoplastic olefin (TPO) which is a blend of polypropylene and an elastomer, typically ethylene-propylene rubber (EPR).
If you want a complete range of polypropylene grades, resume your PP resin selection »
Polyethylene vs Polypropylene – Material Properties Face-off
Physical properties – Unveiling the major differences
| Property Name |
Polyethylene |
Polypropylene |
| Density |
0.91-0.94 g/cm³ |
0.90 to 0.91 g/cm³ |
| Melting point |
115-135°C |
130-171°C (266-340°F) |
| Transparency |
Transparent or translucent |
Mostly translucent |
| Flexibility |
Partially flexible |
Mostly rigid & stiff |
| Chemical resistance |
Resistant to acids, alkalis, and solvents |
Resistant to acids, alkalis, and solvents including strong bases & organic solvents |
| Dielectric constant |
2.25 to 2.35 |
1.78 to 2.20 |
| Electrical resistivity |
1016 to 1018 ohm-cm |
1014 to 1016 ohm-cm |
| Water absorption rate |
0.01% to 0.5% |
0.01% to 0.8% |
| Temperature resistance |
115°C |
170°C |
Mechanical properties – Detailed comparison between polyolefins
| Property Name |
Polyethylene |
Polypropylene |
| Tensile strength |
- LDPE: 1050 to 2100 psi
- HDPE: 4550 to 6100 psi
|
4000 to 5800 psi |
| Impact resistance |
Better impact resistance absorbs impacts and deforms without breaking |
Slightly less than PE |
| Flexural strength |
- LDPE: 6 to 26 MPa
- HDPE: 10 to 50 MPa
|
10 to 20 MPa |
| Clarity retention |
Less than PP |
Better than PE, less prone to yellowing or discoloration when exposed to UV radiation or prolonged use |
| Creep resistance |
Higher than PP because of fewer branches and linear structure |
Less than PE due to branching which reduces its packing density |
| Flash point |
Higher flammability than PP
|
260°C |
| Barrier properties |
Against moisture, water vapor, oxygen, and carbon dioxide |
More against grease, oil, and chemicals than PE |
| Moisture vapor transmission rate |
0.5 to 5 g/m2/day |
1 to 10 g/m2/day, Higher tendency to absorb moisture than PE |
| Get more information about polymer properties here » |
Cost – Which polymer wins the budget battle?
The cost of both can vary on the basis of region, supplier, application, etc. Generally, PE is slightly more expensive than PP mostly because of its manufacturing.
- The cost of HDPE ranges from $0.50 to $1.50 per pound. LDPE ranges from $0.30 to $0.70 per pound. LLDPE is in between the two, costing between $0.40 to $1.00 per pound.
- Homopolymer polypropylene which is a more commonly used type is priced at around $0.30 to $0.35 per pound. Copolymer polypropylene costs around $0.35 to $0.40 per pound.
Sun-savvy polyoelfins – Unmasking the UV stability
Polyethylene is more susceptible to degradation from prolonged exposure to ultraviolet (UV) radiation. Polypropylene has better UV stability, making it more suitable for outdoor applications such as outdoor furniture, garden equipment, and automotive exterior parts. Adding stabilizers such as HALS or UVAs can
improve resistance to UV radiation in both PE and PP.
Food contact safety & migration testing
Both polyethylene and polypropylene are approved by the FDA for use in food contact applications and are considered safe for use with all types of food products.
- PE is also resistant to moisture and chemicals, thus making it an ideal choice for packaging products that require a high level of protection.
- PP is often used for packaging products that require high-temperature sterilization.
In terms of migration, both PE and PP are considered to be low-risk materials as they have low levels of
migration into food products. However, PP has a slightly lower level of migration compared to PE due to its higher density and lower porosity.
Weldability, weld strength & welding techniques
Both PE and PP can be welded using various techniques, including butt fusion, socket fusion, or hot plate welding. These welding methods allow for strong and leak-proof joints, making them useful in applications such as piping systems or tanks.
- Polyethylene has better weldability, especially using techniques like hot plate welding and butt fusion welding. Polypropylene can be welded as well, but it may require additional steps or specialized techniques for achieving strong weld joints. View weldable Polyethylene grades »
- Polypropylene is known to have better weld strength compared to polyethylene. Weld strength refers to the strength and integrity of welded joints. Polypropylene’s higher weld strength makes it suitable for applications that require strong and durable welds, such as piping systems or tanks. View weldable Polypropylene grades »
Recycling techniques – Sustainable solutions for a greener future
1. Recycling with polyethylene
It can be recycled through mechanical recycling which involves:
- STEP 1: Collecting and sorting the plastic by type and color
- STEP2: Cleaning and shredding it into small pieces
- STEP 3: Melting and reforming it into new products
However, not all types of polyethylene can be recycled this way, as some may contain additives. Moreover, contamination due to food or other materials can also affect the recyclability of polyethylene.
2. Recycling with polypropylene
PP can also be recycled through mechanical recycling. The process is like that of PE, however, like polyethylene, not all types of polypropylene can be recycled this way due to the presence of additives or contamination.
Other recycling techniques for both polyolefins may also include:
- Chemical recycling which breaks down the thermoplastic into its chemical components for reuse,
- Feedstock recycling which converts the plastic into fuel or other chemicals, and
- Pyrolysis which uses heat to break down the plastic into a gas or oil that can be used as fuel.
Processing Methods: From Raw to Finished Product
Processing of polyethylene
Most popular technique for polyethylene is
injection molding. In this process, thermoplastic in molten form is injected into a mold under high pressure, then cooled to solidify it into the desired shape. With this method, complex parts for automotive components can be made.
It can also be processed via
extrusion. This involves pushing molten polyethylene through a die to create a continuous shape, such as a tube or sheet. Pipes and films can be produced with this.
Get in-depth knowledge about extrusion process here »
Processing of polypropylene
Polypropylene can be processed in similar fashion with the two techniques namely
injection molding and
extrusion. But one technique unique to PP is
thermoforming. It involves heating a sheet of plastic until it becomes soft and pliable, and then shaping it over a mold using vacuum pressure. Food packaging trays and interiors for automotives can be produced this way.
PP requires higher processing temperatures and has a narrower processing window vs PE. It tends to have a
higher shrinkage rate during cooling, which can affect the dimensional accuracy of molded parts.
Typical Applications: The Limitless Possibilities of PE vs PP
Industries served by polyethylene
It is a widely used thermoplastic owing to its low cost, excellent resistance to chemicals, and low density. Here are some known applications:
- Packaging: Used in materials such as plastic bags, shrink wraps, and stretch films.
- Pipes: Used in water supply and drainage systems due to chemical resistance and durability.
- Automotive components: Used in fuel tanks, battery cases, etc., as it is tough.
Industries served by polypropylene
- Packaging: Used in food containers, bottle caps, and closures as it is tough.
- Textiles: Used in carpets, upholstery, and geotextiles due to strength.
- Automotive components: Used in bumpers, interior trim, and battery cases due to their resistance to chemicals.
If PE suits best for your application, discover our exclusive guide on polyethylene, or if PP is the more suitable option, find out the detailed guide on polypropylene.