Polyethylene vs Polypropylene: When to Choose What? -

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 applicati

Polyethylene vs Polypropylene – Chemical Composition & Major Types

What is polyethylene?

Polyethylene is a polymer composed of repeated ethylene monomer units (-CH₂-CH₂-) which are derived from petroleum or natural gas. It is available as:

Low-density polyethylene (LDPE) which is flexible and transparent.
High-density polyethylene (HDPE) which is a rigid polymer with a higher density compared to LDPE.
Linear low-density polyethylene (LLDPE) which is a flexible and tough form that combines the properties of LDPE and HDPE.

What is polypropylene?

Polypropylene is a polymer composed of repeated propylene monomer units (-CH₂-CH(CH₃)-) derived from petroleum. They are well known for their high strength & stiffness. It is mostly available as:

Homopolymer polypropylene which is made solely from propylene monomer units.
Random copolymer polypropylene which contains both propylene and ethylene or other alpha-olefin comonomer units.
Block copolymer polypropylene which consists of blocks of polypropylene and ethylene-propylene rubber.

Another form is called Thermoplastic olefin (TPO) which is a blend of polypropylene and an elastomer, typically ethylene-propylene rubber (EPR)

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	10¹⁴to 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 psiHDPE: 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 MPaHDPE: 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	LDPE: 343°CHDPE: 388°C 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

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.
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.

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.

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.