Polyethylene vs. Polyethylene Terephthalate: Streamlining Material Selection to Suit Your Needs

Polyethylene (PE) and Polyethylene Terephthalate (PET) are both thermoplastics that play a crucial role in various industries due to their versatility, lightweightness, chemical resistance, and electrical insulation properties. Despite these similarities, it’s important to note that PE and PET also have significant differences.

The primary difference lies in their chemical nature, transparency, strength, barrier properties, processing temperature, and recycling techniques. The choice between the two polymers depends on the end application as well:

• PE is used for applications where flexibility and low cost are required
• PET is often preferred for transparent, rigid, and barrier applications

Some common applications of these widely used plastics include packaging, automotive, construction, medical goods, and electrical & electronic devices, to name a few. Manufacturers have been developing variants of both these polymers that can be customized as per user requirements.

Read on to get a clear picture about:

1. How are they chemically different?
2. Which material properties make them unique?
3. What processing methods transform them into finished products?
4. Where do these thermoplastics find their use?

PE vs. PET – How are they chemically different?

What is Polyethylene?

Polyethylene (PE) is a polymer composed of repeating ethylene monomer units (-CH₂-CH₂-). These monomeric units are either steam-cracked or polymerized to form a long-chain structure with only carbon and hydrogen atoms in the backbone. It can be derived from natural gas, crude oil, and even from renewable sources like biomass.


Based on its density and branching, PE is classified into Low-density polyethylene (LDPE), Linear low-density polyethylene (LLDPE), and High-density polyethylene (HDPE).

Looking for a complete range of polyethylene grades, resume your PE resin selection here.

What is Polyethylene Terephthalate?

Polyethylene Terephthalate (PET) belongs to the polyester family of polymers formed from the polymerization of repeating units of terephthalic acid (TPA) or dimethyl terephthalate (DMT) with ethylene glycol monomeric units. The resulting long-chain repeating units are connected by ester linkages.

Reaction 1: Terephthalic acid (TPA) reacts with ethylene glycol to form PET

C₆H₄(COOCH₃)₂ + (CH₂OH)₂  →  (C₁₀H₈O₄)_n + yCH₃OH + xH₂O

Reaction 2: Dimethyl terephthalate (DMT) reacts with ethylene glycol to form PET

The presence of the terephthalate group gives PET its unique characteristics like transparency and barrier properties. The chemical structure of PET is more complex than PE, with oxygen atoms in addition to carbon and hydrogen atoms in the backbone.

Looking for a complete range of polyethylene terephthalate grades, resume your PET resin selection here.

PE vs. PET – Which material properties make them unique?

Physical properties: Analyzing key attributes for the right selection

The physical properties of polyethylene and polyethylene terephthalate vary due to differences in their chemical structure and molecular arrangements. Find below a summary of how these two polymers differ:

Property Name	Polyethylene	Polyethylene Terephthalate
Density	0.91 - 0.97 g/cm³	1.3 - 1.4 g/cm³
Melting point	115 - 135 °C	250 - 260 °C
Transparency	Translucent or opaque	Transparent
Flexibility	Flexible	Rigid
Glass Transition Temperature	LDPE: < -100 °C LLDPE: -120 to -80 °C HDPE: < -90 °C	73 - 78 °C
Shrinkage	1.5 - 4 %	0.2 - 3.0 %
Water absorption	0.01% to 0.5%	0.1% to 0.2%
Crystallinity	Depends on the density LDPE: low crystallinity HDPE: high crystallinity	Semi-crystalline structure gives high degree of rigidity and dimensional stability
UV resistance	Better than PET	UV degradation can lead to color fading and surface cracking. UV stabilizers are added to improve UV resistance.

Mechanical properties: A comprehensive evaluation

PE’s flexibility and impact resistance makes it ideal for applications where toughness and durability are essential. PET’s higher tensile strength and rigidity make it suitable for applications requiring structural integrity and stiffness. These differences between the mechanical properties of both polymers significantly impact their end-user application.

Property Name	Polyethylene	Polyethylene Terephthlate
Tensile strength	LDPE: 7 - 20 MPa LLDPE: 14 - 24 MPa HDPE: 15 - 37 MPa	55 - 85 MPa
Impact resistance	Good impact resistance making it durable and able to withstand stress without cracking	Good impact resistance but less flexible than some forms of PE
Flexural strength	Lower than PET, less rigid, and may flex more easily under load	Higher than PE making it more rigid
Hardness	Low hardness makes it soft	High hardness makes it stiff
Creep resistance	Exhibits creep	Likely to deform under mechanical stress
Fatigue resistance	Good	Good
Fracture toughness	Lower to PET as it may be prone to crack propagation	Higher than PE which means it can resist crack propagation
Barrier properties	Limited against moisture and gases, permeable to oxygen	Better against moisture and gases making it suitable for packaging
Chemical Resistance	Resistance to most organic solvents including alcohols & hydrocarbons Resistant to acids and bases Can degrade in the presence of strong acids, alkalis, and oxidizing agents

Cost considerations: Making financially savvy material choices

The cost of both polymers can vary depending on the type of grade, regional availability, supplier, and market conditions. PET is slightly more costly than PE because of the following reasons.

• Raw Materials: The raw materials involved in the manufacturing of PET are terephthalate and ethylene glycol which are expensive than the raw materials of PE.
• Production process: Furthermore, the production process of PET is more complex than PE which can lead to higher manufacturing costs.
• Higher Tg & Barrier Properties: PET’s glass transition temperature provides better barrier properties against moisture and gases compared to most PE types. This added feature can contribute to the extra cost of PET.
• Specific Applications: PET is used in applications that require unique properties such as clarity, heat resistance, and barrier properties. These additional features can affect the overall pricing of PET grades.

However, the choice between PET and PE should be based on the specific requirements of your end-use application. The user should consider all the above properties before making the right polymer selection.

Comparing food contact and migration testing in food packaging

Both polyethylene and polyethylene terephthalate are considered safe for use in all food contact applications. These materials should meet regulatory requirements set by agencies like the Food and Drug Administration (FDA) in the U.S. and other agencies within other regions.

PE and PET are both commonly used for food packaging applications such as plastic bags, food wraps, and containers. PET is specifically used to make clear plastic bottles for beverages. In terms of migration, both have low levels of migration in food products. However, PE is considered to have better migration resistance than PET because of its simpler structure, low Tg, and chemical inertness.

Recyclability: Unveiling the path to sustainable solutions

Polyethylene and polyethylene terephthalate can be recycled through mechanical recycling which involves:

STEP 1: Collecting the used plastic materials from various sources including recycling bins, drop-off centers, and waste management facilities. These collected items are then transferred to recycling facilities for sorting.

SPET 2: Sorting the collected items based on the type and color of the plastic materials.

STEP 3: Cleaning the sorted plastics to remove contaminants. This is done by washing them with water and detergents.

STEP 4: Shredding the cleaned plastic items into pellets or flakes to increase the surface area of the material making it easier to process.

STEP 5: Melting the smaller pieces of plastic materials into new pellets or flakes.

STEP 6: Reprocessing the melting plastic materials by extruding them into new products.

Both can be recycled by other methods such as chemical recycling, pyrolysis, blending, and upcycling to name a few. The choice of recycling method depends on the type of the polymer, desired end products, and desired environmental considerations. However, PET has higher recycling rates than many plastics.

Browse the range of recyclable PE or PET grades below.

PE vs. PET – What processing methods transform them into finished products?

Conversion mode for polyethylene

Polyethylene can be processed by injection molding and extrusion. PE pipes are commonly produced through pipe extrusion to create long continuous pipes with uniform wall thickness. The thermoforming process is used to create products from thin sheets of PE. The sheets are heated until they become pliable and are then formed into desired molds. For example, containers and trays. Rotational molding involves rotating a mold while heating the PE material, causing it to coat the interior of the mold and form a product. For example, large containers, tanks, and playground equipment.

Conversion mode for polyethylene terephthalate

Polyethylene terephthalate is typically extruded into sheets or films by sheet extrusion equipment. In some cases, PET may be processed through injection molding such as preforms for plastic bottles. These preforms are heated, stretched, and blow molded into the final bottle shape in a stretch blow molding process. PET bottles and films can also undergo a biaxial orientation process where the material is stretched in both longitudinal and transverse directions to enhance mechanical and barrier properties.

PE vs. PET – Where do these thermoplastics find their use?

Industries served by polyethylene

• Packaging: Used in food packaging applications such as plastic bags and food wraps due to its excellent flexibility, durability, and moisture resistance
• Pipes and Fittings: Used in plumbing applications due to excellent corrosion and chemical resistance
• Building and Construction: Used in geomembranes for lining landfills and ponds, thermal and sound insulation, and sheets
• Agriculture: Used in greenhouse coverings, mulching, irrigation systems
• Consumer Goods: Used in toys, furniture, bathroom, and kitchen appliances

Industries served by polyethylene terephthalate

• Packaging: Used in food packaging applications especially beverage bottles due to its excellent clarity and barrier properties
• Textiles: Used in clothing, upholstery, and carpets
• Electronics: Used in flexible printed board circuits and insulation
• Medical: Used in blister packs and trays
• Automotive: Used in interior trim components, headliners, seat fabrics, etc.

If PE suits best for your application, discover our exclusive guide on polyethylene, or if PET is the more suitable option, find out the detailed guide on polyethylene terephthalate.

Conclusion

The choice between PE and PET depends on the specific requirements of the application. PE excels in cost-effectiveness and versatility, while PET stands out in terms of clarity and recyclability. Understanding their respective strengths and weaknesses is crucial for making an informed material selection.

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