In our daily use of plastic bottles and takeaway boxes, we always find a plastic recycling symbol with a triangle of arrows on the bottom.
It’s not hard to notice that the plastic symbols marked with “1” for PET and “5” for PP are especially common.
But what are the characteristics of these two materials? What differences exist between them? And which material is better?
The choice between PP and PET is increasingly shaped by the needs of a dynamic world, where packaging materials must adapt to changing demands and ongoing innovations.
In the following content, we will embark on a technical exploration of “PP vs PET” to find answers to these questions.
What Is Polypropylene？
Polypropylene (PP) is a versatile thermoplastic resin produced through the polymerization of propylene.
It appears as a white, waxy, translucent material with a high melting point of 167°C, which is lower than that of PET, making PP better suited for applications requiring moderate heat resistance.
PP is non-toxic, odorless, and tasteless, characterized by its milky white, highly crystalline structure. With a density of 0.90–0.91 g/cm³, it is one of the lightest plastics available, offering excellent durability and versatility.
Depending on the arrangement of its methyl groups, PP is categorized into three types: isotactic, atactic, and syndiotactic polypropylene, each with distinct properties suited to various applications.
What Is PET Plastic？
PET (Polyethylene Terephthalate) is a member of the polyester family of polymers, classified as a thermoplastic polyester alongside Polybutylene Terephthalate (PBT).
PET is produced through one of two primary methods: transesterification of dimethyl terephthalate with ethylene glycol or esterification of terephthalic acid with ethylene glycol to form bis-hydroxyethyl terephthalate, followed by polycondensation.
This highly crystalline polymer typically appears as a milky white or light yellow material with a smooth, glossy surface.
PET is widely used in everyday products and can be categorized into types such as APET (Amorphous PET), RPET (Recycled PET), and PETG (Glycol-modified PET).
Its versatility makes it ideal for applications like PET injection molding, where it is melted and shaped into precise forms such as bottle preforms, containers, and other packaging solutions, offering clarity, strength, and recyclability.
PP vs PET : Physical Properties And Chemical Properties
This comparison will focus on the key features of PP and PET, such as durability, resistance, and sustainability.
To determine the quality of a material, it is first necessary to understand its properties.Therefore, let’s introduce the characteristics of PP and PET from multiple aspects:
Mechanical Properties
PP exhibits excellent rigidity and resistance to flex fatigue. Hinges made from PP can withstand more than 70 million cycles of folding without breaking, although its impact strength at low temperatures is poor.
PET, on the other hand, has minimal wear and high hardness, possessing the greatest toughness among thermoplastic plastics. It is also tear resistant, making it suitable for demanding applications. It has good creep resistance, fatigue resistance, and dimensional stability.
Thermal Properties
PP has a melting point of approximately 160°C (320°F), making it suitable for household items, everyday containers, and packaging where a lower melting point is acceptable and high temperature resistance is not required.
PET also has decent heat resistance, maintaining good physical and mechanical properties over a wide temperature range, with a service temperature of up to 120°C.
Chemical Resistance
Polypropylene is very chemically stable. Except for being eroded by concentrated sulfuric acid and nitric acid, it does not react with most chemicals. PP is chemically resistant and can withstand exposure to various chemicals, making it suitable for industrial and chemical applications.
However, certain halogenated compounds, aromatics, and high boiling point hydrocarbons can cause it to swell.
PET material is resistant to weak acids and organic solvents. PET has good resistance to water and certain acids, but is less resistant to some solvents, and is often chosen for applications where it must withstand exposure to various chemicals. However, it is not resistant to hot water soaking or alkalis.
Electrical Properties
PP has excellent high-frequency insulation properties. Because it hardly absorbs water, it also maintains good electrical insulation in humid environments.
Although PET has excellent electrical insulation properties, even at high temperatures and frequencies, its resistance to corona discharge is inferior.
Barrier Properties
The barrier properties of PET and PP play a pivotal role in their selection for packaging industry applications. PET stands out for its excellent barrier properties against gases and moisture, thanks to its unique chemical composition involving ethylene glycol and terephthalic acid.
This strong barrier helps preserve the freshness, flavor, and shelf life of food containers and beverage packaging, making PET the preferred choice for products that demand durability and protection from external contaminants.
In contrast, PP offers good moisture resistance, which is highly valued in packaging films and certain food containers where preventing water vapor penetration is essential.
However, PP is less effective than PET at blocking gases, which can be a limitation for some packaging applications.
To clearly demonstrate the performance of PP and PET plastic, we have compiled a data table for your reference:
PP | PET | |
Chemical Formula | (C3H6)n | (C10H8O4)n |
Density (g/cm³) | 0.90 – 0.91 | 1.31 |
Melting Point (℃) | 164 – 170 | 250 – 255 |
Water Absorption (%) | 0.03 – 0.04 | 0.06 – 0.129 |
Tensile Strength (MPa) | 30.0 – 39.0 | 140 – 160 |
Bending Strength (MPa) | 42.0 – 56.0 | 70 – 100 |
Elongation at Break (%) | 200 – 400 | 50 – 150 |
Manufacturing Process of PET and PP Plastic
The manufacturing processes of PET and PP plastics are fundamental to their unique characteristics and suitability for various industries.
PET is synthesized by polymerizing ethylene glycol with terephthalic acid, resulting in a polyester resin known for its high tensile strength, impact resistance, and excellent chemical resistance.
This process can be fine-tuned to produce PET with specific properties, making it ideal for demanding applications such as beverage bottles and food packaging, where both durability and safety are paramount.
On the other hand, PP is produced through the polymerization of propylene monomers, creating a polyolefin material with outstanding chemical resistance, heat resistance, and excellent fatigue resistance.
The flexibility of PP’s manufacturing processes allows for the adjustment of its properties, such as enhancing its thermal stability or making it more suitable for automotive components, packaging materials, and a wide range of consumer products.
Understanding these manufacturing processes is crucial for selecting the right plastic, as factors like chemical exposure, temperature resistance, and environmental considerations can significantly impact product performance and longevity.
PP vs PET: Applications
Both PP and PET plastics find extensive use across various industries due to their unique properties.
PET is predominantly used in beverage bottles, especially soda bottles, and food packaging where its excellent barrier properties and clarity help preserve product quality and ensure safety.
It is also common in packaging applications requiring transparency and durability, such as takeout containers and other food containers. PET’s high tensile strength and chemical resistance make it suitable for these demanding uses.
Polypropylene , on the other hand, is favored for applications requiring flexibility, chemical resistance, and heat resistance. It is widely used in packaging films, automotive parts, and everyday items like living hinges on containers and household products.
PP’s lightweight nature and excellent fatigue resistance make it ideal for products that demand repeated use and durability. Additionally, PP injection molding is often chosen for applications where cost effectiveness and lower energy consumption during manufacturing and recycling are important factors.
The choice between PP and PET depends largely on the specific requirements of the application, including mechanical strength, temperature resistance, chemical exposure, and environmental considerations.
Cost and Availability Of PP and PET
When it comes to cost and availability, both PP and PET are among the most widely used materials in the packaging industry, but they differ in several key aspects.
Polypropylene is generally more cost effective due to the lower price of propylene monomers and the efficiency of its production processes. This makes PP a popular choice for applications where budget constraints are a priority.
Polyethylene terephthalate , while typically more expensive, offers superior barrier properties, transparency, and is highly recyclable—attributes that can justify its higher cost for high-value applications like beverage packaging and food containers.
The availability of both materials is supported by a robust global supply chain and established manufacturing infrastructure.
However, regional differences in recycling infrastructure and recycling programs can influence the cost and supply of recycled PET (rPET), making it a more attractive option in areas with strong recycling initiatives.
These factors should be considered when evaluating the overall cost effectiveness of PP and PET for specific packaging materials and applications.
PP vs PET:Pros And Cons
When evaluating PP and PET for various applications, it’s important to consider their distinct advantages and limitations.
PET is renowned for its excellent chemical resistance, high tensile strength, and superior barrier properties, making it a top choice for beverage packaging and food containers that require long shelf life and product safety.
Its clarity and recyclability further enhance its appeal, especially in markets with strong environmental concerns. However, PET can be more expensive and less flexible than PP, which may limit its use in certain applications.
PP, by contrast, is valued for its excellent fatigue resistance, chemical resistance, and cost effectiveness. It is widely used in automotive parts, packaging materials, and consumer products due to its durability and versatility.
While PP may not match PET’s barrier properties or transparency, its lower cost and adaptability make it a preferred choice for many packaging and industrial uses.
Ultimately, the decision between PP and PET should factor in the specific requirements of the application, including environmental impact, manufacturing processes, and desired material properties.
Health and Safety Concerns Of PP And PET
It is widely acknowledged that “1” PET and “5” PP are the principal materials used in our daily plastic products, which naturally raises questions about their safety: Is PP plastic safe? Is PET toxic?
Therefore, it is essential to thoroughly examine the safety of these two materials to fully understand their impact on our health and well-being.
PP is odorless, tasteless, and non-toxic, making it a safe choice for food containers, baby bottles, and kitchen utensils.
Under normal conditions, PP does not leach harmful substances into food or beverages. Its high heat resistance makes it the only plastic recommended for microwave use, provided it is labeled as microwave-safe.
PET is also considered safe at room temperature, valued for its transparency and stability, which make it ideal for beverage bottles and food packaging.
Both PP and PET are classified as food-grade plastics and undergo rigorous regulatory approval to ensure safety for food contact when manufactured and used according to guidelines.
However, caution is needed with PET. When exposed to high temperatures, used for extended periods, or filled with overheated liquids, PET may release harmful substances like DEHP, a potentially toxic chemical.
To ensure safety, PET bottles should be used within recommended conditions and not subjected to excessive heat or prolonged reuse.
Environmental Considerations Of PET And PP
PET, identified by the recycling code “1,” is a preferred choice for sustainable packaging solutions due to its widespread acceptance in recycling programs.
PET is easily recyclable, with a well-established infrastructure enabling efficient collection, processing, and repurposing into new products, such as bottles, containers, and textiles.
This robust recycling system enhances PET’s eco-friendly reputation, reducing waste and conserving resources, making it a cornerstone of sustainable plastic use.
Polypropylene (PP), marked with the recycling code “5,” is also easily recyclable, though its recycling infrastructure is less developed compared to PET.
PP can be repurposed into items like automotive parts, storage containers, and fibers, but its recycling rates are lower due to limited collection programs and processing facilities in many regions.
Efforts to improve PP recycling are ongoing, with advancements in sorting technologies and increased awareness driving progress. However, challenges such as contamination and inconsistent recycling standards can hinder its environmental benefits.
Both materials contribute to sustainability when properly recycled, but their environmental impact depends on effective waste management systems. PET’s established recycling network gives it an edge, while PP’s potential is growing as infrastructure improves.
To maximize their environmental benefits, consumers should prioritize recycling, support products made from recycled content, and advocate for expanded recycling programs to ensure both PP and PET contribute to a circular economy.
How To Choose Between PET And PP Plastic?
When choosing between PET and PP , consider your specific needs: PET is ideal for transparent, single-use packaging like beverage bottles, offering excellent clarity and recyclability but should avoid high heat or prolonged use to prevent chemical leaching.
PP is better for durable, heat-resistant, reusable products like microwave-safe containers and baby bottles, though its recycling infrastructure is less developed.
Prioritize PET for cost-effective, recyclable packaging in cold or room-temperature applications, and PP for flexible, long-lasting items requiring heat resistance, ensuring compliance with safety standards and local recycling capabilities to balance health, functionality, and environmental impact.
