PET, PP, PE: three types of plastics, three roles
It's not about which type is better than another; each type serves a different layer within the same bag.
Rigid PET, good gas barrier, and heat resistance are often used as the outer layer for printing and protection. Clear, glossy PP has good moisture resistance, making it suitable for dry goods packaging, labels, and microwaveable items. Soft, heat-sealable PE is usually the innermost layer to create a sealed bag. In practice, they are combined in layers rather than replacing each other.
- needs a durable, beautifully printed outer layer that is heat-resistant and made of PET
- need high transparency and moisture resistance for dry goods or microwave use with PP
- needs a flexible, sealed inner layer that withstands folding and cold, made of PE
- consider three mutually interchangeable types while they coordinate by role class
- mixing different types of plastics loses the ability to recycle mono-materials
When you ask "what type of plastic should I use for this bag?", the answer is rarely a single type. PET, PP, and PE often appear together in many packaging bags because they serve different roles in each layer. Understanding the role of each type helps you communicate more accurately with the printer and avoid making incorrect choices right from the specification stage.
Three types of plastic, three roles in the layer structure
The easiest way to remember is to associate each type with a position in your bag, not with "good or bad quality."
- PET (polyethylene terephthalate): Rigid, high tensile strength, good heat resistance, and excellent oxygen and odor barrier. This is a natural candidate for the outermost layer, where strength, sharp printing, and shape stability are required. Products with long shelf lives or needing high temperatures in the packaging process typically specify PET for this layer.
- PP (polypropylene): Clear, glossy, lightweight, with good moisture resistance. PP is suitable for packaging dry goods such as biscuits, seeds, instant noodles, and for labels due to its high gloss. CPP (cast polypropylene, flat-rolled PP film) has good heat sealing properties, so it is also used as the innermost layer in some structures, especially when products need to be microwaved as PP has better microwave heat resistance than PE.
- PE (polyethylene): Soft, flexible, easy to heat seal, good cold resistance. PE is the most common innermost layer, creating a sealed bag after packing. When standing alone, PE has poor gas barrier properties. However, when combined with PET or an outer aluminum layer, PE fulfills its sealing role without needing additional work.
When to choose which direction
Prioritize PP when: the product is dry, requires high clarity and gloss for display, or needs to be microwaveable. CPP in the inner layer is an alternative to PE in a more recyclable structure.
Prioritize PE in the inner layer when: a strong seal is needed, the product requires cold storage, or the bag needs to withstand multiple folds without cracking.
Common mistakes when selecting materials
- Considering the three types as interchangeable: This is the most common misunderstanding. Buyers ask "use PE or PET" as if choosing one of the two, while in reality, the finished bags can contain both in different layers.
- Choose PE as the outer layer and then print: PE is soft, easily deformed, and the print surface is usually duller than PET and PP. The result is less vibrant printed colors, which do not look appealing when displayed.
- Use single-layer PET and ensure a tight seal: PET alone does not seal well with heat. To seal bags, a layer of PE or CPP is needed inside. Skipping this step leads to weak or open seals.
- Combining multiple types of plastics without considering recycling: A structure of PET combined with PE and aluminum provides good barriers but is very difficult to recycle. If your brand is pursuing sustainability, it is important to understand this trade-off early on.
The structure of a type of plastic that is sustainable
The trend is driven by guidelines such as CEFLEX D4ACE (design standards for flexible packaging aimed at a circular economy in Europe) prioritizing mono-material structures (a single type of plastic). For example, all-PE or all-PP bags can consist of multiple layers but are of the same plastic type, making it easier for recycling facilities to process compared to mixed PET and PE.
This does not mean that mono-material is always better in function, as sometimes the barriers of mixed structures are still higher. The decision depends on the level of barrier the product needs, whether the sales market requires recyclable labels, and your brand's direction.
A composite bag is a layered structure with designated roles, not a uniform plastic sheet. The outer layer protects and prints, the inner layer seals, and the middle layer acts as a barrier when needed. Choosing the right type of plastic for each layer is the first technical step before discussing aesthetics.
Practical experience, Sinh Vũ Studio
The viewpoint of Sinh Vũ
When starting to create molds and specifications for packaging, the Sinh Vũ team identifies the front layer, the inner layer, and then discusses the barrier layer if needed. The final material specifications are confirmed with the partner printer, not decided unilaterally. Sinh Vũ acts as the creative director and coordinates the specifications, without committing to printing costs or overriding the technical opinions of the film manufacturer.
When you care about sustainability, the practical question to ask is: can the product maintain the necessary barrier with a mono-PP or mono-PE structure? If so, that is a direction worth considering from the design stage, not after printing is complete.
Decision checklist
Topic: How do PET, PP, and PE differ and what are their applications. Sinh Vũ guide, sinhvu.com
Select each item you find appropriate, then print or save as PDF to take with you.
If you have marked most of the signs above, this is the time to discuss in more detail. Sinh Vũ can help you review and propose a direction.
References
PP vs PE vs PET Films, Packaging Material Guide, newtopmachine.com. A Flexible Films Guide, Packaging Strategies. CEFLEX D4ACE Design for a Circular Economy in Flexible Packaging.
Frequently asked questions
Can I just have a single layer of plastic?
Yes, if the product is simple and requires low barriers. However, when you need it to look good on the outside, be sealed on the inside, and provide gas barriers in between, a single layer may not meet all those needs. A multi-layer structure is designed to address that specific problem. Additionally, a type of plastic structure (mono-PP or mono-PE) can still be multi-layered but of the same type, making recycling mixed materials easier.
How to distinguish between PE and PP?
PE is soft, flexible, and can be folded multiple times without breaking. PP is harder, shinier, and strong folds may leave white marks. The technical specifications will clearly state the type of film. When working with Sinh Vũ, this specification is confirmed with the printing partner before finalizing the mold, so you don't need to classify it by intuition.
Which type should I use for items that need to be microwaved?
PP has better heat resistance than PE in microwave temperatures, so it is often specified for packaging with this feature. PET also has good heat resistance but usually serves as the outer layer. However, specific heat resistance parameters need to be confirmed with the film manufacturer and partner printer, as different grades of materials vary.