It is often asked "What are Mylar bags made of?" "Are they plastic, foil, polyester or another material?" The gold standard of long-term food storage, supplements, coffee, powders, and specialty retail packaging today is Mylar bags.

Mylar bags are made from multiple laminated layers, typically including polyester film (PET), aluminum foil or metallized film, and a polyethylene (PE) inner sealant layer. The polyester provides strength and printability, the foil blocks oxygen, moisture, and light, and the PE layer allows heat sealing for airtight storage.

If you sell or use Mylar bags for storage, you will need to understand what goes in Mylar bags. The structure determines barrier strength, shelf-life, and print quality, as well as how puncture resistant and durable they are.

Let’s look at the layers, materials, and science behind Mylar packaging.

What Is Mylar?

"Mylar" refers to a brand name for one kind of film that is made from stretched polyester known as BoPET. It was created by DuPont in the 1950s and has since become a generic term for the kind of films with high moisture and gas barriers that can be used for packaging.

Essentially, Mylar is:

(1) A polyester film

(2) Extremely strong and flexible

(3) Resistant to moisture

(4) Resistant to oxygen transmission

(5) Resistant to light, with the correct layers applied

Many of the bags made from Mylar will actually include more than one material and so are multilayer laminates.

Short Answer of What Mylar Bags are Made of

Mylar bags are made from laminated layers of polyester film, a metallic barrier such as aluminum foil or metallized PET, and a polyethylene inner lining. This combination creates a strong, airtight, moisture resistant, and light blocking package designed to extend shelf life and protect contents.

Each Layer Explained

Layer 1 - The Core Material: PET (Polyethylene Terephthalate)

The core for a Mylar bag is typically produced from Polyethylene Terephthalate or PET plastic.

Mylar bags made from materials such as PET provide: 

- High degree of structural strength and tear resistance
- High tolerance to temperature variations
- Excellent dimensional stability
- Excellent surface area for custom print graphics
- High chemical resistance

This is the layer that allows customized mylar bags to hold vibrant designs, matte finishes, gloss coatings, spot UV, holographic effects, or metallic sheens.

Without the properties of PET, Mylar bags will lose their original structure and have a lower heat tolerance as well as a lower heat tolerance.

Layer 2 - The Barrier Layer: Aluminum Foil or Metallized Film

The second layer that contributes to Mylar's exceptional barrier characteristics is the metallization (metallized layer). There are two types of metallization used in Mylar films and bags.

Aluminum Foil-Lined Mylar

An aluminum foil sheet is laminated between two plastic films. Foil lined,

Provides:

• Near-zero oxygen transmission
• Near-zero moisture transmission
• Complete blockage of light
• Maximum retention of aroma

This is why foil lined mylar bags are used for storing, freezing, and long shelf life dry goods.

Metallized PET (MPET)

A thin vapor deposited aluminum coating is used as opposed to solid foil. MPET,

Provides:

• Excellent barrier protection
• Lower cost than solid foil
• More flexible than foil
• Slightly higher oxygen transmission than solid foil

The vast majority of retail Mylar bags are made with MPET because it provides a good balance between performance and low cost.

Layer 3 - The Inner Sealant Layer: PE (Polyethylene)

Typically, the inner layer of a Mylar bag is made from polyethylene (PE).

PE provides:

• Capability to heat seal
• Food-grade safety
• Flexibility
• Moisture barrier

When you heat seal (or iron) a Mylar bag, you melt the PE layer together; thus, making an airtight closure.

Without polyethylene, the bag would not seal correctly.

Typical Mylar Bag Structure

A common structure looks like this:

PET / Aluminum Foil or MPET / PE

Or in thicker retail packaging:

PET / MPET / PE

In high end barrier packaging:

PET / Foil / Nylon / PE

Each added layer improves strength, puncture resistance, and barrier performance.

Mils are commonly used to measure thickness. An example would be:

• 3.5 mils - light storage
• 5 to 7 mils - heavy-duty food storage
• 7 to 10 mil - long-term bulk storage

For those who want to know how thick a Mylar bag should be for food storage, you can review How Thick Should Mylar Bags Be for Food Storage? It will help you understand the durability of the material based on what type of material is used.

Why Multiple Layers Are Used

Single material cannot provide:

- Strength
- Printability
- Oxygen barrier
- Moisture barrier
- Heat-sealing performance

Using layers of different materials allows manufacturers to create an ideal combination of:

- Shelf life
- Flexibility
- Aesthetics
- Protection

This is extremely important when considering custom printed mylar bags as both aesthetics and function are important factors.

The Importance of Material Selection in Custom Mylar Bags

For brands that sell edibles, coffee, dietary supplements, spices or dried fruits the composition of materials used affects:

1. Shelf life.
2. Aroma retention.
3. Moisture barrier.
4. Product freshness.
5. Regulatory compliance.
6. Seal integrity.

As an example:

• Strong oxygen and aroma barrier - coffee.
• Moisture barrier - dried fruit.
• Puncture resistance - powders.
• The thick foil laminate - storage for extended periods.

The layer structures used when having mylar bags printed can be engineered to meet your specific product requirements.

"High-performance packaging films rely on layered polymer structures to achieve both mechanical strength and barrier protection. By combining oriented polyester with metallic or barrier layers and sealant films, manufacturers can dramatically reduce oxygen and moisture transmission while maintaining durability." - Dr. Donald R. Paul, Polymer Materials Researcher

Are Mylar Bags Plastic or Foil?

Technically, both. Mylar bags are multilayered products made with:

• Aluminum foil or other metal film
• Polyester (PET) plastic
• Polyethylene (PE) sealant layer

Therefore, the basic construction of Mylar bags contains all three materials and is considered:

Multilayer polyester film laminate with a metallic barrier and an internal polyethylene sealant layer.

Do All Mylar Bags Have Aluminum?

Not all.

Many clear mylar bags are manufactured without a metallic foil component within them, such as those having a clear polyester barrier coating on the outside and have:

1. Clear PET film
2. Clear barrier layer
3. EVOH (ethylene vinyl alcohol) oxygen barrier layer(s)

However, the true long-term food storage bags typically contain an aluminum foil layer to prevent oxygen from entering into the bag.

Many times, oxygen absorbers are used in dry food packaging in order to create a vacuum around the remaining air when they are sealed and help elongate the shelf life.

How The Layers of a Mylar Bag Work Together

The materials used in the construction of Mylar bags create a seal-able, moisture-resistant, light-blocking environment.

Outer PET layer of the Mylar bag:
Provides strength and printability to the outer surface of the bag.

Middle foil or Mylar foil layer:
Provides a barrier to moisture, oxygen, and light.

Inner PE layer:
Allows heat or pressure sealing and is considered to be food safe for the product contained within the Mylar bag.

When combined together, the elements create a vacuum/moisture-proof and light-proof environment. Mylar bags offer a much greater shelf-life than standard plastic storage bags when sealed properly.

Conclusion

Mylar bags are made from a layered laminate of polyester (PET), a metallic barrier such as aluminum foil or metallized film, and a polyethylene sealing layer. These materials work together to create a durable, light blocking, moisture resistant, and airtight packaging solution commonly used for food storage and retail products.

In short, Mylar bags are made from several types of plastic film laminated together. Each component has a specific function, including barrier protection, structural strength, and sealing integrity.

By comprehending the layer construction of your Mylar bags, you will be able to select the appropriate lamination thickness, type of barrier, and type of structure for your product.

If you are storing food for an extended period of time or manufacturing custom retail Mylar bags, the materials used in the manufacture of your Mylar bags will determine the overall performance of your bags.