Direct answer
Metallized PET and aluminum foil can both be used in high-barrier bags, but they are not equivalent. Metallized PET uses a very thin deposited metal layer and often balances barrier, appearance, weight, cost, and flex durability. Aluminum foil provides the strongest light and gas barrier and full opacity, but it can cost more and may be more vulnerable to flex cracking. Choose the complete laminate from the product, shelf-life target, filling and sealing process, distribution, and validation plan—not from the word “Mylar” alone.
What does “Mylar bag” mean in packaging searches?
Mylar® is a trademark associated with biaxially oriented polyester film. Packaging buyers often use “Mylar bag” more broadly for silver, metallized, foil, odor-barrier, or high-barrier pouches. That search term does not identify the finished laminate, thickness, sealant, barrier values, bag format, or food-contact status.
A custom high-barrier bag is normally a multilayer structure. An outer PET layer can provide printability and strength, a metallized film or aluminum foil layer can provide barrier, and an inner sealant layer can provide product contact and heat sealing. Adhesives, inks, coatings, closures, and converting conditions are also part of the package.
How metallized PET provides barrier
Metallized PET is made by depositing a very thin layer of aluminum onto polyester film. It can provide strong oxygen, moisture, aroma, and light protection compared with many transparent films while retaining lower weight and greater flexibility than a conventional foil layer.
Performance depends on metal coverage, film gauge, lamination, pinholes, flexing, seals, and the rest of the structure. A silver appearance is not a barrier specification. Request relevant oxygen-transmission and water-vapor-transmission data for the exact laminate under defined test conditions.
- Often a balanced option for dry foods, snacks, coffee, powders, and supplements
- Metallic appearance or an opaque printed surface
- Generally better flex-crack tolerance than foil in many constructions
- Barrier values must come from the proposed complete structure
When an aluminum foil laminate may be appropriate
Aluminum foil forms a continuous metal layer that can provide extremely low gas and moisture transmission and complete light blockage when it remains intact. It may be evaluated for highly sensitive products, demanding shelf-life targets, and other projects where maximum barrier is a defined requirement.
Foil is not automatically the best choice. Repeated folding, sharp product edges, aggressive handling, or unsuitable converting can create flex cracks or pinholes. Its cost, weight, opacity, metal-detection considerations, recycling pathway, and sealing process should be reviewed with the performance benefit.
Compare barrier using measurable requirements
Start by documenting what degrades the product: oxygen, moisture gain or loss, aroma transfer, light, oils, or another mechanism. Add the target shelf life, storage temperature and humidity, time after opening, distribution route, and acceptable product-change criteria.
OTR and WVTR can help compare candidate laminates, but published numbers only apply to stated materials, gauges, temperature, humidity, and test methods. Finished-bag seals, zippers, fitments, pinholes, and handling can become the limiting pathway even when the flat film has excellent barrier.
- Product composition and deterioration mechanism
- Target unopened and opened shelf life
- Storage temperature, humidity, and light exposure
- Oxygen, moisture, aroma, grease, and puncture targets
- Seal, zipper, fitment, and package-integrity requirements
Choose the sealant, format, print, and finish together
The product-contact and heat-seal layer must suit the product, filling temperature, seal equipment, contamination risk, storage, and applicable documentation. Three-side-seal bags, stand-up pouches, flat-bottom bags, side-gusset bags, and rollstock create different folds, seals, stresses, and consumer-use patterns.
Digital printing can support qualified shorter runs or multiple artworks, while flexographic and gravure printing are commonly evaluated for larger repeat orders. Matte, gloss, soft-touch, registered effects, metallic finishes, and windows need compatibility review; a clear window normally changes the barrier in that area.
Food-contact and odor-control claims need evidence
A bag is not food safe merely because it is called Mylar. Request food-contact documentation for the exact product-contact layer, inks, adhesives, and intended use. Temperature, food type, duration, and regulatory market can change the applicable evaluation.
High-barrier structures can reduce aroma transmission, but “odor proof” is an absolute claim that should not be assumed. Odor performance depends on film, seals, zipper, fitment, product chemistry, storage, and the test method. Define the actual control target and validate the finished package.
Mylar bag RFQ and validation checklist
A useful quotation identifies the product, format, size, fill quantity, barrier and shelf-life target, closure, print, finish, quantity by artwork, filling method, validation needs, destination, and schedule. If the structure is open, state the required performance instead of copying an unrelated laminate.
- Product and product-contact requirements
- Bag format, dimensions, fill weight, and headspace
- Metallized PET, foil, or measurable barrier target
- Closure, tear feature, valve, spout, or hang hole
- Print process, artworks, colors, and finishes
- Quantity per SKU and repeat-order plan
- Filling, sealing, testing, storage, and delivery
General educational information only. Materials, performance, compliance, claims, quantities, and timing require confirmation for the exact packaging project.