How custom mylar bags protect against moisture and oxygen
Custom mylar bags protect against moisture and oxygen by creating an almost impenetrable barrier. The secret lies in the material itself: a polyester film known as BoPET (biaxially-oriented polyethylene terephthalate), which is then metallized. This combination results in a bag with exceptionally low permeability rates. The metallized layer acts as a shield, reflecting heat and blocking the transmission of gases and water vapor, while the strong polyester base provides structural integrity and puncture resistance. For businesses requiring tailored packaging solutions, using a custom mylar bag allows for the integration of specific features like zip locks, degassing valves, and gauged thickness to match the exact sensitivity of the product being stored.
The fundamental science behind this protection is rooted in material permeability. Permeability measures how easily a gas or vapor can pass through a material. Mylar’s metallized layer drastically reduces this permeability. Let’s look at the critical metrics for moisture and oxygen.
Water Vapor Transmission Rate (WVTR) indicates how much moisture can pass through a material over a specific area in a 24-hour period. It’s measured in grams per square meter per day (g/m²/day). For high-quality metallized mylar bags, the WVTR is exceptionally low.
Oxygen Transmission Rate (OTR) indicates the volume of oxygen that can permeate a material, measured in cubic centimeters per square meter per day (cc/m²/day). Again, mylar’s performance is outstanding.
To put this into perspective, the following table compares the typical barrier properties of mylar against other common packaging materials.
| Material | Typical WVTR (g/m²/day) | Typical OTR (cc/m²/day) | Relative Barrier Strength |
|---|---|---|---|
| Metallized Mylar (3.5 mil) | 0.01 – 0.05 | 0.10 – 1.0 | Excellent |
| Transparent PET (Polyester) | 1.0 – 2.0 | 3.0 – 10.0 | Good |
| Low-Density Polyethylene (LDPE) | 10 – 20 | 4,000 – 7,000 | Poor |
| Polypropylene (PP) | 5 – 10 | 1,500 – 2,500 | Fair |
As the data shows, metallized mylar’s barrier properties are orders of magnitude better than those of common plastics. This low transmission rate is what gives products inside a mylar bag a significantly extended shelf life by preventing the two primary causes of degradation: moisture and oxygen.
The role of thickness and layers in barrier strength
The protective capability of a mylar bag is directly influenced by its thickness, which is measured in mils (one mil equals one-thousandth of an inch). A standard, thin mylar balloon might be 0.3 mils thick, while packaging for long-term food storage is typically between 3.5 and 7.0 mils. Thicker bags provide a more robust physical and chemical barrier. However, mylar bags are rarely just a single layer of metallized film. They are often laminates, combining the strengths of different materials.
A common and highly effective structure is a 3-ply laminate:
1. Outer Layer (Polyester / Mylar): This is the biaxially-oriented PET film that provides excellent tensile strength, puncture resistance, and clarity for printing. Its dimensional stability means it won’t easily stretch or warp.
2. Middle Layer (Aluminum): This is the vapor-deposited metallized layer. It is this thin layer of aluminum, measured in angstroms, that is primarily responsible for blocking light, oxygen, and moisture. It’s not a thick foil but a microscopic coating that creates the impermeable shield.
3. Inner Layer (Polyethylene or Cast Polypropylene): This food-grade plastic layer serves several purposes. It provides a sealable surface—when heat is applied, this layer melts to create a hermetic seal. It also adds another barrier and protects the delicate aluminum layer from being scratched off from the inside.
This multi-layer approach is key. The polyester gives it toughness, the aluminum provides the barrier, and the polyethylene allows for a reliable seal. The quality of the heat seal is critical; even the best barrier material is useless if the seal is weak. Industrial heat sealers ensure the inner layers fuse completely, creating a bond that is as strong as the bag material itself.
How oxygen leads to spoilage and how mylar stops it
Oxygen is a highly reactive gas that causes spoilage through oxidation. This process damages products in several ways:
For Food: Oxygen causes fats and oils to become rancid, leading to off-flavors and odors. It degrades vitamins and nutrients, reducing the food’s nutritional value. It also enables the growth of aerobic bacteria and molds, which require oxygen to live.
For Pharmaceuticals and Electronics: Oxygen can degrade active pharmaceutical ingredients, rendering medications less effective or even harmful. In electronics, it can corrode delicate metal components, leading to failure.
Mylar bags combat this by presenting a solid wall to oxygen molecules. The metallized layer is non-porous, meaning there are no microscopic pathways for oxygen gas to diffuse through. When combined with an oxygen absorber packet placed inside the bag before sealing, the protection becomes almost absolute. The oxygen absorber scavenges the small amount of oxygen trapped inside the bag during packaging, reducing the internal oxygen level to below 0.01%. This modified atmosphere packaging (MAP) creates an environment where oxidative reactions cannot occur, dramatically extending the product’s lifespan. For example, white rice stored in a sealed mylar bag with an oxygen absorber can remain edible for 10-15 years or more, whereas in a paper bag it might last only a few months.
How moisture causes degradation and the defense mylar provides
Moisture, or water vapor, is equally destructive. It leads to:
Moisture Gain: For dry goods like crackers, powders, or cereals, absorbing moisture from the air makes them soft, soggy, and unappetizing. It can also cause clumping.
Moisture Loss: For moist products, losing water to the environment causes them to dry out, harden, and lose their intended texture (e.g., bread becoming stale).
Microbial Growth: Moisture is a prerequisite for mold, yeast, and bacterial growth. Most microbes cannot proliferate in environments with a water activity level below 0.6.
The metallized layer in a mylar bag has an extremely low Water Vapor Transmission Rate (WVTR), as shown in the table earlier. It effectively blocks the movement of water molecules in either direction, maintaining a stable internal humidity level. This is why mylar is the gold standard for long-term food storage. It keeps dry goods dry and moist goods moist by preventing any exchange with the external environment. This barrier also protects products from humidity fluctuations, which is crucial in climates with high humidity or for products shipped across different regions.
Additional protective features of custom mylar bags
Beyond the inherent properties of the material, custom mylar bags can be engineered with specific features to enhance their protective capabilities.
Light Blocking: The metallized layer is opaque, blocking 100% of visible light and a significant portion of ultraviolet (UV) light. Light can catalyze oxidation reactions and degrade colors, nutrients, and potency in products like coffee, spices, and vitamins. A mylar bag keeps the contents in darkness, preventing light-induced spoilage.
Puncture and Abrasion Resistance: The biaxially-oriented polyester film is incredibly strong for its weight. This toughness protects the contents from physical damage during shipping and handling and, crucially, maintains the integrity of the barrier. A puncture would compromise the entire package.
Resealable Features: For products not consumed in one sitting, zipper locks or slider tracks can be added. While these are not as hermetic as the original heat seal, high-quality zippers provide a very good secondary barrier against moisture and oxygen, maintaining freshness after opening.
Degassing Valves: For products that release gases after packaging, such as freshly roasted coffee, a one-way degassing valve can be installed. This valve allows carbon dioxide to escape without allowing any external oxygen to enter, preventing the bag from bursting while preserving freshness.
The effectiveness of a mylar bag is a symphony of its material science, its layered construction, and the precision of its sealing. It is not just a bag but a sophisticated preservation system that tackles the root causes of spoilage head-on by creating a stable, controlled micro-environment for the product it contains. This makes it an indispensable tool for industries where product integrity and longevity are paramount.