Flexible Packaging Materials Explained: A Selection Guide

Soft packaging is not “a single layer of plastic”, but a “composite structure”.

Many people think that a soft packaging bag is just a single plastic film. In fact, it is a multi-layer structure formed by “gluing” different functional materials together, each performing its own specific role.

· Outer layer: Responsible for printing, stiffness, and wear resistance. Commonly used materials are BOPP (biaxially oriented polypropylene) or PET (polyester film).
· Middle layer: Responsible for barrier properties. It may be AL (aluminum foil) to block light and gas, or EVOH (ethylene-vinyl alcohol copolymer) to block oxygen.
· Inner layer: Responsible for heat sealing and direct contact with food. Commonly used materials are CPP (extruded polypropylene) or PE (polyethylene), which are flexible, non-toxic, and capable of sealing.

Understanding this “sandwich” structure will help you know exactly what you are buying.

Common food scenarios: Which kind of “sandwich” should be used?

1. High-temperature sterilization bags: “Pressure cookers” for cooked food and sauces
   · Core requirement: Able to withstand 121℃ high-temperature sterilization to ensure the packaging does not break, leak, or deform during the steaming process.
   · Recommended structure: PET/AL/CPP or PET/AL/PE. The aluminum foil (AL) layer provides extremely strong barrier and heat resistance, while the PET outer layer ensures strength. These bags can be directly placed in a steamer or sterilization tank and are suitable for braised dishes, ready-to-eat meals, and seasoning sauces.
   · Warning: Do not use ordinary plastic bags to store cooked food for steaming. At high temperatures, plastic will deform, separate, or even release harmful substances.
2. High-transparency snack bags: Making chips and biscuits “visible”
   · Core requirement: High transparency, high rigidity (to make the bag look full), and moisture-proof.
   · Recommended structure: BOPP/CPP. The BOPP outer layer has good gloss and rigidity, and the printing effect is bright; the CPP inner layer has good heat sealing performance. This structure has relatively low cost and is a standard for puffed foods, biscuits, and candies.
   · Warning: If the snacks contain high oil content (such as spicy strips, fried peanuts), a PVDC (polyvinylidene chloride) coating layer needs to be added between BOPP and CPP to enhance oil and oxygen resistance. Otherwise, it is prone to rancid smell.
3. Frozen food bags: Maintaining flexibility at -18℃
   · Core requirement: Resistant to low temperatures (-18℃ or lower), with sufficient toughness, and not cracking in the freezing environment.
   · Recommended structure: PET/PE or PET/CPP. PET provides strength, while PE or CPP maintains excellent flexibility and impact resistance at low temperatures. Suitable for frozen foods such as dumplings, tamales, and steaks.
   · Warning: Some cheap bags use recycled materials, which will become brittle at low temperatures and crack upon impact. Make sure the inner layer material is virgin material and pay attention to its “low-temperature drop test impact” performance.

Barrier Properties: The “Invisible Guardian” That Determines Food Shelf Life

1. Oxygen Barrier (Preventing Spoilage and Color Change):
   · Champion: Aluminum foil structure, which almost completely blocks oxygen and light.
   · Expert: EVOH material, has excellent oxygen barrier properties and is transparent, commonly used in high-end meat products and coffee packaging.
   · Cost-effective option: Aluminized film (VMPET/VMCPP), has some barrier properties, costs less than aluminum foil, but has slightly poorer resistance to rubbing.
   · Basic model: Ordinary plastic (such as PE, PP), has average oxygen barrier properties.
2. Moisture Barrier (Preventing Moisture and Drying):
   · Aluminum foil and PVDC coating are experts in preventing moisture, can effectively lock in moisture or block external moisture, suitable for potato chips, milk powder, tea.
   Simple rule: For items that need long-term preservation and prevention of oxidation (such as nuts, coffee, cooked food), prefer structures containing aluminum foil or EVOH; for just moisture prevention (such as biscuits), aluminized film or PVDC coating is sufficient.

Production Process: Dry Lamination vs. Solvent-Free Lamination

These two mainstream processes directly affect the safety and performance of the packaging.
· Dry Lamination: Use solvent-based glue to bond each layer of material, then dry it. It has strong adhesion and can combine a wide range of materials (such as complex structures with aluminum foil), and the process is mature. However, during production, solvents evaporate, and thorough curing is required to ensure compliance with residual standards.
· Solvent-Free Lamination: Use 100% solid content glue to laminate at room temperature. The biggest advantage is environmental friendliness and almost no odor, making it more suitable for the inner layer in direct contact with food. It has a fast production speed. However, higher requirements are placed on equipment, materials, and workshop environment.
How to choose: For ordinary snack bags, both are acceptable; for cooked food packaging in direct contact with soup or oil, or for foods with extremely strict odor requirements (such as chocolate, high-end tea), the solvent-free lamination process should be prioritized to reduce risks from the source.