Meaning
Multi-layer barrier system known as functional barrier evoh utilizes a thin layer of ethylene vinyl alcohol copolymer to prevent the passage of gases and aromas through plastic packaging. This polymer is prized for its exceptional oxygen barrier properties, which are significantly higher than those of standard polyolefins. To be effective, the evoh must be protected from moisture, which otherwise degrades its performance.
It is typically sandwiched between layers of polyethylene or polypropylene using specialized adhesive resins.
Structural Configuration
Performance of the barrier is a function of the thickness and the ethylene content of the copolymer. In a typical five-layer or seven-layer coextrusion, the functional barrier evoh is placed in the center to isolate it from external humidity. The outer layers provide mechanical strength and moisture protection, while the evoh stops oxygen from entering and volatile flavors from escaping.
This specific arrangement extends the shelf life of sensitive products like juices and sauces without the need for preservatives.
Adhesion Mechanism
Bonding the barrier layer to the surrounding polyolefins requires the use of tie-layers. These adhesive resins are chemically modified to bridge the gap between the polar evoh and the non-polar polyethylene. Without these layers, the functional barrier evoh would delaminate during the moulding process or in use, causing immediate failure of the package.
The integrity of these bonds is a critical quality check during the production of high-barrier films and containers.
Processing Challenges
Thermal stability of the barrier resin is lower than that of the surrounding materials. If the melt temperature is too high, the functional barrier evoh can degrade and form gels or carbonized particles. These defects appear as streaks or spots in the final part, compromising both appearance and barrier efficiency.
Careful control of the extruder screw design and temperature profile is necessary to maintain a uniform and effective barrier layer.
