Barrier Materials

The two primary barriers used in rigid packaging are Evoh and Saran. When determining the proper barrier, one must have a relative idea on the required barrier properties needed to protect the product. The determining design criteria for the required barrier could be oxygen transmission rate, moisture vapor transmission rate, a combination of both or a completely different gas such as carbon dioxide or nitrogen. When designing multi-layer packaging, barrier protection may be provided from layers in addition to the barrier layer. For example with a PP/Tie/Evoh/Tie/PP structure, the Evoh has relatively poor moisture barrier properties but the thicker layers of PP provide a great deal of moisture barrier.

Barrier polymers

The most common barrier polymer used in the rigid packaging industry is Evoh. Evoh provides very good gas barrier properties including oxygen, carbon dioxide and nitrogen. Evoh does have limitations in some high moisture applications and does not provide good moisture barrier properties. For these select few applications involving high moisture levels, PVDC or Saran® may be the proper choice. The following provides an overview of the strengths and weaknesses of each of these popular barriers.

Advantages of Evoh

• Evoh has superior gas barrier properties when exposed to low-moisture conditions.
• Evoh has very good chemical resistance including most oils, acids or solvents.
• Evoh has very good clarity.
• Evoh provides very good fragrance preservative properties.
• Evoh provides very good organoleptic properties.
• Evoh is reasonable easy to process.
• Evoh can be extruded with many different polymers during the coextrusion process.
• Evoh regrind can be reused and provides good economic and environmental sense.

Disadvantages of Evoh

• Evoh is hygroscopic. This hygroscopic nature can impact both the processability and end use of Evoh.
• Evoh does not perform well in post retort conditions.
• Evoh does not provide good moisture barrier properties when compared to many other polymers.
• Some grades of Evoh can be difficult to process.

Advantages of Saran

• Saran has very good gas barrier properties.
• Saran has excellent moisture barrier properties.
• Saran's gas barrier properties not affected by moisture conditions.
• Saran performs well in retort processes.

Disadvantages of Saran

• Saran is very heat sensitive, which makes it difficult to process.
• Saran regrind cannot be reprocessed.
• Saran has poor clarity in thicker layers, brownish tint when utilized with clear structures.
• Saran has very few processors in coextruded rigid packaging.
• Environmental acceptance low, particularly in Europe.
• Saran packaging expensive because of the difficulty in processing.

When determining barrier performance in rigid packaging, often product sensitivity to oxygen, moisture transmission, or other gases is unknown or uncertain. It can also be difficult to calculate the various package transmission rates because of outside influences. These influences include temperature, relative humidity, and atmospheric pressure as discussed. Theoretical calculations can become more difficult as these parameters change, as would be the case for example with a retort package. Other influences include process consistency during the thermoforming operation. Process variables such as heat and cooling rates, material temperature, and forming parameters influence the package material distribution in the container sidewall and degree of orientation in the final package. These physical parameters can influence the package transmission rate by a factor of three or more. For the above reasons, developing a package is an inexact science. It is generally recommended that theoretical calculation be verified by shelf studies done under typical manufacturing and storage conditions. Often packaging technologist can develop advanced or accelerated testing procedures to predict actual long-term package performance. Often finished containers or sheet are sent to independent labs for Mocon gas and vapor transmission testing. Although Mocon testing can be expensive, often the data aids in optimizing the final product design, ultimately lowering the final product cost.