Barrier Films

Barrier in packaging could be defined in a very simple way as the ability to stop or reduce the exchange of elements such as oxygen, nitrogen, CO2, flavors, aromas, water vapor or even ultra-violet between the inside and outside of the package. However, absolute barrier may be impossible to achieve and probably a more accurate way to refer to barrier in packaging will be Controlled Barriers.

Many products can be classified as barrier films and the reason why they are classified is different for many sectors of the flexible packaging industry and even regions around the world. 

Some materials or structures are identified as “Passive (Functional) Barriers” while others are known as “Active Barriers”(1).

• Passive barriers: One (i.e. glass) or multiple layers preventing the migration of elements or substances to the food.
• Active Barriers: Those systems that are designed to extend the shelf life of a package by releasing or absorbing substances in the package or its environment. Structures including Oxygen scavengers are good examples of these systems. These technologies still in development and some legal issues exist specially in Europe related to its use for packaging.  Italy in particular according to European reports, does not allow the use of

The main objective however when designing barrier films is to control their permeability to elements such as those mentioned before. 
Other useful definitions as per ASTM D1434 are:

• Gas transmission rate: amount of a given gas passing through a unit of the parallel surfaces of a plastic film in a unit time under certain conditions of temperature and pressure at both sides of the film.
• Permeability is the product of the permeance and the thickness of the film.
• Permeance is the ratio of the gas transmission rate to the difference in partial pressure of the gas on both sides of the barrier material.

ASTM D3985 has defined and established procedures to determine the gas permeability characteristics of films but as mentioned before, defining specifically barrier films could be difficult because the different variables involved on them.
Following those procedures different levels of oxygen barrier are commercially available and help to establish guidelines to differentiate the film structures.  However it is necessary to clarify that the criteria to classify the levels of barrier vary from one source to another depending on the conditions and references to compare with.  As an example PCI Films Consulting has presented the following classification: 

Very High Barrier

Maximum permeation:  1 cm3/m2/day (0,06 cm3/100 sq in/day/atm)

High Barrier

Permeation between 1-5 cm3/m2/day  (0,06 – 0.31 cm3/100 sq in/day/atm)
Some of the materials and general structures providing this level of barrier are: 
Polyolefins/Tie/EVOH/Tie/Polyolefins
PVdC/PET
PET/Adhesive/MXD6 PA
Foil
SiOx coatings

Medium Barrier

Average permeation:  6-100 cm3/m2/day/atm  (0.375-6.25 cm3/100 sq in/day/atm)
Materials providing this level of barrier are: PVC, PET, PA, Met BOPP

Low Barrier

Permeation above 100 cm3/m2/day/atm (6.25 cm3/100 sq in/day/atm)
PE and PP are typical materials providing this level of barrier.
The previous parameter have been established following ASTM D 3985 at 23 degrees Celsius, 0% relative humidity, no extra pressure used – just the normal differentiated pressure between the two sides of the film, while in the test chamber.

For Moisture barrier (WVTR) the following classification has been proposed:

Very High barrier: Lower transmission than 1 gm/m2/24 hr
High barrier: between 1 and 5 gm/m2/24 hr
Medium barrier: between 6-100 gm/m2/24 hr
Low barrier: over 100 gm/m2/24 hr

These parameters followed ASTM F1249 at 38 degrees Celsius, 90% relative humidity and the normal differentiated pressure between the two sides of the film, while in the test chamber.

However, these criteria will depend on the specific conditions used to obtain the permeation values.  A structure offering medium barrier under certain conditions may not guarantee the same protection under different relative humidity and temperature.  It is critical and very important to list those parameters and try to simulate the real conditions to which the structure or package will be exposed.

Product Specifications

Product specifications will depend on the final application and how demanding are the conditions at which the package will be exposed.
A complete discussion of the different factors involved on the packaging process, transportation, storage and type of product the films will be protecting,  is necessary to determine the correct film specifications.

Film Extrusion Technology

Coextrusion or lamination became the ideal processes to combine different materials in one structure and the costs involved will determine when to use either one.  The trend during the last years has been to increase the number of layers in coextrusion. However, lamination still being an important process since many of the coextruded films are being laminated afterwards to others such as PET or BOPP for different purposes.  Some of them include protecting the graphics when the film is printed, increase the appareance of the package and increase stiffness of the structure. These films at the same time increase or complement the barrier properties of the coextruded ones.
Debates and inquires about what is better if 5, 7 or 9 layers still occurring. 
Barrier films can be produced by blown or cast film extrusion process and also different arguments exist to support the convenience of using one or the other.  Here are some characteristics of each process:

Blown vs Cast Barrier Films

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