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Brentwood Plastics Blog

Barrier Films depends on the context

Posted by Joel Longstreth on Fri, Sep 07, 2012 @ 01:12 PM

dreamstime xs 25468564Barrier films and barrier packaging definitions depend on how the film is being used.

The term "barrier" is often used loosely ranging from a moisture vapor barrier in construction, barrier to bodily fluids in surgical drapes or fluid collection, controlled permeability in fresh cut packaging to measuing the amount of oxyen which passes across 100 square inches in 24 hours in terms of a few hundreds of a cubic centimeter.

When describing a barrier to your film supplier, it is important to know where you are in this continuum or what caliber of vacuum you need.  Oxygen Transmission Rate, or OTR is the most common benchmark.  Moisture Vapor Transmission Rate is MVTR for short.  CO2TR is not often referenced, but it means Carbon Dioxide Transmission Rate.   The ratios between these are difficult to manipulate; this poses restrictions on design.  I stopped testing for CO2TR over 16 years ago when i noticed a constant ratio of about 4:1 between carbon dioxide (4) and oxygen ( 1 ) permeability.  This is counterintuitive because the CO2 molecule is much larger than the oxygen molecule, right ?  Turns out CO2 is more combinable than oxygen ( remember high school chemistry and valences ? ).   MVTR depends on the solubility of resins selected.  Nylon and EVA are water soluble and provide pathways to conduct H20.  An example of practical application - I used to add EVA to lettuce film to wick out excess moisture and minimize micro loads.

For an extreme barrier usually for meat packaging, the traditional test method is ASTM D 3984.  This method measures a sample about the size of a thumbnail and interpolates to 100 square inches in 24 hours at specified relative humidity and temperature ( after 24 hours of conditioning, of course ). In the 1990's a test method for higher permeability films was needed for cut vegetable packaging.  The specimen size is larger.  For more information, contact MOCON or Illinois Instruments.

To achieve these low values which are expressed as maximum of 0.06 for example, there are two avenues:

EVOH; short for Ethylene Vinyl Alcohol
EVOH must always be coextruded in the core layer of a coextrusion

or

polyvinyl dichloride PVDC, a/k/a Saranex which can be coated on to substrates such as nylon, polypropylene and PET.

Either one of these structures have the same barrier film properties when laminated to a polyethylene sealant layer.

In the 1990's, the advent of fresh-cut produce demanded an entirely new way to measure precision controlled permeability films.  Fresh-cut produce needs relatively poor barriers to allow the vegetable to continue to respire.  A barrier film will rapidly induce unacceptable anaerobic conditions.  The balancing act is matching the permeability of the film to the metabolic rate of the vegetable. 

Measuring relatively high permeability rates requires a larger sampling size which cannot be accurately interpolated with a tiny sample size.

There are two applications which do not need barrier films yet continue spec out barrier films for a security blanket factor are:

Frozen food
Barrier films will not prevent freezer burn or prolong shelf life.  Take it on faith.

Vacuum packaging of inert things like tools, foam and kits.  If the item being packaged is inert, there is no concentration gradient or reason for nature to seek equlibrium.

 

note about the relationship between permeability and thickness in polyethylene:

Whether you measure by OTR or CO2TR, the permeance as it relates to thickness is not a linear relationship.  Under two mils the OTR increases dramaticaly; over two mils it flattens out slowly.

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Topics: PVDC, moisture barrier, EVOH, barrier film, saran, oxygen barrier

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