What effect does a pinhole in the foil layer of a foil composite have on the barrier?
There are a few generalizations that can be made about pinholes and aluminum foil:
• Pinholes exist in thin gauge foils:
It is true that aluminum foil, even in very thin gauges, provides a total water vapor and
gas barrier if in perfect condition (no pinholes or imperfections). However, aluminum foil below 1.0 mil is seldom, if ever, perfect. As the thickness of aluminum foil decreases, the number of pinholes in the aluminum foil typically increases. Typical
pinhole counts in aluminum foil are:
|Foil Thickness (in.)||Typical Pinhole Count/ft2|
|0.00100 and above||0|
While few studies exist regarding the size distribution, inquiries with two foil vendors indicate that holes typically range in size from 10-50 μm in diameter. Small holes tend to be circular, while the largest holes tend to be oval in shape, with dimensions up to 75 μm by 200μm.
Visual identification of pinholes in foil depends upon the darkness of the viewing room and the quality of backlighting. The smallest hole visible with the unaided eye is generally about 10 μm in diameter. This assumes the foil is unsupported, i.e., has not yet been laminated to another web. Light interference effects due to coatings, other plies, and the adhesive/extrudate used to bond plies of a foil laminate can significantly reduce the ability of observing pinholes in foil,
• Pinholes in the foil layer do not have a significant effect on barrier properties:
The presence of a small pinhole or fracture or even several small pinholes or fractures in the foil layer only of a foil laminate or composite will have very little impact on the WVTR or O2TR of the material. However, if the pinhole or fracture goes all the way
through the laminate or composite, both the water vapor, gas, and sterile barriers may be lost depending upon the size of the hole.
The WVTR or O2TR of foil composites which contain pinholes or fractures in the foil layer can be determined by actual barrier testing or estimated by calculations as shown below.
If extremely high barriers are needed, ingress through the sealant may need to be considered. The effect of ingress through the sealant on barrier can be determined by calculations as shown below or by actual barrier testing.
• Flexing will increase the number of pinholes in the foil layer and decrease the barrier:
In actual use, the package may flex and pinholes may develop. Different structures have varied propensities for pinholing upon flexing. This should be taken into account when determining product shelf life.
• Pinholes in the foil layer are not the same as through-holes in the composite:
Although the eye and most vision systems cannot distinguish between a through-hole (a hole through the entire composite) and pinholes (a hole in the foil layer (only)), they have very different effects on the barrier properties of the package. Depending upon the size of the hole, a through-hole can cause the loss of water vapor, gas, and sterile barriers. As discussed above, a pinhole has a negligible effect on the barrier. Through-holes can be distinguished from pinholes via a dye test (SPMC 004) or through internal pressurization (SPMC 005).
There are two addendums to this FAQ. To download a copy of the entire FAQ and its addendums, please click here.
Last updated on 2009-01-13 12:43:14 UTC
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