The Effect of Pinholes in Foil

There are a few generalizations that can be made about pinholes and aluminum foil:

  • Pinholes exist in thin gauge foils
  • Individual pinholes in the foil layer do not have a significant effect on barrier properties.
  • Flexing will increase the number of pinholes in the foil layer and decrease the barrier.
  • Pinholes in the foil layer are not the same as through-holes in the composite.
  • 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 Typical Pinhole Count/ft2
    0.00025 41
    0.00030 20
    0.00035 12
    0.00050 6
    0.00070 1
    0.00100 and above 0

    Individual 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).

    Addendum I. Calculating Barrier Properties of Foil Composites with Pinholes:

    WVT or O2T: The water vapor transmission or oxygen transmission through a pinhole or fracture in the foil layer (only) of a foil laminate or composite can be determined using formula (1a) or (1b) respectively.

    the-effect-of-pinholes-in-foil-formula-01 Ingress-through-seals-formula-02The overall water vapor transmission or overall oxygen transmission of a foil laminate or composite package which contains several pinholes or fractures in the foil layer (only) can be determined by using formula (2a) or (2b) respectively.

    Ingress-through-seals-formula-03 the-effect-of-pinholes-in-foil-formula-04WVTR or O2TR: The water vapor transmission rate or oxygen transmission rate of a foil laminate or composite package which contains a pinhole or fracture in the foil layer (only) can be determined using formula (3a) or (3b) respectively.

    the-effect-of-pinholes-in-foil-formula-05 the-effect-of-pinholes-in-foil-formula-06Likewise, the overall water vapor transmission rate or overall oxygen transmission rate of a foil laminate or composite package which contains several pinholes or fractures in the foil layer (only) can be determined using formula (4a) or (4b) respectively,

    the-effect-of-pinholes-in-foil-formula-07 the-effect-of-pinholes-in-foil-formula-08

    where

    Ap =
    The inside surface area of the package for the foil laminate being studied (square inches)
    Ah =
    The area of the pinhole or fracture in the foil layer only  (square inches)
    SAh =
    The total area of all pinholes or fractures in the foil layer only of a package (square inches)
    O2Th =
    The oxygen transmission through a pinhole or fracture in the foil layer (only) of a foil laminate or composite (cc/day)
    O2Tp =
    The overall oxygen transmission of a foil laminate or composite package which contains pinholes or fractures in the foil layer (only) (cc/day)
    O2TRh =
    The oxygen transmission rate of a laminate or composite material through a pinhole in the foil layer. The oxygen transmission rate is determined for the composite materials assuming no foil is present (cc/100 in2/day)
    O2TRp =
    The oxygen transmission rate of a foil laminate or composite package which contains a pinhole(s) or fracture(s) in the foil layer (only) (cc/100 in2/day)
    WVTh =
    The water vapor transmission through a pinhole or fracture in the foil layer (only) of a foil laminate or composite (g/day)
    WVTp =
    The overall water vapor transmission of a foil laminate or composite package which contains pinholes or fractures in the foil layer only (g/day)
    WVTRh =
    The water vapor transmission rate of a laminate or composite material through a pinhole in the foil layer. The water vapor transmission rate is determined for the composite materials assuming no foil is present (g/100 in2/day)
    WVTRp =
    The water vapor transmission rate of a foil laminate or composite package which contains a pinhole(s) or fracture(s) in the foil layer (only) (g/100 in2/day)

    Note: the above formulas do not apply and cannot be used if the pinhole goes all the way through the laminate or composite (through-hole). These can be evaluated using a dye test (SPMC 004) or through internal pressurization (ASTM F2096 or SPMC 005).

    Example Calculation:

    Assumptions:

    • Foil laminate of 48 ga PET / 0.001″ Foil / 0.002″ low density polyethylene
    • 6 pinholes in the foil layer each 0.005″ x 0.010″ in size
    • WVTRh = 0.8 g/100 in2/day
    • Package size: 6-1/2″ x 12″ ID Chevron Pouch

    Calculation:

    the-effect-of-pinholes-in-foil-formula-09

    Addendum II. The Effect of Ingress through the Seal on the Barrier Properties of the Package.

    WVTR or O2TR: The Water Vapor Transmission Rate or the Oxygen Transmission Rate through the seal can be determined using formula 5(a) and 5(b) respectively.

    the-effect-of-pinholes-in-foil-formula-10 the-effect-of-pinholes-in-foil-formula-11WVT or O2T: The Water Vapor Transmission or Oxygen Transmission through the seal on a package can be determined using formula 6(a) and 6(b) respectively. This assumes that the same material has been used on both sides of the package.

    the-effect-of-pinholes-in-foil-formula-12 the-effect-of-pinholes-in-foil-formula-13

    where

    WVTRs =
    The water vapor transmission rate through the sealant layer of a seal of a given width (g/100in2/day)
    WVTRm =
    The water vapor transmission rate of the sealant material (g/100in2/day/mil)
    WVTs =
    The water vapor transmission through the seal of a package (g/day)
    O2TRs =
    The oxygen transmission rate through the sealant layer of a seal of a given width (cc/100in2/day)
    O2TRm =
    The oxygen transmission rate of the sealant material (cc/100in2/day/mil)
    O2TR =
    The oxygen transmission through the seal of a package (cc/day)
    Ws =
    The width of the seal (inches)
    ts =
    The thickness of the sealant of a laminate or composite material (inches)

    Ps =
    The inside seal perimeter of a package (inches)

    Example Calculation:

    Assumptions:

    • Package Size: 4″ x 7″ I.D. rectangular pouch
    • Material used to construct the pouch: Foil laminate of 48 ga PET/0.001″ Foil / 0.002″ LDPE with no pinholes. Same material used on both sides of the pouch.
    • WVTRm of LDPE = 0.9 g/100in2/day/mil
    • Seal Width (Ws) = 0.375″

    Calculation:

    Ingress of Water Vapor through the Seal:

    the-effect-of-pinholes-in-foil-formula-14