Edge Effect problem can be simply solved by using a special test cell designed to measure coated paper materials.
In its continuous effort to improve barrier properties of packaging materials with green packaging initiative, the company’s R&D group found great barrier coating formulas to make their paperboard product a better barrier for various packaging applications.
However, while assessing their coatings’ barriers OTR and WVTR properties, they found that it difficult to obtain consistent test results; the results are season dependent. It seems, the coating barrier became better in any drier days, but worse in humid days. When this happens, it causes frustration for engineers who develop coatings for the product. They spent a lot of time looking into coating formulas, as well as coating procedures, but the puzzle remained unsolved. Consequently, it wasted a considerable amount of time.
The challenge the company encountered is called “edge effect.” Edge effect occurs when permeant (e.g. air and/or moisture from ambient environment) diffuses or leaks through the edge (cross section) of a test sample, such as coated paper, coated paperboard, or laminate, and influences the test results. Typically, edge effect taints test results when ambient air or moisture increases or dilutes the desired test gas gradient. Whereby, the test results become environmentally dependent (Fig 1.).
Fig 1. Illustration of Edge Effect
In order to solve this challenge widely affecting the paper and paperboard industry, MOCON designed a special Edge Effect Test Cell. The theoretical concept of the design is described in the following setup:
OTR of laminated 1mil PET/paper/1milPET is used to simulate polymer coated paper
Known 1mil PET’s OTR = 62 cc/(m2*day). Paper is porous material and its OTR is near ∞
Theoretically the lamination should have a total OTR:
OTR total = (1/62 + 1/∞ + 1/62)-1 = 31 cc/(m2*day)
With regular test cell, result is OTR = 12.7 cc/(m2*day). It’s wrong due to room air gets into paper layer from the edge of lamination.
With Edge Effect Cell, result is OTR = 31cc/(m2*day). This matches the theoretical result.
The Edge-Effect Test Cell (Fig 2.) eliminates edge effect by sealing off the edge during testing to prevent oxygen and/or moisture from migrating into or out of the test cell via cross section of the test samples. This ensures more precise and repeatable data on transmission rates through the material, not the edge.
It can be used for testing samples up to 125 mil (1/8”) or 3.175mm thick. There are two types of MOCON Edge Effect Cells:
One type is designed to test OTR and/or WVTR with precise relative humidity (RH). This type has two small holes for test gas inlet and outlet in the test gas chamber.
The other type of Edge Effect Cell is designed to test WVTR with 100% RH. This type has one small hole to vent pressure in the test gas chamber.
For your test, make sure you specify the appropriate Edge Effect Cell for testing at precise RH or 100% RH.
When used with OX-TRAN® Model 2/22 or PERMATRAN-W® Model 3/34, Edge Effect Cells provide accurate and consistent data so customers can make better, more informed decisions about the packaging materials that will protect their products.
The following test results illustrate the variability in WVTR using a standard cell compared to an Edge Effect Cell.
Product: Coating501/Paperboard D
Obtaining accurate transmission rate results enables companies to select the most effective coatings and lightweight packaging that reduce costs and deliver desired shelf life while protecting the OEM brand contents.