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Monday, November 24, 2014

A Story from the Testing Lab: Can Opacity be sided?


It is possible to achieve a measured opacity on one side of a sheet of paper which is different than the measured opacity on the other side of the same sheet.  Differences are usually very small e.g. 0.5% or less from one side to the other.  However, there are instances when the values could be as different as 2.0%.  This normally occurs in papers which are heavily coated only on one side.

The most commonly used methods for the measurement of opacity address this issue slightly differently:

TAPPI T 425, Opacity of paper (15/d geometry, illuminant A/2°, 89% reflectance backing and paper backing)
There is a note in the Procedure section that says, "Usually neither the side nor direction of the grain of the paper makes any significant difference. If either effect exceeds 0.2, report each side and/or direction separately."

TAPPI T 519, Diffuse opacity of paper (d/0 paper backing)
The method instructs measurements to be made on the felt side and then the wire side.  The report section says, "If the measured opacity from each side differs by 0.5% or more, report the opacity for each side separately."

ISO 2471, Paper and board - Determination of opacity (paper backing) - Diffuse reflectance method
The procedure actually includes directions to measure both sides.  There are instructions to "Calculate the mean opacity for each side and the standard deviation.  If the differences between the two means are greater than 0,2%, the sides should be identified and the results reported separately.  If the difference is equal to or less than 0,2%, the overall average shall be reported."