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Monday, April 21, 2014

Smoothness Question


Recently, a customer asked the question: If the smoothness test result is a high number like 80, does it mean that the paperboard is more open or closed?

The first thing to determine is what measurement scale the number "80" is referenced.  There is a big difference in the meaning of the test value if you are measuring surface properties or air permeance. At least the reported test value of "80", can be used to open this discussion.

Bendtsen Roughness Tester Measuring Head
There have been changes in the titles of some of the test methods that measure this property; i.e., "roughness" or "smoothness".  When an instrument reports an increasing test value as the surface gets rougher, it is designated to be a "roughness tester".  Examples of this would be the Sheffield method and the Bendtsen method.  When these "roughness" instruments are connected to a measuring head that uses a gravity deadweight to load a polished circular measuring land against the paper surface, with the bottom of the paper supported by a flat glass backing, the test values increase as the surface is rougher.

When these same Sheffield and Bendtsen instruments are connected to measuring heads with rubber rings, which clamp the paper and define a test area, the instruments measure air permeance (the reciprocal of air resistance), Since your question made a reference to the paperboard as being "open or closed", I mention the air permeance topic because papermakers commonly reference the terms "open or closed" to the relationship of airflow that can pass through the paperboard.  A closed sheet allows less airflow to pass through than an open sheet, under the prescribed test parameters.

Since you are testing paperboard, another topic that frequently arises is the influence of highly porous paper on the surface air-leak test measurements. If the test value of "80" is either a Sheffield or Bendtsen surface roughness test value, perhaps you are testing a coated board, or a calendared board grade. In order to get a surface test value as low as 80 on a paperboard grade, most likely there is not a significant amount of air flow through the sheet, as compared to many of the rougher linerboard packaging grades.

Bekk Smoothness Tester Measuring Head
To finish the discussion, there is an air-leak smoothness tester, the Bekk tester, which gives an increasing test value as the surface gets smoother.  In this test, the reported test value is the time it takes to "leak air into the system" to reduce vacuum intensity, and the leak occurs between the paper surface and a polished flat glass plate.  Rough paper, which leaks air at a faster rate, takes less time to break the vacuum than a smooth paper surface. This same instrument is sometimes fitted with rubber rings to measure air resistance.

In addition to letting us know just what instrument and technique the "80" value comes from, it would be interesting to know the end use of your product.  The surface properties, and air permeance properties are measured for different reasons.