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Monday, September 28, 2015

Why the Cobb test provides insufficient data

The Cobb size test measures the amount of water a specified area of paper will absorb in a given length of time.

Why does the COBB test (ISO 535 & TAPPI T 441) often provide insufficient or even incorrect information for predicting the behavior of paper/board in the converting process? The below schematic examples show the reason: 

1. Sizing distribution in z-direction: sample 1 and sample 2

high – surface sizing – low
low – internal sizing – high
high – surface sizing – low

2. Water absorption speed into the two different samples (penetration depth over time)

3. Cobb test (water penetration depth = volume over time) 

Cobb 1 ≈ Cobb 2
= wrong!

Why can't the Cobb test give this kind of information? The Cobb test gives an average indication about the surface sizing plus internal sizing. No information about surface pore structure or/and surface sizing is possible.

How is this important information about surface and internal properties attained? The Emtec PDA is capable of gathering this information.  In a test cell, the PDA contacts a paper sample with liquid. From the moment of contact, low-energy high-frequency ultrasound is transmitted through the sample in the z-direction.  While the paper absorbs the liquid, the attenuation of the ultrasound undergoes characteristic changes.  The ultrasonic signals are received by a high-sensitivity sensor for processing in the PDA.C 02 from where they are transmitted to a personal computer and displaced as an intensity vs. time diagram.

Example: testing results (time point MAX represents the surface hydrophobic/surface sizing)

MAX (1) >> MAX (2)
= right!

From the shape of the curve the following conclusions may be derived:
  • All quality parameters of paper and especially those surface properties which influence the wetting and absorption of an appropriate test liquid.
  • Quality parameters of liquids that may be characterized by means of a test paper.
  • Converting behavior of industrial paper / paperboard grades and process fluids.

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