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Combining Really Low Basis Weight Boards

By AICC Staff

December 4, 2017

This is like déjà vu. It was in the early ’90s when the industry was moving from Mullen-based linerboards to the new lighter and stronger compression-based grades (then 35 and 56#) that are so common today. Well, it’s time to review the past and make corrugator and starch and process adjustments going forward.

It was almost 30 years ago that the industry was struggling to combine the new lighter-weight, “high-performance” liners and mediums versus the historical 42# and above grades. The new containerboards were made in a different way. And so today, there is another technology breakthrough in which some mills are targeting to replace 35# with 33# and below. These boards are also different from the conventional 35# that we have all become too comfortable with running through the corrugator. So, it’s back to basics in a new world.

TAPPI Test Method T-821 sets the protocol for using a pin apparatus based on the number of flutes per foot and a crush tester to measure quantitatively the strength of a bond. However, there are many other aspects to consider. Most pin jigs have to be custom made because flutes per foot are not what they used to be; we have a dozen more flutes to consider than A, B, and C; and flute profiles have changed. The old numeric values that we targeted in the past no longer serve us well.

The work in developing this standard cautioned against using this method for lightweight (below 33#, which I would expand to 35#) because the liners will bend around the pins and deliver false results, as much as 29 percent below the actual strength. So, what is a quality process team at a corrugator to do? I reached out to two respected associates in Europe to ascertain how they know they have good bonds. Since they have been making these low-grammage linerboards for more than 15 years, this was the place to go for guidance.

Working with some combiners recently, those who perform pin adhesion testing have been concerned about low values. So, what I heard from across the pond follows: One said that they are no longer conducting pin testing, yet there is an active FEFCO method much like our TAPPI one. The other associate said that they monitor bonding by glue-line widths at the soak tank and fiber pull by board separation by a skilled operator who has the “feel” for the bond and can also observe and analyze the fiber distribution between the liner and the medium. Contact me if you would like to learn more about this.

One of the additional concerns has been the thought that lower pin results could mean lower ECT and box performance. There is a very small correlation between increasing pin adhesion and increasing ECT, and it is maxed out at lower levels than before.

What is critical to achieving the best bonds with low grammage (18–23# now available in North America) and high-density sheets is to run with lower-​starch solids, maybe with an additive, and certainly with lower corrugator zone temperatures than midweight combinations. It is imperative that we drive the moisture in the adhesive toward the liner, as they generally have about 7 percent moistures and mediums around 9 percent. These are averages. Starch penetrates only 1 mil into the surface of the liner, so it’s not a matter of going deeper. I suggest that you coordinate with your paper suppliers’ technical representatives and starch and additive suppliers. You do not need to suffer alone. Get the advice of those who should know how to guide you to better bonds and stronger boards.

I have recommended over the years the use of a Chalmers DST to measure medium degradation in the single facers and combined board crush at other points in the corrugating and converting operations. While I am not privy to any research that relates DST test results to bond strength, the machine will tell you how well you have maintained the integrity of the board, which might include the bond.

Editor’s note: Used with permission from Kruger.


PortraitRalph Young is the principal of Alternative Paper Solutions and is AICC’s technical advisor. Contact Ralph directly about technical that impact our industry at askralph@aiccbox.org.