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Matter mystery: why is corrugated board weakened by humidity?

There is still mystery in an everyday material like corrugated board. Anyone with moving boxes in the attic knows they lose their stiffness, their shape. They are not exposed to rain, they are in a fairly protected environment, still they soften. Why is that? What mechanisms are at play? Can the solution be found with a synchrotron?

Lennart Salmen
Lennart Salmén inserts the paper into the environmentally controlled stretcher for exposure to the synchrotron X-ray beams.

An area of larger concern than moving boxes is transportation and storage of groceries and other goods. An area that is in constant growth, both as food is to be distributed to more and more people, and as growing purchasing power enable more and more people around the world to buy things. To avoid irreversible environmental impact, transports and packaging need to be more effective. Boxes made of corrugated board are light and protects groceries and other products well. The material is perfect - until exposed to altering humidity. The box turns soft, it can’t be stapled, fruits and vegetables get squeezed, fragile products break. At BillerudKorsnäs they produce the flat containerboard that the corrugated board is built from, that in turn is cut and folded into different boxes.

– Reducing humidity impact is the holy grail for containerboard manufacturers, says Robert Nilsson, research responsible at BillerudKorsnäs.

Invisible creep

The mechanism that will be examined in synchrotron tests has a scientific term, creep, more specifically mechano-sorptive creep. If you grab a piece of containerboard and try to stretch it, you will quickly reach a limit where nothing more appears to happen. However, if the force is maintained, the material continues to stretch but so slowly that it can only be measured with instruments. The board material creeps. At stable humidity, even when it is very high, creep happens slowly. If the humidity changes periodically, for example from day to night or from one week to another, the material deforms much faster, it loses its stiffness and boxes start to collapse. Why?

New material could reduce climate impact of transports

The exact mechanisms behind creep and moisture changes within containerboard and other wood-based materials have eluded scientists for decades. Does moisture cause changes in structure at nano-scale, or is it playing tricks at molecular level? And how come it is not the humidity in itself that has effect but the changing humidity? Knowing this would make it possible to change the fibre properties to counteract the negative effects.

– Transportation of goods, and the steadily growing e-trade, are global activities with high logistic demands on all levels. Weight and sturdiness are paramount to both individual and bundling packages. As a containerboard supplier we see that the demands increase. A trend is to use the original package from the producer all the way to the buyer. If we can solve this creep problem, we can supply containerboard for packages that producers and logistics companies can depend on under all conditions. The new material would outperform heavier or more complex and thus expensive solutions, explains Robert Nilsson.

Synchrotron generates spreading patterns for analysis

X-rays are great to investigate structure in materials, as anyone who has been to the dentist can confirm. With the extremely intense X-rays generated at a synchrotron, it is possible to use techniques that are not possible in a regular lab.

– The smaller X-ray scattering instruments we have used so far have less strong radiation. A test requires exposure times of fifteen minutes to one hour. They merely allowed us to compare the structure of the fibres before and after the creep phenomenon, and only in vacuum conditions, says Lennart Salmén, senior scientist at RISE and continues.

– To solve our mystery, we need to measure a process that only occurs during tenths of a second, in an atmosphere with varying humidity. We can accomplish this with the synchrotron. The synchrotron generates X-ray lights of such high intensity that the smallest fibre building blocks, depending on the sample, can be examined in minutes or seconds, or even below. In extreme cases molecular reaction has been observed in real time. Our samples will spread the X-rays in different directions. When the sample changes, so will the spreading pattern. By recording and analysing these changes, we can draw conclusions about what happens inside the material down to the molecular level. Our goal now is to establish that we see changes related to creep. It would be a dream come true to finally explain this obtrusive mechanism, says Lennart Salmén.

Basic research that attracts enterprises

The synchrotron experiments are performed as part of the Bioeconomy Research Programme at RISE. Projects are to a large extent funded by industrial partners. There are suppliers that want their bio-based raw material to be of better use or used for products of higher value. There are companies that want to switch from using fossil raw material to renewable bio-based material and there are companies that want to increase their use of sustainable raw material.

– It is very exciting to walk this road together with BillerudKorsnäs. Many companies would categorise synchrotron tests as expensive basic research and thus hard for them to fund. With a long-term research partnership like this, where a company sees a future value of the research, ground-breaking research can be performed and new materials can be brought out for commercial exploitation. Materials that are more efficient and reduce environmental footprint, says research leader Claes Holmqvist from RISE.

BillerudKorsnäs is very active in research but with a history of enhancing existing processes and products. With the dive into material behaviour on nano and molecule scale they expect to break new ground.

– Research of today does not look like research did a decade ago. This disruptive material exploration would have been difficult without the close co-operation with RISE. Our combined knowledge about wood, cellulose and fibres at the smallest scales makes these synchrotron experiments possible and economically viable, finishes Robert Nilsson.

The ambition of the synchrotron tests performed in April was to set references in material and different fixed loads. The next session will be run in May. The research is being carried out with support from Vinnova and, in addition to RISE and BillerudKorsnäs, includes InBev, Georgia Pacific, Metsä Board, Miller Graphics, Mondi Group and Stora Enso.


PARTNERS

Research and funding: BillerudKorsnäs, AB InBev, Georgia Pacific, Metsä Board, Miller Graphics. Mondi Powerflute, RISE and Stora Enso.

Additional funding: Vinnova

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Astrid Glasenapp

Senior Project Manager

+46 10 228 44 98
astrid.glasenapp@ri.se

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