Tiny fibril bridges help hold paper together, study finds
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Powerful microscopes have allowed researchers to see what exactly holds paper together. Tiny strands of cellulose known as fibril bridges seem to play a crucial role, according to a study published Thursday in the journal Scientific Reports.
Austrian researchers wanted to figure out how to make paper stronger by identifying the molecular bonds most important for connecting its dense network of cellulose fibers. Although earlier studies had revealed that many types of bonds are involved, including attractive forces between charged atoms in cellulose and adhesive forces between cellulose and liquids, scientists weren’t sure which ones predominated.
To find out, a team of materials scientists from the University of Leoben in Austria and other institutions compared unrefined paper pulp with refined pulp that had been passed through a system of rotating blades, since the latter is known to yield stronger paper. Left unrefined, the outer walls of cellulose fibers are smooth, making it hard for them to mesh together. Refining the cellulose pulp unravels the outer wall and makes the fibers hairier, so to speak, allowing them to form tighter bonds.
Using a cantilever attached to a high-powered atomic force microscope, the researchers tugged on the pulp samples — each as tiny as an arm hair — until they ripped apart. Then they photographed what happened.
The team saw many tiny fibril threads and bundles dangling from the rip site, which indicated that fibrils had bridged the cellulose fibers together, forming so-called mechanical interlocking bonds.
Compared with unrefined pulp, refined samples had more of these hanging fibril bundles, suggesting further that “fibril bundles play a crucial role in fiber-fiber bonding,” the researchers wrote.
Measurements revealed that tearing apart refined pulp fibers required about twice the force needed to rip unrefined samples. They also showed that tearing the samples caused the fibril bridges to fall one by one. “You get a picture of what happens when the paper is actually failing,” said Martin Hubbe, a materials scientist at North Carolina State University who wasn’t involved in the study.
But Hubbe was skeptical about the importance of fibril bundles in paper strength, since the researchers didn’t examine other types of interactions. “They’re only focusing on one thing,” he said, adding that earlier studies have suggested that the bonding between the hydrogen and oxygen atoms in cellulose plays “a dominant role.”
Still, understanding the damage that occurs at a microscopic level could help paper companies make stronger products, especially packaging materials, said Christian Teichert, a materials scientist at the University of Leoben who worked on the study.
“We can optimize the number of dangling fibrils now that we know they’re so important,” he said. Paper manufacturers may be able to strengthen their products by adjusting the refining process to increase the number of fibril bridges, rather than using more raw material.
The study highlights that, in most cases, damage to paper “occurs at a nanoscale,” Hubbe said. “It’s important to see what’s happening at the finest detail that can be measured.”
Twitter: @mmpandika
