Cellulose Nanofibrils as Paper Additive and Coating Material: Properties, Distribution and Interaction Effects

Abstract

Cellulose is well-known as the most common bio-polymer on earth. In the cell wall of most plants, it is organized in nanoscopic structures with interesting properties for a wide range of applications, among them as an additive to paper and board - or as an applied coating. One socioeconomic perspective on these applications of the nanomaterial is associated environmental benefits; cellulose nanofibrils (CNF) as an additive to paper and board can improve tensile properties, enabling not just higher filler levels, but also increased potential fraction of recycled fibers in the product. As a coating, given its excellent oxygen barrier properties, CNF also has potential as a replacement for plastics in certain food packaging applications or, conceivably, aluminum in liquid packaging board.

In the current work, we have addressed several questions pertaining to these potential applications and some fundamental questions of interest regarding the properties of CNF as individual fibrils and as fibril aggregates. Central questions in this current work, and brief answers later expounded upon, are;

• Do fibril properties like crystallinity affect CNF film ductility?

– Reducing crystallinity index (CI) is shown to correlate with an increased yield point and decreased Young’s modulus.

• How does CNF behave within the paper or board matrix?

– CNF appears to coat the surface of the furnish fraction with which it was added or premixed.

– When filler particles are encased in CNF, it appears to form hollow capsules encasing the particles, and not - as one would expect - adhere very tightly.

• Can CNF be successfully traced and studied within a paper matrix?

– Using high resolution FIB/SEM, this has been demonstrated.

– Attained resolution: 13×16×15 nm using back-scattered electron (BSE) and 7×9×15 nm for secondary electron (SE).

• Are there benefits associated with premixing CNF with different furnish components?

– Some benefit can be seen from premixing CNF with the filler fraction.

– The effect depends on filler levels and weight fraction of added CNF.

• Can CNF be applied as a barrier coating in a roll-to-roll scenario?

– Highly fibrillated CNF, oxidized or not, can be used in a roll-to-roll coating situation.

– Rheology plays a role in quality of applied coatings.

– The mode of adhesion to the base board appears to be exclusively Physico-chemical, not mechanical.

The above list gives an overview, a somewhat more expansive summary follows. Fundamentals: Crystallinity. By treatment with liquid anhydrous ammonia CI of cotton linter was reduced prior to homogenization and fibrillation. Films cast from the resulting CNF suspensions were shown to have a lower Young’s modulus and higher yield point, although crystallinity did not appear to have a significant effect on strain at break. Films of CNF treated with liquid ammonia in the same way shrank, cockled, lost mass and showed a strong increase in strain at break in addition to reduced Young’s modulus and increased yield point. Films treated with ammonia saw very large changes in mechanical properties. We conclude this is due to the cockling and shrinkage of the film itself, and not due to changes in crystallinity.

Fundamentals: CNF in a paper matrix. FIB/SEM dual beam system was employed to perform investigations of CNF distribution within the paper matrix. We found the attainable resolution appears adequate to qualitatively trace CNF in volumes as large as 103 μm3 using SE. While BSE delivers reduced contrast and resolution, reduced artefacts due to surface unevenness and other effects made quantitative analysis more easily achieved using this higher-energy signal. Achieved resolutions were approximately 13×16×15 nm using BSE and 7×9×15 nm for SE.

Fundamentals: Swelling of individual fibrils. Using atomic force microscopy (AFM) in liquid media (de-ionized water) and air individual fibrils were imaged in air and, later, water. As the same area was imaged the swelling of individual fibrils could be studied in detail. After 30 minute submersion, a mean swelling of 48.9 % was observed, with a standard deviation of 27.8 %.

CNF-Capsule Formation. Super-resolution investigation of paper with added CNF shows the CNF appearing to form hollow capsules around filler particles. We hypothesize this is due in part to non-isometric drying. We believe it is conceivable that the surface of these capsules is drying faster, having a lower moisture content at any given time than its interior, leading to a force balance during drying favoring separation over tight conformation to the encased particle. Further investigation into the formation of such complexes by attempting to reproduce the phenomenon was begun in 2017 and continues into 2018.

CNF-Premixing Strategies: Effects. We premixed mechanically produced CNF and retention aid with the long fiber fraction (LFF), the filler fraction or added them straight to (otherwise complete) furnish. We found that for high filler levels and high CNF addition levels there is a beneficial effect to dewatering times and tensile strength. Air permeability was affected similarly to dewatering times. Optical properties were not found to differ between the chosen strategies. Scanning electron microscopy (SEM) investigations of paper surfaces appears to suggest that filler particles adhere to the surface of the fraction to which CNF and retention aids were premixed.

CNF as a barrier coating: Roll-to-roll application. Three qualities of CNF, TEMPO-oxidized CNF (CNF-T), carboxymethylated CNF (CNF-C) and mechanically produced CNF (CNF-M), all with and without added carboxymethylcellulose (CMC), were applied as a coating to a rough, recycled quality board. Using a single applied coating, an even, smooth surface was achieved with air resistance correlated with CNF quality, coating thickness and CMC addition. We found that lower fibril diameter correlates with improved coating. For the CNF qualities with lowest mean fibril diameter addition of CMC reduced viscosity, and appears to have created a smoother coat. We showed that SEM can be used to quantitatively investigate coating quality and gain insight into the foundation for observed air resistance values.