From Laboratory Processing to Continuous Processing of Cellulose Nanofibril (CNF) Sheets for Structural and Packaging Applications

Sami M El Awad Azrak, Purdue University

Abstract

First reported in the early 1980s, cellulose nanomaterials (CNMs) like cellulose nanofibrils (CNFs) have become a very attractive option to reinforce, replace, or reduce the need of oil-derived synthetic polymers. This is largely due to their inherent sustainability, abundance in nature, high crystalline contents, and ample surface chemistries possible (e.g., carboxylic acid, hydroxyls, esters, acetyls). Over the years, improved mechanical isolation processes (e.g., disk refining for isolation of the fibril-like CNFs) and acid hydrolysis processes (e.g., improved acid recovery for isolation of the rod-like cellulose nanocrystals (CNCs)) have increased industrial production capacity and allowed for CNMs to be readily accessible in both academic and industrial research settings. This, in turn, has accelerated research efforts regarding their processing into usable forms (e.g., films, fibers, coatings, etc.) which attain significantly higher properties when compared to synthetic polymer analogs. Although the reported CNM processing efforts are promising, further advances are still needed in order to galvanize the current polymer industry into processing CNMs and possibly ease their adoption in major markets like food packaging and construction. Out of the different CNM types, this work focuses specifically on CNFs and encompasses four main projects, each targeted at solving a different CNF processing related challenge or limitation. The first project addresses the challenge of processing thick CNF sheets (thickness > 100 um) through the development of a wet-stacking lamination technique. The results showed that high-strength multi-layer CNF structures with a thickness of up to ~1.7 mm can be produced. The second project addresses the challenge of bulk continuous processing of CNF sheets through conventional single-screw extrusion. The results showed that near pure CNF sheets (comprised of ~91 wt.% CNFs and ≤ ~9 wt.% of a processing aid like carboxymethyl cellulose (CMC)) can be continuously extruded and calendered, and that traditional polymer compounders (e.g., Banbury mixer) can be used to prepare highly loaded CNF pastes (~25 wt.% solids) for extrusion. The third project addresses the susceptibility of CNFs to humid conditions through the use of different chemical treatments. The results showed that the redispersion and setting behavior of CNF with adsorbed CMC can be controlled and that new crosslinks were effectively formed due to the chemical treatment. Lastly, in hopes of further improving the mechanical performance and expand the possible end uses for CNFs and other CNMs, the fourth project presents preliminary research efforts into processing fiber reinforced CNF composites (FR-CNF).

Degree

Ph.D.

Advisors

Howarter, Purdue University.

Subject Area

Analytical chemistry|Atmospheric sciences|Chemistry|Nanotechnology|Packaging|Physics|Polymer chemistry

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