Tuning physical, mechanical and barrier properties of cellulose nanofibril films through film drying techniques coupled with thermal compression

Cellulose nanofibrils (CNFs) have already been proved to be a potential candidate as one of the next-generation renewable and sustainable packaging materials. However, the mechanical and barrier properties of CNF films are not yet up to the mark for certain applications, especially at higher relative humidity. Those properties can be controlled by the degree of fibrillation of fibers and drying methods of films. Here we prepared CNF films from CNF suspensions with two different degrees of fibrillation- standard CNF (90% fine) and high-fine CNF (97% fine) by casting and filtration. These were dried in four different ways (air, oven, heat gun, and hot press drying) to better understand how these methods affect the physical, mechanical as well as oil, water vapor and oxygen barrier properties of the films. The CNF films made by hot press drying showed the highest tensile strength (98.82 MPa) and lowest water vapor permeability (13.91 g.mm/m² day kPa). Hot press compaction on the dried films further improved the tensile strength by 13.1% and reduced the water vapor and oxygen permeability by 22.3% and 43%, respectively. The average value of oxygen permeability after hot press compaction was found to be 403.2 cc µm/m² day atm, which can be considered as high oxygen barrier at 80% relative humidity. All prepared films showed maximum oil resistance value with kit number ‘12’, regardless of their preparation techniques. The result of folding a representative CNF film showed that the CNF film retained its oxygen barrier properties after a single line folding, but failed after two crossline folding.