Cellulose nanocrystal/amino-aldehyde biocomposite films

Abstract

From the suspensions of cellulose nanocrystals (CNCs) derived from cotton and flax by acidic hydrolysis, transparent and smooth films were produced with different plasticizers and an amino-aldehyde based cross-linking agent in a wide composition range by a simultaneous casting and wet cross-linking process. The effect of cross-linker concentration on the optical and tensile properties and on the morphology of CNC films was investigated by various measurements. The interaction of films with liquid water and water vapour was also characterized by water sorption and water contact angle as well as performing a sinking test. Cross-linking improved the transparency, reduced the porosity and surface free energy, and prevented the delamination of CNC films in water at a concentration of 10% or higher. The surface of CNC films is basic in character and has an electron donor property. The CNC/amino-aldehyde films had a high tensile strength (45 MPa) and modulus (11 GPa).

Heat-induced changes in cellulose nanocrystal/amino-aldehyde biocomposite systems

“…Furthermore, evaporation of the water content and elimination of glycerol can also contribute to an additional increase in porosity and as a result, to a more pronounced degradation of the glycerol-plasticized films. Furthermore, the cross-linking agent can fill in the porous part of CNC films, decreasing the porosity [19]. Thus, the conclusion is that the heat-induced yellowing of the CNC-based films with higher porosity was more favourable than those with a denser and less porous structure.…”

“…The rate of process depends not only on the AA concentration, but surprisingly on the type of plasticizer. It was proven that at low concentration (10%), AA acts "only" as a plasticizer [19], softening the films, as well as filling in the voids within the aggregates of CNCs. At a concentration of 30%, however, AA can wrap the CNC nanocrystals like an envelope and improve their interaction by chemical cross-linking.…”

“…The presence of cross-linking in CNC films was proven indirectly by several methods. One of them revealed that water can disintegrate the composite films at lower cross-linker concentration (10%), whereas at higher AA content the films retained their original film structure [19]. Similarly, the fact that CNC-based films retained their structural integrity in water provided evidence for the presence of cross-linking between the CNC whiskers [30].…”

“…Additionally, incorporation of amino-aldehyde compounds such as melamine-formaldehyde [18] and urea-formaldehyde (e.g. dimethylol dihydroxy ethylene urea, DMDHEU) [19] into the nanocellulose-based thin films was reported.…”

“…Cellulose nanocrystals-DMDHEU-based films-comparable to wood coating systems-possessed excellent characteristics, like high tensile strength (up to 45 MPa) and modulus (9)(10)(11), high density (ca. 1.35 g cm -3 ) and low water sorption [19]. Since AA compounds as finishing agents can improve the thermal stability of cotton fabric, we assumed them to have a similar effect in improving the thermal stability of CNC-based films by cross-linking.…”