Acetylated nanocellulose as a reinforcement for cellulose acetate

Abstract

Current research has shown that the use of cellulose nanofibres (CNFs) can be used in the reinforcement of biodegradable plastics. Problems arise with certain polymers, such with cellulose acetate (CA), due to agglomeration of the nanofibres. Hydrogen bonds will form between cellulose fibres in a process called hornification, which results in a poor-quality material and a waste of CNFs. It is thus necessary to modify the fibres such that they disperse into the CA matrix. The partial acetylation of CNFs can achieve this without destroying the fibrous network. However, there is a problem with this solution. The fibres when made can contain above 75 % water and have to remain in suspension or else irreversible agglomeration will occur. A recent discovery shows that xanthan gum can prevent this from happening by acting as a capping agent for the fibres preserving the fibrillated network during drying. Since the acetylation process can be affected by the presence of water this allows for a simple solvent swap to a solvent that does not affect the reaction. It is first necessary to demonstrate that acetylated CNFs will perform as expected without complicating the methodology. CNF acetylation was carried out through the use of acetic anhydride after the nanocellulose solvent was swapped for acetic acid. Azeotropic distillation was used to ensure that as much water as possible is removed from the CNF. Acetylation was confirmed through FTIR analysis quantitatively compared the peaks from the double bonded oxygen in the acetyl group. In testing, various CNF contents and degrees of acetylation were used and in total sixteen different combinations were incorporated into CA films which were made through the solvent casting. All films were plasticized to 25 % with triacetin and solvent cast in watch glasses. Microscopy images of the films revealed that acetylation of the CNF can reduce the agglomeration of the fibres by 400 % in the CA matrix, a clear indication that acetylation limits the hornification of the CNF. TEM imaging also shows the improved dispersion of the fibres iii in deliberately collapsed CNF samples. Other experimental procedures showed that the modified films will have improved optical transparency and mechanical properties. UV-vis showed an acetylated CNF had 35 % less absorbance compared to its unmodified counterpart. The effect on the Youngs modulus from the CNF can also be increased by over 250 % through modification. Improvements in the tensile strength were limited due to CA already having a high tensile strength. Effects on the rheology and viscosity of the modified CNF still need to be researched to gain a better understanding of the process on the materials shear thinning ability. This research has demonstrated that a biodegradable fibre reinforcement material can be used in certain polymers which previously posed a problem due to poor dispersion. This opens the door for more biodegradable polymers to be used in industry without the worry of the materials not being tough enough. There is potential for further development in this area, especially around polyhydroxyalkanoates, but the ability to minimise the fibre diameter is a big step in the right direction.