Nanocelluloses, their modification and application to reinforce poly(butylene adipate-co-terephthalate) bioplastic
Creators: name: Dhali, Kingshuk
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 300604
subject: Food packaging, preservation and processing

description: Synthetic polymers, especially those derived from polyolefins, are widely used as packaging materials. However, their non-biodegradable nature and prevailing low rate of recycling means that there is a pressing need to develop fully biodegradable packaging materials to address the environmental and sustainability concerns. The development of biodegradable packaging materials is one of the most feasible ways to address this issue. Some polymers such as thermoplastic starch (TPS), poly(lactic acid) (PLA), polyhydroxylalkanoates (PHA), polycaprolactone (PCL) and poly(butylene adipate-co-terephthalate) (PBAT) are considered as alternatives  of conventional plastics.  However, these polymers have certain limitations (e.g., low tensile strength, highly permeable to water vapour and oxygen and high cost) which needs further improvement. Creating composites of these polymers reinforced with suitable additives is the most pragmatic approaches to overcome their limitations. While conventional plastic composites typically use inorganic fillers such as silica, alumina and clay, biodegradable plastics benefit from biodegradable fillers such as nanocellulose and plant derived fibres. Nanocellulose-enriched polymer nanocomposites exhibit improved mechanical, thermal, barrier and functional properties. Nanocellulose is hydrophilic due to the presence of a large number of hydroxyl groups and comes with inherent incompatibility when used in non-polar (hydrophobic) plastics. The surface of nanocellulose needs to be modified, through chemical approaches, in order to make it more compatible and ascertain its homogenous distribution in the non-polar plastic matrix. When used as reinforcing materials, adequately modified nanocellulose is expected to either preserve or improve the biodegradability of the resulting composites. Evaluation of the degradation process of nanocellulose-containing polymers (i.e., composites) under industrial composting conditions gives confidence in their viability as biodegradable packaging materials. 

In the above context, the first experimental chapter focused on extraction of nanocellulose from renewable jute bast fibers and determining their physicochemical characteristics. The multistep extraction process began with chemical pre-treatment to extract nanocellulose from the jute bast fibers and yielded a highly purified microfibrillated cellulose with a 56.5% yield.  Cellulose nanofibers (CNFs) and cellulose nanocrystals (CNCs) were isolated through mechanical and chemical processes providing yields of 55% and 28%, respectively based on the initial dry mass of untreated jute fibers. The length of CNFs extended to micron scale with an average diameter of 14.9 nm. The CNCs were needle shaped with an average width of 10.0 nm and a length of 196.5 nm. The CNCs had much higher crystallinity (75.6%) than that of CNFs (67.5%).

In the second experimental chapter, the surface of CNCs were modified using a facile and environmentally friendly method that partially substituted the surface hydroxyl groups of CNCs with an alkoxysilane, forming a polysiloxane coating. This modification was intended to impart hydrophobic properties. The coupling of the alkoxysilane with CNCs occurred through covalent bonding and physical anchoring. Surface modification of CNCs was carried out at different triethoxyvinylsilane (TEVS) loading. Up to 17.9% of silicon (Si) concentration was observed on the surface of the modified CNC (m-CNC) at 5wt% loading of TEVS. These m-CNCs preserved their nano scale size and morphology. Homogeneous and interfacially compatible composites were prepared from poly(butylene adipate-co-terephthalate) (PBAT) reinforced with m-CNCs through solvent casting.  

The research undertaken in third experimental chapter involved design of CNC containing composite plastics (PBAT as the matrix) using melt blending. Initially, the surface of CNCs was modified using vinyl silane (VS) and phenyl silane (PhS) groups. The atomic concentration of Si was found to be 12.40% in vinyl silane modified CNC (CNC-VS) and 5.41% in phenyl silane modified CNC (CNC-PhS), indicating the successful incorporation of silicon through the modification process. The findings showed that surface treatment enhanced the interfacial compatibility of CNC-PhS in PBAT matrix. The homogeneity of dispersion of these silanes modified CNCs in PBAT matrix improved the mechanical performance, barrier properties, and water repellence of the composites. Significantly lower water vapour and oxygen transmission rates were observed in these nanocomposites when compared with the pristine PBAT. The films containing 5wt% phenyl modified nanocrystals had the lowest WVTR and OTR values measuring 38.3 g m−2 day−1 and 496 cc m−2 day−1 respectively. Higher water repellence (water contact angle = 79.1°) was achieved in the films produced using phenyl silane CNCs reinforced PBAT. Compared to the modulus of neat PBAT films, PBAT/5CNCPhS showed ~58% increase in modulus and 12% increase in tensile strength through reinforcing.

The fourth experimental chapter studied the environmental fate of the PBAT-silane modified CNC composite materials and films in order to understand the extent and nature of biodegradation under industrial composting condition. The structural disintegration and mineralisation of the PBAT-CNC films were higher than that of pristine PBAT. Similarly, PBAT films reinforced with unmodified CNCs had a higher degradation rate followed by the films produced using silane modified CNCs. The rate of degradation was slowest in case of pristine PBAT. Upon 90 days, the extent of degradation was 38%, 45% and 41% for PBAT, PBAT-CNC and PBAT-CNCPhS films, respectively indicating that the reinforcing of PBAT with CNC improved PBAT’s biodegradability.
This thesis advanced the underpinning science and associated technologies towards improving the mechanical, barrier and biodegradation (composting) performance of PBAT-based bioplastics by reinforcing them with phenyl silane modified CNCs. It also contributed towards optimisation of the process of isolating CNCs from jute bast fibers and further modification of  their surface using vinyl and phenyl silanes. Thus, the outcome of this thesis contributes towards mitigating the negative impact of conventional plastics. It also contributes to the sustainable use of agricultural fibers by providing an improved and easy-to-implement pathway for value adding to jute bast fibers.