Effect of pH on water durability of cellulose nanofiber-reinforced starch film

Plastic packaging has played an important role in preservation, protection, handling, and consumer convenience in several industries. Most of the commercial plastic packaging materials are made from petrochemical polymers, namely polyolefin, polyester, polyvinyl chloride, and polystyrene [1‐4]. These plastics commonly serve their single-use functions and are thrown out after use [5]. However, because of their inert nature and very slow degradation, there is a buildup of these plastics globally, along with the amount of microplastics worldwide from the plastics’ fragmentation [6‐8]. Extensive studies into the potential replacement of these conventional polymers have been explored. Biopolymers extracted from renewable biomass resources have attracted much interest for their use as single-use packaging materials, mainly because of their biodegradability and renewability [9, 10].

Cellulose and starch-based films are considered suitable for single-use packaging owing to enzymatic hydrolysis by cellulase and amylase [11, 12]. On top of cellulose and starch being the most naturally abundant biomass resources available, their ability to degrade by bacteria, E. coli, and fungus pushes their suitability as components for the production of bioplastics [13‐16]. However, their application is limited by their opacity, poor mechanical properties, and poor freshwater durability.

In a previous study, a water-stable transparent film was prepared by introducing aldehyde moieties to polysaccharides which prepared environmentally friendly cross linkers, that have been found to improve the films’ overall properties [17]. Starch films cross-linked by oxidized sucrose (di-aldehyde sucrose) [18], di-aldehyde starch [19], and di-aldehyde cellulose (~ 100% oxidized, amorphous) [20] exhibited water stability from hemiacetal crosslinking between the polysaccharides. Studies that have used modified starch to blend with (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO)-oxidized cellulose nanofibers (TCNF) produced films with high optical transparency (~ 90%) and a wet strength of ~ 300 kPa owing to the hemiacetal binding between the TCNF and starch [21‐23]. A study by Soni et al. [24], showed that through the introduction of aldehyde moieties on the cellulose nanofiber surface, the films’ wet strength increased from ~ 2 MPa to ~ 35 MPa after blending with starch. The di-aldehyde moieties on cellulose reduced marine-microbial degradability compared to native cellulose but through the incorporation of starch, the microbial attachment and degradability was improved. Anionic TCNF was blended with cationic starch to prepare poly-ion complex (PIC) based films, which exhibited rapid disintegration in the marine environment owing to the presence of counterions [25]. These kinds of starch blended TCNF films have higher optical transparency, mechanical strength, water durability, and degradability. However, the large-scale production of cellulose-nanofiber is economically expensive, hazardous to the environment, and dangerous for forestry.

Recycling of cellulose has been reported to be possible by dissolving in different chloride salts, such as LiCl [26, 27], ZnCl₂ [28], CaCl₂ [29], and FeCl₃ [30]. Moreover, NaOH/Urea solution [31‐33] and ionic liquid [34‐36] have also been widely reported to be used for recycling cellulose. However, such a recycling process breaks down the hydrogen bonding between the cellulose chains for dissolution, thus diminishes cellulose’s crystallinity during the process [37]. Therefore, recycling of cellulose nanofibers while maintaining its properties as aspect ratio, strength, and crystallinity, is proven to be a challenge with previously reported procedures. According to earlier studies that have stated the presence of hemiacetal crosslinking within the fiber network [38, 39], this chemical crosslinking has been found to alter its strengths with the regulation of different pH conditions [40‐42]. So, through the introduction of inter- and intra-molecular hemiacetal crosslinking, a potential method of recycling the cellulose nanofibers while maintaining its properties can be explored.

In this study, a cellulose nanofiber-reinforced starch film that exhibited adequate freshwater durability and pH-responsive weakening and disintegration was prepared. Mechanically fibrillated cellulose nanofibers (MCNF) were blended with di-aldehyde starch (Di-aldS) to develop a MCNF/Di-aldS film. The film was kept under different pH conditions (4, 7, and 9) in both freshwater and seawater to analyze the freshwater durability and pH-responsive weakening. Moreover, rheology and Fourier transform infrared (FTIR) spectroscopy were performed to confirm the pH-responsive formation and disintegration of hemiacetal bonding between cellulose and starch. Thus, this study presents a novel and low-cost approach to developing pH-responsive disintegratable films that will increase the reusability of cellulose nanofibers.

The film was prepared by mixing MCNF with dialdehyde starch (Di-aldS) at a weight ratio of 1.5:1. A general experimental scheme is illustrated in Fig. 1. These films were then immersed in both freshwater and seawater at pH values of 4, 7, and 9 and were then evaluated for their swelling behavior and wet strength, as shown in Fig. 2 and tabulated in Table 2.

To confirm the strengthening and weakening of intramolecular hemiacetal bonding in different pH conditions, the swelling ratio and tensile strengths of each film were measured. The MCNF/Di-aldS films reached saturated swelling in freshwater with a pH of 7 after a 2-h immersion with a swelling ratio of 243%. The swollen films were also subjected to the UTM with a 10-N load cell at a strain rate of 5 mm/min. The stress–strain curves were used to determine their wet tensile strength. The resulting specimens for the films with a pH of 7 had a value of 4.31 MPa. However, once placed in an acidic solution with a pH of 4, their swelling ratios reduced to 204%. With this reduction in swelling, the films displayed a higher wet strength of 7.03 MPa. While in an alkaline solution with a pH of 9, the films exhibited more swelling with a value of 302%. This resulted in a wet strength of 1.60 MPa for the film.

The film was also kept in artificial seawater under different pH conditions (pH 4, 8, and 9) to investigate the weakening of the films owing to the pH in marine environments. The swelling behavior and wet strength are shown in Fig. 2b, d and are summarized in Table 2. The films reached saturation in artificial seawater (pH 8) after being immersed for 5 h with a swelling ratio of 312%. Increasing the pH to 9 also increased the film’s swelling to 364%, while decreasing the pH resulted in a reduction in swelling. The changes in pH resulted in similar trends being observed compared with the films’ immersion in freshwater. With respect to the wet strength of the films, for a pH of 9, the value decreased to 2.66 MPa, then to 6.98 MPa for a pH of 4.

This phenomenon can be explained through the intramolecular hemiacetal crosslinking that occurs between the modified moieties of the starches and the hydroxyl groups of the fibers within the film’s network [21]. The incorporation of di-aldehyde starch, which acts as a flexible binder, improves the films’ water durability through hemiacetal crosslinking [21], therefore exhibiting less swelling in general. As mentioned before, the formation of hemiacetal bonding depends on pH conditions. The additional H⁺ ions available in acidic conditions allows the hemiacetal crosslinking to be more readily made as H⁺ ions function as catalysts for the interacting groups to convert to hemiacetal groups; this results in the films’ heightened integrity in water, thus taking up less water. The films became more rigid and had a higher wet tensile strength as a result. This was also the case for basic conditions, whereby with the relatively low number of H⁺ ions present, the aldehyde moieties of the network were not able to convert to hemiacetal crosslinks, thus resulted in a higher water uptake. The softer, swollen films would then lack rigidity and integrity, therefore, led to a weaker wet tensile strength. A rheology analysis was also performed to investigate the pH responsive disintegration of the hemiacetal crosslinking in MCNF/Di-aldS suspension. The results of this test as storage moduli (G′) and loss moduli (G″) are displayed in Fig. 3 and tabulated in Table 2. With the increase in pH, both the storage and loss moduli of the suspension decreased, confirming the effect of pH on the formation and disintegration of hemiacetal crosslinking, and an illustrative representation is shown in Fig. 4. Moreover, FTIR analysis of the lyophilized suspension was performed to analyze reversal of hemiacetal bonding in alkaline conditions. With the increase in the C=O peak (1732 cm⁻¹) intensity observed for a pH of 9 and the decrease in intensity of the same peak at a pH of 4, this result confirms that there is a reversal of hemiacetal bonding corresponding to higher pH conditions.

Previous reports have shown use of metal chlorides, ionic liquid, and strong basic conditions for weakening of cellulose nanofiber reinforced films. However, these processes are expensive, hazardous and diminishes cellulose’s properties. MCNF/Di-aldS film with introduction of hemiacetal crosslinking has shown weakening of the films at higher pH conditions. Also, the film lost its strength, with a reduction to 1.60 MPa after 2 h immersion in alkaline water. Such weakened films can completely be disintegrated via mechanical stirring for reusability. In addition, the film rapidly loses strength in seawater compared to other packaging materials [46]. These polysaccharide-based, freshwater-durable, and pH-responsive disintegrable films have the potential to be next-generation packaging materials that can help to realize the reusability of cellulose nanofibers and reduce deforestation.

This study investigated the effects of pH on films made of mechanically fibrillated cellulose nanofibers (MCNF) blended with dialdehyde starch (Di-aldS). MCNF/Di-aldS film strength reduced to 1.60 MPa at pH 9 and disintegration under alkaline conditions. Moreover, rheology and FTIR confirmed the reversal of hemiacetal bonding in basic environments. This phenomenon will enhance the recyclability of cellulose nanofiber compared to existing methods. Further studies of MCNF/Di-aldS films will provide insight for further research on pH-responsive disintegrating, reusable and greener packaging films.