Spray-Coated Cellulose Nanofiber Films: Preparation, Characterization and Application

Abstract

Spraying cellulose nanofibers on the polished stainless-steel plate is a novel approach for the fabrication of free-standing cellulose nanofiber film (CNF). Recently, free-standing cellulose nanofiber film has gained attention as an alternative to synthetic plastic film. Free-standing/self-standing CNF film can be used as a potential barrier, in packaging application, as membranes for waste water application, in fabrication of biomedical film for wound repair and many more such applications in the fabrication of functional materials. To hasten the production of free-standing CNF film, spraying process is a considerable process-intensified method for large-scale production of film in a rapid manner. Spraying CNF on the stainless-steel plate produces the film with unique surfaces, namely a rough surface exposed to air and a smooth surface from the steel surface. The smooth surface of the film is very shiny and glossy and provides a platform for utilizing this smoothness for fabricating the functional materials such as a base substrate for flexible electronics and solar cells, etc. This chapter summarizes the production of free-standing CNF film via spraying and its characterization linked to its application.

1. Introduction

Plastic pollution is one of the serious threats to the environment in the current scenario. Packaging is the main source for these plastics’ pollution and it is an alarming status to replace the synthetic plastics with biopolymers. Biopolymers are good alternatives for synthetic plastics, as they have biodegradability, serve as eco-friendly material, and possess good mechanical and barrier properties for the development of various functional materials [1]. Recently, cellulose nanofiber (CNF) has been getting a predominant place in the list of biopolymers [2].

Cellulose is the most important biorenewable, biodegradable and biopolymer that is available in plenty in nature and plays an excellent feed stock for the development of various sustainable materials on an industrial-scale production [3]. From the past decade, cellulose nanofiber is used as one of the pioneering feed stocks for the development of various functional materials. It is produced by the disintegration and delamination of cellulose fibrils from pulp that is, in turn, produced from a variety of green sources like wood, potato tuber, hemp and flax. It has a dimension diameter ranging from 5 to 60 nanometers (nm) and length of several micrometers [4]. Moreover, having a smaller dimension in cellulose nanofibrils in CNF gives the larger surface of CNF, which is why, there is a great opportunity for developing more functional materials for various applications [4, 5].

The films made from cellulose nanofiber (CNF) have various outstanding mechanical, optical and structural properties and these properties are played to fabricate various functional materials, such as cellulose nanocomposite [4], microfibrillated film [6], inorganic nanocomposite [7], organic transistors and conducting materials [8] and immunoassays and diagnostic materials [9]. Moreover, another advantage of the nanofibrillated cellulose is that it is easy to tailor its surface properties and mechanical properties. As a result, it is used in the field of photonics, surface modifications, nanocomposites, biomedical scaffold and optoelectronics [10]. Due to barrier and colloidal properties of CNF, it is widely used in paper-making, packaging and coatings to enhance its barrier surface and then in automotive industries [11]. Recently, CNF sheets turned out to be one of the most promising high-performance functional materials potentially used as filters [12], adsorbents, catalysts [13], cell culture substrates, thermal insulators and drug carriers [14].

On top of abovementioned excellent properties, these materials are biodegradable and recyclable. Hence, they have the potential to replace some of the synthetic polymeric materials that cause serious environmental problems [1]. However, persisting problems faced during CNF film preparation include low-energy consumption and rapidity in fabrication [15]. CNF films were prepared by vacuum filtration, casting and spray coating. However, these methods are time-consuming processes, and the films are low in weight and thickness [15, 16]. Even though these processes could be efficient in the way of producing better quality of films, they have constraints in the scaling-up process in large-scale production for technology transfer and commercialization of the free-standing films [15].

In the vacuum filtration method, the CNF film formation required a high dewatering time, which shows up as a major constraint for an industrial-scale process. Furthermore, Varanasi and Batchelor [17] reported on the rapid preparation of nanofibrillated sheet using a British hand sheet maker in 10 min. However, they achieved only the mass per unit area of 57.4 g/m² and thickness of 68.9 ± 8.90 μm. This is why the current investigation was motivated to develop a rapid and scalable spray coating technique to produce the nanocellulose film to replace the time-consuming conventional techniques for cellulose film [18]. Beneventi et al. [19] reported on the spray coating of the microfibrillated cellulose on the nylon fabric to prepare the nanopaper with maximum mass of the film of 124 g/m². In this approach, the experimental setup has a conveyor system with a speed of 0.5 m/min to achieve 124 g/m² of film’s basis weight. However, it failed to explain the uniformity of the film through the thickness distribution and surface morphology. This chapter reveals the rapid preparation of a CNF film using a developed laboratory-scale spray coating system to produce high basis weight film in a short span of time.

2. Free-standing CNF film fabrication via spray coating process

Spraying cellulose nanofibers on the polished stainless-steel plate is a rapid and novel process for the fabrication of a CNF wet film [20]. Previously, spraying microfibrillated cellulose on three-dimensional (3D) brass metal was attempted to develop a film. However, the cracks and shrinkage on the film were formed. But this was the basic idea to develop a spray coating process to fabricate CNF film [21]. In the improved process, the microfibrillated cellulose suspension was sprayed on the fabric surface and then it was followed by vacuum filtration to remove the excess water present in the wet film [19]. The concept of spraying nanofibers has been generated from these concrete works. In addition to that, spraying provides contour coating and contactless coating with the solid surface. So that, the surface topography and morphology of the solid surface did not influence the coating process [15]. It has been reported that spraying process produces a CNF film with high basis weight without any change in the operation time [20]. The spraying CNF on the fabric surface and paper substrates was already developed for free-standing films and barrier coating on the papers’ surface [19, 22]. This chapter reveals the spraying CNF on the stainless-steel plates for the fabrication of free-standing films.

The following experimental system on spray coating to fabricate CNF film was developed.

Figure 1 shows the experimental system for spraying CNF on the stainless-steel plate. It reveals the spraying CNF suspension on the metal plate that is kept on the conveyor. In this experimental system, there are two important parameters for tailoring the properties of a CNF film. CNF suspension consistency and velocity of the conveyor are the parameters used for tailoring the thickness and basis weight of the films. At constant velocity of the conveyor, the CNF suspension can be varied for spraying operation to get the CNF film. Normally, spraying low CNF% suspension produces the least thickness and basis weight of the film, whereas high basis weight and thickness of the film can be fabricated by spraying high CNF% suspension on the polished metal surface. Similarly, the CNF suspension concentration has been fixed and varying the velocity of the conveyor helps to tailor the thickness and basis weight of the film. It means that high basis weight and thickness of the film were fabricated at the lowest velocity of the conveyor. At that moment, a high amount of suspension gets deposited and fibers concentrated to form a high basis weight CNF film. Similarly, thin CNF films were fabricated spraying CNF suspension on the metal plate at high velocity of the conveyor. In this case, less amount of fibers were deposited on the steel plate to form a thin CNF film due to fast movement of the conveyor. Apart from these important parameters in a spray system to show their effect on CNF film’s properties, the spray distance between the spray gun to the base surface, spray nozzle and spray gun position is indirectly controlling the film’s properties, such as uniformity, thickness and basis weight [18, 20].

Figure 1.

Experimental setup for lab-scale spray coating system for the preparation of a nanocellulose (NC) film. (A) Rough surface of the NC film. (B) Smooth surface of the NC film.

In this spray system, the CNF wet film can be fabricated and should be subjected to a drying process to remove the excess water in the wet film. The drying process can be carried out by different methods such as drying the wet film in an air oven at a temperature of 105°C and drying the wet sheets in a laminar flow chamber or fume hood with a constant flow of air under standard laboratory practice. The dried CNF film can be subjected to various characterizations and applications. The dried spray-coated CNF film has two unique but compact surfaces, namely rough surface and smooth surface. The rough surface is exposed to the air side when spraying CNF suspension on the metal plates. The smooth side of the CNF film is from the stainless-steel side and it has a shiny and glossy surface as one of the finishing qualities in this process. The surface smoothness of the CNF film is replicated from the stainless-steel plate. This smooth side of the film is used for the fabrication of numerous functional materials such as substrates for flexible electronics and printed electronics [16].

3. Analysis of spray-coated CNF films and their characterization

Spraying cellulose nanofiber suspension on the polished stainless-steel plates is a rapid process for the fabrication of a wet film of CNF. This method produces a compact film of cellulose nanofibrils having two unique surfaces, namely rough surface on the free side and smooth surface on the metal side. The operation time to form a 15.9-cm diameter CNF film consumes less than a minute and is independent of CNF suspension concentration. However, this method produces a wet CNF film and is subjected to a drying process to evaporate the water in a spray-coated CNF suspension. The drying of spray-coated wet film can be performed by keeping the wet film in an air oven at 105°C or air-drying in a laminar flow chamber under the standard laboratory conditions. The dried film on the stainless-steel plate can be easily peeled off from the plate and the CNF becomes free standing/self-standing films for various applications [20].

Figures 2 and 3 show the spray-coated CNF film. The spraying CNF on circular stainless-steel plate and square stainless-steel plate was achieved to fabricate the circular and square sheets. The operation time in spraying CNF suspension to form a 15.9-cm diameter film was less than a minute. Unlike vacuum filtration, the CNF concentration to fabricate film was independent of their operation time in a spraying process. The spraying CNF on the metal surface produces an ultrasmooth film for various applications [18].

Figure 2.

Circular sheets of a CNF film via spray coating process.

Figure 3.

Square sheets of a CNF film via spray coating process.

Figure 4 reveals the cross-section of the scanning electron microscopy (SEM) micrographs of a CNF film prepared via spray coating. The SEM micrograph confirms the complex cellulose nanofibril layers intertwined through the hydrogen bonding between the hydroxyl groups of the CNF. This also increases tortuosity of the film and shows this effect on the barrier performance of the CNF film [16].

Figure 4.

Cross-sectional view of spray-coated CNF films.

Figures 5 and 6 show the SEM micrographs of the spray-coated CNF film and comparison with the CNF film prepared via vacuum filtration. The CNF film via spraying has compactness and roughness on the free side and smoothness on the other side. The rough surface of the film is very porous due to various sizes of the fibre distribution. The smooth side of the film has a shiny and glossy surface and its smoothness is replicated from the surface of the stainless-steel plate [18, 20]. The mechanism of replication of the smoothness from the stainless-steel plate remains obscure [16]. The rough and smooth surface of the spray-coated CNF film has an importance in the fabrication of various functional materials [16]. For example, in the construction of flexible electronics and printed electronics, the conductive ink on the cellulose substrates should be penetrated well on the surface of the substrates. To achieve this, the sufficient roughness/smoothness of the substrates are required for spreading the conductive ink. Similarly, the roughness and smoothness of the film can be used in the fabrication of solar cells [16].

Figure 5.

Rough side of spray-coated CNF film and the filter side of the vacuum-filtered film.

Figure 6.

Smooth side of a spray-coated CNF film and the free side of the vacuum-filtered film.

The rough side of the spray-coated CNF film and the filter side of the vacuum-filtered CNF film are shown in Figure 5. The surfaces of both sides of the films are very porous and have a good surface roughness. Figure 6 reveals the smooth side of the CNF film and the free side of the vacuum-filtered CNF film.

4. Surface roughness of a free-standing CNF film

As discussed earlier, the surface roughness of the film is one of the important criteria for the construction of functional materials. The surface roughness of the CNF film is evaluated by optical profilometry.

Figure 7 shows the optical profilometry image of the rough side of the CNF film prepared via spray coating. The rough side of the film was very porous and it showed high surface roughness on the film. This was due to various fiber size distributions in cellulose nanofibrils. The mean surface roughness on the rough side of the CNF film was found to be 1654 nm and the root mean square (RMS) value of the surface roughness on this side was reported to be 2087 nm. Figure 8 reveals the optical profilometry image of the smooth side of a CNF film. The Ra and Rq values from the image confirm that the surface was very smooth and shiny and glossy. The Ra and Rq values from the image were evaluated to be 278 nm and 389 nm, respectively. Figures 9 and 10 show the optical profilometry images of free and filter sides of the CNF film prepared via vacuum filtration. The Ra and Rq values on the free side of the filtered CNF film were evaluated to be 2150 nm and 2673 nm, respectively. Similarly, the Ra and Rq values on the filter side of the vacuum-filtered CNF film were evaluated to be 3015 nm and 3751 nm, respectively. When comparing the surface roughness of the CNF film with that from filtration, the spray-coated CNF film has smooth and less porous surface [18, 20].

Figure 7.

Optical profilometry image of the rough side of a CNF film via spraying.

Figure 8.

Optical profilometry image of the smooth side of the CNF film via spraying.

Figure 9.

Optical profilometry image of the free side of the CNF film via filtration.

Figure 10.

Optical profilometry image of the filter side of the CNF film via filtration.

Spraying CNF suspension on the polished metal surface like stainless steel produces a more smooth film than that of standard procedure such as vacuum filtration. In the filtration process, the film has rough surface on the filter side and free side and is also porous, depending on the type of cellulose nanofiber used. In spraying, the film has unique surfaces, such as smooth side from the metal side and rough side from the air side.

5. Atomic force microscopy (AFM) studies

The AFM studies on the spray-coated CNF film were investigated to analyze the nanoscale surface, roughness of the rough and smooth surfaces of the film. The visual examination of the CNF film via spraying was rough on the free side of the film and smooth on the metal side. In addition to that, the smooth side was very shiny and glossy as one of the finishing qualities of the film. The RMS surface roughness of the CNF film from the AFM micrographs was evaluated to be 51.4 nm on the rough side and 16.7 nm on the smooth side in an inspection area of 2 μm × 2 μm. In the case of a vacuum-filtered CNF film, the RMS surface roughness of the CNF film was found to be 102.3 nm on the free side and 70.64 nm on the filter side on the same area of inspection. By this way, the nanoscale roughness of the CNF film was evaluated and it is implemented on these surfaces for the construction of printed and flexible electronics substrates. Figures 11 and 12 show the surface roughness of the CNF film and RMS value evaluated from these AFM micrographs [16].

Figure 11.

Rough surface of the spray-coated CNF film.

Figure 12.

Smooth surface of the CNF film via spraying.

6. Thickness mapping of CNF film via spraying

Thickness of the film is one of the main parameters for controlling the barrier performance of the film. Figure 13 shows the thickness mapping of spray-coated CNF film and its comparison with that of a vacuum-filtered CNF film (Figure 14). The thickness mapping of the CNF film was evaluated from 1.5 wt.% CNF film via spraying to 1.5 wt.% CNF on the stainless-steel plate. The basis weights of the film produced by vacuum filtering and spray coating, respectively, are 100.5 ± 3.4 g/m² and 95.2 ± 5.2 g/m², respectively. Vacuum filtering consumes a substantially longer dewatering time of 15 minutes to form the film. In the spraying operation, the operation time to form a film is independent of CNF suspension concentration. Even after accounting for the little variation in basis weight, the spray-coated CNF film is somewhat thicker when compared to the vacuum-filtered film. The apparent densities of the vacuum-filtered and spray-coated films were 793 and 834 kg/m³, respectively. Additionally, the thickness of the spray-coated film is distributed across a somewhat larger range. Figures 13 and 14 reveal the uniform thickness of the CNF film fabricated via spraying and filtration processes. It seems that the spray-coated CNF film has better uniformity and is comparable with a filtered CNF film [20].

Figure 13.

Thickness mapping of the spray-coated CNF film.

Figure 14.

Thickness mapping of the vacuum-filtered CNF film.

7. Thickness and basis weight of the film

Figure 15 reveals the linear relationship between thickness and basis weight of the CNF film via spraying. It demonstrates that the central composite design (CCD) model may be used to scale up the spraying process since it matches the actual experimental data well. The thickness and basis weight of the CNF film were tailored by increasing spraying CNF suspension from 1 wt.% to 2 wt.%. The operation time for spraying CNF suspension was independent of fiber content in the CNF suspension. The following models have been developed to scale up the process. These models reveal that the basis weight and thickness of the CNF film were found to be highly influenced by the CNF suspension concentration, as opposed to conveyor speed and spray distance based on the testing’s findings.

Figure 15.

Linear relationship between thickness and basis weight of the spray-coated CNF film.

$$\begin{array}{ll}\text{Basis weight}=& -64.45+122.43\ast \mathrm{CNF}\text{suspension concentration}-17.28\\ & \ast \text{Velocity of the conveyor}+0.34\ast \text{Spray distance}\end{array}$$E1

$$\begin{array}{ll}\text{Thickness}=& -0.106+0.111\ast \mathrm{CNF}\text{suspension concentration}-0.000017\\ & \ast \text{Velocity of the conveyor}+0.002\ast \text{Spray distance}\end{array}$$E2

These are linear models confirming the direct relationship between thickness and basis weight of the CNF film.

From the linear models, there were two important parameters controlling the thickness and basis weight of the CNF film. Mainly, CNF suspension concentration and velocity of the conveyor in the experimental setup were deciding parameters for tailoring the CNF film’s properties [23].

8. Mechanical performance of spray-coated CNF film

Figure 16 reveals the tensile index of the spray coated cellulose nanofiber film and its comparison with that of a vacuum filtration film. The modified configuration of an experimental spray system produces the CNF film that has tensile indices higher than that of the CNF film via vacuum filtration. This is because of the high uniformity of the CNF film that was fabricated in the modified configuration of the spray system. Generally, spraying CNF suspension on the polished metal plate is controlled by numerous parameters, mainly CNF suspension’s consistency, process variables in the spray system, such as the spray distance, nozzle diameter, type of spray system and sprayability of the CNF suspension. These parameters indirectly control the uniformity of the film, which is linked with barrier and mechanical properties of the film [18].

Figure 16.

Tensile index of the spray-coated cellulose nanofiber film and its comparison with that of a vacuum-filtered film.

9. Cost and environmental analysis

The free-standing CNF film has not been commercialized so far and it is in the researching and development (R&D) stage. The patents on free-standing CNF films and their composites produced via various methods have been increased. The cost of Diacel KY 100S from Diacel Chemical Pvt. Ltd. was 2500 Australian dollars (AUD) per 50 kg of nanocellulose. The cost of a spraying system was around 4000 AUD for the construction of an experimental system to spray CNF on the polished metal surface. The sizes of the film are 220 mm x 220 mm for square sheet and 159 mm for circular sheet. The basis weight of the film was assumed to be 100 g/m². For 1 kg of KY100S, 10 sheets can be fabricated and each sheet consumes 5 AUD for the fabrication. The operation and maintenance costs were not considered in this study. The operation time for the fabrication of a 220 mm × 220 mm square sheet and 159 mm circular sheet was less than a minute. When compared with vacuum filtration (VF), a laboratory version of the paper-making machine, spraying is a process intensified for the fabrication of free-standing cellulose nanofiber films and their composites. In the case of the spraying method, there are few steps, such as spraying CNF suspension on the metal plates, followed by drying under standard approach. It has required less fixed capital and operating cost and labour cost. In the case of vacuum filtration, there are many steps, such as the agitation of CNF suspension, mixing of CNF suspension, dewatering, couching, sheet removal, drying and then pressing. It confirms that filtration requires good fixed capital and high operating cost and labour cost. Spraying operation can be integrated with other coating methods such as roll to roll (R2R) for giving a high performance in the rate of production of a CNF film. So that, the cost of CNF film will be reduced for commercializing in the market.

Under controlled composting circumstances, the biodegradability and compostability of nanofibrillar cellulose-based (NFC) products, such as films, concentrated NFC, and paper products incorporating NFC, were assessed. All of the NFC products that were evaluated met the criteria for biodegradability outlined in European Standard EN 13432. NFC even increased the biodegradability of paper that had 1.5% NFC added to it. The modified pilot-scale composting test EN 14045 was used to assess disintegration during composting. In 3 weeks of composting, NFC films entirely decomposed, and NFC had no effect on how easily paper products containing NFC degraded. Using a bioluminescence test using Vibrio fischeri, ecotoxicity during the biodegradation of NFC products in a compost environment was assessed. For any of the samples, there was no evidence of acute toxicity [24].

10. Comparison with other coating process and validation

Spraying cellulose nanofibers on the polished stainless-steel plate is a rapid process to form a compact CNF film. It is a new process and should be compared with other conventional methods to confirm the efficacy of spray coating method [25]. The current spraying method has the capacity to handle the CNF suspension from 1.0 wt.% to 2.0 wt.% to produce the thickness and basis weight of CNF film from ~60 μm to ~200 μm and ~55 g/m² to ~199 g/m² [20]. The performance of the spray coating process can be improved by the high-performance spray system that can handle the CNF suspension of more than 2.00 wt.% [16, 18]. In addition to that, the rheology modifier, such as montmorillonite (MMT) clay, can be added into the CNF suspension for spraying to avoid any interruption in forming spray jet for fabrication of the films [16]. When comparing spraying microfribillated cellulose on the 3D structures [21], this method is quite reproducible and produces the CNF film without any cracks and homogeneous film with a high degree of uniformity [18]. The earlier method of spraying microfibrillar cellulose (MFC) on a 3D brass structure produces the MFC film having the basis weight and thickness from 59 to 118 g/m² and 46 to 68 μm, respectively. The spray-coated 3D structure consists of cracks and wrinkles formed on the surface. However, the reported method handled high solid MFC suspension, such as 4.5 wt.% and 9 wt.% MFC suspensions, resulting in the formation of a disturbed spray jet and also resulting in the formation of cracks and wrinkles on the film. A high MFC suspension behaves as a gel-like fluid and makes the spray systems lose the spraying ability [21].

Similar to the current spray coating method, the earlier spraying MFC on the nylon fabric was attempted to fabricate the free-standing CNF film. In this method, spraying MFC on the nylon fabric consumed time from 10 min to 20 min and then the wet sprayed film was subjected to water removal from the CNF suspension via applying vacuum, which is similar to vacuum filtration. The time consumed for filtration was from 15 sec to 90 sec and after that vacuum dried under standard temperature. The spray-coated MFC film from this spraying process has the basis weight that varied from 13.7 g/m² to 124 g/m², with the thickness of the film varying from 10 μm to 72 μm. It was also reported that the imprints of nylon fabric were marked on the spray-coated MFC film [19]. Spraying was more efficient in the fabrication of free-standing CNF film while compared with solvent casting, vacuum filtration and hot pressing. These processes were problematic in the evaporation or removal of solvent from CNF film and a time-consuming process and also limitation in the basis weight of the film. Spin coating is a laboratory approach for the fabrication of free-standing thin CNF film for the study of biomolecules’ interaction. It is not a scalable method due to the removal of water from the suspension via spinning to form ultrathin films. This method can be used to coat the substrates for laboratory-scale studies. Roll-to-roll (R2R) coating is another approach for the fabrication of CNF films and capacity for large-scale production of the film. In this method, CNF was coated on the pre-treated substrates such as plastic films. The spreading of CNF on the substrates was a challenging task and then coated and dried under pressing after peeled from the substrates. The basis weight of the CNF film can be achieved from 0.1 to 400 g/m² [15].

Given this analysis, the spraying CNF on the base substrates is more advantageous in the fabrication of free-standing CNF film. Spraying on the stainless-steel plates produces the film with unique surfaces, mainly smooth on the steel side and it can be used for the fabrication of functional materials. When comparing with the other coating process, the operation time for spray coating in the current practice was less than a minute to fabricate a 15.9-cm diameter film and was independent of CNF suspension concentration. The integration of roll-to-roll coating with spray system is another approach for large-scale production of free-standing CNF films.

11. Application of spray-coated CNF films

Spray-coated CNF films have been utilized in various fields and applications as a substrate for developing functional materials. Figure 17 shows various applications for CNF films in various fields. Due to the rapid process of spraying, it can be used as a barrier material to replace the synthetic plastics in the packaging sector. Generally, cellulose nanofibers are good as oxygen barrier and show a performance greater than that of synthetic plastics. However, the water vapor permeability (WVP) of CNF was not equalizing the water vapor barrier performance of synthetic plastics. Spraying CNF on the metal plates produces a film with compactness acting as a good barrier against water vapor and its performance was better than that of the vacuum filtered film and comparable with that of synthetic plastics. Furthermore, the water vapor barrier of the CNF film was improved by incorporating nano-inorganics/antimicrobial inorganics into cellulose nanofibril matrix in the CNF suspension [15].

Figure 17.

Application of cellulose nanofiber films via spraying and filtration.

In the case of fabrication of nanocellulose-montmorillonite composite, the time taken for dewatering in vacuum filtration process was exponentially increased with MMT concentration and time was consumed from 3 hours to 24 hours, depending on the MMT content in cellulose nanofiber suspension. To mitigate this problem, spray coating has been implemented to fabricate CNF-MMT composite and here the operation time for spraying CNF-MMT suspension was independent of MMT concentration in CNF suspension. The spray-coated CNF MMT composite was a good barrier material for replacing synthetic plastics in the packaging materials. The antimicrobial inorganics incorporated into cellulose nanofiber suspension were fabricated as composite via spray coating process. This free-standing composite can be used in anti-microbial packaging and bioactive packaging. Similarly, the free-standing CNF film prepared via spraying can be used as the membrane for waste water treatment. In this composite, titanium dioxide was also impregnated into the film and its becomes a photocatalyst for waste water treatment application [15].

The membranes were also developed from spray-coated CNF film to separate oil and water mixture. In addition to that, various composites from CNF—inorganics can be fabricated via spray coating process for various applications. The spray-coated CNF films have unique surfaces, namely rough surface and smooth surface. The smooth surface of the CNF film can be used for the development of printed and flexible electronics. Figure 18 shows the CNF film as substrates for printed electronics and flexible electronics [16].

Figure 18.

The printed circuits on the spray-coated nanocellulose films.

The spray-coated CNF film can be used as a base biomaterial for the development of tissue engineering material and drug-delivery vehicle. The silver nanoparticle (AgNP) and MMT were coated on the spray-coated CNF film via a laboratory spraying method to develop a drug-delivery vehicle composite for the treatment of wound. The silver nanoparticle present on the surface of CNF film can eradicate the wound pathogens at the wound site and the CNF film can act as a template for skin regeneration. In addition to spray coating to prepare free-standing CNF films and composites, this methodology can be used for developing CNF barrier layers on the paper and paper board substrates for enhancing their barrier potential against air and water vapor. Furthermore, the spraying CNF suspension was implemented to coat the CNF layers for membrane development for water treatment applications [15].

12. Spray-coated CNF film in packaging

The most important application of spraying CNF on the base surface was for the fabrication of CNF film, which can be used as a barrier material and as a good alternative for synthetic plastics. Figure 19 demonstrates the spray-coated CNF film’s capability as a reliable water vapor barrier and comparison to synthetic plastics. However, the barrier efficacy of the packing film against water vapor is also determined by its thickness. Because of this, the film’s water vapor permeability—a number that was determined by normalizing the thickness of the film with its water vapor transmission rate (WVTR) values—was used to characterize the performance of the water vapor barrier. Figure 12 compares synthetic plastics with spray-coated CNF film in terms of WVP. This graphic demonstrates how comparable the WVP of spray-coated CNF film is to that of synthetic polymers. Beyond this benefit, CNF is an environmentally benign nanomaterial with the ability to break down in the environment [25].

Figure 19.

Water vapor permeability of the CNF film and its comparison with that of synthetic plastics.

13. Conclusion

In order to meet the speed of production of film, a rapid process is required to fabricate the free-standing film of cellulose nanofibers/nanocellulose material. To answer this need, spraying/spray coating is a rapid process to fabricate the CNF film in a free-standing manner/self-standing sheets for various applications. The current spraying process produces the free-standing film in a rapid manner, taking up operation time less than a minute in forming the spray-coated wet film. However, the drying of a spray-coated wet film consumes a time of more than 24 hours in an air-drying process under standard laboratory conditions and a couple of hours in an oven drying at 105°C under standard practice. Unlike the vacuum filtration process, the operation time/film formation time of spraying process was independent of CNF suspension concentration and a potential for scaling up. The spray-coated film has unique surfaces, such as rough and smooth, and these surfaces lead to the development of various functional materials, such as packaging, membrane and drug-delivery devices.