Introduction

Since last two decades, there is a dire need of sustainable and renewable resources to be used for in the society as well as industrial production. In recent times, “green polymers” has become a trending topic. Green polymers are produced from sustainable and renewable resources and in the future would be commonly used in industrial processes (Aziz et al., 2024; Shouket et al., 2023). Globally, agricultural waste, because of its biodegradability, is now used as a substitute for petrochemicals. Hence, at present, agricultural waste has become a burning issue (Aziz et al., 2023; Gismatulina et al., 2018). Agro-waste has immense potential for extracting high-value cellulose, especially crop residues rich in their cellulose content. Apart from agri-food waste, exploring non-food competing crops on marginal lands offers exciting possibilities for maximizing biomass utilization and creating valuable products (Kopania et al., 2012) and contributes to sustainable agriculture and the environment (Qiu et al., 2023). Numerous plants are used for producing cellulose, such as cotton, hemp, wheat straw, corn stalks, pineapple leaves, and even nettle. Also, banana leaves, an unexplored area, have shown similar potential (Kopania et al., 2012).

Cellulose is special because each glucose unit has three hydroxyl groups. This makes cellulose a prime candidate for extensive esterification. In theory, all three hydroxyl groups on each glucose unit can be replaced with nitrate groups, creating a highly nitrated cellulose nitrate (cellulose trinitrate). The degree of nitration significantly affects the properties of cellulose nitrate. Highly nitrated forms (over 12.5% of nitrogen content) are highly inflammable and unstable, making them ideal for propellants such as guncotton. Less nitrated forms are more stable and have applications in films, lacquers, and even early (synthetic) plastic (Alinat et al., 2014; Yi et al., 2022).

This research aimed to explore the potential of banana stem waste by extracting cellulose and crafting nitrocellulose from it. Furthermore, the effect of composition and reaction time of nitrating acid on nitrocellulose yield along with solubility of banana stems was also targeted. Hence, the research aimed to transform agro waste into valuable resources. Banana stems offer a sustainable and cost-effective alternative to traditional nitrocellulose feedstocks, eventually promoting circular economy and combating resource depletion. Because nitration affects flammability and solubility, this research focused on finding the optimum level of nitration to produce plant-based nitrocellulose for optimum yield with safety measures and a good source of calcium and potassium (Zhang et al., 2023).

Materials and Methods

Fresh samples of banana pseudo stems were collected from Agricultural Research Farm, Malakhandher, Peshawar, Pakistan. The samples were washed with distilled water to remove impurities, followed by oven-drying at 55°C for 10 h. The samples were further homogenized and sieved with 70-mesh sieve.

Sample preparation

Cellulose extraction

Cellulose extraction was carried out using the protocol followed by Lohmousavi et al. (2020) and Sundarraj et al. (2018). A mixture of 100-mL (80%) CH₃COOH and 10-mL (70%) HNO₃ was added to 5 g of sample. The mixture was kept on an oil bath at 120°C for 20 min, followed by cooling. Oil bath was used to have temperature higher than 99°C, which is the upper limit of water bath in a biochemical laboratory. The mixture was filtered after addition of 60-mL distilled water. The filtrate was rinsed with ethanol till the removal of acidic residues. The filtrate was oven-dried at 60°C. After acidic hydrolysis of samples, the filtrate was subjected to removal of lignins and hemicellulose by soaking in 4% NaOH solution with continuous stirring with magnetic stirrer at 75 rpm, followed by vacuum filtration. After filtrate was subjected to basic treatment, it was washed with distilled water and dried at 80°C for 5 h. The powder was then bleached with 20% H₂O₂ and stored at 85°C. Finally, the bleached cellulose filtrate was washed for several times with de-ionized water to remove all residues. Hence, the resultant pure filtrate was oven-dried at 50°C for 10 h.

Synthesis of cellulose nitrate via different nitrating mix

HNO₃+H₂SO₄ Method (cellulose nitrate A)

The protocol adopted by Cheung (2014) was followed to prepare cellulose nitrate A. Cellulose sample, 175 g, was added to acidic medium comprising 49-mL HNO₃ and 91-mL H₂SO₄. The reaction was sustained for 2 h in thermostat-fixed ice water at 0°C. White precipitates were simply filtered and washed for several times until neutralized.

HNO₃+P₂O₅ Method (cellulose nitrate B)

Mixture of HNO₃–P₂O₅ was prepared in a ratio of 3:5 (HNO₃:P₂O₅) according to a protocol followed by Cheung (2014). The mixture was initially stirred at 0°C in an ice bath. The agglomerates of polyphosphoric acid were eliminated by filtration; 1.75-g of sample was added to a mixture comprising 100-mL P₂O₅. The mixture was again kept in ice bath at 0°C for 3 h. The reaction was stopped after the appearance of white precipitate, which was filtered and washed for several times until neutralized.

Degree of substitution

Concerning the current research, degree of substitution referred to an average number of hydroxyl (OH) groups in a cellulose molecule that were replaced by nitrate (NO₂) groups:

Effect of nitration time on cellulose nitrate yield, solubility, and nitrogen content

In order to find out the effect of time on nitrating ratio and solubility, the procedure adopted by Adekunle (2010) was followed. Nitrating acid–cellulose ratios of 20:1, 40:1, 50:1, 60:1, 80:1, and 100:1 were used at an interval of 30, 60, and 90 min to determine the effect of time on acid mixtures. For determining stability, the nitrated material was repeatedly boiled in water/methanol after each filtration to eliminate adsorbed residues. The resulting solid was rinsed with water, filtered after 10 min, followed by centrifugation at 1,400 rpm. Afterwards, it was boiled for 3 h in refluxing water. Finally, it was rinsed then suction- and oven-dried at 50°C. Standard micro-Kjeldahl technique was followed for determining yield, solubility, and nitrogen content; solubility tests were also performed.

Statistical analysis

The effect of nitrating time on nitrocellulose synthesis was analyzed using one-way ANOVA in SPSS, version 24 (IBM Corporation, Armonk, NY).

Results and Discussions

Isolation of cellulose from banana stem

In order to prepare cellulose nitrate, crude cellulose from banana stem was prepared by chemical hydrolysis. The resultant material was pale yellow in color, which indicated the presence of other lignocelluloses. The material post-acid hydrolysis was treated with alkali and bleaching agent to remove hemicellulose and lignin. The study determined 45.2% of pure cellulose, 26% hemicellulose, and 28.8% lignin from banana stem.

The change in color from pale yellow to cream white indicated the removal of hemicellulose and lignin. Abraham et al. (2011) explored the potential of banana stems as an agricultural waste for production of cellulose fiber. The authors investigated the use of banana stems combined with other agricultural residues, such as rice straw (reportedly containing 33% cellulose), soybean hulls (with 43.7% cellulose), and sugarcane bagasse (having 40% cellulose). This approach aligned with the existing literature, suggesting that banana stems have comparatively higher cellulose content.

Production of cellulose nitrate by using combination of nitrating acids

Two nitrating mixtures, HNO₃+H₂SO₄ and HNO₃+P₂O₅, were explored in the current study. As per the literature, the potential explosiveness of nitrating mixture was assessed by nitrogen contents (Table 1).

The highest nitrogen (8.64%) was found with HNO₃+ H₂SO₄ mixture whereas HNO₃+ P₂O₅, mixture was found to have low (5.68%) nitrogen. Hence, this suggested a higher potential of H₂SO₄ to nitrate for cellulose, compared to P₂O_5.

Nitrocellulose from both mixtures exhibited varying degree of nitration, which was a measure of enhanced reactivity. Along with nitrogen content, degree of substitution was also determined to choose optimal nitrating agent based on the desired level of nitration. Degree of substitution was determined by the above-mentioned standard equation.

Mattar et al. (2020) suggested the effectiveness of H₂SO₄ and P₂O₅ as nitrating agents for cellulose hinges on the degree of substitution. Our findings revealed a low degree of substitution (0.80) for P₂O₅, compared to H₂SO₄ (1.38). At a lower degree of substitution, H₂SO₄ readily substituted hydroxyl groups with nitro groups, while increase in degree of substitution exhibited a contrasting behavior.

Stability of nitrocellulose A and nitrocellulose B

The stability of both nitrocellulose A and nitrocellulose B was followed as per Cheung (2014). Nitrocellulose prepared from both methods exhibited lower nitrogen content (13%), being into nonexplosive category. The findings were in a safe range as higher nitrogen content leads to higher flammability. The resultant nitrocellulose was washed extensively for removal of adsorbed acids from cellulose fibers. Nitrocellulose cannot be stored dry due to potential deflagration hazards causing rapid burning. Additionally, stabilization of resulting nitrocellulose was accomplished in boiled water to remove completely acid residues. During the stability process, sodium bicarbonate (NaHCO₃) was added to adjust pH. It was observed in the current study that addition of NaHCO₃ decreased mass by 2.59% in nitrocellulose A and by 3.57% in nitrocellulose B. The pH can efficiently stabilize the nitrocellulose.

Effect of nitrating time on nitrocellulose A and nitrocellulose B

The current study also aimed to determine the effect of nitrating time on yield. It was revealed that nitrating time of 90 min for both nitrocellulose A and nitrocellulose B resulted in the highest yield, compared to nitrating period of 30 and 60 min. This suggested that longer exposure optimized extraction of cellulose nitrate. The retained exposure time impacted the conversion of active sites in cellulose to nitrates. Hence, the dependence of effectiveness of nitrating agent on the accessibility of these sites can be measured. Nitrating mixtures are quite exothermic, enabling cellulose conversion to nitrates at lower temperatures, thereby forming “nitronium” ions, which then react with cellulose. The nitrating time was limited to 90 min, as further exposure could be hazardous. All the findings against different periods of nitrating cellulose are depicted in Table 3.

Effect of nitrocellulose ratios

Effect of different ratios of nitrating acid mixtures and cellulose on nitrocellulose

The current study also determined the yield because of different ratios of HNO₃-H₂SO₄–cellulose, that is, 20:1, 40:1, 50:1, 60:1, 80:1, and 100:1. The results are shown in Figure 2. Maximum yield (91.13) was obtained with HNO₃-H₂SO₄–cellulose ratio of 60:1, while minimum yield (45.26) was obtained with HNO₃-H₂SO₄–cellulose ratio of 40:1. It was evident from the current research that yield increased when subjected to the following order of mixture ratios: 60:1 > 100:1 > 80:1 > 20:1 > 50:1 > 40:1. A significant linear relationship of 70% was found. Similarly, yield was also determined when exposed to different ratios of nitrating mixture (HNO₃-H₂PO₄–cellulose). The variation ranged from 45.23% to 97.78%. The highest yield (97.78%) was discovered with HNO₃-H₂PO₄–cellulose ratio of 100:1, followed by 96.57% with a nitrating acid mixture and cellulose ratio of 50:1. In contrast, minimum yield (46.23%) was discovered with a nitrating acid mixture and cellulose ratio of 20:1. The following trend with different nitrating acid mixture and cellulose ratios was observed: 100:1 > 50:1 > 60:1 > 80:1 > 40:1 > 20:1. The correlation was found as 60%.

Figure 1. Effect of nitrating time on nitrocellulose A and nitrocellulose B.

Figure 2. Effect of different ratios of nitrating acid mixture and cellulose on nitrocellulose.

Effect of nitrating acid composition and solubility tests

The results revealed that both synthesized nitrocellulose A and nitrocellulose B were insoluble in solvents such as ethanol and benzene but sparingly soluble in acetone (cold) and 1,4-dioxane (cold). Additionally, these were found soluble in hot ester but sparingly soluble in cold ester. Also, these were found soluble in pyridine.

The current study also determined solubility along with factors affecting the solubility of cellulose nitrate and its potential in industrial application. It was observed that solubility of nitrocellulose directly depended on nitrogen content. Higher the nitrogen content, higher the solubility proportion with selected solvents, such as acetone, esters, pyridine, and 1,4-dioxane. It was also noted that addition of water affected the rate of solubility.

Nitrocellulose was found insoluble in 1,4-dioxane with water but soluble in heated acetone. Similarly, cellulose nitrate was soluble in esters and pyridine but not in heated acetone with addition of water in nitrating mixture. The findings further confirmed that acetone and esters are suitable storage solvents for cellulose nitrate because of their better solvency, hence could be used in different industries, such as plastics, paints, etc., as reported by Jesuet et al. (2019); John et al. (2024); Sandewicz (2016); and Tonkinson and Stillman (2002).

Conclusion and Recommendations

The current findings revealed successful extraction and isolation of cellulose from banana stems after removal of impurities, resulting in 45% yield. Additionally, two types of nitrocellulose were prepared with separate nitrating mixtures. Maximum nitrogen content was observed for nitrocellulose A. Maximum yield of nitrocellulose was recorded for nitrocellulose B. Furthermore, nitrocellulose B also exhibited maximum solubility in various solvents along with shorter reaction time.

This research suggests production of nitrocellulose in Pakistan from banana stems, considered as a cheap and abundant source of cellulose. This approach not only offers economic benefits but also manages agricultural waste. The resulting nitrocellulose exhibits promising properties for industrial applications, such as paints, plastics, detergents, and films. Additionally, its potential use in eco-friendly food packaging warrants further exploration. Optimizing the exothermic nitration process and acid regeneration during neutralization through further research could enhance the overall feasibility and sustainability of this method.