From Waste to Strength: Unveiling the Mechanical Properties of Peanut-Shell-Based Polymer Composites

Abstract

Peanut-shell-based polymer composites have gained significant attention as sustainable and cost-effective materials with potential applications as food packaging films, ceiling tiles, insulation panels, supercapacitors, and electrodes in various industries like the packaging industry, construction, furniture, and electronics. This review article presents a systematic roadmap of the mechanical properties of peanut-shell-based polymer composites, analyzing the influence of factors such as filler content, surface modification techniques, interfacial adhesion, and processing methods. Through an extensive literature review, we highlight the mechanical properties of peanut-shell-based polymer composites. Furthermore, challenges and ongoing research efforts in this field are discussed. This comprehensive review provides valuable insights for researchers, industry professionals, and policymakers, promoting the development and utilization of peanut-shell-based polymer composites for various applications.

1. Introduction

In recent years, there has been a growing demand for advanced materials that can offer unique and superior properties beyond those of conventional ceramics, metals, and polymers. This has led researchers in the field of materials science and engineering to shift their focus towards composite materials [1]. Composites are characterized by the combination of two or more constituents on a macroscopic scale, with each constituent retaining its distinct properties [2]. The reinforcing phase, which can take the form of particles or fibers, is integrated with a matrix material such as a polymer, ceramic, or metal [3,4]

Polymer composites, in particular, have gained significant attention and found wide-ranging applications due to their remarkable properties. These composites exhibit enhanced characteristics such as increased specific stiffness, specific strength, impact resistance, abrasion resistance, corrosion resistance, and chemical resistance [5]. These enhanced characteristics make them attractive for various industries such as automotive, aerospace, construction, electronics, and sports equipment [6].

In the automotive sector, composite materials have enabled the development of lightweight vehicles with improved fuel efficiency and reduced emissions [7]. In aerospace, composites offer weight savings, high strength-to-weight ratios, and resistance to extreme temperatures, making them ideal for aircraft structures [8,9,10]. The construction industry benefits from composites’ durability, corrosion resistance, and design flexibility, allowing for innovative architectural designs [11]. In electronics, composites provide thermal management, electrical insulation, and structural integrity [12]. Moreover, composite materials find applications in sports equipment, offering improved performance, durability, and safety [13].

Polymer composites have traditionally relied on inorganic fillers and petroleum-based polymer matrices, which have led to environmental concerns and resource depletion [14,15]. To minimize the above environmental issues, scientists are focusing more on environmentally friendly and sustainable materials known as biobased polymer composites. Biobased polymer composites are the composites in which either the matrix or the reinforcement, or both, are generated from biological sources [16]. These biocomposites have the same or better characteristics than conventional polymer composites, and they also have the potential to degrade naturally.

Organic micro- and nanofillers derived from biomass offer several advantages over traditional fillers, including a higher surface-to-volume ratio, lower density, greater aspect ratio, and improved environmental friendliness [17]. These biomass-derived fillers, when reinforced in polymer matrices, have shown exceptional mechanical, thermal, and chemical properties, making them suitable for a broad range of applications in industries including aerospace, automobile manufacturing, sports equipment, and construction [2].

The use of biomass derived fillers not only reduces the reliance on nonrenewable resources but also addresses environmental challenges associated with waste disposal and greenhouse gas emissions [18]. By utilizing agro-waste materials as fillers, the concept of turning waste into income is realized, contributing to a more sustainable and circular economy [5].

The incorporation of biomass-derived fillers in polymer composites presents exciting opportunities for the development of cost-effective, environmentally friendly, and high-performance materials [19]. However, further research is needed to optimize the processing techniques, enhance the interfacial adhesion between the filler and matrix, and ensure consistent and reproducible material properties. Additionally, the development of recycling and disposal strategies for these biomass-based composites will be crucial to minimize their environmental impact and ensure their long-term sustainability [18]. Lignocellulosic biomass derived from agricultural by-products, such as nut shells [20,21,22,23] and rice husks (RHs) [24,25], has gained attention due to its affordability, abundant availability, and potential for transforming waste into valuable materials [26].

Groundnuts, which are commonly known as peanuts, are a variety of legumes cultivated for their edible seeds. The scientific name for groundnut, Arachis hypogaea Linn, originates from two Greek terms: Arachis, denoting a leguminous plant, and hypogaea, indicating the development of pods beneath the soil surface [27]. Groundnuts are a highly nutritious and versatile crop that is widely used in food products such as peanut butter, cooking oil, and snacks. In addition to their value as a food crop, groundnuts also have the potential as a biomass feedstock for various applications. It is the world’s sixth-most-important oilseed crop. According to the circular of March 2023 released by the United States Department of Agriculture (USDA) on world agricultural production, the global acreage dedicated to groundnut cultivation is estimated at 29.63 million ha, and its average yield per hectare is 1.69 MT/ha, with a total production of 50.11 million metric tonnes. It is cultivated only in tropical regions between the latitudes 40° N to 40° S. The world’s top groundnut-producing countries are China, India, Nigeria, the USA, and the Sudan. The production capacity of these countries is shown in

Table 1. Ranking of top-producing countries and their corresponding production capacities [28].

Rank	Country	World Production	Production (1000 MT)
1	China	37%	18,300
2	India	13%	6650
3	Nigeria	9%	4500
4	USA	5%	2526
5	Sudan	5%	2500

.

Groundnut shells, the outer covering of the groundnut seeds, are a byproduct of the processing of groundnuts for food use. The shells contribute around 30% of the total weight of the legume and approximately 200–300 g of shells are produced per kg of groundnuts [29]. Around 11,000,000 tonnes of this waste from the peanut industry are produced worldwide each year, but its uses are still developing [30]. These shells contain high amounts of cellulose, hemicellulose, and lignin, making them a suitable source of cellulosic biomass for energy production. They can be used for energy generation through combustion, gasification, or pyrolysis [31].

In recent years, there has been growing interest in the use of groundnut shells as a biomass feedstock, due to their abundance and potential to provide a sustainable source of energy. Using groundnut shells as a biomass feedstock could potentially be economically beneficial for groundnut farmers, while also reducing the amount of waste produced by the food processing industry. Various useful compounds can be extracted from groundnut shell powder, including biochar [32], cellulose nanofibers [33], biofuels such as bioethanol or biodiesel [34,35,36,37,38], carbon nanoparticles [39,40], and silica nanoparticles [41]. Some of these extracts, including biochar, carbon nanoparticles, silica nanoparticles, and peanut shell powder/fibers, can be used as filler material for developing polymer composites.

In this review article, we present a detailed review on the synthesis of various extracts from peanut shell waste, such as peanut shell fiber, cellulose, and biochar, which are widely used in various potential applications. In addition to that, we also discuss the development of peanut-shell-powder-reinforced polymer composites and biocarbon-reinforced composites. A systematic analysis of the mechanical properties of these composites, focusing on key factors such as filler content, surface modification techniques, interfacial adhesion, and processing methods, is presented. We discuss the effects of different parameters on mechanical behavior and highlight the advancements made in this field. Additionally, we address current challenges and ongoing research efforts, providing a comprehensive overview of the state-of-the-art knowledge in the field. To the best of our knowledge, we believe that a similar review on the synthesis, fabrication and analysis of polymer composites with reinforced peanut shell extracts has not been presented in the literature.

2. Composition of Peanut Shell

Peanut shells, an abundant agricultural waste, have emerged as a valuable source for producing various derivatives with potential applications in polymer composites. These derivatives offer advantageous properties that can contribute to the development of high-performance materials. In this subsection, we provide a comprehensive overview of the key derivatives obtained from peanut shells, highlighting their properties and potential applications in polymer composites.

Cellulose, hemicellulose, lignin, protein, and ash are all components of the chemical structure of peanut shells [42]. The primary structural element of plant cell walls is cellulose. Cellulose is a glucose molecule with beta (1–4) linked chain [43]. Hemicellulose, a complex carbohydrate, gives plant cell walls strength and stability. Plant cell walls are made of lignin, an intricate organic polymer that gives them stiffness and strength. Ash is an inorganic substance that is still present after organic material has been burned. Protein is an energy-rich organic molecule that provides a source of fuel for the plant. The peanut shells and significant amounts of the above components present in the peanut shell are shown in Figure 1a,b.

3. Preparation of Peanut Shell Powder

The preparation of peanut shell powder involves a series of steps, including washing, drying, grinding, and milling. Initially, the peanut shells are thoroughly washed to remove any dirt, sand, or other impurities. Following the washing process, the shells can be dried either by exposure to sunlight or by using an oven. Once dried, the shells are ground and milled to reduce the particle size, thereby increasing the available surface area [44]. The prepared peanut shell powder can be used for synthesizing biocarbon, cellulose, and peanut shell ash.

For instance, Usman et al. [45,46] prepared two types of peanut shell powders: untreated and treated with NaOH. To prepare the untreated peanut shell powder, the peanut shells were washed with water and sun-dried for three days. Subsequently, the dried peanut shells were ground in a pulverizing machine and sieved to obtain particles of two different sizes, namely, 0–250 μm and 250–420 μm. In contrast, for the treated peanut shell powder, one gram of peanut shell powder was mixed with 10 mL of NaOH and stirred continuously for four hours. The solution was then filtered, neutralized, and washed with distilled water. The resulting powder was vacuum-dried at 80 °C for 12 h. Another approach was used by Jacob et al. [47,48], who prepared peanut shell powder by grinding and sieving it to a size of 150 μm. This powder was continuously stirred in a 10% NaOH solution for six hours. The resulting solution was decanted, and the shells were washed several times with distilled water until neutral. Finally, the fibers were dried in an oven at 80 °C for six hours. Similarly, Zabba et al. [49] prepared peanut shell powder with an average particle diameter of about 66.84 μm for the synthesis of recycled polypropylene/peanut-shell-powder composites. The peanut shells were ground, and the resulting particles were vacuum-dried for three hours at 70 °C. The obtained powder was then modified with hydrolyzed polyvinyl alcohol (PVOH) by blending it with PVOH (6 wt%) powder. The mixture was heated to 80 °C with the addition of ethanol to completely dissolve the PVOH. After 24 h at room temperature, the two separate layers were decanted, and the precipitate was dried in an oven at 70 °C.

In a different study, Zabba et al. [50,51] used untreated peanut shell powder prepared by grinding dried peanut shells using a kitchen grinder and sieving, resulting in particles ranging from 50 to 110 μm in size. Similarly, Yamoum et al. [52] washed peanut shells with water, dried them in an oven at 80 °C for three days, and then ground and sieved them to obtain uniform-sized particles (200 mesh) for reinforcement in PLA/PSP composites. Hwazen et al. [53] prepared peanut shell powder by thoroughly washing the shells with distilled water to remove dirt and sand, followed by sun drying for several days. The dried shells were milled using a hammer mill for one hour, resulting in a particle size of 121.6 μm. Prabhakar et al. [54] prepared peanut shell powder for reinforcing epoxy composites. The process described by Prabhakar et al. involves several steps in the preparation of peanut shell powder. First, the shells are soaked in water for 1–2 h to eliminate dust, sand, and contaminants, followed by a thorough washing with distilled water. Next, the cleaned shells undergo a 30 min soak in a sodium hydroxide (NaOH) solution to remove greasy materials and lignin. They are then washed again with distilled water. After the washing step, the shells are sun-dried for two days to reduce moisture content. Subsequently, they are further dried in an oven at 120 °C for one day to ensure complete removal of moisture. Once dried, the shells are finely ground, and the resulting powder is sieved to obtain particles within the range of 200–300 μm. It was observed that, by increasing the concentration of NaOH in the preparation process of peanut shell powder, the mechanical and thermal properties of the epoxy composites derived from it were improved. Sareena et al. [55] chemically modified peanut shell powder by stirring it for five hours in a 10% NaOH solution using a mechanical stirrer, followed by a 24 h soaking period. The solution was rinsed and washed with water until it became neutral. The powder was then separated and vacuum-dried for 24 h at 70 °C for subsequent usage. Chatterjee et al. [56] prepared peanut shell powder of sizes ranging between 100–200 μm. They cleaned the shells, oven-dried them at 80 °C for 24 h, and ground them using a mixer grinder. The resulting powder was then used as a filler for polypropylene/PSP composite. Bai et al. [57] prepared biochar from peanut shells. The shells were washed with tap water and rinsed with deionized water under ultrasonic treatment to remove dust and impurities. Then, the shells were dried in an oven at 60 °C for 10 h. The dried shells were ground into powder and sieved to obtain uniform-sized particles. Yallappa et al. [39] prepared peanut shell powder for synthesizing carbon nanoparticles. The groundnut shells were cleaned by soaking them in 70% ethanol to remove dirt and impurities, followed by oven drying at 60 °C overnight to remove moisture. The dried shells were ground into a powder and sieved to obtain particles of 60 μm in size. Similarly, Wu et al. [58] reported the preparation of peanut shell powder for synthesizing biocarbon. The peanut shells were thoroughly washed with deionized water to remove impurities and then oven-dried at 90 °C for 24 h. The dried shells were finely ground using a pulverizer operating at 30,000 rpm.

4. Various Extracts of Peanut Shell

4.1. Synthesis of Peanut Shell Fiber

Peanut shell fibers are natural fibers extracted from the outer shell of peanuts. These fibers possess remarkable mechanical properties, including high tensile strength, modulus, and aspect ratio. They offer numerous advantages such as low cost, biodegradability, and a renewable nature. Peanut shell fibers can be effectively used as reinforcing agents in polymer composites, enhancing their mechanical strength and stiffness [29]. The incorporation of these fibers into polymer matrices improves the load-bearing capacity and resistance to deformation of the resulting composites. Furthermore, surface modification techniques can be employed to enhance the interfacial adhesion between peanut shell fibers and polymer matrices, leading to improved mechanical properties and better dispersion within the composite material. The use of peanut shell fibers in polymer composites presents an environmentally friendly alternative to synthetic fibers and can find applications in lightweight structures, automotive components, and packaging materials [59]. Adeosun et al. [60] synthesized treated and untreated peanut shell fibers from boiled peanut shells. In the given process, the initial step involved collecting the peanut fruits and washing them with water and detergent to eliminate any dirt or impurities. The peanut shells were then boiled with a mixture of alum, salt, and water. For the untreated peanut shell powder, the boiled shells were left to sun dry for three days at a temperature of 33 °C. After drying, the shells were crushed and sieved to obtain particles with a size of 150 μm. In the case of treated groundnut shell powder, the sun-dried shells were cut into small pieces and passed through a mechanical crusher with a 10 mm screen to achieve the desired size reduction. Subsequently, the crushed shells underwent several treatments. This included dewaxing using a mixture of benzene and ethanol, followed by washing and oven drying at 45 °C. The resulting treated peanut shell powder then went through a series of additional processing steps, including steam explosion, enzymatic hydrolysis, alkaline hydrolysis, bleaching, and acid hydrolysis. After each treatment, the powder samples were washed, centrifuged in ethanol, dried in ambient air, and ground to a particle size of 150 μm.

4.2. Synthesis of Biocarbon

Biocarbon, a sustainable carbonaceous filler, is derived from the thermochemical conversion of biomass in an oxygen-limited environment [61]. Various thermal conversion processes such as pyrolysis, carbonization, hydrothermal carbonization, gasification, and torrefaction can be employed to synthesize biocarbon from different types of biomass or biomass waste [56]. Pyrolysis, an endothermic reaction, involves the thermal degradation of biomass in the absence of oxygen or in an inert gas atmosphere. This process yields solid biochar, liquid bio-oil, and fuel gas products. The heat requirement for pyrolysis varies depending on the agricultural biomass type, ranging from 207 to 434 kJ/kg [62]. During pyrolysis, the cellulose, hemicellulose, and lignin polymer fragments present in the biomass are converted into organic vapor in the inert environment. These vapors can be condensed to produce bio-oil, while non-condensable gases are released and can be utilized for heat generation. The remaining carbon-rich residue is known as biochar or biocarbon [63,64]. The yield and quality of biocarbon are influenced by crucial parameters such as heating rate, final temperature, residence time, pressure, and feedstock. Lower temperatures favor the production of solid products, while higher temperatures and shorter residence times promote the formation of condensable products. On the other hand, higher temperatures and longer residence times tend to encourage the formation of non-condensable gaseous products due to secondary reactions [65,66].

In a study by Yallappa et al. [39], peanut shell powder was pyrolyzed to produce carbon nanoparticles. The powder was subjected to pyrolysis in a tube furnace under a nitrogen atmosphere. After cooling and purification steps, the carbonized material was obtained for further use. Bai et al. [57] conducted a study on the preparation of biomass charcoal using peanut shell powder. The powder was heated at different temperatures, ranging from 300 °C to 1000 °C, in a porcelain crucible covered with a lid in an electric muffle furnace. The heating rate was set at 20 °C/min, and the residence time was maintained at 6 h. The synthesized charcoal exhibited a porous structure with excellent adsorptive properties and consisted of oxygen functional groups. In another study by Varma et al. [67], the pyrolysis of peanut shell powder was carried out using a thermogravimetric analyzer under an inert (N₂) atmosphere. Three different heating rates of 10, 20, and 30 °C/min were employed. Jung et al. [68] prepared biochar from peanut shells for phosphate removal using a muffle furnace in the absence of air. The pyrolysis temperature was maintained at 700 °C for a duration of 3 h. The biochar derived from peanut shells exhibited a phosphate removal efficiency of 61.3%. Liu et al. [44] employed a fixed-bed pyrolysis system to produce biochar from peanut shell powder. Approximately 30 g of the powder was placed in a quartz boat and heated to a temperature of 500 °C with a heating rate of 10 °C/min. The sample was held at this temperature for 1 h in the furnace. The study investigated the effects of five influencing factors on the quality of biochar: pyrolysis temperature, retention time, heating rate, gas flow rate, and particle size. The study’s findings indicated that, among the considered parameters, the pyrolysis temperature had the most notable influence on the production and properties of the biochar derived from peanut shells. It was determined that a temperature exceeding 400 °C was appropriate for the slow pyrolysis process of peanut shells. The reduction in particle size had a significant impact on various properties of the biochar. The specific surface area ranged from 15.88 to 20.96 m²/g, the specific pore volume ranged from 0.185 to 1.241 cm³/g, the micropore volume ranged from 0.019 to 0.049 cm³/g, and the bio-oil yield ranged from 21.39 to 32.28%. The optimal values were obtained for a particle size of 0.075 mm. The moisture content of the biochar ranged from 1.14 to 2.25%, the fixed carbon content ranged from 71.23 to 78.70%, the micropore surface area ranged from 2.55 to 9.47 m²/g, and the pore size ranged from 9.839 to 11.248 nm. These properties were significantly affected by the slower heating rates ranging from 1 to 10 °C/min. Pawar et al. [69] developed a screw auger pyrolysis reactor for the continuous production of biochar from peanut shells. Biochar was produced at three different temperatures (400, 450, and 500 °C) with a residence time of 4 min. Yaro et al. [40] synthesized biochar from groundnut shells by placing the powder in a graphite crucible and heating it in a muffle furnace at a temperature of 1200 °C for 5 h. The resulting ash particles were then ball milled using a planetary ball mill to reduce their size. In another study by Picard et al. [70], biocarbon was prepared from peanut shells using a vertical tube pyrolyzer as shown in Figure 2. Approximately 60 g of ground peanut shell powder was introduced into a vessel, which was then sealed and placed in a pyrolyzer. The sample was heated gradually to a temperature of 500 °C at a rate of 10 °C per minute and held at that temperature for 15 min. Throughout this process, a nitrogen atmosphere was maintained. Afterward, the samples were cooled to room temperature and subjected to milling using a Fritsch Pulverisette ball mill for one hour at a speed of 300 rpm. Following milling, the samples were dried extensively to attain a minimal moisture content.

Activated carbon can be prepared through the chemical modification of biocarbon, which improves its physical and chemical properties [71,72]. These enhanced qualities make biocarbon suitable for various applications, including its use as an electrode material for supercapacitors and for the adsorption of heavy metals, dyes, and organic compounds. In the study conducted by Bendane et al. [73], activated carbon (AC) was prepared from peanut shells. The shells were crushed and mixed with ZnCl₂ at a mass ratio of 1:5. The mixture was then carbonized by heating it from 25 °C to 200 °C for 30 min, followed by further heating at 480 °C for 90 min under a nitrogen flow. After the carbonization process, the sample was cooled inside the furnace in the presence of nitrogen gas. To remove residual chloride ions, the samples were washed several times with deionized water. For the preparation of activated carbon specifically for supercapacitor applications using peanut shells, a process described by Guo et al. [74] was followed. In the study conducted by Wu et al. [58], activated carbon for supercapacitor applications was prepared using peanut shell powder. The peanut shell powder was subjected to carbonization in a tube furnace at 800 °C for 3 h under an argon atmosphere. After carbonization, the resulting carbonized powder was further processed. In order to reduce the particle size, the carbonized powder underwent milling using a ball milling machine. Subsequently, the activated carbon was prepared using both a physical method and an impregnation method. For the physical method, the carbonized powder was combined with a KOH solution and stirred for 30 min using a magnetic stirrer. In the impregnation method, KOH was dissolved in an alcohol solution, and then the carbon powder was mixed with the solution. The mixture was heated at 65 °C for 24 h until the alcohol solution evaporated completely. Throughout the process, the mixture was stirred using a magnetic stirrer to ensure proper blending of KOH and carbon powder. The dried powder obtained from both methods was subjected to heating at three different temperatures: 700 °C, 800 °C, and 900 °C for a duration of 3 h under an argon atmosphere. Following the heating process, the powder was ball milled again to achieve particles of uniform size. These particles were then washed multiple times using a 10 wt% HCl solution and deionized water. The resulting carbonized and activated peanut shell particles obtained through the carbonization and activation processes possess a high surface area and exhibit a hierarchical structure suitable for energy storage. To further enhance the electrode’s performance, nitrogen doping and graphene oxide can be introduced. The electrode demonstrates a specific capacitance of 289.4 F/g, which remains acceptable even at high scanning rates. Xiao et al. [75] conducted a study where they prepared activated carbon from peanut shells through the activation process using ZnCl₂. The peanut shells were dried, ground into a powder, and sieved to obtain particles of uniform size. The powder was then mixed with a zinc chloride solution in a specific mass ratio and continuously stirred. The resulting mixture was placed in a Teflon-lined stainless-steel autoclave and heated at a certain temperature for a specific duration. Afterward, the treated mixture underwent calcination in a tube furnace under a controlled environment, followed by cooling to room temperature. The end products were rinsed with hydrochloric acid solution or deionized water before being dried. Nuilek et al. [76] conducted a study where they prepared two types of activated carbon from peanut shell powder using different methods: KOH solid and KOH solute. In the first method, KOH (solid) and pre-carbonized peanut shell were combined and milled together. The mixture was then heated at a specific temperature in a tube furnace under an argon atmosphere. Subsequently, ethyl acetate was used to extract carbon from the mixture. In the second method, the pre-carbonized peanut shell was activated with KOH solute. The shell was ground and mixed with aqueous KOH (solute) for a specific duration, followed by drying. The resulting mixture was then heated in a tube furnace under an argon atmosphere. Similarly, ethyl acetate was used to extract carbon from the mixture. The activated peanut shell was further treated with a 10% H₂SO₄ solution, agitated, washed, and dried. After treating the peanut shell with KOH, it was successfully converted into carbon nanosheets by carbonization at 800 °C for one hour. When activated with KOH (solute), carbon nanosheets have flat surfaces and are more evenly dispersed than when activated with KOH (solid). The resulting carbon nanosheets made from peanut shells have a minimum thickness of less than 50 nm.

Hydrothermal carbonization is another method for synthesizing biocarbon. This method involves the treatment of peanut shells with high pressure and temperature in the presence of water, leading to the formation of biocarbon. Hydrothermal carbonization of peanut shells resulted in a high-quality biocarbon with improved properties such as high surface area and low ash content [77]. Jiang et al. [77] successfully synthesized graphitic porous biomass carbon from peanut shells using chemical processing, hydrothermal carbonization, and ZnCl₂ activation in a CO₂ environment. Different pretreatment methods were tested, and NH₄OH pretreatment showed the highest specific capacitance due to its favorable pore size distribution and high surface area. By increasing the activation temperature, the porosity and electrochemical performance of the carbon material can be further improved. In a three-electrode system, the carbon sample treated with NH₄OH, hydrothermal carbonization, and ZnCl₂/CO₂ activation at 800 °C exhibited a high specific capacitance and surface area. A symmetric supercapacitor constructed using this carbon sample demonstrated excellent cycling stability and energy density. The biocarbon and activated carbon produced through pyrolysis and chemical modification is then characterized for its composition, physical and chemical properties, and its potential applications. Biocarbon produced from peanut shell powder has potential applications in fields such as agriculture, energy [78,79], water purification [64,73,80,81], and environmental remediation. It can also be used as reinforcement for preparing biocomposites.

4.3. Synthesis of Peanut Shell Ash

Peanut shell ash is a byproduct obtained after the combustion of peanut shells. It consists of various inorganic compounds, including silica, potassium, calcium, and magnesium [82]. By incorporating peanut shell ash into polymer composites, it can serve as a filler or additive, enhancing their thermal stability, flame retardancy, and mechanical properties. The peanut shell ash is used as a replacement for ordinary Portland cement [. The inclusion of peanut shell ash in polymer matrices improves the ability of the resulting composites to withstand high temperatures and reduces their susceptibility to burning. Additionally, the presence of inorganic compounds in the ash contributes to increased stiffness and a reduced coefficient of thermal expansion in the composites, thereby improving their dimensional stability. The utilization of peanut shell ash in polymer composites offers a sustainable solution to enhance their performance while utilizing what is otherwise a waste material. To strengthen a locally accessible, highly compressible clay soil, different percentages of groundnut shell ash were used as soil stabilizers [82]. Naidu et al. [83] prepared a hybrid composite by reinforcing banana fiber and peanut shell ash in an epoxy resin. The composite laminate was prepared by a hand lay-up process. With the addition of peanut shell ash, the mechanical properties were improved.

4.4. Synthesis of Cellulose

Cellulose is a fibrillated linear polysaccharide biopolymer synthesized from plants. Because of its renewable and biodegradable properties, it makes for an excellent natural filler. It is the main structural component of plant cell walls and is a complex carbohydrate. As the peanut shell consists of 44.8% cellulose, this makes it attractive as a source of cellulosic material for the creation of nanocrystals and gives it commercial worth in waste management ]. For synthesizing cellulose from peanut shell, first, the peanut shells may be pre-treated by cleaning, grinding, and drying to get rid of contaminants. Then, cellulose is extracted by a chemical process while leaving behind the insoluble lignin, hemicellulose, and other impurities. To accomplish this, acid or alkali may be used to dissolve the other compounds and separate out the cellulose [84,85]. The separated cellulose residue is dried after being cleaned to get rid of any impurities. To attain the appropriate particle or fiber size, mechanical operations like grinding, milling, or sieving are applied to the cellulose-rich material that is left over after chemical treatment [86]. The obtained cellulose can be used as a filler or binder in pharmaceutical tablets, as a dietary fiber ingredient in food items, or it can be converted into cellulose-based products including paper, films, and textiles. Bano et al. [84] synthesized cellulose nanocrystals (CNCs) from peanut shell particles by chemical treatment and an acid hydrolysis process with an overall yield of 12%. The resulting CNCs displayed good crystallinity (74%), thermal stability (>200 °C), and rod-shaped morphology with an average aspect ratio of 12. These results suggest that using groundnut-shell-derived CNCs as an organic filler in composite materials is worthwhile. By using acid hydrolysis, Liu et al. [85] synthesized CNCs from the fibers of peanut shells. He examined the use of response surface methodology to optimize the preparation of CNCs from peanut shells. It was found that the sulfuric acid concentration, reaction temperature, and reaction time all affected the yield of CNCs. The results showed that the yield of cellulose nanocrystals was 44.94% under the optimum conditions of 64.6% sulfuric acid concentration, 49.5 °C reaction temperature, and 28.5 min of reaction time. Punnadiyil et al. [87] isolated microcrystalline cellulose (MCC) and nano cellulose (NC) from peanut shell powder (PSP). The MCC was isolated using alkali treatment and bleaching. The NCs were isolated from MMC by using acid hydrolysis.

5. Preparation of PSP-Reinforced Polymer Composites

5.1. PSP-Reinforced Thermoplastic Composites

The mechanical properties of reinforced polymer composites are greatly influenced by the processing method employed. One of the commonly used methods for fabricating reinforced polymer composites is melt blending, also known as melt extrusion. Researchers have also explored techniques such as solvent casting and resin curing [88] for the preparation of polymer composites with peanut shell powder. Melt blending is a highly suitable method for processing thermoplastic polymers, offering effective ways to work with these materials. In this process, polymer pellets are melted to form a viscous liquid. Amorphous polymers can be processed by heating them above their glass transition temperature, while semi-crystalline polymers require higher temperatures to achieve sufficient softening. During melt blending, various reinforcements, including macro-, micro-, or nanoparticles, can be incorporated into the liquid through shear mixing. This results in a homogeneous mixture. The molten blend can then be shaped into desired samples using techniques like compression molding, injection molding, or extrusion. These shaping methods allow for the creation of final products with the desired form and properties. These processing techniques enable the incorporation of peanut shell powder and other reinforcing materials into polymer matrices, leading to the development of polymer composites with superior mechanical properties.

Obasi et al. [89] synthesized peanut-shell-powder-reinforced low-density polyethylene (LDPE) composite using an extrusion machine. The temperature of the machine ranged between 120 and 150 °C and the speed of the screw was maintained at 50 rpm to obtain the LDPE/peanut-shell-powder composite. The loading of the peanut shell powder ranged between 0 and 25 wt%. Maleated polyethylene (MAPE) was added in the melt, which acted as a compatibilizer. MAPE was added at a concentration of 5% by weight, relative to the amount of filler used. The mixture of LDPE and peanut shell powder was melted in an extrusion machine, and the molten material was extruded into thin sheets. These sheets were then subjected to oven drying at 70 °C overnight to remove any moisture present. After drying, the sheets were stored in an airtight container for a minimum of 40 h in accordance with ASTM D618 standards. In their study, E. Garcia et al. [90] focused on preparing a composite material consisting of high-density polyethylene (HDPE) and peanut shell powder. The researchers utilized a single-screw extruder to blend and process the composite. Different weight percentages (2–10 wt.%) of peanut shell powder were incorporated into the HDPE matrix and melted within the extruder. During the extrusion process, the temperature at the die was maintained at 190 °C, while a rotational speed of 40 rpm was employed. The mixture of HDPE and peanut shell powder was melted and extruded into granules using the extruder. Subsequently, these granules were transferred to an injection molding machine, where dumbbell-shaped samples were prepared, as illustrated in Figure 3a, bof the study. This method of melt blending and extrusion enables the efficient dispersion of peanut shell powder within the HDPE polymer matrix, achieving a composite material with potential applications in various industries. The specific composition and processing parameters used were aimed at optimizing the mechanical properties and performance of the HDPE–peanut-shell-powder composite. The tribological properties were carried out using reciprocating sliding wear test as shown in Figure 3c. The tribological properties HDPE/peanut shell powder composite was not changed with the addition of peanut shell fiber.

Yamoum et al. [52] prepared a biocomposite pellet of polylactic acid (PLA) and peanut shell powder using a twin-screw extruder. The PLA pellets and peanut shell powder were dried at 60 °C for 24 h before processing. The peanut shell powder at 10, 20, 30, and 40 wt.% along with a stabilizer was mixed with PLA in a Bossco mixer for 15 min. The mixture was melt-compounded using a corotating twin-screw extruder. The temperature range of 120–160 °C was maintained from the feed throat to the die with the screw speed of 40 rpm. The compounded pellets were injected into the injection molding machine with a pressure of 80 MPa and injection speed of 40 mm/s to prepare the samples for mechanical testing. The temperature at the nozzle of the injection molding machine and the mold temperature were maintained as 160 °C and 40 °C, respectively.

In the study conducted by Jacob et al. [47,48], recycled HDPE and groundnut shell powder were utilized to create a composite material. The compounding process was performed using a two-roll mill, which consisted of two horizontally opposed stainless-steel rolls rotating towards each other at different speeds. This mill facilitated the mixing of the recycled polymer and reinforcement, resulting in the formation of the composite.

Recycled HDPE from water bottle caps with resin code “2” was used as matrix material. The HDPE was melted and mixed with groundnut-shell-powder filler using a two-roll mill. Different weight fractions of the filler material were tested. The resulting mixture was cured using a hydraulic press, cooled, and machined for characterization tests to evaluate its properties. A similar procedure was adopted by Usman et al. [45,46], where the consolidation of groundnut shell powder and recycled polyethylene involved mixing and compounding it into composites. The mixing and compounding of the materials were carried out according to ASTM standards. The recycled plastic was fed into the two-roll mills, where it melted and formed a band around the front roll. The groundnut-shell-powder filler was then introduced, and cross mixing was performed to achieve uniform dispersion. The compounded material was sheeted out to the desired thickness before further processing.

Chatterjee et al. [56] prepared peanut shell flour (PNSF)-reinforced polypropylene composite by injection molding. In their work, polypropylene, PNSF, and a coupling agent were mixed in a kinetic mixer and transferred to an injection molding machine to prepare a composite. Five samples were prepared by varying PNSF loading by 10 wt% to 50 wt%. With the addition of PNSF, the mechanical properties were reduced, but there was an improvement in the mechanical properties with incorporation of the coupling agent maleic anhydride polypropylene (MAPP).

In the research conducted by Guna et al. [91], a sandwich-type prepreg was prepared using polypropylene (PP) webs, rice husk, and groundnut shell (GNS). The process involved incorporating measured amounts of GNS and RH into the PP web to create various combinations: RH/PP, GNS/PP, and RH/GNS/PP prepregs.

Once the prepregs were prepared, they were placed between aluminum foils and subjected to a hot-pressing process. The hot pressing was conducted at a temperature of 170 °C and a pressure of 3000 psi for a duration of 120 s. After the specified time, cold water was run over the compression mold to facilitate the cooling process, and the resulting specimens were then removed. The purpose of this sandwich-type prepreg is to create a composite material with enhanced properties by incorporating the natural fibers from RH and GNS into the polypropylene matrix. The hot-pressing process helps to facilitate the bonding between the fibers and the matrix, resulting in a consolidated composite structure.

The specific parameters of temperature, pressure, and time chosen for the hot-pressing process are critical to achieving the desired consolidation and mechanical properties of the composite. The use of aluminum foils helps in distributing the pressure uniformly across the prepreg during the hot-pressing process, ensuring proper consolidation.

5.2. PSP-Reinforced Thermosetting Composites

The preparation of peanut-shell-powder-reinforced composite materials involves mixing the desired amount of the powder with various types of resins. The resin and peanut shell powder are combined in a container and stirred thoroughly to achieve a homogeneous mixture. Then, a hardener is added to the mixture and stirred again to ensure proper integration. The prepared mixture is poured into a mold and left to cure, either at room temperature for 24 h or in an oven, based on the specific curing specifications [54]. Different types of resins can be utilized for the composite preparation, such as epoxy resin [53,92], unsaturated polyester/styrene-mixture resin [93], vinyl ester resin [94], and others. Each resin type offers specific properties and characteristics that can be tailored to the desired application.

In a study conducted by Patnaik et al. [95], a hybrid composite was prepared using a conventional hand lay-up technique. The composite consisted of different weights of groundnut shell particulates ranging from 0% to 20% by weight, while a constant load of jute fiber at 30 wt.% was maintained. The resin and hardener were mixed in a mass ratio of 10:1. The filler material was added to the mixture and then impregnated into fabric using a paint brush. A roller was used to remove any trapped air and excess polymer. This process of creating laminate layers continued until the desired thickness was achieved. The composite was subsequently allowed to cure either at ambient temperature or at a specific temperature depending on the curing requirements.

Table 2. Fabrication methods for peanut shell micro- and nanoparticle-reinforced polymer composites.

Polymer Matrix	Filler	Filler Size	Filler %	Fabrication Method	Reference
Recycled polyethylene	Alkali-treated groundnut shell powder	0–300 and 300–600 µm	20 and 25	Melt mixing using two-roll mill	[45]
Recycled HDPE	Alkali-treated groundnut shell powder	150 µm	0–25	Melt mixing	[48]
PLA	Peanut shell with stabilizer	13.46 ± 6.8 µm of diameter and 66.8 ± 13.2 µm	10–40	Melt compounding	[51]
Epoxy resin	Groundnut shell powder	121.6 µm	2–8	Hand lay-up	[53]
Epoxy resin	Alkali-treated peanut shell powder	200–300 µm	5–15 gm	Mixing and curing	[54]
Polypropylene	Peanut shell husk + maleated polyethylene (MAPE)	<300 µm	10–50	Kinetic mixer followed by injection molding	[56]
HDPE	Peanut shell fiber	<127 µm	2–10%	Melt extrusion	[90]
Unsaturated polyester/styrene-mixture resin	Untreated peanut shell particles Alkali-treated Silane-treated	<0.5 mm	0–40 and 25–55	Mixing and curing	[93]
Vinyl ester resin	Alkali-treated peanut shell powder	600 µm	20–60	Mixing and curing	[94]
Epoxy resin	Groundnut shell particles + jute fabric	100–200 µm	5–20	Hand lay-up	[95]
Recycled polypropylene	Groundnut shell particles	66.84 µm	10–40	Melt blending, compression molding, and irradiated with electron beam	[96]
PLA + polyethylene glycol (PEG)	Groundnut-shell-powder ash	50–100 nm	10–30	Melt extrusion	[97]
Unsaturated polyester resin	Groundnut shell powder	0.10–0.25 mm	0–30	Stirring and curing	[98]

provides a compilation of research works focusing on peanut shell micro- and nanoparticle-reinforced polymer composites, which can serve as a reference for further exploration and analysis of the topic.

6. Preparation of PSP-Derived Biocarbon-Reinforced Polymer Composites

Picard et al. [70] prepared a green composite by incorporating 20 wt.% peanut shell biocarbon into 80 wt.% poly (trimethylene terephthalate) (PTT), a high-temperature engineering thermoplastic with 35% biobased content. The composite was fabricated using an Xplor DSM micro-injector with a corotating twin-screw configuration, employing a temperature of 250 °C, a screw speed of 100 rpm, a mixing time of 2 min, and a mold temperature of 40 °C. On the other hand, Balaji et al. [99] conducted a study where they produced annealed biochar particles from waste peanut shells. The shells were pyrolyzed at 400 °C and the resulting biochar underwent an annealing process. The biochar particles were ball milled for 300 min to achieve uniform size. Annealing was performed in a vacuum electric furnace under a nitrogen environment with a heating rate of 100 °C/h. The process involved a dwell time of 90 min at a maximum temperature of 1000–1050 °C, using [Fe(NO₃)₃] catalyst to enhance graphitization. The resulting annealed biocarbon particles had a size of 800 nm. These particles were then mixed with aloe vera fibers and incorporated into an epoxy resin to prepare the composite.

7. Mechanical Properties

When incorporating fillers, such as fibers or powders, into polymers, it can greatly impact the mechanical properties of the resulting composite. The addition of fillers has the potential to either enhance or diminish the tensile strength of the composite material. Fibers, specifically, play a crucial role in improving strength by effectively transferring stresses from the polymer to the reinforcement.

Two critical factors that significantly influence the mechanical properties of fiber-reinforced composites are the volume fraction of the filler and the quality of the interfacial adhesion between the filler and the matrix. The strength of the bond at the interface is influenced by various factors, including the nature of the filler and polymer components, the aspect ratio of the filler, the chosen processing method, and the treatment applied to the filler material.

Numerous research studies have consistently indicated that the incorporation of natural fillers into polymers tends to decrease the tensile strength of the composite. Furthermore, as the filler content increases, the tensile strength typically experiences a further reduction. This can be attributed to an increase in the interfacial area between the filler and the polymer matrix, which often leads to inadequate bonding at the interface. The insufficient bonding weakens the overall composite structure, resulting in a decrease in tensile strength.

Obasi et al. [89] investigated the impact of incorporating peanut shell powder into LDPE (low-density polyethylene) and observed a decrease in tensile strength as the amount of peanut shell powder increased. However, they also observed that other mechanical properties such as Young’s modulus, flexural strength, elastic modulus, toughness, and hardness exhibited an increase with the addition of peanut shell powder.

The poor compatibilization between the peanut shell powder and the polymer matrix resulted in decreased tensile strength, as stress propagation between the two materials was compromised. However, the addition of a compatibilizer, such as maleated polyethylene (MAPE), improved the dispersion of the peanut shell powder in the polymer matrix and enhanced the interfacial bonding between the filler and the matrix. This led to an improvement in the tensile strength and overall mechanical properties of the composite. Similar results were observed when peanut shell powder was added to polypropylene [56], where the addition of a coupling agent (PP-g-MA) improved the mechanical properties. The concentration of PP-g-MA in the composites played a crucial role in enhancing the mechanical properties, with composites containing 4 wt.% PP-g-MA showing the best results. For instance, the flexural modulus increased by 243% (1154 MPa) at a filler concentration of 50 wt.% and 4 wt.% PP-g-MA, as compared to pure PP (474 MPa).

The mechanical properties of peanut-shell-reinforced composites can be enhanced by surface modification of the peanut shell powder. This is typically achieved through chemical treatment using substances like polyvinyl alcohol, ethylene/acrylic acid copolymer, and NaOH. Research has shown that chemically modified peanut shell powder in recycled polypropylene (RPP) composites exhibits improved tensile strength, elongation, and modulus of elasticity compared to unmodified peanut shell in recycled PP composites [45,46,49,50,100].

The improved mechanical properties can be attributed to the enhanced adhesion at the interface between the filler and the polymer matrix resulting from the chemical modification. The hydroxyl groups present in the peanut shell powder interact with the modifying chemical, leading to better compatibility between the filler and the matrix.

For instance, Jacob et al. [47] reported that chemically modified peanut shell powder was used as reinforcement in recycled high-density polyethylene (HDPE) composites. The addition of the modified peanut shell powder improved the thermo-mechanical properties of the composites. Optimum values were achieved at specific weight fractions of the reinforcement. The tensile strength initially increased with the weight percentage of the reinforcement, reaching a maximum at a certain point before decreasing. The elastic modulus and hardness also showed improvements with the increasing percentage of reinforcement. Similar trends have been observed in thermoset polymer matrices, where the mechanical properties can be improved by incorporating chemically treated peanut shell powder as reinforcement. Ikladious et al. [93] conducted a study using unsaturated polyester resin (UPE) as the matrix and compared the performance of composites reinforced with alkali-treated peanut shell powder (ATP) and silane-treated peanut shell powder (STP). The results showed that the silane-treated peanut-shell-powder-reinforced composites (STP) exhibited superior mechanical properties compared to the alkali-treated peanut-shell-powder-reinforced composites (ATP). As the filler content increased in both types of composites, the compressive strength increased, surpassing the values obtained for the neat UPE. The maximum compressive strength was achieved at a filler loading of 35%. Furthermore, even with a further increase in the amount of treated filler to 55 wt.%, the strength values remained satisfactory. This indicated that both alkali-treated and silane-treated peanut shell powder effectively improved the compressive strength of the thermoset polymer composites. These findings emphasize the significance of surface modification in enhancing interfacial adhesion and reinforcing properties of the peanut shell powder within the polymer matrix, ultimately resulting in improved mechanical properties. A summary of results of mechanical properties of peanut-shell-reinforced polymer composites are tabulated in

Table 3. Mechanical properties of peanut-shell-reinforced polymer composites.

Polymer Matrix	Peanut Shell Filler	Optimum Filler wt.%	Tensile Strength MPa	Elongation at Break %	Young’s Modulus	Flexural Strength	Impact Strength (kJ/m2)	Reference
Recycled polyethylene	Alkali-treated groundnut shell powder	20	7.7	-	120			[46]
Recycled HDPE	Alkali-treated peanut shell powder	20	25–30	-	77.73			[48]
PLA	Peanut shell with stabilizer	30 10	67.02 ± 0.46	7.83 ± 1.09	2261.42 ±31.73		8.40 ± 0.50	[52]
Epoxy resin	Groundnut shell powder	6 4				150–160	6	[53]
Epoxy resin	Alkali-treated peanut shell powder	15 gm	35.56		1960			[54]
Polypropylene	Peanut shell husk + maleated polyethylene (MAPE)	10	26			29		[56]
LDPE	Peanut husk Compatibilized peanut husk Peanut husk Compatibilized peanut husk	5 5 20 20	15 19	11 13 -	2 GPa 2.3 GPa	27 MPa 31 MPa		[89]
HDPE	Peanut shell fiber	8 2	23.53 ± 0.20	-	1260.91 ± 256.25	-		[90]
Unsaturated polyester/styrene mixture resin	Alkali-treated peanut shell particles Silane-treated Alkali-treated	35				35–40 >40		[93]
Vinyl ester resin	Alkali-treated peanut shell powder	40 50	28.09		1872.65		42.91	[94]
Epoxy resin	Groundnut shell particles + jute fabric	15	24.92			35.04		[95]
Recycled polypropylene	Groundnut shell particles	10 40	25	4	1300			[96]
PLA + polyethylene glycol (PEG)	Groundnut-shell-powder ash	20	114.87	8.61	2305.361 MPa			[97]

.

8. Biodegradability

Biodegradability refers to the ability of a substance or material to undergo biodegradation, a natural process where microorganisms break down the material into simpler compounds. These compounds can then be further metabolized by other organisms, or integrated into the natural carbon cycle, reducing the environmental impact of the original material. Biodegradability is an essential factor in assessing the environmental impact of materials and products. It is very important to check the biodegradability of peanut-shell-powder-reinforced composites to ensure safe disposal in the environment. Usman et al. [45] performed the biodegradability test on groundnut-shell-powder-reinforced recycled-polyethylene composites by inoculating the samples with Aspergillus niger (A. niger) on a potato dextrose agar media and incubating at 25 °C for 21 days. Evidently, fungal growth was observed on the composite surface after only 3 days, indicating that the groundnut shell powder actively supported fungal growth. This presence of fungi on the composite sample provides strong evidence for the biodegradability of these polymer composites, suggesting that the groundnut-shell-powder-reinforced composites promoted the fungal growth during the testing period. In another study, Yamoum et al. [52] conducted a biodegradability test on a peanut-shell-powder-reinforced PLA composite sample. The sample was buried in soil for 8 weeks at a temperature of 60 °C. The biocomposites had higher weight loss than pure PLA. The weight loss increased as the loading of peanut shell powder increased because of the absorption of moisture during composting by the peanut shell powder. The addition of PNS improves the biodegradation of composites.

9. Current Challenges and Future Directions

The use of peanut shell powder as a filler in polymer composites poses ongoing challenges that warrant further exploration. One key challenge revolves around establishing robust interfacial adhesion between the peanut shell powder and the polymer matrix. Inadequate bonding at the interface can result in diminished mechanical properties and compromised composite performance. To tackle this challenge, scientists are actively investigating different strategies. These include employing surface modification techniques and incorporating compatibilizers to improve interfacial adhesion and facilitate efficient stress transfer between the filler and the matrix. For instance, Obasi et al. [89] reported a study on fabricating a biocomposite comprised of peanut husk filler and low-density polyethylene (LDPE) matrix by utilizing a compatibilization modification process. The interfacial adhesion between the filler and matrix was reported to be enhanced in the presence of the compatibilizer maleated polyethylene (MAPE). These approaches aim to enhance the overall performance of composites. Another significant challenge is ensuring uniform dispersion of the peanut-shell-powder particles within the polymer matrix. Inadequate dispersion can result in agglomeration or clustering of the filler, leading to localized stress concentrations and reduced mechanical properties. Cañigueral et al. [101] concluded that good filler dispersion in the composite matrix led to increased composite modulus. Developing effective processing techniques and optimizing the mixing parameters are essential for achieving a homogeneous distribution of the filler throughout the composite matrix [102]. Determining the optimal filler content is also a challenge in peanut-shell-powder composites. While higher filler loading can enhance certain properties, such as stiffness and hardness, excessive filler content may negatively impact other mechanical properties and processing characteristics. For example, Zaman et al. [103] studied the performance of polypropylene composites that were doped with compatibilizers including nanoclay and peanut shell flour. It was concluded that the addition of excess nanoclay did not improve the final properties of polymer composites, which could be due to further agglomeration and poor dispersion of the nanoclay. Consequently, this led to increased voids and cracks in polypropylene/peanut-shell-flour composites. Finding the right balance between filler content and polymer matrix is crucial to achieve the desired performance for specific applications.

In addition to technical challenges, economic feasibility and sustainability considerations play a vital role in the widespread adoption of peanut-shell-powder composites. Assessing the cost-effectiveness of incorporating peanut shell powder, including its collection, processing, and compatibility with different polymer matrices, is crucial for industrial scalability. Okonji et al. [104] reported a study on effective replacement of expensive binder in foundry with cost-effective, eco-friendly sand materials comprised of silica sand and peanut-shell-ash powder. Utilizing peanut shell powder as a filler to mitigate environmental regulatory concerns and achieve cost reduction is crucial for ensuring the long-term sustainability of these composites [105].

Lastly, tailoring the peanut-shell-powder composites for specific applications is a challenge that necessitates a deeper understanding of the relationships between processing parameters, filler content, and the targeted performance characteristics. This includes investigating the influence of variables such as the particle size, aspect ratio, and surface chemistry of the peanut shell powder on the final composite properties. Addressing these ongoing issues and challenges will contribute to the advancement and widespread adoption of high-performance and sustainable peanut-shell-powder composites, enabling their utilization in various industries and applications.

10. Potential Applications

Peanut-shell-powder composites have garnered significant attention in various industrial sectors due to their unique properties and sustainable nature. In the automotive industry, these composites find applications in interior components such as door panels, dashboard trims, and seat structures. They offer excellent mechanical properties, including high strength, impact resistance, and dimensional stability, making them suitable for lightweight and eco-friendly automotive parts. Furthermore, their natural fibers provide aesthetic appeal and can contribute to reducing the overall weight of vehicles, improving fuel efficiency.

In the packaging industry, peanut-shell-powder composites are utilized for the production of food packaging films. These composites offer desirable barrier properties, such as resistance to moisture, gases, and UV radiation, making them suitable for food packaging applications. Additionally, their biodegradability and renewable nature make them an environmentally friendly alternative to conventional packaging materials. A recent study by Usman et al. reported that the addition of peanut shell powder, either treated or untreated, promotes the biodegradability of recycled PP. Meng et al. [106] prepared starch–chitosan film by adding peanut shell and skin extracts. The peanut shell and skin powder was used to prepare the film-forming solution. The film-forming solution was mixed with starch and chitosan and prepared a film. With the addition of peanut shell and skin extracts, the viscosity of the starch–chitosan film-forming solution was reduced. The water vapor permeability and swelling of the film was decreased with the addition of peanut shell and peanut skin extracts. The hydrophilicity of the film was greatly influenced by peanut shell extracts compared to peanut skin extracts. The color and opacity of the film were both improved by both extracts. And the starch–chitosan film infused with peanut skin extracts had greater antioxidant power than the composite film infused with peanut shell extracts. The tensile strength of the film was decreased by the addition of peanut shell extract and the tensile strength of the film was improved with the addition of peanut skin extract. In order to create bio-nanocomposite food-packaging film, Raj et al. [107] synthesized cellulose microfiber from peanut shell. Three types of films were prepared by using agar solution (AS), peanut shell powder (PSP), and cellulose microfiber (CMF) synthesized from peanut shell powder. When compared to the other two films, the CMF film exhibited good mechanical characteristics. The tensile strength and Young’s modulus of CMF film were found to be 49.4 ± 4.3 and 1.71 ± 0.07, respectively, which were greater than the values for AS film and PSP film. The CMF films exhibited improvement in water solubility and opacity compared with the other two films. Due to their advantageous benefits, CMF films can be used as a substitute for petroleum products, as a natural filler, and as a way to turn agricultural waste into a useful product. They can also be utilized in a variety of applications, including food packaging and tissue engineering, among others. Jayaraj et al. [86] prepared a green composite film for food packaging application by using nanocellulose synthesized from peanut shell. The composite film was prepared using chitosan, iron nanoparticles, and nanocellulose. The biodegradable polymer-based film prepared from CNCs showed high degradation temperature, which demonstrated high thermal stability. Due to this, these films can be successfully used as packaging material to extend the shelf life of food products.

The construction sector benefits from peanut-shell-powder composites in various applications. These composites can be used for manufacturing decking materials, flooring, ceiling tiles [108], and insulation panels [109,110]. They possess excellent thermal insulation properties, fire resistance, and durability, contributing to energy-efficient and sustainable building solutions. Moreover, their lightweight nature simplifies construction processes and reduces transportation costs. The electronics and electrical sectors find applications for peanut-shell-powder-derived biocarbon, which can be used for manufacturing super capacitors [109] and electrodes [111].

The industrial applications of peanut-shell-powder composites are extensive and span across the automotive, packaging, construction, furniture, electronics, agriculture, aerospace, and sports sectors. Their unique combination of mechanical properties, sustainability, and versatility makes them a promising material choice for various industrial applications.

11. Conclusions

In summary, this review article offers a comprehensive overview of the mechanical properties of polymer composites reinforced with peanut shell powder extracts. By conducting a thorough analysis of the existing literature, we have examined how various factors, including filler content, surface modification techniques, interfacial adhesion, and processing methods, influence the mechanical performance of these composites. Our findings highlight the significant impact of incorporating peanut shell derivatives as a reinforcing filler on properties such as tensile strength, flexural modulus, elongation, and hardness. We have also identified key challenges and ongoing research endeavors in this field, such as optimizing processing techniques, exploring novel surface modification methods, and investigating different polymer matrices and fillers. Addressing these challenges will enable researchers and industry professionals to further improve the mechanical properties of peanut-shell-based polymer composites and broaden their industrial applications.