Assessment of eggshell-based material as a green-composite filler: Project milestones and future potential as an engineering material

2.1 Preparation of ES particles

The preparation of ES waste for different applications involves relatively similar stages, that is, waste collection, flushing, drying, and milling. ES waste is relatively easy to collect from the food industry [32], the waste industry [33], poultry farmers [34], the cake industry [35], local markets [36,37,38,39,40], hotels [4], the egg and chicken processing industry [41], food industry [42], and household waste [43].

The washing of ESs is carried out to remove dirt or foreign particles, using warm water and soap [44], fresh water [31], and sodium hypochlorite (NaOCl) solution [45]. To remove the ES membrane, Lumlong et al. [46] soaked it in a vinegar solution for 30 min. The ES membrane floats and can be removed, whereas the deposited particles are calcium carbonate, which is a filler material. The separation of the inner membrane from the ES can be performed by mixing the eggshell powder (ESP) in NaOCl solution, stirring for 30 min, and then storing it at room temperature until two layers are formed. The top layer is removed, and the precipitated layer is washed 10–12 times with distilled water. The powder is then stirred in a mixture of 6% isophthalic acid in ethanol for 1 h [45]. In addition, Nayak et al. [47] treated ESs with sodium chloride solution at 70°C.

Drying is carried out to remove water, odor, organic membrane contaminants, and egg remnants from the shell. The moisture in ESs can be dried naturally or by heating. Forced drying has been carried out in heating ovens at various temperatures and with various heating times, such as 60°C for 60 min [48], 80°C for 24 h [40], 100°C for 2 h [46], 100°C for 4 h [45,47], 105°C for 2 h [49], 105°C ± 5°C for 24 h [50,51], and at a temperature of 140°C to obtain a constant weight [45]. Meanwhile, the natural heating approach uses the sun’s heat [31]. To keep the ES particles dry and avoid contact with outside air, they should be stored in a desiccator [47].

Milling and refining ES particles can be carried out using a ball mill, grinding, and blender machine. Various speed settings for the ball mill machine have been used in previous studies, such as a rotating speed of 6,000 rpm for 6 h [44]; a speed of 750 rpm for 1 h [52]; and a low speed of 250 rpm, sieving for 20 min, and then repeating this process three times to obtain particles with a size of 106 μm [53]. The manufacturing of ES particles can also be carried out with a grinding machine [31,40,46]. The densities obtained are in the range of 2.5–2.6 g/cm³ [40]. A blender machine can also be used to grind the ES particles to obtain an average particle size of 100 μm [45].

Separating the membranes from ES particles can be carried out using precipitation. The most widely used mechanism involves adding distilled water to the ES particles and stirring them manually. After that, natural deposition is allowed to occur; the denser ES settles, whereas the lighter membrane floats. Thus, this membrane is easily removed by draining it out of its container [51].

2.2 Pre-treatment of ESs

Pre-treatment of ESs can increase the composite’s bond strength by modifying the particle surface. This surface modification can be carried out by soaking the ES in a specific solution. In one study, raw ESs immersed in lard and palm oil showed increased hydrophobic properties, although the level of oil concentration had no significant effect on changes in their hydrophobic properties [46]. ES particles were also separated via immersion in a 10% NaOH solution for 6 min. The heavier ES particle layer was stirred in a mixture of 6% isophthalic acid and ethanol for 1 h and dried at 80°C. As a coupling agent, isophthalic acid exhibited better strength and interactions in LDPE/ESP composites [54].

Another method for modifying the surface of ES particles is soaking with stearic acid. ES particles, ethanol, and distilled water were mixed in a ratio of 1:1:3 with a magnetic stirrer for 2 h at room temperature. The next step was heating the mixture at 80°C for 30 min and adding 2 wt% stearic acid solution. The mixture was continuously stirred for 2.5 h to ensure a homogeneous mixture. Particles treated with 2 wt% stearic acid were dried at 105°C for 24 h [51].

ES particles, either untreated or with calcination treatment, exhibited good interfacial bonds with various matrices, including both metallic and polymeric matrices. However, the calcination treatment of ESs resulted in better interfacial bonding when compared to that of raw ESs [10]. Calcination is the process of heating solids at high temperatures to remove volatile substances, oxidize some of the mass, or make the material more brittle. Calcination is also known as carbonation. The temperature and time of calcination treatment play important roles in changing the ESs’ structure. The use of an appropriate carbonation temperature can make the carbon porous. The formation of pores on carbon was obtained using CaCO₃. These conditions allow carbon to act as an adsorbent to remove various water and non-aqueous contaminants [38].

The carbonation of ESs at 500°C for 3 h was shown to be capable of removing carbon-containing materials. The carbon released during carbonation leaves through the pores. The use of carbonated ES particles as fillers in metal composites increased the volume of porosity and the light density, along with the percentage of ES added [7]. ESs that were calcined at temperatures of 600, 700, 800, 900, and 1,000°C for 2 h resulted in a more variable weight reduction in samples. The weight loss of each sample at these temperatures was 5, 5, 26, 46, and 52.8%. Temperatures of 600 and 700°C for 2 h did not have enough energy to cause the decomposition of CaCO₃ [15], but the surface area increased to 276 m²/g [38]. When the temperature was increased continuously to 800°C for 2 h, the CaO and CO₂ phases began to form, although in small amounts [15], but there was a significant increase in the surface area, to 626 m²/g [38]. Increasing the temperature to 900 and 1,000°C for 2 h caused the complete decomposition of CaCO₃ into CaO and CO₂ [15].

The optimum calcination temperature to produce pure CaCO₃ was observed at 700°C for 1 h. Calcium carbonate under these conditions was very effectively used to synthesize hydroxyapatite (HAP) [55]. When the treatment time was increased to 5 h, CaCO₃ was observed to change into an elongated structure [56]. The decomposition of calcium carbonate began at a calcination temperature of 800°C for 1 h. In this condition, the CaO phase began to appear, with a microstructure resembling a flower and with a specific surface area of 8.71 m²/g. The phase formed at this temperature is β-TCP, and this phase predominates [55]. When the duration of calcination was increased to 4 h with nitrogen media, a phase change to CaO occurred gradually, although in limited quantities, with CaCO₃ as the main component [16].

ESs calcined in a crucible at 900°C for 1 h left traces of Fe in the form of Ca₂Fe₂O₁₁, and showed a decreased calcium content and an increased oxygen content. ES calcination treatment resulted in a decrease in the flexural modulus, tensile strength, tensile modulus, hardness, and wear index, but the impact strength increased [57]. In duck ESs, researchers observed a change in the rhombohedral structure of CaCO₃ to CaO in the face-centered cubic form. Calcined duck ESs exhibited good dispersion and electrical properties and high porosity. Thus, duck ESs could potentially be used as dielectric materials, absorbents, and fillers [5].

Calcination at 900°C for 4 h with atmospheric and nitrogen media showed that a complete structure of CaO was formed. Calcination temperatures higher than 900°C are not appropriate carbonation temperatures because endothermic activation reactions can occur [44]. The thermal decomposition process of ESs is shown in Eq. (1). CaO is very hygroscopic by nature and quickly absorbs water from the atmosphere, thus forming Ca(OH)₂, as shown by Eq. (2).

The decomposition of protein-containing sulfur in the egg white produces H₂S. H₂S cannot react directly with CaCO₃ to form CaS because H₂S is a weak acid. However, H₂S can react with CaO through H₂O, as shown by Eqs. (2) and (3). The CO₂ produced by reaction (1) can drive the activation process, as shown in Eq. (4), which leads to the formation of a pore network on the carbon.

ES particles in the form of Ca(OH)₂ were found to be very suitable for use as catalysts and additives when compared to CaCO₃ particles [56]. The Gibbs energy equation can be used to estimate the minimum thermal decomposition temperature. In the calcination process of chicken ESs, the thermal decomposition temperature occurs at a temperature of 848°C [15]. Calcination temperature has a great effect on the formation of porous carbon. This porous carbon has been effectively used as an adsorbent to remove contaminants from water and non-aqueous solutions. Research has also shown that ES can be used as an activating agent in the manufacturing of carbon, and that it is cheap, effective, and environmentally friendly [38].

ESs treated with calcination up to a temperature of 1,200°C contain carbon in the form of graphite. ES particles obtained via calcination have a much smaller grain size, resulting in higher fineness. These conditions resulted in a relatively uniform distribution of particles in the metal matrix, and no agglomeration of ES particles occurred. The interface wettability between the ES particles and the metal matrix was the reason for the increase in the mechanical properties of the composite [58].

3.1 Manufacturing of metal matrix composites

3.2 Mechanical behavior

The utilization of ESs as a reinforcement in MMCs had been widely carried out by researchers in recent years. ESs offer exciting prospects in reinforcements and fillers because of the characteristics of ESs observed in both untreated and carbonated conditions. This organic material is available in abundance, cheap, and non-toxic. The characteristics of the matrix determine the quality of the resulting composite. The mechanical properties studied include hardness, tensile strength, compressive strength, impact strength, and density.

3.3 Thermal behavior

The presence of ESs tends to increase the TC and the CTE. The addition of ESP to the AZ31B matrix alloy confirmed the improvements in the TC and CTE. An increase in the ESP percentage and an increase in the temperature from 50 to 400°C were observed to decrease TC and CTE. ES particles increased the formation of solid interfacial compounds in Mg₂Ca and Mg₂SiO₄, as identified by means of XRD analysis, with ES particles acting as a thermal barrier, thereby decreasing TC [66].

A study showed that the thermal stability of nanocomposite ESP (NCESP) was higher than that of regular ES particles. NCESP with silver-based nanoparticles generated in situ could be used as an antibacterial cleaning powder in household appliances. ESs could also be used as an inexpensive antibacterial filler with a polymer matrix to manufacture antibacterial nanocomposites [71].

3.4 Tribological behavior

The wear resistance of Al–1.5Mg–1.5Sn was increased up to 65% with the addition of 3 wt% ES particles. Based on the wear morphology, identified through SEM-EDX, the types of wear were abrasion, adhesive, delamination, and oxidative wear. The addition of ESs tends to lead to a higher porosity, which improves the composite surface’s wear rate [7]. This result contradicts that the one reported by Agunsoye et al. [69], who found that the higher the content of ES particles (2–10 wt%), the lower the wear rate. Lower wear rates correlate with increased hardness and strength. Ononiwu et al. [72] showed that different carbonation temperatures of the ESs over a period of 2 h had differing impacts on the tribological behavior of the composite. Carbonated ESs at temperatures of 900 and 1,200°C as composite fillers showed excellent wear resistance compared to untreated ESs. This increased wear resistance was due to the ability of the ES particles to resist shear flow in the AA6063 Al alloy.

3.5 Corrosion behavior

Corrosion behavior is influenced by many factors, including water content, corrosive elements in the air, temperature gradients, and physical and chemical stability [19]. The addition of certain elements can reduce the corrosion rate of steel or other metal composites. Recent trends indicate that researchers have started to use organic materials for corrosion control. The method involves adding organic material to the metal composite or utilizing organic material as a coating material. Balan et al. [61] used ANOVA to determine the optimal composition of glass fiber, ES particles, and stirring time in the manufacturing of Al matrix composites. Their study showed that the use of 6 wt% ES and 15 wt% glass fibers as additional elements in the Al7075 composite, with a stirring time of 5 min, could significantly reduce the corrosion rate. Other studies have shown that corrosion resistance can be increased by adding ESs [70,73,74]. The addition of ES particles could reduce the corrosion rate of Al alloys. The corrosion rate reached the lowest value at 12.5% ES with the addition of ESs without treatment or with carbonation. The corrosion rate decreased from 5.79 to 5.24 mmpy and from 5.79 to 4.60 mmpy for untreated and carbonation treatment of ESs, respectively. The combination of heat treatment and ES carbonation resulted in the lowest corrosion rate, which was 4.28 mmpy [19]. Corrosion control was also carried out by depositing ES particles using the electrophoretic deposition method. Bakar et al. [75] experimented with depositing calcium carbonate from ESs on gray cast iron. Their study showed that the calcium carbonate layer formed prevented oxygen diffusion, inhibiting corrosion on the steel surface.

4.1 Manufacturing of polymer matrix composites

4.2 Mechanical behavior

5.1 ES–silicon carbide (SiC)

ESs were treated with carbonation at 500°C for 3 h to remove the ES membrane. The secondary reinforcement of the hybrid composite used SiC with the aim of increasing its strength; modulus; abrasive wear resistance; thermal stability; and resistance to acids, alkalis, and salts up to 800°C. Carbonated ES and silica particles could be evenly distributed in the Al alloy AA2014. The maximum specific strength, thermal expansion, porosity, density, and fabrication cost reached the minimum values at 7.5 wt% ES and 2.5 wt% SiC [63]. Girimurugan et al. [88] used an Al6061 Al matrix with the same hybrid reinforcement. This research showed that adding a hybrid reinforcement increased the tensile strength, percentage of elongation, yield strength, compressive strength, and hardness by more than 50%. However, the impact energy and impact strength decreased. This decrease was due to the weakening of the interfacial bond between the reinforcement and matrix.

5.2 ES–SiC–Al₂O₃

ESs increased the fatigue strength of the Al7075–T6 hybrid composite material containing ES reinforcement, SiC, and Al₂O₃. Using the Taguchi method, researchers found that the stirring and composition parameters obtained with the addition of 1.5 wt% ESs, 1.5 wt% SiC, and 1.5 wt% Al₂O₃, and a mechanical stirring time of 360 s produced the highest fatigue strength in light-load applications [60].

5.3 ES–glass fiber type-C

The stir casting method was used to manufacture aluminum matrix composite with type-C glass fiber reinforcement and ES particles. Process parameters were optimized using the Taguchi method. Hybrid reinforcement significantly contributed to its corrosive behavior and tensile strength. This research showed that for the reinforcing composition of 15 wt% glass fiber and 6 wt% ES particles, a stirring time of 15 min produced the optimum tensile strength. A stirring time of 5 min was required for the corrosive rate to be low [61].

5.4 ES–mengkuang fiber

The combination of mengkuang fiber and ESs increased the tensile strength and impact strength of the NR/HDPE hybrid composite. The strong bond between the ES filler and the matrix and the more even distribution of the ES particles were the main reasons for the increased mechanical properties. The higher the weight fraction of the ES, the better the tensile strength and impact of the composite. The same was observed for the tensile modulus, which reached its maximum value at the fiber and filler ratio of 15/5. The high fiber content in the composite led to high water absorption as well. This was due to the hydrophilic nature of the fiber. The treated fibers exhibited better interfacial bonds and water absorption, thus preventing the dispersion of water molecules [89].

5.5 Bio-composites

The hand lay-up method was used to make a thin film from a bio-composite material. The bio-composite thickness swelling increased with the increase in the banana peel/ES filler ratio, for both banana peels that were given treatment or for those without treatment. The lower the ES content, the greater the thickness swelling of the bio-composite [90]. Coconut jute fiber and ES particles in a different bio-composite were bonded with gelatin gel. The weight fraction of 99% ES and 1% coconut jute fiber showed the optimal hardness. This study showed that gelatin gel could form an interfacial bond between the filler and the fiber [91].

5.6 ESs–Bauhinia racemosa fiber

Balan et al. optimized the effect of ES content and Bauhinia racemosa fiber on hybrid composites using ANOVA. ES filler significantly affects the hardness and water absorption of polymer composites. The composition of the fiber and ES must be designed appropriately according to the desired conditions. The maximum composite hardness was observed when the fiber and filler composition reached the maximum level. However, the amount of fiber must be minimal to ensure that there is minimal water absorption. This is due to the nature of the fiber, which absorbs water upon exposure to water. A small ES particle size could lead to reduced porosity. Chemical treatment of the fiber reduced its hydrophilic properties [43].

5.7 ES–water hyacinth–basalt fiber

A hybrid composite reinforced with basalt fiber with ESs and a water hyacinth filler was created. Manikandan et al. compared the characteristics of hybrid composites using an epoxy matrix with the addition of ES and water hyacinth particles. The 3% ES and 3% water hyacinth composition showed excellent flexural strength, tensile strength, and impact strength. This research also confirmed that adding ESs or water hyacinth improved the composites’ observed mechanical properties [92].

5.8 ES–rice husk–coir fiber

A composite of coir fiber, rice husk, and ESs was made using the hand lay-up method. The composition of 40 wt% coir fiber, 48 wt% epoxy resin, 12 wt% ES particles, and rice husk offered the best mechanical properties. The composites’ tensile strength, flexural strength, and impact strength increased with an increase in the filler content from 0 to 12 wt% [93].

5.9 ES–montmorillonite nanoclay (MMT-NC)jute fiber–coir fiber

Ganesan et al. compared the behavior of hybrid composites in dry and wet conditions (with exposure to seawater and river water). The composites reinforced with jute fiber and coir fiber showed increased mechanical properties with the addition of ES filler and nano clay. This study showed that a composition of 1.5 wt% ES, 1.5 wt% MMT-NC, 10 wt% jute fiber, 10 wt% coir, and 77 wt% polyester matrix showed an increase in tensile strength, tensile modulus, flexural strength, flexural modulus, and impact strength under all environmental conditions under study. In general, the addition of filler led to reduced water absorption [78].

5.10 ES–rice husk–cassava peel–palm kernel shell

Particleboard hybrid composites provide a means of using agricultural waste for the creation of indoor furniture with good mechanical properties. Composites with a composition of 50% rice husks by weight, 25% ESs by weight, and 25% cassava peels by weight showed the best density, thickness expansion, and water absorption values [94].

5.11 ESs–Achatina fulica snail shells

Snail shells and ESs were crushed with a ball mill to 50 nm. Composites were made using the conventional resin casting method. Tensile strength, impact strength, stiffness, hardness, and water absorption reached the optimum values with the addition of conch shell particles, ESs, or a combination, at 5–10 wt%. An increase in the ES content beyond 10 wt% tended to decrease the mechanical properties of the epoxy composite [95].

5.12 ES–glass fiber–burlap fiber

Ramamurthy et al. [96] compared the mechanical behavior of glass and jute fibers-reinforced hybrid composites with and without ESs. The ultimate tensile stress, flexural strength, and composite hardness decreased with the addition of ES particles to the hybrid composite, although the density increased.

5.13 ES–glass fiber–kenaf fiber

Adding ES particles into these hybrid composites led to significant improvements in their mechanical properties. The best composition was 5 wt% glass fiber, 15 wt% kenaf fiber, and 9 wt% ESs, resulting in a composite with excellent tensile and flexural strength. Water absorption increased as the number of ESs increased. An increased kenaf fiber content led to increased water absorption [36].

5.14 ES–talc

ESs positively affected the properties of PP/ES/talc hybrid composites. ESs were highly compatible with PP composites and provided a composite strengthening effect compared to the traditional calcium carbonate applications. Another interesting fact was that ESs were able to replace the role of talc without decreasing the mechanical properties of PP–talc composites [97].

7.1 Structure and morphology of ES particles

Calcination treatments with different temperatures and times have been shown to result in particles with different sizes and structures. The higher the temperature and time of calcination, the larger the size of the ES particles. The morphology of the ES changes sequentially from oval, to elongated or to bean-shaped with changes in temperature [56]. The intensity of CaCO₃ decreases with the increase in the calcination temperature and the formation of CaO [16]. The structure of the calcined ES resembles a wasp’s nest. Boiled ESs contain very low calcium concentrations and very high oxygen contents. Calcination treatment allows the formation of many pores. This is indicated by the ES particles’ increasing circumference, area, and pore diameter. The increase in pore size occurs due to the incorporation of smaller pores and the growth of CaO grains in high-temperature treatments. Larger pores can provide space for the accumulation of moisture and dirt [17]. Awogbemi et al. showed that the different treatments of ES particles affected their calcium, carbon, and oxygen content concentrations, as shown in Table 1.

Risso et al. [102] obtained contrasting findings showing that ESs calcined at a temperature of 900°C for 4 h had a pore volume of 0.0020 cm³/g, a pore diameter of 26 nm, a crystallite size of 44 nm, and an activation energy of 18.1 mol/min kg. SEM observations showed that the differences in the particle sizes of ESs with calcination treatment were more significant than those of ES without calcination [57]. A comparison of changes in pore dimensions in various treatments is presented in Table 2. Commey and Mensah [15] showed that calcined ESs were suitable for pH modifiers. PH is an essential parameter in the electrochemical reduction process of carbon dioxide and carbon monoxide. Schouten et al. investigated the effect of pH on copper electrodes. Their study indicated that methane formation was strongly influenced by pH, whereas ethylene had a different dependence on pH [103].

The treatment of ESs affected the density of the composite, although the change in density was not very significant. Bose et al. [19] calcined ESs at 650°C for 3 h and used them as a reinforcement for MMC material. An increase in the ES percentage decreased the density of the composite. This trend differs from that reported by Hassan et al. [10], who found that the density of polyester composites increased with an increase in the ES percentage. The composite’s density became progressively lower in the ESs subjected to calcination treatment [10,19].

7.2 Mechanical characteristics

The characteristics of carbonated ES particles have been found to be superior to those of raw eggshells. The higher the ES percentage, the higher the hardness and tensile strength of the composite, but the impact energy and density decrease, as shown in Figure 3 [58]. Oladele et al. [57] observed a slightly different phenomenon in measuring the tensile strength of the composite. Figure 4 shows that the tensile strength of the composite reinforced with calcined ES particles decreased. In general, the mechanical properties of the composite reached the optimum value at the addition of ESs at 9 wt%. ES particles with calcination treatment improved impact properties and wear resistance [57]. Several other studies have shown that the hardness of composites reinforced with carbonated ESs exhibited better values compared to non-calcined ESs [10,84].

Figure 3

Comparison of tensile strength and impact energy values of raw ES particles and particles subjected to carbonation. Figures are produced based on the data in [58].

Figure 4

Comparison of tensile strength and impact energy values between raw ES particles and particles subjected to calcination. Figures were produced based on the data in [57].

Stress–strain curves are used to show the tensile strength behavior and Young’s moduli of polylactic acid (PLA) composites. The addition of calcined ESs at a low level (10 wt%) resulted in good tensile strength and moduli. At a large percentage, the presence of calcined ES reduced the composites’ tensile strength [104]. The addition of calcined ESs up to 7 wt% increased the degree of crystallinity of the PLA composite. An even distribution of ES particles increased Young’s modulus but decreased the tensile strength, fracture elongation, and impact strength. The presence of ESs lowered the glass transition temperature, thus making the movement of molecular chains more manageable at low temperatures [105].

The differences in the various discussions of the tensile strength and impact strength of composites obtained using carbonated ESs and untreated ESs were fascinating to study. Changes in the phase, structure, and particle size of ESs from CaCO₃ to CaO need to be studied more deeply. This recommendation is based on several studies which stated that ES particles could capture CO₂, which would enable environmentally friendly composites to be developed in the future.

7.3 Characteristics of tribology

Wear resistance is indicated by a wear index. The wear indices of hybrid composites were found to increase with the addition of ES particles and sisal fiber. The increased wear resistance of the composite was due to the very soft molecular nature of the epoxy and the presence of Ca₂Fe₇O₁₁. Figure 7 shows that the addition of calcined ESs increased the resistance of the composite surface to wear. Adding ES particles at 3 wt% offered the highest wear resistance, with 0.38 mg of calcined ES particles [57].

Oladele et al. [106] investigated bio-composites using HAP synthesized from chicken ESs. As shown in Figure 5 (right), they determined that the higher HAP loading in HDPE composites could increase the wear resistance of composites by up to 40 wt% HAC.

Figure 5

Wear indices of uncalcined and calcined ESs [57] and weight losses of composites [106]. Figures are produced based on the data in [57] and [106].

7.4 Thermal characteristics

With the calcination treatment, the thermal stability of ES particles was found to be excellent because the weight loss only reached 5.5% at a temperature of 450°C and then stabilized up to 1,000°C [17]. The cold crystallization temperature at 112.83°C exhibited a perfectly crystalline structure. The ES phase lowered the glass transition temperature, T _g. T _cc and T _m decreased when the ES increased because the perfection of the crystalline structure could be induced more quickly when the ES particles were dispersed in the PLA matrix [105].

ESs with calcination treatment, at a specific temperature and holding time, form CaO, and when this comes into contact with water, it forms Ca(OH)₂. Increasing the temperature increases the value of TC and specific heat but decreases thermal diffusion at temperatures up to 363 K. Thermal diffusivity increases when the temperature is above 363 K. An increase in temperature triggers an increase in the kinetic energy of the phonons; therefore, the TC increases [107].

Figure 6(a) shows the division of the weight losses. At temperatures 200–400°C, a weight loss of 4.46% occurs due to the loss of water content and organic matter. The weight loss of 40.84% observed between 600–800°C is caused by the phase change of CaCO₃ to CaO. Figure 6(b) shows the active CaO content using full carbonation under N₂ atmosphere. This finding showed that there was a minute difference between the actual CaO fraction and the full carbonation test [99]. Another study showed that the weight losses of calcined ES samples were relatively shallow [17].

Figure 6

(a) TGA of fresh ESs and (b) carbonation test for the detection of the actual CaO fraction [99].

7.5 Water absorption characteristics

In one study, HAP, synthesized from ES particles, was used to close the pores in the bio-composite, thereby reducing water absorption [106]. This study was in line with that conducted by Gbadeyan et al., who showed that water absorption decreased along with the increased content of ES and snail shell particles. This decrease was due to the shell particles’ hydrophobic nature and the filler’s interconnecting bonds. ES particles also acted as a barrier to crack propagation, so water absorption decreased. However, an excessively high number of shell particles would cause agglomeration and stress concentrations would occur, which would ultimately weaken the bond between the particles and the matrix [95].

Panchal et al. demonstrated a different phenomenon: water absorption increased with the increase in the addition of the particulate filler. The highest increase in water absorption occurred in salt water compared to fresh water and moist soil. This finding was due to the presence of amine and carboxylic functional groups in ES particles [11]. A higher filler content and higher surface area showed better water absorption ability. The higher the filler content, the higher the agglomeration formation, so the dispersion of filler in the matrix became challenging. This led to an increase in water absorption in the composite. The smaller the filler, the higher the water absorption [39].

Panchal et al. [11] compared the effect of the ES content in mineral water, salt water, and moist soil environments. Figure 7 shows that the higher the ES particle content, the higher the swelling thickness under various conditions. However, the increase in thickness was significant in saltwater and moist soil environments. Yusuf et al. [90] studied the effect of silane treatment on ES particles in terms of water absorption properties and thickness development. Thickness swelling and water absorption increased with ES content, but ESs with silane treatment performed better. This increase was due to the hydrophilic nature of the ES filler. Arsene et al. [108] showed that when in aqueous media, silane was partially or entirely hydrolyzed to form dimers or oligomers.

Figure 7

Thickness swelling of ES filler composites under different environmental conditions. Figures are produced based on the data in [11].

7.6 Antibacterial characteristics

ESs can inhibit the growth of bacteria, and this effect was found to increase with an increase in the percentage of ES particles in a PLA composite bio-filler [104]. CaO, in its pure state, was a very effective antibacterial agent. The presence of CaO in the composite was able to kill bacteria. ESs calcined at a temperature of 800°C for 2 h were very effective as a chromium heavy metal releaser in wastewater [109]. ESs in a raw state were not effective at killing bacteria. Meanwhile, ESs calcined at a temperature of 900°C for 2 h form CaO, which is a potential source of antibacterial material [110].

8.1 Pre-treatment of ESs

Pre-treatment of ESs could improve their mechanical properties, thermal properties, tribological properties, water absorption, and thickness swelling properties. The studies discussed above have shown that calcination treatment can improve composite performance, although decreasing some material properties. This review makes it possible to observe the changes in the behavior of materials caused by the presence of ES particles, both in their untreated form and with calcination treatment. Previous studies showed that ES particles subjected to calcination treatment showed an increase in contact area, which resulted in an increase in their mechanical properties. Silane treatment of ES particles also showed an increase in the composite performance, especially in terms of thickness swelling. Thick swelling indicates high water absorption by the fiber and the filler. The ability of fibers and fillers to absorb water is determined by their hydrophilic properties. This weakness can be improved through silane treatment. The addition of a silane layer on the particles or filler can limit the water absorption rate so that the degradation of the composite material properties is minimized.

ES particles showed good distributions in composites, especially in MMCs obtained using the stir casting method, which showed improved material performance. However, an excessive ES particle content leads to agglomeration. Agglomeration was observed to cause stress concentration and to weaken the bonds between particles in the composite. The pre-treatment of ESs can be divided into two categories. The first of these is pre-treatment at room temperature, such as the immersion of raw ES particles in NaOH, or silane solution, which has not been studied in depth. Research on the effect of the solution concentration and time of immersion of ESs in the solution in regard to composite performance is still limited. The second category of pre-treatment is pre-treatment at high temperatures, such as temperature and time calcination, which have also been carried out by researchers. However, this form of treatment of calcined ESs has not been carried out. These gaps in the research must be explored so that the utilization of ESs as an environmentally friendly green material can be achieved. Composites using calcined ESs as a catcher for greenhouse gas emissions have also not been studied in detail.

8.2 ESs as friction materials

In recent decades, researchers have observed that particulate matter (PM) induced by brake linings can have a harmful effect on human health. PM 10 and PM 2.5 air particles were identified as being particularly hazardous. These particles can be formed of dust, dirt, soot, or smoke. PM 2.5 particles are produced by ash, volcanic ash, burning coal, burning forests, and burning biomass. The concentration of these particles is limited to a maximum of 65 μg/m³, whereas that of PM 10 is limited to 80 μg/m³.

PM 10 particles have been found in building construction, garbage disposal, agriculture, forest fires, dust, and bacterial fragments. PM 10 emissions in relation to heavy vehicles have been observed to be caused by exhaust emissions (41%), the suspension of road dust (38%), and brake wear (21%). For light vehicles, exhaust and brake wear were identified as the greatest contributors, at 63 and 33%, respectively [111]. A reduction in this form of emissions could be achieved by formulating and modifying brake linings and discs. Substances that are harmful to human health should be reduced.

One of the materials that is not good for human health is copper. Copper has been widely used due to its excellent TC. Wei et al. [112] stated that the TC of disc materials can influence the formation and destruction of contact planes and thus affect the particle size distribution in the air due to heat accumulation at the shear interface. The substitution of copper with other materials with equivalent TC could lead to a decrease in PM 10 emissions.

Using ESs, whether calcined or untreated, would undoubtedly have an effect on PM 10 emissions. Composites containing ESs produce relatively lower PM 10 emissions, which are relatively safe for health. CaO has demonstrated an excellent ability to absorb CO₂ in high-temperature conditions. Hsieh et al. [100] showed that CaO synthesized from ESs could absorb CO₂, while reducing environmental waste. The utilization of ESs as a friction material has not been widely carried out, and the literature on this topic remains still limited. It is known that particles from hazardous materials can interfere with health and cause disease in humans or pollute the soil, which can interfere with soil fertility. For this reason, it may be beneficial to replace the hazardous components of friction materials with ESs, which are more environmentally friendly. For this reason, observations of their tribological characteristics must be carried out.