Optimization of Epoxy Resin: An Investigation of Eggshell as a Synergic Filler

Epoxy resin based on bisphenol A diglycidyl ether/anhydride methyl tetrahydrophthalic/2,4,6-tris(dimethylaminomethyl)phenol (DGEBA/MTHPA/DEH 35) was produced by magnetic stirring; chicken eggshell (ES) was added as cure improver. Thermal stability, cure parameters, mechanical properties, and fracture surface were investigated by thermogravimetry (TGA), differential scanning calorimetry (DSC), tensile experiments, and scanning electron microscopy (SEM). In general, the addition of ES slightly decreased the thermal stability, being T0.05 5% lower than that of the reference sample. The cure rate increased with the heating rates, while best results were obtained upon addition of neat membrane (M) from ES. Surprisingly, the mechanical properties were significantly improved with ES as well as with M, being the Young’s modulus 18% higher, the tensile strength 50% higher, and the deformation 35% higher than those of epoxy resin. SEM images showed that the synthetic compounds presented a smooth fracture surface, while the compounds with ES and M had a rougher surface with multiplane fractures, suggesting a fracture with higher energy absorption. In conclusion, epoxy/ES composites with better performance were produced, and effective tools are provided to control and attain in the future even better properties with ecological features.


Introduction
Epoxy is the thermosetting polymer most widely used in the industry, mainly in electronics and aerospace, due to its characteristics such as adhesiveness, low shrinkage, high strength, excellent electrical insulation, and corrosion resistance [1,2]. The most common epoxy resins are the rigid aromatic ones made from bisphenol A diglycidyl ether (DGEBA), such as D.E.R. 331, which is the most largely produced epoxy resin for industrial sites. The tough, insoluble, and infusible epoxy is normally formed via a cross-linking reaction (also called the curing process or solidification) of liquid epoxy resins with hardeners (also called catalysts or curing agents) including polyfunctional amines, acids (or acid anhydrides), phenols, alcohols, and thiols. Nevertheless, brittleness and low impact strength [2,3] are drawbacks that prevent wider applications. Aiming at the improvement of these properties, reinforcements have been incorporated; however, high costs, difficulty of production, and the propensity to agglomeration prevent them from being unattractive products. In addition, the growing concern over sustainability and the environmental impact of petroleum-based polymer  On the basis of the thermal properties determined from DSC analysis, specimens of selected compounds were produced according to ASTM D638-14 standard and by applying thermal profiles as presented in Table 2. Their mechanical behavior and morphological characters were investigated.

Characterization of Epoxy Compound
Thermal stability was investigated by thermogravimetry (TGA). The tests were carried out in TGA Pyris-1 from Perkin Elmer (Waltham, MA, USA). Samples of approximately 5 mg were heated from 30 to 900 • C at a heating rate of 10 • C/min under synthetic air with gas flow of 20 mL/min. The curing process was analyzed by differential scanning calorimetry (DSC), using a DSC Q20 from TA Instruments (New Castle, DE, USA). Samples of approximately 5 mg were tested in a standard closed aluminum pan, under a nitrogen gas flow 50 of mL/min. The samples were heated from 30 • C to 400 • C, at heating rates of 1, 2, 5, 10, and 20 • C/min.
Tensile tests were performed in triplicate using a universal test machine, Instron 3366 (Norwood, MA, USA), according to ASTM D638-14 at ambient temperature (~23 • C) and deformation rate of 5 mm/min.
The fracture surface and filler dispersion of the epoxy/anhydride/catalyzer system were analyzed using a scanning electron microscope (SEM) from World Phenom Pro X800-08334 (Eindhoven, The Netherlands).

Results and Discussion
3.1. Evaluation of Thermal Stability by TGA Figure 1 presents TGA plots of S x compounds and their decomposition rates. The benchmark S 0 100:87:0 system has four decomposition steps, which range from 78 to 287 • C, from 287 to 425 • C, from 425 to 512 • C, and from 512 to 639 • C. It is worth of mention that the curing took place during heating, as also the decomposition. According to Montserrat et al. [23], in the absence of a catalyzer, the thermal degradation of DGEBA with anhydride as a hardener begins at temperatures higher than 300 • C. Therefore, the first and second weight loss suggest the decomposition of unreacted hardener (MTHPA) and non-cross-linked epoxy (additional details may be checked in Supplementary Materials). The third and fourth steps are related to the decomposition of cross-linked epoxy and carbon residue generating two products of degradation, CO 2 and H 2 O [23][24][25].
TGA Pyris-1 from Perkin Elmer (Waltham, MA, USA). Samples of approximately 5 mg were heated from 30 to 900 °C at a heating rate of 10 °C/min under synthetic air with gas flow of 20 mL/min.
The curing process was analyzed by differential scanning calorimetry (DSC), using a DSC Q20 from TA Instruments (New Castle, DE, USA). Samples of approximately 5 mg were tested in a standard closed aluminum pan, under a nitrogen gas flow 50 of mL/min. The samples were heated from 30 °C to 400 °C, at heating rates of 1, 2, 5, 10, and 20 °C/min.
Tensile tests were performed in triplicate using a universal test machine, Instron 3366 (Norwood, MA, USA), according to ASTM D638-14 at ambient temperature (~23 °C) and deformation rate of 5 mm/min.
The fracture surface and filler dispersion of the epoxy/anhydride/catalyzer system were analyzed using a scanning electron microscope (SEM) from World Phenom Pro X800-08334 (Eindhoven, The Netherlands). Figure 1 presents TGA plots of Sx compounds and their decomposition rates. The benchmark S0 100:87:0 system has four decomposition steps, which range from 78 to 287 °C, from 287 to 425 °C, from 425 to 512 °C, and from 512 to 639 °C. It is worth of mention that the curing took place during heating, as also the decomposition. According to Montserrat et al. [23], in the absence of a catalyzer, the thermal degradation of DGEBA with anhydride as a hardener begins at temperatures higher than 300 °C. Therefore, the first and second weight loss suggest the decomposition of unreacted hardener (MTHPA) and non-cross-linked epoxy (additional details may be checked in Supplementary Materials). The third and fourth steps are related to the decomposition of cross-linked epoxy and carbon residue generating two products of degradation, CO2 and H2O [23][24][25].

Evaluation of Thermal Stability by TGA
Upon addition of DEH 35, three decomposition steps occurred verified. According to Meadows et al. [26], during DER 331 curing with MTHPA and DEH 35, hydroxyl is released as a by-product, which is volatilized in the first step of decomposition, ranging from 76 to 272 °C for 5 parts of DEH 35. The second and third steps, which range from 164 to 529 °C and from 529 to 689 °C for 5 parts of DEH 35, are related to the decomposition of epoxy releasing CO2 and H2O [23][24][25]. In contrast to the S0 system, compounds with DEH 35 had no decomposition associated with non-cross-linked epoxy; in fact, the increase of DEH 35 resulted in higher weight loss in the third step, suggesting the amount of catalyst significantly influenced epoxy cross-link. The TGA plots of epoxy/eggshell systems (E10, EM10, and M10) presented a similar thermal behavior to that of benchmark S0, as shown in Figure 2. For these systems, four decomposition steps Upon addition of DEH 35, three decomposition steps occurred verified. According to Meadows et al. [26], during DER 331 curing with MTHPA and DEH 35, hydroxyl is released as a by-product, which is volatilized in the first step of decomposition, ranging from 76 to 272 • C for 5 parts of DEH 35. The second and third steps, which range from 164 to 529 • C and from 529 to 689 • C for 5 parts of DEH 35, are related to the decomposition of epoxy releasing CO 2 and H 2 O [23][24][25]. In contrast to the S 0 system, compounds with DEH 35 had no decomposition associated with non-cross-linked epoxy; in fact, the increase of DEH 35 resulted in higher weight loss in the third step, suggesting the amount of catalyst significantly influenced epoxy cross-link.
The TGA plots of epoxy/eggshell systems (E 10 , EM 10 , and M 10 ) presented a similar thermal behavior to that of benchmark S 0 , as shown in Figure 2. For these systems, four decomposition steps suggested the decomposition of unreacted hardener, non-cross-linked epoxy, cross-linked epoxy, and carbon residue [23][24][25]. From individual analyses of the decompositions steps of S 0 , E 10 , EM 10 , and M 10 , in the first step of S 0 there was an increase of approximately 10% in weight loss of MTHPA in comparison with the epoxy/eggshell system, suggesting that a smaller amount of hardener had reacted in the curing process. The decomposition in the second and third steps was competitive, since one was associated with the decomposition of the non-cross-linked epoxy, and the other with the decomposition of the cross-linked portion. Thus, M 10 presented a more pronounced weight loss in the third stage, suggesting this system had a greater fraction of reticulation, i.e., was more effectively cured. reacted in the curing process. The decomposition in the second and third steps was competitive, since one was associated with the decomposition of the non-cross-linked epoxy, and the other with the decomposition of the cross-linked portion. Thus, M10 presented a more pronounced weight loss in the third stage, suggesting this system had a greater fraction of reticulation, i.e., was more effectively cured. Table 3 presents thermogravimetric parameters of epoxy compounds, additionally to weight loss data, onset (T0), and final (Tf) decomposition temperatures, as well as decomposition rates. The thermal stability of epoxy was evaluated by the parameter τ1/2-time at the 50% decomposition conversion degree-which increased with higher amounts of DEH 35, suggesting the catalyzer improved the thermal stability of epoxy. In regard to the epoxy/eggshell compounds, E10 presented a higher value for τ1/2 than EM10 and M10. Nevertheless, the S5 compounds showed an increase of 29% (10.7 min) in τ1/2 in comparison with E10, indicating that a better cross-link reaction-present in Sx compounds-produced an enhanced thermal decomposition.    Table 3 presents thermogravimetric parameters of epoxy compounds, additionally to weight loss data, onset (T 0 ), and final (T f ) decomposition temperatures, as well as decomposition rates. The thermal stability of epoxy was evaluated by the parameter τ 1/2 -time at the 50% decomposition conversion degree-which increased with higher amounts of DEH 35, suggesting the catalyzer improved the thermal stability of epoxy. In regard to the epoxy/eggshell compounds, E 10 presented a higher value for τ 1/2 than EM 10 and M 10 . Nevertheless, the S 5 compounds showed an increase of 29% (10.7 min) in τ 1/2 in comparison with E 10 , indicating that a better cross-link reaction-present in S x compounds-produced an enhanced thermal decomposition.   Figure 3 shows DSC scans of S X compounds. The uncatalyzed system S 0 does not have a complete exothermic peak in the analyzed temperature range for heating rates higher than 2 • C/min. This corresponds to the results of the TGA curves, i.e., the cure process without catalyzer occurred at temperatures higher than 300 • C at the applied heating rates [23][24][25].

DSC Measurements
Upon addition of DEH 35 catalyzer, an exothermic peak appeared, which ranged from 94 to 175, 90 to 186, 78 to 158, for 1, 2, and 5 parts of DEH 35 at 5 °C/min (temperatures and cure parameters associated with the exothermic peaks are displayed in Table 4). The exothermic peak presenting a bell shape suggests curing taking place in one reaction model, whereas no discontinuities were observed [27][28][29]. These peaks were displaced to higher temperatures, increasing the heating rates. Regarding DEH 35 addition, the peaks appeared at lower temperatures upon its increase, evidencing a faster curing.
Ručigaj et al. [30] also observed a similar influence of the catalyzer content and heating rate during investigation of the cure kinetics of an epoxided soy bean oil (ESO)/anhydride/triazole system, in which the presence of a catalyzer displaced the onset temperature of the exothermic peak to ~120 °C in comparison with the pure epoxy (240 °C); in contrast, the heating rates shifted the exothermic peak to higher temperatures. Upon addition of DEH 35 catalyzer, an exothermic peak appeared, which ranged from 94 to 175, 90 to 186, 78 to 158, for 1, 2, and 5 parts of DEH 35 at 5 • C/min (temperatures and cure parameters associated with the exothermic peaks are displayed in Table 4). The exothermic peak presenting a bell shape suggests curing taking place in one reaction model, whereas no discontinuities were observed [27][28][29]. These peaks were displaced to higher temperatures, increasing the heating rates. Regarding DEH 35 addition, the peaks appeared at lower temperatures upon its increase, evidencing a faster curing.
Ručigaj et al. [30] also observed a similar influence of the catalyzer content and heating rate during investigation of the cure kinetics of an epoxided soy bean oil (ESO)/anhydride/triazole system, in which the presence of a catalyzer displaced the onset temperature of the exothermic peak to~120 • C in comparison with the pure epoxy (240 • C); in contrast, the heating rates shifted the exothermic peak to higher temperatures. The integration of exothermic peaks presented in Figure 3 provides information related to the cure, i.e., the maximum cure rate (c max ) and peak temperatures (T 0.01 , T p , T 0.999 ), which are shown in Table 4. Figure 4a presents the effect of heating rates on c max of S x compounds. This parameter can be understood as the reaction speed. Looking at Figure 4a, a linear trend is observed, i.e., the cure was faster for higher heating rates, and this increase was greater for S 5 compound which presented c max of 0.688 min −1 , higher than those of other compounds. The integration of exothermic peaks presented in Figure 3 provides information related to the cure, i.e., the maximum cure rate (cmax) and peak temperatures (T0.01, Tp, T0.999), which are shown in Table 4. Figure 4a presents the effect of heating rates on cmax of Sx compounds. This parameter can be understood as the reaction speed. Looking at Figure 4a, a linear trend is observed, i.e., the cure was faster for higher heating rates, and this increase was greater for S5 compound which presented cmax of 0.688 min −1 , higher than those of other compounds.
Regarding the influence of DEH 35 on cmax, increasing its content provided higher cmax, and this trend was more pronounced for higher heating rates; for instance, at 20 °C/min, cmax of S5 was 16% higher than those of other compounds with lower content of DEH 35, suggesting the catalyzer addition accelerated the cure.
Concerning the effect of heating rates and DEH 35 content on Tp shown in Figure 4b, at higher heating rates, Tp assumed higher values, while, after the addition of DEH 35, Tp decreased, i.e., the cure took place at lower temperatures. Therefore, both heating rate and DEH 35 content can be used as safe tools to control the curing as required by the industrial process.    Regarding the influence of DEH 35 on c max , increasing its content provided higher c max , and this trend was more pronounced for higher heating rates; for instance, at 20 • C/min, c max of S 5 was 16% higher than those of other compounds with lower content of DEH 35, suggesting the catalyzer addition accelerated the cure.
Concerning the effect of heating rates and DEH 35 content on T p shown in Figure 4b, at higher heating rates, T p assumed higher values, while, after the addition of DEH 35, T p decreased, i.e., the cure took place at lower temperatures. Therefore, both heating rate and DEH 35 content can be used as safe tools to control the curing as required by the industrial process. Figure 5 shows plots of cure conversion (α) versus temperature for S X compounds; these data were computed from DSC scans. All plots presented a sigmoidal shape without discontinuities, suggesting the cure progressed as an autocatalytic reaction [27,31,32], agreeing with the DSC scans indicating that it took place according to a single "reaction model" as indicated by the observed single peaks. In general, the "S" plots may be divided in three stages: initially, around 0 to 5% of process conversion, they have a slow rate probably due to catalyzation reactions/production of active centers; during the second step-around 5 to 90%-the cure quickly increases, as a result of the availability of functional groups and easy molecular movement; afterwards, the cure proceeds with a delayed rate, because of competitiveness between high viscosity and decreased presence of functional groups. Finally, the viscosity significantly increases, and there is a decrease of reactive groups near the cure end [27,29,32]. Regarding the effect of the heating rates, the "S" plots corresponded to higher temperatures using higher heating rates; this effect was associated with the time dependence of the cure reaction, showing an opposite trend compared to that observed upon DEH 35 addition, which had an accelerator influence over the cure according to the DSC curves.  Figure 5 shows plots of cure conversion (α) versus temperature for SX compounds; these data were computed from DSC scans. All plots presented a sigmoidal shape without discontinuities, suggesting the cure progressed as an autocatalytic reaction [27,31,32], agreeing with the DSC scans indicating that it took place according to a single "reaction model" as indicated by the observed single peaks. In general, the "S" plots may be divided in three stages: initially, around 0 to 5% of process conversion, they have a slow rate probably due to catalyzation reactions/production of active centers; during the second step-around 5 to 90%-the cure quickly increases, as a result of the availability of functional groups and easy molecular movement; afterwards, the cure proceeds with a delayed rate, because of competitiveness between high viscosity and decreased presence of functional groups. Finally, the viscosity significantly increases, and there is a decrease of reactive groups near the cure end [27,29,32]. Regarding the effect of the heating rates, the "S" plots corresponded to higher temperatures using higher heating rates; this effect was associated with the time dependence of the cure reaction, showing an opposite trend compared to that observed upon DEH 35 addition, which had an accelerator influence over the cure according to the DSC curves.   Figure 6 illustrates the DSC scans of Ex/EMx/Mx. The compounds containing the membrane displayed exothermic peaks at all heating rates applied in the investigated temperature ranges. This positive response was probably linked to the fact that the membrane contains higher amounts of reactive functional groups such as sulfur constituents than other parts of the chicken eggshell [7].  Figure 6 illustrates the DSC scans of E x /EM x /M x . The compounds containing the membrane displayed exothermic peaks at all heating rates applied in the investigated temperature ranges. This positive response was probably linked to the fact that the membrane contains higher amounts of reactive functional groups such as sulfur constituents than other parts of the chicken eggshell [7].
Thus, the membrane is the best structure for catalyzing the cure reaction of DER 331. Besides that, these data justify the higher weight loss in the third step, related to the cross-linked decomposition content of M 10 , as seen in the TGA plots (Figure 2), confirming the best cure process in this system. As already verified in the S x system, M x presented an exothermic peak with a bell shape, suggesting a single cure reaction model without discontinuities. The cure peaks appeared at higher temperatures with increasing heating rates; as presented above for S x compounds, the thermal transitions depended on the process time. Additionally, when increasing the membrane content, the cure occurred at lower temperatures, suggesting the membrane contains more reactive functional groups in its structure.
effect of the heating rates and filler addition on ΔH, interesting trends were observed, as shown in Table 4, i.e., for the parameters measured for synthetic composites, there was a decrease in ΔH when increasing the heating rates, suggesting a lack of time for full polymerization (all reactive groups) leading to lower ΔH values (energy related to exotherm reaction). On the other hand, according to Table 5, for composites containing the membrane (M), ΔH increased with the heating rates: in this situation, functional groups of M exerted a synergic effect, leading to higher rates and higher degree of polymerization.
The improvement of the cure reaction upon filler addition in the epoxy matrix was investigated by Shanmugharaj and Ryu, [28] who studied the influence of the cure characteristics in epoxy/pristine and superficially modified montmorillonite. These authors verified a displacement of the temperature peak to lower temperatures with montmorillonite addition, as also an increasing of ΔH, indicating a better cure characteristic. Saad et al. [33] evaluated the cure parameters of epoxy containing barium ferrite/polyaniline (PANI) fillers; both DSC scans and cure conversion plots were shifted to lower temperatures when increasing the filler content; the authors suggested this was due to the epoxy group opening by the amine present in the PANI structure.

Mechanical Behavior of Epoxy Compounds
The tensile properties Young's modulus, tensile strength, and maximum deformation were evaluated for S5, E10, EM10, M10 compounds. The data are presented in Table 6 and Figure 7. The effect The temperature ranges and cure parameters of M x are presented in Table 5, as well as the influence of heating rates and M content on the cure parameters c max , T p , and overall reaction heat (∆H). Following a similar trend as observed for S x compounds (S 1 , S 2 , and S 5 ), increasing the heating rates resulted in a higher value of c max , being more significant for M 10 , with a difference of 0.407 min −1 (487%). Among the investigated M contents, for all the heating rates used, M 10 presented a gain of 91% (0.167 min −1 ). Concerning the influence of the heating rates and M amount on T p , the increase of the heating rate resulted in higher T p ; on the other hand, M 10 had lower T p than M 5 . Regarding the effect of the heating rates and filler addition on ∆H, interesting trends were observed, as shown in Table 4, i.e., for the parameters measured for synthetic composites, there was a decrease in ∆H when increasing the heating rates, suggesting a lack of time for full polymerization (all reactive groups) leading to lower ∆H values (energy related to exotherm reaction). On the other hand, according to Table 5, for composites containing the membrane (M), ∆H increased with the heating rates: in this situation, functional groups of M exerted a synergic effect, leading to higher rates and higher degree of polymerization. The improvement of the cure reaction upon filler addition in the epoxy matrix was investigated by Shanmugharaj and Ryu, [28] who studied the influence of the cure characteristics in epoxy/pristine and superficially modified montmorillonite. These authors verified a displacement of the temperature peak to lower temperatures with montmorillonite addition, as also an increasing of ∆H, indicating a better cure characteristic. Saad et al. [33] evaluated the cure parameters of epoxy containing barium ferrite/polyaniline (PANI) fillers; both DSC scans and cure conversion plots were shifted to lower temperatures when increasing the filler content; the authors suggested this was due to the epoxy group opening by the amine present in the PANI structure.

Mechanical Behavior of Epoxy Compounds
The tensile properties Young's modulus, tensile strength, and maximum deformation were evaluated for S 5 , E 10 , EM 10 , M 10 compounds. The data are presented in Table 6 and Figure 7. The effect of ES and M were clearly identified: the compounds with these fillers displayed higher tensile properties. containing graphene oxide showed an improvement in both tensile strength and Young's modulus, which was greater with larger fillers, leading to 47.57 MPa (99%) and 0.91 GPa (33.2%), respectively. The great advantages of ES compounds produced in this work depend on the fact that they are bio-based and ecofriendly composites, since eggshell waste was used, and higher thermal as well as mechanical properties were achieved.   E 10 and EM 10 presented higher Young's modulus than S 5 and M 10 , with an increase of 18% (0.23 GPa) in comparison with the neat epoxy. This effect was associated with the higher stiffness of the CaCO 3 particles from the eggshell which are able to absorb higher stress with reduced deformation. Meanwhile, M 10 had the lowest Young's modulus, nevertheless, it displayed a higher tensile strength and deformation with increases of 47% (8.31 MPa) and 35% (0.82 GPa), respectively. This character could be associated with a better interaction between particle and matrix, as a result of the chemical bonds between the membrane and epoxy established in the cure reaction, which, therefore, can be translated in a good stress transfer [34].
The influence of the fillers on the mechanic behavior of epoxy compounds was investigated by Saeb et al. [34] in epoxy/multiwalled carbon nanotubes (MWCNT). They also analyzed the effect of different surfactants, i.e., polyoxyethylene octyl phenyl ether (Triton X-100), sodium dodecyl sulfate (SDS), and hexadecyl-trimethyl-ammonium bromide (CTAB). According to them, the compounds without surfactant presented a tensile strength reduction of 7.3% (3.3 MPa); on the other hand, the presence of anionic surfactant (SDS) caused an increase of 10.3% (4.7 MPa), while the epoxy/MWCNT system presented higher Young's modulus than the neat epoxy. Kim et al. [35] reported the influence of graphene oxide filler size on the tensile properties of epoxy/diethytolenediamine; the composites containing graphene oxide showed an improvement in both tensile strength and Young's modulus, which was greater with larger fillers, leading to 47.57 MPa (99%) and 0.91 GPa (33.2%), respectively.
The great advantages of ES compounds produced in this work depend on the fact that they are bio-based and ecofriendly composites, since eggshell waste was used, and higher thermal as well as mechanical properties were achieved. Figure 8 shows the SEM images captured from the fracture surface of S 5 , E 10 , EM 10 , and M 10 . The SEM image of S 5 displays a smooth fracture surface, characteristic of brittle materials with fast failure propagation and low energy absorption. The crack nucleation and its propagation follow a linear direction path, commonly observed with brittle thermosets as epoxy.

Fracture Surface Analysis by SEM
In contrast, E 10 , EM 10 , and M 10 - Figure 8b-d presented rougher and multiplane fracture paths, with deviation of the fracture course caused by the presence of eggshell particles which acted as a barrier for the crack propagation. This pattern suggests higher energy absorption during specimen fracture, corroborating the tensile data presented above ( Figure 7 and Table 6).
A similar fracture pattern was observed by Saeb et al. [34] in epoxy/MWCNT nanocomposites. The authors observed a flat fracture surface in the neat epoxy; on the other hand, the epoxy/MWCNT/ Triton X-100 composite-which had a better particle dispersion-presented a rougher surface than that observed when SDS and CTAB surfactant were present in the epoxy/MWCNT systems, proving that the filler dispersion influenced the fracture mechanism. Zieleniewska et al. [21] studied epoxy/alumoxane and epoxy/bohemite nanocomposite and classified the fracture surface of neat epoxy in three zones: in the first zone, crack propagation was slow, and the surface had was smooth; in the second zone, there was a transition from smooth surface to rough surface, as a result of the increase of crack formation speed; finally, in the third zone, the cracks reached a speed limit, originating new cracks and forming a rougher surface. For the nanocomposite system, the authors observed a high dispersion for the alumoxane system than for the epoxy/bohemite system. Vu and Choi [36] studied an epoxy/microfibril cellulose composite and found that the neat epoxy exhibited a mirror-like surface; however, the fracture surface of the composite was rough, because of the crack deviation inflicted by the filler which acted as a barrier.

Conclusions
Epoxy composites with addition of chicken eggshell, chicken eggshell plus membrane, and membrane powders were successfully produced in this work, and optimized dispersion was reached as a result of well-designed processing parameters, as observed by SEM images. The thermal stability of eggshell composites was evaluated using TGA, and benchmark and eggshell systems (E, EM, and M) presented a similar behavior. According to DSC data, in general, higher heating rates led to lower ΔH values, while in M composites, a different trend was observed, mostly due to functional groups of M which provided a synergic effect, leading to higher rates and higher degrees of polymerization. The great advantages of eggshell epoxy composites lie on the fact that they are bio-based and ecofriendly composites, since eggshell waste was used, and higher thermal as well as mechanical properties were achieved. Additionally, this work provides safe parameters to control thermal stability as well as cure rate. This paper suggests a large range of topics for new researches, i.e., the use of different epoxy resins, degradation studies, as well as kinetic ones applying theoretical models, with the goal of producing cleaner systems.
Supplementary Materials: The following are available online at www.mdpi.com/xxx/s1, Figure S1: TGA and DTG plots of the raw materials; Figure S2: DSC scans of SX benchmark compounds at indicated heating rates.
(Effect of heating rates); Figure S3: DSC scans of SX benchmark compounds at indicated heating rates. (Effect of DEH 35 content); Figure S4: Degree of conversion of SX at indicated heating rates; Figure S5: Degree of conversion

Conclusions
Epoxy composites with addition of chicken eggshell, chicken eggshell plus membrane, and membrane powders were successfully produced in this work, and optimized dispersion was reached as a result of well-designed processing parameters, as observed by SEM images. The thermal stability of eggshell composites was evaluated using TGA, and benchmark and eggshell systems (E, EM, and M) presented a similar behavior. According to DSC data, in general, higher heating rates led to lower ∆H values, while in M composites, a different trend was observed, mostly due to functional groups of M which provided a synergic effect, leading to higher rates and higher degrees of polymerization. The great advantages of eggshell epoxy composites lie on the fact that they are bio-based and ecofriendly composites, since eggshell waste was used, and higher thermal as well as mechanical properties were achieved. Additionally, this work provides safe parameters to control thermal stability as well as cure rate. This paper suggests a large range of topics for new researches, i.e., the use of different epoxy resins, degradation studies, as well as kinetic ones applying theoretical models, with the goal of producing cleaner systems.
Supplementary Materials: The following are available online at http://www.mdpi.com/1996-1944/12/9/1489/s1, Figure S1: TGA and DTG plots of the raw materials; Figure S2: DSC scans of S X benchmark compounds at indicated heating rates. (Effect of heating rates); Figure S3: DSC scans of S X benchmark compounds at indicated heating rates. (Effect of DEH 35 content); Figure S4: Degree of conversion of S X at indicated heating rates; Figure S5: Degree of conversion of S X at indicated heating rates. (Effect of DEH 35 content); Figure S6: Conversion rate of S X compounds at indicated heating rates; Figure S7: Conversion rate of S X with the same heating rate at different compositions; Figure S8: DSC scans of E X /EM X /M X , at indicated heating rates; Figure S9: DSC scans of E X /EM X /M X , at indicated heating rates; Figure S10: Degree of conversion of M X at indicated heating rates; Figure S11: Conversion rate of M X at indicated heating rates. Funding: The authors would like to acknowledge the financial support from the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) and from CNPq (Conselho Nacional de Desenvolvimento Científico e Tecnológico).