Development of Eco‐Friendly Thermosetting Resins From Zein and Diglycidyl Ether of Vanillyl Alcohol. A Step Toward Sustainable Materials

Abstract

Herein, bisphenol A‐free, biobased, and sustainable thermosets were obtained for the first time by using two naturally derived compounds. Zein, a renewable biopolymer extracted from maize, exhibited excellent reactivity and versatility, while the diglycidyl ether of vanillyl alcohol (DGEVA), a biobased epoxy monomer derived from vanillin, served as a sustainable alternative to petrochemical epoxides. By chemically integrating Zein into DGEVA thermosetting networks, environmentally friendly resins with excellent thermomechanical properties were developed. A comprehensive analysis of seven formulations, incorporating up to 30 wt.% Zein, provided critical insights into the relationship between protein content and resin properties. Differential scanning calorimetry (DSC) analysis highlighted a strong interaction between Zein and DGEVA's epoxy groups, resulting in significantly higher polymerization enthalpies compared to neat DGEVA homopolymerization. This indicates the formation of robust crosslinked networks driven by Zein's multifunctional groups. Dynamic mechanical analysis (DMA) demonstrated substantial improvements in the thermosets’ glass transition (T _g), with values increasing from 52°C in neat DGEVA to 73°C in DGEVA/30% Zein systems, indicating enhanced stiffness and thermal performance. Furthermore, thermogravimetric analysis (TGA) confirmed good thermal stability of the bioresins, with degradation temperatures exceeding 300°C. Beyond these technical achievements, this study underscores the broader significance of incorporating Zein, a byproduct of corn agriculture, into biobased thermosets.

This study represents a groundbreaking achievement in material science by introducing Zein, a natural and renewable protein, as an active crosslinker in biobased DGEVA epoxy thermosetting resins. Its uniqueness lies in demonstrating how a plant‐derived byproduct can be effectively integrated into synthetic polymer matrices, resulting in materials with enhanced thermal stability, improved mechanical properties, and tunable shape memory effects. Beyond its chemical innovation, the study aligns with global sustainability goals, offering a compelling alternative to petroleum‐based resins and reducing environmental impact.

1. Introduction

Epoxy thermosets are crucial resins with widespread use in various industrial domains, such as aerospace, automotive, and construction^{[ 1} , ² , ^{3 ]} being employed as adhesives,^{[ 4} , ⁵ , ^{6 ]} coatings,^{[ 7} , ⁸ , ^{9 ]} structural materials, or composite matrices.^{[ 10} , ^{11 ]} Their broad applicability is primarily due to their outstanding thermal and mechanical performance, strong chemical resistance, and good adhesion to a wide range of substrates.^{[ 12 ]} The ability of epoxy resins to form durable, crosslinked networks makes them particularly suitable for structural applications that require high strength, dimensional stability, and resistance to environmental degradation. Moreover, the versatility of epoxy resins is well demonstrated in their role as composite matrices, where their ability to bond with fibers like carbon, glass, and aramid significantly enhances the performance of composite materials. These exceptional properties are mainly attributed to the intrinsic network formed during polymerization, which imparts enhanced heat resistance, high tensile and flexural strength, and excellent adhesion to various substrates. However, the growing need for sustainable materials and the environmental concerns regarding the use of petroleum‐derived chemicals, particularly bisphenol A (BPA), have catalyzed research into biobased alternatives to conventional epoxy systems. To address these demands, biobased crosslinking agents, such as proteins^{[ 13} , ^{14 ]} and polysaccharides,^{[ 15} , ^{16 ]} are being explored to improve the thermomechanical properties of epoxy resins while reducing the environmental impact.

Currently, Zein has attracted peculiar attention in scientific research as a biopolymer obtained from natural and sustainable sources. Zein is a water‐insoluble, hydrophobic protein found in corn, categorized into four classes: α‐Zein (19 and 22 kDa), β‐Zein (14 kDa), γ‐Zein (16 and 27 kDa), and δ‐Zein (10 kDa).^{[ 17} , ^{18 ]} Recent scientific interest has focused on biomedical applications of Zein, including targeted and controlled delivery of bioactive molecules and tissue engineering. Its unique structure, particularly its high content of non‐polar amino acids such as leucine, proline, and glutamine, gives Zein remarkable resistance to heat, moisture, and abrasion.^{[ 18} , ¹⁹ , ^{20 ]} In addition, Zein can help extend the shelf‐life of biomolecules.^{[ 19 ]} Zein is typically extracted using aqueous alcohol extraction, though other methods, like dry milling, dry grind processing, and alkaline treatments, are also employed to isolate this protein from maize.^{[ 20 ]}

The high performance and versatility of Zein make it a promising biopolymer for creating sustainable thermosetting materials. The presence of multiple functional groups (amine, carboxyl, and hydroxyl), within Zein's molecular structure enhances its reactivity with other compounds, facilitating the formation of crosslinked networks with improved thermomechanical properties and resulting in environmentally friendly composite materials.^{[ 21 ]} Zein's potential role as a reactive crosslinking agent in biobased epoxy thermosets is of particular interest in this study. Several studies have demonstrated Zein's positive influence on the mechanical properties of composite materials. For instance, Whitacre et al.^{[ 22 ]} studied the impact of Zein treatments on flax fiber/ thermoset resin composites, showing that Zein addition improved tensile strength by 8%, flexural strength by 17%, and short beam shear strength by 30%. Similarly, Souzandeh and Netravali^{[ 23 ]} developed green thermosets based on Zein toughened with epoxidized natural rubber fibers. The obtained composites showed improved mechanical properties with a notable increase in resin's toughness rating from 0.14 (control) to 5.20 MPa. In addition, the developed materials presented high Young's modulus and tensile strength. These findings underscore the ability of Zein to enhance the mechanical properties of thermosetting resins and contribute to the development of more sustainable composite materials.

Conventional epoxy resins primarily utilize diglycidyl ether of bisphenol A (DGEBA) as a key monomer. DGEBA is responsible for most of the epoxy resins used in commercial applications, contributing to approximately 75% of the global market.^{[ 24 ]} Nevertheless, the use of BPA has been proven to be toxic to the reproductive system of humans, therefore, the use of renewable resources and the replacement of this material emerged.^{[ 25 ]} One of the few aromatic building blocks that may be obtained commercially from renewable resources is vanillin which has great prospects as a biobased building component for polymer chemistry. Notwithstanding this promise, there is an increased use of vanillin in the field of polymers, especially axed on the synthesis of epoxy monomers based on vanillin.^{[ 26 ]} Vanillin can be synthesized from lignin, a waste product from the paper and pulp industry, making it an environmentally friendly and sustainable option. Diglycidyl ether of vanillyl alcohol (DGEVA) has already been utilized as a monomer in epoxy resin formulations.^{[ 27} , ²⁸ , ²⁹ , ^{30 ]} DGEVA exhibits excellent reactivity and serves as a monomer in the synthesis of epoxy thermosets with superior sustainability attributes. The use of DGEVA represents a significant step towards more sustainable materials, reducing reliance on fossil resources and enabling advanced reprocessing functionalities.^{[ 31} , ^{32 ]}

Emphasizing the role of Zein in the design of thermosetting materials is a key goal of this research. The novelty of this study lies in the use of Zein powder as a crosslinking agent for DGEVA biobased thermosetting resin, without employing solvents. The methodology involves reacting Zein biopolymer with DGEVA biobased epoxy monomer. Seven formulations with varying Zein content (up to 30 wt.%) were developed, and their polymerization behavior was systematically analyzed using several characterization techniques. Fourier‐transform infrared (FT‐IR) spectroscopy was employed to monitor the curing reactions and analyze the chemical interactions between Zein and DGEVA. In addition, differential scanning calorimetry (DSC) was utilized to study the polymerization exotherm and determine the optimal DGEVA/Zein ratio. Dynamic mechanical analysis (DMA) was conducted to assess the thermo‐mechanical properties of the resulting resins. The hardness of the thermosets was evaluated using Shore D hardness testing, while the thermal stability was assessed through thermogravimetric analysis (TGA), which provided valuable data on the decomposition temperature and weight loss of the materials at elevated temperatures. These combined techniques allowed for a comprehensive evaluation of the thermomechanical performance of the biobased thermosets, in order to prove the feasibility of using Zein as a crosslinker to produce high‐performance, sustainable epoxy resins.

2. Results And Discussion

2.1. Thermal Behavior and Influence of Zein on DGEVA Reactivity

Dynamic DSC analyses were conducted to investigate the thermal behavior of the designed DGEVA‐based formulations, with a particular focus on elucidating the influence of Zein on the DGEVA polymerization. The pristine DGEVA system, without any Zein content, was used as a reference. The results obtained are presented in Table 1 and Figure 1. In the case of the neat DGEVA system, the DSC thermogram exhibits two distinct exothermic events corresponding to different stages of the polymerization mechanism (Figure 1).

Table 1.

DSC data obtained during heating at 10°C·min⁻¹ DGEVA and DGEVA/ Ze formulations.

Samples	T peak (°C)	ΔH (J·g−1)
DGEVA/ 0%Ze	127 254	316
DGEVA/ 5%Ze	252	430
DGEVA/ 10%Ze	248	462
DGEVA/ 15%Ze	245	422
DGEVA/ 20%Ze	200	409
DGEVA/ 25%Ze	182	415
DGEVA/ 30%Ze	180	430

Figure 1.

(a) Dynamic DSC thermograms obtained during heating DGEVA and DGEVA/Ze mixtures at 10°C·min⁻¹; (b) variation of normalized enthalpy of polymerization in function of Zein content.

The first exothermal event, detected in the neat DGEVA system within the temperature range of 105°C–152°C, can be associated with the initiation step of chain‐wise polymerization, involving the generation of the zwitterion intermediate. This step is crucial as it marks the initiation of the chemical transformation of the monomer into a polymerized structure. The main exothermal event takes place between 166°C and 298°C and is ascribed to crosslinking reactions during which, for example, the DGEVA/0%Ze system undergoes homopolymerization through successive nucleophilic attacks of the formed oxyanions on the epoxy rings leading to the progressive formation of a thermoset material. In contrast to neat DGEVA, the incorporation of Zein into the DGEVA matrix induces significant modifications in the overall thermal behavior, as depicted in the DSC thermograms of the DGEVA/Zein formulations (Figure 1). This change in thermal profile indicates a shift in the polymerization kinetics due to the presence of Zein. While the neat DGEVA system shows two distinct exothermic events, the DGEVA/Zein formulations exhibit a single, broader exothermic peak. This broader peak suggests that the polymerization reactions occur over a wider temperature range, which is indicative of a more complex reaction mechanism involving interactions between DGEVA and Zein. Furthermore, increasing the Zein content results in a shift of the maximum temperature of the reaction (T _peak) to lower values, indicating an enhanced overall reactivity of the system. This temperature shift suggests that the presence of Zein facilitates an earlier onset of polymerization reactions. For example, at a Zein loading of 20 wt,%, the T _peak decreases by approximately 54°C from 254°C (neat DGEVA) to 200°C. The lower T _peak of polymerization reactions exhibits higher reactivity of the DGEVA/Zein mixtures, highlighting a pronounced catalytic effect induced by the protein content. These variations in polymerization behavior are corroborated with the corresponding reaction enthalpies (ΔH) of the systems, as summarized in Table 1. The reaction enthalpy, which reflects the overall energy released during the polymerization process, is markedly increased in the presence of Zein. Specifically, neat DGEVA exhibits a homopolymerization enthalpy of approximately ≈316 J·g⁻¹, characteristic of its homopolymerization reactions. Upon incorporation of Zein, the enthalpy values rise substantially, ranging from 409 to 462 J·g⁻¹, depending on the protein content. For example, the formulation containing 5 wt.% Zein exhibits a reaction enthalpy of 430 J·g⁻¹, while the system with 10 wt.% Zein reaches a maximum of 462 J·g⁻¹, suggesting that Zein actively contributes to the polymerization process, likely due to the stronger interactions between the Zein protein and the epoxy groups of DGEVA. With 15 wt.% Zein, the enthalpy decreases slightly, to 409 J·g⁻¹, followed by a subsequent increase to approximately 430 J·g⁻¹ in the 30 wt.% Zein formulation. This trend suggests a complex interplay between Zein and DGEVA, where the protein might form a dense crosslinked network at certain concentrations, leading to variations in the polymerization enthalpy. Overall, the increase in reaction enthalpy in the presence of Zein, combined with the shift of T _peak to lower values, indicates that Zein not only acts as a catalyst but also actively participates in the crosslinking process. These effects can be attributed to multi‐modal interactions between Zein and DGEVA at different stages of reactions, including potential secondary intermolecular forces (hydrogen bonding) and also covalent bonding which contribute to the development of polymer structure.

This enhanced enthalpic contribution underscores the active contribution of Zein in the copolymerization reactions and curing processes of the DGEVA. To further monitor the thermosets formation, FT‐IR spectroscopy was employed, comparing the spectra of the formulation initial components (Zein, DGEVA monomer, 1‐MI) with those of the neat DGEVA/0%Ze and DGEVA/ Ze thermosets (Figure 2).

Figure 2.

FT‐IR spectra of Zein, DGEVA monomer, 1‐MI initial compounds, and that of DGEVA/ 0% Ze and DGEVA/ Ze thermosets.

Thus, in the FT‐IR spectrum of Zein, the most significant absorbance bands correspond to the C═O stretching of the amide group (amide I) at 1640 cm⁻¹, the angular deformation vibrations of the N─H bond (amide II) at 1515 cm⁻¹ and the axial deformation vibrations of the C─N bond (amide III) at 1232 cm⁻¹. These bands are representative of the peptide backbone of Zein and are essential for its interaction with the epoxy monomer. The infrared spectrum of the DGEVA monomer shows characteristic bands at 1593 cm⁻¹, corresponding to the C═C stretching vibration of the aromatic ring, and between 1512 and 1418 cm⁻¹ for the C─C stretching vibration of the aromatic ring. A band at 1335 cm⁻¹ is attributed to the in‐plane deformation of the C─H bond. Additional bands include 1157–1024 cm⁻¹, assigned to the asymmetrical C─O─C stretching vibrations of ether groups, and the bands at 908 and 854 cm⁻¹ corresponding to the stretching vibration of the oxirane ring. In the FT‐IR spectrum of 1‐MI, the band at 1515 cm⁻¹ is attributed to the C═C stretching of the imidazole ring, and the signals at 1074 and 908 cm⁻¹ correspond to the C─N─C bending‐in‐plane. Also, other significant bands are 1419 cm⁻¹ for C═N stretching, 1283 cm⁻¹ for C─N stretching, and 735 cm⁻¹ for C─H bending‐out‐of‐plane. In the spectra of the DGEVA/ 0%Ze thermoset, the band at 1725 cm⁻¹ is attributed to the carbonyl groups that can emerge from the tautomerization stage of zwitterion, after Hofmann elimination (Scheme 1c). In addition, in the epoxy region, there is a band shifting for the epoxy functions from 909 to 930 cm⁻¹, showing their polymerization. For the DGEVA/ 5%Ze system (Figure S3), a significant shift is observed in the FT‐IR spectrum. The amide I band typically situated at 1640 cm⁻¹ in the Zein spectrum, shifts to 1658 cm⁻¹, indicating the polyaddition reactions between the ─NH₂ of Zein with epoxide groups of DGEVA. Also, the presence of a signal at 1730 cm⁻¹ corresponding to the ester groups, confirms the reaction between epoxide and the carboxyl functional groups of Zein. As the Zein content increases, the absorbance in the epoxy region decreases, signaling that the epoxy groups are being consumed as they participate in the reaction with Zein. Furthermore, the peak absorbance at 1658 cm⁻¹ attributed to the C═O stretching of amide I in Zein, increases proportionally with Zein content (Figure 2), further confirming the interaction between Zein and DGEVA. These spectral changes reinforce the idea that Zein is actively participating in the crosslinking and polymerization processes, facilitating the formation of a more complex and robust thermoset network.

Scheme 1.

Schematical representation of possible reactions of DGEVA in presence of 1‐MI (a–c) and in the presence of Zein (d).

The reaction mechanism between Zein and DGEVA in the presence of 1‐MI, as illustrated in Scheme 1, involves several key steps.

First, the initiation step begins with the nucleophilic attack of 1‐MI on the epoxy ring of DGEVA, leading to the formation of an active oxyanion. This oxyanion is crucial for initiating the polymerization process. The active species formed in the initiation step serves as the driving force for the subsequent propagation phase. Next comes the propagation phase involving the oxyanions and the DGEVA monomer with the growth of polyether chains. Furthermore, the oxyanion species can react with Zein carboxyl groups forming carboxylate functions as new active species that can continue the copolymerization reaction with DGEVA. Moreover, Zein itself, being a biopolymer with multiple functional groups, offers several other sites for interaction. Other reactions that have the potential to occur are the polyaddition reactions between the ─NH₂, ─COOH, or ─OH functional groups from Zein with the epoxy groups of the DGEVA, forming additional covalent bonds and further contributing to the growth of the copolymer network. The combined effect of these reactions—nucleophilic attack, propagation with DGEVA monomers, reaction with Zein's functional groups, and polyaddition processes—leads to the formation of a highly crosslinked, complex copolymer network. This network is what ultimately results in the creation of a thermoset material. The crosslinked structure enhances the material's stability, mechanical strength, and thermal properties, giving it superior characteristics compared to the individual components (DGEVA and Zein).

In conclusion, the detailed DSC and FT‐IR analyses demonstrate that Zein plays a critical role in both the thermal and chemical behavior of DGEVA. Zein accelerates the polymerization process, as evidenced by the shift in the reaction temperature and the increased reaction enthalpy. Moreover, Zein actively participates in the crosslinking process, contributing to the formation of a more robust and thermally stable copolymer network. These findings highlight Zein's potential as a biopolymer for the development of sustainable thermosets and composites, providing a promising pathway for advancing biobased materials that combine the advantageous properties of both biopolymer and epoxy‐based chemistry.

2.2. Thermogravimetric Analysis

The thermal stability of the thermoset materials incorporating varying percentages of Zein in the DGEVA matrix was evaluated through thermogravimetric studies. The TGA thermograms of neat DGEVA thermoset, its combinations with varying Zein percentages, and raw Zein, are presented in Figure 3.

Figure 3.

Thermogravimetric analysis (TGA) thermograms of neat Zein and of developed thermosets heated from 25°C to 1000°C at 10°C·min⁻¹, under air.

Raw Zein exhibited three distinct stages of thermal degradation under oxidative conditions. As shown in Figure 3, the first decomposition stage of Zein is characterized by a 2% weight loss at ∼80°C. This initial mass loss is consistent with findings reported in the literature,^{[ 33} , ^{34 ]} where Zein is known to lose volatile compounds and moisture at relatively low temperatures. The second stage of thermal degradation occurs in the temperature range of 240°C–430°C. During this phase, Zein experiences significant weight loss due to the breakdown of amino acids and peptide bonds, leading to a mass loss of approximately 66%. This substantial degradation phase is critical as it reflects the thermal decomposition of the protein structure within the Zein. The third stage of thermal degradation begins at temperatures above 430°C, culminating in a further mass loss of about 28%. This final phase of thermo‐oxidative degradation marks the complete breakdown of the remaining organic material in Zein, resulting in the formation of ash and inorganic residues. Considering that Zein was used in its commercial form without prior purification or chemical modification, the residual ash detected after thermal degradation does not originate from the intrinsic molecular structure of Zein itself. Instead, it can be attributed to trace amounts of inorganic impurities, likely residual mineral content remaining from the extraction and purification processes (e.g., sodium, potassium, calcium salts, phosphates), or unintentional additives introduced during commercial processing and handling of Zein.^{[ 18} , ^{35 ]}

In contrast, the thermal degradation behavior of the DGEVA‐Zein thermosets differs notably from that of raw Zein. The TGA thermograms of these bioresins show two main stages of degradation, with the initial mass loss characteristic of the evaporation of moisture from the Zein being absent. The first significant thermal decomposition stage occurs between 200°C and 470°C, with a maximum degradation rate observed in the range of 378°C–386 °C. This stage reflects the primary pyrolysis process, which involves the breakdown of the polymer network. The addition of Zein content slightly shifts the starting temperature of this first degradation stage to lower values. At the end of this first stage, ∼450°C, the DGEVA‐Zein thermosets had lost between 63% and 61% of mass while the neat DGEVA resin exhibits in this stage a mass loss of ∼71% and the Zein ∼68%. As shown in Figure S4, the mass loss in this first stage of degradation is influenced by the Zein content. The derivative DTG thermograms show a complex mechanism of degradation in this stage, with the presence of two consecutive peaks, one at lower temperatures characteristic of the thermal decomposition of Zein and a second one at higher temperatures, related to the thermal degradation of the DGEVA resin. This result suggests that the DGEVA‐Zein thermosets have enhanced thermal stability due to the formation of a stable cross‐linked network. As observed in Figure 3, the second stage of degradation, associated with the thermo‐oxidative process, occurs between 470°C and 800°C, with a maximum degradation temperature ranging from ∼597°C to 645°C. During this stage, the mass loss is around 29%–36%. This stage represents the final breakdown of the polymeric network under oxidative conditions. Similar to the first stage, increasing the Zein content shifts the onset of this stage to lower temperatures while increasing the mass loss (from 29% for the neat resin to 36% for the system with 30% Zein). The higher thermal stability observed in this stage for the DGEVA‐Zein thermosets, compared to both neat DGEVA and neat Zein, can be attributed to several synergistic effects arising from the molecular interactions and crosslinked network formation between DGEVA and Zein during the curing process. Based on DSC and FTIR analysis, it is evident that Zein takes part in the polymerization of DGEVA, leading to the formation of a chemically bonded network. This network restricts the mobility of polymer chains and delays the onset of thermal decomposition, particularly at elevated temperatures. Furthermore, the enhanced thermal stability of the DGEVA‐Zein resins within this temperature range can be ascribed to the development of a network, which is stabilized by strong intermolecular interactions. The presence of nitrogen‐containing moieties from Zein may contribute to the formation of thermally stable covalent bonds with the DGEVA that hinder further oxidative degradation.

The thermal stability of the designed thermosets, defined as the temperature at which they lose 5% of their mass (T _5%), is given in Table 2. Zein itself exhibits relatively low thermal stability with a T _5% value of ∼245°C, whereas the neat DGEVA resin shows better stability with a T _5% value above 320°C. The addition of Zein slightly influences the temperature of degradation of bioresins. For instance, incorporating 5% Zein reduces this temperature by only 3°C. Materials with 10%–25% Zein maintain a constant T _5% around 300°C. Even with 30% Zein, the T _5% value remains close to 295°C, reflecting good thermal stability.

Table 2.

Thermal properties of biobased systems.

Samples	T 5% [ a ] (°C)	T 30% [ b ] (°C)	T dmax1 [ c ] (°C)	T dmax2 [ d ] (°C)	T s [ e ] (°C)
Zein	245	315	320	595	141
DGEVA/ 0%Ze	320	380	386	615	175
DGEVA/ 5%Ze	317	380	386	605	174
DGEVA/ 10%Ze	300	375	385	605	169
DGEVA/ 15%Ze	300	370	382	635	168
DGEVA/ 20%Ze	300	365	378	645	166
DGEVA/ 25%Ze	300	360	383	635	165
DGEVA/ 30%Ze	295	355	382	595	162

^[a] T _5%—the temperature at which materials lose 5% of their mass.

^[b] T _30%—the temperature at which materials lose 30% of their mass.

^[c] T _dmax1—the maximum temperature of degradation for the first stage.

^[d] T _dmax2—the maximum temperature of degradation for the second stage.

^[e] T _s—the statistic heat resistance index.

For a comprehensive assessment of the thermal behavior of the designed systems, the statistic heat resistance index (T _s) was investigated. This index serves to indicate the physical heat tolerance limit temperature of the polymeric systems. The T _s values were calculated using Equation (1):^{[ 36} , ^{37 ]}

$$T_{\mathrm{s}}=0.49\left[T_{5\%}+0.6\left(T_{30\%}-T_{5\%}\right)\right]$$ (1)

where T _30% represents the temperature at which the material loses 30% of its mass. The calculated T _s values, listed in Table 2, consistently range between 162°C and 175°C for the designed materials. According to industrial criteria, these values classify these developed biobased resins as heat‐resistant. This analysis underscores the effectiveness of incorporating Zein into the bioresin as even with varying Zein content, the T _s values remain consistent and indicate their thermal performance. The slight decrease in T _s with Zein content reflects nuanced changes in thermal stability, yet the bioresins exhibit sufficient resilience for practical applications.

Although the addition of Zein slightly decreases the thermal stability at lower temperatures, as evidenced by the reduced T _s values (Table 2), a remarkable improvement in high‐temperature stability (450°C–600°C) is observed. This suggests that Zein, while thermally labile so then decreasing the onset of thermal degradability, contributes to the formation of a network with DGEVA resin that is thermally stable at elevated temperatures, resulting in higher thermal resistance in the final decomposition stages.

These results suggest that moderate Zein content enhances the thermo‐oxidative stability of the thermosets at high temperatures while higher Zein content introduces more complex degradation behavior, overall maintaining robust thermal performance suitable for various applications. Overall, these findings validate the suitability of the designed systems for industries requiring reliable heat‐resistant materials, further supporting their potential in diverse fields where thermal durability is asked.

2.3. Thermo‐Mechanical Behavior Through Dynamic Mechanical Analysis

The evolution of the materials' viscoelastic characteristics and the effect of Zein's contribution on the thermo‐mechanical behavior of the proposed thermosets were investigated by DMA studies. The evolution of storage modulus (E′) and the damping factor (tanδ) as functions of temperature are displayed in Figure 4, and the thermo‐mechanical parameters are presented in Table 3. The storage modulus variation with temperature, alongside the development of three viscoelastic regions: glassy, glassy‐to‐rubbery transition, and rubbery plateau, are shown in Figure 4a. The stiffness of the materials was estimated based on the storage modulus at room temperature, with the obtained values provided in Table 3.

Figure 4.

Dynamic mechanical analysis (DMA) of biobased thermosets: (a) evolution of storage moduli as function of temperature; (b) tanδ curves versus temperature.

Table 3.

Thermo‐mechanical properties of the designed Zein‐based thermosets

System	E′ @ 25°C (GPa)	E′ @150°C (MPa)	T g‐DSC (°C)	T α (°C) (interval)	υ (mmol·cm−3)	Mc (g·mol−1)	Hardness tests (Shore D)	Apparent density (g·cm−3)
DGEVA/ 0%Ze	2.23	4.72	48	52 (30–80)	0.44	2701	83	1.20 ± 0.04
DGEVA/ 5%Ze	1.98	3.21	51	45 (24–85)	0.30	3900	90	1.18 ± 0.05
DGEVA/ 10%Ze	1.73	1.77	49	51 (22–95)	0.17	7370	92	1.22 ± 0.04
DGEVA/ 15%Ze	1.46	1.60	53	62 (27–115)	0.16	7372	93	1.14 ± 0.04
DGEVA/ 20%Ze	1.73	1.61	57	63 (27–120)	0.15	7583	95	1.17 ± 0.03
DGEVA/ 25%Ze	1.97	1.11	60	67 (30–125)	0.11	11,066	94	1.20 ± 0.05
DGEVA/ 30%Ze	1.10	0.59	61	73 (30–130)	0.06	18,073	95	1.03 ± 0.03

In the glassy region, the neat DGEVA bioresin has a storage modulus of ∼2.2 GPa, reflecting its inherent rigidity and structural integrity at ambient temperatures. The addition of Zein into the thermosets’ composition results in a gradual decrease in the storage modulus, ranging between 1.46 and 1.98 GPa for systems containing 5%–25% Zein, and even halving for the system with 30% Zein (E′ at 25°C ∼1.1 GPa). This drop in storage modulus indicates that the protein decreases the rigidity of the material as its content increases. This behavior is consistent with the elastic response of the bioresins in the rubbery region. The E′ moduli gradually decrease with increasing Zein content, reaching a minimum of about 0.6 MPa for the system with 30% Zein. The reduction in storage modulus across both the glassy and rubbery regions suggests that Zein acts as a plasticizer, reducing the overall stiffness of DGEVA resin by facilitating greater chain mobility.

The major chains move collectively and cooperatively during the α relaxations, which are characterized by a dramatic reduction in elastic modulus before reaching the rubbery plateau. This α relaxation (T _α) is associated with the macroscopic glass transition (T _g) of the materials and is recognized as the maximum value of the damping factor (tanδ). The evolution of tanδ as a function of temperature is graphically illustrated in Figure 4b. For all the tested bioresins, the tanδ peak appears well‐defined and narrow. These well‐defined narrow tanδ peaks suggest that despite the incorporation of Zein proteins, the produced copolymers have homogeneous structures of the network. The neat DGEVA‐based resin exhibits a T _g of ∼52°C. The introduction of Zein into the resin matrix results in notable changes in the T _g, depending on the amount of Zein incorporated. With the addition of 5% Zein, the T _g decreases to approximately 45°C. This initial decrease suggests that at lower concentrations, Zein acts as a plasticizer, reducing the rigidity of the polymer network and thereby lowering the glass transition value. As the Zein content increases to 10%, the T _g begins to rise slightly, returning to a value close to that of the neat DGEVA resin. This indicates that the plasticizing effect of Zein is diminishing and other interactions, possibly due to increased crosslinking or intermolecular interactions, are becoming more significant. Further increases in Zein content led to a more pronounced increase in T _g, with the system containing 30% Zein reaching a T _g of ∼73°C. This higher T _g at elevated Zein concentrations can be attributed to the protein contributing to additional crosslinking and intermolecular interactions within the epoxy matrix. We can also notice that with the increase of the Zein content, the α relaxation phenomenon is larger indicating the greatest distribution of the relaxation times (Table 4). Also, in contrast with the neat DGEVA resin, the DGEVA/Ze thermosets exhibit increased amplitude of the tanδ peaks, indicating higher damping capacity and ease of chain motions so a plasticizing effect by Zein, as previously discussed.

Table 4.

Recovery time to the initial shape for the obtained thermosets.

	Recovery time (s)
System	Shape 1	Shape 2
DGEVA/ 0%Ze	63	71
DGEVA/ 5%Ze	71	88
DGEVA/ 10%Ze	97	120
DGEVA/ 15%Ze	91	90
DGEVA/ 20%Ze	92	103
DGEVA/ 25%Ze	110	172
DGEVA/ 30%Ze	153	142

The crosslink densities (υ) and the molar mass of the segments between crosslinks (M _c) of the developed bioresins were calculated based on Equation (2):^{[ 38} , ^{39 ]}

(2)

where E′ is the storage modulus in the rubbery plateau at T _g + 90°C, R is the value of the gas constant, T is the absolute temperature (K), and ρ the calculated density. The crosslink density of the neat DGEVA resin was found to be ∼0.44 mmol·cm⁻³. This value gradually decreases with the increasing protein content, reaching a value of 0.06 mmol·cm⁻³ for the system containing 30% Zein. This decrease in crosslink density with higher Zein content indicates the incorporation of protein chains between the crosslinks, which correlates with the observed decrease in storage modulus. These results corroborate with the values of the mass of the segments between crosslinks (M _c), the highest M _c value was obtained for the DGEVA/ 30% Ze system, exceeding 18,000 g/mol. This confirms the incorporation of Zein protein within the thermoset network.

From a technical perspective, the observed reduction in stiffness and enhanced flexibility at higher Zein content positions these thermosets as versatile materials for applications requiring tailored mechanical properties. The ability to modulate T _g and E′ through Zein incorporation provides significant design flexibility for industrial applications. Overall, the DMA results highlight the dual role of Zein as both a plasticizer and a structural modifier, offering tunable mechanical and thermal properties to meet diverse application needs. The interplay between plasticization and crosslinking observed in these systems is critical for developing bioresins with optimal performance in targeted end‐use scenarios.

The apparent density of the thermosets was also calculated, with the obtained values reported in Table 3. As observed, the addition of Zein slightly influences the density of the copolymers compared to the neat DGEVA‐resin, 1.20 g·cm⁻³. The Zein has a very close density of 1.226 g·cm⁻³. The densities of the copolymers with varying Zein content fluctuate between 1.03 and 1.22 g·cm⁻³. This small variation reflects the complex interplay between the resin matrix and the incorporated protein. Notably, the system with 30% Zein shows a clear decrease in density to 1.03 g·cm⁻³. The apparent density of a material reflects not only its intrinsic density but also its internal structure, including packing efficiency, voids, porosity, etc. The random fluctuations observed in the apparent density of DGEVA/Zein thermosets may arise from these structural imperfections together with the effect of dispersion quality and processing factors. The lower density at higher Zein content makes these materials promising for applications where weight reduction is critical, such as lightweight components in automotive, aerospace, or portable electronic devices. The Shore hardness of the thermosets, evaluated on the Shore D (SD) scale, revealed a distinct enhancement with the incorporation of Zein. The neat DGEVA bioresin exhibited a hardness of 83 SD, reflecting its baseline rigidity. The addition of Zein progressively increased the hardness, with values ranging from 90 to 95 SD across the copolymers. This observed improvement in hardness is likely due to the reinforcing effect of Zein within the polymer matrix. As a rigid biomolecule, Zein contributes to a denser crosslinked structure and improved intermolecular interactions at lower concentrations, thereby increasing resistance to indentation. This enhancement in hardness, coupled with the observed changes in density, suggests that the addition of Zein not only impacts the thermal and mechanical properties but also contributes to making these materials more suitable for lightweight applications.

2.4. Shape Memory

The shape memory behavior of the developed thermosets, including neat DGEVA and its copolymers with varying Zein content, demonstrates the potential of these materials for applications requiring adaptive functionality. This property is illustrated qualitatively in Figure 5 and quantitatively analyzed through recovery time measurements, detailed in Table 4.

Figure 5.

Shape memory performance evaluation in various forms for:

(a) DGEVA/ 0% Ze and (b) DGEVA/ 5% Ze thermosets.

To evaluate the shape memory properties, thermoset samples were subjected to a shape deformation‐recovery cycle. The test involved heating the samples on a hot plate at a temperature of 70°C, above their T _g, distorting them into two different shapes, and allowing them to cool until the shape was stabilized. Subsequently, the deformed materials were placed again on the hot plate and the time in which they returned to their original form was measured. The recorded recovery times, presented in Table 4, reveal a clear relationship between the Zein content in the thermosets and the speed of shape recovery. The neat DGEVA resin exhibited the shortest recovery time, attributed to its relatively dense and uniform crosslinked network that facilitates rapid structural realignment. As the Zein content increases, the recovery times progressively lengthen. This delay can be attributed to the incorporation of larger, flexible protein chains between the crosslinks. These chains introduce greater molecular mobility and a less rigid network structure, which hinders the rapid reorganization required for shape recovery.

The slower recovery times observed in Zein‐modified systems are indicative of the influence of protein chains on the dynamics of the shape memory process. At lower Zein concentrations, the protein chains are sparsely distributed within the polymer matrix, allowing the thermoset to retain much of its original rigidity and facilitating a relatively fast recovery time. In contrast, at higher concentrations, the network structure undergoes significant modification due to the incorporation of flexible Zein chains. These chains introduce greater molecular mobility and entanglement, thereby reducing the efficiency of elastic recovery and resulting in slower shape recovery. This tunable shape memory behavior highlights the potential of these bioresins for a wide range of advanced applications. In medical devices, such as stents, orthodontic tools, and surgical implants, the ability to adapt and recover controlled shapes under specific conditions can significantly enhance functionality and patient outcomes. Similarly, these materials are well‐suited for wearable electronics and responsive clothing, where their capability to regain specific configurations under thermal activation aligns with the dynamic needs of smart textile applications. Furthermore, the shape memory effect can be leveraged for self‐healing structures, enabling materials to recover from deformation or damage autonomously. This feature is particularly beneficial for critical components in automotive and aerospace industries, where the ability to adapt to environmental changes or correct structural deformations ensures durability and safety under demanding conditions.

In conclusion, the incorporation of Zein not only introduces a degree of flexibility into the thermoset matrix but also broadens the scope of applications by providing a means to control recovery times. This balance between adaptability and mechanical performance underscores the versatility of these bioresins, paving the way for their integration into multifunctional, high‐performance material systems.

3. Conclusion

This study reveals for the first time the use of Zein protein as an active crosslinker for a biobased epoxy thermosetting resin. Seven formulations with different Zein content were developed to investigate the influence of the protein on the thermoset properties. The polymerization reaction enthalpies from DSC analysis display that the protein interacts with DGEVA monomer. TGA analysis shows that the DGEVA/ Zein thermosets present good thermal stability and a thermal behavior similar to DGEVA with a degradation temperature superior to 300°C. The DMA results prove that the addition of Zein results in an increase of around 40% of the glass transition values. Furthermore, the density and hardness of the biocomposites are influenced by the addition of Zein. The density of the composites fluctuates between 1.1 and 1.26 g·cm⁻³ with different Zein contents and the hardness increases from 83 Shore D for the neat bioresin to 90–95 Shore D for the biocomposites. Finally, all the obtained thermosets exhibited shape memory effects triggered by temperature.

This study represents a groundbreaking achievement in material science by introducing Zein, a natural and renewable protein, as an active crosslinker in biobased epoxy thermosetting resins. Its uniqueness lies in demonstrating how a plant‐derived byproduct can be effectively integrated into synthetic polymer matrices, resulting in materials with enhanced thermal stability, improved mechanical properties, and tunable shape memory effects. Beyond its chemical innovation, the study aligns with global sustainability goals, offering a compelling alternative to petroleum‐based resins and reducing environmental impact.

By employing Zein—a renewable, biodegradable, and plant‐derived protein—this study contributes to reducing the reliance on petroleum‐derived resins, which are often associated with significant environmental costs. The ecological benefits of this approach are numerous, including a lower carbon footprint, reduced generation of non‐degradable waste, and alignment with circular economy principles. Furthermore, using Zein as a sustainable feedstock supports global efforts to strengthen agricultural economies. As a byproduct of corn production, a widely cultivated crop, Zein offers the opportunity to valorize agricultural residues and transform them into high‐value industrial materials. For communities dependent on farming, this technological innovation can translate into economic empowerment, creating demand for biobased raw materials and fostering sustainable agricultural practices.

Supporting Information

Detailed experimental procedures, Figure S1: FT‐IR spectra of synthesized DGEVA; Figure S2: ¹H‐NMR spectra in CDCl3 of synthetized DGEVA; Figure S3: FT‐IR spectra and characteristic peaks for DGEVA‐based raw materials and thermosets without and with 5% Zein; Table S2: Assignment of bands in FT‐IR spectra of the initial components of the systems; Figure S4. DTG thermograms of neat Zein and of developed thermosets during heating heated from 25°C to 1000°C at 10°C·min⁻¹, under air.

Conflict of Interests

The authors declare no conflicts of interest.

Supporting information

Supporting Information

Dedicated to the memory of Acad. Prof. Dr. Bogdan C. Simionescu

Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.