A Review on the Aging Behavior of BADGE-Based Epoxy Resin

Abstract

Epoxy adhesives derived from bisphenol A diglycidyl ether (BADGE) are widely utilized in segmental construction—particularly in precast concrete structures—and in building structural strengthening, owing to their outstanding adhesion properties and long-term durability. These materials constitute a significant class of polymeric adhesives in structural engineering applications. However, BADGE-based epoxy adhesives are susceptible to aging under service conditions, primarily due to environmental stressors such as thermal cycling, oxygen exposure, moisture ingress, ultraviolet radiation, and interaction with corrosive media. These aging processes lead to irreversible physicochemical changes, manifested as degradation of microstructure, mechanical properties, and dynamic mechanical properties to varying degrees, with performance deterioration becoming increasingly significant over time. Notably, for the mechanical properties of concern, the decline can exceed 40% in accelerated aging tests. A comprehensive understanding of the aging behavior of BADGE-based epoxy resin under realistic environmental conditions is essential for predicting long-term performance and ensuring structural safety. This paper provides a critical review of existing studies on the aging behavior of BADGE-based epoxy resins. This paper summarizes the findings of various aging tests involving different influencing factors, identifies the main degradation mechanisms, and evaluates current methods for predicting long-term durability (such as the Arrhenius method, Eyring model, etc.). Furthermore, this review provides recommendations for future research, including investigating multifactorial aging, conducting natural exposure tests, and establishing correlations between laboratory-based accelerated aging and field-exposed conditions. These recommendations aim to advance the understanding of long-term aging mechanisms and enhance the reliability of BADGE-based epoxy resins in structural applications.

1. Introduction

Epoxy resins represent a crucial category of amorphous thermosetting polymers, characterized by the presence of two or more epoxy groups in their molecular composition. When reacted with curing agents, these resins undergo a crosslinking process that forms a three-dimensional network structure. Among the various types of epoxy resins, bisphenol A diglycidyl ether (BADGE)-based epoxy resin dominates the market, accounting for over 75%, and is the primary focus of contemporary research. This epoxy resin system primarily comprises BADGE (Part A) and an amine-based curing agent (Part B). The chemical structure of BADGE is shown in Figure 1. This resin is renowned for its exceptional thermal properties, adhesive performance, and workability. Due to the presence of polar functional groups—such as epoxy, hydroxyl, and ether bonds—in its chemical structure, the cured epoxy resin exhibits outstanding mechanical strength, chemical resistance, and toughness. Furthermore, it boasts advantages including zero volatile emissions, low shrinkage, and dimensional stability. These combined attributes have led to the widespread use of epoxy resins, making them the preferred choice for various applications, including adhesion, structural reinforcement, protective coatings, printed circuit boards, aerospace applications, and more [1,2].

The synthesis of epoxy polymers was completed in the late 19th century, and due to their outstanding properties, they have since attracted the attention of researchers worldwide, leading to extensive studies across multiple countries [3]. In 1946, the first batch of epoxy resin adhesives was showcased at an exhibition in Switzerland, marking the beginning of epoxy resin’s commercial entry into the market. Although research on epoxy resins in China began relatively late, it gradually developed during the 1970s and 1980s. Today, this material is widely used across various sectors in China. As shown in Figure 2 [4], from 2008 to 2022, the production volume of epoxy resin in China increased to 2.3 times the original amount, and the usage volume nearly rose to 2.5 times the initial amount. The production and consumption data over the past 15 years indicate that the market demand for this material has maintained a continuous growth trend.

Epoxy resins are often exposed to various environmental factors during their service life, including air, water, temperature, light, and corrosive media. These exposures can lead to the degradation of the resin’s crosslinked structure, causing changes in its microstructure and morphology and resulting in both physical and chemical aging. Consequently, this may lead to performance degradation [5]. As epoxy resin polymers are increasingly used, it is crucial to study their aging properties to understand their durability and longevity. Additionally, research by scholars worldwide has shown that composite materials made by adding fillers and fibers [6,7] to epoxy resin matrices can improve the resin’s properties to some extent. As a result, the aging performance of resin composites with added fillers and fibers has garnered significant attention from researchers.

Current research on epoxy polymers is predominantly focused on BADGE-based epoxy resin, yet there is insufficient attention paid to their aging performance and durability prediction. Additionally, the aging behavior of composites using this resin as a matrix has been overlooked. Therefore, this paper reviews the research progress on the aging performance and post-aging performance prediction of fiber-reinforced BADGE-based epoxy resin matrix composites and filled BADGE-based epoxy resin materials under the action of natural environmental factors. It focuses on comparing and analyzing the changing trends of various material properties under different environmental factors. Meanwhile, considering that the post-aging properties of materials have received considerable attention but have been less thoroughly analyzed, this paper also summarizes the research progress on predicting various post-aging material properties. These studies can draw attention to the durability of BADGE-based epoxy polymers and their composites, help understand the development of their durability, and provide support for their larger-scale and more durable applications.

2. Artificial Accelerated Aging Test

In the research on the post-aging durability of BADGE-based epoxy resin, aging tests are the most convenient method for reflecting the performance changes of materials, which helps to understand the trend of material properties changing with time. When BADGE-based epoxy resin is exposed to a specific environment for extended periods, it undergoes degradation processes [8], which can result in irreversible changes to the material’s properties and performance. When evaluating the service life of a material, predictions must be made based on the conditions to which it will be exposed during use.

Long-term performance tests for materials are primarily divided into natural exposure testing and artificially accelerated aging [9], with the latter being the most authentic aging test results under actual environmental conditions. The approach to studying the long-term aging performance of materials can often be impractical and costly due to the extensive time required for observation. As a result, according to current aging test standards [10,11,12,13,14,15,16], most studies on the long-term aging performance of materials are based on accelerated aging tests. These tests aim to achieve long-term performance in a relatively short time by adjusting test parameters. In these tests, environmental factors are intensified to replicate the natural aging process, allowing for researchers to determine the properties of materials after aging and predict their service life [17]. A summary of the primary artificial aging test methods and parameters used in durability research on BADGE-based epoxy resin is provided in

Table 1. Summary of aging tests on durability of BADGE and its composites [17,18,19].

Aging Test Category	Main Test Environmental Parameters	Test Purpose or Application
Thermo-oxidative aging	Temperature: 70–180 °C; oxygen concentration: around 21%.	Evaluate the durability and life of materials in high-temperature and oxygen environments.
Hygrothermal aging	Temperature: 40–85 °C; relative humidity (RH): 60–95%.	To evaluate the durability and service life of materials under a high-temperature and high-humidity environment.
Photo-oxidation aging	Oxygen concentration: around 21%; light intensity and wavelength: variable; temperature: room temperature.	Study of material performance changes under solar light and oxygen exposure.
Physical and chemical aging	Stress and chemical substances are determined based on practical applications.	Evaluate the durability and service life of epoxy resins and their composites.
Electrical–thermal aging	Temperature and current: range should be determined according to the application environment.	The durability of materials in an electrothermal environment is simulated with different temperatures and currents.
Salt fog aging	Temperature: 35–50 °C; RH: ≥95%; salt spray concentration: 5–10%.	Simulate salt spray corrosion in marine environments to test the material’s corrosion resistance.
Thermal cycling aging	Temperature cycle: high-temperature and low-temperature cycle.	Study the material’s relevant performance changes under temperature cycling.
Oxygen exposure aging	High-oxygen environment, according to the application environment.	Evaluate the material’s oxidation stability in a high-oxygen environment.
Electron beam irradiation	Irradiation using high-energy electron beams.	Study the performance changes in materials under a radiation environment.

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3. Aging Performance of BADGE-Based Epoxy Resin

Aging and degradation are unavoidable processes that affect all materials, including epoxy resin. When BADGE-based epoxy resin is exposed to various environmental factors during use, it undergoes aging and deterioration. After aging, the resin surface may yellow, darken, and even develop cracks in certain areas [20]. Additionally, the internal crosslinked structure of the resin can be disrupted, leading to a decline in material properties and impacting its overall service life. Therefore, it is crucial to study the aging of BADGE-based epoxy resin under different usage conditions and analyze the extent of its impact on the material. This research is essential for assessing the long-term durability of the resin and predicting its service life.

3.1. Thermal Oxidative Aging

BADGE-based epoxy resin can oxidize in high-temperature environments, disrupting its crosslinked structure and leading to degradation, which results in decreased performance [21]. In service environments, the resin inevitably undergoes thermo-oxidative aging, causing a deterioration in its properties.

In service environments, oxygen and temperature are common influencing factors that can cause BADGE-based epoxy resin to undergo oxidation, leading to changes in its chemical structure and initiating thermal degradation processes. This results in gradual alterations to the resin’s properties. Gao et al. [22] studied the thermo-oxidative aging behavior of BADGE-based epoxy resin and found that oxygen is a critical factor in thermal degradation. The shear performance of the material initially increased but then decreased over time, with the rate of decline being more pronounced at higher temperatures. Yang et al. [23] studied the effects of thermo-oxidative aging on BADGE-based epoxy resin at temperatures ranging from 130 °C to 160 °C for 30 days. The study revealed that aging results in surface oxidation, molecular chain rearrangements, and a significant decrease in the apparent free volume fraction. Bending tests revealed that aging significantly reduced the fracture strain but only had a slight effect on bending strength. Moussa et al. [24] examined the long-term physical and mechanical thermo-oxidative aging performance of structural adhesives. The study, which included both natural and controlled indoor testing, demonstrated that the cured material exhibited a nearly linear increase in the glass transition temperature (Tg) under varying indoor and outdoor conditions at different temperatures. Due to the post-curing effect, tensile strength and stiffness increased during the early stages of aging. Jeong et al. [25] studied the impact of thermo-oxidative aging on the bonding interface of BADGE-based epoxy adhesive joints under different curing and holding times. They found that aging reduced the bending strength of the resin and weakened the bonding strength at the interface. Materials that aged after shorter curing times exhibited a faster decline in performance than those cured for longer durations.

Additionally, Yi et al. [26] used molecular dynamics simulations to construct models of BADGE with diethanolamine (DEA) with varying degrees of crosslinking. They analyzed the effects of thermo-oxidative aging on the Tg and Young’s modulus of BADGE-based epoxy resin with different crosslinking densities. The results showed that as the degree of crosslinking increased, both the Tg and Young’s modulus of the system increased. However, as the oxidation degree increased, these properties decreased. This is attributed to the increased chain scission caused by oxidation, which enhances molecular mobility and thus reduces the system’s rigidity. An orthogonal simulation and range analysis were conducted on the Tg and mechanical properties of the BADGE/DEA system under the combined effects of oxidation degree and crosslinking density. The results indicate that at higher crosslinking degrees, the influence of oxidation degree on the Tg of the BADGE/DEA system is greater than that of the crosslinking degree itself, and the impact on the mechanical properties of the BADGE/DEA system is the opposite.

Under the influence of temperature, BADGE-based epoxy resin undergoes post-curing (increased crosslinking), which enhances the material’s crosslinking and consolidation, leading to improved performance in the early stages. However, as oxidation causes chain scission, performance begins to deteriorate, resulting in an aging process characterized by an initial improvement and a decline in material properties. The oxidation reaction between the resin and oxygen primarily occurs at the material’s surface in the early stages, affecting its surface morphology. Over time, the reaction progresses internally, resulting in a decline in performance, with temperature further accelerating the aging process.

3.2. Hot and Humid Aging

Due to the polar groups (such as amine and hydroxyl groups), BADGE-based epoxy resin tends to absorb moisture from the environment. The effect of temperature further promotes moisture absorption, making the material susceptible to hydrothermal aging. Compared to thermo-oxidative aging, the presence of water also leads to resin plasticization and hydrolysis. Under the combined influence of moisture and temperature, significant changes in the material’s properties can occur [27,28,29].

Table 2. BADGE-based epoxy resin hygrothermal aging performance.

Component	Environment	Water Absorption	Performance Change	Ref.
BADGE/PEI	70 °C, RH 75% for 34 d	The water absorption increased rapidly in the first 14 d and then stabilized, and the water absorption rate is 1.55%	Tg decreased by 5 °C; the flexural modulus decreased by 35% in the first 7 d and then stabilized; the bending strength was reduced by 31%.	[30]
BADGE/DEA	Wet–heat cycle 625 times at 25, 100, 180 °C and RH 100%	—	The flexural strength decreases by 16.5% and 39% at 100 °C and 180 °C, respectively; the stiffness changes are similar, approximately 11–33% lower.	[31]
BADGE/AA	20 °C, RH 100%, 2 years	The water absorption is fast in the early stage and then tends to be stable; the maximum water absorption is 5%	The tensile properties decrease with time and reduce by 25% after 2 years.	[32]
BADGE/DDS	80 °C, RH 90%, 900 h	In the early stage, the growth rate is fast, with a balance of 5.15%	The tensile strength decreased by 40%, and the storage modulus decreased by 38.8%.	[33]
BADGE/PA	One-year atmospheric environmental aging experiment	—	The shear strength decreases exponentially, with a maximum decrease of 53.1%. The tensile strength decreases linearly and decreases by 50% after one year.	[34]

Note: PEI—polyethylenimine; AA—aliphatic amine hardener; DDS—diaminodiphenyl sulfone; PA—polyamide.

summarizes the changes in the performance of BADGE-based epoxy resin in a hydrothermal environment, focusing on the aging behavior of mechanical properties such as bending strength and tensile strength under the combined influence of temperature and humidity. In a hydrothermal environment, the moisture absorption of BADGE-based epoxy resin increases over time, resulting in a significant reduction in both bending and tensile strength. The tensile properties are most adversely affected, showing a linear decrease throughout the aging process. In contrast, bending strength is more influenced by temperature, and elevated temperatures exacerbate the degradation of its strength. The moisture absorption behavior of the resin generally exhibits a rapid increase in the early stages, followed by a slow rise, and eventually stabilizes, which aligns with Fick’s second law.

On the other hand, to understand the aging behavior of BADGE-based epoxy resin in terms of properties such as modulus and Tg under different temperature and humidity conditions, De’Nève et al. [35] conducted dynamic mechanical analysis (DMA) on BADGE-based epoxy resin. They found that water absorption reduced Tg, and both Young’s modulus in the glassy and rubbery states decreased, with changes in the rubbery modulus being related to molecular chain scission. Similarly, Wang et al. [36] employed DMA to investigate the impact of hydrothermal aging on the dynamic mechanical properties of this type of resin. The results indicated that moisture absorption significantly impacted the material’s high-temperature modulus and its relaxation behavior. In contrast, its effect was minimal on the elastic modulus and fracture toughness at low temperatures. The primary reason for the reduction in the storage modulus and relaxation of the material was identified as the plasticization of the resin matrix by water. Additionally, increasing the water content did not influence the material’s fracture strain.

In practical applications, BADGE-based epoxy resin used as a structural adhesive is typically evaluated for aging performance based on changes in shear strength after hydrothermal aging. Peng Bo et al. [37] conducted a 90-day hydrothermal aging test (50 °C, RH 95%) on steel-to-steel shear specimens bonded with BPA epoxy structural adhesive. They found a 25.3% reduction in shear strength. Additionally, they performed aging tests using the boiling method. They discovered a correlation between shear strength degradation and aging time in both test modes, suggesting that the boiling method can be used to analyze the aging resistance of structural adhesives. Yang Lei [38] conducted experimental and numerical simulation studies on the joint performance of BADGE-based epoxy adhesives. After 7 days of aging in a constant-temperature water bath (80 °C), the shear strength of steel-to-steel specimens decreased by 39%, indicating poor resistance to hydrothermal aging. The discrepancy with the results in Reference [37] is attributed to the higher temperature and humidity used, which caused more significant performance degradation.

Temperature and moisture are significant factors that influence the aging performance of BADGE-based epoxy resins. After hydrothermal aging, the polymer experiences water plasticization and chain scission, resulting in a nearly linear decrease in tensile strength. The combined effects of water and temperature result in a slight decrease in Tg, and other properties exhibit some degradation. Hydrothermal aging is commonly observed in epoxy resins during use, and the changes in performance after aging are closely related to the material’s service life. Studying the hydrothermal aging of BADGE-based epoxy resin is essential for understanding its durability.

3.3. Photochemical Oxidative Aging

Ultraviolet (UV) radiation induces photo-oxidative aging in BADGE-based epoxy resin, a mechanism similar to thermo-oxidative aging, with the key difference being the presence of a chain initiation process in the latter. BADGE-based epoxy resin is susceptible to photo-oxidative aging when exposed to light due to polar groups and unsaturated bonds, such as ether bonds. The absorption of UV light causes the oxidation and cleavage of chemical bonds within the aromatic structure, resulting in a reduction in molecular weight and material degradation. Oxidation can initiate free radical reactions, which, in turn, accelerate the degradation process [39,40,41].

When exposed to light and oxygen, the surface of BADGE-based epoxy resin first undergoes oxidation, gradually losing its gloss. Over time, cracks, peeling, and other phenomena appear on the resin surface, leading to severe degradation of the outer layer [42,43]. The carbonyl group in the resin is highly susceptible to oxidation and degradation caused by UV light and oxygen, resulting in a reduction in the number of carbonyl groups [42]. Since the carbonyl group plays a crucial role in maintaining the resin’s strength, thermal stability, and adhesion properties, photo-oxidative aging leads to a decline in the resin’s durability. According to the study by Delor-Jestin et al. [44], using photoacoustic FTIR spectroscopy to examine the photo-oxidative aging behavior of BADGE and two types of curing agents (amine and anhydride), it was found that after aging and degradation, the spectra rapidly changed in the hydroxyl and carbonyl absorption regions, with the carbonyl group content decreasing over time (with a more significant reduction for amine-based curing agents). Unlike amine-based curing agents, anhydride curing agents reduce the degradation of BADGE-based epoxy resin and enhance stability.

Recently, Fu Chenyang [45] conducted a study on the photo-oxidative aging of BADGE-based epoxy resin and found that as the aging process progressed, chemical bonds in the resin were destroyed, hydroxyl groups decreased, and the resin turned yellow, accompanied by a loss in mass (Figure 3). Dynamic mechanical analysis revealed a decrease in the storage modulus, loss modulus, and Tg. Mechanical properties such as tensile strength, modulus, and bending strength initially increased and then decreased over time. Additionally, as shown in Figure 3, the degree of cure of BADGE-based epoxy resin varied with aging time, indicating that the material was influenced by post-curing, which aligns with the results in [22,24]. However, a study by Liu Jie on the photo-oxidative aging of BADGE-based epoxy resin revealed that as the aging duration increased, tensile strength, bending strength, and elastic modulus decreased slightly without exhibiting post-curing behavior [46], indicating that not all epoxy resins undergo the post-curing process.

Under aerobic conditions, BADGE-based epoxy resin polymers undergo degradation reactions upon exposure to light radiation, yielding a series of structurally complex oxidation products, including hydroperoxides, alcohols, and esters. Meanwhile, the aging process is accompanied by the cleavage of chemical bonds, leading to the destruction of carbonyl structures. As aging progresses, the properties of the resin material deteriorate to varying degrees, manifesting as a gradual loss of mass and a significant reduction in surface gloss.

In addition to the aging processes mentioned above, BADGE-based epoxy resin undergoes various forms of aging in specific use environments, including physical aging, electrical aging, and thermo-oxidative aging [47,48,49]. These aging mechanisms similarly affect the resin’s durability. As the widespread use of this type of epoxy resin continues, attention should also be given to the aging phenomena in specific environments.

In summary, BADGE-based epoxy resin exhibits post-curing during its aging phase [50].

Table 3. Influence of post-curing of BADGE-based epoxy resin on its performance.

Component	Aging Form	Performance Changes Due to Post-Curing	Ref.
Epoxy resins BADGE/PA	Thermo-oxidative aging	The shear strength increases in the first 5 days of aging and then decreases continuously; the higher the temperature, the greater the decrease.	[22]
BADGE/DETA	Thermo-oxidative aging	The tensile strength and stiffness at 60 °C increase by 7 days before aging, and the stiffness increases by about 15%.	[24]
BADGE/DDS	Thermo-oxidative aging	When the temperature is within 40–140 °C, the post-curing increases the bending strength.	[51]
BADGE/DEA	Photo-oxidation aging	The curing degree reaches the maximum after 7 d of aging, and the tensile and flexural strength reach the peak on the 7th day, which increase by 6.2% and 6.7%, respectively; the tensile modulus rises to the peak on the 3rd day, increasing by 4.1%, and then shows a decreasing trend.	[45]
BADGE/EDA	Thermo-oxidative aging	A post-curing phenomenon at higher temperatures (80, 90 °C) strengthens the flexural strength.	[52]

Note: DETA—diethylene triamine; EDA—ethylenediamine.

summarizes the occurrence of post-curing in this type of epoxy resin during aging. The primary cause of this phenomenon is that, after the initial curing process, the material is subjected to environmental factors such as temperature, which, in the early stages of aging, causes crosslinking reactions to dominate over degradation reactions. As a result, the degree of curing initially increases and then decreases, with specific material properties following the trend of curing degree changes. Additionally, the curing time during resin application also influences its degree of curing. If the curing time is too short, post-curing may occur, resulting in an increased curing degree. BADGE-based epoxy resin operates in complex environments, and its aging process is influenced by more than one environmental factor, often involving multiple factors. When studying the aging performance of a material, the coupling effects of various factors must be considered to accurately assess its long-term performance in real service conditions, thereby improving the accuracy of predicted service life.

4. Aging Performance of Filled BADGE-Based Epoxy Resin

During the long-term service of BADGE-based epoxy resin, the material may undergo irreversible aging phenomena over time. These include oxidation, microcavity expansion, polymer network relaxation, cracking, debonding, and discoloration [53,54]. These aging processes gradually accumulate within the material, leading to a gradual decline in performance, significantly negatively impacting its durability. Fillers are often incorporated into BADGE [55,56] to meet particular demands and improve some properties. In BADGE-based epoxy resin filling systems, the properties and morphology of fillers significantly influence the performance of composite materials. In terms of chemical properties, inorganic fillers such as SiO₂ and Al₂O₃ require surface modification to improve compatibility with the resin. As an industrial solid waste-derived filler, fly ash, with its silico-aluminate composition, exhibits weak alkalinity and pozzolanic activity, which can reduce costs and enhance the durability of materials. Organic fillers, due to their similar chemical structure to the resin, can impart special functions to materials. Although metal-based fillers have excellent electrical and thermal conductivity, they are prone to oxidation. As functional fillers, flame retardants, including halogen-based, phosphorus-based, and nitrogen-based types, exert flame retardancy through mechanisms such as gas-phase dilution and condensed-phase barrier formation. Their acidity or alkalinity must be tailored to the resin system. Morphologically, particulate fillers affect the viscosity and filling amount of the system, while fibrous and flaky fillers can enhance mechanical properties and barrier properties. Porous fly ash can achieve weight reduction and adsorption functions, and the particle size and morphology of flame retardants are crucial for their dispersibility and flame retardant efficiency. Therefore, in practical applications, it is necessary to comprehensively regulate the chemical properties and morphology of fillers according to the target performance and optimize the performance of BADGE-based epoxy resin composites through surface treatment, flame retardant synergistic design, and compounding technology.

Compared to pure BADGE-based epoxy resin, the curing structure of filled resins is altered, leading to changes in their performance, with aging performance becoming a critical area of focus [57,58,59] to ensure reliability and durability under various environmental conditions. Panchal et al. [60] studied the performance of cellulose nanocrystal–BADGE-based epoxy resin composites. They found that the composites exhibited superior UV absorption and color change compared to the pure resin. Moreover, the interface interaction between the filler and resin was strong, providing greater stability than the BADGE-based epoxy resin. However, due to the inherent hydrophilicity of the filler, the composite material had a higher water absorption rate than the pure resin. Wang Wenjun et al. [61] investigated the moisture–heat aging resistance of BADGE-based epoxy resin modified with inorganic fillers. They found that acidic fillers promoted resin degradation, while neutral, spherical, and smaller-sized fillers improved the resin’s resistance to moisture–heat aging. Li Qing et al. [62] modified BADGE-based epoxy resin with recycled phenolic resin powder, resulting in improvements of 9.4% in tensile strength and 2.1% in bending strength compared to the unmodified resin. Dynamic mechanical analysis showed that the composite’s stiffness and Tg increased, and its heat deformation resistance improved.

On the other hand, adding fillers can also decrease the durability of resin composites. Brun et al. [63] performed thermogravimetric and dynamic mechanical analyses on SiO₂-filled BADGE-based epoxy resin. They found that under 80 °C, 80% RH humidity conditions, the filled samples exhibited significant moisture absorption, degrading after 50 days, whereas the unfilled samples showed no significant changes. After aging, the filled samples exhibited a reduction in elastic modulus and Tg, resulting in material plasticization and degradation of the interface properties between the BADGE-based epoxy resin and the filler due to hydrolysis.

By incorporating fillers with higher thermal stability, the thermal stability of BADGE-based epoxy resin can be improved, enhancing the aging performance of its composites in high-temperature environments [64,65]. Goyanes et al. [66] studied the dynamic mechanical properties of quartz powder–BADGE composites at different environmental temperatures. They found that the storage modulus of the composites was positively correlated with filler content, and this effect was more pronounced when the temperature was above the glass transition temperature (Tg). Below Tg, the fillers did not affect the relationship between storage modulus and frequency. An increase in filler content caused the loss of the tangent peak corresponding to relaxation to shift to higher temperatures, thereby improving the thermal stability of the composite. Khotbehsara et al. [67,68] analyzed the mechanical response and microstructure of BADGE-based epoxy resin-containing fly ash (FA) and flame retardant (FR) under high-temperature and moisture–heat aging conditions. The study on high-temperature performance showed that, compared to pure BADGE-based epoxy resin, adding fillers improved the retention of high-temperature mechanical performance in the material, increasing Tg by at least 5 °C and reducing weight loss. The addition of fillers significantly enhanced the thermal stability of the composite. In a moisture–heat environment, the fillers reduced the moisture absorption of the composite, and Tg was similarly increased. As filler content increased, the compressive and tensile strengths of the composite decreased slightly.

In contrast, the bending strength rapidly declined, with a maximum reduction of approximately 85% (for a filler content of 60% by volume). This was because the composite’s bending strength depended on the strength of the resin matrix, and the increased filler content resulted in a significant loss of bending strength.

Table 4. The retention of mechanical properties under hygrothermal aging for filled and unfilled BADGE-based epoxy resin [68].

MIX ID	Conditioned (RH 98%)
	RT			40 °C			60 °C
	1000	2000	3000	1000	2000	3000	1000	2000	3000
Compressive strength retention (%)
F0	100	98.03	96.17	97.59	96.39	93.87	94.97	92.78	91.14
F20	100	97.11	96.00	98.11	95.77	94.44	96.77	94.99	92.66
F40	100	99.22	97.76	98.65	97.76	96.86	97.65	96.52	95.40
F60	100	97.72	96.29	99.29	96.58	95.44	97.29	95.58	93.44
Tensile strength retention (%)
F0	100	97.64	95.54	97.64	96.06	93.18	94.49	93.18	89.76
F20	100	96.89	96.19	96.19	95.50	92.73	94.46	92.73	91.00
F40	100	98.25	95.80	96.75	95.10	93.01	94.76	91.96	91.26
F60	100	98.55	96.62	97.10	94.20	93.82	94.69	92.27	91.30
Flexural strength retention (%)
F0	100	97.33	93.93	98.62	94.85	91.82	95.77	92.92	88.88
F20	100	95.57	94.41	98.46	95.57	92.68	95.38	92.87	89.02
F40	100	97.41	94.81	99.53	94.58	94.10	96.93	94.10	90.57
F60	100	96.21	94.95	98.11	94.64	94.01	95.27	94.32	91.17

Note: Fi (i = 0, 20, 40, 60)—the volume of filler in epoxy resin composite is i%, where i = 0 represents pure epoxy resin; RT—room temperature (23 °C); 1000, 2000, and 3000—hygrothermal aging time (hours).

summarizes the mechanical performance retention of filled and unfilled BADGE-based epoxy resin in a moisture–heat environment. After aging, the mechanical performance retention of the composite materials was generally better than that of the pure BADGE-based epoxy resin, with this trend becoming more pronounced at higher temperatures and longer aging times. SEM images reveal that the filled BADGE-based epoxy resin exhibits superior high-temperature aging performance, enhancing the interlocking effect between the filler and resin matrix in a moisture–heat aging environment. BADGE filled with 40% of particles exhibited a biphasic morphology, consisting of dispersed spherical particles (FA, FR) and a rigid continuous phase (resin). As the temperature increased, the particle distribution improved due to increased flowability and reorientation of the filler, resulting in a denser microstructure that enhanced interlocking between the resin and the filler particles, thereby contributing to the maintenance of mechanical strength.

The aging performance of filled BADGE-based epoxy resin depends not only on the type of filler but also significantly on the working environment. Incorporating fillers can enhance specific properties of the epoxy resin composites and reduce costs; however, it may also diminish the physical and mechanical properties of the resin matrix. Therefore, maintaining a balance between thermal stability, physical and mechanical performance, durability, and cost is essential. Additionally, compared to pure BADGE-based epoxy resin, the interface between the matrix and filler in filled resins is more susceptible to aging, making conducting in-depth studies on the interfacial aging phenomena crucial.

5. Aging Performance of BADGE-Based Composite (FRP)

BADGE-based epoxy resin exhibits excellent intrinsic properties, making it an ideal matrix material. When composite materials reinforced with fibers are incorporated into this resin, they offer high strength, a favorable strength-to-weight ratio, good impact resistance and fatigue performance, and ease of manufacturing. Additionally, these composites exhibit strong resistance to thermal oxidation and damp–heat aging, which contributes to their widespread use in various fields, including construction, aerospace, and the automotive industry [69,70,71,72,73].

Table 5. Durability of BADGE-based FRP in different environments.

Fiber Type	Dosage	Aging Form	Aging Properties of BADGE	Aging Properties of Fiber-BADGE	Ref.
Glass fiber	47 wt% glass fiber	Chemical aging	The tensile strength decreases with an increase in aging time; the E change is insignificant at approximately 0.45 GPa.	The tensile strength has been dramatically improved and remains superior to that of the resin after aging. The E is 2.8 GPa.	[74]
Carbon nanofibers	0.25–2 wt% carbon nanofibers	Hygrothermal aging	Temperature promotes moisture absorption, and the trend conforms to Fick’s law. The tensile strength, E, and δ decrease gradually; Tg decreases by 25.6%.	Water absorption is lower than that of the resin. The tensile strength, E, and δ after aging at 0.5 wt% are 18.4%, 7.5%, and 11.6% higher than those of the resin, and the Tg is 65.3 °C.	[75]
Glass fiber	The fiber volume content is 55 ± 5%.	Photo-oxidation aging	The color changes noticeably, and the mass loss rate is 0.32%. The tensile and flexural strengths of the aged material at 28 days are 54.9 and 89.8 MPa, respectively, which represent a decrease of 2.3% and 2%.	Fiber delays the oxidation of materials, and its tensile and flexural strength are much higher than those of epoxy resin. Tg decreased less than that of resin.	[45]
Carbon fiber	The fiber volume content is 58%	Hygrothermal aging	Water absorption conforms to Fick’s law and stabilizes at 1.1%; as the aging process develops, the density gradually decreases.	Flexural and impact strength are strengthened, with values of 760 MPa and 310 J/m2, respectively, after aging, which decreases by 5.4% and 19%.	[76]

Note: wt%—percentage by weight; E—Young’s modulus; δ—elongation at break.

summarizes the aging performance of BADGE-based FRPs. It can be observed that in specific aging environments, their long-term performance surpasses that of pure BADGE-based epoxy resin. Therefore, in some applications, BADGE-based FRPs can yield better results.

Similar to BADGE-based epoxy resin, the aging performance of this type of epoxy resin-based FRP is crucial to the material’s service life, highlighting the need for focused attention on its aging behavior in different environments [77,78].

Carbon fiber is a high-performance fiber with a carbon content of over 90%, commonly used as a reinforcement material for epoxy resin-based composites, forming CFRP (carbon fiber-reinforced polymer). The microstructure of CFRP differs from that of BADGE-based epoxy resin, resulting in significant changes in its properties, with aging performance being a particular area of concern [79]. Water absorption by epoxy resin leads to hydrolytic plasticization, causing a decline in performance. Similarly, BADGE-based CFRP is water-sensitive, making the resin–fiber interface susceptible to degradation [80]. Abanilla et al. [81] conducted a 100-week wet–heat aging test on BADGE-based CFRP. The material’s moisture absorption followed the trend of Fick’s law, and SEM images showed that water affected the fiber–resin interface, thereby decreasing the material’s tensile properties. After the aging test, the tensile strength was reduced by 20%, and the material stiffness decreased with increasing aging time, although the change was minor compared to the reduction in tensile strength. DMA analysis revealed that the glass transition temperature (Tg) decreased gradually as moisture was absorbed.

Furthermore, CFRP is prone to UV degradation during service. Qiao Kun et al. [82] studied the UV aging performance of BADGE-based CFRP. They found that the fiber–resin interface weakened as aging progressed, resulting in an exponential decrease in the CFRP’s bending strength. The impact strength significantly reduced within the first 15 days of aging, and the composite material’s Tg increased after aging. Considering the thermal oxidative effects during service, Jia Yaoxiong et al. [83] analyzed the thermal oxidative aging performance of BADGE-based CFRP. After 40 days of aging, it was found that at temperatures of 70 °C and 130 °C, the material’s mass loss rate gradually stabilized, while at 190 °C, it continued to increase. At 190 °C, the resin–fiber interface was damaged, leading to crack formation and an acceleration of the resin oxidation rate. After aging, the CFRP’s shear strength increased by 13.7% at 130 °C, indicating good heat resistance of the CFRP.

Meanwhile, there is a synergistic effect between UV and humidity on BADGE epoxy resin-based FRP. UV radiation damages the chemical bonds of the resin through photo-oxidation, weakening the fiber–resin interface. Humidity, on the other hand, impairs the matrix and interface via hydrolysis reactions, penetration, and swelling. The two factors promote each other, so this synergistic effect should also be given significant attention.

Glass fiber is an inorganic, non-metallic material known for its high tensile strength, impact resistance, and stiffness. It is an essential modifier for BADGE-based epoxy resin, drawing significant attention to the aging performance of BADGE-based GFRP (glass fiber-reinforced polymer). Kajorn et al. [74] conducted a 5-month aging test on BADGE-based GFRP in chemical media. The results showed that the composite material exhibited a significant increase in tensile strength and stiffness compared to the resin, and its performance remained high after aging. The mechanical properties of the composite were most severely damaged by acidic media, with higher temperatures exacerbating the damage. Silva et al. [84] analyzed the degradation of GFRP laminates by accelerated testing, using mechanical strength as an indicator. The study found that due to the strong interface between the resin and fibers, the composite’s tensile strength remained high during the early stages of exposure to saltwater. However, as the water temperature and degradation increased, the composite’s water absorption increased, resulting in a decrease in tensile strength.

BADGE-based epoxy resin exhibits a post-curing phenomenon during aging, which affects the material’s performance when used as a matrix for FRP (fiber-reinforced polymer). Marouani et al. [85] conducted a study on the aging performance of BADGE-based CFRP (carbon fiber-reinforced polymer), revealing that the composite material’s mechanical properties exhibited a two-phase trend as the exposure time increased. The resin matrix’s post-curing reaction improved the composite’s performance in the initial phase. However, as the temperature increased, the degradation of the matrix led to a weakening of the resin–fiber interface, resulting in a decline in material properties. Similarly, Wang Guojian et al. [86] studied the changes in the properties of resin-based GFRP under conditions of humid heat and ultraviolet radiation. The results showed that the composite material exhibited discoloration, cracking, and warping after aging. FTIR spectroscopy revealed that the material’s chemical structure changed due to photo-oxidation. Additionally, the post-curing of the resin matrix caused an increase in the composite’s Tg, with tensile strength, flexural strength, and elastic modulus initially increasing before declining.

Based on previous research, it can be concluded that adding fibers improves the moisture and heat aging resistance of BADGE-based composites. Their moisture absorption, tensile properties, and thermal stability are enhanced compared to pure BADGE-based epoxy resin. Additionally, the post-curing phenomenon observed during the aging of BADGE-based epoxy resin is also present in resin-based FRP, and its impact on performance can be referenced from the post-curing behavior in BADGE-based epoxy resin. The aging of resin-based FRPs typically begins with the degradation of the resin matrix, which subsequently affects the resin–fiber interface. The interface properties are critical to the durability of FRPs and should, therefore, be given particular attention. Furthermore, while carbon and glass fibers are the primary reinforcement materials, the aging performance of other fiber–BADGE composites warrants further investigation.

6. Aging Durability Prediction

Whether through natural aging or artificial accelerated aging tests, studying the aging performance of BADGE-based epoxy resin and epoxy resin-based composites involves analyzing how changes in the material’s physical and chemical structure lead to performance variations over time. Currently, researchers use various methods to predict the material’s durability, primarily analyzing the decline in relevant properties until they reach a critical threshold, which serves as the failure criterion to assess the material’s longevity and, ultimately, its service life.

6.1. Arrhenius Method

The Arrhenius equation, proposed by the Swedish physical chemist Svante Arrhenius and based on extensive experimental data, is a formula that describes the temperature dependence of reaction rates. It is commonly used to predict the aging performance and lifespan of materials under the effects of temperature [87,88]. The equation is expressed as

$$K=A{e}^{-{\displaystyle \frac{E_a}{RT}}}$$

(1)

In Equation (1), K is the degradation rate, A is a constant related to the material and the degradation process, E_a is the activation energy of the reaction, R is the universal gas constant, and T is the absolute temperature.

Based on aging test data, the Arrhenius equation can be used to predict the performance of BADGE-based epoxy resins and their composites. Anderson [89] determined the thermal degradation results of BADGE-based epoxy resin adhesives under different temperature ranges (with varying gradients of temperature for each range) using the weight loss method and adhesive loss method, respectively. He simulated the weight and adhesive loss due to thermal degradation using a self-catalyzed rate expression and validated the thermal degradation pattern of the adhesive using the Arrhenius model. The degradation kinetic parameters and predicted service life of the adhesive obtained by both methods were similar. Eldridge [90] employed the Arrhenius model to predict the durability of BADGE-based GFRP (glass fiber-reinforced polymer) in various service environments. His findings showed that after 100 years of aging at average annual temperatures of 3 °C, 10 °C, and 20 °C, the composite material’s tensile strength retention would be 65%, 61%, and 50%, respectively. Similarly, Reference [68] used the Arrhenius equation to predict the mechanical strength of BADGE-based epoxy resin with four different filler contents under wet–heat conditions. The results indicated that after 100 years of environmental use, the composite material would retain at least 70% of its initial strength.

In addition, Aiello et al. [91] employed this method to predict the creep behavior of other resin-based materials over 40 years, further demonstrating that, compared with traditional empirical formulas or short-term test extrapolation methods, this method has higher accuracy and scientific validity to a certain extent.

The reliable prediction of a material’s long-term performance is determined through accelerated aging exposure, with the Arrhenius equation providing a method for predicting the service life of materials at the required performance retention level. However, it is essential to note that certain limitations are associated with its application due to the limited number of factors this method considers.

6.2. Other Aging Durability Prediction Methods

Using the Arrhenius equation to analyze material degradation has limitations, as it often fails to account for other factors that influence material aging. The Eyring model, derived from the laws of quantum mechanics, can be applied to predict the aging performance of materials where factors other than temperature are involved in the aging process [92]. Its expression is

$$L\left(V\right)={\displaystyle \frac{1}{V}}{e}^{\left[-\left(C-{\displaystyle \frac{B}{V}}\right)\right]}$$

(2)

In Equation (2), L is the lifetime scale, V is the stress value in absolute units (e.g., relative humidity), and B and C are the undetermined model parameters.

The combination of the Arrhenius and Eyring models enables the prediction of material performance under various aging conditions. Zhang Xiaojun et al. [93] and Li Xiuji et al. [94] integrated these two models to develop a lifespan model for materials undergoing hygrothermal aging under the combined effects of temperature and humidity (Equation (3)). Using peel strength and tensile strength as evaluation metrics, they fitted model parameters based on experimental data. Subsequently, they used Equation (3) to predict the storage lifespan of materials under hygrothermal aging conditions.

$$L\left(H,T\right)={\displaystyle \frac{a}{H}}{e}^{{\displaystyle \frac{b}{H}}+{\displaystyle \frac{c}{T}}}$$

(3)

In Equation (3): L(H, T) represents the accelerated hot and humid aging lifetime; H is the relative humidity; a, b, and c are the undetermined model parameters, respectively.

The generalized Eyring model can reflect the relationship between material degradation rate and time under the influence of different factors, and it can predict material lifespan based on aging test data. Huang Chao et al. [95] conducted experimental analysis on the aging behavior of carbon fiber-reinforced BADGE-based epoxy resin composites in a humid environment, using the generalized Eyring model to predict the storage lifespan of the composite materials. The model’s predictions aligned well with the component’s design lifespan. Gao Jianye et al. [96] studied the aging process of BADGE-based epoxy resin under varying temperature and humidity conditions. They established an aging lifespan model based on experimental results, the generalized Eyring model, and the inverse power law model that could predict the service life of epoxy resin in different temperature and humidity environments. This demonstrates that the generalized Eyring model can effectively predict the performance of BADGE-based materials under varying temperatures and humidity conditions.

In addition, Meshgin [97] conducted experiments on the adhesive interface performance of BADGE and found that shear stress, the ratio of ultimate shear strength, and the adhesive’s curing time are key factors influencing interface creep. High shear stress and thicker adhesive layers led to creep and interface failure. Based on experimental data, a rheological model for the adhesive interface was developed to simulate the long-term behavior of epoxy resin at the interface. Kim et al. [98] designed double-shear specimens for dry–wet and freeze–thaw cycle tests to investigate the bonding performance of BADGE-based epoxy resin in cold regions. They used a finite element model that accounted for interface debonding characteristics to predict the performance and found that post-curing increased the specimen’s load-bearing capacity.

In contrast, interface performance deteriorated more rapidly with aging cycles. The model predictions were in good agreement with the experimental data, allowing for the prediction of the long-term bonding performance of the epoxy resin. Yu Dongchao [99] conducted durability tests on BADGE-based epoxy resin adhesive joints under freeze–thaw, carbonation, and freeze–thaw–carbonation coupling conditions. Based on the experimental data, freeze–thaw and carbonation damage models were established. The models demonstrated high fitting accuracy, enabling accurate predictions of life expectancy.

Table 6. Prediction of the durability of BADGE-based epoxy resin and its composites.

Material	Aging Form	Model	Index	Prediction Result	Ref.
BADGE and filled BADGE	Hygrothermal aging	Arrhenius	Tensile, flexural, and compressive strength	After aging at 30 °C for 100 years, the strength retention rate is above 70%.	[68]
BADGE-based GFRP	Hygrothermal aging	Arrhenius	Tensile strength	After 100 years of service at 3, 10, and 20 °C, the strength retention rates are 65%, 61%, and 50%, respectively.	[90]
BADGE-based CFRP	Hygrothermal aging	Generalized Eyring	Shear strength	Based on the failure criterion of a 30% loss of shear strength, the storage life at 20 °C and 60% RH is 31 years.	[95]
BADGE-based CFRP	Hygrothermal aging	Generalized Eyring	Flexural strength	Taking a 30% decrease in flexural strength as the failure criterion, the life at 20 °C and RH 90% is 11.4 years.	[96]
BADGE-based GFRP	Chemical aging	Arrhenius	Tensile strength	After 200 years of aging in an alkaline solution at 10 °C and 30 °C, the tensile strength retention rates are 83% and 69%, respectively.	[100]
BADGE-based CFRP	Hygrothermal aging	Arrhenius	Shear strength	After aging at 8.1, 15.8, and 21.8 °C, the shear strength retention rate remains stable at approximately 73%. When the shear strength retention rate reaches 90%, it takes 1442, 881, and 611 days.	[101]

summarizes several material performance prediction methods and evaluation indicators. Based on Table 6 and the previous discussion, it can be seen that performance prediction enables a quantitative analysis of material aging properties, providing a direct assessment of durability and predicting service life. However, limited performance prediction methods are available for BADGE-based epoxy resins and their composites, with most studies focusing on long-term performance predictions under various temperature and humidity conditions. As a result, there can be significant discrepancies in forecasts. Therefore, future research could involve natural aging tests that consider multiple factors, reflecting the actual conditions of the materials, and provide more accurate performance prediction methods based on these findings.

7. Conclusions

Based on extensive research conducted by scholars on the aging properties of BADGE-based epoxy resin, this paper provides a comprehensive review of the current state of research on aging performance, aging durability, and prediction methods for unfilled, filled, and fiber-reinforced epoxy resin-based composites. This comprehensive study draws the following conclusions:

• During service, BADGE-based epoxy resin is susceptible to various environmental factors (such as temperature, air, rain, ultraviolet light, and corrosive media), leading to different aging phenomena. These factors result in changes to the microstructure, mechanical properties, and dynamic mechanical performance—for example, molecular chain breakage and weakening of the crosslinked structure after aging; the tensile strength, flexural strength, etc., may decrease by more than 30%; and the Tg shows a decline—thereby affecting its durability. Researching different forms of aging provides valuable insights into assessing the service life of BADGE-based epoxy resin. However, current research has predominantly considered aging factors in isolation, and there is a scarcity of studies investigating the aging performance of materials under the coupling effects of multiple factors.
• Adding fillers to BADGE-based epoxy resin as a matrix can improve the aging performance of composites, such as increasing the Tg and reducing weight loss, and also reduce costs. However, this often leads to a decrease in mechanical properties. For instance, the addition of fillers may increase the water absorption rate of the material and reduce its elastic modulus, compressive strength, and tensile strength. The performance changes are related to the type and characteristics of the fillers. Therefore, exploring the optimal filler content that maintains the material’s aging performance within acceptable limits is essential. Currently, the fillers used are primarily inorganic materials such as SiO₂, Al₂O₃, and fly ash. In future research, other types of fillers should also be investigated to expand the scope of material improvement.
• Compared to pure BADGE-based epoxy resin, fiber-reinforced BADGE composites show improved aging resistance, with enhanced tensile properties and stiffness. The interfacial bonding effect between fibers and the resin matrix exhibits a significant positive correlation with the composites’ anti-aging performance. However, there has been limited focus on the aging of the resin–fiber interface. It is crucial to understand the degradation of the internal interface that occurs during the aging process.
• The prediction of the performance of BADGE-based epoxy resin after aging is primarily based on accelerated aging tests and empirical formulae to establish aging models; however, these models lack validation through long-term natural aging test data. Therefore, there may be significant errors in the prediction.

Currently, research on the aging of BADGE-based epoxy resins has been limited to relatively few factors, with most studies relying on artificial aging tests. Additionally, there is limited literature on the modification methods for BADGE-based epoxy resin and the interface performance between the resin matrix and the added fillers after modification. Based on the current state of research, the author suggests that future studies should focus on the following areas:

• Conduct natural aging tests on BADGE-based epoxy resin and epoxy resin-based composites under multiple factors. Most studies focus on the aging behavior of materials under heat–oxygen, wet–heat, and photo-oxygen conditions. However, the real-world environments of materials are far more complex, involving multiple conditions acting together. Therefore, aging studies incorporating multifactor coupling more accurately reflect the actual aging process of materials, and the results can provide a more precise analysis of their durability.
• Design natural aging tests under real environmental conditions. Artificial aging tests still differ significantly from natural environments, meaning the material degradation results obtained through these tests may have some degree of error. The aging models built from such data must also be validated and adjusted using long-term natural aging data. However, there is a lack of research on natural aging, and it is essential to conduct long-term studies on natural aging to analyze the aging process and predict the durability of materials, such as BADGE-based epoxy resin, more systematically.
• Establish a relationship between artificial and natural aging tests. Natural aging tests are unsuitable for large-scale aging studies. It is possible to compare the data of material degradation under similar aging conditions in both natural and accelerated aging environments. This comparison could help establish a correlation between the two methods, allowing for quick, convenient, and accurate determination of material aging performance.
• Research modification methods for BADGE-based epoxy resin and the interface performance between the modified matrix and the added materials. As aging progresses and the interface degrades, the material’s durability can be affected. However, there has been limited research on the interface between the matrix and added materials, so this area should be prioritized. Additionally, other effective modification methods should be explored to promote the development of BADGE-based epoxy resins.