Characterization of Mechanical Properties and Surface Wettability of Epoxy Resin/Benzoxazine Composites in a Simulated Acid Rain Environment

Abstract

Despite the robustness of thermosetting coatings in various applications, prolonged exposure to acidic environments can cause gradual deterioration, leading to structural or functional damage. This study investigates composite materials comprised of cycloaliphatic epoxy resin (CER) and benzoxazine (BZ) at three different weight ratios: 50:50, 25:75, and 0:100. These composites were exposed to nitric acid, simulating acid rain, for durations ranging from 1 to 5 h. The specimens were characterized for weight change, mechanical properties (flexural strength and short beam strength), and surface properties (contact angle and contact angle hysteresis). Although minimal changes in the physical and mechanical properties of both homopolymer and copolymer composites were detected after short acid exposure (up to 5 h), surface wettability analysis via static contact angle and contact angle hysteresis revealed more pronounced deterioration. The static contact angle decreased by 24.96% and 28.32% for homopolymer BZ and copolymer BZ-CER composites, respectively. Contact angle hysteresis increased by 19.39% and 27.80% for 5 h acid-exposed homopolymer BZ and copolymer CER, respectively. This study underscores the utility of surface wettability analysis as a valuable tool for monitoring deterioration from acidic aging in polymers, particularly in BZ-CER systems used in structural and high-performance applications.

1. Introduction

Carbon fiber reinforced polymers (CFRPs) are extensively utilized in the aerospace, marine, and automotive sectors due to their exceptional properties. These materials are highly esteemed among lightweight composites for their impressive specific strength, high thermal and electrical conductivity, resistance to high temperatures, and corrosion resistance [1]. However, CFRPs are susceptible to significant quality degradation caused by the shortening of average fiber length and fiber loss due to weathering. This issue can be mitigated by applying suitable reinforcement [2]. The selection of an appropriate reinforcement material is crucial, as an incompatible choice can lead to matrix cracking and difficulties in achieving robust matrix–fiber adhesion, potentially resulting in structural failures. These attributes and limitations necessitate careful consideration in advanced materials research and engineering applications [3].

Numerous strategies were examined to address this issue. One notable approach is the integration of epoxy resin with CFRPs to enhance their mechanical performance. Epoxy resin, widely used in adhesives, coatings, and encapsulation, is a reactive prepolymer and polymer containing epoxide groups in its molecular structure [4]. By combining carbon fiber with epoxy resin, a carbon fiber-reinforced composite is created, which not only strengthens the material, but also mitigates some of the inherent deficits of carbon fiber. This combination offers compelling advantages, including exceptional adhesion, minimal shrinkage rates, moisture resistance, and the ability to withstand varying mechanical and electrical loads [5,6,7]. Various types of resins can be used with carbon fiber to create different polymers, each providing unique benefits such as increased adhesion, minimal shrinkage, moisture resistance, higher strength, fatigue resistance, stiffness, and the capacity to endure diverse mechanical and electrical stresses [6,8].

In the pursuit of further enhancing material properties, the incorporation of thermosetting benzoxazine (BZ) resin emerged as a significant development. BZ is an excellent choice for corrosion protection due to its low water absorption, hydrophobicity, and low dielectric constant, making it an effective barrier for coating applications [9]. Incorporating cycloaliphatic epoxy resin (CER) into epoxy coatings not only reduces the overall viscosity of the resin system, facilitating the resin transfer molding process, but also increases crosslinking density, raises the glass transition temperature ($T_g$), and enhances hydrophobic properties. These improvements are crucial for enhancing the coating’s corrosion resistance [6,10,11,12].

One of the primary purposes of resin coatings on aerospace vehicles is to minimize drag, which can significantly affect a plane’s speed and fuel consumption. The contact angle and contact angle hysteresis are crucial factors in determining the extent to which drag can be reduced. Enhanced hydrophobicity discourages the adhesion of liquid droplets to surfaces, resulting in increased contact angles and reduced contact angle hysteresis. This improvement facilitates the effortless shedding of droplets from aircraft surfaces, thereby reducing drag and fuel consumption. However, any alterations in surface properties can potentially compromise these critical attributes, impacting the overall effectiveness of the coatings [13].

Harsh environmental conditions, such as exposure to acid rain, can significantly exacerbate issues by causing failures in polymeric coatings used in aerospace applications. This exposure triggers processes such as leaching, hydrolysis, and oxidation, which predominantly affect materials such as epoxy resin. Such degradation not only diminishes operational efficiency, but also alters the physical structure of the coating, impacting pore volumes and distribution [6,14,15]. When exposed to acid, epoxy resin undergoes transformations, including material deformation, crack formation, and the emergence of carbonyl groups (C=O), which are indicative of aging and deformation. Both epoxy resin and benzoxazine experience a decline in their mechanical properties over time, thereby reducing their effectiveness as coatings [16,17].

Considering that rainwater typically falls within the pH range of 5 to 5.6, primarily due to dissolved carbon dioxide, materials invariably encounter acidic conditions when exposed to rainfall [17]. In heavily polluted regions, the pH of rainwater can drop to approximately 4.3. This phenomenon is further exacerbated by pollution from the production of epoxy resin and benzoxazine, where atmospheric constituents such as sulfur dioxide (SO₂) and nitrogen oxides (NO_x) contribute to the formation of acid rain [18,19]. The inherent acidity of rain, intensified by atmospheric emissions, underscores the necessity for meticulous engineering practices to preserve the integrity of equipment and vehicles [20,21,22,23,24,25].

Numerous investigations explored the repercussions of corrosion and aging on diverse materials, emphasizing the consequences of modifications in chemical composition and surface texture [15]. The existing body of research primarily centers on metals [21,22]. Previous studies involving sulfuric acid demonstrated crack formation and the emergence of carbonyl groups, which are indicative of aging and deformation in epoxy resin [17]. Similar effects are observed with nitric acid, where surface damage and the presence of lactone and carbonyl groups increase with acid exposure [26].

To date, no prior experimental works examined a copolymer comprising benzoxazine and cycloaliphatic resin within an environment simulating acid rain conditions [7,10,27,28,29]. Therefore, it is necessary to address this gap to enhance our understanding of these materials and their interaction with acid rain. One method to observe the degradation over time of a carbon fiber-reinforced composite is by using wettability science combined with acid treatment. Prolonged aging and examination of the plate’s surface and mechanical characteristics will be pivotal in determining the extent of damage acid will cause to the carbon fiber-reinforced composites, particularly in terms of weight loss, contact angle, contact angle hysteresis, and tensile strength [12].

Our previous study demonstrated that carbon fiber and benzoxazine copolymerized with cycloaliphatic epoxy resin, when exposed to prolonged high temperatures, could prompt degradation and aging of the carbon fiber-reinforced composite [12]. Simulated acid rain, particularly when treated with nitric acid, is considered to have a profound effect on the surface chemistry, suggesting that a combination of such experiments can be conducted. Since the benzoxazine and cycloaliphatic epoxy resin copolymer exhibited increased degradation to thermo-oxidation compared with a benzoxazine homopolymer, building on prior research can aim to determine if the copolymer will perform worse in a nitric acid environment compared with the homopolymer.

This study aimed to utilize surface wettability analysis combined with mechanical property evaluation to uniquely assess the degradation caused by nitric acid-stimulated acid rain on the copolymer over varying exposure times. Different copolymers of carbon fiber with benzoxazine and cycloaliphatic epoxy resin were synthesized and subjected to varying durations in a nitric acid bath, with different concentrations of benzoxazine and cycloaliphatic epoxy resin. Building on our previous research, the study involved analyzing the copolymer surface post-wetting treatment using contact angle and contact angle hysteresis measurements, along with mechanical testing of the plates to evaluate the degradation of aged samples. This comprehensive approach provides valuable insights into the nuanced impacts of acid rain simulation on material characteristics, ultimately aiding in identifying optimal epoxy and benzoxazine combinations.

2. Materials and Methods

2.1. Composite Fabrication and Acid Rain Simulation

The composite samples were fabricated using the vacuum-assisted resin transfer molding (VARTM) method, as illustrated in Figure 1a. In this process, dry carbon fiber plies measuring 30 cm by 30 cm were impregnated with a copolymer resin composed of benzoxazine (BZ) (Huntsman Corp., Woodlands, TX, USA) and cycloaliphatic epoxy resin (CER) (Lindau Chemicals, Columbia, SC, USA) at three different BZ:CER weight ratios: 100:0, 75:25, and 50:50. After impregnation, the composites were cured at 180 °C for 2 h, 200 °C for 2 h, and 220 °C for 2 h.

To evaluate the impact of acid rain, a simulated acid rain solution was prepared by diluting nitric acid in Milli-Q water to achieve a pH of 5, matching the typical acidity of acid rain, as shown in Figure 1b. Each sample was immersed in the acid solution for a total of five hours. During this period, contact angle (CA) and contact angle hysteresis (CAH) measurements were taken hourly. Each measurement was conducted three times for accuracy, and the average value was reported.

2.2. Weight Change

Each sample was placed in a desiccator to prevent moisture absorption before weighing. A microbalance with a precision of 0.0001 g was used to weigh each sample, and the weight change was calculated using the following equation:

$${\mathrm{W}}_{\mathrm{t}}\left(\mathrm{\%}\right)=\left({\displaystyle \frac{{\mathrm{w}}_{\mathrm{t}}}{{\mathrm{w}}_0}}\right)\times 100\mathrm{\%}$$

(1)

where ${\mathrm{W}}_{\mathrm{t}}\left(\mathrm{\%}\right)$ represents the relative weight change percentage of the sample, ${\mathrm{w}}_{\mathrm{t}}$ is the weight of the sample after time interval $\mathrm{t}$, and ${\mathrm{w}}_0$ is the initial weight of the sample.

2.3. Characterization of Flexural and Short Beam Properties

The flexural strength was characterized according to ASTM D790-17 [30] using a three-point bending test, as illustrated in Figure 2a. The samples were cut to dimensions of 14 mm (width) × 115 mm (length) × 2.55 mm (thickness). A universal testing machine (AGS-X, Shimadzu, Kyoto City, Japan) with a 10 kN load was used for the test. The span length was set at 81.6 mm, and the crosshead motion rate was 1.27 mm/min. Five samples were tested for each set, and the average value was reported.

The short beam strength was evaluated according to ASTM D2344-22 [31]. Test samples were prepared with dimensions of 5 mm (width) × 38 mm (length) × 2.55 mm (thickness). The same universal testing apparatus used for the flexural test was utilized for evaluating the short beam strength. The span length was set at six times the specimen’s thickness, and the crosshead speed was 1 mm/min. Five samples were tested for each set, and the average value was reported.

2.4. Optical Microscopy

A 5X-trinocular boom stand stereomicroscope (AmScope, Irvine, CA, USA) was used to examine the samples, focusing on the impact of thermal aging on the surface characteristics of various CFRPs. The analysis specifically targeted cracking phenomena, including crack initiation, density, and the number of cracks, observed at 5× magnification over an area of 1 µm².

2.5. Characterization of Contact Angle and Contact Angle Hysteresis

The contact angle ($\theta$) is a crucial metric for evaluating surface wettability. It quantifies the angle formed between the tangent of a droplet’s edge and the surface under scrutiny, offering valuable insights. Notably, a higher contact angle indicates increased hydrophobicity, a characteristic influenced by several factors, including surface chemistry and roughness [13,22,23]. To obtain the contact angle, a static image of a droplet in equilibrium on the surface was captured for analysis, as shown in Figure 2b,c.

Additionally, our study explores the concept of contact angle hysteresis, which relates to the difference between the advancing (${\theta }_A$) and receding contact angles (${\theta }_B$) observed during the initial phases of droplet motion. The droplet’s motion was driven by aerodynamic forces generated by controlled airflow. This approach was selected for its effectiveness in studying ice accretion mechanisms on aircraft surfaces and water flow on automotive surfaces, which are relevant to the composite system analyzed in this study [32]. The experimental setup, depicted in Figure 2d, included a tube connected to an air compressor and a pneumatic valve, allowing precise control of air release onto a water droplet positioned on the plate. Tests were performed at room temperature (25 °C), with regulated air velocity used to initiate droplet movement, replicating the plate tilting method from our previous study [12]. A high-speed camera (Photron Fastcam, San Diego, CA, USA) positioned laterally recorded the droplet’s response to the released air. The proprietary software, Photron Fastcam Viewer 4 (version 4.2.0.0), was used to record and store visual data. Synchronization of air release and recording was achieved using an Arduino-based programming solution. This synchronized action was initiated using a button mechanism integrated with a device from prior research efforts. This approach ensured precise coordination of air release and camera recording, enabling accurate observations of the droplet’s behavior in response to controlled air movement.

After recording all samples, the Photron Fastcam Viewer 4 software was used to determine the tangential line and calculate the contact angle for each recorded instance. To deduce the contact angle hysteresis, the moment of initial droplet motion was pinpointed by carefully reviewing the video frames. Once identified, the video was reversed by a single frame, and the contact angles for both the advancing and receding edges were measured. The disparity between these two angles was then calculated, yielding the contact angle hysteresis value, which correlates with the droplet’s mobility across the surface. A lower contact angle hysteresis value indicates increased droplet mobility [33]. It should be noted that five measurements were analyzed for each contact angle and contact angle hysteresis, and average values were reported.

3. Results and Discussion

3.1. Weight Change

Figure 3 illustrates the progression of weight loss in specimens with varying proportions of BZ and CER over time when exposed to acid rain. All samples exhibited a minimal weight reduction of less than 0.04 g after 5 h of acid exposure, indicating that only a small amount of material was corroded, primarily at the surface. While nitric acid is commonly used to decompose the matrices of CFRP composites, including benzoxazine and epoxy, for recycling purposes, a significant reduction in weight was not observed until after 25 h of exposure [34,35,36]. Therefore, exposure to acid for up to 5 h resulted in minimal weight loss.

3.2. Flexural Properties

Figure 4 presents a comparative analysis of the flexural strength of specimens containing varying proportions of BZ and CER, subjected to different acid exposure durations. Prior to acid exposure (at 0 h), the homopolymer BZ sample exhibited the highest flexural strength. The flexural strength decreased with increasing CER content. At 50 wt.% CER content, the flexural strength decreased by 15.83%, which aligned with our previous finding that the addition of CER to BZ reduced flexural performance [12]. This reduction could be attributed to the incorporation of CER, which decreased the amount of rigid BZ in the mixture. The newly formed crosslinks between the epoxide groups of CER and the hydroxyl groups of BZ could disrupt the strong intermolecular interactions between BZ backbones, leading to an increase in free volume and a decrease in molecular packing density [11].

All samples, regardless of the BZ:CER weight ratios, showed a reduction in flexural strength with prolonged acid exposure. The homopolymer BZ exhibited the highest reduction in flexural strength, approximately 15% after 5 h, as shown in Figure 4a. In contrast, the addition of CER at 25 wt.% and 50 wt.% resulted in lesser reductions of 2.62% and 3.91%, respectively, as shown in Figure 4b,c. Micrographs of the cracking cross-sectional surfaces after various durations of acid exposure of the homopolymer BZ and copolymer BZ-CER specimens are shown in Figure 4d. It can be observed that all specimens, regardless of CER content, exhibited both intraply (red arrows) and interply (yellow arrows) delamination. The interply delamination in samples with higher CER content was more extensive and severe than in the homopolymer BZ samples, resulting in lower flexural strength. This finding aligns with previous studies [37,38], which attributed the decline in flexural strength to hydrolysis, plasticization, and swelling effects on the resin matrix. Additionally, the acidic environment can damage the polymer matrix surface, creating cracks that allow further moisture diffusion into the CFRP composites. This results in composite degradation, leading to debonding and crack progression at the fiber–matrix interface.

3.3. Short Beam Properties

Figure 5 compares the short beam strength of homopolymer BZ and copolymer BZ-CER samples. Without acid exposure, the homopolymer BZ sample exhibited the highest short beam strength at 99.48 MPa, followed by the copolymer BZ-CER with 75 wt.% BZ and 50 wt.% BZ, showing values of 94.45 MPa and 94.34 MPa, respectively, as shown in Figure 5a–c. This trend mirrors the flexural properties discussed previously, where increasing CER content in the BZ system reduced the composite’s mechanical properties. Under acid exposure, short beam strength decreased with longer exposure durations. After 5 h, all systems showed a similar reduction of approximately 9%. Figure 5d illustrates the cross-sectional surfaces of homopolymer BZ and copolymer BZ-CER at a 50:50 weight ratio, highlighting delamination in both systems (yellow and blue arrows), with significantly more severe delamination in the copolymer system.

3.4. Surface Properties

Figure 6 presents microscopic images of the surface conditions of samples exposed to acid for varying durations, up to 5 h. Prior to acid exposure, regardless of the copolymerization concentration with CER, the specimen surfaces exhibited minimal signs of deterioration with no fiber exposure. However, with prolonged acid exposure, surface degradation and increased fiber exposure became more apparent, as shown in Figure 6a. Additionally, Figure 6b indicates that the color of the sample panels turned redder upon contact with the acid. The color change could be attributed to the oxidation process and chemical reactions occurring when the CFRPs were exposed to nitric acid. As a strong oxidizing agent, nitric acid oxidized the composite’s surface upon contact with the carbon fibers, introducing various oxygen-containing functional groups, such as carboxyl (−COOH), carbonyl (C=O), and hydroxyl (−OH) groups. These functional groups may react with the CER and BZ resins, leading to the formation of chromophores that absorb light in the visible spectrum, resulting in a red color [26,39].

FTIR analysis was employed to examine the chemical changes on the surface of homopolymer BZ and copolymer BZ-CER samples due to acid exposure. Figure 6c displays the FTIR scan of the homopolymer BZ samples before acid exposure (red), after 3 h of exposure (yellow), and after 5 h of exposure (purple). In comparison, Figure 6d shows the FTIR scan of the copolymer BZ-CER at a 50:50 weight ratio. Comparing the unexposed samples of homopolymer and copolymer composites, an increase in the peak at 2400 cm⁻¹ in the copolymer sample corresponds to the intermolecular hydrogen bonds between hydroxyl and oxygen due to the hybrid network formation when CER was added to BZ [6]. Furthermore, the increase in hydrogen bonding between the carbonyl group on CER and the hydroxy dangling group on BZ resulted in an increased peak intensity at 1730 cm⁻¹. This could affect the surface properties of the composite by increasing its surface energy and hydrophilicity. Similar to the thermal aging observations reported in our previous study [6], the FTIR data show the disappearance of a peak at ~1176 cm⁻¹, the carbon double-bond stretching peak at ~1498 cm⁻¹, and the N-H wagging−twisting peak at ~858 cm⁻¹ in both BZ and BZ-CER samples, indicating the cleavage of the Mannich bridge upon acid exposure. Additionally, the loss of the medium peak of C−H bonding between 1440 and 1480 cm⁻¹ in both samples indicates another cleavage on the main polymer backbone. The formation of broad carbonyl peaks in the range of 1500–1700 cm⁻¹ in both samples suggests the development of a hydrophilic surface with increasing acid exposure duration.

3.5. Contact Angle and Contact Angle Hysteresis

The contact angle results quantify the wettability changes induced by acid exposure. Static contact angle measurements provide direct insight into a surface’s wettability, reflecting its interaction with water. Figure 7a illustrates the process of measuring contact angle and contact angle hysteresis. The static contact angle was determined at both end tips of the droplet where it contacts the surface in a dome configuration. With the aid of compressed air blowing from one side, the droplet began to change shape, allowing the determination of contact angle hysteresis by measuring the difference between the angles at the droplet’s end tips.

Figure 7b shows the change in the contact angle of sessile droplets on the composite sample surfaces, highlighting how this angle evolves with increasing acid exposure duration. Prior to acid exposure (at hour 0), the contact angle of the copolymer BZ-CER was lower than that of the homopolymer BZ. Initially, BZ exhibited hydrophobicity, but when CER was incorporated, the surface became more hydrophilic. This change occurred because cycloaliphatic epoxies contained polar groups, such as hydroxyl (−OH) and ether (−O−) groups. When these epoxies were added to benzoxazine, they introduced more polar functional groups to the surface, forming hydrogen bonds with water molecules. This increased the surface’s affinity for water and the overall surface energy of the composite [40,41]. This observation was also consistent with the FTIR results discussed in the previous section. Additionally, both homopolymer BZ and copolymer BZ-CER composites showed a decrease in contact angle with prolonged acid exposure. The change in the contact angle values of the BZ-CER samples surpassed those of the BZ samples when exposed to acid. Similar to our previous study [12], this discrepancy was likely due to greater intermolecular hydrogen bonding within the copolymer composite, which increased its surface energy and hydrophilicity. Specifically, after 5 h of acid exposure, the static contact angle of the homopolymer BZ decreased by 24.96%, while the addition of CER up to 50 wt.% further decreased the static contact angle of the composite by 28.32%.

Figure 7c describes the effects of copolymerization of BZ with CER and acid exposure duration on the mobility of the droplet over the composite’s surface via characterization of contact angle hysteresis. An upward trend in contact angle hysteresis was observed with increasing acid exposure duration, indicating that prolonged acid contact created surfaces with reduced droplet mobility. After 5 h of acid exposure, the contact angle hysteresis of the homopolymer BZ increased by 19.39%, while a further increase up to 27.80% was observed on the composite sample’s surface with 50 wt.% CER incorporation. This result aligned with our previous study [12], indicating that the effect of acid exposure on the CFRP’s surface wettability was similar to the effect of thermal aging, where liquid mobility on the copolymer composite was lower than that of the homopolymer composite. Moreover, characterizing the composite’s surface wettability proves to be an effective method for assessing advanced composite conditions even at an early stage when mechanical and physical property changes are not yet detectable.

4. Conclusions

This study highlights the use of surface wettability analysis as a novel method to monitor CFRP composites exposed to acid rain, detecting minor degradation not captured by mechanical and physical inspections. Short-term exposure to diluted nitric acid (up to 5 h) was examined for its effects on the physical condition, mechanical properties, and surface wettability of homopolymer BZ and copolymer BZ-CER composites. Minimal weight change and small reductions in flexural strength (up to 15%) and short beam strength (up to 9%) were observed. However, surface wettability analysis revealed more pronounced deterioration, with static contact angle decreasing by up to 28.32% and contact angle hysteresis increasing by up to 27.80%. These findings underscore the importance of surface wettability analysis for early-stage monitoring of composite conditions, ensuring long-term performance and reliability. Further investigation is required to assess prolonged acid exposure using fog to simulate real acid rain conditions.