Degradation evolution and mechanism of sheet molding compound with variable composition exposed to acid solution environment

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The water absorption of SMC composites with various compositions was measured to estimate the influence of water on mechanical properties deterioration. Hardness and flexural properties tests were performed to investigate the degradation evolution. The degradation mechanism was revealed by analyzing the change of molecule configuration and evaluating the thermal stability. A minimum reduction of the flexural strength (3.21%) was observed on the SMC composites with 11.3 wt% hollow glass microspheres (HGMs). The chemical resistance and addition amount of the fillers had significant impacts on the acid resistance of SMC composites. A great flexural property and a minor degradation of flexural strength (6.29%) and modulus (7.86%) was obtained in SMC composites with the mixed resin. The resin characteristics, molecules weight, free volume size and polar groups number, had an important impact on the water absorption and acid resistance of SMC composites. A high flexural property and minor degradation of flexural strength (5.12%) and modulus (7.66%) was observed in SMC composites with 55 wt% glass fibers (GFs). Exposed to 25 ℃, 20 wt% sulfuric acid solution for 28 days, the SMC composites exhibited a minor degradation of HGMs and GFs. In this condition, the deterioration of mechanical properties was dominated by the resin matrix plasticization and decomposition, along with the interface degradation. It can be concluded that the original defects and weak interacted regions in the composites system initiated the degradation of SMC composites, while the microstructure and composition of SMC composites dominated the degradation progress. SMC composites composition acid resistance mechanical properties degradation mechanism Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 Figure 10 Figure 11 Figure 12 1. Introduction Structural components exposed to a range of harsh conditions through their serve life. Irreversible properties degradation of structural component leads to a severe failure risk, even causing the security accidents. It is critical to determinate the performance deterioration and understand the degradation mechanism, beneficial for designing the components applied in aggressive environments. Fiber reinforced polymer (FRP) composites offer a desired balance of performance and cost for replacement of traditional metallic materials. Owing to the plenty of advantages, FRP composites have drawn greater attention in the field of automotive, infrastructure, aircraft, aerospace and chemical industry. Nonetheless, the application of FRP composites is influenced by combination effect of environment due to their long-term exposure [ 1 , 2 ]. FRP composites may work in certain harsh environment, including high temperature and humidity, acid and alkali, cyclic loading, ultraviolet radiation and the combinations of these effects [ 3 , 4 ]. Debonding and degradation of fiber/resin interface, degradation of resin and fiber, microcracks on the material surface, deteriorated the performance of the FRP composites, ultimately reducing the service life of the structures and components [ 5 , 6 ]. In recent decades, a few investigations have been performed to evaluate the properties of FRP composites under water, high humidity, hot and humidity or acid solution [ 7 – 10 ]. A study demonstrated that the corrosion rate of glass fiber/vinyl ester resin in 40 wt% H 2 SO 4 solution increased with immersion time and temperature [ 11 ]. Research indicated that the fluorine- and boron-free glass fibers can significantly enhance the acid resistance of composites [ 12 ]. A study identified that a particular high performance and general glass fibers reinforced composites shows almost the same deterioration of mechanical properties under hot and humid conditions, while the high-performance fiber reinforced composites showed a better corrosion resistance under acid and salt spray conditions [ 13 ]. A study demonstrated that the alkaline solution promotes higher decrease of the flexural properties than the acid solution [ 14 ]. The glass fiber/epoxy composite showed better resistance to the acidic solution than that of CNF-filled glass fiber/epoxy composite [ 15 ]. The static strength of the basalt fiber reinforced polymer (BFRP) shows negligible degradation after aging in the salt solution, whereas the detrimental effect of saltwater becomes apparent under fatigue loading [ 16 ]. Sheet molding compound (SMC) has become a very attractive FRP composites using as pipes and storage vessels, providing its product with lightweight, high strength, dimensional stability, anti-corrosion, and industrial manufacturing [ 17 , 18 ]. Generally, the corrosion resistance of the SMC composites is much higher than that of steel. SMC composites shows obvious properties deterioration subjected to harsh conditions. Research suggested that interfacial properties might affect flexural strength of SMC composites [ 19 ]. The thermomechanical properties of carbon/epoxy SMC were influenced by hygrothermal aging and the moisture intake effects on Tg value [ 9 ]. Hygrothermal aging has an influence on the chemical composition and mechanical properties of SMC composites [ 20 ]. SMC composites are ideal candidates as structural components for Lead/Acid battery containers application. A comprehensive understanding of degradation evolution and mechanism have not been sufficiently investigated due to its complex ingredients. Therefore, the study of environmental effects on the performance of SMC composites and the degradation mechanism is of great value for the production of corrosion-resistant SMC composites and the evaluation of the reliability of SMC composites products. Little literature was reported on acid resistance of the SMC composites. A major objective of this work has been to study the effect of composition on the acid resistance of SMC composites. The water absorption of SMC composites with various compositions was measured to estimate the influence of water on mechanical properties deterioration. Hardness and flexural properties tests were performed to investigate the degradation evolution. The degradation mechanism was revealed by analyzing the change of molecule configuration and evaluating the thermal stability. The present work can serve as a foundation for designing the SMC composites with high performance applied for Lead/Acid battery containers. 2. Experimental details 2.1 Materials The vinyl ester resin crosslinked by styrene (VE) used as the matrix resin was obtained from Shanghai Showa Polymer Co., Ltd., China. The 892 and 902 epoxy resin used as corporations for vinyl ester resin was produced by Shanghai King chemical Co., Ltd. The ECT55NS-4800 short-cut glass fibers (GFs) with a 10 ± 1 mm single filament diameter was provided by Changzhou Hualike New Materials Co., Ltd., China. Hollow glass microspheres (HGMs, iM16K, diameter 12–30 µm, wall thickness 1–2 µm) used as the filler, were produced by Foshan Lanling Chemical Co., Ltd., China. The commercially available calcium carbonate powder (CCP) and silica fume (SF) was supplied by Sichuan Langtian Resource Comprehensive Utilization Co., Ltd., China and Guangzhou Chaoshun Chemical Co., Ltd., China, respectively. Methyl ethyl ketone peroxide (MEK) provided by Shandong Xiaya Chemical Industry Co., Ltd., China, was used as curing agent. Zinc Stearate produced by Tianjin Guangfu Fine Chemical Research Institute, was used as release agent. Magnesium oxide paste used as thickening agent was prepared from magnesium oxide powder (Tianjin Guangfu Fine Chemical Research Institute). Coupling agent KH550 was provided by Nanjing Chuangshi Chemical Additives Co., Ltd., China. 2.2 Device XLB-D350X350X2 hydraulic press was manufactured by Jiangdu Mingzhu Testing Machinery Co., Ltd., China. LX-D Shore hardness tester was provided by Wenzhou Haibao Instrument Co., Ltd., China. The CMT5000 electronic universal testing machine was from Shenzhen SANS Measurement Technology, Co., Ltd., China. JSM-6510LA scanning electron microscope (SEM) was manufactured by Hitachi Ltd., Japan. Nexus-670 Fourier transform infrared spectroscopy was produced by Shimadzu Ltd., Japan. Q600 simultaneous thermal analyzer was produced by TA, America. 2.3 Fabrication of SMC composites The SMC composites were prepared with the following method. First, the resin slurry was prepared by mixing all the compositions except for the fibers. Then, the the precursor slurry was mold into an SMC sheet. The GFs were impregnated into the resin slurry to form a precursor slurry, which was cast on a thin plastic conveyor film. Next, the precursor slurry holding by the conveyor film matured at 25 ℃ for a week. The matured precursor slurry was moved into a mold with a determined size and compression molded by a hydraulic press. 2.4 Exposure procedure SMC composites sheets were completely immersed in 20 wt% sulfuric acid solution at 25 ℃. The exposure condition needed to be maintained during the whole exposure period. The specimens were removed from the sulfuric acid solution after every exposure period 7, 14, and 28 days. The hardness and flexural properties were tested after each exposure period. 2.5 Water absorption measurement The water absorptions were determined for specimens cut out of the SMC composites sheets. The dry specimens were weighted at 25 ℃ and 50%RH. The specimens were immersed in distilled water for 24 h and then the weight is tested at 25 ℃ and 50%RH. At least five duplicates per condition were tested and the average value is reported. The percentage of weight gain is calculated as water absorption ( w i ): $${w}_{i}=\frac{{m}_{t}-{m}_{i}}{{m}_{i}}\times 100\%$$ Here, m t is the weight of the water-absorbed specimen, and m i is its initial weight before water immersion. 2.6 Characterization The Shore hardness was tested by a Shore D-type hardness tester. The thickness of the long strip specimens is not less than 6 mm. At least three duplicates per condition were tested and the average hardness value is reported. Three-point flexural tests were performed on an electronic universal testing machine. The flexural strength and modulus were measured at constant 10 mm/min rate with a support span of 104 mm, which is approximately 17-fold the sample thickness. At least five duplicates per condition were tested and the average value is reported. The cross-section of SMC composites was observed by a SEM with 10 kV accelerating voltage in vacuum mode and a magnification of 400X. The section surfaces were sputtered with a thin gold layer to minimize charging. The thermal properties of SMC composites were evaluated by thermogravimetric analysis (TGA). TGA was conducted in the nitrogen at a heating rate of 15 ℃/min from room temperature to 800 ℃. FTIR curves of the specimens were recorded in the range between 4000 and 500 cm − 1 in transmittance mode. 3. Results and discussion 3.1 The effect of fillers composition on acid solution resistance of SMC composites The water absorption is closely associated with degradation of polymer composites in water or high moisture environments [ 21 ]. The penetrated water molecules can damage the multiple level microstructures of the polymer composites, such as space between resin polymer chains and interface of fiber and matrix [ 22 ]. If the material shows high water absorption, the degradation effect is more pronounced. [ 15 ] The water absorption of SMC composites with various compositions is determined. Fillers shows a prominent influence on the performance of SMC composites, possessing a large proportion in SMC composites [ 23 ]. More interface and interphase are created when large amounts of fillers added into the SMC composites, which play an important role in its degradation behavior. Hollow glass microsphere is employed as the major fillers for SMC composites in this work, due to its weight reduction, hydrophobicity, and corrosion resistance [ 24 ]. Other fillers that adopted frequently in actual production, CCP and SF, were introduced to investigate the correlation between the filler composition and water absorption effect. Figure 1 (a) exhibited that the water absorption of SMC composites increased with the increasing amount of HGMs. It is believed that the water absorption behavior of the composites is related with its micro-structure [ 25 , 26 ]. The water molecules trends to active along the weak regions, such as defects, voids and interfaces between resin matrix with fillers or fibers [ 27 , 28 ]. More interfaces and interphase cause a higher water absorption for SMC composites with multiple ingredients. The increased HGMs amount resulted in an increased number of organic/inorganic interfaces and interphases, expanding the active areas for water molecules. In addition, more HGMs trends to break during the SMC processing, creating more defects and voids in the composites system and expanding the active areas of water molecules. As a result, the water absorption is higher for the SMC composites with more HGMs. As shown in Fig. 1 (b), when the total amount of fillers is fixed (20 wt%), the incorporation of CCP reduced the water absorption of SMC composites. It can be explained that the more interfaces and interphases between filler and resin acted as the paths for water molecules to move in composites system, leading to a prominent tendency to the increased water absorption. The CPP shows much higher density than HGMs. The volume fraction of CPP is much lower than that of HGMs when the mass percentage is same. As a result, the number of filler and resin interfaces in SMC composites HGMs and CPP both added is much less than the SMC composites only HGMs added. With the amount of CCP increasing, the water absorption of SMC composites increased. It was possible attributed to the hydrophilicity of CCP. CaCO 3 is an ionic bonded compound with the polar group, inherently hydrophilic. Especially, appearing in powder, CCP exhibited greater hydrophilicity. The water absorption of SMC composites with 20 wt% filler total amount and 1:1 mass ratio of HGMs to CCP is higher than that of SMC composites with 10 wt% HGMs, meaning that the extra 10 wt% CCP promoted the water penetration for SMC composites. The resin amount of SMC composites with 20 wt% filler total amount and 1:1 mass ratio of HGMs to CCP is lower than that of SMC composites with 10 wt% HGMs. Besides, more fillers addition into SMC composites suggesting more interface and interphase in composites system. Figure 1 (c) shows that with the same total filler content, the water absorption of SMC composites incorporated SF is lower comparing with the SMC composites only HGMs added. With the increased mass percentage of SF in total fillers content, the water absorption of SMC composites increased. The influence of SF on water absorption of SMC composites is similar with the CCP. It can be explained by the number of the interface and interphase formed during resin and fillers blending. Less interfaces and interphases created in SF incorporated SMC composites system, so the active region is smaller for water molecules. The preserving preference of water molecules was reduced, the water absorption decreased as a result. Hardness properties of composites expresses the capacity against to local deformation, reflecting the plasticity and flexibility of the composites. For a defect-free ideal crystal model, the hardness is proportional to elastic stiffness [ 29 ]. Therefore, the status of the defects in the composites, including number and distribution, has a major influence on the hardness properties [ 30 ]. Examining of hardness revealed that sulfuric acid solution has detrimental effects on the mechanical performance of SMC composites. In Fig. 2 , it can be observed that the hardness value is decreased with the exposure time for the SMC composites with varied filler composition, when exposed to the sulfuric acid solution. The properties degradation of SMC composites exposed to acid solution can be attributed to various damage mechanisms, such as matrix swelling, chemical degradation and interface debonding. The resin matrix may swell by water molecules and hydrolyze in the presence of acid, leading to polymer matrix plasticization and degradation. Fillers, fibers and other additive may be attacked by the water molecules and acid ions, voids, cracks and defects appearing. The water molecules and acid ions attack the interphase, region between the bulk filler and bulk matrix, leading to debonding between polymer with fibers or fillers. The above chemical and physical degradation caused a substantial change on the microstructure associated with the hardness properties of SMC composites. The hardness degradation of SMC composites incorporated CCP and SF is more aggressive than that of SMC composites only HGMs added. In addition, more HGMs may suppress the deterioration of hardness properties for SMC composites in acid condition. SMC composites with 24.2 wt% HGMs shows a minor decline on hardness properties (2.84%). It can be contributed to the greater acid resistance of HGMs superior to CCP and SF. A dramatic degradation on hardness properties (9.17%) is observed in the SMC composites with 1:3 mass ratio of HGMs to CCP. The CCP reacted with hydrogen ions in acid solution, forming voids and defects inner composites. It caused the SMC composites failure to fight with deformation, resulting in hardness degradation. It was distinct that a clear deterioration on the flexural strength and modulus of SMC composites with varied filler composition exposed to acid solution. As shown in Fig. 3 (a), the SMC composites with 24.2 wt% HGMs represented the greatest degradation on the flexural strength (6.43%) and modulus (13.41%) after exposure to acid solution for 28 days. A minimum reduction of the flexural strength (3.21%) and modulus (7.85%) was observed on the SMC composites with 11.3 wt% HGMs. This can be attributed to the detrimental effect of excessive HGMs added in SMC composites. When too much HGMs were added into the SMC composites, a trend of crack and break is more prominent for HGMs. The broken HGMs created more voids, cracks or other defects in the composites system. More interfaces between the fillers and resin matrix were formed, which had a poor interaction with each other. When exposed to the acid and water condition, water molecules, hydrogen ions and sulphate ions penetrated the SMC composite through voids, cracks and other defects, further accelerating the matrix swelling, chemical degradation, and micro-crack spread [ 31 ]. This affected significantly the SMC composites resistance to loading stresses, leading to a pronounced damage for flexural properties of SMC composites. Therefore, defects effect is regarded as the main cause of large reductions in mechanical properties. From Fig. 3 (b), for the SMC composites with a 1:2 mass ratio of HGMs to CCP, the flexural strength and modulus was reduced by 7.68% and 7.87% respectively, which was the minimum reduction for CCP incorporated specimens. From Fig. 3 (c), for the SMC composites with a 1:1 mass ratio of HGMs to SF, the flexural strength and modulus was reduced by 4.41% and 4.07% respectively, which was the minimum reduction for SF incorporated specimens. The CCP incorporated SMC composites exhibited a more pronounced flexural properties degradation than SF incorporated SMC composites, contributing to the poor resistance to acid of CCP. The reaction between the CCP and acid allowed more voids and cavities forming, providing more travel path for water molecules and acid ions. The weaken interacted region expanded, causing polymer hydrolysis and interfaces degradation developed. When HGMs were replaced by SF with half amount, the break of HGMs was suppressed. The weaken interacted regions and defects number decreased, leading to a minor degradation of flexural properties. It can be concluded the chemical resistance and addition amount of the fillers had significant impacts on the acid resistance of SMC composites. Appropriate addition amount and excellent chemical resistance of fillers can enhance the acid solution resistance of SMC composites. 3.2 The effect of resin composition on acid solution resistance of SMC composites The features of resin influence the water absorption behavior and acid resistance of SMC composites. VE was chosen as the resin matrix of SMC composites. Two types of epoxies were employed to incorporate with epoxy to investigate the correlation of resin composition with acid resistance of SMC composites. Figure 4 shows the water absorption of SMC composites with varied resin composition. 892 epoxy incorporated SMC composites exhibited a higher weight gain than 902 epoxy incorporated ones after immersion in water. The viscosity of 902 epoxy is 1300–1500 Pa∙s, while viscosity of 892 epoxy is low as 0.30–0.50 Pa∙s. The GFs can impregnate easily for both type of epoxy incorporated resin slurry. There were little defects at fiber and resin interfaces for both type of epoxy added SMC composites. In this case, the defects were not the major reason to induce the water penetration. The lower water absorption for SMC composites with 902 epoxy is attributed to its small free volume between the polymer chains. The water absorption is related with the size and structure of intermolecular-space holes and amount of polar groups [ 32 , 33 ]. The water molecules trends to filler into the free volume holes at initial diffusion stage [ 34 ]. 902 epoxy shows higher viscosity, meaning a higher molecular weight than 892 epoxy. After curing, the 892 epoxy resin cast showed larger free volume holes than 902 epoxy, which caused the water molecules trended to penetrate. Due to the polar groups, the water absorption of epoxy is higher than that of VE. Therefore, with the epoxy amount increasing, the water absorption of SMC composites increased. It can be concluded that the free volume characteristics of resin matrix, including sizes and distributions, prominently influence the water absorption of SMC composites with varied resin composition rather than the number of defects. Figure 5 shows the hardness reduction of 892 epoxy incorporated SMC composites is greater than that of 902 epoxy incorporated specimens. In Fig. 5 (a), the hardness of SMC composites with 1:2 mass ratio of 902 to VE decreased by 3.10%, showing a slightly higher reduction comparing with the other two 902 epoxy incorporated specimens. In Fig. 5 (b), the hardness of SMC composites with 1:2 mass ratio of 892 to VE decreased by 4.06%, slight greater reduction in contrast to the other two 892 epoxy incorporated specimens. The degradation of hardness properties is mainly attributed to the plasticization result from the swelling and hydrolysis of polymer matrix by water molecules. Exposed to sulfuric acid solution, water molecules penetrate the polymer matrix, resulting in a free volume increasing and polymer plasticization. The plasticization of 892 epoxy is more aggressive than 902 epoxy, due to its low molecular weight. In addition, VE hydrolyze in the presence of acid, promoting the hardness degradation of SMC composites. The relationship between resin characteristics and hardness properties degradation can be illustrated by the combination effect of matrix plasticization and chemical degradation. As shown in Fig. 6 , an obvious deterioration can be observed in the flexural properties of SMC composites with varied resin composition exposed to sulfuric acid solution. The 902 epoxy incorporated SMC composites exhibited more excellent flexural properties and greater resistance to acid solution than that of 902 epoxy incorporated specimen. It demonstrated that a relatively high molecular weight and proper viscosity provided 902 epoxy prominent mechanical properties and strong bonding with fibers, offering the SMC composites great flexural properties and resistance to acid solution. In Fig. 6 (a), SMC composites with 1:2 mass ratio of 902 to VE without exposure to acid solution showed a higher flexural strength by 151.0 MPa and a higher flexural modulus by 7.89 GPa. This specimen showed a minor degradation of flexural strength (6.29%) and modulus (7.86%). In Fig. 6 (b), SMC composites with 1:3 mass ratio of 892 to VE without exposure to acid solution showed a higher flexural by 130.8 MPa and a higher flexural modulus by 7.46 GPa. This specimen showed a minor degradation of flexural strength (6.50%) and modulus (8.44%). Unsuitable mass ratios of 902 to VE may lead to weak interaction between resin and fibers or different resin, exhibiting relatively low flexural properties. More defects occurred at the interface, providing the active paths for water molecules and acid ions, resulting in matrix plasticization and fiber-matrix interface decohesion [ 15 , 35 ]. The defects effect and weaken interacted regions play an important role in the SMC composites mechanical properties degradation. Mechanical degradation is the result of polymer plasticization and chemical degradation inducing stresses large enough to pull the matrix away from the fiber [ 36 ]. The structure of polymer chains, such as molecules weight and polar groups amount, have an important impact on the resistance of SMC composites. The polymer with low molecules weight and a lot of polar groups are prone to absorb water and react with sulfuric acid solution, resulting in a more pronounced damage on resin matrix, which can effectively transfer stresses in the composites under external loading. The flexural properties of composites were mainly dominated by the matrix and interface. Therefore, reduction extent of SMC composites flexural properties was related with defects effect and resin characteristics. 3.3 The effect of fiber composition on acid solution resistance of SMC composites Figure 7 showed that water absorption of SMC composites increased with the GFs content. When the GFs content reached 60 wt%, water absorption tends to remain constant. This can be attributed to the greater hydrophilicity of GFs than resin matrix and HGMs, resulting from the hydrogen bonding on the GFs surface. It demonstrated that the absorbed water by GFs occupied the interface of resin and fibers, diminishing resin-fiber interaction and facilitating separation of fibers and resin under external force. Moreover, the number of interfaces increased with GFs content, leading to an increase of water absorption. The hardness of SMC composites with varied fiber composition was reduced exposed to sulfuric acid solution, as shown in Fig. 8 (a). The hardness properties reflect the plasticity and flexibility of composites, considering as a surface contribution. Hardness of composites is generally associated with the fiber amount [ 37 ]. It is evident that the SMC composites with 40 wt% GFs shows a slightly higher hardness than the SMC composites with other fiber contents. When the fiber content is relatively low, below 40 wt%, the hardness increased with the fibers amount. As fiber content continued to increase, the hardness did not change significantly. Figure 8 (b) showed that the hardness reduction rate did not change obviously with the fibers amount, suggesting that the hardness reduction of SMC composites exposed to acid solution for 28 days may be independent with the fiber content in case of the high fiber content. It can be explained by that the obvious change was not appear on fibers for 28 days exposure to sulfuric acid solution. It suggested that the hardness reduction was associated with the resin matrix swelling and interface degradation, rather than the fiber crack for short-term exposure. As shown in Fig. 9 (a), SMC composites with 55 wt% GFs exhibited the highest flexural strength and modulus. The flexural properties were enhanced by GFs reinforcement. The flexural strength and modulus were improved with the GFs content increasing. However, when the GFs content reached to 60 wt%, poor impregnation of fibers occurred due to the decreased resin content, leading to a reduction of flexural properties. Lowest reduction rate of flexural properties was obtained on SMC composites with 55 wt% GFs exposed to sulfuric acid solution, represented in Fig. 9 (b). It can be illustrated by the degradation mechanism of SMC composites short-term exposure to acid solution. In the present work, the 28 days exposure period was short and the 20 wt% sulfuric acid concentration is low. The damage and fracture of fibers did not take place in the relatively short exposure period. In this case, the deterioration of flexural properties is dominated by the resin matrix plasticization and decomposition, along with the interface degradation, which agreed with the conclusions drawn in hardness tests. The fractured section of SMC composites with 55 wt% GFs, 11.3 wt% HGMs, and 1:2 mass ratio of 902 to VE was examined after different exposure period. The failure modes of fracture were analyzed to describe the degradation mechanisms of SMC composites in acid solution for short-term. In Fig. 10 (a), the fractured section of the unexposed specimen displayed a strong interface between resin matrix with HGMs and fibers. In this case, the failure of fracture supposed to be a localized and brittle failure mode, meaning the fiber-matrix and filler-matrix interface were robust. The fracture occurred subjected to quite high stress. In Fig. 10 (b), parts of resin remained on the surface of fiber after failure, indicating that the attachment of resin matrix on fibers maintained a moderate level for specimen exposed to acid solution for 7 days. No obvious damage of resin, HGMs or fibers was observed in the specimen exposed for 7 days. Figure 10 (c) showed a smooth fracture surface appeared at the interface between resin and HGMs, indicating a prominent debonding between resin and HGMs. Traces of resin residues were noticed on the fibers, suggesting that the interaction between matrix and fiber became considerably weak due to the interface degradation. Exposed to acid solution for 28 days, the fiber-resin debonding developed aggressively, by leaving lots of smooth surfaces when fibers separated with resin shown in Fig. 10 (d). In addition, the matrix crack appeared, demonstrating that resin matrix had been damaged severely by physical swelling and chemical decomposition. Cracks in the matrix indicated the degradation of the polymer due to acid and water. The interface degradation and matrix damage of SMC composites exposed to sulfuric acid solution can be confirmed by analyzing the fracture manner. However, no obvious fiber damage can be observed in fractured section, which was agreed with the results of flexural properties reduction. 3.4 The degradation mechanism analysis The degradation mechanism of SMC composites in acid solution was clarified by analyzing the change of molecule configuration and evaluating the thermal stability. According to Fig. 11 (a), the exposed specimens showed a decreased intensity of peak at 1725 cm − 1 belonging to carbonyl groups. The intensity of peaks at 1091 cm − 1 and 1251 cm − 1 , belonging to aliphatic ether groups and aromatic ether groups respectively, decreased slightly for exposed specimens, suggesting that the esters groups hydrolyze in the absence of water and acid. It can be concluded that chemical decomposition occurred in the resin matrix of SMC exposed to sulfuric acid solution. According to Fig. 11 (b), the thermal stability decreased for exposed specimens, indicating a macromolecule chains scission, agreed with the FTIR results. Exposed to 25 ℃, 20 wt% sulfuric acid solution for 28 days, the SMC composites exhibited a minor degradation of HGMs and GFs. Even though, a minor damage of GFs and HGMs may occur in acid solution environment, the degradation of polymer matrix and interphase was the main cause to induce the deterioration of mechanical properties. The matrix swelling, chemical degradation, and interfaces debonding were responsible for the deterioration of mechanical properties for SMC composites. It can be concluded that the SMC composites with various constituent displayed multiple degradation mechanisms under the combination effect of water and acid. The evolution of degradation was influenced by exposure period. It can be descripted schematically in Fig. 12 . In the first stage, water molecules together with acid ions penetrated the resin matrix and interphase of composites through diffusion and capillary action [ 38 ]. A three-dimensional region between the bulk fiber or filler and bulk matrix is referred as interphase [ 39 ]. Some voids, cracks and other defects may exist in the interphase, forming a weaken interacted regions in composites system. Water and other small molecules prefer to travel along the weaken interacted regions [ 40 ]. The penetrated water molecules and acid ions induced swelling of the matrix and propagation of microcracks on interphase [ 41 ]. accelerating the degradation of matrix and interphase. Absorbed water molecules filled into voids and cracks in the matrix and weaken interacted regions acting as a plasticiser, causing the composites more flexible, which is the major reason to reduce the hardness of SMC composites. In the second stage, the penetrated water and acid further attacked the GFs-matrix and HGMs-matrix interphases, finally fiber-matrix and filler-matrix debonding [ 42 ]. The penetrated water molecules reacted with ester groups on resin chains in the presence of acid, the hydrolysis of matrix occurring. The polymer decomposition reduced the interaction between the resin chains and bonding of resin and fillers or fibers, promoting the interphase delamination and water molecules and acid ions diffusion additionally. In the third stage, the HGMs and GFs were attacked by water molecules and acid ions, cracks appearing on the GFs surface and break of HGMs taking place. It can be concluded that the original defects and weak interacted regions in the composites system initiated the degradation of SMC composites, while the microstructure and composition of SMC composites dominated the degradation progress. 4. Conclusions In the present work, the SMC composites with variable filler, resin and fiber composition were prepared to investigate the effects of composition on acid resistance of SMC composites. The water absorption of SMC composites with various compositions was measured to estimate the influence of water on mechanical properties deterioration. The degradation of mechanical properties was evaluated by hardness and flexural strength and modulus results. The acid solution degradation mechanism was revealed by analyzing the change of molecule configuration and evaluating the thermal stability of the unexposed and exposed specimens. A minimum reduction of the flexural strength (3.21%) was observed on the SMC composites with 11.3 wt% HGMs. The chemical resistance and addition amount of the fillers had significant impacts on the acid resistance of SMC composites. A great flexural property and a minor degradation of flexural strength (6.29%) and modulus (7.86%) was obtained in SMC composites with 1:2 mass ratio of 902 to VE. The resin characteristics, molecules weight, free volume size and polar groups number, have an important impact on the water absorption and acid resistance of SMC composites. A high flexural property and minor degradation of flexural strength (5.12%) and modulus (7.66%) was observed in SMC composites with 55 wt% GFs. Exposed to 25 ℃, 20 wt% sulfuric acid solution for 28 days, the SMC composites exhibited a minor degradation of HGMs and GFs. In this condition, the deterioration of mechanical properties was dominated by the resin matrix plasticization and decomposition, along with the interface degradation. It can be concluded that the original defects and weak interacted regions in the composites system initiated the degradation of SMC composites, while the microstructure and composition of SMC composites dominated the degradation progress. This paper provided a better understanding of the degradation evolution and mechanisms of SMC composites to meet the requirements of strong acid environments application. Declarations Declaration of competing interest The authors declare that they have no competing financial interests or personal relationships that could have influenced the work reported in this paper. Author contribution Jian Li prepared the materials and conducted most of the measurements and data analysis. Chao Fu, Ruifeng Ming, Minxian Shi, Wenhao Dong, Jiang Guo, and Xingkui Guo contributed to the data analysis. Duo Pan and Xiufang Zhu conceived the idea, wrote the paper, and coordinated the overall project. Dalal A. Alshammari and Saad Melhi revised the paper. Mufang Li provided supervision and resources. Hamdy Khamees Thabet reviewed and revised the manuscript. All authors reviewed the manuscript. Funding We acknowledge the financial support from the Outstanding Youth Project of Natural Science Foundation of Hubei Province of China (2021CFA068), the Outstanding Young and Middle-aged Innovation Team of Hubei Province of China (T2021007). The authors extend their appreciation to the Deanship of Scientific Research at Northern Border University, Arar, KSA for funding this research “work through the project number “NBU-FPEJ-2024-ID-XXX. Data availability The authors confirm that the data supporting the findings of this study are available within the article. Raw data that support the findings of this study are available from the corresponding author, upon reasonable request. 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HGMs, and 1:2 mass ratio of 902 to VE exposed to sulfuric acid solution for each exposure period.\u003c/p\u003e","description":"","filename":"image10.png","url":"https://assets-eu.researchsquare.com/files/rs-4112494/v1/25168e53b3ad1afb95afb763.png"},{"id":54032509,"identity":"bc761381-4dbf-4d9a-b596-c2fb31f74cb2","added_by":"auto","created_at":"2024-04-03 16:20:35","extension":"jpeg","order_by":10,"title":"Figure 10","display":"","copyAsset":false,"role":"figure","size":395616,"visible":true,"origin":"","legend":"\u003cp\u003eSEM micrograph of fracture section of SMC composites with 55 wt% GFs, 11.3 wt% HGMs, and 1:2 mass ratio of 902 to VE exposed to sulfuric acid solution for 0 day (\u003cstrong\u003ea\u003c/strong\u003e), 7 days (\u003cstrong\u003eb\u003c/strong\u003e), 14 days (\u003cstrong\u003ec\u003c/strong\u003e), 28 days (\u003cstrong\u003ed\u003c/strong\u003e)\u003c/p\u003e","description":"","filename":"image11.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-4112494/v1/817d7f32532d6a0d207a00b3.jpeg"},{"id":54032508,"identity":"8f9e2dd1-eb4d-49cf-b81c-0aca6b845e10","added_by":"auto","created_at":"2024-04-03 16:20:35","extension":"jpeg","order_by":11,"title":"Figure 11","display":"","copyAsset":false,"role":"figure","size":116699,"visible":true,"origin":"","legend":"\u003cp\u003eFTIR (\u003cstrong\u003ea\u003c/strong\u003e) and TGA (\u003cstrong\u003eb\u003c/strong\u003e) curves of SMC composites with 55 wt% GFs, 11.3 wt% HGMs, and 1:2 mass ratio of 902 to VE exposed to sulfuric acid solution for each exposure period\u003c/p\u003e","description":"","filename":"image12.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-4112494/v1/724f7dfdd25cc74049f8b136.jpeg"},{"id":54033007,"identity":"c0de5892-cbdb-44e5-83de-52e59a288199","added_by":"auto","created_at":"2024-04-03 16:28:35","extension":"jpeg","order_by":12,"title":"Figure 12","display":"","copyAsset":false,"role":"figure","size":283715,"visible":true,"origin":"","legend":"\u003cp\u003eSchematical degradation mechanisms of SMC composites exposed to acid solution.\u003c/p\u003e","description":"","filename":"image13.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-4112494/v1/9d9aa4fa97bed4c98f0c113d.jpeg"},{"id":59532705,"identity":"c8027815-a76e-402a-8cbf-c009d4b42af5","added_by":"auto","created_at":"2024-07-03 00:49:46","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":4407844,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-4112494/v1/69c7f243-ec46-483d-8e4a-a32b460b4dec.pdf"},{"id":54033006,"identity":"7966f0c4-efa8-440f-9606-ceec767418ce","added_by":"auto","created_at":"2024-04-03 16:28:35","extension":"jpeg","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":325513,"visible":true,"origin":"","legend":"","description":"","filename":"GraphicalAbstract.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-4112494/v1/843470252c7b5e7c1b24feb4.jpeg"}],"financialInterests":"No competing interests reported.","formattedTitle":"Degradation evolution and mechanism of sheet molding compound with variable composition exposed to acid solution environment","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eStructural components exposed to a range of harsh conditions through their serve life. Irreversible properties degradation of structural component leads to a severe failure risk, even causing the security accidents. It is critical to determinate the performance deterioration and understand the degradation mechanism, beneficial for designing the components applied in aggressive environments. Fiber reinforced polymer (FRP) composites offer a desired balance of performance and cost for replacement of traditional metallic materials. Owing to the plenty of advantages, FRP composites have drawn greater attention in the field of automotive, infrastructure, aircraft, aerospace and chemical industry. Nonetheless, the application of FRP composites is influenced by combination effect of environment due to their long-term exposure [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e, \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e]. FRP composites may work in certain harsh environment, including high temperature and humidity, acid and alkali, cyclic loading, ultraviolet radiation and the combinations of these effects [\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e, \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e]. Debonding and degradation of fiber/resin interface, degradation of resin and fiber, microcracks on the material surface, deteriorated the performance of the FRP composites, ultimately reducing the service life of the structures and components [\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e, \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eIn recent decades, a few investigations have been performed to evaluate the properties of FRP composites under water, high humidity, hot and humidity or acid solution [\u003cspan additionalcitationids=\"CR8 CR9\" citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e]. A study demonstrated that the corrosion rate of glass fiber/vinyl ester resin in 40 wt% H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e solution increased with immersion time and temperature [\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e]. Research indicated that the fluorine- and boron-free glass fibers can significantly enhance the acid resistance of composites [\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e]. A study identified that a particular high performance and general glass fibers reinforced composites shows almost the same deterioration of mechanical properties under hot and humid conditions, while the high-performance fiber reinforced composites showed a better corrosion resistance under acid and salt spray conditions [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e]. A study demonstrated that the alkaline solution promotes higher decrease of the flexural properties than the acid solution [\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e]. The glass fiber/epoxy composite showed better resistance to the acidic solution than that of CNF-filled glass fiber/epoxy composite [\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e]. The static strength of the basalt fiber reinforced polymer (BFRP) shows negligible degradation after aging in the salt solution, whereas the detrimental effect of saltwater becomes apparent under fatigue loading [\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eSheet molding compound (SMC) has become a very attractive FRP composites using as pipes and storage vessels, providing its product with lightweight, high strength, dimensional stability, anti-corrosion, and industrial manufacturing [\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e, \u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e]. Generally, the corrosion resistance of the SMC composites is much higher than that of steel. SMC composites shows obvious properties deterioration subjected to harsh conditions. Research suggested that interfacial properties might affect flexural strength of SMC composites [\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e]. The thermomechanical properties of carbon/epoxy SMC were influenced by hygrothermal aging and the moisture intake effects on Tg value [\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e]. Hygrothermal aging has an influence on the chemical composition and mechanical properties of SMC composites [\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eSMC composites are ideal candidates as structural components for Lead/Acid battery containers application. A comprehensive understanding of degradation evolution and mechanism have not been sufficiently investigated due to its complex ingredients. Therefore, the study of environmental effects on the performance of SMC composites and the degradation mechanism is of great value for the production of corrosion-resistant SMC composites and the evaluation of the reliability of SMC composites products. Little literature was reported on acid resistance of the SMC composites.\u003c/p\u003e \u003cp\u003eA major objective of this work has been to study the effect of composition on the acid resistance of SMC composites. The water absorption of SMC composites with various compositions was measured to estimate the influence of water on mechanical properties deterioration. Hardness and flexural properties tests were performed to investigate the degradation evolution. The degradation mechanism was revealed by analyzing the change of molecule configuration and evaluating the thermal stability. The present work can serve as a foundation for designing the SMC composites with high performance applied for Lead/Acid battery containers.\u003c/p\u003e"},{"header":"2. Experimental details","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003e2.1 Materials\u003c/h2\u003e \u003cp\u003eThe vinyl ester resin crosslinked by styrene (VE) used as the matrix resin was obtained from Shanghai Showa Polymer Co., Ltd., China. The 892 and 902 epoxy resin used as corporations for vinyl ester resin was produced by Shanghai King chemical Co., Ltd. The ECT55NS-4800 short-cut glass fibers (GFs) with a 10\u0026thinsp;\u0026plusmn;\u0026thinsp;1 mm single filament diameter was provided by Changzhou Hualike New Materials Co., Ltd., China. Hollow glass microspheres (HGMs, iM16K, diameter 12\u0026ndash;30 \u0026micro;m, wall thickness 1\u0026ndash;2 \u0026micro;m) used as the filler, were produced by Foshan Lanling Chemical Co., Ltd., China. The commercially available calcium carbonate powder (CCP) and silica fume (SF) was supplied by Sichuan Langtian Resource Comprehensive Utilization Co., Ltd., China and Guangzhou Chaoshun Chemical Co., Ltd., China, respectively. Methyl ethyl ketone peroxide (MEK) provided by Shandong Xiaya Chemical Industry Co., Ltd., China, was used as curing agent. Zinc Stearate produced by Tianjin Guangfu Fine Chemical Research Institute, was used as release agent. Magnesium oxide paste used as thickening agent was prepared from magnesium oxide powder (Tianjin Guangfu Fine Chemical Research Institute). Coupling agent KH550 was provided by Nanjing Chuangshi Chemical Additives Co., Ltd., China.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003e2.2 Device\u003c/h2\u003e \u003cp\u003eXLB-D350X350X2 hydraulic press was manufactured by Jiangdu Mingzhu Testing Machinery Co., Ltd., China. LX-D Shore hardness tester was provided by Wenzhou Haibao Instrument Co., Ltd., China. The CMT5000 electronic universal testing machine was from Shenzhen SANS Measurement Technology, Co., Ltd., China. JSM-6510LA scanning electron microscope (SEM) was manufactured by Hitachi Ltd., Japan. Nexus-670 Fourier transform infrared spectroscopy was produced by Shimadzu Ltd., Japan. Q600 simultaneous thermal analyzer was produced by TA, America.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec5\" class=\"Section2\"\u003e \u003ch2\u003e2.3 Fabrication of SMC composites\u003c/h2\u003e \u003cp\u003eThe SMC composites were prepared with the following method. First, the resin slurry was prepared by mixing all the compositions except for the fibers. Then, the the precursor slurry was mold into an SMC sheet. The GFs were impregnated into the resin slurry to form a precursor slurry, which was cast on a thin plastic conveyor film. Next, the precursor slurry holding by the conveyor film matured at 25 ℃ for a week. The matured precursor slurry was moved into a mold with a determined size and compression molded by a hydraulic press.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec6\" class=\"Section2\"\u003e \u003ch2\u003e2.4 Exposure procedure\u003c/h2\u003e \u003cp\u003eSMC composites sheets were completely immersed in 20 wt% sulfuric acid solution at 25 ℃. The exposure condition needed to be maintained during the whole exposure period. The specimens were removed from the sulfuric acid solution after every exposure period 7, 14, and 28 days. The hardness and flexural properties were tested after each exposure period.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec7\" class=\"Section2\"\u003e \u003ch2\u003e2.5 Water absorption measurement\u003c/h2\u003e \u003cp\u003eThe water absorptions were determined for specimens cut out of the SMC composites sheets. The dry specimens were weighted at 25 ℃ and 50%RH. The specimens were immersed in distilled water for 24 h and then the weight is tested at 25 ℃ and 50%RH. At least five duplicates per condition were tested and the average value is reported. The percentage of weight gain is calculated as water absorption (\u003cem\u003ew\u003c/em\u003e\u003csub\u003e\u003cem\u003ei\u003c/em\u003e\u003c/sub\u003e):\u003cdiv id=\"Equa\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equa\" name=\"EquationSource\"\u003e\n$${w}_{i}=\\frac{{m}_{t}-{m}_{i}}{{m}_{i}}\\times 100\\%$$\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003eHere, \u003cem\u003em\u003c/em\u003e\u003csub\u003e\u003cem\u003et\u003c/em\u003e\u003c/sub\u003e is the weight of the water-absorbed specimen, and \u003cem\u003em\u003c/em\u003e\u003csub\u003e\u003cem\u003ei\u003c/em\u003e\u003c/sub\u003e is its initial weight before water immersion.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec8\" class=\"Section2\"\u003e \u003ch2\u003e2.6 Characterization\u003c/h2\u003e \u003cp\u003eThe Shore hardness was tested by a Shore D-type hardness tester. The thickness of the long strip specimens is not less than 6 mm. At least three duplicates per condition were tested and the average hardness value is reported. Three-point flexural tests were performed on an electronic universal testing machine. The flexural strength and modulus were measured at constant 10 mm/min rate with a support span of 104 mm, which is approximately 17-fold the sample thickness. At least five duplicates per condition were tested and the average value is reported. The cross-section of SMC composites was observed by a SEM with 10 kV accelerating voltage in vacuum mode and a magnification of 400X. The section surfaces were sputtered with a thin gold layer to minimize charging. The thermal properties of SMC composites were evaluated by thermogravimetric analysis (TGA). TGA was conducted in the nitrogen at a heating rate of 15 ℃/min from room temperature to 800 ℃. FTIR curves of the specimens were recorded in the range between 4000 and 500 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e in transmittance mode.\u003c/p\u003e \u003c/div\u003e"},{"header":"3. Results and discussion","content":"\u003cdiv id=\"Sec10\" class=\"Section2\"\u003e\n\u003ch2\u003e3.1 The effect of fillers composition on acid solution resistance of SMC composites\u003c/h2\u003e\n\u003cp\u003eThe water absorption is closely associated with degradation of polymer composites in water or high moisture environments [\u003cspan class=\"CitationRef\"\u003e21\u003c/span\u003e]. The penetrated water molecules can damage the multiple level microstructures of the polymer composites, such as space between resin polymer chains and interface of fiber and matrix [\u003cspan class=\"CitationRef\"\u003e22\u003c/span\u003e]. If the material shows high water absorption, the degradation effect is more pronounced. [\u003cspan class=\"CitationRef\"\u003e15\u003c/span\u003e] The water absorption of SMC composites with various compositions is determined.\u003c/p\u003e\n\u003cp\u003eFillers shows a prominent influence on the performance of SMC composites, possessing a large proportion in SMC composites [\u003cspan class=\"CitationRef\"\u003e23\u003c/span\u003e]. More interface and interphase are created when large amounts of fillers added into the SMC composites, which play an important role in its degradation behavior. Hollow glass microsphere is employed as the major fillers for SMC composites in this work, due to its weight reduction, hydrophobicity, and corrosion resistance [\u003cspan class=\"CitationRef\"\u003e24\u003c/span\u003e]. Other fillers that adopted frequently in actual production, CCP and SF, were introduced to investigate the correlation between the filler composition and water absorption effect.\u003c/p\u003e\n\u003cp\u003eFigure\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003e(a) exhibited that the water absorption of SMC composites increased with the increasing amount of HGMs. It is believed that the water absorption behavior of the composites is related with its micro-structure [\u003cspan class=\"CitationRef\"\u003e25\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e26\u003c/span\u003e]. The water molecules trends to active along the weak regions, such as defects, voids and interfaces between resin matrix with fillers or fibers [\u003cspan class=\"CitationRef\"\u003e27\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e28\u003c/span\u003e]. More interfaces and interphase cause a higher water absorption for SMC composites with multiple ingredients. The increased HGMs amount resulted in an increased number of organic/inorganic interfaces and interphases, expanding the active areas for water molecules. In addition, more HGMs trends to break during the SMC processing, creating more defects and voids in the composites system and expanding the active areas of water molecules. As a result, the water absorption is higher for the SMC composites with more HGMs.\u003c/p\u003e\n\u003cp\u003eAs shown in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003e(b), when the total amount of fillers is fixed (20 wt%), the incorporation of CCP reduced the water absorption of SMC composites. It can be explained that the more interfaces and interphases between filler and resin acted as the paths for water molecules to move in composites system, leading to a prominent tendency to the increased water absorption. The CPP shows much higher density than HGMs. The volume fraction of CPP is much lower than that of HGMs when the mass percentage is same. As a result, the number of filler and resin interfaces in SMC composites HGMs and CPP both added is much less than the SMC composites only HGMs added. With the amount of CCP increasing, the water absorption of SMC composites increased. It was possible attributed to the hydrophilicity of CCP. CaCO\u003csub\u003e3\u003c/sub\u003e is an ionic bonded compound with the polar group, inherently hydrophilic. Especially, appearing in powder, CCP exhibited greater hydrophilicity. The water absorption of SMC composites with 20 wt% filler total amount and 1:1 mass ratio of HGMs to CCP is higher than that of SMC composites with 10 wt% HGMs, meaning that the extra 10 wt% CCP promoted the water penetration for SMC composites. The resin amount of SMC composites with 20 wt% filler total amount and 1:1 mass ratio of HGMs to CCP is lower than that of SMC composites with 10 wt% HGMs. Besides, more fillers addition into SMC composites suggesting more interface and interphase in composites system.\u003c/p\u003e\n\u003cp\u003eFigure\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003e(c) shows that with the same total filler content, the water absorption of SMC composites incorporated SF is lower comparing with the SMC composites only HGMs added. With the increased mass percentage of SF in total fillers content, the water absorption of SMC composites increased. The influence of SF on water absorption of SMC composites is similar with the CCP. It can be explained by the number of the interface and interphase formed during resin and fillers blending. Less interfaces and interphases created in SF incorporated SMC composites system, so the active region is smaller for water molecules. The preserving preference of water molecules was reduced, the water absorption decreased as a result.\u003c/p\u003e\n\u003cp\u003eHardness properties of composites expresses the capacity against to local deformation, reflecting the plasticity and flexibility of the composites. For a defect-free ideal crystal model, the hardness is proportional to elastic stiffness [\u003cspan class=\"CitationRef\"\u003e29\u003c/span\u003e]. Therefore, the status of the defects in the composites, including number and distribution, has a major influence on the hardness properties [\u003cspan class=\"CitationRef\"\u003e30\u003c/span\u003e]. Examining of hardness revealed that sulfuric acid solution has detrimental effects on the mechanical performance of SMC composites. In Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003e, it can be observed that the hardness value is decreased with the exposure time for the SMC composites with varied filler composition, when exposed to the sulfuric acid solution. The properties degradation of SMC composites exposed to acid solution can be attributed to various damage mechanisms, such as matrix swelling, chemical degradation and interface debonding. The resin matrix may swell by water molecules and hydrolyze in the presence of acid, leading to polymer matrix plasticization and degradation. Fillers, fibers and other additive may be attacked by the water molecules and acid ions, voids, cracks and defects appearing. The water molecules and acid ions attack the interphase, region between the bulk filler and bulk matrix, leading to debonding between polymer with fibers or fillers. The above chemical and physical degradation caused a substantial change on the microstructure associated with the hardness properties of SMC composites.\u003c/p\u003e\n\u003cp\u003eThe hardness degradation of SMC composites incorporated CCP and SF is more aggressive than that of SMC composites only HGMs added. In addition, more HGMs may suppress the deterioration of hardness properties for SMC composites in acid condition. SMC composites with 24.2 wt% HGMs shows a minor decline on hardness properties (2.84%). It can be contributed to the greater acid resistance of HGMs superior to CCP and SF. A dramatic degradation on hardness properties (9.17%) is observed in the SMC composites with 1:3 mass ratio of HGMs to CCP. The CCP reacted with hydrogen ions in acid solution, forming voids and defects inner composites. It caused the SMC composites failure to fight with deformation, resulting in hardness degradation.\u003c/p\u003e\n\u003cp\u003eIt was distinct that a clear deterioration on the flexural strength and modulus of SMC composites with varied filler composition exposed to acid solution. As shown in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003e(a), the SMC composites with 24.2 wt% HGMs represented the greatest degradation on the flexural strength (6.43%) and modulus (13.41%) after exposure to acid solution for 28 days. A minimum reduction of the flexural strength (3.21%) and modulus (7.85%) was observed on the SMC composites with 11.3 wt% HGMs. This can be attributed to the detrimental effect of excessive HGMs added in SMC composites. When too much HGMs were added into the SMC composites, a trend of crack and break is more prominent for HGMs. The broken HGMs created more voids, cracks or other defects in the composites system. More interfaces between the fillers and resin matrix were formed, which had a poor interaction with each other. When exposed to the acid and water condition, water molecules, hydrogen ions and sulphate ions penetrated the SMC composite through voids, cracks and other defects, further accelerating the matrix swelling, chemical degradation, and micro-crack spread [\u003cspan class=\"CitationRef\"\u003e31\u003c/span\u003e]. This affected significantly the SMC composites resistance to loading stresses, leading to a pronounced damage for flexural properties of SMC composites. Therefore, defects effect is regarded as the main cause of large reductions in mechanical properties.\u003c/p\u003e\n\u003cp\u003eFrom Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003e(b), for the SMC composites with a 1:2 mass ratio of HGMs to CCP, the flexural strength and modulus was reduced by 7.68% and 7.87% respectively, which was the minimum reduction for CCP incorporated specimens. From Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003e(c), for the SMC composites with a 1:1 mass ratio of HGMs to SF, the flexural strength and modulus was reduced by 4.41% and 4.07% respectively, which was the minimum reduction for SF incorporated specimens. The CCP incorporated SMC composites exhibited a more pronounced flexural properties degradation than SF incorporated SMC composites, contributing to the poor resistance to acid of CCP. The reaction between the CCP and acid allowed more voids and cavities forming, providing more travel path for water molecules and acid ions. The weaken interacted region expanded, causing polymer hydrolysis and interfaces degradation developed. When HGMs were replaced by SF with half amount, the break of HGMs was suppressed. The weaken interacted regions and defects number decreased, leading to a minor degradation of flexural properties. It can be concluded the chemical resistance and addition amount of the fillers had significant impacts on the acid resistance of SMC composites. Appropriate addition amount and excellent chemical resistance of fillers can enhance the acid solution resistance of SMC composites.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec11\" class=\"Section2\"\u003e\n\u003ch2\u003e3.2 The effect of resin composition on acid solution resistance of SMC composites\u003c/h2\u003e\n\u003cp\u003eThe features of resin influence the water absorption behavior and acid resistance of SMC composites. VE was chosen as the resin matrix of SMC composites. Two types of epoxies were employed to incorporate with epoxy to investigate the correlation of resin composition with acid resistance of SMC composites. Figure\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003e shows the water absorption of SMC composites with varied resin composition. 892 epoxy incorporated SMC composites exhibited a higher weight gain than 902 epoxy incorporated ones after immersion in water. The viscosity of 902 epoxy is 1300\u0026ndash;1500 Pa∙s, while viscosity of 892 epoxy is low as 0.30\u0026ndash;0.50 Pa∙s. The GFs can impregnate easily for both type of epoxy incorporated resin slurry. There were little defects at fiber and resin interfaces for both type of epoxy added SMC composites. In this case, the defects were not the major reason to induce the water penetration. The lower water absorption for SMC composites with 902 epoxy is attributed to its small free volume between the polymer chains. The water absorption is related with the size and structure of intermolecular-space holes and amount of polar groups [\u003cspan class=\"CitationRef\"\u003e32\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e33\u003c/span\u003e]. The water molecules trends to filler into the free volume holes at initial diffusion stage [\u003cspan class=\"CitationRef\"\u003e34\u003c/span\u003e]. 902 epoxy shows higher viscosity, meaning a higher molecular weight than 892 epoxy. After curing, the 892 epoxy resin cast showed larger free volume holes than 902 epoxy, which caused the water molecules trended to penetrate. Due to the polar groups, the water absorption of epoxy is higher than that of VE. Therefore, with the epoxy amount increasing, the water absorption of SMC composites increased. It can be concluded that the free volume characteristics of resin matrix, including sizes and distributions, prominently influence the water absorption of SMC composites with varied resin composition rather than the number of defects.\u003c/p\u003e\n\u003cp\u003eFigure\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e5\u003c/span\u003e shows the hardness reduction of 892 epoxy incorporated SMC composites is greater than that of 902 epoxy incorporated specimens. In Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e5\u003c/span\u003e(a), the hardness of SMC composites with 1:2 mass ratio of 902 to VE decreased by 3.10%, showing a slightly higher reduction comparing with the other two 902 epoxy incorporated specimens. In Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e5\u003c/span\u003e(b), the hardness of SMC composites with 1:2 mass ratio of 892 to VE decreased by 4.06%, slight greater reduction in contrast to the other two 892 epoxy incorporated specimens. The degradation of hardness properties is mainly attributed to the plasticization result from the swelling and hydrolysis of polymer matrix by water molecules. Exposed to sulfuric acid solution, water molecules penetrate the polymer matrix, resulting in a free volume increasing and polymer plasticization. The plasticization of 892 epoxy is more aggressive than 902 epoxy, due to its low molecular weight. In addition, VE hydrolyze in the presence of acid, promoting the hardness degradation of SMC composites. The relationship between resin characteristics and hardness properties degradation can be illustrated by the combination effect of matrix plasticization and chemical degradation.\u003c/p\u003e\n\u003cp\u003eAs shown in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e6\u003c/span\u003e, an obvious deterioration can be observed in the flexural properties of SMC composites with varied resin composition exposed to sulfuric acid solution. The 902 epoxy incorporated SMC composites exhibited more excellent flexural properties and greater resistance to acid solution than that of 902 epoxy incorporated specimen. It demonstrated that a relatively high molecular weight and proper viscosity provided 902 epoxy prominent mechanical properties and strong bonding with fibers, offering the SMC composites great flexural properties and resistance to acid solution. In Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e6\u003c/span\u003e(a), SMC composites with 1:2 mass ratio of 902 to VE without exposure to acid solution showed a higher flexural strength by 151.0 MPa and a higher flexural modulus by 7.89 GPa. This specimen showed a minor degradation of flexural strength (6.29%) and modulus (7.86%). In Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e6\u003c/span\u003e(b), SMC composites with 1:3 mass ratio of 892 to VE without exposure to acid solution showed a higher flexural by 130.8 MPa and a higher flexural modulus by 7.46 GPa. This specimen showed a minor degradation of flexural strength (6.50%) and modulus (8.44%). Unsuitable mass ratios of 902 to VE may lead to weak interaction between resin and fibers or different resin, exhibiting relatively low flexural properties.\u003c/p\u003e\n\u003cp\u003eMore defects occurred at the interface, providing the active paths for water molecules and acid ions, resulting in matrix plasticization and fiber-matrix interface decohesion [\u003cspan class=\"CitationRef\"\u003e15\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e35\u003c/span\u003e]. The defects effect and weaken interacted regions play an important role in the SMC composites mechanical properties degradation. Mechanical degradation is the result of polymer plasticization and chemical degradation inducing stresses large enough to pull the matrix away from the fiber [\u003cspan class=\"CitationRef\"\u003e36\u003c/span\u003e]. The structure of polymer chains, such as molecules weight and polar groups amount, have an important impact on the resistance of SMC composites. The polymer with low molecules weight and a lot of polar groups are prone to absorb water and react with sulfuric acid solution, resulting in a more pronounced damage on resin matrix, which can effectively transfer stresses in the composites under external loading. The flexural properties of composites were mainly dominated by the matrix and interface. Therefore, reduction extent of SMC composites flexural properties was related with defects effect and resin characteristics.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec12\" class=\"Section2\"\u003e\n\u003ch2\u003e3.3 The effect of fiber composition on acid solution resistance of SMC composites\u003c/h2\u003e\n\u003cp\u003eFigure\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e7\u003c/span\u003e showed that water absorption of SMC composites increased with the GFs content. When the GFs content reached 60 wt%, water absorption tends to remain constant. This can be attributed to the greater hydrophilicity of GFs than resin matrix and HGMs, resulting from the hydrogen bonding on the GFs surface. It demonstrated that the absorbed water by GFs occupied the interface of resin and fibers, diminishing resin-fiber interaction and facilitating separation of fibers and resin under external force. Moreover, the number of interfaces increased with GFs content, leading to an increase of water absorption.\u003c/p\u003e\n\u003cp\u003eThe hardness of SMC composites with varied fiber composition was reduced exposed to sulfuric acid solution, as shown in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e8\u003c/span\u003e(a). The hardness properties reflect the plasticity and flexibility of composites, considering as a surface contribution. Hardness of composites is generally associated with the fiber amount [\u003cspan class=\"CitationRef\"\u003e37\u003c/span\u003e]. It is evident that the SMC composites with 40 wt% GFs shows a slightly higher hardness than the SMC composites with other fiber contents. When the fiber content is relatively low, below 40 wt%, the hardness increased with the fibers amount. As fiber content continued to increase, the hardness did not change significantly. Figure\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e8\u003c/span\u003e(b) showed that the hardness reduction rate did not change obviously with the fibers amount, suggesting that the hardness reduction of SMC composites exposed to acid solution for 28 days may be independent with the fiber content in case of the high fiber content. It can be explained by that the obvious change was not appear on fibers for 28 days exposure to sulfuric acid solution. It suggested that the hardness reduction was associated with the resin matrix swelling and interface degradation, rather than the fiber crack for short-term exposure.\u003c/p\u003e\n\u003cp\u003eAs shown in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e9\u003c/span\u003e(a), SMC composites with 55 wt% GFs exhibited the highest flexural strength and modulus. The flexural properties were enhanced by GFs reinforcement. The flexural strength and modulus were improved with the GFs content increasing. However, when the GFs content reached to 60 wt%, poor impregnation of fibers occurred due to the decreased resin content, leading to a reduction of flexural properties. Lowest reduction rate of flexural properties was obtained on SMC composites with 55 wt% GFs exposed to sulfuric acid solution, represented in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e9\u003c/span\u003e(b). It can be illustrated by the degradation mechanism of SMC composites short-term exposure to acid solution. In the present work, the 28 days exposure period was short and the 20 wt% sulfuric acid concentration is low. The damage and fracture of fibers did not take place in the relatively short exposure period. In this case, the deterioration of flexural properties is dominated by the resin matrix plasticization and decomposition, along with the interface degradation, which agreed with the conclusions drawn in hardness tests.\u003c/p\u003e\n\u003cp\u003eThe fractured section of SMC composites with 55 wt% GFs, 11.3 wt% HGMs, and 1:2 mass ratio of 902 to VE was examined after different exposure period. The failure modes of fracture were analyzed to describe the degradation mechanisms of SMC composites in acid solution for short-term. In Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e10\u003c/span\u003e(a), the fractured section of the unexposed specimen displayed a strong interface between resin matrix with HGMs and fibers. In this case, the failure of fracture supposed to be a localized and brittle failure mode, meaning the fiber-matrix and filler-matrix interface were robust. The fracture occurred subjected to quite high stress. In Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e10\u003c/span\u003e(b), parts of resin remained on the surface of fiber after failure, indicating that the attachment of resin matrix on fibers maintained a moderate level for specimen exposed to acid solution for 7 days. No obvious damage of resin, HGMs or fibers was observed in the specimen exposed for 7 days. Figure\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e10\u003c/span\u003e(c) showed a smooth fracture surface appeared at the interface between resin and HGMs, indicating a prominent debonding between resin and HGMs. Traces of resin residues were noticed on the fibers, suggesting that the interaction between matrix and fiber became considerably weak due to the interface degradation. Exposed to acid solution for 28 days, the fiber-resin debonding developed aggressively, by leaving lots of smooth surfaces when fibers separated with resin shown in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e10\u003c/span\u003e(d). In addition, the matrix crack appeared, demonstrating that resin matrix had been damaged severely by physical swelling and chemical decomposition. Cracks in the matrix indicated the degradation of the polymer due to acid and water. The interface degradation and matrix damage of SMC composites exposed to sulfuric acid solution can be confirmed by analyzing the fracture manner. However, no obvious fiber damage can be observed in fractured section, which was agreed with the results of flexural properties reduction.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec13\" class=\"Section2\"\u003e\n\u003ch2\u003e3.4 The degradation mechanism analysis\u003c/h2\u003e\n\u003cp\u003eThe degradation mechanism of SMC composites in acid solution was clarified by analyzing the change of molecule configuration and evaluating the thermal stability. According to Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e11\u003c/span\u003e(a), the exposed specimens showed a decreased intensity of peak at 1725 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e belonging to carbonyl groups. The intensity of peaks at 1091 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e and 1251 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, belonging to aliphatic ether groups and aromatic ether groups respectively, decreased slightly for exposed specimens, suggesting that the esters groups hydrolyze in the absence of water and acid. It can be concluded that chemical decomposition occurred in the resin matrix of SMC exposed to sulfuric acid solution. According to Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e11\u003c/span\u003e(b), the thermal stability decreased for exposed specimens, indicating a macromolecule chains scission, agreed with the FTIR results. Exposed to 25 ℃, 20 wt% sulfuric acid solution for 28 days, the SMC composites exhibited a minor degradation of HGMs and GFs. Even though, a minor damage of GFs and HGMs may occur in acid solution environment, the degradation of polymer matrix and interphase was the main cause to induce the deterioration of mechanical properties.\u003c/p\u003e\n\u003cp\u003eThe matrix swelling, chemical degradation, and interfaces debonding were responsible for the deterioration of mechanical properties for SMC composites. It can be concluded that the SMC composites with various constituent displayed multiple degradation mechanisms under the combination effect of water and acid. The evolution of degradation was influenced by exposure period. It can be descripted schematically in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e12\u003c/span\u003e. In the first stage, water molecules together with acid ions penetrated the resin matrix and interphase of composites through diffusion and capillary action [\u003cspan class=\"CitationRef\"\u003e38\u003c/span\u003e]. A three-dimensional region between the bulk fiber or filler and bulk matrix is referred as interphase [\u003cspan class=\"CitationRef\"\u003e39\u003c/span\u003e]. Some voids, cracks and other defects may exist in the interphase, forming a weaken interacted regions in composites system. Water and other small molecules prefer to travel along the weaken interacted regions [\u003cspan class=\"CitationRef\"\u003e40\u003c/span\u003e]. The penetrated water molecules and acid ions induced swelling of the matrix and propagation of microcracks on interphase [\u003cspan class=\"CitationRef\"\u003e41\u003c/span\u003e]. accelerating the degradation of matrix and interphase. Absorbed water molecules filled into voids and cracks in the matrix and weaken interacted regions acting as a plasticiser, causing the composites more flexible, which is the major reason to reduce the hardness of SMC composites. In the second stage, the penetrated water and acid further attacked the GFs-matrix and HGMs-matrix interphases, finally fiber-matrix and filler-matrix debonding [\u003cspan class=\"CitationRef\"\u003e42\u003c/span\u003e]. The penetrated water molecules reacted with ester groups on resin chains in the presence of acid, the hydrolysis of matrix occurring. The polymer decomposition reduced the interaction between the resin chains and bonding of resin and fillers or fibers, promoting the interphase delamination and water molecules and acid ions diffusion additionally. In the third stage, the HGMs and GFs were attacked by water molecules and acid ions, cracks appearing on the GFs surface and break of HGMs taking place. It can be concluded that the original defects and weak interacted regions in the composites system initiated the degradation of SMC composites, while the microstructure and composition of SMC composites dominated the degradation progress.\u003c/p\u003e\n\u003c/div\u003e"},{"header":"4. Conclusions","content":"\u003cp\u003eIn the present work, the SMC composites with variable filler, resin and fiber composition were prepared to investigate the effects of composition on acid resistance of SMC composites. The water absorption of SMC composites with various compositions was measured to estimate the influence of water on mechanical properties deterioration. The degradation of mechanical properties was evaluated by hardness and flexural strength and modulus results. The acid solution degradation mechanism was revealed by analyzing the change of molecule configuration and evaluating the thermal stability of the unexposed and exposed specimens. A minimum reduction of the flexural strength (3.21%) was observed on the SMC composites with 11.3 wt% HGMs. The chemical resistance and addition amount of the fillers had significant impacts on the acid resistance of SMC composites. A great flexural property and a minor degradation of flexural strength (6.29%) and modulus (7.86%) was obtained in SMC composites with 1:2 mass ratio of 902 to VE. The resin characteristics, molecules weight, free volume size and polar groups number, have an important impact on the water absorption and acid resistance of SMC composites. A high flexural property and minor degradation of flexural strength (5.12%) and modulus (7.66%) was observed in SMC composites with 55 wt% GFs. Exposed to 25 ℃, 20 wt% sulfuric acid solution for 28 days, the SMC composites exhibited a minor degradation of HGMs and GFs. In this condition, the deterioration of mechanical properties was dominated by the resin matrix plasticization and decomposition, along with the interface degradation. It can be concluded that the original defects and weak interacted regions in the composites system initiated the degradation of SMC composites, while the microstructure and composition of SMC composites dominated the degradation progress. This paper provided a better understanding of the degradation evolution and mechanisms of SMC composites to meet the requirements of strong acid environments application.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eDeclaration of competing interest\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors declare that they have no competing financial interests or personal relationships that could have influenced the work reported in this paper.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthor contribution\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eJian Li prepared the materials and conducted most of the measurements and data analysis. Chao Fu, Ruifeng Ming, Minxian Shi, Wenhao Dong, Jiang Guo, and Xingkui Guo contributed to the data analysis. Duo Pan and Xiufang Zhu conceived the idea, wrote the paper, and coordinated the overall project. Dalal A. Alshammari and Saad Melhi revised the paper. Mufang Li provided supervision and resources. Hamdy Khamees Thabet reviewed and revised the manuscript. All authors reviewed the manuscript.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFunding\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eWe acknowledge the financial support from the Outstanding Youth Project of Natural Science Foundation of Hubei Province of China (2021CFA068), the Outstanding Young and Middle-aged Innovation Team of Hubei Province of China (T2021007). The authors extend their appreciation to the Deanship of Scientific Research at Northern Border University, Arar, KSA for funding this research \u0026ldquo;work through the project number \u0026ldquo;NBU-FPEJ-2024-ID-XXX.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eData availability\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors confirm that the data supporting the findings of this study are available within the article. Raw data that support the findings of this study are available from the corresponding author, upon reasonable request.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eLiu X, Liu T, Feng P (2022) Long-term performance prediction framework based on XGBoost decision tree for pultruded FRP composites exposed to water, humidity and alkaline solution. Compos. Struct. 284: 115184. https://doi.org/10.1016/j.compstruct.2022.115184\u003c/li\u003e\n\u003cli\u003eSethi S, Ray BC (2015) Environmental effects on fibre reinforced polymeric composites: Evolving reasons and remarks on interfacial strength and stability. Adv. Colloid Interface Sci. 217: 43-67. https://doi.org/10.1016/j.cis.2014.12.005\u003c/li\u003e\n\u003cli\u003eRay BC, Rathore D (2014) Durability and integrity studies of environmentally conditioned interfaces in fibrous polymeric composites: Critical concepts and comments. Adv. 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Compos Part A 78: 311-317. https://doi.org/10.1016/j.compositesa.2015.08.027\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"advanced-composites-and-hybrid-materials","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"achm","sideBox":"Learn more about [Advanced Composites and Hybrid Materials](https://link.springer.com/journal/42114)","snPcode":"42114","submissionUrl":"https://submission.nature.com/new-submission/42114/3","title":"Advanced Composites and Hybrid Materials","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"SMC composites, composition, acid resistance, mechanical properties, degradation mechanism","lastPublishedDoi":"10.21203/rs.3.rs-4112494/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-4112494/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eIn the present work, the sheet molding compound (SMC) composites with variable filler, resin and fiber composition were prepared to investigate the effects of composition on acid resistance of SMC composites. The water absorption of SMC composites with various compositions was measured to estimate the influence of water on mechanical properties deterioration. Hardness and flexural properties tests were performed to investigate the degradation evolution. The degradation mechanism was revealed by analyzing the change of molecule configuration and evaluating the thermal stability. A minimum reduction of the flexural strength (3.21%) was observed on the SMC composites with 11.3 wt% hollow glass microspheres (HGMs). The chemical resistance and addition amount of the fillers had significant impacts on the acid resistance of SMC composites. A great flexural property and a minor degradation of flexural strength (6.29%) and modulus (7.86%) was obtained in SMC composites with the mixed resin. The resin characteristics, molecules weight, free volume size and polar groups number, had an important impact on the water absorption and acid resistance of SMC composites. A high flexural property and minor degradation of flexural strength (5.12%) and modulus (7.66%) was observed in SMC composites with 55 wt% glass fibers (GFs). Exposed to 25 ℃, 20 wt% sulfuric acid solution for 28 days, the SMC composites exhibited a minor degradation of HGMs and GFs. In this condition, the deterioration of mechanical properties was dominated by the resin matrix plasticization and decomposition, along with the interface degradation. It can be concluded that the original defects and weak interacted regions in the composites system initiated the degradation of SMC composites, while the microstructure and composition of SMC composites dominated the degradation progress.\u003c/p\u003e","manuscriptTitle":"Degradation evolution and mechanism of sheet molding compound with variable composition exposed to acid solution environment","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2024-04-03 16:20:30","doi":"10.21203/rs.3.rs-4112494/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2024-04-24T17:40:55+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2024-04-24T15:59:20+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2024-04-24T14:18:06+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2024-04-24T10:26:45+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2024-04-21T13:45:04+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"371a44b6-7517-4807-af13-4c63d20a6c42","date":"2024-04-16T15:07:19+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"a04aa271-706d-492d-bf78-5c60c7fdbc1d_SNPRID","date":"2024-04-16T12:16:28+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"e5ca6878-86a0-4362-9df0-5639e86d9073","date":"2024-04-15T11:10:32+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"652ec1d0-3bdc-437b-9c01-8e32dfb714e2","date":"2024-04-15T02:24:11+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"5128becc-11b6-44bd-9a84-eb1fc67955a1","date":"2024-04-14T14:09:30+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2024-04-14T08:51:35+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2024-04-02T17:16:06+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2024-03-29T09:28:15+00:00","index":"","fulltext":""},{"type":"submitted","content":"Advanced Composites and Hybrid Materials","date":"2024-03-16T09:57:14+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"advanced-composites-and-hybrid-materials","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"achm","sideBox":"Learn more about [Advanced Composites and Hybrid Materials](https://link.springer.com/journal/42114)","snPcode":"42114","submissionUrl":"https://submission.nature.com/new-submission/42114/3","title":"Advanced Composites and Hybrid Materials","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"e0ea7069-a048-4d03-b1a6-b65fa8caee7e","owner":[],"postedDate":"April 3rd, 2024","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[],"tags":[],"updatedAt":"2024-07-03T00:49:35+00:00","versionOfRecord":{"articleIdentity":"rs-4112494","link":"https://doi.org/10.1007/s42114-024-00925-3","journal":{"identity":"advanced-composites-and-hybrid-materials","isVorOnly":false,"title":"Advanced Composites and Hybrid Materials"},"publishedOn":"2024-07-03 00:49:35","publishedOnDateReadable":"July 3rd, 2024"},"versionCreatedAt":"2024-04-03 16:20:30","video":"","vorDoi":"10.1007/s42114-024-00925-3","vorDoiUrl":"https://doi.org/10.1007/s42114-024-00925-3","workflowStages":[]},"version":"v1","identity":"rs-4112494","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-4112494","identity":"rs-4112494","version":["v1"]},"buildId":"qtupq5eGEP_6zYnWcrvyt","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}