Improved Tribological Properties of Epoxy Cement Reinforced with Impact-Resistant Core-Shell Structured Polymer Nanoparticles

Abstract

Traditional cement epoxy pavements suffer from inherent limitations such as terrible tribological properties, poor wear resistance, and weak impact resistance, presenting significant challenges to ensure the safety and continuous operation of urban roads. As a solution, high-performance cement epoxy composite grouting materials have emerged as the preferred option for engineering construction and road maintenance. In this study, CSP/epoxy cement (CSEC) composite materials were prepared by emulsion polymerization. The thermal properties of the materials were characterized, revealing that CSP enhances the thermal properties of epoxy cement (EC) to a certain extent. Furthermore, the frictional properties of CSEC composite materials and pure epoxy cement under different normal loads were investigated. The results indicated that the CSEC composite material exhibited a slight increase in friction coefficient and a notable decrease in wear rate compared to pure epoxy cement (EC). Specifically, the wear rate of CSEC decreased by 14.4% at a load of 20 N, highlighting the enhanced frictional performance facilitated by CSP. Mechanistic analysis attributed the improvement to the unique core-shell structure of CSP, which imparted higher impact resistance and eliminated alleviate residual stresses at the friction interface. This structural advantage further enhanced the wear resistance of materials, making it a promising choice for improving the durability and safety of urban road surfaces.

1. Introduction

With the rapid development of urbanization, cement epoxy composite grouting materials have become important materials in engineering construction and pavement repair due to their various excellent properties [1]. High-performance repair materials are required to repair and reinforce ground cracks, damaged surfaces, etc., in order to improve structural stability and extend the service life of pavements [2]. Traditional cement pavements were characterized by low hardness, poor abrasion resistance, weak impact resistance, and short service life, making it difficult to adapt to the increasing traffic volume and traffic load, posing challenges to the safety and continuous operation of urban roads [3,4]. Extensive wear accelerates the aging and damage of pavements; thus, a simultaneous emphasis on new construction and repair has become the main strategic direction of the concrete construction industry. Short service life leads to frequent construction and replacement projects, wasting a large number of resources and increasing the burden on the environment. Therefore, to improve the situation it is necessary to apply modern science to seek new materials and technologies. Furthermore, skid resistance and abrasion resistance of pavements are among the most influential factors affecting vehicle safety and the maintenance of surface functionality during their service life [5]. Since 1929, researchers abroad have been conducting a series of studies on the skid resistance of pavements. Later, with the rapid development of highway construction in various countries, more and more researchers gradually began to pay attention to and focus on the research of pavement skid resistance and abrasion resistance. Currently, the main method to enhance the skid resistance and abrasion resistance of pavements is by increasing the pavement roughness. This immediately enhances the skid-resistant structure of the pavement and significantly improves the friction coefficient in the short term, enhancing the skid resistance of the pavement. However, the abrasion resistance of the pavement is still limited, leading to a relatively rapid decay in skid-resistance performance [6]. Therefore, it is essential for the construction industry to design a multifunctional, high-performance, skid-resistant, durable ground material [7]. Common cement-based grouting materials include cement slurry, concrete regenerators, concrete repair agents, and epoxy cement composite materials. Cement-based grouting materials play an important role in the repair and reinforcement of buildings, providing guarantee of the overall strength and stability of buildings [8,9]. Epoxy resin grouting material is a polymer grouting material composed of epoxy resin, which can firmly bond to cement concrete and can be used as a new type of ground material [10,11]. Due to its excellent bonding strength, chemical resistance, durability, and flowability, epoxy resin has been widely used in various engineering applications, such as repairing and filling structural cracks, repairing and protecting concrete surfaces, coating floors and pavements, and corrosion prevention. However, there is still room for improvement in the tribological properties of epoxy resin [12,13]. To further enhance the tribological performance of epoxy resin, adding fillers or modifying its formulation can enhance its skid resistance and abrasion resistance.

By adding fillers of different types, shapes, sizes, and concentrations to epoxy cement, its coefficient of friction and wear rate can be effectively altered. Additionally, incorporating toughening agents with different properties into the epoxy cement, such as thermoplastic resins [14,15], rubber [16,17], rigid particles [18], and nanoparticles [19,20], enhances the material’s toughness and abrasion resistance. Peng et al. improved the toughness of resin-toughened cement by using 2-aminomethylpropane sulfonic acid (SEA) hydrophilic modified epoxy resin (E-44) based on the mechanism [21]. Zhang et al. prepared three-dimensional graphene/epoxy composite materials, successfully enhancing tribological and thermal management properties, leading to a significant reduction in coefficient of friction and wear rate [22]. Rittin et al. enhanced the performance of polymer composite materials by adding multi-walled carbon nanotubes (MWCNTs) to epoxy resin, improving the interfacial bonding strength between the resin and nanoparticles as well as the tribological properties [23]. Zhang et al.’s research indicated that the addition of core-shell nano-spheres significantly reduced the coefficient of friction and wear rate of polyamide-6/epoxy cement composite materials, while also enhancing abrasion resistance and fatigue resistance, demonstrating the significant influence of particle size and proportion on performance [24]. Core-shell structured polymer nanoparticles (CSP) are an excellent type of elastomer-toughening modifier. When blended with epoxy resin, the particles can be uniformly dispersed to form an “island” structure [25,26]. In this state, the particles exhibit outstanding compatibility with the matrix, forming a high-strength interface. When the elastomer modifier is subjected to external impact, it triggers shear yielding of the material or generates tiny voids, absorbing a large amount of energy and improving toughness and impact resistance [27,28,29]. Furthermore, CSP undergoes deformation due to external frictional forces and energy absorption. The deformation generates frictional resistance, enhancing the material’s anti-slip and wear-resistant properties.

In this study, hard-soft core-shell structured impact-resistant nano-particles (CSP) were prepared using butyl acrylate as the core and methyl methacrylate as the shell. These particles were added to E51 epoxy cement as additives for slip resistance, wear resistance, and impact resistance, creating a novel cement-based epoxy resin anti-slip, wear-resistant, and shock-absorbing pavement material. Additionally, the influences of impact resistance of core-shell nanoparticles on the tribological properties of epoxy cement were investigated, developing a new type of epoxy resin cement pavement material as a reference for the future promotion and application of new construction materials.

2. Materials and Methods

2.1. Materials

Methyl methacrylate (MMA, Energy Chemical, 99%) and butyl acrylate (BA, Energy Chemical, stabilized with MEHQ, 99%) were used as monomers after purifying by activated Al₂O₃ column chromatography. Ethylene glycol dimethacrylate (EGDMA, Macklin, stabilized with MEHQ, 98%) was used as a crosslinking monomer after purifying by activated Al₂O₃ column chromatography. Sodium dodecyl sulfate (SDS, Macklin, 92.5%) was used as the emulsifier. Potassium persulfate (KPS, Aladdin, 99.5%) was the heat-induced initiator. The E51 and polyether amine (D400) were purchased from Shanxi Lanxin Chemical Co., Ltd.; cement was purchased from Shandong Jinan Xinglong Chemical Co., Ltd. GCIR balls were purchased from Zhejiang Tai zhou Lihong stainless steel products Co., Ltd.

2.2. Synthesis of Core–Shell Structured Nanoparticles (CSP)

First, 100 mL of n-butyl acrylate, 2 mL of cross-linker EGDMA, and 60 mL of surfactant deionized aqueous solution containing 1.0 g of sodium dodecyl sulfate were subjected to ultrasonic shear emulsification for 10 min to obtain a volume of about 162 mL of core monomer pre-emulsion.

Then, 1/3 of the volume of the above freshly prepared core monomer pre-emulsion was taken and mixed with 60 mL of deionized aqueous solution containing 0.125 g of KPS. Nitrogen was pumped 3 times, and heating and stirring was carried out at 82 °C. After bluing, 5 mL of aqueous deionized water solution containing 0.125 g of KPS was then added. The remaining 2/3 of the core monomer pre-emulsion was added dropwise within 1.5 h. Subsequently, the reaction was kept warm for 1 h. The reaction was finally warmed to 90 °C for 1 h to obtain the nuclear latex.

The above 50 g was weighed and mixed with 60 mL of an aqueous solution containing 0.08 g of KPS; nitrogen was pumped 3 times, and heated and stirred at 82 °C. Subsequently, a pre-emulsion of shell monomers pre-emulsified by 15 mL of methyl methacrylate, and 0.1 g of sodium dodecyl sulfate and 10 mL of deionized water were added within 1 h. The reaction was subsequently held for 1 h to obtain the core-shell particle latex. Finally, the core-shell particles were obtained by spray-drying the diluted core-shell nanoparticles. The corresponding preparation process is illustrated in Figure 1.

2.3. Preparation Procedure of Core-Shell Epoxy Cement (CSEC)

First, 10 g of E51 epoxy resin, 3 g of cement, 1 g of CSP, and 4.2 g of polyether amine D400 were placed in a sample bottle and mixed thoroughly with a homogenizer. As shown in Figure 1, the mixed slurry was poured into a mold of PTFE and placed in a vacuum oven at 60 °C for 2 h of pre-polymerization, and then the temperature was raised to 120 °C and continued for 6 h so that the epoxy resin was fully cured and the epoxy cement containing a core-shell structure was obtained and named CSEC. The same method was used to prepare epoxy cement without adding CSP, and it was named as the blank control group EC.

2.4. Tribological Property Tests

Frictional performance of EC and CSEC was evaluated using reciprocating ball-on-plate module with a UMT-3, Bruker. A GCR 15 ball with a diameter (Φ) of 6.35 mm and a Vickers hardness of 207–269 HV was selected for the dry reciprocating experiments. Friction experiments were conducted at a frequency of 1 Hz and an amplitude of 4 mm under 30 min 5 N, 15 N, and 20 N loads to investigate the effect of CSP content on the friction coefficient, wear rate, and surface morphology of epoxy cement [27]. Each experiment was repeated more than 3 times. The wear rate (k) was calculated using Formula (1):

$$k={\displaystyle \frac{V_{\mathit{EC}/\mathit{CSEC}}}{X·F_z}}$$

(1)

The wear volume (V) of the epoxy cement plate, sliding distance (X), and normal load (F_z) were inputted for calculation.

The coefficient of friction (COF) was determined by measuring the ratio of horizontal friction force F_x to the vertical load force F_z during reciprocating motion, with the formula as follows:

$$\mathit{COF}={\displaystyle \frac{F_x}{F_z}}$$

(2)

The average value of the horizontal friction force (F_x) was measured throughout the entire reciprocating cycle, and the applied constant vertical load was F_z.

2.5. Characterization

Dynamic light scattering (DLS) measurements were performed on CSP particle diameters using a Malvern Nano Zetasizer 3000 (Malvern Instruments LTD, Malvern, UK), while their nanostructures were observed via transmission electron microscopy (TEM) with a Hitachi HT-7700. Epoxy cement samples were prepared using spray dryer model LP-2200G (procured from Shanghai Yuming Instrument Co., Ltd., Shanghai, China). The dispersion of precursors was facilitated by the vacuum defoaming mixer ZYMC-350VS (from Suzhou Zhongyi Precision Technology Co., Ltd.,Suzhou, China). Surface wear and three-dimensional profiles of the samples were analyzed by the white light interferometer (WLI, Bruker, Contour GT-K0, Germany) and scanning electron microscope (SEM, FEI Nova 450, USA, at 15 kV). The depth and volumes of the wear scars of CSEC and EC samples were visually examined and quantified by 3D micrographs. DSC measurements were carried out on a DSC Q100 V9.7 Build 291 instrument (USA). The powder samples were weighed and placed in aluminum pans before being heated from −70 °C to 100 °C at a rate of 10 °C min⁻¹ under a nitrogen atmosphere. The thermal stability analysis was conducted using a TA Instruments Q600 SDT thermogravimetric analyzer (TGA, USA) under the nitrogen. Each sample (~5 mg) was heated from ambient temperature to 700 °C with a heating rate of 20.0 °C/min.

3. Results and Discussion

3.1. The Characterization of CSP

The morphology and structure of core-shell structured nanoparticles (nano-pass structure, CSP) can be confirmed through transmission electron microscopy (TEM). It can be clearly observed from Figure 2a that CSP nanoparticles with a diameter of about 100 nm had an obvious core-shell structure, and the particles were mostly clustered. The core was a region of higher density, while the shell was relatively lighter. Additionally, dynamic light scattering (DLS) was employed to analyze the intensity fluctuations of scattered light from particles undergoing Brownian motion, allowing for the measurement of particle-size distribution in the solution. As shown in Figure 2b, it can be seen that the peak of the size distribution was around 220 nm, exhibiting a shape close to a normal distribution. This distribution indicated a high degree of uniformity in particle size, consistent with the control standards during the preparation process. This uniform size distribution was consistent with the results observed by transmission electron microscopy, which indicated the good practicability of the material and further confirmed the successful preparation of CSP.

3.2. Thermogravimetric Analysis TGA and Differential Scanning Calorimetry DSC Tests

The thermal stability, decomposition temperature, glass transition temperature (T_g), and thermal behavior of the material were primarily determined through thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) to evaluate the impact of CSP on the thermal stability and behavior of epoxy cement composite materials. By measuring the change in sample mass with increasing temperature, the thermal stability of organic components and the residual content of inorganic fillers in the composite materials were determined. Figure 3a,b, respectively, present the thermogravimetric analysis (TGA) curves for pure epoxy cement and CSP-reinforced epoxy cement composite materials. From the TGA curve in Figure 3a and DSC results in Figure 3b, it can be found that the thermal mass loss of epoxy cement occurs in three stages. The first stage in the temperature range of 50–225 °C primarily involved the evaporation of unreacted free water and a small amount of epoxy resin and dehydrated CSP curing. The mass loss of the composite material was only 2.465%, reflecting the stable curing of epoxy cement and excellent thermal stability in the low-temperature range. The second stage occurs in the temperature range of 225–350.78 °C, mainly due to the evaporation of water produced by the decomposition of calcium hydroxide in the cement. There was almost no volatile component release from the composite material in this temperature range. The third stage occurred in the temperature range of 350.78–600 °C, where a significant mass drop was observed as the temperature rose to 350.78 °C, indicating substantial thermal decomposition of the main organic component, CSP, in the composite material. This clear thermal decomposition characteristic revealed that the introduction of CSP directly affected the thermal decomposition behavior of the epoxy cement matrix, enhancing the thermal stability and high-temperature resistance of the epoxy cement.

Figure 4 illustrates the differential scanning calorimetry (DSC) curves comparing pure epoxy cement (EC) and the composite material (CSEC), detailing the thermal flux variation within the temperature range of −60 °C to 130 °C. Differential scanning calorimetry was used to assess the operational temperature range of materials. In Figure 4a,b, there was minimal change in the glass transition temperature after modification, indicating that the addition of CSP did not alter the network structure and thermodynamic properties of the material. CSEC retained the excellent characteristics of epoxy resin, with improvements in tribological and mechanical properties warranting further discussion. Additionally, from Figure 4a to Figure 4b, within the range of −20 °C to 40 °C, the normalized curves exhibited consistently gentle slopes, suggesting minimal thermal efficiency fluctuations for both EC and CSEC, ensuring stable performance. Therefore, the stability and reliability of CSEC within typical temperature ranges ensure its adaptability to various conditions in road applications.

3.3. Frictional Performance Testing

The frictional performance under different load conditions was tested and the data for unmodified epoxy cement (EC) and CSP−modified epoxy cement (CSEC) were analyzed. Figure 5 shows the comparison of the coefficient of friction (COF) between pure epoxy cement and composite epoxy cement containing CSP under three different loads of 10 N, 15 N, and 20 N, demonstrating the change in COF over time. In Figure 5a−c, it can be observed that the COF of the epoxy cement samples with added CSP was higher than that of the unmodified epoxy cement, indicating the occurrence of a friction-increasing effect due to the addition of CSP. This may be attributed to the nanoscale spheres of the CSP, which result in a more uniform mixture within the epoxy cement and increase its surface roughness and microfriction points.

Figure 6 showed the average coefficient of friction (ACOF) and wear rate (k) of EC and CSEC under different dry friction load conditions. All data were obtained through multiple experiments. In Figure 6a, it can be observed that under the three loads of 10 N, 15 N, and 20 N, the average coefficient of friction (ACOF) of CSEC was higher than that of unmodified EC, consistent with the results in Figure 5. In Figure 6b, the wear rate of CSEC is lower than the wear rate of EC, indicating that the addition of CSP had successfully improved the wear resistance of epoxy cement. Under a 10 N load, the wear rate of the composite material containing CSP decreased by approximately 12.3%. With the load increase to 15 N and 20 N, the wear rate decreased, respectively, by 7.4% and 14.4%, reaching the requirements for friction reduction and wear resistance.

3.4. The Analysis of Wear Surface

Through 3D white light interferometry and scanning electron microscopy (SEM), the wear scars on the sample surface were characterized. Figure 7(a1–a4,b1–b4) shows the wear surface analysis of EC and CSEC under the 10 N load, respectively. From Figure 7(a1,b1), it can be seen that the wear depth of EC and CSEC was consistent. The surface of EC exhibited scratches, fragments, and significant tearing (Figure 7(a2–a4)), while the degree of scratching and tearing on the surface of CSEC was noticeably reduced (Figure 7(b2–b4)). The wear surface images of EC and CSEC under 15 N load are shown in Figure 8(a1–a4,b1–b4). The difference in wear depth in Figure 7(b1) and Figure 8(a1) were not significant. However, the SEM images revealed that the epoxy cement (EC) undergoes severe tearing and delamination, with many fragments observed at the wear location, and the internal structure of the sample experiences significant wear (Figure 8(a2–a4)). In Figure 8(b2−b4), evidence of tearing and the formation of layers due to wear can be observed, indicating that the addition of CSP significantly enhanced the wear resistance of the epoxy cement. The 3D contour images of unmodified epoxy cement (EC) and CSP-modified epoxy cement (CSEC) under the 20 N load showed almost the same wear depth values, while the SEM images were significantly different. The sample of EC clearly showed many voids (Figure 9(a2)), and the occurrence of tearing and delamination indicated severe wear. Conversely, Figure 9(b2–b4) showed alleviated evidence of tearing and wear, possibly due to the improvement in wear resistance of the epoxy cement through the addition of CSP, which acted as an external hard/inner soft impact-resistant small ball. In summary, the addition of CSP slowed down the wear of the epoxy cement, enhancing its wear resistance.

4. Friction Mechanism Analysis

Through the above analysis of the tribological performance, it can be found that the addition of CSP has great potential in increasing the friction coefficient and wear resistance of the samples. The structure of CSP with the shell of methyl methacrylate and an inner shell of butyl acrylate exhibits impact resistance, and the nanoballs make the distribution of epoxy cement more uniform. It can be observed that the increase in the coefficient of friction (COF) and average coefficient of friction (ACOF) was not significant, which may be due to the impact resistance of the core-shell polymer nanoballs (Figure 5 and Figure 6a). In the SEM images in Figure 7(a4–b4), Figure 8(a4–b4) and Figure 9(a4–b4), there were a large number of fragments and tears, proving the occurrence of abrasive wear and adhesive wear. Furthermore, a significant reduction in wear rate with the addition of CSP can be observed, reducing the curling of tear and wear surfaces caused by friction. Attributed to the uniform dispersion of impact-resistant nano-sized balls (CSP) within the epoxy resin matrix, an “island” structure with high-strength interfaces was formed. The rigid shell was worn and exposed the elastic inner surface, with external impacts inducing shear yielding of the composite material, absorbing significant energy, and enhancing toughness and impact resistance (Figure 10a,b) [30]. The tribological performance was affected by both the normal force generated at the interface junction and the shear force needed to overcome the resistance of rough surfaces, as follows [31]:

$$\mu ={\displaystyle \frac{F_f}{F_n}}$$

(3)

$$F_f=F_{f,\mathit{adh}}+F_{f,\mathit{pl}}$$

(4)

$$F_{f,\mathrm{adh}}=\tau ·A_r$$

(5)

Research has suggested that the shear force surpasses the cohesive energy of the polymer matrix balls’ wear [31]. As shown in Figure 10b, the hard methyl methacrylate shell surrounded the nano-balls fractures, revealing the inner ethyl acrylate elastomer on the surface. The exposed internal elastic body influenced frictional resistance by increasing the contact area of friction and simultaneously reducing some shear force. Consequently, rigidity decreased while flexibility increased, resulting in elevated friction resistance and friction coefficient (Figure 5 and Figure 6a). These were also the reason for the limited increase in the friction coefficient. Furthermore, a significant number of chemical bonds in the polymer shell of the nano-balls broke, absorbing energy and reducing wear (Figure 6b) [30]. As the rigid exterior of the CSP wore down, resistance arose from the deformation of its naked, flexible interior, achieving the effect of increasing tri and reducing wear.

5. Conclusions

This paper successfully prepared CSP impact-resistant nanoballs with a diameter of approximately 100–225 nm. The composite CSP/epoxy cement (CSEC) has excellent thermal stability, meeting the ambient temperature demands generated by daily road surface conditions and heavy friction. Additionally, compared to pure epoxy cement (EC), CSEC epoxy cement shows increased friction coefficients of 2.89%, 9.68%, and 2.28%, respectively, at 10 N, 15 N, and 20 N loads. The wear rate also decreases to varying degrees. Specifically, at a 20 N load, the wear rate of CSEC decreased by 14.4%, highlighting the enhanced frictional performance facilitated by CSP. This indicates that CSP impact-resistant nanospheres play a role in the microfriction processes. Mechanistic analysis attributes this enhancement to its inherent hard-outer-soft-inner structure and internal material deformation that cushions shear forces, thereby leading to higher friction coefficients and reduced wear. The increase of friction coefficient is insufficient, and the decrease of wear rate is insufficient. Further development is needed to improve frictional properties and wear resistance.