Study on the Mechanical Characteristics, Heat Resistance, and Corrosion Resistance of Unsaturated Polyester Resin Composite

Abstract

In this study, the researchers aimed to broaden the applications of unsaturated polyester resin (UPR) composites in the building field by incorporating surface-modified titanium dioxide (TiO₂) nanoparticles and reinforced chopped glass fibers within the UPR matrix. Specifically, the composites containing 4 wt.% TiO₂ nanoparticles and 30 wt.% chopped glass fiber exhibited remarkable properties of 46.1 kJ/m² in impact strength, 90.1 MPa in flexural strength, 78.8 MPa in tensile strength, and 121.5 °C in heat deflection temperature, which improve 207.5%, 80.3%, 62.5%, and 73.6%, respectively, when compared with the blank sample. Furthermore, the results of the corrosion resistance test showed that the composites performed poorly against polar organic and alkaline solutions, presented good corrosion resistance against acidic media, and behaved relatively stable in the salt solution.

1. Introduction

Composite materials are created by blending multiple materials with distinct properties to enhance their physical characteristics, surpassing those of the individual components [1]. Unsaturated polyester resin (UPR) and its composite materials have been widely used in agriculture, industry, the construction field, as well as the military industry, due to their ease of processing, high chemical resistance, and comparatively low cost, which contribute to the popularity of composite materials [2,3]. The considerable degree of cross-linking among individual polymer chains in UPR imparts a range of advantageous characteristics, including a higher glass transition temperature, elevated modulus, high specific strength, creep resistance, and excellent tolerance to solvents [4,5].

However, like other thermosetting materials, UPR has some shortcomings, such as brittleness, low modulus, and cracking deformation caused by volumetric shrinkage; these characteristics restrict their usage in products that demand exceptional impact and fracture strength. Research has been undertaken to modify UPR to improve its mechanical, thermal, and chemical properties.

Incorporating fibers into the UPR matrix significantly improves the overall mechanical strength of the composites as compared to the pure polymer. The mechanical properties of fiber-reinforced combined materials are influenced by a variety of factors, including the type and quantity of fibers, fiber distribution, fiber orientation, and void content, which collectively contribute to the material’s performance. Currently commonly used reinforcement fibers include glass fiber, basalt fiber, carbon fiber, and natural fiber. Among them, glass fiber composites are used in the widest range of application among fiber-reinforced materials due to their favorable mechanical performance, excellent corrosion resistance, weight-saving impact, higher impact, greater tensile strength, and lower cost [6]. In addition to these characteristics, the kind of interfacial connections and the load transfer processes at the interphase are crucial. Therefore, good interfacial adhesion is of great importance to ensure the full strength of chopped glass fiber [7,8,9,10,11]. Varga et al. [6] explored enhancing the mechanical properties of glass fiber-reinforced polyester composites through modifications to the fiber surface. Their research identified new additives that address the compatibility issues commonly associated with glass fiber-reinforced polyester composites.

In recent years, research has been conducted on the thermal properties of glass fiber-reinforced unsaturated polyester composites. The results show that, compared with the continuous fiber, the chopped fiber has better isotropy, while the glass microcrystals in the continuous glass fiber are oriented to different degrees along the fiber axis, and the axial thermal conductivity is much larger than the radial thermal conductivity, showing obvious anisotropy. Moreover, the hybridization of natural fiber with glass allows a significantly better heat transport ability of the composites; the mechanical strength of the composites is improved under 100 °C, while the strength and ductility loss over 250 °C as the higher temperature causes some fractures of the fiber–matrix interfaces [12,13]. Errajhi et al. [14] conducted a study on the water absorption behavior of aluminized E-glass fiber-reinforced unsaturated polyester composites. While these composites demonstrated outstanding resistance to water absorption at room temperature, their water absorption characteristics changed significantly when exposed to elevated temperatures of 60 °C. The presence of the aluminum coating affected the bond interface at higher temperatures, resulting in altered water absorption behavior.

Although fiber-reinforced UPR composites reveal high mechanical properties, the interface debonding defect between the fiber and the matrix will still occur, caused by the mismatch of different component responses to out-loading and heat [15]. In recent years, nanoparticles have been used as advanced fillers to improve the mechanical and physical properties of the UPR. Different types of nanoparticles, such as TiO₂, Al₂O₃, graphene, and SiO₂, are used to improve the properties of the UPR matrix. Efrat et al. [16] carried out the study about the dielectric behavior of thin films of unsaturated polyester-resin/carbon nanotube semiconductor composites. Nidhin et al. [17], using the modified Hummers method to synthesize Graphene oxide to modify UP, found that the fabricated nanocomposites improved tensile strength augmentation by 75.2% with 0.08 wt% f-GO. S. C. Qi et al. [18] found that when the filling amount of nano-Al₂O₃ particles increased from 6% to 8%, there was an obvious brittleness and toughness transition, and the tensile strength and bending strength of the composite gradually increased with the increase in the filling amount of nano-Al₂O₃, reaching the maximum at 8%. In terms of thermodynamics, the glass transition temperature of the composite material gradually increased with the increase in the filling amount of nano-Al₂O₃ and reached a peak when the filling amount reached 8%. Y. Zhang et al.’s [19] research showed that the addition of nano-SiO₂ particles to the UPR matrix could significantly improve the tensile and bending properties of the composite. When the content of nano-SiO₂ was 6%, the tensile and bending properties of the composite were the best, and then tended to decline with the increase in the filling amount. When the amount of modified nano-SiO₂ increased from 4% to 6%, the impact strength showed an obvious brittleness and toughness transition, and then the impact strength gradually decreased with the increase in the filling amount. Inorganic nanofillers are highly promising due to their availability, low fabrication cost, and the ability to optimize mechanical and thermal properties during the design stage. Incorporating nanoparticles in the matrix enhances mechanical performance by reducing matrix cracking and delamination, thanks to improved interfacial adhesion between particles and the matrix [20]. The resulting composite material exhibits desirable hardness, impact resistance, flexural properties, and thermal stability that differ significantly from conventional composites. The use of small particle sizes in epoxy matrices influences the deformation mechanism and is believed to contribute to toughening epoxy resins through a combination of mechanisms [21]. Among various nanoparticles used to improve epoxy resin properties, TiO₂ shows particular promise. Hamad et al. [22] demonstrated that TiO₂-epoxy composites with particle sizes of 17 nm, 50 nm, and 220 nm, at a TiO₂ fraction of 1 wt.%, exhibited respective increases of 22%, 17%, and 14% in flexural strength compared to the absence of TiO₂. The maximum improvement in flexural properties was observed at a lower TiO₂ fraction of 1 wt.%. Y. H. Xiao et al. [23] studied nano-TiO₂ as applied to the modification of unsaturated polyester resin. The research results showed that the mechanical properties of UP were improved. When the amount of TiO₂ was 4 wt.%, tensile strength and bending strength increased by 43% and 173%, respectively. The tensile and the bending elasticity modulus increased by 22% and 12%. More C.V. et al. [24] prepared a new nano-TiO₂ enhanced UPR composite material which could be used as a nuclear radiation shielding material. It was found that the mechanical properties, structural properties, and nuclear shielding properties of the composite were improved with the addition of TiO₂ and increased with the increase in TiO₂ content. When the addition of TiO₂ reached 20%, the composite had the best shielding ability for gamma rays. Mirabedini et al. [25] investigated the surface modification of TiO₂ nanoparticles using a silane coupling agent. Ribeiro et al. [26] examined a new kind of unsaturated polyester-based combined materials using enhanced fire retardancy through a modified polymer matrix with nano/micro-oxide particles, considering their fire and flexural mechanical behaviors. They found that, compared to the neat or control formulation, the modified formulations exhibited an average decrease of 29% and 54% in flexural and impact strengths, respectively, while also observing an average increase of 19% in the elastic modulus. Chieruzzi et al. [27] discovered that incorporating clay particles in unsaturated polyester resin reduces the coefficients of thermal expansion. In recent years, there has been an increasing number of studies focusing on the use of nano-TiO₂ and fiber-reinforced unsaturated polyester resin. Fan et al. [28] proposed that adding nanofillers to glass fiber-reinforced unsaturated polyester resin composites could potentially address pore/void closure and enhance the interface/interphase strength. Nayak et al. [29] investigated the water absorption behavior, mechanical properties, and thermal properties of TiO₂ nanoparticle-enhanced glass fiber-reinforced polymer composites. Their findings revealed that the addition of TiO₂ effectively decreased water diffusion coefficient, increased flexural strength, and improved interlaminar shear strength in a hydrothermal environment.

The brief review shows that research works about toughening and reinforced UPR have been carried out to expand its application range by overcoming its shortcomings, such as brittleness, low modulus, product warpage, and cracking deformation caused by volume shrinkage. However, there is a scarcity of studies investigating the impact of varying TiO₂ nanoparticle and chopped glass fiber content on the mechanical, thermal, and corrosion resistance properties of the composite material, as indicated by the existing literature. Therefore, the objective of this paper was to examine the influence of key parameters, specifically the content of TiO₂ nanoparticles and glass fiber, on mechanical and thermal properties such as impact strength, flexural strength, and heat deflection temperature. Additionally, the chemical corrosion resistance of the composite material in different corrosive environments was investigated and analyzed.

2. Materials and Method

2.1. Materials

Rutile TiO₂ nanoparticles with an average size of 30 nm, a melting point of 1970 °C, a weight of 300 GSM, and a density of 5.58 g/cm³ were provided by Nanjing Haitai Nano Material Co. Ltd. (Nanjing, China). The particle size distribution curve of nano-TiO₂ is showed in Figure 1. The concentrated distribution range is 2540 nm. Unsaturated polyester resin (density: 1.1~1.2 g/cm³) containing 42% styrene branded as “Number 191”, was provided from Jinan Oasis Composite Material Co., Ltd. (Jinan, China) Chopped glass fiber with a 15 mm length, 13 ± 1 μm diameter, and 2.7 g/m³ density was provided by Shandong Taishan Fiberglass Co., Ltd. (Taian, China) Titanate coupler NXT-102, as a surfactant, was provided by Nanjing Xiangfei Chemical Research Institute (Nanjing, China).

2.2. Sample Preparation

The nano-titanium dioxide was first modified by titanate during the sample preparation process. The nano-TiO₂ modified by adding 3% titanate with the best modification effect was selected [30]. Then, varying weights of modified TiO₂ nanoparticles were added to a specific amount of UPR matrix. The modified TiO₂ nanoparticles were thoroughly mixed with the UPR matrix through stirring to ensure uniform distribution. The chopped glass fiber was added into the mixture and mixed fully to ensure uniform dispersion of TiO₂ and glass fibers by mechanical stirring. Afterward, 1% curing agent, 0.2% accelerator, and defoamer were added to the mixture and thoroughly mixed. The resulting mixture was then cast into a mold and covered with cellophane, ensuring a tight seal. The composite specimens were cured at room temperature for 24 h, followed by a post-curing process. The sample was put into the oven in the laboratory at 45 °C for 2 h, at 80 °C for 3 h, and then at 95 °C for 2 h. Five replicate experiments were performed to get the average data in this study. After taking the above procedures, the sample was cooled naturally for different tests.

2.3. Test Method

2.3.1. Impact Strength

The impact strength test of the nano-TiO₂/chopped glass fiber/UPR composites was conducted in accordance with the standard GB/T2567-2021 [31]. The dimensions of the composites, including the width and thickness, were measured as 120 mm and 70 mm, respectively. The impact strength of the composite materials was determined using a calculation method specified by the standard.

$${\alpha }_k=\frac{A}{b\times h} ,$$

(1)

where A represents the energy consumed of the impact specimens (measured in Jm), b corresponds to the width of the middle of the notch sample (measured in mm), h represents the thickness of the middle of the notch sample (measured in mm).

2.3.2. Flexural Properties

The flexural strength test of the composites is conducted in accordance with the standard GB/T2567-2021 [31]. The length and span of the composite sample are measured as 100 mm and 120 mm, respectively. The test speed for the composite sample is set at 3 mm/min. The flexural strength of the composite materials is calculated using a specific formula as outlined in the standard.

$${\mathsf{\sigma }}_f=\frac{3P\times L}{b{h}^2}$$

(2)

where σ_f represents the flexural strength (MPa); P is the maximum load (N); L is specimen’s span (mm); b is specimen’s width (mm); h represents specimen’s thickness (mm).

2.3.3. Tensile Strength

The tensile strength of the composites is determined following the standard GB/T2567-2021 [31]. The width of the composites is measured as 120 mm, and the thickness is measured as 70 mm. The tensile strength of the composite materials is calculated using a specific method described in the standard.

$${\mathsf{\sigma }}_t=\frac{P}{bh}$$

(3)

where σ_t means the tensile strength (MPa); P means the failure load (N); b means specimen’s width (mm); h is specimen’s thickness (mm).

2.3.4. Heat Deflection Temperature

The thermal deformation temperature of the composite sample was measured according to GB/T 1634.2-2019 [32]. After being applied the load, the maximum flexural normal stress of the specimens is recorded as 18.5 kg/cm², and the heating rate during the test is set at 2 °C/min.

2.3.5. Corrosion Resistance

The corrosion resistance of samples was measured in accordance with GB/T 3857-2017 [33]. Test items included the Barcol hardness and flexural strength retention rate. The hardness of samples was determined in accordance with GB/T 3854-2017 [33]. The sample based on the best compound is filled with 4 wt.% TiO₂ nanoparticles, 30 wt.% chopped glass fiber with a 15 mm length and 4 wt.% coupling agent. The sample was placed in various mediums, including 30% sulfuric acid, 5% hydrochloric acid, 10% sodium hydroxide, distilled water, acetone, and sodium chloride saturated solution. The test temperature was 80 °C.

2.3.6. SEM Test

The microstructure of the composite was examined using a JSM-5900 scanning electron microscope (SEM) were provided by JEOL (BEIJING) CO., LTD. (Beijing, China). The impact sections of the sample untreated, treated with TiO₂, 30% glass fiber, and 40% glass fiber were tested separately.

3. Results and Discussion

3.1. Mechanical Properties

3.1.1. Impact Strength

The impact strength of the composite material was analyzed in relation to the content of TiO₂ nanoparticles and chopped glass fiber, as depicted in Figure 2. The findings indicate that the impact strength initially increases and then decreases with the increasing TiO₂ content. In particular, the impact strength of unsaturated polyester resin reaches its maximum when the content of the nanoparticles is 4%. When a certain amount of TiO₂ nanoparticles is added into the UPR matrix as a stress concentration point, the composite material can absorb more energy. One possible reason for this trend is attributed to the small size and large specific surface area of nanoparticles. These characteristics provide numerous binding opportunities with the polymer chains, thereby enhancing the strength of the composite. Moreover, after surface treatment with titanate, a flexible interface layer with interfacial adhesion is formed between the nano-TiO₂ surface and the matrix resin UPR, which endows the matrix with flexibility and stress transfer function, thereby improving the impact strength of the composite system [30]. However, when the TiO₂ content exceeds a certain threshold, particle agglomeration may occur, making it challenging to achieve proper dispersion. As a result, the strength of the composite decreased.

As the chopped glass fiber and TiO₂ proportions increase, the impact strength shows a notable increase until reaching its peak value of 46 kJ/m² with a glass fiber content of 30%. However, beyond this point, with a 40% glass fiber content, the impact strength of the composites decreases. The significant improvement in impact strength with increasing chopped glass fiber content can be attributed to the increased stress borne by the individual glass fiber filaments within the composite. This leads to a substantial enhancement in strength. However, when the chopped glass fiber content exceeds a certain threshold, the amount of resin present becomes insufficient to adequately fill the voids created by the glass fibers. As a result, the interfacial bonding strength between the chopped glass fiber and unsaturated polyester resin weakens, leading to a decrease in the impact strength of the composites.

The best composition is the content of 4 wt.% TiO₂ nanoparticles and 30 wt.% chopped glass fiber with a 15 mm length, whose maximum impact strength is 46.1 kJ/m², which is an improvement of 207.5% compared to that without chopped glass fiber. Therefore, under the combined effect of chopped glass fiber and TiO₂ nanoparticles, the impact strength of the ternary composite system has been greatly improved.

3.1.2. Flexural Properties

Figure 3 illustrates the impact of TiO₂ nanoparticle and chopped glass fiber content on the flexural strength of the composite material. These findings show an initial increase followed by a decrease in flexural strength as the TiO₂ content increases. Specifically, the unsaturated polyester resin exhibits maximum flexural strength at a nanoparticle content of 4%. The initial enhancement in strength can be attributed to the favorable interaction between UPR and nanoparticles, which leads to the formation of a stronger interface bonding with the fiber, thereby improving the overall strength and toughness of the matrix. After modification with titanate, the dispersion of nano-TiO₂ is more uniform, reducing the agglomeration phenomenon, which is conducive to interface bonding and thus improving the bending strength of the composite material [30]. However, as the TiO₂ content increases, the Van der Waals force between nanoparticles becomes stronger, resulting in reduced dispersion of nanoparticles within the matrix. This aggregation of nanoparticles decreases the active surface area available for interaction with the UPR matrix, leading to a decline in flexural strength. Additionally, it hinders the transfer of load from the matrix to the fiber, further contributing to the decrease in flexural strength [26].

The flexural strength of the composites shows a significant increase with an increase in the percentage of chopped glass fiber, reaching its maximum at a content of 30%. However, when the content of glass fiber exceeds 30% and reaches 40%, the flexural strength of the composites starts to decrease. During the flexural test, the stress is distributed to each crack branch, resulting in a reduction of stress concentration at the main crack tip. This reduced stress concentration at the main crack tip leads to fiber extraction, fiber fracture, and matrix cracking being the main mechanisms of energy absorption in the fracture behavior of the chopped glass fiber-reinforced unsaturated polyester resin composites. Therefore, the increase in the content of chopped glass fiber makes it more challenging for the fibers to be pulled out, ultimately resulting in an increase in flexural strength.

The stress–strain curve of the flexural test of the sample with the optimal TiO₂ dosage of 4% is shown in Figure 4. It can be seen that within the range of 10~30% glass fiber content, as glass fiber content increases, the strain of the specimen under the same stress gradually decreases, while its flexural elastic modulus, namely toughness, gradually increases. When the stress is 60 MPa, the strain of the specimen with 30% glass fiber content is nearly 90% lower than that with 10%, indicating that the 30% glass fiber specimen has greater resistance to elastic deformation under the action of external force. However, when the glass fiber content is 40%, the flexural strength of the specimen decreases obviously. This shows that suitable glass fibers can be fully infiltrated by resin and evenly dispersed in UPR to assume and transfer flexural stress, thus improving the overall flexural strength of composite materials; while excessive glass fiber is added, the fibers cannot be fully infiltrated by resin, resulting in a decrease in flexural strength.

The best compound is the content of 4 wt.% TiO₂ nanoparticles and 30 wt.% chopped glass fiber with a 15 mm length, whose impact strength is 90.1 MPa, which is an improvement of 80.3% compared to that without chopped glass fiber. Under the combined effect of chopped glass fiber and TiO₂ nanoparticles, the flexural strength of the ternary composite system has been greatly improved.

3.1.3. Tensile Strength

Figure 5 illustrates the impact of different contents of TiO₂ particles and chopped glass fibers on the tensile strength of the composites. It is evident that the addition of TiO₂ and chopped glass fiber leads to an improvement in the tensile strength of the unsaturated polyester resin (UPR). As the TiO₂ content increases from 2% to 5%, the tensile strength of the composites shows a gradual increase, followed by a sharp decrease. At a TiO₂ content of 4% by weight, the composites achieve the highest tensile strength of 48.5 MPa, which is 19.6% higher than that of pure UPR. This enhancement can be attributed to the small size and large specific surface area of the TiO₂ particles, which possess numerous defects which further promote the physical combination between particles and UPR polymer chains. This will improve the adhesive force between them, thus enhancing the tensile strength of the resin. However, as the TiO₂ particles are small in size, they might agglomerate easily. When the concentration of TiO₂ particles increases, the particles agglomerate and the dispersion effect is poor, which cannot play the role of the stress concentration center and induce the silver shear band, thus reducing the overall tensile strength. After adding glass fiber, the tensile strength of the composite is greatly improved. When the TiO₂ content is 4%wt and glass fiber content is 30%, the tensile strength attains 78.8 MPa, which is the maximum value with 62.5% higher than that without glass fiber.

3.1.4. SEM Analysis

SEM analysis was conducted on the cross-section of the fractured impact samples after gold spraying. Figure 6 presents the SEM image capturing the fracture surface of the chopped fiber/TiO₂/UPR composites. It can be seen that after TiO₂ is added, the section of the TiO₂/UPR system become rough. Lamellated fault zones appear. There are many spots and a few clumpy TiO₂ particles evenly dispersed in the UPR matrix. The absence of large TiO₂ aggregates indicates that TiO₂ has a good dispersion effect in the UPR composite. Moreover, it can also be observed that the fracture of impact samples was added with TiO₂. The pinning effect of particles and the role of passivating the crack tip can be seen everywhere. The fracture has a certain roughness and the material shows a special comb-like tearing failure. Due to the stress concentration, the matrix yields obvious plastic deformation and shear bands appear. Figure 6b,c display SEM images of the fractured surfaces of the chopped glass fiber/TiO₂/UPR composite samples. These images were captured at different magnifications, showcasing the microstructural details of the composites; the samples used for analysis had a length of 15 mm and a glass fiber dosage of 30%. It can be seen that the glass fibers in the matrix are distributed randomly in the form of monofilament, instead of existing in the form of bundles. However, with content less than or equal to 30%, the glass fibers are still mainly enhanced. Figure 6d is the SEM photo of the section of the chopped glass fiber/TiO₂/UPR composite sample with a length of 15 mm and a dosage of 40%. It can be seen that the gaps between glass fibers can no longer be filled by resin due to the large content of chopped glass fiber. Therefore, the interface is defective, which will seriously affect the bonding strength of the interface and in turn, lead to more glass fibers being pulled out. Thus, the strength of the composite system decreases as the amount of glass fiber increases when the limit value is exceeded.

3.2. Heat Deflection Temperature

Figure 7 illustrates the impact of the TiO₂ nanoparticle and chopped glass fiber content on the heat deflection temperature of the composite. It can be seen that the optimal values of the heat deflection temperature are 121.5 °C when the content of the TiO₂ nanoparticles is 4% and chopped glass fiber is 30%, which improves 73.6% compared to that without chopped glass fiber.

The presence of nano-TiO₂ particles in the composite introduces defects on their surface. These particles are small in size and large in specific surface area, which allows more interactions with the polymer chain. As a result, the polymer chain becomes more rigid and less mobile due to the increased number of cross-linking points. This enhanced rigidity restricts the movement of the polymer chains, leading to improved mechanical properties, such as increased stiffness and strength in the composite material.

The heat distortion temperature tends to rise. The higher percentage of chopped glass fiber hinders the thermal motion of the UPR molecular chain, requiring an increase in temperature for the polymer chain to move freely. Consequently, the heat deflection temperature of the UPR matrix increases as the temperature rises, indicating greater resistance to thermal distortion. Furthermore, with the high glass transition temperature of the nano-TiO₂ and the high softening point temperature of the chopped glass fiber, the addition of nano-TiO₂ and chopped glass fiber into the UPR matrix increased the heat deflection temperature as well.

3.3. Corrosion Resistance

3.3.1. The Retention Rate of Flexural Strength

Figure 8 illustrates the decline in flexural strength of the composite samples across various media. The sample is relatively stable in the saturated NaCl solution and the flexural strength decreases slightly. For reinforced composite, in the acid medium, the flexural strength decreases and the corrosion process is slow because the degradation of the resin in the acid medium is acid hydrolysis; the flexural strength reduces most significantly in the acetone and alkaline medium such as the NaOH solution. The biggest decrease is in the sodium hydroxide solution, while the composite is the most stable in the saturated sodium chloride solution. The decrease in the sodium hydroxide solution is 100% at 28 days compared with the saturated sodium chloride. In addition, it decreases rapidly in acetone within a month. The residual flexural strength decreases by 22.9% at 28 days in the saturated NaCl solution, which is relatively stable. The fact is that the salt solution does not react chemically with the UPR; there is only slight physical swelling. The sample soaked in the HCl and H₂SO₄ solution shows a slow decrease in strength and hardness. The main reason is that the degradation of resin in an acid medium is acid hydrolysis; it is a reversible reaction and will reach equilibrium finally. The sample soaked in the NaOH solution exhibits a dramatic decrease in strength and hardness, and degradation of the resin in an alkaline medium, which produces a stable acidion.

The retention rates of flexural strength of non-reinforced samples have a more significant decrease compared with reinforced composites, especially in Sodium hydroxide solution, which are 4.5% and 8.4%, respectively. Due to water molecules penetrating and diffusing into the interior of the resin through non-uniform structures, voids, microcracks, and other defects, which will lead to the generation and propagation of microcracks, the strength of non-reinforced samples also has a significant decrease in distilled water. The residual flexural strength of reinforced resin and non-reinforced resin in distilled water is 82.2% and 69.1%, respectively, at 7 d. The residual flexural strength of the reinforced resin decreases sharply in acetone due to swelling of the sample, and the retention rates of flexural strength of both are 58.6% and 34.2% at 3 d, respectively.

3.3.2. The Retention Rate of Barcol Hardness

Figure 9 presents the Barcol hardness of composite samples in all different media. It can be seen that the sample is relatively stable in the saturated NaCl solution and the Barcol hardness decreases slightly. In the acid medium, the Barcol hardness decreases, and the corrosion process is slow. The main factor behind this phenomenon is the resin’s degradation through acid hydrolysis, resulting in a significant reduction in Barcol hardness. The most substantial decrease in Barcol hardness is observed in acetone and alkaline mediums, such as the NaOH solution. Samples soaked in water show a decrease in strength and hardness. This can be explained, as microcracks generate and expand when the water penetrates the resin matrix through the non-uniform structure of the resin, voids, microcracks, and other defects. The resin ester bond hydrolyzes and the molecular chain breaks with the infiltration of water molecules; the infiltration of water molecules diminishes the bond strength between the resin and the fiber, as well as between the resin and the nanoparticles at the interface. As a result, the interface experiences debonding, leading to a decrease in bond strength.

Samples soaked in acetone show a decrease in strength and hardness. When polar groups such as hydroxyl and carboxyl groups in UPR interact with acetone, the infiltration and diffusion of acetone lead to the swelling of the sample and physical corrosion. Acetone has the ability to permeate to the interface between the matrix and fillers, leading to the dissolution of the coupling agent present on the surface of TiO₂ nanoparticles and chopped glass fiber.

There is little tendency to react with the alcohol and the reaction is irreversible. Therefore, the alkali resistance of the resin is very poor, and the surface corrosion reaction is very fast. The medium infiltrates the interface between the chopped glass fiber and the resin, causing damage to the resin matrix and reducing the adhesion between the resin and the fiber. The medium further infiltrates through the interfacial voids via capillary action, leading to the occurrence of interface debonding.

Before and after reinforcement, the hardness retention of specimens in alkali solution was very low, and the Barcol values are 9.1% and 20.2% at 3 d, respectively, due to poor alkaline resistance of the resin; its surface corrodes quickly. The retention rates of Barcol hardness of the reinforced and nonreinforced resin are 75.1% and 68.2% at 7 d in distilled water because of the hydrolysis of resin ester bonds caused by the infiltration of water molecules. Due to the coupling agent layer on the surface of nano-TiO₂, and chopped fibers being soluble in acetone, the interfacial bonding is severely disrupted. The retention rates of Barcol hardness of the reinforced and nonreinforced resin decrease sharply, which are 35.1% and 21.2% at 3 d.

4. Conclusions

The experimental investigation into the properties of UPR reinforced with TiO₂ nanoparticles and chopped glass fiber yields the following findings:

(1)

When incorporating TiO₂ nanoparticles and chopped glass fiber into the URP matrix, the impact strength, flexural strength, and heat deflection temperature of the composite material are enhanced. Specifically, when the TiO₂ nanoparticle content is 4% and the chopped glass fiber content is 30% of the UPR matrix, with a length of 15 mm, the maximum impact strength of the URP composite reaches 46.1 kJ/m², representing a significant improvement of 207.5%; the flexural strength reaches a maximum of 90.14 MPa, which is an improvement of 80.3%; the maximum tensile strength is 78.8 MPa, which is an improvement of 62.5%; and the thermal deflection temperature reaches its maximum of 121.5 °C, which is an improvement of 73.6% compared to that without chopped glass fiber. Moreover, within the range of 10~30% glass fiber content, as glass fiber content increases, the strain of the specimen under the same stress gradually decreases, while its elastic modulus, namely toughness, gradually increases. SEM analysis shows that the fracture of the impact samples has good roughness after adding nano-TiO₂, and the material exhibits special comb tearing damage. Within a certain proportion, the glass fiber mainly plays a reinforcing role.

(2)

The chemical corrosion resistance of the TiO₂ nanoparticles/chopped glass fiber/UPR composite against polar organic and the alkaline solution is poor; the URP composite presents good corrosion resistance against water and acidic media and behaves relatively stable in the salt solution. The main reason is that the degradation of resin in an acid medium is acid hydrolysis; the sample soaked in an alkaline solution exhibits a dramatic decrease in strength and hardness.

(3)

The incorporation of chopped glass fiber and TiO₂ nanoparticles enhances the overall performance of URP, resulting in improved mechanical and durability properties. In the case of chopped glass fiber-reinforced resin, the stress exerted on the resin is transferred to the fiber through the fiber–resin interface, making the fiber the primary load-bearing component. On the other hand, the toughening mechanism associated with inorganic nanoparticles involves the generation of stress concentration during deformation, which leads to localized yielding of the resin matrix surrounding the particles. This yielding process absorbs significant amounts of deformation energy, thereby contributing to the toughening effect.