Preparation of High-Performance Polyaspartic Polyurea and Application in Hydraulic Concrete Protection

TL;DR

This study develops a high-performance polyurea coating that protects hydraulic concrete from water, chloride ions, and carbonation, showing great potential for infrastructure durability.

Contribution

The novel design of PAE-PTMEG coating with tailored soft segments significantly improves reactivity, mechanical strength, and environmental resistance.

Findings

PAE-PTMEG coating achieved high tensile strength (43.8 MPa) and elongation (646.1%) due to its microphase-separated structure.

The coating showed excellent water resistance (<1% absorption) and exceptional chloride ion resistance (1.3 × 10−4 mg·cm−2·d−1).

PAE-C paint demonstrated complete carbonation resistance and high frost resistance (200 cycles).

Abstract

The long-term durability of hydraulic concrete infrastructure is severely compromised by water penetration, carbonation, and chloride ion erosion, necessitating the development of high-performance protective coatings. This study designed two polyaspartic ester polyurea coatings, PAE-PTMEG and PAE-PPG, derived from isocyanate prepolymers with polytetramethylene ether glycol (PTMEG) and polypropylene glycol (PPG) soft segments, respectively. The results demonstrated that the PTMEG-based prepolymer exhibited higher reactivity, leading to shorter curing times. The resulting PAE-PTMEG coating showed outstanding mechanical properties (tensile strength: 43.8 Mpa; elongation: 646.1%) and excellent water resistance (<1% absorption), attributable to its well-defined microphase-separated structure. When formulated into a practical paint (PAE-C), it surpassed mechanical standards for waterproofing and demonstrated exceptional resistance to chloride ion penetration (1.3 × 10−4 mg·cm−2·d−1), complete carbonation resistance, and high frost resistance (200 cycles). This work confirms that tailoring the soft segment structure is a crucial strategy for developing durable polyurea coatings, with PAE-PTMEG showing significant potential for protecting critical water conservancy infrastructure.

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Full text

1. Introduction

The concrete structures of water conservancy infrastructure (e.g., dams, sluice gates, and water conveyance tunnels) face multiple threats during long-term service in harsh environments, including water penetration [1,2], carbonation [3,4], and chloride ion erosion [5,6,7]. These factors lead to concrete neutralization and internal reinforcement corrosion, significantly compromising the structural load-bearing capacity and durability [8]. Therefore, applying high-performance protective coatings on concrete surfaces is widely recognized as an economical and effective strategy to block harmful agents and extend structural service life [9].

An ideal protective coating for concrete, especially in hydraulic engineering, must meet multiple stringent requirements. First, it must possess excellent barrier properties to effectively resist the penetration of water, chloride ions, and carbon dioxide [10,11]. Second, it requires outstanding mechanical properties, including high strength, high toughness, and good adhesion, to withstand water flow erosion, sediment abrasion, and structural deformation [12]. Furthermore, superior durability is essential to resist long-term exposure to ultraviolet radiation [13], hygrothermal alternation, freeze–thaw cycles [14], and chemical media. Finally, a suitable application window is crucial for ensuring film-forming quality and efficiency in large-scale construction [15].

Polyaspartic ester polyurea has emerged as a novel high-performance coating material following epoxy resins, traditional polyurethanes, and first-generation polyurea, successfully overcoming many limitations of conventional materials. Its core technology lies in the tunable molecular structure of the polyaspartic ester curing agent. By introducing steric hindrance through substitution on the amine group, the reaction rate with isocyanates can be precisely controlled from several minutes to hours. This enables the material to retain the rapid curing and good weather resistance characteristic of polyurea, while providing sufficient pot life for convenient application [16]. Moreover, molecular design enables the formulation of solvent-free PAE-polyurea systems, meeting environmental requirements and making it an ideal raw material for eco-friendly coatings. Simultaneously, PAE-polyurea has demonstrated excellent protective performance in several major projects in recent years. For instance, the “water-immunity polyurea” concrete surface protection material developed by the Changjiang River Scientific Research Institute has been successfully applied in numerous large- and medium-sized water conservancy projects such as the Danjiangkou Reservoir, the Middle Route of the South-to-North Water Diversion Project, and the Xiluodu Hydropower Station [17]. This material was “immune to degradation by water in its service environment, possessing water immunity”, significantly reducing the impact of the service environment on material application and performance.

Despite significant advancements in PAE-polyurea technology, current research primarily focused on modifications of the curing agent [18] or filler composites [19]. However, the final coating properties depended not only on the curing agent but also critically on the structure of the isocyanate prepolymer, particularly the chemical structure of the soft segment polyol forming its backbone. Among various polyols, polytetramethylene ether glycol (PTMEG) and polypropylene glycol (PPG) are the two most commonly used soft segment raw materials with distinctly different structures [20]. Although comparisons between PTMEG and PPG exist in the broader polyurethane/urea literature, systematic studies within the context of PAE-polyurea-a system prized for its rapid cure and weatherability-are limited. More importantly, prior work often focuses on general material properties, leaving a significant gap in understanding how these soft segments govern the curing kinetics, structure–property relationships, and ultimately [21], the critical barrier performance (e.g., against chlorides and carbonation) required for durable hydraulic infrastructure protection. These intrinsic molecular-level differences were expected to lead to significant distinctions in the following aspects. Nevertheless, systematic comparative studies and mechanistic analyses on the specific effects of PTMEG versus PPG soft segments on the PAE-polyurea system, spanning from prepolymer reactivity and microphase separation to final barrier properties and long-term protective efficacy, remain insufficient. Research that closely integrates this understanding with the specific needs of water conservancy infrastructure protection is particularly scarce.

Based on this, the present study aimed to design and synthesize two IPDI-based isocyanate prepolymers using PTMEG and PPG as soft segments, respectively, and subsequently prepared high-performance polyurea coatings with a polyaspartic ester curing agent. The work focused on the following aspects: (1) Characterizing the fundamental properties and reactivity of the two prepolymers, and evaluating their application windows; (2) Systematically comparing the mechanical properties, water resistance, and weatherability of the two cured coatings, and explaining the origin of the performance differences from the perspective of molecular structure and microphase separation; (3) Selecting the coating formulation with the optimal comprehensive performance and evaluating its key capabilities in resisting carbonation and chloride ion penetration when applied for concrete protection, thereby validating its practical application potential in the waterproofing and protection of water conservancy infrastructure.

2. Materials and Methods

2.1. Materials

Poly(tetrahydrofuran) ether polyol (PTMEG), analytical grade, Scoray New Materials Technology Co., Ltd., Linyi, Shandong, China; Polypropylene glycol ether polyol (PPG), analytical grade, Guangzhou Xuxiang Chemical Co., Ltd., Guangzhou, Guangdong, China; Isophorone diisocyanate (IPDI), analytical grade, Wanhua Chemical Group Co., Ltd., Yantai, Shandong, China; Dibutyltin dilaurate (DBTDL), analytical grade, Nantong Bonna Chemical Technology Co., Ltd., Nantong, Jiangsu, China; Polyaspartic acid ester (F421), analytical grade, Zhuhai Feiyang New Materials Co., Ltd., Zhuhai, Guangdong, China. All reagents listed above were used without further purification.

2.2. Preparation of Novel Curing Agents

The preparation of the highly water-resistant curing agent is described in Figure 1a. Add PTMEG or PPG (0.5 mol) to a dry reactor and dehydrate under vacuum at 120 °C for 2 h. Subsequently, the temperature was lowered to 60 °C, the vacuum was relieved with nitrogen gas, and IPDI (1.0 mol) was added. The -NCO: -OH ratio was controlled at 2.0:1. A small amount of DBTDL was added dropwise, and the temperature was slowly raised to 80 °C. The mixture was reacted for 2 h, and additional IPDI (0.5 mol) was added to achieve a total -NCO: -OH ratio of 3.0:1. The temperature was raised to 90 °C and reacted for 3 h until the -NCO content reached the theoretical value. Pump the prepolymer into a rotary evaporator under controlled parameters. The product (discharged from the bottom, cooled to 50 °C for storage) constitutes the novel curing agents, namely PTMEG-IPDI and PPG-IPDI.

2.3. Preparation of Polyaspartic Ester Polyurea Films

F421 resin was separately blended with two curing agents: PTMEG-IPDI and PPG-IPDI. Each mixture underwent vacuum degassing to eliminate bubbles. The uniformly dispersed prepolymers were then cast and cured, followed by 14 days of curing at room temperature to yield polyaspartic ester polyurea films designated as PAE-PTMEG and PAE-PPG.

2.4. Preparation of Polyaspartic Ester Polyurea Coating

The polyaspartic ester polyurea coating employs a two-component system comprising a base agent (Component A) and a curing agent (Component B), as detailed in Table 1. The polyaspartic ester polyurea coating was obtained by uniformly mixing Component A and Component B in a 1:1 ratio. This mixture was applied to the pre-treated concrete surface to form a polyaspartic ester polyurea topcoat system, denoted as PAE-C.

2.5. Methods

Viscosity was measured in accordance with ASTM D2196 using a Brookfield DV2T rotary viscometer (Brookfield Engineering Laboratories, Middleborough, MA, USA), at a constant temperature of 25.0 ± 0.1 °C. Each sample was measured three times, with the average value recorded.

-NCO content was determined by titration using the di-n-butylamine method. A precise weight of prepolymer was dissolved in anhydrous toluene and reacted with an excess of di-n-butylamine/toluene solution. This was back-titrated with a standard hydrochloric acid solution to calculate the percentage of unreacted -NCO groups.

Solid content was determined according to ASTM D2369. Approximately 1.0 g of sample (denoted as m_1_) was accurately weighed onto a pre-weighed aluminum foil pan (denoted as m_0_) and heated in a forced-air oven at 105 °C for 2 h. After removal, the sample was cooled to room temperature in a desiccator and reweighed (denoted as m_2_). The solid content was calculated using the following formula: solid content (%) = [(m_2_ − m_0_)/(m_1_ − m_0_)] × 100%. The result is the average of three parallel tests.

The functional group structure of the compound was qualitatively analyzed using Fourier Transform Infrared Spectroscopy (Nicolet IS10 FTIR) from Thermo Fisher Scientific (Waltham, MA, USA). The chemical structure of the material was further characterized using ATR-FTIR.

Pot life was determined according to ISO 9514 [22]. The mixed sample was placed in a cup, and its viscosity change was measured periodically using a rotary viscometer at 25 °C. The pot life was defined as the time taken for the viscosity to increase to 200% of its initial value.

Specific gravity was determined according to ASTM D1475 [23]. using the hydrometer cup method (weight cup method) at a constant temperature of 25.0 ± 0.5 °C. Employ a clean, dry metal hydrometer cup and accurately weigh its mass (denoted as m_0_). Fill the pycnometer with the test sample (prepolymer liquid), ensuring no air bubbles are present. Secure the stopper and allow excess liquid to overflow through the stopper hole. Wipe the outer surface of the pycnometer clean and precisely weigh the total mass (denoted as m_1_). The sample’s specific gravity (ρ) was calculated using the following formula: ρ = (m_1_ − m_0_)/V. The result is the average of three parallel tests.

Surface drying time and full cure time were determined using the cotton-ball method per national standard GB/T 1728-1979 (1989) [24]. Under standard conditions (23 ± 2 °C, 50 ± 5% RH), the sample was applied by scraping onto a tinplate (wet film thickness 150 μm). Using tweezers, a cotton ball (approximately 5–6 mm diameter) was gently placed upon the coating surface. The tester blows horizontally upon the cotton ball from a distance of 100–150 mm. The time elapsed from application completion until the cotton ball was blown off without leaving fibers on the coating surface was recorded. Dry time: Determined using the cotton ball pressure method per National Standard GB/T 1728-1979 (1989). Under the same environmental conditions as above, place a cotton ball on the coating surface and lightly press a dryness tester onto it. After holding for 30 s, remove the weight and cotton ball. Record the time from coating application completion until no lint residue or gloss loss was visible on the coating surface.

Gel time was determined according to ASTM D2471 [25] using a gel timer. A thoroughly mixed sample was placed in a test cup maintained at 25 °C. The instrument oscillates or rotates a probe at a fixed frequency, recording the time from mixing commencement until the resin loses fluidity, the probe amplitude increases significantly, or the probe breaks.

UV-vis transmittance was measured using a Shimadzu UV-2600 UV-vis spectrophotometer (Shimadzu Corporation, Kyoto, Japan). Scanning was performed within the 200–800 nm wavelength range, with air as the reference medium.

X-ray diffraction analysis was conducted using a Rigaku Smart Lab XRD instrument (Rigaku Corporation, Tokyo, Japan). The scanning speed was set at 5° min^−1^, with the typical scanning range (2θ angle) spanning 5–60°.

Tensile properties were evaluated according to ASTM D412 Type V [26] using a universal testing machine to determine tensile strength and elongation at break. The tensile speed was typically set at 50 mm·min^−1^. At least three valid specimens per formulation were tested, with results averaged.

The fracture energy test was conducted by the universal testing machine using the single-edge notched sample (length of the slit is 1 mm).

Tear resistance was tested on a universal testing machine per ASTMD624 standard [27] using a right-angle specimen.

Water resistance was achieved by immersing a film specimen of known weight (W_0_) and dimensions completely in deionized water and maintain at room temperature for a specified duration. After removal, gently blot surface moisture with filter paper and immediately weigh (W_1_). Subsequently, dry the sample in a vacuum oven at 60 °C until constant weight (W_2_). Calculate mass change rate and volume swelling rate: Water absorption rate (%) = [(W_1_ − W_2_)/W_2_] × 100%. The result is the average of three parallel tests.

UV aging was conducted per ASTM G154 [28] using an accelerated aging chamber. Employing UVA-340 lamps (Q-Lab Corporation, Westlake, OH, USA), the cyclic conditions were as follows: 8 h of UV exposure at 60 °C followed by 4 h of condensation at 50 °C, repeated cyclically. Samples were periodically inspected for appearance changes and tested for retention of mechanical properties.

For concrete protective coatings, in accordance with the relevant standards of JTJ 275-2000 [29], the penetration of chloride ions within the coating and the carbonation process shall be assessed.

3. Results

3.1. Preparation and Characterization of Novel Isocyanate Curing Agents

This study successfully synthesized two isocyanate prepolymers (curing agents) based on distinct soft segment structures: the PTMEG-IPDI prepolymer based on PTMEG and the PPG-IPDI prepolymer based on PPG. As depicted in Figure 1a, IPDI reacted with the terminal -OH groups of PTMEG or PPG. This addition reaction yielded prepolymers linked via urethane bonds and bearing terminal NCO groups. Both synthesized curing agents exhibited excellent transparency at room temperature, yet their rheological properties differed markedly (Figure 1a inset). The PTMEG-IPDI prepolymer was a highly viscous transparent liquid, while the PPG-IPDI prepolymer exhibited superior fluidity. The chemical structures of both prepolymers were verified using FTIR, with results shown in Figure 1b. Distinctly observable features included a broad characteristic N-H stretching vibration peak at approximately 3370 cm^−1^. A sharp, intense C=O stretching vibration peak of the urethane bond appeared around 1720 cm^−1^. Concurrently, a pronounced characteristic absorption peak for the isocyanate group (-NCO) persisted at approximately 2270 cm^−1^. Notably, comparison with the spectrum of the original polyol revealed the complete disappearance of the characteristic broad hydroxyl (-OH) peak at approximately 3450 cm^−1^. Collectively, these spectroscopic dates confirmed the thorough completion of the urethane reaction, successfully synthesizing the target prepolymer with terminal -NCO groups.

Systematic testing of key physicochemical parameters for both prepolymers yielded detailed data presented in Table 1. Quantitative characterization of the prepolymers’ flow behavior was performed using a rotary viscometer at 25 °C. The PTMEG-IPDI prepolymer exhibited an exceptionally high viscosity (22,420 mPa·s), while the PPG-IPDI prepolymer displayed a viscosity one order of magnitude lower (1780 mPa·s). This directly corroborates prior observations and quantifies the disparity between them. The -NCO contents of both were remarkably similar (6.64% and 6.50%), indicating comparable reaction completion and controllable molecular weight design. This provided an accurate metrological foundation for subsequent equivalent ratio reactions with aspartate resins, ensuring fair performance comparisons of the resulting polyurea materials. Solid content data revealed both were above 90%, aligning with the trend towards high-solid, low-VOC, and environmentally friendly coatings [30]. Their relative densities were essentially identical, indicating no significant volumetric differences.

In summary, we successfully synthesized two isocyanate prepolymers with similar structures yet markedly distinct physicochemical properties, particularly viscosity. The PTMEG core endowed the prepolymer with stronger intermolecular forces, manifesting as extremely high viscosity; conversely, the PPG core reduced segment regularity due to steric hindrance, yielding a low-viscosity, easily workable prepolymer. These fundamental structural differences laid the groundwork for subsequent investigations into the final properties of the polyurea materials obtained through reaction with aspartate esters.

3.2. Reaction Activity of Novel Curing Agents

The reaction kinetics of the isocyanate prepolymers with the polyaspartic resin dictated key application parameters like pot life [31]. Thus, they ultimately dictated the architecture of the final coating network. This study employed F421 resin (a typical polyaspartic resin with secondary amine functionality) to systematically evaluate the curing behaviors of PTMEG-IPDI and PPG-IPDI prepolymers regarding gel time, surface dry time, through-dry time, and pot life. The objective was to elucidate the influence of soft segment structure on reactivity.

As illustrated in Figure 2a,b, significant differences existed in the curing kinetic parameters between the two systems. The PTMEG-IPDI/F421 system exhibited a faster curing reaction rate, with a surface dry time of merely 2.1 h, a full cure time of 12.3 h, and a relatively short gel time of 3.1 h. In contrast, the curing times for the PPG-IPDI/F421 system were markedly extended: surface dry time increased to 3.3 h, through-dry time to 18.2 h, and gel time to 5.2 h.

This marked disparity in reactivity stems from differing diffusion and steric effects arising from the molecular structures of the prepolymers. The regular soft segments of PTMEG form robust hydrogen-bond networks which, whilst restricting overall motion, enhance the local effective collision frequency with amine groups. Conversely, the side methyl groups of PPG generated steric hindrance, shielding the -NCO groups, whilst its weak intermolecular forces result in a loosely distributed molecular arrangement, diminishing initial collision efficiency. Consequently, PTMEG-IPDI exhibited superior reactivity owing to stronger intermolecular interactions and reduced steric hindrance, resulting in shorter skin and gel times and enhanced application efficiency. PPG-IPDI, conversely, offered extended working time, rendering it suitable for complex application scenarios.

3.3. Structural Characterization of Polyaspartic Ester Polyurea

To investigate the influence of different soft segment structures in isocyanate prepolymers on the microstructure of the final material, we first synthesized two polyaspartic ester polyurea materials: the material prepared from PTMEG-IPDI prepolymer was denoted as PAE-PTMEG, while that prepared from PPG-IPDI prepolymer was denoted as PAE-PPG. Figure 3a clearly illustrates the chemical reaction pathway for this one-step polyurea synthesis. Its core mechanism involved the addition reaction between the terminal -NCO group of the isocyanate prepolymer and the secondary amine group (-NH-) in the aspartate ester resin, forming a urea bond (-NH-CO-NH-) and thereby establishing a crosslinked network structure.

The chemical structure of the cured film was verified using FTIR, with results presented in Figure 3b. The most critical evidence was the complete disappearance of the characteristic absorption peak attributed to isocyanate (-NCO) at ~2270 cm^−1^, indicating full consumption of the -NCO groups. Concurrently, distinct broad absorption peaks and intense absorption peaks appeared at ~3320 cm^−1^ and ~1640 cm^−1^, respectively. These correspond to the stretching vibration of -N-H in the urea bond and the stretching vibration of the carbonyl group (C=O, amide I band). Moreover, the absorption peak observed at ~1530 cm^−1^ (amide II band), arising from the coupling of N-H bending and C-N stretching vibrations, further confirmed the successful formation of the urea bond. Notably, the -N-H stretching peak in the PAE-PTMEG spectrum exhibited a slight shift towards lower wavenumbers, suggesting the potential formation of stronger intermolecular hydrogen bonds. This phenomenon correlated with the PTMEG’s regular chain segments, which favored the establishment of ordered structures. Collectively, these spectral features conclusively validated the successful construction of the target polyurea network.

From a macroscopic perspective, both materials exhibit excellent transparency (Figure 3c inset). UV-vis transmittance revealed that at the visible wavelength of 550 nm, both PAE-PTMEG and PAE-PPG exhibited transmittance exceeding 90% (Figure 3c). This high transparency directly reflected the materials’ exceptional internal homogeneity, with no discernible phase separation or scattering centers, indicating that both polyureas had formed uniform amorphous structures.

To investigate the aggregated structure of the materials in greater depth, XRD testing was conducted, with results shown in Figure 3d. The XRD patterns of both materials exhibited a characteristic broad peak without any sharp crystalline diffraction peaks. This feature unequivocally confirmed that both PAE-PTMEG and PAE-PPG possessed entirely amorphous structures. Although the PTMEG soft segment itself exhibited a certain crystallization tendency, its molecular chain motion was severely restricted after chemically bonding with the rigid aspartate ester hard segments to form a crosslinked network. This prevented regular arrangement into crystalline domains [32]. The PPG soft segment was inherently amorphous due to steric hindrance from its side methyl groups, resulting in PAE-PPG exhibiting a broader amorphous diffuse peak.

3.4. Mechanical Properties of Polyaspartic Ester Polyurea

As illustrated in Figure 4a, PAE-PTMEG exhibited remarkable strength and toughness, capable of effortlessly supporting loads exceeding 20,000 times its own weight. Consequently, tensile testing of the prepared PAE was conducted using a universal testing machine to investigate the influence of PTMEG and PPG structures on the mechanical properties of polyaspartic ester polyurea. As depicted in Figure 3b, tensile testing revealed markedly distinct mechanical behaviors between PAE-PTMEG and PAE-PPG (Figure 4b). PAE-PTMEG exhibited synergistic high strength and high toughness, achieving a tensile strength of 43.8 MPa, an elongation at break of 646.1%, and a fracture toughness of 95.9 MJ·m^−3^. In contrast, PAE-PPG behaved as a typical elastomer, exhibiting a tensile strength of 21.6 MPa but significantly higher elongation at break (1173.4%) and toughness of 73.0 MJ·m^−3^. The Young’s modulus of PAE-PTMEG (6.9 MPa) was substantially higher than that of PAE-PPG (1.4 MPa), indicating superior rigidity (Figure 4c). PAE-PTMEG’s outstanding mechanical properties make it ideal for waterproofing applications, offering significant advantages over conventional polyurethane/polyurea waterproofing materials, particularly its ultra-high strength of 43.8 MPa [33,34,35]. Damage tolerance was crucial for practical applications. Uniaxial notched tensile tests (Figure 4d) revealed that PAE-PTMEG specimens with a 1 mm notch could still be stretched to 400% strain, exhibiting a fracture energy of 88.9 kJ·m^−2^, significantly surpassing PAE-PPG’s 47.6 kJ·m^−2^. Tear testing further corroborated this trend (Figure 4e), with PAE-PTMEG exhibiting tear energy (88.7 N mm^−1^) approximately double that of PAE-PPG (43.3 N mm^−1^), demonstrating superior crack resistance. In puncture resistance testing (Figure 4f), the 0.4 mm thick PAE-PTMEG sample withstood a maximum puncture force of 220 N and a puncture energy of 485.6 mJ, whereas the PAE-PPG sample exhibited a maximum force of only 80 N and a puncture energy of 377.4 mJ.

The performance disparity stemmed from the distinct microphase separation dictated by the soft segment structures. The regular PTMEG chains facilitated more complete separation, yielding a robust network with effective hard-segment reinforcement and a dense hydrogen-bonding network. In contrast, the steric hindrance from PPG’s methyl side groups disrupted segmental order, resulting in inferior phase separation. This diminished the hard segments’ role as physical crosslinks, imparting high ductility but lower strength and toughness. Consequently, PAE-PTMEG proved more suitable for demanding applications requiring high mechanical strength.

3.5. Water Resistance and Weatherability of Polyaspartic Ester Polyurea

To systematically evaluate the water resistance and weatherability of the prepared asparagine polyurea materials, water contact angle testing, water absorption analysis, chemical medium immersion experiments, and xenon lamp accelerated aging studies were conducted on both PAE-PTMEG and PAE-PPG samples.

As shown in Figure 5a, the initial water contact angle of PAE-PTMEG was 49.5°, decreasing slightly to 46.5° after 30 s, indicating a more wettable surface. In contrast, PAE-PPG exhibited an initial contact angle of 88.3°, remaining at 84.5° after 30 s, indicating a less wettable surface. This difference in surface wettability was primarily governed by the surface chemistry of the soft segments: PTMEG chains contain polar ether bonds readily interacting with water molecules, whereas the introduction of methyl groups in PPG enhances hydrophobic behavior. Furthermore, the long-term water resistance, a more critical metric for protective coatings, was evaluated by water absorption. After immersion for 72 h (Figure 5b), PAE-PTMEG exhibited a lower saturated water absorption (~1.0%) than PAE-PPG (~1.5%), demonstrating superior overall water resistance despite its more wettable surface. This apparent contrast highlights that surface wettability and water resistance are distinct properties governed by different factors. The exceptional bulk resistance of PAE-PTMEG was attributed to its denser and more cohesive crosslinked network structure, which effectively restricts the diffusion and permeation of water molecules, a key advantage for durable concrete protection. Additionally, compared to common epoxy water-resistant materials, less 1% water absorption rate is also relatively superior [36,37].

Consequently, the PAE-PTMEG sample, demonstrating superior comprehensive properties, was selected for more systematic evaluation of its resistance to media and weathering. Samples were immersed in aqueous solutions of 5% H_2_SO_4_, 5% NaCl, and 5% NaOH for 7 days, after which their macroscopic morphology was observed. As shown in Figure 5c, no swelling, cracking, or deformation occurred, indicating the material possesses good chemical stability across a wide pH range and in saline media. To quantitatively assess the impact of medium immersion on mechanical properties, tensile tests were conducted on the treated samples. Results, as depicted in Figure 5d, reveal no significant alterations in tensile strength or elongation at break for PAE-PTMEG after 7 days of immersion in various media compared to the original samples. This demonstrates the network structure’s stability under harsh chemical conditions, further validating the material’s exceptional water resistance and media tolerance. Finally, PAE-PTMEG samples underwent a 7-day xenon lamp accelerated aging test to simulate combined outdoor climatic factors, including ultraviolet radiation and humid heat, and thermal cycling (between 50 °C and 60 °C). As shown in Figure 5e, post-testing, no significant yellowing, chalking, or cracking was observed on the sample surfaces. Mechanical property tests revealed minimal deviation in tensile performance from the original samples, indicating the material’s molecular structure effectively resists degradation caused by photo-oxygen aging and possesses good weather resistance.

In summary, PAE-PTMEG asparagine polyurea not only exhibited extremely low water absorption and outstanding resistance to media even with a hydrophilic surface, but also demonstrated excellent light stability in accelerated aging tests. This indicated its broad potential for applications requiring long-term weather resistance, such as protective coatings and outdoor building materials.

3.6. Application Evaluation of Polyaspartic Ester Polyurea Coating for Concrete Protection

Building upon the aforementioned findings, PAE-PTMEG demonstrated outstanding comprehensive properties, particularly its balanced mechanical performance and excellent water resistance, endowing it with significant application potential for concrete protection in harsh environments. To validate its practical engineering applicability, PAE-PTMEG was formulated into a gray colored coating, designated as PAE-C, and its key protective characteristics were systematically evaluated, including mechanical properties, freeze–thaw resistance, chloride ion penetration resistance, and carbonation resistance.

Figure 6a visually presented the appearance of the prepared PAE-C coating, showing the film formed after curing on a concrete substrate. The surface was smooth, dense, and uniformly colored. To ensure the coating can withstand external stress and deformation during service, the cured PAE-C film was subjected to uniaxial tensile testing. As shown in Figure 6b, the tensile strength of this colored coating reached 20 MPa, with an elongation at break as high as 415%. This combination of mechanical properties not only surpassed that of traditional rigid coatings like epoxy resin but also fully meets the technical requirements for Type I products (strength ≥ 10 MPa, elongation at break ≥ 300%) specified in the GB/T 23446-2009 [38] This indicated that the PAE-C coating possesses sufficient strength to resist physical damage while exhibiting remarkable toughness to accommodate potential micro-cracks and deformations in the concrete substrate, providing fundamental mechanical assurance for long-term and effective protection.

Furthermore, the primary threat to the durability of concrete structures, especially in coastal and de-icing salt environments, was the corrosion of steel reinforcement induced by chloride ion penetration. The chloride ion penetration resistance test conducted on the PAE-C coating yielded a remarkably low chloride ion permeability rate of 1.3 × 10^−4^ mg·cm^−2^·d^−1^. This value was an order of magnitude lower than the conventional technical indicator for protective coatings (typically 1.0 × 10^−3^ mg·cm^−2^·d^−1^), demonstrating the coating’s exceptional barrier properties. This value is an order of magnitude lower than the conventional technical indicator (1.0 × 10^−3^ mg·cm^−2^·d^−1^) and significantly outperforms typical values reported for many epoxy and aromatic polyurethane coatings in similar test conditions [39,40]. This was primarily attributed to the dense crosslinked network structure and well-defined microphase-separated morphology of PAE-C, which effectively hindered the diffusion and migration of corrosive small molecules like Cl^−^.

For hydraulic engineering in northern regions and cold climates, the freeze–thaw resistance of the coating is crucial. After subjecting concrete blocks coated with PAE-C to 200 rigorous freeze–thaw cycles, their macroscopic morphology is shown in Figure 6c. It can be observed that the coating surface showed no signs of failure, such as blistering, delamination, or cracking, and the adhesion between the coating and the concrete substrate remains intact. This confirmed the coating’s excellent weatherability, dimensional stability, and compatible coefficient of thermal expansion with concrete, enabling it to maintain its structural integrity and protective function in environments with severe temperature fluctuations.

Additionally, the barrier effect of the coating against carbon dioxide was evaluated through an accelerated carbonation test. Additionally, the barrier effect of the coating against carbon dioxide was evaluated through an accelerated carbonation test. As shown in Figure 6d, the calculated carbonation depth after the specified test period was 0 mm. The coating maintained integrity after 200 freeze–thaw cycles and exhibited complete resistance to carbonation, outperforming many conventional epoxy coatings [41,42] and meeting or exceeding the long-term durability expectations for high-performance protective systems in harsh environments. This result clearly showed that carbon dioxide was completely unable to penetrate the PAE-C coating, and the alkaline environment within the concrete was perfectly preserved, thereby entirely avoiding the risk of passivation film destruction on the steel reinforcement due to carbonation.

4. Conclusions

This study successfully elucidated the decisive influence of soft segment structure on the properties of polyaspartic ester polyurea. The PTMEG soft segment conferred higher reactivity and shorter curing times to the prepolymer. More importantly, its regular chain structure facilitated the formation of a well-defined microphase separation, resulting in the PAE-PTMEG coating exhibiting exceptional mechanical properties (strength of 43.8 MPa, elongation of 646.1%), excellent water resistance, and outstanding barrier performance. When formulated, PAE-C application tests confirmed its effectiveness in providing concrete with superior resistance to chloride ion penetration, carbonation, and freeze–thaw cycles. In summary, PAE-PTMEG integrated rapid curing, high-strength-toughness, and durable protection, presenting a highly promising material for safeguarding water conservancy infrastructure.