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Article

Ester Exchange Modification for Surface-Drying Time Control and Property Enhancement of Polyaspartate Ester-Based Polyurea Coatings

by
Xiandi Yang
1,
Yiqing Deng
2,
Peini Li
1,
Kaixuan Guo
1 and
Qiang Zhao
1,*
1
School of Chemical Engineering, Sichuan University, Chengdu 610065, China
2
College of Polymer Science and Engineering, Sichuan University, Chengdu 610065, China
*
Author to whom correspondence should be addressed.
Coatings 2025, 15(2), 244; https://doi.org/10.3390/coatings15020244
Submission received: 24 January 2025 / Revised: 15 February 2025 / Accepted: 17 February 2025 / Published: 19 February 2025
(This article belongs to the Special Issue Synthesis and Application of Functional Polymer Coatings)
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Abstract

:
In recent years, polyurea (PUA) systems have drawn considerable attention in the coatings industry for their superior performance. Among these systems, polyaspartate ester-based polyurea (PAE-PUA) stands out for its excellent comprehensive properties, and the structure of the diamines used in polyaspartate ester (PAE) significantly influences key performance attributes, such as gel time, mechanical properties, and thermal stability. To investigate the influence of diamine structures on PAE-PUA properties, this study synthesized PAEs through ester exchange reactions involving diamines and monohydric alcohols with varied chain lengths and structural types (linear or cyclic). The effects of four diamines (D230, DMH, IPDA, PACM) and four monohydric alcohols (CA, DDA, OD, CHOL) on polyurea coating properties were systematically examined. The results demonstrated that adjusting the structural regularity of PAEs via ester exchange reactions effectively regulated their viscosity, maintaining it below 1500 mPa·s. These reactions also enabled simultaneous regulation of surface-drying time, mechanical properties, and thermal performance. Notably, introducing 1-octadecanol (OD) significantly improved surface-drying time and thermal stability, whereas cyclic structures in diamines or alcohols resulted in higher glass transition temperatures (Tg). Additionally, the mechanical properties and reaction rates of modified PAEs can be tailored to meet specific application requirements, offering an effective strategy for developing polyurea materials optimized for the coatings industry.

1. Introduction

With the rapid development of polymer materials, polyurea has emerged as a high-performance coating material and has gained a significant position in the coatings industry. Polyurea is an elastic polymer formed by the chemical reaction between diamines and diisocyanates [1]. It is characterized by its environmental friendliness [2], excellent corrosion resistance, shear strength [3], thermal performance, and outstanding film properties [4], making it a versatile coating material. Consequently, it is widely applied in engineering fields, including corrosion protection [5,6,7], waterproofing [8,9,10], abrasion resistance, and explosion-proof applications [11,12]. The mechanical properties of polyurea largely depend on the formation of intermolecular hydrogen bonds, a type of physical crosslinking [3,13,14]. Chemical crosslinking can enhance material rigidity, extensibility, and chemical resistance [15], but studies on its mechanisms in polyurea remain limited and are largely focused on polyurethane systems [16,17]. Thus, optimizing polyurea performance through chemical crosslinking remains a critical area of research. Despite its excellent properties, polyurea faces notable challenges in practical applications. For instance, the reaction between primary amines and isocyanates proceeds excessively fast, leading to short leveling times, and the application process requires specialized spraying equipment [18,19]. This rapid reaction often results in surface defects, including pinholes or orange peel textures. Additionally, the rapid reaction rate causes concentrated heat release, high shrinkage, and significant internal stress, ultimately reducing adhesion to the substrate. These challenges underscore the need for further research into polyurea application techniques and performance optimization to meet practical demands.
PAE-PUA materials, a novel class of aliphatic, slow-reacting, high-performance coating materials in the field of polyurea, are often referred to as the third-generation polyurea [20]. Compared to traditional polyurea, PAEs utilize secondary amines instead of primary amines as raw materials, effectively reducing their reaction rate with diisocyanates. This adjustment enables the gel time to be tailored from several minutes to several tens of minutes [13], thereby effectively addressing issues such as complex coating formation, reduced performance, and poor adhesion caused by the overly rapid reactions of traditional polyurea [21,22,23]. PAE-PUA offers numerous advantages, including lower curing temperature, slower curing speed, higher hardness, and excellent toughness. It is also well-suited for thick coating applications, offering strong substrate adhesion as well as superior weather and chemical corrosion resistance [24]. In terms of mechanical properties, diisocyanates function as hard segments, imparting rigidity to the polyurea, while PAEs serve as soft segments, providing flexibility. At room temperature, the soft segments exhibit high elasticity, characterized by low modulus, outstanding flexibility, and an amorphous, curled state. In contrast, the hard segments remain in a glassy state at room temperature, exhibiting high modulus but limited plasticity, thereby providing strength and rigidity to the material. Thus, the ratio of hard to soft segments is a critical factor influencing the mechanical properties and applications of PAE-PUA [25,26,27]. While the application of PAE-PUA continues to expand globally, the increasing demands on PAE-PUA performance have rendered the properties of conventional PAE-PUA relatively insufficient.
In the practical preparation of coatings, large quantities of PAEs are typically mixed with compatible diisocyanates. Under solvent-free conditions, the mixture’s viscosity increases rapidly, generating significant heat during stirring, which further accelerates the gelation process. Once PAEs are mixed with diisocyanates, the mixture becomes unusable when its viscosity exceeds acceptable levels [28,29]. Currently, the curing time for PAEs and diisocyanates mixtures typically ranges from 10 to 60 min. As a result, users must apply the coating while its viscosity remains below 2000 cps. However, the short workable time and the requirement for specialized equipment significantly complicates the application process. To address these challenges, this study synthesized PAEs using various diamines and diethyl maleate as reactants, serving as the amine component in polyurea. Additionally, ester exchange reactions were employed to modify PAEs with varying structures, enabling the tailoring of PAE-PUA performance to meet specific application requirements. For scenarios requiring high leveling properties or thick protective coatings, PAE-PUA can be applied via rolling or casting methods to meet current performance demands.
Currently, extensive research has been conducted on the composites of PAE-PUA with other materials, with most studies focusing on enhancing specific properties. For example, Yizhi Du et al. [30] incorporated modified TiO2 particles into PAE polyurea and subsequently crosslinked it with a curing agent, leading to enhanced mechanical stability and anti-icing properties of the coating. Ref. [31] synthesized a polyurethane prepolymer from poly(propylene carbonate) diol and 4,4′-dicyclohexylmethane diisocyanate (HMDI). The prepolymer was subsequently chain-extended with PAE to produce polyurethane-urea (PPCD–PUU). By adjusting the -NCO content in the prepolymer and the prepolymer-to-PAE ratio, the mechanical properties of PPCD-PUU were optimized. Additionally, Maolian Guo et al. [32] achieved self-healing functionality in an epoxy matrix by microencapsulating diisocyanates and PAEs via in situ polymerization. Fragiadakis et al. [33] investigated the influence of the soft-to-hard segment ratio in polyurea on its tensile and impact resistance properties. Their findings revealed that increasing the hard segment content by just 3% significantly enhanced the tensile strength of polyurea elastomers, transforming the material from a highly deformable, elastic state to a rigid and brittle one. Guanlin Ren et al. [34] demonstrated that the molecular chain length of aliphatic amines and the viscosity of base resins significantly affect the physicochemical properties of polyurea. However, limited studies have explored how the structure of PAE affects its reactivity and the performance of PAE polyurea coatings.
To address the challenges of excessive viscosity and short application time during PAE-PUA construction, and to investigate the influence of different PAE structures on the properties of polyurea coatings, we conducted a study synthesizing PAEs via Michael addition reactions using four structurally distinct diamines and diethyl maleate. Additionally, four monohydric alcohols were employed in ester exchange reactions to prepare 20 distinct PAE samples. The structures of the synthesized PAEs were confirmed by FT-IR and 1H NMR characterization. These PAEs were then reacted with HDI trimer to produce PAE-PUA, whose surface-drying time, mechanical properties (hardness, tensile strength, elongation at break), and thermal stability (TGA, DSC) were evaluated.
The results revealed that the performance of PAE-PUA coatings varied depending on the structure of the diamines. This study effectively improved the application conditions of polyurea coatings while simultaneously enhancing their overall performance. The preparation process is mild, the raw materials are readily accessible, and this approach offers a feasible pathway for the industrial-scale production of precursors. An overview of the research is illustrated in Scheme 1.

2. Experimental

2.1. Materials

Diethyl maleate (DEM) and 1-dodecanol (DDA) were purchased from Chengdu Dingsheng Times Technology Co., Ltd. (Chengdu, China). Polyetheramine D-230 (D230), isophorone diamine (IPDA), n-octanol (CA), and titanium isopropoxide (Ti(OPr-iso)4) were obtained from Shanghai Aladdin Biochemical Technology Co., Ltd. (Shanghai, China). 4,4′-Diaminodicyclohexylmethane (PACM) was supplied by Shanghai Macklin Biochemical Co., Ltd. (Shanghai, China). 1-Octadecanol (OD), cyclohexanol (CHOL), deuterated chloroform (CDCl3), and 1,6-hexanediamine (DMH) were purchased from Chengdu Zopu Instruments Co., Ltd. (Chengdu, China). The aliphatic polyisocyanate Desmodur 3300 (HDI trimer) was provided by Guangzhou Haoyi New Materials Technology Co., Ltd. (Guangzhou, China).

2.2. Synthesis of PAEs

Four structurally distinct diamines (D230, DMH, IPDA, PACM) and DEM were selected as reactants. The selection of different diamine structures aimed to facilitate subsequent comparisons of the effects of chain length and alicyclic structures on the performance of PAE-PUA coatings. The synthesis procedure for PAEs is as follows: A three-necked flask, equipped with a mechanical stirrer, a constant-pressure dropping funnel, and a nitrogen inlet, was charged with a measured amount of diamine. The flask was placed in a thermostatic oil bath, and an equimolar amount of DEM was added dropwise via the constant-pressure dropping funnel. The reaction was conducted under a nitrogen atmosphere at 50 °C with continuous stirring and heating. After the addition was complete, the temperature was raised to 80 °C, and the reaction was maintained for 24 h. After completion, the product was cooled to room temperature and stored for further use. The resulting PAEs were denoted as D230-DEM, DMH-DEM, IPDA-DEM, and PACM-DEM. The detailed synthesis process of PAEs is shown in Scheme S1.

2.3. Ester Exchange Modification of PAEs

Four structurally distinct monohydric alcohols (CA, DDA, OD, CHOL) were selected to undergo ester exchange reactions with PAEs to investigate the influence of PAE side chain structures on the performance of PAE-PUA coatings. The reaction steps were as follows: Ester exchange reactions were conducted via a one-pot method. The monohydric alcohols and synthesized PAEs were added to a flask at a molar ratio of 0.7~1, with titanium isopropoxide (Ti(OPr-iso)4) as a catalyst. Under vacuum conditions, the mixture was heated to 100 °C for 1 h, followed by raising the temperature to 115 °C and maintaining it for 3 h. The reaction was continued at 130 °C for an additional 2 h to complete the process. After completion, the product was cooled to room temperature and stored for further use. The resulting products of the D230 series were designated as D230-DEM-CA, D230-DEM-DDA, D230-DEM-OD, and D230-DEM-CHOL, with similar naming conventions applied to other series. The detailed synthesis process of modified PAEs is shown in Scheme S1.

2.4. Preparation of PAE-PUA Coatings

PAEs and the isocyanate curing agent (HDI) were mixed at a molar ratio of n(-NCO):n(-NH) = 3:2 and stirred thoroughly to prepare 20 distinct PAE-PUAs. The prepared PAE-PUA mixtures were poured into polytetrafluoroethylene molds for shaping.
To eliminate air bubbles and ensure resin uniformity, the mixtures were degassed under vacuum conditions and cured at room temperature. The reaction equation between PAEs and isocyanates is illustrated in Scheme 1. The polyurea coatings of the D230 series were named D230-DEM/PUA, D230-DEM-CA/PUA, D230-DEM-DDA/PUA, D230-DEM-OD/PUA, and D230-DEM-CHOL/PUA, with similar naming conventions applied to other series. The detailed synthesis process of PAE-PUA is shown in Scheme S1.

2.5. Characterization of PAEs Before and After Ester Exchange Modification

The chemical structures of the synthesized and modified PAEs were identified by analyzing their infrared characteristic peaks using a Fourier transform infrared spectrometer (NEXUS 670, Thermo Fisher Scientific, Waltham, MA, USA). The IR spectra were recorded within a wavelength range of 400 cm−1 to 4000 cm−1. The chemical structures of the synthesized and modified PAEs were further analyzed using 1H NMR spectroscopy at 400 MHz and 600 MHz (Avance II-400 MHz, Avance II-600 MHz, Bruker, Fällanden, Switzerland), with deuterated chloroform (CDCl3) as the solvent. The viscosity of the synthesized and modified PAEs was measured using a digital viscometer (NDJ-85, Shanghai Lichen-BX Instrument Technology Co., Ltd., Shanghai, China) at 25 °C. Each sample was tested in triplicate, and the average value was recorded. The viscosities of the samples were compared to determine the changes after the ester exchange reaction.

2.6. Performance Characterization of PAE-PUA Coatings

Surface-Drying Time: Following GB/T 23446-2009 [35], approximately 6 g of the sample was weighed at a molar ratio of n(-NCO):n(-NH) = 3:2 under standard conditions, mixed thoroughly, and stirred uniformly. The time from mixing to the point when the coating surface became non-sticky was recorded as the surface-drying time.
Hardness: As specified in GB/T 23446-2009 [35], the dried and shaped polyurea samples were cut into pieces measuring 100 mm × 25 mm, with a minimum thickness of 6 mm, and the surface was smoothed. A Shore A hardness tester was employed for the measurement. The sample was placed on the base plate of the hardness tester stand, ensuring it was flat and in full contact with the plate. The presser foot was lowered parallel to the sample surface, with the needle perpendicular to the surface. Uniform pressure was applied, held for 1.5 s, and the reading was recorded. Five different positions on the same sample surface were measured, maintaining a minimum distance of 6 mm between two positions.
Following GB/T 528-2009 [36], the dried and shaped polyurea samples were cut into dumbbell-shaped specimens using a standard dumbbell cutter. The thickness at the midpoint of the testing section was measured using a vernier caliper, and the data were recorded and input into a universal testing machine. The tensile test was performed at a stretching speed of 10 mm/min. For each material, at least three specimens were tested, and the arithmetic mean of the results was calculated.
TGA: Thermogravimetric analysis (TGA, TG 209, NETZSCH, Selb, Germany) was employed to study the relationship between the mass of PAE polyurea and temperature. The test was conducted under a nitrogen atmosphere with a temperature range of 30 °C to 600 °C and a heating rate of 10 °C/min.
DSC: The glass transition temperature (Tg) of PAE-PUA was determined using a differential scanning calorimeter (DSC, TA Q250, TA Instrument, New Castle, DE, USA). The test was conducted under a nitrogen atmosphere with a temperature range of −50 °C to 100 °C and a heating rate of 10 °C/min. The Tg values were determined using the midpoint method.

3. Result and Discussion

3.1. Structural Characterization of PAEs Before and After Ester Exchange Modification

In traditional polyurea, the amino component is typically a primary amine, which reacts extremely rapidly with isocyanates with reaction times of only a few seconds. This rapid reaction adversely affects coating performance and imposes strict requirements on application conditions. The use of PAEs synthesized via the reaction of maleate esters with primary amines effectively mitigates this issue. The reaction mechanism involves a Michael addition reaction between the double bond of the maleate ester and the active hydrogen atom in the primary amine, transforming the primary amine into an aliphatic secondary amine with increased steric hindrance. This modification reduces the reactivity of the hydrogen atom with isocyanates and significantly extends the gel time.
Furthermore, the reaction activity is primarily influenced by the substituent groups adjacent to the secondary amine, indicating that the types of primary amine and maleate ester directly affect the gel time of the resulting PAEs. By selecting different types of primary amines and maleate esters, it is possible to obtain PAEs with varying gel times.
The reaction equation for the Michael addition reaction between maleate esters and primary amines is illustrated in Scheme 1.
The mechanism of acid-catalyzed ester exchange reactions involves the activation of the ester carbonyl group by a protonic acid, which increases the electrophilicity of the carbonyl carbon. This activation allows the carbonyl carbon to be attacked by an alcohol, forming a tetrahedral transition state, which is followed by the elimination of an alcohol molecule and the proton, yielding the ester exchange product. The mechanism is depicted in Scheme 2.
Titanium isopropoxide was selected as the catalyst because its titanium atom acts as a Lewis acid with strong electron-accepting capability. The titanium atom enhances the electrophilicity of the ester group by coordinating with the oxygen atom of the ester molecule. During the reaction, the hydroxyl group (-OH) in the alcohol molecule (R″OH) acts as a nucleophile, attacking the electrophilic carbon atom of the ester molecule. This nucleophilic attack breaks the C-O bond of the ester, producing a new ester and an alcohol molecule. The departing ester group is released as a leaving alcohol (R′OH).
Fourier transform infrared (FT-IR) spectroscopy was conducted to characterize the PAEs and modified PAEs after the Michael addition and ester exchange reactions. The results, presented in Figure 1, indicate that the synthesized D230-DEM, DMH-DEM, IPDA-DEM, and PACM-DEM lack the characteristic C=C stretching vibration peak of diethyl maleate (DEM) at 1620–1680 cm−1, suggesting that the C=C bond of DEM has reacted with the primary amine to form a secondary amine. The FT-IR spectrum of DEM is provided in the Supporting Material (Figure S1). The stretching vibration absorption peaks of the secondary amine (-NH-) are observed at 3326–3330 cm−1 for each sample. The peaks between 2860 and 2990 cm−1 correspond to the stretching vibrations of aliphatic C-H bonds. The strong peaks at 1729–1732 cm−1 correspond to the stretching vibration of the carbonyl group (C=O) in the ester group. The peaks around 1090–1200 cm−1 are attributed to the stretching vibrations of C-O-C, while the peak at 1024 cm−1 corresponds to the stretching vibration of C-N bonds. Comparing the spectra before and after the ester exchange reaction across all series, certain changes in peak shapes were observed corresponding to the substitution of the ester group during the ester exchange reaction. However, the positions of the main functional group peaks remained largely unchanged, indicating that the ester exchange reaction modified the carbon chain structure of the PAEs without altering their primary functional groups. These observations preliminarily confirm the occurrence of the ester exchange reaction and suggest that no significant byproducts were generated.; however, further analysis with 1H NMR is required to elucidate the final product structure. FT-IR analysis thus preliminarily verifies the successful synthesis and modification of PAEs.
Figure S2 presents the 1H NMR spectra of PAEs and modified PAEs, with deuterated chloroform (CDCl3) as the solvent. The 1H NMR spectra confirm that the ester exchange reaction successfully introduced a monohydric alcohol onto one of the ester groups in the PAEs, altering the ester structure. For the D230-DEM series, Figure S2a shows the 1H NMR spectrum of D230-DEM. The peak at 4.18 ppm (H11, H18, H23, H30) corresponds to the hydrogen (COOCH2CH3) on the first carbon of the ester group, while the peak at 3.72 ppm (H14, H27) confirms that the primary amine in D230 has reacted to form a secondary amine (NH-CH-COOEt). The 1H NMR spectra of PAEs modified with different monohydric alcohols exhibit similar patterns, with differences observed only in the substituted ester chain segments. Characteristic peaks include δ(ppm): 1.25–1.30 (–CH2–, Figure S2b), 1.25–1.29 (–CH2–, Figure S2c), 1.25–1.33 (–CH2–, Figure S2d), and 1.49–1.70 (–CH2–, Figure S2d), 1.43–1.68 (Cycloaliphatic H, Figure S2e), confirming that the four monohydric alcohols were successfully grafted onto D230-DEM. Figure S2f presents the 1H NMR spectrum of DMH-DEM. Among the four unmodified PAEs, only the secondary amine structures differ, while other chemical shifts remain generally similar. Characteristic peaks for the DMH series are observed at δ (ppm) = 1.27–1.30 (–CH2–, Figure S2f), 1.43–1.77 (–CH2–, Figure S2g), 1.41–1.72 (–CH2–, Figure S2h), 1.41–1.76 (–CH2–, Figure S2i), and 1.40–1.80 (Cycloaliphatic H, Figure S2j), confirming the successful ester exchange modification of DMH-DEM. For the IPDA series, characteristic peaks are observed at δ (ppm) = 1.40–1.79 (–CH2–, Figure S2k), 1.43–1.69 (–CH2–, Figure S2l), 1.45–1.90 (–CH2–, Figure S2m), 1.39–1.80 (–CH2–, Figure S2n), and 1.40–1.81 (Cycloaliphatic H, Figure S2o), confirming the successful modification of IPDA-DEM. For the PACM series, characteristic peaks are observed at δ (ppm) = 1.35–1.81 (–CH2–, Figure S2p), 1.30–1.61 (–CH2–, Figure S2q), 1.31–1.59 (–CH2–, Figure S2r), 1.33–1.80 (–CH2–, Figure S2s), and 1.30–1.79 (Cycloaliphatic H, Figure S2t), confirming the successful modification of PACM-DEM. The combined analysis of FT-IR and 1H NMR spectra confirm the structures of the PAEs and modified PAEs.

3.2. Viscosity Characterization of PAEs Before and After Ester Exchange Modification

Table 1 compares the viscosities of PAEs and modified PAEs at 24 °C, emphasizing the critical role of viscosity in the spraying and application of polyurea coatings. Among the four unmodified PAEs, D230-DEM exhibits the lowest viscosity due to its long chain length and high molecular flexibility. The high viscosity of PACM-DEM is attributed to its two cyclic structures, which increase molecular rigidity and restrict molecular mobility. The viscosity of modified PAEs is primarily influenced by the molecular weight and structure of the substituted monohydric alcohols. The incorporation of more rigid cyclic structures leads to higher viscosity. To achieve lower viscosity, it is crucial to avoid incorporating rigid structures in the molecules. The viscosities of the D230-DEM and DMH-DEM series remain relatively low, with minor differences among different alcohols. However, substituting ethanol with monohydric alcohols slightly increases viscosity, making it less favorable for viscosity reduction. For the IPDA-DEM and PACM-DEM series, DDA resulted in the most effective viscosity reduction. DDA’s smaller molecular weight increases the chain length upon substitution, thereby reducing viscosity. From the perspective of viscosity reduction, larger molecular weight monohydric alcohols are preferable substitutes for ethanol. Overall, the viscosities of the synthesized modified PAEs remain below 1500 mPa·s, ensuring their practical applicability.

3.3. Performance Testing of PAE-PUA Coatings

3.3.1. Surface-Drying Time

The curing time of PAE-PUA is characterized by the surface-drying time, which plays a critical role in the application process of polyurea coatings. Therefore, the surface-drying time must align with the requirements of the specific application process.
The surface-drying time of the modified PAEs varies, enabling tailored selection based on specific process requirements. Polyurea coatings were prepared using PAEs with different branching structures and molecular weights as starting materials, following the method in Section 2.4. The surface-drying time was observed and recorded, with results presented in Figure 2. Overall, the surface-drying time of PAEs increased after ester exchange modification. The D230-DEM series exhibited relatively long surface-drying times due to the presence of long chains in the D230 molecular structure. These long chains increased steric hindrance around the secondary amine group and enhanced the internal plasticization effect, reducing the reaction activity. The PACM-DEM series exhibited the longest surface-drying time after modification with OD, showing a more pronounced effect compared to other monohydric alcohol modifications. The steric hindrance from the cyclic structures in PACM and the grafted OD extended the surface-drying time to as long as 408 min, which is significantly slower than the fast-reacting traditional polyurea (5–10 s). The surface-drying time of the IPDA-DEM series showed only minor variations but reached its maximum after modification with OD. The DMH-DEM series exhibited the shortest surface-drying time, with little variation, remaining close to the control group. The shorter molecular chains in DMH had minimal impact on the original steric hindrance, resulting in the fastest reaction rate.

3.3.2. Mechanical Properties

PAEs derived from different diamines exhibit varying reactivity and significant differences in the mechanical properties of the resulting polyurea, directly influencing flexibility, hardness, tensile strength, and elongation at break. Figure 3a presents the Shore A hardness test results of the PAE-PUA coatings. Overall, the hardness progressively increases from the D230-DEM series to the PACM-DEM series. The D230-DEM series exhibited relatively low hardness due to the internal plasticizing effect of its long molecular chains, which reduced crosslinking density and increased flexibility. After the introduction of cyclohexanol, a rigid group, the hardness increased due to its contribution to a denser crosslinked structure. The rigid cycloaliphatic structure of the IPDA-DEM series enhances crosslinking density and restricts chain mobility, leading to increased hardness. In contrast, the PACM-DEM series exhibited significantly higher hardness than other samples, peaking after modification with cyclohexanol. This increase is attributed to the cyclic structure in PACM, which promotes the formation of a denser crosslinked network. However, with increasing side chain length and molecular flexibility, the hardness of the PAE decreased.
Figure 3b,c illustrate the effects of different diamine structures and monohydric alcohol modifications on the tensile strength and elongation at the break of PAE-PUA coatings. Polyurea derived from the PACM-DEM series, which contains two cyclic structures, exhibits the highest tensile strength among all series. The rigidity of the two cyclic structures restricts molecular deformation, resulting in higher tensile strength.
The combination of the two cyclic rings enhances intermolecular interactions, reducing the likelihood of breakage or slippage under external force. Furthermore, tensile strength increases with the introduction of monohydric alcohols, with OD showing the most pronounced effect. OD acts as a chain extender, and its inclusion in the polyurea formulation brings the urea functional groups closer, enhancing hydrogen bonding [37].
For the D230-DEM series, which contains polyether chains, tensile strength is relatively low due to the longer polyether chains, but flexibility is improved. The elongation at break reached its maximum, significantly higher than that of samples with cyclic structures, and further increased with the introduction of long-chain monohydric alcohols. The DMH-DEM series, containing short-chain diamine segments, exhibited lower elongation at break than the D230-DEM series but higher tensile strength. As the molecular weight of the introduced long-chain monohydric alcohols increased, elongation at break and tensile strength both improved. For the IPDA-DEM series, the relatively low tensile strength and elongation at break are attributed to the rigid cyclic structure in IPDA molecules, which reduces molecular flexibility. External force restricts molecular chain movement, resulting in brittle fracture and lower tensile strength. After modification with monohydric alcohols, tensile strength remained largely unchanged, while elongation at break slightly improved. In summary, the observed trends arise from the structural diversity introduced by different diamines and ester exchange modifications, which allows for the simultaneous optimization of strength and elongation. Therefore, PAEs synthesized from different diamines and modified with various monohydric alcohols allow for both the adjustment of surface-drying time and the tuning of mechanical properties of polyurea coatings.

3.3.3. Thermal Stability

Temperature variations significantly influence the tensile stress–strain behavior of PAE-PUA [38]. Therefore, TGA and DSC tests were conducted under identical temperature conditions. TGA was performed to analyze the thermal degradation behavior of PAE-PUA in a nitrogen atmosphere. Figure 4a–d presents the TGA curves for each series, with the corresponding thermal degradation data summarized in Tables S1–S5. Most samples exhibited two distinct stages of thermal degradation within the temperature range of 150–500 °C. The first weight loss (150–350 °C) corresponds to the decomposition of ester bonds with low crosslinking density, undergoing thermal cleavage via a β-elimination mechanism [39]. The second weight loss (350–500 °C) corresponds to polymer chain decomposition. The urea groups in the PAE-PUA molecular chain act as rigid hard segments, contributing to crosslinking [40]. Variations in thermal performance primarily arise from the substitution of different monohydric alcohols. For the D230-DEM series, the temperatures at 5% and 10% weight loss for D230-DEM-CA/PUA and D230-DEM-DDA/PUA are lower than those of unmodified D230-DEM/PUA, while the temperature at 50% weight loss increases. D230-DEM-CHOL/PUA, modified with cyclohexanol, shows minimal change compared to unmodified D230-DEM/PUA, whereas D230-DEM-OD/PUA exhibits the best thermal performance in the series. For the DMH-DEM series (Figure 4b), DMH-DEM-CA/PUA exhibits lower thermal performance. According to Table S2, n-octanol and cyclohexanol significantly reduce thermal stability, resulting in lower thermal degradation temperatures. The introduction of long-chain DDA and OD improves the thermal performance of the material, albeit modestly. In this series, long-chain alkyl groups enhance thermal stability, primarily due to their larger molecular weight and improved packing efficiency, which result in a more stable crystalline structure. Conversely, short-chain alkyl groups and rigid ring structures lead to earlier thermal degradation. For the IPDA-DEM series (Figure 4c), substitution with n-octanol and OD improves thermal stability, whereas substitution with DDA and cyclohexanol lowers the temperatures at 5% and 10% weight loss but raises the temperature at 50% weight loss. In this series, IPDA-DEM-OD/PUA modified with OD exhibits the best thermal stability.
Similar trends were observed for the PACM-DEM series (Figure 4d). The introduction of DDA and cyclohexanol lowers the temperatures at 5% and 10% weight loss but raises the temperature at 50% weight loss. The introduction of n-octanol and OD improves thermal stability, with OD resulting in the highest temperature at 50% weight loss.
In summary, incorporating OD into the ester side chain of PAEs significantly enhances the thermal stability of polyurea coatings. The thermal performance of the four unmodified PAEs is shown in Table S5. As the diamine molecular structures vary, the thermal performance changes correspondingly. The TGA curve of D230-DEM/PUA is relatively stable, indicating good thermal stability, primarily attributed to its longer molecular chains and stronger intermolecular interactions.
The effect of ester exchange modification and various diamine structures on the glass transition temperature (Tg) of PAE-PUA coatings was studied using DSC. Figure 5 presents the DSC spectra of each sample, and the corresponding Tg values are summarized in Tables S6–S10. In the D230 series (Figure 5a), the introduction of straight-chain monohydric alcohols (except cyclohexanol) slightly increases the Tg of the material, primarily due to their plasticizing effect, which reduces intermolecular interactions. The melting peak of D230-DEM-OD/PUA is associated with residual OD. For the DMH series (Figure 5b), the unmodified DMH-DEM/PUA (without monohydric alcohols) exhibits the highest Tg. In the IPDA series (Figure 5c), the relatively high Tg of each sample is attributed to the rigid groups in the molecular backbone. As monohydric alcohols are introduced, Tg decreases with increasing molecular weight of the alcohol—a trend also observed in the PACM series (Figure 5d). To verify the effect of various diamine structures on the final material, the four unmodified PAEs were compared. Table S10 demonstrates that polyurea coatings with cyclic structures exhibit higher Tg values than those with straight-chain diamines, as the cyclic structures increase chain rigidity.

4. Conclusions

This study systematically investigated the effects of various diamine structures and ester side chain substitutions on the performance of polyurea coatings by synthesizing PAEs with distinct structures and performing ester exchange modifications. The structures of the synthesized PAEs were characterized using FT-IR and 1H NMR spectroscopy.
Viscosity measurements revealed that among the four unmodified PAEs, D230-DEM exhibited the lowest viscosity, while the addition of DDA achieved the most effective viscosity reduction in the IPDA-DEM and PACM-DEM series. Substituting monohydric alcohols with larger molecular weights and lower rigidity effectively reduced the viscosity, keeping the modified PAEs below 1500 mPa·s, ensuring their practicality.
The curing performance of PAE-PUA coatings was evaluated, revealing that the curing rate is closely related to the unique molecular structures of PAEs. Ester exchange modification generally extended the surface-drying time of PAEs. The D230-DEM series exhibited a longer surface-drying time. Ester exchange modification generally extended the surface-drying time of PAEs. PAEs modified with PACM-DEM and OD exhibited a surface-drying time of 408 min, far exceeding that of traditional polyurea. The modification effects of DDA and OD were significant, with larger side chain groups of maleate esters leading to slower reaction rates.
The molecular structure of PAEs directly influences the mechanical properties of PAE-PUA. The hardness of the PACM-DEM series was significantly higher than that of other samples, peaking after modification with cyclohexanol. The D230-DEM series, characterized by longer polyether chain segments, exhibited lower tensile strength but superior flexibility, achieving the highest elongation at break. Moreover, the introduction of long-chain monohydric alcohols significantly improved elongation at break.
Regarding thermal properties, the introduction of OD significantly enhances the thermal stability of polyurea coatings, whereas short-chain alkyl groups and rigid cyclic structures lower the thermal degradation temperatures. Among the modified PAEs, polyurea coatings with cyclic structures exhibited significantly higher Tg than those with straight-chain diamines.
Overall, ester exchange as a resin modification method for PAE-PUA coatings effectively addresses the issues of short curing time and high viscosity while enabling the performance of PAE-PUA to be tailored to specific application requirements. This offers a feasible approach for developing more applicable polyurea resins, particularly for solvent-free, high-solids content coatings.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/coatings15020244/s1, Scheme S1. Detailed synthesis process diagram of PAEs, modified PAEs and PAE-PUA; Figure S1: FTIR spectra of D230-DEM; Figure S2: 1H NMR Spectra of PAEs Before and After Modification: (a) D230-DEM (b) D230-DEM-CA (c) D230-DEM-DDA (d) D230-DEM-OD (e) D230-DEM-CHOL (f) DMH-DEM (g) DMH-DEM-CA (h) DMH-DEM-DDA (i) DMH-DEM-OD (j) DMH-DEM-CHOL (k) IPDA-DEM (l) IPDA-DEM-CA (m) IPDA-DEM-DDA (n) IPDA-DEM-OD (o) IPDA-DEM-CHOL (p) PACM-DEM (q) PACM-DEM-CA (r) PACM-DEM-DDA (s) PACM-DEM-OD (t) PACM-DEM-CHOL; Table S1: Thermal Degradation Performance of D230 Series Polyurea Coatings; Table S2: Thermal Degradation Performance of DMH Series Polyurea Coatings; Table S3: Thermal Degradation Performance of IPDA Series Polyurea Coatings; Table S4: Thermal Degradation Performance of PACM Series Polyurea Coatings; Table S5: Thermal Degradation Performance of Unmodified PAE Polyurea Coatings; Table S6: Tg of D230 Series Polyurea Coatings; Table S7: Tg of DMH Series Polyurea Coatings; Table S8: Tg of IPDA Series Polyurea Coatings; Table S9: Tg of PACM Series Polyurea Coatings; Table S10: Tg of Unmodified PAE Polyurea Coatings.

Author Contributions

Conceptualization, methodology, writing—original draft preparation, X.Y.; formal analysis, investigation, Y.D., P.L. and K.G.; writing—review and editing, supervision, funding acquisition, Q.Z. All authors have read and agreed to the published version of the manuscript.

Funding

This work was funded by Sichuan University-Beixin Carpoly Coating Group Co., Ltd. Cooperation Project (No. 24H1454).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data are contained within the article and Supplementary Materials.

Conflicts of Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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Coatings 15 00244 sch001
Scheme 1. Synthetic routes and characterization diagram of PAEs and PAE-PUA.
Scheme 1. Synthetic routes and characterization diagram of PAEs and PAE-PUA.
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Scheme 2. Mechanism of acid-catalyzed ester exchange reaction.
Scheme 2. Mechanism of acid-catalyzed ester exchange reaction.
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Figure 1. (a) FTIR spectra of D230-DEM and modified D230-DEM. (b) FTIR spectra of DMH-DEM and modified DMH-DEM. (c) FTIR spectra of IPDA-DEM and modified IPDA-DEM. (d) FTIR spectra of PACM-DEM and modified PACM-DEM.
Figure 1. (a) FTIR spectra of D230-DEM and modified D230-DEM. (b) FTIR spectra of DMH-DEM and modified DMH-DEM. (c) FTIR spectra of IPDA-DEM and modified IPDA-DEM. (d) FTIR spectra of PACM-DEM and modified PACM-DEM.
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Figure 2. Relationship between surface-drying of PAE-PUA coatings before and after modification and diamine structure.
Figure 2. Relationship between surface-drying of PAE-PUA coatings before and after modification and diamine structure.
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Figure 3. (a) Shore hardness of PAE-PUA coatings before and after modification and diamine structure. (b) Tensile strength of PAE-PUA coatings before and after modification and diamine structure. (c) Elongation at break of PAE-PUA coatings before and after modification and diamine structure.
Figure 3. (a) Shore hardness of PAE-PUA coatings before and after modification and diamine structure. (b) Tensile strength of PAE-PUA coatings before and after modification and diamine structure. (c) Elongation at break of PAE-PUA coatings before and after modification and diamine structure.
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Figure 4. TGA curves of (a) D230 series coatings. (b) DMH series coatings. (c) TGA curve of IPDA series coatings. (d) PACM series coatings.
Figure 4. TGA curves of (a) D230 series coatings. (b) DMH series coatings. (c) TGA curve of IPDA series coatings. (d) PACM series coatings.
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Figure 5. DSC curves of (a) D230 series coatings. (b) DMH series coatings. (c) IPDA series coatings. (d) PACM series coatings.
Figure 5. DSC curves of (a) D230 series coatings. (b) DMH series coatings. (c) IPDA series coatings. (d) PACM series coatings.
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Table 1. Viscosity (mPa·s) of PAEs before and after modification at 24 °C.
Table 1. Viscosity (mPa·s) of PAEs before and after modification at 24 °C.
D230-DEMDMH-DEMIPDA-DEMPACM-DEM
Control88 ± 5132 ± 7787 ± 301283 ± 150
CA181 ± 5151 ± 7842 ± 40957 ± 125
DDA134 ± 5171 ± 5512 ± 35717 ± 100
OD297 ± 15507 ± 35987 ± 1251172 ± 125
CHOL474 ± 30535 ± 303781 ± 3004388 ± 500
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MDPI and ACS Style

Yang, X.; Deng, Y.; Li, P.; Guo, K.; Zhao, Q. Ester Exchange Modification for Surface-Drying Time Control and Property Enhancement of Polyaspartate Ester-Based Polyurea Coatings. Coatings 2025, 15, 244. https://doi.org/10.3390/coatings15020244

AMA Style

Yang X, Deng Y, Li P, Guo K, Zhao Q. Ester Exchange Modification for Surface-Drying Time Control and Property Enhancement of Polyaspartate Ester-Based Polyurea Coatings. Coatings. 2025; 15(2):244. https://doi.org/10.3390/coatings15020244

Chicago/Turabian Style

Yang, Xiandi, Yiqing Deng, Peini Li, Kaixuan Guo, and Qiang Zhao. 2025. "Ester Exchange Modification for Surface-Drying Time Control and Property Enhancement of Polyaspartate Ester-Based Polyurea Coatings" Coatings 15, no. 2: 244. https://doi.org/10.3390/coatings15020244

APA Style

Yang, X., Deng, Y., Li, P., Guo, K., & Zhao, Q. (2025). Ester Exchange Modification for Surface-Drying Time Control and Property Enhancement of Polyaspartate Ester-Based Polyurea Coatings. Coatings, 15(2), 244. https://doi.org/10.3390/coatings15020244

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