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. 2026 Sep 25;16(45):57664–57675. doi: 10.1039/d6ra06498a

Synergistic effects of perfluoropolyether alcohol modification on the hydrophobicity and flexibility of waterborne nano-polyurea coatings

Bingbing Bu a, Rui Zhang b, Huixuan Wu a, Shujie Liu a, Chen Gao a, Shihan Wu a, Dongliang Wu a, Shucheng Xie c, Fanhou Kong a,✉
PMCID: PMC13614022  PMID: 42799415

Abstract

Waterborne nano-polyurea coatings have attracted increasing attention as eco-friendly anti-corrosion materials, but their practical application is limited by insufficient hydrophobicity and flexibility. To overcome these limitations, a fluorinated grafted MDI, synthesized using 4,4′-methylenediphenyl diisocyanate (MDI) and monohydroxyl-terminated perfluoropolyether (PFPE-OH), was introduced into a waterborne nano-polyurea system. The modified coating was comprehensively evaluated through structural characterization, analysis of physical properties, and application performance testing. The effects of PFPE-OH content on the structure and performance of the coatings were systematically investigated. The FT-IR and 1H NMR results demonstrated successful incorporation of PFPE-OH into the polyurea network through the formation of urethane linkages, while 19F NMR confirmed the introduction of PFPE side chains. The fluorinated groups were uniformly dispersed in the modified coatings as revealed by SEM-EDS, and XPS results verified their enrichment at the coating surface, with the F atomic percentage reaching 38.92 at% for the modified sample (JM-4). With increasing PFPE-OH content, the water contact angle increased from 63.3° to 108.4°, the elongation at break first increased from 2.40% to 3.98% and then decreased to 0.69% at excessive PFPE-OH loadings, and the Shore hardness decreased from 61 to 42, indicating significant improvements in both hydrophobicity and flexibility. An appropriate PFPE-OH content also contributed to enhanced thermal stability, abrasion resistance, neutral salt spray resistance, and adhesion performance. In particular, when the MDI/PFPE-OH feed ratio was 1 : 0.08, the coating exhibited an adhesion strength of 5.33 MPa and superior overall performance. These findings demonstrate that PFPE-OH modification is an effective strategy for achieving simultaneous enhancement of hydrophobicity and flexibility in waterborne nano-polyurea coatings.

After perfluoropolyether alcohol hydroxyl groups react with diphenylmethane diisocyanate to form polyurea coating with both anti‑corrosion and hydrophobic properties.graphic file with name d6ra06498a-ga.webp

1. Introduction

Waterborne coatings have attracted extensive attention owing to their low volatile organic compound (VOC) emissions, high application safety, and environmental friendliness. In particular, this development is further promoted by increasingly stringent environmental regulations and the continuous advancement of carbon peak and carbon neutrality strategies.1 Conventional solvent-based coatings with high VOC contents pose potential risks to both the environment and human health, thereby limiting their practical applications.2 Polyurea coatings are polymeric materials synthesized via a polyaddition reaction between isocyanates and amines, resulting in a polymer with flexible soft segments and rigid hard segments.3,4 The soft segments, generally derived from polyamines or polyols, provide excellent flexibility and extensibility. Contrastingly, the hard segments formed by the reaction between isocyanates and chain extenders exhibit high polarity and rigidity, enabling strong intermolecular hydrogen-bonding interactions. This microphase-separated structure enables polyurea coatings to maintain high mechanical strength while providing excellent impact resistance and deformation tolerance.5,6 Consequently, polyurea coatings possess outstanding mechanical properties along with wear, corrosion, and weather resistance. Owing to these, they demonstrate broad application prospects in anti-corrosion protection, waterproofing engineering, energy storage protection, and infrastructure protection.7–9

The incorporation of nanomaterials into waterborne systems not only reduces the consumption of organic solvents to maintain ultra-low VOC emissions but also enhances coating compactness and overall performance through the unique structural characteristics of nanomaterials. Therefore, waterborne nano-polyurea coatings have gradually become an important research frontier in the field of high-performance, environmentally friendly coatings.10–15 Waterborne nano-polyurea coatings are environmentally friendly protective coatings in which water serves as the dispersion medium and nanoparticles are incorporated into the polyurea matrix.

However, waterborne nano-polyurea coatings still suffer from several limitations during practical applications. Due to the relatively high content of polar groups, the surface free energy of the coatings tends to increase, resulting in enhanced water absorption and reduced long-term water resistance and service stability.16 In addition, the rigid hard-segment structure and strong hydrogen-bonding interactions within the polyurea matrix increase the rigidity of the coatings. Although this improves the mechanical strength, it inevitably sacrifices flexibility and elongation at break, thereby weakening the coating's ability to accommodate substrate deformation and increasing the risk of cracking or delamination under complex service conditions. Therefore, regulating the soft–hard segment structure of polyurea systems through the introduction of flexible chain segments has become an effective strategy for improving the flexibility and strain-coordination capability of waterborne nano-polyurea coatings.17,18 Nevertheless, solely adjusting the soft–hard segment structure has limited effects on improving surface hydrophobicity.19 Accordingly, achieving simultaneous enhancement of hydrophobicity and flexibility while maintaining protective performance has become an important direction for development in waterborne nano-polyurea coatings.

Fluorinated materials have been widely employed in functional material systems owing to their inherently low surface energy, excellent chemical stability, and superior weather resistance.20–22 In recent years, fluorinated polyurea and related waterborne coating systems have demonstrated promising enhancements in hydrophobicity, antifouling, and corrosion resistance, providing valuable insights into the incorporation of fluorinated chain segments in waterborne polyurea systems.23–26 Perfluoropolyether alcohol (PFPE-OH), a fluorinated polymeric alcohol compound, exhibits excellent hydrophobic and oleophobic properties, and improves material flexibility due to the presence of ether bonds within the molecular structure and relatively long molecular chains.27–29 Compared with conventional small-molecule fluorinated modifiers, PFPE-OH possesses superior thermal stability, chemical stability, and molecular chain flexibility, making it more suitable for the functional modification of polyurea systems.30,31 However, studies focusing on the simultaneous regulation mechanism of hydrophobicity and flexibility in PFPE-OH-modified waterborne nano-polyurea systems remain relatively limited.

In this study, PFPE-OH was employed as a fluorinated modifier for the waterborne nano-polyurea coatings. The incorporation of this fluorinated modifier with flexible chain segments was expected to improve overall coating performance. Fourier transform infrared spectroscopy (FT-IR), Raman spectroscopic analysis, 1H and 19F nuclear magnetic resonance spectroscopy (NMR), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), scanning electron microscopy (SEM), mechanical properties, thermogravimetric analysis (TGA), shore hardness, water contact angle, taber abrasion resistance, neutral salt spray resistance, and adhesion test measurements were conducted to systematically investigate the effects of PFPE-OH modification on the hydrophobicity, flexibility, corrosion resistance and wear resistance of waterborne nano-polyurea coatings. This work is expected to provide a theoretical basis for the design and application of high-performance waterborne nano-polyurea coatings.

2. Experimental section

2.1. Raw materials and reagents

4,4′-Methylenediphenyl diisocyanate (MDI, analytical grade) was supplied by Wanhua Chemical Group Co., Ltd. Isopropylated triphenyl phosphate (IPPP, chemical grade), monohydroxyl-terminated perfluoropolyether (PFPE-OH, analytical grade, MW = 1000, fOH = 1), and dibutyltin dilaurate (DBTDL, analytical grade) were purchased from Macklin Biochemical Co., Ltd. Tributyl phosphate (TBP, analytical grade) was obtained from Xilong Scientific Co., Ltd. Potassium silicate (analytical grade) and Defoamer 6800 (industrial grade) were provided by Jiangsu Yannai New Materials Technology Co., Ltd and Shandong Baozhongbao New Materials Co., Ltd, respectively. Titanium dioxide (TiO2, industrial grade) was purchased from Tianjin Zhiyuan Chemical Reagent Co., Ltd.

2.2. Preparation of coating samples

All synthesis and preparation steps were carried out at room temperature unless otherwise stated. PFPE-OH-modified grafted MDIs were synthesized using MDI and PFPE-OH as raw materials. The feed ratios are listed in Table 1. Predetermined amounts of MDI and PFPE-OH were added to a beaker, and DBTDL (0.1 g) as a catalyst was introduced during stirring at 100 rpm for 2 min using an electric stirrer to obtain the PFPE-OH-modified grafted MDI. After the reaction is completed, the content of –NCO is detected by titration. Predetermined amounts of IPPP (100 g) and TBP (20 g) were placed into a grinding tank together with TiO2 (10 g) and zirconia balls. The planetary ball mill was operated at 600 rpm for 30 min. After ball milling, the slurry was filtered to obtain a homogeneous dispersion. Subsequently, predetermined amount of MDI (160 g) was added to the slurry (80 g), and the mixture was stirred at 600 rpm for 3 min to obtain the unmodified non-fluorinated component A (Fig. 1).

Table 1. Feed ratios of MDI and PFPE-OH and corresponding NCO/OH equivalent.

Sample MDI(mol) PFPE-OH (mol) NCO/OH equivalent ratio NCO% (mass) Description
JM-1 1 0 — 32.7% Unmodified coating
JM-2 1 0.05 40 26.5%
JM-3 1 0.1 20 21.7% Fluorinated coating
JM-4 1 0.15 13.3 18.3%
JM-5 1 0.2 10 15.3%
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Fig. 1. Reaction scheme for the preparation of fluorinated coating.

Fig. 1

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The slurry was first prepared following the same procedure described. Subsequently, the PFPE-OH-modified grafted MDI (160 g) was introduced into the slurry (80 g) and stirred at 600 rpm for 3 min to obtain the modified fluorinated Component A. Potassium silicate solution with a mass fraction of 30 wt% was prepared and used as Component B. Component B (50 g) was slowly added into the unmodified non-fluorinated Component A (80 g) under stirring at 700 rpm for 3 min using an electric stirrer. Defoamer 6800 (1 g) was then added to this mixture under stirring. After stirring, the unmodified waterborne nano-polyurea coating was obtained. The fluorinated Component A (80 g) was placed in a beaker, and Component B (50 g) was slowly added under stirring at 800 rpm for 2 min. Defoamer 6800 (1 g) was simultaneously introduced during mixing. After stirring, the PFPE-OH-modified waterborne nano-polyurea coating was successfully prepared. The prepared coating samples were coated onto polytetrafluoroethylene (PTFE) plates cleaned with anhydrous ethanol at a coating thickness of 1.5 mm. After surface drying, dumbbell-shaped specimens were cut using a standard cutter. The cut specimens were then cured for 24 h. The cured films were then peeled off for subsequent characterization and performance testing.

2.3. Characterization methods

FT-IR spectra were recorded using a Fourier transform infrared spectrometer (Nicolet iS50, Thermo Fisher Scientific, USA) in the wavenumber range of 4000–400 cm−1. Raman spectra were recorded using a Raman spectrometer (LabRAM HR Evolution, HORIBA, Japan). 1H and 19F NMR spectra were recorded using a nuclear magnetic resonance spectrometer (AVANCE NEO 400 MHz, Bruker, Germany). XPS spectra were recorded using an X-ray photoelectron spectrometer (K-Alpha, Thermo Fisher Scientific, USA). XRD patterns were recorded using an X-ray diffractometer (XRD-6100, Shimadzu, Japan). The microscopy was observed using a field-emission scanning electron microscopy (FE-SEM, HITACHI Regulus 8100, Japan) equipped with an energy-dispersive spectroscopy detector. Mechanical properties were measured using a universal testing machine (Zwick Z250, Zwick/Roell Group, Germany). Simultaneous TG-DTA measurements were performed using a simultaneous thermal analyzer (HCT-3, Beijing Hengjiu Experimental Equipment Co., Ltd, China). Shore hardness was determined using a Shore D hardness tester (Shanghai Yizong Precision Instrument Co., Ltd, China). Water contact angles were determined using a contact angle analyzer (JY-82B Kruss DSA, Chengde Dingsheng Testing Equipment Co., Ltd, China). Adhesion strength was evaluated using a pull-off adhesion tester (DeFelsko, USA). Salt spray resistance was measured using a salt spray test chamber (LYW-025, Shanghai Yiheng Scientific Instrument Co., Ltd, China). Abrasion resistance was measured using a taber abrasion tester (Taber Industries, USA).

3. Results and discussion

3.1. Structural characterization of the coatings

As shown in Fig. 2(a), a distinct absorption peak at 2246 cm−1 observed in the MDI spectrum corresponds to the asymmetric stretching vibration of the isocyanate group (–N Created by potrace 1.16, written by Peter Selinger 2001-2019 C Created by potrace 1.16, written by Peter Selinger 2001-2019 O). In the spectrum of PFPE-OH, the characteristic peaks located at 1119 cm−1 and 1196 cm−1 are attributed to the ether bond (C–O–C) and C–F bonds, respectively. Due to the strong intermolecular and intramolecular hydrogen bonding interactions, the characteristic absorption peak of the hydroxyl group in PFPE-OH could not be clearly detected.

Fig. 2. Component analysis. (a) The FT-IR spectra of MDI, PFPE-OH, JM-1, and JM-4. (b) Raman spectra of JM-1 and JM-4. (c) 1H NMR spectrum of JM-4. (d) 19F NMR spectrum of JM-4. (e) XRD spectra of JM-1 and JM-4. (f) XPS spectrum of JM-1 and JM-4.

Fig. 2

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For the unmodified sample JM-1, the absorption peaks at 3322 cm−1, 1655 cm−1, and 1536 cm−1 correspond to N–H, C Created by potrace 1.16, written by Peter Selinger 2001-2019 O, and C–N bonds, respectively, confirming the formation of urea linkages (–NH–CO–NH–) in the polyurea structure. The characteristic absorption peaks at 3392 cm−1, 1717 cm−1, and 1520 cm−1, corresponding to N–H, C Created by potrace 1.16, written by Peter Selinger 2001-2019 O, and C–N bonds, respectively, were observed in JM-4. The noticeable shift in absorption peak positions compared with JM-1 indicates the formation of new urethane linkages (–NH–COO–) after PFPE-OH modification. Furthermore, characteristic peaks assigned to the C–F bond and ether bond (C–O–C) appear at 1231 cm−1 and 1104 cm−1, respectively. By comparing the key characteristic absorption peaks of MDI, PFPE-OH, JM-1, and JM-4, it can be concluded that the fluorinated modifier PFPE-OH reacted with MDI, forming fluorinated grafted MDIs, and was successfully incorporated into the existing polyurea system, thereby confirming the feasibility of the proposed modification strategy.32

The Raman spectrum of JM-1 (Fig. 2(b)) exhibited distinct peaks at 1616 cm−1 and 1524 cm−1, corresponding to the amide I and amide I vibrations of the polyurea structure, respectively. The presence of these peaks indicates that the unmodified sample possesses typical characteristics of the polyurea structure. After modification with PFPE-OH, JM-4 retained the characteristic amide I and amide II bands of the polyurea backbone. In addition, new bands at 1268 and 1714 cm−1, assigned to the C–F stretching vibration and the carbamate C Created by potrace 1.16, written by Peter Selinger 2001-2019 O stretching vibration, respectively, were observed. These Raman results are consistent with the FT-IR findings, further corroborating the successful incorporation of PFPE segments and the presence of carbamate groups in the modified polyurea system. The modification is expected to provide a structural basis for maintaining a balance between the coating's hydrophobicity and flexibility.

The 1H NMR spectrum of JM-4 is presented in Fig. 2(c). The signals at 7.0 to 7.4 ppm are attributed to the aromatic protons of MDI, and the peak at 3.7 to 3.8 ppm corresponds to the Ar–CH2–Ar methylene bridge. These signals confirm that the MDI backbone structure is retained after PFPE-OH modification. The peak at 4.0 ppm is assigned to the methylene protons in the CH2–O–CO–NH structure, which is characteristic of the urethane linkage formed by the reaction between the hydroxyl group of PFPE-OH and the isocyanate group of MDI. The broad peak at 7.55 to 7.65 ppm corresponds to the hydrogen-bonded NH proton of the urethane group, and the broad peak shape is typical of NH protons involved in hydrogen bonding in polyurea systems, indicating the formation of urethane linkages between PFPE-OH and MDI.

As shown in Fig. 2(d), the solid-state 19F NMR of JM-4 exhibits chemical shifts in the range of −150 to −50 ppm. There are three groups of characteristic fluorine signals. The peak with the highest intensity at −81 ppm is assigned to the terminal –CF3 group, while the characteristic peak at −131 ppm corresponds to the fluorine atoms of the –CF2–O– difluoromethyleneoxy units. The signal at −145 ppm is due to the tertiary fluorine of the –CF (CF3)– backbone units. The three resonance peaks are consistent with the reported chemical shifts of the terminal trifluoromethyl, difluoromethyleneoxy, and pendant trifluoromethyl groups in the PFPE side chains. This confirms the successful introduction of the fluorinated side chains into the polyurea system.33

As shown in Fig. 2(e), both JM-1 and JM-4 exhibited distinct diffraction peaks near 2θ values of 22°, 27°, 30°, 31°, 34°, 39°, and 43°. The peak positions are consistent with those of the standard SiO2 diffraction pattern (PDF# 51-1380), indicating that SiO2 is the primary crystalline phase in the coating. After PFPE-OH modification, the diffraction peak positions of the JM-4 sample did not show significant shifts, and no new diffraction peaks appeared.

The XPS survey spectra of the coatings are shown in Fig. 2(f). No F 1s signal is detected in JM-1, while a strong F 1s peak appears at approximately 689 eV in JM-4. As summarized in Table 2, the F atomic percentage on the surface of JM-4 reaches 38.92 at%, while no fluorine is detected on the surface of JM-1. In addition, the C 1s content decreases from 77.26 at% for JM-1 to 39.15 at% for JM-4, further indicating that the outermost surface of the modified coating is covered by fluorinated segments.34

Table 2. Surface elemental composition of the cured coatings determined by XPS.

Sample C 1s O 1s N 1s F 1s Si 2p K 2p
JM-1 (at%) 77.26 15.33 3.68 0 2.11 1.63
JM-4 (at%) 39.15 12.83 5.47 38.92 0.58 3.05
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The SEM images and corresponding EDS elemental mapping results of sample JM-1 are shown in Fig. 3(a). The coating exhibited a typical composite structural morphology with clear interfaces between the organic phase and inorganic fillers. The inorganic particles are approximately spherical and uniformly dispersed throughout the matrix, indicating a relatively homogeneous particle size distribution. Element C was continuously distributed throughout the coating, forming the primary framework of the organic matrix. The distribution region of the N element is highly consistent with that of the C element, indicating the successful formation of the polyurea structure within the coating. The distribution of the O element was also uniform across the scanned area, reflecting the presence of stable oxygen-containing functional groups in the system. The Si element was distributed as discrete dots corresponding to the locations of inorganic particles, and no obvious particle agglomeration was observed. Although the signals of the P and K elements were relatively weak, they were still distributed uniformly within the coating system.

Fig. 3. (a) SEM and EDS images of JM-1. (b) SEM and EDS images of JM-4.

Fig. 3

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The SEM images and corresponding EDS elemental mapping results of the sample JM-4 are shown in Fig. 3(b). The modified coating exhibited a composite structure dominated by spherical particles. However, the blurred interfaces between some of the particles suggested the occurrence of interparticle fusion after the introduction of PFPE-OH, indicating improved interfacial compatibility within the coating system. The F element is clearly detected in sample JM-4 and exhibits a relatively uniform distribution throughout the scanned region. Moreover, partial enrichment of fluorine can be observed on particle surfaces and interfacial regions, indicating that the fluorinated modifier PFPE-OH was successfully incorporated into the coating system and tends to migrate toward the coating surface or phase interfaces. For the remaining elements, no obvious changes in elemental distribution behavior are observed after PFPE-OH incorporation.

3.2. Determination of physical properties of the coatings

As shown in Fig. 4(a)–(c), the incorporation of the fluorinated modifier PFPE-OH significantly reduced the stress level of the coatings, thereby improving their flexibility and decreasing their rigidity. Furthermore, with the increasing PFPE-OH content, the stress showed a gradually decreasing trend under the same strain conditions, indicating that the incorporation of PFPE-OH enhances the flexibility of the coatings. As summarized in Table 3, the tensile strength and tensile modulus of the coatings showed a gradually decreasing trend with the increasing PFPE-OH content. However, the elongation at break first increased and then decreased, reaching the maximum value at JM-2. Compared with the unmodified sample JM-1, the tensile strength and tensile modulus of JM-2 decreased from 15.296 MPa and 648.787 MPa to 11.057 MPa and 495.801 MPa, respectively, while the elongation at break increased from 2.40% to 3.98%. When the PFPE-OH content continued to increase, the mechanical properties deteriorated significantly. The tensile strength of JM-5 dropped to 5.30 MPa, and the elongation at break decreased to 0.69%, even lower than that of the pure sample. These results indicate that the introduction of flexible fluorinated chain segments weakens the rigid structure of the coating system and enhances the coating flexibility within an appropriate modification range, while excessive PFPE-OH leads to severe phase separation and mechanical property deterioration. As shown in Fig. 4(d), all five coating samples exhibited an initial weight loss within the temperature range of 0–300 °C, which is mainly attributed to the decomposition of soft segments and certain unstable functional groups in the polyurea structure.

Fig. 4. (a) Stress–strain curves of coatings with different PFPE-OH feeding ratios, (b) tensile stress at break, (c) tensile strain at break, (d) thermogravimetric (TG) curves of coatings with different PFPE-OH feeding ratios.

Fig. 4

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Table 3. Mechanical properties of coatings with different PFPE-OH feeding ratiosa.

Sample Maximum force (N) Tensile strength (MPa) Tensile stress at break (MPa) Tensile strain at break (%) Tensile modulus (MPa)
JM-1 163.540 ± 9 15.296 ± 1.0 15.294 ± 1.0 2.40 ± 0.2 648.787 ± 56
JM-2 165.007 ± 10 11.057 ± 0.8 10.122 ± 0.7 3.98 ± 0.6 495.801 ± 43
JM-3 181.848 ± 10 11.672 ± 0.8 11.616 ± 0.8 2.91 ± 0.5 613.105 ± 52
JM-4 143.728 ± 7 10.304 ± 0.6 10.278 ± 0.6 2.61 ± 0.5 458.413 ± 43
JM-5 96.162 ± 5 5.301 ± 0.3 3.835 ± 0.4 0.69 ± 0.1 388.299 ± 31
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a

Tensile properties were measured according to ASTM D638 using dumbbell-shaped specimens (nominal gauge length 25 mm, width 6 mm, thickness 1.5 mm) at a strain rate of 50 mm min−1. The actual cross-sectional area of each specimen was measured by the testing machine and used for stress calculation. Each value is the mean ± standard deviation of three independent specimens.

This weight-loss behavior is more clearly resolved in the DTG curves presented in Fig. 5(a) and Table 4, where two distinct decomposition stages can be identified. The first stage occurs in the range of 100–250 °C and is associated with the volatilization of small molecules and cleavage of weak bonds, showing peak temperatures of 172–247 °C and mass losses of 2.58–3.20%. The second stage takes place between 250 and 400 °C and corresponds to the decomposition of the polyurea main chain, exhibiting peak temperatures of 258–327 °C and mass losses of 3.02–4.29%. The apparent activation energies estimated by the single-heating-rate Coats–Redfern method with a first-order reaction assumption are consistent with this distinction, ranging from 35.2 to 43.2 kJ mol−1 for the first stage and from 61.7 to 80.4 kJ mol−1 for the second stage, indicating that main-chain decomposition requires overcoming a higher energy barrier. Above 300 °C, the sample JM-2 exhibited a relatively slower weight-loss trend and a noticeably higher mass retention rate as compared with the sample JM-1, suggesting that the incorporation of a small amount of PFPE-OH contributes to improving the thermal stability of the coating system.

Fig. 5. (a) Derivative thermogravimetric (DTG) curves of coatings with different PFPE-OH feeding ratios. (b) Differential thermal analysis (DTA) curves of coatings with different PFPE-OH feeding ratios.

Fig. 5

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Table 4. Thermal decomposition parameters of coatings with different PFPE-OH feed ratios.

Sample Stage T i (°C) T p (°C) T f (°C) Δm (%) E a (kJ mol−1)
JM-1 1st 100 247.0 250 3.20 43.2
2nd 250 257.8 400 4.29 61.7
JM-2 1st 100 174.3 250 2.87 38.1
2nd 250 327.4 400 3.40 80.4
JM-3 1st 100 176.2 250 3.03 38.2
2nd 250 319.4 400 3.32 76.5
JM-4 1st 100 172.3 250 2.85 35.2
2nd 250 301.6 400 3.02 76.4
JM-5 1st 100 231.6 250 2.58 38.5
2nd 250 314.3 400 3.80 76.6
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The DTA curves shown in Fig. 5(b) corroborate this observation. The exothermic peak temperature of the pure sample JM-1 is 592 °C, whereas the modified samples show generally higher exothermic peak temperatures of 604–659 °C, reflecting an enhanced thermal oxidative stability after PFPE-OH modification. However, with the further increase in PFPE-OH content, the differences in mass retention among the modified samples in the medium- and high-temperature regions became relatively small, and the TG curves gradually approached each other, and neither the DTG decomposition parameters nor the DTA exothermic peak temperatures showed a continuous trend with increasing PFPE-OH content. This indicates that incorporation of fluorinated chain segments beyond a certain concentration has a limited effect on further improving the thermal stability.

As shown in Table 5, the shore hardness (Type D) of the coatings gradually decreased with increasing PFPE-OH content. The hardness of the unmodified sample JM-1 is 61, classifying it as a hard material. After introducing the fluorinated modifier PFPE-OH, the hardness values of JM-2, JM-3, and JM-4 decreased to 59, 56, and 51, respectively, which correspond to semi-rigid materials. When the PFPE-OH content was the highest (JM-5), the hardness further decreased to 42, indicating a transition toward a soft plastic material with potential applications in flexible protective or heavy-duty anti-corrosion coating systems. The introduction of PFPE-OH weakens the rigid structure of the coating system, resulting in increased flexibility and reduced hardness of the material. Additionally, the increased flexibility of the polymer chains contributes to relieving internal stress and improving crack resistance.

Table 5. Shore hardness values of coatings with different PFPE-OH feeding ratiosa.

Sample Shore hardness (type D) Material type
JM-1 61 ± 1 Hard type
JM-2 59 ± 1
JM-3 56 ± 2 Semi-rigid type
JM-4 51 ± 1
JM-5 42 ± 2 Soft plastic type
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a

Shore D hardness was measured according to ASTM D2240. Each value is the mean ± standard deviation of at least three measurements at different positions on each sample.

3.3. Application performance testing of the coatings

The water contact angles of the five coating samples are presented in Fig. 6. The unmodified sample JM-1 exhibited a contact angle of 63.3 ± 1°, indicating obvious hydrophilic characteristics. With increasing PFPE-OH content, the contact angle of JM-2, JM-3, and JM-4 gradually increased to 96.2 ± 1°, 104.3 ± 2°, and 107.9 ± 3°, respectively, demonstrating a continuous enhancement in hydrophobicity. Among all samples, JM-5 exhibited the highest contact angle of 108.4 ± 3°, representing an increase of 71.25% compared with the unmodified coating, JM-1. All contact angle values are reported as the mean ± standard deviation of at least three measurements at different positions on each sample. These results indicate that the fluorinated chain segments of the incorporated PFPE-OH tend to migrate and enrich at the coating surface. Such surface enrichment suppresses the spreading behavior of water droplets on the coating surface, and also effectively reduces the surface free energy of the coating, thereby significantly altering its wettability behavior. Therefore, the incorporation of PFPE-OH can markedly improve the hydrophobic properties of waterborne nano-polyurea coatings, leading to the transformation of the coatings from hydrophilic to hydrophobic.

Fig. 6. Water contact angles of the coatings: (a) JM-1; (b) JM-2; (c) JM-3; (d) JM-4; (e) JM-5. Contact angles were measured by the sessile drop method using 5 µL deionized water droplets at room temperature.

Fig. 6

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Table 6 summarizes the adhesion test results of the prepared coatings. The content of the fluorinated modifier PFPE-OH had a significant influence on the interfacial bonding performance between the coating and the concrete substrate. Sample JM-3 exhibited the highest pull-off adhesion strength (5.33 MPa) among all samples, representing an increase of 10.58% compared with the unmodified sample JM-1 (4.82 MPa). This result demonstrates that PFPE-OH incorporation at an appropriate amount can effectively improve the interfacial bonding performance between the coating and substrate.

Table 6. Adhesion test results of coatings with different PFPE-OH feeding ratiosa.

Sample Adhesion strength (MPa) Failure description
JM-1 4.82 ± 0.3 50% A/B; 50% B/Y
JM-2 4.10 ± 0.2 100% A/B
JM-3 5.33 ± 0.3 70% A/B; 30% B/Y
JM-4 4.04 ± 0.1 100% B/Y (retested)
JM-5 2.56 ± 0.1 100% B/Y (retested)
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a

Pull-off adhesion strength was measured according to ASTM D4541 on concrete substrates using a 20-mm-diameter dolly at a loading rate of 1 MPa s−1. Each value is the mean ± standard deviation of three replicates.

However, comparison of the five samples indicates that the adhesion strength can only be effectively enhanced within a suitable PFPE-OH content range. As shown in Fig. 7(a), samples JM-4 and JM-5 both exhibited a 100% B/Y failure mode during testing, indicating that failure occurred at the interface between the primer layer and adhesive. This phenomenon suggests that excessive PFPE-OH addition alters the surface characteristics of the coating, thereby weakening the adhesion ability of the adhesive toward the primer surface. Therefore, based on the comprehensive performance evaluation, sample JM-3 exhibited the optimal overall performance, and the corresponding feed ratio effectively enhanced the interfacial adhesion between the coating and substrate.

Fig. 7. (a) Adhesion failure morphologies of coatings with different PFPE-OH feeding ratios. (b) Neutral salt spray test morphologies of coatings with different PFPE-OH feeding ratios.

Fig. 7

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As shown in Table 7, the abrasion loss of the coatings first decreased and then increased, while the relative abrasion resistance showed the opposite trend. Compared with the unmodified coating sample JM-1, the abrasion loss of JM-2 decreased from 0.022 g to 0.015 g, and the relative abrasion resistance increased from 1.00 to 1.47, showing the best abrasion resistance. However, when the PFPE-OH content continued to increase, the abrasion loss gradually rose. JM-5 had the largest abrasion loss of 0.026 g, with a relative abrasion resistance of only 0.85. The variation in abrasion resistance is the result of the combined effects of surface energy and mechanical properties. The incorporation of PFPE-OH increases the contact angle from 63.3° to 108.4°, indicating a decrease in surface energy, which helps to reduce the friction coefficient during the abrasion process. However, excessive PFPE-OH leads to a significant decrease in mechanical properties. The tensile strength of JM-5 drops to 5.30 MPa, and the Shore D hardness decreases to 42. This change outweighs the friction-reducing effect of low surface energy, resulting in a decrease in abrasion resistance.

Table 7. Taber abrasion test results of coatings with different PFPE-OH feeding ratiosa.

Sample Abrasion loss (g) Relative abrasion resistance
JM-1 0.022 ± 0.003 1.00 ± 0.19
JM-2 0.015 ± 0.001 1.47 ± 0.22
JM-3 0.016 ± 0.001 1.38 ± 0.21
JM-4 0.019 ± 0.002 1.16 ± 0.20
JM-5 0.026 ± 0.003 0.85 ± 0.15
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a

Taber abrasion resistance was measured according to ASTM D4060 using CS-10 wheels under a 500 g load for 1000 cycles. Each value is the mean ± standard deviation of three replicates.

The standard for neutral coating salt spray test is GB/T 1771-2007(ISO 7253:1996). The base material is selected as Q235 sandblasted steel plate, 150 mm × 70 mm × 1 mm. Sodium chloride is dissolved in deionized water, with a mass concentration of (50 ± 5) g L−1. The pH of the test solution was 6.8. Three parallel samples were set for each sample. The temperature inside the salt spray chamber is set at (35 ± 1) °C, and the average collection rate of the salt spray solution should be 1.5 mL h−1. The coatings (no lines on the surface) with different PFPE-OH feeding ratios are summarized in Table 8, and the corresponding surface morphologies after the test are shown in Fig. 7(b). For the unmodified sample JM-1, severe rusting and coating delamination were observed after 2400 h of exposure, approximately 2% of patchy rust spots. There are also scattered rust spots and the edges and corners of the base structure have corroded and become exposed. While JM-2 exhibited filamentous corrosion with partial delamination at the edges after 2880 h. With increasing PFPE-OH content, the salt spray resistance of the coatings improved gradually. JM-3 and JM-4 maintained intact surface morphology after 3000 h of salt spray treatment, exhibiting the best corrosion resistance. The surface of the board is free of bubbles and rust. The improved corrosion resistance is attributed to the introduction of fluorinated segments from PFPE-OH, which lowers the surface energy of the coating and enhances hydrophobicity, thereby hindering the penetration of corrosive media such as Cl− and H2O. However, for JM-5, the salt spray resistance decreased to 2800 h, accompanied by pronounced blistering and edge delamination on the coating surface. There are numerous pinholes, and there are long strip-shaped rust stains in the middle of the right side, covering an area of approximately 4%. Excessive PFPE-OH may cause phase separation in the polyurea matrix and increase internal defects, weakening the barrier property of the coating.35

Table 8. Neutral salt spray test results of coatings with different PFPE-OH feeding ratios.

Sample JM-1 JM-2 JM-3 JM-4 JM-5
Average thickness 86.4 µm 83.7 µm 85.8 µm 86.5 µm 85.2 µm
Time (h) 2400 2880 3000 3000 2800
Main phenomenon S-spot Filamentous corrosion Good condition Good condition Numerous pinholes
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4. Conclusions

In this study, waterborne nano-polyurea coating with enhanced properties was developed by incorporating the fluoro modifier, PFPE-OH. This modification resulted in a simultaneous improvement in hydrophobicity and flexibility of the coating. The FT-IR and 1H NMR results confirmed a successful reaction between PFPE-OH and MDI, forming urethane linkages in the fluorinated grafted MDIs, while the 19F NMR results confirmed the introduction of PFPE side chains, which was further supported by the Raman results. The XPS results verified the enrichment of fluorinated segments at the coating surface, with the F atomic percentage reaching 38.92 at% for the modified sample (JM-4), and the uniform distribution of the F element displayed by SEM-EDS analysis demonstrated the feasibility of introducing fluorine-containing flexible segments into the polyurea matrix. With the increase in PFPE-OH content, the water contact angle of the coatings increased from 63.3° (for JM-1) to 108.4° (for JM-5), and the coating changed from hydrophilic to hydrophobic. At the same time, the elongation at break increased from 2.40% (for JM-1) to 3.98% (for JM-2), although the elongation at break decreased to 0.69% (for JM-5) when the PFPE-OH content was excessive, and the Shore hardness decreased from 61 to 42, indicating that the fluorine-containing low surface energy segments and the flexible ether functional groups in PFPE-OH could effectively reduce the free energy of the coating surface and weaken the rigidity of the system, thereby improving the deformation resistance and flexibility of the coating. An appropriate amount of PFPE-OH enhanced the thermal stability of the coating, as evidenced by the higher main-chain decomposition temperature and activation energy, whereas excessive PFPE-OH provided only limited additional improvement. Moreover, JM-3 exhibited the highest adhesion strength of 5.33 MPa, which was 10.58% higher than that of the unmodified coating (JM-1), indicating enhanced interfacial bonding. The application tests further showed that the relative abrasion resistance increased from 1.00 (JM-1) to 1.47 (JM-2), and the neutral salt spray resistance was extended from 2400 h (JM-1) to 3000 h (JM-3 and JM-4). The PFPE-OH modification could effectively improve the hydrophobicity, flexibility, abrasion resistance, salt spray resistance, and thermal stability of the waterborne nano-polyurea coatings. Excessive incorporation of PFPE-OH led to a decrease in mechanical strength, abrasion resistance, and adhesion performance. When the ratio of MDI to PFPE-OH was 1 : 0.08, the coating showed a good balance between hydrophobicity, flexibility, mechanical strength, corrosion resistance and adhesion, providing a reference for the application of waterborne nano-polyurea coatings in waterproofing, anti-corrosion, and complex substrate deformation environments.

Author contributions

Bingbing Bu: writing – original draft, methodology, investigation. Rui Zhang: data curation, formal analysis. Huixuan Wu: software, methodology. Shujie Liu: formal analysis. Chen Gao: writing – review & editing. Shihan Wu: data curation, validation. Dongliang Wu: conceptualization, resources, project administration. Shucheng Xie: validation. Fanhou Kong: funding acquisition, conceptualization, supervision.

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.

Acknowledgments

This work was supported by the Natural Science Foundation Project of Dongying (grant no. 2024ZR02), Science and Development Fund of Dongying (grant no. DJ2024006) and Shandong Institute of Petroleum and Chemical Technology (grant no. 2023kyqd015).

Data availability

Data will be made available on request.

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Associated Data

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Data Availability Statement

Data will be made available on request.

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