Mechanical, Structural, and Electrochemical Performance of Polyurethane Coatings for Corrosion Protection in Wind Energy Systems

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

Paper discusses properties of polyurethane modified with TiO2 addition as protective layer in big blades operating in the wind energy farms. From the practical point of view this is important paper and for the better acceptance I propose to make several corrections:

1. Chapter 1. Clarify what the SPIFT method is meaning
2. Chapter 1. Is it correct term: … Similarly, Liang et al. developed multifunctional carbon nanofiber (CNF) paper-based nanocomposite coatings incorporating Al₂O₃ and TiO₂ nanoparticles, short carbon fibers (SCFs), and graphite…. Is it really paper-based nanocomposite discussed ?
3. Add Chapter 2.3 with better described water-based polyurethane system: what were the solvents, what was viscosity, what was resin concentration.
4. Be more precise in Chapter 2.3 – what was concentration of added TiO2 particles 1, 3, 5 wt%. Was it referred to the all weight of polyurethane composition or was it related to the PU solid resin only and should be determined in the phr units (per hundred resin)
5. Interesting discussion of the FTIR spectra (Chapter 3.2) in Figure 7 and Table 1, with deconvoluted N-H stretching band. This part of paper is the most interesting this discussion can be more stressed.
6. Add something new in the research methods, even only in the discussion form, for example an highly accelerated aging procedures or crack progagation studied with the dye ingress.
7. There are several easy to correct editorial mistakes, which should be corrected:
8. Chapter 1: In sentence … TiO₂ particles have been widely employed in polymeric matrices for optical, photo-catalytic, and UV-protection applications, as well as in wastewater treatment, owing to their chemical stability, optical properties, UV absorption capacity, and low cost…. should be: TiO₂ particles have been widely employed in polymeric matrices for optical, photo-catalytic, and UV-protection applications, as well as in wastewater treatment, owing to their chemical stability, optical properties, UV absorption capacity and low cost.
9. Chapter 2.3. Change …30 PSI… to …30 psi….
10. How the films were prepared for the research described in the Chapter 3.5 ? Add more experimental details.
11. Correct line in the Chapter 3.5…. High TiO₂ concentrations (5%) exhibited poor compatibility with the PU matrix, restricting pol-ymer chain mobility…to the …. High TiO₂ concentrations (5%) exhibited poor compatibility with the PU matrix, restricting polymer chain mobility…

Author Response

 Thank you very much for your positive feedback on our manuscript.

1. Chapter 1. Clarify what the SPIFT method is meaning

 

It has been defined.

 

2. Chapter 1. Is it correct term: … Similarly, Liang et al. developed multifunctional carbon nanofiber (CNF) paper-basednanocomposite coatings incorporating Al₂O₃ and TiO₂ nanoparticles, short carbon fibers (SCFs), and graphite…. Is it really paper-based nanocomposite discussed ?

 

 

 

3. Add Chapter 2.3 with better described water-based polyurethane system: what were the solvents, what was viscosity, what was resin concentration.

It has been improved:

The TiO₂ particles (Ps) were incorporated into a water-based polyurethane (PU) resin (Polyform^Ò 3000, Pinturas Comex, Mexico). Gravimetric drying confirmed that the polyurethane (PU) resin contains approximately 40 wt% non-volatile solids, with typical viscosity of 80-120 KU and a density of 1.00-1.05 g/mL, consistent with aliphatic PU dispersions. TiO₂ particles was added at nominal concentrations of 1, 3, and 5 wt% relative to the as-received polyurethane (PU) resin, corresponding to the formulations PU/TiO₂ 1 %, PU/TiO₂ 3 %, and PU/TiO₂ 5 %, respectively.

 

4. Be more precise in Chapter 2.3 – what was concentration of added TiO2 particles 1, 3, 5 wt%. Was it referred to the all weight of polyurethane composition or was it related to the PU solid resin only and should be determined in the phr units (per hundred resin)

It has been improved:

Gravimetric drying confirmed that the polyurethane (PU) resin contains approximately 40 wt% non-volatile solids, with typical viscosity of 80-120 KU and a density of 1.00-1.05 g/mL, consistent with aliphatic PU dispersions. TiO₂ particles was added at nominal concentrations of 1, 3, and 5 wt% relative to the as-received polyurethane (PU) resin

 

5. Interesting discussion of the FTIR spectra (Chapter 3.2) in Figure 7 and Table 1, with deconvoluted N-H stretching band. This part of paper is the most interesting, this discussion can be more stressed.

It has been improved:

The neat PU exhibits a high fraction of hydrogen-bonded N–H (~58.8%) due to due to intrinsic interactions between N–H and C=O groups within the polymer chains (see Figure 7a). When 1 wt% TiO₂ is incorporated, the fraction of hydrogen-bonded N-H decreases to ~40.0% (see Figure 7b). At this loading, the TiO₂ particles provide only a limited number of surfaces –OH sites available to form hydrogen bonds, and they may partially disrupt existing N–H···C=O hydrogen bonds in the PU matrix.

A markedly different behavior is observed at 3% wt% TiO₂, where the hydrogen-bonded N-H fraction increases to ~64.7% (see Figure 7c). This enhancement is attributed to improved particle dispersion and a large effective TiO₂ surface area. Which introduces a greater density of -OH groups capable of forming hydrogen bonds with N-H groups in the PU. However, at 5 wt%, the bonded N-H fraction decreases again to ~53.6% (see Figure 7d). This reduction is consistent with the onset of TiO₂ agglomeration, which lowers the accessible surface area and limits the number of active -OH sites available to interact with N-H groups. Consequently, the hydrogen-bonding equilibrium shifts back toward lower interfacial interactions [33].

In conclusion, the PU/TiO₂ 3% coating exhibits the highest hydrogen-bonded N–H fraction (~64.7%), attributed to enhanced interactions between the –OH groups on the TiO₂ surface and the N–H groups in the PU. These results highlight the central role of hydrogen bonding in dictating dispersion quality, polymer chain mobility, and ultimately the microstructural cohesion of the PU/TiO₂ coatings [34,35].

 

6. Add something new in the research methods, even only in the discussion form, for example an highly accelerated aging procedures or crack progagation studied with the dye ingress.

It has been improved:

Other complementary techniques, such as dye ingress crack propagation analysis or periodic EIS monitoring, can also reveal damage that is not captured by short-term test [55]. To extend the present insights toward long-term performance, these accelerated aging protocols may be incorporated in future work. Importantly, they do not replace the present evaluation but rather complement it by simulating extreme service conditions relevant to offshore environments.

 

7. There are several easy to correct editorial mistakes, which should be corrected:

Chapter 1: In sentence … TiO₂ particles have been widely employed in polymeric matrices for optical, photo-catalytic, and UV-protection applications, as well as in wastewater treatment, owing to their chemical stability, optical properties, UV absorption capacity, and low cost…. should be: TiO₂ particles have been widely employed in polymeric matrices for optical, photo-catalytic, and UV-protection applications, as well as in wastewater treatment, owing to their chemical stability, optical properties, UV absorption capacity and low cost.

It has been corrected.

 

8. Chapter 2.3. Change …30 PSI… to …30 psi….

It has been changed.

 

9. How the films were prepared for the research described in the Chapter 3.5 ? Add more experimental details.

It has been improved.

 

10. Correct line in the Chapter 3.5…. High TiO₂ concentrations (5%) exhibited poor compatibility with the PU matrix, restricting pol-ymer chain mobility…to the …. High TiO₂ concentrations (5%) exhibited poor compatibility with the PU matrix, restricting polymer chain mobility…

It has been corrected.

Reviewer 2 Report

Comments and Suggestions for Authors

The manuscript examines the mechanical, structural, and electrochemical performance of polyurethane (PU) coatings reinforced with TiO₂ particles, with the aim of improving the protection of wind turbine blades. The study addresses a relevant and timely problem, particularly given the increasing industrial focus on mitigating leading-edge erosion in modern wind energy systems. The paper is clearly structured and employs a broad range of characterization techniques, including XRD, FTIR, EIS, hardness testing, tensile testing, and adhesion measurements.

Despite these strengths, some aspects require further clarification. Therefore, major revisions are required. The main concerns involve:

1. The manuscript states that the TiO₂ particles used have sizes between 74 and 150 µm:

“The TiO₂ particles, with diameters ranging from 74 to 150 µm, … were supplied by LJQMETAL…”

These are large particles, orders of magnitude bigger than those typically used in PU/TiO₂ composite literature, which usually relies on nano-TiO₂. Yet, the discussion repeatedly interprets data in terms of nanoparticle-like dispersion and surface –OH interactions:

“The incorporation of TiO₂ improved the mechanical and electrochemical performance… FTIR and XRD confirmed that low TiO₂ loadings were well dispersed due to hydrogen bonding…”

Such molecular-scale interaction mechanisms are not plausible with 100 µm particles unless surface area is dramatically increased or the particles are porous. This contradiction must be addressed. The discussion should be rewritten to reflect the actual particle dimensions, or the authors must clarify whether the supplied material contained a large fraction of fines.

2. The introduction emphasizes erosion as the key performance requirement:

“Leading edge erosion (LEE) … one of the most severe forms of damage…”

“Composite coatings… absorb and diffuse impact energy generated by repeated strikes… resulting in strong erosion resistance.”

However, no erosion experiments (liquid or solid particle) were conducted. The conclusions claim that PU/TiO₂ coatings “are promising candidates for mitigating erosion,” yet this is inferred only from mechanical/electrochemical tests—not actual erosion data.

This must be explicitly acknowledged, or a minimal erosion test must be added. Otherwise, conclusions regarding erosion protection are overstated.

3. The coating thickness is stated as “approximately 300 µm” using a 60-second spray:

“The application time was 60 seconds, which allowed for an approximate thickness of 300 µm…”

No information is provided on:

• variation between samples,
• whether thickness was equalized across formulations,
• whether the SEM shown is representative.

Since thickness strongly influences corrosion resistance, a statistical summary is required.

4. The authors argue that the 3% TiO₂ sample has the best dispersion, citing FTIR:

“PU/TiO₂ 3% displays the highest fraction of hydrogen-bonded N–H (~64.7%), attributed to better dispersion…”

However, for hardness, they claim the same formulation suffers from agglomeration:

“PU/TiO₂ 3% exhibited a slight decrease in hardness, likely due to the presence of localized TiO₂ agglomerates…”

These interpretations contradict each other. Either 3% TiO₂ is well dispersed or it is agglomerated—both cannot be simultaneously true. The authors must reconcile the mechanical and spectroscopic interpretations with a coherent microstructural explanation.

5. The EIS data fitting uses the equivalent circuit R(QR), but the rationale is not explained. The manuscript states:

“The EIS data were fitted using the R(QR) equivalent circuit…”

But no justification is given for why:

• only one time constant is assumed,
• no coating degradation or water uptake is modeled,
• the 5% sample allegedly presents a second time constant (not visible in the figures provided).

Furthermore, the strong claim:

“PU/TiO₂ 3% exhibits the highest Rc, confirming… significantly enhanced corrosion resistance.”

should be supported by including Bode phase plots or visual evidence of the second time constant mentioned later in the text.

6. The manuscript concludes:

“PU/TiO₂ 1% sample exhibited the highest adhesion strength (15.3 MPa) … The behavior can be attributed to improved dispersion…”

But the trend (1% > 3% > 5%) is likely influenced not only by dispersion but also by:

• surface roughness changes,
• stress concentration from large particles,
• local debonding due to 100 µm inclusions.

These effects are absent from the discussion and should be incorporated for a more realistic interpretation.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

The authors have addressed the suggestions I made in my initial review. I consider the manuscript to be ready for publication.