# Self-stratifying epoxy/thermoplastic polyurethane coatings designed for aerospace applications

Adrien Lebeau, Mathilde Casetta, Séverine Bellayer, A. Walter, S. Suel, Maude Jimenez

► To cite this version:

Adrien Lebeau, Mathilde Casetta, Séverine Bellayer, A. Walter, S. Suel, et al. Self-stratifying epoxy/thermoplastic polyurethane coatings designed for aerospace applications. Progress in Organic Coatings, 2025, Progress in Organic Coatings, pp.109478. 10.1016/j.porgcoat.2025.109478. hal-05223874

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# Self-stratifying epoxy/thermoplastic polyurethane coatings designed for aerospace applications

A. Lebeau<sup>a,b</sup>, M. Casetta<sup>a</sup>, S. Bellayer<sup>a</sup>, A. Walter<sup>b</sup>, S. Suel<sup>b</sup>, M. Jimenez<sup>a,c,*</sup>

## ARTICLE INFO

### Keywords:

- Self-stratifying coatings

- Epoxy/thermoplastic polyurethane

- Aerospace applications

- Mechanical performance

- Environmental sustainability

## ABSTRACT

Coatings with various functionalities (e.g., adhesive, antibacterial, flame-retardant) usually require multiple layers, such as a primer and a finish layer, to deliver a specific surface function. However, these multi-layer systems involve complex application and curing processes that generate more by-products, consume extra energy, and extend processing times which are impractical for industrial applications such as aeronautics and lead to increased environmental impacts. Furthermore, issues such as interlayer adhesion failure and aging may arise due to the distinct chemical nature of the layers, that can lead to severe defects and induce corrosion on airplanes. Reducing the number of layers while maintaining or enhancing performance is therefore critical. Self-stratifying coatings offer a promising solution by forming multilayer structures in a single application and curing step directly on various substrates. This approach simplifies the process and improves eco-efficiency by lowering material waste, energy consumption, and production time. In this paper, a self-stratified epoxy/thermoplastic polyurethane coating is designed for the first time. The use of a sulfur-containing compound as curing agent in the system leads to a less energy consuming process thanks to the reduction of the curing temperature compared to traditional amine curing agents. The presence of sulfur atoms also facilitates the layers differentiation in the self-stratified coating. Finally, excellent adhesion and mechanical properties are obtained, out-performing a conventional step-by-step bi-layer system. This self-stratifying coating is therefore very promising for aeronautical applications.

## 1. Introduction

To provide protection to a material via its surface without affecting its bulk properties, different layers of coatings — such as a primer, a functional coating and a topcoat — are usually successively applied. These multi-layer systems involve intricate application and curing processes. The numerous formulations, application, and processing steps not only generate waste and contribute to environmental pollution but also demand substantial energy consumption to produce a solid film. Furthermore, these extensive procedures result in prolonged application times, which are non-viable for industrial operations, and in particular for aerospace applications. Additionally, issues such as adhesion failure and aging may arise due to the different chemical compositions of these complex structures. Consequently, minimizing the number of layers while maintaining or exceeding the performance of current systems is highly desirable, enabling the creation of multilayer coatings from a

single-layer formulation.

In this context, the self-stratifying approach emerges as a promising solution for the future, enabling the formation of complex multilayer or gradient coating structures in a single step directly on various substrates (metal, plastic, composite, etc.) (Fig. 1) [1,2].

This approach combines optimized surface and adhesion properties within a single coating composition. Each layer serves a distinct purpose: the bottom layer enhances adhesion and protects the substrate, while the top layer offers aesthetic appeal and shields against environmental conditions [3]. The lower layer is referred to as the base resin, and the upper one as the stratifying resin. The benefits of self-stratifying systems include: (i) Simplified application: installation and cleaning efforts are reduced, potentially increasing production speed, (ii) Cost and time efficiency: efforts are focused on minimizing coating process costs using faster curing methods, reducing solvent usage, and lowering energy consumption, (iii) Multifunctional capabilities: systems easy to

<footnote><sup>a</sup> Univ. Lille, CNRS, INRAE, Centrale Lille, UMR 8207 - UMET - Unité Matériaux et Transformations, 59000 Lille, France</footnote>

<footnote><sup>b</sup> Airbus Central R&T, Nantes, France</footnote>

<footnote><sup>c</sup> Institut Universitaire de France (IUF), Paris, France</footnote>

<footnote>* Corresponding author at: Univ. Lille, CNRS, INRAE, Centrale Lille, UMR 8207 - UMET - Unité Matériaux et Transformations, 59000 Lille, France.
E-mail address: maude.jimenez@univ-lille.fr (M. Jimenez).</footnote>
apply, multifunctional, and compatible with various substrates can be
developed, (iv) Enhanced coating performance: inter-layer adhesion
defects are reduced compared to traditional multi-layer systems,
improving overall coating durability and functionality.

These kinds of coatings are formed from the mixing of incompatible
polymers in a common solvent blend, followed by their spontaneous
separation during application and curing [4]. Compared to traditional
multilayered systems, self-stratification reduces application steps and
chemical usage, offering both economic and environmental advantages
[5]. The self-stratification phenomenon has been explored by several
researchers. In the 1970s, Funke introduced the concept of blending
incompatible powder polymers to create multifunctional self-layered
coatings, initially attributing stratification to density differences but
later emphasizing viscosity and surface energy [6,7]. In the 1980s,
Verkholantsev linked phase diagrams and solvent evaporation to strat-
ification, identifying additional influences such as layer thickness and
curing agents [8]. More recently, Baghdachi proposed that stratification
in thermosetting systems is primarily governed by kinetic factors [5].
Lemesle et al. concluded that the surface energy and polarity of the
selected resins influence their separation. Specifically, (i) a large dif-
ference in surface energy between the two resins and (ii) a base resin
more polar than the stratifying resin enhance their incompatibility,
promoting phase separation and, consequently, stratification. The most
significant result is that curing temperature is a critical parameter for
achieving phase separation and self-stratification. Additionally, resin
incompatibility is enhanced by an increasing molecular weight (MW)
difference between the two resins during the cross-linking of the
thermoset.

The self-stratification process offers a promising alternative to
multilayer coating systems. However, it remains a complex phenome-
non, influenced by numerous parameters, including solvent evapora-
tion, curing reactions, surface tension, coating thickness, and
temperature. Optimizing these factors and understanding their role is
critical for the successful development of self-stratifying systems, which
requires in-depth research. In fact, studies often yield contradictory
conclusions, and a comprehensive, systematic approach to determine
the parameters governing self-stratification is necessary.

In this study, an epoxy/thermoplastic polyurethane (TPU) system was selected for its potential in the aeronautical sector. Aerospace coatings experience mechanical stresses from aerodynamic forces, vibrations, and impacts from debris. They also have to withstand extreme environmental conditions, including UV radiation, temperature fluctuations, humidity, and exposure to aggressive chemicals such as hydraulic fluids and de-icing agents. TPU, being resistant to scratches, to UV degradation, oxidation, and weathering, can thus be considered as ideal for the top layer of the coating, bringing long-term outdoor performance. Epoxy resins, on the other hand, were selected as the base coat because of their excellent adhesion to a wide variety of substrates, as well as their high flexibility and superior mechanical and chemical resistance. For aerospace applications, the integration of self-stratifying coatings could offer a significant advantage by streamlining application processes and meeting stringent requirements in terms of resistance to various fluids and to weathering conditions.

In this study, the self-stratification approach is being applied for the
first time in the case of an epoxy/TPU system. The development of a

functional epoxy/thermoplastic polyurethane (TPU) self-stratifying
coating will first be described. Then, the adhesion performance of the
coating will be compared to that of a traditional multilayered system.

## 2. Experimental

### 2.1. Chemicals

To formulate the epoxy/thermoplastic polyurethane self-stratifying
coating, DGEBA (DER 332 Bisphenol-A epoxide, from Sigma-Aldrich)
and Desmolac® 2100 (Covestro) were selected as the thermosetting
epoxy and the thermoplastic polyurethane respectively. To crosslink the
epoxy resin and create a 3D network, the catalysis of a polymercaptan by
an amine was considered: Pentaerythritol tetrakis(3mercaptopropio-
nate) (PETMP) was selected as the polymercaptan and triethyl amine
(TEA) as the catalyst. PETMP was added in stoichiometric quantity and
TEA was added in a 3 wt% ratio to the final blend. Other curing agents
(Isophorone diamine (IPDA), diethylene triamine (DETA), both pur-
chased from Sigma-Aldrich) were also tested to select the best curing.
The approach based on the Hansen Solubility Parameter was applied to a
range of 38 commercial organic solvents to determine the couple of low
toxicity solvents adapted to the two-resin system. A blend of ethyl
lactate/cyclohexanone (50/50) (both purchased from Sigma-Aldrich)
was used without any purification.

### 2.2. Substrate

Aluminum plates (aluminum grade 2024, 120 × 80 × 1.4 mm³) coated with a mandatory yellow primer for aeronautical applications were used as substrates.

### 2.3. Formulation and application

To prepare the coating, each resin (epoxy and TPU) – the epoxy and
polythiol system being considered as a single resin – was individually
dissolved at 30 wt% in the ethyl lactate/cyclohexanone solvent blend.
Then, the two solutions were combined at a 1:1 ratio and mixed together
for 5 min. The catalyst was then added to the mixture in a 3 wt% ratio
relative to the total mass. The final solution was stirred for an additional
5 min before application. Coatings were sprayed on the substrate in
order to obtain a 190 µm thick humid film thanks to an HVLP spray gun
(Fpro G Airspay from SamesKremlin, France) with a compact buzzard
and stainless-steel nozzle of 1.2 mm using an air pressure of 3 bars in a
spray booth (Labo 600 from Tricolor Industries, France). After applica-
tion, the coating was left for 1 h at room temperature to prevent the
premature cross-linking of the thermoset polymer and to allow the sol-
vents to start evaporating. The coating was then cured in an oven
(XU058 from France Etuves, France) for 2 h at different temperatures,
based on the DSC results described in Section 3.2. Formulation, appli-
cation and curing steps are summarized in Fig. 2.

### 2.4. Preparation of classical multilayer coating

The two resins were also applied separately, layer-by-layer. The aim
is to assess the mechanical performance of the coating prepared using a

<figure><img src="image_2.png" /><figcaption>**Fig. 1.** Classic multilayered coating VS. self-stratifying coating concept.</figcaption></figure>
<figure><img src="image_3.png" /><figcaption>Fig. 2. Self-stratifying coating elaboration process.</figcaption></figure>

2-step process so as to compare it with the self-stratified system. The two solutions containing each of the resins dissolved in the solvent blend were prepared as described for the self-stratifying coatings. The epoxy solution was applied by spraying in the same conditions as for the self-stratifying coatings and then cured for 2 h at 110 °C. The TPU solution was applied using a bar coater due to its very high viscosity, and then dried in an oven for 2 h at 90 °C. All steps are summarized in Figs. 3 and 4.

## 2.5. Differential Scanning Calorimetry (DSC)

Enthalpy measurements were conducted using Differential Scanning Calorimetry (DSC) with a DSC Discovery, TA. Prior to analysis, 1 g of epoxy resin was mixed with the corresponding weight of hardener, calculated based on the epoxy equivalent weight (EEW). Approximately 10 mg of this mixture was placed into an aluminum pan that was then sealed with a lid. The degree of conversion (%) was calculated using equation (Eq. (1)).

$$ \alpha = \frac{\Delta H_{\text{react}} - \Delta H_{\text{post}}}{\Delta H_{\text{react}}} \times 100\% \qquad (1) $$

The total heat of curing, ΔH<sub>react</sub> (J.g<sup>-1</sup>), was determined via a non-isothermal scan, while ΔH<sub>post</sub> (J.g<sup>-1</sup>) represents the enthalpy of reaction after 1 h of isothermal heating at specific curing temperatures (between 50 and 110 °C). To measure ΔH<sub>react</sub> samples were heated from -40 °C to 230 °C at a rate of 5 °C/min, cooled back to 40 °C, and then subjected to the same temperature ramp again. The second ramp ensures complete cross-linking, and ΔH<sub>react</sub> is calculated by integrating the signal from the first ramp. To determine ΔH<sub>post</sub>, samples were first heated isothermally at the target temperature (between 50 °C and 110 °C) for 1 h, followed by the two successive temperature ramps done to determine the first enthalpy ΔH<sub>react</sub>. The temperature corresponding to the maximum of exothermic peak represents the temperature of the maximum heat flow of the epoxy/hardener reaction.

## 2.6. Characterization

### 2.6.1. Stratification assessment

To determine the stratification level, coating cross sections were prepared for electronic microscopy. The samples were embedded in an epoxy resin (DBF 010 005 001 resin/hardener HY 956 010 005 002) mixed at a 4:1 ratio (ESCIL, France). After curing for 48 h at room temperature, the samples were polished with SiC polishing sheets (ESCIL, France) from coarse (grade 80) to fine (grade 4000) grit. Finally, they were carbon-coated using a Bal-Tec SCD sputter coater to produce clear and conductive cross-sections suitable for microscopic analysis. A Cameca SX100 electron probe microanalyzer (EPMA) was used to perform elemental analysis. Back scattered electron images were carried out at 15 kV, 15 nA and X-ray mappings were carried out at 15 kV, 40 nA. For mappings, a PC2 crystal was used to detect the O Kα X-ray, a TAP crystal to detect the Al Kα X-ray, a LiF crystal to detect the Fe Kα X-ray and a PET crystal to detect the S and Ti Kα X-ray. The determination of coatings stratification levels is made according to the classification defined by Toussaint [9]. The ratings are ranging from Type I for a perfectly stratified coating to Type IV for a coating composed of large islands of resin.

### 2.6.2. Coating performance assessment

The properties of the coating were mechanically evaluated in terms of adhesion and scratch resistance. The cross-hatch cutter test (ISO 2409) and the pull-off test (ISO 4624) were used for adhesion assessment. The cross-hatch cutter test, evaluates coating adhesion by scoring a grid pattern through the coating to the substrate. Adhesion is rated from grade 0 (optimal adhesion) to grade 5 (poor adhesion), providing a quick qualitative measure of coating's performance. Each self-stratifying sample was tested for adhesion seven days after application. The samples were then immersed in water for 14 days, was evaluated again. The pull-off test, measures coating adhesion by applying a force to detach a bonded dolly from the coating surface. The maximum tensile stress required for detachment is recorded, providing a quantitative measure of adhesion strength. The failure profile is mainly analyzed as it gives important information about the mode of failure: adhesive or cohesive. The ISO 1518 test was used to determine the scratch resistance. It consists in the application of a maximum load of 20 N to a stylus and the subsequent observation of the resistance to penetration to the substrate. These standardized methods enable a comprehensive characterization of the mechanical performance of the coatings.

<figure><img src="image_4.png" /><figcaption>Fig. 3. Classical bi-layer coating elaboration steps.</figcaption></figure>
<figure><img src="image_5.png" /><figcaption>Fig. 4. Application of the TPU layer using a bar coater.</figcaption></figure>

# 3. Results and discussion

## 3.1. Hansen solubility parameter

The first studies on solubility parameters were conducted by Hildebrand and Scott [10,11] to define hydrogen and polar bonding interactions. A key question was whether hydrogen bonding significantly influences solvent retention. Solvents, polymers, resins, emulsifiers, pigments, plasticizers and dyes were characterized under this framework to evaluate solubility, mutual solubility, and the compatibility of polymers, resins, and phases in pigment dispersion. These experiments introduced a new empirical concept that incorporates all the energies involved in the process: the Hansen solubility parameter (E), also known as the “three-dimensional solubility” parameter. This approach describes phenomena with significant solvent effects and characterizes materials based on their interactions with a series of solvents. By studying solvent properties from this perspective, the method enabled predictions of paint behavior during manufacturing and drying processes.

### 3.1.1. Prediction of stratification

The solubility behavior of chemicals was evaluated using Hansen Solubility Parameters (HSP) in two steps. First, the solubility sphere of each resin was defined based on its interaction with a range of solvents, with good solvents positioned inside the sphere and poor ones outside. A mathematical model determined the sphere's minimum radius, though some contradictions may occur due to the theoretical nature of the model. Secondly, resins compatibility was assessed by comparing their solubility spheres: the degree of overlap predicts compatibility, with minimal overlap indicating partial compatibility and aiding in stratification predictions [12].

It has been determined that the overlap between the two solubility spheres should not exceed 80 % of the sphere volume to ensure stratification [3,13–15]. In this study, no overlap is observed (Fig. 5), predicting strong incompatibility between the two resins. According to the model, this suggests a potential for system stratification. Additionally, the literature highlights the critical role of solvent selection in the stratification process [9,14–17].

### 3.1.2. Selection of solvents

The selected solvent or solvent blend must dissolve both resins to enable formulation preparation and substrate application, while also meeting specific polarity and volatility requirements. The TPU is reported to be soluble in MEK only as mentioned in its technical data sheet [18]. A study was conducted to find out other usable solvents fitting the best parameters leading to stratification. A preliminary screening was carried out by dissolving the TPU at approximately 1 wt% at 25 °C in 38 different commercial organic solvents. This initial step was carried out

<figure><img src="image_6.png" /><figcaption>Fig. 5. Solubility sphere of Desmolac 2100 (green) and DER 332 (blue). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>

to quickly remove ineffective solvents from the list without using an excessive amount of polymer for dissolution tests. Among the 38 solvents tested, 16 were effective at 1 wt%. The TPU concentration was then increased to approximately 30 wt%. This concentration has been chosen considering successful self-stratified systems described in the literature. The solvents dissolving TPU at 30 wt% are listed in Table 1, with the corresponding technical information. According to the literature, the chosen solvents should neither be too volatile nor too heavy for the self-stratification to occur [1]. Additionally, for environmental and health considerations, toxicity criteria based on the ACS ranking [19] were considered. ACS ranking is based on (i) a safety score, (ii) a health score and (iii) an environmental score, leading to 4 different designations: recommended, problematic, hazardous and highly hazardous.

The final selection of the solvent mixture was made following two criteria: (i) a vapor pressure (representative of volatility) ranging between 2 and 10 hPa at 20 °C [2] and (ii) solvents classified as hazardous or highly hazardous must be banned. Previous work done in the laboratory on DGEBA by Lemesle et al. [2,20] also guided the selection of
<table><caption>**Table 1**
Solubility results at 30 wt% of TPU Desmolac 2100 in various solvents.</caption><thead><tr><th>Name</th><th>CAS</th><th>Vapor pressure (hPa, 20 °C)</th><th>ACS ranking</th></tr></thead><tbody><tr><td>1,3-Dioxolane</td><td>646-06-0</td><td>93</td><td>Hazardous</td></tr><tr><td>Acetone</td><td>67-64-1</td><td>245</td><td>Recommended</td></tr><tr><td>Acetophenone</td><td>98-86-2</td><td>0.45</td><td>Problematic</td></tr><tr><td>Carbon disulfide</td><td>75-15-0</td><td>274</td><td>Highly hazardous</td></tr><tr><td>Cyclohexanone</td><td>108-94-1</td><td>7</td><td>Recommended</td></tr><tr><td>Dimethyl sulfoxide (DMSO)</td><td>67-68-5</td><td>0.55</td><td>Recommended</td></tr><tr><td>Ethyl lactate</td><td>97-64-3</td><td>3</td><td>Problematic</td></tr><tr><td>Isophorone</td><td>78-59-1</td><td>0.3</td><td>Problematic</td></tr><tr><td>Methyl isobutyl ketone (MIBK)</td><td>108-10-1</td><td>20</td><td>Recommended</td></tr><tr><td>N,N-Dimethyl formamide (DMF)</td><td>68-12-2</td><td>3.77</td><td>Hazardous</td></tr><tr><td>n-Methyl-2-pyrrolidone (NMP)</td><td>872-50-4</td><td>0.32</td><td>Hazardous</td></tr><tr><td>Pyrrole</td><td>109-97-7</td><td>8.7</td><td>Recommended</td></tr><tr><td>Tetrahydrofuran (THF)</td><td>109-99-9</td><td>170</td><td>Problematic</td></tr><tr><td>γ-Butyrolactone (GBL)</td><td>96-48-0</td><td>0.34</td><td>Problematic</td></tr></tbody></table>

solvents to be tested, specifically those that effectively solubilize DGEBA at 30 wt% and 25 °C. Ultimately, the two selected solvents were ethyl lactate and cyclohexanone. In fact, ethyl lactate is classified as problematic only due to the environmental criteria.

## 3.2. Epoxy cross-linking

Epoxy resins, classified as thermoset polymers, are valued for their low-pressure production, minimal cure shrinkage, and low residual stresses. Their versatility allows operation across a wide temperature range, depending on the curing agent used to control cross-linking. They are available in both low-viscosity liquid and solid (powder) forms [21]. It was also demonstrated in the literature by Lemesle et al. that a conversion rate higher than 90 % was needed to obtain a stratified system [2].

### 3.2.1. Classical approach: epoxy/amine reactivity

Classically, epoxy resins are cross-linked following a well-documented epoxy/amine reaction mechanism. The three steps are as follows: (i) a primary amine reacts with an epoxy group to form a secondary amine; (ii) the secondary amine then reacts with another epoxy group to create a tertiary amine; (iii) finally, a reacted epoxy group and an unreacted epoxy group react together to form an ether bridge [22]. These steps are illustrated in Fig. 6.

This technique is advantageous due to its ease of use, as many di- or tri-amines effectively facilitate the reaction [23].

### 3.2.2. Polymercaptan curing agents

Polymercaptans and polysulfides are aliphatic oligomers containing sulfhydryl (-SH) groups that react with epoxy groups at relatively low temperature (generally from -20 °C to 70 °C) [23] to form cured epoxy structures.

The chemistry of epoxy/mercaptan systems involves a tertiary amine catalyst forming a salt with the mercaptan, thereby generating a mercaptide anion, which acts as a strong nucleophile. The mercaptide anion readily opens the epoxy ring, and its reaction with another mercaptan group regenerates the mercaptide anion as shown in Fig. 7.

<figure><img src="image_7.png" /><figcaption>**Fig. 6.** Chemical reactions between epoxy and amine monomers to create a cured epoxy resin. (a) Primary amine reaction, (b) Secondary amine reaction and (c) Etherification (adapted from [22]).</figcaption></figure>

**Activation**

$$RSH + R'^{3}N \leftrightarrow RS^{-} + R'^{3}N^{+}H$$

**Propagation**

$$RS^{-} + epoxy \rightarrow RSCH_{2}CH(O^{-})CH_{2}OR'' \\ RSCH_{2}CH(O^{-})CH_{2}OR'' + RSH \rightarrow RSCH_{2}CH(OH)CH_{2}OR'' + RS^{-}$$

<caption>**Fig. 7.** Polymercaptan-epoxy chemistry accelerated with the use of a tertiary amine (adapted from [24]).</caption>

Curing an epoxy with a polymercaptan offers two significant advantages: (i) The curing temperature is lower compared to traditional epoxy/amine curing systems leading potentially to a less energy consuming process; (ii) The incorporation of sulfur into the thermoset's 3D network provides a marker detectable via EDX analysis, simplifying the identification of stratified layers. The sulfur-containing layer corresponds to the epoxy, while the sulfur-free layer corresponds to the TPU.

### 3.2.3. Thermal assessment

As previously mentioned, curing an epoxy molecule with a polysulfide enables the process to occur at relatively low temperature (from -20 °C to 60 °C). Literature review indicates that this reaction can take place at room temperature, depending on the selected curing agent [23].

In order to evaluate the difference in the curing temperature between amino-curing and polysulfide curing, the reaction between the DGEBA epoxy resin and different curing agents was studied by DSC. The temperature of the exothermic peak of epoxy/amine or epoxy/polysulfide reaction and the reaction rate were more specifically investigated for different temperatures during a 1-hour curing period. More precisely, using the method described in Section 2.5, the conversion rates of the different curing agents versus the temperature were drawn in Fig. 8.

The curing behavior of the amines (IPDA and DETA) is really different from that of PETMP. At lower temperature (around 50 °C), amines demonstrate greater efficiency with a conversion rate superior to 80 % compared to a value of 75 % for PETMP. On the contrary, a 95 % conversion rate is achieved after 1 h of curing at 90 °C with IPDA and DETA whereas PETMP reaches the same conversion rate at only 70 °C. This result confirms that the curing of an epoxy with a polysulfide can be performed at lower temperature than with amine curing agents, allowing to save energy and thus improving the eco-efficiency of the process, particularly when high conversion rates are needed. As literature studies showed that a conversion rate higher than 90 % is necessary to obtain a stratified coating, PETMP has been used as curing agent with the epoxy/TPU system.
<figure><img src="image_8.png" /><figcaption>Fig. 8. Conversion rate of different curing agents versus temperature.</figcaption></figure>

### 3.3. Application to epoxy/TPU system

The following step was to determine the stratification level of the two resins system. Mappings of sulfur on cross-section samples allow assessing whether the epoxy has migrated to the top or bottom of the coating. Based on the separation behavior of the two resins, the stratification level can then be ranked according to the commonly used classification system [2].

#### 3.3.1. Proof of concept

The incorporation of polymercaptans into the thermoset network facilitates the characterization of the observed layer. Coatings were prepared varying the curing temperature: 110 °C and 90 °C were tested successively.

When cured at 110 °C (Fig. 9b), a high sulfur concentration is observed at the bottom of the coating, indicating that the epoxy blend started to migrate downward, while the TPU blend remained at the top, resulting in a Type I stratification. However, between these regions, a

uniform sulfur concentration is observed, indicating that the coating primarily exhibits a Type II stratification or some kind of mix between Type I and II. While the majority of sulfur is positioned in the intermediate zone, the higher concentration at the bottom suggests that further improvement is possible to achieve a more distinct separation. Additionally, this gradient-like distribution avoids abrupt interfaces between layers, potentially improving mechanical performance while maintaining the functional characteristics of a dual-layer structure.

When the coating is cured at 90 °C, a different trend is observed (Fig. 10b). The coating can be divided into two distinct sections: the bottom half of the coating exhibits a homogeneous and relatively high sulfur concentration, while the top half shows a homogeneous and relatively low sulfur concentration. A Type I coating is formed in these conditions, suggesting that the decrease of the curing temperature promotes the separation of the resins. In fact, this temperature decrease can have two effects: (i) the curing of the thermoset epoxy is slowed down, leaving enough time for the two resins to separate properly; (ii) the slower evaporation of solvents leads to a more gradual increase of

<figure><img src="image_9.png" /><figcaption>Fig. 9. BSE image (a) and EDX mapping (b) of sulfur on a cross-section of an epoxy/TPU self-stratifying coating cured at 110 °C.</figcaption></figure>
<figure><img src="image_10.png" /><figcaption>Fig. 10. BSE image (a) and EDX mapping (b) of sulfur on a cross-section of an epoxy/TPU self-stratifying coating cured at 90 °C.</figcaption></figure>

the viscosity, facilitating phase separation. The solvent evaporation rate and the curing kinetics have been identified as critical factors influencing the stratification process. Langer et al. showed that phase separation happens immediately after the coating is applied, driven by solvent evaporation [17]. Similarly, Lemesle et al. found that solvent volatility plays a key role in self-stratification. Using a solvent that is too volatile or not volatile enough can prevent stratification. They identified an optimal range of solvent vapor pressure that allows stratification to occur in their specific epoxy/silicon system [2].

Thus, the conditions of the self-stratifying process can be optimized to achieve either a Type I or a Type II coating, depending on the requirements.

The next step consists in evaluating the mechanical performance of the coatings and comparing them to those of conventional multilayer systems. The conditions used to prepare the coatings were: a 1-hour curing at room temperature followed by a 1-hour curing at 90 °C. Three samples were tested for each characterization.

### 3.3.2. Comparison to conventional multilayer system

#### 3.3.2.1. Cross-hatch cutter test.
Adhesion properties were tested on both self-stratified and conventional bi-layer systems before and after water immersion as described in Section 2.6.2. Adhesion properties of the self-stratified coatings before and after immersion in water are the same: a grade 0 rating is systematically obtained, demonstrating a perfect adhesion to the substrate (Fig. 11a). The coating remained intact, with no peeling or degradation.

On the contrary, a significant peeling off of the coating is observed for the conventional bilayer system, indicating poor TPU-epoxy adhesion. Fig. 11b shows the poor result for the conventional bi-layer system without water immersion. The coating has peeled off, resulting in a very low adhesion rating. According to ISO 2409, this system would be rated between grade 2 and grade 3.

The primary goal of this experiment was to compare the performance of the self-stratifying system against the conventional bi-layer reference sample. Since the reference sample already demonstrated significantly inferior adhesion under dry conditions, it was not immersed in water.

This result was sufficient to confirm the superiority of the self-stratifying coating, which, in contrast, maintained a grade 0 rating even after 14 days of immersion in water.

#### 3.3.2.2. Pull-off test.
The pull-off test measures adhesion by applying force to detach a bonded dolly, focusing on the failure profile. The analysis was conducted in accordance with the routine verification conditions of Airbus, which prioritizes the analysis of the failure profile over the quantitative force value. The mode of failure, i.e., cohesive (within a layer) versus adhesive (between layers) is the critical performance indicator as it provides strong evidence of the weakest link in the coating system. Pictures of the sample and dolly sides after the pull-off test are presented in Table 2. For all tested self-stratified samples, the presence of yellow primer is observed on both the sample and the dolly, in a relatively uniform layer and with a matte aspect. Thus, a cohesive failure is obtained within the yellow primer layer, i.e. in the layer between the substrate and the self-stratified coating, confirming that adhesion is optimal both inside the coating and between the coating and the primer. On the contrary, the conventional bi-layer coating samples exhibit adhesive failure between the TPU and epoxy layers, as shown in

<table><caption>Table 2
Pull-off results.</caption><thead><tr><th></th><th>Self-stratifying coating</th><th>Conventional bi-layer</th></tr></thead><tbody><tr><th>Sample side</th><td></td><td></td></tr><tr><th>Dolly side</th><td></td><td></td></tr></tbody></table>

<figure><img src="image_11.png" /><figcaption>(a) Cross-hatch cutter test on self-stratifying coating sample (b) Cross-hatch cutter test on conventional bi-layer coating</figcaption></figure>

<caption>Fig. 11. Cross-hatch cutter tests results.</caption>
**Table 2.** Two main factors support this conclusion: (i) the adhesive failure surface has a glossy finish on the sample side, unlike the matte appearance of cohesive failure on both sides, and (ii) the thickness of the remaining film on the sample and dolly sides was measured after the test and allowed confirming the exact location of failure.

3.3.2.3. *Scratch resistance.* In line with performance requirements for aeronautical standards, the primary evidence of success for this test is the absence of any penetration through the coating down to the substrate when a specified load is applied. Multiple tests were performed on the designed coating, applying forces up to a maximum load of 20 N. As shown in Fig. 12, for all tests, no penetration to the substrate was observed. This result confirms that the coating meets this critical performance standard. The excellent scratch resistance was the expected outcome, given that the TPU, known for its durability, successfully migrated to the top layer of the coating during stratification.

The comparison with the conventional bi-layer coating system is not relevant for this test, as TPU is already positioned at the surface, being applied in the second stage.

Thus, for all adhesion and mechanical tests, a performance equivalent or even superior to that of the conventional bi-layer system is obtained, demonstrating the benefits of the self-stratifying process.

## 4. Conclusion

This study demonstrates the successful development of a self-stratifying epoxy/thermoplastic polyurethane (TPU) coating for aeronautical applications, combining the functional properties of traditional multilayer systems with a single-step, eco-efficient application process. By reducing the number of layers sprayed and simplifying the curing steps, this approach minimizes material waste, energy consumption, and production time, making it a more sustainable alternative to conventional coatings. The use of PETMP as curing agent allowed reducing the curing temperature to 1 h at 90 °C instead of 2 h at 110 °C, leading to a less energy consuming process. In those conditions, a type I self-stratified coating was obtained with a high amount of epoxy resin in the bottom layer as evidenced by the high concentration of sulfur. The performance in terms of adhesion properties is generally superior to that of the conventional step-by-step bi-layer system. With the cross-hatch cutter test, a perfect adhesion to the substrate is observed before and

after immersion in water whereas portions of the bi-layer coating peel off even without water immersion. In the case of the pull-off test, a cohesive failure within the primer layer is obtained contrary to an adhesive failure between TPU and epoxy layers for the bi-layer system. These results are very promising if aeronautical applications are considered.

The use of PETMP as a curing agent perfectly fulfilled its role of reducing curing temperature in favor of eco-efficiency. Through systematic characterization, the coating exhibits excellent adhesion and mechanical performance, with stratification profiles confirmed by cross-sectional microscopy. These performances are especially appreciated when compared to a conventional step-by-step bi-layer system. The self-stratified system designed demonstrates superior performance, particularly in the pull-off test, where adhesive failure is observed at multiple interfaces.

The use of alternative bio-based resins could also be explored to align with growing sustainability goals in the coatings industry. It is important to note that the primary objective of this study was to establish the proof-of-concept for the epoxy/TPU self-stratifying system. While materials were selected for their potential in the aerospace sector – including the known UV resistance of TPU – the current formulations do not yet contain the specialized pigments and additives necessary to provide full barrier properties for aerospace use. Therefore, a crucial next step will be to investigate the influence of the incorporation of functional additives and pigments.

## CRediT authorship contribution statement

A. Lebeau: Writing – original draft, Visualization, Methodology, Investigation, Formal analysis, Data curation. M. Casetta: Writing – review & editing, Writing – original draft, Validation, Supervision, Methodology, Conceptualization. S. Bellayer: Writing – review & editing, Investigation, Data curation. A. Walter: Writing – review & editing, Validation, Supervision, Resources, Methodology, Conceptualization. S. Suel: Writing – review & editing, Methodology, Investigation, Data curation. M. Jimenez: Writing – review & editing, Writing – original draft, Validation, Supervision, Resources, Project administration, Methodology, Funding acquisition, Conceptualization.

<figure><img src="image_12.png" /><figcaption>**Fig. 12.** Scratch test resistance.</figcaption></figure>
Declaration of competing 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.

Acknowledgements

The authors would like to acknowledge Airbus CRT, Nantes, France for funding the project, the ANRT for the CIFRE PhD funding and Van Loon Chemical Innovations, Amsterdam, The Netherlands for their support and expertise in the Hansen study. The Chevreul Institute is thanked for its help in the development of this work through the ARCHI-CM project supported by the "Ministère de l'Enseignement Supérieur de la Recherche et de l'Innovation", the region "Hauts-de-France", the ERDF program of the European Union and the "Métropole Européenne de Lille". The electron probe microanalyser (EPMA) facility in Lille (France) is supported by the European Regional Development Fund (ERDF).

Data availability

Data will be made available on request.

References

[1] A. Beaugendre, S. Degoutin, S. Bellayer, C. Pierlot, S. Duquesne, M. Casetta, M. Jimenez, Selfstratifying coatings: a review 110 210–241. doi:https://doi.org/10.1016/j.porgcoat.2017.03.011.

[2] C. Lemesle, S. Bellayer, S. Duquesne, A.-S. Schuller, L. Thomas, M. Casetta, M. Jimenez, Selfstratified bio-based coatings: formulation and elucidation of critical parameters governing stratification 536 147687. doi:https://doi.org/10.1016/j.apsusc.2020.147687.

[3] V. Verkholantsev, Self-stratifying coatings for industrial applications 32 (5) 300–306. doi:https://doi.org/10.1108/03699420310497445.

[4] V. Verkholantsev, Heterophase and self-stratifying polymer coatings 26 (1) 31–52. doi:https://doi.org/10.1016/0300-9440(95)00552-8.

[5] J. Baghdachi, H. Perez, P. Talapatcharoenkit, B. Wang, Design and development of self-stratifying systems as sustainable coatings 78 464–473. doi:https://doi.org/10.1016/j.porgcoat.2014.06.017.

[6] W. Funke, New aspects of paint films with inhomogeneous structures 2 (4) 289–313. doi:https://doi.org/10.1016/0300-9440(74)85002-2.

[7] W. Funke, Preparation and properties of paint films with special morphological structure, J. Oil Colour Chem. Assoc., Vol. 59 pp. 398–403.

[8] V. Verkholantsev, Nonhomogeneous-in-layer coatings 13 (2) 71–96. doi: https://doi.org/10.1016/0033-0655(85)80019-4.

[9] A. Toussaint, Self-stratifying coatings for plastic substrates (BRITE EURAM PROJECT RI 1B 0246 C(H)) 28 (3) 183–195. doi:https://doi.org/10.1016/0300-9440(95)00597-8.

[10] J.H. Hildebrand, R.L.R.L. Scott, Regular solutions, Prentice-Hall International Series in Chemistry, Prentice-Hall. URL, https://cir.nii.ac.jp/crid/1130000797 015522048.

[11] J. H. Hildebrand, R. L. Scott, The solubility of nonelectrolytes, 3rd edition, Monograph Series, Reinhold Pub. Corporation. URL https://cir.nii.ac.jp/crid/1130000794721627264.

[12] S. Benjamin, C. Carr, D. Walbridge, Self-stratifying coatings for metallic substrates 28 (3) 197–207. doi:https://doi.org/10.1016/0300-9440(95)00596-X.

[13] H. C. M, Three dimensional solubility parameter and solvent diffusion coefficient. Importance in Surface Coating Formulation. URL https://cir.nii.ac.jp/crid/1573105974193799808.

[14] P. Vink, T. Bots, Formulation parameters influencing self-stratification of coatings 28 (3) 173–181. doi:https://doi.org/10.1016/0300-9440(95)00595-1.

[15] I. Hopkinson, M. Myatt, Phase separation in ternary polymer solutions induced by solvent loss 35 (13) 5153–5160. doi:https://doi.org/10.1021/ma012244p.

[16] A. Abbasian, R. Ghaffarian, Investigation of Factors Affecting Stratification Phenomenon in Epoxyacrylic Coatings.

[17] E. Langer, H. Kuczyn'ska, E. Kamin'ska-Tarnawska, J. L ukaszczyk, G. Kamin'ska-Bach, Changes of solubility parameters during evaporation of solvents as a factor influencing the self-stratification of epoxy/acrylic systems 66 (3) 228–234. doi: http://doi.org/10.1016/j.porgcoat.2009.07.008.

[18] Desmolac® 2100, URL, https://solutions.covestro.com/en/products/desmolac/de smolac-2100_00479020-00010062.

[19] J. Clark, Green and sustainable medicinal chemistry: methods, tools and strategies for the 21st century pharmaceutical industry, The Royal Society of Chemistry. doi: https://doi.org/10.1039/9781782625940-00001.

[20] C. Lemesle, J. Frémiot, A. Beaugendre, M. Casetta, S. Bellayer, S. Duquesne, A.-S. Schuller, M. Jimenez, Life cycle assessment of multi-step versus one-step coating processes using oil or biobased resins 242 118527. doi:https://doi.org/10.1016/j. jclepro.2019.118527.

[21] R. Drake, D. Egan, Elastomer-Modified Epoxy Resins in Coatings Applications 1–20.

[22] R. Ramsdale-Capper, J. P. Foreman, Internal antiplasticisation in highly crosslinked amine cured multifunctional epoxy resins 146 321–330. doi: https:// doi.org/10.1016/j.polymer.2018.05.048.

[23] E. M. Petrie, Epoxy Adhesive Formulations, McGraw Hill Professional. arXiv: 738MPf05FEkC.

[24] C. Frihart, A. Natesh, U. Nagorny, Less odor and skinning with stabilized mercaptans for curing epoxies, Adhes. Sealants Ind. 8 (1) (2001) 26, copyright - Copyright Business News Publishing Company Feb 2001. URL, http://ressources-e-lectroniques.univ-lille.fr/login?url=https://www.proquest.com/trade-journals /less-odor-skinning-with-stabilized-mercaptans/docview/232918348/se-2.