# Identification of polymeric binders in anticorrosive coatings via FTIR spectroscopy

Laurine Andrieu, Edgar Espinosa-Rodriguez, Manel Taam, Maud Villard, Olivier Boyron

► To cite this version:

Laurine Andrieu, Edgar Espinosa-Rodriguez, Manel Taam, Maud Villard, Olivier Boyron. Identification of polymeric binders in anticorrosive coatings via FTIR spectroscopy. Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy [1994-..], 2025, 343, pp.126493. 10.1016/j.saa.2025.126493. hal-05448805

HAL Id: hal-05448805

https://hal.science/hal-05448805v1

Submitted on 8 Jan 2026

HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_1.png" /></figure>

Distributed under a Creative Commons CC BY 4.0 - Attribution - International License
<figure><img src="image_2.png" /></figure>

# Identification of polymeric binders in anticorrosive coatings via FTIR spectroscopy

Laurine Andrieu<sup>a</sup>, Edgar Espinosa-Rodriguez<sup>a</sup>, Manel Taam<sup>a</sup>, Maud Villard<sup>b</sup>, Olivier Boyron<sup>a,* ID</sup>

## HIGHLIGHTS

* Identification of characteristic bands in anticorrosive paints using ATR-FTIR.

* Differentiation of key components like acrylic, epoxy, polyester, and polysilane.

* Thermal analysis with FTIR reveals spectral changes for component identification.

* Simple and effective method for coatings industry, quality control, and certification.

## GRAPHICAL ABSTRACT

<figure><img src="image_3.png" /></figure>

## ARTICLE INFO

### Keywords:

FTIR spectroscopy

Anticorrosive coatings

Polymeric binders

Paint identification

ATR-FTIR

Protective polymers

## ABSTRACT

Analytical spectroscopy techniques have long been invaluable tools for rapid exploration and targeted monitoring in industrial and academic environments. Among these techniques, Fourier Transfrom Infrared (FTIR) spectroscopy is particularly noteworthy for its effectiveness in real-time characterization. In particular, the Attenuated Total Reflectance (ATR) mode enables detailed analysis of complex formulations in a relatively short time. However, the increasing diversity of materials used in anticorrosion coatings, ranging from one- to two-component systems, presents significant challenges for their identification. This study demonstrates the effectiveness of FTIR in meeting these challenges by illustrating its ability to discern and characterize the components of various coating systems. Using a wide selection of paints, we illustrate the versatility and accuracy of FTIR spectroscopy as a valuable tool for the analysis of protective coatings.

## 1. Introduction

The corrosion of metallic materials is a natural phenomenon caused

by their interaction with environmental factors. It leads to gradual degradation, reducing the lifespan of structures and posing safety and profitability challenges [1–3]. The global cost of corrective maintenance

<footnote><sup>a</sup> Université de Lyon, CPE Lyon, CNRS, UMR 5128, Laboratoire CP2M, Equipe PCM, Villeurbanne 69100, France</footnote>

<footnote><sup>b</sup> Cerema, Centre d'Études et de Recherche de l'Équipement et des Méthodes d'Analyse, Bron 69500, France</footnote>

<footnote>* Corresponding author.
E-mail address: olivier.boyron@univ-lyon1.fr (O. Boyron).</footnote>
for metal structure is estimated at several billions of dollars annually [4]. To mitigate these impacts, various protection techniques have been developed. Among them, anticorrosive paints play a key role by providing an effective barrier against corrosive agents [5]. The application of coatings allows control corrosion through one of the following three mechanisms:

(1) The barrier effect, which prevents both contact between the corrosive agent and the material and the migration of ions within the coating.

(2) The cathodic protection, where the coating acts as a sacrificial anode. It's more anodic than the metal and thereby it has a more negative electrochemical potential. This sacrificial anode cor- rodes first, releasing electrons that are transferred to the metal, turning it into a cathode and so protecting it from corrosion [6–9].

(3) The use of inhibitor coating generates a passivation layer on the metal they protect, which prevents contact with water, chemicals and other corrosion causing materials. Passivation reduces the reactivity by electrochemical polarization [10,11].

Composed of binders, pigments, and solvents, anticorrosive paints are formulated to meet the specific requirements of corrosive environ- ments. Binders ensure adhesion and cohesion of the pigments, while solvents facilitate application [12]. A typical paint system includes multiple layers: (1) the primary layer: provides anticorrosive properties, (2) the intermediate layers: protect the primary layer and enhance durability, (3) the finishing layer: shields the coating from UV radiation and chemical aggressions.

The evolution of environmental regulations has driven the devel-
opment of safer and more sustainable formulations. Researchers are
focusing on eco-friendly formulations, integrating less polluting binders
and additives while improving the performance of paints [13,14]. The
replacement of chromate-based coatings is another example of recent
advancements in coating technology [15].

It is of great importance to ascertain the composition of a given paint in order to classify it according to the binder present. A range of characterisation techniques can be used, including spectoscopy (FTIR, NIR, NMR) [16,17], thermal analysis (DSC, TGA) [18,19] and chromatography [20–22]. Among the FTIR techniques, attenuated total reflectance (ATR) FTIR is particularly advantageous due to its rapid analysis, minimal sample preparation, and ability to directly analyze solid samples. These features make ATR-FTIR a valuable tool for on-site or laboratory rapid analysis of paint. By analyzing the chemical shifts observed in the spectrum, this technique can classify paints based on their primary components. However, the complexity of IR spectra increases as the number of components in the sample rises, posing challenges in interpretation. The aim of this study is to identify the key absorption peaks, enabling the quick and reliable identification of all existing anticorrosive paints. In this context, two types of paint components, corresponding to different layers in anticorrosive coating systems, were analyzed: primers, which are the first applied layers and typically provide adhesion and corrosion protection, and hardeners (or topcoats), which are the outermost layers offering mechanical and chemical resistance. These components were selected as they represent the most relevant sources of polymeric binders in protective coatings. The findings have applications in various fields, such as quality control, material preservation, and forensic investigations.

## 2. Experimental

### 2.1. *Sample preparation*

The binder of the hardener required no specific preparation and thus
was directly analyzed. Whereas, for the primer, the powdered form
needed to be separated from the binder. The separation was performed

by centrifugation according to the method NFT30074. This standard
describes the separation of the organic binder from the pigment matrix
using solvent-based extraction and centrifugation, and is commonly
applied in the characterization of paint samples. In each tube, 1g of the
sample was mixed with 7 mL of methyl ethyl ketone, followed by
centrifugation for 15 min at 4 °C and 15,000 rpm. After centrifugation,
two distinct phases were observed in each tube: the powdered phase at
the bottom and the binder phase on top. The binder was carefully
separated and allowed to dry overnight in an oven at 70 °C. Once dried,
a thin film of the binder was obtained and subsequently analyzed using
infrared spectroscopy.

The paint samples analyzed in this study originate from commercial formulations submitted for COFRAC certification prior to their market release. Specific brands are not disclosed in order to preserve the neutrality and methodological focus of the study.

2.2. Infrared spectroscopy analysis

Infrared spectra were recorded using a Thermo Fisher Scientific Nicolet iS50 FTIR spectrometer equipped with a diamond crystal ATR accessory, a deuterated triglycine sulphate (DTGS) detector and KBr optics. Background and sample spectra were collected with 32 scans at a spectral resolution of 4 cm⁻¹, covering the range from 4000 to 400 cm⁻¹. A new background spectrum was acquired every 60 min to account for any drift. Before each measurement, the diamond crystal was carefully cleaned with ethanol and dried in ambient air. A small quantity of sample was applied directly to the diamond crystal, ensuring a constant pressure of 27 kg during pressing. Spectral data were acquired, processed and analyzed using OMNIC software (Thermo Fisher Scientific).

2.3. TGA analysis

Thermogravimetric analyses (TGA) were conducted using a TGA2 thermogravimetric analyzer (Mettler-Toledo) under the following conditions: temperature range from 25 to 700 °C, heating rate of 10 K/min, and an air atmosphere with a gas flow rate of 30 mL/min to ensure optimal gas circulation and prevent any disturbance during the analysis. Sample masses ranging from 5 to 20 mg were carefully weighed and placed in 70 µL alumina crucibles to ensure good thermal contact and stability during heating. The thermogravimetric curves (TGA) were evaluated for the onset and endpoint temperatures, as well as the maximum temperatures (peak mass loss) associated with each thermal degradation step. Data were processed using STARE software (version 18.00), allowing precise determination of degradation characteristics and enhancing the interpretation of experimental results.

3. Results and discussion

3.1. Alkyd paint

Alkyd-based anticorrosion paints are widely used to protect metal structures, thanks to their cost-effectiveness, long-lasting and good protective properties in moderately corrosive environments [23,24]. Alkyd resins are synthetic polymers obtained by the polycondensation of polyols (such as glycerol) and dicarboxylic acids. Their chemical structure (Fig. 1), which combines ester groups and acid chains, gives them good adhesion to metal surfaces and a flexibility that helps them absorb mechanical stress without cracking. So the use of an expanded gamma of anticorrosive pigments is a valuable quality [25–27].

The FTIR spectrum of an alkyd paint (Fig. 1) offers insightful information about its molecular structure, particularly regarding the identification of characteristic functional groups. Such spectral features have been thoroughly discussed in previous studies focused on the chemical characterization of alkyd-based materials in commercial and artistic paints [28–30]. The main absorption bands and their interpretation are
<figure><img src="image_4.png" /><figcaption>Fig. 1. FTIR spectrum of an alkyd paint.</figcaption></figure>

detailed below.

The prominent band at 1730 cm⁻¹ is characteristic of the carbonyl (C=O) stretch. This peak typically corresponds to ester and ketone functional groups, both of which are commonly found in alkyd paints. Esters are formed during the polymerization process of alkyd paints, linking fatty acids and polyols, while ketones may appear as part of additives or degradation products in some cases.

The bands at 1255 cm⁻¹ and 1150 cm⁻¹ are assigned to the C=O stretching vibrations. These peaks are consistent with ester linkages within the alkyd resin, where the ester bonds form the backbone of the resin structure, linking polyols with fatty acids. These ester bonds provide flexibility to the alkyd paint film, allowing it to adapt to various environmental stresses without cracking.

Peaks at 1600 cm⁻¹ and 1580 cm⁻¹ are indicative of the aromatic C=C stretching vibrations. These bands suggest the presence of an aromatic ring structure within the paint composition, likely introduced from the aromatic components in the alkyd resin or added stabilizers. Aromatic rings provide increased stability and UV resistance, essential for protective coatings.

The band observed at 740 cm⁻¹ corresponds to the C–H out-of-plane bending vibration, a typical marker for aromatic rings.

### 3.2. Alkyd silicone paint

An alkyd silicone coating is a special type of coating in which alkyd resins are modified by adding silicone-based additives (Fig. 2). These paints have excellent durability, toughness, good resistance to cracking and abrasion resistance caused by severe temperature changes. Compared to other high-performance coatings, alkyd silicone paints remain relatively easy to apply (by brushing, rolling, or spraying) and are more cost-effective. They are often used as a more affordable alternative to fully silicone or epoxy-based coatings in projects that still require considerable weather and heat resistance [31,32].

Highly used as maintenance paints for steel and concrete, or as a coating protection of wood and as a fast dry coating. Identifying easily alkyd silicone may be valuable, especially for degradation tests [33].

The FTIR spectrum of alkyd silicone paint is displayed in Fig. 2.

The peak observed at 1730 cm⁻¹ corresponds to the C=O stretching vibration, which could indicate the presence of ketone or ester groups—both commonly found in alkyd resins. Peaks at 1600 cm⁻¹ and 1580 cm⁻¹ are characteristic of the C=C bonds within aromatic rings, and the aromatic nature is further confirmed by the C–H out-of-plane bending vibration at 830 cm⁻¹. The presence of C–O bonds is indicated by the band at 1105 cm⁻¹.

<figure><img src="image_5.png" /><figcaption>Fig. 2. FTIR spectrum of an alkyd silicon paint.</figcaption></figure>

Silicone-specific bonds are also detected: the broad band at 1030 cm⁻¹ corresponds to the Si–O–Si (siloxane) linkage, a defining feature of silicone structures, while the Si–CH₃ stretching vibration observed at 1248 cm⁻¹ further verifies the presence of silicone functionality within the paint matrix. This combination of spectral features reflects the dual composition of alkyd and silicone.

### 3.3. Acrylic and polyester paint

Acrylic and polyester paints are both commonly used in various industrial and architectural applications due to their durability, versatility, and performance characteristics. Their ageing behavior, thermal stability, and spectral characteristics have been the focus of several studies [34–36].

Acrylic paints dries quickly, are very flexible and waterproof once dry. They're also incredibly versatile since they adhere to virtually any surface, plus they are UV resistant which is making them ideal for architectural and decorative applications. Polyester paints are favored because they offer great coverage and excellent performance for polish systems. They are often used to coat high-end furnishings, such as boat interiors. Used for some applications at the same time then epoxy, epoxy-polyamide, or alkyd resins, the identification of these systems can be required [37].

Both acrylic and polyester samples exhibit similar IR spectra, with identical characteristic peaks corresponding to the same functional groups. However, thermogravimetric analysis (TGA) provided distinct thermal decomposition profiles for the two materials. The first significant weight loss for acrylics occurs between 100 °C and 200 °C, whereas polyester samples show their initial decomposition starting at around 400 °C, as shown in Fig. 3. This temperature difference is indicative of the differing thermal stabilities between the two polymers, reflecting their distinct chemical structures and thermal behaviors.

Acrylic and polyester binders exhibit very similar FTIR spectra (Fig. 4 et Fig. 5), making their distinction based solely on ATR-FTIR challenging. In this context, TGA analysis was essential to group the samples and uncover a subtle but consistent spectral difference. Specifically, TGA revealed two distinct degradation behaviors: a major weight loss between 100 °C and 200 °C for one group, and a second around 400 °C for the other. The early weight loss is characteristic of acrylics, which degrade via a *retro*-depolymerization mechanism (monomer release) at lower temperatures. Conversely, the degradation around 400 °C is indicative of polyesters, whose ester bonds require more energy to cleave. Once this classification was established using TGA, the corresponding FTIR spectra could be grouped accordingly. This
<figure><img src="image_6.png" /><figcaption>Fig. 3. TGA profiles of an acrylic and a polyester paint.</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>Fig. 4. FTIR spectrum of a polyester paint.</figcaption></figure>

regrouping made it possible to identify a meaningful spectral difference: the carbonyl stretching band appears at ~1730 cm⁻¹ for acrylics and ~1715 cm⁻¹ for polyesters. This 15 cm⁻¹ shift, although small, becomes reliable.

In an IR spectrum, the key distinction between acrylic and polyester paint lies in the chemical shift of the C=O bond. For polyester paints, the C=O stretching vibration typically appears around 1715 cm⁻¹ (as shown in Fig. 4), whereas for acrylic paints, it shifts to a slightly higher frequency of approximately 1730 cm⁻¹ (as shown in Fig. 5). This difference in the carbonyl stretching frequency is a result of the distinct chemical environments of the ester groups in each polymer. The carbonyl group in polyester is influenced by the polymer's structure, which typically results in the lower shift, while the acrylic ester group exhibits a higher shift due to its different bonding environment.

The difference in the C=O stretching vibration between acrylic and polyester paints is primarily influenced by the electronic environment surrounding the ester functional group in each polymer. This difference in wavenumbers, around 1715 cm⁻¹ for polyester and 1730 cm⁻¹ for acrylic, can be explained by the following factors:

Polyester resins typically feature a more rigid backbone, where the

<figure><img src="image_8.png" /><figcaption>Fig. 5. FTIR spectrum of an acrylic paint.</figcaption></figure>

ester groups are directly attached to aromatic rings or other bulky groups. This configuration can lead to stronger electron-withdrawing effects on the carbonyl group, which lowers its stretching frequency (around 1715 cm⁻¹).

In contrast, acrylic resins often have a more flexible structure with ester groups attached to aliphatic chains. The lack of strong electron-withdrawing groups leads to less polarization of the carbonyl group, and hence, the C=O stretching vibration appears at a higher frequency, typically around 1730 cm⁻¹.

These distinct shifts in the C=O bond stretching frequency are useful for differentiating between the two types of paints in FTIR analysis. The presence of adjacent functional groups, such as aromatic rings in polyesters or alkyl chains in acrylics, also contributes to these slight variations in wavenumbers. However, given the small magnitude of the shift (~15 cm⁻¹), such differentiation could not be confidently established using FTIR alone. The use of TGA was instrumental in identifying clear thermal degradation patterns specific to each binder family, which in turn enabled the regrouping of FTIR spectra and the validation of this subtle spectral distinction.

### 3.4. Styrene acrylic paint

Acrylic styrene-based paints are highly valued for their versatility across a broad range of applications, from automotive and industrial coatings to architectural and decorative finishes. Their ability to form smooth, durable surfaces while maintaining color stability under UV light makes them an excellent choice for decorative finishes and aesthetic applications [28,38]. In addition, their relatively low VOC content and ease of application contribute to their growing adoption in environmentally conscious formulations.

The FTIR spectrum of styrene acrylic paint (Fig. 6) reveals key functional groups, providing insights into its molecular composition. The band at 1730 cm⁻¹ corresponds to the stretching vibration of the carbonyl (C=O) bond, which is characteristic of ester groups in the acrylic structure. The two peaks around 1600 cm⁻¹ indicate the presence of the aromatic C=C bond, a characteristic feature of styrene, a monomer used in the formulation of the paint. These peaks are typical for aromatic compounds, where the C=C stretching vibration in the benzene ring contributes to the observed frequencies. The peak at 698 cm⁻¹ is attributed to the out-of-plane C-H bending vibration of the styrene ring, confirming the presence of the aromatic styrene unit in the paint. The broad band around 1150 cm⁻¹ is indicative of the C=O stretching vibration, typical of ester groups.
<figure><img src="image_9.png" /><figcaption>Fig. 6. FTIR spectrum of a styrene acrylic paint.</figcaption></figure>

### 3.5. Epoxy and modified epoxy paint

Epoxy coatings are widely used to protect metals from corrosion. Their strong adhesion to substrates, resistance to moisture and durability in extreme conditions make them ideal for steel, iron and aluminium surfaces. Modified epoxies, which are more flexible or UV-resistant, are often used in marine environments to protect ships, boats and other craft from the corrosive effects of sea water [39,40]. Modified epoxy coatings are also used to create conductive films for electronic and electrical applications [41].

The FTIR spectrum of the epoxy paint, shown in Fig. 7, reveals two characteristic peaks associated with the epoxy structure, confirming the presence of the cyclic C–O–C bond typical of epoxies. The first peak is observed around $1240 \text{ cm}^{-1}$, which corresponds to the stretching vibration of the C–O bond in the epoxy group. The second peak, although weaker, appears at $915 \text{ cm}^{-1}$, which is attributed to the deformation vibration of the C–O–C bond in the epoxy ring.

When the epoxy is modified with siloxane, additional features appear in the FTIR spectrum, as shown in Fig. 8. The $915 \text{ cm}^{-1}$ peak associated with the epoxy ring undergoes a slight shift to $910 \text{ cm}^{-1}$ due to the interaction between the epoxy group and the siloxane structure. This shift indicates a subtle change in the bonding environment of the epoxy

<figure><img src="image_10.png" /><figcaption>Fig. 7. FTIR spectrum of an epoxyde paint.</figcaption></figure>

<figure><img src="image_11.png" /><figcaption>Fig. 8. FTIR spectrum of an epoxy siloxane paint.</figcaption></figure>

resin upon modification, possibly due to the introduction of siloxane chains.

In addition to the characteristic peaks of epoxy, the presence of siloxane introduces new bands that are distinct and confirms the modification. The Si–O–Si bond stretch, typical of siloxane groups, is observed around $1050 \text{ cm}^{-1}$, which is indicative of the formation of a siloxane network. Furthermore, the Si–C bonds are confirmed by peaks at $1430 \text{ cm}^{-1}$ and $1270 \text{ cm}^{-1}$, both of which are characteristic of the siloxane structure. These new peaks validate the presence of siloxane units in the modified epoxy resin, providing additional properties such as improved flexibility, moisture resistance, and enhanced thermal stability.

### 3.6. Ethyl silicate paint

Ethyl Silicate is an essential binder in zinc-based inorganic paints, valued for its high corrosion resistance and ability to form durable protective coatings. A key feature of ethyl silicate is its ability to hydrolyze into silica ($SiO_2$) during curing, a process that enhances the durability of the coating. The hydrolyzed ethyl silicate forms a robust network that offers excellent resistance to weathering, high temperatures, UV degradation, and chemical exposure. This makes it particularly valuable for industries that require advanced protection for metal surfaces exposed to harsh and aggressive conditions. Ethyl silicate-based coatings are commonly used to protect steel structures such as bridges, offshore platforms, and storage tanks [42,43]. These paints create a hard, inorganic layer that adheres strongly to metal substrates, providing long-lasting protection even in severe environments. Additionally, the silicate network's ability to bond tightly with zinc particles allows ethyl silicate paints to be combined with zinc-rich formulations, creating highly protective coatings that act as sacrificial layers, further enhancing the corrosion resistance of the metal surface [44].

The FTIR spectrum of an ethyl silicate paint is shown in Fig. 9. The chemical shift corresponding to the siloxane Si–O–Si bond appears at $1050 \text{ cm}^{-1}$, characterized by a rather intense and broad peak, indicative of the presence of siloxane linkages in the network structure of the paint. Additionally, the Si–O bond is further confirmed by a peak at $798 \text{ cm}^{-1}$.

### 3.7. PVC acrylic paint

PVC–acrylic copolymer-based paints are commonly employed in protective and decorative coatings for both indoor and outdoor applications, offering a good balance of adhesion, UV resistance, and mechanical performance. These systems benefit from the flexibility and
<figure><img src="image_12.png" /><figcaption>Fig. 9. FTIR spectrum of an ethyl silicate paint.</figcaption></figure>

weatherability of acrylic resins combined with the chemical resistance and low permeability of poly(vinyl chloride) [45]. FTIR spectroscopy provides a reliable analytical method for identifying this type of copolymer. The FTIR spectrum of a PVC-acrylic paint, shown in Fig. 10, displays characteristic absorption bands that correspond to both components.

For the acrylic component, the band at 1730 cm⁻¹ corresponds to the C=O stretch, while the peaks at 1255 cm⁻¹ and 1150 cm⁻¹ are attributed to the C-O stretching vibrations. For the PVC component, the peak at 610 cm⁻¹ is characteristic of the C-Cl bond in the polymer. These distinct features provide the basis for identifying the PVC-acrylic copolymer in the paint.

### 3.8. Polyisocyanate paint

Polyisocyanate-based anticorrosive paints are widely used in various industries to protect metal surfaces against corrosion [46]. These systems are typically two-component formulations, combining a polyisocyanate resin with a polyol or amine-based hardener. The resulting cross-linking reactions lead to the formation of dense polymer networks, providing outstanding chemical resistance, mechanical strength, and weathering stability [47,48]. In the FTIR spectrum of

<figure><img src="image_13.png" /><figcaption>Fig. 10. FTIR spectrum of a PVC acrylic paint.</figcaption></figure>

polyisocyanate paint (Fig. 11), a peak around 2260 cm⁻¹ is observed, corresponding to the asymmetric stretching vibration of the isocyanate group (—N=C=O). This distinct and isolated band serves as a reliable signature for unambiguously identifying this type of paint.

### 3.9. Polysiloxane paint

Polysiloxanes consist of a silicon-oxygen (Si-O) backbone with organic groups attached. These organic groups can be tailored to enhance specific properties of the paint. The stable chemical structure of polysiloxanes provides excellent resistance to UV radiation, moisture, temperature fluctuations, and corrosion [49–51]. Polysiloxane paints are simple to apply on various surfaces, making them ideal for large-scale projects like building façades [52].

The FTIR spectrum of the polysiloxane paint is shown in Fig. 12. The prominent peak at 1073 cm⁻¹ corresponds to the stretching vibration of the Si-O-Si bond, a hallmark of the polysiloxane backbone. The peaks at 1272 cm⁻¹ and 1430 cm⁻¹ confirm the presence of the Si-C bond, indicating organic groups attached to the silicon atoms. These bonds are characteristic of methyl or other alkyl-substituted polysiloxanes. Peaks near 2960–2850 cm⁻¹ are typically observed and attributed to the C-H stretching and bending vibrations of methyl groups (CH₃) attached to the silicon atoms. These confirm the presence of organic functionalities in the siloxane structure. A broad peak around 3200–3600 cm⁻¹ may indicate the presence of residual Si-OH (silanol) groups, which are often found in unreacted or partially cured polysiloxanes.

### 3.10. Polysilane paint

Polysilane coatings are typically used when applications require films with strong adhesive properties and hydrophobicity. Recent research is focusing on modifying polysilanes to make them more eco-friendly. This includes finding solutions to replace organic solvents in their formulation and reducing their environmental impact while maintaining their performance [53–55].

The FTIR spectrum of the polysilane paint is shown in Fig. 13. This compound is characterized by the presence of a single distinct functional group, which is reflected by two notable peaks. The Si-Si bond exhibits characteristic absorptions at 1075 cm⁻¹ and 955 cm⁻¹, corresponding to the stretching vibrations of the Si-Si bond.

### 3.11. Polyamidoamine, polyamine and adduct

Polyamidoamines (PAMAM) are dendritic polymers widely used in

<figure><img src="image_14.png" /><figcaption>Fig. 11. FTIR spectrum of a polyisocyanate paint.</figcaption></figure>
<figure><img src="image_15.png" /><figcaption>Fig. 12. FTIR spectrum of a polysiloxane paint.</figcaption></figure>

<figure><img src="image_16.png" /><figcaption>Fig. 13. FTIR spectrum of a polysilane paint.</figcaption></figure>

various anticorrosive paints applications due to their unique chemical properties, such as a large specific surface area, a branched structure, and reactive functional groups. These features allow them to interact specifically with metallic surfaces, improving adhesion in coatings [56,57].

Polyamidoamine and polyamine only differs by one bond, the C=O bond which appears as a peak at 1650 cm⁻¹. An adduct refers to the reaction between an epoxy group and an amine, where the epoxy ring opens, forming an alcohol group. However, both PAMAM and polyamine, when exposed to air, can undergo either hydrolysis or degradation through the *retro*-aza-Michael reaction resulting in the formation of an OH group. This hypothesis has been confirmed by TGA analysis. An adduct undergoes three steps of mass loss, whereas PAMAM or polyamine only undergo two. However, if PAMAM or polyamine undergoes hydrolysis or degradation, it also exhibits three weight loss stages, as shown in Fig. 14.

This modification can also be seen on a FTIR spectrum, degraded PAMAM or polyamine exhibits a broader peak for the N–H bond at 3300 cm⁻¹, as the O–H bond shifts to 3600 cm⁻¹, resulting in a single large peak instead of two, as shown in Fig. 15. In contrast, an undegraded polymer shows a narrower peak, with only the N–H bond shifting, as

<figure><img src="image_17.png" /><figcaption>Fig. 14. TGA profiles of a degraded PAMAM and an adduct.</figcaption></figure>

<figure><img src="image_18.png" /><figcaption>Fig. 15. FTIR spectrum of a degraded PAMAM or an adduct.</figcaption></figure>

<figure><img src="image_19.png" /><figcaption>Fig. 16. FTIR spectrum of a non-degraded PAMAM.</figcaption></figure>
depicted in Fig. 16.

While it is not possible to distinguish between an adduct and a non-adduct component, it is possible to differentiate between a PAMAM in Fig. 15 and a polyamine in Fig. 17. The C=O bond, which shifts at 1650 cm⁻¹ for a PAMAM is absent for a polyamine.

### 3.12. Phenalkamine paint

Phenalkamine-based anticorrosive paints are a specific type of coating that combines the properties of phenolic resins and alkylamines. These paints offer unique benefits in terms of corrosion resistance and are particularly suitable for protecting metal surfaces exposed to harsh environmental conditions. Phenalkamine is a modified resin made by the reaction of phenol or a phenolic derivative with an amine compound. This modification provides a structure with both phenolic and amine functional groups, which significantly enhances the coating's performance [58].

The FTIR spectrum of a phenalkamine is displayed in Fig. 18. The O-H and N-H bonds appear in the spectrum as a single peak at 3300 cm⁻¹, the thickness and the shape of the peak indicating that both are present. The two peaks around 1600 cm⁻¹ correspond to the aromatic C=C stretching vibration. The presence of additional peaks between 850 cm⁻¹ and 650 cm⁻¹ provides further evidence to support the identification of the aromatic ring. Finally, the phenalkamine is characterized by the Ph-OH bond, which appears at 1248 cm⁻¹.

The results presented above are summarized in Table 1 and Table 2. The measurement errors reported in the tables were calculated based on the variability of peak positions observed across different spectra attributed to the same binder category. These errors were evaluated by repeating FTIR measurements multiple times on the same sample to assess instrumental and procedural consistency.

### 3.13. Identification of adduct by combining thermal degradation and FTIR

Adducts and degraded polyamidoamine (PAMAM) cannot be easily distinguished in their initial state. However, PAMAM is more susceptible to thermal degradation than an adduct. After both samples were left in a 140 °C oven for 2 h, the FTIR spectra before and after heating were compared. The FTIR spectrum of the PAMAM sample after heating exhibited differences from the pre-heating spectrum, indicating structural changes due to the heat. In contrast, the spectrum of the adduct remained unchanged, as it requires much higher temperatures to degrade. Therefore, distinguishing between an adduct and PAMAM is

<figure><img src="image_20.png" /><figcaption>Fig. 17. FTIR spectrum of a degraded or adduct amine.</figcaption></figure>

<figure><img src="image_21.png" /><figcaption>Fig. 18. FTIR spectrum of a phenalkamine paint.</figcaption></figure>

possible by subjecting the samples to thermal degradation and comparing the spectra before and after heating. Additionally, thermal degradation can be confirmed through TGA analysis, where the first weight loss typically observed around 100 °C is absent after heating, indicating that the sample has undergone degradation.

## 4. Conclusion

FTIR spectroscopy has proven to be an effective technique for identifying the characteristic bands of anticorrosive paints, enabling the differentiation of key components such as acrylic, epoxy, polyester, and polysilane, among others. Thermal treatment further facilitated this differentiation, with significant spectral changes observed in the FTIR spectra of PAMAM, confirming its higher susceptibility to heat-induced degradation compared to adducts. TGA analysis complemented the FTIR data, offering additional insights into the thermal stability and degradation patterns of the samples. Measurement errors were quantified through the analysis of multiple samples, providing a reliable framework for precise identification. The results presented here offer a robust methodology for the identification of unknown anticorrosive paint samples based solely on FTIR spectra, requiring only a few key chemical shifts for accurate classification.

The results of this study on anticorrosive paints have numerous applications in key areas such as the coatings industry, infrastructure maintenance, regulatory compliance, and the protection of public infrastructure. In the coatings industry, this work facilitates the implementation of rigorous quality control measures and the verification of paint products' compliance with established standards. This is a vital undertaking in sectors such as building, automotive, aerospace, and marine. Finally, the protection of public infrastructure, such as bridges and reservoirs, benefits from this approach. By quickly detecting the degradation of anticorrosive coatings, corrective measures can be taken before significant damage occurs, thereby ensuring the longevity of the structures and reducing long-term maintenance costs.

## 5. Disclosure

The authors declare that no generative AI and AI-assisted technologies were used in the writing process, and we take full responsibility for the content of publication.

### Funding

This research did not receive any specific grant from funding
<table><caption>**Table 1**

Characteristic infrared shifts in primer coatings.</caption><thead><tr><th></th><th>Band 1</th><th>Band 2</th><th>Band 3</th><th>Band 4</th><th>Band 5</th></tr></thead><tbody><tr><td>Acrylic</td><td>C=O (carbonyle): 1730 ± 5 cm<sup>-1</sup></td><td>C=O-C: often 2 peaks at 1150 ± 40 cm<sup>-1</sup> and 1255 ± 15 cm<sup>-1</sup></td><td></td><td></td><td></td></tr><tr><td>Alkyd</td><td>C=O (carbonyle): 1730 +/-6 cm<sup>-1</sup></td><td>C=O-C: often 2 peaks at 1150 ± 40 cm<sup>-1</sup> and 1255 ± 15 cm<sup>-1</sup></td><td>C=C (aromatic): 1580 ± 20 cm<sup>-1</sup><br>and C-H aromatic at 740 ± 20 cm<sup>-1</sup></td><td></td><td></td></tr><tr><td>Alkyd silicone</td><td>Si-O-Si (siloxane): 1030 ± 8 cm<sup>-1</sup></td><td>Si-C: 1248 ± 15 cm<sup>-1</sup></td><td>C=O (carbonyle): 1730 +/- 6 cm<sup>-1</sup></td><td>C=O-C: often 2 peaks at 1150 ± 40 cm<sup>-1</sup> and 1255 ± 15 cm<sup>-1</sup></td><td>C=C (aromatic): 1580 ± 20 cm<sup>-1</sup> and C-H aromatic at 740 ± 20 cm<sup>-1</sup></td></tr><tr><td>Styrene-acrylic</td><td>C=O (carbonyle): 1730 ± 6 cm<sup>-1</sup></td><td>C=O-C: often 2 peaks at 1150 ± 40 cm<sup>-1</sup> and 1255 ± 15 cm<sup>-1</sup></td><td>C=C (aromatic): 1580 ± 20 cm<sup>-1</sup><br>and C-H aromatic at 740 ± 20 cm<sup>-1</sup></td><td>C-H (styrene): 698 ± 1 cm<sup>-1</sup></td><td></td></tr><tr><td>Epoxy</td><td>C=O-C (epoxy): 2 peaks at 915 ± 1 cm<sup>-1</sup> and 1240 ± 10 cm<sup>-1</sup></td><td></td><td></td><td></td><td></td></tr><tr><td>Polyester</td><td>C=O (carbonyle): 1715 ± 3 cm<sup>-1</sup></td><td>C=O-C: often 2 peaks at 1150 ± 40 cm<sup>-1</sup> and 1255 ± 15 cm<sup>-1</sup></td><td></td><td></td><td></td></tr><tr><td>Ethyl silicate</td><td>Si-O-Si (siloxane): 1030 ± 8 cm<sup>-1</sup></td><td>Lack of the Si-C bond in comparison to a silicone component</td><td></td><td></td><td></td></tr><tr><td>Acrylic-PVC</td><td>C=O (carbonyle): 1730 ± 5 cm<sup>-1</sup></td><td>C=O-C: often 2 peaks at 1150 ± 40 cm<sup>-1</sup> and 1255 ± 15 cm<sup>-1</sup></td><td>C-Cl: 610 ± 6 cm<sup>-1</sup></td><td></td><td></td></tr><tr><td>Modified epoxy (epoxy siloxane only)</td><td>C=O-C (epoxy): 2 peaks at 910 ± 1 cm<sup>-1</sup> and 1240 ± 10 cm<sup>-1</sup></td><td>Only if the modification is carried with a siloxane: Si-O-Si (siloxane): large peak at 1050 ± 45 cm<sup>-1</sup></td><td>Only if the modification is carried with a siloxane: Si-C: 2 peaks at 1272 ± 100 cm<sup>-1</sup> and 1430 ± 40 cm<sup>-1</sup></td><td></td><td></td></tr></tbody></table>

<table><caption>**Table 2**

Characteristic infrared shifts in hardeners.</caption><thead><tr><th></th><th>Band 1</th><th>Band 2</th><th>Band 3</th></tr></thead><tbody><tr><td>Polyisocyanate</td><td>N=C=O: 2260 ± 10 cm<sup>-1</sup></td><td></td><td></td></tr><tr><td>Polysilane</td><td>Si-Si: 3 peaks at 955 ± 3 cm<sup>-1</sup>, 1075 ± 3 cm<sup>-1</sup> and 1100 ± 1 cm<sup>-1</sup></td><td></td><td></td></tr><tr><td>Polysiloxane</td><td>Si-O-Si (siloxane): large peak at 1050 ± 45 cm<sup>-1</sup></td><td>Si-C: 2 peaks at 1272 ± 100 cm<sup>-1</sup> and 1430 ± 40 cm<sup>-1</sup></td><td></td></tr><tr><td>Polyamidoamine</td><td>N-H: 3294 ± 10 cm<sup>-1</sup> (peak begin at 3400 cm<sup>-1</sup> if it is not degraded, otherwise the peak begin at 3550 cm<sup>-1</sup>)</td><td>C=O: 1650 ± 2 cm<sup>-1</sup></td><td></td></tr><tr><td>Polyamine</td><td>N-H: 3294 ± 10 cm<sup>-1</sup> (peak begin at 3400 cm<sup>-1</sup> if it is not degraded, otherwise the peak begin at 3550 cm<sup>-1</sup>)</td><td>Lack of peak at 1650 ± 2 cm<sup>-1</sup></td><td></td></tr><tr><td>Adduct</td><td>Indiscernible from PAMAM or amine that are degraded</td><td></td><td></td></tr><tr><td>Phenalkamine</td><td>Ph-OH: 1248 ± 8 cm<sup>-1</sup></td><td>C=C (aromatic): 1590 ± 10 cm<sup>-1</sup> and O-H: 3294 ± 20 cm<sup>-1</sup></td><td>N-H and 10 cm<sup>-1</sup></td></tr></tbody></table>

agencies in the public, commercial, or not-for-profit sectors. Funding the authors report no conflicts of interest and no financial support for this study.

### 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.

### Acknowledgments

The authors would like to thank Franck Collas of Mettler Toledo for his help with the thermal analysis.

### Data availability

Data will be made available on request.

### References

[1] P.R. Roberge, *Handbook of Corrosion Engineering*, McGraw-Hill, New York, 2012.

[2] P. R. Roberge, *Corrosion Engineering: Principles and Practice*, McGraw Hill LLC, 2008.

[3] T. Kirchgeorg, I. Weinberg, M. Hörnig, R. Baier, M.J. Schmid, B. Brockmeyer, *Mar. Pollut. Bull.* 136 (2018) 257-268.

[4] G. Koch, J. Varney, N. Thompson, O. Moghissi, M. Gould, J. Payer, 2016.

[5] P.A. Sørensen, S. Kiil, K. Dam-Johansen, C.E. Weinell, J. Coat. Technol. Res. 6 (2009) 135-176.

[6] X. Chen, X.G. Li, C.W. Du, Y.F. Cheng, *Corros. Sci.* 51 (2009) 2242-2245.

[7] A. Yabuki, S. Tanabe, I.W. Fathona, *Surf. Coat. Technol.* 341 (2018) 71-77.

[8] H. Cho, H. Chang, B. Lim, H. Park, Y. Kim, *Corros. Sci. Technol.* 14 (2015) 177-183.

[9] F. Mahdavi, M. Forsyth, M.Y.J. Tan, *Prog. Org. Coat.* 105 (2017) 163-175.

[10] A. Kakaroglou, M. Domini, I. De Graeve, *Surf. Coat. Technol.* 303 (2016) 330-341.

[11] R. Figueira, C. Silva, E. Pereira, J. Coat. Technol. Res. (2014) 12, https://doi.org/10.1007/s11998-014-9595-6.

[12] O.Ø. Knudsen, A. Forsgren, *Corrosion Control Through Organic Coatings*, CRC Press, Boca Raton, 2017.

[13] E. Almeida, D. Santos, J. Uruchurtu, *Prog. Org. Coat.* 37 (1999) 131-140.

[14] I. Díaz, B. Chico, D. de la Fuente, J. Simancas, J.M. Vega, M. Morcillo, *Prog. Org. Coat.* 69 (2010) 278-286.

[15] F. Andreatta, L. Paussa, P. Aldighieri, A. Lanzutti, D. Raps, L. Fedrizzi, *Prog. Org. Coat.* 69 (2010) 133-142.

[16] O. Boyron, B. MacQueron, M. Taam, J. Thuilliez, C. Boisson, *Macromol. Chem. Phys.* 219 (2018) n/a.

[17] O. Boyron, M. Taam, C. Boisson, *Macromol. Chem. Phys.* 220 (2019) 1900376.

[18] R. Chiriac, F. Toche, O. Boyron, *Thermal Analysis and Calorimetry: Versatile Techniques* (2023) 245-291.

[19] O. Boyron, T. Marre, A. Delauzun, R. Cozic, C. Boisson, *Macromol. Chem. Phys.* 220 (2019) n/a.

[20] E. Armelin, C. Ocampo, F. Liesa, J.I. Iribarren, X. Ramis, C. Alemán, *Prog. Org. Coat.* 58 (2007) 316-322.

[21] B. Raeissi, M.A. Bashir, J.L. Garrett, M. Orlandic, T.A. Johansen, T. Skramstad, J. Coat. Technol. Res. 19 (2022) 559-574.

[22] M.P. Mateo, V. Piñon, G. Nicolas, *Surf. Coat. Technol.* 211 (2012) 89-92.

[23] B. Chico, J. Simancas, J.M. Vega, N. Granizo, I. Díaz, D. de la Fuente, M. Morcillo, *Prog. Org. Coat.* 61 (2008) 283-290.

[24] M.I. Abdou, H. El-Sayed Ahmed, M.A. Wahab Gaber, A.M. Fadl, *Cogent Eng.* 5 (2018) 1427844.

[25] G.S. Gonçalves, A.F. Baldissera, L.F. Rodrigues, E.M.A. Martini, C.A. Ferreira, *Synth. Met.* 161 (2011) 313-323.

[26] K. Jeyasubramanian, V.S. Benitha, V. Parkavi, *Prog. Org. Coat.* 132 (2019) 76-85.

[27] L.S. Hernandez, B. Del Amo, R. Romagnoli, *Anti-Corros. Methods Mater.* 46 (1999) 194-204.

[28] G. Germinario, I.D. van der Werf, L. Sabbatini, *Microchem. J.* 124 (2016) 929-939.
[29] C. Duce, V. Della Porta, M.R. Tiné, A. Spepi, L. Ghezzi, M.P. Colombini, E. Bramanti, Spectrochim. Acta A Mol. Biomol. Spectrosc. 130 (2014) 214-221.

[30] R. Ploeger, D. Scalarone, O. Chiantore, J. Cult. Herit. 9 (2008) 412-419.

[31] S.S. Pawar, R. Baloji Naik, S.K. Rath, T.K. Mahato, B. Kandasubramanian, J. Coat. Technol. Res. 17 (2020) 719-730.

[32] M.C. Mejía, J. Palacio, E.A. Murillo, Prog. Org. Coat. 105 (2017) 336-341.

[33] I. Sekine, M. Yuasa, N. Hirose, T. Tanaki, Prog. Org. Coat. 45 (2002) 1-13.

[34] W.R. Zhang, S.J. Hinder, R. Smith, C. Lowe, J.F. Watts, J. Coat. Technol. Res. 8 (2011) 329-342.

[35] L. Ortiz-Herrero, I. Cardaba, S. Setien, L. Bartolomé, M.L. Alonso, M.I. Maguregui, Talanta 205 (2019) 120114.

[36] B. Ormsby, 2009.

[37] O. Radovici, P. Drob, E. Vasilescu, M. Popa, J. Mirza-Rosca, R. Serban, Rev. Roum. Chim. (1997) 805-812.

[38] M. Melchiorre Di Crescenzo, E. Zendri, M. Sánchez-Pons, L. Fuster-López, D. J. Yusá-Marco, Polym. Degrad. Stab. 107 (2014) 285-293.

[39] E. Armelin, C. Alemán, J.I. Iribarren, Prog. Org. Coat. 65 (2009) 88-93.

[40] Y.S. Salim, C.H. Chan, C.H. Ong, Macromol. Symp. 365 (2016) 209-222.

[41] P. Deshpande, S. Vagge, S. Jagtap, R. Khiarnar, S. Kelkar, M. More, Bul. Chem. Commun. 44 (2012) 318-323.

[42] G. Parashar, D. Srivastava, P. Kumar, Prog. Org. Coat. 42 (2001) 1-14.

[43] S. Vahur, A. Teearu, P. Peets, L. Joosu, I. Leito, Anal. Bioanal. Chem. 408 (2016) 3373-3379.

[44] S.K. Ghosh, G. Waghoo, A. Kalita, D. Balgude, K.R. Kumar, Prog. Org. Coat. 73 (2012) 70-75.

[45] I. Sekine, T. Suzuki, M. Yuasa, K. Handa, K. Takaoka, L. Silao, Prog. Org. Coat. 31 (1997) 185-191.

[46] I. FelhHosi, L. Molnárné Nagy, S. Horváth, T. Pozman, J. Bognár, T. Szabó, Z. Keresztes, Materials 16 (2023) 2753.

[47] S. Gaumet, N. Siampiringue, J. Lemaire, B. Pacaud, Surf. Coat. Int. 80 (1997) 367-372.

[48] Z.H. Gao, J.Y. Gu, X. Wang, Z.G. Li, X.D. Bai, Pigm. Resin Technol. 34 (2005) 282-289.

[49] K. Satoh, M.W. Urban, Prog. Org. Coat. 29 (1996) 195-199.

[50] E. Cappelletto, E. Callone, R. Campostrini, F. Girardi, S. Maggini, C. Della Volpe, S. Siboni, R. Di Maggio, J. Sol.-Gel. Sci. Technol. 62 (2012) 441-452.

[51] S. Ahmad, S.M. Ashraf, E. Sharmin, A. Mohomad, M. Alam, J. Appl. Polym. Sci. 100 (2006) 4981-4991.

[52] M. Arioli, A. Manfredi, J. Alongi, P. Ferruti, E. Ranucci, Polymers 12 (2020) 1376.

[53] W.-G. Ji, J.-M. Hu, J.-Q. Zhang, C.-N. Cao, Corros. Sci. 48 (2006) 3731-3739.

[54] F. Deflorian, S. Rossi, L. Fedrizzi, Electrochim. Acta 51 (2006) 6097-6103.

[55] S.G. Moghadam, S. Pazokifard, S.M. Mirabedini, Prog. Org. Coat. 156 (2021) 106235.

[56] M.A. Mikailov, U.J. Yolchuyeva, Y.J. Aghazada, C.G. Salmanova, F.T. Alizadeh, Processes of Petrochemistry and Oil Refining (2022) 23.

[57] S. Chi, D. Zhang, R. Fang, M. Liu, Q. Lin, Prog. Org. Coat. 166 (2022) 106779.

[58] M. Kathalewar, A. Sabnis, Prog. Org. Coat. 84 (2015) 79-88.