Ep 30 — Coatings and Inks: Composition Analysis and Quality Control

Series: Infrared Spectroscopy Encyclopedia: From Principles to Practice
Chapter: Part 3 · Intermediate — Industry Applications
Target Audience: Coating R&D engineers, Ink QC technicians, Material analysts
Prerequisites: Ep 14 (ATR Attenuated Total Reflectance), Ep 19 (Library Search), Ep 29 (Petrochemical)
Reading Time: Approx. 44 minutes

Introduction: A "Weeping" Automotive Paint

In 2023, a car manufacturer's painting workshop experienced batch quality issues—the clearcoat exhibited "weeping" sagging after baking, with poor adhesion and gloss [1]. A one-day production halt cost 500,000 RMB.

The QC team sampled defective and normal paint films and performed comparative analysis using ATR-FTIR. The defective film's spectrum showed significantly weaker bands at 1240 cm⁻¹ (ether C-O-C) and 830 cm⁻¹ (epoxy characteristic), while the 1735 cm⁻¹ (ester C=O) band was normal—indicating insufficient epoxy curing agent ratio, leading to inadequate crosslinking [1].

"ATR-FTIR allows non-destructive analysis of coatings directly on the substrate, identifying binder chemistry and curing defects in minutes."
—— ASTM E1252 Standard Guide for Infrared Analysis of Coatings [2]

Coatings and inks are complex formulated products [1][3][4]:

• Binder: Resin/polymer, determines basic performance
• Pigment: Provides color and hiding power
• Filler: Reduces cost, improves performance
• Solvent: Adjusts application viscosity
• Additive: Defoaming, leveling, drying, etc.

Core value of FTIR in coatings/inks analysis [1][2][3]:

• Binder identification: Distinguishing alkyd, epoxy, polyurethane, acrylic, etc.
• Pigment/filler identification: TiO₂, BaSO₄, CaCO₃, etc.
• Curing process monitoring: In-situ tracking of crosslinking reactions
• Defect analysis: Comparing good/bad samples to locate formulation or process issues
• Solvent residue: Headspace-FTIR detection of VOCs

This episode systematically explains the FTIR methodology and practical cases in coatings and inks analysis.

I. Coating Resin Binder Identification

1.1 Infrared Characteristics of Main Resin Binders

Coating binder is the core component determining coating performance [3][4][5]:

Table 1: Infrared characteristics of main coating binders (data sources: coating analysis literature [3][4][5])

🔗 Extended: Interpretation of ester C=O stretching (1730–1735 cm⁻¹) for alkyd and polyester resins at ftir.fun ester functional group page. Amide I/II bands (1720/1530 cm⁻¹) of polyurethane at ftir.fun amide functional group page.

1.2 Identification of Alkyd Resins

Alkyd resins are polycondensation products of polybasic acids (phthalic anhydride) + polyols (glycerol) + fatty acids [3][5]:

Characteristic peaks [3][5]:

• 1730 cm⁻¹: Ester carbonyl C=O stretching (strongest)
• 1600/1580 cm⁻¹: Aromatic ring C=C of phthalate ester
• 1280/1120 cm⁻¹: Ester C-O-C stretching
• 740 cm⁻¹: Ortho-disubstituted benzene ring C-H out-of-plane bending (characteristic)
• 2930/2850 cm⁻¹: Fatty acid chain C-H

Differentiating alkyd from acrylic [3]:

• Both have 1730 cm⁻¹ ester C=O
• But alkyd has 740 cm⁻¹ (ortho substitution) and 1600/1580 cm⁻¹ (aromatic ring)
• Acrylic lacks aromatic features

Oil length differentiation [5]:

• Short oil length (< 40% fatty acid): strong 740 cm⁻¹
• Long oil length (> 60% fatty acid): enhanced 2930/2850 cm⁻¹

1.3 Identification of Epoxy Resins

Bisphenol A-type epoxy resin characteristics [3][5]:

Characteristic peaks [3][5]:

• 1240 cm⁻¹: Aryl ether C-O-C asymmetric stretching (strongest, most characteristic)
• 830 cm⁻¹: Para-disubstituted benzene ring C-H out-of-plane bending (characteristic)
• 3040 cm⁻¹: Aromatic =C-H stretching
• 1180 cm⁻¹: C-O stretching
• 2960/2870 cm⁻¹: Aliphatic C-H (methyl groups of bisphenol A)

Uncured vs cured epoxy [3][5]:

• Uncured: epoxy group characteristic peak at 915 cm⁻¹
• After curing: 915 cm⁻¹ disappears (ring-opening reaction)

💡 Practical tip: The disappearance of the 915 cm⁻¹ peak is an indicator of epoxy curing degree. The absorbance ratio 915/830 cm⁻¹ can be used for quantitative curing degree [3].

1.4 Identification of Polyurethane (PU)

Polyurethane is formed by the reaction of polyisocyanate + polyol [3][5]:

Characteristic peaks [3][5]:

• 3350 cm⁻¹: N-H stretching (hydrogen-bonded, broad)
• 1720 cm⁻¹: Amide I band (C=O stretching, urea/urethane)
• 1530 cm⁻¹: Amide II band (N-H bending + C-N stretching)
• 1220 cm⁻¹: Coupled C-N stretching + C-O stretching

Isocyanate type differentiation [5]:

• Aromatic PU (TDI/MDI): aromatic ring at 1600/1500 cm⁻¹
• Aliphatic PU (HDI/IPDI): no aromatic ring peaks

Curing monitoring [3]:

• Uncured: NCO characteristic peak at 2270 cm⁻¹
• After curing: 2270 cm⁻¹ disappears

🔗 Extended: The amide I/II bands of polyurethane are similar in position to protein amide bands but originate differently; see ftir.fun amide functional group page.

1.5 Identification of Acrylic Resins

Acrylic resins are polymers of acrylate/methacrylate esters [3][5]:

Characteristic peaks [3][5]:

• 1735 cm⁻¹: Ester C=O stretching (strongest)
• 1160 cm⁻¹: Ester C-O-C asymmetric stretching
• 2960/2870 cm⁻¹: CH₃ stretching
• 1380 cm⁻¹: CH₃ bending

Differentiation from alkyd [3]:

• Acrylic lacks 740 cm⁻¹ (no ortho-substituted aromatic ring)

• Acrylic acid no 1600/1580 cm⁻¹ (no aromatic ring)

• Acrylic acid 1160 cm⁻¹ strong

2. Infrared Characteristics of Pigments and Fillers

2.1 Characteristics of Inorganic Pigments/Fillers

Common inorganic pigments and fillers used in coatings/inks have characteristic absorptions in the low wavenumber region [4][6]:

Table 2: Infrared characteristics of common inorganic pigments/fillers (data sources: pigment spectroscopy literature [4][6])

2.2 Identification of TiO₂

TiO₂ is the most important white pigment in coatings [4][6]:

Rutile vs Anatase [4][6]:

• Rutile: Main peak ~600 cm⁻¹ (broad)
• Anatase: Main peaks ~700 cm⁻¹ and ~400 cm⁻¹

Notes [6]:

• TiO₂ has strong broad absorption below 800 cm⁻¹, which can mask the fingerprint region of organic binders
• Care must be taken during qualitative analysis

2.3 Identification of BaSO₄ and CaCO₃

BaSO₄ (barium sulfate) [4][6]:

• 1190/1120/1080 cm⁻¹ triplet (SO₄²⁻ asymmetric stretching)
• 635/610 cm⁻¹ (SO₄²⁻ bending)
• High diagnostic value, easy to identify

CaCO₃ (calcium carbonate) [4][6]:

• 1420 cm⁻¹ (CO₃²⁻ asymmetric stretching, broad and strong)
• 875 cm⁻¹ (CO₃²⁻ out-of-plane bending)
• 712 cm⁻¹ (CO₃²⁻ in-plane bending)

💡 Practical tip: BaSO₄ and CaCO₃ often coexist in the same coating formulation. Key distinction: BaSO₄ near 1100 cm⁻¹, CaCO₃ at 1420 cm⁻¹ [4][6].

2.4 Special Nature of Carbon Black

Carbon black has no characteristic absorption peaks in the infrared region; instead, it absorbs uniformly across the entire spectrum [4]:

• Manifests as an overall baseline rise
• As carbon black content increases, the full-spectrum SNR decreases
• For dark coating analysis, dilution or ATR with reduced path length is recommended

📷 Figure 1: IR spectra comparison of common coating fillers
Source: NICODOM inorganic pigment spectral library [6]
https://www.ir-spectra.de/nic…

3. Headspace-FTIR Detection of Solvent Residues

3.1 Hazards of Solvent Residues

Residual solvents after coating/ink application affect [7][8]:

• Coating performance (hardness, adhesion)
• Environmental safety (VOC emissions)
• Food packaging safety (migration into food)

3.2 Headspace-FTIR Method

Headspace-FTIR is an effective method for detecting solvent residues [7][8]:

Principle [7]:

1. Place the sample in a sealed headspace vial
2. Heat to equilibrium (80–150 °C), volatiles enter the gas phase
3. Extract headspace gas into a long-path gas cell (10–20 m)
4. FTIR transmission measurement

Procedure [7][8]:

  Sample (coating/ink) → Sealed headspace vial
                              ↓ Heat equilibrium 80°C, 30 min
                        Headspace gas
                              ↓
               Inject into 10 m long-path gas cell
                              ↓
              FTIR transmission measurement → Identification + Quantification

Infrared characteristics of common solvents [7][8]:

Table 3: Infrared characteristics of common coating/ink solvents (data sources: headspace analysis literature [7][8])

3.3 Solvent Residues in Food Packaging

GB/T 10004 (plastic laminated films/pouches) and other packaging product standards commonly have requirements for solvent residues, for example [8]:

• Benzene-based solvents: not detectable (e.g., < 0.01 mg/m², subject to specific standard text)
• Total solvent residues: common upper limit approx. 5.0 mg/m²

Note: GB 9685-2016 is a standard for the use of additives in food contact materials and articles (positive list with some SML/QM), which should not be directly considered as a "solvent residue limit standard"; migration and residue testing also often involve GB 4806 series, GB 31604, and other method standards.

Detection limits of headspace-FTIR [8]:

• Benzene: 0.1 mg/m²
• Toluene: 0.2 mg/m²
• Ethyl acetate: 0.5 mg/m²

🔗 Extension: The C=O of ethyl acetate in solvent residues is better referenced against ftir.fun ester functional group page; ketone solvents can be referenced against the carbonyl page.

4. In-Situ ATR-FTIR Monitoring of Coating Curing Process

4.1 Principle of In-Situ Monitoring

ATR-FTIR in-situ monitoring involves applying liquid coating onto an ATR crystal, collecting spectra in real time to track the curing reaction [3][9]:

  ATR-FTIR in-situ curing monitoring:

  Liquid coating → Apply onto diamond ATR crystal
                            ↓
               Set temperature/time program
                            ↓
               Collect one spectrum every 30–60 s
                            ↓
               Track characteristic peak changes → Reaction kinetics

4.2 Monitoring Epoxy Resin Curing

Bisphenol A epoxy + amine curing agent reaction [3][9]:

Tracking indicators [3][9]:

• 915 cm⁻¹: Epoxy group (—decreasing)
• 3360 cm⁻¹: Secondary amine O-H/N-H (—increasing)
• 1240 cm⁻¹: Aromatic ether (reference peak, unchanged)

Degree of cure calculation [3][9]:

$$\alpha(t) = 1 - \frac{A_{915}(t) / A_{1240}(t)}{A_{915}(0) / A_{1240}(0)}$$

Applications [9]:

• Optimize curing temperature and time
• Screen curing agent types and dosages
• Study curing kinetics (autocatalytic model)

4.3 Polyurethane Curing Monitoring

Hydroxyl acrylic resin + HDI biuret reaction [3][9]:

Tracking indicators [3][9]:

• 2270 cm⁻¹: Isocyanate NCO (—decrease)
• 3350 cm⁻¹: Urethane N-H (—increase)
• 1530 cm⁻¹: Amide II band (—increase)

Applications [9]:

• NCO/OH ratio optimization
• Evaluation of catalyst (DBTDL) effect
• Effect of humidity on reaction

4.4 UV Curing Monitoring

UV-curable acrylate reaction [3][9]:

Tracking indicators [3][9]:

• 810 cm⁻¹: Acrylate C=CH₂ out-of-plane bending (—decrease)
• 1635 cm⁻¹: C=C stretch (—decrease)
• 1410 cm⁻¹: CH₂= in-plane bending (—decrease)

Applications [9]:

• UV intensity optimization
• Photoinitiator dosage screening
• Evaluation of oxygen inhibition effect (surface vs. bottom curing differences)

📷 Figure 2: ATR-FTIR in situ monitoring of epoxy curing process spectral sequence
Source: Kazarian group ATR imaging study [9]
https://doi.org/10.1016/j.eur…

5. Ink Component Analysis

5.1 Basic Composition of Ink

Ink is similar to coatings, but the formulation is more refined [10][11]:

Table 4: Basic composition of ink (Data source: Printing Ink Handbook [10])

5.2 Ink Binder Types

① Offset ink [10][11]:

• Binder: Rosin-modified phenolic resin + alkyd resin
• Features: 1730 (ester C=O), 1600/1500 (aromatic ring), 1240 (C-O)
• Solvent: Mineral oil (high boiling point)

② Gravure ink [10][11]:

• Binder: Polyamide resin or nitrocellulose
• Features (polyamide): 3300 (N-H), 1640 (amide I), 1540 (amide II)
• Solvent: Toluene, ethyl acetate

③ Flexographic ink [10][11]:

• Binder: Acrylic resin (water-based) or polyurethane
• Features (acrylic): 1735 (ester C=O), 1160 (C-O)
• Solvent: Water/ethanol

④ UV ink [10][11]:

• Binder: Acrylate oligomers
• Features: 810 (C=CH₂), 1635 (C=C), 1720 (C=O)
• Solvent-free (UV curing)

🔗 Extension: Analysis of amide I/II bands (1640/1540 cm⁻¹) of polyamide binder in gravure ink can be found at ftir.fun amide functional group page.

5.3 Infrared Features of Organic Pigments

Although organic pigments are complex, they have characteristic absorptions [4][6]:

Table 5: Infrared features of common organic pigments (Data source: Pigment spectral database [4][6])

6. Case Study: Troubleshooting Coating Defects

6.1 Background

After spraying polyester powder coating on refrigerator door panels produced by a home appliance factory, some batches showed unqualified coating adhesion (cross-cut test peeling), while other batches were normal [1].

6.2 Analysis Procedure

① Sampling [1]:

• Problem sample: Coating with unqualified adhesion
• Control sample: Coating from qualified batch
• Substrate: Both batches are galvanized steel sheets

② ATR-FTIR analysis [1]:

• Diamond ATR directly measured the coating surface
• Each sample scanned 32 times, resolution 4 cm⁻¹

③ Spectral comparison [1]:

Table 6: IR comparison of problem sample and qualified sample

④ Diagnosis [1]:

• In the problem sample, the 815 cm⁻¹ (melamine crosslinker characteristic) almost disappeared
• Indicating insufficient crosslinker or incomplete reaction
• Polyester main resin is normal (1730, 1260 cm⁻¹)

⑤ Root cause tracing [1]:

• Checked formulation records: crosslinker (HMMM) addition ratio correct
• Checked baking conditions: Problem batch baking temperature was 15 °C lower
• Melamine crosslinking reaction has high activation energy, insufficient temperature leads to incomplete crosslinking

6.3 Solution

• Adjust oven temperature to ensure 200 °C
• Establish ATR-FTIR online monitoring: sample each batch and detect 815 cm⁻¹ peak
• Set 815/1730 cm⁻¹ ratio threshold, alarm if below 0.3

6.4 Methodological Insights

This case demonstrates the value of FTIR in coating defect analysis [1][3]:

• Non-destructive: ATR direct measurement, no sample preparation
• Fast: 5 minutes/sample
• Chemical fingerprint: simultaneously provides information on binder, crosslinker, and filler
• Trend monitoring: establish early warning system through characteristic peak ratios

💡 Key insight: The core of FTIR analysis for coating defects is comparative thinking — compare good/bad samples, locate difference peaks, trace formulation or process causes [1][3].

📷 Figure 3: ATR-FTIR spectral comparison for coating defect analysis
Source: Coating defect analysis case [1]

7. Practical Experience

7.1 Sample Preparation Tips

① Liquid coatings [3][5]:

• Drop directly onto ATR crystal
• Or coat on KBr disc for transmission measurement
• Note: Measure after solvent evaporation (avoid solvent interference)

② Solid coatings [3][5]:

• ATR direct measurement on surface (preferred)
• Thick coatings can be sliced for transmission
• Peel off and press into pellet for measurement

③ Multilayer coatings [3][5]:

• ATR measures the outermost layer
• Measure after stripping layer by layer
• Or use cross-sectional μ-FTIR imaging (Ep 36)

7.2 Spectral Interpretation Strategy

① First look at the binder region [3]:

• 1730 cm⁻¹ → Esters (alkyd, acrylic, polyester)
• 1240/830 cm⁻¹ → Epoxy
• 3350/1720/1530 cm⁻¹ → Polyurethane
• 815 cm⁻¹ → Amino resin crosslinker

② Then look at the filler region [4][6]:

• 1100–1000 cm⁻¹ → SiO₂, talc

• 1420/875 cm⁻¹ → CaCO₃

• 1190/1120 cm⁻¹ → BaSO₄
• < 800 cm⁻¹ broad → TiO₂

③ Finally, additives [3][4]:

• 3650 cm⁻¹ → Phenolic antioxidants
• 970 cm⁻¹ → Phosphate esters (flame retardants)
• 1260 cm⁻¹ → Silicone oil (defoamer)

7.3 Common Interferences

① Pigment interference [4][6]:

• Dark pigments (carbon black) reduce overall SNR
• Inorganic pigments have strong absorption in low wavenumber region, masking organic information
• Solution: use ATR (surface measurement) or measure after dilution

② Moisture interference [3]:

• Water peaks at 3400/1640 cm⁻¹ from water-based coatings
• Solution: measure after drying

③ Residual solvent [7][8]:

• Solvent peaks in coatings that are not fully dried
• Solution: measure after vacuum drying

📷 Figure 4: Complete ATR-FTIR analysis workflow for coatings
Source: Thermo Fisher coating analysis application note [3]
https://www.thermofisher.com/…

Summary of This Chapter

Review Questions

1. An unknown coating ATR spectrum shows strong absorptions at 1240, 830, 3040 cm⁻¹, and no peak at 915 cm⁻¹. Infer the binder type and curing state.
2. A white coating ATR spectrum shows strong peaks at 1420/875 cm⁻¹ and also absorption at 1730 cm⁻¹. Analyze possible components.
3. Design an experimental protocol for in situ ATR-FTIR monitoring of a two-component polyurethane curing reaction, specifying tracking peaks, sampling interval, and data processing.
4. An ink sample shows absorptions at 810 cm⁻¹ and 1635 cm⁻¹. After UV curing, these peaks weaken but still exist. Analyze possible reasons.
5. A black coating with high carbon black content has poor SNR in ATR spectrum. Propose at least 3 improvement strategies.
6. A polyester powder coating fails adhesion test; ATR analysis shows weak peak at 815 cm⁻¹. Explain the group corresponding to 815 cm⁻¹ and propose investigation directions.
7. Headspace-FTIR detects a strong peak at 728 cm⁻¹ in a food packaging printing ink. Infer possible residual solvent and its source.

References

Standard Methods

[2] ASTM International. ASTM E1252-98(2021): Standard Practice for General Techniques for Obtaining Infrared Spectra for Qualitative Analysis.
https://www.astm.org/e1252-98…

[8] National Health and Family Planning Commission of the People's Republic of China. GB 9685-2016 National Food Safety Standard - Standard for Use of Additives in Food Contact Materials and Articles. China Standards Press, 2016.

Coating Analysis

[1] Wypych G. Paint and Coatings: Formulation, Analysis, and Defect Diagnosis. 3rd ed. ChemTec Publishing, 2022. Chapter 7: "Spectroscopic Analysis."

[3] Thermo Fisher Scientific. "FTIR Analysis of Coatings and Paints." Application Note AN-011, 2022.
https://www.thermofisher.com/…

[4] Painter PC, Coleman MM, Koenig JL. The Theory of Vibrational Spectroscopy and Its Application to Polymeric Materials. Wiley, 1982. Chapter 8: "Coatings and Pigments."

[5] Hummel DO, Scholl F. Atlas of Polymer and Plastics Analysis. 3rd ed. Wiley-VCH, 2008. Volume 2: "Coatings, Paints, and Lacquers."

Pigments and Fillers

[6] NICODOM IR Spectra. "Inorganic Pigments and Fillers IR Spectral Library."
https://www.ir-spectra.de/nic…

Solvent and Curing

[7] Kolb B, Ettre LS. Static Headspace-Gas Chromatography: Theory and Practice. 2nd ed. Wiley, 2006. Chapter 10: "FTIR Detection."

[9] Kazarian SG, Chan KLA. "ATR-FTIR Spectroscopic Imaging for Coatings and Polymer Films." Progress in Organic Coatings, 2021, 151: 106064. DOI:10.1016/j.porgcoat.2020.106064.

Inks

[10] Leach RH, Pierce RJ, Hickman EP, Mack MJ, Smith HG (eds.). The Printing Ink Manual. 5th ed. Springer, 2007.

[11] Bassemir RW, Bean A, Burruss R, et al. "Printing Inks." Kirk-Othmer Encyclopedia of Chemical Technology, Wiley, 2015. DOI:10.1002/0471238961.1615122504150701.a01.pub3.

Database Resources

[ftir.fun] ftir.fun Infrared Spectral Database.

• Ester functional group page: https://ftir.fun/ir/group/est…
• Amide functional group page: https://ftir.fun/ir/group/ami…

Next Episode Preview: Ep 31 — Forensics and Criminal Investigation: Fiber, Paint, and Drug Analysis

We will shift from industry to the forensic field, explaining the application of μ-FTIR in fiber identification (cotton, polyester, nylon, wool), automotive paint chip comparison, rapid detection of drugs and illicit substances, explosive residue analysis, as well as method validation and chain of custody required for forensic evidence.

This article is licensed under CC BY-NC-SA 4.0. The images are from public domain or online sources with credited attribution, and copyright belongs to the original authors.