AN EVALUATION OF REFLECTANCE-ABSORPTION INFRA RED SPECTROSCOPY FOR IN SITU INVESTIGATION OF ORGANIC COATINGS ON CORRODED METALS

S. C. BOYATZIS1*, V. ARGYROPOULOS1, A. SIATOU1, K. POLIKRETI1, D. CHARALAMBOUS1 and A. M. DOUVAS2

1 TEI of Athens, Dept. of Conservation of Antiquities & Works of Art, Egaleo 12210, Greece2 Institute of Microelectronics, NCSR “Demokritos”, 15310 Aghia Paraskevi, Greece

Kramers-Kronig Transformation (KKT)RAIR spectra carry both the transmittance and the specular reflectance component. When specular reflectance component dominates the spectrum, spectral lines appear with a derivative-like distortion

1. Typical application of protective coatings on metal surfaces generally involves non-ideal films of high, non-uniform thickness on non-ideal metal surfaces. RAIR spectra in these cases suffer from band distortions due to anomalous dispersion of refractive index and/or saturation due to high absorbances that fall out of the Beer law regime.

2. In films of low-medium thickness, both the transmitted and reflected beam reach the detector. Optimization involves use of the suitable polarizing angle and angle of incidence, which can eliminate the above problems. Use of p-polarized beam at Brewster’s angle, and film thicknesses where linearity in absorbance is obeyed, results in good spectra quality, in which all features carry entirely chemical information.

3. In cases of films so thick, that the transmitted beam is almost zero (because it is fully absorbed in the medium), or with metal surface is so rough that the transmitted beam is poorly reflected, the specular reflectance beam becomes important. In these cases the Kramers-Kronig transformation renders a spectral line that corresponds to an absorption spectrum.

Reflectance spectroscopy: Thin or thick films?

The Reflection-Absorption FTIR Technique

references

This question illustrates the geometrical aspects of a coating, basically controlled by the film application practice. The effectiveness of metal protection as well as the chemical information acquired spectroscopically is dependent on the film thickness, as this controls reactive species migration, light penetration, etc. The chemical spectroscopic information is averaged through the IR beam path in thin or thick films and reflects the behavior of polymeric coating chains and their functional groups with the metal and/or, if present, corrosion products.

Reflection-Absorption Infra Red Spectrometry (RAIRS) along with Attenuated Total Reflectance spectroscopy (ATR) have been widely used as characterization and monitoring tools that can be used to evaluate the effectiveness of coatings on metal[1]. Increasingly, these techniques are being applied to test coatings for cultural property made of metals, such as outdoor bronze monuments [2]. The advantage is that the RAIRS technique can carry out an in-depth characterization of layers from the bulk coating to the metal surface analyzing changes in the chemical bonds.

This work evaluates the application of RAIRS to investigate Paraloid®B-72 and Poligen® ES 91009® coatings of variable thickness on corroded iron and copper alloy coupons under the auspices of the EC project PROMET. Furthermore, it highlights how a simple FTIR system (PE Spectrum GX) can be adapted to carry out reflectance measurements coupling with a reflectance accessory (PE, 12.5˚ fixed angle) at a relatively low cost. Such adaptability makes it attractive for conservation research, since most Conservation facilities, such as ours have a simple FTIR system.

For an accurate interpretation of the spectra throughout the coating layers, the following properties are significant: thickness and uniformity of the coating, glossiness of its surface, and the nature of the metal surface. In conservation research, the effectiveness of coatings often needs to be evaluated on corroded metals which are far from being considered as ideal cases.

In thin films the transmitted beam carries basically chemical information through the small path of a few nanometers, as it records orientationalphenomena of polymeric chains towards the metal surface. Orientation is lost in thicker films, as chains tend to be more “random”, away from the metal surface.

Paraloid® B-72 on bronze and iron

μFTIR on corrosion products on iron coated with Paraloid +benzotriazole-based corrosion inhibitor

Poligen® ES910009 on bronze and iron

1. J. Umemura, Reflection–Absorption Spectroscopy of Thin Films on Metallic Substrates, Handbook Of Vibrational Spectroscopy, vol. 2, John Wiley & Sons Ltd, 2002,1016-1032.2. L. B. Brostoff, Coating strategies for the protection of outdoor bronze art and ornamentation, Ph.D. dissertation. Netherlands, 2003.3. V. Argyropoulos et al, Testing of a new wax coating Poligen ES91009 and corrosion inhibitor additives used for improving coatings for historic iron alloys, in: eds., C. Degrigny et al, METAL2007, (2007), 10-15.

Significant changes were observed in reflectance mode of Poligen films, most notably the sharp decrease of the original 1535 cm-1 carboxylate band , the increase of the 1700 cm-1 carboxylic acid band and the formation of a new band at 1605 cm-1 (only on bronze), possibly due to a copper-coordinated carboxylate phase. These features were correlated with those in model coupons incorporating the same coating materials with controlled film thicknesses and surface glossiness. Thus, our examination of infra-red RAIRS bands for the coated corroded coupons is entirely attributed to chemical changes.

Oriented acrylate

chains

random acrylate

chains

‘thin film’, (a few monolayers)

0 – 50 nm

Thick film: >50 nm

Surface roughness

Physical information can be dominant in IR reflectance in thick films, where anomalous dispersion of refractive index is important. This results in numerous distortions of the recorded chemical peaks, depending on the nature of coating medium (i.e. refractive index), the film thickness, surface smoothness, IR beam polarization and the angle of incidence. Predicting these distortions can lead to a spectrum with “pure” chemical information.

IR beam geometry during the reflectance-

absorption technique

Beam polarization and

reflectionPolarization of the

electrical IR component can be either parallel

(p-polarized) or perpendicular

(s-polarized) to the plane normal.

When angle of incidence equals the Brewster’sangle of the coating material, the p-polarized beam is totally transmitted through the material.

Three-layered system:

layer 1 (air, refr. index n1)layer 2 (coating, refr. index n2)layer 3 (metal, refr. index n3)

coating

metal

mirrors

3996,3 3000 2000 1500 1000 399,2

cm-1

%T

1699

1558

1466

1399 1171 720

7619239951085

1234600

539642

2915 28501695

1467

1253720

3083

1605

15481405

1381

1171

1083

927780

642598

543

Poligen/ bronze coupon / 0 min bakingPoligen/ bronze coupon / 5 min baking 50degPoligen/ bronze coupon / +5 min baking 60degPoligen/ bronze coupon / +5 min baking 70deg

Carboxylate peak diminished with baking/ water evaporation

Carboxylic acid peak increasing with baking /

water evaporation

new peak at 1605 cm-1

formed upon baking/ water evaporation

Poligen ES91009 on bronze

RAIR spectrum of Poligen ES91009 on Si wafer (13⁰ incidence angle). 3400(br) OH (carboxylic acid)2942(s), 2860(s) vC-H; 2649 (sh) dimeric carboxyls1706(s) C=O (acidic) 1556(s) C=O (carboxylate) 1468(m-s) δCH2, 1383(w) δCH3, 1407(w) δC-O-H1259 v αsC-O-H1179 v sC-O-C940(m, br) o.o.p. δ(O-H … O) hydrogen bonded 721 ρCH2 (long alkyl)

3996,3 3600 3200 2800 2400 2000 1800 1600 1400 1200 1000 800

cm-1

1. Original spectrum exhibiting specular derivative-like distortion2. Baseline-corrected spectrum3. KKT on original spectrum (in Transmittance mode)4. KKT on baseline-corrected spectrum (in Transmittance mode)

Effect of KKT on reflectance spectra (%T mode): carbonyl band of Paraloid B72 on Si wafer

Paraloid® B-72 Transmittance and RAIR spectra BASF Poligen® ES910009 RAIR spectra

KKT transformation on RAIR spectrum (Absorbance mode) of Paraloid on corroded bronze coupon. The result is a mostly distortionless absorbance spectrum.Comparison in IR peaks of coating and corrosion products may be done in the 1750 – 400 cm-1 region.

1800.6 1600 1400 1200 1000 800 600 395 .50 .49

0 .6

0 .8

1 .0

1 .2

1 .4

1 .6

1 .8

2 .0

2 .2

2 .4

2 .6

2 .8

3 .0

3 .19

cm-1

%T

1733

1473

1448

1386

1236

1166

1147

1025

964 858 749643

515 465

398

KKT-transformedPMTR_318

Original spectrum 74.C1PMTR_318

Comparison of the KKT- derived spectra (absorbance mode) from Paraloid on bronze coupons, brown area (artificially aged and reference). Major changes in Copper oxides and hydroxides (increase in 642, 515 and 465 cm-1 peaks.

1801.5 1600 1400 1200 1000 800 600 399 .4cm-1

A

598

1733

1448 1386

1236

1147

1025

858 749 643515

4651195

1169

600

515

465

642

1196

Artificially aged, KKT spectrum 74.C1 PMTR_318

Reference, KKT spectrum, 73.C1 PMTR_315

Difference spectrum

Introduction

Poligen ES91009 is sold commercially by BASF and reported to be a “water emulsion prepared from polyethylene wax”. FTIR spectra actually show hydrocarbon-like structure as well as carboxylic acid functionality. RAIR spectra of a film cast from water emulsion on a smooth surface (Si wafer, or metal: bronze, iron) further show carboxylate ion functionality depending upon the water content of the film.

4000,0 3000 2000 1500 1000 401,6

cm-1

%T

3738

29222853

2363

1700

1469

14081382

1259

939 721610

1173

2650

1556

761884

995

1082

(a)

(b)

CH2

CH2CH

CH2CH2

CH2CH2

CH2CH

CH2CH2

CH2CH2

C CO OH O O

-

Polyethylene-type backbone

carboxylatecarboxylic acid

Spectrum (a), recorded after evaporation of most water on cast film, shows both carboxylic acid (1700 cm-1) and carboxylate (1560 cm-1) functionalities as the structure is partly ionized. Spectrum (b) recorded one month after film casting ensures “complete” dryness of the film as carboxylate is mostly removed and the polymer is in ‘non-ionized’ state. This behavior is also confirmed by transmittance spectra of Poligen films on Si wafer.

y = 0,0197x + 3,7259R² = 0,9942

0,00

20,00

40,00

60,00

80,00

100,00

120,00

0 1000 2000 3000 4000 5000 6000

Abs

area

s (C

-H)

thickness, nm

Transmittance spectraPoligen/Si wafer: Abs(C-H) vs thickness before baking

Σειρά1

Γραμμική (Σειρά1)

The linear relationship of absorbances (measured as areas) of v(C-H) vs. film thickness assures that quantitative spectra for thick films (at least up to 5 μm) can lead to reliable Transmittance measurements for Poligen . This is crucial esp. for the carbonyl band of acrylics like Paraloid B72, which is a generally accepted marker for chemical alterations.

Reflectance spectra: angle of incidenceLinearity fails when reflectance IR bands exhibit distorted lineshapes and saturation (very high absorbance that does not obey Beer’s law), as is commonly the case in thick films. Band lineshapes can be optimized choosing Brewster’s angle (55⁰ for Paraloid) as incidence angle.

angle of incidence

4000.0 3000 2000 1500 1000 370.0cm-1

%T 2985

2954

1732

14751448

1387

1368

1236

1145

1025

970 860

76⁰ p-polarized

55⁰ p-polarized

13⁰

spin coated films on bronze; thickness 5.92 1 μm

Application of the Kramers –Kronig algorithm transforms the distorted spectral line (1, 2) line to an absorption spectrum of the film (3, 4).

12

3

4

Specular reflectance

Transmitted beam

In a typical reflection-absorption FTIR experiment the infra red beam impinges in the coating at a specified angle of incidence. A part of this beam is reflected onthe coating surface (specular reflectance). Another part is transmitted inside the film, accordingly reflects on the metal surface, re-transmits and finally, exits the film travelling to the detector (see figure). This is the absorption part of the RAIRS technique, recording the average information throughout the entire beam path. Both beam paths are detected within the same experiment: different physical information is recorded in each case, along with the chemical information of the involved materials.

conclusions

x

x

γ-FeOOH

3999,2 3000 2000 1500 1000 500 399,2

cm-1

%T

2921

2851

1698

1540

1466

1405

13811255

937 7202641

11712922

28511698

16041541

1466

1406

1381

1257938 720

1171

2918

2850

1698

14671259 11711405

1379719

1. RAIR spectrum of Poligen on iron2. RAIR spectrum of Poligen on bronze3. Transmission spectrum of Poligen on Si wafer (film thickness 1.48 μm)

film thickness 2.1 μm

2

1

3

Poligen ES91009. All spectra: incidence angle 55⁰

Paraloid B72 spectra are here recorded in micro-scale resolution. The quality of spectra varies between points. Corrosion products are also detected.

0,00

1,00

2,00

3,00

4,00

5,00

6,00

7,00

8,00

0 50 C, 5 min 50 C, 5 min+ 60 C 5 min

50 C 5 min+ 60 C 5 min+70C 5min

Abs

area

v(C

=O)

baking conditions

acidic COanionic COacidic+anionic CO

Poligen on polished bronze coupon. Film thickness 1.480 μm

TRANSMITTANCE on Si wafer: effect of baking on Abs of acidic and anionic carbonyls.

y = 0,0063x + 0,9781R² = 0,9995

y = 0,0092x + 5,791R² = 0,9962

0,00

20,00

40,00

60,00

80,00

100,00

0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000

abso

rban

ce a

reas

thickness (nm)

Absorbances of v(C-H) and vC=O vs. film thickness

vC-H vCO Transmittance spectraThe carbonyl band and the methyl/methylene stretch are markers for chemical alterations of organic coatings.

Reliable absorbance measurements of bands v(C-H) and v(C=O) for Paraloid B72 can be taken for films up to 8-9 μm where linearity in absorbance vs. thickness is obeyed.