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Recent advances in the analysis and characterization of organic coatings
Progress
in Organic
Coatings,
14 (1986)
67
- 86
67
RECENT ADVANCES IN THE ANALYSIS AND CHARACTERIZATION OF ORGANIC COATINGS MASAO Toray
TAKAHASHI* Research
Center,
Inc.,
Sonoyama,
Otsu, Shiga,
520
(Japan)
Contents Introduction. ................................... ...... Analysis of the surfaces of organic coatings ..................... 2.1 Analysis of impurities adhering on the surface ................. 2.2 Analysis of the chemical changes arising in the surfaces of organic coatings after outdoor exposure .......................... 2.3 Analysis of changes in organic coatings brought about by immersion in seawater ........................................ 2.4 Analysis of changes in organic coatings brought about by heating .... Depth profiling of coatings ................................. Problemsrelatedtointerfaces ............................... 4.1 Observation of the interface between the organic coating and the substrate ........................................ 4.2 Modification of the surfaces of polymer materials by plasma treatment. 4.3 Adhesive properties and peeling between the topcoat and the undercoat ............................................ 4.4 The distribution and chemical state of a nickel component in the chemically modified surface layer of a steel substrate ............ 4.5 Analysis of the materials in a very thin layer on the surface of a steel substrate ........................................ Analysis of small inclusions on coating surfaces. ................... New applications of infra-red and Raman spectroscopies to analysis of surfaces, interfaces and small areas on surfaces .................... References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . _. .
67 69 69 69 74 14 75 76 77 79 80 80 81 81 81 86
1 Introduction Problems related to surfaces and interfaces are very often important in the field and technology of organic coatings. In many other industrial fields, surface science and technology also have become increasingly important year by year. Examples include the controlled growth of crystals at surfaces to form solid-solid junctions in microelectronic devices and the chemical transformation of molecules on the surfaces of catalysts in heterogeneous catalytic chemistry; both involve fundamental problems concerning surfaces. The technologies and instruments for the analysis and characterization of surfaces have shown remarkable developments. Over the past decade *Present
address:
0033-0655/86/$7.50
Toray
Techno
Co., Ltd.,
Sonoyama,
0 Elsevier
Otsu,
Shiga,
Sequoia/Printed
520
(Japan).
in The Netherlands
68
surface analytical tools and technologies with very high resolutions have been developed. Some examples are Auger electron spectroscopy (AES), secondary ion mass spectrometry (SIMS) and X-ray photoelectron spectrometry (XPS or ESCA), and a number of unique developments related to the analysis and characterization of surfaces and interfaces have been achieved by means of IR and Raman spectroscopies. The various analytical tools now available, including the above, are listed in Table 1 [2, 81. High-resolution transmission electron microscopy has also contributed greatly towards the characterization of surfaces, and should also be included as an effective tool. In the science and technology of organic coatings, a number of problems exist which are related to surfaces and interfaces since organic coatings are composed of very thin layers formed on a variety of substrates. It is almost impossible to list all the related practical problems, but some are as follows: Adhesion or peeling at the interface, ;;V) Corrosion of the substrate. (iii) Surface appearance and its change after weathering. (iv) The existence of very small amounts of foreign substance on the surface. (v) Bleeding out of the coating components on the surface. In this paper, recent advances in analysis and characterization of organic coatings will be reviewed. However it is not possible to cover all the related published articles in the limited space available, and for this reason the review will be limited to recent advances in Japan, especially recent work undertaken at the Toray Research Center, Inc. TABLE
1
Tools for the analysis
and characterization
of surfaces
and interfaces
[ 2, 81
LEED RHEED SEM EPMA AES SAM APS ESD ISS RBS SIMS IEX SCANIIR XPS UPS LMA CL ATR FT-IR IR
Low energy electron diffraction Reflection high-energy electron diffraction Scanning electron microscopy Electron probe micro-analysis Auger electron spectroscopy Scanning auger micro-probe Appearance potential spectroscopy Electron-stimulated desorption Ion-scattering spectroscopy Rutherford back-scattering Secondary ion mass spectrometry Ion excited X-ray diffraction Surface composition by analysis of neutral and ion-impact X-ray photoelectron spectroscopy Ultraviolet photoelectron spectroscopy Laser micro-analysis Cathode ray luminescence Attennuated reflectance spectroscopy Fourier transformation infrared spectroscopy Infrared spectroscopy
radiation
69
2 Analysis of the surfaces of organic coatings ESCA or XPS methods provide a very high sensitivity for the analysis of the outermost surface layers, and the ease with which inorganic elements may be detected together with FT-IR spectroscopy provides plenty of information about the chemical composition and structure. A combination of these various techniques should therefore provide a very effective tool for analyzing surfaces. 2.1. Analysis of impurities adhering on the surface [ 1, 10, 111 Silicone oil is a substance which is very often found on the surfaces of coatings and causes various problems. Silicone oil layers of cu. 0.2 pm thickness coated on polypropylene can be readily detected by means of XPS and FT-IR spectroscopy. Figure 1 illustrates the XPS spectra arising from a thin silicone layer (0.2 pm thickness) coated on a polypropylene surface. Silicone may be detected with a very high degree of sensitivity by means of XPS, with the chemical structure of the silicone oil being identified through the chemical shifts arising from the peaks. No peak was apparent for the polypropylene substrate. Figure 2 illustrates the FT-IR spectra for the same sample; however in this case, the measurable depth is large (cu. 2 pm) so that the spectrum of the polypropylene substrate is now apparent and mixed with that of the silicone oil. However, it is still possible to separate the peaks arising from the oil through the use of a difference spectrum.
5ooev
Nal;zw Scan
1000
1500KE
01s
AsAm 1200ev
J&+ 1204
952
956
Fig. 1. XPS spectra of a thin silicone propylene film surface.
1363
1387 K.E
oil layer
(cu. 0.2 pm thickness)
coated
on a poly-
2.2 Analysis of the chemical changes arising in the surfaces of organic coatings after outdoor exposure [l, 6,121 After outdoor exposure, changes in the appearance, brightness and physical properties of exposed coatings are usually observed, butthe investigations of more fundamental changes such as those in chemical composition and structure are equally important.
70
z 5
b)
Polypropylene
*
0 v)
z c) Difference
4cuJ
spectrumh-b)
ICW
3an
Wave number(cm-’ > Fig. 2. FT-IR spectra of a thin silicone propylene film surface.
oil layer
(ca. 2.0 pm thickness)
coated
on a poly-
When the surfaces of various organic coatings are investigated by means of XPS and FT-IR spectroscopy after outdoor exposure the following changes are commonly observed: (i) hydrolysis, degradation and other chemical changes which occur in polymer compositions depending on the types of polymer binders and inorganic components such as pigments which become exposed at the surfaces of the coatings [ 1,6, 121; (ii) very small changes in chemical composition and structure which may be detected by subtracting the FT-IR spectra obtained before and after exposure from each other. The durability or life of organic coatings may be more reasonably estimated through the use of studies of this type. The changes in the XPS spectra of a coated surface brought about by weathering are shown in Fig. 3, which indicates that many more elements than may be expected are detected on the surface. From such spectra it is possible to evaluate changes in the elemental compositions on the surfaces of the coatings arising from weathering of the type listed in Tables 2, 3 and 4. Tables 3 and 4 list the assignments of the chemical species on the surfaces of coatings derived from paints A and C, respectively, as determined by XPS chemical shift measurements, and Table 2 shows changes observed in the relative peak strengths of these chemical species due to weathering. In the case of paint A (acrylic-urethane type paint), no change in the chemical species was observed after weathering, but the amounts of inorganic chemical species such as Ti, Pb and Al arising from inorganic additives increased, while the amounts of nitrogen and functional groups which probably arise from the urethane component decreased as a result of weathering. These
0.06 0.05
NI,
0.02 0.03
S&n
0.33 0.62
01s
0.05 0.03
N1S
0.01 0.05
Si2*
0.02 0.14
TiZp
Peak area relative to Cl,
0.61 0.96
01,
Peak area relative to Cr,
0.01 0.08
Pbf
0.04
‘hB
0.01 0.03
Al2P
0.003 0.02
Al2p
0.05 0.06
Fe2p3,2
Ca2,
0.02
0.01 0.01
zn2P3/2
0.02 0.02
Cazp
12 10
24 19
$0
7 7
.-;4
‘O-
0
20 17
$&N
$0
Ratio of carbon components (%I
0.01 0.01
Cl2*
73 76
Neutral carbon
64 71
Neutral carbon
Ratio of carbon components (%I
brought about by weathering of the surfaces of coatings arising from various paints
aPaint A was an acrylic-urethane type paint whose acrylic component consisted of methyl methacrylate copolymer with a hydroxyl group-containing component and whose urethane component contained the isocyanate ‘Desmodur N-75’. The OH/NC0 ratio in the paint was 1 ,O, and the main component of the pigment was Ti02. bPaint C was an acrylic-epoxy type paint commercially available as ‘Epicomarine AE’. The epoxy in this point was of the bisphenol A type, the hardener of the amine type and the main component of the pigment was TiOa.
control weathered
Sample
Paint Cb
control weathered
Sample
Paint Aa
Changes in the elemental composition
TABLE 2
5cQ
*
-
’
’
lcm
*
.
.
eV
.
L
15m
K.EFig. 3. Changes in the XPS spectra of a coated surface brought about by weathering (paint A: an acrylic-urethane paint). TABLE 3 Assignment of chemical species on the surface of paint A (an acrylic-urethane from XPS chemical shift measurements Element
type paint)
Control and weathered samples
--ago o-
0 N Si Ti Pb Al Fe Zll Ca Cl
organic, inorganic urethane silicone TiOs Pbn AlaI I?@ zn= Ca” ?
data indicate that weathering leads to the decomposition of the polymer component and that the components of the pigment leach out on to the surface as a result of exposure outdoors. Such changes were also observed by means of scanning electron microscopy. Similar results were observed in the case of paint C (an acrylic-epoxy type paint). Figure 4 illustrates the FT-IR-ATR spectra of the control and weathered samples. Individually, IR spectra have insufficient sensitivity, so that it is hardly surprising that little difference is apparent between these two spectra. However, by using the difference spectrum, peaks may be assigned
73 TABLE
4
Assignment of chemical species on the surface from XPS chemical shift measurements Element
Control
and weathered
C
neutral,
>&O,
>&N,
of paint
C (an acrylic-epoxy
type
paint)
samples
-a<’ O-
0
* , C-O-, amine SiO2 -
N Si Ti Al Ca
4ollo
SiO2, Al203
Al203 -
3aoo
Fig. 4. FT-IR-ATR surface developed
1600 2000 WAVE NUHBERS
1200
spectra illustrating the structural from paint A by weathering.
000
changes
brought
about
in a coated
to increases in the alcohol, amine and acid concentrations while the concentrations of amide II and III as well as the size of the peak for C-O have decreased. Takeshima et al. have studied the chemical changes occurring in organic coatings after various types of weathering test by following the variations in concentration of selected functional groups by means of FT-IR-ATR spectroscopy. Figure 5 shows a typical example of the results obtained in these studies. The pattern-type illustration of the changes in various functional groups provides a good picture of the deterioration brought about by outdoor exposure of a thermosetting polyester/melamine type paint coated
74
Non-weathered
& outdoor weathering
test
Dew cycle weatherometer Fig. 5. FT-IR-ATR steel sheet (sample pigment).
spectral consisted
test
analysis of the deterioration of a paint film coated on to a of a thermosetting polyester/melamine resin containing TiOz
on to a steel sheet. The decomposition of the melamine resin, oxidation of the resin and appearance of the pigment on the surface are all well illustrated in this figure. Figure 5 also shows that of the nine types of accelerated weathering tests undertaken in parallel on the sample, the dew cycle weatherometer test gave the pattern closest to that observed for outdoor weathering. 2.3 Analysis of changes in organic coatings brought about by immersion in sea water [I] Changes in organic coatings after immersion in sea water have been analyzed by XPS methods. Figure 6 shows the change observed in the XPS Cl peak of a poly(vinyl chloride) coated surface after immersion in sea water. In this case, Cl ions were detected on the surface. However, whether these Cl ions came from the sea water or from the decomposition of the poly(viny1 chloride) coatings is undetermined at present and should be one of the points to be studied further. By using the difference spectrum as shown in Fig. 7, very small changes occurring on immersion in sea water may be detected, and from these the extent of the oxidative degradation of the coating and the adherence of bio-materials may be estimated. 2.4 Analysis of changes in organic coatings brought about by heating [l] Figure 8 shows the changes in the elemental composition of the coating surface brought about by heating as measured by XPS methods. After such treatment, components of the organic coatings or additives such as insecticides emerged on the surface and could be observed.
75
I
1
,
!
12ea
1265
1230
Kinetic
Energy
( eV )
Fig. 6. Change in the XPS Clzp peak of a coated surface after immersion in sea water * (paint B: commercially available poly(viny1 chloride) type marine paint).
3 Depth profiling of coatings [l] It is useful to observe the depth profiling of the compositions of organic coatings to enable an understanding of the uniformity of composition and structure as well as the extent of diffusion of the components in the coated films. Figures 9 - 12 illustrate the depth profiles of organic coatings studied after outside weathering tests. As shown in Fig. 9, the diffusion of oxygen, oxidative degradation, changes in composition arising from chemical reactions and changes in concentration of the components could all be observed by means of XPS combined with a depth-profiling technique. The FT-IR-ATR spectra of the same samples at different depths are illustrated in Fig. 10. In this case, no definitive change could be observed on comparing with the spectra of the control sample. However, small differences in composition and structure may be observed by means of the difference spectrum as shown in Fig. 11. The depth profile of an Sn compound obtained by this method is shown in Fig. 12, which correlates well with the result obtained by XPS methods as shown in Fig. 9.
76
Control
Difference
I
Spectrum
Normalized to Hydrocarbon
I 1200 Kinetic
(1202.0
eV)
1 1205
Energy
(eV)
Fig. 7. Change in the XPS C1, peak of a coated surface after immersion in sea water as revealed by the difference spectrum (paint B).
cts
I
Paint A
I
500
1
I
I
I
I
1
Kxo
KE(eV) Fig. 8. Changes in the elemental composition heating as observed by XPS methods (paint B).
I
I
ml
of a coated surface brought about by
4 Problems related to interfaces Many practical problems such as the occurrence of rust and interfacial peeling may be related to interfaces.
77
0
I.0
40
Depth
(PI
’
20
LO
Fig. 9. Depth profiles obtained from the relative XPS intensity ratios of the control and immersed coatings (paint B).
0
Fig. 10. Depth profiles of weathered coatings obtained by means of FT-IR-ATR studies (paint B).
spectral
4.1 Observation of the interface between the organic coating and the substrate [l, 91 A coating on a steel substrate may be peeled off by immersion in liquid nitrogen. The interfaces on both sides of the coating thus obtained have been studied by XPS methods. Table 5 lists the various elemental quantities on
78
I Sn cnmp
Surface
- inner layer(20p)
Fig. 11. Difference spectrum between the spectra of the surface and an inner layer (at 20 pm depth) of a weathered coating (paint A).
Fig. 12. Depth profiles of inorganic additives in a coated surface as obtained by means of FT-IR-ATR methods.
the peeled surfaces obtained from such measurements. Both sides contained zinc phosphate and iron, while nitrogen arising from the urethane coating could be detected on the substrate side. From this it was therefore deduced that the location of the peeled interface had shifted towards the substrate and interdiffusion of the components had occurred.
19
TABLE 5 Elemental quantities on peeled surfaces before and after weatheringa Weathered for 24 month
Non-weathered
Substrate side
Coating side
Substrate side
Coating side
0.92 0.10 0.20 0.06 0.07
Sizp
-
0.44 0.33 0.03 0.15 0.03 0.03
0.62 0.10 0.06
Feza Pan
1.67 0.03 0.54 0.30 0.23
01s
Nls zn2P3/2
0.02 -
aMeasured as a ratio relative to Cl,
4.2 Modification of the surfaces of polymer materials by plasma treatment 13951 Polar groups such as carbonyl and carboxyl formed via plasma treatment of the surfaces of polyethylene, polypropylene and a fluoro polymer have been analyzed by means of XPS. Such treatment was found to improve the adhesive properties of the modified surfaces. Figure 13 shows an example of such an analysis. From this it may be deduced that plasma treatment leads to the formation of carbonyl groups on the surface.
295
290
285
Binding energy (eV)
540
535
530
Eb (eV)
Untreated
295.
290
285
Binding energy (eV)
535 540 Ea(eV)
530
Fig. 13. XPS spectra of the surfaces of a polypropylene treatment.
film both before and after plasma
80
4.3 Adhesive properties and peeling between the topcoat and the undercoat [3,7,81 To estimate the extent of peeling between the topcoat and the undercoat, the interface was analyzed by means of XPS as shown in Fig. 14, which indicates the disappearance of carboxylic groups and bleeding out of the component on to the surface of the film when the latter was cured at elevated temperatures (200 “C) (showing poor adhesion) in comparison to the film cured at lower temperatures (80 “C) (showing good adhesion). It was found that residual polar groups such as carboxyl on the surface of an undercoat and ones arising from the melamine resin contribute to the adhesive properties, and that removal of the additive components from the undercoat leads to inter-facial peeling.
cts 1200cps
NIS IlOOcps Coating peeled at interlayer 501 from surface surface
5Ol
from surface
surface
50ii from surface surface
Fig. 14. XPS Cl, and N1, spectra for various coatings both at the surface layer and 50 from the surface (the type of the paint used was not described in the original paper).
A
4.4 The distribution and chemical state of a nickel component in the chemically modif~d surface layer of a steel substrate [3,4) Depth profiles of a nickel component providing nuclei for zinc phosphate crystals in the zinc phosphate treated surface layer of a steel prepared by the so-called dipping chemical treatment process have been analyzed by XPS methods as shown in Fig. 15. In this case it was found that the concentration of nickel became larger as the distance from the surface increased, thus supporting the formation mechanism for the zinc phosphate layer as being based on nickel acting as a nucleating agent on steel surfaces. It was
81
100 -
S + k? -
Coating surface
2
c
Expanded spectrum
.o....
- 6” : : ,#’ 0 10
Steel/coating interface _,___---a 4
-../860 20
30
40
BindIng energy
& (W
Fig. 15. Depth profile of a nickel component in a zinc phosphate coating layer on steel as determined by XPS measurements employing the Ar+ etching method. Fig. 16. The expanded Niz, XPS peak associated with the Ni component in the zinc phosphate layer of Fig. 15 and separation of the peak into two components (peak 1 = 861.55 eV, peak 2 = 858.85 eV).
also found that two types of nickel compounds were formed in the layer as shown in Fig. 16. 4.5 Analysis of the materials in a very thin layer on the surface of a steel substrate [lo, 111 A very thin layer of an alkyd resin located on the surface of steel has been analyzed by means of infrared emission spectroscopy, with the same sensitivity being obtained as with infrared reflectance spectroscopy. Figure 17 shows a typical result obtained, indicating that as the measuring angle increases the sensitivity also increases.
5 Analysis of small inclusions on coating surfaces [ 11 Small inclusions on coating surfaces very often give rise to a variety of problems. These inclusions can usually be investigated and analyzed by means of SEM-XMA techniques. By utilizing the micro-analysis technique of FT-IR spectroscopy, very small inclusions of the type which were most difficult to analyze can now be studied. Examples of such studies are shown in Figs. 18 and 19 which illustrate the analysis of a white inclusion and a granular particle on a coating surface as being SiOZ and an aggregate of an insecticide, respectively [ 11.
6 New applications of infrared and Raman spectroscopies to the analysis of surfaces, interfaces and small areas on surfaces [ 131 New applications of IR and Raman spectroscopy to the analysis of surfaces, interfaces and small areas on surfaces are being developed. New techniques involving IR spectroscopy include Ft-IR-DRIFT, -ATR, -RAS,
Fig. 17. IR emission spectra of a thin layer of an alkyd resin on a steel surface, Measurements were made on a film of 150 nm thickness at a temperature of 150 “C. The continuous line depicts the reflectance spectrum obtained at room temperature, while the dotted lines and the dot-dashed line depict the spectrum obtained with parallel and vertical polarized light, respectively. The measuring angles employed were A = 76”, B = 60”, C = 45”, D = 30” and E = O”, respectively.
I
N-H
Si-0
C-H
3000
2000
1000
hn-3
Fig. 18. Spectrum of a white spot (100 x 200 pm) in a weathered coating as analyzed by the FT-IR microsampling technique.
83
-EMS and -PAS. The application of surface-enhanced Raman scattering (SERS) to surface analysis is another field which is showing remarkable development. The following are some examples of applications in these fields. Figure 20 shows a schematic illustration of the principles of FT-IRPAS, which is very effective for the non-destructive analysis of surfaces and interfaces, and for the analysis of non-flat and black samples. Figure 21 shows one example using the FT-IR-PAS technique. When a multilayer ?le of the type illustrated in Fig. 19 was analyzed by means of FT-IRC-H
1
’
Sn-0 (H)
6
I
1
C-H
1
co2 c-n
Fig. 19. Spectrum of an inclusion sampling technique.
(100
pm) in a coating
as analyzed
by the FT-IR
PAS Cell
Interferometer
... b .,‘... ) (,~ i. is cl
!F
. . .. ( ,’ .
L .‘...
‘..
\ Sample ? Stage PAS Spectrum
Source
FT AmpUfier
Fig. 20. Schematic
illustration
of the principles
of FT-IR-PAS
analysis.
micro-
84
PAS, it was possible to observe the spectrum arising from the polyurethane layer in a non-destructive manner. Thus, for example, the reaction occurring in an inner layer cc&d be followed by means of this technique. Figure 22 is a schematic diagram depicting the new FT-‘-IR-ATR technique while Fig* 23 shows that a molecular layer of the Cd salt of eicosanoic acid on a glass substrate may be detected by using this particular technique. Figure 24 indicates that the Raman spectrum of a surface can be considerably enhanced and hence measured to a very high degree of sensitivity by means of silver-i&and film deposition_ Figure 25 illustrates the spe&ral changes in highly oriented pyrolytic mphite (HOPG) braught about by
PP Thickness Fig.
21,
A : 4t.im
El : rJim
Polwopyrene
( PP 1
SlllCOlV3
t3umf
C : 15pm
An example of depth analysis capable with the FT-IR-PAS
Fig. 22. Schematic diagram depicting the FT-IR-ATR
technique.
D:22vm_
technique.
85
RES = 4
Fig. ‘23. FT-IR-ATR on a glass substrate.
spectra
of the mono-
Raman shift VA
and multi-layer
films of cadmium
eicosanoate
Ag-island film
A~~(50-100% ,‘.
..,
,“;,: ..,.
Fig. 24. Application
B
B; of SERS
‘(‘,’ to surface
..i
analysis.
argon-ion etching. A broad Raman band (1350 cm-‘) arising from the disordered carbon structure appears in the spectrum. When a silver-island film is placed upon the etched HOPG, the resulting Raman spectrum is remarkably different. In this case the outermost disordered surface has been enhanced exclusively by contact with the silver-island film and this enhanced spectrum may be measured to a very high degree of sensitivity.
I
I
I
I
I
I
I
I 1000
1500
Raman
shift
cm-’
Fig. 25. Enhanced Raman scattering arising from an argon-ion etched HOPG surface covered with a silver-island film.
References 1 2 3 4 5 6 7 8 9 10 11 12 13
A. Ishitani, J. Jpn. Sot. Colour Mater., 53 (1980) 468. T. Tanaka, Zairyo to Seko (Materials & Finishing, in English), I (1982) 112. N. Sato, J. Jpn. Sot. Colour Mater., 57 (1984) 206. N. Sato, Corros. Eng., 32 (1983) 379. Y. Taru and K. Takaoka, Toso Gijutsu (Coating Technology, in English), 1 (1980) 106. E. Takeshima, T. Kawano and H. Mizuki, J. Zron Steel Inst. Jpn., 68 (1982) S1049. N. Sato, Toso Kogaku (Finishing Eng., in English), 18 (1983) 226. T. Tanaka, Toso Kogaku (Finishing Eng., in English), 19 (1984) 26. S. Maeda, Corros. Eng., 31 (1982) 621. T. Yokoyama, Buseki (Analysis, in English), (1983) 201. K. Makinouchi, K. Agatsuma and H. Suetaka, J. Spectrosc. Sot. Jpn., 29 (1980) 23. E. Takeshima, T. Kawano and H. Mizuki, Nisshin Steel Tech. Rep. No. 47, (1982) 37. H. Ishida and A. Ishitani, F’repr. Seminar Kansai Branch Sot. Fiber Sci. Technol., Jpn., Otsu, Feb. 1985, p. 1.
in Organic
Coatings,
14 (1986)
67
- 86
67
RECENT ADVANCES IN THE ANALYSIS AND CHARACTERIZATION OF ORGANIC COATINGS MASAO Toray
TAKAHASHI* Research
Center,
Inc.,
Sonoyama,
Otsu, Shiga,
520
(Japan)
Contents Introduction. ................................... ...... Analysis of the surfaces of organic coatings ..................... 2.1 Analysis of impurities adhering on the surface ................. 2.2 Analysis of the chemical changes arising in the surfaces of organic coatings after outdoor exposure .......................... 2.3 Analysis of changes in organic coatings brought about by immersion in seawater ........................................ 2.4 Analysis of changes in organic coatings brought about by heating .... Depth profiling of coatings ................................. Problemsrelatedtointerfaces ............................... 4.1 Observation of the interface between the organic coating and the substrate ........................................ 4.2 Modification of the surfaces of polymer materials by plasma treatment. 4.3 Adhesive properties and peeling between the topcoat and the undercoat ............................................ 4.4 The distribution and chemical state of a nickel component in the chemically modified surface layer of a steel substrate ............ 4.5 Analysis of the materials in a very thin layer on the surface of a steel substrate ........................................ Analysis of small inclusions on coating surfaces. ................... New applications of infra-red and Raman spectroscopies to analysis of surfaces, interfaces and small areas on surfaces .................... References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . _. .
67 69 69 69 74 14 75 76 77 79 80 80 81 81 81 86
1 Introduction Problems related to surfaces and interfaces are very often important in the field and technology of organic coatings. In many other industrial fields, surface science and technology also have become increasingly important year by year. Examples include the controlled growth of crystals at surfaces to form solid-solid junctions in microelectronic devices and the chemical transformation of molecules on the surfaces of catalysts in heterogeneous catalytic chemistry; both involve fundamental problems concerning surfaces. The technologies and instruments for the analysis and characterization of surfaces have shown remarkable developments. Over the past decade *Present
address:
0033-0655/86/$7.50
Toray
Techno
Co., Ltd.,
Sonoyama,
0 Elsevier
Otsu,
Shiga,
Sequoia/Printed
520
(Japan).
in The Netherlands
68
surface analytical tools and technologies with very high resolutions have been developed. Some examples are Auger electron spectroscopy (AES), secondary ion mass spectrometry (SIMS) and X-ray photoelectron spectrometry (XPS or ESCA), and a number of unique developments related to the analysis and characterization of surfaces and interfaces have been achieved by means of IR and Raman spectroscopies. The various analytical tools now available, including the above, are listed in Table 1 [2, 81. High-resolution transmission electron microscopy has also contributed greatly towards the characterization of surfaces, and should also be included as an effective tool. In the science and technology of organic coatings, a number of problems exist which are related to surfaces and interfaces since organic coatings are composed of very thin layers formed on a variety of substrates. It is almost impossible to list all the related practical problems, but some are as follows: Adhesion or peeling at the interface, ;;V) Corrosion of the substrate. (iii) Surface appearance and its change after weathering. (iv) The existence of very small amounts of foreign substance on the surface. (v) Bleeding out of the coating components on the surface. In this paper, recent advances in analysis and characterization of organic coatings will be reviewed. However it is not possible to cover all the related published articles in the limited space available, and for this reason the review will be limited to recent advances in Japan, especially recent work undertaken at the Toray Research Center, Inc. TABLE
1
Tools for the analysis
and characterization
of surfaces
and interfaces
[ 2, 81
LEED RHEED SEM EPMA AES SAM APS ESD ISS RBS SIMS IEX SCANIIR XPS UPS LMA CL ATR FT-IR IR
Low energy electron diffraction Reflection high-energy electron diffraction Scanning electron microscopy Electron probe micro-analysis Auger electron spectroscopy Scanning auger micro-probe Appearance potential spectroscopy Electron-stimulated desorption Ion-scattering spectroscopy Rutherford back-scattering Secondary ion mass spectrometry Ion excited X-ray diffraction Surface composition by analysis of neutral and ion-impact X-ray photoelectron spectroscopy Ultraviolet photoelectron spectroscopy Laser micro-analysis Cathode ray luminescence Attennuated reflectance spectroscopy Fourier transformation infrared spectroscopy Infrared spectroscopy
radiation
69
2 Analysis of the surfaces of organic coatings ESCA or XPS methods provide a very high sensitivity for the analysis of the outermost surface layers, and the ease with which inorganic elements may be detected together with FT-IR spectroscopy provides plenty of information about the chemical composition and structure. A combination of these various techniques should therefore provide a very effective tool for analyzing surfaces. 2.1. Analysis of impurities adhering on the surface [ 1, 10, 111 Silicone oil is a substance which is very often found on the surfaces of coatings and causes various problems. Silicone oil layers of cu. 0.2 pm thickness coated on polypropylene can be readily detected by means of XPS and FT-IR spectroscopy. Figure 1 illustrates the XPS spectra arising from a thin silicone layer (0.2 pm thickness) coated on a polypropylene surface. Silicone may be detected with a very high degree of sensitivity by means of XPS, with the chemical structure of the silicone oil being identified through the chemical shifts arising from the peaks. No peak was apparent for the polypropylene substrate. Figure 2 illustrates the FT-IR spectra for the same sample; however in this case, the measurable depth is large (cu. 2 pm) so that the spectrum of the polypropylene substrate is now apparent and mixed with that of the silicone oil. However, it is still possible to separate the peaks arising from the oil through the use of a difference spectrum.
5ooev
Nal;zw Scan
1000
1500KE
01s
AsAm 1200ev
J&+ 1204
952
956
Fig. 1. XPS spectra of a thin silicone propylene film surface.
1363
1387 K.E
oil layer
(cu. 0.2 pm thickness)
coated
on a poly-
2.2 Analysis of the chemical changes arising in the surfaces of organic coatings after outdoor exposure [l, 6,121 After outdoor exposure, changes in the appearance, brightness and physical properties of exposed coatings are usually observed, butthe investigations of more fundamental changes such as those in chemical composition and structure are equally important.
70
z 5
b)
Polypropylene
*
0 v)
z c) Difference
4cuJ
spectrumh-b)
ICW
3an
Wave number(cm-’ > Fig. 2. FT-IR spectra of a thin silicone propylene film surface.
oil layer
(ca. 2.0 pm thickness)
coated
on a poly-
When the surfaces of various organic coatings are investigated by means of XPS and FT-IR spectroscopy after outdoor exposure the following changes are commonly observed: (i) hydrolysis, degradation and other chemical changes which occur in polymer compositions depending on the types of polymer binders and inorganic components such as pigments which become exposed at the surfaces of the coatings [ 1,6, 121; (ii) very small changes in chemical composition and structure which may be detected by subtracting the FT-IR spectra obtained before and after exposure from each other. The durability or life of organic coatings may be more reasonably estimated through the use of studies of this type. The changes in the XPS spectra of a coated surface brought about by weathering are shown in Fig. 3, which indicates that many more elements than may be expected are detected on the surface. From such spectra it is possible to evaluate changes in the elemental compositions on the surfaces of the coatings arising from weathering of the type listed in Tables 2, 3 and 4. Tables 3 and 4 list the assignments of the chemical species on the surfaces of coatings derived from paints A and C, respectively, as determined by XPS chemical shift measurements, and Table 2 shows changes observed in the relative peak strengths of these chemical species due to weathering. In the case of paint A (acrylic-urethane type paint), no change in the chemical species was observed after weathering, but the amounts of inorganic chemical species such as Ti, Pb and Al arising from inorganic additives increased, while the amounts of nitrogen and functional groups which probably arise from the urethane component decreased as a result of weathering. These
0.06 0.05
NI,
0.02 0.03
S&n
0.33 0.62
01s
0.05 0.03
N1S
0.01 0.05
Si2*
0.02 0.14
TiZp
Peak area relative to Cl,
0.61 0.96
01,
Peak area relative to Cr,
0.01 0.08
Pbf
0.04
‘hB
0.01 0.03
Al2P
0.003 0.02
Al2p
0.05 0.06
Fe2p3,2
Ca2,
0.02
0.01 0.01
zn2P3/2
0.02 0.02
Cazp
12 10
24 19
$0
7 7
.-;4
‘O-
0
20 17
$&N
$0
Ratio of carbon components (%I
0.01 0.01
Cl2*
73 76
Neutral carbon
64 71
Neutral carbon
Ratio of carbon components (%I
brought about by weathering of the surfaces of coatings arising from various paints
aPaint A was an acrylic-urethane type paint whose acrylic component consisted of methyl methacrylate copolymer with a hydroxyl group-containing component and whose urethane component contained the isocyanate ‘Desmodur N-75’. The OH/NC0 ratio in the paint was 1 ,O, and the main component of the pigment was Ti02. bPaint C was an acrylic-epoxy type paint commercially available as ‘Epicomarine AE’. The epoxy in this point was of the bisphenol A type, the hardener of the amine type and the main component of the pigment was TiOa.
control weathered
Sample
Paint Cb
control weathered
Sample
Paint Aa
Changes in the elemental composition
TABLE 2
5cQ
*
-
’
’
lcm
*
.
.
eV
.
L
15m
K.EFig. 3. Changes in the XPS spectra of a coated surface brought about by weathering (paint A: an acrylic-urethane paint). TABLE 3 Assignment of chemical species on the surface of paint A (an acrylic-urethane from XPS chemical shift measurements Element
type paint)
Control and weathered samples
--ago o-
0 N Si Ti Pb Al Fe Zll Ca Cl
organic, inorganic urethane silicone TiOs Pbn AlaI I?@ zn= Ca” ?
data indicate that weathering leads to the decomposition of the polymer component and that the components of the pigment leach out on to the surface as a result of exposure outdoors. Such changes were also observed by means of scanning electron microscopy. Similar results were observed in the case of paint C (an acrylic-epoxy type paint). Figure 4 illustrates the FT-IR-ATR spectra of the control and weathered samples. Individually, IR spectra have insufficient sensitivity, so that it is hardly surprising that little difference is apparent between these two spectra. However, by using the difference spectrum, peaks may be assigned
73 TABLE
4
Assignment of chemical species on the surface from XPS chemical shift measurements Element
Control
and weathered
C
neutral,
>&O,
>&N,
of paint
C (an acrylic-epoxy
type
paint)
samples
-a<’ O-
0
* , C-O-, amine SiO2 -
N Si Ti Al Ca
4ollo
SiO2, Al203
Al203 -
3aoo
Fig. 4. FT-IR-ATR surface developed
1600 2000 WAVE NUHBERS
1200
spectra illustrating the structural from paint A by weathering.
000
changes
brought
about
in a coated
to increases in the alcohol, amine and acid concentrations while the concentrations of amide II and III as well as the size of the peak for C-O have decreased. Takeshima et al. have studied the chemical changes occurring in organic coatings after various types of weathering test by following the variations in concentration of selected functional groups by means of FT-IR-ATR spectroscopy. Figure 5 shows a typical example of the results obtained in these studies. The pattern-type illustration of the changes in various functional groups provides a good picture of the deterioration brought about by outdoor exposure of a thermosetting polyester/melamine type paint coated
74
Non-weathered
& outdoor weathering
test
Dew cycle weatherometer Fig. 5. FT-IR-ATR steel sheet (sample pigment).
spectral consisted
test
analysis of the deterioration of a paint film coated on to a of a thermosetting polyester/melamine resin containing TiOz
on to a steel sheet. The decomposition of the melamine resin, oxidation of the resin and appearance of the pigment on the surface are all well illustrated in this figure. Figure 5 also shows that of the nine types of accelerated weathering tests undertaken in parallel on the sample, the dew cycle weatherometer test gave the pattern closest to that observed for outdoor weathering. 2.3 Analysis of changes in organic coatings brought about by immersion in sea water [I] Changes in organic coatings after immersion in sea water have been analyzed by XPS methods. Figure 6 shows the change observed in the XPS Cl peak of a poly(vinyl chloride) coated surface after immersion in sea water. In this case, Cl ions were detected on the surface. However, whether these Cl ions came from the sea water or from the decomposition of the poly(viny1 chloride) coatings is undetermined at present and should be one of the points to be studied further. By using the difference spectrum as shown in Fig. 7, very small changes occurring on immersion in sea water may be detected, and from these the extent of the oxidative degradation of the coating and the adherence of bio-materials may be estimated. 2.4 Analysis of changes in organic coatings brought about by heating [l] Figure 8 shows the changes in the elemental composition of the coating surface brought about by heating as measured by XPS methods. After such treatment, components of the organic coatings or additives such as insecticides emerged on the surface and could be observed.
75
I
1
,
!
12ea
1265
1230
Kinetic
Energy
( eV )
Fig. 6. Change in the XPS Clzp peak of a coated surface after immersion in sea water * (paint B: commercially available poly(viny1 chloride) type marine paint).
3 Depth profiling of coatings [l] It is useful to observe the depth profiling of the compositions of organic coatings to enable an understanding of the uniformity of composition and structure as well as the extent of diffusion of the components in the coated films. Figures 9 - 12 illustrate the depth profiles of organic coatings studied after outside weathering tests. As shown in Fig. 9, the diffusion of oxygen, oxidative degradation, changes in composition arising from chemical reactions and changes in concentration of the components could all be observed by means of XPS combined with a depth-profiling technique. The FT-IR-ATR spectra of the same samples at different depths are illustrated in Fig. 10. In this case, no definitive change could be observed on comparing with the spectra of the control sample. However, small differences in composition and structure may be observed by means of the difference spectrum as shown in Fig. 11. The depth profile of an Sn compound obtained by this method is shown in Fig. 12, which correlates well with the result obtained by XPS methods as shown in Fig. 9.
76
Control
Difference
I
Spectrum
Normalized to Hydrocarbon
I 1200 Kinetic
(1202.0
eV)
1 1205
Energy
(eV)
Fig. 7. Change in the XPS C1, peak of a coated surface after immersion in sea water as revealed by the difference spectrum (paint B).
cts
I
Paint A
I
500
1
I
I
I
I
1
Kxo
KE(eV) Fig. 8. Changes in the elemental composition heating as observed by XPS methods (paint B).
I
I
ml
of a coated surface brought about by
4 Problems related to interfaces Many practical problems such as the occurrence of rust and interfacial peeling may be related to interfaces.
77
0
I.0
40
Depth
(PI
’
20
LO
Fig. 9. Depth profiles obtained from the relative XPS intensity ratios of the control and immersed coatings (paint B).
0
Fig. 10. Depth profiles of weathered coatings obtained by means of FT-IR-ATR studies (paint B).
spectral
4.1 Observation of the interface between the organic coating and the substrate [l, 91 A coating on a steel substrate may be peeled off by immersion in liquid nitrogen. The interfaces on both sides of the coating thus obtained have been studied by XPS methods. Table 5 lists the various elemental quantities on
78
I Sn cnmp
Surface
- inner layer(20p)
Fig. 11. Difference spectrum between the spectra of the surface and an inner layer (at 20 pm depth) of a weathered coating (paint A).
Fig. 12. Depth profiles of inorganic additives in a coated surface as obtained by means of FT-IR-ATR methods.
the peeled surfaces obtained from such measurements. Both sides contained zinc phosphate and iron, while nitrogen arising from the urethane coating could be detected on the substrate side. From this it was therefore deduced that the location of the peeled interface had shifted towards the substrate and interdiffusion of the components had occurred.
19
TABLE 5 Elemental quantities on peeled surfaces before and after weatheringa Weathered for 24 month
Non-weathered
Substrate side
Coating side
Substrate side
Coating side
0.92 0.10 0.20 0.06 0.07
Sizp
-
0.44 0.33 0.03 0.15 0.03 0.03
0.62 0.10 0.06
Feza Pan
1.67 0.03 0.54 0.30 0.23
01s
Nls zn2P3/2
0.02 -
aMeasured as a ratio relative to Cl,
4.2 Modification of the surfaces of polymer materials by plasma treatment 13951 Polar groups such as carbonyl and carboxyl formed via plasma treatment of the surfaces of polyethylene, polypropylene and a fluoro polymer have been analyzed by means of XPS. Such treatment was found to improve the adhesive properties of the modified surfaces. Figure 13 shows an example of such an analysis. From this it may be deduced that plasma treatment leads to the formation of carbonyl groups on the surface.
295
290
285
Binding energy (eV)
540
535
530
Eb (eV)
Untreated
295.
290
285
Binding energy (eV)
535 540 Ea(eV)
530
Fig. 13. XPS spectra of the surfaces of a polypropylene treatment.
film both before and after plasma
80
4.3 Adhesive properties and peeling between the topcoat and the undercoat [3,7,81 To estimate the extent of peeling between the topcoat and the undercoat, the interface was analyzed by means of XPS as shown in Fig. 14, which indicates the disappearance of carboxylic groups and bleeding out of the component on to the surface of the film when the latter was cured at elevated temperatures (200 “C) (showing poor adhesion) in comparison to the film cured at lower temperatures (80 “C) (showing good adhesion). It was found that residual polar groups such as carboxyl on the surface of an undercoat and ones arising from the melamine resin contribute to the adhesive properties, and that removal of the additive components from the undercoat leads to inter-facial peeling.
cts 1200cps
NIS IlOOcps Coating peeled at interlayer 501 from surface surface
5Ol
from surface
surface
50ii from surface surface
Fig. 14. XPS Cl, and N1, spectra for various coatings both at the surface layer and 50 from the surface (the type of the paint used was not described in the original paper).
A
4.4 The distribution and chemical state of a nickel component in the chemically modif~d surface layer of a steel substrate [3,4) Depth profiles of a nickel component providing nuclei for zinc phosphate crystals in the zinc phosphate treated surface layer of a steel prepared by the so-called dipping chemical treatment process have been analyzed by XPS methods as shown in Fig. 15. In this case it was found that the concentration of nickel became larger as the distance from the surface increased, thus supporting the formation mechanism for the zinc phosphate layer as being based on nickel acting as a nucleating agent on steel surfaces. It was
81
100 -
S + k? -
Coating surface
2
c
Expanded spectrum
.o....
- 6” : : ,#’ 0 10
Steel/coating interface _,___---a 4
-../860 20
30
40
BindIng energy
& (W
Fig. 15. Depth profile of a nickel component in a zinc phosphate coating layer on steel as determined by XPS measurements employing the Ar+ etching method. Fig. 16. The expanded Niz, XPS peak associated with the Ni component in the zinc phosphate layer of Fig. 15 and separation of the peak into two components (peak 1 = 861.55 eV, peak 2 = 858.85 eV).
also found that two types of nickel compounds were formed in the layer as shown in Fig. 16. 4.5 Analysis of the materials in a very thin layer on the surface of a steel substrate [lo, 111 A very thin layer of an alkyd resin located on the surface of steel has been analyzed by means of infrared emission spectroscopy, with the same sensitivity being obtained as with infrared reflectance spectroscopy. Figure 17 shows a typical result obtained, indicating that as the measuring angle increases the sensitivity also increases.
5 Analysis of small inclusions on coating surfaces [ 11 Small inclusions on coating surfaces very often give rise to a variety of problems. These inclusions can usually be investigated and analyzed by means of SEM-XMA techniques. By utilizing the micro-analysis technique of FT-IR spectroscopy, very small inclusions of the type which were most difficult to analyze can now be studied. Examples of such studies are shown in Figs. 18 and 19 which illustrate the analysis of a white inclusion and a granular particle on a coating surface as being SiOZ and an aggregate of an insecticide, respectively [ 11.
6 New applications of infrared and Raman spectroscopies to the analysis of surfaces, interfaces and small areas on surfaces [ 131 New applications of IR and Raman spectroscopy to the analysis of surfaces, interfaces and small areas on surfaces are being developed. New techniques involving IR spectroscopy include Ft-IR-DRIFT, -ATR, -RAS,
Fig. 17. IR emission spectra of a thin layer of an alkyd resin on a steel surface, Measurements were made on a film of 150 nm thickness at a temperature of 150 “C. The continuous line depicts the reflectance spectrum obtained at room temperature, while the dotted lines and the dot-dashed line depict the spectrum obtained with parallel and vertical polarized light, respectively. The measuring angles employed were A = 76”, B = 60”, C = 45”, D = 30” and E = O”, respectively.
I
N-H
Si-0
C-H
3000
2000
1000
hn-3
Fig. 18. Spectrum of a white spot (100 x 200 pm) in a weathered coating as analyzed by the FT-IR microsampling technique.
83
-EMS and -PAS. The application of surface-enhanced Raman scattering (SERS) to surface analysis is another field which is showing remarkable development. The following are some examples of applications in these fields. Figure 20 shows a schematic illustration of the principles of FT-IRPAS, which is very effective for the non-destructive analysis of surfaces and interfaces, and for the analysis of non-flat and black samples. Figure 21 shows one example using the FT-IR-PAS technique. When a multilayer ?le of the type illustrated in Fig. 19 was analyzed by means of FT-IRC-H
1
’
Sn-0 (H)
6
I
1
C-H
1
co2 c-n
Fig. 19. Spectrum of an inclusion sampling technique.
(100
pm) in a coating
as analyzed
by the FT-IR
PAS Cell
Interferometer
... b .,‘... ) (,~ i. is cl
!F
. . .. ( ,’ .
L .‘...
‘..
\ Sample ? Stage PAS Spectrum
Source
FT AmpUfier
Fig. 20. Schematic
illustration
of the principles
of FT-IR-PAS
analysis.
micro-
84
PAS, it was possible to observe the spectrum arising from the polyurethane layer in a non-destructive manner. Thus, for example, the reaction occurring in an inner layer cc&d be followed by means of this technique. Figure 22 is a schematic diagram depicting the new FT-‘-IR-ATR technique while Fig* 23 shows that a molecular layer of the Cd salt of eicosanoic acid on a glass substrate may be detected by using this particular technique. Figure 24 indicates that the Raman spectrum of a surface can be considerably enhanced and hence measured to a very high degree of sensitivity by means of silver-i&and film deposition_ Figure 25 illustrates the spe&ral changes in highly oriented pyrolytic mphite (HOPG) braught about by
PP Thickness Fig.
21,
A : 4t.im
El : rJim
Polwopyrene
( PP 1
SlllCOlV3
t3umf
C : 15pm
An example of depth analysis capable with the FT-IR-PAS
Fig. 22. Schematic diagram depicting the FT-IR-ATR
technique.
D:22vm_
technique.
85
RES = 4
Fig. ‘23. FT-IR-ATR on a glass substrate.
spectra
of the mono-
Raman shift VA
and multi-layer
films of cadmium
eicosanoate
Ag-island film
A~~(50-100% ,‘.
..,
,“;,: ..,.
Fig. 24. Application
B
B; of SERS
‘(‘,’ to surface
..i
analysis.
argon-ion etching. A broad Raman band (1350 cm-‘) arising from the disordered carbon structure appears in the spectrum. When a silver-island film is placed upon the etched HOPG, the resulting Raman spectrum is remarkably different. In this case the outermost disordered surface has been enhanced exclusively by contact with the silver-island film and this enhanced spectrum may be measured to a very high degree of sensitivity.
I
I
I
I
I
I
I
I 1000
1500
Raman
shift
cm-’
Fig. 25. Enhanced Raman scattering arising from an argon-ion etched HOPG surface covered with a silver-island film.
References 1 2 3 4 5 6 7 8 9 10 11 12 13
A. Ishitani, J. Jpn. Sot. Colour Mater., 53 (1980) 468. T. Tanaka, Zairyo to Seko (Materials & Finishing, in English), I (1982) 112. N. Sato, J. Jpn. Sot. Colour Mater., 57 (1984) 206. N. Sato, Corros. Eng., 32 (1983) 379. Y. Taru and K. Takaoka, Toso Gijutsu (Coating Technology, in English), 1 (1980) 106. E. Takeshima, T. Kawano and H. Mizuki, J. Zron Steel Inst. Jpn., 68 (1982) S1049. N. Sato, Toso Kogaku (Finishing Eng., in English), 18 (1983) 226. T. Tanaka, Toso Kogaku (Finishing Eng., in English), 19 (1984) 26. S. Maeda, Corros. Eng., 31 (1982) 621. T. Yokoyama, Buseki (Analysis, in English), (1983) 201. K. Makinouchi, K. Agatsuma and H. Suetaka, J. Spectrosc. Sot. Jpn., 29 (1980) 23. E. Takeshima, T. Kawano and H. Mizuki, Nisshin Steel Tech. Rep. No. 47, (1982) 37. H. Ishida and A. Ishitani, F’repr. Seminar Kansai Branch Sot. Fiber Sci. Technol., Jpn., Otsu, Feb. 1985, p. 1.