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Research ArticleReceived: 14 August 2007 Revised: 18 December 2007 Accepted: 20 December 2007 Published online in Wiley Interscience: 10 March 2008

(www.interscience.com) DOI 10.1002/sia.2767

Surface characterization of polyester resinsformulated with different cross-linking agentsPaolo Marino,a∗ Chris Lowe,b Marie-Laure Abela and John F. Wattsa

Three thermally cured coatings, formulated on a low Tg isophthalic-based polyester, were investigated by XPS and ToF-SIMS.A model formulation was employed for all three coatings investigated; however, the cross-linking agents used were varied foreach formulation. Hexamethoxymethyl melamine (HMMM), tris-isocyanurate (TIC), and a combination of HMMM and TIC wereincluded as the cross-linking agents. The use of TIC alone required a tin-based catalyst to promote the curing reaction.

The aim of this work was to investigate the difference in the surface compositions of the three coatings and the distributionof the different cross-linking agents used. This was in preparation for further studies which will involve interfacial analysis inorder to elucidate the mechanism responsible for intercoat adhesion.

The XPS analysis of the coating surfaces revealed a nitrogen concentration consistent with the concentration expected fromthe formulation for the coating containing HMMM. In the other two formulations a lower concentration than calculated wasobserved. The surface concentration of the two cross-linking agents was not influenced by the presence of the others; indeed,the formulation containing both cross-linking agents was, in terms of nitrogen concentration, merely a simple combinationof the other two coatings. Peaks diagnostic of the cross-linking agents were observed in ToF-SIMS spectra acquired fromthe coating surfaces. By using XPS and ToF-SIMS analysis, we could determine that the HMMM and the TIC have a differentdistribution at the coating surface, that is not affected by the presence of the other. Copyright c© 2008 John Wiley & Sons, Ltd.

Keywords: coil coating; polyester; XPS; ToF-SIMS; cross-linker

Introduction

Coil coating is an industrial process widely used to apply a paintlayer to metal coil stock, in order to improve the resistance tocorrosion as well as the aesthetics of the surface. The purpose inapplying a coating is to improve the resistance to external agentsof the metal substrate to which the coating is applied to, and tomodify its aesthetic properties as well. A coating is expected tolast for a very long period, at least 20 years in case of coil coatings.In order to achieve this, all the processing conditions, temperatureand time of curing process, in case of thermally cured coatings,have to be settled properly and specific components have tobe included in the formulation. A crucial component in everythermally cured coating formulation is the cross-linking agent.

The role of the cross-linking agent is to cross-link the polymerchains of the basic resin, to give consistency to the final coating. Inprevious works,[1,2] the coating surfaces of several samples basedon hexamethoxymethyl melamine (HMMM) have been extensivelyinvestigated. The aim of this work was to study a coating curedusing different cross-linking agents, tris-isocyanurate (TIC), HMMMand a combination of the two, to develop cross-linked coatings, inorder to understand the effect of the cross-linking agents on thesurface chemistry. Future work will be aimed at understanding theinterfacial chemistry responsible for adhesion.

Experimental

Sample preparation

Three thermally cured coatings, formulated on a low Tgisophthalic-based polyester, Fig. 1, were studied. A model formula-tion was employed for all three coatings, however, the cross-linkingagent used (Fig. 2), varied for each formulation.

Apart from the cross-linking agent, the only difference betweenthe three samples was the presence of a tin-based catalyst in thesample with TIC as the only cross-linking agent. All formulationsincluded a poly(acrylic) flow aid agent.[1] The curing time andtemperature were the same for all the systems. Hereinafter, thesamples will be called with the name of the cross-linking agentused, namely HMMM, TIC, and COMB (combination) where bothare used.

Surface analysis

The XPS analyses were performed using a modified VG ScientificESCALAB Mk II electron spectrometer equipped with a ThermoAlpha 110 electron energy analyser and a Thermo XR3 digitaltwin anode source, which were operated using the AlKα at 300 W.Survey spectra (0–1350 eV) were recorded using a pass energyof 100 eV and a step size of 0.4 eV. High-resolution spectra wererecorded using a pass energy of 20 eV and a step size of 0.1 eV forthe C1s peak and of 0.2 eV for other corelines. The spectrometerwas controlled by a Thermo Avantage datasystem, that was alsoused for subsequent data processing.

Time-of-flight (ToF)-SIMS spectra were acquired on an ToF.SIMS5 (ION-TOF GmbH), using Bi3

+ as primary ion beam, operating inthe high current bunched mode, over an area of 100 × 100 µm,

∗ Correspondence to: Paolo Marino, The Surface Analysis Laboratory, Faculty ofEngineering and Physical Sciences, University of Surrey, Guildford, Surrey, GU27XH, UK. E-mail: p.marino@surrey.ac.uk

a The Surface Analysis Laboratory, Faculty of Engineering and Physical Sciences,University of Surrey, Guildford, Surrey, GU2 7XH, UK

b Becker Industrial Coatings Ltd, Goodlass Road, Speke, Liverpool, L24 9HJ, UK

Surf. Interface Anal. 2008; 40: 137–141 Copyright c© 2008 John Wiley & Sons, Ltd.

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COO

COO

OOCCOO OH

OOC COO OH

Figure 1. Structure of the basic resin.

at a resolution of 64 × 64 pixels. During the analysis, a floodgun was used to compensate for the samples charging. All theanalyses were performed under the SIMS static limit, i.e. ion dose<1013 ions cm−2.

Results

XPS analysis

A survey spectrum was recorded for every coating: C1s, 285 eV;O1s, 532 eV; and N1s, 400 eV are the main peaks in the surveyspectra. The Sn3d doublet, at 487 and 495 eV, is a feature of thesurvey spectrum of the TIC. The high-resolution peaks were usedfor the quantification of the samples. The surface compositions ofthe three coatings are given in Table 1.

Peak-fitting was used to identify all the components in thecarbon spectra. The same full width at half-maximum (FWHM)was used for all the components in the fitting of the three C1speaks, at a value of 1.36 eV. It has been noted that the resolution

Table 1. Coating surface compositions (at.%)

C O N Sn

HMMM 69.6 29.0 1.4 –

TIC 71.3 27.4 1.0 0.3

COMB 69.6 28.0 2.4 –

obtainable with the Alpha 110 and monochromated X-rays was farsuperior to that achievable on the standard ESCALAB MkII. For thefitting of the high-resolution spectra, the relative ratios betweencomponents, and their binding energies, taking for reference thebinding energies established by Beamson and Briggs,[3] that wereconsistent with our previous work,[2] were used to ensure a goodquality work. Knowing the proportions of the different kinds ofcarbon present in the material, such as the carbon atoms inthe HMMM ring, careful peak-fitting may be performed. Thosefittings are consistent with respective nitrogen concentrationdemonstrating their good quality. In Fig. 3 the C1s of COMB isshown. Eleven different components are used for the fitting. Fivecomponents originated from the basic resin, three from eachcross-linking agent. The fitting of the other two samples requiredthe use of eight components. The peaks used in the fitting, andtheir surface concentrations are reported in Table 2.

ToF-SIMS analysis

Positive and negative spectra were acquired for the three samples,in the 0–800 mass/u range. The general pattern of the spectra,

N N

N OO

O

(CH2)6 (CH2)6

NHC

(B)

O NH

C

(B)

O

CH3-(C=NOH)-CH2-CH3

N

NN

NN

NCH2OCH3CH3OCH2

CH2OCH3

CH2OCH3CH3OCH2

CH3OCH2

(CH2)6

HN C O

(B)B = Methyl Ethyl Keto Oxime

Figure 2. The structure of the two cross-linking agents used. On the left the TIC, on the right the HMMM.

0

200

400

600

800

1000

1200

1400

1600

1800

280281282283284285286287288289290291292293294

Binding Energy

C aliphatic

C aromatic

*C-C=O

*C-O-C=O O-*C=O C-O-C

N-*C-O

NN-*C=O *C-N

NO-*C=O

Cou

nts

/ s

C in HMMM

Figure 3. Fitting of the C1s high-resolution spectrum of the COMB formulation.

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Table 2. Carbon component surface concentrations (at.%)

COMB TIC HMMM

Aliphatic C 21.4 27.0 21.2

Aromatic C 11.2 11.9 11.5

C –C O 12.2 12.1 12.2

C –O–C O 10.0 8.9 10.5

O–C O 9.6 9.8 10.4

C –O–C 1.4 – 1.4

N–C –O 1.4 – 1.4

C in HMMM ring 0.6 – 0.7

C –N 1.0 1.0 –

NN–C O 0.5 0.5 –

NO–C O 0.5 0.5 –

in Fig. 4, the positive spectrum of the HMMM, both in positiveand negative, is the same for the three coatings; however, it is

possible to identify some peaks that characterize the two differentcross-linking agents used. These peaks can be identified moreeasily by checking the high-resolution peaks. Peaks present in theTIC spectra are not in those of HMMM, and vice versa. All of thesepeaks can be found also in the COMB sample, which exhibits allthe peaks yielded by the resin and the cross-linking agents, butnot the peaks due to the tin catalyst employed in the TIC coating.Peaks due to this catalyst appear only in the TIC spectra.

Discussion

The only source of nitrogen was the cross-linking agent, or agentsused, so using its concentration values, the distributions of thecross-linking agents on the coating surface were studied. Byknowing the precise concentration of the cross-linking agentincluded into the formulation and the N percentage in its structure,expected nitrogen values have been calculated for the threesamples. The observed value is lower than the one expectedboth for the HMMM, observed 1.4%, expected 1.9%, and the TIC,

Figure 4. Positive ToF-SIMS spectrum of the HMMM formulation in the mass range m/z 0–300 u.

Surf. Interface Anal. 2008; 40: 137–141 Copyright c© 2008 John Wiley & Sons, Ltd. www.interscience.wiley.com/journal/sia

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Figure 5. High resolution peaks characteristic of: flow aid agent (a), HMMM (b), TIC (c) and resin (d). Top spectrum corresponds to HMMM coating, middleto Tic and bottom to COMB.

1.0% versus 2.4%. The difference is due to the presence of a layerin the surface formed by the flow aid agent included into theformulation, which partially covers the counts due to the otheragents. The thickness of this layer is likely to be influenced bythe small amount of the flow aid in the formulation, 0.5%, and itsdiffusion through the coating.

Previous studies into coatings cured with melamine haddemonstrated the migration of the flow aid agent to the coatingsurface, with the formation of a layer, whose thickness wasestimated to be less than 1 nm.[1] According to the data, wecan assume that the flow aid migrates in the coating surface even

for the coating cured with TIC and with the combination of thetwo cross-linking agents. Its migration is not influenced by thecross-linking agent included in the formulation.

The ToF-SIMS spectra confirm the presence of the cross-linkingagent in the first atomic layer of the coatings. In all the positivespectra, a very strong peak at 99 m/z can be observed. This peak,due to the C4H3O3

+ fragment, is characteristic of the flow aidincluded in the formulation. Considering the SIMS informationdepth, about 2 nm, seeing peaks due to the resin and the cross-linking agents is further evidence of our assumption about theflow aid layer thickness.

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When the two cross-linking agents were used in combination,the nitrogen concentration was equal to the addition of the valuesof two when they were alone. This seems to show that the twocross-linking agents do not influence one another. The results ofthe fitting of the C1s peaks, Table 2, seems to confirm this. Theconcentrations of the components originating from the cross-linking agents do not change when they are alone and when theyare used in combination.

According to Hinder et al.,[4] the higher the nitrogen concentra-tion the better the adhesion of a coating with a top coat, so, thecoating cured with TIC is expected to yield a worse adhesion thanthe coating cured with HMMM, if this hypothesis is correct.

Conclusions

XPS and SIMS were used to investigate the influence of a cross-linking agent on the composition of the external surface ofvarious coatings. The two cross-linking agents used showed adifferent distribution, with the HMMM, being more concentratedon the coating surface than the TIC. When used together, theirdistributions do not change, as showed by the comparison of thenitrogen concentration values and the fitting of the C1s spectra.

The flow aid included in the formulation migrates to the coatingsurface in all the samples, and it is not influenced by the cross-linking agent used. The observation of peak characteristics of thebasic resin and of the cross-linking agent in all the spectra recordedindicates that the flow aid layer is thinner than 1 nm.

Acknowledgements

Paolo Marino thanks the ESPRC and Becker Industrial Coatings forthe financial support. All authors acknowledge Dr Steve Hinderwith XPS and ToF-SIMS measurements.

References

[1] Perruchot C, Abel M-L, Watts JF, Lowe C, Maxted JT, White RG. Surf.Interface Anal. 2002; 34: 570.

[2] Leadley SR, Watts JF, Blomfield CJ, Lowe C. Surf. Interface Anal. 1998;26: 444.

[3] Beamson G, Briggs D. High Resolution XPS of Organic Polymer. TheScienta Database. John Wiley: Chichester, 1992.

[4] Hinder SJ, Lowe C, Maxted JT, Perruchot C, Watts JF. Prog. Org. Coat.2005; 54: 20.

Surf. Interface Anal. 2008; 40: 137–141 Copyright c© 2008 John Wiley & Sons, Ltd. www.interscience.wiley.com/journal/sia