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The Interface Chemistry Of Adhesion: Segregation Phenomena Of Minor Components Of A Model Coil Coating Formulation  

 

S.R. Leadley1, J. F. Watts1, C.J. Blomfield2 and B. Tielsch2 .

1 Department of Materials Science & Engineering, University of Surrey, Guildford, Surrey, GU2 5XH, UK..
2 Kratos Analytical, Manchester, UK.

 
 
Note: The Figure "thumbnails" in the text are links to separate pages (displayed in a separate browser window) which will display larger reproductions of the spectra for closer examination.

Aim

  • Use high resolution XPS to investigate the surface chemistry of an unpigmented model stoving primer for use in coil coating.
  • Investigate the effect of a minor component on the surface chemistry of the primer.
  • Investigate the effect of curing on the surface chemistry of the primer.
  • Investigate the effect of the substrate on the surface chemistry of the primer.

 
AXIS ULTRA analysis chamber

Introduction

Coil coatings are extensively used for a variety of different purposes, and they are considered one of the most technically advanced paints. Conventional thermally cured coatings consist of complex mixtures of organic resins and inorganic pigments, dissolved in non-reactive volatile solvents. The concept of surface segregation of components within coatings and adhesives is now recognised.  
   

Experimental

Materials

Two formulations of a thermally cured primer were under investigation.

Formulation A Formulation B
  • Epoxy Resin
  • Melamine-Formaldehyde Resin
  • Non-reactive solvents
  • Epoxy Resin
  • Melamine-Formaldehyde Resin
  • Flow Agent
  • Non-reactive Solvents
The epoxy resin based on the diglycidyl ether of bisphenol A, which has the structure shown below.

 

The epoxy resin is cross-linked via the melamine-formaldehyde resin, which has the following structure:

  Both the epoxy and melamine-formaldehyde resins have poor wetting properties. To aid the application of the coating, less than 1% of a polyester flow agent is usually added to the formulation. The flow agent under investigation is a low molecular weight modified methacrylate polymer.
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Substrates

0.6 gauge hot-dipped galvanised steel (HDGS)
0.6 gauge HDGS + alkaline surface conditioning + chromate rinse (Bonder 1303) (Parcolene 62)

Samples

Formulations A & B were applied to the substrates using a draw bar, which gave a 24 µm wet film thickness. All the samples were then placed in an oven at 80°C for 30 minutes, so that the non-reactive volatile solvents were removed. The samples were then removed from the oven and allowed to cool to room temperature. The coatings were cured by heating from room temperature to a peak metal temperature of 230°C, for 40 seconds.

Curing Mechanism

The epoxy/melamine-formaldehyde coatings are cured at high temperature.
The epoxy resin cross-links with the melamine-formaldehyde by trans etherification.
transetherification
The cured resin has the cross-linked structure below.
cured resin
 

 

XPS Analysis

All the XPS analysis was performed on a Kratos Axis Ultra electron spectrometer. The instrument is equipped with a unique spherical mirror analyser, 165 mm mean radius HSA, integral automatic charge neutraliser and magnetic lens.
10mm discs were stamped from the treated HDGS sheet and mounted on a sample strip bar. A monochromatic Al K X-ray source was used at a nominal power of 450W.
Charge compensation was required for all of the samples. The self regulating nature of the Kratos charge neutralisation system meant that each sample was run under identical neutraliser parameters in a hands-off manner.

The following conditions were applied to each region:

Region
 		Pass
Energy eV 		Acquisition
Time min 		Step
Size eV
Survey 160 5 1
O 1s 10 3 0.05
C 1s 10 3 0.05
N 1s 10 10 0.05
Valence 20 30 0.1
.

Table 1 Elemental concentrations detected by XPS

 
Sample %C %O %N
HDGS + Formulation A (uncured) 81.3 16.2 2.4
HDGS + Formulation A (cured) 75.3 17.4 2.8
HDGS + Formulation B (uncured) 79.3 19.0 1.7
HDGS + Formulation B (cured) 80.0 18.56 1.5
Chromated HDGS + Formulation A (uncured) 80.8 16.3 2.9
Chromated HDGS + Formulation A (cured) 79.1 16.8 2.3
Chromated HDGS + Formulation B (uncured) 81.1 17.7 1.3
Chromated HDGS + Formulation B (cured) 79.7 18.5 1.8


RESULTS

Both the epoxy and melamine-formaldehyde resins have poor wetting properties. To aid the application of the coating, less than 1% of a polyester flow agent is usually added to the formulation. The flow agent under investigation is a low molecular weight modified methacrylate polymer.

peak fit 1

Figure 1: (alongside) C1s feature from Formulation A uncured.
The C 1s component is fitted with 8 peaks, peaks (5,7,8) are associated with melamine formaldahyde.

Formulation A

Peaks fitted to the C 1s spectra of samples coated with Formulation A:
Five peaks associated with the epoxy resin (1,2,3,4,6)
Three peaks associated with the melamine-formaldehyde (5,7,8)

Formulation B peak fit

Figure 2 C1s feature from Formulation B uncured.
The C 1s component is fitted with 11 peaks, peaks (9,10,11) are associated with polyesters.

Formulation B

Peaks fitted to the C 1s spectra of samples coated with Formulation B:
Five peaks associated with the epoxy resin (1,2,3,4,6)
Three peaks associated with the melamine-formaldehyde (5,7,8)
Three peaks associated with polyesters (9,10,11)

 

Valence band spectra were determined for Formulation A uncured and Formulation B uncured. The subtle differences evident in the O2s, C2s and C2p signals reflect the changes observed in the fitted C1s regions above.

link to valence band spectrum
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Discussion


Chlorine contamination observed in the survey scan (approx. 0.1%) is associated with epichlorohydrin.

Formulation A

Assumption:
All of the volatile solvents are removed by heating the samples at 80°C. Formulation A should contain 96% epoxy resin and 4% melamine formaldehyde.

%C %O %N
85.3 - 85.4 14.6 -14.5   0.1  

Surface nitrogen concentrations are observed greater than would be expected from the theoretical ratio above.
The peaks associated with melamine-formaldehyde (C4 -O, O-C7-N, & N-C8=N), had areas greater than expected, which suggests surface enrichment of melamine-formaldehyde
No changes observed as a result of curing.
No changes as a result of as a result of substrate pre-treatment.

link to cmposition chart

Formulation B

Surface nitrogen concentrations lower than samples coated with Formulation A but still higher than predicted (Figure 4)
The C1s spectra of the samples coated by Formulation B were different to the C1s spectrum of samples coated by Formulation A.
Additional peaks were fitted that are associated with polyesters.
Evidence that the polyester flow agent added to Formulation B is observed at the surface.
No changes observed as a result of curing
No changes observed as a result of substrate pre-treatment.

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Conclusions


All of the components in the coated samples were identified by high resolution XPS. Surface enrichment of the melamine-formaldehyde resin is observed. The flow agent added to Formulation B surface segregates. The surface segregation phenomena are not affected by curing at high temperatures. The surface segregation phenomena are not affected by substrate pre-treatment. Valence band spectra showed subtle changes between Formulation A and B which related to the C 1s feature.

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Acknowledgements

The authors would like to thank the following:

Dr C. Lowe (Beckers Industrial Coatings Ltd), for preparing the coated samples.
EPSRC for funding

 

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