Adhesion: A sticky problem for Industry
Prof JF Watts
Professor Watts open the technical session
by explaining what adhesion science means to him …
the examination of the organic/metal or organic/organic
interface. Uses of surface analysis in this area are :-
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Surface characterisation.
Looking for contamination or pre-treatment.
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Failure analysis.
Key areas of analysis examples include the Concorde
tail plane.
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Interphase chemistry.
How to probe this layer. Also real life problems…environmental
failures.
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Locus-of-Failure
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Prof.
John Watts
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Early work was OM or SEM based,
then the analyst tried to guess as to whether it was an
adhesive or cohesive failure. The first example shown by
Professor Watts was a coating failure in Bodington beer
cans. Conclusion from work was that there was a cohesive
failure in the epoxy lacquer on the tin free steel. The
work was completed in 40 minutes using a VG Sigma probe;
it took longer to do the overheads and data analysis! The
work clearly could not have been done using a SEM since
the film was too thin.
Professor Watts then talked about component
segregation in adhesives due to the use of excessive levels
of low-level components in the formulation. The example
used was the Zeneca adhesive work completed by Alison Taylor
in the mid nineties. The work highlighted the limitations
of old instruments, which were unable to show the structure
of the C1s line.
Using the ESCA300 they were able to show
that there was an aromatic component in the fracture surface
on the bulk side, this was shown to be active diluents in
the adhesive. This layer was probably only a monolayer thick!
The XPS measurements were supported by the use of molecular
dynamics calculations on the interface. Removing the diluents
the environmental performance improved but the possibility
was compromised.
Generation of Adsorption isotherms
For the adsorption of cations onto a surface,
in general, XPS is used, while ToF-SIMS is used for the
adsorption of organic systems. The later has been used extensively
to study the adsorption of DEGBA onto treated and untreated
carbon fibres. Treated fibres will adsorb more DEGBA than
untreated materials. Interestingly, the amine-hardening
agent does not follow the same trend.
High Spectral Resolution XPS studies
on thin organic films
Using mono sources it is possible to see
small shifts due to the adsorption of thin organic films
on the surface of silicon or Aluminium. This can be done
in order to determine the acid-base characteristics of the
surface. The work requires a detailed knowledge of the surface
and considerable levels of faith in the small peak shifts
(~0.1 eV).
High Mass Resolution ToF-SIMS studies
on thin organic films
The work has been developed to study the
interaction of silane molecules with Aluminium surfaces.
The technique has been used to show the formation of Al-O-Si
bonds, which is not possible using other techniques such
as XPS and is difficult to do with older T0F instruments
due to the overlap of other ions at the 71 M/z position.
High Mass ToF-SIMS studies on thin organic films
More recently using high transmission instruments
it has been possible to see the adsorption of the epoxy
molecule from FM73 onto the surface of an Aluminium substrate,
via the silane adhesion promoter (gamma GPS). The peak in
question is m/z 277. The work clearly shows the formation
of a covalent bond with the surface and helps explain the
environmental stability of the AlOx/GPS/FM73 system.
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