How coatings bond to
and disbond from metals
Hello, everyone. Last month I discussed several natural
defects in polymer coatings and films that could lead to
coating corrosion and corrosion of the metal under the
coating. This month, I’m going to discuss the prominent theories
of how coatings bond to metals and disbond (delaminate) from
metals. I’m going to again use “coating” to include both polymer
coatings on metals or laminate films bonded to metals.
Several types of coatings are used for spray packaging:
• Thermoset coatings
• Lacquer coatings
• Laminate films
Thermoset coatings are those cured at high temperatures. These
coatings, such as epoxy and powder coatings, are typically deposited
on a container metal as a liquid then cured at a high temperature.
Lacquer coatings are also applied to container metals as liquids.
However, the polymer in this case is dissolved in a solvent and
heat is applied to remove the solvent from the liquid, leaving the
coating behind on the metal surface (much like painting). Polyacrylamide
(PAM) is an example of a lacquer coating.
Laminate films are fabricated by bonding polymer films to thin
metal foils with adhesives. Typically there is either a film on both
sides of the metal foil or two separate films on one side of the
metal foil and one film on the other side.
There are two prominent theories for how polymers bond to
metals: key bonding or Lewis acid-base bonding.
The key bonding theory envisions polymers covering a roughened
metal surface. The polymer flows into microscopic cracks
and divots on the rough metal surface producing mechanical bonds
that hold the polymer coating on the metal. In other words, the
cracks and divots on the metal surface are microscopic keys that
mechanically bond the polymer to the metal.
A Lewis acid is an electron pair acceptor and a Lewis base is an
electron pair donor. Metal surface atoms have extra valence electrons
and thus can function as a Lewis base. Polymer molecules
often have unsaturated groups that can function as a Lewis acid.
The metal and the coating share the metal’s valence electrons,
creating the polymer-metal bonds. Thus, Lewis acid-base bonds
are chemical bonds.
Breaking polymer-metal bonds leads to different types of coating
delamination. There are three prominent theories for how
coatings disbond (delaminate) from metals:
1. Anodic undermining
2. High pH breakage of the metal-polymer bonds
3. Water hydration of coating-metal bonds, also referred to as
wet adhesion loss
Figure 1 is an example of anodic undermining for an epoxy
coating on tinplated steel. The dark area around the light circle
is epoxy coating that is bonded to the metal and the light circle is
delaminated (disbonded) epoxy.
Figure 1: Anodic undermining
W. Stephen tait, ph.D.
Chief Science Officer & principal Consultant,
pair O Docs professionals, LLC
Anodic undermining occurs when the metal substrate under
the coating corrodes, leaving unattached sections of coating. The
metal in this example is a tin coating on top of steel. Anodic
undermining is common in tinplated steel aerosol containers and
coated tinplated steel aerosol valves when detinning (corrosion)
Coating blisters are hypothesized to be caused by a ring of high
pH liquid at the edges of a blister. Figure 2 is an example of a
coating blister. The arrows indicate where the pH is high at the
edge of the blister.
Figure 2: An example of high pH coating delamination
26 Spray June 2018