Corrosion Corner
Polymer coatings and
laminated films as
corrosion barriers—Part I
Hello, everyone. Synthetic polymers, such as epoxies,
Micoflex, polyethylene terephthalate (PET) and laminate
films are used for spray package coatings as barriers between
a formula and the underlying metal or metal foil. Polymers
have voids and channels in them that allow formula ingredients
to permeate the coating and laminate film, thereby changing
them from a dry polymer to a wet polymer.
I’m going to start a two-part discussion on the permeation
of coatings and laminate films by formula ingredients and how
permeation could lead to both polymer and metal corrosion.
Part 1 this month will discuss chemical modification of coatings
and films by permeation and Part 2 next month will discuss how
temperature interacts with the chemical modifications to reduce
a coating’s and laminate film’s ability to be a barrier between the
underlying metal and a formula.
For the sake of simplicity, I will refer to coatings and laminate
films as polymers, new unexposed coatings and laminate films as
dry-polymers and polymer coatings and laminate films exposed to
formula liquids and gases as wet-polymers.
It sometimes comes as a surprise that polymers are corroded
by formula ingredients. Human skin is a natural polymer and
provides a good analogy for the corrosion of synthetic polymers
used for spray package coatings and laminate films.
W. Stephen tait, ph.D.
Chief Science Officer & principal Consultant,
pair O Docs professionals, LLC
rustdr@pairodocspro.com
Many consumer/packaged goods formula ingredients—such
as water, emollients, surfactants and fragrances—absorb into and
modify skin. Skin properties could also be modified by formula
properties such as pH. For example, certain emollients soften
skin, but a pH of two or 14 might cause a burning sensation on
skin.
Consequently, by analogy it is reasonable to conclude that the
properties of synthetic polymer coatings and films might also
be modified by formula ingredients, depending on the type of
polymer, chemical composition of the formula and its physical
properties. Modification of polymers could lead to:
• The polymer becoming a semi-permeable membrane,
• The loss of polymer barrier properties and/or
• The lowering of the polymer glass transition temperature (Tg).
Polymer absorption by formula ingredients could cause the
polymer to become a semi-permeable membrane on top of
the underlying metal instead of a barrier layer. The chemical
composition of the liquid-permeate would be significantly different
from the chemical composition of a liquid formula and the
liquid-permeate might cause corrosion of the underlying metal
and delamination of the polymer from the metal. The chemical
composition plus the pH of the permeate-liquid
determines which of these two forms or corrosion
will occur or if both forms of corrosion will
occur together.
Absorption of formula ingredients could also
cause the polymer to partially or completely lose
its barrier properties. Loss of barrier properties
often leads to coating and laminate film
delamination from the package substrate metal
and/or corrosion of the substrate metal under
the delaminated areas.
Polymer delamination could be either localized,
such as blisters, or widespread. Corrosion
of the substrate metal could also either be localized
under blisters or under widespread areas
of delaminated polymer. Widespread corrosion
often also accelerates the polymer delamination.
Absorption of formula ingredients also lowers
the glass transition temperature (Tg) of the
polymer. We will continue this discussion next
month on how the act of lowering the Tg affects
polymer performance as a barrier between your
formula and the metal under the coating.
Thanks for reading Corrosion Corner and I’ll
see you in September for Part 2. Spray
Human skin is a natural polymer and provides a good analogy for the corrosion of synthetic
polymers used for spray package coatings and laminate films. Skin properties can be modified
by formula properties such as pH. While certain emollients soften skin, a pH of two or 14
might cause a burning sensation on skin.
August 2018 Spray 25