W. Stephen Tait, Ph.D.
Chief Science Officer & Principal Consultant,
Pair O Docs Professionals, LLC
Happy new year, everyone. In the September 2017 edition
of Corrosion Corner, I began a five-part series on the
corrosive behavior of the various metals and coated
metals used in fabricating spray packages and valves. This month
completes the series with a discussion on the structures and
corrosive behavior of laminated metal and laminated foil spray
package materials.
What are laminated metals and laminated foils?
Laminated metals and laminated metal foil structures are
fabricated by bonding independent polymer films to sheet metal
and metal foil. Adhesives and heat are typically used to bond the
different layers together.
Laminated metal/foil structures could be one polymer
film bonded on either one or both sides of the sheet metal or
bonded onto the metal foil. They could additionally be one film
material bonded to the metal/metal foil with a second different
film material on top of the first film. A single polymer film is
typically bonded to the opposite side of metal foils with a double
film layer on the other side.
Valves, metal container tops, bodies and bottoms typically
have single layer films bonded to one side of the metal. In some
instances, the other side of these components has a polymer
coating.
There are three types of spray packaging components that
might be fabricated from laminated metals:
1. Spray package valves
2. Aerosol container tops, bodies and bottoms
3. Bags, pouches and tubes
Aerosol valves are the oldest spray package component
fabricated from laminated metals, followed by laminated aerosol
container tops, bodies and bottoms.
22 Spray January 2018
Laminated valves, aerosol container tops,
bodies and bottoms
Figure 1 has a photograph of a laminated aerosol valve cross
section. The laminate structure in Figure 1 is similar to those
of container tops, bodies and bottoms, while the differences are
typically in polymer film thickness. Consequently, I’ll combine
the discussion on valves with container components.
Figure 1 is not a photomicrograph, so the adhesive layer is too
thin to be seen. Notice that the laminate film is slightly thicker
than the valve metal. Polypropylene was the original laminate
film material, but today there is a greater variety in laminate film
materials, including polypropylene, PET and nylon.
We’ve observed several different cases of laminated spray package
corrosion:
• Severe film delamination from the substrate metal/metal foil
• Film delamination with metal corrosion underneath
• Valve metal perforation under a delaminated film with no
evidence of damage to the film, such as holes and cracks that
expose the metal to a formula
In this last case, the aerosol container used the specified pressure
and sprayed normally. In other words, the laminate film
maintained container pressure and allowed the container to
spray as specified despite a perforated valve cup.
Laminated metal foils
As mentioned previously, laminated foils for spray packaging are
newer than the laminated spray valves and container components.
Bags fabricated from laminated foils are typically used
inside traditional aerosol containers.
Figure 2 provides a diagram for one type of laminated metal
foil structure possessing two polymer films on the product side
of a bag and one polymer film on the side not exposed to the
product. The adhesives between layers are not shown
in Figure 2. Both films for each layer could be different
types of polymers as indicated by the different
colors in Figure 2.
Laminated metal foils are not immune to polymer
film and metal foil corrosion, with delamination being
the most common form of laminated foil corrosion.
However, general and pitting foil corrosion can also
occur when a film delaminates from its substrate foil.
We’ve also observed cases of film delamination in the
welds between laminate sheets. In these cases, weld
failure caused bag leakage.
As with all spray package materials, the corrosion
resistance for both laminated metals and laminated
foils is determined by the chemical composition of the
formula. This corresponds with how formula chemistry
determines the corrosion behavior the other spray
package metals, coated metals and polymer coatings.
Consequently, corrosion testing is also needed to
determine if a given formula will cause laminated
valve, laminated container components and laminated
Corrosion Corner
Spray Package Metallurgy and Corrosion—
Part V: Laminated Metals and Foils
Figure 1: Laminated spray valve cross section