Hello, Everyone. We have a paradox—both docs from Pair O Docs Professionals LLC are authoring the June and July editions of Corrosion Corner. Those of you who have worked with Pair O Docs know Sue from our Elements of Spray Package Corrosion short course, as well as her work in the Pair O Docs laboratory operations and on corrosion test design.
This issue, we begin a two-part discussion on the corrosion of refillable multiple-use (RMu) packaging.
The RMu package platform has been much in the news and appears to be a promising new element for “Green” packaging. Indeed, some of the articles we’ve seen indicate that RMu packages might replace traditional, disposable single-use packages for a variety of spray products and pump products (see SPRAY, March 2019, p.35).
Based on what we’ve read, it appears that the main types of materials being considered for RMu packaging are coated aluminum, glass and stainless steel. All of these materials will eventually fail and the failure modes could be either structural or cosmetic. The possible structural failure modes are:
• Fracturing (cracking and breaking)—glass and internal polymer coatings
• Aging/Memory—glass and internal polymer coatings
• Corrosion—metals and coated metals
The possible cosmetic failure modes are:
• Etching (glass)
• Fracturing (glass and polymer coatings)
• Denting (metals) and scratching (all materials)
Scratching does not lead to metal failure, but it could subsequently lead to glass fracturing or a breach in a polymer coating that allows the environment (e.g., your formula) to contact the metal under the coating.
Etching is glass corrosion brought about by chemical attack. It causes the glass to have a frosted look.
Glass, polymer coatings and polymer films have amorphous molecular structures instead of the orderly, metal, repeating geometrical structure. Amorphous materials typically become more brittle with age and thus become more susceptible to cracking, chipping and fracture. Consequently, aging ultimately leads to glass and polymer failures.
We have all experienced the manifestations of long-term material memory. Remember that favorite glass or ceramic mug that you used over and over? You dinged it here and there, plus probably knocked it over a few times. Then one day when you were using it, you dinged it just a little and the mug broke. Glass and ceramics are brittle and micro-cracks formed each time you bumped your favorite mug, dropped it and knocked it over. The micro-cracks accumulated and grew until finally the last bump was enough to cause the mug to fail.
Internal polymer coatings in aluminum packages also develop memories each time they are subjected to different chemicals, such as the detergents used for cleaning with water in combination with high temperatures. Polymer memory could lead to increasing degradation of a coating’s or film’s barrier properties, fracture of the polymer coating/film and delamination of the coating or film from the metal, such as blistering.
Metal denting is cosmetic and does not lead to metal package failure. A lot of force is typically needed to dent metals, so they are essentially dent-resistant. This statement gains credence from the fact that around 300-plus billion aerosol, food and beverage containers are used globally each year with a minimum number of dented packages. Resistance to cosmetic metal denting can be increased by using harder metals such as steel and stainless steel alloys and using thicker metals for the RMu packages.
All three of the RMu packaging materials (coated aluminum, glass and stainless steel) will corrode. The issue with corrosion is whether the corrosion rate is low enough so that the packages will have the desired service life and if the concentration of metal ions from corrosion is low enough to not adversely affect product performance and efficacy. We’ll continue the discussion of RMu packaging in the July issue.
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Thanks for reading and we will see you in July. SPRAY