W. Stephen Tait, Ph.D. Chief Science Officer & Principal Consultant, Pair O Docs Professionals, LLC Corrosion Corner What factors determine the length of spray package service lives? Hello, everyone. In the February 2015 edition of Corrosion Corner I provided an overview of the relationship between spray package commercial life and spray package service life. These two lifetimes can be used to determine if corrosion is too severe to market a formula-package system or slow enough that failures will not occur before packages are emptied and recycled. Last month, I expanded the discussion on commercial life, and this month I will expand the discussion on spray package service life. Package service life is the package age after filling and before it leaks product or propellant and/or stops spraying. Package life is determined/governed by three factors: 1. The chemical composition of a formula 2. The type of materials used in a package • Tinplated steel • Tin-free-steel • Aluminum • Laminated polymer films on aluminum foil o Polyethylene terephthalate (PET) films o Polypropylene films o Nylon films • Epoxy coatings • Polyamide imide (PAM) coatings • Polyester coatings 3. The commercial variability of: • A formula’s chemical composition from batch-to-batch • The materials used to fabricate the spray package components o Metal composition (rarely the cause for corrosion) o Weld quality on traditional aerosol containers and laminated foil bags o Thickness of polymer coatings 30 Spray April 2015 Figure 1 illustrates commercial life when it is longer and shorter than package service life. Commercial life is defined as the time when all packages from a given manufacturing batch are recycled, and it is indicated in Figure 1 as the horizontal line with vertical bars at the ends. Slow corrosion is not an issue when the package life is longer than the commercial life as illustrated by the right arrow at the bottom of Figure 1. Thus, corrosion is not an issue when service life is longer than commercial life and commercial variability does not increase corrosion. However, changes in market dynamics could shorten commercial life and individual packages could fail in customer’s facilities or during consumer use when commercial life is too near or shorter than package service life (failure is when a package leaks or it ceases to spray). A product-package system is not commercially viable when corrosion is fast and the package service life is shorter than commercial life (left arrow at the bottom of Figure 1). Package failures are more numerous in customer’s facilities and/or during consumer use in this situation. Corrosion causes spray package failure risk and each company decides what level of risk is acceptable. A no-corrosion criterion for new spray products and line extensions is probably the lowest risk. However, a no-corrosion criterion eliminates products that are commercially viable with the appropriate market dynamics. A wider range of products and line extensions can be commercialized when slow corrosion is acceptable. However, products with slow corrosion require determining if corrosion adversely affects product efficacy/stability, plus: • More corrosion testing to determine which variability factors might increase the corrosion rate • More stringent control of the factors that increase corrosion • More monitoring to ensure marketing dynamics don’t shift the commercial age to near or shorter than package service life Corrosion testing is used to estimate spray package lifetime and to assess the effect of the three variability factors on corrosion rates, and thus package service life. Corrosion tests are either a one-year storage test or a shorter electrochemical test. Temperature typically does not accelerate metal and polymer corrosion. However, higher temperatures could cause spray package corrosion when a formula is not thermally stable. We’ll talk more about temperature-effects in the May edition of Corrosion Corner. We would be happy to teach our Elements of Spray Package (Aerosol Container) Corrosion short course at your R&D facility. Want a specific topic discussed in an issue of Corrosion Corner? Please send your suggestion/questions/comments to rustdr@ pairodocspro.com or visit www.pairodocspro.com. Back articles of Corrosion Corner are available from Spray. Thanks for your interest and I’ll see you in May. SPRAY Figure 1 Two illustrations: package service life exceeds commercial life (1) and package service life is shorter than commercial life (2).
Spray April 2015
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