W. Stephen tait, ph.D. Chief Science Officer & principal Consultant, pair O Docs professionals, LLC Corrosion Corner Common mistakes associated with corrosion stability tests on spray packaging 40 Spray March 2016 test time increases the risk of not discovering that a product is incompatible with the chosen spray package. Consequently, we recommend conducting storage tests for at least one year prior to making a decision to commercialize new or derivative products. Understandably, reducing risk might conflict with faster to market needs. Faster results with lower risk can be obtained through electrochemical corrosion testing as illustrated by the red box and green triangle in the lower left corner of Figure 1. 2Make package corrosion go faster Another common mistake is to try and make the spray package corrosion go faster by increasing the storage temperature. This concept is based on the Arrhenius law that states the rate of a chemical reaction doubles for each 20º increase in temperature (in degrees Kelvin). The Arrhenius law is valid only when the reaction has first order kinetics and the rate is controlled by the reaction’s activation energy. Metal corrosion does not follow the Arrhenius law because a) corrosion is a hybrid chemical reaction involving electron transfer between the container metal and your formula ingredients; b) corrosion does not have first order reaction kinetics (or pseudo first order) and; c) corrosion rates are not controlled by the activation energy. The Arrhenius law is also not valid for polymers. In addition, increasing temperature could also significantly change corrosion results when: • Higher storage temperatures are at or above the temperature for the surfactant cloud point, causing product instability and package corrosion • The wet polymer glass transition temperature is lower than the storage test temperature, causing the polymer to lose its corrosion protection property • Thermally induced ingredient degradation at higher test temperatures leads to package polymer and metal corrosion 3 Applying an electrical voltage to a spray package to make corrosion go faster At one time, there was an attempt to make aerosol container corrosion go faster by applying an electrical voltage to the container. The theory was that corrosion could be accelerated by an electrical voltage because corrosion is an electrochemical reaction. However, corrosion currents do not follow Ohms law: voltage = (resistance) (current). Applied voltages Hello, everyone. The storage stability test is one of the oldest types of tests for spray packaging corrosion and product stability. As most know, the test consists of storing spray packages at room temperature and one or more higher temperatures. Samples are periodically removed from each temperature room for destructive examinations. There are six common mistakes that are made with storage tests with respect to corrosion: 1Insufficient test length In our age of “faster-to-market” there are significant pressures to compress development timetables for new and derivative products. One method of compression is shorter storage stability tests. Figure 1 is a graph of how pitting corrosion risk decreases with increasing test length. The Y-axis is the percent of approximately 7,500 containers that had pitting corrosion and the X-axis is the storage test length. Figure 1 also contains the risks associated with electrochemical corrosion testing. The graph and data in Figure 1 are aggregate probabilities from decades of storage and electrochemical test data collected on a wide variety of consumer and pharmaceutical products with steel or aluminum aerosol containers. We have not developed similar data for spray packages that use laminated foil bags welded to aerosol valves. Notice in Figure 1 that the initial risk for pitting corrosion is on the order of 60% and the risk decreases to around 7% after one year of storage testing. In other words, Figure 1 illustrates that reducing the test time increases the risk. Thus, shortening lifetime Figure 1: Corrosion risk as a function of storage test length
Spray March 2016
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