Corner The importance of using W. Stephen tait, ph.D. different storage temperatures pair O Docsrincipal Consultant,rofessionals, LLCppChief Science Officerand for spray package stability testing. Happy New Year everyone. in your product lines will be exposed to temperatures higher and Testing spray packages at several different storage temperatures lower than room temperature (20°C). Higher and lower tempera- is a common practice. The typical objectives for using different ture storage stability tests are necessary to determine: temperatures are to determine product stability and product-pack- •product stability at temperatures higher and lower than room age compatibility for the range of temperatures to which your spray temperature products will be subjected during everyday storage and usage. •if product-temperature instability causes corrosion of spray Higher temperatures are often also used in an attempt to accelerate package materials material corrosion. The origins for this practice are from the Ar- • if higher temperatures degrade active ingredients rhenius law, which states that the rate of a chemical reaction doubles for each 20 degree increase in temperature. For example, higher and lower temperatures could break an emulsion However, the rates of corrosion for spray package materials and produce a package that sprays water or a corrosive free-water (i.e., polymers and metals) do not follow the conditions for which phase that corrodes the package. Higher temperatures could also the Arrhenius law is valid. These conditions are 1) the chemical degrade active intermediates and reduce the effective concentration reaction is a first order reaction or a pseudo first order reaction, and to where the product no longer performs as specified. 2) the kinetics of the chemical reaction is controlled by the energy How long should spray packages be tested at temperatures higher of activation for the reaction. and lower than room temperature? The graph shows historical Metallic corrosion is not a chemical process. Corrosion does involve metal atoms changing their chemical state from atoms to ions. However, this change of state is in response to the transfer of electrons from the metal to chemicals in the environment (i.e., your formula ingredients). Hence, metal cor- rosion 1) is not a first order chemical reaction and 2) the rate of corrosion (kinetics) is not controlled by the energy of activation for the metal changing its chemical state. Indeed, I’ve seen numerous instances where higher storage temperatures arrested corrosion that normally occurs at room tem- perature and caused corrosion that did not occur at room temperature. Polymer corrosion is also not a first order chemical reaction. Thus, polymer corrosion also does not follow the Arrhenius law. In other words, raising temperature does not ac- celerate either the rate of metal corrosion nor the rate of polymer corrosion. Please don’t conclude from the discussion so far that I’m saying higher temperature stor- age stability tests should not be conducted to qualify spray packages. Some of the packaging Temperature-Time profile for a 2009 summer day in New Orleans, LA 36 Spray January 2013
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