Happy New Year everyone.
Testing spray packages at several different storage temperatures is a common practice. The typical objectives for using different temperatures are to determine product stability and product-package compatibility for the range of temperatures to which your spray products will be subjected during everyday storage and usage.
Higher temperatures are often also used in an attempt to accelerate material corrosion. The origins for this practice are from the Arrhenius law, which states that the rate of a chemical reaction doubles for each 20 degree increase in temperature.
However, the rates of corrosion for spray package materials (i.e., polymers and metals) do not follow the conditions for which the Arrhenius law is valid. These conditions are 1) the chemical reaction is a first order reaction or a pseudo first order reaction, and 2) the kinetics of the chemical reaction is controlled by the energy of activation for the reaction.
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 corrosion 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 temperature 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 accelerate 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 storage stability tests should not be conducted to qualify spray packages. Some of the packaging in your product lines will be exposed to temperatures higher and lower than room temperature (20°C). Higher and lower temperature storage stability tests are necessary to determine:
•product stability at temperatures higher and lower than room temperature
•if product-temperature instability causes corrosion of spray package materials
• if higher temperatures degrade active ingredients
For example, higher and lower temperatures could break an emulsion and produce a package that sprays water or a corrosive free-water phase that corrodes the package. Higher temperatures could also degrade active intermediates and reduce the effective concentration to where the product no longer performs as specified.
How long should spray packages be tested at temperatures higher and lower than room temperature? The graph shows historical temperature data for a 2009 summer day in New Orleans, LA. The data for this graph were obtained from the U.S. National Oceanic & Atmospheric Administration web site.
Notice in the graph below that:
•The time for the peak indoor, non-air conditioned ware house temperature lags the time for the peak for the outside temperature
•The indoor temperatures are lower than the outdoor tem peratures
•Temperatures above 90°F (32°C) last only a few hours per day
A survey I conducted on peak weather temperatures for several cities around the world indicated that peak temperatures for most cities are typically below 105°F (41°C). Higher temperatures were, or course, observed in desert cities.
We can use the graph to estimate approximately how many days per year the outside air temperature is above 90°F (32°C) in Louisiana. First, let’s assume summer lasts approximately 5 months in this city (June through October—150 days).
Next, use the graph to estimate the amount of time temperatures are above 90°F—typically around two hours. We then estimate that spray products are exposed to temperatures above 90°F (32°C) for approximately 13 days each year (two hours per day x 150 days).
Consequently, 39 days of storage testing at 90°F (or slightly above) would simulate three years. A similar estimation could be made for storage temperatures below room temperature.
The majority of your spray package products will be stored and used at room temperature. However, a fraction of the containers produced each year will also be exposed to temperatures above and below room temperature. Thus, we recommend the following guidelines for storage stability testing above and below room temperature:
•Do not use higher temperature to accelerate the corrosion rate for metal and polymer spray package materials
•Use historical temperature-time data to determine maxi mum and minimum temperatures to which your spray pack age products might be exposed
•Use this historical data to estimate how many days per year your spray package products are exposed to temperatures above and below room temperature
•Use your estimates to determine the test length for storage temperatures above and below room temperature
•Use the majority of your test samples for room temperature storage, and a smaller number of test units for storage tempera tures above and below room temperature
Following these guidelines will also increase the statistical confidence in the storage-stability test results at room temperature—most likely the most common temperature where the majority of your spray package products are stored and used by consumers.
Please send your questions/comments/suggestions to
[email protected] Back issues of Corrosion Corner are available on CD from ST&M. Thanks for your interest and I’ll see you in February.