The Effects of Chlorine on Very Low Density Thermoplastic Olefins

In 2007 we became aware of a number of failures of Thermoplastic Olefin Materials (TPOs) that led us to re-evaluate our use of these polymers. We were witnessing the failure of a series of floating covers in both North America and Australia where we had been asked to provide repair and replacement of floating covers and liners. Although the research at that time was concentrating on UV light resistance as a key failure factor; examples of liners failing in the absence of light led us to look at chlorine resistance as an alternative factor. A key success in our research was the development of a simple test that clearly replicated the failures that we had seen in service. This new test subjected the liner material to a strong chlorine solution at elevated temperature while under stress. This chlorine stress cracking test mirrored the observed steps in the field degradation of TPO materials from loss of stabilizers; to surface cracking; to full failure in a test period of less than 1000 hours. In our evaluation of failed materials we discovered that antioxidant stabilizers were not retaining their effectiveness in the presence of chlorine. Once the stabilizers lost their effectiveness the specimens progressed through predictable degradation stages until cracking occurred. Once we had developed a background of what was causing the problems in these TPOs we started work on the development of an improved formulation. Through the use of our chlorine stress cracking test we were able to clearly evaluate individual ingredients. This gave us the opportunity to determine which ingredients were causing problems; and, more importantly, gave us a method to optimize the formulation of a replacement material. This paper outlines the development of a chlorine stress cracking test, the results of testing from this test, and the development of an alternative chlorine-resistant TPO formulation as a result of the guidelines provided by this test.