Color inhibition of phenolic antioxidants in Ziegler‐Natta polyethylene. II. In‐situ solution studies

Part I of this series of papers showed that color suppression (in some cases superior to that with established phosphites) of polyethylenes (PEs) containing a highly discoloring hindered phenolic antioxidant, Lowinox 22IB46 [2,2′‐isobutylidenebis(4,6‐dimethylphenol)] (L22IB46), and titanium catalyst residues could be achieved by using a range of relatively low cost multifunctional compounds, among which dipentaerythritol (DPE) and triisopropanolamine (TIPA) performed notably well. It was speculated that the latter compounds in some way blocked the formation of highly chromophoric species, which were thought to be titanium phenolates. The studies conducted in Part II further investigate this effect via a solution‐based model‐compound approach, with decahydronaphthalene and titanium and aluminum isopropoxides as models for the PE and Ti‐ and Al‐based catalyst residues, respectively. This approach enabled the use of second‐derivative UV spectroscopy of refluxing solutions for the detection of chromophoric species. The level of antioxidant consumption was determined by using UV spectroscopy, and L22IB46 – (color suppressant) reaction products were characterized by using infrared spectroscopy. Titanium(IV) was found to play a major role in discoloration (via production of titanium phenolates) and the consumption of L22IB46 relative to that of aluminum, which displayed little color formation activity. This observation shows that L22IB46 oxidation products are not necessarily the major cause of discoloration. The DPE and TIPA also performed well in this solution‐based study, though interesting chain‐length‐related (and possibly solubility‐related) effects were apparent with poly(ethylene glycol). J. VINYL ADDIT. TECHNOL., 2009. © 2009 Society of Plastics Engineers