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

Although the level of transition-metal catalyst residues in polyethylene (PE) has been drastically reduced over the years, they can still give rise to discoloration, particularly when associated with other additives such as antioxidants. This first of this series of papers screens a variety of candidate color suppressants featuring a range of functional groups, including alcohols, amine/ sulfur compounds, and acid-containing species. These candidate color suppressants were melt-blended into a Ziegler-Natta linear low-density PE in combination with 2,2 0 -isobutylidenebis(4,6-dimethylphenol) (a highly discoloring hindered bisphenol antioxidant) and zinc stearate antacid. Yellowness index measurements made after multiple extruder passes indicated that dipentaerythritol (DPE) and triisopropylamine (TIPA) gave good color inhibition and, in some cases, outperformed established phosphites. The DPE and TIPA were found (via melt flow rate measurement) not to affect melt stability, and hydroperoxide determination revealed that DPE had no peroxide decomposition activity. The latter results indicate that the color-suppression mechanism of DPE and TIPA is different from that associated with phosphites.