Color inhibition of phenolic antioxidants in Ziegler‐Natta polyethylene. I. In‐situ polymer studies

Abstract

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.

Effects of polymerization catalyst technology on the melt processing stability of polyethylenes. part 2. single stabilizer performance

“…Phosphites are known to reduce color development in polyolefins whereas hindered phenols cause discoloration. Such effects have been mainly attributed to formation of colored transformation products, including quinoid/stilbene and carbonyl species, and also to colored complexes formed via chelation of metallic catalyst residues with phenolic AOs and associated transformation products [18]. The formulation showing the most severe yellowing was that containing V‐4010 which itself is actually brown in color to start with, though the good melt stabilization activity also suggests formation of colored transformation products.…”

Perspectives on additives for polymers. 1. Aspects of stabilization

“…[ 50,51 ] In situ studies in Zeigler‐Natta polyethylene confirm the findings that both the use of dipentaerythritol (DPE) and triisopropylamine (TIPA) gave excellent color inhibition when used with highly color forming phenolic antioxidants. [ 51,52 ] When the study was extended to the solution phase, [ 53 ] it was confirmed that the actual color was due to the formation of titanium‐based phenolates and that the DPE and TIPA operated through competitive complexation with the phenol for the titanium ions.…”

Discoloration Effects of High-Dose γ-Irradiation and Long-Term Thermal Aging of (U)HMW-PE

“…However, it is possible that oxidation is concentrated in the black spots that were formed in the material (see Figure 8) and that could not be examined by color, UV-Vis, and FTIR measurements. The black spots are probably formed when catalyst residues from polymer synthesis, like titanium chloride (e.g., TiCl 3 ) and chromium (Cr) particles, react with the phenolic antioxidant to produce chromophoric species, such as titanium phenolates [34][35][36][37]. As these catalyst residues promote hydroperoxide decomposition [38], oxidation preferentially starts at the polymercatalyst interface.…”