Comparative evaluation of the efficiency of a series of commercial antioxidants studied by kinetic modeling in a liquid phase and during the melt processing of different polyethylenes

An antioxidant response in condensed polymeric environments is often ambiguous and may vary strongly depending on the nature of the polymer and the conditions of polymer storage, processing, and use. The impact of polymeric environments during melt processing on the intrinsic efficiency of a set of commercial antioxidants was studied. The antioxidative activity of primary antioxidants Lowinox CPL, Lowinox 22IB46, Naugard 445, hydroxylamine Genox EP, and secondary phosphite Weston TNPP were determined by using two versions of the model reaction of cumene initiated (2,2-azobisisobutyronitrile, AIBN, and cumyl hydroperoxide, ROOH) oxidation. The melt stabilizing efficiency of the antioxidants was also studied during multipass extrusion testing in HDPE (Phillipstype), metallocene LLDPE, and (Ziegler-Natta) LLDPE. The kinetic measurements showed that each of the three functional hydroxyl groups of Lowinox CPL is consumed in the model reaction (version 1) with the same high inhibition rate constant (k7), whereas the two functional groups of Lowinox 22IB46 have different activity stipulated by hydrogen bonding between the hydroxyls. All the primary stabilizers involved afforded transformation products with additional antioxidative activity. For phenolic Lowinox CPL and amine Naugard 445, these products exhibited lower inhibition rate constants than that of the main functionality, but for Lowinox 22IB46, the discrepancy was not observed. Genox EP revealed three inhibition centers with different rate constants which, however, have low values of the inhibition coefficients (f). This effect is presumably due to the versatility of the inhibition pathways for the antioxidant and its intermediates, including the path of active interception of cumylalkyl (R) radicals. The secondary stabilizer Weston TNPP, tested by means of the second version of the model system, along with the expected decomposition of hydroperoxide appeared to be an effective radical scavenger. Kinetic parameters of the antioxidizing activity of the stabilizers - inhibition rate constants, coefficients of the oxidation chain termination, and total antioxidative activity {A = [k7(i) (fn[InH])(i)]} - were determined for each functional group and for the whole antioxidant molecule. The phenolic stabilizers manifested powerful antioxidative activity. Their strongest functional groups have very high inhibition rate constant values: (log k7) = 5.4 ± 0.15 (Lowinox 22IB46) and 5.2 ± 0.1 M-1s-1 (Lowinox CPL). In terms of the total inhibiting activity in the liquid system the antioxidants can be ordered as: Lowinox CPL > Lowinox 22IB46 > Naugard 445 > Genox EP > Weston TNPP. The effect of stabilizers during multipass extrusion experiments was assessed via melt flow rate and yellowness index measurements conducted as a function of the number of passes. Phenolic antioxidants and Genox EP significantly improved the melt stability of the polyethylenes in terms of melt viscosity retention and in partial compliance with the data from kinetic modeling measurements. According to the melt stabilizing efficiency data, the antioxidants can be arranged as: Lowinox 22IB46 > Lowinox CPL > Genox EP > Naugard 445 > Weston TNPP. The Lowinox 22IB46 with relatively lower molecular weight exhibited the best results among the primary stabilizers because of the unrestricted molecular dynamics in the viscous-flow state of the polymer. Yellowness index measurements made after multiple extruder passes indicated that Weston TNPP effectively decreased the color development caused by the phenolic antioxidants. Genox EP displayed high efficiency as an antioxidant and melt-processing stabilizer and additionally provided good color protection. Generally, we received a good correlation between the activity of the antioxidants in the model system and their melt stabilization performance in HDPE, metallocene LLDPE, and LLDPE. The model reaction of cumene-initiated oxidation has demonstrated excellent applicability as an effective tool for preliminary quantitative assessment of antioxidant radical-scavenging efficiency