Two‐step degradation of high‐density polyethylene during multiple extrusion

Abstract

Ensuing our recent studies on polyethylene stabilization, 1,2 further multiple extrusion experiments were carried out with a high-density polyethylene (HDPE) polymer containing various amounts of a sterically hindered phenolic antioxidant. Discoloration, thermoxidative stability, and melt flow index (MFI) retention were measured by standard techniques; the functional group content of the polymer was determined by Fourier transform infrared (FTIR) spectroscopy, and rheological, as well as mechanical properties, were also measured. The results indicated that degradation and stabilization reactions take place according to two different mechanisms in the first and subsequent processing steps, respectively. Color development could be described well by the simple first-order overall reaction kinetics proposed earlier, and color change could be related to the stability of the polymer. The existence of general correlations among the properties proved that all chemical reactions are interrelated. The reactions of the stabilizer lead to color development, while those of the polymer to a modification of its molecular architecture, which determines the rheological and mechanical characteristics of the product.

Processability and mechanical properties of extensively recycled high density polyethylene

“…increasing of MFI with number of reprocessing cycles [6,26], decreasing [20,23,27,28], or they did not observe significant change [21,24,28]. Many authors concluded that intensity of MFI change can be related to the content of stabilizers [6,23,28].…”

“…Many authors concluded that intensity of MFI change can be related to the content of stabilizers [6,23,28].…”

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

“…Color development in polyolefin formulations is widely thought to be due to transformation products of hindered phenolic AO that include highly chromophoric quinoidal structures. Thus, consequently we must assume that discoloration is proportional to the amount of stabilizer which reacts during processing of the polymer [10, 11]. Stilbenequinone species are reported to be the major contributor to phenolic AO derived color formation in polymers [6, 12, 13].…”

Effects of type of polymerization catalyst system on the degradation of polyethylenes in the melt state. Part 1: Unstabilized polyethylenes (including metallocene types)

“…degradation occurs most rapidly during melt processing, when the PE is typically at temperatures between 180°C and 280°C and also subjected to high mechanical shear stresses. In closed conditions such as in an extruder barrel, there is sufficient oxygen dissolved in the polymer to cause thermo‐oxidative degradation [1]. Oxygen availability in the system and its concentration also has an important role in the thermo‐mechanical degradation [2–5].…”

“…Different polymerization routes give rise to differences in type and relative concentration of vinyl, trans ‐vinylene and vinylidene unsaturated species. These variations interfere with the primary degradation mechanism, which involves chain scission and chain branching followed by crosslinking [6–8]. Both types of reactions (chainscission and cross‐linking (leading to decreased melt viscosity and increased melt viscosity, respectively [9])) occur simultaneously but one type may predominate depending on factors including molar mass, shape of molar mass distribution, polymerisation catalyst system (and associated residues) and the level of branching.…”