Competition Between Chain Scission and Branching Formation in the Processing of High-Density Polyethylene: Effect of Processing Parameters and of Stabilizers R. Scaffaro, 1 F.P. La Mantia, 1 L. Botta, 1 M. Morreale, 1 N. Tz. Dintcheva, 1 P. Mariani 2 1 Dipartimento di Ingegneria Chimica dei Processi e dei Materiali, Universita ` di Palermo, Viale delle Scienze, Ed. 6, 90128 Palermo, Italy 2 Centro Ricerche Polimeri Europa, Via Taliercio 14, 46100 Mantova (MN), Polimeri Europa S.p.A, Italy Two samples of high-density polyethylene with differ- ent molecular weight were processed in a batch mixer and the rheological and structural properties were investigated. In particular, the effect of different pro- cessing parameters and the eventual presence of differ- ent stabilizers were evaluated. Actually, two reactions may occur during processing: branching/crosslinking or chain scission. The results indicate that when the pro- cessing conditions promote a scarce mobility of the macromolecular chains (lower temperatures, lower mix- ing speed, and higher molecular weight), branching is more favored than chain scission. On increasing the mobility of the chain (higher temperature, higher mixing speed, and lower initial molecular weight), branching appears still the predominant reaction, but the chain scission becomes progressively more important. In both cases, no crosslinking occurs. The presence of stabilizers has different effects on the relative extent of branching or chain scission reactions depending on the kind and of the amount of stabilizer and on the process- ing condition adopted. The correct choice of a stabilizer system will, therefore, depend on the desired final structure. POLYM. ENG. SCI., 49:1316–1325, 2009. ª 2009 So- ciety of Plastics Engineers INTRODUCTION All the polymeric materials display properties that are dependent on the molecular structure and on the interac- tion among the macromolecular chains. These properties can change dramatically when the chemical characteristics of the polymer undergo even modest modifications [1]. In other words, any chemical or physical action able to influ- ence the molecular weight, its distribution, or the chemi- cal structure may lead to alterations, sometimes dramatic and irreversible, of the properties. Typical example of these phenomena is the degradation of a polymeric material due to its melt-processing. In this case, the concurrence of several driving forces, such as oxy- gen concentration, temperature, mechanical stresses, and processing time, may act in an unpredictable way on the materials and, in this case, the knowledge of the final struc- ture achieved by the polymer and, consequently, on its mac- roscopical properties is somehow difficult. In this sense, it is critical to be able to predict the final characteristics of a polymer, especially for industrial applications [2–9]. The mechanism and the chemistry of the thermal and thermo-oxidative degradation of polyethylene have been studied for many years [10–16] and a complete review is out of the scopes of this work. It can be synthesized that, in the conditions adopted for melt-processing, a combination of different mecha- nisms may occur with considerable increase or decrease of the molecular weight and with a major change in the structure, with the formation of branching or crosslinks. In addition, a broadening of the molecular weight distri- bution by only a few amount of long chain branching may have dramatic implications in the processing of the material [1, 17]. High-density polyethylene (HDPE) shows an interest- ing feature. As well known, depending on the conditions adopted, both molecular weight increase and chain scis- sion may occur simultaneously depending on both temper- ature (hence melt viscosity) and unsaturation (vinyl-end groups) [11, 12, 18]. Despite the great academic and industrial interest for polyethylenes, only in the last decade some work appeared to try to provide a direct correlation between different processing operations, structure and properties, for prediction use especially for industrial purposes. Correspondence to: R. Scaffaro; e-mail: scaffaro@dicpm.unipa.it Contract grant sponsor: Polimeri Europa. DOI 10.1002/pen.21317 Published online in Wiley InterScience (www.interscience.wiley.com). V V C 2009 Society of Plastics Engineers POLYMER ENGINEERING AND SCIENCE—-2009