Core-Crosslinked Star Copolymers as Viscosity Index Improvers for Lubricants
Katia Sparnacci, Tommaso Frison, Edoardo Podda, Diego Antonioli, Michele Laus, Marcello Notari, Giulio Assanelli, Marco Atzeni, Gianfranco Merlini, Riccardo Pó
Abstract
Core cross-linked star (CCS) copolymers were prepared by reversible addition–fragmentation chain-transfer polymerization according to the arm-first technique and studied as viscosity index improvers for the application in hydraulic lubricants. A one-pot two-step synthetic procedure was optimized: starting from PMA arms with a precisely controlled structure and three different molecular weights and using different amounts of either divinylbenzene or ethylene glycol dimethacrylate as cross-linking agents, CCS copolymers with a high star yield (63 to 88%) were obtained. CCS copolymers present weight average molecular weight values ranging from 130,000 to 420,000 g mol–1 and a number of arms comprised between 4 and 13. All samples were employed, without any purification step, in the preparation of model lubricant formulations for hydraulic fluid applications. A good compromise between viscosity index improvement and capability to resist chain breaking reactions was obtained for CCS copolymers with target molecular weight for the PMA arms of 20,000 g mol–1. Improved performances with respect to a commercial additive were obtained.