Vulcanization of polychloroprene rubber. I. A revised cationic mechanism for ZnO crosslinking

Abstract

Abstract Data relating to the vulcanization of mercaptan‐grade polychloroprene (CR) by ZnO and MgO (alone or in combination) are examined. Compounds were vulcanized through the isothermal heating of samples at 140°C in a laboratory press and at programmed rates in a differential scanning calorimeter. The reaction was stopped at various points during the heating process. The crosslink densities were determined via swelling. Extractable ZnCl 2 and MgCl 2 were analyzed by atomic absorption spectrometry. Three different crosslinking processes were identified. The first crosslinking process involved the activation of the highly reactive tertiary allylic 1,2‐units along the polymer chain, whereas the second and third crosslinking processes were attributed to the activation toward crosslinking of 3,4‐ and 1,4‐units, respectively. The crosslinking reactions of the 1,2‐units comprised three distinct steps: isomerization (promoted by ZnO), dechlorination, and crosslinking. ZnCl 2 (which formed during compounding and upon crosslinking) promoted crosslinking, and its addition to formulations decreased but did not eliminate the induction period before crosslinking. MgO retarded the crosslinking process by limiting the formation of ZnCl 2 during mixing. The results of the CR/ZnO system are discussed, and a modified cationic mechanism for crosslinking is proposed. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007