Phase transfer study on the effects of metal ions on storage hardening of natural rubber

Abstract

The storage hardening of natural rubber (NR) has long been investigated; however, its mechanism remains unclear. Most work has focused on proteins and phospholipids, whereas the role of metal ions is poorly understood. This study, therefore, examined the metal-ion effect under real storage conditions. Phase transfer (PT) experiments were used to evaluate their influence. PTNR samples showed that about 2.6 % of Mg²⁺, 63.2 % of Ca²⁺, and 31.1 % of Fe²⁺ were transferred to the organic phase with NR. Except for Fe²⁺, the metal ions added were hardly transferred. PT samples contained 0.02–0.05 % nitrogen, indicating that bonded proteins moved with NR, whereas free ones remained in the aqueous phase. The storage modulus (G′) measurements showed pronounced storage hardening in WNR, with G′ increasing by 25–40 kPa over 2 months. In contrast, PTNR and metal-PTNR exhibited only minimal changes (±10 kPa), indicating stable viscoelastic behavior. The gel content of PT samples (1.5–1.9 %) was lower than that of WNR (6.0–6.2 %), and showed no significant change during storage. These findings showed that the residual bonded proteins and metal ions (Mg²⁺, Ca²⁺, and Fe²⁺) did not contribute significantly to the storage hardening, suggesting that the phenomenon originated from interactions involving other non-rubber constituents rather than intrinsic polyisoprene properties.