Abstract
Zinc oxide nanoparticles (N-ZnO) and zinc complexes with 1,3-diketones of different structures were applied instead of microsized zinc oxide (M-ZnO) to activate the sulfur vulcanization of styrene-butadiene rubber (SBR). The influence of vulcanization activators on the cure characteristics of rubber compounds, as well as crosslink density and functional properties of SBR vulcanizates, such as tensile properties, hardness, damping behavior, thermal stability and resistance to thermo-oxidative aging was explored. Applying N-ZnO allowed to reduce the content of zinc by 40% compared to M-ZnO without detrimental influence on the cure characteristic and performance of SBR composites. The activity of zinc complexes in vulcanization seems to strongly depend on their structure, i.e., availability of zinc to react with curatives. The lower the steric hindrance of the substituents and thus the better the availability of zinc ions, the greater was the activity of the zinc complex and consequently the higher the crosslink density of the vulcanizates. Zinc complexes had no detrimental effect on the time and temperature of SBR vulcanization. Despite lower crosslink density, most vulcanizates with zinc complexes demonstrated similar or improved functional properties in comparison with SBR containing M-ZnO. Most importantly, zinc complexes allowed the content of zinc in SBR compounds to be reduced by approximately 90% compared to M-ZnO.
Highlights
Sulfur vulcanization is the oldest and one of the most widely exploited method for the crosslinking of rubbers having unsaturated double bonds in their macromolecules, such as natural rubber (NR) and synthetic rubbers, e.g., styrene-butadiene rubber (SBR) [1]
HeBPP-Zn allowed to obtain significantly higher ∆S and crosslink density of the vulcanizate compared to other zinc complexes, ∆S of SBR compound with HeBPP-Zn was approximately 3 dNm lower compared to rubber compounds with zinc oxides
SBR compounds with zinc complexes containing methoxy groups (MBPP-Zn, tBuMBPP-Zn and MMBPP-Zn) demonstrated approximately 6 dNm less ∆S than rubber compounds with microsized zinc oxide (M-zinc oxide (ZnO)) due to the lower crosslinking degree of the elastomer
Summary
Sulfur vulcanization is the oldest and one of the most widely exploited method for the crosslinking of rubbers having unsaturated double bonds in their macromolecules, such as natural rubber (NR) and synthetic rubbers, e.g., styrene-butadiene rubber (SBR) [1]. The use of crown ethers [14], alkylammonium salts or ionic liquids [21] was reported to effectively improve the dispersion of N-ZnO in an elastomer matrix and, its activity in the vulcanization process These additives affect the final properties of the vulcanizates, e.g., they deteriorate the thermal stability [22,23]. The overall zinc content in this activator was 25.6% Regarding vulcanization kinetics, this activator was a good alternative to ZnO/stearic acid system, which allowed to reduce the level of zinc by approximately 75% without detrimental influence on the crosslink density of the vulcanizates and their dynamic mechanical properties. The organo-modified LDH allowed to replace the ZnO and stearic acid in rubber compounds leading to significant reduction of zinc level in the final elastomer composites, without deterioration of their crosslinking degree and mechanical properties. The onset curing temperature was determined following the ISO 11357-1 [36] standard using the STARe software
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