The role of water in making rubber

Tyre company researchers explore fundamental question about vulcanization

Rubber is everywhere. Tyres, shoe soles, electrical insulation, sports balls, surgical gloves, hoses, belts…. more than 40,000 everyday products use it as an ingredient.  Natural rubber – a milky liquid produced by a variety of plants – was discovered and used by the peoples of Mesoamerica millennia ago. In the mid-1800s, a chemical process called vulcanization transformed it into the robust, thermoset material we’re familiar with. By 2020, the world was producing 27.4 million tonnes of natural and synthetic rubber each year.

Despite this, there are still some open questions around vulcanization; particularly, the role that water plays in it. Researchers from tyre manufacturer, The Yokohama Rubber Co., set out to try and explore this, and they’ve reported their results in the latest issue of Polymer Testing [DOI: 10.1016/j.polymertesting.2023.108030].

They started by preparing samples from deproteinized natural rubber (DPNR). They chose this material because the proteins typically present in rubber affect the vulcanization process and exhibit strong affinity for water, making it difficult to separate out their respective influences. To investigate the influence of water, they made DPNR samples with varying moisture content, using three different conditions (all at 25°C). ‘DRY’ involved drying DPNR under reduced pressure for 4 days; ‘RH100’ was DPNR stored at 100% relative humidity for 3 weeks; and ‘WET’ which involved immersing the DPNR in water for 3 days. The resulting water content of the samples was 0.019, 0.82, and 4.2 w/w%, respectively.

The DRY, RH100, and WET samples were each masticated and mixed with stearic acid and ZnO. They were then milled with sulphur and TBBS (N-tert-butylbenzothiazole-2-sulfenamide) to form a rubber compound. Vulcanization was performed at 150°C using a hydraulic press. Cure timing ranged from 1 to 20 mins in 1 min intervals, and from 20 to 30 min in 5 min intervals. The samples were then immersed in ice water to terminate the vulcanization process, surface dried, and stored in a freezer, to prepare them for testing.

The focus of the paper was the low-molecular weight products that result from ionic reactions between accelerators, activators, and sulphur during vulcanization. By analysing the concentration of these products over time in the three samples, the researchers could elucidate the effect of water in the process. They were specifically interested in quantifying the time-dependent concentrations of MBT (2-mercaptobenzothiazole), ZnMBT (zinc-MBT), zinc complexes, sulphur linked to rubber, and MBTS (2,2′-dibenzothiazole disulfide).

MBT and ZNMBT content was analysed using UV–visible spectroscopy and Zn²⁺ was analysed by inductively coupled plasma atomic emission spectroscopy. The amount of sulphur linked to the rubber was determined using combustion analysis, and the MBTS and TBBS were quantified by high-performance liquid chromatography.

Cure curves, which plot torque vs cure time, showed that the presence of water shortens the induction period of the vulcanization (sometimes called the scorching time, before crosslinking begins) and increases the crosslink density of the resultant vulcanized natural rubber. Increasing water content was also seen to shorten the onset time of the abrupt increase in the torque, and the inflection time. Analysis of the products showed that increasing water content also activated the formation of MBTS, but supressed formation of MBT and ZnMBT. The onset of the increase in the amount of sulphur linked to rubber (during induction) was delayed as the water content increased.

The authors say that this reflects “…the suppression of the formation of crosslink precursors and crosslinking junctions,” and suggest that “the water promotes the formation of the C-C linkages, and then it sequentially promotes the formation of the C-S linkages.” They conclude, “the water present in natural rubber strongly affects the production of the low-molecular weight compounds in the accelerated sulfur vulcanization of natural rubber.”

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Yayoi Akahori and Seiichi Kawahara. “Effect of water on the accelerated sulfur vulcanization of natural rubber,” Polymer Testing 123 (2023) 108030. DOI: 10.1016/j.polymertesting.2023.108030