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This paper investigated the formation of crosslinks in natural rubber compounds in the vulcanization systems: conventional (CV), semi-efficient (SEV), and efficient (EV), processed with three types of accelerators: MBTS (dibenzothiazole disulfide), TMTD (tetramethylthiuram disulfide) and CBS (n-cyclohexyl 2-benzothiazole sulfenamide). The cross-linked densities were determined by organic solvent swelling, dynamic mechanical analysis (DMA), stress vs strain, and low-field nuclear magnetic resonance, the latter being the reference technique for comparison with the other results. It was found that the choice of accelerator type influences the processing time and the cross-linked density of the vulcanizate. The four techniques showed close values of cross-linked density for natural rubber compounds, demonstrating that the analytical techniques studied can be applied to determine crosslinked density.

The tanning industry generates millions of tons of waste every year, which can cause environmental damage as well as problems to human health. In this article, we analyzed the use of leather shavings (20 to 80 parts per hundred rubber) as fillers in Natural Rubber (NR) composites. For this purpose, the interfacial interaction of the filler was quantitatively evaluated based on the cross-link density value calculated by the Flory-Rehner methodology using the Lorenz-Park equation (swelling). Then, the results of this evaluation were compared with those obtained by the Mooney-Rivlin method (by a stressstrain test). According to our findings, the tensile strength at rupture of the composites increased by 100%, that is, from 7.0 MPa (NR) to 14.0 MPa, when 80 phr of leather waste were incorporated. In addition, their final deformation decreased around 50% compared to pure gum (no filler). According to the Lorentz-Park equation, values over 0.7 indicate a strong interaction between the leather shavings and the NR matrix. Likewise, a dynamic mechanical analysis confirmed a strong interfacial interaction based on the B parameter. Scanning electron microscopy and Fourier transform infrared spectroscopy were employed to examine the morphology and chemical properties of the composites.

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