Effect of carbon black on the properties of concentrated and skim natural rubber blends for potential tire applications

Designing raw natural rubber (NR) that meets tire-grade specifications has long been a central topic in NR processing. In this study, concentrated NR (CNR) latex and skim NR (SNR) latex were obtained by centrifuging fresh NR (FNR) latex. Raw rubber samples were then prepared by blending CNR and SNR latex at various ratios and compounded with carbon black (CB) to evaluate their potential for tire applications. The results showed that the samples with a higher SNR portion exhibited higher cross-linking density, which was attributed to the higher protein content in SNR. CNR/CB displayed superior tensile strength and lower compression heat buildup compared to the control sample (FNR/CB). However, with increasing SNR portion, mechanical properties deteriorated progressively. Dynamic mechanical analysis revealed that SNR/CB had a higher storage modulus but also a higher tan δ, indicating poor elastic recovery due to poor CB dispersion. This was attributed to the interaction between protein in SNR and CB, which promoted CB agglomeration. Furthermore, 1 H DQ NMR analysis confirmed that the incorporation of SNR led to uneven cross-linking and the formation of network defects. The SNR protein might act as a filler, exceeding the dispersion threshold with CB and degrading performance. Reducing CB loading improves dispersion, but performance remains inferior to that of FNR/CB, indicating that the protein merely interacts with CB without a significant filler effect. Therefore, it is recommended that raw NR such as CNR, or CNR/SNR blend with lower SNR ratio, is more suitable for tire production.