Thermally induced structural changes of resorcinol formaldehyde latex adhesive used in cord-rubber composites

In this study, the accelerated thermal exposure of resorcinol formaldehyde latex (RFL) adhesive used in cord rubber composites was investigated. In particular, the structural changes of the RFL with and without curatives (sulfur, zinc oxide, accelerator and stearic acid) during thermal treatment at 100 °C were identified by a multiscale approach using dynamic mechanical analysis (DMA), atomic force microscopy (AFM), and solid-state nuclear magnetic resonance (ssNMR). The RFL samples with curatives show an approximately four-fold increase in the overall elastic modulus at room temperature during thermal treatment of up to 10 days. At the same time, DMA results indicated an increase of Tg of the latex phase from −39 °C to −3 °C. However, for the RFL samples without curatives, there was no change in the Tg of the latex phase. At the nanoscale, AFM mappings on samples with curatives have also confirmed an increase in the nanoscale modulus in the latex phase (50 ± 10 MPa to 140 ± 20 MPa) and in the RF resin phase (330 ± 100 MPa to 620 ± 200 MPa) for the same thermal treatment duration. Finally, solid-state NMR analysis showed a decrease in the intensity of the vinyl groups (noted at 115 ppm and 145 ppm) in the latex phase indicating the possibility of further sulfur cross-linking during the 10 days of thermal treatment for samples containing curatives. Our studies have highlighted the prominent role of the curatives in the RFL adhesive, as well as its initial structure and evolution during thermal treatment. Accordingly, the RFL structure was drastically modified by the accelerated thermal exposure.