STUDY OF RESORCINOL FORMALDEHYDE LATEX ADHESIVE IN FLEXIBLE RUBBER COMPOSITES: Multiscale characterization of initial structure and its evolution upon thermal treatment

Polymeric cord-rubber composites play a vital role in the performance of a tire. They provide dimensional stability, strength to the sidewall, absorb shock, etc. in the tires. The polymer cords used in these composites are dipped in resorcinol–formaldehyde–latex (RFL) adhesive to improve the cord-rubber adhesion. Not enough studies have been performed to fully understand the RFL interfacial region and its structural changes in course of tire usage. Moreover, its detailed structure and its evolution during thermal exposure has not been fully resolved. This study has been dedicated in understanding the structural properties of the RFL interfacial layer and its evolution upon thermal treatment in cord-rubber composites. Firstly, pure RFL adhesive properties were investigated when subjected to accelerated thermal exposure. The DMA and AFM measurements highlighted an increase of the modulus of the latex phase and the resin phase during thermal treatment in the presence of curatives. However, no changes occurred in the latex phase in RFL without curatives samples during thermal treatment. Such results demonstrated that the increase of modulus of the latex phase during thermal treatment was mainly due to the presence of curatives through the co-vulcanization process highlighted by NMR. Then, a model composite system composing RFL dipped polyamide monofilaments embedded in a rubber matrix was developed. A multiscale methodology was implemented to study the RFL interfacial region when the model composite was subjected to thermal treatment. While the macroscopic interfacial adhesion properties decreased with thermal treatment, an increase in the local modulus in RF resin and latex phases of the interfacial region was observed by AFM. The SEM-EDX results indicated the presence of oxygen in the RFL region which was facilitating the resin hardening in the RF phase. The further crosslinking in the latex phase due to the presence of sulfur curatives resulted in the increase of the latex phase modulus. Finally, the developed methodology on the model system was applied to the multifilament tire composites (polyamide cords embedded in rubber matrix) to have a multiscale connection between macroscale adhesion behavior and local RFL interfacial region evolution.