Facile exfoliation method for improving interfacial compatibility in montmorillonite-natural rubber nanocomposites

Nanoscale fillers have exhibited great potential for use as an efficient alternative to conventional reinforcement fillers like carbon black.  For instance, nanoscale fillers have been widely used to reinforce various polymeric materials such as natural rubber (NR) to enhance their mechanical properties. In particular, layered silicates exhibiting remarkable anisotropy and swelling properties such as montmorillonite (Mt) have attracted significant research attention as promising nanostructured reinforcement for natural rubber. A recent study has reported that in order to achieve a high degree of reinforcement, Mt layers should be exfoliated within the rubber matrix to obtain high surface area and strong interfacial interactions between the two components. However, obtaining high interfacial properties between the rubber matrix and inorganic fillers has remained a research challenge mainly due to lack of appropriate methods for making stable Mt dispersions.

In an effort to address the above challenges, a group of researchers: Dr. Malindu Alwis, and Dr. Upul Ratnayake at Rubber Research Institute of Sri Lanka, in collaboration with Professor Nilwala Kottegoda from the University of Sri Jayewardenepura developed a novel facile exfoliation method for improving interfacial compatibility in montmorillonite-natural rubber nanocomposites. The main aim of their study was to realize a high degree of exfoliation through enhanced compatibility between the two components and high surface area for interactions between the filler and rubber matrix. Their research work is currently published in the journal, Applied Clay Science.

Briefly, the new approach comprised of two main steps: formation of a charge inverted NR latex and coagulation using the exfoliated Mt layers. First, a charge inversion approach involving the NR latex treatment with quaternary ammonium surface-active substance was used to prepare the stable cationic latex of NR. Secondly, the prepared cationic NR latex was directly coagulated using Mt aqueous dispersion to facilitate the formation of Mt-NR nanocomposite comprising exfoliated and intercalated clay mineral layers. Lastly, the feasibility of the method was validated by studying and comparing the interfacial and stress behavior of the un-vulcanized nanocomposite with those prepared by the conventional latex compounding method.

The research team observed the occurrence of exfoliation of Mt platelets within the NR matrix, thus resulting in improved reinforcement. As evidenced by the X-ray diffraction and transmission electron microscopy images, the Mt platelets exhibited a higher degree of exfoliation accompanied by significantly enhanced physicochemical and mechanical properties. This was attribute to the increased computability between the NR latex particles and nanostructured Mt platelets. Furthermore, studies carried out using a combination of Mooney viscosity and green strength measurements revealed that the interfacial interaction between the cation latex and Mt was much improved than that of nanocomposites prepared using conventional latex compounding method.

In summary, the study reported a novel, scalable, and simple method for reinforcing natural rubber with Mt. Results showed that the presented approach was suitable for improving the interfacial compatibility in Mt-natural rubber nanocomposites. Moreover, the un-vulcanized Mt-NR prepared using this method exhibited enhanced mechanical properties, including high strength and stiffness. In a statement to Advances in Engineering, Dr. Upul Ratnayake said that the charge inversion approach is a promising technology for preparing clay-based rubber nanocomposites with improved interfacial compatibility.