Sustainable Plasticizer from Agroindustrial Waste for Natural Rubber Compounds: Influence on the Curing System and Compound Properties

The industry has pursued efforts to minimize the use of fossil fuel-based processing oils in favor of naturally occurring nontoxic oils from renewable sources, such as vegetable oils. In this study, the feasibility of replacing naphthenic oil (NAPH)-based plasticizers in natural rubber (NR) compounds with oil originating from cashew nut shells was evaluated. Cardanol oil (Cdn), which is derived from decarboxylated cashew nut shell liquid, a byproduct of cashew nut production, was chemically modified by acetylation and named Cdn-A. The successful acetylation of Cdn with acetic anhydride was confirmed. The effects of Cdn and Cdn-A as plasticizers on the curing characteristics, thermal aging, and physical, mechanical, and dynamic properties of carbon black-filled NR compounds were then evaluated. Cdn acted as a co-activator for rubber vulcanization; by contrast, Cdn-A showed negligible effects on this process. Vulcanized Cdn- and Cdn-A-containing NR compounds exhibited improved abrasion and tear resistance compared with the reference compound containing NAPH, as well as antioxidant activity. The Cdn-added NR compound also demonstrated high elongation at break and toughness. The results collectively suggest that NR compounds containing Cdn exhibit properties suitable for tire tread applications requiring high grip characteristics.