Recycling, upcycling, and reuse of scrap rubber: Novel approaches and applications

The discovery of vulcanization and advancements in crosslinking technology shaped the modern rubber industry, fueling the rapid growth of rubber production, particularly in the automotive sector. While vulcanization offers significant technical benefits, it also presents considerable environmental challenges. The stable crosslinked structure of vulcanized rubber, formed by irreversible covalent bonds, resists natural degradation, making it difficult to recycle,
repair, reprocess, or upcycle. Besides, as rubber consumption increases, so does rubber waste, especially in the form of discarded and damaged items. Each year, over 10 million tons of rubber waste are generated, with tires being the primary contributor. By the year 2030, the number will have reached 1.2 billion every year, and if the tire is kept, it will be 5 billion tires that are frequently dumped, creating a very significant danger to the environment (Thomas and Gupta, 2016). Therefore, managing the increasing number of discarded tires is crucial for environmental sustainability.

Concerns about the environmental impact of discarded rubber have persisted for decades, with the search for effective recycling methods intensifying after the discovery of vulcanization. Recycling strategies began soon after Charles Goodyear’s discovery of the vulcanization process. Early methods used sodium hydroxide as a reclaiming agent, which destroyed both crosslink bonds and rubber backbones. The resulting reclaimed rubber (RR) was reused in rubber compounds, serving as a processing aid and improving the properties of rubber extrudates, such as reducing shrinkage and providing smoother surfaces.

Despite commercial interest in rubber waste processing, modern vulcanized rubber is more resistant to recycling due to the introduction of new additives and the strong interactions between rubber and fillers. This creates a significant gap in our ability to effectively manage rubber waste, as current recycling methods remain inadequate. As a result, there is an urgent need for new, innovative approaches to prevent environmental pollution and establish sustainable solutions. Several attempts have been made to develop recycling methods that are both economically and environmentally viable.

Rubber waste management has been proposed not only for environment protection and waste reduction but also for circular economy and sustainability concepts. The circular economy approach envisions a closed-loop system that optimizes value extraction from end-of-life products. This concept means cradle-to-cradle, but not cradle-to-grave. Hence, rubber waste is a major consideration for circular economy due to the extensive rubber utilization and rubber waste accumulation.

An overview of the life cycle of a rubber product and waste with the reuse, recycling, and upcycling approaches has been depicted in Fig. 14.1. The rubber waste recycling can be divided into three levels: (1) Reuse rubber waste approach is giving a second life to rubber items without significant modification, mainly with reduced market value compared to the initial product. Most reuse approach is actually a downcycling method and is far from closed-loop recycling, leading to quality and value reduction over time. (2) Recycling involves processing and transforming rubber waste into new products without significant value-added. Recycling rubber waste is achieved by converting crosslinked rubbers into reprocessable rubber intermediates that can be mixed with new additives and revulcanized into new products. (3) Upcycling rubber waste involves transforming discarded and recycled items into new products with increased value or function. This chapter focuses on providing a comprehensive update on reuse, recycling, and upcycling methodologies for a wide range of conventionally vulcanized rubber materials.