A Study of Scrap Rubber Devulcanization and Incorporation of Devulcanized Rubber into Virgin Rubber Compound

Abstract

In this era of rapid development and economic growth, waste management is of high concern for humans. According to statistics, a great amount of rubber waste materials are produced annually. The major amount of these waste rubber material comes from scrap tires. Only a few percent of the waste rubber materials are recycled and made into new products. Recycling of waste rubber has a certain problem: the crosslink structure of thermoset rubber. Due to the presence of three dimensional crosslink structures in rubber products, they cannot be reprocessed by application of heat, like we do for thermoplastic materials. In order to recycle and reuse the vulcanized rubber we have to find a way to cleave the crosslink bonds. This technique which can break down the crosslink bonds in the rubber is called devulcanization. Different devulcanization processes are utilizing heat, chemicals, ultrasound, and mechanical stresses in order to break down the crosslink structure of vulcanized rubber and convert the insoluble thermoset rubber into a soft and sticky material which can be processed and vulcanized again. Comparing various devulcanization processes, the thermo-mechanical devulcanization process which applies mechanical forces and heat to break down the crosslink network of rubber is very effective. One of the common ways to maintain the required shear stresses and mechanical forces to cleave and break down the crosslink structure of vulcanized rubber is utilizing the extrusion process. In this research, twin screw extruders are preferred due to their modular designs which enable us to adjust the different levels of shearing on the material. In order to facilitate the extrusion process supercritical carbon dioxide is used. Supercritical carbon dioxide diffuses into the rubber particles during the devulcanization process and facilitates the process by expanding and softening the rubber particles. In this research we have established a high throughput devulcanization process to produce devulcanized scrap tire rubber at an industrial scale. The effect of different process parameters on obtained devulcanized rubber properties, such as network structure, rheological properties, and physical and mechanical properties are studied. Results showed that in the range of our experiment design, the only effective parameter on crosslink density and Mooney viscosity of the devulcanized scrap tire is feed rate. Using Horikx theory, it was shown that the devulcanization mechanism for our scrap tire devulcanization process is a selective crosslink cleavage rather than a random chain scission. The obtained devulcanized rubber was then incorporated in a virgin tire rubber compound and the impact of devulcanized rubber utilization on tire compound performance was studied. Results showed that incorporation of devulcanized rubber into a tire tread compound up to 30 percent does not deteriorate the tire tread properties in a significant manner. Finally, a high throughput devulcanization process was established for scrap ethylene-diene-propylene (EDPM) rubber devulcanization. The effects of main process variables on the properties of the devulcanized EPDM rubber were studied. It was observed that crosslink density and Mooney viscosity of the devulcanized EPDM rubber are affected by the feed rate and screw speed in a significant manner.