Disposal of used tires and rubber wastes is one of the biggest challenges of the 21st century waste management. One of the environmentally friendly possibilities of recycling this type of material is to go through the breaking of their three-dimensional structure. This treatment, called devulcanization, can be defined as a process that causes the selective breakup of the sulfur-sulfur (S-S) and carbon sulfur (C-S) chemical bonds without breaking the backbone network and without degrading the material. The devulcanized rubber can be mixed with virgin rubber or with other kinds of matrices to give new compounds without generating a significant decrease in mechanical and physical properties. Many devulcanization process types are presented in the literature: chemical, ultrasound, microwave, thermomechanical, etc. The thermomechanical devulcanization based on extrusion seems to be the more suitable to be applied on an industrial scale. The supercritical CO2 has been proposed as a green atmosphere that can be used to improve this type of devulcanization. In fact, it seems that in supercritical conditions, the CO2 swells the rubber and stretches the sulfide links, making them easier to break.
Nearly 70% of the rubber produced in the world is used in tires. There are abbot 1.475 billion vehicles worldwide. Each such vehicle replaces all its four tires every three to four years. This comes down to a tire a year, on average, which means that abot 1.5B passenger tires are scrapped each year. If we do nothing about it, we will face 3B scrap tires the following year, 4.5B scrap tires the year after, and so on. Still, this problem is worsening with electric vehicles, as they are heavier, more vehicles on roads, cheaper tires that last less, etc. Add to that, tires from off-the-road vehicles, trucks, buses, construction machinery, etc.
Strategies to cater to retiring tires in North America include incineration (45%), crumb rubber generation (34%), landfilling (19%), and a good deal of illegal dumping – – none of which being a good strategy. Rubber cooks (vulcanizes, with a fraction of a vulcanization agent, heat and pressure) to turn into tires, like dough turning into bread (with yeast and heat). This makes rubber hard to recycle.
Some applications of the waste rubbers are sports and playing surfaces, floor and walkways tiles, concrete, thermal isolation, acoustic isolation, footwear, road and rail equipment, activated carbon production, etc. However, the best way to carry out the disposal of rubber wastes is through recycling, including all methods where rubber waste is converted to make new commodities.
Because of the three-dimensional structure network of rubbers, the vulcanization process which produces strong bonds and the specific composition that includes several additives, the recycling of such materials is a current technological challenge. In fact, a tire is a composite made up of several types of synthetic and natural rubbers, particulate fillers (silica, carbon black, etc.), chemical additives (sulfur, oils, etc.), and textile and/or metal reinforcements.
Regarding the useful life, there are two types of tires: the reusable tires and the non-reusable tires. The first ones are tires which are sent to retreading companies that gives them a second life. The second ones are tires that cannot be retreaded due to advanced damage, structural deformation or high degradation. These tires are starting materials for recycling.
With the aim of recycling and reusing the vulcanized rubbers, it is crucial to find a safety way for their devulcanization, i.e. a way to cleave their crosslink bonds. Rubber devulcanization is a process in which the vulcanized waste rubber is transformed into the state in which it can be revulcanized, after its mixing and processing. Strictly, devulcanization can be defined as a process where poly-, di-, and mono-sulfidic bonds, formed during vulcanization, are totally or partially broken. This means that devulcanization can be defined as a process that causes the selective breakup of sulfidic bonds without main chain scissions and degradation of the polymer.
Regardless of the devulcanization method, some of the main polymer chains are broken during devulcanization and, thus, the properties of the treated material are different compared to the original rubber. The use of devulcanized rubber can reduce the cost of final products. Depending on the desired application, the devulcanized rubber can be reused as obtained or mixed in different amounts with virgin rubber or other polymers.
Dr. Ben Chouchaoui
Operations Manager
WIDL Inc.
April 1, 2024
Monday – Friday
8:00 am – 5:00 pm
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Disclaimer - Dr. Chouchaoui and WIDL do not do engineering services regulated by PEO (Professional Engineers Ontario). If you need services as such please contact PEO for referrals to an engineer who does such.
