Waste tire rubber in polymer blends: a review on the evolution, properties and future

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 the 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. This paper presents a literature review on rubbers recycling by devulcanization. It is focused on the different devulcanization techniques used in the last decades. Particular attention is paid to the thermomechanical method, those in the presence of supercritical CO2 and, finally, the combined thermomechanical process with supercritical CO2 atmosphere.

Recycling of rubber wastes by devulcanization

Recycling and recovery of waste tires is a serious environmental problem since vulcanized rubbers require several years to degrade naturally and remain for long periods of time in the environment. This is associated to a complex three dimensional (3D) crosslinked structure and the presence of a high number of different additives inside a tire formulation. Most end-of-life tires are discarded as waste in landfills taking space or incinerated for energy recovery, especially for highly degraded rubber wastes. All these options are no longer acceptable for the environment and circular economy. However, a great deal of progress has been made on the sustainability of waste tires via recycling as this material has high potential being a source of valuable raw materials. Extensive researches were performed on using these end-of-life tires as fillers in civil engineering applications (concrete and asphalt), as well as blending with polymeric matrices (thermoplastics, thermosets or virgin rubber). Several grinding technologies, such as ambient, wet or cryogenic processes, are widely used for downsizing waste tires and converting them into ground tire rubber (GTR) with a larger specific surface area. Here, a focus is made on the use of GTR as a partial replacement in virgin rubber compounds. The paper also presents a review of the possible physical and chemical surface treatments to improve the GTR adhesion and interaction with different matrices, including rubber regeneration processes such as thermomechanical, microwave, ultrasonic and thermochemical producing regenerated tire rubber (RTR). This review also includes a detailed discussion on the effect of GTR/RTR particle size, concentration and crosslinking level on the curing, rheological, mechanical, aging, thermal, dynamic mechanical and swelling properties of rubber compounds. Finally, a conclusion on the current situation is provided with openings for future works.

Recycling Waste Tires into Ground Tire Rubber (GTR)/Rubber Compounds: A Review

This work is focused on the recycling of natural rubber industry waste by means of thermo-mechanical devulcanization. With that aim, tests were carried out in an industrial twin-screw extruder, at different barrel temperatures, ranging from 80 to 220 °C. The extrusion was done with a screw profile specifically designed for the devulcanization process. The extent and quality of devulcanization were evaluated through the measurements of crosslink density, soluble fraction and Mooney viscosity, and by using the Horikx diagram. Results showed that a high degree of reclaiming (˜90%) was obtained, independently of the barrel temperature. The samples with the best devulcanization quality, i.e. the samples with a more selective sulfur bond scission, were found to be those treated at a lower input temperature. This was explained by the effect of the rubber homogenous self-heating which contributes to the local increase of the material temperature, during the devulcanization process. Moreover, it was found that the properties of the devulcanized rubber/virgin rubber blends were not significantly affected by the addition of the treated rubber. Results suggest that the entire natural rubber industry waste could be recycled into new competitive products, with low energy consumption. This would present a real contribution to the industrial recycling and thus a noticeable improvement of the environment.

Thermo-mechanical devulcanization and recycling of rubber industry waste

Polymeric products such as plastics and rubbers are in widespread use, and the recycling of these polymeric materials has not kept pace with the increased production. There are serious environmental consequences to disposing of waste polymeric materials in landfills or incinerating them. Their low biodegradability makes it unpopular for both large and small industries to recycle them. This study reviews recycling strategies for waste thermoplastic and thermoset materials undertaken by industries and researchers. The primary objective is to review these techniques and identify those that can be applicable to the construction industry. There are three conclusions from this review: (1) Recycling of polymeric waste materials can be helpful to limit landfilled or incinerated waste plastic. (2) Proper selection of a recycling method for waste plastics and rubbers can effectively address the recycling problem without compromising the mechanical properties of the construction material. (3) The majority of the published articles in construction materials research have overlooked the importance of pre-treatment of waste polymeric materials to ensure compatibility with construction materials. Accordingly, there is an urgent need for understanding the science of functionalization of polymers to be able to tune surface properties of the recycled granules for targeted applications. This paper highlights critical research needs to facilitate the recycling of thermosets and thermoplastics to promote resource conservation and sustainability.

State of the art in recycling waste thermoplastics and thermosets and their applications in construction

Four different reclaiming methods involving important reclaiming factors such as temperature, shear force, and atmosphere were used to reclaim ground tire rubber. The structure and performance of the reclaimed rubber were investigated. The reclaimed samples were all found to be mixtures of three parts: the sol part, a loosely crosslinked gel part, and low molecular substances. For a reclaimed product to have both good processability and mechanical properties, the ideal structure should be that the sol fraction and its molecular weight (Mn) are as high as possible. However, the high sol fraction and high Mn cannot be reached at the same time because of the nonselective scission of the main chain and crosslink bonds. Thus, for a reclaimed rubber to have high quality, the presence of some amount of gel fraction is essential. Our preliminary results showed that the recommended reclaiming method would be a process under oxygen-free atmosphere, without severe shear force, and at relative low temperature. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013

Structure and performance of reclaimed rubber obtained by different methods

The invention relates to a method for continuously preparing liquid reclaimed rubber by the aid of a screw extruder, which belongs to the field of waste rubber recycling. The method is characterized in that a cross-linked network of pretreated waste rubber powder is crushed to obtain the liquid reclaimed rubber under the combined action of temperature, pressure, shearing and desulfurizer in the screw extruder by means of a physical and chemical linkage method and by the aid of the screw extruder. The method is simple in technical process and has the advantages of high desulfurization and depolymerization degree, energy conservation, environmental protection and continuity. The obtained liquid reclaimed rubber can be used with asphalt or replaces the asphalt for paving, and can be used with pure rubber to manufacture waterproof anticorrosion materials and the like.

Method for continuously preparing liquid reclaimed rubber by aid of screw extruder

The invention provides a method for regenerating a cross-linked network through low-temperature selectivity rupture of vulcanized rubber, belonging to the field of recycling of waste rubber resources. According to the method provided by the invention, firstly, after the vulcanized rubber and de-cross-linking assistants are mixed, solvent swelling is carried out, and the de-cross-linking temperature is kept; the vulcanized rubber is de-cross-linked through the interaction and matching of the de-cross-linking assistants, and a good selectivity rupture effect on sulfur-sulfur bonds in the waste rubber can be achieved; and at relatively low temperature, great rupture of the cross-linked network can be realized, thereby obtaining regenerated rubber with high sol content and high molecular weight.

Method for regenerating cross-linked network through low-temperature selectivity rupture of vulcanized rubber

The invention relates to a method for realizing rapid waste rubber powder depolymerization by using ultraviolet rays and belongs to the field of recycling of renewable resources. According to the method, a high-energy ultraviolet generator is used, a physical ray irradiation method is adopted, in the oxygen environment, different kinds of waste rubber powder are subjected to high-energy ultraviolet irradiation treatment, and a rubber 3D cross-linked network is broken in a short time, so that green depolymerization generation of the waste rubber powder is realized. The high-energy ultraviolet generator is used, and the method has the simple process and has the advantages that the depolymerization temperature is low, the depolymerization process is rapid, the recovery method is energy-saving and environment-friendly, the depolymerization degree is controllable and the performance of reclaimed rubber products is good.

Method for realizing low-temperature and rapid waste rubber powder depolymerization by using ultraviolet rays

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Structure and performance of reclaimed rubber obtained by different methods

Method for continuously preparing liquid reclaimed rubber by aid of screw extruder

Method for regenerating cross-linked network through low-temperature selectivity rupture of vulcanized rubber

Method for realizing low-temperature and rapid waste rubber powder depolymerization by using ultraviolet rays