Waste Rubber Recycling: A Review on the Evolution and Properties of Thermoplastic Elastomers

Abstract

Currently, plastics and rubbers are broadly being used to produce a wide range of products for several applications like automotive, building and construction, material handling, packaging, toys, etc. However, their waste (materials after their end of life) do not degrade and remain for a long period of time in the environment. The increase of polymeric waste materials' generation (plastics and rubbers) in the world led to the need to develop suitable methods to reuse these waste materials and decrease their negative effects by simple disposal into the environment. Combustion and landfilling as traditional methods of polymer waste elimination have several disadvantages such as the formation of dust, fumes, and toxic gases in the air, as well as pollution of underground water resources. From the point of energy consumption and environmental issues, polymer recycling is the most efficient way to manage these waste materials. In the case of rubber recycling, the waste rubber can go through size reduction, and the resulting powders can be melt blended with thermoplastic resins to produce thermoplastic elastomer (TPE) compounds. TPE are multi-functional polymeric materials combining the processability of thermoplastics and the elasticity of rubbers. However, these materials show poor mechanical performance as a result of the incompatibility and immiscibility of most polymer blends. Therefore, the main problem associated with TPE production from recycled materials via melt blending is the low affinity and interaction between the thermoplastic matrix and the crosslinked rubber. This leads to phase separation and weak adhesion between both phases. In this review, the latest developments related to recycled rubbers in TPE are presented, as well as the different compatibilisation methods used to improve the adhesion between waste rubbers and thermoplastic resins. Finally, a conclusion on the current situation is provided with openings for future works.

Keywords: compatibilisation; recycling; rubber; thermoplastic elastomer; waste polymers.

Figure 1

Chemical structure of isoprene and natural rubber (NR) (polyisoprene). Adapted with permission from [12]; copyright 2019 Elsevier Ltd.

Figure 2

Chemical structure of styrene-butadiene rubber (SBR). Adapted with permission from [12]; copyright 2019 Elsevier Ltd.

Figure 3

Monomers and polymer structure of nitrile-butadiene rubber (NBR). Adapted with permission from [12]; copyright 2019 Elsevier Ltd.

Figure 4

Chemical structure of ethylene-propylene-diene monomer (EPDM) containing 5-ethylidene-2-norborene (ENB) as a diene. Adapted with permission from [15]; copyright 2019 John Wiley and Sons Ltd.

Figure 5

Schematic representation of polyurethane (PU) and its monomers. Adapted with permission from [18]; copyright 2019 RSC Publishing.

Figure 6

The four groups making polysiloxanes: “M” is trimethylsiloxychlorosilanes (Me₃SiO), “D” Me₂SiO₂, “T” MeSiO₃, and “Q” silicate (SiO₄). For “P”, replace Me by phenyl side groups, while for “V”, replace Me by vinyl side groups. Adapted with permission from [12]; copyright 2019 Elsevier Ltd.

Figure 7

Morphology of a block copolymer thermoplastic elastomer (TPE). Adapted with permission from [33]; copyright 2020 Elsevier Ltd.

Figure 8

Morphology of rubber/plastic blend thermoplastic elastomer (TPE). Adapted with permission from [32]; copyright 2020 Elsevier Ltd.

Figure 9

Thermoplastic vulcanizates (TPV) morphology with continuous plastic phase and discrete rubber particles. Adapted with permission from [32]; copyright 2020 Elsevier Ltd.

Figure 10

Processing steps to produce thermoplastic vulcanizates (TPV) compounds. Adapted with permission from [42]; copyright 2019 Elsevier Ltd.

Figure 11

Different structures of linear copolymers: (a) alternating, (b) random, (c) gradient, and (d) block copolymers. Adapted with permission from [45]; copyright 2019 Elsevier Ltd.

Figure 12

Notched Izod impact strength of high-density polyethylene (HDPE)/ground tire rubber (GTR) composites as a function of rubber content. Adapted with permission from [50]; copyright 2019 John Wiley and Sons Ltd.

Figure 13

The three possible cases for nanoparticles’ (NP) localisation in an immiscible binary polymer blend: (a) in the dispersed phase, (b) at the interface (ideal case), or (c) in the continuous phase. Adapted with permission from [58]; copyright 2019 Elsevier Ltd.

Figure 14

Schematic representation of the devulcanisation and reclamation process. Adapted with permission from [1]; copyright 2020 Elsevier Ltd.

Figure 15

Elongation at break as a function of composition for ground tire rubber (GTR)/polypropylene (PP) blends. Adapted with permission from [73]; copyright 2019 Taylor & Francis Ltd.

Figure 16

Effect of ground tire rubber (GTR) particle size on the mechanical properties of thermoplastic blends. Adapted with permission from [1]; copyright 2020 Elsevier Ltd.

Figure 17

SEM micrographs of ethylene-vinyl acetate (EVA) blends with different ground tire rubber (GTR) contents: (a) 10 wt.%, (b) 20 wt.%, (c) 50 wt.%, and (d) 70 wt.%. Adapted with permission from [74]; copyright 2019 SAGE Publications Ltd.

Figure 18

Torque evolution for polypropylene (PP)/waste tire dust (WTD) blends (250–500 μm). Adapted with permission from [73]; copyright 2019 Taylor & Francis Ltd.

Figure 19

Complex viscosity as a function of angular frequency for thermoplastic natural rubber (TPNR) based on different natural rubber (NR)/high-density polyethylene (HDPE) ratios. Adapted with permission from [82]; copyright 2019 Elsevier Ltd.

Figure 20

Storage modulus (G′) as a function of angular frequency for thermoplastic natural rubber (TPNR) based on different natural rubber (NR)/high-density polyethylene (HDPE) ratios. Adapted with permission from [82]; copyright 2019 Elsevier Ltd.

Figure 21

Hardness of high-density polyethylene (HDPE) as a function of reclaimed rubber (RR) content: (1) H-R10, (2) HR10-C, (3) H-R10-P. Adapted with permission from [77]; copyright 2019 Springer Nature Ltd.

Figure 22

Tensile stress-strain curves of high-density polyethylene (HDPE) and HDPE/ground tire rubber (GTR) compounds. Adapted with permission from [50]; copyright 2019 John Wiley and Sons Ltd.

Figure 23

Reaction mechanism for low-density polyethylene (LDPE)/natural rubber (NR) modified with maleic anhydride (MA). Adapted with permission from [85]; copyright 2019 Taylor & Francis Ltd.

Figure 24

X-ray diffraction patterns of: (a) Cloisite 15A and TPE nanocomposites based on polypropylene (PP) with: (b) 60%, (c) 40%, and (d) 20% ethylene-propylene-diene monomer (EPDM). Adapted with permission from [87]; copyright 2019 John Wiley and Sons Ltd.

Figure 25

SEM micrographs of TPE based on: (a) unfilled polypropylene (PP)/ethylene-propylene-diene monomer (EPDM) (60/40), (b) nanoclay-filled PP/EPDM (60/40), (c) unfilled PP/EPDM (40/60), and (d) nanoclay-filled PP/EPDM (40/60) blends. Adapted with permission from [87]; copyright 2019 John Wiley and Sons Ltd.

Figure 26

SEM of the ground tire rubber (GTR) particles surface: (a) untreated and treated with: (b) perchloric acid (HClO₄), (c) nitric acid (HNO₃), and (d) sulphuric acid (H₂SO₄). Adapted with permission from [89]; copyright 2019 Elsevier Ltd.

Figure 27

Compatibilisation mechanism of thermoplastic/ground tire rubber (GTR) blends using an elastomer as a modifier. Adapted with permission from [80]; copyright 2019 Elsevier Ltd.

Figure 28

Tensile strength and elongation at break of dynamically cured devulcanized rubber (DR)/copolyester (COPE) blends as a function of the devulcanisation time at 180 °C. Adapted with permission from [91]; copyright 2019 Elsevier Ltd.

Figure 29

Schematic representation of the microstructure differences between the thermoplastic vulcanizates (TPV) based on devulcanized rubber (DR)/copolyester (COPE) and undevulcanised rubber (DR)/copolyester (COPE) blends. Adapted with permission from [91]; copyright 2019 Elsevier Ltd.