Material PropertiesPolyethylene/ground tyre rubber blends: Influence of particle morphology and oxidation on mechanical properties
Introduction
For many years, the management of end-of-life tyres is a concern. For example, more than 250 million tyres are scrapped every year in the United States. In consequence, most developed countries have imposed regulations to forbid land filling of tyres. Waste management of end-of-life tyres requires profitable recycling solutions. The grinding of tyres and the incorporation of ground tyre rubber (GTR) powders in polymers or mortars seems to be one of the more advantageous ways. Nevertheless, in the case of a simple re-incorporation in polymer matrices, the mechanical properties of the pure polymer tend to decrease drastically with the powder content, because of the lack of interfacial interactions due to the crosslinked structure of GTR particles [1], [2], [3].
In order to promote GTR compatibility with the matrix, modification of the powder seems to be required. A first approach is the control of the GTR particle size and morphology.
The influence of particle mean size (typically about several hundreds of micrometers) was studied by several authors [3], [4], [5]. Goncharuk et al. [4] observed that elongation at break of LDPE/GTR blends decreased with increasing particle mean size, while tensile strength was not affected by this parameter. Liu et al. [3] noted the same tendency in the case of PP/EPDM powder blends. Rajalingam et al. [5] showed that a decrease of particle size from 600 to 74 μm had a weak effect on impact energy of NR/GTR blends.
The influence of the particle shape and of the powder's specific surface area, which are controlled by the grinding process, was also investigated [4], [6], [7]. Goncharuk et al. [4] studied three methods of grinding: roller grinding, elastic-strain grinding and cryogenic grinding. The blends containing powders prepared by roller grinding exhibited the best properties. This was ascribed to a higher specific surface area. Liu et al. [7] observed that the elongation at break of PP/EPDM powder blends was higher with an ambient ground powder than with a cryogenic ground powder because the particles surface was less developed in this latter case.
However, all the authors agree about the necessity to perform chemical modification on GTR to obtain good adhesion with the polymer matrix and to improve mechanical properties.
According to this objective, numerous processes of devulcanization were considered, such as chemical [2], [8], [9], biological [10], ultrasonic [11], [12] or microwave [13] processes. However, all these treatments take a long time and/or have pollution problems. Thermomechanical devulcanization of rubber powder in a counterrotating twin screw [14], [15] is highly interesting because it is a fast and ecological method, but only poor results were obtained by the authors [15].
A third possibility for compatibilization consists of the oxidation of the GTR surface. Adhesion between rubber-like SBR and several polymers was highly enhanced by oxidation or chlorination treatments, according to chemical [16], [17] or plasma [18] methods. Nevertheless, the incorporation of oxidized or chlorinated GTR in PE [19], [20], PVC [21] or NR [22] matrix resulted in poor mechanical properties. No thermoplastic elastomer-like behaviour was reported. The improvement of Young's modulus, tensile strength and hardness was attributed to a rigid layer of degraded rubber on the particle surface. At the same time, elongation at break decreased.
Liu et al. proposed oxidization of EPDM powder by KMnO4 to break unsaturated CC bonds of the rubber and to create hydroxyl groups. Then, the modified powder was incorporated into a PP matrix containing a small amount of maleic anhydride (MA) grafted chains [3], [7]. These authors noted a significant improvement of elongation at break while tensile strength remained constant. They explain these results considering that polar groups on the EPDM surface reacted with maleic anhydride to form covalent bonds.
Gamma Irradiation in air could provide another way of oxidation. Numerous studies show that the radiation energy can induce macromolecular chain scission [23]. Some of the created free radicals can also recombine together to crosslink [24], [25], or with oxygen in air to create polar groups [26]. Nevertheless, no radiation method had been performed on GTR as far as we know.
In the present work, the objective was to evaluate the possibility of recycling high percentages of GTR into a recycled polyethylene matrix to obtain a 100% recycled blend with acceptable mechanical performance. The influence of GTR particle mean size and specific surface area on PE/GTR blend properties was first investigated. In a second step, two oxidation treatments were carried out to improve the compatibility between rubber particles and the recycled PE matrix, in which maleic anhydride grafted polyethylene (MAgPE) chains were added. The first treatment used KMnO4 according to the earlier works carried on by Liu et al. through a wet process. The second was γ-irradiation in air according to a dry process.
Section snippets
Materials
The LDPE and HDPE used were recycled materials and contained small amounts of PP impurities. The MAgPE was OREVAC 18302 from ATOFINA (0.5 wt% of maleic anhydride). GTR powders were provided by different European firms (Holland, Italy, Spain and France) and are designated in this study G, T5PL, T5, T6, T8, R8 and E. All powders are blends of car and truck tyres and contain a small amount of fibres, except T5PL which comes only from pure truck tyres and which is fibre free. Two of the GTR powders
Mechanical properties
As can be seen in Fig. 1, the tensile properties are strongly damaged by the incorporation of GTR (powder G, powder mean size=380 μm) in LDPE and these properties decrease proportionally with the powder content. Nevertheless, the elongation at break levels off at high loading, which could be ascribed to percolation phenomena of GTR particles. This drastic decrease of mechanical performances could be ascribed to poor interfacial adhesion, as can be observed on ESEM fracture surface (Fig. 2). A
Conclusion
First, it has been shown that the incorporation of a ground tyre powder into recycled polyethylene matrix led to a drastic decrease of the mechanical performance because of a too weak interfacial adhesion. Moreover, the mean size and the surface roughness of particles seem to have no significant influence on these properties.
In order to improve the interfacial adhesion, ground tyre powders were treated on one hand by potassium permanganate (wet process) and on the other hand by γ-irradiation
Acknowledgements
We acknowledge financial support from ADEME (French Agency for the environment and energy management). The authors would also like to thank Mr Jean-Marie Taulemesse and Mr Marc Longerey for their help in Scanning Electron Microscopy observations and mechanical measurements respectively.
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