ground tyre rubber blends by γ irradiation

ground tyre rubber blends by γ irradiation

Polymer Degradation and Stability 91 (2006) 2375e2379 www.elsevier.com/locate/polydegstab Compatibilisation of polyethylene/ground tyre rubber blends...

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Polymer Degradation and Stability 91 (2006) 2375e2379 www.elsevier.com/locate/polydegstab

Compatibilisation of polyethylene/ground tyre rubber blends by g irradiation R. Sonnier, E. Leroy*, L. Clerc, A. Bergeret, J.M. Lopez-Cuesta Centre des Mate´riaux de Grande Diffusion, Ecole des Mines d’Ale`s, 6, avenue de Clavie`res, 30319 Ale`s Cedex, France Received 10 February 2006; received in revised form 27 March 2006; accepted 2 April 2006 Available online 15 May 2006

Abstract In the present work, g irradiation is used for the in situ compatibilisation of blends of recycled high density polyethylene (rHDPE) and ground tyre rubber (GTR) powder. The expected compatibilisation mechanism involves the formation of free radicals, leading to chain scission within rubber particles, crosslinking of polyethylene matrix and co-crosslinking between the two blend components at the interface. While uncompatibilised rHDPE/GTR blends show poor mechanical properties, especially for elongation at break and Charpy impact strength, irradiation leads to a significant increase of these mechanical performances. Such behaviour is attributed to the development of an adhesion between GTR particles and the surrounding thermoplastic matrix. This conclusion is supported by in situ scanning electron microscopy observations during microtensile tests, showing strong elongation of GTR particles upon deformation of irradiated blends. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Ground tyre rubber; Recycling; Blends; Compatibilisation; HDPE; Irradiation

1. Introduction The reutilization of ground tyre rubber (GTR) powder as a dispersed elastomeric phase in a thermoplastic matrix offers an opportunity to design second generation materials which would be recyclable due to the thermoplastic matrix and which potentially could present thermoplastic elastomer (TPE)-like mechanical behaviour [1]. Indeed, a particular family of TPE, called thermoplastic vulcanisates (TPVs), consists in dispersing an uncrosslinked rubber phase into a thermoplastic matrix by melt blending, and to dynamically crosslink that rubber phase in the melt [2]. Recycling end-of-life GTR powder as a functional filler in a thermoplastic matrix with the aim of obtaining materials of similar morphology and behaviour is particularly interesting since it turns into an advantage the three dimensional network nature of rubber, while this structure is generally a problem for recycling (compared to

* Corresponding author. Fax: þ33 4 66 78 53 65. E-mail address: [email protected] (E. Leroy). 0141-3910/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymdegradstab.2006.04.001

thermoplastics) due to the insolubility and non-melting associated properties. Nevertheless, the main difficulty is the generation of a sufficient adhesion between the two phases. Various examples of compatibilisation of GTR particles with different polymer matrices, involving interfacial coupling reactions, can be found in literature [1,3e7]. In most cases [3e6], the introduction of an additional reactive component (such as an uncrosslinked rubber phase which is dynamically crosslinked at the interface between GTR and thermoplastic, or, maleic anhydride grafted chains in the case of a polyolefin matrix) and/or a surface treatment of the GTR prior to blending are needed. Recent studies propose more valuable approaches in which such additives and/or surface treatments can be avoided: Wiessner et al. [1] achieved in situ compatibilisation between GTR particles and a polypropylene matrix by using a peroxide as a catalyst of dynamic co-crosslinking at the interface, the materials obtained showing TPE-like behaviour. A different approach, without any catalyst or additive, has been proposed by Scaffaro et al. [7], who showed the possibility to obtain recycled polyethylene/GTR blends with good

R. Sonnier et al. / Polymer Degradation and Stability 91 (2006) 2375e2379

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Variation of property (%)

120%

P

Young's modulus Yield stress

100%

Elongation at break Charpy impact strength

80% 60% 40% 20% 0% Pure rHDPE (reference)

10

30

50

70

GTR Weight content % Fig. 1. Mechanical properties of uncompatibilised rHDPE/GTR blends. Pure rHDPE only partially breaks during Charpy impact tests.

mechanical properties by twin screw extrusion only by varying the process parameters (screw rotation speed, temperature and residence time) in order to promote controlled degradation of the three dimensional structure of GTR, allowing to reduce incompatibility with PE matrix. Nevertheless, no TPE-like mechanical properties for the resulting materials were reported by the authors [7]. In the present work, our aim is to develop in situ compatibilisation of recycled polyethylene/GTR blends by g irradiation. The interaction of such radiations with polymers leads to the formation of free radicals [8,9], which can result in chain scission, as well as chain branching and crosslinking. Gamma irradiation under inert atmosphere is commonly used for the crosslinking of polyethylene [10], this effect being predominant for this kind of polymer. In the presence of air, oxidation also occurs, leading to lower crosslinking [10]. As an opposition, for rubbers, chain scission dominates, this domination over crosslinking being enhanced under air atmosphere [11]. In a GTR/polyethylene blend, it can be expected that this radiation induced chain scissions within the rubber phase will reduce the incompatibility with the thermoplastic matrix.

Radiation induced crosslinking between PE and GTR at the interface is also expected to occur. 2. Experimental 2.1. Materials Recycled high density polyethylene (rHDPE), containing a small amount of polypropylene impurities, was supplied by VALCO SAS (France). Ground tyre rubber (GTR) powder was purchased from Granuband BV (Holland) under the trade name Granuband 0e0.5 mm. It contains approximately 70% of rubber from car tyres and 30% of rubber from lorry tyres, as well as a small amount of fibre impurities. The specific surface area and mean particle diameter are 0.09 m2/g and 380 mm, respectively. 2.2. Processing and g irradiation rHDPE/GTR blends were compounded using a twin screw extruder (Clextral BC 21, 180  C, 250 rpm) and pelletized.

200% 180%

Young's modulus

Variation of property (%)

Yield stress 160% 140%

Elongation at break Charpy impact strength

120% 100% 80% 60% 40% 20% 0% No irradiation (Reference)

15 kGy

25 kGy

50 kGy

100 kGy

Fig. 2. Effect of irradiation on the mechanical properties of 50/50 w/w% rHDPE/GTR blends.

R. Sonnier et al. / Polymer Degradation and Stability 91 (2006) 2375e2379

Irradiation of neat rHDPE matrix and rHDPE/GTR blends was performed by Ionisos SA (France) directly on the pellets, using a 2  106 Ci 60Co source, under air atmosphere. Irradiation dose was controlled with a precision of 5%. Standard ISO 527-2 type 1A tensile test specimens were injection-moulded (95 tons Sandretto AT press, 170  C) from the pellets. 2.3. Thermal analysis The melting temperature and crystallinity of all samples were measured by differential scanning calorimetry (DSC Setaram 92, 10 K/min, under air). Crystallinity was calculated considering a melting enthalpy of 295 J/g for a 100% crystalline high density polyethylene [12]. 2.4. Mechanical testing Tensile properties and Charpy impact strength were determined at least 3 days after injection moulding. Ten samples were tested for each formulation. Tensile tests were carried out using a ZWICK Z010 apparatus at 1 mm/min for the determination of Young’s modulus, and at 100 mm/min for the determination of yield stress and elongation at break. Charpy impact strength was measured on un-notched ISO 179 standard specimens using a ZWICK 5101 pendulum (4 J or 7.5 J (for samples which did not break with the smallest pendulum)). 2.5. In situ scanning electron microscopy observations Interfacial adhesion between GTR particles and surrounding rHDPE matrix in both uncompatibilised and irradiated blends was investigated by in situ scanning electron microscopy observations under tensile strain: a Quanta 200 FEG environmental scanning electron microscope (FEI), equipped with

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a microtensile test module (Microtest 5000, Gatan) was used. This device allows to perform tensile tests on 4  10  10 mm3 samples at 0.5 mm/min and to observe the evolution of material’s morphology. The samples were notched before testing in order to promote the propagation of a crack. SEM beam is focused around a GTR particle, on the crack propagation direction, in order to observe the behaviour of the particle during the tensile test. Micrographs are taken using retro-diffused electrons in order to avoid the influence of topography. 3. Results and discussion The melting temperature of neat rHDPE matrix was 138  2  C, with a crystallinity of 49  1%. No significant variations of these values were observed upon blending with GTR and/or after irradiation, except for samples having received the highest irradiation dose (100 kGy), for which the crystallinity rate of the rHDPE matrix decreased to 40% due to crosslinking. The mechanical properties of the neat rHDPE matrix are the following: Young’s modulus (958  19 MPa); yield stress (29.0  0.2 MPa); elongation at break (29  3%), and Charpy impact strength (13  1 kJ/m2). These values are taken as a reference in Fig. 1, showing the evolution of mechanical properties of uncompatibilised rHDPE/GTR blends as a function of GTR weight content. It can be seen that blending strongly decreases all the mechanical performances. Given the typical mechanical properties of the added rubber phase (hyperelasticity with several hundred % elongation at break and Young’s modulus of the order of 1 MPa), such behaviour is not surprising for Young’s modulus and yield stress values, but clearly unexpected for what concerns elongation at break and impact strength values. Fig. 2 shows the effect of g irradiation on the mechanical properties of a 50/50 (w/w) rHDPE/GTR blend, the Young's modulus

170%

Yield stress Elongation at break

Variation of property (%)

150%

Charpy impact strength

130% P 110% 90% 70% 50% 30% 10% -10% No irradiation (Reference)

25 kGy

50 kGy

100 kGy

Fig. 3. Effect of irradiation on the mechanical properties of pure rHDPE. When no irradiation has been applied, pure rHDPE only partially breaks during Charpy impact tests.

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uncompatibilised blend being taken as a reference: for increasing irradiation dose up to 50 kGy, the progressive increase of both elongation at break and Charpy impact strength values, as well as the simultaneous decrease of Young’s modulus, indicates a compatibilisation of the blend. This conclusion is supported by Fig. 3, showing the variation of the mechanical properties of pure rHDPE matrix as

a function of irradiation dose: radiation induced crosslinking of the polymer leads to a strong decrease of elongation at break and impact strength values, counterbalanced by an increase of Young’s modulus and yield stress values. Therefore, the behaviour observed for the irradiated blends (Fig. 2), can be attributed to the development of a significant adhesion between rHDPE matrix and GTR particles. In

Fig. 4. SEM micrographs for uncompatibilised 50/50 w/w% rHDPE/GTR blend under tensile deformation: (a) 0% strain, (b) 13% strain, and (c) 17% strain.

Fig. 5. SEM micrographs for an irradiated (25 kGy) 50/50 w/w% rHDPE/GTR blend under tensile deformation: (a) 0% strain, (b) 22% strain, and (c) 24% strain.

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addition, the crosslinking of rHDPE matrix probably tends to limit the decrease of Young’s modulus that could be expected to be more important in compatibilised blends. Blend compatibilisation is confirmed by SEM observations performed during a microtensile test: Fig. 4 shows the behaviour of an uncompatibilised 50/50 (w/w) rHDPE/GTR blend, focusing on a single GTR particle embedded in the rHDPE matrix. Initially, no strain is applied (Fig. 4a). For a strain of 13% along the vertical axis, a lack of cohesion can be observed below the GTR particle (Fig. 4b). Finally, Fig. 4c, corresponding to a strain of 17%, shows that the GTR particle does not participate in blend’s deformation. A significantly different behaviour can be observed in Fig. 5, for a 50/50 (w/w) rHDPE/GTR blend having received a 25 kGy irradiation dose. A strong elongation of the GTR particles can be seen in Fig. 5b and c, corresponding to strains of 22% and 24% along the vertical axis, respectively. 4. Conclusions Gamma irradiation allows achieving in situ compatibilisation of recycled HDPE/ground tyre rubber powder blends, leading to an improvement of their mechanical properties. Elongation at break and Charpy impact strength of the blends are significantly increased for irradiation doses of 25e50 kGy, thanks to the involvement of GTR dispersed phase in material deformation. Blends’ Young’s modulus values are only slightly decreased by this compatibilisation due to the fact that in the same time, the radiation induced crosslinking of the rHDPE matrix leads to an increase of both its Young’s modulus and yield stress. Nevertheless, for higher irradiation doses (100 kGy), such crosslinking of the matrix was too important, leading to lower mechanical performances for the blends. Acknowledgments We acknowledge financial support from ADEME (French Agency for the environment and energy management). The

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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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