Xinxing Zhang, Dong Tian, Wei Zhang, Jian Zhu, and Canhui Lu*

Transcription

1 Morphology, Foaming Rheology and Physical Properties of Ethylene-Propylene Diene Rubber/Ground Tyre Rubber (GTR) Composite Foams: Effect of Mechanochemical Devulcanisation of GTR Morphology, Foaming Rheology and Physical Properties of Ethylene- Propylene Diene Rubber/Ground Tyre Rubber (GTR) Composite Foams: Effect of Mechanochemical Devulcanisation of GTR Xinxing Zhang, Dong Tian, Wei Zhang, Jian Zhu, and Canhui Lu* State Key Laboratory of Polymer Materials Engineering, Polymer Research Institute of Sichuan University, Chengdu , China Received: 21 May 2012, Accepted: 14 September 2012 SUMMARY In this study, an attempt was made to enhance the foamability and physical properties of ethylene-propylene diene (EPDM) rubber/ground tyre rubber (GTR) composite foams, thereby providing a potential new route to recycle discarded tyres. Pan-mill type mechanochemical reactor made in our lab was applied to realise the partial devulcanisation and ultrafine pulverisation of GTR. Fluorescence microscopy and scanning electron microscopy (SEM) studies indicated that the dispersion and interfacial adhesion between devulcanised GTR and EPDM matrix was much better than that of the untreated GTR and EPDM. The enhanced foamability through devulcanisation of GTR was confirmed by foaming rheological analysis. The physical properties of the composite foams were significantly improved through the devulcanisation of GTR. For the EPDM/ GTR (100/30) composite foams, the rebound resistance was enhanced from 37% to 51% while the compression set was decreased from 43% to 33% via the devulcanisation of GTR. It has been proved that the mechanochemically devulcanised GTR can be used as a low cost but effective functional additive for enhancing the rebound resistance properties of EPDM foams. * Corresponding author. Tel.: (+86) ; Fax: (+86) address: canhuilu@263.net Smithers Rapra Technology, 2013 Progress in Rubber, Plastics and Recycling Technology, Vol. 29, No. 2,

2 Xinxing Zhang, Dong Tian, Wei Zhang, Jian Zhu, and Canhui Lu INTRODUCTION Ethylene-propylene diene (EPDM) rubber foam plays an important role in packaging, sealing, vibration and isolation applications because of its good design flexibility, lightweight, excellent weathering and aging characteristics [1]. In order to improve the modulus, extend the application range and reduce the cost of the foams, various kinds of fillers such as whiting, talc, carbon black etc. [2] have been added into the EPDM foams. However, the introduction of these fillers into EPDM foams usually resulted in the deterioration of its rebound resistance properties, which could be ascribed to the rigidity nature of the inorganic fillers. Replacing the inorganic fillers by reclaimed tyre rubbers is a potentially attractive way to achieve a desirable combination of high rebound resistance and low cost. The use of ground rubbers in polymer industry not only protects the environment but also saves the valuable rubber resources [3-6]. However, directly compounding ground tyre rubbers (GTR) with matrix materials without modification led to significant decline in mechanical properties due to the inert surface of GTR. It could be ascribed to the three dimensional crosslinked structure of GTR, which results in poor dispersion and weak interfacial adhesion between rubber particles and polymer matrix. Compatibilisation of the GTR-filled polymer blend by applying compatibilisers is a typical way to obtain composites with superior mechanical properties. Kim and coworkers [7-10] have carried out extensive studies on the application of various compatibilisers to prepare high performance polypropylene/gtr composite foams. However, lack of suitable compatibiliser and additional cost limit this method to be used in GTR-filled rubber foams. To date, few studies reported the preparation of GTR-filled rubber foams. The aim of this work is to prepare EPDM/GTR composite foams with excellent mechanical properties. We report for the first time that the reclaimed rubbers can be used as a low cost but effective functional additive for enhancing the rebound resistance properties of EPDM foams. To achieve this, a self-designed pan-mill type mechanochemical reactor was applied to realise the partial devulcanisation and ultrafine pulverisation of GTR simultaneously. As a result, better dispersion and improved interfacial adhesion between GTR particles and EPDM matrix were realised. Derived from the traditional Chinese stonemill, the innovative pan-mill equipment is efficient in mechanical pretreatment owing to its unique structure. The mechanochemical reactor can exert a fairly strong squeesing force in normal direction and shearing forces in both radial and tangential directions, functioning like a pair of three-dimensional scissors. Our previous work demonstrated that it has excellent devulcanisation effect on GTR [11-15], natural rubber vulcanisates [16] and fluoroelastomers [17, 18] at ambient temperature without the use of any chemicals. Compared with existing devulcanisation methods like chemical devulcanisation [19], ultrasonic 82 Progress in Rubber, Plastics and Recycling Technology, Vol. 29, No. 2, 2013

3 Morphology, Foaming Rheology and Physical Properties of Ethylene-Propylene Diene Rubber/Ground Tyre Rubber (GTR) Composite Foams: Effect of Mechanochemical Devulcanisation of GTR devulcanisation [20], microwave devulcanisation [21] and thermo-mechanical devulcanisation [22], the pan-milling devulcanisation technique has obvious advantages. It has high capacity, relatively low cost and no need to use any chemicals. Furthermore, the pan-milling process is an energy efficient process since it is operated at ambient temperature. In this study, the influence of stress-induced mechanochemical devulcanisation of GTR on the morphology, foaming rheology and physical properties of EPDM/GTR composite foams will be discussed in detail. EXPERIMENT Materials EPDM 4045 (ethylene content: 46%, ENB content: 7.6%, ML (1+4) at 100 C=45, density: 860 kg/m 3 ) manufactured by Mitsui Chemicals, Inc. (Japan) was used in this study. GTR powder (60 mesh) was generated from passenger car and light truck tyres, manufactured by Sichuan Tianlimin Rubber Co. Ltd (China). It composed of: hydrocarbon, 59.9%; carbon black, 36.4%; ash, 3.7%; acetone extractable volatiles, 8.6%. Blowing reagent, N,N -dinitrosopentam (H) with decomposable temperature about 205 C and blowing gas 260 ml per gram, was produced by Shandong Yilong Industry Co. Ltd (China). Other compounding ingredients, such as sulfur (S, as a crosslinking agent), zinc oxide (ZnO, lower the decomposition temperature of the blowing agent), stearic acid (SA, as a blowing co-agent), and N-cyclohexyl benzthiazyl sulphenamide (CBS, as a accelerator agent) were of reagent grade and obtained commercially. Sample Preparation The mechanochemical devulcanisation of GTR was achieved through high shearing and squeesing forces generated by the pan-mill mechanochemical reactor. The details of the pan-mill equipment and operation procedure can be found in our previous publications [11, 17]. The resulting material was blended with EPDM rubber on a two-roll mixing mill (diameter, 150 mm; working distance, 320 mm; speed of the slow roll, 15 rpm; friction ratio, 1:1.4) at the processing temperature of about 50 C. The compound recipes are listed in Table 1. GTR powder was masticated with virgin EPDM and then mixed with several compounding ingredients. The mixing time was 10 min. The prepared EPDM/GTR blends were then put in a mold and pressed in a hydraulic press at 160 C and 8 MPa for 15 min. After removing the pressure, Progress in Rubber, Plastics and Recycling Technology, Vol. 29, No. 2,

4 Xinxing Zhang, Dong Tian, Wei Zhang, Jian Zhu, and Canhui Lu expansion took place immediately and the foamed structure was achieved. The obtained foams were left at room temperature to cool down. Table 1. Compounding formulations used to prepare EPDM/GTR composite foams Ingredients (phr) Formulation code EPDM Devulcanised GTR Untreated GTR Zinc oxide Stearic acid CBS Blowing agent H Sulfur Characterisations The sol fractions of GTR before and after pan-milling were measured by the Soxhlet extraction method using toluene as a solvent. The samples were refluxed in toluene for 24 h and then dried in a vacuum oven at 60 C for 6 h. The weight of the samples before and after the treatment was recorded. The soluble part of the pan-milled GTR derived from Soxhlet extraction process was analysed by Fourier transform infrared (FTIR) spectroscopy and compared with a virgin natural rubber sample. The FTIR spectra were recorded on a Nicolet 560 spectrometer (USA). The spectra were recorded from 4000 to 400 cm -1 at a resolution of 4 cm -1 over 20 scans. Curing characteristics of EPDM/GTR compounds were analysed by a rubber curometer (R100E from Beijing Youshen Electronic Co., Ltd., China) at 160 C. Fluorescence microscopy (IX-71, Olympus, Japan) was used to investigate the dispersion state of the GTR in EPDM matrix. When subjected to ultraviolet light, the GTR rich phase appeared dark or black, whereas the EPDM matrix rich phase appeared light. Foaming rheology studies were carried out on a foam rheometer (GT- M2000F, Gotech, Taiwan, China), using the mold pressure of 7.6 MPa and the temperature of 160 C. Testing time was 1 min (for foaming pressure) and 15 min (for foaming elasticity) respectively. SEM (JEOL JSM-5600, Japan) studies were performed to characterise the following items: particle size of GTR before and after pan-milling, interfacial 84 Progress in Rubber, Plastics and Recycling Technology, Vol. 29, No. 2, 2013

5 Morphology, Foaming Rheology and Physical Properties of Ethylene-Propylene Diene Rubber/Ground Tyre Rubber (GTR) Composite Foams: Effect of Mechanochemical Devulcanisation of GTR adhesion between GTR and the rubber matrix and the cell morphology of the obtained composite foams. The samples were fractured in liquid nitrogen and gold coated before testing. The stress-strain properties of the composite foams were measured according to ASTM D specification by an Instron 5567 Universal Testing Machine at a crosshead speed of 100 mm min -1. At least five measurements for each sample were made in order to eliminate experimental error. All the tests were conducted at the room temperature of 23 C. The rebound resilience test was performed according to ASTM D using a Schob pendulum (TY2001). The samples were cut into cylinders with diameter of 2.8 cm, 1 cm in thickness. In the measurement, the pendulum was released from a horizontal position and struck the samples at a vertical point. The compression set was determined using specimens with diameter of 1 cm and a thickness of 1 cm. The test was performed according to ASTM D First we put the specimens between two boards, compressed 50% of the thickness and left them in ambient environment for 24 h. Then took the specimens out, placed them in room temperature for 1 h to measure the thickness. Compression set was calculated according to the equation: Compression set = (L 1 L 2 ) / L 1 100% Where: L 1 : the thickness before compression L 2 : the thickness after compression RESULTS AND DISCUSSION Morphological Development of GTR During Pan-Milling The morphological development of GTR during pan-milling was estimated by SEM observation. Figure 1a is the SEM micrograph of raw GTR powder without milling, which consists of some large particles with smooth surface. As shown in the particle size distribution diagram, the average diameter of the raw GTR is about 300 µm. Figure 1b is the SEM micrograph of GTR after 5 cycles of milling, in which the GTR powders began to deformation and ground into small irregular particles. Particles with rough surface were also obtained. Figure 1c is the SEM micrograph of GTR after 20 cycles of milling, the smooth surface of GTR was destroyed and fluffy powder was Progress in Rubber, Plastics and Recycling Technology, Vol. 29, No. 2,

6 Xinxing Zhang, Dong Tian, Wei Zhang, Jian Zhu, and Canhui Lu obtained due to the effects of strong shearing and squeesing stresses during pan-milling. The ultrafine GTR powder with a particle size of ~35 μm or less was obtained after 20 cycles of milling. Furthermore, rougher surface and larger specific surface area was acquired after milling, which could improve the binding ability and compatibility with the matrix rubber. Figure 1. Morphological development of GTR powders during pan-milling: (a) before milling; (b) after 5 cycles of milling; (c) after 20 cycles of milling 86 Progress in Rubber, Plastics and Recycling Technology, Vol. 29, No. 2, 2013

7 Morphology, Foaming Rheology and Physical Properties of Ethylene-Propylene Diene Rubber/Ground Tyre Rubber (GTR) Composite Foams: Effect of Mechanochemical Devulcanisation of GTR Sol Fraction Variation of GTR during Pan-Milling Sol fraction measurements were conducted to monitor breakage of the threedimensional crosslinked structure of GTR during pan-milling. It is well known that the crosslinked rubber vulcanisates cannot dissolve in any solvents. The sol fraction of the unprocessed GTR used in this study was found to be 8.6%, which consists of the plasticising agent such as aromatic oil. Figure 2 illustrates that the sol fraction of GTR increases with the increase of milling time. The highest sol fraction value of 27.7% was obtained after 20 cycles of milling. It could be attributed to the breakage of crosslinked networks in the vulcanised rubber during the pan-milling process where the rubber granulates were subjected to strong shearing and squeesing stresses. The breakage of the sulfur bonds is called devulcanisation, that is the reverse of the process of vulcanisation [23]. Figure 2. Effect of pan-milling on the sol fraction of GTR FTIR Analysis of the Sol Component of Pan-Milled GTR To ascertain the devulcanisation of GTR during pan-milling, the extracted sol components derived from GTR before and after 20 cycles of milling were recovered by evaporating the solvent. The sol component of unprocessed GTR in solid state was difficult to obtain, because it is mainly composed of aromatic oil. On the other hand, the solid rubber-like material was easy to obtain from the sol fraction of the GTR after 20 cycles of pan-milling. This Progress in Rubber, Plastics and Recycling Technology, Vol. 29, No. 2,

8 Xinxing Zhang, Dong Tian, Wei Zhang, Jian Zhu, and Canhui Lu phenomenon agrees well with the sol fraction measurements. FTIR analysis was employed to further verify the structure of the obtained solid material. The result is compared with the virgin natural rubber as shown in Figure 3. All the characteristic peaks of natural rubber can be found in the spectrum of the devulcanised GTR. The produce of soluble rubber components during panmilling effectively confirms the realisation of stress-induced mechanochemical devulcanisation of GTR. Figure 3. FTIR spectra of the virgin natural rubber and the soluble component of GTR after 20 cycles of pan-milling Curing Characteristics of EPDM/GTR Compounds The inclusion of reclaimed rubbers in a virgin rubber formulation typically negatively affects the curing rheology of the compounds. For example, the minimum torque was usually remarkably increased with the addition of reclaimed rubbers. This is undesirable since the minimum torque value represents an index of rubber processing. The lower the minimum torque is, the better the processability of the rubber compounds is. In order to evaluate the effect of mechanochemical devulcanisation on the curing characteristics of EPDM/reclaimed GTR compounds, the curing behavior was tested by a rubber curometer. Figure 4 shows typical curing rheographs of those samples. As can be seen, the 30 phr devulcanised GTR-filled EPDM compound exhibits a cure rheology similar to that of virgin EPDM compound. However, the unprocessed GTR dramatically affects the curing behavior of EPDM compounds by increasing the curing torque. The results indicate that the devulcanised GTR prepared through pan-milling is a suitable filler of EPDM compounds without the deterioration of their processability. 88 Progress in Rubber, Plastics and Recycling Technology, Vol. 29, No. 2, 2013

9 Morphology, Foaming Rheology and Physical Properties of Ethylene-Propylene Diene Rubber/Ground Tyre Rubber (GTR) Composite Foams: Effect of Mechanochemical Devulcanisation of GTR Figure 4. Typical curing rheographs of EPDM/GTR compounds: (a) pristine EPDM; (b) 30 wt% untreated GTR-filled EPDM compound; (c) 30 wt% devulcanised GTR (after 5 cycles of milling)-filled EPDM compound; (d) 30 wt% devulcanised GTR (after 20 cycles of milling)-filled EPDM compound Morphology of EPDM/GTR Blends The fluorescent images (at 400 times of magnification) of EPDM/GTR blends with the GTR content of 30 wt% are shown in Figure 5. GTR powder without mechanochemical pretreatment has a larger size and is poorly dispersed in the EPDM matrix. Some obvious agglomerates are observed in Figure 5a, which is attributed to its three-dimensional crosslinked structure and poor processing property. In the devulcanised GTR-filled sample with the same blend ratio, the particle size of GTR in the rubber matrix is sharply reduced to achieve excellent distribution (Figures 5b and c). In order to further investigate the dispersion and interfacial adhesion state of the EPDM/GTR blends, SEM studies were conducted. Figure 6 shows the fractured surface micrographs of EPDM/GTR blends at the GTR loading of 30 wt%. It is obvious that the dispersion of the rubber particles in the blend prepared from mechanochemically milled GTR is much better than that of the unprocessed GTR. The stress-induced devulcanisation of GTR during pan-milling was well confirmed by the sol fraction measurements and FTIR analysis. The devulcanised molecular chains of GTR facilitated its macromolecular entanglement with polymer matrix. Furthermore, it acquired rougher surface and larger specific surface area, which improved the binding ability and compatibility with the matrix rubber. As a result, better dispersion Progress in Rubber, Plastics and Recycling Technology, Vol. 29, No. 2,

10 Xinxing Zhang, Dong Tian, Wei Zhang, Jian Zhu, and Canhui Lu (a) (b) (c) Figure 5. Fluorescent micrographs of EPDM/GTR (100/30) blends prepared from untreated GTR (a) and devulcanised GTR [(b): after 5 cycles of milling; (c): after 20 cycles of milling], respectively (a) (b) (c) Figure 6. SEM micrographs of the fractured surface of EPDM/GTR (100/30) blends prepared from untreated GTR (a) and devulcanised GTR [(b): after 5 cycles of milling; (c): after 20 cycles of milling], respectively 90 Progress in Rubber, Plastics and Recycling Technology, Vol. 29, No. 2, 2013

11 Morphology, Foaming Rheology and Physical Properties of Ethylene-Propylene Diene Rubber/Ground Tyre Rubber (GTR) Composite Foams: Effect of Mechanochemical Devulcanisation of GTR and interfacial adhesion between GTR and EPDM matrix were realised. However, the GTR powder without mechanochemical pretreatment could not be well-mixed with EPDM matrix and as a result, there was obvious phase separation between GTR particles and EPDM matrix. In summary, the devulcanised GTR-filled EPDM blends were homogeneously dispersed due to the ultrafine pulverisation and partial devulcanisation through pan-milling. Foaming Rheological Studies In order to investigate the foamability of EPDM/GTR blends, foaming rheological studies were performed. Figure 7a shows foaming pressure of EPDM/GTR (100/40) blends prepared from unprocessed and devulcanised GTR. The foaming pressure of the devulcanised GTR-filled sample is much higher than Figure 7. Foaming pressure (a) and foaming elasticity (b) curves of EPDM/GTR (100/40) blends prepared from untreated GTR and devulcanised GTR (after 20 cycles of milling), respectively Progress in Rubber, Plastics and Recycling Technology, Vol. 29, No. 2,

12 Xinxing Zhang, Dong Tian, Wei Zhang, Jian Zhu, and Canhui Lu that of the unprocessed GTR, indicating the foamability was enhanced after mechanochemical pretreatment. According to Dutta et al. [24], the weak interfacial adhesion might lead to the deterioration of foamability. The poor adhesion between untreated GTR particles and EPDM matrix resulted in the formation of some defects during foaming process, which was easier to cause gas escape. Therefore, bubbles were difficult to grow in the untreated GTR-filled sample. On the other hand, the ultrafine pulverisation and partial devulcanisation of GTR were achieved after pan-milling. The enhancement of interfacial adhesion between devulcanised GTR and EPDM matrix could help to maintain higher and more stable foaming pressure. That was why the foamability of the composites prepared from mechanochemically pretreated GTR was improved. It is worth noting that we failed to prepare the foamed sample of the 40 wt% untreated GTR-filled EPDM composite foam. As indicated in Figure 7a, its foaming pressure is as low as 300 kg/cm 2, thus the physical properties of this composite foam could not be given out. The effect of mechanochemical pretreatment on the foaming elasticity of EPDM/ GTR (100/40) blends is shown in Figure 7b. The foaming elasticity decreases at the initial stage of foaming process due to the softening behavior at elevated temperature. After that the foaming elasticity increases rapidly owing to the bubble growth, which is a typical behavior of foaming process. It is worth noting that the foaming elasticity of the devulcanised GTR-filled sample is slight lower than that of the unprocessed GTR. It could be attributed to the decreased crosslink density of the devulcanised GTR-filled EPDM sample. It is known that at a given blowing agent concentration, the higher the crosslink density the higher the foaming elasticity [24]. Pan-milling contributed to the partial devulcanisation of GTR, thereby resulting in the decrease of crosslink density of the GTR-filled EPDM vulcanisates. As a result, the foaming elasticity decreases after the mechanochemical pretreatment of GTR. The relatively low elasticity is beneficial to bubbles growth. Cell Morphology of EPDM/GTR Composite Foams Figure 8 reveals the cell morphology of EPDM/GTR (100/30) composite foams prepared from unprocessed and mechanochemically devulcanised GTR. The results indicate that compared with the foam prepared from the GTR without pretreatment, the foam prepared from devulcanised GTR has more uniform closed-cell structures and thinner cell walls. Some larger cells (>1000 μm) and structure defects are observed in the untreated GTR-filled sample (Figure 8a). In the untreated GTR-filled EPDM blends, the weak interfacial adhesion between GTR particles and EPDM matrix led to the cell-to-cell gas diffusion. As a result, the obtained foam has huge holes with interconnected cell structures. 92 Progress in Rubber, Plastics and Recycling Technology, Vol. 29, No. 2, 2013

13 Morphology, Foaming Rheology and Physical Properties of Ethylene-Propylene Diene Rubber/Ground Tyre Rubber (GTR) Composite Foams: Effect of Mechanochemical Devulcanisation of GTR (a) (b) (c) Figure 8. SEM micrographs of EPDM/GTR (100/30) composite foams prepared from untreated GTR (a) and devulcanised GTR [(b): after 5 cycles of milling; (c): after 20 cycles of milling], respectively On the contrary, the mechanochemical pretreatment of GTR through panmilling realised the ultrafine pulverisation and partial devulcanisation of GTR simultaneously, which contributed to the enhancement of the interfacial adhesion between GTR particles and EPDM matrix. The strength of cell walls was high enough to prevent rupture due to the strong interfacial adhesion. Therefore, the uniform cellular structure with smaller average cell size was obtained through the mechanochemical pretreatment of GTR (Figures 8b and c). Stress-Strain Properties of EPDM/GTR Composite Foams The devulcanised GTR-filled EPDM composite foams with enhanced foamability and uniform cell morphology are expected to show excellent mechanical properties. The stress-strain curves shown in Figure 9 indicate that the tensile strength and elongation at break of the EPDM/GTR (100/20) composite foams Progress in Rubber, Plastics and Recycling Technology, Vol. 29, No. 2,

14 Xinxing Zhang, Dong Tian, Wei Zhang, Jian Zhu, and Canhui Lu increase significantly through the mechanochemical pretreatment of GTR, demonstrating the compatibility between GTR particles and EPDM matrix is improved. The untreated GTR-filled composites have 0.27 MPa of tensile strength and 290.7% of elongation at break, respectively. After pan-milling for 20 cycles, the tensile strength and elongation at break of the devulcanised GTR-filled EPDM composite foam significantly increase to 0.72 MPa and 748.2%, respectively. Thus, the tensile strength was enhanced by 166.7% and the elongation at break was more than 2.5 times that of the untreated sample. The enhancement in mechanical properties could be attributed to the stress-induced mechanochemical devulcanisation of GTR and enhanced interfacial interaction between GTR particles and EPDM matrix. The results demonstrate that mechanochemical pretreatment of GTR by pan-milling is an efficient and industrially scalable method for the preparation of EPDM/GTR composite foams with enhanced mechanical properties, thereby providing a novel strategy for rubber recycling. Figure 9. Stress-strain curves of EPDM/GTR (100/20) composite foams prepared from untreated GTR and devulcanised GTR after various cycles of pan-milling Rebound Resilience and Compression Set of EPDM/GTR Composite Foams Since microcellular EPDM foams are usually applied in the sealing materials fields, physical properties such as rebound resilience and compression set are also important parameters. Figures 10a and b show the rebound resilience 94 Progress in Rubber, Plastics and Recycling Technology, Vol. 29, No. 2, 2013

15 Morphology, Foaming Rheology and Physical Properties of Ethylene-Propylene Diene Rubber/Ground Tyre Rubber (GTR) Composite Foams: Effect of Mechanochemical Devulcanisation of GTR Figure 10. Effect of mechanochemical pretreatment of GTR on the rebound resistance (a) and compression set (b) of EPDM/GTR composite foams Progress in Rubber, Plastics and Recycling Technology, Vol. 29, No. 2,

16 Xinxing Zhang, Dong Tian, Wei Zhang, Jian Zhu, and Canhui Lu and compression set performance of EPDM/GTR composite foams with various GTR content, respectively. It is interesting to note that the rebound resilience of the composite foams increases with the increase of devulcanised GTR content up to 30 wt% while the rebound resilience decreases with the increase of untreated GTR content. The ultrafine pulverisation and partial devulcanisation of GTR was realised through pan-milling. As a result, the dispersion and interfacial adhesion between GTR particles and EPDM matrix were improved. Stress could be easily transferred from EPDM matrix to GTR particles under pressure. Thus, the elastic GTR particle functioned like a resilient sphere and resulted in the enhancement of rebound resilience of the composite foams. However, the poor adhesion between untreated GTR particles and EPDM matrix hindered the stress transfered between them. Therefore, the aggregates of untreated GTR particles are just like defects and lead to the deterioration of the rebound resilience properties. Similar phenomenon is found in the compression set results. The mechanochemical pretreatment of GTR significantly decreases the compression set of the EPDM/ GTR composite foams. CONCLUSIONS This study examined the effect of stress-induced mechanochemical devulcanisation of GTR on the morphology, foaming rheology and physical properties of EPDM/GTR composite foams. The following conclusions could be drawn: 1. The partial devulcanisation and ultrafine pulverisation of GTR were realised simultaneously during pan-milling. 2. The dispersion and interfacial adhesion between GTR particles and EPDM matrix were significantly improved through the mechanochemical pretreatment of GTR, as indicated by the fluorescence microscopy and SEM studies. 3. The foamability of EPDM/GTR blends was greatly enhanced through the mechanochemical pretreatment of GTR. As a result, more uniform and well-defined cell morphology was obtained. 4. The physical properties of EPDM/GTR composite foams were notably improved through the devulcanisation of GTR. Moreover, the devulcanised GTR could be used as a low-cost but effective functional filler to enhance the rebound resistance of EPDM foams. 96 Progress in Rubber, Plastics and Recycling Technology, Vol. 29, No. 2, 2013

17 Morphology, Foaming Rheology and Physical Properties of Ethylene-Propylene Diene Rubber/Ground Tyre Rubber (GTR) Composite Foams: Effect of Mechanochemical Devulcanisation of GTR ACKNOWLEDGMENTS The authors would like to thank the National Science Foundation of China ( and ) and National High Technology Research and Development Program (863 Program, SS2012AA062613) for financial support. REFERENCE 1. Yamsaengsung W., and Sombatsompop N., Composites: Part B, 40 (2009) Lawindy A.M.Y.E., El-Kade K.M.A., Mahmoud W.E., and Hassan H.H., Polymer International, 51 (2002) Macsiniuc A., Rochette A., and Rodrigue D., Progress in Rubber, Plastics and Recycling Technology, 26 (2010) Premachandra J.K., Edirisinghe D.G., and De Silva M.I.A., Progress in Rubber, Plastics and Recycling Technology, 27 (2011) Li H., Zhang X.X., Lu C.H., and Liang M., Progress in Rubber, Plastics and Recycling Technology, 28 (2012) Zhang X.X., Zhu X.Q., Liang M., and Lu C.H., Journal of Applied Polymer Science, 114 (2009) Zhang Z.X., Zhang S.L., Lee S.H., Kang D.J., Bang D., and Kim J.K., Materials Letters, 62 (2008) Xin Z.X., Zhang Z.X., Pal K., Byeon J.U., Lee S.H., and Kim J.K., Materials and Design, 31 (2010) Zhang Z.X., Fan J.L., Pal K., Kim J.K., and Xin Z.X., Journal of Vinyl and Additive Technology, 17 (2011) Zhang Z.X., Li L., Xin Z.X., and Kim J.K., Cellular Polymers, 30 (2011) Zhang X.X., Lu C.H., and Liang M., Journal of Applied Polymer Science, 103 (2007) Zhang X.X., Lu C.H., and Liang M., Journal of Polymer Research, 16 (2009) Zhang X.X., Lu C.H., and Liang M., Journal of Applied Polymer Science, 122 (2011) Zhang X.X., Chen C., and Lu C.H., Progress in Rubber, Plastics and Recycling Technology, 28 (2012) Zhang X., Zhou Z., He X., Li J., and Lu C., International Polymer Processing, 27 (2012) Progress in Rubber, Plastics and Recycling Technology, Vol. 29, No. 2,

18 Xinxing Zhang, Dong Tian, Wei Zhang, Jian Zhu, and Canhui Lu 16. Zhang X.X., Lu C.H., and Liang M., Plastics Rubber and Composites, 36 (2007) Zhang X.X., Lu C.H., Zheng Q.Y., and Liang M., Polymers for Advanced Technologies, 22 (2011) Zhang X.X., Li H., Tian D., He X., and Lu C.H., Materials Research Innovations, 16 (2012) Edirisinghe D.G., De Silva M.I.A., and Premachandra J.K., Progress in Rubber, Plastics and Recycling Technology, 27 (2011) Lee S.H., Hwang S.H., Kontopoulou M., Sridhar V., Zhang Z.X., Xu D., and Kim J.K., Journal of Applied Polymer Science, 112 (2009) Bani A., Polacco G., and Gallone G., Journal of Applied Polymer Science, 120 (2011) Yazdani H., Karrabi M., Ghasmi I., Azizi H., and Bakhshandeh G.R., Journal of Vinyl and Additive Technology, 17 (2011) Bilgili E., Arastoopour H., and Bernstein B., Powder Technology, 115 (2001) Dutta S. and De S.K., Polymer, 37 (1996) Progress in Rubber, Plastics and Recycling Technology, Vol. 29, No. 2, 2013