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Mechanical and tribological properties of natural rubber reinforced with carbon blacks and Al2O3 nanoparticles
Materials and Design 49 (2013) 336–346
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Mechanical and tribological properties of natural rubber reinforced with carbon blacks and Al2O3 nanoparticles Ji-Fang Fu a,⇑, Wen-Qi Yu a,b, Xing Dong a, Li-Ya Chen a, Hai-Sen Jia a, Li-Yi Shi a,b, Qing-Dong Zhong a, Wei Deng a a b
Nano-Science & Technology Research Center, School of Materials Science and Engineering, Shanghai University, 99 Shanghai Road, Shanghai 200444, PR China College of Science, Shanghai University, Shanghai 200444, PR China
a r t i c l e
i n f o
Article history: Received 17 November 2012 Accepted 19 January 2013 Available online 4 February 2013 Keywords: Al2O3 nanoparticles Natural rubber Tire tread Nanocomposites Abrasion resistance
a b s t r a c t A novel Natural rubber (NR) nanocomposites filled with carbon blacks (CBs) and organically modified Al2O3 nanoparticles were prepared and the synergistic effect of CBs and Al2O3 nanoparticles on the properties of the nanocomposites was investigated. The microstructure, tensile strength, elongation at break, modulus, tearing strength and abrasion resistance of the NR nanocomposites were evaluated. The nanoAl2O3 played an important reinforcement effect on the mechanical properties, abrasion resistance and thermal stability of the nanocomposites when it was introduced combined with CBs. Especially, the NR nanocomposites containing hyperbranched polyester modified nano-Al2O3 and CBs gave the best abrasion resistance and thermal stability. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Natural rubber (NR) is widely used in many fields, due to its distinct properties such as high elongation and excellent resilience. To obtain improvement in properties such as tear strength, abrasion resistance, stiffness and hardness, NR was generally reinforced with carbon blacks (CBs) and silica [1–3]. Over the last two decades, research has focused on development of other reinforcing agents to be complete or partial replacement of CBs in rubber composites. Nanomaterials such as clay [4–8], SiO2 [9], CaCO3 [10] and CNTs [11] were used to reinforce rubber with high performance and special functionality [12], including high mechanical properties, abrasion resistance, erosion resistance and multifunctional properties [5–8]. However, it has been remaining challenges to control the microstructure of the resulting materials, especially the degree of fillers dispersion, which often restrict the fabrication of rubber nanocomposites. Surfactants, coupling agent, and surface-grafting modification were usually employed to improve the compatibility between nanoparticles and NR [13–15]. A coupling agent has two functionally active end groups, an alkoxy group can react with the hydroxyl group on the surface of nanoparticles and another functional groups are compatible with rubber, leading
⇑ Corresponding author. Address: Nano-Science & Technology Research Center, Shanghai University, P.O. Box 111, No. 99 Shanghai Road, Shanghai 200444, PR China. Tel.: +86 21 66133800; fax: +86 21 66135215. E-mail addresses: [email protected], [email protected] (J.-F. Fu). 0261-3069/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.matdes.2013.01.033
to strong physical or chemical linkage between nanoparticles and rubber molecules [13–15]. Macromolecular coupling agent and surface-grafting modification are more efficient, because the organic coating layer grafted on the nanoparticles by macromolecular coupling agent brings about more strong electrostatic and steric stabilization [13]. The surface functionalization of nanomaterials by grafting of polymer or hyperbranched polymer is expected to play important roles in the chemical modification of nanostructural materials [16–24]. A great interest has been focused on ‘‘hyperbranched polymers’’, which have fundamental building blocks, controllable molecular weight, controllable branching and versatility in modification of terminal groups [20]. Many researches have been reported that the surface treatment of nanomaterials by grafting of hyperbranched polymer, such as carbon nanotubes [25], silica [21,26], and CBs [24,27,28]. In our previous work [29], Al2O3 nanoparticles were singly used to reinforce NR vulcanizates, and the resulting nanocomposites exhibited significantly enhanced aging resistances, acid and alkaline resistances. This work describes a novel tire tread nanocomposites based on NR filled with CBs and Al2O3 nanoparticles. Here the Al2O3 nanoparticles were grafted with hyperbranched aliphatic polyester (HBP) via a melt polycondensation of 2,2bis(hydroxymethyl) propionic acid (bis-MPA) and treated with silanes respectively. The nanoparticles grafted with hyperbranched aliphatic polyester and silanes were used to replace partial of CBs to reinforce NR. The reinforcement effect on the mechanical properties, abrasion properties, and thermal stability of the nanocomposites was investigated.
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2. Experimental details 2.1. materials The a-Al2O3 nanoparticles with mean diameter of 100 nm were provided by Dalian Luming Co. Ltd., Dalian, China. c-methacryloxypropyl trimethoxy silane (KH570) and vinyl triethoxysilane (A151) were purchased from Nanjing Shuguang Co. Ltd., China. c-aminopropyltriethoxysilane (APTES) was obtained from Shanghai Yaohua Chemical Plant and was used without further purification. 2,2bis(hydroxymethyl)propionic acid (bis-MPA) was analytical grade, and used as received from Quzhou Mingfeng Chemical CO., Ltd. Acetone, ammonia solution, ethanol, p-toluenesulfonic (p-TSA) and N,N-Dimethylformamide (DMF) were all analytical grade and used as received from Sinopharm Chemical Rragent Co., Ltd. Natural rubber SCR-5 was purchased from Hainan Nongken groups Co. Ltd. CBs (N220) were commercial products and purchased from Shanghai Cabot Speciality Chemicals Co., Ltd. Zinc oxide (ZnO) (activator), Stearic acid (SA), sulfur (S) and 2-(4-Morpholinothio)-benzothiazole (NOBS) (accelerator) were commercial products. Poly (1,2-dihydro-2,2,4-trimethyl-quinoline) (antioxidant RD), N-1, 3-dimethylbutyl-N0 -phenyl-p-phenylenediamine (antioxidant 4020) were commercial products. The used basic recipes were as follows (in parts): natural rubber (100 phr), CBs (N220) (45 phr), ZnO (5.0 phr), SA (2.0 phr), RD (1.5 phr), 4020(1.5 phr), NOBS (1.6 phr), S (1.6 phr). And Al2O3 nanoparticles are various amounts (1, 3, 5, 7, 10 phr).
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The mixture was decentralized ultrasonically for 30 min and then agitated at 80 °C for 4 h. Next the mixture was heated to 120 °C and stirred for additional 2 h. After ethanol was completely removed, the product was triturated in mortar, and then agitated at 500 rpm at room temperature for 12 h to get tiny particles. Then the sample was dried in a vacuum at 120 °C. Then the AB2 monomer, bis-MPA, was melt polycondensation according to the method reported previously [28]. The details were as follows: 10 g amino-Al2O3, 2 g bis-MPA and 0.5 g p-TSA were triturated in mortar to achieve a well proportioned mixture. The mixture were added into a three-necked flask with a reflux condenser and agitated at 200 °C for 1 h in argon flow. After that the mixture cooled to room temperature, the mixture was dispersed ultrasonically for 30 min and washed throughout with DMF and next with deionized water until no polymer was found in the filtrate, and then dried in a furnace at 110 °C. The treated Al2O3 is denoted as HBP-Al2O3. 2.3. Sample preparation Compounding was performed in an open two-roll mixing mill (laboratory size) and the compounded rubber was left overnight before vulcanization. The vulcanization was carried out in hydraulic press at 150 °C under a pressure of 15 MPa for the optimum cure time (T90) to get square sheet of 13 cm 12 cm 0.2 cm sizes.
2.2. Surface treatment of the nanoparticles
2.4. Characterization
The Al2O3 nanoparticles were dried in an oven at 110 °C under vacuum for 24 h prior to surface treatment. The Al2O3 nanoparticles were treated with KH570 and A151 according to our previous work [30]. The treated Al2O3 is denoted as KH570-Al2O3 and A151Al2O3 respectively. The mechanism of the introduction of amino and hyperbranched polymer onto Al2O3 nanoparticles surface was shown in Scheme 1. The introduction of amino onto Al2O3 nanoparticles surface was achieved by the treatment of surface hydroxyl groups on alumina with APTES. Then the AB2 monomer, bis-MPA, was melt polycondensated cored with the amino-alumina with surface active site, amino groups. Firstly, the c-aminopropyl alumina (amino-alumina) was prepared according to the method reported previously [18]. The details were as follows: 10.0 g of Al2O3 nanoparticles, 2.0 mL APTES, 2.0 mL ammonia solution, and 40 mL ethanol were added into a 500 ml three-necked round-bottom flask with a reflux condenser.
The Fourier transform infrared spectrum (FTIR) of the particles was performed on a Nicolet 3200 FTIR instrument (Thermo Electron, Waltham, MA) in the range from 4000 to 400 cm1. The morphology of the nanoparticles was observed using a JEOL 2000FX transmission electron microscope (TEM, JEOL Ltd., Japan) at an accelerating voltage of 200 kV. Morphology of fracture and abrasion surfaces of samples were examined with a JSM-6700F scanning electron microscope (SEM, JEOL Ltd., Japan) at an activation voltage of 15.0 kV after coated with thin layers of gold. Cure characteristics were determined by a MDR 2000 rheometer (Shanghai D&G Machinery Equipment Co., Ltd., China) at 150 °C according to ISO 3417: 2008 [31].Tensile strength was measured according to ISO 37: 2005 [32] and tear strength was measured according to ISO 34-1: 2004 [33] (an unnicked 90° angle
Scheme 1. Preparation procedure for the HBP-Al2O3 nanoparticles.
Fig. 1. FTIR spectrum of 100 nm a-Al2O3 (a) and treated with KH550 (b) and HBP (c).
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Fig. 2. SEM and TEM of Al2O3 (a and c) and Al2O3-HBP (b and d).
Fig. 3. Cure characteristic of nano-Al2O3/CBs/NR: (a) T10, (b) T90, (c) curing rate and (d) crosslink densities.
specimen) both by Reger 3010 Universal Testing Machine (Shenzhen Reger Instrument Co., Ltd., China) at a speed of 500 mm/min. The average value of at least three measurements for each sample was recorded.
Abrasion experiments were performed in an air atmosphere at room temperature using a WML-76 Akelon abrasion testing machine (Wuxi Liyuan Electronic & Chemical Equipments Co., Ltd., China) according to GB/T 1689–1998 [34]. The rotation speed of
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the sample wheel and the empery wheel were 76 and 33 rpm, respectively, and the angle between the shafts of the two related wheels was 15°. The applied normal load was 2.72 kg (26.7 N). Thermal stability was performed from the ambient temperature to 800 °C at the heating rate of 10 °C/min in an air atmosphere using A NetzchSta449c thermogravimetric analyzer (NETZSCH, Bavaria, Germany). 3. Results and discussion 3.1. Characteristics of Al2O3 nanoparticles treated by HBP Fig. 1 shows the infrared spectrum of untreated Al2O3 (a), amino-Al2O3 (b), and HBP-Al2O3 (c). The infrared spectrum of amino-Al2O3 show some new absorption at 2931.95 cm1 ascribing to C@H stretching vibration, and 1139.54 cm1 ascribing to CAN stretching vibration, which are characteristic of APTES [25– 28]. This suggests that amino group is introduced onto the Al2O3 surface. The infrared spectrum of HAPE-Al2O3 show new absorption peak at 1734.72 cm1 ascribing to C@O of ACOOA stretching vibration [25–28], which is characteristic of bis-MPA. This suggests that hyperbranched polymer is successfully grafted onto the amino-Al2O3 nanoparticles surface. Fig. 2 shows the SEM and TEM of untreated Al2O3 and HBPAl2O3 nanoparticles. The large bulks in Fig. 2a and c indicate that the untreated Al2O3 nanoparticles were apparently agglomerated, which presents a powerful contrast to the monodisperse modified nanoparticles in Fig. 2b. The thin film surrounding nanoparticles in Fig. 2d indicates that HBP are successfully grafted onto nanoparticles and HBP macromolecules have penetrated into the agglomerated nanoparticles. Those agglomerated particles were separated by the strong inorganic–organic interactions and formed a relatively uniform distribution.
Fig. 5. Elongation at break of nano-Al2O3/CBs/NR with different nano-Al2O3 loading.
3.2. Curing behavior Fig. 3 shows the cure characteristic of NR nanocomposites filled with CBs and Al2O3 nanoparticles. As shown in Fig. 3a and b, the optimum cure time (T90) and scorch time (T10) of the nanocomposites decrease with the addition of fillers and change little in range of 1–10 phr filler content, which indicates that the nanoparticles have accelerated the cure process of the NR nanocomposites to some extent. On the whole, the scorch time change slightly and the cure time decrease obviously, which indicate the addition of nanoparticles can reduce the product development cycles and
Fig. 6. Modulus at 100% elongation and modulus at 300% elongation of nano-Al2O3/ CBs/NR with different nano-Al2O3 loading.
Fig. 7. Hardness of nano-Al2O3/CBs/NR filled with different loading content of different modified Al2O3.
Fig. 4. Tensile strength of nano-Al2O3/CBs/NR with different nano-Al2O3 loading.
make processing safe. At the same filler content, the T90 and T10 of the NR nanocomposites are as follows: HBP-Al2O3/CBs/ NR > A151-Al2O3/CBs/NR > KH570-Al2O3/CBs/NR. As shown in Fig. 3c, with the addition of particles, the values of 1/(T90–T10), represent cure rate of NR nanocomposites, increase with increasing the filler content. This may be attributed to the high surface-to-vol-
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Fig. 8. SEM of the tensile fracture surface of unfilled NR (a) and nano-Al2O3/CBs/NR with 3 phr Al2O3 loading at low magnification, (b) A151-Al2O3/CBs/NR, (c) KH570-Al2O3/ CBs/NR and (d) HBP-Al2O3/CBs/NR.
ume ratio of nanoparticles and the increasing filler-rubber interaction, which result in an increase in activity and cure rate. At the same time, this may be also due to the relatively higher thermal conductivity of nanoparticles, which result in an increase in thermal conductivity of NR nanocomposites [35]. The value of MHML can be taken as a measurement of crosslinking density [4]. Fig. 3d suggests that HBP-Al2O3/CBs/NR displays higher crosslink density than A151-Al2O3/NR and KH570-Al2O3/NR, which is contrary to the cure rate of NR nanocomposites. This was attributed to the carboxyl groups, hydroxyl group and special structure of hyperbranched polyester, which participated in rubber’s crosslinking and resulted in more complete curing. This was in accordance with De and Rajeev’s work [36].
3.3. Mechanical properties As shown in Fig. 4, the tensile strength of the NR nanocomposites filled with Al2O3 nanoparticles and CBs is improved. The tensile strength of the NR nanocomposites increases initially, attains a maximum value, and then decreases with increasing the filler content. From Fig. 4, the optimum filler content is 1–3 phr, a further addition will decrease the tensile strength slightly. On the whole, KH570-Al2O3/CBs/NR and A151-Al2O3/CBs/NR show relatively higher tensile strength, HBP-Al2O3/CBs/NR shows relatively lower tensile strength. As presented in Fig. 5, the elongation at break of nano-Al2O3/ CBs/NR is improved. The elongation increases firstly and then de-
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creases with increasing Al2O3 filler content in range of 0–10 phr. As a whole, KH570-Al2O3/CBs/NR shows relatively higher elongation at break than A151-Al2O3/CBs/NR, which are in accordance with its relatively higher strength. High tensile strength is often prerequisites for NR to gain high elongation at break. Moreover, macromolecules’ movement are limited by nanoparticles [37], and more macromolecules twist around each other and are curled with increasing filler content, the curled macromolecules will uncurl under stretching and result in an increase in elongation at break [4,8]. In this case, effects of nanoparticles on elongation at break are as follows: the modified nanoparticles not only enhance the tensile strength and interfacial bonding along with good dispersion [10], but restrict the macromolecule chains’ movement in the meantime, and both which result improvement in elongation at break under stretching. Fig. 6 presents modulus at 100% and modulus at 300% of the NR nanocomposites filled with CBs and Al2O3 nanoparticles. With the addition of nanoparticles, the modulus of the NR nanocomposites containing nano-Al2O3 increase with increasing the filler content. This was may be due to the restriction of molecular chain mobility or an increase in the cross-kink density [9]. The significant improvement in modulus is also ascribed to the addition of rigid nano-Al2O3 and their addition to a relatively soft matrix will result in the development of local stretching in the matrix, thus the nanocomposites will respond with a higher strength when deformed.
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100% modulus is followed: KH570-Al2O3/CBs/NR > A151-Al2O3/ NR > HBP-Al2O3/CBs/NR. At the same filler content, the 300% modulus of A151-Al2O3/CBs/NR and HBP-Al2O3/CBs/NR are higher than that of KH570-Al2O3/CBs/NR. These indicate the addition of nanoparticles in natural rubber is beneficial to improving rigidity and resistance to extensional deformation of the NR nanocomposites. A151 and hyperbranched polyester grafting modification onto nanoparticles are more beneficial to improving stiffness and resistance to extensional deformation, and KH570 modification is conducive to resilience. As shown in Fig. 7, the hardness of NR vulcanizates filled with CBs and nano-Al2O3 is improved and increase with increasing the filler content. At the same filler content, the hardness shows a same trend as 300% modulus, A151-Al2O3/CBs/NR shows the highest hardness, and then followed with HBP-Al2O3/CBs/NR and KH570-Al2O3/CBs/NR, respectively. The improvement in hardness is attributed to the effect factors that lead to an increase in polymer chain rigidity and crosslink density or interaction [9]. The modified nanoparticles penetrated through the void in rubber matrix and interacted with rubber matrix, acting as physical cross-linking points and resulting in an increase in crosslink densities [29,37]. The SEM taken from the tensile fracture surface morphology of nano-Al2O3/CBs/NR nanocomposites is shown in Fig. 8. The unfilled NR vulcanizates exhibit smooth morphology with river-like fracture feature surface at low magnification; however the
Fig. 9. SEM of the tensile fracture surface of unfilled NR (a and b) and nano-Al2O3/CBs/NR with 3 phr nano-Al2O3 loading at high magnification: (c and d) A151-Al2O3/CBs/NR and (e and f) HBP-Al2O3/CBs/NR.
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Fig. 10. Tearing strength of unfilled NR and nano-Al2O3/CBs/NR with different loading content of different modified Al2O3.
and some cracks with wide space between cracks. However, the fracture surface of nano-Al2O3/CBs/NR presents a more complicated and rougher morphology without particles pulled-out or decohesion. Moreover, the fracture surface is full of hierarchical structure and much regulated micro-crack with minute grain, which indicates that much energy is absorbed under tearing process, resulting in an improvement in tearing strength. As shown in Fig. 12, a large quantity of individual particles are well dispersed and embedded in the NR matrix, and the interface between the particles and matrix are obscure, which indicate that the compatibility between the particles and matrix are enhanced. The improvement in properties depends on the reinforcement of filler in the rubber matrix. Combination of modified nanoAl2O3 and CBs strongly affects the mechanical properties of Al2O3/CBs/NR. The enhancement in the mechanical properties is attributed to the uniform dispersion of fillers in the NR matrix and good polymer-filler interaction [5–8].
3.4. Tribological behaviors nano-Al2O3/CBs/NR nanocomposites show a more complicated and rougher morphology with crack and massive plastic deformation, suggesting a reinforcement behavior of nano-Al2O3/CBs. KH570Al2O3/CBs/NR shows a more rougher morphology, which indicates its relatively higher tensile properties. In order to investigate the nanoparticles dispersion in Al2O3/CBs/NR nanocomposite, the SEM images were further magnified in Fig. 9. It can be seen that a large quantity of individual particles are well dispersed and embedded in NR matrix without agglomeration. Fig. 10 gives the tearing strength of the NR nanocomposites. With the addition of Al2O3 nanoparticles, the tearing strength of NR nanocomposites is improved. At 7 phr filler content, A151Al2O3/CBs/NR, KH570-Al2O3/CBs/NR, HBP-Al2O3/CBs/NR show optimum values of 145.65, 148.50 and 146.93 MPa, respectively. The tearing strength of the three systems is close and shows a same trend with increasing the filler content. The tearing behavior of nano-Al2O3/CBs/NR can be explained in term of the SEM morphology taken from the tearing fracture surface. As shown in Fig. 11, the unfilled NR shows a clear surface
As shown in Fig. 13a, with the addition of nanoparticles, the abrasive wear resistance of Al2O3/CBs/NR vulcanizates is improved. This may be attributed to the addition of nano-Al2O3, which reinforce the rubber matrix and can hinder the deformation of the matrix during abrasion process. At the same filler content, the abrasion loss of HBP-Al2O3/CBs/NR is the lowest; this indicates HBP-Al2O3/CBs/NR shows the highest abrasion resistance. At 7 phr filler content or below, KH570-Al2O3/CBs/NR shows lower abrasion loss and higher abrasion resistance than A151-Al2O3/ CBs/NR, which is in accordance with its higher comprehensive mechanical properties such as high tearing strength, 100% modulus and tensile strength. This indicates that higher comprehensive mechanical properties are beneficial to improvement in wear resistance, especially high tearing strength and 100% modulus. The abrasion resistance of NR nanocomposites depends on its strength, resilience, fatigability and frictional behavior [9,12,38]. In general, the abrasion resistance of rubber increases with the increase in tensile strength and tearing strength, especially abrading on rough
Fig. 11. SEM of the tearing fracture surface of nano-Al2O3/CBs/NR containing 7 phr Al2O3: (a) NR, (b) KH570-Al2O3/CBs/NR, (c) A151-Al2O3/CBs/NR and (d) HBP-Al2O3/CBs/NR.
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Fig. 12. SEM of the tearing fracture surface of nano-Al2O3/CBs/NR containing 7 phr Al2O3 at high magnification: (a) NR, (b) KH570-Al2O3/CBs/NR, (c) A151-Al2O3/CBs/NR and (d) HBP-Al2O3/CBs/NR.
Scheme 2. Microstructure development of NR filled with HBP-Al2O3 and CBs.
Fig. 13. Abrasion loss of nano-Al2O3/CBs/NR (a) and nano-Al2O3 /NR (b).
surface. NR nanocomposites filled with HBP-Al2O3 and CBs exhibit 20–46% improvement in the abrasion resistance compared with
unfilled NR. This is as opposed to HBP-Al2O3-filled NR which exhibit the lowest abrasion resistance among three different treated nano-Al2O3 singly filled NR systems without CBs. It can be seen in Fig. 13b the NR containing single HBP-Al2O3 shows the highest abrasion loss. For comparison and investigating the synergy effect on the abrasion resistance of NR filled with CBs and nanoparticles, the NR is singly reinforced with nano-Al2O3. Based on the analysis above, the highest abrasion resistance of HBP-Al2O3/CBs/NR could be attributed to the synergistic effect of HBP-Al2O3 and CBs. The structure of NR filled with HBP-Al2O3 and CBs can be illustrated in Scheme 2. CBs with high activity are interacted with rubber and periphery groups of hyperbranched polyester grafted on nanoparticles through chemical and physical reaction. Moreover, hyper-
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Fig. 14. SEM of the worn surface of nano-Al2O3/CBs/NR with different Al2O3 loading: NR (a and a1), KH570-Al2O3/CBs/NR (b and b1), A151-Al2O3/CBs/NR (c and c1) and HBPAl2O3/CBs/NR (d and d1).
branched polyester and rubber macromolecule twine each other, which result in an increase in cross-linking densities and bonded rubber content as well as the filler-rubber interaction. In addition, hyperbranched polyester grafted on nanoparticles can improve nanoparticles dispersion in matrix and compatibility with NR matrix. In all, the hyperbranched grafted nanoparticles and CBs show synergy in Al2O3/CBs/NR, thus, the abrasion resistance, tearing strength, hardness and modulus are improved. SEM photomicrographs of the worn surface of NR and Al2O3/ CBs/NR at different magnifications are shown in Fig. 14. On the worn surfaces of NR in Fig. 14a, parallel ploughed furrows, the so-called schallamach-type waves [38] appear with some fragments, cluster from debris and deep sharpened scratch, which are produced in the abrading direction. In its magnified SEM in
Fig. 14a1, some pull-off fragments, fatigue-induced debris and spherical particles can be observed clearly. These suggest that high abrasion loss and low abrasion resistance are found in fact. On the worn surfaces of Al2O3/CBs/NR, the schallamach type waves and debris deposition are still characteristic, but no deep sharpened scratch can be recognized. Moreover, on KH570-Al2O3/CBs/NR in Fig. 14b, the depths of ploughed furrows, debris and fragments decrease with the addition of nanoparticles. A151-Al2O3/CBs/NR shows fine and grained schallamach type waves with shallow ploughed furrows in Fig. 14c. With the addition of A151-Al2O3, the space between the neighboring waves and the depths of ploughed furrows are reduced. Its magnified SEM in Fig. 14c1 exhibits less debris and smooth surface. On the worn surface of HBP-Al2O3/CBs/NR in Fig. 14d, the depths of ploughed furrows de-
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(3) The particles are uniformly dispersed in NR matrix. (4) Compared with neat NR, NR nanocomposites show improvement in tensile strength, elongation at break, modulus and tearing strength. The optimum filler content for tensile strength is 1–3 phr. Hardness, modulus and tearing strength of NR nanocomposites increase with increasing the filler content. (5) Among three filled systems, NR nanocomposites containing both CBs and hyperbranched polyester modified nanoAl2O3 show best abrasion resistance and thermal stability. Acknowledgments
Fig. 15. TGA curves of nano-Al2O3/CBs/NR with different loading content of different modified Al2O3.
crease, more rolling formations and debris appear; in its magnified SEM in Fig. 14d1, there is less pull-off or cut-off debris with particles embedded in matrix compared with neat NR, which is in accordance with its high abrasion resistance. It is worth noting that the basic wear mechanisms changes and the abrasion resistance is improved with the addition of reinforcement of a rubber independent of the type of the active filler used [38]. 3.5. Thermal stability As shown in Fig. 15, compared with that of the neat NR, the thermal stability of NR nanocomposites filled with nano-Al2O3 is improved to an optimum value at 1 phr content. At the same filler content, HBP-Al2O3/CBs/NR nanocomposites show higher thermal stability than KH570-Al2O3/CBs/NR and A151-Al2O3/CBs/NR. This may be due to the synergistic effect of alumina nanoparticles grafted with HBP and CBs, among which there are strong crosslinking networks and interactions. There aren’t direct relations between thermal stability and crosslinking densities. Thermal stability of materials mainly depends on its chemical structure and chemical properties. In the temperature range 200–270 °C both scission and crosslinking occur though no loss of unsaturation of the bulk rubber [39]. The decomposition of natural rubber is accompanied by the volatilization ofisoprene, dipentene and small amounts of p-menthene [39]. The enhancement in thermal stability is due to the uniform dispersion of nanoparticles in the matrix that keeps the rubber chains intact on cross-linking and improves degree of vulcanization [10]. The monodispersed nanoparticles in NR hinder the permeability of volatile degradation products out from the material [39] and absorb free radical during decomposition process of rubber, consequently delaying the decomposition of the nanocomposites. The molecular mobilization of rubber matrix in composites was affected due to the interactions between the fillers and rubber molecules [35]. The results manifests the function groups introduced on the surface of nanoparticles through the treatment with silanes and HBP could result in physical adsorption and chemical interaction between nano-Al2O3 and rubber molecules [35]. 4. Conclusions (1) A novel NR nanocomposites filled with organically modified nano-Al2O3 and CBs were prepared. The synergistic effect of nanoparticles and CBs in NR nanocomposites was presented. (2) The addition of nano-Al2O3 accelerates the curing reaction of NR.
The authors acknowledge the Special Project for Nano-Science of Shanghai Science and Technology Committee (0852nm00202), Key Project of Chinese Ministry of Education (No. 208182), Shanghai Leading Academic Discipline Project (S30107), Shanghai University Development Foundation (A.10-0407-11-002), Program for Professor of Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher Learning (B.39-0411-10-001), Shanghai Pujiang Program, and Key subject of Shanghai Municipal Education Commission. The authors would like to thank Mr. Y.L. Chu and Mr. W.J. Yu from Instrumental Analysis & Research Center of Shanghai University for help with the SEM and TEM measurement. The authors would like to thank Dr. S.F. Li from Hubei polytechnic university, Dr. F.C. Ding from Texas State University at San Marcos and Dr. G.Z. Zhao from Xiamen Tungsten CO., Ltd. for language assistance. References [1] Adhikari B, Ghosh AK, Maiti S. Developments in carbon black for rubber reinforcement. J Polym Mater 2000;17:101–25. [2] Maiti M, Bhattacharya M, Bhowmick AK. Elastomer Nanocomposites. Rubber Chem Technol 2008;81:384–469. [3] Edwards KL. A designers’ guide to engineering polymer technology. Mater Design 1998;19:57–67. [4] Pal K, Rajasekar R, Kang DJ, Zhang ZX, Kim JK, Das CK. Effect of epoxidized natural rubber-organoclay nanocomposites on NR/high styrene rubber blends with fillers. Mater Des 2009;30:4035–42. [5] Bhattacharya M, Bhowmick AK. Synergy in carbon black-filled natural rubber nanocomposites. Part I: Mechanical, dynamic mechanical properties, and morphology. J Mater Sci 2010;45:6126–38. [6] Praveen S, Chattopadhyay PK, Albert P, Dalvi VG, Chakraborty BC, Chattopadhyay S. Synergistic effect of carbon black and nanoclay fillers in styrene butadiene rubber matrix: development of dual structure. Compos Part A – Appl Sci Manuf 2009;40:309–16. [7] Rajasekar R, Pal K, Heinrich G, Das A, Das CK. Development of nitrile butadiene rubber-nanoclay composites with epoxidized natural rubber as compatibilizer. Mater Des 2009;30:3839–45. [8] Jia QX, Wu YP, Xiang P, Ye X, Wang YQ, Zhang LQ. Combined effect of nano-clay and nano-carbon black on properties of NR nanocomposites. Polym Polym Compos 2005;13:709–19. [9] Pal K, Rajasekar R, Kang DJ, Zhang ZX, Pal SK, Das CK, et al. Effect of fillers on natural rubber/high styrene rubber blends with nano silica: morphology and wear. Mater Des 2010;31:677–86. [10] Mishra S, Shimpi NG, Patil UD. Effect of nano CaCO3 on thermal properties of styrene butadiene rubber (SBR). J Polym Res 2007;14:449–59. [11] Lorenz H, Fritzsche J, Das A, Stockelhuber KW, Jurk R, Heinrich G, et al. Advanced elastomer nano-composites based on CNT-hybrid filler systems. Compos Sci Technol 2009;69:2135–43. [12] Wang QL, Yang FY, Yang QA, Chen JH, Guan HY. Study on mechanical Properties of nano-Fe3O4 reinforced nitrile butadiene rubber. Mater Des 2010;31:1023–8. [13] Tai Y, Miao J, Qian J, Xia R, Zhang Y. An effective way to stabilize silicon nitride nanoparticles dispersed in rubber matrix by a one-step process. Mater Chem Phys 2008;112:659–67. [14] Ansarifar A, Azhar A, Ibrahim N, Shiah SF. The use of a silanised silica filler to reinforce and crosslink natural rubber. Int J Adhes Adhes 2005;25:77–86. [15] Rooj S, Das A, Thakur V, Mahaling RN, Bhowmick AK, Heinrich G. Preparation and properties of natural nanocomposites based on natural rubber and naturally occurring halloysite nanotubes. Mater Des 2010;31:2151–6. [16] Liu P, Wang TM. Adsorption properties of hyperbranched aliphatic polyester. Grafted attapulgite towards heavy metal ions. J Hazard Mater 2007;149:75–9. [17] Yokoyama K, Nishizawa N, Yamaguchi T, Yamauchi T, Shirai K, Tsubokawa N. Effect of postgrafted alkyl chains on the wettability of hydrophilic poly(amidoamine)-grafted nano-sized silica. Compos Interface 2005;12: 617–27.
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[28] Liu P. Hyperbranched aliphatic polyester grafted attapulgite via a melt polycondensation process. Appl Clay Sci 2007;35:11–6. [29] Fu JF, Chen LY, Yang H, Zhong QD, Shi LY, Deng W, et al. Mechanical properties, chemical and aging resistance of natural rubber filled with nano-Al2O3. Polym Compos 2012;33:404–11. [30] Fu JF, Shi LY, Zhang DS, Zhong QD, Chen Y. Effect of nanoparticles on the performance of thermally conductive epoxy adhesives. Polym Eng Sci 2010;50:1809–19. [31] ISO 3417:2008. Rubber – measurement of vulcanization characteristics with the oscillating disc curemeter. ISO; 2008-12-01. [32] ISO 37:2005. Rubber, vulcanizedorthermoplastic-determination of tensile stress–strain properties. ISO; 2005-07-15. [33] ISO 34-1:2004. Rubber, vulcanized or thermoplastic – determination of tear strength – Part 1: Trouser, angle and crescent test pieces. ISO; 2004-03-15. [34] GB/T1689-1998. Rubber vulcanized-determination of abrasion resistance (Akron machine). China Zhijian Publishing House (China Standard Publishing House), being approved in 1998-11-04, being implemented; 1999-06-01. [35] Sui G, Zhong WH, Yang XP, Zhao SH. Processing and material characteristics of a carbon-nanotube-reinforced natural rubber. Macromol Mater Eng 2007;292:1020–6. [36] Rajeev RS, De SK. Crosslinking of rubbers by fillers. Rubber Chem Technol 2002;75(3):475–509. [37] Kueseng K, Jacob KI. Natural rubber nanocomposites with SiC nanoparticles and carbon nanotubes. Euro Polym J 2006;42:220–7. [38] Xu D, Karger-Kocsis J, Major Z, Thomann R. Unlubricated rolling wear of HNBR/ FKM/MWCNT compounds against steel. J Appl Polym Sci 2009;112:1461–70. [39] Sharif J, Yunus W, Dahlan K, Ahmad MH. Preparation and properties of radiation crosslinked natural rubber/clay nanocomposites. Polym Test 2005;24:211–7.
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Materials and Design journal homepage: www.elsevier.com/locate/matdes
Mechanical and tribological properties of natural rubber reinforced with carbon blacks and Al2O3 nanoparticles Ji-Fang Fu a,⇑, Wen-Qi Yu a,b, Xing Dong a, Li-Ya Chen a, Hai-Sen Jia a, Li-Yi Shi a,b, Qing-Dong Zhong a, Wei Deng a a b
Nano-Science & Technology Research Center, School of Materials Science and Engineering, Shanghai University, 99 Shanghai Road, Shanghai 200444, PR China College of Science, Shanghai University, Shanghai 200444, PR China
a r t i c l e
i n f o
Article history: Received 17 November 2012 Accepted 19 January 2013 Available online 4 February 2013 Keywords: Al2O3 nanoparticles Natural rubber Tire tread Nanocomposites Abrasion resistance
a b s t r a c t A novel Natural rubber (NR) nanocomposites filled with carbon blacks (CBs) and organically modified Al2O3 nanoparticles were prepared and the synergistic effect of CBs and Al2O3 nanoparticles on the properties of the nanocomposites was investigated. The microstructure, tensile strength, elongation at break, modulus, tearing strength and abrasion resistance of the NR nanocomposites were evaluated. The nanoAl2O3 played an important reinforcement effect on the mechanical properties, abrasion resistance and thermal stability of the nanocomposites when it was introduced combined with CBs. Especially, the NR nanocomposites containing hyperbranched polyester modified nano-Al2O3 and CBs gave the best abrasion resistance and thermal stability. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Natural rubber (NR) is widely used in many fields, due to its distinct properties such as high elongation and excellent resilience. To obtain improvement in properties such as tear strength, abrasion resistance, stiffness and hardness, NR was generally reinforced with carbon blacks (CBs) and silica [1–3]. Over the last two decades, research has focused on development of other reinforcing agents to be complete or partial replacement of CBs in rubber composites. Nanomaterials such as clay [4–8], SiO2 [9], CaCO3 [10] and CNTs [11] were used to reinforce rubber with high performance and special functionality [12], including high mechanical properties, abrasion resistance, erosion resistance and multifunctional properties [5–8]. However, it has been remaining challenges to control the microstructure of the resulting materials, especially the degree of fillers dispersion, which often restrict the fabrication of rubber nanocomposites. Surfactants, coupling agent, and surface-grafting modification were usually employed to improve the compatibility between nanoparticles and NR [13–15]. A coupling agent has two functionally active end groups, an alkoxy group can react with the hydroxyl group on the surface of nanoparticles and another functional groups are compatible with rubber, leading
⇑ Corresponding author. Address: Nano-Science & Technology Research Center, Shanghai University, P.O. Box 111, No. 99 Shanghai Road, Shanghai 200444, PR China. Tel.: +86 21 66133800; fax: +86 21 66135215. E-mail addresses: [email protected], [email protected] (J.-F. Fu). 0261-3069/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.matdes.2013.01.033
to strong physical or chemical linkage between nanoparticles and rubber molecules [13–15]. Macromolecular coupling agent and surface-grafting modification are more efficient, because the organic coating layer grafted on the nanoparticles by macromolecular coupling agent brings about more strong electrostatic and steric stabilization [13]. The surface functionalization of nanomaterials by grafting of polymer or hyperbranched polymer is expected to play important roles in the chemical modification of nanostructural materials [16–24]. A great interest has been focused on ‘‘hyperbranched polymers’’, which have fundamental building blocks, controllable molecular weight, controllable branching and versatility in modification of terminal groups [20]. Many researches have been reported that the surface treatment of nanomaterials by grafting of hyperbranched polymer, such as carbon nanotubes [25], silica [21,26], and CBs [24,27,28]. In our previous work [29], Al2O3 nanoparticles were singly used to reinforce NR vulcanizates, and the resulting nanocomposites exhibited significantly enhanced aging resistances, acid and alkaline resistances. This work describes a novel tire tread nanocomposites based on NR filled with CBs and Al2O3 nanoparticles. Here the Al2O3 nanoparticles were grafted with hyperbranched aliphatic polyester (HBP) via a melt polycondensation of 2,2bis(hydroxymethyl) propionic acid (bis-MPA) and treated with silanes respectively. The nanoparticles grafted with hyperbranched aliphatic polyester and silanes were used to replace partial of CBs to reinforce NR. The reinforcement effect on the mechanical properties, abrasion properties, and thermal stability of the nanocomposites was investigated.
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2. Experimental details 2.1. materials The a-Al2O3 nanoparticles with mean diameter of 100 nm were provided by Dalian Luming Co. Ltd., Dalian, China. c-methacryloxypropyl trimethoxy silane (KH570) and vinyl triethoxysilane (A151) were purchased from Nanjing Shuguang Co. Ltd., China. c-aminopropyltriethoxysilane (APTES) was obtained from Shanghai Yaohua Chemical Plant and was used without further purification. 2,2bis(hydroxymethyl)propionic acid (bis-MPA) was analytical grade, and used as received from Quzhou Mingfeng Chemical CO., Ltd. Acetone, ammonia solution, ethanol, p-toluenesulfonic (p-TSA) and N,N-Dimethylformamide (DMF) were all analytical grade and used as received from Sinopharm Chemical Rragent Co., Ltd. Natural rubber SCR-5 was purchased from Hainan Nongken groups Co. Ltd. CBs (N220) were commercial products and purchased from Shanghai Cabot Speciality Chemicals Co., Ltd. Zinc oxide (ZnO) (activator), Stearic acid (SA), sulfur (S) and 2-(4-Morpholinothio)-benzothiazole (NOBS) (accelerator) were commercial products. Poly (1,2-dihydro-2,2,4-trimethyl-quinoline) (antioxidant RD), N-1, 3-dimethylbutyl-N0 -phenyl-p-phenylenediamine (antioxidant 4020) were commercial products. The used basic recipes were as follows (in parts): natural rubber (100 phr), CBs (N220) (45 phr), ZnO (5.0 phr), SA (2.0 phr), RD (1.5 phr), 4020(1.5 phr), NOBS (1.6 phr), S (1.6 phr). And Al2O3 nanoparticles are various amounts (1, 3, 5, 7, 10 phr).
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The mixture was decentralized ultrasonically for 30 min and then agitated at 80 °C for 4 h. Next the mixture was heated to 120 °C and stirred for additional 2 h. After ethanol was completely removed, the product was triturated in mortar, and then agitated at 500 rpm at room temperature for 12 h to get tiny particles. Then the sample was dried in a vacuum at 120 °C. Then the AB2 monomer, bis-MPA, was melt polycondensation according to the method reported previously [28]. The details were as follows: 10 g amino-Al2O3, 2 g bis-MPA and 0.5 g p-TSA were triturated in mortar to achieve a well proportioned mixture. The mixture were added into a three-necked flask with a reflux condenser and agitated at 200 °C for 1 h in argon flow. After that the mixture cooled to room temperature, the mixture was dispersed ultrasonically for 30 min and washed throughout with DMF and next with deionized water until no polymer was found in the filtrate, and then dried in a furnace at 110 °C. The treated Al2O3 is denoted as HBP-Al2O3. 2.3. Sample preparation Compounding was performed in an open two-roll mixing mill (laboratory size) and the compounded rubber was left overnight before vulcanization. The vulcanization was carried out in hydraulic press at 150 °C under a pressure of 15 MPa for the optimum cure time (T90) to get square sheet of 13 cm 12 cm 0.2 cm sizes.
2.2. Surface treatment of the nanoparticles
2.4. Characterization
The Al2O3 nanoparticles were dried in an oven at 110 °C under vacuum for 24 h prior to surface treatment. The Al2O3 nanoparticles were treated with KH570 and A151 according to our previous work [30]. The treated Al2O3 is denoted as KH570-Al2O3 and A151Al2O3 respectively. The mechanism of the introduction of amino and hyperbranched polymer onto Al2O3 nanoparticles surface was shown in Scheme 1. The introduction of amino onto Al2O3 nanoparticles surface was achieved by the treatment of surface hydroxyl groups on alumina with APTES. Then the AB2 monomer, bis-MPA, was melt polycondensated cored with the amino-alumina with surface active site, amino groups. Firstly, the c-aminopropyl alumina (amino-alumina) was prepared according to the method reported previously [18]. The details were as follows: 10.0 g of Al2O3 nanoparticles, 2.0 mL APTES, 2.0 mL ammonia solution, and 40 mL ethanol were added into a 500 ml three-necked round-bottom flask with a reflux condenser.
The Fourier transform infrared spectrum (FTIR) of the particles was performed on a Nicolet 3200 FTIR instrument (Thermo Electron, Waltham, MA) in the range from 4000 to 400 cm1. The morphology of the nanoparticles was observed using a JEOL 2000FX transmission electron microscope (TEM, JEOL Ltd., Japan) at an accelerating voltage of 200 kV. Morphology of fracture and abrasion surfaces of samples were examined with a JSM-6700F scanning electron microscope (SEM, JEOL Ltd., Japan) at an activation voltage of 15.0 kV after coated with thin layers of gold. Cure characteristics were determined by a MDR 2000 rheometer (Shanghai D&G Machinery Equipment Co., Ltd., China) at 150 °C according to ISO 3417: 2008 [31].Tensile strength was measured according to ISO 37: 2005 [32] and tear strength was measured according to ISO 34-1: 2004 [33] (an unnicked 90° angle
Scheme 1. Preparation procedure for the HBP-Al2O3 nanoparticles.
Fig. 1. FTIR spectrum of 100 nm a-Al2O3 (a) and treated with KH550 (b) and HBP (c).
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Fig. 2. SEM and TEM of Al2O3 (a and c) and Al2O3-HBP (b and d).
Fig. 3. Cure characteristic of nano-Al2O3/CBs/NR: (a) T10, (b) T90, (c) curing rate and (d) crosslink densities.
specimen) both by Reger 3010 Universal Testing Machine (Shenzhen Reger Instrument Co., Ltd., China) at a speed of 500 mm/min. The average value of at least three measurements for each sample was recorded.
Abrasion experiments were performed in an air atmosphere at room temperature using a WML-76 Akelon abrasion testing machine (Wuxi Liyuan Electronic & Chemical Equipments Co., Ltd., China) according to GB/T 1689–1998 [34]. The rotation speed of
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the sample wheel and the empery wheel were 76 and 33 rpm, respectively, and the angle between the shafts of the two related wheels was 15°. The applied normal load was 2.72 kg (26.7 N). Thermal stability was performed from the ambient temperature to 800 °C at the heating rate of 10 °C/min in an air atmosphere using A NetzchSta449c thermogravimetric analyzer (NETZSCH, Bavaria, Germany). 3. Results and discussion 3.1. Characteristics of Al2O3 nanoparticles treated by HBP Fig. 1 shows the infrared spectrum of untreated Al2O3 (a), amino-Al2O3 (b), and HBP-Al2O3 (c). The infrared spectrum of amino-Al2O3 show some new absorption at 2931.95 cm1 ascribing to C@H stretching vibration, and 1139.54 cm1 ascribing to CAN stretching vibration, which are characteristic of APTES [25– 28]. This suggests that amino group is introduced onto the Al2O3 surface. The infrared spectrum of HAPE-Al2O3 show new absorption peak at 1734.72 cm1 ascribing to C@O of ACOOA stretching vibration [25–28], which is characteristic of bis-MPA. This suggests that hyperbranched polymer is successfully grafted onto the amino-Al2O3 nanoparticles surface. Fig. 2 shows the SEM and TEM of untreated Al2O3 and HBPAl2O3 nanoparticles. The large bulks in Fig. 2a and c indicate that the untreated Al2O3 nanoparticles were apparently agglomerated, which presents a powerful contrast to the monodisperse modified nanoparticles in Fig. 2b. The thin film surrounding nanoparticles in Fig. 2d indicates that HBP are successfully grafted onto nanoparticles and HBP macromolecules have penetrated into the agglomerated nanoparticles. Those agglomerated particles were separated by the strong inorganic–organic interactions and formed a relatively uniform distribution.
Fig. 5. Elongation at break of nano-Al2O3/CBs/NR with different nano-Al2O3 loading.
3.2. Curing behavior Fig. 3 shows the cure characteristic of NR nanocomposites filled with CBs and Al2O3 nanoparticles. As shown in Fig. 3a and b, the optimum cure time (T90) and scorch time (T10) of the nanocomposites decrease with the addition of fillers and change little in range of 1–10 phr filler content, which indicates that the nanoparticles have accelerated the cure process of the NR nanocomposites to some extent. On the whole, the scorch time change slightly and the cure time decrease obviously, which indicate the addition of nanoparticles can reduce the product development cycles and
Fig. 6. Modulus at 100% elongation and modulus at 300% elongation of nano-Al2O3/ CBs/NR with different nano-Al2O3 loading.
Fig. 7. Hardness of nano-Al2O3/CBs/NR filled with different loading content of different modified Al2O3.
Fig. 4. Tensile strength of nano-Al2O3/CBs/NR with different nano-Al2O3 loading.
make processing safe. At the same filler content, the T90 and T10 of the NR nanocomposites are as follows: HBP-Al2O3/CBs/ NR > A151-Al2O3/CBs/NR > KH570-Al2O3/CBs/NR. As shown in Fig. 3c, with the addition of particles, the values of 1/(T90–T10), represent cure rate of NR nanocomposites, increase with increasing the filler content. This may be attributed to the high surface-to-vol-
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Fig. 8. SEM of the tensile fracture surface of unfilled NR (a) and nano-Al2O3/CBs/NR with 3 phr Al2O3 loading at low magnification, (b) A151-Al2O3/CBs/NR, (c) KH570-Al2O3/ CBs/NR and (d) HBP-Al2O3/CBs/NR.
ume ratio of nanoparticles and the increasing filler-rubber interaction, which result in an increase in activity and cure rate. At the same time, this may be also due to the relatively higher thermal conductivity of nanoparticles, which result in an increase in thermal conductivity of NR nanocomposites [35]. The value of MHML can be taken as a measurement of crosslinking density [4]. Fig. 3d suggests that HBP-Al2O3/CBs/NR displays higher crosslink density than A151-Al2O3/NR and KH570-Al2O3/NR, which is contrary to the cure rate of NR nanocomposites. This was attributed to the carboxyl groups, hydroxyl group and special structure of hyperbranched polyester, which participated in rubber’s crosslinking and resulted in more complete curing. This was in accordance with De and Rajeev’s work [36].
3.3. Mechanical properties As shown in Fig. 4, the tensile strength of the NR nanocomposites filled with Al2O3 nanoparticles and CBs is improved. The tensile strength of the NR nanocomposites increases initially, attains a maximum value, and then decreases with increasing the filler content. From Fig. 4, the optimum filler content is 1–3 phr, a further addition will decrease the tensile strength slightly. On the whole, KH570-Al2O3/CBs/NR and A151-Al2O3/CBs/NR show relatively higher tensile strength, HBP-Al2O3/CBs/NR shows relatively lower tensile strength. As presented in Fig. 5, the elongation at break of nano-Al2O3/ CBs/NR is improved. The elongation increases firstly and then de-
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creases with increasing Al2O3 filler content in range of 0–10 phr. As a whole, KH570-Al2O3/CBs/NR shows relatively higher elongation at break than A151-Al2O3/CBs/NR, which are in accordance with its relatively higher strength. High tensile strength is often prerequisites for NR to gain high elongation at break. Moreover, macromolecules’ movement are limited by nanoparticles [37], and more macromolecules twist around each other and are curled with increasing filler content, the curled macromolecules will uncurl under stretching and result in an increase in elongation at break [4,8]. In this case, effects of nanoparticles on elongation at break are as follows: the modified nanoparticles not only enhance the tensile strength and interfacial bonding along with good dispersion [10], but restrict the macromolecule chains’ movement in the meantime, and both which result improvement in elongation at break under stretching. Fig. 6 presents modulus at 100% and modulus at 300% of the NR nanocomposites filled with CBs and Al2O3 nanoparticles. With the addition of nanoparticles, the modulus of the NR nanocomposites containing nano-Al2O3 increase with increasing the filler content. This was may be due to the restriction of molecular chain mobility or an increase in the cross-kink density [9]. The significant improvement in modulus is also ascribed to the addition of rigid nano-Al2O3 and their addition to a relatively soft matrix will result in the development of local stretching in the matrix, thus the nanocomposites will respond with a higher strength when deformed.
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100% modulus is followed: KH570-Al2O3/CBs/NR > A151-Al2O3/ NR > HBP-Al2O3/CBs/NR. At the same filler content, the 300% modulus of A151-Al2O3/CBs/NR and HBP-Al2O3/CBs/NR are higher than that of KH570-Al2O3/CBs/NR. These indicate the addition of nanoparticles in natural rubber is beneficial to improving rigidity and resistance to extensional deformation of the NR nanocomposites. A151 and hyperbranched polyester grafting modification onto nanoparticles are more beneficial to improving stiffness and resistance to extensional deformation, and KH570 modification is conducive to resilience. As shown in Fig. 7, the hardness of NR vulcanizates filled with CBs and nano-Al2O3 is improved and increase with increasing the filler content. At the same filler content, the hardness shows a same trend as 300% modulus, A151-Al2O3/CBs/NR shows the highest hardness, and then followed with HBP-Al2O3/CBs/NR and KH570-Al2O3/CBs/NR, respectively. The improvement in hardness is attributed to the effect factors that lead to an increase in polymer chain rigidity and crosslink density or interaction [9]. The modified nanoparticles penetrated through the void in rubber matrix and interacted with rubber matrix, acting as physical cross-linking points and resulting in an increase in crosslink densities [29,37]. The SEM taken from the tensile fracture surface morphology of nano-Al2O3/CBs/NR nanocomposites is shown in Fig. 8. The unfilled NR vulcanizates exhibit smooth morphology with river-like fracture feature surface at low magnification; however the
Fig. 9. SEM of the tensile fracture surface of unfilled NR (a and b) and nano-Al2O3/CBs/NR with 3 phr nano-Al2O3 loading at high magnification: (c and d) A151-Al2O3/CBs/NR and (e and f) HBP-Al2O3/CBs/NR.
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Fig. 10. Tearing strength of unfilled NR and nano-Al2O3/CBs/NR with different loading content of different modified Al2O3.
and some cracks with wide space between cracks. However, the fracture surface of nano-Al2O3/CBs/NR presents a more complicated and rougher morphology without particles pulled-out or decohesion. Moreover, the fracture surface is full of hierarchical structure and much regulated micro-crack with minute grain, which indicates that much energy is absorbed under tearing process, resulting in an improvement in tearing strength. As shown in Fig. 12, a large quantity of individual particles are well dispersed and embedded in the NR matrix, and the interface between the particles and matrix are obscure, which indicate that the compatibility between the particles and matrix are enhanced. The improvement in properties depends on the reinforcement of filler in the rubber matrix. Combination of modified nanoAl2O3 and CBs strongly affects the mechanical properties of Al2O3/CBs/NR. The enhancement in the mechanical properties is attributed to the uniform dispersion of fillers in the NR matrix and good polymer-filler interaction [5–8].
3.4. Tribological behaviors nano-Al2O3/CBs/NR nanocomposites show a more complicated and rougher morphology with crack and massive plastic deformation, suggesting a reinforcement behavior of nano-Al2O3/CBs. KH570Al2O3/CBs/NR shows a more rougher morphology, which indicates its relatively higher tensile properties. In order to investigate the nanoparticles dispersion in Al2O3/CBs/NR nanocomposite, the SEM images were further magnified in Fig. 9. It can be seen that a large quantity of individual particles are well dispersed and embedded in NR matrix without agglomeration. Fig. 10 gives the tearing strength of the NR nanocomposites. With the addition of Al2O3 nanoparticles, the tearing strength of NR nanocomposites is improved. At 7 phr filler content, A151Al2O3/CBs/NR, KH570-Al2O3/CBs/NR, HBP-Al2O3/CBs/NR show optimum values of 145.65, 148.50 and 146.93 MPa, respectively. The tearing strength of the three systems is close and shows a same trend with increasing the filler content. The tearing behavior of nano-Al2O3/CBs/NR can be explained in term of the SEM morphology taken from the tearing fracture surface. As shown in Fig. 11, the unfilled NR shows a clear surface
As shown in Fig. 13a, with the addition of nanoparticles, the abrasive wear resistance of Al2O3/CBs/NR vulcanizates is improved. This may be attributed to the addition of nano-Al2O3, which reinforce the rubber matrix and can hinder the deformation of the matrix during abrasion process. At the same filler content, the abrasion loss of HBP-Al2O3/CBs/NR is the lowest; this indicates HBP-Al2O3/CBs/NR shows the highest abrasion resistance. At 7 phr filler content or below, KH570-Al2O3/CBs/NR shows lower abrasion loss and higher abrasion resistance than A151-Al2O3/ CBs/NR, which is in accordance with its higher comprehensive mechanical properties such as high tearing strength, 100% modulus and tensile strength. This indicates that higher comprehensive mechanical properties are beneficial to improvement in wear resistance, especially high tearing strength and 100% modulus. The abrasion resistance of NR nanocomposites depends on its strength, resilience, fatigability and frictional behavior [9,12,38]. In general, the abrasion resistance of rubber increases with the increase in tensile strength and tearing strength, especially abrading on rough
Fig. 11. SEM of the tearing fracture surface of nano-Al2O3/CBs/NR containing 7 phr Al2O3: (a) NR, (b) KH570-Al2O3/CBs/NR, (c) A151-Al2O3/CBs/NR and (d) HBP-Al2O3/CBs/NR.
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Fig. 12. SEM of the tearing fracture surface of nano-Al2O3/CBs/NR containing 7 phr Al2O3 at high magnification: (a) NR, (b) KH570-Al2O3/CBs/NR, (c) A151-Al2O3/CBs/NR and (d) HBP-Al2O3/CBs/NR.
Scheme 2. Microstructure development of NR filled with HBP-Al2O3 and CBs.
Fig. 13. Abrasion loss of nano-Al2O3/CBs/NR (a) and nano-Al2O3 /NR (b).
surface. NR nanocomposites filled with HBP-Al2O3 and CBs exhibit 20–46% improvement in the abrasion resistance compared with
unfilled NR. This is as opposed to HBP-Al2O3-filled NR which exhibit the lowest abrasion resistance among three different treated nano-Al2O3 singly filled NR systems without CBs. It can be seen in Fig. 13b the NR containing single HBP-Al2O3 shows the highest abrasion loss. For comparison and investigating the synergy effect on the abrasion resistance of NR filled with CBs and nanoparticles, the NR is singly reinforced with nano-Al2O3. Based on the analysis above, the highest abrasion resistance of HBP-Al2O3/CBs/NR could be attributed to the synergistic effect of HBP-Al2O3 and CBs. The structure of NR filled with HBP-Al2O3 and CBs can be illustrated in Scheme 2. CBs with high activity are interacted with rubber and periphery groups of hyperbranched polyester grafted on nanoparticles through chemical and physical reaction. Moreover, hyper-
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Fig. 14. SEM of the worn surface of nano-Al2O3/CBs/NR with different Al2O3 loading: NR (a and a1), KH570-Al2O3/CBs/NR (b and b1), A151-Al2O3/CBs/NR (c and c1) and HBPAl2O3/CBs/NR (d and d1).
branched polyester and rubber macromolecule twine each other, which result in an increase in cross-linking densities and bonded rubber content as well as the filler-rubber interaction. In addition, hyperbranched polyester grafted on nanoparticles can improve nanoparticles dispersion in matrix and compatibility with NR matrix. In all, the hyperbranched grafted nanoparticles and CBs show synergy in Al2O3/CBs/NR, thus, the abrasion resistance, tearing strength, hardness and modulus are improved. SEM photomicrographs of the worn surface of NR and Al2O3/ CBs/NR at different magnifications are shown in Fig. 14. On the worn surfaces of NR in Fig. 14a, parallel ploughed furrows, the so-called schallamach-type waves [38] appear with some fragments, cluster from debris and deep sharpened scratch, which are produced in the abrading direction. In its magnified SEM in
Fig. 14a1, some pull-off fragments, fatigue-induced debris and spherical particles can be observed clearly. These suggest that high abrasion loss and low abrasion resistance are found in fact. On the worn surfaces of Al2O3/CBs/NR, the schallamach type waves and debris deposition are still characteristic, but no deep sharpened scratch can be recognized. Moreover, on KH570-Al2O3/CBs/NR in Fig. 14b, the depths of ploughed furrows, debris and fragments decrease with the addition of nanoparticles. A151-Al2O3/CBs/NR shows fine and grained schallamach type waves with shallow ploughed furrows in Fig. 14c. With the addition of A151-Al2O3, the space between the neighboring waves and the depths of ploughed furrows are reduced. Its magnified SEM in Fig. 14c1 exhibits less debris and smooth surface. On the worn surface of HBP-Al2O3/CBs/NR in Fig. 14d, the depths of ploughed furrows de-
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(3) The particles are uniformly dispersed in NR matrix. (4) Compared with neat NR, NR nanocomposites show improvement in tensile strength, elongation at break, modulus and tearing strength. The optimum filler content for tensile strength is 1–3 phr. Hardness, modulus and tearing strength of NR nanocomposites increase with increasing the filler content. (5) Among three filled systems, NR nanocomposites containing both CBs and hyperbranched polyester modified nanoAl2O3 show best abrasion resistance and thermal stability. Acknowledgments
Fig. 15. TGA curves of nano-Al2O3/CBs/NR with different loading content of different modified Al2O3.
crease, more rolling formations and debris appear; in its magnified SEM in Fig. 14d1, there is less pull-off or cut-off debris with particles embedded in matrix compared with neat NR, which is in accordance with its high abrasion resistance. It is worth noting that the basic wear mechanisms changes and the abrasion resistance is improved with the addition of reinforcement of a rubber independent of the type of the active filler used [38]. 3.5. Thermal stability As shown in Fig. 15, compared with that of the neat NR, the thermal stability of NR nanocomposites filled with nano-Al2O3 is improved to an optimum value at 1 phr content. At the same filler content, HBP-Al2O3/CBs/NR nanocomposites show higher thermal stability than KH570-Al2O3/CBs/NR and A151-Al2O3/CBs/NR. This may be due to the synergistic effect of alumina nanoparticles grafted with HBP and CBs, among which there are strong crosslinking networks and interactions. There aren’t direct relations between thermal stability and crosslinking densities. Thermal stability of materials mainly depends on its chemical structure and chemical properties. In the temperature range 200–270 °C both scission and crosslinking occur though no loss of unsaturation of the bulk rubber [39]. The decomposition of natural rubber is accompanied by the volatilization ofisoprene, dipentene and small amounts of p-menthene [39]. The enhancement in thermal stability is due to the uniform dispersion of nanoparticles in the matrix that keeps the rubber chains intact on cross-linking and improves degree of vulcanization [10]. The monodispersed nanoparticles in NR hinder the permeability of volatile degradation products out from the material [39] and absorb free radical during decomposition process of rubber, consequently delaying the decomposition of the nanocomposites. The molecular mobilization of rubber matrix in composites was affected due to the interactions between the fillers and rubber molecules [35]. The results manifests the function groups introduced on the surface of nanoparticles through the treatment with silanes and HBP could result in physical adsorption and chemical interaction between nano-Al2O3 and rubber molecules [35]. 4. Conclusions (1) A novel NR nanocomposites filled with organically modified nano-Al2O3 and CBs were prepared. The synergistic effect of nanoparticles and CBs in NR nanocomposites was presented. (2) The addition of nano-Al2O3 accelerates the curing reaction of NR.
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