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silica composites
Applied Surface Science 423 (2017) 43–52
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Full Length Article
Influence of acetone extract from natural rubber on the structure and interface interaction in NR/silica composites Tiwen Xu a , Zhixin Jia a,∗ , Lianghui Wu a , Yongjun Chen a , Yuanfang Luo a , Demin Jia a , Zheng Peng b a b
College of Materials Science and Engineering, South China University of Technology, Guangzhou 510640, China Agricultural Product Processing Research Institute, Chinese Academy of Tropical Agriculture Sciences, Zhanjiang 524001, China
a r t i c l e
i n f o
Article history: Received 18 March 2017 Received in revised form 10 June 2017 Accepted 14 June 2017 Available online 15 June 2017 Keywords: Acetone extract Natural rubber Silica Interfacial interaction Dynamical properties
a b s t r a c t It is well known that the coupling reagents as the additional modifiers were often used to improve the reinforcement effect of silica filled natural rubber. Actually, the commercial raw NR is a mixture consisting of polyisoprene and non-isoprene, where the latter one might have impact on the properties of NR/silica composites as an inartificial modifier inside. Thus, investigating the effect of non-isoprene compounds on the structure and properties of NR/silica composites is a novel approach to disclose the peculiarity of NR, which is meaningful to the assessment of NR quality. In this paper, the influences of acetone extract (AE) from natural rubber on the structure and mechanical properties of NR/silica composites were studied. Then the interfacial interactions between AE and silica were also illustrated through Fourier transform infrared spectroscopy (FTIR), thermogravimetic analysis (TGA), X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM). Results demonstrated the existence of hydrogen bond between silica and AE, also the covalent bond induced by esterification reaction between COOH and Si OH, which resulted in an increase of constrained regions around silica surface leading to the promotions on mechanical and dynamical properties of NR/silica composites significantly. © 2017 Published by Elsevier B.V.
1. Introduction Natural rubber (NR), which is coagulated from Hevea brasiliensis latex, is widely used for its 42% proportion of global elastomer consumption in industry. 70% of NR consumption are used in tyre and anti-vibration industries because of its remarkable properties. It is well known that the rolling of tire would generate hysteresis energy loss leading to a large consumption of gasoline, which could produce much more CO2 to enhance greenhouse effect and also make dust particles pollution. Therefore, a high performance tire should avoid these disadvantages, where it could be done via constructional design and optimizing natural rubber composites. Normally, materials researchers concern additives, such as silane coupling agent, which can be used to improve dynamic properties of natural rubber composites, rarely observing the role of non-isoprene compounds of natural rubber itself in filled natural rubber composites. Reportedly, natural rubber is composed of cis1,4-polyisoprene (95% w/w) and non-isoprene compounds which
∗ Corresponding author. E-mail address: [email protected] (Z. Jia). http://dx.doi.org/10.1016/j.apsusc.2017.06.150 0169-4332/© 2017 Published by Elsevier B.V.
includes proteins or polypeptides (1–2% w/w), acetone extract (1–2% w/w), carbohydrates (0.4% w/w) and minerals (0.3–0.7% w/w)[1], all of which are determined by climate, territory, strain and so forth. Although the contents of non-isoprene compounds are relatively less, they are essential for the stability of natural rubber latex, as well as the processing property, curing behaviors and so on. Several studies have shown that non-isoprene compounds influence the mechanical properties of NR [2–6], but the effects of non-isoprene compounds on dynamical properties of filled or unfilled NR composites are still unclear which might be more crucial than static properties in tyre application field. Specially, the influences of non-isoprene compounds on the interface interaction between filler and rubber matrix are rarely reported. As one of the non-isoprene compounds, acetone extract (AE) is important for the quality of NR. It is reported [7] that AE is consist of oleic acid, linoleic acid, stearic acid, sterol ester and so on. Normally, AE has been considered as an ingredient, which has an effect on the vulcanization or anti-aging resistance of unfilled natural rubber [8–10]. For example, some accelerants and sterol antioxygen that are contained in AE could make positive functions on curing rate and anti-aging resistance to ozone. Unfortunately, there are no reports about the effects of AE on dynamic and mechanical
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properties of NR/filler composites till now, which might be more significant for the application of NR. Silica, as a common filler, is widely used in passenger tire for its good performance on rolling resistance [11–13]. Due to its drastic polarity ascribed from silicon hydroxyl, it should be modified by coupling reagent [14–16] to diminish agglomeration of particles before using. Thus, considering its active groups, such as carboxyl, aldehyde, carbonyl and alkyl chain, it might be possible to employ AE as a natural multifunctional coupling reagent to improve the interaction between silica and NR matrix. In this work, NR/silica composites with different contents of AE were prepared on double rolls open mill. Dynamic properties of NR/silica composites were studied. Simultaneously, the reinforced mechanism and interface interactions between silica and AE were illustrated through DMA, DSC, FTIR, TGA, XPS and TEM. The results made the impacts of AE on the structure and dynamic properties of NR/silica composites clearly, which could offer a valuable information to the meaning of controlling the content of acetone solutes in natural rubber latex decided by the Hevea brasiliensis planting. Our study illustrated a novel way to change or improve the interaction between NR and silica from the natural rubber inside. Simultaneously, the conclusions are useful to the application of natural rubber products. 2. Material and methods 2.1. Materials Natural rubber (NR) with a trade name SCR-WF was friendly supplied by Hainan Sinochem Rubber Company, Ltd (Hainan, China). Precipitated silica (type 518) was kindly offered by Wanzai Huiming Chemical Company, Ltd (Jiangxi, China). BET was determined to be 180 m2 /g from nitrogen adsorption method, pH value was 6.5–7.5 (5% wt aqueous solution), average diameter was 8 m. Other ingredients are commercially available industrial products and used as received. 2.2. Preparation of NR/silica composites with different AE Firstly, natural rubber was extracted by acetone for 48 h through Soxhlet extraction method to obtain pure NR, which was used to prepare NR/silica composites and NR/silica model compound. The residual solution was concentrated using Rotary Evaporator and dried in an oven until constant weight was obtained, which was confirmed to be AE. Extracted NR and other ingredients were mixed on a X(S)K-160 open-mill (Dongguan, China) at room temperature with the speed ratio of 1:1.25. Basic formulation (mass parts): NR 100, silica (0 or 30), AE (0–5), zinc oxide 5, stearic acid 2, sulfur 1.5, N-cyclohexyl2-benzothiazolesulfenamide (accelerator CZ) 1.5, benzothiazole disulfide (accelerator DM) 0.5. Optimum curing time (T90 ) was determined at 143 ◦ C by UR-2030 oscillating disk rheometer (UCAN Technology Company, Ltd,Taiwan). The compounds were vulcanized on a KSH R100 flat vulcanizing machine (Kesheng Machinery Co., Ltd, Dongguan, China) after their accommodation at room temperature for 4 h. 2.3. Preparation of NR/silica model compound NR/silica/AE and NR/silica model compounds with the mass ratio of 100/30/5, 100/30, respectively were made on a LRMR-S150/0 (Lab Tech Engineering Company LTD.) open-mill at room temperature for 15 min. The NR/silica compound was taken as a referential sample and no other additives were added in the whole process. Then, they were extracted in Soxhlet extractor by acetone for 72 h firstly to wipe the free moving components of AE,
and then extracted by toluene for another 72 h to remove the NR molecules from silica surface that were adsorbed weakly. The extracted residuums were analyzed through FTIR, TGA, XPS and TEM to achieve the interfacial interaction mechanisms between AE and Silica. 2.4. Testing and characterization 2.4.1. Mechanical properties of NR/silica composites Tensile properties and tear strength of vulcanizates were determined according to ISO 37-2005 standard (dumbbell A shape), ISO 34-1:2004 (nicked angle), respectively, at 25 ◦ C using a U-CAN UT2060 instrument. Shore A hardness was performed following ISO 7619-1:2004 using a XY-1 durometer (Shanghai). Strain sweeps of raw natural rubber and unvulcanized NR/silica composites were conducted by Rubber Processing Analyzer (RPA) (RPA 2000, Alpha, USA) in the range of 0.7%–200% at 100 ◦ C, 1 Hz. Strain sweeps of vulcanized NR/silica composites were also done by RPA from 0.7% to 200% at 60 ◦ C, 10 Hz. Dynamic mechanical thermal analyzer (DMA) (NETZSCH DMA 242) was used to evaluate damping factors (tan␦) of vulcanized composites. The rectangular specimens (10 mm × 6 mm × 1 mm) were conducted in tensile mode at 10 Hz, 40 um (amplitude). The ◦ temperature was swept from −100 to 80 C at a speed of 3 ◦ C/min. Rolling resistance of composites was obtained through a RSSII rolling resistance testing machine (Beijing Wan Hui technology development co., LTD, China) under a load of 15 kg for 30 min followed by a rotate speed of 600 rpm. Abrasion resistance of vulcanized composites was determinated using a rotating cylindrical drum device (DIN) according to ISO 4649:2002. The sliding distance of the sample was 200 mm. 2.4.2. Differential scanning calorimetry (DSC) The glass transition behaviors of NR/silica composites were detected by a NETZSCH DSC 204 F. The temperature program involved 5 min isothermal hold at −80 ◦ C followed by heating at 3 ◦ C/min to 10 ◦ C under nitrogen. The heat capacity step Cpn and weight fraction of immobilized polymer layer im [17,18] were calculated as follows: Cpn = Cp /(1 − w)
(1)
im = (Cp0 − Cpn )/Cp0
(2)
Where, Cp is the heat capacity jump at Tg . Cpn is normalized to the polymer weight fraction. w is the weight fraction of filler. Cp0 referrers to the heat capacity jump at Tg of the unfilled polymer matrix. im is the weight fraction of immobilized polymer layer. Tg was determined according to ASTM E1356-08, and Cp was calculated by the software NETZSCH Thermal Analysis. 2.4.3. Field emission scanning electron microscopy (SEM) The vulcanized NR/silica composites were made into a shape (15 mm × 6 mm × 1 mm) and then cracked through liquid nitrogen brittle failure. The morphology of fractured surface sprayed by platinum was observed using a ZEISS Merlin SEM (EVO18 German) machine at an acceleration voltage of 10.0 kV. 2.4.4. Fourier transformation infrared spectroscopy (FTIR) FTIR analysis was carried out by a Vector 33 Fourier transform infrared spectrometer (Bruker Co.) in the range of 4000–400 cm−1 . The sample was compressed into platelets with KBr. 2.4.5. Thermal gravity analysis (TGA) Thermal Gravity Analysis was executed using a NETZSCH TG 209 F1 in the range of 35 ◦ C–700 ◦ C under nitrogen at a heating rate of ◦ 20 C/min.
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Table 1 Mechanical properties of NR/silica composites. AE (phr)a
Properties
Modulus at 100% elongation (Mpa) Modulus at 300% elongation (Mpa) Modulus at 500% elongation (Mpa) Tensile strength (Mpa) Elongation at break (%) Tear strength (KN•m−1 ) Shore A hardness a
0
1
2
3
4
5
1.03 ± 0.06 2.43 ± 0.16 6.53 ± 0.31 12.07 ± 0.59 641 ± 24 24.70 ± 0.04 44 ± 1
1.17 ± 0.07 2.85 ± 0.10 7.45 ± 0.21 15.87 ± 0.61 686 ± 15 25.72 ± 0.47 47 ± 1
1.18 ± 0.08 2.95 ± 0.08 7.89 ± 0.26 19.13 ± 0.89 724 ± 14 27.95 ± 0.55 48 ± 0
1.14 ± 0.13 3.03 ± 0.12 8.38 ± 0.16 21.89 ± 0.81 725 ± 14 28.56 ± 0.11 48 ± 1
1.06 ± 0.07 2.78 ± 0.10 7.68 ± 0.30 21.12 ± 0.98 733 ± 26 31.05 ± 0.46 46 ± 1
1.05 ± 0.12 2.74 ± 0.11 7.56 ± 0.18 21.02 ± 0.81 764 ± 14 26.22 ± 0.48 45 ± 1
phr is the part per hundred part of rubber.
2.4.6. X-ray photoelectron spectroscopy (XPS) XPS spectrum was recorded using a X-ray photoelectron spectrometer (Kratos Axis Ultra DLD, England). Operating voltage and electric current were 15 kV and 5 mA, respectively. The vacuum degree of analysis room was 5 × 10−9 torr and analysis area was 0.7 mm × 0.3 mm. A monochromated aluminum K␣ source (1486.6 eV) was used and all XPS spectra were individually calibrated to its reference C1s component at 284.8 eV [19]. Shirley type background and Gaussian functions were employed in the fitting program to distinguish the delicate difference of the chemical environment with XPS Peak 4.1 software [19,20]. 2.4.7. Transmission electron microscopy (TEM) The extracted NR/silica/AE model compound was dispersed firstly at 0.25 wt% concentration in ethanol, and then treated using a KQ3200 ultrasonic dispersion apparatus for 2 h. The suspension was dropped into a copper net for observation. A JEM-2100F TEM machine (Japan) was used to observe the morphology of the sample at the accelerating voltage of 80 kV.
cal properties of NR/silica composites by AE indicate an enhanced interfacial interaction between silica and rubber phase. 3.2. Dynamical mechanical properties of NR/silica composites DMA measurement can be applied in detecting the variation of molecular mobility for polymer segments in the vicinity of silica surface. Fig. 1(a) performs the effect of AE on the tan␦ values of NR/silica composites. Comparing with the unfilled NR vulcanizate, the glass transition temperature of NR shifts from −65.5 ◦ C to −52.9 ◦ C, and the height of transition peak decreases via adding silica. This means a formation of constrained region in composites. Simultaneously, the transition peak of NR/silica composites exhibits no distinct shift while increasing AE, but a slight decline for the peak height, which means an increase of the volume percent of constrained region [21]. To investigate the constrained region of NR/silica composites further, the volume fraction of constrained region was calculated through the equations [22–24] below: W=
tan␦ tan␦ + 1
3. Results and discussion C=1− 3.1. Mechanical properties of NR/silica composites Table 1 presents the mechanical properties of NR/silica composites with different AE. As it can be seen, the modulus at 100%, 300%, 500% elongation, tensile strength and shore A hardness all are increased gradually by increasing AE until they reach a threshold value when AE is 3 phr. Tear strength also is increased firstly by loading AE and gets its optimum value when AE is 4 phr. Simultaneously, the elongation at break shows a climbing tendency by increasing AE, which might be due to the small molecules in AE that act as plasticizers. The significant improvements on mechani-
(1 − C0 ) W W0
(3) (4)
Where, W is the energy loss fraction at the peak point, C0 is the volume fraction of constrained region for unfilled NR and is taken to be 0 here. W0 is the energy loss fraction of unfilled NR. C is the volume fraction of constrained region. The column in Fig. 1(a) lists the values of C of NR/silica composites. It is observed that the volume fraction of constrained region is increased via adding AE until it obtains the maximum value at 2 phr, which induces an enhanced mechanical properties of NR/silica composites. Then, excess AE might make a plasticization effect [25] signally to rubber molecular chains leading to an improvement of molecular mobility.
Fig. 1. Influences of AE on DMA curves of tan␦ vs temperature (a) and RPA strain sweep results (b) of NR/silica composites.
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Table 2 Rolling resistance and DIN abrasion volume of NR/silica composites. AE (phr)
0 1 2 3 4 5
Properties ◦
Rolling power loss (J/r)
Rolling temperature rise ( C)
DIN abrasion volume (cm3 )
4.17 ± 0.08 3.22 ± 0.01 3.05 ± 0.05 2.95 ± 0.03 2.58 ± 0.01 3.12 ± 0.04
38.2 ± 1.4 30.8 ± 1.1 29.0 ± 0.6 28.8 ± 0.4 25.0 ± 0.1 29.4 ± 0.9
0.164 ± 0.018 0.148 ± 0.003 0.145 ± 0.004 0.144 ± 0.001 0.134 ± 0.007 0.118 ± 0.017
In the range of 50–70 ◦ C, tan␦ value at the same temperature is decreased markedly by increasing AE and it achieves the minimum value when AE is 4 phr. Taking the tan␦ value at 60 ◦ C as an example, it is reduced by 37.2% at 4 phr AE comparing with that of unmodified NR/silica composites. The variation tendency of tan␦ at 60 ◦ C by DMA is in line with the results of RPA strain sweep what we see in Fig. 1(b). Synchronously, the tan␦ value around 0 ◦ C shows no apparent change with the addition of AE. According to viscoelastic theory, an ideal material which is needed by high performance tire should perform a low tanı value at 60 ◦ C to reduce rolling resistance or save energy, and also demonstrate a high hysteresis at 0 ◦ C to obtain high wet skid resistance or wet grip [26]. The results indicate that the rolling resistance and wet skid resistance of NR/silica composites could be improved or stable via adding appropriate AE, respectively. The optimized content of AE in these experiments is 4 phr. Rolling power loss and temperature rise of NR/silica composites modified by AE are conducted in Table 2. It can be seen that the rolling power loss and temperature rise both decrease memorably before reaching a critical value with the increase of AE. The lowest values are obtained at 4 phr AE, where they were diminished by 38.1%, 34.6% respectively in contrast with that of the unmodified NR/silica composites. The results imply that appropriate AE could improve the rolling resistance of NR/silica composites observably. DIN abrasion volume of composites is minished following the increase of AE, which means an improvement of the abrasion resistance. These phenomenons should be caused by the components in AE, where alkyl ester, carboxylic acid and aldehyde compounds [9] are contained that enhanced the interfacial interactions between silica and rubber phase.
3.3. Glass transition behaviors of NR/silica composites The mobility of polymer molecules are sensitive to the local environment because of the characteristic long-chain morphology. Therefore, Tg and normalized change of heat capacity (Cp ) of a polymer at the glass transition region are important parameters that could offer information about the structural variation during transition[21]. Fig. 2 depcits the glass transition behaviors of unfilled NR and NR/silica composites modified by AE. The normalized heat capacity(Cpn ) as a direct evidence of polymer chain mobility [21,27] is displayed in Fig. 2. The greater value of Cpn at the glass transition region means a better chain mobility. As seen in Fig. 2, taking unfilled NR as the reference, normalized Cpn of NR/silica composites all decrease and they manifest a reductive change with the increase of AE, which suggests more and more polymer chains are confined among filler galleries. The results are also certified by the increased weight fraction of immobilized polymer layer (im ). The consequence about constrained region detected by DSC demonstrates a parallel relationship with that obtained by DMA. The results can be interpreted with the components of AE, where alkyl acid, alkyl ester, aldehyde compound, etc exist. These polar oxygenous groups could react with the silicon hydroxyl on silica surface through hydrogen-bond or covalent bond
interaction. Consequently, silica surface was encapsulated gradually by organic hydrocarbon chains that improved the compatibility between rubber and silica. Thus, a thick crosslinked immobilized polymer layer around silica surface was formed after vulcanization. The immobilized polymer layer would prevent the macro-phase separation between rubber and silica, leading to a constrained compatibility and strong rubber-silica interfacial interaction. Meanwhile, Tg parameter of NR/silica composites is determined to shift to a lower temperature via adding AE comparing with that of unfilled NR. The result is in accordance with other researchers’ works [28,29]. This can be explained in terms of two points: one is the average size of constrained polymer region is greater than the length scale of segmental motion, so the relaxation is independent from composition [30]. The other might attribute to the small molecular weight alkylates in AE which show up as the plasticizers [25], that is why the rolling power loss of NR/silica composites was increased when superfluous AE was added.
3.4. Reinforcement mechanisms of silica in NR matrix modified by AE The schematic diagram of silica reinforced NR matrix modified by AE is presented in Figs. 3 and 4 shows the relevant evidences of structural variation of NR through RPA strain sweep. Before the preparation of NR/silica/AE composites, the natural rubber was extracted by acetone for 48 h to remove acetone solutes, then extracted raw NR and a battery of experiments were designed. Extraction treatment by acetone solvent influenced the raw NR network significantly and made a disentanglement phenomenon appear which results in a decrease of the number of physical entanglement points. The viewpoint could be proved by the reduction of shear modulus (G’) of raw NR after extraction in Fig. 4(b). When silica was added into the NR matrix, AE could improve the compatibility among the two phases by linking silica aggregates and NR matrix together through their alkyl chains and oxygen-containing groups, which promote the mobility of filler particles and make them to be the new physical entanglement points. These effects were more pronounced than that of unmodified silica system, that is why a larger constrained layer was formed in NR/silica/AE composites. In Fig. 4(a), due to the enhanced formation of physical entanglement points, Payne effects of unvulcanized NR/silica/AE composites display a higher G’ value than that of NR/silica composites, it is not the same conception as we understand usually for the agglomeration of filler [31]. So an improved compatibility resulted in an enhanced interfacial interaction between silica and NR matrix, also made an increase of G’, which is in line with other report [30]. Meantime, G’ is increased with the increase of AE until it obtained a threshold value at 2 phr. The diminish of G’ for composites might be ascribed to the plasticizing effect [25] of AE. That means AE could act as both interfacial modifier and plasticizer in NR/silica composites.
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Fig. 2. Effects of AE on glass transition behaviors of unfilled NR and NR/silica composites.
Fig. 3. Reinforcement mechanism of NR/silica composites with and without AE.
Fig. 4. Shear modulus (G’) versus shear strain (e): a) unvulcanized NR/silica composites modified by AE with different contents; b) raw natural rubber with or without extraction by acetone (the two samples were homogenized on open mill for 10 times before RPA test).
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Fig. 5. SEM images of NR/silica composites modified by AE: a) 0phr, b) 1phr, c) 2phr, d) 3phr, e) 4phr, f) 5phr.
3.5. SEM morphology of NR/silica composites To investigate the dispersion of silica in NR matrix, SEM observations of brittle fracture surfaces of composites are performed in Fig. 5. As we can see, none of apparent silica aggregates are found before and after modification by AE in NR/silica composites, which implies a good dispersion of silica with nanoscale in NR matrix. This result should be a direct evidence to support the interpretation of Fig. 4a). Simultaneously, with the increase of AE, few silica particles are presented in sight until AE was added at 5 phr that indicates an improved compatibility between silica and NR that made silica imbeded into NR matrix. The enhancements on modulus and elongation at break in Table 1 should be caused by these reasons. 3.6. Mechanism of interfacial interaction in NR/silica/AE model compound To study further the mechanism of interfacial interaction between silica and AE, NR/silica/AE model compounds were prepared. Then, TGA, FTIR, XPS and TEM were used to characterize the specimens. The results are shown as below. 3.6.1. Thermal gravity analysis (TGA) TGA curves of silica and residuums of extracted NR/silica, NR/silica/AE model compounds are displayed in Fig. 6. It can be seen that residuum of extracted NR/silica/AE model compound demonstrates better thermal stability and larger weight loss than
Fig. 6. TGA curves of silica (A), residuum of extracted NR/silica model compound (B), residuum of extracted NR/silica/AE model compound (C).
that of residuum of extracted NR/silica model compound. The improved thermal stability with a higher onset temperature should be ascribed to the enhanced interfacial interaction between silica and NR modified by AE, which leads to an increase of constrained regions. Besides, a larger weight loss at 700 ◦ C suggests more rubber chains were absorbed on silica surface, which also provides a direct evidence of enhanced interface interaction between silica and NR matrix.
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Fig. 7. FTIR spectra of specimens: a) silica and Acetone extract (AE), b) residuums of extracted NR/silica and NR/silica/AE model compounds.
3.6.2. Fourier transform infrared spectroscopy (FTIR) FTIR spectrum was used to explore the chemical environment changes on silica surface and the graphs of silica, acetone extract (AE), residuums of extracted NR/silica, NR/silica/AE model compounds, respectively, are viewed in Fig. 7. In Fig. 7a), the absorption peaks for Si-OH and Si-O-Si asymmetrical stretching vibration of silica are observed at 3458 cm−1 , 1104 cm−1 , respectively. 3649 cm−1 is assigned to alcohol hydroxy [32] and 2925 cm−1 [33] stands for the C H asymmetrical stretching vibration of CH2 which means the provided silica had been modified before. In spectrum of acetone extract (AE), the absorption peak at 2970 cm−1 is the asymmetrical stretching vibration of CH3 and its symmetrical deformation vibration is observed at 1378 cm−1 [34]. Peaks at 2924 cm−1 , 2854 cm−1 are typical of C H asymmetrical stretching vibration, symmetrical stretching vibration of CH2 , respectively, also the peak at 1461 cm−1 for the deformation vibration of CH2 [34,35]. 3407 cm−1 is deduced by the stretching vibration of OH [36] of carboxylic acid and 1714 cm−1 ascribes to the stretching vibration of C O of aliphatic acid [37] and 1742 cm−1 is due to the stretching vibration of C O of aliphatic acid ester [35]. Besides, the peak at 2729 cm−1 is due to the C H [38,39] stretching vibration of aldehyde group. 954 cm−1 might attribute to the C H out-ofplane bending vibration of alkene [35]. The results indicate that AE must contain organic aliphatic acid, aliphatic acid ester, aldehyde, alkene and so on. In Fig. 7b), peaks at 1660 cm−1 , 2963 cm−1 ,2929 cm−1 stand for the stretching vibration absorption of C C , CH3 , CH2 of natural rubber [1,40]. Taking the spectrum of residuum of extracted NR/silica model compound as the reference, the intensity ratio of CH3 /Si O Si is increased from 0.1786 to 0.2891 (increased by 61.9%) after modification by AE, which means more rubber chains had been absorbed on silica surface and an enhanced interaction was formed in NR/silica/AE model compound. The appearance of peaks at 2728 cm−1 , 1694 cm−1 in NR/silica spectrum give the existence of aldehyde, ester carbonyl, respectively that means extracted natural rubber still contains aldehyde group and ester group. The former can be interpreted as the existence of aldehyde on rubber chain end [41,42]. The latter might be due to ester which can not be extracted. But, the aldehyde should be thimbleful for its low relative absorption peak intensity. Thus, hydrogen bond could be built between carbonyl and silicon hydroxyl that induces the shift of silicon hydroxyl from 3458 cm−1 to 3295 cm−1 . Comparing with the spectrum of NR/silica, the relative content of ester carbonyl in NR/silica/AE has been changed for its rela-
tive intensity ( C O/ CH3 ) decreased from 0.64 (1694 cm−1 ) to 0.58 (1696 cm−1 ), which means the adsorption of carbonyl compound from AE on silica surface successfully. According to the infrared spectrum of AE, the main carbonyl comes out of carboxylic acid and aliphatic acid ester. Therefore, the variation of ester carbonyl in NR/silica/AE should be in relation to carboxylic acid or aliphatic acid ester. Simultaneously, combining the disappearance of carboxy hydroxyl at 3407 cm−1 in AE, the change of intensity of ester carbonyl at 1696 cm−1 should be due to the reaction between COOH and Si OH. The position of peak at 1696 cm−1 is ascribed to the redshift effect of the newly formed ester carbonyl for its hydrogen-bond interaction with the residual silicon hydroxyl, because the absorption peak of carbonyl in alkyl ester is at 1730 cm−1 normally[35]. The hydrogen-bond also made a shift of silicon hydroxyl from 3458 cm−1 to 3312 cm−1 . On the other hand, ester carbonyl existed before and unreacted carboxyl might contribute to the absorption of 1696 cm−1 peak as well. Besides, the shift of alcohol hydroxy peak at 3649 cm−1 in silica might be in association with its reactivity and the adsorption of rubber phase. Consequently, hydrogen-bond interaction and covalent bond improve the interaction between silica and NR matrix. 3.6.3. X-ray photoelectron spectroscopy (XPS) analysis X-ray photoelectron spectroscopy is an effective method to investigate the constitution and structure of material surface [32]. Fig. 8 presents the low-resolution XPS spectras of silica and residuums of extracted NR/silica, NR/silica/AE model compounds, also the informations of elements composition on surface of particle are shown in Table 3. As it can be seen, original silica has been modified before experimenting according to the high content of carbon on particle surface, which is in line with the FTIR results. Simultaneously, the mass ratio of O/Si on silica surface is increased from 0.97 to 1.41 when mixing NR and silica together, which means oxygenated chemicals that still exist in rubber phase had been absorbed on silica surface successfully. That is because the hydrogen bond interaction between oxygen and Si-OH. And a larger value of O/Si shows up in NR/silica/AE system, which is caused by the fixation of carbonyl compounds from AE on silica surface. This implies an improved hydrogen bond interaction might be formed between silica and AE compositions resulting in a raise of mechanical and dynamical properties of NR/silica composites. To investigate further the variation of chemical circumstance around silica surface, high- resolution XPS spectra of O 1s are presented in Fig. 9. The binding energy of O 1s for O-Si-O and Si-OH
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Fig. 8. Low-resolution XPS spectra of a) silica, b) residuum of extracted NR/silica model compound, c) residuum of extracted NR/silica/AE model compound.
Table 3 Elements composition of silica, residuums of NR/silica and NR/silica/AE model compounds after extraction. Samples
Silica Residuum of extracted NR/silica model compound Residuum of extracted NR/silica/AE model compound
Element mass content (%)
Mass ratio
C
Si
O
N
O/Si
N/Si
73.85 58.81 59.32
12.75 16.47 10.49
12.38 23.29 30.01
1.03 1.43 0.19
0.97 1.41 2.86
0.08 0.08 0.02
are at 534.3 eV [43], 532.2 eV [44], respectively. The value of O 1 s at 533.2 eV [45] might be caused by hydroxy coming from the original modifier on silica surface, which could be confirmed by FTIR. When mixing with natural rubber, the BE values of O 1 s for O-Si-O, Si-OH and hydroxy shift to 533.6 eV, 532.3 eV, 532.7 eV, respectively, as well as the peak of O1 s for alkyl ester that show up at 532.0 eV ( C O)[46], 533.5 eV ( O ) [47]. This indicates the variation of circumstance around silica surface. After the treatment by AE, the five initial BE peaks of O 1s for O-Si-O, Si-OH, −OH, C O (ester), O (ester) are still present at 533.6 eV, 532.5 eV, 533.0 eV, 532.0 eV, 533.5 eV, severally. Besides, three new peaks at 532.1 eV, 533.4 eV, 531.1 eV show up, indicating the BE of O 1 s for carbonyl of COOH [48], hydroxy of COOH [48], Si O C [49], respectively. These manifest the adsorption of AE on silica surface firmly, which is consistent with the result by FTIR, and the formation of esterification reaction between COOH and Si OH was proved as well. Simultaneously, the hydrogen bond could be formed between carboxyl or ester group and silicon hydroxyl, which influences the chemical environment around oxygen atom inducing the shifts of binding energy of O 1s. In Fig. 9c), a higher peak intensity of O 1s for Si OH comparing with that of Si O C implies slight Si OH had reacted with COOH which was affected by the processing conditions.
3.6.4. Transmission electron microscopy (TEM) TEM images can provide direct information about the structure of fillers in the composites. Fig. 10 illustrates the TEM images of silica and residuums of extracted NR/silica, NR/silica/AE model compounds, respectively. Spherical silica particles can be viewed
clearly with nanoscale dispersion. In Fig. 10b) and c), adsorbed rubber phases which are marked with white circles could be observed obviously. Thus, the images prove the existence of bond rubber directly. Simultaneously, comparing with the brunet rubber phase in Fig. 10b), it seems to be a more sequential rubber phase exists in Fig. 10c). That indicates, as a modifier, AE could promote the reinforcement effect of silica to NR.
4. Conclusions In summary, acetone extract (AE) from natural rubber did a significant improvement on mechanical and dynamical properties of NR/silica composites which was ascribed to the increased constrained regions and enhanced interfacial interactions. The modulus, tensile strength and elongation at break of NR/silica composites were promoted remarkably via adding AE that indicates the reinforcement and plasticization effects both for AE on NR, as well as the environmental protection feature. Dynamical properties test illustrated that the decreased rolling power loss, temperature raise and DIN abrasion volume of NR/silica composites were obtained notably when adding AE at 4 phr, which means an improvement of rolling resistance and an enhanced abrasion resistance. And the decline of rolling power loss was in line with the decrease of tan␦ value at 60 ◦ C tested by DMA. The interfacial interactions between AE and silica were revealed through FTIR, TGA, XPS and TEM to be the hydrogen bond between C O and Si OH, also the covalent bond induced by esterification reaction of COOH/Si OH, that results in the increase of constrained regions around silica surface. Finally, an excellent application performance of NR/silica com-
T. Xu et al. / Applied Surface Science 423 (2017) 43–52
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Fig. 9. High-resolution XPS spectra of O 1s of a) silica, b) residuum of extracted NR/silica model compound, c) residuum of extracted NR/silica/AE model compound.
Fig. 10. TEM images of silica and NR/silica model compounds: a) silica, b) residuum of NR/silica model compound after extraction, c) residuum of NR/silica/AE model compound after extraction.
posites was achieved in AE system, where AE acted as a natural multifunctional coupling reagent. Our findings make the effects of AE on the interface structure and properties of NR/silica composites clearly, also offer a valuable reference to comprehend the importance of controlling the content of acetone solutes for Hevea brasiliensis planting. The results are meaningful to the application of natural rubber on low rolling resistance tire. Acknowledgments This work was supported by the National Basic Research Program of China [grant numbers 2015CB654700 (2015CB674703)]; the Special Fund for Agro-scientific Research in the Public Interest from the Ministry of Agriculture, China [grant number 201403066];
National Natural Science Foundation of China [grant number 51573051] and the National Key Technology R & D Program of China [grant number 2012BAE01B03]; Strategic New Industry Core Technology Research Project of Guangdong Province (2012A090100017).
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Full Length Article
Influence of acetone extract from natural rubber on the structure and interface interaction in NR/silica composites Tiwen Xu a , Zhixin Jia a,∗ , Lianghui Wu a , Yongjun Chen a , Yuanfang Luo a , Demin Jia a , Zheng Peng b a b
College of Materials Science and Engineering, South China University of Technology, Guangzhou 510640, China Agricultural Product Processing Research Institute, Chinese Academy of Tropical Agriculture Sciences, Zhanjiang 524001, China
a r t i c l e
i n f o
Article history: Received 18 March 2017 Received in revised form 10 June 2017 Accepted 14 June 2017 Available online 15 June 2017 Keywords: Acetone extract Natural rubber Silica Interfacial interaction Dynamical properties
a b s t r a c t It is well known that the coupling reagents as the additional modifiers were often used to improve the reinforcement effect of silica filled natural rubber. Actually, the commercial raw NR is a mixture consisting of polyisoprene and non-isoprene, where the latter one might have impact on the properties of NR/silica composites as an inartificial modifier inside. Thus, investigating the effect of non-isoprene compounds on the structure and properties of NR/silica composites is a novel approach to disclose the peculiarity of NR, which is meaningful to the assessment of NR quality. In this paper, the influences of acetone extract (AE) from natural rubber on the structure and mechanical properties of NR/silica composites were studied. Then the interfacial interactions between AE and silica were also illustrated through Fourier transform infrared spectroscopy (FTIR), thermogravimetic analysis (TGA), X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM). Results demonstrated the existence of hydrogen bond between silica and AE, also the covalent bond induced by esterification reaction between COOH and Si OH, which resulted in an increase of constrained regions around silica surface leading to the promotions on mechanical and dynamical properties of NR/silica composites significantly. © 2017 Published by Elsevier B.V.
1. Introduction Natural rubber (NR), which is coagulated from Hevea brasiliensis latex, is widely used for its 42% proportion of global elastomer consumption in industry. 70% of NR consumption are used in tyre and anti-vibration industries because of its remarkable properties. It is well known that the rolling of tire would generate hysteresis energy loss leading to a large consumption of gasoline, which could produce much more CO2 to enhance greenhouse effect and also make dust particles pollution. Therefore, a high performance tire should avoid these disadvantages, where it could be done via constructional design and optimizing natural rubber composites. Normally, materials researchers concern additives, such as silane coupling agent, which can be used to improve dynamic properties of natural rubber composites, rarely observing the role of non-isoprene compounds of natural rubber itself in filled natural rubber composites. Reportedly, natural rubber is composed of cis1,4-polyisoprene (95% w/w) and non-isoprene compounds which
∗ Corresponding author. E-mail address: [email protected] (Z. Jia). http://dx.doi.org/10.1016/j.apsusc.2017.06.150 0169-4332/© 2017 Published by Elsevier B.V.
includes proteins or polypeptides (1–2% w/w), acetone extract (1–2% w/w), carbohydrates (0.4% w/w) and minerals (0.3–0.7% w/w)[1], all of which are determined by climate, territory, strain and so forth. Although the contents of non-isoprene compounds are relatively less, they are essential for the stability of natural rubber latex, as well as the processing property, curing behaviors and so on. Several studies have shown that non-isoprene compounds influence the mechanical properties of NR [2–6], but the effects of non-isoprene compounds on dynamical properties of filled or unfilled NR composites are still unclear which might be more crucial than static properties in tyre application field. Specially, the influences of non-isoprene compounds on the interface interaction between filler and rubber matrix are rarely reported. As one of the non-isoprene compounds, acetone extract (AE) is important for the quality of NR. It is reported [7] that AE is consist of oleic acid, linoleic acid, stearic acid, sterol ester and so on. Normally, AE has been considered as an ingredient, which has an effect on the vulcanization or anti-aging resistance of unfilled natural rubber [8–10]. For example, some accelerants and sterol antioxygen that are contained in AE could make positive functions on curing rate and anti-aging resistance to ozone. Unfortunately, there are no reports about the effects of AE on dynamic and mechanical
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properties of NR/filler composites till now, which might be more significant for the application of NR. Silica, as a common filler, is widely used in passenger tire for its good performance on rolling resistance [11–13]. Due to its drastic polarity ascribed from silicon hydroxyl, it should be modified by coupling reagent [14–16] to diminish agglomeration of particles before using. Thus, considering its active groups, such as carboxyl, aldehyde, carbonyl and alkyl chain, it might be possible to employ AE as a natural multifunctional coupling reagent to improve the interaction between silica and NR matrix. In this work, NR/silica composites with different contents of AE were prepared on double rolls open mill. Dynamic properties of NR/silica composites were studied. Simultaneously, the reinforced mechanism and interface interactions between silica and AE were illustrated through DMA, DSC, FTIR, TGA, XPS and TEM. The results made the impacts of AE on the structure and dynamic properties of NR/silica composites clearly, which could offer a valuable information to the meaning of controlling the content of acetone solutes in natural rubber latex decided by the Hevea brasiliensis planting. Our study illustrated a novel way to change or improve the interaction between NR and silica from the natural rubber inside. Simultaneously, the conclusions are useful to the application of natural rubber products. 2. Material and methods 2.1. Materials Natural rubber (NR) with a trade name SCR-WF was friendly supplied by Hainan Sinochem Rubber Company, Ltd (Hainan, China). Precipitated silica (type 518) was kindly offered by Wanzai Huiming Chemical Company, Ltd (Jiangxi, China). BET was determined to be 180 m2 /g from nitrogen adsorption method, pH value was 6.5–7.5 (5% wt aqueous solution), average diameter was 8 m. Other ingredients are commercially available industrial products and used as received. 2.2. Preparation of NR/silica composites with different AE Firstly, natural rubber was extracted by acetone for 48 h through Soxhlet extraction method to obtain pure NR, which was used to prepare NR/silica composites and NR/silica model compound. The residual solution was concentrated using Rotary Evaporator and dried in an oven until constant weight was obtained, which was confirmed to be AE. Extracted NR and other ingredients were mixed on a X(S)K-160 open-mill (Dongguan, China) at room temperature with the speed ratio of 1:1.25. Basic formulation (mass parts): NR 100, silica (0 or 30), AE (0–5), zinc oxide 5, stearic acid 2, sulfur 1.5, N-cyclohexyl2-benzothiazolesulfenamide (accelerator CZ) 1.5, benzothiazole disulfide (accelerator DM) 0.5. Optimum curing time (T90 ) was determined at 143 ◦ C by UR-2030 oscillating disk rheometer (UCAN Technology Company, Ltd,Taiwan). The compounds were vulcanized on a KSH R100 flat vulcanizing machine (Kesheng Machinery Co., Ltd, Dongguan, China) after their accommodation at room temperature for 4 h. 2.3. Preparation of NR/silica model compound NR/silica/AE and NR/silica model compounds with the mass ratio of 100/30/5, 100/30, respectively were made on a LRMR-S150/0 (Lab Tech Engineering Company LTD.) open-mill at room temperature for 15 min. The NR/silica compound was taken as a referential sample and no other additives were added in the whole process. Then, they were extracted in Soxhlet extractor by acetone for 72 h firstly to wipe the free moving components of AE,
and then extracted by toluene for another 72 h to remove the NR molecules from silica surface that were adsorbed weakly. The extracted residuums were analyzed through FTIR, TGA, XPS and TEM to achieve the interfacial interaction mechanisms between AE and Silica. 2.4. Testing and characterization 2.4.1. Mechanical properties of NR/silica composites Tensile properties and tear strength of vulcanizates were determined according to ISO 37-2005 standard (dumbbell A shape), ISO 34-1:2004 (nicked angle), respectively, at 25 ◦ C using a U-CAN UT2060 instrument. Shore A hardness was performed following ISO 7619-1:2004 using a XY-1 durometer (Shanghai). Strain sweeps of raw natural rubber and unvulcanized NR/silica composites were conducted by Rubber Processing Analyzer (RPA) (RPA 2000, Alpha, USA) in the range of 0.7%–200% at 100 ◦ C, 1 Hz. Strain sweeps of vulcanized NR/silica composites were also done by RPA from 0.7% to 200% at 60 ◦ C, 10 Hz. Dynamic mechanical thermal analyzer (DMA) (NETZSCH DMA 242) was used to evaluate damping factors (tan␦) of vulcanized composites. The rectangular specimens (10 mm × 6 mm × 1 mm) were conducted in tensile mode at 10 Hz, 40 um (amplitude). The ◦ temperature was swept from −100 to 80 C at a speed of 3 ◦ C/min. Rolling resistance of composites was obtained through a RSSII rolling resistance testing machine (Beijing Wan Hui technology development co., LTD, China) under a load of 15 kg for 30 min followed by a rotate speed of 600 rpm. Abrasion resistance of vulcanized composites was determinated using a rotating cylindrical drum device (DIN) according to ISO 4649:2002. The sliding distance of the sample was 200 mm. 2.4.2. Differential scanning calorimetry (DSC) The glass transition behaviors of NR/silica composites were detected by a NETZSCH DSC 204 F. The temperature program involved 5 min isothermal hold at −80 ◦ C followed by heating at 3 ◦ C/min to 10 ◦ C under nitrogen. The heat capacity step Cpn and weight fraction of immobilized polymer layer im [17,18] were calculated as follows: Cpn = Cp /(1 − w)
(1)
im = (Cp0 − Cpn )/Cp0
(2)
Where, Cp is the heat capacity jump at Tg . Cpn is normalized to the polymer weight fraction. w is the weight fraction of filler. Cp0 referrers to the heat capacity jump at Tg of the unfilled polymer matrix. im is the weight fraction of immobilized polymer layer. Tg was determined according to ASTM E1356-08, and Cp was calculated by the software NETZSCH Thermal Analysis. 2.4.3. Field emission scanning electron microscopy (SEM) The vulcanized NR/silica composites were made into a shape (15 mm × 6 mm × 1 mm) and then cracked through liquid nitrogen brittle failure. The morphology of fractured surface sprayed by platinum was observed using a ZEISS Merlin SEM (EVO18 German) machine at an acceleration voltage of 10.0 kV. 2.4.4. Fourier transformation infrared spectroscopy (FTIR) FTIR analysis was carried out by a Vector 33 Fourier transform infrared spectrometer (Bruker Co.) in the range of 4000–400 cm−1 . The sample was compressed into platelets with KBr. 2.4.5. Thermal gravity analysis (TGA) Thermal Gravity Analysis was executed using a NETZSCH TG 209 F1 in the range of 35 ◦ C–700 ◦ C under nitrogen at a heating rate of ◦ 20 C/min.
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Table 1 Mechanical properties of NR/silica composites. AE (phr)a
Properties
Modulus at 100% elongation (Mpa) Modulus at 300% elongation (Mpa) Modulus at 500% elongation (Mpa) Tensile strength (Mpa) Elongation at break (%) Tear strength (KN•m−1 ) Shore A hardness a
0
1
2
3
4
5
1.03 ± 0.06 2.43 ± 0.16 6.53 ± 0.31 12.07 ± 0.59 641 ± 24 24.70 ± 0.04 44 ± 1
1.17 ± 0.07 2.85 ± 0.10 7.45 ± 0.21 15.87 ± 0.61 686 ± 15 25.72 ± 0.47 47 ± 1
1.18 ± 0.08 2.95 ± 0.08 7.89 ± 0.26 19.13 ± 0.89 724 ± 14 27.95 ± 0.55 48 ± 0
1.14 ± 0.13 3.03 ± 0.12 8.38 ± 0.16 21.89 ± 0.81 725 ± 14 28.56 ± 0.11 48 ± 1
1.06 ± 0.07 2.78 ± 0.10 7.68 ± 0.30 21.12 ± 0.98 733 ± 26 31.05 ± 0.46 46 ± 1
1.05 ± 0.12 2.74 ± 0.11 7.56 ± 0.18 21.02 ± 0.81 764 ± 14 26.22 ± 0.48 45 ± 1
phr is the part per hundred part of rubber.
2.4.6. X-ray photoelectron spectroscopy (XPS) XPS spectrum was recorded using a X-ray photoelectron spectrometer (Kratos Axis Ultra DLD, England). Operating voltage and electric current were 15 kV and 5 mA, respectively. The vacuum degree of analysis room was 5 × 10−9 torr and analysis area was 0.7 mm × 0.3 mm. A monochromated aluminum K␣ source (1486.6 eV) was used and all XPS spectra were individually calibrated to its reference C1s component at 284.8 eV [19]. Shirley type background and Gaussian functions were employed in the fitting program to distinguish the delicate difference of the chemical environment with XPS Peak 4.1 software [19,20]. 2.4.7. Transmission electron microscopy (TEM) The extracted NR/silica/AE model compound was dispersed firstly at 0.25 wt% concentration in ethanol, and then treated using a KQ3200 ultrasonic dispersion apparatus for 2 h. The suspension was dropped into a copper net for observation. A JEM-2100F TEM machine (Japan) was used to observe the morphology of the sample at the accelerating voltage of 80 kV.
cal properties of NR/silica composites by AE indicate an enhanced interfacial interaction between silica and rubber phase. 3.2. Dynamical mechanical properties of NR/silica composites DMA measurement can be applied in detecting the variation of molecular mobility for polymer segments in the vicinity of silica surface. Fig. 1(a) performs the effect of AE on the tan␦ values of NR/silica composites. Comparing with the unfilled NR vulcanizate, the glass transition temperature of NR shifts from −65.5 ◦ C to −52.9 ◦ C, and the height of transition peak decreases via adding silica. This means a formation of constrained region in composites. Simultaneously, the transition peak of NR/silica composites exhibits no distinct shift while increasing AE, but a slight decline for the peak height, which means an increase of the volume percent of constrained region [21]. To investigate the constrained region of NR/silica composites further, the volume fraction of constrained region was calculated through the equations [22–24] below: W=
tan␦ tan␦ + 1
3. Results and discussion C=1− 3.1. Mechanical properties of NR/silica composites Table 1 presents the mechanical properties of NR/silica composites with different AE. As it can be seen, the modulus at 100%, 300%, 500% elongation, tensile strength and shore A hardness all are increased gradually by increasing AE until they reach a threshold value when AE is 3 phr. Tear strength also is increased firstly by loading AE and gets its optimum value when AE is 4 phr. Simultaneously, the elongation at break shows a climbing tendency by increasing AE, which might be due to the small molecules in AE that act as plasticizers. The significant improvements on mechani-
(1 − C0 ) W W0
(3) (4)
Where, W is the energy loss fraction at the peak point, C0 is the volume fraction of constrained region for unfilled NR and is taken to be 0 here. W0 is the energy loss fraction of unfilled NR. C is the volume fraction of constrained region. The column in Fig. 1(a) lists the values of C of NR/silica composites. It is observed that the volume fraction of constrained region is increased via adding AE until it obtains the maximum value at 2 phr, which induces an enhanced mechanical properties of NR/silica composites. Then, excess AE might make a plasticization effect [25] signally to rubber molecular chains leading to an improvement of molecular mobility.
Fig. 1. Influences of AE on DMA curves of tan␦ vs temperature (a) and RPA strain sweep results (b) of NR/silica composites.
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Table 2 Rolling resistance and DIN abrasion volume of NR/silica composites. AE (phr)
0 1 2 3 4 5
Properties ◦
Rolling power loss (J/r)
Rolling temperature rise ( C)
DIN abrasion volume (cm3 )
4.17 ± 0.08 3.22 ± 0.01 3.05 ± 0.05 2.95 ± 0.03 2.58 ± 0.01 3.12 ± 0.04
38.2 ± 1.4 30.8 ± 1.1 29.0 ± 0.6 28.8 ± 0.4 25.0 ± 0.1 29.4 ± 0.9
0.164 ± 0.018 0.148 ± 0.003 0.145 ± 0.004 0.144 ± 0.001 0.134 ± 0.007 0.118 ± 0.017
In the range of 50–70 ◦ C, tan␦ value at the same temperature is decreased markedly by increasing AE and it achieves the minimum value when AE is 4 phr. Taking the tan␦ value at 60 ◦ C as an example, it is reduced by 37.2% at 4 phr AE comparing with that of unmodified NR/silica composites. The variation tendency of tan␦ at 60 ◦ C by DMA is in line with the results of RPA strain sweep what we see in Fig. 1(b). Synchronously, the tan␦ value around 0 ◦ C shows no apparent change with the addition of AE. According to viscoelastic theory, an ideal material which is needed by high performance tire should perform a low tanı value at 60 ◦ C to reduce rolling resistance or save energy, and also demonstrate a high hysteresis at 0 ◦ C to obtain high wet skid resistance or wet grip [26]. The results indicate that the rolling resistance and wet skid resistance of NR/silica composites could be improved or stable via adding appropriate AE, respectively. The optimized content of AE in these experiments is 4 phr. Rolling power loss and temperature rise of NR/silica composites modified by AE are conducted in Table 2. It can be seen that the rolling power loss and temperature rise both decrease memorably before reaching a critical value with the increase of AE. The lowest values are obtained at 4 phr AE, where they were diminished by 38.1%, 34.6% respectively in contrast with that of the unmodified NR/silica composites. The results imply that appropriate AE could improve the rolling resistance of NR/silica composites observably. DIN abrasion volume of composites is minished following the increase of AE, which means an improvement of the abrasion resistance. These phenomenons should be caused by the components in AE, where alkyl ester, carboxylic acid and aldehyde compounds [9] are contained that enhanced the interfacial interactions between silica and rubber phase.
3.3. Glass transition behaviors of NR/silica composites The mobility of polymer molecules are sensitive to the local environment because of the characteristic long-chain morphology. Therefore, Tg and normalized change of heat capacity (Cp ) of a polymer at the glass transition region are important parameters that could offer information about the structural variation during transition[21]. Fig. 2 depcits the glass transition behaviors of unfilled NR and NR/silica composites modified by AE. The normalized heat capacity(Cpn ) as a direct evidence of polymer chain mobility [21,27] is displayed in Fig. 2. The greater value of Cpn at the glass transition region means a better chain mobility. As seen in Fig. 2, taking unfilled NR as the reference, normalized Cpn of NR/silica composites all decrease and they manifest a reductive change with the increase of AE, which suggests more and more polymer chains are confined among filler galleries. The results are also certified by the increased weight fraction of immobilized polymer layer (im ). The consequence about constrained region detected by DSC demonstrates a parallel relationship with that obtained by DMA. The results can be interpreted with the components of AE, where alkyl acid, alkyl ester, aldehyde compound, etc exist. These polar oxygenous groups could react with the silicon hydroxyl on silica surface through hydrogen-bond or covalent bond
interaction. Consequently, silica surface was encapsulated gradually by organic hydrocarbon chains that improved the compatibility between rubber and silica. Thus, a thick crosslinked immobilized polymer layer around silica surface was formed after vulcanization. The immobilized polymer layer would prevent the macro-phase separation between rubber and silica, leading to a constrained compatibility and strong rubber-silica interfacial interaction. Meanwhile, Tg parameter of NR/silica composites is determined to shift to a lower temperature via adding AE comparing with that of unfilled NR. The result is in accordance with other researchers’ works [28,29]. This can be explained in terms of two points: one is the average size of constrained polymer region is greater than the length scale of segmental motion, so the relaxation is independent from composition [30]. The other might attribute to the small molecular weight alkylates in AE which show up as the plasticizers [25], that is why the rolling power loss of NR/silica composites was increased when superfluous AE was added.
3.4. Reinforcement mechanisms of silica in NR matrix modified by AE The schematic diagram of silica reinforced NR matrix modified by AE is presented in Figs. 3 and 4 shows the relevant evidences of structural variation of NR through RPA strain sweep. Before the preparation of NR/silica/AE composites, the natural rubber was extracted by acetone for 48 h to remove acetone solutes, then extracted raw NR and a battery of experiments were designed. Extraction treatment by acetone solvent influenced the raw NR network significantly and made a disentanglement phenomenon appear which results in a decrease of the number of physical entanglement points. The viewpoint could be proved by the reduction of shear modulus (G’) of raw NR after extraction in Fig. 4(b). When silica was added into the NR matrix, AE could improve the compatibility among the two phases by linking silica aggregates and NR matrix together through their alkyl chains and oxygen-containing groups, which promote the mobility of filler particles and make them to be the new physical entanglement points. These effects were more pronounced than that of unmodified silica system, that is why a larger constrained layer was formed in NR/silica/AE composites. In Fig. 4(a), due to the enhanced formation of physical entanglement points, Payne effects of unvulcanized NR/silica/AE composites display a higher G’ value than that of NR/silica composites, it is not the same conception as we understand usually for the agglomeration of filler [31]. So an improved compatibility resulted in an enhanced interfacial interaction between silica and NR matrix, also made an increase of G’, which is in line with other report [30]. Meantime, G’ is increased with the increase of AE until it obtained a threshold value at 2 phr. The diminish of G’ for composites might be ascribed to the plasticizing effect [25] of AE. That means AE could act as both interfacial modifier and plasticizer in NR/silica composites.
T. Xu et al. / Applied Surface Science 423 (2017) 43–52
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Fig. 2. Effects of AE on glass transition behaviors of unfilled NR and NR/silica composites.
Fig. 3. Reinforcement mechanism of NR/silica composites with and without AE.
Fig. 4. Shear modulus (G’) versus shear strain (e): a) unvulcanized NR/silica composites modified by AE with different contents; b) raw natural rubber with or without extraction by acetone (the two samples were homogenized on open mill for 10 times before RPA test).
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Fig. 5. SEM images of NR/silica composites modified by AE: a) 0phr, b) 1phr, c) 2phr, d) 3phr, e) 4phr, f) 5phr.
3.5. SEM morphology of NR/silica composites To investigate the dispersion of silica in NR matrix, SEM observations of brittle fracture surfaces of composites are performed in Fig. 5. As we can see, none of apparent silica aggregates are found before and after modification by AE in NR/silica composites, which implies a good dispersion of silica with nanoscale in NR matrix. This result should be a direct evidence to support the interpretation of Fig. 4a). Simultaneously, with the increase of AE, few silica particles are presented in sight until AE was added at 5 phr that indicates an improved compatibility between silica and NR that made silica imbeded into NR matrix. The enhancements on modulus and elongation at break in Table 1 should be caused by these reasons. 3.6. Mechanism of interfacial interaction in NR/silica/AE model compound To study further the mechanism of interfacial interaction between silica and AE, NR/silica/AE model compounds were prepared. Then, TGA, FTIR, XPS and TEM were used to characterize the specimens. The results are shown as below. 3.6.1. Thermal gravity analysis (TGA) TGA curves of silica and residuums of extracted NR/silica, NR/silica/AE model compounds are displayed in Fig. 6. It can be seen that residuum of extracted NR/silica/AE model compound demonstrates better thermal stability and larger weight loss than
Fig. 6. TGA curves of silica (A), residuum of extracted NR/silica model compound (B), residuum of extracted NR/silica/AE model compound (C).
that of residuum of extracted NR/silica model compound. The improved thermal stability with a higher onset temperature should be ascribed to the enhanced interfacial interaction between silica and NR modified by AE, which leads to an increase of constrained regions. Besides, a larger weight loss at 700 ◦ C suggests more rubber chains were absorbed on silica surface, which also provides a direct evidence of enhanced interface interaction between silica and NR matrix.
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Fig. 7. FTIR spectra of specimens: a) silica and Acetone extract (AE), b) residuums of extracted NR/silica and NR/silica/AE model compounds.
3.6.2. Fourier transform infrared spectroscopy (FTIR) FTIR spectrum was used to explore the chemical environment changes on silica surface and the graphs of silica, acetone extract (AE), residuums of extracted NR/silica, NR/silica/AE model compounds, respectively, are viewed in Fig. 7. In Fig. 7a), the absorption peaks for Si-OH and Si-O-Si asymmetrical stretching vibration of silica are observed at 3458 cm−1 , 1104 cm−1 , respectively. 3649 cm−1 is assigned to alcohol hydroxy [32] and 2925 cm−1 [33] stands for the C H asymmetrical stretching vibration of CH2 which means the provided silica had been modified before. In spectrum of acetone extract (AE), the absorption peak at 2970 cm−1 is the asymmetrical stretching vibration of CH3 and its symmetrical deformation vibration is observed at 1378 cm−1 [34]. Peaks at 2924 cm−1 , 2854 cm−1 are typical of C H asymmetrical stretching vibration, symmetrical stretching vibration of CH2 , respectively, also the peak at 1461 cm−1 for the deformation vibration of CH2 [34,35]. 3407 cm−1 is deduced by the stretching vibration of OH [36] of carboxylic acid and 1714 cm−1 ascribes to the stretching vibration of C O of aliphatic acid [37] and 1742 cm−1 is due to the stretching vibration of C O of aliphatic acid ester [35]. Besides, the peak at 2729 cm−1 is due to the C H [38,39] stretching vibration of aldehyde group. 954 cm−1 might attribute to the C H out-ofplane bending vibration of alkene [35]. The results indicate that AE must contain organic aliphatic acid, aliphatic acid ester, aldehyde, alkene and so on. In Fig. 7b), peaks at 1660 cm−1 , 2963 cm−1 ,2929 cm−1 stand for the stretching vibration absorption of C C , CH3 , CH2 of natural rubber [1,40]. Taking the spectrum of residuum of extracted NR/silica model compound as the reference, the intensity ratio of CH3 /Si O Si is increased from 0.1786 to 0.2891 (increased by 61.9%) after modification by AE, which means more rubber chains had been absorbed on silica surface and an enhanced interaction was formed in NR/silica/AE model compound. The appearance of peaks at 2728 cm−1 , 1694 cm−1 in NR/silica spectrum give the existence of aldehyde, ester carbonyl, respectively that means extracted natural rubber still contains aldehyde group and ester group. The former can be interpreted as the existence of aldehyde on rubber chain end [41,42]. The latter might be due to ester which can not be extracted. But, the aldehyde should be thimbleful for its low relative absorption peak intensity. Thus, hydrogen bond could be built between carbonyl and silicon hydroxyl that induces the shift of silicon hydroxyl from 3458 cm−1 to 3295 cm−1 . Comparing with the spectrum of NR/silica, the relative content of ester carbonyl in NR/silica/AE has been changed for its rela-
tive intensity ( C O/ CH3 ) decreased from 0.64 (1694 cm−1 ) to 0.58 (1696 cm−1 ), which means the adsorption of carbonyl compound from AE on silica surface successfully. According to the infrared spectrum of AE, the main carbonyl comes out of carboxylic acid and aliphatic acid ester. Therefore, the variation of ester carbonyl in NR/silica/AE should be in relation to carboxylic acid or aliphatic acid ester. Simultaneously, combining the disappearance of carboxy hydroxyl at 3407 cm−1 in AE, the change of intensity of ester carbonyl at 1696 cm−1 should be due to the reaction between COOH and Si OH. The position of peak at 1696 cm−1 is ascribed to the redshift effect of the newly formed ester carbonyl for its hydrogen-bond interaction with the residual silicon hydroxyl, because the absorption peak of carbonyl in alkyl ester is at 1730 cm−1 normally[35]. The hydrogen-bond also made a shift of silicon hydroxyl from 3458 cm−1 to 3312 cm−1 . On the other hand, ester carbonyl existed before and unreacted carboxyl might contribute to the absorption of 1696 cm−1 peak as well. Besides, the shift of alcohol hydroxy peak at 3649 cm−1 in silica might be in association with its reactivity and the adsorption of rubber phase. Consequently, hydrogen-bond interaction and covalent bond improve the interaction between silica and NR matrix. 3.6.3. X-ray photoelectron spectroscopy (XPS) analysis X-ray photoelectron spectroscopy is an effective method to investigate the constitution and structure of material surface [32]. Fig. 8 presents the low-resolution XPS spectras of silica and residuums of extracted NR/silica, NR/silica/AE model compounds, also the informations of elements composition on surface of particle are shown in Table 3. As it can be seen, original silica has been modified before experimenting according to the high content of carbon on particle surface, which is in line with the FTIR results. Simultaneously, the mass ratio of O/Si on silica surface is increased from 0.97 to 1.41 when mixing NR and silica together, which means oxygenated chemicals that still exist in rubber phase had been absorbed on silica surface successfully. That is because the hydrogen bond interaction between oxygen and Si-OH. And a larger value of O/Si shows up in NR/silica/AE system, which is caused by the fixation of carbonyl compounds from AE on silica surface. This implies an improved hydrogen bond interaction might be formed between silica and AE compositions resulting in a raise of mechanical and dynamical properties of NR/silica composites. To investigate further the variation of chemical circumstance around silica surface, high- resolution XPS spectra of O 1s are presented in Fig. 9. The binding energy of O 1s for O-Si-O and Si-OH
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Fig. 8. Low-resolution XPS spectra of a) silica, b) residuum of extracted NR/silica model compound, c) residuum of extracted NR/silica/AE model compound.
Table 3 Elements composition of silica, residuums of NR/silica and NR/silica/AE model compounds after extraction. Samples
Silica Residuum of extracted NR/silica model compound Residuum of extracted NR/silica/AE model compound
Element mass content (%)
Mass ratio
C
Si
O
N
O/Si
N/Si
73.85 58.81 59.32
12.75 16.47 10.49
12.38 23.29 30.01
1.03 1.43 0.19
0.97 1.41 2.86
0.08 0.08 0.02
are at 534.3 eV [43], 532.2 eV [44], respectively. The value of O 1 s at 533.2 eV [45] might be caused by hydroxy coming from the original modifier on silica surface, which could be confirmed by FTIR. When mixing with natural rubber, the BE values of O 1 s for O-Si-O, Si-OH and hydroxy shift to 533.6 eV, 532.3 eV, 532.7 eV, respectively, as well as the peak of O1 s for alkyl ester that show up at 532.0 eV ( C O)[46], 533.5 eV ( O ) [47]. This indicates the variation of circumstance around silica surface. After the treatment by AE, the five initial BE peaks of O 1s for O-Si-O, Si-OH, −OH, C O (ester), O (ester) are still present at 533.6 eV, 532.5 eV, 533.0 eV, 532.0 eV, 533.5 eV, severally. Besides, three new peaks at 532.1 eV, 533.4 eV, 531.1 eV show up, indicating the BE of O 1 s for carbonyl of COOH [48], hydroxy of COOH [48], Si O C [49], respectively. These manifest the adsorption of AE on silica surface firmly, which is consistent with the result by FTIR, and the formation of esterification reaction between COOH and Si OH was proved as well. Simultaneously, the hydrogen bond could be formed between carboxyl or ester group and silicon hydroxyl, which influences the chemical environment around oxygen atom inducing the shifts of binding energy of O 1s. In Fig. 9c), a higher peak intensity of O 1s for Si OH comparing with that of Si O C implies slight Si OH had reacted with COOH which was affected by the processing conditions.
3.6.4. Transmission electron microscopy (TEM) TEM images can provide direct information about the structure of fillers in the composites. Fig. 10 illustrates the TEM images of silica and residuums of extracted NR/silica, NR/silica/AE model compounds, respectively. Spherical silica particles can be viewed
clearly with nanoscale dispersion. In Fig. 10b) and c), adsorbed rubber phases which are marked with white circles could be observed obviously. Thus, the images prove the existence of bond rubber directly. Simultaneously, comparing with the brunet rubber phase in Fig. 10b), it seems to be a more sequential rubber phase exists in Fig. 10c). That indicates, as a modifier, AE could promote the reinforcement effect of silica to NR.
4. Conclusions In summary, acetone extract (AE) from natural rubber did a significant improvement on mechanical and dynamical properties of NR/silica composites which was ascribed to the increased constrained regions and enhanced interfacial interactions. The modulus, tensile strength and elongation at break of NR/silica composites were promoted remarkably via adding AE that indicates the reinforcement and plasticization effects both for AE on NR, as well as the environmental protection feature. Dynamical properties test illustrated that the decreased rolling power loss, temperature raise and DIN abrasion volume of NR/silica composites were obtained notably when adding AE at 4 phr, which means an improvement of rolling resistance and an enhanced abrasion resistance. And the decline of rolling power loss was in line with the decrease of tan␦ value at 60 ◦ C tested by DMA. The interfacial interactions between AE and silica were revealed through FTIR, TGA, XPS and TEM to be the hydrogen bond between C O and Si OH, also the covalent bond induced by esterification reaction of COOH/Si OH, that results in the increase of constrained regions around silica surface. Finally, an excellent application performance of NR/silica com-
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Fig. 9. High-resolution XPS spectra of O 1s of a) silica, b) residuum of extracted NR/silica model compound, c) residuum of extracted NR/silica/AE model compound.
Fig. 10. TEM images of silica and NR/silica model compounds: a) silica, b) residuum of NR/silica model compound after extraction, c) residuum of NR/silica/AE model compound after extraction.
posites was achieved in AE system, where AE acted as a natural multifunctional coupling reagent. Our findings make the effects of AE on the interface structure and properties of NR/silica composites clearly, also offer a valuable reference to comprehend the importance of controlling the content of acetone solutes for Hevea brasiliensis planting. The results are meaningful to the application of natural rubber on low rolling resistance tire. Acknowledgments This work was supported by the National Basic Research Program of China [grant numbers 2015CB654700 (2015CB674703)]; the Special Fund for Agro-scientific Research in the Public Interest from the Ministry of Agriculture, China [grant number 201403066];
National Natural Science Foundation of China [grant number 51573051] and the National Key Technology R & D Program of China [grant number 2012BAE01B03]; Strategic New Industry Core Technology Research Project of Guangdong Province (2012A090100017).
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