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halloysite nanotubes nanocomposites modified by methacrylic acid
Applied Surface Science 255 (2008) 2715–2722
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Styrene–butadiene rubber/halloysite nanotubes nanocomposites modified by methacrylic acid Baochun Guo *, Yanda Lei, Feng Chen, Xiaoliang Liu, Mingliang Du, Demin Jia Department of Polymer Materials and Engineering, South China University of Technology, Guangzhou 510640, China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 22 July 2008 Received in revised form 30 July 2008 Accepted 31 July 2008 Available online 8 August 2008
Methacrylic acid (MAA) was used to improve the performance of styrene–butadiene rubber (SBR)/ halloysite nanotubes (HNTs) nanocomposites by direct blending. The detailed interaction mechanisms of MAA and the in situ formed zinc methacrylate (ZDMA) were revealed by X-ray diffraction (XRD), surface area and porosity analysis, X-ray photoelectron spectroscopy (XPS) together with crosslink density determination. The strong interfacial bonding between HNTs and rubber matrix is resulted through ZDMA and MAA intermediated linkages. ZDMA connects SBR and HNTs via grafting/complexation mechanism. MAA bonds SBR and HNTs through grafting/hydrogen bonding mechanism. Significantly improved dispersion of HNTs in virtue of the interactions between HNTs and MAA or ZDMA was achieved. Effects of MAA content on the vulcanization behavior, morphology and mechanical properties of the nanocomposites were investigated. Promising mechanical properties of MAA modified SBR/HNTs nanocomposites were obtained. The changes in vulcanization behavior, mechanical properties and morphology were correlated with the interactions between HNTs and MAA or ZDMA and the largely improved dispersion of HNTs. ß 2008 Elsevier B.V. All rights reserved.
Keywords: Styrene–butadiene rubber Halloysite nanotubes Methacrylic acid Interface Reinforcing mechanism
In the past decade, rubber nanocomposites with layered clay such as montmorillonite have exhibited their unusual performance such as significant reinforcing effect and impressive lowered permeability compared with those of the conventional rubber vulcanizates reinforced by carbon black or silica [1–7]. However, in order to obtain desired performance, sophisticated control on the exfoliation and intercalation should be done and this greatly increases the fabrication cost and uncertainty of the performance [8–14]. Consequently, exploitation on the facile process for rubber nanocomposites is highly desired. Fibrillar silicate such as attapulgite or palygorskite was found to have much weaker interactions among the single crystals than interlayer force of montmorillonite clay. Consequently, through conventional polymer melt blending, fibrillar silicate could be decohered into single crystals or crystal bundles with diameters less than 100 nm in polymeric matrix [15–19]. Alternatively, halloysite nanotubes (HNTs) with large L/D ratio also offer great opportunities for fabricating polymer nanocomposites with promising performance [20–24]. HNTs with molecular formula of Al2Si2O5 (OH)4nH2O, are multiwalled kaolinite nanotubes. The tubular
* Corresponding author. Fax: +86 20 2223 6688. E-mail address: [email protected] (B. Guo). 0169-4332/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2008.07.188
halloysite is formed by rolling of kaolin sheet in preference to tetrahedral rotation to correct misfit of the octahedral and tetrahedral sheets [25]. Comparing with carbon nanotubes (CNTs), the naturally occurring HNTs are much cheaper and easily available. An ideal HNTs crystal consists of a nonexpansive, layered structure that contains octahedrally coordinated Al3+ and tetrahedrally coordinated Si4+ in a 1:1 stoichiometric ratio [26]. Different from most of clay, most aluminols located in the inner of the HNTs, while in the outer of the HNTs are primary siloxane and few silanols and aluminols which are exposed in the edges of the sheet. It is well recognized that tight interfacial bonding and fine dispersion of the clay are two crucial factors to determine the performance of the rubber/clay nanocomposites [27]. For less active surface property [24,28], however, HNTs can hardly be effective filler for rubber because of the unsatisfied interfacial bonding and agglomeration in the rubber matrix. Dispersion of clay by means of the substances bearing hydrogen bonding functionalities has been widely reported. For instances, the dispersion of clay such as montmorillonite could be drastically improved in the matrices such as carboxyl methyl cellulose sodium salt, polyacrylic acid sodium salt [29] or poly(2-methoxyethyl acrylate) (PMEA) [30] or poly(vinyl acetate) [31]. Also, methacrylic acid (MAA) was used as interfacial modifier for polymer/inorganics composites [32,33].
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The affinity of MAA to the clay is due to the formation of hydrogen bonds between clay and the acid. In addition, MAA is reactive to polymer chains such as unsaturated rubber chains or saturated polyolefins via free radical grafting. Specially, MAA is reactive to zinc oxide (ZnO) or magnesium oxide (MgO) to yield unsaturated metal methacrylates which show unusual reinforcing effects for rubber compounds. In situ generated metal methacrylates were reported to reinforce rubber compounds during the compounding [34–39]. To obtain satisfied dispersion of HNTs and strong interfacial bonding between HNTs and styrene–butadiene rubber (SBR), in the present study, MAA was utilized to modify the SBR/HNTs composites. The interactions between HNTs and MAA or in situ formed zinc methacrylate (ZDMA) were characterized fully. The significantly improved mechanical properties were correlated to the largely improved interfacial bonding between HNTs and rubber and significantly enhanced dispersion of HNTs.
solvent and the surface toluene was quickly blotted off. The samples were immediately weighed and then dried in a vacuum oven for 36 h at 80 8C to remove all the solvent and reweighed. The volume fraction of SBR in the swollen gel, yr, was calculated by the following equation [40]:
yr ¼
m0 fð1 aÞ=rr m0 ð1 aÞ=rr þ ðm2 m1 Þ=rs
(1)
where m0 is the sample mass before swelling, m1 and m2 are sample masses before and after drying, respectively, f is the mass fraction of rubber in the vulcanizate, a is the mass loss of the gum SBR vulcanizate during swelling, and rr and rs are the rubber and solvent density, respectively. The elastically active network chain density, ye, which was used to represent the whole crosslink density, was then calculated by the well-known Flory–Rehner equation [41]: lnð1 yr Þ þ yr þ xyr 2 ys ðyr 1=3 yr =2Þ
1. Experiments
ye ¼
1.1. Materials
where yr is the volume fraction of the polymer in the vulcanizate swollen to equilibrium and ys is the solvent molar volume (107 cm3/mol for toluene). x is the SBR–toluene interaction parameter and is taken as 0.0653 calculated according to reference [42]. As mentioned above, the vulcanizates contained both covalent and ionic crosslinks. To distinguish ionic crosslinks from covalent crosslinks, samples were swollen again in the mixture of toluene and dichloroacetic acid (90:10 in mass) for 120 h to destroy ionic crosslinks, swollen in toluene for 72 h and weighed, and then vacuum-dried and reweighed. yr1 calculated by Eq. (1) represents the extent of swelling after destroying ionic crosslinks. ye1 calculated by Eq. (2) represented the covalent crosslink density. ye2 calculated by subtracting ye1 from ye represented ionic crosslink density.
Styrene–butadiene rubber, with trademark SBR1502 (styrene content 23.5 wt.%), was manufactured by Jilin Chemical Industry Company, China. Halloysite nanotubes (HNTs) were mined from Hubei province, China and purified according to the reported procedure [23]. Methacrylic acid was analytical grade from Guangzhou Reagent Factory. Chemical grade zinc methacrylate (ZDMA) was provided by Xi’an Chemical Factory. Other rubber additives were industrial grade and used as received. 1.2. Preparation of the SBR/HNTs nanocomposites SBR and HNTs were compounded with rubber additives with a two-roll mill and then compression molded at 170 8C. The compositions of the rubber compounds were tabulated in Table 1. The compounds were vulcanized at 170 8C T90. 2. Characterizations 2.1. X-ray diffraction (XRD) for rubber compounds and vulcanizates When recording XRD patterns of the uncured compounds, only necessary ingredients, SBR, HNTs, ZnO and MAA, were included in the formula. The X-ray diffraction experiments for the uncured compounds and the vulcanizates were conducted at ambient temperature on a Rigaku Dmax/III diffractometer using Cu Ka radiation (l = 1.54 A˚). The generator was operated at 40 kV and 30 mA. The samples of different shapes were scanned from 28 to 508 with a step length of 0.028.
(2)
2.3. Surface area and pore structure analysis In order to explore the possible interactions between HNTs and MAA (or ZDMA), two model compounds were prepared. The blend of HNTs/ZDMA/DCP (weight ratio of 73/27/0.04) and that of HNTs/ MAA/DCP (weight ratio of 72/28/0.4) were pre-mixtured and treated at 170 8C for 30 min in sealed vessels. The heated mixtures were grinded and sieved in fine powders for surface area determination and pore structure analysis. Pore analysis of the HNTs and the two model compounds were investigated using nitrogen adsorption–desorption isotherms with Micromeretics ASAP 2020 analyzer. The pore-size distributions were computed by applying the Barrett–Joyner–Halenda (BJH) methods [43]. 2.4. X-ray photoelectron spectroscopy (XPS)
2.2. Determination of crosslink density Equilibrium swelling method was used to determine the crosslink density of the vulcanizates. Samples were swollen in toluene at room temperature for 72 h and then removed from the
XPS spectra of the HNTs and the two compounds were recorded by using an X-ray photoelectron spectrometer (Kratos Axis Ultra DLD) with an aluminum (mono) Ka source (1486.6 eV). The aluminum Ka source was operated at 15 kV and 10 mA. For all the
Table 1 Composition of SBR/HNTs with MAA nanocompositesa Sample code
SBR
SBR2A
SBR3A
SBR4A
SBR5A
SBR7A
SBR9A
SBR12A
SBR15A
SBR HNTs MAA
100 40 0
100 40 2
100 40 3
100 40 4
100 40 5
100 40 7
100 40 9
100 40 12
100 40 15
a
Rubber intergradient: zinc oxide, 5 phr; stearic acid, 1 phr; dicumyl peroxide (DCP), 1.0 phr; N-isopropyl-N0 -phenyl-p-phenylenediamine (4010NA), 1.5 phr.
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samples, a high-resolution survey (pass energy = 48 eV) was performed at spectral regions relating to silicon, aluminum and zinc atoms. All core level spectra were referenced to the C 1s neutral carbon peak at 285.0 eV. 2.5. Vulcanization characteristics The curing characteristics of the SBR compounds were determined at 170 8C by U-CAN UR-2030 vulcameter, Taiwan. 2.6. Morphological observations The tensile fractured surfaces were plated with a thin layer of gold before any observations. The SEM observations were then performed using a LEO1530 VP SEM machine. The specimens were ultramicrotommed into thin pieces of about 120 nm in thickness with Leica EM UC6. Then the TEM observations were done using a Philips Tecnai 12 TEM machine at an accelerating voltage of 30 kV. 2.7. Mechanical properties Tensile tests were performed following ISO standard 37-2005 at 25 8C. Tensile strength, modulus and elongation at break were measured using U-CAN UT-2060 (Taiwan) instrument. Shore A hardness was performed following ISO standard 7619-1986 using a XY-1 sclerometer (Shanghai). 3. Results and discussion 3.1. Formation of ZDMA and its reactivity towards rubber and HNTs ZDMA may be formed through the reaction between ZnO and MAA during the rubber compounding [34,35,39]. This process is revealed in Fig. 1. As shown, the intensity of the peaks at 31.68 and 36.18, characterizing the (1 0 0) and (1 0 1) diffractions of ZnO [44], are consistently decreased with MAA loading to eventually invisible at 7 phr of MAA. Noticeably, with the decreasing of ZnO, the peaks at 9.88 and 10.98, for (2 0 0) and (1 0 1) diffractions of orthogonal ZDMA crystals, do not appear until ZnO is completely exhausted. This indicates that at low concentration ZDMA could be dissolved in SBR matrix. During vulcanization, it is well recognized that ZDMA undergoes self-polymerization to yield reinforcing nanoparticles and grafting onto rubber chains [34–39,45]. This process may be
Fig. 1. X-ray diffraction of uncured SBR compounds.
Fig. 2. X-ray diffraction of vulcanizates with different MAA loading.
verified in Fig. 2, in which the peaks for ZDMA are disappeared for all the samples, suggesting complete conversion of ZDMA during the vulcanization. The co-curing of SBR and ZDMA leads to the formation of ionic crosslinks between the SBR chains. Fig. 3 shows the dependence of the ionic crosslink density (ye2) and total crosslink density (ye) on the MAA concentration. It can be seen that ye2 is increased with the MAA content while too much MAA leads to decreased ye2. This trend may be due to the change of the content of formed ZDMA. More MAA leads to more yield of ZDMA until all ZnO is depleted. Besides the self-polymerization and grafting onto SBR, the interactions between ZDMA and HNTs are also important in the present system. Adsorption of metal ions onto mineral substrates is an important process in soil chemistry. Adsorption is a complex process, usually involving much more than simple ion exchange on the mineral surface [46–49]. Miyazaki et al. presented experimental data that showed that Zn2+ ions are adsorbed onto amorphous aluminosilicate by forming a complex with the surface OH groups [50,51]. This complexation process may be described as follows: 2fAl; SiOHg þ Zn2þ ¼ fðAl; SiOÞ2 Zng þ 2Hþ
(3)
where {Al, Si–OH} represents aluminol and/or silanol groups on the surface of amorphous aluminosilicate. In the present work, HNTs belong to the class of aluminosilicate and are believed to interact with the Zn2+ in ZDMA via the above complexation during
Fig. 3. Effect of MAA content on crosslink density.
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Fig. 5. XPS results of HNTs/ZDMA model compound (Zn 2p1/2 and Zn 2p3/2).
Fig. 4. Isothermal adsorption–desorption curve (a) and pore distribution (b) of model compounds and HNTs.
compounding. The adsorption of ZDMA on HNTs may be characterized by adsorption experiment. The porosity analysis of HNTs before and after adsorption of ZDMA was performed. The adsorption–desorption curves and the pore distribution curve of HNTs and HNTs/ZDMA model compounds are depicted in Fig. 4. For HNTs, the three peaks of the pore volume vs. pore width curve around 3, 15 and 35 nm are attributed to surface defects, the lumens of the nanotubes and pores among the tubes, respectively. For the HNTs/ZDMA complex, however, all the three peaks are disappeared, suggesting the adsorption of ZDMA on HNTs. This adsorption also leads the decrease of BET area from 50.4 to 33.8 m2/g. XPS survey was performed in order to further substantiate the complexation between ZDMA and HNTs. The complexation between Zn2+ and aluminols or silanols on HNTs may change the chemical environment of Zn2+ due to the changed density of the electron cloud. The result of XPS high resolution survey on Zn2+ is depicted in Fig. 5. For ZDMA, the binding energies for Zn 2p1/2 and Zn 2p3/2 are 1021.8 and 1044.9 eV, respectively. After the complexation, however, the two bands are increased to 1022.3 and 1045.3 eV, respectively, suggesting obvious changes in Zn2+ chemical environment. Consequently, ZDMA acts as special interfacial modifier between rubber matrix and HNTs via dual mechanisms of grafting and complexation. 3.2. Interactions between MAA and HNTs In the present systems, apart from acting as reactant of the formation of ZDMA, MAA is excessive when its concentration is
higher than 7 phr. The remained MAA and its polymer are important in promoting the dispersion of HNTs and strengthening the interface of HNTs and SBR. It is well known that as vinyl monomer MAA may graft onto SBR during vulcanization [52]. The pendent carboxyl groups on the polymerized MAA (or grafted MAA) are reactive to HNTs via hydrogen bonding. This is also substantiated by mains of adsorption and XPS experiments. The adsorption–desorption curves and the pore distribution curve of the HNTs/MAA model compound are also depicted in Fig. 4. It can be seen the BET area of the HNTs/MAA model compound is drastically decreased to 7.7 m2/g. The dramatic decrease in BET area together with the complete disappearing of the pores indicates intensive interaction between HNTs and MAA. It is believed that the formation of hydrogen bonding lead to the variation of the chemical environment for the hydrogen bonding functionalities, which can be characterized by the variation of bonding energy of the related atoms via XPS survey [53–55]. As shown in Fig. 6, there is decrease in the bonding energy of silicon atom and aluminum atom, which is connected to the oxygen atom in the hydrogen bond. The decreases in bonding energy for silicon and aluminum are 0.3 and 0.2 eV, respectively. This confirms the formation of hydrogen bonding in the HNTs/MAA compound. As a consequence, the interface between SBR and HNTs may also be strengthened by MAA via grafting and hydrogen bonding mechanisms. 3.3. Effects of MAA on vulcanization behavior of SBR/HNTs compounds The vulcanization curves were shown in the Fig. 7. It can be seen that the incorporation of MAA leads to significantly improved torque of the vulcanizates. The toque of the vulcanizate is increased with MAA loading when the content of MAA is lower than 12 phr. Overloading of MAA leads to decreased torque. More detailed characteristics were summarized and depicted in the Fig. 8. As shown, the scorch time (Ts2) was practically independent of MAA loading. The vulcanization time (T90) is initially decreased with MAA content. However, when the MAA content is sufficiently high (higher than 12 phr), the vulcanization time starts to increase. The curing rate index (CRI), which indicates the rate of cure of the compounds, is defined as 100/(T90 Ts2). A higher value means a higher rate of vulcanization. The inclusion of MAA accelerates the vulcanization process and the curing rate increases with MAA content. Again, while the MAA content is sufficiently high (higher than 12 phr), the curing rate is to be lowered in some extent. As discussed above, very complicated interactions among rubber matrix and HNTs and the additives are presented in the present systems. These interactions have pronounced influences on the curing characteristics of the MAA included compounds. Once MAA is compounded, the yielded ZDMA and the above mentioned interactions effectively promote the vulcanization. Consequently the vulcanization time is shortened and curing rate
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Fig. 8. Vulcanization characteristics of rubber compounds.
with high MAA loading is decreased and the vulcanization is slowed down. 3.4. Effects of MAA on morphology of the composites
Fig. 6. XPS results of HNTs/MAA model compound (Al 2p and Si 2p).
is increased. However, over loading of MAA weakens the complexation between HNTs and zinc ions. In addition, the excessive MAA consumes the peroxide that is also consumed for SBR crosslinking reactions and in situ polymerization of ZDMA. The weakened complexation between HNTs and zinc ions and reduced crosslinking of SBR and ZDMA lead to decreased crosslinking density of the vulcanizate, which is consistent with the result shown in Fig. 3. Consequently, the torque of the vulcanizate
Fig. 9 shows the SEM photos of the tensile fractured surface of the SBR/HNTs nanocomposite with variable MAA loading. It is shown clearly that the fractured surface of the MAA containing compound is much rugged compared with that of the control sample. In addition, the ruggedness of the fractured surface is increased with MAA content, suggesting increasing interfacial bonding between HNTs and SBR. The increased interfacial bonding is attributed to the interactions between HNTs and ZDMA or MAA. More detailed information about dispersion of HNTs is observed in TEM photos in Fig. 10. Inclusion of MAA into SBR/HNTs compounds leads to significantly improved dispersion of HNTs. Significantly, when the MAA content is relatively low (7 phr), the HNTs are individually dispersed. The significantly improved dispersion by MAA is attributed to the intensive interactions between HNTs and ZDMA or MAA. Fig. 11 shows the evolution of the transparency of 0.5 mm composite sheet with variable MAA concentration. As shown, when MAA content is higher than 9 phr, the vulcanizates with 40 phr silicate still show high transparency, indicating excellent dispersion achieved in the nanocomposites. Noticeably, although the observation of nanosized ZDMA aggregates in range of 10–20 nm in rubber was reported [56], the aggregates of polymerized ZDMA in the present system are not observed under TEM. This result is consistent with that of NBR/ ZDMA/montmorillonite systems in which ZDMA was also generated in situ [57]. The failure of direct observation of polymerized ZDMA may be attributed to the much smaller aggregates than 10– 20 nm and the low contrast between such small aggregates and rubber matrix under TEM. 3.5. Mechanical performance of MAA modified SBR/HNTs nanocomposites
Fig. 7. Vulcanization curves of SBR/HNTs/MAA compounds.
The dependences of the mechanical properties of the SBR/HNTs nanocomposite on MAA loading are shown in Figs. 12 and 13. It is revealed that the overall mechanical properties are increased with MAA loading. However too much MAA deteriorates tensile strength and modulus. Based on the two figures, it is found that when loaded with 12 phr MAA, optimal mechanical properties are obtained. Three to four folds increases in tensile strength and tear strength are achieved compared with those of the control sample. Fig. 14 is the stress–strain curves of SBR/HNTs composites. One can see that low MAA loading is benefit to improving the modulus of the composites while higher loading MAA effectively increases the
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Fig. 9. SEM photos of SBR/HNTs nanocomposites with different MAA loading. (a) SBR, (b) SBR2A, (c) SBR7A and (d) SBR15A.
elongation at break. Although the vulcanizate of SBR with 12 phr MAA only (exclude HNTs) shows increased tensile strength (13.0 MPa), the 300% stress and tear strength are still very low (1.29 MPa and 18.47 kN/m, respectively). Consequently, the
formation of the reinforcing ZDMA does not completely account for the improvement in the mechanical properties. The significantly enhanced mechanical properties could be attributed to the below origins. First, the strong interfacial bonding
Fig. 10. TEM photos of SBR/HNTs nanocomposites with different MAA loading. (a) SBR, (b) SBR2A, (c) SBR7A and (d) SBR15A.
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Fig. 11. The appearance photos of the vulcanizates.
between HNTs and rubber matrix is resulted from the ZDMA and MAA intermediated linkages. ZDMA connects SBR and HNTs via grafting/complexation mechanism. MAA bonds SBR and HNTs through grafting/hydrogen bonding mechanism. Secondly, in situ formed ZDMA undergoes self-polymerization to yield reinforcing nanoparticles. At last but not the least, significantly improved dispersion of HNTs in virtue of the interactions between HNTs and MAA or ZDMA largely increases the L/D ratio of the particles and the interfacial volume between HNTs and the matrix, which are also important in improving the performance.
Fig. 14. Effect of MAA content on stress–strain curve.
4. Conclusions
Fig. 12. Effect of MAA content on tensile properties.
Methacrylic acid was used to improve the performance of styrene–butadiene rubber /halloysite nanotubes nanocomposites by direct blending. The detailed interaction mechanisms of MAA and the in situ formed ZDMA were revealed. The strong interfacial bonding between HNTs and rubber matrix was resulted from the ZDMA and MAA-intermediated linkages. ZDMA-connected SBR and HNTs via grafting/complexation mechanism. MAA bonded SBR and HNTs through grafting/hydrogen bonding mechanism. Significantly improved dispersion of HNTs in virtue of the interactions between HNTs and MAA or ZDMA was achieved. Promising mechanical properties of MAA-modified SBR/HNTs nanocomposites were obtained. The changes in vulcanization behavior, mechanical properties and morphology were correlated with the interactions between HNTs and MAA or ZDMA and the largely improved dispersion of HNTs. Acknowledgment We are grateful for the financial support by the National Natural Science Foundation of China with grant number of 50603005. References [1] [2] [3] [4] [5]
Fig. 13. Effect of MAA content on tear property and Shore A hardness.
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Styrene–butadiene rubber/halloysite nanotubes nanocomposites modified by methacrylic acid Baochun Guo *, Yanda Lei, Feng Chen, Xiaoliang Liu, Mingliang Du, Demin Jia Department of Polymer Materials and Engineering, South China University of Technology, Guangzhou 510640, China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 22 July 2008 Received in revised form 30 July 2008 Accepted 31 July 2008 Available online 8 August 2008
Methacrylic acid (MAA) was used to improve the performance of styrene–butadiene rubber (SBR)/ halloysite nanotubes (HNTs) nanocomposites by direct blending. The detailed interaction mechanisms of MAA and the in situ formed zinc methacrylate (ZDMA) were revealed by X-ray diffraction (XRD), surface area and porosity analysis, X-ray photoelectron spectroscopy (XPS) together with crosslink density determination. The strong interfacial bonding between HNTs and rubber matrix is resulted through ZDMA and MAA intermediated linkages. ZDMA connects SBR and HNTs via grafting/complexation mechanism. MAA bonds SBR and HNTs through grafting/hydrogen bonding mechanism. Significantly improved dispersion of HNTs in virtue of the interactions between HNTs and MAA or ZDMA was achieved. Effects of MAA content on the vulcanization behavior, morphology and mechanical properties of the nanocomposites were investigated. Promising mechanical properties of MAA modified SBR/HNTs nanocomposites were obtained. The changes in vulcanization behavior, mechanical properties and morphology were correlated with the interactions between HNTs and MAA or ZDMA and the largely improved dispersion of HNTs. ß 2008 Elsevier B.V. All rights reserved.
Keywords: Styrene–butadiene rubber Halloysite nanotubes Methacrylic acid Interface Reinforcing mechanism
In the past decade, rubber nanocomposites with layered clay such as montmorillonite have exhibited their unusual performance such as significant reinforcing effect and impressive lowered permeability compared with those of the conventional rubber vulcanizates reinforced by carbon black or silica [1–7]. However, in order to obtain desired performance, sophisticated control on the exfoliation and intercalation should be done and this greatly increases the fabrication cost and uncertainty of the performance [8–14]. Consequently, exploitation on the facile process for rubber nanocomposites is highly desired. Fibrillar silicate such as attapulgite or palygorskite was found to have much weaker interactions among the single crystals than interlayer force of montmorillonite clay. Consequently, through conventional polymer melt blending, fibrillar silicate could be decohered into single crystals or crystal bundles with diameters less than 100 nm in polymeric matrix [15–19]. Alternatively, halloysite nanotubes (HNTs) with large L/D ratio also offer great opportunities for fabricating polymer nanocomposites with promising performance [20–24]. HNTs with molecular formula of Al2Si2O5 (OH)4nH2O, are multiwalled kaolinite nanotubes. The tubular
* Corresponding author. Fax: +86 20 2223 6688. E-mail address: [email protected] (B. Guo). 0169-4332/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2008.07.188
halloysite is formed by rolling of kaolin sheet in preference to tetrahedral rotation to correct misfit of the octahedral and tetrahedral sheets [25]. Comparing with carbon nanotubes (CNTs), the naturally occurring HNTs are much cheaper and easily available. An ideal HNTs crystal consists of a nonexpansive, layered structure that contains octahedrally coordinated Al3+ and tetrahedrally coordinated Si4+ in a 1:1 stoichiometric ratio [26]. Different from most of clay, most aluminols located in the inner of the HNTs, while in the outer of the HNTs are primary siloxane and few silanols and aluminols which are exposed in the edges of the sheet. It is well recognized that tight interfacial bonding and fine dispersion of the clay are two crucial factors to determine the performance of the rubber/clay nanocomposites [27]. For less active surface property [24,28], however, HNTs can hardly be effective filler for rubber because of the unsatisfied interfacial bonding and agglomeration in the rubber matrix. Dispersion of clay by means of the substances bearing hydrogen bonding functionalities has been widely reported. For instances, the dispersion of clay such as montmorillonite could be drastically improved in the matrices such as carboxyl methyl cellulose sodium salt, polyacrylic acid sodium salt [29] or poly(2-methoxyethyl acrylate) (PMEA) [30] or poly(vinyl acetate) [31]. Also, methacrylic acid (MAA) was used as interfacial modifier for polymer/inorganics composites [32,33].
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The affinity of MAA to the clay is due to the formation of hydrogen bonds between clay and the acid. In addition, MAA is reactive to polymer chains such as unsaturated rubber chains or saturated polyolefins via free radical grafting. Specially, MAA is reactive to zinc oxide (ZnO) or magnesium oxide (MgO) to yield unsaturated metal methacrylates which show unusual reinforcing effects for rubber compounds. In situ generated metal methacrylates were reported to reinforce rubber compounds during the compounding [34–39]. To obtain satisfied dispersion of HNTs and strong interfacial bonding between HNTs and styrene–butadiene rubber (SBR), in the present study, MAA was utilized to modify the SBR/HNTs composites. The interactions between HNTs and MAA or in situ formed zinc methacrylate (ZDMA) were characterized fully. The significantly improved mechanical properties were correlated to the largely improved interfacial bonding between HNTs and rubber and significantly enhanced dispersion of HNTs.
solvent and the surface toluene was quickly blotted off. The samples were immediately weighed and then dried in a vacuum oven for 36 h at 80 8C to remove all the solvent and reweighed. The volume fraction of SBR in the swollen gel, yr, was calculated by the following equation [40]:
yr ¼
m0 fð1 aÞ=rr m0 ð1 aÞ=rr þ ðm2 m1 Þ=rs
(1)
where m0 is the sample mass before swelling, m1 and m2 are sample masses before and after drying, respectively, f is the mass fraction of rubber in the vulcanizate, a is the mass loss of the gum SBR vulcanizate during swelling, and rr and rs are the rubber and solvent density, respectively. The elastically active network chain density, ye, which was used to represent the whole crosslink density, was then calculated by the well-known Flory–Rehner equation [41]: lnð1 yr Þ þ yr þ xyr 2 ys ðyr 1=3 yr =2Þ
1. Experiments
ye ¼
1.1. Materials
where yr is the volume fraction of the polymer in the vulcanizate swollen to equilibrium and ys is the solvent molar volume (107 cm3/mol for toluene). x is the SBR–toluene interaction parameter and is taken as 0.0653 calculated according to reference [42]. As mentioned above, the vulcanizates contained both covalent and ionic crosslinks. To distinguish ionic crosslinks from covalent crosslinks, samples were swollen again in the mixture of toluene and dichloroacetic acid (90:10 in mass) for 120 h to destroy ionic crosslinks, swollen in toluene for 72 h and weighed, and then vacuum-dried and reweighed. yr1 calculated by Eq. (1) represents the extent of swelling after destroying ionic crosslinks. ye1 calculated by Eq. (2) represented the covalent crosslink density. ye2 calculated by subtracting ye1 from ye represented ionic crosslink density.
Styrene–butadiene rubber, with trademark SBR1502 (styrene content 23.5 wt.%), was manufactured by Jilin Chemical Industry Company, China. Halloysite nanotubes (HNTs) were mined from Hubei province, China and purified according to the reported procedure [23]. Methacrylic acid was analytical grade from Guangzhou Reagent Factory. Chemical grade zinc methacrylate (ZDMA) was provided by Xi’an Chemical Factory. Other rubber additives were industrial grade and used as received. 1.2. Preparation of the SBR/HNTs nanocomposites SBR and HNTs were compounded with rubber additives with a two-roll mill and then compression molded at 170 8C. The compositions of the rubber compounds were tabulated in Table 1. The compounds were vulcanized at 170 8C T90. 2. Characterizations 2.1. X-ray diffraction (XRD) for rubber compounds and vulcanizates When recording XRD patterns of the uncured compounds, only necessary ingredients, SBR, HNTs, ZnO and MAA, were included in the formula. The X-ray diffraction experiments for the uncured compounds and the vulcanizates were conducted at ambient temperature on a Rigaku Dmax/III diffractometer using Cu Ka radiation (l = 1.54 A˚). The generator was operated at 40 kV and 30 mA. The samples of different shapes were scanned from 28 to 508 with a step length of 0.028.
(2)
2.3. Surface area and pore structure analysis In order to explore the possible interactions between HNTs and MAA (or ZDMA), two model compounds were prepared. The blend of HNTs/ZDMA/DCP (weight ratio of 73/27/0.04) and that of HNTs/ MAA/DCP (weight ratio of 72/28/0.4) were pre-mixtured and treated at 170 8C for 30 min in sealed vessels. The heated mixtures were grinded and sieved in fine powders for surface area determination and pore structure analysis. Pore analysis of the HNTs and the two model compounds were investigated using nitrogen adsorption–desorption isotherms with Micromeretics ASAP 2020 analyzer. The pore-size distributions were computed by applying the Barrett–Joyner–Halenda (BJH) methods [43]. 2.4. X-ray photoelectron spectroscopy (XPS)
2.2. Determination of crosslink density Equilibrium swelling method was used to determine the crosslink density of the vulcanizates. Samples were swollen in toluene at room temperature for 72 h and then removed from the
XPS spectra of the HNTs and the two compounds were recorded by using an X-ray photoelectron spectrometer (Kratos Axis Ultra DLD) with an aluminum (mono) Ka source (1486.6 eV). The aluminum Ka source was operated at 15 kV and 10 mA. For all the
Table 1 Composition of SBR/HNTs with MAA nanocompositesa Sample code
SBR
SBR2A
SBR3A
SBR4A
SBR5A
SBR7A
SBR9A
SBR12A
SBR15A
SBR HNTs MAA
100 40 0
100 40 2
100 40 3
100 40 4
100 40 5
100 40 7
100 40 9
100 40 12
100 40 15
a
Rubber intergradient: zinc oxide, 5 phr; stearic acid, 1 phr; dicumyl peroxide (DCP), 1.0 phr; N-isopropyl-N0 -phenyl-p-phenylenediamine (4010NA), 1.5 phr.
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samples, a high-resolution survey (pass energy = 48 eV) was performed at spectral regions relating to silicon, aluminum and zinc atoms. All core level spectra were referenced to the C 1s neutral carbon peak at 285.0 eV. 2.5. Vulcanization characteristics The curing characteristics of the SBR compounds were determined at 170 8C by U-CAN UR-2030 vulcameter, Taiwan. 2.6. Morphological observations The tensile fractured surfaces were plated with a thin layer of gold before any observations. The SEM observations were then performed using a LEO1530 VP SEM machine. The specimens were ultramicrotommed into thin pieces of about 120 nm in thickness with Leica EM UC6. Then the TEM observations were done using a Philips Tecnai 12 TEM machine at an accelerating voltage of 30 kV. 2.7. Mechanical properties Tensile tests were performed following ISO standard 37-2005 at 25 8C. Tensile strength, modulus and elongation at break were measured using U-CAN UT-2060 (Taiwan) instrument. Shore A hardness was performed following ISO standard 7619-1986 using a XY-1 sclerometer (Shanghai). 3. Results and discussion 3.1. Formation of ZDMA and its reactivity towards rubber and HNTs ZDMA may be formed through the reaction between ZnO and MAA during the rubber compounding [34,35,39]. This process is revealed in Fig. 1. As shown, the intensity of the peaks at 31.68 and 36.18, characterizing the (1 0 0) and (1 0 1) diffractions of ZnO [44], are consistently decreased with MAA loading to eventually invisible at 7 phr of MAA. Noticeably, with the decreasing of ZnO, the peaks at 9.88 and 10.98, for (2 0 0) and (1 0 1) diffractions of orthogonal ZDMA crystals, do not appear until ZnO is completely exhausted. This indicates that at low concentration ZDMA could be dissolved in SBR matrix. During vulcanization, it is well recognized that ZDMA undergoes self-polymerization to yield reinforcing nanoparticles and grafting onto rubber chains [34–39,45]. This process may be
Fig. 1. X-ray diffraction of uncured SBR compounds.
Fig. 2. X-ray diffraction of vulcanizates with different MAA loading.
verified in Fig. 2, in which the peaks for ZDMA are disappeared for all the samples, suggesting complete conversion of ZDMA during the vulcanization. The co-curing of SBR and ZDMA leads to the formation of ionic crosslinks between the SBR chains. Fig. 3 shows the dependence of the ionic crosslink density (ye2) and total crosslink density (ye) on the MAA concentration. It can be seen that ye2 is increased with the MAA content while too much MAA leads to decreased ye2. This trend may be due to the change of the content of formed ZDMA. More MAA leads to more yield of ZDMA until all ZnO is depleted. Besides the self-polymerization and grafting onto SBR, the interactions between ZDMA and HNTs are also important in the present system. Adsorption of metal ions onto mineral substrates is an important process in soil chemistry. Adsorption is a complex process, usually involving much more than simple ion exchange on the mineral surface [46–49]. Miyazaki et al. presented experimental data that showed that Zn2+ ions are adsorbed onto amorphous aluminosilicate by forming a complex with the surface OH groups [50,51]. This complexation process may be described as follows: 2fAl; SiOHg þ Zn2þ ¼ fðAl; SiOÞ2 Zng þ 2Hþ
(3)
where {Al, Si–OH} represents aluminol and/or silanol groups on the surface of amorphous aluminosilicate. In the present work, HNTs belong to the class of aluminosilicate and are believed to interact with the Zn2+ in ZDMA via the above complexation during
Fig. 3. Effect of MAA content on crosslink density.
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Fig. 5. XPS results of HNTs/ZDMA model compound (Zn 2p1/2 and Zn 2p3/2).
Fig. 4. Isothermal adsorption–desorption curve (a) and pore distribution (b) of model compounds and HNTs.
compounding. The adsorption of ZDMA on HNTs may be characterized by adsorption experiment. The porosity analysis of HNTs before and after adsorption of ZDMA was performed. The adsorption–desorption curves and the pore distribution curve of HNTs and HNTs/ZDMA model compounds are depicted in Fig. 4. For HNTs, the three peaks of the pore volume vs. pore width curve around 3, 15 and 35 nm are attributed to surface defects, the lumens of the nanotubes and pores among the tubes, respectively. For the HNTs/ZDMA complex, however, all the three peaks are disappeared, suggesting the adsorption of ZDMA on HNTs. This adsorption also leads the decrease of BET area from 50.4 to 33.8 m2/g. XPS survey was performed in order to further substantiate the complexation between ZDMA and HNTs. The complexation between Zn2+ and aluminols or silanols on HNTs may change the chemical environment of Zn2+ due to the changed density of the electron cloud. The result of XPS high resolution survey on Zn2+ is depicted in Fig. 5. For ZDMA, the binding energies for Zn 2p1/2 and Zn 2p3/2 are 1021.8 and 1044.9 eV, respectively. After the complexation, however, the two bands are increased to 1022.3 and 1045.3 eV, respectively, suggesting obvious changes in Zn2+ chemical environment. Consequently, ZDMA acts as special interfacial modifier between rubber matrix and HNTs via dual mechanisms of grafting and complexation. 3.2. Interactions between MAA and HNTs In the present systems, apart from acting as reactant of the formation of ZDMA, MAA is excessive when its concentration is
higher than 7 phr. The remained MAA and its polymer are important in promoting the dispersion of HNTs and strengthening the interface of HNTs and SBR. It is well known that as vinyl monomer MAA may graft onto SBR during vulcanization [52]. The pendent carboxyl groups on the polymerized MAA (or grafted MAA) are reactive to HNTs via hydrogen bonding. This is also substantiated by mains of adsorption and XPS experiments. The adsorption–desorption curves and the pore distribution curve of the HNTs/MAA model compound are also depicted in Fig. 4. It can be seen the BET area of the HNTs/MAA model compound is drastically decreased to 7.7 m2/g. The dramatic decrease in BET area together with the complete disappearing of the pores indicates intensive interaction between HNTs and MAA. It is believed that the formation of hydrogen bonding lead to the variation of the chemical environment for the hydrogen bonding functionalities, which can be characterized by the variation of bonding energy of the related atoms via XPS survey [53–55]. As shown in Fig. 6, there is decrease in the bonding energy of silicon atom and aluminum atom, which is connected to the oxygen atom in the hydrogen bond. The decreases in bonding energy for silicon and aluminum are 0.3 and 0.2 eV, respectively. This confirms the formation of hydrogen bonding in the HNTs/MAA compound. As a consequence, the interface between SBR and HNTs may also be strengthened by MAA via grafting and hydrogen bonding mechanisms. 3.3. Effects of MAA on vulcanization behavior of SBR/HNTs compounds The vulcanization curves were shown in the Fig. 7. It can be seen that the incorporation of MAA leads to significantly improved torque of the vulcanizates. The toque of the vulcanizate is increased with MAA loading when the content of MAA is lower than 12 phr. Overloading of MAA leads to decreased torque. More detailed characteristics were summarized and depicted in the Fig. 8. As shown, the scorch time (Ts2) was practically independent of MAA loading. The vulcanization time (T90) is initially decreased with MAA content. However, when the MAA content is sufficiently high (higher than 12 phr), the vulcanization time starts to increase. The curing rate index (CRI), which indicates the rate of cure of the compounds, is defined as 100/(T90 Ts2). A higher value means a higher rate of vulcanization. The inclusion of MAA accelerates the vulcanization process and the curing rate increases with MAA content. Again, while the MAA content is sufficiently high (higher than 12 phr), the curing rate is to be lowered in some extent. As discussed above, very complicated interactions among rubber matrix and HNTs and the additives are presented in the present systems. These interactions have pronounced influences on the curing characteristics of the MAA included compounds. Once MAA is compounded, the yielded ZDMA and the above mentioned interactions effectively promote the vulcanization. Consequently the vulcanization time is shortened and curing rate
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Fig. 8. Vulcanization characteristics of rubber compounds.
with high MAA loading is decreased and the vulcanization is slowed down. 3.4. Effects of MAA on morphology of the composites
Fig. 6. XPS results of HNTs/MAA model compound (Al 2p and Si 2p).
is increased. However, over loading of MAA weakens the complexation between HNTs and zinc ions. In addition, the excessive MAA consumes the peroxide that is also consumed for SBR crosslinking reactions and in situ polymerization of ZDMA. The weakened complexation between HNTs and zinc ions and reduced crosslinking of SBR and ZDMA lead to decreased crosslinking density of the vulcanizate, which is consistent with the result shown in Fig. 3. Consequently, the torque of the vulcanizate
Fig. 9 shows the SEM photos of the tensile fractured surface of the SBR/HNTs nanocomposite with variable MAA loading. It is shown clearly that the fractured surface of the MAA containing compound is much rugged compared with that of the control sample. In addition, the ruggedness of the fractured surface is increased with MAA content, suggesting increasing interfacial bonding between HNTs and SBR. The increased interfacial bonding is attributed to the interactions between HNTs and ZDMA or MAA. More detailed information about dispersion of HNTs is observed in TEM photos in Fig. 10. Inclusion of MAA into SBR/HNTs compounds leads to significantly improved dispersion of HNTs. Significantly, when the MAA content is relatively low (7 phr), the HNTs are individually dispersed. The significantly improved dispersion by MAA is attributed to the intensive interactions between HNTs and ZDMA or MAA. Fig. 11 shows the evolution of the transparency of 0.5 mm composite sheet with variable MAA concentration. As shown, when MAA content is higher than 9 phr, the vulcanizates with 40 phr silicate still show high transparency, indicating excellent dispersion achieved in the nanocomposites. Noticeably, although the observation of nanosized ZDMA aggregates in range of 10–20 nm in rubber was reported [56], the aggregates of polymerized ZDMA in the present system are not observed under TEM. This result is consistent with that of NBR/ ZDMA/montmorillonite systems in which ZDMA was also generated in situ [57]. The failure of direct observation of polymerized ZDMA may be attributed to the much smaller aggregates than 10– 20 nm and the low contrast between such small aggregates and rubber matrix under TEM. 3.5. Mechanical performance of MAA modified SBR/HNTs nanocomposites
Fig. 7. Vulcanization curves of SBR/HNTs/MAA compounds.
The dependences of the mechanical properties of the SBR/HNTs nanocomposite on MAA loading are shown in Figs. 12 and 13. It is revealed that the overall mechanical properties are increased with MAA loading. However too much MAA deteriorates tensile strength and modulus. Based on the two figures, it is found that when loaded with 12 phr MAA, optimal mechanical properties are obtained. Three to four folds increases in tensile strength and tear strength are achieved compared with those of the control sample. Fig. 14 is the stress–strain curves of SBR/HNTs composites. One can see that low MAA loading is benefit to improving the modulus of the composites while higher loading MAA effectively increases the
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Fig. 9. SEM photos of SBR/HNTs nanocomposites with different MAA loading. (a) SBR, (b) SBR2A, (c) SBR7A and (d) SBR15A.
elongation at break. Although the vulcanizate of SBR with 12 phr MAA only (exclude HNTs) shows increased tensile strength (13.0 MPa), the 300% stress and tear strength are still very low (1.29 MPa and 18.47 kN/m, respectively). Consequently, the
formation of the reinforcing ZDMA does not completely account for the improvement in the mechanical properties. The significantly enhanced mechanical properties could be attributed to the below origins. First, the strong interfacial bonding
Fig. 10. TEM photos of SBR/HNTs nanocomposites with different MAA loading. (a) SBR, (b) SBR2A, (c) SBR7A and (d) SBR15A.
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Fig. 11. The appearance photos of the vulcanizates.
between HNTs and rubber matrix is resulted from the ZDMA and MAA intermediated linkages. ZDMA connects SBR and HNTs via grafting/complexation mechanism. MAA bonds SBR and HNTs through grafting/hydrogen bonding mechanism. Secondly, in situ formed ZDMA undergoes self-polymerization to yield reinforcing nanoparticles. At last but not the least, significantly improved dispersion of HNTs in virtue of the interactions between HNTs and MAA or ZDMA largely increases the L/D ratio of the particles and the interfacial volume between HNTs and the matrix, which are also important in improving the performance.
Fig. 14. Effect of MAA content on stress–strain curve.
4. Conclusions
Fig. 12. Effect of MAA content on tensile properties.
Methacrylic acid was used to improve the performance of styrene–butadiene rubber /halloysite nanotubes nanocomposites by direct blending. The detailed interaction mechanisms of MAA and the in situ formed ZDMA were revealed. The strong interfacial bonding between HNTs and rubber matrix was resulted from the ZDMA and MAA-intermediated linkages. ZDMA-connected SBR and HNTs via grafting/complexation mechanism. MAA bonded SBR and HNTs through grafting/hydrogen bonding mechanism. Significantly improved dispersion of HNTs in virtue of the interactions between HNTs and MAA or ZDMA was achieved. Promising mechanical properties of MAA-modified SBR/HNTs nanocomposites were obtained. The changes in vulcanization behavior, mechanical properties and morphology were correlated with the interactions between HNTs and MAA or ZDMA and the largely improved dispersion of HNTs. Acknowledgment We are grateful for the financial support by the National Natural Science Foundation of China with grant number of 50603005. References [1] [2] [3] [4] [5]
Fig. 13. Effect of MAA content on tear property and Shore A hardness.
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