Article pubs.acs.org/Biomac
Simple Green Approach to Reinforce Natural Rubber with Bacterial Cellulose Nanofibers Eliane Trovatti,†,‡ Antonio J. F. Carvalho,‡ Sidney J. L. Ribeiro,§ and Alessandro Gandini*,†,‡ †
Instituto de Química de São Carlos, Universidade de São Paulo, PB 780, 13560-970, São Carlos, SP, Brazil Departamento de Engenharia de Materiais/Escola de Engenharia de São Carlos, Universidade de São Paulo, 13566-590, São Carlos, SP, Brazil § Instituto de Química, Universidade Estadual Paulista Júlio de Mesquita Filho, UNESP, 14801-970, Araraquara, SP, Brazil ‡
ABSTRACT: Natural rubber (NR) is a renewable polymer with a wide range of applications, which is constantly tailored, further increasing its utilizations. The tensile strength is one of its most important properties susceptible of being enhanced by the simple incorporation of nanofibers. The preparation and characterization of natural-rubber based nanocomposites reinforced with bacterial cellulose (BC) and bacterial cellulose coated with polystyrene (BCPS), yielded high performance materials. The nanocomposites were prepared by a simple and green process, and characterized by tensile tests, dynamical mechanical analysis (DMA), scanning electron microscopy (SEM), and swelling experiments. The effect of the nanofiber content on morphology, static, and dynamic mechanical properties was also investigated. The results showed an increase in the mechanical properties, such as Young’s modulus and tensile strength, even with modest nanofiber loadings.
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fiber, because it provides nanocomposites with enhanced mechanical properties, even at very low loadings. Here, the reinforcement is related to both chemical structure and morphological aspects. With respect to the cellulose chemical structure, its OH functionality enables good interactions with compatible macromolecular matrices, i.e., polar polymers, because of hydrogen bond formation and/or other dipole− dipole interactions. The morphology of the nanofibers, when included into a polymeric matrix, enhances the mechanical behavior of the ensuing composites provided that the stress transfer from the matrix to the reinforcing fibers is ensured, leading to an improvement in Young’s modulus and in the stress at break.19 Additionally, the use of a crystalline reinforcement enhances the mechanical strength of the composite. Whereas these mechanical advantages are indeed observed with polar matrices, composites based on cellulose and hydrophobic matrices such as polyolefins encounter the classical problems of low interfacial compatibility associated with the high polar character of the fibers. Their incorporation in hydrophobic matrices has therefore been achieved by specific chemical modifications that decrease the hydrophilic character of the nanofiber surfaces.20 This investigation deals with the preparation and characterization of NR/BC composites and includes a study of the in situ surface modification of the nanofibers in their pristine aqueous suspension in order to improve the quality of their
INTRODUCTION Natural rubber (NR) is a renewable polymer endowed with remarkable properties such as high elasticity, high film forming capacity, and high hydrophobicity and is hence used worldwide in a wide range of applications, including tires, the most important NR-based material.1 Some properties of NR, mainly its tensile strength must often be tailored, in order to elaborate products with improved specific performances. The use of nanoscopic additives is the oldest technology for reinforcing NR. The most common among these fillers are carbon black and silica nanoparticles,2 but the search for new fillers has led, in the last few decades, to the use of discontinuous fibers, which constitutes a promising alternative to the conventional fillers, especially in terms of mechanical reinforcement. Some successful examples of fibers used to reinforce NR are aramide,3polyester,3 nylon,4 Kenaf,5 glass fibers,6 as well as ordinary cellulose fibers,7 whereas the use of nanofibers for NR reinforcement is just beginning to be investigated. The most important nanofibers used for this purpose are carbon nanotubes8,9 and microfibrillated cellulose (MFC).10 The use of cellulose nanofibers as fillers for reinforcing natural or synthetic polymeric matrices is a technology that is rapidly developing after the recent rise in interest in their processing and characterization.11 Besides the vegetal nanofibers, namely, MCF and cellulose nanowhiskers (CNs), fibers from bacterial cellulose (BC)12−14 have also been used as reinforcing agents for natural or synthetic polymers.15,16 All these nanocelluloses are renewable, biodegradable, have a low density, and are becoming progressively cheaper and available in larger quantities.11,17,18 Their nanoscale dimensions and properties have promoted their use as viable alternatives to conventional © 2013 American Chemical Society
Received: April 12, 2013 Revised: June 5, 2013 Published: June 19, 2013 2667
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Figure 1. Surface tension of HPC solution and HPC−BC suspension (A) and adsorption of HPC at the surface of BC (B).
adhesion to the matrix. The advantages of this process are to attain a suspension of modified nanocelluloses in their natural aqueous medium, thus avoiding the laborious operations used in other modification approaches, where organic solvents are required.17
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MATERIALS AND METHODS
Materials. NR (clone RRYV 600, 2012) was kindly supplied by Cambuhy, Matão, SP, Brazil. All reagents, solvents and other chemicals were purchased from Sigma-Aldrich and were used as received, except when otherwise stated. The inhibitor in commercial 99% styrene (St) was removed by the standard treatment with a 0.5 M aqueous NaOH solution. BC Production and Purification. BC membranes were produced in polypropylene trays using a modified Hestrin Shran liquid culture medium21composed (g/L) of glucose (20), yeast extract (4), KH2PO4 (2), at 28 °C, without agitation. The membranes were purified following standard procedure,22 with a 0.25 M NaOH solution at 80 °C and crushed in an Ultraturrax disperser at 10.000 rpm for 20 min, generating a suspension of BC nanofibers used as such for the composite preparation. Critical Micellar Concentration (CMC) of Hexadecylpyridinium Chloride (HPC). The CMC of HPC was determined by the surface tension of 0.2, 0.4, 0.6, 0.8, 1.0, 1.4, 2.0, and 3.0 mM aqueous solutions by the ring method using a DuNouy Kruss Tensiometer. Determination of HPC Adsorbed at the Nanofibre Surface. HPC solutions (0.2, 0.4, 0.6, 0.8, 1.0, 1.4, 2.0, and 3.0 mM) were equilibrated in the presence of 1% w/w BC under magnetic stirring for 24 h, at 25 °C. The dispersions were centrifuged at 6000 rpm for 10 min, and the equilibrium concentration of surfactant in the aqueous phase was determined by UV spectroscopy (200 to 280 nm), after appropriate dilution. The extent of adsorption was calculated by the difference between the initial and the equilibrium concentration of HPC. Admicelar Polymerization of Styrene at the Surface of the BC Nanofibers. The dispersion of 1 wt % BC in 0.6 mM HPC was equilibrated for 24 h under magnetic stirring for adsorption of surfactant at the surface of the fibers. St (0.5% v/v with respect to the aqueous medium) was then added, and the system was gently stirred for 48 h at room temperature in order to promote the adsolubilization of the monomers into the surfactant layer adsorbed on the fiber surface. The ammonium persulfate initiator (1% w/w with respect to the added monomer) was added, and the suspension was purged with nitrogen. The polymerization was carried out at 80 °C for 3 h. After cooling, the suspension was centrifuged at 7000 rpm for 10 min, washed with a mixture of 70/30 (v/v) water/ethanol, followed by washing with distilled water and centrifugation until total removal of the surfactant, as monitored by UV spectroscopy. The ensuing BC nanofibers decorated with polystyrene (PS) were characterized and used to reinforce NR.
Figure 2. SEM micrographs of BC (A) and BCPS fibers (B) and their respective FTIR spectra (C). Characterization of BC Decorated with PS. The PS-decorated nanofibers were characterized using Fourier transform infrared (FTIR) spectroscopy, thermogravimetry (TGA), scanning electron microscopy (SEM) and water contact angle measurements. FTIR spectra were acquired using a Perkin-Elmer Spectrum 100 FT-IR Spectrometer equipped with a single horizontal Golden Gate ATR cell, the resolution being 4 cm−1 after 32 scans. Spectra were collected from 4000 to 650 cm−1. SEM micrographs of the fiber composite surface coated with evaporated carbon were obtained using a FEI Philips XL50 Scanning Electron Microscope. Contact angle measurements were carried out by depositing a water drop onto a film of nanofibers prepared by solvent casting in a ventilated oven, using a KSV LTD Can 101 instrument. Each reported value was the average of three determinations. TGA analyses were performed on a Perkin-Elmer Pyris 1, equipped with a platinum cell. Samples were heated at a constant rate of 10 °C·min−1 from room temperature to 600 °C under a nitrogen flow of 20 mL·min−1. The area of the initial mass/min derivative plot was used to calculate the corresponding amount of cellulose and St in each sample. Mixtures of BC and PS (Mv = 250.000 Da) at defined proportions were used to validate the method. Sample Compounding. BC nanofibers and BC nanofibers decorated with PS were added to the NR latex at 1.0, 2.5, 5.0, and 10.0 wt %, and stirred for dispersion. The viscous suspension was then dried at room temperature. The vulcanized NR (VNR) was prepared following the same procedure, but the vulcanization agent was added and homogenized into the latex before adding the fibers. The dried films were cured at 160 °C for 10 min. The vulcanization agent was composed of a water suspension containing ZnO (5 weight%), elemental sulfur (2), tetramethylthiuram disulfide (TMTD) (2), sodium dodecylbenzenesulphonate (1), bentonite (5), Irganox (1), 2668
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of 100 mm·min−1. Five specimens (dimensions 90 × 10 × 0.7 mm) of each sample were tested, and the Young’s moduli were calculated using the secant modulus method. The elongation of the samples was calculated based on the measured data by the crosshead movement of the machine. Toluene Uptake. Composite specimens (dimensions 20 × 10 × 0.7 mm) were immersed in toluene at room temperature to study their solvent uptake by swelling. The weight increase was assessed at 5, 15, 30, and 60 min. Samples were taken out, their surfaces wiped dry, weighed, and then reimmersed. The mass gain at time t, Wuptake, was calculated as
and casein (1), which was added to latex 5phr. The NR/BC samples were called NR-BC1, NR-BC2.5, NR-BC5, and NR-BC10 for untreated BC and NR-BCPS1, NR-BCPS2.5, NR-BCPS5 and NRBCPS10 for the PS-coated BC fibers. The vulcanized samples were called NRV, NRV-BC1, NRV-BC2.5, NRV-BC,5 NRV-BC10, NRVBCPS1, NRV-BCPS2.5, NRV-BCPS5, and NRV-BCPS10, following the same criteria. Composite Characterization. The materials were characterized using dynamical mechanical analysis (DMA), tensile tests, SEM and toluene absorption tests. SEM micrographs of the composite fractured surfaces were obtained using a FEI (XL50, Philips) Scanning Electron Microscope. Samples were coated with evaporated carbon. DMA measurements were carried out with a Perkin-Elmer 8000 DMA equipment working in the tension mode. The samples were kept at 43% relative humidity and 25 °C for 48 h before being tested. Tests were performed at 1 Hz and the temperature was varied from −80 to 60 °C in 5 °C steps . Tensile tests were performed at room temperature (25 °C) using an EMIC DL universal testing machine equipped with a load cell of 0.5kN and working at a deformation rate
Wuptake = [(Wt − W0)/W0]·100 where W0 is the specimen’s initial mass and Wt is the specimen’s mass after immersion time t.
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RESULTS AND DISCUSSION Polymerization of Styrene at the Surface of BC Fibers. This work focuses on the use of BC and BCPS nanofibers to
Figure 3. Thermogravimetric curves for BC and BCPS.
Figure 4. Mechanical tests of NR and VNRBC-based composites. 2669
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Figure 5. Mechanical tests of NR and VNRBC-based composites.
albeit in variable proportions depending on the fiber source and processing. The CMC of HPC solution and BC suspension in HPC were determined by surface tension measurements, as shown in Figure 1A. The CMC was 1.0 mM and 1.5 mM, for HPC solution and fiber suspension, respectively. The higher concentration of HPC to reach the CMC in the presence of fibers indicates its adsorption at their surface. The values of CMC in solution and in cellulose suspension were used to calculate the amount of HPC adsorbed at the fiber surface (Figure 1B). The concentration of HPC (0.6 mM), well below the CMC, was used to avoid the presence of free micelles of surfactant in the medium, which would have sequestered the styrene preferentially to the HPC bilayer at the surface of the fibers. The subsequent addition of St and its free-radical admicellar polymerization was followed by the isolation of the sleeved nanofibers and their characterization, before their incorporation into the NR matrix.
reinforce NR comparing the best way to prepare the composites, based on the principle that the BCPS fibers should be more readily dispersed into the NR latex. The main objective of the in situ polymerization was to decrease the hydrophilicity and polarity of the nanofibers, and hence to increase their adhesion to the NR matrix. To the best of our knowledge, the in situ admicellar polymerization of styrene at the surface of the BC nanofibers had not been previously reported. The principle of admicellar polymerization involves the formation of a double layer of a cationic surfactant on the surface of the fibers, followed by the incorporation of a hydrophobic monomer in the aqueous medium. After the monomer migration to the surfactant sleeve, the polymerization is carried out using a water-soluble initiator. The modification is thus attained by the physical sleeving of the polymer, generated in situ.23 A cationic surfactant was used because its adsorption to cellulose fibers is favored by the electrostatic interaction with the negative charges always present on cellulose macromolecules, 2670
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Figure 6. DMA of VNRBC-based composites.
Characterization of the Nanofibers. The typical morphology of BC (Figure 2A) was preserved after the in situ polymerization of styrene (Figure 2B), and both BC and BCPS showed a homogeneous tridimensional network of nanofibrils with 40−70 nm width. With respect to the coated fibers, a clearly visible layer of homogeneously distributed PS with occasional bulging, due to local excess of polymer deposit, was generated at their surface (Figure 2B), forming a thin sleeve, which confirms the occurrence of the admicellar polymerization. The surface energy of the films showed a considerable enhancement of the hydrophobic nature of the BCPS surface, through the corresponding increase in the water contact angle, which were 43.2 (±1) and 59.5° (±0.9) for BC and BCPS, respectively. The decrease in the surface energy of fibers was essentially due to the reduction in its polar component because of the masking of part of the hydroxyl groups of cellulose by the thin PS sleeve. However, even after abundant washing, the presence of traces of residual surfactant probably influenced these results by decreasing the hydrophobicity of the fibers, which were not hot pressed. In other words, one would have expected a more substantial increase in contact angle if PS were the only structure present at the modified fiber surface. The success of the polymerization of styrene was confirmed by FTIR spectroscopy, based on the typical bands of the polymer added to the bands of cellulose. Figure 3 shows the transmission FTIR spectra of PS-modified BC fibers. The FTIR spectrum of BC (Figure 2C) presented strong bands at 3300, 2880, and 1100 cm−1, associated with the vibrations of its O−H, C−H and C−O−C groups, respectively. The typical PS
bands related to the monosubstituted phenyl ring absorption can clearly be found at 700 and 750 cm−1. The content of polystyrene at the surface of BC fibers was assessed by TGA using the degradation profiles of BC and PS for each sample. The TGA tracing of BCPS exhibited two separated weight-loss steps, showing the typical degradation curves of cellulose and polystyrene respectively, with a maximum decomposition rate at 368 °C (Figure 3) for cellulose and 450 °C for PS, respectively. The proportion of BC−PS, estimated by the integration of the rate of the decomposition (mg/min) of BC and PS, gave 38 wt % of PS. Characterization of BCPS Nanocomposites. The composite films were characterized by tensile experiments, thermo-mechanical analysis, and SEM. Mechanical Behavior of NR-Based Composites. The Young’s modulus, the tensile strength, and the elongation at break were determined from the stress−strain curves. The result of tension assays shown in Figure 4 for the NR matrix is a nonlinear stress−strain curve, typical of this material, in which the tension increases slowly, reaching high elongation values (around 700 and 870% for NR and VNR, respectively), followed by a gradual increase until break. For the composites, the increase in the fiber content led, in general, to a substantial increase in the mechanical properties (Figure 4A−D). The NR-based composites (NRBC, NRBCPS, VNR, and VNRBCPS) with low fiber content (1 and 2.5 wt %) preserved the typical features of NR or VNR and were poorly influenced by the presence of BC or BCPS. Those with high amounts of fibers (5 and 10 wt %) were instead highly influenced by the nanofiber presence, by the occurrence of considerable 2671
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observations discussed above in the case of the stress−strain results. Also, the Tg tracing (Figure 6C) of VNR indicated a behavior of a viscoelastic material, whereas those of VNR composites incorporating BC and BCPS displayed a behavior more typical of an elastic material. SEM. The composite samples were approximately 0.7 mm thick films and appeared homogeneous to the naked eye, indicating a good dispersion of the fibers into the matrix. The compatibility between the polymer matrix and the reinforcing phase and the proper dispersion of the reinforcing phase into the matrix are essential parameters to reach a high-performance composite. SEM micrographs of the fractured surface of NRbased composites (Figure 7) were inspected in order to assess
changes in the stress−strain profile, indicating a more fragile behavior and a less-stretchable material, in which the tensile strength and Young’s modulus increased with increasing fiber concentration, both with BC and BCPS. The increment in the mechanical behavior of the composites caused by loading the NR with BC fibers is shown, for instance, for NRBC samples with 1 and 10% of BC, where Young’s modulus of NRBC composites increased from 2 × 104 to 2 × 105 Pa (Figure 5A), while the tensile strength increased from 2.5 × 106 to 4.5 × 106 (Figure 5B). This behavior, exhibited by all the composites prepared in this study, is the result of the efficient transfer of charge from the matrix to the filler, which certainly forms a continuous structure resulting in a percolation effect.24 Also, the OH groups of cellulose exerted an important role with respect to the interaction among fibers, possibly due to the formation of intra- and intermolecular hydrogen bonds, which ensured the integrity of the continuous structure inside the matrix. A better result was found with the VNRBCS samples, in which Young’s modulus jumped from 6 × 105 to 6 × 106 Pa in the composites with 1 and 7 wt %, respectively (Figure 5D). The tensile strength increased from 2 × 106 to 6 × 106 Pa for the same composites (Figure 5E). In this case, the increase in the mechanical properties can be attributed almost exclusively to the percolation effect, since the hydrogen bonds of cellulose were masked by the PS sleeve. The incorporation of BC or BCPS in the NR and VNR matrix caused, in agreement with the tensile strength and Young’s modulus results, a considerable decrease in the elongation at break in the composites. These results are in good agreement with previous work related to the reinforcement of NR with carbon nanotubes, in which the tensile strength was around 7 MPa for composites with 8.3% of nanofibers.8 The BC or BCPS reinforced VNR displayed similar profiles, but with higher moduli and strengths at break, as expected. For instance, the maximum values of Young’s modulus and strength at break were reached for VNRBC10 at around 6 × 105 Pa and 1 × 107 Pa, respectively (Figure 5E). In general, these results indicate an excellent compatibility between BC or BCPS and NR. A very important effect to be highlighted was the improvement of these properties in BC-filled nonvulcanized NR, which reached values close to those of vulcanized rubber without BC. These results clearly show that the reinforcement with BC or BCPS represent a promising alternative approach to increase the mechanical properties of NR, with the important additional advantages of the use of a natural renewable fiber as the loading phase, using a green approach and giving totally biodegradable materials. Dynamic Mechanical Measurements. Figure 6 shows the results of the tension modulus (E′/Pa) as a function of temperature for VNR and its composites filled with BC or BCPS at different concentrations. VNR showed a typical curve of a cross-linked elastomer, which is glassy below its Tg and shows the classical major drop in its elastic modulus around the temperature corresponding to the glass−rubber transition, thereafter remaining constant with increasing temperature. Upon incorporation of both BC and BCPS nanofibers, the strengthening effect appeared in a very substantial fashion, as shown in Figure 6A,B, with the additional quantitative feature related to the actual percentage of added nanofibers. Interestingly, the gains in modulus on the elastomeric plateau were found to be very similar for both types of reinforcement, confirming similar
Figure 7. SEM micrographs of fractured zones of NRBC10 and VNRBC10 composites.
these features. Figure 7A,B showed the general aspect of the NRBC10 and VNRBCPS10 composites, respectively, revealing a good dispersion of the nanofibers, even in such highly loaded samples. Figure 7C,D (scale of 30 μm) showed the good dispersion of the fibers into the matrix, visualized as homogeneously dispersed points (indicated by arrows). The high compatibility between the two phases was evidenced in Figure 7E,F (scale of 5 μm), which shows both types of broken fibers sticking to the NR matrix, with no evidence of pull out. All the other composites showed very similar morphology. The high compatibility and homogeneous dispersion of the fibers into the matrix explain the improvement in the mechanical 2672
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All composite samples displayed a lower extent of toluene absorption, as shown in Figure 9. Moreover, the composites based on NR and BC showed lower mass gains when compared with those loaded with BCPS because of the good affinity of PS for toluene. Similar trends were reported previously for NR composites incorporating carbon nanotubes.9
properties discussed above. To achieve a more accurate evaluation of the interfacial region, the cross sections of samples broken with a tensile machine were also submitted to SEM inspection. The results were in total agreement with those obtained from the sample broken under liquid nitrogen, as shown by the typical images in Figure 8A,B.
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CONCLUSIONS
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AUTHOR INFORMATION
The polymerization of styrene took place at the surface of BC nanofibers in their pristine aqueous medium and was assessed by SEM, FTIR, contact angle, and TGA. These results represent an important advance since cellulose nanofibers find growing applications in a wide range of areas, other than polymer matrix reinforcement. The use of both BC and BCPS nanofibers as fillers resulted in substantial improvements in the mechanical properties of NR and VNR matrices. These materials represent a promising alternative to the conventional reinforcement for NR and potential applications include biomedical materials such as catheters, gloves, elastic dentistry, among others in more conventional realms. In more general terms, their mechanical properties can be tuned as a function of the nanofiber load, going from typical elastomeric to stiff thermoplastic features.
Figure 8. SEM micrographs of the cross sections of NRBC10, broken with a tensile machine at room temperature, at 30 μm (A) and 5 μm (B) scale.
Toluene Uptake. The mass gain of toluene as a function of immersion time for all the NRBC-based composites is shown in Figure 9. NR, VNR, and their respective composites absorbed toluene following different patterns. At the beginning, the absorption was fast, reaching high mass increases, even after 5 min of immersion, and all samples showed thereafter a tendency to level off after 30 min. As expected, VNR showed a higher resistance to toluene absorption when compared to NR, because of the cross-linked nature of the former. Moreover, the NR samples started to break down after 30 min of immersion, corresponding to an 800% weight gain, which, in fact, begun forming a concentrated polymer solution.
Corresponding Author
*E-mail:
[email protected]. Author Contributions
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes
The authors declare no competing financial interest.
Figure 9. Mass gain for NRBC based composites dipped in toluene, as a function of time. 2673
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ACKNOWLEDGMENTS E.T. thanks FAPESP for a Postdoctoral grant (2012/05184-0). The authors thank Professor José Manoel Marconcini from EMBRAPA Instrumentaçaõ , São Carlos, Brazil, for providing the facilities for the mechanical tests, and FAPESP and CNPq for financial support. A.G. thanks CAPES for a visiting professorship (707/2012).
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