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Improved natural rubber composites reinforced with a complex filler network of biobased nanoparticles and ionomer
Accepted Manuscript Improved natural rubber composites reinforced with a complex filler network of biobased nanoparticles and ionomer
Lei Jong PII:
S0254-0584(17)30768-X
DOI:
10.1016/j.matchemphys.2017.09.067
Reference:
MAC 20034
To appear in:
Materials Chemistry and Physics
Received Date:
06 June 2017
Revised Date:
13 September 2017
Accepted Date:
29 September 2017
Please cite this article as: Lei Jong, Improved natural rubber composites reinforced with a complex filler network of biobased nanoparticles and ionomer, Materials Chemistry and Physics (2017), doi: 10.1016/j.matchemphys.2017.09.067
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Highlights
Hydrophilic filler and ionomer form a complex filler network The complex filler network greatly improves the properties of rubber composites Modulus, tensile stress and reinforcement factor were significantly improved Great potential for improving rolling resistance in tire tread applications
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Graphical abstract
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Improved natural rubber composites reinforced with a complex filler network of biobased nanoparticles and ionomer Lei Jong United States Department of Agriculture, Agricultural Research Service, National Center for Agricultural Utilization Research†, 1815 N. University St., Peoria, IL 61604, United States ABSTRACT: Biobased rubber composites are renewable and sustainable. Significant improvement in modulus of rubber composite reinforced with hydrophilic filler was achieved with the inclusion of ionomers. Soy particles aided with ionomer, carboxylated styrene-butadiene (CSB), formed a strong complex filler network in rubber. The effect of CSB caused Young’s modulus and the tensile stress at 100-300% elongation to increase significantly while maintaining good tensile strength. The optimum concentration of CSB in natural rubber that produced the greatest improvement in tensile stress at higher elongation ratios and shear elastic modulus was determined. CSB improves the cohesion within the composites. The increase of reinforcement factor with increasing CSB and filler concentration in the composites indicates that the effect of CSB increased the reinforcement of soy nanoparticles more significantly at higher than lower filler concentrations. The mechanical properties of the composites were also compared to carbon black reinforced natural rubber composites. The result indicates great potential for tire tread applications.
Keywords: Natural rubber; mechanical properties; composites; ionomer. †Mention
of trade names or commercial products in this publication is solely for the purpose of providing specific information and does not imply recommendation or endorsement by the U.S. Department of Agriculture. USDA is an equal opportunity provider and employer. Correspondence to: L. Jong (Email: [email protected])
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1. Introduction Renewable materials are important for a sustainable future. Biobased composites reinforced with nanoparticles have potential to be used as molded components for various damping applications. Rubber composites is one type of particle reinforced composites. Major factors in improving the strength of rubber composites are small particle size, anisotropic particles, and interface adhesion between particles and polymer matrix [1-3]. Carbon black and silica are currently the major fillers for rubbers, but renewable fillers are also showing potentials in recent years. Nanoparticles from starch [4-7], cellulose [8-11], and lignin [12-14] have been developed for composite reinforcement. However, bio-fillers when compared to carbon black have lower modulus. The control of rigidity of filler network is one way of increasing the modulus of rubber composites. Another factor that has a negative impact on rubber reinforcement is the coagulation of hydrophilic fillers at higher filler concentrations because of the hydrophilic functional groups on the bio-filler surface. These functional groups tend to interact through hydrogen and ionic bonding and lead to aggregation of these filler particles into larger particles, which can reduce reinforcement effect. One approach to prevent such aggregation of particles is to use an ionic polymer capable of interacting with hydrophilic nanoparticles. In a previous study, nanoparticle dispersion of carboxylated styrene-butadiene (CSB) showed a strong interaction with soy protein particles [15] and produced rubber composites with high shear elastic modulus, but the composites could not be melt-processed because of the strong interactions. However, the composites can be melt processed by blending CSB with natural rubber. At low CSB concentration in natural rubber, the adhesion between these two polymers is sufficient to produce good mechanical properties although they are not miscible at molecular level [16]. In the present study, a flexible CSB consisted of 25% styrene and 75% butadiene content is used to complex
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with the filler network of soy protein nanoparticles. The composites were prepared by blending natural rubber latex, CSB nanoparticles and soy protein nanoparticles. The composites were then melt-processed using traditional rubber compounding process. These modified natural rubber composites were characterized to obtain optimum concentration of CSB and their static and dynamic mechanical properties. The composite structure is also discussed.
2. Materials and Methods
2.1. Materials
The carboxylated styrene-butadiene rubber (CSB) emulsion (Rovene 9410) was from Mallard Creek Polymers, Charlotte, NC. The styrene/butadiene ratio of the latex is 25/75. The latex received had ~50.5% solids and a pH ~8.6. The volume weighted mean particle size of the CSB latex was ~150 nm. Soy protein (SP) (trade name Ardex F) as dried powder contained ~90% protein, ~5% ash, and ~5% fat (Archer Daniels Midland Company, Decatur, IL). The NR latex (Centex LATZ) with ~61 % solids and a pH ~10 was from Centrotrade Rubber USA, Inc. (Chesapeake, VA). Calcium hydroxide was from Fisher Scientific (Waltham, MA). Ncyclohexyl-2-benzothiazolesulfenamide (CBTS) as vulcanization accelerator and Carbon black (CB) N339 were from Akrochem Co. (Akron, OH). 2,2’-Methylenebis(6-tert-butyl-4methylphenol) as antioxidant and stearic acid were from Sigma-Aldrich (St. Louis, MO). Zinc oxide and sulfur were from Alfa Aesar (Ward Hill, MA).
2.2. Size Reduction of Soy Protein and rubber composites
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The protein was hydrolyzed by heating 9.3 % concentration of SP dispersion with 2.2% calcium hydroxide based on the dry weight of SP in distilled water. After heating at 60 oC under stirring for 1 h, the dispersion was homogenized for 0.5 h at 8,000 rpm for 30 min before it was fed to a microfluidizer (M-100P, Microfluidics, Newton, MA). The dispersion passed through a diamond interaction chamber (200 μm) and a ceramic interaction chamber (80 μm) three times. The operating pressure was 159 MPa and the temperature of the exiting dispersion was cooled to 30 oC by cold water. The SP dispersion was blended with NR latex and CSB latex at pH 10 and ~15% concentration for 30 min at room temperature. The water was then removed by freezedrying. Four composites containing 10, 20, 30, and 40% SP were prepared for each blend of NR and CSB latexes composed of 5, 10, 15, 20, 25, and 30 % by weight of CSB in NR. The processing conditions described above were obtained empirically based on general principles of compatibility, ease of processing, and low cost. The dried composites were processed in a Brabender mixer (ATR Plasti-corder, C.W. Brabender Instruments, Inc., South Hackensack, NJ). The volume of the composites is 70% of the mixing bowl. The formulation was the same for all composites: 100 phr natural rubber, 1 phr anti-oxidant, 2 phr stearic acid, 5 phr zinc oxide, 2 phr accelerator, and 2 phr sulfur. The phr is referred to parts per hundred parts of rubber. The dried NR or NR-CSB filled with different amount of SP was mixed with antioxidant, stearic acid, and zinc oxide at 80 oC and 60 rpm for 15 minutes. The rubber compound was then compounded with sulfur and accelerator for 3 minutes. The composites with 10, 20, 30, and 40 wt% filler in terms of total weight of the filler and rubber were prepared. The final compounds were compression molded in a window-type mold at 5 MPa and 160 oC for 15 min.
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One objective of this study is to know the processes and compositions required for the composites reinforced with the hydrophilic filler to have mechanical properties that are comparable to the traditional carbon black filled NR composites. Therefore, the carbon black composites were prepared by traditional method of compounding dry carbon black powder with dry NR using the same rubber formulation and compounding process described above. The carbon black composites with 10, 20, 30, and 40 wt% carbon black in terms of total weight of the CB and NR were prepared. CSB is not used in carbon black composites because traditional carbon black composites do not include CSB.
2.3. Particle Size Measurement
The mean particle size and distribution of soy protein aggregates were measured using a Horiba LA-930 laser scattering particle size analyzer (Horiba Instruments, Irvine, CA) equipped with both 632.8 nm and 405 nm wavelength. A few drops of an emulsion sample were added to circulating distilled water with the same pH as that of the emulsion. An average of 20 acquisitions was obtained to calculate volume and number weighted mean diameters and size distribution of particles. The fraction and particle size of the dispersions that were less than 0.7 µm were determined by filtration using a 0.7 µm glass microfiber filter (WHATMAN GF/F). The solid contents of the dispersions before and after filtration were also measured by using a moisture analyzer.
2.4. Swelling Measurements and Fractured Surface
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The change in the dimension of composites in a good solvent was measured by immersing weighted samples in toluene for 72 h at 25 oC. Excess solvent on the surface of a swollen composite was removed and the swollen weight of the samples was measured. The samples were then dried in a vacuum oven for 24 h at 50 oC. Volume fraction of the rubber in a swollen sample was determined by both swollen and dry weight of the samples. A scanning electron microscope (SEM) was used to obtain images of tensile fractured surface. The fractured surface was placed an aluminum stubs and coated with Au-Pd, and then examined under vacuum at ambient temperature using a SEM (JOEL JSM-6010LA, JEOL USA, Inc., Peabody, MA).
2.5. Mechanical Property Measurements
Dynamic mechanical properties were measured with a strain-controlled rheometer, ARES-G2 rheometer (TA Instruments, Piscataway, NJ). A sample has a dimension of 50 X 12.5 X 5 mm was used in a torsion rectangular geometry and temperature ramp experiments were carried out at a heating rate of 1 oC/min from -68 to 140 oC. The measurements based on four repeats show that the uncertainty of the measurements is less than 10% for the values listed in Table 2. For tensile properties, dumbbell specimens were cut based on the specifications in ISO 37-2. The specimens have a thickness of ~2 mm. The average of five repeats was obtained for each composite at crosshead speed of 500 mm/min. These tensile tests were conducted with an Instron
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tensile testing machine (Instron, Norwood, MA) equipped with an Instron AutoX750 automatic extensometer and a 1 KN load cell.
3. Results and Discussion 3.1. Characterization of SP and CSB Nanoparticles
The particle size of soy protein was reduced by hydrolysis with calcium hydroxide at the elevated temperature and then microfluidized to obtain a homogeneous dispersion. Figure 1 shows the number- and volume-averaged size of SP particles, which in comparison is smaller than NR latex particles. SP particles after three cycles of microfluidization showed a double distribution in its volume-averaged size distribution. A fraction of large particles was centered at ~6 µm and a fraction of small particles centered at ~400 nm. Although more cycles of microfluidization can reduce the remaining larger particles to smaller particles, microfluidization is a slow and energy intensive process. For all practical purposes, the number of cycles should be kept to a minimum. To determine the amount of larger particles, a filtration with 0.7 µm glass microfiber filter was conducted to remove the fraction of larger SP particles. The result showed that ~93% of particles are smaller than 0.7 µm, and had a volume-averaged size of 260 ± 60 nm and a number-averaged size of 210 ± 60 nm. Compared with NR latex that had a volumeaveraged size of 760 ± 230 nm and a number-averaged size of 550 ± 200 nm, SP particles were much smaller. Soy protein nanoparticle aggregates are non-spherical as observed from TEM images of a previous study [17] and therefore have greater reinforcing effect than spherical particles. The particle size of CSB emulsion is the smallest among these particles measured, with
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a volume-averaged size of 150 ± 40 nm and a number-averaged size of 120 ± 30 nm. At a given weight of CSB, the smaller the size of CSB particles are, the greater the numbers of CSB particles are available at interfaces between SP and NR particles.
3.2. Stress-strain Properties
Figure 2 shows stress-strain curves of the composites with 0 to 40% filler. This comparison is made between the NR and NR-CSB composites with 20% CSB. For the composites without filler, the addition of CSB suppressed the strain-induced crystallization in NR as observed from the tensile stress at elongations greater than 500%, where tensile stress of NR composite is greater than that of NR-CSB composite. For the filled composites, an increase of tensile stress at larger strains was observed from the NR-CSB composites. This significant improvement is reflected in the tensile stress at 100 to 300% elongation. It is also observed that the addition of CSB has smaller effect at lower filler concentration, but become more significant at higher filler concentration. This effect is shown in Figure 3 and Table 1. The tensile stress at 200% and 300% elongation in Figure 3 is plotted against the amount of CSB in NR for different filler concentrations ranging from 10 to 40%. Although the general trend is that the tensile stress increased with the increasing concentration of CSB in NR, the highest tensile stress at these elongations occurs when ~20% CSB is incorporated into NR. CSB contains carboxylic acid functional groups that can interact strongly with amine and carboxylic acid functional groups on SP particles. This was observed from a previous study on the composites of SP particles and CSB, which had a high shear elastic modulus of 100-200 MPa for the composites with 40% of SP particles [18]. The addition of CSB into NR will therefore cause the complex formation
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between CSB and SP. This will prevent the coagulation of SP particles at higher filler concentrations and promote the formation of a more extensive rigid filler network through the connectivity between CSB and SP particles. Although CSB and NR do not mix at molecular level [19], CSB has greater affinity towards NR than SP because of hydrophobic styrenebutadiene segments compared with hydrophilic functional groups on SP particle surface. Figure 3 also shows that the increase of tensile stress is small in the rubber blends of NR and CSB without any filler when the CSB content is increased. However, the tensile stress increased significantly when SP particles were incorporated into the composites. This is another indication that interactions between CSB and SP contribute significantly to the reinforcement effect. The tensile stress of the composites starts to show a decrease when CSB content is greater than ~20% in NR. Table 1 also summarizes the characteristics of these composites. Without SP nanoparticles, the tensile strength of NR and CSB blend is 10-13 MPa. With the addition of 10% SP nanoparticles, the tensile strength of the composites increased significantly to 22-25 MPa. With the increase of filler concentration, the tensile strength decreased, but retained useful tensile strength for many applications of molded rubber objects. The elongation at break generally decreased with increasing content of CSB and filler concentration. Young’s modulus generally increased with increasing CSB and filler concentration. Compared to carbon black composites, the overall tensile properties of NR composites with SP and 10-20% CSB are closer to that of CB composites. The effect of CSB can also be observed from Figure 4, where the fractured surfaces of NR composites with 30% SP and 0-30% of CSB were obtained from the tensile tests. Without CSB, the fractured surface of the composite had more detached fragments. With the inclusion of CSB
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in NR, the fractured surfaces show better cohesion within the composites. NR latex contains ~2% of NR protein, which is responsible for the latex stability in water. NR latex is the aqueous dispersion of poly(isoprene) particles coated with NR protein at alkali pH. When NR latex is mixed with CSB, carboxyl functional group in CSB can form ionic and hydrogen bonds with NR protein upon the water removal. Therefore, the interface between CSB and NR is the NR protein, which acts as compatibilizer between CBS and NR. The higher resolution micrographs in Figure 4(e) and 4(f) show good adhesion between SP and rubber matrix with or without CBS and are unable to differentiate different extent of interfacial interactions between SP particles and rubber matrix. The increase of interactions between SP and CSB is also reflected in the extent of swelling when the composites were immersed in a good solvent. Figure 5 shows the Kraus plot [20] of swollen composites based on Eq. (1), which relates the degree of swelling to the effect of filler-rubber interactions.
Vr 0 m 1 Vr (1 )
(1)
Where Vr 0 and Vr are the volume fraction of the rubber in the swollen NR and the swollen SP reinforced NR, respectively. is filler volume fraction and m 3C (1 Vr10/ 3 ) Vr 0 1 , where C is a characteristic constant of the filler and is independent of rubber matrix. The composites with different amount of CSB in Figure 5(a) have almost the same slope and have a value of C ~1.04, which is within the range of C values (0.92-1.35) for different types of carbon black [20]. It is also noted that the intercept of lines in Figure 5(a) is not 1, which indicates the vulcanization is affected by the filler and the crosslinking density of the polymer matrix in the composites
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without the effect of filler is not the same as that of the unfilled rubber [20]. The C value of 1.04 can then be used in Eq. (1) to calculate Vr 0 for each and the corrected Vr 0 can be used to calculate the crosslinking density of the matrix in the composites without the effect of filler using Flory-Rehner equation assuming tetrafunctional junction points.
[ln(1 2 ) 2 22 ] nV1 21 / 3 2 2
(2)
where χ is toluene-NR interaction parameter and is taken as 0.36 [21] and is 0.46 for SBR and toluene [22]. For the blend, χ is taken as the volume average. n is the crosslinking density defined as the number of network chain between crosslinks. V1 is the molar volume of toluene, 106.3 mole/cm3. ν2 is the rubber volume fraction in a swollen filled rubber composite after the correction of filler volume because the filler is not swollen by toluene. To calculate the crosslinking density of the matrix in the composites without the effect of filler, ν2 is equal to the corrected Vr 0 and the crosslinking densities are shown in Figure 5(b). The trends show that functional groups on the protein particles retard the vulcanization, but CSB can complex with the protein particles and reduces its retarding effect. Carboxylic acid functional groups in CSB and protein have similar retarding effect to those from traditional scorch retarders such as benzoic acid, salicylic acid, and phthalic anhydride. Kraus plots in Figure 5(a) shows that composites with 20% and 30% CSB had a similar extent of swelling, while the composites with 0% and 10% of CSB had greater extent of swelling. This indicates that additional CSB at a concentration greater than ~20% did not contribute to the
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reduction of the swelling in a good solvent. 20% CSB in NR is therefore the optimum concentration for the modulus enhancement.
3.3. Dynamic Mechanical Properties
Shear elastic moduli of the composites with 20% CSB is shown in Figure 6. The elastic moduli were measured in the linear viscoelastic region (small strain region). SP particles have a shear elastic modulus of 1-2 GPa [15]. The shear elastic modulus increases with the increasing filler content in the composites for all temperatures measured. At higher temperatures, the G’ decreased slowly with the increasing temperature especially for the composites with higher filler fractions. Such decrease is caused by the decrease of G’ of SP particles with the increasing temperature and is reflected in the increase of loss tangent of SP shown in the inset of Figure 6, which also shows that tan δ of the unfilled rubber with 20% CSB does not increase at the higher temperature region. The increase of tan δ of SP particles is also reflected in the composites with 30% filler in Figure 7. Figure 7 shows the glass transition temperatures of composites filled with 30% SP nanoparticles in rubber matrix containing 0-30% of CSB. Glass transition temperature defined by Tan δ shows that NR has a Tg (glass transition temperature) at -54 oC, while CSB has two glass transition temperatures at -38 oC and 14 oC. The transition temperature at -38 oC is from styrene-butadiene segments and the transition at 14 oC is associated with aggregation of carboxylic acid functional groups. These glass transition temperatures also indicate that the NR and CSB do not mixed at molecular level. Tg of NR at -54 oC did not shift with the inclusion of SP particles or CSB. Tg of CSB is not sufficiently distinct and its shifting caused by the presence of other components in the composites cannot be determined correctly because its fraction is too
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small compared to that of NR. Within this temperature range, the composites are thermally stable as shown in the inset of Figure 7, which shows the thermal degradation occurs above 200 oC.
Figure 8 shows that the reinforcement factor of the composites changed with both CSB and
filler concentrations at 140 oC. The reinforcement factor is defined as the elastic modulus of composite normalized by the elastic modulus of rubber matrix. At 140 oC, the composites are devoid of Tan δ transitions as shown in Figure 7. Therefore, the effect of CSB concentration can be observed more clearly. The general trend shows that the reinforcement factor increased with the increasing CSB and filler concentrations. The effect peaked at ~20% CSB content, similar to the observations in tensile stress at 200 and 300% elongations (Figure 3). To understand the reinforcement effect of these composites, reinforcement factors for these composites at different filler volume fractions at 140 oC are shown in Figure 9. The curves show a rapid increase of G’ with increasing filler concentration. This type of curves cannot be fitted with simple particle inclusion theory such as The Einstein equation. Halpin-Tsai model [23] that describes the anisotropic effect of filler in a matrix also does not describe these curves well because it does not take into account of particle-particle association at higher filler concentrations. Empirical Guth model [24] for non-spherical fillers is adequate to describe these curves. Guth model has the following form.
Gc Gm 1 0.67 p 1.62 p 2 2
(3)
where p is the aspect ratio of a reinforcement element and is the volume fraction of filler. p is obtained by fitting Eq. (3) to the experimental values of Gc / Gm and is an indicator of anisotropic character of the reinforcement elements. The fitted curves indicate that reinforcement factor increased with increasing CSB content in the composites. This may imply that the addition of CSB provides additional connectivity between SP particles and therefore increases the
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anisotropic character of the reinforcement elements, which are composed of reinforcement particles and restricted polymer chains immediately adjacent to the particles. The fitting also shows that the composites with 20% CSB have higher reinforcement factor than that with 30% CSB, similar to the observation from tensile stress at 200% and 300% elongation despite G’ was measured at small strain (0.05%). This indicates that the connectivity between SP and CSB is rigid and can be detected even at small strain. As the concentration of CSB increased from 0 to 30% (Figure 9), it was also observed that the difference between reinforcement factors from the composites with different amount of CSB was greater at higher filler fractions (30 and 40% SP) than at lower filler fractions (10 and 20% SP). This indicates that CSB had greater effect at higher filler concentrations, likely providing a more rigid connectivity between SP particles, improving SP particle dispersion and preventing aggregation of SP particles. At ambient temperature of 25 oC, the reinforcement factors are significantly greater than that at 140 oC (Figure 10). Because of an even more rapid rise of reinforcement factor with the increasing filler concentration, Guth model does not fit the data as well as that at 140 oC. However, it is used here to indicate the trend that reinforcement factors are proportional to the increasing CSB concentration. As shown in Figure 7, reinforcement factors at 25 oC is within the CSB glass transition region that involved the ionic interactions of carboxylic acid functional groups. Any excess CSB that is not interacting with SP particles forms domains with ionic interactions that also contribute to the reinforcement effect of the rubber matrix. This can be seen from Young’s moduli listed in Table I. For the composites without any SP particles, the Young’s modulus increased from 1.4 to 1.8 MPa as the CSB concentration increased from 0 to 30%. At 25 oC, shear elastic modulus of CSB is 1.62 MPa, while it is 0.79 MPa for NR. This means that
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ionic CSB domains also behave like weak reinforcing fillers and contribute to the rubber reinforcement. Another interesting comparison with CB composites is listed in Table 2. Since CB is the major filler for rubber and ~70% of worldwide CB consumption is used in tires, CB with the same filler fraction in NR is used as a comparison standard. In rubber industry, dynamic mechanical properties are frequently used to predict tire tread performance at different temperatures [25]. For such application, the filler content is usually at 30-40 wt% in the total weight of filler and rubber, which corresponds to a filler concentration of 43-66 phr. The values in Table II indicate that NR composite reinforced with SP and 20% CSB is likely to have better low temperature and energy performance, but less dry handling performance. This comparison also indicates certain advantages of using SP nanoparticles with CB in NR composites to obtain optimum properties.
4. Conclusions Synergistic coordination of SP and CSB nanoparticles were used to increase significantly the modulus of NR. CSB has a number-averaged size of 120 nm. After microfluidization, 93% of SP particles produced have a number-averaged size of 210 nm. The addition of SP and CSB into NR caused the tensile stress to increase significantly at 100-300% elongation. The optimum concentration of CSB in NR is ~20%, which produced the greatest improvement in tensile stress at higher elongation ratios. SP nanoparticles improve the tensile strength of rubber matrix significantly from 10-13 MPa to 22-25 MPa with the addition of 10% SP particles. Young’s modulus of the composites increased with increasing CSB content in NR. SEM indicates an improvement of cohesion within the composites with the addition of CSB. The Swelling studies
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also show the reduction of swelling with increasing concentration of CSB up to ~20%. Reinforcement factors from shear elastic modulus shows that the highest reinforcement factors occur at ~20% CSB in NR. Increasing reinforcement factor with increasing filler concentration in the composites was described by Guth model and indicates the addition of CSB increased the reinforcement effect, likely through the connectivity and formation of rigid filler network. These experiments also indicate the addition of CSB improved reinforcement more significantly at higher than lower filler concentrations. The tensile properties of NR composites with SP and 1020% CSB were also found to be closer to that of carbon black reinforced NR composites. Loss tangents of these composites indicate a great potential for tire tread application.
Acknowledgements The author would like to thank A. Thompson for the sample imaging with scanning electron microscope.
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Table 1 Tensile Properties of the composites Filler concentration (%)
CSB
0
10
20
30
40
Tensile stress at
0%
0.9 ± 0.1
1.1 ± 0.03
1.6 ± 0.1
3.1 ± 0.3
5.3 ± 0.1
100% elongation (MPa)
10 %
0.9 ± 0.1
1.2 ± 0.1
2.0 ± 0.1
3.5 ± 0.3
6.5 ± 0.4
20 %
1.0 ± 0.04
1.4 ± 0.1
2.6 ± 0.1
5.3 ± 0.1
7.6 ± 0.2
30 %
1.1 ± 0.04
1.5 ± 0.03
2.5 ± 0.1
3.9 ± 0.2
7.8 ± 0.2
CB
0.9 ± 0
1.2 ± 0.1
2.0 ± 0.1
3.3 ± 0.1
7.0 ± 0.4
Tensile stress at
0%
2.4 ± 0.2
4.6 ± 0.2
8.0 ± 0.8
13.0 ± 0.8
15.5 ± 0.03
300% elongation (MPa)
10 %
2.7 ± 0.3
4.8 ± 0.4
9.3 ± 0.3
13.6 ± 0.9
---
20 %
2.7 ± 0.2
5.4 ± 0.3
11.2 ± 0.5
16.8 ± 0.2
---
30 %
3.0 ± 0.1
5.8 ± 0.1
10.4 ± 0.4
14.1 ± 0.6
---
CB
2.4 ± 0.2
4.4 ± 0.3
10.1 ± 0.4
16.3 ± 0.3
---
0%
1.4 ± 0.3
1.9 ± 0.1
2.7 ± 0.2
4.0 ± 0.3
6.5 ± 0.3
10 %
1.5 ± 0.2
2.1 ± 0.4
3.1 ± 0.3
4.2 ± 0.5
8.8 ± 0.3
20 %
1.6 ± 0.04
2.7 ± 0.2
4.1 ± 0.3
6.9 ± 0.2
11.1 ± 0.4
30 %
1.8 ± 0.1
2.6 ± 0.3
4.3 ± 0.4
6.2 ± 1.0
15.5 ± 0.4
CB
1.4 ± 0.2
2.0 ± 0.2
3.1 ± 0.4
5.9 ± 0.2
12.3 ± 0.5
0%
13.2 ± 2
24.8 ± 0.7
22.2 ± 0.6
19.1 ± 0.6
15.5 ± 0.4
10 %
11.9 ± 2.7
24.3 ± 0.8
21.5 ± 0.4
18.6 ± 1.0
15.2 ± 0.7
20 %
12.5 ± 2.1
21.7 ± 0.7
22.5 ± 0.8
18.0 ± 0.6
14.8 ± 0.6
30 %
9.8 ± 1.1
22.5 ± 0.7
21.3 ± 0.7
18.2 ± 0.8
13.4 ± 0.3
CB
13.2 ± 2
22.5 ± 0.1
24.7 ± 1.4
23.8 ± 0.5
21.4 ± 2
0%
540 ± 17
594 ± 13
539 ± 17
426 ± 11
302 ± 10
Young’s Modulus (MPa)
Tensile Strength (MPa)
Elongation at break (%)
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10 %
542 ± 28
598 ± 23
498 ± 6
402 ± 11
273 ± 9
20 %
558 ± 35
564 ± 19
476 ± 3
324 ± 14
218 ± 6
30 %
505 ± 8
570 ± 10
484 ± 8
383 ± 19
198 ± 8
CB
540 ± 17
569 ± 15
487 ± 8
400 ± 4
256 ± 31
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Table 2 Predictive performance from dynamic mechanical properties Filler concentration (%)
0
10
20
30
40
Tanδ at -10 oC (higher is better)
20% CSB
0.080
0.053
0.057
0.069
0.084
Ice traction
CB
0.014
0.030
0.039
0.061
0.075
Tanδ at 10 oC (higher is better)
20% CSB
0.042
0.037
0.046
0.060
0.076
Wet traction
CB
0.014
0.021
0.026
0.052
0.066
Tanδ at 60 oC (lower is better)
20% CSB
0.027
0.032
0.033
0.041
0.054
Rolling resistance
CB
0.014
0.019
0.036
0.063
0.084
G’ (MPa) at -20 oC (lower is better)
20% CSB
1.1
2.0
3.5
7.8
14.7
Winter traction
CB
0.7
1.0
2.5
10.3
24.9
G’ (MPa) at 30 oC (higher is better)
20% CSB
0.8
1.5
2.6
4.9
7.8
Dry handling
CB
0.8
1.0
2.2
6.9
14.4
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FIGURES Fig. 1. Particle size distributions of NR, CSB, SP, and 0.7 um filtered SP.
Fig. 2. Comparison of tensile properites from NR composites and NR-CSB composites with 20% CSB reinforced with 10 to 40% SP nanoparticles. Filled symbols represent NR composites and unfilled symbols represent NR-CSB composites.
Fig. 3. Comparison of tensile stress at 200% and 300% elongation from NR composites with 0% to 30% CSB.
Fig. 4. Comparison of tensile fractural surfaces from the composites reinforced with 30% SP nanoparticles and different amount of CSB: (a) 0% CSB (b) 10% CSB (c) 20% CSB (d) 30% CSB (e) 0% CSB - high resolution (f) 20% CSB – high resolution.
Fig. 5. (a) Kraus plot of swollen composites (b) crosslinking densities of polymer matrices in the composites without the effect of filler.
Fig. 6. Shear elastic moduli of NR composites with 0 to 40% SP nanoparticles and 20% CSB. The inset shows the variation of loss tangent with temperature for SP and the unfilled rubber with 20% CSB.
Fig. 7. Loss tangent of NR composites with 30% SP nanoparticles and 0 to 30% CSB. The inset is the thermal degradation analysis of the 30% filled composite with 20% CSB.
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Fig. 8. Reinforcement factor (G’/G’o) of NR composites with 0 to 40% SP nanoparticles for different amount of CSB in NR at 140 oC.
Fig. 9. Shear elastic moduli of NR composites with 0 to 40% soy nanoparticles for different amount of CSB in NR at 140 oC. The data is fitted with Guth model.
Fig. 10. Shear elastic moduli of NR composites with 0 to 40% soy nanoparticles for different amount of CSB in NR at 25 oC. The data is fitted with Guth model.
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Fig. 1. Particle size distributions of NR, CSB, SP, and 0.7 um filtered SP.
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Fig. 2. Comparison of tensile properites from NR composites and NR-CSB composites with 20% CSB reinforced with 10 to 40% SP nanoparticles. Filled symbols represent NR composites and unfilled symbols represent NR-CSB composites.
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Fig. 3. Comparison of tensile stress at 200% and 300% elongation from NR composites with 0% to 30% CSB.
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Fig. 4. Comparison of tensile fractural surfaces from the composites reinforced with 30% SP nanoparticles and different amount of CSB: (a) 0% CSB (b) 10% CSB (c) 20% CSB (d) 30% CSB (e) 0% CSB - high resolution (f) 20% CSB – high resolution.
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Fig. 5. (a) Kraus plot of swollen composites (b) crosslinking densities of polymer matrices in the composites without the effect of filler.
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Fig. 6. Shear elastic moduli of NR composites with 0 to 40% SP nanoparticles and 20% CSB. The inset shows the variation of loss tangent with temperature for SP and the unfilled rubber with 20% CSB.
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Fig. 7. Loss tangent of NR composites with 30% SP nanoparticles and 0 to 30% CSB. The inset is the thermal degradation analysis of the 30% filled composite with 20% CSB.
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Fig. 8. Reinforcement factor (G’/G’o) of NR composites with 0 to 40% SP nanoparticles for different amount of CSB in NR at 140 oC.
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Fig. 9. Shear elastic moduli of NR composites with 0 to 40% soy nanoparticles for different amount of CSB in NR at 140 oC. The data is fitted with Guth model.
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Fig. 10. Shear elastic moduli of NR composites with 0 to 40% soy nanoparticles for different amount of CSB in NR at 25 oC. The data is fitted with Guth model.
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Lei Jong PII:
S0254-0584(17)30768-X
DOI:
10.1016/j.matchemphys.2017.09.067
Reference:
MAC 20034
To appear in:
Materials Chemistry and Physics
Received Date:
06 June 2017
Revised Date:
13 September 2017
Accepted Date:
29 September 2017
Please cite this article as: Lei Jong, Improved natural rubber composites reinforced with a complex filler network of biobased nanoparticles and ionomer, Materials Chemistry and Physics (2017), doi: 10.1016/j.matchemphys.2017.09.067
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Highlights
Hydrophilic filler and ionomer form a complex filler network The complex filler network greatly improves the properties of rubber composites Modulus, tensile stress and reinforcement factor were significantly improved Great potential for improving rolling resistance in tire tread applications
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Graphical abstract
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Improved natural rubber composites reinforced with a complex filler network of biobased nanoparticles and ionomer Lei Jong United States Department of Agriculture, Agricultural Research Service, National Center for Agricultural Utilization Research†, 1815 N. University St., Peoria, IL 61604, United States ABSTRACT: Biobased rubber composites are renewable and sustainable. Significant improvement in modulus of rubber composite reinforced with hydrophilic filler was achieved with the inclusion of ionomers. Soy particles aided with ionomer, carboxylated styrene-butadiene (CSB), formed a strong complex filler network in rubber. The effect of CSB caused Young’s modulus and the tensile stress at 100-300% elongation to increase significantly while maintaining good tensile strength. The optimum concentration of CSB in natural rubber that produced the greatest improvement in tensile stress at higher elongation ratios and shear elastic modulus was determined. CSB improves the cohesion within the composites. The increase of reinforcement factor with increasing CSB and filler concentration in the composites indicates that the effect of CSB increased the reinforcement of soy nanoparticles more significantly at higher than lower filler concentrations. The mechanical properties of the composites were also compared to carbon black reinforced natural rubber composites. The result indicates great potential for tire tread applications.
Keywords: Natural rubber; mechanical properties; composites; ionomer. †Mention
of trade names or commercial products in this publication is solely for the purpose of providing specific information and does not imply recommendation or endorsement by the U.S. Department of Agriculture. USDA is an equal opportunity provider and employer. Correspondence to: L. Jong (Email: [email protected])
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1. Introduction Renewable materials are important for a sustainable future. Biobased composites reinforced with nanoparticles have potential to be used as molded components for various damping applications. Rubber composites is one type of particle reinforced composites. Major factors in improving the strength of rubber composites are small particle size, anisotropic particles, and interface adhesion between particles and polymer matrix [1-3]. Carbon black and silica are currently the major fillers for rubbers, but renewable fillers are also showing potentials in recent years. Nanoparticles from starch [4-7], cellulose [8-11], and lignin [12-14] have been developed for composite reinforcement. However, bio-fillers when compared to carbon black have lower modulus. The control of rigidity of filler network is one way of increasing the modulus of rubber composites. Another factor that has a negative impact on rubber reinforcement is the coagulation of hydrophilic fillers at higher filler concentrations because of the hydrophilic functional groups on the bio-filler surface. These functional groups tend to interact through hydrogen and ionic bonding and lead to aggregation of these filler particles into larger particles, which can reduce reinforcement effect. One approach to prevent such aggregation of particles is to use an ionic polymer capable of interacting with hydrophilic nanoparticles. In a previous study, nanoparticle dispersion of carboxylated styrene-butadiene (CSB) showed a strong interaction with soy protein particles [15] and produced rubber composites with high shear elastic modulus, but the composites could not be melt-processed because of the strong interactions. However, the composites can be melt processed by blending CSB with natural rubber. At low CSB concentration in natural rubber, the adhesion between these two polymers is sufficient to produce good mechanical properties although they are not miscible at molecular level [16]. In the present study, a flexible CSB consisted of 25% styrene and 75% butadiene content is used to complex
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with the filler network of soy protein nanoparticles. The composites were prepared by blending natural rubber latex, CSB nanoparticles and soy protein nanoparticles. The composites were then melt-processed using traditional rubber compounding process. These modified natural rubber composites were characterized to obtain optimum concentration of CSB and their static and dynamic mechanical properties. The composite structure is also discussed.
2. Materials and Methods
2.1. Materials
The carboxylated styrene-butadiene rubber (CSB) emulsion (Rovene 9410) was from Mallard Creek Polymers, Charlotte, NC. The styrene/butadiene ratio of the latex is 25/75. The latex received had ~50.5% solids and a pH ~8.6. The volume weighted mean particle size of the CSB latex was ~150 nm. Soy protein (SP) (trade name Ardex F) as dried powder contained ~90% protein, ~5% ash, and ~5% fat (Archer Daniels Midland Company, Decatur, IL). The NR latex (Centex LATZ) with ~61 % solids and a pH ~10 was from Centrotrade Rubber USA, Inc. (Chesapeake, VA). Calcium hydroxide was from Fisher Scientific (Waltham, MA). Ncyclohexyl-2-benzothiazolesulfenamide (CBTS) as vulcanization accelerator and Carbon black (CB) N339 were from Akrochem Co. (Akron, OH). 2,2’-Methylenebis(6-tert-butyl-4methylphenol) as antioxidant and stearic acid were from Sigma-Aldrich (St. Louis, MO). Zinc oxide and sulfur were from Alfa Aesar (Ward Hill, MA).
2.2. Size Reduction of Soy Protein and rubber composites
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The protein was hydrolyzed by heating 9.3 % concentration of SP dispersion with 2.2% calcium hydroxide based on the dry weight of SP in distilled water. After heating at 60 oC under stirring for 1 h, the dispersion was homogenized for 0.5 h at 8,000 rpm for 30 min before it was fed to a microfluidizer (M-100P, Microfluidics, Newton, MA). The dispersion passed through a diamond interaction chamber (200 μm) and a ceramic interaction chamber (80 μm) three times. The operating pressure was 159 MPa and the temperature of the exiting dispersion was cooled to 30 oC by cold water. The SP dispersion was blended with NR latex and CSB latex at pH 10 and ~15% concentration for 30 min at room temperature. The water was then removed by freezedrying. Four composites containing 10, 20, 30, and 40% SP were prepared for each blend of NR and CSB latexes composed of 5, 10, 15, 20, 25, and 30 % by weight of CSB in NR. The processing conditions described above were obtained empirically based on general principles of compatibility, ease of processing, and low cost. The dried composites were processed in a Brabender mixer (ATR Plasti-corder, C.W. Brabender Instruments, Inc., South Hackensack, NJ). The volume of the composites is 70% of the mixing bowl. The formulation was the same for all composites: 100 phr natural rubber, 1 phr anti-oxidant, 2 phr stearic acid, 5 phr zinc oxide, 2 phr accelerator, and 2 phr sulfur. The phr is referred to parts per hundred parts of rubber. The dried NR or NR-CSB filled with different amount of SP was mixed with antioxidant, stearic acid, and zinc oxide at 80 oC and 60 rpm for 15 minutes. The rubber compound was then compounded with sulfur and accelerator for 3 minutes. The composites with 10, 20, 30, and 40 wt% filler in terms of total weight of the filler and rubber were prepared. The final compounds were compression molded in a window-type mold at 5 MPa and 160 oC for 15 min.
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One objective of this study is to know the processes and compositions required for the composites reinforced with the hydrophilic filler to have mechanical properties that are comparable to the traditional carbon black filled NR composites. Therefore, the carbon black composites were prepared by traditional method of compounding dry carbon black powder with dry NR using the same rubber formulation and compounding process described above. The carbon black composites with 10, 20, 30, and 40 wt% carbon black in terms of total weight of the CB and NR were prepared. CSB is not used in carbon black composites because traditional carbon black composites do not include CSB.
2.3. Particle Size Measurement
The mean particle size and distribution of soy protein aggregates were measured using a Horiba LA-930 laser scattering particle size analyzer (Horiba Instruments, Irvine, CA) equipped with both 632.8 nm and 405 nm wavelength. A few drops of an emulsion sample were added to circulating distilled water with the same pH as that of the emulsion. An average of 20 acquisitions was obtained to calculate volume and number weighted mean diameters and size distribution of particles. The fraction and particle size of the dispersions that were less than 0.7 µm were determined by filtration using a 0.7 µm glass microfiber filter (WHATMAN GF/F). The solid contents of the dispersions before and after filtration were also measured by using a moisture analyzer.
2.4. Swelling Measurements and Fractured Surface
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The change in the dimension of composites in a good solvent was measured by immersing weighted samples in toluene for 72 h at 25 oC. Excess solvent on the surface of a swollen composite was removed and the swollen weight of the samples was measured. The samples were then dried in a vacuum oven for 24 h at 50 oC. Volume fraction of the rubber in a swollen sample was determined by both swollen and dry weight of the samples. A scanning electron microscope (SEM) was used to obtain images of tensile fractured surface. The fractured surface was placed an aluminum stubs and coated with Au-Pd, and then examined under vacuum at ambient temperature using a SEM (JOEL JSM-6010LA, JEOL USA, Inc., Peabody, MA).
2.5. Mechanical Property Measurements
Dynamic mechanical properties were measured with a strain-controlled rheometer, ARES-G2 rheometer (TA Instruments, Piscataway, NJ). A sample has a dimension of 50 X 12.5 X 5 mm was used in a torsion rectangular geometry and temperature ramp experiments were carried out at a heating rate of 1 oC/min from -68 to 140 oC. The measurements based on four repeats show that the uncertainty of the measurements is less than 10% for the values listed in Table 2. For tensile properties, dumbbell specimens were cut based on the specifications in ISO 37-2. The specimens have a thickness of ~2 mm. The average of five repeats was obtained for each composite at crosshead speed of 500 mm/min. These tensile tests were conducted with an Instron
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tensile testing machine (Instron, Norwood, MA) equipped with an Instron AutoX750 automatic extensometer and a 1 KN load cell.
3. Results and Discussion 3.1. Characterization of SP and CSB Nanoparticles
The particle size of soy protein was reduced by hydrolysis with calcium hydroxide at the elevated temperature and then microfluidized to obtain a homogeneous dispersion. Figure 1 shows the number- and volume-averaged size of SP particles, which in comparison is smaller than NR latex particles. SP particles after three cycles of microfluidization showed a double distribution in its volume-averaged size distribution. A fraction of large particles was centered at ~6 µm and a fraction of small particles centered at ~400 nm. Although more cycles of microfluidization can reduce the remaining larger particles to smaller particles, microfluidization is a slow and energy intensive process. For all practical purposes, the number of cycles should be kept to a minimum. To determine the amount of larger particles, a filtration with 0.7 µm glass microfiber filter was conducted to remove the fraction of larger SP particles. The result showed that ~93% of particles are smaller than 0.7 µm, and had a volume-averaged size of 260 ± 60 nm and a number-averaged size of 210 ± 60 nm. Compared with NR latex that had a volumeaveraged size of 760 ± 230 nm and a number-averaged size of 550 ± 200 nm, SP particles were much smaller. Soy protein nanoparticle aggregates are non-spherical as observed from TEM images of a previous study [17] and therefore have greater reinforcing effect than spherical particles. The particle size of CSB emulsion is the smallest among these particles measured, with
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a volume-averaged size of 150 ± 40 nm and a number-averaged size of 120 ± 30 nm. At a given weight of CSB, the smaller the size of CSB particles are, the greater the numbers of CSB particles are available at interfaces between SP and NR particles.
3.2. Stress-strain Properties
Figure 2 shows stress-strain curves of the composites with 0 to 40% filler. This comparison is made between the NR and NR-CSB composites with 20% CSB. For the composites without filler, the addition of CSB suppressed the strain-induced crystallization in NR as observed from the tensile stress at elongations greater than 500%, where tensile stress of NR composite is greater than that of NR-CSB composite. For the filled composites, an increase of tensile stress at larger strains was observed from the NR-CSB composites. This significant improvement is reflected in the tensile stress at 100 to 300% elongation. It is also observed that the addition of CSB has smaller effect at lower filler concentration, but become more significant at higher filler concentration. This effect is shown in Figure 3 and Table 1. The tensile stress at 200% and 300% elongation in Figure 3 is plotted against the amount of CSB in NR for different filler concentrations ranging from 10 to 40%. Although the general trend is that the tensile stress increased with the increasing concentration of CSB in NR, the highest tensile stress at these elongations occurs when ~20% CSB is incorporated into NR. CSB contains carboxylic acid functional groups that can interact strongly with amine and carboxylic acid functional groups on SP particles. This was observed from a previous study on the composites of SP particles and CSB, which had a high shear elastic modulus of 100-200 MPa for the composites with 40% of SP particles [18]. The addition of CSB into NR will therefore cause the complex formation
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between CSB and SP. This will prevent the coagulation of SP particles at higher filler concentrations and promote the formation of a more extensive rigid filler network through the connectivity between CSB and SP particles. Although CSB and NR do not mix at molecular level [19], CSB has greater affinity towards NR than SP because of hydrophobic styrenebutadiene segments compared with hydrophilic functional groups on SP particle surface. Figure 3 also shows that the increase of tensile stress is small in the rubber blends of NR and CSB without any filler when the CSB content is increased. However, the tensile stress increased significantly when SP particles were incorporated into the composites. This is another indication that interactions between CSB and SP contribute significantly to the reinforcement effect. The tensile stress of the composites starts to show a decrease when CSB content is greater than ~20% in NR. Table 1 also summarizes the characteristics of these composites. Without SP nanoparticles, the tensile strength of NR and CSB blend is 10-13 MPa. With the addition of 10% SP nanoparticles, the tensile strength of the composites increased significantly to 22-25 MPa. With the increase of filler concentration, the tensile strength decreased, but retained useful tensile strength for many applications of molded rubber objects. The elongation at break generally decreased with increasing content of CSB and filler concentration. Young’s modulus generally increased with increasing CSB and filler concentration. Compared to carbon black composites, the overall tensile properties of NR composites with SP and 10-20% CSB are closer to that of CB composites. The effect of CSB can also be observed from Figure 4, where the fractured surfaces of NR composites with 30% SP and 0-30% of CSB were obtained from the tensile tests. Without CSB, the fractured surface of the composite had more detached fragments. With the inclusion of CSB
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in NR, the fractured surfaces show better cohesion within the composites. NR latex contains ~2% of NR protein, which is responsible for the latex stability in water. NR latex is the aqueous dispersion of poly(isoprene) particles coated with NR protein at alkali pH. When NR latex is mixed with CSB, carboxyl functional group in CSB can form ionic and hydrogen bonds with NR protein upon the water removal. Therefore, the interface between CSB and NR is the NR protein, which acts as compatibilizer between CBS and NR. The higher resolution micrographs in Figure 4(e) and 4(f) show good adhesion between SP and rubber matrix with or without CBS and are unable to differentiate different extent of interfacial interactions between SP particles and rubber matrix. The increase of interactions between SP and CSB is also reflected in the extent of swelling when the composites were immersed in a good solvent. Figure 5 shows the Kraus plot [20] of swollen composites based on Eq. (1), which relates the degree of swelling to the effect of filler-rubber interactions.
Vr 0 m 1 Vr (1 )
(1)
Where Vr 0 and Vr are the volume fraction of the rubber in the swollen NR and the swollen SP reinforced NR, respectively. is filler volume fraction and m 3C (1 Vr10/ 3 ) Vr 0 1 , where C is a characteristic constant of the filler and is independent of rubber matrix. The composites with different amount of CSB in Figure 5(a) have almost the same slope and have a value of C ~1.04, which is within the range of C values (0.92-1.35) for different types of carbon black [20]. It is also noted that the intercept of lines in Figure 5(a) is not 1, which indicates the vulcanization is affected by the filler and the crosslinking density of the polymer matrix in the composites
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without the effect of filler is not the same as that of the unfilled rubber [20]. The C value of 1.04 can then be used in Eq. (1) to calculate Vr 0 for each and the corrected Vr 0 can be used to calculate the crosslinking density of the matrix in the composites without the effect of filler using Flory-Rehner equation assuming tetrafunctional junction points.
[ln(1 2 ) 2 22 ] nV1 21 / 3 2 2
(2)
where χ is toluene-NR interaction parameter and is taken as 0.36 [21] and is 0.46 for SBR and toluene [22]. For the blend, χ is taken as the volume average. n is the crosslinking density defined as the number of network chain between crosslinks. V1 is the molar volume of toluene, 106.3 mole/cm3. ν2 is the rubber volume fraction in a swollen filled rubber composite after the correction of filler volume because the filler is not swollen by toluene. To calculate the crosslinking density of the matrix in the composites without the effect of filler, ν2 is equal to the corrected Vr 0 and the crosslinking densities are shown in Figure 5(b). The trends show that functional groups on the protein particles retard the vulcanization, but CSB can complex with the protein particles and reduces its retarding effect. Carboxylic acid functional groups in CSB and protein have similar retarding effect to those from traditional scorch retarders such as benzoic acid, salicylic acid, and phthalic anhydride. Kraus plots in Figure 5(a) shows that composites with 20% and 30% CSB had a similar extent of swelling, while the composites with 0% and 10% of CSB had greater extent of swelling. This indicates that additional CSB at a concentration greater than ~20% did not contribute to the
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reduction of the swelling in a good solvent. 20% CSB in NR is therefore the optimum concentration for the modulus enhancement.
3.3. Dynamic Mechanical Properties
Shear elastic moduli of the composites with 20% CSB is shown in Figure 6. The elastic moduli were measured in the linear viscoelastic region (small strain region). SP particles have a shear elastic modulus of 1-2 GPa [15]. The shear elastic modulus increases with the increasing filler content in the composites for all temperatures measured. At higher temperatures, the G’ decreased slowly with the increasing temperature especially for the composites with higher filler fractions. Such decrease is caused by the decrease of G’ of SP particles with the increasing temperature and is reflected in the increase of loss tangent of SP shown in the inset of Figure 6, which also shows that tan δ of the unfilled rubber with 20% CSB does not increase at the higher temperature region. The increase of tan δ of SP particles is also reflected in the composites with 30% filler in Figure 7. Figure 7 shows the glass transition temperatures of composites filled with 30% SP nanoparticles in rubber matrix containing 0-30% of CSB. Glass transition temperature defined by Tan δ shows that NR has a Tg (glass transition temperature) at -54 oC, while CSB has two glass transition temperatures at -38 oC and 14 oC. The transition temperature at -38 oC is from styrene-butadiene segments and the transition at 14 oC is associated with aggregation of carboxylic acid functional groups. These glass transition temperatures also indicate that the NR and CSB do not mixed at molecular level. Tg of NR at -54 oC did not shift with the inclusion of SP particles or CSB. Tg of CSB is not sufficiently distinct and its shifting caused by the presence of other components in the composites cannot be determined correctly because its fraction is too
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small compared to that of NR. Within this temperature range, the composites are thermally stable as shown in the inset of Figure 7, which shows the thermal degradation occurs above 200 oC.
Figure 8 shows that the reinforcement factor of the composites changed with both CSB and
filler concentrations at 140 oC. The reinforcement factor is defined as the elastic modulus of composite normalized by the elastic modulus of rubber matrix. At 140 oC, the composites are devoid of Tan δ transitions as shown in Figure 7. Therefore, the effect of CSB concentration can be observed more clearly. The general trend shows that the reinforcement factor increased with the increasing CSB and filler concentrations. The effect peaked at ~20% CSB content, similar to the observations in tensile stress at 200 and 300% elongations (Figure 3). To understand the reinforcement effect of these composites, reinforcement factors for these composites at different filler volume fractions at 140 oC are shown in Figure 9. The curves show a rapid increase of G’ with increasing filler concentration. This type of curves cannot be fitted with simple particle inclusion theory such as The Einstein equation. Halpin-Tsai model [23] that describes the anisotropic effect of filler in a matrix also does not describe these curves well because it does not take into account of particle-particle association at higher filler concentrations. Empirical Guth model [24] for non-spherical fillers is adequate to describe these curves. Guth model has the following form.
Gc Gm 1 0.67 p 1.62 p 2 2
(3)
where p is the aspect ratio of a reinforcement element and is the volume fraction of filler. p is obtained by fitting Eq. (3) to the experimental values of Gc / Gm and is an indicator of anisotropic character of the reinforcement elements. The fitted curves indicate that reinforcement factor increased with increasing CSB content in the composites. This may imply that the addition of CSB provides additional connectivity between SP particles and therefore increases the
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anisotropic character of the reinforcement elements, which are composed of reinforcement particles and restricted polymer chains immediately adjacent to the particles. The fitting also shows that the composites with 20% CSB have higher reinforcement factor than that with 30% CSB, similar to the observation from tensile stress at 200% and 300% elongation despite G’ was measured at small strain (0.05%). This indicates that the connectivity between SP and CSB is rigid and can be detected even at small strain. As the concentration of CSB increased from 0 to 30% (Figure 9), it was also observed that the difference between reinforcement factors from the composites with different amount of CSB was greater at higher filler fractions (30 and 40% SP) than at lower filler fractions (10 and 20% SP). This indicates that CSB had greater effect at higher filler concentrations, likely providing a more rigid connectivity between SP particles, improving SP particle dispersion and preventing aggregation of SP particles. At ambient temperature of 25 oC, the reinforcement factors are significantly greater than that at 140 oC (Figure 10). Because of an even more rapid rise of reinforcement factor with the increasing filler concentration, Guth model does not fit the data as well as that at 140 oC. However, it is used here to indicate the trend that reinforcement factors are proportional to the increasing CSB concentration. As shown in Figure 7, reinforcement factors at 25 oC is within the CSB glass transition region that involved the ionic interactions of carboxylic acid functional groups. Any excess CSB that is not interacting with SP particles forms domains with ionic interactions that also contribute to the reinforcement effect of the rubber matrix. This can be seen from Young’s moduli listed in Table I. For the composites without any SP particles, the Young’s modulus increased from 1.4 to 1.8 MPa as the CSB concentration increased from 0 to 30%. At 25 oC, shear elastic modulus of CSB is 1.62 MPa, while it is 0.79 MPa for NR. This means that
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ionic CSB domains also behave like weak reinforcing fillers and contribute to the rubber reinforcement. Another interesting comparison with CB composites is listed in Table 2. Since CB is the major filler for rubber and ~70% of worldwide CB consumption is used in tires, CB with the same filler fraction in NR is used as a comparison standard. In rubber industry, dynamic mechanical properties are frequently used to predict tire tread performance at different temperatures [25]. For such application, the filler content is usually at 30-40 wt% in the total weight of filler and rubber, which corresponds to a filler concentration of 43-66 phr. The values in Table II indicate that NR composite reinforced with SP and 20% CSB is likely to have better low temperature and energy performance, but less dry handling performance. This comparison also indicates certain advantages of using SP nanoparticles with CB in NR composites to obtain optimum properties.
4. Conclusions Synergistic coordination of SP and CSB nanoparticles were used to increase significantly the modulus of NR. CSB has a number-averaged size of 120 nm. After microfluidization, 93% of SP particles produced have a number-averaged size of 210 nm. The addition of SP and CSB into NR caused the tensile stress to increase significantly at 100-300% elongation. The optimum concentration of CSB in NR is ~20%, which produced the greatest improvement in tensile stress at higher elongation ratios. SP nanoparticles improve the tensile strength of rubber matrix significantly from 10-13 MPa to 22-25 MPa with the addition of 10% SP particles. Young’s modulus of the composites increased with increasing CSB content in NR. SEM indicates an improvement of cohesion within the composites with the addition of CSB. The Swelling studies
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also show the reduction of swelling with increasing concentration of CSB up to ~20%. Reinforcement factors from shear elastic modulus shows that the highest reinforcement factors occur at ~20% CSB in NR. Increasing reinforcement factor with increasing filler concentration in the composites was described by Guth model and indicates the addition of CSB increased the reinforcement effect, likely through the connectivity and formation of rigid filler network. These experiments also indicate the addition of CSB improved reinforcement more significantly at higher than lower filler concentrations. The tensile properties of NR composites with SP and 1020% CSB were also found to be closer to that of carbon black reinforced NR composites. Loss tangents of these composites indicate a great potential for tire tread application.
Acknowledgements The author would like to thank A. Thompson for the sample imaging with scanning electron microscope.
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11. S. Beck-Candanedo, M. Roman, D. Gray, Effect of reaction conditions on the properties and behavior of wood cellulose nanocrystal suspensions, Biomacromolecules 6 (2005) 10481054. 12. C. Jiang, H. He, H. Jiang, L. Ma, M. Jia, Nano-lignin filled natural rubber composites: preparation and characterization, Express Polym. Lett. 7 (2013) 480-493. 13. C. Frangville, M. Rutkevicius, A. Richter, O. Velev, S. Stoyanov, V. Paunov, Fabrication of environmentally biodegradable lignin nanoparticles, ChemPhysChem 13 (2012) 4235-4243. 14. Y. Qian, Y. Deng, X. Qiu, H. Li, D. Yang, Formation of uniform colloidal spheres from lignin, a renewable resource recovered from pulping spent liquor, Green Chem. 16 (2014) 2156-2163. 15. L. Jong, Characterization of soy protein/styrene-butadiene rubber composites, Composites Part A 36 (2005) 675-682. 16. R. Stephen, K. Raju, S. Nair, S. Varghese, Z. Oommen, S. Thomas, mechanical and viscoelastic behavior of natural rubber and carboxylated styrene-butadiene rubber latex blends, J. Appl. Polym. Sci. 88 (2003) 2639-2648. 17. L. Jong, Influence of protein hydrolysis on the mechanical properties of natural rubber composites reinforced with soy protein particles, Ind. Crops Prod. 65 (2015) 102–109. 18. L. Jong, Aggregate structure and effect of phthalic anhydride-modified soy protein on the mechanical properties of styrene-butadiene copolymer, J. Appl. Polym. Sci. 119 (2011) 1992-2001. 19. R. Stephen, C. Ranganathaiah, S. Varghese, K. Joseph, S. Thomas, Gas transport through nano and micro composites of natural rubber (NR) and their blends with carboxylated styrene butadiene rubber (XSBR) latex membranes, Polymer 47 (2006) 858-870.
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Table 1 Tensile Properties of the composites Filler concentration (%)
CSB
0
10
20
30
40
Tensile stress at
0%
0.9 ± 0.1
1.1 ± 0.03
1.6 ± 0.1
3.1 ± 0.3
5.3 ± 0.1
100% elongation (MPa)
10 %
0.9 ± 0.1
1.2 ± 0.1
2.0 ± 0.1
3.5 ± 0.3
6.5 ± 0.4
20 %
1.0 ± 0.04
1.4 ± 0.1
2.6 ± 0.1
5.3 ± 0.1
7.6 ± 0.2
30 %
1.1 ± 0.04
1.5 ± 0.03
2.5 ± 0.1
3.9 ± 0.2
7.8 ± 0.2
CB
0.9 ± 0
1.2 ± 0.1
2.0 ± 0.1
3.3 ± 0.1
7.0 ± 0.4
Tensile stress at
0%
2.4 ± 0.2
4.6 ± 0.2
8.0 ± 0.8
13.0 ± 0.8
15.5 ± 0.03
300% elongation (MPa)
10 %
2.7 ± 0.3
4.8 ± 0.4
9.3 ± 0.3
13.6 ± 0.9
---
20 %
2.7 ± 0.2
5.4 ± 0.3
11.2 ± 0.5
16.8 ± 0.2
---
30 %
3.0 ± 0.1
5.8 ± 0.1
10.4 ± 0.4
14.1 ± 0.6
---
CB
2.4 ± 0.2
4.4 ± 0.3
10.1 ± 0.4
16.3 ± 0.3
---
0%
1.4 ± 0.3
1.9 ± 0.1
2.7 ± 0.2
4.0 ± 0.3
6.5 ± 0.3
10 %
1.5 ± 0.2
2.1 ± 0.4
3.1 ± 0.3
4.2 ± 0.5
8.8 ± 0.3
20 %
1.6 ± 0.04
2.7 ± 0.2
4.1 ± 0.3
6.9 ± 0.2
11.1 ± 0.4
30 %
1.8 ± 0.1
2.6 ± 0.3
4.3 ± 0.4
6.2 ± 1.0
15.5 ± 0.4
CB
1.4 ± 0.2
2.0 ± 0.2
3.1 ± 0.4
5.9 ± 0.2
12.3 ± 0.5
0%
13.2 ± 2
24.8 ± 0.7
22.2 ± 0.6
19.1 ± 0.6
15.5 ± 0.4
10 %
11.9 ± 2.7
24.3 ± 0.8
21.5 ± 0.4
18.6 ± 1.0
15.2 ± 0.7
20 %
12.5 ± 2.1
21.7 ± 0.7
22.5 ± 0.8
18.0 ± 0.6
14.8 ± 0.6
30 %
9.8 ± 1.1
22.5 ± 0.7
21.3 ± 0.7
18.2 ± 0.8
13.4 ± 0.3
CB
13.2 ± 2
22.5 ± 0.1
24.7 ± 1.4
23.8 ± 0.5
21.4 ± 2
0%
540 ± 17
594 ± 13
539 ± 17
426 ± 11
302 ± 10
Young’s Modulus (MPa)
Tensile Strength (MPa)
Elongation at break (%)
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10 %
542 ± 28
598 ± 23
498 ± 6
402 ± 11
273 ± 9
20 %
558 ± 35
564 ± 19
476 ± 3
324 ± 14
218 ± 6
30 %
505 ± 8
570 ± 10
484 ± 8
383 ± 19
198 ± 8
CB
540 ± 17
569 ± 15
487 ± 8
400 ± 4
256 ± 31
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Table 2 Predictive performance from dynamic mechanical properties Filler concentration (%)
0
10
20
30
40
Tanδ at -10 oC (higher is better)
20% CSB
0.080
0.053
0.057
0.069
0.084
Ice traction
CB
0.014
0.030
0.039
0.061
0.075
Tanδ at 10 oC (higher is better)
20% CSB
0.042
0.037
0.046
0.060
0.076
Wet traction
CB
0.014
0.021
0.026
0.052
0.066
Tanδ at 60 oC (lower is better)
20% CSB
0.027
0.032
0.033
0.041
0.054
Rolling resistance
CB
0.014
0.019
0.036
0.063
0.084
G’ (MPa) at -20 oC (lower is better)
20% CSB
1.1
2.0
3.5
7.8
14.7
Winter traction
CB
0.7
1.0
2.5
10.3
24.9
G’ (MPa) at 30 oC (higher is better)
20% CSB
0.8
1.5
2.6
4.9
7.8
Dry handling
CB
0.8
1.0
2.2
6.9
14.4
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FIGURES Fig. 1. Particle size distributions of NR, CSB, SP, and 0.7 um filtered SP.
Fig. 2. Comparison of tensile properites from NR composites and NR-CSB composites with 20% CSB reinforced with 10 to 40% SP nanoparticles. Filled symbols represent NR composites and unfilled symbols represent NR-CSB composites.
Fig. 3. Comparison of tensile stress at 200% and 300% elongation from NR composites with 0% to 30% CSB.
Fig. 4. Comparison of tensile fractural surfaces from the composites reinforced with 30% SP nanoparticles and different amount of CSB: (a) 0% CSB (b) 10% CSB (c) 20% CSB (d) 30% CSB (e) 0% CSB - high resolution (f) 20% CSB – high resolution.
Fig. 5. (a) Kraus plot of swollen composites (b) crosslinking densities of polymer matrices in the composites without the effect of filler.
Fig. 6. Shear elastic moduli of NR composites with 0 to 40% SP nanoparticles and 20% CSB. The inset shows the variation of loss tangent with temperature for SP and the unfilled rubber with 20% CSB.
Fig. 7. Loss tangent of NR composites with 30% SP nanoparticles and 0 to 30% CSB. The inset is the thermal degradation analysis of the 30% filled composite with 20% CSB.
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Fig. 8. Reinforcement factor (G’/G’o) of NR composites with 0 to 40% SP nanoparticles for different amount of CSB in NR at 140 oC.
Fig. 9. Shear elastic moduli of NR composites with 0 to 40% soy nanoparticles for different amount of CSB in NR at 140 oC. The data is fitted with Guth model.
Fig. 10. Shear elastic moduli of NR composites with 0 to 40% soy nanoparticles for different amount of CSB in NR at 25 oC. The data is fitted with Guth model.
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Fig. 1. Particle size distributions of NR, CSB, SP, and 0.7 um filtered SP.
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Fig. 2. Comparison of tensile properites from NR composites and NR-CSB composites with 20% CSB reinforced with 10 to 40% SP nanoparticles. Filled symbols represent NR composites and unfilled symbols represent NR-CSB composites.
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Fig. 3. Comparison of tensile stress at 200% and 300% elongation from NR composites with 0% to 30% CSB.
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Fig. 4. Comparison of tensile fractural surfaces from the composites reinforced with 30% SP nanoparticles and different amount of CSB: (a) 0% CSB (b) 10% CSB (c) 20% CSB (d) 30% CSB (e) 0% CSB - high resolution (f) 20% CSB – high resolution.
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Fig. 5. (a) Kraus plot of swollen composites (b) crosslinking densities of polymer matrices in the composites without the effect of filler.
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Fig. 6. Shear elastic moduli of NR composites with 0 to 40% SP nanoparticles and 20% CSB. The inset shows the variation of loss tangent with temperature for SP and the unfilled rubber with 20% CSB.
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Fig. 7. Loss tangent of NR composites with 30% SP nanoparticles and 0 to 30% CSB. The inset is the thermal degradation analysis of the 30% filled composite with 20% CSB.
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Fig. 8. Reinforcement factor (G’/G’o) of NR composites with 0 to 40% SP nanoparticles for different amount of CSB in NR at 140 oC.
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Fig. 9. Shear elastic moduli of NR composites with 0 to 40% soy nanoparticles for different amount of CSB in NR at 140 oC. The data is fitted with Guth model.
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Fig. 10. Shear elastic moduli of NR composites with 0 to 40% soy nanoparticles for different amount of CSB in NR at 25 oC. The data is fitted with Guth model.
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