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Reinforcement effect of soy protein nanoparticles in amine-modified natural rubber latex
Industrial Crops & Products 105 (2017) 53–62
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Reinforcement effect of soy protein nanoparticles in amine-modified natural rubber latex
MARK
Lei Jong United States Department of Agriculture, Agricultural Research Service, National Center for Agricultural Utilization Research1, Plant Polymer Research, 1815 N. University St., Peoria, IL 61604, United States
A R T I C L E I N F O
A B S T R A C T
Keywords: Mechanical properties Natural rubber Rubber filler Soy protein
Mechanical properties of natural rubber reinforced with soy protein nanoparticles are useful for various rubber applications. However, the properties is further improved by improving interactions between soy protein and rubber. A novel method is used to modify particle surface of natural rubber latex by grafting with diallylamine. The improved polymer-filler interactions caused tensile stress at 300% elongation to match that of carbon black, which is a breakthrough for organic fillers. The grafting was demonstrated with infrared and particle size measurements. The size of natural rubber latex particles increased 8% after the grafting and the infrared spectra showed an increase of amine groups in coagulated NR latex. The curing rate of modified NR reinforced with soy protein matches that of carbon black composite. With the addition of 10% soy protein nanoparticles, the tensile strength of natural rubber and modified natural rubbers increased from 9 to 15 MPa to 19–25 MPa. Both dynamic mechanical strain and swelling measurements indicate modified natural rubber composites have a higher degree of polymer-filler interaction. The change of reinforcement factors with filler fraction can be described by modified Mooney model that includes anisotropic reinforcement elements. Tanδ at 60 °C indicates the modified composite has a better rolling resistance than carbon black.
1. Introduction Natural rubber (NR) composites have many applications in molded objects such as tire treads, seals, automobile belts, and hoses. Fillers used in NR include different types of fillers that have different particle size and surface energy for different applications. To improve the rubber strength, nano-sized filler and suitable interactions between filler and rubber matrix usually lead to an improvement in the modulus of rubber (Wang, 1998; Leblanc, 2002). Both carbonized organic materials and inorganic particles have been used as fillers in rubber products. The major fillers are carbon black and silica of various aggregate size and surface treatments. Carbon black is currently the dominant filler for rubbers. It is produced by burning nonrenewable sources such as petroleum oil or natural gas with undesirable emissions as byproducts. The advantages of using natural fillers are well known as sustainable, biodegradable, and light weight. A number of efforts have indicated that the mechanical properties of NR can be improved with natural fillers such as fibers (Favier et al., 1995; Cao et al., 2007), cellulose (Jacob et al., 2004; Geethamma et al., 2005), biomass carbon
(Jong, 2013a), and starch (Wu et al., 2004). Lignin was also used to reinforced rubber and showed practically useful mechanical properties (Jiang et al., 2013; Datta et al., 2017; Ikeda et al., 2017). We have studied the effect of soy protein nanoparticles in NR and showed its reinforcement effect (Jong, 2015). Dry soy protein is rigid with a storage modulus of ∼1 GPa (Jong, 2005) and therefore can reinforce rubbers. With global soybean production approaching 340 million metric tons, it provides a stable material source for industrial applications. However, NR composites reinforced with hydrophilic bio-fillers (Jiang et al., 2013; Datta et al., 2017; Ikeda et al., 2017) are often soft compared with carbon black and are reflected in lower tensile stress at 100–300% strain. Although the increase of modulus can be achieved by increasing crosslinking density and filler concentration, they often have adverse effect on tensile strength and elongation. To solve this problem, interactions between rubber and filler can be increased to restrict the movement of polymer chains, but still allow the mobility under high stress so that concentrated local stress under larger load can be redistributed to prevent premature rupture of rubber composites. To this end, NR can be modified to contain some hydrophilic functional
E-mail address: [email protected]. 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. 1
http://dx.doi.org/10.1016/j.indcrop.2017.05.007 Received 25 October 2016; Received in revised form 31 March 2017; Accepted 4 May 2017 0926-6690/ Published by Elsevier B.V.
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groups to improve the interactions between NR and hydrophilic fillers. Direct modification of natural rubber latex with hydrophilic monomers has been reported in several publications. Dimethylaminoethyl methacrylate has been used to modify natural rubber (Kangwansupamonkon et al., 2005; Lamb et al., 2001; Kangwansupamonkon et al., 2004) and the modified NR was used to improve compatibility with starch particles (Rouilly et al., 2004). Methacrylamide (Burfield and Ng, 1978a,b,c) has also been used to modify natural rubber latex. However, the grafting of diallylaime to NR to improve the mechanical properties of soy protein nanocompoaites has not been studied. In this study, rubber particles in NR latex was modified using diallylamine and free-radical initiator to impart hydrophilic amine functional groups onto a portion of NR on the surface of latex particles so that improved coupling through hydrogen and ionic bonds between NR and hydrophilic bio-fillers such as soy protein nanoparticles can be achieved. The composites were melt-processed using traditional rubber compounding process. These latex and modified natural rubber composites were characterized to understand their structures and evaluate their properties. With the improved mechanical properties of these rubber composites, they have potential to be used in tire tread, seals, rubber belts, and damping applications.
tion based on the dry weight of SP was used in the hydrolysis. Before using microfluidizer to reduce the particle size of SP, the dispersion was homogenized for 30 min at 8000 rpm. The pressure used in the operating the microfluidizer (M-100P, Microfluidics, Newton, MA) was 159 MPa. The dispersion was passed through a diamond (200 μm) and a ceramic interaction chamber (80 μm) three times. Cold flowing water was used to keep the exiting dispersion at about 30 °C. To prepared mixture of SP particles and NR latex, they were mixed at pH 10 for 30 min at room temperature with a solid content of ∼15%. Water was removed from the mixture by freeze-drying. To compare the differences between the modified and unmodified NR latexes, four composites containing 10, 20, 30, and 40% SP were prepared with each latex. The dry mixture of SP and NR was compounded with rubber chemicals in a Brabender mixer (ATR Plasti-corder, C.W. Brabender Instruments, Inc., South Hackensack, NJ). Rubber chemicals added were weighted in terms of phr (parts per hundred parts of rubber). The same formulation was used for all samples: 100 phr natural rubber, 1 phr anti-oxidant, 5 phr zinc oxide, 2 phr stearic acid, 2 phr sulfur, and 2 phr accelerator. The material volume is 70% of the mixing bowl volume. All components were added at once except sulfur and accelerator, and then mixed for 15 min at 60 rpm. After the mixing, sulfur and accelerator were added and mixed for 3 min at 80 °C. The composites with 10, 20, 30, and 40 wt% SP filler in the composites were compression molded at 5 MPa and 160 °C for 15 min, which was beyond full cured times for all samples.
2. Experimental 2.1. Materials Soy protein (SP) (trade name Ardex F) in powder form was from Archer Daniels Midland Company (Decatur, IL). The composition of SP is consisted of ∼90% protein, ∼5% ash, and ∼5% fat. The NR latex with ∼61% solids and a pH ∼10 was from Centrotrade Rubber USA, Inc. (Chesapeake, VA). Calcium hydroxide was from Fisher Scientific (Waltham, MA). Diallyamine was purchased from Alfa Aesar (Ward Hill, MA). tert-butyl hydroperoxide as initiator and tetraethylene pentamine as activator were from Sigma-Aldrich (St. Louis, MO). Carbon black (CB) N339 and rubber curing accelerator, N-cyclohexyl2-benzothiazolesulfenamide, were from Akrochem Co. (Akron, OH). Sulfur, zinc oxide, 2,2′-Methylenebis(6-tert-butyl-4-methylphenol), and stearic acid were from Alfa Aesar (Ward Hill, MA).
2.4. Characterization of particles and latex modification SP particle size was measured in water with a Horiba LA-930 laser scattering particle size analyzer (Horiba Instruments, Irvine, CA), which is equipped with both long (632.8 nm) and short wavelength (405 nm). An accumulation of 20 scans was obtained for the measurement of volume and number-weighted mean diameters and size distribution. The SP dispersion was also filtered through a 0.7 μm glass microfiber filter (WHATMAN GF/F) in order to measure the fraction and particle size of the dispersions less than 0.7 μm. The solid contents of the dispersions before and after filtration were also measured to determine the amount of particles passed through the filter. The latex modification was monitored with infrared absorption of the NR and modified NR films using ATR (Attenuated Total Reflectance) mode (Thermo Nicolet iS10 FT-IR, Waltham, MA). The measurement range was 650–4000 cm−1 for 100 scans at a spectral resolution of 4 cm−1.
2.2. Modification of natural rubber A method by Hourston and Romaine (Hourston and Romaine, 1990) to modify NR latex with unsaturated monomer was used to modify NR latex with diallylamine. The NR latex (490 g) was diluted with 100 g of a 0.26% ammonium hydroxide solution in a closed glass reactor under nitrogen atmosphere. Diallylamine at 5 wt% of dry rubber in 300 g of a 0.26% ammonium hydroxide solution was added to the reactor and mixed for 30 min. tert-butyl hydroperoxide as initiator at 1 wt% of rubber was then added and mixed for 15 min. Finally, tetraethylene pentamine as activator at 0.5 wt% of rubber was added and the temperature was raised to 50 °C. The reaction under constant stirring was allowed to proceed for 24 h. A different batch using the same condition was also prepared, but with the initiator at 0.3 wt% of rubber and the activator at 0.2 wt% of rubber. To examine the NR crosslinking by the free radical initiator, NR latex was also modified with tert-butyl hydroperoxide at 1 wt% of rubber under the same reaction condition. The modified rubber with the higher amount of initiator is designated as H and that with the lower amount of initiator is designated as L in this report. The NR modified with the initiator only is designated as Init.
2.5. Measurements of mechanical properties An Instron tensile testing machine (Instron, Norwood, MA) was used to measured stress-strain properties with a 1 KN load cell and at the crosshead speed of 500 mm/min. Dumbbell test specimens was based on ISO 37-2. The samples have a thickness of ∼2 mm and the test was repeated five times for each composite. For elastic materials, the use of extensometer is required. The elongation was measured by using an Instron AutoX750 automatic extensometer. Linear viscoelastic properties were studied with a strain-controlled rheometer (ARES-G2, TA Instruments, Piscataway, NJ). The samples had a dimension of 50 × 12.5 × 5 mm and used in a torsion rectangular geometry. The variation of modulus with temperature was measured at a heating rate of 1 °C/min and within a range from −68 to 140 °C. For curing studies, a rubber compound before curing was compressed to form a circular disk of 25 mm in diameter and storage modulus was measured over time using serrated parallel plate fixture at 160 °C. The strain sweep experiments were conducted with a sample size of 25 × 12.5 × 5 mm at four temperatures (−15, 0, 25, and 60 °C) in a strain range of 0.01% to 100% and at a frequency of 1 Hz.
2.3. Preparation of soy protein particles and composites SP dispersion was prepared by hydrolyzing SP powder in distilled water at 9.3% concentration, and heated at 60 °C for 1 h under stirring. The batch weight was 1.93 kg. Calcium hydroxide at 2.2% concentra54
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2.6. Fractured surface and swollen composites
Table 1 Particle size and swelling of modified NR.
A scanning electron microscope (JOEL JSM-6010LA, JEOL USA, Inc., Peabody, MA) was used to obtain images of tensile fractured surface, which was coated with Au-Pd on an aluminum stub and examined under vacuum at ambient temperature. The degree of composite swelling in a good solvent was measured by immersing weighted samples in toluene for 72 h at 25 °C. The weight of swollen composites was measured by removing the excess solvent on the surface. The swollen composites were dried in a vacuum oven for 24 h at 50 °C. By comparing the swollen and dry composites, volume fraction of NR in a swollen sample was determined.
Volume-averaged Particle size Crosslinking density (mole/m3)
NR
H
L
760 ± 9 nm 185 ± 3
823 ± 6 nm 231 ± 6
761 ± 5 nm 193 ± 2
*Average value from triplicate measurements.
occurred between 1500 and 1700 cm−1, where 1660 cm−1 is assigned to C]C stretching in poly(isoprene). 1600 cm−1 in the modified NR with low amount of initiator (sample L) is related to oligomeric diallylamine. With high amount of initiator (sample H), this absorbance is not obvious. Particle size measurements in Table 1 showed volumeaveraged particle size for NR, H, and L latexes. The particle size for NR and L are similar, whereas the particle size for H is larger. The size distribution curve in Fig. 2 shows that the distribution curves are almost identical for both NR and L latexes. The entire distribution curve of H latex is shifted towards larger size and is an indication that the sizes of all particles have increased. This may also indicate that the modified NR latex with low amount of initiator formed mostly oligomers with little grafting to NR particles, while the modified NR with high amount of initiator has diallylamine units attached to NR particles. Additional acid coagulation experiments were conducted and showed that H latex had a clear supernatant after the coagulation, indicating a more complete coagulation than both NR and L latexes. Both NR and the sample L had a similar coagulation behavior and gave a cloudy supernatant. Because oligomeric diallylamine is more water soluble,
3. Results and discussion 3.1. Characterization of modified NR particles and SP nanoparticles Natural rubber latex is poly(isoprene) particles stabilized by ∼2% of protein (Posch et al., 1997; Kurup et al., 1996) in water. Compared with previous studies on the modification of NR with unsaturated monomers such as methyl methacrylate (Hourston and Romaine, 1990), diallylamine is more stable and often only form low molecular weight oligomers (Zubov et al., 1979; Litt and Eirch, 1960; Kabanov and Topchiev, 1988; Shcherbina et al., 1970). IR spectra of diallylaminemodified NR casted as films from their latexes are shown in Fig. 1. The inclusion of diallylamine increases the IR absorbance of NeH stretching vibrations at 3280 cm−1 in additional to the NeH stretching vibrations of the protein already present in NR latex. The notable difference
Fig. 1. FTIR spectra of dried rubbers from NR, H, and L latexes.
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Fig. 2. Volume-averaged size distribution of NR, H, and L latexes.
Fig. 3. FTIR spectra of acid-coagulated rubber from NR, H, and L latexes.
amine (3220 cm−1), SeO stretching in sulfate and CeN stretching in amine (1090 cm−1), and SeO bending in sulfate (617 cm−1). The absorbance intensity indicates that H latex has a greater extent of grafting than L latex, consistent with the particle size measurement in Fig. 2. Fig. 4 shows the number and volume averaged size of SP particles. A
acid-coagulated rubber should contain least amount of oligomeric diallylamine. The IR spectra of acid-coagulated rubber are shown in Fig. 3. Compared with NR and L rubbers, H rubber has greater absorbance at 3220, 1090, and 617 cm−1 related to the grafting of diallylamine. Because these modified natural rubbers were coagulated with dilute sulfuric acid, these absorbance bands are related to NeH of 56
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composite. The use of free radical initiator in the grafting reaction can also induce crosslinking reaction in NR, but its extent is significantly suppressed in the presence of diallylamine. Table 2 shows that without the presence of diallylamine and with the same amount of initiator, the Young’s modulus significantly increased from 4.1 to 6.3 MPa, the tensile strength significantly decreased from 18.7 to 7.8 MPa, elongation decreased from 352% to 100%, and 100% modulus increased from 3.4 to 7.4 MPa. These results indicate that although increasing the crosslinking density of NR can improve 100–300% modulus, the tensile strength and elongation were negatively impacted. With the presence of diallylaime, the Young’s modulus, tensile strength, and 100% modulus remained unchanged as the initiator concentration increased from 0.3% to 1%. The elongation showed a small decrease to reflect the effect of increased polymer-filler interactions. Therefore, the crosslinking of NR by the initiator is controlled and suppressed by the presence of unsaturated monomers. Fig. 6 shows stress-strain curves of the composites with 0–40% filler. The comparison is made between the NR and modified NR composites filled with 10–40% SP particles. An increase of modulus at larger strain was observed from the modified NR composites with the high amount of initiator. This significant improvement is reflected in the tensile stress at 100–300% elongation shown in Table 3. These moduli of the composites increased as the filler fraction increased from 10% to 40%. Generally, the inclusion of diallylamine increased the tensile stress at 100–300% elongations compared with the composites without it because of an increase in filler-rubber interactions. Compared with the SP composites, Young’s moduli are higher only at 0% and 10% filler fractions for the H composites, and they are lower at 0–20% filler fractions for the L composites. The effect of diallylamine is not significant at small strains, but become significant at large strains. This is caused by the increase of compatibility between filler and NR because of the presence of diallylamine, either in the form of oligomer or graft. However, grafted diallylamine is more effective in increasing tensile stress at higher elongation than oilgomeric diallylamine as shown in Fig. 7, which shows that H composites have greater tensile stress at higher elongation ratio than L composites although the difference become smaller as the filler concentration increases. For L composites at small strain (Young’s modulus in Table 3), the increase of the compatibility means a better dispersion of filler particles in the polymer matrix, which in turn prevents the filler–filler interaction and favors filler-rubber interactions. As a result, filler network is softer because of the inclusion of more rubber in the filler network. This effect is more pronounced at lower filler fractions, but become not significant at higher filler fractions because of particle–particle jamming effect that dominates the filler network. For H composites at small strain (Young’s modulus in Table 3), the higher Young’s modulus of the composites with 0–10% filler fraction compared with the SP composites is caused by the higher crosslinking density, both physical and chemical crosslinks, of the modified NR matrix. The crosslinking density is usually estimated with Flory-Rehner equation assuming the crosslinks are tetrafunctional.
Fig. 4. Aggregate size distribution of soy protein (SP), 0.7 um filtered SP, and NR.
double distribution was observed in its volume-averaged size distribution. A larger particle fraction is centered at ∼6 μm and a smaller particle fraction centered at ∼400 nm. Although the portion of larger particles can be reduced to smaller particles (Jong, 2013b), for all practical purposes, the number of cycles should be kept to a minimum because microfluidization is a slow and energy intensive process. The fractions of smaller and larger particles were determined by filtration to remove the fraction of larger SP particles. The measurement showed that ∼93% of particles had a volume-averaged size of 258 nm and a number-averaged size of 210 nm. Compared to NR latex with a volumeaveraged size of 760 nm and a number-averaged size of 550 nm, SP particles are much smaller.
3.2. Curing and stress-strain properties Fig. 5 shows the curing behavior of NR, CB composite, SP composite, SP composites with diallyamine modified NR, and SP composite with initiator modified NR. CB composite and SP composite with diallylamine modified NR had similar initial curing rate, but SP composite with diallyamine modified NR showed a decrease of storage modulus overtime. SP composite with initiator modified NR showed the slowest curing rate. NR is known to show degradation overtime at high temperature. The bio-fillers with carboxylic acid groups are known to interfere with sulfur crosslinking reaction (Jong, 2016). Therefore, the decrease of storage modulus overtime for SP containing composites is the combination of both NR degradation and retardation of the crosslinking reaction. These curing kinetics show that the use of diallylamine grafted NR increased the curing rate and reduced the interference of crosslinking reaction by soy protein. The initial curing rate of modified NR reinforced with soy protein also matches that of carbon black
⎛ ν ⎞ − [ln(1 − ν2 ) + ν2 + χν22] = nV1 ⎜ν21/3 − 2 ⎟ ⎝ 2⎠
(1)
where 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. χ is toluene-NR interaction parameter and is taken as 0.36 (Brandrup et al., 1999). V1 is the molar volume of toluene, 106.3 mol/cm3. n is the crosslinking density defined as the number of network chain between crosslinks. Table 1 shows that H rubber has the highest crosslinking density followed by L rubber and then NR. This is explained as the interaction of grafted amine groups with soy protein filler by forming hydrogen and ionic bonds, which can restrict the swelling of rubber. The crosslinking densities of the composites are 57
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Fig. 5. Curing behavior of NR and composites. NR = NR without filler. CB = NR with 43 phr of carbon black. Init = NR modified with initiator only and with 43 phr of soy protein. SP = NR with 43 phr of soy protein only. H = NR modified with diallylamine and with 43 phr of soy protein. The highest point of each curve is taken as 100% cure.
unmodified NR composites, this means that more small particles are present in the composites of higher filler fractions. These particles can hinder the sulfur crosslinking reaction because of steric effect. The compatibility effect of the modified NR can also be observed from SEM images of tensile fractured surface shown in Fig. 9, where H composites with 10% and 30% filler are compared with the SP composites. Few surface particles are observed on the H composites than the SP composites, indicating better compatibility between rubber and filler in H composites.
Table 2 Tensile properties of the composites with 43 phr of soy protein.
SP L H Init
Young’s Modulus (MPa)
Tensile Strength (MPa)
Elongation at break (%)
Tensile stress at 100% elongation (MPa)
4.0 4.1 4.1 6.3
19.1 ± 0.6 18.6 ± 0.8 18.7 ± 1.1 7.8 ± 1.6
426 378 352 100
3.1 3.5 3.4 7.4
± ± ± ±
0.3 0.5 0.2 0.2
± ± ± ±
11 33 24 18
± ± ± ±
0.3 0.5 0.2 0.0
*Init = NR modified with initiator only.
3.3. Dynamic mechanical properties One of ways to investigate polymer-filler interactions is to use dynamic strain measurements to look at interaction forces. The change in dynamic modulus with increasing strain is known as Payne effect (Payne, 1963). The decrease of dynamic shear modulus with increasing dynamic strain was interpreted by Maier-Goritz (Maier and Goritz, 1996) as the dissociation between polymer chains and the filler surface. The polymer-filler interactions were divided into two types. Stable interactions do not dissociate even at very high strain, while unstable interactions dissociate with increasing strain and is identified as Payne effect. For the current composites, Maier-Goritz model does not fit the experimental data well at the high strain region, a modified equation is used.
G′(γ ) = Gst′ +
GI′ 1 + cγ m
(2)
where G’I is the modulus contribution from unstable polymer-filler interactions. G’st is the modulus contribution from the stable polymerfiller interactions. G’ is equal to the difference between G’(γ) at very small strain and very large strain. G’st is equal to G’(γ) at very large strain and c is a constant related to adsorption and desorption rate of polymer chains from the filler surface. m is a fitting parameter related to filler aggregate structure in Kraus model (Kraus, 1984). Eq. (2) has been used to adequately describe a few rubber composites (Maier and Goritz, 1996; Meera et al., 2009). By fitting strain sweep experimental data with equation 2, G’I can be obtained and related to activation energy of desorption through the Arrhenius equation, G’I = [NIoo exp (-EI/kBT)]kBT, where NIoo is a constant characterizing the density of unstable bonds independent of temperature and deformation amplitude and EI is activation energy of polymer detachment from filler surface. KB and T are the Boltzmann constant and temperature, respectively. The
Fig. 6. Comparison of tensile properites from NR composites (SP) and modified NR composites (H). The H composites are the modified NRcomposites with a high amount of initiator.
shown in Fig. 8. For the L and SP composites, the crosslinking densities of the rubber matrix are similar. For the H composites, the crosslinking density is higher at lower filler fractions compared with the SP and L composites, but the difference became smaller as the filler fraction increased. This indicates that the rubber crosslinking density is not independent from filler fraction and the filler may have effect on the crosslinking density of diallylamine modified NR. Since the same filler is used in these composites, the effect must come from the diallylamine modified NR, which interacts more with filler, therefore can prevent coagulation of filler particles at higher filler fractions. Compared with 58
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Table 3 Tensile Properties of the composites. Filler concentration (phr)
0
11
25
43
67
Tensile stress at 100% elongation (MPa)
SP H L CB
0.9 1.2 0.9 0.9
± ± ± ±
0 0 0 0
1.1 1.6 1.1 1.2
± ± ± ±
0 0 0.1 0.1
1.6 1.8 1.7 2.0
± ± ± ±
0.1 0.1 0 0.1
3.1 3.4 3.5 3.3
± ± ± ±
0.3 0.2 0.5 0.1
5.3 6.3 5.9 7.0
Tensile stress at 200% elongation (MPa)
SP H L CB
1.5 2.3 1.6 1.5
± ± ± ±
0.1 0.1 0.1 0.1
2.3 3.7 2.3 2.3
± ± ± ±
0.1 0.1 0.3 0.2
4.1 5.0 4.4 5.0
± ± ± ±
0.4 0.2 0.2 0.2
7.9 9.4 9.1 8.9
± ± ± ±
0.7 0.4 1.1 0.3
11.0 13.2 12.0 16.8
Tensile stress at 300% elongation (MPa)
SP H L CB
2.4 4.2 2.6 2.4
± ± ± ±
0.2 0.2 0.2 0.2
4.6 7.4 4.8 4.4
± ± ± ±
0.2 0.2 0.6 0.3
8.0 ± 0.8 10.1 ± 0.4 8.7 ± 0.3 10.1 ± 0.4
13.0 16.4 14.7 16.3
Young’s Modulus (MPa)
SP H L CB
1.4 1.7 1.3 1.4
± ± ± ±
0.2 0.1 0.3 0.2
1.9 2.3 1.6 2.0
± ± ± ±
0.1 0 0.4 0.2
2.7 2.7 2.4 3.1
4.0 4.1 4.1 5.9
Tensile Strength (MPa)
SP H L CB
13.2 ± 2 9.4 ± 1.4 15.1 ± 3.4 13.2 ± 2
24.8 19.3 24.8 22.5
± ± ± ±
0.7 1 0.7 0.1
22.2 21.8 22.1 24.7
± ± ± ±
0.6 0.8 1.6 1.4
19.1 18.7 18.6 23.8
± ± ± ±
0.6 1.1 0.8 0.5
15.5 16.2 17.0 21.4
± ± ± ±
0.4 0.4 1.5 2
Elongation at break (%)
SP H L CB
540 423 578 540
594 470 600 569
± ± ± ±
13 11 25 15
539 477 510 487
± ± ± ±
17 7 27 8
426 352 378 400
± ± ± ±
11 24 33 4
302 253 299 256
± ± ± ±
10 4 14 31
± ± ± ±
17 25 25 17
± ± ± ±
0.2 0.1 0.3 0.4
± ± ± ±
± ± ± ±
0.8 0.5 1.3 0.3
0.3 0.2 0.5 0.2
± ± ± ±
0.1 0.3 0.7 0.4
± ± ± ±
0.1 0.5 1.3 0.8
15.5 ± 0 – 15.3 ± 0.1 – 6.5 ± 0.3 6.6 ± 0.3 6.5 ± 1.1 12.3 ± 0.5
Arrhenius plot is shown in Fig. 10. The estimated activation energy expressed in electron volts (eV) indicates that polymer-filler interactions in modified NR composite is greater than the unmodified NR composite. However, the stable interactions may include strong interactions such as ionic interactions in addition to covalent bonds. Such strong interactions may not dissociate under dynamic strain experiments. Therefore, this method necessarily underestimate the polymerfiller interactions. Shear elastic moduli of H composites are shown in Fig. 11. The elastic moduli were measured in the linear viscoelastic region (small strain region). SP particles have a shear elastic modulus of 1–2 GPa. The shear elastic modulus increases with the increasing filler content in the composites for all temperatures measured. To understand the reinforcement effect of these composites, reinforcement factors for these composites at different filler volume fractions at 25 °C are shown in Fig. 12. The reinforcement factor is defined as the elastic modulus of the composite normalized by the elastic modulus of NR. The curves show a rapid increase of G’ with increasing filler fraction. This type of curves cannot be fitted with simple particle inclusion theory such as The Einstein equation. They also cannot be fitted with Halpin-Tsai model that describes the anisotropic effect of filler in a matrix. Guth model (Guth, 1945) for non-spherical fillers was also found to be inadequate to describe these curves. It was found that empirical modified Mooney equation for non-spherical particles can be used to fit these data. The modified Mooney equation has the following form (Ahmed and Jones, 1990; Brodnyan, 1959).
Fig. 7. Comparison of tensile properites from NR composites with high (1%) and low (0.3%) amount of initiator used in the modification of NR latex.
⎛ 2.5φ + 0.407(p − 1)1.508φ ⎞ Gc = Gm exp ⎜ ⎟ 1 − Sφ ⎠ ⎝
(3)
where Gc is the modulus of the composites, Gm is the modulus of the matrix, and φ is the volume fraction of filler. p is the aspect ratio of a reinforcement element with a value of 1 < p < 15. S is the crowding factor and is defined as the volume occupied by the filler divided by the true volume of the filler. For close packed spheres, S is equal to 1.35. The rapid increase of G’ at higher filler fractions indicates the presence of filler network from inter-particle interactions in addition to simple filler reinforcement because of particle crowding effect. G’ was measured at 0.05% strain. At such small strain, current NR modification
Fig. 8. Crosslinking density of composites.
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Fig. 9. Comparison of tensile fractural surfaces from the composites of (a) 10% SP (b) 10% H (c) 30% SP (d) 30% H.
Fig. 10. Amount of unstable bonds between polymer and filler from Maier-Goritz model.
Fig. 11. Shear elastic moduli from H composites.
does not show significant difference in the reinforcement factors at ambient temperature. This indicates a better compatibility between filler and matrix for the current composite system does not contribute to small strain modulus. The presence of oligomeric diallylamine is similar to the presence of solvent that can make composites softer. The presence of graft has a similar effect and therefore makes the filler network softer despite H composites have a higher crosslinking density at 10–30% filler fractions. This would indicate that the shear elastic modulus at small strain is determined by the rigidity of filler network. As the temperature increases to 140 °C, Fig. 13 shows that reinforcement factor decreases compared with that at 25 °C because of the
softening of rubber that is a part of filler network. The reinforcement factors at this temperature also do not increase as rapidly with the increasing filler fractions. Therefore, the curves in Fig. 13 can be described by Guth model with anisotropic reinforcement elements. Guth model (Guth, 1945) has the following form.
Gc = Gm (1 + 0.67pφ + 1.62p 2 φ 2 )
(4)
where p is the aspect ratio of a reinforcement element and φ is the volume fraction of filler. By fitting the Eq. (4) to the experimental data, p was determined and shown in Fig. 13. These results show that reinforcement element formed from the 60
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connected filler particles and rubber are anisotropic and change with temperature. No significant difference was observed between modified and unmodified NR composites at small strain measurements. The difference becomes more obvious at higher elongations as shown in the stress-strain measurements. This can be explained as the presence of oligomeric diallylamine and graft preventing a close bonding between poly(isoprene) polymer chains and soy nanoparticles. Therefore, the effect is not detected at small strains, but only at higher elongations. To explore possible application in tire tread, loss tangent at different temperatures are listed in Table 4. The values of tan δ were used to relate the dynamic mechanical properties to tire performance (Bao et al., 2015). The results indicate the diallylamine modified NR reinforced with soy protein nanoparticles may improve rolling resistance and winter traction, but with less wet traction compared to carbon black reinforced NR.
4. Conclusions NR latex was modified to include amine functional groups by reacting NR latex particles with diallylamine using free-radical initiator. FTIR shows different absorption bands when different amount of initiator was used in the reaction. The NR particles was found to become larger when high amount of initiator was used in the reaction, while the NR particle size remains the same when low amount of initiator was used. These results indicate the presence of oligomeric diallylamine when 0.3% of initiator was used, whereas diallylamine units were attach to NR particles when 1% of initiator was used in the reaction. For reinforcement particles, 93% of SP particles have a number-averaged size of 210 nm. The curing kinetics show that the use of diallylamine grafted NR increased the curing rate and reduced the interference of crosslinking reaction by soy protein. For tensile properties, 26–75% increase of tensile stress was found at 300% elongation for the H composites compared with SP composites. Swelling experiments indicate the amine-modified NR films have 25% higher crosslinking density than the unmodified NR. For the composites, SP and L composites have similar crosslinking density while H composites have higher crosslinking density at 10–30% filler fractions. Compared with NR modified with the initiator only, the presence of diallylamine significantly suppressed the crosslinking of polyisoprene. Compared with NR, SEM images indicate the modified NR has better adhesion towards soy nanoparticles. Dynamic mechanical properties indicate no difference between modified and unmodified NR composites when measured in linear viscoelastic region. Polymer-filler interactions by dynamic mechanical method indicate diallylamine modified NR has 33% greater interactions than unmodified NR. At 25 °C, the increase of reinforcement factor with increasing filler fraction can be described by modified Mooney equation to account for rigid filler network at higher filler fractions. At 140 °C, filler network is softened because of the rubber contribution in the filler network, and the reinforcement factor can be described by Guth equation that takes into account of the anisotropic effect of reinforcement elements. This study indicates that amine-modified NR improves the compatibility between hydrophilic soy nanoparticles and hydrophobic NR and leads to improvement in the tensile stress at 100–300% elongation ratios. Tanδ at 60 °C indicates the modified NR reinforced with soy protein nanoparticles has possible better rolling resistance than carbon black reinforced NR for tire tread application.
Fig. 12. Shear elastic moduli at 25 °C from SP, H, and L composites. The data is fitted with modified Mooney model.
Fig. 13. Shear elastic moduli at 140 °C from SP, H, and L composites. The data is fitted with Guth model. Table 4 Storage modulus and loss tangent at different temperatures. Filler concentration (phr) Tanδ at 10 °C (higher is better) Wet traction Tanδ at 60 °C (lower is better) Rolling resistance G’ (MPa) at −20 °C (lower is better) Winter traction
H CB H CB H CB
0
11
25
43
67
0.007 0.014 0.018 0.014 1.1 0.7
0.023 0.021 0.016 0.019 1.1 1.0
0.030 0.026 0.025 0.036 1.8 2.5
0.031 0.052 0.031 0.063 3.6 10.3
0.046 0.066 0.040 0.084 7.0 24.9
Acknowledgement The author would like to thank A. Thompson for the sample imaging with the scanning electron microscope.
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Jong, L., 2015. Influence of protein hydrolysis on the mechanical properties of natural rubber composites reinforced with soy protein particles. Ind. Crops Prod. 65, 102–109. Jong, L., 2016. Particle size and particle–particle interactions on tensile properties and reinforcement of corn flour particles in natural rubber. Eur. Polym. J. 74, 136–147. Kabanov, V.A., Topchiev, D.A., 1988. Kinetics and mechanism of the radical polymerization of N,N-dialkyl-N,N-diallyl ammonium halides. Rev. Polym. Sci. USSR 30 (4), 667–679. Kangwansupamonkon, W., Fellows, C.M., Lamb, D.J., Gibert, R.G., Kiatkamjornwong, S., 2004. Kinetics of surface grafting on polyisoprene latexes by reaction calorimetry. Polymer 45, 5775–5784. Kangwansupamonkon, W., Gibert, R.G., Kiatkamjornwong, S., 2005. Modification of natural rubber by grafting with hydrophilic monomers. Macromol. Chem. Phys. 206, 2450–2460. Kraus, G., 1984. Mechanical losses in carbon-black-filled rubbers. J. Appl. Polym. Sci. Appl. Polym. Symp. 39, 75–92. Kurup, V.P., Alenius, H., Kelly, K.J., 1996. A two-dimensional electrophoretic analysis of latex peptides reacting with IgE and IgG antibodies from patients with latex allergy. Int. Arch. Allergy Immunol. 109, 58–67. Lamb, D.J., Anstey, J.F., Fellows, C.M., Monteiro, M.J., Gibert, R.G., 2001. Modification of natural and artificial polymer colloids by topology-controlled emulsion polymerization. Biomacromolecules 2, 518–525. Leblanc, J.L., 2002. Rubber-filler interactions and rheological properties in filled compounds. Prog. Polym. Sci. 27, 627–687. Litt, M., Eirch, F.R., 1960. Polymerization of allyl acetate. J. Polym. Sci. 45 (146), 379–396. Maier, P.G., Goritz, D., 1996. Molecular interpretation of the Payne effect. Kautsch. Gummi Kunstst. 49, 18–21. Meera, A.P., Said, S., Grohens, Y., Thomas, S., 2009. Nonlinear viscoelastic behaviour of silica-filled natural rubber nanocomposites. J. Phys. Chem. C 113, 17997–18002. Payne, A.R., 1963. Dynamic properties of heat-trated butyl vulcanizates. J. Appl. Poly. Sci. 7, 873–885. Posch, A., Chen, Z., Wheeler, C., Dunn, M.J., Raulf-Heimsoth, M., Baur, X., 1997. Characterization and identification of latex allergens by two-dimensional electrophoresis and protein microsequencing. J. Allergy Clin. Immunol. 99, 385–395. Rouilly, A., Rigal, L., Gilbert, R.G., 2004. Synthesis and properties of composites of starch and chemically modified natural rubber. Polymer 45, 7813–7820. Shcherbina, F.F., Fedorova, I.I., Gorlov, Y.I., 1970. Chain transfer during the polymerization of allylamine and its acyl derivatives. Polym. Sci. USSR 12 (9), 2317–2321. Wang, M.J., 1998. Effect of polymer-filler and filler–filler interactions on dynamic properties of filled vulcanizates. Rubber Chem. Technol. 71, 520–589. Wu, Y.P., Ji, M.Q., Qi, Q., Wang, Y.Q., Zhang, L.Q., 2004. Preparation, structure, and properties of starch/rubber composites prepared by co-coagulating rubber latex and starch paste. Macromol. Rapid Commun. 25, 565–570. Zubov, V.P., Kumar, M.V., Masterova, A.M., Kabanov, V.A., 1979. Reactivity of allyl monomers in radical polymerization. J. Macromol. Sci. Part A − Chem. 13, 111–131.
References Ahmed, S., Jones, F.R., 1990. A review of particulate reinforcement theories for polymer composites. J. Mater. Sci. 25, 4933–4942. Bao, Z., Flanigan, C., Beyer, L., Tao, J.J., 2015. Processing optimization of latexcompounded montmorillonite/styrene-butadiene rubber-polybutadiene rubber. J. Appl. Polym. Sci. 132, 41521. Brandrup, J., Immergut, E.H., Grulke, E.A., 1999. Polymer Handbook, 4th ed. John Wiley & Sons New York, New York. Brodnyan, J.G., 1959. The concentration dependence of the newtonian viscosity of prolate ellipsoids. Trans. Soc. Rheol. 3, 61–68. Burfield, D.R., Ng, S.C., 1978a. Persulphate initiated graft copolymerization of methacrylamide in natural rubber latex–I The influence of non-rubber components. Eur. Polym J. 14, 789–792. Burfield, D.R., Ng, S.C., 1978b. Persulphate initiated graft copolymerization of methacrylamide in natural rubber latex–II A kinetic study. Eur. Polym J. 14, 793–797. Burfield, D.R., Ng, S.C., 1978c. Persulphate initiated graft copolymerization of methacrylamide in natural rubber latex–III Characterization of graft copolymer. Eur. Polym J. 14, 799–802. Cao, X., Dong, H., Li, C.M., 2007. New nanocomposites materials reinforced with flax cellulose nanocrystals in waterborn polyurethane. Biomacromolecules 8, 899–904. Datta, J., Parcheta, P., Surowka, J., 2017. Softwood-lignin/natural rubber composites containing novel plasticizing agent: preparation and characterization. Ind. Crops Prod. 95, 675–685. Favier, F., Chanzy, H., Cavaille, J.Y., 1995. Polymer nanocomposites reinforced by cellulose whiskers. Macromolecules 28, 6365–6367. Geethamma, V.G., Kalaprasad, G., Groeninckx, G., Thomas, S., 2005. Dynamic mechanical behavior of short coir fiber reinforced natural rubber composites. Composites Part A 36, 1499–1506. Guth, E., 1945. Theory of filler reinforcement. J. Appl. Phys. 16, 20–25. Hourston, D.J., Romaine, J., 1990. Modification of natural rubber latex: II. Natural rubber poly(methyl methacrylate) composite latexes synthesized using an amine-activated hydroperoxide. J. Appl. Polym. Sci. 39, 1587–1594. Ikeda, Y., Phakkeeree, T., Junkong, P., Yokohama, H., Phinyocheep, P., Kitano, R., Kato, A., 2017. Reinforcing biofiller Lignin for high performance green natural rubber nanocomposites. RSC Adv. 7, 5222–5231. Jacob, M., Thomas, S., Varughese, K.T., 2004. Mechanical properties of sisal/oil palm hybrid fiber reinforced natural rubber composites. Compos. Sci. Technol. 64, 955–965. Jiang, C., He, H., Jiang, H., Ma, L., Jia, D.M., 2013. Nano-lignin filled natural rubber composites: preparation and characterization. Express Polym. Lett. 7 (5), 480–493. Jong, L., 2005. Rubber composites reinforced by soy spent flakes. Polym. Int. 54 (11), 1572–1580. Jong, L., 2013a. Reinforcement effect of biomass carbon and protein in elastic biocomposites. Polym. Compos. 34, 697–706. Jong, L., 2013b. Characterization of soy protein nanoparticles prepared by high shear microfluidization. J. Dispersion Sci. Technol. 34 (4), 469–475.
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Reinforcement effect of soy protein nanoparticles in amine-modified natural rubber latex
MARK
Lei Jong United States Department of Agriculture, Agricultural Research Service, National Center for Agricultural Utilization Research1, Plant Polymer Research, 1815 N. University St., Peoria, IL 61604, United States
A R T I C L E I N F O
A B S T R A C T
Keywords: Mechanical properties Natural rubber Rubber filler Soy protein
Mechanical properties of natural rubber reinforced with soy protein nanoparticles are useful for various rubber applications. However, the properties is further improved by improving interactions between soy protein and rubber. A novel method is used to modify particle surface of natural rubber latex by grafting with diallylamine. The improved polymer-filler interactions caused tensile stress at 300% elongation to match that of carbon black, which is a breakthrough for organic fillers. The grafting was demonstrated with infrared and particle size measurements. The size of natural rubber latex particles increased 8% after the grafting and the infrared spectra showed an increase of amine groups in coagulated NR latex. The curing rate of modified NR reinforced with soy protein matches that of carbon black composite. With the addition of 10% soy protein nanoparticles, the tensile strength of natural rubber and modified natural rubbers increased from 9 to 15 MPa to 19–25 MPa. Both dynamic mechanical strain and swelling measurements indicate modified natural rubber composites have a higher degree of polymer-filler interaction. The change of reinforcement factors with filler fraction can be described by modified Mooney model that includes anisotropic reinforcement elements. Tanδ at 60 °C indicates the modified composite has a better rolling resistance than carbon black.
1. Introduction Natural rubber (NR) composites have many applications in molded objects such as tire treads, seals, automobile belts, and hoses. Fillers used in NR include different types of fillers that have different particle size and surface energy for different applications. To improve the rubber strength, nano-sized filler and suitable interactions between filler and rubber matrix usually lead to an improvement in the modulus of rubber (Wang, 1998; Leblanc, 2002). Both carbonized organic materials and inorganic particles have been used as fillers in rubber products. The major fillers are carbon black and silica of various aggregate size and surface treatments. Carbon black is currently the dominant filler for rubbers. It is produced by burning nonrenewable sources such as petroleum oil or natural gas with undesirable emissions as byproducts. The advantages of using natural fillers are well known as sustainable, biodegradable, and light weight. A number of efforts have indicated that the mechanical properties of NR can be improved with natural fillers such as fibers (Favier et al., 1995; Cao et al., 2007), cellulose (Jacob et al., 2004; Geethamma et al., 2005), biomass carbon
(Jong, 2013a), and starch (Wu et al., 2004). Lignin was also used to reinforced rubber and showed practically useful mechanical properties (Jiang et al., 2013; Datta et al., 2017; Ikeda et al., 2017). We have studied the effect of soy protein nanoparticles in NR and showed its reinforcement effect (Jong, 2015). Dry soy protein is rigid with a storage modulus of ∼1 GPa (Jong, 2005) and therefore can reinforce rubbers. With global soybean production approaching 340 million metric tons, it provides a stable material source for industrial applications. However, NR composites reinforced with hydrophilic bio-fillers (Jiang et al., 2013; Datta et al., 2017; Ikeda et al., 2017) are often soft compared with carbon black and are reflected in lower tensile stress at 100–300% strain. Although the increase of modulus can be achieved by increasing crosslinking density and filler concentration, they often have adverse effect on tensile strength and elongation. To solve this problem, interactions between rubber and filler can be increased to restrict the movement of polymer chains, but still allow the mobility under high stress so that concentrated local stress under larger load can be redistributed to prevent premature rupture of rubber composites. To this end, NR can be modified to contain some hydrophilic functional
E-mail address: [email protected]. 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. 1
http://dx.doi.org/10.1016/j.indcrop.2017.05.007 Received 25 October 2016; Received in revised form 31 March 2017; Accepted 4 May 2017 0926-6690/ Published by Elsevier B.V.
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groups to improve the interactions between NR and hydrophilic fillers. Direct modification of natural rubber latex with hydrophilic monomers has been reported in several publications. Dimethylaminoethyl methacrylate has been used to modify natural rubber (Kangwansupamonkon et al., 2005; Lamb et al., 2001; Kangwansupamonkon et al., 2004) and the modified NR was used to improve compatibility with starch particles (Rouilly et al., 2004). Methacrylamide (Burfield and Ng, 1978a,b,c) has also been used to modify natural rubber latex. However, the grafting of diallylaime to NR to improve the mechanical properties of soy protein nanocompoaites has not been studied. In this study, rubber particles in NR latex was modified using diallylamine and free-radical initiator to impart hydrophilic amine functional groups onto a portion of NR on the surface of latex particles so that improved coupling through hydrogen and ionic bonds between NR and hydrophilic bio-fillers such as soy protein nanoparticles can be achieved. The composites were melt-processed using traditional rubber compounding process. These latex and modified natural rubber composites were characterized to understand their structures and evaluate their properties. With the improved mechanical properties of these rubber composites, they have potential to be used in tire tread, seals, rubber belts, and damping applications.
tion based on the dry weight of SP was used in the hydrolysis. Before using microfluidizer to reduce the particle size of SP, the dispersion was homogenized for 30 min at 8000 rpm. The pressure used in the operating the microfluidizer (M-100P, Microfluidics, Newton, MA) was 159 MPa. The dispersion was passed through a diamond (200 μm) and a ceramic interaction chamber (80 μm) three times. Cold flowing water was used to keep the exiting dispersion at about 30 °C. To prepared mixture of SP particles and NR latex, they were mixed at pH 10 for 30 min at room temperature with a solid content of ∼15%. Water was removed from the mixture by freeze-drying. To compare the differences between the modified and unmodified NR latexes, four composites containing 10, 20, 30, and 40% SP were prepared with each latex. The dry mixture of SP and NR was compounded with rubber chemicals in a Brabender mixer (ATR Plasti-corder, C.W. Brabender Instruments, Inc., South Hackensack, NJ). Rubber chemicals added were weighted in terms of phr (parts per hundred parts of rubber). The same formulation was used for all samples: 100 phr natural rubber, 1 phr anti-oxidant, 5 phr zinc oxide, 2 phr stearic acid, 2 phr sulfur, and 2 phr accelerator. The material volume is 70% of the mixing bowl volume. All components were added at once except sulfur and accelerator, and then mixed for 15 min at 60 rpm. After the mixing, sulfur and accelerator were added and mixed for 3 min at 80 °C. The composites with 10, 20, 30, and 40 wt% SP filler in the composites were compression molded at 5 MPa and 160 °C for 15 min, which was beyond full cured times for all samples.
2. Experimental 2.1. Materials Soy protein (SP) (trade name Ardex F) in powder form was from Archer Daniels Midland Company (Decatur, IL). The composition of SP is consisted of ∼90% protein, ∼5% ash, and ∼5% fat. The NR latex with ∼61% solids and a pH ∼10 was from Centrotrade Rubber USA, Inc. (Chesapeake, VA). Calcium hydroxide was from Fisher Scientific (Waltham, MA). Diallyamine was purchased from Alfa Aesar (Ward Hill, MA). tert-butyl hydroperoxide as initiator and tetraethylene pentamine as activator were from Sigma-Aldrich (St. Louis, MO). Carbon black (CB) N339 and rubber curing accelerator, N-cyclohexyl2-benzothiazolesulfenamide, were from Akrochem Co. (Akron, OH). Sulfur, zinc oxide, 2,2′-Methylenebis(6-tert-butyl-4-methylphenol), and stearic acid were from Alfa Aesar (Ward Hill, MA).
2.4. Characterization of particles and latex modification SP particle size was measured in water with a Horiba LA-930 laser scattering particle size analyzer (Horiba Instruments, Irvine, CA), which is equipped with both long (632.8 nm) and short wavelength (405 nm). An accumulation of 20 scans was obtained for the measurement of volume and number-weighted mean diameters and size distribution. The SP dispersion was also filtered through a 0.7 μm glass microfiber filter (WHATMAN GF/F) in order to measure the fraction and particle size of the dispersions less than 0.7 μm. The solid contents of the dispersions before and after filtration were also measured to determine the amount of particles passed through the filter. The latex modification was monitored with infrared absorption of the NR and modified NR films using ATR (Attenuated Total Reflectance) mode (Thermo Nicolet iS10 FT-IR, Waltham, MA). The measurement range was 650–4000 cm−1 for 100 scans at a spectral resolution of 4 cm−1.
2.2. Modification of natural rubber A method by Hourston and Romaine (Hourston and Romaine, 1990) to modify NR latex with unsaturated monomer was used to modify NR latex with diallylamine. The NR latex (490 g) was diluted with 100 g of a 0.26% ammonium hydroxide solution in a closed glass reactor under nitrogen atmosphere. Diallylamine at 5 wt% of dry rubber in 300 g of a 0.26% ammonium hydroxide solution was added to the reactor and mixed for 30 min. tert-butyl hydroperoxide as initiator at 1 wt% of rubber was then added and mixed for 15 min. Finally, tetraethylene pentamine as activator at 0.5 wt% of rubber was added and the temperature was raised to 50 °C. The reaction under constant stirring was allowed to proceed for 24 h. A different batch using the same condition was also prepared, but with the initiator at 0.3 wt% of rubber and the activator at 0.2 wt% of rubber. To examine the NR crosslinking by the free radical initiator, NR latex was also modified with tert-butyl hydroperoxide at 1 wt% of rubber under the same reaction condition. The modified rubber with the higher amount of initiator is designated as H and that with the lower amount of initiator is designated as L in this report. The NR modified with the initiator only is designated as Init.
2.5. Measurements of mechanical properties An Instron tensile testing machine (Instron, Norwood, MA) was used to measured stress-strain properties with a 1 KN load cell and at the crosshead speed of 500 mm/min. Dumbbell test specimens was based on ISO 37-2. The samples have a thickness of ∼2 mm and the test was repeated five times for each composite. For elastic materials, the use of extensometer is required. The elongation was measured by using an Instron AutoX750 automatic extensometer. Linear viscoelastic properties were studied with a strain-controlled rheometer (ARES-G2, TA Instruments, Piscataway, NJ). The samples had a dimension of 50 × 12.5 × 5 mm and used in a torsion rectangular geometry. The variation of modulus with temperature was measured at a heating rate of 1 °C/min and within a range from −68 to 140 °C. For curing studies, a rubber compound before curing was compressed to form a circular disk of 25 mm in diameter and storage modulus was measured over time using serrated parallel plate fixture at 160 °C. The strain sweep experiments were conducted with a sample size of 25 × 12.5 × 5 mm at four temperatures (−15, 0, 25, and 60 °C) in a strain range of 0.01% to 100% and at a frequency of 1 Hz.
2.3. Preparation of soy protein particles and composites SP dispersion was prepared by hydrolyzing SP powder in distilled water at 9.3% concentration, and heated at 60 °C for 1 h under stirring. The batch weight was 1.93 kg. Calcium hydroxide at 2.2% concentra54
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2.6. Fractured surface and swollen composites
Table 1 Particle size and swelling of modified NR.
A scanning electron microscope (JOEL JSM-6010LA, JEOL USA, Inc., Peabody, MA) was used to obtain images of tensile fractured surface, which was coated with Au-Pd on an aluminum stub and examined under vacuum at ambient temperature. The degree of composite swelling in a good solvent was measured by immersing weighted samples in toluene for 72 h at 25 °C. The weight of swollen composites was measured by removing the excess solvent on the surface. The swollen composites were dried in a vacuum oven for 24 h at 50 °C. By comparing the swollen and dry composites, volume fraction of NR in a swollen sample was determined.
Volume-averaged Particle size Crosslinking density (mole/m3)
NR
H
L
760 ± 9 nm 185 ± 3
823 ± 6 nm 231 ± 6
761 ± 5 nm 193 ± 2
*Average value from triplicate measurements.
occurred between 1500 and 1700 cm−1, where 1660 cm−1 is assigned to C]C stretching in poly(isoprene). 1600 cm−1 in the modified NR with low amount of initiator (sample L) is related to oligomeric diallylamine. With high amount of initiator (sample H), this absorbance is not obvious. Particle size measurements in Table 1 showed volumeaveraged particle size for NR, H, and L latexes. The particle size for NR and L are similar, whereas the particle size for H is larger. The size distribution curve in Fig. 2 shows that the distribution curves are almost identical for both NR and L latexes. The entire distribution curve of H latex is shifted towards larger size and is an indication that the sizes of all particles have increased. This may also indicate that the modified NR latex with low amount of initiator formed mostly oligomers with little grafting to NR particles, while the modified NR with high amount of initiator has diallylamine units attached to NR particles. Additional acid coagulation experiments were conducted and showed that H latex had a clear supernatant after the coagulation, indicating a more complete coagulation than both NR and L latexes. Both NR and the sample L had a similar coagulation behavior and gave a cloudy supernatant. Because oligomeric diallylamine is more water soluble,
3. Results and discussion 3.1. Characterization of modified NR particles and SP nanoparticles Natural rubber latex is poly(isoprene) particles stabilized by ∼2% of protein (Posch et al., 1997; Kurup et al., 1996) in water. Compared with previous studies on the modification of NR with unsaturated monomers such as methyl methacrylate (Hourston and Romaine, 1990), diallylamine is more stable and often only form low molecular weight oligomers (Zubov et al., 1979; Litt and Eirch, 1960; Kabanov and Topchiev, 1988; Shcherbina et al., 1970). IR spectra of diallylaminemodified NR casted as films from their latexes are shown in Fig. 1. The inclusion of diallylamine increases the IR absorbance of NeH stretching vibrations at 3280 cm−1 in additional to the NeH stretching vibrations of the protein already present in NR latex. The notable difference
Fig. 1. FTIR spectra of dried rubbers from NR, H, and L latexes.
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Fig. 2. Volume-averaged size distribution of NR, H, and L latexes.
Fig. 3. FTIR spectra of acid-coagulated rubber from NR, H, and L latexes.
amine (3220 cm−1), SeO stretching in sulfate and CeN stretching in amine (1090 cm−1), and SeO bending in sulfate (617 cm−1). The absorbance intensity indicates that H latex has a greater extent of grafting than L latex, consistent with the particle size measurement in Fig. 2. Fig. 4 shows the number and volume averaged size of SP particles. A
acid-coagulated rubber should contain least amount of oligomeric diallylamine. The IR spectra of acid-coagulated rubber are shown in Fig. 3. Compared with NR and L rubbers, H rubber has greater absorbance at 3220, 1090, and 617 cm−1 related to the grafting of diallylamine. Because these modified natural rubbers were coagulated with dilute sulfuric acid, these absorbance bands are related to NeH of 56
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composite. The use of free radical initiator in the grafting reaction can also induce crosslinking reaction in NR, but its extent is significantly suppressed in the presence of diallylamine. Table 2 shows that without the presence of diallylamine and with the same amount of initiator, the Young’s modulus significantly increased from 4.1 to 6.3 MPa, the tensile strength significantly decreased from 18.7 to 7.8 MPa, elongation decreased from 352% to 100%, and 100% modulus increased from 3.4 to 7.4 MPa. These results indicate that although increasing the crosslinking density of NR can improve 100–300% modulus, the tensile strength and elongation were negatively impacted. With the presence of diallylaime, the Young’s modulus, tensile strength, and 100% modulus remained unchanged as the initiator concentration increased from 0.3% to 1%. The elongation showed a small decrease to reflect the effect of increased polymer-filler interactions. Therefore, the crosslinking of NR by the initiator is controlled and suppressed by the presence of unsaturated monomers. Fig. 6 shows stress-strain curves of the composites with 0–40% filler. The comparison is made between the NR and modified NR composites filled with 10–40% SP particles. An increase of modulus at larger strain was observed from the modified NR composites with the high amount of initiator. This significant improvement is reflected in the tensile stress at 100–300% elongation shown in Table 3. These moduli of the composites increased as the filler fraction increased from 10% to 40%. Generally, the inclusion of diallylamine increased the tensile stress at 100–300% elongations compared with the composites without it because of an increase in filler-rubber interactions. Compared with the SP composites, Young’s moduli are higher only at 0% and 10% filler fractions for the H composites, and they are lower at 0–20% filler fractions for the L composites. The effect of diallylamine is not significant at small strains, but become significant at large strains. This is caused by the increase of compatibility between filler and NR because of the presence of diallylamine, either in the form of oligomer or graft. However, grafted diallylamine is more effective in increasing tensile stress at higher elongation than oilgomeric diallylamine as shown in Fig. 7, which shows that H composites have greater tensile stress at higher elongation ratio than L composites although the difference become smaller as the filler concentration increases. For L composites at small strain (Young’s modulus in Table 3), the increase of the compatibility means a better dispersion of filler particles in the polymer matrix, which in turn prevents the filler–filler interaction and favors filler-rubber interactions. As a result, filler network is softer because of the inclusion of more rubber in the filler network. This effect is more pronounced at lower filler fractions, but become not significant at higher filler fractions because of particle–particle jamming effect that dominates the filler network. For H composites at small strain (Young’s modulus in Table 3), the higher Young’s modulus of the composites with 0–10% filler fraction compared with the SP composites is caused by the higher crosslinking density, both physical and chemical crosslinks, of the modified NR matrix. The crosslinking density is usually estimated with Flory-Rehner equation assuming the crosslinks are tetrafunctional.
Fig. 4. Aggregate size distribution of soy protein (SP), 0.7 um filtered SP, and NR.
double distribution was observed in its volume-averaged size distribution. A larger particle fraction is centered at ∼6 μm and a smaller particle fraction centered at ∼400 nm. Although the portion of larger particles can be reduced to smaller particles (Jong, 2013b), for all practical purposes, the number of cycles should be kept to a minimum because microfluidization is a slow and energy intensive process. The fractions of smaller and larger particles were determined by filtration to remove the fraction of larger SP particles. The measurement showed that ∼93% of particles had a volume-averaged size of 258 nm and a number-averaged size of 210 nm. Compared to NR latex with a volumeaveraged size of 760 nm and a number-averaged size of 550 nm, SP particles are much smaller.
3.2. Curing and stress-strain properties Fig. 5 shows the curing behavior of NR, CB composite, SP composite, SP composites with diallyamine modified NR, and SP composite with initiator modified NR. CB composite and SP composite with diallylamine modified NR had similar initial curing rate, but SP composite with diallyamine modified NR showed a decrease of storage modulus overtime. SP composite with initiator modified NR showed the slowest curing rate. NR is known to show degradation overtime at high temperature. The bio-fillers with carboxylic acid groups are known to interfere with sulfur crosslinking reaction (Jong, 2016). Therefore, the decrease of storage modulus overtime for SP containing composites is the combination of both NR degradation and retardation of the crosslinking reaction. These curing kinetics show that the use of diallylamine grafted NR increased the curing rate and reduced the interference of crosslinking reaction by soy protein. The initial curing rate of modified NR reinforced with soy protein also matches that of carbon black
⎛ ν ⎞ − [ln(1 − ν2 ) + ν2 + χν22] = nV1 ⎜ν21/3 − 2 ⎟ ⎝ 2⎠
(1)
where 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. χ is toluene-NR interaction parameter and is taken as 0.36 (Brandrup et al., 1999). V1 is the molar volume of toluene, 106.3 mol/cm3. n is the crosslinking density defined as the number of network chain between crosslinks. Table 1 shows that H rubber has the highest crosslinking density followed by L rubber and then NR. This is explained as the interaction of grafted amine groups with soy protein filler by forming hydrogen and ionic bonds, which can restrict the swelling of rubber. The crosslinking densities of the composites are 57
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Fig. 5. Curing behavior of NR and composites. NR = NR without filler. CB = NR with 43 phr of carbon black. Init = NR modified with initiator only and with 43 phr of soy protein. SP = NR with 43 phr of soy protein only. H = NR modified with diallylamine and with 43 phr of soy protein. The highest point of each curve is taken as 100% cure.
unmodified NR composites, this means that more small particles are present in the composites of higher filler fractions. These particles can hinder the sulfur crosslinking reaction because of steric effect. The compatibility effect of the modified NR can also be observed from SEM images of tensile fractured surface shown in Fig. 9, where H composites with 10% and 30% filler are compared with the SP composites. Few surface particles are observed on the H composites than the SP composites, indicating better compatibility between rubber and filler in H composites.
Table 2 Tensile properties of the composites with 43 phr of soy protein.
SP L H Init
Young’s Modulus (MPa)
Tensile Strength (MPa)
Elongation at break (%)
Tensile stress at 100% elongation (MPa)
4.0 4.1 4.1 6.3
19.1 ± 0.6 18.6 ± 0.8 18.7 ± 1.1 7.8 ± 1.6
426 378 352 100
3.1 3.5 3.4 7.4
± ± ± ±
0.3 0.5 0.2 0.2
± ± ± ±
11 33 24 18
± ± ± ±
0.3 0.5 0.2 0.0
*Init = NR modified with initiator only.
3.3. Dynamic mechanical properties One of ways to investigate polymer-filler interactions is to use dynamic strain measurements to look at interaction forces. The change in dynamic modulus with increasing strain is known as Payne effect (Payne, 1963). The decrease of dynamic shear modulus with increasing dynamic strain was interpreted by Maier-Goritz (Maier and Goritz, 1996) as the dissociation between polymer chains and the filler surface. The polymer-filler interactions were divided into two types. Stable interactions do not dissociate even at very high strain, while unstable interactions dissociate with increasing strain and is identified as Payne effect. For the current composites, Maier-Goritz model does not fit the experimental data well at the high strain region, a modified equation is used.
G′(γ ) = Gst′ +
GI′ 1 + cγ m
(2)
where G’I is the modulus contribution from unstable polymer-filler interactions. G’st is the modulus contribution from the stable polymerfiller interactions. G’ is equal to the difference between G’(γ) at very small strain and very large strain. G’st is equal to G’(γ) at very large strain and c is a constant related to adsorption and desorption rate of polymer chains from the filler surface. m is a fitting parameter related to filler aggregate structure in Kraus model (Kraus, 1984). Eq. (2) has been used to adequately describe a few rubber composites (Maier and Goritz, 1996; Meera et al., 2009). By fitting strain sweep experimental data with equation 2, G’I can be obtained and related to activation energy of desorption through the Arrhenius equation, G’I = [NIoo exp (-EI/kBT)]kBT, where NIoo is a constant characterizing the density of unstable bonds independent of temperature and deformation amplitude and EI is activation energy of polymer detachment from filler surface. KB and T are the Boltzmann constant and temperature, respectively. The
Fig. 6. Comparison of tensile properites from NR composites (SP) and modified NR composites (H). The H composites are the modified NRcomposites with a high amount of initiator.
shown in Fig. 8. For the L and SP composites, the crosslinking densities of the rubber matrix are similar. For the H composites, the crosslinking density is higher at lower filler fractions compared with the SP and L composites, but the difference became smaller as the filler fraction increased. This indicates that the rubber crosslinking density is not independent from filler fraction and the filler may have effect on the crosslinking density of diallylamine modified NR. Since the same filler is used in these composites, the effect must come from the diallylamine modified NR, which interacts more with filler, therefore can prevent coagulation of filler particles at higher filler fractions. Compared with 58
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Table 3 Tensile Properties of the composites. Filler concentration (phr)
0
11
25
43
67
Tensile stress at 100% elongation (MPa)
SP H L CB
0.9 1.2 0.9 0.9
± ± ± ±
0 0 0 0
1.1 1.6 1.1 1.2
± ± ± ±
0 0 0.1 0.1
1.6 1.8 1.7 2.0
± ± ± ±
0.1 0.1 0 0.1
3.1 3.4 3.5 3.3
± ± ± ±
0.3 0.2 0.5 0.1
5.3 6.3 5.9 7.0
Tensile stress at 200% elongation (MPa)
SP H L CB
1.5 2.3 1.6 1.5
± ± ± ±
0.1 0.1 0.1 0.1
2.3 3.7 2.3 2.3
± ± ± ±
0.1 0.1 0.3 0.2
4.1 5.0 4.4 5.0
± ± ± ±
0.4 0.2 0.2 0.2
7.9 9.4 9.1 8.9
± ± ± ±
0.7 0.4 1.1 0.3
11.0 13.2 12.0 16.8
Tensile stress at 300% elongation (MPa)
SP H L CB
2.4 4.2 2.6 2.4
± ± ± ±
0.2 0.2 0.2 0.2
4.6 7.4 4.8 4.4
± ± ± ±
0.2 0.2 0.6 0.3
8.0 ± 0.8 10.1 ± 0.4 8.7 ± 0.3 10.1 ± 0.4
13.0 16.4 14.7 16.3
Young’s Modulus (MPa)
SP H L CB
1.4 1.7 1.3 1.4
± ± ± ±
0.2 0.1 0.3 0.2
1.9 2.3 1.6 2.0
± ± ± ±
0.1 0 0.4 0.2
2.7 2.7 2.4 3.1
4.0 4.1 4.1 5.9
Tensile Strength (MPa)
SP H L CB
13.2 ± 2 9.4 ± 1.4 15.1 ± 3.4 13.2 ± 2
24.8 19.3 24.8 22.5
± ± ± ±
0.7 1 0.7 0.1
22.2 21.8 22.1 24.7
± ± ± ±
0.6 0.8 1.6 1.4
19.1 18.7 18.6 23.8
± ± ± ±
0.6 1.1 0.8 0.5
15.5 16.2 17.0 21.4
± ± ± ±
0.4 0.4 1.5 2
Elongation at break (%)
SP H L CB
540 423 578 540
594 470 600 569
± ± ± ±
13 11 25 15
539 477 510 487
± ± ± ±
17 7 27 8
426 352 378 400
± ± ± ±
11 24 33 4
302 253 299 256
± ± ± ±
10 4 14 31
± ± ± ±
17 25 25 17
± ± ± ±
0.2 0.1 0.3 0.4
± ± ± ±
± ± ± ±
0.8 0.5 1.3 0.3
0.3 0.2 0.5 0.2
± ± ± ±
0.1 0.3 0.7 0.4
± ± ± ±
0.1 0.5 1.3 0.8
15.5 ± 0 – 15.3 ± 0.1 – 6.5 ± 0.3 6.6 ± 0.3 6.5 ± 1.1 12.3 ± 0.5
Arrhenius plot is shown in Fig. 10. The estimated activation energy expressed in electron volts (eV) indicates that polymer-filler interactions in modified NR composite is greater than the unmodified NR composite. However, the stable interactions may include strong interactions such as ionic interactions in addition to covalent bonds. Such strong interactions may not dissociate under dynamic strain experiments. Therefore, this method necessarily underestimate the polymerfiller interactions. Shear elastic moduli of H composites are shown in Fig. 11. The elastic moduli were measured in the linear viscoelastic region (small strain region). SP particles have a shear elastic modulus of 1–2 GPa. The shear elastic modulus increases with the increasing filler content in the composites for all temperatures measured. To understand the reinforcement effect of these composites, reinforcement factors for these composites at different filler volume fractions at 25 °C are shown in Fig. 12. The reinforcement factor is defined as the elastic modulus of the composite normalized by the elastic modulus of NR. The curves show a rapid increase of G’ with increasing filler fraction. This type of curves cannot be fitted with simple particle inclusion theory such as The Einstein equation. They also cannot be fitted with Halpin-Tsai model that describes the anisotropic effect of filler in a matrix. Guth model (Guth, 1945) for non-spherical fillers was also found to be inadequate to describe these curves. It was found that empirical modified Mooney equation for non-spherical particles can be used to fit these data. The modified Mooney equation has the following form (Ahmed and Jones, 1990; Brodnyan, 1959).
Fig. 7. Comparison of tensile properites from NR composites with high (1%) and low (0.3%) amount of initiator used in the modification of NR latex.
⎛ 2.5φ + 0.407(p − 1)1.508φ ⎞ Gc = Gm exp ⎜ ⎟ 1 − Sφ ⎠ ⎝
(3)
where Gc is the modulus of the composites, Gm is the modulus of the matrix, and φ is the volume fraction of filler. p is the aspect ratio of a reinforcement element with a value of 1 < p < 15. S is the crowding factor and is defined as the volume occupied by the filler divided by the true volume of the filler. For close packed spheres, S is equal to 1.35. The rapid increase of G’ at higher filler fractions indicates the presence of filler network from inter-particle interactions in addition to simple filler reinforcement because of particle crowding effect. G’ was measured at 0.05% strain. At such small strain, current NR modification
Fig. 8. Crosslinking density of composites.
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Fig. 9. Comparison of tensile fractural surfaces from the composites of (a) 10% SP (b) 10% H (c) 30% SP (d) 30% H.
Fig. 10. Amount of unstable bonds between polymer and filler from Maier-Goritz model.
Fig. 11. Shear elastic moduli from H composites.
does not show significant difference in the reinforcement factors at ambient temperature. This indicates a better compatibility between filler and matrix for the current composite system does not contribute to small strain modulus. The presence of oligomeric diallylamine is similar to the presence of solvent that can make composites softer. The presence of graft has a similar effect and therefore makes the filler network softer despite H composites have a higher crosslinking density at 10–30% filler fractions. This would indicate that the shear elastic modulus at small strain is determined by the rigidity of filler network. As the temperature increases to 140 °C, Fig. 13 shows that reinforcement factor decreases compared with that at 25 °C because of the
softening of rubber that is a part of filler network. The reinforcement factors at this temperature also do not increase as rapidly with the increasing filler fractions. Therefore, the curves in Fig. 13 can be described by Guth model with anisotropic reinforcement elements. Guth model (Guth, 1945) has the following form.
Gc = Gm (1 + 0.67pφ + 1.62p 2 φ 2 )
(4)
where p is the aspect ratio of a reinforcement element and φ is the volume fraction of filler. By fitting the Eq. (4) to the experimental data, p was determined and shown in Fig. 13. These results show that reinforcement element formed from the 60
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connected filler particles and rubber are anisotropic and change with temperature. No significant difference was observed between modified and unmodified NR composites at small strain measurements. The difference becomes more obvious at higher elongations as shown in the stress-strain measurements. This can be explained as the presence of oligomeric diallylamine and graft preventing a close bonding between poly(isoprene) polymer chains and soy nanoparticles. Therefore, the effect is not detected at small strains, but only at higher elongations. To explore possible application in tire tread, loss tangent at different temperatures are listed in Table 4. The values of tan δ were used to relate the dynamic mechanical properties to tire performance (Bao et al., 2015). The results indicate the diallylamine modified NR reinforced with soy protein nanoparticles may improve rolling resistance and winter traction, but with less wet traction compared to carbon black reinforced NR.
4. Conclusions NR latex was modified to include amine functional groups by reacting NR latex particles with diallylamine using free-radical initiator. FTIR shows different absorption bands when different amount of initiator was used in the reaction. The NR particles was found to become larger when high amount of initiator was used in the reaction, while the NR particle size remains the same when low amount of initiator was used. These results indicate the presence of oligomeric diallylamine when 0.3% of initiator was used, whereas diallylamine units were attach to NR particles when 1% of initiator was used in the reaction. For reinforcement particles, 93% of SP particles have a number-averaged size of 210 nm. The curing kinetics show that the use of diallylamine grafted NR increased the curing rate and reduced the interference of crosslinking reaction by soy protein. For tensile properties, 26–75% increase of tensile stress was found at 300% elongation for the H composites compared with SP composites. Swelling experiments indicate the amine-modified NR films have 25% higher crosslinking density than the unmodified NR. For the composites, SP and L composites have similar crosslinking density while H composites have higher crosslinking density at 10–30% filler fractions. Compared with NR modified with the initiator only, the presence of diallylamine significantly suppressed the crosslinking of polyisoprene. Compared with NR, SEM images indicate the modified NR has better adhesion towards soy nanoparticles. Dynamic mechanical properties indicate no difference between modified and unmodified NR composites when measured in linear viscoelastic region. Polymer-filler interactions by dynamic mechanical method indicate diallylamine modified NR has 33% greater interactions than unmodified NR. At 25 °C, the increase of reinforcement factor with increasing filler fraction can be described by modified Mooney equation to account for rigid filler network at higher filler fractions. At 140 °C, filler network is softened because of the rubber contribution in the filler network, and the reinforcement factor can be described by Guth equation that takes into account of the anisotropic effect of reinforcement elements. This study indicates that amine-modified NR improves the compatibility between hydrophilic soy nanoparticles and hydrophobic NR and leads to improvement in the tensile stress at 100–300% elongation ratios. Tanδ at 60 °C indicates the modified NR reinforced with soy protein nanoparticles has possible better rolling resistance than carbon black reinforced NR for tire tread application.
Fig. 12. Shear elastic moduli at 25 °C from SP, H, and L composites. The data is fitted with modified Mooney model.
Fig. 13. Shear elastic moduli at 140 °C from SP, H, and L composites. The data is fitted with Guth model. Table 4 Storage modulus and loss tangent at different temperatures. Filler concentration (phr) Tanδ at 10 °C (higher is better) Wet traction Tanδ at 60 °C (lower is better) Rolling resistance G’ (MPa) at −20 °C (lower is better) Winter traction
H CB H CB H CB
0
11
25
43
67
0.007 0.014 0.018 0.014 1.1 0.7
0.023 0.021 0.016 0.019 1.1 1.0
0.030 0.026 0.025 0.036 1.8 2.5
0.031 0.052 0.031 0.063 3.6 10.3
0.046 0.066 0.040 0.084 7.0 24.9
Acknowledgement The author would like to thank A. Thompson for the sample imaging with the scanning electron microscope.
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