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SBR blends prepared from latex system
Polymer Testing 32 (2013) 852–861
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Analysis method
Raman spectroscopy and thermal analysis of gum and silica-filled NR/SBR blends prepared from latex system Sarawut Prasertsri a, Fabienne Lagarde b, Nittaya Rattanasom a, c, *, Chakrit Sirisinha a, c, Philippe Daniel b a
Department of Chemistry, Faculty of Science, Mahidol University, Bangkok 10400, Thailand LUNAM Université, Université du Maine, Institut des Molécules et des Matériaux du Mans (IMMM), UMR CNRS 6283, Av. O. Messiaen, 72085 Le Mans Cedex 9, France c Rubber Technology Research Centre, Faculty of Science, Mahidol University, Salaya, Nakhonpathom 73170, Thailand b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 5 March 2013 Accepted 16 April 2013
Natural rubber/styrene-butadiene rubber (NR/SBR) blends, with and without silica, were prepared by co-coagulating the mixture of rubber latices and various amounts of welldispersed silica suspension. An attempt to predict blend compositions was made using Raman spectroscopy in association with differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). It was found that the intensity of each Raman characteristic peak was strongly dependent on the blend composition, but there was no significant evolution with the presence of silica. Also, TGA results revealed an improvement in thermal stability of NR/SBR blends with increasing both SBR and silica contents due to the dilution effect. Two distinct glass transition temperatures (Tg) were observed in DSC thermograms of all blends, and their Tg values were independent on both blend composition and silica content. This indicated a physical blend formation, which agreed well with no shifts in Raman peaks of the blends in comparison with those of the individual rubbers. Linear regression with R2 quality factor close to 0.99 was achieved when plotting intensity ratio at 1371/1302 cm1 versus blend ratios. On the other hand, the peak height ratio and heat capacity ratio from TGA and DSC analysis, respectively, yielded quadratic equations as a function of blend ratios. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: Rubber blends Silica Latex coagulation Raman spectroscopy Thermal analysis
1. Introduction Styrene-butadiene rubber (SBR) is blended with natural rubber (NR) in order to achieve balance of their properties such as heat build-up, abrasion and aging resistance [1–3]. Although dry blending is commonly used to prepare the blends, it has been reported that latex blending offers the possibility of finer scale dispersion leading to improvement in processing as well as the ultimate properties of the
* Corresponding author. Rubber Technology Research Centre, Faculty of Science, Mahidol University, Salaya, Nakhonpathom 73170, Thailand. Tel.: þ66 2441 9816x1137; fax: þ66 2441 0511. E-mail addresses: [email protected], [email protected] (N. Rattanasom). 0142-9418/$ – see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.polymertesting.2013.04.007
blends [4]. Also, addition of filler into the latices followed by co-coagulating the mixture leads to greater dispersion of filler than by conventional dry blending. However, loss of either rubber or filler may occur if the co-coagulating procedure is not appropriate. Therefore, it is necessary to confirm the amounts of rubber and filler in the premix or masterbatch before further compounding by conventional mixing. Thermogravimetric analysis (TGA) is one of the useful techniques to measure the thermal stability and to differentiate the components of the blends [5–7]. Sircar and Lamond [5] could identify the composition of NR/butadiene rubber (BR) blends by using the ratio of peak height of derivative TGA results for neat NR and butadiene rubber at 365 C and 465 C, respectively. Among other techniques,
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vibrational spectroscopy, such as Fourier transform infrared (FTIR) and Raman spectroscopy, have been extensively used to characterize chemical structure and blend compositions of rubber blends [8–10]. Ghebremeskel and Shield [8] employed FTIR to determine the composition of SBR/nitrile rubber (NBR) blends by plotting the absorbance at 1602 cm1 for SBR and 2237 cm1 for NBR as a function of SBR content in the blends. They found that the absorbance ratio linearly depends on SBR content. Furthermore, the blend composition of SBR/NBR obtained from a calibration curve is in good agreement with the results obtained from TGA, which was used as a reference technique. Appel and co-workers [9] also applied Raman spectroscopy to monitor the blend composition of BR/brominated isobutylene-co-para-methylstyrene (BIMS) blends. They found that the relative intensity ratio (714 cm1 and 1648 cm1 for BIMS and BR, respectively) is proportional to the concentration ratio of these two components. Furthermore, this correlation can be used to examine the phase segregation by Raman mapping technique. The purpose of this work was to apply Raman spectroscopy and thermal techniques, including TGA and differential scanning calorimetry (DSC), for analyzing blend composition of NR/SBR blends, with and without silica, prepared from latex. The suitability for constructing calibration curves from each characterization method was investigated. Thermal stability and compatibility of the blends were also evaluated by using TGA and DSC, respectively. In addition, the actual amount and degree of silica dispersion in the blends were examined by using TGA and scanning electron microscopy (SEM), respectively. 2. Experiment 2.1. Materials Concentrated NR latex (High ammonia grade) was purchased from Thai Rubber Latex Corporation (Thailand) Public Co. Ltd. (Chonburi, Thailand). SBR latex (Emulsion SBR1502) was kindly provided by BST Elastomer Co. Ltd. (Rayong, Thailand). Characteristics of both latices are compared in Table 1. Commercial precipitated silica (TokusilÒ233) was supplied by Tokuyama Siam Silica Co., Ltd. (Rayong, Thailand). 2.2. Preparation of NR/SBR blends, with and without silica The steps for preparing NR/SBR blends, with and without silica, are schematically depicted in Fig. 1. Various amounts of NR and SBR latices were firstly mixed together such that the dried blends would contain 100/0, 70/30, 50/50, 30/70 and 0/100% w/w NR/SBR. For the blends containing silica, 10% w/w well-dispersed silica suspension prepared as described in previous work [11,12] was added to the blended latices such that the dried rubber blends would contain 10, 20 and 30 phr of solid silica. After stirring for 30 min, the mixture of NR/SBR latices and silica suspension was cocoagulated using 2% w/v calcium chloride. The rubber coagulum was then sheeted with a creping machine, washed with water and dried in an oven at 50 C for 48 h. Finally, the dried or uncompounded NR/SBR blends at various ratios,
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with and without silica, were obtained. It should be noted that dried SBR containing silica could not be prepared in this experiment because the silica/SBR latex mixture did not cocoagulate to form coagulum. Thus, the results of silica-filled 0/100 NR/SBR were not reported. 2.3. Characterization of NR/SBR blends, with and without silica Since the NR/SBR blends, with and without silica, were prepared from a latex system, there might be loss of rubber and/or silica during the coagulating procedure. Therefore, blend composition and silica content have to be confirmed prior to utilization of the blends for compounding and vulcanization. In this work, Raman scattering, thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) techniques were used to determine the composition of NR and SBR in the dried blends. In addition, dispersion of silica in the blends was examined by using scanning electron microscopy. 2.3.1. Raman spectroscopy The Raman scattering experiments were performed using a Jobin-Yvon T64000 spectrometer. In order to minimize the fluorescence effect, uncompounded NR/SBR blends, with and without silica, were illuminated with an argon-krypton ion laser (Coherent model Innova 70C) operating at the 647.1 nm (red line). The laser beam was focused by a 100 magnification objective of a confocal microscope (Olympus BX40). Each spectrum was collected in the frequency range 400–3200 cm1 over 60 s and with 10 accumulations to avoid electronic peaks and average background. The SBR Raman data, which up to now are partially reported in the literature, were fully assigned and interpreted. 2.3.2. Thermogravimetric analysis (TGA) TGA experiments were carried out using a TGA/SDTA 851 (Mettler Toledo). Approximately, 8 mg of NR/SBR blends, with and without silica, were heated from 40 C to 600 C at a rate of 20 C/min under nitrogen atmosphere. Then, the gas was automatically changed to oxygen and heating of the sample was continued up to 800 C. Both TGA and derivative TGA (DTGA) results were used to identify the composition of NR/SBR blends. The actual amount of silica in the blends was also calculated from TGA curves using Equation (1) [12].
Silica content ðphrÞ ¼
Rf Fr Ru 100 100 Rf Fr Ru
(1)
where
Fr ¼ weight fraction of rubber in the blends
Ru ¼ residual weight (%) at 800 C of unfilled blends Rf ¼ residual weight (%) at 800 C of silica-filled blends
2.3.3. Differential scanning calorimetry (DSC) DSC measurements were carried out using a DSC Q100 (TA Instruments). Approximately, 15 mg of the samples
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be noted that the Raman peaks of SBR are only partially assigned in the literature [21,22]. This work gives a full attribution for which Raman study of polystyrene [23] and butadiene rubber [24] is good guidance. Both symmetric and asymmetric –CH2 and –CH3 stretching vibrations typically appear in the 2800–3000 cm1 region. Evidently, C]C stretching vibrations of NR and SBR are observed at 1666 and 1668 cm1, respectively. In addition, there are several overlapped signals of NR and SBR which cannot be used to distinguish their contributions, thus only the characteristic peaks allowing differentiation will be focused on. Raman spectra of unfilled NR/SBR samples at various blend ratios are illustrated in Fig. 3. The results show that intensity of Raman peaks depends on the blend composition. As can be seen, the intensities of characteristic signals at 1002, 1302, 1602 and 3058 cm1 tend to increase with increasing SBR content in the blends, whereas the intensities of the isolated characteristic bands of NR at 1371, 1128 and 496 cm1 decrease. The peak at 1666 cm1 is not dependent on NR/SBR ratio and is used as an internal standard. From this correlation, it is possible to confirm composition in the blends by plotting the intensity ratio of characteristic signals of each rubber versus blend ratio. It could be noted here that the characteristic signals at 1371 and 1302 cm1 of –CH3 asymmetric deformation for NR and ]C–H in-plane deformation for SBR, respectively, were chosen for constructing a calibration curve. This is because these chosen peaks have the best resolution and the characteristic peak at 1371 cm1 is not overlapped. Fig. 4 illustrates Raman spectra of NR filled with silica at various contents. Raman characteristic signals of NR are not significantly changed after addition of silica. This observation is probably due to the conjunction of low quantum efficiency of the CCD detector in this frequency range (647.1 nm) and the weak Raman intensity of silica particles which have an amorphous character. It is evident from Fig. 5 that the characteristic bands of silica are rather poor when the sample is excited at 647.1 nm. Moreover, these signals are not improved by increasing the number of scans and accumulation time. When using excitation wavelength at 514.5 nm, the silica sample shows a Raman spectrum having prominent signals at 456, 796 and 994 cm1 corresponding to Si–O–Si and Si–OH stretching vibration. Unfortunately, peaks of NR assigned to ]CC2 rocking and C–CH2 stretching are located in the same frequency region
Table 1 Characteristics of NR and SBR latices. Property
NR latex
SBR latex
Total solid content (%wt) Dry rubber content (%wt) pH NH3 content (%wt) Styrene content (%)
61.68 60.10 10.5 0.69 –
23.35 22.56 10.3 – 23.5
were capsulated in aluminum pans. Each experiment was done under nitrogen atmosphere at a heating rate of 10 C/ min within the temperature range from 80 C to 0 C. Data were plotted as heat flow versus temperature. The inflection point of the slope in the thermogram was taken as the glass transition temperature (Tg). Heat capacity (Cp) was calculated by integrating the area under each transition step. 2.3.4. Scanning electron microscopy (SEM) Dispersion of silica in NR/SBR blends was examined by using a scanning electron microscope (model S-2500, Hitachi) at an accelerating voltage of 15 kV and under vacuum atmosphere. Newly cryogenically fractured surfaces of the rubber specimens were prepared before coating with Pt–Pd to prevent the charging on the surface. 3. Results and discussion 3.1. Raman scattering results According to the literature, the presence of impurity and addition of crosslinking agents, fillers and other rubber ingredients into rubber are the main reasons of fluorescence parasite signals in Raman scattering spectrum, especially with the laser excitation lines in the visible region [13–15]. The quantum yield of the fluorescence process is much higher than that of the Raman process, and thus the main spectroscopic information is overlapped. In order to clearly identify the characteristic bands of NR and SBR, the coagulated rubber without silica was firstly selected to perform Raman spectroscopy. The spectra of neat NR and SBR are compared in Fig. 2 and band assignments are done based on comparison to literature spectra, as tabulated in Table 2. These Raman bands of NR are in agreement with the literature [15–20], however it should
NR/SBR latices (100/0, 70/30, 50/50, 30/70, 0/100% w/w solid rubber) Stir for 20 min 1) Co-coagulate with 2% w/v CaCl2 2) Dry in an oven
Unfilled NR/SBR blends
1) Add 10% w/w silica suspension (10, 20, 30 phr solid silica) 2) Stir for 30 min 3) Co-coagulate with 2% w/v CaCl2 4) Dry in an oven
Silica-filled NR/SBR blends
Fig. 1. The schematic diagram for preparing NR/SBR blends, with and without silica.
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Fig. 2. Typical Raman spectra of neat NR and SBR.
Table 2 Positions and assignments of bands observed in Raman spectra of neat NR and SBR. Raman shift (cm1) NR
Assignment
SBR
Literature This Literature This work work – – 3055 [23] 3058 (S) 3033 [18] 3041 3003 [24] 3000 (B) 2965 [18] 2964 – – 2940 2911 2850 1664 – – – 1452 1375
[18] [18] [18] [18]
2934 2912 2850 1666 – – – [18] 1448 [19] 1371
2906 – 2846 1664 1639 1601 1583 1431 –
[22] 2910 – [24] 2845 [24] 1668 [24] 1642 [23] 1602 [23] 1582 [24] 1437 –
(B) (B) (S) (S)
1357 [18] 1360 – – 1327 [18] 1327 1301 [24] 1302 (B) 1314 1286 1243 1215 –
[18] [18] [18] [20]
1309 1285 1236 1208 –
– – – – 1267 [24] 1273 – – 1201 [24] 1199 (B)
1130 [18] 1128 – – 1039 [18] 1031 – – – – 1002 [23] 1002 (S) 1000 [18] 997 – – – 620 [21] 491 [18] 496 –
– 622 (S) –
]C–H ring stretching ]C–H stretching CH3 asymmetric stretching CH2 asymmetric stretching CH3 symmetric stretching CH2 symmetric stretching C]C stretching C]C stretching ]C–H ring stretching C]CH ring stretching CH2 deformation CH3 asymmetric deformation CH2 deformation ]C–H in-plane deformation CH2 twisting CH bending CH2 twisting ]C–H in plane bending Symmetric stretching of trans C]C CH2 wagging CH3 rocking Symmetric ring breathing C–CH2 stretching C]CH ring bending ]CC2 rocking and scissoring
B ¼ butadiene segments in SBR and S ¼ styrene segments in SBR.
at 496 and 1000 cm1, respectively, [16,17]. Thus, it is difficult to distinguish the Raman signals of precipitated silica from those of NR. Similar to silica/NR samples, the spectra of various NR/SBR blends containing 30 phr of silica are not significantly different in comparison to those of unfilled blends. Fig. 6 shows the plot of intensity ratio at 1371/ 1302 cm1 as a function of SBR content for both unfilled and silica-filled NR/SBR blends. As can be seen, both unfilled and silica-filled blends give a straight line with R2 quality factor close to 0.99. This result indicates that very little loss of NR and SBR occurs during the coagulation process for preparing dried blends. Furthermore, these calibration curves could be applied for analysis of blend composition in the NR/SBR blends.
3.2. TGA results Fig. 7(a) and (b) shows TGA and DTGA results, respectively, of various NR/SBR blends without silica. From TGA curves (Fig. 7(a)), there are two main steps of decomposition in all samples. The first weight loss in region 350– 500 C is about 98.5% of the total weight loss, corresponding to decomposition of the blended rubbers. The small decomposition having weight loss of about 0.7% occurs at about 600 C after switching to oxygen atmosphere caused by oxidative degradation of carbon char from the rubber decomposition. The residue about 0.8% relates to ash and inorganic substances. As shown in Fig. 7(b), only one decomposition peak is observed at approximately 390 C and 473 C, denoted as decomposition temperature (Tmax), for neat NR and SBR, respectively. Because SBR contains partially aromatic styrene groups, it possesses higher thermal stability than NR [5]. When blending NR
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Fig. 3. Raman spectra of unfilled NR/SBR blends at various ratios.
Fig. 4. Raman spectra of uncompounded NR containing various silica loadings.
with SBR, two decomposition peaks are observed and correspond to characteristic Tmax of the individual rubbers. In the decomposition region of the rubbers (350–500 C), it can be noted that (i) the weight loss at a specified temperature reduces when SBR content in the blends is increased (Fig. 7(a)) and (ii) the peak height at Tmax strongly depends on the blend composition (Fig. 7(b)). TGA and DTGA curves of NR/SBR blends containing silica are illustrated in Fig. 8. After adding silica, Tmax of the blends does not significantly change when compared to that of unfilled blends, but its peak height reduces with increasing silica content due to the dilution effect with respect to volume fraction of rubber. Furthermore, the
residue weight from the TGA curve is found to increase with respect to the combination of ash and silica. Thus, it is necessary to separate them by subtraction in order to know the actual amount of silica in the coagulated blends. The results are summarized in Table 3 and discussed below. Since the Tmax of NR and SBR phases in the blends is significantly different (a difference of about 80 C), the ratios of their peak heights are criterion to distinguish them apart. In this work, the ratio of peak height at Tmax of 473 C and 390 C of SBR and NR, respectively, is plotted against the content of SBR in the NR/SBR blends in order to construct the calibration curve. Fig. 9 shows the relationship between weight of SBR and peak height ratio (HSBR/
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2.00 Intensity ratio at 1371/1302 cm
-1
Unfilled blends Silica-filled blends Linear (Unfilled blends) Linear (Silica-filled blends)
1.50
y = -0.0172x + 1.6434
1.00
2
R = 0.9869 y = -0.0166x + 1.5903 2
R = 0.9946
0.50
0.00 0
20
40
60
80
100
% SBR in NR/SBR Fig. 6. Plot of intensity ratio (1371/1302 cm1) versus SBR content in unfilled and 30 phr of silica-filled NR/SBR blends.
Fig. 5. Raman spectra of as-received precipitated silica recorded at different excitation wavelengths; (a) lexc ¼ 647.1 nm and (b) lexc ¼ 514.5 nm.
transition steps of NR and SBR in the blends were correlated with the blend composition. Figs. 10(a) and (b) show, respectively, DSC thermograms of unfilled NR/SBR blends at various blend ratios and silica-filled 50/50 NR/SBR blend at different silica contents. Tg values of NR, SBR and their blends are tabulated in Table 3. Tgs of pure NR and SBR appear at 62 C and 50 C, respectively. After blending, two transitions are observed in the thermograms at around Tg of each elastomer. They are coded as Tg1 and Tg2 for NR and SBR phases, respectively. It can be noted that neither Tg1 nor Tg2 show any deviation from the values of neat NR
HNR) for NR/SBR blends, with and without silica. It is found that polynomial regression is a good fit and yields a correlation coefficient of at least 98% for NR/SBR blends, with and without silica. This could be applicable for analyzing the blend composition of an unknown NR/SBR sample. 3.3. DSC results DSC was employed to determine glass transition temperature (Tg) of NR/SBR blends. Also, heat capacity ratios at
b1
100
Deriv. weight (%/ C)
a1 Weight (%)
o
80
0.0
60 NR/SBR 100/0 70/30 50/50 30/70 0/100
40 20 0
-0.5
-1.5 -2.0
50 150 250 350 450 550 650 750 Temperature (oC)
b2
100
0.0
o
80 Weight (%)
50 150 250 350 450 550 650 750 Temperature (oC)
Deriv. weight (%/ C)
a2
NR/SBR 100/0 70/30 50/50 30/70 0/100
-1.0
60 40 20 0
-0.5 -1.0 -1.5 -2.0
300
350 400 450 500 Temperature (oC)
550
300
350 400 450 500 Temperature (oC)
550
Fig. 7. (a) TGA and (b) DTGA curves of various NR/SBR blends without silica (subscripts 1 and 2 respectively represent the temperature range from 50–750 C and 300–550 C (enlarged curve)).
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0.0 Deriv. weight (%/ºC)
100
Weight (%)
80 60 40
0 phr 10 phr 20 phr 30 phr
20 0
-0.2 -0.4 -0.6
-1.0 -1.2
50 150 250 350 450 550 650 750 Temperature (ºC)
a
50 150 250 350 450 550 650 750 Temperature (ºC)
70/30 NR/SBR 0.0 Deriv. weight (%/ºC)
100 Weight (%)
80 60 40
0 phr 10 phr 20 phr 30 phr
20 0
-0.2 -0.4 -0.6 0 phr 10 phr 20 phr 30 phr
-0.8 -1.0 -1.2
50 150 250 350 450 550 650 750 Temperature (ºC)
50 150 250 350 450 550 650 750 Temperature (ºC)
b
50/50 NR/SBR 0.0 Deriv. weight (%/ºC)
100 80 Weight (%)
0 phr 10 phr 20 phr 30 phr
-0.8
60 40
0 phr 10 phr 20 phr 30 phr
20 0
-0.2 -0.4 -0.6 -0.8
0 phr 10 phr 20 phr 30 phr
-1.0 -1.2 -1.4
50 150 250 350 450 550 650 750 Temperature (ºC)
c
50 150 250 350 450 550 650 750 Temperature (ºC)
30/70 NR/SBR
Fig. 8. TGA and DTGA curves of NR/SBR blends containing different silica contents.
Table 3 Glass transition temperature of various NR/SBR samples measured by DSC. NR/SBR (% w/w)
Silica content (phr)
100/0 70/30 50/50 30/70 0/100 50/50 50/50 50/50
0 0 0 0 0 10 20 30
Glass transition temperature ( C) Tg1
Tg2
62.3 62.1 62.9 62.7 – 62.8 63.4 63.5
– 50.5 52.8 51.8 49.9 53.9 53.7 53.8
and SBR. This infers that there is only physical interlocking, without any chemical interaction, between the molecular chains of NR and SBR in the coagulated blends. This is consistent with the Raman result where no shifts of the assigned bands are observed. Similarly, by adding silica into the NR/SBR blend, two transitions can be seen at approximately the same temperatures as for the neat NR and SBR, indicating immiscible blends. The correlation of DSC results with blend composition was also done. Heat capacity at each transition step was calculated and plotted as a function of SBR content in the blend. As evidenced in Fig. 11, heat capacity data of unfilled and 30 phr of silica-filled NR/SBR blends versus SBR content in the blends can be fitted with polynomial functions. From
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3.0
2.0
1.50
2
y = 0.0003x + 0.0076x + 0.0262
1.5
Unfilled blends Silica-filled blends Poly. (Unfilled blends) Poly. (Silica-filled blends)
1.75 Cp,SBR /C p,NR
HSBR /HNR
2.00
Unfilled blends Silica-filled blends Poly. (Unfilled blends) Poly. (Silica-filled blends)
2.5
859
2
R = 0.9844
1.0
1.25 2
y = 0.0004x - 0.0066x + 0.0138
1.00
2
R = 0.9913
0.75 0.50
2
y = 0.0003x - 0.0039x + 0.0094
2
0.5
y = 0.0003x + 0.0071x + 0.0124
0.25
2
2
R = 0.9948
R = 0.9954
0.00
0.0 0
20
40 60 % SBR in NR/SBR
80
0
100
20
40
60
80
100
% SBR in NR/SBR
Fig. 9. Plot of the ratio of the peak heights (HSBR/HNR) versus SBR content in unfilled and 30 phr of silica-filled NR/SBR blends.
Fig. 11. Plot of the ratio of the heat capacity (Cp,SBR/Cp,NR) versus SBR content of unfilled and 30 phr of silica-filled NR/SBR blends.
this result, it should, therefore, also be possible to determine the composition of NR/SBR blends from this calibration curve.
Table 4 Silica content determined from the results of TGA measurement of silicafilled NR/SBR blends prepared from latex stage. NR/SBR (% w/w)
3.4. Actual amount of silica in NR/SBR blends 100/0
The amounts of silica in the blends calculated from TGA results are summarized in Table 4. At a given blend ratio, it is found that loss of silica increases with increasing silica content. Furthermore, loss of silica is found to increase with increasing SBR content in the blends. The highest silica loss of about 1.3% wt is found in 30/70 NR/SBR blends having 30 phr of silica. The low coagulation efficiency could be due to the lower dry rubber content (DRC) of SBR latex. DRC of SBR latex is 22.56% wt which is approximately three times lower than that of concentrated NR latex. Hence, the higher silica loss would be possible during coagulation of silica/ latex mixture which contains high proportions of both SBR latex and silica suspension in NR/SBR latices. 3.5. Dispersion morphology of silica in NR/SBR blends Figs. 12 and 13 show SEM micrographs of silica-filled NR/SBR blends. The original micrographs are depicted in
70/30
50/50
30/70
Silica content (phr) Theoretical value
Actual value
0 10 20 30 0 10 20 30 0 10 20 30 0 10 20 30
N/A 9.62 0.01 19.48 0.02 29.12 0.06 N/A 9.37 0.08 19.35 0.15 28.78 0.28 N/A 8.97 0.18 18.73 0.17 28.35 0.31 N/A 8.90 0.20 18.32 0.33 27.97 0.42
the upper row of each figure where white spots represent silica, and black areas are rubber matrix. In order to clearly differentiate between the feature of silica structures (aggregates and agglomerates) and rubber phase, the original
Fig. 10. DSC thermograms of (a) unfilled NR/SBR blends at various ratios, and (b) silica-filled 50/50 NR/SBR blend at various silica contents.
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Fig. 12. SEM micrographs of fracture surface of 50/50 NR/SBR blends containing different silica contents (upper; original images and lower; inverse images).
SEM images were converted to negative images (by using ImageJ software) as presented in the lower row of each figure. In contrast to the original micrographs, silica and rubber matrix in the negative micrographs are presented as
black spots and gray areas, respectively [25]. Dispersion morphologies of 50/50 NR/SBR blends containing different silica loadings (0–30 phr) are shown in Fig. 12(a)–(d). At low silica loading, it is observed that silica particles
Fig. 13. SEM micrographs of fracture surface of 30 phr of silica-filled NR/SBR blends at various ratios; (a) 70/30, (b) 50/50 and (c) 30/70 NR/SBR (upper; original images and lower; negative images).
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distribute and disperse well after being processed for preparing the coagulated silica-filled NR/SBR blends. When silica loading is increased, large aggregates can be observed in the blend containing the highest silica loading (30 phr). The morphologies of 30 phr of silica-filled NR/SBR blends at different blend ratios are presented in Fig. 13. It can be seen that the distribution and dispersion state of silica depend on the blend composition. Better distribution and dispersion of silica in rubber matrix can be observed in NR rich blends, as illustrated in Fig. 13(a). For SBR rich blends shown in Fig. 13(c), a few micrometers of silica agglomerates are observed. Additionally, it could be noted that the uneven distribution of silica in 30/70 NR/SBR sample would give more variation or higher standard deviation in the actual amount of silica calculated from TGA results. 4. Conclusions Binary blends of NR and SBR, with and without silica, were prepared by latex co-coagulation. SEM results reveal that very little self-agglomeration of silica occurs during co-coagulation. DSC result shows that NR/SBR blends are immiscible, displaying two Tgs over the entire composition range. This result agrees with the Raman results in which there are neither no new peaks nor significant shifts in the spectra, confirming no chemical moieties were generated in the blending process. In addition, the incorporation of silica into the blends does not affect the characteristic Raman peaks and Tgs of NR and SBR but gives improvement in thermal stability. Raman peaks of NR, SBR and their blends are fully assigned in this paper. It is also found that the quantitative blend compositions can be practically determined from the height of each characteristic peak. Linear regression with R2 quality factor close to 0.99 of the plot between intensity ratio at 1371/1302 cm1 and blend ratios is achieved. Meanwhile, the peak height ratio and heat capacity ratio from TGA and DSC analysis, respectively, yield quadratic equations as a function of blend ratios. All fitting curves could be potentially used either for quality control when coagulating the NR/SBR latices or for quantitative analysis of unknown composition of NR/SBR blends.
Acknowledgements Financial support from the Thailand Research Fund through the Royal Golden Jubilee Ph.D. Program (Grant No. PHD/0286/2549) is deeply acknowledged. The authors would like to express their gratitude to the Rubber Technology Research Centre (RTEC), Mahidol University and the Institute for Molecules and Materials of Le Mans – UMR CNRS 6283, Université du Maine for supporting the instruments used in this work. Sincere appreciation is extended to NETZSCH (Thailand) Ltd. and Mr. Krit Suthacheva for supporting a laboratory agitator bead mill used for preparing silica suspension. The authors are also grateful to Miss Jirawadee Thanatnit, Center of
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Nanoimaging, Faculty of Science, Mahidol University for carrying out the SEM measurement. References [1] K. Jones, A. Tinker, Blends of Natural Rubber: Novel Techniques for Blending with Speciality Polymers, Chapman & Hall, London, 1997. [2] L.L.Y. Visconte, A.F. Martins, J.C.M. Suarez, R.C.R. Nunes, Different preparative modes for the incorporation of additives in NR/SBR blends, J. Appl. Polym. Sci. 93 (2004) 483–489. [3] M. Shundo, M. Imoto, Y. Minoura, Studies on polymer blends: blending methods for natural rubber and styrene-butadiene rubber, J. Appl. Polym. Sci. 10 (1966) 939–953. [4] A. Boudenne, L. Ibos, Y. Candau, S. Thomas, Handbook of Multiphase Polymer Systems, John Wiley and Sons Ltd., Chichester, 2011. [5] A.K. Sircar, T.G. Lamond, Identification of elastomers in tire sections by total thermal analysis. I. Tread and black sidewall, Rubber Chem. Technol. 48 (1975) 301–309. [6] J.J. Maurer, Advances in thermogravimetric analyses of elastomer systems, J. Macromol. Sci. A Chem. 8 (1974) 73–82. [7] I.A. Amraee, A.A. Katbab, S. Aghafarajollah, Qualitative and quantitative analysis of SBR/BR blends by thermogravimetric analysis, Rubber Chem. Technol. 69 (1996) 130–136. [8] G.N. Ghebremeskel, S.R. Shield, Characterization of binary/tertiary blends of SBR, NBR and PVC by IR spectroscopy, Rubber World 227 (2003) 26–30. [9] R. Appel, T.W. Zerda, W.H. Waddell, Raman microimaging of polymer blends, Appl. Spectrosc. 54 (2000) 1559–1566. [10] J.K. Clark, R.A. Scott, Characterization of sulfur-cured styrene-butadiene copolymer rubbers and their polybutadiene blends, J. Appl. Polym. Sci. 14 (1970) 1–9. [11] S. Prasertsri, N. Rattanasom, Mechanical and damping properties of silica/natural rubber composites prepared from latex system, Polym. Test. 30 (2011) 515–526. [12] S. Prasertsri, N. Rattanasom, Fumed and precipitated silica reinforced natural rubber composites prepared from latex system: mechanical and dynamic properties, Polym. Test. 31 (2012) 593– 605. [13] B. Schrader, Special Techniques and Applications: Infrared and Raman Spectroscopy, Wiley-VCH Verlag GmbH, Weinheim, 2007. [14] J.L. Koenig, Spectroscopy of Polymers: Raman Spectroscopy of Polymers, Elsevier Science Inc., New York, 1999. [15] K.D.O. Jackson, M.J.R. Loadman, C.H. Jones, G. Ellis, Fourier transform Raman spectroscopy of elastomers: an overview, Spectrochim. Acta A Mol. Spectrosc. 46 (1990) 217–226. [16] S.W. Cornell, J.L. Koenig, Raman spectra of polyisoprene rubbers, Macromolecules 2 (1969) 546–549. [17] P.J. Hendra, K.D.O. Jackson, Applications of Raman spectroscopy to the analysis of natural rubber, Spectrochim. Acta A Mol. Spectrosc. 50 (1994) 1987–1997. [18] J. Samran, P. Phinyocheep, P. Daniel, D. Derouet, J.Y. Buzare, Raman spectroscopic study of non-catalytic hydrogenation of unsaturated rubbers, J. Raman Spectrosc. 35 (2004) 1073–1080. [19] P. Nallasamy, S. Mohan, Vibrational spectra of cis-1,4-polyisoprene, Arab. J. Sci. Eng. 29 (2004) 17–26. [20] V. Arjunan, S. Subramanian, S. Mohan, Fourier transform infrared and Raman spectral analysis of trans-1,4-polyisoprene, Spectrochim. Acta A Mol. Biomol. Spectrosc. 57 (2001) 2547–2554. [21] G. Martinez-Barrera, H. Lopez, V.M. Castano, R. Rodriguez, Studies on the rubber phase stability in gamma irradiated polystyrene-SBR blends by using FT-IR and Raman spectroscopy, Radiat. Phys. Chem. 69 (2004) 155–162. [22] J. Guilment, L. Bokobza, Determination of polybutadiene microstructures and styrene-butadiene copolymers composition by vibrational techniques combined with chemometric treatment, Vib. Spectrosc. 26 (2001) 133–149. [23] S. Prakanrat, P. Phinyocheep, P. Daniel, Spectroscopic investigation of polystyrene surface grafting on natural rubber, Appl. Spectrosc. 63 (2009) 233–238. [24] S.W. Cornell, J.L. Koenig, The Raman spectra of polybutadiene rubbers, Macromolecules 2 (1969) 540–545. [25] Y. Konishi, M. Cakmak, Nanoparticle induced network self-assembly in polymer-carbon black composites, Polymer 47 (2006) 5371– 5391.
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Analysis method
Raman spectroscopy and thermal analysis of gum and silica-filled NR/SBR blends prepared from latex system Sarawut Prasertsri a, Fabienne Lagarde b, Nittaya Rattanasom a, c, *, Chakrit Sirisinha a, c, Philippe Daniel b a
Department of Chemistry, Faculty of Science, Mahidol University, Bangkok 10400, Thailand LUNAM Université, Université du Maine, Institut des Molécules et des Matériaux du Mans (IMMM), UMR CNRS 6283, Av. O. Messiaen, 72085 Le Mans Cedex 9, France c Rubber Technology Research Centre, Faculty of Science, Mahidol University, Salaya, Nakhonpathom 73170, Thailand b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 5 March 2013 Accepted 16 April 2013
Natural rubber/styrene-butadiene rubber (NR/SBR) blends, with and without silica, were prepared by co-coagulating the mixture of rubber latices and various amounts of welldispersed silica suspension. An attempt to predict blend compositions was made using Raman spectroscopy in association with differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). It was found that the intensity of each Raman characteristic peak was strongly dependent on the blend composition, but there was no significant evolution with the presence of silica. Also, TGA results revealed an improvement in thermal stability of NR/SBR blends with increasing both SBR and silica contents due to the dilution effect. Two distinct glass transition temperatures (Tg) were observed in DSC thermograms of all blends, and their Tg values were independent on both blend composition and silica content. This indicated a physical blend formation, which agreed well with no shifts in Raman peaks of the blends in comparison with those of the individual rubbers. Linear regression with R2 quality factor close to 0.99 was achieved when plotting intensity ratio at 1371/1302 cm1 versus blend ratios. On the other hand, the peak height ratio and heat capacity ratio from TGA and DSC analysis, respectively, yielded quadratic equations as a function of blend ratios. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: Rubber blends Silica Latex coagulation Raman spectroscopy Thermal analysis
1. Introduction Styrene-butadiene rubber (SBR) is blended with natural rubber (NR) in order to achieve balance of their properties such as heat build-up, abrasion and aging resistance [1–3]. Although dry blending is commonly used to prepare the blends, it has been reported that latex blending offers the possibility of finer scale dispersion leading to improvement in processing as well as the ultimate properties of the
* Corresponding author. Rubber Technology Research Centre, Faculty of Science, Mahidol University, Salaya, Nakhonpathom 73170, Thailand. Tel.: þ66 2441 9816x1137; fax: þ66 2441 0511. E-mail addresses: [email protected], [email protected] (N. Rattanasom). 0142-9418/$ – see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.polymertesting.2013.04.007
blends [4]. Also, addition of filler into the latices followed by co-coagulating the mixture leads to greater dispersion of filler than by conventional dry blending. However, loss of either rubber or filler may occur if the co-coagulating procedure is not appropriate. Therefore, it is necessary to confirm the amounts of rubber and filler in the premix or masterbatch before further compounding by conventional mixing. Thermogravimetric analysis (TGA) is one of the useful techniques to measure the thermal stability and to differentiate the components of the blends [5–7]. Sircar and Lamond [5] could identify the composition of NR/butadiene rubber (BR) blends by using the ratio of peak height of derivative TGA results for neat NR and butadiene rubber at 365 C and 465 C, respectively. Among other techniques,
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vibrational spectroscopy, such as Fourier transform infrared (FTIR) and Raman spectroscopy, have been extensively used to characterize chemical structure and blend compositions of rubber blends [8–10]. Ghebremeskel and Shield [8] employed FTIR to determine the composition of SBR/nitrile rubber (NBR) blends by plotting the absorbance at 1602 cm1 for SBR and 2237 cm1 for NBR as a function of SBR content in the blends. They found that the absorbance ratio linearly depends on SBR content. Furthermore, the blend composition of SBR/NBR obtained from a calibration curve is in good agreement with the results obtained from TGA, which was used as a reference technique. Appel and co-workers [9] also applied Raman spectroscopy to monitor the blend composition of BR/brominated isobutylene-co-para-methylstyrene (BIMS) blends. They found that the relative intensity ratio (714 cm1 and 1648 cm1 for BIMS and BR, respectively) is proportional to the concentration ratio of these two components. Furthermore, this correlation can be used to examine the phase segregation by Raman mapping technique. The purpose of this work was to apply Raman spectroscopy and thermal techniques, including TGA and differential scanning calorimetry (DSC), for analyzing blend composition of NR/SBR blends, with and without silica, prepared from latex. The suitability for constructing calibration curves from each characterization method was investigated. Thermal stability and compatibility of the blends were also evaluated by using TGA and DSC, respectively. In addition, the actual amount and degree of silica dispersion in the blends were examined by using TGA and scanning electron microscopy (SEM), respectively. 2. Experiment 2.1. Materials Concentrated NR latex (High ammonia grade) was purchased from Thai Rubber Latex Corporation (Thailand) Public Co. Ltd. (Chonburi, Thailand). SBR latex (Emulsion SBR1502) was kindly provided by BST Elastomer Co. Ltd. (Rayong, Thailand). Characteristics of both latices are compared in Table 1. Commercial precipitated silica (TokusilÒ233) was supplied by Tokuyama Siam Silica Co., Ltd. (Rayong, Thailand). 2.2. Preparation of NR/SBR blends, with and without silica The steps for preparing NR/SBR blends, with and without silica, are schematically depicted in Fig. 1. Various amounts of NR and SBR latices were firstly mixed together such that the dried blends would contain 100/0, 70/30, 50/50, 30/70 and 0/100% w/w NR/SBR. For the blends containing silica, 10% w/w well-dispersed silica suspension prepared as described in previous work [11,12] was added to the blended latices such that the dried rubber blends would contain 10, 20 and 30 phr of solid silica. After stirring for 30 min, the mixture of NR/SBR latices and silica suspension was cocoagulated using 2% w/v calcium chloride. The rubber coagulum was then sheeted with a creping machine, washed with water and dried in an oven at 50 C for 48 h. Finally, the dried or uncompounded NR/SBR blends at various ratios,
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with and without silica, were obtained. It should be noted that dried SBR containing silica could not be prepared in this experiment because the silica/SBR latex mixture did not cocoagulate to form coagulum. Thus, the results of silica-filled 0/100 NR/SBR were not reported. 2.3. Characterization of NR/SBR blends, with and without silica Since the NR/SBR blends, with and without silica, were prepared from a latex system, there might be loss of rubber and/or silica during the coagulating procedure. Therefore, blend composition and silica content have to be confirmed prior to utilization of the blends for compounding and vulcanization. In this work, Raman scattering, thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) techniques were used to determine the composition of NR and SBR in the dried blends. In addition, dispersion of silica in the blends was examined by using scanning electron microscopy. 2.3.1. Raman spectroscopy The Raman scattering experiments were performed using a Jobin-Yvon T64000 spectrometer. In order to minimize the fluorescence effect, uncompounded NR/SBR blends, with and without silica, were illuminated with an argon-krypton ion laser (Coherent model Innova 70C) operating at the 647.1 nm (red line). The laser beam was focused by a 100 magnification objective of a confocal microscope (Olympus BX40). Each spectrum was collected in the frequency range 400–3200 cm1 over 60 s and with 10 accumulations to avoid electronic peaks and average background. The SBR Raman data, which up to now are partially reported in the literature, were fully assigned and interpreted. 2.3.2. Thermogravimetric analysis (TGA) TGA experiments were carried out using a TGA/SDTA 851 (Mettler Toledo). Approximately, 8 mg of NR/SBR blends, with and without silica, were heated from 40 C to 600 C at a rate of 20 C/min under nitrogen atmosphere. Then, the gas was automatically changed to oxygen and heating of the sample was continued up to 800 C. Both TGA and derivative TGA (DTGA) results were used to identify the composition of NR/SBR blends. The actual amount of silica in the blends was also calculated from TGA curves using Equation (1) [12].
Silica content ðphrÞ ¼
Rf Fr Ru 100 100 Rf Fr Ru
(1)
where
Fr ¼ weight fraction of rubber in the blends
Ru ¼ residual weight (%) at 800 C of unfilled blends Rf ¼ residual weight (%) at 800 C of silica-filled blends
2.3.3. Differential scanning calorimetry (DSC) DSC measurements were carried out using a DSC Q100 (TA Instruments). Approximately, 15 mg of the samples
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be noted that the Raman peaks of SBR are only partially assigned in the literature [21,22]. This work gives a full attribution for which Raman study of polystyrene [23] and butadiene rubber [24] is good guidance. Both symmetric and asymmetric –CH2 and –CH3 stretching vibrations typically appear in the 2800–3000 cm1 region. Evidently, C]C stretching vibrations of NR and SBR are observed at 1666 and 1668 cm1, respectively. In addition, there are several overlapped signals of NR and SBR which cannot be used to distinguish their contributions, thus only the characteristic peaks allowing differentiation will be focused on. Raman spectra of unfilled NR/SBR samples at various blend ratios are illustrated in Fig. 3. The results show that intensity of Raman peaks depends on the blend composition. As can be seen, the intensities of characteristic signals at 1002, 1302, 1602 and 3058 cm1 tend to increase with increasing SBR content in the blends, whereas the intensities of the isolated characteristic bands of NR at 1371, 1128 and 496 cm1 decrease. The peak at 1666 cm1 is not dependent on NR/SBR ratio and is used as an internal standard. From this correlation, it is possible to confirm composition in the blends by plotting the intensity ratio of characteristic signals of each rubber versus blend ratio. It could be noted here that the characteristic signals at 1371 and 1302 cm1 of –CH3 asymmetric deformation for NR and ]C–H in-plane deformation for SBR, respectively, were chosen for constructing a calibration curve. This is because these chosen peaks have the best resolution and the characteristic peak at 1371 cm1 is not overlapped. Fig. 4 illustrates Raman spectra of NR filled with silica at various contents. Raman characteristic signals of NR are not significantly changed after addition of silica. This observation is probably due to the conjunction of low quantum efficiency of the CCD detector in this frequency range (647.1 nm) and the weak Raman intensity of silica particles which have an amorphous character. It is evident from Fig. 5 that the characteristic bands of silica are rather poor when the sample is excited at 647.1 nm. Moreover, these signals are not improved by increasing the number of scans and accumulation time. When using excitation wavelength at 514.5 nm, the silica sample shows a Raman spectrum having prominent signals at 456, 796 and 994 cm1 corresponding to Si–O–Si and Si–OH stretching vibration. Unfortunately, peaks of NR assigned to ]CC2 rocking and C–CH2 stretching are located in the same frequency region
Table 1 Characteristics of NR and SBR latices. Property
NR latex
SBR latex
Total solid content (%wt) Dry rubber content (%wt) pH NH3 content (%wt) Styrene content (%)
61.68 60.10 10.5 0.69 –
23.35 22.56 10.3 – 23.5
were capsulated in aluminum pans. Each experiment was done under nitrogen atmosphere at a heating rate of 10 C/ min within the temperature range from 80 C to 0 C. Data were plotted as heat flow versus temperature. The inflection point of the slope in the thermogram was taken as the glass transition temperature (Tg). Heat capacity (Cp) was calculated by integrating the area under each transition step. 2.3.4. Scanning electron microscopy (SEM) Dispersion of silica in NR/SBR blends was examined by using a scanning electron microscope (model S-2500, Hitachi) at an accelerating voltage of 15 kV and under vacuum atmosphere. Newly cryogenically fractured surfaces of the rubber specimens were prepared before coating with Pt–Pd to prevent the charging on the surface. 3. Results and discussion 3.1. Raman scattering results According to the literature, the presence of impurity and addition of crosslinking agents, fillers and other rubber ingredients into rubber are the main reasons of fluorescence parasite signals in Raman scattering spectrum, especially with the laser excitation lines in the visible region [13–15]. The quantum yield of the fluorescence process is much higher than that of the Raman process, and thus the main spectroscopic information is overlapped. In order to clearly identify the characteristic bands of NR and SBR, the coagulated rubber without silica was firstly selected to perform Raman spectroscopy. The spectra of neat NR and SBR are compared in Fig. 2 and band assignments are done based on comparison to literature spectra, as tabulated in Table 2. These Raman bands of NR are in agreement with the literature [15–20], however it should
NR/SBR latices (100/0, 70/30, 50/50, 30/70, 0/100% w/w solid rubber) Stir for 20 min 1) Co-coagulate with 2% w/v CaCl2 2) Dry in an oven
Unfilled NR/SBR blends
1) Add 10% w/w silica suspension (10, 20, 30 phr solid silica) 2) Stir for 30 min 3) Co-coagulate with 2% w/v CaCl2 4) Dry in an oven
Silica-filled NR/SBR blends
Fig. 1. The schematic diagram for preparing NR/SBR blends, with and without silica.
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Fig. 2. Typical Raman spectra of neat NR and SBR.
Table 2 Positions and assignments of bands observed in Raman spectra of neat NR and SBR. Raman shift (cm1) NR
Assignment
SBR
Literature This Literature This work work – – 3055 [23] 3058 (S) 3033 [18] 3041 3003 [24] 3000 (B) 2965 [18] 2964 – – 2940 2911 2850 1664 – – – 1452 1375
[18] [18] [18] [18]
2934 2912 2850 1666 – – – [18] 1448 [19] 1371
2906 – 2846 1664 1639 1601 1583 1431 –
[22] 2910 – [24] 2845 [24] 1668 [24] 1642 [23] 1602 [23] 1582 [24] 1437 –
(B) (B) (S) (S)
1357 [18] 1360 – – 1327 [18] 1327 1301 [24] 1302 (B) 1314 1286 1243 1215 –
[18] [18] [18] [20]
1309 1285 1236 1208 –
– – – – 1267 [24] 1273 – – 1201 [24] 1199 (B)
1130 [18] 1128 – – 1039 [18] 1031 – – – – 1002 [23] 1002 (S) 1000 [18] 997 – – – 620 [21] 491 [18] 496 –
– 622 (S) –
]C–H ring stretching ]C–H stretching CH3 asymmetric stretching CH2 asymmetric stretching CH3 symmetric stretching CH2 symmetric stretching C]C stretching C]C stretching ]C–H ring stretching C]CH ring stretching CH2 deformation CH3 asymmetric deformation CH2 deformation ]C–H in-plane deformation CH2 twisting CH bending CH2 twisting ]C–H in plane bending Symmetric stretching of trans C]C CH2 wagging CH3 rocking Symmetric ring breathing C–CH2 stretching C]CH ring bending ]CC2 rocking and scissoring
B ¼ butadiene segments in SBR and S ¼ styrene segments in SBR.
at 496 and 1000 cm1, respectively, [16,17]. Thus, it is difficult to distinguish the Raman signals of precipitated silica from those of NR. Similar to silica/NR samples, the spectra of various NR/SBR blends containing 30 phr of silica are not significantly different in comparison to those of unfilled blends. Fig. 6 shows the plot of intensity ratio at 1371/ 1302 cm1 as a function of SBR content for both unfilled and silica-filled NR/SBR blends. As can be seen, both unfilled and silica-filled blends give a straight line with R2 quality factor close to 0.99. This result indicates that very little loss of NR and SBR occurs during the coagulation process for preparing dried blends. Furthermore, these calibration curves could be applied for analysis of blend composition in the NR/SBR blends.
3.2. TGA results Fig. 7(a) and (b) shows TGA and DTGA results, respectively, of various NR/SBR blends without silica. From TGA curves (Fig. 7(a)), there are two main steps of decomposition in all samples. The first weight loss in region 350– 500 C is about 98.5% of the total weight loss, corresponding to decomposition of the blended rubbers. The small decomposition having weight loss of about 0.7% occurs at about 600 C after switching to oxygen atmosphere caused by oxidative degradation of carbon char from the rubber decomposition. The residue about 0.8% relates to ash and inorganic substances. As shown in Fig. 7(b), only one decomposition peak is observed at approximately 390 C and 473 C, denoted as decomposition temperature (Tmax), for neat NR and SBR, respectively. Because SBR contains partially aromatic styrene groups, it possesses higher thermal stability than NR [5]. When blending NR
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Fig. 3. Raman spectra of unfilled NR/SBR blends at various ratios.
Fig. 4. Raman spectra of uncompounded NR containing various silica loadings.
with SBR, two decomposition peaks are observed and correspond to characteristic Tmax of the individual rubbers. In the decomposition region of the rubbers (350–500 C), it can be noted that (i) the weight loss at a specified temperature reduces when SBR content in the blends is increased (Fig. 7(a)) and (ii) the peak height at Tmax strongly depends on the blend composition (Fig. 7(b)). TGA and DTGA curves of NR/SBR blends containing silica are illustrated in Fig. 8. After adding silica, Tmax of the blends does not significantly change when compared to that of unfilled blends, but its peak height reduces with increasing silica content due to the dilution effect with respect to volume fraction of rubber. Furthermore, the
residue weight from the TGA curve is found to increase with respect to the combination of ash and silica. Thus, it is necessary to separate them by subtraction in order to know the actual amount of silica in the coagulated blends. The results are summarized in Table 3 and discussed below. Since the Tmax of NR and SBR phases in the blends is significantly different (a difference of about 80 C), the ratios of their peak heights are criterion to distinguish them apart. In this work, the ratio of peak height at Tmax of 473 C and 390 C of SBR and NR, respectively, is plotted against the content of SBR in the NR/SBR blends in order to construct the calibration curve. Fig. 9 shows the relationship between weight of SBR and peak height ratio (HSBR/
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857
2.00 Intensity ratio at 1371/1302 cm
-1
Unfilled blends Silica-filled blends Linear (Unfilled blends) Linear (Silica-filled blends)
1.50
y = -0.0172x + 1.6434
1.00
2
R = 0.9869 y = -0.0166x + 1.5903 2
R = 0.9946
0.50
0.00 0
20
40
60
80
100
% SBR in NR/SBR Fig. 6. Plot of intensity ratio (1371/1302 cm1) versus SBR content in unfilled and 30 phr of silica-filled NR/SBR blends.
Fig. 5. Raman spectra of as-received precipitated silica recorded at different excitation wavelengths; (a) lexc ¼ 647.1 nm and (b) lexc ¼ 514.5 nm.
transition steps of NR and SBR in the blends were correlated with the blend composition. Figs. 10(a) and (b) show, respectively, DSC thermograms of unfilled NR/SBR blends at various blend ratios and silica-filled 50/50 NR/SBR blend at different silica contents. Tg values of NR, SBR and their blends are tabulated in Table 3. Tgs of pure NR and SBR appear at 62 C and 50 C, respectively. After blending, two transitions are observed in the thermograms at around Tg of each elastomer. They are coded as Tg1 and Tg2 for NR and SBR phases, respectively. It can be noted that neither Tg1 nor Tg2 show any deviation from the values of neat NR
HNR) for NR/SBR blends, with and without silica. It is found that polynomial regression is a good fit and yields a correlation coefficient of at least 98% for NR/SBR blends, with and without silica. This could be applicable for analyzing the blend composition of an unknown NR/SBR sample. 3.3. DSC results DSC was employed to determine glass transition temperature (Tg) of NR/SBR blends. Also, heat capacity ratios at
b1
100
Deriv. weight (%/ C)
a1 Weight (%)
o
80
0.0
60 NR/SBR 100/0 70/30 50/50 30/70 0/100
40 20 0
-0.5
-1.5 -2.0
50 150 250 350 450 550 650 750 Temperature (oC)
b2
100
0.0
o
80 Weight (%)
50 150 250 350 450 550 650 750 Temperature (oC)
Deriv. weight (%/ C)
a2
NR/SBR 100/0 70/30 50/50 30/70 0/100
-1.0
60 40 20 0
-0.5 -1.0 -1.5 -2.0
300
350 400 450 500 Temperature (oC)
550
300
350 400 450 500 Temperature (oC)
550
Fig. 7. (a) TGA and (b) DTGA curves of various NR/SBR blends without silica (subscripts 1 and 2 respectively represent the temperature range from 50–750 C and 300–550 C (enlarged curve)).
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0.0 Deriv. weight (%/ºC)
100
Weight (%)
80 60 40
0 phr 10 phr 20 phr 30 phr
20 0
-0.2 -0.4 -0.6
-1.0 -1.2
50 150 250 350 450 550 650 750 Temperature (ºC)
a
50 150 250 350 450 550 650 750 Temperature (ºC)
70/30 NR/SBR 0.0 Deriv. weight (%/ºC)
100 Weight (%)
80 60 40
0 phr 10 phr 20 phr 30 phr
20 0
-0.2 -0.4 -0.6 0 phr 10 phr 20 phr 30 phr
-0.8 -1.0 -1.2
50 150 250 350 450 550 650 750 Temperature (ºC)
50 150 250 350 450 550 650 750 Temperature (ºC)
b
50/50 NR/SBR 0.0 Deriv. weight (%/ºC)
100 80 Weight (%)
0 phr 10 phr 20 phr 30 phr
-0.8
60 40
0 phr 10 phr 20 phr 30 phr
20 0
-0.2 -0.4 -0.6 -0.8
0 phr 10 phr 20 phr 30 phr
-1.0 -1.2 -1.4
50 150 250 350 450 550 650 750 Temperature (ºC)
c
50 150 250 350 450 550 650 750 Temperature (ºC)
30/70 NR/SBR
Fig. 8. TGA and DTGA curves of NR/SBR blends containing different silica contents.
Table 3 Glass transition temperature of various NR/SBR samples measured by DSC. NR/SBR (% w/w)
Silica content (phr)
100/0 70/30 50/50 30/70 0/100 50/50 50/50 50/50
0 0 0 0 0 10 20 30
Glass transition temperature ( C) Tg1
Tg2
62.3 62.1 62.9 62.7 – 62.8 63.4 63.5
– 50.5 52.8 51.8 49.9 53.9 53.7 53.8
and SBR. This infers that there is only physical interlocking, without any chemical interaction, between the molecular chains of NR and SBR in the coagulated blends. This is consistent with the Raman result where no shifts of the assigned bands are observed. Similarly, by adding silica into the NR/SBR blend, two transitions can be seen at approximately the same temperatures as for the neat NR and SBR, indicating immiscible blends. The correlation of DSC results with blend composition was also done. Heat capacity at each transition step was calculated and plotted as a function of SBR content in the blend. As evidenced in Fig. 11, heat capacity data of unfilled and 30 phr of silica-filled NR/SBR blends versus SBR content in the blends can be fitted with polynomial functions. From
S. Prasertsri et al. / Polymer Testing 32 (2013) 852–861
3.0
2.0
1.50
2
y = 0.0003x + 0.0076x + 0.0262
1.5
Unfilled blends Silica-filled blends Poly. (Unfilled blends) Poly. (Silica-filled blends)
1.75 Cp,SBR /C p,NR
HSBR /HNR
2.00
Unfilled blends Silica-filled blends Poly. (Unfilled blends) Poly. (Silica-filled blends)
2.5
859
2
R = 0.9844
1.0
1.25 2
y = 0.0004x - 0.0066x + 0.0138
1.00
2
R = 0.9913
0.75 0.50
2
y = 0.0003x - 0.0039x + 0.0094
2
0.5
y = 0.0003x + 0.0071x + 0.0124
0.25
2
2
R = 0.9948
R = 0.9954
0.00
0.0 0
20
40 60 % SBR in NR/SBR
80
0
100
20
40
60
80
100
% SBR in NR/SBR
Fig. 9. Plot of the ratio of the peak heights (HSBR/HNR) versus SBR content in unfilled and 30 phr of silica-filled NR/SBR blends.
Fig. 11. Plot of the ratio of the heat capacity (Cp,SBR/Cp,NR) versus SBR content of unfilled and 30 phr of silica-filled NR/SBR blends.
this result, it should, therefore, also be possible to determine the composition of NR/SBR blends from this calibration curve.
Table 4 Silica content determined from the results of TGA measurement of silicafilled NR/SBR blends prepared from latex stage. NR/SBR (% w/w)
3.4. Actual amount of silica in NR/SBR blends 100/0
The amounts of silica in the blends calculated from TGA results are summarized in Table 4. At a given blend ratio, it is found that loss of silica increases with increasing silica content. Furthermore, loss of silica is found to increase with increasing SBR content in the blends. The highest silica loss of about 1.3% wt is found in 30/70 NR/SBR blends having 30 phr of silica. The low coagulation efficiency could be due to the lower dry rubber content (DRC) of SBR latex. DRC of SBR latex is 22.56% wt which is approximately three times lower than that of concentrated NR latex. Hence, the higher silica loss would be possible during coagulation of silica/ latex mixture which contains high proportions of both SBR latex and silica suspension in NR/SBR latices. 3.5. Dispersion morphology of silica in NR/SBR blends Figs. 12 and 13 show SEM micrographs of silica-filled NR/SBR blends. The original micrographs are depicted in
70/30
50/50
30/70
Silica content (phr) Theoretical value
Actual value
0 10 20 30 0 10 20 30 0 10 20 30 0 10 20 30
N/A 9.62 0.01 19.48 0.02 29.12 0.06 N/A 9.37 0.08 19.35 0.15 28.78 0.28 N/A 8.97 0.18 18.73 0.17 28.35 0.31 N/A 8.90 0.20 18.32 0.33 27.97 0.42
the upper row of each figure where white spots represent silica, and black areas are rubber matrix. In order to clearly differentiate between the feature of silica structures (aggregates and agglomerates) and rubber phase, the original
Fig. 10. DSC thermograms of (a) unfilled NR/SBR blends at various ratios, and (b) silica-filled 50/50 NR/SBR blend at various silica contents.
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Fig. 12. SEM micrographs of fracture surface of 50/50 NR/SBR blends containing different silica contents (upper; original images and lower; inverse images).
SEM images were converted to negative images (by using ImageJ software) as presented in the lower row of each figure. In contrast to the original micrographs, silica and rubber matrix in the negative micrographs are presented as
black spots and gray areas, respectively [25]. Dispersion morphologies of 50/50 NR/SBR blends containing different silica loadings (0–30 phr) are shown in Fig. 12(a)–(d). At low silica loading, it is observed that silica particles
Fig. 13. SEM micrographs of fracture surface of 30 phr of silica-filled NR/SBR blends at various ratios; (a) 70/30, (b) 50/50 and (c) 30/70 NR/SBR (upper; original images and lower; negative images).
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distribute and disperse well after being processed for preparing the coagulated silica-filled NR/SBR blends. When silica loading is increased, large aggregates can be observed in the blend containing the highest silica loading (30 phr). The morphologies of 30 phr of silica-filled NR/SBR blends at different blend ratios are presented in Fig. 13. It can be seen that the distribution and dispersion state of silica depend on the blend composition. Better distribution and dispersion of silica in rubber matrix can be observed in NR rich blends, as illustrated in Fig. 13(a). For SBR rich blends shown in Fig. 13(c), a few micrometers of silica agglomerates are observed. Additionally, it could be noted that the uneven distribution of silica in 30/70 NR/SBR sample would give more variation or higher standard deviation in the actual amount of silica calculated from TGA results. 4. Conclusions Binary blends of NR and SBR, with and without silica, were prepared by latex co-coagulation. SEM results reveal that very little self-agglomeration of silica occurs during co-coagulation. DSC result shows that NR/SBR blends are immiscible, displaying two Tgs over the entire composition range. This result agrees with the Raman results in which there are neither no new peaks nor significant shifts in the spectra, confirming no chemical moieties were generated in the blending process. In addition, the incorporation of silica into the blends does not affect the characteristic Raman peaks and Tgs of NR and SBR but gives improvement in thermal stability. Raman peaks of NR, SBR and their blends are fully assigned in this paper. It is also found that the quantitative blend compositions can be practically determined from the height of each characteristic peak. Linear regression with R2 quality factor close to 0.99 of the plot between intensity ratio at 1371/1302 cm1 and blend ratios is achieved. Meanwhile, the peak height ratio and heat capacity ratio from TGA and DSC analysis, respectively, yield quadratic equations as a function of blend ratios. All fitting curves could be potentially used either for quality control when coagulating the NR/SBR latices or for quantitative analysis of unknown composition of NR/SBR blends.
Acknowledgements Financial support from the Thailand Research Fund through the Royal Golden Jubilee Ph.D. Program (Grant No. PHD/0286/2549) is deeply acknowledged. The authors would like to express their gratitude to the Rubber Technology Research Centre (RTEC), Mahidol University and the Institute for Molecules and Materials of Le Mans – UMR CNRS 6283, Université du Maine for supporting the instruments used in this work. Sincere appreciation is extended to NETZSCH (Thailand) Ltd. and Mr. Krit Suthacheva for supporting a laboratory agitator bead mill used for preparing silica suspension. The authors are also grateful to Miss Jirawadee Thanatnit, Center of
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Nanoimaging, Faculty of Science, Mahidol University for carrying out the SEM measurement. References [1] K. Jones, A. Tinker, Blends of Natural Rubber: Novel Techniques for Blending with Speciality Polymers, Chapman & Hall, London, 1997. [2] L.L.Y. Visconte, A.F. Martins, J.C.M. Suarez, R.C.R. Nunes, Different preparative modes for the incorporation of additives in NR/SBR blends, J. Appl. Polym. Sci. 93 (2004) 483–489. [3] M. Shundo, M. Imoto, Y. Minoura, Studies on polymer blends: blending methods for natural rubber and styrene-butadiene rubber, J. Appl. Polym. Sci. 10 (1966) 939–953. [4] A. Boudenne, L. Ibos, Y. Candau, S. Thomas, Handbook of Multiphase Polymer Systems, John Wiley and Sons Ltd., Chichester, 2011. [5] A.K. Sircar, T.G. Lamond, Identification of elastomers in tire sections by total thermal analysis. I. Tread and black sidewall, Rubber Chem. Technol. 48 (1975) 301–309. [6] J.J. Maurer, Advances in thermogravimetric analyses of elastomer systems, J. Macromol. Sci. A Chem. 8 (1974) 73–82. [7] I.A. Amraee, A.A. Katbab, S. Aghafarajollah, Qualitative and quantitative analysis of SBR/BR blends by thermogravimetric analysis, Rubber Chem. Technol. 69 (1996) 130–136. [8] G.N. Ghebremeskel, S.R. Shield, Characterization of binary/tertiary blends of SBR, NBR and PVC by IR spectroscopy, Rubber World 227 (2003) 26–30. [9] R. Appel, T.W. Zerda, W.H. Waddell, Raman microimaging of polymer blends, Appl. Spectrosc. 54 (2000) 1559–1566. [10] J.K. Clark, R.A. Scott, Characterization of sulfur-cured styrene-butadiene copolymer rubbers and their polybutadiene blends, J. Appl. Polym. Sci. 14 (1970) 1–9. [11] S. Prasertsri, N. Rattanasom, Mechanical and damping properties of silica/natural rubber composites prepared from latex system, Polym. Test. 30 (2011) 515–526. [12] S. Prasertsri, N. Rattanasom, Fumed and precipitated silica reinforced natural rubber composites prepared from latex system: mechanical and dynamic properties, Polym. Test. 31 (2012) 593– 605. [13] B. Schrader, Special Techniques and Applications: Infrared and Raman Spectroscopy, Wiley-VCH Verlag GmbH, Weinheim, 2007. [14] J.L. Koenig, Spectroscopy of Polymers: Raman Spectroscopy of Polymers, Elsevier Science Inc., New York, 1999. [15] K.D.O. Jackson, M.J.R. Loadman, C.H. Jones, G. Ellis, Fourier transform Raman spectroscopy of elastomers: an overview, Spectrochim. Acta A Mol. Spectrosc. 46 (1990) 217–226. [16] S.W. Cornell, J.L. Koenig, Raman spectra of polyisoprene rubbers, Macromolecules 2 (1969) 546–549. [17] P.J. Hendra, K.D.O. Jackson, Applications of Raman spectroscopy to the analysis of natural rubber, Spectrochim. Acta A Mol. Spectrosc. 50 (1994) 1987–1997. [18] J. Samran, P. Phinyocheep, P. Daniel, D. Derouet, J.Y. Buzare, Raman spectroscopic study of non-catalytic hydrogenation of unsaturated rubbers, J. Raman Spectrosc. 35 (2004) 1073–1080. [19] P. Nallasamy, S. Mohan, Vibrational spectra of cis-1,4-polyisoprene, Arab. J. Sci. Eng. 29 (2004) 17–26. [20] V. Arjunan, S. Subramanian, S. Mohan, Fourier transform infrared and Raman spectral analysis of trans-1,4-polyisoprene, Spectrochim. Acta A Mol. Biomol. Spectrosc. 57 (2001) 2547–2554. [21] G. Martinez-Barrera, H. Lopez, V.M. Castano, R. Rodriguez, Studies on the rubber phase stability in gamma irradiated polystyrene-SBR blends by using FT-IR and Raman spectroscopy, Radiat. Phys. Chem. 69 (2004) 155–162. [22] J. Guilment, L. Bokobza, Determination of polybutadiene microstructures and styrene-butadiene copolymers composition by vibrational techniques combined with chemometric treatment, Vib. Spectrosc. 26 (2001) 133–149. [23] S. Prakanrat, P. Phinyocheep, P. Daniel, Spectroscopic investigation of polystyrene surface grafting on natural rubber, Appl. Spectrosc. 63 (2009) 233–238. [24] S.W. Cornell, J.L. Koenig, The Raman spectra of polybutadiene rubbers, Macromolecules 2 (1969) 540–545. [25] Y. Konishi, M. Cakmak, Nanoparticle induced network self-assembly in polymer-carbon black composites, Polymer 47 (2006) 5371– 5391.