- Email: [email protected]
SWNT composite
Polymer Degradation and Stability 83 (2004) 383–388 www.elsevier.com/locate/polydegstab
Thermal analysis of an acrylonitrile–butadiene–styrene/SWNT composite Shuying Yanga, Jose Rafael Castillejaa, E.V. Barrerab, Karen Lozanoa,* a Department of Mechanical Engineering, University of Texas Pan American, Edinburg, TX 78539, USA Department of Mechanical Engineering and Materials Science, Rice University, Houston, TX 77005, USA
b
Received 12 May 2003; received in revised form 28 July 2003; accepted 2 August 2003
Abstract Thermogravimetric analysis and differential thermal analysis were used to study the thermal degradation of acrylonitrile–butadiene–styrene/single-walled carbon nanotube composites under static air and nitrogen. For the dynamic analysis, two-step degradation was observed. Low concentrations of SWNTs (0.5 wt.%) tend to destabilize the first degradation step of ABS but reinforce it at the second step degradation. Further addition of SWNT, destabilizes the whole system by lowering both degradation peak temperatures. The activation energies of isothermal degradation were evaluated by using Freeman’s model. Under both static air and nitrogen, the activation energy was lowered by the SWNTs. DSC analysis showed that the introduction of SWNTs increases the glass transition temperature of the composites and low concentration of SWNTs act as nucleating agents to the crystallization of ABS. # 2003 Elsevier Ltd. All rights reserved. Keywords: Thermal degradation; ABS; SWNT; Carbon nanotube; Polymer composite
1. Introduction Acrylonitrile–butadiene–styrene (ABS) is an important engineering copolymer widely used in industry due to superior mechanical properties, chemical resistance, ease of processing and recyclability. Various applications of ABS are in building and construction, personal care products, toys, computer and business equipment, medical devices and in automotive interior components [1]. ABS is also used with fiber-reinforcements to enhance the mechanical properties. Given the remarkable mechanical, electrical, and thermal properties of carbon nanotubes, their use as reinforcements for polymer composites has receieved considerable attention [2–6]. Several studies are now being conducted on carbon nanotube reinforced ABS composites and fundamental understanding of the tube–matrix interactions are needed to better understand and manipulate macro properties [7]. Thermogravimetric analysis (TGA) and differential thermal analysis (DTA) are common methods to investigate the thermal stability of polymers and composites [8]. With proper experimental procedures, information about the degradation kinetics of decomposition can be * Corresponding author. Fax.: 956-381-3527. E-mail address: [email protected] (K. Lozano). 0141-3910/$ - see front matter # 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymdegradstab.2003.08.002
obtained. There are several reports about the thermal analysis (TGA, DTA, FTIR, DSC) of ABS as well its component polymers under various conditions (purging gases, heating rates, etc.) [9–18]. The degradation of ABS is a radical process where end-chain and random scissions occurs. Based on different characterizing conditions, the degradation of ABS was found to occur in either one step or two steps with different kinetic parameters (degradation starting temperature, ending temperature, peak temperature, reaction order, preexponential factor and activation energy) by assuming different kinetic models. Differential scanning calorimetry (DSC) is another common method used to characterize the thermal properties of polymers. Here the TGA/DTA and DSC results on acrylonitrile-butadienestyrene/single wall nanotube (ABS/SWNT) composites with different weight percentages of SWNTs are reported.
2. Experimental section 2.1. Materials ABS in powder form with approximately 50% of butadiene content was purchased from Aldrich Chemical Company (No. 180882), with density of 1.04 g/ml.
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The single-walled carbon nanotubes (SWNT) were purchased in toluene slurry from Tubes@rice. These were purified laser grown carbon nanotubes, with concentrations ranging from 2.5 to 7.8 g/l. The sample preparation of the ABS/SWNT composites is published elsewhere [7]. ABS composites of 0.0, 0.5, 1.0, 3.5, 5.0, and 10.0 wt.% of SWNTs were analyzed. 2.2. Characterization The thermogravimetric analysis was performed using a Model 2960 of TA Instruments under static air and nitrogen with a flow rate of 25 ml/min. The samples were ramped from room temperature till 1000 C at a scanning rate of 10 C/min for the dynamic analysis. The isothermal degradation at 370, 400, 430, and 560 C were also studied. Sample weight was about 5–10 mg. Platinum pans were used. DSC analysis was performed on a Model 2010 of TA Instruments with closed Aluminum pans. The samples were heated from room temperature to 200 C with a heating rate of 5 C/min.
3. Results and discussion 3.1. TGA dynamic thermal stability analysis
evolution of monomeric acrylonitrile began at 400 C [9]. Dong et al. investigated the degradation of ABS with different TGA testing conditions, the evolution of volatile substances was observed throughout the whole process [13]. In the present study, for the second step degradation, there is only one peak in the DTA curve, which means there is only one major degradation reaction. As for the thermal degradation of ABS, some researchers observed one step [8–10, 12–16,18], others observed two steps [10,18]. The discrepancy comes from the sample itself (molecular weight, acrylonitrile, butadiene, and styrene component ratios), sample weight, sample volume, heating rate, purging gases, purging gas rate, etc. All of these factors affect the actual degradation of ABS. To investigate the effect of SWNT on the degradation of ABS, the TGA degradation curves of 330–450 and 455–630 C under static air of various composites with different SWNT loading are plotted in Fig. 2(a) and (b) respectively. All the samples degraded in two major steps, similar to the pure ABS material. A closer look at the curves finds that the addition of SWNTs accelerates the initial degradation of ABS composites. This is an unexpected result since it is known that nanotubes have very high thermal stability as reported by Pang et al. where TGA analysis showed that carbon nanotubes start to degrade at 550 C in air atmosphere [19]. The
The degradation of pure ABS under static air was analyzed and the TGA and DTA curves are presented in Fig. 1. It is clear that ABS degrades in two steps. The first step initiates at 180 C and ends at 480 C and the second step from 480 until 620 C. From the DTA curve, at least three peaks can be observed overlapping each other for the first degradation step, this means that the first degradation is not a simple chemical reaction but several reactions occurring simultaneously. Suzuki et al. applied thermogravimetry/Fourier transform infrared (TGA/FTIR) to investigate the degradation of ABS and they found that the evolution of butadiene commenced at 340 C and styrene at 350 C, while the
Fig. 1. TGA curve of pure ABS under static air showing the two-step degradation.
Fig. 2. Dynamic TGA of ABS/SWNT composites under static air. Note the earlier initial degradation of composites with SWNTs.
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maximum rate of weight loss occurred at 695 C at a heating rate of 1 C/min and the nanotubes do not completely disappeared until about 740 C [19]. Since the degradation of ABS is known as a radical process [13], and the butadiene-rich domains are the most vulnerable sites for degradation [17], SWNTs may have participated in the radical initiation process. In the second degradation step, the degradation of ABS and SWNTs overlapped and importantly, at around 600 C, the majority of SWNTs have already degraded. This is quite unexpected considering Pang et al.’s work [19]. SWNTs take part in the initiation of the degradation of ABS and degrade themselves along with ABS. The summary of the TGA and DTA results for the various composites in static air is presented in Table 1. Compared to the pure ABS sample, the first degradation step was lowered 9 C with just 0.5% SWNTs. The maximum lowering here is 17 C with 10% SWNTs. The weight loss percentages of ABS of various composites are listed in the last column in Table 1. Compared to pure ABS, it is clear that all the ABS/SWNT composites have about 4% higher ABS weight loss, which is consistent to the former claim that SWNTs catalyze the degradation of ABS at the initial step. For the second degradation step, relatively small amount of SWNTs (see 0.5%) raised the peak temperature 40 C, with the further increase of SWNTs, the amount of peak temperature rise gets smaller. There is about 17 C peak temperature rise at 3.5% SWNTs. The rise of peak temperature shows a reinforcement effect. However, with the further increase of SWNTs (10 wt.%), the peak temperature was shifted from 554 to 536 C, an 18 C change. The weight loss in the second step is much smaller than the first step, about 10–19% compared to 80–89% for the first step. The final residue as observed in Table 1 for all the samples is small, generally below 1%, except for the 10% SWNTs having a residue of 1.6%. The standard deviation of the residue % is 0.46%. For the degradation behavior of the composites under nitrogen, the TGA curves are presented in Fig. 3. Fig. 3(a) represents the curves from 330 to 450 C and Fig. 3(b) from 450 to 750 C. The curves have similar shape as those under static air. The degradation is also a two-step process with a major weight loss happening in
the first step. Compared to the degradation under air, the curves here have longer and larger tails because without the participation of oxygen, the thermal degradation goes slower. For a closer analysis, the summary of TGA and DTA is listed in Table 2. Similar as in static air, the addition of SWNTs initiates the first step degradation at lower temperatures than for pure ABS. The catalytic effect of SWNT is more obvious as SWNT concentration increases: with 5% of SWNT, the peak temperature is lowered from 443 to 416 C. It is 27 C degrees lower. For the second degradation step, SWNTs have similar behavior as in static air; a very small
Fig. 3. Dynamic TGA of ABS/SWNT composites under nitrogen. Note the earlier initial degradation of composites with SWNTs.
Table 1 Results of TGA and DTA analysis of ABS/SWNT composites under static air SWNT (%)
T1 ( C)
T2 ( C)
WL1(%)
WL2(%)
Residue (%)
WL% of ABS in step 1
0.0 0.5 1.0 3.5 5.0 10.0
445 436 434 438 430 428
554 594 575 571 544 536
85.6 89.1 87.7 86.9 85.4 78.7
13.8 10.9 12.0 12.6 13.8 19.7
0.6 0.0 0.3 0.5 0.8 1.6
85.6 89.5 88.6 90.1 89.9 87.4
Where T1 ( C) and T2 ( C) are the temperatures corresponding to maximum degradation rate in step 1 and step 2, respectively. WL1(%) and WL2(%) are the corresponding composite weight loss percentage. The standard deviation of temperature is 0.47 C and that of residue is 0.46%.
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Table 2 Results of TGA and DTA analysis of ABS/SWNT composites under nitrogen SWNT (%)
T1( C)
T2( C)
WL1(%)
WL2(%)
Residue (%)
WL% of ABS in step 1
0.0 0.5 1.0 2.0 3.5 5.0 10.0
443 443 421 443 437 416 414
623 674 612 566 597 585 587
93.0 97.6 91.1 95.4 90.4 88.8 89.1
6.7 1.9 8.7 4.3 9.4 11.0 5.6
0.3 0.5 0.2 0.3 0.2 0.2 5.3
93 98 92 97 94 93 99
Where column labels have the same meaning as in Table 1. The standard deviation of temperature is 0.47 C and that of residue is 0.46%.
Fig. 6. Thermogravimetric curves of ABS at different isothermal temperatures under nitrogen.
3.2. TGA Isothermal thermal degradation analysis Fig. 4. Thermogravimetric curves of Pure ABS at different isothermal temperatures in air.
Fig. 5. Thermogravimetric curves of 5% SWNT/ABS at different isothermal temperatures in air.
amount of SWNT (0.5%) reinforces the composites by raising the peak temperature by 50 C. However, further increases of SWNT results in earlier degradation of the system, the peak temperature was continuously lowered to 585 C with 5% of SWNT. The final residue is small, about 0.3–0.5% with the exception of the 10 wt.% SWNT sample where the residue is 5.3%.
TGA curves of the ABS/SWNT composites at various isothermal temperatures, 370, 400, 430, 560 C under static air and nitrogen are shown in Figs. 4–8. The Y-axis is the ln (Residue wt.%). Specifically, the composites with 0–3.5 wt.% SWNTs under static air have very similar TGA curves as the one shown in Fig. 4. The TGA curve of the composite with 5 wt.% SWNTs under air is shown in Fig. 5. The most obvious difference is that the sample with 5 wt.% SWNTs has a steeper tail at 560 C. By checking the time needed to degrade to zero 1n(wt.%) for the various samples at 560 C isothermal degradation, it is found that the addition of SWNTs destabilize the system: all the samples with SWNTs have a shorter time than pure ABS (8.4 min). Figs. 6–8 present the TGA of the composites under nitrogen with 0, 0.5 and 2 wt.% SWNTs, respectively. Again the most obvious difference is observed for 560 C isothermal degradations. As shown in Figs. 6 and 7, by adding only 0.5 wt.% SWNTs, the time needed to degrade to zero 1n(wt.%) is almost doubled, from 8.7 to 16 min, indicating that a small amount of SWNTs stabilize ABS under nitrogen. This is consistent to the dynamic result that under nitrogen, the residue is larger for the 0.5 wt.% SWNTs sample than pure ABS (0.5 vs. 0.3%). However, further increase of SWNTs
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Table 3 Activation energies obtained from isothermal analysis for ABS/SWNT composites
Fig. 7. Thermogravimetric curves of 0.5% SWNT/ABS at different isothermal temperatures under nitrogen.
Fig. 8. Thermogravimetric curves of 2% SWNT/ABS at different isothermal temperatures under nitrogen.
does not help stabilizing the system, the time needed to degrade to zero 1n(wt.%) decreases, as shown in Fig. 8. In summary, as shown in Figs. 4–8, the residue weight decreased with an increasing of the heating time and expectedly at higher isothermal temperature, the weight loss is larger. The kinetic parameters of the thermal degradation can be obtained from a single TGA curve. The method of Freeman and Carroll has widely been used to determine the order and the activation energy of the degradation [8]. The rate of degradation may be written as:
dW ¼ kd W n dt
ð1Þ
Where kd is the rate constant; W is the weight remaining,%; n is the order of the reaction;. If assuming first order decomposition, then lnW0 lnW ¼ kd t
ð2Þ
lnW ¼ lnW0 kd t
ð3Þ
where W0 is the initial weight. From Figs. 4–8, it is found that for the isothermal at 370 C, a straight line is obtained which means that the degradation at 370 C can be regarded as a first order decomposition with a steady rate constant. However, for the isothermal at 400, 430 C, all the TGA curves have a concave shape
SWNT (%)
Ea, under nitrogen (Kcal/mol)
Ea, under static air (Kcal/mol)
0.0 0.5 1.0 2.0 3.5 5.0 10.0
32.1 31.2 30.4 28.9 29.1 28.9 29.4
37.3 32.4 31.9 31.6 30.9 31.3
with the reaction rate getting smaller and smaller meaning that either the order of reaction is not one, or the rate constant is not a constant, or both. Finally, for the isothermal at 560 C, as shown in the figures, after the initiation of degradation, all of the samples have a straight line till around 5 min, when the curves deviate. The degradation at 560 C can be considered as a first order reaction with different rate constants since the slopes of the lines are different. Using Freeman and Carroll’s model [8] as stated in Eqs. (1)–(3), the kinetic parameters of the degradation under various conditions, were calculated assuming a first order decomposition. The rate constants, kd were obtained as the slopes of the curves in Figs. 4–8. Additionally, kd ¼ AexpðEa =RTÞ
ð4Þ
where A is the pre-exponential factor, Ea is the activation energy (Kcal/mol), R is the universal gas constant (Kcal/Kg-molK); and T is the absolute temperature (K). From this relationship, the activation energies were evaluated. The results were summarized in Table 3. Compared to pure ABS, the addition of SWNTs decreased the activation energies under both static air and nitrogen. The decrease is more obvious in the situation of static air. This is consistent to the former finding that SWNTs destabilize ABS. 3.3. DSC results DSC results on ABS/SWNT composites evaluated in the temperature range of room temperature to 200 C showed that the addition of SWNTs increased the glass transition temperature (Tg) of the composites from 101 C to about 107 C. All of the samples, except for the two composites with 0.5 and 1 wt.% SWNTs did not show melting peaks, indicating that the composites are amorphous. However, for the two samples with 0.5 and 1 wt.% of SWNTs, small melting peaks were observed indicating nucleation ability of the SWNT as reported by several researchers that have worked with semicrystalline
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matrices [20]. For samples containing more than 1 wt.% SWNTs, the composites did not show a melting peak, suggesting that the rigidity of SWNTs interferes with the mobility of the polymer chains and crystallization is not able to occur.
4. Conclusion Thermogravimetry analysis and differential thermal analysis were used to study the thermal degradation of ABS/SWNT composites under static air and nitrogen. For the dynamic analysis, a two-step degradation of the ABS was observed. It was found that the addition of SWNTs destabilized ABS and the composites began to degrade at lower temperatures. However, a small amount of SWNTs reinforce the second degradation step and the peak temperature increased. Further addition of SWNTs again destabilizes the system lowering the peak temperature by 38 C compared to the pure ABS. The analysis of activation energy further supports the claim that the addition of SWNTs destabilize ABS under both static air and nitrogen, the activation energy was lowered by the addition of SWNTs. DSC analysis showed that the addition of SWNTs increases the glass transition temperature of the composites and a small amount of SWNTs might act as nucleating agents to the crystallization of ABS. Further investigation is needed to fully explain the role of SWNTs in the radical degradation initiation.
Acknowledgements Financial supports for this work from National Science Foundation under grant numbers: CMS 0092621 and CMS 0078990 are gratefully acknowledged.
References [1] Salamone J, editor. Concise polymeric materials encyclopedia. Boca Raton: CRC Press; 1999. [2] Thess A, Lee R, Nikolaev P, Dai H, Petit P, Robert J, et al. Crystalline ropes of metallic carbon nanotubes. Science 1996; 273(5274):483. [3] Hone J, Whitney M, Zettl A. Thermal conductivity of singlewalled carbon nantubes. Synthetic Metals 1999;103:2498–9.
[4] Yu MF, Files BS, Arepalli S, Ruoff RS. Tensile loading of ropes of single wall carbon nanotubes and their mechanical properties. Physical Review Letters 2000;84(24):5552–5. [5] Walters DA, Ericson LM, Casavant MJ, Liu J, Colbert DT, Smith KA, et al. Elastic strain of freely suspended single-wall carbon nanotube ropes. Applied Physics Letters 1999;74(25): 3803–5. [6] Rinzler G, Liu J, Dai H, Nikolaev P, Huffman CB, RodriguezMacias FJ, et al. Large-scale purification of single-wall carbon nanotubes: process, product, and characterization. Appl Phys A: Materials Science and Processing 1998;67(1):29–37. [7] Shofner ML, Rodrı´guez-Macı´as FJ, Vaidyanathan R, Barrera EV. Composites Part A: Applied Science and Manufacturing, (Submitted for publication). [8] Yang MH. Thermal degradation of acrylonitrile-butadiene-styrene terpolymer under various gas conditions. Polymer Testing 2000;19(1):105–10. [9] Suzuki M, Wilkie CA. Thermal degradation of acrylonitrilebutadiene-styrene terpolymer as studied by TGA/FTIR. Polymer Degradation and Stability 1995;47(2):217–21. [10] Shapi MM. TG and DSC studies of some thermal properties and stability aspects of poly(acrylonitrile butadiene styrene), polysterene and poly(acrylonitrile styrene) plastics. Thermochimica Acta 1991;175:25–34. [11] Suzuki M, Wilkie CA. Thermal degradation of acrylonitrilebutadiene-styrene terpolymer grafted with methacrylic acid. Polymer Degradation and Stability 1995;47(2):223–8. [12] Luda di Cortemiglis HP, Camino G, Costa L, Guaita M. Thermal degradation of ABS. Thermochimica Acta 1985;93:187–90. [13] Dong D, Tasaka S, Aikawa S, Kamiya S, Inagaki N, Inoue Y. Thermal degradation of acrylonitrile-butadiene-styrene terpolymer in bean oil. Polymer Degradation and Stability 2001;73:319– 26. [14] Tjong SC, Jiang W. Mechanical and thermal properties of Poly(acrylonitrile-butadiene styrene) copolymer reinforced with potassium titanate whiskers. Journal of Applied Polymer Science 1999; 73:2985–91. [15] Deacon C, Wilkie CA. Graft copolymerization of acrylic acid on to acrylonitrile-butadiene-styrene terpolymer and thermal analysis of the copolymers. Eur Polym J 1996;32(4):451–5. [16] Klaric I, Roje U, Stipanelov N. Kinetic investigation of thermooxidative degradation of poly(vinyl chloride)/ acrylonitrile-butadiene-styrene blends by isothermal thermogravimetric analysis. Journal of Applied Polymer Science 1999;71:833–9. [17] Krzystof K, Motyakin MV, Schlick S. ID and 2D electron spin resonance imaging (ESRI) of nitroxide radicals in stabilized poly(acrylonitrile-butadiene-styrene)(ABS): UV vs. thermal degradation. J Phys Chem B 2000;104:3387–92. [18] Fatu D, Geambas G, Segal E, Budrugeac P, Ciutacu S. Thermochimica Acta 1989;149:181–7. [19] Pang LSK, Saxby JD, Chatfield SP. Thermogravimetric analysis of carbon nanotubes and nanoparticles. The Journal of Physical Chemistry 1993;97(27):6941–2. [20] Bhattacharyya AR, Sreekumar TV, Liu T, Kumar S, Ericson LM, Hauge RH, et al. Crystallization and orientation studies in polypropylene/single wall carbon nanotube composite. Polymer 2003;44(8):2373–7.
Thermal analysis of an acrylonitrile–butadiene–styrene/SWNT composite Shuying Yanga, Jose Rafael Castillejaa, E.V. Barrerab, Karen Lozanoa,* a Department of Mechanical Engineering, University of Texas Pan American, Edinburg, TX 78539, USA Department of Mechanical Engineering and Materials Science, Rice University, Houston, TX 77005, USA
b
Received 12 May 2003; received in revised form 28 July 2003; accepted 2 August 2003
Abstract Thermogravimetric analysis and differential thermal analysis were used to study the thermal degradation of acrylonitrile–butadiene–styrene/single-walled carbon nanotube composites under static air and nitrogen. For the dynamic analysis, two-step degradation was observed. Low concentrations of SWNTs (0.5 wt.%) tend to destabilize the first degradation step of ABS but reinforce it at the second step degradation. Further addition of SWNT, destabilizes the whole system by lowering both degradation peak temperatures. The activation energies of isothermal degradation were evaluated by using Freeman’s model. Under both static air and nitrogen, the activation energy was lowered by the SWNTs. DSC analysis showed that the introduction of SWNTs increases the glass transition temperature of the composites and low concentration of SWNTs act as nucleating agents to the crystallization of ABS. # 2003 Elsevier Ltd. All rights reserved. Keywords: Thermal degradation; ABS; SWNT; Carbon nanotube; Polymer composite
1. Introduction Acrylonitrile–butadiene–styrene (ABS) is an important engineering copolymer widely used in industry due to superior mechanical properties, chemical resistance, ease of processing and recyclability. Various applications of ABS are in building and construction, personal care products, toys, computer and business equipment, medical devices and in automotive interior components [1]. ABS is also used with fiber-reinforcements to enhance the mechanical properties. Given the remarkable mechanical, electrical, and thermal properties of carbon nanotubes, their use as reinforcements for polymer composites has receieved considerable attention [2–6]. Several studies are now being conducted on carbon nanotube reinforced ABS composites and fundamental understanding of the tube–matrix interactions are needed to better understand and manipulate macro properties [7]. Thermogravimetric analysis (TGA) and differential thermal analysis (DTA) are common methods to investigate the thermal stability of polymers and composites [8]. With proper experimental procedures, information about the degradation kinetics of decomposition can be * Corresponding author. Fax.: 956-381-3527. E-mail address: [email protected] (K. Lozano). 0141-3910/$ - see front matter # 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymdegradstab.2003.08.002
obtained. There are several reports about the thermal analysis (TGA, DTA, FTIR, DSC) of ABS as well its component polymers under various conditions (purging gases, heating rates, etc.) [9–18]. The degradation of ABS is a radical process where end-chain and random scissions occurs. Based on different characterizing conditions, the degradation of ABS was found to occur in either one step or two steps with different kinetic parameters (degradation starting temperature, ending temperature, peak temperature, reaction order, preexponential factor and activation energy) by assuming different kinetic models. Differential scanning calorimetry (DSC) is another common method used to characterize the thermal properties of polymers. Here the TGA/DTA and DSC results on acrylonitrile-butadienestyrene/single wall nanotube (ABS/SWNT) composites with different weight percentages of SWNTs are reported.
2. Experimental section 2.1. Materials ABS in powder form with approximately 50% of butadiene content was purchased from Aldrich Chemical Company (No. 180882), with density of 1.04 g/ml.
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S. Yang et al. / Polymer Degradation and Stability 83 (2004) 383–388
The single-walled carbon nanotubes (SWNT) were purchased in toluene slurry from Tubes@rice. These were purified laser grown carbon nanotubes, with concentrations ranging from 2.5 to 7.8 g/l. The sample preparation of the ABS/SWNT composites is published elsewhere [7]. ABS composites of 0.0, 0.5, 1.0, 3.5, 5.0, and 10.0 wt.% of SWNTs were analyzed. 2.2. Characterization The thermogravimetric analysis was performed using a Model 2960 of TA Instruments under static air and nitrogen with a flow rate of 25 ml/min. The samples were ramped from room temperature till 1000 C at a scanning rate of 10 C/min for the dynamic analysis. The isothermal degradation at 370, 400, 430, and 560 C were also studied. Sample weight was about 5–10 mg. Platinum pans were used. DSC analysis was performed on a Model 2010 of TA Instruments with closed Aluminum pans. The samples were heated from room temperature to 200 C with a heating rate of 5 C/min.
3. Results and discussion 3.1. TGA dynamic thermal stability analysis
evolution of monomeric acrylonitrile began at 400 C [9]. Dong et al. investigated the degradation of ABS with different TGA testing conditions, the evolution of volatile substances was observed throughout the whole process [13]. In the present study, for the second step degradation, there is only one peak in the DTA curve, which means there is only one major degradation reaction. As for the thermal degradation of ABS, some researchers observed one step [8–10, 12–16,18], others observed two steps [10,18]. The discrepancy comes from the sample itself (molecular weight, acrylonitrile, butadiene, and styrene component ratios), sample weight, sample volume, heating rate, purging gases, purging gas rate, etc. All of these factors affect the actual degradation of ABS. To investigate the effect of SWNT on the degradation of ABS, the TGA degradation curves of 330–450 and 455–630 C under static air of various composites with different SWNT loading are plotted in Fig. 2(a) and (b) respectively. All the samples degraded in two major steps, similar to the pure ABS material. A closer look at the curves finds that the addition of SWNTs accelerates the initial degradation of ABS composites. This is an unexpected result since it is known that nanotubes have very high thermal stability as reported by Pang et al. where TGA analysis showed that carbon nanotubes start to degrade at 550 C in air atmosphere [19]. The
The degradation of pure ABS under static air was analyzed and the TGA and DTA curves are presented in Fig. 1. It is clear that ABS degrades in two steps. The first step initiates at 180 C and ends at 480 C and the second step from 480 until 620 C. From the DTA curve, at least three peaks can be observed overlapping each other for the first degradation step, this means that the first degradation is not a simple chemical reaction but several reactions occurring simultaneously. Suzuki et al. applied thermogravimetry/Fourier transform infrared (TGA/FTIR) to investigate the degradation of ABS and they found that the evolution of butadiene commenced at 340 C and styrene at 350 C, while the
Fig. 1. TGA curve of pure ABS under static air showing the two-step degradation.
Fig. 2. Dynamic TGA of ABS/SWNT composites under static air. Note the earlier initial degradation of composites with SWNTs.
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S. Yang et al. / Polymer Degradation and Stability 83 (2004) 383–388
maximum rate of weight loss occurred at 695 C at a heating rate of 1 C/min and the nanotubes do not completely disappeared until about 740 C [19]. Since the degradation of ABS is known as a radical process [13], and the butadiene-rich domains are the most vulnerable sites for degradation [17], SWNTs may have participated in the radical initiation process. In the second degradation step, the degradation of ABS and SWNTs overlapped and importantly, at around 600 C, the majority of SWNTs have already degraded. This is quite unexpected considering Pang et al.’s work [19]. SWNTs take part in the initiation of the degradation of ABS and degrade themselves along with ABS. The summary of the TGA and DTA results for the various composites in static air is presented in Table 1. Compared to the pure ABS sample, the first degradation step was lowered 9 C with just 0.5% SWNTs. The maximum lowering here is 17 C with 10% SWNTs. The weight loss percentages of ABS of various composites are listed in the last column in Table 1. Compared to pure ABS, it is clear that all the ABS/SWNT composites have about 4% higher ABS weight loss, which is consistent to the former claim that SWNTs catalyze the degradation of ABS at the initial step. For the second degradation step, relatively small amount of SWNTs (see 0.5%) raised the peak temperature 40 C, with the further increase of SWNTs, the amount of peak temperature rise gets smaller. There is about 17 C peak temperature rise at 3.5% SWNTs. The rise of peak temperature shows a reinforcement effect. However, with the further increase of SWNTs (10 wt.%), the peak temperature was shifted from 554 to 536 C, an 18 C change. The weight loss in the second step is much smaller than the first step, about 10–19% compared to 80–89% for the first step. The final residue as observed in Table 1 for all the samples is small, generally below 1%, except for the 10% SWNTs having a residue of 1.6%. The standard deviation of the residue % is 0.46%. For the degradation behavior of the composites under nitrogen, the TGA curves are presented in Fig. 3. Fig. 3(a) represents the curves from 330 to 450 C and Fig. 3(b) from 450 to 750 C. The curves have similar shape as those under static air. The degradation is also a two-step process with a major weight loss happening in
the first step. Compared to the degradation under air, the curves here have longer and larger tails because without the participation of oxygen, the thermal degradation goes slower. For a closer analysis, the summary of TGA and DTA is listed in Table 2. Similar as in static air, the addition of SWNTs initiates the first step degradation at lower temperatures than for pure ABS. The catalytic effect of SWNT is more obvious as SWNT concentration increases: with 5% of SWNT, the peak temperature is lowered from 443 to 416 C. It is 27 C degrees lower. For the second degradation step, SWNTs have similar behavior as in static air; a very small
Fig. 3. Dynamic TGA of ABS/SWNT composites under nitrogen. Note the earlier initial degradation of composites with SWNTs.
Table 1 Results of TGA and DTA analysis of ABS/SWNT composites under static air SWNT (%)
T1 ( C)
T2 ( C)
WL1(%)
WL2(%)
Residue (%)
WL% of ABS in step 1
0.0 0.5 1.0 3.5 5.0 10.0
445 436 434 438 430 428
554 594 575 571 544 536
85.6 89.1 87.7 86.9 85.4 78.7
13.8 10.9 12.0 12.6 13.8 19.7
0.6 0.0 0.3 0.5 0.8 1.6
85.6 89.5 88.6 90.1 89.9 87.4
Where T1 ( C) and T2 ( C) are the temperatures corresponding to maximum degradation rate in step 1 and step 2, respectively. WL1(%) and WL2(%) are the corresponding composite weight loss percentage. The standard deviation of temperature is 0.47 C and that of residue is 0.46%.
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Table 2 Results of TGA and DTA analysis of ABS/SWNT composites under nitrogen SWNT (%)
T1( C)
T2( C)
WL1(%)
WL2(%)
Residue (%)
WL% of ABS in step 1
0.0 0.5 1.0 2.0 3.5 5.0 10.0
443 443 421 443 437 416 414
623 674 612 566 597 585 587
93.0 97.6 91.1 95.4 90.4 88.8 89.1
6.7 1.9 8.7 4.3 9.4 11.0 5.6
0.3 0.5 0.2 0.3 0.2 0.2 5.3
93 98 92 97 94 93 99
Where column labels have the same meaning as in Table 1. The standard deviation of temperature is 0.47 C and that of residue is 0.46%.
Fig. 6. Thermogravimetric curves of ABS at different isothermal temperatures under nitrogen.
3.2. TGA Isothermal thermal degradation analysis Fig. 4. Thermogravimetric curves of Pure ABS at different isothermal temperatures in air.
Fig. 5. Thermogravimetric curves of 5% SWNT/ABS at different isothermal temperatures in air.
amount of SWNT (0.5%) reinforces the composites by raising the peak temperature by 50 C. However, further increases of SWNT results in earlier degradation of the system, the peak temperature was continuously lowered to 585 C with 5% of SWNT. The final residue is small, about 0.3–0.5% with the exception of the 10 wt.% SWNT sample where the residue is 5.3%.
TGA curves of the ABS/SWNT composites at various isothermal temperatures, 370, 400, 430, 560 C under static air and nitrogen are shown in Figs. 4–8. The Y-axis is the ln (Residue wt.%). Specifically, the composites with 0–3.5 wt.% SWNTs under static air have very similar TGA curves as the one shown in Fig. 4. The TGA curve of the composite with 5 wt.% SWNTs under air is shown in Fig. 5. The most obvious difference is that the sample with 5 wt.% SWNTs has a steeper tail at 560 C. By checking the time needed to degrade to zero 1n(wt.%) for the various samples at 560 C isothermal degradation, it is found that the addition of SWNTs destabilize the system: all the samples with SWNTs have a shorter time than pure ABS (8.4 min). Figs. 6–8 present the TGA of the composites under nitrogen with 0, 0.5 and 2 wt.% SWNTs, respectively. Again the most obvious difference is observed for 560 C isothermal degradations. As shown in Figs. 6 and 7, by adding only 0.5 wt.% SWNTs, the time needed to degrade to zero 1n(wt.%) is almost doubled, from 8.7 to 16 min, indicating that a small amount of SWNTs stabilize ABS under nitrogen. This is consistent to the dynamic result that under nitrogen, the residue is larger for the 0.5 wt.% SWNTs sample than pure ABS (0.5 vs. 0.3%). However, further increase of SWNTs
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Table 3 Activation energies obtained from isothermal analysis for ABS/SWNT composites
Fig. 7. Thermogravimetric curves of 0.5% SWNT/ABS at different isothermal temperatures under nitrogen.
Fig. 8. Thermogravimetric curves of 2% SWNT/ABS at different isothermal temperatures under nitrogen.
does not help stabilizing the system, the time needed to degrade to zero 1n(wt.%) decreases, as shown in Fig. 8. In summary, as shown in Figs. 4–8, the residue weight decreased with an increasing of the heating time and expectedly at higher isothermal temperature, the weight loss is larger. The kinetic parameters of the thermal degradation can be obtained from a single TGA curve. The method of Freeman and Carroll has widely been used to determine the order and the activation energy of the degradation [8]. The rate of degradation may be written as:
dW ¼ kd W n dt
ð1Þ
Where kd is the rate constant; W is the weight remaining,%; n is the order of the reaction;. If assuming first order decomposition, then lnW0 lnW ¼ kd t
ð2Þ
lnW ¼ lnW0 kd t
ð3Þ
where W0 is the initial weight. From Figs. 4–8, it is found that for the isothermal at 370 C, a straight line is obtained which means that the degradation at 370 C can be regarded as a first order decomposition with a steady rate constant. However, for the isothermal at 400, 430 C, all the TGA curves have a concave shape
SWNT (%)
Ea, under nitrogen (Kcal/mol)
Ea, under static air (Kcal/mol)
0.0 0.5 1.0 2.0 3.5 5.0 10.0
32.1 31.2 30.4 28.9 29.1 28.9 29.4
37.3 32.4 31.9 31.6 30.9 31.3
with the reaction rate getting smaller and smaller meaning that either the order of reaction is not one, or the rate constant is not a constant, or both. Finally, for the isothermal at 560 C, as shown in the figures, after the initiation of degradation, all of the samples have a straight line till around 5 min, when the curves deviate. The degradation at 560 C can be considered as a first order reaction with different rate constants since the slopes of the lines are different. Using Freeman and Carroll’s model [8] as stated in Eqs. (1)–(3), the kinetic parameters of the degradation under various conditions, were calculated assuming a first order decomposition. The rate constants, kd were obtained as the slopes of the curves in Figs. 4–8. Additionally, kd ¼ AexpðEa =RTÞ
ð4Þ
where A is the pre-exponential factor, Ea is the activation energy (Kcal/mol), R is the universal gas constant (Kcal/Kg-molK); and T is the absolute temperature (K). From this relationship, the activation energies were evaluated. The results were summarized in Table 3. Compared to pure ABS, the addition of SWNTs decreased the activation energies under both static air and nitrogen. The decrease is more obvious in the situation of static air. This is consistent to the former finding that SWNTs destabilize ABS. 3.3. DSC results DSC results on ABS/SWNT composites evaluated in the temperature range of room temperature to 200 C showed that the addition of SWNTs increased the glass transition temperature (Tg) of the composites from 101 C to about 107 C. All of the samples, except for the two composites with 0.5 and 1 wt.% SWNTs did not show melting peaks, indicating that the composites are amorphous. However, for the two samples with 0.5 and 1 wt.% of SWNTs, small melting peaks were observed indicating nucleation ability of the SWNT as reported by several researchers that have worked with semicrystalline
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matrices [20]. For samples containing more than 1 wt.% SWNTs, the composites did not show a melting peak, suggesting that the rigidity of SWNTs interferes with the mobility of the polymer chains and crystallization is not able to occur.
4. Conclusion Thermogravimetry analysis and differential thermal analysis were used to study the thermal degradation of ABS/SWNT composites under static air and nitrogen. For the dynamic analysis, a two-step degradation of the ABS was observed. It was found that the addition of SWNTs destabilized ABS and the composites began to degrade at lower temperatures. However, a small amount of SWNTs reinforce the second degradation step and the peak temperature increased. Further addition of SWNTs again destabilizes the system lowering the peak temperature by 38 C compared to the pure ABS. The analysis of activation energy further supports the claim that the addition of SWNTs destabilize ABS under both static air and nitrogen, the activation energy was lowered by the addition of SWNTs. DSC analysis showed that the addition of SWNTs increases the glass transition temperature of the composites and a small amount of SWNTs might act as nucleating agents to the crystallization of ABS. Further investigation is needed to fully explain the role of SWNTs in the radical degradation initiation.
Acknowledgements Financial supports for this work from National Science Foundation under grant numbers: CMS 0092621 and CMS 0078990 are gratefully acknowledged.
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