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Improving stabilization degree of stabilized fibers by pretreating polyacrylonitrile precursor fibers in nitrogen
Materials Letters 76 (2012) 162–164
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Improving stabilization degree of stabilized fibers by pretreating polyacrylonitrile precursor fibers in nitrogen Xianying Qin, Yonggen Lu ⁎, Hao Xiao, Yunpeng Song State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, College of Materials Science and Engineering, Donghua University, Songjiang District, Shanghai 201620, China
a r t i c l e
i n f o
Article history: Received 1 October 2011 Accepted 22 February 2012 Available online 28 February 2012 Keywords: Polyacrylonitrile Carbon materials Oxidation Microstructure FTIR
a b s t r a c t Polyacrylonitrile fibers were heat-treated in nitrogen before oxidation to make the nitrile groups cyclized partially, and then were sufficiently stabilized and oxidated in air atmosphere. Fourier transform infrared spectroscopy showed that the stabilization degree of the stabilized fibers was improved by 2.9% and more conjugated nitrile groups were formed by heating precursor fibers in nitrogen, when the resultantly modified and conventionally stabilized fibers were stabilized to the same density of 1.39 g/cm3. Thermal gravimetric analysis of the resultant stabilized fibers showed the carbon yield at carbonization of 900 °C was enhanced by 1.7% attributed to the higher stabilization degree. The crystallite width and mechanical properties of the resultant carbon fibers were improved compared to the carbon fibers manufactured by the conventional method. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Polyacrylonitrile (PAN) fibers have to be subjected to stabilization prior to carbonization in order to develop into high performance carbon fibers (CFs). The stabilization in air atmosphere is intended to prevent the fibers from melting or fusing, and to avoid excessive volatilization of elemental carbon in subsequent carbonization by forming ladder structures [1–5]. When PAN fibers are heated in air, unsaturated nitrile groups, i.e., the conjugated and β-amino nitrile groups, are generated by dehydrogenation and termination of cyclization reaction, respectively. Since residual nitrile groups and unsaturated nitrile groups will be lost at higher temperatures, chain scission is usually introduced during carbonization [6–9]. Gupta and Harrison [10,11] concluded that, when PAN fibers were heat treated in inert gases, the intramolecular cyclization was the dominant reaction, which resulted in fully aromatic cyclized structure. Sivy et al. [12] reported that polyene structure without pendant nitriles, which was formed due to the reaction with oxygen and elimination of HCN, was prone to form β-amino nitrile groups rather than ladder structures. Our previous work found that the preferred orientation of both oxidized fibers and CFs was improved by a pretreatment of heating and stretching the PAN precursor fibers in nitrogen (HSN) [13]. In present study, our efforts have been focused on the increase of
⁎ Corresponding author. Tel./fax: + 86 21 67792936. E-mail address: [email protected] (Y. Lu). 0167-577X/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2012.02.103
stabilization degree (SD) during stabilization, and resultantly its influence on the properties of CFs. 2. Experimental The PAN precursor fibers (supplied by Mitsubishi Co, Japan) were wet-spun from a copolymer of acrylonitrile/acrylamide/methacrylic acid in a ratio of 97.9/1.5/0.6 (mol%), and had 3000 filaments/tow, 11.3 μm average diameter, 870 MPa tensile strength, 13.3 GPa Young's modulus, 13.9% elongation, and 50.5% crystallinity. The whole stabilization process was conducted continuously in a ten-zone tube furnace. The partially stabilized fibers (PSFs) and sufficiently stabilized fibers (SFs) were obtained at the end of zone 4 and zone 10, respectively. The pretreatment of HSN was carried out in nitrogen from zone 1 to zone 4 with temperatures of 180–210–220– 230 °C and stretching ratios of 2–2–2–3%. The PSFs produced by HSN were named N-PSFs and were exposed to further oxidation in air from zone 5 to zone 10 with temperatures of 205–225–240– 255–258–270 °C and stretching ratios of 0–0–0–0–(−1)–(−1) % to obtain the SFs (N-SFs). With the same temperatures and stretching ratios as N-PSF preparation, the PSFs generated in air were named A-PSFs. Correspondingly, A-SFs were obtained by heating A-PSFs in air from zone 5 to zone 10 with temperatures of 235–240–250– 258–260–270 °C and the same stretching ratios as N-SFs. Densities of A-SFs and N-SFs were maintained at 1.39 g/cm 3. Finally, carbonization was carried out in a temperature range of 350–480–600–800– 1300 °C in nitrogen with a stress of 15 MPa. According to A-SFs and N-SFs, CFs were classified into A-CFs and N-CFs.
X. Qin et al. / Materials Letters 76 (2012) 162–164
Fourier transform infrared (FTIR) spectroscopy was conducted on a Nicolet-8700 FTIR spectrometer in 400–4000 cm − 1 range at 2 cm − 1 resolution using KBr disks pressed by mixing KBr with fiber sample containing a certain amount of KSCN as the internal standard. The mass ratio of the fiber sample to KSCN was 3:1. Fiber's density was measured by the sink-float method. Thermogravimetry (TG) for SFs was carried on a TG 209 F1 thermogravimetric analyzer in nitrogen at 20 °C/min. X-ray diffraction (XRD) for CFs was carried out on a D/Max-2550 PC XRD apparatus (Cu Kα, 0.154056 nm, 40 kV, 250 mA). The crystallite width parallel to the fiber axis La//(100) was calculated using Scherrer formula based on the meridional scan. Mechanical properties of CF tows were measured
163
Table 1 Peak fitting results of FTIR spectra and SD values for A-SFs and N-SFs. Sample
A-SFs N-SFs
Area fraction by peak deconvolution Unreacted nitrile: S1 (%)
Conjugated nitrile: S2 (%)
β-amino nitrile: S3 (%)
19.7 24.6
44.9 48.6
35.4 26.8
S2/S3
SD (%)
1.27 1.81
76.3 79.2
using a WDW3020 electronic universal testing machine according to the Chinese Standard CNS-GB-3362/3366-82. 3. Results and discussion The chemical structures and their differences for PSFs and SFs were observed by FTIR spectroscopy as shown in Fig. 1. The PeakFit was used to analyze the unreacted (2243 cm − 1), conjugated (2210 cm − 1) and β-amino (2190 cm − 1) nitrile groups by fixing the peak position and varying the peak width and intensity. The optimum peak fitting of various nitrile groups for A-SFs and N-SFs was shown in Fig. 1(B) and (C), and the corresponding data were listed in Table 1. It was noted that the area fraction of conjugated nitrile groups for N-SFs (48.6%) was larger than that for A-SFs (44.9%), while the area fraction of β-amino nitrile groups for N-SFs (26.8%) was smaller than that for A-SFs (35.4%). More importantly, the ratio of S2 to S3 for N-SFs was 1.81, much larger than 1.27 for A-SFs. The results indicated that there were more conjugated and fewer β-amino nitrile groups in N-SFs than in A-SFs. The value of SD for SFs is an important parameter for the conversion from PAN fiber to CFs. The higher value of SD, the higher carbon yield during carbonization [14]. As shown in Fig. 1(A), the absorbances of –SCN groups at 2057 cm − 1 (I2057) in FTIR spectra for all the samples were standardized to compare the relative content of nitrile groups of different samples. Since the shape of the peak was uniform and narrow, we assumed that the area of unreacted nitrile was proportional to the intensity of absorption peak. Based on the information from Fig. 1, the SD value was calculated as follows: SD ¼
S þ S2 þ S3 1− 1 100% S0
ð1Þ
′ S0 I2243 ¼ S1 I2243
SD ¼
Fig. 1. (A) FTIR spectra of PAN-based fibers mixed with KSCN: (a) KSCN, (b) PAN precursor fibers, (c) A-PSFs, (d) N-PSFs, (e) A-SFs, and (f) N-CFs; (B) nitrile band peak fitting results of A-SFs; (C) nitrile band peak fitting results of N-SFs.
ð2Þ
S þ S2 þ S3 I2243 0 1− 1 S1 I2243
! 100%
Fig. 2. TG curves of A-SFs and N-SFs.
ð3Þ
164
X. Qin et al. / Materials Letters 76 (2012) 162–164
groups would generate intramolecular cyclization and form a longer ladder polymer with more aromatic repeat structures and fewer cyclization terminations [6,7]. Thus, it was concluded that more planar zigzag sequences were formed and converted to longer ladder structures during stabilization for N-SFs. The tensile strength and Young's modulus of A-CFs were 2.89 ± 0.11 GPa and 242 ± 4 GPa, respectively; and those for N-CFs were 3.32 ± 0.13 GPa and 248 ± 5 GPa, respectively. It was noted that the tensile strength and Young's modulus of N-CFs were higher than those of A-CFs. The better mechanical properties of N-CFs were mainly attributed to the fewer interior and edge defects for crystallites and fewer microvoid defects in fibers due to the higher carbon yield and larger crystallite width. 4. Conclusions Fig. 3. XRD and peak fitted patterns of CFs.
where I2243 is the absorption intensity of unreacted nitrile groups for PSFs and SFs, and I′2243 is that for the precursor fibers. S1, S2 and S3 are the area fractions of unreacted, conjugated and β-amino nitriles, respectively. S0 is a relative area of unreacted nitriles for precursor fibers based on S1. For PSFs, N-PSFs had a higher SD (42.0%) than A-PSFs (37.2%), indicating more nitrile groups cyclized in nitrogen than in air at the same temperature. After sufficient stabilization, the N-SFs had a higher SD value of 79.2% than A-SF3 with a value of 76.3%, and even both of them had the same density (1.39 g/cm 3). Based on these results, we confirmed that the HSN pretreatment was a simple and effective method to enhance the SD value of SFs. Since the residual nitrile groups are supposed to fail to convert to carbon layers, the carbon yield of SFs with a higher SD is expected to be higher. The TG curves of A-SFs and N-SFs during carbonization were shown in Fig. 2. The carbon yield of N-SFs (67.5%) was higher than that of A-SFs (65.8%) at 900 °C, which was mainly attributed to the higher SD and fewer structural ends of ladder polymer in N-SFs. The XRD and peak fitted patterns of A-CFs and N-CFs by meridional scan were shown in Fig. 3, which were used to calculate the crystallite width La//(100). The La//(100) of N-CFs (2.3 nm) was larger than that of A-CFs (2.1 nm). Since La//(100) of carbon fibers was strongly determined by the length of ladder structure of SFs, it was deduced that N-SFs had longer ladder structures compared to A-SFs. It was evidenced that the planar zigzag conformation of conjugated nitrile
PAN fibers were heat-treated appropriately in nitrogen to make the nitrile groups partially cyclized. A further reaction in air made the fibers sufficiently stabilized. The resultant N-SFs showed a higher SD and more conjugated nitrile groups compared with the conventional A-SFs, leading to a higher carbon yield, larger crystallite width and better mechanical properties for N-CFs. Acknowledgments The financial supports of this work by the National Basic Research Program of China (“973 Program”, Grant Nos.: 2011CB605602 and 2011CB605603) and the Cultivation Fund of the Key Scientific and Technical Innovation Project from Ministry of Education of China are gratefully acknowledged. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]
Watt W, Johnson W. Nature 1975;257:210–2. Bashir Z. Carbon 1991;29:1081–90. Fitzer E, Müller DJ. Carbon 1975;13:63–9. Dalton S, Heatley F, Budd PM. Polymer 1999;40:5531–43. Fitzer E, Frohs W, Heine M. Carbon 1986;24:387–95. Ouyang Q, Cheng L, Wang HJ, Li KX. Polym Degrad Stab 2008;93:1415–21. Chae HG, Minus ML, Rasheed A, Kumar S. Polymer 2007;48:3781–9. Xue TJ, Mckinney MA, Wilkie CA. Polym Degrad Stab 1997;58:193–202. Beltz LA, Gustafson RR. Carbon 1996;34:561–6. Gupta A, Harrison IR. Carbon 1996;34:1427–45. Gupta A, Harrison IR. Carbon 1997;35:809–18. Sivy GT, Gordon III B, Coleman MM. Carbon 1983;21:573–8. Qin X, Lu Y, Xiao H, Hao Y, Pan D. Carbon 2011;49:4598–600. Deurbergue A, Oberlin A. Carbon 1991;29:621–8.
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Improving stabilization degree of stabilized fibers by pretreating polyacrylonitrile precursor fibers in nitrogen Xianying Qin, Yonggen Lu ⁎, Hao Xiao, Yunpeng Song State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, College of Materials Science and Engineering, Donghua University, Songjiang District, Shanghai 201620, China
a r t i c l e
i n f o
Article history: Received 1 October 2011 Accepted 22 February 2012 Available online 28 February 2012 Keywords: Polyacrylonitrile Carbon materials Oxidation Microstructure FTIR
a b s t r a c t Polyacrylonitrile fibers were heat-treated in nitrogen before oxidation to make the nitrile groups cyclized partially, and then were sufficiently stabilized and oxidated in air atmosphere. Fourier transform infrared spectroscopy showed that the stabilization degree of the stabilized fibers was improved by 2.9% and more conjugated nitrile groups were formed by heating precursor fibers in nitrogen, when the resultantly modified and conventionally stabilized fibers were stabilized to the same density of 1.39 g/cm3. Thermal gravimetric analysis of the resultant stabilized fibers showed the carbon yield at carbonization of 900 °C was enhanced by 1.7% attributed to the higher stabilization degree. The crystallite width and mechanical properties of the resultant carbon fibers were improved compared to the carbon fibers manufactured by the conventional method. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Polyacrylonitrile (PAN) fibers have to be subjected to stabilization prior to carbonization in order to develop into high performance carbon fibers (CFs). The stabilization in air atmosphere is intended to prevent the fibers from melting or fusing, and to avoid excessive volatilization of elemental carbon in subsequent carbonization by forming ladder structures [1–5]. When PAN fibers are heated in air, unsaturated nitrile groups, i.e., the conjugated and β-amino nitrile groups, are generated by dehydrogenation and termination of cyclization reaction, respectively. Since residual nitrile groups and unsaturated nitrile groups will be lost at higher temperatures, chain scission is usually introduced during carbonization [6–9]. Gupta and Harrison [10,11] concluded that, when PAN fibers were heat treated in inert gases, the intramolecular cyclization was the dominant reaction, which resulted in fully aromatic cyclized structure. Sivy et al. [12] reported that polyene structure without pendant nitriles, which was formed due to the reaction with oxygen and elimination of HCN, was prone to form β-amino nitrile groups rather than ladder structures. Our previous work found that the preferred orientation of both oxidized fibers and CFs was improved by a pretreatment of heating and stretching the PAN precursor fibers in nitrogen (HSN) [13]. In present study, our efforts have been focused on the increase of
⁎ Corresponding author. Tel./fax: + 86 21 67792936. E-mail address: [email protected] (Y. Lu). 0167-577X/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2012.02.103
stabilization degree (SD) during stabilization, and resultantly its influence on the properties of CFs. 2. Experimental The PAN precursor fibers (supplied by Mitsubishi Co, Japan) were wet-spun from a copolymer of acrylonitrile/acrylamide/methacrylic acid in a ratio of 97.9/1.5/0.6 (mol%), and had 3000 filaments/tow, 11.3 μm average diameter, 870 MPa tensile strength, 13.3 GPa Young's modulus, 13.9% elongation, and 50.5% crystallinity. The whole stabilization process was conducted continuously in a ten-zone tube furnace. The partially stabilized fibers (PSFs) and sufficiently stabilized fibers (SFs) were obtained at the end of zone 4 and zone 10, respectively. The pretreatment of HSN was carried out in nitrogen from zone 1 to zone 4 with temperatures of 180–210–220– 230 °C and stretching ratios of 2–2–2–3%. The PSFs produced by HSN were named N-PSFs and were exposed to further oxidation in air from zone 5 to zone 10 with temperatures of 205–225–240– 255–258–270 °C and stretching ratios of 0–0–0–0–(−1)–(−1) % to obtain the SFs (N-SFs). With the same temperatures and stretching ratios as N-PSF preparation, the PSFs generated in air were named A-PSFs. Correspondingly, A-SFs were obtained by heating A-PSFs in air from zone 5 to zone 10 with temperatures of 235–240–250– 258–260–270 °C and the same stretching ratios as N-SFs. Densities of A-SFs and N-SFs were maintained at 1.39 g/cm 3. Finally, carbonization was carried out in a temperature range of 350–480–600–800– 1300 °C in nitrogen with a stress of 15 MPa. According to A-SFs and N-SFs, CFs were classified into A-CFs and N-CFs.
X. Qin et al. / Materials Letters 76 (2012) 162–164
Fourier transform infrared (FTIR) spectroscopy was conducted on a Nicolet-8700 FTIR spectrometer in 400–4000 cm − 1 range at 2 cm − 1 resolution using KBr disks pressed by mixing KBr with fiber sample containing a certain amount of KSCN as the internal standard. The mass ratio of the fiber sample to KSCN was 3:1. Fiber's density was measured by the sink-float method. Thermogravimetry (TG) for SFs was carried on a TG 209 F1 thermogravimetric analyzer in nitrogen at 20 °C/min. X-ray diffraction (XRD) for CFs was carried out on a D/Max-2550 PC XRD apparatus (Cu Kα, 0.154056 nm, 40 kV, 250 mA). The crystallite width parallel to the fiber axis La//(100) was calculated using Scherrer formula based on the meridional scan. Mechanical properties of CF tows were measured
163
Table 1 Peak fitting results of FTIR spectra and SD values for A-SFs and N-SFs. Sample
A-SFs N-SFs
Area fraction by peak deconvolution Unreacted nitrile: S1 (%)
Conjugated nitrile: S2 (%)
β-amino nitrile: S3 (%)
19.7 24.6
44.9 48.6
35.4 26.8
S2/S3
SD (%)
1.27 1.81
76.3 79.2
using a WDW3020 electronic universal testing machine according to the Chinese Standard CNS-GB-3362/3366-82. 3. Results and discussion The chemical structures and their differences for PSFs and SFs were observed by FTIR spectroscopy as shown in Fig. 1. The PeakFit was used to analyze the unreacted (2243 cm − 1), conjugated (2210 cm − 1) and β-amino (2190 cm − 1) nitrile groups by fixing the peak position and varying the peak width and intensity. The optimum peak fitting of various nitrile groups for A-SFs and N-SFs was shown in Fig. 1(B) and (C), and the corresponding data were listed in Table 1. It was noted that the area fraction of conjugated nitrile groups for N-SFs (48.6%) was larger than that for A-SFs (44.9%), while the area fraction of β-amino nitrile groups for N-SFs (26.8%) was smaller than that for A-SFs (35.4%). More importantly, the ratio of S2 to S3 for N-SFs was 1.81, much larger than 1.27 for A-SFs. The results indicated that there were more conjugated and fewer β-amino nitrile groups in N-SFs than in A-SFs. The value of SD for SFs is an important parameter for the conversion from PAN fiber to CFs. The higher value of SD, the higher carbon yield during carbonization [14]. As shown in Fig. 1(A), the absorbances of –SCN groups at 2057 cm − 1 (I2057) in FTIR spectra for all the samples were standardized to compare the relative content of nitrile groups of different samples. Since the shape of the peak was uniform and narrow, we assumed that the area of unreacted nitrile was proportional to the intensity of absorption peak. Based on the information from Fig. 1, the SD value was calculated as follows: SD ¼
S þ S2 þ S3 1− 1 100% S0
ð1Þ
′ S0 I2243 ¼ S1 I2243
SD ¼
Fig. 1. (A) FTIR spectra of PAN-based fibers mixed with KSCN: (a) KSCN, (b) PAN precursor fibers, (c) A-PSFs, (d) N-PSFs, (e) A-SFs, and (f) N-CFs; (B) nitrile band peak fitting results of A-SFs; (C) nitrile band peak fitting results of N-SFs.
ð2Þ
S þ S2 þ S3 I2243 0 1− 1 S1 I2243
! 100%
Fig. 2. TG curves of A-SFs and N-SFs.
ð3Þ
164
X. Qin et al. / Materials Letters 76 (2012) 162–164
groups would generate intramolecular cyclization and form a longer ladder polymer with more aromatic repeat structures and fewer cyclization terminations [6,7]. Thus, it was concluded that more planar zigzag sequences were formed and converted to longer ladder structures during stabilization for N-SFs. The tensile strength and Young's modulus of A-CFs were 2.89 ± 0.11 GPa and 242 ± 4 GPa, respectively; and those for N-CFs were 3.32 ± 0.13 GPa and 248 ± 5 GPa, respectively. It was noted that the tensile strength and Young's modulus of N-CFs were higher than those of A-CFs. The better mechanical properties of N-CFs were mainly attributed to the fewer interior and edge defects for crystallites and fewer microvoid defects in fibers due to the higher carbon yield and larger crystallite width. 4. Conclusions Fig. 3. XRD and peak fitted patterns of CFs.
where I2243 is the absorption intensity of unreacted nitrile groups for PSFs and SFs, and I′2243 is that for the precursor fibers. S1, S2 and S3 are the area fractions of unreacted, conjugated and β-amino nitriles, respectively. S0 is a relative area of unreacted nitriles for precursor fibers based on S1. For PSFs, N-PSFs had a higher SD (42.0%) than A-PSFs (37.2%), indicating more nitrile groups cyclized in nitrogen than in air at the same temperature. After sufficient stabilization, the N-SFs had a higher SD value of 79.2% than A-SF3 with a value of 76.3%, and even both of them had the same density (1.39 g/cm 3). Based on these results, we confirmed that the HSN pretreatment was a simple and effective method to enhance the SD value of SFs. Since the residual nitrile groups are supposed to fail to convert to carbon layers, the carbon yield of SFs with a higher SD is expected to be higher. The TG curves of A-SFs and N-SFs during carbonization were shown in Fig. 2. The carbon yield of N-SFs (67.5%) was higher than that of A-SFs (65.8%) at 900 °C, which was mainly attributed to the higher SD and fewer structural ends of ladder polymer in N-SFs. The XRD and peak fitted patterns of A-CFs and N-CFs by meridional scan were shown in Fig. 3, which were used to calculate the crystallite width La//(100). The La//(100) of N-CFs (2.3 nm) was larger than that of A-CFs (2.1 nm). Since La//(100) of carbon fibers was strongly determined by the length of ladder structure of SFs, it was deduced that N-SFs had longer ladder structures compared to A-SFs. It was evidenced that the planar zigzag conformation of conjugated nitrile
PAN fibers were heat-treated appropriately in nitrogen to make the nitrile groups partially cyclized. A further reaction in air made the fibers sufficiently stabilized. The resultant N-SFs showed a higher SD and more conjugated nitrile groups compared with the conventional A-SFs, leading to a higher carbon yield, larger crystallite width and better mechanical properties for N-CFs. Acknowledgments The financial supports of this work by the National Basic Research Program of China (“973 Program”, Grant Nos.: 2011CB605602 and 2011CB605603) and the Cultivation Fund of the Key Scientific and Technical Innovation Project from Ministry of Education of China are gratefully acknowledged. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]
Watt W, Johnson W. Nature 1975;257:210–2. Bashir Z. Carbon 1991;29:1081–90. Fitzer E, Müller DJ. Carbon 1975;13:63–9. Dalton S, Heatley F, Budd PM. Polymer 1999;40:5531–43. Fitzer E, Frohs W, Heine M. Carbon 1986;24:387–95. Ouyang Q, Cheng L, Wang HJ, Li KX. Polym Degrad Stab 2008;93:1415–21. Chae HG, Minus ML, Rasheed A, Kumar S. Polymer 2007;48:3781–9. Xue TJ, Mckinney MA, Wilkie CA. Polym Degrad Stab 1997;58:193–202. Beltz LA, Gustafson RR. Carbon 1996;34:561–6. Gupta A, Harrison IR. Carbon 1996;34:1427–45. Gupta A, Harrison IR. Carbon 1997;35:809–18. Sivy GT, Gordon III B, Coleman MM. Carbon 1983;21:573–8. Qin X, Lu Y, Xiao H, Hao Y, Pan D. Carbon 2011;49:4598–600. Deurbergue A, Oberlin A. Carbon 1991;29:621–8.