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A technique for determining fiber content in FRP by thermogravimetric analyzer
Polymer Testing 24 (2005) 376–380 www.elsevier.com/locate/polytest
Test Method
A technique for determining fiber content in FRP by thermogravimetric analyzer Cho-Rok Moona,*, Bo-Rae Bangb, Won-Jong Choia, Gil-Ho Kanga, Sang-Yoon Parka a
Department of Materials Engineering, Hankuk Aviation University, 200-1, Hwajon-Dong, Deokyang-Gu, Goyang-City, Gyeonggi-Do, South Korea b Division of Materials Science and Engineering, Hanyang University, 17 Haengdang-Dong, Seongdong-Gu, Seoul, South Korea Received 16 August 2004; accepted 6 October 2004
Abstract A new technique was developed to determine the fiber content of composites using a thermogravimetric analyzer (TGA). The optimum condition for the TGA technique was predicted from various isothermal and dynamic scans, and the decomposition behavior of epoxy composites reinforced with glass and carbon fiber was analyzed. The results of the TGA method showed good agreement with those obtained by conventional methods such as standard digestion and ignition loss. q 2004 Elsevier Ltd. All rights reserved. Keywords: Fiber content; TGA method
1. Introduction Fiber content is an important characteristic to control mechanical properties of fiber reinforced plastics (FRP) such as strength and stiffness. Widely used methods for determining fiber content in FRP are standard digestion and ignition loss. However, the standard digestion method produces toxic waste which results in negative consequences for the working environment [1]. The ignition loss method is suitable only for glass fiber reinforced plastics (GFRP), but is not used for carbon fiber reinforced plastics (CFRP) because of fiber oxidation in air [2,3]. These methods also require an established quantity and take several hours to measure. Therefore, a new method for determining the fiber content is needed for more efficient and accurate quality control of FRP. The specific aim of this work is to apply thermogravimetric analysis (TGA) to determining fiber content of epoxy * Corresponding author. Tel.: C82 2 300 0058; fax: C82 2 3158 3770. E-mail address: [email protected] (C.-R. Moon). 0142-9418/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymertesting.2004.10.002
composites reinforced with glass and carbon fibers. The optimum condition for this technique was predicted from various isothermal and dynamic scans, and the decomposition behavior of composites was analyzed based on the TGA profile obtained. Finally, the results of the TGA technique were compared with standard digestion and ignition loss methods.
2. Experimental procedure In this experiment, fiber content was determined by three methods: standard digestion, ignition loss and the TGA method. For statistical evaluation, five samples of each composite batch were tested. 2.1. Materials The materials used in this study were HG120 prepreg for GFRP and T-300 prepreg for CFRP. Both resin systems are 120 8C cure epoxy. All prepregs were produced by Hankuk
C.-R. Moon et al. / Polymer Testing 24 (2005) 376–380
377
Table 1 Densities of the fiber and FRP (g/cm3)
Df Dc
GFRP
CFRP
2.550 1.503
1.730 1.512
fiber Co. Ltd. The dimensions of the laminates were 300! 300 mm with 10 plies. Laminates were cured under the recommended cure cycle for the material with consolidation pressure of 3.04!105 Pa in an autoclave. 2.2. Standard digestion method A sample was weighed and treated with nitric acid in a hot digestion medium. After heating for 5 h, the residue was filtered and weighed [4]. Density values of the fiber and FRP were measured based on ASTM D-792 and the results are listed in Table 1. The weight % of fiber (Fwt) and the volume % of fiber (Fvol) can be calculated by the following equation m mf rc Fwt ð%Þ Z f 100; Fvol ð%Þ Z 100 (1) mc rf mc where mc is the weight of FRP, mf is the weight of fiber, rc is the density of FRP and rf is the density of fiber. 2.3. Ignition loss method The dried samples in crucibles were placed in a furnace and burnt until only ash remains. Once the matrix was completely burnt-out, the residue was placed in a desiccator and allowed to cool before re-weighing [3]. The fiber content can be calculated using Eq. (1). 2.4. Thermogravimetric analyzer (TGA) method A thermogravimetric analyzer is an equipment that measures mass of a substance according to a controlled temperature program [5]. In this experiment, a Shimadzu TGA-50 instrument was used. Measurements were performed at various temperatures and atmospheres. The gas flow rate was 50 ml/min based on ASTM E 1131-93. The mass change of the sample was recorded continuously over the temperature and time intervals.
Fig. 1. Weight loss as a function of temperature for GFRP under air atmosphere.
decomposition started at 300 8C, and the char decomposition was observed in the range 330–530 8C. The glass fiber reached a constant residual weight above 530 8C, therefore, fiber content in GFRP can be determined at an isothermal temperature of 550 8C for 60 min without considering the amount of fiber weight loss. 3.1.2. Carbon fiber reinforced plastic The TGA profile obtained by a dynamic scan of CFRP is presented in Fig. 2. CFRP was heated up to 1000 8C in air at a heating rate of 5 8C/min. Unlike GFRP, carbon fiber did not achieve a constant residual weight. Therefore, it is necessary to investigate the optimum condition for determining fiber content. For this reason, isothermal scans of CFRP were performed at four different temperatures: 450, 500, 550 and 600 8C. The sample was heated up to the isothermal temperature at a rate of 5 8C /min, then kept at each temperature for 300 min as shown in Fig. 3.
3. Results and discussion 3.1. Thermal analysis method 3.1.1. Glass fiber reinforced plastic Dynamic scans were performed through a heating program from room temperature to 1000 8C at a heating rate of 5 8C/min in air as shown in Fig. 1. The resin
Fig. 2. Weight loss as a function of temperature for GFRP and CFRP under air atmosphere.
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C.-R. Moon et al. / Polymer Testing 24 (2005) 376–380
Fig. 3. Weight loss as a function of time for CFRP in isothermal temperature of 450, 500, 550 and 600 8C under air atmosphere.
At isothermal temperatures of 450 and 500 8C, the char was not completely separated. The weight loss of the sample was 28% at 450 8C and 30% at 500 8C. The remaining char was observed by scanning electron microscope as shown in Fig. 4(A) and (B). At 600 8C it could be seen that the fiber oxidation was more severe than 550 8C, as shown in Fig. 4(C) and (D). Based on this analysis, it was decided that
an isothermal temperature of 550 8C is the optimum for determining fiber content in CFRP. The four decomposition mechanisms are illustrated in the TGA profile shown in Fig. 5. In this figure, the initial weight loss occurred due to solvent or water vaporization, and second stage is related to the decomposition of epoxy resin. The char decomposition occurred prior to the last inflection which corresponded to fiber degradation. The last stage is an unstable phase in which carbon fiber loses its weight by fiber oxidation [6,7]. Therefore, the fiber content should be considered between the char decomposition and fiber degradation. At this point of inflection on the TGA profile, the carbon fiber degraded with the epoxy matrix when it was exposed to an isothermal temperature of 550 8C. To quantify the specific fiber weight loss, carbon fibers only were heated from room temperature to 1000 8C under air and nitrogen atmospheres, respectively (Fig. 6). Under nitrogen atmosphere, a weight loss of 0.62% corresponded to loss of sizing materials. On the other hand, in air, a weight loss of 1.83% was due to fiber oxidation and the reduction of sizing materials. Here, fiber content can be determined in air if the actual fiber weight loss is known. In this experiment, the fiber weight loss fraction (Floss wt) was supposed to be 1.21%, the difference in residual weights between nitrogen and air atmospheres. Therefore, fiber content in FRP can be
Fig. 4. Surface topology of the carbon fiber after isothermal TGA experiment at: (A) 450, (B) 500, (C) 550, and (D) 600 8C.
C.-R. Moon et al. / Polymer Testing 24 (2005) 376–380
379
expressed by the following equation Fwt Z ð100 K Rwt Þ C Floss wt
(2)
where Fwt is the weight % of fiber and Rwt is the weight % of resin. After the five samples had been kept at 550 8C for 300 min, the fiber content was calculated using Eqs. (1) and (2). The results of the fiber content obtained by standard digestion, ignition loss and TGA method are summarized in Table 2.
4. Conclusions A TGA technique for determining fiber content of the composites has been demonstrated in this study. Employing this method, immediate results could be obtained without reweighing, and a small sample-size (15 mg) is required in comparison with the standard digestion and ignition loss method (0.3 and 6 g). It is also a reliable quality control technique for epoxy composites reinforced with glass and carbon fibers. The following conclusions are drawn:
Fig. 5. Typical TGA profile illustrating thermal effects on CFRP in the isothermal temperature of 550 8C under air atmosphere: (A) initial weight loss, (B) resin decomposition, (C) char decomposition, (D) fiber decomposition.
(1) The optimum condition decided on for the TGA method to determine fiber contents of GFRP and CFRP was an isothermal temperature of 550 8C. In the case of CFRP, the carbon fiber degrades with the epoxy matrix when it was exposed to this optimized temperature, therefore, a parameter for the fiber weight loss fraction (Floss wt) should be considered. This parameter can be derived by comparison of fiber degradation at air and nitrogen atmospheres. (2) Test results of the TGA technique have good consistency within 1% and agreement with those obtained by standard digestion and ignition loss methods. In the case of GFRP, the mean values and standard deviation of fiber weight fraction (Fwt) are 52.83% (G0.191%, standard digestion), 52.65% (G0.427%, ignition loss) and 52.65% (G0.572%, TGA). The values of CFRP are 64.33% (G0.080%, standard digestion) and 64.60% (G0.164%, TGA).
Fig. 6. Weight loss as a function of temperature for carbon fiber under air and nitrogen atmosphere.
Table 2 Fiber weight and volume fractions (%) determined by various methods Method
Standard digestion
Ignition loss
Material
GFRP
Test
Fwt
Fvol
Fwt
Fvol
Fwt
Fvol
Fwt
Fvol
Fwt
Fvol
Fwt
Fvol
T-1 T-2 T-3 T-4 T-5 Average
52.94 52.56 52.96 53.01 52.72 52.83
31.20 30.97 31.21 31.24 31.07 31.14
64.28 64.39 64.29 64.27 64.45 64.33
56.17 56.27 56.18 56.17 56.32 56.22
52.46 52.59 52.77 53.56 52.87 52.65
30.92 90.99 31.10 30.97 31.16 31.03
– – – – – –
– – – – – –
52.43 52.76 52.83 53.94 52.86 52.65
30.90 31.09 31.13 31.20 31.74 31.03
64.47 64.56 64.45 64.83 64.72 64.60
56.34 56.42 56.32 56.66 56.56 56.46
CFRP
GFRP
TGA CFRP
GFRP
CFRP
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C.-R. Moon et al. / Polymer Testing 24 (2005) 376–380
References [1] M.D. Seale, B.T. Smith, W.H. Prosser, J.N. Zalameda, Lamb wave assessment of fiber volume fraction in composites, J. Acoust. Soc. Am. 104 (3) (1998) 1399–1403. [2] R.Y. Yee, T.S. Stephens, A TGA technique for determining graphite fiber content in epoxy composites, Thermochim. Acta 272 (1996) 191–199. [3] American Society for Testing and materials, Standard test method for ignition loss of cured reinforced resin, ASTM D 2584-94.
[4] American Society of Testing and Materials, Standard test method for fiber content of resin matrix composites by matrix digestion, ASTM D 3171-76. [5] D.M. Price, D.J. Hourston, Thermogravimetry of polymers in: R.A. Meyers (Ed.),, Encyclopedia of Analytical Chemistry (2000), pp. 8094–8105. [6] W.A. Siger, Ablation characteristics of graphite epoxy, SAMPE Q. 17 (1986) 25. [7] G.A. Matzkanin, Nondestructive characterization of heat damage in graphite/epoxy composite: a stage of the art report, Nondestructive Testing Information Analysis Center (NTIAC), Texas.
Test Method
A technique for determining fiber content in FRP by thermogravimetric analyzer Cho-Rok Moona,*, Bo-Rae Bangb, Won-Jong Choia, Gil-Ho Kanga, Sang-Yoon Parka a
Department of Materials Engineering, Hankuk Aviation University, 200-1, Hwajon-Dong, Deokyang-Gu, Goyang-City, Gyeonggi-Do, South Korea b Division of Materials Science and Engineering, Hanyang University, 17 Haengdang-Dong, Seongdong-Gu, Seoul, South Korea Received 16 August 2004; accepted 6 October 2004
Abstract A new technique was developed to determine the fiber content of composites using a thermogravimetric analyzer (TGA). The optimum condition for the TGA technique was predicted from various isothermal and dynamic scans, and the decomposition behavior of epoxy composites reinforced with glass and carbon fiber was analyzed. The results of the TGA method showed good agreement with those obtained by conventional methods such as standard digestion and ignition loss. q 2004 Elsevier Ltd. All rights reserved. Keywords: Fiber content; TGA method
1. Introduction Fiber content is an important characteristic to control mechanical properties of fiber reinforced plastics (FRP) such as strength and stiffness. Widely used methods for determining fiber content in FRP are standard digestion and ignition loss. However, the standard digestion method produces toxic waste which results in negative consequences for the working environment [1]. The ignition loss method is suitable only for glass fiber reinforced plastics (GFRP), but is not used for carbon fiber reinforced plastics (CFRP) because of fiber oxidation in air [2,3]. These methods also require an established quantity and take several hours to measure. Therefore, a new method for determining the fiber content is needed for more efficient and accurate quality control of FRP. The specific aim of this work is to apply thermogravimetric analysis (TGA) to determining fiber content of epoxy * Corresponding author. Tel.: C82 2 300 0058; fax: C82 2 3158 3770. E-mail address: [email protected] (C.-R. Moon). 0142-9418/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymertesting.2004.10.002
composites reinforced with glass and carbon fibers. The optimum condition for this technique was predicted from various isothermal and dynamic scans, and the decomposition behavior of composites was analyzed based on the TGA profile obtained. Finally, the results of the TGA technique were compared with standard digestion and ignition loss methods.
2. Experimental procedure In this experiment, fiber content was determined by three methods: standard digestion, ignition loss and the TGA method. For statistical evaluation, five samples of each composite batch were tested. 2.1. Materials The materials used in this study were HG120 prepreg for GFRP and T-300 prepreg for CFRP. Both resin systems are 120 8C cure epoxy. All prepregs were produced by Hankuk
C.-R. Moon et al. / Polymer Testing 24 (2005) 376–380
377
Table 1 Densities of the fiber and FRP (g/cm3)
Df Dc
GFRP
CFRP
2.550 1.503
1.730 1.512
fiber Co. Ltd. The dimensions of the laminates were 300! 300 mm with 10 plies. Laminates were cured under the recommended cure cycle for the material with consolidation pressure of 3.04!105 Pa in an autoclave. 2.2. Standard digestion method A sample was weighed and treated with nitric acid in a hot digestion medium. After heating for 5 h, the residue was filtered and weighed [4]. Density values of the fiber and FRP were measured based on ASTM D-792 and the results are listed in Table 1. The weight % of fiber (Fwt) and the volume % of fiber (Fvol) can be calculated by the following equation m mf rc Fwt ð%Þ Z f 100; Fvol ð%Þ Z 100 (1) mc rf mc where mc is the weight of FRP, mf is the weight of fiber, rc is the density of FRP and rf is the density of fiber. 2.3. Ignition loss method The dried samples in crucibles were placed in a furnace and burnt until only ash remains. Once the matrix was completely burnt-out, the residue was placed in a desiccator and allowed to cool before re-weighing [3]. The fiber content can be calculated using Eq. (1). 2.4. Thermogravimetric analyzer (TGA) method A thermogravimetric analyzer is an equipment that measures mass of a substance according to a controlled temperature program [5]. In this experiment, a Shimadzu TGA-50 instrument was used. Measurements were performed at various temperatures and atmospheres. The gas flow rate was 50 ml/min based on ASTM E 1131-93. The mass change of the sample was recorded continuously over the temperature and time intervals.
Fig. 1. Weight loss as a function of temperature for GFRP under air atmosphere.
decomposition started at 300 8C, and the char decomposition was observed in the range 330–530 8C. The glass fiber reached a constant residual weight above 530 8C, therefore, fiber content in GFRP can be determined at an isothermal temperature of 550 8C for 60 min without considering the amount of fiber weight loss. 3.1.2. Carbon fiber reinforced plastic The TGA profile obtained by a dynamic scan of CFRP is presented in Fig. 2. CFRP was heated up to 1000 8C in air at a heating rate of 5 8C/min. Unlike GFRP, carbon fiber did not achieve a constant residual weight. Therefore, it is necessary to investigate the optimum condition for determining fiber content. For this reason, isothermal scans of CFRP were performed at four different temperatures: 450, 500, 550 and 600 8C. The sample was heated up to the isothermal temperature at a rate of 5 8C /min, then kept at each temperature for 300 min as shown in Fig. 3.
3. Results and discussion 3.1. Thermal analysis method 3.1.1. Glass fiber reinforced plastic Dynamic scans were performed through a heating program from room temperature to 1000 8C at a heating rate of 5 8C/min in air as shown in Fig. 1. The resin
Fig. 2. Weight loss as a function of temperature for GFRP and CFRP under air atmosphere.
378
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Fig. 3. Weight loss as a function of time for CFRP in isothermal temperature of 450, 500, 550 and 600 8C under air atmosphere.
At isothermal temperatures of 450 and 500 8C, the char was not completely separated. The weight loss of the sample was 28% at 450 8C and 30% at 500 8C. The remaining char was observed by scanning electron microscope as shown in Fig. 4(A) and (B). At 600 8C it could be seen that the fiber oxidation was more severe than 550 8C, as shown in Fig. 4(C) and (D). Based on this analysis, it was decided that
an isothermal temperature of 550 8C is the optimum for determining fiber content in CFRP. The four decomposition mechanisms are illustrated in the TGA profile shown in Fig. 5. In this figure, the initial weight loss occurred due to solvent or water vaporization, and second stage is related to the decomposition of epoxy resin. The char decomposition occurred prior to the last inflection which corresponded to fiber degradation. The last stage is an unstable phase in which carbon fiber loses its weight by fiber oxidation [6,7]. Therefore, the fiber content should be considered between the char decomposition and fiber degradation. At this point of inflection on the TGA profile, the carbon fiber degraded with the epoxy matrix when it was exposed to an isothermal temperature of 550 8C. To quantify the specific fiber weight loss, carbon fibers only were heated from room temperature to 1000 8C under air and nitrogen atmospheres, respectively (Fig. 6). Under nitrogen atmosphere, a weight loss of 0.62% corresponded to loss of sizing materials. On the other hand, in air, a weight loss of 1.83% was due to fiber oxidation and the reduction of sizing materials. Here, fiber content can be determined in air if the actual fiber weight loss is known. In this experiment, the fiber weight loss fraction (Floss wt) was supposed to be 1.21%, the difference in residual weights between nitrogen and air atmospheres. Therefore, fiber content in FRP can be
Fig. 4. Surface topology of the carbon fiber after isothermal TGA experiment at: (A) 450, (B) 500, (C) 550, and (D) 600 8C.
C.-R. Moon et al. / Polymer Testing 24 (2005) 376–380
379
expressed by the following equation Fwt Z ð100 K Rwt Þ C Floss wt
(2)
where Fwt is the weight % of fiber and Rwt is the weight % of resin. After the five samples had been kept at 550 8C for 300 min, the fiber content was calculated using Eqs. (1) and (2). The results of the fiber content obtained by standard digestion, ignition loss and TGA method are summarized in Table 2.
4. Conclusions A TGA technique for determining fiber content of the composites has been demonstrated in this study. Employing this method, immediate results could be obtained without reweighing, and a small sample-size (15 mg) is required in comparison with the standard digestion and ignition loss method (0.3 and 6 g). It is also a reliable quality control technique for epoxy composites reinforced with glass and carbon fibers. The following conclusions are drawn:
Fig. 5. Typical TGA profile illustrating thermal effects on CFRP in the isothermal temperature of 550 8C under air atmosphere: (A) initial weight loss, (B) resin decomposition, (C) char decomposition, (D) fiber decomposition.
(1) The optimum condition decided on for the TGA method to determine fiber contents of GFRP and CFRP was an isothermal temperature of 550 8C. In the case of CFRP, the carbon fiber degrades with the epoxy matrix when it was exposed to this optimized temperature, therefore, a parameter for the fiber weight loss fraction (Floss wt) should be considered. This parameter can be derived by comparison of fiber degradation at air and nitrogen atmospheres. (2) Test results of the TGA technique have good consistency within 1% and agreement with those obtained by standard digestion and ignition loss methods. In the case of GFRP, the mean values and standard deviation of fiber weight fraction (Fwt) are 52.83% (G0.191%, standard digestion), 52.65% (G0.427%, ignition loss) and 52.65% (G0.572%, TGA). The values of CFRP are 64.33% (G0.080%, standard digestion) and 64.60% (G0.164%, TGA).
Fig. 6. Weight loss as a function of temperature for carbon fiber under air and nitrogen atmosphere.
Table 2 Fiber weight and volume fractions (%) determined by various methods Method
Standard digestion
Ignition loss
Material
GFRP
Test
Fwt
Fvol
Fwt
Fvol
Fwt
Fvol
Fwt
Fvol
Fwt
Fvol
Fwt
Fvol
T-1 T-2 T-3 T-4 T-5 Average
52.94 52.56 52.96 53.01 52.72 52.83
31.20 30.97 31.21 31.24 31.07 31.14
64.28 64.39 64.29 64.27 64.45 64.33
56.17 56.27 56.18 56.17 56.32 56.22
52.46 52.59 52.77 53.56 52.87 52.65
30.92 90.99 31.10 30.97 31.16 31.03
– – – – – –
– – – – – –
52.43 52.76 52.83 53.94 52.86 52.65
30.90 31.09 31.13 31.20 31.74 31.03
64.47 64.56 64.45 64.83 64.72 64.60
56.34 56.42 56.32 56.66 56.56 56.46
CFRP
GFRP
TGA CFRP
GFRP
CFRP
380
C.-R. Moon et al. / Polymer Testing 24 (2005) 376–380
References [1] M.D. Seale, B.T. Smith, W.H. Prosser, J.N. Zalameda, Lamb wave assessment of fiber volume fraction in composites, J. Acoust. Soc. Am. 104 (3) (1998) 1399–1403. [2] R.Y. Yee, T.S. Stephens, A TGA technique for determining graphite fiber content in epoxy composites, Thermochim. Acta 272 (1996) 191–199. [3] American Society for Testing and materials, Standard test method for ignition loss of cured reinforced resin, ASTM D 2584-94.
[4] American Society of Testing and Materials, Standard test method for fiber content of resin matrix composites by matrix digestion, ASTM D 3171-76. [5] D.M. Price, D.J. Hourston, Thermogravimetry of polymers in: R.A. Meyers (Ed.),, Encyclopedia of Analytical Chemistry (2000), pp. 8094–8105. [6] W.A. Siger, Ablation characteristics of graphite epoxy, SAMPE Q. 17 (1986) 25. [7] G.A. Matzkanin, Nondestructive characterization of heat damage in graphite/epoxy composite: a stage of the art report, Nondestructive Testing Information Analysis Center (NTIAC), Texas.