THERMAL PROPERTIES OF CARBON FIBER-EPOXY COMPOSITES WITH DIFFERENT FABRIC WEAVES R. Joven, R. Das, A. Ahmed, P. Roozbehjavan, B. Minaie*
Department of Mechanical Engineering Wichita State University Wichita, KS 67260, USA *Corresponding author:
[email protected] (316) 978-5613
ABSTRACT This paper presents results for thermal properties of carbon fiber-epoxy composites fabricated using prepregs with different fabric weaves including unidirectional, eight-harness satin, and plain weave. Results corresponding to fully cured composites indicated that weaves in carbon fibers may affect thermal properties. Thermal diffusivity and conductivity values measured in fiber direction from samples without weaves (unidirectional) were more than twice as those obtained for eight-harness and plain weave. Similar behavior was observed for samples tested in through-thickness direction. To determine the changes in thermal properties during manufacturing, thermal diffusivity for the three weave configurations was characterized using uncured prepregs. Such results indicated that diffusivity did not change considerably as a function of degree of cure. Finally, thermal properties of a tetrafluoroethylene (TFE) release film were measured to determine the tool-part heat transfer. This release film showed thermal conductivity values three times lower than carbon fiber-epoxy indicating that the film is an important contributor to thermal lag between tool and part [18].
1. INTRODUCTION Thermal properties of thermoset composite materials such as carbon fiber reinforced polymers (CFRP) depend on the amount and configuration of constituent materials. Since carbon fibers have higher thermal conductivity than polymeric matrices (24.0 W/(m·K) for graphite carbon fibers and 0.17 - 0.79 W/(m·K) for epoxy matrices [1, 2]) fiber orientation, configuration, and volume fraction are factors that may affect the heat propagation in composite parts. Hence, the characterization of thermal properties for composites with different fiber configuration is important to predict gradients of cure since parts made of CFRP are commonly cured in ovens or autoclaves. During processing, the predominant heat transfer methods are convection and conduction as shown in Figure 1. Convection is generated by the oven or autoclave gas circulation while conduction is related to tool-part heat transfer. Although tools are commonly heated by convection, tool-part heat transfer mode is conduction because the tool has considerably higher mass (thermal inertia) and thermal conductivity than the part. In addition, there is a significant amount of internal heat generated by the exothermic curing reaction of the resin. The combination of these heat transfer methods and the orthotropic thermal properties of
the composite may result in irregular heat flow and temperature distribution inside the part. Since temperature governs the cure kinetics of thermoset resins, gradients of temperature may cause uneven degree of cure, which results in irregular part shapes and reduction of mechanical properties. As these defects are not easily reversible, curing gradients are commonly predicted by numerical methods [3]. These methods require several sub-models of physical properties, including thermal conductivity, diffusivity, and specific heat (Cp).
Figure 1. Heat transfer mechanisms during composite processing. Previous studies have used conduction methods to measure thermal properties of CFRP composites at different temperatures. Sweeting et al. [4] implemented a customized device to measure the thermal conductivity under vacuum conditions in which temperature sensors were placed along and through the thickness of the samples. Their results indicated conductivity values of 2-3 W/(m⋅K) in fiber direction and 0.5-0.6 W/(m⋅K) in through-the-thickness direction. Scott et al. [5] proposed an alternative method to characterize simultaneously volumetric heat flux and thermal conductivity of composite materials during cure. Using modulated heat, the authors found an increase of thermal conductivity as a function of degree of cure. Friis et al. [6] implemented a technique to determine thermal properties (Cp , diffusivity, and conductivity) of epoxy composites using the Ångström method. This method consists in measuring temperature response to heat fluctuations induced in one end of the sample. The authors found a relation between thermal properties and resin vitrification since their measurements showed an important variation in thermal properties when the degree of cure was higher than 0.7. Garnier et al. [7] proposed another method to calculate the thermal conductivity during cure. Using signal analysis and the Ångström method, the thermal conductivity was calculated by deconvolution of the measured heat signal into two signals corresponding to thermal conductivity and internal heat generation. In this study, the thermal diffusivity and conductivity of carbon fiber-epoxy composites was characterized using a light radiation method. Samples made of prepregs with different fiber configurations including unidirectional, plain weave, and eight-harness were analyzed and the influence of fiber configuration on thermal properties was identified. The methodology utilized was the light flash method, which represents a fast and precise way to measure the thermal properties by heat radiation. Such tests were performed for uncured and fully cure d samples at temperatures between 20 and 177 °C. In addition, the thermal conductivity at the tool part interface was characterized by measuring the thermal properties of a tetrafluoroethylene (TFE) release film.
2. MATERIALS AND METHODS 2.1 Materials Thermal conductivity of autoclave and out-of-autoclave carbon fiber-epoxy prepregs with different woven fabrics was measured. The configuration of such materials is shown in Figure 2 and corresponds to Cytec IM7/977-2 unidirectional tape (UD), Cytec T650 3k/5320 eightharness satin weave (8HS), and Cytec T300 3k/977-2 plain weave (PW).
(a)
(b)
(c)
Figure 2. Fiber configuration of (a) unidirectional tape, (b) eight-harness satin weave, and (c) plain weave [8]. The thermal properties were measured in fiber and through-the-thickness directions. Samples tested in fiber direction consisted of five strips of 10 x 2 mm2 with 2 mm in thickness while squared samples of 10 x 10 mm2 with 2 mm in thickness were used for through-the-thickness measurement. In order to avoid variations due to emissivity of the surfaces, all the samples were coated with a dry graphite film (dgf-123 by Miracle Power). Similarly, squared samples of 10 x 10 mm2 were used to determine the thermal conductivity of the TFE release film (WL-5200 blue by Airtech). 2.2 Methodology The thermal properties were measured utilizing a light flash analyzer (LFA, Nanoflash 447 by Netzsch) following the ASTM E1461. This analyzer operated a xenon flash light that induced a pulse of energy on one side of the sample. Such pulse increased the sample temperature and an indium antimonide (InSb) infrared detector measured the temperature response time to the pulse of energy on the other side of the sample. The response time was used to calculate thermal diffusivity and conductivity. For instance, Parker/adiabatic method (ideal case) consists in calculating the thermal diffusivity as follows [9]:
0.13879
l2 t1 / 2
(1)
where α is the thermal diffusivity, l is the thickness, t1/2 is half of the response time. Then, the thermal conductivity k was calculated by multiplying α, Cp , and the sample density ρ:
k (T ) C p (T ) (T )
(2)
2.2.1 Diffusivity Model Using the software LFA Analysis v. 4.8.5 (by Netzsch), the post-processing of data acquired with the LFA consisted in finding a thermal diffusivity model by solving the Fourier equation for heat conduction in one dimension (z direction) [10]:
T ( z, t ) 2T ( z, t )
(3)
where T(z,t) represents the difference between sample and surrounding temperatures. To solve α from equation (3), four mathematical models of thermal diffusivity were evaluated including adiabatic/Parker model (equation (1)) [9], heat loss correction/Cowan model [11], radiationdiathermic/Mehling et al. model [12], and Cape-Lehman model [13]. Finally, thermal diffusivity was recalculated for all the samples using the model that presented the lowest standard deviation.
2.2.2 Cp Measurement The specific heat (Cp) was calculated by measuring temperature change of the samples and a reference material when subjected to the same pulse of energy (equation (4)). Q Ts lC p s
Q Tr lC p r
and
(4)
where ∆T is temperature change, and suffixes s and r are sample and reference, respectively. The reference material used for this purpose was AMX-Q5 poco graphite, provided by Netzsch and the samples were the previously mentioned CFRP composites or TFE release film. Q is the energy corresponding to a light pulse of 0.18 ms of the LFA xenon lamp. Since Q was the same for sample and reference, Cp of sample was calculated as follows: C ps
r lr C p Tr s ls Ts r
(5)
2.2.3 CTE Measurement The variation in density as a function of temperature was calculated with the linear coefficient of thermal expansion (CTE). This CTE (Figure 3) was measured using a TMA Q400 of TA instruments. Notice that the CTE for uncured samples was not measured since such analysis was performed by measuring the thermal diffusivity only.
Figure 3. CTE values for cured samples in fiber direction (d11) and through-the-thickness direction (d33). 2.3 Thermal Conductivity of TFE Release Film Thermal conductivity of TFE was calculated using a gravimetric method. Due to low thickness and high transparency of the sample, a layer of graphite was deposited on the release film as illustrated in Figure 4. However, the graphite layer distorts considerably the thermal conductivity because a thin layer of graphite can represent up to 20% of the total mass of the sample. For this reason, five samples were fabricated with different amounts of graphite (between 0.96 to 5.6 mg) and the thermal conductivity of TFE was estimated by extrapolation.
Figure 4. Thermal conductivity setup for the release film.
3. RESULTS AND DISCUSSION 3.1 Diffusivity Model The thermal diffusivity was calculated using the four aforementioned models (Mehling, CapeLehman, Cowan, and Adiabatic) for each material. The occurrence in which each model showed the lowest standard deviation was identified using a Pareto diagram (Figure 5) where radiation/Mehling model was the best fit for the three composites. The material that behaved closest to the Mehling model was 5320 8HS (relative frequency: 0.9 - 1.0) while 977-2 PW depicted the lowest relative frequency (0.45 - 0.75). A visual inspection of the tested samples suggested that the epoxy matrix was a semitransparent body while the carbon fibers reduced the transparency of the material. Therefore, such properties agreed with the theory of Mehling
(radiation) model since in the literature is mentioned that this model can be used to calculate the thermal conductivity of semitransparent materials with low optical thickness [12].
(a) 977-2 UD
(b) 5320 8HS
(c) 977-2 PW Figure 5. Frequency in which each model showed the lowest standard deviation values of thermal diffusivity for (a) 977-2 UD, (b) 5320 8HS, and (c) 977-2 PW. 3.2 Specific Heat Cp was characterized for fully cured samples made of unidirectional, eight-harness, and plain weave prepregs as shown in Figure 6. Results indicated that Cp of 977-2 UD and PW was around
4% lower than 5320 8HS. Such difference may correspond to the resin properties since 5320 was an out-of-autoclave resin while 977-2 was formulated for autoclave processing. Since the two composites made of 977-2 resin presented a negligible Cp difference, results sugested that fiber configuration did not have a relevant influence on Cp. For the three materials, Cp showed a linear dependence on temperature where the Cp values were around ~0.85 J/(gr·K) at 25 °C and ~1.30 J/(gr·K) at 200 °C. LFA results agreed with those reported by Friis et al. [6] since in their findings was measured Cp of 1.25 J/(gr·K) at 115 °C for a fully cured epoxy composite characterized by differential scanning calorimeter (DSC).
Figure 6. Cp calculated by comparison method for 977-2 UD, 5320 8HS, and 977-2 PW. 3.3 Thermal Diffusivity and Conductivity of Cured Samples Thermal diffusivity and conductivity were measured for the three prepregs in fiber (d11) and through-the-thickness (d33) directions. For these materials, results in Figure 7 showed that thermal diffusivity decreased linearly as temperature increased. In d11, diffusivity of the unidirectional composite (977-2 UD) was more than twice as 5320 8HS and 977-2 PW. As well, 5320 8HS showed diffusivity values 18% higher than 977-2 PW. These results of d11 were up to 10 times higher than d33 where 977-2 UD presented the highest diffusivity and 977-2 PW the lowest one.
Figure 7. Thermal diffusivity in fiber (d11) and through-the-thickness (d33) directions. Figure 8 shows the thermal conductivity calculated with equation (2) for samples of Figure 7. In d11, results corresponding to 977-2 UD showed a thermal conductivity around two times higher than 5320 8HS, and three times than 977-2 PW. On the other hand, results corresponding to d33 showed thermal conductivity values for 977-2 UD around 1.16 times higher than 5320 8HS and 1.33 higher than 977-2 PW. For the three prepregs, results also showed a linear increase as a function of temperature, in agreement with the findings of Sweeting et al [4]. As well, the thermal conductivity values of the plain weave composite agreed with the values reported by Sweeting et al (d11: 2-3 W/(m⋅K) - d33: 0.5-0.6 W/(m⋅K)).
Figure 8. Thermal conductivity of the different prepregs in fiber (d11) and through-the-thickness (d33) directions.
Results of Figure 7 and Figure 8 also implied that the amount of weaves in the carbon fibers decreased the thermal conductivity and diffusivity because the unidirectional composite showed the highest thermal properties. As well, eight-harness fabric depicted higher diffusivity and conductivity than plain weave in both directions. Note that in Figure 2, eight-harness configuration consisted of one weave every eight tows while plain weave had alternated interweave tows. Hence, the fabric with the highest amount of weaves (plain weave) depicted the lowest thermal properties. In the literature was found that the amount of weaves might be related to fiber volume fraction (Vf) since typical values of Vf corresponding to unidirectional composites was 50-70% [14] and for woven fabrics was 35-55% [15, 16]. Therefore, results in Figure 8 indicated that composites with lower amount of weaves (high Vf) like 977-2 UD had higher thermal conductivity and diffusivity in both d11 and d33. Notice that although 5320 8HS was an out-of-autoclave material, its thermal conductivity did not show any irregular behavior compared with 977-2 UD and PW indicating that cured autoclave and out-of-autoclave materials have similar thermal diffusivity and conductivity behavior. 3.4 Thermal Properties during Cure Thermal diffusivity of 977-2 UD (d33) was measured during cure between 25 to 177 °C, using three temperature intervals: 1, 2, and 3 °C. In the LFA, such intervals corresponded to temperature ramps of 0.353 °C/min, 0.667 °C/min, and 0.969 °C/min respectively (see Figure 9). Using these temperature ramps, results indicated that thermal diffusivity of 977-2 UD presented negligible changes during cure since such result did not depict changes higher than 0.1mm2/s with respect to the initial value.
Figure 9. Thermal diffusivity of 977-2 UD during cure (d33) at three different ramp rates: (1) 0.969 °C/min, (2) 0.667 °C/min, and (3) 0.353 °C/min. Since thermal diffusivity did not seem to depend on temperature rate, results of Figure 9 corresponding to 0.667 °C/min were used to compare the thermal conductivity of 977-2 UD with
5230 8HS and 977-2 PW as a function of degree of cure. The thermal diffusivity in d33 direction was measured in the LFA and compared with the degree of cure as depicted in Figure 10. The degree of cure was determined using a differential scanning calorimeter (DSC, Q200 by TA instruments). These results showed that thermal diffusivity of the three prepregs fabrics did not change more than 15% during cure. However, Scott et al. found changes in thermal conductivity during cure of thermoset composites [5]. Therefore, this change in conductivity may be attributed to density and Cp variations during cure according to equation (2).
Figure 10. Comparison of thermal diffusivity during cure for 977-2 UD, 5320 8HS, and 977-2 PW. 3.5 Thermal Conductivity of TFE Release Film Thermal conductivity of TFE release film was measured using different amounts of graphite coating in order to reduce the transparency of this material. For highly transparent material as the release film, the thermal properties characterization cannot be performed directly because the flash radiation cannot be absorbed in the sample. However, a graphite coating may represent 20% of sample weight since the thickness of the film was 50 µm. Hence, five different samples were tested with graphite amounts between 1.5 and 5.58 mg as depicted in Figure 11. In this graph is shown that thermal conductivity depicted a linear trend as a function of graphite mass, with a minimum R2 of 0.95. Thus, the conductivity of TFE release film was calculated by extrapolation. Results indicated a thermal conductivity of 0.23 - 0.24 W/(m⋅K) which agreed with the theoretical values of fluoropolymers: 0.25 W/(m⋅K) [17]. The experiments were replicated at temperatures between 30 and 100 °C and it was found that temperature changes were not strong contributors to thermal properties. Note that these values of thermal conductivity indicated that release film may be an important factor of thermal lag at the tool-part interface since the thermal conductivity of release film was more than two times lower than composites in (Figure 8, d33).
Figure 11. Thermal conductivity of TFE film measured by gravimetric method.
4. CONCLUSIONS Thermal properties of carbon fiber-epoxy composites and TFE release film were characterized using the light flash method. The specific heat (Cp), thermal conductivity, and thermal diffusivity were analyzed for three prepregs: IM7/977-2 unidirectional, T650/5320 eight-harness satin weave, and T300/977-2 plain weave. Results of fully cured samples showed that Cp of 5320 8HS was 5% higher than 977-2 UD and PW indicating that autoclave materials have lower Cp than out-of-autoclave. However, thermal conductivity and diffusivity seemed influenced mostly by the amount of weaves in the fabric and fiber volume fraction (Vf). 977-2 UD, which had the highest Vf, showed up to three times higher thermal conductivity than 5320 8HS and 977-2 UD in fiber direction. In through-the-thickness direction, results showed that thermal properties of 977-2 UD were 1.16 and 1.33 times higher than 5320 8HS and 977-2 PW respectively. For the three materials, thermal diffusivity during cure did not show changes higher than 15% during cure. Thermal conductivity of the TFE release film was more than two times lower than composites. Hence, results indicated that the release film is an important contributor to thermal lag between the tool and the part.
5. ACKNOWLEDGMENTS The authors gratefully acknowledge financial support from the National Aeronautics and Space Administration (Grant No. NNX09AO58A).
6. REFERENCES 1.
2.
3. 4.
5. 6.
7.
8. 9.
10.
11. 12.
13.
14.
15.
MatWeb. "Graphite, Carbon, C". 2012. May, 2012 . MatWeb. "Overview of materials for Epoxy, Cast, Unreinforced". 2012. May, 2012 . Bogetti, T. A. and Gillespie, J. W. "Two-Dimensional Cure Simulation of Thick Thermosetting Composites". Journal of Composite Materials 25(3) (1991): 239-273. Sweeting, R. D. and Liu, X. L. "Measurement of thermal conductivity for fibre reinforced composites". Composites Part A: Applied Science and Manufacturing 35(7–8) (2004): 933-938. Scott, E. P. and Beck, J. V. "Estimation of thermal properties in carbon/epoxy composite materials during curing ". Journal of Composite Materials 26(1) (1992): 20 - 36. Friis-Pedersen, H. H., Pedersen, J. H., Haussler, L., and Storm, B. K. "Online measurement of thermal diffusivity during cure of an epoxy composite". Polymer Testing 25(8) (2006): 1059-1068. Garnier, B. and Sommier, A. "Thermal property measurements during curing of thermoset resins using steady periodic conditions". Journal of Reinforced Plastics and Composites 21(13) (2002): 1193-1203. Hexcel. "Reinforcements for Composites", in Technical Fabrics Handbook. 2012. Parker, W. J., Jenkins, R. J., Butler, C. P., and Abbott, G. L. "Flash Method of Determining Thermal Diffusivity, Heat Capacity, and Thermal Conductivity". Journal of Applied Physics 32(9) (1961): 1679-1684. Pinto, C. S. C., Massard, H., Couto, P., Orlande, H. R. B., Cotta, R. M., and Ambrosio, M. C. R. "Measurement of thermophysical properties of ceramics by the flash method". Brazilian Archives of Biology and Technology 49 (2006): 31-40. Cowan, R. D. "Pulse Method of Measuring Thermal Diffusivity at High Temperatures". Journal of Applied Physics 34(4) (1962): 926 - 928. Mehling, H., Hautzinger, G., Nilsson, O., Fricke, J., Hofmann, R., and Hahn, O. "Thermal Diffusivity of Semitransparent Materials Determined by the Laser-Flash Method Applying a New Analytical Model". International Journal of Thermophysics 19(3) (1998): 941-949. Cape, J. A. and Lehman, G. W. "Temperature and Finite Pulse-Time Effects in the Flash Method for Measuring Thermal Diffusivity". Journal of Applied Physics 34(7) (1963): 1909-1914. Koushyar, H., Alavi-Soltani, S., Minaie, B., and Violette, M. "Effects of variation in autoclave pressure, temperature, and vacuum-application time on porosity and mechanical properties of a carbon fiber/epoxy composite". Journal of Composite Materials (2011). Sun, B., Liu, F., and Gu, B. "Influence of the strain rate on the uniaxial tensile behavior of 4-step 3D braided composites". Composites Part A: Applied Science and Manufacturing 36(11) (2005): 1477-1485.
16. 17.
18.
Department of Defence, MIL-HDBK-17-1F Composite materials handbook Vol. 1. Polymer matrix composites guidelines for characterization of structural materials. 2002. Duncan M. Price, M. J. "Thermal conductivity of PTFE and PTFE composites", in Proceedings of the twenty-eight conference of the North American thermal analysis society. 2000: Orlando, FL. Joven, R., Tavakol, B., Rodriguez, A., and Minaie, B. "Experimental Investigation of Tool/Part Interface during Curing of Composites," Proceedings of the SAMPE 2010 Conference and Exhibition "New Materials and Processes for a New Economy," May 17, 2010
