Thermal Degradation of Carbon Fiber/Cyanate Ester Resin Composites Filled With Clay Silicate Nanoparticles Authors: S.P. Doherty, M. Takimori, J.M. Deitzel, D. Heider, J.W. Gillespie Jr., A. Shah, A. Giaya Presenter: Shawn P. Doherty Title: Research Associate Location: University of Delaware, Center for Composite Materials Address: 201 Composites Manufacturing Science Laboratory, Newark, DE 19716-3144 USA Email address: [email protected]Telephone: 1-302-831-0272 Presented at a meeting of the Thermoset Resin Formulators Association at the Hyatt Regency Montreal in Montreal, Quebec, Canada, September 11 through 12, 2006. This paper is presented by invitation of TRFA. It is publicly distributed upon request by the TRFA to assist in the communication of information and viewpoints relevant to the thermoset industry. The paper and its contents have not been reviewed or evaluated by the TRFA and should not be construed as having been adopted or endorsed by the TRFA.
Transcript
Thermal Degradation of Carbon Fiber/Cyanate Ester Resin Composites Filled With Clay Silicate Nanoparticles
Authors: S.P. Doherty, M. Takimori, J.M. Deitzel, D. Heider, J.W. Gillespie Jr., A. Shah, A. Giaya Presenter: Shawn P. Doherty Title: Research Associate Location: University of Delaware, Center for Composite Materials Address: 201 Composites Manufacturing Science Laboratory, Newark, DE 19716-3144 USA Email address: [email protected] Telephone: 1-302-831-0272 Presented at a meeting of the Thermoset Resin Formulators Association at the Hyatt Regency Montreal in Montreal, Quebec, Canada, September 11 through 12, 2006. This paper is presented by invitation of TRFA. It is publicly distributed upon request by the TRFA to assist in the communication of information and viewpoints relevant to the thermoset industry. The paper and its contents have not been reviewed or evaluated by the TRFA and should not be construed as having been adopted or endorsed by the TRFA.
1 Introduction Since the pioneering work at Toyota research, there has been a great deal of
interest in polymer-organoclay nanocomposites1. The desire for these composites is due to their improved properties compared to the polymer matrix2. Many methods have been proposed for preparing these polymer-clay nanocomposites. While many of the initial studies have dealt with thermoplastics, there has been some work done on thermosets3,4,5. Cyanate ester resin has been a focus of the thermoset studies due to its superior properties, including high glass-transition temperature, low moisture absorption, good thermal stability, low shrinkage, and relative ease in processing compared to other high temperature materials like polyimides. However, there is a desire to use these materials in extreme conditions, such as temperatures above 250 ºC, for use in engine components and aircraft components. One of the difficulties with using cyanate esters in these conditions is that long term use in extreme environments can lead to a decrease in the composite properties due to resin degradation and microcracking in the composite structure. By adding organically modified clay nanoparticles to the cyanate ester resin, it is possible to reduce the degradation of the resin properties at high temperatures.
In polymeric resins, the resin degrades through a thermo-oxidative process. Oxygen can diffuse into the sample and cleave chemical bonds in the resin. This cleavage leads to the formation of a surface degraded layer. As the surface degrades, mass is lost from the surface and microcracks form. Oxygen is able to penetrate deeper into the resin through these microcracks, which accelerates the resin degradation. This process will cause mass loss in the resin and degradation of mechanical and other material properties.
The purpose of this work is to use nanoparticle additives, specifically organically modified clays, to reduce the rate of thermo-oxidative decomposition of high temperature polymer resins. Organoclay particles should help prevent degradation in several ways. First, it is possible that the dispersed nanoparticles in the matrix will present a barrier to the diffusion of oxygen, which would slow the decomposition of the resin for long-term exposures. Second, the presence of the inorganic nanoparticle additives will help overcome induced stresses during resin degradation, preventing microcracking, which accelerates thermo-oxidative degradation. Third, the nanoclay particles help prevent microcracks from growing rapidly in the resin. All of these factors allow the organoclay composites to resist thermal degradation and allow the material to remain tough after exposures to high temperatures. First, we will examine the effects of the nanoclay particles on the cure behavior of the cyanate ester resin. Without understanding the effects of the nanoclay on the processability of the material, the material will not be commercially viable, regardless of improved properties. Second, we will examine the effects of the clay nanoparticles on the structure and properties of the resin and determine if they are having the desired effects at high temperatures.
2 Experimental
2.1 Materials and Preparation The base resin was a proprietary modified polycyanate blend (RS-9D). It has a
high Tg (>350 ºC) and a maximum service temperature of 280 ºC. The organoclays used
in the study were organically modified montmorillonite (Triton Systems, Chelmsford, MA). The organic modifiers were developed by Triton Systems specifically for high temperature applications. The modifiers were selected based on their thermal stability and solubility with cyanate ester. The selected organic modifiers were two phosphonium-based modifiers (aromatic and aliphatic) and two amine-based modifiers (cyclic and heterocyclic) formulated by Triton Systems. Table 1 lists the solubility parameter and decomposition temperature for each organic modifier.
Table 1. Solubility and decomposition temperature for organic modifiers
Each of the modifiers has a decomposition temperature above 275 ºC and a solubility parameter (∆δ) less than 2.8. These organoclays were synthesized by Triton Systems by treating the sodium montmorillonite with cationic surfactants in an aqueous medium. The organic modifiers attach to the clay through an ion exchange reaction. After the exchange, the organoclays are cleaned, dried, and ground. The cyanate ester-organoclay composite resin was formed using a high-shear mixing setup, where different weight percentages of the organoclay were mixed into the cyanate ester resin at 10,000 rpm and 115 ºC for 10 minutes. For each of the 4 organoclays, two resin mixtures were formed: 2.5 wt% and 5 wt% clay, except the heterocyclic amine, which only had the lower concentration. In order to promote curing of the cyanate ester resin, a commercial cobalt-based catalyst was added to each sample at 1.5 wt% prior to curing. These resin system were used to characterize the curing behavior of the cyanate ester-organoclay mixtures.
These organoclay resins were also processed into carbon fiber composites for aging and mechanical testing. In order to produce a resin-fiber composite with a distribution of organoclays throughout the material, a prepreg was made from the organoclay resin and IM7 carbon fiber. The prepreg was pressed into ½” thick panels with unidirectional carbon fibers included at a volume fraction of ~57%.
2.2 Characterization Techniques
2.2.1 Rheology Rheological measurements were carried out using a rheometer. A 40 mm parallel
plate fixture with a continuous flow of resin was used for all measurements. To determine the range of working temperatures for each resin, a temperature ramp of 2 ºC/min was conducted for each of the organoclay-cyanate ester resin mixtures. The temperature ranged from 70 to 160 ºC; however, the ramp was stopped prior to 160 ºC if the resin had fully cured. In addition, each resin system had its viscosity measured isothermally at different temperatures to determine the length of processing time at elevated temperatures. Each system was tested for 180 minutes at 100, 110, and 120 ºC to measure the effects of temperature on the bulk viscosity.
2.2.2 Calorimetry Calorimetric measurements were performed using a differential scanning
calorimeter (DSC). For each of the organoclays, the heat flow of the sample that contained 2.5 wt% of clay was measured. The samples cured with the cobalt catalyst and without the catalyst were analyzed. Samples were heated at 2 ºC/min from 50 to 300 ºC.
2.3 Fracture Toughness Measurement
To measure the fracture toughness of the organoclay composite systems, panels were cut into notched samples according to ASTM D-5045 and work done by Hansen et al.6 Each panel was cut into multiple test replicates having dimensions of 3” x 0.5” x 0.25”, with the fiber direction parallel to the shortest dimension (Fig. 1)6. A ~0.25” deep notch was cut into the center of the long direction parallel to the fiber direction. The width of the notch was 0.020” and the tip of the notch was cut with a sharp razor blade to initiate a crack 0.010” deep at the tip.
Figure 1. Fracture toughness sample testing according to ASTM D5045
The sample was tested using a three-point bend test to measure the displacement of the sample as load was applied to the cracked region. For each set of test conditions, five samples were tested. Based on these measurements, the fracture toughness (KIC) of each sample was calculated. Sample results not meeting the criteria for plane strain fracture: W-(N+C) > 2.5*(KIC/σy)2
Where: N=Notch length C=Crack tip diameter W=Sample width KIC=Fracture toughness σy=Failure stress of the resin
(ASTM D-5045) were discarded.
2.4 Thermal Aging To determine how the inclusion of the organoclays affects the thermal
decomposition of the cyanate ester resin, the organoclay composites were thermally aged and tested. Some of the test strips that were cut for fracture toughness measurements were aged prior to being tested for fracture toughness. The samples were cut into the proper test size and notched prior to thermal aging. For each organoclay and organoclay concentration, five samples each were aged in a forced convection oven at 260 ºC for 250 and 500 hours. Samples were rested on non-metallic supports to maximize surface area exposure to air. After aging, the samples were tested to determine fracture toughness. In addition, a sample of each composition was aged up to 51 days (1221 hours). At incremental periods during the aging, samples were weighed to determine the weight loss of the composite over time. The weight loss was calculated based on the initial weight of the cyanate ester resin, not including the carbon fiber or clay.
3 Results
3.1 Rheological Behavior The rheological behavior of the organoclay resin mixture strongly depends on the
type of organic modifier on the clay and weakly depends on the concentration of the clay in the resin. Figure 2 shows the viscosity profile for each of the clay systems when heated from 70 to 160 ºC. In all of the clay systems, the cure temperature (indicated by the sharp increase in viscosity) was lower than for the neat resin (~ 151 ºC). The cyclic amine and aromatic phosphonium-modified clays had the smallest decreases in cure onset temperature (~ 146 ºC), while the heterocyclic amine-modified clay was depressed to ~136 ºC. For each of those systems, the minimum viscosity during the heating was less than 0.20 Pa·s. In addition, there was only a small decrease in cure temperature for the 5 wt% samples compared to the 2.5 wt% (∆T~2ºC). For the clay modified with aliphatic phosphonium, the change in viscosity was more significant. The cure temperature for the 2.5 wt% aliphatic phosphonium was 135 ºC with the 5 wt% sample curing at 126 ºC. In addition, the minimum viscosities for the 2.5 and 5 wt% samples were 0.30 and 0.69 Pa·s.
Figure 2. Viscosity profile for temperature ramp of organoclay resin systems
Based on the temperature ramp data, the most processable clay systems are the ones modified with the cyclic amine and the aromatic phosphonium, since they had lower minimum viscosities and higher cure onset temperatures. The clay system modified with the aliphatic phosphonium had higher minimum viscosity and lower cure onset temperature, radically reducing the processing window. The change in viscosity was so significant for the aliphatic amine sample it was excluded from further testing.
To evaluate the amount of time that each clay system was processable, isothermal rheological measurements were taken for the remaining clay systems at three different temperatures (100, 110, and 120 ºC). Figures 3-5 show the change in viscosity versus time for the clay systems at each temperature. All of the resins cured the slowest at the lowest temperature. The cure time decreased with increasing cure temperature. In addition, at each temperature, the neat resin had the longest cure time followed by the cyclic amine, the aromatic phosphonium, and finally the heterocyclic amine. Unlike the temperature ramp data, the isothermal data indicated that there is a difference in cure behavior between the cyclic amine and the aromatic phosphonium, with the latter curing sooner than the former.
Figure 5. Viscosity-time plot for isothermal rheology at 120 °C
In each case, the organoclay appears to act as a catalyst, causing the resin to cure at lower temperatures and in shorter times. This is consistent with what has been reported in previous work with commercially available amine functionalized clays7. At elevated temperatures, the organic modifiers will begin to break down where the cation exchange occurred. The products of this decomposition are groups that are known to initiate cyanate ester polymerization. The aliphatic phosphonium may have had the greatest effect on the polymerization due the more flexible alkyl chain compared to the aromatic phosphonium.
3.2 Calorimetric Measurements To better understand how the organoclays act as catalysts in the resin system, the resin systems were tested in the DSC to determine their heat flow during the curing process. These tests were conducted on organoclay resin systems with and without the cobalt catalyst. In each system, the resin with organoclay and catalyst was compared to the resin with only the organoclay. Figure 6 shows the thermograms for each of the clay systems.
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Figure 6. DSC Thermograms for cyanate ester clay systems: (a) Neat resin; (b) Aromatic Phosphonium; (c) Heteroyclic Amine; (d) Aliphatic Phosphonium; and (e) Cyclic Amine. In the neat resin sample, the addition of the catalyst lowered the initial cure temperature and broadened the peak. The addition of the organoclay to the catalyzed system lowered the initial cure temperature further and narrowed the temperature range over which curing can occur. The addition of the clay particles to the catalyst-free resin lowered the peak temperature. Table 2 shows the heat of cure (∆H0) and peak maximum temperature for each of the cyanate ester systems. The heat of cure listed here has been adjusted for the mass of the clay and refers only to the polymer component. Table 2. Heat of cure and peak maximum temperature for cyanate ester systems.
Tmax(C) ∆H0 (J/g) Tmax(C) ∆H0 (J/g) Organoclay type Without catalyst With catalyst
These DSC results are consistent with rheological data in that it is clear that the
clay silicates are acting as catalysts, reducing the cure temperature and increasing the cure rate. However, the DSC also indicates that the addition of the clay reduces the heat of cure in the catalyzed systems compared to the neat resin, by as much as 24% (for heterocyclic amine). The change in heat of cure between the neat and clay systems for the catalyst-free systems was present, but not as significant. These results are similar to those reported in the literature7. It has been proposed7 that the exchange cation surfactant in the organomodified clays degrades at elevated temperatures, resulting in a number of degradation products. Some of the degradation products (ammonium compounds in particular), can act as catalysts in the polymerization process, while others act as chain terminators7. The end result is that the polymerization reaction is initiated at a lower temperature and occurs at a faster rate, but is prevented from going to complete conversion due to premature termination of the chain growth/crosslinking.
This explanation is consistent with the results reported in Figure 6 and Table 2. These changes in the cure kinetics of the resin systems may lead to a reduction in mechanical properties for the organoclay systems, since the amount of cross-linking will be reduced. The data clearly indicates that great care must be taken when interpreting data comparing the results of different polymer/clay systems, since the presence of the clay clearly has an effect on the polymer crosslink density. Furthermore, care must be taken to clearly determine the processing window for novel clay/resin systems to prevent auto acceleration of the polymerization when manufacturing large composite parts.
3.3 Microscopy
3.3.1 SEM and EDS In order to determine if clay particles were present in the organoclay composite
panels, Energy Dispersive Spectroscopy (EDS) measurements were conducted on composite samples made from Cyanate ester/carbon fiber/clay prepreg. Using an SEM, the surface of a cyclic amine composite panel was examined. Several EDS scans were done, looking at individual particles and larger regions to determine if elements from the clay were present. The first region that was scanned contained group of particles ~ 1 micron in diameter (Fig. 7).
Figure 7. SEM image of Resin/clay surface with pronounced particle Figure 8 is the EDS spectrum indicating the presence of aluminum, silicon, carbon, and oxygen. The silicon and aluminum peaks indicate the presence of clay.
Figure 8. EDX spectra of resin surface indicated in Figure 7 A second spectrum (Fig. 9) was taken of a spot on the largest particle in Figure 7. This spectrum had a lower carbon peak indicating that it was not measuring much resin or carbon fiber.
Figure 9. EDX spectra of a clay particle in Figure 7 In addition, the strong silicon peak and lesser aluminum peaks indicate that this particle was contained clay. Additional regions of the composite surface were scanned and additional regions containing clay particles were found. Based on these observations and measurements, the organoclay particles are distributed throughout the composite panel.
3.3.2 TEM Some of the organoclay resin systems were examined using TEM to determine the
distribution of clay particles within the resin. Samples of the 5 wt% cyclic amine and aromatic phosphonium resins were microtomed and examined in the TEM under magnification. At lower magnification, agglomeration of clay particles could be seen (Fig. 10).
Figure 10. Low magnification TEM image of cyclic amine clay particles They did not appear to be much exfoliation of the clay layers. At higher magnification of the regions, clay agglomerates can be seen along with the layers of the clay particles (Fig. 11). The clay particles are grouped together and have not exfoliated.
a) b) Figure 11. High magnification TEM image of: a) Cyclic amine clay particles; b) Aromatic Phosphonium clay particles.
3.4 Effect of Thermal Aging on Fracture Toughness
3.4.1 Weight Loss Samples of each of the organoclay composites were aged for over 50 days at 260
ºC to determine the amount of resin that was lost when aged at high temperatures. Figure 12 shows the weight loss for each of the clay systems.
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Figure 12. Plot of weight loss for the resin/clay systems vs. time heated to 260 ºC
After about 3 days (78 hours), each sample had lost ~0.8% of the initial weight. After one month of aging (696 hours), there was some noticeable differences in weight loss between the samples. The neat resin sample had the highest weight loss (~5.6%), while the others had lost about 4.5%. After 51 days (1221 hours), there were more variations in weight loss. The neat resin system had a 12.0% loss, which was the greatest weight loss of the samples. The 5 wt% cyclic amine sample had the next largest weight loss (10.5%). The 2.5 wt% cyclic amine and 2.5 wt% heterocyclic amine had less weight loss, while the aromatic phosphonium samples had the least amount of weight loss. Based on the weight loss measurements, the clay particles are having an effect in reducing the rate of degradation of the resin system compared to the neat resin. In addition, for longer times, there is a difference between the clays in the amount of weight loss prevention. The aromatic phosphonium exhibits the greatest resistance to thermal degradation, while the cyclic and heterocyclic amines seem to retard the rate of degradation to a lesser degree.
3.4.2 Fracture Toughness The fracture toughness of each of the clay systems were measured at room
temperature. For the thermally aged samples, the crack in the notch tip was initiated after thermally aging them. For each system, the conditions for which these samples were measured include control specimens, 250 hours at 260 ºC, and 500 hours at 260 ºC. Figure 13 shows the fracture toughness for each of the clay systems.
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Figure 13. Fracture Toughness measurements for clay/resin composites after 0 hours (blue), 250 hours (purple), and 500 hours (yellow) of aging at 260 ºC. It is noted that the KIC for the control specimens shows a slight degree of variation dependent on the clay modification type. It is likely that this variation is due at least in
part to the variation in the cure behavior of each system discussed in the previous section. Of greater interest is the comparison of the same clay systems that have undergone different aging conditions. In this case all the aged specimens share the same initial state, so comparison of the aged specimens to the control sample is much more straightforward.
After 250 hours, each of the clay filled composite systems showed a decrease in toughness. The neat resin showed a 16% decrease in toughness after 250 hours. The cyclic amine systems had the largest decreases (> 20%), the aromatic phosphonium had some decrease, and the heterocyclic amine had almost no decrease in KIC. The large decreases in fracture toughness for the neat resin and cyclic amine systems are consistent with the weight loss measurements indicating that the loss of mechanical properties is the result of oxidation of the resin.
For each of the samples, there was little significant change in fracture toughness between 250 hours of thermal aging and 500 hours. This result is consistent with the idea that the presence of the clay slows the diffusion of oxygen into the composite either by acting directly as a barrier to the oxygen, or by preventing the formation of cracks through which the oxygen can penetrate. Given that the TEM results indicate that the clay layers were at best intercalated rather than completely exfoliated, the second mechanism seems more likely. A study of the effect of the clay systems on crack propagation in these composites is currently in progress to verify this hypothesis and will be the subject of a future publication. Initial observations of the aged surfaces show that there are microcracks penetrating the surface of the system. For both the neat resin and the aromatic phosphonium, there were no visible surface cracks prior to aging (Fig. 14).
(a)
(b) Figure 14. Micrographs of: (a) control cyanate ester composite surface; and (b) aromatic phosphonium cyanate ester composite surface prior to thermal aging. Figure 15 shows a micrograph of the control sample after 500 hour aging. This region, which was near the notch of the sample, had microcracks across the surface.
Figure 15. Micrograph of control cyanate ester composite surface after 500 hours of aging. By comparison, the aromatic phosphonium sample aged for 500 hours had surface cracking, but the surface cracks did not appear to be as large or as numerous the control sample (Fig. 16).
Figure 16. Micrograph of aromatic phosphonium cyanate ester composite surface after 500 hours of aging. Further work is currently in progress to quantify the penetration of cracks into the composite bulk as a function of aging time for each of the composite systems in both the in-plane and transverse directions.
4 Conclusions The addition of organically modified nanoclay particles to a cyanate ester resin
has significant effects on the properties and processability of the material. The clay particles decrease the processing window of the resin prior to curing in terms of viscosity, temperature and time. The particles decrease the cure temperature and shrink the amount of time before the resin will cure at a fixed temperature. Both of these factors make the resin system can significantly alter the processing window of the resin. The DSC results clearly indicate that the clay particles act as a catalyst, decreasing the max cure temperature, and increasing the cure rate. A review of the literature suggests that the reason for this is the degradation of the cations in the organically modified clay systems.
The clay particles have been shown to improve the thermal stability of the resin system. The addition of the nanoclays decreased the mass loss and degradation of the cyanate ester resin at elevated temperatures. In addition, clay systems functionalized with heterocyclic amines helped prevent loss in fracture toughness for samples aged at elevated temperature. It is likely that additional improvement can be made by improving the distribution and exfoliation of the nanoclay particles within the resin system. By increasing the interactions of the resin system and the organoclays, there should be increased improvement in the properties of the composite.
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