Ablative nanocomposites were prepared by incorporating multiwall carbon nanotubes (MWCNT) into phenolic resin and then impregnating them into rayon-based carbon fabric. MWCNT were blended into phenolic resin at 0.5, 1, and 2 wt% loadings using a combination of sonication and high shear mixing to insure uniform dispersion of MWCNT. The composite test specimens were tested by using an oxyacetylene test bed (OTB) applying a heat flux of 1000 W/cm2 for duration of 45 seconds. Composite specimens with 2 wt% MWCNT showed reduction in mass loss, recession in length, and in situ temperatures compared to control composites.
Ablation is a process of material removal from a surface or other erosive process and usually associated with materials for space reentry vehicles and rocket nozzles. The ablative materials are used as thermal protection materials for rocket nozzles, space vehicles, and combustion chambers of rocket motors. These materials should withstand very high temperatures in the order of thousands of degrees Celsius, high thrust, and high impact. The final material should be able to form complex shapes and be as light as possible. Currently the main consumers of ablative materials are military, NASA, and commercial space launching company.
Intensive research on ablative materials began during the space race between the USSR and the USA in 1950s. Some of these research materials were made by universities, while some by private companies, such as Cytec Industries. The most popular material used in the United States, which is the standard material for NASA and other organizations, is MX-4926 manufactured by Cytec Engineered Materials. MX-4926 is a composite material composed of woven rayon-based carbon fiber, carbon black filler, and a phenolic resin matrix. Many research groups, such as Ho et al. [
Ablative materials have progressed with the introduction of new materials and technologies. Since the late 1990s, nanotechnology has been a new frontier of the scientific community. Nanotechnology deals with particles which have at least one dimension on the nanometer scale [
Patton et al. modified carbon fiber-reinforced phenolic composites with carbon nanofibers (CNF). It was observed that the nanoscale dimensions of the vapor-grown CNF caused major changes in the heat transfer rates and affected the resultant combustion chemistry [
For obtaining desired improvement in ablative properties, higher loading of MMT organoclays and CNF has to be employed. As wt% of nanoparticle increases, the viscosity of polymer resin increases rapidly. There is significant challenge in processing fiber reinforced composites using highly viscous polymer resins. On the other hand, lower loadings of MWCNT and POSS in polymer resin provide similar performance as that of higher loadings of MMT organoclays and CNF [
The major objective of this research is to develop commercially viable ablative material that has significant improvement compared to current state-of-the-art (SOTA) ablative materials. The important factors for commercial success are cost, processability, and availability of all constituent materials in the composites. MWCNT are affordable. They are easy to process in polymer resin and available in sufficient commercial quantity. This paper describes the manufacturing and ablative performance of rayon-based carbon fiber reinforced composites using MWCNT modified phenolic resin. Ablative composites in this research are produced using exactly the same constituent materials: rayon-based carbon fabric and SC-1008 phenolic resin and exactly same manufacturing process as that of SOTA ablative material traded name as MX-4926.
This was a collaborative research with Cytec Engineered Materials (CEM). Carbon fiber reinforced nanocomposites were manufactured using processes and materials similar to those used by CEM for current SOTA ablative material, MX-4926. According to CEM description, MX-4926 MC (molding compound) is rayon precursor-based carbon fabric impregnated with MIL-R-9299 phenolic resin and carbon black as filler.
Ablative test panels were manufactured using rayon-based 8-harness satin carbon fabric supplied by CEM. The fabric had a weight of 261 g/m2, specific gravity of 1.84, and thickness of 0.48 mm. Break strength was 0.496 MPa and 0.599 MPa in the warp and fill directions, respectively. It had 1.96 picks/mm in warp direction and 2 picks/mm fill direction. The 8-harness satin weave provided conformability essential for producing complex geometries.
The matrix used in this research was SC-1008 phenolic resin which was manufactured according to MIL-R-9299 and supplied by Momentive (formerly Hexion Specialty Chemicals). The viscosity was in the range of 180–300 cP depending on storage conditions. SC-1008 contained roughly 20%–25% isopropyl alcohol (IPA) as a solvent. This is a phenolic-laminating varnish specifically designed for applications where components are exposed up to 260°C for extended duration of time.
Nanomodification in ablative composites was achieved by incorporating Graphistrength MWCNT supplied by Arkema, Inc. They had a typical diameter of 10–15 nm and length between 1–10
In the previous studies by the same research team, 1 wt% MWCNT was dispersed into phenolic resin by using three different techniques: sonication (SN), high shear mixing (HS), and combination of sonication and high shear mixing (SNHS). To analyze the effect of dispersion of MWCNT in phenolic resin with respect to these different mixing techniques thermogravimetric analysis (TGA) was performed on all the samples. Characterization was carried out according to ASTM E 2550, the test method for thermal stability, and ASTM E 1131, the test method for compositional analysis. Samples were taken from a fractured surface of the phenolic coupons and had weights of 11.6–21.8 mg. All samples were placed in aluminum pans and heating was carried at a heating rate of 20°C/min, using argon as a purge gas at the rate of 100 mL/min. All samples were heated to a peak temperature of 800°C. Use of inert atmosphere in TGA analysis is the key as MWCNT would oxidize in open air above temperatures 650°C.
TGA showed that the majority of the mass loss for SN and HS samples was in the range of 200°C to 500°C, while this range was 300°C to 550°C for SNHS samples. SNHS samples also showed better thermal stability for higher temperatures above 550°C as compared to other samples. Figure
TGA analysis of phenolic resin samples containing 1 wt% MWCNT using different mixing techniques: sonication (SN), high shear mixing (HS), and combination of sonication and high shear mixing (SNHS).
Sonication was used to disperse MWCNT in IPA. Since SC-1008 contained IPA as the solvent, its usage as a dispersion medium was ideal. The amplitude of sonication varied between 25%–45% to compensate for subsequent increase in viscosity. The power of the sonicator was kept below 40 watts to avoid damaging the MWCNT and to prevent rapid build up of heat. Mixing could only proceed for two minutes before viscosity of the mixture increased, resulting in cessation of sonication. Then the resulted mixture was mixed with phenolic resin in a high shear mixer for 15 minutes at 40 Hz which provided shear rate of 77,000 s−1. A water cooling system was used to avoid overheating of the mixer and resin. The temperatures of the resin and mixing chamber were maintained below 50°C. Three different loadings of MWCNT were introduced in the phenolic resin: 0.5, 1, and 2 wt%. After high shear mixing, the resultant mixture was kept in a vacuum oven for 24 hours at 80°C, so that excess IPA that was used for dispersion of MWCNT using sonication, got evaporated. It was observed that the viscosity increased very rapidly as the wt% loading of MWCNT increased. Highly viscous resin imposes tremendous challenges in making prepregs using even hand lay-up technique. Carbon fabric must be completely soaked in the resin in order to maintain desired fiber volume percentage and density of the composite. Therefore, MWCNT loading was not increased beyond 2 wt%.
The second step of the manufacturing process was the production of prepregs using hand lay-up technique. Carbon fabric and resin were taken in proportion to achieve desired fiber volume fraction and density. Desired fiber volume fraction was 50% by volume. Carbon fabric was soaked with nanomodified phenolic resin using hand rollers. Prepregs were weighed after impregnation and then placed into a preheated oven at 100°C for 30 minutes to achieve B-staging. The prepregs were again weighed in order to evaluate mass loss. To prevent premature curing of the material, prepregs were stored into the freezer. Further these prepregs were manually cut into 12 mm by 12 mm squares using a cutting board.
Ablative test panels were produced by compression molding using a Wabash Automatic Hydraulic Compression Press. Two-part compression mold with cavity of 120 × 120 × 12.7 mm
Ablative test specimens of size 12.7 mm × 12.7 mm × 12.7 mm cube were cut using a tile saw. Six specimens were cut in each category. Two holes of diameter 1.5 mm were made at depths 2.7 mm and 7.7 mm from the rear surface to place thermocouples during testing. The distance between the holes was 2.5 mm as shown in Figure
Placement of thermocouples in the ablative specimens.
The oxyacetylene test bed (OTB) is a small scale ablation testing device at the University of Texas at Austin, that is, capable of producing a heat flux of 1450 W/cm2 and flame temperature up to 3000°C using a calibrated oxyacetylene welding torch. Similar types of devices have been developed at Sapienza University of Rome [
OTB setup contains a data acquisition system to measure the in situ temperature of the test specimens using embedded thermocouples. The distance of the torch nozzle from the test specimen was used to calibrate the heat flux output of the OTB. A slug calorimeter was used to calculate these values in which a copper slug of known mass was exposed to the torch, and the power into the slug was derived from the measured time versus temperature curve of the slug [
For this experiment, all materials were tested for duration of 45 seconds at a distance of 6 mm and applying approximately heat flux of 1000 W/cm2. Six identical specimens were tested in each category. Masses and lengths of all test specimens were measured before and after the ablation test in order to measure the percentage mass loss and recession in length. Peak temperature at depths 2.7 mm and 7.7 mm from the rear face (which is 10 mm and 5 mm from flame front face) was obtained from the data acquisition system using thermocouples. Figure
Scanning electron microscopy (SEM) analysis was performed on all test specimens after ablation testing. High resolution SEM images were obtained at different orientations of the char microstructures in order to understand the effects of ablative testing on materials. SEM images were taken using Leo SRV-32 scanning electron microscope, under an argon gas atmosphere at different magnifications.
Ablative test specimens were exposed to a heat flux of 1000 W/cm2 for approximately 45 seconds. Results of percentage mass loss, recession, and peak in-situ temperatures presented herein are averages of six identical specimens tested in each category.
All ablative samples were weighed before and after the ablation testing in order to measure mass loss. It was observed that percentage mass loss for the ablative test specimens decreased with increasing MWCNT weight percentage. Results for percentage mass loss are shown in Table
Average percentage mass loss after ablation testing.
MWCNT wt% | Percentage mass loss (%) |
---|---|
|
26 |
|
25.5 |
|
25 |
|
23 |
Note: values in brackets indicate standard deviation.
Decrease in percentage mass loss with increase in MWCNT wt%.
The thickness of each specimen measured before and after the test was used to determine average recession. The thickness of samples measured after the ablation test was only thickness of virgin material, excluding char thickness. Recessions for nanocomposites were decreased with increasing MWCNT wt%. Results for the recession are shown in Table
Average recession after ablation testing.
MWCNT wt% | Recession (mm) |
---|---|
|
0.83 |
|
0.59 |
|
0.42 |
|
0.38 |
Note: values in brackets indicate standard deviation.
Decrease in recession with increase in MWCNT wt%.
During ablation testing, two K-type thermocouples, T1 and T2, were placed inside the specimens at depths of 2.7 mm and 7.7 mm from the rear face, respectively. Figure
Figure
Peak in-situ temperatures at 5 mm and 10 mm depths from flame front surface.
After ablation testing, test specimens were cut into two halves, and SEM images on flame front and the top of cut surface were obtained. Flame front surface was also observed at certain degree tilt. Multiple images were taken on the top surface at distances of 2, 4, 6, 8, and 10 mm from flame front surface. Figure
SEM Images on specimens after ablation testing.
SEM images of control ablative samples.
SEM images of 0.5 wt% MWCNTs ablative nanocomposite samples.
SEM images of 1 wt% MWCNTs ablative nanocomposite samples.
SEM images of 2 wt% MWCNTs ablative nanocomposite samples.
Ablative panels were successfully manufactured with phenolic resin, rayon precursor based carbon fabric, and MWCNT using similar procedure as that of Cytec Engineered Materials’ MX-4926 MC (molding compound) ablative panels. The combination of sonication and high shear mixing was used for uniform dispersion and separation of individual MWCNT. Ablative test specimens were tested at a heat flux of 1000W/cm2 using the oxyacetylene test bed for 45 seconds. The test specimens were compared on the basis of percentage mass loss, recession, and peak in-situ temperatures at depths of 10 mm and 5 mm from flame front. The percentage mass loss for control test specimen was 26%, whereas it was 23% for nanocomposite specimens containing 2 wt% MWCNT. Average recession was 0.83 mm for control test specimens, while it was reduced to 0.39 mm for nanocomposite specimens 2 wt% MWCNT. The peak in-situ temperatures at depths 10 mm and 5 mm from flame front showed decreasing trends as the MWCNT wt% increased. Based on these results, it was concluded that the increase in wt% of MWCNT improved ablation and insulation performance of nanocomposites.
None of the authors has any conflict of interests with these commercial companies, such as financial gain.
The authors would like to thank Mr. Jon Weispfenning of Cytec Engineered Materials for providing technical support, Mr. Aram Mekjian of Mektech Composites for supplying SC-1008 phenolic resin, Mr. Evan J. Silo and Mr. Mark D. Finn of McLube for providing mold release agents, Mr. Blake Johnson of the University of Texas at Austin for helping in conducting ablation testing, and Mr. Ray Cook and Mr. Shane Arabie of Ingram School of Engineering at Texas State University for making compression molds. In this research, authors have used commercially available multiwall carbon nanotubes (MWCNT) from Arkema, Inc. The references of MMT organoclay, POSS, and carbon nanofibers (CNF) in this paper are primarily to provide the readers with what has been done. In the past, the authors also have used different nanoparticles, such as MMT organoclay, POSS, and CNF from commercial suppliers to evaluate their effects on ablative performance of composites.