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Elastomeric ablative nanocomposites used in hyperthermal environments.

INTRODUCTION

Inorganic/organic nanofillers, nanorods, nanotubes, and nanofibers have been used to fabricate nanocomposites for specific applications. These nanoscale reinforcements enhance thermal, electrical, magnetic, and mechanical properties of the host matrixes due to their large specific surface area and nanoscale interaction with the recipient (1). Polymer nanocomposites (PNCs) are being used in aerodynamics, sports, vehicles, medicines, building structures, etc. as they have the capability to fulfill the required needs of the hour (2), (3). PNCs have usually low density with high strength to weight ratio as compared to metal and ceramic matrix composites (1). Carbon nanotube, a homogeneous benzene structure of carbon atoms that turn themselves into a tube in the presence of a suitable catalyst and under specific pressure/temperature conditions, is a remarkable invention by Illjima in 1991 (4). This carbon phase has opened new horizons of study in almost all fields of research and development. Multiwalled carbon nano-tubes are used as reinforcing filler in PNCs due to its outstanding strength, thermal stability, electromagnetic effects, and fire retardant capability (5), (6). Ethylene propylene diene monomer (EPDM) is a polymer with good thermal/ electrical insulation, tensile strength, aging resistance, air tightness, and low density (7), (8).

Ablative composites are used as thermal insulating materials for intercontinental ballistic missiles (ICBM) and space shuttles to protect their structures from hypersonic and hyperthermal environments encountered during their mission (9-11). EPDM composites have good thermal, mechanical, and ablation characteristics, and are used for low as well as high temperature insulations (10), (12), (13). The incoming heat flux having velocity and temperature up to Mach 10 and 3000 K, respectively, during the ablation is encountered by the aerodynamic surface of a space vehicle or ICBM as illustrated in the schematic ablation mechanism in Fig. 1. This heat flux is blocked, dissipated, and reflected back by the endothermic heat quenching phenomena occurred within the ablative polymer composite, i.e., transpirational, reradiational, vaporizational, and charring heat fluxes (14), (15). A protective solid layer is developed due to the char--reinforcement reactions during ablation that helps to reduce thermal/mechanical erosion of the ablator (2), (16).

Besides many general purpose applications of EPDM composites, they are also used for the thermal protection of rocket motors. The novelty of this work resides in the ablation investigation of multiwalled carbon nanotube (MWCNT)/EPDM composites and to execute the potentials of the nanotubes to enhance the ablation resistance and to reduce the backface temperature elevation during the ultrahigh temperature flame exposure on the surface of the fabricated ablative nanocomposites. In addition, thermal decomposition/transport and mechanical properties of the nanocomposites having variant MWCNTs concentrations were also carried out in this article.

EXPERIMENTAL

Materials

Carbon black (N330) was supplied by Hebei Dag-uangming Juwuba, Carbon Black Co., Ltd. Sulfur, zinc oxide, and stearic acid were supplied by Merck, Germany. Mercaptobenzthiazole Disulfide (MBTS) and cyclohexyl benzthiazyl sulfenamide (HBS) were purchased from Dalian Richon Chemical Co. Ltd, China. Aromatic oil was purchased from International petrochemicals Pvt., Ltd, Pakistan. MWCNTs (fabricated through chemical vapor deposition (CVD) method with Fe catalyst, purity > 95%, outer diameter 20-30 nm, inner diameter 5-10 nm, and length 10-30 [mu]m, average aspect ratio 2400:1) were received from Nanoport Co., Ltd, China. EPDM (KELTAN 4331A) rubber was received from Technical Rubber Products, China. The contents of ethylene (53 wt%), propylene (4 wt%), and diene third monomer (33-44 wt%) are present in the EPDM rubber, used as matrix for ablative composite fabrication. Silane coupling agent (SCA-98) was supplied by STRUKTOL, USA.

Formulations of Nanocomposites

Basic composition of EPDM composite (El) is illustrated in Table 1. MWCNTs were impregnated with five diverse concentrations as 0.1 wt% (E2), 0.3 wt% (E3), 0.7 wt% (E4), and 1 wt% (E5) in the basic formulation El.

TABLE 1. Basic formulation of EPDM composite.

EPDM    Nano  carbon         ZnO  Stearic    HBS  Sulfur
(wt%)  (wt%)   (wt%)  acid (wt%)    (wt%)  (wt%)   (wt%)

100      40        5         2.5        2     2.5    2.5

MBTS       Aromatic    SCA    Wax
oil (wt%)     (wt%)  (wt%)  (wt%)

2.5              10      4    2.5

BBS: cyclohexyl benzthiazyl sulfenamide;
MBTS: mercaptobenzthiazole disulphide;
SCA: silane coupling agent.


Fabrication of Ablative Nanocomposites Specimens

Five diverse loadings (0-1 wt%) of MWCNTs were incorporated along with the aforementioned filler SCA (17), (18) and processing aids into the EPDM rubber matrix using dispersion kneader at 110[degrees]C for 30 min and two roller mixing mill at temperature 70[degrees]C and 40 rpm roller's speed for 20 min. The nanocomposite specimens for the ablation and mechanical investigation were fabricated on the hot isostatic hot press at 130[degrees]C and 1500 psi for 40 min. The specimens for ablation characteristics have 100 mm x 100 mm x 10 mm dimensions, thermal transport properties have 25 mm X 25 mm X 2 mm dimension, and tensile testing composite samples were fabricated according to the ASTM D412-98A (19).

Ablation Testing of the Nanocomposites

Backface Temperature Monitoring. Ablation testing of the composite specimens was accomplished according to the ASTM E285-08 (19), (20). Experimental setup for ablation testing is displayed in Fig. 2, in which oxy-acetylene (0-A) torch is exposed on the surface of the ablator, i.e. engraved in a domestically made ablative fixture.

Three K-type thermocouples were attached with an adhesive tape at the backface central region of the nanoablator. These thermocouples were connected to the data logger TECPEL 319 (0.1[degrees]C temperature resolution), i.e. also in link with the laptop through RS-232 data cable. Both oxygen and acetylene gases have the same flow rate, i.e. 0.35 [m.sup.3]/h during the ablation test. 0-A torch was kept at a distance of 10 mm far from the surface of the ablator. Temperature evolution with time was monitored and displayed on the laptop, synchronously during the high temperature flame exposure for 200 s on the surface of the polymer composite. Scanning electron microscopy (SEM, JSM 6490 A Jeol, Japan) along with the energy dispersive X-ray spectroscopy (EDS) was used to elucidate the uniform dispersion of MWCNTs in the EPDM rubber matrix and char morphology/composition of the ablated nano-composite specimens.

Ablation Rates. Ablation rates, insulation indexes, and percent char yields of the MWCNT/EPDM composites were measured according to the following formulae (21), (22):

Linear ablation rate = [v.sub.1] = [[T.sub.1]-[T.sub.2]]/t (1)

Mass ablation rate = [v.sub.m] = [[M.sub.1]-[M.sub.2]]/t (2)

% char yield = Y = [[M.sub.1]-[M.sub.2]] x 100 / [M.sub.1](3)

Insulation index = [I.sub.T] = [t.sub.T] / d (4)

where [[T.sub.1], [[M.sub.1] and [[T.sub.2], [[M.sub.1] are the thickness and mass of the ablator before and after O-A flame exposure, respectively, and t is the ablation testing duration. [t.sub.T] is the time required to elevate the backface temperature at a specific temperature (7) and d is the specimen thickness.

Thermal Gravimetric/Differential Thermal Analysis

Perkin Elmer diamond Thermogravimetric/differential thermal analyzer (TG/DTA) was used to analyze the thermal degradation and heat flow response of the rubber ablative composites within the temperature range 25-830[degrees]C in air environment with temperature elevation rate 10 [degrees]C/min.

Thermal Conductivity/Impedance

Thermal conductivity ([[lambda].sub.N]) of the nanocomposite specimens were carried out using ASTM E 1225-99. The tested specimen has 1 inch2 area and 3 mm thickness. Copper was used as a relativistic material for the measurement of ([[lambda].sub.N]) and R. Schematic illustration of comparative guarded longitudinal heat flow system in Fig. 3 shows the possible locations of temperature sensors, heating source, water heat sink, temperature data logger, and a laptop. Time--temperature contours of all thermocouples located at specific positions were monitored on the laptop screen through 0Q610 6 Channel Thermocouple data logger ([[lambda].sub.N]) of the EPDM nanocomposite specimens was measured using Eq. 5.

Thermal conductivity of specimen (W/m K)

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (5)

where

Heat flow at top bar = [Q.sub.T]' = [[lambda].sub.s]([T.sub.2]-[T.sub.1])/([D.sub.2]-[D.sub.1])

Heat flow at bottom bar = [Q.sub.S]' = [[lambda].sub.s]([T.sub.6]-[T.sub.5])/([D.sub.6]-[D.sub.5])

[[lambda].sub.s] = Thermal conductivity of the copper meter bar.

[T.sub.1], [T.sub.2], [T.sub.3], [T.sub.4], [T.sub.5] and [T.sub.6] are the temperatures of six thermocouples and [D.sub.1], [D.sub.2], [D.sub.3], [D.sub.4], [D.sub.5], and [D.sub.6] are the corresponding positions of these six thermocouples.

Thermal impedance (I) of a material is its ability to resist thermal/temperature fluctuations in a variable heating environment. To evaluate I of the rubber nanocompo-sites, four thermocouples have been used instead of six in aforementioned thermal conductivity experimental setup. I was measured according to ASTM D5470-03 and Eq. 6.

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (6)

where

Heat flow (Q) = voltage (V) x current (I)

Temperature of upper bar surface in contact with the sample (K) Ta = (T1-T2)

Temperature of lower bar surface in contact c with the sample (K) = Td = (T3-T4)/4.73

(K) = upper temperature of upper bar;

T2 (K) = lower temperature of upper bar

T3 (K) = upper temperature of lower bar; T4 (K) = lower temperature of lower bar

d1 (m) = distance between Ti and T1; d2 (m) = distance between T, and S sample.

d3 (in) = distance between T3 and T4; da (m) = distance between Sample and T3.

Tensile Testing

Ultimate tensile strength, elongation at break, and modulus of elasticity of the nanocomposite specimens were appraised on the universal tensile testing machine (AG-20KNXD Plus, Shirnadzu) according to the ASTM D412-98A and Shore A hardness of the rubber composites was evaluated on Torsee, Tokyo testing machine.

RESULTS AND DISCUSSION

Dispersion of MWNTs

The heated environment in the internal dispersion kneader makes the diffusion of nanotubes easy for spreading them within the polymer matrix. The presence of the SCA in the rubber formulation enhances the bond strength among the matrix and the nanotubes that has a positive effect on the thermal, ablation, and mechanical characteristics of the fabricated composites [17, 18]. An even distribution of MWCNTs in the EPDM rubber matrix has been achieved by tri-axial flow of the material during its passage through the heated twin roll nip of two roller mixer. The dispersion of nanotubes in the rubber matrix is analyzed in the SEM micrographs at different magnifications, i.e. portrayed in Fig. 4a-d. Spot elemental analysis of carbon nanotube is depicted in Fig. 4e that show the presence of 100% carbon in the MWCNT. The evenly dispersed MWCNTs have efficiently enhanced the ablation resistance; reduced the backface temperature elevation during the O-A flame flow on the surface of the fabricated nanocomposites; augmented the thermal stability/ insulation characteristics; and remarkably improved the mechanical properties of the nanoablatives which are clearly observed in the proceeding investigation outcomes.

Ablation Testing of the Nanocomposites

Backface Temperature Monitoring. High temperature/ velocity 0-A flame was exposed on the surface of the nanocomposites and simultaneously temperature elevation at the back facet of the ablator with respect to exposure time was monitored. Figure 5a elucidates the temperature evolution trend of all nanocomposites collectively that exhibits a low temperature elevation rate up to 80s due to the effective transpirational, vaporizational, and reradiational cooling effects generated within the ablator during ablation test. In the leading 60s, maximum conductive heat flux is transported through the ablator that enhances the temperature evolution rate (TER) due to thermal/mechanical erosion of the nanocomposites. A protective strong solid char layer is developed due to the char--rein-forcement reactions that outstandingly reduce the TER, thermal conduction through the ablator, and mechanical erosion rates of the polymer composites. Due to the polymer melting and char reinforcement reactions, a protective char layer is developed during the high temperature flame impingement on the surface of the ablator. Consequently, it reduces the ablation or surface receding rate of the testing specimen (23-25). The backface temperature profiles of the testing composite specimens under the ultrahigh temperature 0-A flame exposure on their surfaces, were monitored online using a temperature data logging system. The variation in backface temperature elevation up to 140s was owing to the uneven heat atmosphere at the surface of the ablator. But after the formation of a char-reinforcement layer, heat transport through the composite specimens was adopted a discrete level of backface temperature, i.e. El > E2 > E3 > E4 > E5. Due to the low thermal conductivity of MWCNTs compared to the carbon black, progressive incorporation of the nanotubes effectively reduced the backface temperature elevation accordingly, during the ablation investigation (26).

Backface temperature evolution rate (BTER) and peak back facet temperatures (PBT) were measured from time--temperature contours of the ablators and depicted in Fig. 5b. BTER and PBT were reduced with increasing MWCNTs concentration in the rubber matrix due to the high thermal stability, large surface area, and nanoscale interaction of nanotubes with the polymer chains of the rubber matrix that restricted the thermal motion of the molecular chains during flame exposure (27), (28). The minimum BTER and PBT were measure for E5 ablator, i.e. 0.41 [degrees]C and 112[degrees]C, respectively as compared to El ablator that have 0.58 [degrees]C/s and 144[degrees]C, which means that heat conduction through the composite specimens is reduced with increasing the nanotubes loading in the rubber matrix.

Insulation indexes ([I.sub.T]) of the PNCs at 60[degrees]C, 80[degrees]C, and 110[degrees]C backface temperatures were measured using Eq. 4 and displayed in Fig. Sc. The data simulate that [I.sub.T] is progressed with increasing the nanotubes concentrations up to 0.5 wt% loading at 60 and 80[degrees]C backface temperature of the nanocomposite and there is approximately no [I.sub.T] further elevation noticed for MWCNTs extra loadings. The 1 wt% MWCNTs loaded ablative nanocomposite has the maximum IT at 110[degrees]C backface temperatures that elucidates the potential of nanotubes to enhance the capability of the E5 ablator to stay in the hyperthermal/sonic environment for a prolong duration compared with the pristine rubber matrix, counterpart.

Ablation Resistance. Linear ablation rates ([v.sub.1]) in mm/s, mass ablation rates in g/s ([v.sub.m]), and % char yield (Y) of the ablated composite specimens were determined using Eqs. 1-3. E4 ablator has the minimum [v.sub.1], [v.sub.m] and Y, i.e., 0.04 mm/s, 0.27 g/s, and 17%, respectively as shown in Fig. 6. The 1 wt% addition of MWCNTs in the EPDM rubber matrix improves the linear ablation resistance up to 125%, progresses the mass erosion resistance up to 74%, and reduces Y up to 9%, respectively compared with the El ablator. Ablation resistance is augmented with increasing nanotubes concentration in the rubber matrix due to excellent thermal stability of MWCNTs (29). A strong protective char layer is developed during the ablation test due to char--reinforcement reaction that reduces the ablation rates as well as peak backface temperature evolution which is clear from SEM micrographs of E5 ablated specimen depicted in Fig. 7a-c. The microlevel porosity in the ablated char layer scoops up the transpira-tional and vaporizational heat fluxes that cools down the back facet of the polymer composite during ablation test. C, S. Si, Zn, 0, Mg, and Ca were found in the EDS analysis of the E5 ablated char, i.e. depicted in Fig. 7d. The element carbon (C) comes from the polymer, reinforced black carbon, and MWCNTs; sulfur (S) is used as a crosslinker; zinc (Zn) is used as an activator; CaO, MgO, and Cl are present as impurities in the composite formulation (19).

Thermal Decomposition

Thermogravimetric analysis of the nanocomposites in the temperature range 25-830[degrees]C is displayed in Fig. 8a that simulates a two-step thermal degradation. The first weight loss can be scrutinized in the temperature range 200-450eC due to the evaporation of aromatic oil, wax, and other volatile products. The second major thermal degradation is observed in the temperature range 450-520[degrees]C due to the polymer matrix pyrolysis. The incorporation of MWCNTs in the rubber matrix enhances the thermal stability of the nanocomposites as clear from Fig. 8a and b due to high thermal endurance and nanoscale interaction of the nanotubes that restricts the thermal motion of molecular polymer chains in the heating environment. Figure 8b illustrates the improvement in thermal stability at 600[degrees]C of the composite specimens with increasing nanotubes concentration in the host matrix (2), (15).

Differential thermal analysis of the nanocomposite specimens is depicted in Fig. 8b. It is observed that with increasing the MWCNTs concentration the polymer matrix, melting temperatures of the fabricated composites are preceded due to the effective thermal resistance offered by the nanotubes to constraint polymer melting/ flow (30), (31).

Thermal Conductivityllinpedance

Thermal conductivity and thermal impedance of the MWCNT/EPDM nanocomposite were measured at 373 K according to the Eqs. 5 and 6 and the accumulated data are presented in Fig. 9. The aforementioned property is diminished with increasing filler to matrix ratio and the maximum reduction is observed, i.e., 95% for E5 relative to base composite formulation (El). The 1 wt% MWCNTs incorporated rubber nanocomposite has 29% improved thermal impedance compared to the El composition due to the excellent thermal stability, heat quenching through phonon entrapping, and the uniform dispersion of the nanotubes in the host polymer matrix (32-34). Thermal transport through a solid material strongly depends upon the anisotropic factor, [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] and are the parallel and perpendicular thermal conductivity factors respectively). Pyrolytic form of graphite has the highest value of the anisotropy factor of thermal conductivity among all natural existing materials [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (35). Thermal conductivity through the carbonaceous materials depends upon the particle size, form, and shape of carbon (32). Regardless of many similarities of heat flow mechanism in graphite and MWCNTs, a high density hexagonally packed MWCNT structure must have a much lower anisotropy of thermal and electrical conductivity due to a contribution of the circumferential bypass pathway to perpendicular thermal conductivity. The transfer rate between the nonparallel carbon nanotubes is extremely low due to the presence van der Waals forces among themselves (26). Thermal conductivity of a polymer filled with high particles loading largely decrease with increasing temperature (36), (37).

Mechanical Properties

Stress--strain contours of the nanocomposites are displayed in Fig. 10a that shows an augmentation in tensile strength while elongation at break suffers with the progressive addition of carbon nanotubes in the EPDM rubber matrix.

Figure 10b elucidates that 1 wt% incorporation of MWCNTs in the rubber matrix elevates the tensile strength and elastic modulus up to 41% and 111%, respectively of the nanocomposite due to even dispersion of nanotubes in the polymer matrix, strong MWCNT--polymer interaction, and outstanding mechanical strength of the carbon nanotubes (38). MWCNTs have extremely high ultimate tensile strength (50-100 GPa) and elastic modulus (500-1000 GPa). On their even incorporation into the polymer matrix, efficiently enhance the mechanical properties of the composite (39), (40).

A progress in Shore A hardness of the composite specimens is observed in Fig. 10c with increasing filler contents in the rubber matrix. The addition of MWCNTs in the host polymer matrix augments the rubber hardness due to the high modulus and hardness of the nanotubes. Additionally, the progressive incorporation of carbon nanotubes reduces the polymeric flow ability of the host matrix that eventually enhances the rubber hardness of the fabricated composites (39).

CONCLUSION

MWNTs were evenly dispersed in the EPDM rubber matrix using dispersion kneader and two roller mixing mill. The progressive incorporation of nanotubes into the polymer matrix has remarkably enhanced the anti-ablation performance of the nanocomposites. Linear/mass ablation resistance have been augmented up to 125% and 74% while % char yield is reduced up to 9% and peak back-face temperature during the ablation test has been diminished up to 35[degrees]C while insulation index is enhanced up to 51% with 1 wt% addition of carbon nanotubes in the host matrix. Thermal stability and heat quenching capability of the PNCs have been elevated with increasing nanotubes concentration in the rubber matrix. Thermal transport study reveals that the incorporation of MWCNTs into the rubber matrix has effectively enhanced the thermal resistance of the fabricated composite specimens up to 95%. Ultimate tensile strength and Shore A rubber hardness of the rubber composites have been improved while elongation at break is reduced with increasing filler to matrix ratio due to the even dispersion, durable filler to matrix bonding, and excellent mechanical strength of MWCNTS.

ACKNOWLEDGMENTS

The authors have greatly acknowledged Sheikh Zolfi-qar Ahmed, MD Longman mills for providing facilities regarding fabrication of nanoablative specimens; Pakistan Railways Carriage Factory, Islamabad for ablation testing, and Dr. Mohammad Mujahid, Principal SCME, MUST for SEM/EDS analysis.

Correspondence to: Nadeem lqbal; e-mail: nadeemiqbal@scme.nust. edu.pk

DOI 10.1002/pen.23573

Published online in Wiley Online Library (wileyonlinelibrary.com) [c] 2013 Society of Plastics Engineers

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Nadeem lqba1, (1) Sadia Sagar,1 Mohammad Bilal Khan, (2) Hafiz Muhammad Rafique (3)

(1) School of Chemical & Materials Engineering (SCME), NUST, Islamabad, Pakistan

(2) Centre for Energy Systems (CES), NUST, Islamabad, Pakistan

(3) Department of Physics, University of the Punjab, Lahore, Pakistan
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Author:Iqbal, Nadeem; Sagar, Sadia; Khan, Mohammad Bilal; Rafique, Hafiz Muhammad
Publication:Polymer Engineering and Science
Article Type:Report
Geographic Code:9PAKI
Date:Feb 1, 2014
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