Ž . Applied Clay Science 15 1999 67–92 Polymerrlayered silicate nanocomposites as high performance ablative materials Richard A. Vaia a, ) , Gary Price b , Patrick N. Ruth c , Hieu T. Nguyen d , Joseph Lichtenhan d,1 a Air Force Research Laboratory, AFRL r MLBP, Bldg. 654, 2941 P St., WPAFB, OH 45433-7750, USA b UniÕersity of Dayton Research Institute, Dayton, OH 45469, USA c Raytheon STX, Air Force Research Laboratory, AFRL r PRSM, 10 E. Saturn BlÕd., Edwards AFB, CA 93524-7680, USA d Air Force Research Laboratory, AFRL r PRSM, 10 E. Saturn BlÕd., Edwards AFB, CA 93524-7680, USA Received 18 October 1998; received in revised form 10 March 1999; accepted 10 March 1999 Abstract Ž .Ž . The ablative performance of poly caprolactam nylon 6 nanocomposites is examined. A relatively tough, inorganic char forms during the ablation of these nanocomposites resulting in at Ž . least an order-of-magnitude decrease in the mass loss erosion rate relative to the neat polymer. Ž . This occurs for as little as 2 wt.% ;0.8 vol.% exfoliated mica-type layered silicate. The presence of the layers does not alter the first-order decomposition kinetics of the polymer matrix. Instead, the nanoscopic distribution of silicate layers leads to a uniform char layer that enhances the ablative performance. The formation of this char is only minutely influenced by the type of organic modification on the silicate surface or specific interactions between the polymer and the aluminosilicate surface, such as end-tethering of a fraction of the polymer chains through ionic interaction to the layer surface. Thus, the enhancement in ablative performance should be general for the class of exfoliated layered silicaterpolymer nanocomposites. q 1999 Elsevier Science B.V. All rights reserved. Ž . Keywords: nanocomposites; poly caprolactam ; ablation; decomposition; pyrolysis; intumescent ) Corresponding author. Tel.: q1-937-255-9184; Fax: q1-937-255-9157; E-mail: [email protected]1 Current address: Hybrid Plastics, 18237 Mount Baldy Circle, Fountain Valley, CA, USA. 0169-1317r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. Ž . PII: S0169-1317 99 00013-7
Transcript
Ž .Applied Clay Science 15 1999 67–92
Polymerrlayered silicate nanocomposites as highperformance ablative materials
Richard A. Vaia a,), Gary Price b, Patrick N. Ruth c,Hieu T. Nguyen d, Joseph Lichtenhan d,1
a Air Force Research Laboratory, AFRLrMLBP, Bldg. 654, 2941 P St., WPAFB,OH 45433-7750, USA
b UniÕersity of Dayton Research Institute, Dayton, OH 45469, USAc Raytheon STX, Air Force Research Laboratory, AFRLrPRSM, 10 E. Saturn BlÕd.,
Edwards AFB, CA 93524-7680, USAd Air Force Research Laboratory, AFRLrPRSM, 10 E. Saturn BlÕd., Edwards AFB,
CA 93524-7680, USA
Received 18 October 1998; received in revised form 10 March 1999; accepted 10 March 1999
Abstract
Ž . Ž .The ablative performance of poly caprolactam nylon 6 nanocomposites is examined. Arelatively tough, inorganic char forms during the ablation of these nanocomposites resulting in at
Ž .least an order-of-magnitude decrease in the mass loss erosion rate relative to the neat polymer.Ž .This occurs for as little as 2 wt.% ;0.8 vol.% exfoliated mica-type layered silicate. The
presence of the layers does not alter the first-order decomposition kinetics of the polymer matrix.Instead, the nanoscopic distribution of silicate layers leads to a uniform char layer that enhancesthe ablative performance. The formation of this char is only minutely influenced by the type oforganic modification on the silicate surface or specific interactions between the polymer and thealuminosilicate surface, such as end-tethering of a fraction of the polymer chains through ionicinteraction to the layer surface. Thus, the enhancement in ablative performance should be generalfor the class of exfoliated layered silicaterpolymer nanocomposites. q 1999 Elsevier ScienceB.V. All rights reserved.
1 Current address: Hybrid Plastics, 18237 Mount Baldy Circle, Fountain Valley, CA, USA.
0169-1317r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved.Ž .PII: S0169-1317 99 00013-7
( )R.A. Vaia et al.rApplied Clay Science 15 1999 67–9268
1. Introduction
Ablative materials are critical for insulation applications in space and launchsystems, protecting aerodynamic surfaces, propulsion structures, payloads andground equipment from the severe effects of very high temperatures and incidentheating rates. For example, during firing of a solid rocket motor, internalinsulation positioned between the case and the propellant is subjected totemperatures that may exceed 20008C and pressures of 1000 psi or greater. In
Žaddition, the insulation experiences a variety of operational stresses compres-.sive, tensile, shear and a chemically oxidizing atmosphere with gas and
particulate velocities that may range from Mach 0.01 to 10q .Thermal protection via ablation is achieved through a self-regulating heat and
mass transfer process involving an insulator with low thermal conductivity andthe sacrificial pyrolysis and concomitant formation of a tough refractory char on
Žthe insulator surface D’Alelio and Parker, 1971; Youren, 1971; Kershaw andStill, 1975; Deuri et al., 1988; Katzman et al., 1995; Cho, 1996; Cho et al.,
.1993; Kahn, 1996; Oyumi, 1998 . Above the decomposition temperature, theinsulator produces pyrolysis gases in the reaction zone and degrades to a char
Ž .layer at higher temperatures Fig. 1 . The majority of mass loss occurs in thereaction zone. The presence of the char layer regulates penetration of heat from
the surface and produces a steep temperature gradient. The pyrolysis gases andthermal expansion generate stresses in the char layer and if not reinforced, thedegraded material easily breaks away increasing the surface erosion rate.
ŽTherefore, these ablative materials generally consist of a polymeric matrix suchŽ . Ž .as butadiene-acrylonitrile NBR , ethylene-propylene diene EPDM or pheno-
. Ž . Žlic reinforced with a high volume fraction )50% of fibers chopped or. Žwoven fabric of graphite, carbon, glass or Kevlare or fillers silica, metal
.oxides, asbestos or silicates .Several limitations associated with these composite ablatives have motivated
current efforts to identify next generation materials. Many times, the resultingchars are structurally weak and susceptible to mechanical erosion, severelyreducing the lifetime of the insulator or necessitating additional insulationthickness, which reduces the volume available for propellant. Additionally,provided that the char remains intact, resistance to thermal and chemicalablation ultimately limits lifetime and performance. Most critical though is thatthe composite-like nature of these ablatives severely limits their processibilityand requires them to be pattern cut, and laid into place by hand. This not onlysignificantly increases overall system production costs, but also generates signif-
Ž .icant amounts of scrap material as much as 50% that cannot be reused. Inaddition, a number of environmental issues have recently become of concernregarding the disposal of hazardous scrap and the use of solvents and adhesivesrequired to glue composite insulation to the motor case and propellant.
.et al., 1997, 1998 , encourages the examination of this unique class of evolvingmaterials as potential ablatives. In contrast to conventional composites contain-ing micron-scale or larger reinforcing constituents, nanocomposites containultrafine phase dimensions, typically on the order of a few nanometers, and thusexhibit unique combination of properties typically not found in traditionalcomposites. Thermoplastic PLS nanocomposites exhibit substantial improve-
Žments in mechanical Kojima et al., 1993; Usuki et al., 1993a,b; Yano et al.,1993; Burnside and Giannelis, 1995; Messersmith and Giannelis, 1995; Okada
. Ž .and Usuki, 1995 , barrier Yano et al., 1993; Messersmith and Giannelis, 1995Ž .and solvent up-take Burnside and Giannelis, 1995 properties relative to the
neat parent resin while retaining melt processibility.The nanoscale structure of PLS nanocomposites is associated with the 0.96
nm thick aluminosilicate layers of an organically modified mica-type layeredŽ .silicate e.g., montmorillonite . In contrast to pristine mica-type silicates which
contain alkali metal and alkali earth charge-balancing cations, organically
2 See also Nanocomposites Showing Promise in Automotive and Packaging Roles, ModernPlastics, Feb. 1998, pp. 26–27.
( )R.A. Vaia et al.rApplied Clay Science 15 1999 67–9270
Ž .modified layered silicates OLS contain alkyl ammonium or phosphoniumŽ .cations Lagaly, 1981, 1986; Vaia et al., 1994 . The presence of these organic-
modifiers in the galleries renders the originally hydrophilic silicate surface,organophilic. The mesostructure of the nanocomposite depends on the degree ofpenetration of the polymer into this OLS framework, producing structuresranging from intercalated to exfoliated. Polymer penetration resulting in finiteexpansion of the silicate layers produces intercalated hybrids consisting ofordered multilayers of alternating polymerrsilicate with a repeat distance of afew nanometers. Extensive polymer penetration resulting in delamination of thesilicate layers produces exfoliated nanocomposites consisting of individualnanometer-thick layers suspended in a polymer matrix. The greatest combinationof property improvements have been observed for the exfoliated PLS nanocom-
Žposites. Detailed discussion of different techniques solution, in situ polymeriza-.tion and melt processing to fabricate the various types of PLS nanocomposites
Žcan be found in the literature Theng, 1979; Kojima et al., 1993; Usuki et al.,1993a,b; Vaia et al., 1993; Yano et al., 1993; Burnside and Giannelis, 1995; Lan
.et al., 1995; Messersmith and Giannelis, 1995; Okada and Usuki, 1995 .PLS nanocomposites show excellent potential as ablative materials because
upon pyrolysis, the organic–inorganic nanostructure reinforcing the polymer canbe converted into a uniform ceramic char, which may lead to significantlyincreased resistance to oxidation and mechanical erosion compared to currentchar forming carbon-based ablative materials. In addition, the low volumefraction of inorganic and subsequent melt processibility will enable the design ofcompatible interfaces between the ablative insulation and the polymeric propel-lants used in solid rocket motors. This capability manifests in the potential forelimination of bondlines in solid rocket motor systems. Considering that a largefraction of known solid rocket motor failures result from bondlinerinterfacial
Ž .failures Quinn, 1995 , the impact that PLS nanocomposites may have onsystem reliability and performance would be considerable.
In the current paper, the ablative performance and thermal decomposition of aŽ .series of exfoliated layered silicaterpoly caprolactam nanocomposites are ex-
amined. To understand the most important characteristics of the nanocompositeswith respect to ablation, the affect of concentration and size of the silicate layersas well as specific bonds between the polymer and the silicate on degradationkinetics and on the microstructure of the char layer are determined.
2. Experimental
2.1. Materials
Ž . Ž .The ablative behavior of poly caprolactam nylon 6, N; Ube was comparedŽ . Ž . Ž .to a 2 wt.% NCH2 and 5 wt.% NCH5 layered silicaterpoly caprolactam
Ž .nanocomposite Ube . These materials were synthesized by in situ polymeriza-tion of ´-caprolactam in the presence of dispersed organically modified mont-
Ž .morillonite Kojima et al., 1993; Usuki et al., 1993b; Okada and Usuki, 1995 .The organically modified montmorillonite was prepared by a cation exchangereaction with 12-aminolauric acid. The presence of the carboxylic acid at thesilicate surface initiates ´-caprolactam ring opening. For the NCH2 and NCH5,
Ž .this results in approximately 30 and 50%, respectively, of the poly caprolactamchains end-tethered through the 12-aminolauric acid to the silicate surfaceŽ .Usuki et al., 1993b .
Additionally, the ablative behavior of a nominally 5 wt.% layeredŽ . Ž .silicaterpoly caprolactam nanocomposite NLS5 formed during melt process-Ž . Ž .ing of poly caprolactam Capron B135WP, Allied Signal and Cloisite 30A
Ž .organically modified montmorillonite, Southern Clay Products was examined.The Cloisite 30A was used as received. This exfoliated nanocomposite was
Ž .fabricated using a co-rotating twin-screw extruder MPCrV-30, Haake withzone 1 at 2408C, zone 2 at 2208C and the die at 2458C. A premix of nominally 5
Ž .wt.% Cloisite 30A and nylon 6 dried 808C under vacuum for 24 h was fed intothe extruder running at 180 rpm resulting in a residence time of 2–3 min.Inhomogeneities in the premix arising from the gross size difference of powderand pellets resulted in a nanocomposite that is approximately 3.4 wt.% Cloisite30A via residue from thermalgravimetric analysis in air. In contrast to in situpolymerization of NCH2 and NCH5, melt processing of NLS5 is not expected to
Ž .result in a large fraction of poly caprolactam chains tethered through covalentbonds to the organic-modifiers on the silicate surface. Instead, weak secondaryinteractions, such as van-der-Waals, will dominate polymer–silicate interactions,resulting in a drastically different polymer–silicate interphase region within thenanocomposite.
2.2. Ablation test
Ž .The ablation rate erosion rate of the materials was determined fromexposure to combustion gases in a mock solid rocket motor firing rig. Fifty
Y Ž . Y Žgrams of pelletized resin were compression molded into 7 180 mm by 3 75. Y Ž .mm triangular sections approximately 0.2 5 mm thick in an autoclave at
2468C for 5 min with 200 kPa pressure and 15 Torr vacuum. To enhanceretention of material during the test, a second series of plaques were producedby consolidating the resins into an 1r8Y polyimide mat. In addition to the
Ž . Ž .poly caprolactam nanocomposites, ethylene-propylene diene EPDM with 30%Žsilica filler, EPDM with 15% chopped Kevlare fiber state-of-the-art internal
.insulation for solid rocket motors and phenolic impregnated canvas were alsoexamined. Table 1 summarizes the samples.
Y Ž .The ablation tests were performed in a 4 diameter char motor Fig. 2 . Twotriangular plaques were arranged along the inside of a tapered, rectangular
( )R.A. Vaia et al.rApplied Clay Science 15 1999 67–9272
Table 1Summary of char motor ablation rate resultsnwmr: no virgin material remaining.
aŽ .Erosion rate milrs , "15%b cLow mass flux High mass flux
chamber, aft to the propellant. The initial throat diameter between the propellantY Ž . Y Ž .and sample was 0.275 7 mm and increased to 3.5 89 mm at the widest end
of the sample. This corresponds to a maximum Mach number range from 0.25 toŽ . 2 Ž .0.012 and a mass flux range from 2.0 13.8 to 0.10 lbrin. s 0.69 kPars ,
Ž .respectively, for an operating chamber pressure of 630 psi 4.4 MPa . Thepropellant consisted of 88% ammonium perchlorate with hydroxylated polybuta-diene binder. Temperature of the combustion gases was approximately 30008C.
Fig. 2. Schematic of char motor used for ablation tests. The samples experience decreasing massflux at measurement zones farther removed from the propellant.
The ablation rate of the materials was determined from the decrease in samplethickness after an 8-s exposure to the combustion environment. The thickness of
Ž .the remaining virgin material after removal of the degraded char layer wasdetermined from the average of at least four measurements with the accuracydefined as the standard deviation of the data. The tapered geometry of the testrig allowed the ablation rate to be determined in 10 zones representative ofdifferent mass fluxes.
2.3. Characterization
Ž . ŽThermalgravimetric analysis TGA of the virgin materials N, NCH2, NCH5. Ž y1.and NLS5 were carried out at four heating rates 2, 7, 10 and 208C min
Ž .under a flow of dry air and helium 50 mlrmin using TA Instruments High-Res2950 Thermogravimetric Analyzer. Samples were in the form of pellets. Kineticanalysis of the thermalgravimetric data was carried out according to the Flynn
Ž .and Wall method Flynn and Wall, 1966 . The initial decomposition rate of apolymer may be described by a reaction order model as:
da tŽ . nsk 1ya t , 1Ž . Ž .Ž .
d t
where k is the rate coefficient, t is time, n is the apparent reaction order andŽ . Ž . Ža t is the degree of decomposition which is defined as a t s m y0Ž .. Ž . Ž .m t r m ym with m being the initial mass, m t the actual mass and m0 ` 0 `
the final mass. Assuming a single decomposition event and an ArrheniusŽ .expression for the rate coefficient, Eq. 1 can be re-expressed for first-order
Ž .reactions ns1 as
yR d log bE s , 2Ž .A b d 1rTŽ .
Ž .where E is the activation energy, b is the heating rate bsdTrd t , T isAŽ .temperature and b is a constant bsy0.457; Flynn and Wall, 1966 . Thus, for
first-order reactions, the logarithm of heating rate is proportional to the recipro-cal of the absolute temperature for equivalent mass loss within the decomposi-tion region.
Wide-angle X-ray diffraction was performed on residual surface char fromŽthe ablation tests using a Statton camera and Cu Ka radiation graphite crystal
.monochrometer from a rotating anode. Samples were ground and placed inŽ1-mm diameter quartz capillaries Glas Berlin-West, Charles Supper, Natick,
.MA . Patterns were collected on Kodak stage-phosphor screens and read using aMolecular Dynamics Storm 820 image plate reader. One-dimensional diffractionspectrums were calculated by radially averaging digitized two-dimensionalpatterns with 50 mm pixel resolution. Scattering associated with the capillary
( )R.A. Vaia et al.rApplied Clay Science 15 1999 67–9274
was removed via direct subtraction after the sample data was normalized forexposure time and corrected for relative transmission. Small-angle X-ray scatter-
Ž .ing SAXS experiments were performed at the National Synchrotron Light˚ŽSource, Brookhaven National Laboratory beamline X27C, ls1.307 A, D ErE
. Ž .s0.0011 using a linear position sensitive detector Mbraun . Spectra werecorrected for sample absorption, background scatter and incident beam fluctua-tion.
Ž .Freeze fractured surfaces of the ablation test specimens AurPd coated wereexamined using a Leica 360 scanning electron microscope. Because of therelatively fragile nature of the ablative char layer, material was lost duringspecimen preparation and initial thickness of the char layer could only beroughly estimated. Relative elemental analysis was performed on a 100 mm2
area using an attached energy disperse spectrometer.For transmission electron microscopy, virgin material and ablation char were
Ž .mounted in Spurr epoxy Spurr, 1969 . 50–70 nm thick sections were micro-tomed at room temperature using a Reichert–Jung Ultracut Microtome equippedwith a 458 diamond knife and mounted on 200 mesh copper grids. Bright field
Žtransmission electron images were obtained using low dose techniques Vaia et.al., 1996 on a Phillips CM200 Transmission Electron Microscopy with a LaB6
filament operating at 200 kV. Silicate layer dimensions were determined byaveraging the length of the observed edges over four micrographs. Distributionof layer size corresponds to the standard deviation of the statistics.
3. Results and discussion
3.1. AblatiÕe performance
The most distinguishing feature of PLS nanocomposites is their mesostruc-ture, consisting of individual silicate layers dispersed in a polymer matrix. Fig. 3
Ž .shows the small angle X-ray scattering SAXS of NCH5 and NLS5. ScatteringŽ .from neat poly caprolactam is included for comparison. The scattering profiles
were independent of sample orientation. The absence of a basal reflection in theSAXS patterns of NCH5 or NLS5 indicates a disruption of the stacking order ofthe silicate layers and is generally indicative of an exfoliated nanocomposite
˚Ž .Vaia et al., 1996 . Nominally, d is ;18.5 A in unintercalated Cloisite 30A001˚and is between 30 and 35 A in most intercalated polymer-organically modified
˚ y1Ž .silicate complexes Vaia, 1995 . The reflection around qs0.05 A in neatŽ .poly caprolactam is associated with the long-spacing of the crystal lamella. The
drastic suppression of this reflection in NCH5 and NLS5 indicate that thedispersed silicate layers disrupt lamella formation. However, wide angle X-raydiffraction and differential scanning calorimetry of NCH5 and NLS5 indicate
Ž .Fig. 3. Small angle X-ray scattering of NCH5 and NLS5. Scattering from neat poly caprolactamŽ . Ž . Ž .nylon 6 is included for comparison. log I ylog q inset is included to enhance features withweak intensity or very small q.
that polymer crystallites are still present. The small features apparent in theŽ . Ž . Ž Ž . .log I y log q inset of NLS and NCH5 at very low q log q ;y1.5 are
believed to be associated with layer–layer spacing. Calculated layer–layerseparation for a uniform distribution of oriented silicate layers at 5–7 wt.% is in
˚ Ž .agreement with the observed values of 200–250 A Giannelis et al., 1992 .Finally, the SAXS intensity from NLS5 and NCH5 scales as qy2 over the rangeof reciprocal space examined, agreeing with idealized scattering from a distribu-
Ž .tion of isolated layers Glatter and Kratky, 1992 .Absolute verification of the exfoliated structure necessitates electron mi-
croscopy though. Fig. 4 shows bright field transmission electron micrographs ofŽ . Ž .the mesostructure of a NCH5 and b NLS5. The dark lines are individual
silicate layers oriented perpendicular to the sample surface. The average dimen-Žsions of the layers are approximately twice as large in NCH5 ;0.16"0.03
. Ž . Ž .mm and NCH2 than in NLS5 ;0.07"0.02 mm . Note that despite similarX-ray spectra, microscopy indicates that layer distribution is generally moreuniform in NCH5 than in NLS5 where a fraction of aggregates containing two to
()
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Ž . Ž . Ž .Fig. 4. Bright field transmission electron micrographs of poly caprolactam nanocomposites: a NCH5 and b NLS5.
three silicate layers are observable. Even though, the layer dispersion in NLS5 isvery extensive and the mesostructure can be considered as exfoliated.
This mesoscopic mixing of inorganic and organic drastically alters theablative properties. Fig. 5 shows the erosion rate at the mass fluxes experienced
Ž . Žin the ablation test for the poly caprolactam nanocomposites NCH2, NCH5.and NLS5 with and without the polyimide mat. The results of current state-of-
the-art ablatives, EPDMrKevlare and phenolic impregnated canvas, are in-Ž .cluded for comparison. Pure poly caprolactam did not survive the experimental
conditions. Thus, the erosion rate of the neat polymer is estimated to be at leastŽ .60 milrs 1.5 mmrs . Table 1 summarizes the ablation results in high and low
mass flux regimes for the nanocomposites with and without the polyimide matas well as EPDM with 30% silica, EPDMrKevlare and canvas phenolic.
In general, the ablation performance of the nanocomposites is comparable tocurrent state-of-the-art EPDM ablatives for the mass fluxes examined. This is at
Ž .least an order of magnitude less than that of pure poly caprolactam . TheŽ .marked reduction in erosion rate is observed with as little as 2 wt.% 0.8 vol.%
exfoliated silicate. The presence of the polyimide mat ostensibly improvesretention of the surface char layer thereby slightly decreasing the erosion rate
Ž .further. Increased concentration of the exfoliated silicate NCH2 vs. NCH5 alsoslightly reduces mass loss. A weaker polymer–silicate interaction, slight nonuni-
Žformity in layer dispersion andror smaller lateral dimensions of the silicate as.exemplified in NLS5 only alters ablation performance to a minor degree in the
low mass flux region. The largest impact is observed in the high flux regionwhere NLS5 does not perform as well as NCH2 or NCH5. In general, increasedmass flux increases the erosion rate of all the nanocomposites, indicating thatmechanical erosion of the surface char occurs. Thus, the increased erosion rateof NLS5 relative to NCH5 and NCH2 indicates that polymer–silicate interac-tions, layer dispersion andror layer size impact the toughness of the char.However, the major factor leading to improved ablation performance and chartoughness relative to the neat resin or traditional micron-scale filled polymers is
Žthe nanoscopic distribution of the silicate layers present in all the nanocompos-.ites examined . Comparable micron-scale composite ablatives such as EPDM
with silica require in excess of 30 wt.% inorganic filler to achieve the samedegree of performance enhancement relative to the neat polymer as demon-strated by these exfoliated layered silicate nanocomposites.
In general, the erosion rate of an ablation system depends on a complex heatand mass balance at the surface. This is influenced by conditions within the
Ž .combustion gas temperature, pressure and composition , by the structure,mechanical integrity and thermal conductivity of the char layer, and by thevolatilization rate of pyrolysis gases. Extensive reactions are known to occurbetween mica-type layered silicates and organic polymers at elevated tempera-
Ž .tures Grim, 1968; Thomas, 1982 , potentially altering decomposition productsand kinetics in the reaction zone or increasing carbonaceous char yield. In
( )R.A. Vaia et al.rApplied Clay Science 15 1999 67–9278
ŽFig. 5. Erosion rate at mass fluxes experienced in the ablation test for the nanocomposites NCH2,.NCH5, NLS5 without polyimide mat and with polyimide mat as well as the EPDMrKevlare and
addition, the enhanced barrier properties of nanocomposites may influencedecomposition kinetics and retard escape of pyrolysis gases. Thus, an examina-tion of the impact of layer size, layer concentration and strength of polymer–silicate interaction on the thermal decomposition and on the composition andmicrostructure of the surface char are necessary for a detailed understanding ofthe factors contributing to the enhanced ablation performance of the nanocom-posites.
( )3.2. Thermal decomposition of poly caprolactam nanocomposites
The ablative process begins with the decomposition of the polymer matrix inthe reaction zone. Table 2 summarizes the weight loss, degradation temperaturesand char yield for thermal decomposition of the nanocomposites and neat
Ž .poly caprolactam in oxidizing and inert atmospheres at 108rmin. Figs. 6 and 7show the differentiated TGA curves for oxidizing and inert atmospheres,respectively. Fig. 8 displays the estimated activation energies associated with the
Table 2Summary of thermogravimetric analysis at 108CrminDw : initial weight loss from 40–2408C.i
T : initial decomposition temperature defined at 4% weight loss.i
T : temperature at maximum decomposition rate.M
T : temperature at maximum of secondary decomposition.M2
T : final decomposition temperature.f
DT sT yT : decomposition temperature range.f i
Dw : weight percentage of residual char.r
D E : activation energy of nonisothermal decomposition for 0.2 - a -0.7.A
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Ž .Fig. 6. Differentiated TGA curves for oxidizing air atmosphere at 108Crmin. Curves verticallyoffset for clarity.
nonisothermal decomposition of the nanocomposites in inert atmosphere atvarious degrees of decomposition, a . The activation energies were determined
Žfrom the slope of a linear least-squares fit of the logarithm of the heating rate 2,y1.7, 10 and 208C min vs. reciprocal of the absolute temperature at a given
Ž Ž .. 2residual weight Eq. 2 . The square of the product moment correlation, R , ofall the liner fits were greater than 0.997. The average activation energy andstandard deviation over 0.2-a-0.7 are included in Table 2.
In general, the concentration of the exfoliated silicate layers or other specificŽcharacteristics of the nanocomposite layer size, distribution or strength of
.polymer–silicate interphase do not alter the temperature of the maximum rateŽ .of weight loss T s457"38C in inert or oxidizing atmospheres. The pres-M
ence of the silicate only slightly decreases the onset and broadens the tempera-Ž .ture range of primary poly caprolactam decomposition. The presence of the
silicate, however, does retard secondary decomposition in oxidizing atmospheresas demonstrated by an increase in T . After complete decomposition, if theM2
initial concentration of silicate is considered, no additional retention of carbona-ceous residue can be attributed to the mesostructure of the nanocomposites. Note
Ž .Fig. 7. Differentiated TGA curves for inert helium atmosphere at 108Crmin. Curves verticallyoffset for clarity.
that the lower than anticipated char yield for NCH5 reflects the uncertaintyassociated with the absolute silicate content in the nanocomposite and not loss ofinorganic material.
The thermal analyses agree with previous studies comparing the degradationŽ . Žof milligram quantities of poly caprolactam and NCH5 nanocomposite Gilman
.et al., 1997; Gilman et al., 1998 . These studies also identified the majordecomposition products as ´-caprolactam with traces of CO and CO in2
Žagreement with previous studies of polyamide decomposition Ballistreri et al.,.1988; Bockhorn et al., 1996; Shimasaki et al., 1997; Siat et al., 1997 .
The activation energy associated with the nonisothermal decomposition re-Ž .veals additional details of polymer degradation. Poly caprolactam decomposes
in one step with an activation energy that progressively increases with theadvancement of the reaction. In general, the activation energies determinedŽ . Ž .190–210 kJrmole agree well with previous studies of poly caprolactamŽ .Reardon and Barker, 1974; Bockhorn et al., 1996; Levchick et al., 1992, 1997 .
Ž .This corresponds to chain scission through the alkyl amide bond CH –NH–CO2Ž .via a free radical mechanism Levchick et al., 1992 . The changes in the
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Ž .Fig. 8. Activation energy associated with nonisothermal decomposition helium atmosphere ofŽ .poly caprolactam and the nanocomposites at various degrees of decomposition, a .
activation energy associated with the presence of the silicate is substantially lessŽ .than observed for other inorganic and organic additives to poly caprolactam
Žwhich is generally y30 to y50 kJrmole Reardon and Barker, 1974; Levchick.et al., 1997 . This implies that the organically modified silicate does not
participate chemically in the degradation reaction. Minor effects associated withthe silicate–polymer interphase structure, silicate layer size, concentration orother impurities from the polyamide synthesis are manifested in the initialdecomposition stages and require further examination. However, the proportion-
Ž . Ž .ality postulated in Eq. 2 Fig. 8 holds for all the nanocomposites implying thatdegradation follows first-order reaction kinetics irrespective of specific charac-teristics of the mesostructure or presence of the layers. This behavior contrastsother nanocomposites which exhibit improved nonisothermal decompositionŽ .Burnside and Giannelis, 1995 . Thus, the role of the dispersed silicate dependscritically on the specific mechanisms associated with the polymer degradationreaction.
Ž .The enhanced barrier properties of the exfoliated poly caprolactam nanocom-Ž .posites Yano et al., 1993; Messersmith and Giannelis, 1995 relative to the neat
polymer may also be anticipated to influence polymer degradation. Since smallmolecule diffusion is retarded, the escape of decomposition products or perme-ation of oxidizing reactants should also be retarded, thus altering degradation
kinetics. This effect will be especially pronounced for the slowest heating ratesor isothermal decomposition. For rapid heating rates, however, temperatureincreases within the sample will influence reaction kinetics to a greater extentthan the degree to which small molecule transport is retarded. This effect caneven be observed in unfilled polymers. For example, 108Crmin is sufficiently
Žrapid that the presence of an oxidizing atmosphere known to decrease theŽ ..temperature for isothermal decomposition of poly caprolactam does not de-
Ž .crease the temperature for nonisothermal decomposition Table 2 . Therefore, atfast heating rates or near surface degradation, which are characteristic of theablation process, retardation of small molecule transport at most only minutelyaffects polymer degradation kinetics or ablation performance.
The presence of the silicate layers, though, is not without impact on theŽ .decomposition of poly caprolactam . Exfoliated silicate layers do alter sec-
Ž . Ž .ondary decomposition of poly caprolactam oxidation products T . PreviousM2Ž .work Gilman et al., 1997; Gilman et al., 1998 as well as ours discussed below
Ž .indicates that at these temperatures T)5008C , the original exfoliatedmesostructure collapses into an intercalated structure. In contrast to the exfoli-ated mesostructure, the confined environment of the intercalated structureŽ .highly anisotropic with 1–3 nm confinement extremely retards diffusion ofoxidizing reactants and products and thus alters nonisothermal stability. Thus,the formation of the intercalated structure will influence secondary reactionswithin the char layer.
3.3. Microstructure of the ablatiÕe char
Since the presence of the layered silicate does not alter primary polymerdecomposition in the reaction zone, the initial nanostructure of the dispersedsilicate must influence the microstructure and composition of the char layer,which in turn enhances the ablation performance.
Figs. 9–11 show scanning electron micrographs of the surface char region ofNCH2, NCH5 and NLS5 ablative samples, respectively. There was no recover-
Ž .able material of the neat poly caprolactam for examination. The characteristicŽ .ablation regions virgin material, porous reaction zone and dense char layer are
apparent in all the samples. In general, the residual char is porous with solidŽ .regions exhibiting a platy habit e.g., Fig. 10b and Fig. 11 . Elemental analysis
of the surface char on NCH5 and NLS5 indicated an approximate 13"1 foldenhancement of Si and Al relative to the virgin material. This is anticipatedgiven the relative residue observed in the thermogravimetric studies. Addition-ally, the SirAl ratio in the char was the same as in the virgin nanocompositeŽ .2.5"0.1 , indicating that Si or Al is not preferentially depleted during ablation.
The microstructural features of these chars imply that increasing silicatecontent results in a tougher char. A relatively weak, sub-micron lamellar
Ž .structure is most prevalent in NCH2 char Fig. 9b , but to a lesser extent in
( )R.A. Vaia et al.rApplied Clay Science 15 1999 67–9284
Ž . ŽFig. 9. Scanning electron micrographs of NCH2 ablative sample. a Surface region sample tilted. Ž . Žto observe cross-section and surface features and b char layer. VM: virgin material, RZ:
Ž . ŽFig. 10. Scanning electron micrographs of NCH5 ablative sample. a Surface region sample. Ž . Žtilted to observe cross-section and surface features and b char layer. VM: virgin material, RZ:
.reaction zone, CL: char layer.
( )R.A. Vaia et al.rApplied Clay Science 15 1999 67–9286
ŽFig. 11. Scanning electron micrographs of surface region sample tilted to observe cross-section. Žand surface features of NLS5 ablative sample. VM: virgin material, RZ: reaction zone, CL: char
.layer.
Ž .NCH5 and NLS5 char. In these chars, a nanoscopic 10–20 nm thick denselyŽ .packed platy morphology is observed Fig. 12 . Increased silicate content results
qualitatively in a spatially more uniform and thicker char. Additionally, handlingof the samples indicated that the char layer adheres more to the underlyingvirgin material with increased silicate content. Specific characteristics of thesilicate such as layer size, uniformity of layer distribution and strength of the
Žinterphase result in a finer surface texture for NCH5 char than NLS5 char Figs..9 and 10 . The rougher surface in NLS5 reflects nonuniform mass loss and
agrees with the greater erosion rate relative to NCH5.In general, the spatially uniform arrangement of the silicate layers on an
Ž .ultra-fine nanometer level facilitates the formation of a spatially uniforminorganic char. At 5 wt.%, the layers are on average only 20–25 nm apart. Incontrast, micron-scale fillers require much higher loadings to achieve this spatialuniformity. Thus, the efficient distribution of the inorganic precursors necessaryto form a uniform char layer is the key structural characteristic of the nanocom-posites leading to the enhanced ablative performance at remarkable low addi-tions of a reinforcing phase.
Additional details on the microstructure and composition of the char weredetermined using X-ray diffraction. Fig. 13 shows the one-dimensional X-raydiffraction patterns of the NCH2, NCH5 and NLS5 char layer. In addition to
Ž .amorphous scattering 2u;258 , various broad reflections are observed andsummarized in Table 3. Many of the higher angle reflections are associated with
Fig. 12. Bright field transmission electron micrograph of NCH5 char.
a relatively few crystallites as evidenced by the texture along the diffractionrings depicted in the two-dimensional pattern of NLS5 char in Fig. 14.
During the ablation experiment, temperature within the char layer exceeds10008C and approaches 2000–25008C at the surface. At these temperatures, any
ŽŽ .carbonaceous residue from the polymer will contain graphite 001 reflections˚ .3.35 A . Additionally, mica-type layered silicates, such as montmorillonite,
irreversibly transform into other aluminosilicate phases. Between 600 and10008C, montmorillonite dehydroxylates and has been observed to initially
Ž . Žtransform into spinel, cristobolite, mullite andror pyroxenes enstatite Grim,.1968 . At temperatures greater than 13008C, mullite, cristobolite and cordierite
Žform and subsequently melt at temperatures in excess of 15008C mullite. Ž18508C, pure cristobolite 17288C and cordierite ;15508C Kingery et al.,
.1976 . The presence of an inorganic that transforms into a high viscosity melt onthe surface of the char will improve ablation resistance by flowing to self-heal
Žsurface flaws. This is known to occur in silica-filled ablatives D’Alelio and.Parker, 1971 .
Various aluminosilicate, silica andror alumina containing phases will bepresent in the final char based on the steep temperature gradient between thereaction zone and the char surface. As indicated by the similarity of thediffraction patterns, the transformation of montmorillonite during ablation is
Ž .comparable in all the nanocomposites Fig. 13 . However, unique phase identifi-cation of the reflections was unsuccessful and will require more detailed
( )R.A. Vaia et al.rApplied Clay Science 15 1999 67–9288
Ž .Fig. 13. a One-dimensional wide-angle X-ray diffraction patterns of the NCH2, NCH5 andNLS5 char. Scattering from capillary included for comparison. Curves vertically offset for clarity.Ž .b One-dimensional wide-angle X-ray diffraction patterns for the chars corrected for scatteringfrom the capillary. Curves vertically offset for clarity.
Table 3Summary of X-ray reflections from chars: strong, m: medium, w: weak, –: absent.
Ž .2u d nm NCH2 NCH5 NLS5
7.44 1.19 w m –20.86 0.426 m s s
a22.62 0.393 – – wa24.92 0.357 – – w
26.58 0.335 s s sa28.67 0.311 m m ma30.76 0.291 m m ma32.08 0.279 m m ma34.94 0.257 w w ma35.82 0.251 w m ma40.60 0.222 – – m
Fig. 14. Two-dimensional wide-angle X-ray diffraction pattern of NLS5 char.
chemical analysis and higher angle diffraction data. The main difference be-Ž .tween the chars is the absence of the low angle reflection 2us7.48 in NLS5.
This low angle reflection, present in NCH2 and NCH5, is thought to beassociated with a layered structure, consisting of alternating oxidergraphitic
Ž .layers Gilman et al., 1998 . This structure may develop from the presence oflarge amounts of organic or carbonaceous residue between the aluminosilicatelayers originating from the organic surfactants and polymer chains. This residuewill impede layer–layer consolidation during high temperature phase transfor-mation and potentially alter the subsequent high temperature phases formed,resulting in a portion of the inorganic char exhibiting a nanoscale-layered
Ž .structure 1.2–1.4 nm . In contrast, the relatively small amount of organicmaterial within aggregates containing two to three silicate layers in NLS5 willnot impede interlayer consolidation. Thus, the final char will not contain asmuch of this nanoscale-layered structure. Note that a large fraction of individualsilicate layers are also present in NLS5 and these apparently do not yield thealternating oxidergraphitic layers. Thus, the composition and size of thealuminosilicate also appear to influence the nanoscale structure and compositionof the insulative char.
4. Conclusion
The mesoscopic structure of exfoliated PLS nanocomposites results in asubstantial improvement in ablative performance relative to the neat polymer. A
Ž .relatively tough, inorganic char forms during ablation of the poly caprolactam
( )R.A. Vaia et al.rApplied Clay Science 15 1999 67–9290
nanocomposites. This refractory char results in at least an order-of-magnitudedecrease in the mass loss rate relative to the neat polymer, even for as little as 2
Ž .wt.% ;0.8 vol.% exfoliated layered silicate. This enhanced ablative perfor-mance is not related to an alteration of decomposition kinetics associated withthe presence of the silicate layers. In fact, the presence of the exfoliated silicate
Ž .only minutely alters nonisothermal decomposition kinetics in poly caprolactam ,which is dominated by chain scission through the alkyl amide bond via a freeradical mechanism. This contrasts other nanocomposites which exhibit improvednonisothermal decomposition. Thus, the role of the dispersed silicate in thermaldecomposition of a polymer depends critically on the specific mechanismsassociated with the polymer degradation reaction.
The improvement in ablative performance of these nanocomposites relative tothe neat polymer or traditional filled systems with a comparable inorganicfraction is associated with the char-forming characteristics of the nanocompos-ites. The spatially uniform distribution of aluminosilicate layers on the nanoscaleresults in the formation of a uniform inorganic char layer at a relatively lowfraction of inorganic. This nanoscopic morphology is comparable to the lengthscale of the decomposition and char-forming reactions determined by thetemperature profile and the diffusivities of the reactants and products. Thus, auniform supply of inorganic precursor to the char is available during decomposi-tion. In contrast, the localization of inorganic on the micron-scale associatedwith traditional filled systems requires a higher inorganic loading for theformation of a uniform inorganic char at the surface. At loading fractionscomparable to the nanocomposites, large regions void of inorganic precursor arepresent resulting in locally, nonuniform erosion rates leading to rough surfacesmore susceptible to mechanical erosion. Thus, beyond the role of compatibilize,the aluminosilicate with the polymer resin, the type of organic modification onthe silicate surface and specific interactions between the polymer and the silicate
Žsurface such as end-tethering of a fraction of the polymer chains through ionic.interaction to the layer surface , at most, only minutely alters the ablative or
char-forming behavior of the nanocomposites. Layer concentration, size anddegree of dispersion are the dominant factors. Therefore, the enhancement inablative performance should be a general observation for exfoliated PLSnanocomposites and may also be observed for other nanocomposite systems
Ž .such as dispersed nanoparticulate e.g., TiO , SiO –polymer blends.2 2
Acknowledgements
This work was partially funded by the Air Force Office of Scientific Researchand Air Force Research Laboratory Propulsion and Advanced Concepts Direc-
Ž .torate Propulsion Materials Applications, EAFB and benefited from the centralfacilities of the Air Force Research Laboratory Materials and Manufacturing
Ž .Directorate WPAFB . The authors are grateful to Mr. W. Ragland for preparingthe TEM samples and Mrs. M. Holtz for performing the thermalgravimetricexperiments. Additionally, the authors are grateful to Southern Clay Products,for fabricating the melt-processed nanocomposites.
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