epdm nanocomposites using polyimide as ablator: morphology ... · was vulcanized at 150oc for 30...

Columbia International Publishing American Journal of Macromolecular Science (2014) Vol. 1 No. 1 pp. 1-16

Research Article

______________________________________________________________________________________________________________________________ *Corresponding e-mail: *[email protected] and **[email protected] 1* Rubber Technology Center, Indian Institute of Technology Kharagpur, West Bengal, India, 721302. 2 Kalpana Chawla Space Technology Cell, Indian Institute of Technology Kharagpur,

West Bengal, India, 721302.

1

EPDM Nanocomposites using Polyimide as Ablator: Morphology and Thermophysical Properties

Sangita Singh1, P K Guchhait2, Nikhil K Singha1* and T K Chaki1**

Received 29 October 2013; Published online 19 April 2014 © The author(s) 2014. Published with open access at www.uscip.us

Abstract Launching of space craft liberates intense heat and in order to protect the rocket system from immense temperature and pressure during launching, multilayered insulation is necessary on solid rocket motor case. Elastomer, being better candidates for heat and electrical insulation are used over the metal casing in rocket motor system. The development of light weight EPDM insulation compound with suitable nanofillers can change the conventional insulation technology. The present work deals with an introduction of thermally stable polyimide based EPDM nanocomposites of different formulations for insulation in space applications. The insulators are prepared by melt mixing of polyimide (PI) powder and different nanofillers (nanosilica, nanoclay and carbon nanofiber) in ethylene propylene diene elastomer (EPDM) matrix. Polyimide provides good ablative properties and nanofillers provide good reinforcement as well as optimum thermal properties. In this study EPDM was compounded with polyimide and nanosilica/nanoclay/carbon nano fiber in Haake mixer followed by mixing of curatives and accelerator in a two roll open mill. The degradation of the elastomeric insulator has been investigated by thermogravimetric analysis (TGA). Glass transition temperature and specific heat were determined by differential scanning calorimetry (DSC) analysis. Apart from physical and mechanical properties, thermal conductivity, FTIR analysis and XRD studies were also carried out. Morphology and the study of dispersion of nanofillers in EPDM matrix were analyzed by SEM and HR-TEM. Keywords: EPDM; Nanocomposites; Polyimide; Insulation properties; Rocket motor insulation

1. Introduction Multilayered insulation compounds are necessary to protect the solid propellant metal case during lift of space vehicles. These thermal insulation layers are used between the rocket motor case and propellant of a space craft to protect the whole system from immense heat and pressure. Rocket fuel or the solid propellant is a mixture of high energy particles which produces high speed gases

Sangita Singh, P K Guchhait, N K Singha, and T K Chaki / American Journal of Macromolecular Science (2014) Vol. 1 No. 1 pp. 1-16

2

with immense heat and pressure during launching. The heat accumulated near the rocket motor leads tremendous increase in temperature near to 2500oC - 2700oC which may seriously cause a blast damaging the whole rocket system. This is well understood that the solid propellant tank of a rocket must be protected from the tremendous heat and high pressure generated during launch of a space vehicle. Different elastomer/polymer, having good insulation property is used as an insulating layer on multi layered metal case of rocket motor (Fan et al, 2007; David Sayal, 1990; Marks, 1991; Sauae Chen, 1991, Martin Jon and Richard, 1993; and Guillot David, 1994). But, elastomer itself is not sufficient to provide desirable mechanical strength to the insulating material. Hence, incorporation of different fillers is necessary to reinforce the rubber matrix. Conventionally, insulation fillers namely asbestos fiber, cork powder, aluminium hydroxide, antimony trioxide and organic fire retardant materials etc. were used (Graham et al, 1998; Liles G. Herring, 1985;

Gajiwala M. Himansu, 2004; and Rogwaski, 1990). The technologies in aerospace insulation application can now be improved by light weight polymers having high specific strength, low modulus, high ablation and good thermal properties. Ethylene propylene diene ter-polymer (EPDM) with all these above mentioned properties is emerging as a dominant elastomer in rocket motor insulation and various applications particularly in applications that demand excellent chemical and thermal stability. Literature survey and some patents reported that EPDM possesses better ageing property for rocket motor insulation application (Urayel, 1996; Deuri A. Saha et al, 1986 and 1987; Harvey and Ellertson, 2003; Guillot and Harvey, 2004; Hartz and Meyer, 1967;

Tucker and Waco, 1971; Gardon 1972; Chase Michael John et al, 1976; Roger et al, 1978; Kenneth et al, 1985; Liles G. herring, 1985; Wendel , 1987; L. G. Herring, 1987 and 1989). EPDM has good mechanical property, good tear strength and high thermal resistance. It also has good retention of properties even after ageing because of saturated main chain. EPDM finds increasing application in automobile and aerospace sector due to its good ageing properties and high filler loading capacity. At the same time nanofillers are incorporated into the EPDM rubber matrix to reinforce the mechanical strength. Present study deals with the use of polyimide (PI) powder as new filler in EPDM matrix along with nanosilica (NS), nanoclay (NC) and carbon nanofiber (CNF) respectively. Because of its stiff aromatic backbone, polyimide shows high thermal stability and high strength (Barikani Mehdi, 2002 and Ryo Tamaki 2003). Apart from the flexibility and good creep resistance, the powdery state of PI helps in mixing and uniform dispersion inside the EPDM matrix. Polyimide being the main constituent in this study was kept constant in all the cases. Introduction of nanoparticles (nanosilica, nanoclay and carbon nanofiber) into the rubber-polyimide matrix exhibit better performance characteristics compared to the conventional fillers. Nanosilica is known to be good erosion and heat-resistant, electrical and thermal insulator, whereas high thermal efficiency of nanoclay shows reduced permeability for gases. CNF has been chosen because of its good fire retardant property, lower smoke emission and high char formation (Jacobsen et al; David et al; Shimamira; Gibson and Lee, 2007; Lake and Tibbetts; Harvy et al, 2004). The objective of this investigation is to develop a low density material with high thermal insulation performance based on EPDM. Present work describes the effect of ablative polyimide (PI)/nanosilica (NS), polyimide (PI)/nanoclay (NC) and polyimide (PI)/carbon nanofiber (CNF) fillers on the various properties of EPDM nanocomposites for rocket motor insulation application. PI has very good compatibility with nanosilica and this combination (PI and NS) is used to prepare transparent membranes for various applications (Xiangyi and Shung, 2005; Wu Guang-Yu et al, 2011; Jovanovic et al, 2011; Tang, 2007; Deng Yuyuan, 2010). Dispersion of nanoclay in polyimide


3

matrix also shows very good compatibility with polyimide (Landon, 2007; Wang, 2012; Iroh, 2012). Likewise nanosilica and nanoclay, carbon nanofiber also gets dispersed in polyimide and EPDM uniformly to make an insulation compound (Harvey et al, 2004; Gibson 2007). Interaction of PI/nanofillers with EPDM matrix will be an important factor which affects all the properties of the EPDM nanocomposites. There is very good interaction between the fillers and EPDM matrix through physical bonding that is also reflected in TEM analysis discussed in section 3.6.2. At the same time presence of ENB (Ethylidene norbornene) in EPDM main chain may provide some chemical bonding with the polar fillers.

2. Materials EPDM rubber of 55.0 wt % ethylene content and 4.3 wt % ENB content was purchased from BP Chemicals Ltd., Mumbai, India. Powder P84 NT, a commercial polyimide was obtained from Lenzing Inc, Austria. Nanosilica filler (Aerosil, size-10-14 nm) was purchased from Sigma Aldrich, India. Nanoclay (nanofil 5) was procured from Sud-Chemie, India and Carbon nanofiber (CNF grade PR-24-XT-HHT) was purchased from Pyrograph Products Inc, US. The average diameter for the CNF was 100 nm. All other necessary rubber processing chemicals viz. zinc oxide, stearic acid, 2,2,4-trimethyl-1,1,2-dihydroquinoline (TQ), sulphur (S), 2-mercaptobenzothiazole (MBT) and tetramethylthiauram disulphide (TMTD) of laboratory grades (E-Merck, Mumbai, India) were used directly for the processing of the EPDM nanocomposites. A coupling agent, Bis-(triethoxysilylpropyl)tetrasulphide (Silane A-189) was procured by M/s Connell Bros. Co. Ltd.(Mumbai, India) to add in nanosilica composites. 2.1. Testing methodology Mixing of EPDM with PI, nanosilica / nanoclay / CNF and other ingredients was carried out in Haake internal mixer at 100ºC, maintaining the rotor speed at 60-80 rpm for 10-12 minutes. The vulcanization was carried out at 150ºC under pressure of 6 MPa (Moore press) as per OCT (optimum cure time). Mechanical properties were measured by HOUNSFIELD (Model No.-H10KS) (ASTM D412). CV Shore Durometer (Model No- Digital A/RS232/DSAS001) was used to measure Shore ‘A’ hardness (ASTM2240). Density measurement was carried out by water displacement method (ASTM 792). Dispersion uniformity of PI/ nanosilica/ nanoclay / CNF in the elastomeric nanocomposites was analyzed by scanning electron microscopy (SEM) (Model- JSM-5800) and HR-TEM (Model- JEM2100). TGA and DSC analysis were carried out in Q50 TA instruments Inc, USA. Thermal conductivity was investigated by guarded hot plate method (ASTM C177) in. Ageing studies were done in hot air oven for 24hrs and 72hrs at 120ºC. X’pert pro PANalytical was used for the XRD analysis of nanofillers and rubber nanocomposites. FTIR & ATR spectra were recorded on Perkin-Elmer, Spectrum RX; FTIR spectrometer. 2.2. Compounding of the EPDM nanocomposites EPDM was masticated at 100oC in Haake internal mixer with shear rate of 60-80 rpm. The mass became more soften after the addition of stearic acid. PI powder was then added to the soft mass of EPDM followed by the addition of nanofillers. Coupling agent (Silane A-189, 0.1 phr) was well mixed with nanosilica before adding to the mixing chamber. No coupling agent was used in case of nanoclay or CNF. Antioxidant (TQ) and accelerator activator (ZnO) were added to the melt mixing chamber. The same procedure was repeated for all three nanosilica/nanoclay/CNF composites. Compounding of EPDM with fillers and other processing aids took 10-12 minutes to mix in haake


4

internal mixer maintaining the cam rotor speed for 60-80 rpm. The master mix thus obtained was thereafter taken into two-roll mixing mill to mix with the curatives. Sulphur with MBT and TMTD accelerator [Efficient vulcanization (EV) system] has been used for EPDM/PI/nanofillers composite. The mix batch was kept for 24 hours for effective chemical interactions and later the compound mix was vulcanized at 150oC for 30 minutes by compression molding at 6 MPa pressures (Deuri Saha et al, 1987). The filler loading and the designation of the samples coupled with PI/nanofillers are shown in Table1. The digits in subscripts described their loadings in the corresponding material, as explained in the footnote of the table. Table 1 Sample designation and filler loadings for EPDM/PI/nanofiller composites

*E= EPDM, NF= Nanofiller, NS= Nanosilica, NC= Nanoclay, CNF= Carbon nanofiber (For example, EP1NS5 means EPDM matrix with 1 phr of polyimide and 5 phr of nanosilica)

3. Results and discussion

3.1. Mechanical properties Stress-strain curves of the polyimide based EPDM nanocomposites are shown in the Figure 1 (A, B, C and D). The mechanical and physical test results are shown in Table 2. Four dumbbell shaped samples were cut from each 2mm rubber sheet and their average value was considered for tensile strength. The incompatibility of nonpolar EPDM and polar polyimide leads very small increase in mechanical properties. Though, the incompatible composite respond well to the insulation part, there was always a scope for further modification of EPDM to enhance the polarity by chemical means. In present study, it was found that tensile strength, elongation at break and shore A hardness values increase progressively with increase in filler loadings. Addition of polyimide does not enhance the tensile strength, rather the tensile strength of the material decreases. This is because of the less polymer-filler interaction and difference in polarity. Incorporation of nanofillers (nanosilica / nanoclay and CNF) is responsible for the small enhancement in mechanical properties. EPDM with polyimide (EP5NF0) reduces the tensile property by 6 % compared to the neat EPDM. Difference in polarity of EPDM and polyimide might be a reason for the reduction in properties. However, there is an increase of 75 %, 19 % and 25 % tensile strength (compared to neat EPDM)

Sample Designation

EPDM (phr) Polyimide Powder (phr)

Nanosilica (phr) Nanoclay (phr) Carbon nanofiber (phr)

EP0NF0 100.0 0.0 0.0 0.0 0.0 EP5NF0 100.0 5.0 0.0 0.0 0.0 EP5NS1 100.0 5.0 1.0 - - EP5NS3 100.0 5.0 3.0 - - EP5NS5 100.0 5.0 5.0 - - EP5NS7 100.0 5.0 7.0 - - EP5NC1 100.0 5.0 - 1.0 - EP5NC3 100.0 5.0 - 3.0 - EP5NC5 100.0 5.0 - 5.0 - EP5NC7 100.0 5.0 - 7.0 - EP5CNF1 100.0 5.0 - - 1.0 EP5CNF3 100.0 5.0 - - 3.0 EP5CNF5 100.0 5.0 - - 5.0 EP5CNF7 100.0 5.0 - - 7.0


5

with the incorporation of maximum (7 phr) loading of nanosilica, nanoclay and CNF respectively. It is clear from Figure 1(A), (B) and (C) that the increasing loading of nanofillers from 1 phr to 7 phr increases the tensile strength and elongation at break whereas the modulus decreases with higher loadings of nanofillers. Initially, with the addition of 1 phr of nanofiller in the base matrix, there is a decrease in the tensile strength because of the less polymer filler interaction. But this interaction increases again with the increasing amount of filler and hence the mechanical strength. The case is similar for all three nanofillers. Figure 1(D) gives the comparative study of neat EPDM and EPDM nanocomposites discussed above having maximum loadings of 7 phr of nanoclay, nanosilica and carbon nano fiber respectively. It is well known that nanosilica as a filler shows very strong filler networking and the synergistic effect of polyimide and nanosilica (EP5NS7) showed maximum increase (75 %) in tensile strength compared to the control sample. CNF composites reflected better mechanical properties than nanoclay composites. Although, higher loading of nanofillers leads to higher density, the addition of 5 phr of polyimide and 7 phr of nanofillers in EPDM matrix restricted its density to less than 1 g/cm3. The hardness value is more in case of CNF, least in case of nanosilica and nanoclay lies in between. It has been found that, there is no significant increase in hardness with higher loadings of nanofillers.

Fig. 1. (A), (B) and (C): Stress-Strain curve of PI based EPDM nanosilica (NS) / nanoclay(NC)/ CNF composite with varying loadings of nanofillers. (D): Stress-Strain plot of neat EPDM, EPDM/PI and EPDM/PI/nanofillers composite.


6

Table 2 Mechanical properties of polyimide (PI) based EPDM nanocomposites with different loadings of nanosilica (NS), nanoclay (NC) and carbon nanofiber (CNF)

3.2. Aging studies Aging studies of PI based EPDM nanocomposites were carried out for 24 hours and 72 hours at 120oC in air atmosphere. The results were shown in Figure 2(a), Figure 2(b) and in Figure 2(c) for nanosilica, nanoclay and carbon nanofiber composites respectively. Aging affects the mechanical properties in all three cases. The effects of hot air aging were shown in bar diagram for tensile strength, elongation and modulus at 100 % elongation. It was quite interesting that in all three cases (except EP5NS5 and EP5NC5) tensile strength increases with the aging time. After 72 hours of aging there was 51 % increase in tensile strength in case of EP5NS7. Similarly, in EP5NC7 and EP5CNF7 tensile strength increases up to 100 % and 77 % in comparison with un-aged samples respectively. Unlike to the other samples the tensile strength of EP5NS5 and EP5NC5 decreases, because of over curing (Choi Sung-Seen, 2006) after 72 hours of aging. There was a progressive increase in modulus @ 100 % elongation from 24 hours to 72 hours at 120oC in air for all three cases. Due to oxidative degradation and chain scission, PI based EPDM nanocomposites exhibited considerable increase in tensile strength with further decrease in elongation at break. The other major reason involved the breakage of polysulphidic linkage [C – (S) n – C] into mono-sulphidic [C – S – C] linkage into the matrix which provided very strong bond strength. (Joseph Rani, 1987)

Sl No. Sample Designation

Tensile Strength (MPa)

Elongation @% break

Modulus @ 100% (MPa)

Hardness Shore A

Density g/cm3

1. EP0NF0 1.6 263 1.0 44 0.96 2. EP5NF0 1.5 405 0.7 44 0.93 3. EP5NS1 1.3 280 0.7 44 0.94 4. EP5NS3 1.4 357 0.7 44 0.97 5. EP5NS5 2.4 413 0.9 45 0.95 6. EP5NS7 2.8 662 0.6 47 1.00 7. EP5NC1 1.3 495 0.5 44 0.95 8. EP5NC3 1.2 312 0.6 46 0.92 9. EP5NC5 1.5 363 0.7 47 0.95 10. EP5NC7 1.9 500 0.7 50 1.07 11. EP5CNF1 1.0 235 0.7 45 0.95 12. EP5CNF3 1.6 233 1.1 52 0.96 13. EP5CNF5 1.8 275 1.0 55 0.98 14. EP5CNF7 2.0 682 0.7 55 0.99


7

(a)

(b)

(c)

Fig. 2. (a). Effect of aging on mechanical properties for PI/EPDM/NS composites, (b). Effect of aging on mechanical properties for PI/EPDM/NC composites (c). Effect of aging on mechanical properties for PI/EPDM/CNF composites


8

3.3. Thermal properties A. Thermal conductivity and thermal diffusivity

Thermal conductivity of PI based EPDM nanocomposites were measured from room temperature to 85oC. The results were described in Table 3. Thermal conductivity of EPDM was measured as 0.17 W/mK. It exhibited high degree of insulation and even, incorporation of polyimide and nanofillers in EPDM matrix were not participating much in the enhancement of thermal conductivity as shown in Figure 3. Nanosilica and nanoclay is bad conductor of heat but CNF is having better thermal conductivity compare to the formers. However, incorporation of 7 phr of CNF in EPDM matrix was still sufficient to keep the optimum insulation property. Moreover, high char yield in EPDM/CNF composite would be an additional advantage in thermal insulation. The conductivity curve for CNF nanocomposite lies in between 0.5-0.6 W-m/K. Thermal conductivity of nanosilica composite was found lowest in the range 0.1-0.2 W-m/K, even less than the unfilled EPDM matrix. However, for nanoclay composite, it is found to be at 0.45 W-m/K in PI/EPDM matrix. Consequently, all the composites of PI/EPDM/nanosilica, PI/EPDM/nanoclay and PI/EPDM/carbon nanofiber showed good insulation property. Specific heat was calculated from DSC analysis as discussed in the section 3.3 (C) below. The change in specific heat values were insignificant from unfilled EPDM to the PI based EPDM nanocomposites as shown in Table 3. Thermal diffusivity is the measure of thermal inertia and that can be measured by the following equation (1) using thermal conductivity and specific heat of the material. Small free path length of phonons in neat EPDM provides lower thermal conductivity but higher thermal diffusivity. In nanosilica composite, very small particles of nanosilica have a greater tendency to agglomerate, however, due to higher interfacial area the higher phonon scattering brought higher thermal diffusivity comparatively (Gwaily S. E., 1998). The case is different with nanoclay and carbon nanofiber as the thermal inertia showed 0.2 x10-8 m2/s and 0.2 x10-8 m2/s respectively.

α = K / (ρ x C) (1) where α is the thermal diffusivity, K is the thermal conductivity, ρ is the density and C is the specific heat of the sample.

Fig. 3. Thermal conductivity of PI / EPDM / nanosilica, PI / EPDM / nanoclay and PI / EPDM / carbonanofiber composites.


9

Table 3 Measurement of thermal conductivity, thermal diffusivity and specific heat of PI/EPDM/nanofiller composites

Sl No.

Sample designation

Thermal Conductivity (K) (W/mK) at 85oC

Thermal Diffusivity (α) x 10-8 (m2/s)

Specific heat (C) (cal/g/oC)

1. EP0S0 0.17 9.1 0.46 2. EP5NS7 0.10 4.7 0.50 3. EP5NC7 0.44 0.2 0.50 4. EP5CNF7 0.54 0.3 0.47

B. Thermogravimetric analysis (TGA)

Fig. 4. TGA and DTG curve of EPD / PI / NC, EPDM / PI / NS and EPDM / PI / CNF nanocomposites

TGA was carried out in Q50 TA Instruments Inc, USA, from room temperature to 600oC. Nearly 10 mg of sample was place under nitrogen atmosphere at a heating rate of 10oC/min to measure thermal efficiency. The nitrogen flow (60 ml/min) was kept constant in all the cases. Figure 4 showed the TGA curves for all three EPDM nanocomposites with same loadings of polyimide/nanosilica, polyimide/nanoclay and polyimide/carbon nanofiber respectively. The maximum degradation temperature for unfilled EPDM (EP0NF0) compound was found 460oC and that for silica nanocomposites (EP5NS5), clay nanocomposite (EP5NC5) and carbon fiber nanocomposite (EP5CNF5) were found at 484oC, 482oC and 480oC respectively. There was nearly 20-22 % char residue in case of CNF composites. High char yield would be an additional advantage as the char itself is highly insulating and the probability of heat transfer is evens less. Compared to the silica and carbon fiber nanocomposites, nanoclay composite shows inferior thermal stability.

C. Differential scanning calorimetry (DSC)

DSC analysis of EPDM nanocomposites were done in TA instruments DSC 100. EPDM samples (6 mg) were analyzed under nitrogen atmosphere using a flow of 50 cm3 per minute and temperature ranges from -80oC to 400oC at heating rate of 10oC per minute. Figure 5 showed the DSC analysis of EPDM/PI/nanosilica, EPDM/PI/nanoclay and EPDM/PI/CNF nanocomposites respectively. Glass transition temperature (Tg) of neat EPDM was found to be at -51oC and there was very small shift


10

found with the incorporation of polyimide and different nanofillers like nanosilica, nanoclay and carbon nanofiber at -52oC, -52oC and -56oC respectively. Heat capacity was also measured from DSC (Table 3) as an absolute value of the heat flow as per equation (2).

(2) Where:

Cp= Specific heat (J/g/ oC) E= Calibration constant (for Indium=1) H= Heat Flow (m/w) 60= Conversion constant (minSec) Hr= Heating rate (oC/min) M= Sample mass (mg)

Fig. 5. DSC analysis of EPDM/PI/NC, EPDM/PI/NS and EPDM/PI/CNF nanocomposi

3.4 X-Ray diffraction (XRD) analysis X-ray diffraction (XRD) studies have been carried for all major constituents used in the compounding of insulation material. The broad pattern in XRD was found because of the amorphous nature of polyimide, nanosilica, nanoclay and carbon nanofiber. Along with the amorphous pattern of nanofillers some exceptionally sharp crystalline peaks were also recognized for the presence of activator zinc oxide (ZnO) in all three composites. It was also analyzed that the broad patterns of base rubber and fillers are merged together without shifting much which confirms the absence of chemical interactions between the rubber-filler and filler-filler matrix. Furthermore, area under the curve analysis of XRD patterns investigated 0.5 % crystallinity that can be attributed because of the addition of ZnO to the insulation material. Figure 6 shows the amorphous nature of EPDM nanocomposites in XRD.

E x H x 60

Hr x M Cp =


11

Fig. 6. XRD patterns of EPDM/PI/NC, EPDM/PI/NS and EPDM/PI/CNF nanocomposite

3.5 FTIR and ATR analysis

Thin films (~ 150 μm) were molded at vulcanization condition (at OCT) through which infra red light can pass easily to record FTIR data. Figure7 (a) showed the FTIR analysis of neat EPDM and polyimide. EPDM rubber showed the characteristic hydrocarbon absorbance at 2925 cm-1 and 2859 cm-1 (C-H asymmetric and symmetric stretching vibration). Absorption at 1436 cm-1, 1371 cm-1 and 722 cm-1 (-CH2, -CH3 angular deformation) confirmed the hydrocarbon chain. Polyimide characteristic spectra were at 1722 cm-1 (C=O str) for cyclic imide carbonyl and 1092 cm-1 (C-O-C str). There was an intense spectrum at 1359 cm-1 for C-N-C stretching in tertiary amine functional group. A less intense but broad spectra was found at 3487 cm-1 for N-H stretching. And a prominent spectra of imide five ring deformation appeared at 715 cm-1. Figure 7 (b) exhibited the absorption spectra of nanoclay and EPDM/PI/NC nanocomposite. The absorption spectra at 3628 cm-1 in nanoclay are attributed to the –OH stretching. Spectra of small intensity at 1655 cm-1 was due to absorption of –OH bending hydration. Spectra at 1126 cm-1 and 1037 cm-1 signified for Si-O stretching out of plane and in plane respectively. The disappearance of broad spectra of nanoclay merged into sharp peaks of polyimide in EPDM/PI/NC composite which reflected the physical interactions between polyimide and nanoclay. The broad peak of Si-O-Si disappeared and sharp intense spectra due to absorption of C-O-C appeared at 1032 cm-1. In Figure 7 (c) an intense broad absorbance for nanosilica was found at 3432 cm-1 (OH-stretching vibration of free silanol). Intense spectra of Si-O-Si network were found at 1082 cm-1 and a small band spectrum at 1634 cm-1 (O-H bending) was characterized for nanosilica. The decreased intensity of characteristic spectra of polyimide (cyclic imide C=O str) at 1722 cm-1 confirms the imidisation reaction in between polyimide and nanosilica in the composite sample (EPDM/PI/NS). Also, the broad spectrum of silanol group near 3432 cm-1 has disappeared leaving behind a small residual absorption in EPDM/PI/NS nanocomposite. In case of carbon nano fiber (CNF) and its composite ATR studies have been done. Figure 8 explained the absorption spectra of CNF and EPDM/PI/CNF nanocomposite. The broad absorption spectra of CNF were observed at 1087 cm-1 (C-O bond in alcohol). The C=C stretching was observed in the range 1400 – 1500 cm-1 and the main functional group C=O (carboxyl, lactone, conjugated ketone) were appeared as a broad peak in the range 1558 –1700 cm-1. The good compatibility of CNF with polyimide might be confirmed by the disappearance and shifting of many small peaks in EPDM/PI/CNF nanocomposite. The characteristic absorbance of C-O (in alcohol) has merged with the broad spectra of polyimide. It was found that in EPDM/PI/CNF nanocomposite, the absorbance due to the C=C has shifted from the range (1400 – 1500 cm-1) to the lower range (1360 – 1460 cm-1 ). There was disappearance of C=O groups which confirmed the interactions of CNF functional groups with polyimide.


12

Fig. 7. (a). FTIR absorption spectra of neat EPDM and polyimide (b). FTIR absorption peaks of nanoclay and EPDM/PI/nanoclay composite (c). FTIR absorption peaks of nanosilica and EPDM/PI/nanosilica composite.

Fig. 8. ATR absorption spectra of carbon nano fiber and EPDM/PI/CNF composite.

3.6 Scanning electron microscopy analysis and high resolution transmission electron microscopy

analysis 3.6.1. Scanning electron microscopy (SEM) analysis Morphology of the cured and uncured EPDM/PI nanocomposites (nanosilica, nanoclay and carbon nano fiber) were studied using scanning electron microscopy (SEM) at lower magnification. The gold coated samples were analyzed under SEM. Figure 9 (a) illustrated the non-continuous, layered morphology of uncured EPDM/PI/NS, EPDM/PI/NC and EPDM/PI/CNF nanocomposites. The melt matrix showed undefined distribution of polyimide and nanofillers in the matrix of EPDM. But, the morphology has changed in the cured samples which undergone heat treatment at 150ºC for 30 minutes and then cooled at room temperature. Depending on the vulcanization time and cooling conditions morphology of a material changes. It was found that with the increase of the vulcanization time, the interfacial adhesion between the rubber vulcanizates also increases. The SEM micrographs of cryo-fractured EPDM nanocomposites (cured or crosslinked) were illustrated in Figure 9 (b). The flexibility is restricted in uncured nanocomposites system which after vulcanization led to flexible crosslinked morphology in cured composites. The dispersion and


13

distribution of polyimide particulates and all three nanofillers were well defined throughout the matrix as continuous phases in vulcanized EPDM nanocomposite.

(a)

(b)

Fig. 9. (a). SEM micrograph of uncured EPDM/PI/NS, EPDM/PI/NC and EPDM/PI/CNF (b). SEM

micrograph of cured EPDM/PI/NS, EPDM/PI/NC and EPDM/PI/CNF 3.6.2. High resolution transmission electron microscopy (HR-TEM) analysis

EPDM nanocomposites (EP5NS7, EP5NC7, EP5CNF7) were investigated under high resolution transmission electron microscopy. HR-TEM micrographs were exhibited in Figure 10 for main ingredients (Polyimide / nanosilica / nanoclay / carbon nanofiber) and their respective nanocomposites. The composite samples were prepared by ultra cryo-microtomy. TEM micrograph of polyimide appeared uniform for very fine powder. Because of its very small particle size, nanosilica appeared as in agglomerate, whereas nanoclay was found in layered structured. TEM picture showed beautiful bamboo like structure of carbon nanofiber. Application of huge stress during mixing breaks this arrangement of CNF and in the composite of EPDM/PI/CNF it appeared as a hollow core in between the catalytic layer and CVD layer. Very good alignment of carbon fibers confirmed the better processing of the composite material. Dispersion and distribution of CNF is also uniform in the matrix of EPDM/PI. Orientation of nanoclay in the matrix of EPDM/PI/NC illustrated better processing of nanofillers. Dispersion and distribution also appeared good. However, there was some small agglomeration because of very small particle dimension of nanosilica in the matrix of EPDM/PI/nanosilica.


14

Fig. 10. TEM micrographs of polyimide (PI), nanosilica (NS), nanoclay (NC) and carbon nanofiber

(CNF) and TEM micrographs of EPDM/PI/NS, EPDM/PI/NC and EPDM/PI/CNF nanocomposites.

4. Conclusions Polyimide based nanosilica/ nanoclay/carbon nano fiber filled EPDM elastomer nanocomposites were prepared by melt mixing technique. Morphology of the nanocomposites samples by SEM and HR-TEM revealed that there was fine dispersion and distribution of all three nanofillers in the EPDM-polyimide matrix, though nanosilica showed small but insignificant agglomeration. Low density (≤ 1g/cm3) and enhanced mechanical properties were obtained from EPDM/PI/nanocomposites. Mechanical properties were better in case of nanosilica and carbon nanofiber composites with the compromise of density nearly equal to one. Polyimide based EPDM/nanosilica composite showed good retention of properties even after ageing. Thermal properties with all three fillers (nanosilica, nanoclay and carbon nanofiber) lead to approximately same maximum degradation temperature for the insulator material. Same composition of nanosilica composite participated for least thermal conductivity providing good candidature for thermal insulation application. All these properties at laboratory level showed that EPDM / polyimide / nanosilica is a promising composite which performed well for rocket motor insulation application compared to EPDM / polyimide / carbon nanofiber composites.


15

References Barikani Mehdi (2002). Polyimide derived from diisocyanates, Iranian Polymer Journal, 11, 215-236. Chase Michael John, Smith D. A., Dudly M. A. (1976). Rocket motor with ablative insulating casing liner, US

Patent 3973397. Choi Sung-Seen, Ha Sung-Ho, Woo Chang-Su (2006). Bulletin of the Korean Chemical Society, 27(3), 429-431. http://dx.doi.org/10.5012/bkcs.2006.27.3.429 David Burton, Patrick Lake, and Andrew Palmer, Properties and application of carbon nanofibers (CNFs)

synthesized using vapor-grown carbon fiber (VGCF) manufacturing technology. Applied Sciences, Inc., Cedarville, OH.

David C. Sayle (1990). Siloxane-based elstomeric interceptor motor insulation, US Patent 4953476. David G. Guillot (1994). Methods of insulating a rocket motor, US Patent 5352312. Deng Yuyuan, Wu Min, liu Xiangyang, Gu. Yi (2010). Novel inorganic morphologies formed in polyimide/silica

hybrid films, European Polymer Journal, 46, 2255-2260. http://dx.doi.org/10.1016/j.eurpolymj.2010.09.039 Deuri A. Saha, Bhowmick A. K. (1986). Aging of rocket insulator compound based on EPDM, Polymer

Degradation and Stability, 16, 221-239. http://dx.doi.org/10.1016/0141-3910(86)90028-5 Deuri A. Saha, Bhowmick A. K. (1987). Degradation of rocket insulator at high temperature, Journal of

Thermal Analysis, 32, 755-770. http://dx.doi.org/10.1007/BF01913761 Deuri A. Saha, Bhowmick A. K., John B., Ram T. S. (1987). Effect of moulding pressure, cooling rate and curing

temperature on the properties of rocket insulator compound, Journal of Material Science and Letters, 6, 1117- 1122.

http://dx.doi.org/10.1007/BF01729155 Fan J. L., Tsai Shr-Hau, Tu Fu-Hua, Tu Yao-Tsai (2007). Low density rocket motor insulation, US Patent

0112091 A1. Gajiwala M. Himansu (2004). Low density ablative rubber insulator for rocket motor, US Patent 0209987. Gibson W. Phillip, Lee Calveen (2007). Application of nanofiber technology to nonwoven thermal insulation,

Journal of Engineered fibers and fabrics, 2(2), 32-40.+- Gordon Sulherland (1972). EPDM rubber insulation composition, US Patent 3637576. Graham et al (1998). Durable motor insulation, US Patent 5821284. Guillot David J, Harvey Albert R. (2004). EPDM rocket motor insulation, US Patent 6787586 B2. Gwaily S. E., Abdel-Aziz M. M. and Madani M. (1998). Thermal and electrical studies on irradiated silica-

ethylene-propylene diene monomer (SiO2/EPDM) composites, Polymer Testing, 17, 265-287. http://dx.doi.org/10.1016/S0142-9418(97)00048-2 Hartz A. Walter, Meyer A. Daniel (1967). Elastomeric composition for use as rocket insulation, US Patent

3347047. Harvey Albert R, Ellertson Jhon W. (2003). Rocket motor insulation containing hydrophobic particles, US

Patent 6,606,852 B1. Harvey et al (2004). Fiber reinforced rocket motor insulation, US patent no. 6691505 B2. Iroh J. O., Longun J. (2012). Viscoelastic properties of Montmorillonite clay/ polyimide composite membrane

and thin films, Journal of Inorganic and Organometallic Polymers, 22, 653-661. http://dx.doi.org/10.1007/s10904-011-9613-4 Jacobsen L. Ronaldo, Applied Sciences, Inc, Carbon nanofiber filled materials for charge dissipation. Joseph Rani, George K. E., Francis Joseph (1987). Studies on 50/50 natural rubber/ styrene butadiene

copolymer blend, Die Angewandte Makromolekulare Chemie, 152, 107-119. http://dx.doi.org/10.1002/apmc.1987.051520110

http://dx.doi.org/10.5012/bkcs.2006.27.3.429

http://dx.doi.org/10.1016/j.eurpolymj.2010.09.039

http://dx.doi.org/10.1016/0141-3910(86)90028-5

http://dx.doi.org/10.1007/BF01913761

http://dx.doi.org/10.1007/BF01729155

http://dx.doi.org/10.1016/S0142-9418(97)00048-2

http://dx.doi.org/10.1007/s10904-011-9613-4

http://dx.doi.org/10.1002/apmc.1987.051520110


16

Jovanovic Samarzija S., Jovanovic V., Markovic G., Konstantinovic S., Marinovic-Cincovic M. (2011). Nanocomposites based on silica-reinforced ethylene-propylene-diene monomer/acrylonitrile-butadiene rubber blends, Composite: part B-engineering, 42, 1244-1250.

Kenneth E. Junior, Madison, Byrd D. James (1985). Aramid polymer and powder filler reinforced elastomeric composition for use as rocket motor insulation, US Patent 4492779.

Landon J. Shayne (2007). Insulated glass unit with sealant composition having reduced permeability to gas, US Patent 0178256A1.

Liles G. Herring (1985). Phenolic blast tube insulators for rocket motors, US Patent 4504532. Liles G. Herring (1985). Elastomer insulation composition for rocket motor, US Patent 4507165. Liles G. Herring (1987). Elastomer insulation compositions for rocket motors, US Patent 4663065. Liles G. Herring (1989). Elastomeric insulating material for rocket motor, US Patent 4878431. Lake L. Max, Tibbetts G. Gary. CNF re-inforced polymer composite, AIP conf. proc, 455-459. Marks D. John (1991). Rocket motor insulation and like made with thin tacky ribbon, US Patents 5007343. Martin W. Jon, Griese A Richard (1993). Carbon and silicon polymer ablation liner material, US Patent

5212944. Qiao Xiangyi, Chung Tai-Shung (2005). Fundamental characteristics of sorption, swelling, and permeation of

P84 co-polyimide membrane for pervaporation dehydration of alcohols, Industrial and Engineering Chemistry Research, 44, 8938-8943.

http://dx.doi.org/10.1021/ie050836g Roger J. Eldered, S. A. Lobst, Abu-Isa (1978). Ethylene-propylene elastomer and ethylene propylene diene

elastomer with improved heat and oil resistance, US Patent 4066590. Ryo Tamaki, Jiwon Choi, Richard M. Laine (2003). A polyimide nanocomposite from Octa (aminophenyl)

silsesquioxane, Chemistry of Materials, 15, 793-797. http://dx.doi.org/10.1021/cm020797o Rogwaski S. Gregory, Davidson T. F. (1990). Insulating liner for solid rocket motor containing vulcanizable

elastomer and a bond promoter which is a NOVOLAC epoxy or a resole treated cellulose, US Patent 4956397. Shimamura Y., Chiba T., Tohgo K. and Araki H., Thermal and mechanical properties of unidirectionally aligned carbon nanofiber/epoxy composites by using AC electrical field.

Suae-chen Chang (1991). Rocket motor insulation using phosphonitrilic elastomeric compositions, US Patent 5024860.

Tang J. C., Lin G. L., Yang H. C., Jiang G. J., Chen-Yang Y. W. (2007). Polyimide silica nanocomposite exhibiting low thermal expansion and water absorption, Journal of Applied Polymer Science, 104, 4096-4105.

http://dx.doi.org/10.1002/app.26041 Tucker Jerry, Waco (1971). Ablative composition of material, US Patent 3562304. Uyarel A. Y., Pektas I. (1996). A thermal analysis investigation of new insulator compositions based on EPDM

and phenolic resin, Journal of Thermal Analysis, 46, 163-176. http://dx.doi.org/10.1007/BF01979956 Wang Jia, Iroh Jude O., Long Amy (2012). Controlling the structure and rheology of polyimide/ nanoclay

composite by condensation polymerization, Journal of Applied polymer Science, 125, E486-E494. http://dx.doi.org/10.1002/app.36242 Wendel M. Gary (1987). Thrust nozzle with insulation, US Patent 4649701. Wu Guang-Yu, Hai-bo, Yao Wang Cheng, Qiu Feng-Xian (2011). Preparation, morphological characteristics

and electro-optic property of polyimide and hybrid, Material Science Forum, 663-665, 808-811.

http://dx.doi.org/10.1021/ie050836g

http://dx.doi.org/10.1021/cm020797o

http://dx.doi.org/10.1002/app.26041

http://dx.doi.org/10.1007/BF01979956

http://dx.doi.org/10.1002/app.36242

epdm nanocomposites using polyimide as ablator: morphology ... · was vulcanized at 150oc for 30...

Documents