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SUPPLEMENTARY MATERIAL
Thermal Ablation and Laser Shielding Characteristics of Ionic
liquid Microseeded Functionalized Nanoclay/Resorcinol
Formaldehyde Nanocomposites for Armor Protection
Vijay Kumar1, Sunith Singh2, Balasubramanian Kandasubramanian1*
1Department of Materials Engineering, Defence Institute of Advanced Technology,
Girinagar, Pune- 411025, India
2Indian Institute of Technology (Banaras Hindu University), Varanasi, India
*Corresponding author: [email protected] ,
Fax: +91 020 2438-9509; Tel: +91 020-24304207
1. SYNTHESIS
Synthesis of a resole type RF takes place by the polycondensation of resorcinol and
formaldehyde with a basic catalyst (fig.1a); the overall experimental procedure
employed in the synthesis process is represented in fig.1 (b). The resole type RF
samples so synthesized were highly cross-linked with molecular weights ranged above
50000; GPC used to evaluate the molecular weight before cross-linking wherein post 25
minutes run using Styragel HR3 column (molecular weight range 500 to 50000), the
solution eluted first and no peak was observed. The GPC equipment used for evaluation
of molecular weight of RF was Waters (Waters 2414 Refractive Index Detector, 515
HPLC Pump and Styragel HR3 THF column).
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Fig.1.(a) Resorcinol Formaldehyde (Resole) formation reaction chemistry. (b)
Schematic illustration of in-situ polymerization process used for synthesis of NC-RF
nanocomposites.
2. OXY-ACETYLENE FLAME TEST
The Oxyacetylene Flame Test carried out as per ASTM E285-08 (2015)
standards. The test specimen with 40mm (dia) x 20mm (thickness) dimensions was
placed in an enclosed chamber, which was slowly filled with an inert gas (argon) to
prevent oxidation during the test. Argon gas being heavier than air filled up the
chamber, displacing the air upwards to exit from the outlet at the top of the chamber,
which was confirmed by the oxygen sensing gauge. The hot oxy-acetylene flame was
directed perpendicular to the specimen surface for 60 seconds, maintaining volume
ratio of oxygen to acetylene as 1.2. The back-face temperature was measured with a
thermocouple, bonded on rear surface of each sample. The back-face temperature was
observed till 300 seconds, though the oxy-acetylene flame was switched off after 60
seconds. Five samples of each formulation were tested and the average of the
measurements was taken as the final value. The schematic of test configuration is
depicted in fig.2.
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Fig.2. Schematic of the test configuration for Oxyacetylene Flame Test.
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3. DISPERSION AND SURFACE MORPHOLOGY
3-D images of the resin, filler and nanocomposites developed by image
metrology software showing the comparative surface morphology are shown in fig.3.
Fig.3. 3-D images – (a-b) RF: (a) at room temperature (b) post ablation; (c) Nanoclay,
surface modified; (d-e) NC-RF Nanocomposite (10 wt %): (d) at room temperature (e)
post ablation.
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4. FOURIER–TRANSFORM INFRARED (FTIR) SPECTROSCOPY
Fig.4. FT-IR spectra of (a) pristine RF (b) NC-RF nanocomposite (c) NC-RF
nanocomposite with IL (d) NC-RF nanocomposite, post ablation.
The FT-IR spectra of pristine RF and NC–RF nanocomposites are shown in fig.4.
The peaks appearing at 2942, 2842 and 1479 cm -1 in the spectrum of pristine RF at
room temperature are assigned to –CH2– bonds, while a broad band in the region
3000- 3600 cm-1 is associated with the –OH group of resorcinol[1]. This broad band is not
observed in the spectra of NC-RF nanocomposites. The band at 1602 cm -1 can be
attributed to the aromatic -C-C- stretches present due to resorcinol. The pronounced
sharp absorptions in 2900–3000 cm-1 region of RF are assigned to the aliphatic –CH2–
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stretches[2], whereas the peak at 3650 cm-1 is associated with the stretching of –OH
bond. The spectrum of the nanocomposite with IL, shows peaks at 871 cm -1 and 1057
cm-1 which correspond to B-F stretching[3] while the 1062 cm-1 peak is assigned to the
asymmetric stretching vibrations in BF4- anion [4]. The small peak at 801 cm-1 in the
spectrum of ablated NC-RF nanocomposite sample is assigned to a Si–C stretching
vibration[5], confirming the formation of SiC on the ablated surfaces. The spectrum of
NC-RF nanocomposite shows the presence of the characteristic absorptions of the
nanoclay at 1018 cm-1 and 3637 cm-1 which correspond to Si-O and Si-OH stretches
respectively[6,7]. The presence of these absorption bands in the ablated sample spectra
as well indicate the presence of clay-rich protective barrier network over the ablated
surface as seen in the SEM images.
5. SURFACE WETTABILITY
The increase in the concentration of surface modified nanoclay from 1 to 10 wt%
in RF resin induces hydrophobic character and the hydrophobicity further increases with
the addition of 3 wt% IL, implying improved intercalation of the polymer inside clay
galleries to form an intercalated and/or exfoliated structure. This phenomenon can be
explained by Cassie-Baxter regime equation:
cosθCB= f scosθ+ (1−f s )=f s[ γsv−γ slγlv+1]−1 (1)
where fs is the fraction of the surface contact with liquid and the remaining (1-fs) contact
with air associated with interfacial energy of solid-liquid ( γsl), solid-vapor (γsv) and liquid-
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vapor (γlv) and (θCB) is the contact angle involved. The equation (1) illustrates that the
key parameter affecting contact angle measurement is surface roughness. The
roughness created on the surface with nanoclay might be sufficient to trap air inside the
voids of the surface. Consequently, a heterogeneous surface composed of both air and
solid evolves, which reduces the adhesive force between solid surface and the
solvent[8]. In this case, the pores on the top surface of the NC-RF sample get filled with
air and enhance the contact angle. For better understanding of the wettability of NC-RF
nanocomposites, the Cassie and Baxter contact area fraction fC between the liquid and
the rough hydrophobic surface is given in equation (2) where θ’ is the contact angle for
the smooth solid surface and θ is the contact angle for the same roughened surface[9]:
f=f c=cosθ'+1cosθ+1
(2)
The Cassie’s equation applies to a surface composed of well-separated and distinct
domains while further work in this field by Israelachvili and Gee’s [10] focussed on the
chemical heterogeneities of atomic or molecular scale. In equation (3), where fR is the
contact fraction between the liquid and the nanorough hydrophobic surface, θ’ is the
contact angle for the nano-roughened surface and θ is the contact angle for the original
smooth solid surface:
f=f R=[ cosθ '+1cosθ+1 ]2
(3)
To exemplify this aspect for a starting hydrophobic smooth surface with contact
angle 950, to increase θ to 1410, fC will be 0.24 while fR will be 0.06, which implies that
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the liquid drop makes contact with only 6% of the solid surface (Equation 2 & 3). This is
due to the transition from the Wenzel regime to Cassie region.
REFERENCES
[1] Badhe, Y.; Balasubramanian, K. Novel hybrid ablative composites of resorcinol
formaldehyde as thermal protection systems for re-entry vehicles. RSC Adv. 2014, 4,
28956-28963.
[2] Mulik, S.; Leventis, C. S.; Leventi, N. Polym. Prepr. 2006, 47(2), 364-365.
[3] Shukla, M.; Srivastava, N.; Saha, S. Interactions and transitions in imidazolium
cation based ionic liquids. Prof. Scott Handy (Ed.), 2011, ISBN: 978-953-307-634-8.
[4] Thanganathan, U.; Nogami, M. Investigations on effects of the incorporation of
various ionic liquids on PVA based hybrid membranes for proton exchange membrane
fuel cells. Int. J. Hydrogen Energy 2015, 40(4), 1935–1944.
[5] Mendelovici, E.; Frost, R. L.; Kloprogge, J. T. Modification of chrysotile surface by
organosilanes: an IR–photoacoustic spectroscopy study. Journal of Colloid and
Interface Science 2001, 238, 273–278.
[6] Zanetti, M.; Bracco, P.; Costa, L. Thermal degradation behaviour of PE/clay
nanocomposites. Polymer Degradation and Stability 2004, 85, 657- 665.
[7] Loo, L. S.; Gleason, K. K. Fourier transform infrared investigation of the deformation
behavior of montmorillonite in nylon-6/nanoclay nanocomposite. Macromolecules 2003,
36, 2587-2590.
[8] Sahoo, B. N.; Balasubramanian, k. Facile synthesis of nano cauliflower and nano
broccoli like hierarchical superhydrophobic composite coating using PVDF/carbon soot
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particles via gelation technique. Journal of Colloid and Interface Science 2014, 436,
111- 121.
[9] Sahoo, B N.; Sabarish, B.; Balasubramanian, K. Controlled fabrication of non-fluoro
polymer composite film with hierarchically nano structured fibers. Prog Org Coat. 2014,
77(4), 904-907.
[10] Israelachvili, J. N.; Gee, M. L. Contact angles on chemically heterogeneous
surfaces. Langumir 1989, 5, 288-289.