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: meetkbs@gmail.com,

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).

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.

Fig.2. Schematic of the test configuration for Oxyacetylene Flame Test.

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.

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–

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-

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

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.

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