Keywords

Introduction

Materials for Coating Application in Automobile Industry

In the present scenario, the heat transfer or dissipation mechanism in any automobile is very important so that it can run for a long time efficiently and effectively. Thermal protective coating (acronym as TPC) has always been a dynamic research domain which employs material’s heat sinking characteristic and versatility for its enhanced performance. These necessitated properties are accomplished by utilizing polymer composites as heat sinking materials with organic/inorganic reinforcements. Following which inorganic nanoparticles reinforced in polymer composites (or nanocomposites) has not only captivated scientists but also industries due to significant enhancement in their mechanical, frictional, electrical, and thermal performances (Zou et al. 2008; Ma et al. 2010; Sengupta et al. 2011; Sinha Ray and Okamoto 2003; Daniel et al. 2016, 2019; Sanoj and Kandasubramanian 2014; Kumar and Kandasubramanian 2016; Hussain 2018; Hussain and Mishra 2018). These consequential properties have boosted competitiveness and innovation in automobile sectors to redesign or replace old products with new and advanced materials.

Among these, coating technologies have manifested improved energy efficiencies, enhanced performance, and extended lifetimes for wider state-of-the-art applications (e.g., automobile components having more susceptibility to heat). More efforts have been implemented to use advance materials in vehicle bonnet, radiators, car battery casings, etc. Coatings for vehicles have been the most researched domain where innovations can be frequently perceived through improvement in heat transfer mechanism and weight reduction effect. Among the existing advanced materials, composite materials have emerged as a potential candidate for thermal applications due to its low thermal conductivity. Excellent insulative properties and lightweight composite materials have inducted them a prominent material for coating purpose in automobiles (Fig. 1). But a direct replacement of an existing metal system with composite materials may not be feasible owing to weaknesses associated with composite materials. This challenge has led to an intensified research on strengthening mechanism of composites for enhancing thermal insulative properties (Yadav et al. 2016; Panwar et al. 2016; Kumar and Balasubramanian 2016; Katiyar and Balasubramanian 2014, 2015; Badhe and Balasubramanian 2014).

Fig. 1
figure 1

Advanced coating materials for automotive applications

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Heat Transfer Mechanism

Heat sink functions as heat exchanger, lowering down the temperature of an entity with heat dissipation into the surrounding, and therefore, the design incorporates maximization in surface area for an enhanced interaction with its proximity environment, e.g., air, water, oil, or refrigerant. However, factors such as material selection, protrusion design, air velocity, and finally surface treatment are pivotal to steer its performance. Thus, these heat sinks are an effectual source for absorbing and abandoning superfluous heat into the surrounding by conduction, radiation, or convection. Therefore, potential is employed in electronic industries owing to its heat loss (<100% efficiency) which deteriorates device functioning. Amidst that, heat sink performance is compromised when it fails to adhere to heat spreader and thus necessitates attachment methods and interfacial materials such as grease and adhesives which prevent air-gap formation (Ho and Chen 2013; Irvine et al. 2015).

Introduction to Polymer Matrix Composites

Composite material unites matrix which functions as a binding agent and enfolds another phase called reinforcement, either of them having a different set of properties merged together to engender its enhanced operation in specific application. The key factor for deciding fabrication method depends on matrix properties, the effect of reinforcement in polymer matrix, and its inertness or non-reactivity to chemicals (Akonda et al. 2012).

Figure 2 represents the schematic of constituent elements in composite material owing to its negation in dissolution or blending which shows their physical distinctness in each other (Karasek et al. 1996). Among various kinds of composites with polymer as a matrix and fly ash cenosphere (FAC), FS, graphene, POSS, etc. as inorganic fillers, polycarbonate-based has attracted the modern sector due to its excellent insulating properties (Das and Prusty 2013; Han and Fina 2011).

Fig. 2
figure 2

Schematic representation of interaction of polymer and filler particles

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Normally, reinforced fibers are strong albeit brittle and are less resistive to sharp bending; therefore, support/protection/load sharing has been imparted by plastic matrix. Thus optimized combination of matrix and reinforcement bestows excellent functional properties, but materials are limited in terms of expensive precursors which act as lockage to their wide applications (Lee et al. 2014). Consequently, nanofiller-reinforced polymer composites have attracted a great scientific interest as they significantly enhance the properties of the composites.

Polymer Matrix-Based Nanocomposites

Nanocomposites are engineered by nano-size reinforcements like nanoparticles, nanotubes, and nanolayers (based on geometries) in matrix which significantly affects the crystallinity, mobility, and other structural aspects (Balasubramanian and Tirumalai 2013). The most common materials used as matrix in polymer nanocomposites are polycarbonates, epoxy, nylon, polypropylene, and polyetherimides (Nakagaito and Yano 2005; Suryanegara et al. 2009). Significant improvement in properties of nanocomposite is due to interphase formation near nanofillers as they have large specific surface area to affect host properties (Suryanegara et al. 2011).

The strong interfacial interaction between two phases in nanocomposites significantly aids in heat transfer from matrix to filler, and size along with morphology of the fillers has appreciable impact on its stability, e.g., ultrafine size filler materials with higher aspect ratio show excellent dispersion without any agglomeration. The potential application of nanocomposites is in aerospace, marine structures, automobile sectors, wind energy, military applications, electronic devices, and medical applications (Balasubramanian et al. 2017; Tirumali et al. 2017; Prasad and Kandasubramanian 2019; Gonte et al. 2014, 2016, 2019; Ashish et al. 2019; Ramdayal 2014; Gore and Kandasubramanian 2018a, b; Jain et al. 2018; Mishra and Balasubramanian 2014; Balasubramanian 2012; Magisetty et al. 2019).

Polycarbonate and Its Nanocomposites

Polycarbonate (PC) (Fig. 3) possesses a long chain of linear polyesters from carbonic acids and dihydric phenols, which entails intermolecular attraction among phenyl groups in molecular chain. This interaction contributes molecular immobility and stiffness in chains, thereby conferring thermal stability to polycarbonates (Sweileh et al. 2010; Banerjee and Khaira 2014; Yadav et al. 2017; Banerjee and Balasubramanian 2015). Owing to its exceptional shatter and heat resistance, a vast area of research awaits for enhancing thermal properties through filler reinforcements. Among macro and micro dimensional additions, nanometer-sized fillers are preferred for enhancing the thermal properties on account of their higher specific surface area (1000–1500 m2/gm), less stress concentration, minimal wt% requirements, and reduced interparticle distance (Hofmann et al. 2014). Escalation of nanotechnology research especially in composite study (PC, etc. as matrix with nano-size fillers, e.g., polyhedral oligomeric silsesquioxane, graphene, montmorillonite, and other inorganic nanoparticles) has led to augmentation in varied properties like thermostability, flame retardancy, etc. for industrial-oriented applications of PC were reinforced after adding nano-inorganic fillers (Parvaiz et al. 2010; Moon et al. 2014; Poręba et al. 2011; Jindal et al. 2014; Song et al. 2008; Gedler et al. 2012; Feng et al. 2010; Hakimelahi et al. 2010).

Fig. 3
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Chemical and 3D structure of polycarbonate

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Moon et al. (2014) investigated electrospinning and compression molding-manufactured PC-reinforced carbon nanotube (CNT) composite for its influence on mechanical, flame retardant (thermal), and electrical properties. This enhancement was accounted to uniform dispersion of nanomaterials and, therefore, proposed huge potential for automobile and construction (Abbasi et al. 2010; Hornbostel et al. 2006).

Additionally, Parvaiz et al. (2010) reinforced mica in PC to study dispersion, thermal, mechanical, and rheological properties. Results delineated that vinyltrimethoxysilane and 3-aminopropyltriethoxysilane-modified mica had rendered uniform dispersion in polymer matrix as compared to raw mica. Limiting oxygen index (LOI) was measured which corroborated improvement of 11% thermal stability and 29% flexural modulus, viscosity, and dynamic modulus along with declination in flammability for 5 wt% silane-treated mica incorporation.

Conclusively, polycarbonate had been previously investigated with a number of reinforced fillers to enhance the mechanical, rheological, and thermal properties, but current document presents FAC, FS, and POSS reinforcements for enhancement of the same.

Fly Ash Cenosphere (FAC)

Fly ash is the discharged product of aluminosilicate-rich mineral from coal-fired power plants whose disposal has grown as a challenge and concern (Deepthi et al. 2010). However, emission of fly ash from pulverized combustion of coal embodies hollow aluminosilicate microspheres, which had bulk density range of 0.2–0.8 g/cm3, which are known as cenosphere. Proportion of the same fly ash varies from 0.01 to 4.80 wt% and constitutes SiO2 = 50–65 wt%, Al2O3 = 20–37 wt%, and Fe2O3 = 1–11 wt% (Fomenko et al. 2011; Raask 1968, 1969; Ismail et al. 2007). Specific formulation of alumina and silica in cenospheres renders mullite content (3Al2O.2SiO2) which equipped considerable thermal stability, high creep resistance, low thermal expansion, and high thermal shock resistance (Varadarajan et al. 2001; Cao et al. 2004; Bartuli et al. 2007).

Its compatibility with polymers (e.g., latex, urethanes, thermoplastic, epoxy, etc.) and specialty materials (e.g., cement, composite, coatings, etc.) is dependent on interface bonding of cenosphere with surrounding matrix which further is a function of degree of surface treatment. Though properties manifest its wide-range utilization in marines, fire/heat protection devices, insulations, etc., low-cost availability of cenosphere and associated environmental dangers have challenged the integration of fillers.

Das and Satapathy (2011) investigated thermal, mechanical, and structural properties of cenosphere-filled polypropylene composites. Results explicated 30% enhancement in storage modulus and shift of onset temperature by 8–10 °C (by TGA) for 10 wt% addition of cenosphere in polymer matrix. Deepthi et al. (2010) had researched on high-density polyethylene (HDPE)-FAC composites and discovered enhancement in char residue and onset degradation temperature by 28%, 30 °C, respectively.

Fumed Silica (FS) Composites

FS particles, due to inherent structure, cease porosity and empower hydrophobic character, thus facilitating its utilization as reinforcements, pigments, and polishing materials (Ettlinger et al. 2000; Gun’ko et al. 2000; Gupta and Balasubramanian 2016). Olmos et al. (2011) had reported the consequence of alumina and silica nanoparticles on coefficient of thermal expansion in low-density polyethylene (LDPE). Results revealed that silica nanoparticles had curtailed the coefficient of thermal expansion value about 40% higher than the composites containing alumina particles. Authors had also reaffirmed the abovementioned by validating the experimental value to that of theoretical results. However, Feng et al. (2014) had aimed on thermal degradation kinetics and mechanisms of PC/silica composites which were fabricated by melt blending of pristine and modified silica particles. The thermal analysis data revealed increment in char residue of composite when 1 wt% of FS was utilized. They also reaffirmed the results of thermal degradation by employing Kissinger-Akahira-Sunose (KAS) as well as Flynn-Wall-Ozawa (FWO).

POSS Composites

POSS has allured thermal domain on account of its outer organic nanostructure (multiple terminal functionalities which impart compatibility with polymers) encapsulating inorganic cores which are engineered such as to fabricate hybrid reinforced nanomaterials. POSS is defined by formula, in general, (R-SiO1.5)n, where n is the repeatability of unit and R is an alkyl chain (Haddad and Lichtenhan 1996; Fu et al. 2000; Striolo et al. 2005; Hany et al. 2005; Ye et al. 2006; Zheng et al. 2001; Yadav et al. 2019). POSS incorporation resulted in modulation of thermal stability, mechanical and thermomechanical property, flame resistance, electrical insulation, shape-memory property, etc. with further dependency on employed method, dispersibility, and degree of compatibility with polymer, e.g., the presence of phenyl group at corners in POSS (Ph-POSS) enhances its reaction with PC (Xu and Song 2010; Li et al. 2002). Evidently, Hao et al. (2007) had prepared Ph-POSS/PC (reinforcement/matrix) composite by solution casting method and found that reinforcement was miscible in matrix but only up to ~7 wt% due to phase separation at higher additions. Additionally, Li et al. (2001) and Phillips et al. (2004) had studied the influence on glass transition temperatures (Tg) of composite with POSS nanofiller addition (i.e., proportional relation) which were accounted to reinforcement aggregation, thus hindering molecular/segmental rotation and dipole interaction potential which contributed to polymer strengthening. Moreover, anhydride-cured POSS/epoxy composites had incremented storage moduli but lowered coefficient of thermal expansion furnishing stability at high temperature but could match the flame retardancy as aforementioned with PC or with other heat-resistant thermoplastics. Thus, this reinforcement due to its multifunctional advantages in polymers explores automobile and space domain and thermal print heads used in rapid prototyping.

CNT, carbon fibers, etc. are some universally acknowledged nanofillers which bestow the final entity with thermal property, but these reinforcements are restrained by material cost and therefore real-life applications (Tirumali et al. 2018; Badhe et al. 2015). However, FAC proposes a cost-effective reinforcement, but FS and POSS fillers lack a rigorous investigation for improving thermal properties of polycarbonate. Therefore, this chapter depicts fabrication of polycarbonate composites using aforesaid fillers with three varied processes such as solvent casting, melt blending, and spray coating. Effectiveness of the composite has been affirmed through XRD, SAXS, FTIR, AFM, FESEM and HRTEM, viscosity, contact angle, TGA, WAXRD, and AFM. Finally, the chapter discusses optimized results of reinforcements and also comparative analysis between the three with a proposed application in car battery casing.

Experimental

Preparation of PC Solution

Mixing of pre-dried PC pellets was commenced in dichloromethane (DCM) with further addition of FAC/FS/POSS particles individually in 20 ml prepared PC solution to form three different composites. Homogenization was accomplished by ultrasonication with 20 kHz frequency emanating from 13 mm diameter titanium (Ti) probe for a period of 4 h (20/5 on/off cycle with 40% input energy) followed by 1 h stirring. However, to prohibit DCM from being vaporizing (boiling point = 39 °C), temperature was kept at <35 °C with ice/water during the complete course of mixing.

Functionalization of Cenosphere

Before PC composite formation, raw FAC was functionalized by dissolving 50 g of filler in 32:8 v/v ethanol/water solution at 15 °C with subsequent addition of silane coupling agent tetraethoxysilane (3 wt% of cenosphere). Resultant amalgam was kept under stirring at 40 °C for 60 min to proceed with uniform distribution and hydrolysis reaction. Lastly, the stirred mixture was vacuum dried in an oven at 80 °C for a period of 24 h and obtained functionalized FAC as an output was utilized as reinforcement in PC for composite production (Kang et al. 1990). Figure 4 shows the silane grafting of cenosphere.

Fig. 4
figure 4

Depicting FAC silane grafting

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Fabrication of Composite: Polycarbonate – FAC/FS/POSS Composites

Solvent Casting Method

The first method employed for composite development is solvent casting in which solid fillers (PAC/FS/Ph-POSS) (Fig. 5) were stirred or sonicated for a period of time to attain homogeneous dispersion in solvent. Concentration of all fillers was varied, tested, and finally optimized at 30 wt% for both FAC and Ph-POSS while 10 wt% for FS (influential in controlling thermal insulation and permeability of composite than the other two) which resulted in film thickness uniformity. Otherwise (beyond selected concentrations), each of the solution witnessed elevated viscosity (thixotropic behavior conferred by fillers), brittleness, and irregular film surface. After finalizing reinforcement quantification, as-prepared solution was casted on flat substrate or base followed by evaporation without interference of thermal/mechanical stresses (Chen et al. 2010). Thereafter, evaporation of solvent rendered film of thickness 0.2 mm (after peeling) as measured by digital thickness gauge; furthermore, these fillers imparted thermal stability to PC composite (Wang et al. 2013).

Fig. 5
figure 5

The mechanism of solvent casting method PC/NFC and schematic of solvent casting process of PC/POSS composite (Katiyar and Balasubramanian 2015)

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Melt Mixing

Second processing method commenced by pre-drying of PC polymer in oven at 120–140 °C for duration of 2 h to eliminate any of the adsorbed moisture on its surface. Meanwhile, all reinforcing particles (i.e., FAC/FS/Ph-POSS) were differentiated by sieve size 45 mm, and, consequently, obtained fillers were compounded in a corotating twin screw extruder (Co-TSE) having a screw (full intermeshing and distributive mixing Favis and Therrien 1991; Goffreda et al. 1998) of 20 mm diameter with 40:1 L/D ratio. Extrusion facilitated blending of FS up to 30 wt% in contrast to 10 wt% by other discussed techniques, but 10 wt% FS had been homogenized for comparative analysis. Filament was extruded out of die engineered with 50 mm diameter at 280 °C or 290 °C and screw speed of 20–25 rpm which were subsequently cooled, pelletized, dried (~90 °C for 12 h), and conditioned to 24 h at 23–27 °C.

Spraying Method

This method had deposited a continuous and uniform film on surface by spraying a composite solution (PC mixed with FS/Ph-POSS) (Fig. 6). For obtaining the aforementioned, parameters were adjusted such as pressure of airstream forcing solution (in micro-droplets form) onto substrate which was kept at 0.6 MPa, substrate (e.g., glass slide) and gun/nozzle head gap 20 cm, viscosity of solution 25 cP, and spraying angle 45°, and 1.5 ml volumetric solution was coated on each substrate. Before performing spraying, solution was prepared by dissolving 2 g PC in 20 mL dichloromethane (DCM) along with addition of fillers having same concentration as that used with other techniques (i.e., 10 wt% each). Spraying facilitated uniform distribution of particles across the surface rendering conformal film and, thereafter, dried (solvent evaporation allows filler interaction for obtaining dry film); nevertheless, consistency was determined by rate of solution emanation. Inside spray nozzle plate was housed with multitude pores in range 10–15 nm for equip uniformity to solution on surface However, this process caused entrapment of air in layer as well as evaporation of solvent while spraying sourced viscosity increment affecting film evenness. Equations (1), (2), (3), and (4) (Eq. 2 is based on 1D heat conduction model) are defined which assist in optimization of spray parameters and solidification or cooling rate of the film which are further characterized for property evaluation (Ettlinger et al. 2000; Gun’ko et al. 2000; Olmos et al. 2011; Feng et al. 2014; Haddad and Lichtenhan 1996; Fu et al. 2000; Armster et al. 2002).

$$ P=\frac{1}{2}\uprho {v}_s^2 $$
(1)
$$ {R}_c=\frac{h{T}_{pa}}{L_{pa}\uprho} $$
(2)
$$ F=\uprho a{v}_s^2 $$
(3)
$$ K=W{e}^{\frac{1}{2}}+R{e}^{\frac{1}{4}} $$
(4)

where P = pressure (with which droplets undergo collision on glass or any other substrate-energy conservation), adjusting its magnitude will regulate spray velocity vs, ρ = particle density, Rc = film-cooling rate, Lpa = solidification latent heat, Tpa = temperature of particle, h = interface heat transfer coefficient, F = momentum change rate with force direction, ρavs = mass spray rate, Re = Reynolds number, K = Sommerfeld number (value determines particle behavior during impact on substrate, i.e., K < 3 ➔ rebound, 3 < K < 58 ➔ deposition, K > 58 ➔ splashing and deposition), and We = Weber number.

Fig. 6
figure 6

Schematic representation of spray coating process

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Performance Evaluation of Different Reinforcements

Morphological Analysis

Filler particles were characterized by FESEM and HRTEM which depicted morphological response to different processing techniques.

Figure 7a–c portrays morphology transition from untreated FAC to grafted particles; former had smooth and globular particles while latter displayed roughness on its surface. Additionally, outer skin of particles, after treatment, transited to small and granular reaffirming the modification and also favored adhesion between matrix (PC) and filler (FAC). Moreover, Fig. 7d also presented weak attachment and loose dispersion of particles into matrix. Figure 7e, f illustrated FAC (after treatment) mechanical interlocked structure in matrix and retained spherical nature even after composite extrusion; however, Fig. 7g, h displayed composite 50 wt% morphology containing numerous FAC particles uniformly distributed and dispersed accompanied with low quantity of unbound filler.

Fig. 7
figure 7

FESEM images of FAC and PC/FAC composites: (a) raw FAC, (b) raw FAC at 15.00 kV, (c) silane-grafted FAC, (d) 5 wt% PC/FAC, (e) 5 wt% PC/grafted FAC, (f) 50 wt% PC/FAC pellets, (g) 50 wt% PC/FAC, and (h) 50 wt% PC/grafted FAC (Katiyar and Balasubramanian 2014)

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FESEM Fig. 8a and HRTEM Fig. 8b depicted spherical, nanoscale chain-like and bridging morphology of FS particles with sizes in range of 40–80 nm. Morphology of sample (i.e., solvent containing PC) in its pristine form was illustrated in Fig. 9a. Further, for spray-coated films (Fig. 9b), composite morphology showed spherical and smooth (similar to abovesaid for pure FS), while Fig. 9c, d are solvent casting and melt-mixed composite structure. Roughness on composite surface also delineated about entrapped air while spraying (jet flight time) which engendered porosity and symbolized adequate wettability between contents (PC and FS) as also observed by Chrissafis et al. (2009a, b) and Tenjimbayashi and Shiratori (2014).

Fig. 8
figure 8

(a) FESEM image of nano-fumed silica particles and (b) HRTEM image of nano-fumed silica particles (Katiyar and Balasubramanian 2015)

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Fig. 9
figure 9

FESEM micrographs of (a) pristine PC, (b) spray-coated film, (c) solvent-casted film, and (d) melt-blended FS-PC composite (Katiyar and Balasubramanian 2015)

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Similarly, Fig. 10a demonstrated agglomeration of pure Ph-POSS particles owing to its size effect (nano-dimensional structures having large surface/volume ratio are susceptible to clustering) and thereby necessitated homogeneous dispersion by mechanical energy. Figure 10b, a HRTEM picture depicting Ph-POSS filler brick-like arrangement (Kolay and Singh 2001) while with more magnification and resolution. Figure 10c single particle was analyzed which exhibited thickness of ~29 nm. Figure 11a displayed nonuniformity in film morphology of solvent-casted composite in contrast to smooth and rectangular surface from spray coating (Fig. 11b) and even distribution from mix blend method (Fig. 11c). Challenge of air entrapment was similar as described above, but manifested distribution had surpassed results obtained by other authors (Periyasamy et al. 2015; Leng et al. 2014).

Fig. 10
figure 10

(a) FESEM image of Ph-POSS particles, (b) HRTEM image of Ph-POSS particles, and (c) HRTEM image of Ph-POSS particles at higher magnification

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Fig. 11
figure 11

FESEM images of (a) solvent-casted film, (b) spray-coated film, and (c) reactive blended POSS-PC composite

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FTIR Analysis

FAC, possessing organic properties, hampers molecular-dimensional adhesion of fillers and matrix and thus requisites chemical modification for miscibility. FTIR graph (Fig. 12) displayed 2970, 2330, and 2950 cm−1 peaks for C–H stretching in pure PC, PC/FAC, and PC/grafted FAC composite, respectively. Later two peaks were attributed to silane bonding and hydroxyl group vibration, respectively, which evidenced efficacious coupling between FAC and silane. Weak methyl stretching was perceived at 2970 cm−1 due to non-existence of other peak (Wang et al. 2013) nevertheless, fingerprint peaks of 1508 cm−1 (for carbonate ion stretching) and 1152, 1188, & 1227 cm−1 triplets had characterized PC. Moreover, FTIR analysis had also reflected peaks of 1772 cm−1 for C=O stretch, strongest peak among C-O i.e 1016 cm−1 , and an intense 1506 cm−1 for aromatic ring breathing modes. Furthermore, 1009 cm−1 delineated presence of Si-O-Si asymmetric stretches which are attributed to tetrahedral SiO structure in FAC and similarly, in grafted-FAC composites, 757 cm−1 depicted Si-O vibration originating from Si(OC H )).

Fig. 12
figure 12

(a) FTIR spectra for FAC and grafted FAC. (b) FTIR spectra for PC composites (Katiyar and Balasubramanian 2014)

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Figure 13 characterized peaks at 1094 cm−1 and 3821 cm−1 attributed to O-Si-OH in PC/FS composite for deformation mode and methylene stretching, respectively. Additional peaks at 3742 and 1772 cm−1 belonged to free surface of hydroxyl group (–SiOH); these also indicated about existence of –C=O bond from pyrolyzed (–CO3) and filler (FS)-matrix (PC) exquisite interaction

Fig. 13
figure 13

FTIR spectra of NFS, pristine PC and PC/NFS (10 wt%) (Katiyar and Balasubramanian 2015)

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The FTIR analysis Fig. 14a of pristine ph-POSS was found around 1090 cm−1 for Si-O framework accompanied by other signals at 695 and 738 cm−1 due to phenyl groups out-of-plane deformation; 1425 cm−1 appertained to deformation of Si-phenyl group bonds, 1583 cm−1 for C–C bond stretching, and bands about 3000 cm−1 for C–H stretching (Lin-Vien et al. 1991; Eklund et al. 1995). The formation of new peaks in the PC/POSS FTIR spectra (Fig. 14b), i.e., 1090 cm−1 for Si-O framework in the PC/POSS composite while peak at 3000 cm−1 depicted phenyl C–H bond and, therefore, confirmed interaction between PC and POSS (Galvez et al. 2002)

Fig. 14
figure 14

(a) FTIR spectra of pristine Ph-POSS (b) FTIR spectra of PC/POSS composite

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XRD Analysis

Figure 15a displayed XRD spectrum for FAC (raw and silane grafted) which consisted of crystalline mineral phases, namely, (1) tobermorite (Ca5(OH)2Si6O16.5H2O), (2) mullite (3Al2O32-SiO2), (3) quartz (SiO2), (4) calcite (CaCO3), and (5) lime (CaO)0.36 with (2) and (3) being in larger content. Therefore, XRD signal in FAC comprised mainly mullite content which asserted thermal stability of structure. In addition, no perceptible variation in spectra was observed for raw and silane-treated FAC barring peak at 2θ = 11°, which symbolized silane coupling. For pure PC, Fig. 15b displayed three leading peaks at 27.42°, 41.65°, and 61.58° which were accredited to interference along the axis of main chain segment and interference between chains and short-range interaction between phenyl groups on neighboring chains, respectively. Pertaining to amorphous structure of PC (no sharp peak), composite was highly susceptible to concentration of FAC (crystallinity), and, therefore, XRD plot for composite approximated more like filler spectra. Decomposed composite XRD plot could be discerned in Fig. 15c which delineated peaks for turbostratic carbon and evidenced crystallinity in char residue. Reflections from planes (002), (100), and (110) evidenced turbostratic carbon (a structure intermediate to graphite and amorphous) presence in char. However, a peak associated with (100) plane can be regarded as merger of peaks disguising amorphous char into crystallinity (Das and Satapathy 2011; Li et al. 2007).

Fig. 15
figure 15

(a) XRD spectra of FAC. (b) XRD spectra for PC/FAC composite. (c) XRD spectra for char (Katiyar and Balasubramanian 2014)

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Figure 16a depicted XRD spectra of POSS with high-intensity reflections at 8.9°, 10.0°, 13.2°, and 19.6°; however, diffraction peak at 2θ = 8.9° corresponded to strong diffraction of the pristine POSS powders and also harmonize with other POSS of similar structure (Zheng et al. 2004; Yoon et al. 2005). Figure 16b, c represented pure PC and PC/POSS composites in which three additional peaks at 10°, 19.1°, and 27.6° were embedded on previous POSS profile signifying crystallinity of sample. Peak at 10° appeared to be slightly broader which supported the presence of separate POSS domains in PC/POSS composite.

Fig. 16
figure 16

XRD spectra of (a) Ph-POSS powder. (b) Pristine PC film (c). XRD spectra of PC/POSS composite film

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Topographical Analysis and Contact Angle Measurement

AFM and contact angle study utilize surface examination for describing the roughness of coating therefore profiles related to PC coated, and its composite containing 10 wt% FS was displayed in Fig. 17. Pristine PC shown in Fig. 17a (AFM) manifested low surface roughness having an average value of 4.5 nm. Similarly, Fig. 17b displayed surface behavior, i.e., asymmetric spikes accounted to FS in solvent-casted PC/FS composite; however, the figure also demonstrated agglomeration and roughness of ~6.3 nm. In contrast to above, spray-coated composite, owing to gun’s powerful jet impact on glass slide, showcased uniformity on surface and roughness of 12 nm, while melt-blended composite presented spikes on its surface with roughness magnitude of 15 nm. Above results also explained that presence of spikes on surface introduced impact tolerance property in composite but stimulates turbulent transitions of heat flow causing a shoot in the surface recession (Reda 1981; Batt and Legner 1983; Kashiwagi et al. 2005) which validate PC/NFS composite is most feasible for thermal appliances that produce extreme dissipation of heat. Regarding contact angle on surface, pure PC exhibited 76° solvent-casted composite 95° and spray-coated 141°, thus validating spraying as an efficient coating method (also enhancement in hydrophobic character that other techniques).

Fig. 17
figure 17

AFM and contact angle measurement for (a) pristine PC, (b) solvent-casted PC/NFS film, (c) spray-coated PC/NFS membrane, and (d) melt-blended PC/NFS composite (Katiyar and Balasubramanian 2015)

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When considering POSS as reinforcement, film prepared with solvent casting displayed symmetric spikes but irregular distribution and, therefore, surface roughness of 6.6 nm (Fig. 18a). As similar to above AFM study with FS, POSS also made the uniformity more prominent when composite was coated with spraying rendering surface roughness of 8.2 nm (Fig. 18b) and melt-blended film to have higher impact tolerance with 12 nm roughness (Fig. 18c). When these pristine and composite coatings were studied for hydrophobic and hydrophilic characteristics, solvent casted achieved 85°, spray-coated 96°, and melt-blended 108° contact angle with surface which is in contrast with aforementioned FS composite (spraying had highest hydrophobicity than others). In both types of fillers, hydrophobicity enhancement was appertained to entrapping of air particles (surface engineering) which manifested heterogeneity to surface, thus decreasing adhesive forces with liquid.

Fig. 18
figure 18

AFM and contact angle measurement for (a) solvent-casted films of PC/POSS, (b) spray-coated films of PC/POSS, and (c) melt-blended PC/POSS composite

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SAXS Analysis

To understand the crystalline behavior of PC/NFS composites, SAXS was employed as depicted in Fig. 19 for pure PC and PC/FS composite prepared by three different methods. PC is an amorphous polymer as also depicted by SAXS pattern having no peaks, while at small scattering vector, there was a dominant peak for all the composites of PC/NFS which might belong to FS. This peak represented distribution of NFS in PC matrix, and the most intense peak was displayed by a melt-mixed composite of PC/NFS. And also SAXS analysis predicted the shape of FS particles from the curvature carried by the graph (spherical shape as also reaffirmed by FESEM) and decrement in the intensity of peak attributed to presence of some stretch in bonds present in PC system.

Fig. 19
figure 19

SAXS spectra of (a) pure PC, (b) PC/NFS composite from solvent casting, (c) PC/NFS composite from spray coating, and (d) PC/NFS composite from melt blending

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Similar results as abovesaid for FS were also applicable for POSS-reinforced composite, i.e., the most intense peak was obtained from melt blending, etc. shown in Fig. 20.

Fig. 20
figure 20

SAXS spectra of PC/POSS composite

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Thermogravimetric Analysis

Thermal stability of PC/FAC composite was validated by thermal decomposition analysis of samples prepared by different methods. Flame retardancy as observed for FAC incorporated composite deduced escalation in PC thermal degradation rate which resulted in inhibition of flammable gas and heat transference due to insulating carbon layer formed on surface (Baglari et al. 2011). Thermal graph shown in Fig. 21a, b can be partitioned into three sections with first, being reaction zone (from 25 °C to 360 °C) which expelled out all volatile, unreacted, and small organic groups like CH4, ethylene acetone, etc. Second zone was from 350 °C to 480 °C in which pyrolysis onset (350 °C – decomposition commencement – defined at 5 wt% weight loss of sample), and weight loss can be predicted. Lastly, third stage was ranged 480–700 °C which assisted in determination of direct relationship between FAC concentration and thermal stability of composite. TGA graph for melt-blended composite also evidenced residual char (varying from 22% to 70%) (Table 1) as a function of FAC content (0–50 wt%) and, thus, aided in the augmentation of thermal insulation behavior. However, results did not reveal much distinction in the char quantity when composite incorporated untreated and treated FAC.

Fig. 21
figure 21

(a) TGA thermograph of PC with raw FAC. (b) TGA thermograph of PC with silane-grafted FAC (Katiyar and Balasubramanian 2014)

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Table 1 Char residue of PC/cenosphere composite (Katiyar and Balasubramanian 2014)
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TGA graph (Fig. 22) for solvent casting presented dependency of residual char production on concentration of FAC utilized for forming composite which had also been explained in Table 2 along with its respective critical temperature (Tc). Results predicted enhancement in thermal stability with resultant char, i.e., from 22% char for pure PC; it had increased to 36% for 30 wt% when examined at 800 °C which was ascribed to decomposition of main carbonate (–O–(C=O)–O) group at 360 °C (which was delayed after FAC reinforcement). Furthermore, accretion in Tc from 0 to 30 wt% grafted FAC also affirmed increased temperature sustainability of composite without undergoing any internal combustion.

Fig. 22
figure 22

TGA analysis showing pure PC and composites with filler in varying concentrations

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Table 2 Tabulation of results obtained from TGA of PC and its composites
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Similar study was carried for FS- and POSS-loaded PC composite which also displayed increment in thermal stability as presence of inorganic filler impeded thermal degradation and resulting residual char (concentration ∝ residual char) was effective in prevention of fire spreading at higher loading (Sánchez-Soto et al. 2009; Cai et al. 2011; Yadav et al. 2017).

Thermal Conductivity Models

Composite’s thermal conductivity is a pivotal characteristic for its application as insulating materials. Therefore, Maxwell was the first to theorize a model based on thermal conductivity of two-phase composite consisting of matrix-dispersed spherical fillers. An assumption was laid that spherical particles were thermally noninteractive and following which two mathematical models, i.e., parallel (Eq. 5) and series (Eq. 6) conduction models, were considered for validating the results with that to experimental (Dawson and Briggs 1981).

$$ C=\left(1-a\right)M+ aF $$
(5)
$$ \frac{1}{C}=\frac{1-a}{M}+\frac{a}{F} $$
(6)

Correspondingly, effective (geometric mean) and Maxwell-modified (relationship resembles with electrical resistance) thermal conduction models were explained by Eqs. 7 and 8, respectively (Dawson and Briggs 1981; Tong et al. 2016).

$$ C={F}^a.{M}^{\left(1-a\right)} $$
(7)
$$ C=M\left[\frac{F+2M-2a\left(M-F\right)}{F+2M+a\left(M-F\right)}\right] $$
(8)

where F, C, and M represent value for thermal conductivity of filler, composite, and matrix, respectively, while a = filler volume fraction.

Evaluating these equations for FAC, i.e., F and M = 0.11 and 0.19 W/m°C, respectively, Table 3 was designed for every composition used in PC composite.

Table 3 Thermal conductivity values theoretically calculated for each model (Katiyar and Balasubramanian 2014)
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This modeled results manifested strong influence of spherical FAC on thermal conductivity of PC matrix and displayed an inverse relation (i.e., from 5 to 50 wt% FAC ➔ average conductivity varied as 0.18–0.l3 W/m°C). The same had also been validated with above-explained experimental TGA results by witnessing an increment in residual char with increasing FAC loading, thus stabilizing the composite (thermal insulation) and decreasing its conductivity (Puri and Khanna 2016).

Integral Procedural Decomposition Temperature (IPDT)

PDT is an empirically determined temperature for ascertaining about thermal stability of composite and relied on sensing accuracy along with surrounding conditions in which analysis is carried out. However, its subclassification IPDT is defined as shape cumulative of area under the TGA curve (Doyle 1961). Pure PC polymer has an IPDT in range of 710 °C and for FS-filled composites; values are predicted in Table 4 (values calculated by taking in account the areas associated with TGA curve, initial and final temperature of experimentation).

Table 4 Char residue and IPDT values for the composite
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Thus, Table 4 also validated that increase in FS content resulted in production of more inorganic content and free carbon which contributed to enhancement in IPDT, therefore, by definition increased thermal stability. Similar results, i.e., higher IPDT, were also realized for PC/POSS composite which implied higher residual ceramic and carbon char and, therefore, elevated thermal stability which can be utilized for insulators for 3D print heads. Thermal conductivity was calculated by model = 0.13 W/mK, i.e., 32% depression, while experimental it was 0.15 W/mK.

Numerical Analysis Method

The finite element method (FEM) is a persuasive tool used in numerical methods to calculate approximate solutions to mathematical problems so that it can simulate the responses of physical systems to various forms of excitation. For analyzing thermal behavior of abovementioned composition, a numerical tool known as finite element method (FEM) enabled simulation of physical system response into excitation forms (Agrawal and Satapathy 2015). However, authors had employed ABAQUS 6.12 to analyze the effective insulation of composite for coating application in 3D print heads.

Temperature Loads and Boundary Conditions

This numerical model considered the cylinder with an inner surface to have surface heat flux of 1 kJ/m2 and temperature = 400 K for heat conduction. Mesh element size was taken as 0.1 and 1 mm along thickness and length, respectively; thus, total 5544 elements with 9306 nodes of DC3D8 element were created for heat transfer analysis. Directions of heat flow and boundary condition of composite prototype for analyzing transient flow 3D heat conduction problem were illustrated in Fig. 23.

Fig. 23
figure 23

Prototype used for numerical modeling of heat conduction in composite component

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The inner surface of the cylinder is subjected to a surface heat flux of 1 kJ/m2 and 400 K temperature to analyze the heat flow through heat conduction. The model is meshed with element size 0.1 mm along the thickness of the cylinder and 1 mm along the length creating a total number of 5544 elements with 9306 nodes of DC3D8 element (an 8-node linear heat transfer brick) for heat transfer analyses.

In this analysis, assuming the material to be homogeneous with thermal conductivity value as material property, temperature profiles along thickness of cylinder were obtained from numerical results for two materials, i.e., pristine PC and PC/POSS composite. The temperature variation along the thickness as obtained in the numerical analysis is depicted in Fig. 24.

Fig. 24
figure 24

Temperature profile along the thickness of PC/POSS composite

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Effective insulation along thickness of cylinder was analyzed by plotting the variation of temperature for three materials as embellished in Fig. 25. PC/POSS attained a temperature of 28.8 °C, whereas pristine PC attained 48.3 °C, respectively; thus, it can be inferred that PC/POSS provides better insulating properties than pristine PC.

Fig. 25
figure 25

Temperature variation along the thickness

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Conclusion

This chapter had delved into understanding of varying reinforcement (FAC, FS, and POSS) effects on PC composite fabricated with different methods (solvent casting, melt mixing, and spraying). Thereafter, prepared films were tested with various characterizing tools to delineate structural, shape, size, distribution, dispersion, and other features in composites. The present chapter commenced with FAC and its grafting to render reinforced composite whose structure and interactions were validated with FESEM, HRTEM, FTIR, XRD, and SAXS. However, thermal conductivity and analysis reflected residual char increment to 70% from merely 22% which also indicated enhancement in resistivity or hindrance to heat conduction, thereby increasing isolative property of composite (FAC/PC). While utilizing FS as filler, hydrophobic character was bestowed to the film or coating along with elevated thermal insulation by its composite with PC. However, in POSS reinforcement, results displayed 32% boost in hydrophobic melt-mixed PC/POSS composite. TGA analysis depicted 62% residual char which is comparable (70%) to FAC filler, and results were also validated with numerical modeling tool to establish credence over the obtained results. Thus, all reinforcements had displayed enhanced thermal behavior and, therefore, can be visualized for its utilization in high-temperature applications in automotives, tiles in aircraft for heat insulation, casing for car battery, aircraft canopy, transparent armor for military, locomotive windshield laminates, etc. while also utilizing their hydrophobic characteristics.