Multifunctional nano-engineered and bio-mimicking smart superhydrophobic reticulated ABS/fumed silica composite thin films with heat-sinking applications

Abstract

There are increasing requirements for engineered surfaces with distinct properties such as superhydrophobicity, self-cleaning, high thermal stability, and anti-corrosion. A number of materials for various applications have been created by adding fumed silica as a filler in polymeric materials. In the present work, we demonstrate that an engineering thermoplastic, acrylonitrile butadiene styrene (ABS), which has a high thermal stability and superior mechanical properties, can be ameliorated with nano fumed silica (FS) for applications requiring superhydrophobicity and heat resistance. An efficacious and cost effective combination of solvent casting and spray coating methods was used to prepare superhydrophobic thin films (0.1 mm thickness) of the ABS/FS composite. These composite thin films exhibited a superhydrophobic rough surface with a favourable water contact angle of 174° and an adequate resistance to UV irradiation. The reticulated superhydrophobic surface morphology of the ABS/FS composite thin films was confirmed by FE-SEM analysis. The water repellency of the superhydrophobic surfaces was demonstrated by the Wenzel, Cassie–Baxter, and McCarthy models. The elevated glass transition temperature (T_g) of 112 °C and the thermal stability of the thin films up to 200 °C, confirmed by DSC and TGA thermograms, respectively, make the ABS/FS composite a strong contender for heat-sinking applications in diverse electronic devices which have complex integrated circuits.

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1. Introduction

Cutting-edge innovations and continuous improvements in superhydrophobic surfaces are attracting great interest as they cater to the growing range of applications requiring specific characteristics such as slippery surfaces,¹ self-cleaning,² anti-icing,³ anti-corrosion,⁴ anti-microbial activity⁵ and thermal stability.⁶ Coatings possessing the above characteristics have applications in organic photovoltaics,⁷ phase change materials,⁸ thermal barrier coatings,⁶ and in the automotive⁹ and construction industries.¹⁰ Superhydrophobic surfaces notably have water contact angles greater than 150°, which is achieved by the effects of their hydrophobic surface chemistry and hierarchical roughness.¹¹ The synergy between the surface roughness and capillary phenomenon during the wetting of a solid surface leads to conglomerate effects. The best-understood effect is the Lotus effect, which involves superhydrophobicity and self-cleaning.¹² Zhang et al. reported that the water contact angle for superhydrophobic surfaces, such as a lotus leaf, was greater than 150°. In order to create a superhydrophobic surface, the surface energy should be as low as possible, and the surface should have some degree of roughness.¹³

By mimicking the Lotus effect, various cost effective methodologies have been reported for the fabrication of superhydrophobic surfaces, such as the spin coating method, which uses a spin coater,¹⁴ electrospinning, which exploits the large active surface area of nanofibers,¹⁵ the sol–gel method, which uses a colloidal suspension of dispersed solid particles,⁴ plasma etching, which generally uses oxygen plasma,¹⁶ the layer by layer method, which uses different layers of materials,¹⁷ electroless plating,¹⁸ and chemical vapour deposition, which uses a mixture of reactive components.¹⁹ In one such application, Liu et al. have described a one-step electrodeposition process for producing a superhydrophobic surface based on a magnesium alloy, which had corrosion-resistant properties. This superhydrophobic coating provided a contact angle of 159.8°, with a sliding angle of less than 2° and electrodeposition time of less than 1 minute.²⁰ Xue et al. have created colourful superhydrophobic surfaces with long lifetimes by chemically etching PET fiber surfaces following the diffusion of fluoroalkylsilanes into fibers.²¹ Boscher et al. mentioned that PDMS is nontoxic and is used as a candidate of choice for the preparation of superhydrophobic surfaces.²² The improvement of the thermal, mechanical and electrical properties of carbon-based materials (CNTs, CNFs, fullerene, graphene, carbon soot) has recently received particular attention.²³ Jiang et al. fabricated a PTFE coating mesh which was superoleophilic and superhydrophobic and could be utilized for oil/water disseverment.²⁴ In a review article, Ragesh et al. have discussed the importance of self-cleaning coatings and materials, their various methods of fabrication and their use in extreme applications.²⁵ Balasubramanian et al. have demonstrated a PVDF/graphite composite having a porous superhydrophobic nature coupled with an induced nano-cellular patterned surface, giving a water contact angle of 153° with a roll-off angle of 4°. They have also synthesized a superhydrophobic composite coating of PVDF/carbon soot particles with a water contact angle of 172°.^26,27 Similarly, Zha et al. discussed the effect of graphene in graphene/PVDF composites, in which the graphene produces a rough surface containing micropores; a water contact angle of 153° was obtained by increasing the graphene content.²⁸

Sohn et al. have explored anti-frosting as an application of superhydrophobic coatings for safety and energy conservation. They mentioned that in cold-climate regions these safety measures have great use because of the immense temperature fluctuations occurring on a daily basis.²⁹ Saito et al. were the first to test a superhydrophobic material for anti-ice applications and several other groups followed with further innovations in anti-icing properties.³⁰

Balasubramanian et al. have demonstrated the use of a polycarbonate/fumed silica composite for hydrophobic and heat-sink applications with a 10% loading of fumed silica. They reported an 85% increment in the water contact angle and their TGA results showed that the char residue increased from 22% to 49%, confirming the prospect of using PC/FS composites for heat-sink applications. They used spray coating and solvent casting methods to efficiently prepare uniform films of the PC/FS composite. They concluded that the addition of fumed silica enhanced the surface roughness, as determined by the Cassie–Baxter equation.³¹ Wang et al. have reported the use of polydivinylbenzene (PDVB) colloidal films for heat-resistant and superhydrophobic applications. The PDVB colloidal films possessed a contact angle of more than 151°, showing their superhydrophobic characteristics. The contact angle and rough surface morphology of the PDVB films remained almost unaffected by treatment up to a temperature of 150 °C for 24 hours. These PDVB films did not show any cracks up to a thickness of 8.1 μm. According to their results, the high heat resistance of the PDVB films was due to the highly cross-linked network structure.³²

Qu et al. have reported the flame retarding application of fumed silica in a blend of ethylene-vinyl acetate/magnesium hydroxide (EVA/MH). Their morphological and TGA data confirmed the synergistic flame retardant mechanism of fumed silica in the EVA/MH blend. They argued that the physical presence of the fumed silica, which acted as the char/silica interface in the condensed phase, prevented heat and mass transfer in the event of fire.³³ Letterman et al. have described the use of fumed silica and other silicate derivatives for the potential application of thermal dissipation using organic polymer coatings for a packaged semiconductor component.³⁴

In pursuit of a superhydrophobic surface and heat resistant properties for potential applications, the present work utilizes the concept of bio-mimicry of the nano-pillars found naturally on lotus leaves. ABS, which is a terpolymer of acrylonitrile, butadiene and styrene, combines the advantages of all three monomers in its composition. Each monomer imparts its own beneficial properties to the polymer matrix, such that acrylonitrile provides chemical resistance, tensile strength, aging resistance and heat stability; butadiene provides impact strength, toughness, and low-temperature stability; and styrene provides gloss, processability and rigidity.^35,36 The filler chosen in our work is fumed silica, which consists of microscopic droplets of amorphous silica fused into branches, and is created by heat treatment. Because of its fineness, thinness, non-carcinogenicity and low density, fumed silica is advantageous for use as a thickener and a reinforcing filler.³⁷ ABS/FS composite thin films have been fabricated in previous studies by the cost effective and facile methods of solvent casting and spray coating. Spray coating has been used as the method in this study, since it offers the advantages of easy handling, uniform film thickness and optimum hydrophobicity. In an earlier application of the surface metallization of polymers, the spray coating technique was combined with the layer by layer (LbL) deposition method to produce adhesion-enhanced metallic coatings. In the LbL method, prior to the metallic coating, Ag is plated onto the modified polymer substrates.¹⁷ In another application, Mitra et al. have reported the fabrication of an antibacterial coating via the spray coating method using polyvinyl fluoride (PVF) and stainless steel as the substrates, along with quaternized chitosan (QCS) and a multivalent agent such as sodium tri-polyphosphate (TPP). In this method, they spray coated TPP solution onto a plasma etched PVF film followed by curing at 70 °C for 3 hours. Then, the solution of QCS was spray coated onto the TPP layer followed by curing at 50 °C for 5 hours.³⁸ In another study, Li et al. reported the fabrication of tunable adhesive superhydrophobic surfaces based on ZnO nanoparticles. In their method, a dispersed solution of ZnO nanoparticles in ethanol could be spray coated onto any desired substrate.³⁹ Elsewhere, Nigam et al. have reported the fabrication of a metallic coating on an ABS plastic substrate via electric arch thermal spray coating. In this method, the ABS plastic substrate is treated by sand blasting for surface activation, then the primary coating of zinc is applied, followed by the final coating of a copper layer.⁴⁰ In our method, a dispersed polymer solution can be directly sprayed onto an etched metal substrate, with manual control for maintaining uniform film thickness, in one single step, which represents a simple, cost effective and time saving methodology for the fabrication of heat sinking superhydrophobic coatings.

2. Experimental section

2.1 Materials

Acrylonitrile butadiene styrene (ABS) (Styrolution ABS India Ltd., ABSOLAC 920 grade, melt flow rate = 27 g/10 min at 220 °C/10 kg, and density = 1.04 g cm⁻³) was used for the preparation of the thin films. Fumed silica powder (particle size = 0.007 μm, bulk density = 0.0368 g cm⁻³ at 25 °C), dichloromethane (DCM) (purity = 99.7%, B.P. = 39.8–40 °C/760 mmHg), and stearic acid (purity = 99%, mol. wt. = 284.49) were purchased from Sigma-Aldrich, USA. All reagents and solvents involved were of analytical grade, and were used without any further purification.

2.2 Synthesis of homogeneous solution and thin film fabrication

A homogeneous solution of ABS was prepared by dissolving it in dichloromethane solvent. Homogeneous dispersions of fumed silica in dichloromethane were prepared with different loadings, ranging from 10% to 30%, along with 1% stearic acid in the same solvent. The ABS solution and fumed silica dispersion were mixed together at a ratio of 1 : 1 and the solution was transformed into a homogeneous dispersion using a Vibracell Ultrasonicator (Model VCX 750, Sonics and Materials Inc., USA) with a frequency of 20 kHz and a titanium alloy probe tip (13 mm in diameter) for 2 hours. The evaporation of DCM was controlled using an ice bath which maintained the temperature below room temperature. A pictorial representation of the preparation of the ABS/fumed silica composite solution is shown in Fig. 1.

Fig. 1 Synthesis of ABS/fumed silica composite solution.

The fabrication of ABS/fumed silica composite thin films involves using the terpolymer, ABS, as a polymeric continuous phase and the reinforcing filler, fumed silica, as the discontinuous phase. Several fabrication techniques have been practiced previously, viz. in situ polymerization, melt blending, solution blending, sol–gel method etc. Here, the fabrication of thin films of ABS/fumed silica was accomplished using the cost effective and facile methods of solvent casting and spray casting. In the solvent casting process, the composite thin film is made by the evaporation of solvent from a solution which is cast on a substrate; hence, no stress or energy is required, only the evaporation of the solvent.⁴¹ DCM is a common solvent used for casting ABS. Stearic acid was used here as a dispersing agent. Owing to the rapid evaporation and high volatility of DCM solvent, its use results in the formation of cracks resembling those in drought-stricken land. To overcome this drawback, the fabrication was carried out in a water bath so that a controlled evaporation took place. After the film was formed, it was stripped off from the substrate and its thickness was quantified using a digital thickness gauge as 0.1 mm. In spray coating, the pre-prepared solution is sprayed onto a substrate (metal bar) using a spray gun with a high pressure airstream of 0.6 MPa. A homogeneous dispersion of the solution is ejected through the spray gun nozzle in the form of micro-droplets. The metal bar used here was made of dural, which had been etched using a solution of 2 mL HF and 6 mL HNO₃ in 92 mL distilled water.⁴³ The dural bar was sprayed from the nozzle of a spray gun kept at a distance of approximately 10 cm and at an angle of 45°. The viscosity retained by the solution was between 220 cP and 650 cP, measured by using a Brookfield Viscometer (Brookfield DV-E, Brookfield Engineering Laboratories, Inc., USA) at 28.7 °C and 200 rpm. Despite the high power consumption involved in the spray coating process as compared to solvent casting, the former gives a more uniform film thickness, because the latter gives non-uniformities at high filler loadings. Considering its ability to form a uniform film thickness, shorter drying cycle time, easy handling and fast controllability, spray coating was chosen as the most reliable fabrication technique for preparing the ABS/fumed silica composites. The mechanisms of the fabrication techniques are represented in Fig. 2.^42–45

Fig. 2 Mechanisms of fabrication techniques.

Homogeneous suspensions can be generated by the combination of a polymeric matrix, inorganic filler and solvent along with a binder to create uniform film morphologies using the spray coating technique.⁴⁴ The double step mechanism of spray coating includes the spraying and evaporation of the solvent, which brings together solution droplets containing large quantities of micron-scale fillers. The distance between the dural substrate and the nozzle tip of the spray gun was maintained at 10 cm, causing the solution droplets to fall onto the substrate. The effect of the substrate was to help the micron-sized fillers to aggregate and undergo rearrangement during the evaporation of the solvent. The addition of fumed silica lowered the surface energy, and allowed the generation of an air-solution interface, resulting in a rough superhydrophobic surface, the formation of which depends on factors such as the solvent properties, solution viscosities, air pressure, gun tip geometry and distance between nozzle and substrate.^31,44 The filler particles irregularly adhered together onto the substrate, resulting in a rough, superhydrophobic surface. In the spray coating technique, the highly volatile solvent evaporates and in turn forms non-uniformities on the substrate coating layer. The maintenance of viscosity is an important factor and the maximum viscosity of the solution here was 650 cP.⁴⁴ The optimum filler content was found to be 30 wt%: on further increasing the filler loading, the brittleness of the film increased, forming cracks, without any increase in the water contact angle (Fig. 3).

Fig. 3 Pictorial representation of the mechanism of spray coating.

3. Characterization techniques

The surface morphology as well as the homogeneous dispersion of the fumed silica in ABS were confirmed by FE-SEM analysis (Carl Zeiss AG, JSM-6700F, Germany) at 2 kV and BET analysis (Microtrac SSA, India). Vacuum drying at 90 °C for 2 hours was carried out to quantify the porosity of the sample. The water contact angle was quantified using DI water on a contact angle goniometer (DSA100, Krüss GmbH, Germany) at room temperature. High resolution transmission electron microscopy of the fumed silica was performed with an FEI TECNAI TF20 using lanthanum hexaboride as the filament at 300 kV. FT-IR spectral analysis of the ABS/FS composite was conducted between 4000 cm⁻¹ and 400 cm⁻¹ at room temperature, in KBr pellets, utilizing a PerkinElmer Spectrum BX FTIR system (PerkinElmer Inc., USA). Glass transition temperatures were analyzed by differential scanning calorimetry (DSC) (Q20, TA Instruments Inc., USA). The thermal stability of the samples was analyzed by thermogravimetric analysis (TGA) (STA 6000, Perkin-Elmer Inc., USA).

4. Results and discussion

4.1 Morphological analysis

The effect of filler incorporation on hydrophobicity was studied in the present work by comparing samples in which the loading of fumed silica in the ABS matrix was 10%, 20%, 30% and 40% by weight. The surface morphology as well as the uniform distribution of the fumed silica in the ABS/FS composite thin films were analyzed by FE-SEM at different loadings (10%, 20% and 30%) of fumed silica in the spray coated film (Fig. 4a and b). Although the fumed silica particles were homogeneously dispersed, the FE-SEM images showed their inherent chain-like structural morphology.²⁰ In the FE-SEM images of Fig. 4, patterned hierarchical surfaces containing nano-pillars are visible, which increase in prominence on the incremental loading of fumed silica up to 30%. These marked changes indicated the transformation of the surface morphology with increased loading, making it increasingly superhydrophobic.

Fig. 4 FE-SEM images of (a) 10%, (b) 20%, and (c) 30% filler loadings along with their respective water contact angles.

Balasubramanian et al. have demonstrated that an increase in surface roughness as well as a decrement in surface free energy can be attributed to the effect of fumed silica on superhydrophobic coatings.³¹ In that study, the HR-TEM images showed that the spherically shaped particles of fumed silica possessed innate chain-like structures like those shown in Fig. 5.³¹ It could clearly be observed that the fumed silica created a pattern of pin-like structures on the surface, which held the water some distance above the surface in the form of spherical droplets.

Fig. 5 HR-TEM images of fumed silica at different magnifications.

4.2 Water contact angle measurements

The water contact angles on the spray coated thin film surfaces were measured to evaluate their hydrophilic or hydrophobic nature. The water droplets exhibited a lotus effect, becoming increasingly spherical on the incremental loading of fumed silica. The variation of the water contact angle with different weight percent loadings of fumed silica via spray coating is graphically represented in Fig. 6. The water contact angle increased from a value of 90.7° up to 174° on the incremental loading of fumed silica up to 30 wt%. On further increment of the filler loading, no enhancement of the water contact angle could be detected and the uniformity of the film was completely lost, with cracks appearing on the film. Percolation was achieved at 30% loading of fumed silica (Fig. 6). Stearic acid was used for better processability.⁴⁶ Due to the presence of an –OH group in the 18-carbon chained stearic acid, it acts as a compatibilizer which, in combination with the fumed silica filler and the ABS polymer matrix, endows flexibility to the composite thin films.⁴⁶ Synthesis of the ABS/FS composite thin films was accomplished using DCM solvent, which is highly volatile, and this volatility helped to form a surface composed of connected protrusions. If a solvent with a lower evaporation rate is used, the protrusions formed are flatter and less clearly separated.^42,44 A comparison of the water contact angles for 30% loaded ABS/FS composite thin films prepared by spray coating and solvent casting is shown in Fig. 7. A thickness of 0.1 mm was maintained for both films.

Fig. 6 Variation of water contact angle with respect to filler loading for spray coated films.

Fig. 7 Variation of water contact angle with respect to filler loading for the two fabrication techniques.

4.3 Effect of sunlight ultraviolet irradiation

The spray coated ABS/FS composite thin films provided higher water repellency compared to the solvent casted composite films. Herein we have investigated the effect of ultraviolet rays on the ABS/FS composite thin films. Seven samples of ABS/FS composite thin films were made by spray coating on dural substrates. All these samples were exposed to UV light for 7 days, with the average temperature maintained at 38 °C and an average humidity of around 60%, and their contact angles were periodically measured throughout this time, as shown in Fig. 8. Then, the samples were kept in a desiccator overnight, and on the following morning the same samples were taken outdoors again for exposure to sunlight UV irradiation. A slight decrease in the water contact angle was observed on exposure to sunlight after the first day (173°). From day 1 to day 7, there was no further variation in the water contact angle, which shows the strong resistance to UV sunlight offered by the ABS/FS composite thin films. The slight decrement in the water contact angle after day 1 can be attributed to small entrapped traces of DCM solvent and the butadiene phase present in the ABS terpolymer, which are sensitive towards UV irradiation. UV radiation softens the butadiene phase, allowing a rearrangement within the thin film following exposure.^47–49 A pictorial representation of the changes occurring in the butadiene phase of the ABS polymer after exposure to sunlight is shown in Fig. 9. These spray coated thin film samples were soaked in water for half a day and no changes were observed in the contact angle value. This implies that the films can efficiently maintain the water repellency imparted by their superhydrophobic surfaces.

Fig. 8 Variation in water contact angle with exposure to sunlight.

Fig. 9 Effect of UV radiation on ABS polymer.

The effect of sunlight on the ABS/FS composite thin films was also investigated by FT-IR analysis, as shown in Fig. 10.⁵⁰ This analysis was performed by recording the FT-IR absorption spectra from day 0 to day 7. Among the peaks that were present in the FT-IR absorption spectra of the ABS/FS composite thin films, variations in absorbance intensity on different days were observed for the peaks appearing at 3297 cm⁻¹, 998 cm⁻¹ and 755 cm⁻¹, which correspond to O–H stretching, =C–H bending and C–Cl stretching. Note that the peak appearing at 998 cm⁻¹ represents the butadiene phase of the ABS polymer. The above results demonstrate that the synthesized ABS/FS composite thin films efficiently maintained their superhydrophobicity and possessed good resistance towards UV sunlight radiation.^47–50

Fig. 10 FT-IR spectra of ABS/FS composite thin films exposed to sunlight for 7 days.

4.4 Thermal analysis

The TGA thermogram in Fig. 11 shows the thermal stability of the ABS/FS composite thin film samples with 0%, 10%, 20%, and 30% loadings along with the Pure ABS sample. In the temperature range of 140 °C to 220 °C, fractional weight loss was observed. This weight loss is attributed to the entrapped volatile traces of DCM solvent and entrapped air molecules. For Pure ABS, the curve is stable up to almost 300 °C, with only a slight weight loss. Since ABS is a non-crystalline polymer, it does not have a true melting point, but it is processed in the temperature range of 250 °C to 260 °C.^51,52 Around this temperature, the internal and external Brownian motions, also known as segmental mobility and molecular mobility, respectively, start to occur due to the presence of sufficient kinetic energy in the form of heat, thereby loosening the polymer chains.^53–55 After crossing the glass transition temperature, the entangled polymer chains start to relax, and the deeply penetrated and entrapped solvent molecules begin to evaporate at this temperature.^54–57

Fig. 11 TGA thermograms of ABS/FS composite samples at different loadings.

Then, further weight loss in the ABS/FS composite occurs, with an onset temperature of 365 °C and continuing up to 485 °C, indicating substantial degradation of the ABS polymer. Finally, from 500 °C to 850 °C, the curve is almost flat. The residual weight at 850 °C consists of un-combusted inorganic material. Inorganic materials do not burn, i.e. combust, on heating. Since, in the ABS/FS composite, the only inorganic material is silica, we can conclude that the residual weight consists of unburned silica. This implies that due to the presence of silica, the ABS/FS composite has good prospects in heat resistant and flame retardant applications.

The DSC thermogram in Fig. 12 shows the glass transition temperature (T_g) values for the ABS/FS composite thin film samples with 0%, 10%, 20%, and 30% weight loadings of fumed silica. The Pure ABS and Pristine ABS samples, both of which have 0% loading of fumed silica, show T_g values of 107 °C and 108 °C, respectively. (The ABS pristine sample was treated with DCM solvent, whereas ABS pure was not.) It is evident that the glass transition temperatures of the pure ABS and pristine ABS samples are almost identical, meaning that T_g was unaffected by the solvent treatment. However, in most cases polar solvents have been found to affect the glass transition temperatures of polymers.^56,57 With a loading of 10% and 20% fumed silica, the glass transition temperatures were observed at 92 °C and 86 °C, respectively. This indicates that the 10% and 20% loading of fumed silica decreased the onset temperature of segmental mobility, i.e. internal Brownian motion.^53–55 Generally, in any amorphous or semi-crystalline polymer, the segments of an entangled polymer chain begin to vibrate with increasing kinetic energy, which directly corresponds to the onset of the glass transition.^54,55 At 30% loading of fumed silica, T_g increased to 112 °C. This raising of the glass transition temperature can be attributed to the suppressed onset of segmental mobility. It can be concluded that, at 30% loading, the fumed silica particles penetrated deeply into the ABS polymer chains, forming a network-like structure, which decreased the mobility of the segments of polymer chains, thereby increasing the glass transition temperature.^56–59

Fig. 12 DSC analysis of ABS/FS composite samples at different loadings.

4.5 Theories on superhydrophobicity

The wetting of a horizontal flat surface can be described by Young's equation. This equation describes the wetting behavior of a liquid with respect to a solid phase (S), liquid phase (L), and vapour phase (V), and the interfacial energies associated with them. Young's equation is written as follows:⁶⁰where θ_Y is the Young's contact angle, and γ_sv, γ_lv and γ_sl are the interfacial energies of solid–vapour, liquid–vapour, and solid–liquid phases, respectively.

The hydrophobicity of a homogeneous surface with a given roughness can be predicted by Wenzel's theory, which posits the surface roughness as the main factor contributing to hydrophobicity. This is explained by Wenzel's equation as follows:⁶¹

where θ_W denotes the contact angle on a rough surface, θ represents the Young's contact angle formed on an ideal smooth surface, and r represents the surface roughness factor, which can be described as the ratio of the true surface area to the projected surface area.

Wenzel's theory describes the hydrophobicity of a rough homogeneous surface, but it does not explain the hydrophobicity of heterogeneous rough surfaces. Cassie and Baxter explained the superhydrophobicity of heterogeneous rough surfaces having a porous nature. The Cassie–Baxter equation is given as:⁶²

cos θ_c = f₁ cos θ₁ − f₂ (3)

where θ_c is the apparent contact angle, f₁ and f₂ are the surface fractions of phase 1 and phase 2 respectively, and θ₁ is the contact angle on phase 1.

The increment in hydrophobicity in the Cassie–Baxter regime is due to the porous surface, which traps air in the grooves of the surface. In this regime, an amalgamated interface of liquid–vapour and liquid–solid is formed, instead of only a liquid–surface interface.⁶²

Balasubramanian et al. have established that the increment in the contact angle can be completely determined by the Cassie–Baxter model. They have found that the porosity and roughness are the major factors affecting the increment in the contact angle, and that materials with highly porous structure become superhydrophobic due to entrapped air pockets on their surfaces.⁴⁴ These pores become interconnected and the immense gaps formed between the layers containing the air pockets are the main reason for the enhancement in the superhydrophobicity.^44,45

However, McCarthy and coworkers determined that the contact angle is not dependent on the surface roughness alone, but on the contact line, due to the pinning of water droplets.^63–65 A three-phase contact line particularly favors the pinning of water droplets. Here, they derived an expression for the contact line during the receding and advancing movements of a droplet. The pinning of the water droplet and the contact angle hysteresis greatly depend on factors which are determined by the contact line. The dynamics of the contact line motions during the receding and advancing of the droplet are the main factors influencing the pinning forces and in turn the superhydrophobicity.^68–71 Fig. 13 shows the movement of water molecules on the heterogeneous surface of the ABS/FS composite thin film, depicting the pinning of a water droplet, which implies that the movement occurs only on the liquid molecules that are near to the contact line and not on the water molecules of the inner region where the solid–liquid interface exists.

Fig. 13 Movement of a water droplet pinned on a heterogeneous surface of ABS/FS composite thin film.

When fumed silica is incorporated into the ABS matrix, its wt% loading becomes the critical factor in the formation of composite thin films. Beyond 30 wt% loading, the high agglomeration of fumed silica in the ABS matrix leads to cracks on the surface, which consequently destroys the thin film. The reduction in surface tension is attributed to the abundant polymer phase, while the enhancement in the surface area is ascribed to the increasing filler loading. The synergistic effect of low surface energy and high roughness helps in the formation of hierarchical structures, which in turn helps in improving the superhydrophobicity.^44,45 The noticeable increment in superhydrophobicity in the ABS/FS composite thin films is also attributed to the pinning of water droplets on the hierarchical surface formed on the contact line, and not only to the contact area. Cassie also states that the observed adhesive behavior of superhydrophobic surfaces compared to non-patterned hydrophobic planar surfaces is attributed to the contact line pinning. This effect of contact line pinning entirely depends on the geometry of the superhydrophobic patterns of the surface, specifically, the chemical heterogeneity, physical morphology and interfacial wetting states.^65–69 In terms of chemical heterogeneity, surfaces which contain highly hydrophobic flat spots or defects are conducive to the dynamics created by a three-phase contact line of a superhydrophobic surface. As a droplet moves across a hydrophobic chemical defect on the surface, the defect causes the pinning of the contact line with a localized pinning force (F), which can be estimated by Hooke's law (eqn (4))⁶⁹

where k is the spring stiffness, γ denotes the surface tension of the liquid, θ₀ is the square of the ideal surface quasi-equilibrium contact angle without any defect, L denotes the cut-off length scale of either droplet diameter or capillary length and d denotes the diameter of the circular defect. The deformed or pinned contact line exerts a pinning force on the defect as given by eqn (5)⁶⁹

F = kx_m (5)

where x_m denotes the maximum amplitude of distortion of the contact line. Away from the defect, the pinning force decreases to zero. The value of the pinning force can be precisely calculated via distortions of the contact line.⁶⁹

As the number density of flat spots or defects on the surface increases due to the higher loading of fumed silica in the ABS matrix, the defects become closer to each other and they collectively act to deform the contact line, causing the water droplet to be pinned to the surface.^68,69 The force required for the movement of water droplets on the superhydrophobic surface as the contact line advances or recedes over the defects can be estimated by eqn (6) and (7).^68,69

nW_R = γ(cos θ_R − cos θ₀) (6)

nW_A = γ(cos θ₀ − cos θ_A) (7)

where the pinning force per unit length at a single defect is represented by W_A and W_B at an advancing contact angle of θ_A and a receding contact angle of θ_R, respectively, and the total number of defects encountered by the contact line during its movement is denoted n.^69,70 The contact angle hysteresis is estimated by an equation formed by combining eqn (6) and (7) (eqn (8))⁷¹

n(W_R + W_A) = γ(cos θ_R − cos θ_A) (8)

4.6 Heat resistance in ABS/FS composite

The study of the thermal conductivity of ABS/FS composites is essential for exploring their application as heat resistant materials. The first theoretical model for calculating the thermal conductivity of a two phase system was proposed by Maxwell. In this system, he studied the effect of spherical fillers which are dispersed in a continuous matrix. In this two phase system model, Maxwell assumed that thermal interaction between the filler particles was absent.⁷² The Maxwell model correctly describes the spherical shape of fumed silica in ABS polymer matrices. Experimental results have been established and validated using this model.

Maxwell proposed the following model for calculating the thermal conductivity of a two phase system having a homogeneous matrix, i.e. medium, with randomly dispersed spheres:⁷²

Thermal conductivity can also be estimated by the parallel conduction model:⁷³

K_c = (1 − ∅)K_m + ∅K_f (10)

Hamilton and Crosser have proposed a model which calculates the thermal conductivity of heterogeneous two-component systems. This model contains an empirical shape factor n which depends on the thermal conductivity of the matrix and the filler particle shape. The Hamilton–Crosser model is given as:⁷⁴

In the above three equations, K_c = thermal conductivity of the system, i.e. ABS/FS composite (W m⁻¹ °C), ∅ = volume fraction of filler, i.e. fumed silica (wt%), K_m = thermal conductivity of the matrix, i.e. ABS polymer (0.17 W m⁻¹ °C). K_f = thermal conductivity of the filler, i.e. fumed silica (0.000198 W m⁻¹ °C), n = empirical constant (n = 3 for spheres; n = 6 for cylinders).

Thermal conductivity values calculated for different loadings of fumed silica in the ABS/FS composites using the above models are shown in Table 1.

Table 1 Thermal conductivity values for ABS/FS composite samples

Composition	Maxwell model (W m^−1 °C)	Parallel conduction model (W m^−1 °C)	Hamilton–Crosser model (W m^−1 °C)
ABS/FS 10%	0.1459 ± 0.01	0.1532 ± 0.01	0.1459 ± 0.01
ABS/FS 20%	0.1239 ± 0.01	0.1362 ± 0.01	0.1239 ± 0.01
ABS/FS 30%	0.1038 ± 0.01	0.1192 ± 0.01	0.1038 ± 0.01

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To account for the possibility of error during the experiments, a correction factor of ±0.01 has been applied during the thermal conductivity calculations. There is a possibility of air becoming entrapped in the ABS/FS thin film composites prepared using the spray coating method. Therefore, the thermal conductivity of air, i.e. 0.00024 W m⁻¹ °C, has also been considered during the calculations.

From the above calculations, it can be concluded that the thermal conductivity of ABS/FS composites is greatly influenced by the addition of fumed silica. The above results show that the addition of fumed silica efficaciously reduced the thermal conductivity of the ABS/FS composites by up to 60% when compared with the Pristine ABS polymer. This strongly endorses the ABS/FS composite as a promising candidate in various heat resistance applications.^75–77

5. Conclusion

Superhydrophobic thin films of an acrylonitrile butadiene styrene/fumed silica composite were successfully fabricated by the cost effective and facile methods of solvent casting and spray coating. The addition of fumed silica greatly enhanced the water contact angle of the spray coated films, with the optimum filler content of 30% producing a water contact angle of 174°. The surface morphology and homogeneous dispersion were determined by FE-SEM imaging and the enhancement in water repellency was studied with reference to the Cassie–Baxter, Wenzel, and McCarthy models. These latter results showed that the surface morphology and the contact line dynamics were the critical factors allowing the pinning of water droplets above the surface and in turn the superhydrophobicity of the films. An impressive 60% reduction in the thermal conductivity of the ABS/FS composites was measured, and thermal stabilities up to 300 °C were confirmed by thermogravimetric analysis, while the enhanced glass transition temperature of 112 °C strongly supports the use of the ABS/FS composites in heat resistance applications.

Acknowledgements

The authors would like to acknowledge Dr Surendra Pal, Vice Chancellor, DIAT-DU for continuous encouragement and support and DIAT DRDO Nano Project Program (EPIPR/1003883/M/01/908/2012/D(R&D))/1416 Dated: 28.03.2012, DRDO HQ, New Delhi for financial assistance. Also authors would like to thank Mr Ram Dayal (DIAT), Dr Anoop Anand, and Mr Avinash Ecka (R&DE (Engineers), Dr Chandra Shekhar Pant, and Mr Ashish Jauhari (ACEM), DRDO Labs for continuous technical support.

References

1. M. Nosonovosky, Nature, 2011, 477, 412–413.
2. M. K. Kim, H. Cha, P. Birbarah, S. Chavan, C. Zhong, Y. Xu and N. Miljkovic, Enhanced Jumping-Droplet Departure, Langmuir, 2015, 31, 13452–13466.
3. L. L. Cao, A. K. Jones, V. K. Sikka, J. Z. Wu and D. Gao, Anti-Icing Superhydrophobic Coating, Langmuir, 2009, 25, 12444–12448.
4. A. V. Rao, S. S. Latthe, S. A. Mahadik and C. Kappenstein, Mechanically Stable and Corrosion Resistant Superhydrophobic Sol–Gel Coatings on Copper Substrate, Appl. Surf. Sci., 2011, 257, 5772–5776.
5. R. Dastjerdi and M. Montazer, A review on the application of inorganic nano-structured materials in the modification of textiles: focus on anti-microbial properties, Colloids Surf., B, 2010, 79(1), 5–18.
6. T. M. Pollocka, D. M. Lipkina and K. J. Hemker, Multifunctional coating interlayers for thermal-barrier systems, MRS Bull., 2012, 37(10), 923–931.
7. P. Šiffaloviča, M. Jergela, M. Benkovičová, A. Vojtkoa, V. Nádaždya, J. Ivančoa, M. Bodíkb, M. Demydenkoa and E. Majková, Towards new multifunctional coatings for organic photovoltaics, Sol. Energy Mater. Sol. Cells, 2014, 125, 127–132.
8. W. Liang, G. Zhang, H. Sun, P. Chen, Z. Zhu and A. Li, Graphene–nickel/n-carboxylic acids composites as form-stable phase change materials for thermal energy storage, Sol. Energy Mater. Sol. Cells, 2015, 132, 425–430.
9. M. Groenewol, Highly scratch resistant coatings for automotive applications, Prog. Org. Coat., 2008, 61(2–4), 106–109.
10. J. Lee, S. Mahendra and P. J. J. Alvarez, Nanomaterials in the Construction Industry: A Review of Their Applications and Environmental Health and Safety Considerations, ACS Nano, 2010, 4(7), 3580–3590.
11. C. H. Xue, Z. D. Zhang, J. Zhang and S. T. Jia, Lasting and Self-Healing Superhydrophobic Surfaces by Coating of Polystyrene/SiO₂ Nanoparticles and Polydimethylsiloxane, J. Mater. Chem. A, 2014, 2, 15001.
12. M. Nosonovsky and B. Bhushan, Curr. Opin. Colloid Interface Sci., 2009, 14, 270–280.
13. X. Zhang, F. Shi, J. Niu, Y. Jiang and Z. Wang, Superhydrophobic surfaces: from structural control to functional application, J. Mater. Chem., 2008, 18, 621–633.
14. H. Zhang, X. Zeng, Y. Gao, F. Shi, P. Zhang and J. Chen, Facile Method To Prepare Superhydrophobic Coatings by Calcium Carbonate, Ind. Eng. Chem. Res., 2011, 50(6), 3089–3094.
15. S. Wang, Y. Li, X. Fei, M. Sun, C. Zhang, Y. Li, Q. Yang and X. Hong, Preparation of a durable superhydrophobic membrane by electrospinning poly(vinylidene fluoride) (PVDF) mixed with epoxy-siloxane modified SiO₂ nanoparticles: a possible route to superhydrophobic surfaces with low water contact angle, J. Colloid Interface Sci., 2011, 359, 380–388.
16. C. H. Barshilia and N. Gupta, Superhydrophobic polytetrafluoroethylene surfaces with leaf-like micro-protrusions through Ar + O₂ plasma etching process, Vacuum, 2014, 99, 42–48.
17. D. Chen, Y. Zhang, T. Bessho, J. Sang, H. Hirahara, K. Mori and Z. Kang, Layer by layer electroless deposition: an efficient route for preparing adhesion-enhanced metallic coatings on plastic surfaces, Chem. Eng. J., 2016, 303, 100–108.
18. R. Yadav and K. Balasubramanian, Metallization of electrospun PAN nanofibers via electroless gold plating, RSC Adv., 2015, 5, 24990.
19. H. Liu, L. Feng, J. Zhai, L. Jiang and D. Zhu, Reversible wettability of a chemical vapor deposition prepared ZnO film between superhydrophobicity and superhydrophilicity, Langmuir, 2004, 20, 5659–5661.
20. Q. Liu, D. Chen and Z. Kang, One-step electrodeposition process to fabricate corrosion-resistant superhydrophobic surface on magnesium alloy, ACS Appl. Mater. Interfaces, 2015, 7(3), 1859–1867.
21. C. H. Xue, P. Zang, J. Z. Ma, P. T. Ji, Y. R. Li and S. T. Jia, Long-lived superhydrophobic colorful surfaces, Chem. Commun., 2013, 49, 3588.
22. N. D. Boscher, V. Vaché, P. Carminati, P. Grysan and P. Choquet, A Simple and Scalable Approach towards the Preparation of Superhydrophobic Surfaces – Importance of the Surface Roughness Skewness, J. Mater. Chem. A, 2014, 2, 5744.
23. K. Balasubramanian, B. N. Sahoo and B. Sabarish, Controlled Fabrication of Superhydrophobic Hierarchical Structures Using EPF/Carbon Based Composites, Indian Patent, 289/MUM/2013, 2013.
24. L. Jiang, L. Feng, Z. Zhang, Z. Mai, Y. Ma, B. Liu and D. Zhu, A Super-Hydrophobic and Super-Oleophilic Coating Mesh Film for the Separation of Oil and Water, Angew. Chem., Int. Ed., 2004, 43, 2012.
25. P. Ragesh, V. A. Ganesh, S. V. Nair and A. S. Nair, A Review on Self-cleaning and Multifunctional Materials, J. Mater. Chem. A, 2014, 2, 14773.
26. B. N. Sahoo and K. Balasubramanian, A Nanocellular PVDF-Graphite Water-Repellent Composite Coating, RSC Adv., 2015, 5, 6743.
27. K. Balasubramanian and B. N. Sahoo, Facile synthesis of nano cauliflower and nano broccoli like hierarchical superhydrophobic composite coating using PVDF/carbon soot particles via gelation technique, J. Colloid Interface Sci., 2014, 436, 111–121.
28. D. Zha, S. Mei, Z. Wang, H. Li, Z. Shi and Z. Jin, Superhydrophobic Polyvinylidene fluoride/Graphene Porous Materials, Carbon, 2011, 49, 5166–5172.
29. Y. Sohn, D. Kim, S. Lee, M. Yin, J. Y. Song, W. Hwang, S. Park, H. Kim, Y. Ko and I. Han, Anti-frost Coatings Containing Carbon Nanotube Composite with Reliable Thermal Cyclic Property, J. Mater. Chem. A, 2014, 2, 11465.
30. H. Saito, K. Takai and G. Yamauchi, Water- and ice-repellent coatings, Surf. Coat. Int., 1997, 80, 168–171.
31. N. Katiyar and K. Balasubramanian, Nano-heat-sink Thin Film Composite of PC/Three-dimensional Networked Nano-Fumed Silica with Exquisite Hydrophobicity, RSC Adv., 2015, 5, 4376–4384.
32. W. Ye, X. Luo and Z. Wang, Heat-Resistant Crack-Free Superhydrophobic Polydivinylbenzene Colloidal Films, Langmuir, 2016, 32, 3079–3084.
33. M. Fu and B. Qu, Synergistic flame retardant mechanism of fumed silica in ethylene-vinyl acetate/magnesium hydroxide blends, Polym. Degrad. Stab., 2004, 85, 633–639.
34. J. P. Letterman, R. K. Asher, K. A. Reginald, M. Sanduja and F. B. Dragnea, Method of manufacturing semiconductor component, US Pat. No. 5, 785,79.
35. N. K. Shrivastava, S. Suin, S. Maiti and B. B. Khatua, An Approach to Reduce the Percolation Threshold of MWCNT in ABS/MWCNT Nanocomposites through Selective Distribution of CNT in ABS Matrix, RSC Adv., 2014, 4, 24584–24593.
36. A. Masoumi, K. Hemmati, M. Ghaemy, A. Masoumi, K. Hemmati and M. Ghaemy, Structural Modification of Acrylonitrile–Butadiene–Styrene waste as an Efficient Nanoadsorbent for Removal of Metal Ions from Water: Isotherm, Kinetic and Thermodynamic Study, RSC Adv., 2015, 5, 1735–1744.
37. J. Y. Ying, Z. Zhang, L. Zhang and M. S. Dresselhaus, A Process for Fabricating an Array of Nanowires, US Pat. No. 6,231,744.
38. D. Mitra, M. Li, R. Wang, Z. Tang, E. T. Kang and K. G. Neoh, Scalable Aqueous-Based Process for Coating Polymer and Metal Substrates with Stable Quaternized Chitosan Antibacterial Coatings, Ind. Eng. Chem. Res., 2016, 55(36), 9603–9613.
39. J. Li, Z. Jing, F. Zha, Y. Yang, Q. Wang and Z. Lei, Facile Spray-Coating Process for the Fabrication of Tunable Adhesive Superhydrophobic Surfaces with Heterogeneous Chemical Compositions Used for Selective Transportation of Microdroplets with Different Volumes, ACS Appl. Mater. Interfaces, 2014, 6, 8868–8877.
40. S. Nigam, S. K. Patel and S. S. Mahapatra, Copper plating on ABS plastic by thermal spray, Proceedings of 25th IRF International Conference, ISBN: 978-93-85465-00-0, Pune, India, 26th April 2015.
41. B. S. Banerjee and K. Balasubramanian, Nanotexturing of PC/n-HA Nanocomposites by Innovative and Advanced Spray System, RSC Adv., 2015, 5, 13653.
42. B. N. Sahoo, K. Balasubramanian and M. Sucheendran, Thermally Triggered Transition of Superhydrophobic Characteristics of Micro- and Nanotextured Multiscale Rough Surfaces, J. Phys. Chem. C, 2015, 119(25), 14201–14213.
43. B. N. Sahoo and K. Balasubramanian, An Experimental Design for the Investigation of Water Repellent Property of Candle Soot Particles, Mater. Chem. Phys., 2014, 148, 134–142.
44. B. N. Sahoo and K. Balasubramanian, Recent Progress in Fabrication and Characterisation of Hierarchical Biomimetic Superhydrophobic Structures, RSC Adv., 2014, 4, 22053–22093.
45. B. N. Sahoo and K. Balasubramanian, Photoluminescent Carbon Soot Particles Derived from Controlled Combustion of Camphor for Superhydrophobic Applications, RSC Adv., 2014, 4, 11331.
46. N. Saleema and M. Farzaneh, Thermal Effect on Superhydrophobic Performance of Stearic Acid Modified ZnO Nanotowers, Appl. Surf. Sci., 2008, 254, 2690–2695.
47. J. G. Bakira and S. Schlick, Spatial effects in the photodegradation of poly (acrylonitrile-butadiene-styrene): a study by ATR-FTIR, Polymer, 2002, 43(11), 3239–3246.
48. M. E. Adams, D. J. Buckley, R. E. Colborn, w. P. England and D. N. Schissel, Acrylonitrile-Butadiene-Styrene Polymers, A Review Report published by RAPRA Technology Ltd, Shawbury, England, 1993.
49. K. Chen, S. Zhou and L. Wu, Facile fabrication of self-repairing superhydrophobic coatings, Chem. Commun., 2014, 50, 11891–11894.
50. R. Gedye, F. Smith, K. Westaway, H. Ali, L. Baldisera, L. Laberge and J. Rousell, The use of microwave ovens for rapid organic synthesis, Tetrahedron Lett., 1986, 27(3), 279–282.
51. Y. Yao, J. Hou, Z. Xu, G. Li and Y. Yang, Effects of Solvent Mixtures on the Nanoscale Phase Separation in Polymer Solar Cells, Adv. Funct. Mater., 2008, 18, 1783–1789.
52. H. Sanaeepur, A. E. Amooghin, A. Moghadassi and A. Kargari, Preparation and characterization of acrylonitrile–butadiene–styrene/poly(vinyl acetate) membrane for CO₂ removal, Sep. Purif. Technol., 2011, 80, 499–508.
53. J. A. Brydson, Plastics Materials, Butterworth-Heinemann, 7th edn, 1999, pp. 441–447.
54. V. R. Gowarikar, N. V. Viswanathan and J. Sreedhar, Polymer Science, New Age International (P) Ltd, Publishers, 1st edn, 1986, pp. 150–172.
55. T. Hatakeyama and F. X. Quinn, Thermal Analysis: Fundamentals and Applications to Polymer Science, John Wiley & Sons, 2nd edn, 1999, pp. 90–107.
56. J. H. Zanten, W. E. Wallace and W. Wu, Effect of strongly favorable substrate interactions on the thermal properties of ultrathin polymer films, Phys. Rev. E, 1996, 53, R2053(R).
57. A. Rios De Anda, L. A. Fillot, S. Rossi, D. Long and P. Sotta, Influence of the Sorption of Polar and Non-Polar Solvents on the Glass Transition Temperature of Polyamide 6,6 Amorphous Phase, Polym. Eng. Sci., 2011, 51(11), 2129–2135.
58. H. Assaedia, F. U. A. Shaikhb and I. M. Lowa, Effect of nano-clay on mechanical and thermal properties of geopolymer, Journal of Asian Ceramic Societies, 2016, 4(1), 19–28.
59. T. Kashiwagi, A. B. Morgan, J. M. Antonucci, M. R. Van Landingham, R. H. Harris, W. H. Awad and J. R. Shields, Thermal and Flammability Properties of a Silica–Poly(methylmethacrylate) Nanocomposite, J. Appl. Polym. Sci., 2003, 89, 2072–2078.
60. T. Young, An Essay on the Cohesion of Fluids, Phil. Trans. R. Soc. Lond, 1805, 95, 65–87.
61. R. N. Wenzel, J. Ind. Eng. Chem., 1936, 28, 988.
62. A. B. D. Cassie and S. Baxter, Trans. Faraday Soc., 1944, 40, 546.
63. L. Gao and T. J. McCarthy, An Attempt to Correct the Faulty Intuition Perpetuated by the Wenzel and Cassie “Laws”, Langmuir, 2009, 25(13), 7249–7255.
64. L. Gao, T. J. McCarthy, H. Wenzel and C. W. Wrong, Langmuir, 2007, 23, 3762–3765.
65. L. Gao and T. J. McCarthy, Contact Angle Hysteresis Explained, Langmuir, 2006, 22, 6234–6237.
66. K. A. Wier and T. J. McCarthy, Condensation on Ultrahydrophobic Surfaces and Its Effect on Droplet Mobility: Ultrahydrophobic Surfaces Are Not Always Water Repellant, Langmuir, 2006, 22, 2433–2436.
67. L. Gao and T. J. McCarthy, Wetting 101°, Langmuir, 2009, 25(24), 14105–14115.
68. M. A. Sarshar, W. Xu and C. H. Choi, Advances in Contact Angle, Wettability and Adhesion, Scrivener Publishing, Wiley, 2013, vol. 1, pp. 3–18.
69. P. S. H. Forsberg, C. Priest, M. Brinkmann, R. Sedev and J. Ralston, Contact Line Pinning on Microstructured Surfaces for Liquids in the Wenzel State, Langmuir, 2009, 26, 860–865.
70. M. Reyssat and D. Quere, Contact Angle Hysteresis Generated by Strong Dilute Defects, J. Phys. Chem. B, 2009, 113, 3906–3909.
71. C. Lv, C. Yang, P. Hao, F. He and Q. Zheng, Sliding of Water Droplets on Micro-Structured Hydrophobic Surfaces, Langmuir, 2010, 26, 8704–8708.
72. M. James Clark, Electricity and Magnetism, Clarendon Press, Oxford, 1873.
73. D. Serhat, A numerical study on the effective thermal conductivity of composite material, PhD thesis, Graduate School of Natural and Applied Sciences of Dokuz Eylul University, January 2004.
74. R. L. Hamilton and O. K. Crosser, Thermal conductivity of heterogeneous two-component systems, Ind. Eng. Chem. Fundam., 1962, 1(3), 187–191.
75. H. Ma, L. Tong, Z. Xu and Z. Fang, Intumescent flame retardant-montmorillonite synergism in ABS nanocomposites, Appl. Clay Sci., 2008, 42, 238–245.
76. G. Zu, J. Shen, L. Zou, W. Wang, Y. Lian, Z. Zhang and A. Du, Nanoengineering Super Heat-Resistant, Strong Alumina Aerogels, Chem. Mater., 2013, 25(23), 4757–4764.
77. B. Wicklein, A. Kocjan, G. Salazar-Alvarez, F. Carosio, G. Camino, M. Antonietti and L. Bergström, Thermally insulating and fire-retardant lightweight anisotropic foams based on nanocellulose and graphene oxide, Nat. Nanotechnol., 2015, 10(3), 277–283.

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