Hybrid Hemp Particles as Functional Fillers for the Manufacturing of Hydrophobic and Anti-icing Epoxy Composite Coatings

Abstract

 

The development of hydrophobic composite coatings is of great interest for several applications in the aerospace industry. Functionalized microparticles can be obtained from waste fabrics and employed as fillers to prepare sustainable hydrophobic epoxy-based coatings. Following a waste-to-wealth approach, a novel hydrophobic epoxy-based composite including hemp microparticles (HMPs) functionalized with waterglass solution, 3-aminopropyl triethoxysilane, polypropylene-graft-maleic anhydride, and either hexadecyltrimethoxysilane or 1H,1H,2H,2H-perfluorooctyltriethoxysilane is presented. The resulting epoxy coatings based on hydrophobic HMPs were cast on aeronautical carbon fiber-reinforced panels to improve their anti-icing performance. Wettability and anti-icing behavior of the prepared composites were investigated at 25 °C and −30 °C (complete icing time), respectively. Samples cast with the composite coating can achieve up to 30 °C higher water contact angle and doubled icing time than aeronautical panels treated with unfilled epoxy resin. A low content (2 wt %) of tailored HMPs causes an increase of ∼26% in the glass transition temperature of the coatings compared to pristine resin, confirming the good interaction between the hemp filler and epoxy matrix at the interphase. Finally, atomic force microscopy reveals that the HMPs can induce the formation of a hierarchical structure on the surface of casted panels. This rough morphology, combined with the silane activity, allows the preparation of aeronautical substrates with enhanced hydrophobicity, anti-icing capability, and thermal stability.

1. Introduction

In the last decades, superhydrophobic surfaces have drawn great attention both in academia and in industry due to their self-cleaning,¹ anti-icing,² antifouling,³ antifogging,⁴ and anticorrosive⁵ properties. In particular, superhydrophobic surfaces can be useful in many industrial fields, such as automotive,⁶ aerospace,⁷ textiles,⁸ medical diagnostics, and sensing,⁹ plus they can be combined with smart functionalities derived from nanoparticles.¹⁰⁻¹² The surface wettability is determined by the strength of the cohesive forces within the water molecules and the interaction of the water with the surface itself. When water droplets roll off, keeping the surface dry, the cohesive forces within the water molecules are greater than those on the surface.¹³ A hydrophobic surface provides a water contact angle (CA) larger than 90°, while a superhydrophobic surface exhibits a water CA larger than 150° and a contact angle hysteresis (CAH) lower than 10°.¹⁴ Low surface energy and micro-/nano-hierarchical roughness of the solid substrate are the two fundamental parameters for designing a superhydrophobic surface.¹³ Inorganic oxide nanoparticles (e.g., silica, zinc oxide, titania) can provide rough surfaces with a microscale/nanoscale architecture.¹⁵⁻¹⁸ For the development of the hydrophobic/superhydrophobic coatings, different fabrication techniques can be utilized, for example, electrochemical deposition,¹⁹ electro-spinning and electro-spraying,^20,21 chemical vapor deposition,²² layer-by-layer deposition,²³ sol–gel processing, and solution casting.^24,25 Among these, the sol–gel method and water glass route are widely used, as they operate under mild and low-cost conditions to produce a variety of nanostructured materials^26,27 and superhydrophobic layers on various substrates.^24,28 Superhydrophobic coatings derived from silica sols and hydrophobic compounds have been widely investigated. Silica can be functionalized by alkyl silanes and fluorinated alkyl silanes, obtaining hydrophobic and superhydrophobic coatings showing low surface free energy owing to the surface hydroxyl groups.²⁹⁻³² Among hydrophobic compounds, the fluorinated ones exhibit high effectiveness; however, they have some drawbacks, including high costs and risks for human health and the environment.^33,34 The US Environmental Protection Agency and the European Chemicals Agency are considering the restriction of several long-chain linear per- and polyfluoroalkyl substances (PFAS).^35,36 Nevertheless, PFAS are still widely used as functional additives to prepare hydrophobic products. Hence, the use of non-fluorinated silanes, such as long-chain alkyl silanes, is encouraged for fabricating sustainable coatings owing to their lower toxicity and low costs.

Epoxy resins have been widely used as a matrix to prepare hydrophobic nano-/micro-composite coatings³⁷⁻³⁹ due to their easy processing, strong adhesion to many substrates, and excellent chemical resistance.⁴⁰ The high surface energy of epoxy resins strongly limits their use as the polymer matrix in the manufacturing of water-proof coatings. To overcome such limitations, it is possible to disperse hydrophobic nanoparticles into the epoxy matrix,⁴¹⁻⁴³ before applying the coating onto a substrate (e.g., carbon fiber panels, plastic films, glass slides, metals, etc.).⁴⁴⁻⁴⁷ Recently, natural fibers have become more attractive as potential sustainable and eco-friendly reinforcement for epoxy composites, encouraging the use of surplus cellulose fibers derived from agricultural waste.^48,49 However, due to their hydrophilic chemical nature, the use of cellulose-based particles as platforms and functional fillers for the manufacturing of hydrophobic epoxy-based coatings has not yet been investigated. The few works on the wettability of epoxy composites containing natural microfibers reported an unaltered⁵⁰ or decreased⁵¹ hydrophobic character. Moreover, the treatment of jute fibers with an epoxy resin solution caused only a slight increase in water CA (from about 67° to 74°),⁵² confirming that offsetting the marked hydrophilicity of these materials is a challenging task. In the context of the circular economy, the production of modified hemp particles would represent a sustainable and easy solution for the recovery of waste hemp rugs, as epoxy resins and fabrics are largely used in the aerospace sector for the fabrication of multifunctional fiber-reinforced composites.^53,54 In this perspective, it would be highly desirable to exploit properly functionalized hemp-based particles as fillers for manufacturing composite coatings with enhanced hydrophobicity and anti-icing performances without any detrimental impact on the viscoelastic behavior and the thermal stability of the polymer matrix.

In the present study, hydrophobic composite epoxy coatings were cast onto typical aeronautical panels, based on carbon-fiber-reinforced (CFR) polymers, to assess their feasibility in aerospace applications. In particular, the possibility of obtaining innovative fiber-reinforced epoxy coatings reusing waste hemp as particles, functionalized with waterglass (i.e., sodium metasilicate) solution, 3-aminopropyl triethoxysilane (APTES), polypropylene-graft-maleic anhydride (PPgMA) and silanes, either hexadecyltrimethoxysilane (HDTMS) or 1H,1H,2H,2H-perfluorooctyltriethoxysilane (PFOTES), was deeply investigated, and the influence of the molecular structure of these silanes on the wettability and anti-icing performances of treated aeronautical panels was evaluated. Through this approach, hemp particles’ surface chemistry and morphological characteristics can improve the interphase between the filler and the polymer matrix and confer rough morphology to the surface, leading to a higher glass transition temperature and reduced wettability compared to the virgin epoxy coating.

2. Experimental Section

2.2. Methods

 

2.2.1. Synthesis and Functionalization of Hemp Fabric Microparticles

Hemp fabric rugs are generally used in the manufacturing of fiber-reinforced epoxy composites and are one of the most abundant waste materials resulting from the fabrication process. In Scheme 1, the overall procedure for the synthesis of hydrophobic hemp particles is shown.

Scheme 1

Overall Procedure for the Synthesis of Hydrophobic Hemp Particles

Starting from waste hemp fabric rugs, HMPs were obtained through a sol–gel route previously established.^55,56 Briefly, hemp fabric rugs were submitted to iterated soaking–drying cycles (see stage 1 of Scheme 1) in a waterglass (Na₂SiO₃, 0.01 M) solution acidified up to pH = 2.5 with hydrochloric acid (HCl). The sol–gel methodology allows for the deposition of a silica gel on the surface of hemp rugs, which makes them brittle after drying.⁵⁶ Then, the dry fabrics could be reduced to silica-coated hemp powder (HEMPSi) particles by a low-power (350 W) mixer (IMETEC S.p.A., Azzano San Paolo, Bergamo, Italy). In the second step (see stage 2 of Scheme 1), the functionalization with amino groups was performed by laying HEMPSi particles in an EtOH/water solution acidified up to pH = 5 with acetic acid and containing APTES. The final product (HEMPN) was washed with an EtOH/water solution through centrifugation cycles.⁵⁵

In the present study, to confer hydrophobic character to HEMPN microparticles, the following strategy was adopted. The primary amino groups were left to react with maleic anhydride molecules grafted along polypropylene chains of PPgMA (see stage 3 of Scheme 1). For a typical synthesis, this reaction was performed as follows: 0.25 g of PPgMA was added into a vessel containing 50 mL of xylene solution at 80 °C and left stirring till the complete dissolution of the polypropylene-based compatibilizer. Then, 1 g of HEMPN particles, fully dried overnight at 60 °C, was added to the vessel and left reacting under reflux for 3 h at 80 °C. The final product (HEMPP) was washed with pure xylene by three centrifugation cycles to remove unreacted PPgMA and finally with a solution of EtOH/water (80:20 v/v %).⁵⁷

 

2.2.2. Manufacturing of Fiber-Reinforced Epoxy Coatings

The bulk sample of blank epoxy (EPO) was manufactured by mixing a certain amount of unfilled DGEBA with an amine hardener (26 wt % of the epoxy resin). After stirring at room temperature, the mixture was poured into a silicone rubber mold (5 × 5 × 0.3 cm³), with a thickness of 0.1 cm, cured at 60 °C for 24 h, and then post-cured at 80 °C for 4 h. By following a similar procedure, functionalized HMPs were used to fabricate DGEBA-based epoxy composite coatings with the same dimensions (5 × 5 × 0.1 cm³). In the preparation of a typical formulation, a specific amount of DGEBA resin was added with 2 wt % of HEMPP particle loading (this content was selected on the basis of tan δ measurements previously performed on a similar system by some of the authors⁵⁵), 10 wt % of acetone, and 1 wt % of HDTMS or PFOTES (see stage 4 of Scheme 1) silanizing agent. These percentage values for the synthesis all refer to the epoxy matrix cured with 26 wt % of hardener. These bulk composite coatings were cured at the same conditions as the EPO sample.

The unfilled epoxy coating and composite ones were also deposited by the drop-casting method from one-pot formulations on aeronautical panels based on the carbon fiber-reinforced material. Before the deposition, the formulations were strongly stirred to obtain homogeneous systems and then weakly sonicated to remove the bubbles. The final products were cast on CFR panels (7.5 × 2.5 × 0.1 cm³) previously cleaned with ethanol. The coated substrates were cured at 60 °C for 24 h and then post-cured at 80 °C for 4 h. Untreated CFR panels and coated ones are shown in Figure S1. It can be observed that the deposition of the coatings does not alter the chromatic characteristics of the substrates, confirming a uniform distribution of the filler at a macroscopic scale. In the next sections, the epoxy coating samples containing HEMPP particles modified with HDTMS and PFOTES will be named EH_HDTMS and EH_PFOTES, respectively.

 

2.2.3. Wettability and Anti-icing Tests

The wettability of composite coatings was evaluated by measuring the CA and the CAH. The CA measurement allows the evaluation of the surface interaction of the investigated system with three phases (solid, liquid, and vapor). The CA is the angle arising from the intersection of liquid–vapor and solid–liquid interfaces (see Figure S2a).⁵⁸ This angle forms when a liquid drop lies on a horizontal plane. The CA of distilled water was evaluated by a high-resolution camera (iPhone 13 Pro Max-12 MP, f/1.5, 26 mm-12 MP, f/2.8, 77 mm-12 MP, f/1.8, 13 mm-LiDAR ToF 3D, Apple Inc.) at room temperature using the sessile drop method.⁵⁸ The CA was measured with an open-source image processing software ImageJ (v.1.52t, NIH, 2020). The CAH was evaluated as follows: a distilled water drop was placed on the substrate, firmly anchored to an inclined plane, and the CAs (advancing and receding) were collected when the droplet started sliding down (Figure S2b). The difference between the advancing (θ_A) and receding (θ_R) contact angles corresponds to the CAH (eq 1).

1

 

All the CAs were estimated by placing water drops of volume in the range of 30–40 μL on the coating surface with a micropipette.

The anti-icing test was performed by dripping water droplets (30–40 μL) on the surfaces of overcooled samples (−30 °C).² First, the samples were placed in a refrigerator at a temperature of −30 °C for 10 min. Then, the deionized water droplet was dripped onto the coating’s surface. Meanwhile, a high-speed camera (pco. dimax cs1, PCO) was used for recording the freezing process. Figure S3 shows the apparatus used for the anti-icing tests.

 

2.2.4. Structural, Morphological, and Thermal Analysis

 

2.2.4.1. Attenuated Total Reflectance–Fourier Transform Infra-Red

Attenuated total reflectance–Fourier transform infra-red (ATR–FTIR) analysis was performed on the functionalized HMPs and epoxy coatings by using a Nicolet 5700 spectrometer (Thermo Fisher, Waltham, MA, USA) with a single reflection ATR accessory. The instrument has a resolution of 4 cm^–1, and the collected spectra was the result of 32 running scans. The analysis software used was Thermo Scientific OMNIC Software Suite (v7.2, Thermo Fisher, Waltham, MA, USA, 2005).

 

2.2.4.2. Atomic Force Microscopy

Atomic force microscopy (AFM) analysis was performed by means of a Bruker NanoScope V multimode AFM (Digital Instruments, Santa Barbara, CA, USA) apparatus to quantify the surface roughness parameters and analyze the nanoscale surface morphology of samples EPO, EH_HDTMS, and EH_PFOTES. Topographic height images were acquired at room temperature and processed using a Bruker software Nanoscope Analysis 1.80 (BuildR1.126200). The measurements were performed in tapping mode, in which the sharp tip of the probe scans the sample surface intermittently by oscillating up and down as the cantilever is vibrated near its resonance frequency. The tip is characterized by a radius of 5–10 nm, a nominal spring constant of 20–100 N/m, and resonance frequencies of 200–400 kHz. For each analyzed sample, several AFM images were acquired at different locations to evaluate the trend of the roughness parameters and verify if these are reproducible on different scanned areas of the samples. The scanning rate was 0.500 Hz per scan line, with 512 pixels per line. In order to evaluate the surface roughness, different roughness parameters are generally estimated and applied. The magnification of the scanned area during the AFM acquisition greatly influences the roughness parameters. This means that the roughness value measured for a large section of the surface will be very different from that calculated for a smaller section. In fact, AFM images having the same scan size were compared for the three samples EPO, EH_HDTMS, and EH_PFOTES in order to effectively quantify the roughness values. In this work, to derive the quantitative roughness, two of the most relevant height parameters, namely, the roughness average (R_a) and the root mean square roughness (R_q), have been considered. More precisely, R_a represents the arithmetic mean of the absolute values of the height of the surface profile, and R_q is analogous to the roughness average (R_a), with the only difference that R_q is more sensitive to peaks and valleys than R_a, due to the squaring of the amplitude in its calculation. These amplitude parameters, which characterize the surface based on the vertical deviations of the roughness map from the mean surface, are extensively used in the literature.^59,60 For all the samples, the R_a and R_q parameters were evaluated according to eqs 2 and 3

2

3

where l_r is the length of the line, z is the height, and x is the position.

The surface morphology of the functionalized HMPs and coated aeronautical panels was observed via scanning electron microscopy (SEM) using a Leica Stereoscan 440 microscope (20 kV) (Cambridge Ltd., Cambridge, UK), coupled with an energy-dispersive X-ray (EDX) analytical system (Inca Energy 200). The Aztec Energy EDS software (v2.1, Oxford Instruments, Abingdon, UK, 2006) was used.

 

2.2.4.3. Scanning Electron Microscopy

SEM of hemp particles was performed at each functionalization step using a Leica Stereoscan 440 microscope (20 kV) (Leica Microsystems Cambridge Ltd., Cambridge, UK), coupled with an EDX system (Inca Energy 200). The EDX system used an Aztec Energy EDS Software (v2.1, Oxford Instruments, Abingdon, UK, 2006).

 

2.2.4.4. Differential Scanning Calorimetry

Differential scanning calorimetry (DSC) was carried out to evaluate the thermal behavior of EPO, EH_HDTMS, and EH_PFOTES samples. The analysis was performed by a Mettler DSC 822/400 thermal analyzer (Mettler-Toledo Columbus, OH, USA) at a heating rate of 10 °C/min by running two repeating cycles (from 20 °C to 300 °C) under a N₂ flow (50 mL/min). The T_g value was determined at the inflection point on the second heating curve.

 

2.2.4.5. Thermogravimetric Analysis

Thermogravimetric analysis (TGA) was performed on a Mettler TGA/SDTA 851 (Mettler-Toledo Columbus, OH, USA) instrument under air and N₂ atmospheres with a flow rate of 50 mL/min. The weight loss as a function of the temperature was recorded at 10 °C/min from 25 °C to 800 °C.

3. Results and Discussion

3.1. Structural Analysis of Functionalized Hemp Particles and Epoxy Composite Coatings

It is known that the hydrophobicity of a surface depends on two main effects: the texture morphology of the surface and its chemistry. A polymer coating exhibiting a micro-structured surface is more hydrophobic than a flat surface.⁶¹ Hence, to make hemp particles a suitable functional filler for lowering the wettability of epoxy-based coatings, their surface chemistry needs to be modified to increase their hydrophobicity and affinity toward the polymer matrix. HEMPSi powders were composed of microparticles of diameter ranging from tens of nanometers to tens of microns, as shown by SEM images (Figure​Figure11a).⁵⁵ Such a morphology derives from the web-like structure formed by the fibrils and microfibrils of hemp.^57,62

Figure 1

SEM micrographs at different magnifications of (a) silica gel-functionalized hemp particles (HEMPSi), (b) amino-functionalized hemp particles (HEMPN), and (c) HEMPN particles functionalized with PPgMA. The yellow circles indicate the areas where EDX analysis was performed.

The analysis of the ATR–FTIR spectrum of the HEMPSi sample reveals the presence of a layer of silica gel rich in silanol groups, as attested by the broad bands at about 3340 cm^–1 (ν_O–H) and 1100 cm^–1 (ν_Si–O) (Figure S4). The formation of a silicon-based layer is confirmed by EDX analysis (Table 1). Silanol groups on the surface of HMPs can react through condensation with APTES, a well-known coupling agent for epoxy systems.^63,64Figure​Figure11b reports SEM images of amino-functionalized hemp particles (HEMPN). The presence of primary amino groups on these fibers is clearly shown by the appearance of IR bands at 1554 cm^–1 (δ_N–H) and 1407 cm^–1 (ν_C–N) (see Figure S4) and is confirmed by EDX measurements (Table 1), which reveal a significant amount of nitrogen on the hemp surface. The amino groups on HEMPN particles can react with the oxirane rings of the epoxy chain; therefore, the resulting material can be used to prepare fiber-reinforced epoxy composites with tailored interphase. However, as observed in a previous work,⁵⁵ HEMPN particles tend to segregate in the polymer matrix as their surface chemistry limits the migration of such fillers to the surface of epoxy coatings. Conversely, PPgMA-functionalized (HEMPP) particles show a non-polar character and are not able to react with oxirane rings. SEM micrographs at different resolutions (Figure​Figure11c) reveal that HEMPP particles still exhibit the characteristic unregular rugged morphology,^57,62 and their surface appears slightly waxy and different compared to the ones of HEMPSi and HEMPN samples. This may be ascribed to a thin layer of PP covering the primary wall of hemp particles. The presence of this polymer coating is further supported by a higher amount of carbon recorded by EDX investigation for HEMPP particles with respect to HEMPSi (Table 1). It also makes nitrogen undetectable by EDX, probably because the nitrogen signal is covered by PPgMA after the reaction of primary amino groups with maleic anhydride moieties.^57,62 The successful grafting of PPgMA on the surface of the final product (HEMPP) is assessed by the characteristic bands attributed to C–H bending at 1376 cm^–1 (ρ_CH3) and 1460 cm^–1 (δs_CH2) and by the increase of the C–H stretching bands in the region 2900–3000 cm^–1 (Figure S4).

Table 1

Elemental Analysis (EDX) of Surface Chemical Composition (wt %) of Functionalized Hemp Particles

composition (%)	C	O	N	Si
HEMPSi	39.5	57.3		3.2
HEMPN	37.9	53.2	6.7	2.2
HEMPP	46.5	45.6		7.9

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The functionalization of hemp particles by PPgMA completely changes the chemical characteristics of their surface, enhancing their hydrophobicity and causing them to appear much more similar to the epoxy resin from a chemical point of view. Thus, due to their chemical and morphological characteristics, HEMPP particles appear highly suitable to be incorporated into epoxy systems to manufacture polymer coatings with hierarchical micro-structured surface textures. To combine this aspect with an even more hydrophobic chemistry for HEMPP, these particles were additionally functionalized with two silanes with non-polar chains, HDTMS and PFOTES (see Scheme 1). As shown in Table 1, HEMPP particles display a residual amount of silicon on their surface due to some free silanol groups that have remained unreacted. These silanol groups can condensate with HDTMS and PFOTES (see Scheme 1) during the curing process (cure: 60 °C/24 h, post-cure: 80 °C/4 h) to form tailored hemp particles with a rugged morphology, high hydrophobicity, and enhanced compatibility with the epoxy matrix.^65,66

The chemical structure of the pristine epoxy resin (EPO) and the composites containing the functionalized hemp particles (EH_HDTMS and EH_PFOTES) was investigated by ATR–FTIR spectra (Figure S5). All the samples appear completely cured, as attested by the disappearance of the oxirane ring vibration band at 913 cm^–1 and by the rise of the O–H stretching band between 3500 and 3100 cm^–1.⁶⁷⁻⁶⁹ It is known that the position of this band is very sensitive to the strength of H-bonding in which OH groups are involved, shifting toward lower frequency and increasing the magnitude of H-bonding.⁷⁰ For pure epoxy, two features at about 3470 and 3322 cm^–1 are seen, which can be related to free and H-bonded hydroxyls, respectively. For the EH_HDTMS and EH_PFOTES composites, the O–H stretching band shows a single feature at about 3380 cm^–1, indicating weaker H-bonding interactions between the epoxy matrix and the functional groups of hemp particles.^68,71 Furthermore, in the spectra of EH_HDTMS and EH_PFOTES, despite the low concentration of both HEMPP particles and silanes, some of their main functional groups can be detected, for example, the Si–O bonds, whose stretching lies between 1030 and 1080 cm^–1, while other features are overlapped with the several characteristic vibrations of the polymer matrix.

3.2. Wettability and Anti-icing Properties

The water CA and CAH values of the uncoated and treated aeronautical panels are displayed in Table 2 and Figure​Figure22. The intrinsic water CA value of the pristine substrate was 62° ± 3°, which denotes a hydrophilic behavior of the carbon fiber-reinforced panel. In coated samples, in the absence of a functionalized hemp filler, the deposition of EPO does not change the hydrophilic character (water CA = (84° ± 2°) < 90°). This hydrophilicity can be explained by the presence of hydroxyl groups, arising from the curing of the resin, which can form H-bonds with water molecules,⁷² as confirmed by the ATR–FTIR investigation. A slight hydrophobicity (water CA = (96° ± 3°) > 90°) is observed for the EH_HDTMS coating. As previously demonstrated, HEMPP particles still exhibit free silanol groups, making them amphiphilic and able to migrate to the surface of the epoxy matrix, establishing a micro-structured and rough texture (see Section 3.4). The functionalization of hemp particles with PPgMA plays a key role in the above-described migration phenomenon and, at the same time, increases the hydrophobicity of the surface and the compatibility with the polymer matrix. The anchoring of HDTMS causes the exposure of alkyl chains at the solid–air interface, which, combined with the suitable morphology, provides higher CA and lower CAH than EPO (see Table 2). Concerning the panel coated with EH_PFOTES, the presence of fluorinated alkyl chains boosts the hydrophobic response of the surface [water CA = (115° ± 3°), see Table 2 and Figure​Figure22], due to the well-known function of fluorine-based silanes in lowering the wettability of coatings.¹³Figure​Figure22 shows the progressive increase in CA and a relative decrease in CAH moving from EPO to EH_PFOTES, highlighting the effects of structural and chemical contributions on surface wettability. The rough morphology and non-polar character of the functionalized hemp particles exert a synergistic action in enhancing the hydrophobicity of the epoxy composite coatings, even using a low content of the waste-derived filler (2 wt %) in a simple one-pot procedure with mild operating conditions.

Figure 2

CA, CAH, and water droplet representations of untreated and coated CFR panels.

Table 2

Wettability and Anti-icing Parameters of Untreated and Coated CFR Panels: CA, CAH, and Freezing Time^a

sample	CA (deg)	CAH (deg)	freezing time (s)
panel	62 ± 3		35 ± 5
EPO	84 ± 2	55 ± 3	54 ± 2
EH_HDTMS	96 ± 3	43 ± 2	78 ± 3
EH_PFOTES	115 ± 3	33 ± 4	113 ± 5

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^aCAH was not measured for the pristine panel as the droplet could not slide down.

The potential effectiveness of epoxy composite coatings as protective layers against ice formation was evaluated by measuring the freezing time of water droplets deposited on the surfaces of the aeronautical panels at −30 °C. Figure​Figure33 shows the shape of the droplets as deposited at room temperature and their change after the freezing process on pristine CFR panels and substrates coated with pure epoxy (EPO), EH_HDTMS, and EH_PFOTES coatings.

Figure 3

Photographs of water droplets frozen at −30 °C (under the yellow dot line), water CAs, and freezing time of water droplets on bare CFR panels and substrates treated with pristine resin (EPO), EH_HDTMS, and EH_PFOTES coatings.

The water droplets on the CFR panel and on EPO were semicircular in shape and froze within 35 and 54 s, respectively. In contrast, the water droplet on the EH_HDTMS and EH_PFOTES coatings remained more spherical due to the higher hydrophobicity, and the freezing time was delayed up to 113 s (see Table 2 and Figure​Figure33). These results can be explained by the reduction of the contact area between the water droplet and the hydrophobic surface, which leads to a lower heat transfer rate.⁷³⁻⁷⁵ The increase of CA and reduction of CAH values caused by the presence of the silanized hemp particles in the epoxy coatings (EH_HDTMS and EH_PFOTES) are clearly related to the anti-icing performances, as the observed variation of these parameters agrees with a reduced ice adhesion strength on the coated panels.^76,77 Based on the values of the collected CAs, the formation of ice on EH_HDTMS and EH_PFOTES coatings occurs by a heterogenous nucleation mechanism.⁷⁸ This low adhesion, combined with the decreased contact area, increases water droplet freezing time. Although PFOTES appears to be the most effective functionalizing agent, HDTMS represents a valuable greener alternative, still improving wettability and anti-icing properties.

4. Conclusions

The current study focuses on the possibility of using functionalized particles based on waste hemp to obtain hydrophobic epoxy coatings for aeronautical carbon fiber-reinforced laminates. The proposed chemical modification procedure allows turning an intrinsically hydrophilic material, such as hemp, into hydrophobic particles suitable as fillers in a non-polar polymer matrix to give rise to surfaces with reduced wettability. It was found that, by embedding the functionalized HMPs within the epoxy matrix, a great improvement in terms of thermal behavior, hydrophobicity, and anti-icing performance could be achieved. In particular, the incorporation of only 2 wt % of silane-modified hemp particles into the epoxy resin resulted in both an increased glass transition temperature (up to ∼26%) and an enhanced thermo-oxidative stability because of the good chemical affinity between the functionalized fiber particles and the polymer matrix. Aeronautical panels treated by the composite epoxy coatings exhibited water CA up to 115° and a freezing time of 1.8 min, unlike samples cast with pristine resin showing ∼84° and 0.9 min, respectively. AFM analysis of casted aeronautical laminates proved that the presence of hemp fillers in the epoxy coating led to a rougher surface morphology with a hierarchical structure. The boosted hydrophobicity and anti-icing properties are due to the synergistic effects of chemical and morphological features of the microparticles with anchored silanes. Furthermore, these properties can be tuned using either the high-performing fluorinated silane (PFOTES) or the greener and cheaper alkyl silane (HDTMS). This research may inspire the design and development of sustainable multifunctional composite epoxy coatings containing waste-derived fibers as fillers for aircraft components.

• Supporting InformationPhotographs of CFR panels bare and treated with coatings of pristine resin (EPO), EH_HDTMS, and EH_PFOTES; schematic representation of water drop on a plane; schematic of the apparatus used for the anti-icing tests; ATR-FTIR spectra; results of TGA in air atmosphere for pristine resin and epoxy composites; and DSC curves for the samples EPO, EH_HDTMS, and EH_PFOTES (PDF)

Acknowledgments

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.3c01415.

• (PDF)

Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Notes

This research is funded by the University of Salerno, project code: 300395CIC22RAIMONDO.

Notes

The authors declare no competing financial interest.

References