Formation of Icephobic Surface with Micron-scaled Hydrophobic

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Applications of Polymer, Composite, and Coating Materials

Formation of Icephobic Surface with Micron-scaled Hydrophobic Heterogeneity on Polyurethane Aerospace Coating Yeap Hung Ng, Siok Wei Tay, and Liang Hong ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b13403 • Publication Date (Web): 04 Oct 2018 Downloaded from http://pubs.acs.org on October 9, 2018

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ACS Applied Materials & Interfaces

Formation of Icephobic Surface with Micron-scaled Hydrophobic Heterogeneity on Polyurethane Aerospace Coating

Yeap-Hung Ng a Siok-Wei Tay,b Liang Hong*a

a

Department of Chemical & Biomolecular Engineering, National University of Singapore, 4 Engineering Drive 4, Singapore 117585, Singapore b

Institute of Materials Research and Engineering, 2 Fusionopolis Way, Innovis, #08-03, Singapore 138634, Singapore

* Corresponding author E-mail address: [email protected]

Keywords: icephobic coating, segment copolymer, hydrophobic heterogeneity, polyurethane, aerospace coating, ice adhesion strength, washing scrubbing.

ACS Paragon Plus Environment

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ABSTRACT: Development of an anti-icing surface on a desired industrial coating patch/object has been the persistent challenge to several industries, such as aviation and wind power. For this aim, performing surface modification to implement the ice-phobic property on existing commercial coatings is important for practical applications. This work accomplishes an icephobic coating overlying a PPG aerospace polyurethane (PU) coating. It manifests a clear capability to delay formation of frost as well as to reduce adhesion strength of ice. This icephobic coating is sustained by a unique hydrophobic heterogeneity in micron-scale of segregation, which is realized through solution casting of a specific copolymer consisting of random rigid and soft segments, namely poly(methyl methacrylate) and poly(lauryl methacrylate-2-hydroxy-3-(1-amino dodecyl)propyl methacrylate), respectively. A wrinkled pattern developed over the coating is observed because of the diverse traits between these two segments. Besides, the OH/NH groups of the soft segment are crosslinked by a diisocyanate monomer upon drying and curing to strengthen the coating. More importantly, integration of a small dose of paraffin wax into the copolymer induces a spread of soft microdomains on the winkled pattern surface. It is hypothesized that this dual heterogeneous assemblies are responsible to the icephobicity since they instigate distinct interactions with condensed water droplets. Lastly, the thermoelectric cooling (Peltier effect) and the adhesion strength of ice on the typical coatings were assessed. This investigation also includes examination on the icephobic durability of coating, which is enhanced when a small amount of polyethylene oligomer is incorporated into the coating.


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1. INTRODUCTION

Formation of ice on airfoils of aircraft and wind turbine is of most concern since it adversely affects the aerodynamics of equipment causing safety concerns and also compromises performance. In contrast to the existing ice-removing approaches utilizing fusion, scraping, ultrasonic actuation and dissolution to remove ice1-3, fabrication of an intrinsically icephobic surface is obviously more advantageous because of its low cost and convenience. To date, two typical types of surface structures have been identified to be icephobic: (i) Superhydrophobic surface consisting of lowenergy molecular chains such as perfluoro-carbon or long hydrocarbon chains and rough topography in micron and nano scales, known as lotus-leaf inspired superhydrophobic surfaces (LLISS),4-5 (ii) Slippery liquid-infused porous surfaces (SLIPS) comprising a porous substrate and a non-volatile lubricating liquid layer held by the surface pores.6-8

An anti-icing surface of LLISS type can be realized by several strategies: (i) incorporation of nanoparticles with a hydrophobic shell, such as long aliphatic chain-anchored nanoparticles, into a coating matrix9-10 or distribution of a layer of hybrid hairy Janus sub-micron particles with hydrophilic and hydrophobic half shells on a substrate,11 (ii) incorporation of hydrophobic organic molecular moieties into a sol-gel coating matrix,12 and (iii) fabrication of a rough/hierarchical hydrophobic coating by the approaches of top-down or/and bottom-up and self-assembly.4,13-14 Compared with the demand for low surface energy, surface roughness is more critical in attaining an icephobic surface.15 However, the complex topographical features of a LLISS might encourage ice nucleation due to the curvature effect16 and hence, facilitate ice adhesion compared to a smooth


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surface with similar surface composition in humid surrounding. There is a lack of understanding on the impact and accumulation of super-cooled water droplets on LLISS surfaces poses challenges to the development of icephobic coatings.17-20 On the other hand, inspired by Nepenthes pitcher plant, Aizenberg et al.21-22 first realized an icephobic SLIPS, in which a stable and low hysteresis lubricant over-layer is maintained by infiltration and subsequent confinement of a waterimmiscible liquid in a porous nanostructured surface. The SLIPS surface allows condensed water droplets to slide off before freezing and lowers ice adhesion strength down to 15 kPa at -10 °C. Various research groups have reported similar tactics, involving typically silicone oil-infused polydimethylsiloxane (PDMS) coating,23 lubricant-impregnated periodic silicon microposts,24 hydrophilic/ hydrophobic sorbent loaded with water-glycerin mixture,25 rough amphiphilic coating with infused polyol,26 anti-icing polyurethane coating with an aqueous lubricating layer27 and etc. Following the pursuit for low energy composition and hierarchical nano surface structure, the durability of performance of an icephobic polymer coating has been an important goal, for which the self-lubricating aqueous layer is an inspiring concept.28 A very recent study has realized such a non-frozen water layer through interfacial hydrogen bonding to intervene freezing of water.29 Nevertheless, both LLISS and SLIPS designs have yet to manifest feasibility for pilot testing by far. This is mainly because a practical aerospace icephobic coating must satisfy not only with easy maintenance and repair operation (MRO) but also regulations for aerospace paint, such as Aircraft Painting and Finishing by FAA.30


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Scheme 1. Synthesis of the segment copolymer (upper) Graphical illustration of the formation of the soft hydrophobic microdomain induced by paraffin wax molecules (bottom).

At present, the commercial aerospace coatings generally use PU-based material because of its excellent performance properties.31-32 Nevertheless, the typical aerospace PU topcoat presents a water contact angle hysteresis (CAH) around 30° and hence is weakly ice-proof. Therefore, to realize an icephobic PU aerospace topcoat, it is rational to modify the surface of an existing aerospace PU topcoat. This study implements ice repellency to the surface of a commercial aerospace PU topcoat (Desothane HS, PPG) through overlying it with a thin coating. The coating formulation is made of a segment copolymer, a diisocyanate curing agent and a small amount of paraffin wax as icephobic promoter. The segment copolymer (SCP) is made by free radical copolymerization of methyl methacrylate (MMA), lauryl methacrylate (LMA) and an adduct monomer (GMA⊥DDA) of glycidyl methacrylate (GMA) and 1-dodecylamine (DDA) (Scheme 1). As MMA has an apparently higher copolymerization reactivity ratio than the other two rather similar monomers, this difference results in random PMMA and P(LMA–GMA⊥DDA) segments in the copolymer obtained. The PMMA segments provide dimensional stability of the coating made by solution casting, whereas the P(LMA-GMA⊥DDA) segments constitute, together with a small dose ( totally < 5 wt.%) of wax/PE oligomer incorporated, soft hydrophobic domains. In these domains, wax/PE are dissolved in the long aliphatic side chains, which has been proved by the differential scanning calorimetry. This trait is achieved because of the thermodynamic compatibility between the long aliphatic side chains and wax/PE species.33 In consequence, the resulting coating surface, possessing soft hydrophobic microdomains, P(LMA-GMA⊥DDA), and rigid PMMA agglomeration (Scheme 1, bottom graphic), demonstrates a significant improvement


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in delaying icing of the condensed water-droplets. Besides, we also examined the critical shear stress between ice and coating surface in contrast to the pristine PU topcoat by pulling the sample from a bulk of ice at a constant pulling rate, which is similar to a testing method summarized in an up-to-date review on the measurement of ice adhesion to solid substrates.34

2. EXPERIMENTAL

2.1 Materials: 1-Dodecylamine (DDA, Acros Organics, 98%), 2,2'-azobis(isobutyronitrile) (AIBN, > 98.0%(N), Tokyo Chemical Industry Co., Ltd.), 1,1-azobis(cyclohexanecarbonitrile) (ACHN, 98%, Aldrich), paraffin wax (Merck, granular, mp. 56-58 ºC), polyethylene oligomer (PE, average Mw~4,000 by GPC, average Mn~1,700 by GPC, Aldrich), ethanol (Absolute for analysis, EMSURE®, Merck KGaA), toluene (99.99%, Analytic reagent grade, Fisher Scientific), dibutyltin dilaurate (DBTDL, 95%, Aldrich), isophorone diisocyanate (IPDI, >99.0%, Tokyo Chemical Industry Co., Ltd.) were used as received. Glycidyl methacrylate (GMA, 97%, Aldrich), methyl methacrylate (MMA, 99%, Aldrich) and lauryl methacrylate (LMA, 96%, Aldrich) were passed through a short column of neutral alumina to remove the inhibitor before used. Aluminum alloy plate (7075-T6), polyurethane topcoat (Desothane HS 8000J0053, Gloss Green, PPG Industries), topcoat activator (8000B, PPG), thinner (8000C1, PPG), primer coat (Urethane compatible high solids primer, 7755A, PPG), primer activator (7755BE, PPG) were used as received.

2.2 Synthesis of Adduct Monomer, 2-hydroxy-3-(1-amino dodecyl)propyl methacrylate: DDA (8.15 g, 43.98 mmol) was added to a one-neck round bottom flask equipped with a magnetic stirrer


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and then sealed with a rubber septum. The solid was heated to 45 °C in an oil bath for about 20 min to form a clear liquid. GMA (6 mL, 43.98 mmol) was then introduced using a syringe and the mixture was stirred at 45 °C for 8 h to complete the synthesis of the adduct monomer, which is dubbed by a simple short form, GMA⊥DDA. 2.3 Synthesis of the Copolymer Coating Solution and the Related Control Solutions: For a typical synthesis, GMA⊥DDA (2.04 g, 6.24 mmol), MMA (1.56 g, 15.60 mmol), LMA (0.40 g, 1.56 mmol), paraffin wax (0-155.9 mg), PE (0-41.4 mg), AIBN (1 mol% with respect to total monomers) and toluene (20 g) were added to a one-neck round bottom flask equipped with a magnetic stirrer and then sealed with a rubber septum. The mixture was purged with argon for 20 minutes, and then heated in an oil-bath at 70 ˚C to initiate the free radical polymerization. After reaction for 24 h, the polymer solution obtained was used directly for coating without further purification. There were negligible monomers in the polymer solution formed according to chromatography analysis, implying that the monomer ratio in the copolymer formed is about the same as that in the feed although individual polymer chains have somewhat different composition from this ratio. With the exception of different wax paraffin/PE loadings (Table 1), three control coating solutions were also prepared by varying the polymerization feed. They are homopolymer PMMA, copolymer poly(PMMA-LMA), and the segment copolymer poly(MMA-LMAGMA⊥DDA), which are free of wax paraffin-PE additives.


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Table 1. Recipe for the Preparation of Copolymer Coating Solution

Sample

GMADDA/LMA/ MMA (mol ratio) a

Wax/(GMADDA +LMA) (wt.%)

PE/(GMADDA +LMA) (wt.%)

Wax (mg)

PE (mg)

S0 (or SCP)

4/1/10

-

-

-

-

S0.8/0

4/1/10

0.8

-

9.7

-

S3.2/0

4/1/10

3.2

-

39.0

-

S12.8/0 b

4/1/10

12.8

-

155.9

-

S0/1.7

4/1/10

-

1.7

-

20.7

S3.2/1.7

4/1/10

3.2

1.7

39.0

20.7

S3.2/0.85

4/1/10

3.2

0.85

39.0

10.4

(a) GMADDA/LMA/MMA = 4/1/10 (mol/mol) = 13/2.5/10 (mass/mass) and the total mass of the three monomers (2 g) was charged to the polymerization system. (b) The samples are labelled by Sx/y where x: wt.% of paraffin wax and y: wt.% of PE with respect to the amount of the two soft monomers. Sample S12.8/0 was prepared only for conducting DSC thermal analysis.

2.4 Implementation of an Ice-Repellent Surface on the PU Topcoat: 1 g copolymer coating solution (Table 1) was weighed in a dried 1.5-mL vial, followed by addition of isophorone diisocyanate (IPDI, 0.4 –OH equiv. with respect to the GMA⊥DDA unit) and dibutyltin dilaurate (DBTDL, 0.5 mol% with respect to IPDI) using micropipette. After mechanical agitation for 30 s, the mixture was immediately applied evenly onto the PU topcoat on Al plate, subsequently the sample was subjected to thermal curing at 80 °C for 20 h.


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2.5 Structural and Property Characterizations: Icephobic coating samples for differential scanning calorimeter (DSC) and atomic force microscopy (AFM) were prepared on microscope glass slide. DSC analysis was performed on a TA Instrument (DSC Q100 Photo Differential Scanning Calorimeter) using 2-3 mg sample and a cyclic scanning profile: 25 °C to 120 °C at 5 °C /min, 120 °C to -20 °C at 5 °C /min and -20 °C to 120 °C at 5 °C /min. The second heating cycle was recorded accordingly. The surface contour and roughness of the coatings were determined by AFM (Bruker Dimension ICON) using tapping mode with scanning frequency of 0.6-1.0 Hz. The 2-D depth profile and phase image were acquired from build-in software (NanoScope Analysis 1.40). Lastly, the morphology of the PU topcoat and the icephobic coating developed on it were recorded on a Field emission Scanning Electron Microscope (FE-SEM, JOEL JSM-6700).

2.6 Contact Angle Measurements: The apparent water contact angles (CA) on different surface were measured by the sessile drop method35 under ambient atmosphere using a contact angle goniometer (Ramé-Hart, Model 100-00). Deionized water droplets (5 µL) were dropped onto the sample surface using a built-in dispenser, and the average value of five measurements made on different locations was recorded. The CAH range was determined through measurement of the advancing and receding angles (θA and θR) as normally defined.36

2.7 Determination of the Time Delay of Heterogeneous Ice Nucleation: The testing samples were placed horizontally on a Peltier cold plate (TeCa, AHP-1200CP, America Corporation) in the research laboratory where the temperature is in the range of 22 °C- 23 °C and relative humidity of 70% or slightly below. To avoid the gravitational effect, we aligned the testing samples (3.5 × 3.5 cm) horizontally on the Peltier cooling stage. The plate is therefore gradually cooled to -10 °C by


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taking about 15 min. A thermocouple is fixed near the testing samples on the cooling plate. The condensation of humid air and the subsequent freezing process of the water droplets was recorded and monitored by using a video camera (Canon IXUS 1000 HS) or a digital USB microscope (ViTiny UM12 USA).

2.8 Determination of the Critical Shear Stress between Ice and Coating Surface: The critical shear stress between ice and coating surface is determined by using a setup shown in Figure 1, with double coaxial metal cylindrical chambers. 250 mL DI water was placed in the inner cylindrical chamber and a rectangular sample with double-sided coating was then immersed in water in the chamber in an upright position to let water cover the coating. Subsequently, this chamber was subjected to freezing in a fridge for 24 h. This metal chamber was then quickly placed in the outer chamber and the gap between two cylindrical chambers was filled with dry ice to assure the freezing state in the inner chamber. The sample entrapped in the ice at the top of the inner chamber was mounted to a mechanical testing machine (Instron 5569 Table Universal testing machine). The Universal testing machine was then started to slowly pull the sample out of the ice block at a rate of 5 mm/min. The graph of load against shift distance was then plotted (Bluehill software). The shear stress  is computed by the formula: 𝜎 = 𝑙𝑜𝑎𝑑⁄(𝐹𝑅 − 𝑙) × 𝑤 × 2 (𝑠𝑖𝑑𝑒𝑠), where FR (full removal) stands for the length of the rectangular coating embedded in the ice block prior to the pulling test, which can be read from the load-shift diagram, l is the distance shifted at the moment when a load reading is taken, and w is the width of sample.


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Figure 1. In-house setup for the determination of the critical shear stress between ice and coating surface, left: Instron 5569 Table Universal testing machine coupled with icing device, right: dissection of icing device metal chambers, where the outer chamber holds dry ice for keeping the icing in the inner chamber.

2.9 Sample Washing and Scrubbing Test: There is no direct ASTM procedure for testing and quantifying the durability of an icephobic coating thus far. We have devised a test protocol, adapted from ASTM D4828-94,37 which is the standard testing method to check practical washability of an organic coating. The sample plate (reference PU, Sample S3.2/0 and S3.2/0.85, 3.5 × 3.5 cm) was mounted onto the middle of rotating platen of Buehler MetaServ® 2000 Grinder Polished Unit, using double-sided adhesive tape. Heavy duty Scoth-Brite® scrub sponge (10 × 7.5 × 3.5 cm, 3M Singapore Pte. Ltd) was placed on top of the testing sample. A uniform compressive force of 2.4 N was imposed on the sponge by a pair of pellet dies, which result in an approximately constant normal stress of 320 Pa during the washing process. Tap water was supplied to moisten


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the sponge during the course of washing scrubbing test. The rotating platen was set at 40 rpm, while the sample was scrubbed against the wet sponge for 3 min, to simulate an effective washing motion of 120 strokes. After washing scrubbing, the coating surfaces were dried by and kept under ambient condition for one day before further examination.

3. RESULTS AND DISCUSSION

3.1 Essential of Segment Copolymer (SCP) for the Development of Soft Hydrophobic Micro-Domains

Of the three monomers, MMA, LMA and GMA⊥DDA (that are copolymerized with the molar ratio of 10:1:4), the latter two have apparently longer alkoxyl groups than MMA and should possess lower copolymerization tendencies with MMA. This size effect has been identified in several previous studies on the free-radical copolymerization of MMA (r1 1.38) with LMA (r2 0.68).38 On the contrary, LMA and GMA⊥DDA are structurally analogous and hence, the copolymer segments formed by them have homogeneous composition. As such, the resulting tricomponent copolymer possesses the following characteristics: (1) It consists of randomly arrayed PMMA and P(LMA–GMA⊥DDA) segments in each copolymer chain, (2) There are a pendant amine (-NH2) and a pendant hydroxyl (-OH) groups at each monomer unit of GMA⊥DDA, which are close to the backbone. Hence, the diisocyanate monomer, IPDI, was employed to crosslink with these two functional groups with the aim to secure the soft segment agglomerations in each soft microdomain. When the coating film is formed of the copolymer P(MMA/LMA–


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GMA⊥DDA) alone, i.e. S0 in Table 1, on the PU topcoat, it exhibits a pattern of wrinkles according to the comparison of SEM images (Fig. 2 a vs. b). This surface morphology could be attributed to the segmented chain structure since it drives uneven local contractions, due to inconsistent thermodynamic traits between rigid and soft segments as well as their different mechanical properties, over the coating film while drying. It is presumed that the solvent vaporization during drying accompanies faster coalescences of the rigid segments than the soft segments.39 As a result, there are more soft segments near crinkles surrounding aggregates of the PMMA segments. The occurrence of PMMA aggregates can be verified by AFM and DSC analysis (Fig. 3 and Fig. 4), which retrospectively verifies the presence of PMMA segments in the copolymer. On top of this observation, when paraffin wax (0.8 wt.%) was first incorporated into the coating formulation (S0.8/0, Table 1), the resulting surface reveals soft micro-domains owing to association of paraffin wax molecules with some aliphatic side chains of the soft segments (Fig. 2c). These soft microdomains are embedded on the wrinkled surface. Raising the dose of paraffin wax aggravates the trend to form soft micro-domains and uneven local contraction extent as shown in coating S3.2/0 (Fig. 2d), where both complex and extensive wrinkled patterns are found. Reciprocally, the paraffin molecules prompt aggregation of aliphatic side chains to form the soft micro-domains as illustrated in Scheme 1. Meanwhile, such a surface pattern is fixed through the subsequent thermal curing that facilitates crosslinking of IPDI with pendant hydroxyl and amine groups. To verify the miscibility of paraffin wax with the soft segment of SCP, the pristine SCP and the blends of it with different amounts of paraffin wax were examined using DSC (Fig. 3). SCP exhibits a broad glass transition regime of 40-60 °C. It supports the presence of MMA segment since if a random copolymer is formed its Tg will be around 8 to 10 oC according to Gordon-Tylor equation.40 When SCP is blended with 12.8 wt.% paraffin wax, an independent wax phase exists manifesting a strong


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melting peak at ~ 57 °C (S-Table 1). However, when the loading of paraffin wax is reduced to 3.2 wt.%, the crystalline phase disappears, indicating that the reciprocal chain association between the long (C12) aliphatic side chains and wax molecules becomes predominant; namely the wax molecules are softened because they no longer crystallize. This dissolution-enhanced stability can be well justified by parafilmM in which paraffin wax is homogeneously distributed in polyolefin.


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(a)

(b)

(c)

(d)

(e)

(f)

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Figure 2. The scanning electron micrographs of the coatings: (a) the PU topcoat, (b) S0, (c) S0.8/0 in which a soft micro-domain is labelled by a dash circle, (d) S3.2/0, (e) S0/1.7, and (f) S3.2/1.7.


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S0.8/0 S3.2/0 S12.8/0 S0

Figure 3. DSC profiles labelled by the four respective polymer samples. The detailed analytical procedure is provided in section 2.5.

With the exception of using paraffin wax (with an average of 25 carbons) as a hydrophobic enhancing agent, the PE oligomer (ca. 120 carbons) was employed to combine with SCP, leading to coating S0/1.7 (Fig. 2e). Remarkably, no clear soft micro-domains display in S0/1.7 whereas the surface shows more obvious and crowded wrinkles in submicron scale. This morphology is proposed to be the role of long PE chains in extensively interlocking the soft hydrophobic segments. As a result, extra stability of the coating could be gained and the contraction of SCP


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chains takes place at more spots. Therefore, the combination of wax and PE gives rise to a topography (Fig. 2f) that integrates surface characteristics of Figure 2c and 2e.

(a)

(b)

(c)

Soft Domain Rigid Boundary

Figure 4. AFM surface topology characterization with 2-D depth profiles (upper row) and phase images (middle) and 3-D depth profile (lower): (a) S0 (brown dots: PMMA segments, yellow dots: P(LMA-GMA⊥DDA) segment) shown in the phase image, (b) S0/1.7, and (c) S3.2/0.


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With respect to the water contact angle (WCA) measurement of the above coating surfaces, pristine coating (S0) on the PU topcoat just depicts a slightly higher WCA but a larger contact angle hysteresis (CAH) compared with the PU topcoat (Table 2). With regards to this difference, AFM topography exhibits different roughness levels between PU topcoat (15.6 nm) (S-Fig. 1a) and S0 (1.77 nm) (Fig. 4a). The increase in CAH suggest that S0 lacks Cassie-Baxter-state41 as compared with the PU topcoat. The AFM phase image of S0 first displays the aggregated PMMA phase in sub-micron scale. Once a very small amount of paraffin wax is added into S0 to form S0.8/0, we observed an increase in WCA and a significant decrease in CAH but a slight increase in RMS roughness. The formation of soft micro-domains over the surface (Fig. 2c) is responsible to the change. This outcome is consistent with the known fact that icephobicity requires a high WCA and a low CAH as well as flat coating surface.42 It is also found that coating S3.2/0 displays a higher distribution of soft micro-domains and more wrinkled surface (Fig. 2d and Fig. 4c), and in consequence, the highest WCA and lowest CAH. Compared to S0, S3.2/0 shows a clear 3-D pattern (Fig. 4C) corresponding to its SEM surface morphology (Fig. 2d). On the other hand, coating S0/1.7 presents a trivial hydrophobic enhancement implying formation of Wenzel-wetting regime, i.e. a low WCA and a large CAH. This inference is supported by the AFM phase and 3-D depth images of S0/1.7 (Fig. 4b) that shows the highest RMS roughness (~23 nm) attributed to interlocking between PE and the soft segments, which jacks up segregation of PMMA. Clearly, Cassie-Baxtertype coating S3.2/0 and Wenzel-type coating S0/1.7 are derived, respectively, from S0, depending upon the chain length of the alkane added. Specifically, increasing surface roughness is undesirable to improve hydrophobic heterogeneity. In short, the hydrophobic heterogeneity of the coating, caused by the separate aggregations of rigid and soft segments as well as the dissolution of wax in the soft aggregates, sustains higher WCA and smaller CAH. Furthermore, aiming to augment the


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stability of the hydrophobic enhancement, PE is assimilated with wax while the synthesis of SCP, forming S3.2/1.7 and S3.2/0.85. Compared to S0/1.7, these two coatings demonstrate significantly improved hydrophobic heterogeneity due to the incorporation of wax (Table 2 and S-Fig. 1b). Meanwhile, the effect of PE on stabilizing icephobic performance will be evaluated in the last section. It could therefore be concluded that the hydrophobic heterogeneity, relying on the soft microdomains spread and embedded over the wrinkled surface, is an essential topography to grant icephobicity.

Table 2. Characterization of Samples Roughness and Hydrophobicity

Sample

Equivalent RMS roughness a (nm)

Water contact angles (CA)

Advancing CA (θA)

Receding CA (θR)

CA Hysteresis (θA - θR)

PU

15.6

80.6 ± 0.4

82.1

54.2

27.9

S0

1.77

84.6 ± 0.9

89.2

41.3

47.9

S0.8/0

2.57

108.9 ± 0.4

114.6

103.5

11.1

S3.2/0

6.96

109.7 ± 0.4

114.7

106.3

8.4

S0/1.7

23.3

83.2 ± 1.1

88.1

43.9

44.2

S3.2/1.7

10.5

108.6 ± 0.6

112.2

100.3

11.9

S3.2/0.85

14.5

109.5 ± 0.6

114.4

102.2

12.2

(a) The root-mean-square (RMS) surface roughness level is obtained from the depth profile image (upper row Fig. 4) by applying the power spectral density analysis.


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Figure 5. Cooling dynamics of Peltier cooling stage for 30 minutes averaged over 4 repetitive runs. Inset: a photo of the cooling plate on which thermal couple attached is adjacent to testing samples to display the temperature change of the plate.


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(a)

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(b)

S0

S3.2/0

S0.8/0

S0/1.7

S3.2/1.7

S3.2/0.85

(c)

(d)

PU

10 min

15 min

20 min

30 min

Figure 6. A study on delay heterogeneous ice nucleation process over a period of (a) 10 min, T: ~ -6.3 °C, with pertinent notation for samples arrangement, (b) 15 min, T: ~ -8.7 °C, (c) 20 min, T: ~ -9.9 °C, and (d) 30 min, T: ~ -11.4 °C. The bottom row provides frosting on the aerospace PU topcoat. Icing starts from the edges in all samples.


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3.2 Impact of the Micron-scale Hydrophobic Heterogeneity on the Delay of Condensation Frosting

The cooling dynamics imposed by the cooling plate was reasonably consistent under constant ambient condition as detailed in Section 2.7. The plot of temperature-time profile (Fig. 5) was first established through averaging over four cooling runs. The condensation of humid air and the subsequent freezing process of the water droplets were recorded using a video camera mounted above the cooling plate, over a period of 30 min. The relative humidity (RH) in the environment where the test of icing on different coatings was conducted is close to 70%. The higher the RH the closer to the cooling plate as RH is the ratio of condensation rate to vaporization rate. To assure exactly the same cooling dynamics, we put the samples listed in Table 1 together to carry out the test (Fig. 6a). The onset of condensation frosting can be observed within the field-of-view, as evidenced by the rapid onset of opacity. PU shows a good propensity towards frost formation within 10 to 15 min in which the actual icing procedure commences from the edges of each sample, followed by invasion of frost wave front migrating towards the center of the sample. More noticeably, relative to the PU topcoat, coating S0 and S0/1.7 display no justifiable delay of icing. This phenomenon can be interpreted from the WCA and CAH data of these three coatings since they are weak in hydrophobicity. Contrary to this surface trait, the presence of soft micro-domains over the rest coating surfaces effectively defers frosting in condensation droplets to different extents. This observation was examined by repeating four rounds of the cooling test, consisting of cooling, warming, and drying steps.


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To visualize microscopically the course of icing on the different coatings in question, the central region of each sample was scrutinized under a digital USB microscope using a magnification of 50 and field-of-view dimension of 1.8 × 2.5 mm. Taking moisture condensation and frost formation on coating S3.2/0 as example (Fig. 7), the spherical condensed droplets can growth up to 50 µm without any prominent aggregation of droplets at -1 °C (7c), manifesting a Cassie-Baxterstate wetting regime. More noticeably, this growth of highly spherical water droplets spans over a long period (21.7 min) until onset of icing in the area of surveillance at ~ -10.3 °C (7c). At this moment, the selected water droplets (the yellow dashed circle) started turning opaque, which marks the onset of frost formation. The same surveillances at the moisture condensation and icing on the PU topcoat (with onset of frosting at ~ -5.1 °C ) and S3.2/0.85 (with onset of frosting at ~ 10.0 °C) were conducted as well (Supporting Information S-Fig. 2 and 3). Video (Image_Icing.mpS3) records icing courses on these three coatings. They display different capabilities in terms of duration to retain supercooled water droplets.


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(a)

(b)

(c)

(d)

(e)

(f)

Figure 7. Microscopic observation of frost formation on S3.2/0 by choosing field-of-view dimension of 1.8 × 2.5 mm, Scale bar: 500 µm. (a-b) visualizing the progress of water condensation at t = 2 min (T~ 9 °C) and t = 5 min (T~ -1 °C), respectively, (c) onset of frost formation at t = 20 min (T~ -9.7 °C), (d) spreading of frosting wave front over the entire field-of-view at t = 21.4 min (T~ -10.3 °C), and (e-f) subsequent build-up of ice crystals from t = 25 to 30 min (T~ -10.8 to 11.4 °C).

From the semi-quantitative perspective, this cooling test can be divided into two stages: the liquid-holding time 𝑡1 , describing the duration from the moment when condensate droplets are formed to the formation of very first opaque frost crystal on the plate; the freezing time 𝑡𝑓 , describing the subsequent duration required for the propagation of icing from edge to center over each sample. Both 𝑡1 and 𝑡𝑓 of each sample are displayed in Figure 8, where 𝑡2 is defined as the


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length of the entire testing course. The uncertainty ranges of all the duration are determined on the basis of the four rounds of cooling test. Moreover, spreading of the freezing front is useful to elucidate the anti-icing property of a coating, the propagation velocity 𝑣𝑓 can be approximated as 𝑣𝑓 = 𝐿𝑓 ⁄𝑡𝑓 = 0.5 × (2 × 𝐿2 )0.5 ⁄𝑡𝑓 (refer to the caption of Fig. 8). According to 𝑡𝑓 and 𝑣𝑓 values (the last two columns), the icephobic behavior increases by following the sequence: PU and S0 < S0/1.7 < S3.2/1.7 and S3.2/0.85 < S3.2/0 and S0.8/0. It is in accordance with the tier of WCA and CAH (the opposite order) presented in Table 2.

Figure 8. Delay icing evaluated macroscopically on the basis of four cooling cycles. Left figure, t1: the liquid-holding time, t2: time to develop full ice coverage, 𝑡𝑓 = 𝑡2 − 𝑡1 : icing duration; 𝑣𝑓 spreading velocity of freezing wave front: 𝑣𝑓 = 𝐿𝑓 ⁄𝑡𝑓 = 0.5 × (2 × 𝐿2 )0.5 ⁄𝑡𝑓 , where Lf is the half of the diagonal of the square sample and L is its width. Right figure, temperatures based on interpolated data of imposed Peltier cooling profile (Fig. 5).


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The above icing delay order reflects the vital role of the soft microdomains as well as the wrinkled patterns underneath. The basis of this conclusion could be concentrated just to coatings S3.2/0 and S3.2/1.7. Although they show similar contact angles, S3.2/0 demonstrates a lower surface hysteresis and surface roughness, signifying better hydrophobic heterogeneity or Cassie–Baxter state as described in section 3.1 and thus, stronger icephobicity. It could therefore be envisioned that on coating S3.2/0 super-cooled liquid water droplets could remain because of the interfacial perturbation effect. Namely, these water droplets undergo icing mainly via the heterogeneous nucleation mechanism. Several recent publications

43-44

have considered that the local chemical

composition of polymer affects icing of the overlying water droplet because of their seeding effect. In the present case, each droplet covers numerous soft microdomains as well as the wrinkled patterns surrounding them. As these surface micro-pitches are physically and chemically dissimilar, it can be assumed that the nucleation of water near the interface is disturbed at temperatures below the normal freezing point. In contrast to S3.2/0, coating S3.2/1.7 possesses rougher surface (Fig. 2f vs. 2d and S-Fig. 1b vs. Fig. 4c) that shows more Wenzel-wetting characteristic, it would therefore wage a higher nucleation rate relative to the Cassie–Baxter state.45 It is also noteworthy that as a control, a uniform paraffin wax on PU topcoat shows no improvement on delaying frost formation because the pristine wax is neither soft at the testing temperature nor heterogeneous. 3.3

Reduced Ice Adhesion on Icephobic Coating and the MRO Assessment

It has been identified from the previous examination on the delayed icing that coating S3.2/0 demonstrates the strongest icephobicity among the testing samples. It was therefore selected to conduct ice adhesion test using the device as shown in Fig. 1. In parallel, the PU topcoat with


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exactly the same testing area as S3.2/0 was the control of this examination. The measurement outcome (Fig. 9) is presented by the profiles of sample load ~ shift distance as well as shear stress ~ shift distance, which reflect the resistance against moving of sample. The first steep peak is regarded as the adhesion strength of ice on coating. After that, there is a residual stress-motion trace, which describes the fact that there are some tough ice knobs remaining on the coating. Thus, the friction of these lumps with the ice block leaves behind a residual stress trace. The pristine PU topcoat shows an adhesion-strength of 740 kPa and an obvious residual stress trace dragging until 51 mm, defined as the distance after which the resistance is free. It should be underscored that Fig. 9 presents the true rather than the engineering shear stress, leading to the highest final peak due to the least area as explained in section 2.8. The residual work spent according to the load ~ shift distance curve is roughly 7.5 J. On the contrary, S3.2/0 shows an adhesion-strength of 390 kPa and requires 2.25 J of residual work, which are about 53% and 30%, respectively, the PU topcoat incurs. The above test manifests that coating S3.2/0 possesses not only delayed frost formation but also a largely reduced ice adhesion strength as well as ice-slippery surface.


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FR

FR

Figure 9. The load ~ shift distance profile (upper row); The true shear stress ~ shift distance profile (lower), where the full removal (FR) stands for the shift distance when the bottom coating brink is lifted off the ice bulk. The true shear stress is calculated using the coating area remaining in ice bulk throughout the entire test. Left: the PU topcoat, Right: the S3.2/0 coating.


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Figure 10. The impact of washing scrubbing on delay icing on the three selected coatings: row (a): the as-prepared coatings, and row (b): the washing-scrubbed coatings. Left column and right column compare two cooling temperatures, -8.9 and -9.7 oC, reached, respectively.

Development of a realistic aerospace icephobic coating has to include surface stability against MRO operations. This is because any deterioration in the coating layer would lead to continuous loss of anti-icing capability. In the last part of this study, the PU topcoat, S3.2/0 and S3.2/0.85 coatings were chosen to justify the icephobic stabilization role of PE, in which the PU topcoat serves as benchmark. By using the amended ASTM procedure as mentioned in Section 2.9, the three coatings were then subjected to the delayed icing test. We select the images of coatings observed at icing durations of 15 and 20 min, respectively, for comparison (Fig. 10). The first row displays the icing situation of the pristine coatings whereas the second row displays the icing situation after washing and scrubbing. The test reveals a reduction in the delayed icing capability on the scrubbed coating S3.2/0 because its surface ice coverage was comparable to the PU topcoat after 15 min icing. Nonetheless, coating S3.2/0.85 emerged clearly a better capability of delayed icing than S3.2/0. This


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implies the role of 0.85% PE in retaining the surface heterogeneity and hence, enhance its resilience to washing and scrubbing albeit a slightly weaker performance as compared with its unrubbed counterpart. On the basis of this proxy MRO treatment, we also found that annealing the scrubbed S3.2/0 and S3.2/0.85 coatings at the temperature close to the melting point of wax (60 oC) for a certain duration, e.g. 12 h, could resume their delayed icing capability as displayed in S-Figure 4. This result also demonstrates that the icephobic surface sustained by hydrophobic heterogeneity as defined above could be resumed since recovering this surface heterogeneity is thermodynamically spontaneous once soft chains gain sufficient kinetic energy through annealing. Besides, this test also suggests persistence of the paraffin wax and PE constituents in the coating since the initial icephobic property cannot be resumed otherwise. This positive feature suggests that this unique surface structure be potentially attractive towards an actual aerospace coating.

CONCLUSION

This investigation explores an alternative icephobic coating surface typified by a spread of soft hydrophobic microdomains on wrinkled patterns in majority, namely hydrophobic heterogeneity in micron scale. This surface microstructure, although is neither analogous to lotus-leaf inspired superhydrophobic surfaces nor to the slippery liquid-infused porous surfaces, requires similarly a low surface energy state. The icephobic surface developed in the present work is realized through a specific coating constituted by a copolymer consisting of rigid and soft segments and a low dose of paraffin wax (0.8 ~ 3.2 wt. %). The coating of this copolymer and wax formulation forms a hydrophobic and heterogeneous surface over a commercial aerospace polyurethane (PU) topcoat.


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The coating layers achieved with various wax contents display enhanced icephobicity characterized by the delayed icing through decelerating the icing propagation wave front across the surface after the onset of icing nucleation. More essentially, the best-performed coating manifests 47% decrease in ice adhesion strength and a 70% reduction in energy required to detach ice from the coating as compared with the pristine PU topcoat. The stability of the coating against water washing and scrubbing test was examined by measuring its impact on the delayed icing capability. It is found that incorporating a small dose of polyethylene oligomer with wax enhances the stability of the hydrophobic heterogeneity against washing scrubbing. Finally, the disturbed hydrophobic coating due to washing scrubbing could be recovered by annealing.

ACKNOWLEDGMENT: We thank the financial support of A*Star Singapore, Aerospace Programme (Project No.: 112 155 0706, 2014).

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Abstract Graphic


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Formation of Icephobic Surface with Micron-scaled Hydrophobic

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