Robust hyper-branched polyester-based anti-smudge coatings for self

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Applications of Polymer, Composite, and Coating Materials

Robust hyper-branched polyester-based anti-smudge coatings for self-cleaning, anti-graffiti and chemical shielding Ximing Zhong, Hengfeng Hu, Lei Yang, Jie Sheng, and Heqing Fu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b22447 • Publication Date (Web): 14 Feb 2019 Downloaded from http://pubs.acs.org on February 17, 2019

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ACS Applied Materials & Interfaces

Robust hyper-branched polyester-based anti-smudge coatings for self-cleaning, anti-graffiti and chemical shielding

Ximing Zhong,a Hengfeng Hu,a Lei Yang,b Jie Sheng,c Heqing Fu*a (a School of Chemistry and Chemical Engineering, Guangdong Provincial Key Lab of Green Chemical Product Technology, South China University of Technology, Guangzhou 510640, P.R. China; b College of Chemistry and Environmental Engineering, Shaoguan University, Shaoguan 512005, P.R. China; c School of Chemistry and Chemical Engineering, Guangzhou University, Guangzhou 510006, P.R. China) *Corresponding author. Tel: +86 020 87114919; Email: [email protected].

KEYWORDS: hyper-branched polyester, anti-smudge coatings, self-cleaning, anti-graffiti, chemical shielding, mechanical robustness

ABSTRACT˖ ˖In this work, a novel anti-smudge coating system was developed by using hydroxyl-terminated hyper-branched polyester (HBPE) as coating precursor, mono-hydroxyl-terminated polydimethylsiloxane (PDMS) as anti-smudge agent, and hexamethylene diisocyanate trimer (HDIT) as curing agent. The resultant coatings with 0.5 wt% PDMS content incorporated are highly transparent and liquid-repellent. They exhibit striking repellency against various liquids, and display remarkable self-cleaning performance. Water, hexadecane, salt solution, strong alkali solution,

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strong acid solution, pump oil and crude oil could slide off the coating surface without leaving any traces, and the dirt on coating surface could be readily removed by water or oil. Besides, these coatings show potential application for anti-fingerprint and anti-graffiti due to the exceptional repellency of coating surface against artificial fingerprint liquid, oil-based ink, paint and water-based smudge. Furthermore, they also possess superb chemical shielding ability and thus endow substrates with remarkable protection against exposure to harsh chemical conditions. Moreover, these coatings are mechanically robust against extensive abrasion, impact and bending without compromising anti-smudge properties, and they also exhibit excellent adhesion to various substrates. Therefore, these newly developed coatings have tremendous potential for widespread applications.

INTRODUCTION Anti-smudge coatings exhibit remarkable repellency against water- and oil-based liquids and prevent them from spreading over the coating surface, and, under certain conditions, make them roll or slide off the surface without leaving any residues. Therefore, these coatings are of great use in practical applications and highly desirable. When applied on smartphones, they reduce the deposition of fingerprint liquid and facilitate screen cleaning. On skyscraper windows, they help to inhibit stain formation and readily shed off the dirt on a rainy day, significantly facilitating our daily life. Several effective approaches are adopted for the preparation of anti-smudge


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coatings. The first approach is to create micro- and nanoscale rough surfaces with fluorinated moieties incorporated to provide superomniphobicity.1-6 On these rough surfaces, although oil and water droplets show exceptional contact angles (above 150°) and roll off readily,7-9 the inherent drawbacks resulted from the textured structures, including poor wearing resistance and optical transparency, limit their widespread applications and long-term use.10-13 To the best of our knowledge, on a smooth surface, the contact angles toward liquid droplets never exceed 120°,14,15 while droplets could slide off an anti-smudge surface without leaving any traces. As for slippery liquid-infused porous surfaces (SLIPS),14,16-18 that generally prepared by generating a porous surface and then filling the pores with a fluorinated lubricant, they exhibit remarkable repellency against water- and oil-based contaminants. However, the evaporation or the leaking of unbound lubricants is still a challenge that needed to be addressed, and the use of fluorinated liquids is potentially detrimental to environment and human beings. Compared with SLIPS, liquid-like monolayers19,20 seem to be more practical because they are prepared by covalently grafting liquid chains, such as polydimethylsiloxane (PDMS), on the pretreated substrates, which effectively avoids the loss of anti-smudge agents resulted from evaporation or leaking. Although they also render outstanding liquid sliding performance, they are vulnerable to wearing. Different from above coatings, smooth polymer-based anti-smudge coatings become more advantageous in practical applications because they possess tunable thickness and transparent appearance, bind well to various substrates and have anti-smudge agents firmly grafted in coating matrices, efficaciously addressing the



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challenges of poor optical transparency, inferior wearing tolerance, and the evaporation or leaking of anti-smudge agents as mentioned above. Similarly, such polymer-based anti-smudge coatings generally have low-surface-tension moieties incorporated to endow the surfaces with low surface energy, such as fluorinated components or PDMS containing components. Although using fluorinated components as anti-smudge agents is popular and effective,21-23 the drawbacks of expensiveness, degradation-resistance, bioaccumulation and high toxicity limit their widespread applications.24,25 As a counterpart, PDMS is widely used for its chemical inertness,

biocompatibility,

moderate

cost,

excellent

optical

property

and

environmental friendliness.26-28 Also, to prepare fluorine-free anti-smudge coatings, one end of PDMS chains covalently bound to coating matrices is more favorable than the one with both ends of chains bound because the unbound side chains facilitate to enrich and form a PDMS brush layer on coating surface, consequently providing superior liquid sliding capability.29-31 Aside from low surface energy, heavy crosslinking is indispensable for polymer-based anti-smudge coatings because insufficient crosslinking tends to result in surface reconstruction and thus deteriorates liquid-repellency.25 Therefore, in such coating system, coating precursor should bear high density of functional groups, then cured by specific curing agents or techniques to achieve heavy crosslinking.22,25,29,31 At the same time, we note that hyper-branched polymers are attractive for academic and industrial researchers because they possess the unique merits of highly-branched structures and multi-functionality,32-34 and show widespread application in the fields


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of drug carriers,35,36 catalysis37,38 and coatings39,40. Therefore, we envision that using hyper-branched polymers as coating precursors for smooth anti-smudge coatings would be favorable because they possess high density of functional end-capping groups, which could serve as crosslinking sites to be crosslinked by curing agents to form desirably sufficient crosslinking. And such highly crosslinked coatings are expected to exhibit remarkable mechanical robustness. To verify our speculations, in this study, mono-hydroxyl-terminated PDMS was used as low-surface-tension component to render the coatings with low surface energy, hydroxyl-terminated hyper-branched polyester (HBPE) with 12 hydroxyl groups per molecule was employed as coating precursor to provide sufficient crosslinking sites, hexamethylene diisocyanate trimer (HDIT) was selected as curing agent to crosslink the hydroxyl groups in this coating system to form heavy crosslinking. As proof-of-concept, we found that the as-prepared coatings exhibited remarkable self-cleaning performance, various liquids, such as water, hexadecane, salt solution, strong acid solution, strong alkali solution, pump oil and crude oil, glided down the coating smoothly with no trace left, and the dirt on coating surface could be readily removed by water or oil. Besides, they showed potential application for anti-fingerprint and anti-graffiti, and also endowed substrates with protection against extreme chemical corrosions. Moreover, they displayed mechanical robustness against extensive abrasion, impact, and bending, and demonstrated excellent adhesion to various substrates. Therefore, these versatile hyper-branched polyester-based anti-smudge coatings have great promise for widespread applications.



EXPERIMENTAL SECTION Materials. Hexamethylene diisocyanate trimer (HDIT, with a NCO content of 19.6 ± 0.3%) was purchased from Bayer and used without further purification. Mono-hydroxyl-terminated poly(dimethysiloxane) (PDMS) was kindly supplied from FOSHAN SILCHEM TRADING Co., Ltd. Hydroxyl-terminated hyperbranched polyester (HBPE) was supplied by Wuhan Hyperbranched Polymer Science & Technology Co., Ltd. Dibutyltin dilaurate (DBTDL), propylene glycol monomethyl ether acetate (PGMEA), dimethyl formamide (DMF), 1-methoxy-2-propanol, and acetic acid were provided by Aladdin. Concentrated sulfuric acid (H2SO4), anhydrous cupric sulfate (CuSO4), sodium chloride (NaCl), sodium hydroxide (NaOH), lactic acid, sodium hydrogen phosphate, methylene blue and hexadecane were procured from Tianjin Damao Chemical Reagent Factory and of analytical reagent grade. Red oil O used to dye hexadecane was purchased from Shanghai Yuanye Bio-Technology Co., Ltd. Paint was epoxy resin (E-44) supplied from Guangzhou Zhujiang Chemical Group Co., Ltd. Glass plates and tin plates with different dimensions were provided by local suppliers. Preparation of the anti-smudge coatings. HBPE (1.00 g) was dissolved in DMF (1.00 g), and then HDIT (2.26 g), DBTDL (0.01g), PGMEA (2.00 g) and different weight ratios of PDMS (0 wt%, 0.5 wt%, 1.0 wt%, 1.5 wt%, 2.0 wt% with respect to the total weight of HBPE and HDIT) were added and mixed thoroughly. Thereafter, the mixture was heated at 80 ℃ for 90 s and then was cast onto various substrates, such as glass plates and tin plates. The coated substrates were cured at 120 ℃


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for 1 h, and the cured samples were used for further investigations. Characterization and measurement. Fourier transform infrared (FT-IR) spectroscopy was measured with a Tensor-27 spectrometer (Bruker Optics, Germany) to confirm the structure of sample and KBr was utilized as the sample matrix. The optical transmittances of the coatings were measured on coated glass plates at three different places using a Hitachi U-3010 UV-vis spectrometer at the wavelength of 500 nm and a pristine glass plate was used as reference. Bruker atomic force microscopy (AFM) and Hitachi S-4800 scanning electron microscope (SEM) were applied to examine the surface morphology, and the root-mean-square (RMS) roughness of the coating surface was calculated from AFM image. Thermo Fisher Scientific ESCALAB 250Xi X-ray photoelectron spectroscopy (XPS) was employed to analyze the surface compositions of the coating prior to abrasion and after 5000 abrasion cycles. The wettability of the coatings was assessed by contact angles and sliding angles that were measured using a Dataphysics OCA40 Micro instrument at room temperature. Specifically, water droplet and hexadecane droplet used for contact angle measurements was in a volume of 5.0 μL, while water volume for sliding angle measurements at different abrasion cycles was 40.0 μL, and each reported value represented the average of five measurements on different places. To investigate whether there were free reagents existing in coating, a coating piece with a dimension of 8.0 × 5.0 × 1.0 mm3 peeled from a Teflon mold was immersed in acetone (0.5 mL) for 1 h in a sealed vial, and then the resultant acetone was used for FT-IR analysis. A PHS-3C pH meter was used to measure accurate pH value for strong acid solution and



strong alkali solution. Because corrosive agents applied would cause corrosion to polished tin plates, glass plates were used as substrates to investigate the liquid repellency of the coatings subjected to chemical shielding test. Specifically, the coated glass plates were immersed in 5.0 wt% NaCl solution, 0.5 M CuSO4 solution, H2SO4 solution (pH = 0), and NaOH solution (pH = 14) for 15 h respectively, rinsed with deionized water, heated at 50 ℃ for 2 h to dry the coated substrates before related measurements were implemented. A portable thick gauge (LZ-990 Kett, Japan) was used to determine the coating thickness. The coating hardness was determined by pencil test according to ASTM D3363. Pencils at different hardness from 9H to 9B were flattened and pressed against the coating surface at 45° until the pencil left no scratch on coating surface was found, and the one was the hardness of the coating. In the case of abrasion test, a piece of cotton fabric was placed on coating surface and a weight (200 g) was placed onto the fabric to enhance the effectiveness of abrasion. And one abrasion cycle included a back and forth movement crossed a distance of 12 cm at a moving speed of 4 cm s-1. Impact experiment was carried out using a membrane impact tester (QCJ-50, Shanghai Modern Environmental Engineering Technology). A hammer with a weight of about 1000 g fell down from a height of 50 cm to cause an impact on the coating. According to ASTM adhesion standard,41 adhesion test was implemented on coated glass plate and coated tin plate. Cross-cutting pattern (10 × 10 lines) was cut on coating surface with a blade, and the distance between each line was about 1 mm. Thereafter, a 3M Scotch tape was firmly pressed onto the damaged surface and removed quickly via peeling. The loss fraction


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of the coating was used to determine the ranking of adhesion according to ASTM protocol.

RESULTS AND DISCUSSION Insert Figure 1 Effect of the PDMS content on coating properties. Figure 1 demonstrates the approach to preparing hyper-branched polyester-based anti-smudge coatings. In this coating system, HBPE was adopted as coating precursor to provide sufficient crosslinking sites, PDMS was used to render the coatings with low surface energy, while HDIT was selected as curing agent to form heavy crosslinking. Above agents were mixed to form a homogenous coating solution, and then the solution was cast onto substrates. After thermal curing, the resultant coatings would be liquid-repellent. Insert Figure 2 As an effective low-surface-tension component, PDMS was introduced into a hyper-branched polyester-based coating matric cured by hexamethylene diisocyanate trimer, and the unbound PDMS chains were expected to enrich onto the coating surface during curing process to form a layer of PDMS brush to provide anti-smudge performance. Therefore, PDMS content is significantly important, and the effect of PDMS content on coating properties was investigated. As shown in Figure 2, in the case of the coating without grafting PDMS, the contact angles toward water and hexadecane droplets (with a surface tension of ~72 mN m

-1

and ~27 mN m

-1

respectively) were 83.7 ± 1.7° and 7.0 ± 1.1° respectively. In contrast, 0.5 wt% PDMS



content brought about a sharp increase of contact angle for water and hexadecane droplets to 103.5 ± 1.9° and 25.4 ± 1.5° respectively, while further increasing PDMS content had little influence on liquid contact angles, indicating 0.5 wt% PDMS content in this coating system was adequate for the resistance against water and hexadecane. Also, the optical transmittance at 500 nm for the coatings at a thickness of 40.0 ± 1.2 μm as a function of PDMS content was investigated. The result revealed that the incorporation or the increase of PDMS content resulted in the decrease of optical clarity, and the coatings with 0.5 wt% PDMS content were highly transparent and possessed an optical transmittance above 98.0%. Such high transparency was attributed to two reasons. First, although it is known that PDMS shows poor compatibility with various coating matrices,25,29,41 such low content of PDMS could be dispersed in coating solution without causing significant macrophase separation and thus showed less detrimental to transparency compared with the ones with higher contents. Second, according to SEM analysis (Figure S1a), the coating surface was fairly smooth, and the coating roughness was only 0.36 nm (characterized via AFM, Figure S1b), which was impossible to scatter light and consequently caused little adverse effect on coating transparency. Due to high transparency and liquid-repellency, the samples with 0.5 wt% PDMS content incorporated was selected for further investigations, and the resultant FT-IR is provided in Figure S2. Besides, we also found that, as increasing the coating thickness, contact angles of water and hexadecane on coating surface showed little variation, while the optical transmittance decreased slightly (Figure S3). When the coating thickness reached 90.0


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± 2.7 μm, the coating remained visually transparent and possessed an optical transmittance above 97.0%. These results are meaningful because they could provide guidance value for actual applications. Insert Figure 3 Self-cleaning performance. From XPS analysis, the Si content on coating surface reached 17.82%, indicating the enrichment of PDMS chains on coating surface. And such high Si content on coating surface significantly rendered substrates with striking liquid-repellency. Therefore, apart from high transparency, on coating surface, liquids could slide off as the coating was slightly tilted. As we know, as increasing the test liquid volume applied, corresponding sliding angle would decrease. For 5.0 μL hexadecane droplet, sliding angle on coating surface was 4.5 ± 0.8°; when the volume increased to 10.0 μL, sliding angle decreased to 3.0 ± 1.0°, while 10.0 μL water droplet demonstrated a sliding angle of 33.0 ± 2.0°. As the volume of water droplet increased to 40.0 μL, sliding angle decreased to 8.5 ± 1.0°. The reason for the higher sliding angle of water droplet than that of hexadecane with the same volume was attributed to the miscibility between PDMS and hexadecane, and the PDMS chains on coating surface reduced the sliding friction of hexadecane, leading to a smooth sliding toward hexadecane at a lower tilting angle.41 Due to the excellent liquid-repellency of the coating, self-cleaning performance could be demonstrated on coating surface. As seen in Figure 3a, a blue-dyed water droplet readily slid off without leaving any traces on a coated glass plate, while water droplet elongated and contaminated the uncoated one (Figure S4a). Similarly, on



coating surface, a red-dyed hexadecane droplet smoothly glided down (Figure 3b), whereas the hexadecane droplet spread on pristine glass plate and left noticeable red traces (Figure S4b). Besides, as a common oily contaminant, pump oil easily pinned on untreated surface and gradually spread down with an obvious elongated trace left (Figure S4c), while it contracted on coating surface and slid down without leaving any traces (Figure 3c), indicating that these anti-smudge coatings show huge potential in many places that are vulnerable to oil-based contaminants to endow them with self-cleaning property to maintain surface clean, such as industrial equipment or kitchens. As we know, crude oil was difficult to be cleaned away once it contacted upon substrates due to its high viscosity and complicated components. Although it showed no contraction behavior on uncoated surface and left an apparent black trace (Figure S4d), it shrunk on coating surface and glided down gradually without leaving any traces (Figure 3d). The repellency against crude oil and the sliding property of the coating make it a good candidate to impart crude oil transport pipelines with self-cleaning surface to reduce transport friction and oil deposition, which will help to improve transport efficiency and cut down energy consumption. Insert Figure 4 Moreover, we found that dirt on coating surface could be cleaned away by liquids. Therefore, dirt-removal test was implemented on coating surface to further demonstrate self-cleaning performance of the coating. As seen in Figure 4a, dirt (methylene blue powder) scattered on coating surface was carried away by water


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droplets, and no traces left on coating surface after the removal of dirt. It was noteworthy that the dirt employed was water-soluble and used to dye water, leading to the dark blue water after cleaning process, and the undissolved dirt was floating on water (Figure 4a3). Besides, as aforementioned, hexadecane droplet could slide off coating surface with no traces left. Although showing insolubility in hexadecane, the dirt was also cleaned away by hexadecane droplets with no markings left on coating surface (Figure 4b). Above results indicated that dirt on coating surface could be removed by water or oil (hexadecane). Furthermore, dust was selected for dirt-removal test. As seen in Figure S5, dust scattered on coating surface could also be removed by water or hexadecane droplets, indicating that the coating possessed striking self-cleaning ability. Therefore, due to exceptional self-cleaning performance, these coatings can be used in skyscraper windows, windshields, solar panels and communal facilities that exposed to dust and air to keep surface clean. Insert Figure 5 Ink contraction test and durability. However, high contact angles and cleanly liquid sliding on coating surface do not imply that the coating possesses anti-smudge properties because oil-based ink contraction on coating surface is more challenging.29 On insufficiently crosslinked coating surface, except for surface reconstruction, ink exhibited little or insignificant contraction behavior that resulted in persistent and noticeable markings on coating surface after wiping.31 In hyper-branched polyester-based coating system, high density of hydroxyl groups was heavily crosslinked by HDIT. To investigate whether there were free reagents existing in



coating, a coating piece was immersed in acetone for 1 h in a sealed vial, and then the resultant acetone was used for FT-IR analysis. However, no corresponding peaks were found in FT-IR analysis because the coating was highly crosslinked and no free reagents existing in polymer network. Owing to heavy crosslinking in this coating system, the coating possessed a hardness of 3H and exhibited striking ink-repellency. As seen in Figure 5a, oil-based ink formed a uniform and distinct trace on the pristine glass plate, and the ink trace remained clear even after wiping with a tissue. In sharp contrast, on the coating surface, ink readily contracted into discrete ink droplets and easily removed by wiping with a tissue without leaving any markings (Figure 5b). Therefore, these anti-smudge coatings restrained ink deposition and conduced to ink removal. Besides, the durability of ink contracting ability was further investigated. The coating was subjected to 50, 100, 150, 200, 250, 300 writing and erasing cycles respectively on the same marked region, and corresponding ink shrinking behavior is shown in Figure 5c. Compared with the reported 30 cycles of ink contraction test,29 the coating we fabricated exhibited superior durability, indicative of its robustness and availability for practical application. Insert Figure 6 Anti-fingerprint and anti-graffiti tests. Besides striking ink-repellency, these coatings also demonstrated exceptional repellency against artificial fingerprint liquid, water-based smudge and oil-based paint, suggesting that they can be used for anti-fingerprint and anti-graffiti. To first evaluate anti-fingerprint ability, a rubber


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stamp bearing concentric patterns wetted with an artificial fingerprint liquid composed of acetic acid, lactic acid, 1-methoxy-2-propanol, sodium hydrogen phosphate, hydroxyl-terminated PDMS, sodium chloride, and deionized water42 was firmly pressed onto the coated glass plate and the uncoated one for comparison. As shown in Figure 6a,b, artificial fingerprint liquid wetted and spread on the pristine substrate and showed no liquid contraction behavior, while the liquid shrunk into distinct droplets on the coating surface, indicating that the coating strikingly rejected the fingerprint liquid. Similarly, as seen in Figure S6, the fingerprint liquid pressed by finger also contracted into discrete droplets on coating surface, while liquid wetted and spread on the uncoated one, indicative of the remarkable anti-fingerprint ability of the coating. Therefore, with the advantage of high optical transparency, these coatings show potential applications in smartphones, tablets, and display panels for anti-fingerprint purpose. Moreover, these coatings also exhibited remarkable anti-graffiti ability. To evaluate the repellency of the coating against water-based smudge or contaminants, commercially available water-based black ink was used to wipe multiple times on coating surface, and the black smudge simultaneously contracted instead of spreading over the surface (Figure 6c), indicating the coating exhibited remarkable repellency against water-based smudge. Furthermore, yellow-dyed oil-based paint was used to apply on coating surface, and the paint gradually shrunk together (Figure 6d) due to the striking anti-paint ability. As for the diluted oil-based paint (diluted the paint to 80 wt% solid content with propylene glycol methyl ether acetate), it contracted into



discrete patches on coating surface within 10 s (Figure S7). Therefore, it was impossible for paint and water-based smudge to spread on coating surface, and the marker ink on coating surface would also contract into faint patches (Figure 5b), suggesting these anti-smudge coatings could be used for anti-graffiti purpose. Insert Figure 7 Chemical shielding test. Due to the chemical inertness of PDMS and the liquid-repellency of coating surface, these coatings also exhibited remarkable chemical shielding performance.43,44 As seen in Figure 7a, sodium chloride solution (NaCl, 5.0 wt%), copper sulfate solution (CuSO4, 0.5 M), sulfuric acid solution (H2SO4, pH = 0), sodium hydroxide solution (NaOH, pH = 14) on coating surface showed a contact angle of 107.2°, 105.9°, 100.7°, and 88.1°, respectively. Although exhibiting different wettability, on coating surface, all of above liquid droplets slid off smoothly without leaving any traces or causing any damages, and the pH-indicator papers placed on the bottom of coated glass plate turned yellow, light orange, red and dark violet respectively upon contact with above liquids to visually exhibit respective nature (Figure 7b). To evaluate the chemical shielding durability of these coatings, liquid droplets were placed respectively onto the coated section and the uncoated section of a half-coated tin plate under various times for comparison. As seen in Figure 7c, all liquid droplets used were contracted on coating surface while spread over the uncoated region. It was noteworthy that, upon contact with copper sulfate solution, the uncoated polished tin plate was readily covered with a layer of copper because of


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the occurrence of replacement reaction. After 2 h of sitting, liquid volume diminished due to water evaporation, and coating surface showed no evidence of corrosions or damages, while these liquids caused different degree of corrosion on the uncoated tin plates. More strikingly, even after subjected to 15 h of corrosion, coating surface remained intact, whereas the uncoated surface visually demonstrated severe corrosion. Furthermore, to evaluate the chemical shielding ability of the coating, the liquid repellency of the coatings subjected to immersion in above corrosive agents for 15 h respectively was investigated. As shown in Figure S8, the contact angles and sliding angles toward water and hexadecane on coating surface treated by different corrosive agents respectively exhibited insignificant variations except the water sliding angle on the surface treated by strong alkali solution increased obviously to 16.2°, and test liquids could also slide off the treated coating surface without leaving any traces, indicating that the coating maintained remarkable liquid repellency even after subjected to severe chemical shielding test. Therefore, these coatings endowed substrates with superior chemical shielding ability, and bestowed them with remarkable protection against exposure to extreme chemical conditions. Insert Figure 8 Mechanical robustness test. Although the as-made smooth polymer-based anti-smudge coatings did not provide exceptionally high contact angles that available for rough surfaces, they were mechanically robust due to the absence of frail textures or particles that prone to wearing in coating matrices. To investigate the mechanical robustness of the coating, abrasion, adhesion, impact, and bending tests were



implemented. First, the coating was subjected to abrasion test. As seen in Figure 8a, the contact angles toward water and hexadecane decreased as increasing the abrasion cycles, whereas the sliding angles increased along with the cycles of abrasion. After 5000 abrasion cycles, liquid droplets could also slid off the worn surface, although the sliding angle of water increased to 17.5°, and that of hexadecane went up to 7.2°. Besides, from XPS analysis (Figure 8b, Table S1), after suffering from 5000 abrasion cycles, the Si content on coating surface decreased from 17.82% to 10.97%, which was due to the wearing of the enriched PDMS layer,41 and it mainly contributed to the decrease of contact angles and the increase of sliding angles. And the worn surface remained ink-repellency even after subjected to 5000 abrasion cycles (Figure S9). Furthermore, adhesion test was carried out according to ASTM standard to investigate the adhesion of the coating to substrates.41 The coating was cut into a grip-pattern by a blade, and a 3 M Scotch tape was firmly applied onto the cross-cut surface and was peeled off quickly. The loss fraction of the coating was used to determine the ranking of adhesion. As demonstrated in Figure 8c and Figure S10, the coating bound securely to tin plate and glass plate, and showed no evidence of coating loss even after subjected to adhesion test, indicating that the coating adhered well to various substrates with a top adhesion ranking of 5B. More strikingly, the adhesion test had little adverse effect on the ink-repellency of the coating. Impact test was implemented by freeing down a hammer (1000 ± 1g) that had been elevated to a height of 50 cm to cause an impact on the coating. Figure 8d and Figure 8e respectively demonstrate the front side and the opposite side of the coating


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subjected to an impact experiment. Therefore, although the coating underwent extreme impact damage, the coating maintained intact and exhibited no sigh of fractures, showing remarkable adhesion to the substrate. Besides, oil-based ink was applied on the impact sites, and the ink contracted into faint patches, indicating the impact test caused little effect on the ink-repellency of the coating. Furthermore, bending test was carried out on the coated tin plate. As seen in Figure 8f, after being severely bended, the ink applied on the bending sites shrunk, indicating that the coating exhibited no evidence of cracking or damages and remained tightly adhering to the substrate. Therefore, these anti-smudge coatings are mechanically robust against abrasion, adhesion, impact and bending tests, showing huge potential for practical applications.

CONCLUSIONS In summary, hyper-branched polyester-based anti-smudge coatings were prepared by using hydroxyl-terminated hyper-branched polyester as coating precursor to provide sufficient crosslinking sites, mono-hydroxyl-terminated polydimethylsiloxane to render low surface energy, and hexamethylene diisocyanate trimer to form high crosslinking. The coatings with 0.5 wt% PDMS content incorporated possessed high transparency and showed outstanding liquid-repellency. Water, hexadecane, pump oil, crude oil and corrosive solutions could slide off the coating surface without leaving any traces, and the dirt on coating surface could be cleaned away by water or oil, indicative of the striking self-cleaning performance of these coatings. Besides, these



anti-smudge coatings retained ink contracting ability even after 300 writing and erasing cycles of oil-based ink on coating surface. Also, they rejected oil-based paint, water-based smudge, and artificial fingerprint liquid, showing potential utilization for anti-graffiti and anti-fingerprint in smartphones, tablets, and display panels. Furthermore, these coatings exhibited remarkable chemical shielding performance and endowed substrates with striking protection against exposure to extreme chemical conditions. Moreover, they demonstrated excellent adhesion to various substrates, and withstood 5000 abrasion cycles, impact and bending tests without compromising anti-smudge properties, suggesting that these anti-smudge coatings are mechanically robust. Besides, numerous advantages, including the use of commercially available and environmentally-friendly agents, the need of extremely low PDMS content, and simple fabrication procedure, make this coating system more attractive for practical applications.

ASSOCIATED CONTENT Supporting Information SEM and AFM images of the coating surface; FT-IR spectrum of the anti-smudge coating; variations of the optical transmittance and the contact angle toward water and hexadecane on coating surface as a function of coating thickness; photographs of various liquids on uncoated glass plates; photographs of the removal of dust scattered on coating surface; anti-fingerprint test; contraction of the diluted oil-based paint on coated glass plate; liquid repellency of the coating subjected to immersion in different


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corrosive agents for 15 h respectively; atom percentages of the coating surface before and after abrasion test; ink contraction on the coating after subjected to 5000 abrasion cycles; ink contraction on the coated glass plate after subjected to adhesion test. (PDF)

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected].

Phone: +86 020 87114919

ORCID Heqing Fu: 0000-0002-1298-8306 Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS We appreciate the financial support from the National Natural Science Foundation of China under grant No. 21878108.

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Shang,

B.;

Chen,

M.;

Wu,

L.

One-Step

Synthesis

of

Statically

Amphiphilic/Dynamically Amphiphobic Fluoride-Free Transparent Coatings. ACS Appl. Mater. Interfaces 2018, 10, 41824-41830.



Figures Captions Figure 1. Synthesis route to the hyper-branched polyester-based anti-smudge coatings, and the chemical structures of PDMS, HDIT, and HBPE. Figure 2. Contact angle variation toward water and hexadecane as a function of PDMS content (blue curve) and transmittance variation of the coatings as a function of PDMS content (black curve). Figure 3. Photographs of the sliding of various liquids on coated glass plates. (a) Blue-dyed water droplet. (b) Red-dyed hexadecane droplet. (c) Pump oil droplet. (d) Crude oil droplet. Water droplet applied was 40.0 μL, and the tilting angle of the coated glass plate was about 25.0°. (scale bar, 5 mm) Figure 4. Photographs of the removal of dirt scattered on coating surface. (a) Dirt removed by dropping water droplets. (b) Dirt removed by dropping hexadecane droplets. The dirt used was methylene blue powder, and the tilting angle of the coating was about 38.0°. (scale bar, 5 mm) Figure 5. Ink contraction test and durability. (a) Ink traces left obviously on pristine glass plate even after wiping with a tissue. (b) Ink contracted on coating surface and being wiped off without leaving any traces. (c) Status of the ink contraction on coating surface after 50, 100, 150, 200, 250, 300 writing and erasing cycles respectively. (scale bar, 5 mm) Figure 6. Anti-fingerprint and anti-graffiti tests. (a) Artificial fingerprint liquid wetted and spread on the uncoated glass plate. (b) Artificial fingerprint liquid contracted on the coated glass plate. (c) Behavior of water-soluble smudge on the coated glass plate.


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(d1,d2,d3) Status after the application of yellow-dyed paint onto the coated glass plate. (scale bar, 5 mm) Figure 7. Chemical shielding ability of the coating against salt solution, strong acid solution and strong alkali solution. (a) Contact angles toward 5.0 wt% NaCl solution, 0.5 M CuSO4 solution, H2SO4 solution (pH = 0), and NaOH solution (pH = 14) on coating surface. The insets are corresponding droplet images. (b) Above solutions sliding off the coating surface respectively without leaving any traces. The tilting angle of the coated glass plate was about 25.0°. The pH-indicator papers placed on the bottom of coated glass plate were used to demonstrate the nature of above solutions, and they turned yellow, light orange, red and dark violet respectively upon contact with NaCl solution, CuSO4 solution, H2SO4 solution, and NaOH solution. (c) Above solution droplets (0.5 mL) placing on half-coated polished tin plates for 0 h, 2 h, and 15 h, respectively. (scale bar, 10 mm) Figure 8. Mechanical robustness tests. (a) Variations of contact angle and sliding angle toward water and hexadecane as increasing abrasion cycles. The water volume for sliding angle measurements was 40.0 μL. (b) XPS analysis on coating surface before and after 5000 abrasion cycles. (c) Adhesion test on a coated tin plate. (d) Front side of a coated tin plate after impact test. (e) Opposite side of a coated tin plate after impact test. (f) A coated tin plate subjected to severe bending. Tin plates were polished before being coated. (scale bar, 5 mm)



Table of Content (TOC) graphic


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Figure 1 Synthesis route to the hyper-branched polyester-based anti-smudge coatings, and the chemical structures of PDMS, HDIT, and HBPE. 119x91mm (300 x 300 DPI)



Figure 2 Contact angle variation toward water and hexadecane as a function of PDMS content (blue curve) and transmittance variation of the coatings as a function of PDMS content (black curve). 75x52mm (300 x 300 DPI)


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Figure 3 Photographs of the sliding of various liquids on coated glass plates. (a) Blue-dyed water droplet. (b) Red-dyed hexadecane droplet. (c) Pump oil droplet. (d) Crude oil droplet. Water droplet applied was 40.0 μL, and the tilting angle of the coated glass plate was about 25.0°. (scale bar, 5 mm) 114x81mm (300 x 300 DPI)



Figure 4 Photographs of the removal of dirt scattered on coating surface. (a) Dirt removed by dropping water droplets. (b) Dirt removed by dropping hexadecane droplets. The dirt used was methylene blue powder, and the tilting angle of the coating was about 38.0°. (scale bar, 5 mm) 99x64mm (300 x 300 DPI)


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Figure 5 Ink contraction test and durability. (a) Ink traces left obviously on pristine glass plate even after wiping with a tissue. (b) Ink contracted on coating surface and being wiped off without leaving any traces. (c) Status of the ink contraction on coating surface after 50, 100, 150, 200, 250, 300 writing and erasing cycles respectively. (scale bar, 5 mm) 144x57mm (300 x 300 DPI)



Figure 6 Anti-fingerprint and anti-graffiti tests. (a) Artificial fingerprint liquid wetted and spread on the uncoated glass plate. (b) Artificial fingerprint liquid contracted on the coated glass plate. (c) Behavior of water-soluble smudge on the coated glass plate. (d1,d2,d3) Status after the application of yellow-dyed paint onto the coated glass plate. (scale bar, 5 mm) 114x65mm (300 x 300 DPI)


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Figure 7 Chemical shielding ability of the coating against salt solution, strong acid solution and strong alkali solution. (a) Contact angles toward 5.0 wt% NaCl solution, 0.5 M CuSO4 solution, H2SO4 solution (pH = 0), and NaOH solution (pH = 14) on coating surface. The insets are corresponding droplet images. (b) Above solutions sliding off the coating surface respectively without leaving any traces. The tilting angle of the coated glass plate was about 25.0°. The pH-indicator papers placed on the bottom of coated glass plate were used to demonstrate the nature of above solutions, and they turned yellow, light orange, red and dark violet respectively upon contact with NaCl solution, CuSO4 solution, H2SO4 solution, and NaOH solution. (c) Above solution droplets (0.5 mL) placing on half-coated polished tin plates for 0 h, 2 h, and 15 h, respectively. (scale bar, 10 mm) 120x96mm (300 x 300 DPI)



Figure 8 Mechanical robustness tests. (a) Variations of contact angle and sliding angle toward water and hexadecane as increasing abrasion cycles. The water volume for sliding angle measurements was 40.0 μL. (b) XPS analysis on coating surface before and after 5000 abrasion cycles. (c) Adhesion test on a coated tin plate. (d) Front side of a coated tin plate after impact test. (e) Opposite side of a coated tin plate after impact test. (f) A coated tin plate subjected to severe bending. Tin plates were polished before being coated. (scale bar, 5 mm) 124x74mm (300 x 300 DPI)


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Robust hyper-branched polyester-based anti-smudge coatings for self

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