In-Situ Incorporation of Alkyl-Grafted Silica into Waterborne ... - MDPI

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In-Situ Incorporation of Alkyl-Grafted Silica into Waterborne Polyurethane with High Solid Content for Enhanced Physical Properties of Coatings Yanting Han 1 , Jinlian Hu 1, * 1 2

*

ID

and Zhongyin Xin 2

Institute of Textiles & Clothing, The Hong Kong Polytechnic University, Hung Hom, Kowloon 999077, Hong Kong, China; [email protected] National Engineering Laboratory for Clean Technology of Leather Manufacture, Sichuan University, Chengdu 610044, China; [email protected] Correspondence: [email protected]; Tel.: +852-2766-6486

Received: 7 April 2018; Accepted: 8 May 2018; Published: 9 May 2018

Abstract: Waterborne polyurethane (WPU) with high solid content (45%) was obtained by utilizing dimethylol propionic acid (DMPA) and ethoxylated capped polymeric diol as complex hydrophilic groups. Alkyl-grafted silica was incorporated into polymer matrix through in situ polymerization to improve the performance of coatings casted from WPU dispersions. The addition of alkyl-grafted silica enlarged the particle size distribution whilst increased emulsion viscosity, which showed little influence on attainment of high solid content for WPU. The properties of obtained WPU/Silica coatings were investigated. Results showed that the functionalized surface of silica provides good compatibility with the WPU matrix, which promoted the homogeneous dispersion of silica particles. This facilitated the formation of nanosized silica papillae on coatings, contributing to surface roughness and hydrophobicity. Solvent resistance of WPU was enhanced with existence of alkyl-grafted silica particles. The WPU/Silica coatings also displayed improved thermal stability due to the thermal insulation ability and tortuous path effect of silica. Besides this, valid interactions between silica and WPU resulted in hybrid microphase of which the synergistic effect imparted superior mechanical properties at relatively low loadings of silica (2%). The facile technique presented here will provide an effective and promising method for preparing WPU hybrids with enhanced performance. Keywords: waterborne polyurethane; silica; hybrid; high solid content; coatings; microphase

1. Introduction Compared with traditional solvent-based polyurethane, waterborne polyurethane (WPU) has the advantages of being non-toxic, non-flammable, and non-polluting to the environment [1]. They have been widely used in leather finishing, automotive painting, fiber processing, coatings, and adhesives [2]. In WPU dispersions, polyurethane exists in water in the form of latex particles. The viscosity of WPU dispersions increases with the increase of solid content since, when the particles are crowded, the interactions between neighboring particles become strong. As high viscosity will lead to more burden of production and product handling equipment, conventional WPU usually has a low solid content in the range of 20% to 40%. The low solid content results in inferior drying rates and slower development of adhesion. These shortcomings can be largely overcome by increasing the solid content of WPU [3]. Hence, high solid content (40~55%) of WPU has received much attention from academic and industry fields [4–6]. For example, Kim et al. [7] have found that high solid content (45%) of WPU can be obtained at a low ionic content (2%) by incorporating the ionic groups to the soft segments.

Polymers 2018, 10, 514; doi:10.3390/polym10050514

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Shan et al. [4] used nonionic monomers (polyethylene glycol, PEG) together with anionic monomers (dimethylolpropionic acid, DMPA) as hydrophilic groups to synthesize WPU. The introduction of nonionic hydrophilic chains helps to decrease required dosage of DMPA, and thus to reduce the interaction force and hydration layer. Similarly, Wei et al. [6] proved that the complex hydrophilic chain-extending agent led to the multimodal distribution of latex particles. As a result, large particles and small particles can compact tightly, increasing the maximum packing factor of particles. Although most of the literature claim that high solid content of WPU can be achieved by molecular design, their results revealed that the stability of the WPU may be reduced; especially, the performance of casting films could be impaired [6]. However, it is addressed that the excellent properties of organic/inorganic hybrids can provide efficient methods and approaches for high performance and functionalization of various materials based on the synergistic effect [8]. Particularly, as one of the important reinforcing fillers, hybrid with silica has been reported as an alternative approach to improving the properties of final products. For example, Malaki et al. [9] added silica into acrylic polyurethane to fabricate hybrid coatings. Their results showed that the addition of silica is beneficial for the adhesive strength and erosion resistance. Li et al. [10] concluded that the silica additives significantly improved the hardness and abrasion with uniform dispersion of silica nanoparticles. Wang et al. [11] prepared the polyurethane/silica hybrids via chemical reaction between urethane groups of PU prepolymer and hydroxyl groups at the surfaces of silica. They found that silica can work as inorganic hard-segment which contributes to the enhanced mechanical properties of hybrids. As all these literatures confirmed the reinforcement effect of silica, we can propose that adding silica to WPU should be an effective way to improve the performance of high solid content WPU. In this case, the challenge is to adopt appropriate methods to incorporating silica into WPU dispersions without affecting the achievement of high solid content. Generally, there are three ways for adding silica into PU matrix: blending method, sol-gel method, and in situ polymerization. The blending method is the simplest method for preparing nanocomposites. However, silica particles are easily aggregate due to the large number of hydroxyl groups, making it difficult to evenly disperse silica in PU matrix [12]. To solve this problem, sol-gel methods consisting of hydrolysis and polycondensation were developed. Generally, the precursor is first dissolved in the monomer or prepolymer solution, then gradually hydrolyzed to generate nanosized silica particles with the aid of a catalyst [13]. This process can lead to an organic-inorganic network with uniformly dispersed silica particles. Nevertheless, the inevitable use of toxic precursors and solvents increases the production cost and causes environmental pollution. Most notably, the requirement of large amounts of solvent during sol-gel process violates the aim of high solid content for WPU. In contrast, in situ polymerization does not have strict requirements for the solvent. Since silica particles are first uniformly dispersed with prepolymers, the subsequent polymerization can occur around the nanoparticles, which facilitates the interactions between silica and polymer molecule [14,15]. This in turn can prevent the aggregation of silica particles. Moreover, research has proved that grafting alkyl monomers to the surface of silica can promote its miscibility with polymer components because of its decreased hydrophilicity and enhanced interfacial interaction between silica and PU matrix, through entanglement of the grafting alkyls attached to the silica particles with the polymer molecules [16,17]. Therefore, this study aims to improve the properties of coatings casted from WPU with high solid content through addition of alkyl-grafted silica as nanofillers. Here, WPU with high solid content (45%) was obtained by utilizing dimethylol propionic acid (DMPA) and ethoxylated capped polymeric diol as complex hydrophilic groups. To improve the performance of final coatings, alkyl-grafted silica was incorporated into the WPU matrix through in situ polymerization. The effect of alkyl-grafted silica on the resultant WPU/Silica dispersions as well as the morphology, solvent resistance, thermal stabilities, and mechanical properties of the hybrid coatings were investigated. This research should contribute to the synthesis of environment-friendly coatings with high performance.

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2. Experiments 2.1. Materials Isophorone diisocyanate (IPDI), purchased from Bayer (Leverkusen, Germany). Polytetrahydrofurfuryl glycol (PTMG-1K: 1000 g/mol; PTMG-2K: 2000 g/mol), dimethylol propionic acid (DMPA), pentaerythritol (PE), and ethoxylated capped polymeric diol (YmerTM N120) were obtained by Perstorp Specialty Chemicals (Perstorp, Sweden). Alkyl-grafted silica (Aerosil® R816) was offered by Degussa Chemical (Essen, Germany). Dibutyltin dilaurate (T-12), Ethylenediamine (EDA), N,N-dimethylformamide (DMF) were produced from Chengdu Kelong Chemical Reagent Factory (Chengdu, China). DMPA and PE were dried in vacuum at 100 ◦ C for 6 h before use. Laboratory-made distilled water and other reagents were used without treatment. 2.2. Preparation of Waterborne Polyurethane/Silica (WPU/Silica) Hybrid Dispersions with High Solid Content PTMG-1K (0.0450 mol), PTMG-2K (0.0225 mol), DMPA (0.0138 mol), ethoxylated capped polymeric diol (0.0120 mol), and quantitative long-chain alkyl modified silica were added to a three-necked flask equipped with a stirrer and a temperature controller. The mixture was dispersed at a high speed stirring with temperature raising to 110 ◦ C. The dehydration was performed under a negative pressure of 0.08 MPa for 90 min. After cooling to 40 ◦ C, IPDI (0.200 mol) was added followed by the addition of catalyst (0.5 wt %). Then, the temperature was raised to 90 ◦ C for full reaction. During the reaction, the free –NCO content was monitored by the standard dibutylamine back-titration method. Upon reaching the desired –NCO value, DMPA was added and the reaction was continued at 90 ◦ C until the NCO content reached the theoretical value. Subsequently, the temperature was lowered to 70 ◦ C and 1 wt % PE (0.0111 mol) was added, which is followed by slowly raising the temperature to 90 ◦ C. When the –NCO content reached the theoretical value, the pre-polymer was cooled down to ~60 ◦ C, followed by the addition of triethylamine (0.0130 mol). After neutralization for 10 mins, the temperature of pre-polymer was decreased to ~40 ◦ C. Then, quantitative deionized cold water (5 ◦ C) was rapidly added for emulsification. This process was done in an ice bath for 10 min under vigorous stirring. The pH value of dispersion was ~7. Diluted EDA (0.0598 mol) was then slowly added dropwise for chain extension for 1.5 h at 35 ◦ C. The high solid content (~45%) was measured according to the method described by Gong et al. [3]. Coatings from WPU/Silica were prepared by casting dispersions onto the glass plate at ambient temperature for 5 days and 60 ◦ C for 24 h The obtained coatings with thickness of ~0.4 mm was stored in a desiccator (Hangzhou Kebo Instrument CO., LTD, Hangzhou, China) for later testing. Figure 1 outlines the in situ polymerization of waterborne polyurethane/Silica (WPU/Silica) dispersions and preparation of coatings. According to the mass fraction of the silica, the samples were designated as WPU/Silica-0.5, WPU/Silica-1.0, WPU/Silica-1.5, WPU/Silica-1.8, WPU/Silica-2.0, and WPU/Silica-2.3. For example, WPU/Silica-1.0 indicates that the addition of silica accounts for 1.0% of the total mass of the resin. Meanwhile, as comparison, pure WPU was blended with 1.0% silica to prepare a composite which was named as WPU/Silica-1.0-B. 2.3. Characterization The mean particle size, particle size distribution, and zeta potential of dispersions were investigated by Zetasizer Nano S90 laser particle analyzer (Malvern Instrument, Malvern, UK) in polystyrene statistical model. Samples were diluted with deionized water before testing. The viscosities of dispersions were measured on NDJ-8S digital readout viscometer at 25 ◦ C. The rotate speeds used were 3, 6, 12, 30, and 60 rpm. Fourier transform infrared (FTIR) spectra of the film samples were performed on a NEXUS 670 spectrophotometer (Nicolet, Madison, WI, USA) in transmission mode with a resolution of 4 cm−1 and 32 scanning times. The cross-section morphology of specimens was observed under a Phenom (Kunming, China) scanning electron microscopy (SEM) with an accelerating voltage of 5 kV. Atom force microscopy (AFM) was performed on the surface of the film prepared by casting 0.02 mL dispersions on mica plate. The measurement was conducted using the SPA-400 Atomic

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the film prepared by casting 0.02 mL dispersions on mica plate. The measurement was conducted using the SPA-400 Atomic force microscope (Seiko Instruments Inc., Chiba, Japan) in tapping mode. force microscope (Seiko Instruments Inc., Chiba, Japan) in tapping mode. Surface roughness analysis Surface roughness analysis and particle size distribution were carried out using NanoScope-Analysis and particle size distribution were carried out using NanoScope-Analysis software (Veeco Instruments, software (Veeco Instruments, Plainview, NY, USA). Contact angles of films with deionized water and Plainview, NY, USA). Contact angles of films with deionized water and glycol were determined glycol were determined using the OCAH200 high-speed video contact angle measurer (DataPhysics using the OCAH200 high-speed video contact angle measurer (DataPhysics Instruments GmbH Ger, Instruments GmbH Ger, Filderstadt, Germany). Water and acetone swelling ratio of prepared Filderstadt, Germany). Water and acetone swelling ratio of prepared coatings were examined by coatings were examined by following equation: following equation: Swelling (%) (%) == (W − −W 0 × (1) Swelling W0)/W × 100% (1) 0 )/W 0 100% × 20 where W 00 is the original weight of dried sample (20 mm × 20 mm mm in in size) size) and W is the weight of swollen sample after immersion in solvent at 25 25 ◦°C for 24 h. C Differential Scanning Scanning Calorimetric Calorimetric (DSC) performed on DSC200 (NETZSCH (NETZSCH Instruments, Differential (DSC) was was performed on DSC200 Instruments, ◦ C under Ahlden, Germany) Germany) at at aa heating heating rate rate of of 55◦°C/min from − −100 Ahlden, C/min from 100 to to 150 150 °C under nitrogen nitrogen atmosphere. atmosphere. Thermogravimetric analysis (TGA) was carried out by a differential thermogravimetric analyzer Thermogravimetric analysis (TGA) was carried out by a differential thermogravimetric analyzer ◦ ◦ (NETZSCH Instruments, Instruments, Germany) Germany) at at aa heating heatingrate rateof of10 10 °C/min (NETZSCH C/min from from 100 100 to to 550 550 °C C under under nitrogen nitrogen atmosphere. Dynamic viscoelastic properties of samples were investigated using DMA 242 C atmosphere. Dynamic viscoelastic properties of samples were investigated using DMA 242 C analyzer ◦ ◦ analyzer (NETZSCH Instruments, Germany) with a heating rate 5 °C/min from −100~150 °C at 1 Hz (NETZSCH Instruments, Germany) with a heating rate 5 C/min from −100~150 C at 1 Hz in in stretching mode a amplitude 30 Mechanical μm. Mechanical properties of samples a rectangle stretching mode withwith a amplitude of 30ofµm. properties of samples with awith rectangle size of mm × 25 mm were measured using GT-AI-7000S tensile testing (Gotech machineTesting (Gotech Testing 5size mmof×5 25 mm were measured using GT-AI-7000S tensile testing machine Machines Machines Inc., Guangzhou, China) with a stretching speed of 100 under mm/min under room temperature. Inc., Guangzhou, China) with a stretching speed of 100 mm/min room temperature.

Figure 1. dispersions with with high solid 1. Preparation Preparation of of waterborne waterbornepolyurethane/Silica polyurethane/Silica (WPU/Silica) (WPU/Silica) dispersions content and their coatings.

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3. Results and Discussion 3. Results and Discussion 3.1. Characterization of the Waterborne Polyurethane/Silica Dispersions (WPU/Silica) with High Solid Content 3.1. Characterization of the Waterborne Polyurethane/Silica Dispersions (WPU/Silica) with High Solid Content

The particle particle size size distributions distributions of of the theWPU/Silica WPU/Silica dispersions are shown in Figure Figure 2a. 2a. The The mean mean The particle size size of of the the WPU/Silica WPU/Silica decreased at low content of Silica Silica (2.0%). The initial decline of maymay be because thethe strong rigidity of of silica PDI be because strong rigidity silicananoparticle nanoparticlecan canaccelerate accelerate the the dispersion efficiency through the the physical physical friction friction effect effectwhen whenthe the prepolymer prepolymermatrix matrixisisemulsified emulsified[18]. [18]. It It is is suggested suggested that that through the hydrophobic hydrophobic parts parts of of WPU WPU chains chains will will be be surrounded surrounded by by hydrophilic hydrophilic segments segments due due to to energetic energetic the reasonsin inWPU WPUdispersions dispersions [4,6]. Hence, alkyl-grafted silicas are supposed be shielded by reasons [4,6]. Hence, thethe alkyl-grafted silicas are supposed to beto shielded by more more hydrophilic of WPU. Thus,incorporation with incorporation of the higher silica, the particle hydrophilic chainschains of WPU. Thus, with of the higher contentcontent silica, the particle size of size of WPU/Silica enlarges, which dueincreased to the increased silica packaged WPU WPU/Silica enlarges, which may bemay due be to the numbernumber of silicaofpackaged by WPUbychains. chains. widesize particle size distribution is also for beneficial forcontent, high solid content, small The wideThe particle distribution is also beneficial high solid because smallbecause particles can particles pack into the the voids among the large particles. Inmaximum this case, the maximum factor pack into can the voids among large particles. In this case, the packing factorpacking of particles in of particles in water can be improved [5]. water can be improved [5].

Figure 2. properties of high solid content of WPUofand WPU/Silica dispersions:dispersions: (a) particle Figure 2. Emulsion Emulsion properties of high solid content WPU and WPU/Silica size distribution; (b) average particle size and polydispersity index (PDI); (c) Zeta values; (d) (a) particle size distribution; (b) average particle size and polydispersity index (PDI); (c) Zeta values; rheological properties. (d) rheological properties.

Viscosity of prepolymer is also reported to influence the particle size [19]. During our Viscosity of prepolymer is also reported to influence the particle size [19]. During our preparation preparation of WPU/Silica, the prepolymer viscosity increased as the content of alkyl-grafted silica of WPU/Silica, the prepolymer viscosity increased as the content of alkyl-grafted silica increased. increased. Byung et al. [20] found that the polymer viscosity directly contributed to the finer breakup Byung et al. [20] found that the polymer viscosity directly contributed to the finer breakup of of the dispersed phase during phase version with water addition, in agreement with our results. The the dispersed phase during phase version with water addition, in agreement with our results. viscosities of WPU/Silica dispersions at different rotate speeds are shown in Figure 2c. Results The viscosities of WPU/Silica dispersions at different rotate speeds are shown in Figure 2c. Results showed that WPU/Silica dispersions were non-Newtonian fluids with viscosity increased with rotate showed that WPU/Silica dispersions were non-Newtonian fluids with viscosity increased with rotate speed, and the viscosity of WPU/Silica increased as the content of silica increases. With larger particle

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speed, and the viscosity of WPU/Silica increased as the content of silica increases. With larger particle size containing higher content of silica, dispersions should give greater viscosity, owing to the greater volume fraction of fillers according to [21]: ln (η/η1 ) = (KE /ϕ)/(1 − ϕ/ϕm )

(2)

where η is the viscosity of component containing fillers, η1 is s the viscosity of neat component. KE is the Einstein constant (2.5), ϕ is the volume fraction of component containing fillers, and ϕm is maximum value of ϕ. ϕ increases as the mass fraction of filler increases. This leads to the increased viscosity of the WPU containing silica particles. On the other hand, the large specific surface area of silica nanoparticles can promote the adsorption of water by silica to increase the volume of the dispersed phase, as a result, the viscosity of the emulsion increases. Zeta potential is one of the key parameters for evaluation of the stability of emulsions [7]. The higher value of zeta potential confers the stability due to the strong repellant between emulsion particles. It can be seen from Figure 2d that the absolute value of the zeta potential of WPU/Silica showed an increase with increasing the silica content to 1.0%, indicating the enhanced stability of WPU/Silica dispersion. Nevertheless, the decrease of zeta potential is probably because the incorporation of silica increases the particle size of the emulsion, hence, the density of carboxyl groups per unit volume of emulsion particle decreases. In other words, the number of charge per unit volume decreases, thereby causing the decline of zeta potential. 3.2. Structure and Morphology of WPU/Silica Coatings FTIR spectra of alkyl-grafted silica, pure WPU, and WPU/Silica coatings are presented in Figure 3. In the infrared spectrum of alkyl-grafted silica, stretching absorption peaks of Si–O–Si were observed at 1100 and 808 cm−1 . The absorption peaks in the range of 3200~3750 cm−1 belongs to the vibration absorption of Si–OH and absorbed water while the peak at 940 and 2850 cm−1 attribute to stretching vibrations of –CH2 and –CH3 respectively [22]. These peaks confirmed the silica structure with alkyl groups on surface. Notably, there was a peak at 480 cm−1 on spectrum of WPU/Silica specimens due to the vibration of Si–O–Si. This peak was also found on the spectrum of WPU/Silica specimens while WPU had no characteristic peak of Si–O–Si. It can be seen from the enlarged part that the intensity of the peak enhanced with increasing silica content. Compared to WPU/Silica-1.0, WPU/Silica-1.0-B had a weak peak for Si–O–Si at 480 cm−1 . This may be because the physical blending method causes the unstable silica particles in WPU dispersions. Hence, silica particles may agglomerate in a block rather than be evenly distributed during the formation process of coatings. Moreover, the peak of Si–OH disappeared in spectrum of WPU/silica specimens. This may be due to the bonding effect of Si–OH to the polyurethane segments [23,24]. At the same time, the intensity of peak at 1100 cm−1 showed an increase with incorporation of silica due to the overlapping absorption peak of C–O–C with Si–O–Si [9,11]. Hence, it can be concluded that alkyl-grafted silica was successfully incorporated into WPU matrix by in situ polymerization. If the fillers are not uniformly dispersed in the emulsion, agglomeration will occur inside the formed coating. Hence, the presence and distribution of fillers in emulsions matrix can be evaluated by their dispersion in coating through SEM study [25]. Figure 4 shows the SEM micrographs of the cross sections of the prepared samples. Compared to neat WPU, alkyl-grafted silica particles were observed in WPU/Silica coatings regardless of the preparation method. However, the blending method basically caused obvious aggregation at 1% loading of silica (see samples WPU/Silica-1.0-B). On the contrary, the in situ method resulted in an even dispersion of silica for WPU/Silica-1.0 and WPU/Silica-2.0, which indicates that in situ incorporation is conducive to uniform dispersion of silica particles. Such an even and uniform distribution of the fillers in the matrix was reported to play an important role in improving the mechanical performance of the composite coatings [9].

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Figure3. 3. FTIR FTIR spectra spectra of of alkyl-grafted alkyl-grafted silica, silica,WPU, WPU,and andWPU/Silica. WPU/Silica. Figure Figure 3. FTIR spectra of alkyl-grafted silica, WPU, and WPU/Silica.

Figure 44 SEM SEM images of of cross cross sections sections of of WPU WPU and and WPU/Silica WPU/Silica coatings. Figure Figure 4. SEM images images of cross sections of WPU and WPU/Silica coatings. coatings.

AFM phase phase images images (Figure (Figure S1) S1) illustrate illustrate the the typical typical microphase microphase separation separation structure structure of of AFM AFM phase images (Figure S1) illustrate the typicalthe microphase separation structure of WPU/Silica coatings. The 3D topography can demonstrate surface properties of coatings. As WPU/Silica coatings. The 3D topography can demonstrate the surface properties of coatings. As WPU/Silica coatings. The 3D topography can demonstrate the surface surface properties of coatings. As shown shown in in Figure Figure 5, there there were dots protruding protruding out of of the the for WPU/Silica WPU/Silica coatings. These shown 5, were dots out surface for coatings. These in Figure 5, there were dots protruding out of the surface for WPU/Silica coatings. These papillae papillae attribute to the alkyl-grafted silica particles embedded in WPU matrix, which indicates that papillae attribute to the alkyl-grafted silica particles embedded in WPU matrix, which indicates that attribute to the can alkyl-grafted silica particles embedded in WPU matrix, which indicates thatmatrix silica silica particles migrate to the film surface rather than totally deposit inside the WPU silica particles can migrate to the film surface rather than totally deposit inside the WPU matrix particles canfilm migrate to the process. film surface rather totally deposit inside the WPUcoatings matrix during during the the formation Hence, thethan surface properties of WPU/Silica WPU/Silica shouldthe be during film formation process. Hence, the surface properties of coatings should be film formation process. Hence, the surface properties of WPU/Silica coatings should be determined determined by the synergy of WPU and silica particles. With increasing silica content, the number of determined by the synergy of WPU and silica particles. With increasing silica content, the number of by thedots synergy of WPUnotably, and silica particles. With increasing silica content,ranged the number of silica dots silica increased, the average particle size for WPU/Silica from 100~200 nm silica dots increased, notably, the average particle size for WPU/Silica ranged from 100~200 nm increased, the average particle size for WPU/Silica ranged from 100~200 nmIn(Figure 5b) as (Figure 5b) 5b)notably, as analyzed analyzed by NanoScope-Analysis NanoScope-Analysis software (Veeco (Veeco Instruments, USA). comparison, (Figure as by software Instruments, USA). In comparison, silica particles particles in in WPU/Silica-1.0-B WPU/Silica-1.0-B had had diameters diameters larger larger than than 300 300 nm. nm. This This is is evidence evidence that that in in situ situ silica

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analyzed by10, NanoScope-Analysis Polymers 2018, x FOR PEER REVIEW software (Veeco Instruments, USA). In comparison, silica particles 8 of in 15 WPU/Silica-1.0-B had diameters larger than 300 nm. This is evidence that in situ polymerization can polymerization can promote the the interactions between chains ofwhich silica stabilizes and prepolymers, promote the interactions between alkyl chains of silicathe andalkyl prepolymers, the silica which stabilizes silica particles in preventing WPU dispersions, thereby preventing silica to from cohesion and particles in WPUthe dispersions, thereby silica from cohesion and leading better dispersion leading to betterThe dispersion and uniformity. The results were consistent with thesilica SEMcontent analysis. In and uniformity. results were consistent with the SEM analysis. In addition, also addition, silica contentdistribution. also influences therecent particle distribution. recent research Xue et al. [26] influences the particle In the research of XueIn et the al. [26] about silica of hybrid coatings, about silica hybrid coatings, although of particles were homogeneous with a load of 3% although distributions of particles weredistributions homogeneous with a load of 3% silica, distinct agglomeration silica,observed distinct in agglomeration observedwould in theimpair hybrids. defection would of impair the were the hybrids. were This defection the This mechanical properties coatings, mechanical properties of coatings, however, not discussed in their research. Taking which, however, was not discussedwhich, in their research.was Taking a closer look at Figure 5b, there area closer lookand at Figure 5b,boundaries there are nobetween cracks and obvious boundaries alkyl-grafted silicagood and no cracks obvious alkyl-grafted silica andbetween WPU matrix due to their WPU matrix due to their goodfrom compatibility, which benefits from theon existence of polymer on compatibility, which benefits the existence of polymer chains the surface of silica.chains Surface the surface(R ofa )silica. Surface roughness (Ra) of by coatings were also analyzed by AFM. Expectedly, roughness of coatings were also analyzed AFM. Expectedly, surface roughness of coatings surface roughness of coatings after addition of silica as to Ra 3.481 increased from 0.850 of WPU to increased after addition of silicaincreased as Ra increased from 0.850 of WPU of WPU/Silica-2.0. Even so, 3.481 of WPU/Silica-2.0. Even so,showed the addition of alkyl-grafted silicaand showed little influence on the addition of alkyl-grafted silica little influence on appearance transparency of coatings appearance (Figure S2). and transparency of coatings (Figure S2).

Figure of WPU WPU and and WPU/Silica WPU/Silica coatings; (b) particle particle distribution distributionof ofWPU/silica. WPU/silica. Figure 5. 5. (a) (a) 3D 3D topography topography of coatings; (b)

3.3. Contact Tension of of WPU/Silica WPU/Silica Coatings Coatings 3.3. Contact Angle Angle and and Surface Surface Tension Figure 66shows showsthe thewater water and glycol contact angles on coatings. the content of alkylFigure and glycol contact angles on coatings. WhenWhen the content of alkyl-grafted ◦ to grafted silica increased from 0% to 2%, the contact angle (CA) of water on coating increased silica increased from 0% to 2%, the contact angle (CA) of water on coating increased from 72.0from ◦ ◦ ◦ 72.0° to 87.3° whilst the CA of ethylene glycol increased from 63.5° to 70.5°. This improved 87.3 whilst the CA of ethylene glycol increased from 63.5 to 70.5 . This improved hydrophobicity of hydrophobicity of WPU/Silica coating may be because of the presence of nanosized silicon papillae WPU/Silica coating may be because of the presence of nanosized silicon papillae on the surface creating on the surface creating the “lotus effect” [27]. Although WPU/Silica-1.0 has same silica contentitswith the “lotus effect” [27]. Although WPU/Silica-1.0 has same silica content with WPU/Silica-1.0-B, CA WPU/Silica-1.0-B, its CA of water ethylene glycol were larger than According that of WPU/Silica-1.0-B. of water and ethylene glycol were and larger than that of WPU/Silica-1.0-B. to the Wenzel According to surface the Wenzel relationcould [28], lead surface roughness hydrophilicity could lead to enhanced hydrophilicity of relation [28], roughness to enhanced of hydrophilic film. Since hydrophilic film. Since WPU/Silica-1.0 possesses smaller R a than that of WPU/Silica-1.0-B, WPU/Silica-1.0 possesses smaller Ra than that of WPU/Silica-1.0-B, WPU/Silica-1.0 should show WPU/Silica-1.0 should less hydrophilicity than Surface free energy (γ) is less hydrophilicity thanshow WPU/Silica-1.0-B. Surface freeWPU/Silica-1.0-B. energy (γ) is another important parameter another important parameter to evaluate the properties of coating such as self-cleaning, adhesion, to evaluate the properties of coating such as self-cleaning, adhesion, and anti-corrosion. Here, γ of and anti-corrosion. Here, γ of coatings were calculated by substituting the contact angle values obtained using water and ethylene glycol in Owens-Wendt equation [29]. The results indicate the difference in the dispersive γd and the polar component γp of the WPU/Silica coatings (Table 1). These variations ultimately lead to the decreased γ after the addition of silica. As shown in Table 1, by

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coatings were calculated by substituting the contact angle values obtained using water and ethylene glycol in Owens-Wendt equation [29]. The results indicate the difference in the dispersive γd and the polar component γp of the WPU/Silica coatings (Table 1). These variations ultimately lead to Polymersγ2018, 10, xthe FOR PEER REVIEWof silica. As shown in Table 1, by increasing the content 9 of 15 of silica the decreased after addition from 0% to 2%, γ decreased from 32.87 to 21.54 mJ/m2 , implying the enhanced hydrophobicity of increasing the content of silica from 0% to 2%, γ decreased from 32.87 to 21.54 mJ/m2, implying the WPU/Silica coatings. enhanced hydrophobicity of WPU/Silica coatings.

Figure 6. Contact angle of WPU and WPU/Silica coatings: (a) water and (b) glycol.

Figure 6. Contact angle of WPU and WPU/Silica coatings: (a) water and (b) glycol. Table 1. Surface free energy of WPU and WPU/Silica coatings.

Table 1. Surface of WPU andEnergy WPU/Silica Sample free energySurface Free (mJ/m2)coatings. γd γp γ 2 Surface Free Energy (mJ/m 5.23 ± 0.88 27.64 ± 4.43 32.87 ± 3.55) WPU/Silica-1.0-B 9.84 26.64 ± 0.78 γ p γd ± 0.51 16.80 ±γ0.76 WPU/Silica-1.0 9.57 ± 2.16 15.70 ± 4.68 25.27 ± 2.52 WPU ± 0.88 27.64 ± 4.4321.54 ± 32.87 WPU/Silica-2.0 5.23 12.17 ± 0.93 9.37 ± 1.01 1.02 ± 3.55

Sample WPU

WPU/Silica-1.0-B WPU/Silica-1.0 3.4. Solvent Resistant Properties WPU/Silica-2.0

9.84 ± 0.51 9.57 ± 2.16 12.17 ± 0.93

16.80 ± 0.76 15.70 ± 4.68 9.37 ± 1.01

26.64 ± 0.78 25.27 ± 2.52 21.54 ± 1.02

Figure 7 shows the water and acetone swelling of WPU coatings as a function of alkyl-grafted silica content. The incorporation of silica decreased the swelling ratio arises from both water and 3.4. Solvent Resistant Properties acetone. Specifically, the value of water and acetone swelling ratio decreased by 27% and 15%, respectively, as the content of silica was up to 2.0%. The results demonstrate that the presence of Figure 7 shows the and swelling WPU coatings as aenhances function alkyl-grafted impermeable silicawater particles in acetone WPU matrix reducesofthe swelling ratio and theof solvent silica content. TheThis incorporation of silica decreased ratio arises from water and resistance. phenomenon may be attributed to that the silicaswelling can work as cross-linking pointsboth in WPU matrix through interactions with polyurethane molecules [10,11]. Thus, the increased crosslinking acetone. Specifically, the value of water and acetone swelling ratio decreased by 27% and 15%, density can make it difficult for solvent molecules to penetrate and diffuse into the coatings, therefore respectively, as the content of silica was up to 2.0%. The results demonstrate that the presence of improving the solvent resistance. The enhancement of solvent resistance in other PU composite impermeable silica particles in WPU matrix reduces the swelling ratio and enhances the solvent systems containing inorganic nanofillers such as nano-ZnO has also been reported [30].

resistance. This phenomenon may be attributed to that silica can work as cross-linking points in WPU matrix through interactions with polyurethane molecules [10,11]. Thus, the increased crosslinking density can make it difficult for solvent molecules to penetrate and diffuse into the coatings, therefore improving the solvent resistance. The enhancement of solvent resistance in other PU composite systems containing inorganic nanofillers such as nano-ZnO has also been reported [30].

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Figure 7. Water and acetone swelling ratio of coatings. Figure 7. Water and acetone swelling ratio of coatings.

3.5. Thermal Properties

3.5. Thermal Properties

Figure 7. Water and acetone swelling ratio of coatings.

Previous research reported that the polar surface of fillers can influence the crystallization behavior of PU blocky segments [31]. Thus,surface DSC was used tocan investigate thethe interactions of alkylPrevious research reported that the polar of fillers influence crystallization behavior 3.5. Thermal Properties grafted silica with WPU. As presented in Figure 8, all investigated samples showed a pronounced of PU blocky segments [31]. Thus, DSC was used to investigate the interactions of alkyl-grafted silica reported that the polar surface the crystallization glassPrevious transitionresearch behavior in the temperature range from of −70fillers to −40can °C.influence This transition is due to the with WPU. As presented in Figure 8,[31]. all investigated samples showed athe pronounced glass transition behavior of of PU blocky segments Thus, DSC was used to investigate interactions of higher alkylmovement soft-segment chains. Meanwhile, a broad endothermic peak can be observed at ◦ behavior in the temperature range −70 to of −8,hard 40 C. This transition is duethe to the movement of grafted silica with As presented in Figure all investigated samples showed a alkyl-grafted pronounced temperature, whichWPU. is related to from crystallization segments. With increasing soft-segment chains. Meanwhile, a broad endothermic peak can be observed at higher temperature, glass transition behavior in the temperature range from −70 to −40 °C. This transition is to the silica content, the glass transition temperature (Tg) of the soft-segment was increased due whilst the of crystallization soft-segment Meanwhile, broad endothermic canalkyl-grafted be observedthat at silica higher endothermic peak of the chains. hard-segment wasa expanded. Naik et peak al. the [32] presented weakcontent, which ismovement related to of hard segments. With increasing which isfiller related of reduce hard segments. increasing theendothermic alkyl-grafted interactions and would the increased Tg ofWith composite. et al. [33] peak of the glasstemperature, transitionbetween temperature (Tto )crystallization ofmatrix the soft-segment was whilstHassanajili the gPU silica content, the glass transition crystalline temperature (Tg) of contribute the soft-segment was increased whilst the suggested that organic-inorganic domains to the increased T g and melting the hard-segment was expanded. Naik et al. [32] presented that weak interactions between filler and endothermic the ofhard-segment wasincreases expanded. Naik et al. peaks [32] presented that weak point. Hence,peak in theofcase WPU/Silica, the of endothermic may be attributed to PU matrix would reduce the of PU composite. Hassanajili et Tal. [33] suggested that organic-inorganic interactions between fillerTgand matrix would the g of composite. Hassanajili al.hand [33] strong interfacial interaction between silica and reduce polyurethane molecules, which on the et one crystalline domains contribute to the increased Tdomains melting point. Hence, inmovement the WPU/Silica, g andthe suggested organic-inorganic crystalline contribute toofthe Tgcase and of melting restrict thethat movement of soft segment and reduces free volume theincreased chain [34], on point. Hence, in the case of WPU/Silica, the increases of endothermic peaks may be attributed to silica the increases of endothermic peaks may be attributed to strong interfacial interaction between the other hand create new hybrid domain with higher stiffness. Thus, the endothermic peak of hybrid strong interfacial interaction between silica and polyurethane molecules, which on the one hand domain maymolecules, superimpose with that of the polyurethane phase, causing the peak shifting and polyurethane which on the oneoriginal hand restrict the movement of soft segment andtoreduces ofmovement soft segment[34], and on reduces the free volume of the chain movement [34], on higher the highthe temperature region. the freerestrict volume ofmovement the chain the other hand create new hybrid domain with the other hand create new hybrid domain with higher stiffness. Thus, the endothermic peak of hybrid stiffness. Thus, the endothermic peak of hybrid domain may superimpose with that of the original domain may superimpose with that of the original polyurethane phase, causing the peak shifting to polyurethane causing the peak shifting to the high temperature region. the highphase, temperature region.

Figure 8. DSC curves of WPU and WPU/Silica coatings.



Thermal stability is one of the references to determine the life of coatings applied in an environment with high temperature. Thermal degradation behavior for selected samples were demonstrated in TGA and DTGA as shown in Figure 9. The weight loss is summarized in Table 2.


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Thermal stability is one of the references to determine the life of coatings applied in an environment with high temperature. Thermal degradation behavior for selected samples were Polymers 2018, 10, 514 11 of 15 demonstrated in TGA and DTGA as shown in Figure 9. The weight loss is summarized in Table 2. Apart from a slight weight loss at 200 °C, which can be assignable to the degradation of alkyl chains, no significant massweight loss was observed when alkyl-grafted silica was heated up to 500of°C, indicating Apart from a slight loss at 200 ◦ C, which can be assignable to the degradation alkyl chains, ◦ its significant intrinsic high thermal stability. As for WPU coatings, the firstwas peakheated loss atup 280to°C attributed to the no mass loss was observed when alkyl-grafted silica 500 C, indicating ◦ degradation of hard-segment whilst the second major loss (about 60%) occurred from 360 its intrinsic high thermal stability. As for WPU coatings, the first peak loss at 280 C attributed °C to corresponded to of thehard-segment decomposition of soft-segments as reported previously The incorporation the degradation whilst the second major loss (about 60%) [15]. occurred from 360 ◦ C of alkyl-grafted silica induced thermal stabilizationasofreported WPU/silica coatings. For example, with the corresponded to the decomposition of soft-segments previously [15]. The incorporation of load of 2.0% silica, the temperature for 50% weight loss was increased by about 30 °C compared to alkyl-grafted silica induced thermal stabilization of WPU/silica coatings. For example, with the load ◦ neat WPU. The stability attributed to thermal of of 2.0% silica, theimprovement temperature in forthermal 50% weight loss can wasbe increased by about 30 Cinsulation comparedeffect to neat silica lead to the delay of molecule chains degradation. Besides this, the interaction between the silica WPU. The improvement in thermal stability can be attributed to thermal insulation effect of silica lead particles andofmolecules lead toBesides increasing energybetween needed for to the delay molecule chains chains would degradation. this,potential the interaction the decomposition silica particles of polymer chains. It would is also lead presented that thepotential tortuousenergy path effect of for fillers in polymer of matrix can and molecules chains to increasing needed decomposition polymer retard the escape of volatile degradation products and char formation [35]. Moreover, compared to chains. It is also presented that the tortuous path effect of fillers in polymer matrix can retard the WPU, aofnew small peak in theproducts temperature range of 335 to[35]. 355 Moreover, °C appeared in the DTGA curves of escape volatile degradation and char formation compared to WPU, a new ◦ Cpeak WPU/Silica. increasing therange content of silica, point shifted to higher temperature, which small peak inWith the temperature of 335 to 355the appeared in the DTGA curves of WPU/Silica. indicates the existing of a new hybrid With increasing the content of silica, thedomain. peak point shifted to higher temperature, which indicates the existing of a new hybrid domain.

Table 2. TG data of WPU and WPU/Silica coatings.

Table 2. TG data of WPU and WPU/Silica coatings.

Samples Samples WPU

WPU/Silica-0.5 WPU WPU/Silica-1.0 WPU/Silica-0.5 WPU/Silica-1.5 WPU/Silica-1.0 WPU/Silica-2.0 WPU/Silica-1.5

Temperature (°C) 30% Weight Loss 50% Weight (Loss Tmax1 ◦ C) Temperature 326.3 361.8 315.8 30% Weight Loss 50% Weight Loss Tmax1 331.0 370.1 319.8 326.3 361.8 315.8 334.8 383.4 320.0 331.0 370.1 319.8 339.5 386.3 321.8 334.8 383.4 320.0 339.5 388.2 322.6 339.5 386.3 321.8

Tmax2 393.2 Tmax2 404.0 393.2 413.2 404.0 416.5 413.2 419.8 416.5

WPU/Silica-2.0 339.5 322.6 419.8 Abbreviations: Tmax1, temperature at first major weight 388.2 loss; Tmax2, temperature at first major Abbreviations: Tmax1 , temperature at first major weight loss; Tmax2 , temperature at first major weight loss. weight loss.

Figure9. 9. (a) (a) TGA TGA and and (b) (b) DTGA DTGA curves curvesof ofalkyl-grafted alkyl-graftedsilica, silica,WPU, WPU,and andWPU/Silica WPU/Silica coatings. coatings. Figure

3.6. Mechanical Properties 3.6. Mechanical Properties Figure 10a demonstrates temperature dependence of storage modulus (E′) for different samples. Figure 10a demonstrates temperature dependence of storage modulus (E0 ) for different samples. It can be seen that the introduction of alkyl-grafted silica significantly increased the E′ of WPU/Silica It can be seen that the introduction of alkyl-grafted silica significantly increased the E0 of WPU/Silica coatings in the region where hard-segments were in the glassy state (Figure 10a). This enhanced coatings in the region where hard-segments were in the glassy state (Figure 10a). This enhanced stiffness indicates that the presence of silica preferentially resided in the soft segment, which hinders stiffness indicates that the presence of silica preferentially resided in the soft segment, which hinders the macromolecular chain segmental movements and results in tougher hybrid coatings. Figure 10b the macromolecular chain segmental movements and results in tougher hybrid coatings. Figure 10b shows that WPU/Silica coatings had broad peaks of loss factor (tanδ) at low temperature (−45~−60 °C) shows that WPU/Silica coatings had broad peaks of loss factor (tanδ) at low temperature (−45~−60 ◦ C) and high temperature regions (0~125 °C), which corresponded to the glass transition temperature of and high temperature regions (0~125 ◦ C), which corresponded to the glass transition temperature of soft-segment (Tg-s ) and hard-segment (Tg-h ), respectively. This temperature range of transition is consistent with the results of DSC. Compared to neat WPU, the Tg-h of hard-segment for WPU/Silica


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soft-segment (Tg-s) and hard-segment (Tg-h), respectively. This temperature range of transition is Polymers 2018, 514 results of DSC. Compared to neat WPU, the Tg-h of hard-segment for WPU/Silica 12 of 15 consistent with10,the first decreased at 0.5% silica loading and then gradually shifted to higher temperature region when further increasingat silica content. Thisand kind silica-content variationsregion of Tg when are also first decreased 0.5% silica loading thenofgradually shifteddetermined to higher temperature Polymers 2018, 10, x FOR PEER REVIEW 12 of 15 reported other literature [36]. On one hand, the incorporation silicavariations particle may furtherinincreasing silica content. This kind of silica-content determined of Tgweaken are also the interactions between the hard-segment and increase the free volume of molecular chains; on other reported in other(Tliterature [36]. On one the incorporation silica particle weaken soft-segment g-s) and hard-segment (Tg-hhand, ), respectively. This temperature range ofmay transition isthethe hand, silica can be compounded with polyurethane segments form hybridchains; domains with high interactions between hard-segment and increase the free the volume molecular on the other consistent with thethe results of DSC. Compared to neat WPU, Tg-hto ofof hard-segment for WPU/Silica first decreased at 0.5% silica loading and then gradually shifted to higher temperature region when hand, silica can be compounded with polyurethane segments to form hybrid domains with high modulus. These two effects are in a competitive relationship. Hence, the results indicate that when further increasing silica are content. This silica-content determined variations ofdominates, Tgthat are when also leading These two effects relationship. Hence, the indicate the themodulus. alkyl-grafted silica content isinataacompetitive low kind levelof(0.5%), the influence of results the former reported in other literature [36]. On one hand, the incorporation silica particle may weaken the to alkyl-grafted silica content is at a low level (0.5%), the influence of the former dominates, leading to the decreased of Tg-h. After increasing the silica content (>1%), the effect of the latter governs, thus interactions the hard-segment andsilica increase the free volume of effect molecular chains; ongoverns, the other thus the decreased ofbetween T . After increasing the content (>1%), the ofhigher the latter g-h increasing the T g-h . In the case of soft-segment, T g-s steadily shifted to the temperature region hand, silica can be compounded with polyurethane segments to form hybrid domains with high increasing the T . In the case of soft-segment, T steadily shifted to the higher temperature region g-s g-h with silica content astwo evidenced results. This may be because have long modulus. These effects are in in aDSC competitive relationship. Hence, the resultssoft-segments indicate that when with silica content as evidenced in DSC results. This may be because soft-segments have long molecular the alkyl-grafted contentmobility is at a low which level (0.5%), the influence of restricted the former dominates, leading of silica molecular chains withsilica inherent would be easily in the vicinity chains with inherentofmobility would be easily restricted in effect the vicinity of silica particles due the decreased Tg-h. Afterwhich increasing silica content the of the latter governs, thus particlestodue to the entanglement betweenthe alky chain on(>1%), surface of silica and molecules chains. The to theincreasing entanglement between alky chain on surface of silica and molecules chains. The peak values Tg-h. In the case of soft-segment, Tg-s steadily shifted to the higher temperature region peak values and the width of loss factor (tanδ) can be used to predict the damping behavior which is and width of loss factoras(tanδ) can be predict themay damping behavior which ishave important with silica content evidenced in used DSC to results. This be because soft-segments long for important for special coatings with vibration damping, thermal insulation, and sealing performance. molecular inherent mobilitythermal which would be easily in the vicinity As of silica special coatingschains with with vibration damping, insulation, andrestricted sealing performance. shown in As Figure shown in Figure by incorporating silica, values of of tanδ ofand coatings increased, indicating particles to10b, the entanglement between alkyofpeak chain silica molecules chains. The 10b, bydue incorporating silica, peak values tanδon of surface coatings increased, indicating the enhanced thedamping enhanced damping capacity. This verifies that properties of WPU cancontrolling be enhanced peak capacity. values andThis width of lossthat factor (tanδ) can bedamping used to the be damping behavior which is the by verifies damping properties of predict WPU can enhanced by important for special coatings with vibration damping, thermal insulation, and sealing performance. controlling the content of incorporated silica. content of incorporated silica. As shown in Figure 10b, by incorporating silica, peak values of tanδ of coatings increased, indicating the enhanced damping capacity. This verifies that damping properties of WPU can be enhanced by controlling the content of incorporated silica.

0

Figure Storagemodulus modulus(E′) (E )and and (b) (b) loss loss factor and WPU/Silica coatings. Figure 10.10. (a)(a) Storage factor(tanδ) (tanδ)ofofWPU WPU and WPU/Silica coatings. Figure 10. (a) Storage modulus (E′) and (b) loss factor (tanδ) of WPU and WPU/Silica coatings.

Representative stress-straincurves curves forWPU/Silica WPU/Silicacoatings coatingsare areshown shown in in Figure 11. 11. When When the Representative stress-strain Representative stress-strain curvesfor for WPU/Silica coatings are shown in FigureFigure 11. When the the content of alkyl-grafted silica increased from 0% to 2.0%, the breaking strength of WPU/Silica content content of alkyl-grafted silica increased to 2.0%, 2.0%,thethe breaking strength of WPU/Silica of alkyl-grafted silica increasedfrom from 0% 0% to breaking strength of WPU/Silica coatings increased from 11.10 toto 17.2 MPa. The results indicatethat that the presence of silica particle coatings increased from 11.10 17.2 MPa. The results indicate the presence of silica particle can cancan coatings increased from 11.10 to 17.2 MPa. The results indicate that the presence of silica particle act asact a as physical netpoint, contributing tothe thehigh high strength of WPU/Silica coatings. Nevertheless, a physical netpoint, contributing to strength of WPU/Silica coatings. Nevertheless, the act as a physical netpoint, contributing to the high strength of WPU/Silica coatings. Nevertheless, the the presence of silica led to some decline of elongation property. By increasing silica content from presence of silica led to some decline of elongation property. By increasing silica content from 0% to 0% presence of silica led to some decline of elongation property. By increasing silica content from 0% to strainatatthe thebreak breakdecreased decreased from implying thethe reduced ductility of coatings to 2%,2%, thethe strain from480% 480%toto350%, 350%, implying reduced ductility of coatings 2%,which the which strain at the break decreased from 480% to 350%, implying the reduced ductility result from restriction effect silica on the chains [9]. [9]. of coatings maymay result from thethe restriction effect ofofsilica on the mobility mobilityofofmolecular molecular chains

which may result from the restriction effect of silica on the mobility of molecular chains [9].

Figure Stress-straincurves curves of of WPU WPU and coatings. Figure 11.11. Stress-strain andWPU/Silica WPU/Silica coatings.

Figure 11. Stress-strain curves of WPU and WPU/Silica coatings.

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4. Conclusions After addition of alkyl-grafted silica, the average particle size of waterborne polyurethane (WPU) dispersions enlarged whilst the particle size distribution broadened. The emulsion viscosity increased with increasing content of alkyl-grafted silica. Results verify that the in situ incorporation of alkyl-grafted silica does not impair the synthesis of high solid content of WPU. FTIR study confirmed the successful incorporation of alkyl-grafted silica into polyurethane matrix and their preferred molecular interactions. Evidenced by AFM and SEM results, alkyl-grafted silica showed good dispersion and compatibility with WPU. The nanosized silica papillae on surface contributed to the enhanced surface roughness and hydrophobicity of coatings. Solvent resistance of WPU coatings was improved with existence of alkyl-grafted silica particles. Tg of WPU/Silica coatings shifted to the higher temperature region with increasing silica content. The combination of thermal insulation ability and tortuous path effect of silica particles led to better thermal stability. Compared to neat WPU, WPU/Silica coatings showed higher storage modulus, Young’s modulus, and tensile strength without much loss of ductility. Hence, the resultant hybrid coatings present superior comprehensive properties, which will contribute to the coating applications of WPU with high solid content. Supplementary Materials: The following are available online at http://www.mdpi.com/2073-4360/10/5/514/s1, Figure S1: Phase images of WPU and WPU/Silica coatings, Figure S2: Photos of WPU and WPU/Silica coatings. Author Contributions: Y.H. and Z.X. conceived and designed the experiments. Y.H. carried out the laboratory experiments, analyzed the data, interpreted the results, and prepared the figures. Y.H. and J.H. wrote the manuscript. Acknowledgments: The authors of this paper would like to acknowledge the financial support from projects Research Grants Council (RGC) of Hong Kong (Project Number: 500810), Development of Environmental Friendly and Self-Adaptive Breathable Polyurethane Film (GHX/001/16GD), and the Science and Technology Planning Project of Guangdong Province, China (Project Number: 2016A050503013). Conflicts of Interest: The authors declare no conflict of interest.

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