materials

Article Waterborne Polyurethane Coatings with Covalently Linked Black Dye Sudan Black B

Tao Wang ID , Wei Sun, Xingyuan Zhang * ID , Haiyan Xu and Fei Xu

CAS Key Laboratory of Soft Matter Chemistry, Department of Science and Engineering, University of Science and Technology of China, Hefei 230026, China; [email protected] (T.W.); [email protected] (W.S.); [email protected] (H.X.); [email protected] (F.X.) * Correspondence: [email protected]; Tel.: +86-0551-6360-7484

Received: 17 September 2017; Accepted: 26 October 2017; Published: 28 October 2017

Abstract: Colored waterborne polyurethanes have been widely used in paintings, leathers, textiles, and coatings. Here, a series of black waterborne polyurethanes (WPUs) with different ratios of black dye, Sudan Black B (SDB), were prepared by step-growth polymerization. WPU emulsions as obtained exhibit low particle sizes and remarkable storage stability at the same time. At different dye loadings, essential structural, statistical and thermal properties are characterized. FTIR (fourier transform infrared) spectra indicate that SDB is covalently linked into waterborne polyurethane chains. All of the WPUs with covalently linked SDB show better color fastness and resistance of thermal migration than those with SDB mixed physically. Besides, WPUs incorporated SDB covalently with different polymeric diols, polytetramethylene ether glycol (PTMG), polypropylene glycol (PPG), poly-1, 4-butylene adipate glycol (PBA) and polycaprolactone glycol (PCL), were prepared to obtain different properties to cater to a variety of practical demands. By a spraying method, the black WPUs can be directly used as metal coatings without complex dyeing process by simply mixing coating additive and other waterborne , which exhibit excellent coating performance.

Keywords: Sudan black B; waterborne polyurethane; polymeric dye; metal coatings

1. Introduction Recently, colored polymeric dyes, especially back dyes, have been widely used in textiles, ink, coatings, leathers and photoelectric materials [1–6]. Generally, the strategy on developing polymeric dyes is usually to physically mix micromolecular dyes with polymeric matrices by ionic bonds, hydrogen bonds or Van der Waals force. However, there is a thorny problem that, with time elapsing, the dyes may migrate and aggregate, leading to color fading of materials due to the noncovalent bond interaction between matrices and dyes. Moreover, plenty of black dyes containing benzidine groups have a potential risk to humans and environment [7–9]. Therefore, low-toxic and environmentally friendly materials are of great importance. An effective method to solve the problem is to chemically link micromolecular dyes to polymeric main chains [10,11] or side chains [12] by various chemical reactions. Generally speaking, polymeric dyes are safe and nontoxic for humans because they cannot be absorbed by skin owing to their large molecular dimension, excellent chemical and thermal stability. Moreover, polymeric dyes with tunable molecular structures exhibit great compatibility and strong binding force with fibers. In the past decades, many researchers have been devoted to investigating polymeric dyes. For example, in the 1980s, Marechal et al. studied systemically on polymeric dyes for the first time [13–15]. Recently, many polymeric dyes have been prepared by incorporating chromophores into common polymeric materials, such as polyacrylates, polyethlene, polyamide and polymaleic acid, to enlarge the application fields [12,16–19]. Waterborne polyurethanes (WPUs) as a kind of highly versatile polymeric material

Materials 2017, 10, 1247; doi:10.3390/ma10111247 www.mdpi.com/journal/materials Materials 2017, 10, 1247 2 of 13 with excellent environment-friendly and low-toxic properties have been widely used as coatings, leathers, , and paints [20–22]. By a facile polycondensation reaction, a lot of colored WPUs are developed by chemically incorporating micromolecular dyes into polyurethane matrices, which could show great migration resistance and color fastness while not obviously changing the intrinsic characteristic of the polymer materials [23–26]. However, there are few of reports on black dyes because of incomplete purity of black for purely organic dyes. Here, A series of novel black WPU dyes with different ratios of black dye, Sudan Black B (SDB), were prepared by polycondensation reaction, which exhibit good migration resistance, storage stability and heat resistance, while, at the same time, not causing significant aggregation/phase separation between micromolecular dyes and WPU matrices. Besides, Black WPUs with different polymeric diols are also investigated serving as black metal coatings, which exhibit excellent coating performance.

2. Experimental

2.1. Materials Isophorone diisocyanate (IPDI) was purchased from Bayer Co., Ltd. (Leverkusen, Germany) Polytetramethylene ether glycol (PTMG, Mn = 2000) and polypropylene glycol (PPG, Mn = 2000) were supplied by Mitsubishi Co., Ltd. (Tokyo, Japan). Poly-1,4-butylene adipate glycol (PBA, Mn = 2000) was obtained from Qingdao Xinyutian Chemical Co., Ltd. (Qingdao, China). Polycaprolactone glycol (PCL, Mn = 2000) was supplied by Daicel Corporation (Kobe, Japan). All of the glycols were thoroughly dehydrated at 110 ◦C before use. 2, 2-dimethylolpropionic acid (DMPA) and Sudan black B (SDB, C.I.26150) were purchased form Aladdin Reagent Co., Ltd. (Shanghai, China). 1,4-Butanediol (BDO), Dibutyltin dilaurate (DBTDL), triethylamine (TEA), were purchased from Sinopharm Chemical Reagent Co., Ltd. (Hefei, China). BYK-025 and BYK-331 were acquired from BYK-Chemie GmbH Co., Ltd. (Wesel, Germany). ACRYSOLTM RM-8W was provided by Rohm&Haas Co., Ltd. (Philadelphia, America). Other reagents were obtained from Energy Reagent Co., Ltd. (Shanghai, China), and used as received.

2.2. Methods Fourier transform infrared (FTIR) spectra were recorded on a Bruker Tensor27 FTIR spectrometer (Bruker Co., Ltd., Karlsruhe, Germany) in the range of 4000–500 cm−1 using the thin WPU films (less than 20 µm in thickness) prepared by coating SDB-WPU emulsions on a potassium bromide (KBr) flake and then evaporating by heating under an infrared lamp. -visible (UV-vis) spectra of SDB and WPU dispersion were measured by UV-3600 spectrophotometer (Shimadzu Co., Ltd., Kyoto, Japan) in dimethylformamide (DMF) and water at 298 K. UV-vis spectra of WPU films were recorded on UV-vis-NIR spectrometer (Shimadzu Co., Ltd., Kyoto, Japan) ranging from 240 nm to 1200 nm at 298 K. Dynamic light scattering (DLS): The particle size distribution of WPU emulsion were carried out on a Zetasizer Nano ZS-90 (Malvern Co., Ltd., Worcestershire, the United Kongdom) by dynamic light scattering (DLS) at room temperature. Gel permeation chromatography (GPC) analyses were investigated by a GPC instrument system (Waters Co., Ltd., Milford, Massachusetts, America) and calibrated with linear polystyrene, at a constant column temperature of 35 ◦C using tetrahydrofuran (THF) as eluent with a flow rate of 0.6 mL/min. Differential scanning calorimetric (DSC) curves were recorded via a Mettler-Toledo DSC (Mettler-Toledo Co., Ltd., Zurich, Switzerland) at a constant heating rate of 10 ◦C/min from −65 ◦C ◦ –180 C under N2 atmosphere. Thermogravimetric analysis (TGA) curve was collected by Shimadzu TGA-50 (Shimadzu Co., ◦ ◦ ◦ Ltd., Kyoto, Japan) under N2 atmosphere at a constant heating rate of 10 C/min from 25 C to 700 C. Materials 2017, 10, 1247 3 of 13

Materials 2017, 10, 1247 3 of 13 WPU films were prepared by pouring WPU emulsions onto some polytetrafluoroethylene panels, ◦ and theWPU panels films were were left preparedat 25 C for by 48 pouring h to obtain WPU a series emulsions of homogeneous onto some thin polytetrafluoroethylene films with a thickness ofpanels 1 mm., and The the as panels obtained were films left were at 25 characterized °C for 48 h to without obtain a further series of annealing homogeneous process. thin films with a thicknessThe migration of 1 mm. The ratio as (Mp) obtained was used films to were evaluate characterized migration without resistance further of SDB-WPU. annealing Aprocess. glass plate was coatedThe migration with SDB-WPU ratio (Mp) latex was to formused uniformto evaluate film migration and divided resistance into two of areas SDB (Area-WPU. A A and glass Area plate B), aswas shown coated in with Scheme SDB1-.WPU Area latex A was to clampedform uniform by a watchfilm and glass divided tightly, into while two Areaareas B(Area was exposedA and Area to ◦ theB), air.as show Then,n thein Scheme glass plate 1. Area was A kept was at clamp 60 Ced for by 24 a h. watch The films glass in tightly, Area A while and AreaArea BB werewas exposed selected andto the dissolved air. Then, into the DMF glass at plate the same was kept concentration at 60 °C for and 24 the h. absorbanceThe films in was Area measured A and Area by UV-3600 B were spectrophotometer.selected and dissolved Mp into was DMF calculated at the same using concentration the followed formula:and the absorbance was measured by UV- 3600 spectrophotometer. Mp was calculated using the followed formula: Mp = [(AmaxB − AmaxA)/AmaxA] × 100% (1) Mp=[(AmaxBAmaxA)/AmaxA]100% (1) where AmaxA and AmaxB are the maximum absorbance (Amax) of the SDB-WPU in DMF in Area A and where AmaxA and AmaxB are the maximum absorbance (Amax) of the SDB-WPU in DMF in Area A and Area B, respectively. Area B, respectively.

Scheme 1. Evaluation model for thermal migration of the dye. Scheme 1. Evaluation model for thermal migration of the dye.

PreparationPreparation ofof thethe blackblack coatings:coatings: SDB-WPU,SDB-WPU, defoamer, thickener,thickener, levelingleveling agentagent andand deionizeddeionized waterwater werewere addedadded intointo aa dispersiondispersion machine.machine. AfterAfter stirringstirring atat highhigh speedspeed untiluntil aa well-blendedwell-blended latexlatex formsforms andand thenthen beingbeing leftleft toto standstand forfor 3030 min,min, thethe latexlatex waswas sprayedsprayed onon thethe surfacesurface ofof steelsteel plateplate andand ◦ keptkept atat 100100 °CC forfor 2020 min.min. TheThe physicalphysical properties properties of of the the SDB-WPU SDB-WPU black black coatings coatings were were evaluated evaluated according according to standard to standard test methodstest methods (impact (impact strength strength GB/T GB/T 1732-93, 1732-93, Gloss Gloss GB/T GB/T 1743-89, 1743-89, adhesion adhesion force force GB/T GB/T 1720-89,1720-89, pencilpencil hardnesshardness GB/TGB/T 6739 6739-2006,-2006, and water resistance GB/T 23999 23999-2009).-2009).

2.3. Synthesis of SDB-WPUs 2.3. Synthesis of SDB-WPUs TheThe preparationpreparation processesprocesses ofof SDB-WPUsSDB-WPUs areare shownshown inin SchemeScheme2 2.. IPDI IPDI and and PTMG PTMG (Mn (Mn = = 2000)2000) were added to a three-neck round-bottom flask (specific mass ratios are presented in Table1). Then, the were added to a three-neck◦ round-bottom flask (specific mass ratios are presented in Table 1). Then, mixture was heated at 90 C for 2 h under an N2 atmosphere (NCO content was determined using a the mixture was heated at 90 °C for 2 h under an N2 atmosphere (NCO content was determined using standard di-n-butylamine titration test method [27]). Subsequently, the mixture was cooled down to a standard◦ di-n-butylamine titration test method [27]). Subsequently, the mixture◦ was cooled down 80to 80C; °C a; specified a specified amount amoun oft DMPAof DMPA was was added; added and; and it was it was again again heated heated to 80 to 80C for°C approximatelyfor approximately 2 h until the content of the NCO group in the mixture reached the expected theoretical value. The calculated 2 h until the content of the NCO group in the◦ mixture reached the expected theoretical value. The amountscalculated of amounts BDO and of SDB BDO were and addedSDB were at 70 addedC and at allowed70 °C and to allowed react for to 4 hreact with for a trace4 h with amount a trace of DBTDLamount (0.05–0.1of DBTDL wt (0.05 %)– as0.1 a wt catalyst. %) as a A catalyst. moderate A moderate amount of amount acetone of was acetone required was required at this stage at this to stage to reduce the viscosity in the course of polymerization (the weight ratio of prepolymer and acetone is 7:3). TEA as a neutralization agent (neutralization ratio is 100%) was added at 40 °C to react with carboxyl group for 5 min to form a NCO-terminated WPU prepolymer (the molar ratio of –NCO

Materials 2017, 10, 1247 4 of 13 reduce the viscosity in the course of polymerization (the weight ratio of prepolymer and acetone is 7:3). ◦ TEAMaterials as a 2017 neutralization, 10, 1247 agent (neutralization ratio is 100%) was added at 40 C to react with carboxyl4 of 13 group for 5 min to form a NCO-terminated WPU prepolymer (the molar ratio of –NCO and –OH is aroundand –OH 1–1.1). is around Finally, 1– specific1.1). Finally, deionized specific water deionized was poured water into was the poured mixture into with the themixture shearing with rate the ofshearing 3000 r/min rate toof form3000 r/min WPU to emulsion. form WPU A blackemulsion aqueous. A black dispersion aqueous was dispersion obtained was after obtained the acetone after wasthe removedacetone was from removed the WPU from emulsion the WPU using emulsion a rotary using evaporator a rotary in vacuum.evaporator The in solid vacuum. content The of solid the obtainedcontent of WPU the obtained emulsion WPU was approximatelyemulsion was approximately 30 wt %. By this 30 method,wt %. By athis series method, of SDB-WPUs a series of were SDB- synthesized,WPUs were assynthesize illustratedd, inas Tableillustrated1. in Table 1.

Scheme 2. Synthetic processes of SDB-WPU (Sudan Black B-waterborne polyurethanes). Scheme 2. Synthetic processes of SDB-WPU (Sudan Black B-waterborne polyurethanes).

Materials 2017, 10, 1247 5 of 13

MaterialsTable2017 , 1.10 , 1247The specific ratio of each component in SDB-WPUs (Sudan Black B-waterborne5 of 13 polyurethanes).

Polymeric SDB TableSample 1. The specific ratio of each componentPolymeric inD SDB-WPUsiol /g IPDI (Sudan /g DMPA Black B-waterborne/g SDB /g polyurethanes).BDO /g Diol (wt %) SDB-WPU1 PolymericPTMG Polymeric25 12 2.3 0.42 1.8 SDB1% Sample IPDI /g DMPA /g SDB /g BDO /g SDB-WPU2 PTMGDiol Diol /g25 12 2.3 0.83 1.7 (wt2% %) SDB-WPU4 PTMG 25 12 2.3 1.7 1.55 4% SDB-WPU1 PTMG 25 12 2.3 0.42 1.8 1% SDBSDB-WPU2-WPU6 PTMG 2525 1212 2.32.3 0.832.6 1.71.4 2%6% SDBSDB-WPU4-WPU-PTMG PTMG 2525 1212 2.32.3 1.70.83 1.551.7 4%2% SDBSDB-WPU6-WPU-PPG PTMGPPG 2525 1212 2.32.3 2.60.83 1.41.7 6%2% SDB-WPU-PTMGSDB-WPU-PCL PTMGPCL 2525 1212 2.32.3 0.830.83 1.71.7 2%2% SDBSDB-WPU-PPG-WPU-PBA PPGPBA 2525 1212 2.32.3 0.830.83 1.71.7 2%2% SDB-WPU-PCL PCL 25 12 2.3 0.83 1.7 2% SDB-WPU-PBA PBA 25 12 2.3 0.83 1.7 2% 3. Results and Discussion

3.3.1. Results UV-Vis and Spectra Discussion of SDB 3.1. UV-VisSudan Spectra black B of (SDB) SDB is a highly coloration-efficient and cost-effective dye, with high molar absorptivity. The UV-vis spectra of SDB with different concentrations are measured in N, N- Sudan black B (SDB) is a highly coloration-efficient and cost-effective dye, with high dimethylformamide (DMF). As Figure 1a shows, SDB has an almost total absorption in the visible molar absorptivity. The UV-vis spectra of SDB with different concentrations are measured in region with an intense absorbance around 621 nm and a minor absorbance centered at 428 nm, N, N-dimethylformamide (DMF). As Figure1a shows, SDB has an almost total absorption in the indicating that SDB solutions are nearly blue-black in dilute solution. Obviously, the maximum visible region with an intense absorbance around 621 nm and a minor absorbance centered at 428 nm, absorbance (Amax) at 621 nm intensifies from 0.159 to 1.235 as the concentration of SDB in DMF indicating that SDB solutions are nearly blue-black in dilute solution. Obviously, the maximum increases from 2 mg/L to 20 mg/L (Figure 1b), which lines up with Beer-Lambert Law that can be absorbance (Amax) at 621 nm intensifies from 0.159 to 1.235 as the concentration of SDB in DMF used to calculate the SDB concentration in SDB-WPU. increases from 2 mg/L to 20 mg/L (Figure1b), which lines up with Beer-Lambert Law that can be used to calculate the SDB concentration in SDB-WPU.

Figure 1. (a) UV-vis absorption spectra of SDB (Sudan Black B) with different dye concentrations in Figure 1. (a) UV-vis absorption spectra of SDB (Sudan Black B) with different dye concentrations DMF (dimethylformamide). (b) The linear relationship of the Amax (621 nm) with different dye in DMF (dimethylformamide). (b) The linear relationship of the Amax (621 nm) with different concentration. dye concentration. 3.2. Structural Analysis of SDB-WPUs 3.2. Structural Analysis of SDB-WPUs SDB-WPUs were synthesized from the reaction between -NH/-OH and -NCO groups according SDB-WPUs were synthesized from the reaction between –NH/–OH and –NCO groups according to previously reported polycondensation reaction (Scheme 2) [20,28]. Briefly, there are two main steps to previously reported polycondensation reaction (Scheme2)[ 20,28]. Briefly, there are two main in the course of polymerization. One is the reaction of diols and diisocyanates to obtain prepolymers steps in the course of polymerization. One is the reaction of diols and diisocyanates to obtain containing SDB. The other is the emulsifying process of the prepolymers with a high shear speed prepolymers containing SDB. The other is the emulsifying process of the prepolymers with a high after trimethylamine is added. Figure 2 shows the FTIR spectra of SDB-WPUs with different ratios of shear speed after trimethylamine is added. Figure2 shows the FTIR spectra of SDB-WPUs with SDB (0–6%). Compared to WPU without SDB (WPU0), the characteristic absorption peak at 1595 cm−1 different ratios of SDB (0–6%). Compared to WPU without SDB (WPU0), the characteristic absorption (N=N) appears and enhances with increasing SDB loadings. Meanwhile, the representative N–H peak at 1595 cm−1 (N=N) appears and enhances with increasing SDB loadings. Meanwhile, the vibration at 3430 cm−1 disappears in all of the WPU films, suggesting that SDB has been covalently representative N–H vibration at 3430 cm−1 disappears in all of the WPU films, suggesting that SDB linked to the WPU chains. Other characteristic absorption peak assignments for SDB-WPUs include: has been covalently linked to the WPU chains. Other characteristic absorption peak assignments for −1 ν −1 −1 ν ν −1 ν SDB-WPUs include: 3325 cm ( NH), 2860 cm and 2940 cm ( CH2 and CH3), 1701 cm ( C=O), Materials 2017, 10, 1247 6 of 13

MaterialsMaterials 2017 2017, ,10 10, ,1 1242477 66 of of 13 13 −1 ν −1 ν 1240 cm−1 ( NHC–O in carbamate−1 group) and−1 CH2 1110 cm CH3( C–O–C in−1 PTMG),C=O indicating−1 thatC-O SDB-WPUs 33253325 cm cm−1 (ν (νNH),), 2860 2860 cm cm−1 and and 2940 2940 cm cm−1 (ν (νCH2 and and ν νCH3),), 1701 1701 cm cm−1 (ν (νC=O),), 1240 1240 cm cm−1 (ν (νC-O in in carbamate carbamate were synthesized successfully.−1 C-O-C group)group) and and 1110 1110 cm cm−1 (ν(νC-O-C in in PTMG), PTMG), indicating indicating that that SDB SDB--WPUsWPUs were were synthesized synthesized successfully. successfully.

3438 3438 SDBSDB

WPU0WPU0

SDB-WPU1SDB-WPU1

SDB-WPU2SDB-WPU2

SDB-WPU4SDB-WPU4

SDB-WPU6SDB-WPU6 33253325 1595 17011595 1110 1701 12401240 1110 35003500 30003000 20002000 15001500 10001000 -1 Wavenumber (cm-1 ) Wavenumber (cm )

FigureFigure 2. Figure2. FTIR FTIR spectra 2.spectraFTIR of spectraof SDB SDB and ofand SDB SDB SDB and--WPUWPU SDB-WPU with with various various with variousdye dye contents contents dye contents on on KBr KBr plates. onplates. KBr plates.

To further demonstrate that SDB is chemically linked to polymeric main chains, the investigation ToTo further further demonstrate demonstrate that that SDB SDB is is chemically chemically linked linked to to polymeric polymeric main main chains, chains, the the investigation investigation on macroscopic migration of the SDB-WPU between water and phases is conducted. The onon macroscopic macroscopic migration migration of ofthe the SDB SDB-WPU-WPU between between water water and and solvent solvent phases phases is conducte is conducted.d. The alteration of the color in two phases is recorded by digital camera at different moments, as illustrated alterationThe alteration of the ofcolor the in colortwo phases in two is phasesrecorded is by recorded digital camera by digital at different camera moments, at different as illustrated moments, in Figure 3. To water emulsion are added chloroform to illustrate whether SDB can be extracted. As inas Figure illustrated 3. To in water Figure emulsion3. To water are emulsionadded chloroform are added to chloroform illustrate whether to illustrate SDB whethercan be extracted. SDB can As be Figure 3 shows, after stirring, chloroform does not immediately turn black. However, after 24 hours Figureextracted. 3 shows, As Figure after 3stir shows,ring, chloroform after stirring, do chloroformes not immediately does not turn immediately black. However, turn black. after However, 24 hours standing, there is no obvious change in chloroform with slight black, which are caused by few SDB- standing,after 24 hours there standing,is no obvious there change is no obvious in chloroform change inwith chloroform slight black, with which slight are black, caused which by arefew caused SDB- WPUs transferring from water to organic phase. As we all know, SDB as micromolecular dye has WPUsby few transferring SDB-WPUs transferringfrom water to from organic water phase. to organic As we phase. all know, As we SDB all know, as micromolecular SDB as micromolecular dye has high mobility, which should migrate into water immediately after intense stirring because SDB is highdye hasmobility, high mobility,which should which migrate should into migrate water into immediately water immediately after intense after stir intensering stirringbecause becauseSDB is hydrophobic. Therefore, the slight change in chloroform ascribed to migration of SDB-WPU is hydrophobic.SDB is hydrophobic. Therefore, Therefore, the slight the slight change change in chloroform in chloroform ascribed ascribed to tomigration migration of of SDB SDB-WPU-WPU is is reasonable, further indicating that SDB is covalently incorporated into WPUs. The mechanical reasonable,reasonable, further further indicating indicating that that SDB SDB is covalently is covalently incorporated incorporated into WPUs. into WPUs. The mechanical The mechanical stability stability in Table 2 also suggests that SDB is reacted into WPUs due to no obvious alteration in stabilityin Table 2 in also Table suggests 2 also that suggests SDB is that reacted SDB into is reacted WPUs due into to WPUs no obvious due to alteration no obvious in emulsion alteration state, in emulsion state, which means that no hydrophobic SDB aggregates precipitates after centrifugal emulsionwhich means state, that which no hydrophobic means that SDB no hydrophobic aggregates precipitates SDB aggregates after centrifugal precipitates sedimentation. after centrifugal sedimentation.sedimentation.

Figure 3. Graphs of: SDB-WPU2 (a–d); and SDB+WPU (physical mix of 2 wt % SDB and pure WPU) (e) FigureFigure 3. 3. Graphs Graphs of of: :SDB SDB--WPWPU2U2 ( (aa––dd)); ;and and SDB+WPU SDB+WPU ( (physicalphysical mix mix of of 2 2 wt wt % % SDB SDB and and pure pure WPU) WPU) migration between aqueous phase and CH Cl phase at different moments (t ). (a) t = 0 h; (b) t = 6 h; (e) migration between aqueous phase and 3CH3Cl phase at different momentss (ts). (sa) ts = 0 h; (bs) ts = 6 (e) migration between aqueous phase and CH3Cl phase at different moments (ts). (a) ts = 0 h; (b) ts = 6 (c) t = 12 h; (d) t = 24 h; and (e) t = 5 min. h; (cs) ts = 12 h; (ds) ts = 24 h; and (e)s ts = 5 min. h; (c) ts = 12 h; (d) ts = 24 h; and (e) ts = 5 min. 3.3. Statistical Properties of SDB-WPUs 3.3.3.3. Statistical Statistical P Propertiesroperties of of SDB SDB--WPUsWPUs The specific ratio of each component is shown in Table1. The particle diameters measured by TheThe specificspecific ratioratio ofof eacheach componentcomponent isis shownshown inin TableTable 1.1. TheThe particleparticle diametersdiameters measuredmeasured byby DLS of emulsions are shown in Table2. All of the WPU dispersions exhibit tiny average particle size DLSDLS of of emulsion emulsionss are are shown shown in in Table Table 2. 2. All All of of the the WPU WPU dispersions dispersions exhibit exhibit tiny tiny average average particle particle size size (<50 nm). As the SDB ratio increases from 1% to 6%, the average particle size increases from 19.79 nm (<(< 5050 nm).nm). AsAs thethe SDBSDB ratioratio increasesincreases fromfrom 1%1% toto 6%,6%, thethe averageaverage particleparticle sizesize increasesincreases fromfrom 19.7919.79 to 43.32 nm, ascribed to the enhanced hydrophobicity of SDB-WPU due to multi-aromatic rings in SDB. nmnm toto 43.3243.32 nm,nm, ascribedascribed toto thethe enhancedenhanced hydrophobicityhydrophobicity of of SDBSDB--WPUWPU duedue toto multimulti--aromaticaromatic ringsrings Besides, multi-aromatic structures and large size of SDB may prevent polymeric chains from entangling inin SDB. SDB. Besides, Besides, multi multi--aromaticaromatic structures structures and and large large size size of of SDB SDB may may prevent prevent polymeric polymeric chains chains from from to small radius. To the best of our knowledge, smaller particle sizes show better emulsion stability. entanglingentangling to to small small radius. radius. To To the the best best of of our our knowledge, knowledge, smaller smaller particle particle size sizess show show better better emulsion emulsion stability.stability. After After more more than than six six months, months, the the dispersion dispersion remain remainss homogeneous homogeneous without without any any precipitate precipitatess

Materials 2017, 10, 1247 7 of 13

After more than six months, the dispersion remains homogeneous without any precipitates (Table2). Moreover, there is no obvious change in emulsion state after 30 min centrifugation at 3000 r/min (Table2), which demonstrates that SDB-WPUs are mechanically stable.

Table 2. The various statistical properties of SDB-WPUs.

Average Storage Mechanical Sample Mn a PDI b Particle Size Stability Stability SDB-WPU1 19.79 nm Unchanged Unchanged 16,100 3.61 SDB-WPU2 21.38 nm Unchanged Unchanged 16,300 3.70 SDB-WPU4 41.69 nm Unchanged Unchanged 14,400 3.63 SDB-WPU6 43.32 nm Unchanged Unchanged 13,200 3.17 SDB-WPU-PTMG 21.38 nm Unchanged Unchanged 16,300 3.70 SDB-WPU-PPG 19.21 nm Unchanged Unchanged 16,000 2.79 SDB-WPU-PCL 13.40 nm Unchanged Unchanged 18,900 2.85 SDB-WPU-PBA 13.97 nm Unchanged Unchanged 17,400 2.67 a b Number, average molecular weights of SDB-WPUs measured in THF; Polydispersity index (Mw/Mn).

The molecular weight information of SDB-WPUs is characterized by gel-permeation chromatography (GPC, Table2). In Table2, all of the SDB-WPUs show broad molecular weight distributions with polydispersity indices around 2–4. It is worth noting that molecular weight information of SDB-WPUs is not very reliable due to the presence of extremely polar carboxylate and ammonium groups.

3.4. DSC and TG Analyses Figure4 shows the thermal properties of SBD-WPUs with different contents. From Differential Scanning Calorimetry (DSC) curves (Figure4a) (all of the samples were measured without the elimination of the thermal history), a broad endothermic peak appears obviously in the range of 50–100 ◦C for all of the samples, which is not a typical glass transition, which usually shows a slope between two platforms [29]. The broad peak is due to the disappearance of short-range ordered structures in the hard segments [30]. There is a new endothermic peak around 125 ◦C for SDB-WPU6, which is due to the damage of microcrystalline structures organized in the process of film formation. Thermogravimetric (TG) analysis is shown in Figure4b. All films show three thermal decomposition courses. The slow descending slopes from the 80 to 250 ◦C due to the disappearance of small molecules such as triethylamine and water. The steep slopes showing in the 250 ◦C–350 ◦C region are the breakage of allophanate and carbamate bonds. The last stage appears in the 350 ◦C–460 ◦C region where C–C bonds in the soft segment (PTMG) of WPUs decompose. It is noteworthy that SDB-WPU6 exhibits more clear decrease in the first decomposition stage. To make SDB-WPU6 (solid content is ~32%) disperse homogenously, more deionized water was added due to the incorporation of more rigid and hydrophobic SDB compared to other samples. Therefore, there may be more residual water in the film after being left at 25 ◦C for 48 h. With increasing SDB loadings, the maximum hard-segment decomposition temperature rises from 306 ◦C to 314 ◦C by a derivative thermogravimetric analysis (DTGA) (Figure4c), which may be caused by more and more rigid phenyl rings in the hard segments, while the maximum soft-segment decomposition temperature remains at 418 ◦C, indicating that there is microphase separation to some extent between soft segments and hard segments. Materials 2017, 10, 1247 8 of 13

Materials 2017, 10, 12471247 8 of 13

) 0.00 c)

C

 SDB-WPU1 -0.05 SDB-WPU2 SDB-WPU4

100%/

) c)  SDB-WPU6

( 0.00

C

 -0.10 SDB-WPU1 -0.05 SDB-WPU2 SDB-WPU4 100%/ -0.15  SDB-WPU6

( -0.10 -0.20

Weight loss rate -0.15 -0.25 0 100 200 300 400 500 600 700 -0.20 Weight loss rate Temperature (C) -0.25 Figure 4. Thermal characterization of 0SDB100-WPUs200 300 with400 500 various600 700 SDB contents via: DSC (a); TG (b); Temperature (C) and DTGA (c) measurements. Figure 4. Thermal characterizationcharacterization ofof SDB-WPUs SDB-WPUs with with various various SDB SDB contents contents via: via DSC: DSC (a); TG(a); (TGb); and(b); 3.5. UVDTGAand-V DTGAis (Sc)pectra measurements. (c) measurements. Analysis of SDB -WPUs Figure 5 shows the UV-vis spectra of SDB-WPUs. Compared to the UV-vis spectra of pure SDB 3.5.3.5. UV UV-Vis-Vis S Spectrapectra A Analysisnalysis of SDB SDB-WPUs-WPUs in DMF solution, it is clearly found that, when SDB is introduced into the WPU chains, the maximum absorptionFigure 5 of5 shows showsSDB shifts the the UV-vis UV from-vis 623 spectra spectra nm ofto of SDB-WPUs.603 SDB nm,-WPUs. which Compared Compared likely results to to the thefrom UV-vis UV the-vis spectra influence spectra of pureofof pureconjugate SDB SDB in DMFineffect DMF solution,between solution, itaromatic isit is clearly clearly amino found found in that, SDBthat when, andwhen allophanyl SDB SDB is is introduced introduced in WPUs, into into which the the WPUWPUweakens chains,chains, electron thethe maximum-donating absorptionability of aromatic of SDB shifts amino from in SDB. 623 It nm can to be 603 illustrated nm, which by likelydissolving results SDB from -WPU the filmsinfluence influence in DMF of conjugate solution, effectin which between maximum aromatic absorption amino in SDB wavelength and allophanyl is almost in WPUs,WPUs, consistent which with weakens the SDB electron-donatingelectron-WPU- donating aqueous abilityemulsion, of aromatic excluding amino the possible in SDB. It impact can can be be induced illustrated illustrated by theby by dissolving dissolvingpolarity of SDB SDB-WPUsolvent.-WPU Figure films films 5bin in showsDMF DMF solution, solution, the UV- invis which whichspectra maximum maximum of SDB-WPU absorption absorption films. wavelengthAll wavelengthof the SDB is almost- WPU is almost consistentfilms exhibit consistent with complete the with SDB-WPU , theintense SDB aqueous absorption-WPU emulsion, aqueous in the excludingemulsion,visible region theexcluding possiblein comparison the impact possible to induced solution impact by inducedstate, the polarity which by the is of almostpolarity solvent. pure of Figure solvent. black5b, shows asFigure observed the 5b UV-visshows by the the spectra naked UV- ofviseye. SDB-WPU spectra of films.SDB-WPU All of films. the SDB-WPU All of the films SDB exhibit-WPU films complete, exhibit intense complete absorption, intense in absorption the visible regionin the invisible comparison region in to comparison solution state, to solution which is state, almost which pure is black, almost as observedpure black by, as the observed naked eye. by the naked eye.

FigureFigure 5. 5. (a(a)) UV-vis UV-vis absorption absorption spectra spectra of SDB of SDB in DMF, in DMF, SDB-WPU2 SDB-WPU2 aqueous aqueous emulsion emulsion and SDB-WPU2 and SDB-

inWPU2 DMF in (blue) DMF with(blue) the with same the concentration same concentration of SDB. of ( bSDB.) UV-vis (b) UV absorption-vis absorption spectra spectra of the of SDB-WPU the SDB- Figure 5. (a) UV-vis absorption spectra of SDB in DMF, SDB-WPU2 aqueous emulsion and SDB- filmsWPU dispersedfilms dispersed in BaSO in4 BaSO. 4. WPU2 in DMF (blue) with the same concentration of SDB. (b) UV-vis absorption spectra of the SDB- 3.6.3.6. AnalysesWPU films of SDB-WPUsdispersedSDB-WPUs in BaSO withwith Different4D. ifferent PolymericPolymeric DiolsDiols

3.6. AnalysesToTo investigateinvestigate of SDB - thetheWPUs influenceinfluence with D ifferent ofof softsoft P segmentssegmentsolymeric D ononiols physicalphysical propertiesproperties ofof blackblack WPUs,WPUs, differentdifferent polymericpolymeric diols (PTMG, (PTMG, PBA, PBA, PPG, PPG, PCL) PCL) are are used used to toprepare prepare WP WPUsUs with with constant constant SDB SDBcontent content (2%, To investigate the influence of soft segments on physical properties of black WPUs, different (2%,wt/wt). wt/wt). All of All the of WPU the WPU emulsions emulsions show show the particle the particle diameters diameters around around 10–20 10–20 nm, nm, suggesting suggesting the polymeric diols (PTMG, PBA, PPG, PCL) are used to prepare WPUs with constant SDB content (2%, wt/wt). All of the WPU emulsions show the particle diameters around 10–20 nm, suggesting the

Materials 2017, 10, 1247 9 of 13 Materials 2017, 10, 1247 9 of 13

excellent dispersion, storage and mechanical stability (Table 2). All of the samples exhibit no clear thechange excellent after dispersion,centrifuging storage for 30 andmin mechanicalat 3000 r/min stability or sitting (Table for2 ).six All months. of the samples exhibit no clear changeThe after FT- centrifugingIR spectra of for WPUs 30 min with at different 3000 r/min polymeric or sitting diols for sixare months.presented in Figure 6. Consistent withThe the FT-IRdescription spectra above, of WPUs SDB with is covalently different attached polymeric into diols WPU are chains presented due into Figurethe disappearance6. Consistent of withN-H thestretching description vibration above, of SDBSDB isand covalently the appearance attached of into the representative WPU chains due N- toH thevibration disappearance at 3325 cm of−1 N–Hin polyether stretching-based vibration WPUs of SDB (SDB and-WPU the-PTMG appearance and ofSDB the-WPU representative-PPG) and N–H at 3350 vibration cm−1 atin 3325 polyester cm−1- inbased polyether-based WPUs (SDB WPUs-WPU- (SDB-WPU-PTMGPCL and SDB-WPU and-PBA). SDB-WPU-PPG) The difference ands between at 3350 cm polyether−1 in polyester-based-based WPUs WPUsand polyester (SDB-WPU-PCL-based WPUs and SDB-WPU-PBA). may be ascribed The to differences their differ betweenent microenviro polyether-basednments, such WPUs as and the polyester-baseddifferent degree WPUsof microphase may be ascribedseparation to and their amounts different of microenvironments, hydrogen bonds. A suchsimilar as thephenomenon different degreeis presented of microphase in stretching separation vibration andamounts of C=O of(1730 hydrogen cm−1 in bonds. polyester A similar-based phenomenon WPUs and 1701 is presented cm−1 in inpolyether stretching WPUs). vibration Other of C=O characteristic (1730 cm− 1absorptionin polyester-based peak assignment WPUs ands 1701for SDB cm−-1WPUsin polyether with different WPUs). Otherpolymeric characteristic diols are absorptionsimilar to Figure peak assignments 2: 2860 cm−1 for and SDB-WPUs 2940 cm−1 with(νCH2 different and νCH3 polymeric), 1240 cm diols−1 (νC- areO in −1 −1 −1 −1 similarcarbamate to Figure group)2: 2860 and cm 1110and cm 2940(νC cm-O-C in(ν CH2soft and segmentsνCH3),), 1240 suggesting cm (νC–O thatin carbamate SDB-WPUs group) were −1 andsynthesized 1110 cm successfully.(νC–O–C in soft segments), suggesting that SDB-WPUs were synthesized successfully.

SDB-WPU-PTMG

SDB-WPU-PPG 3325 1701 SDB-WPU-PCL

SDB-WPU-PBA 3350 1730

35003000 2000 1500 1000 -1 Wavenumber (cm )

Figure 6.Figure FT-IR spectra 6. FT-IR of spectra SDB-WPUs of SDB-WPUs with various with polymeric various polymeric diols on KBr diols plates. on KBr plates.

The thermal properties of SDB-WPUs with various polymeric diols are characterized by DSC The thermal properties of SDB-WPUs with various polymeric diols are characterized by DSC and TG curves (Figure 7). Similar results line up with the discussion above. The difference in DSC and TG curves (Figure7). Similar results line up with the discussion above. The difference (Figure 7a) is that a typical endothermic peak appears at 40–50 °C in SDB-WPU-PCL and SDB-WPU- in DSC (Figure7a) is that a typical endothermic peak appears at 40–50 ◦C in SDB-WPU-PCL PBA due to the melting of crystalline structure of the polyester in PCL and PBA. There is a clear and SDB-WPU-PBA due to the melting of crystalline structure of the polyester in PCL and PBA. difference in third decomposition stage caused by different thermal resistance of polymeric diols and There is a clear difference in third decomposition stage caused by different thermal resistance of degree of microphase separation (Figure 7b) [31–33]. According to the TG curves, SDB-WPU-PTMG, polymeric diols and degree of microphase separation (Figure7b) [ 31–33]. According to the TG curves, with the highest starting and soft-segment decomposition temperatures, shows the best thermal SDB-WPU-PTMG, with the highest starting and soft-segment decomposition temperatures, shows the resistance, while SDB-WPU-PBA exhibits the poorest thermal resistance. The remaining two kinds of best thermal resistance, while SDB-WPU-PBA exhibits the poorest thermal resistance. The remaining WPUs have similar thermal resistances. two kinds of WPUs have similar thermal resistances. As we all know, dyes are physically mixed into polymeric coatings influenced by dispersants in that dyes tend to migrate from the coatings interior to aggregate on the coatings surface due to weak binding force. To evaluate the migration property of SDB-WPUs, the same experiments were carried out with the sample formed by physically mixing WPU with SDB (WPU+SDB) as a control. As provided in Table3, Mp values of SDB-WPUs and WPU+SDB are 7%–8% and 31.0%, respectively, manifesting that SDB-WPUs exhibit greater thermal-migration resistance than WPU+SDB. Shown in Scheme3 are the models of thermal migration of dye-doped WPU and dye-incorporated WPU. SDB tends to migrate and aggregate to a certain area due to enhanced free volumes and weak binding force (Scheme3a), which may cause inhomogeneous color distribution. Migration process occurs in both the interior and surface of SDB-WPU films. For example, SDB in the interior may migrate to surface and then aggregate to an area in the surface leading to the inhomogeneity of black. However, the mobility of SDB is confined due to the covalent binding of –NH in SDB and –NCO, resulting in homogeneous dispersity of SDB in WPU. The black SDB-WPU obtained by covalent link with excellent

Materials 2017, 10, 1247 9 of 13

excellent dispersion, storage and mechanical stability (Table 2). All of the samples exhibit no clear change after centrifuging for 30 min at 3000 r/min or sitting for six months. The FT-IR spectra of WPUs with different polymeric diols are presented in Figure 6. Consistent with the description above, SDB is covalently attached into WPU chains due to the disappearance of N-H stretching vibration of SDB and the appearance of the representative N-H vibration at 3325 cm−1 in polyether-based WPUs (SDB-WPU-PTMG and SDB-WPU-PPG) and at 3350 cm−1 in polyester- based WPUs (SDB-WPU-PCL and SDB-WPU-PBA). The differences between polyether-based WPUs and polyester-based WPUs may be ascribed to their different microenvironments, such as the different degree of microphase separation and amounts of hydrogen bonds. A similar phenomenon is presented in stretching vibration of C=O (1730 cm−1 in polyester-based WPUs and 1701 cm−1 in polyether WPUs). Other characteristic absorption peak assignments for SDB-WPUs with different polymeric diols are similar to Figure 2: 2860 cm−1 and 2940 cm−1 (νCH2 and νCH3), 1240 cm−1 (νC-O in carbamate group) and 1110 cm−1 (νC-O-C in soft segments), suggesting that SDB-WPUs were synthesized successfully.

SDB-WPU-PTMG

SDB-WPU-PPG 3325 1701 SDB-WPU-PCL

SDB-WPU-PBA 3350 1730

35003000 2000 1500 1000 -1 Wavenumber (cm ) Figure 6. FT-IR spectra of SDB-WPUs with various polymeric diols on KBr plates.

The thermal properties of SDB-WPUs with various polymeric diols are characterized by DSC and TG curves (Figure 7). Similar results line up with the discussion above. The difference in DSC (Figure 7a) is that a typical endothermic peak appears at 40–50 °C in SDB-WPU-PCL and SDB-WPU- PBA due to the melting of crystalline structure of the polyester in PCL and PBA. There is a clear difference in third decomposition stage caused by different thermal resistance of polymeric diols and Materials 2017 10 degree of microphase, , 1247 separation (Figure 7b) [31–33]. According to the TG curves, SDB-WPU-PTMG10 of 13, with the highest starting and soft-segment decomposition temperatures, shows the best thermal migration-resistanceresistance, while SDB property-WPU-PBA can exhibits be applied the poorest effectively thermal in polyurethane resistance. The coatings, remaining while two traditional kinds of preparationWPUs have methodsimilar thermal via mixing resistance polyurethanes. with micromolecular dyes cannot easily achieve such remarkable migration resistance.

Materials 2017, 10, 1247 10 of 13

Figure 7. Thermal characterization of SDB-WPUs with various polymeric diols via: DSC (a); and TG (b) measurements.

As we all know, dyes are physically mixed into polymeric coatings influenced by dispersants in that dyes tend to migrate from the coatings interior to aggregate on the coatings surface due to weak binding force. To evaluate the migration property of SDB-WPUs, the same experiments were carried out with the sample formed by physically mixing WPU with SDB (WPU+SDB) as a control. As provided in Table 3, Mp values of SDB-WPUs and WPU+SDB are 7%–8% and 31.0%, respectively, manifestingFigure 7.thThermalat SDB-WPUs characterization exhibit greater of SDB-WPUs thermal with-migration various polymericresistance diols than via: WPU+SDB. DSC (a); and Shown TG in Scheme (b) 3 measurements. are the models of thermal migration of dye-doped WPU and dye-incorporated WPU. SDB tends to migrate and aggregate to a certain area due to enhanced free volumes and weak binding force (Scheme 3a), whichTable 3.mayThermal-migration cause inhomogeneous property color of SDB-WPUs distribution. and WPU+SDB. Migration process occurs in both the interior and surface of SDB-WPU films. For example, SDB in the interior may migrate to Samples A a A b M (%) c surface and then aggregate to an area in the surface1 leading to2 the inhomogeneityp of black. However, the mobility of SDB is confinedWPU+SDB due to the covalent 0.332 binding 0.435 of –NH in SDB 31.0% and –NCO, resulting in SDB-WPU-PTMG 0.309 0.332 7.4% homogeneous dispersity of SDB in WPU. The black SDB-WPU obtained by covalent link with SDB-WPU-PPG 0.276 0.298 8.0% excellent migration-resSDB-WPU-PCListance property can 0.327be applied effectively 0.351 in polyurethane 7.3% coatings, while traditional preparationSDB-WPU-PBA method via mixing 0.252 polyurethane 0.271 resin with micromolecular 7.5% dyes cannot easilya achievemaximum such absorbance remarkable of SDB-WPUs migration in area A;resistance.b maximum absorbance of SDB-WPUs in area B; c migration ratio of SDB-WPUS.

Scheme 3. Thermal migration models of: dye-doped (not covalently bonded) WPUs (WPU+SDB) (a); Scheme 3. Thermal migration models of: dye-doped (not covalently bonded) WPUs (WPU+SDB) (a); and dye-linked WPUs (SDB-WPUs) (b). and dye-linked WPUs (SDB-WPUs) (b). 3.7. Application in Metal Coatings Table 3. Thermal-migration property of SDB-WPUs and WPU + SDB. Black SDB-WPU coatings can easily be prepared by spraying the mixture on the metal surface after Samples A1 a A2 b Mp (%) c compounded with other waterborne resins and coatings additives (Table4). The coating properties of WPU+SDB 0.332 0.435 31.0% the SDB-WPUsSDB-WPU are-PTMG illustrated in Table0.3095. All of the film coatings0.332 show smooth and bubble-free7.4% surfaces with glossinessSDB-WPU around-PPG 80–90. Because0.276 of the chemically linking0.298 reaction, SDB is uniformly8.0% distributed into theSDB polyurethanes-WPU-PCL without dyes0.327 aggregating and phase separation0.351 (Figure8), resulting7.3% in excellent coatingsSDB performance.-WPU-PBA To further investigate0.252 the properties0.271 of SDB-WPU coatings,7.5% adhesion force, pencil hardness, impact strength and water resistance are tested. Slight difference can be observed in a maximum absorbance of SDB-WPUs in area A; b maximum absorbance of SDB-WPUs in area B; c migration ratio of SDB-WPUS.

3.7. Application in Metal Coatings Black SDB-WPU coatings can easily be prepared by spraying the mixture on the metal surface after compounded with other waterborne resins and coatings additives (Table 4). The coating properties of the SDB-WPUs are illustrated in Table 5. All of the film coatings show smooth and bubble-free surfaces with glossiness around 80–90. Because of the chemically linking reaction, SDB is uniformly distributed into the polyurethanes without dyes aggregating and phase separation (Figure 8), resulting in excellent coatings performance. To further investigate the properties of SDB-WPU

Materials 2017, 10, 1247 11 of 13 these properties, attributed to the change of polymeric diols, which are able to significantly influence degree of microphase separation, flexibility, rigidity and crystallinity. Consequently, different black WPU coatings with SDB linked chemically may be developed by changing the kind of polymeric diol to cater to a specific applied situation.

Table 4. Formulation of black SDB-WPU coatings.

Entry Function wt % SDB-WPU Resin 95 BYK-025 Defoamer 0.5 BYK-331 Leveling additive 0.3 ACRYSOLTM RM-8W Thickener 1.2 Deionized water - 3 Total - 100

Table 5. The properties of black waterborne polyurethane coatings with various polymer diols.

Materials 2017Property, 10, 1247 SDB-WPU-PTMG SDB-WPU-PPG SDB-WPU-PCL SDB-WPU-PBA11 of 13 Smooth, Smooth, Smooth, Smooth, Appearance coatings, adhesion force, pencilNo hardness,bubbles impactNo bubbles strength and waterNo bubbles resistance areNo tested. bubbles Slight differenceGloss can (60be ◦observed) in these 83 properties, attribut 83ed to the change 86 of polymeric diols, 87 which are Adhesion force (grade) 0 1 0 0 able to significantlyPencil hardness influence degree 2B of microphase 3Bseparation, flexibility, B rigidity and crystallinity. 2B Consequently,Impact strength different (kg/cm) black WPU 50 coatings with 50 SDB linked chemically 50 may be developed 50 by changingWater the resistance kind (24of polymeric h) Unchanged diol to cater to a Unchangedspecific applied situation. Unchanged Unchanged

Figure 8. Black SDB-WPUSDB-WPU coatings on the surface of steel plate withwith differentdifferent softsoft segments:segments: PTMG (polytetramethylene(Polytetramethylene ether glycol);glycol); PPG (polypropylene glycol);glycol); PCL (polycaprolactone(polycaprolactone glycol);glycol); and PBA (poly-1,(poly-1, 4-butylene4-butylene adipate glycol).glycol).

4. Conclusions Table 4. Formulation of black SDB-WPU coatings. In summary,Entry a series of black WPUs fromFunction Sudan black B (SDB) were prepared.wt % The obtained SDB-WPU emulsionsSDB-WPU show remarkable storage andResin mechanical stability. Compared95 to the traditional BYK-025 Defoamer 0.5 method by physically mixing dyes with polymeric substrates, SDB-WPUs also exhibit excellent BYK-331 Leveling additive 0.3 thermal-migrationACRYSOLTM resistance RM-8W and color fastness,Thickener in that SDB is chemically linked1.2 to WPU chains. Besides, incorporationDeionized water of SDB does not obviously change- the thermal properties of3 WPUs. By altering different polymericTotal diols, all of the SDB-WPUs show- some differences in glossiness100 and adhesion force but maintain their excellent storage and mechanical stability, and great thermal resistance. Therefore, differentTable black 5. WPUThe properties coatings of with black SDB waterborne linked chemically polyurethane may coatings be developed with various by changingpolymer diols. the kind of polymeric diol to cater to a specific applied situation. Property SDB-WPU-PTMG SDB-WPU-PPG SDB-WPU-PCL SDB-WPU-PBA Smooth, Smooth, Smooth, Smooth, Appearance No bubbles No bubbles No bubbles No bubbles Gloss (60°) 83 83 86 87 Adhesion force (grade) 0 1 0 0 Pencil hardness 2B 3B B 2B Impact strength 50 50 50 50 (kg/cm) Water resistance (24 h) Unchanged Unchanged Unchanged Unchanged

4. Conclusions In summary, a series of black WPUs from Sudan black B (SDB) were prepared. The obtained SDB-WPU emulsions show remarkable storage and mechanical stability. Compared to the traditional method by physically mixing dyes with polymeric substrates, SDB-WPUs also exhibit excellent thermal-migration resistance and color fastness, in that SDB is chemically linked to WPU chains. Besides, incorporation of SDB does not obviously change the thermal properties of WPUs. By altering different polymeric diols, all of the SDB-WPUs show some differences in glossiness and adhesion force but maintain their excellent storage and mechanical stability, and great thermal resistance. Therefore, different black WPU coatings with SDB linked chemically may be developed by changing the kind of polymeric diol to cater to a specific applied situation.

Materials 2017, 10, 1247 12 of 13

Acknowledgments: The authors would like to thank the National High Technology Research and Development Program of China (No.2015AA033903) for financial support. Author Contributions: Tao Wang and Xingyuan Zhang conceived and designed the experiments; Tao Wang and Wei Sun carried out synthesis and analyzed the data; Haiyan Xu and Fei Xu contributed to reagents and laboratory equipment; and Tao Wang and Xingyuan Zhang wrote the manuscript. All authors took part in the discussion and revisions. Conflicts of Interest: The authors declare no conflict of interest.

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