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Effects of fluorine and silicon components on the hydrophobicityfailure behavior of acrylic polyurethane coatings
Fei Yang, Liqun Zhu, Dongxiao Han, Weiping Li, Yichi Chen, Xianming Wang, Liang Ning
� American Coatings Association 2017
Abstract Hydrocarbon acrylic copolymer was synthe-sized by a radical polymerization route. Fluorine-containing copolymer was prepared via a post-fluorinemodification method. Hydrocarbon, fluorine-contain-ing, silicon-containing, and fluorosilicone acrylic poly-urethane coatings were prepared by curing reaction ofthe curing agents and mixtures of the synthesizedcopolymers and amino silicone, respectively. Indooratmospheric environment, hygrothermal environment,different temperature environment, as well as xenon
arc aging environment were employed to investigate thehydrophobicity failure behavior of the coatings. Chem-ical structure of the coatings was characterized byattenuated total reflectance Fourier transform infraredspectrum. Water contact angles of the coatings weremonitored during the failure process. Thermostability ofthe coatings was explored by thermo gravimetric anal-ysis. Surface morphology of the coatings was investi-gated by scanning electron microscopy. Elementalcomposition of the coating surface was analyzed bya X-ray photoelectron spectrometer. The resultsshowed that the acrylic polyurethane coatings wereprepared as expected. In addition, the modificationmechanism determined the performance of the coatingsin different environments. The fluorine-containing coat-ing performed with better hydrophobicity in the indooratmospheric environment, hygrothermal environmentand low-temperature environment, but failed faster inthe high temperature and xenon arc aging environ-ments. The silicon-containing coating exhibited rela-tively stable hydrophobicity in the high temperature andthe xenon arc aging environments. The hydrophobicityof the fluorosilicone coating fell in between that of thefluorine- and silicon-containing coatings under all of theexperimental conditions. The fluorine-containing com-ponents improved the hydrophobicity of the coatingsmore effectively, while the silicon-containing compo-nents contributed more to the thermo and weatherresistance property of the coatings.
Keywords Preparation, Failure behavior,Hydrophobicity, Acrylic polyurethane, Coating,Fluorine, Silicon, Fluorosilicone
Introduction
Polyurethanes have been famous for their excellentabrasion resistance; low-temperature flexibility; excel-
Electronic supplementary material The online version of thisarticle (doi:10.1007/s11998-016-9887-0) contains supplementarymaterial, which is available to authorized users.
F. Yang, L. Zhu, D. Han, W. Li (&)Key Laboratory of Aerospace Advanced Materials andPerformance (Ministry of Education), School of MaterialScience and Engineering, Beihang University,Beijing 100191, Chinae-mail: [email protected]
D. Hane-mail: [email protected]
F. YangWuhan the Second Ship Design and Research Institute,Wuhan 430064, China
D. HanBeijing XinLi Machinery Co., LTD, Beijing 100039, China
Y. Chen (&)Key Laboratory of Bio-Inspired Smart Interfacial Scienceand Technology of Ministry of Education, School ofChemistry and Environment, Beihang University,Beijing 100191, Chinae-mail: [email protected]
X. Wang, L. NingMarine Chemical Research Institute, Qingdao 266071,China
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DOI 10.1007/s11998-016-9887-0
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lent chemical, mechanical and physical properties; andbroad application in the coatings industries.1–4 Two-pack polyurethane coatings have been extensively usedfor plastics, wood, aircraft topcoats, and automotivetopcoats, etc., due to their flexibility in formulations ascompared with the one-pack system.3,4 Acrylic poly-urethane, prepared with hydroxyl functionalized ac-rylic copolymer and isocyanate curing agents, is one ofthe most widely used high-performance coatings,combining excellent properties of polyurethanes andsufficient stability against weathering.1,4,5
Fluorinated, silicone, and fluorosilicone coatings playan important roleamong low-surfaceenergycoatings.3,6–8
Fluorine-containing coatings have been studied andapplied widely due to their outstanding hydrophobicity,oleophobicity, antifouling properties, etc.9–14 Coatingsprepared with silicon-containing polymers have beencharacterized by their exceptional hydrophobicity, flex-ibility, chemical and thermo stabilities.15–17 As a combi-nation of fluorine-containing coatings and silicon-containing coatings, the fluorosilicone coatings possessthe excellent properties of fluorine- and silicon-contain-ing polymers.6–8,18–22Much attention has beenpaid to theresearch studies on the preparation of such coatings andtheir hydrophobicity. However, the properties of thecoatings are usually characterized as they are newlyprepared, rather than under application environments.Coatings will fail under application conditions such astemperature, oxygen, water, pollutants, humidity, andsalt spray. Therefore, it is significant to study the failurebehavior of the coatings under service conditions. More-over, the effects of the fluorine-containing, silicon-containing, and fluorosilicone components on thehydrophobicity failure behavior of the coatings in differ-ent application conditions need further study.
In this work, a hydrocarbon acrylic copolymer wassynthesized via a radical polymerization route withmethyl methacrylate (MMA), butyl acrylate (BA),styrene (St), hydroxyethyl methacrylate (HEMA), andacrylic acid (AA). A fluorine-containing copolymerwas prepared via a post-fluorine modification method.9
The hydrocarbon and fluorine-containing acrylic poly-urethane coatings were prepared with the synthesizedhydrocarbon and fluorine-containing copolymers andtrimer of hexamethylene diisocyanate curing agent,respectively. Mixtures of amino silicone and thesynthesized hydrocarbon copolymer, amino silicone,and the synthesized fluorine-containing copolymerwere used to prepare the silicon-containing and fluo-rosilicone acrylic polyurethane coatings with trimer ofhexamethylene diisocyanate curing agent, respectively.An indoor atmospheric environment, hygrothermalenvironment, different temperature environment, aswell as xenon arc aging environment were employed toinvestigate the hydrophobicity failure behavior of thecoatings. Chemical structure of the coatings wasconfirmed by attenuated total reflectance Fouriertransform infrared spectrum (ATR-FTIR). Watercontact angles of the coatings were monitored duringthe failure process. Surface morphology of the coatings
was investigated by scanning electron microscopy(SEM). Elemental composition of the coating surfacebefore and after failure experiment was analyzed by anX-ray photoelectron spectrometer (XPS). The effectsof the fluorine-containing, silicon-containing and fluo-rosilicone components on the hydrophobicity failurebehavior of the coatings were discussed.
Experimental
Materials
Methyl methacrylate (MMA), butyl acrylate (BA),styrene (St), hydroxyethyl methacrylate (HEMA), andacrylic acid (AA) were purchased from DongfangYakeli Chemicals Limited Corporation (Beijing, Chi-na), and used as common monomers. 1H,1H,2H,2H-Perfluoro-1-decanol (FOH) was obtained fromGuangzhou Liyuan Industrial Materials Co., Ltd.(Guangzhou, China). Toluene diisocyanate (TDI)was supplied by Tianjin Dengke Chemical ReagentCo., Ltd. (Tianjin, China). Amino silicone (N323) withan amino value of 0.5�0.6 was obtained from Shan-dong Dayi Chemical Co., Ltd. (Shandong, China).Butyl acetate and xylene were purchased from BeijingChemical Works (Beijing, China) and used as solvents.Benzoyl peroxide (BPO) was used as initiator andobtained from Xilong Chemicals Limited Corporation(Shantou, China). The hexamethylene diisocyanatetrimer (N3375, Bayer) was used as curing agent. Allthe reactants were used as received.
Synthesis of copolymers and coating preparation
The hydrocarbon copolymer was synthesized with aradical polymerization route. The reaction was con-ducted in a four-neck flask equipped with a mechanicalstirrer, a reflux condenser, a thermometer and anaddition funnel. The solvent was composed of butylacetate and xylene with a weight ratio of 2:3. St, MMA,BA, HEMA and AA with a weight ratio of20:20:35:20:5 were used as the monomers. BPO wasused as the initiator and accounted for 1 wt% of thewhole monomers. At first, 2/5 of the solvent was addedinto the flask and heated to 100�C under continuousstirring. Then 1/2 of the solvent, all the monomers, and9/10 of the BPO solution were added dropwise into theflask for about 3 h. The reaction was carried out at100�C for another 2 h after feeding. After that, the restof the solvent and BPO were added dropwise into theflask for about 0.5 h. The polymerization was contin-ued for another 2 h to promote the conversion of theresidual monomers. Finally, the reaction mixture wascooled down to room temperature and the hydrocar-bon acrylic copolymer was prepared.
A kind of fluorine-containing monomer (FTDI) wassynthesized using 1H,1H,2H,2H-Perfluoro-1-decanol(FOH) and toluene diisocynate (TDI) to prepare the
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fluorine-containing copolymer.9 A certain amount ofFTDI was added into the synthesized hydrocarbonacrylic copolymer solution under continuous stirring at70�C. The mole ratio of –NCO to –OH was 1:10, wherethe –NCO group was from FTDI monomer and –OHwas from HEMA that copolymerized into the hydro-carbon copolymer. The reaction was conducted foranother 1 h after feeding, and the fluorine-containingcopolymer was prepared.
Five weight percent of amino silicone (N323,R1(CH3)2SiO[(CH3)2SiO]m[(R2)(CH3)SiO]n Si(CH3)2R1,R1 was –CH3 or –OH, and R2 contained amine group)was dispersed into the hydrocarbon and fluorine-containing copolymers to obtain the silicon-containingand fluorosilicone copolymers, respectively. Hexam-ethylene diisocyanate tripolymer (N3375, Bayer) wasemployed as a curing agent to react with the hydroxyland amino groups in the copolymers and form thecorresponding acrylic polyurethane coatings. Thecopolymers were mixed with stoichiometric N3375and spun on tin plates. The anticipated acrylicpolyurethane coatings were obtained followed bydrying at 50�C for 10 h.
Environment experiments
The indoor atmospheric environment with the tem-perature ranging from 20 to 30�C and the relativehumidity ranging from 20% to 40% was employed toexplore the failure behaviors of the coatings.
The hygrothermal environment experiment wasconducted in a chamber with the temperature of38�C and relative humidity of 97%.
Different temperature environments, including 50,100 and 150�C, were chosen to test the failure behaviorof the coatings. The coated samples were put into dryovens with the setting temperature. The relativehumidity ranged from 20% to 40%.
Xenon lamp exposure and weathering equipment(Nanjing Wuhe Testing Equipment Co. Ltd, China)were employed to test the failure behaviors of thecoatings under the artificial weathering environment.The xenon lamp irradiation intensity was 550 W m2.The measured wavelength range was 400–1000 nmbased on the detector of the employed equipment. Anda boro/boro filter combination was used on the xenonarc lamp. The black-standard temperature was 65 ±2�C, and the test chamber temperature was 38 ± 3�C,respectively. A wetting/drying cycle composed of18 min wetting time and 102 min dry period wasadopted. The surfaces of coated samples were sprayedwith de-ionized water in the wetting period, while therelative humidity was 60% in the dry period.
Characterization
The chemical structure of the acrylic polyurethanecoatings was characterized by attenuated total reflec-
tance Fourier transform infrared spectrum (ATR-FTIR, Thermo Nicolet AVATAR, USA).
Static water contact angle was used to reflect thehydrophobicity of the coatings and measured by sessiledrop method using DSA 20 equipment (KRUSS,Germany) at 23 ± 1�C. Each sample was tested atthree different positions and measured for more thanfive times to calculate the average value.
Thermostability of the coatings was explored bythermo gravimetric analysis (TGA, TA instrumentsQ5000, USA). The temperature ranged from 35 to500�C at a heating rate of 10�C/min in nitrogenatmosphere.
The surface morphologies of the coatings wereobserved by field emission scanning electron micro-scopy (FESEM, JSM-7500F, JEOL, Japan). The filmswere sprayed with platinum (Pt) before observation.
X-ray photoelectron spectrometer (XPS, ESCA-LAB 250 Xi, Thermo Fisher Scientific, USA) wasemployed to analyze the elemental composition at thesurface of the copolymer films.
Results and discussion
Preparation of the acrylic polyurethane coatings
The chemical structure of the four kinds of coatingswas confirmed by ATR-FTIR and is shown in Fig. 1. Itis clear that all four kinds of coatings exhibited thecharacteristic stretching vibration of asymmetrical andsymmetrical stretching vibration of –CH3 at 2940 and2868 cm�1, characteristic vibration of C=O at1725 cm�1 from ester and 1688 cm�1 from urethanelinkage, deformation vibration of N-H from urethane
a
b
c
d
2940
2868
1725
1688
1535
1461 13
7912
4211
6412
5411
5910
9810
2782
0
Wavenumbers/cm–1
Tran
smitt
ance
/%
4000 3500 3000 2500 2000 1500 1000
Fig. 1: ATR-FTIR spectra of the acrylic polyurethane coat-ings. (a) Hydrocarbon coating; (b) fluorine-containingcoating; (c) silicon-containing coating; (d) fluorosiliconecoating
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linkage at 1535 cm�1, asymmetrical and symmetricaldeformation vibration of –CH3 at 1461 and 1379 cm�1,stretching vibration of C-N at 1242 cm�1, characteristicvibration of C-O-C at 1164 cm�1. In the spectrum offluorine-containing coating (Curve b), the characteris-tic vibration of perfluoroalkyl resulted in the peakenhancement of 1254 and 1159 cm�1, suggesting thatthe fluorine-containing component has been intro-duced into the coating. Curve c shows the spectrumof the silicon-containing coating, which is prepared bythe reaction of –NCO from N3375 and amine groupfrom N323 amino silicone, as well as hydroxyl groupfrom the hydrocarbon copolymer. The characteristicpeaks at 1098 and 1027 cm�1 were assigned to the -SiO(CH3)- structure unit from N323 amino silicone,while that at 820 cm�1 was assigned to Si-CH3. Thespectrum of the fluorosilicone coating (Curve d)exhibits all the fluorine-containing and silicon-contain-ing characteristic peaks, indicating that the fluorine-and silicon-containing components were introducedinto the coatings. In all the above curves, there was nocharacteristic peak of –OH, -NH2, and –NCO ob-served, suggesting that all the copolymers have reactedwith the curing agent and the expected coatings wereprepared.
The FTIR spectra, DSC curves, as well as the GPCtrace of the copolymers are shown in the supplementalinformation.
Hydrophobicity failure behavior of the coatings
Indoor atmospheric environment
Water contact angle has been employed to reflect thehydrophobicity of the coatings and better hydropho-bicity corresponds to higher water contact angle.23,24
Figure 2 displays water contact angles of the coatingsas a function of exposure time in indoor atmospheric
environment. It is clear that all four kinds of coatingsperformed with stable hydrophobicity during theexposure time (500 h). The fluorine-containing acrylicpolyurethane coating exhibited higher water contactangle than the other three kinds of coatings, suggestingthe best hydrophobicity of the four kinds of coatings.The fluorosilicone coating exhibited a high contactangle value above 110� throughout the test time, whichwas only a little lower than the fluorine-containing one.The water contact value of the silicon-containingcoating was above 100� during the test, which was alsomuch higher than that of the hydrocarbon one. It iseasily found that the fluorine-containing componentplayed the most significant role in the hydrophobicityof the coating. The silicon-containing component wasalso helpful for improving the hydrophobicity of thecoating. The hydrophobicity of the fluorosiliconecoating fell in between the above two coatings.
Hygrothermal environment
Figure 3 shows the water contact angles as a functionof the testing time in hydrothermal conditions. Ahigher temperature of 38�C and relative humidity of97% environment was employed to detect thehydrophobicity of the coatings. The result was almostthe same as that of the indoor atmosphere environmenttest. The hydrophobicity of the coatings was stable dur-ing the test. The fluorine-containing coating exhibitedthe best hydrophobicity. The fluorosilicone coatingalso showed better hydrophobicity. The silicon-con-taining coating showed better hydrophobicity than thehydrocarbon one.
It can be concluded that the indoor atmosphere andthe hydrothermal environments had a slight influenceon the four kinds of coatings. The fluorine-containingand the silicon-containing components were bothhelpful to the hydrophobicity of the acrylic polyur-
0 100 200 300 400 50070
80
90
100
110
120
130
140
Time/h
Con
tact
ang
le/°
HydrocarbonFluorine-containing
FluorosiliconeSilicon-containing
Fig. 2: Water contact angles of the coatings in indooratmospheric environment
HydrocarbonFluorine-containingSilicon-containingFluorosilicone
70
80
90
100
110
120
130
140
Con
tact
ang
le/°
1000 200 300
Time/h400 500
Fig. 3: Water contact angles of the coatings in hygrother-mal environment
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ethane coatings. Although there was less fluorine-containing component in the coating, the fluorine-containing coating exhibited higher contact angle valuesthan the other coatings. This might be attributed to thelong perfluoroalkyl side chains containing seventeenfluorine atoms in each fluoroalkyl segment, and theirpreferential orientation behavior toward the film-airsurface during film-forming process. The addition ofthe silicon-containing components also improved thehydrophobicity of the hydrocarbon coatings. However,the water contact angle of the fluorosilicone coating wasa little lower than the fluorine-containing one. Thismight be due to the synergy of the silicon- and fluorine-containing components. The migration and distributionof the fluorine-containing segments were influenced bythe silicone segments.
Different temperature environments
The failure behavior of the coatings was explored indifferent temperature environments. Table 1 displaysthe water contact angles of the coatings at differenttemperatures for 96 h. For the hydrocarbon coating, thewater contact angle increased as the test time extendedat 50 and 100�C, while that increased a little at the first24 h and then decreased as the exposure time at 150�C.For the fluorine-containing coating, the hydrophobicitywas relative stable at 50�C and 100�C during theexposed 96 h, while the water contact angle decreaseddrastically from 117.7� to 80� over 96 h at 150�C, whichwas almost the same as that of the hydrocarbon oneafter 96 h test at 150�C. The silicon-containing coatingexhibited stable hydrophobicity in all the three exper-iment temperatures, suggesting that the silicon-contain-ing components were helpful to the thermal resistanceof the coatings. As for the fluorosilicone coating, thewater contact angle exhibited almost the same trendwith that of the fluorine-containing one at 50�C. Whenthe experiment temperature was raised to 100 and150�C, the water contact angle of the fluorosiliconecoating dropped to values similar to that of the silicon-containing coating. In particular, the water contact angledecreased more quickly at 150 than 100�C.
Figure 4 displays the water contact angles of thecoatings at 150�C for 300 h. The water contact angle ofthe hydrocarbon coating increased a little during thefirst 24 h, but decreased slowly as the exposure timeextended. The silicon-containing coating exhibited
stable hydrophobicity during the exposure period of300 h. As for the fluorine-containing and fluorosiliconecoatings, the water contact angles decreased sharply inthe first 24 h and then were similar with those of thehydrocarbon and silicon-containing coatings, indicatingthat the fluorine-containing segments might be de-stroyed in the high-temperature environment. More-over, the silicon-containing segments could provide thecoating with excellent thermal resistance.
TGA test was employed to explore the thermosta-bility of the copolymer coatings, and the results areincluded in Fig. 5. It can be seen that 1.7 wt.% of thehydrocarbon coating degraded at 150�C, while thosefor the fluorine-containing, silicon-containing and flu-orosilicone coatings were 2.3, 0.9, and 0.5 wt%. More-over, 10 wt% of the hydrocarbon coating degraded at304.5�C, while those were at 270.8, 327.5 and 303�C forthe fluorine-containing, silicon-containing, and fluo-rosilicone coatings. It is clear that the fluorine-con-taining coating began to degrade at 150�C, and 10 wt%of the coating degraded when the temperatureincreased to 270.8�C, which was much lower than theother three kinds of coatings. The fast degradation rateof the fluorine-containing coating resulted in the sharpdecrease in the coating hydrophobicity. In contrast, thesilicon-containing component contributed better ther-mostability to the coatings.
This was relevant to the different modificationmechanism. The fluorine-containing copolymer was
Table 1: Water contact angles of the coatings under different temperatures
Hydrocarbon Fluorine-containing Silicon-containing Fluorosilicone
50�C 100�C 150�C 50�C 100�C 150�C 50�C 100�C 150�C 50�C 100�C 150�C
0 h 81.6 82.3 81.4 116.9 118.1 117.7 100.5 100.7 101.1 116.9 114.5 116.424 h 83.2 87.2 83.9 116.3 114.4 85.1 102.7 102.4 103.3 116.5 108.3 100.372 h 85.5 88.1 81.8 115.2 112.8 80.5 102.7 104.1 103.4 115.1 106.0 103.796 h 86.7 89.4 80.6 114.8 112.5 80.0 103.4 104.2 103.6 114.9 105.1 103.0
HydrocarbonFluorine-containingSilicon-containingFluorosilicone
0 50 100 150 200 250 300
Time/h
70
80
90
100
110
120
130
Con
tact
ang
le/°
Fig. 4: Water contact angles of the coatings at 150�C
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prepared by a fluorine modification method of post-polymerization. The fluorine-containing side chains wereincorporated into the copolymer via a linker of toluenediisocyanate (TDI), and mostly distributed at the coatingsurface. When the segments are damaged, fluorinecontent at the coating surface will decrease, resulting insubstantial dropping of water contact angle of thecoating. It has been reported that degradation of theacrylic urethane coating mostly proceeds through a director radical induced scission of the urethane linkage.2,25,26
The urethane linkage linking the perfluoroalkyl segmentsand the copolymer side chains got ruptured whenexposed to heating at 150�C for 24 h. So that thefluorine-containing alkyl chains with extremely lowsurface free energy were isolated, resulting in a decreasein the coating hydrophobicity. Therefore, water contactangle of the fluorine-containing and fluorosilicone coat-ings decreased to almost the same as that of thehydrocarbon and silicon-containing coatings. On theother hand, however, the silicon-containing segmentswere incorporated by the ureido group resulting from thereaction of amine group and –NCO group. The ureidobonds are more stable than the urethane linkage at hightemperature; thus, the silicon-containing segments weremore difficult to keep isolated than the fluorine-contain-ing components. Consequently, the silicon-containingcoating exhibited good thermal resistance and stayedhydrophobic after exposure to heat at 150�C for at least300 h. The fluorosilicone coating exhibited nearly thesame water contact angles with the silicon-containingcoating after the exposure experiment at 150�C.
Xenon arc aging environment
Xenon lamp exposure and weathering equipment wereused to provide the artificial weathering comprehen-sive environment to investigate the failure behavior ofthe coatings. The water contact angle of the coatings as a
function of test time under xenon arc aging environmentis shown in Fig. 6. All the coatings failed more quickly inthe artificial weathering environment than in the otherenvironments. During the testing time of the four kindsof coatings, the silicon-containing coating showed rela-tive stable hydrophobicity compared with the othercoatings. The water contact angle of the fluorine-containing coating decreased sharply during the first20 h, and then it was similar to that of the hydrocarbonone at the later part the test. The fluorosilicone one fellin between the above two coatings.
It is believed that hydrophobicity of polymeric films isgenerally determined by the film surface compositionand the surface roughness.9,27,28 Therefore, to explorethe failure mechanism of the coatings, the surfacemorphology and the surface chemical composition ofthe coatings were investigated by SEM, FTIR and XPS.
SEM of the fluorosilicone coating before and afterxenon arc aging environment
Figure 7 exhibits the SEM images of the fluorosiliconeacrylic polyurethane coating before and after xenon arcaging test. It is clear that the coating surface was flat,and there were no flaws observed on the surface beforethe aging test (Fig. 7a), while apparent defects appearedand the coating surface became rough after the test. Itindicated that the coating was destroyed during thexenon arc aging test. The variation of the chemicalcomposition and roughness of the coating surfaceresulted in a decrease in the water contact angles.
ATR-FTIR of the fluorosilicone coating before andafter xenon arc aging environment
Figure 8 displays the ATR-FTIR spectra of the fluo-rosilicone acrylic polyurethane coating before and
100 200 300 400 500
Temperature/°C
Mas
s/%
0
20
40
60
80a
cb
d
100
Fig. 5: TGA curves of the copolymer coatings. (a) Hydro-carbon coating; (b) fluorine-containing coating; (c) silicon-containing coating; (d) fluorosilicone coating
0 20 40 60 80 100
Time/h
70
80
90
100
110
120
Con
tact
ang
le/°
HydrocarbonFluorine-containing
FluorosiliconeSilicon-containing
Fig. 6: Water contact angles of the coatings in xenon arcaging environment
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after the xenon arc aging test. It can be seen that thestrength of the characteristic vibration of C=O1688 cm�1 from urethane linkage, and the character-istic vibration of perfluoroalkyl at 1254 cm�1 and1159 cm�1 decreased after the xenon arc aging test,indicating that the acrylic polyurethane coating wasdestroyed and the fluorine-containing segments wereisolated from the coating surface. However, thestrength of the characteristic vibration of the siliconesegments remained and was even enhanced after thexenon arc aging test, indicating that the siliconesegments still distributed on the coating surface afterthe test. This was helpful for maintaining thehydrophobicity of the silicon-containing and fluorosil-icone coatings.
The presence of fluorine- and silicon-containingchains and their degradation behavior can be quanti-fied by surface characterization of XPS. The elementalcomposition of the fluorosilicone acrylic polyurethanecoating before and after xenon arc aging test wasfurther investigated via XPS analysis. Figure 9 displays
the XPS spectra of the fluorosilicone acrylic polyur-ethane coating before and after the xenon arc agingtest. Table 2 shows the atomic ratio data of the films.The variation of the elemental composition of thecoating surface can be easily observed from the XPSresults. The F/C ratio decreased from 0.310 before thefailure test to 0.009 after the failure test, suggestingthat the fluorine content decreased sharply after thexenon arc aging test. The N/C ratio decreased from0.076 to 0.032; while the Si/C ratio increased from 0.115before the failure test to 0.277 after the failure test, andthe O/C ratio increased from 0.323 to 0.410.
The XPS data reflected the chemical composition ofthe outermost of the coating surface, which directlydetermined the hydrophobicity of the coatings. It canbe seen that the fluorine-containing component wasdestroyed during the failure test, and the silicon-containing component was stable during the test.Therefore, the hydrophobicity of the fluorine-contain-ing and fluorosilicone coatings decreased drasticallyduring the test, while the silicon-containing oneperformed better.
Fig. 7: Surface morphology of the fluorosilicone acrylic polyurethane coating before (a) and after (b) xenon arc aging test
a
b
4000 3500 3000 2500 2000 1500 1000
Wavenumbers/cm–1
Tran
smitt
ance
/%
2940 28
68
1728
1685
1528
1461 12
50 1160 10
9810
2580
5
Fig. 8: ATR-FTIR spectra of the fluorosilicone acrylicpolyurethane coating before (a) and after (b) xenon arcaging test
a
b
1200 1000 800 600 400 200 0
Binding energy/eV
Inte
nsity
/a.u
.
F1sO1s
N1s
C1s
Si2p
Fig. 9: XPS spectra of the fluorosilicone acrylic polyur-ethane coating before (a) and after xenon arc aging test (b)
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In our characterization, the test depth of ATR FTIRwas about 0.3–2 lm according to the different wave-length of the incident ray, which was much deeper thanthat of the XPS test. Therefore, the change in fluorinecontent via ATR was smaller than XPS.
Combined with the hydrophobicity failure behaviorof the four kinds of coatings in all the above environ-ments, it can be concluded that the fluorine-containingand fluorosilicone coatings exhibited better hydropho-bicity in moderate environments, but failed morequickly in high temperature and xenon arc agingenvironments. The silicon-containing coating exhibitedbetter hydrophobicity failure behavior in the hightemperature and xenon arc aging environments. Thiswas determined by the fluorine and silicon modifica-tion method. The fluorinated alkyl side group wasincorporated into the fluorine-containing copolymervia a linker of toluene diisocyanate (TDI) and formedurethane linkages. The silicon-containing segmentswere incorporated by the reaction of amino groupfrom the silicone and the –NCO from the curing agent,in which ureido groups were generated. During the filmformation process, the –NCO from the curing agentreacted with the hydroxyl from the copolymers andamino group from the silicone, and produced urethaneand ureido linkages, respectively.
It has been reported that the degradation of theacrylic urethane coating mostly proceeds through adirect or radical induced scission of the urethanelinkage.2,25 The perfluoroalkyl segment was incorpo-rated into the copolymer via urethane linkage, and theperfluoroalkyl side chain was preferentially distributedon the coating surface. When exposed to severeenvironments, the urethane linkage became ruptured,and then the fluorine-containing component withextremely low surface free energy was isolated, resultingin a substantial decrease in water contact angle of thefluorine-containing and fluorosilicone coatings. Unlikethe bonding way of the fluorine-containing component,the silicon-containing component was incorporated byureido linkage, which was much more stable than theurethane linkage. So the silicon-containing coatingshowed relatively stable hydrophobicity in such severeconditions. Therefore, after the failure experiment in150�C environment, the water contact angles of thefluorine-containing and fluorosilicone coatings de-creased to almost the same as those of the hydrocarbonand silicon-containing coating, respectively.
The photoirradiation aging plays a significant rolein the degradation of the acrylic urethane coating thanthe other climatic conditions, and the photocleavageof the urethane linkage will result in the loss ofphysical properties of the acrylic urethane coating.29
Therefore, the four kinds of acrylic polyurethanefailed more quickly under the xenon arc agingcondition than in the other environments. The betterhydrophobicity of the silicon-containing and fluorosil-icone coatings resulted from the silicon-containingsegment, suggesting that the incorporation of thesilicon-containing component was helpful for improv-ing the weather resistance property of the acrylicpolyurethane coatings.
Conclusion
Hydrocarbon, fluorine-containing, silicon-containing, andfluorosilicone acrylic polyurethane coatings were pre-pared. The hydrophobicity failure behaviors of thecoatings were explored in an indoor atmospheric envi-ronment, hygrothermal environment, different tempera-ture environment, and xenon arc aging environment.ATR-FTIR confirmed that the acrylic polyurethanecoatings were prepared as expected. The fluorine-con-taining coating exhibited better hydrophobicity in theindoor atmospheric environment, hygrothermal environ-ment and low-temperature environments than the othercoatings, but failed faster in high temperature and xenonarc aging environments. The silicon-containing coatingexhibited relative stable hydrophobicity in high temper-ature and xenon arc aging environments. The hydropho-bicity of the fluorosilicone coating fell in between thefluorine- and silicon-containing coatings. The fluorine-containing components could migrate and distribute onthe coating surface to improve the hydrophobicity of thecoatings more effectively, while the silicon-containingcomponents played a more significant role in the weatherresistance property of the coatings. The fluorine andsilicon modification method determined the performanceof the coatings in different environments.
Acknowledgments This work was financially supportedby the National Natural Science Foundation of China(51173006).
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Table 2: XPS data of the fluorosilicone acrylic poly-urethane coating before and after xenon arc aging test
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C1s 54.81 57.88O1s 17.71 23.72F1s 16.99 0.54Si2p 6.32 16.03N1s 4.17 1.83
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