纳米氧化锌--聚氨酯--防腐---英文

Progress in Organic Coatings53(2005)91–98Effects of P/B on the properties of anticorrosivecoatings with different particle sizeL.H.Yang,F.C.Liu,E.H.Han∗Environmental Corrosion Center,Institute of Metal Research,The Chinese Academy of Sciences,Shenyang110016,ChinaReceived10July2004;accepted11January2005AbstractThe effect of the pigment/binder ratio(P/B)on the corrosion resistance properties of polyurethane paints on carbon steel was investigated by ac impedance techniques.Painted steel electrodes were studied during exposure to3.5%NaCl solution for up to1000h.Two series of polyurethane paints were used:conventional ZnO or nano-ZnO as pigments.The results showed that the corrosion resistance is affected by the P/B ratio,the optimum amount of zinc oxide of conventional zinc oxide(P/B=1)is more than that of nano-zinc oxide(P/B=0.3).Therefore, better anticorrosive protection is achieved by the coatings pigmented with nano-ZnO and a lesser amount of nano-ZnO is required.©2005Elsevier B.V.All rights reserved.Keywords:Pigment/binder ratio;Nano-ZnO;Polyurethane;Anticorrosion properties;Fick’s second law1.IntroductionThe appearance and appliance of nano-materials brings new opportunities to the coating industry[1,2].Due to their tiny particle size,nano-materials have special properties; adding nano-materials to the coatings improves the proper-ties of the conventional coatings and produces new multi-functional coatings[3].Multi-functionality results from the characteristic structure of the nano-ZnO,including optical, chemical,biological,and semi-conductor properties;more-over,ZnO is a non-toxic material,which complies well with current environmental protection ing the spe-cial properties of nano-ZnO,environmentally friendly coat-ings can be developed.While the fundamental function of the coatings is anticorrosion,few studies have considered the attractive anticorrosion properties resulting from the addition of nano-ZnO to the coatings.The weight ratio of the pigment/base(P/B)is an impor-tant measure by which the properties of coatings are deter-mined[4],and relates to the transport of corrosive species of electrolytes through coatings.The purpose of the study was∗Corresponding author.E-mail address:ehhan@(E.H.Han).to correlate the size of particles with the anticorrosion effi-ciency.Polyurethane coatings pigmented with nano-ZnO and conventional ZnO at different P/B were prepared by a direct dispersion method to form multiple coatings.The appraisal of the anticorrosive property,the ascertainment of the opti-mum concentration of additives,and the comparison of their anticorrosive properties were determined by electrochemical impedance spectroscopy(EIS).2.Experimental procedures2.1.Preparation of coatingsPolyurethane resin was used as a matrix and the size of the particles of pigment in the coatings was on the scale of micrometers or nanometers.The morphology and size of the two pigments in the resin matrix are shown in Fig.1.Fig.1 shows that the mean particle size of nano-ZnO was about 20nm and the mean particle size of micro-ZnO(the usual size)was about400nm.The polyurethane coatings containing nano-ZnO were pigmented with nano-ZnO with values of P/B of0.05,0.3, 0.6,and1.0.The conventional polyurethane coatings were pigmented with micro-ZnO with values of P/B of0.05,0.3,0300-9440/$–see front matter©2005Elsevier B.V.All rights reserved. doi:10.1016/j.porgcoat.2005.01.00392L.H.Yang et al./Progress in Organic Coatings53(2005)91–98Fig.1.TEM photographs of different coating:(a)nano-composite coat and(b)conventional coat.0.6,1.0,and the phase structures of the two pigments were identified by X-ray diffraction as shown in Fig.2,results showed that both the micro-and nano-ZnO particles exhib-ited the same crystal structure.2.2.Preparation of substratesPrewashed cold rolled steel test panels(45#)were used in order to ensure uniform surfaces for testing.They were mechanically polished,degreased with acetone,washed with alcohol,and kept in the desiccator prior to testing.2.3.Electrochemical testingA three-electrode cell was used for measuring electro-chemical impedance spectroscopy.The substrate of the work-ing electrode was coated with an exposed area of12.56cm2,a saturated calomel electrode(SCE)was used as the reference electrode,and the counter electrode was stainless steel.EISFig.2.XRD pattern of ZnO and nano-ZnO powder. experiments were conducted at the corrosion potential.Sam-ples were exposed to3.5%NaCl solution,and experimental data were acquired at different immersion times.The EIS experiments were performed with an EG&G M263potentio-stat and M5210lock-in amplifier,the frequencies included the range from100kHz to10mHz,and the amplitude of the signal was20mV.The EIS data were analyzed by Equivert as developed by Boukamp.3.Results3.1.EIS analysis of nano-ZnO/polyurethane coatingsFig.3shows the EIS of polyurethane coatings with dif-ferent P/B immersed in3.5%NaCl solution.As shown in Fig.3(b),the coating with P/B=0.3exhibited relatively high-resistance even after1000h of immersion,this long time in-dicated good anticorrosion performance.This good perfor-mance could result from the absence of a corrosion process or a path in the coating that would have allowed the substrate underneath the coating to communicate readily with the sur-rounding electrolyte solution.The resistance of the coating (R p)with P/B=0.3was about1010 cm2,which was much higher than that of other coatings,thereby indicating its ex-cellent protective property.Fig.3(a)shows the EIS of the coating with P/B=0.05. After130h of immersion,the semi-circles in EIS changed from one to two.The low-frequency part of the impedance plot is related to corrosion processes occurring on the metal surface,while the high-frequency range is determined by the coating.This change could reflect the diffusion pro-cess of corrosive electrolyte through the coating.In thefirst 130h,the corrosive electrolyte solution did not arrive at the metal/coating interface,hence there was no sub-coating corrosion and only one semi-circle in the EIS.However, after130h of exposure,water and corrosive ions arrivedL.H.Yang et al./Progress in Organic Coatings 53(2005)91–9893Fig.3.Nyquist plots of polyurethane coatings containing nano-ZnO immersed in 3.5%NaCl solution.(a)P/B =0.05,(b)P/B =0.3,(c)P/B =0.6,and (d)P/B =1.0.at the interface,and corrosion of the base material oc-curred.Fig.3(c and d)show EIS of the coating with P/B =0.6and 1.It is evident that the two spectra exhibit many simi-larities,such as the two semi-circles at the beginning of im-mersion,indicating that the diffusion processes of the cor-rosive electrolyte solution were similar for these two coat-ings.Observing the resistance values in more detail,the re-sistance values of the coating were quite low (on the order of 105–107 cm 2),thereby confirming the reduced barrier properties of these coatings.Fig.4shows coating resistance and capacitance parame-ters as a function of time.The coating resistance (R p ),which was influenced by the presence of defects and conductivity through the pores,decreased with time as shown in Fig.4a.These changes refer to the pore formation and deterioration of paints due to the interaction with the corrosive electrolyte so-lution.The general trend was the same,but clear differences were evident when comparing the four kinds of samples,the coating capacitance growth,which was related to water up-take caused by diffusion of water molecules in the matrix waslowest in the case of P/B =0.3.Also,the coating resistance R p with P/B =0.3decreased slowest while maintaining the highest resistance.These simultaneous patterns of resistance and capacitance shows that P/B =0.3is optimal.The capacitance of the coating (C p ),which is related to water uptake,increased with time in all cases,as shown in Fig.4(b).Many studies [4,5]show that the transportation of water through coatings is a diffusion process,which can be divided into two stages:the initial stage and saturation stage.In this work,in the initial stage,the coating capacitance,C p increased with immersion time.When the saturation stage was attained,C p remained approximately constant.In the initial stage,the diffusion process follows Fick’s second law in some cases and in others not depending on the P/B of the coatings.If the diffusion of water through coatings follows Fick’s law,there will be a relationship among the coating capacitance (C p ),diffusion coefficient (D ),coating thickness (d ),and immersion time (t )as follows [5,6]:log(C p /C o )log(C s /C o )=4Dd 2π×t 1/2(1)94L.H.Yang et al./Progress in Organic Coatings53(2005)91–98Fig.4.Pore resistance(a)and coating capacitance(b)vs.immersion time for nano-ZnO polyurethane coatings at different P/B ratios.The plots of log(C p)versus t1/2and the linearfitting results (coefficients correlation)are shown in Fig.5.It is evident that,for the four coatings in the initial period of the diffusion process,the curvesfit the lines well(this depends on the evaluations of the coefficients of correlation).In the3.5% NaCl solution,the diffusion of water through polyurethane coatings at different P/B in the initial period follows Fick’s second ing Eq.(1),the effect of P/B on the diffusion coefficients are shown in Fig.6.It is obvious that the rate of water diffusion through nano-ZnO polyurethane coatings with P/B=0.3is much slower than through the polyurethane coatings than with nano-ZnO of other values of P/B.In summary,from the results in Figs.3and4and Fig.6, the nano-ZnO/polyurethane coating with P/B=0.3is about optimum and exhibits the best protective properties.3.2.EIS analysis of conventional ZnO/polyurethane coatingsElectrochemical impedance responses obtained from the painted electrodes,which were pigmented with conventional ZnO,after1000h immersion in the sodium chlorideelec-Fig.5.Linearfitting results for water diffusion through polyurethane coat-ings with nano-ZnO particles with different P/B in the initial period. trolyte are shown in Fig.7.These Bode plots,log(R p)ver-sus log(f),exhibit several important features[4].First,for the pigmented versions with P/B=1,the curves obtained at 1000h show two distinctive segments.The high-frequency part of each curve exhibits a linear increase of log(R p)with a decrease in the values of log(f).This is the response expected of a pure capacitor.However,at the lower frequencies,a tran-sition towards constant log(R p)versus log(f)values occurs. The invariance of log(R p)with frequency is indicative of the expected response due to the coating and can be modeled in terms of a simple parallel resistance–capacitance(RC)elec-trical equivalent circuit.In practice,as the coatings degrade, the coating capacitance increases,and the resistance of the paintfilm(R p)decreases.These effects may result from the development of ionic conduction through the coating to the metal substrate.From the data in Fig.7,it is clear that,within the testing time,the substrate under the coating with P/B=1 did not corrode significantly and the coating retained its in-tegrity.Fig.6.Diffusion coefficient of through nano-ZnO polyurethane coatings vs. the P/B ratio.L.H.Yang et al./Progress in Organic Coatings53(2005)91–9895Fig.7.Electrochemical impedance(a)and phase angle(b)after1000h im-mersion in3.5%NaCl for steel electrodes coated with polyurethane coatings containing micro-ZnO at values0.05,0.3,0.6,1,and1.3.Except for P/B=1,all of the conventional ZnO coat-ings exhibited capacitance/resistances that were displaced towards significantly higher frequencies(i.e.,>102Hz);a further curve component displayed a linear relationship be-tween log|Z|and decreasing values of log(f),while,simulta-neously,the phase angle,shown in Fig.7(b),moved to higher frequencies and a second time constant appeared.This addi-tional low-frequency component of the impedance suggests that corrosion of the substrates under these paints was signif-icant after1000h testing.Fig.8is comparable to Fig.4for the nano-ZnO,and shows the resistance and capacitance versus time for the conventional ZnO coatings.The resistance of the coatings (R p)decreased relatively rapidly initially,and then decreased more slowly.The general trend was the same,but some differences were evident when comparing thefive values of P/B:the coating resistance R p with P/B=1decreased most slowly,and maintained the highest value.Thisshows Fig.8.Resistance(a)and capacitance(b)vs.time of immersion for conven-tional ZnO polyurethane coatings at different P/B ratios.that this concentration of the conventional ZnO is opti-mum.The capacitance(C p)increased with time in all cases,and these are shown in Fig.8(b),these results are consistent with the trends observed for the R p data shown in Fig.8(a),since an increased uptake of the electrolyte solution would decrease the resistance of the paintfilm.For the coating with P/B=1.3, the electrolyte uptake was extensive;and this occurs because the paint structure is relatively open and porous.For the con-ventional coatings with P/B≤1,the accumulation of water depends little on the value of P/B.For the conventional ZnO coatings,the diffusion of wa-ter through the coatings can be divided into two stages:for thefirst stage,the plots of log(C p)versus t1/2and the lin-earfitting are shown in Fig.9,where the diffusion of water through the polyurethane coatings at different values of P/B for thefive coatings exposed to the3.5%NaCl solution fol-lows Fick’s second law for the initial period.Calculating the diffusion coefficients from Eq.(1),the effect of P/B on the diffusion coefficients is shown in Fig.10.Here,the rate of dif-fusion of water through polyurethane coatings with P/B=1is96L.H.Yang et al./Progress in Organic Coatings 53(2005)91–98Fig.9.Linear fitting results for water diffusion through polyurethane coat-ings with different P/B in the initial period.slower than through polyurethane coatings with other values of P/B.4.DiscussionResults from investigating the properties and mechanism of ZnO on the effectiveness of coatings when the ZnO is added as conventional particles or nano-particles are de-scribed in Sections 3.1and 3.2.These data were obtained using EIS for measuring the resistance and capacity of the coatings as a function of the types of particles,the concentra-tion of ZnO,and time of exposure.These experiments were remarkable for their unambiguous measurement of mecha-nistic details.Important observations and comparisons from measuring the resistance and capacitance of coatings result-ing from adding the two types of particles are shown in Fig.11and are asfollows.Fig.10.Diffusion coefficient of water vs.P/B for the diffusion of water through conventional ZnO-polyurethanecoatings.Fig.11.Coating resistance (a)and calculated capacitance (b)vs.P/B af-ter1000h exposure to 3.5%NaCl solutions.Plotting the low-frequency R f values as a function of P/B for these coatings gives a clear indication as to the relative corrosion resistance properties at the different P/B loadings.From the data in Fig.11(a),it is apparent that the P/B ratio significantly affects R f .The highest R f values were obtained at P/B =0.3for nano-coatings and at P/B =1for conventional coatings,respectively.The former was much higher than the latter.From the EIS data,the capacitance was calculated accord-ing to the following relationship [4]:C =12πfZ imag(2)The capacitance results are compared in Fig.11(b).These results are consistent with the trends observed for the R f data shown in Fig.11(a).The data of Figs.1–10in Sections 3.1and 3.2as well as the comparisons in Fig.11shows that the P/B ratio significantly affects the performance of the coating.These results suggest that the role of the ZnO for the two types of particles is relatedL.H.Yang et al./Progress in Organic Coatings53(2005)91–9897to packing of particles and the respective access to wetting by the resin.When the nano-ZnO was added in excess,the pigment stacked too tightly,and the amount of resin was in-sufficient to wet all the pigments;therefore,the coating did not form a continuousfilm,thereby resulting in extensive de-fects throughout the coating.When the nano-ZnO was added deficiently,the volume of air in the matrix increased which caused the anticorrosive property of the coating to decrease.Thus,an increased uptake of the electrolyte solution would lead to decreased paintfilm resistance values.Except for the nano-coating with P/B=0.3and conventional coating with P/B=1,the uptake of electrolyte was relatively extensive. This is due to the fact that the paint structure is relatively open and porous.Therefore,the nano-coatings have a better capability to prevent the electrolyte from passing through the coatings than conventional coatings.Moreover,because the transportation coefficient of the nano-coating with P/B=0.3 is smaller than that of the conventional coating with P/B=1, the coating pigmented with nano-ZnO has better anticorro-sive property as shown by the comparative data of Fig.11.It is evident that the nano-ZnO/polyurethane coating with P/B=0.3and the conventional ZnO/polyurethane coating with P/B=1have much improved protective properties.Fur-ther,the conventional ZnO requires more mass than nano-ZnO in order to be optimally effective.In order to rationalize the effects of particle size,the coating should be considered as a system composed of pigment,resin,and air according to Eq.(3):V p+V b+V o=F(3) where V p is the volume of the pigment,V b the volume of binder,and V o is the volume of air.When an optimum state is reached,the trace air is replaced and the space between pigments isfilled with resin;then Eq.(3)becomes V p+V b=F[7].Because the nano-pigment has greater surface activity,it can adsorb more resin on its surface.The resistance to the transport of the electrolyte solu-tion through the coating of nano-ZnO/polyurethane coating is greater than that of the conventional ZnO/polyurethane coating as shown in Fig.11.This trend may be interpreted by the clogging of pores between particles during transition from electrochemical action mode to the formation of the final barrier mechanisms.Free space between the particles is likely to be more frequent with large particles than with small particles.For larger particlesfilling the pores by means of an oxide is insufficient and the resulting space facilitates penetration through the coating[7].Another study pointed out that the active surface area of nano-materials is larger than that of the conventional mate-rial.This is due to the dimension effect of nano-material. The smaller is the particle size,the larger is the proportion of the active part,such as borders and corners.The comparison shown in Fig.11can be interpreted by applying this interpre-tation.Because the nano-ZnO has the special afore mentioned surface trait that conventional ZnO lacks due to the factthat Fig.12.Effect of the particle size of pigment on the DSC spectrum of the coating.there are many more active molecules on the nano-ZnO sur-face than that on conventional ZnO surface,it can absorb large amount of resin on its surface.Fig.12shows that the surface energy(calculated by apex area)of nano-ZnO(107.3J/g)is greater than that of the conventional ZnO(43.8J/g),which indicates that the attraction of the interface is greater for the nano-ZnO.5.Conclusions1.The performance of the coatings is largely affected bythe amount of ZnO,as expressed by the ratio P/B and by the particle size of the ZnO.The optimum amounts of ZnO in the polyurethane coatings is about P/B=1for conventional ZnO and about P/B=0.3for nano-ZnO. 2.The transport of water through coatings followed Fick’ssecond law.The transport coefficient of the nano-coating with P/B=0.3is1.47×10−11and the transportation coefficient of the conventional coating with P/B=1is4.5×10−11,which is almost three times more than theformer.3.The size of zinc oxide particles considerably affects theanticorrosion properties of polyurethane coatings.Due to its high-activity and smaller size,the addition of nano-ZnO can enhance the density of the coatings,reduce the transport paths through which the corrosive electrolyte solution can penetrate,and improve the protective perfor-mance of coatings.References[1]S.X.Zhou,L.M.Wu,Discussion on the development of nanometercompound coatings,Coat.Ind.9(2001)28–35.[2]D.R.Baer,P.E.Burrows,A.A.El-Azab,Enhancing coating functional-ity using nanoscience and nanotechnology,.Coat.47(2003) 342–355.98L.H.Yang et al./Progress in Organic Coatings53(2005)91–98[3]T.Xu,C.S.Xie,Tetrapod-like nano-particle ZnO/acrylic resin com-posite and its multi-function property,.Coat.46(2003) 297–301.[4]B.S.Skerry,C.-T.Chen,C.J.Ray,Pigment volume concentration andits effect on the corrosion resistance properties of organic paintfilms, J.Coat.Technol.46(1992)806–819.[5]C.Perez, A.Collazo,M.Izquierdo,Characterisation of the bar-rier properties of different paint systems.Part I.Experimental set-up and ideal Fickian diffusion,.Coat.36(1999)102–108.[6]F.Bellucci,L.Nicodemo,T.Monetta,Corros.Sci.33(1992)1203–1236.[7]A.Kalendova,Effects of particles sizes and shapes of zinc metal onthe properties of anticorrosive coatings,.Coat.46(2003) 324–332.。