This article appeared in a journal published by Elsevier. The attached copy is furnished to the author for internal non-commercial research and

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2 Progress in Organic Coatings 71 (2011) Contents lists available at ScienceDirect Progress in Organic Coatings journal homepage: Nano zinc oxide as a UV-stabilizer for aromatic polyurethane coatings M. Rashvand a, Z. Ranjbar a,, S. Rastegar b a Department of Surface Coatings and Corrosion, Institute for Color Science and Technology, No. 55, Vafamanesh St., Hossein Abaad Sq., Pasdaran, Tehran, Iran b Faculty of Polymer Engineering and Color Technology, Amirkabir University of Technology, Hafez Street, Tehran, Iran article info abstract Article history: Received 23 January 2011 Received in revised form 5 April 2011 Accepted 7 April 2011 Keywords: Nano zinc oxide UV stabilizer Polyurethane Electrocoating Nano zinc oxide (ZnO) was co-deposited together with a cathodic electrodeposition paint onto phosphated normal steel panels. The films containing nano zinc oxide were compared with blank films regarding their stability against UV radiation. SEM micrographs show that nano-zno can stop the formation of cracks in the film. On the other hand, AFM, surface roughness and loss of gloss studies showed that the presence of nano zinc oxide particles reduces the photo-degradation of the aromatic polyurethane binder. It was also found that the presence of the nano-sized ZnO particles in the films reduces the tendency of the films to yellowing Elsevier B.V. All rights reserved. 1. Introduction Polyurethanes are well known coatings in a large variety of commercial and technical applications, including adhesives, sealants, coatings, fibers, thermoplastic parts, elastomers and both rigid and flexible foams, due to their high flexibility, good process ability, excellent water resistance, good resistance to acids and solvents, better alkaline resistance than most of other polymers, good abrasion resistance and good mechanical properties [1 3]. Aromatic based polyurethane materials are sensitive to UV light that is the major limitation for their use them as exterior surface coatings. They show good resistance to chemicals and solvent and they are less expensive than their aliphatic counterparts [3]. Many researchers have studied the changes of the chemical structural of polyurethanes upon exposure to UV radiation as a destructive factor for their physical and mechanical properties [4 11]. One of the major application fields of aromatic polyurethane chemistry in the automotive coatings sector is in cathodic electrodeposition (CED) paints. The present CED technology utilizes aromatic polyurethanes based on epoxy polyols. In this way, very high adhesion and saponification resistance is obtained and therefore the corrosion resistance of the electrocoat is guaranteed. The reaction is catalyzed by the basic nature of amines in the epoxy-amine adduct polyol. After cross-linking a molecular network containing urethane linkages are formed, which sup- Corresponding author. Tel.: ; fax: addresses: roshanakranjbar@yahoo.com, ranjbar@icrc.ac.ir (Z. Ranjbar). port primer adhesion. The schematic reaction is illustrated in Scheme 1. In CED process the positively charged resin and pigment particles migrate to the cathode (substrate) and deposit at its surface [12]. The film that is formed by electrodeposition is homogeneous, smooth and covers all parts of the object to virtually the same thickness, even at the edges and hollow boxes. CED primers were introduced in mid-1970s in an attempt to generally improve the corrosion resistance of car bodies. These primers offered very good corrosion resistance, improved throwing power, and good cross-linking efficiency [12]. In an automotive coating system, the CED coating film is overcoated by a primer-surfacer followed by a metallic base-coat and a clear-coat. One of the recent approaches to car body painting is to reduce the space and footprint of the paint shops. This can be done for example by eliminating primer surfacer layer. In this case, the base-coat and clear-coat layers are not able to filter-out the UV part of the sunlight and it reaches the CED film, where it can deteriorate the film and form a weak boundary layer resulting in loss of intercoat adhesion. A certain degree of deterioration of corrosion resistance and discoloration of the CED film could also be expected [13]. In order to reduce the negative effects of UV radiation on the performance properties of CED films, one can protect it by using UV absorbers [14]. Generally organic UV absorbers are used in coating formulations but they tend to migrate either to the coating surface or into the substrate [14 16]. Recently inorganic nano-particles were also used as efficient UV absorbers in these systems [17]. Zinc oxide is one of inorganic these UV absorbers. It has a wide band gap (3.37 ev) and a large excitation binding energy of /$ see front matter 2011 Elsevier B.V. All rights reserved. doi: /j.porgcoat

3 M. Rashvand et al. / Progress in Organic Coatings 71 (2011) Scheme 1. (I) Reaction of epoxy resins with secondary amines, (II) reaction of polydiisocyanate with alcohols, and (III) curing of epoxy amine adducts with blocked diisocyanate. 60 mev. Therefore, it can absorb light that matches or passes their band gap energy, which lies in the UV range of the solar spectrum [18,19]. In this study, we elucidate the effect of UV light on electrodeposited films of a polyurethane-based CED material and for the first time we describe co-deposition of nano-sized zinc oxide particles in these films and their potential to perform as a light stabilizer for these systems. 2. Experimental 2.1. Materials Nano-ZnO (Oxylink 3102) with a mean particle size of less than 50 nm as zinc oxide dispersion in water (40 ± 1 wt%), density (g/ml) of , ph value of , was purchased from Bühler Company. The waterborne resin dispersion and pigment paste were commercial grade from BASF Company, in the mixing ratio of 1:6, respectively. The CED resin dispersion had a solid content of 40 ± 2% and the solid content of the pigment paste was 66 ± 2% Equipments and procedures Electrocoating cell and nano-composite coating deposition The electrocoating cell consisted of a cubic 1000 ml plastic vessel containing 800 ml of depositable dispersion. The anode was a stainless steel panel with an efficient area of 24 cm 2. The anode to cathode ratio was 1:6. The anode and the cathode were connected to the respective terminals of DC power supply (0 450 V, 4 A, from Eamen Tablo Co.). The distance between anode and cathode was about 10 cm. The dispersion was magnetically stirred during the 3 min electro-coating process to facilitate electrolysis gasses to get out from the CED bath. The bath temperature was controlled at 28 ± 1 C. The paint films, 20 ± 1 m, were deposited on tricationically phosphated cold-rolled steel panels at 190 Volts and baked for 15 min at 165 C. The panels were conditioned for 1 week before further tests Accelerated weathering tests The accelerated weathering tests were performed by exposing the coated panels in a QUV chamber (QUV weathering Tester-model QUV/spray from Q-Lab Co.) during a period of 1000 h. In the QUV chamber, the panels were cyclically exposed to UV-B radiation

4 364 M. Rashvand et al. / Progress in Organic Coatings 71 (2011) (315 nm) at 50 C for 8 h followed by water condensation at 40 C for 4 h according to ASTM G53 standard Instruments The nano particles were dispersed in the electrodeposition media using a Hielscher UP400S ultrasonic processor. A 450 V/4 A rectifier (Eamen Tablo Co.) was used to deposit the dispersion onto the test panels. The morphology of the films was investigated using a LEO 1455VP scanning electron microscope. The UV visible absorption spectra of the electrodeposited films were obtained using a CE9200 UV/VIS spectrophotometer from BUCK Scientific Inc. in reflection mode. Color difference ( E) was measured using a MA-68 spectrophotometer from X-Rite in face mode under a D65 light source. The color difference calculation was performed based on CIE L*a*b* 1976 color difference equation. Gloss values were recorded at 60 using a Micro-TRI-Gloss from BYK Gardner. Surface roughness of the coatings was measured using a TR100 Surface Roughness Tester. A Dual scope DS E Atomic force microscope from Micro Photonics Inc. was used to determine the topography of the coatings. Adhesion of the coatings were measured using pull-off technique in accordance with ASTM 4541 by a Positest AT adhesion tester. Fig. 1. Scanning electron micrograph of the electrodeposited resin containing 3 wt% nano-zinc oxide particles. 3. Results and discussion 3.1. Characterization of non-exposed coatings Micro-structure analysis Fig. 1 shows the SEM surface micrographs of the electrodeposited resin films containing 3 wt% of nano-zno particles before exposure to UV radiation. As can be seen, the nano-zno particles are well dispersed in the coating film. While most of the particles are present as single particle, an aggregate of two particles is also found in the right hand side of the image. This shows that a certain degree of nano-zno particles aggregation can occur during electrodeposition. Electrodeposition is performed at a ph-value of 6.0 which lies well below the iso-electric point of zinc oxide surface ( ). Therefore, the surface of the zinc oxide particles acquires positive net electrical charge and move toward the cathode together with the resin dispersion particles. As the migrating particles get closer to the cathode surface they are exposed to hydroxyl ions generated by the electrolysis of water, their charges are drained and their stability is lost. This co-deposition process leads to a very uniform distribution of the nano-zinc oxide particles throughout the film. Fig. 2. UV visible absorption spectra of nano ZnO stabilized coatings. The equations for the electrode reactions are described in Scheme 2 [12 19] UV absorption spectra In order to evaluate the UV absorption characteristics of the films containing nano-zinc oxide and compare them with the blank films, reflection-mode UV visible spectroscopy was used. Fig. 2 illustrates the results of this evaluation. As can be seen, both neat and nano-zno containing films absorb UV light in the range of nm. The presence of 3 wt% of nano-zno in the coating films has resulted in about 50% increase in the absorption of the Scheme 2. Electrode reactions that occur during electrodeposition of the coatings.

5 M. Rashvand et al. / Progress in Organic Coatings 71 (2011) Fig. 3. SEM micrograph of the surface of (a) unexposed non-stabilized coating, (b) unexposed nano-zno stabilized coatings, (c) non-stabilized coating after 1000 h of UV exposure and (d) nano ZnO stabilized coatings after 1000 h of UV exposure. UV wavelengths measured in the reflection mode. Since zinc oxide particles have a higher density than the medium they can sink into the film and therefore their surface concentration could be less than their bulk concentration. In other words, the efficiency of the nano ZnO could be higher if their sedimentation in the film could be decreased. Fig. 4. Variation of the surface roughness factor as a function of the time of exposure in QUV chamber Characterization of UV-exposed coatings Micro-structure analysis Fig. 3 shows the SEM micrographs of the coating surface before and after exposure to UV light. As it can be seen before exposure to UV radiation the resin engulfs all the particles and the solid particles could not be seen clearly (Fig. 3a and b). Fig. 3c and d shows the SEM micrograph of the surface of the UV-exposed coating after 1000 h of exposure. In both cases bare particles could be seen at the surface of the exposed coatings, which is a result of photo-degradation of the binder. It can be seen that the presence of nano-zinc oxide in the film has caused a more uniform degradation of the resin to occur. On the other hand, cracking has been occurred in the non-stabilized film while no crack could be seen in the nano-zinc oxide stabilized film. The formation of these cracks could be an evidence for the concentration of degradation-induced stresses in specific places of the film. As nano-zinc oxide absorbs UV light, it reduces the UV intensity at the higher depth of the film and therefore reduces the probability of stress generation in the bulk of the coating. In this way nano-zinc oxide could help the polymeric matrix of the films to remain intact for a longer period of time. Because of semi-conductor nature of nano ZnO and its wide band gap, electrons absorb the energy from UV light and then jump from a valence band to an empty conduction band with the creation of a positive hole in the valence band and mobile or trapped electrons in the conduction band. A fast charge recombination of positive holes and electrons

6 366 M. Rashvand et al. / Progress in Organic Coatings 71 (2011) Fig. 5. AFM topographic height images of unexposed (a) non-stabilized coating and (b) nano ZnO-stabilized and 1000 h exposed (c) non-stabilized coating and (d) nano ZnO-stabilized coating. in the conduction band will quench the charge-separated state of nano ZnO particles and decay the destructive energy of UV radiation [20] Surface roughness Surface roughness values of the samples during different times of exposure to accelerated weathering are shown in Fig. 4. Here we define a roughness factor, which is described as Eq. (1): Roughness factor = R a,t R a,0 (1) where R a,t is the mean surface roughness after exposure for t hours and R a,0 is the initial mean surface roughness.

7 M. Rashvand et al. / Progress in Organic Coatings 71 (2011) Scheme 3. Photo-degradation reaction of aromatic polyurethane (Eq. (1)), chain-scission reactions that liberate CO and CO 2 (Eqs. (2) and (3)) [25]. As it is evident, the surface roughness of the coating increases during the period of exposure in both stabilized and non-stabilized coatings but the rate of photo-induced roughening of the blank coating is much more than the nano-zno stabilized coating. It is also possible to conclude that the presence of nano zinc oxide in the film has caused the exposure time corresponding to roughness jump to shift from 360 to 720 h, which confirms its stabilizing effect. AFM studies have also confirmed these results. As can be seen from Fig. 5, the surface profile of the nano zinc oxide-stabilized coating is much more uniform than the non-stabilized coating. On the other hand, the maximum height difference of the profile is about 5 m for nano-stabilized coating while it undertakes a value of more than 9 m in case of the non-stabilized coating Loss of gloss Gloss is an appearance characteristic, which is related to the surface roughness. Therefore, its values can be used as another evidence for the variation of the surface roughness of the coatings during exposure to UV radiation. As the surface of the film is degraded, the film losses its gloss and tend to lower gloss values. Finally, at a very high level of surface roughness the gloss value is reached to its minimum value and then levels off. Bulcke et al. [26] developed theoretical models which can predict practical gloss values. The numerical values of loss of 60 gloss after different periods of exposure to UV radiation is illustrated in Fig. 6. As can be seen, the gloss of the blank coating decreases much quicker than the coating stabilized with nano zinc oxide. The nonstabilized coating loses 95% of its gloss after 360 h of exposure to UV radiation while this happens for the nano zinc oxide-stabilized coating after 720 h of exposure. This is the same trend observed for the variation of the surface roughness of the films with exposure time Color difference Exposure of the aromatic polyurethanes to UV light could result in discoloration of the films. Therefore color measurement is a suitable mean for studying the photo-induced chemical changes in these systems.

8 368 M. Rashvand et al. / Progress in Organic Coatings 71 (2011) Fig. 6. Loss of 60 gloss of the samples during exposure to UV radiation. The results of UV-induced color changes in both blank and nano zinc oxide-stabilized coatings are reported in Fig. 7. As can be seen after 72 h of exposure, color difference ( E) has reached to a value of 11 for the blank sample which is quite high for such a short duration. Color difference of the blank sample, after 168 h (7 days) reaches to a value of about 13 and after that it changes very slightly. This can be related to the extreme susceptibility of these systems to yellowing and the quick and extensive extent of yellowing. This yellowing is caused by the oxidation reactions in the backbone of the polymer. Irradiation changes physical and chemical characteristics of the coating surface which results in rapid color change and degradation. Studies have supported [11,21 25] two mechanisms for photo-degradation in polyurethanes based on aromatic diisocyanate (MDI), involving photo-oxidation of the aromatic functional groups and direct photolytic cleavage of the urethane group. Photo-oxidation of the aromatic isocyanate may produce a diquinone imide. Quinone imides are the chromophore which are responsible for the rapid yellowing of MDI based polyurethanes (Scheme 3, Eq. (1)). Subjecting aromatic radicals to oxygen results in the formation of semi-quinone groups, which have strong UV absorption and act as a photostabilizing surface layer that protects the bulk of the coating. In addition to reaction 1, which results in discoloration of the coating, Osawa [10] has recommended UV-induced chain-scission reactions that release CO and CO 2, which can also be resulted in direct photolytic cleavage of the urethane group (Scheme 3, Eqs. (2) and (3)). This reaction can happen in the absence of oxygen. It has been shown that the cleavage of the urethane group can result in a photo-fries rearrangement. The alkoxy, amino and alkyl radicals in Eqs. (2) and (3) start further degradation reactions. For example, two amino radicals can combine to give an intermediate that then reacts with an alkoxy radical to form diazo compounds. These diazo compounds are also brownish in color. Azo products can also be created through light-initiated photo-fries rearrangement [25]. The alkyl radicals formed in reaction (3) can also react with oxygen to produce aldehydes, peroxy radicals, and hydroxy radicals [25]. Similar to the semi-quinone and quinone groups, the photo-fries product is also an efficient UV absorber and is capable of acting as an internal filter to inhibit later photo oxidation. As Fig. 7 shows, adding nano ZnO particles decrease color change considerably but it can be seen that in this case the yellowing process does not reach to its equilibrium value during 1000 h of exposure. 4. Conclusion The effect of nano-zno particles on the photo-degradation of the aromatic polyurethane-based electro-coatings was investigated by exposing the films to humidity and UV radiation. It was found that the presence of nano-zinc oxide particles could reduce the photo-degradation tendency of the film and protect it against deteriorating effects of UV radiation to a certain value. Therefore, co-deposition of nano-zinc oxide particles in cathodic electrodeposition paints could improve their UV resistance. References Fig. 7. E values during exposure to UV radiation. [1] D. Rosu, L. Rosu, N. Cascaval, Polym. Degrad. Stab. 94 (2009) [2] H. Sanders, C.K. Frish, Polyurethanes chemistry and technology. Part I. Chemistry, Wiley, New York, 1962, pp [3] A. Forsgren, Corrosion Control Through Organic Coatings, CRC Press Taylor & Francis Group, 2006, pp [4] A. Kachan, N. Kargan, N. Kulik, G. Boyarskii, Theor. Exp. Chem. 4 (2004) [5] P. Davies, G. Evrard, Polym. Degrad. Stab. 92 (2007) [6] X.F. 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