Progress in Organic Coatings

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1 Progress in Organic Coatings 65 (2009) Contents lists available at ScienceDirect Progress in Organic Coatings journal homepage: The influence of surface chemistry of nano-silica on microstructure, optical and mechanical properties of the nano-silica containing clear-coats Zahra Ranjbar a,, Saeed Rastegar b a Department of Surface Coatings and Corrosion, Institute for Colorants, Paint and Coatings, No. 59, Vafamanesh St., Hosainabad Sq., Lavizan, Tehran, Iran b Farayand Rang Khodro Co. Ltd., Automotive Coatings Department, Tehran, Iran article info abstract Article history: Received 3 April 2008 Accepted 28 October 2008 Keywords: Nano-silica Clear-coat Morphology SEM Different nano-silicas were incorporated in an automotive OEM clear-coat based on acrylic-melamine chemistry. The morphological characteristics of the heat-cured films were investigated using scanning electron microscopy. It was found that there is a close relationship between the interfacial interactions of binder-silica nano-particles and mechanical and optical properties of the baked films Elsevier B.V. All rights reserved. 1. Introduction 2. Experimental The use of nano-particles to modify specific properties of thin composite films such as organic coatings is getting more and more popular [1 4]. Nano-particles lying on their large specific surface area, hardness, small particle size and/or binder-matching refractive index can render specific properties to organic coating films, which were not possible to obtain using common modification strategies [5]. Nano-silica particles have refractive indices close to that of the common binders used in formulating coating materials. Therefore it is theoretically possible to incorporate them into the clear-coat formulations without damaging the optical properties of their baked films. But in practice the films containing nano-silica particles show a more or less degree of haziness. The goal of this study was to investigate the impact of the surface chemistry of nano-silica particles on morphological, optical and mechanical properties of nano-particle containing clear-coats. It was also aimed on finding other possible mechanism(s) of hazing of clear-coats due to presence of nano-silica particles Materials The clear-coat was formulated based on an acrylic-polyol and a butylated melamine-formaldehyde resin. Xylene, butanol, ethoxy propyl acetate and butyl glycol acetate were used as solvents. A block copolymer-type dispersing agent was incorporated in the nano-silica grinding vehicle. The UV-absorber and the hindered amine light-stabilizer were Tinuvin 400 and Tinuvin 292, respectively. The clear-coat was formulated according to Table 1. Different nano-silicas were incorporated into the clear-coat formulation, the properties of which are shown in Table 2. Two different types of hydrophilic silicas were used, which can be distinguished by very different specific surface area and primary particle size. Three hydrophobic silica grades were also used, which were modified using different silanes, namely dimethyl dichlorosilane, hexamethyldisilane and methacryl silane. These silicas also vary in specific surface area and primary particle size. The acidity of the surface of the hydrophilic silicas was the same but this parameter varies between different hydrophobic nano-silica grades Methods Corresponding author. Tel.: ; fax: address: Ranjbar@icrc.ac.ir (Z. Ranjbar). Grinding of nano-silica dispersions were performed in a sandmill. After letting down, the nano-silica containing clear-coats were diluted to 50 vol% and the mixture was applied on glass and normalsteel panels using a four-sided film applicator at a wet film thickness of 120 m. The films were baked for 20 min at 140 C /$ see front matter 2008 Elsevier B.V. All rights reserved. doi: /j.porgcoat

2 126 Z. Ranjbar, S. Rastegar / Progress in Organic Coatings 65 (2009) Table 1 Clear-coat formulation. Mill base Hydroxy acrylic resin 16.3 Dispersing agent 5 Xylene 5 Nano-silica 5 Let down Hydroxy acrylic resin 42.1 Xylene 4.5 Melamine resin 24.1 Butyl glycol acetate 2.5 UV absorber/catalyst 2 Total Fig. 2. Surface roughness of the films containing different nano-silicas. Fig. 1. Specular gloss value of blank sample and samples containing different nanosilicas. Fig. 3. Haze values of the films containing different nano-silicas. The gloss values of the films were determined using a glossmeter (Byk-gardner, Micro-tri gloss). The haze values of the films were calculated from the data obtained by a Gretag Macbeth, Color-Eye 7000A spectrophotometer based on instructions of ASTM D Pendulum hardness (Sheen Instrument Ltd.), impact resistance and conical mandrel bending resistance (Braive Instruments) were also measured on test samples. Micro-indentation hardness measurements were performed using a Leica VMHT MOT. The surface roughness of the samples was measured using a Roughness tester TR 100. Scanning electron microscopic studies were performed on free surface and fracture surface of free films using a Link Opal LEO 440 SEM instrument. The free films were obtained by immersing glassborne films in de-ionized water for 20 min and drying the film for 24 h at room temperature. The films were fractured at room temperature for taking the images of the bulk of the films. A transmission electron microscope (Philips Co.) was used to clarify the state of dispersion and film morphology in a more detailed manner. Fig. 4. Koenig hardness values of the films containing nano-silica additives. 3. Results and discussion 3.1. Optical properties Fig. 1 shows the specular gloss values of blank sample and samples containing different nano-silicas. As can be seen in all cases Table 2 Nano-silicas specification were incorporated into the clear-coat formulation. Sample code Surface area (m 2 /g) Particle size (nm) Surface treatment ph Purity (%) Carbon content 1 50 ± ± ± MS ± 25 7 HMDS ± 2 16 DMDCS

3 Z. Ranjbar, S. Rastegar / Progress in Organic Coatings 65 (2009) Fig. 5. Direct impact resistance of the films containing nano-silica additives. Fig. 6. Flexibility of the films in conical mandrel bending containing nano-silica additives. a reduction in specular gloss value is observed by incorporating nano-silicas. This reduction is more pronounced in case of Add. 3. Since specular gloss is a surface-related phenomenon, it can be concluded that the structure of the surface of the films is nearly the same in case of different nano-silicas, except Add. 3. Values of the surface roughness of the samples also confirm this result (Fig. 2). It could be seen that the roughness is nearly the same in all cases except Add. 3. The film containing Add. 3 also show the biggest haze value compared to other nano-silicas. In contrast to the specular gloss, the haze value varies severely with variation of nano-silica grade due to different bulk structures of the films (Fig. 3). Fig. 7. Scanning electron micrographs of the free surface of the blank and nano-silica containing clear-coats: (a) blank, (b) Add. 1, (c) Add. 2, (d) Add. 3, (e) Add. 4 and (f) Add. 5.

4 128 Z. Ranjbar, S. Rastegar / Progress in Organic Coatings 65 (2009) Fig. 8. SEM micrographs of the fracture surfaces of the blank and samples containing different nano-silica particles: (a) blank, (b) Add. 1, (c) Add. 2, (d) Add. 3, (e) Add. 4 and (f) Add Mechanical properties Koenig hardness values of the films show an increase as nanosilica particles are incorporated (Fig. 4). Direct impact resistance of the films however, show a more complicated behavior (Fig. 5). Incorporation of nano-silicas improves this property in some cases (Add. 5, 4 and 1) and worsens it in other cases (Add. 3 and 2). As Fig. 6 shows, the flexibility of the films in conical mandrel bending is not influenced by some nano-silicas and is worsened by some other grades (Add. 3 and 2). In order to find an explanation for this behavior, we have studied the morphology of the baked films Morphology of baked films Fig. 7a f show the scanning electron micrographs of the free surface of the blank and nano-silica containing clear-coats. As can be seen the blank clear-coat forms films with very smooth surface. Adding nano-silica particles to the base formulation causes some particles to be present on the free surface of the films. The presence of these particles results in reduced specular gloss and surface roughness as are observed from Figs. 1 and 2. As can be seen this roughness is more pronounced in case of Add. 3. Comparing the primary particle size and the size of nano-particles at the surface of the films, it is seen that the nano-silica particles show some degree of aggregation in all cases. SEM micrographs of the fracture surfaces of the samples containing different nano-silica particles show a more or less homogeneous dispersion of nano-silica particles (Fig. 8a f). A very interesting feature of the morphology of the films is the existence of very small cracks in the bulk of the films containing Add. 1, Add. 3 and Add. 4. The extents by which the cracks exist depend on nano-silica surface treatment. Fig. 9a c shows the above-mentioned cracks at a higher magnification. As can be seen the width and number-density of the cracks follows the ranking: Add. 1 > Add. 3 > Add. 4. These cracks could be a reason for hazing of the clear-coat films. The trend in the haze value is Add. 3 > Add. 1 > Add. 4. This shows that the presence of the cracks is not the only factor which causes hazing. The other factor maybe the degree of aggregation of the nano-silica particles. On the other hand no cracks are visible in the bulk of films containing Add. 2 and Add. 5. So the main reason of hazing may be the aggregation of nano-silica particles.

5 Z. Ranjbar, S. Rastegar / Progress in Organic Coatings 65 (2009) Fig. 9. The cracks in the bulk of the films containing (a) Add. 1, (b) Add. 3 and (c) Add. 4 at a higher magnification Cracks formation A glance at Fig. 8a reveals that the blank sample does not contain any cracks in the bulk area. As nano-silica particles are incorporated into the film, cracks are formed. To find an explanation for the crack formation, one must go through the chemistry of the cross-linking reaction and the effect of nano-silica on these reactions. Cross-linking hydroxyl-functional acrylic resins using alkylated melamine resins results in formation of ether linkages and the release of an alkanol (butanol in our case). The butylated melamine resin may also react with other acidic protons, such as those of hydroxyl groups existing on the surface of the nano-silica particles. In other words there could be a competition between the hydroxyl groups of the acrylic resin and those of the silica surface to donate their protons to the melamine resin. This means that incorporation of nano-silica particles may result in side reactions which could change the distribution of the cross-links throughout the polymeric matrix. On the other hand the cross-linking reaction is catalyzed Fig. 10. Transmission electron micrograph of the sample containing Add. 1. by acidic conditions. Nano-silicas could change the chemical composition of the bulk of the film to a more acidic one, at least in the vicinity of their surface. This may also result in changing the spatial distribution of the cross-links in the film. The cross-linking by poly-condensation reactions results in volume reduction. If the shrinkage is more intensive in the neighbourhood of nano-silica particles it may result in formation of very small cracks. The width and the number-density of these cracks depend on the extent of volume shrinkage in the vicinity of the particles. The extent of volume shrinkage is influenced by the catalytic action of acidic conditions. As can be seen from Table 2, the acidic nature of the nano-silica particles are: Add. 1 > Add. 3 > Add. 4. This is while the specific surface area of the nano-silica shows a reverse trend. This means that in these cases the acidity of the particles is more important than their specific surface area, which is an indication of the catalytic nature of these nano-particles. The question now is why no crack is observed in films containing Add. 2 and Add. 5? As can be seen from Table 2, Add.2isa high surface area nano-silica without any surface modification. This means that the surface of this additive is acidic and the hydroxyl groups are easily available. High concentration of acidic protons in an acidic environment could result in a highly substoichiometric reaction between the melamine resin and the hydroxyl protons. The cracks are formed in the bulk of the film if the material which exists in the inter-particle zone behaves as a rigid solid. But if the material is not rigid no crack could be observed. This is what happens in case of Add. 2. In this case the melamine resin preferentially reacts with the surface hydroxyl groups of the Add. 2, causing a low crosslink density gel to be formed in farther distance from the nano-silica surface. This plastic/elastic gel tolerates more elongation and therefore no crack is observed. The presence of plastic area in this film is observable from Fig. 8b, in which high plastic deformation area are shown by circles. Looking at Table 2, one will see that Add. 9 has the same ph-value as that of Add. 1 and 2 but the specific surface area is twice that of Add. 1 and half that of Add. 2. In this case, observation of some cracks is expectable. To explain the absence of cracks in this sample,

6 130 Z. Ranjbar, S. Rastegar / Progress in Organic Coatings 65 (2009) one has to consider the concept of ph-value of solid particles. This is the ph-value of the water in which particles are dispersed. If a silane-coated and an un-coated silica have the same ph, it means that their hydroxyl groups are to same extent available for water molecules. It must be noted that these available species are not necessarily available to other molecules such as melamine resins. Therefore, while the Add. 5 is acidic in nature and helps the crosslinking reaction to take place easier, it does not influence the spatial distribution of the cross-links to a high extent which is necessary for crack formation. On the other hand, larger aggregate size and smaller interface between the nano-silica particles and the binder could be an additional reason for the absence of cracks. Transmission electron microscopy: Fig. 10 shows a transmission electron micrograph of the sample containing Add. 1. It can be seen that the particles are present as aggregates. 4. Conclusion The degree of dispersion and the interaction between the nanosilica particles and the cross-linking matrix of the acrylic-melamine clear-coats are very important factors determining the final microstructure of the nano-silica containing clear-coats. The acidity of the hydroxyl groups of the surface of nano-silica and their availability to reactive species of the matrix affect the final film morphology in these systems. Nano-silica particles may behave either as catalyst or as co-reactant. If they behave as a catalyst, the system may show some cracks, the size and extent of which depend on the surface chemistry of the particles. In some cases nano-silica could react with melamine part of the matrix and leave highly plastic area in the film which results in reduced mechanical performance of the baked film. References [1] R.A. Ryntz, D. Britz, Journal of Coatings Technology 74 (925) (2002) 77. [2] W. Shen, L.M.B. Jiang, Tribology International 39 (2006) [3] U. Schulz, V. Wachtendorf, T. Klimmasch, P. Alers, Progress in Organic Coatings 42 (1 2) (2001) [4] P. Bertrand-Lambotte, J.L. Loubet, C. Verpy, S. Pavan, Thin Solid Films 420 (2002) [5] S.Z. Zhou, L.M. Wu, J. Sun, W.D. Shen, Progress in Organic Coatings 45 (2002) 33.