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Investigation into the effect of nano-silica on the protective properties of polyurethane coatings
Surface & Coatings Technology 209 (2012) 137–142
Contents lists available at SciVerse ScienceDirect
Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat
Investigation into the effect of nano-silica on the protective properties of polyurethane coatings D.J. Mills, S.S. Jamali ⁎, K. Paprocka School of Science and Technology, University of Northampton, St George's Avenue, Northampton, NN2 6JD, UK
a r t i c l e
i n f o
Article history: Received 27 February 2012 Accepted in revised form 26 August 2012 Available online 1 September 2012 Keywords: Nano-silica PU coating Ionic resistance Cross-linking density Protective properties
a b s t r a c t The effect of nano-silica particles on the protective properties of polyurethane (PU) coatings has been investigated. Current PU clear coats have shown promising scratch, abrasion and UV resistance properties, however their corrosion resistance has not been much investigated. This study focuses on the effect of non-polar nano-silica particles on electrochemical properties of 2-pack polyurethane matrix. Nano silica was incorporated at different levels into acrylic polyol/HDI polyisocyanate polyurethane matrix and cured at three different temperatures (20 °C, 70 °C and 110 °C). DC resistance technique and Electrochemical Impedance Spectroscopy (EIS) have been employed to evaluate protective properties. Free films were prepared to determine the water-uptake using coating capacitance function measured in the early stages of exposure. Also the effect of nanoparticles on cross-linking density and glass transition temperature (Tg) was investigated by Dynamical Mechanical Thermal Analysis (DMTA) technique. Abrasion resistance was evaluated using Taber abrasion tester by measuring the weight loss from a coated panel. Electrochemical results showed a positive effect on the permeability properties for PU coatings with 5% of embedded nano-silica particles. Also coatings cured at higher temperatures showed improved protective properties. The Tg also increased with elevated curing temperature which was attributed to an increase in cross-linking density. The Taber abrasion test indicates that both, nano-silica particles and higher curing temperatures enhance abrasion resistance. © 2012 Elsevier B.V. All rights reserved.
1. Introduction It is estimated, that organic coatings constitute about 90% of total corrosion protection systems. Polyurethane resins have gained an increasing attraction in paint and coating market due to their good abrasion resistance, low temperature flexibility, excellent chemical, mechanical and physical properties [1]. Currently polyurethane coatings are being used in many industrial applications such as: automotive industry (OEM, on line repair, refinish), general industry (protective, defense coatings and coil-coatings), transportation (aerospace, trains, commercial vehicles), plastics (car bumpers, dash-boards, and computers), industrial wood (kitchen furniture, parquet flooring), and adhesives (sealants and sealers). The automotive industry uses about 30% of total volume of PU coatings [2]. With the increasing attraction of nano-materials, a variety of nanoparticles e.g. TiO2, ZnO, Al2O3, CaCO3, CeO2, have been added into polymeric matrixes to modify different properties of PU coatings. The extraordinarily small size of nanoparticles, less than 100 nm, makes them invisible when incorporated into a transparent polymeric matrix and so they impose no unpleasant side effect on optical properties. In ⁎ Corresponding author at: Intelligent Polymer Research Institute, AIIM Facility, Innovation Campus, University of Wollongong, Wollongong, NSW 2522, Australia. Tel.: + 61 426 736 630. E-mail address: [email protected] (S.S. Jamali). 0257-8972/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.surfcoat.2012.08.056
some cases, even increase of light fastness and transparency has been reported [3]. Also their enormous specific surface area makes them desirable for a variety of surface treatments. It has been reported that even small variations in concentration of nano-silica may remarkably change the coating characteristics and optimize mechanical, chemical and optical properties when there is a strong chemical interaction between nano-silica and polymer matrix [4]. In other studies nanoparticles of SiO2, Zn and Fe2O3 have shown promise as anti-corrosive agents as well as mechanical properties' modifiers for epoxy [5] and silane based sol–gel [6] coatings. Such unique properties of nanoparticles have been utilized in the coating industry to improve physical, mechanical and chemical properties of organic coatings [7–10]. Consequently, the need for higher mechanical and optical properties has resulted in incorporation of nano-fillers e.g. nano-silica, nano-alumina into PU coatings to improve scratch, mar and UV resistances [7]. The compatibility of silica structure was utilized to incorporate desired electromagnetic properties into the polymer matrix through addition of core–shell Fe–silica nanoparticles [11]. Similar nanoparticles were also used to improve the barrier properties and the corrosion protection afforded by PU coating [12]. Gue et al. have used reactive sites introduced onto iron oxide [13] and barium titanate [14] nanoparticles to polymerize PU and obtain improved dispersion and electromagnetic properties. Polyurethane nanocomposites with enhanced thermal stability and improved mechanical and electrical properties have demonstrated excellence as structural and functional materials [15,16]. Also addition of
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nano-silica has been shown to assist in providing a remarkable barrier to gas and moisture, evidenced by a strong decrease in water absorption [9]. They have also shown promise as filler in PU anti-corrosive coatings with the aim of reducing the number of coating layers [17]. In order to improve the protectiveness by adding nano-materials, it is important to understand how nanoparticles interact with the polymeric matrix. As a general rule, slowing down the ionic conduction enhances the anti-corrosive properties. Bacon, Smith and Rugg [18], after measuring the DC resistances of over 300 coating systems, determined a direct correlation between these resistances and the ability of the coating to protect the underlying steel from corrosion. It is suggested that the ionic conduction becomes the rate determining step for corrosion reactions when a certain level of adhesion is provided. High ionic resistance prevents ions from getting to the substrate and also hinders the movement of reactants between anodes and cathodes. The main aim of this work is to determine whether the use of nano-silica enhances protective properties of 2-pack polyurethane coatings. This will be carried out by electrochemical characterization of nano-silica added systems during long term exposure to corrosive media. Also interaction between nano-silica and the resin and the effect of elevated curing temperature and the consequences on physico-mechanical properties have been investigated. 2. Experimental 2.1. Materials
To obtain water-uptake information detached films were mounted into U glass cells and EIS measurements were performed using a PC controlled potentiostat (model Gill AC manufactured by ACM Instruments) in a 3 electrode arrangement with SCE electrode as reference and Pt as auxiliary electrode over a frequency range from 10 4 Hz to 10−1 Hz. 3% NaCl was used as electrolyte. The measurements were taken after 5 min, then every 20 min during 4 h and after 24 h. The capacitance coefficient was extracted by ZSimpWin program. Water-uptake was calculated via Brasher–Kingsbury equation (Eq. (1)): Vw ¼
ð logCt =C0 Þ logεw
ð1Þ
where, Vw = volume fraction of absorbed water at time t, Ct = coating capacitance at time t, C0 = coating capacitance at time zero, and εw = dielectric constant of water (80) [19]. 2.3.2. Dynamic Thermo-Mechanical Analysis (DMTA) Measurements were performed using the Dynamic Mechanical Analyzer (DMA) Tritec 2000 (Triton Technology Ltd.). Small samples of 4 × 1 cm of polymeric film with 100 ± 2 μm thickness were examined. Measurement was conducted in tension mode with a 1 Hz oscillation frequency and a 0.02 mm displacement. The heating rate was carefully adjusted at 5 °C/min ending at 100 °C. Mafi et al. examined epoxy and polyester clear powder coatings and concluded that the rate of heating affects the measured Tg [20]. The maximum in the damping peak of the “tan δ” curve was taken as Tg.
NanoBYK-3650N which is a non-polar, surface modified with polysiloxane nano-silica with particle size 20 nm in pre-dispersed form in methoxypropylacetate/methoxypropanol was supplied by BYK Chemie GmbH. 2-Pack acrylic polyol/polyisocyanate based coating was used as the polymeric matrix. An acrylic polyol SETALUX 1196 VV-60 YA with 2.6% OH was supplied by Nuplex Co. The isocyanate part TOLONATE HDT LV was provided by Perstorp. Laboratory grade of xylene was used as the diluent.
2.3.3. Taber abrasion test Abrasion resistance was measured by Taber Abrasion tester model 5130. CS-10 abrasive wheels were used under the 500 g load for 500 cycles. Before each measurement the weight of each sample was measured to the nearest 0.0001 g. After the test, the weight of each sample was measured again and Taber wear index was calculated as the loss in weight in milligrams per thousand cycles of abrasion according to Eq. (2):
2.2. Sample preparation
Taber wear index ¼
All polyurethanes have been prepared at 1:1 isocyanate (NCO) and hydroxide (OH) ratio. In order to reduce the viscosity and eliminate cavities xylene was added and the mixture was stirred until homogenized. 5 wt.% and 10 wt.% of nano-silica were added into polyurethanes and thoroughly mixed. Since nano-silica was provided in pre-dispersed form, mechanical mixing provides adequate dispersion within the polymeric matrix. As a control specimen, a sample without nanoparticle additions was also prepared. The coatings were applied using spreader bar on 10 × 15 cm Q-panels (the ground finish side of the type S, 8 mm thick, Q-panel) and on non-stick PTFE (Polytetrafluoroethylene) sheets to prepare detached films. All coatings were about 100 μm thick. Coated panels and free films were cured at 3 different temperatures; 20 °C, 70 °C and 110 °C. 2.3. Test methods 2.3.1. Electrochemical measurements To make measurements on the coated Q panels a plastic cell affixed to the surface using siloxane glue was half filled with 3% NaCl. DC electrolytic resistance measurement was conducted in a two electrode arrangement using a Keithley Electrometer model 610C. Saturated Calomel Electrode (SCE) with negligible internal resistance served as the second electrode. Measurements were taken after 1 h, 24 h and then daily for a week, and over the next 5 weeks with a one week interval.
weight loss : number of test cycles=1000
ð2Þ
3. Results and discussion 3.1. Electrochemical results 3.1.1. Ionic resistance measured by DC technique The data presented in Figs. 1–3 are DC resistance over a period of 1008 h contact with 3% NaCl electrolyte. To ensure that results are reproducible, measurement was conducted on a number of samples and an average of 3 similar values has been reported. However in some cases up to four orders of magnitude differences in resistance between samples of the same type was observed. This is attributed to inhomogeneous nature of organic coating causing differentiation of electrochemical properties from point to point. A number of phenomena such as non-functional polymer chains, solvent entrapment, inappropriate pigment incorporation etc. may produce small areas with locally higher permeability. Micro capillaries present in these areas are easier pathways for ion transport compared to the bulk of polymer film. This so-called electrochemical inhomogeneity was first addressed by Kinsella and Myane [21]. Further works by Mills and Mayne [22,23] proposed that lack of cross-linking or non-functional (dead) polymer chains may play a key role in producing defective areas with several orders of magnitude lower resistance. Areas of high resistance around 1010–1012 Ω cm2 are called “I” (inverse) and areas with low resistance around 10 6–108 Ω cm2 are called “D” (direct). However, all values reported here are associated with the samples exhibiting resistances
D.J. Mills et al. / Surface & Coatings Technology 209 (2012) 137–142
139 o
1011
0% 5% 10%
Curing temperature: 110 C
0% 5% 10%
o
Curing temperature: 20 C
11
10
1010 10
10 109
10
108
107
0
200
400
600
800
1000
t [h]
10
9
8
0
200
400
600
800
1000
t [h]
Fig. 1. Effect of time on DC resistance in 3% NaCl of PU coatings cured at 20 °C containing varying amount of nano-silica.
Fig. 3. Effect of time on DC resistance in 3% NaCl of PU coatings cured at 110 °C containing varying amount of nano-silica.
higher than 108 Ω cm2 to avoid confusion between the effect of D area and of nano-silica. This means that all alterations in results are due to the nano-silica incorporation rather than turning “I” areas into “D” areas. The effect of nano-silica on creating/eliminating “D” areas could be subject for further studies. Comparison between Figs. 1 and 3 shows a general improvement of ionic resistance with elevated curing temperature. This might be associated with higher mobility of polymer chains at higher temperatures leading to improved cross-linking. At higher temperature it is less likely to leave functional groups non-reacted, resulting in a more uniform chemistry and homogenous electrochemical properties. All figures (Figs. 1–3) suggest significant improvement of ionic resistance as a result of adding 5% nano-silica. This can be either due to the non-polar surface treatment on nano-silica particle and/or network reinforcement. As a general rule the more the coating is inherently non-polar, the less water it absorbs. Although there is not always a direct correlation between water-absorption and ionic resistance there is evidence that ions use the aqueous pathways to reach the substrate [24]. Therefore, higher hydro affinity and bigger water clusters lead to larger pathways and decreased ionic resistance. On the other hand, as has been reported before, the chemical interaction between nanoparticle and functional groups on the polymer matrix can effectively improve barrier properties and ionic resistance [25]. Thus network reinforcement might be a significant factor that contributes to the enhanced
ionic resistance. Chemically treated nanoparticles, like NanoBYK-3650N, are capable of making strong bonds with appropriate reactive polymer chains and play an essential role in linking chains together effectively and hence reinforcing the network. Further structural and analytical investigation is required to identify which of these hypotheses apply. Further increase of nano-silica from 5% to 10% leads to a worsening in the protective properties when the coating is cured at 20 °C and 110 °C. At 70 °C curing temperature there is no significant difference between samples. The non-polar nanoparticle when added into a highly polar matrix has a remarkable tendency to form agglomerates in order to minimize their interface with the incompatible surrounding. Therefore, there is a critical concentration for any given nanoparticle and polymeric matrix which has to be avoided in order to eliminate agglomeration. In this case, it appears that 10% is above the critical concentration leading to local imperfections which may later turn into the permeable area with lower ionic resistance. Also it has been reported that in some cases nano-silica may temporarily block the functional groups of reactive polymers and result in lack of cross-linking [26]. This is due to the higher activation energy required for bonding between silica and the reactive polymer. There is evidence in Fig. 2 that increasing the curing temperature from 20 °C to 70 °C has efficiently provided the bonding activation energy which leads to improved compatibility. Consequently ionic resistance has been significantly improved. However there is no advantage in doubling the nano-silica from 5% to 10% as each affords similar protection. Fig. 3 reveals that increasing curing temperature up to 110 °C has had a detrimental consequence on protective properties of PU containing 10% nano-silica. Considering the improved protectiveness of the same recipe at 70 °C, this may not be a direct effect of nano-silica. Instead the nano-silica dispersion media, methoxypropylacetate/methoxypropanol, may play the key role. At very high curing temperatures, due to rapid solvent evaporation, a thin layer on the outer surface of the coating dries quickly and entraps the remaining solvent underneath. Eventually the entrapped solvent bursts out due to the high vapor pressure at high temperature and leaves the coating with numerous capillaries (in the extreme situation, it leaves pinhole marks). These capillaries can serve as ionic pathways when coating is in contact with the corrosive media. In industry this problem can be eliminated by giving the coating a flash-off time at a lower temperature before exposing them to high temperature stove. This way all the solvent will be removed before chemical reactions happen at high temperature. However, this seems to be the major source of low ionic resistance for 10% nano-silica containing PU. Fluctuations of ionic resistance value, particularly in the early stages of contact with corrosive media, are connected with the repeated
0% 5% 10%
o
Curing temperature: 70 C
1011
1010
109
0
200
400
600
800
1000
t [h] Fig. 2. Effect of time on DC resistance in 3% NaCl of PU coatings cured at 70 °C containing varying amount of nano-silica.
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75
T g [ oC]
70
65
0% 5% 10%
60
55
50 Fig. 4. Effect of temperature of cure on the volume fraction of absorbed water into PU coating with varying nano silica content.
blocking of capillaries with corrosion product and dissolution of the oxide in contact with chloride ion. The so-called “pore-plugging mechanism” occurs due to solubility of primary corrosion product, normally ferrous (Fe+2) oxides. The further these oxides move away from the substrate through the ionic pathways of coating, the higher the concentration of oxygen is available to oxidize them further to the higher oxidation state, ferric (Fe+3), which is an insoluble oxide form. This insoluble corrosion product can temporarily block the ionic pathway and raise the resistance until the chloride ion reaches the insoluble corrosion product and dissolves it. 3.1.2. Polymer capacitance and water-uptake measurement The water absorption has been quantified for the control and 5% nano-silica containing samples using Brasher–Kingsbury equation (Eq. (1)). Results are shown in Fig. 4. Sample's thicknesses should be taken into consideration as the differences in free film thicknesses affect the time of saturation. So effort has been made to keep the coating thickness strictly within 100 ± 2 μm. Results indicate a remarkable drop of water absorption at 5% nano-silica concentration when the temperature is raised to 70 °C or higher. This indicates the presence of non-reacted functional groups at ambient temperature. These have completed their reactions at elevated temperatures when the required activation energy was supplied. Most generic types of organic coatings are highly polar and so they have a remarkable tendency to absorb water. Polarity is an essential characteristic of organic coatings which provides electrostatic bonding between coating and substrate. In the meantime, the coating's polarity attracts water to be absorbed and accumulated into the coating which subsequently builds up pathways for ions. So the higher the water-uptake, the bigger the pathways are. The water molecule when absorbed into the coating, tends to produce clusters and absorb more water through “osmosis” mechanism. Here is when the non-reacted functional groups and loose network come into the picture by providing possible space for water accumulation. Also results at 20 °C suggest that the hydrophobicity of nanoparticle has an insignificant role compared to the effect of lack of cross-linking. Higher curing temperature has shown little influence on water absorption of the control sample with a minor increase seen at 110 °C. This might be related to the quick solvent evaporation from the surface and solvent entrapment at the highest temperature.
10
20
30
40
50
60
70
80
90
100
110
120
T of curing [ oC] Fig. 5. Effect of temperature of cure on the Tg of PU coatings with varying nano-silica content.
chemical reactions and results in enhanced cross-linking. Theoretically, smaller permeation and water accumulation is anticipated at higher network density. However compared to the other factors which dominate the water-uptake (including solvent evaporation/entrapment and nanoparticle incorporation) network density plays a minor role and was not observed (Fig. 4) as a significant factor. Fig. 5 reveals no significant influence of nano-silica on Tg of PU coatings. A previous investigation had found the same result [27]. This investigation confirmed that nano-silica particles positively affect cross-linking; however the changes (up to 3.6 °C) were relatively small.
3.3. Taber abrasion test The primary purpose of adding nano-silica in PU systems is to improve the abrasion and UV resistances. Fig. 6 shows that the non-polar nano-silica fulfills this purpose very well. Ideal dispersion of nanoparticles and effective bonding with the functional groups on polymeric chains are essential. This will uniformly reinforce the polymer network which enhances abrasion resistance. Results also indicate that the higher curing temperature generally improves the abrasion resistance. This is a result of enhanced crosslinking at higher temperatures resulting in higher elasticity and resistance against deformation.
3.2. Dynamic Mechanical Thermal Analysis (DMTA) Tg values were taken as the maximum in damping peak of the tan δ. All measurements were repeated twice and the average is reported in Fig. 5. All three recipes show increased Tg at elevated curing temperatures. This is connected with higher mobility of polymer chains and functional groups at higher temperature which increases the probability of
Fig. 6. Taber abrasion resistance of PU coatings cured at different temperatures with varying nano-silica content.
D.J. Mills et al. / Surface & Coatings Technology 209 (2012) 137–142
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no significant agglomeration is shown on the surface of coatings containing nanoparticles. This suggests an appropriate compatibility of the nanoparticles with polymer matrix which results in good dispersion inside the coating. 4. Conclusions The effect of nano-silica particles on the protective properties of polyurethane coating was investigated. Three levels of nano-silica (0%, 5% and 10%) were added into PU pre-polymer followed by curing at three different curing temperatures (20 °C, 70 °C and 110 °C). DC resistance measurements have proven that addition of 5% of nano-silica significantly increases ionic resistance due to the polymer network reinforcement. Water-uptake measurement has evidenced reduced water absorption in PU coating containing 5% nano-silica. This goes together with higher ionic resistance which is connected with a denser network with reduces possible spaces for water accumulation. 10% nano-silica appears to be over the critical concentration causing particle agglomeration and localized defects which result in lower ionic resistance. General improvement has been observed at elevated curing temperature. Perfection of chemical reactions at higher temperature and enhanced uniformity were suggested as explanations. Thermo-mechanical analysis has confirmed the improved cross-linking and higher glass transition temperature at elevated temperatures. Also from Taber abrasion test it can be concluded that both nanoparticles and higher curing temperatures enhance abrasion resistance of PU coatings. Acknowledgment Authors wish to thank Perstorp and Nuplex for kindly providing the polyisocyanate and acrylic resins and BYK-Chemie Gmbh for supplying the nano-silica. Also authors thankfully appreciate the British School of Leather Technology at the University of Northampton for providing the laboratory space and facilities. Thanks are also expressed to Kasia Schaefer working at the Department of Electrochemistry, Corrosion Materials Engineering at the Gdansk University of Technology for producing the AFM images. References
Fig. 7. a. AFM image of PU coating without nano-silica, control sample (50 × 50 μm). b. AFM image of PU coating with 5% nano-silica (50 × 50 μm). c. AFM image of PU coating with 10% nano-silica (50 × 50 μm).
3.4. Morphological characterization by Atomic Force Microscopy (AFM) Images of PU coatings with 0, 5% and 10% of nano-silica embedded can be seen in Fig. 7a–c, respectively. Fig. 7a shows the substrate preparation pattern printed on the coating surface which can be due to poor leveling properties on the coating. Small amount of nanoparticles with
[1] V.V. Gite, P.P. Mahulikar, G.G. Hundiwale, U.R. Kapadi, J. Sci. Ind. Res. 63 (2004) 348. [2] C. Varron, E. Charrière-Peroud, P. Ardaud, J.M. Bernard, M. Cowley, T. Jeanette, J. Maxted, G. Rayner, B. Storer, A. Shields, P. Thomas, N. Wheat, Waterborne & Solvent Based Surface Coating Resins and their Applications: Volume III, Polyurethanes, SITA technology Ltd., London, UK, 1998. [3] T. Sawitowska, in: International Congress of Nanotechnology (ICNT), San Francisco, USA, 2005. [4] A. Dashtizadeh, M. Abdouss, H. Mahdavi, M. Khorassani, Appl. Surf. Sci. 257 (2011) 2118. [5] X. Shi, T.A. Nguyen, Z. Suo, Y. Liu, R. Avci, Surf. Coat. Technol. 204 (2009) 237. [6] A. Phanasgaonkar, V.S. Raja, Surf. Coat. Technol. 203 (2009) 2260. [7] M.M. Jalili, S. Moradian, H. Dastmalchian, A. Karbasi, Prog. Org. Coat. 59 (2007) 81. [8] E. Amerio, P. Fabbri, G. Malucelli, M. Messori, M. Sangermano, R. Taurino, Prog. Org. Coat. 62 (2008) 129. [9] M. Sangermano, G. Malucelli, E. Amerio, A. Priola, E. Billi, G. Rizza, Prog. Org. Coat. 54 (2005) 134. [10] E. Barna, B. Bommer, J. Kursteiner, A. Vital, O. Trzebiatowski, W. Koch, B. Schmid, T. Graule, Compos. Part A: Appl. Sci. Manuf. 36 (2005) 473. [11] J. Zhu, S. Wei, N. Haldolaarachchige, D.P. Young, Z. Guo, J. Phys. Chem. C 115 (2011) 15304. [12] J. Zhu, S. Wei, I.Y. Lee, S. Park, J. Willis, N. Haldolaarachchige, D.P. Young, Z. Luo, Z. Guo, RSC Adv. 2 (2012) 1136. [13] Z. Guo, S. Park, S. Wei, T. Pereira, M. Moldovan, A.B. Karki, D.P. Young, H.T. Hahn, Nanotechnology 18 (2007) 335704. [14] Z. Guo, S.E. Lee, H. Kim, S. Park, H.T. Hahn, A.B. Karki, D.P. Young, Acta Mater. 57 (2009) 267. [15] Z. Guo, T.Y. Kim, K. Lei, T. Pereira, J.G. Sugar, H.T. Hahn, Compos. Sci. Technol. 68 (2008) 164. [16] Z. Guo, S. Park, H.T. Hahn, S. Wei, M. Moldovan, A.B. Karki, D.P. Young, J. Appl. Phys. 101 (2007) 09M511.
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Contents lists available at SciVerse ScienceDirect
Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat
Investigation into the effect of nano-silica on the protective properties of polyurethane coatings D.J. Mills, S.S. Jamali ⁎, K. Paprocka School of Science and Technology, University of Northampton, St George's Avenue, Northampton, NN2 6JD, UK
a r t i c l e
i n f o
Article history: Received 27 February 2012 Accepted in revised form 26 August 2012 Available online 1 September 2012 Keywords: Nano-silica PU coating Ionic resistance Cross-linking density Protective properties
a b s t r a c t The effect of nano-silica particles on the protective properties of polyurethane (PU) coatings has been investigated. Current PU clear coats have shown promising scratch, abrasion and UV resistance properties, however their corrosion resistance has not been much investigated. This study focuses on the effect of non-polar nano-silica particles on electrochemical properties of 2-pack polyurethane matrix. Nano silica was incorporated at different levels into acrylic polyol/HDI polyisocyanate polyurethane matrix and cured at three different temperatures (20 °C, 70 °C and 110 °C). DC resistance technique and Electrochemical Impedance Spectroscopy (EIS) have been employed to evaluate protective properties. Free films were prepared to determine the water-uptake using coating capacitance function measured in the early stages of exposure. Also the effect of nanoparticles on cross-linking density and glass transition temperature (Tg) was investigated by Dynamical Mechanical Thermal Analysis (DMTA) technique. Abrasion resistance was evaluated using Taber abrasion tester by measuring the weight loss from a coated panel. Electrochemical results showed a positive effect on the permeability properties for PU coatings with 5% of embedded nano-silica particles. Also coatings cured at higher temperatures showed improved protective properties. The Tg also increased with elevated curing temperature which was attributed to an increase in cross-linking density. The Taber abrasion test indicates that both, nano-silica particles and higher curing temperatures enhance abrasion resistance. © 2012 Elsevier B.V. All rights reserved.
1. Introduction It is estimated, that organic coatings constitute about 90% of total corrosion protection systems. Polyurethane resins have gained an increasing attraction in paint and coating market due to their good abrasion resistance, low temperature flexibility, excellent chemical, mechanical and physical properties [1]. Currently polyurethane coatings are being used in many industrial applications such as: automotive industry (OEM, on line repair, refinish), general industry (protective, defense coatings and coil-coatings), transportation (aerospace, trains, commercial vehicles), plastics (car bumpers, dash-boards, and computers), industrial wood (kitchen furniture, parquet flooring), and adhesives (sealants and sealers). The automotive industry uses about 30% of total volume of PU coatings [2]. With the increasing attraction of nano-materials, a variety of nanoparticles e.g. TiO2, ZnO, Al2O3, CaCO3, CeO2, have been added into polymeric matrixes to modify different properties of PU coatings. The extraordinarily small size of nanoparticles, less than 100 nm, makes them invisible when incorporated into a transparent polymeric matrix and so they impose no unpleasant side effect on optical properties. In ⁎ Corresponding author at: Intelligent Polymer Research Institute, AIIM Facility, Innovation Campus, University of Wollongong, Wollongong, NSW 2522, Australia. Tel.: + 61 426 736 630. E-mail address: [email protected] (S.S. Jamali). 0257-8972/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.surfcoat.2012.08.056
some cases, even increase of light fastness and transparency has been reported [3]. Also their enormous specific surface area makes them desirable for a variety of surface treatments. It has been reported that even small variations in concentration of nano-silica may remarkably change the coating characteristics and optimize mechanical, chemical and optical properties when there is a strong chemical interaction between nano-silica and polymer matrix [4]. In other studies nanoparticles of SiO2, Zn and Fe2O3 have shown promise as anti-corrosive agents as well as mechanical properties' modifiers for epoxy [5] and silane based sol–gel [6] coatings. Such unique properties of nanoparticles have been utilized in the coating industry to improve physical, mechanical and chemical properties of organic coatings [7–10]. Consequently, the need for higher mechanical and optical properties has resulted in incorporation of nano-fillers e.g. nano-silica, nano-alumina into PU coatings to improve scratch, mar and UV resistances [7]. The compatibility of silica structure was utilized to incorporate desired electromagnetic properties into the polymer matrix through addition of core–shell Fe–silica nanoparticles [11]. Similar nanoparticles were also used to improve the barrier properties and the corrosion protection afforded by PU coating [12]. Gue et al. have used reactive sites introduced onto iron oxide [13] and barium titanate [14] nanoparticles to polymerize PU and obtain improved dispersion and electromagnetic properties. Polyurethane nanocomposites with enhanced thermal stability and improved mechanical and electrical properties have demonstrated excellence as structural and functional materials [15,16]. Also addition of
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nano-silica has been shown to assist in providing a remarkable barrier to gas and moisture, evidenced by a strong decrease in water absorption [9]. They have also shown promise as filler in PU anti-corrosive coatings with the aim of reducing the number of coating layers [17]. In order to improve the protectiveness by adding nano-materials, it is important to understand how nanoparticles interact with the polymeric matrix. As a general rule, slowing down the ionic conduction enhances the anti-corrosive properties. Bacon, Smith and Rugg [18], after measuring the DC resistances of over 300 coating systems, determined a direct correlation between these resistances and the ability of the coating to protect the underlying steel from corrosion. It is suggested that the ionic conduction becomes the rate determining step for corrosion reactions when a certain level of adhesion is provided. High ionic resistance prevents ions from getting to the substrate and also hinders the movement of reactants between anodes and cathodes. The main aim of this work is to determine whether the use of nano-silica enhances protective properties of 2-pack polyurethane coatings. This will be carried out by electrochemical characterization of nano-silica added systems during long term exposure to corrosive media. Also interaction between nano-silica and the resin and the effect of elevated curing temperature and the consequences on physico-mechanical properties have been investigated. 2. Experimental 2.1. Materials
To obtain water-uptake information detached films were mounted into U glass cells and EIS measurements were performed using a PC controlled potentiostat (model Gill AC manufactured by ACM Instruments) in a 3 electrode arrangement with SCE electrode as reference and Pt as auxiliary electrode over a frequency range from 10 4 Hz to 10−1 Hz. 3% NaCl was used as electrolyte. The measurements were taken after 5 min, then every 20 min during 4 h and after 24 h. The capacitance coefficient was extracted by ZSimpWin program. Water-uptake was calculated via Brasher–Kingsbury equation (Eq. (1)): Vw ¼
ð logCt =C0 Þ logεw
ð1Þ
where, Vw = volume fraction of absorbed water at time t, Ct = coating capacitance at time t, C0 = coating capacitance at time zero, and εw = dielectric constant of water (80) [19]. 2.3.2. Dynamic Thermo-Mechanical Analysis (DMTA) Measurements were performed using the Dynamic Mechanical Analyzer (DMA) Tritec 2000 (Triton Technology Ltd.). Small samples of 4 × 1 cm of polymeric film with 100 ± 2 μm thickness were examined. Measurement was conducted in tension mode with a 1 Hz oscillation frequency and a 0.02 mm displacement. The heating rate was carefully adjusted at 5 °C/min ending at 100 °C. Mafi et al. examined epoxy and polyester clear powder coatings and concluded that the rate of heating affects the measured Tg [20]. The maximum in the damping peak of the “tan δ” curve was taken as Tg.
NanoBYK-3650N which is a non-polar, surface modified with polysiloxane nano-silica with particle size 20 nm in pre-dispersed form in methoxypropylacetate/methoxypropanol was supplied by BYK Chemie GmbH. 2-Pack acrylic polyol/polyisocyanate based coating was used as the polymeric matrix. An acrylic polyol SETALUX 1196 VV-60 YA with 2.6% OH was supplied by Nuplex Co. The isocyanate part TOLONATE HDT LV was provided by Perstorp. Laboratory grade of xylene was used as the diluent.
2.3.3. Taber abrasion test Abrasion resistance was measured by Taber Abrasion tester model 5130. CS-10 abrasive wheels were used under the 500 g load for 500 cycles. Before each measurement the weight of each sample was measured to the nearest 0.0001 g. After the test, the weight of each sample was measured again and Taber wear index was calculated as the loss in weight in milligrams per thousand cycles of abrasion according to Eq. (2):
2.2. Sample preparation
Taber wear index ¼
All polyurethanes have been prepared at 1:1 isocyanate (NCO) and hydroxide (OH) ratio. In order to reduce the viscosity and eliminate cavities xylene was added and the mixture was stirred until homogenized. 5 wt.% and 10 wt.% of nano-silica were added into polyurethanes and thoroughly mixed. Since nano-silica was provided in pre-dispersed form, mechanical mixing provides adequate dispersion within the polymeric matrix. As a control specimen, a sample without nanoparticle additions was also prepared. The coatings were applied using spreader bar on 10 × 15 cm Q-panels (the ground finish side of the type S, 8 mm thick, Q-panel) and on non-stick PTFE (Polytetrafluoroethylene) sheets to prepare detached films. All coatings were about 100 μm thick. Coated panels and free films were cured at 3 different temperatures; 20 °C, 70 °C and 110 °C. 2.3. Test methods 2.3.1. Electrochemical measurements To make measurements on the coated Q panels a plastic cell affixed to the surface using siloxane glue was half filled with 3% NaCl. DC electrolytic resistance measurement was conducted in a two electrode arrangement using a Keithley Electrometer model 610C. Saturated Calomel Electrode (SCE) with negligible internal resistance served as the second electrode. Measurements were taken after 1 h, 24 h and then daily for a week, and over the next 5 weeks with a one week interval.
weight loss : number of test cycles=1000
ð2Þ
3. Results and discussion 3.1. Electrochemical results 3.1.1. Ionic resistance measured by DC technique The data presented in Figs. 1–3 are DC resistance over a period of 1008 h contact with 3% NaCl electrolyte. To ensure that results are reproducible, measurement was conducted on a number of samples and an average of 3 similar values has been reported. However in some cases up to four orders of magnitude differences in resistance between samples of the same type was observed. This is attributed to inhomogeneous nature of organic coating causing differentiation of electrochemical properties from point to point. A number of phenomena such as non-functional polymer chains, solvent entrapment, inappropriate pigment incorporation etc. may produce small areas with locally higher permeability. Micro capillaries present in these areas are easier pathways for ion transport compared to the bulk of polymer film. This so-called electrochemical inhomogeneity was first addressed by Kinsella and Myane [21]. Further works by Mills and Mayne [22,23] proposed that lack of cross-linking or non-functional (dead) polymer chains may play a key role in producing defective areas with several orders of magnitude lower resistance. Areas of high resistance around 1010–1012 Ω cm2 are called “I” (inverse) and areas with low resistance around 10 6–108 Ω cm2 are called “D” (direct). However, all values reported here are associated with the samples exhibiting resistances
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139 o
1011
0% 5% 10%
Curing temperature: 110 C
0% 5% 10%
o
Curing temperature: 20 C
11
10
1010 10
10 109
10
108
107
0
200
400
600
800
1000
t [h]
10
9
8
0
200
400
600
800
1000
t [h]
Fig. 1. Effect of time on DC resistance in 3% NaCl of PU coatings cured at 20 °C containing varying amount of nano-silica.
Fig. 3. Effect of time on DC resistance in 3% NaCl of PU coatings cured at 110 °C containing varying amount of nano-silica.
higher than 108 Ω cm2 to avoid confusion between the effect of D area and of nano-silica. This means that all alterations in results are due to the nano-silica incorporation rather than turning “I” areas into “D” areas. The effect of nano-silica on creating/eliminating “D” areas could be subject for further studies. Comparison between Figs. 1 and 3 shows a general improvement of ionic resistance with elevated curing temperature. This might be associated with higher mobility of polymer chains at higher temperatures leading to improved cross-linking. At higher temperature it is less likely to leave functional groups non-reacted, resulting in a more uniform chemistry and homogenous electrochemical properties. All figures (Figs. 1–3) suggest significant improvement of ionic resistance as a result of adding 5% nano-silica. This can be either due to the non-polar surface treatment on nano-silica particle and/or network reinforcement. As a general rule the more the coating is inherently non-polar, the less water it absorbs. Although there is not always a direct correlation between water-absorption and ionic resistance there is evidence that ions use the aqueous pathways to reach the substrate [24]. Therefore, higher hydro affinity and bigger water clusters lead to larger pathways and decreased ionic resistance. On the other hand, as has been reported before, the chemical interaction between nanoparticle and functional groups on the polymer matrix can effectively improve barrier properties and ionic resistance [25]. Thus network reinforcement might be a significant factor that contributes to the enhanced
ionic resistance. Chemically treated nanoparticles, like NanoBYK-3650N, are capable of making strong bonds with appropriate reactive polymer chains and play an essential role in linking chains together effectively and hence reinforcing the network. Further structural and analytical investigation is required to identify which of these hypotheses apply. Further increase of nano-silica from 5% to 10% leads to a worsening in the protective properties when the coating is cured at 20 °C and 110 °C. At 70 °C curing temperature there is no significant difference between samples. The non-polar nanoparticle when added into a highly polar matrix has a remarkable tendency to form agglomerates in order to minimize their interface with the incompatible surrounding. Therefore, there is a critical concentration for any given nanoparticle and polymeric matrix which has to be avoided in order to eliminate agglomeration. In this case, it appears that 10% is above the critical concentration leading to local imperfections which may later turn into the permeable area with lower ionic resistance. Also it has been reported that in some cases nano-silica may temporarily block the functional groups of reactive polymers and result in lack of cross-linking [26]. This is due to the higher activation energy required for bonding between silica and the reactive polymer. There is evidence in Fig. 2 that increasing the curing temperature from 20 °C to 70 °C has efficiently provided the bonding activation energy which leads to improved compatibility. Consequently ionic resistance has been significantly improved. However there is no advantage in doubling the nano-silica from 5% to 10% as each affords similar protection. Fig. 3 reveals that increasing curing temperature up to 110 °C has had a detrimental consequence on protective properties of PU containing 10% nano-silica. Considering the improved protectiveness of the same recipe at 70 °C, this may not be a direct effect of nano-silica. Instead the nano-silica dispersion media, methoxypropylacetate/methoxypropanol, may play the key role. At very high curing temperatures, due to rapid solvent evaporation, a thin layer on the outer surface of the coating dries quickly and entraps the remaining solvent underneath. Eventually the entrapped solvent bursts out due to the high vapor pressure at high temperature and leaves the coating with numerous capillaries (in the extreme situation, it leaves pinhole marks). These capillaries can serve as ionic pathways when coating is in contact with the corrosive media. In industry this problem can be eliminated by giving the coating a flash-off time at a lower temperature before exposing them to high temperature stove. This way all the solvent will be removed before chemical reactions happen at high temperature. However, this seems to be the major source of low ionic resistance for 10% nano-silica containing PU. Fluctuations of ionic resistance value, particularly in the early stages of contact with corrosive media, are connected with the repeated
0% 5% 10%
o
Curing temperature: 70 C
1011
1010
109
0
200
400
600
800
1000
t [h] Fig. 2. Effect of time on DC resistance in 3% NaCl of PU coatings cured at 70 °C containing varying amount of nano-silica.
140
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75
T g [ oC]
70
65
0% 5% 10%
60
55
50 Fig. 4. Effect of temperature of cure on the volume fraction of absorbed water into PU coating with varying nano silica content.
blocking of capillaries with corrosion product and dissolution of the oxide in contact with chloride ion. The so-called “pore-plugging mechanism” occurs due to solubility of primary corrosion product, normally ferrous (Fe+2) oxides. The further these oxides move away from the substrate through the ionic pathways of coating, the higher the concentration of oxygen is available to oxidize them further to the higher oxidation state, ferric (Fe+3), which is an insoluble oxide form. This insoluble corrosion product can temporarily block the ionic pathway and raise the resistance until the chloride ion reaches the insoluble corrosion product and dissolves it. 3.1.2. Polymer capacitance and water-uptake measurement The water absorption has been quantified for the control and 5% nano-silica containing samples using Brasher–Kingsbury equation (Eq. (1)). Results are shown in Fig. 4. Sample's thicknesses should be taken into consideration as the differences in free film thicknesses affect the time of saturation. So effort has been made to keep the coating thickness strictly within 100 ± 2 μm. Results indicate a remarkable drop of water absorption at 5% nano-silica concentration when the temperature is raised to 70 °C or higher. This indicates the presence of non-reacted functional groups at ambient temperature. These have completed their reactions at elevated temperatures when the required activation energy was supplied. Most generic types of organic coatings are highly polar and so they have a remarkable tendency to absorb water. Polarity is an essential characteristic of organic coatings which provides electrostatic bonding between coating and substrate. In the meantime, the coating's polarity attracts water to be absorbed and accumulated into the coating which subsequently builds up pathways for ions. So the higher the water-uptake, the bigger the pathways are. The water molecule when absorbed into the coating, tends to produce clusters and absorb more water through “osmosis” mechanism. Here is when the non-reacted functional groups and loose network come into the picture by providing possible space for water accumulation. Also results at 20 °C suggest that the hydrophobicity of nanoparticle has an insignificant role compared to the effect of lack of cross-linking. Higher curing temperature has shown little influence on water absorption of the control sample with a minor increase seen at 110 °C. This might be related to the quick solvent evaporation from the surface and solvent entrapment at the highest temperature.
10
20
30
40
50
60
70
80
90
100
110
120
T of curing [ oC] Fig. 5. Effect of temperature of cure on the Tg of PU coatings with varying nano-silica content.
chemical reactions and results in enhanced cross-linking. Theoretically, smaller permeation and water accumulation is anticipated at higher network density. However compared to the other factors which dominate the water-uptake (including solvent evaporation/entrapment and nanoparticle incorporation) network density plays a minor role and was not observed (Fig. 4) as a significant factor. Fig. 5 reveals no significant influence of nano-silica on Tg of PU coatings. A previous investigation had found the same result [27]. This investigation confirmed that nano-silica particles positively affect cross-linking; however the changes (up to 3.6 °C) were relatively small.
3.3. Taber abrasion test The primary purpose of adding nano-silica in PU systems is to improve the abrasion and UV resistances. Fig. 6 shows that the non-polar nano-silica fulfills this purpose very well. Ideal dispersion of nanoparticles and effective bonding with the functional groups on polymeric chains are essential. This will uniformly reinforce the polymer network which enhances abrasion resistance. Results also indicate that the higher curing temperature generally improves the abrasion resistance. This is a result of enhanced crosslinking at higher temperatures resulting in higher elasticity and resistance against deformation.
3.2. Dynamic Mechanical Thermal Analysis (DMTA) Tg values were taken as the maximum in damping peak of the tan δ. All measurements were repeated twice and the average is reported in Fig. 5. All three recipes show increased Tg at elevated curing temperatures. This is connected with higher mobility of polymer chains and functional groups at higher temperature which increases the probability of
Fig. 6. Taber abrasion resistance of PU coatings cured at different temperatures with varying nano-silica content.
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no significant agglomeration is shown on the surface of coatings containing nanoparticles. This suggests an appropriate compatibility of the nanoparticles with polymer matrix which results in good dispersion inside the coating. 4. Conclusions The effect of nano-silica particles on the protective properties of polyurethane coating was investigated. Three levels of nano-silica (0%, 5% and 10%) were added into PU pre-polymer followed by curing at three different curing temperatures (20 °C, 70 °C and 110 °C). DC resistance measurements have proven that addition of 5% of nano-silica significantly increases ionic resistance due to the polymer network reinforcement. Water-uptake measurement has evidenced reduced water absorption in PU coating containing 5% nano-silica. This goes together with higher ionic resistance which is connected with a denser network with reduces possible spaces for water accumulation. 10% nano-silica appears to be over the critical concentration causing particle agglomeration and localized defects which result in lower ionic resistance. General improvement has been observed at elevated curing temperature. Perfection of chemical reactions at higher temperature and enhanced uniformity were suggested as explanations. Thermo-mechanical analysis has confirmed the improved cross-linking and higher glass transition temperature at elevated temperatures. Also from Taber abrasion test it can be concluded that both nanoparticles and higher curing temperatures enhance abrasion resistance of PU coatings. Acknowledgment Authors wish to thank Perstorp and Nuplex for kindly providing the polyisocyanate and acrylic resins and BYK-Chemie Gmbh for supplying the nano-silica. Also authors thankfully appreciate the British School of Leather Technology at the University of Northampton for providing the laboratory space and facilities. Thanks are also expressed to Kasia Schaefer working at the Department of Electrochemistry, Corrosion Materials Engineering at the Gdansk University of Technology for producing the AFM images. References
Fig. 7. a. AFM image of PU coating without nano-silica, control sample (50 × 50 μm). b. AFM image of PU coating with 5% nano-silica (50 × 50 μm). c. AFM image of PU coating with 10% nano-silica (50 × 50 μm).
3.4. Morphological characterization by Atomic Force Microscopy (AFM) Images of PU coatings with 0, 5% and 10% of nano-silica embedded can be seen in Fig. 7a–c, respectively. Fig. 7a shows the substrate preparation pattern printed on the coating surface which can be due to poor leveling properties on the coating. Small amount of nanoparticles with
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