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Studies on silicone–acrylic hybrid aqueous dispersions and corresponding silicone–acrylic nanopowders designed for modification of powder coatings and plastics
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Contents lists available at ScienceDirect
Progress in Organic Coatings journal homepage: www.elsevier.com/locate/porgcoat
Studies on silicone–acrylic hybrid aqueous dispersions and corresponding silicone–acrylic nanopowders designed for modification of powder coatings and plastics Part II – Effect of modification with silicone–acrylic nanopowders and of composition of silicone resin contained in those nanopowders on properties of epoxy-polyester and polyester powder coatings b ´ Janusz Kozakiewicz a,∗ , Izabela Ofat a , Joanna Trzaskowska a , Helena Kuczynska a b
Industrial Chemistry Research Institute, 02-724 Warsaw, Poland Institute for Engineering of Polymer Materials and Dyes, Paints and Plastic Department, 50A, Chorzowska Street, 44-100 Gliwice, Poland
a r t i c l e
i n f o
Article history: Received 6 May 2013 Received in revised form 8 January 2014 Accepted 9 January 2014 Available online xxx Keywords: Epoxy-polyester powder coatings Polyester powder coatings Core–shell nanoparticles Impact modifiers Silicones
a b s t r a c t Epoxy-polyester and polyester coatings were modified with 3% of NP-DASI “nanopowders” consisting of core–shell nanoparticles having silicone resin of very low glass transition temperature (Tg ) in the core and poly(methyl methacrylate) of high Tg in the shell. The nanopowders were obtained through spray drying of corresponding aqueous dispersions with core–shell particle structure which were synthesized in a process of emulsion polymerization of methyl methacrylate monomer in aqueous dispersions of silicone resins with different compositions resulting from different compositions of silicone monomers used for their synthesis. A designed experiment was conducted where the effect of composition of silicone resin on the properties of cured coatings was studied. It was confirmed that modification with that “nanopowder” significantly affected the properties of cured coatings, in particular impact resistance and cupping as well as surface properties. Regarding mechanical properties the influence of modification with “nanopowder” was generally more distinct for epoxy-polyester coatings than for polyester coatings. Specifically, great increase in impact and cupping resistance was observed for modified coatings and was explained by absorption of mechanical stresses by particles of low modulus silicone resin which were released from core–shell nanoparticles in the process of curing of the coatings. Other mechanical properties of cured modified and unmodified coatings (abrasion resistance, hardness, scratch resistance, elasticity and adhesion to steel) as well as appearance of the coatings (gloss and whiteness) did not differ much. Water resistance and salt fog resistance were slightly better for modified coatings. Testing the surface properties of modified coatings by XPS and AFM revealed the presence of silicone resin on coating surface what was reflected in higher contact angle and lower SFE as compared to unmodified coatings. That phenomenon was explained by migration of silicone resin to the coating surface. © 2014 Elsevier B.V. All rights reserved.
1. Introduction It is well known that nanoparticles containing low modulus polymers, specifically silicones, can significantly increase fracture toughness of polymer composites and that good dispersion of nanoparticles in the body of the composite are of great importance [1,2]. Following that concept silicone–acrylic core–shell nanoparticles have been applied as effective impact modifiers for powder coatings and plastics [3–8]. In our laboratory such impact modifiers
∗ Corresponding author. Tel.: +48 22 5682378. E-mail address: [email protected] (J. Kozakiewicz).
were obtained in a form of “nanopowder” consisting of agglomerated silicone–acrylic core–shell nanoparticles by spray-drying of silicone–acrylic hybrid aqueous dispersions synthesized from silicone resin aqueous dispersions using a recently patented process [9]. Our preliminary studies confirmed [5,10–12] that addition of only 3% of such nanopowder to epoxy-polyester powder coating resulted in a very distinct increase in impact resistance, elasticity and cupping resistance without any significant change in other important coating properties (hardness, abrasion resistance). It could be explained by excellent distribution of silicone resin having very low Tg (ca. −120 ◦ C) in the body of coating what provided resistance to stresses originated from impact or from other mechanical forces imposed on the coating during testing. As it can be seen in
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Please cite this article in press as: J. Kozakiewicz, et al., Studies on silicone–acrylic hybrid aqueous dispersions and corresponding silicone–acrylic nanopowders designed for modification of powder coatings and plastics. Part II – Effect of modification with silicone–acrylic nanopowders and of composition of silicone resin contained in those nanopowders. . . , Prog. Org. Coat. (2014), http://dx.doi.org/10.1016/j.porgcoat.2014.01.010
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to the resulting silicone resin composition. The experiment was designed in the form of a triangle where each corner corresponded to 100% of one of the monomers. Detailed plan of the designed experiment showing the actual situation of experimental points in that triangle was presented in Part I of our paper, so it is not repeated here. 2. Experimental 2.1. Starting materials Epoxy resin (Epidian 012) from Chemical Company OrganikaSarzyna Poland Carboxylated polyester resin (Policen 3000T) from PPG Polifarb Cieszyn Poland, acid value – 33 – used as hardener for epoxy-polyester coatings and as main binder for polyester coatings Primid XL 552 (from EMS-Chemie AG) – used as hydroxyalkylamide crosslinker for polyester coatings TiO2 (Rutile from Chemical Company Police Poland) – used as pigment. Benzoin from DSM and Resiflow PV 88 from Worlee Chemie GmbH – used as standard additives for powder coatings. Silicone–acrylic nanopowders applied as impact modifiers were NP-DASI samples obtained in the designed experiment described in [16]. 2.2. Preparation and curing of powder coatings Fig. 1. Simplified scheme of the process of powder coating production with NP-DASI nanopowder added as impact modifier.
Fig. 1 presenting schematically the steps of powder coating modification with such nanopowders, during the preparation of powder coating masterbatch in the extruder at temperatures significantly lower than Tg of the methacrylic polymer core, the core–shell particles are released from agglomerates due to the shear forces without any damage of the core and become nicely distributed in the body of powder coating particles. When the powder coating is sprayed onto the metal surface and cured at a temperature of over 160 ◦ C, i.e. significantly higher than Tg of the methacrylic polymer shell, the shell flows and the silicone resin core is released, but since the core–shell particles are separated in the body of coating there is only limited possibility of gluing of the silicone resin cores. It was also found in our preliminary studies that TiO2 -filled coatings modified with such nanopowders showed enhanced whiteness. This unexpected benefit achieved through modification of powder coatings with silicone–acrylic nanopowders can be explained by migration of silicone resin to the coating surface resulting in changing the light reflection on the coating surface. The phenomenon of silicone resin migration to the surface of powder coatings modified with our silicone–acrylic nanopowders was fully proved by the results of XPS, AFM and SEM-EDS investigations [12–15]. It provides unique opportunity to obtain coatings of silicone-like surface, but high hardness and abrasion resistance combined with enhanced impact resistance and elasticity. The results of our studies on the effect of starting silicone resin composition on the properties of silicone silicone–acrylic hybrid dispersions (DASI) and corresponding nanopowders (NPDASI) obtained by spray-drying of DASI were discussed earlier [16] while in this paper the results of using those NP-DASI nanopowders of different composition of silicone resin as impact modifiers for epoxy-polyester and polyester powder coatings will be presented. As it was explained in [16] the designed experiment was conducted in which the independent variables were contents (in wt%) of each of the compounds, i.e. octamethylcyclotetrasiloxane (D4), methyltrimethoxysilane (METMS) and methacryloyltrimethoxysilane (MATMS) in the monomers mixture used in synthesis of silicone dispersion which were assumed to correspond roughly
For preparation of epoxy-polyester coatings epoxy resin (30 parts), carboxylic polyester hardener (70 parts), TiO2 (30.0 parts), standard additives (1.0 parts) and, if appropriate, the NP-DASI nanopowder (3% per coating mass) were mixed in a grinder at 1000 rpm for 3 min. The resulting powder mix of starting materials was added to the Buss Ko-Knether PR-46 extruder (temp. of the screw and adapter 82 ◦ C and 102 ◦ C, respectively, screw rotational speed: 3.5 rpm) For preparation of polyester coatings polyester resin (56.05 parts), hardener (2.95 parts), TiO2 (37.0 parts), standard additives (1.0 parts) and, if appropriate, the NP-DASI nanopowder (3% per coating mass) were mixed at 1000 rpm for 3 min. The resulting powder mix was added to the extruder as described above. It was found that the process of extrusion was clearly facilitated when the masterbatches contained NP-DASI nanopowder, indicating that the silicone resin acted as additional flow aid. The extruded masterbatches were crushed and then pulverized to yield fine powder of particle size from 5 to 85 . When the SEM pictures of the powder coating particles obtained using NP-DASI as additive and not containing that additive were compared it could be concluded that the particle surface was more smooth in the case of modified powder coating particles which contained NP-DASI (see Fig. 2) what confirmed our earlier observations [10,12]. This phenomenon can be explained by migration of silicone resin to the particle surface. Powder coatings obtained as described above were deposited on both sides of steel plates using electrostatic spraying and the coated plates were placed in an oven at 180 ◦ C for 15 min to produce smooth good looking glossy white hard coats of thickness ranging from 50 to 70 . More than 30 specimens were produced for each coating composition. 2.3. Testing of cured coatings Specimens of similar coating thickness were selected for testing. The following standard properties of cured coatings were tested: - Gloss – at 60◦ and 20◦ – according to EN ISO 2813, using Erichsen PICO GLOSS 503 apparatus - Whiteness – using X-Rite 968 spectrometer at 0/45◦ geometry and at D65 /10◦ illuminant value. The following parameters were measured: L* – brightness (0–100), a* – red/green balance
Please cite this article in press as: J. Kozakiewicz, et al., Studies on silicone–acrylic hybrid aqueous dispersions and corresponding silicone–acrylic nanopowders designed for modification of powder coatings and plastics. Part II – Effect of modification with silicone–acrylic nanopowders and of composition of silicone resin contained in those nanopowders. . . , Prog. Org. Coat. (2014), http://dx.doi.org/10.1016/j.porgcoat.2014.01.010
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Fig. 2. Comparison of appearance of epoxy-polyester powder coating particles. (a) Unmodified coating and (b) coating modified with 3% of NP-DASI-293 nanopowder. Pictures were taken by SEM.
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(positive = red, negative = green) and b* – yellow/blue balance (positive = yellow, negative = blue) Contact angle (water and CH2 I2 ) – according to EN 828:2000, using KRUSS DSA 100E apparatus. For calculation of surface free energy (SFE) and its polar and dispersive components OwensWendt method was applied. Pendulum hardness (Persoz) – according to PN ISO 1522 Pencil hardness – according to EN 13523-4, using Erichsen Scratch Hardness Tester Model 291 Adhesion – according to EN ISO 2409 Abrasion resistance (Taber) – according to ISO 7784-2, CS-10, 1000 cycles, 1000 g weight Elasticity – according to EN ISO 6860 Impact resistance (direct and reverse) – according to EN ISO 62721, using Erichsen Variable Impact Tester Model 304 Cupping – according to EN ISO 1520, using Erichsen Cupping Tester Scratch resistance – according to EN ISO 1518 Water resistance – % of rusty spots on the surface (test conducted according to EN ISO 2812-2 and assessment made according to EN ISO 4628-1) after 34 days immersion in distilled water followed by 24 h drying at room temperature.
- Salt fog resistance – according to EN ISO 9227, 8 cycles (192 h). Damage to the specimen surface caused by salt fog was estimated by assessing the appearance of the cross-cuts which were made before the test.
Moreover, the coatings surfaces were examined by X-ray Photoelectron Spectroscopy (XPS) – ESCALAB-210 apparatus and Atomic Force Microscopy (AFM) – TopoMetrix, Discoverer TMX2010 apparatus). Scanning Electron Microscopy (SEM) (Jeol JSM – 6490LV microscope) was applied to compare appearance of unmodified and modified powder coating particles and differential scanning calorimetry (TA Instruments Q2000 apparatus) was used for Tg determinations of those particles in heat–cool–heat cycle which allowed for curing of the resin used as coating binder.
3. Results and discussion Results of testing the appearance and surface properties as of cured epoxy-polyester and polyester coatings are presented in Tables 1, 2 and 4, respectively.
Table 1 Appearance and surface properties of cured epoxy-polyester powder coatings modified with 3% of NP-DASI nanopowder containing silicone–acrylic core–shell nanoparticles of different composition of silicone resin constituting the core. Sample designation
Unmodified coating NP-DASI 289 NP-DASI 293 NP-DASI 299 NP-DASI 301 NP-DASI 297 NP-DASI 295 NP-DASI 291 NP-DASI 289 NP-DASI 286a NP-DASI 305a a
Silicone resin composition D4/MATMS/METMS wt%
– 83.99/6.53/9.48 92.07/2.49/5.44 79.95/2.49/17.56 79.95/14.61/5.44 88/12/0 88/0/12 75.88/12.06/12.06 83.99/6.53/9.48 83.99/6.53/9.48 83.99/6.53/9.48
Properties of epoxy-polyester coatings Gloss 60◦ /20◦
Whiteness L*/a*/b* Contact angle water/CH2 I2 o
SFE (mJ/m2 ) SFE dispersive component (mJ/m2 )
SFE polar component (mJ/m2 )
94.6/68.3 88.8/50.6 87.9/49.9 89.6/55.1 94.1/74.3 87.4/51.6 80.9/42.9 91.8/66.9 88.8/50.6 90.6/58.9 89.7/65.9
93.94/−1.05/3.29 94.86/−1.08/3.56 94.90/−1.15/3.18 95.39/−1.18/3.48 93.90/−1.29/2.59 94.95/−1.11/3.55 95.24/−1.11/3.40 93.98/−1.29/3.10 94.86/−1.08/3.56 94.91/−1.06/3.48 94.42/−1.06/3.49
45.47 43.05 37.74 41.53 44.20 41.98 37.67 42.88 43.05 42.63 43.13
0.79 0.98 0.88 0.89 0.99 0.30 0.07 0.79 0.98 0.96 0.76
88.2/27.6 88.5/33.1 92.1/43.9 89.8/36.5 87.8/30.5 93.7/36.6 99.2/45.8 89.6/33.7 88.5/33.1 88.8/34.1 89.7/33.2
44.68 42.07 36.86 40.64 43.21 41.68 37.60 42.09 42.07 41.67 42.37
Silicone resin composition – the same as that in NP-DASI 289.
Please cite this article in press as: J. Kozakiewicz, et al., Studies on silicone–acrylic hybrid aqueous dispersions and corresponding silicone–acrylic nanopowders designed for modification of powder coatings and plastics. Part II – Effect of modification with silicone–acrylic nanopowders and of composition of silicone resin contained in those nanopowders. . . , Prog. Org. Coat. (2014), http://dx.doi.org/10.1016/j.porgcoat.2014.01.010
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Table 2 Appearance and surface properties of cured polyester powder coatings modified with 3% of NP-DASI nanopowder containing silicone–acrylic core–shell nanoparticles of different composition of silicone resin constituting the core. Sample designation
Unmodified coating NP-DASI 289 NP-DASI 293 NP-DASI 299 NP-DASI 301 NP-DASI 297 NP-DASI 295 NP-DASI 291 NP-DASI 286a NP-DASI 305a a
Silicone resin composition D4/MATMS/METMS wt%
– 83.99/6.53/9.48 92.07/2.49/5.44 79.95/2.49/17.56 79.95/14.61/5.44 88/12/0 88/0/12 75.88/12.06/12.06 83.99/6.53/9.48 83.99/6.53/9.48
Properties of polyester coatings Gloss 60◦ /20◦
Whiteness L*/a*/b* Contact angle water/CH2 I2 o
SFE (mJ/m2 ) SFE dispersive component (mJ/m2 )
SFE polar component (mJ/m2 )
81.7/41.6 87.6/58.3 87.3/57.5 79.2/34.7 88.0/64.4 85.0/51.0 82.4/47.2 86.6/50.1 87.7/61.0 84.3/49.7
95.40/−1.16/3.27 95.18/−1.15/2.86 96.67/−1.05/3.23 95.27/−1.12/2.94 95.65/−1.01/3.33 95.56/−1.11/3.55 95.17/−1.18/2.38 95.20/−1.15/2.77 94.81/−1.13/2.53 95.87/−1.06/3.30
45.04 43.32 40.89 42.22 43.88 42.60 38.68 43.77 44.06 43.24
1.15 1.21 1.25 1.30 0.91 0.90 0.08 1.16 1.23 0.99
86.5/28.3 87.2/32.3 88.4/37.5 87.5/34.7 88.4/31.3 89.2/34.2 98.5/43.9 87.2/31.4 86.7/30.6 88.4/32.7
43.89 42.11 39.64 40.92 42.97 41.70 38.60 42.61 42.83 42.25
Silicone resin composition – the same as that in NP-DASI 289.
Table 3 Mechanical properties of cured epoxy-polyester powder coatings modified with 3% of NP-DASI nanopowder containing silicone–acrylic core–shell nanoparticles of different composition of silicone resin constituting the core. Sample designation
Unmodified coating NP-DASI 289 NP-DASI 293 NP-DASI 299 NP-DASI 301 NP-DASI 297 NP-DASI 295 NP-DASI 291 NP-DASI 286a NP-DASI 305a a
Silicone resin composition D4/MATMS/METMS wt%
– 83.99/6.53/9.48 92.07/2.49/5.44 79.95/2.49/17.56 79.95/14.61/5.44 88/12/0 88/0/12 75.88/12.06/12.06 83.99/6.53/9.48 83.99/6.53/9.48
Properties of epoxy-polyester coatings
Impact resistance direct/rev (J)
Cupping (mm0
Abrasion resistance (mg)
Hardness (Persoz) (s)
Pencil hardness
Adhesion to steel
Scratch resistance (g)
Water resistance (%)
4/0 6/0 14/10 16/4 18/10 10/8 6/2 8/8 4/0 6/1
4 7.8 5.8 7.7 9.3 6 7.2 8.3 5 8.4
16.33 31.67 21.67 21.33 14.00 15.00 22.33 21.33 12.33 20.33
185 189 186 175 174 190 190 196 190 182
3B – 2B 3B – 2B 3B – 2B 3B – 2B HB – B 3B – 2B 3B – 2B 7B – 6B 3B – 2B 6B – 5B
0 0 0 0 0 0 0 0 0 0
450 400 475 400 375 400 425 375 425 400
10.00 10.00 0.50 10.00 20.00 6.00 3.00 5.00 5.00 3.00
Silicone resin composition – the same as that in NP-DASI 289.
Results of testing mechanical properties of cured epoxypolyester and polyester coatings are presented in Tables 3 and 4, respectively. It can be seen from those results that – as it could be expected – the properties of cured epoxy-polyester coatings were much more affected by modification with NP-DASI nanopowder than the properties of cured polyester coatings. Therefore, discussion of the effect of composition of silicone resin (i.e. silicone part of silicone–acrylic
core–shell particles contained in NP-DASI nanopowder) on properties of cured coatings will be based mainly on the results obtained for epoxy-polyester coatings. Nevertheless, it should be noted that the substantial coating properties like impact resistance or cupping were improved due to NP-DASI nanopowder addition also for cured polyester coatings. Below, the effect of modification with NP-DASI and of the composition of silicone resin contained in NP-DASI on various properties of the cured coatings will be analyzed based on
Table 4 Mechanical properties of cured polyester powder coatings modified with 3% of NP-DASI nanopowder containing silicone–acrylic core–shell nanoparticles of different composition of silicone resin constituting the core. Sample designation
Unmodified coating NP-DASI 289 NP-DASI 293 NP-DASI 299 NP-DASI 301 NP-DASI 297 NP-DASI 295 NP-DASI 291 NP-DASI 286a NP-DASI 305a a
Silicone resin composition D4/MATMS/METMS (%)
– 83.99/6.53/9.48 92.07/2.49/5.44 79.95/2.49/17.56 79.95/14.61/5.44 88/12/0 88/0/12 75.88/12.06/12.06 83.99/6.53/9.48 83.99/6.53/9.48
Properties of polyester coatings
Impact resistance direct/rev (J)
Cupping (mm)
Abrasion resistance (mg)
Hardness (Persoz)
Pencil hardness
Adhesion Scratch to steel resistance (g)
Water resistance (%)
20/8 20/12 20/12 20/8 20/12 20/12 20/8 20/10 20/12 20/12
5.70 9.30 11.10 5.55 5.35 5.75 6.50 5.53 12.15 9.20
15.33 24.00 20.00 31.33 27.33 19.67 29.67 25.33 24.67 26.00
160 170 172 161 165 174 175 165 167 165
3B – 2B 3B – 2B 3B – 2B 4B – 3B 3B – 2B 6B – 5B 3B – 2B 7B – 6B 3B – 2B 6B – 5B
0 0 0 0 0 0 0 0 0 0
10 5 1 1 5 10 1 5 5 2
425 400 400 450 375 450 400 350 400 450
Silicone resin composition – the same as that in NP-DASI 289.
Please cite this article in press as: J. Kozakiewicz, et al., Studies on silicone–acrylic hybrid aqueous dispersions and corresponding silicone–acrylic nanopowders designed for modification of powder coatings and plastics. Part II – Effect of modification with silicone–acrylic nanopowders and of composition of silicone resin contained in those nanopowders. . . , Prog. Org. Coat. (2014), http://dx.doi.org/10.1016/j.porgcoat.2014.01.010
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the results presented in Tables 1–4. For explanation of the effect of silicone resin composition, triangular graphs obtained from Statistica computer program after analysis of data shown in Tables 1–4 will be used. Those graphs clearly show the combined effect of the content of each of the three components of silicone monomers mixture used to synthesize silicone resin which correspond to the share of units originating from those monomers in the silicone resin structure, i.e. to silicone resin composition. That combined effect is also shown on the graphs in the form of relevant Yi = f(X1, X2, X3) functions where Yi is particular factor characterizing the coating and X1–X3 are shares of the respective three silicone monomers in the monomers mixture used in silicone resin synthesis. 3.1. Effect of modification with NP-DASI and of composition of silicone resin contained in NP-DASI on gloss and whiteness of cured coatings 3.1.1. Effect on gloss As it can be seen from Tables 1 and 2 gloss measured at 60◦ for both epoxy-polyester and polyester coatings did not change much after modification with NP-DASi though some general declining tendency for epoxy-polyester coatings and some general improving tendency for polyester coating can be noted when the results obtained for modified and unmodified samples are compared. Those tendencies are even more clearly marked when the results obtained for 20◦ gloss are analyzed and it seems clear that the silicone resin composition influenced the gloss value – see Fig. 3. A positive effect of NP-DASI modification on gloss in case of polyester coatings and a negative effect in case of epoxy-polyester coatings can be explained by poor miscibility of epoxy resins with silicone polymers [17,18] what could result in greater haze observed when gloss was measured at 20◦ angle. In polyester coatings miscibility of silicone resin with the polyester resin hardener was much better, so the haze level was much lower. In Fig. 4 the effect of silicone resin composition in NP-DASI on 20◦ gloss measured for cured polyester powder coatings is presented. Based on the results presented in Fig. 4 it can be assumed that the silicone resin hydrophobicity was a prevailing factor in affecting the gloss of the cured coatings modified with NP-DASI because more METMS and D4 in the mixture of monomers used for silicone resin synthesis led to decrease in 20◦ gloss.
Fig. 4. Effect of composition of silicone resin contained in NP-DASI nanopowder on 20◦ gloss of cured polyester powder coatings modified with 3% of NP-DASI.
balance (a*) and yellow/blue balance (b*) of the coatings modified with NP-DASI and unmodified coatings were quite small and no specific effect of silicone resin composition on whiteness can be observed. 3.2. Effect of modification with NP-DASI and of composition of silicone resin contained in NP-DASI on surface properties of cured coatings The significant effect of modification with silicone–acrylic core–shell nanoparticles on surface properties of powder coatings has been reported in our earlier papers [10–12,15] and by other authors [13]. It was clearly proved based on the results of XPS and AFM studies that increasing the content of such nanoparticles in the coating formulation led to increase in silicone content on the coating surface [10–12,15]. Such phenomenon can be explained by migration of silicone resin to the surface due to very low surface free energy and great flexibility of polysiloxane segments resulting from very high value of O Si O bond angle (145◦ ) and low interactions between segments. The results obtained in the present study fully confirmed our previous findings with regard to the effect of modification with silicone–acrylic nanopowder on surface properties of powder coatings. 3.2.1. Effect of Si content close to the coating surface In Fig. 5 the changes in silicon (Si) content in the surface layers of unmodified epoxy-polyester coating and the same coating 30
epoxy-polyester coating modified with 3% NP-DASI 295
25
Si content [%]
Fig. 3. Effect of modification with 3% of NP-DASI nanopowder (NP-DASI-293) on gloss of cured epoxy-polyester coatings and cured polyester coatings measured at 20◦ and 60◦ angle.
epoxy-polyester coating modified with 3% NP-DASI 301
20 15 10 5 0 0
3.1.2. Effect on whiteness The results presented in Tables 1 and 2 show that modification with NP-DASI had only little effect on whiteness of the powder coatings studied. The differences in brightness (L*), red/green
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2
4
6
8
10
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Distance from the surface [nm] Fig. 5. Changes in Si concentration close to the coating surface determined by XPS for cured epoxy-polyester coating modified with two NP-DASI nanopowders containing silicone resin of different composition.
Please cite this article in press as: J. Kozakiewicz, et al., Studies on silicone–acrylic hybrid aqueous dispersions and corresponding silicone–acrylic nanopowders designed for modification of powder coatings and plastics. Part II – Effect of modification with silicone–acrylic nanopowders and of composition of silicone resin contained in those nanopowders. . . , Prog. Org. Coat. (2014), http://dx.doi.org/10.1016/j.porgcoat.2014.01.010
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6 20
polyester coating modified with 3% NP-DASI 289
Si content [%]
15
polyester coating modified with 3% NP-DASI 301
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0
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4
6
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Distance from the surface [nm] Fig. 6. Changes in Si concentration close to the coating surface determined by XPS for cured polyester coating modified with two NP-DASI nanopowders containing silicone resin of different composition.
modified with 3% of two different nanopowders (NP-DASI- 295 and NP-DASI-301) are shown. The former NP-DASI nanopowder contained silicone resin that was lacking structural elements originating from MATMS (see Table 1) and therefore was considered to be more hydrophobic and the latter contained silicone resin with structural elements originating from all there silicone monomers and thus considered to be less hydrophobic. It can be noted from Fig. 5 that Si content increased significantly with diminishing distance from the coating surface reaching very high values (close to 20–25%) at ca. 4 nm from the surface and that it was distinctly higher for the coating modified with NP-DASI-295 containing highly hydrophobic silicone resin. However, when polyester coatings modified with 3% of the same NP-DASI nanopowders were examined concentration of silicon on the surface appeared to be much lower – see Fig. 6. This difference can again be explained (as it was explained with regard to gloss in Section 3.1) by difference in silicone resin compatibility with other coating ingredients in case of both types of coatings. 3.2.2. Effect on surface image obtained from AFM The occurrence of silicone resin on the epoxy-polyester coating surface could also be assumed based on comparison of AFM images of the surfaces of unmodified and modified epoxy-polyester coatings. As it can be seen in Fig. 7 those images differ much and some nano-sized structures (tiny white “flakes” of ca. 100–300 nm in size) could be seen on the surface of modified coating.
Taking into account the results of our previous findings showing that the increase in silicone–acrylic nanopowder content in the coating resulted in increase in the number of those tiny particles on the surface [10,12] it can be anticipated that they were very small agglomerates of silicone resin originating from the cores released at high temperature applied in the curing process of the coating from silicone–acrylic core–shell nanoparticles contained in NP-DASI. 3.2.3. Effect on contact angle and SFE The effect of modification of powder coatings investigated in this study with NP-DASI nanopowder on surface properties was also examined by contact angle determinations. The results presented in Tables 1 and 2 clearly show that the contact angle was in general higher for modified coatings. That increase was more distinct for epoxy-polyester coatings than for polyester coatings what confirmed XPS findings. Based on the results of contact angle determinations surface free energy (SFE) was calculated and the SFE values were distinctly lower for coatings modified with NP-DASI nanopowder. Based on contact angle and SFE values presented in Tables 1 and 2 it was also possible to assess the effect of composition of silicone resin contained in NP-DASI on those two parameters characterizing the coating surface. The effect of silicone resin composition on contact angle and on SFE of epoxy-polyester coating modified with 3% NP-DASi nanopowder is presented in Figs. 8 and 9, respectively. It can be concluded from the results presented in Figs. 8 and 9 that inside the area of the designed experiment the contact angle showed a minimum and SFE showed a maximum for roughly the same silicone resin composition. Very high values of contact angle (reaching even 98◦ ) and very low values of SFE (indicating the most hydrophobic surface) were observed for silicone resin structures with very high share of segments originating from D4 and METMS what could be expected taking into account that less MATMSoriginating units in the resin structure makes it more hydrophobic. 3.3. Effect of modification with NP-DASI and of composition of silicone resin contained in NP-DASI on impact resistance, cupping and elasticity of cured coatings 3.3.1. Effect on impact resistance Based on the results presented in Tables 3 and 4 it is clear that modification with 3% NP-DASI nanopowder resulted in great increase in impact resistance of cured coatings (both direct and reverse) and that the observed effect was much more distinct for
Fig. 7. AFM images of the surfaces of unmodified cured epoxy-polyester coating (a) and for cured epoxy-polyster coating modified with 3% NP-DASI nanopowder (b).
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Fig. 8. Effect of composition of silicone resin contained in NP-DASI nanopowder on contact angle (water) of cured epoxy-polyester powder coatings modified with 3% of NP-DASI.
Fig. 9. Effect of composition of silicone resin contained in NP-DASI nanopowder on SFE of cured epoxy-polyester powder coatings modified with 3% of NP-DASI.
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Fig. 10. Effect of modification with 3% of NP-DASI nanopowder (NP-DASI-293) on impact resistance (direct and reverse) of cured epoxy-polyester coatings and cured polyester coatings.
epoxy-polyester coatings than for polyester coatings. When impact test was made for epoxy-polyester coating in a reverse mode the unmodified coating could not withstand even quite low impact while impact value determined for the modified coating was quite reasonable. That effect is illustrated in Fig. 10, and in Fig. 11 the photos of coating surfaces examined after impact test for unmodified and modified epoxy-polyester coating is presented. As it was explained already earlier in this paper the observed effect can be attributed to the ability of silicone resin that is originating from of the core–shell nanoparticles which are evenly distributed in the coating body to absorb effectively mechanical energy of impact. Similar effect was also reported earlier for modification of powder coatings and plastics with silicone–acrylic and other types of core–shell nanoparticles containing polymers characterized by very low Tg s [3,4,7,8,18–28]. When the Tg s of unmodified epoxy-polyester powder coating particles and the same powder coating particles modified with 3% of NP-DASI were examined by DSC in heat-cool-heat mode (so curing could proceed during heating part of the cycle) it was found that Tg determined for unmodified coating particles was higher than Tg observed for coating particles modified with 3% of NP-DASI nanopowder what means that even so small concentration of silicone resin in the coating formulation resulted in decrease in its Tg (see Fig. 12). More detailed information on thermal properties of
Fig. 11. Photos of coating surfaces of unmodified epoxy-polyester coating (a) and the same coating modified with 3% of NP-DASI-293 nanopowder (b) examined after impact test (reverse).
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Fig. 12. Effect of modification with 3% of NP-DASI nanopowders containing silicone resins of different composition on Tg of particles of epoxy-polyester coatings cured in heat-cool-heat cycle in DSC apparatus. (1) Unmodified. (2) Modified with 3% of NP-DASI-293. (3) Modified with 3% of NP-DASI-291.
coatings modified with NP-DASI can be found in our paper that has recently been published [29]. From the point of view of optimization of NP-DASI nanopowder formulation it was especially interesting to examine the effect of composition of silicone resin contained in NP-DASI nanopowder on impact resistance of powder coatings investigated in this study. In Figs. 13 and 14 that effect is presented for epoxy-polyester coatings tested for direct and reverse impact resistance, respectively. Based on the results presented in Figs. 13 and 14 it can be noted that impact resistance (direct) was generally much higher than impact resistance (reverse) what is the phenomenon usually observed for powder coatings. Moreover, in both cases impact values showed clear minimum at roughly the same silicone resin composition. High impact values corresponded to high share of linear segments originating from D4 in the resin structure what could be expected since that means lower crosslinking density and higher elasticity modulus of the resin that would be reflected in its better ability to relieve mechanical stresses resulting from impact. The results of detailed studies of the effect of structure of silicone part
of silicone-based core–shell impact modifiers on impact resistance of PVC led to similar conclusions [8].
Fig. 13. Effect of composition of silicone resin contained in NP-DASI nanopowder on impact resistance (direct) of cured epoxy-polyester powder coatings modified with 3% of NP-DASI.
Fig. 14. Effect of composition of silicone resin contained in NP-DASI nanopowder on impact resistance (reverse) of cured epoxy-polyester powder coatings modified with 3% of NP-DASI.
3.3.2. Effect on cupping Cupping test is a very important tool used to assess ability of a coating to withstand stresses resulting from mechanical forming of a coated steel sheet in order to obtain the desired shape. As it is shown in Tables 3 and 4 coatings investigated in this study modified with 3% NP-DASI behaved in that test much better than the unmodified coatings. However, unlike it was observed in the case of impact test, the effect of NP-DASI modification was more distinct for polyester coatings. That effect is shown in Fig. 15. Based on the results presented in Tables 3 and 4 it can be also noted that while modification with NP-DASI nanopowder generally resulted in enhanced cupping resistance the level of that increase was very much depending on the composition of silicone resin contained in NP-DASI. In Fig. 16 the effect of the composition of silicone resin contained in NP-DASI on cupping resistance of epoxypolyester coatings modified with 3% NP-DASI is presented.
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3.4. Effect of modification with NP-DASI and of composition of silicone resin contained in NP-DASI on hardness, abrasion resistance, scratch resistance and adhesion of cured coatings
Fig. 15. Effect of modification with 3% of NP-DASI nanopowder (NP-DASI-293) on cupping resistance of cured epoxy-polyester coatings and cured polyester coatings.
Results of the relevant tests presented in Tables 3 and 4 confirmed that modification with 3% NP-DASI nanopowder did not distinctly affect hardness, abrasion resistance, scratch resistance and adhesion of the coatings investigated in this study. Persoz hardness was slightly enhanced for modified polyester coatings and did not change much for modified epoxy-polyester coatings. Pencil hardness seemed to decrease slightly for both types of modified coatings. Abrasion resistance varied depending on the composition of silicone resin contained in NP-DASI, but in general was slightly diminished for epoxy-polyester coatings and slightly enhanced for modified polyester coatings. Slight decrease in abrasion resistance in the case of epoxy polyester coatings can be explained by weakening of surface layers of the coating due to migration of silicone resin to the coating surface. Since in the case of polyester coatings silicone resin content on the surface was found to be much lower (see Section 3.1) decrease in abrasion resistance was not observed. Slight differences between samples tested for scratch resistance were observed, but those can be neglected taking into account quite low accuracy of that test. Adhesion to steel was excellent for both modified and unmodified coatings.
3.5. Effect of modification with NP-DASI and of composition of silicone resin contained in NP-DASI on water and salt fog resistance of cured coatings
Fig. 16. Effect of composition of silicone resin contained in NP-DASI nanopowder on cupping resistance of cured epoxy-polyester powder coatings modified with 3% of NP-DASI.
As it can be seen in Fig. 16, cupping resistance depended mostly on D4 share in the silicone resin structure and decreased significantly with increase in that share. 3.3.3. Effect on elasticity Regarding the results of elasticity test conducted for modified and unmodified coatings it can only be concluded from Tables 3 and 4 that modification with NP-DASI clearly led to improvement of elasticity in case of epoxy-polyester coatings and did not affect elasticity of polyester coatings that was already excellent.
3.5.1. Effect on water resistance Though water resistance of both unmodified coatings and coatings modified with NP-DASI nanopowder was very good (for most of the coatings first visual changes appeared only after more than 14 days of immersion in water), those changes – if any – were more distinct for unmodified coatings. Final assessment of damage of coatings surface resulting from immersion in water was made after only 34 days of immersion followed by 24 h drying. As it can be seen in Tables 3 and 4 for coatings modified with NP-DASI which contained silicone resin of certain composition almost no changes occurred. Better water resistance of coatings modified with NPDASI can be explained by higher hydrophobicity of coating surface caused by silicone resin – see discussion on surface properties of the coatings contained in Section 3.1.
3.5.2. Effect on salt fog resistance Like water resistance test, also salt fog resistance test did not reveal very significant differences between modified and unmodified coatings – in both cases there were only traces of corrosion
Fig. 17. Photos of coating surfaces of unmodified epoxy-polyester coating (a) and the same coating modified with 3% of NP-DASI-293 nanopowder (b) examined after salt fog test.
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under the coating, though again those traces were more visible for unmodified coatings – see Fig. 17.
and XPS determinations, respectively, and Ms A. Lukomska and Ms S. Kowalska from Industrial Chemistry Research Institute for making SEM pictures is also acknowledged.
4. Conclusions It was confirmed that modification of epoxy-polyester and polyester powder coatings with 3% of nanopowder containing silicone–acrylic core–shell nanoparticles significantly affected the properties of cured coatings. Regarding mechanical properties that effect was generally more distinct for epoxy-polyester coatings than for polyester coatings. Specifically, great increase in impact and cupping resistance was observed for modified coatings and was explained by absorption of mechanical stresses by particles of low modulus silicone resin which were released from core–shell nanoparticles in the process of curing of the coatings. Other mechanical properties of cured modified and unmodified coatings (abrasion resistance, hardness, scratch resistance, elasticity and adhesion to steel) as well as appearance of the coatings (gloss and whiteness) did not differ much. Water resistance and salt fog resistance were slightly better for modified coatings. Testing the surface properties of modified coatings by XPS and AFM revealed the presence of silicone resin on coating surface what was reflected in higher contact angle and lower SFE as compared to unmodified coatings. That phenomenon was explained by migration of silicone resin to the coating surface. Based on the results of a designed experiment it can be stated that silicone resin structure is a very important factor influencing both mechanical and surface-related properties of the cured powder coatings modified with silicone–acrylic nanoparticles containing silicone resin in the core. Optimization of that structure through appropriate design of monomers used for silicone resin synthesis can lead to significant improvement of coating properties, in particular impact resistance. Acknowledgements The authors wish to acknowledge Polish State Center for Research and Development for providing financial support for the studies described in this paper that were conducted under the project N R08-0004 10. Assistance of Prof. R. Nowakowski and Dr J. W. Sobczak from Polish Academy of Sciences for conducting AFM
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Studies on silicone–acrylic hybrid aqueous dispersions and corresponding silicone–acrylic nanopowders designed for modification of powder coatings and plastics Part II – Effect of modification with silicone–acrylic nanopowders and of composition of silicone resin contained in those nanopowders on properties of epoxy-polyester and polyester powder coatings b ´ Janusz Kozakiewicz a,∗ , Izabela Ofat a , Joanna Trzaskowska a , Helena Kuczynska a b
Industrial Chemistry Research Institute, 02-724 Warsaw, Poland Institute for Engineering of Polymer Materials and Dyes, Paints and Plastic Department, 50A, Chorzowska Street, 44-100 Gliwice, Poland
a r t i c l e
i n f o
Article history: Received 6 May 2013 Received in revised form 8 January 2014 Accepted 9 January 2014 Available online xxx Keywords: Epoxy-polyester powder coatings Polyester powder coatings Core–shell nanoparticles Impact modifiers Silicones
a b s t r a c t Epoxy-polyester and polyester coatings were modified with 3% of NP-DASI “nanopowders” consisting of core–shell nanoparticles having silicone resin of very low glass transition temperature (Tg ) in the core and poly(methyl methacrylate) of high Tg in the shell. The nanopowders were obtained through spray drying of corresponding aqueous dispersions with core–shell particle structure which were synthesized in a process of emulsion polymerization of methyl methacrylate monomer in aqueous dispersions of silicone resins with different compositions resulting from different compositions of silicone monomers used for their synthesis. A designed experiment was conducted where the effect of composition of silicone resin on the properties of cured coatings was studied. It was confirmed that modification with that “nanopowder” significantly affected the properties of cured coatings, in particular impact resistance and cupping as well as surface properties. Regarding mechanical properties the influence of modification with “nanopowder” was generally more distinct for epoxy-polyester coatings than for polyester coatings. Specifically, great increase in impact and cupping resistance was observed for modified coatings and was explained by absorption of mechanical stresses by particles of low modulus silicone resin which were released from core–shell nanoparticles in the process of curing of the coatings. Other mechanical properties of cured modified and unmodified coatings (abrasion resistance, hardness, scratch resistance, elasticity and adhesion to steel) as well as appearance of the coatings (gloss and whiteness) did not differ much. Water resistance and salt fog resistance were slightly better for modified coatings. Testing the surface properties of modified coatings by XPS and AFM revealed the presence of silicone resin on coating surface what was reflected in higher contact angle and lower SFE as compared to unmodified coatings. That phenomenon was explained by migration of silicone resin to the coating surface. © 2014 Elsevier B.V. All rights reserved.
1. Introduction It is well known that nanoparticles containing low modulus polymers, specifically silicones, can significantly increase fracture toughness of polymer composites and that good dispersion of nanoparticles in the body of the composite are of great importance [1,2]. Following that concept silicone–acrylic core–shell nanoparticles have been applied as effective impact modifiers for powder coatings and plastics [3–8]. In our laboratory such impact modifiers
∗ Corresponding author. Tel.: +48 22 5682378. E-mail address: [email protected] (J. Kozakiewicz).
were obtained in a form of “nanopowder” consisting of agglomerated silicone–acrylic core–shell nanoparticles by spray-drying of silicone–acrylic hybrid aqueous dispersions synthesized from silicone resin aqueous dispersions using a recently patented process [9]. Our preliminary studies confirmed [5,10–12] that addition of only 3% of such nanopowder to epoxy-polyester powder coating resulted in a very distinct increase in impact resistance, elasticity and cupping resistance without any significant change in other important coating properties (hardness, abrasion resistance). It could be explained by excellent distribution of silicone resin having very low Tg (ca. −120 ◦ C) in the body of coating what provided resistance to stresses originated from impact or from other mechanical forces imposed on the coating during testing. As it can be seen in
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to the resulting silicone resin composition. The experiment was designed in the form of a triangle where each corner corresponded to 100% of one of the monomers. Detailed plan of the designed experiment showing the actual situation of experimental points in that triangle was presented in Part I of our paper, so it is not repeated here. 2. Experimental 2.1. Starting materials Epoxy resin (Epidian 012) from Chemical Company OrganikaSarzyna Poland Carboxylated polyester resin (Policen 3000T) from PPG Polifarb Cieszyn Poland, acid value – 33 – used as hardener for epoxy-polyester coatings and as main binder for polyester coatings Primid XL 552 (from EMS-Chemie AG) – used as hydroxyalkylamide crosslinker for polyester coatings TiO2 (Rutile from Chemical Company Police Poland) – used as pigment. Benzoin from DSM and Resiflow PV 88 from Worlee Chemie GmbH – used as standard additives for powder coatings. Silicone–acrylic nanopowders applied as impact modifiers were NP-DASI samples obtained in the designed experiment described in [16]. 2.2. Preparation and curing of powder coatings Fig. 1. Simplified scheme of the process of powder coating production with NP-DASI nanopowder added as impact modifier.
Fig. 1 presenting schematically the steps of powder coating modification with such nanopowders, during the preparation of powder coating masterbatch in the extruder at temperatures significantly lower than Tg of the methacrylic polymer core, the core–shell particles are released from agglomerates due to the shear forces without any damage of the core and become nicely distributed in the body of powder coating particles. When the powder coating is sprayed onto the metal surface and cured at a temperature of over 160 ◦ C, i.e. significantly higher than Tg of the methacrylic polymer shell, the shell flows and the silicone resin core is released, but since the core–shell particles are separated in the body of coating there is only limited possibility of gluing of the silicone resin cores. It was also found in our preliminary studies that TiO2 -filled coatings modified with such nanopowders showed enhanced whiteness. This unexpected benefit achieved through modification of powder coatings with silicone–acrylic nanopowders can be explained by migration of silicone resin to the coating surface resulting in changing the light reflection on the coating surface. The phenomenon of silicone resin migration to the surface of powder coatings modified with our silicone–acrylic nanopowders was fully proved by the results of XPS, AFM and SEM-EDS investigations [12–15]. It provides unique opportunity to obtain coatings of silicone-like surface, but high hardness and abrasion resistance combined with enhanced impact resistance and elasticity. The results of our studies on the effect of starting silicone resin composition on the properties of silicone silicone–acrylic hybrid dispersions (DASI) and corresponding nanopowders (NPDASI) obtained by spray-drying of DASI were discussed earlier [16] while in this paper the results of using those NP-DASI nanopowders of different composition of silicone resin as impact modifiers for epoxy-polyester and polyester powder coatings will be presented. As it was explained in [16] the designed experiment was conducted in which the independent variables were contents (in wt%) of each of the compounds, i.e. octamethylcyclotetrasiloxane (D4), methyltrimethoxysilane (METMS) and methacryloyltrimethoxysilane (MATMS) in the monomers mixture used in synthesis of silicone dispersion which were assumed to correspond roughly
For preparation of epoxy-polyester coatings epoxy resin (30 parts), carboxylic polyester hardener (70 parts), TiO2 (30.0 parts), standard additives (1.0 parts) and, if appropriate, the NP-DASI nanopowder (3% per coating mass) were mixed in a grinder at 1000 rpm for 3 min. The resulting powder mix of starting materials was added to the Buss Ko-Knether PR-46 extruder (temp. of the screw and adapter 82 ◦ C and 102 ◦ C, respectively, screw rotational speed: 3.5 rpm) For preparation of polyester coatings polyester resin (56.05 parts), hardener (2.95 parts), TiO2 (37.0 parts), standard additives (1.0 parts) and, if appropriate, the NP-DASI nanopowder (3% per coating mass) were mixed at 1000 rpm for 3 min. The resulting powder mix was added to the extruder as described above. It was found that the process of extrusion was clearly facilitated when the masterbatches contained NP-DASI nanopowder, indicating that the silicone resin acted as additional flow aid. The extruded masterbatches were crushed and then pulverized to yield fine powder of particle size from 5 to 85 . When the SEM pictures of the powder coating particles obtained using NP-DASI as additive and not containing that additive were compared it could be concluded that the particle surface was more smooth in the case of modified powder coating particles which contained NP-DASI (see Fig. 2) what confirmed our earlier observations [10,12]. This phenomenon can be explained by migration of silicone resin to the particle surface. Powder coatings obtained as described above were deposited on both sides of steel plates using electrostatic spraying and the coated plates were placed in an oven at 180 ◦ C for 15 min to produce smooth good looking glossy white hard coats of thickness ranging from 50 to 70 . More than 30 specimens were produced for each coating composition. 2.3. Testing of cured coatings Specimens of similar coating thickness were selected for testing. The following standard properties of cured coatings were tested: - Gloss – at 60◦ and 20◦ – according to EN ISO 2813, using Erichsen PICO GLOSS 503 apparatus - Whiteness – using X-Rite 968 spectrometer at 0/45◦ geometry and at D65 /10◦ illuminant value. The following parameters were measured: L* – brightness (0–100), a* – red/green balance
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Fig. 2. Comparison of appearance of epoxy-polyester powder coating particles. (a) Unmodified coating and (b) coating modified with 3% of NP-DASI-293 nanopowder. Pictures were taken by SEM.
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(positive = red, negative = green) and b* – yellow/blue balance (positive = yellow, negative = blue) Contact angle (water and CH2 I2 ) – according to EN 828:2000, using KRUSS DSA 100E apparatus. For calculation of surface free energy (SFE) and its polar and dispersive components OwensWendt method was applied. Pendulum hardness (Persoz) – according to PN ISO 1522 Pencil hardness – according to EN 13523-4, using Erichsen Scratch Hardness Tester Model 291 Adhesion – according to EN ISO 2409 Abrasion resistance (Taber) – according to ISO 7784-2, CS-10, 1000 cycles, 1000 g weight Elasticity – according to EN ISO 6860 Impact resistance (direct and reverse) – according to EN ISO 62721, using Erichsen Variable Impact Tester Model 304 Cupping – according to EN ISO 1520, using Erichsen Cupping Tester Scratch resistance – according to EN ISO 1518 Water resistance – % of rusty spots on the surface (test conducted according to EN ISO 2812-2 and assessment made according to EN ISO 4628-1) after 34 days immersion in distilled water followed by 24 h drying at room temperature.
- Salt fog resistance – according to EN ISO 9227, 8 cycles (192 h). Damage to the specimen surface caused by salt fog was estimated by assessing the appearance of the cross-cuts which were made before the test.
Moreover, the coatings surfaces were examined by X-ray Photoelectron Spectroscopy (XPS) – ESCALAB-210 apparatus and Atomic Force Microscopy (AFM) – TopoMetrix, Discoverer TMX2010 apparatus). Scanning Electron Microscopy (SEM) (Jeol JSM – 6490LV microscope) was applied to compare appearance of unmodified and modified powder coating particles and differential scanning calorimetry (TA Instruments Q2000 apparatus) was used for Tg determinations of those particles in heat–cool–heat cycle which allowed for curing of the resin used as coating binder.
3. Results and discussion Results of testing the appearance and surface properties as of cured epoxy-polyester and polyester coatings are presented in Tables 1, 2 and 4, respectively.
Table 1 Appearance and surface properties of cured epoxy-polyester powder coatings modified with 3% of NP-DASI nanopowder containing silicone–acrylic core–shell nanoparticles of different composition of silicone resin constituting the core. Sample designation
Unmodified coating NP-DASI 289 NP-DASI 293 NP-DASI 299 NP-DASI 301 NP-DASI 297 NP-DASI 295 NP-DASI 291 NP-DASI 289 NP-DASI 286a NP-DASI 305a a
Silicone resin composition D4/MATMS/METMS wt%
– 83.99/6.53/9.48 92.07/2.49/5.44 79.95/2.49/17.56 79.95/14.61/5.44 88/12/0 88/0/12 75.88/12.06/12.06 83.99/6.53/9.48 83.99/6.53/9.48 83.99/6.53/9.48
Properties of epoxy-polyester coatings Gloss 60◦ /20◦
Whiteness L*/a*/b* Contact angle water/CH2 I2 o
SFE (mJ/m2 ) SFE dispersive component (mJ/m2 )
SFE polar component (mJ/m2 )
94.6/68.3 88.8/50.6 87.9/49.9 89.6/55.1 94.1/74.3 87.4/51.6 80.9/42.9 91.8/66.9 88.8/50.6 90.6/58.9 89.7/65.9
93.94/−1.05/3.29 94.86/−1.08/3.56 94.90/−1.15/3.18 95.39/−1.18/3.48 93.90/−1.29/2.59 94.95/−1.11/3.55 95.24/−1.11/3.40 93.98/−1.29/3.10 94.86/−1.08/3.56 94.91/−1.06/3.48 94.42/−1.06/3.49
45.47 43.05 37.74 41.53 44.20 41.98 37.67 42.88 43.05 42.63 43.13
0.79 0.98 0.88 0.89 0.99 0.30 0.07 0.79 0.98 0.96 0.76
88.2/27.6 88.5/33.1 92.1/43.9 89.8/36.5 87.8/30.5 93.7/36.6 99.2/45.8 89.6/33.7 88.5/33.1 88.8/34.1 89.7/33.2
44.68 42.07 36.86 40.64 43.21 41.68 37.60 42.09 42.07 41.67 42.37
Silicone resin composition – the same as that in NP-DASI 289.
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Table 2 Appearance and surface properties of cured polyester powder coatings modified with 3% of NP-DASI nanopowder containing silicone–acrylic core–shell nanoparticles of different composition of silicone resin constituting the core. Sample designation
Unmodified coating NP-DASI 289 NP-DASI 293 NP-DASI 299 NP-DASI 301 NP-DASI 297 NP-DASI 295 NP-DASI 291 NP-DASI 286a NP-DASI 305a a
Silicone resin composition D4/MATMS/METMS wt%
– 83.99/6.53/9.48 92.07/2.49/5.44 79.95/2.49/17.56 79.95/14.61/5.44 88/12/0 88/0/12 75.88/12.06/12.06 83.99/6.53/9.48 83.99/6.53/9.48
Properties of polyester coatings Gloss 60◦ /20◦
Whiteness L*/a*/b* Contact angle water/CH2 I2 o
SFE (mJ/m2 ) SFE dispersive component (mJ/m2 )
SFE polar component (mJ/m2 )
81.7/41.6 87.6/58.3 87.3/57.5 79.2/34.7 88.0/64.4 85.0/51.0 82.4/47.2 86.6/50.1 87.7/61.0 84.3/49.7
95.40/−1.16/3.27 95.18/−1.15/2.86 96.67/−1.05/3.23 95.27/−1.12/2.94 95.65/−1.01/3.33 95.56/−1.11/3.55 95.17/−1.18/2.38 95.20/−1.15/2.77 94.81/−1.13/2.53 95.87/−1.06/3.30
45.04 43.32 40.89 42.22 43.88 42.60 38.68 43.77 44.06 43.24
1.15 1.21 1.25 1.30 0.91 0.90 0.08 1.16 1.23 0.99
86.5/28.3 87.2/32.3 88.4/37.5 87.5/34.7 88.4/31.3 89.2/34.2 98.5/43.9 87.2/31.4 86.7/30.6 88.4/32.7
43.89 42.11 39.64 40.92 42.97 41.70 38.60 42.61 42.83 42.25
Silicone resin composition – the same as that in NP-DASI 289.
Table 3 Mechanical properties of cured epoxy-polyester powder coatings modified with 3% of NP-DASI nanopowder containing silicone–acrylic core–shell nanoparticles of different composition of silicone resin constituting the core. Sample designation
Unmodified coating NP-DASI 289 NP-DASI 293 NP-DASI 299 NP-DASI 301 NP-DASI 297 NP-DASI 295 NP-DASI 291 NP-DASI 286a NP-DASI 305a a
Silicone resin composition D4/MATMS/METMS wt%
– 83.99/6.53/9.48 92.07/2.49/5.44 79.95/2.49/17.56 79.95/14.61/5.44 88/12/0 88/0/12 75.88/12.06/12.06 83.99/6.53/9.48 83.99/6.53/9.48
Properties of epoxy-polyester coatings
Impact resistance direct/rev (J)
Cupping (mm0
Abrasion resistance (mg)
Hardness (Persoz) (s)
Pencil hardness
Adhesion to steel
Scratch resistance (g)
Water resistance (%)
4/0 6/0 14/10 16/4 18/10 10/8 6/2 8/8 4/0 6/1
4 7.8 5.8 7.7 9.3 6 7.2 8.3 5 8.4
16.33 31.67 21.67 21.33 14.00 15.00 22.33 21.33 12.33 20.33
185 189 186 175 174 190 190 196 190 182
3B – 2B 3B – 2B 3B – 2B 3B – 2B HB – B 3B – 2B 3B – 2B 7B – 6B 3B – 2B 6B – 5B
0 0 0 0 0 0 0 0 0 0
450 400 475 400 375 400 425 375 425 400
10.00 10.00 0.50 10.00 20.00 6.00 3.00 5.00 5.00 3.00
Silicone resin composition – the same as that in NP-DASI 289.
Results of testing mechanical properties of cured epoxypolyester and polyester coatings are presented in Tables 3 and 4, respectively. It can be seen from those results that – as it could be expected – the properties of cured epoxy-polyester coatings were much more affected by modification with NP-DASI nanopowder than the properties of cured polyester coatings. Therefore, discussion of the effect of composition of silicone resin (i.e. silicone part of silicone–acrylic
core–shell particles contained in NP-DASI nanopowder) on properties of cured coatings will be based mainly on the results obtained for epoxy-polyester coatings. Nevertheless, it should be noted that the substantial coating properties like impact resistance or cupping were improved due to NP-DASI nanopowder addition also for cured polyester coatings. Below, the effect of modification with NP-DASI and of the composition of silicone resin contained in NP-DASI on various properties of the cured coatings will be analyzed based on
Table 4 Mechanical properties of cured polyester powder coatings modified with 3% of NP-DASI nanopowder containing silicone–acrylic core–shell nanoparticles of different composition of silicone resin constituting the core. Sample designation
Unmodified coating NP-DASI 289 NP-DASI 293 NP-DASI 299 NP-DASI 301 NP-DASI 297 NP-DASI 295 NP-DASI 291 NP-DASI 286a NP-DASI 305a a
Silicone resin composition D4/MATMS/METMS (%)
– 83.99/6.53/9.48 92.07/2.49/5.44 79.95/2.49/17.56 79.95/14.61/5.44 88/12/0 88/0/12 75.88/12.06/12.06 83.99/6.53/9.48 83.99/6.53/9.48
Properties of polyester coatings
Impact resistance direct/rev (J)
Cupping (mm)
Abrasion resistance (mg)
Hardness (Persoz)
Pencil hardness
Adhesion Scratch to steel resistance (g)
Water resistance (%)
20/8 20/12 20/12 20/8 20/12 20/12 20/8 20/10 20/12 20/12
5.70 9.30 11.10 5.55 5.35 5.75 6.50 5.53 12.15 9.20
15.33 24.00 20.00 31.33 27.33 19.67 29.67 25.33 24.67 26.00
160 170 172 161 165 174 175 165 167 165
3B – 2B 3B – 2B 3B – 2B 4B – 3B 3B – 2B 6B – 5B 3B – 2B 7B – 6B 3B – 2B 6B – 5B
0 0 0 0 0 0 0 0 0 0
10 5 1 1 5 10 1 5 5 2
425 400 400 450 375 450 400 350 400 450
Silicone resin composition – the same as that in NP-DASI 289.
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the results presented in Tables 1–4. For explanation of the effect of silicone resin composition, triangular graphs obtained from Statistica computer program after analysis of data shown in Tables 1–4 will be used. Those graphs clearly show the combined effect of the content of each of the three components of silicone monomers mixture used to synthesize silicone resin which correspond to the share of units originating from those monomers in the silicone resin structure, i.e. to silicone resin composition. That combined effect is also shown on the graphs in the form of relevant Yi = f(X1, X2, X3) functions where Yi is particular factor characterizing the coating and X1–X3 are shares of the respective three silicone monomers in the monomers mixture used in silicone resin synthesis. 3.1. Effect of modification with NP-DASI and of composition of silicone resin contained in NP-DASI on gloss and whiteness of cured coatings 3.1.1. Effect on gloss As it can be seen from Tables 1 and 2 gloss measured at 60◦ for both epoxy-polyester and polyester coatings did not change much after modification with NP-DASi though some general declining tendency for epoxy-polyester coatings and some general improving tendency for polyester coating can be noted when the results obtained for modified and unmodified samples are compared. Those tendencies are even more clearly marked when the results obtained for 20◦ gloss are analyzed and it seems clear that the silicone resin composition influenced the gloss value – see Fig. 3. A positive effect of NP-DASI modification on gloss in case of polyester coatings and a negative effect in case of epoxy-polyester coatings can be explained by poor miscibility of epoxy resins with silicone polymers [17,18] what could result in greater haze observed when gloss was measured at 20◦ angle. In polyester coatings miscibility of silicone resin with the polyester resin hardener was much better, so the haze level was much lower. In Fig. 4 the effect of silicone resin composition in NP-DASI on 20◦ gloss measured for cured polyester powder coatings is presented. Based on the results presented in Fig. 4 it can be assumed that the silicone resin hydrophobicity was a prevailing factor in affecting the gloss of the cured coatings modified with NP-DASI because more METMS and D4 in the mixture of monomers used for silicone resin synthesis led to decrease in 20◦ gloss.
Fig. 4. Effect of composition of silicone resin contained in NP-DASI nanopowder on 20◦ gloss of cured polyester powder coatings modified with 3% of NP-DASI.
balance (a*) and yellow/blue balance (b*) of the coatings modified with NP-DASI and unmodified coatings were quite small and no specific effect of silicone resin composition on whiteness can be observed. 3.2. Effect of modification with NP-DASI and of composition of silicone resin contained in NP-DASI on surface properties of cured coatings The significant effect of modification with silicone–acrylic core–shell nanoparticles on surface properties of powder coatings has been reported in our earlier papers [10–12,15] and by other authors [13]. It was clearly proved based on the results of XPS and AFM studies that increasing the content of such nanoparticles in the coating formulation led to increase in silicone content on the coating surface [10–12,15]. Such phenomenon can be explained by migration of silicone resin to the surface due to very low surface free energy and great flexibility of polysiloxane segments resulting from very high value of O Si O bond angle (145◦ ) and low interactions between segments. The results obtained in the present study fully confirmed our previous findings with regard to the effect of modification with silicone–acrylic nanopowder on surface properties of powder coatings. 3.2.1. Effect of Si content close to the coating surface In Fig. 5 the changes in silicon (Si) content in the surface layers of unmodified epoxy-polyester coating and the same coating 30
epoxy-polyester coating modified with 3% NP-DASI 295
25
Si content [%]
Fig. 3. Effect of modification with 3% of NP-DASI nanopowder (NP-DASI-293) on gloss of cured epoxy-polyester coatings and cured polyester coatings measured at 20◦ and 60◦ angle.
epoxy-polyester coating modified with 3% NP-DASI 301
20 15 10 5 0 0
3.1.2. Effect on whiteness The results presented in Tables 1 and 2 show that modification with NP-DASI had only little effect on whiteness of the powder coatings studied. The differences in brightness (L*), red/green
5
2
4
6
8
10
12
Distance from the surface [nm] Fig. 5. Changes in Si concentration close to the coating surface determined by XPS for cured epoxy-polyester coating modified with two NP-DASI nanopowders containing silicone resin of different composition.
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polyester coating modified with 3% NP-DASI 289
Si content [%]
15
polyester coating modified with 3% NP-DASI 301
10
5
0
0
2
4
6
8
10
12
Distance from the surface [nm] Fig. 6. Changes in Si concentration close to the coating surface determined by XPS for cured polyester coating modified with two NP-DASI nanopowders containing silicone resin of different composition.
modified with 3% of two different nanopowders (NP-DASI- 295 and NP-DASI-301) are shown. The former NP-DASI nanopowder contained silicone resin that was lacking structural elements originating from MATMS (see Table 1) and therefore was considered to be more hydrophobic and the latter contained silicone resin with structural elements originating from all there silicone monomers and thus considered to be less hydrophobic. It can be noted from Fig. 5 that Si content increased significantly with diminishing distance from the coating surface reaching very high values (close to 20–25%) at ca. 4 nm from the surface and that it was distinctly higher for the coating modified with NP-DASI-295 containing highly hydrophobic silicone resin. However, when polyester coatings modified with 3% of the same NP-DASI nanopowders were examined concentration of silicon on the surface appeared to be much lower – see Fig. 6. This difference can again be explained (as it was explained with regard to gloss in Section 3.1) by difference in silicone resin compatibility with other coating ingredients in case of both types of coatings. 3.2.2. Effect on surface image obtained from AFM The occurrence of silicone resin on the epoxy-polyester coating surface could also be assumed based on comparison of AFM images of the surfaces of unmodified and modified epoxy-polyester coatings. As it can be seen in Fig. 7 those images differ much and some nano-sized structures (tiny white “flakes” of ca. 100–300 nm in size) could be seen on the surface of modified coating.
Taking into account the results of our previous findings showing that the increase in silicone–acrylic nanopowder content in the coating resulted in increase in the number of those tiny particles on the surface [10,12] it can be anticipated that they were very small agglomerates of silicone resin originating from the cores released at high temperature applied in the curing process of the coating from silicone–acrylic core–shell nanoparticles contained in NP-DASI. 3.2.3. Effect on contact angle and SFE The effect of modification of powder coatings investigated in this study with NP-DASI nanopowder on surface properties was also examined by contact angle determinations. The results presented in Tables 1 and 2 clearly show that the contact angle was in general higher for modified coatings. That increase was more distinct for epoxy-polyester coatings than for polyester coatings what confirmed XPS findings. Based on the results of contact angle determinations surface free energy (SFE) was calculated and the SFE values were distinctly lower for coatings modified with NP-DASI nanopowder. Based on contact angle and SFE values presented in Tables 1 and 2 it was also possible to assess the effect of composition of silicone resin contained in NP-DASI on those two parameters characterizing the coating surface. The effect of silicone resin composition on contact angle and on SFE of epoxy-polyester coating modified with 3% NP-DASi nanopowder is presented in Figs. 8 and 9, respectively. It can be concluded from the results presented in Figs. 8 and 9 that inside the area of the designed experiment the contact angle showed a minimum and SFE showed a maximum for roughly the same silicone resin composition. Very high values of contact angle (reaching even 98◦ ) and very low values of SFE (indicating the most hydrophobic surface) were observed for silicone resin structures with very high share of segments originating from D4 and METMS what could be expected taking into account that less MATMSoriginating units in the resin structure makes it more hydrophobic. 3.3. Effect of modification with NP-DASI and of composition of silicone resin contained in NP-DASI on impact resistance, cupping and elasticity of cured coatings 3.3.1. Effect on impact resistance Based on the results presented in Tables 3 and 4 it is clear that modification with 3% NP-DASI nanopowder resulted in great increase in impact resistance of cured coatings (both direct and reverse) and that the observed effect was much more distinct for
Fig. 7. AFM images of the surfaces of unmodified cured epoxy-polyester coating (a) and for cured epoxy-polyster coating modified with 3% NP-DASI nanopowder (b).
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Fig. 8. Effect of composition of silicone resin contained in NP-DASI nanopowder on contact angle (water) of cured epoxy-polyester powder coatings modified with 3% of NP-DASI.
Fig. 9. Effect of composition of silicone resin contained in NP-DASI nanopowder on SFE of cured epoxy-polyester powder coatings modified with 3% of NP-DASI.
7
Fig. 10. Effect of modification with 3% of NP-DASI nanopowder (NP-DASI-293) on impact resistance (direct and reverse) of cured epoxy-polyester coatings and cured polyester coatings.
epoxy-polyester coatings than for polyester coatings. When impact test was made for epoxy-polyester coating in a reverse mode the unmodified coating could not withstand even quite low impact while impact value determined for the modified coating was quite reasonable. That effect is illustrated in Fig. 10, and in Fig. 11 the photos of coating surfaces examined after impact test for unmodified and modified epoxy-polyester coating is presented. As it was explained already earlier in this paper the observed effect can be attributed to the ability of silicone resin that is originating from of the core–shell nanoparticles which are evenly distributed in the coating body to absorb effectively mechanical energy of impact. Similar effect was also reported earlier for modification of powder coatings and plastics with silicone–acrylic and other types of core–shell nanoparticles containing polymers characterized by very low Tg s [3,4,7,8,18–28]. When the Tg s of unmodified epoxy-polyester powder coating particles and the same powder coating particles modified with 3% of NP-DASI were examined by DSC in heat-cool-heat mode (so curing could proceed during heating part of the cycle) it was found that Tg determined for unmodified coating particles was higher than Tg observed for coating particles modified with 3% of NP-DASI nanopowder what means that even so small concentration of silicone resin in the coating formulation resulted in decrease in its Tg (see Fig. 12). More detailed information on thermal properties of
Fig. 11. Photos of coating surfaces of unmodified epoxy-polyester coating (a) and the same coating modified with 3% of NP-DASI-293 nanopowder (b) examined after impact test (reverse).
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Fig. 12. Effect of modification with 3% of NP-DASI nanopowders containing silicone resins of different composition on Tg of particles of epoxy-polyester coatings cured in heat-cool-heat cycle in DSC apparatus. (1) Unmodified. (2) Modified with 3% of NP-DASI-293. (3) Modified with 3% of NP-DASI-291.
coatings modified with NP-DASI can be found in our paper that has recently been published [29]. From the point of view of optimization of NP-DASI nanopowder formulation it was especially interesting to examine the effect of composition of silicone resin contained in NP-DASI nanopowder on impact resistance of powder coatings investigated in this study. In Figs. 13 and 14 that effect is presented for epoxy-polyester coatings tested for direct and reverse impact resistance, respectively. Based on the results presented in Figs. 13 and 14 it can be noted that impact resistance (direct) was generally much higher than impact resistance (reverse) what is the phenomenon usually observed for powder coatings. Moreover, in both cases impact values showed clear minimum at roughly the same silicone resin composition. High impact values corresponded to high share of linear segments originating from D4 in the resin structure what could be expected since that means lower crosslinking density and higher elasticity modulus of the resin that would be reflected in its better ability to relieve mechanical stresses resulting from impact. The results of detailed studies of the effect of structure of silicone part
of silicone-based core–shell impact modifiers on impact resistance of PVC led to similar conclusions [8].
Fig. 13. Effect of composition of silicone resin contained in NP-DASI nanopowder on impact resistance (direct) of cured epoxy-polyester powder coatings modified with 3% of NP-DASI.
Fig. 14. Effect of composition of silicone resin contained in NP-DASI nanopowder on impact resistance (reverse) of cured epoxy-polyester powder coatings modified with 3% of NP-DASI.
3.3.2. Effect on cupping Cupping test is a very important tool used to assess ability of a coating to withstand stresses resulting from mechanical forming of a coated steel sheet in order to obtain the desired shape. As it is shown in Tables 3 and 4 coatings investigated in this study modified with 3% NP-DASI behaved in that test much better than the unmodified coatings. However, unlike it was observed in the case of impact test, the effect of NP-DASI modification was more distinct for polyester coatings. That effect is shown in Fig. 15. Based on the results presented in Tables 3 and 4 it can be also noted that while modification with NP-DASI nanopowder generally resulted in enhanced cupping resistance the level of that increase was very much depending on the composition of silicone resin contained in NP-DASI. In Fig. 16 the effect of the composition of silicone resin contained in NP-DASI on cupping resistance of epoxypolyester coatings modified with 3% NP-DASI is presented.
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3.4. Effect of modification with NP-DASI and of composition of silicone resin contained in NP-DASI on hardness, abrasion resistance, scratch resistance and adhesion of cured coatings
Fig. 15. Effect of modification with 3% of NP-DASI nanopowder (NP-DASI-293) on cupping resistance of cured epoxy-polyester coatings and cured polyester coatings.
Results of the relevant tests presented in Tables 3 and 4 confirmed that modification with 3% NP-DASI nanopowder did not distinctly affect hardness, abrasion resistance, scratch resistance and adhesion of the coatings investigated in this study. Persoz hardness was slightly enhanced for modified polyester coatings and did not change much for modified epoxy-polyester coatings. Pencil hardness seemed to decrease slightly for both types of modified coatings. Abrasion resistance varied depending on the composition of silicone resin contained in NP-DASI, but in general was slightly diminished for epoxy-polyester coatings and slightly enhanced for modified polyester coatings. Slight decrease in abrasion resistance in the case of epoxy polyester coatings can be explained by weakening of surface layers of the coating due to migration of silicone resin to the coating surface. Since in the case of polyester coatings silicone resin content on the surface was found to be much lower (see Section 3.1) decrease in abrasion resistance was not observed. Slight differences between samples tested for scratch resistance were observed, but those can be neglected taking into account quite low accuracy of that test. Adhesion to steel was excellent for both modified and unmodified coatings.
3.5. Effect of modification with NP-DASI and of composition of silicone resin contained in NP-DASI on water and salt fog resistance of cured coatings
Fig. 16. Effect of composition of silicone resin contained in NP-DASI nanopowder on cupping resistance of cured epoxy-polyester powder coatings modified with 3% of NP-DASI.
As it can be seen in Fig. 16, cupping resistance depended mostly on D4 share in the silicone resin structure and decreased significantly with increase in that share. 3.3.3. Effect on elasticity Regarding the results of elasticity test conducted for modified and unmodified coatings it can only be concluded from Tables 3 and 4 that modification with NP-DASI clearly led to improvement of elasticity in case of epoxy-polyester coatings and did not affect elasticity of polyester coatings that was already excellent.
3.5.1. Effect on water resistance Though water resistance of both unmodified coatings and coatings modified with NP-DASI nanopowder was very good (for most of the coatings first visual changes appeared only after more than 14 days of immersion in water), those changes – if any – were more distinct for unmodified coatings. Final assessment of damage of coatings surface resulting from immersion in water was made after only 34 days of immersion followed by 24 h drying. As it can be seen in Tables 3 and 4 for coatings modified with NP-DASI which contained silicone resin of certain composition almost no changes occurred. Better water resistance of coatings modified with NPDASI can be explained by higher hydrophobicity of coating surface caused by silicone resin – see discussion on surface properties of the coatings contained in Section 3.1.
3.5.2. Effect on salt fog resistance Like water resistance test, also salt fog resistance test did not reveal very significant differences between modified and unmodified coatings – in both cases there were only traces of corrosion
Fig. 17. Photos of coating surfaces of unmodified epoxy-polyester coating (a) and the same coating modified with 3% of NP-DASI-293 nanopowder (b) examined after salt fog test.
Please cite this article in press as: J. Kozakiewicz, et al., Studies on silicone–acrylic hybrid aqueous dispersions and corresponding silicone–acrylic nanopowders designed for modification of powder coatings and plastics. Part II – Effect of modification with silicone–acrylic nanopowders and of composition of silicone resin contained in those nanopowders. . . , Prog. Org. Coat. (2014), http://dx.doi.org/10.1016/j.porgcoat.2014.01.010
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ARTICLE IN PRESS J. Kozakiewicz et al. / Progress in Organic Coatings xxx (2014) xxx–xxx
under the coating, though again those traces were more visible for unmodified coatings – see Fig. 17.
and XPS determinations, respectively, and Ms A. Lukomska and Ms S. Kowalska from Industrial Chemistry Research Institute for making SEM pictures is also acknowledged.
4. Conclusions It was confirmed that modification of epoxy-polyester and polyester powder coatings with 3% of nanopowder containing silicone–acrylic core–shell nanoparticles significantly affected the properties of cured coatings. Regarding mechanical properties that effect was generally more distinct for epoxy-polyester coatings than for polyester coatings. Specifically, great increase in impact and cupping resistance was observed for modified coatings and was explained by absorption of mechanical stresses by particles of low modulus silicone resin which were released from core–shell nanoparticles in the process of curing of the coatings. Other mechanical properties of cured modified and unmodified coatings (abrasion resistance, hardness, scratch resistance, elasticity and adhesion to steel) as well as appearance of the coatings (gloss and whiteness) did not differ much. Water resistance and salt fog resistance were slightly better for modified coatings. Testing the surface properties of modified coatings by XPS and AFM revealed the presence of silicone resin on coating surface what was reflected in higher contact angle and lower SFE as compared to unmodified coatings. That phenomenon was explained by migration of silicone resin to the coating surface. Based on the results of a designed experiment it can be stated that silicone resin structure is a very important factor influencing both mechanical and surface-related properties of the cured powder coatings modified with silicone–acrylic nanoparticles containing silicone resin in the core. Optimization of that structure through appropriate design of monomers used for silicone resin synthesis can lead to significant improvement of coating properties, in particular impact resistance. Acknowledgements The authors wish to acknowledge Polish State Center for Research and Development for providing financial support for the studies described in this paper that were conducted under the project N R08-0004 10. Assistance of Prof. R. Nowakowski and Dr J. W. Sobczak from Polish Academy of Sciences for conducting AFM
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Please cite this article in press as: J. Kozakiewicz, et al., Studies on silicone–acrylic hybrid aqueous dispersions and corresponding silicone–acrylic nanopowders designed for modification of powder coatings and plastics. Part II – Effect of modification with silicone–acrylic nanopowders and of composition of silicone resin contained in those nanopowders. . . , Prog. Org. Coat. (2014), http://dx.doi.org/10.1016/j.porgcoat.2014.01.010