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Improving the mechanical properties of waterborne nitrocellulose coating using nano-silica particles
Progress in Organic Coatings 109 (2017) 110–116
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Progress in Organic Coatings journal homepage: www.elsevier.com/locate/porgcoat
Improving the mechanical properties of waterborne nitrocellulose coating using nano-silica particles
MARK
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Fatemeh Fallah, Manouchehr Khorasani , Morteza Ebrahimi Department of Polymer Engineering and Color Technology, Amirkabir University of Technology Tehran Polytechnic, 424 Hafez Ave, Tehran 15875-4413, Iran
A R T I C L E I N F O
A B S T R A C T
Keywords: Nanocomposite Waterborne nitrocellulose Mechanical properties Catastrophic phase inversion Gel emulsification
In this study several nitrocellulose emulsions containing different percentage of nano-silica particles were prepared, and the effect of different content of nano-silica particles on the drop size, morphology and stability of nitrocellulose emulsions as well as optical and mechanical properties of the resultant films was investigated. Incorporation of nano-silica particles up to 2% not only did not adversely affect the properties of nitrocellulose emulsions and appearance of the formed films but also the glass transition temperature (Tg), storage modulus, tensile strength, elongation at break, Young’s modulus and pendulum hardness all increased with increasing nano-silica particles content. High content of nano-silica particles (3% and 4%) surprisingly resulted in emulsion phase inversion from oil in water emulsion (O/W) to water in oil emulsion (W/O). Theses phase inversions were attributed to the presence of the nano-silica particles at the oil-water interface which conducted the large variation in the affinity of stabilizers. Formation of multiple drops prior to phase inversion indicated that catastrophic phase inversion was responsible for these inversions. In comparison with bare nitrocellulose film, at 2% nano-silica particles, the Young’s modulus increased by 91.1%, the tensile strength increased by 46.6%, elongation at break increased by 12.7%, the Tg increased from −7.3 °C to 6.8 °C, and pendulum hardness increased from 31 to 40. The results demonstrated a homogenous dispersion of nano-silica particles in the nitrocellulose resin matrix and strong interfacial interaction of the nano-silica particles with the nitrocellulose resin matrix which caused improvement in mechanical performance of waterborne nitrocellulose coating.
1. Introduction As a film former, nitrocellulose (NC) is used widely in different industries such as leather, food packaging, inks and wood coating due to rapid drying, compatibility with other polymers, good mechanical properties and pigment dispersion and its degradability by microorganisms [1,2]. Moreover, regarding to environmental issues in recent years, the biodegradability of nitrocellulose highlights its great potential as an alternative for non-biodegradable polymers [3–8]. Nowadays, however, using nitrocellulose emulsions (waterborne nitrocellulose) has attracted much attention because of restriction on the use of solvents and the implementation of VOC (volatile organic solvent) directive. In addition, emulsions provide several advantages such as independency of viscosity to molecular weight, non-flammable, very low odor, easy application using conventional equipment, ease of clean up, and improved recoatability [9,10]. However, waterborne systems have some drawbacks, such as poor storage stability, poor mechanical properties and high water sensitivity of films, which have restricted its extensive application [11,12].
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In recent years, enormous efforts have been made to develop new waterborne coating with better performance. Recent findings have shown that incorporation nano-particles can play a significant role in the modification of waterborne coating [13–18]. Various nano-particles, such as SiO2, TiO2, ZnO, Fe2O3 and Al2O3 are the examples of the nano-particles which have been used in the coatings [18]. It has been found that nano-silica particles can significantly enhance the performance of waterborne coating from the stand point of mechanical strength and durability. Ahmad Dashtizadeh et al. studied the mechanical and optical properties of acrylic based waterborne coatings containing nano-silica particles [19,20]. It was shown that addition of nano-silica particles caused an improvement in the hardness and solvent resistance of acrylic based waterborne coating. Limin Cheng et al. reported that waterborne polyurethane/nanosilica composites exhibited a remarkable improvement in the mechanical strength, thermostability, water resistance, and UV absorbance compared with conventional waterborne polyurethane coatings [21]. Dongmei Wu et al. modified mechanical performance of conventional waterborne polyurethane by adding nano-silica particles [22]. Zhaofeng Wu et al.
Corresponding author. E-mail addresses: [email protected] (F. Fallah), [email protected] (M. Khorasani), [email protected] (M. Ebrahimi).
http://dx.doi.org/10.1016/j.porgcoat.2017.04.016 Received 18 May 2016; Received in revised form 25 March 2017; Accepted 13 April 2017 0300-9440/ © 2017 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/BY/4.0/).
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master sizer 2000 with range of 0.02 μm − 2000 μm. The mean drop size was calculated from the particle size distribution. The specimens were dissolved in a 0.6 wt% SDS (sodium dodecyl sulfate) solution before measurement to reduce drop concentration and stabilize the dilute emulsion. The mean volume-surface diameter, d32, defined as:
showed that both water resistance and mechanical properties of hybrid films of waterborne polyurethane and fluorinated polymethacrylate were improved significantly in the presence of the nano-silica particles [23]. To the best of our knowledge, there is no systematic research concerning the mechanical properties of waterborne nitrocellulose/ silica composites. In the previous study, stable waterborne nitrocellulose (with nano size drops) was successfully prepared using gel emulsification method [24]. The aim of this work is studying the effects of nano-silica particles on mechanical and optical properties of waterborne nitrocellulose clear coats. In this regard, nanocomposites were prepared using various loadings of nano-silica particles. First, we investigated the effect of nano-silica particles content on the drop size and stability of nitrocellulose emulsions and then morphological and mechanical properties of the resultant films.
d32 = (∑ Ni di3)/(∑Ni di2)
(1)
where Ni is the number of drops with diameter di. 2.4.2. Stability Final emulsions were poured into measuring cylinders and were stored in oven at 40° C (to accelerate breakdown process). Stability of emulsions was assessed by visual observing samples as a function of time. The release of the oil phase was used as an indicator of the end of the stability. The period of stability (hours or days) was considered as a criterion to measure emulsion stability.
2. Materials and methods
2.4.3. Emulsion type Emulsion type was determined by the conductivity of the emulsions which was determined using a digital conductivity meter and also by observing what happened when a drop of each emulsion was added to a volume of either pure oil or pure water. Oil in water (O/W) emulsions dispersed in water and remained as drops in oil, while water in oil (W/ O) emulsions dispersed in oil and remained as drops in water [25].
2.1. Materials Nitrocellulose flake containing 11 ± 0.5% of nitrogen provided by Parchin Co. n-butyl acetate (products of Merck) was used as the solvent of nitrocellulose. The nonionic surfactant NP-40 (Nonylphenol Polyethylene Glycol Ether 6) used here was chemical grade (product of Kimyagaran Emrooz chemical IND.CO. Iran). Hydrophobic nanosilica particles (Aerosil R 972) having a specific surface area of 90–130 m2/g and particle diameter of 16 nm were supplied by Degussa Co.
2.5. Preparation of nanocomposite films Portions of formed emulsions were applied onto the surface of Preplex plates. Films were dried under controlled conditions (25 ° C and 50% relative humidity), and dried films were peeled and stored at 25 ° C and 50% relative humidity. The thickness of the free films was randomly measured at three different locations using a micrometer (Mitutoyo, Tokyo,Japan). All films approximately had the same thickness (0.4 ± 0.05 mm).
2.2. Preparation of nitrocellulose/silica composites (dispersed phase) i To prepare nitrocellulose solution, 50 g nitrocellulose was soaked in 115 ml n-butyl acetate for 24 h and then they were mixed and stirred thoroughly at speed of 2000 rpm ii To prepare the pre-dispersed nano-silica particles (dispersed in nbutyl acetate solvent), different weight contents (1%, 2%, 3% and 4% w/w nitrocellulose) of hydrophobic nano-silica particles were dispersed in 115 ml n-butyl acetate and then homogenized by ultrasonic (Hielscher, UP400S model), operating at 45 kHz, for 20 min iii The pre-dispersed nano-silica particles (dispersed in n-butyl acetate solvent) were added to the nitrocellulose solution and then were mixed by conventional mixer at 2000 rpm for 60 min. This represented dispersed phase.
2.6. Characterization of nanocomposite films 2.6.1. Morphological characterization of the films The quality of nano-silica dispersion in the nitrocellulose coating matrix was studied by SEM microscope (LEO 1455VP). The fracture surface of the samples was sputter coated by gold prior to analysis. 2.6.2. Thermal characterization of the films Tg and storage modulus of the nanocomposite films were determined by dynamic mechanical thermal analyzer (Tritec 2000). Samples were made by cutting films into strips about 1.5–2 cm long and 7.5–8 mm wide. Samples were tested in tension while being heated from −20 to 100° C at frequency of 1 Hz and heating rate of 5 ° C/min according to ASTM E1640(2002).
2.3. Preparation of waterborne nitrocellulose/silica composites In this study gel emulsification method (as an energy efficient method) was used to prepare waterborne nitrocellulose [24]. This approach encompasses two steps; in the first step, 280 ml of dispersed phase was added to the small fraction of aqueous phase (31 ml water containing NP-40 (6% w/w of the whole water)) under continuous stirring to make concentrated emulsion with high dispersed phase volume fraction (Ф = 90). The concentrate emulsion was then diluted with 89 ml water containing no surfactant in the second step to make target emulsion with Ф = 70. Emulsions were made in 1-lit glass reactor (10 cm in diameter). A mechanical mixer was used to rotate 4-blade impeller, 5 cm in diameter at 2000 rpm during emulsification and the addition rate of dispersed phase to the continuous phase was 5 ml/min. Emulsification was carried out at 25 ± 2 ° C. Fig. 1 shows schematic representation of emulsification process.
2.6.3. Tensile test Tensile tests were performed using an Instron dynamometer instrument at 25 ° C and 50% relative humidity according to ASTM D882-02 (2002) on free films, having length and width of 120 mm, 25 mm respectively, at a crosshead speed of 2 mm/min. 2.6.4. Hardness The hardness of surfaces on the glass plate was tested with a BYKGardner GMBH pendulum hardness tester according to ASTM-D 4366 (2002) standard as follows: the panel table was moved to its limit stop. The pendulum deflected through 6∘ and locked in its position. Then it was released and the number of oscillations identifies the damping time of surfaces by changing to the second unit. The damping time required to slow the oscillation down from 6∘ to 3∘ by using the adjusting glass plate, should be 250 ± 10s.
2.4. Stability and characterization of emulsions 2.4.1. Drop size measurement Drop size was measured using laser diffraction method by Malvern 111
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Fig. 1. Schematic representation of emulsification process.
Fig. 2. Variation in conductivity of emulsions containing 3 and 4% nano-silica particles as a function of dispersed phase volume fraction (Ф).
Fig. 4. Simplified-Phase map for the systems containing different nano-silica particles during the first step of gel emulsification method.
Fig. 5. Variation of drop size (μm) as a function of nano-silica particles content.
Fig. 6. Stability of emulsions as a function of nano-silica particles content.
Fig. 3. Emulsion morphology prior to inversion for the system containing a) 3% nanosilica particles b) 4% nano-silica particles.
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Fig. 7. FE-SEM micrograph of the cross section of (a) the bare nitrocellulose film, and (b) the 1% and (c) 2% hydrophobic nano-silica particles incorporated nitrocellulose films.
Fig. 8. Tan Delta as a function of temperature for the 0% (the bare nitrocellulose film) and the 1%, and 2% hydrophobic nano-silica particles incorporated nitrocellulose films.
Fig. 10. Stress-strain curves for the 0% (the bare nitrocellulose film) and the 1%, and 2% hydrophobic nano-silica particles incorporated nitrocellulose films. Table 1 Mechanical properties of the 0% (the bare nitrocellulose film) and the 1%, and 2% hydrophobic nano-silica particles incorporated nitrocellulose films. Nano-silica particles (wt%)
Tensile strength (MPa)
Elongation at break (%)
Young’s modulus (MPa)
0 1 2
3.15 ± 0.43 4.30 ± 0.16 4.62 ± 0.21
68.20 ± 1.1 71.82 ± 1.8 76.90 ± 2.9
0.45 ± 0.08 0.71 ± 0.13 0.86 ± 0.09
2.6.5. Gloss The gloss of films were measured by means of a portable Braive Super Gloss meter instrument at a specular angle of 60° and 85° according to ASTM-D 2457 (2002) standard.
Fig. 9. Storage modulus as a function of temperature for the 0% (the bare nitrocellulose film) and the 1%, and 2% hydrophobic nano-silica particles incorporated nitrocellulose films.
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Sajjadi has reported that, the type of formulation variable cannot determine the mechanism of phase inversion, so the features of the two types of inversions should be considered to identify the mechanism of inversion [33]. According to previous studies, formation of multiple drops and the increase of drop size as a result of further inclusion of the continuous phase into the dispersed drops prior to phase inversion can be considered as the features of catastrophic phase inversion [33–35]. Figs. 3a and b show emulsion morphology prior to inversion for the systems containing 3 and 4% nano-silica particles respectively. Formation of multiple W/O/W drops in a pre-inversion region implies that, the inversion mechanism is catastrophic. In order to better understand the inversion mechanism, we have developed the phase map for different routs (Fig. 4). The transitional inversion line, where the hydrophilicity and hydrophobicity of the stabilizers are balanced, is shown by a solid line and catastrophic inversion line by the dashed line. The inversion points are shown as black dots and the concentrate emulsions as white points on the phase map. As it is illustrated in Fig. 4, catastrophic phase inversion delayed in terms of oil volume fraction with decreasing nano-silica particles concentration, which can be attributed to the weakened inclusion of the water phase by oil drops with decreasing nano-silica particles content. Since the oil in water (O/ W) emulsion is desirable in this study, more emphasis is laid upon the systems containing 0%, 1% and 2% nano-silica particles in further study. Fig. 5 shows the effect of nano-silica particles’ content on the drop size. According to this figure, increasing the amount of nano-silica particles up to 2% did not lead to significant variations in the drop size. In addition, polydispersity Index of emulsions was not strongly affected by incorporation of nano-silica particles, PDI ≈ 1.2 (data not shown). As expected, the stability of emulsions followed the same trend as that of the drop size (Fig. 6). In fact incorporation of nano-silica particles had no effect on the stability of resulting emulsions. These results are important because they imply it is possible to incorporate nano-silica particles (up to 2%) without causing extreme side effects on the drop size and emulsion stability.
Fig. 11. Variation of gloss reflection as a function of nano-silica particles content.
Fig. 12. Variation of pendulum hardness as a function of nano-silica particles content.
2.7. Statistical analysis All experiments were repeated for at least three times using freshly prepared samples. Means and standard deviations were calculated from measurements using Microsoft Office Excel. 3. Results and discussions 3.1. Effects of various contents of nano-silica particles on the emulsions’ properties
3.2. Morphological characterization In practical application the shelf life of emulsion products is very important. Due to this reason the effect of nano-silica particles’ contents on the drop size and storage stability of nano-emulsions produced by gel emulsification method was initially investigated. For the systems containing 3 and 4% nano-silica particles, phase inversion from oil in water emulsion (O/W) to water in oil emulsion (W/O) was surprisingly observed in the first step of emulsification process at Ф = 70 and Ф = 60 respectively (before reaching concentrate emulsion Ф = 90). Phase inversion was detected by a jump in emulsion conductivity; Fig. 2 shows the variations of emulsions’ conductivity with dispersed phase volume fraction; moreover these emulsions could not be diluted with water. The origin of the phase inversion can be explained by considering the theories about Pickering emulsions which are stabilized by solid particles instead of surfactants. According to these theories, solid particles can act as stabilizer and relative affinity of solid particles for aqueous and organic phases determines the type of the emulsions, so that O/W (W/O) emulsions are produced with hydrophilic (hydrophobic) solid particles [25–30]. It is notable that in the present system the phase inversion from O/W emulsion to W/O emulsion occurred when a silica concentration higher than 2% was used. These inversions can be attributed to the large variation in the affinity of stabilizers as a result of increasing nano-silica particles content which enhanced the presence of hydrophobic nano-silica particles at the oil-water interface. This phase inversion represented catastrophic phase inversion. Catastrophic phase inversion (CPI) has been attributed to an increase in the rate of drop coalescence so that the balance between the rate of drop coalescence and drop break-up can no longer be maintained (as opposed to transitional phase inversion (TPI), which has been attributed to any large variations in the stabilizers affinity for the two phases) [31–33].
The SEM images of the 0% (the bare nitrocellulose film), 1%, and 2% nano-silica particles incorporated nitrocellulose films are shown in Figs. 7a to 7c respectively. It is clear from Fig. 7 that nano-silica particles were dispersed in the bulk of the nitrocellulose films uniformly. The average size of the particles is less than 100 nm indicating the good compatibility and dispersibility of nano-silica particles in the nitrocellulose resin matrix. 3.3. Thermal characterization The evolution of Tan Delta as a function of temperature for the 0% (the bare nitrocellulose film), the 1%, and 2% hydrophobic nano-silica particles incorporated nitrocellulose films are shown in Fig. 8. As seen in Fig. 8 Tan Delta curves possess one peak of transition for all samples which indicates the presence of a single phase in these nanocomposite films [20,36]. The maximum of Tan Delta curves shift to the higher temperature upon increasing hydrophobic nano-silica particles’ content. As illustrate in Fig. 8, Tg of bare nitrocellulose film is −7.3 ± 0.4 which increases to −0.1 ± 0.3 and 6.8 ± 0.5 for the system containing 1%, and 2% hydrophobic nano-silica particles, respectively. This means that Tg of samples increases systematically by increasing the percentage of hydrophobic nano-silica particles owning to the stronger interfacial interaction of hydrophobic nano-silica particles with the nitrocellulose resin matrix which changes the polymer chain mobility [37–39]. Moreover, as seen in Fig. 8, the value of Tan Delta decreases as the amount of hydrophobic nano-silica particles increases, this means that the mechanical performance of the films has improved by increasing nano-silica particles’ content [21]. This could be explained 114
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silica particles up to 2% did not adversely affect the emulsions stability, but higher content of nano-silica particles (3% and 4%) resulted in catastrophic phase inversion from O/W emulsion to W/O emulsion which was not preferred type. On the other hand, incorporation of nano-silica particles (up to 2%) significantly improved the mechanical properties of the resultant films. The Tg, storage modulus, tensile strength, elongation at break, Young’s modulus and pendulum hardness all increased with increasing nano-silica particles’ content, indicating good reinforcement of the waterborne nitrocellulose coating. The results present excellent potential of nano-silica particles for reinforcing weak mechanical properties of waterborne coatings.
by the strong interaction of hydrophobic nano-silica particles with the nitrocellulose resin matrix which caused more efficient stress transfer across the interface [21]. According to Fig. 9, storage modulus increases with increasing nano-silica particles’ content. These trends in storage modulus suggest the strong interfacial interaction of nano-silica particles with the nitrocellulose resin matrix which causes more efficient stress transfer across the interface. 3.4. Mechanical characterization Fig. 10 shows stress-strain curves for the bare nitrocellulose film and the films containing 1%, and 2% nano-silica particles. A significant increase in tensile strength and elongation at break and Young’s modulus as compared to the bare nitrocellulose film are obtained due to incorporation of hydrophobic nano-silica particles (Table 1). The best mechanical properties (maximum increase in tensile strength (46.6%) and elongation at break (12.7%) and Young’s modulus (91.1%)) are acquired for the film containing 2% nano-silica particles. This can be explained by fine dispersion of hydrophobic nano-silica particles and a strong adhesion and interaction between the nano-silica particles and the nitrocellulose resin matrix which results in greater possibility of the applied stress transfer from nitrocellulose resin matrix to the rigid nano-silica particles [21,36]. Mechanical properties would likely be improved in case higher nano-silica particles incorporation ( > 2%) did not lead to phase inversion, (because of the trend of improvement until hydrophobic nano-silica particles’ content reaches by 2%).
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3.5. Gloss evaluation of the films Nitrocellulose is widely used in leather and coating industries in which the glossiness of the final films is important. As the gloss is considered as an important aspect of visual perception, we examined the effect of nano-silica particles incorporation on the gloss of the films. As seen in Fig. 11 all the samples are semi gloss and the gloss of the films in both the 60 ° and 85 ° angels slightly decreased by increasing nano-silica particles’ content. This can be attributed to the presence of the nano-silica particles on the film surfaces that reduces the gloss by reducing the smoothness of surfaces and so the optical uniformity of films was reduced [19]. In fact an increase of nano-silica content leads to lower gloss because of the presence of a larger amount of nano-silica particles on the surface. 3.6. Hardness evaluation of the films Increasing surface hardness results in improvement of the abrasion and scratch resistance [20,21,40] which are considered as the important concerns in leather and coating industries. The variation in surface hardness (after 1 and 3 days) with nano-silica particles content could be observed in Fig. 12. As seen in Fig. 12, as the percentage of nano-silica particles increases, the surface hardness of the samples increases. The increased hardness can be attributed to the increase of Tg of the films [22]. It should be noted that the presence of nano-silica particles at surface also improves the surface hardness [20,22], but as mentioned before in the system under study the migration of nano-silica particles from the matrix to the surface of the film is not considerable due to strong interactions between nano-silica particles and nitrocellulose matrix resin. 4. Conclusion In this study a waterborne nitrocellulose/silica composites were prepared through emulsification of nitrocellulose polymer containing different percentage of nano-silica particles. The effect of nano-silica particles content on the properties of formed emulsions and mechanical properties of the resultant films was examined. Incorporation of nano115
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Progress in Organic Coatings journal homepage: www.elsevier.com/locate/porgcoat
Improving the mechanical properties of waterborne nitrocellulose coating using nano-silica particles
MARK
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Fatemeh Fallah, Manouchehr Khorasani , Morteza Ebrahimi Department of Polymer Engineering and Color Technology, Amirkabir University of Technology Tehran Polytechnic, 424 Hafez Ave, Tehran 15875-4413, Iran
A R T I C L E I N F O
A B S T R A C T
Keywords: Nanocomposite Waterborne nitrocellulose Mechanical properties Catastrophic phase inversion Gel emulsification
In this study several nitrocellulose emulsions containing different percentage of nano-silica particles were prepared, and the effect of different content of nano-silica particles on the drop size, morphology and stability of nitrocellulose emulsions as well as optical and mechanical properties of the resultant films was investigated. Incorporation of nano-silica particles up to 2% not only did not adversely affect the properties of nitrocellulose emulsions and appearance of the formed films but also the glass transition temperature (Tg), storage modulus, tensile strength, elongation at break, Young’s modulus and pendulum hardness all increased with increasing nano-silica particles content. High content of nano-silica particles (3% and 4%) surprisingly resulted in emulsion phase inversion from oil in water emulsion (O/W) to water in oil emulsion (W/O). Theses phase inversions were attributed to the presence of the nano-silica particles at the oil-water interface which conducted the large variation in the affinity of stabilizers. Formation of multiple drops prior to phase inversion indicated that catastrophic phase inversion was responsible for these inversions. In comparison with bare nitrocellulose film, at 2% nano-silica particles, the Young’s modulus increased by 91.1%, the tensile strength increased by 46.6%, elongation at break increased by 12.7%, the Tg increased from −7.3 °C to 6.8 °C, and pendulum hardness increased from 31 to 40. The results demonstrated a homogenous dispersion of nano-silica particles in the nitrocellulose resin matrix and strong interfacial interaction of the nano-silica particles with the nitrocellulose resin matrix which caused improvement in mechanical performance of waterborne nitrocellulose coating.
1. Introduction As a film former, nitrocellulose (NC) is used widely in different industries such as leather, food packaging, inks and wood coating due to rapid drying, compatibility with other polymers, good mechanical properties and pigment dispersion and its degradability by microorganisms [1,2]. Moreover, regarding to environmental issues in recent years, the biodegradability of nitrocellulose highlights its great potential as an alternative for non-biodegradable polymers [3–8]. Nowadays, however, using nitrocellulose emulsions (waterborne nitrocellulose) has attracted much attention because of restriction on the use of solvents and the implementation of VOC (volatile organic solvent) directive. In addition, emulsions provide several advantages such as independency of viscosity to molecular weight, non-flammable, very low odor, easy application using conventional equipment, ease of clean up, and improved recoatability [9,10]. However, waterborne systems have some drawbacks, such as poor storage stability, poor mechanical properties and high water sensitivity of films, which have restricted its extensive application [11,12].
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In recent years, enormous efforts have been made to develop new waterborne coating with better performance. Recent findings have shown that incorporation nano-particles can play a significant role in the modification of waterborne coating [13–18]. Various nano-particles, such as SiO2, TiO2, ZnO, Fe2O3 and Al2O3 are the examples of the nano-particles which have been used in the coatings [18]. It has been found that nano-silica particles can significantly enhance the performance of waterborne coating from the stand point of mechanical strength and durability. Ahmad Dashtizadeh et al. studied the mechanical and optical properties of acrylic based waterborne coatings containing nano-silica particles [19,20]. It was shown that addition of nano-silica particles caused an improvement in the hardness and solvent resistance of acrylic based waterborne coating. Limin Cheng et al. reported that waterborne polyurethane/nanosilica composites exhibited a remarkable improvement in the mechanical strength, thermostability, water resistance, and UV absorbance compared with conventional waterborne polyurethane coatings [21]. Dongmei Wu et al. modified mechanical performance of conventional waterborne polyurethane by adding nano-silica particles [22]. Zhaofeng Wu et al.
Corresponding author. E-mail addresses: [email protected] (F. Fallah), [email protected] (M. Khorasani), [email protected] (M. Ebrahimi).
http://dx.doi.org/10.1016/j.porgcoat.2017.04.016 Received 18 May 2016; Received in revised form 25 March 2017; Accepted 13 April 2017 0300-9440/ © 2017 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/BY/4.0/).
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master sizer 2000 with range of 0.02 μm − 2000 μm. The mean drop size was calculated from the particle size distribution. The specimens were dissolved in a 0.6 wt% SDS (sodium dodecyl sulfate) solution before measurement to reduce drop concentration and stabilize the dilute emulsion. The mean volume-surface diameter, d32, defined as:
showed that both water resistance and mechanical properties of hybrid films of waterborne polyurethane and fluorinated polymethacrylate were improved significantly in the presence of the nano-silica particles [23]. To the best of our knowledge, there is no systematic research concerning the mechanical properties of waterborne nitrocellulose/ silica composites. In the previous study, stable waterborne nitrocellulose (with nano size drops) was successfully prepared using gel emulsification method [24]. The aim of this work is studying the effects of nano-silica particles on mechanical and optical properties of waterborne nitrocellulose clear coats. In this regard, nanocomposites were prepared using various loadings of nano-silica particles. First, we investigated the effect of nano-silica particles content on the drop size and stability of nitrocellulose emulsions and then morphological and mechanical properties of the resultant films.
d32 = (∑ Ni di3)/(∑Ni di2)
(1)
where Ni is the number of drops with diameter di. 2.4.2. Stability Final emulsions were poured into measuring cylinders and were stored in oven at 40° C (to accelerate breakdown process). Stability of emulsions was assessed by visual observing samples as a function of time. The release of the oil phase was used as an indicator of the end of the stability. The period of stability (hours or days) was considered as a criterion to measure emulsion stability.
2. Materials and methods
2.4.3. Emulsion type Emulsion type was determined by the conductivity of the emulsions which was determined using a digital conductivity meter and also by observing what happened when a drop of each emulsion was added to a volume of either pure oil or pure water. Oil in water (O/W) emulsions dispersed in water and remained as drops in oil, while water in oil (W/ O) emulsions dispersed in oil and remained as drops in water [25].
2.1. Materials Nitrocellulose flake containing 11 ± 0.5% of nitrogen provided by Parchin Co. n-butyl acetate (products of Merck) was used as the solvent of nitrocellulose. The nonionic surfactant NP-40 (Nonylphenol Polyethylene Glycol Ether 6) used here was chemical grade (product of Kimyagaran Emrooz chemical IND.CO. Iran). Hydrophobic nanosilica particles (Aerosil R 972) having a specific surface area of 90–130 m2/g and particle diameter of 16 nm were supplied by Degussa Co.
2.5. Preparation of nanocomposite films Portions of formed emulsions were applied onto the surface of Preplex plates. Films were dried under controlled conditions (25 ° C and 50% relative humidity), and dried films were peeled and stored at 25 ° C and 50% relative humidity. The thickness of the free films was randomly measured at three different locations using a micrometer (Mitutoyo, Tokyo,Japan). All films approximately had the same thickness (0.4 ± 0.05 mm).
2.2. Preparation of nitrocellulose/silica composites (dispersed phase) i To prepare nitrocellulose solution, 50 g nitrocellulose was soaked in 115 ml n-butyl acetate for 24 h and then they were mixed and stirred thoroughly at speed of 2000 rpm ii To prepare the pre-dispersed nano-silica particles (dispersed in nbutyl acetate solvent), different weight contents (1%, 2%, 3% and 4% w/w nitrocellulose) of hydrophobic nano-silica particles were dispersed in 115 ml n-butyl acetate and then homogenized by ultrasonic (Hielscher, UP400S model), operating at 45 kHz, for 20 min iii The pre-dispersed nano-silica particles (dispersed in n-butyl acetate solvent) were added to the nitrocellulose solution and then were mixed by conventional mixer at 2000 rpm for 60 min. This represented dispersed phase.
2.6. Characterization of nanocomposite films 2.6.1. Morphological characterization of the films The quality of nano-silica dispersion in the nitrocellulose coating matrix was studied by SEM microscope (LEO 1455VP). The fracture surface of the samples was sputter coated by gold prior to analysis. 2.6.2. Thermal characterization of the films Tg and storage modulus of the nanocomposite films were determined by dynamic mechanical thermal analyzer (Tritec 2000). Samples were made by cutting films into strips about 1.5–2 cm long and 7.5–8 mm wide. Samples were tested in tension while being heated from −20 to 100° C at frequency of 1 Hz and heating rate of 5 ° C/min according to ASTM E1640(2002).
2.3. Preparation of waterborne nitrocellulose/silica composites In this study gel emulsification method (as an energy efficient method) was used to prepare waterborne nitrocellulose [24]. This approach encompasses two steps; in the first step, 280 ml of dispersed phase was added to the small fraction of aqueous phase (31 ml water containing NP-40 (6% w/w of the whole water)) under continuous stirring to make concentrated emulsion with high dispersed phase volume fraction (Ф = 90). The concentrate emulsion was then diluted with 89 ml water containing no surfactant in the second step to make target emulsion with Ф = 70. Emulsions were made in 1-lit glass reactor (10 cm in diameter). A mechanical mixer was used to rotate 4-blade impeller, 5 cm in diameter at 2000 rpm during emulsification and the addition rate of dispersed phase to the continuous phase was 5 ml/min. Emulsification was carried out at 25 ± 2 ° C. Fig. 1 shows schematic representation of emulsification process.
2.6.3. Tensile test Tensile tests were performed using an Instron dynamometer instrument at 25 ° C and 50% relative humidity according to ASTM D882-02 (2002) on free films, having length and width of 120 mm, 25 mm respectively, at a crosshead speed of 2 mm/min. 2.6.4. Hardness The hardness of surfaces on the glass plate was tested with a BYKGardner GMBH pendulum hardness tester according to ASTM-D 4366 (2002) standard as follows: the panel table was moved to its limit stop. The pendulum deflected through 6∘ and locked in its position. Then it was released and the number of oscillations identifies the damping time of surfaces by changing to the second unit. The damping time required to slow the oscillation down from 6∘ to 3∘ by using the adjusting glass plate, should be 250 ± 10s.
2.4. Stability and characterization of emulsions 2.4.1. Drop size measurement Drop size was measured using laser diffraction method by Malvern 111
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Fig. 1. Schematic representation of emulsification process.
Fig. 2. Variation in conductivity of emulsions containing 3 and 4% nano-silica particles as a function of dispersed phase volume fraction (Ф).
Fig. 4. Simplified-Phase map for the systems containing different nano-silica particles during the first step of gel emulsification method.
Fig. 5. Variation of drop size (μm) as a function of nano-silica particles content.
Fig. 6. Stability of emulsions as a function of nano-silica particles content.
Fig. 3. Emulsion morphology prior to inversion for the system containing a) 3% nanosilica particles b) 4% nano-silica particles.
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Fig. 7. FE-SEM micrograph of the cross section of (a) the bare nitrocellulose film, and (b) the 1% and (c) 2% hydrophobic nano-silica particles incorporated nitrocellulose films.
Fig. 8. Tan Delta as a function of temperature for the 0% (the bare nitrocellulose film) and the 1%, and 2% hydrophobic nano-silica particles incorporated nitrocellulose films.
Fig. 10. Stress-strain curves for the 0% (the bare nitrocellulose film) and the 1%, and 2% hydrophobic nano-silica particles incorporated nitrocellulose films. Table 1 Mechanical properties of the 0% (the bare nitrocellulose film) and the 1%, and 2% hydrophobic nano-silica particles incorporated nitrocellulose films. Nano-silica particles (wt%)
Tensile strength (MPa)
Elongation at break (%)
Young’s modulus (MPa)
0 1 2
3.15 ± 0.43 4.30 ± 0.16 4.62 ± 0.21
68.20 ± 1.1 71.82 ± 1.8 76.90 ± 2.9
0.45 ± 0.08 0.71 ± 0.13 0.86 ± 0.09
2.6.5. Gloss The gloss of films were measured by means of a portable Braive Super Gloss meter instrument at a specular angle of 60° and 85° according to ASTM-D 2457 (2002) standard.
Fig. 9. Storage modulus as a function of temperature for the 0% (the bare nitrocellulose film) and the 1%, and 2% hydrophobic nano-silica particles incorporated nitrocellulose films.
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Sajjadi has reported that, the type of formulation variable cannot determine the mechanism of phase inversion, so the features of the two types of inversions should be considered to identify the mechanism of inversion [33]. According to previous studies, formation of multiple drops and the increase of drop size as a result of further inclusion of the continuous phase into the dispersed drops prior to phase inversion can be considered as the features of catastrophic phase inversion [33–35]. Figs. 3a and b show emulsion morphology prior to inversion for the systems containing 3 and 4% nano-silica particles respectively. Formation of multiple W/O/W drops in a pre-inversion region implies that, the inversion mechanism is catastrophic. In order to better understand the inversion mechanism, we have developed the phase map for different routs (Fig. 4). The transitional inversion line, where the hydrophilicity and hydrophobicity of the stabilizers are balanced, is shown by a solid line and catastrophic inversion line by the dashed line. The inversion points are shown as black dots and the concentrate emulsions as white points on the phase map. As it is illustrated in Fig. 4, catastrophic phase inversion delayed in terms of oil volume fraction with decreasing nano-silica particles concentration, which can be attributed to the weakened inclusion of the water phase by oil drops with decreasing nano-silica particles content. Since the oil in water (O/ W) emulsion is desirable in this study, more emphasis is laid upon the systems containing 0%, 1% and 2% nano-silica particles in further study. Fig. 5 shows the effect of nano-silica particles’ content on the drop size. According to this figure, increasing the amount of nano-silica particles up to 2% did not lead to significant variations in the drop size. In addition, polydispersity Index of emulsions was not strongly affected by incorporation of nano-silica particles, PDI ≈ 1.2 (data not shown). As expected, the stability of emulsions followed the same trend as that of the drop size (Fig. 6). In fact incorporation of nano-silica particles had no effect on the stability of resulting emulsions. These results are important because they imply it is possible to incorporate nano-silica particles (up to 2%) without causing extreme side effects on the drop size and emulsion stability.
Fig. 11. Variation of gloss reflection as a function of nano-silica particles content.
Fig. 12. Variation of pendulum hardness as a function of nano-silica particles content.
2.7. Statistical analysis All experiments were repeated for at least three times using freshly prepared samples. Means and standard deviations were calculated from measurements using Microsoft Office Excel. 3. Results and discussions 3.1. Effects of various contents of nano-silica particles on the emulsions’ properties
3.2. Morphological characterization In practical application the shelf life of emulsion products is very important. Due to this reason the effect of nano-silica particles’ contents on the drop size and storage stability of nano-emulsions produced by gel emulsification method was initially investigated. For the systems containing 3 and 4% nano-silica particles, phase inversion from oil in water emulsion (O/W) to water in oil emulsion (W/O) was surprisingly observed in the first step of emulsification process at Ф = 70 and Ф = 60 respectively (before reaching concentrate emulsion Ф = 90). Phase inversion was detected by a jump in emulsion conductivity; Fig. 2 shows the variations of emulsions’ conductivity with dispersed phase volume fraction; moreover these emulsions could not be diluted with water. The origin of the phase inversion can be explained by considering the theories about Pickering emulsions which are stabilized by solid particles instead of surfactants. According to these theories, solid particles can act as stabilizer and relative affinity of solid particles for aqueous and organic phases determines the type of the emulsions, so that O/W (W/O) emulsions are produced with hydrophilic (hydrophobic) solid particles [25–30]. It is notable that in the present system the phase inversion from O/W emulsion to W/O emulsion occurred when a silica concentration higher than 2% was used. These inversions can be attributed to the large variation in the affinity of stabilizers as a result of increasing nano-silica particles content which enhanced the presence of hydrophobic nano-silica particles at the oil-water interface. This phase inversion represented catastrophic phase inversion. Catastrophic phase inversion (CPI) has been attributed to an increase in the rate of drop coalescence so that the balance between the rate of drop coalescence and drop break-up can no longer be maintained (as opposed to transitional phase inversion (TPI), which has been attributed to any large variations in the stabilizers affinity for the two phases) [31–33].
The SEM images of the 0% (the bare nitrocellulose film), 1%, and 2% nano-silica particles incorporated nitrocellulose films are shown in Figs. 7a to 7c respectively. It is clear from Fig. 7 that nano-silica particles were dispersed in the bulk of the nitrocellulose films uniformly. The average size of the particles is less than 100 nm indicating the good compatibility and dispersibility of nano-silica particles in the nitrocellulose resin matrix. 3.3. Thermal characterization The evolution of Tan Delta as a function of temperature for the 0% (the bare nitrocellulose film), the 1%, and 2% hydrophobic nano-silica particles incorporated nitrocellulose films are shown in Fig. 8. As seen in Fig. 8 Tan Delta curves possess one peak of transition for all samples which indicates the presence of a single phase in these nanocomposite films [20,36]. The maximum of Tan Delta curves shift to the higher temperature upon increasing hydrophobic nano-silica particles’ content. As illustrate in Fig. 8, Tg of bare nitrocellulose film is −7.3 ± 0.4 which increases to −0.1 ± 0.3 and 6.8 ± 0.5 for the system containing 1%, and 2% hydrophobic nano-silica particles, respectively. This means that Tg of samples increases systematically by increasing the percentage of hydrophobic nano-silica particles owning to the stronger interfacial interaction of hydrophobic nano-silica particles with the nitrocellulose resin matrix which changes the polymer chain mobility [37–39]. Moreover, as seen in Fig. 8, the value of Tan Delta decreases as the amount of hydrophobic nano-silica particles increases, this means that the mechanical performance of the films has improved by increasing nano-silica particles’ content [21]. This could be explained 114
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silica particles up to 2% did not adversely affect the emulsions stability, but higher content of nano-silica particles (3% and 4%) resulted in catastrophic phase inversion from O/W emulsion to W/O emulsion which was not preferred type. On the other hand, incorporation of nano-silica particles (up to 2%) significantly improved the mechanical properties of the resultant films. The Tg, storage modulus, tensile strength, elongation at break, Young’s modulus and pendulum hardness all increased with increasing nano-silica particles’ content, indicating good reinforcement of the waterborne nitrocellulose coating. The results present excellent potential of nano-silica particles for reinforcing weak mechanical properties of waterborne coatings.
by the strong interaction of hydrophobic nano-silica particles with the nitrocellulose resin matrix which caused more efficient stress transfer across the interface [21]. According to Fig. 9, storage modulus increases with increasing nano-silica particles’ content. These trends in storage modulus suggest the strong interfacial interaction of nano-silica particles with the nitrocellulose resin matrix which causes more efficient stress transfer across the interface. 3.4. Mechanical characterization Fig. 10 shows stress-strain curves for the bare nitrocellulose film and the films containing 1%, and 2% nano-silica particles. A significant increase in tensile strength and elongation at break and Young’s modulus as compared to the bare nitrocellulose film are obtained due to incorporation of hydrophobic nano-silica particles (Table 1). The best mechanical properties (maximum increase in tensile strength (46.6%) and elongation at break (12.7%) and Young’s modulus (91.1%)) are acquired for the film containing 2% nano-silica particles. This can be explained by fine dispersion of hydrophobic nano-silica particles and a strong adhesion and interaction between the nano-silica particles and the nitrocellulose resin matrix which results in greater possibility of the applied stress transfer from nitrocellulose resin matrix to the rigid nano-silica particles [21,36]. Mechanical properties would likely be improved in case higher nano-silica particles incorporation ( > 2%) did not lead to phase inversion, (because of the trend of improvement until hydrophobic nano-silica particles’ content reaches by 2%).
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3.5. Gloss evaluation of the films Nitrocellulose is widely used in leather and coating industries in which the glossiness of the final films is important. As the gloss is considered as an important aspect of visual perception, we examined the effect of nano-silica particles incorporation on the gloss of the films. As seen in Fig. 11 all the samples are semi gloss and the gloss of the films in both the 60 ° and 85 ° angels slightly decreased by increasing nano-silica particles’ content. This can be attributed to the presence of the nano-silica particles on the film surfaces that reduces the gloss by reducing the smoothness of surfaces and so the optical uniformity of films was reduced [19]. In fact an increase of nano-silica content leads to lower gloss because of the presence of a larger amount of nano-silica particles on the surface. 3.6. Hardness evaluation of the films Increasing surface hardness results in improvement of the abrasion and scratch resistance [20,21,40] which are considered as the important concerns in leather and coating industries. The variation in surface hardness (after 1 and 3 days) with nano-silica particles content could be observed in Fig. 12. As seen in Fig. 12, as the percentage of nano-silica particles increases, the surface hardness of the samples increases. The increased hardness can be attributed to the increase of Tg of the films [22]. It should be noted that the presence of nano-silica particles at surface also improves the surface hardness [20,22], but as mentioned before in the system under study the migration of nano-silica particles from the matrix to the surface of the film is not considerable due to strong interactions between nano-silica particles and nitrocellulose matrix resin. 4. Conclusion In this study a waterborne nitrocellulose/silica composites were prepared through emulsification of nitrocellulose polymer containing different percentage of nano-silica particles. The effect of nano-silica particles content on the properties of formed emulsions and mechanical properties of the resultant films was examined. Incorporation of nano115
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