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Surface coating with poly(trifluoroethyl methacrylate) through rapid expansion of supercritical CO2 solutions
J. of Supercritical Fluids 89 (2014) 106–112
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Surface coating with poly(trifluoroethyl methacrylate) through rapid expansion of supercritical CO2 solutions Ornwaree Ratcharak a , Amporn Sane a,b,c,∗ a
Department of Packaging and Materials Technology, Faculty of Agro-Industry, Kasetsart University, Bangkok 10900, Thailand Center for Advanced Studies in Nanotechnology and Its Applications in Chemical, Food and Agricultural Industries, Kasetsart University, Bangkok 10900, Thailand c NANOTEC Center of Excellence, National Nanotechnology Center, Kasetsart University and Center of Nanotechnology, Kasetsart University Research and Development Institute, Kasetsart University, Bangkok 10900, Thailand b
a r t i c l e
i n f o
Article history: Received 4 August 2013 Received in revised form 23 November 2013 Accepted 27 February 2014 Available online 12 March 2014 Keywords: Supercritical RESS Fluoropolymer Coating Hydrophobicity
a b s t r a c t Rapid expansion of supercritical solutions (RESS) of poly(trifluoroethyl methacrylate), poly(TFEMA), was performed to produce ultrafine particles for spray coating application to improve the hydrophobicity of moisture-sensitive biodegradable materials. Carbon dioxide (CO2 ) was used as the RESS solvent. Thermoplastic starch/poly(butylene adipate-co-terephthalate) (TPS/PBAT, 60:40 wt/wt) blend was used as the coating substrate. The objectives of this work were to determine the capacity of the RESS process for coating TPS-based material with poly(TFEMA), and to investigate the effect of RESS parameters – i.e. pre-expansion pressure and temperature (Ppre , Tpre ) and poly(TFEMA) concentration – on the surface morphology and hydrophobicity of the coated materials. It was found that RESS produced poly(TFEMA) particles precipitated onto the surface of the TPS/PBAT substrate, with particle sizes ranging from 30 nm to several microns, depending on processing parameters. Rapid expansion of fluoropolymer solutions (0.3–1.0 wt%) with Ppre of 331 bar initiated from unsaturated conditions produced nanoparticles with a narrow size distribution of ∼30–70 nm; whereas larger particles with broader size distributions and a lower degree of agglomeration were obtained when supersaturated solutions were expanded with Ppre of 172 bar, especially at Tpre (80 ◦ C) – higher than the glass transition temperature (73 ◦ C) of poly(TFEMA). The surface coverage by the fluoropolymer increased with increasing Ppre and poly(TFEMA) concentration, but decreased with increasing Tpre . In addition, the hydrophobicity of the coated substrate, determined by water contact angle and water vapor transmission rate measurements, increased with increasing surface coverage. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Poly(trifluoroethyl methacrylate), poly(TFEMA), has been used in various coating applications due to its outstanding thermal stability, chemical inertness, and water resistance [1,2]. Poly(TFEMA) is one of the fluoropolymers that are soluble in supercritical carbon dioxide (CO2 ) [2], and is considered to be an environmental friendly carrier medium with a high potential for use in coating processes because of its low critical temperature (31.1 ◦ C), moderate critical pressure (73.8 bar), nontoxicity, nonflammability, and low cost. In addition, the removal of CO2 from coatings is rather simple, without
∗ Corresponding author at: Department of Packaging and Materials Technology, Faculty of Agro-Industry, Kasetsart University, Bangkok 10900, Thailand. Tel.: +66 2562 5099; fax: +66 2562 5092. E-mail addresses: [email protected], [email protected] (A. Sane). http://dx.doi.org/10.1016/j.supflu.2014.02.020 0896-8446/© 2014 Elsevier B.V. All rights reserved.
requiring complicated drying and solvent removal steps, because it remains in gas phase at ambient conditions [3,4]. Rapid expansion of supercritical solutions (RESS) is a wellknown technique for producing nano- to micron-sized particles. In this process, a supercritical solution containing a solute and a supercritical solvent (typically CO2 ) is rapidly (10−6 s) expanded across a micro-orifice, causing a sudden decrease in solvent density and hence a dramatic increase in solution degree of saturation, leading to precipitation of the solute in the form of fine particles. The RESS process has been used for preparing particles from a variety of substances, including organic, polymeric, and inorganic materials. Generally, RESS products vary in size (from nano- to micron-size range) and shape (from particles to fibers), depending upon operating conditions [5–13] and material properties [10,11]. Several research groups have investigated the effect of RESS processing conditions – e.g. solute concentration, pre-expansion temperature and pressure (Tpre , Ppre ), nozzle geometry, and spraying
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distance – on the product size and morphology [5–7,9,10,14]. RESS technique has also been extended to coating application [14–25]. Coating materials previously used in the RESS process include perfluoropolyether diamide [14], paraffin [15,18,19], poly(2-ethylhexyl acrylate) [16], poly(dimethylsiloxane) [17], alkyl ketene dimer [23,24], poly(-caprolactone), and poly(vinyl acetate-co-vinyl pivalate) [25]. The coatings were applied to different substrates, such as catalyst particles [15], surface acoustic sensors [17], silica and glass particles [18–20], and paper [23], as well as silica wafers [25]. Furthermore, RESS has been used in combination with fluidized bed technology [18,19,22] and electrostatic collection [21] to achieve uniform deposition of fine polymer particles onto substrate surfaces with complex geometries. To date, limited work has been reported on the effect of RESS operating conditions on coating surface morphology and hydrophobicity of coated substrates. Quan et al. [23] investigated the influence of RESS parameters on surface morphology and hydrophobicity of paper coated with alkyl ketene dimer. The authors found that the coating surface consisted of flakelike particles (∼1–2 m), and that the particle sizes decreased with increasing pre-expansion temperature and pressure and decreasing spraying distance. In addition, the presence of flake-like structures significantly increased the hydrophobicity of paper as the measured water contact angle increased above 150◦ . Recently, Ovaskainen et al. [25] reported that coating silica wafers with poly(vinyl acetate-co-vinyl pivalate) via RESS process was more effective at improving hydrophobicity compared with coating with poly(-caprolactone). The water contact angle of silica wafers coated with poly(-caprolactone) (89–104◦ ) was considerably lower than when coated with poly(vinyl acetate-co-vinyl pivalate) (120–156◦ ). The authors explained that the higher contact angle obtained with poly(vinyl acetate-co-vinyl pivalate) coating was probably due to the higher solubility in a solvent mixture containing acetone and supercritical CO2 (10:90 v/v), hence providing higher surface coverage. To the knowledge of the authors, there are no previous reports on RESS of poly(TFEMA) for coating of moisture-sensitive materials. Recently there has been increasing interest in the use of thermoplastic starch (TPS) in packaging applications because of its biodegradability and because it is produced from starch which is inexpensive, abundant and renewable. However, the moisture sensitivity of this material has limited its application. Consequently, several attempts have been made to improve the moisture resistance of TPS by melt blending with a hydrophobic biodegradable polymer, e.g. polyethylene, poly(lactic acid), or poly(butylene adipate-co-terephthalate) [26–29]. In the present work, a TPS-based polymer blend consisting of TPS and poly(butylene adipate-co-terephthalate) (60:40 wt/wt) was chosen as the coating substrate because of its moisture sensitivity due to TPS being the primary component. The objectives of this work were to determine the capacity of the RESS process for coating a TPS-based substrate with poly(TFEMA), and to investigate the effect of RESS processing conditions (i.e. pre-expansion pressure and temperature, and fluoropolymer concentration) on the surface morphology and hydrophobic properties of the TPS-based substrate.
2. Experimental 2.1. Materials Poly(butylene adipate-co-terephthalate) (PBAT; Ecoflex® F BX 7011) was purchased from BASF Corp. (USA). Cassava starch (13.2% inherent moisture) was supplied by Tong Chan (Thailand). Glycerol (99.5% purity) was obtained from Siam Chemicals Solutions
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(Thailand). High-purity grade (≥99.98%) carbon dioxide (CO2 ) was purchased from Chattakorn Lab Center (Thailand). Poly(2,2,2trifluoroethyl methacrylate), poly(TFEMA), with a weight-average molecular weight of 36,500 g/mol and polydispersity of 1.46, was supplied by Polymer Source (Canada). Thermal transitions (i.e. glass transition and melting) of poly(TFEMA) were determined using a differential scanning calorimeter (DSC 1 STARe ; Mettler Toledo, Switzerland) by heating from −60 to 250 ◦ C at a rate of 10 ◦ C/min. Only glass transition was observed at a temperature of 73 ◦ C, indicating that poly(TFEMA) was in the amorphous state. 2.2. Phase-behavior measurements The phase behaviors of poly(TFEMA) in supercritical CO2 were examined before performing rapid expansion experiments, because the phase state of a solute-solvent mixture prior to rapid expansion has been found to affect the size and morphology of precipitated products obtained from rapid expansion of supercritical solutions [7,10,30,31]. A schematic of the apparatus used for phase-behavior measurements is shown in Fig. 1. Note that the same apparatus was used for both phase behavior and RESS experiments; the procedures are described in detail elsewhere [30]. Briefly, a variable-volume view cell was connected to a syringe pump (500HP; Teledyne Isco, USA). Supercritical CO2 from the syringe pump was used as the working fluid to move the piston inside the view cell to compress and depressurize the polymer solution on the other side of the piston. For a typical experiment, a view cell was charged with 0.038–0.132 g of poly(TFEMA) and 12.30 g of CO2 such that the polymer concentration range was 0.3–1.0 wt%. The mixture was then compressed to ∼340 bar using pure CO2 , delivered by a syringe pump, and heated to 35 ◦ C under continuous mixing with a magnetic stirrer until a homogeneous solution was obtained. To determine the cloud-point (L to LL phase transition) pressure, the polymer solution was slowly depressurized (1.4 bar/min) until the homogeneous solution became slightly hazy. This transition was observed through a sapphire window located in front of the view cell using a borescope connected to a chargecoupled device (CCD) camera and a personal computer. In this work, the cloud-point pressures of poly(TFEMA) solutions (0.3 and 1.0 wt%) were measured at a temperature range of 35–60 ◦ C. All cloud-point experiments were carried out in duplicate. 2.3. Coating experiments 2.3.1. Preparation of TPS/PBAT blend sheets Cassava starch was mixed with glycerol (plasticizer) in a weight ratio of 70:30 for 30 min using a dough mixer. The obtained mixture was then extruded using a co-rotating, fully intermeshing twinscrew extruder (LTE 20-40; Labtech Engineering, Thailand) with a screw diameter of 20 mm and a screw length to diameter ratio of 40:1. Extrusion was carried out at a temperature range of 70–135 ◦ C and a screw speed of 170 rpm. Before melt blending, both TPS and PBAT pellets were dried at 45 ◦ C overnight using a hot-air oven. TPS/PBAT blend with a weight ratio of TPS to PBAT of 60:40 was produced using the same twin-screw extruder previously used for preparing TPS, with processing temperatures of 75–170 ◦ C and a screw speed of 90 rpm. The blend was prepared in the form of sheets with a thickness of 0.90 mm. The samples were then stored in a desiccator at room temperature. 2.3.2. Rapid expansion of supercritical solutions RESS technique was utilized to deposit poly(TFEMA) onto the prepared TPS/PBAT substrate. To perform a coating experiment, the view cell was charged with ∼0.098–0.330 g of poly(TFEMA) and 32.00 g of CO2 , in the same manner as the phase-behavior measurements. The contents of the cell were compressed with CO2 from the
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Fig. 1. Schematic of the phase behavior and RESS apparatus.
syringe pump to the desired pressure. The mixture was then heated and stirred until the fluoropolymer was completely dissolved in the supercritical solvent. Before expanding the poly(TFEMA) solution, steady-state conditions were established by flowing pure CO2 under heating from the syringe pump, bypassing the high-pressure cell, to the nozzle (50 m dia., L/D = 4) in order to obtain a constant pre-expansion temperature and pressure (Tpre , Ppre ) upstream of the nozzle (Fig. 1). Note that the geometry and assembly of nozzle used in this work are similar to those previously reported by Sane and Thies [9]. The flow of pure CO2 was then redirected to the cell to indirectly push the poly(TFEMA) solution out of the cell by means of the movable piston located inside the cell. The fluid in the tubing during flow from the view cell to the nozzle was heated to the desired pre-expansion temperature using two cable heaters. The solution was subsequently expanded through the nozzle into the coating chamber where the TPS/PBAT substrate and copper grid were located 15 cm below the nozzle exit. The samples were coated for 140 s. The length, width, and height of the chamber were 27, 27, and 30 cm, respectively. During coating, chamber temperature ranged between 22 and 24 ◦ C. All coating experiments were carried out in duplicate. 2.4. Characterization
apparatus (Mocon, USA) according to ASTM 398-03 standard test method [32]. Prior to the measurements, the samples were conditioned in a closed chamber containing a saturated aqueous solution of calcium chloride at 25 ◦ C (30% RH) for ∼2 days. All measurements were carried out in triplicate.
3. Results and discussion 3.1. Phase behaviors of poly(TFEMA) in supercritical CO2 Cloud-point pressures of 0.3 and 1.0 wt% poly(TFEMA) solutions in supercritical CO2 were measured as a function of temperature. The obtained results are shown as a pressure-temperature diagram in Fig. 2. The cloud-point pressure of a 0.3 wt% poly(TFEMA) solution increased from ∼195 to 314 bar when temperature increased from 30.5 to 60.2 ◦ C. During cloud-point transition, the poly(TFEMA) homogeneous solution (L) separated into two liquid phases: polymer-rich and solvent-rich phases (L1 L2 ) [2,7]. Increasing poly(TFEMA) concentration from 0.3 to 1.0 wt% resulted in an increase in cloud-point pressure of ∼20 bar. All cloud-point curves exhibited a typical lower critical solution temperature (LCST) [2,6,7]. The slopes of cloud-point curves were ∼3.4–3.6 bar/◦ C, in good agreement with those reported by Kwon et al. [2].
2.4.1. Scanning electron microscopy Surface morphologies of TPS/PBAT substrates and copper grids coated with poly(TFEMA) by the RESS process were characterized using a field-emission scanning electron microscope (FESEM) (S4700; Hitachi, Japan). The samples were dried under vacuum (1 bar) at room temperature for at least 2 days and then sputter-coated with a ∼1.5 nm thick platinum layer before imaging. 2.4.2. Water contact angle Static water contact angles of poly(TFEMA)-coated samples were measured with a contact angle analyzer (OCA 20; Dataphysics Instruments, Germany). The static contact angle was measured from a 2.0 L water droplet within 1 s after deposition of the liquid droplet on the surface to avoid the effect of liquid penetrating into the polymer substrate. For each sample, measurements were repeated at least five times on three individual samples. 2.4.3. Water vapor transmission rate Water vapor transmission rate (WVTR) of poly(TFEMA)coated samples was measured by a Mocon PERMATRAN-W® 398
Fig. 2. Cloud-point curves for 0.3 and 1.0 wt% poly(TFEMA) solutions in supercritical CO2 , and RESS conditions shown in relation to the cloud-point curves.
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Fig. 3. FESEM micrograph of an uncoated TPS/PBAT sheet.
3.2. Surface coating by the RESS process In this work, spray coating by the RESS process was performed with a poly(TFEMA) concentration of 0.3–1.0 wt%, Tpre of 40–80 ◦ C, and Ppre of 172–331 bar. In Fig. 2, the selected pre-expansion conditions are shown in relation to cloud-point curves of 0.3 and 1.0 wt% poly(TFEMA) solutions. The pre-expansion conditions chosen in this work were both above and below the cloud-point curves, such that the expansion path initiates from supercritical solutions with different degrees of saturation: (i) S < 1 at Tpre = 40 ◦ C, Ppre = 331 bar; and (ii) S > 1 at Tpre = 40 ◦ C, Ppre = 172 bar and Tpre = 80 ◦ C, Ppre = 331 bar. Surface morphology of TPS/PBAT substrates before and after coating with poly(TFEMA) are illustrated in Figs. 3–5 . Figs. 4 and 5 (left) show the surface morphology of TPS/PBAT sheets after coating by RESS of 0.3 and 1.0 wt% poly(TFEMA) solutions, respectively. In general, the substrates were covered with small particles and agglomerates. Because TPS/PBAT substrate has poor electrical conductivity, the surface morphology of TPS/PBAT sheets at higher magnification cannot be directly observed. Accordingly, supercritical solutions were also sprayed onto copper grids in the same manner as TPS/PBAT sheets to investigate the effect of poly(TFEMA) concentration, Tpre and Ppre on the morphology of precipitates on the coating surface. Figs. 4 and 5 (middle and right) show the surface morphology of copper grids after coating by RESS of 0.3 and 1.0 wt% poly(TFEMA) solutions. 3.2.1. Effect of pre-expansion pressure The effect of Ppre on surface morphology was evaluated by rapidly expanding supercritical solutions at Ppre of 172 and 331 bar with a constant fluoropolymer concentration (0.3 and 1.0 wt%) and Tpre (40 ◦ C). It was found that surface structures of poly(TFEMA) typically comprised both individual spherical particles and agglomerates. For RESS of 0.3 wt% poly(TFEMA) solutions with Ppre of 172 bar, the substrate surface was covered by particles with two size ranges: nanosize (30–70 nm) and submicron size (∼0.1–1.5 m). In addition, the coating layer was covered with small agglomerates (∼1–3 m) composed of nanoparticles. Increasing Ppre from 172 to 331 bar decreased the size of primary particles from ∼0.03–1.5 m to ∼30–70 nm but increased the size and quantity of agglomerates in thread-like structures (compare Fig. 4a and b). The larger primary particles were present at lower Ppre , possibly because the expansion path begins from supersaturated states (S > 1), causing early precipitation of the solution before entering the nozzle [7,10]. Similar trends were also observed for RESS of 1.0 wt% poly(TFEMA) solutions (compare Fig. 5a and b). In addition, increasing Ppre resulted in both increasing agglomerate
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size and surface coverage. This could be explained by the fact that expansion at higher pressure resulted in a higher flow rate (see Table 1), which increased both the number of particles and the rate of particle collision [10]. The increase in particle size with decreasing pre-expansion pressure agrees with results previously reported by Türk et al. [8] and Hezave and Esmaeilzadeh [12] for the formation of benzoic acid and cephalexin particles, respectively. The observed relationship between degree of saturation and particle size is consistent with (i) the phase-separation kinetics and (ii) the nucleation and growth theory. We observed that poly(TFEMA)-CO2 phase separation and the solution flow through the pre-expansion section occurred within 1 s and ∼5–10 s, respectively. If RESS is carried out such that the solution is supersaturated at pre-expansion conditions, the solution has already phase separated (L to LL) upstream of the nozzle, and the poly(TFEMA)-rich droplets have already formed and even coalesced in the CO2 -rich phase during flow from the view cell to the nozzle. Nuclei of fluoropolymer are formed in the polymer-rich droplets and then continue to grow by condensation and coagulation [7,33,34]. The remaining polymer in the solvent-rich phase will eventually precipitate possibly inside the nozzle or in the free jet. As a result, particles with a bimodal size distribution are usually obtained (larger particles are formed due to early precipitation and smaller ones are obtained by late precipitation). On the other hand, if rapid expansion is initiated from unsaturated conditions, smaller particles with a narrower size distribution are obtained because nucleation will not occur until there is a significant pressure drop, that is, inside the nozzle or possibly not even until the free jet. 3.2.2. Effect of pre-expansion temperature To examine the effect of Tpre on surface morphology, RESS experiments with poly(TFEMA) solutions were performed at two different Tpre (40 and 80 ◦ C) with a constant Ppre (172 bar, S > 1). It was found that increasing Tpre from 40 to 80 ◦ C dramatically increased the size of particles, as the size of the majority of particles increased from 30–70 nm to submicron size (compare Fig. 4a to 4c). However, particle agglomeration and surface coverage decreased with increasing Tpre . Similar results were also observed for RESS of 1.0 wt% poly(TFEMA) solutions, as the particle size considerably increased from <100 nm to up to ∼2 m (compare Fig. 5a to 5c). Moreover, the coating layer was mainly composed of individual particles uniformly distributed over the surface. Larger particles with less particle agglomeration were obtained at higher Tpre (80 ◦ C), possibly because rapid expansion of supercritical solutions performed at Tpre higher than the glass transition temperature of poly(TFEMA) (73 ◦ C) facilitates the movement of polymer molecules between the initially formed particles and the coalescence of particles upon collision [10,12]. The increase in particle size with increasing Tpre agrees with previous reports by Türk et al. [8], Hezave and Esmaeilzadeh [12] and Lee et al. [13] on the particle formation of benzoic acid, cephalexin, and cyclotrimethylenetrinitramine, respectively. 3.2.3. Effect of poly(TFEMA) concentration The effect of poly(TFEMA) concentration on surface morphology was investigated by expanding solutions of 0.3 and 1.0 wt% poly(TFEMA) at a constant Tpre (40 and 80 ◦ C) and Ppre (172 and 331 bar). It was found that increasing poly(TFEMA) concentration from 0.1 to 0.3 wt% at Tpre of 40 ◦ C had no effect on the size of fluoropolymer particles (compare Fig. 4a to 5a and Fig. 4b to 5b), as particles with the same size ranges were still obtained. However, increasing poly(TFEMA) concentration resulted in increased extent of particle agglomeration as well as surface coverage. Unlike RESS of supercritical solutions with higher Tpre (80 ◦ C), increasing the fluoropolymer concentration from 0.1 to 0.3 wt% dramatically increased the size of particles from submicron to micron size
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Fig. 4. FESEM micrographs of poly(TFEMA) particles deposited onto TPS/PBAT sheets (left) and copper grids (middle and right) obtained from RESS of a 0.3 wt% poly(TFEMA) solution in supercritical CO2 with Ppre of 172–331 bar and Tpre of 40–80 ◦ C.
ranges (compare Figs. 4c and 5c). This could be explained by: (i) increasing solute concentration increased the rate of particle collision [34,35], and (ii) the initially formed particles tended to coalesce upon collision and form larger particles when RESS was performed at Tpre higher than the glass transition temperature of poly(TFEMA). 3.3. Hydrophobic properties of poly(TFEMA)-coated substrates The hydrophobicity of coated samples was assessed by water contact angle and water vapor transmission rate measurements. The obtained results showed that coating with poly(TFEMA) increased the water contact angle of TPS/PBAT sheets from 76.2◦ to 86.0–113.3◦ (Table 1), indicating increased hydrophobicity of the coated substrates. It was found that the contact angle of the coated substrate increased slightly, from 98.6 to 101.9◦ , when increasing Ppre from 172 to 331 bar in RESS of 0.3 wt%. However,
the contact angle decreased from 98.6 to 91.1◦ when increasing Tpre from 40 to 80 ◦ C. The results indicate improved water resistance with increasing Ppre and decreasing Tpre ; this was in good agreement with the surface morphology observed through SEM, as the surface coverage with poly(TFEMA) increased with increasing Ppre and decreased with increasing Tpre (Fig. 4). Similar trends were also obtained for RESS of 1.0 wt% fluoropolymer solutions: the contact angle increased slightly, from 105.1 to 113.3◦ , when increasing Ppre from 172 to 331 bar, but decreased from 105.1 to 86.0◦ when increasing Tpre from 40 to 80 ◦ C (Table 1). Furthermore, the obtained results showed that the contact angle of TPS/PBAT substrate increased with increasing poly(TFEMA) concentration, indicating that the hydrophobicity of the substrate increased with increasing surface coverage by the fluoropolymer; this was in agreement with the results obtained from coating silica wafers with poly(vinyl acetate-co-vinyl pivalate) previously reported by Ovaskainen et al. [25]. However, the contact angle of TPS/PBAT
Table 1 Contact angle and water vapor transmission rate of TPS/PBAT sheets before and after coating with poly(TFEMA) using RESS process with different operating conditions. Poly(TFEMA) (wt%) Control
Tpre (◦ C)
Ppre (bar)
S
Flow rate (mL/min)
Contact angle (◦ )
WVTR (g/m2 day)
76.2 ± 3.8
15.4 ± 0.4
10.2 15.1 6.4
98.6 ± 1.7 101.9 ± 2.5 91.1 ± 4.3
9.7 ± 0.2 9.2 ± 0.3 10.1 ± 0.4
10.0 14.7 6.1
105.1 ± 2.5 113.3 ± 1.9 86.0 ± 5.0
8.3 ± 0.4 7.7 ± 0.2 9.9 ± 0.5
–
–
–
–
0.3
40 40 80
172 331 172
>1 <1 >1
1.0
40 40 80
172 331 172
>1 <1 >1
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Fig. 5. FESEM micrographs of poly(TFEMA) particles deposited onto TPS/PBAT sheets (left) and copper grids (middle and right) obtained from RESS of a 1.0 wt% poly(TFEMA) solution in supercritical CO2 with Ppre of 172–331 bar and Tpre of 40–80 ◦ C.
sheets coated with poly(TFEMA) was lower than that of poly(vinyl acetate-co-vinyl pivalate)-coated silica wafers (88–156◦ ), possibly because the substrate surfaces were not completely covered with fluoropolymer particles. WVTR of TPS/PBAT sheets after coating with poly(TFEMA) decreased from 15.4 to 7.7–10.1 g/m2 day (Table 1), indicating improved moisture resistance of the substrates; this was in agreement with the results obtained from contact angle measurements. It is possible that there are both (i) particles loosely sitting on top of the substrate surface and (ii) particles adhering to the surface. It should be noted that the surface of TPS/PBAT substrate is rather rough (Fig. 3). It has been reported that spray coating with powder on a rough surface provides better adhesion strength and coating, as compared to coating on a smooth surface [36]. As a result, it is possible that fluoropolymer particles produced by rapid expansion process can adhere to the substrate surface. The decrease in WVTR value is most likely caused by the increase in the amount of fluoropolymer particles adhering to the substrate surface and preventing water vapor permeation through the substrate. Our results show that WVTR of the coated substrates decreased slightly, from 9.7 to 9.2 g/m2 day, when increasing Ppre from 172 to 331 bar in RESS of 0.3 wt% poly(TFEMA) solutions. However, the transmission rate of the coated sheets increased slightly, from 9.7 to 10.1 g/m2 day, when increasing Tpre from 40 to 80 ◦ C. For RESS of 1.0 wt% fluoropolymer solutions, WVTR of coated substrates decreased from 8.3 to 7.7 g/m2 day when increasing Ppre from 172 to 331 bar, while WVTR increased from 8.3 to 9.9 g/m2 day when increasing Tpre from 40 to 80 ◦ C. The
increase in WVTR with decreasing Ppre and increasing Tpre was due to the decrease in amount of fluoropolymer particles adhering to the substrate surface. In addition, the obtained results also showed that the transmission rate of the coated sheets decreased with increasing poly(TFEMA) concentration (Table 1). Consequently, the increase in moisture barrier properties of TPS/PBAT sheets was primarily due to the increase in surface coverage with increasing Ppre and fluoropolymer concentration. 4. Conclusions RESS is a promising, environmentally friendly alternative to existing conventional coating techniques to improve the hydrophobicity of moisture-sensitive substrates. In this work we have demonstrated the feasibility of the RESS process to improve the hydrophobicity of a moisture-sensitive substrate with poly(TFEMA) surface coating. The sizes of poly(TFEMA) particles precipitated from supercritical solutions onto TPS/PBAT substrate ranged from 30 nm to several microns, depending on processing conditions. The sizes of fluoropolymer particles increased with increasing S and Tpre , while the degree of particle agglomeration increased with increasing Ppre and fluoropolymer concentration. Coating by RESS with higher Ppre and poly(TFEMA) concentration increased the surface coverage and resulted in increased water contact angle and decreased water vapor transmission rate, thus increasing the hydrophobicity of the TPS/PBAT material. Our results illustrated that poly(TFEMA) particles were distributed on the substrate surface; however, complete surface coverage could not be
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achieved. Therefore, further experimental investigations are necessary to optimize the coating process. Acknowledgements This research was financially supported in part by: (i) the Research, Development and Engineering (RD&E) Fund through the National Nanotechnology Center (NANOTEC), the National Science and Technology Development Agency (NSTDA), Thailand (Project P-10-10775); (ii) the NSTDA (NSTDA Chair Professor, funded by the Crown Property Bureau); and (iii) the Commission on Higher Education, Ministry of Education, Thailand (National Research University of Thailand). The authors are grateful to Prof. Mark C. Thies (Clemson University, USA) for kindly providing a variable-volume view cell and rapid expansion nozzles. References [1] F. Ciardelli, M. Aglietto, L. Montagnini di Mirabello, E. Passaglia, S. Giancristoforo, V. Castelvetro, G. Ruggeri, New fluorinated acrylic polymers for improving weatherability of building stone materials, Progress in Organic Coatings 32 (1997) 43–50. [2] S. Kwon, W. Bae, K. Lee, H.-S. Byun, H. Kim, High pressure phase behavior of carbon dioxide + 2,2,2-trifluoroethyl methacrylate and + poly(2,2,2-trifluoroethyl methacrylate) systems, J. Chemical & Engineering Data 52 (2007) 89–92. [3] J.N. Hay, A. Khan, Environmentally friendly coatings using carbon dioxide as the carrier medium, J. Materials Science 37 (2002) 4743–4752. [4] L.N. Nikitin, M.O. Gallyamov, E.E. Said-Galiev, A.R. Khokhlov, V.M. Buznik, Supercritical carbon dioxide: a reactive medium for chemical processes involving fluoropolymers, Russian J. General Chemistry 79 (2009) 578–588. [5] A.K. Lele, A.D. Shine, Morphology of polymers precipitated from a supercritical solvent, AIChE J. 38 (1992) 742–752. [6] S. Mawson, K.P. Johnston, J.R. Combes, J.M. DeSimone, Formation of poly(1,1,2,2-tetrahydroperfluorodecyl acrylate) submicron fibers and particles from supercritical carbon dioxide solutions, Macromolecules 28 (1995) 3182–3191. [7] A. Blasig, C. Shi, R.M. Enick, M.C. Thies, Effect of concentration and degree of saturation on RESS of a CO2 -soluble fluoropolymer, Industrial & Engineering Chemistry Research 41 (2002) 4976–4983. [8] M. Türk, P. Hils, B. Helfgen, K. Schaber, H.-J. Martin, M.A. Wahl, Micronization of pharmaceutical substances by the Rapid Expansion of Supercritical Solutions (RESS): a promising method to improve bioavailability of poorly soluble pharmaceutical agents, J. Supercritical Fluids 22 (2002) 75–84. [9] A. Sane, M.C. Thies, The formation of fluorinated tetraphenylporphyrin nanoparticles via rapid expansion processes: RESS vs RESOLV, J. Physical Chemistry B 109 (2005) 19688–19695. [10] A. Sane, M.C. Thies, Effect of material properties and processing conditions on RESS of poly(l-lactide), J. Supercritical Fluids 40 (2007) 134–143. [11] E. Breininger, M. Imran-ul-haq, M. Türk, S. Beuermann, Effect of polymer properties on poly(vinylidene fluoride) particles produced by rapid expansion of CO2 + polymer mixtures, J. Supercritical Fluids 48 (2009) 48–55. [12] A.Z. Hezave, F. Esmaeilzadeh, Investigation of the rapid expansion of supercritical solution parameters effects on size and morphology of cephalexin particles, J. Aerosol Science 41 (2010) 1090–1112. [13] B.-M. Lee, D.S. Kim, Y.-H. Lee, B.-C. Lee, H.-S. Kim, H. Kim, Y.-W. Lee, Preparation of submicron-sized RDX particles by rapid expansion of solution using compressed liquid dimethyl ether, J. Supercritical Fluids 57 (2011) 251–258. [14] Y. Chernyak, F. Henon, R.B. Harris, R.D. Gould, R.K. Franklin, J.R. Edwards, J.M. DeSimone, R.G. Carbonell, Formation of perfluoropolyether coatings by the rapid expansion of supercritical solutions (RESS) process. Part 1: Experimental results, Industrial & Engineering Chemistry Research 40 (2001) 6118–6126.
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Surface coating with poly(trifluoroethyl methacrylate) through rapid expansion of supercritical CO2 solutions Ornwaree Ratcharak a , Amporn Sane a,b,c,∗ a
Department of Packaging and Materials Technology, Faculty of Agro-Industry, Kasetsart University, Bangkok 10900, Thailand Center for Advanced Studies in Nanotechnology and Its Applications in Chemical, Food and Agricultural Industries, Kasetsart University, Bangkok 10900, Thailand c NANOTEC Center of Excellence, National Nanotechnology Center, Kasetsart University and Center of Nanotechnology, Kasetsart University Research and Development Institute, Kasetsart University, Bangkok 10900, Thailand b
a r t i c l e
i n f o
Article history: Received 4 August 2013 Received in revised form 23 November 2013 Accepted 27 February 2014 Available online 12 March 2014 Keywords: Supercritical RESS Fluoropolymer Coating Hydrophobicity
a b s t r a c t Rapid expansion of supercritical solutions (RESS) of poly(trifluoroethyl methacrylate), poly(TFEMA), was performed to produce ultrafine particles for spray coating application to improve the hydrophobicity of moisture-sensitive biodegradable materials. Carbon dioxide (CO2 ) was used as the RESS solvent. Thermoplastic starch/poly(butylene adipate-co-terephthalate) (TPS/PBAT, 60:40 wt/wt) blend was used as the coating substrate. The objectives of this work were to determine the capacity of the RESS process for coating TPS-based material with poly(TFEMA), and to investigate the effect of RESS parameters – i.e. pre-expansion pressure and temperature (Ppre , Tpre ) and poly(TFEMA) concentration – on the surface morphology and hydrophobicity of the coated materials. It was found that RESS produced poly(TFEMA) particles precipitated onto the surface of the TPS/PBAT substrate, with particle sizes ranging from 30 nm to several microns, depending on processing parameters. Rapid expansion of fluoropolymer solutions (0.3–1.0 wt%) with Ppre of 331 bar initiated from unsaturated conditions produced nanoparticles with a narrow size distribution of ∼30–70 nm; whereas larger particles with broader size distributions and a lower degree of agglomeration were obtained when supersaturated solutions were expanded with Ppre of 172 bar, especially at Tpre (80 ◦ C) – higher than the glass transition temperature (73 ◦ C) of poly(TFEMA). The surface coverage by the fluoropolymer increased with increasing Ppre and poly(TFEMA) concentration, but decreased with increasing Tpre . In addition, the hydrophobicity of the coated substrate, determined by water contact angle and water vapor transmission rate measurements, increased with increasing surface coverage. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Poly(trifluoroethyl methacrylate), poly(TFEMA), has been used in various coating applications due to its outstanding thermal stability, chemical inertness, and water resistance [1,2]. Poly(TFEMA) is one of the fluoropolymers that are soluble in supercritical carbon dioxide (CO2 ) [2], and is considered to be an environmental friendly carrier medium with a high potential for use in coating processes because of its low critical temperature (31.1 ◦ C), moderate critical pressure (73.8 bar), nontoxicity, nonflammability, and low cost. In addition, the removal of CO2 from coatings is rather simple, without
∗ Corresponding author at: Department of Packaging and Materials Technology, Faculty of Agro-Industry, Kasetsart University, Bangkok 10900, Thailand. Tel.: +66 2562 5099; fax: +66 2562 5092. E-mail addresses: [email protected], [email protected] (A. Sane). http://dx.doi.org/10.1016/j.supflu.2014.02.020 0896-8446/© 2014 Elsevier B.V. All rights reserved.
requiring complicated drying and solvent removal steps, because it remains in gas phase at ambient conditions [3,4]. Rapid expansion of supercritical solutions (RESS) is a wellknown technique for producing nano- to micron-sized particles. In this process, a supercritical solution containing a solute and a supercritical solvent (typically CO2 ) is rapidly (10−6 s) expanded across a micro-orifice, causing a sudden decrease in solvent density and hence a dramatic increase in solution degree of saturation, leading to precipitation of the solute in the form of fine particles. The RESS process has been used for preparing particles from a variety of substances, including organic, polymeric, and inorganic materials. Generally, RESS products vary in size (from nano- to micron-size range) and shape (from particles to fibers), depending upon operating conditions [5–13] and material properties [10,11]. Several research groups have investigated the effect of RESS processing conditions – e.g. solute concentration, pre-expansion temperature and pressure (Tpre , Ppre ), nozzle geometry, and spraying
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distance – on the product size and morphology [5–7,9,10,14]. RESS technique has also been extended to coating application [14–25]. Coating materials previously used in the RESS process include perfluoropolyether diamide [14], paraffin [15,18,19], poly(2-ethylhexyl acrylate) [16], poly(dimethylsiloxane) [17], alkyl ketene dimer [23,24], poly(-caprolactone), and poly(vinyl acetate-co-vinyl pivalate) [25]. The coatings were applied to different substrates, such as catalyst particles [15], surface acoustic sensors [17], silica and glass particles [18–20], and paper [23], as well as silica wafers [25]. Furthermore, RESS has been used in combination with fluidized bed technology [18,19,22] and electrostatic collection [21] to achieve uniform deposition of fine polymer particles onto substrate surfaces with complex geometries. To date, limited work has been reported on the effect of RESS operating conditions on coating surface morphology and hydrophobicity of coated substrates. Quan et al. [23] investigated the influence of RESS parameters on surface morphology and hydrophobicity of paper coated with alkyl ketene dimer. The authors found that the coating surface consisted of flakelike particles (∼1–2 m), and that the particle sizes decreased with increasing pre-expansion temperature and pressure and decreasing spraying distance. In addition, the presence of flake-like structures significantly increased the hydrophobicity of paper as the measured water contact angle increased above 150◦ . Recently, Ovaskainen et al. [25] reported that coating silica wafers with poly(vinyl acetate-co-vinyl pivalate) via RESS process was more effective at improving hydrophobicity compared with coating with poly(-caprolactone). The water contact angle of silica wafers coated with poly(-caprolactone) (89–104◦ ) was considerably lower than when coated with poly(vinyl acetate-co-vinyl pivalate) (120–156◦ ). The authors explained that the higher contact angle obtained with poly(vinyl acetate-co-vinyl pivalate) coating was probably due to the higher solubility in a solvent mixture containing acetone and supercritical CO2 (10:90 v/v), hence providing higher surface coverage. To the knowledge of the authors, there are no previous reports on RESS of poly(TFEMA) for coating of moisture-sensitive materials. Recently there has been increasing interest in the use of thermoplastic starch (TPS) in packaging applications because of its biodegradability and because it is produced from starch which is inexpensive, abundant and renewable. However, the moisture sensitivity of this material has limited its application. Consequently, several attempts have been made to improve the moisture resistance of TPS by melt blending with a hydrophobic biodegradable polymer, e.g. polyethylene, poly(lactic acid), or poly(butylene adipate-co-terephthalate) [26–29]. In the present work, a TPS-based polymer blend consisting of TPS and poly(butylene adipate-co-terephthalate) (60:40 wt/wt) was chosen as the coating substrate because of its moisture sensitivity due to TPS being the primary component. The objectives of this work were to determine the capacity of the RESS process for coating a TPS-based substrate with poly(TFEMA), and to investigate the effect of RESS processing conditions (i.e. pre-expansion pressure and temperature, and fluoropolymer concentration) on the surface morphology and hydrophobic properties of the TPS-based substrate.
2. Experimental 2.1. Materials Poly(butylene adipate-co-terephthalate) (PBAT; Ecoflex® F BX 7011) was purchased from BASF Corp. (USA). Cassava starch (13.2% inherent moisture) was supplied by Tong Chan (Thailand). Glycerol (99.5% purity) was obtained from Siam Chemicals Solutions
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(Thailand). High-purity grade (≥99.98%) carbon dioxide (CO2 ) was purchased from Chattakorn Lab Center (Thailand). Poly(2,2,2trifluoroethyl methacrylate), poly(TFEMA), with a weight-average molecular weight of 36,500 g/mol and polydispersity of 1.46, was supplied by Polymer Source (Canada). Thermal transitions (i.e. glass transition and melting) of poly(TFEMA) were determined using a differential scanning calorimeter (DSC 1 STARe ; Mettler Toledo, Switzerland) by heating from −60 to 250 ◦ C at a rate of 10 ◦ C/min. Only glass transition was observed at a temperature of 73 ◦ C, indicating that poly(TFEMA) was in the amorphous state. 2.2. Phase-behavior measurements The phase behaviors of poly(TFEMA) in supercritical CO2 were examined before performing rapid expansion experiments, because the phase state of a solute-solvent mixture prior to rapid expansion has been found to affect the size and morphology of precipitated products obtained from rapid expansion of supercritical solutions [7,10,30,31]. A schematic of the apparatus used for phase-behavior measurements is shown in Fig. 1. Note that the same apparatus was used for both phase behavior and RESS experiments; the procedures are described in detail elsewhere [30]. Briefly, a variable-volume view cell was connected to a syringe pump (500HP; Teledyne Isco, USA). Supercritical CO2 from the syringe pump was used as the working fluid to move the piston inside the view cell to compress and depressurize the polymer solution on the other side of the piston. For a typical experiment, a view cell was charged with 0.038–0.132 g of poly(TFEMA) and 12.30 g of CO2 such that the polymer concentration range was 0.3–1.0 wt%. The mixture was then compressed to ∼340 bar using pure CO2 , delivered by a syringe pump, and heated to 35 ◦ C under continuous mixing with a magnetic stirrer until a homogeneous solution was obtained. To determine the cloud-point (L to LL phase transition) pressure, the polymer solution was slowly depressurized (1.4 bar/min) until the homogeneous solution became slightly hazy. This transition was observed through a sapphire window located in front of the view cell using a borescope connected to a chargecoupled device (CCD) camera and a personal computer. In this work, the cloud-point pressures of poly(TFEMA) solutions (0.3 and 1.0 wt%) were measured at a temperature range of 35–60 ◦ C. All cloud-point experiments were carried out in duplicate. 2.3. Coating experiments 2.3.1. Preparation of TPS/PBAT blend sheets Cassava starch was mixed with glycerol (plasticizer) in a weight ratio of 70:30 for 30 min using a dough mixer. The obtained mixture was then extruded using a co-rotating, fully intermeshing twinscrew extruder (LTE 20-40; Labtech Engineering, Thailand) with a screw diameter of 20 mm and a screw length to diameter ratio of 40:1. Extrusion was carried out at a temperature range of 70–135 ◦ C and a screw speed of 170 rpm. Before melt blending, both TPS and PBAT pellets were dried at 45 ◦ C overnight using a hot-air oven. TPS/PBAT blend with a weight ratio of TPS to PBAT of 60:40 was produced using the same twin-screw extruder previously used for preparing TPS, with processing temperatures of 75–170 ◦ C and a screw speed of 90 rpm. The blend was prepared in the form of sheets with a thickness of 0.90 mm. The samples were then stored in a desiccator at room temperature. 2.3.2. Rapid expansion of supercritical solutions RESS technique was utilized to deposit poly(TFEMA) onto the prepared TPS/PBAT substrate. To perform a coating experiment, the view cell was charged with ∼0.098–0.330 g of poly(TFEMA) and 32.00 g of CO2 , in the same manner as the phase-behavior measurements. The contents of the cell were compressed with CO2 from the
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Fig. 1. Schematic of the phase behavior and RESS apparatus.
syringe pump to the desired pressure. The mixture was then heated and stirred until the fluoropolymer was completely dissolved in the supercritical solvent. Before expanding the poly(TFEMA) solution, steady-state conditions were established by flowing pure CO2 under heating from the syringe pump, bypassing the high-pressure cell, to the nozzle (50 m dia., L/D = 4) in order to obtain a constant pre-expansion temperature and pressure (Tpre , Ppre ) upstream of the nozzle (Fig. 1). Note that the geometry and assembly of nozzle used in this work are similar to those previously reported by Sane and Thies [9]. The flow of pure CO2 was then redirected to the cell to indirectly push the poly(TFEMA) solution out of the cell by means of the movable piston located inside the cell. The fluid in the tubing during flow from the view cell to the nozzle was heated to the desired pre-expansion temperature using two cable heaters. The solution was subsequently expanded through the nozzle into the coating chamber where the TPS/PBAT substrate and copper grid were located 15 cm below the nozzle exit. The samples were coated for 140 s. The length, width, and height of the chamber were 27, 27, and 30 cm, respectively. During coating, chamber temperature ranged between 22 and 24 ◦ C. All coating experiments were carried out in duplicate. 2.4. Characterization
apparatus (Mocon, USA) according to ASTM 398-03 standard test method [32]. Prior to the measurements, the samples were conditioned in a closed chamber containing a saturated aqueous solution of calcium chloride at 25 ◦ C (30% RH) for ∼2 days. All measurements were carried out in triplicate.
3. Results and discussion 3.1. Phase behaviors of poly(TFEMA) in supercritical CO2 Cloud-point pressures of 0.3 and 1.0 wt% poly(TFEMA) solutions in supercritical CO2 were measured as a function of temperature. The obtained results are shown as a pressure-temperature diagram in Fig. 2. The cloud-point pressure of a 0.3 wt% poly(TFEMA) solution increased from ∼195 to 314 bar when temperature increased from 30.5 to 60.2 ◦ C. During cloud-point transition, the poly(TFEMA) homogeneous solution (L) separated into two liquid phases: polymer-rich and solvent-rich phases (L1 L2 ) [2,7]. Increasing poly(TFEMA) concentration from 0.3 to 1.0 wt% resulted in an increase in cloud-point pressure of ∼20 bar. All cloud-point curves exhibited a typical lower critical solution temperature (LCST) [2,6,7]. The slopes of cloud-point curves were ∼3.4–3.6 bar/◦ C, in good agreement with those reported by Kwon et al. [2].
2.4.1. Scanning electron microscopy Surface morphologies of TPS/PBAT substrates and copper grids coated with poly(TFEMA) by the RESS process were characterized using a field-emission scanning electron microscope (FESEM) (S4700; Hitachi, Japan). The samples were dried under vacuum (1 bar) at room temperature for at least 2 days and then sputter-coated with a ∼1.5 nm thick platinum layer before imaging. 2.4.2. Water contact angle Static water contact angles of poly(TFEMA)-coated samples were measured with a contact angle analyzer (OCA 20; Dataphysics Instruments, Germany). The static contact angle was measured from a 2.0 L water droplet within 1 s after deposition of the liquid droplet on the surface to avoid the effect of liquid penetrating into the polymer substrate. For each sample, measurements were repeated at least five times on three individual samples. 2.4.3. Water vapor transmission rate Water vapor transmission rate (WVTR) of poly(TFEMA)coated samples was measured by a Mocon PERMATRAN-W® 398
Fig. 2. Cloud-point curves for 0.3 and 1.0 wt% poly(TFEMA) solutions in supercritical CO2 , and RESS conditions shown in relation to the cloud-point curves.
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Fig. 3. FESEM micrograph of an uncoated TPS/PBAT sheet.
3.2. Surface coating by the RESS process In this work, spray coating by the RESS process was performed with a poly(TFEMA) concentration of 0.3–1.0 wt%, Tpre of 40–80 ◦ C, and Ppre of 172–331 bar. In Fig. 2, the selected pre-expansion conditions are shown in relation to cloud-point curves of 0.3 and 1.0 wt% poly(TFEMA) solutions. The pre-expansion conditions chosen in this work were both above and below the cloud-point curves, such that the expansion path initiates from supercritical solutions with different degrees of saturation: (i) S < 1 at Tpre = 40 ◦ C, Ppre = 331 bar; and (ii) S > 1 at Tpre = 40 ◦ C, Ppre = 172 bar and Tpre = 80 ◦ C, Ppre = 331 bar. Surface morphology of TPS/PBAT substrates before and after coating with poly(TFEMA) are illustrated in Figs. 3–5 . Figs. 4 and 5 (left) show the surface morphology of TPS/PBAT sheets after coating by RESS of 0.3 and 1.0 wt% poly(TFEMA) solutions, respectively. In general, the substrates were covered with small particles and agglomerates. Because TPS/PBAT substrate has poor electrical conductivity, the surface morphology of TPS/PBAT sheets at higher magnification cannot be directly observed. Accordingly, supercritical solutions were also sprayed onto copper grids in the same manner as TPS/PBAT sheets to investigate the effect of poly(TFEMA) concentration, Tpre and Ppre on the morphology of precipitates on the coating surface. Figs. 4 and 5 (middle and right) show the surface morphology of copper grids after coating by RESS of 0.3 and 1.0 wt% poly(TFEMA) solutions. 3.2.1. Effect of pre-expansion pressure The effect of Ppre on surface morphology was evaluated by rapidly expanding supercritical solutions at Ppre of 172 and 331 bar with a constant fluoropolymer concentration (0.3 and 1.0 wt%) and Tpre (40 ◦ C). It was found that surface structures of poly(TFEMA) typically comprised both individual spherical particles and agglomerates. For RESS of 0.3 wt% poly(TFEMA) solutions with Ppre of 172 bar, the substrate surface was covered by particles with two size ranges: nanosize (30–70 nm) and submicron size (∼0.1–1.5 m). In addition, the coating layer was covered with small agglomerates (∼1–3 m) composed of nanoparticles. Increasing Ppre from 172 to 331 bar decreased the size of primary particles from ∼0.03–1.5 m to ∼30–70 nm but increased the size and quantity of agglomerates in thread-like structures (compare Fig. 4a and b). The larger primary particles were present at lower Ppre , possibly because the expansion path begins from supersaturated states (S > 1), causing early precipitation of the solution before entering the nozzle [7,10]. Similar trends were also observed for RESS of 1.0 wt% poly(TFEMA) solutions (compare Fig. 5a and b). In addition, increasing Ppre resulted in both increasing agglomerate
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size and surface coverage. This could be explained by the fact that expansion at higher pressure resulted in a higher flow rate (see Table 1), which increased both the number of particles and the rate of particle collision [10]. The increase in particle size with decreasing pre-expansion pressure agrees with results previously reported by Türk et al. [8] and Hezave and Esmaeilzadeh [12] for the formation of benzoic acid and cephalexin particles, respectively. The observed relationship between degree of saturation and particle size is consistent with (i) the phase-separation kinetics and (ii) the nucleation and growth theory. We observed that poly(TFEMA)-CO2 phase separation and the solution flow through the pre-expansion section occurred within 1 s and ∼5–10 s, respectively. If RESS is carried out such that the solution is supersaturated at pre-expansion conditions, the solution has already phase separated (L to LL) upstream of the nozzle, and the poly(TFEMA)-rich droplets have already formed and even coalesced in the CO2 -rich phase during flow from the view cell to the nozzle. Nuclei of fluoropolymer are formed in the polymer-rich droplets and then continue to grow by condensation and coagulation [7,33,34]. The remaining polymer in the solvent-rich phase will eventually precipitate possibly inside the nozzle or in the free jet. As a result, particles with a bimodal size distribution are usually obtained (larger particles are formed due to early precipitation and smaller ones are obtained by late precipitation). On the other hand, if rapid expansion is initiated from unsaturated conditions, smaller particles with a narrower size distribution are obtained because nucleation will not occur until there is a significant pressure drop, that is, inside the nozzle or possibly not even until the free jet. 3.2.2. Effect of pre-expansion temperature To examine the effect of Tpre on surface morphology, RESS experiments with poly(TFEMA) solutions were performed at two different Tpre (40 and 80 ◦ C) with a constant Ppre (172 bar, S > 1). It was found that increasing Tpre from 40 to 80 ◦ C dramatically increased the size of particles, as the size of the majority of particles increased from 30–70 nm to submicron size (compare Fig. 4a to 4c). However, particle agglomeration and surface coverage decreased with increasing Tpre . Similar results were also observed for RESS of 1.0 wt% poly(TFEMA) solutions, as the particle size considerably increased from <100 nm to up to ∼2 m (compare Fig. 5a to 5c). Moreover, the coating layer was mainly composed of individual particles uniformly distributed over the surface. Larger particles with less particle agglomeration were obtained at higher Tpre (80 ◦ C), possibly because rapid expansion of supercritical solutions performed at Tpre higher than the glass transition temperature of poly(TFEMA) (73 ◦ C) facilitates the movement of polymer molecules between the initially formed particles and the coalescence of particles upon collision [10,12]. The increase in particle size with increasing Tpre agrees with previous reports by Türk et al. [8], Hezave and Esmaeilzadeh [12] and Lee et al. [13] on the particle formation of benzoic acid, cephalexin, and cyclotrimethylenetrinitramine, respectively. 3.2.3. Effect of poly(TFEMA) concentration The effect of poly(TFEMA) concentration on surface morphology was investigated by expanding solutions of 0.3 and 1.0 wt% poly(TFEMA) at a constant Tpre (40 and 80 ◦ C) and Ppre (172 and 331 bar). It was found that increasing poly(TFEMA) concentration from 0.1 to 0.3 wt% at Tpre of 40 ◦ C had no effect on the size of fluoropolymer particles (compare Fig. 4a to 5a and Fig. 4b to 5b), as particles with the same size ranges were still obtained. However, increasing poly(TFEMA) concentration resulted in increased extent of particle agglomeration as well as surface coverage. Unlike RESS of supercritical solutions with higher Tpre (80 ◦ C), increasing the fluoropolymer concentration from 0.1 to 0.3 wt% dramatically increased the size of particles from submicron to micron size
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Fig. 4. FESEM micrographs of poly(TFEMA) particles deposited onto TPS/PBAT sheets (left) and copper grids (middle and right) obtained from RESS of a 0.3 wt% poly(TFEMA) solution in supercritical CO2 with Ppre of 172–331 bar and Tpre of 40–80 ◦ C.
ranges (compare Figs. 4c and 5c). This could be explained by: (i) increasing solute concentration increased the rate of particle collision [34,35], and (ii) the initially formed particles tended to coalesce upon collision and form larger particles when RESS was performed at Tpre higher than the glass transition temperature of poly(TFEMA). 3.3. Hydrophobic properties of poly(TFEMA)-coated substrates The hydrophobicity of coated samples was assessed by water contact angle and water vapor transmission rate measurements. The obtained results showed that coating with poly(TFEMA) increased the water contact angle of TPS/PBAT sheets from 76.2◦ to 86.0–113.3◦ (Table 1), indicating increased hydrophobicity of the coated substrates. It was found that the contact angle of the coated substrate increased slightly, from 98.6 to 101.9◦ , when increasing Ppre from 172 to 331 bar in RESS of 0.3 wt%. However,
the contact angle decreased from 98.6 to 91.1◦ when increasing Tpre from 40 to 80 ◦ C. The results indicate improved water resistance with increasing Ppre and decreasing Tpre ; this was in good agreement with the surface morphology observed through SEM, as the surface coverage with poly(TFEMA) increased with increasing Ppre and decreased with increasing Tpre (Fig. 4). Similar trends were also obtained for RESS of 1.0 wt% fluoropolymer solutions: the contact angle increased slightly, from 105.1 to 113.3◦ , when increasing Ppre from 172 to 331 bar, but decreased from 105.1 to 86.0◦ when increasing Tpre from 40 to 80 ◦ C (Table 1). Furthermore, the obtained results showed that the contact angle of TPS/PBAT substrate increased with increasing poly(TFEMA) concentration, indicating that the hydrophobicity of the substrate increased with increasing surface coverage by the fluoropolymer; this was in agreement with the results obtained from coating silica wafers with poly(vinyl acetate-co-vinyl pivalate) previously reported by Ovaskainen et al. [25]. However, the contact angle of TPS/PBAT
Table 1 Contact angle and water vapor transmission rate of TPS/PBAT sheets before and after coating with poly(TFEMA) using RESS process with different operating conditions. Poly(TFEMA) (wt%) Control
Tpre (◦ C)
Ppre (bar)
S
Flow rate (mL/min)
Contact angle (◦ )
WVTR (g/m2 day)
76.2 ± 3.8
15.4 ± 0.4
10.2 15.1 6.4
98.6 ± 1.7 101.9 ± 2.5 91.1 ± 4.3
9.7 ± 0.2 9.2 ± 0.3 10.1 ± 0.4
10.0 14.7 6.1
105.1 ± 2.5 113.3 ± 1.9 86.0 ± 5.0
8.3 ± 0.4 7.7 ± 0.2 9.9 ± 0.5
–
–
–
–
0.3
40 40 80
172 331 172
>1 <1 >1
1.0
40 40 80
172 331 172
>1 <1 >1
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Fig. 5. FESEM micrographs of poly(TFEMA) particles deposited onto TPS/PBAT sheets (left) and copper grids (middle and right) obtained from RESS of a 1.0 wt% poly(TFEMA) solution in supercritical CO2 with Ppre of 172–331 bar and Tpre of 40–80 ◦ C.
sheets coated with poly(TFEMA) was lower than that of poly(vinyl acetate-co-vinyl pivalate)-coated silica wafers (88–156◦ ), possibly because the substrate surfaces were not completely covered with fluoropolymer particles. WVTR of TPS/PBAT sheets after coating with poly(TFEMA) decreased from 15.4 to 7.7–10.1 g/m2 day (Table 1), indicating improved moisture resistance of the substrates; this was in agreement with the results obtained from contact angle measurements. It is possible that there are both (i) particles loosely sitting on top of the substrate surface and (ii) particles adhering to the surface. It should be noted that the surface of TPS/PBAT substrate is rather rough (Fig. 3). It has been reported that spray coating with powder on a rough surface provides better adhesion strength and coating, as compared to coating on a smooth surface [36]. As a result, it is possible that fluoropolymer particles produced by rapid expansion process can adhere to the substrate surface. The decrease in WVTR value is most likely caused by the increase in the amount of fluoropolymer particles adhering to the substrate surface and preventing water vapor permeation through the substrate. Our results show that WVTR of the coated substrates decreased slightly, from 9.7 to 9.2 g/m2 day, when increasing Ppre from 172 to 331 bar in RESS of 0.3 wt% poly(TFEMA) solutions. However, the transmission rate of the coated sheets increased slightly, from 9.7 to 10.1 g/m2 day, when increasing Tpre from 40 to 80 ◦ C. For RESS of 1.0 wt% fluoropolymer solutions, WVTR of coated substrates decreased from 8.3 to 7.7 g/m2 day when increasing Ppre from 172 to 331 bar, while WVTR increased from 8.3 to 9.9 g/m2 day when increasing Tpre from 40 to 80 ◦ C. The
increase in WVTR with decreasing Ppre and increasing Tpre was due to the decrease in amount of fluoropolymer particles adhering to the substrate surface. In addition, the obtained results also showed that the transmission rate of the coated sheets decreased with increasing poly(TFEMA) concentration (Table 1). Consequently, the increase in moisture barrier properties of TPS/PBAT sheets was primarily due to the increase in surface coverage with increasing Ppre and fluoropolymer concentration. 4. Conclusions RESS is a promising, environmentally friendly alternative to existing conventional coating techniques to improve the hydrophobicity of moisture-sensitive substrates. In this work we have demonstrated the feasibility of the RESS process to improve the hydrophobicity of a moisture-sensitive substrate with poly(TFEMA) surface coating. The sizes of poly(TFEMA) particles precipitated from supercritical solutions onto TPS/PBAT substrate ranged from 30 nm to several microns, depending on processing conditions. The sizes of fluoropolymer particles increased with increasing S and Tpre , while the degree of particle agglomeration increased with increasing Ppre and fluoropolymer concentration. Coating by RESS with higher Ppre and poly(TFEMA) concentration increased the surface coverage and resulted in increased water contact angle and decreased water vapor transmission rate, thus increasing the hydrophobicity of the TPS/PBAT material. Our results illustrated that poly(TFEMA) particles were distributed on the substrate surface; however, complete surface coverage could not be
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