Development of a solvent selection guide for CO2 spray coating

Development of a solvent selection guide for CO2 spray coating

The Journal of Supercritical Fluids 130 (2017) 172–175 Contents lists available at ScienceDirect The Journal of Supercritical Fluids journal homepag...

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The Journal of Supercritical Fluids 130 (2017) 172–175

Contents lists available at ScienceDirect

The Journal of Supercritical Fluids journal homepage: www.elsevier.com/locate/supflu

Development of a solvent selection guide for CO2 spray coating a,⁎

a

a

a

Yoshiyuki Sato , Tomoki Shimada , Kohei Abe , Hiroshi Inomata , Shin-ichiro Kawasaki

MARK b

a

Research Center of Supercritical Fluid Technology, Graduate School of Engineering, Tohoku University, Aoba 6-6-11-403, Aramaki, Aoba-ku, Sendai 980-8579, Japan Research Institute for Chemical Process Technology, National Institute of Advanced Industrial Science and Technology (AIST), Nigatake 4-2-1, Miyagino, Sendai 9838551, Japan b

A R T I C L E I N F O

A B S T R A C T

Keywords: Solubility parameter Phase equilibria Polymer solution

The use of CO2 for spray coating is a promising technique for volatile organic compound reduction in the paint industry. However, the polymer precipitation might be induced by the addition of CO2 into solvent, since CO2 is a poor solvent for most polymer solutions. In this work, a solvent selection guide for CO2 spray coating is proposed based on the solubility parameter from the viewpoint of avoiding the polymer precipitation. The precipitation tests were carried out for polymer (acrylic resin) + solvent (eight kinds) + CO2 systems. The solubilities of CO2 in the polymer + solvent (diethyl ketone, butanol, and methanol) varied from 7 to 33 wt.% CO2 at 40 °C for pressures from 0.9 to 6.4 MPa. It was found that the solubility parameter of the mixture at the precipitation point had similar values that are due to dissolution of the CO2 in the solution.

1. Introduction

square root of the cohesive energy density. Some researchers [7–9] have applied the solubility parameter to the temperature and pressure ranges including the supercritical region, by using equations of state to calculate the solubility parameter. Williams et al. [9] proposed temperature and pressure effects of the three components of the solubility parameter of Hansen (HSP) [10] and gave CO2 HSP values as a function of temperature and pressure. On the other hand, it should be noted that solubility parameter of solvents and CO2 mixtures is important for paint solvent selection in the CO2 spray coating systems. The solubility parameter of mixtures has been investigated by some researchers [11,12]. Since solubility parameter is based on the cohesive energy density (cohesive energy of unit volume), the volume average solubility parameter is usually used for mixture. However, there have not been reported the effective methods of the solubility parameter evaluation of solvent mixtures containing supercritical fluids, which enable us to design the solvent selection for paint formulations in CO2 spray coating systems. Some progress has been made in redefining solubility parameters that contain hydrogen bonding effects that are denoted as partial solvation parameters (SPS) [13,14]. This method was considered in this work, but the practical system studied makes application of SPS uncertain. Therefore, a simple approach was taken for the acrylic resin system. In this work, a solvent selection guide (methodology) for CO2 spray coating systems is proposed via evaluating the solubility parameters of the solvent + CO2 mixture and polymer. Acrylic resin was chosen as a polymer component and eight solvents were used. Phase behavior and phase equilibria of polymer + solvent + CO2 system were

The paint industry is the largest source of volatile organic compound (VOC) emissions in Japan [1]. It has become an urgent issue to reduce VOC emissions [2]. In 1990, the UNICARB® CO2 spray system [3] was developed by Union Carbide with the aim of reducing the concentration of VOCs in coating formulations. Lewis et al. [4] reported that CO2 spray systems had technological advantages such as superior leveling compared with conventional ones. Recently, Kawasaki et al. [5] proposed a CO2 spray system equipped with a micromixer that enables CO2 to dissolve into organic paint solutions rapidly and homogeneously. Conventional paint consists of polymer, solvent, and diluents with a large portion being organic solvents. A CO2 spray coating system is expected to reduce the amount of VOCs as well as energy consumption by using CO2 as a diluent. However, precipitation of the polymer is induced by CO2 dissolution into some paints in terms of CO2 concentration, because CO2 can act as anti-solvent for a polymer. It is important to prevent precipitation of the polymer in paint solution during mixing of CO2 with the organic solvent-polymer solution and during transport of the solution to the target surface. Since it is undesirable to replace proven polymers used in coating processes, the solvents should be selected to prevent precipitation in CO2 spray coating systems. Therefore, development of an effective and practical solvent selection methodology for paint formulations is important. The solubility parameter [6] is one of the most useful tools for solvent selection. The solubility parameter is originally defined as the



Corresponding author. E-mail address: [email protected] (Y. Sato).

http://dx.doi.org/10.1016/j.supflu.2017.07.041 Received 17 April 2017; Received in revised form 13 July 2017; Accepted 31 July 2017 Available online 14 August 2017 0896-8446/ © 2017 Elsevier B.V. All rights reserved.

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Table 1 Characteristics and composition of acrylic resin used in this work. component

fraction [wt%]

acrylic resin butyl acetate methyl methacrylate acid value hydroxyl number

45–55 45–55 1.0 3.5 mgKOH/g 73.3 mgKOH/g

Table 2 Purity and solubility parameter of diluents used in the CO2 spray coating system. The source of all chemicals was Wako Pure Chemical Industries (Osaka).

experimentally studied at 40 °C. Furthermore, the relationship between solubility parameter of the solvent and CO2 mixtures and the phase behavior was investigated.

Chemical name

Purity (wt%)a

Solubility Parameter (MPa0.5) [10]

n-hexane diisobutyl ketone (DIBK) diethyl ketone (DEK) 1-pentanol 1-butanol ethanol methanol ethylene glycol

96.0 90.0 98.0 98.0 99.0 99.85 99.8 99.5

14.9 16.9 18.2 21.6 23.2 26.5 29.6 33.0

a

as reported by the supplier.

2. Experimental 2.1. Sample Acrylic resin (ACRYDIC 52-666-BA, DIC Corporation, Japan) was used as a polymer component in paints. This polymer is used for isocyanate curing and was supplied as a butyl acetate solution. The number average molecular weight and the weight average molecular weight of the acrylic resin were obtained by SEC with a differential refractive index detector using polystyrene standards and determined to be M n (14.9 kg/mol) and M w (16.8 kg/mol). Characteristics and composition of the acrylic resin obtained by the manufacturer are shown in Table 1. The chemical structure of the polymer was estimated from the information given in Table 1 as shown in Fig. 1. Solubility parameter of the acrylic resin was estimated to be 21.9–23.2 MPa0.5 by using Fedors’group contribution method [15]. Eight diluents were used for precipitation tests described below. The data on source, purity, and solubility parameter [10] of the diluents are given in Table 2. The sample (acrylic resin dissolved in butyl acetate) was mixed with one of the diluents (sample/diluent = 5/3 wt.) listed in Table 2. The diluted solutions, which are composed of the polymer sample and a diluent solvent, were prepared so as to have a polymer concentration of 10–20 wt% when CO2 dissolved into the solution. The diluted solutions were used for precipitation tests and for CO2 solubility measurements. The CO2 (purity > 99.5 vol%) was obtained from Showa Denko Gas Products Co., Ltd. All chemicals were used as received.

Fig. 2. Phase behavior observation apparatus.

the test tube with a cathetometer. In the experiments, equilibration was generally reached in about 10 min. 2.3. CO2 solubility measurement Solubility of CO2 in the diluted solutions was measured with a synthetic method apparatus for evaluation of the solubility parameter. The apparatus and the details of measuring bubble point pressure (BP) are described in a previous paper [16]. BP was determined from the relationship between the pressure and the inner volume of the cell based on compressibility difference of the liquid phase and the vaporliquid phase. Estimated uncertainties of temperature and BP are ± 0.06 K and ± 0.028 MPa, respectively. A sample of known composition was prepared gravimetrically. Estimated uncertainties of CO2 weight fraction for the BP experiments are less than 0.0005.

2.2. Precipitation test 3. Results and discussion

Precipitation of polymer was observed with the phase behavior apparatus shown in Fig. 2. The observation cell, equipped with a cylindrical sapphire window (i.d ϕ10, o.d. ϕ14, length 18 mm) and a test tube (i.d. ϕ8.3, length 27 mm), was set in a thermostated water bath. Diluted solution (0.3 g) was filled into the test tube. CO2 was supplied with a regulator (≤5 MPa) or a syringe pump (> 5 MPa, ISCO model 260D) and CO2 pressure was increased at 0.5 MPa interval. When polymer precipitation was observed, the pressure (Krone Corporation, Tokyo, KDM30, accuracy 0.05 MPa) was recorded. A Teflon-coated stirring bar (ϕ3 × 8 mm) was used to promote equilibration via a magnetic stirrer that was located under the cell. Equilibration of gas dissolution was confirmed with height change of the liquid meniscus in

3.1. Precipitation test Fig. 3 shows examples of the precipitation test. When the diluted solution was homogeneous, the back cell wall behind the test tube could be clearly seen as shown in Fig. 3(a). When polymer precipitation occurred during CO2 dissolution by pressurization, the solution became cloudy as shown in Fig. 3(b). The images shown in Fig. 3 were typical for all solvent systems. Experimental results of the precipitation test of the diluted solutions are shown in Fig. 4. Precipitation pressure in the diluted solutions increased as the diluent solubility parameter was close to that of the acrylic resin. It should be noted that the precipitation pressures for DIBK and DEK cases were fairly low compared with those for ethanol and methanol for which the solubility parameter differences from the resin were comparable or slightly larger than those of DIBK and DEK. This is attributed to CO2 dissolution into the solvent phase. The solubility parameter of CO2 [9] was calculated as 3.4 MPa0.5 at temperature of 40 °C and pressure of 7 MPa so that the solubility parameter of the diluted solution decreased as CO2 dissolution amount increased. Solubility parameter of the diluents which have a higher solubility

Fig. 1. Estimated chemical structure of the acrylic resin used in the CO2 spray coating study.

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Fig. 3. Photographs of precipitation test. (a) homogeneous, (b) heterogeneous.

parameter than that of acrylic resin decreases and become close to that of acrylic resin. Therefore, methanol exhibited a wide soluble pressure range. From these results, it is clear that the chosen solvent should have a solubility parameter that is higher than that of the polymer component, because it will be more likely to remain single phase in a CO2 spray painting system. To discuss this issue quantitatively, CO2 solubility in the diluted solutions was measured as described in the next section. 3.2. CO2 solubility measurement Solubilities of CO2 in the diluted solutions were measured at 40 °C to estimate the solubility parameter of the mixtures. The diluted solutions of DEK, butanol, and methanol were used for the solubility measurements. Experimental results of the solubility are shown in Fig. 5 and Table S1 (Supplementary Materials). The solubility of CO2 in these diluted solutions was directly proportioned the pressure. The solubility of CO2 in butanol solution was slightly lower than that in the other diluted solutions. The solubility data were correlated with Henry’s law as shown in Fig. 5. Weight-fraction Henry's constant and average relative deviations of the correlation are shown in Table S2.

Fig. 5. Solubility of CO2 in diluted solution of acrylic resin butyl acetate solution with butanol, methanol, and diethyl ketone (DEK) at temperature of 40 °C.

for the precipitation test results. Thus, the solubility parameter of the mixture, δmixture, was determined from Eq. (1) [17] as:

3.3. Solubility parameter

δmixture = ϕ1 δ1 + ϕ2 δ2 + ϕ3 δ3

Solubility parameter was used to evaluate the precipitation results. Namely, the solubility parameter of the mixture was considered as a good indicator to accounting for the amounts of solvent, diluent, and also the measured CO2 solubility and could provide a possible reason

where ϕ is the volume fraction. Subscripts 1, 2, 3 denote butyl acetate (solvent), diluent, and CO2, respectively. The volume fractions were evaluated from the (saturated) liquid density at 25 °C and at mixture composition. The solubility parameter of the pure components was

(1)

Fig. 4. Experimental results of the precipitation test for polymer (acrylic resin) + solvent (butyl acetate) + diluent + CO2 (40 °C), ○: dissolution; × : precipitation.

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solubility parameter of solvent and CO2 mixture differs by about 6 MPa0.5. Similar experiments with different polymers will be needed to generalize the conclusions of this work. Acknowledgment This research was supported by JSPS (Japan Society for the Promotion of Science) KAKENHI (No: JP15K06534). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.supflu.2017.07.041. References

Fig. 6. Solubility parameter of mixture of CO2 saturated liquid and shaded precipitation region for solvent selection guide for CO2 spry coating system.

[1] Ministry of Environment Japan, VOC emission inventoryreport 2015 (2016). [2] for example, web site of the California Environmental Protection Agency, https:// www.arb.ca.gov/consprod/regs/regs.htm, 2015 (Accessed 17.02.23). [3] H.F. Bok, K.L., Hoy, K.A. Nielsen, Methods and apparatus for obtaining a feathered spray when spraying liquids by airless techniques, Patent US 5 057 342 (1989). [4] J. Lewis, J.N. Argyropoulos, K.A. Nielson, Supercritical carbon dioxide spray systems, Met. Finish. 98 (2000) 254–262. [5] S.-I. Kawasaki, K. Ishida, Y. Sakurai, T. Fujii, A. Suzuki, CO2 atomizing technology CAT for green coating process using high pressure micromixer, 14th European Meeting on Supercritical Fluids, MS21, Marseilles (2014). [6] J.H. Hildebrand, R.L. Scott, The Solubility of Nonelectrolytes, 3rd ed., Reinhold, New York, 1950. [7] S.R. Allada, Solubility parameters of supercritical fluids, Ind. Eng. Chem. Process Des. Dev. 23 (1984) 344–348. [8] C. Panayiotou, Solubility parameter revisited: an equation-of-state approach for its estimation, Fluid Phase Equilib. 131 (1997) 21–35. [9] L.L. Williams, J.B. Rubin, H.W. Edwards, Calculation of hansen solubility parameter values for a range of pressure and temperature conditions, including the supercritical fluid region, Ind. Eng. Chem. Res. 43 (2004) 4967–4972. [10] C.M. Hansen, Hansen Solubility Parameters: A User’s Handbook, 2nd edn, CRC Press, Boca Raton, 2007. [11] J.S. Dickmann, J.C. Hassler, E. Kiran, Modeling of the volumetric properties and estimation of the solubility parameters of ionic liquid + ethanol mixtures with the Sanchez-Lacombe and Simha-Somcynsky equations of state: [EMIM]Ac + ethanol and [EMIM]Cl + ethanol mixtures, J. Spercrit. Fluids 98 (2015) 86–101. [12] T. Sato, S. Araki, M. Morimoto, R. Tanaka, H. Yamamoto, Comparison of hansen solubility parameter of asphaltenes extracted from bitumen produced in different geographical regions, Energy Fuels 28 (2014) 891–897. [13] C. Panayiotou, Partial solvation parameters and mixture thermodynamics, J. Phys. Chem. B 116 (2012) 7302–7321. [14] C. Panayiotou, Inverse gas chromatography and partial solvation parameters, J. Chromatogr. A 1251 (2012) 194–207. [15] R.F. Fedors, A method for estimating both the solubility parameters and molar volumes of liquids, Polym. Eng. Sci. 14 (1974) 147–154 (and 472). [16] Y. Sato, N. Hosaka, K. Yamamoto, H. Inomata, Compact apparatus for rapid measurement of high-pressure phase equilibria of carbon dioxide expanded liquids, Fluid Phase Equilib. 296 (2010) 25–29. [17] J.M. Prausnitz, R.N. Lichtenthaler, E.G. de Azevedo, Chapter 7, Molecular Thermodynamics of Fluid-Phase Equilibria, 3rd ed., Prentice-Hall, Upper Saddle River, NJ, 1999. [18] R. Span, W. Wagner, A new equation of state for carbon dioxide covering the fluid region from the triple-point temperature to 1100 K at pressure up to 800 MPa, J. Phys. Chem. Ref. Data 25 (1996) 1509–1596.

obtained from literature data [10] at 25 °C. For the case of CO2, the solubility parameter (=9.70 MPa0.5) was estimated with Eq. (2) [9] and using the Span-Wagner equation of state [18] for the saturated liquid state at 25 °C. This is because it was assumed that the dissolved CO2 acted as a liquid in solution.

∂P δ2 ≈ T ⎛ ⎞ − P ⎝ ∂T ⎠V

(2)

Fig. 6 shows plots of the solubility parameter of the mixture versus CO2 solubility. The solubility parameters clearly decreased with increasing CO2 concentration as expected. Open symbols in Fig. 6 denote extrapolated values at the precipitation pressure. It should be noted that the solubility parameter values of the mixture at the precipitation point were located in the limited range of 14.2–16.5 MPa0.5. This means that the precipitation occurs when the solubility parameter difference between the solvent mixture and acrylic resin is about 6 MPa0.5 regardless of diluted solvents. If a generalized rule is found for the value of the solubility parameter difference between resin and CO2 dissolved mixture, it can be a useful guide (methodology) for selecting a solvent in CO2 spray painting systems for given a paint and resin condition. 4. Conclusions A solvent selection guide was developed for CO2 spray coating systems. The solubility parameter of the mixture is helpful in the solvent selection and polymer precipitation conditions. It is important for solvent selection to choose a solvent that has a higher solubility parameter than that of the polymer component. Solubility parameter of solvent and CO2 mixture was estimated by using the solubility of CO2 and also density and solubility parameter of CO2 in the saturated liquid state of 25 °C. Precipitation of the polymer (acrylic resin) from the diluent solution (methanol, butanol, and diethyl ketone) will occur, when

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