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Ind. Eng. Chem. Res. 2002, 41, 4750-4757
Aqueous Latexes Formed from Polymer/CO2 Suspensions. 2. Hydrophilic Surfactants in Water Jae-Jin Shim† and Keith P. Johnston*,‡ School of Chemical Engineering and Technology, Yeungnam University, 214-1 Tae-dong, Kyongsan City, Kyongbuk 712-749, Korea, and Department of Chemical Engineering, The University of Texas at Austin, Austin, Texas 78712
Stable aqueous latexes were made by transporting viscous liquid poly(2-ethylhexyl acrylate) suspensions in carbon dioxide into water containing hydrophilic surfactants. The surfactants were composed of poly(ethylene oxide) hydrophilic blocks and either poly(propylene oxide) or poly(butylene oxide) anchor blocks. The polymer suspensions were synthesized in supercritical CO2 utilizing poly(dimethylsiloxane)-g-pyrrolidonecarboxylic acid as a stabilizer. Rapid expansion of these suspensions through a nozzle into the aqueous solution produces stable latexes. Alternatively, CO2 may be vented from the polymer suspensions, and the resulting viscous polymer may be redispersed in an aqueous surfactant solution with mild stirring. Polymer concentrations up to 40 g in 100 g of water were stabilized with low concentrations of hydrophilic surfactant. The latexes may be utilized as environmentally benign coatings and adhesives without the need for organic solvents. I. Introduction Since the mid-1980s, supercritical fluid technology has been extended to polymer processes such as impregnation,1-5 purification,3 and fractionation6 and later to polymer synthesis.7-14 Unlike conventional polymerization in water or an organic solvent, polymerization in supercritical carbon dioxide (CO2) does not produce wastewater or organic emissions. CO2 is essentially nontoxic and environmentally benign. However, CO2 is usually a poor solvent for polymers, and a small amount of cosolvent is often required.15 Only a few polymers with high molecular weights are soluble in CO2 without a cosolvent.16-19 Highly soluble poly(1,1,2,2-tetrahydroperfluorodecyl acrylate) may be expanded through a nozzle to produce submicron to several micron-sized particles and fibers.20 A variety of processes have examined polymers that are not soluble. The viscosity of polymers maybe lowered by dilution with CO2 in order to spray solutions to produce powders for powder coating and composite material applications.21 In addition, polymer particles may be stabilized in supercritical CO2 with surfactants that have a CO2philic block(s) and an anchor block(s) that adsorbs to the polymer.7 Polymethacrylate latexes in CO2 have been formed by dispersion polymerization with various macromonomer, homopolymer, and block copolymer stabilizers.7-9,21,22 Submicron poly(methyl methacrylate) (PMMA) particles that were synthesized with a triblock surfactant in supercritical CO2 have been redispersed in water to form up to 40 wt % stable aqueous latexes.23 The CO2-philic poly(dimethylsiloxane) (PDMS) block provides steric stabilization for PMMA particles in CO2, while the hydrophilic methacrylic acid or acrylic acid groups ionize to produce electrostatic stabilization in * To whom correspondence should be addressed. E-mail:
[email protected]. † Yeungnam University. ‡ The University of Texas at Austin.
water. The lipophilic acrylate group anchors to the PMMA polymer surface in both media. The surfactant is “ambidextrous” in that it stabilizes latexes in both media. In another latex application, suspensions of poly(2-ethylhexyl acrylate) (PEHA) in supercritical CO2 formed by dispersion polymerization with a PDMSbased surfactant have been sprayed to form uniform films on a glass substrate.10 In our previous study13 we produced stable aqueous emulsions from PEHA suspensions synthesized in supercritical CO2. Two surfactants were present during the synthesis in CO2. A CO2-philic surfactant was utilized to stabilize the polymer suspension in CO2. A CO2-soluble hydrophilic surfactant was also present. After this suspension was expanded rapidly through a nozzle into water, the hydrophilic surfactant provided stabilization. Both surfactants were inexpensive compared to fluorinated-polymer surfactants or other triblock copolymer surfactants utilized previously.7,23 Although this method requires only a small amount of hydrophilic surfactant, many of these surfactants are poorly soluble in CO2. To overcome this solubility limitation, it would be desirable to design a process that does not require a hydrophilic surfactant in the CO2 phase. The objective of this study was to produce a stable aqueous latex by two methods: (1) expanding a polymer suspension in CO2 into an aqueous surfactant solution containing a hydrophilic surfactant [polymerization/ rapid expansion into supercritical solution (RESS) method] and (2) venting CO2 from the polymer suspensions and redispersing the resulting polymer in an aqueous surfactant solution with mild stirring (transfer method). A CO2-philic surfactant was used to form the suspension by dispersion polymerization. Unlike the case above, a hydrophilic surfactant was not present in the CO2 phase. Upon spraying of the suspension into an aqueous solution, the hydrophilic surfactant diffused to the particle surface to provide stabilization. The effects of surfactant structure, sonication of the aqueous dispersion, and pH on the droplet size and stability of
10.1021/ie011000q CCC: $22.00 © 2002 American Chemical Society Published on Web 08/17/2002
Ind. Eng. Chem. Res., Vol. 41, No. 19, 2002 4751 Table 1. Molecular Formulas (Average Number of Repeat Units) and Performances of Various Surfactants Tested in This Study commercial name
molecular formula
PEO
PPO
HLBa
emulsion stabilizationb
Pluronic 17R2c Pluronic 25R4c Pluronic 31R1c Pluronic L31c Pluronic L43c Pluronic L61c Pluronic L62c Pluronic P103c Pluronic P123c Tetronic 90R4c SAM 185d SAM 187d SAM 211-80
(PPO)14.7-b-(PEO)10.2-b-(PPO)14.7 (PPO)20.2-b-(PEO)28.6-b-(PPO)20.2 (PPO)26.0-b-(PEO)5.2-b-(PPO)26.0 (PEO)1.1-b-(PPO)16.5-b-(PEO)1.1 (PEO)7.4-b-(PPO)20.7-b-(PEO)7.4 (PEO)1.7-b-(PPO)31.9-b-(PEO)1.7 (PEO)6.8-b-(PPO)32.8-b-(PEO)6.8 (PEO)17.0-b-(PPO)59.5-b-(PEO)17.0 (PEO)18.2-b-(PPO)71.6-b-(PEO)18.2 {(PPO)19.4-b-(PEO)15.6}2-NCH2CH2N-{(PEO)15.6-b-(PPO)19.4}2 (PBO)12-b-(PEO)25 (PBO)12-b-(PEO)35 alkylene poly(alkoxy sulfate)
425 1260 230 100 650 150 600 1500 1600 2750 1100 1540 N/A
1700 2340 3020 950 1200 1850 1900 3450 4150 4490 960e 960e N/A
6 8 1 5 12 3 7 9 8 7 8 12 N/A
fair fair fairly good bad bad good very good fairly good fairly good fair good fair fairly good
a The HLB numbers are from the data in the BASF’s Web Page. b The grades are based on the visual observation of the PEHA emulsion in a surfactant solution. The sample is marked “very good” when it is stable over 96 h and “bad” when it is completely settled within 24 h. c The molecular formulas of these surfactants are only approximate, calculated based on the information from the manufacturer. d The molecular formulas are from O’Neill et al.31 The number of PEO segments appears to be somewhat low for the reported HLB values.e The number indicates the molecular weight of PBO.
the PEHA emulsions in water are reported. The use of two separate surfactants, many of which are commercially available, offers flexibility in designing emulsions in contrast with the use of a single ambidextrous surfactant.23 The polymer particles that are covered with only a CO2-philic surfactant can potentially be more stable in CO2 than those covered with an ambidextrous surfactant or two surfactants, with one being CO2-philic and the other hydrophilic. Also, the ability to add large concentrations of a hydrophilic (CO2insoluble) surfactant to the aqueous phase affords the opportunity to produce highly concentrated aqueous latexes. Applications of this study include environmentally friendly paints, coatings, and adhesives. II. Experimental Section Materials. 2-Ethylhexyl acrylate (2-EHA) of purity of 98% was purchased from Aldrich. The inhibitor and dissolved oxygen in the monomer were removed by alumina (Aldrich) and pure nitrogen, respectively. Monasil PCA (PDMS-g-pyrrolidonecarboxylic acid) containing three grafted carboxylic acid groups (molecular weight of approximately 8500) was obtained from Mona Industries and deoxygenated by flowing nitrogen. Hydrophilic surfactants including Pluronic 17R2, Pluronic 25R4, Pluronic 31R1, Pluronic L43, Pluronic L61, Pluronic L62, Pluronic P123, SAM 185, SAM 187, SAM 211-80, and Tetronic 90R4 were obtained from BASF. These surfactants were used without further purification; molecular formulas are shown in Table 1. The initiator 2,2′-azobis(isobutyronitrile) (AIBN; Aldrich) had a purity of 98% and was further purified by recrystallization in methanol. The instrument-grade CO2 (99.99% in purity) contained only trace amounts of impurities such as oxygen (
