Ind. Eng. Chem. Res. 1999, 38, 3655-3662
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Polymer Coatings by Rapid Expansion of Suspensions in Supercritical Carbon Dioxide Jae-Jin Shim,† Matthew Z. Yates, and Keith P. Johnston* Department of Chemical Engineering, The University of Texas at Austin, Austin, Texas 78712-1062
Suspensions of poly(2-ethylhexyl acrylate) in supercritical CO2 formed by dispersion polymerization with a poly(dimethylsiloxane)-based surfactant were sprayed to form uniform films. The viscosity reduction of the dispersed phase caused by dissolved CO2 is crucial for atomization to produce fine droplets, and for coalescence and leveling on the surface to form a uniform film. Re-suspension of the polymer after de-pressurization and re-pressurization led to modestly larger droplets in the suspension which produced a film nearly as uniform as the original one. Inferior films were produced by suspensions without surfactant. Unlike previous studies of rapid expansion of homogeneous polymer solutions and related techniques, films were produced from concentrated polymer mixtures without any organic solvent. I. Introduction Carbon dioxide is an interesting alternative to both organic solvents and water in coating applications, as it is nonflammable, essentially nontoxic, and very volatile. In the UNICARB process, CO2 is used as a diluent for forming acrylic polyester, nitrocellulose, silicone, vinyl, epoxy, phenolic, and urethane coatings.1 The coating formulation consists of a polymer, an organic solvent, and CO2. By using CO2 instead of an organic solvent as the diluent, volatile organic carbon emission may be reduced by 2/3. It would be desirable to find a way to totally eliminate the organic solvent in the CO2-based spray process.2 We are grateful to George Keller for introducing us to the UNICARB process, and we all benefited from his remarks over 10 years ago concerning the potential for applications of supercritical fluid technology in specialty materials. Polymer particles and films may be produced by rapid expansion from supercritical solution. Very few polymers are soluble in CO2 without a cosolvent.3-7 In previous studies of RESS, a homogeneous solution8 in some cases with a cosolvent9 was expanded to atmospheric pressure. In a recent study, polymer particles were produced by RESS with a cosolvent which becomes a nonsolvent after expansion.10 One of the cosolvents studied, ethanol, is more environmentally acceptable than many organic solvents. RESS of a crystalline fluoropolymer, poly(1,1,2,2-tetrahydroperfluorodecyl acrylate), which is highly soluble in carbon dioxide at temperatures near ambient, produces submicron-sized to several micron-sized particles and fibers.11 Polymers diluted with CO2 have been sprayed for powder-coating applications.12 Poly(methyl methacrylate) (PMMA) latexes in CO2 have been formed by dispersion polymerization with various macromonomer, homopolymer, and block copolymer stabilizers.13-16 For this relatively high Tg polymer, polymer latexes with particle sizes in the 1-µm range are stable for hours even without stirring for * To whom correspondence should be addressed. † Current address: School of Chemical Engineering and Technology, Yeungnam University, Kyongsan, Kyongbuk 712749 Korea.
poly(fluoroacrylate)-based stabilizers. For low Tg materials such as poly(vinyl acetate), the particle sizes are much larger, about 7 µm, due to slower nucleation kinetics.17 For poly(dimethylsiloxane) (PDMS) based stabilizers, the particles flocculate and sediment during the polymerization as the monomer concentration becomes small. To date, the morphology produced by spraying latexes in CO2 onto a surface has not been addressed. Our objective is to produce coatings from polyacrylate suspensions in CO2, specifically poly(2-ethylhexyl acrylate) (PEHA), without using an organic solvent or cosolvent. To our knowledge, the use of RESS of heterogeneous polymer suspensions to form polymer films has not been reported. The viscosity of high molecular weight PEHA containing dissolved CO2 is too large for forming a uniform coating by RESS as shown in this study. Instead of lowering the viscosity with an organic solvent, our approach is to reduce the viscosity by forming a suspension of small polymer droplets. The coating is then produced by rapid expansion of this polymer suspension in supercritical CO2. The suspensions are formed by dispersion polymerization with a graft copolymer stabilizer. The results and discussion section begins with spray experiments for these stabilized suspensions to examine the effects of nozzle size, fluid velocity, spray distance, and spray duration. We next describe spray coatings for polymer that was resuspended after stirring was stopped or after the mixture was de-pressurized to remove the CO2, and then re-pressurized. To place these results in perspective, the last section examines suspensions formed without surfactant. Coatings made from latexes or suspensions in CO2 would be extremely different from those made from aqueous latexes. The time for drying of the solvent is far shorter for CO2 than for water. Unlike water, CO2 is soluble in the polyacrylate and may be expected to have a large effect on the viscosity, based upon studies for related polymers.18 The reduction in viscosity is likely important for droplet coalescence and film formation on the surface. Finally, the geometry of the spray may be expected to be considerably different for RESS of a suspension in CO2 compared with that for an aqueous solution.
10.1021/ie990039g CCC: $18.00 © 1999 American Chemical Society Published on Web 08/14/1999
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Ind. Eng. Chem. Res., Vol. 38, No. 10, 1999 Table 1. Average Molecular Weight and Polydispersity of PEHA Produced by Dispersion Polymerization with Monasil PCA Surfactant with the Ratio of Reactants of CO2/2-EHA/Surfactant/AIBN 5/1/0.05/0.01 polymer sample
Mw
rxn time (h)
polydispersity
P1 P2 P3 P4a
124 300 120 500 119 100 102 100
6.3 24 30 29.3
2.20 4.80 4.62 4.42
a
Figure 1. Molecular structure of Monasil PCA.
Figure 2. Schematic diagram of experimental apparatus.
II. Experimental Section Materials. 2-Ethylhexyl acrylate (2-EHA) with a purity of 98% was purchased from Aldrich. The inhibitor was removed by passing the monomer through a column packed with alumina particles (Aldrich). Dissolved oxygen was removed by passing pure nitrogen through the monomer. Once purified, the monomer was kept at 0 °C until it was added to the reaction vessel. Monasil PCA (PDMS-g-pyrrolidone carboxylic acid), obtained from Mona Industries, has a molecular structure shown in Figure 1. From GPC, Mw ) 12 100 and Mn ) 6 600, and the NMR analysis showed that the average value of n is about 3. The surfactant was deoxygenated by flowing nitrogen. The initiator AIBN (2,2′-azobis(isobutyronitrile)) (Aldrich) had a purity of 98% and was purified by re-crystallization from methanol. The instrument grade carbon dioxide (99.99% in purity) contained only trace amounts of impurities such as oxygen (
