Direct Force Measurements between Surfaces Coated with Hydrophobic Polymers in Aqueous Solutions and the Separation of Mixed Plastics by Flotation Nini Ma Thesis submitted to the Faculty of the Virginia Tech in partial fulfillment of the requirements for the degree of Master of Science in Mining and Minerals Engineering Roe-Hoan Yoon, Chair Gregory T. Adel Gerald H. Luttrell Aug.27, 2008 Blacksburg, Virginia Keywords: surface force, hydrophobic polymers, DLVO theory, froth flotation, contact angle, plastics recycling, flotation of plastics Direct Force Measurements between Surfaces Coated with Hydrophobic Polymers in Aqueous Solutions and the Separation of Mixed Plastics by Flotation Nini Ma ABSTRACT Froth floatation is an important process used in the mining industry for separating minerals from each other. The separation process is based on rendering a selected mineral hydrophobic using an appropriate hydrophobizing reagent (collector), so that it can selectively attach onto the surfaces of a rising stream of air bubbles. Thus, controlling the hydrophobicity of the minerals to be separated from each other is of critical importance in flotation. If one wishes to separate plastics from each other by flotation, however, it would be necessary to render a selected plastic hydrophilic and leave the others hydrophobic. In the present work, the possibility of separating common plastics from each other by flotation has been explored. While water contact angle is the most widely used measure of the hydrophobicity of a solid, it does not give the information on the kinetics of flotation. Therefore, the forces acting between the surfaces coated with different hydrophobic polymers (or plastics) in water were measured using the Atomic Force Microscope (AFM). The results obtained with polystyrene, poly(methyl methacrylate) (PMMA), polypropylene (PP), and Teflon showed the existence of long-range attractive forces (or hydrophobic force) that cannot be explained by the classical DLVO theory. The surface force measurements were conducted in pure water and in solutions of surfactant (alkyltrimethylammonium chloride) and a salt (NaCl). In pure water, the attractive forces were much stronger than van der Waals force. In the presence of the surfactant and NaCl, the long-range attraction decreased with increasing concentration and the alkyl chain length. A series of contact angle measurements were conducted to determine the hydrophobicity of polystyrene (PS), polyvinyl chlorite (PVC), and polymethylmethacrylate (PMMA) in the presence of different wetting agents (surfactants). The results show the possibility of separating plastics from each other by flotation, and a series of microflotation tests conducted on PS and PVC showed promising results. TABLE OF CONTENTS I - INTRODUCTION ................................................................................................ 1 1.1 Hydrophobic Forces .................................................................................... 1 1.2 Plastics Flotation .......................................................................................... 4 1.3 Research Layout .......................................................................................... 6 II - HYDROPHOBIC FORCES BETWEEN PLASTIC SURFACES IN AQUEOUS SOLUTIONS ............................................................................................. 7 2.1 Materials and Experiments ......................................................................... 8 2.1.1 Plastic Spheres ........................................................................................ 8 2.1.2 Synthesis of PS, PMMA and PP Substrates Through Spin-coating .......... 9 2.1.3 Water and Surfactant Solutions ............................................................... 9 2.1.4 Surface Force Measurements on PS Surface ......................................... 10 2.1.5 Contact Angle Measurement ................................................................. 10 2.1.6 Zeta-potential Measurement.................................................................. 11 2.2 Results and Discussion ............................................................................... 11 2.2.1 AFM Image .......................................................................................... 11 2.2.2 Zeta-potential ....................................................................................... 11 2.2.3 Surface Forces Measured in Nanopure Water ....................................... 11 2.2.4 Surface Forces Measured in Surfactant Solutions .................................. 14 2.2.5 Surface Forces Measured in Salt Solution ............................................. 16 2.3 Summary .................................................................................................... 17 2.4 Figures and Tables ..................................................................................... 19 III – SEPARATION OF MIXED PLASTICS BY FLOTATION ......................... 31 3.1 Materials and Experiments ....................................................................... 31 3.1.1 Plastics ................................................................................................. 31 3.1.2 Surfactants ............................................................................................ 31 3.1.3 Water .................................................................................................... 32 3.1.4 Contact Angle Measurement and Captive Bubble Technique ................ 32 iii 3.1.5 Micro-flotation ..................................................................................... 32 3.2 Results ........................................................................................................ 33 3.2.1 Effect of Different Wetting Agents on Contact Angles of PS ................ 33 3.2.2 Effect of Different Wetting Agents on Contact Angles of PVC ............. 36 3.2.3 Effect of Different Wetting Agents on Contact Angles of PS and PVC . 37 3.2.4 Flotation Test Results ........................................................................... 40 3.3 Summary .................................................................................................... 41 REFERENCES ....................................................................................................... 43 iv INDEX OF FIGURES AND TABLES Figure 2.1 Schematic representation of the captive bubble technique. ...................... 19 Figure 2.2.a AFM image of PS substrate immersed in water..................................... 19 Figure 2.2.b AFM image of PMMA substrate immersed in water. ............................ 20 Figure 2.2.c AFM image of PP substrate immersed in water. ................................... 20 Figure 2.3 Zeta-potential value of PS, PMMA and PP measured at different pH. ..... 21 Figure 2.4 AFM force measurements conducted between a PS sphere and plate in water at neutral pH. ...................................................................................................... 22 Figure 2.5 AFM force measurements conducted between a PS sphere and plate in water at neutral pH. ...................................................................................................... 22 Figure 2.6 AFM force measurements conducted between a PMMA sphere and plate in water at a neutral pH. ................................................................................................... 23 Figure 2.7 AFM force measurements conducted between a polymethyl methacrylate (PMMA sphere and plate in water at a neutral pH......................................................... 23 Figure 2.8 AFM force measurements conducted between a PP sphere and plate in water at a neutral pH. ................................................................................................... 24 Figure 2.9 AFM force measurements conducted between a polypropylene (PP) sphere and plate in water at a neutral pH. ................................................................................ 24 Figure 2.10 AFM force measurements conducted between a Teflon sphere and plate in water at a neutral pH. ................................................................................................... 25 Figure 2.11 AFM force measurements conducted between a Teflon sphere and plate in water at a neutral pH. ................................................................................................... 25 Figure 2.12 Surface force measurements conducted between a PS sphere and plate in the presence of different concentrations of C14TACl....................................................... 26 Figure 2.13 Surface force measurements conducted between a PS sphere and plate in the presence of different concentrations of aC16TACl..................................................... 26 Figure 2.14 Surface force measurements conducted between a PS sphere and plate in the presence of different concentrations of a C18TACl.................................................... 27 Figure 2.15 AFM surface force measurements conducted between a PS sphere and plate in 0.1% sodium lignosulfonate solution. ................................................................ 27 v Figure 2.16 Surface force measurements conducted between a PMMA sphere and plate in the presence of different concentrations of C14TACl. ......................................... 28 Figure 2.17 Surface force obtained between a PMMA sphere and plate in salt solutions. 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