Polymers and Supercritical

Polymers and Supercritical CO2 - An invention to new polymer processing technologies - On-line Number 5017 Masahiro Ohshima Department of Chemical Engineering, Kyoto University, Nishikyo-ku, Kyoto, 615-8510, Japan E-mail: [email protected] ABSTRACT Supercritical CO2 (sc-CO2) is being utilized for various polymers processing such as polymer blending, additive impregnation and plastic foaming. When CO2 dissolves into polymers, the polymer chain mobility is enhanced and rheological and thermal properties of the polymer are changed. In this talk, the solubility and the diffusivity of CO2 in the polymer as well as the CO2-induced polymer property (i.e., plasticization effects), such as reduction of glass transition temperature and viscosity, are shown with experimental data. Then, CO2 microcellular polymeric foaming, a CO2 aided additive infusion and polymer surface modification schemes are introduced to illustrate the potential of CO2 creating new polymer processing techniques. KEYWORDS Super critical fluid, carbon dioxide, plasticization effect, polymer, foams, and surface modification INTRODUCTION Carbon dioxide (CO2) becomes supercritical at pressures and temperatures above 7.38 MPa and o 31.1 C. At the supercritical condition, the CO2 has a higher diffusion coefficient and a lower viscosity than liquids and the surface tension becomes absent, which allows a rapid penetration of molecule into the pores of heterogeneous matrices. Taking advantage of these characteristics together with its nontoxicity, nonflammability and inexpensiveness, CO2 has been used in a variety of industries, i.e., food, pharmaceutical, fiber, chemical and plastic industries (Berens et al, 1992; Schnitzler et al, 1999). At the beginning of the research on the supercritical fluids, the focus was mainly on extraction and separation of small molecules by the fluids. Then, research interest gradually shifted toward complex molecules. In polymer reaction engineering, a famous application of CO2, in which sc-CO2 was used as a solvent for tetrafluoroethylene (TFE) during the production of polytetrafluoroethylene (PTFE), was reported by DuPon in 1999 (McCoy, 1999). There is another successful application in material processing fields, in which sc-CO2 was used as a physical foaming agent (PFA) to create the microcellular plastic foams (Okamoto, 2003). In addition to the microcellular foaming, many potential applications of CO2, such as polymer blending, additive impregnation and surface modifications, have been investigated for polymer 1 processing purposes. In this talk, we will focus on the applications of CO2 to materials processing, especially polymer processing, and discuss the potentials of CO2 creating new materials processing technologies. CO2 Dissolving in Polymers One major advantage of applying sc- CO2 to polymers from the viewpoint of material processing is that the processing conditions and the morphology of polymers are significantly affected by CO2. The presence of CO2 in polymer changes the polymer’s properties in both the molten and solid sates and realizes unique material processing scheme for creating new functional materials. Solubility of CO2 into polymer The solubility and the diffusivity of CO2 in molten polymer are fundamental properties for CO2 aided polymer processing. Figure 1 shows the solubility data of CO2 into several polymers, such as low-density polyethylene (LDPE), polypropylene (PP), poly(lactic acid) (PLA), Ethylene Propylene Diene Monomer (EPDM) and polystyrene (PS). They were measured by a magnetic suspension o o balance at temperature 200 C, except 150 C for o Fig.1 Solubility of CO2 in polymers (200 C) Polyethylene glycol (PEG) (Areerat et al., 2004). (Areerat et al., 2003, 2004) For all thermoplastic polymers, the solubility increases in proportion to the increase of pressure and it decreases as temperature increases. Diffusivity of CO2 into polymer Figure 2 shows the mutual diffusion coefficients of CO2 in those polymers against the initial concentration of CO2 in polymer. The diffusivity is a function of temperature, pressure and concentration of dissolved CO2. The diffusivity increases as temperature increase and decreases as pressure increases. When CO2 dissolves, the polymer swells and the free volume of the polymer Fig. 2 Mutual diffusion coefficients of increases. Then, the diffusion coefficient becomes o CO2 in polymers @ 200 C at different pressures larger as the dissolved CO2 concentration becomes (Areerat et al., 2003,2004;Funami et al 2004) higher. Although the dependence of diffusion 2 coefficient on dissolved CO2 concentration was not clearly observed at these temperatures, the effect of dissolved CO2 concentration becomes prominent with decreasing the temperature. Koga et al investigated the CO2 sorption into poly(methyl methacrylate) PMMA as well as PS polymers using in situ neutron reflectivity (Koga et al., 2002). They observed that PMMA and PS swell by approximately 30% and anomalous diffusion of CO2 in these polymers at temperatures below their original glass transition o temperature (Tg ≈ 100 C ). The dissolved CO2 enhances the mobility of the polymer chains and changes the polymer properties, such as glass transition temperature, viscosity and so on. Some of the CO2 induced property changes are introduced in the next section. CO2 induced property change CO2 induced reduction of glass transition temperature, Tg The glass transition temperature, Tg, of the polymer 0 ○：PET decreases when CO2 dissolves into polymer. Chiou et al. △ ：cyclic olefin (ARTON) -5 ×： cyclic olefin (ZEONEX) showed that 5 wt% CO2 reduces Tg of polyethylene ：Polyimide（Kapton) o -10 terephthalate (PET) about 25 C (Chiou et al., 1985). ] ] Figure 3 shows the CO -induced reduction of T of other -15 2 g ℃ ℃ [ [ g g polymers, which were measured by using a high-pressure T T ∆ ∆ -20 differential scanning calorimeter (Perkin-Elmer, Pyris 1 with -25 N520 high pressure cell). In Fig. 3, the magnitude of -30 depressions is plotted against CO2 pressure. -35 The CO2-induced Tg depression shows an 0 10203040interesting nonlinear behavior against CO2 pressure at CO pressure [atm] 2 PMMA, acrylonitrile- butadiene- styrene (ABS) and ∆ T g =Tg under pressurized CO2 － original Tg poly(ethyl methacrylate) (PEMA) (Condo et al., 1992; Fig. 3 CO2-induced depression of Tg Nawaby et al., 2003). Figure 4 shows the CO2-induced Tg depression of ABS. This nonlinear depression is called retrograde behavior, whereby two transitions under a constant CO2 pressure exist: a rubber to glass transition at a lower temperature and a glass to rubber transition at a higher temperature. The existence of the lower (rubber to glass) transition temperature is due that the solubility of CO2 increases with decreasing temperature and enhances the mobility of polymer chains. Using dielectric relaxation spectroscopy, Saito et al measured the variation in molecular motion of PMMA under CO2 dissolution and observed the increase of β relaxation with dissolving CO2 (Saito et al., Fig.4 Depression of Tg of ABS (Nawaby et al, 2003). 3 2003). The CO2-enhancement of polymer chain mobility changes several polymer properties. CO2 induced reduction of shear viscosity Figure 5 shows the CO2– induced viscosity reductions of PP and PS polymers. The viscosities of the polymer/CO2 mixtures were measured by a capillary rheometer equipped with a foaming extruder. As can be seen in Fig. 5, the viscosity of PS was reduced by 40% by dissolving CO2 3.5 wt % and that of PP was reduced by 25%. The CO2-induced viscosity reductions can be predicted by the following models with the free volume and viscosity data of the neat polymer (Nagata et al,2000;Areerat et al, 2003): (n −1) n ⎛ γ ⎞ η(γ) ≅ ηo ⎜ ⎟ (1) ⎝τ ⎠ ⎛ B ⎞ η = A exp⎜ 3 ⎟ (2) o 3 ⎜ ⎟ ⎝ f (T, P, wg ) ⎠ f (T, P, wg ) = fr +(1− fr)α(T−Tr)−(1− fr)β(P−Pr)+ζ(wg −wgr) (3) f (T,P,wg) denotes the free volume fraction Fig. 5 CO2 induced viscosity reduction in PP and PS of polymer/CO2 mixture. It is a function of temperature, T, pressure, P, and weight fraction of dissolved CO2, wg. A3, B3 and n are constant parameters determined by the viscosity data of the polymer alone. α and β are thermal expansion and isothermal compressibility coefficients, respectively. The values of these coefficients are determined by the PVT data of the polymer alone. The gas expansion coefficient, ζ , is determined by solubility measurements, The models with these parameter values can predict the viscosity of polyme/CO2 single phase mixtures. The solid lines in Fig.5 represent the estimates of the models. Plastic deformation The dissolved CO2 can also change the mechanical property of polymers. Beyond the elastic limit, the polymer undergoes plastic deformation. The dissolved CO2 can make brittle polymers ductile and allows large deformation. Figure 6 shows a strain-stress curve of PMMA measured at the room temperature. The PMMA was treated under pressurized CO2 for 1 hr before the measurement. As long Fig. 6 Strain-Stress curve of PMMA/CO2 system 4 as CO2 dissolves in the PMMA, the PMMA could be elongated to a large extent with a smaller stress. Micro/Nano Cellular Plastic Foaming In the mid 1980s, microcellular foaming with sc-CO2, which is a physical foaming method, was developed in the United States (Martini et al, 1982). The foaming showed a specific structure with a cell size of about 10 µm and cell density of about 108-109 cells/cm3, which provided improved mechanical properties, e.g., higher impact strength and toughness, than conventional foaming, while reducing

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