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 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, , 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, - - 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 material cost. Since then, the physical foaming with CO2 and/or N2 has attracted much attention and has led to exploration of large number of innovative applications (Baldwin et al, 1996, Park, et al, 1993) . Figure 7 shows a micrograph of commercially available micro cellular foams, which are made of PET by foaming at a furnace after dissolving CO2 (Furukawa, 2004).

------30μm Fig.7 Microcellular PET Fig. 8 Series of micrographs of batch foaming foams (Courtesy of Furukawa o Electric Co. Ltd) (Polypropylene-CO2 at 180 C, 11MPa)

The procedure of CO2-physical foaming is as follows; dissolving CO2 in polymers as a physical foaming agent and then releasing pressure or increasing the temperature induces phase separation of the

CO2 in polymer. The foaming procedure is simple, however, the creation of the fine cellular structure is not easy. The foaming involves CO2 dissolution, bubble nucleation, growth and coalescence. One has to understand the cause-and-effect relationship among polymer properties, processing conditions and foaming mechanism at these four processes in order to produce fine (micro/nano) cellular structures. There have been many researches on physical foaming both from experimental and theoretical viewpoints. A visual observation of batch foaming using a high-pressure autoclave equipped quarts windows was conducted to observe the bubble nucleation, growth and coalescence behavior in situ (Taki et al, 2002).

5 Bubble Nucleation and Growth in Foam Polymers Figure 8 shows a series of micrographs obtained through experiments. The pictures were taken every 0.4 second over 2 seconds from 6.4 second after the pressure release. They show the dynamic behavior of bubble nucleation and growth in polypropylene, where it was foamed at 180 oC by releasing the pressure from 10 to 0 MPa at the rate of 0.48 MPa/s after dissolving CO2 for 30 minutes. The bubbles were observed as black dots in the micrographs due that the bubbles reflected the light coming from a window. These pictures show that the bubble growth and nucleation occurred simultaneously and the nucleation rate around the existing large bubbles was lower. The diffusion of CO2 from polymer matrix to bubble occurs in the course that the bubble is growing. Thus, the concentration of dissolved

CO2 around the existing bubble becomes lower when the bubble grows, and the lower concentration suppresses the nucleation around the bubble. The image analysis of these micrographs could reveal several other aspects of polymeric physical foaming: 800 ] 1) the bubble nucleation is strongly dependent on 2 700 2.4 MPa/s 600 1.8 MPa/s operating conditions, especially, the pressure release 500 0.97MPa/s rate or pressure gradient. Figure 9 shows the 400 300

cumulative number of nucleated bubbles against time at 200 various pressure release rates. The solid line represents 100 0 number density of bubbles [1/mm bubbles of density number the cumulative number of nucleated bubbles and the 0123456 time elapsed after pressure release [s] error bar indicates the standard deviation. As the pressure release rate increases, the number of nucleated Fig.9 Effect of pressure release rate bubble increases at the batch physical foaming. for bubble nucleation 2) the wettability of nucleating agents to polymer controls the nucleation rate, drastically. (Details will be given by 35 N2 foaming CO foaming other papers) 2 30

3) the bubble growth in polymer is a combination of mass m] µ 25 transfer controlled and viscosity controlled processes: 20 At the initial stage of foaming, the bubble growth is controlled by polymer viscosity, and then followed by Bubbleradius [ 15 mass transfer controlled growth. When the bubbles 10

0.0 0.1 0.2 0.3 0.4 0.5 0.6 grow in polymer, the CO2 diffuses into bubbles from time [s] the surrounding polymer matrix. Thus, higher diffusivity and higher concentration of CO2 results in Fig.10 Bubble growth rate in the

higher bubble growth rate, and higher viscosity induces initial stage of N2 and CO2 foaming lower growth rate. Figure 10 shows the bubble growth behaviors obtained from the visual observation of both

N2 and CO2 foaming experiments, where. N2 and CO2 were individually dissolved into PP at 10 MPa o and librated at 180 C by releasing the pressure at the rate of 0.64 MPa. N2, whose solubility to

6 polymer is lower than that of CO2, depressed the bubble growth rate greatly and consequently tended to create smaller bubbles. This experimental result shows that the bubble growth is a diffusion-controlled or mass transfer controlled process. Figures 11 and 12 show numerical simulation results of growth behavior and internal pressure of

a bubble born at time 3.4 second, where the five polypropylene samples were foamed by CO2. The viscosities of five samples were different, four of them correspond to the actual viscosity of the polypropylene samples and the other, 65,700 Pa s, was one hypothetically assumed. After the bubble grows to a certain diameter, about 20-30 µm, the bubble growth profiles of all five samples showed linearity against the square root of the time. This means that the bubble growth in this region is a mass transfer controlled growth. However, before entering in to the mass transfer controlled region, the polymer viscosity controls the bubble growth. As can be seen in Fig. 11, the higher viscosity makes the bubble growth slower at the initial stage. However, at a certain stage, the bubble growth of the higher viscosity polymer is accelerated. The growth rate of the higher viscosity polymer is accelerated owing that the higher viscosity polymer keeps the pressure difference between bubble and polymer matrix higher. Namely, the bubble growth in high viscosity polymer is the viscosity-controlled growth at the initial stage of foaming and moves to a mass transfer controlled growth when the bubble internal pressure approach to the matrix pressure.

250 11 PP-K1 (2590 Pa s) 200 PP-K3 (4640 Pa s) PP-K1 (2590 Pa s) m] PP-K5 (12 500 Pa s) µ 10 PP-K3 (4640 Pa s) PP-K9 (419 Pa s) PP-K5 (12 500 Pa s) 150 η=65 700 Pa s PP-K9 (419 Pa s) 9 η=65 700 Pa s

100

Pressure [MPa] 8 -9 2

Bubble diameter [ diameter Bubble D=8.1x10 m /s 50 H=1.3x10-4 mol/(m3 Pa) -6 2 7 γ=21-6.2x10 xP mJ/m P D C P =P =11 MPa 0 c0 D 6 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 3.03.54.04.55.05.56.06.57.07.5 Time [s] Time [s]

Fig. 11 Simulation result of change in diameter Fig. 12 Simulation results of change in pressures

of a growing bubble (PD: bubble, Pc: Polymer matrix)

4) Interaction bubble growth and nucleation exists. It is often observed in physical foaming that, when the bubble density increases, the average diameter of the bubble decreases. This is due to the mass

balance of dissolved CO2. That is, when the number of bubble (bubble density) increases, the

amount of CO2 available to grow a bubble becomes less. Then, the size of the bubbles becomes smaller as the number of bubble increases. This is a statistical interaction between bubble growth and

nucleation caused by the total mass balance of dissolved CO2. The mass balance also affects the relation between bubble growth and nucleation rates, dynamically. Several PS samples with

7 different molecular weights were foamed by CO2 in the visual observation of the batch foaming and the maximum nucleation rate observed at the batch foaming experiment was recorded as a representative nucleation rate for an experiment. The foaming experiments were conducted by varying the foaming temperature. Figure 13 shows the maximum nucleation rate of PS foam with respect to foaming temperature. The nucleation rate shows a local minimum against the temperature. The classical nucleation theory can explain the increase of nucleation rate with increasing the foaming temperature but cannot explain the increase of nucleation rate with decreasing the temperature. It

could be explained by viscosity controlled bubble growth behavior and CO2 mass balance relationship. Namely, the bubble growth rate at temperature below the minimum point was depressed owing to

viscosity increase and the concentration of dissolved CO2 in polymer matrix was kept high. Consequently, the bubble nucleation increases with decreasing the temperature. 5) Shear and elongational viscosities dominate bubble coalescence. The bubble coalescences in foaming 1800

s)] PS-3K 3 1600 were classified into several patterns. The most PS-20K 1400 PS-30K PS-210K interesting behavior is that, when two bubbles are 1200 coalesced, a bubble sticks into the inside of the other 1000

bubble and breaks up. This behavior was observed 800 specifically in the polymers with high elongational 600 400 viscosity. The shear and the elongational 200 80 100 120 140 160 180 200

viscosities strongly affect the coalescing time, which [1/(mm rate nucleation Bubble Foaming Temperture [oC] is the time until coalescing after contacting each other. The higher shear viscosity and/or higher Fig. 13 Maximum nucleation rate vs elongational viscosity the polymer has, the longer temperature at physical foaming of PS. the coalescence time becomes. Therefore, the high (3K:Mw3500, 20K:24000,30K:32800,210K292000) elongational viscosity is effective to protect the bubble coalescence.

Cellular Structures in Foam Materials

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Fig. 15 Cellular structure of Fig. 16 Cellular structure of Fig.14 Cellular structure of PEG/PLA blend foam PEG/PS blend foam PP/EPR blend foam

8 Using the above-mention characteristics positively, several cellular structures can be produced by the CO2 physical foaming. Figures 14-16 show micrographs of some of the interesting cellular structures created by the physical foaming in our laboratory. Figure 14 is foam of PP/EPR blend where the bubbles are localized in rubber (EPR) phase. In the figure, the white dots in black portion represent the bubbles in the rubber. Figure 15 shows a unique bimodal (large and small) cellular structure of PEG/PLA blend, where the large-size cells embrace a PEG particle. Figure 16 illustrates an open-cell structure observed at foams of PS/PEG blend.

The physical foaming technique, where CO2 is dissolved in materials and phase separation is induced by pressure or temperature conditions to create voids in materials, can be applicable to producing other foam materials. Figure 17 shows foam ceramics, which was made by foaming a mixture of aluminum oxide particles and polymeric binder and sintering the foam.

Fig.17 Foam alumina ceramics CO2 aided Polymer Processing (CO2 foaming) Additive impregnation (Coating)

As described in the previous sections, CO2 enhances the mobility of polymer, plasticizes the polymers and dissolves several organic chemicals in supercritical state. Taking advantages of these characteristics, CO2 is being utilized as an efficient and safe solvent in polymer processing. The CO2 aided additive impregnation (CO2 aided infusion) is one of the promising material processing techniques(Berens et al., 1992). It is normally performed in the following way: polymer is exposed to both sc-CO2 and the additives under pressure for a certain period so as Fig. 18 Infusion schemes to diffuse CO2 as well as additives into polymer. Then, depressurization causes the CO2 to rapidly diffuse out from the polymer, while keeping additives in the polymer owing to the slower their diffusivities. One of the drawbacks of this CO2 aided infusion was that the additives insoluble to sc-CO2 could not be infused into polymer. Recently, Mizutani et al found that immersing polymer in additive-rich solution of an equilibrium state of

CO2/addiive system can infuse the additive in polymer more efficiently (Mizutani et al, 2004). Their method can Fig. 19 Infusion of PEG 1000 in PS

9 infuse the Poly(ethylene grychol) (PEG) polymer whose molecular weights were 1000 and 4000 into PS, which is hard to soluble to sc-CO2. Figure 19 shows weight changes of the PS sample after infusing PEG 1000 in additive-rich solution phase of the equilibrium states established by varying equilibrium temperature. At the same time, for the comparison, a polymer sample was also treated in CO2-rich gas

(sc-CO2) phase. The infusion in CO2-rich gas phase of the equilibrium states is equivalent to the conventional infusion method. The PEG with the molecular weight 1000 as well as that of 4000 could be infused in PS by immersing the polymer in the additive-rich solution as shown in Fig. 19.

Surface Modification of Polymer(Injection Molding) By combining the plasticization and the solvation effects, a new injection-molding scheme of modifying the surfaces of products can be also invented: Two basic schemes, direct injection and core-back (compression) moldings, are proposed. Direct injection molding

In the direct injection-molding scheme, CO2 (scf-CO2) is introduced into a mold cavity of an injection molder. On the gas line of introducing the CO to the cavity, 2 Fig. 20 Concept of CO2 aided surface low molecular substances, such as dye pigments, alcohols, monomers, and/or low molecular polymers are dissolved into the sc-CO2. The cavity is pressurized and filled with the CO2 and the molten polymer is then injected into the pressurized cavity. The scheme can be regarded as a counter-pressure injection-molding scheme. As illustrated in Fig. 20, when the polymer is flowing in the cavity, the CO2 and the low molecules solutes diffuse into the melt front of the flowing polymer. In the mold cavity, the polymer shows so-called fountain flow behavior, i.e., the Fig. 21 Experimental result of dyeing of polymer polymer at the melt front moves from the surface centerline of the stream to the cavity wall. The surface polymer at the melt front containing CO2 as well as low molecular substances flows toward the cavity walls on the fountain flow. The dissolved CO2 in polymer eventually diffuses out to atmosphere while low molecular substance remains the products. The scheme can produce a plastic product whose surface is modified by the low molecular substances. Figure 21 shows a result of surface modification injection molding experiments, where a 35 tons

10 injection molder was used to modify the PP and LDPE polymers with disperse red dye pigment, Oil Red.

By varying the scf-CO2 pressure at the buffer tank on the CO2 injection line, the surface modification was performed. The color of dyeing samples was evaluated using the x-y-z color system. The X-value indicates the degree of red color. As can be seen, the red color of the plastic products’ surface becomes darker as the pressure of scf-CO2 increases over critical temperature. A Core back (Compression) molding In the direct injection method, the contact time, which is a period of contacting the polymer with sc-CO2, cannot be made longer due to the processability of polymers. In order to manipulate the contact time and control the thickness of the layer modified by a low molecular substance, a core-back molding method is available. After injecting the polymer or placing the polymer in a cavity, a part of the mold is shifted back so as to make a narrow space. Then, the mixture of

CO2 and the low molecular substance is introduced into the space while keeping the pressure sealing. Then, the part of the mold is push back and the pressure of CO2 with solutes in space is increased so as to dissolve both CO2 and low molecular solutes into the polymer. By varying the volume of the space and the pressurizing time, the thickness of the modifying layer is controlled. The scheme can Fig. 22 Scheme of core-back (compression) be used for realizing the developed additive molding impregnation scheme at industrial scale.

Conclusion As described in the foaming section, not only selecting chemicals of materials but also incorporating the chemicals, materials properties with processing scheme can attain a creation of new functional materials. Utilization of sc-CO2 is one of the promising material processing schemes and has a lot of potential of inventing a new processing technology. As the polymer processing using CO2 has steadily been advanced and industrialized during the past decade, sc-CO2 aided polymer processing will be progressing in the future.

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