Sustainable polymer foaming using high pressure carbon dioxide: a review on fundamentals, processes and applications

Volume 10 | Number 7 | 2008 RSC Database and Current Awareness Products  Cutting-edge research for a greener sustainable future www.rsc.org/greenchem Easy to use search functions  Clearly displayed results  Volume 10 | Number 7 | July 2008 | Pages 721–812 Green Chemistry  Abstracted from high quality sources Spanning the chemical sciences for quick and easy searching Specialist Databases Analytical Abstracts present search results in both text and graphical form. Titles include Catalysts & Catalysed Reactions, Methods in Organic Synthesis and Natural Product Updates. review both academic and industrial literature on a wide range of hard to reach and unique information. Titles include Chemical Hazards in Industry and Laboratory Hazards Bulletin. 06020718 Graphical Databases is the first stop for analytical scientists. Offering coverage on all areas of analytical and bioanalytical science. With a fresh new look, including improved search and results features, Analytical Abstracts offers an excellent online service. Find out more at Registered Charity Number 207890 Pages 721–812 www.rsc.org/databases ISSN 1463-9262 Jacobs et al. Sustainable polymer foaming using carbon dioxide Bhaskar et al. Debromination of flame retardant plastics with microwave irradiation 1463-9262(2008)10:7;1-B www.rsc.org/greenchem | Green Chemistry CRITICAL REVIEW Sustainable polymer foaming using high pressure carbon dioxide: a review on fundamentals, processes and applications Leon J. M. Jacobs,* Maartje F. Kemmere and Jos T. F. Keurentjes Received 6th February 2008, Accepted 8th May 2008 First published as an Advance Article on the web 6th June 2008 DOI: 10.1039/b801895b In recent years, carbon dioxide (CO2 ) has proven to be an environmentally friendly foaming agent for the production of polymeric foams. Until now, extrusion is used to scale-up the CO2 -based foaming process. Once the production of large foamed blocks is also possible using the CO2 -based foaming process, it has the potential to completely replace the currently used foam production process, thus making the world-wide foam production more sustainable. This review focuses on the polymer–CO2 -foaming process, by first addressing the principles of the process, followed by an overview of papers on nucleation and cell growth of CO2 in polymers. The last part will focus on application of the process for various purposes, including bulk polymer foaming, the production of bioscaffolds and polymer blends. 1. Introduction The discovery of Bakelite1 followed by the mass production of synthetic polymers a few decades later has had a major impact on today’s society. By the end of the 20th century plastics had become one of the most important construction materials for consumer goods. The highly viscous nature of polymers brought about processing difficulties which led to the development of plasticization technology. From plasticizing agent to blowing agent is only a small step, and this resulted in the discovery and the large scale production of polymeric foams. Due to the increasing demand for light weight, insulating, shock and sound absorbing materials, the production and variety of polymeric foams has increased dramatically and has become a very important part of the annual polymer production. One of the most commonly used production processes for polymeric foams is the so-called thermally induced phase separation (TIPS) process, where the foaming agent, which is dissolved in the polymer, induces a phase separation upon heating, followed by nucleation and cell growth. The foaming agent is usually a low boiling organic liquid, such as pentane and hydrochlorofluorocarbons (HCFCs), which is dissolved into the polymer at concentrations of about 7 wt%. Foaming via the TIPS process usually takes place in two steps. In the first step, polymer pellets with blowing agent are partly foamed with steam. These foamed pellets are then transferred into a mold and exposed to steam again, resulting in further foaming of the pellets. Due to the expansion, the pellets stick together and take the shape of the mold. This makes it relatively easy to produce large blocks of foamed material, which can then be cut into any shape. Furthermore, the density of the produced block can easily be controlled by the amount of partly foamed pellets added to the mold. Process Development Group, Department of Chemical Engineering and Chemistry, Eindhoven University of Technology, P.O. Box 513, 5600 MB, Eindhoven, The Netherlands. E-mail: l.j.m.jacobs@tue.nl; Fax: +31 40 2446104; Tel: +31 40 2472884 This journal is © The Royal Society of Chemistry 2008 A variation on the TIPS process is dissolving the polymer in a solvent at elevated temperatures, after which a temperature quench will induce a phase separation. Removal of the solvent results in the polymeric foam.2–4 Next to the TIPS process, several other methods of preparing polymeric foams are available (Fig. 1). One of these methods involves foaming by means of a chemical foaming agent (CFA). The CFA is a thermally unstable component, which is added to the polymer. Upon heating the CFA decomposes into gaseous components, resulting in the desired foam. The gaseous CFA can also be formed by reaction of two polymeric components, which is the case in polyurethane (PUR) foams.5,6 Fig. 1 Schematic overview of methods for preparing polymeric foams. Polymeric foams can also be produced by casting and leaching. This method consists of dissolving the polymer in a highly volatile solvent and casting the solution into a mold containing a solid porogen. The porogen is usually a water soluble salt, such as NaCl or KCl, which is washed out after the solvent has evaporated, leaving a highly porous polymeric structure. The advantage of this method is that the pore size and morphology can be controlled by the size and distribution of the porogen and the amount added.7–9 The main problem with the above mentioned methods is, however, that they either lead to unwanted contaminations in the polymeric foams, such as residual solvents and salts, or lead to the emission of harmful substances, which cannot be recovered. For example, the European expandable polystyrene demand for 2002 was 3 million tons and is expected to grow to a demand of Green Chem., 2008, 10, 731–738 | 731 approximately 3.7 million tons in 2010.10 Since 7 wt% of blowing agent is used for foam production, this will result in an estimated emission of 256 thousand tons of low boiling organic liquids in 2010 in Europe alone. Problems concerning the environment and the need for environmentally benign foaming agents have triggered researchers to start working on this topic. Universities as well as industry are focusing their research on developing “greener” foaming processes using a non-toxic foaming agent. Despite the fact that companies have come up with the idea of an extrusion process using gases such as nitrogen and carbon dioxide as a blowing agent some time ago,11–14 only in the early nineties were the first articles on foaming of polymers using gases published.15 Although nitrogen can be used as a blowing agent in the polymer foaming process,16,17 most publications on this topic address the foaming of polymers using carbon dioxide, because carbon dioxide also affects other polymer properties, thereby enhancing the processability of the polymer.18 However, both nitrogen and carbon dioxide (CO2 ) are considered to be sustainable alternatives for the replacement of the currently used blowing agents. Because the foaming of polymers using CO2 has been a ‘hot topic’ and most probably will be for some time, here we review the most relevant literature on the polymer–CO2 foaming process. First, the principles of the polymer–CO2 foaming process are addressed, followed by a short overview of papers addressing the nucleation of CO2 in polymers. Batch and continuous foaming processes will be discussed, after which the foamability of bio-based and synthetic polymers will be addressed. The last part of this review will focus on applications, such as bulk polymer foaming, bioscaffolds and polymer blends. Finally, conclusions and a future outlook will be given. 2. The CO2 -foaming process The CO2 -based foaming process can roughly be divided into two steps. In the first step, the polymer is saturated with CO2 , which is followed by an expansion step. Both steps, together with some important foaming parameters are schematically depicted in Fig. 2. During saturation of the polymer the glass transition temperature (T g ) will decrease and the polymer is plasticized. This means that the T g of the polymer decreases to a value below the temperature at which the polymer is saturated (i.e. the saturation temperature). The T g is the temperature at which the polymer matrix becomes brittle upon cooling, or soft upon heating. The state below T g is called the glassy state and the state above T g is called the rubbery state. Above T g , polymers are capable of plastic deformation without fracture.19 Furthermore, the polymer matrix will swell and the viscosity of the polymer decreases, allowing processing of the polymer–CO2 mixture at lower temperatures. The diffusivity inside the polymer is also enhanced, enabling the use of CO2 as a medium to add additives to the polymer matrix. The type of polymer, together with the applied pressure and temperature determine to a large extent the amount of CO2 that can be dissolved in the polymer. Once the polymer is saturated, a rapid decrease in pressure will induce a shift in the thermodynamic equilibrium, leading to an oversaturation of CO2 in the polymer. However, this does not necessarily lead to nucleation and cell growth. If the temperature at which the polymer has been saturated is relatively low, the polymer can still be in the glassy state, because T g has not been sufficiently depressed by the sorption of CO2 . Therefore, phase separation and nucleation will only occur once the saturated sample is heated to a temperature above the glass transition temperature. This will lead to cell growth and the final formation of the polymeric foam. Below the glass transition temperature, foaming cannot occur, because the polymer matrix is too rigid. If the saturation temperature is high enough and the polymer– CO2 mixture is in the rubbery state due to sufficient depression of T g , phase separation and nucleation will occur instantaneously upon depressurization. Cell growth will stop once the polymer matrix returns to the glassy state, either due to a decrease in temperature or a decrease in the CO2 concentration in the polymer and the polymer is no longer plasticized. The gas phase can separate from the polymer phase by two mechanisms. If the pressure drop results in a metastable state, nucleation will be the dominant mechanism for foam formation. In the unstable state, spinodal decomposition will be the dominant mechanism.20 This is schematically depicted in Fig. 3. Both mechanisms are diffusion driven, although the direction of diffusion is opposite. In spinodal decomposition, diffusion moves from low concentration to high concentration, due to a high driving force. This review, however, will focus on nucleation as the mechanism for polymer foaming since it is usually suggested as the dominant phase separation mechanism. Fig. 3 Schematic phase diagram of the composition vs. the gas pressure. 3. Nucleation and cell growth Fig. 2 Schematic representation of the CO2 -foaming process. 732 | Green Chem., 2008, 10, 731–738 The classical nucleation theory is often used as a basis for modeling the nucleation process. The theory is based on the Gibbs free energy required to create a void in a liquid, resulting This journal is © The Royal Society of Chemistry 2008 in a critical bubble, which is in mechanical and thermodynamic equilibrium with the surrounding fluid.21,22 However, the classical theory only holds for describing the boiling of low molecularweight-liquids and does not apply to the nucleation of gases in viscous liquids such as polymers. For that reason, many extensions of the classical nucleation theory have been proposed, to include amongst others free volume effects and surface tension reduction due to the dissolved gases and additives,23,24 polymer-solvent interactions and supersaturation of the blowing agent.25 Colton et al.26 have validated their adaptation of the theory with experiments, yielding a good qualitative description of nucleation behavior of microcellular bubbles in amorphous thermoplastic polymers. Next to the modeling of nucleation, many theories have been published concerning cell growth in polymers. These models are mainly based on the diffusion-driven growth of a spherical bubble,27–29 incorporating effects such as the dependencies on temperature and CO2 concentration of the density, diffusivity and viscosity, respectively.30 Leung et al.31 have successfully developed a model to accurately describe the bubble growth of experimentally observed data for the polystyrene–CO2 system. Although numerous papers discuss either the nucleation behavior or cell growth, only few have tried to model both effects simultaneously. One of these models has been developed by Joshi et al.32 As a base case for their model, parameters of the low density polyethylene–N2 system have been used. However, only a numerical analysis has been performed without any experimental validation. More recently, Feng et al.33 have integrated nucleation and cell growth models into a consistent theory, predicting the cell size distribution during the foaming process. Even though their results are in reasonable agreement with experimental data, the authors are very critical about their results and acknowledge the opportunities for further improvements. The formation of nuclei in a viscous liquid followed by cell growth is a very complex mechanism, which is influenced by many parameters, such as temperature, viscosity, CO2 concentration, depressurization rate and pressure drop. Despite the significant research efforts, it will probably take a considerable time before the complete process is fully understood. 4. Batch versus continuous Polymeric foams can be produced batch-wise or continuously. For both processes the general foaming steps of Fig. 2 can be applied. The batch process is usually applied in the research and development field where new materials are foamed or the foaming behavior is studied. In order to make the foaming process economically feasible and possible on a larger scale, a continuous process based on extruders is commonly used. In general, the extrusion process consists of a mixing step, where the polymer is mixed with the additives and is pressurized with CO2 . Due to sorption of CO2 , T g and viscosity will decrease, allowing processing of the polymer–CO2 mixture at lower temperatures. This means that different zones in the extruder can be operated at different temperatures, making it possible to add or use temperature sensitive components or polymers. This could make the CO2 -based foaming process applicable to biobased polymers and/or additives. At the die of the extruder the This journal is © The Royal Society of Chemistry 2008 pressure is released and the polymer foam is produced. Several patents have claimed the process where an extruder is used in combination with a gas as a blowing agent.11,12 Since the nineties, more and more patents claim specific parts of the extrusion foaming process, e.g. the way the gaseous blowing agent is added to the extruder,34,35 the specific zones of the extruder,36 as well as the dimensions of the die and pressure release zone.37 Next to patents claiming (parts of) the extrusion process, the extrusion process has also been investigated in academia. A division can be made between literature using the extruder as a means for mixing during the foaming process and literature optimizing and modeling the extruder and the extrusion process itself. In the former, only the processing conditions in the extruder are changed, such as saturation pressure and screw rotation speed38–43 or the type and amount of additive added, such as carbon nanofibres44 or nanoclays.45 The effect of the die geometry and temperature on the nucleation rate or the expansion ratio of the produced foams has also been investigated.46–48 A proper die design plays a crucial role in improving the quality of the extruded foams, since the geometry of the die determines the pressure decay rate and absolute pressure drop, by which both cell density and cell morphology are dominantly affected. Baldwin et al.49 have developed generalized design models of nucleated solutions in an extruder. It approximates the pressure drop and flow rate experienced by a two-phase polymer/gas solution flow through a die channel, with the goal to capture the major physical attributes of the complicated flow and provide means of quickly estimating the required flow channel geometry and evaluating the feasibility of a flow channel design. Additionally, Stephen et al.50 have refined the model to incorporate more realistic features of gas nonideality and viscosity reduction. 5. Bulk polymer foaming The main requirement of the CO2 -foaming process is that a sufficient amount of CO2 will dissolve in the polymer. This excludes the use of polymers such as cellulose and polyethylene, which have a very low affinity for CO2 . Nevertheless, a full range of polymers is still available that can be used in this process, including polystyrene, poly(methyl methacrylate) and biopolymers such as poly(lactic-co-glycolic). The properties of the polymer determine to a large extent the properties of the foam and, therefore, the field of application: for insulation purposes different requirements are needed as compared to biomedical applications. For the latter, bio-based polymers are preferably used, as these polymers are generally biocompatible and/or biodegradable. 5.1 Micro- and macrocellular foams Polymeric foams can be divided into two groups: microcellular foams (MCFs) and macrocellular foams. The latter have typical cell sizes of 50 lm or larger and are mainly used as insulation and packaging materials, due to their relatively poor mechanical properties. MCFs have a typical cell size of 0.1–10 lm and cell densities ranging from 109 –1015 cells cm−3 . The main idea for producing these materials has been materials savings, by creating Green Chem., 2008, 10, 731–738 | 733 voids without compromising material properties too much. This has been accomplished by keeping the cell size below a critical size, smaller than the pre-existing flaws in the polymeric matrix. In that way the cells would act as crazing initiation sites and toughen the material. Suh et al.51,52 were one of the first to produce MCFs in the early 1980s, followed by other publications on this topic in the late 1980s and after.26,53,54 Initially, MCFs were the main subject studied in the literature, produced by saturating a polymeric sample at room temperature with a gas at a certain pressure, followed by heating to a temperature above the glass transition temperature. The latter induces nucleation and gives the polymer matrix the flexibility needed for cell growth.55,56 In the early 1990s, Goel and Beckman57,58 were one of the first to describe the pressure quench method for producing MCFs, which is similar to the procedure described in paragraph 2. Both procedures are now used to produce microcellular as well as macrocellular foams. Based upon the results published in the literature,53,57,59–66 several general conclusions about the polymer foaming process can be drawn. These conclusions have been illustrated with experimental results, in which poly(styrene-co-methyl methacrylate) (SMMA) has been foamed at different temperatures, pressures and depressurization rates.66 The results can be found in Fig. 4, 5 and 7. For both foaming methods, the number of nuclei that will be formed upon fast depressurization increases with increasing saturation pressure. Fig. 4 clearly shows that the cell size decreases with increasing pressure resulting in more cells per unit volume. For the pressure quench method, it can be generalized that a higher saturation temperature results in larger cells and an overall lower bulk density, as can be seen in Fig. 5. The polymer matrix will be less rigid at higher temperatures, resulting in less resistance to cell growth. Furthermore, the time available for cell growth is also longer at higher temperatures, since vitrification of the polymer matrix will occur at a later stage. This is schematically depicted in Fig. 6. A decrease in depressurization rate also has an effect on polymer foaming. As this rate decreases, the degree of oversaturation per unit time decreases as well and fewer nuclei will be formed. Therefore, more time is available for diffusion of CO2 from the polymer Fig. 4 A sequence of SEM pictures (100×) of SMMA foamed at 119 ◦ C and (A) 100 bar, (B) 150 bar and (C) 175 bar, where a decrease in cell size with increasing pressure is clearly visible. These results are typically obtained for both the pressure quench foaming method and the foaming method induced by an increase in temperature. 734 | Green Chem., 2008, 10, 731–738 Fig. 5 A sequence of SEM pictures (100×) of SMMA foamed at 100 bar and (A) 93 ◦ C, (B) 110 ◦ C and (C) 119 ◦ C, where an increase in cell size with increasing temperature is clearly visible. These results are typically obtained for both the pressure quench foaming method and the foaming method induced by an increase in temperature. Fig. 6 Schematic representation of the decrease in foaming time at lower temperatures of saturation. matrix into the cells, which results in larger cells. Fig. 7 clearly illustrates the effect of a decreasing depressurization rate on the cell size. If depressurization occurs in two steps, a foam with a bimodal cell size distribution will be produced. The first Fig. 7 A sequence of SEM pictures (100×) of SMMA foamed at 119 ◦ C, 200 bar and (A) < 1 s, (B) 35 s and (C) 230 s. These figures clearly show the effect of decreasing depressurization rate, which results in an increase in cell size. This journal is © The Royal Society of Chemistry 2008 step will induce nucleation and some cell growth. The second depressurization step results in secondary nucleation and further growth of the primary nucleated cells. The secondary nuclei have less time to grow and will therefore be smaller. An example of such a foam is given in Fig. 8. Fig. 8 A typical bimodal cell size distribution which can be obtained when depressurization of the foaming process occurs in two steps. Molecular weight and polydispersity do not appear to have a significant effect on the foam morphology.67 However, the presence of even a few percent of low molecular weight compounds will increase the cell size and decrease the nucleation density in microcellular foaming processes. The effect of the low molecular compound cannot be explained by the classical nucleation theory. It is suggested that spinodal decomposition causes this effect. In almost all CO2 foaming literature the formation of a skin at the surface of the foamed sample is described. The main reason for skin formation is the rapid diffusion of CO2 from the surface of the sample upon depressurization. Especially in the foaming of thin films, where the surface/thickness ratio is unfavorable, the skin can be relatively thick. To overcome this problem, Siripurapu et al.68 have proposed to confine the polymeric film between two CO2 -impermeable surfaces, so CO2 can only escape through the film edges. This resulted in polymeric films with very thin skins (approximately 1 cell diameter thick), or even without any skin, making them interesting materials for application in molecular separation processes, biotechnology and microelectronics. 5.2 Nanocomposite foams One of the problems with polymeric foams for the use as a construction material is that the mechanical strength decreases as the cell size increases. By introducing nanoparticles in the polymeric matrix, not only can the mechanical properties of nanocomposite foams be enhanced, but also the physical properties, such as the fire resistance of the polymer. One of the most widely used particles is montmorillonite (MMT) clay, but also carbon nanofibres (CNF), spherical silica particles, nanocrystals, gold and other metal nanoparticles can be used.69–73 Because of the small dimension of the nanoparticles, they are especially beneficial for reinforcing cell walls of the foams, since the thickness of the cell walls is in the micron and submicron range. Lee et al.74 have shown that PS foamed with CO2 containing 1 wt% of CNF increased the tensile modulus by 28%, as compared to neat PS foams with a similar density of 0.6 g cm−3 . The tensile modulus of PS foams with 5 wt% of CNF increased by 45% to a This journal is © The Royal Society of Chemistry 2008 value of 1.07 GPa, which is close to that of bulk PS (1.26 GPa). The compression modulus of both the CNF foams was even higher than the compression modulus of bulk PS. Another advantage of the nanoparticles is that they act as very effective heterogeneous nucleation sites. The lowered energy barrier for nucleation in combination with a high nucleant density can result in a high nucleation rate and therefore a high cell density with a small cell size, which makes these nanocomposite materials especially suited for the production of microcellular foams.75 Furthermore, the obtained cell size distribution is much more homogeneous. Zhai et al.76 have produced polycarbonate (PC) foams with nano-silica particle content ranging from 1 to 9 wt% and have found that the homogeneity of the foams increased with an increasing amount of nano-silica particles. PC foams with 1 wt% of nano-silica particles had a cell size ranging from 0.2–1.5 lm which means a dramatic increase in homogeneity, as compared to the cell size range of neat PC foams (from 0.7 lm up to 6 lm). The cell size distribution of the PC foams with 9 wt% nano-silica particles is even narrower: 0.1–0.8 lm. A new trend in polymer composite foaming is the addition of so called molecular composites to the polymeric matrix. Contrary to the addition of nanoparticles, rigid-rod polymers are dispersed on the molecular level, so the polymeric matrix is directly reinforced with the rigid-rod chains. An example of such a rigid-rod polymer is polybenzimidazole (PBI). However, miscibility of PBI with other polymers can be an issue. Sulfonated and aminated polysulfone (PSF), polyphenylsulfone (PPSF) and carboxylated polysulfone (C-PSF) have been reported to form miscible blends with PBI. Furthermore, these composites have been successfully foamed with CO2 .77–79 The reason why it is interesting to foam composites with rigid-rod polymers is that there are no defects in a single molecular chain. Therefore, these molecular ‘fibers’ can possibly act as reinforcements of the struts and walls of polymeric foams, which could greatly improve the compression modulus and strength of the foam. Furthermore, most rigid-rod polymers also have very good thermostability. The combination of mechanical strength and improved thermal properties would make these type of nanocomposite foams suitable for a full range of high-tech applications in, for example, military and commercial aircraft. 6. Bioscaffolds Porous biodegradable polymer matrices have received increasing attention because of their potential application within the field of tissue engineering and guided tissue regeneration. These materials can act as a temporary support for in vitro cell growth and can encourage cellular growth in vivo. As the cells grow, the support of the matrix is no longer needed and over time, the polymer matrix degrades into chemically benign components, which are not harmful to the surrounding cells. One of the most commonly used biopolymers is poly(lactic-co-glycolic) acid (PLGA), because it biodegrades into lactic and glycolic acid, which are relatively harmless to the growing cells. Furthermore, its use in other in vivo applications, such as resorbable sutures, has been approved by the Food and Drug Administration.80 Also, the degradation rate of PLGA can be controlled by varying the ratio of its co-monomers, lactic acid and glycolic Green Chem., 2008, 10, 731–738 | 735 acid. Several techniques have been reported to produce porous PLGA, such as casting and leaching, fiber waving and phase separation.81–84 Although scaffolds with high porosity and large cells have been produced, the main drawback of these methods that they use organic solvents in the fabrication process, which can remain in the polymer after processing. These substances may be harmful to the cells and can inactivate many biologically active compounds (e.g. growth factors). To overcome this problem, CO2 has been used in order to produce porous bioscaffolds. Singh et al.85 have studied the generation of 85/15 PLGA foams for biomedical applications at temperatures up to 40 ◦ C and CO2 -pressures ranging from 100 to 200 bar. The obtained porosity was 89% with a pore size ranging from 30 to 100 lm. Mooney et al.86 obtained higher porosities of approx. 97% for 50/50 PLGA foamed at ambient temperatures and 55 bar. These results are confirmed by Sheridan et al.87 who have found similar results at similar saturation temperatures and pressures. A patent by De Ponti et al.88 also describes the foaming of PLGA using scCO2 . In general, it appears that milder process conditions are favorable for a high porosity. Due to the fact that scaffolds have very large pores, mercury intrusion porosimetry (MIP) is generally regarded as an unsuitable method for scaffold characterization. SEM and micro-CT images yield results for the pore size and pore size distribution. Fig. 9 shows an example of a micro-CT image of a poly(ethyl methacrylate)/tetrahydrofurfuryl methacrylate (PEMA/THFMA) scaffold produced with CO2 .89 MIP, however, can be used to measure the pore apertures and permeability of scaffolds. Fig. 9 Micro-CT image of scaffolds processed at (a) rapid depressurization; and (b) very slow depressurization. This figure is taken from Barry et al.89 and has been reproduced with kind permission of Springer Science and Business Media. difference in solubility and diffusivity of CO2 in both polymer phases provides an additional means to influence the foam morphology. Of course, the ratio of the blend and the degree of mixing will have an effect on the morphology of the foam as well. Han et al.70 have demonstrated the latter. Well-mixed and poorly mixed PS/9 wt% PMMA blends were foamed with CO2 . The well-mixed blends were rather homogeneous, whereas the poorly mixed blends clearly showed a dominant small cell phase and larger cells spread as stripes through the foamed sample. Interestingly enough, the smaller cells are formed in the PS phase and the larger cells in the PMMA phase. These results are opposite of what would be expected based upon the results of foaming experiments of both pure polymers, where foamed PMMA in general forms smaller cells as compared to foamed PS at similar conditions. These opposite results have been attributed to the diffusion of CO2 from the PMMA phase to the PS phase. As a result, the CO2 concentration in the PMMA phase decreases, which leads to a lower degree of oversaturation in this phase, resulting in fewer nuclei and larger cells. Related to the situation of poorly mixed blends is the case of blending non-miscible polymers. Foaming of these “blends” can result in remarkable foam morphologies. Taki et al.93 have produced foams with a bimodal cell size distribution of the “blends” of polyethylene glycol (PEG) and PS, where the PEG particles are dispersed in a PS matrix. Due to a higher diffusivity and solubility of CO2 in the PEG-phase, nucleation and cell growth are faster in this phase. Furthermore, cell coalescence occurs more easily in the PEG-phase, resulting in a bimodal cell size distribution in the produced foams, with cells ranging from 40 lm to 500 lm dispersed in cells of less than 20 lm (see Fig. 10). The mechanism of this type of cell formation is schematically depicted in Fig. 11. Similarly to the results described by Han et al.,70 the cell size in the PS phase is also smaller than the cell size in neat PS, foamed at similar conditions. In this case, however, the smaller cell size is attributed to the faster growing PEG cells, which cause a depletion of CO2 in the PS-phase. Due to this decrease in CO2 concentration in the PS phase, the plasticization effect of CO2 becomes less. Therefore, the PSmatrix vitrifies which results in a suppression of cell growth and, hence, a smaller cell size. Teng et al.90 have combined the CO2 -foaming process with salt leaching in order to increase the porosity and interconnectivity of poly(D,L)lactic acid PDLLA)/hydroxyapatite (HA) scaffolds. PDLLA/HA composite and NaCl have been mixed in a heated mold after which the mixture is foamed with CO2 and the salt is washed out. Next to the previously described polymers, PEMA/ THFMA,91,92 poly(isoprene-co-styrene)/THFMA91 and polycaprolactone (PCL)62 also have also been successfully foamed using CO2 and have a high potential to be used as bioscaffolds. 7. Polymer blends So far, the foaming of only one type of polymer, with or without a non-polymeric additive has been discussed. In these cases, CO2 dissolves only in the polymeric phase and the additive acts as sites for heterogeneous nucleation. It is also possible to mix two types of polymers and use CO2 to foam the blends. The 736 | Green Chem., 2008, 10, 731–738 Fig. 10 Cellular structure of a PEG/PS blend foam, prepared at 110 ◦ C and 100 bar. This figure is taken from Taki et al.93 and has been reproduced with kind permission of John Wiley & Sons, Inc. Because of the immiscibility of PS and PEG, the resulting foam morphology is largely dependent on the initial dispersion This journal is © The Royal Society of Chemistry 2008 Fig. 11 Schematic diagram of the formation of the bimodal cellular structure observed in Fig. 10: (a) initial state, (b) bubble nucleation and growth, (c) bubble coalescence and (d) particle formation. This figure is taken from Taki et al.93 and has been reproduced with kind permission of John Wiley & Sons, Inc. of PEG in the PS-matrix. This provides an extra means of controlling the resulting foam morphology. For example, a better and more homogeneous dispersion of smaller PEG particles will lead to a homogeneous bimodal cell structure, which can result in an open cellular structure, due to the fact that the cell walls between separate PEG cells will rupture once these cells come into contact. Furthermore, dispersing PEG as “fibers” in the polymer matrix can result in the formation of open channels, which provide a whole new range of applications for these materials. 8. Conclusion and outlook As demonstrated in this review, the foaming of polymers using scCO2 has been a topic of interest for many years and most probably will be for many years to come. In the early years, patents and research focused mainly on how to produce the foam itself. Later on, research shifted towards understanding and controlling the foaming process followed by an interest into the foaming of biocompatible or biodegradable polymers for the use as bioscaffolds. Recently, more papers on the production of microcellular nanocomposite foams have been published because of the high potential of these composite foams as hightech, light weight and strong construction materials. Empirically, the foaming process can be described quite well. The influence of different parameters, such as pressure, temperature and depressurization rate, are well known for many polymers. However, the theory behind polymer foaming is rather complex. Despite many efforts to modify classical nucleation and crystallization theories to include polymer–gas properties and interactions, these theories still fail to give an accurate description of nucleation experiments. Moreover, nucleation and cell growth are modeled separately in many studies, even though in the polymer foaming process both nucleation and growth are fully integrated and can occur simultaneously. Research on this topic will need to continue for many years to come to deliver accurate and predictive models that describe nucleation and cell growth. Such a model will provide the tools to really predict and control the polymer foaming process. This will help in evaluating polymers for their foamability without having to test all foaming This journal is © The Royal Society of Chemistry 2008 parameters experimentally, making the choice of a polymer for a specific application much easier. So far, the only way to scale-up the CO2 -based foaming process is by means of extrusion. Some companies are already using an extruder based foaming process, where CO2 is used as a blowing agent (e.g. Styrodur R , extruded polystyrene (XPS) produced by BASF). 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