The synergy of supercritical CO2 and supercritical N2 in foaming of polystyrene for cell nucleation
Graphical abstract
Introduction
Plastics have become one of the world's most commonly used manufacturing materials today, with applications that range from common household items to advanced engineering products. Within the last two decades, there has been a heightened interest in studying the processing of plastic foams. This is attributed to the increasing prices of plastic resins, as well as the many unique features of foamed plastics. For example, a great deal of research has been directed towards microcellular plastic foams, characterized by cell densities between 109 and 1012 cells/cm3 and cell sizes between 10 and 0.1 μm [1]. This is due to their superior performance in mechanical strengths-to-weight ratio [2], [3], [4], [5], [6], thermal [7] and acoustical [8] insulation, as well as optical properties [9] when compared to their solid, unfoamed counterparts. Previously, chlorofluorocarbons (CFCs) were widely used as physical blowing agents (BAs) in plastic foaming processes, but were eventually replaced by hydrochlorofluorocarbons (HCFCs) due to serious concerns about the ozone depleting effect of CFCs. Even though HCFCs are relatively less harmful than CFCs to the Earth's ozone layer and the environment, HCFCs were eventually phased out in Europe for the production of foams in 2004 [10]. Furthermore, the Montreal Protocol dictates that the use of HCFCs in all applications be reduced in stages (e.g., in 2010, 65% reduction from the production level in 1996), and completely phased out by 2020 in all developed countries [11].
Currently, hydrofluorocarbons (HFCs) [12] and hydrocarbons (HCs) [13] are also being used as BAs; however, many of them are flammable and/or have other environmental/safety concerns. As a result, more attention has been shifted towards using more green, environmentally safe BAs to foam plastics, such as supercritical CO2 [14], [15], [16], [17], [18], [19], supercritical nitrogen N2 [20], [21], [22], and supercritical argon (Ar) [23], [24]. These BAs are more volatile than the aforementioned HFC/HC/CFC BAs, which might result in better cell nucleating performance, but their solubility in polymers is also much lower. Therefore, these BAs are typically used in their supercritical states due to the enhanced solubility and plasticization effect. Although the nature of the interactions of the gas-gas molecules and polymer–gas molecules is not fully understood, it has been previously demonstrated that these supercritical fluids are effective in plastic foaming processes [1], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25]. Nevertheless, even at the supercritical state, their solubility is still significantly lower than those of HCFCs, HFCs or HCs [26], [27], [28], [29], [30]. As a result, better distributive and dispersive mixing techniques and higher system pressure are often needed to fully dissolve these supercritical BAs into the polymer melt prior to the foaming process. Also, the high diffusivity of these supercritical BAs causes rapid gas loss during plastic foaming process, which hinders foam expansion. These characteristics pose technical challenges to the production of foams with high expansion where higher BA concentration is needed.
Currently, many PS foaming processes utilize a blend of supercritical CO2 and an alcohol or a hydrocarbon as plasticizing co-blowing agents to improve the foamability [31], [32], [33]. Foaming of other polymers (e.g., poly(methyl methacrylate) (PMMA) and polycaprolactone (PCL)) with similar BA blends has also been studied. Similar to the PS cases, improved foams were observed when compared to those blown with 100% supercritical CO2 [34], [35]. For example, with the intention of finding a suitable replacement for HCFC-142b, Daigneault and Gendron used a gas mixture of supercritical CO2 and 2-ethyl hexanol (2-EH) to foam PS and found that 2-EH worked as an effective additional plasticizer [31]. In addition, it was theorized that 2-EH might have increased the solubility and lowered the diffusivity of supercritical CO2. In a subsequent study, Gendron and Moulinié [35] used an autoclave to foam PMMA using a mixture of supercritical CO2 dissolved into liquid isopropanol and found that premixed BA blends had a higher plasticization effect than separately injected gas blends. Another study by Gendron et al. [33] examined extrusion foaming of PS blown with supercritical CO2 and ethanol, and found that ethanol eliminated blow holes or pores in foams associated with supercritical CO2 dissolution issues and rupture of cell walls. Foaming of biodegradable PCL with supercritical CO2/ethanol mixtures was also investigated by Tsivintzelis et al. [34]. It was found that the addition of ethanol generate more uniform cell morphology, compared to the case when only supercritical CO2 was used.
Most of the previously mentioned studies attributed the improved foaming behaviour to increases in BA solubility, permeability and plasticization effects due to the addition of an alcohol and/or a hydrocarbon. The additional cooling effect from the rapidly evaporated alcohol (or hydrocarbon) upon depressurization also helped to stabilize the cell structure. This cooling will also increase the local pressure variations favourable for cell nucleation through the increase of the viscosity and/or elasticity, and thereby increases the cell density [36], [37], [38]. So for some BA combinations, the cell density can dramatically increase by adding a small amount of co-blowing agent [39]. However, alcohols and hydrocarbons are flammable, and therefore potentially hazardous if these BAs are not diffused out of the foams prior to usage [40]. Therefore, it is imperative to reduce their usage by introducing a safer alternative which provides greater ease in handling and storage. When an alcohol is used to enhance the cell density, one possible replacement of the alcohol is supercritical N2.
Various researchers have suggested that the nucleating power of supercritical N2 was higher than supercritical CO2 per wt% of BA [22], [41], [42]. However, supercritical N2 has only been used for manufacturing high-density foams. This is due to the difficulty in dissolving a large amount of supercritical N2 because its solubility in polymers is extremely low. On the other hand, the solubility of supercritical CO2 is much higher than that of supercritical N2 [43], so foams with much lower density could be manufactured. Therefore, by blending these two BAs, it might be possible to produce foams with high cell density and high volume expansion. For example, in the study conducted by Di Maio et al. [41], it was demonstrated that PCL foams blown with a blend of supercritical CO2 and supercritical N2 resulted in high cell density while maintaining a low overall foam density. These results were later used to create porous PCL scaffolds for tissue engineering [44]. It is believed that the high nucleating power of supercritical N2 and the high foam expanding ability of supercritical CO2 produced positive synergistic effects. Also, in a recent solubility study by Hasan [45], it was found that the presence of supercritical CO2 could increase the solubility of supercritical N2 in PS, so the concentration of dissolved supercritical N2 gas could be higher than the prediction by the simple mixing rule. The improved supercritical N2 solubility might further improve the cell nucleating behaviours.
Despite the earlier studies on blowing agent blends, a thorough understanding of the fundamental mechanisms of plastic foaming using BA blends, with supercritical CO2–supercritical N2 or otherwise, has not yet been achieved. In typical foaming processes, a melted plastic is foamed at a relatively high temperature and is subsequently cooled and stabilized. Cell morphology is characterized after foam stabilization. However, during these foaming processes, cell coalescence, coarsening and collapse could occur after the cells are nucleated, all of which would significantly affect the final cell morphology. Therefore, it would be very difficult to accurately evaluate the effectiveness of each BA in generating cells and inducing cell growth. To fill this gap, the current study has investigated the effectiveness of various compositions of supercritical CO2–N2 BA blends, as well as pure supercritical CO2 and supercritical N2, via direct observation of the PS foaming processes. This study has been conducted using a foaming visualization system developed by Guo et al. [46], where a plastic sample was foamed in a batch chamber under static conditions. This minimizes subjecting the plastic melt to any shear and extensional stresses that have been shown to affect foaming behaviour [47], [48], [49], [50]. Therefore, the effectiveness of the supercritical blowing agents and blends could be studied in an isolated manner.
Section snippets
Theoretical background
The classical nucleation theory (CNT) [51] is a well-accepted theory for explaining the cell nucleation processes in fluids, and has been employed in many studies examining plastic foaming processes. According to the theory, nucleated cells that are larger than the critical radius (Rcr) grow spontaneously, whereas those that are smaller than Rcr collapse. Past studies have demonstrated that Rcr could be expressed as [36], [51], [52]:where Pbub,cr is the pressure inside the
Experimental materials
The polymer used for the foaming experiments was PS (Styron PS685D, Dow Chemical Ltd.), which has a melt flow rate (MFR) of 1.5 g/10 min (200 °C/5 kg) and a density of 1.04 g/cm3. Polymer pellets were compression moulded into films 200 μm in thickness by using a hot press. Five BAs with various supercritical CO2–N2 compositions (Linde Gas Inc.) were used: N2 (99.998% pure), 75% N2–25% CO2 blend (99.99% pure), 50% N2–50% CO2 blend (99.99% pure), 25% N2–75% CO2 blend (99.99% pure) and CO2 (99.8% pure).
Experimental setup and procedure
Results and discussions
Fig. 2 shows the timeline from a video of a PS sample foamed at 100 °C with the 75% CO2–25% N2 gas blend. It was observed that the cell nucleation rate and the cell density were significantly higher in the region where PS was in contact with the PET; in addition, there was an earlier onset of cell nucleation. This demonstrated that the heterogeneous nucleating effect was substantial, which agreed with Eq. (3) and the results obtained by Guo et al. [42]. Similar phenomena were observed for the
Conclusion
In summary, a blend of supercritical CO2 and supercritical N2 can be used in order to achieve good foams. Supercritical CO2 has higher solubility and plasticization effects. At the same time, supercritical N2, which has been shown to exhibit better cell nucleating power than supercritical CO2, was also needed to generate higher cell densities. The 75% CO2–25% N2 BA blend appeared to have the best foaming performance: it yielded the highest cell densities and cell growth rates over the widest
Acknowledgments
The authors of this paper are grateful to the Consortium of Cellular and MicroCellular Plastics (CCMCP), and NSERC DG154279-2010 for their financial support of this project.
References (60)
- et al.
Solubilities and diffusion coefficients of carbon dioxide and nitrogen in polypropylene, high-density polyethylene, and polystyrene under high pressures and temperatures
Fluid Phase Equilibria
(1999) - et al.
Biodegradable polymer foams prepared with supercritical CO2–ethanol mixtures as blowing agents
J. Supercritical Fluids
(2007) - et al.
Change in the critical nucleation radius and its impact on cell stability during polymeric foaming processes
Chemical Engineering Science
(2009) - et al.
The effects of extensional stresses on the foamability of polystyrene–talc composites blown with carbon dioxide
Chemical Engineering Science
(2012) - et al.
Mechanism of extensional stress-induced cell formation in polymeric foaming processes with the presence of nucleating agents
J. Supercritical Fluids
(2012) - et al.
A batch foaming visualization system with extensional stress-inducing ability
Chemical Engineering Science
(2011) - et al.
A visualization system for observing plastic foaming processes under shear stress
Polymer Testing
(2012) - et al.
Fundamental mechanisms of cell nucleation in polypropylene foaming with supercritical carbon dioxide—effects of extensional stresses and crystals
J. Supercritical Fluids
(2013) - et al.
A thermodynamic model for ternary mixture systems—gas blends in a polymer melt
Fluid Phase Equilibria
(2008) - et al.
Gas solubility, diffusivity and permeability in poly(ethylene oxide)
J. Membrane Science
(2004)