Microcellular foaming of polysulfones in supercritical CO2 and the effect of co-blowing agent

https://doi.org/10.1016/j.supflu.2018.05.017Get rights and content

Highlights

  • The microcellular foaming of PPSU and PSU were investigated in supercritical CO2.

  • The interaction energy of the blowing agent with polymer chains increases with the solvent content.

  • The expansion ratio of foams has an evident increase when adding co-blowing agents.

  • The foaming temperature window expands upon the addition of co-blowing agents.

Abstract

The microcellular foaming of two kinds of polysulfones, polyphenylsulfone (PPSU) and polysulfone (PSU), were investigated in supercritical CO2. The expansion ratio, which ranges from 1.10–2.45 for PPSU and 1.10–3.72 for PSU obtained under different conditions, is not sufficient due to the limited solubility of CO2. To improve the porosity of foams, the effect of co-blowing agents on foaming was studied by molecular modeling and foaming experiments. Ethanol has a more favorable interaction with CO2, followed by water, acetone and ethyl acetate. Additionally, ethanol significantly increases the interaction of CO2 with polymer chains. The expansion ratio of foams has an evident increase, and the foaming temperature window expands upon the addition of co-blowing agents. Consistently, the addition of ethanol has the highest expansion ratio, 5.02 for PPSU and 6.54 for PSU. Meanwhile, it broadens the foaming temperature dramatically, decreasing the value by 50 °C for PPSU and 70 °C for PSU.

Introduction

High-temperature polymers including polysulfones, polyetherimide (PEI) and poly(ether-ether-ketone) (PEEK) have attracted attention because of the recent rapid development of aerospace and aeronautic, medical and automobile industries [[1], [2], [3]]. Polysulfone (PSU) and polyphenylsulfone (PPSU) are amorphous thermoplastic polymers that have high glass transition temperatures (Tg), good mechanical performances and excellent chemical stabilities, which can be long-term used at temperatures above 150 °C [[3], [4], [5]]. However, the high costs limit their extended application.

The foaming of plastic materials is an effective way to reduce material weight and cost while maintaining good heat insulation and sound insulation properties. On the basis of the good thermal stability, low thermal conductivity and light weight, polysulfones foams can be used as aircraft interior insulation parts. Currently, the green preparation of lightweight and high-temperature materials using supercritical CO2 foaming technology is of interest because of its environmental friendliness, nonflammability and nontoxicity [[6], [7], [8], [9]].

The microcellular foams can be fabricated via increasing temperatures or rapid depressurization. Solid-state CO2 foaming is a common method to fabricate high-temperature polymer foams [10,11] through the temperature rising process. Krause et al. [11] systematically studied the foaming behavior of PSU and polyethersulfone (PES). Foams with an expansion ratio below 3 were obtained, and the influence of Tg on the foaming behavior were discussed; it was shown that the nucleation and growth of cells starts at the Tg of the polymer/gas mixture and that a linear relationship exists between Tg and the CO2 content. Miller et al. [2,12] investigated the foaming process of microcellular and nanocellular PEI foams and obtained foam products with an expansion ratio of up to 2.5 and cell sizes ranging from 30 nm to 4 μm.

Solid-state CO2 foaming also be applied in the foaming of polysulfones through the temperature rising method. According to the work of Guo et al. [4], nanocellular PPSU foams were fabricated by increasing the CO2 concentration through increasing the saturation pressure to 7 MPa or decreasing the saturation temperature to -10 °C. The cell size could reach 20-40 nm, with an expansion ratio ranging from 1 to 2.6. Generally, the temperature rising process must be improved to prepare polymer foams efficiently because of the long saturation time at relatively low temperatures. As reported by Sun [5], the saturation times of PPSU and PSU sheets with a thickness of 1.6 mm were more than 60 h at 5.7 MPa and room temperature.

A different solid-state foaming process was proposed by Goel et al. [13], where polymers were first saturated at a relatively high temperature followed by a rapid pressure quench. In this rapid depressurization foaming process, polymers can be saturated at a temperature over Tg, which is beneficial to reducing the saturation time because of the higher diffusion rate of CO2. However, the solubility of CO2 also decreases with the temperature. As is well known, the solubility of supercritical CO2 in a polymer matrix is one of the most important physical properties during CO2 foaming. It is a challenge to prepare PPSU and PSU foams with a high expansion ratio at high temperatures because of the rather low solubility of CO2.

To improve the foaming ability of plastic materials, some solvents such as ethanol [14,15], water [16,17], and acetone [18] were introduced as a co-blowing agent to enhance the solubility of CO2. Tsivintzelis et al. [15] used CO2-ethanol mixtures as a blowing agent for the foaming of polycaprolactone (PCL). With increasing weight fractions of ethanol in CO2, larger bubbles were formed, and as a result, the foams had larger expansion ratios. The additive solvents can act as a plasticizer, which decrease the melt temperature and glass transition temperature of the polymers. For crystalline polymers, this is also beneficial for cell uniformity because of the decrease in crystallinity [15]. Zhao et al. [19] investigated the foaming of PVOH using water and CO2 as co-blowing agents. With the addition of 37 wt% water, the melting temperature and glass transition temperature decreased by approximately 70 °C; this resulted in the polymer chains becoming more flexible, which is conducive to the improvement in foamability. CO2-ethyl lactate mixtures were used by Salerno et al. [20] to foam PCL and polylacticacid (PLA). The addition of ethyl lactate decreased the Tg of the polymer/solvent system to decrease the operating temperature and improve the morphology uniformity of the foams. Similarly, in a different work, extruded PS foams were prepared by using CO2 and 2-ethyl hexanol [21], which acted as an efficient additive plasticizer. To identify a suitable co-blowing agent in CO2-foaming polysulfones, molecular modeling can been applied to calculate the binding energy between CO2 and the solvents of interest [22,23]. Meanwhile, an effective evaluation of the interaction between polymer chains and the CO2/co-blowing agent via molecular modeling provides the possibility to predict the compatibility and solubility of the blowing agent with the polymers.

The main aims of this work are to investigate the foaming behaviors of microcellular PPSU and PSU by using supercritical CO2 and to increase the expansion ratio of PSU and PPSU by using a co-blowing agent. The supercritical CO2 foaming of PPSU and PSU was carried out by using the batch rapid depressurization method to study the basic effects of foaming temperature, pressure and depressurization rate on the cell morphology of the foams. Then, molecular modeling was introduced to evaluate the binding energies between the potential co-blowing agent (ethanol, water, acetone and ethyl acetate) and CO2 and the interaction energies of the CO2/co-blowing agent mixture with the polymer chains. Meanwhile, the solvents were added to CO2 as a co-blowing agent to enlarge the porosity of PPSU and PSU. The effects of solvents and their contents on the foaming temperature window and expansion ratio were also explored to gain a deeper understanding of the supercritical CO2 foaming of high-temperature polymers assisted by a co-blowing agent.

Section snippets

Materials

The polymers, PPSU (R-5000) and PSU (P-1700), were supplied by Solvay Advanced Polymer. The densities of PPSU and PSU are 1.29 g/cm3 and 1.34 g/cm3, respectively. The glass transition temperatures (Tg) of PPSU and PSU are 220 °C and 185 °C, respectively, which were measured by differential scanning calorimetry (DSC, NETZSCH 204 HP). The samples were dried for 12 h at 110 °C in a vacuum oven before being used. Ethanol, acetone and ethyl acetate (purity≥99.5%) were purchased from Shanghai

Ab initio method

To evaluate the compatibility between the co-blowing agent and CO2, an ab initio method, a high-level quantum mechanical approach, was applied to calculate the interaction energy between the candidate solvents and CO2 to screen for one that was suitable for CO2 foaming. The calculations were implemented in the Gaussian 09 software. 34 Four candidate solvents (ethanol, water, acetone and ethyl acetate) were chosen as co-blowing agent candidates to evaluate their interaction with CO2. In a

Gas saturation process

The solubility of a blowing agent will affect the foam structure. The first step of the foaming process is to saturate a polymer with a blowing agent, similar to CO2. The sorption curves were experimentally measured in a pressure vessel filled with a CO2 15 MPa atmosphere at 200 °C to obtain the time required to achieve full gas saturation. Fig. 1 shows the absorbed amount of CO2 in PPSU and PSU for different saturation times under 15 MPa CO2 at a temperature of 200 °C. The CO2 concentration is

Conclusion

The foaming behavior of PPSU and PSU was investigated using the batch rapid depressurization method with supercritical CO2 as the blowing agent. The solubility of CO2 in PSU is higher than that in PPSU under the same conditions. Meanwhile, PSU shows a lower complex viscosity, which is expected to form foams with a larger cell size. Microcellular foams of PPSU and PSU with different average cell diameters and cell densities were prepared at different temperatures, pressures and depressurization

Acknowledgements

The authors are grateful to National Natural Science Foundation of China (21706063, 21676092), National Key R&D Program of China (2016YFB0302200), and the Fundamental Research Funds for the Central Universities (22221714005).

References (36)

  • H. Zhong et al.

    Integrated process of supercritical CO2 -assisted melt polycondensation modification and foaming of poly(ethylene terephthalate)

    J. Supercrit. Fluids

    (2013)
  • J.B. Bao et al.

    Oriented foaming of polystyrene with supercritical carbon dioxide for toughening

    Polymer

    (2012)
  • C. Wan et al.

    Rheological properties of HDPE and LDPE at the low-frequency range under supercritical CO2

    J. Supercrit. Fluids

    (2017)
  • H. Wu et al.

    Evaluation of solubility enhancement of carbon dioxide in polystyrene via introduction of water

    Fluid Phase Equilibr.

    (2017)
  • H. Guo et al.

    Fabrication of high temperature polyphenylsulfone nanofoams using high pressure liquid carbon dioxide

    Cell. Polym.

    (2016)
  • H. Sun et al.

    Preparation, characterization, and mechanical properties of some microcellular polysulfone foams

    J. Appl. Polym. Sci.

    (2002)
  • C.A. Eckert et al.

    Supercritical fluids as solvents for chemical and materials processing

    Nature

    (1996)
  • B. Krause et al.

    Microcellular foaming of amorphous high-Tg polymers using carbon dioxide

    Macromolecules

    (2001)
  • Cited by (0)

    View full text