Enhancement of the gas separation properties of polyurethane membrane by epoxy nanoparticles

https://doi.org/10.1016/j.jiec.2016.08.012Get rights and content

Highlights

  • Polyurethane/epoxy nanocomposite membranes were prepared up to 30 wt% of epoxy.

  • Polyurethane was synthesized based on PPG2000-HDI-BDO.

  • PU–epoxy hybrid membranes were characterized using FTIR, SEM, WAXD and DSC analyses.

  • The gas permeation decreased by epoxy content, but gas selectivities increased.

Abstract

In this study, we report highly selective CO2 polyurethane incorporated with cured epoxy nanoparticles. PU–epoxy composite membranes were prepared via solution casting method. The obtained SEM micrographs confirmed the nano-scale distribution of epoxy particles in the polymer matrix. DSC and FTIR spectra showed different phase separation for the PU composites compared to the pure PU. The effect of epoxy nanoparticles on the gas permeability of CO2, CH4, O2 and N2 was studied at 25 °C and 10 bar. The selectivity of CO2/N2 increased from 25 for the pure PU membrane to 55.5 for the PU–EP10, while the CO2 permeability was unchanged.

Graphical abstract

CO2 capture is one of the most important issues in the worldwide. Using CO2 philic epoxy particles embedded in the polyurethane results a high CO2 selective gas separation membrane. Also, good interactions between the epoxy particles with polyurethane chains improve the gas separation performance.

Introduction

Polymeric membrane-based gas separation is gaining a great deal of interest because of its high energy efficient, non-environmental effect, low maintenance costs, a modular process, and a compact system. However, the industrial applications of the polymeric membranes are still limited by the trade of between the gas permeability and selectivity [1], [2], [3], [4], [5].

The concept of mixed matrix membrane (MMM) is an effective way to improve the gas separation performance of polymeric membranes. Over the last few years, inorganic particles, e,g. metal oxides, silica [6], zeolite imidazolium framework [7], metal organic frameworks (MOFs) [8] have been utilized to prepare mixed matrix membranes. However, poor compatibility and the density differences between the inorganic and organic phases limit the membrane fabrication process [9]. The lack of good adhesion between carbon molecular sieves (CMS) and polyimide chains at high filler loadings results unselective voids and decreases the selectivity [10].

Some efforts are made to design some functionalized particles in order to improve the interactions with the polymer chains and the solubility of CO2 in the MMMs [11], [12]. For example, the amine modification of UiO66 and MOF-199 is used to enhance the CO2/CH4 gas separation performance of the polyimide composite membrane [13]. But, decreasing the thermal stability and using too much solvent limit the commercial applications of these particles. Therefore, the particle synthesis without using organic solvent has been considered in our groups. In our previous publication, MCM-41 mesoporous silica was modified by 3-aminopropyltrimetoxysilane (APTMS) and trimethylchlorosilane (TMCS) by the two different methods with and without using solvents [14]. Moreover, we synthesized silica nanoparticles via hydrolysis of tetraethoxysilane (TEOS) in ethanol, which is environmentally friendly [15]. In another study, the epoxy particles were synthesized by using the emulsion technique based on water in oil water (W/O/W) [16].

The gas separation properties of various MMMs have been studied extensively in our group. The CO2 and CH4 gas permeabilities increase with the incorporation of silica in ethylene vinyl acetate and polybenzimidazole membranes [15], [17]. However, the adverse results are reported for the PU–silica MMMs, where the gas permeability decreases with the silica contents [18], [19], [20]. This behavior is commonly observed for PU MMMs [21], [22], [23]. It is believed that the functions of particles in glassy or rubbery polymer membranes are different for gas separation performance [5], [24], [25], [26], [27].

The limitation of using inorganic fillers within the polymeric membranes induces using organic particles to obtain a good interaction between the polymer chains and particles. PIM-1 is used as an organic filler in the polyetherimide-based MMMs [28]. The CO2 permeability increases at the low concentration (up to 10 wt%) without much compromising gas selectivity. The effect of trimethylsilylglucose (TMSG) particles in the high free volume poly(trimethylsilylpropane) (PTMSG) MMMs is studied. The gas permeability reduction probably relates to the reduced free volume of PTMSG after filling of the pores by TMSG particles [29].

Although many studies on the effects of inorganic particles in the polymeric membranes have been made, there is a lack of research on using organic particles as fillers in the mixed matrix membranes. Here, the nano-sized organic epoxy particles were embedded in the PU matrix to enhance the CO2 gas separation performance. In our previous publication [16], [30], the synthesis and properties of the epoxy nano particles were explained. The particle synthesis process is an environmentally friendly technique as no solvent used and there is no emission of volatile products in the process. Moreover, the cured epoxy particles showed good thermal and dimensional stability, excellent chemical and corrosion resistance and superior mechanical properties [31], [32]. The chemical structure and amine curing mechanism of the epoxy resin is depicted in Scheme 1. The gas separation of PU membranes has been studied extensively by our groups [33], [34], [35]. The surface of the cured epoxy particles contains a lot of polar OH groups (Scheme 1), which can be useful not only as an active site for CO2 absorption, but also providing a good interaction with the PU chains [15], [36]. Therefore, we investigate the gas separation performance of polyurethane–epoxy nanocomposite membranes at various epoxy contents.

Section snippets

Materials and synthesis

Polypropylene glycol, PPG, (Mw = 2000), were obtained from Aldrich and dried at 80 °C under vacuum for 24 h to remove any residual water before use. 1,4-Butane diol (BD) chain extender, hexamethylenediisocyanate (HDI), N,N-dimethylformamide (DMF), and dibutyltindilaurate (DBTDL) were purchased from Merck (New Jersey, USA). The CO2, N2 and O2 (purity 99.99) that used for gas permeation tests were purchased from Roham Gas Co., Tehran, Iran and also CH4 (purity 99.95) was purchased from Air Products

ATR-FTIR characterization

ATR-FTIR spectra were used to investigate the structure of the synthesized pure PU and PU composite. The results are depicted in Fig. 1. The absence of the NCO peak at 2270 cm−1 indicates completion of the reaction. Nsingle bondH stretching of urethanes and Cdouble bondO peaks are the most important peaks for characterizing the PUs structure which appear at around 3300 cm−1 and 1670–1730 cm−1, respectively.

Normally, the major differences of the ATR-FTIR spectra for different PU samples can be found in the Nsingle bondH and single bondCdouble bondO

Conclusion

In this study, the effect of epoxy particles on permeability of CO2, CH4, O2 and N2 in the PU membranes was investigated. The DSC and FTIR results showed more phase mixed structure in the PU-based MMMs with increasing the epoxy nanoparticles contents. The gas permeability of the PU composite membranes decreased with increasing the epoxy particles whilst the selectivity increased. The gas permeability reduction resulted from both the incorporation of particles and lower phase separation of the

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