1. Introduction
The membrane gas separation industry has increased in size over the past four decades. This growth is related to the rapid development of more efficient membranes with higher perm-selectivity properties. The first industrial membrane for gas separation systems was tested in 1979 for the separation of H
2 from N
2, as well as CH
4 and Ar [
1]. In addition, the increasing demand for biogas production/purification, lower costs, and more efficient membrane separation are the main factors driving this market growth, especially for polymer-based membranes. Membranes for gas separation are also used for other applications, such as nitrogen generation and oxygen enrichment, hydrogen recovery, steam/gas separation, steam/steam separation, and air dehydration [
2]. Today, a significant amount of work is dedicated to the development of polymeric membranes for gas separation, which is related to the numerous advantages of these membranes, such as low cost, low weight, and high efficiency, as well as easy production and simple operation/maintenance [
3]. Today, membrane-based processes, especially porous polymeric films, receive a great deal of attention due to their simple use in separation and purification [
4], and as solid supports for sensors and catalysts [
5].
The membranes for gas separation can be classified into compact or porous structures. For the latter, specific morphological properties, such as pore size, shape, density, and internal surface area-to-volume ratio must be optimized to control the gas transport properties (diffusivity, permeability, selectivity, etc.), which are important for each membrane with respect to the gases (solubility) and separation conditions (composition, pressure, temperature, etc.). Although porous materials have been used, including mesoporous silica and alumina, polymers are of high interest because they are more ductile (less fragile) over a wide range of conditions (temperature) depending on the final application, e.g., gas separation, air/water filtration, and biological/biomedical purification [
6].
Some studies reported on different methods to produce porous polymer membranes. For example, porous polyethylene membranes were successfully prepared from low-density polyethylene (LDPE)/tapioca starch using acidic and enzymatic leaching techniques. It was found that the formation of the porous structure was directly related to the amount of starch removed from the blends, as starch particles were most effectively removed by an aqueous solution of 5 N HNO
3 at 65 °C, leading to a removal efficiency of 85% [
7]. In another case, PE microporous membranes were also prepared using ultrahigh-molecular-weight polyethylene (UHMWPE) and liquid paraffin via thermally induced phase separation (TIPS). The quenching temperature and annealing time were the main parameters controlling the final porous structure with pore sizes of 3–5 μm and porosity of 50–60% [
8]. Lastly, several works used salt leaching to generate the porosity. For example, highly hydrophobic microporous LDPE hollow-fiber membranes used for CO
2 capture in gas–liquid membrane contactors were prepared using melt extrusion of LDPE/NaCl blends followed by salt leaching via immersion in water at 60 °C for 160 min. The membrane porosity reached 51% using 60–68 wt.% of salt with a pore diameter size in the range of 2–5 μm [
9]. In another study, a simple and efficient method was used to produce a microporous LDPE support for a polydimethylsiloxane (PDMS) active layer by continuous extrusion and salt leaching (68 wt.% NaCl) via immersion in water at 50 °C. The results showed a pore size in the range of 8–11 μm, and the porosity was calculated by the volume of salt removed from the polymer (47%) [
10]. Recently, scaffolds based on polylactic acid (PLA) reinforced with cellulose nanofibers (CNF) and salt were immersed in deionized water for 3 days to leach out the salt (porogen). The samples had interconnected pores with an average pore size in the range of 67–137 µm and porosities above 76% using solvent casting and particle leaching techniques [
11].
Considering the information available in the literature, the main objective of this work was to produce low-cost porous polymer membranes with an easy-to-control simple process based on different formulations (concentration/composition) and extrusion parameters (temperature profile, screw speed, flow rate, etc.). To have a more sustainable product, a biobased material (corn starch) was combined with an easy-to-recycle polymer (polyethylene).
4. Conclusions
In this work, a simple methodology was presented to produce a flat porous polymer membrane without chemical agents or toxic solvents. As a first step, virgin LDPE was used as the matrix with corn starch as a biobased leachable particle. Nevertheless, to reduce the production cost and keep the sustainability of the process, recycled LDPE (or any other thermoplastic resin) and/or other soluble biobased particles can be used.
The process is a continuous extrusion–calendering blending/forming step coupled with an immersion leaching step to create a porous structure. The latter can be optimized/controlled by a careful selection of the particle size and concentration. In our case, a 10 μm commercial corn starch was used at 50 wt.% as a proof of concept. After optimization, an aqueous solution of diluted acetic acid (20%) was selected to remove the CS with high efficiencies (over 91%), which was confirmed via TGA. Overall, the proposed methodology is simple and economical since it is based on low-cost materials and can be continuously operated. It was also shown via DSC that the CS particles can improve the LDPE crystallinity, leading to a more tortuous path for the gas molecules.
Lastly, the permeation results for CO2, CH4, O2, and N2 were measured under standard conditions (25 °C and 17 psi). Although the ideal selectivities were not very high (1.6–3.1), the permeability was impressive (7 × 104–2 × 105 Barrer) leading to conditions above or very close to the 2008 Robeson upper bounds.
With these properties, it is expected that these membranes could achieve reasonable separation performance, at least at a laboratory scale for applications such as biogas sweetening (CO2/CH4), oxygen-enriched air (O2/N2), and natural gas purification (N2/CH4). These membranes can be used alone in contactors with recycling streams to improve gas separation efficiency or serve as a porous support to coat with an active layer. There is also the possibility to produce hollow fibers using similar conditions by changing the die geometry. All these aspects will be investigated in a future study.