Polyvinyl chloride-based membranes: A review on fabrication techniques, applications and future perspectives

The global challenges and problems on water scarcity, energy shortage and environmental pollution have led to the increasing development of membrane technology in recent years by replacing traditional separation methods or integrating with them. The main advantages of the membrane technology are selective permeation, reduced energy consumption and non-thermal processing of sensitive compounds. Nowadays, among various membrane materials, polymeric membranes are the main research object due to their diversities, good film-forming properties, suitable flexibility, simple fabrication methods and ease of scale-up. The most commonly used polymeric materials for membrane fabrication include cellulose acetate (CA), polysulfone (PSf), polyvinylidene fluoride (PVDF), polyethersulfone (PES), polyacrylonitrile (PAN), polyetherimide (PEI), polypropylene (PP), and polyvinyl chloride (PVC) [1]. Among these polymers, PSf, PVDF, and PES cannot be considered as a cost-effective material to fabricate universal membranes. On the other hand, membranes prepared from relatively low cost polymers such as PP suffers from low chemical resistance when organic solvents are used [2]. Therefore, it is of great importance to consider both economic aspects and structural properties when choosing a polymer for membrane fabrication.

PVC is a relatively cheap polymer with appropriate chemical characteristics and physical and thermal stability for use as a marketable membrane material. Some favorable characteristics of PVC are: film-forming ability, high resistance to solvents, bases, and acids, proper solubility in various organic solvents, and long lifespan. In terms of economy, PVC can be considered as an interesting material for the membrane fabrication. Because of the challenges such as fouling, membranes should be replaced after a period of time. Since PVC is comparatively cheap polymer, it is so economical to use PVC-based membranes in the membrane separation processes. So, the PVC-based membranes have attracted attentions for application in different membrane separation processes, i.e. liquid filtration [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], ion exchange [19], [20], [21], [22], [23], [24], [25], gas separation [26], [27], [28], pervaporation [29], [30], [31], [32], [33], electromembrane extraction and PVC-based polymer inclusion membranes [34], [35], [36]. Moreover, PVC-based polymers have been widely used as the template to form porous inorganic materials. For example, PVC-g-POEM graft copolymers have been used as the structure directing agent to generate organized mesoporous TiO₂ films for solar cell and photocatalytic applications [37], [38], [39].

It should be mentioned, the PVC is considered to be susceptible to strong bases such as NaOH and thermally unstable. But, since the PVC membranes do not operate at very high pH values or high temperatures, these susceptibilities are not considered as issue. A number of benefits and challenges of PVC for membrane fabrication are schematically shown in Fig. 1.

Fig. 2 shows the number of PVC-related membrane articles have been published during the past 20 years for separation purposes. As shown, a significant increasing trend is observed after 2009 for PVC membranes and the application of PVC membranes is progressing approving the importance of this polymer for fabrication of separation membranes. However, the broader application of the PVC membranes in pressure-driven membrane processes was limited in past decades due to the intrinsic hydrophobicity of the PVC polymer. Hydrophobicity is a serious drawback for polymeric membranes used in liquid filtration because of high fouling tendency and flux decline during filtration processes. So, several techniques have been recently developed for modification of physicochemical properties of the PVC membranes to improve their hydrophilicity and fouling resistance. The main techniques are surface coating, surface grafting, and bulk modification, which incorporates inorganic or organic additives into the polymer matrix.

The purpose of this paper is to critically review the application of PVC polymer in membrane fabrication. In this regard, different PVC-based membranes are reviewed along with their preparation methods and applications. Also, the reported modification techniques of PVC membranes are investigated. Then, a brief look at commercial PVC membranes is taken and finally, concluding remarks and future perspective of the subject are discussed.

Polyvinyl chloride polymers

The historical background and development of the PVC polymer is well described in literature [40], [41]. The production of vinyl chloride monomer by the reaction of ethylene dichloride and alcoholic potash in 1835 can be considered as the start point of PVC story. However, it is generally accepted that PVC was discovered in 1912 [37]. During more than a century, PVC has become the world's third-most widely produced synthetic plastic polymer, after polyethylene and polypropylene, owing to its

Application of PVC in membrane fabrication

After the pioneering work of Hirose and co-workers for development of a method for preparation of a porous PVC membrane [43], the initial works on this membrane performed for immobilization of enzymes and microorganisms [44], [45], [46], gas separation [47], pervaporation [48] and ultrafiltration (UF) [49], [50], [51]. Hirose et al. also published a worth mentioning study to investigate the relationship between PVC membrane structure and phase separation process using time-turbidity curves [52]

Filtration membranes (MF, UF, NF, RO)

There are a large number of articles published on the PVC membranes used for liquid filtration including microfiltration (MF), UF and nanofiltration (NF). In addition to the benefits listed for PVC in the Introduction section, it has high solubility in some organic solvents such as DMAc, NMP, THF and DMF [54], [55], [56]. These membranes were usually fabricated by the non-solvent induced phase separation (NIPS) [55], [56] and electrospun nanofiber techniques [57]. In NIPS method, a dope

Commercial PVC membranes

As previously mentioned, according to the properties and advantages of the PVC, it can be considered as a potential candidate for fabricating polymeric membranes. As a result, PVC membranes currently are widely applied for different membrane processes. In this section, a number of commercial PVC membranes fabricated for different membrane processes are provided in Table 6. As shown, PVC is applied for preparation of commercial UF membranes in hollow fiber configuration. However, it has

Concluding remarks and future perspective

The membranes research area and market are broad and extremely diverse. But, any membrane is designed for application in specific technology with particular structure and composition. In most of the applications, the main requirements to membrane materials are selectivity, productivity, and stability. Moreover, there are other important parameters including strength, cost, etc. In the case of large-scale applications, such as water treatment, the cost of membranes is critical and, thus,

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

The authors thank the Kharazmi University (Iran) and the Azarbaijan Shahid Madani University for all the support provided. The Kharazmi University financially supported from Kharazmi membrane research core (Grant number: H/4/360).