Membranes synthesized via interfacial polymerization using 'new chemistry' monomers.
Membrane applications have gained an important place in separation processes and have been gradually combined with or have been substituting more traditional, industrial separation processes, such as distillation, adsorption and extraction. The majority of today’s industrial membranes for wastewater treatment and/or desalination are synthesized via interfacial polymerization, a technique which enables the synthesis of thin, dense top-layers on top of a porous support, hence high permeability and retention can be achieved. The development of membranes with high chemical stability, however, remains necessary to achieve the real breakthrough of industrial solvent resistant nanofiltration applications. This relatively new technique allows separation of solutes in organic solvent down to a molecular level and finds applications in the food, pharmaceutical, fine chemical and (petro)chemical industry. Today, some commercially available SRNF membranes consist of a polyamide (PA) selective layer on top of a diamine crosslinked polyimide (XL-PI) support-layer. Even though the number of successful SRNF application screenings increases steadily, many challenges remain unsolved, like dealing with more aggressive organic feeds, or when purifying metals from highly acidic extraction solutions in e.g. mining. Indeed, an urgent need for high-performance, acid-stable membranes still exists.
A first part of this dissertation explores and introduces the use of epoxide-curing via an interfacial approach for SRNF by the implementation of a nucleophilic ring opening polymerization, creating a poly(β-alkanolamine) top-layer, applying the most commonly used epoxide-system: bisphenol-A-diglycidyl ether with 1.6-hexamethylenediamine. A thorough parameter study of the system leads to a better understanding of this poly(β-alkanolamine) top-layer and the effects of the synthesis parameters, such as monomer concentrations, reaction time, reaction temperature and used solvent system. Based on Carothers and Flory-Stockmayer theories, higher epoxide or amine functionalities could lead to faster gelling, accelerated film formation and densification. Supported by these theories, the curing of tetra-functional epoxide EPON Resin 1031 with various amines, differing in terms of diffusivity, functionality and thus reactivity is investigated next. Indeed, faster and more dense film formation can be observed. Considering the aim to synthesize exceptionally stable top-layers, the influence of several solvents and cleaning conditions on the TFC-membranes is investigated as well. The poly(β-alkanolamine) top-layers proved stable in highly acidic conditions (pH = 0), though stability in DMF remained an issue, possibly due to different chemistries in support- and top-layer. Also very basic conditions (pH = 13.5) rendered the TFC-membrane instable, which is inherently linked to the formed poly(β-alkanolamine) structure.
In a second part of this research, the focus is on the anionic ring opening polymerization of epoxides, resulting in a poly(epoxyether). Application of TFC-synthesis via interfacial in-situ initiation, realizes molecular structures which are unparalleled in terms of chemical stability, even compared to the nucleophilic ring opening of epoxides. Again the correlation between the polymerization parameters and matching membrane performance and/or chemical stability is made. This chemical stability is proven by an unchanged to even improved membrane performance after immersion in challenging conditions, like 1 M HCl and 400 ppm NaOCl, in comparison to complete performance loss of a commercial NF 90 in these conditions.
Support stability remains an issue for TFC-membrane synthesis, limiting its application in harsh conditions. Different swelling in support- and top-layer implies formation of cracks, hence destroying membrane performance. A last part in this thesis tackles this problem by the synthesis of a new crosslinked full-epoxy-based support for SRNF via phase inversion. This way, a support with exceptional stability at extreme pH and in challenging solvent streams, is obtained which not only enables the application of the epoxy-based TFC-membranes without restrictions, but also shapes the TFC-membrane for solvent activation, inducing increased performance. A combination between 20 wt% EPONOL Resin 53-BH-53 and 25 wt% EPON Resin 164 resulted in DMF-stable SRNF-membranes (0.3 L m-2 h-1 bar-1, 76.6 % RB retention, after 24h DMF treatment) which could be successfully crosslinked, although heterogeneity remained an issue. After some further optimization, the obtained membranes could have clear industrial significance.