Dual-layer spinneret

Figure 31.9 Structure and flow channels of a dual-layer spinneret (Li et al., 2002).

Figure 31.9 illustrates the dual-layer spinneret design in our lab (Li et al., 2002). The outer-layer dope, inner-layer dope, and bore fluid are delivered to the orifice by passing through three independent channels. The most difficult parts of spinneret design arc precision and channel alignment. To run the dual-layer hollow-fiber spinneret, two metering pumps were employed to deliver inner and outer dopes. 

FIGURE 7.12 Schematic of the dual-layer spinneret and its flow channels for bore fluid as well as polymer solutions. (Data from D.F. Li, T.S. Chung, R. Wang, and Y. Liu, J. Memb. ScL, 198, 211-223, 2002.)... 

Figure 31.12 displays the desirable skin morphologies at various locations of dual-layer membranes. Briefly, both the inner and outer skins of the inner layer are porous with intimate adhesion between them. When using two different materials, one can control the interlayer diffusion as a function of spinning conditions. Table 31.2 summarizes the elemental analysis data of the interfacial layers of hollow fibers spun from PES as the inner layer and Matrimid polyimide as the outer layer at different spinneret temperatures (Jiang et al., 2004). After being extruded from the spinneret, the two dopes should make contact with each other under normal conditions. When the spinneret temperature is low (i.e., 25°C), their viscosities are high, the diffusion rate of the polymer molecules between the two layers will not be fast therefore, no sulfur can be found in the outer layer, as shown in this table. When the spinneret temperature is increased to 60°C, the interlayer diffusion between the two polymers apparently occurs, as evidenced in Table 31.2, in which it is... 

It is possible to produce dual-layer hollow fibers, for several applications, using a triple orifice spinnerets. The production of dual-layer fibers is more complex due to the presence of two dope solutions. One of the most critical issues is delamination of the outer layer. A modified spinneref, with an indented middle tube, has been proposed to induce interdiffusion between the two fiber layers and prevent delamination [49]. 

The dual-layer hoUow-fiber precursors were prepared by a phase inversion-based co-extrusion technique. Prior to the co-extrusion, both spinning suspensions were degassed while stirring at room temperature to fully remove the air trapped inside the suspensions. Both spinning suspensions were then loaded into two stainless steel containers and forced simultaneously through a triple-orifice spinneret. The co-extrusion process is represented schematically in Figure 11.17. 

Principles of dual-layer manbrane fabrication for hollow-fiber [152,154] and flat-sheet [148,150] membranes are similar they are prepared by a single-step coextrusion or casting of two different polymer solutions based on NIPS. Simultaneous formation of the dual-layer structiire can be done using a triple-orifice spinneret for hollow-fiber membranes and co-casting using a double-blade casting machine for flat-sheet membranes, as illustrated in Figme 15.26 [150,154]. 

FIGURE 15.26 (a) Schematic diagram of a dual-layer hollow-fiber spinning process (b) cross section of triple-orifice spinneret (Adapted from L. Setiawan et al.. Journal of Membrane Science, 423-424, 73-84, 2012.) and (c) fabrication process of a dual-layer flat-sheet membrane using a double-blade casting machine. (Adapted from S.A. Hashemifard et al. Journal of Membrane Science, 375, 258-267, 2011.)... 

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