About innovations in support materials that enable separation membranes to achieve higher performance

Separation membranes are indispensable in fields such as water treatment and medical applications. However, since the membranes themselves are mechanically weak, they require a support layer to provide physical strength and ensure proper functionality. Nonwoven fabrics have long played a crucial role as support materials, but conventional technologies have struggled to balance precise pore size control with sufficient mechanical strength and thickness.

This article introduces the fundamentals of separation membranes and the necessity of support layers, and then describes in detail a newly developed dual-layer nonwoven fabric composed of heterogeneous fiber types, designed to overcome the limitations of conventional support materials.

What Is a Separation Membrane?

A separation membrane is a type of filter used to selectively isolate specific components from a mixture. This is achieved by leveraging differences in solubility and the membrane’s fine pore structure, allowing only certain substances to pass through. Separation membranes are applied in processes such as removing fine particles from liquids, concentrating or purifying solutes, and a range of other liquid and gas separation operations.

These membranes are primarily used in industrial applications, including water treatment, gas separation, and medical fields. Typical examples include water purification, seawater desalination, oxygen enrichment or CO₂ removal from air, and blood purification through dialysis. Depending on the specific application and target substance, various membrane types are used, including ultrafiltration (UF), reverse osmosis (RO), ion-exchange membranes, and gas separation membranes.

A typical separation membrane features a composite two-layer structure comprising a thin, dense selective layer and a thicker, porous support layer. The selective layer performs the actual separation, while the support layer provides essential mechanical strength. Since the separation membrane is extremely thin and susceptible to pressure and mechanical stress, a robust support structure is essential for maintaining its integrity. Thus, the support layer is a critical component that ensures both durability and functionality.

Functional Requirements and Technical Challenges of Membrane Support Layers

Traditionally, single-layer nonwoven or woven fabrics have been used as support layers for separation membranes. However, such single-layer structures have difficulty achieving both precise pore size control and adequate strength or thickness.

For instance, using coarser fibers to provide necessary strength, thickness, or airflow results in larger pores, causing the membrane-forming resin solution to penetrate through the support layer. This prevents the formation of a stable separation membrane on the surface.

Conversely, reducing the fiber diameter to create smaller pores with higher density compromises the strength, thickness, or breathability of the support, potentially leading to reduced flow rates and inadequate mechanical durability.

To address this trade-off, several approaches have been attempted. These include calendaring nonwovens made from coarser fibers to flatten and densify the surface; laminating ultrathin spunbond nonwovens or paper to reduce pore size; and incorporating low-melting-point fibers for thermal bonding to shrink the pores.

While these methods have achieved some success, they still face limitations in fully preventing resin breakthrough. Moreover, they tend to increase production costs and may reduce permeability.

Technological Breakthrough: Dual-Layer Nonwoven Fabric with Different Fiber Types

To overcome these limitations, Hirose Paper Mfg. Co., Ltd. and the National Institute for Materials Science (NIMS) jointly developed a dual-layer nonwoven fabric composed of different fiber types.

This innovative nonwoven consists of two distinct layers: a surface layer made of fine fibers and a backing layer made of coarser fibers. The surface layer forms a dense, small-pore structure to retain resin, while the backing layer ensures overall thickness and mechanical strength (Figure 1).

This structure results in a smooth surface with fine pores that effectively retain the membrane-forming resin solution, while the backing layer ensures the necessary strength and pressure resistance. The two layers are integrally bonded to provide strong adhesion without the risk of delamination.

For example, the surface layer may consist of a blend of ultra-fine fibers (~0.1 dtex) and medium-fine fibers (~0.6 dtex), while the backing layer is composed of coarse fibers in the range of 6–12 dtex.

This configuration enables the fine-fiber surface to retain resin effectively, while the coarser-fiber backing layer provides robust structural support—making the fabric highly suitable for membrane formation.

Figure 2 shows a cross-sectional comparison of how resin behaves when applied to conventional nonwoven fabric (composed only of coarse fibers) versus the dual-layer structure. Conventional fabrics have large inter-fiber gaps, allowing resin to leak through to the bottom. In contrast, the fine-fiber surface layer in the dual-layer fabric retains the resin, preventing such breakthrough.

Applications of Dual-Layer Nonwoven Fabrics

The dual-layer nonwoven fabric can be used not only in water filtration and purification but also in sterile filtration for pharmaceutical production, food industry filtration processes, and organic solvent-based separations. In these applications, a resin solution is applied to the surface to form a separation membrane. The structure is also applicable to gas separation membranes.

This material offers superior performance compared to single-layer supports in thin-film membrane applications. From a membrane-type perspective, it is particularly well-suited for liquid separation uses, such as RO membranes for seawater desalination and NF/UF membranes for industrial wastewater treatment or potable water production. These membranes typically involve polymer-based separation layers formed on nonwoven substrates, making the dual-layer structure an ideal support.

For instance, in manufacturing RO membranes, the nonwoven support is first coated with a resin solution, which then undergoes the Non-solvent Induced Phase Separation (NIPS) method to form a porous support layer. A thin selective barrier layer, such as polyamide, is then added. The dual-layer nonwoven is especially effective in this process by preventing resin breakthrough and forming a uniform porous support layer—resulting in high-performance composite membranes.

Manufacturing Method and Future Development of Dual-Layer Nonwovens

Figure 4 presents a schematic flow of the manufacturing process. The dual-layer nonwoven is primarily produced using a wet-laid (papermaking) method, with two paper machines arranged in sequence. First, a dispersion of fine fibers is used to form the surface layer into a wet sheet. Then, a dispersion of coarse fibers is applied on top to form the backing layer. Since both layers are laminated in a wet state, interfiber entanglement occurs at the interface, resulting in strong integration. The laminated sheet is then dried and thermally pressed—for example, using a Yankee dryer that heats and compresses the sheet.

Additional calendaring may be performed to adjust the thickness, density, and surface smoothness. Thermal treatment fuses fibers, further stabilizing the layered structure. Binder fibers with low melting points may also be incorporated to enhance bonding during heat treatment.

This process allows large-scale, continuous roll-to-roll production of composite nonwoven materials. While two layers provide sufficient functionality in most cases, additional layers can be added by extending the papermaking process to introduce intermediate layers if necessary.

Suitable Materials for Dual-Layer Nonwovens

The dual-layer nonwoven can be made from any fiber materials traditionally used for nonwovens. For separation membrane applications, synthetic resin fibers that offer chemical resistance and mechanical durability are preferred. For example, polyethylene terephthalate (PET) is widely used in RO membrane supports due to its durability and chemical resistance.

Other suitable materials include polyolefins such as polypropylene and polyethylene, as well as nylon, vinylon, PPS, polyamide, and aramid. Different fiber types can be blended or used separately in the surface and backing layers, depending on application needs.

To suit wet-forming, fibers are cut to a few millimeters in length (e.g., 3 mm). For the surface layer, ultra-fine fibers (~0.1 dtex) and medium-fine fibers (0.6–1.2 dtex) can be combined. Ultra-fine fibers help form micro-pores and support resin films, while medium fibers assist bonding and structural integrity. For the backing layer, coarse fibers (6–12 dtex) provide strength and thickness. Binder fibers can also be added to enhance adhesion during thermal processing.

Conclusion

The dual-layer nonwoven fabric—featuring an ultra-fine fiber surface and a coarse-fiber backing layer—effectively prevents resin breakthrough during membrane formation. This structure improves adhesion between the membrane resin and the support, enables thinner membrane formation, and may improve both separation efficiency and mechanical strength. It is particularly promising for enhancing membrane quality and production efficiency in processes such as the NIPS method.

We offer this dual-layer nonwoven technology to support your separation membrane applications. If you are interested, please feel free to contact us. Our expert team will be happy to assist you.

Reference: Japanese Patent No. 7474431 – “Nonwoven Fabric for Separation Membranes and Its Manufacturing Method”