Plastic materials are lightweight, resistant to corrosion, and easy to process, so they are used in a wide range of applications, from everyday items to industrial uses. However, they cannot replace metal materials in applications where durability and high strength are required.

Fiber materials such as ropes and wires are known for their tensile strength, and by combining these with plastics, it is possible to create materials that have the best of both worlds: tensile strength equivalent to or greater than that of fibers, while maintaining the flexibility of resin.

The development of fiber-reinforced plastic (FRP) has led to the creation of a new material that is lightweight and strong like wood or bamboo, and can be produced industrially.

Carbon nanotubes (CNTs) were first discovered by Japanese researchers and have since been extensively studied worldwide.

Like diamonds and black smoke, CNTs are made from carbon. Their structure is a rolled-up cylindrical shape of a flat graphene sheet, in which carbon atoms are bonded at each vertex of a hexagon. There are two main types of CNTs: single-walled nanotubes (SWNTs), which have a single layer, and multi-walled nanotubes (MW-CNTs), which are made up of multiple layers.

CNTs are nanoscale in diameter (1 nanometer = 1/1,000,000 mm), but can reach millimeter-scale in length, and have a long, thin shape.

Compared to other materials, CNTs are known for having the highest specific strength (tensile strength per unit weight), making them one of the most promising advanced materials. They are even considered a candidate material for futuristic applications such as space elevators, earning them the title of a "dream material."

Carbon fiber (CF), as its name suggests, is a fiber made of carbon. It is industrially produced as CF tow, consisting of bundles of thin fibers with diameters ranging from 5 to 7 μm.

These CF tows are typically supplied in the form of regular tow, comprising 10,000–30,000 fibers, or large tow, with even more fibers. CF tow is often used as a reinforcing fiber in combination with plastic materials such as epoxy resin to create carbon fiber-reinforced plastics (CFRP). CFRP is highly valued for its light weight and high strength, and is used in a wide range of applications, from sports equipment to aircraft and satellite components.

Namd™ is a technology that coats a uniform CNT film on the surface of each individual CF. This CNT coating enhances the interfacial properties between CF and resin, significantly improving the performance of CFRP.

In particular, the second-generation 2G-Namd™ improves the strength of the fiber/resin interface by forming it into a nonwoven fabric-like structure, which prevents damage from friction during processing and resin flow-induced delamination during molding.

Despite using a minimal amount of CNTs—less than 0.3% of the total CFRP— it is able to uniformly coat the entire CF surface, and this is Namd™'s unique performance.

In the process of molding CFRP, it is essential to impregnate resin into the spaces between the fibers within a bundle consisting of 10,000–30,000 filaments, each with a diameter of 5–7 μm. The development of an intermediate material called prepreg (PPG), which is made by impregnating uncured resin into the interior of a bundle of fibers to form a sheet, has made it possible to widely apply CFRP to sports equipment, aircraft, and other products.

Namd™-CF is also provided in the form of prepreg (PPG), where uncured epoxy resin or other uncured resins are sheeted.

The following table shows the typical Namd™-PPG products currently available.

In standard CFRP, the resin penetrates into the inside of the CF bundle and fills the space between the CFs. In the case of Namd™-CFRP, which is made by composite with Namd™ yarn, the resin not only fills the space between the CFs but also penetrates into the CNT film formed on the surface of the CFs.

As a result, Namd™-CFRP forms an additional “CNT + resin” layer at the interface between the CF and the resin, giving it a “layered structure” of CF / CNT + resin / resin layers.

In standard CFRP, the CF-resin interface has a high modulus CF and a significantly lower modulus resin, so when stress is applied from the outside, stress concentration occurs at the interface, and delamination failure may occur at the CF/resin interface.

In the Namd™-CFRP, the composite of the high elastic modulus CNT and the low elastic modulus resin at the interface forms a layer with an elastic modulus between those of the CF and the resin, which is thought to be effective in avoiding stress concentration.

Normal CF can have many microscopic defects on its surface during the manufacturing process and the process of combining it with resin.

Although CF is a material with high tensile strength, under tensile stress, cracks can develop from the defective areas on the surface of the CF and extend to the inside, resulting in breakage. In the case of Namd™, a film of CNT, which has higher tensile strength than CF, is formed on the surface, so the risk of breakage due to surface defects can be reduced.

In fact, when comparing the tensile strength of normal CF and Namd™-CF, which is a grade of CF used in industry, it has been confirmed that Namd™-CF is stronger by several dozen percent.

When the cross-section of CFRP is observed under an enlarged microscope, it is confirmed that there is variation in the distribution of CF in standard CFRP.

On the other hand, the distribution of CF in Namd™-CFRP is uniform. This is thought to be achieved by the gaps between the CFs being filled with CNT film, resulting in a uniform distribution. This structure is similar to the cross-sectional structure of bamboo and wood that exist in nature, and it appears to be an even more ideal composite material structure.

In fact, when a rod-shaped CFRP test specimen is created by arranging the CF in one direction using the pultrusion molding method, and the bending failure of standard CFRP and Namd™-CFRP is compared, standard CFRP shows a failure mode similar to bamboo strips with many splinters. On the other hand, Namd™-CFRP is confirmed to break with a clean cross-section with few splinters after significantly bending and deforming.

In standard CFRP, when transverse compressive stress is applied to the fibers, cracks may develop along the interface between the CF and resin, leading to failure.

On the other hand, in Namd™-CFRP, the CNT film on the CF surface suppresses delamination at the interface, and it tends to exhibit high strength. In particular, when the fiber volume fraction (Vf) reaches around 70%, neighboring CFs come into close proximity, and the CNT+resin layer effectively bonds the fibers together.

In bending tests of CFRP laminates made by layering and heating prepreg, the process and mechanism of failure differ between standard CFRP and Namd™-CFRP.

In the case of standard CFRP, when a bending load is applied to the test piece, a phenomenon called buckling occurs on the inside, and after a convex portion is formed, failure occurs. This phenomenon is caused by delamination between the layers of CFRP, which causes bulging on the inside.

On the other hand, Namd™-CFRP suppresses delamination between the layers. As a result, it undergoes significant bending deformation before failure, with less pronounced buckling. Due to this characteristic, Namd™-CFRP shows higher deformation resistance.

The CNT film in Namd™-CFRP is thought to play a role in inhibiting crack propagation. Therefore, it is predicted that the structure of Namd™-CFRP, in which the CNT film is present at the interface where cracks are most likely to spread, will show high resistance to fatigue failure caused by crack growth.

In fact, we conducted a test using standard CFRP and Namd™-CFRP test specimens, fixing one end and applying a repeated load to the other end. As a result, it was confirmed that Namd™-CFRP had 1 to 2 orders of cycles more before breaking than standard CFRP. This demonstrates the superior fatigue resistance of Namd™ CFRP.

In recent years, in the field of mobility, such as electric vehicles and drones, there has been a demand for higher efficiency in motors in order to ensure a longer operating range. One solution being considered is increasing motor rotational speeds to several tens of thousands of rpm. However, at such high rotational speeds, conventional metallic materials cannot withstand the centrifugal force, so there is a trend towards the adoption of CFRP, which has a high specific strength.

When applying Namd™-CFRP to the sleeve that covers the surface of the rotor, depending on the molding conditions, it is possible to expect an improvement in strength of over 20% compared to standard CFRP, making it possible to further increase the rotational speed. Additionally, the structure of Namd™-CFRP offers enhanced resistance to fatigue failure caused by crack propagation, contributing to improved product reliability and durability of the product.