In the segment 'A Look Around Innovation,' we introduce R&D sites that have led to social implementation. In this installment, we introduce Assistant Professor Masaki Kakiage of the Division of Molecular Science in the Graduate School of Science and Technology at Gunma University, who has produced high-strength fibers from ultrahigh-molecular-weight polymers by combining two technologies, namely, 'melt-spinning' and 'melt-drawing,' in a cost-effective and environment-friendly manner.
Research on polymer materials in 'Shokuto Kiryu' (capital of textiles): An alternative to the 'gel-spinning method'
Known as 'Nishijin in the West and Kiryu in the East,' Kiryu City in Gunma Prefecture has flourished as a center of silk woven fabrics since the Nara period. Kakiage was born in Kiryu City, known as a 'Shokuto (capital of textiles),' which is steeped in history and tradition, and has been engaged in research on polymeric materials since his student days. "At that time, I was working in this same laboratory on the mechanism of structure formation during polymer deformation processes. In 2015, I moved to the Institute for Fiber Engineering of the Interdisciplinary Cluster for Cutting Edge Research at Shinshu University (IFES), where I began my research on fibers in earnest," he said.
His current research focuses on the production of ultrahigh-molecular-weight polyethylene (UHMW-PE) fibers, which are a type of high-strength fiber. Whereas ordinary polyethylene has a molecular weight in the tens to hundreds of thousands, UHMW-PE has a molecular weight in excess of one million and a very long molecular chain. The molecular chains must be regularly arranged in one direction and their length must be longer to increase the strength of a fiber. The longer the molecular chain, the fewer breaks in the fiber and the higher its strength (Fig. 1).
Fig. 1: Elemental technologies required to increase fiber strength.
However, long molecular chains become entangled. A common method for disentangling the molecular chains is the 'gel-spinning method.' In this method, the polymer is dissolved in an organic solvent such as decalin, xylene or paraffin, and then disentangled and laid out in their chains. However, this conventional method uses 10 to 100 times as much organic solvent as the weight of the polymer to be disentangled, resulting in a high environmental impact. It also requires complex processes and large facilities, which add to manufacturing costs.
"If this production method, which is burdensome not only for the environment but also for producers and consumers, can be changed, products can be obtained at lower prices." Explaining the background of his research, Kakiage explained, "As someone who was born, raised, and educated in a textile city, I wanted to strengthen the local industry through my research." In 2020, the project was adopted by JST's A-STEP 'Establishment of High-Strength and High-Functional Fiber Production System by Innovative Green Processing' and initiated.
UHMW-PE Fiber production by 'melt-processing'
To produce high-strength fibers without the use of organic solvents, Kakiage proposed a method called 'melt-processing,' which combines two techniques: 'melt-spinning' and 'melt-drawing.' Melt spinning is a process in which polymers are melted by heat, extruded through a spinneret into an inert cooling medium such as air or nitrogen and then cooled and solidified to form fibers. It is mainly used to produce nylon and other fibers but has been difficult to apply to UHMW-PE. Because of the high melt viscosity of UHMW-PE due to its long molecular chains, melting it into fibers using conventional melt-spinning is challenging. Moreover, the entanglements of the molecular chains are difficult to completely disentangle.
When UHMW-PE is melted, the molecular chains remain entangled and do not flow, resulting in melt-spun fibers with a bumpy appearance. Such a structure causes thin fiber sections to easily break. In addition, if the entanglements are not disentangled, the molecular chains will not arrange properly, and the strength of the fiber will not increase. Kakiage recalled, "The main challenges in my research were how to make the UHMW-PE fibers by melt-spinning, how to disentangle the molecular chains, and how to increase the strength of the fibers."
The key was melt-drawing. This method is used to stretch polymer films by disentangling the entanglements of the molecular chains. "Arranging molecular chains neatly by melt-spinning alone is very difficult. Based on my experience on the in-process structural analysis of the melt-drawing process for polymer materials, I knew that melt-drawing must be combined with melt-spinning," he said. For Kakiage, who had been studying melt-drawing since his student days, the solution came naturally.
The first step in his research was to establish a melt-spinning process to obtain UHMW-PE fibers. Melting requires temperatures above the melting point of the material. However, excessively high temperatures will cause the molecular chains to break. In addition, because the gel-spinning process is well established, few examples of melt-spinning that could be used as a reference were available. "We went through a lot of trial and error to find the right temperature for fiber formation," said Kakiage. The melt-spinning process for producing melt-drawable UHMW-PE fibers was developed by controlling the melt-flow properties of UHMW-PE through repeated experiments (Fig. 2, left).
Fig. 2: Production of UHMW-PE fibers by melt-processing.
Furthermore, because the melting of UHMW-PE leads to extensive molecular chain entanglements, the melt-drawing method was considered that uses the entanglements as stress-transfer points to increase the strength of the fibers. "Having scientific evidence to support why strong fibers are produced is essential to prove this. We employed in-process measurements of the drawing process used in the production of polymer films to find conditions suitable for increasing the strength of the fiber." The combination of melt-spinning and melt-drawing led to the successful production of UHMW-PE fibers with a fiber diameter of approximately 150 micrometers (micrometer: one millionth of a meter) and a tensile strength of more than 1 gigapascal (giga: one billion) without the use of organic solvents (Fig. 2, right ).
Fibers with smaller diameters can be used in clothing and other products — Stimulating local industry through research
Kakiage is also working on further reducing the diameter of the UHMW-PE fibers. Even if the fiber is drawn blindly, it may break or lose its strength. Therefore, a 'multistage melt-drawing process' in which the drawing process was divided into different stages to achieve finer, and stronger fibers was performed (Fig. 3). "We achieved fiber fineness and strength by optimizing the conditions for the stage of drawing, which makes them finer, and the stage of drawing, which makes them stronger." Future applications are expected to include clothing and outdoor products that combine high strength and flexibility, safety products and playground equipment with excellent abrasion resistance, and medical products with excellent hygiene.
Fig. 3: Reducing fiber diameter by multistage melt-drawing.
A joint research system with Shinshu University, which was the starting point for Kakiage's fiber research, and the Gunma Prefecture Textile Research Institute has been established at A-STEP. "The melt-process we have developed makes it easy for small and medium-sized enterprises without large factories or equipment to enter the market, and small batch production and high value-added textile products can be expected. I would be happy if my research could revitalize the local industry," he said, looking forward to the future. Through further industry-academia collaboration and joint research, initiatives to improve the functionality of the fibers and industrialize them are also being considered. Future developments in this research will be anticipated.
(Article: Manami Yokoi, Photography: Hideki Ishihara)