The University of Fukui develops new recycling method using supercritical fluid to decolorize fabric

Older clothes are eventually thrown away, even if they are taken to a recycle store. Irrespective of being recycled as fabrics or raw materials, the dye in older clothes must be completely removed. Existing methods used for this removal are costly and damage the fabric. Professor Kazumasa Hirogaki of the Division of Engineering Frontier Fiber Technology and Science at the University of Fukui and Visiting Professor Teruo Hori of the Headquarters for Innovative Society-Academia Cooperation of the same university developed a technology that enables almost 100% decolorization using a supercritical fluid of carbon dioxide. Using the proposed technology, carbon dioxide and dyes can be easily recovered. As no water or other solvents are used, it is expected to lead to environmentally friendly recycling.

Hirogaki and his colleagues have been developing a dyeing technology that uses a supercritical fluid under the NEDO project. Dyeing uses 5.8 billion tons of water, accounting for 20% of the total industrial wastewater, and consumes a lot of chemicals and energy. By changing the dyeing solvent from water to supercritical carbon dioxide, the researchers have developed an energy-saving dyeing method that does not use water or chemicals. This technology was developed further under the JST COI-NEXT program.

"We thought that in principle, it should be possible to apply this dyeing method in reverse for decolorization, so we went back to basics. Through trial and error under various conditions, we achieved nearly 100% decolorization of polyester. Additionally, we are experimenting with other materials and feel confident that we can expand the application scope of the technology," said Hirogaki.

When a co-solvent or an adsorbent with a solubility parameter (SP) value close to the SP value of the dye is added to supercritical carbon dioxide, almost 100% of the dye is transferred from the fabric to the co-solvent or adsorbent at a low temperature of 120℃ and in a short period of 30 min. Carbon dioxide is then converted back into gas and captured, allowing the recovery of the decolorized dry fabric and dye.

"I believe that together with companies and local governments, we can change the current value system to one in which clothes are recycled rather than thrown away and create new industries and businesses," added Susumu Yonezawa, director of the Headquarters for Innovative Society-Academia Cooperation.