The world’s first horizontal chemical recycling technology for super absorbent polymers used for disposable diapers

SAP is a resin that can absorb water 200 to 1,000 times its dry weight and retain most of the absorbed liquid in gel form even when pressure is applied. It can absorb urine several tens of times its dry weight, and its ability to keep urine from seeping out even under pressure makes it the perfect material for use as a diaper absorbent.

Disposable diapers are expected to account for as much as 7% of municipal waste (general garbage) by fiscal 2030. In response, the Ministry of the Environment has intensified efforts to promote the recycling of disposable diapers. Sumitomo Seika, a leading producer of SAP, views the development of recycling technology as part of its corporate social responsibility and began working on SAP recycling technology three years ago.

In 1978, a Japanese company pioneered the world’s first commercial production of SAP and, in 1983, a Japanese company was first to commercialize disposable diapers made of SAP. Many people in their 30s and 40s, as well as younger generations, grew up using disposable diapers.

In the 1990s, pants-type disposable diapers debuted. While initially designed for babies, the launch of pants-type disposable diapers for adults in 1994 sparked a surge in demand beyond nursing care facilities.

A Japanese company was also the first to develop pants-type disposal diapers. Indeed, Japanese companies have been responsible for many other “world’s firsts” related to disposable diapers. For example, synthetic rubber adhesives, used to bond different components of disposable diapers through heat without hardening when cooled, were first developed in Japan. Additionally, Japan pioneered the development of cellulose nanofiber, a highly absorbent material that contains large quantities of silver ions to neutralize odors.

With a view to recycling disposable diapers as raw materials, research has been underway to extract pulp from the fibrous portion and refine it for use as building material or to reuse it as diaper material. However, many difficulties were encountered in attempts to utilize recycled materials derived from SAP.

Virgin SAP can absorb urine about 50 times its dry weight but when SAP is recycled, its absorption capacity drops to around 10 times. Sumitomo Seika explored potential applications of recycled SAP beyond disposable diapers. For materials such as pulp and plastics, various product markets can make use of recycled materials even if their performance is degraded after recycling. However, SAP is primarily used in sanitary products such as disposable diapers, and Sumitomo Seika was unable to find commercially viable applications for recycled SAP beyond its use in diapers.

Sumitomo Seika, having reached a dead end, shifted its R&D focus around 2022 toward achieving chemical recycling. The goal was to recycle SAP in such a way that it would have same water-absorption performance level as virgin SAP.

SAP is typically produced through the cross-linking of acrylate monomers via ester bonding. Attempting to recycle SAP in its cross-linked state results in significant deterioration of its water absorption performance. To address this issue, Sumitomo Seika developed a technology that decompose only the cross-links of SAP retrieved from disposable diapers by hydrolysis, converting them back into polyacrylic acid, an intermediate polymer.

Impurities, such as urine, are removed from the polymer to create a purified polymer, which can then be re-cross-linked to produce recycled SAP. Consequently, recycled SAP’s water absorption performance and water retention under pressure are comparable to those of virgin SAP.

At its Himeji Works (Himeji City, Hyogo Prefecture), Sumitomo Seika intends to construct a pilot plant where these innovative technologies will be introduced The pilot plant is scheduled to be operational by the end of FY2026. This project was recently selected by the Ministry of the Environment as a fiscal 2024 “Program to promote establishment of decarbonized and circular economy”. Sumitomo Seika is aiming for full-scale social implementation.

Several hurdles must be cleared before this initiative can be realized. To begin with, how will disposable diapers be collected? Currently, in most municipalities disposable diapers are treated as general waste (burnable garbage) and not sorted. Large amounts of used disposable diapers for babies and infants might be collected from nursery schools and kindergartens, and those for adults from nursing homes and other facilities. However, collection of disposable diapers from households would require establishment of infrastructure for separate collection. Municipalities are responsible for waste collection and cooperation with them nationwide would be necessary.

In the first place, as a materials manufacturer, Sumitomo Seika found it difficult to collect used SAP for R&D purposes. To address this issue at the R&D phase of SAP chemical recycling, the company sought cooperation from partner companies, requesting them to provide used SAP.

With a view to creating a recycling framework, Sumitomo Seika aims to be recycling about 5,000 tons of used disposable diapers annually by 2030. Whereas the volume of disposable diapers discarded in Japan was around 2 million tons in fiscal 2015, this figure is projected to increase to 2.5-2.6 million tons by fiscal 2030. With recycling of 5,000 tons as an initial target, there is still a long way to go, but even so, paving the way for recycling disposable diapers, which will account for as much as 7% of general waste in fiscal 2030, will be a big step forward.

Support from national and local government to encourage the use and purchase of disposable diapers made of recycled materials may be desirable in the future. This is because the cost of recycled SAP will inevitably be higher than that of virgin SAP. Measures, such as converting pulp, another material used in the production of disposable diapers, and human waste absorbed by SAP into biomass fuel, should be considered.

How to determine the cost burden associated with recycling and the diffusion of recycled materials remains an issue, but it is gratifying to see the advent of yet another world’s first technology from Japan in the field of disposable diapers. In Asian countries, where birthrates are declining and populations are aging rapidly, disposable diapers have become commonplace, widely regarded as a daily necessity. Similarly, SAP recycling technology promises to spread as an indispensable technology for the world.