Improving the sustainability of disposable diapers has been a concern for Swedish global hygiene and forest company, SCA, and Europe’s largest market pulp producer, Södra, for some time. In 2008, together with Chalmers University of Technology in Sweden, they launched WooDi, an R&D project to produce commercially viable alternatives to the oil-based materials used in disposable diapers, based on cellulose as a renewable resource.

The 14-member team included four doctorate students and had a budget of SEK 35 million ($4.6 million), of which SEK 20 million was from Vinnova, the Swedish government agency for innovation.

WooDi’s target is to reduce fossil fuel-based materials by 50 percent with a focus on the absorbent core in an incontinence product. While the outcome has not resulted in a commercially viable alternative to oil-based materials as yet, project leader Cécile Sandin, material specialist superabsorbents, SCA Hygiene Products, said that much has been learned along the way, which the team hopes to follow through in the future.

She admitted that they have yet to find a renewable material, which will perform to the same standard as oil-based equivalents currently in use. A wood-based diaper was first introduced to the market in the 1950s, and there have been various subsequent attempts to launch a diaper from renewable sources since, but none has performed to the stringent standards today’s consumers expect. Diapers today have far greater liquid retention, much less leakage, and far less risk of causing nappy rash than they did a few decades ago, and all in a highly price-sensitive market.

Elderly people, active people with light bladder weakness, and babies are the target market for diaper producers. Fast absorption, high capacity, and storage of urine are of prime importance; the product has to be able to withstand multiple soakings, prolonged contact with the skin and, in many cases, lots of movement.

The current trend is to concentrate absorption capacity in the crotch area of the diaper, in a fluff pulp mixed with oil-based SAPs (super absorbent polymers). The latter have excellent absorption properties, trapping and locking in urine quickly and efficiently. When introduced at the end of the 1980s, SAPs had a dramatic impact on the disposable diaper business and most baby diapers now contain more oil-based material than fluff pulp, some are even 100 percent oil-based.

WooDi has been investigating more sustainable options, using Södra softwood pulp as a starting point. In a reaction vessel, pulp water and chemicals were mixed and filtered to form a cake and then, after further processing, a jelly-type material. Once freeze dried, the end result is very light and airy super-absorbent foam which can reabsorb moisture effectively.

The cellulose-based foam can be used either on its own or in varying amounts added to a pulp stock before drying and could have interesting potential within a range of applications. However, the foam needs to be freeze dried to hold its structure, which is expensive; if air dried, it collapses and fails to maintain its foamy structure. When introduced into the pulp mix, entrance retention levels are immediately increased. The new material has been called SAW (for superabsorbent wood) and when mixed with fiber, it’s known as SAM (superabsorbent mix).

In tests so far, SAW has been found to hold up to 10 times its weight in saline solution after centrifugation. For SAM, the calculations are done in reverse, as a part of SAM. SAW can raise retention to 20 grams or 20 times its weight. “Fibers are added to reduce the load on the foam and an optimum was found for a mix of 85 percent fibers and 15 percent foam,” said Sandin. “Absorption capacity is amazing, but in terms of function versus price, it’s very difficult to compete with the SAPs.”

Linda Echardt, project leader for Södra Innovation, said that both SAW and SAM show good potential for new applications if their water content before freeze drying could be reduced. Water content is currently 99 percent before drying, which means the process is not currently cost effective enough for mass-produced markets.

“If we really want to find renewable solutions, we could consider developing new high-value applications, but we need enough volumes to make these products viable,” said Echardt. “The freeze-drying step is a bottleneck because it’s expensive, slow and can only be done a little at a time.”

“We’re not there yet,” said Sandin. “While cellulose is a polymer, it can’t yet match the SAPs in terms of function. Still, some very interesting materials have come out of this project and we believe we’ll be able to build some functioning products from them for the future although at this stage there may be some sacrifice to efficiency in terms of comfort parameters. We already have the theoretical process to manufacture them and we intend to conduct some environmental LCAs comparing the use of renewable forest materials to oil-based, including social aspects. Our hope is still to reduce the amount of oil-based components, little by little, but we can’t sacrifice performance, the market just won’t accept it.”

Although the team can’t say all the goals of the project have been met, it has two public patent applications pending for oxidized cellulose, a process which produces a very finely ground cellulose. Patents are pending for both the product and process.

From Echardt’s perspective, WooDi may not have generated an immediate answer to a more sustainable diaper core, but it has given Södra a whole new area to explore. The new material, called ThirstPulp, has been introduced via the company’s interactive website for innovation, PulpLabs (www.sodrapulplabs.com), and is being shown in both SAW and SAM form, undried and air-dried as well as freeze-dried.

“We hope to introduce new ideas and applications for ThirstPulp and see where the journey takes us,” said Echardt. “Students from Chalmers University of Technology have just collaborated in a workshop with us and come up with some really innovative, smart ideas. Who knows where these will take ThirstPulp in the future.”