Mile­stone in diaper recyc­ling: NMI suc­ceeds in degrad­ing cel­lu­lose in com­pos­ite materials

Paper, glass, plastic, even con­crete: many mater­i­als are now recycled. For diapers, on the oth­er hand, this is still almost impossible. World­wide, less than one per­cent of all diapers are recycled — and this is des­pite the fact that diapers make up around ten per­cent of house­hold waste (in care homes, this fig­ure can rise to 70 per­cent). The chal­lenge is sep­ar­at­ing the cel­lu­lose in the diapers from the super­ab­sorb­ent mater­i­al that absorbs the liquid. A team of research­ers led by Dr. Anne Zeck, group lead­er at the NMI Nat­ur­al and Med­ic­al Sci­ences Insti­tute in Reut­lingen, has now achieved a break­through with the help of enzymes as part of the Invest BW pro­ject Encycling.

Mix­ture of cel­lu­lose and the superabsorber

Diapers are a so-called com­pos­ite mater­i­al. Inside them is a mix­ture of cel­lu­lose, which in this case is like absorb­ent cot­ton and is neces­sary for the most com­fort­able wear­ing exper­i­ence pos­sible, and the so-called super­ab­sorber, which con­sists of the sodi­um salt of poly­ac­ryl­ic acid and can absorb many times its own weight in liquid. Both mater­i­als are eas­ily recyc­lable on their own. In the case of diapers, how­ever, they must first be sep­ar­ated. This is com­plic­ated — and unfor­tu­nately not pos­sible for all man­u­fac­tur­ers.
Extremely vis­cous mass

“When a liquid is added to this mix­ture of cel­lu­lose and super­ab­sorb­ent mater­i­al, the res­ult is a gel-like mass. This makes it impossible to mix the mass prop­erly and treat it with enzymes,” explains pro­ject man­ager Anne Zeck. By adding cal­ci­um chlor­ide, the super­ab­sorber releases the bound liquid again. The shred­ded diaper mass can now be mixed thoroughly.

Cel­lu­lose degrad­a­tion is not new — but it is in diapers

The next step was to search for enzymes that can break down cel­lu­lose. ​“Two cri­ter­ia are par­tic­u­larly import­ant in this search: from a huge range of enzymes, we had to find those that are reli­ably able to con­vert cel­lu­lose into sol­uble sug­ar molecules in the very spe­cif­ic envir­on­ment of a diaper. At the same time, how­ever, these enzymes had to be not too expens­ive so that the tech­no­logy had a real­ist­ic chance of suc­ceed­ing on the mar­ket,” says Anne Zeck, describ­ing the chal­lenges. The com­pany Can­didum provided sup­port in this search.

0.7 grams of enzymes break down 800 grams of cellulose

Anne Zeck­’s team added a total of 0.7 grams of the enzymes that were ulti­mately selec­ted to 5.8 kilo­grams of diaper mater­i­al, which they had pre­vi­ously shred­ded and soaked in water — because diapers end up in the trash in a used, i.e. damp, state. They shook this mix­ture for sev­er­al days at 50 degrees Celsi­us. Dur­ing this time, the enzymes broke down 800 grams of cel­lu­lose and pro­duced sug­ar from it, which was removed with the aqueous phase. Five kilo­grams of the shred­ded diaper mater­i­al, which mainly con­tains super­ab­sorb­ents, remained.

A mile­stone — to be fol­lowed by others

In the next step, ARCUS Green­cyc­ling will test on a pilot plant scale (i.e. lar­ger than a labor­at­ory exper­i­ment, but smal­ler than an indus­tri­al pro­duc­tion pro­cess) wheth­er the low-cel­lu­lose com­pos­ite mater­i­al can be chem­ic­ally recycled sig­ni­fic­antly bet­ter. Put simply, the ques­tion is wheth­er this so-called pyro­lys­is (i.e. the split­ting of the com­pos­ite mater­i­al into its ori­gin­al raw mater­i­als) and thus the return of the raw mater­i­als to the cycle is more suc­cess­ful. Ways of using the sug­ar-con­tain­ing liquid as a mater­i­al in the future are to be invest­ig­ated. Water con­sump­tion also needs to be optim­ized. Only then will recyc­ling be truly suc­cess­ful and make both eco­lo­gic­al and eco­nom­ic sense. Nev­er­the­less, the suc­cess­ful use of enzymes in a dif­fi­cult envir­on­ment is a mile­stone on the way there.