Disposable diapers are made of materials that consist of wood pulp, a chemical called Super Absorbent Polymer" Or SAP for short.
The sap is used to to absorb the urine and expands as it absorbs that's why diapers swell up when wet.
Other materials such as plastic are used for the plastic outer shell and for the tapes or Velcro as the tabs on cloth backed adult disposable diapers or cloth backed baby diapers.