Absorption properties of foam formed cellulose fiber structures & effect of bio-based additives

Disposable hygiene products, such as diapers, wipes and pads, are the largest class of single used nonwovens with increasing consumption volumes. Approximately 70 % of the materials used in the hygiene products are plastic-based, including plastic fibers and/or acryl-based super absorbent polymers (SAP)1. However, the rising expectations towards more sustainable solutions have raised a demand for bio-based alternatives with high functionality. In the case of hygiene products, one of the most important functionality is the liquid absorption capacity.

Wipes and diapers already contain some cellulose fibers (wood fibers, cotton, regenerated fibers), which are bio-based, biodegradable and have moderate liquid absorption properties (cellulose fibers 1 – 2 g/g and cellulose fiber webs 5 – 15 g/g). Diaper materials are on totally different level with 100 – 1000 g/g of liquid absorption, but majority of the capacity comes from the SAPs and cellulose fibers provide mainly the supportive structure.

The material absorption in general (without SAPs) is defined by the structural features, such as density and porosity. For example, porous low density nanocellulose aerogel structures produced by freeze drying have shown increased absorption potential (100 – 200 g/g)2. However, freeze drying is rather energy consuming process. Foam forming technology3, on the other hand, utilizes aqueous foam in the production of cellulose-fiber based materials. With foam forming, materials of different density levels can be prepared, which provides possibilities for large-scale production of low density fiber based materials with reasonable energy consumption.

In this study, the absorption performance of foam formed cellulose fiber materials was demonstrated in laboratory scale. Fiber webs of 60 g/m2 in basis weight and below 30 kg/m3 in density were prepared using wood fibers and bio-based additives with laboratory foam forming device and oven drying (Fig 1). Bio-based additives included Tempo-oxidized nanocellulose, chitosan and carboxymethyl cellulose. The absorption, strength and porosity of the structures were measured.

Free water absorption capacity of the foam formed low density structures varied between 20 – 30 g/g, while absorption under load was 16 – 20 g/g. Dry and wet tensile strengths of the structures were generally low. However, especially Tempo-oxidized nanocellulose (5 % addition) showed clear improvement for both dry and wet strength (130 N/m, 17 N/m respectively) with good free water absorption capacity of 30 g/g and very soft haptic feel. Laboratory demonstrations indicated that foam forming could be utilized in production of hygiene materials of moderate absorption capacity. However, more work would be needed to further enhance the material absorption and strength.