Boosting the absorption of foam formed structures - preliminary studies

Summary
• The goal of the work was to study the absorption capacity of foam formed cellulose materials made in laboratory scale.
• The effects of structure density, additives, fibre- and surfactant type on material absorption capacity, density and strength were investigated.
• Low material density resulted increased absorption but had negative effect on strength. Investigations around effective strength additives will be continued.

Introduction
Liquid absorption capacity of cellulose fibre materials can be divided in two effects: 1) fibre absorption and 2) fibre structure absorption. Hydrophilic and porous cellulose fibres have absorption capacity usually around 1 g/g whereas fibre structures, such as towels, absorb 5-10g/g. Water absorption in structures is caused by the capillary forces acting among the walls of the pores. Thus, it is possible to affect the material absorption properties by modifying the fibre surfaces or affecting the structure of the formed fibre network.
To ensure good absorption, the fibre structure needs to have suitable bulk and porosity. With conventional water forming the preparation of bulky structures is restricted as the structure is easily densified during water removal. With foam forming, fibre structure density can be adjusted on a totally different level. High air content and stability of foam enables manufacturing of fibre-foams that have densities under 100 kg/m3 and the bulky structure is preserved after drying. For example, absorption capacity of 17g/g has been measured for foam formed softwood (kraft) structure with density of 60kg/m3 and basis weigh of 1200g/m2. Materials with even lower densities and basis weights can be prepared, however; there is no previous research around the absorption properties of low weight foam formed materials.
The aim of this work is to demonstrate the performance of foam formed cellulose based materials in absorption structures. The development has been started with very basic laboratory investigations. These include studies on the effects of structure density, fibre- and surfactant type and additives on absorption and tensile strength properties of foam formed handsheets. Handsheets of 60 g/m2 in basis weight were prepared using laboratory foam forming device. The structures contained only one layer and the main raw materials were wood fibres.

Results
Sheets of three different density levels were produced by altering the vacuum during sheet preparation. So called thick sheets had densities between 10-30 kg/m3 and thin sheets had densities between 90-200 kg/m3 depending on the fibre type. Stiff mechanical fibres had lower density compared to chemical fibres. Thin structures had absorption around 10g/g and dry tensile index between 3-5 Nm/g without additives. Thick sheets were very low in strength (dry tensile index below 1 Nm/g) and absorption could not be measured. Basic paper making additives were tested to increase the material strength. Especially fibrillated cellulose materials, carboxymethyl cellylose (CMC) and polyamide epichlorohydrin (PAE) increased dry- and wet strength of the structures (Figure 1). With additives the strength of the thick sheets was increased so that absorption could be measured reliably. Absorption capacities for thick sheets were 20-30g/g. Also the effect of surfactant type was investigated. It was observed that surfactant affected especially the haptic properties. Nonionic surfactants made the sheets softer when compared to anionic ones.

Conclusions
Structure density plays the main role in material absorption. The lower the density the better the absorption. However, low density has a negative effect on material strength. First tests have shown that basic paper making additives can be used to increase the strength but further work is still needed to get the values on desired level depending on the application. Future tests will include pilot scale trials, layered structures and screening of more efficient strength additives.