Mid Sweden University

The hot-pressing technology has proven to have the potential for manufacturing of strong, wet stable materials based on eco-friendly renewable and recyclable lignocellulose. The purpose of this work was to study how the pulp characteristics and the hot-pressing conditions affect the dry and wet strength properties of paper. Two different devices for hot-pressing were used. One using felted nip and a heated cylinder with a temperature limit at 200°C and one new design using a hard nip and an IR-heated steel belt with a temperature limit of 300°C.

The results showed that dry strength can increase up to 150% for high yield pulp (HYP) based sheets at pressing temperatures well above the softening temperature of lignin. The maximum dry tensile strength obtained was 70 kNm/kg at 200°C pressing temperature and the corresponding value for a lignin-rich kraft pulp was about 130 kNm/kg, an increase of 30%. For all lignin-rich pulps the dry strength increased linearly with density up to 200°C whereafter it levelled off and was reduced.

The wet tensile strength for paper based on HYP increase from 2 to 28 kNm/kg and for paper based on unbleached kraft pulp from 5 up to 60 kNm/kg in the temperature interval 20-270°C. The increase in wet strength independently of pulp grade seemed to be exponential to the pressing temperature with the steepest slope above 150°C. For unbleached kraft pulp a lignin content of minimum 7% seemed to be necessary for improved wet strength but 12% gave the highest value within the studied interval. In HYPs the lignin content is 25-28% depending on the pulping process but the level of wet strength was lower which is probably related to the lower density and lower dry strength compared to unbleached kraft pulps.

Dry strength of lignin-rich paper is enhanced by improved fibre-fibre contact that can be improved by compression at high temperature, well above softening temperature (Tg) of moist lignin, native or chemically modified. It is known that sulfonation of lignin lowers the Tg in moist conditions. It was observed that at 150°C temperature the dry strength increased by 15% to a level of 71 kNm/kg for the high sulfonated pulp compared to the lower sulfonated pulp that had a dry strength of 60 kNm/kg at the same density. The level of wet strength was however not found to be affected by the sulfonation.

Paper strength is to a large extent related to pulp fibre morphology and fines content. In this work studied these aspects where briefly studied with respect to hot-pressing and the results indicate that the relative influence of fibre morphology seems to be reduced with increasing pressing temperature. Hot-pressed sheets based on a coarse fines free fibre fraction showed 100% dry strength increase and wet strength increase up to 20 kNm/kg. The dry and wet strength were however also shown to be favoured by the presence of fines fraction.

Wet strength development as a function of temperature was fitted to an Arrhenius type of equation and activation energies were found to be similar for very different pulp grades provided that the lignin content is above 7%. This could indicate that the process(es) giving wet strength were similar.

It was found that the ratio wet:dry strength was about 35-60% for all lignin containing pulp grades. A rule of thumb for an efficient wet strength resin is that the wet: dry strength ratios are 15%. This means that it should be possible to manufacture wet-strong paper from lignin-rich pulps by means of hot-pressing without using wet strength chemicals. The concern regarding repulpability of such material led to an initial test to disintegrate this paper showing that re-pulping under vigorous mixing at room temperature is possible.

The connection between dry and wet strength, high pressing temperature, and lignin content of pulp fibres is suggested to be related to some redistribution mechanisms of surface lignin between adjacent fibres. The improved wet strength and water resistance could be due to intermixing of lignin polymers across the interface between contacting fibre surfaces, or it could be lignin sufficient to cover the fibre-fibre bonds and/or chemical modifications, but these remain open questions.