18. Up-scaling hot-pressing systems to manufacture strong and water-resistant products from biobased raw material

New technology is required to produce bio-based materials solely from green sources and which can replace plastic in waterproof packaging. It is also important to maximize the use of the tree to increase resource efficiency. This could be possible with high yield pulp (HYP) in a hot-pressing process that utilizes the rheology and thermoplastic behaviour of all wood components. Pilot scale trials showed that when a moist paper web or molded pulp passes a press nip at temperatures above Tg, the density, tensile strength and stiffness increase significantly due to compressed fibre network. In addition, lignin-rich pulps showed remarkably high wet strength when pressed at high temperature up to 270°C. Wet strength of 50% of dry strength has been achieved.
To create hydrophobic surfaces, these were treated with betulin from birch bark. This was done by producing a water suspension with betulin which was sprayed on the surface, whereafter a hardening took place in the hot-pressing machine at a temperature up to 260°C, i.e. in the range of the melting point of betulin. Very hydrophobic paper surfaces are obtained with a contact angle of around 115°.

The process we developed was based on the synergy effect of hot-pressing of HYP and surface treatment with betulin without any additives and strengthening agents. Achieved properties such as high dry strength, high wet strength and water repellency are at the level of requirements for plastic replacement materials for several applications.