AIC's 52nd Annual Meeting

From the mid-19th century until about 1990, acidic paper-based materials were produced in large quantities throughout the world. Unfortunately, the use of acidic paper reduced the paper's strength due to chemical reactions during long-term storage. Although efforts have been made to mitigate this degradation through deacidification such as the Bookkeeper (BK) method, it remains difficult to restore the strength of degraded paper. The authors have developed an innovative coating method using fine cellulose fibers (FCF) as a strength-enhancing treatment after deacidification of degraded paper. FCF are defined as nano or submicron fibers prepared from cellulose fibers by miniaturization, and are characterized by high optical transparency and chemical affinity with the cellulose. This method was patented and registered as a Japanese patent in February 2022.

Prior to FCF coating, the BK method was conducted on naturally degraded wood-free paper, after which the paper was wetted and excess of water was removed on a vacuum suction table. Until now, FCF coating process has been done manually using a coating bar. In this study, we developed a compact coating machine that enables continuous coating on both sides of degraded paper by passing through two rolls in sequence. As optimized conditions, a coating speed of 4 m/min and a gap of 1500 μm between the rolls were selected for the coating of commercial FCF. Freeze drying, thermal drying, and vacuum drying were attempted as drying conditions for the paper after coating, with vacuum drying being the most appropriate. In this experiment, vacuum drying was performed at 40°C. Under this condition, the coating amount was approximately 1.2 g/m2.

While BK treatment of degraded paper did not change the tearing strength of the paper, FCF coating treatment increased the tear strength of BK-treated paper by 1.2 times. Comparing the tear strength of papers after accelerated aging showed that BK-treated paper was 1.2 times stronger than the untreated paper, indicating that degradation was inhibited. Furthermore, BK-treated paper coated with FCF was 1.4 times stronger than untreated degraded paper. This indicates that the combination of BK treatment and FCF coating treatment can achieve both degradation suppression and strength improvement. The legibility of the original paper remained unchanged after the FCF coating treatment, and the increase in thickness due to FCF coating was about 1% of the original thickness.

Good experimental results were also obtained in the possibility of lowering the drying temperature from 40°C to 30°C and in the preparation from raw materials ( Hardwood bleached karft pulp ) of FCF suitable for the coating.