Esterification of pulp fibres have been performed in pyridine and toluene. The influence on swelling and the mechanical properties of sheets are shown to be different at the same level of esterification obtained by the two methods. This is interpreted to be due to the topological position of the acetyl group in the cell wall. Thus, esterification in toluene tends to allocate the esterification to the surface while in pyridine the interior of the cell wall is made accessible. At low degree of esterification in pyridine an increase in swelling is obtained. Thus, the hydrophobic groups may be seen as structure-breakers primarily by keeping the carbohydrate chains apart which otherwise would tend to form water-inaccessible regions, due to extensive hydrogen bonding. This maximum in swelling is maintained even in the dry state. Thus, the equilibrium moisture content is higher in a fibre moderately esterified in pyridine than a nontreated fibre. The introduction of acetyl groups is also shown to have an influence on the reswelling of the fibre after drying. The loss in swelling after drying is considerably less after drying for a moderately esterified fibre than for a nontreated fibre. Consequently the retention of the strength of the sheet has been improved. Essentially moderately esterified fibres which have been once dried yield a sheet with a strength equal to never dried virgin fibres.
The experiments with the fibres esterified in toluene did not yield the same effect indicating that the presence of hydrophoric groups inside the cell wall is necessary in order to maintain the swelling properties of the fibres. The results indicate new ways of permanenting the strength potential of recycled fibres and have illustrated the fundamental mechanism leading to the decrease of strength potential due to the drying of virgin fibres.