A thesis submitted in partial fulfillment of the requirements for the degree of Master of Engineering, School of Environment, Resources and Development

Thesis (M.Eng.) - Asian Institute of Technology, 2001

Paper recycling causes major changes in the strength properties of pulps. The magnitude of change upon recycling is different for different types of pulps (mechanical, kraft, sulfite, etc.). Since mechanical pulps do not seem to suffer from the same problems as chemical (low yield) pulps, difficulties in paper recycling seem to be related to the chemical composition of free cell wall. It was concluded that the presence of lignin is a major obstacle for extensive hydrogen bonding between microfibrils. Besides, hemicellulose has also been proposed to play an important role on paper strength loss due to recycling. However, a systematic work on these proposals is scarce and needs to be explored. The present study was developed to examine the impact of chemical compositions of the fiber wall on strength properties of recycled paper. Fine-free CTMP were used to remove lignin, selectively at different degree, 15%, 35%, 45%, 50%, 70% and 90% respectively. Delignified pulps were recycled and tested for tensile strength. The results show that hemicellulose, which is unchanged in chlorite treatment, might not give any influences on paper strength loss due to recycling. It was concluded that elimination of lignin as low as 16% from the cell wall reduces recycling potential of the fibers. This was proposed that the lignin should be removed first from the surface of the fibers. Loss of surface lignin was discussed as a source of hornification.