Chemical treatment of recycled pulp fibres for property development: Part 1. Effects on bleached kraft pulps

It was observed that sodium hydroxide, formamide, and ethyl acetate treatments enhance fibre swelling (re-swelling) abilities, especially after the first recycling stage. The highest water retention values (WRV) of 2.76 g/g were found at the second recycling stage of 7.5% formamide treatment (7.5Fa2). However, the highest air permeability value of 8.83 m2/sec. was found at the third recycling stage of 5.0% in ethyl acetate treated samples (5Et3). The optical properties of the whiteness and brightness of the sheets presented a variation in the recycled sheets and with further recycling, usually resulted in marginal changes. The sodium hydroxide treatment usually resulted in a 1.2%–24.6% improvement of tensile strength. However, the highest burst strength improvement of 49.6% was achieved at a 7.5% NaOH charge at the first recycling stage (7.5Na1). The formamide treatments resulted in approx. 5.3% to 53.2% tensile strength improvement. The highest burst of 4.48 kPa m2/g was found at the first recycling stage of 7.5% (7.5Fa1) conditions with formmaide, which indicates 88.2% higher than control (C1). Moreover, the highest tensile (38.8%) and burst strength (87.8%) improvements were found in the ethyl acetate treatment at the first recycling stage of 7.5% chemical charge level, while the highest tear strength improvement (102.6%) was found at the second recycling stage of 7.5% treatment level. It was realised that increasing the ethyl acetate concentration is usually positively correlated with all three strength properties of the sheets.