The Mechanism Of Fiber Hornification Of Recycled Plant Fiber During Recycling And Modification Of Fiber Properties

Fiber qualities undergo a series of irreversible changes when recycled plant fiber is exposed to cycles of wetting and drying. This process is known as"hornification". Although the researchers gave many mechanisms of hornification, the exact mechanism behind hornification is still disputed. In addition, the hornification of the fiber restricts the use of recycled fiber. Till now, there is no appropriate method to improve its properties. In order to increase the use of rcycled plant fiber, this work systematically studied the mechanism of fiber hornification in drying and beating processes during recycling, studied the effect of noncellulosic constituents and carboxyl group content on fiber properties of recycled fiber, studied laccase and laccase mediator system biologically treated the recycled fiber to improve fiber properties.The mechanism of fiber hornification was studied during drying. The results showed that the crystallinity of cellulose and L002 increased, WRV decreased. The main reason of fiber hornification during drying was the formation of lactones. When the drying temperature increased from 60 oC to 120 oC, WRV was decreased by 24.9%, crystallinity of cellulose and L002 were increased by 5.0% and 19.2%, average pore size was decreased by 26.2%, and the lactones content was increased by 46.3%. The co-crystallization of crystallites occurred in fibrils during drying. Unbleached wheat straw fiber, analyzed by transmission electron microscope (TEM), showed a deformed and collapsed surface, and a less porous surface after drying and rewetting. Moreover, the deformation and collapse were irreversible.The mechanism of fiber hornification was studied during recycling. The results showed that the main reason of fiber hornifiation during recycling was the irreversible closure of the pore. WRV of wheat straw fiber was decreased by 16.2% after five rounds, compared with the 0 round. The crystallinity of cellulose, L002, lateral fibril dimension and lateral fibril aggregate dimension, analyzed by XRD and NMR, increased with the increase in the number of recycling rounds. Moreover, the increase of crystallinity was caused by co-crystallization of the fibril. The decrease of pore size and pore size distribution during the recycling process was due to the decrease of hemicelluloses content and the effect of drying and pressing processes on pore structure. The effect of beating on the properties of recycled fiber was studied. The results showed that beating can improve the strength properties of recycled fiber at some degree, but the improvement was just limited to the first cycle of the fiber. The degree of hornification of unbeaten and beaten pulps was 16.2% and 37.6% after five rounds, respectively. It showed that the beaten pulp had higher degree of hornification. The external fibrillation of the beaten pulp was significant characterized by ESEM. The crystallinity of cellulose of wheat straw fiber increased, analyzed by NMR, increased with the degree of beating. Fiber pore size and surface area increased with the increase in beating degree. The tensile index was increased by 10.4% and 42.0%, respectively when the beating revolutions was 500 and 2000, compared with the unbeaten pulp.In order to evaluate the effects of the noncellulosic constituents on recycled plant fiber recycling potential, the contents of hemicellulose and lignin were changed individually. The results showed the amount of hemicelluloses in fibers played a crucial role in the recycling potential of pulps. The degree of the decrease of tensile, burst strength increased with the decrease in the hemicelluloses content after recycling, and the degree of the increase of tear strength. When the hemicelluloses content was 18.8%, 13.5% and 9.7%, compared with un-recycled pulp, tensile strength was decreased by 10.3%, 17.5% and 25.6% after five rounds, respectively. When the hemicelluloses content decreased from 22.1% to 13.5%, average pore size and BET specific surface area was decreased by 11.6% and 22.9%, respectively.There was a positive correlation between WRV and carboxyl content, but the degree of hornification increased with an increase in carboxyl content. Crystallinity of cellulose decreased with the increase in carboxyl content. When the carboxyl content increased from 35.6 mmol/kg to 315.7 mmol/kg, crystallinity of cellulose was decreased by 11.8%. ESEM images showed the bonding between fibers was tight, and fibrillation was significant after beating and carboxymethylation treatment, resulting in the improvement of fiber swelling. FTIR spectra showed that the peak strength at 1633.73 cm-1 increased after carboxymethylation treatment, which showed that the carboxyl content of the pulp increased. The increase in carboxyl content not only improved the strength properties of the fiber, but also increased the recycling time of the fiber. The optimum conditions for laccase-histidine modification of ONP fiber were: the concentration of laccase 0.1% dry pulp, the concentration of histidine 1 % relative to the dry pulp, room temperature, reaction time 1.5 h, pH 7, the pulp consistency 5% and O₂ atmosphere. Compared with the control pulp, WRV of ONP treated with laccase and histidine was increased 45.7%. ESEM images showed that more collapse and more fibrillation were observed on the laccase-histidine treated fiber surface than the control samples, which led to form better bonding between fibers in handsheets resulting in the increase of the paper strength of laccase-histidine treated ONP pulp. The results of industry pilot experiment showed that WRV, longitudinal ring rush index and longitudinal tensile index of OCC after laccase-histidine treatment was increased by 45.8%, 28.1% and 55.3%.The optimum conditions for laccase biological modification of OCC fiber were: the laccase dose 160 U/g, reaction time20 h, room temperature, pH 7, the pulp consistency 5% and O₂ atmosphere. Compared with the control pulp, WRV of OCC treated with laccase and HBT was increased 21%. FTIR spectra showed that lac-HBT treated pulp, compared with the control pulp and laccase-treated, was the appearance of the band at cm-1 due to C=O stretching in conjugated carbonyl groups and carboxylic acid groups, which showed that new conjugated carbonyl groups and carboxylic acid groups were formed during lac-HBT treatment. NMR spectra showed that the signal became weaker of G-lignin and S-lignin of OCC pulp after laccase or lac-HBT treatment, compared with the control pulp, which indicated that lignin was oxidized after treatment resulting in the lignin content decreased.