Light-induced Yellowing Mechanism Of Poplar APMP Pulps And Its' Inhibition Measures

The pulping properties of several poplar species by APMP pulping processwere investigated and compared. The mechanism of light-induced yellowing of poplar APMP pulps was studied, the prohibition measures from light-induced yellowing for poplar APMP pulps were investigated. Populus deltoides, Populus tomentosa and triploid clones of Populus tomentosa were examined, by means of analysis of wood properties, chemical components of wood and fiber anatomy as well as APMP pulping properties. It may be concluded that fast-grown poplar wood will be good potential for new fiber resource for pulping industry using chemi-mechanical pulping process, because of its proper basic density, excellent fiber dimension and initial brightness of wood. It was proved that the application of APMP pulping process on fast-grown poplar wood has many advantages such as easy to be pulped, lower specific refining energy consumption and lower chemical charges to produce bleached chemimechanical pulps with high yield , good brightness, stronger pulp strength. Therefore, the APMP pulping process should be a preferred pulping technology for utilization of fast-grown poplar wood.The modern spectrum analytical technologies were applied to study the lignin structure of poplar wood and its APMP pulps. The results show that the lignin of Populus deltoid wood is a GS type lignin (Residual lignin in its APMP pulps is the same type) with a content of S-56.81%, G-42.42% and H-0.77%. There are large amounts of phenyl hydroxyl groups and a-carbonyl available in the residual lignin of poplar APMP pulps by using 'H-NMR spectra analysis. Lignin macro-molecules could be able to de-gradated by UV light irradiation to form phenoxyl free radicals which were easily to be oxidized by oxygen into lignin peroxide radicals to rape the hydrogen protons causing the breaking out of -O-4 linkage. More phenoxyl radicals and a-carbonyl structures were formed. The further oxidation converted the phenoxyl into quinoid structures which are chromophores. Therefore the mechanism of light-induced yellowing of poplar APMP pulps could be described as the pathways with formation of those functional groups as quinoid, a-carbonyl, conjugated , - propylene in lignin.Referring to the mechanism mentioned above, several measures preventing poplar APMP pulps from light-induced yellowing were studied including reducing of pulps, chemical modification of lignins as well as application of UV screeners, anti-oxidants and radical trappers onto surface of papers. The results show that the brightness stabilizing effect of pulps could be improved by reducing with sodium borohydride alone or the mixtures of sodium borohydride and sodium bisulfite with very sufficient effects comparing with other type mechanical pulps. The optimum conditions for the reduction were optimized as NaHB4/NaHSO3: ~0.8%/1.2%, temperature 70~75癈 for 20-30 minutes (or at room temperature for 80 minutes more) with pH 10-11 and pulp consistency of 20%. Acetylation also could obviously stabilize the brightness of poplar APMP pulps. The moisture content of pulps during acetylation will influence on the brightness stabilizing effects. If using acetone as reaction mediator, the wet pulp can be used with a slight decrement of pulp brightness. There are light bleaching phenomena for acetylated pulps at acetone solvent. Acetylation carried out at gas phase seems to be easily for application in industrial practice, the further investigation should be required. The brightness of papers from poplar APMP pulps would be stabilized efficiently by applying UV screens, anti-oxidants and free radical trappers with an appropriate mixing rate. By adding of dodecanol-1, the brightness stabilizing effect could be enhanced. It is found that UVa-1, dodecanol, radical trapper (0.1% or no) and anti-oxidant(0.5%) in aquatic solution are able to stabilize the brightness of reduced poplar APMP pulps (0.5% NaHB4) efficiently. This method could be used for industrial operation.