| Wood dyeing is an effective way to increase the added value of low-quality raw materials.In the process of wood dyeing,there are still problems such as insufficient dyeing depth and uneven dyeing.The pretreatment can effectively improve the permeability of wood.The effects of various factors on extract content,volume shrinkage and dye-uptake and dye penetration rateof Acid Scarlet 3R dyed wood after pretreatment with sodium hydroxide were analyzed.The effects of freezing time,thawing time and cycles on wood dyeing rate and permeability were investigated.On this basis,response surface methodology was used to optimize the sodium hydroxide-freeze-thaw cycle treatment process,and single acid,reactive dye and their mixed dyes were used to verify the experiment.The changes of wood micro-structure and pore structure before and after treatment were observed and analyzed by field emission scanning electron microscopy（FE-SEM）and mercury intrusion porosimetry（MIP）.Finally,the effects of pretreatment on light fastness and water fastness of dyed materials are studied.The main conclusions are as follows:（1）With the increase of sodium hydroxide concentration,extraction time and extraction temperature,the content of wood extracts increased and the volume shrinkage rate increased.Compared with untreated wood,the maximum dye-uptake,longitudinal,radial tangential dye penetration rate increased by 1.6,10.6,30.6,37.0 times and 1.9,12.2,34.5 and 43.24time;after sodium hydroxide treatment and sodium hydroxide-freeze-thaw cycles pretreatment,respectively.According to the two linear fitting results,sodium hydroxide concentration,extraction time,freezing time and cycle times were selected as response surface methodology factors.（2）The optimal process of sodium hydroxide-freeze-thaw cycles treatment was optimized by response surface methodology.The process was sodium hydroxide concentration 0.42%,extraction time 24 h 15 min,freezing time 18 h 40 min and cycle times 2 times.The predicted values of dyeing uptake,longitudinal,radial,tangential dye penetration rate were8.90%,77.57%,48.29%and 27.45%respectively.The average dye-uptake,longitudinal,radial and tangential dye penetration rate of the three repeated validation tests were 8.45%,76.63%,47.25%and 26.46%respectively,which were not significantly different from the predicted values.Therefore,the model was reasonable and effective.The dyeing tests of single acid,reactive dye and their respective mixed dyes and the water absorption and weight gain tests further verified the reliability of the process.Scanning electron microscopy（SEM）of sodium hydroxide-freeze-thaw cycle treated wood showed that the extracts from longitudinal tracheid wall decreased in radical section,and the pits were clearly visible.The extracts from ray parenchyma cells were dissolved in tangential section,and the longitudinal tracheid walls were separated from ray parenchyma cells.Mercury intrusion analysis showed that the total pore volume and porosity of pretreated wood increased from 35.93%to 4.75%compared with untreated wood.（3）After 108 hour UV exposure,the color difference of dyed wood with sodium hydroxide and sodium hydroxide-freeze-thaw cycles were lower than that of untreated dyed wood,which indicated that pretreatment could improve the light fastness of dyed wood.The wash resistance test of dyeing materials showed that pretreatment had little effect on their water color fastness.The FTIR infrared spectrum showed that the carbonyl peak at 1739 cm-1 disappeared of the sodium hydroxide-freeze-thaw cycles treated materials,and the other peak shapes were basically unchanged,which were consistent with the changes of light fastness and water fastness of the dyed wood after pretreatment. |
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