| The flattening technology without nicked in curved bamboo strips is a new technology for efficient processing and utilization of bamboo after the flattening technology without nicked of bamboo.It has the advantages of fewer processing procedures,high utilization rate of bamboo and low product cost,and is of great significance for promoting the development of bamboo flattening technology.In this paper,curved bamboo strips are taken as the research object,microwave as the softening medium,through the direct microwave softening,boiling-microwave softening and saturated steam-microwave softening three methods,the influence of different microwave treatment methods on the softening effect of bamboo and the rule of temperature change.By comparing the water content and mechanical properties of curved bamboo strips after softening,the microwave softening method of curved bamboo strips without nicked was optimized;Then taking curved bamboo strips with different physical characteristics as the research object,the effects of microwave softening technology and physical characteristics of curved bamboo strips on the softening efficiency,appearance quality,physical and chemical properties and production cost of bamboo exhibition plate were carried out,the mechanism of microwave softening and flattening curved bamboo strips was revealed,and the flattening without nicked technology of curved bamboo strips was further optimized to provide theoretical technical support for industrial production of enterprises.The main research conclusions are as follows:(1)After direct microwave softening,the temperature of each layer of curved bamboo strips increased continuously,and the rising rate decreased at 100℃.The highest temperature of bamboo green,bamboo meat and bamboo yellow could reach 111.2℃,138.5℃ and 129.5℃ respectively.At the same time,the water content showed a downward trend,and the MOE and MOR showed a downward trend and then an upward trend.When the microwave softening power and time were40 KW and 16 min,respectively,the MOE and MOR were the lowest,which were 3109 MPa and66.2MPa,respectively,and decreased by 54.13% and 30.17% compared with untreated materials,but the water content was lower.With the extension of boiling time and the increase of temperature,the water content of curved bamboo strips was increased.After boiling at 80℃ for 3h,microwave power was 40 KW and time was 20 min,the MOE and MOR of curved bamboo strips decreased the most,which were 2391.32 MPa and 60.93 MPa,respectively.Compared with untreated wood,it decreased by 64.72% and 35.73%,which is conducive to improving the ductility of curved bamboo pieces,but the water is polluted greatly because of the long time of boiling.The moisture content,MOE and MOR of curved bamboo strips softened by saturated steam-microwave combined with saturated steam pretreatment tended to decrease with the increase of saturated steam pretreatment time and temperature,and the MOE and MOR of curved bamboo strips decreased the most when the microwave power was 40 KW and the time was 20 min.They are 1692.49 MPa and 23.3MPa respectively,which are reduced by 75.03% and 75.42% compared with untreated materials.The treatment time is short,which can effectively reduce the mechanical strength while maintaining high water content,and improve the plasticity is conducive to the flattening of curved bamboo pieces.In conclusion,saturated steam and microwave softening is the best softening method,followed by direct microwave,which can be used for softening and flattening narrow curved bamboo strips.At the same time,the saturated steam-microwave combined softening method has higher continuity than the traditional softening method,and can flatten quickly after softening,while maintaining a higher temperature at normal time is more conducive to ensuring the quality of bamboo flattening.(2)Saturated steam pretreatment temperature,microwave power,microwave time,temperature of curved bamboo strips,removing the bamboo green treatment and number of bamboo segments all have different degrees of influence on the flattening quality of arc bamboo strips.In the test range(150～160℃),the success rate and surface quality of bamboo flattening can be effectively improved by increasing the saturated steam pretreatment temperature.Microwave softening and flattening curved bamboo strips pretreated with saturated steam at 150℃ and 180℃produced several penetrating cracks and poor surface quality.The curved bamboo strips pretreated by saturated steam at 160℃ and 170℃ showed relatively good flattening effect after microwave softening.However,a penetrating crack was occasionally produced after microwave softening at170℃ for 12 min and 20 min.The curved bamboo strips after microwave softening at 160℃ for20 min had the best flattening effect.The success rate of flattening is 100%,and the surface quality of Zhuzhan plate is good without surface cracks.As the temperature of bamboo decreases,it can lead to cracking and reduce the success rate of flattening.The removal of bamboo green is helpful to improve the appearance quality,but has little effect on the success rate of flattening.The curved bamboo strips softened by microwave for 20 minutes after 160℃ saturated steam pretreatment with microwave were flattened with green and with green,and the success rate of flattening was about 85%,but there were few cracks on the surface of the slab after green treatment.The number of arc-shaped bamboo segments has a certain effect on its flattening quality.The closer the arcshaped bamboo segments are to the root of the bamboo,the worse the flattening effect is,and the farther away from the root,the less the effect is.Under the same microwave softening technology,the flattening success rate of the third section curved bamboo slice is 20% higher than that of the first section.At the same time,the softening efficiency of saturated steam and microwave is slightly lower than that of traditional softening efficiency,but its energy cost is relatively low,and the continuity between the former and the non-scratch flattening machine is higher than the latter,and the temperature of bamboo sheet is easy to control.Due to the use of saturated steam and microwave treatment,the softening effect is excellent,the flattening quality is high,the cracking rate is low,the material yield is high,has a good application prospect.(3)With the increase of softening temperature and the extension of softening time,the number of hydroxyl and carbonyl groups of bamboo decreased,the crystallinity increased,the dimensional stability increased,and the content of cellulose and hemicellulose decreased,which was beneficial to the flattening of curved bamboo strips.After the saturated steam-microwave softening for 16 min and 20 min flattening,the radial air-drying shrinkage decreased by 51.84% and 63.34%,respectively,and the stringed air-drying shrinkage decreased by 34.63% and 61.97%,respectively.The MOR and MOE of the saturated steam-microwave softening and flattening bamboo increased with the extension of microwave time.The MOR and MOE of the plate are 93.8MPa and6685.4MPa respectively when microwave is applied for 20 min,showing good mechanical strength.Scanning electron microscopy(SEM)showed that after microwave softening,the bamboo was subjected to pressure and temperature.Meanwhile,the hemicellulose in the cell wall of the bamboo was decomposed and the cellulose was degraded.The thin-walled cell tissue was slightly crumpled and stratified,but the whole structure was not obviously damaged,showing irregular circle.After flattening,the parenchyma cells squeeze and deform and fit together,destroying the original shape. |
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