大片薄刨花采用常规滚筒干燥工艺变形显著，影响施胶、铺装及热压等一系列后续工艺，最终降低生产的刨花板力学性能。低温网带式干燥工艺作为一种新型刨花干燥工艺被提出来解决大片薄刨花干燥的变形问题。然而，此方面的研究非常少，阻碍了该工艺的应用和推广。研究了单片辐射松大片刨花在相对较低的140℃恒定干燥温度下，其纹理方向、厚度和长宽比对刨花变形的影响，为后续大片刨花低温网带式干燥工艺的优化奠定理论基础。刨花纹理方向与长边角度具体控制为0°～30°，31°～60°和61°～90°;平均厚度控制为0。37，0。45，0。55和0。65 mm;长宽比控制为3。5，4。5，5。5和6。5。干燥结束后刨花的变形程度通过自定义的变形高度:刨花长宽比、变形高度:刨花长度及变形高度:刨花宽度定量评定。此外，利用X射线计算机断层成像(X-ray computed tomography，X-CT)技术探索了单片刨花干燥过程中含水率分布情况，利用低场核磁共振(low-field nuclear magnetic resonance，LF-NMR)技术探索了辐射松早材和晚材试样干燥过程中水分分布的差异，基于此分析了刨花变形的机理。结果表明，140℃干燥温度下辐射松刨花的纹理方向、厚度、长宽比均显著影响刨花干燥后的变形。刨花纹理方向与长边夹角越大、刨花越薄，干燥后刨花变形程度越大。刨花长宽比对干燥变形的影响随变 形评价指标的变化而变化。干燥过程中刨花内部与边缘的含水率差异以及早晚材差异是导致刨花变形的主要原因。
Deformation of large thin radiata pine flakes under low temperature drying and deformation mechanisms
Large thin flakes are deformed significantly by the conventional drum drying process,which affects a series of follow-up processes such as sizing,paving,and hot-pressing,and downgrades the mechanical properties of produced particleboards.A low-temperature net belt drying process has been proposed as a new drying process for flakes to solve the deformation problem of large thin flakes.However,there is very limited research in this area,which hinders the wide applications of this process.The effects of the texture direction,thickness,and length to width ratio on the deformation of individual radiation pine flakes were studied at a relatively low constant drying temperature of 140 ℃,which laid a theoretical foundation for optimizing the low-temperature net belt drying process of bulk flakes.The texture direction and long edge angle of the flakes were 0°-30°,31°-60° and 61°-90°,the average thickness was 0.37,0.45,0.55 and 0.65 mm,and the length to width ratio was 3.5,4.5,5.5 and 6.5.The evaluation of flakes'deformation post-drying involved assessing their self-defined deformation height:flakes'length to width ratio,deformation height:flakes'length and deformation height:flakes'width.In addition,the moisture content distribution of individual flakes during drying and the difference of moisture distribution between radiation pine earlywood and latewood during drying were explored by the X-ray computed tomography(X-CT)and the low-field nuclear magnetic resonance(LF-NMR)technology,respectively.The deformation mechanism of flakes was further analyzed.The results showed that the texture direction,thickness,and length to width ratio of radiation pine flakes significantly affected the deformation of dried flakes at 140 ℃.The larger the angle between the texture direction and the long edge of the flakes,the thinner the flakes were,and the greater the flakes'deformation after drying.The effect of the length to width ratio on flake deformation depended on the evaluation variables chosen.During the drying process,the difference in moisture contents between the interior and edge of the flakes and the difference between earlywood and latewood were the leading causes of flakes'deformation.The higher MC of the flake interior and lower MC of the flake edges during drying caused the inconsistent drying of the interior and edges,resulting in the deformation of the four corners of the flakes bending inward after drying.There were apparent differences in the drying rate between the earlywood and latewood areas of radiata pine flakes,and the distribution of bound and free water in the middle and late stages of drying.Additionally,the differences in the shrinkage characteristics of the earlywood and latewood themselves resulted in deformation at the boundary of the earlywood and latewood of the flakes after drying.
large thin flakelow-temperature dryingflake deformationX-ray computed tomography(X-CT)low-field nuclear magnetic resonance(LF-NMR)
万方数据
维普
