纖維體積分數對孟宗竹自膠合粒片板製程與機械性質影響

Abstract

竹材具有纖維體積分數隨縱向及徑向漸進變化之特性，竹子之各項強度、組織及化學組成與此梯度功能特性密切相關，斷面影像分析為測量竹材纖維體積分數的方法之一。自膠合技術因利用材料本身既有的木質素作為天然膠合劑無須另外添加，近年來愈發受到關注，然而目前尚未有竹材纖維體積分數對竹粒片自膠合板製程與性質之相關探討，故本試驗以三年生孟宗竹為材料，分部位記錄其纖維體積分數，並比較對應的自膠合板基礎性質及化學組成。為了解熱壓後之粒片型態，拍攝電子顯微鏡影像，觀察取自不同纖維體積分數之竹材部位的粒片，比較其型態差異。結果顯示，竹基部外側具有最高纖維體積分數39%，竹梢次之為35%，竹中為31%，竹基部內側具最低纖維體積分數19%。觀測板芯溫度經時變化結果發現，180oC，二階段式熱壓，最大壓力3MPa，30分鐘板芯溫度約達175oC。內聚強度結果顯示，竹基部內側自膠合板具最佳強度0.272MPa，為四者中最良好之膠合品質，其餘三者內聚強度不具顯著差異。主成分定量結果顯示，竹基部外側粒片具最高木質素含量27.5%，竹梢與竹中同為26.9%，竹基部內側之木質素含量最低25.5%。木質素含量與纖維體積分數具有相同趨勢，隨著纖維占比提升，木質素含量隨之上升，兩者具顯著相關。熱壓後所有部位之全纖維素比例皆因高溫造成半纖維素降解而下降。壓縮比對缺乏膠合劑固化的自膠合板之膠合效果影響甚鉅。電子顯微鏡拍攝自膠合板板面與板芯狀態結果顯示，由於竹自膠合粒片型態為針狀且長度分布跨度較大，導致板面粒徑較大孔隙較多，板芯粒片則較為緻密。竹基部內側粒片的自膠合板其板材粒片具有明顯壓潰，另外三者則粒片形狀較為立體且完整。180oC，3MPa之熱壓條件對竹基部內側竹粒片具較好的自膠合效果。取自較高纖維體積分數部位之粒片，需要較高溫高壓之熱壓條件方可達到足夠的自膠合效果。

Bamboo exhibits a characteristic of gradual variation in fiber volume fraction along both longitudinal and radial directions. The strengths, structure, and chemical composition are related to this gradient functional feature. Image analysis of cross section is one of the methods used for measuring the fiber volume fraction of bamboo. Self-bonding technology, which utilizes the inherent lignin of the material as a natural adhesive without the need for additional additives, has gained increasing attention in recent years. However, there has been limited investigation into the correlation between the fiber volume fraction of bamboo and the process and properties of bamboo particle self-bonding boards. Therefore, in this study, three-year-old Moso bamboo was used as the material. The fiber volume fractions were recorded for different sections of the bamboo, and the corresponding basic properties and chemical composition of the self-bonding boards were compared. To understand the particle morphology after hot pressing, electron microscope images were taken to observe the particles from bamboo sections with different fiber volume fractions, and their morphological differences were compared. The experimental results reveal that the outer part of the bamboo base has the highest fiber volume fraction at 39%, followed by 35% in the bamboo tip, 31% in the middle portion, and the inner side of the bamboo base has the lowest fiber volume fraction at 19%. Observation of the temporal evolution of the core temperature during the two-stage hot-press process at 180°C with a maximum pressure of 3 MPa for 30 minutes indicates that the core temperature reaches approximately 175°C. Regarding the internal bond strength, the bamboo self-bonding board from the inner side of the bamboo base exhibits the optimal strength of 0.272 MPa, showcasing the highest adhesive quality among the four samples. The internal bond strengths of the other three samples do not exhibit significant differences. Quantitative analysis of the main components reveals that the bamboo particles from the outer part of the bamboo base have the highest lignin content of 27.5%, while both the bamboo tip and middle part possess 26.9% lignin content. The inner side of the bamboo base has the lowest lignin content at 25.5%. Lignin content follows a similar trend as the fiber volume fraction, with an increase in fiber proportion leading to higher lignin content. The two variables are significantly correlated. Post hot-pressing, the cellulose content of all sections decreased due to the degradation of hemicellulose caused by the high temperature. The compression ratio significantly affects the bonding effectiveness of self-bonding boards lacking adhesive curing. Electron microscope images of the surface and core of the self-bonding boards revealed that the needle-like and widely distributed morphology of bamboo self-bonding particles results in larger pores and more voids on the surface, while the core particles are denser. The self-bonding board from the inner side of the bamboo base exhibited evident particle collapse, whereas the other three showed more three-dimensional and intact particle shapes. Under the conditions of 180°C and 3 MPa, the self-bonding effect on bamboo particles from the inner side of the bamboo base was notably favorable. Particles from sections with higher fiber volume fractions require higher temperature and pressure during hot-pressing to achieve sufficient self-bonding effects.