Bamboo drying is an indispensable part of the industrial utilization of bamboo. Due to the anisotropic nature of the bamboo itself and its inherent internode structure, such as dryness, it is bound to cause various defects such as cracking. Therefore, in order to make rational and efficient utilization of bamboo resources, it is necessary and important to actively carry out basic theoretical research and applied research of bamboo.
1 Bamboo drying characteristics and hygroscopic expansion characteristics
The drying characteristics of bamboo mainly refer to the moisture movement characteristics, dry shrinkage deformation, drying defects and their formation reasons during the drying process of bamboo, the influence of bamboo structural characteristics on drying, and the influence of bamboo inclusions on drying. Walter Liese [3~6] studied the biological and utilization properties (chemistry, physics and mechanics) of bamboo. The results show that bamboo drying is different from wood drying. When bamboo begins to shrink, it shrinks. When the moisture content reaches 40%, it stops shrinking, 40%. It may continue to shrink below, but mainly in the radial direction. Wang Jianhe [7] determined the variation of the expansion and contraction of bamboo plywood (the bamboo): the radial direction is larger than the chord direction than the longitudinal direction; the bamboo joint is larger than the no bamboo joint; the moisture content and density of the bamboo are related, and the bamboo The age and site conditions are quite different; the bamboo chips should be dried by hot air circulation to keep them flat. Since the temperature and humidity vary with the natural environment, it is generally difficult to keep the moisture content of the bamboo stalk stable.
KT wu [8] studied the influencing factors of the crack resistance of Mengzong bamboo bamboo under high temperature drying. The results show that the temperature has a significant effect on the crack resistance of the bamboo. Xu Bin [] used end-pressure injection method to prevent bamboo from being cracked and insect-repellent by injection of water-soluble inorganic salts, polymer compounds and insect repellents under high water content. : This kind of treatment has certain effect on the crack prevention of the bamboo.
Sun Zhaobin et al [10] studied the drying characteristics of Dendrocalamus giganteus, a typical bamboo plant in Yunnan, and compared the drying speed, drying shrinkage and deformation of bamboo at 100 °C and 60 °C. The drying speed of bamboo in three directions of longitudinal, radial and chord directions. The results show that: high temperature drying, low temperature and fast drying speed, but the bamboo shrinkage rate is larger and the deformation is larger; the radial shrinkage rate of the bamboo without joints is greater than the chord direction shrinkage ratio; the radial shrinkage rate of the bamboo section is less than the string To the dry shrinkage rate; in the same direction, the dry shrinkage rate of the knot portion is smaller than the dry shrinkage ratio of the joint portion; the longitudinal drying speed of the untested specimen in the unit time is faster than the chord direction and the radial direction, and the section test piece is in the unit time. The inner radial drying speed is faster than the longitudinal chord direction.
The hygroscopic swelling property of bamboo mainly refers to the characteristic that bamboo absorbs moisture from the surrounding humid air and expands. The hygroscopicity of bamboo can lead to unstable bamboo size, and may also be accompanied by deformation, which affects the utilization of bamboo. Zhou Fangchun [11] elaborated on the water swelling characteristics of 36 kinds of bamboo in his discussion. The water absorption speed of bamboo is proportional to the weight of the public volume and inversely proportional to the time of water immersion. When bamboo absorbs water, its length, width, thickness and volume will expand, and its expansion rate is closely related to the amount of water absorbed. The expansion rate of bamboo immersed in water after drying is lower than that of air-dried bamboo, and the expansion speed is also slow. There are few reports on the hygroscopic swelling characteristics of bamboo.
2 Bamboo drying method and drying T art
Bamboo drying is usually done by natural drying and kiln drying.
Wang Lianxi [12] has been tested as saying that the drying temperature of bamboo yellow is preferably within 105 °C. Du Fuyuan et al [13] reported the drying process of bamboo rafts in the production of bamboo mats, and compared several special drying chambers. The conclusion is that bamboo rafts can achieve high temperature (80 ° C) rapid drying and continuous heating The section is to increase the drying standard of the hot air volume. When the final moisture content reaches 8.0%, the drying period is 14 h.
Zhang Qisheng [14] expounded the drying process of bamboo chips in the discussion of the production process of bamboo laminated timber and bamboo flooring: after the bamboo chips are cooked or charred, the water content is generally close to 35% to 50%. Since the bamboo fibers are arranged neatly and have a small thickness, distortion and cracking like wood are not generated in the convection drying process. Therefore, the bamboo drying process is much simpler than wood drying, neither steaming nor humidification nor complex temperature curves are required. Generally, the temperature is continuously dried at a temperature of about 60 to 70 ° C for 72 to 84 hours, and the water content can be within 10%. However, it is not advisable to use temperatures above 70 °C, otherwise the bamboo will be warped due to excessive drying speed. The shaping drying adopts the hot pressing method, the bamboo piece is heated and dehumidified under appropriate pressure conditions, and the pressure is released intermittently, so that the bamboo pieces are wetted and freely contracted, so as to accelerate the evaporation of the moisture of the bamboo piece and prevent the drying due to drying. Lateral cracking due to stress.
Yi Songlin et al [15] made a preliminary study on the drying process of bamboo flooring. The conclusion is: under the premise of ensuring the dry quality of bamboo flakes, the critical temperature of the bleached flakes (white flakes) does not appear uneven light brown spots. °C, the critical temperature of the carbonized sheet does not appear severe shrinkage is 55 ° C, and the bamboo drying time reference is proposed.
Zhang Qinli [16] proposed that the bamboo chips should be cooked after coarse shaving to remove part of the extract. The cooking time is 3~4 h. The drying temperature of the bamboo slices should not exceed 75 °C, and the temperature cannot be raised too fast. The bamboo flooring after hot pressing is prone to warp deformation during the cooling process, and needs to be placed in a cold press to be cooled and shaped under restraint to ensure the floor is flat.
Taiwan's Wu Xuedan [17] studied the artificial drying of Taiwan's economic bamboo species, and concluded that the thickness of the bamboo wall is different, and the drying speed is different, which should be grouped and dried according to the thickness of the bamboo wall. Opening the bamboo bamboo section can speed up the drying process and reduce surface cracking.
Shan Zhiqing et al. [18] studied the drying of bamboo. Drying tests were carried out on bamboo bamboo and bamboo slices using two drying standards. One initial dry bulb temperature was 45 ° C, and the final dry bulb temperature was 70 ° C. The other initial dry bulb temperature was 60 ° C. The final dry bulb temperature was 70 ° C. The results showed that the bamboo chips with bamboo green and bamboo yellow had faster drying speed than the bamboo chips without bamboo green and bamboo yellow; the drying speed of the bamboo leaves was improved after removing the bamboo joints; when the second standard was dry, the bamboo leaves were separated and The water content is inconsistent when the joint is not split, and the crack is cracked at a higher water content (29.4% to 47.5%), and the crack is cracked at a low water content (6.9% to 23.3%) without the partition.
Sharama [19] tested the air-drying and kiln drying of nine bamboo species in India. As far as the whole bamboo is dry, the surface of the bamboo stalk is more prone to cracking, internal cracking and deformation than the dry air. Dry the whole bamboo. Yellow bamboo is very easy to dry, sometimes there are small cracks on the surface at the beginning of drying, but then healed; there is no big change in the drying of Indian thorn bamboo, the drying of mature bamboo stalk is slow; B.hamlitonii bamboo is fast drying, the effect is good; Long, mature bamboo drying results are satisfactory. When the bamboo is dry and dry, the bamboo part is prone to deformation and shrinkage (immature material), and the surface of the thick mature material is easy to crack, end crack and shrink; the round bamboo gas needs about 1.5 to 3.5 months. The length of time depends on the thickness of the bamboo wall. The research results of the dry process of the round bamboo kiln show that even under mild dry conditions (35 °C ~ 40 °C), the bamboo can not be used in the kiln drying process, mainly the cracking, shrinkage and deformation of the round bamboo when the kiln is dry. It is more serious.
Taiwan KT Wu [8] studied the effect of high temperature drying on the crack resistance of bamboo. The bamboo samples were dried at a high temperature. When the temperature was raised from 60 ° C to 120 ° C, the bamboo material (Guizhu) did not crack. The reason is that high temperature drying reduces the strength of the cells inside the bamboo, especially the soft tissue cells, causing the bamboo to shrink and the stress to be released, so that surface cracking does not occur. Taiwan Lin Hongji [20] studied the effect of bamboo drying temperature on the mechanical properties of bamboo. The bamboo samples were dried by four single drying standards of 60 ° C, 80 ° C, 100 ° C and 120 ° C. The results showed that the strength of the bamboo increased with the increase of the temperature of the drying kiln, and the upper section > the middle section > the lower section, Mazhu is the strongest at 80 °C.
Weng Wenzeng [21] has experimentally concluded that bamboo drying (referring to bamboo veneers, bamboo rafts, and bamboo flooring for bamboo flooring) should be slowly dried at medium and low temperatures, and the temperature should not exceed 70 °C, otherwise it will lead to hardness. High and difficult to process; relative humidity can be lower; no intermediate treatment is required, and the final treatment should be carried out. The drying time of bamboo chips is better at 2.5 to 3 days, and the drying of bamboo bundles takes 3 to 4 days. The difference between bamboo drying and wood drying is mainly the loading.
3 drying equipment
Sheng Binghua and Zhang Ke'an [22] designed a bamboo drying and setting machine to solve the problem of drying and shaping of bamboo in the production process of plywood. The function of the drying setting machine is to make the bamboo sheet meet the specified moisture content and surface flatness standards, and provide qualified bamboo sheets for the bamboo plywood which is further subjected to the assembly and hot pressing. In addition to continuous production to meet production efficiency, the machine can also adapt to the needs of bamboo chips of different lengths and thicknesses, so that bamboo chips of different specifications can meet the drying and setting standards. It has been used in more than 10 bamboo plywood enterprises for many years. It solves the problem of drying and shaping of bamboo in the production process of plywood. According to the effect of use, its shaping ability is superior to drying ability.
Zhang Qisheng [14] also explained the bamboo drying and setting method and shaping equipment in the book "Industrial Utilization of Bamboo in China". For the bamboo piece below 4 mm, the pre-drying and shaping drying of the bamboo piece can be completed once on the machine, but the thick bamboo piece still needs two steps of pre-drying and shaping drying.
At present, in actual production, the bamboo drying equipment used by most enterprises is the same as the wood drying kiln, but the capacity is relatively small, and a small drying kiln of 20-40 m3 is generally used [21].
4 Bamboo drying related parameters
When drying bamboo and drying bamboo heat and mass transfer model, it involves some physical properties of bamboo, such as bamboo specific heat, thermal conductivity, temperature coefficient, fiber saturation point of bamboo, equilibrium moisture content, vitrification. Change temperature, etc. Some domestic scholars have measured and studied these parameters. Zhou Fangchun [] summarized the specific heat, thermal conductivity and temperature coefficient of bamboo in his theory: the specific heat of dried bamboo is basically not affected by bamboo species and bulk density, with an average of 0.327 kCal/(kg.°C). The average dry bamboo material is 0.41 kcaL/(kg.°C), and the specific heat of bamboo increases with the increase of water content. Bamboo is made up of bamboo material, moisture and air. The thermal conductivity of bamboo material is 0.25-0.34 kCal/(m•h•clC); the thermal conductivity of moisture is 0.5 kcal/(m•h•°C), and the thermal conductivity of air is 0.02l kcal/(m•h•°C) ). Take bamboo air dry density 639 kg/m3, bamboo specific heat 0.4l kCal/, (kg•°C), thermal conductivity 0.154 4 kcal/(m•h•°C), then the bamboo temperature coefficient is 5.89×104m2/h [ Generally, the wood temperature coefficient is about 5×104 m2/h (Xu Yongji, 1994)].
Xia Jingguo [] determined the thermal conductivity of bamboo broken panels to be 0.180 2 w/(m•°C). Wu Shuxue et al [24] used the flash method, the comparison method and the straight line method to measure several thermodynamic characteristic parameters of bamboo and bamboo along the direction of vertical bamboo fiber: thermal diffusivity A, specific heat cp and bamboo density Q, and according to formula K =418.68ACpQ gives the thermal conductivity K. The results show that the specific heat Cp and thermal conductivity K of bamboo are all at 70 c (= near the maximum value, that is, near the temperature of 70 ° C, bamboo has the best heat transfer. This result has guidance for the heat treatment of bamboo Significance, such as drying at around 70 ° C, because of its high thermal conductivity, heat easily penetrates into the interior of the material, so that the inside and outside are evenly dried, can better ensure the quality of drying, while the use of heat is sufficient, thus saving energy Consumption.
In 1998, Taiwan used a non-invasive method to measure the saturation point of bamboo fiber. The fiber saturation points of the three sections of the upper, middle and lower parts of the bamboo were different. The total average values ​​were 15%-28%, 15%-25%, 13 respectively. %~23%. Zhou Fangchun [11] "Bamboo Cultivation and Utilization" mentioned that the fiber saturation point of bamboo is 30% to 35%, Hui Chaomao and Yang Yuming [25] in the book "Industrial Utilization of Bamboo Resources" The formula for calculating the fiber saturation point is calculated from the measured absolute dry shrinkage ratio and the nominal bulk density. According to the fiber saturation point=volume dry shrinkage ratio/common volume weight, 7 species of Yunnan (Dragon Bamboo, Wild Dragon Bamboo) are calculated. The fiber saturation points of bamboo, sweet dragon bamboo, oil bamboo, yellow bamboo, dragon and tin gold are 14%~26.81%. The difference between fiber saturation points between bamboo and segment is larger, and the lower segment is higher than middle and upper. The fiber has the highest saturation point (26.81%) and the lowest (15.23%) of the yellow bamboo. The total average of the seven bamboo fiber saturation points is 22.33%.
Guan Mingjie [26] used the dry shrinkage method, flexural modulus of elasticity and thermogravimetric analysis to determine the fiber saturation points of three species of bamboo (Dragon Bamboo, Yellow Bamboo, Sweet Bamboo) and Bamboo. The dry shrinkage method and the flexural modulus test method are used to determine the fiber saturation point. The water content of the bamboo material conversion point is used as the definition of the fiber saturation point. The fiber saturation point measured by this definition varies with the bamboo part, bamboo age, and bamboo green. There is a difference between bamboo and yellow. The variation of the shrinkage method ranged from 13.94% to 50.34%, and the variation of the flexural modulus of elasticity ranged from 29.85% to 42.55%. The fiber saturation point of 32.32% was measured by thermogravimetry at 20 °C. Neither of the first two methods clearly defines the state of complete evaporation of free water in the cell cavity. Through thermogravimetric analysis, the moisture content of the bamboo, that is, the fiber saturation point, when the free water is completely evaporated can be accurately determined. It can be seen that the value of the fiber saturation point varies greatly with different measurement methods.
Jiang Zhihong [27,28] tested the equilibrium moisture content of bamboo and the equilibrium moisture content of bamboo wood-based panels. The results showed that the equilibrium moisture content of undried bamboo was dried at 100 °C ± 3 °C to dry bamboo. The equilibrium moisture content differs by about 3%.
Jiang Jingyan [29] used the dynamic method and the differential scanning calorimetry method to determine the glass transition temperature of the bamboo lignin from the dry bamboo. The values ​​determined by the two methods have a certain gap. The heat softening temperature of bamboo green is 208.3~2l1.3 °C, the bamboo meat is 198.7 °C, and the bamboo yellow is 217.8 °C. Bamboo thermoplasticity increases with increasing temperature. The thermal softening temperature of bamboo with a certain water content needs to be further determined, which is of great significance for determining the softening and bending process parameters of bamboo.
5 Conclusion
At home and abroad, research on bamboo drying has been reported at present, especially the research on the moisture movement characteristics of bamboo drying process, the stress strain during bamboo drying, the permeability of bamboo, the drying mechanism of bamboo and the mechanism of plastic deformation. Blank, in-depth study is needed to improve the bamboo drying technology level and bamboo utilization rate, produce more high value-added products, and increase the economic and social benefits of enterprises. The research on the drying characteristics and drying mechanism of bamboo has positively promoted and promoted the development of bamboo application research. It can be used for the research of the relationship between various properties and processing and utilization of bamboo materials, such as bamboo retorting technology, protective treatment, chemical treatment, dyeing and gluing. The processing technology provides theoretical basis and provides theoretical guidance for actual production.
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