Poplar wood defects and their impact on processing and utilization (10)

First, poplar wood defects

Poplar wood defects mainly include knots, should be pulled wood, wet heartwood and so on.

(A) Setsuko Setsuko is <br> <br> poplar natural growth, it is part of the branches embedded in the xylem. The knot destroys the uniformity of the wood structure, causing the surrounding cells to deform and texture to be skewed, which not only affects the appearance of the wood surface, but more importantly reduces the strength of the wood (especially the maximum reduction of bending strength), which is not conducive to the utilization of wood. There are many kinds of festivals, which can be divided into sound festivals, unsound festivals, decay festivals and live festivals; they can be divided into needle-shaped sections, small sections, medium sections, large sections, etc. according to the size of the knot. The knots of poplar wood can be eliminated by the method of pruning and nurturing, and the no-frills for plywood production are cultivated.

(2) Should pull the wood

In broad-leaved trees, it is usually the case that trees should be pulled when they are eccentric or curved. However, in the case of poplar, there is no eccentricity and bending growth defects. Especially in the case of intensive planting, the clones with fast growth rate contain a large amount of pullwood. The puller should be characterized by the inclusion of colloidal fibers. Some of the poplar glial fibers in the wood tissue ratio can be as high as 45% or more. Such a large and common glial fiber is difficult to come out with an intuitive method. In the laboratory, it is usually distinguished by double staining of red and solid green. Because the gelatinous layer is soft and porous, in the process of slicing and dyeing, the metal salts can be absorbed more quickly, so that the cell wall is deeper than other cell walls.

Compared with the normal fibers of poplar, the glial fibers appear coarse and the wall thickness is even more than 4-5 times. There is a loose bond between the colloidal layer in the cell wall of the glial fiber and the S2 layer in the secondary wall, and only weak fibrils are connected. Under the action of wood stress, the microfibrils connected to the S2 layer are easily broken, and the slip surface and broken crack are observed on the surface of the gel layer.

Compared with normal wood, should be pulled wood due to the presence of colloidal fibers, the density of wood is larger than the normal material (3-1).

Among the chemical components, the lignin content is low and the cellulose content is high due to the severe lack of lignification of the gum layer (Table 3-2).

The presence of a large amount of wood in the poplar wood causes a series of problems in the processing and utilization of wood. When the sawn timber is sawn, the saw blade is heated, the sawn surface is raised, the longitudinal shrinkage is abnormal during drying, and shrinkage occurs. Bow bending; when the veneer is cut, the surface is rough and curved. The use of chemical and semi-chemical pulping in pulp and paper can increase the pulp yield, but reduce the strength of the sheet; the pulping and papermaking by the kraft method, the paper is loose and porous, and all the strength indexes are relatively low. Therefore, minimizing the formation of Yingmu should be an important indicator for the directed cultivation and intensive management of poplar.

(3) Poplar Wet Heartwood (false heartwood, red heartwood)

1. Discoloration symptoms

Poplar is one of the important tree species for short-cycle industrial materials. Because of its fast-growing and high-yield production, it has become one of the main raw materials for China's plywood industry. However, due to the serious phenomenon of “red hearts” or “black hearts” of poplar, it has a great impact on the utilization of wood such as plywood production.

Poplar is originally a sapwood species, but due to external environmental factors and biological factors, its heartwood changes, causing the heartwood to exhibit a red color that is significantly different from the sapwood, and the water content of the heartwood is high, the heartwood is usually high. Known as wet wood. The moisture content of poplar wet heartwood is 40-50% higher than that of normal sapwood.

According to the investigations of Liaoning, Hubei, Hebei, Jiangsu, Shandong, and Beijing, according to the survey of Liaoning, Hubei, Hebei, Jiangsu, Shandong, and Beijing, the poplar tree species with wet heartwood in China are: Populus tomentosa, I-69, I-72, Zhonglin 46, Faku Xiaoyan Yang, Xiaomei Yangyang, Shanhaiguan Yang, Mei X Qingyang and many other poplars. The incidence of poplar heartwood was 100% except for Populus tomentosa, which was 100%.

The proportion of wet heartwood in the plant from the base is gradually reduced, and the wet heartwood does not exist in the top, and the wet heartwood has no obvious correlation with the growth rate of the tree.

Wet heartwood is a discoloration phenomenon. There are two types of this kind of discoloration: one is water immersion, which is rare, and is caused by fungi. It is not the early stage of decay of poplar wood, and it does not affect the strength of wood. The interior of the wood is lightly discolored, and after the wood is dried, the color naturally disappears without adversely affecting the use of the wood. The other type of poplar wood is dark black inside, and after the wood is dry, it remains dark and black. There are many different reasons for the formation of variant defects. Magyar believes that the Hungarian poplar "red heartwood" is caused by fungal infection. The Belgian poplar "black heartwood" is due to bacterial infection. The black heart of Chinese poplars is also more common, which has a negative impact on the use of wood, especially for the plywood and pulp industry.

2. Type and mechanism of color change (1) Type of color change

Poplar wet heartwood is a type of microbial discoloration. The bacteria that cause discoloration from wet heartwood and sap, respectively, are Gram-negative bacillus, Erwinia and Bacillus.

Jiang Xiaomei and other studies on poplar wet heartwood in China showed that the color-changing bacteria are the carrot subspecies (Erwinia carotovora subsp.carotovora), E. rhapontici Millard Burkhold, E. chrysanthemi Burkholder, and waxy Bacillus cereus Frankland, B. sphaericus Neide. Erwinia and Bacillus licheniformis are facultative anaerobic bacteria, and Bacillus globiformis is an aerobic bacterium.

(2) Discoloration mechanism

Foreign studies have suggested that wet heartwood is caused by the activity of bacteria. Carter isolated Erwinia from the eucalyptus wet heartwood and backed it on the healthy tree to increase the moisture content of the trees. Schink studied Populus deltoids Marsh, Ulmus Americanan L, and Abies alba Mill. The formation of wet heartwood in woods such as Picea spp., Quercus spp., and Tsuga spp. is related to aerobic and facultative anaerobic bacterial communities, and the wet heartwood is divided into two types. One is alkaline and weakly alkaline, and the other is acidic.

Jiang Xiaomei et al.'s research on poplar wet heartwood in China shows that I-69 poplar and I-72 poplar heartwood are alkaline. In living trees, due to excessive water, anoxic environment, these anaerobic and facultative anaerobic bacteria A mixed community that produces acid or alkali during the fermentation process, degrading pectin, starch and some hemicelluloses with a single lignin. Some bacteria also produce pectinase to destroy the pit membrane. Due to the metabolic activities of these bacteria, the structural, chemical, and physical and mechanical properties of the wet heartwood have changed, affecting its processing and utilization. When the wet heartwood is formed, the wood cells secrete phenolic compounds to resist the invasion of the bacteria. The discoloration of the wood has already occurred in the stem of the living tree, and becomes deeper after encountering the air after felling. Schink et al. believe that wet heartwood under alkaline conditions develops more rapidly than under acidic conditions.

Wet heartwood is also accompanied by wood discoloration.

Bauch et al. explained the cause of wood discoloration, one caused by the physiological action of trees, one caused by bacterial action and chemical changes. Different trees have different causes of discoloration, but the prerequisites for discoloration are temperature and water content changes and The role of bacteria and microorganisms. Jiang Xiaomei and other studies have shown that the moisture content of poplar wetwood is extremely high, and a large number of bacteria exist. The cause of discoloration of I-69 poplar and I-72 poplar belongs to the second one mentioned above. In summary, studies have shown that the presence and discoloration of wet heartwood is caused by bacterial activity, and three species of Erwinia and two species of Bacillus are identified from wet heartwood and sap.

3. Influence of wet heartwood on processing and utilization The acidity of poplar heartwood is lower than that of sapwood, and it is alkaline. The extract content is higher than normal material and the permeability is poor. These have an impact on poplar drying, plywood production and planing processes. If the wet heartwood is dry, it is easy to crack in the radial and chord directions, and it is also prone to shrinkage, honeycomb cracking, and the like.

Wet heartwood causes serious problems in plywood production and product quality, such as ring cracking, frozen cracking material sliding on the cutting machine, and veneer tearing; the moisture content of the veneer falls below the required value during the gluing of the composite; the moisture content of the wet heartwood Abnormalities lead to poor curing of the glue, penetration, lack of glue, bubbling during hot pressing, shooting and so on. The influence of wet heartwood on the bonding strength is related to the uneven distribution of the moisture content of the veneer. The oxidative discoloration of the panel causes the adhesive to adhere to the wood and affects the bonding strength. Wet heartwood has an adverse effect on the strength properties of particleboard and fiberboard. The strength of poplar wet heartwood waffle plate is lower than that of normal material, and the thickness expansion is much larger. The elastic modulus, fracture strength and internal bond strength of wet fiberboard are all produced. Lower than normal. However, the hard board produced from poplar wet heartwood is better in dimensional stability than the sapwood.

Poplar wet heartwood is not suitable for papermaking because of the high cost of bleaching.
When the poplar wet heartwood is planed, various layer peeling phenomena are likely to occur, which affects the planing process. This is mainly due to early ring cracking or shrinkage caused by wet heartwood.

4. Prevention of poplar wet heartwood

There is no effective prevention method for wet heartwood. It is still to be further studied. It is hoped that the forest genetic breeding and forest cultivation measures will be fundamentally improved, and it is expected to select and cultivate new poplar varieties or stand with a small proportion of wet heartwood.

The use of wet heartwood materials should be used differently for sapwood and wet heartwood.

Second, the problems in the processing and utilization of poplar wood 1. The processing and utilization of poplar and the cultivation of poplars are interdependent and mutually reinforcing between poplar wood processing and poplar cultivation in Europe and the United States, and play a decisive role in the development of poplar. In Europe and the United States, poplar is an indispensable raw material for wood processing plants and paper mills. Almost all poplars are processed in factories, and logs are rarely used directly. The Po River Basin in Italy and the Garonne River Basin in France are European poplar concentrated cultivation areas, where each county or city has a wood processing plant that uses poplar as a raw material, and a paper mill is built on a large scale. . China's current situation is still far behind Italy, France and the United States.

2. Poplar cultivars are numerous, and the line is updated too fast, which makes it difficult to process and utilize wood.

Since the 1950s, Chinese forestry breeders have been breeding and introducing a large number of new varieties and clones for decades of developing poplar trees in China. These new varieties and clones have undoubtedly played a significant role in improving the productivity of poplar plantations. However, with the economic reform and the opening up of the outside world in the past 10 years, the selection and introduction of the improved varieties of poplars in China has entered a new era. Everyone has introduced them, everyone has been selected, and everyone has been identified. New varieties and clones have appeared in large numbers, replacing the past strains and becoming new members of the poplar family. There are many varieties, small cultivation areas, and the wood properties and quality have not been thoroughly studied, which have brought certain difficulties to wood processing. In contrast, the clones used in Italy are relatively stable. They have established a strict legal system for the review and identification of new poplar varieties. They have only participated in various tests and compared them with I-214 Yang as a reference system, and then It is worth mentioning that their new varieties must undergo a plywood rotary cutting test to determine whether they can be used for plywood production. My country still needs a lot of work to do in this regard.

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