First, the main chemical composition of wood Cellulose is the main component of the wood cell wall and belongs to polysaccharides, accounting for 40%-50% (α-cellulose) of the total amount of poplar wood. It consists of three chemical elements: C (44.2%), H (6.3%), and O (49.5%). Odorless, odorless, insoluble in water, dilute acid, dilute alkali and general solvents. However, it is soluble in 72% sulfuric acid (H2SO4), 41% hydrochloric acid (HCl) and 85% phosphoric acid (H3PO4). Cellulose is microfibrillar in the cell wall of wood, and a long bundle of cellulose molecules, which are arranged in parallel in a bundle, is one of the main characteristics of cellulose. In the microfibrils of cellulose, there are crystalline regions and amorphous regions (amorphous regions), and the percentage of crystalline regions in the whole cellulose is called the crystallinity of cellulose. There are many methods for determining the crystallinity of cellulose, such as X-ray diffraction, density, and hydrolysis. It is difficult to purify the cellulose from the wood and keep it natural. Only the wood powder can be made into non-oriented powder flakes and fixed on the support of the X-ray diffractometer to determine the relative crystallinity of the cellulose (Table 2). -10). Generally, as the crystallinity of cellulose increases, the tensile strength, modulus, hardness, density and dimensional stability of the fiber increase, and the water retention value, elongation, dye sorption, swelling, and softness Sexual and chemical reactivity is reduced. At the same time, the crystallinity of cellulose is related to the age of the tree, the type of fiber, and the dielectric constant of the fiber. Therefore, understanding the relative crystallinity of cellulose has practical significance for the utilization of forest by-products and papermaking and hydrolysis. 2. Hemicellulose 3. Lignin (Lingnin) Lignin is a natural polymer compound composed of phenylpropane structural units. It cannot exist alone in nature, but coexists with cellulose and hemicellulose in the cell wall of wood. It is the crust material of cells. In poplar wood, the lignin content is generally below 30%. The secondary chemical constituents of poplar wood are mainly extracts, and the extracts can be extracted with water, alcohol, benzene, ethanol, ether and other organic solvents, or by dilute alkali extraction or steam distillation. The extraction of the extract does not cause a change in the structure and composition of the wood. Wood extracts are a general term for many types of organic compounds, including common polyphenols, terpenoids, resin acids, lipids and carbohydrates. These substances are not only closely related to the color, aroma, taste and durability of wood, but also have an important influence on processing and utilization. Second, the chemical composition of the main poplar wood in China The chemical composition of wood is closely related to the use of wood. The chemical composition of the main poplar wood in China is shown in Table 2-12. According to the analysis and comparison of gram-bey cellulose, it can be seen from Table 2-12 that the average cellulose content of the measured tree species is 57.20%, and the maximum value is Populus euphratica, 65.22%. The minimum value is the Shalanyang produced in Luoning, Henan. , for 48.66%. Lignin is an important indicator affecting pulp and paper. When the lignin content is high, the chemicals consumed during pulping are large, the production cost is high, and the low lignin content reduces the use of chemicals, reduces costs, and reduces environmental pollution. Heald cellulose is the sum of cellulose and hemicellulose, and the larger the content, the more favorable for papermaking. It can be seen from Table 2-14 that the average cellulose content of the measured tree species is 79.43%, the maximum value is I-69 and I-214 poplar (partial), 85.96%, and the minimum value is 72.92% of Middle Eastern poplar. Third, the acidity and alkalinity of poplar wood Acidity and alkalinity The acidity of wood comes from its chemical composition, some are salts and extracts, and hemicellulose uronic acid. The chemical composition of various parts of wood varies greatly. When the sapwood is converted into a heartwood, the death of the sapphire ray parenchyma cells and the formation of the heartwood also occur simultaneously. The formation of the hardwood invaders, the stored starch is exhausted, and the acidity of the wood often increases toward the center of the tree. Due to the gradual hydrolysis of the acetyl group and the formation of acetic acid. The alkalinity of wood may be formed by certain special inclusions, such as the deposition of polyphenols. The total amount of polyphenols increases from the center of the tree to the junction of the heart and the sapwood, and the polyphenol in the heartwood has a higher molecular weight than the polyphenol in the sapwood portion. 2. Buffer capacity The buffering effect of wood is manifested in its ability to counteract acid and alkali, while the buffer capacity is an indicator of the size of the external acid-base buffer. Understanding the pH and buffer capacity of wood is important for the processing and utilization of wood. In the industrial production of particleboard, plywood, fiberboard, etc., it affects the degree of curing of the binder and the optimum ratio of binder to catalyst. The gel time of urea-formaldehyde resin glue is positively correlated with the pH value of wood, but negatively correlated with the buffer capacity. Some poplar wood heartwoods have a high pH value, and urea-formaldehyde resin glue is used to produce plywood, and the hot pressing curing period is prolonged, and the bonding strength is lowered. Peng Haiyuan and Li Jian studied the "pH of the Northeast economical materials, the buffer capacity and its effect on the gelation time of urea-formaldehyde resin", and obtained the pH value, buffer capacity and urea-formaldehyde resin gelation of several poplar woods such as Daqingyang. The relationship of time. Coffee Table,Modern Coffee Table,Folding Coffee Table,Adjustable Coffee Table DUOBUY TECHNOLOGY SHANDONG CO., LTD. , https://www.dbtfurniture.com
The chemical composition (component) of wood can be divided into main components and secondary components. The main components of wood are cellulose, hemicellulose and lignin, which mainly constitute the cell wall and intercellular layer of wood. Pectin, starch, ash, extracts, etc., are secondary components of wood, mainly found in wood cell cavities and sometimes in wood cell walls.
(1) The main chemical components of wood 1. Cellulose
Hemicellulose is all carbohydrates except cellulose (except for a small amount of pectin and starch), ie non-cellulosic carbohydrates, also known as pentosans, which account for 25%-35% of the total amount of wood, most of which are insoluble. In water, but soluble in lye, it is easy to decompose in case of acid. Unlike cellulose, hemicellulose is a heterogeneous polysaccharide composed of two or more monosaccharide groups, mostly with short side chains. When using chemical pulp to produce ordinary paper in the pulp and paper industry, it is necessary to retain more hemicellulose, which can improve the pulping yield and have a good influence on the pulping performance and paper-forming properties of the pulp.
(2) Secondary chemical constituents of poplar wood
1, cellulose
2, lignin
The average lignin content of the poplar species was 22.76%, the maximum value was Xinjiang Yang, 35.18%, and the minimum value was 19.97%.
3. Hemicellulose (pentanose)
As shown in Table 2-12, the average value of hemicellulose of the tested species was 23.12%, the maximum value was plus poplar, 32.47%, and the minimum value was Middle Eastern poplar, which was 19.33%.
4, heald cellulose
The acidity and alkalinity of wood is expressed by pH. It reflects the concentration of hydrogen ions in wood. In hardwood, it is more acidic, while poplar wood is more alkaline or neutral, and the heartwood is higher than the sapwood. This creates some difficulties for the use of poplar wood (plywood, fiberboard, etc.). The pH value should be monitored at any time during the production of plywood and fiberboard (Table 2-13).