Textile Dyeing and Finishing Technology

Author: Chief Editor: Fan Xuerong
Publisher:
Publish Date: 2005-02-01
Features: This book is written based on the characteristics of the textile engineering major included in the newly published engineering professional directory by the Ministry of Education, as well as the recent development of dyeing and finishing technology. It covers the basic structure and properties of common fibers in the textile industry, the fundamental principles, basic processes, and commonly used dyeing and finishing equipment for various textile dyeing and finishing processes. It also provides a concise introduction to the basic knowledge of surfactants and the rapidly developing functional finishing of textiles in recent years. This book can be used as a teaching material for dyeing and finishing technology or an introduction to dyeing and finishing in textile engineering programs, and can also be referenced by textile industry engineers and management personnel.
Excerpt: When exposed to acid, the hand feel becomes hard, and the strength is severely reduced. This is because the acid catalyzes the hydrolysis of glycosidic bonds in the cellulose macromolecule, reducing its degree of polymerization and damaging the fibers. The factors affecting the hydrolysis of cellulose fibers are primarily the nature of the acid, the temperature of the hydrolysis reaction, and the duration of the reaction. In actual production, if the acid process is properly applied, it will not cause severe damage to the fabric. Generally, the stronger the acid, the faster the hydrolysis rate. Strong inorganic acids such as hydrochloric acid, sulfuric acid, and nitric acid have a particularly strong hydrolyzing effect on cellulose fibers, while weak acids like phosphoric acid and boric acid have weaker catalytic activity, and organic acids are even more moderate. The higher the acid concentration, the faster the hydrolysis rate. Temperature has a significant impact on the hydrolysis of cellulose; the higher the temperature, the faster the hydrolysis rate. When the acid concentration is constant, the hydrolysis rate of cellulose increases by 2 to 3 times for every 10°C increase in temperature. Under the same conditions, the degree of cellulose hydrolysis is proportional to the reaction time—the longer the reaction time, the more severe the hydrolysis. Additionally, the rate of cellulose hydrolysis also depends on the type of cellulose, such as flax, cotton, bleached cotton, and viscose fiber, which have progressively increasing hydrolysis rates. This is mainly due to the increasing proportion of amorphous regions in their fiber structures. In actual production, cotton fabrics are generally treated with very dilute acids, and the temperature does not exceed 50°C. After treatment, thorough rinsing is necessary, especially to avoid drying under acidic conditions. Although acid has harmful effects on cellulose fibers, it can also be utilized if controlled properly. For example, after bleaching with chlorine-containing bleaching agents, treating with dilute acid can further enhance the bleaching effect. Acid is also used to neutralize excess alkali on the fabric, and acid treatment is applied to cotton fabrics to produce (butterfly yarn) and products in polyester-cotton fabrics.
3. The Interaction Between Cellulose and Oxidizing Agents
Cellulose is generally unaffected by reducing agents but is susceptible to oxidation by oxidizing agents, forming oxidized cellulose, which causes fiber degradation and damage. Cellulose is stable in air but is prone to oxidative degradation in the presence of alkali, so it should be avoided from contact with air during high-temperature alkali boiling. When using oxidizing agents such as sodium hypochlorite, sodium chlorite, or hydrogen peroxide for bleaching, strict control of process conditions is necessary to ensure the strength of the fabric or yarn. The oxidation of cellulose primarily occurs on the three hydroxyl groups of the glucose residue and the potential aldehyde group at the end of the macromolecule. The primary hydroxyl group is oxidized to an aldehyde group, which is further oxidized to a carboxyl group; the secondary hydroxyl group is oxidized to a ketone group, which is further oxidized to an open-chain aldehyde and carboxyl group; and the potential aldehyde group at the end of the macromolecule is oxidized to a carboxyl group. Under different conditions, oxidized cellulose can be obtained, containing a high proportion of aldehyde or carbonyl groups (reduced-type oxidized cellulose) or a high proportion of carboxyl groups (acidic-type oxidized cellulose). Although reduced-type oxidized cellulose does not involve the breakage of the cellulose macromolecular chain, it still carries potential damage. Under alkaline conditions, the cellulose macromolecular chain breaks, leading to a decrease in degree of polymerization and fiber strength.
(V) Cellulose Symbionts
During the growth of cotton fibers, the content of cellulose increases with the maturity of the cotton. Additionally, there is a certain amount of protective substances that play a role during cotton fiber growth, as well as impurities generated during biological metabolism, which grow alongside cellulose. These substances are referred to as cellulose symbionts. The main cellulose symbionts include pectic substances, nitrogen-containing substances, waxy substances, natural pigments, and cottonseed shells, which are impurities introduced during the separation of fibers. The proportion of symbionts decreases as cotton fibers mature. Symbionts affect the water absorption, dyeing, and whiteness of cotton fabrics during dyeing and finishing processes, so they need to be removed in the pretreatment stage to meet the requirements of dyeing and finishing and end use.
1. Pectic Substances
Pectin is present in both cotton and flax fibers, with higher content in flax. In cotton fibers, pectic substances are mainly found in the primary cell wall, with small amounts in the secondary cell wall. The main components of pectin are polygalacturonic acid, calcium pectate, magnesium pectate, methyl pectate, and polysaccharides. The hydrophilicity of calcium pectate, magnesium pectate, and methyl pectate is lower than that of cellulose, making them difficult to remove with hot water. However, if treated with an appropriate concentration of caustic soda at a certain temperature, the esters can be hydrolyzed into carboxyl groups and converted into sodium salts, significantly increasing the solubility of pectin in water and facilitating its removal.
2. Nitrogen-Containing Substances
Nitrogen-containing substances in cotton fibers are mainly present as proteins and simple nitrogen-containing inorganic salts (such as nitrates and nitrites) in the lumen of the fibers, with some also present in the primary and secondary cell walls. Nitrogen-containing inorganic salts can dissolve in 60°C warm water or weak acid/weak alkali solutions at room temperature. Proteins cannot be completely removed even after prolonged boiling in sodium hydroxide solution. Sodium hypochlorite can break the amide bonds in protein macromolecules, forming a series of water-soluble chloro-amino acid sodium salts that can be removed.
3. Waxy Substances
Waxy substances in cotton fibers are water-insoluble but extractable with organic solvents, primarily found in the primary cell wall. The waxy substances in cotton fibers are a mixture containing fat-soluble high-chain aliphatic monohydroxyl alcohols, free fatty acids, sodium salts of fatty acids, esters of high-chain monohydroxyl alcohols, and solid and liquid hydrocarbons. In the dyeing and finishing of cotton fabrics, the removal of waxy substances is achieved through saponification and emulsification. Fatty acid substances undergo saponification with alkali during boiling and are removed. High-chain alcohols and hydrocarbons can be removed using saponification products or by adding emulsifiers through emulsification.
4. Ash
The ash in cotton fibers consists of potassium, sodium, calcium, magnesium, and manganese salts of silicic acid, carbonic acid, hydrochloric acid, sulfuric acid, and phosphoric acid, as well as iron oxide and aluminum oxide. Among these, potassium and sodium salts account for 95% of the total ash. The ash in cotton fibers can dissolve in acid and can be reduced in content through acid washing during bleaching.
5. Natural Pigments
Natural pigments in cotton fibers are colored substances, including creamy, brown, and grayish-green colors. Currently, the structural research on natural pigments is not yet sufficient. Some pigments can dissolve in boiling water. During bleaching, bleaching agents can break down these pigments, removing them.
6. Cottonseed Shells
Cottonseed shells are not cellulose symbionts but rather the seed coats attached to cotton fibers, which are impurities introduced during fiber separation. They have a dark color and a hard texture, which is unfavorable for the surface smoothness of fabrics and dyeing and finishing processes. Their composition is also complex, mainly consisting of lignin, cellulose, tannins, polysaccharides, as well as small amounts of proteins, oils, and minerals. During boiling of cotton fabrics, under the action of caustic soda, high temperature, and prolonged treatment, the ether bonds in lignin are broken, the lignin macromolecules degrade, making the cottonseed shells loose and essentially disintegrate. They are then removed through thorough squeezing and washing. During bleaching, lignin also undergoes chlorination and dissolves in alkali.
II. The Structure and Main Chemical Properties of Viscose Fiber
Viscose fiber is a regenerated cellulose fiber made from materials containing cellulose, such as wood, cotton linters, and bagasse, through dissolution and regeneration. It belongs to the category of chemical fibers among regenerated cellulose fibers.