4) I saperi tradizionali della gondola veneziana
1.3 L’approccio contemporaneo nello studio delle comunità: Habermas, Bauman e Giddens
Objective:
After reading this unit, you should able to Know what dyeing process is.Know how dye can be clarified.
Know the dyeing process keywords.
READING 1
Dyes may be classified according to chemical structure or by their usage or application method. The former approach is adopted by practicing dye chemists, who use terms such as azo dyes, anthraquinone dyes, and phthalocyanine dyes. The latter approach is used predominantly by the dye user, the dye technologist, who speaks of reactive dyes for cotton and disperse dyes for polyester. Very often, both terminologies are used, for example, an azo disperse dye for polyester and a phthalocyanine reactive dye for cotton.
Chemical Classification. The most appropriate system for the classification of dyes is by chemical structure, which has many advantages. First, it readily identifies dyes as belonging to a group that has characteristic properties, for example, azo dyes (strong, good all-round properties, cost-effective) and anthraquinone dyes (weak, expensive). Second, there are a manageable number of chemical groups (about a dozen). Most importantly, it is the classification used most widely by both the synthetic dye chemist and the dye technologist. Thus, both chemists and technologists can readily identify with phrases such as an azo yellow, an anthraquinone red, and a phthalocyanine blue. The classification used here maintains the backbone of the Color Index classification, but attempts to simplify and update it. This is done by showing the structural interrelationships of dyes that are assigned to separate classes by the Color Index, and the classification is chosen to highlight some of the more recent discoveries in dye chemistry.
Usage Classification. It is advantageous to consider the classification of dyes by use or method of application before considering chemical structures in detail because of the dye nomenclature and jargon that arises from this system. Classification by usage or application is the principal system adopted by the
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Colour Index. Because the most important textile fibers are cotton and polyester, the most important dye types are those used for dyeing these two fibers, including polyester-cotton blends. Other textile fibers include nylon, polyacrylonitrile, and cellulose acetate.
TASK 1: TRUE OR FALSE
Decide whether the following statements are true (T) or false (F).
1. Dye chemists classify dye according to their chemical structures.
2. The classification of dyes by method application is the most appropriate system.
3. Azo dyes are cheap while anthraquinone dyes are expensive.
4. The chemical classification is the main system adopted by the Color Index.
5. The term ―a cyanine basic dye for nylon‖ is used by both dye chemists and dye technologists.
TASK 2: COMPREHENSION QUESTIONS
Answer the questions below.1.
How can dye be classified?...
2. What textile fibers are the most crucial?
...
3. Why do dye technologists prefer the classification of dye by use?
...
READING 2
1.
Reactive Dyes. These dyes form a covalent bond with the fiber, usually cotton, although they are used to a small extent on wool and nylon. This class of dyes, first introduced commercially in 1956 by ICI, made it possible to achieve extremely high wash fastness properties by relatively simple dyeing methods.A marked advantage of reactive dyes over direct dyes is that their chemical structures are much simpler, their absorption spectra show narrower absorption bands, and the dyeings are brighter. The principal chemical classes of reactive dyes are azo (including metallized azo), triphendioxazine, phthalocyanine, formazan, and anthraquinone. High-purity reactive dyes are used in the ink-jet printing of textiles, especially cotton.
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2. Disperse Dyes. These are substantially water-insoluble nonionic dyes for application to hydrophobic fibers from aqueous dispersion. They are used predominantly on polyester and to a lesser extent on nylon, cellulose, cellulose acetate, and acrylic fibers. Thermal transfer printing and dye diffusion thermal transfer processes for electronic photography represent niche markets for selected members of this class.
3. Direct Dyes. These water-soluble anionic dyes, when dyed from aqueous solution in the presence of electrolytes, are substantive to, i.e., have high affinity for, cellulosic fibers. Their principal use is the dyeing of cotton and regenerated cellulose, paper, leather, and, to a lesser extent, nylon. Most of the dyes in this class are polyazo compounds, along with some stilbenes, phthalocyanines, and oxazines. After treatments, frequently applied to the dyed material to improve wash fastness properties, include chelation with salts of metals (usually copper or chromium), and treatment with formaldehyde or a cationic dye-complexing resin.
4. Vat Dyes. These water-insoluble dyes are applied mainly to cellulosic fibers as soluble leuco salts after reduction in an alkaline bath, usually with sodium hydrogensulfite. Following exhaustion onto the fiber, the leuco forms are reoxidized to the insoluble keto forms and after treated, usually by soaping, to redevelop the crystal structure. The principal chemical classes of vat dyes are anthraquinone and indigoid.
5. Sulfur Dyes. These dyes are applied to cotton from an alkaline reducing bath with sodium sulfide as the reducing agent. Numerically this is a relatively small group of dyes. The low cost and good wash fastness properties of the dyeing make this class important from an economic standpoint. However, they are under pressure from an environmental viewpoint.
7. Cationic (Basic) Dyes. These water-soluble cationic dyes are applied to paper, polyacrylonitrile (e.g.
Dralon), modified nylons, and modified polyesters. Their original use was for silk, wool, and tannin-mordanted cotton when brightness of shade was more important than fastness to light and washing. Basic dyes are water-soluble and yield colored cations in solution. For this reason they are frequently referred to as cationic dyes. The principal chemical classes are diazahemicyanine, triarylmethane, cyanine, hemicyanine, thiazine, oxazine, and acridine. Some basic dyes show biological activity and are used in medicine as antiseptics.
8. Acid Dyes. These water-soluble anionic dyes are applied to nylon, wool, silk, and modified acrylics.
They are also used to some extent for paper, leather, ink-jet printing, food, and cosmetics.
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9. Solvent Dyes. These water-insoluble but solvent-soluble dyes are devoid of polar solubilizing groups such as sulfonic acid, carboxylic acid, or quaternary ammonium. They are used for coloring plastics, gasoline, oils, and waxes. The dyes are predominantly azo and anthraquinone, but phthalocyanine and triarylmethane dyes are also used.
TASK 3: GAP FILLING
Complete the table below with appropriate words or phrases
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READING 3The goal of every dyeing is a colored textile in the desired shade, homogeneous in hue and depth of shade, produced by an economic process and which exhibits satisfactory fastness properties in the finished state.
Colored textiles are produced today on a large industrial scale. Although modern automation techniques have been introduced for color measurement, metering of dyes and auxiliaries, and automatic control of the dyeing process, much human intervention is still required. Fibers can only be standardized to a limited extent, due to biological and environmental factors, e.g., in growing cotton or raising sheep. And new developments in fashion and application of textiles require constant modifications of finishing procedures.
To remain flexible with regard to fashion and fastness properties, dyeing is carried out at the end of the production process whenever possible.
The textile material generally needs a pretreatment before dyeing. Wool must be washed to remove wax and dirt and is sometimes bleached, cotton must be boiled and bleached to remove pectins and cotton seeds and is mercerized. Sizes and spinning oils must be eliminated. Basically there are three methods of dyeing textiles:
1) Mass dyeing: dyeing of a synthetic polymer before fiber formation.
2) Pigment dyeing: affixing an insoluble colorant on the fiber surface with a binder.
3) Exhaustion dyeing from an aqueous bath with dyes that have an affinity for the fiber.
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Mass and pigment dyeing are not considered here. In exhaustion dyeing, the dye, which is at least partially soluble in the dye bath, is transported to the fiber surface by motion of the dye liquor or the textile. It is then adsorbed on the fiber surface and diffuses into the fiber. Finally, depending on the dye-fiber interaction, it is fixed chemically or physically. The dye can be applied to the textile discontinuously from a dilute solution (exhaustion dyeing from along liquor) or continuously by immersing the textile in a concentrated bath and squeezing-off excess liquor (padding), followed by separate steps for diffusion and fixation in the fiber. Water is almost exclusively the liquid phase which transports the dye in textile dyeing. For most dyeing processes demineralized water should be used; if not available, complexing agents must be added, where necessary, to avoid negative interference of metal ions.
For exhaustion dyeing, recipes are made up in percent relative to the weight of the textile goods. For continuous processes the amounts of dyes and auxiliaries used are given as g/L of the dye liquor and have to be based on a predetermined liquor add-on per unit weight of the textile material. The speed of exhaustion (dyeing rate) of individual dyes can vary widely, depending on their chemical and physical properties and the type of textile material. The dyeing rate depends on factors such as temperature, liquor ratio, dye concentration, and the chemicals and auxiliary products in the dye bath.
High dyeing rates bear the danger of unlevel dyeing. Therefore, dyes have to be carefully selected when used together in one recipe. The dye producers readily communicate their knowledge on dyeing characteristics of their dyes and on the particular requirements for all textile fibers.
The end of the dyeing process is characterized by the equilibrium phase, when dye concentration in the fiber and in the liquor do not change any more. Under standard conditions, the distribution coefficient of the dye between liquor and fiber is constant; in other words, the rates of adsorption and desorption are equal. The equilibrium phase is frequently necessary and may be extended to level out by migration inhomogeneities incurred during the exhaustion phase. When the dyeing is carried out continuously, it is of prime importance to achieve perfectly homogeneous dye application and to avoid migration during subsequent steps, e.g., when drying. Leveling out a dyeing after fixation of the dye is tedious and time-consuming. Stripping a faulty dyeing is difficult in most cases and should be avoided.
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TASK 4: VOCABULARY CHECKFind the words in the text which mean:
1. resistance in the dye of a textile fabric which protects against various effects occurring in production and during practical use.
2. the ability of a textile fibre to receive a given dye
3. the spread of particles through random motion from regions of higher concentration to regions of lower concentration
4. situations in which two phases can coexist in thermodynamic equilibrium with each other
5. the ratio of the concentration of the solute in one phase to the concentration of the solute in the other phase under equilibrium conditions
TASK 5: ORDER
Put the steps below in the correct order in the exhaustion dyeing process of cotton
1. bleaching 2. leveling out 3. fixation 4. diffusion 5. mercerization 6. boiling 7. transportation 8. adsorption 9. drying
TASK 6: COMPREHENSION QUESTIONS
Answer the questions below.1.
What is the purpose of the pretreatment before dyeing?...
2. What is the objective of dyeing?
...
3. What are the three methods of dyeing textiles?
...
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4. How is the dye transported to the fiber surface?...
5. Is the dye fixed to the fiber chemically or physically?
...
6. What kind of water is used for dyeing processes?
...
7. Why must complexing agents be used?
...
8. What does the dyeing rate depend on?
...
9.How is the end of the dyeing process characterized?
...
TASK 7: How to find good keywords
Find the keywords in reading paragraphs 1 and 3 TASK 8: Summary
In about 5 sentences, summarize the main idea in paragraphs 1 and 3
TASK 9: GlossarySearch your knowledge, look up your dictionary, internet or ask your instructor to clarify the definition and Vietnamese meaning of the following terminologies.
No Terminology Definition Vietnamese
1 adsorption 2 azo disperse dye 3 bleaching 4 chelation 5 cotton 6 covalent 7 desorption 8 disperse dyes 9 dispersion
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10 dye11 equilibrium phase 12 fibers
13 fixation 14 hydrophobic 15 inhomogeneities 16 ink-jet
17 insoluble 18 leveling out 19 mercerization 20 nomenclature 21 pretreatment 22 reactive dyes 23 textiles