Dažai

Dažai, medžiaga, naudojama spalvai suteikti tekstilės gaminiams, popieriui, odai ir kitoms medžiagoms, kad dažymas nebūtų lengvai keičiamas dėl skalbimo, karščio, šviesos ar kitų veiksnių, kuriems medžiaga gali būti veikiama. Dažai skiriasi nuo pigmentų, kurie yra susmulkintos kietos dalelės, išskaidytos skystyje, tokiuose kaip dažai ar rašalas, arba sumaišytos su kitomis medžiagomis. Daugelis dažų yra organiniai junginiai (ty juose yra anglies), tuo tarpu pigmentai gali būti neorganiniai junginiai (ty jie neturi anglies) arba organiniai junginiai. Pigmentai paprastai suteikia ryškesnes spalvas ir gali būti dažai, kurie netirpsta naudojamoje terpėje.

Spalva visada žavėjo žmoniją tiek dėl estetinių, tiek dėl socialinių priežasčių. Per visą laiką dažai ir pigmentai buvo pagrindiniai prekybos straipsniai. Praktiškai visų komercinių gaminių gamyba tam tikru etapu apima spalvas, o šiandien naudojama maždaug 9000 dažiklių, turinčių daugiau nei 50 000 prekės pavadinimų. Didelis skaičius yra norimų atspalvių ir atspalvių asortimento, dažomų medžiagų cheminės prigimties ir to, kad spalva yra tiesiogiai susijusi su dažiklio molekuline struktūra, pasekmė.

Dažų istorija

Natūralūs dažai

Iki 1850 m. Beveik visi dažai buvo gaunami iš natūralių šaltinių, dažniausiai iš daržovių, tokių kaip augalai, medžiai ir kerpės, kai kurie iš jų - iš vabzdžių. Tvirtų įrodymų, kad dažymo metodai yra senesni nei 4000 metų, pateikė dažyti audiniai, rasti Egipto kapuose. Senovės hieroglifai apibūdina natūralių dažų ekstrahavimą ir naudojimą. Nesuskaičiuojama daugybė bandymų išgauti dažus iš ryškiaspalvių augalų ir gėlių; vis dėlto tik keliolika natūralių dažų buvo plačiai naudojami. Be abejo, dauguma bandymų nepavyko, nes dauguma natūralių dažų nėra labai stabilūs ir yra sudėtinių mišinių komponentai, kurių sėkmingo atskyrimo vargu ar būtų galima pasiekti naudojant senovėje taikomus neapdorotus metodus. Nepaisant to, šių dažų tyrimai 1800-aisiais sudarė pagrindą sintetinių dažų vystymuisi,kuris dominavo rinkoje iki 1900 m.

Two natural dyes, alizarin and indigo, have major significance. Alizarin is a red dye extracted from the roots of the madder plant, Rubia tinctorium. Two other red dyes were obtained from scale insects. These include kermes, obtained from Coccus ilicis (or Kermes ilicis), which infects the Kermes oak, and cochineal, obtained from Dactylopius coccus, which lives on prickly pear cactus in Mexico. One kilogram (2.2 pounds) of cochineal dye can be obtained from an estimated 200,000 insects. The principal coloured components in these dyes are kermesic and carminic acids, respectively, whose similarity was established by 1920. In their natural state many colorants are rendered water-soluble through the presence of sugar residues. These sugars, however, are often lost during dye isolation procedures.

Probably the oldest known dye is the blue dye indigo, obtained in Europe from the leaves of the dyerswoad herb, Isatis tinctoria, and in Asia from the indigo plant, Indigofera tinctoria. Even by modern standards, both alizarin and indigo have very good dyeing properties, and indigo remains a favoured dye for denim, although synthetic indigo has replaced the natural material.

With a process developed by the Phoenicians, a derivative of indigo, Tyrian purple, was extracted in very small amounts from the glands of a snail, Murex brandaris, indigenous to the Mediterranean Sea. Experiments in 1909 yielded 1.4 grams (0.05 ounce) from 12,000 snails. Historically, this dye was also called royal purple because kings, emperors, and high priests had the exclusive right to wear garments dyed with it, as is well documented in the Hebrew Bible and illustrated for Roman emperors on mosaics in Ravenna, Italy. By the 1450s, with the decline of the Eastern Roman Empire, the Mediterranean purple industry died out.

Natural yellow dyes include louting, from the leaves of weld, Reseda luteola, and quercetin, from the bark of the North American oak tree, Quercus tinctoria. These are in the flavonoid family, a group of compounds occurring almost exclusively in higher plants and producing the colours of many flowers. In fact, these compounds can produce all the colours of the rainbow except green. Luteolin, a yellow crystalline pigment, was used with indigo to produce Lincoln green, the colour associated with Robin Hood and his merry men.

Another group of compounds, the carotenoids, present in all green plants, produce yellow to red shades. Lycopene, from which all carotenoids are derived, produces the red colour of tomatoes. An ancient natural yellow dye, crocetin, was obtained from the stigmas of Crocus sativus; this dye is undoubtedly derived from lycopene in the plant. Few of the flavonoid and carotenoid colorants would have survived ancient extraction processes.

Logwood is the only natural dye used today. Heartwood extracts of the logwood tree, Haematoxylon campechianum, yield hematoxylin, which oxidizes to hematein during isolation. The latter is red but in combination with chromium gives shades of charcoal, gray, and black; it is used mainly to dye silk and leather.

Mordants

Highly skilled craftsmen with closely guarded secret formulas rendered dyeing a well-protected trade. The formation of different colours by mixing red, blue, and yellow dyes was well known in ancient times, as was the use of metal salts to aid the retention of dyes on the desired material and to vary the resultant colours. Natural dyes cannot be applied directly to cotton, in contrast to wool and silk, although cotton can be dyed by vatting or by pretreatment with inorganic salts known as mordants (from Latin mordere, meaning “to bite”). These are adsorbed on the fibre and react with the dye to produce a less soluble form that is held to the fabric. Alum, KAl(SO₄)₂ × H₂O, as well as iron, copper, and tin salts were common ancient mordants. No doubt the secret processes included other ingredients to improve the final results. Mordants also were used to vary the colours produced from a single dye. For example, treatment with aluminum hydroxide, Al(OH)₃, before dyeing with alizarin produces Turkey red, the traditional red of British and French army uniforms. Alizarin gives violet colours with magnesium mordants, purple-red with calcium mordants, blue with barium mordants, and black-violet with ferrous salts. Around 1850, chromium salts, used as mordants, were found to provide superior dye retention and, in time, largely displaced the others; chromium mordants are still widely used for wool and, to some extent, for silk and nylon.

Decline of natural dyes

Until 1857 the dye industry utilized natural dyes almost exclusively; however, by 1900 nearly 90 percent of industrial dyes were synthetic. Several factors contributed to the commercial decline of natural dyes. By 1850 the Industrial Revolution in Europe led to a burgeoning textile industry, which created increased demand for readily available, inexpensive, and easily applied dyes and revealed the important economic limitations of natural dyes. Since most dyes were imported from distant sources, transportation delays were likely to slow the production of dyed materials. Dye quality was affected by the whims of nature and the dye maker’s skills. In addition, inefficient processes were often required for optimum results; for example, Turkey red dyeing could involve more than 20 steps to produce the desired bright, fast colour. Advances in organic chemistry, both practical and theoretical, spurred by studies of the many new compounds found in coal tar, increased interest in finding ways to utilize this by-product of coke production. The dye industry played a major role in the development of structural organic chemistry, which in turn provided a sound scientific foundation for the dye industry.