Color and Design. Tutorial

In addition, antifreezes are introduced into the pastes (glycerin, ethylene glycol). Optical brightening agents are highly soluble in water and are easily absorbed by the fiber.

Optical brightening agents must have certain physical and chemical properties in order to be suitable for their application They should be as colorless as possible; absorb minimally the rays of the visible part of the spectrum; emit pure fluorescent light (from violet-blue to green); dissolve well; have sufficient substantive properties for the susceptibility of its bleaching material; be resistant to moisture and light, as well as to chemical bleaching agents (hypochlorite, hydrogen peroxide, rongalite, hydrosulfite, etc.)

Optical brightening agents should not cause eczema and other inflammatory processes on human skin when working with them or during the application of optically bleached materials.

Optical brightening agents are called colorless fluorescent dyes and the mechanism of their interaction with the hairline is similar to the mechanism of interaction with acid dyes. However, the principle of their action is different from chemical bleaches and fluorescent dyes. Fluorescent dyes, in contrast to optical brightening agents, absorb light in the visible region, like conventional dyes.

Optical brighteners are successfully used for fur processing, along with those presented above, if:

1)optical brighteners provide radiation of the treated hair in the wavelength range of 400–500 nm, which is associated with the physiological characteristics of the human eye;

2)optical brighteners have an affinity for hair;

3)hairline treatment is carried out at the optimal concentration of an optical brightener;

4)before treatment with optical brighteners, the hairline is bleached with chemical restorative bleaches;

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preventing the achievement of the maximum degree of whiteness.

The presence of heavy metals (Fe (III), Cu (II), Cr (III)) in the structure of fur skins, as well as the presence of some aromatic compounds (phenols, anilines, iodobenzene, etc.) in the form of impurities in optical brightening agents can lead to the reduction or complete suppression of fluorescence due to the formation of non-fluorescent complexes (static fluorescence suppression effect) or bimolecular reactions involving excited molecules of fluorescent bleaching agents (dynamic fluorescence quenching effect).

In this case, these substances act as sensitizers that increase the yellowness of wool.

In order to change the shade of the white hairline, optical brighteners are used in combination with aqua washing of the unpigmented hairline with blue or blue-violet dyes of low concentration. Such technique leads to an increase in the absorption coefficient of solar spectrum rays in the region lying to the right of the emission band of fluorescent bleach.

The low light fastness of dyed products and photosensitivity of the optically bleached hairline during wearing of fur products are the main disadvantages of processing skins with optical brighteners.

New technologies of fur bleaching

Companies develop new technologies to replace traditional sulfurbased reducing agents for bleaching pigmented hair and improve the quality of fur products. Hypophosphoric acid is used to bleach karakul. This method of bleaching ensures the stability of disulfide bonds.

The use of non-aqueous media such as tetrachlorethylene or perchlorethylene, can significantly reduce water consumption for technological needs and reduce production effluents. The use of these organic solvents increases the degree of whiteness of the semi-finished product in a shorter time under milder conditions. This method is less

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labor-intensive and energy-consuming. The lack of special sealed equipment limits the use of this method in the fur industry.

Bleaching of fur semi-finished products is carried out in combination with protective treatment. Therefore, the experience of the leather industry in the production of white leather products with a complex of useful properties based on the technology used amino resins is of great interest. Thus, the new tanning agent БМФ-1, developed for the retanning of white and light leathers, is a product of the modification of oligomeric melamine-formaldehyde resin with optical brighteners.

It has been proven that the use of БМФ-1 is effective for processing fur velor, fur sheepskins, and lambskins. БМФ-1 provides good bleaching and better preparation of semi-finished products for subsequent dyeing.

Bleaching of gray astrakhan, ermine, hare, and white fox skins may increase their production in natural form. Reduction bleaching of white fox and ermine skins is carried out after their skinning.

For lambskin and astrakhan skins, dry salting cure is used. This group of skins are bleached and soaked at the same time before pickling. The process of pickling is carried out in the spent reducing solution. Subsequent optical bleaching is combined with pickling and salting.

There are pigmented skins with an unattractive natural color. To produce them in their natural form or to use subsequent dyeing in light or fancy colors, catalytic oxidative bleaching is recommended. It is followed by optical bleaching with preliminary chemical protection of the skin tissue and hairline. To protect the astrakhan skin during its bleaching, it is additionally treated with an alkaline-salt solution at pH = 5–6, and then with formaldehyde at the same pH. This eliminates flow and protects the hair from the damaging effects of hydrogen peroxide.

Dipping of skins is widely used for bleaching. In addition to dipping bleaching, spread bleaching is also recommended to use for American opossum, blue fox, marten, and red fox skins. Spread bleaching consists in applying an ammonia solution of hydrogen peroxide with a brush to

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the surface of the hairline seven times, followed by drying the skins at 40 °C. Spread bleaching is used less frequently than dipping method.

Jos. H. Lowenstein & Sons has achieved significant success in the field of depigmentation of fur skins. The company has developed different bleaches:

•Bleach BL-15 and Bleach M-30 as hydrogen peroxide decomposition stabilizers;

•Bleach assist MB as a keratin-protective material;

•Tan EZN as a protective material for increasing skin

resistance to the effects of oxidizing agents.

Russian Research Institute of Fur Industry produces a set of chemical materials for depigmentation of fur skins with different structures and intensity of natural color. These products are made from Russian raw materials.

Catalytic redox method has been chosen for fur skin depigmentation.

The processing technology of all types of skins includes the main stages: protective preparation of leather tissue and hairline to the effects of oxidizing agents using a chromium-free tanning agent during retanning; introduction of M and MФ tanning agents to protect leather tissue; introduction of Anticolor-1 to prepare skins for depigmentation and protect disulfide bonds of keratin from destruction; introduction of catalysts into the hair structure; oxidative depigmentation using Anticolor-2, which regulates the decomposition rate of hydrogen peroxide; using of Anticolor-3, which protects the hair from the destructive effects of oxidizing agents; recovery using Anticolor-4 and Anticolor-5, which solve the problem of the complete transition of oxidizers and catalysts into inert forms and the extraction of fur skins from their structural elements; washing and retanning with aluminum or chromium compounds.

Russian Research Institute of Fur Industry has developed the chemical products for bleaching non-pigmented hairline of fur skins:

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•Anticolor A – a composition based on optical brightening agents with homogenizing additives;

•Anticolor B – a product based on sodium dithionite with stabilizing additives.

LECTURE QUESTIONS

1.What is the main principle for triad dyeing of fur products?

2.What is the difficulty in triad dyeing compared to traditional method using individual dyes?

3.What fur skins are usually used for imitating more valuable types of fur?

4.What finishing methods are used in the fur industry to create fur imitations?

5.What kinds of fur imitations do you know?

6.What color effects can be obtained by dyeing in combination with bleaching?

7.How do the dye molecule size and the hydrophobicity / hydrophilicity of the dissolving medium affect the dyeing of fur hair and skin tissue?

8.What dyeing method is used to dye the leather of fur velour?

9.What dyeing methods are used to dye fur velour on both sides?

10.What finishing operation is used to produce the double face sheepskin leather with minimum hygroscopicity?

11.What is silkscreen?

12.Why is the main goal of fur lightening and bleaching?

13.What two stages of melanin pigment discoloration do you know?

14.What substance is used as a bleaching agent in the process of catalytic oxidative bleaching?

15.What processes are used to remove the yellowness of unpigmented fur hair?

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LECTURE VOCABULARY LIST

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