Color and Design. Tutorial

A batch of sheepskins is sent for nappalan finishing after the entire cycle of mechanical operations, including toggling. Toggling is a process that straightens the sheepskins and softens the leather surface.

To create nappalan coating, the following operations are carried

out:

•two water-repellent coatings (fluorocarbonate, isopropyl alcohol);

•one color coating (acrylic latex, acrylate copolymer, binder polymer, silicone, water);

•pressing.

Next, a solution including nitrocellulose emulsion, acrylic latex, silicone, matte nitrocellulose emulsion, water, and a dye or pigment is applied in one or two layers.

Then the pressing is carried out again.

There are some conditions for producing nappalan leather:

1.It is essential that the leather fabric has good water repellency before applying the first coating. To check the water

the processed sheepskin, after applying all the coatings, will have cracks and a rough surface. Products from such sheepskins will quickly get wet.

2.The leather should be colored by combining metal-complex dyes and/or natural pigments.

3.The first coating should be applied evenly with a small

consumption of material in several passes.

A large number of film coating designs can be created on sheepskin to meet the needs of buyers.

At the same time, it is very important to show buyers all possible designs of leather.

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Many skins are not suitable for nappalan coating. Therefore, other operations are to be developed for such sheepskins. There are technologies which allow us to create Timber leather (imitations of wood structure or cowhide).

Regardless of the imitation created on the leather fabric, sheepskins should be soft and pleasant to the touch.

Silk printing leather

To obtain new imitations on the leather fabric of sheepskins, an idea borrowed from the textile industry for drawing a pattern using the silk-screen printing method was used. Silk-screen printing is widely used to apply all kinds of patterns, as well as to create various coloristic effects on the leather surface, which can be matte, shiny, contrasting, etc.

In this case, the color and pattern on the skin tissue and the color of the hairline should be harmonious.

Transferring patterns onto leather

This operation is used to transfer the color or pattern to the entire area of the sheepskin or to part of it. Technically, this process consists in applying a painted metal shield with a gilded or silver-plated surface to the sheepskin. Typically, this process is used for sheepskins which are processed for extravagant clothes.

For the same purposes, but on a smaller scale, the transfer of coloring or patterns is made by using special paper. This operation ensures the application of several color coatings of various designs.

To finish the leather fabric by transferring from a metal sheet or from paper, special sublimation dyes are used, which evaporate from the surface of the sheet and are fixed on the skin.

This process is carried out on a conventional press, on the table of which the sheepskin is laid with the leather upside down and the dyed

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paper is laid on it. The smooth plate presses the paper against the sheepskin and the dye is transferred within 5 seconds. When the plate is raised, the paper lags behind the surface of the sheepskin and is easily removed, leaving a pattern or a certain color on the surface.

The type of paper (adhesive) is to be easily removed after pressing. Sublimation dyes should easily and uniformly sublimate at a certain temperature.

This operation can be used to pattern or color finished products to provide a desired effect.

Skin bleaching

The aesthetic properties of fur after final processing of the skins are largely determined by the natural color (pigmentation) of the hairline. Natural coloring is determined by the presence or absence of colored substances (pigments) in the skin tissue and hair.

The whole range of colors of natural fur skins is determined by the concentration of pigment in the hair, the type of pigment, the nature of the distribution of the pigment in each individual hair and in the hairline as a whole. Lack of pigment or weak pigmentation, as well as the presence of air inclusions in the hair, causes different shades of white hair color.

For many types of furs (silver-black, platinum, snow and blackbrown foxes, arctic foxes), the color of the hairline is the main characteristic in quality assessment and cost determination.

According to GOST for sorting, the natural color of skins is divided into the first, second, third color sort. Pigment spots that are not characteristic of the traditional skin color, or yellow spots (more than 3 cm2 in area) are considered as defects.

To improve the aesthetic properties of the fur skin, namely to eliminate the unattractive natural color, yellowness or to prepare for subsequent dyeing in light colors, bleaching or lightening is used in fur finishing operations.

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Bleaching or lightening is a combination of complex chemical processes.

Bleaching aims to eliminate natural coloration, i.e. depigmentation of the hair. Bleaching is mainly used to expand the range of semi-finished fur products. For example, using bleaching, you can get the effect of golden sable on pastel mink skins. Bleaching in combination with subsequent dyeing allows you to obtain a variety of fancy colors on brown mink skins or black and colored astrakhan. Discoloration of skins of muskrat, marmot, and rabbit is carried out in order to imitate valuable types of furs. Bleaching of pigmented fur hair is carried out using oxidizing agents. It is called oxidative bleaching.

If you want to eliminate the yellowness of unpigmented hair or increase its whiteness, the process is called lightening. Lightening is carried out using optical brightening agents. Sometimes optical brightening agents are used in combination with reducing agents. Depending on the ultimate goal, the set of chemical effects may be different.

Thus, bleaching is used to treat a pigmented hairline, while lightening is used to treat a hairline with increased yellowness.

Bleaching of pigmented fur skins

The most effective way to destroy hair pigments is catalytic oxidative bleaching.

In practice, the implementation of catalytic oxidative bleaching is carried out in a gentle mode, with a minimal destructive effect on the hairline and skin tissue. Bleaching is based on the ability of melanin to

Thus, bleaching is used to destruct the melanin chromophore system partially or completely by the directed action of the oxidizing agent

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the pigments of the hairline, in practice, with the most widespread dipping methods of processing fur skins, not only hair, but also leather tissue is exposed to specific effects of aggressive environments.

Therefore, it should be considered that lightening is a complex of various chemical effects on fur skins, including processes aimed at:

•preparing skins for lightening (pickling, neutralizationkilling, protective treatments of leather tissue);

•whitening;

•eliminating the damaging effects of catalytic oxidative bleaching (washing, recovery);

•increasing the whiteness;

•strengthening the hair;

•strengthening connection of a hair with the skin tissue;

•improving the quality of the bleached fur semi-finished product (tanning, fatliquoring, optical bleaching, etc.).

Fur lightening includes a number of complex chemical processes. Companies are more willing to dye fur with unattractive natural pigmentation than to light it. It is possible to light both dressed and undressed skins. However, due to the complexity of lightening processes, the difficulty of maintaining the quality of lighted skins, a limited range of furs is subjected to this operation: arctic fox, mink, ermine, and astrakhan skins. If the cost of raw materials is very large, their lightening can be used as a finishing operation.

However, due to the growing attention to environmental problems, bleaching is one of the ways which can help to reduce the volume of dyed effluents. Masking various color defects and yellowness of the hairline by dyeing in darker tones leads to increased volume of these effluents.

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Nowadays, bleaching is also used as a pre-treatment of skins for dyeing in light and fancy colors (rabbit skins bleaching). The world market provides a large number of different bleaches, including various additional chemicals. This allows companies to simplify the process of bleaching and improve the quality of fur semi-finished products.

The process of melanin pigment bleaching can be divided into two

the pigment are destroyed under the action of oxidizing agents.

The main bleaching agent is hydrogen peroxide H2O2. They have high oxidizing ability and provide ecological purity of its reduction product (H2O).

The fur skins of mink and arctic fox can be bleached using various bleaches based on sodium perborate at a temperature of the working solution which is adopted in the fur industry. It is not higher than 35 °C.

Compared to hydrogen peroxide, sodium perborate is more stable during storage and more convenient to use as an oxidizing agent for the process of bleaching.

Commercial bleaches, along with an oxidizing agent, include various activators (for example, commercial bleach based on sodium perborate), washing additives with a high fat-removing effect, containing enzymes.

Commercial complex bleaches, including solid peroxides (for example, urea monoperoxyhydrate, sodium perpyrphosphate) are easily transported, stored for a long time, and less toxic. They do not contain ballast substances. An example of bleach with complex properties is persyntamm, which includes urea and ethoxylated fatty acid alkylamides.

The use of solid peroxides is limited by their high cost.

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Lightening of non-pigmented hair

The yellowness of non-pigmented hair is not caused by the presence of brown or yellow pigment. Therefore, the catalytic method of oxidative bleaching in such cases is ineffective. It is not widely used, since yellowness is practically not removed. It may be used only for weakly pigmented fur hair. For example, it is used to light gray astrakhan fur, the hairline of which consists of white and black hair. The pigment of black hair can be destroyed.

Skins with natural white or greyish-white hair are lightened in two

stages:

1.Treatment using reducing agents.

2.Treatment using optical brighteners.

The first stage can be carried out as a separate technological process or in combination with soaking which is used before pickling,

borohydrides of alkali and alkaline earth metals are used as reducing agents to eliminate yellowness of fur products.

Dithionite-containing substances are products that are a mixture of a reducing agent with surfactants and stabilizers for the decomposition of reducing agents. Some compositions contain optical brighteners.

When treated with reducing agents, colored products which are finely dispersed in the hair are destroyed and removed.

Potassium tetrahydroborate is used to remove yellowness on the skins of a sea animal. Potassium tetrahydroborates are rarely used due to their high toxicity, volatility, and, consequently, the need for special equipment. Treatment with reducing agents allows you to increase the whiteness of the hairline, but does not provide complete elimination of yellowness. Complete elimination of yellowness is achieved by processing the fur semi-finished product with optical brighteners.

The second stage uses optical brighteners.

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It is known that any white fibers of animal, vegetable or artificial origin are not pure white. They differ in a yellowish tint to some extent. To increase the degree of whiteness of fabrics, paper and other materials, the well-known method of coloring with the addition of a small amount of blue or violet dye is used. When mixing two colors (yellow and blue or yellow and violet), according to the law of mixing colors, the object being painted is perceived as gray without yellowness. This method allows us to neutralize unwanted shades. However, we cannot obtain a pure white color. A color closer to pure white can be obtained by processing materials with yellow hues of OBA. They are called optical brighteners.

Optical brighteners are used to improve the whiteness of unpigmented hair. It is based on their ability to absorb the invisible rays of the near UV part of the solar spectrum and convert the energy into the short-wavelength visible part of the spectrum (in the form of fluorescence).

The fluorescence intensity of optical brighteners depends on the power of UV radiation in the incident light. The energy of the absorbed light is inversely proportional to the wavelength. UV rays have the highest energy in the solar spectrum (λ = 350...390 nm, the lowest limit is ~270 nm, radiation with λ < 270 nm is absorbed by ozone and air and does not reach the earth's surface). Light sources that do not have UV radiation in their spectrum, such as electric incandescent lamps, do not cause fluorescence on a surface treated with optical brighteners.

The absorption region of optical brighteners ranges between the wavelengths of 340 and 400 nm. The fluorescence maximum, which determines its color, depends on the type of optical brighteners. It is located in the interval between wavelengths of 415 and 466 nm (Table).

During fluorescence, the yellowness of the hairline is masked due to the optical mixing of the yellow rays reflected by the hair, as well as the blue, indigo and violet emitted by optical brightening agents.

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Thus, the whiteness of the hairline (luminous factor) increases, i.e. its reflectivity increases. In this case, it does not mean the shine of the hairline, due to the shape of the scales and the geometry of the hair.

Table. Dependence of fluorescent color on maximum emission

It is known that a surface is considered as absolutely white if it reflects all the visible light falling on it. For example, a surface coated with magnesium oxide reflects almost 100 % of light energy or barium sulfate reflects 96 % of light energy. A surface coated with magnesium oxide is used as the standard of whiteness. All materials are compared with it. In practice, preference is given to such a white color, in which radiation with a wavelength of 400–500 nm predominates. This is due to the physiological peculiarity of the human eye, which perceives light blueness as a high degree of whiteness. The same property of vision explains the fact that blue things seem whiter. White color is perceived differently by people. In Europe, preference is given to white with a slight purple or bluish tint, in the USA–with a greenish one.

Whiteness is characterized by a reflection coefficient ζ which is equal to the ratio of the power of the reflected radiation flux to the power of the incident radiation flux. Thus, an increase in the reflectivity of the hairline surface after optical brightening agent treatment is perceived as an increase in whiteness.

Fluorescent optical brightening agents are called colorless fluorescent dyes. They are organic compounds that contain fluorophore systems: luminophores and fluorogens.

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Fluorescence is based on energy phenomena associated with the transition of molecules from the ground state to the excited state and vice versa.

For bleaching a fur semi-finished product, anion-active optical brightening agents are often used. They are fixed on the hair as acid dyes, forming anions containing luminophores in aqueous solutions.

Russian optical brightening agents are called Belofor. To indicate the shade of whiteness, the letters are used:

•Belofor R – reddish;

•Belofor B – bluish;

•Belofor G – greenish;

•Belofor O – no shade.

To specify the material to be dyed, the letters are used:

•Belofor C – for cellulose fibers;

•Belofor A – for acetate fibers;

•Belofor M – for artificial and synthetic fibers;

•Belofor D – for detergents;

•Belofor W – for wool.

In some cases, the scope of Belofor is indicated in the title by the words: for plastics, for fur, etc.

For bleaching fur semi-finished products, Belofor optical brighteners from a mixture based on derivatives of 1,3-diarylpyrozoline and coumarin are used.

Foreign companies produce optical brightening agents with various trade names: blancofor, ultrafor, ultrafix, weisstoper (Germany), tinopali, zvitek, leukophor (Switzerland), reluxe (Czech Republic, Slovakia), brightners (USA).

Optical brightening agents are produced in the form of powders. Some of them are produced in the form of pastes like dispersed dyes. In addition to optical brightening agents, commercial products also contain surfactants, stabilizers, substances that provide better fiber selectivity, etc.

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