Why is the color darker after the fabric is mercerized or treated with concentrated alkali?

The masters in the laboratory and workshop know that after our fabrics are treated with mercerizing or concentrated alkali, the color will be darker after dyeing. What is the reason for this?

An overview of the role of visible light and textiles

The cause of color

Nature is full of colors, from blue sky and rainbow to colorful flowers, minerals, and animals. Color is the reflection of visible light on people’s optic nerves. That is, visible light of a certain wavelength produces a certain color reflection, and the color is only a visual perception of human beings.

  1. The reason for the color

There are many different ways of producing or emitting visible light, and therefore the causes of color are many and varied. Nassau roughly divides the causes of color into the following five types:
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(1) Vibration and simple excitation of electrons: such as the color effects of flames, lightning, aurora, and iodine, most of which are high-temperature plasma emissions.

(2) Transition of electronic coordination field effect: ruby, emerald, turquoise, and metal complex color effects in various metal complex dyes (or pigments) belong to this category.

(3) The transition of electrons between molecular orbitals: the color effects of most organic dyes (or pigments) and some inorganic substances (sapphire) belong to this category.

(4) The transition of electrons in the energy band: the color effects of non-ferrous metals (gold, silver, copper, and iron), semiconductors and color centers (amethyst, smoky crystal) belong to this category.

(5) Geometric and physical optical effects: color effects of dispersive refraction, scattering, interference, and diffraction. Of these five causes, few can be further subdivided into fifteen.

  1. Reasons for color development of textiles

Textile color development (including dyeing and coloring by other means) belongs to the interaction of light and bulk solid matter. There are many ways to make textiles produce color, currently mainly through the application of colored substances (adsorbed dyes or fixed pigments) to produce color.

Moreover, the research on the color of textiles is mainly based on the molecular structure of pigments, that is, the relationship between the molecular structure of dyes or pigments and the absorption spectrum, such as their absorption spectra in aqueous or organic solutions.

Strictly speaking, this absorption spectrum does not fully reflect the color of textiles, because the color of colored textiles depends not only on the molecular structure of dyes and pigments but also on the following factors:
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(1) The adsorption or fixation state of dyes or pigments on textiles. For example, it is a monomolecular adsorption layer or a multi-molecular adsorption layer; it is aggregates or crystals, the structure, and size of aggregates or crystals; whether they are distributed on the surface of fibers or inside fibers, etc.

( 2 ) The structure and morphology of fibers. Fibers with different chemical structures have different effects on light absorption and reflection; different fibers have different supramolecular structures and different effects on light, different molecular orientation, crystallinity and structure size, and reflect, scatter and interfere with light. etc. will not be the same; different fibers have different morphological structures and have different effects on light. The fiber thickness, surface area, different cross-sectional shapes, and the existence of holes and cracks in the fiber will affect the effect of the fiber on light, especially the physical and geometric optical effects of the fiber.

(3) Different substances applied to the fibers have different effects on light. In addition to dyes or pigments, many other substances are applied to the fibers by adsorption or adhesion, such as various surface modifiers, especially darkening, brightening, light-reflecting, and various film-forming substances, which can significantly change the fiber’s ability to respond to light. effect. It can be seen from the above that as a colorant, it is not enough to study the color of fibers only from the molecular structure of dyes or pigments, but should be considered from many aspects, including the absorption, emission, and geometric and physical optical effects of colored fibers.

  1. The way and way of action of visible light and textiles
  2. Reflection and Diffuse

When light hits the surface of the fiber, it will reflect, and when the surface of the fiber is very smooth, the specular reflection will occur, resulting in a strong reflection in a certain direction. However, the fiber surface is not absolutely smooth and has a certain degree of roughness, so a certain degree of diffuse reflection will also occur (not shown in Figure 3-1).

From the point of view of the interaction between light and fiber, specular reflection has a weak interaction with fiber, while diffuse reflection is more closely related and will show a certain color because diffuse reflection light is scattered and reflected after light enters a certain depth in the fiber. Out of the fiber, especially after the dye in the fiber selectively absorbs light. The diffuse reflected light is mainly the complementary light of absorbed light, which has a strong color effect.

The absorption of light by the dye mainly occurs inside the fiber, but in some cases, when the dye, especially the pigment in the coating, is mainly located on the surface of the fiber, not only simple reflection but also absorption and diffuse reflection occur. When there are coatings, solid particles, and various finishing agents on the fiber surface, the interaction of light and fiber on the surface is more complicated, which will be discussed later.

  1. Absorption, scattering and transmission

The fiber can be regarded as a translucent object, and part of the light can enter the fiber, where absorption, scattering, or fluorescence emission mainly occurs. Absorption is mainly caused by the interaction of dyes and light, that is, selective absorption occurs. Complimentary light is diffusely reflected or transmitted through the fiber to make the fiber appear colored. The unabsorbed light is scattered inside the fiber, and part of the light is also transmitted.

In the presence of fluorescent substances, the fibers emit fluorescence after absorbing light (mainly ultraviolet rays), and the fibers exhibit fluorescence through diffuse reflection or transmission. The absorption and scattering of light in the fiber occur alternately many times. Different supramolecular structures of fibers (crystalline and amorphous regions) have different effects on light.

The morphological structure as well as the voids and cavities in the fibers, such as the cells of cotton and other fibers or the micropores of original bamboo fibers, will increase the scattering of light inside the fibers and make the color lighter, while after mercerization or concentrated alkali treatment, the cells are Or the pores become smaller or disappear, which will reduce the scattering of light inside the fiber and make the color thicker. Therefore, the mercerization of cotton fabrics increases the color depth (or concentration), not only due to the increase of the amorphous area of ​​the fiber, which makes the color darker. The increase in dye uptake was also associated with changes in the supramolecular structure and morphological structure of the fibers. It is worth noting that some multi-component composite fibers that have appeared at present, such as soybean, silkworm pupa protein fibers, etc., also have significant reflection or scattering at the interface of the two components in the fiber, which will make the fabric lighter and lighter. Not bright.

  1. Refraction and Polarization

The amount of light refraction and polarization depends on the speed at which light travels through air and fibers. The size of the refraction angle is related to the refractive index of the fiber. Internal reflection also occurs when light is transmitted from the fiber. Since it is transmitted from the tightly organized fiber to the sparse air medium, the internal reflection intensity is much weaker than the incident reflection.

Since the speed of light propagating in the fiber is not only related to the refractive index of the fiber, but also to the wavelength of the light, the transmitted light may have a certain dispersion effect, so that the color of the reflected light is slightly different from that of the transmitted light. The transmission of light with long wavelengths is strong, and vice versa. Creating different degrees of dispersion changes the color of the transmitted light. Since the fiber is anisotropic and has different optical properties in different directions, that is, pleochroism, the fiber will not only reflect, diffuse, scatter, absorb, and refract light, but also polarize, interfere, and diffract light.

And because textiles are a multi-fiber aggregate, and the structure of yarns and fabrics is different, the optical properties of textiles are very complex, and their changes will directly affect the color of textiles. We can discuss the effect of light on textile color from geometric and physical optics, that is, reflection, scattering, refraction, polarization, interference, and diffraction effects, and analyze ways to achieve structural thermogenesis and methods to control color.

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