Every master craftsman in the laboratory and workshop knows that after our fabrics are treated with mercerization or concentrated alkali, the color after dyeing will be darker. What is the reason for this? In fact, it also involves the knowledge of visible light and textiles. This article is an excerpt from the book “New Fibers and Fabric Dyeing and Finishing” by Professor Song Xinyuan. I hope you can read it patiently.
Overview of the role of visible light and textiles
1. The origin of color
The natural world is colorful, from the blue sky and rainbow to colorful flowers, plants, minerals and animals. Color is the reflection of visible light on people’s visual nerves. That is, visible light of a certain wavelength produces a certain color reflection, and color is just a human visual feeling.
1. Causes of color
There are many ways to produce or emit visible light, and therefore there are many causes of color. Nassau roughly divides the causes of color into the following five types:
(1) Vibration and simple excitation of electrons: such as flame, lightning, aurora and iodine color effects, among which Most of them are high-temperature plasma emissions.
(2) Transition of electronic coordination field effect: The metal complex color effect in ruby, emerald, turquoise and various metal complex dyes (or pigments) are all Belongs to this category.
(3) Transition of electrons between molecular orbitals: The color effects of most organic dyes (or pigments) and some inorganic substances (sapphires) fall into this category.
(4) Transition of electrons in energy bands: color effects of non-ferrous metals (gold, silver, copper and iron), semiconductors and color centers (amethyst, smoky quartz) Belongs 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.
2. Reasons for the color development of textiles
The color development of textiles ( Including dyeing and other methods of coloring) belongs to the interaction of light and large solid matter. There are many ways to produce color in textiles. Currently, color is mainly produced by applying colored substances (adsorbed dyes or fixed pigments).
And the study of 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 their adsorption spectra, such as their absorption spectra in aqueous solutions or organic solutions. .
Strictly speaking, this absorption spectrum does not fully reflect the color of textiles, because the color of colored textiles not only depends on the molecular structure of dyes and pigments, but also includes the following factors :
(1) The adsorption or fixation state of dyes or pigments on textiles. For example, it is a single-molecule adsorption layer or a multi-molecule adsorption layer; it is an aggregate or crystal, the structure and size of the aggregate or crystal; it is distributed on the surface of the fiber or inside the fiber, etc.
(2) Structure and morphology of fiber. 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 fibers have different molecular orientation, crystallinity and structure size, and have different effects on light reflection, scattering and interference. etc. will not be the same; different fibers have different morphological structures and different effects on light. Fiber thickness, surface area, different cross-sectional shapes, and the presence of holes and cracks in the fiber will all affect the fiber’s effect on light, especially the physical and geometric optical effects of the fiber.
(3) Different materials applied to the fiber have different effects on light. In addition to dyes or pigments, many other substances are also applied to the fiber through adsorption or adhesion, such as various surface modifiers, especially deepening, brightening, reflective and various film-forming substances, which will significantly change the light resistance of the fiber. effect. It can be seen from the above that as a coloring body, it is not enough to study the color of the fiber only from the molecular structure of the dye or pigment, but should be considered from many aspects, including the absorption, emission and geometric and physical optical effects of the colored fiber on light.
2. The ways and methods of interaction between visible light and textiles
When When a beam of light interacts with textiles, at least several modes of action will occur as shown in Figure 3-1.
1. Reflection and diffuse reflection
After light hits the fiber surface, it will be reflected. When the fiber surface is very smooth, specular reflection will occur, producing 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 occur (not shown in Figure 3-1).
From the perspective of the interaction between light and fiber, specular reflection has a weak interaction with fiber, while diffuse reflection has a closer interaction and will show a certain color, because diffuse reflection light is After light enters a certain depth into the fiber, it is scattered and reflected out of the fiber, especially after the dye in the fiber selectively absorbs the light. Diffuse reflected light is mainly the complementary light of absorbed light and has a strong color effect.
The absorption of light by dyes mainly occurs inside the fiber, but in some casesUnder this circumstance, when the dye, especially the pigment in the coating, is mainly on the surface of the fiber, not only simple reflection occurs, but also absorption and diffuse reflection occur. When there are coatings, solid particles and various finishing agents on the fiber surface, the interaction between light and fiber on the surface is more complex, which will be discussed later.
2. Absorption, scattering and transmission
Fibers can be seen as It is a translucent object, and part of the light can enter the interior of the fiber, where it mainly absorbs, scatters or emits fluorescence. Absorption is mainly caused by the interaction between dyes and light, that is, selective absorption. The complementary light is diffusely reflected or transmitted through the fiber, causing the fiber to show color. The unabsorbed light is scattered inside the fiber, and part of the light is also transmitted.
When there is a fluorescent substance, the fiber absorbs light (mainly ultraviolet light) and emits fluorescence through diffuse reflection or transmission. , making the fiber fluorescent. The absorption and scattering of light in the fiber occur alternately multiple times. Different supramolecular structures of fibers (crystalline region and amorphous region) have different effects on light.
The morphological structure and the gaps and cavities in the fiber, such as the cell cavity of cotton and other fibers or the micropores of original bamboo fiber, will increase the scattering of light inside the fiber and make the color lighter. After mercerization or concentrated alkali treatment, The reduction or disappearance of the cell cavity or micropores will reduce the scattering of light inside the fiber and make the color thicker. Therefore, the mercerization of cotton fabric increases the color depth (or concentration) not only due to the increase in the amorphous area of the fiber. , the increase in dye uptake is also related to the changes in the supramolecular structure and morphological structure of the fiber. It is worth noting that some of the currently emerging multi-component composite fibers, such as soybeans, silkworm pupa protein fibers, etc., also have significant reflection or scattering at the interface between the two components in the fiber, which will make the color of the fabric lighter and brighter. Not bright.
3. Refraction and polarization
The size of light refraction and polarization depends on The speed of light propagation in air and fibers. The size of the refraction angle is related to the refractive index of the fiber. Internal reflection will also occur when light is transmitted from the fiber. Since it is transmitted from the tightly organized fiber to the loose air medium, the intensity of the internal reflection is much weaker than the reflection at the incident time.
Since the speed of light propagation in the fiber is not only related to the refractive index of the fiber, but also related to the wavelength of the light, the transmitted light may produce a certain dispersion effect, causing The color of reflected light is slightly different from that of transmitted light. Light with a longer wavelength has stronger transmission, and vice versa. Producing varying degrees of dispersion changes the color of the transmitted light. Because the fiber is anisotropic and has different optical properties in different directions, that is, pleochroism, the fiber not only reflects, diffuses, scatters, absorbs, and refracts light, but also polarizes, interferes, and diffracts the light.
And because textiles are a multi-fiber aggregate, and the structures of yarns and fabrics are different, the optical properties of textiles are very complex, and their changes will directly affect the optical properties of textiles. s color. We can discuss the impact of light on the color of textiles from the perspective of geometric and physical optics, that is, reflection, scattering, refraction, polarization, interference and diffraction, and analyze the ways to achieve structural coloration and methods to control color. </p
