When textile materials are dyed, they are coloured by being submerged in a dye liquor solution, which is an aqueous solution of dye. The dye liquid typically contains dye, water, and an auxiliary. Usually, the dye fluid is heated to increase the dye’s efficacy. This interplay between dyes, fibre, water, and dye auxiliary is explained by the general theory of dyeing. In more detail, it explains the repelling forces that form between the water and dye molecules and the forces of attraction that develop between the dye molecules and the fibres. These forces cause the dye molecules to exit the aqueous dye liquid, enter the polymers of the fibre, and bind themselves to them.
The molecule of the dye
Organic compounds, such as dye molecules, fall into the following categories
anionic, in which the dye molecule’s anionic component is what gives the color;
cationic, in which the dye molecule’s cationic component is what gives the color;
disperse, in which the entire molecule is responsible for the color.
Applying the first two types of dye molecules requires an aqueous solution. The third comes from an aqueous dispersion and is applied.
Textile fibres are organic substances that, when submerged in an aqueous solution, acquire a tiny negative surface charge or potential. The dye and the fibre have a propensity to reject one another because, in an aqueous solution, both the dye molecule and the textile fibre become somewhat negatively charged.
Water serves as both a solvent for the dye and a conduit for the dye molecules’ entry into the fiber. In order for the dye to dissolve in water, the polar groups on the dye molecules must attract water molecules. The dye molecules resist leaving the water and entering the fibre because of this affinity between the water and the dye, which is generally unwanted. To guarantee that the fibre is coloured uniformly, it is sometimes preferable to slow down the rate at which the dye leaves the water and enters the fibre.
These chemical substances include wetting agents, detergents, anti-foaming agents, levelling agents, carriers or swelling agents, and dispersion agents.
The most widely accepted theory is that carriers aid in expanding the fibre and facilitating dye molecule entry into the polymer system. In general, carriers are only employed when dispersion colours are being applied to polyester fibres.
The levelling agents that are added to the dye liquid aid in giving textile fibres a more uniform color. Retarding agents, or retarders, are another name for levelling agents that have the tendency to slow down the uptake of colour by the fibres. Leveling agents are surface-active substances that share chemical similarities with wetting agents, synthetic detergents, and soaps.
Although there isn’t a single, all-encompassing theory of dyeing that can fully describe all dyeing occurrences, there are a number of models that can be used as a starting point for research on different dye fibre systems. The characteristics of dye diffusion in solid polymeric fibres are a topic of discussion in dyeing theories. Essentially, they are supported by the pore diffusion model and the free volume or mobile segment model, two significant and essentially distinct theories for colour diffusion in fibre
According to this model, a fibre is a solid structure with a network of connected channels or pores that are stuffed full of dyeing liquid, which is often water, during the dying process. The dye that has been dissolved diffuses into these pores, where it may also become adsorbed on the pore walls. The pore model makes the assumption that the pores are interconnected with one another on the external dye bath and have a large enough diameter to accommodate the dye molecules. The pore model largely supports dye transport in cellulose fibres. Accordingly, a network of water-filled and enlarged pores exists within the fibre, and it is within this network that dye sorption and diffusion take place.
Reactive dyes undergo diffusion first, and then the dye and fibre undergo chemical reactions. It is well known that cellulose fibres generally absorb dye more or less effectively depending on their physical makeup. This explains why various regenerated cellulose fibre types, as well as cotton and rayon, take dye differently.
The free volume model describes the dyeing process as the dye diffusing across the less ordered or amorphous areas of the polymer structure, in contrast to the pore model. The mobility of the polymer chain segment in these areas thus controls the rate of diffusion. The discovery that the temperature dependency of the dyeing speed for a certain variety of fibre is less pronounced above a particular temperature is the strongest argument in favour of this idea. Above this temperature, the fiber’s solid structure offers far less resistance to the dye’s penetration. Since the traditional glass transition temperature may be a metric that’s often measured within the dry state of the fibre, this temperature is known as the glass transition temperature (Tg) of fibre or, more accurately, the glass transition temperature of the fibres during dyeing conditions.
The basic four steps of the dyeing process are as follows
Adsorbed on the fibre surface, the dye molecules from the dye bath form a layer that surrounds the fibre surface. The fastness characteristics and penetration of the dye into the fibre are impacted by dye adsorption. Adsorption could be transient or ongoing.
Diffusion refers to the dyes’ transfer or penetration from the fiber’s surface to its center. Because it affects fastness characteristics and the manufacture of coloured materials, diffusion is crucial.
The dispersed dyes are trapped or form a link with the substrate to ensure wet fastness.There may be primarily two forms of dye-fibre interaction. Both ionic and non-ionic.
To remove unfixed dyestuff and chemicals from the dyed fabrics or yarn, dyed materials must be washed after the dyeing process. It is a necessary technique to guarantee the best dyeing results, such as good colorfastness qualities, uniform dyeing, level dyeing, etc.
To conclude, the flow of dye into the interior of the fibre as well as the interaction between a dye and a fibre are both components of the dyeing process. In addition to direct absorption, dyeing may also entail a chemical reaction with the fibre or the precipitation of dyes inside the fibre (vat dyes; reactive dyes). Printing can be seen as partial dyeing with various colours on fabric to create an attractive pattern from the perspective of coloration. A dyeing or printing process is complex because it involves various fibre types and structures, yarn or fabric structures, dyes and chemical auxiliaries, as well as dyeing technologies. All aspects that may affect the dyeing or printing process must be accurately regulated in order to achieve the desired dyeing or printing quality.
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