Chemical Bonds in Dyeing: A chemical bond is an attraction between atoms that allows the formation of chemical substances that contain two or more atoms. Chemical bond occurs in a chemical reaction between the fiber and the dye. The bond is caused by the electromagnetic force attraction between opposite charges, either between electrons and nuclei, or as the result of a dipole attraction. The dyeing method, the parameters that are needed to be followed during the dyeing, and the auxiliaries that are necessary for dyeing are mainly dependent on the chemical bonds between the fiber and the dye. The reaction between dyes and fibers must take into account the various types of forces exerted by one molecule upon another.
Fig: Chemical bonds
All dyeing mechanisms can be divided into following three groups:
Specific bond between dye and fiber owing to covalent bond, hydrogen bonds or other directed bonds.
Non- specific attraction between dye and fiber owing to ion-exchange or Van der Waals’ forces.
Absence of any interaction, dyes is only mechanically retained. This may be due to insolubilisation of the dye inside the fiber or may be due to self –association into possibly quite large molecular aggregates following their entry into fiber. Aggregation is promoted by a high ratio of molecular weight to ionic group as well as by increase in length of the aromatic structure of the molecule, also by rise in concentration and the presence of inorganic salts, e.g. common salt.
Different Types and Roles of Chemical Bonds in Dyeing:
1. Vander Waal’s Forces: Van der Waals forces include attractive forces between closed shell molecules. This includes attraction between dipoles as well as repulsive forces between molecules, which increase with short distance. These forces are so named because they were first recognized by Van der Waals in 1873. Very weak forces of attraction are always present between the electrons of one atom and the nucleus of another in close enough proximity. Individually these are very weak forces, but collectively they are considered to be of sufficient strength to be the most important attractive forces between dye and fiber. These forces of attraction are known as Van der Waals’ forces. Disperse dyes are held in a polyester fiber by means of Van der Waals’ forces.
2. Hydrogen Bonds: Hydrogen bonds represent a special case of Van der Waals forces. In case a hydrogen atom is bound to a heteroelement with higher electronegativity. These forces of attraction are weak forces set up between certain atoms in the dyestuff molecule when they are close enough to other atoms in the fiber. One of these atoms is the hydrogen atom, hence the term “hydrogen bond”. Some direct and vat dyes are “hydrogen bonded” in cellulose fibers.
The hydrogen bonds exhibit thermal dependency in their length and organisation, while a tetrahedral geometry with 90% intact hydrogen bonds is observed near to the freezing point of water, the length of the hydrogen bonding as well as the statistical probability for a hydrogen bond reduces to 20% at the boiling point.
3. Hydrophobic Bonding: The hydrophobic groups, especially alkyl chains, tend to associate together and escape from aqueous environment. The effect due to two simultaneous causes-the Van der Waals’ forces between the hydrogen groups and the hydrogen bonds between water molecules. Each set of forces causes respective assembly of molecules or groups to associate together and to exclude the other. Hydrophobic bonds occur when both the fiber and dye contain a considerable portion of purely hydrocarbon, aliphatic or aromatic, as with some dyes applied on wool or most dyes applied on polyester. It is strictly not a new type of bond or intermolecular force.
4. Salt Linkages or Ionic Bond: These bonds play an important part in dyeing fiber containing amino groups, i.e. wool, silk and nylon with anionic dyes. In the presence of water or dilute acids the amino groups become protonated:
NH₂CH₂COOH → NH₃⁺CH₂COO⁻
Acid dyes, being anionic in solution, are attracted at the positive site of the fiber. As the fiber forms zwitterions on ionization, a negative charge is also created on the fiber. This negative charge is responsible for attraction towards basic dyes which forms cationic dye ions in solution. However, basic dyes are now mostly applied on acrylic fibers which contain strong acidic sites. Due to ionization in acidic medium, negative charged sites are created in fiber which attracts cationic dye ions.
5. Ion-Dipole Forces: While attracting those of opposite charge, ions in solution can exert attraction upon any polar molecule, giving rise to so-called ion-dipole forces. These forces are largely responsible for aqueous solubility of dyes. The attraction between dipoles on the cellulose ether groups and ionic groups in the dye molecules is also proposed.
6. Covalent Bond: Bonds resulting in very strong chemical forces that are not easy to break expect under serve conditions are called covalent bonds. The classic example is that of the combination of cellulose fibers with reactive dyestuffs, where the hydroxyl group in the cellulose is covalently bonded to a suitable atom in the reactive dye.
Comparison of the Relative Strength of Dye-Fiber Bonds:
Founder & Editor of Textile Learner. He is a Textile Consultant, Blogger & Entrepreneur. Mr. Kiron is working as a textile consultant in several local and international companies. He is also a contributor of Wikipedia.
