Dyes bond to fabrics through several mechanisms, primarily through covalent bonds and other weaker interactions.
Covalent Bonding: The Strongest Link
Reactive dyes, introduced in 1956, form strong, lasting bonds with fibers. This happens via covalent bonds created by chemical reactions between dye molecules and the fiber's substituents. This means the dye chemically attaches to the fabric's structure. [This is explained in several of the provided references](https://www.flinnsci.ca/api/library/Download/707e03bc678c460b810aa65c23e02a44, https://www.flinnsci.com/api/library/Download/c46ad9cfbff64e9cae7caf3aed645692, https://pmc.ncbi.nlm.nih.gov/articles/PMC7002841/). Azo dyes are a common example of this type.
Weaker Interactions: A Variety of Bonds
Many dye-fiber interactions don't involve covalent bonds. Instead, they rely on weaker forces:
- Hydrogen bonding: This is a common interaction, particularly with acid dyes. (Source: Dyeing The Chemistry of Pysankarstvo)
- Ionic interactions: Attraction between oppositely charged dye ions and fiber molecules.
- Van der Waals forces: Weak, short-range attractions between molecules.
These weaker bonds can be influenced by factors like the dye's chemical structure, the fiber's composition (natural fibers like cotton and wool vs. synthetics like polyester and nylon – see Dyes and Chemical Bonding for more information on fiber structures), and the dyeing process itself (temperature, pH, etc.). While less permanent than covalent bonds, these weaker interactions are still crucial for many dyeing applications. Soybean protein, for instance, can form bonds, though the exact nature of these bonds requires further information. Pretreating fabric in soya (soy) milk: FAQs - Rebecca Desnos mentions that it forms a bond between the fabric and dye plant, thus fulfilling the definition of a bonding agent.
The strength and permanence of the dye's bond directly impact the color's fastness, i.e., its resistance to fading or washing out. Covalent bonds generally lead to better color fastness than weaker interactions.
