Chemistry of Dyeing

The chemistry of dyeing is highly complex, and intricate. There are many steps of production, and each is crucial for the final colour to emerge with and retain full intensity.

A dye is an organic compound, which can be a natural or synthetic substance, that is used to add or change the colour of something. In the past, mostly all dyes have come from natural sources such as plants and animals. More recently, chemists have begun to replicate these colours found in nature, to synthetic dyes. These synthetic dyes are generally more intensely coloured, and have better colour fastness.

Molecular Structure of a Dye:

A dye has 2 main parts. The first one is a chromophore. Each dye has one chromophore. It’s a group of atoms that control the colour of the dye and this is their main function. They hold on tightly to their electrons.

Most dyes also contain 2 auxochromes. These are used to intensify the colour and to form chemical bonds with the fibres, so that the dye particles can attach to the fibre. They give away their electrons. Chromophores and auxochromes are connected by a conjugated system.

Types of Dyes:

There are four main types of dyes. Direct dyes form chemical bonds with fabric. You immerse the fabric in a solution and following there is an “uptake” where the dye molecules attach to the fabric molecules. Due to “polar” structure of dye molecule and fabric molecule, sections of the molecules can have a slightly positive or negative charge. The negative part of dye molecule attracts to the positive part of fabric molecule. There is no sharing of electrons, and there is a weaker bond called “Van Der Waals”, and the color can wash away easily. An example of a direct dye, would be tie dye. The second type of dye is a dispersed dye. They are unique in the sense that they are the only insoluble dyes, meaning they are not soluable in water. Disperse dyes, are most effective at dying polyester. The molecules that make up disperse dyes are the smallest of all the dye molecules. These dyes are mostly used with dye bath solutions, and to ensure that the process goes smoothly, dispersing products are used to intensify the water soluability of the dye. The third type of dye is a fibre reactive dye. These dyes can react chemically to create covalent bonds. These are the strongest dyes. The actual chemical reaction can strip the electrons or protons from fabrics. This process opens up sites for new bonds, between the dye molecule and the fabric. This reaction produces very strong, covalent bonds.

The original fibre-reactive dyes were created for cellulose fibres, and until today they are mostly used for that purpose. Fibre-reactive dyes have been anticipated for a long while, and weren’t really created until 1954. Before that, they tried many times to react the dye and fibre, but it only resulted in the material being ruined. The fourth type of dye is the vat dyes. In vat dyes, the colour is formed within the fibre. The colour does not appear unless it’s given the proper treatments. An example of a vat dye is indigo. Another example would be tyrian purple. This colour doesn’t form until the dye is exposed to air and sunlight.

First Synthetic Dye:

The first synthetic dye was created by, William Henry Perkins. It was actually discovered by mistake. Perkins was trying to synthesize quinine, to attempt to find a cure for Malaria, he was using coal tars and when he heated it, it became a black gunk and it didn’t absorb water. He dissolved that in alcohol and discovered the first synthetic dye – purple. This colour is called “mauveine”, or Perkins violet. He went into mass production funded by his family. He built a factory to synthesize Perkin’s violet and other dyes on a large scale.

Types of Fabrics used in Dyeing:

• Silk/wool = proteins, amino acids. Ionic groups, bonding works super well
• Cotton – cellulose, hydroxyl group, polar
• Polyethelyne (CH2) not receptive to dyes

Ways to Dye Material:

1. Aqueous or conventional dying – most important and most widely used method. Process is use of dyestuffs. Treatment of textile material in aqueous water solutions
2. Solution dyeing – this is part of manufactured fibre production. It involves adding micro sized colored pigments to manmade fibre during manufacturing.

A Useful Dye Should Include the Following:

The dye should have an intense colour (how the dye imparts colour to textile materials). Its solubility in water is crucial. This is talking about during the process of dyeing, so that water can carry dye particles towards inner molecular structure of fibre. Another important element is the substantivity to fiber – how the dye molecule will penetrate the inner molecular structure. The dyes durability to wet treatments is another important part. Once the dye is penetrated is inside fibre molecular structure, the kind of chemical bond takes place in between fibre and dye molecules is important. Meaning, the colourfastness and property of dye, STRONG = record durability to further treatment. Like washing. Safe, easy to handle, and reasonably priced.

Why Dyeing With and Without Mordants Produces a Different Colour:

A mordant is basically glue for the dye. It’s a chemical that attaches itself to the molecular bond, between the dye and the fibre. Using a mordant is crucial in achieving the desired colour result when dyeing. If a fabric is dyed without a mordant, the colour will eventually fade with each wash. Soaking the fibre in water with a mordant, will ensure that the colour stays intense even with washing. Without a mordant, the colour will be very dull, and wash out easily. With a mordant, the colour will be intense, and will stay attached to fibre even after washing.

Why Varying Acidity of Dyebaths Affects Final Colour:

Whereas most dyes dye better in alkaline baths, acid dyes bond better in acidic dyebaths. Acidic dyes are highly soluable in water, and they donate their H+ (hydrogen). The solution is acidic so there are lots of positive charges. Acid dyes are used to dye certain protein fibres. This includes feathers, silk and wool. The reason they are called “acid” dyes, is because a very light acid such as lemon juice or vinegar is added to decrease the pH of the dyebath, making it a little acidic, thereby causing the dye to make bonds with the protein fibres. The varying acidity levels, may affect the final colour. In part 1 of the assignment, the pieces of fabric with lemon juice added came out much lighter than those without an acid used.

In conclusion, dyeing is a very useful process, and has made many advances in recent years. The basic process of dyeing is central and it is heavily relied in our everyday lives, without even realizing it.