With the passage of time new researches have led to newer ways to dye that is fast, involves less expenditure, dyeing of blended fibres and conforms to the environmental guidelines. In the pages two of very recent developments have been described here. One is the latest state-of-the art Dyeing with supercritical fluids, second is Union Dyeing and the third is the Pigment Precursor.

Dyeing and the allied industries have often been in the scanner of the environmental watch dogs for large scale pollution of the environment. In fact it has assumed the status of a global problem. Considerable researches has been done in this field to come out with technologies that are environmentally much cleaner. One of the technologies that has evolved is the use of supercritical carbon dioxide as the dye solvent, instead of water.

The diagram above shows an apparatus for dyeing, using supercritical carbon dioxide. The machine consists of the following:

• Temperature controller
• Vessel heater that surrounds the vessel
• Stainless steel dyeing vessel with a built in quick release cap
• Manometer
• Carbon dioxide pump
• Cooler that cools the head of the carbon dioxide pump

The dyeing system using supercritical carbon dioxide is composed of a three-component/three-phase system. The key components are gas, dyestuff and a substrate like fiber polymer. While in the solid state, dyestuff and polymer are present in three separate phases along with the supercritical mixture. The dyestuff is first dissolved in the supercritical fluid, and is then transferred to, the fibre. Which gets subsequently absorbed and diffused into the fiber. The process of supercritical dyeing is shown in the following diagram.

• Contaminated waste water streams are eliminated.
• No need of dispersants for making disperse dyes water soluble.
• Lower viscosities enabling the circulation of dye solutions easier.
• CO₂ causes the polymer fibre to swell slightly that gives a faster diffusion within the polymer.
• Faster penetration of voids between fibres because of no surface tension and the miscibility of CO₂ with air under pressure.
• Higher diffusiveness in the fluid resulting in faster mass transfer in the fluid.
• Supercritical CO₂ is ecologically harmless, non-toxic and nonexplosive.
• No drying process involved so no drying devices required.

Comparison between Conventional Dyeing and Supercritical Dyeing

Dyers have found a new way of dyeing that will make cotton chemically more similar to wool. This will help to dye the fabric blend of the two. In a process, called Union Dyeing, which dyes fibres having different dye affinities and still achieve an uniform appearance and a homogenous colour.



Actually, dyeing a blend of cotton/wool is difficult as the two fibers have a completely different set of chemical makeups. Wool, that is made up of sheep hair, is an animal proteins, while cotton is essentially cellulose, a plant product. What happens is that if wool and cotton are blended together, there is a need for two separate dye baths as it is the wool that takes up most of the dye.

In a new innovation, Union dyeing, a single-bath dyeing procedure was developed. This process goes a long way in helping textile producers to overcome a significant "technological bottleneck"- dyeing of all the blends that has the depth of same shade just in a single step.

How does Union dyeing work
In this new concept of Union Dyeing there is reversal of the chemical charge of cotton. The change is done before dyeing from negative to positive. Now wool is already positive beforehand. This is done by using cationic fixatives, a set of positively charged ions. By using the fixatives before the actual dyeing is able to give both the components of the fibre a positive charge. As the dye is negatively charged. And it is a chemical phenomenon that positive changes attract negative. As a result the cotton and wool is dyed to a uniform shade for the simple reason of the dye being attracted equally to both the fibers. Thus Union-dyeing process makes use of just one dye in a single bath, normally under a single set of conditions.

In the recent years there has been much development in the field of inkjet printings. There has been a new technology that utilizes a method in which a water soluble pigment precursor is able to penetrate the surface of the print medium and then the pigment is released. This particular method is able to combine the properties of a dye based ink like high gloss and resistance to abrasion along with displaying properties of the conventional pigment based ink like superior light stability.

The The following table gives a comparison of the imagesobtained from dyes, conventional pigments and pigment precursor:

A Microscopic view of the comparative images obtained by synthesizing a quinacridone type water soluble pigment precusor, with conventional pigment and dyes are shown in the image here.

As can be seen in the images, though dye images provide features like high gloss and fairly good resistance to surface abrasion, but they suffer from insufficient light stability.



Image obtained for a Conventional Pigment
Pigments provide a high light stability, however the problem lies on the surface of the medium. The average size of the pigment particle is in the range of 100-150 nm. The pigment particle is prevented from penetrating the surface. The results are rough surfaces, limitation in gloss and succeptibility towards abrasion.



Image obtained for a Pigment Precursor
Lastly the pigment precursor deposit the pigments below the surface of the medium. This action give rise to a kind of dual advantage.

• Like Dyes resistance to abrasion and gloss is high.
• Light stability is also higher as a mark of a true pigment.