Green process cuts water use, pollution in textile industry

Liquid, supercritical CO2 is used instead of water; sucrose octaacetate for starch

Both carbon dioxide and the sizing agent can be recycled endlessly, says P. Raveendran (centre).

A completely green method developed by researchers from the University of Calicut, Kerala, can potentially do away with using water for sizing and desizing cotton and polyster yarn. Textile industry is highly water-intensive and also one of the biggest water polluters.

By using liquid and supercritical carbon dioxide instead of water, and sucrose octaacetate in place of starch, team of researchers led by Dr. Poovathinthodiyil Raveendran from the University’s Department of Chemistry has made the sizing and desizing process eco-friendly. The results of the study were published in the journal ACS Sustainable Chemistry & Engineering.

Before the yarn is woven into fabric, it is coated with sizing agents to strengthen the yarn (to decrease breakages on the loom) and protect it from damage and reduce friction. Sizing also removes or smoothens the projecting microfibres that might interfere with the weaving process.

Traditionally, starch mixed in water is used for the sizing process, and this requires plenty of water. The used water is disposed of, leading to water pollution. “So we looked at changing the sizing process from a wet to a completely dry process,” says Dr. Raveendran. The researchers used liquid carbon dioxide as solvent and tested three agents that easily dissolve in carbon dioxide for sizing both cotton and polyester yarn.

Best agent

“Of the three agents tested, we found sucrose octaacetate produced the best results,” says Dr. Raveendran. The yarn had a smooth, glassy coating on the surface and the strength of the yarn (cotton and polyester) nearly doubled and the yarn exhibited improved mechanical properties essential for weaving. All the microfibres that protrude from the yarn were aligned and smoothened. The abrasion resistance also increased upon sizing.

The yarn after sizing has to be dried when water is used, making the entire process energy-intensive. But no drying is needed when liquid carbon dioxide is used as it is an inherently dry process. When the pressure of carbon dioxide is reduced to gas phase pressure, the carbon dioxide changes its state from a liquid to gas leaving the yarn dry. “The yarn becomes dry instantaneously,” he says.

Once the weaving is completed, the sizing agent has to be completely removed from the yarn as it might resist dyes and chemicals commonly used in textile processing. In the conventional desizing process, large volume of water is used for desizing or washing the fabric to remove the sizing agent from the yarn, which generates lots of waste water.

Instead of water, the researchers used supercritical carbon dioxide for desizing. “While both liquid and supercritical carbon dioxide have lower viscosity and surface tension compared with water, the molecular diffusion of supercritical carbon dioxide is 10 times more than liquid carbon dioxide,” says Dr. Raveendran. “The more the molecular diffusion, the faster will be the movement of molecules in the fluid and this determines the efficiency of cleaning.” The sizing agent dissolves in the supercritical carbon dioxide when it comes in contact with it.

As in the case of sizing, the yarn (in the fabric) becomes dry almost instantaneously when the pressure of carbon dioxide is reduced to gas phase pressure after desizing. And the sizing agent separates out from the yarn and settles at the bottom.

“The best part of this process is that it is zero-pollution, zero-waste as both carbon dioxide and the sizing agent (sucrose octaacetate), which is modified cane sugar, can be recycled endlessly,” says Dr. Raveendran. The researchers are next planning to scale up the process and are looking at setting up a pilot plant to test the green process.