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Thursday, January 9, 2014

Dyeing silks from the inside out

Suppose you have a textile factory with a huge market for colored silks, but you prefer not to use the hugely polluting methods of chemically dying your fabrics. How can you satisfy your customers? One way might be to create naturally colored silk threads by feeding dyes directly to your silk worms. That’s the strategy that researchers from the CSIR-National Chemical Laboratory and the Central Sericultural Research and Training Institute have been working on.

Bombyx mori silkworms are responsible for creating the silk we use in textiles. These silkworms have a pleasant life surrounded by all the mulberry leaves they can eat… until they are dunked into pots of hot water and killed for their silken cocoons. 

Before being made into clothing, the outer protein coat (sericin, which gives the industry the name ‘sericulture’) of the silk is usually removed in a process called ‘degumming’. The inner fibroin protein is then spun into textiles.

Most silkworm cocoons are white, though there are pink, yellow, green or brown varieties. However, that color is generally lost during early processing (the aforementioned degumming).

To create permanently dyed fabrics, the researchers experimented with feeding their silkworms mulberry leaves that had been coated with a dye solution. Seven different commercially available dyes were used. Despite being chemically similar, not all the dyes were equally effective at changing the silk colors. Only three of them resulted in cocoons that were at all distinguishable from the ones made by silkworms that ate undyed food and only one of the dyes preferentially colored the fibroin proteins, rather than the discarded sericin. The winner was a dye called ‘Direct Acid Fast Red’.

four images showing dye-fed silkworms, their cocoons and their final silk
a) Bombyx mori larvae feeding on mulberry leaves sprayed with Direct Acid Fast Red
b) Silkworm larvae spinning coccoons
c) colored cocoons
d) Direct Acid Fast Red threads after degumming.
ACS Sustainable Chemistry & Engineering DOI: 10.1021/sc400355k.

None of the tested dyes adversely affected the silkworms. They were perfectly happy to munch on the colored leaves and spin their cocoons. Meanwhile, manufacturing the colored silk did not require the huge amounts of water or hazardous chemicals nor produce the toxic wastewater that conventional silk dying entails. 

The researchers are continuing to experiment, hoping to increase the number of natural silk colors they can create. The more, the better for everyone.

Anuya Nisal, Kanika Trivedy, Hasan Mohammad, Suyana Panneri, Sayam Sen Gupta, Ashish Lele, Ramesh Manchala, Nirmal S. Kumar, Mugdha Gadgil, Harish Khandelwal, Snehal More, & R. Seeta Laxman (2013). Uptake of Azo Dyes into Silk Glands for Production of Colored Silk Cocoons Using a Green Feeding Approach ACS Sustainable Chemistry & Engineering DOI: 10.1021/sc400355k.

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