Silk a natural protein fibre

The origins of silk in textiles trace back to China, where the earliest silk textile dates back to around 3630 BC [1]. During the Han Dynasty, silk weaving emerged as a significant export for China [2]. The Silk Road, spanning through central Asia, facilitated the transportation of Chinese silk to regions such as Syria and Rome. In addition to China, India also boasts a rich history of silk cultivation, with evidence dating back to 2450 BC and 2000 BC [1].

After being limited to China, silk cultivation began to spread to Japan, Persia and Europe [3]. The Industrial Revolution affected the European silk industry as cotton became cheaper [4]. The introduction of new fabrics, such as nylon, in the 20th century, affected the use of silk globally, and nowadays, silk is a luxury item.

Silk is considered a “luxury” fibre with properties unlike any other fibre: a unique natural lustre, high strength and good absorbency. Some artificial fibres, like viscose, initially produced under the name of artificial silk, have sought to replicate the structure of silk fibres.

Silk is a natural protein fibre from the silkworm’s secretions to spin cocoons. The controlled production of silk is called silk farming. The cultivated silk is also called mulberry silk, as these silk larvae feed on mulberry leaves. This silk moth (Bombyx mori) produces the most well-known silk [1]. There is also wild silk whose production is not controlled. The butterflies are of different species and eat leaves other than mulberry leaves [5]. We can find the cocoons of these butterflies in forests, and collecting them is laborious compared to silk cultivation.

Further differences exist between cultivated silk and wild silk regarding the fibres.
Cultivated silk displays a smooth and slightly uneven surface, accompanied by a soft lustre that imparts a smooth fabric appearance. In contrast, wild silk exhibits a larger and rougher surface, resulting in a duller glow and fabrics with a more transparent texture.

Photo: Textile Sphere

In 2018, 75 % of all silk was produced in China and 22 % in India [6]. Of all the fibres produced globally in 2018, silk accounted for only about 0.1 %.

Silk is the only natural fibre that comes naturally in filament length. Silk is a solid, smooth fibre with a simple physical structure, which several artificial fibres try to imitate.

Protein fibres, such as silk, are hygroscopic, capable of absorbing moisture without feeling wet, with silk specifically able to hold up to a third of its weight in moisture [7]. This quality renders silk comfortable in the summer and warm in the winter, akin to wool, as it is a poor conductor of heat. The smooth surface of silk makes it easy to remove dirt, and, in addition to its insulating properties, silk is known for being soft to the touch, moisture-wicking, and breathable, culminating in a comfortable fabric to wear [7].

Silk has a dry feel, high strength, sound absorbency, and a natural lustre and can be draped. It is one of the strongest natural fibres but can lose up to 20% of its strength when wet. Silk is not as elastic as the protein fibre wool; with a slight stretch, silk cannot return to its original length.

Silk has many potential uses. We use it in clothing for everyday and formal occasions, haute couture, lingerie, and much more [1].

In the Swedish language, silk is the name of the fibre, while “siden” is the name of the fabric [10]. Examples of some silk fabrics are satin, habutai, chiffon, shantung and crepe de chine [1]. We also use silk in interior textiles and in technical and medical applications.

Different types of wild silk exist: Tussah silk, Tasar silk, Dupioni silk and Ahimsa silk [5]. Ahimsa silk is also known as Peace silk or cruelty-free silk [5], as the breeding of silk moths and the harvesting of the cocoons follows a peaceful method, where no animals are harmed [9].

Silk can be damaged and yellowed by strong soaps, detergents and high temperatures. It is also weakened and yellowed by sunlight and sweat, and insects can attack the silk. In the longer term, silk can lose its strength if it is not stored correctly and can begin to dissolve if exposed to too much sunlight [7]. Silk is rarely affected by mould.

The production of silk negatively impacts animals, the environment, and humans. In conventional silk farming, most silk caterpillars are killed in their cocoons before they reach the butterfly stage [7]. To extract the silk fibers and produce thread from them, one must boil away the sticky protein binding the cocoons together [10]. When the cocoons are heated, the larvae die. Organizations such as Animal Rights and PETA recommend avoiding silk and choosing other materials that don’t harm animals [11], [12]. A major player that has distanced itself from silk is ASOS, which has added the material to its Animal Welfare Policy, where fur, down, mohair, and cashmere products are already banned [13], [14].

Conventional silk cultivation needs mulberry trees to feed larvae [6]. One mature mulberry tree provides enough leaves to feed 100 larvae. But it takes 3000 cocoons to produce 1 yard of fabric or about 0.9 meters, so we need many trees to produce silk. The trees do not require chemical pesticides or fertilizers, but these may be used anyway. Fertilizing animal feces can lead to eutrophication, which can cause algal blooms and fish kills. Irrigating the trees requires a lot of water and energy. The extraction of fibres is also very energy-intensive, and large volumes of water are needed to unwind the cocoons and clean the fibres. Cleaning can involve toxic chemicals. When we don’t treat wastewater for chemicals, biological waste can be discharged into groundwater, leading to soil degradation and eutrophication.

Regarding social sustainability, the silk industry provides jobs for millions of people, but there is also child labor [15]. In 2003, Human Rights Watch reported that more than 350,000 children were forced to work in slave-like conditions in India’s silk industry, where a work week is 12 hours or more per day, 6-7 days per week [15], [16]. There are also reports of child labor in Uzbekistan [17]. Furthermore, breeding silkworms can cause workers injuries, such as eye infections, back problems, allergies, respiratory problems, and headaches [3].

An alternative to cultivated silk is wild silk, where the larvae can reach the butterfly stage and leave the cocoon [5]. However, the quality of wild silk is lower than conventionally grown silk due to its length, as organic silk is also a more sustainable option. The silk not treated with dyes or dangerous chemicals and mixed with other fibres is biodegradable.

Sources

September 2020, TÄNKOM | Revised May 2024 RETHINK

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