Silk

50-65 usd/kg
Circularity potential
Very high
Strength
Very high
Production energy
Extreme
Stiffness
Medium
Embodied CO2
Extreme
Density
Medium

Silk is a natural protein fibre produced by silkworms. Used for millennia, it is prized for its lustrous surface and excellent dyeing characteristics. The filaments, up to 1,500 m long, are fine and strong.

Most of the worlds silk comes from China and India. And almost all of that is from the domesticated Mulberry silkworm (Bombyx mori). Silk production, known as sericulture, has been practiced in China for millennia and as a result, the moths have evolved to be entirely dependent on humans for their lifecycle – they are blind, mostly flightless and have lost their camouflage. There has been a great deal of genetic engineering and many hundreds of strains exist.

Production starts with the Mulberry tree, which provides the leaves that are fed to the worms. One of the largest impacts is the cultivation of the leaves, which requires pesticides and irrigation. The other half of the impacts are from housing he silkworms, boiling (to kill the silkworm) and unwinding of the silk filament from the hardened cocoon. The silk filament is held in place by sericin, a strong protein, which is heated in water to break it down and allow the fibre to be separated.

It takes around 10 kg of cocoons, which is around 2,500, to produce 1 kg of silk. In addition, composting the waste from production can have a big impact on overall CO2 emissions. Therefore, by incinerating the waste for energy recovery, the CO2 emissions can be dramatically reduced. Nevertheless, the negative environmental impacts of silk production are quite extreme.

Tussah silk, also tussore silk, is produced by several species of silkworm that are either domesticated or living wild in the forests (wild silk). Examples include eri silk (Samia ricini) and muga silk (Antheraea assamensis). Whereas eri silk is off-white and looks more like cotton once converted into fabric, muga is natural bright golden coloured.

If the silkworm is allows to complete its metamorphosis and hatch into a moth, then the cocoon is broken. In this case, it may be referred to as peaceful silk, or vegan silk, because the worm and moth are unharmed. The fibre from these cocoons can be turned into staple fibre suitable for textiles, but is not as desirable as continuous filaments from unbroken cocoons. It may be blended with other fibres to impart some of the strength and softness of silk.

Cruelty free alternatives to Mulberry silk include synthetics, such as polyamide (PA), nylon, and regenerated cellulose (viscose, lyocell, acetate), which can be produced with the same lustrous appearance and fibre cross-section. They are much less expensive, more sustainable, and their properties are highly tailorable. Plastic free options include cotton and the bast fibres, such as hemp and flax (linen). However, these materials behave quite differently.


Sustainability concerns
Sourced from animals


Mulberry silk produces a lustrous and very strong filament. It is prized for it tenacity, fineness, drape, colour and shine. It is highly absorbent and this gives it good moisture wicking properties. It can absorb up to around 30% moisture before it starts to feels wet. This same property means it can be dyed vivid and saturated colours.

The individual filaments are 10-13 microns in diameter, which is about half the thickness of sheep wool, and the only natural filament. All other natural fibres are staple, available as short strands that are twisted into yarn. The fibre is stretched as it is secreted from glands on the silkworm’s head. This aligns the natural protein polymer within the filament, maximising strength to weight. As a result, silk can be woven into very fine fabrics, such as sheer chiffon and organza.

It is used in fabric constructions that have evolved alongside, created to take advantage of its unique qualities. For example, velvet pile fabric makes the most of its soft texture and affinity for dyestuffs – Mulberry silk is naturally white and shiny, providing a very good base for saturated colour. Satin weaves, embroidery and similar fabric constructions with long floats (yarns laying on the surface with few overlaps) exploit the full visual effect of silk and allow it to be packed very densely to maximise its bright colour.

The role of the cocoon is to protect the larvae during pupation. As a result, it has evolved some properties desirable for fashion and interior textiles, such as strength, stiffness, resilience, tenacity, thermal regulation and resistance to chemicals. In each cocoon, two triangular filaments are packed side-by-side, glued together with sericin. Once separated, they are combined with several other filaments to produce a more consistent yarn.

Silk is biocompatible and has very good knot strength, as a result of its combination of elasticity and strength. These properties are utilised in non-absorbable sutures and wound dressings. While synthetic alternatives now dominate, and have largely replaced silk, this natural protein fibre is popular in cardiovascular, ophthalmic, and neurological procedures.


Design properties
Cost usd/kg
50-65
Embodied energy MJ/kg
1350-1850
Carbon footprint kgCO2e/kg
52.5-136
Density kg/m3
1300
Tensile modulus GPa
4.5-16
Tensile strength MPa
370-760
Hardness Mohs
1.5
Thermal conductivity W/mK
0.5
Temperature min-max °C
-40 to 150
Thermal
insulator
Electrical
insulator