Define Silk Yarns | Manufacturing Process for Silk Yarns

Define Silk Yarns | Manufacturing Process for Silk Yarns

 

Define Silk Yarns | Manufacturing Process for Silk Yarns: Silk, the only natural fiber to be produced with a continuous thread, is obtained from the cocoon of Bombyx mori. An insect that feeds on mulberry leaves. It represents the most noble of textile fibers . It is used to make fine fabrics.

What is silk ?

Silk is a protein fiber of natural origin, produced by some insects . It is used by man to make fine yarns and fabrics. Most of the silk is obtained from the cocoons of the Bombyx mori insect , commonly called bomb ice , filugello , mulberry worm or silkworm .

The larva of this insect processes the drool inside two glands (called seritteri or silk glands). Which open to the outside with two orifices located on the lower sides of the mouth. From these openings emerge the two flaps which, in contact with the air, grow and weld together in a single continuous thread called burr. This union is guaranteed by the presence of a rubbery substance, called sericin, produced by glands located near the exit of the seritteri. By moving the head around its body, as if to form an eight, the burr is worked by the worm to build a protective casing, called the cocoon. Which imprisons and protects it during the butterfly metamorphosis.

Within this cocoon, within 2-3 weeks the silkworm first turns into a chrysalis and then into a butterfly. The latter mechanically opens a way out of the cocoon and frees itself in flight. The moth lives a maximum of one week, during which it mates and lays eggs (300 to 700 eggs per butterfly). The following year, in April-May, coinciding with the budding of the mulberry buds, the eggs hatch giving birth to the young worms. For about a month, the young larvae eagerly feed on the mulberry leaves, going through four molts (or ” skin ” changes ).

After the last moult the fourth generation larvae reach twigs where they weave their own cocoon. And the cycle starts again. The cocoon of the silkworm is formed by a single continuous filament, of variable length from a few hundred meters to three kilometers, wrapped in 20-30 concentric layers. The burr is of a proteinic nature and is mainly composed of two proteins , fibroin (72-76%) and sericin (21-25%); in a minimum percentage (2%) there are also waxy fatty substances, mineral salts and natural pigments.

 

Manufacturing Process for Silk Yarns

The silk threads are obtained directly from the cocoons of the Bombyx mori worm . However, not all burrs can be used.Also it is necessary to remove the rubbery component given by the sericin. Therefore, at the end of the production process, approximately 10 kg of silk is obtained from 10 kg of cocoons.

For a particular breed of Japanese worm, the yield is clearly higher (6kg of cocoons are enough to obtain 1kg of silk). Let’s now see the production phases that lead from the cocoon to the silk threads used by the textile industry.

Harvesting :

it is important that at the time of harvesting the cocoons are still intact (in jargon not flickering); therefore, they must be collected before the chrysalis hatches and the consequent puncture by the butterfly. Stripping : consists of removing the outer layer of the cocoons; this layer of lunaggine, called spelaia, is made up of the first meters of burr used by the worm to hang the cocoon on twigs or artificial structures prepared by silkworm growers. Sorting and sieving : it aims to remove imperfect cocoons and to classify the remaining ones based on size and quality (color, grain, etc.). The best selection of cocoons is called “real”.

 

Stripping :

consists of removing the outer layer of the cocoons; this layer of lunaggine, called spelaia, is made up of the first meters of burr used by the worm to hang the cocoon on twigs or artificial structures prepared by silkworm growers. Sorting and sieving : it aims to remove imperfect cocoons and to classify the remaining ones based on size and quality (color, grain, etc.). The best selection of cocoons is called “real”. Stewing : the chrysalis is killed through a treatment of about an hour with water vapor at 70-90 ° C.

 

Sorting and sieving :

it aims to remove imperfect cocoons and to classify the remaining ones based on size and quality (color, grain, etc.). The best selection of cocoons is called “real”.

 Stewing :

the chrysalis is killed through a treatment of about an hour with water vapor at 70-90 ° C.

Maceration :

dry cocoons are immersed in hot water ; the purpose of this treatment is to soften the gummy protein (sericin) that holds the cocoon together, so that the filament of the intermediate layer (the most useful and precious) can be freed (unraveled).

 

Scopinatura :

in this phase the head of the thread that forms the cocoons is sought in the water. With a special brush, the initial part of the bavelle (the leader) is drawn, eliminating the silk burrs that are on the surface. In this way it will be possible to unravel. That is, the unraveling of the burrs from the cocoons on which they are wrapped.

Weaving :

brings together several threads in a single thread, called raw silk , which is rough due to the presence of sericin. This phase is necessary because the filaments drawn from the water are so thin that they break easily; consequently the burrs of several cocoons (from three to ten depending on the desired diameter) are glued together thanks to the glueing effect of the silky softened by the water.

 

spinning :

it is used to eliminate excess humidity and above all to make the filaments more resistant, through an established number of twists and a fixing phase of the single burrs with damp heat; in this way the filaments are made suitable for weaving. After this treatment, the threads are wrapped around a reel to form the skeins.

 DE gumming :

in this phase the rubbery part of the raw silk, the sericin, is eliminated. Which makes it rough, opaque and not very suitable for dyeing operations. Taking advantage of the solubility of sericin in hot water, washing is carried out. Which first transforms it into raw silk (attenuation of the rubberiness by light washing) and then into softened or half-cooked silk through a more thorough washing.  Finally, by means of a final wash, the cooked silk or DE gummed silk is obtained . The latter product is totally rubber-free, is made up of only fibroin, and is particularly shiny.

 

Charge :

To increase the weight of the silk, reduced by 25% by the DE gumming operations, the silk can be weighed down with special baths in vegetable or mineral salts (tannin, ferric hydrate, metal salts, etc.).This is the so-called charge, which gives shine to the sight, body to the touch and crackle to the hearing. With the charge, in addition to undergoing a considerable increase in weight, silk becomes more resistant to washing and mechanical treatments. And can be dyed with greater ease. However, one should not exaggerate with the caricature, since an excessively loaded silk becomes fragile, loses elasticity and tenacity; as a result, cracks and breaks are easily formed on the fabric at the points of greatest wear.

 

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