Silk Fiber

 

    What is silk fiber?                                     

Silk is the filament spun by the caterpillars of various butter flies. Silk is a natural protein filament. Its filament density is 1.34g/cm³ which make it a medium weight fiber. Very light weight silk textile materials may be manufactured form silk filaments.

 

                                                 Different types of silk

 

      Types of silk:

 

a)     Silk: It refers to cultivate silk.

 

b)     Wild or Tussah Silk: Wild or Tussah silk is a tan-colored fibre from the cultivated silk worm which feeds on sorub oak. As the cocoon are always pierced the fibres are shorter than reeled silk. It is different both physically and chemically from ordinary silk. It is brown in color, considerably stiffer and coarser. It is less reactive towards chemical. It is used in the shantung, pongee fabrics.

 

c)     Thrown silk or greg: Thrown silk consists of two or more threads of raw silk reeled tighter and given a slight twist.

 

d)     Organzine silk: Organzine silk is produced from best cocoons. It contains tow or more strands each composed of a number of greges twisted together slightly. These threads are then doubled and retwisted in the opposite direction to the original twist in the strands (Strands mean a number of flexible strings twisted together into a rope). Organzine is used for warp threads when high tensile strength is required.

 

e)     Tram silk: Tram silk si usually made from cocoons of lower grade, like organzine. It is composed of two or more strands of thrown silk lightly twisted together and then doubled.

 

f)      Chapple silk: When silk which is still in the green is spun, the yarn is known as chapple.

 

                                 The manufacturing steps of silk

 

      The manufacturing steps of silk are:

i)        Sericulture,

ii)      Sorting cocoons,

iii)    Softening the sericin,

iv)    Reeling,

v)      Throwing.

 i)       Sericulture: Sericulture the care and nurture of the silk caterpillar is a tedious painstaking business. The silk worm cultivation is called sericulture. The process starts with the silk moth, which lays eggs on specially prepared paper. The eggs are kept in cold storage until the mulberry frees start to bud. Then they are placed in incubators for a ten days period.

 

The next stage is one of the phenomenal growths. When hatched the silk worm is about one quarter of an inch long. When it ahs reached maximum growth it has increased in length to about 3inches and in weight from 3milligrams to 5grms. It eats only fresh, young, tender, dry mulberry leaves. The worms are fed and placed on clean trays several times a day.

 

When the silk worm is grown it spins a fibre cocoon around itself. The cocoon is spun in an almost continuous session of 24 or 72hours. The caterpillar extrudes from two holes in its hard two strands of fibroin (called brins), which are glued together by the ‘secicin’ selected by adjacent glands. The silk worm winds the thread in figures of eight round its body forming an egg-shaped case the cocoon.

 

Sometimes later a moth emerges from this cocoon, bursting the cocoons. In order to obtain the silk thread undamaged the charysales are destroyed by that before they emerge. The cocoons are beaten in hot water to disentangle the filaments. The filaments of several cocoons from a silk thread which is wound up on a reel.

 

ii)    Sorting cocoons:  The cocoons are shorted according to colour, shape and texture.

 

iii)  Softening the sericin: (What is sericin: when the silk worm is grown it spins a double strand of silk fibres surrounded by water soluble substance is called sericin)

 After the cocoons have been sorted they are put through a series of hot and cold immersions as the sericin must be softened to permit the unwinding of the filament as one continuous thread. Raw silk consists of about 80% fibroin and 20% sericin. At this time only about 1% of sericin is removed, because this silk gum is a needed protection during the further handing of the delicate filament.

 

iv)  Reeling:  The process by which filament is taken up from the cocoon is called reeling. The diameter of the filament is so little that if it is reeled, its commercial value will be decrease. More-over double filament is too delicate to handle alone. So several of these double strands from as many cocoons are joined and wound in a skein. The gum which holds the strands of silk together in the cocoons is softened but not removed before reeling. Only about 16% of the weight of the cocoon is silk fibre. About half of which can be reeled. These fibre vary in length 300 to 1000yds.

 

v)     Throwing: The production of yarn from reeled silk know as throwing consists adding twist or of doubling and further twisting these strands into the desired size. When two or three of silk multifilament are twisted together to form heavier threads, this process is called throwing.

It is an operation of making a twisted yarn from reeled silk or putting additional twist into filament yarns of man made fibres or in some cases combining and twisting two separate yarns into one piled yarn.

 

              Thrown or reeled silk yarn are classified as follows:

 

a)      Singles,                                g) Compensne

b)      Tram silk,                             h) Crepe.

c)      Organzine silk,

d)     Voile,

e)      Georgetle

f)       Two by two (Grenadine)

 

                                  The common diseases of silk worms

 

Silk worms’ disease: The silk worm the particularly susceptible to various disease which become more or less epidemic in character. The principle diseases of the silk worm are as follows:

 

i)       Pebrine: This disease is cased by protozoa parasites worm affected with these disease devel9op slowly, irregularly and very unequally and show black spot on their skin.

 

ii)    Flachesie:  Flachesie is caused by digestive deraugment due to atmospheric conditions and lack of ventilation in rearing rooms.

 

iii)  Grasseric: This disease shows itself by the worms becoming restlen, bleated and yellow in coloru and caused by uneven feeding.

 

iv)  Muscardine: This disease is caused by minute fungus. The body becomes raddish in color and dead often 20hors.

 

v)     Fly paraside: The silk worm is affected by pest (hest). Pest lays eggs on the silk worm.

 

                                             Weighting of silk

        

Silk is rarely used without degumming which causes loss in weight 22-25%. To replace this lose by causing the fibre to absorb and retain suitable organic or inorganic substances. This procedure is knows as weighting of silk.

 

It is a process of adding weight or body to fabric or yarn by addition of various materials either chemically or mechanically. The degummed silk is first immersed in an acid solution of tetrachloride of tin. The fibres are permeated to absorb the salt to the point of saturation. Excess solution is now removed by centrifugal hydro extraction. The material is then thoroughly washed with cold water which hydrolise the tetrachloride of tin into stannic oxide and hydrochloride acid. The insoluble oxide remains precipitated in the fibre while the acid carried off in the wash water. A treatment in a hot solution of disodium phosphate radical to the tin, already present in the silk.

 

 

                                              Degumming

 

The process of which the natural gum or sericin is removed from the raw silk is called degumming.

Degumming is needed in order to make a silk fabric soft and glossy. It is basically a scouring operation. The gum is removed from the finished yarns or fabrics usually by boiling with soap at a temperature 200-205⁰F. Without degumming silk can not absorb dye.

 

                                                Scroop

 

Scroop refer to the cracking sound emitted when the fibre is squeezed or pressed. The scroop of silk does not appear to be an inherent property of silk fibre itself but is acquired when the silk is worked in a bath of dilute acid (acetic or tannic) and dried without washing. This produces the resulting property of silk. Scroop does not change the true quality or real value of silk.

 

Chemical composition of Silk:

 

                             Silk gum or Sericin ® 22-25%

                                    Silk or Fibroin ® 62.5-67%

                                                  Water ® 10-11%

                                                    Salts ® 1-1.5%

 

Fibrion is composed of a number of -amino acids in which the most important are;

                                              Glycine ® 38%

                                              Alanine ® 22%

                                                 Serine ® 15%

                                             Tyrocine ® 9%

Other amino acids with bulky subtituents ® 16%

 

 

Polymer System of Silk:

 

Silk polymer is a linear, fibroin polymer. It is composed of sixteen different amino acid, so the repeating unit of silk polymer is amino acids. The silk polymer is created only by the beta-configuration. It is 140nm long and 0.9nm thick. Functional groups of silk polymer are -COOH, -NH₂, -CONH. Hydrogen bonds are formed by peptide bond and salt linkage by carboxyl and amine groups. Silk is composed by 65-70% crystalline and 30-35% amorphous region.

 

Physical properties of Silk:

 

i)        Specific gravity: 1.34 gm/cm³.

                                   

ii)      Moisture: MR%= 11

                           

iii)    Strength:

a) Tenacity = 3-6gm/dtex.

b) Elongation = 13-20%,

c) Dry = 4.3gm/d,

d) Wet = dry ´ 0.92 gm/d.

 

iv)    Elasticity: Breaking extension = 23.4%, Recovery = 52% at 5%  

                           

v)      Appearance and color: 

               Shinning appearance, slipy fabric, and creamy white, browny color etc. Wild silk shinning as natural.

 

vi)    Effect of light:

                  Changes color by sunshine, silk losses weight by degumming about 20%, compensate by weighing.

 

vii)  Resiliency: Moderate.  

                            

viii)      Absorption resistance: Good.

                                               

ix)    Dimensional stability: Good.                  

                                              

Chemical properties of silk:

 

            Silk is a protein fibre of animal origin. The major composing part of silk is silk gum or sericin and silk or fibroin. The fibroin molecules contain only carbon, hydrogen, nitrogen and oxygen. There is no Sulphur in it.

 

Effect of bleaches:

 

Silk fibrion is attacked by oxidizing agents; bleaches such as H₂O₂, NaOCl, calcium hypo- chlorite Ca(OCl)₂ , KMnO₄, K₂Cr₂O₇, O₃, NaCl . Silk becomes yellowish in sodium hypochlorite (NaOCl) and dissolve. It is less harmed by reducing agents; bleaches such as ZnO, SnCl₂, SO₂, H₂S and FeSO4.

 

Effect of acid:

 

Silk is readily soluble in cold concentrated mineral acids. The solubility in cold concentrated HCl may be made use of to determine the percentage of silk present when it is in a mixture. In moderate concentrated acids cause contraction in silk; this shrinkage is used to bring about crape effect in silk fabrics; called ‘scroop of handle’. Dilute acids do not attack silk under mild condition. Acids are readily absorbed into silk filaments and are not easily removed.

 

Effect of alkalis:

 

Silk is less readily damaged by alkali than is wool. Cold concentrated solutions of caustic alkalis appear to have little effect when contact in short duration and when rinsing follows immediately. More concentrated solution of caustic alkalis will destroy the lustre and causes loss of strength. Silk dissolve in solution of concentration caustic alkalis. More resistance than wool dissolves in hot concentrated solution.

 

Effect of water:

 

             Silk does not dissolve in water and it withstands the effect of boiling water better than wool. Prolonged boiling tends to cause a loss of strength.

 

Effect of inorganic solvent:

 

               Silk dissolves in solution of ZnCl₂, CaCl₂ and alkali thiocyanates and ammoniacal solution of copper or nickel.

 

Effect of organic solvents:

               Silk is insoluble in the dry-cleaning solvents in common use.

 

Effect of sunlight and weather:

 

           The resistance to silk to the environment is not as good as of wool. This power resistance is due mainly to the lack of covalent cross links in the polymer system of silk.

  

Thermal properties of Silk:

 

 Silk is more sensitive to heat than wool. The existing peptide bonds, salts, linkage and hydrogen bonds to the silk polymer system tend to break down once the temperature exceeds 100⁰C.

     

Electrical properties of Silk:

 

Silk is a poor conductor of electricity and tend to acquire a static charge when it is handled. This causes difficulties during manufacture in a dry atmosphere. Silk is valuable for insulating materials.