Wool and silk scouring

Wool and silk scouring

1. SCOURING & BLEACHING WOOL

2. SCOURING Scouring of wool differs from cotton in the following respects Raw wool contains anything from 30% to 60% of wool grease, compared with 0.5% of oil and wax in cotton Wool is very rapidly degraded by alkali So the alkali that must be applied should be mild and at temperature well below boiling Sodium hydroxide is never used Mild alkali like sodium carbonate or ammonium carbonate or ammonia is used

3. Raw wool is scoured by counter – current method The machine contains generally four to five bowls containing scouring liquor and water The wool fibres are moved continuously from the 1 st bowl to the last bowl Scouring is done in the 1 st three bowls and washing is done in the 4 th bowl Soap and alkali are added in the 1 st component and detergent in the 2 nd and 3 rd bowls

4. Soap (%) in solution Alkali (Sodium carbonate %) Temperature ( C) First Bowl 0.8 0.2 50 Second Bowl 0.4 - 45-48 Third Bowl 0.37 - 42-45 Fourth Bowl Water only - 40 The pH should never be allowed to rise above 10 and the temperature not above 60 C

5. BLEACHING Bleaching using reducing agents The bleaching action of reducing agents used for bleaching is due to the action of bi-sulphite ions (HSO 3 - ) In practice wool can be bleached either exposing the wool to Sodium bi-sulphite (NaHSO 3 ) , or Sulphur dioxide (SO 2 ) The use of reducing agents used for bleaching wool is limited The whiteness is not permanent due to aerial oxidation

6. Bleaching wool with hydrogen peroxide The whiteness produced by hydrogen peroxide is permanent The pH and temperature is to be controlled, otherwise degradation of wool fibre will take place The pH is kept between 8 – 8.5 and the temperature should not exceed 60 C A mixture of sodium hydroxide and sodium carbonate is used to maintain the pH Wool hydrogen peroxide bleaching also requires stabilizer to control the decomposition of peroxide. Sodium silicate is used as stabilizer

7. Method of Bleaching wool Preparation of bath in stainless steel with 2 volume sol. of H 2 O 2 + 0.5% sodium silicate at 50 C The scoured material is entered in the bath and worked out for 20 – 30 minutes Cooling the bath for 3 – 4 hours Material is taken out and rinsed in water Soured with dilute acetic acid Rinsing with water

8. Degumming & Bleaching SILK

9. Raw silk is composed of ‘fibroins’ connected with a gum like protein known as ‘sericin’ The presence of sericin gives the silk a harsh feeling Therefore, the sericin is to be removed from raw silk to increase the lustre and softness of the silk filaments Again, since the silk filament contain some natural colouring material, hence, it requires removal to get white silk filaments

10. DEGUMMING Silk gum or sericin is removed The process is commonly known as “boiling off’ or ‘degumming’ Degumming is done by using hot soap solution (mild alkali can also be used) Degumming is carried out by treating the silk for 2 hours, or more if necessary at 95 C with liquor ratio of 1:30

11. The solubility of sericin depends upon the alkalinity produced by hydrolysis Silk can also be degummed with a mixture of sodium carbonate and sodium bi-carbonate at pH 10 (this process however is not so efficient as soap) Recipe: Soap solution – 0.5% to 0.75% Temperature – 95 C Time – 2 hours M:L ratio – 1:30 ** It is to be noted that during degumming, vigorous mechanical stresses are to be avoided or else rupture of fibroin filament will take place

12. Bleaching Oxidative or reductive bleaching can be used for silk Bleaching generally follows the degumming For a full-white optical whiteners can be added to the subsequent reducing bleach bath after oxidative pre-bleaching Hydrogen peroxide bleaching is most preferable

13. SCOURING & BLEACHING MANMADE FIBRES

14. SCOURING Manmade fibres are comparatively free from impurities So, milder methods of scouring is sufficient Rayon Scouring is done at 70 – 90 C Scoured with soap and detergent Recipe: Soda ash: 1 – 2 g/L Detergent: 1 – 2 g/L Temperature: 70 C Time: 30 min

15. Cellulose Acetate Chance of hydrolysis if the temperature is too high and if the scouring liquor is more alkaline Only soap is used for scouring (slightly alkaline) Recipe: Detergent: 1 g/L Temperature 80 C Time 30 min Polyester Same recipe as that of rayon

16. BLEACHING Polyester Normally bleaching is not required for the man-made fibres Bleaching is done in exceptional cases Best bleaching agent is NaClO 2 Recipe: Sodium chlorite 1 – 5 g/L Sodium nitrate 1 – 3 g/L Formic acid 1 – 2 g/L Temp At boil (≈ 100 C) Time 60 – 90 min

17. Polyamide (Nylon) bleaching Nylon fabric is rarely bleached If at all bleaching is required, a mild sodium chlorite plus formic acid/acetic acid (for pH 4) bleaching treatment is given at 70 C Recipe: Sodium chlorite 1 g/L acetic acid / formic acid 2 g/L Temperature Boil Time 30 minutes

18. Acrylic Bleaching Does not require bleaching normally For extra white, it requires bleaching Sodium chlorite is used Recipe: Sodium chlorite 2 – 3 g/L Sodium nitrate 2 – 5 g/L pH (Formic acid) 4 Temperature 85 C Time 30 – 45 minutes