by I.R.Anto, T.Ravindran, P.K.Ravi, M.Kumaraswamy Pillai and U.S.Sarma, Proceedings of the International Workshops

on wet processing of Coir 8-9 December 1997

Bleaching of coir fibre/yarn is essential for improving the colour.  In order to ascertain the different chemicals used for bleaching of textile materials, a literature and industrial survey had been conducted.  The cost aspects of bleaching process adopted in different coir firms were analysed. A comparative study of the bleaching system led to the identification of various chemicals used for textile materials which can be suitably adopted for coir materials. The chemicals marketed by different firms were evaluated for their suitability to impact superior colour to the coir materials.  Studies also aimed to estimate the extent of chemicals in spent liquor of the bleaching process and extent of pollution to the environment.

Bleached coir fibre/yarn is widely used in coir Industry for the purpose of ornamentation of coir products.  There are certain shades of dyestuffs to be used in the coir matting/carpets which require bleaching of yarn to improve its penetration and brightness especially in the case of pastel shades.  It is estimated that about 20% of the coir yarn/fibre used for the manufacture of coir mats, mattings, rugs and carpets are made out of bleached coir yarn in the coir Industry.  Bleaching of coir fibre/yarn is generally being carried out using hydrogen peroxide.

  The objects of the study cover the following

             1.      To standardize the bleaching process in the coir industry

2.      To evolve a low cost bleaching process that can be afforded to all sectors of the industry

3.      To evolve a new bleaching process to improve the light fastness

The work of the earlier researchers in the bleaching systems of textile materials revealed that mixture of soap, turpentine oil, ammonia and pine oil could be used for bleaching linen  ⁽¹⁾ Brandy ⁽²⁾ has measured the rate of bleaching action of hypo chlorite and peroxide liquors.  Chemicals such as sodium peroxide, sodium per borate, hydrogen peroxide incorporating the chemicals like Citric acid, soap, sodium carbonate and sodium silicate have been used as bleaching agents for clothes ^(3-5)   

The bleaching with foam technique has been reported by Faber etal ⁽⁶⁾  Bleaching of threads by  treatment in bath containing aluminum sulphate and sodium bisulphate/meta bisulphate followed by bleaching with chlorine ⁽⁷⁾ are also reported.  The use of orthophosphoric acid ⁽⁸⁾ in bleaching and ozone ⁽⁹⁾ as a bleaching agent by continuously circulating a very dilute mixture of ozone in air through slightly wetted and acidified goods have also been reported.

Various bleaching reagents have been reported ^(10-11) for bleaching of wood pulp for making paper.

It is reported ^(12-29) that bleaching of textile material can be effected by the abstraction of the coloring matter either by oxidation or reduction under appropriate conditions. The bleaching system include hypo chlorites, chlorites, peroxides, peracids and the reducing agents like sulphites bisulphites, borohydrides, etc.  The quality of water used in bleaching plays a very important role and useful to incorporate water softeners like calgon, zeolites, ETDA etc. in the bleaching recipes.

It is revealed from the earlier reports that very often the products obtained by reduction method acquire part of the original colour on being  exposed to the atmospheric air for some time which is perhaps due to re oxidation of reduced functional groups present in fibres, The products from oxidation bleaching method turn yellow on storage, particularly in presence of high humidity and sun light.  This phenomenon is common to all lignin from the interior of the fibres to its surface and its subsequent oxidation.

  Industrial Survey

An industrial survey was conducted to collect the details of chemicals and recipes being used in coir industry. There are about 4500 small scale coir units in the coir industry engaged in the production of coir products to cater to this export oriented production sector.  Besides the major exporters, there are a few common facility centres set up in the industry to extend service facilities in bleaching and dyeing.

From the industrial survey it is ascertained that the oxidation process using hydrogen peroxide is in vogue in the coir industry. There are two process followed in the industry viz. Hot and cold processes.  The recipe adopted by different coir industrial units are detailed in Annexure-I

It is also observed that none of the factories are using imported chemicals for bleaching of coir materials.  Generally the chemicals used for bleaching are hydrogen peroxide and sodium silicate.  However, there are wide variations in the recipe of different factories as well as the degree of whiteness.  The costing is therefore based on the standard recipe of Central Coir Research Institute and the recipe collected from the major factories & Societies and furnished in Table I

Basis                          :100kg.of coir yarn/fibre

Material : Liquor ratio: 1:12 for coir yarn

                                    1:20 for coir fibre

Materials: Details of the materials used listed in Annexure II

EXPERIMENTAL AND COMMENTS

1.Bleaching with different recipes including trisodium phosphate in place of soda ash

a) Coir yarn

The soda ash is replaced by trisodium phosphate for stabilizing pH and making hydrogen peroxide more effective.  The ml ratio followed is 1:10 and 1:12 for coir yarn and coir fibre respectively with a treatment time of 16 hours at a room temp. for cold process and 1 hour at boiling temp for hot bleaching process.  The following recipes were used noting the pH of the bleaching solution and spent liquor

 Ingredients

b) Coir Fibre

The retted coir fibre was bleached with receipe A,B,C,D & F under cold and hot process keeping the same conditions as adopted for coir yarn with a difference in m:l ratio of 1:12.  The pH of the bleaching solution noted before and after treatment is detailed below as Table III

On examination of the products, it was seen that the vycome yarn fared better on bleaching with Recipe A than Recipe B whereas it is the reverse for Anjengo coir yarn compared to the control.  The brightness of the Vycome and Anjengo coir yarn bleached with recipe A & B is better than that obtained with conventional bleaching process C to E.  The coir yarn bleached with receipe C has better colour than that of coir yarn bleached with recipe E & E.  It is also observed that the coir yarn bleached with receipe E under cold condition possessed brightness inferior to that obtained with receipe C & D containing soda ash and Lissapol D as the pH of the bleaching solution of recipe E is well below 8.0.  The bleaching of coir could be effective only if the pH of the bleaching solution is 10 or more.

  On examination with the bleached fibres it was observed that the brightness of the fibre increased with the receipe A & B compared to the other recipes.

 2.Bleaching with celidon-L1

The bleaching was conducted on coir fibre replacing soda ash, the conventional stabilizer (pH increasing additive) by CELIDON –L1 made available by an overseas firm viz.Giovanni Bozzeto. The following recipes were used, keeping a ml ratio of 1:20

All the receipes were used under cold conditions except A which is conducted under hot conditions in specific concentrations as recommended by the supplier of Celidon L1.

  On examination of all the treated specimens it was observed that the sample treated by reeipe. A showed little improvement in colour whereas that treated with receipe C showed bright yellow colour and D slight yellowish tint respectively which was removed by subsequent treatment with 1% alum solution for half an hour.  Even though receipe D did not contain caustic soda, it also had slight yellowish tint.

  Bleaching of softened coir fibre/yarn

  Vycome coir yarn softened with NaOH solution @50g/l,  100g/l,  150g/l  and 200g/l for 30 minute in the room temperature maintaining a ml ratio of 1:10. It is then subjected to bleaching process of the following compositions.

  The pH of the bleaching solution before and after the treatment were noted.  It is thereafter subjected to measurement of breaking load and elongation at break.  The results are furnished in the Table IV below.  

On examination of the bleached products, it was seen that the bleaching occurs only if the pH of the bleaching solution is higher than 10  or above. This is particularly evident with recipe F in which no pH increasing additives such as soda ash or trisodium phosphate were added and the colour is dull compared to other recipes. The bleached and softened coir yarn with NaOH solution has yellowish tint than that of bleached and not softened coir yarn

Bleaching with Bleaching powder and Hydrogen peroxide

Since the cost of hydrogen peroxide is increased considerably a recipe was formulated in which the coir is pretreated with bleaching powder solution, washed and then bleached with lesser quantity hydrogen peroxide.

Bleaching powder was suspended in water and kept for 6-8 hours.  The clear supernatant liquid was decanted and made up to the required volume for coir fibre/yarn, for maintaining fixed percentage of available chlorine.  The coir is immersed for 15 minutes.  The fibre is removed, washed and bleached with hydrogen peroxide in the cold process @ cc/l.

  The bleaching process was also conducted on vycome coir yarn after pretreatment with bleaching powder solution (5% available chlorine). A solution was prepared by suspending bleaching powder (120-140 g/l) and kept for 12 hours.  The clear solution was decanted and coir yarn was treated for 30 minutes, keeping the ml ratio of 1:10.  Thereafter it is washed and bleached with hydrogen peroxide in the cold process.  The process was repeated with the recipe containing trisodium phosphate which is used in place of soda ash.  The recipes used for bleaching as follows.

It was observed that the brightness of both the processes was similar. The cost aspect of both the processes were analyzed and it is found that the process containing trisodium phosphate is costing more by Rs.2/- per kg.  The brightness indeed which is most important property in the bleached materials, could not be studied.

Since the hydrogen peroxide is the major factor on the colour of the bleached coir materials, the quality of hydrogen peroxide supplied by different companies were evaluated and selected.  On observation of the bleached coir fibre/yarn, it is inferred that the quantity of hydrogen peroxide marketed by HOCL required for bleaching is one and half times higher than the peroxide of the other two parties due to the low strength of H2O2 marketed by HOCL. The material bleached by H2O2 of HOCL showed superior colour than that obtained with H2O2.

Estimation of H2O2 in spent liquor of bleaching process

The extent of the pollution of the environment due to the disposal of bleaching effluents was assessed by analysing the pent liquor after bleaching using standard receipe.  The spent liquor contained about 30% of the hydrogen peroxide used for bleaching

1.The bleaching with hydrogen peroxide is effective only if the pH is 10 or above

2.A pH stabilizing buffer is necessary to get good bleached materials

3.The cost for bleaching is less for a pretreatment with bleaching powder solution 

   followed by hydrogen peroxide bleaching

4.The softened coir materials requires more hydrogen peroxide for bleaching.

5.The hydrogen peroxide marketed hy HOCL give superior colour to coir materials

   than the hydrogen peroxide of other parties.

6.It is desirable to use hydrogen peroxide of concentration 35% (w/v) for bleaching of

   coir materials

7.Bleaching is advantageous to produce pastel shades of good brightness

8.The hydrogen peroxide content is about 30% in the spent liquor of bleaching.

The authors gratefully acknowledge the financial support given by FAO for conducting the studies.  Thanks are also due to the Coir Board for permitting to publish this paper

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