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