Delustring

Introduction

ð  Rayon fibres are having a bright metallic luster.

ð  The high metallic luster of this fibre is not acceptable by the consumer.

ð  Consumer needs a little luster approximately to that of natural silk.

ð  In order to make the highly lustred rayons appear like natural silk, it is necessary to reduce the luster and that is known as DELUSTRING.

Mechanism

            To delustre the bright metallic lustrous fibre, it is necessary:-

(1)   To scatter the light from the external surface.

(2)   To reflect the scattering light in a diffused manner which penetrate into the interior of the fibre.

Methods

1)      Internal delustring

2)      External delustring

 Internal Delustring

ð  In this method the delustring of rayon is done by adding the delustring agent in the spinning bath during the manufacture of filament itself.

ð  Various substances which have high refractive index (more than 1.5) are added in the spinning solutions of the filaments and then the filaments are extruded.

ð  The filaments are finished before the filament formation itself.

Chemical	Refractive index	Delustring effectiveness
TiO2	2.5	0.5 %
Zinc sulphate	2.37	1 %
Barium	1.637	4 %

Advantages of TiO₂

ð  It has a high refractive index of 2.5

ð  It has very small particle size of 0.75 micron and will not block the spinneret.

External Delustring

ð  The mechanism is precipitating some insoluble substances on the fibre using various delustring agent.

ð  An after treatment to rayon fabrics is carried out which is known as external delustring method.

ð  Here, the refractive index and particle sizes are not important.

ð  External delustring can be done by two methods.

·         One bath method

·         Two bath method

ð  These methods involve external white pigment coating uniformly on the surface of the fabric.

One Bath Method

ð  In this method, matting agents are made in advance and applied in presence of certain additives which brings attachment to the fibre.

ð  China clay, aluminium oxide, zinc oxide, titanium oxide and barium sulphate are widely used.

ð  Among with the above compounds 5 % softener, 15 % of resin may be added.

ð  The fabric becomes unnecessarily harsh and stiff by the use of TiO₂.

Two Bath Method

ð  In this method, the fabric is first impregnated with Barium hydroxide.

ð  Chemicals like stearamide can be added in the bath if higher exhaustion is required.

ð  The impregnated fabric is then impregnated again in a bath containing H₂SO₄.

ð  The H₂SO₄ reacts with Ba(OH)₂ to form BaSO₄ which is insoluble.

Ba(OH)₂  +  H₂SO₄  à BaSO₄  +  2H₂O

Process

                        Pad – Dry – Cure

ð  However, the two bath method is the most effective since the surface deposition will be less. The fabric does not become too harsh and stiff.