Factors Affecting the Rubbing Fastness of Textile Materials

 Intertek India Pvt. Ltd, Bangalore, India
Email: senthilkumar163@gmail.com

All the dyes are not suitable for all kinds of fabric. A dye must have the property of fastness. Among all fastness issues, the most encountered one is undoubtedly rubbing fastness. Rubbing fastness depends on factors like material characteristics, Selection of dyes/depth of shade, Dyeing \ printing procedure .In this article the factors affecting the rubbing fastness of direct, reactive, indigo dyed materials & pigment printed materials are studied and the steps to improve rubbing fastness are discussed.

In dyed and printed textile materials the unfixed dye particles are mechanically held on the surface and these particles are rubbed off easily on the wearer skin or any other cloth of contact. so it is necessary to determine the rubbing fastness of dyed or printed textile materials. Rubbing is the transference of colour from coloured textile material to other surfaces principally by rubbing. The rubbing fastness can be determined by using an instrument called crockmeter and it works on the principle of abrasion. when the crockmeter finger with test fabric slides over the specimen due to friction there is colour transfer from the coloured specimen to the white crocking cloth. Due to the abrasion the deep dyed fibres break into micro fibrils and stick onto the crocking cloth permanently. Rubbing can occur under dry and wet conditions.

In dry rubbing, initially the periphery of the coloured specimen get ruptured, so that the loosely or unfixed dye particles are removed and adhered to the surface fibres of the crocking cloth. Initially the concentration of the dye stuff is more, so more staining occurs and there are more surface fibres in the crocking cloth that can hold dye or pigment particles. As the rubbing cycle continues due to abrasion the surface fibres are removed as deep dyed micro fibrils in dark shade.

In wet rubbing, unfixed dyes will dissolve in water and then it is transferred to the test fabric, hence resulting in poor wet rubbing fastness. In wet crocking both colour and the coloured short fibres are transferred to the crocking cloth. In the standard crock meter about 9N of load is applied with a finger diameter of 16 mm which slides over the track length of 104mm which slides about 10 cycles per 10 sec.

1. Material Characteristics :
The characteristics of the textile materials like the quality of fibre or yarn and the fabric surface structure play an important role in deciding the rubbing fastness .

1.1 The quality of fiber or yarn:
Cotton is the most widely used textile fibres in the world accounting for more than 50% of total consumption. There should be proper selection of cotton fibre to ensure good rubbing fastness.When immature and dead cotton fibres are used, during spinning it undergoes abrasion with metallic parts and the immature fibres may breaks into short fibres. Further in dyeing the dyes will not be properly fixed to the immature and dead fibres. So, mature cotton fibres can be chosen to enhance the rubbing fastness.

The carded yarn have more short fibres, during wet rubbing both colour and coloured short fibres are transferred to the crocking cloth. For this reason we can choose combed yarn where the short fibre content is less.The open end spinning yarns like rotor yarn have more short fibres so we can go for ring \compact yarn.

It has been found that the fabric made out of mercerised cotton shows improved fastness to rubbing. Due to the change in the fibre structure on mercerising , there is a 30% less extent of removal of fiber particles.

These protruding fibres dotted over the surface can be removed by bio polishing. Cellulase enzymes are used to degrade the raised fibre or yarn. When these enzymes are applied to the fabric, they partially digest the protruding fibres and loosened from the fabric. The loosened fibre or yarn is then removed by high speed mechanical agitation.

1.2 Surface characteristics:
Surface structure of the textile materials influences the rubbing fastness to an extent. when the crocking finger slides over the specimen there will be a contact area between the two surfaces (nominal contact area).However all the points of the crocking cloth wont touch the specimen, the contact takes place at the tip of the junction (true contact area).The nominal contact area is same for all kinds of fabrics but the true contact area varies from fabric to fabric. If a dyed cotton fabric is used as a specimen there will be surface fibres in between the warp or weft yarns. so the true contact area is more, therefore there is a possibility of more colour transfer from the specimen to the crocking cloth. For synthetic fibres the amount of surface fibres is less comparatively with cotton fabrics. If the surface is rough the abrasion is more and there is a possibility of poor rubbing fastness. so smooth surface gives good rubbing fastness.
2. Selection of dyes/Depth of shade:
The selection of dye is an essential factor to ensure excellent rubbing fastness. The selection of dye should be done on the basis of the affinity of the dye molecules to the fibre molecules. For cellulosic fibres the dyes of long planar molecules is selected with one or more solubilising groups. For synthetic polymeric fibre, the dye is chosen based on the ionic groups in the fibre molecules. The polyamide fibres are cationic in nature, so dyes of anionic nature are suitable. Where as acrylics are anionic in nature, so the cationic dyes are suitable. For cellulose acetate and polyester fibres the dyes of one or more polar groups must be present. Usually manmade fibres has good affinity to dyes having short alkyl chains.

Solubility is essential for a dye to penetrate into the intermolecular micropores of the fibre. The size of the dye molecules should be smaller than the pores of the fibre molecule so that it enter in to interior of the fibres. Otherwise it will remains on the surface of fibre and get removed while on rubbing. The solubility has also an adverse effect that it causes staining while wet crocking.

The depth of shade determines the ratings of rubbing fastness of the textile materials. The depth is high due to the more amount of dye deposition (or) due to the strength of the optical effect produced by the dye. Due to this reason the darker shade will crock more than the lighter shade.
3. Dyeing/printing procedure (or) fixation of dye:
The incorrect dyeing procedure and improper fixation is the major cause for poor crocking.

Direct dyes:
The direct dyes are mainly used to dye cellulosic fibres such as cotton, viscose & acetate rayon. The direct dye molecules contain groups like amino groups get attached to the polymeric molecules at the hydroxyl groups of cellulose. The size of the dye molecule is such that it can enter the amorphous region of cellulose and it cannot enter in to the crystalline region. When a cellulosic fibre is dyed with a direct dye for a short time the dye molecules remains mainly on the surface, with slight diffusion so it can be easily removed by crocking.

For the direct dyes to have affinity to fibre, the dye molecule should be linear and the benzene ring should be coplanar and the dye molecule should have hydrogen bond forming groups such as amino, hydroxyl (or) azo groups.

In direct dyeing the temperature of dyeing determines the amount of the dye taken. At low temperature slow dyeing takes place, when the temperature

increases the dye molecules rushes in to the fibre. For even dyeing the dyeing process should be started at low temperature and the temperature is raised gradually to necessary temperature. So uniform shade occurs.

Reactive dyes:
Reactive dyeing is the most important dyeing method for the coloration of cellulosic fibres. During dyeing the H atom in the cellulose molecule combines with a Cl atom of the dye molecule and the covalent bond formation occur. The dyes not only react with the cellulose but also react with hydroxyl ions present in the dye bath and causes dye hydrolysis.

After dyeing there will be residual unfixed dyes present over the surface. These residual unfixed dyes and the other dye complexes sticking to the surface can be removed by suitable after cleaning process. By soaping/washing process these unfixed dyes are removed to an extent. During soaping process the unfixed dyes are removed from the surface and it aggregates together. The aggregated dye molecules may redeposit on to the surface. It can be prevented by using suitable soaping agent with anti re-deposition property.

When these soaping agents are used, the removed unfixed dyes are trapped by the soaping molecule and therefore no aggregation occurs. so the chance of re- deposition get reduced.

Even after soaping there will be small amount of unfixed dyes. Owing to the solubility of reactive dyes the unfixed and the surface dyes will dissolve in water during wet rubbing and get transferred to the crocking cloth. To reduce the solubility of unfixed dyes the fixing agents are used.

For anionic reactive dyes cationic fixing agents are used. These fixing agents will neutralise the permanent solubilising group of the unfixed dyes and make them insoluble.

Dyestuffs with only one functional group sometimes have a low degree of fixation. So bifunctional dye stuff like dichloro triazine can be used for better fixation. Addition type reactive dyes have good substantivity for fibres, so dyes like vinyl sulhone type can be used. Due to good fixation and exhaustion properties , Substitution type reactive dyes (mono and dichloro triazine) can be used .Reactive dyestuffs having two different reactive groups (one mono chloro triazine & one vinyl sulphone ) can be used to improve degree of fixation.

Indigo dyes:
Indigo is one of the earliest dyestuff used for about thousand years. Indigo dyes are insoluble in nature, so it reduced to leuko form to assist dyeing and then oxidised to make it insoluble. The reduced form of indigo is blue in colour. Indigo dyeing is carried out in continuous ranges either in sheet or rope form, where the the dye stuff is not allowed to diffuse into the fibre. Due to the presence of more amount of surface dye, Indigo dyed denims shows poor fastness to crocking.

Due to ineffective desizing, during processing the starch film breaks and the cracking of starch occurs. Due to the abrasion, of dyes present at these places are discharged into the processing bath. The discharged indigo dye may redeposit over the surface .This process is called back staining. Because of back staining the re- deposited indigo dye will not be fixed to the surface. So it leads to poor crocking. The back staining can be controlled by
  • The ph of 6.5 to 7.5 should be maintained during processing .
  • Lengthing the rinsing time will reduce the back staining.
  • Dispersing agents can be used to minimize the back staining. Optimum dosage of 0.5 to 2 gpl of fatty alcohols can be used.
  • After treatment with protease based detergents can control back staining.
  • The finishing recipes are available to improve the rubbing fastness of indigo dyed denim.
To improve the dry rubbing fastness, the silicone softeners are used. When the macro emulsions of silicone softeners are used, these macro emulsions of silicone molecules get deposited on the surface and provide surface softness. The finishing layer will prevent the adhesion of the two rubbing surfaces (crocking cloth and the specimen)

Printing procedure:
Pigment printed goods often suffer from colour fastness problems. This is due to lack of affinity of pigment towards the fibre. Usually pigments are insoluble in water and it is applied to the textile materials with the help of the binding adhesive. The application is of simple process pad-dry-cure. After printing and drying the printed fabric should be cured at elevated temperature. The rubbing fastness mainly depends on the curing of the binder. The cured binder act as a film for protecting film. The curing is as follows
  • At 130º for 7-8 min -–cross linking will not be completed at this time
  • At 150º for 4-5 min—standard curing temperature
  • At 170º for 1.5-2.5min—complete evaporation of emulsifier occurs
Rubbing fastness improvers are used to ensure excellent rubbing fastness of pigment printed goods.

Apart from the above discussed factors the testing parameters influence to a least extent. In the testing of rubbing fastness of textile matierals the sample is holded by sample holder to avoid slippage. Based on the condition the sample is holded (tight or slack) the dye migration from the sample to the crocking cloth varies. when it is holded at slack condition even at applied load the crimp will contribute to the true contact area. It require some frictional force to shear the asperities (yarn crown) because of this there will be liberation or removal of dye particles. At the tight or stretched condition the height of the crimp get decreased due to the tension in the yarns and the crimps get compressed towards the fabric surface. so the number of asperities get increased therefore there is a increase in the true contact area, the frictional force required to shear the asperities is more, there is a chance of liberation of dye particles is more .For the same sample the migration of colour to the crocking cloth varies by the condition the sample is holded.

From the above discussion it can be concluded that rubbing fastness depends on factors like material characteristics, Selection of dyes/depth of shade, Dyeing \ printing procedure .It is clear that the selected sample should contain less short fibres , by proper soaping and by good fixation ,we can improve the rubbing fastness of textile matierals. However the rubbing fastness of printed & dyed goods can be improved by carefully handling the above factors.

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