Color Fastness to Crocking or Rubbing
Reactive dyes being water soluble it is difficult to achieve the same level of wet rubbing ratings as compared to insoluble dyestuff and also dry rubbing. With water soluble dyes, apart from the bleeding of the dyestuff from the rubbed area, finely distributed substrate particles also rub off on the surface of crockmeter cloth.
The problem is increasing with higher depths and fabrics like towels, corduroy etc. with rough surface. While evaluating the rating it is necessary to 9gnore such rub-off particles appearing on the crockmeter cloth and consider only the staining of the dye on the rubbed cloth.
Instrument used for checking is the standard crockmeter. However, test is quite sensitive and for getting consistent result, it is necessary to use standard crockmeter cloth, maintain uniform pressure for applying rubbing strokes and number of strokes. Besides, for wet rubbing, % moisture on the crock-cloth has to be kept to uniform level. For ISO-105 x 12 test method, rubbing cloth that has been wetted with water, has to be squeezed to contain its own weight of water. For AATCC 116-1995 methods, wet pick up is to be maintained between 65 ± 5% by squeezing the wet crockmeter cloth using a AATCC blotting paper. Any variation in the moisture content can lead to deviation in the rating. With high amount of moisture i.e.., wet pick up, ratings will be lower. Degree of staining is visually assessed using Grey scale for change of colour with grade of 1-5 where rating of 5 signify negligible change and 1 maximum change.
In order to get maximum achievable wet rubbing rating, with reactive dyes, it is absolutely necessary to remove all unfixed hydrolyzed dyes by proper soaping/washing of the sample before evaluating the ratings. Extraction with pyridine can be done to check the removal of hydrolyzed dyestuff.
1. Effect of depth of shade/selection of Dyes:
For reactive dyes with high solubility and good washing fastness properties, rating will be relatively higher. However, in deep shades, even with dyes with good washing fastness, a fastness rating of 2-3 on the grey scale is achievable and is considered satisfactory and acceptable. The liquid introduced with the wet crocking cloth results in all cases in a deterioration in rub fastness of up to 2 points in comparison to dry rubbing.
2. Effect of Mercerising
The colour transfer is relatively less, for the mercerised cotton and the rubbing fastness grade is higher. Due to change in the fiber structure on mercerising, there is a 30% less extent of removal of fiber particles, during wet rubbing and lesser amount of colour on the fibre for the same visual depth of shade.
3. Effect of finishing treatments
Different types of finishing treatment viz. softness, polysiloxanes, Zr-compounds, fluorocarbon, chitosan and cellulase enzymes etc. applied to dyes, fabrics do not show improvements in wet rubbing ratings. With some of the cross linking agents, rubbing fastness grade is lowered by 1/2 to 1 unit. In one of the recent study it is claimed that for reactive dyed blacks and bordeaux materials, with polyacrylate finishes there is some improvement in the wet rubbing fastness rating.