The Light Fastness of Dyed Fibres— A Statistical Study
Authors: GILES C. H. 
Published: July 2010

About 16,000 published commercial light‐fastness assessments have been examined statistically, and from the results the following general rules are formulated, which apply to most dyes and fibres—

On different fibres the fastness of a dye is higher the higher the regain of the fibre, provided that this is greater than about 4% (i.e. the value for nylon). Thus dyes are usually faster to light on viscose rayon than on cotton, on cotton than on nylon, on wool than on silk, and on secondary cellulose acetate than on cellulose triacetate or nylon
The above rule is reversed with fibres having regains below about 4%, i.e. with the newer synthetic fibres: with these the fastness is usually higher on the fibre of lower regain.
Fastness is often reduced by (i) solid foreign materials in the fibre, e.g. dulling agents or the resinused in crease‐resist finishes; or (ii) either cationic surface‐active agents or powerfully hydrogen‐bonding compounds, e.g. phenols or urea, used as dyeing assistants or aftertreating agents.
The increase in light fastness with increase in depth of any dye is usually greater the higher the regain of the fibre: it tends to be higher with dyes which are insolubilised in the fibre than with water‐soluble ones. The increase in average fastness for a whole class of dyes for a tenfold increase in depth is usually about 1½ units for water‐soluble, disperse, and vat dyes, and about 3 units for azoic and sulphur dyes.

The effects summarised by these general principles are considered to be the result mainly of differences in the physical form of the dyes in the fibres, chemical action being important only in a few special cases, e.g. the effect of titanium dioxide delustrant.There is, however, a more general chemical influence which affects comparisons between certain fibres. This is the difference between the chemical nature of fading on protein and that on non‐protein fibres—reduction and oxidation respectively— which has some effect on the comparative fastness of a given dye on fibres of the two classes, mainly at the upper and lower ends of the scale. Thus, dyes of low fastness on a non‐protein, e.g. cotton, viscose rayon, cellulose acetate, or nylon, are likely to have a fastness about one or two grades higher on a protein, e.g. wool or silk (and perhaps Ardil fibre); and those of high fastness on a non‐protein are likely to be graded one or two units lower on a protein.Some suggestions are made of means by which light fastness might be improved: e.g. better fastness is more likely to be obtained by eliminating all small dye particles in the fibre than by encouraging larger ones to grow if small ones are also present. This is because the smaller particles tend to fade more rapidly than the larger, and therefore their fading rate determines the fastness grade of the dyed material, which is judged in the earlier stages of fading. Some types of dye molecule likely to exhibit high fastness are suggested.The reliability of the fastness data is discussed. Fastness assessments given by different manufacturers for the same dye appear to agree for wool dyes, but Continental assessments for dyes on cotton are usually about one unit higher than those given in this country. This is believed to be due to longer periods of sunshine on the Continent.