Colour strength
K/S value of a dyed material has a close relationship to the amount of dye absorbed by the fabric. K/S values of cotton/nylon dyed samples with acid dyes are shown in Table 2. It was observed that the color measurements of untreated cotton/nylon fabric have the lowest values. This was because cotton fibers when immersed in water produce a negative zeta potential. The negative charge on the fiber repels the acid dye ions and consequently the exhaustion of the dye bath was limited which lead to the decrease of the color measurements. The color measurements of cotton/nylon blends increased with the plasma- nano chitosan pretreatment, it can be concluded that the K/S values of chitosan treated dyed fabrics are higher than that of untreated sample. This enhancement in K/S values of chitosan treated cotton/nylon fabrics shows that the chitosan has an incremental effect in dyeing processes. The improved dye ability is related to the presence of amine groups available from the chitosan. Increasing the number of active functional groups due to plasma activation in the cotton/nylon surface enabled the adsorption of higher amounts of chitosan and, consequently, a higher amount of amino groups responsible for dyeing. It has to be pointed out that several past researches showed that, in some cases, when chitosan interacts with non-activated cellulose, adsorption could also be irreversible (Cakara et al. 2009). That irreversible adsorption of chitosan onto weakly acidic cotton fabric is, under present conditions, predominately driven by a non-electrostatic attraction. Myllyte et al. (2009) evidenced a non-electrostatic interaction between chitosan and cellulose. This may be attributed to specific structural nteraction between chitosan and cellulose (H-bonds and hydrophobic interactions). Under acidic condition, protonation of the carbonyl group oxygen atom of amide groups generates new cationic sites for dye adsorption. The higher amount of amino groups in plasma activated nano chitosan treated samples increased the probability that a protonated amino group met electrostatic bond with acid dye anions under acidic conditions. Once a dye anion with moderate substantively adsorbs onto an ammonium ion site in the cotton nylon, it is quite resistant to displacement. The dye–fiber interaction must involve forces other than the attraction of oppositely charged ions. Obviously, dipole–dipole and hydrophobic interactions between the dye and nylon molecules play an important role in determining the high substantively and good washing fastness of acid dyes.