Application Of Chitosan Derivatives In Wash-off Reactive Cotton Dyeings

The consumption of reactive dyes is on the rise and is expected to remain so in the near future for their acceptable price,good color values and reasonably good fastness properties.However,the reactive dyes suffer the disadvantage of incomplete exhaustion and hydrolysis up to 20～40%during dyeing and hence there is considerable loss of dyes. Additionally,unreacted and hydrolyzed reactive dyes can adhere to the substrate physically decreasing the fastness,because the hydrolyzed dyes are usually similar to the original dye in diffusion and adsorption properties.When applying reactive dyes in full depths,especially inefficient reactive dyes,the effectiveness of fixation may decrease.As a consequence,in order to achieve optimum fastness,the resultant dyeings need to be thoroughly washed-off to remove the unfixed dyes.Water-soluble polymers have been frequently applied in the formulation of soaping agent in wash-off reactive dyeings,in order to inhibit the restaining of released dyes to cotton fabrics.For this scheme,chitosan,a natural and readily biodegradable material, was modified through the introduction of carboxymethyl groups,amino carboxylic acid, hydroxyl carboxylic acid,lactam groups and alkyl groups to prepare N, O-carboxymethyl chitosan(NOCC),N-ethylenediaminetetraacetic acylated chitosan (ETC),N-nitrilotriacetic acylated(NTC),N-citrylated chitosan(CAC),5-methyl pyrrolidinone chitosan(MPC) and N-isobutyl chitosan(IBC),respectively.The six chitosan derivatives were applied to wash-off reactive cotton dyeings and determined the effectiveness in removing unfixed reactive dyes.By comparing the performances of these chitosan derivatives,it showed that the carboxyl substituted derivatives exhibited the higher chelation,dispersion and pH buffering,and the derivatives substituted with lactam and alkyl groups exhibited a general dispersion capacity but poor chelation capacity.For the derivatives with the same substituted groups,increase the molecular weight can strength their chelation and dispersion capacity,but the too high molecular weight leads to the decrease of chelation and dispersion capacity.The chelation and dispersion effectiveness of chitosan derivatives increased with the increase of degree of substitution values of carboxyl groups.Three problems associated with the use of chitosan derivatives in the wash-off process viz.,the substituted groups,degree of substitution and molecular weight,were investigated.The results indicated that the wash-off performance with the carboxyl substituted derivatives of chitosan was better than lactam and alkyl substituted ones. The wash-off effectiveness of chitosan derivatives increases with the increase of the degree of substitution of carboxyl groups,but the too high carboxyl contents lead to increase the electrostatic repulsion between dye molecules and chitosan derivatives and result in a lower wash-off effectiveness.For MPC and IBC,the wash-off effectiveness also increases with increasing the degree of substitution.The results from wash-off tests with different molecular weight of chitosan derivatives suggested that increase in molecular weight of chitosan derivatives can increase the wash-off effectiveness; however,the results also showed that too high molecular weight produced a sharp reduction in wash-off effectiveness because the amount of effective groups of chitosan derivatives to combine with dyes decreased.Surface tension studies on the interactions of different types of surfactants in the presence and absence of NOCC respectively,were also conducted.It was showed that the presence of NOCC slightly increased the surface tension and critical micelle concentration value of dodecyl dimethyl betaine(DDB).This result can illustrate that electrostatic attraction between the molecules of NOCC and DDB exist,forming hydrophilic complexes soluble in the solution.NOCC acted as a polyelectrolyte increase the number of counter ions in the Stern and diffuse layers,screening the electrostatic repulsion between the head groups of SDS,which can illustrate the slight decrease of surface tension and critical micelle concentration value of SDS upon addition of NOCC. Octadecyl polyoxyethylene ether(C₁₈EO₂₀) and NOCC can bind to form hydrophilic complexes due to hydrogen bonding,which can account for the significant decrease of surface tension below critical micelle concentration and critical micelle concentration value of the solution containing NOCC.The surface activity acted in the order: NOCC-SDS＞NOCC-DDB＞NOCC-C₁₈EO₂₀.Spectroscopic studies on the hydrolyzed Reactive Turquoise Blue KN-G(RBE21) in surfactant solutions showed that upon addition of NOCC,synergistic solubilization by NOCC and surfactants on the hydrolyzed RBE21 was observed.In higher concentration,NOCC-DDB showed higher solubiliztion ability to hydrolyzed RBE21 than NOCC-C₁₈EO₂₀,and in lower concentration correspondingly less.However,the hydrolyzed RBE21 was slightly solubilized in NOCC-SDS micelles by adoption of the aggregate forms.In wash-off process,NOCC and surfactants acted on released dyes and cotton fibers and would not deposit on cotton fibers after sufficient wash with water.The effects of inhibiting hydrolyzed RBE21 from staining cotton fabrics were found to be in the order: NOCC-C₁₈EO₂₀＞NOCC-DDB＞NOCC-SDS.The synergy effects of NOCC and surfactants in wash-off tests showed that the wash-off performance was in the same order with solubilization effects.This result confirmed that the wash-off process of reactive dyeings with NOCC-surfactant complexes was dominated by solubilization effect.Investigations have been made of the adsorption of different hydrolyzed reactive dyes to cotton fabrics to establish key factors governing dye transfer in wash-off reactive dyeings.The results showed that for the reactive dyes with same reactive groups, possessing chromophores of azo-copper complex,copper phthalocyanine and anthraquinone,were found to be higher affinity for cotton fabrics.Heterocyclic azo dyes showed higher affinity to cotton fabric than other azo dyes.Adsorption to cotton fabric increased with apolar substituent on dyes,implicating entropic factors dominated the binding process.Dyes with triazine reactive groups tended to increase the adsorption to cotton after they were hydrolyzed,while hydrolysis was likely to be opposing contibutions for dyes with vinyl sulfone reactive groups.These studies help to illustrated that the hydrolyzed dyes with triazine reactive groups increased the difficulty of wash-off process,while the hydrolyzed dyes with vinyl sulfone reactive groups can easily be washed-off.The influence of sulphonate groups in different pH solutions showed that adsorption to cotton was strongly influenced by hydrophobic interactions.These studies help to highlight the role of solubilization effects in wash-off process.Chitosan derivatives were extremely effective at preventing adsorption of released dyes in washing bath to cotton and they tended to bind dyes more by electrostatic,polar or hydrogen bonding interactions rather than hydrophobic interactions.A kind of dye absorbent was prepared from the combination of N-hydroxypropyl trimethylammonium chloride chitosan(CTA-CTS) and Ca-bentonite and was employed as builders in wash-off unfixed dyes from RBE21 dyed cotton fabric.CTA-CTS loaded bentonite was studied by means of IR,XRD and SEM.The factors influencing wash-off effectiveness,including the amount of CTA-CTS and the achievable degree of substitution and molecular weight were investigated.The results show that the optimum weight ratio of chitosan to bentonite was 1:5,DS=0.93,Mw=100.0 kDa.The experiments demonstrated that the reactive dyeings washed with this absorbent can reach the soaping fastness level of ordinary soaping agent;meanwhile the residual washing liquor had ca.1/10 absorbance and COD_Cr values of ordinary ones.